Nanomaterials For Energy and Sensor Applications

Nanomaterials for Energy and Sensor Applications
Nanomaterials for Energy

and Sensor Applications
Editors
Vidya Nand Singh
CSIR-National Physical Laboratory
New Delhi, India
Sunil Singh Kushvaha
CSIR-National Physical Laboratory
New Delhi, India
Cover credit: Image reproduced by kind courtesy of the authors of Chapter 4.
First edition published 2024

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ISBN: 978-1-032-39700-9 (hbk)
ISBN: 978-1-032-39702-3 (pbk)
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DOI: 10.1201/9781003350965
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Preface
The book, “Nanomaterials for Energy and Sensor Applications,” edited

by Dr. Vidya Nand Singh and Dr. Sunil Singh Kushvaha and published
by CRC Press, Taylor & Francis Group, LLC, is the result of joint
efforts of some well-known researchers in the field. Nowadays, most
of the research work focuses on nanomaterials in which one of the
dimensions falls in the range of 1–100 nm. These nanomaterials can
be in the form of thin films, quantum dots, nanowires, nanopyramids,
and nanoclusters. Thus, nanomaterials are touching almost all aspects
of materials for various applications in the field of emerging energy and
sensor devices. The book comprises ten chapters and discusses various
aspects of nanomaterials for their application in the field of energy,
solar cells, water splitting and sensors, etc. The book aims to cater to
the need of budding researchers in the field of synthesis and application
of nanomaterials and post-graduate students.
Chapter 1 provides a brief introduction to nanomaterials used for
energy storage devices such as supercapacitors, pseudo-capacitors,
etc. This chapter summarized various carbon nanofibers based
nanostructured, e.g., hollow, porous, porous-hollow structures to
enhance the electrochemical performance of supercapacitors. Moreover,
it discusses the role of mesopore volume and total pore volume in
determining the electrochemical performance of supercapacitors.
The need for renewable energy sources for the sustainable growth of
human society is one of the key parameters. The various nanocomposite
development tools for energy production have been discussed in
Chapter 2. Furthermore, it presents several methods of harvesting energy
such as hydropower resources, ocean and tide energy, wind energy,
solar power, etc., and summarized the several applications of energy
harvesting devices. Additionally, this chapter also provides insight into
futuristic flexible electronics by using flexible polymer-based materials.
iv Nanomaterials for Energy and Sensor Applications
Chapter 3 explores silicon (Si) based nanostructures for solar energy

conversion applications and also discusses the electronic and optical
properties of Si nanostructures. This chapter provides wide synthesis
procedures to prepare Si nanostructure such as the vapor-liquid-solid
(VLS) method, chemical vapor deposition (CVD), electrochemical
etching, electropolishing, etc. It explores the merit and demerits of
Si nanostructures and provides a possible solution to overcome their
limitations.
The hydrogen production by photoelectrochemical (PEC) water
splitting technique is one of the ideal techniques to utilize the solar
spectrum. Chapter 4 discusses the need for highly efficient photocatalysts
to hunt the PEC technology in a commercial platform. Further, it
explores the role of binary chalcogenides material in PEC application.
The prepared film is characterized by X-ray diffraction (XRD), Raman
spectroscopy, field emission scanning electron microscopy (FESEM), and
X-ray photoelectron spectroscopy (XPS). A few methods to enhance the
PEC performance of Bi2Se3 material by designing various heterostructures,
doping of co-catalyst, Z-scheme structure, and p-n junction etch have
been also discussed. Moreover, flexible photoelectrode can be beneficial
to large-scale roll-to-roll PEC device fabrication and other applications
such as flexible solar cells, flexible thermoelectric generators, etc.
Chapter 5 provides a review of the quantum-cutting phosphors
phenomenon and their application in various solar thermal devices,
such as CRT tubing, plasma display, light bulbs, and X-ray conversion
screens, etc. and also in various biomedical applications (such as tumor
monitoring, diagnosing ischemia, etc.) This chapter highlights various
key point which affects the thermal stability of phosphors and gives
appropriate solution to overcome these limitations.
Chapter 6 is a review of the recent development of a flexible sensor
for their practical use in wearable electronics, artificial intelligence, etc.
Further, the author introduces sensor parameters and the basic working
mechanism of various types of sensors. This chapter also discusses many
fabrication methods and various kinds of flexible substrates polyimide,
PET, PEN, etc. to fabricate flexible sensors to development of futuristic
flexible electronics.
Chapter 7 presents the recent insight into Pb and Pb-free organic-
inorganic metal halide (OIMH) perovskite materials. In this chapter, the
authors present the pros or cons of Pb and Pb free (OIMH) perovskite
materials and give appropriate solutions to tackle their limitations,
such as stability and efficiency, etc. Furthermore, this chapter provides
different methods for improving the stability and quantum efficiency
of Pb-free as compared to Pb-based perovskite inks synthesized by
different solution-processed colloidal methods.
Preface v
The advantage of dye sensitization in solar cell (DSSCs) application

to increase the solar conversion efficiency of photoelectrodes is one of
the innovative approaches. Chapter 8 presents the construction of DSSCs
and details of working electrodes, including conductive substrates,
compact layers, mesoporous active layers, etc. The preparation of TiO2
powders, doping, composite structures, junction formations, and unique
and mixed morphological influences on the DSSC device performance
was also reviewed. At last, the authors ended this chapter by considering
how DSSC devices integrated into modules from small scale to large
scale for real-world applications.
Chapter 9 is mainly focused on the need for renewable energy
sources and their importance for the sustainable development of human
society. It provides a brief introduction to hydrogen production via
photocatalytic (PC) and its working mechanism. Various semiconductors,
e.g., metal oxides, like titanates, niobates, etc., have been discussed
for PC applications. To increase the efficiency of photocatalysts in the
PC process, various engineering processes such as doping, forming
heterojunctions, and co-catalysts have been widely discussed in the
literature.
Chapter 10 deals with a fundamental understanding of the
correlations development of nanofluids for heat transfer systems and
energy applications. It discusses various nanomaterials for widespread
applications of the nanofluidic, the mechanism of heat transfer in
nanofluids, and also discusses key parameters that affect the heat transfer
behavior in nanofluids. Moreover, the challenges and sustainability
aspects of nanofluids in heat transfer applications have been discussed
in detail for large-scale implementation in commercial applications.
We would like to thank all those who kindly contributed chapters to
this book. We are also indebted to the editorial office and the publishing
and production teams at CRC Press for their assistance in preparing and
publishing this book. Finally, we hope that the efforts of various authors
will be helpful to the research community.
Vidya Nand Singh

Acknowledgments
First and foremost, we humbly express our gratitude to the Omnipotent

God for showering us with his blessings throughout this venture.
Without His grace, this book could not have become a reality.
We would like to take this opportunity to thank everyone who
has been instrumental in the successful completion of this book. We
are especially grateful to all the authors who have made significant
contributions to this book. Their timely inputs and willingness to
respond to every query aided us in undertaking and accomplishing
this challenging task. By presenting their work on the latest areas of
nanomaterials and their applications in energy and sensor devices, all
the authors have justified their contribution to this book, for which we
are truly indebted.
We would like to extend our sincere thanks to Prof. Venugopal
Achanta, Director, CSIR-National Physical Laboratory, New Delhi (India),
for his constant support and encouragement in creating an academic and
research environment that fosters higher education. We would also like
to thank Dr. Nahar Singh and Sh. J.C. Biswas for their kind support
whenever we needed it.
We are equally thankful to the editorial team of CRC Press, Taylor
& Francis, for their efforts in bringing the book to its present form and
ensuring its timely publication.
Vidya Nand Singh

Contents
Preface iii
Acknowledgments vii
Chapter 1. Porous and Hollow Carbon Nanofibrous Electrode
Materials from Electrospinning for Supercapacitor
Energy Storage 1
Kingsford Asare, Md Faruque Hasan,
Abolghasem Shahbazi and Lifeng Zhang
Chapter 2. Energy and Sensor Applications of Polymer
Nanocomposites 21
Ankit Kumar Srivastava, Swasti Saxena and
Surendra K. Yadav
Chapter 3. Nanostructured Silicon for Solar Energy Conversion
Applications 54
Ragavendran Venkatesan, Jeyanthinath Mayandi,
Terje G. Finstad, J.M. Pearce and
Vishnukanthan Venkatachalapathy
Chapter 4. Selenium-Based Metal Chalcogenides Thin Films
on Flexible Metal Foils for PEC Water-Splitting
Applications 94
Bheem Singh, Sudhanshu Gautam,
Vishnu Aggarwal, Rahul Kumar,
Vidya Nand Singh and Sunil Singh Kushvaha
Chapter 5. Quantum-Cutting Phosphors for Thermal Sensor
Applications 123
Abhijit Jadhav
x Nanomaterials for Energy and Sensor Applications
Chapter 6. A Review of Flexible Sensors 137

Surendra Maharjan and Ahalapitiya H. Jayatissa
Chapter 7. The Transition from Pb- to Pb-Free Halide-Based
Perovskite Inks for Optoelectronic Applications 154
Sonali Mehra, A.K. Srivastava and
Shailesh Narain Sharma
Chapter 8. Impacts of Working Electrode Parameters on
Dye-Sensitised Solar Cell Performance 189
V. Sasirekha, J. Mayandi, J. Vinodhini, R. Selvapriya,
P. Jayabal, V. Ragavendran and J.M. Pearce
Chapter 9. Nanostructured Metal Oxides for Photocatalytic
Water Splitting 209
Anu Kumari, Shaswati Jyoti and Sonalika Vaidya
Chapter 10. Nanofluidics for Heat Transfer System and Energy
Applications 227
Nagendra S. Chauhan
Index 259
Chapter 1
Porous and Hollow Carbon

Nanofibrous Electrode
Materials from Electrospinning
for Supercapacitor Energy
Storage
Kingsford Asare1, Md Faruque Hasan1,

Abolghasem Shahbazi2* and Lifeng Zhang1*
¹Department of Nanoengineering, Joint School of Nanoscience and
Nanoengineering, North Carolina A&T State University, 2907 E Gate City Blvd,
Greensboro, NC 27401, USA.
2
Department of Natural Resources and Environmental Design, College of
Agriculture and Environmental Sciences, North Carolina A&T State University,
1601 E Market St, Greensboro, NC 27411, USA.
1.1 IntroductIon to SupercapacItor
Electrical energy storage is essential in modern society and spans uses

from consumer electronics to motorized vehicles. Among all electrical
energy storage devices, supercapacitors have demonstrated their great
promises in the energy storage market owing to their high power
*For Correspondence: Lifeng Zhang (lzhang@ncat.edu), Abolghasem Shahbazi

(ash@ncat.edu)
2 Nanomaterials for Energy and Sensor Applications
density, faster charge and discharge rates, and cycle stability [1]. They
are best employed for devices that require high current in a short time.
According to the mechanism of charge storage, supercapacitors can be
classified into three categories: electric double-layer capacitors (EDLC),
pseudocapacitors, and hybrid capacitors. EDLC stores electrical charges at
the interface between the electrode and electrolyte upon ion adsorption
when a voltage is applied. The capacitance of EDLC is proportional to
the accessible surface area of its electrode. EDLC can provide ultrahigh
power and excellent cycle stability due to the fast and non-destructive
process between electrode and electrolyte [2, 3]. Pseudocapacitor stores
electrical charges by making use of fast and reversible redox reactions at
the interface between the electrode surface and electrolyte. Charges that
are associated with the redox reaction transfer across the abovementioned
interface and show pseudocapacitance [4]. Pseudocapacitors can have
greater specific capacitance and energy density than EDLC, but they
have lower power performance and rate capability. The hybrid capacitors
combine both EDLC capacitance and pseudocapacitance with improved
electrochemical performance.
1.2 electroSpun carbon nanofIbrouS

MaterIalS for SupercapacItor
electrode
The electrochemical performance of supercapacitors relies on quite a few
factors like electrode materials, electrolytes, and voltage, among which
the characteristics of electrode material are the most critical. Carbon has
been a choice of supercapacitor electrode material due to its excellent
electrical conductivity, chemical and thermal stability, and low cost [5].
A variety of carbon-based electrode materials have been investigated so
far including activated carbon, carbon nanotube, carbon cloth, carbon
aerogel, carbide-derived carbon, etc.
In recent years, carbon nanofibers from electrospinning have been
extensively studied as promising electrode materials for supercapacitors
[6–8]. Unlike conventional fiber spinning techniques, like wet spinning
and dry spinning, electrospinning utilizes electrical driving force and
has a unique thinning mechanism (whipping instability) [9]. As a result,
fibers with diameters at least one order of magnitude smaller than
conventional fibers are obtained from electrospinning. In general, the
preparation of electrospun carbon nanofibers (ECNFs) follows three
steps: electrospinning, stabilization, and carbonization. Firstly, a carbon
precursor polymer typically polyacrylonitrile (PAN), the most popular
carbon precursor polymer with high carbon yield, is dissolved in N,
Porous and Hollow Carbon Nanofibrous Electrode Materials ... 3
N-dimethyl formamide (DMF) to make an electrospinning solution

(spin dope). The solution is then electrospun to obtain PAN nanofibers.
Secondly, the electrospun PAN nanofibers are heated to 200–300 °C
in air to cyclize PAN molecules. The PAN cyclization reaction can
facilitate its formation of a ladder molecular structure in carbonization
process and thus reduce mass loss and dimension shrinkage therefrom.
Thirdly, the stabilized PAN electrospun nanofibers are further heated
to a temperature over 800 °C in an inert atmosphere to acquire ECNFs
upon H and N extraction. The final ECNFs hold an advantage of
conductive network structure for charge transfer and can be used as
a stand-alone and binder-free electrode material, which warrants better
electrochemical performance of the resultant supercapacitor electrode.
1.3 porouS/Hollow carbon nanofIbrouS

electrode
It is noted that the ECNFs from PAN alone have a solid structure, possess
relatively low specific surface area, and correspondingly present a low
EDLC capacitance as supercapacitor electrode material [8]. In order to
improve the EDLC capacitance of ECNFs, a lot of efforts have been carried
out to improve the specific surface area of ECNFs by creating porous
or hollow structures [10]. For example, porous ECNFs were prepared
by in-situ activation of ECNFs with chemical agents (such as H3PO4
[11]), integration of nanoscale templates (such as CaCO3 nanoparticles
in ECNFs followed by selective removal [12]), inclusion of sacrificial
component under high temperature (such as poly(methyl methacrylate)
(PMMA) with PAN in electrospinning [13]), or use of PAN-based block
copolymers (such as poly(acrylonitrile-block-methyl methacrylate) (PAN-
b-PMMA) for electrospinning and subsequent carbonization [14]). At
the same time, electrode materials with 1D hollow nanostructures have
demonstrated their advantages in supercapacitor applications by serving
as “ion-buffering reservoirs” and reducing the diffusion path of electrons
and ions [15, 16]. Co-axial electrospinning is a common and convenient
electrospinning technique to make core-shell or hollow carbon nanofibers
[17]. In co-axial electrospinning, precursor solutions for core and shell
components are fed separately and simultaneously to the corresponding
inner and outer nozzle, which are concentrically aligned as co-axial
spinneret, and result in core-shell electrospun nanofibers. By selectively
extracting the core component, hollow nanofibers are attained. Hollow
ECNFs, for example, can be acquired by co-axial electrospinning with
PAN solution as a shell solution and sacrificial PMMA solution as a
core solution [18]. In recent years, hollow ECNFs as well as porous

hollow ECNFs have been developed through co-axial electrospinning
and explored as supercapacitor electrode materials [18–20]. Compared to
solid ECNFs, these porous and/or hollow ECNFs exhibited improvement
in electrochemical performance as electrode materials for supercapacitor
applications.
1.4 coMparatIve Study of porouS and

Hollow carbon nanofIbrouS electrode
applIcatIon
It is noteworthy that all other research about the electrochemical

performance of porous and/or hollow ECNFs only reported one type
of carbon nanofibrous structure, i.e. porous, hollow, or hollow porous,
at a time and the electrochemical data from these reports are not
directly comparable due to different electrospinning conditions and
various electrochemical stations for electrochemical measurements. It
is unknown if a certain 1D carbon nanofibrous structure outperforms
another in electrode material for supercapacitors. For example, hollow
ECNFs might outperform porous ECNFs as supercapacitor electrode
material due to the “ion-buffering reservoir” effect while hollow porous
ECNFs might outperform hollow ECNFs or porous ECNFs as electrode
material for supercapacitor due to their chance to take advantages of
both hollow and porous nanostructures. However, there is no such
conclusion or evidence yet. To verify this hypothesis, we prepared
four types of supercapacitor electrode materials with different carbon
nanofibrous structures, i.e. solid, porous, hollow, and hollow porous
ECNFs, respectively, and conducted a side-by-side comparison for
their electrochemical performance as electrode materials for EDLC
supercapacitor. The findings herein are expected to provide an in-depth
understanding of the relationship between 1D carbon nanofibrous
structures from electrospinning including solid, porous, hollow, and
hollow porous nanostructures and their corresponding electrochemical
performance as supercapacitor electrode materials, which can benefit the
advances of electrode materials for high-performance supercapacitors.
1.4.1 preparation of electrode Materials

Based on the fact that PAN and PMMA have phase separation in their
bicomponent nanofibers and PMMA can decompose completely under
heat and generate only volatile products as a sacrificial component
[21], four types of carbon nanofibrous materials including solid carbon

nanofibers (ECNFs), porous carbon nanofibers (P-ECNFs), hollow
carbon nanofibers (H-ECNFs), and hollow porous carbon nanofibers
(HP-ECNFs) were prepared in this research through electrospinning
and co-axial electrospinning followed by stabilization and carbonization
[22]. Specifically, ECNFs were prepared from electrospinning a 12 wt.%
PAN (Mw = 150,000) DMF solution. P-ECNFs were prepared from
electrospinning a 12 wt.% PAN/PMMA (Mw = 120,000) DMF solution
with PAN/PMMA compositions at 90/10, 70/30, and 50/50 and denoted
as P-ECNF-90-10, P-ECNF-70-30, and P-ECNF-50-50, respectively. In
both cases, the electrospinning was conducted at 15 kV with a solution
feed rate of 1 mL/h. To make H-ECNFs and HP-ECNFs, a co-axial
electrospinning setup was used with certain polymer solutions for
shell and core components, respectively. In the co-electrospinning
for H-ECNFs, the shell solution was fixed with 12 wt.% PAN DMF
solution while the core solutions were PMMA DMF solutions with
varied PMMA concentrations at 10 wt.%, 20 wt.%, and 30 wt.% and
labeled as H-ECNF-10, H-ECNF-20, and H-ECNF-30, respectively. In the
co-axial electrospinning for HP-ECNFs, the core solution was fixed with
20 wt.% PMMA DMF solution while the shell solutions were 12 wt.%
PAN/PMMA DMF solutions but with varied PAN/PMMA compositions
at 90/10, 70/30, and 50/50 and labeled as HP-ECNF-90-10, HP-ECNF-70-30,
and HP-ECNF-50-50, respectively. All the co-axial electrospinning was
conducted at 15 kV with a feeding rate of 1.5 mL/h for shell solution
and 1 mL/h for core solution. Stabilization of as-spun nanofibrous mats
was carried out in air at 280 °C for 6 hours with a heating rate of
1 °C/min from room temperature. The stabilized nanofibrous mats were
cooled down to room temperature and then re-heated in a nitrogen
atmosphere to 900 °C at a heating rate of 5 °C/min. The nanofibrous
mats were further maintained at 900 °C for 1 hour to be fully carbonized
before cooling down to room temperature. Herein, PMMA was used as
a sacrificial component and it completely decomposed in the process of
carbonization [21] for hollow and porous nanostructures as described.
1.4.2 electrode Materials characterization

1.4.2.1 Morphology
The respective morphology of the carbon nanofibrous electrode materials
was examined using a Zeiss Auriga field emission scanning electron
microscope (FESEM). ECNFs were solid fibers with an average diameter
of 1034 ± 95 nm (Figure 1.1A). P-ECNFs showed porous structure both
on the surface and inside (Figure 1.1B–1.1D). At PAN/PMMA = 50/50,
elongated pores and/or short channels were observed. With the increase
of PMMA content from 10 wt.% to 50 wt.% in PAN/PMMA bicomponent
fibers, the average fiber diameter reduced from 998 ± 91 nm at 10 wt.%
PMMA to 679 ± 72 nm at 30 wt.% PMMA and to 565 ± 46 nm at 50
wt.% PMMA. H-ECNFs presented a hollow structure as expected. With
the increase of PMMA concentration from 10 wt.% to 30 wt.% in the
core solution of co-axial electrospinning for H-ECNFs, the average fiber
diameter increased from 513 ± 48 nm at 10 wt.% PMMA to 921 ± 89 nm at
20 wt.% PMMA and to 1679 ± 94 nm at 30 wt.% PMMA, respectively
(Figure 1.1E–1.1G). The wall thickness of these hollow fibers was similar,
i.e. 150–180 nm. HP-ECNFs showed a combined hollow and porous
morphology, i.e. hollow fibers with porous walls (Figure 1.1H–1.1J).
With the increase of PMMA content from 10 wt.% to 50 wt.% in the
shell solution of co-axial electrospinning for HP-ECNFs, the average
fiber diameter reduced from 1588 ± 85 nm at 10 wt.% PMMA to
782 ± 52 nm at 30 wt.% PMMA and to 503 ± 59 nm at 50 wt.% PMMA
with wall thickness reduction from ~200 nm to ~100 nm, correspondingly.
It is well known that PMMA completely degrades and gasifies in a
temperature range of 300–400 °C [21]. In hollow carbon nanofibers, the
fiber size, porous structure, hollow channel size, and wall thickness are
determined by both an inflation effect due to the discharge of a relatively
large volume of volatile PMMA degradation/gasification products and a
volume shrinking effect due to PMMA removal and PAN carbonization
in the process of carbonization. Polymer-polymer phase separation in
the PAN/PMMA bicomponent as-electrospun nanofibers occurs and
domains of PMMA in the bicomponent nanofibers can completely
decompose during carbonization. A larger proportion of PMMA would
lead to a larger volume shrinking in carbonization and reduce average
fiber size in the case of P-ECNFs. In the meantime, a larger proportion
of PMMA could form more elongated domains under electrical driving
force in the electrospinning process and result in a “short channel”
structure in the carbonized fibers. In the case of H-ECNFs, a larger
amount of PMMA in the core of the core-shell PAN/PMMA carbon
precursor nanofibers from higher PMMA concentration in the core
solution of the co-axial electrospinning could generate a larger volume
of volatile products upon degradation/gasification in the process of
carbonization, counteract the fiber contraction in that process, and thus
resulted in larger hollow channels. As for HP-ECNFs, the porous wall
structure of hollow nanofibers could reduce the thickness as well as
mechanical strength of the fiber walls and lead to huge size shrinkage/
collapse in the process of carbonization. With PAN/PMMA = 50/50 in
the shell solution of the co-axial electrospinning for HP-ECNFs, some
long and open slits on the fiber surface were observed.
Figure 1.1 Representative SEM images of electrospun carbon nanofibrous

materials: (A) ECNFs; (B) P-ECNF-90-10; (C) P-ECNF-70-30; (D) P-ECNF-50-50;
(E) H-ECNF–10; (F) H-ECNF-20; (G) H-ECNF-30; (H) HP-ECNF-90-10; (I)
HP-ECNF-70-30; (J) HP-ECNF-50-50 [22].
1.4.2.2 Structure
Raman spectroscopy was used to characterize carbon structure of
the electrospun carbon nanofibrous materials and was done at room
temperature using a Horiba Raman Confocal Microscope at an
excitation wavelength of 532 nm. In Raman spectra of all the carbon
nanofibrous materials (Figure 1.2), the “D-band” between 1,297 and
1,355 cm–1 corresponded to the sp3 hybridized disordered carbonaceous
structures while the “G-band” between 1,548 and 1,591 cm–1 indicated
the sp2 hybridized graphitic phase of carbon [23]. The intensity ratio of
G-band to D-band (IG/ID) was used to characterize the carbon structure
of these carbon nanofibrous materials. Among all the studied carbon

nanofibrous materials, ECNFs showed the least IG/ID value, indicating
the least ordered carbon structure. The use of a larger amount of PMMA
for P-ECNFs, H-ECNFs, and HP-ECNFs resulted in a more ordered
carbon structure. Apparently, the addition of PMMA promoted PAN
carbonization. This promotion might be caused by PMMA’s hydroxyl end
groups, which could assist in the cyclization reaction of PAN molecules
via an ionic mechanism [24]. Under this circumstance, the activation
energy of the PAN cyclization reaction is lowered, the cyclization
reaction temperature is reduced, and the enthalpy of the cyclization
reaction is increased, consequently leading to the formation of a more
ordered carbon structure.
IG/ID
HP-ECNF-50-50 1.1391
Intensity (a.u.)
HP-ECNF-70-30 1.0464
HP-ECNF-90-10 1.0279
H-ECNF-30 1.0884
H-ECNF-20 1.0780
H-ECNF-10 1.0154
P-ECNF-50-50 1.0878
P-ECNF-70-30 1.0700
P-ECNF-90-10 1.0226
ECNF 1.0131
1000 1200 1400 1600 1800
Raman Shift (cm-1)

Figure 1.2 Raman spectra of the electrospun carbon nanofibrous materials [22].
BET surface area and porosity analyses of the electrospun carbon

nanofibrous materials were performed by using a Micromeritics ASAP
2020 Surface Area and Porosity Analyzer. The N2 adsorption isotherm
as well as pore size distribution based on BJH adsorption of the
electrospun carbon nanofibrous materials are shown in Figure 1.3. The
N2 adsorption curves of the electrospun carbon nanofibrous materials
can be classified as IUPAC Type II. The increase of N2 adsorption in low-
pressure region (P/P0 < 0.1) indicated N2 adsorption in micropores, while
the large increase of N2 adsorption in high-pressure region (P/P0 > 0.9)
indicated N2 adsorption in mesopores [25]. The N2 adsorption curves,
as well as pore size distribution results, suggested that P-ECNF-90-10,
P-ECNF-50-50, H-ECNF-30, and HP-ENCF-90-10 possessed relatively
large amounts of micropores while all the porous, hollow, and hollow
porous samples had a significant amount of mesopores.
200 0.35
A B ECNF
180 ECNF
P-ECNF-90-10
P-ECNF-90-10 0.30 P-ECNF-70-30
Quantity Adsorbed (cm³/g STP)
160 P-ECNF-70-30
P-ECNF-50-50
P-ECNF-50-50
0.25 H-ECNF-10
140 H-ECNF-10
dV/dlog(w) (cm³/g)
H-ECNF-20
H-ECNF-20
120 H-ECNF-30
H-ECNF-30 0.20 HP-ECNF-90-10
HP-ECNF-90-10
100 HP-ECNF-70-30
HP-ECNF-70-30
0.15 HP-ECNF-50-50
HP-ECNF-50-50
80
60 0.10
40
0.05
20
0 0.00
0.0 0.2 0.4 0.6 0.8 1.0 0 50 100 150 200 250 300
Relative Pressure (P/Po) Pore Width (nm)
Figure 1.3 N2 adsorption isotherms (A) and pore size distributions (B) of
electrospun carbon nanofibrous materials. The pore size distribution was
obtained via the BJH adsorption [22].
According to BET-specific surface area and porosity analysis results

(Table 1.1), ECNFs showed the lowest specific surface area and the smallest
pore volume, indicating a solid fiber structure. P-ECNFs demonstrated
the largest specific surface area of 146 m2/g in the case of PAN/PMMA =
90/10 in electrospinning solution. However, the corresponding pore
volume was the smallest among the three P-ECNF samples. This is
probably caused by the micropore formation in P-ECNF-90-10. At this
low proportion, PMMA formed much smaller domains in PAN/PMMA
bicomponent nanofibers and resulted in the largest micropore volume
upon degradation/gasification. With the increase of PMMA content in
the electrospinning solution, the corresponding micropore volume of
P-ECNFs reduced, while its mesopore and macropore volumes increased.
Although P-ECNF-50-50 possessed the largest pore volume of 0.2763
cm3/g, the largest proportion of its pores was mesopores and its specific
surface area was still less than that of P-ECNF-90-10. Compared to
P-ECNF-90-10, P-ECNF-70-30 contained less micropore volume due to
larger PMMA phase separation domains in the PAN/PMMA bicomponent
precursor fibers. Compared to P-ECNF-50-50, P-ECNF-70-30 produced
less amounts of PMMA volatile products upon degradation/gasification
and created fewer pores, resulting in smaller pore volume (micropore,
mesopore, macropore, and total pore volumes) and thus smaller specific
surface area than that of P-ECNF-50-50. H-ECNFs possessed the least
pore volume compared to P-ECNFs and HP-ECNFs, indicating solid
walls in these hollow nanofibers. Meanwhile, the hollow space did
not contribute much to micro-, meso- or macro-porous structures. The
pore volumes of all H-ECNF samples were close and there were some
increases in total pore volume with the increase of PMMA concentration
in core solution of the co-axial electrospinning for H-ECNFs. The total
pore volumes of H-ECNFs, however, were still more than four times
that of ECNFs, indicating that the PAN/PMMA core-shell structure from

co-axial electrospinning did generate extra pores during carbonization.
This could be ascribed to two aspects: (1) the PAN shell solution and
the PMMA core solution at the interface of the Taylor cone in co-axial
electrospinning may partially mix because of the common DMF solvent.
This mixing led to very fine PAN-PMMA phase separation and resulted
in different levels of pores in final carbon nanofibers after carbonization;
(2) the release of a large volume of volatile products from PMMA
degradation/gasification in the process of carbonization could also create
pores. Compared to H-ECNF-20, the introduction of porous structure in
walls of hollow fibers increased specific surface area and pore volume
of HP-ECNFs. With the increase of PMMA proportion in shell solution
of the co-axial electrospinning, the micropore volume of HP-ECNFs
reduced with increase of mesopore, macropore, and total pore volumes.
The reduction of specific surface area of HP-ECNFs with increase of
PMMA proportion in shell solution of the co-axial electrospinning may
be attributed to shrinkage and collapse of the thinner and more porous
fiber walls in the process of carbonization.
Table 1.1 BET-specific surface area and porosity of electrospun carbon

nanofibrous materials [22]
Carbon BJH Average
S BET Vmicro Vmeso Vmacro Vtotal
Nanofibrous Pore Size
(m 2 /g) (cm3/g) (cm3/g) (cm3/g) (cm3/g)
Materials (nm)
ECNF 0.0006 0.0065 0.0053
9.4 0.0124 16.877
Pore proportion 4.8% 52.4% 42.7%
P-ECNF-90-10 0.0260 0.0637 0.0094
146 0.0991 3.582
P-ECNF-70-30 0.0021 0.1706 0.0476
45.9 0.2204 19.173
P-ECNF-50-50 0.01 0.2031 0.0633
130 0.2763 11.069
H-ECNF-10 0.0014 0.0324 0.0164
18.1 0.0503 13.756
H-ECNF-20 0.0023 0.0333 0.0188
26.3 0.0544 11.572
H-ECNF-30 0.0068 0.0422 0.0103
73.3 0.0592 5.738
HP-ECNF-90-10 0.0062 0.0722 0.0282
78.8 0.1066 8.604
HP-ECNF-70-30 0.0015 0.1580 0.0545
47.7 0.2141 18.132
HP-ECNF-50-50 0.0017 0.1877 0.0933
51.5 0.2827 24.790
Pore proportion 0.7% 66.4% 33%
SBET – BET specific surface area; Vmicro – micropore volume; Vmeso – mesopore volume;
Vmacro – macropore volume; Vtotal – total pore volume
1.4.3 Electrochemical Evaluation

The electrochemical performance of all the electrospun carbon nanofibrous
materials was evaluated by cyclic voltammetry (CV) and galvanostatic
charge-discharge (CD) tests in 6M KOH aqueous electrolyte. Specifically,
a square piece (1 cm × 1 cm) of each as-prepared carbon nanofibrous
mat was cut and weighed. The square piece was then firmly attached
to an Au working electrode using conductive carbon glue and placed in
6M KOH aqueous solution in combination with a reference electrode of
Ag/AgCl and a counter electrode of platinum rod to construct a three-
electrode electrochemical system, which was connected to a CHI660E
electrochemical workstation for electrochemical measurement. CV test
was conducted using a potential range of 0.0 to –0.8 V with a scan rate
of 5, 10, 20, 50, and 100 mV/s, respectively. The galvanostatic CD test
was performed at current densities of 0.5, 1, and 2 A/g, respectively, in
the potential range of 0.0 to –0.8 V.
The specific capacitance (Csp) was calculated based on CV analysis
from the following formula:
V2
Ú I (V )dV
V1
Csp =
2 ¥ m ¥ n ¥ DV
where I is current, m is mass of electrode material, n is scan rate, and
∆V = V2 – V1, which is the sweeping potential window (0.8 V) and CD
analysis from the following formula:
I ¥ Dt
Csp =
m ¥ DV
where I is discharge current, m is mass of electrode material, ∆t is
discharge time, and ∆V is potential window (0.8 V).
1.4.3.1 Specific Capacitance

CV profiles of all the carbon nanofibrous electrode materials at a scan
rate of 5 mV/s as well as their CD profiles at the current density of
0.5 A/g are shown in Figure 1.4. The specific capacitances of these
carbon nanofibrous electrode materials from both CV and CD tests
were compared in Figure 1.5 and these values generally matched
with each other, i.e. specific capacitance from CV (Figure 1.5A) vs.
specific capacitance from CD (Figure 1.5B). Compared to ECNFs, all
the carbon nanofibrous electrode materials with porous and/or hollow
nanostructures demonstrated better electrochemical performance. In
each type of electrospun carbon nanofibrous material, i.e. P-ECNFs,
0.8 0.8 0.8
12
A1 0.6
A2 0.6
A3
0.6
0.4 0.4 0.4
0.2 0.2 0.2
0.0 0.0 0.0
-0.2 -0.2 -0.2
-0.4 -0.4 -0.4

ECNF ECNF
Current density (A/g)

ECNF H-ECNF-10
-0.6 -0.6 -0.6 HP-ECNF-90-10
P-ECNF-90-10
H-ECNF-20 HP-ECNF-70-30
-0.8 P-ECNF-70-30 -0.8 -0.8
P-ECNF-50-50
-1.0 -1.0 -1.0
-0.8 -0.6 -0.4 -0.2 0.0 -0.8 -0.6 -0.4 -0.2 0.0 -0.8 -0.6 -0.4 -0.2 0.0
Potential (V) Potential (V) Potential (V)
0.0
B1 0.0 B2 0.0 B3
-0.2 ECNF -0.2 -0.2 ECNF

P-ECNF-90-10 HP-ECNF-90-10
P-ECNF-70-30 ECNF HP-ECNF-70-30
P-ECNF-50-50 H-ECNF-10 HP-ECNF-50-50
-0.4 -0.4 H-ECNF-20 -0.4
Potential (V)
Potential (V)
Potential (V)
H-ECNF-30
-0.6 -0.6 -0.6
-0.8 -0.8 -0.8

0 50 100 150 200 250 300 350 400 0 50 100 150 200 250 300 350 400 0 50 100 150 200 250 300 350 400
Time (s)
Time (s) Time (s)
Figure 1.4 CV (A) and galvanostatic CD (B) profiles of P-ECNFs (labeled as 1), H-ECNFs (labeled as 2), and HP-ECNFs (labeled as
3). ECNFs are used as a reference in all plots [22].
H-ECNFs, and HP-ECNFs, electrochemical performance increased with

the content of PMMA in electrospinning and co-axial electrospinning
solutions. With the largest PMMA content in electrospinning or co-axial
electrospinning solution for each type of carbon nanofibrous material, the
sequence of electrical performance is P-ECNF-50-50 > HP-ECNF-50-50 >
H-ECNF-30. P-ECNF-50-50 was further scanned at varied scan rates of
5–100 mV/s for CV test and examined with varied current densities at
0.5, 1, and 2 A/g, respectively, for CD test (Figure 1.6). With the increase
of voltage sweep rate, the CV curves retained their quasi-rectangular
shape. All the CD curves were almost linear and exhibited isosceles
triangles. All these results indicated excellent supercapacitor behavior
of P-ECNF-50-50.
120 120
50-50
A 50-50
B
100 100
70-30
Specific Capacitance (F/g)
Specific Capacitance (F/g)
70-30 50-50
30 50-50
80 80 30
60 20 60 20
10 70-30
2 2
10 70-30
90-10 90-10
90-10 90-10
40 40
1 1
20 20
0 0
ECNF P-ECNF H-ECNF HP-ECNF ECNF P-ECNF H-ECNF HP-ECNF
Electrospun carbon nanofibrous materials Electrospun carbon nanofibrous materials
Figure 1.5 Specific capacitances of electrospun carbon nanofibrous materials

including ECNFs, P-ECNFs, H-ECNFs, and HP-ECNFs from CV at 5 mV/s (A) and
galvanostatic CD at 0.5 A/g (B). ECNFs is used as a control of all the electrospun
carbon nanofibrous materials and marked with line 1 while H-ECNF-20 is used as
a control of all the HP-ECNF samples with a fixed core solution of 20 wt.% PMMA
in DMF and marked with line 2 [22].
10 A B
0.0
5
-0.2
Potential (V)
2 A/g
0 1 A/g
0.5 A/g
-0.4
-5
5 mV/s
10 mV/s
-10 20 mV/s -0.6
50 mV/s
100 mV/s
-15
-0.8
-0.8 -0.6 -0.4 -0.2 0.0
0 50 100 150 200 250 300 350 400
Potential(V) Time (sec)
Figure 1.6 CV (A) and galvanostatic CD (B) profiles of P-ECNF-50-50 at

different scan rates (A) and current densities (B) [22].
400 40
A1 A2 ECNF
350 35
P-ECNF-90-10
300 30 P-ECNF-70-30
P-ECNF-50-50
250 25
-Z''(ohm)
-Z''(ohm)
200 20
150 15
ECNF
100 P-ECNF-90-10 10
P-ECNF-70-30
50 5
P-ECNF-50-50
0 0
0 50 100 150 200 250 300 350 400 0 5 10 15 20 25 30 35 40
Z'(ohm) Z'(ohm)
400 40
B1 B2 ECNF
350 35
H-ECNF-10
30 H-ECNF-20
300
H-ECNF-30
250 25
-Z''(ohm)
-Z''(ohm)
200 20
150 15
ECNF
100 H-ECNF-10 10
H-ECNF-20
50 5
H-ECNF-30
0 0
0 50 100 150 200 250 300 350 400 0 5 10 15 20 25 30 35 40
Z'(ohm) Z'(ohm)
400 40
C1 C2 ECNF
350 35 HP-ECNF-90-10
30 HP-ECNF-70-30
300
HP-ECNF-50-50
250 25
-Z''(ohm)
-Z''(ohm)
200 20
150 15
ECNF
100 HP-ECNF-90-10 10
HP-ECNF-70-30
50 5
HP-ECNF-50-50
0 0
0 50 100 150 200 250 300 350 400 0 5 10 15 20 25 30 35 40
Z'(ohm) Z'(ohm)
Figure 1.7 EIS profiles of P-ECNFs (A), H-ECNFs (B), and HP-ECNFs (C) in full
frequency range (labeled as 1) and high-frequency range (labeled as 2). ECNFs are
used as a reference in all plots [22].
1.4.3.2 Electrochemical Impedance Spectroscopy (EIS)

EIS has been extensively used to characterize electrode materials for
supercapacitors [26]. EIS test herein was performed within the frequency
range of 100 kHz to 0.1 Hz. Nyquist plots of all the electrospun carbon
nanofibrous electrodes showed no or much-depressed semi-circles at

high frequency and nearly straight lines at low frequency (Figure 1.7).
For P-ECNFs, the sample from the electrospinning solution with the
largest PMMA content (P-ECNF-50-50) exhibited the smallest electrode
resistance (the intercept at the axis of the real part of complex impedance)
and the smallest electrolyte resistance (the size of the semi-circle at high
frequency), while the sample from the electrospinning solution with
the least PMMA content (P-ECNF-90-10) showed the largest electrode
resistance, the largest electrolyte resistance, and the least capacitive
behavior (the angle between the straight line and the axis of the real
part of complex impedance). For H-ECNFs, the sample from the core
solution of co-axial electrospinning with the largest PMMA content
(H-ECNF-30) exhibited the lowest electrode resistance, the smallest
electrolyte resistance, and the best capacitive behavior. For HP-ECNFs,
the sample from the shell solution of co-axial electrospinning with
the largest PMMA content (HP-ECNF-50-50) demonstrated the smallest
electrolyte resistance but the largest electrode resistance.
1.4.3.3 Cycling Stability

The cycling stability of the electrode materials including P-ECNF-50-50,
H-ECNF-30, and HP-ECNF-50-50 was evaluated by monitoring
the variation of specific capacitance with 3,000 cycles of CD test at
5 A/g within the potential window of 0.0 to –0.8 V (Figure 1.8). It
is observed that there was almost no loss in specific capacitance for
these nanofibrous electrode materials after 3,000 cycles of charging/
discharging, indicating their long-term durability for supercapacitor use.
120
Capacitance Retention (%)
100
80
60
40
P-ECNF-50-50
H-ECNF-30
20
HP-ECNF-50-50
0
0 500 1000 1500 2000 2500 3000
Cycle number
Figure 1.8 Cycling stability of electrospun carbon nanofibrous electrode

materials at current density of 5 A/g [22].
Particularly P-ECNF-50-50 and HP-ECNF-50-50 even showed some

increases in specific capacitance during the cycling test. This could be
attributed to the wettability improvement and activation of the electrode
caused by continuous diffusion of electrolyte ions into previously

inaccessible pores and graphitic layers, which led to an increase of active
charge storage sites of the electrode. P-ECNF-50-50 possessed a relatively
large amount of micropores among the three P-ECNF samples and these
micropores might gradually become accessible for electrolyte ions with
charging/discharging cycles. Therefore P-ECNF-50-50 exhibited the most
increase in specific capacitance during the cycling test.
1.4.3.4 Discussion
The electrochemical performance of these electrospun carbon nanofibrous
materials showed a sequence of porous structure > hollow porous
structure > hollow structure > solid structure. Compared to the porous
structure, the hollow structure did not demonstrate the previously
mentioned “ion-buffering reservoir” effect toward the electrode’s
specific capacitance. Meanwhile, the electrochemical performance of
these electrospun carbon nanofibrous materials did not correlate linearly
with their specific surface area, indicating that multiple factors instead
of just specific surface area determine the electrochemical performance
(specific capacitance) of these electrode materials. These factors may
include overall accessible surface area, electrolyte resistance, electrode
resistance, etc. It is observed that the samples with a larger proportion
of micropore volume such as P-ECNF-90-10 and HP-ECNF-90-10
possessed higher specific areas but lower electrochemical performance
in their respective material groups. They also exhibited the largest
electrolyte resistance. These results indicated that electrolyte ions had
more resistance to access micropores. In the meantime, the samples in
each group, i.e. P-ECNFs, H-ECNFs, and HP-ECNFs, with the largest
PMMA content in electrospinning/co-axial electrospinning solution
exhibited the largest pore volume particularly mesopore volume
and concurrently the best electrochemical performance. When the
specific capacitance of samples from each group is compared with the
corresponding sample’s pore volume including micropore, mesopore,
and macropore volumes as well as total pore volume, it is observed
that the mesopore volume and total pore volume follow the same
sequence as that of specific capacitance for all the samples in each
group (Figure 1.9). Particularly for P-ECNFs, the specific capacitance
is proportional to the amount of mesopore volume (Coefficient of
Determination, R 2 = 0.99998) and the amount of total pore volume
(R 2 = 0.99293) instead of the proportion of mesopore. These results
indicated that mesopore volume and total pore volume are primary in
determining the electrochemical performance of these electrospun carbon
nanofibrous electrode materials. Total pore volume determines the total
number of available ion adsorption sites. Mesopore volume not only
contributes to ion adsorption sites but also determines the transportation
0.30 0.30 0.30

A P-ECNF-90-10 B H-ECNF-10 C HP
P-ECNF-70-30 H-ECNF-20 HP
0.25 0.25 0.25
P-ECNF-50-50 H-ECNF-30 HP
Pore volume (cm3/g)
Pore Volume (cm3/g)

0.20 0.20
Volume (cm3/g)
0.20
0.15 0.15 0.15
0.10 0.10 0.10
0.05 0.05 0.05
0.00 0.00 0.00

Micro Meso Macro Total Micro Meso Macro Total Micro
Pore Structure Pore Structure
0.30
H-ECNF-10 C HP-ECNF-90-10
H-ECNF-30 0.25
HP-ECNF-50-50
Pore Volume (cm3/g)
0.20
0.15
0.10
0.05
0.00
Micro Meso Macro Total Micro Meso Macro Total
Pore Structure Pore Structure
Figure 1.9 Comparison of pore volumes of P-ECNFs (A), H-ECNFs (B), and
HP-ECNFs(C) [22].
difficulty of electrolyte ions to available adsorption sites. The mesopores

may serve as connection paths and temporary reservoirs for electrolyte
ions and assist electrolyte ions in access of micropores for charge storage
(Figure 1.10). The mesopore volume sequence of electrospun carbon
nanofibrous electrode materials matched their electrolyte resistance
sequence from EIS very well (Figure 1.7). P-ECNF-50-50 possessed the
highest mesopore volume and thus exhibited the highest electrochemical
performance. When combining pore volume information with EIS
results, it is also observed that micropores hindered electrolyte ion
transport and increased electrolyte resistance, while hollow structures
facilitated electrolyte transport and reduced electrolyte resistance.
Electrode resistance also played a role in the final electrochemical
performance. A larger amount of PMMA in the electrospinning/co-axial
electrospinning solution led to a more ordered carbon structure and
could result in lower electrode resistance. Nevertheless, the electrode
resistance of HP-ECNF-50-50 increased significantly. This might be due
to the open long slits on HP-ECNF-50-50 nanofibers as observed from
SEM (Figure 1.1J) as well as the largest average pore size, which reduced
short moving paths of electrons.
Figure 1.10 Schematic diagram of electrolyte ion transfer in electrospun

carbon nanofibers [22].
1.5 concluSIonS
In this chapter, electrospun carbon nanofibrous materials with solid,

porous, hollow, and hollow porous nanostructures are comprehensively
compared for their electrochemical performance as electrode materials
for supercapacitors. In contrast to solid nanofibrous structures, all the
carbon nanofibrous electrode materials with porous and/or hollow
nanostructures demonstrate better electrochemical performance.
Despite the variation of morphology, i.e. porous, hollow, and hollow
porous nanostructures, mesopore volume and total pore volume are
primary in determining the electrochemical performance (specific
capacitance) of the nanofibrous electrode materials. Total pore volume
determines the total number of available charge storage sites while
mesopore volume controls electrolyte resistance. Mesopores can serve
as connection paths and temporary reservoirs for electrolyte ions and
assist electrolyte ions in access of micropores for more charge storage.
The hollow nanofibrous structure can benefit electrolyte transport and
reduce electrolyte resistance of the supercapacitor electrode but is not the
decisive factor for its electrochemical performance. Electrode resistance
of these electrospun carbon nanofibrous materials also plays a role in
the final electrochemical performance through corresponding electrical
resistance, which is dependent on carbon internal structure as well as
overall fiber morphology.
acknowledgMentS
This work was performed in whole/part at the Joint School of Nanoscience

and Nanoengineering, a member of the National Nanotechnology
Coordinated Infrastructure (NNCI), which is supported by the National

Science Foundation (Grant ECCS-2025462).
RefeRences
[1] Kim, B.K., S. Sy, A. Yu and J. Zhang. 2015. Electrochemical Supercapacitors
for Energy Storage and Conversion. pp. 1–25. In: J. Yan (ed.). Handbook
of Clean Energy Systems. John Wiley & Sons.
[2] Simon, P. and Y. Gogotsi. 2008. Materials for electrochemical capacitors.
Nat. Mater. 7: 845–854.
[3] Najib, S. and E. Erdem. 2019. Current progress achieved in novel
materials for supercapacitor electrodes: mini review. Nanoscale Adv.
1: 2817–2827.
[4] Schoetz, T., L.W. Gordon, S. Ivanov, A. Bund, D. Mandler and
R.J. Messinger. 2022. Disentangling faradaic, pseudocapacitive, and
capacitive charge storage: A tutorial for the characterization of batteries,
supercapacitors, and hybrid systems. Electrochim. Acta 412: 140072.
[5] Zhang, L.L. and X.S. Zhao. 2009. Carbon-based materials as
supercapacitor electrodes. Chem. Soc. Rev. 38: 2520–2531.
[6] Zhang, L., A. Aboagye, A. Kelkar, C. Lai and H. Fong. 2014.
A review: carbon nanofibers from electrospun polyacrylonitrile and
their applications. J. Mater. Sci. 49: 463–480.
[7] Lai, C., Z. Zhou, L. Zhang, X. Wang, Q. Zhou, Y. Zhao, et al. 2014.
Free-standing and mechanically flexible mats consisting of electrospun
carbon nanofibers made from a natural product of alkali lignin as
binder-free electrodes for high-performance supercapacitors. J. Power
Sources 247: 134–141.
[8] Aboagye, A., Y. Liu, J.G. Ryan, J. Wei and L. Zhang. 2018. Hierarchical
carbon composite nanofibrous electrode material for high-performance
aqueous supercapacitors. Mater. Chem. Phys. 214: 557–563.
[9] Shin, Y.M., M.M. Hohman, M.P. Brenner and G.C. Rutledge. 2001.
Experimental characterization of electrospinning: the electrically
forced jet and instabilities. Polymer 42: 9955–9967.
[10] Zhang, B., F. Kang, J.-M. Tarascon and J.-K. Kim. 2016. Recent
advances in electrospun carbon nanofibers and their application in
electrochemical energy storage. Prog. Mater. Sci. 76: 319–380.
[11] Zhi, M., S. Liu, Z. Hong and N. Wu. 2014. Electrospun activated carbon
nanofibers for supercapacitor electrodes. RSC Advances 4: 43619–43623.
[12] Zhang, L., Y. Jiang, L. Wang, C. Zhang and S. Liu. 2016. Hierarchical
porous carbon nanofibers as binder-free electrode for high-performance
supercapacitor. Electrochim. Acta 196: 189–196.
[13] He, G., Y. Song, S. Chen and L. Wang. 2018. Porous carbon nanofiber
mats from electrospun polyacrylonitrile/polymethylmethacrylate
composite nanofibers for supercapacitor electrode materials. J. Mater.
Sci. 53: 9721–9730.
[14] Zhou, Z., T. Liu, A.U. Khan and G. Liu. 2019. Block copolymer-based
porous carbon fibers. Sci. Adv. 5: eaau6852.
[15] Peng, S., L. Li, Y. Hu, M. Srinivasan, F. Cheng, J. Chen, et al. 2015.
Fabrication of spinel one-dimensional architectures by single-spinneret
electrospinning for energy storage applications. ACS Nano 9: 1945–1954.
[16] Xu, K., S. Li, J. Yang and J. Hu. 2018. Hierarchical hollow MnO2
nanofibers with enhanced supercapacitor performance. J. Colloid
Interface Sci. 513: 448–454.
[17] Han, D. and A.J. Steckl. 2019. Coaxial electrospinning formation of
complex polymer fibers and their applications. ChemPlusChem. 84:
1453–1497.
[18] Shilpa and A. Sharma. 2016. Free standing hollow carbon nanofiber
mats for supercapacitor electrodes. RSC Adv. 6: 78528–78537.
[19] Kim, J.-G., H.-C. Kim, N.D. Kim and M.-S. Khil. 2020. N-doped
hierarchical porous hollow carbon nanofibers based on PAN/PVP@
SAN structure for high performance supercapacitor. Composites Part B
186: 107825.
[20] Le, T.H., Y. Yang, L. Yu, T. Gao, Z. Huang and F. Kang. 2016. Polyimide-
based porous hollow carbon nanofibers for supercapacitor electrode.
J. Appl. Polym. Sci. 133: 43397.
[21] Zhang, L. and Y.-L. Hsieh. 2009. Carbon nanofibers with nanoporosity
and hollow channels from binary polyacrylonitrile systems. Eur. Polym.
J. 45: 47–56.
[22] Asare, K., M.F. Hasan, A. Shahbazi and L. Zhang. 2021. A comparative
study of porous and hollow carbon nanofibrous structures from
electrospinning for supercapacitor electrode material development.
Surf. Interfaces 26: 101386.
[23] Zhou, Z., C. Lai, L. Zhang, Y. Qian, H. Hou, D.H. Reneker, et al.
2009. Development of carbon nanofibers from aligned electrospun
polyacrylonitrile nanofiber bundles and characterization of their
microstructural, electrical, and mechanical properties. Polymer 50:
2999–3006.
[24] Zhang, L. and Y.-L. 2006. Hsieh. Nanoporous ultrahigh specific surface
polyacrylonitrile fibres. Nanotechnology 17: 4416–4423.
[25] Lee, H.-M., H.-R. Kang, K.-H. An, H.-G. Kim and B.-J. Kim. 2013.
Comparative studies of porous carbon nanofibers by various activation
methods. Carbon Lett. 14: 180–185.
[26] Mei, B.-A., O. Munteshari, J. Lau, B. Dunn and L. Pilon. 2018. Physical
interpretations of Nyquist plots for EDLC electrodes and devices.
J. Phys. Chem. C. 122: 194–206.
Chapter 2
Energ y and Sensor Applications

of Polymer Nanocomposites
Ankit Kumar Srivastava1, Swasti Saxena2* and

Surendra K. Yadav3
1
Department of Physics, Indrashil University, Mehsana 382740, India.
E-mail: pushpankit@gmail.com
2
Department of Physics, Sardar Vallabhbhai National Institute of Technology,
Surat 395007, India.
E-mail: swastisaxenaa@gmail.com
3
Department of Physics, Sri Venkateswara College, University of Delhi,
Benito Juarez Marg, 110021, New Delhi.
E-mail: surendraky@gmail.com
2.1 IntroductIon
The development of energy sources that can meet the world’s rising energy
demand while being ecologically benign is a fundamental challenge the
world faces at the beginning of the twenty-first century. The world’s
energy requirements are constantly rising due to population growth and
quickening economic growth. Approximately 60% of the electrical energy
is lost as heat losses during the production process [1]. Also, 8–15% is
lost as heat during the transmission and transformation of electricity [2].
Because of this, only 35% of the energy produced in a power plant makes
it to our houses. Another illustration is transportation efficiency. Overall,
*For Correspondence: E-mail: swastisaxenaa@gmail.com

40% of the energy produced in a car is lost as heat. Another 30% is used to
cool the engine. Thus, 70% of energy is wasted, even without considering
the CO2 emissions caused by the additional 70% of the fuel that must be
used. To put it another way, much low-quality thermal energy must be
produced to sustain daily life, and industrial activity, which is regrettably
wasted [3]. Renewable thermoelectric functional materials can generate
heat and electricity by using the mobility of solid internal carriers, even
at very low-temperature variations from room temperature.
In contrast to conventional new energy technologies, thermal
energy (TE) devices provide several distinctive qualities, including the
absence of moving parts and noise, a long operational lifetime, and the
potential to replace existing energy materials [4–6]. Thermo electrical
materials are frequently used in military, aerospace, and other high-tech
fields, as well as in microsensors, medical thermostats, and other non-
military applications [7–8]. Thermoelectric generators, or devices that
generate electricity from waste heat, can contribute to a more sustainable
environment by using their ability to convert temperature changes into
energy [9–10]. Burning fossil fuels releases significant atmospheric CO2,
a known greenhouse gas that accelerates climate change. To prevent a
catastrophe due to climate change, it is necessary to stabilise CO2 levels
at appropriate goal levels, which can be achieved through the large-scale
development of carbon-free renewable sources.
2.2 EnErgy HarvEsting

Developers had no reason to worry about energy harvesting before
the advent of ultra-low-energy MCUs. But as portable gadgets became
more and more common and battery innovation lagged, real attention
began to be paid to energy harvesting. Without ultra-low-energy
MCUs, wireless sensor networks, for instance, would not be feasible.
Micropower harvesting systems also support these MCUs [11–12]. The
most prevalent energy-harvesting systems use solar, thermal, RF, and
piezoelectric energy sources.
1. Photovoltaic (PV) panels or solar cells convert light energy into
electricity. Photovoltaic cells are among the energy-collection
technologies with the highest power density and output.
2. Thermoelectric energy harvesters turn heat into electricity. When
the temperature difference between their bi-metal junctions is
measured, their thermocouple (TC) arrays—which are what they
are made of—produce a voltage (effect of Seebeck effect). On the
contrary, when power is applied to a thermocouple (TC) junction,
one junction heats up while the other cools, which is how heat
pumps function due to the Peltier Effect.
Energy and Sensor Applications of Polymer Nanocomposites 23
Figure 2.1 shows the Phonon scattering mechanism which

is the best example of thermocouple see beck effect where two
ends of thermocouple maintain the flow of electricity, this is also
called Peltier effect.
Figure 2.1 Schematic diagram illustrating phonon scattering

mechanisms via atomic defects, nanoparticles, and grain boundaries.
Thermoelectric materials convert thermal gradients and

electric fields for power generation and refrigeration, respectively.
Thermoelectric devices currently find only limited applications
because of their poor efficiency, which is benchmarked by the
so-called thermoelectric figure of merit:
S 2s
ZT = T (Dimensionless) (1)
k
S is the Seebeck coefficient, the electrical conductivity s, average
temperature T, and thermal conductivity k. The latter contains
both electronic and phononic contributions. Maximising ZT is
challenging because optimising one physical parameter often
adversely affects another. In metals, electrons contribute equally
to the electrical and thermal transport (Wiedemann–Franz law);
in insulators, only phonons give a non-negligible contribution
to thermal conductivity; in semiconductors, both subsystems
strongly contribute to the thermal transport, while the electrical
conductivity can be dramatically changed via doping. For
materials having similar thermal conductivities, the term power
factor (PF) is used to define the performance of thermoelectric,
PF = S2s (2)
The Arrhenius equation can explain the temperature
dependence of conductivity for conducting substances; accordingly,
the activation energy value is low for the highest conductivity.
(-E / k T)
s = s0e a b (3)
Where s is the conductivity, s 0 is the pre-exponential factor, Ea
is the activation energy and k is the Boltzmann constant.
3. Radiofrequency harvesters gather environmental radiofrequency
radiation, rectify it, amplify it, and use it to power ultra-low-
energy devices. Radiofrequency identification functions similarly
by responding to a strong RF field aimed at the sensor rather
than absorbing ambient RF. Protecting RF energy from the radio
environment and using it in low-voltage electronic devices are the
two main goals of RF energy harvesting. Antenna-like patches
with ultra-wideband characteristics or narrow-band antennas
are needed to detect radio frequency energy emitted by the
radio environment. The other’s use, though, depends on the
frequency ranges that will be picked up. For example, detecting
GSM-900 frequencies necessitate an antenna with narrow-band
characteristics. In order to enhance the functional qualities of
wireless communication systems, multiple-input multiple-output
systems are also used.
4. Piezoelectric transducers are used to transform pressure or
tension into electricity. Roadbed vibrations caused by motors,
airfoils, and piezoelectric energy harvesters are frequently
reported as anomalies. Although other energy-collection systems
are being looked into, the redisplay is now in the lead.
Due to the expansion of battery-powered portable consumer,
commercial, and diagnostic supplies, these four energy-harvesting
businesses will continue to expand quickly for many years.
2.3 EnErgy-HarvEstIng sourcEs

Resources that can provide enough energy entirely or partially to power
sensor networks in smart environments are known as energy-harvesting
resources. Based on their properties, energy-collection sources can be
categorised into two classes: natural resources, such as sun, wind, and
geothermal energy, which are readily available from the environment,
and synthetic sources, which are generated by human or system activity.
They are not a part of the ecosystem naturally. Examples include
human movement, pressure from jogging or walking on floor slab
inserts, and system vibrations during operation. Table 2.1 lists several
energy sources for energy harvesting and information about the source
type and typical harvesting power. System designers must consider
the source of the energy-efficient solution for two reasons. Natural
factors, like weather, temperature, and season, impact natural sources,
while the schedules and impacts of human and mechanical systems

impact artificial sources. For instance, the prediction methods of each
generating source will be impacted by physical conditions.
Second, there is no need for more energy to generate natural
resources. Our research on microscale energy-harvesting systems does
not address the potential environmental effects of large-scale resource
extraction. On the other side, artificial resources demand energy from
human/machine systems to produce atmospheric harvestable energy.
It should not be considered a cost if energy is mainly used for other
purposes, such as lighting a room or powering a computer system.
Consequently, the energy harvested through such a process is merely
a by-product. However, it is considered a cost if energy production is
primarily used to produce harvestable energy. It can occur if a light is
left on for a few more hours to charge a sensor with solar energy or if
a radio spectrum is created to charge an RFID sensor. Renewable energy
thermoelectric (TE) materials can convert heat to electricity directly.
They could be utilised in TE generators for energy collection and
local cooling. Wasted fuel and exceptionally tiny heat losses could
be advantageous as well. Due to synergistic effects that highlight the
advantages of carbon nanoparticles and polymers, organic conducting
polymers carbon nanocomposites, which are employed as TE composites,
have recently attracted much attention. To find alternative energy
sources, thermoelectric generators are being used to help increase the
effectiveness of the real energy system by recovering heat that has been
lost today. TE nanostructure features, ranging from so-called 3D nano
bulk materials to the inclusion of 0D quantum dots in TE structures
[61], are studied.
Table 2.1 Comparison in the study of various forms of energy consumption

Serial No. Sources of Energy Energy Amount (TW)
1. Hydropower resources ≤ 0.50
2. Ocean and tide energy ≤ 2.00
3. Wind energy 2.00 – 4.00
4. Solar power 120,000
According to our extensive literature review, achieving favourable

thermal properties for CNT nanofluids over time requires maintaining
a homogenous dispersion and long-term stability. The goal of preserving
the criteria mentioned above is challenging, given that CNTs are
hydrophobic to most fluids and have a strong van der Waals interaction
with one another. Nevertheless, many academics have made various
attempts to satisfy these requirements. However, several issues need to
be identified and fixed for various CNT nanofluid applications, especially
for solar systems. The commercialisation of this technology is hindered

due to major issues: stability and production expenses. Because of this,
most collector designs must be modified to accommodate the practical
needs of water heating systems used in home and industrial settings.
If these challenges are overcome, nanofluids are predicted to have
a substantial impact not only on companies and technological areas but
also on raising the standard of living for people. Recent studies [13–
15] have shown the viability of using hybrid/composite materials and
magnetic nanofluids to increase heat transmission. The most significant
level of effectiveness of nanofluid in solar thermal engineering devices
must be ensured, although none have been used in solar energy systems.
Researchers have shown that carbon nanotube yarns may be used to
gather energy. These researchers led the group from the University of
Texas in Dallas, and their specialists have been developing yarns made
of carbon nanotubes for over ten years.
Finally, energy-harvesting devices have been created by stretching
and twisting carbon nanotube threads. According to the initial research,
these nanotubes might be used right away to power tiny sensor nodes
for Internet of Things (IoT) applications. Nanotube yarns may produce
a significant amount of energy by flexing and stretching in response to
the motions of breaking waves, according to scientists. The nanotube
appears to utilise the piezoelectric effect.
2.4 EnErgy-HarvEstIng storagE
1. Even if micropower energy is not intermittent, the output is

typically so low that a boost valve is required to control the
build-up of energy.
2. Small rechargeable Li-ion batteries are frequently used in
portable applications with limited space.
3. Micropower energy devices find it challenging to deal with
current spikes, such as when a sensor displays data bursts. In
addition to the need for power regulation, energy harvesters
frequently require a large supercapacitor or capacitor to dampen
rapid surges in output.
4. Electrical double layers capacitors are also known as
supercapacitors due to the great proximity of their conducting
layers. These supercapacitors have a lower energy density
than batteries but a significantly higher power density. They
are ideally suited to handle sudden increases in demand since,
unlike batteries, they may be completely exhausted in a matter
of seconds. They are widely used in combination with ultra-low-
power applications because they have less internal resistance

than thin-film batteries, which enables them to drain more
slowly over time.
2.5 EnErgy collEctIon From conductIng

nanocomposItEs dEvElopmEnt tools
2.5.1 development tool for tE Harvesting

Energy harvesting is a broad category of technologies that are constantly
evolving. The significance of development tools is growing as design
complexity and design cycle lengthen. Designers can evaluate and
become familiar with the most recent power harvesting technologies
and products using development kits and boards in the industry.
The iterative design approach must include development and testing,
and circuit modelling. Engineers are frequently compelled to use the
Breadboard, which is purportedly inescapable. Development kits offer a
simple approach to reaching the development core with little setup time.
In addition to a quicker time to market, development tools can
benefit immediately applicable, tested circuits, widely available printed
circuit layouts, and a similar level from which to build designs. Energy
harvesting is a broad category of technologies that are constantly
evolving. The significance of development tools is growing as design
complexity and design cycle lengthen. Designers can evaluate and
become familiar with the most recent energy-harvesting technologies
and products using developer kits and boards in the field.
2.5.2 a Quick look at carbon nanotubes (cnt)

and graphene
Having a wide variety of allotropes, carbon has historically been the
most adaptable and all-purpose element, with numerous applications
in the world of materials. There are many other forms of carbon, but
carbon nanotubes and graphene have emerged as the most exciting
ones, offering countless research opportunities in every field of science.
After being discovered by the renowned scientist Iijima, the carbon
nanotube has long been a household word in science [16]. CNTs are
typically in the micrometre range in length. CNTs display three different
chirality: armchair, zigzag, and chiral, based on the lattice vectors and
chiral angles [17]. With a wide range of applications from industrial
to nanotechnology, including energy storage, modular electronics,
conductive polymers, and structural composites, CNTs have emerged
as the most anticipated substance in society.
Graphene is a different kind of carbon that has recently been

researched for various uses. Graphene is a relatively new material
but because of its unusual structure, it has already shown excellent
mechanical, thermal, and electrical properties. A hexagonal lattice of
carbon atoms on a planar surface makes up the two-dimensional allotrope
of carbon known as graphene. Graphene is the building block for other
carbon nanostructures, including fullerene and carbon nanotubes. For
the first time, graphene became a fact in 2004. After that, curiosity about
the structure and properties of graphene suddenly soared, and it did
not disappoint. The detailed measurements of graphene’s properties,
including charge transfer and tensile strength, revealed that they are
both relatively high. These distinctive qualities allow graphene to be
employed in a wide range of applications.
Only a few themes covered [18–23] include thin and flexible
displays, solar cells, electronics, medical, pharmaceutical, and industrial
activities. In order to enhance electrical and mechanical properties,
CNTs and graphene have been extensively investigated in various
thermoplastic and thermosetting polymer systems. Research using
polyurethane, polystyrene, polycarbonate, ABS, PMMA, polyethylene,
epoxy, and phenolic systems has been described by several researchers
[24–33]. Aside from being used with polymers to create self-healing
nanocomposites, CNT and graphene have also been used for their
superior heat conductivity and stability [21]. The following sections
provide a brief overview of the self-healing phenomenon achieved with
graphene and the polymers that subsequently used CNTs similarly.
2.5.3 self-Healing polymer composites Based on graphene

Graphene’s potential for healing is its capacity to correct ingrained
faults. The graphene structure’s reconstruction (knitting) produces the
healing action [35]. When a vacancy defect develops, carbon atoms from
the surrounding areas rush in to fill the gaps [36]. Impurities, such
as hydrocarbon contaminants, often cause these extra carbon atoms.
When graphene is scraped in the presence of metals, carbon atoms are
transferred from the nearby hydrocarbon impurities and fill the hole
that is left behind [37]. This unusual reknitting of graphene holes is
the outcome of this process. It is because graphene’s innate ability to
mend itself creates new possibilities for applications that use various
techniques, including the e-beam technique and the etching process. A
stiff C60 molecule that causes nano-damage in a suspended graphene
monolayer is studied using molecular dynamics simulation, and the
results show that the correct heat treatment can effectively repair the
damage [38].
The self-healing mechanism is explained as a two-step process:

(a) the formation of local curvature around the defects brought on
by the damage, and (b) defect rebuilding leading to noise filtering
of the contour brought on by the destruction, ultimately leading to
the destruction being minimised. Two factors dominate the ability of
graphene to self-heal. The first one is the damage’s extent, and the
former is the temperature change. As a result, graphene, being a single
atom layer, is worth highlighting, may preserve the power of moving
current, and thus damage repair in the same manner. Graphene may
readily repair damage in microstructures when combined with polymer
composites. The self-healing in polymer composites using CNTs and
graphene is the primary emphasis of this chapter. Because of their flat
structure and superior thermal contraction, high-performance thermal
interface materials are another excellent use of graphene-based polymer
composites. Much research has found that adding graphene to a material
improves heat conductivity. It will increase the polymer matrix’s heat
transport capabilities, improving self-healing in response to thermal
or other external stimuli. Epoxy and polyurethane are two instances
of polymers that incorporate graphene. There have been numerous
assessments of the self-healing properties.
2.5.4 carbon nanotube-Based as self-Healing polymer

nanocomposites
Due to their exceptional mechanical properties, carbon nanotubes (CNTs)
have been the focus of material research ever since their discovery. There
are several applications for CNT-reinforced materials across numerous
sectors. In addition to the abovementioned applications, CNT has been
thoroughly explored to fully appreciate the self-healing properties of
polymer composites based on CNT.
CNT can be used in self-healing materials in both extrinsic and
intrinsic approaches. On the one hand, it has a 1D tubular structure
with strong reinforcing qualities; on the other, it has excellent thermal
conductivity and heat transfer capabilities.
2.5.5 Extrinsic self-Healing polymers with cnts

Both the fluid overload and capsule-based extrinsic self-healing strategies
can be applied. Due to their efficient 1D nanotubular structure, CNTs
can be used as a nanoreservoir for the healing agent. Additionally,
because it enhances structural integrity, CNTs can be combined with
other polymers as a healing agent in fluid overload self-healing. Finally,
CNTs can be reinforced in polymers with microcapsules to regain
mechanical strength.
2.5.6 carbon nanotubes as nano reservoirs

In microvascular self-healing techniques, single-walled carbon nanotubes
(SWCNTs) can be used as nano reservoirs of healing chemicals [39].
According to the dynamics study, the fluid from a burst SWCNT after
the injury is thoroughly studied, with the fluid resembling a healing
agent. Using SWCNTs as a self-healing container reinforces the overall
mechanical strength of the system. The following are the main factors
that affect how much of a healing agent needs to be retained in the
SWCNT reservoir to facilitate self-healing.
Figure 2.2 Carbon nanotubes act as nano reservoirs in a self-healing mechanism

(D. Ponnamma, K.K. Sadasivuni, M. Strankowski, Q. Guo, S. Thomas, Soft Matter
9(43): 10343–10353).
The final applications of the material will dictate these requirements

for developing a practical CNT-based self-healing system [39]. Using
the self-sustained diffusion method, liquid monomers utilised as
therapeutic treatments were intercalated into CNTs [40]. The healing
agent (dicyclopentadiene (DCPD) or isophorone diisocyanate) was first
coupled with an empty CNT solution that was semisolid and dissolved
in benzene. The benzene in the solution evaporated after sonication,
allowing the solutes to penetrate the CNTs. Fresh benzene was added
after the intercalation and sonicated for 3–4 minutes to sanitise the CNTs’
exterior. Mass production of intelligent composites with integrated self-
healing agents might start if a more practical method for loading self-
healing compounds into CNTs can be created. Different self-healing
capsule and vascular-based polymer systems have three main drawbacks:
• It is challenging to embed capsules containing healing agents into
polymer systems;
• Capsule and vascular implantation reduce the mechanical
strength of the material;
• The mending agent runs out after just one crack.
The vast electrical conductivity and exceptional mechanical properties
of CNTs make them preferable to pure polymers as reinforcing conductive
fillers. As a result, the strength loss brought on by capsule embedment

can be made up for by CNT reinforcement. An epoxy-based self-healing
coating with CNT fillers was produced by microencapsulating the
healing agent inside the polymer system [41–44]. Even though it is
well known that adding microcapsules to a polymer system reduces its
mechanical strength, this problem was fixed by integrating SWCNTs.
Nanoindentation experiments significantly improved the samples’ elastic
modulus and hardness after adding SWCNTs to the system. As a result,
CNTs may easily enhance the physical properties of composites, thereby
expanding their applications and helping to re-establish the mechanical
properties lost due to capsule embedment.
2.5.7 carbon nanotubes as Effective Healing agents

In vascular-based self-healing (SH) polymers, CNTs have also been
used as healing agents. The vacant channels of an epoxy resin system
were filled with a nanocomposite made of SWCNTs and 5 ethylidene-2
norbornene (5E-2N) that was combined with ruthenium Grubb’s catalyst
[45] and used as a healing agent. An example defines a microvascular
SH method.
A mass was applied to raise the impact hole to evaluate the self-
healing behaviour. The damaged sample was treated with the 5E-2N/
SWNT composite healing agent and thermally repaired for 15 minutes
at 60 degrees Celsius. Although it could not measure the epoxy system’s
mechanical healing effectiveness, it recovered its structure after 30
minutes. Similarly, ethyl-phenylacetate and 2.50% carbon nanotubes
were mixed and used as medicinal agents in a capsule-based method
[46]. CNTs boost both electrical and mechanical healing capability when
paired with healing agents. As a result, the role of carbon nanotubes in
extrinsic self-healing materials has been intensively explored. Due to their
tubular shape, CNTs can restore lost mechanical properties in micro-
capsule-based healable polymer nanocomposite and a nanoreservoir
for healing agents. In addition to this study, carbon nanotubes have
been used to create carbon nanotube-based polymer composites with
built-in healing properties. These composites will be discussed in the
following section.
2.5.8 Intrinsic self-Healing using cnts composites

made of polymers
Carbon nanotubes are combined with polymers to make healable
composites, which can be used in applications requiring enhanced
flexibility, durability, and fracture resistance. Some key uses include
shear-stiffening materials for body armour and conducive healable

polymers for robotics. The following sections go through all these
different uses for polymers based on carbon nanotubes with inherent
healing properties.
2.5.9 Healable-conductive polymer composites with

multiple Functions
SH multifunctional conductivity can be used to restore circuit
conductance, prevent damage, and lengthen the lifespan of electronic
appliances. Carbon nanotubes exhibit very high mechanical and
electrical conductivities. A conductive elastomer with autonomic healing
capacity was made utilising a nanocomposite of poly-2-hydroxyethyl
methacrylate and SWNTs connected via interactions [47] by introducing
carbon nanotubes into an elastomer system. Both mechanical and
electrical healing capacities in the sample were identified. An LED
light and a power source were connected to the nanocomposite sample.
Figure 2.3 Sushi-style architecture displays conductance Schematic represen-

tation of the disconnection of HPAMAM/carbon nanotube composite (Copyright
2015). For this reproduction, permission has been obtained from the Royal Society
of Chemistry.
The sample was immediately split in half, and the LED went off due
to a lack of connectivity. However, when kept close to one another and
the LED lamp was turned back on, the test parts repaired themselves
in just five minutes. The electrical healing efficiency was calculated
for all the various sample types, and it was found to be roughly 95%
with different SWNT concentrations. Consequently, using SWCNTs,
an elastomer with electrical conductivity and structural stability was
made. This elastomer might be used to manufacture complex sensing
devices. Another composite with SH characteristics and a 20-minute
perfect electric-conductance recovery time was developed using hyper-
branched polyamidoamine (HPAMAM) polymers covered in carbon
nanotube films (Figure 2.2). The light was on when the sample was
first connected to the circuit. It was off when it was detached, but it
was turned back on when it was reconnected. The light was off when
resistance was measured both before and after separation.
As seen in Figure 2.3, the structural integrity and conductivity were
restored by the sushi-like construction of the SH conductive composite.
The conduction channels are restored because the surfaces at the site of
damage meet thanks to the spiral coiling of the CNT layers.
2.5.10 self-Healing polymer nanocomposites with

shear-stiffening
Shear-thickening materials fall within the topic of intelligent materials.
These materials’ viscosity has an uncommon feature that causes them to
rise sharply when the applied tension exceeds the critical shear rate. S-ST
polymer composites have lately emerged as a significant research area
because of their crucial applications in military body armour. Combining
S-ST with other functionalities will create the best composites for high-
performance body armour. Because of their exceptional mechanical
properties, low density, and electrical conductivity, MWCNTs are
excellent nanofillers for reinforcing polymer materials. Due to their
better dispersity than SWCNTs, MWCNTs can be easily combined with
the polymer matrix to produce multifunctional nanocomposites.
Several pieces of research have already demonstrated that
incorporating MWCNTs into polymers enhances their mechanical
properties. Body armour materials with good shielding and compressing
rate-dependent conductivity may be the MWNT-based S-ST polymer
composite. An MWNT/S-ST composite based on a poly-boron dimethyl
siloxane (PBDMS) derivative has demonstrated electrical self-healing.
On the opposite side, an LED light was connected in a circuit using the
MWNT/S-ST-polymer composite. The LED glowed brightly with a 9 V
power supply when the composite was split into two sections before
abruptly going out. The LED shone brightly once more when the broken
components were put back together. It highlights the material’s capacity
for self-healing and how little conductivity was lost in the repaired
specimen. Because of this, the MWCNT/S-ST polymer composite has an
exceptional capacity for self-healing at room temperature. Due to their
very high specific stiffness and strength, CNTs are intriguing candidates
for the production of composite materials. CNTs have excellent thermal
and electrical conductivities. Due to these characteristics, their high
aspect ratio, one-dimensional (1D) honeycomb lattice, and low density,
CNTs have been extensively researched for the construction of various
composite and smart materials [48].
Graphene, a monolayer of carbon atoms tightly packed into a
two-dimensional (2D) flat form, is the fundamental building block of
graphite materials. Open-ended CNTs are suitable for the construction of

composite materials because the geometrical (2D) and electronic effects
of graphene on their field-emission properties affect their mechanical
and electrical properties [49]. In addition, graphene has the lowest
energy due to the overlap of the 2pz orbitals of carbon atoms, which
gives its composite materials an anisotropic quality.
Due to their distinctive properties and low cost, graphene-based
composites are potential fillers due to the changes in carbon atom
bonding in-plane and out-of-plane, as well as their three-dimensional
(3D) geometrical qualities. Carbon nanotube (CNT)/graphene-filled
organic composites have much potential for developing less expensive
thermoelectric materials for energy-harvesting applications due to
their low cost, low density, straightforward preparation pathways,
variety of process capabilities, and low thermal conductivity. These
characteristics elevate them above other hybrid alloys that have been
previously reported. CNT and graphene are currently the most widely
used nanofillers because of their unique structures and characteristics,
such as superconductivity, low weight, high stiffness, and axial strength.
2.5.11 carbon nanotubes with customised shapes

produce Energy-collecting textile
Numerous pieces of research have examined the viability of extracting
energy from textiles. It makes sense because fabric-based harvesting
presents a promising area for such exploitation. One material used
to accomplish this is carbon nanotubes (CNTs), which have already
demonstrated various distinctive and beneficial qualities in this field.
Researchers from Rice University and Tokyo Metropolitan University
collaborated to create a flexible cotton fabric with improved fibres that
uses carbon nanotubes as a thermoelectric (TE) energy source. Using
the well-known Seebeck effect, it transforms heat into sufficient energy
to illuminate an LED. Using carbon nanotubes has disadvantages.
However, given their one-dimensionality and distinctive qualities, such
as flexibility and lightweight, they seem like appealing options. However,
maintaining the massive energy factor of isolated nanostructures in the
macroscopic assembly has proven challenging due to poor sample shape
and inadequate Fermi energy tuning [50–54].
2.6 EnErgy-collEctIng modEs
The many methods of harvesting energy are described below in the

following sections.
2.6.1 Energy Harvesting for Fossil Fuel alternatives

Also, with Paris Agreement, most world leaders have come together to
pledge to do more to combat the climate change threat. Each nation must
reduce emissions to prevent an increase in world average temperatures
of more than 2 degrees Celsius. If this goal is to be accomplished,
time is running out, and more fresh, clean energy solutions must be
developed. The new economy looks at a few of the fascinating green
energy technologies now under development.
2.6.2 Elephant grass Energy Harvesting

For centuries, biomass energy was the chosen fuel before it was more
convenient to access coal, oil, and gas. Today’s CO2 emissions are causing
havoc on the environment and are once more a significant player in the
world’s energy mix. Any biological material derived from plants or
animals is called biomass, but wood is the most prevalent. A system
using an alternative to wood pellets has been created by Sweden-based
clean-tech startup Next Fuel. With elephant grass, Next Fuel provides a
direct replacement for fossil fuels that emit no greenhouse gases.
Elephant grass is a unique plant that can reach a height of 4 metres
in just 100 days and produces many harvests each year. After the grass
is collected, Next Fuel’s method uses less energy to transform it into
briquettes during manufacturing. Because less CO2 is released into
the environment when the fuel is burned than was taken in from the
atmosphere a few months earlier when the grass was growing, the entire
carbon cycle shifts to a negative state annually.
2.6.3 Energy-Harvesting Hydrogen Fuel cells

One of the most prevalent elements in the world, hydrogen, has long
been used in the power sector, but recent intriguing developments have
reignited interest in it. Clean energy can be produced using hydrogen
fuel cells from a variety of sources. Like lithium-ion batteries, they can
be used in the transportation sector but do not require charging.
1. Electricity can be produced or transported using hydrogen, a
renewable energy source. When hydrogen is consumed, all that
is generated is heat and water.
2. We must first isolate hydrogen from a mixture to acquire pure
hydrogen.
3. A method for dissolving a molecule of water into its hydrogen
and oxygen atoms is electrolysis.
4. An electric current is produced when electrons are forced to
flow across a circuit. After completing the circuits, the electrons
combine with oxygen and hydrogen molecules to generate water

(H2O), which is then heated.
Figure 2.4 Schematic diagram of hydrogen fuel cell.
5. Hydrogen fuel cells can be used to help create a zero-emission

power system because they do not release any CO2 or other
pollutants.
6. A wide range of electrical devices, including cars, aeroplanes,
and buildings, can be powered by fuel cells (Figure 2.4). To
generate the enormous amounts of electricity needed to run
automobiles and other electrical equipment, many fuel cells can
be connected to form a fuel cell stack.
7. The anode electrode allows hydrogen to enter the fuel cell.
Hydrogen atoms are divided into protons with a positive charge
and electrons (Figure 2.5) with a negative charge (positive charge).
Figure 2.5 Fuel cell diagram.
8. The fuel cell membrane allows the positive charge protons

to pass through. Electrons with negative charges cannot pass
through the membrane and must travel through a circuit instead.
9. Electrons travelling through a circuit produce electrical current.
Germany was the first nation in the world to operate passenger

trains fuelled by hydrogen-based fuel cells due to the high cost of
hydrogen technology. European Union has decided to work together to
improve the possibilities for hydrogen in the power and transportation
industries.
2.6.4 solar paint as a source of Energy

One of the most popular sustainable fossil fuel alternatives is solar
panels, but what if one can harness the sun’s energy without being
concerned about the panels’ harmful effects on the environment? After
developing a paint that can generate energy, researchers may have found
the solution. The substance may capture solar energy and humidity
from the environment by fusing synthetic molybdenum-sulphide with
titanium oxide, which is present in many wall paints. A concrete
structure might be converted into a fuel and energy source by adding
a new composition. The process is easy to follow:
The paint is made of synthetic molybdenum-sulphide and titanium
oxide, which are two new chemicals (Figure 2.6). The characteristics of
that mouthful to wick away moisture are comparable to those of the silica
gel package that comes with new pairs of sneakers and other goods.
Figure 2.6 Solar paint applied on the wall of a house (Wikipedia source).
Synthetic molybdenum-sulphide absorbs solar energy before splitting

it into hydrogen and oxygen. After that, the hydrogen can be captured
and used to power a house, car, truck, boat, or all-terrain vehicle (ATV).
2.6.5 Energy Harvesting from Waves

It seems easy to harness the energy of the ocean’s waves. Unfortunately,
it is far more challenging in practice. An optimal design has been the
focus of years of research. A small-scale “wave snake” experiment

operated between 2008 and 2009 off the coast of Portugal, but when
the Scottish company that invented the technology went bankrupt,
Wave Energy Scotland, a public organisation, received the intellectual
property.
Research on wave energy is constantly going on. The most significant
engineering company in the world, Lockheed Martin, disclosed plans.
The EU is working with Wave Generating Scotland on an initiative to
provide open-source software for wave and tidal energy systems to
promote private investment (Figure 2.7).
Figure 2.7 Energy harvesting through wave energy.
Wave energy harvesters outperform other ocean energy harvesting

technologies, like tidal currents, ocean thermal energy conversion
(OTEC), and salinity processes. Waves are better suited for energy
gathering than tides because they are present throughout the ocean.
2.6.6 Energy-Harvesting Whisky

Whisky is a source of energy. Scotland has been making whisky for
centuries, and distilleries have developed strategies to deal with the
trash produced as a by-product, typically selling it to farmers as cow
feed. With 4 million tonnes of waste produced annually, the sector is
looking for a more inventive solution. The market for whisky waste
by-products may be worth £140 million ($184 million), according to a
2015 assessment by the Green Alliance. Certain distilleries use anaerobic
digester facilities to produce bio-gas, which is transformed into steam
energy that powers their operations. Diageo’s Glendullan Distillery
produced 6,000 MW hours of TE in its first year of operations, cutting
down on the need for fossil fuels by 25% [11]. By-products from whisky
may also be used to generate heat and power automobiles.
2.6.7 vehicle Energy-Harvesting system

High electricity consumption and constant traffic are two unrelated
aspects of city living. On the other hand, the Vehicle Energy Harvesting
System (VEHS) has developed a cutting-edge link that uses traffic flow
to increase access to electricity. The VEHS uses an engineered overlay
layer to gather pressure from any new or existing road, which is then
used to drive a traffic turbine and generate electricity [12].
The system might be finished and operational in six months,
enabling quicker access to power. The VEHS’s straightforward design
will permit local assembly, a deliberate move to local manufacture,
and greater electrical access. We anticipate that this will give local
communities the skills and employment opportunities they need and
aid in reviving their ailing economies.
2.6.8 Energy Harvesting from a sustainable power supply

The field of wireless gadgets has expanded exponentially since its
inception. However, the battery life of these gadgets frequently limits
the variety of services they can offer. Therefore, having an independent
power source would help us advance and fully utilise the capabilities
of such devices. The current technological revolution is driven by
miniaturisation, and as devices get smaller, less power is needed. It has
led scientists to wonder if the battery may be enhanced by devices that
continuously capture the environment’s energy that would otherwise be
wasted. Converting otherwise wasted renewable sources into a usable
form is known as energy harvesting or scavenging. Energy harvesting
might even replace the need for unfavourable or challenging-to-reach
battery replacement and maintenance when used with a wireless system’s
long-term power source. Safety monitoring devices, microsensors
incorporated in structures, and implants for treating patients are a few
examples of energy harvesting benefits of environmentally renewable
sources, including waves, heat, light, and water into electricity. It creates
a “battery-free” solution.
2.6.9 Harvesting mechanical Energy

Transportation, fluid flow like air motions, biological locomotion
like walking, and internal body motion, like chest and heart, are all
mechanical energy sources. They also include vibrations and noise from
industrial gear and equipment. The two fundamental techniques for
extracting energy from mechanical sources are kinematic and inertial
[54–56]. The resistance to an acceleration of a mass absorbs inertial
energy. These systems have a single connection point between the base
and the spring-mass-damper system. Due to its inertia, most of the base
vibrates, and these vibrations can turn into electrical energy. Pendulums,
magnetoelastic oscillators, and cantilever beams are examples of
harvesters that use this concept of inertia.
The energy-collecting transducer is directly connected to various
parts of the source in kinematic energy harvesting. The relative
movement between these elements causes the transducer to deform,
ultimately transforming into electrical energy. Two instances of energy
harvesting are bending a tyre wall to check the pressure and flexing
and extending limbs to charge a phone. Electrostatic, electromagnetic,
and piezoelectric transduction methods are being used to transform
mechanical energy into electrical energy [57–60].
2.7 advancE applIcatIons and

tEcHnologIEs oF EnErgy HarvEstIng
There are now several energy-collection devices, and new ones are
on the horizon. The most prevalent forms of energy include light,
heat, vibration, and radio frequency (RF). An enormous and rapidly
expanding power harvesting business has resulted from the invention
of ultralow-energy microcontroller devices. Low-power wireless sensors
were produced due to energy harvesting and are now widely used.
There are many applications of energy harvesting (Figure 2.8) on the
other hand, the ripple effect will spread throughout the commercial,
scientific, and consumer sectors, leading to new uses, such as creating
compact battery-powered devices.
Figure 2.8 Applications of energy harvesting.

2.7.1 mobile phone

One can achieve the low-power consumption needed to run a cell phone
by collecting energy from the environment. Mobile phones require a good
battery life while advancing wireless analogue telephony technologies
to portable computers, including website browsing, movies, sports, and
email capabilities. Low power has always been the most crucial electrical
design goal for at least the previous 10 years. Many industrial, medical,
and commercial purposes use wireless sensor networks with minimal
power consumption.
2.7.2 solar power

Small solar cells are used in consumer and industrial products, such
as calculators, smart watches, entertainment, street light controllers,
transportable energy sources, and satellites. Because light beams are
frequently intermittent, solar energy cells are used on batteries with
long lifespans to provide a continuous energy supply.
2.7.3 thermoelectric
The piezoelectric effect, which occurs when two different metals have
a difference in temperature and produce a voltage, is the basis for
thermoelectric harvesters. TEGs are constructed from several thermo-
couples connected to a heat source, such as a heater, an engine, or even
a solar panel.
2.7.4 piezoelectric
Piezoelectric transducers are excellent for accelerometer sensors with
energy-collecting modules that identify aeroplane wing vibrations and
motor bearing noise because they generate electricity while under
strain (Figure 2.9). When the cantilever vibrates, it produces an AC
output voltage that can be rectified, regulated, or used as a battery’s
thin-film or supercapacitor foundation.
Figure 2.9 Midé VoltureTM piezoelectric energy harvester

(Courtesy of Midé et al.).
2.8 InnovatIvE tEcHnIQuEs and

tEcHnologIEs
Some incredibly innovative energy-harvesting labs may change the

direction of the industry in the upcoming years.
2.8.1 medical and Fitness Equipment

A few cutting-edge uses for piezoelectric energy collection are beginning
to materialise. This might subsequently be implanted or used to control
a pacemaker, perhaps eliminating the need for battery maintenance. The
potential for other implanted devices to be driven by body heat, motion,
or vibration is being researched.
• The patient utilises a low-frequency RF emitter attached to the
chair, which the device picks up, corrects, and stores.
• People who frequent the gym will be delighted to learn that they
can somewhat recover the power they waste there. The creation
of a piezoelectric power harvesting unit that attaches to the knee
and produces electricity while using a treadmill has been a team
effort among three British colleges.
2.8.2 antennas
NEC devices have been effectively prototyped on silicon and HDPE
substrates, but additional funding and work will be needed to achieve
cost-effective mass manufacturing procedures. The researchers foresee
a system that utilises previously untapped infrared energy in addition
to conventional PV solar panels.
2.9 sEnsor applIcatIons oF polymEr

nanocomposItEs
2.9.1 polyaniline
Electrochemical sensors for numerous significant analytes, including
serotonin, ammonia, [61–62] ammonia, and HCl vapours [63], have
integrated polyaniline. Additionally, the fabrication of a sensor using
polyaniline, graphene, and carbon nanotube nanocomposites effectively
identified several environmental hazards, such as Cs, phenol, and
4-aminophenol [64].
2.9.2 polypyrrole
The functionalisation of Au nanoparticles on PPy is known to have an
enhanced response to ammonia [65], and p-polypyrrole—n-tungsten
oxide hybrid nanocomposites were used to detect nitrogen dioxide gas
at ambient temperature [66]. A polycrystalline hybrid nanocomposite
material based on cobalt hexacyanoferrate/carbon nanofibers/polypyrrole
(CoHCF/CNF/PPY) was created to bridge the gap between supercapacitors
and batteries. Using a variety of characterisation techniques, the
structural and elemental characterisations of the nanocomposite
revealed the formation of a PPY layer on the CNF surface to which
CoHCF nanoparticles were attached. Therefore, the proposed CoHCF/
CNF/PPY ternary hybrid nanocomposite can be a strong candidate for
the upcoming energy storage technologies [67].
2.9.3 graphene and Its derivatives

2.9.3.1 Graphene
A promising use for hydrogen sensing was demonstrated by advanced
electrode materials made from multi-layer graphene nanoribbons
functionalised with Pd. The development of a novel method to
increase the energy density of flexible solid-state supercapacitors using
hierarchically designed graphene nanocomposite (GNC) electrode
material and an ionic liquid gel polymer electrolyte also represents a
breakthrough in the field of supercapacitors [68]. This method aims to
provide the next generation of wearable and portable electronic devices
with high energy density, durability, and flexibility of supercapacitors.
Additionally, silicon boron carbon nitride (SiBCN) ceramic composites
filled with nitrogen sulphur dual-doped graphene (NSG) sheet (SiBCN/
NSGs) were designed and synthesised, and their viability to function
as lithium-ion battery anodes were tested [69]. These improvements
significantly increased the Li-ion loading capacity and gave a higher
rate ability.
After flocculation treatment, the MnO2/graphene nanocomposite
was created to remove the remaining tetracycline from pharmaceutical
effluent. Due to the nanocomposite’s strong water solubility, the
tetracycline removal rate was up to 99.4%. Additionally, the addition
of MnO2 nanorods made it possible to prevent the needless loading
of prepared graphene sheets during the adsorption process, giving
the tetracycline molecules more chances to come into contact with the
adsorbents [70]. Additionally, 3D electrochemical GQDs can detect Fe3+
ions optically in a sensitive and focused manner [71]. In addition to
the aforementioned uses, graphene-based fibre exhibits extremely high
sensibility to high tensile deformation and has dazzling bending and

torsion-sensitive capabilities due to a variable microstructure under
various mechanical stimuli [72].
2.9.3.2 Graphene Oxide

The development of rGO-Pt-NiO nanocomposites for enzyme-free glucose
sensing. Additionally, a concave tetrahedral Pd NCs@CuO composite and
reduced graphene oxide was used to create an ultra-low detection limit
glucose sensor [73]. Additionally, many electrochemical sensors based
on reduced graphene oxide have been successfully developed to detect
the incorporation of NO2 gas into ZnO, Fe2O3, and SnO2 nanoparticles
[74–75]. Additionally, a TiO2 nanoparticles adorned reduced graphene
oxide nanocomposite can be employed for ultrasensitive electrochemical
detection of rifampicin [78], in conjunction with CdS, as a non-enzymatic
biosensor for H2O2 [76], and also Cu2O [77].
For the susceptible electrochemical measurement of essential cancer
and oxidative stress biomarker 8-hydroxy-2′-deoxyguanosine (8-HDG),
multi-layer based graphene oxide coated zinc oxide nanoflower (ZnO
NFs @ GOS) was used as an electrode material [79]. Additionally,
silica/graphene oxide nanocomposites were employed as solid-phase
extraction adsorbents in conjunction with high-performance liquid
chromatography to analyse aflatoxins in cereal crops [80].
2.9.3.3 Carbon Nanotubes

In conjunction with metals and metal oxides, carbon nanotubes
serve as a substrate for several electrochemical sensors. When CNT
is assembled atop graphene, a synergistic effect makes it easier for
conductive channels to form, improving electrical conductivity [81].
Additionally, compared to CNT-based and WO3 nano brick-based
sensors used independently, the ammonia gas-sensing characteristics
at low temperatures were synergistically improved by carbon nanotube
and tungsten oxide nano brick sensors [82]. Additionally, a composite
made of multi-walled carbon nanotubes, magnetite nanoparticles, and
poly (2-aminopyrimidine) can perform solid-phase extraction of acidic,
essential, and amphoteric medicines for the quantitative assessment of
these pharmaceuticals in bodily fluids and wastewater. The composite
exhibits a greater adsorption capacity and improves medication
dispersion in aqueous media [83].
An extremely sensitive electrochemical sensor made of Fe3O4 -
SWCNTs/MOCTICl/CPE was successfully made to analyse the anticancer
agent epirubicin in actual samples [84]. Additionally, the devices
that combine electrochemical systems and nanosensors are ideally
suited for quick and accurate trace analysis of dangerous substances
in the environment, food, or health applications [85]. By building

a stereoselective sensor based on multi-walled carbon nanotubes
cross-linked with chiral nanocomposites based on Hydroxypropyl-b-
cyclodextrin, atorvastatin isomers could be discriminated [86]. PCL-CNT
nanocomposites were created using an oil-in-water emulsion solvent
evaporation technique. Applications involving medication delivery make
use of these nanocomposites [87]. Recently, a promising electrode that
was sensitive and selective was modified using MWCNTs/THI/AuNPs
nanocomposites to inhibit the antibodies to 17-estradiol, resulting in
the creation of an easy-to-make, inexpensive, label-free 17-estradiol
immunosensor [88]. Additionally, electrochemical oxidation of BZ at CPE/
nMBZBr/NiO-NPs can be used to simultaneously determine benserazide
and levodopa by voltammetry in tablet formulation and human
urine [89]. Multi-wall carbon nanotubes—poly (p-phenylene
terephthalamide) nanoparticles were produced and described to produce
a nanocomposite with increased electrical conductivity [90].
2.10 conclusIon and FuturE scopE
In this era of polymer and essential flexible electronics, society is striving

for damaged and crack-free materials. In actuality, self-healing material
production on an industrial scale will usher in a new technological era,
with the aerospace and military sectors undergoing profound change. It
will increase flexibility and decrease maintenance costs while extending
the usable life of all manufactured materials. The polymer has better
healing capacity due to the inclusion of graphene and carbon nanotubes.
Although graphene seems to be a viable SH material, incorporating it
into polymers is still challenging. Society is working to develop materials
impervious to breakage and cracking in this era of polymer and crucial
flexible electronics.
The industrial manufacturing of self-healing materials will usher
in a new technological era, with the aerospace and defence industries
experiencing significant upheaval. It will prolong the useful life of
all manufactured materials while increasing flexibility and lowering
maintenance expenses. Because graphene and carbon nanotubes are
present in the polymer, it has a more vital ability to repair. Although
graphene appears to be a potential SH material, adding it to polymers
remains difficult. The impact of CNTs and graphene on an intrinsic
self-healing behaviour in a polymer composite was investigated. Two
other extrinsic strategies are examining how CNT is used in practise
as a nanoreservoir and conducting in-depth self-healing investigations.
We are not far from the day manufactured materials will be able
to restore their structural integrity in the case of a breakdown, even
though self-healing is still a science fiction concept. The term “ohmic

overpotential” refers to the voltage loss caused by the electrolyte’s
resistance, and the term “mass transport overpotential” refers to the
loss caused by the time it takes for ions to move through the electrolyte.
Conductive polymer composites for energy harvesting are materials
that experience volume changes due to electrochemical reactions. If the
length between the electrodes were lowered, the resistance and ohmic
overpotential would also be decreased. The mass transport overpotential
and the amount of time it takes for ions to move between electrodes are
decreased by reducing the distance between them.
Therefore, it is necessary to choose the electrode distance to balance
the two. Conductive polymer composites are ineffective at converting
mechanical energy into electrical energy. It is because a chemical process,
rather than a direct conversion, is used to mediate the conversion
efficiency from mechanical to electrical. They are useless as transducers
in mechanical energy harvesters as a result. Conductive polymers with
carbon nanotube fillers are appealing options for creating electrodes for
thermogalvanic cells because of their low production costs, chemical
and thermal stability, and rapid electron transfer kinetics. All energy
conversion processes are inefficient to some extent. Energy is wasted as
heat in the following situations: motors, power transistors, car engines,
and light bulbs. Radio stations generate megawatts of RF, but the signals
they send to antennas are only microwatts. A fraction of this wasted
energy is captured, converted to power, and used by energy-harvesting
systems. The two most important energy sources nowadays are air
turbines and solar power panels. The most common energy-collecting
collectors are air turbines and large solar power panels. With proper
design, energy-harvesting devices may be capable of replacing batteries
in some applications.
The output of an energy-harvesting unit is often intermittent.
Therefore, a system containing a controller for a thin-film or charging
Li-ion battery, a valve for the MCU or sensors, and a wireless networking
module must be properly constructed. The possibility of an embedded
system becoming battery-free increases with the degree to which an
energy-harvesting device can satisfy all of its requirements. For mass,
motion, temperature, humidity, and light sensing applications, a wide
range of sensing materials, including metals, metal-organic frameworks,
metal oxide, solid electrolytes, sol-gel materials, carbon nanotubes, and
graphene nanocomposites, are utilised to make both direct and complex
sensors. As a result, we have made an effort to include many detection
materials as practical in our research and evaluate the overall data
linked with them, along with their potential interests and applications
in current sensors.
RefeRences
[1] Agency Energy and environment (EE) Report. 2008.
[2] Commission IE Efficient Electrical Energy Transmission and Distribution.
2007.
[3] Tritt, T.M., H. Böttner and L. Chen. 2008. Thermoelectrics: direct solar
thermal energy conversion. MRS Bull. 33(4): 366‒368.
[4] Abhat, A. 1981. Low temperature latent heat thermal energy storage:
heat storage materials. Sol. Energy 30(4): 313–32.
[5] Sharma, S.D., D. Buddhi and R.L. Sawhney. 1999. Accelerated thermal
cycle test of latent heat-storage materials, Sol. Energy 66: 483–490.
[6] Davis, E.S. and R. Bartern, 1975. Stratification in solar water heater storage
tanks. pp. 38–42. In: Proc. Workshop on Solar Energy Storage Subsystems
for Heating and Cooling of Buildings. Charlottesville, Virginia.
[7] George, A. 1989. Hand book of thermal design. In: Guyer, C. (ed.). Phase
Change Thermal Storage Materials. McGraw Hill Book Co.
[8] Heremans, J.P., C.M. Thrush, D.T. Morelli and M.C. Wu. 2002.
Thermoelectric power of bismuth nanocomposites. Phys. Rev. Lett. 88:
4361–5.
[9] He, W., G. Zhang, X. Zhang, J. Ji, G. Li and X. Zhao. 2015. Recent
development and application of thermoelectric generator and cooler.
Appl. Energy. 143: 1–25.
[10] Yang, J. and T. Caillat. 2006. Thermoelectric materials for space and
automotive power generation. MRS Bull. 31(3): 224‒229.
[11] Changyoon, J., J. Chanwoo, L. Seonghwan, Q.F. Maria and B.P. Young.
2020. Carbon nanocomposite based mechanical sensing and energy
harvesting. Int. J. Precis. Eng. Manuf. Green Technol. 1: 85.
[12] Dey, A., O.P. Bajpai, A.K. Sikder, S. Chattopadhyay and M.A.S. Khan.
2016. Recent advances in CNT/graphene based thermoelectric polymer
nanocomposite: A proficient move towards waste energy harvesting.
Renewable Sustainable Energy Rev. 53: 653–671.
[13] Thostenson, E.T., Z. Ren and T.W. Chou. 2001. Advances in the science
and technology of carbon nanotubes and their composites: A review.
Compos. Sci. Technol. 61(13): 1899–1912.
[14] Baughman, R.H., A.A. Zakhidov and W.A. de Heer. 2002. Carbon
nanotubes—the route toward applications. Science. 297(5582): 787–792
[15] Prasek, J., J. Drbohlavova, J. Chomoucka, J. Hubalek, O. Jasek, V. Adam,
et al. 2011. Methods for carbon nanotubes synthesis—review. J. Mater.
Chem. 21(40): 15872–15884.
[16] Iijima, S. 1991. Helical microtubules of graphitic carbon. Nature
354(6348): 56–58.
[17] Farukh, M., R. Dhawan, B.P. Singh and S. Dhawan. 2015. Sandwich
composites of polyurethane reinforced with poly(3,4-ethylene
dioxythiophene)—coated multi-walled carbon nanotubes with
exceptional electromagnetic interference shielding properties. RSC Adv.

5(92): 75229–75238.
[18] Mathur, R., S. Pande, B. Singh and T. Dhami. 2008. Electrical and
mechanical properties of multi-walled carbon nanotubes reinforced
PMMA and PS composites. Polym. Compos. 29(7): 717–727.
[19] Graham, A.P., G.S. Duesberg, W. Hoenlein, F. Kreupl, M. Liebau,
R. Martin, et al. 2005. How do carbon nanotubes fit into the semiconductor
roadmap? Appl. Phys. A 80(6): 1141–1151.
[20] Jindal, P., S. Pande, P. Sharma, V. Mangla, A. Chaudhury, D. Patel,
et al. 2013. High strain rate behavior of multi-walled carbon nanotubes-
polycarbonate composites. Compos. B Eng. 45(1): 417–422.
[21] Pande, S., B.P. Singh and R.B. Mathur. 2014. Processing and properties of
carbon nanotube/polycarbonate composites. pp. 333–364. In: V. Mittal (ed.).
Polymer Nanotubes Nanocomposites: Synthesis, Properties and
Applications, 2nd Ed. Wiley. New Jersey.
[22] Singh, B.P., P. Saini, T.K. Gupta, P. Garg, G. Kumar, I. Pande, et al. 2011.
Designing of multi-walled carbon nanotubes reinforced low density
polyethylene nanocomposites for suppression of electromagnetic
radiation. J. Nanopart Res. 13(12): 7065–7074.
[23] Fim, F.C., N.R. Basso, AP. Graebin, D.S. Azambuja and G.B. Galland.
2013. Thermal, electrical, and mechanical properties of polyethylene–
graphene nanocomposites obtained by in situ polymerization, J. Appl.
Polym. Sci. 128(5): 2630–2637.
[24] Li, B. and, J. Zhang. 2015. Polysiloxane/multi-walled carbon nanotubes
nanocomposites and their applications as ultrastable, healable and super
hydrophobic coatings. Carbon. 93: 648–658.
[25] Wang, Z., Y. Yang, R. Burtovyy, I. Luzinov and M.W. Urban. 2014.
UV-induced self-repairing polydimethylsiloxane–polyurethane (PDMS–
PUR) and polyethylene glycol–polyurethane (PEG–PUR) Cu-catalyzed
networks. J. Mater. Chem. A 2(37): 15527–15534.
[26] Ling, J., M.Z. Rong and M.Q. Zhang. 2012. Photo-stimulated self-healing
polyurethane containing dihydroxyl coumarin derivatives. Polymer
53(13): 2691–2698.
[27] Verma, M., P. Verma, S. Dhawan and V. Choudhary. 2015. Tailored
graphene based polyurethane composites for efficient electrostatic
dissipation and electromagnetic interference shielding applications. RSC
Adv. 5(118): 97349–97358.
[28] Saini, P., V. Choudhary, B. Singh, R. Mathur and S. Dhawan. 2011.
Enhanced microwave absorption behavior of polyaniline-CNT/
polystyrene blend in 12.4–18.0 GHz range. Synth. Met. 161(15): 1522–1526.
[29] Shahzad, F., S. Yu, P. Kumar, J.-W. Lee, Y.-H. Kim, S.M. Hong, et al. 2015.
Sulfur doped graphene/polystyrene nanocomposites for electromagnetic
interference shielding. Compos. Struct. 133: 1267–1275.
[30] Babal, A., R. Gupta, B. Singh, V. Singh, R. Mathur and S. Dhakate. 2014.
Mechanical and electrical properties of high performance MWCNT/
polycarbonate composites prepared by industrial viable twin screw
extruder with back flow channel. RSC Adv. 4: 64649–64658.
[31] Gedler, G., M. Antunes, J. Velasco and R. Ozisik. 2016. Enhanced

electromagnetic interference shielding effectiveness of polycarbonate/
graphene nanocomposites foamed via 1-step supercritical carbon dioxide
process. Mater. Des. 90: 906–914.
[32] Shen, B., W. Zhai, M. Tao, D. Lu and W. Zheng. 2013. Enhanced
interfacial interaction between polycarbonate and thermally reduced
graphene induced by melt blending. Compos. Sci. Technol. 86: 109–116.
[33] Zeng, X., J. Yang and W. Yuan. 2012. Preparation of a poly (methyl
methacrylate)-reduced graphene oxide composite with enhanced
properties by a solution blending method. Eur. Polym. J. 48(10): 1674–1682.
[34] Malinskii, Y.M., V. Prokopenko, N. Ivanova and V. Kargin. 1970.
Investigation of self-healing of cracks in polymers. Polym. Mech. 6(2):
240–244.
[35] Wool. R.P. 1980. Crack healing in semicrystalline polymers, block
copolymers and filled elastomers. pp. 341–362. In: Adhesion and
Adsorption of Polymers. Springer. Berlin.
[36] Wool, R. and K. O’connor. 1981. A theory crack healing in polymers.
J. Appl. Phys. 52(10): 5953–5963.
[37] White, S.R., N. Sottos, P. Geubelle, J. Moore, M.R. Kessler, S. Sriram,
et al. 2001. Autonomic healing of polymer composites. Nature. 409(6822):
794–797.
[38] Kessler, M. 2007. Self-healing: A new paradigm in materials design.
Proceedings of the institution of mechanical engineers. Part G. J. Aerosp.
Eng. 221(4): 479–495.
[39] Lanzara, G., Y. Yoon, H. Liu, S. Peng and W. Lee. 2009. Carbon nanotube
reservoirs for self-healing materials. Nanotechnology 20(33): 335704.
[40] Sinham, R.S., D. Pelot, Z. Zhou, A. Rahman, X.-F. Wu and A.L. Yarin.
2012. Encapsulation of self-healing materials by coelectrospinning,
emulsion electrospinning, solution blowing and intercalation. J. Mater.
Chem. 22(18): 9138–9146.
[41] Mario, L. and N. Ahmed. 2011. Organic thermoelectrics: Green energy
from a blue polymer. Nat. Mater. 10(6): 409–410.
[42] Blaiszik, B., S. Kramer, S. Olugebefola, J.S. Moore, N.R. Sottos and
S.R. White. 2010. Self-healing polymers and composites. Annu. Rev.
Mater. Res. 40: 179–211.
[43] Yuan, Y., T. Yin, M. Rong and M. Zhang. 2008. Self-healing in polymers
and polymer composites. Concepts, realisation and outlook: A review.
Polym. Lett. 2(4): 238–250.
[44] Herbst, F., D. Döhler, P. Michael and W.H. Binder. 2013. Self-healing
polymers via supramolecular forces. Macromol. Rapid. Commun. 34(3):
203–220.
[45] Aissa, B., E. Haddad, W. Jamroz, S. Hassani, R. Farahani, P. Merle,
et al. 2012. Micromechanical characterisation of single-walled carbon
nanotube reinforced ethylidene norbornene nanocomposites for self-
healing applications, Smart. Mater. Struct. 21(10): 105028.
[46] Bailey, B.M., Y. Leterrier, S. Garcia, S. Van Der Zwaag and V. Michaud.
2015. Electrically conductive self-healing polymer composite coatings.
Prog. Org. Coat. 85: 189–198.
[47] Cui, W., J. Ji, Y.-F. Cai, H. Li and R. Ran. 2015. Robust, Anti-fatigue,
and self-healing graphene oxide/hydrophobically associated composite
hydrogels and their use as recyclable adsorbents for dye wastewater
treatment. J. Mater. Chem. A 3(33): 17445–17458.
[48] Jyoti, J., S. Basu, B. Singh and S. Dhakate. 2015. Superior mechanical
and electrical properties of multi-wall carbon nanotube reinforced
acrylonitrile butadiene styrene high performance composites. Compos.
B. Eng. 83: 58–65.
[49] Sharma, S., V. Gupta, R. Tandon and V. Sachdev. 2016. Synergic effect of
graphene and MWCNT fillers on electromagnetic shielding properties of
graphene–MWCNT/ABS nanocomposites. RSC Adv. 6(22): 18257–18265.
[50] Rowe, D.M. 1995. In Handbook of Electrics. CRC Press. Boca Raton. FL.;
b) Zhao D., Tan G. 2014. A review of thermoelectric cooling: materials,
modeling and applications. Appl. Therm. Eng. 66(1‒2): 15‒24.
[51] Riffat, S.B. and X. Ma. 2003. Thermoelectrics: a review of present and
potential applications. Appl. Therm. Eng. 23(8): 913‒935.
[52] DiSalvo, F.J. 1999. Thermoelectric cooling and power generation. Science
285(5428): 703–706.
[53] Chen, G., M.S. Dresselhaus, G. Dresselhaus, J.-P. Fleurial and T. Caillat.
2003. Recent developments in thermoelectric materials. Int. Mater. Rev.
48(1): 45‒66.
[54] Snyder, G.J. and E.S. Toberer. 2008. Complex thermoelectric materials.
Nat. Mater. 7(2): 105–114.
[55] Tee, B.C., C. Wang, R. Allen and Z. Bao 2012. An electrically and
mechanically self-healing composite with pressure-and flexion-sensitive
properties for electronic skin applications. Nat. Nanotechnol. 7(12):
825–832.
[56] Han, Y., T. Wang, X. Gao, T. Li and Q. Zhang. 2016. Preparation of
thermally reduced graphene oxide and the influence of its reduction
temperature on the thermal, mechanical, flame retardant performances
of PS nanocomposites. Compos. A Appl. Sci. Manuf. 84: 336–343.
[57] Garg, P., B.P. Singh, G. Kumar, T. Gupta, I. Pandey, R. Seth, et al. 2010.
Effect of dispersion conditions on the mechanical properties of multi-
walled carbon nanotubes-based epoxy resin composites. J. Polym. Res.
18(6): 1397–1407.
[58] Singh, B.P, K. Saini, V. Choudhary, S. Teotia, S. Pande, P. Saini,
et al. 2014. Effect of length of carbon nanotubes on electromagnetic
interference shielding and mechanical properties of their reinforced
epoxy composites. J. Nanopart. Res. 16(1): 1–11.
[59] Tang, L.-C., Y.-J. Wan, D. Yan, Y.-B. Pei, L. Zhao, Y.-B. Li, et al. 2013. The
effect of graphene dispersion on the mechanical properties of graphene/
epoxy composites. Carbon 60: 16–27.
[60] Mathur, R.B., B.P. Singh, T. Dhami, Y. Kalra, N. Lal, R. Rao, et al. 2010.
Influence of carbon nanotube dispersion on the mechanical properties of
phenolic resin composites. Polym. Compos. 31(2): 321–327.
[61] Ansari, M.O. and F. Mohammad. 2011. Thermal stability, electrical
conductivity and ammonia sensing studies on p-toluenesulfonic acid
doped polyaniline: titanium dioxide (pTSA/Pani: TiO2) nanocomposites.
Sensor and Actuators B 157: 122–129.
[62] Sun T., Y. Fan, P. Fan, F. Geng, P. Chen and F. Zhao. 2019. Use of graphene
coated with ZnO nanocomposites for microextraction in packed syringe
of carbamate pesticides from juice samples. J. Separ. Sci. 42: 2131–2139.
[63] Kondawar, S.B., P.T. Patil and S.P. Agrawal. 2014. Chemical vapour
sensing properties of electrospun nanofibers of polyaniline/ ZnO
nanocomposites. Adv. Mater. Lett. 5: 389–395.
[64] Rahman, M.M., M.A. Hussein, K.A. Alamry, F.M. Al-Shehry and
A.M. Asiri. 2018. Polyaniline/graphene/carbon nanotubes nanocomposites
for sensing environmentally hazardous 4-aminophenol. Nano-Struct.
Nano-Objects. 15: 63–74.
[65] Zhang, J., X. Liu, S. Wu, H. Xu and B. Cao, Sens. 2013. One-pot fabrication
of uniform polypyrrole/Au nanocomposites and investigation for gas
sensing. Sens. Actuators B. 186: 695–700.
[66] Mane, A.T., S.T. Navale, S. Sen, D.K. Aswal, S.K. Gupta and V.B. Patil.
2015. Nitrogen dioxide (NO2) sensing performance of p-Polypyrrole/n-
Tungsten oxide hybrid nanocomposites at room temperature. Org.
Electron. 16: 195–204.
[67] Rawool, C.R., N.S. Punde, A.S. Rajpurohit, S.P. Karna and A.K. Srivastava.
2018. High energy density supercapacitive material based on a ternary
hybrid nanocomposite of cobalt hexacyanoferrate/carbon nanofibers/
polypyrrole. Electrochim. Acta 268: 411–423.
[68] Feng, L. K. Wang, X. Zhang, X. Sun, C. Li, X. Ge, et al. 2018. Flexible solid-
state supercapacitors with enhanced performance from hierarchically
graphene nanocomposite electrodes and ionic liquid incorporated gel
polymer electrolyte. Adv. Funct. Mater. 28: 1704463–1704475.
[69] Idrees, M., S. Batool, J. Kong, Q. Zhuang, H. Liu, Q. Shao, et al. 2019.
Polyborosilazane derived ceramics - Nitrogen sulfur dual doped
graphene nanocomposite anode for enhanced lithium ion batteries.
Electrochim. Acta 296: 925–937.
[70] Song, Z., Y.L. Ma and C.E. Li. 2019. The residual tetracycline in
pharmaceutical wastewater was effectively removed by using MnO2/
graphene nanocomposite. Sci. Total Environ. 651: 580–590.
[71] Ananthanarayanan, A., X. Wang, P. Routh, B. Sana, S. Lim, D.H. Kim,
et al. 2014. Facile synthesis of graphene quantum dots from 3D graphene
and their application for Fe3+ sensing. Adv. Funct. Mater. 24: 3021–3026.
[72] Cheng, Y., R. Wang, J. Sun and L. Gao. 2015. A Stretchable and highly
sensitive graphene-based fiber for sensing tensile strain, bending, and
torsion. Adv. Mater. 27: 7365–7371.
[73] Liu, Q., Y. Tang, X. Yang, M. Wei and M. Zhang. 2019. An ultra-low
detection limit glucose sensor based on reduced graphene oxide-concave
tetrahedral Pd NCs@CuO composite. J. Electrochem. Soc. 166(6): B381–
387.
[74] Neri, G., S.G. Leonardi, M. Latino, N. Donato, S. Baek, D. E. Conte, et al.
2013. Sensing behavior of SnO2/reduced graphene oxide nanocomposites
toward NO2. Sens. Actuators B. 179: 61–68.
[75] Zhang, H., J. Feng, T. Fei, S. Liu and T. Zhang. 2014. SnO2 nanoparticles-
reduced graphene oxide nanocomposites for NO2 sensing at low
operating temperature. Sens. Actuators B 190: 472–478.
[76] An, X., X. Yu, C. Y. Jimmy and G. Zhang. 2013. CdS nanorods/reduced
graphene oxide nanocomposites for photocatalysis and electrochemical
sensing. J. Mater. Chem. A 1: 5158–5164.
[77] Xu, F., M. Deng, G. Li, S. Chen and L. Wang. 2013. Electrochemical
behavior of cuprous oxide–reduced graphene oxide nanocomposites
and their application in nonenzymatic hydrogen peroxide sensing.
Electrochim. Acta 88: 59–65.
[78] Reddy, Y.V.M., S. Bathinapatla, T. Łuczak, M. Osińska and H. Maseed.
2018. An ultra-sensitive electrochemical sensor for the detection of
acetaminophen in the presence of etilefrine using bimetallic Pd–Ag/
reduced graphene oxide nanocomposites. New J. Chem. 42(4): 3137–
3146.
[79] Govindasamy, M., S.F. Wang, B. Subramanian, R.J. Ramalingam,
H. Al-lohedan and A. Sathiyan. 2019. A novel electrochemical
sensor for determination of DNA damage biomarker (8-hydroxy-2′-
deoxyguanosine) in urine using sonochemically derived graphene
oxide sheets covered zinc oxide flower modified electrode. Ultrason.
Sonochem. 58: 104622–104623.
[80] Yu, L., F. Ma, X. Ding, H. Wang and P. Li. 2018. Silica/graphene oxide
nanocomposites: Potential adsorbents for solid phase extraction of
trace aflatoxins in cereal crops coupled with high performance liquid
chromatography. Food Chem. 245: 1018–1024.
[81] Liu, H., J. Gao, W. Huang, K. Dai, G. Zheng, C. Liu, et al. 2016. Electrically
conductive strain sensing polyurethane nanocomposites with synergistic
carbon nanotubes and graphene bifillers. Nanoscale 8: 12977–12990.
[82] Le, X.V., T.L.A. Luu, H.L. Nguyen and C.T. Nguyen. 2019. Synergistic
enhancement of ammonia gas-sensing properties at low temperature by
compositing carbon nanotubes with tungsten oxide nanobricks. Vacuum
168: 108861–108861.
[83] Jalilian, N., H. Ebrahimzadeh and A.A. Asgharinezhad. 2018.
Determination of acidic, basic and amphoteric drugs in biological fluids
and wastewater after their simultaneous dispersive micro-solid phase
extraction using multiwalled carbon nanotubes/magnetite nanoparticles@
poly(2-aminopyrimidine) composite. Microchem. J. 143: 337–349.
[84] Abbasghorbani. M. 2018. Fe3O4 loaded single wall carbon nanotubes and
1-methyl-3-octylimidazlium chloride as two amplifiers for fabrication
of highly sensitive voltammetric sensor for epirubicin anticancer drug

analysis. J. Mol. Liq. 266: 176-180.
[85] Norouzi, P., B. Larijani, T. Alizadeh, E. Pourbasheer, M. Aghazadeh and
M.R. Ganjali. 2019. Application of advanced electrochemical methods
with nanomaterial-based electrodes as powerful tools for trace analysis
of drugs and toxic compounds. Curr. Anal. Chem. 15(2): 143–151.
[86] Upadhyay S.S. and A.K. Srivastava. 2019. Hydroxypropyl b-cyclodextrin
cross-linked multiwalled carbon nanotube-based chiral nanocomposite
electrochemical sensors for the discrimination of multichiral drug
atorvastatin isomers. New. J. Chem. 43: 11178–11188.
[87] Niezabitowska, E., J. Smith, M.R. Prestly, R. Akhtar, F.W. von Aulock,
Y. Lavallée, et al. 2018. Hydroxypropyl b-cyclodextrin cross-linked
multiwalled carbon nanotube-based chiral nanocomposite electrochemical
sensors for the discrimination of multichiral drug atorvastatin isomers.
RSC Adv. 8: 16444–16454.
[88] Wang, Y., J. Luo, J. Liu, X. Li, Z. Kong, H. Jin, et al. 2018. Electrochemical
integrated paper-based immunosensor modified with multi-walled carbon
nanotubes nanocomposites for point-of-care testing of 17b-estradiol.
Biosens. Bioelectron. 107: 47–53.
[89] Miraki, M., H. Karimi-Maleh, M.A. Taher, S. Cheraghi, F. Karimi,
S. Agarwal, et al. 2019. Voltammetric amplified platform based on
ionic liquid/NiO nanocomposite for determination of benserazide and
levodopa. J. Mol. Liq. 278: 672–676.
[90] Mazrouaa, A.M., N.A. Mansour, M.Y. Abed, M.A. Youssif,
M.A. Shenashen and Md R. Awual. 2019. Nano-composite multi-wall
carbon nanotubes using poly(p-phenylene terephthalamide) for enhanced
electric conductivity. J. Environ. Chem. Eng. 7(2): 103002–103003.
Chapter 3
Nanostructured Silicon for

Solar Energ y Conversion
Applications
Ragavendran Venkatesan1, Jeyanthinath Mayandi1*,

Terje G. Finstad2*, J.M. Pearce3 and
Vishnukanthan Venkatachalapathy2,3
1
Department of Materials Science, School of Chemistry,
Madurai Kamaraj University, Madurai – 625 021, India.
2
Department of Physics/Centre for Materials Science and Nanotechnology,
University of Oslo, P.O. Box 1048 Blindern, N-0316 Oslo, Norway.
3
Department of Electrical and Computer Engineering,
Western University, London, ON, Canada.
4
Department of Materials Science, National Research Nuclear University
“MEPhI”, 31 Kashirskoe sh, Moscow, Russian Federation.
3.1 IntroductIon
Climate change and global warming caused by the burning of fossil

fuels, resulting in carbon dioxide (CO2) emissions, have led to an
urgent need for abundant and cost-efficient renewable energy sources.
*For Correspondence: E-mail: jeyanthinath.chem@mkuniversity.org

Email: terje.finstad@fys.uio.no
Nanostructured Silicon for Solar Energy Conversion Applications 55
The available renewable energy sources are wind energy, hydropower,

geothermal, and solar energy. Among various renewable energies, solar
energy is the most abundant and universally available. Sunlight strikes
the Earth’s surface at a rate of ~120,000 TW [1]. Sunlight can be captured
and converted into electricity by solar photovoltaic (PV) cells. It can also
be utilised to produce chemical fuels, such as methanol and hydrogen
via CO2 reduction [2] and water splitting [3] in photoelectrochemical
cells. Solar energy, however, is diffuse (~100 mW cm–2) and diurnal, only
available during daytime [4], requiring large areas of solar collectors
to harvest civilisational significant amounts of energy. PV energy
generation is the fastest-growing energy source [5]. Until now, the high
cost and low efficiency hinder the wide installation of these solar energy
conversion systems. For instance, even though the solar cell industry
has experienced a significant expansion in the past decade, the total
installed capacity is only ~1.1 TW in 2022 [6]. The levelised cost of
electricity from PV is already lower than conventional generation [7],
and the value of solar is much higher than conventional electricity [8]
because of the environmental benefits that make it a truly sustainable
energy source [9]. Even though solar is the lowest-cost source of electric
power [10] according to the EIA [11], the cost of producing electricity
from solar must decrease further to accelerate the investment needed
to eliminate all use of fossil fuels for energy. At present, photovoltaics
is the most elegant method to produce electricity without moving
parts, emissions, or noise, and all this by converting abundant sunlight
without practical limitations. The relevance of solar energy specifically
PV can be justified mainly by the factors like scalability, sustainability,
environmental impact, and the security of the source [12]. The scalability
means the abundant availability of solar radiation to be utilised for
PV. Solar cells are zero-emission electricity generators, which proves
their environmental friendliness. Therefore, it is crucial to develop even
more cost-effective solar energy conversion systems with high light
absorption/conversion efficiency. In order to decrease cost and increase
power output simultaneously, novel processes must be introduced in
the conventional production of solar cells. These processes must result
in an increased power conversion efficiency of the solar cells while
making the fabrication process itself less expensive. At the same time,
only significantly low-cost and abundant materials may be used in cell
production in order to be able to scale-up the production to the order
of magnitude of the global power consumption, which is in the TW
range [13].
Progress made in nanotechnology can be used as a driving tool to
produce less-expensive and more efficient solar cells. Nanostructures
have attracted much interest over the past few years as they offer very
attractive features, which make them good candidates for solar cell
applications [14]. In particular, porous silicon and silicon nanowires

(SiNWs) have remarkable optical properties, such as antireflection and
light-trapping effects as illustrated in Figure 3.1, allowing for efficient
light harvesting capabilities [15]. A low-temperature and high-pressure
oxidation process is done to passivate the formed porous silicon.
These excellent optical properties are partly due to the sub-
wavelength structure offered by the porous silicon and SiNWs, acting
like an antireflective layer. The enhanced absorption is also explained
by the electromagnetic wave interaction with the SiNWs. Many papers
have been published recently on NW-based solar cells. Some attempts
have already been carried out using CdS, CdSe, CdTe, ZnO, or TiO2 NWs
[16–20]. Nevertheless, the toxicity of CdS, CdSe, and CdTe as well as
the high energy gap (Eg > 3 eV) of ZnO and TiO2 nanostructures make
these materials unsuitable for efficient devices. In this chapter, we focus
on silicon nanostructures as they are environmental-friendly, abundant,
well-understood, and show excellent light trapping and antireflection
properties and Si already dominates the PV industry.
Figure 3.1 shows that the light is effectively trapped within the Si
nanostructures. One common strategy to reduce the cost of Si-based
solar cells is to use thinner wafers so less material is necessary. The
thickness reduction, however, is detrimental to the optical absorption.
Indeed, solar cells based on c-Si are generally relatively thick because
of the indirect band gap of Si, which requires large thickness to absorb
most of the incoming light.
Figure 3.1 Illustration of the light-trapping effect of Si nanostructures.
Porous silicon and SiNWs can be used as a driving force to transfer

the bulk c-Si-based solar cells to the thin films technology and cut the
material and process costs of the PV modules.
Indeed, the perfect light-trapping properties of SiNW arrays enable
high absorption in just 1 µm thick long NW arrays, rendering the
transfer to the thin film technology highly desirable. By using NWs, we
are able to produce high-quality materials for the thin film technology,
and therefore high efficiency is expected. The ultimate goal will be to
produce high-efficiency solar cells based on SiNWs.
One of the most important aspects to improve overall cell efficiency
is by reducing the front surface reflectance of crystalline material.
Compared with a 100% reflected mirror, polished silicon wafers have
surface reflectivity from 70% to 30%, depending on the wavelength of
incident light [21]. In shorter wavelength ranges, silicon wafers appear
to have a higher average reflectivity and a higher average absorption
coefficient than compared to longer wavelength ranges. In this case,
reduced reflectivity in wavelength regions with high absorption will
significantly enhance PV cell efficiency.
Reducing front surface reflectivity is a commonly used method in
modern solar cell fabrication. Antireflection (AR) coating and surface
textured structures are the most often employed technologies to reduce
surface reflectivity. AR coating is achieved by depositing a thin layer of
low-reflectivity material, on the front surface of the solar cell to reduce
reflectivity at specific wavelengths [22]. The AR layer can only reduce
reflectivity at specific wavelengths, however, which is not sufficient
when solar cells are excited under the whole solar light spectra. A
flat surface has higher reflectivity than a textured surface since more
light can be instantly reflected from the surface. The surface textured
structure methodology uses a chemical solution to etch the front surface
of the solar cell to reduce reflectivity. Due to precise and controlled
chemical etching, textured structures in the order of nano/micro scale
can be fabricated [23]. Black silicon (b-Si) is one such type of surface
textured structure. Compared with normal micron-sized surface texture
structures, the black silicon structures are smaller to be in a nanometer
scale and have a larger distribution angle to incident light.
3.2 reduced Surface reflectIvIty
Black silicon (b-Si) structures feature needle-like surface structures,

where these needle-like shaped structures are made of crystalline silicon
(c-Si) material. The b-Si solar cell is a c-Si solar cell that has the b-Si
structure on the front surface. The reflectivity of untreated c-Si solar
cells reduces the number of photons injected into the solar cell, which
decreases cell efficiency at the same time. The average reflectivity of an
untreated c-Si solar cell is around 40%. By using the b-Si structures on
the front surface, the front surface reflectivity of the c-Si solar cell can
be controlled below 10%, which means that an additional 20% of photons
from incident light are injected into the solar cell substrate compared to
the conventional c-Si solar cells [24].
The b-Si structures on the c-Si solar cell front surface can reduce
the reflectivity according to diffraction effects and the “moth-eye effect”
occurring under incident light. The reflectivity of c-Si solar cells is
due to the abrupt transition of light from the surrounding medium to
the solar cell. Increased refractivity difference between the solar cell
and the surrounding media will increase reflectivity on the solar cell-
surrounding media interface. The interface reflectivity can be reduced
when the disparity between the two mediums is decreased. The b-Si
structures on the solar cell are constructed at the nanometer scale
so that the geometry of b-Si structures can reduce the reflectivity at
different wavelengths by making a continuous transition between two
materials [25]. When incident light is in a shorter wavelength region,
the diffraction effects occur at the b-Si structures since the dimensions
of the needle-like structures are comparable to the wavelength of
incident light. As the incident light has been diffracted through the
b-Si structures, the incident light can be diffracted to different angles,
which would increase the path length of light in the substrate. When
the incident light is at a longer wavelength region, the moth-eye effect
occurs at the b-Si structures since the dimensions of the needle-like
structures are close to the wavelength of incident light. The surface on
the b-Si structures behaves like an effective medium, and the effective
index of refraction changes continuously from that of the surrounding
medium to that of the needle-like structures. Hence, the reflectivity on
the b-Si structures is lower than the reflectivity on conventional c-Si
solar cells [26]. The best b-Si can produce record PV efficiencies [27]
and use dry etching [28], which can be cost-effective in mass production
compared to conventional optical engineering strategies [29]. There are
several approaches, however, to get to high-efficiency black silicon solar
cells that we will review here.
3.3 SIlIcon nanoStructureS
Silicon nanostructures (SiNS) are classified into different types, but this
chapter will focus on two structures used in solar cell applications:
(i) Porous silicon
(ii) Silicon nanowires
3.3.1 Porous Silicon (PS)

The root of porous silicon (PS) can be traced back to the pioneering work
of Arthur Uhlir Jr. and Ingeborg Uhlir in 1956 [30]. PS was discovered
accidentally when they performed electropolishing of silicon wafers
in hydrofluoric acid-containing electrolytes. This early work, however,

attracted little attention because the PS films were mistakenly suggested
to be silicon sub-oxide. It was not until the early 1970s, the porous
features of Si were observed for the first time on the Si surface. Watanabe
and Sakai demonstrated the first application of P-Si in electronics in
1971 [31]. They oxidised PS to achieve dielectric isolation of silicon
devices. In 1986, Takai and Itoh first introduced silicon-on-insulator
(SOI) technology to the integrated circuit industry [32]. In 1990, Leigh
Canham discovered the tuneable photoluminescence of PS at room
temperature due to quantum-confinement effects [33], which surged a
tremendous amount of research effort to study PS and was believed to
hold great promise in silicon-based integrated optoelectronics. In the
late 1990s, interesting photonic structures (e.g., Bragg stacks, rugate
filters, and microcavities) were conveniently achieved using PS, and
the extraordinary biocompatibility of PS was demonstrated. Then,
the focus of the PS researches slowly extended to other directions
towards applications for antireflective coatings [34], interference filters,
waveguides, solar cells [35], biomedical implants, drug delivery [36],
chemical and biological sensors [37], and many other applications.
3.3.2 Silicon nanowire (SinW)

Nanostructures such as nanotubes and nanowires offer unique access
to low-dimensional physics. Nanowires could be important, as they
can be considered the building blocks of a technology, which harnesses
the quantum size effects for useful device applications. Such one-
dimensional (1-D) nanomaterials can play a key role in nanotechnology,
as well as provide model systems to demonstrate quantum size
effects. Silicon-based nanowires including silicon, silicate, and silicide
nanowires are particularly attractive due to the central role of silicon in
the semiconductor industry. Potential applications include the field of
photonics, interconnects, sensors, and nano-electromechanical systems;
perhaps most importantly, they can also be applied to photovoltaics.
For SiNWs, the carrier type and concentration can be controlled
by doping, as in bulk silicon. Furthermore, silicon turns into a direct
bandgap semiconductor at nanometer size, due to quantum confinement,
opening an entirely new field of optoelectronic applications for such
nano-sized silicon-based devices. For instance, Si-nanowire-based
devices have shown stable electroluminescent at room temperature. In
addition, Si-nanowires have also shown efficiency enhancement in test
photovoltaic applications, arising from enhanced optical absorption in
the nanostructured form of silicon. Even though Si-nanowire research
has led to a clear improvement in device performance in terms of
efficiency, there is still much scope in understanding the growth and
behaviour of this material before viable device production becomes

possible. Theoretical studies suggest that the band-gap character (direct
or indirect) strongly depends on the crystallographic direction of the
nanowire axis. Also, the magnitude of the direct or indirect band-gap
due to quantum confinement depends on the nanowire growth direction
and the nanowire diameter [38].
At the nano-scale, quantum-confinement effects are important, which
affect the properties of materials. So, it is important to see the quantum-
confinement effects and exciton effects at the nano-scale, which affects
the optical properties of the nanowires. The orientation of the SiNWs
grown depends on the diameter of the nanowires. Smaller SiNWs whose
diameter range from 3 nm to 10 nm, are grown primarily along the (110)
direction. Also, medium SiNWs whose diameter ranges from 10 nm to 20
nm, grow primarily along the (112) direction, and large SiNWs, whose
diameter ranges from 20 nm to 30 nm, grow primarily along the (111)
direction [39]. Several possible structures for SiNWs have been reported.
These include fullerene-like cage structures (clathrate), tetrahedral
structures, and polycrystalline nanowires [40–42] as shown in Figure 3.2.
Figure 3.2 Schematic presentations of small-diameter silicon nanowires

(a) tetrahedral (top), (b) Si34-clathrate, (c) Si46-clathrate, and (d) polycrystalline
types of nanowires [42].
3.3.3 Physical Properties of Silicon nanostructures

1-D silicon nanostructures have one free direction in which the electrical
carriers can flow. Their physical properties are significantly different
from those in bulk silicon, as explained below.
3.3.3.1 Electronic and Optical Properties

Silicon nanostructure’s quantum confinement is manifested as a change
in the band gap from the bulk material [43]. It is usually accepted that
the band gap opens as a result of quantum confinement, which pushes

the PL in the visible for crystallite sizes below 5 nm [44]. It has even
been proposed that quantum confinement explains the entire PL from
P-Si. However, many groups have reported that when the crystallite size
decreases to a few nanometers, the PL in the air does not increase much
beyond 2.1 eV, even when the crystallite size drops well below 3 nm [45].
This observation does not coincide with theory, which predicts a much
larger opening of the band gap, in excess of 3 eV for sizes below 2 nm.
For 1-D nanostructures, the band gap is directly related to the diameter
of the nanowires [46].
1
Band gap a ;1£ n£ 2
dn
where d is the diameter of the 1-D nanostructure.

Silicon nanostructures have exhibited band-gap variations from 1.1
to 3.95 eV with diameters ranging from 7 nm to 1.3 nm, respectively [47].
The electronic properties such as the band gap, valley splitting, and
effective mass also scale as a function of the diameter [48]. These affect
the transport properties of the nanostructures like nanowires [49].
Hydrogen and oxygen-terminated SiNWs have been studied to gain an
understanding of their optical and electronic properties. Confinement
in SiNWs renders them optically active (direct band gap) and is
independent of their specific orientation [50]. Quantum confinement in
nanowires could play an important role in photovoltaic devices. SiNWs
could also be used as light-emitting diodes and lasers at small diameters.
3.4 thermal and mechanIcal ProPertIeS
Mesoporous silicon has tuneable thermal properties that can be radically

different from those of bulk silicon. The combination of very low
thermal conductivity and heat capacity of high PS has led to a range
of potential applications, such as thermal isolation of microdevices, chip-
based ultrasound emission, thermoelectrics, and photothermal therapy.
SiNWs could be used in nano-scale thermoelectric power generator
applications [51]. Therefore, it is important to study their thermal
conductivity. Thermal conductivity in thin SiNWs having diameters
between 1.4 and 8.3 nm has been studied. As the nanowire diameter
decreases, the surface-to-volume ratio increases, which increases the
surface scattering effects. This decreases the thermal conductivity of
the nanowires. While at a very small diameter (<1.5 nm), quantum-
confinement effects occur and these increase the thermal conductivity.
The values of Young’s modulus for SiNWs were estimated to be 186 and
207 GPa, respectively, for single- and double-clamped SiNWs and the
values are close to the bulk value of 169 GPa for Si (111) [52].
3.4.1 light trapping

One of the common techniques used to decrease the front-face
reflectance is by texturing the surface and thus enhancing the level
of absorption of incident light by increasing the number of reflections,
optical path length, and total internal reflection. In Si-wafer-based solar
cells, texturing can also improve light absorption within the bulk of
the cell by absorbing photons closer to the collection junction and by
making use of weakly absorbed near band-gap photons. Depending
on the texturing methodology and the crystallographic orientation of
silicon surface planes, there are different types of texture morphologies
produced on silicon surfaces. The most commonly being the upright
pyramid structures demonstrated in Figure 3.3.
Figure 3.3 Pyramids as surface textures.
The surface textures decrease the overall reflectance by three distinct

mechanisms. First, a portion of the light that is incident on the Si wafer
is reflected from and onto the textured surface resulting in multiple
reflections or multiple light bounces (Figure 3.4; case 1).
Figure 3.4 Normally incident light behaviour on a textured surface (on the
right) comparison with a flat surface (on the left).
It increases, the amount of transmitted solar flux and thus the

probability of photon absorption since initially reflected photons will
have at least one other chance to enter the solar cell [53]. The longer
path length of the refracted light carrying photons is the second reason
for increased photon absorption and absorption closer to the collection
junction. Normally incident light propagates within the bulk of the
silicon at an angle in textured solar cells. This results in longer path
lengths that light will travel before reaching the rear surface, as opposed
to the path lengths of normally incident light refracted from a flat
surface (see Figure 3.4, case 2) [54]. This is especially important if the
diffusion lengths are comparable to or less than the cell thickness [55].
Finally, increased total internal reflection probability also increases
the overall transmitted flux since it does not permit the light from
escaping the silicon material (Figure 3.4; case 3). This phenomenon
is also known as light-trapping. Long wavelength photons that the
semiconductor material was unable to absorb are reflected from the
back surface to the front.
Because of the poor optical properties of silicon material (absorption
coefficient), the thickness of solar cells has to be relatively large in order
to absorb 99% of the solar spectrum, which leads to an increase in weight,
production cost, and recombination probability in the bulk. Since light
absorption is enhanced in textured cells, the thickness of such cells can
remain relatively small [56]. Some other techniques, such as antireflection
coatings (ARC) can be used to reduce front-face reflection. ARC is a thin
layer of an optimal thickness of dielectric material. This layer terminates
the reflected beams by interference. In fact, texturing can be used in
combination with an ARC. Such configuration can produce even lower
reflectance than each of these individual layers separately.
3.5 nanoStructured SIlIcon fabrIcatIon

methodS
Nanostructuring of silicon (Si) has received increasing substantial
interest over the past few decades due to its ability to possess unique
material properties that are not possible in bulk form [57, 58]. The
fabrication of the nanostructures is often separated into two categories;
bottom-up and top-down approaches.
3.5.1 Bottom-Up Silicon Nanostructure Formation

3.5.1.1 Vapour-Liquid-Solid (VLS) Method
Since the early 1990s, bottom-up techniques have been the more favorable
single crystalline Si nanostructuring method. In particular, the vapour-
liquid-solid (VLS) growth method has received substantial attention for

the synthesis of Si nanowires (SiNWs). The VLS growth was first described
by Wagner and Ellis [59], and the growth occurs via phase changes that
are mediated through a catalyst particle. This means that the material
to be grown, in this case, silicon is delivered in the gas phase, either
molecularly or in the form of a gas compound into the growth chamber.
Figure 3.5 shows a schematic sketch illustrating the widely adopted
VLS growth. In the case of Si, molecular Si can be obtained either by
evaporating Si in effusion cells, resulting in physical vapour deposition,
or Si gas precursors resulting in chemical vapour deposition. Silicon gas
precursors include mono-silane (SiH4), trichlorosilane (SiHCl3), or higher-
order silanes. The catalyst metal particle acts as a collector of silicon. In
the case of employing gas precursors, the catalytic nature leads to the
lowering of the dissociation energies of the gas. Consequently, a high
concentration of silicon atoms is found at the catalyst particles’ surface,
leading to diffusion into the cluster. For gold catalyst clusters, the gold-
silicon system can become liquid at temperatures above the eutectic
point (363°C), provided that a sufficient fraction of Si is present (e.g.
18.6 at. % at the eutectic temperature). In case of a constant silicon flow,
silicon atoms will continue to diffuse into the Au–Si melt, although this
is thermodynamically unstable. To reduce energies, the super-saturated
or excess silicon atoms condense into the solid-phase. Nucleation of Si
occurs at the footprint of the Au–Si catalyst layer by layer. Consequently,
a silicon monolith or nanowire is formed.
Figure 3.5 Schematic view of VLS growth of silicon nanowires (a) gold parti-
cles formed on the growth substrate and (b) VLS growth using silane as a silicon
precursor.
The advantage of the VLS method is that the growth can be done
on different types of substrates. It has been shown that thermodynamics
dictates the crystal growth direction of the nanowire, even if growth
occurs on amorphous substrates, i.e., without pre-defined lattice
orientation. In addition, a correlation between crystal direction and
nanowire diameter is clearly visible [60, 61]. Schmidt attributed this to

the strong contribution of outer silicon atoms to the overall formation
energies. When a silicon substrate is used, epitaxial growth can be
achieved as well. Moreover, the grown wires can be subsequently
transferred to any substrate even if direct growth on that substrate
is not possible. In that case, however, the original vertical nanowire
arrangement cannot be maintained. Although the VLS method is a
well-developed synthesis method, it suffers from a few limitations and
drawbacks.
3.5.1.2 Chemical Vapour Deposition (CVD)

SiNWs are grown in a chemical vapour deposition (CVD) chamber
through the absorption of a Si-based gas precursor at a liquid catalytic
alloy, typical gold (Au), at a temperature higher than the eutectic
point (300–400°C) [62, 63]. The liquid catalyst serves as the preferred
decomposition site of the Si atoms from the gas precursor until the
catalyst becomes supersaturated with Si, at which point Si crystallises
beneath the catalyst, forming SiNWs.
Figure 3.6 Schematic of a typical CVD film deposition unit.
Figure 3.6 shows the schematic of a typical CVD deposition unit.

The CVD setup and growth process is rather expensive and is typically
limited to small growth areas due to the chamber size. The SiNW
density is relatively low due to the challenge of preventing the close-
packed catalyst particles from merging at the growth temperature [64].
Achieving a high degree of vertical alignment is often a challenge. The
SiNWs can be doped through a variety of methods, including with
the metal catalyst or the addition of a dopant gas; however, accurately
predicting the resulting dopant level is very challenging [60]. During
the high-temperature growth process, the metal catalyst diffuses into
the SiNWs forming crystallographic defects and trap sites that reduce
the minority carrier lifetimes [65].
As an alternative to CVD growth, a wide variety of other growth
methods can be applied using catalysts [66]. Among them are the
annealing in a reactive atmosphere like hydrogen to form the nanowires
directly from a silicon substrate [67], laser ablation [68], and molecular
beam epitaxy [69].
3.5.2 top-down Silicon nanostructure formation

A top-down approach starts with a bulk Si wafer and involves removing
material rather than adding material. These offer the potential to alleviate
some of the limitations and drawbacks associated with the bottom-up
synthesis of Si nanostructures. For instance, since the resulting SiNWs
are etched from a bulk wafer, the doping concentration is based on
the original wafer, and the high-temperature growth conditions are
eliminated, removing the crystallographic defect sites. Furthermore,
precise scalable patterning methods can be used to form highly aligned,
dense Si nanostructures over a large area, making the top-down
technique a very promising method. The top-down methods begin
with the basics of forming porous Si and expanding to highly aligned
SiNWs fabrication and modification methods.
3.5.2.1 Deep Reactive Ion Etching

Vertically aligned Si micro/nano wires are fabricated using the
conventional deep reactive ion etching (DRIE) process, as shown in
Figure 3.7, often referred to as the Bosch process. Prior to DRIE, a Si
wafer was cleaned in a 3:1 (v/v) H2SO4:H2O2 for 10 minutes, followed
by a 10-minute 2% HF dip [70]. The Si wires are patterned with a
photoresist or a silica sphere masking layer. The Si wire arrays are
formed by DRIE with cycles of etching with SF6 followed by passivation
with C4F8 in radio-frequency generated plasma. The etch time will vary
depending on the exposed Si area and the desired Si wire length. The Si
wires are fabricated approximately 10–20 µm in length with diameters
ranging from a few microns to approximately 300 nm depending
on the patterning method. The Si wire diameter is limited to above
300 nm due to the on/off etch/passivation steps forming scalloped sidewalls
along the length of the wire which is too severe in Si wires below
300 nm in diameter.
Si micro wires are patterned using conventional photolithography.
The DRIE masking layer consists of a 1 µm thick photoresist layer that
is exposed and developed, leaving behind an array of masked circular
features.
Figure 3.7 Schematic of a typical reactive ion etching unit [70].
3.6 fabrIcatIon methodS
3.6.1 electrochemical etching

3.6.1.1 PS Formation, Etching Chemistry, and Theory
Porous Si was discovered accidentally in the mid-1950s while attempting
to develop an electrochemical method to polish the surface of silicon (Si)
and germanium (Ge) [71]. Instead of polishing the Si to form a
reflective surface, a brittle dark film was found that contained porous
holes propagating primarily in the <100> direction through the Si
wafer. Although this was recorded in a technical report at Bell Labs,
it did not receive substantial interest until the early 1990s when it
was determined that porous Si can emit visible light [33, 72, 73]. A
plethora of work focused on creating porous Si-based optoelectronics
was reported during the mid-1990s until it slowly calmed down due
to disappointing electroluminescence efficiency, and poor chemical and
structural stability [74]. Since then, the fundamental understanding of
porous Si has been improved and is currently being used in a wide range
of applications from drug delivery systems, optoelectronics, sacrificial
materials, energy storage, and harvesting devices [74–79].
Silicon is thermodynamically unstable in air and water, forming a
native oxide layer. In the presence of hydrofluoric acid (HF), the oxide
is rapidly removed leaving the surface terminated with hydrogen atoms
(Si-H) [80]. The Si-H bonds passivated the surface and thereby prevent
further oxidation in an aqueous HF medium without the presence
of strong oxidising agents. A common and controlled method to
promote the continued oxidation of Si is through anodising the Si in an
HF solution. A typical anodisation cell, shown in Figure 3.8 is composed
of a Teflon chamber with a circular opening where a clamped piece of Si
wafer is exposed to the solution. A power supply is connected to the cell,
supplying a constant current to flow through the Si wafer with metal
back contact acting as an anode and to platinum (Pt) electrode acting
as a cathode. Both electrodes being submerged in the HF electrolyte
solution allow the electrochemical reaction to occur at the interface of
the Si and electrolyte solution.
Figure 3.8 A sketch of the basic etch cell used and the schematics of
PS formed [77].
Although porous Si forms in HF solutions diluted with deionised

water, the hydrophobic properties of the hydrogen-terminated surface
hinder the ability of the solution to infiltrate the pores and therefore
induce lateral and in-depth inhomogeneity [77]. Thus, alcoholic solutions
are commonly used to increase the wettability on the porous Si surface,
allowing the solution to infiltrate the pores [74].
The dissolution of Si is obtained by supplying either an anodic
current or potential; however, supplying a constant current allows for
superior control and reproducibility [81]. The power source supplies
holes to the Si valence band, oxidising the Si surface. The holes are
driven to the surface by diffusion from the electric field generated within
the wafer. At the Si surface, holes break apart the Si-H bonds causing
competition between the formation of Si-H, Si-O, and Si-F bonds [74].
The Si-F bond has the highest electronegativity making it feasible

over the formation of the Si-O bond, being the second most likely
favorable one. The high electronegativity makes the Si-F bond highly
polarised, so as soon a fluoride ion attaches to a Si atom, it becomes
the preferred atom for the subsequent holes to attach to, leading to the
rapid dissolution of the Si atom into the water as soluble SiF62– as shown
in Figure 3.9 [74].
Figure 3.9 Dissolution mechanism of Si in the presence of hydrofluoric acid

[74]. Overall Reaction at Silicon Surface: Si + 6F– + 2H+ + 4h+ → SiF62– + H2.
The preferred crystal plane direction for the removal of Si in

HF electrolyte solution is related to the energy associated with
cleaving the Si back-bonds [82]. The effected number density of
back-bonds increases relative to the crystal plane in the order
of (100) < (110) < (111); therefore, the pores typically propagate
along the <100> direction [83]. The pore morphology, size, and
overall porosity greatly depend on the wafer, solution, and applied
conditions during the porous Si formation process. Varying the wafer
type and doping level will affect the pore formation mechanism,
which in turn affects the size and directionality of the pores [84].
For instance, micropores (pore width ≤ 2 nm) are often formed in low and
moderately-doped p-type Si with the most uniform porosity. Mesopores
(2 nm < pore width ≤ 50 nm) are often formed with highly doped
p- and n-type wafers with pores perpendicular to the wafer surface due
to the enhanced electric field driving the holes toward the base of the
pore. Macropores (pore width > 50 nm) are often formed in low-doped
n-type wafers with pores forming perpendicular to the surface due to
the space charge region depleting the holes around the pore [74, 81]. The
pore size and morphology can also be tuned by varying the solution
or conditions. In general, increasing the current density, increasing the
process temperature, or decreasing the HF concentration has a similar
effect, leading to an increase in the pore size and porosity [82–85].
3.6.2 electropolishing
As mentioned earlier, Uhlir discovered porous Si formed under certain
conditions while trying to develop an electrochemical method to polish
Si and Ge [1]. If the porous Si conditions are modified in a particular
way, electropolishing occurs, which is very beneficial for removing thin
porous layers from bulk Si wafers [86]. The key condition required for
electropolishing is the rate at which the current injects holes into the Si
surface has to exceed the rate at that fluorine ions can be transported
to the pore tip [87–91].
The JV curves for n- as well as for p-type Si can be divided into
two main regions as shown in Figure 3.10.
J mA/cm2
5
Jep
1
0
2 4 6 V
Figure 3.10 J-V characteristics of electro polishing and PS formation.
• Pore formation region.

• Electropolishing region.
Pore formation occurs in the initial rising part of the curve for
0 < V < Vep, with Vep the potential of the small sharp peak. This peak,
also called the electropolishing peak, has intensity Jep, which depends
mostly on the solution composition and little effect from the substrate.
For V > Vep electro polishing occurs since the surface becomes covered
by an oxide layer, whose composition and dielectric properties depend
on the applied potential. By increasing the HF concentration, the first
peak shifts to higher current values (higher electropolishing current),
while increasing the substrate doping concentration shifts the first peak
towards lower voltages.
3.6.3 metal-assisted chemical etching (mace)

One of the earliest and most widely cited works on MACE was reported
by Li and Bohn in 2000 [92], which characterised the chemical etching
of silicon assisted by metal catalyst (gold-Au, platinum-Pt, or gold/

palladium-Au/Pd alloys) to fabricate pores and wires. Later, several
other reports started establishing the legitimacy of this process which
resulted in the early foundation of this technique, which has since
moved to less-expensive catalysts like silver. A notable work by Chartier
et al. [93] characterised the silver-Ag catalyst system and discussed the
relationship between metal-assisted chemical etching (MACE) and the
electrochemistry of silicon. This report helped the researchers working
in this field to approach MACE in a very systematic manner. These early
reports pioneered the establishment of MACE and resulted in increased
acceptance within the broader scientific community [94, 95].
3.6.4 Possible mechanism for mace of Silicon

MACE is a localised electrochemical reaction with the metal region
acting as a microscopic cathode and the metal-semiconductor interface
acting as the anode as shown in Figure 3.11. The oxidant used in the
etching solution gets reduced locally on the surface of the catalyst, where
the role of the metal is to reduce the activation energy required for the
Figure 3.11 Schematic diagram showing the chemical process of MACE. The
reduction of oxidant takes place only on top of the metal catalyst (cathode) and
the dissolution of silicon takes place near the metal-silicon interface (anode).
Hydrogen gas bubbles evolve during the reaction.
reduction of the oxidant (gain of electrons). Any noble metals like Ag,
Au, Pt, etc., can act as a catalyst for the reduction of an oxidant, with
the rate of reduction reaction directly related to the electronegativity of
the metal used. The reduction of the oxidant (e.g. H2O2) in the presence
of protonated hydrogen (H+) coming from the acidic solution results in
the injection of holes into the semiconductor region surrounding the
metal layer. It should be noted that non-noble metals like chromium-Cr,
aluminium-Al, and titanium-Ti [96] could not act as a catalyst for MACE
as these metals do not reduce the activation energy for the reduction
of oxidants.
At Cathode:
H2O2 + 2H+ → 2H2O + 2h+ (Local reduction)
2H+ → H2 + 2h+ (Hydrogen gas formation)
At Anode:
Si + 6HF + 4h+ → H2SiF6 + 4H+ (Dissolution of silicon)
Net reaction:
Si + 6HF + H2O2 → H2SiF6 + 2H2O + H2
In addition, there is a second cathodic reaction resulting in the
formation of hydrogen which is observed as gas bubbles during the
reaction. On the other hand, the injection of holes or removal of electrons
from the neutral silicon atom changes the silicon from a Si0 to a Si4+
state. The oxidised silicon is dissolved by HF into (hexafluorosilicic
acid) H2SiF6, which is a soluble product. Unlike wet chemical etching,
the metal layer sinks to the bottom and travels along with the
semiconductor which helps to achieve higher aspect ratios and novel
surface morphologies. The lateral dimension of the structure formed is
limited by the dimension of the metal mesh made using lithography or
the diameter of the metal particle used and the vertical dimension is
decided by the etch time.
3.7 role of catalySt metalS
The catalyst used in MACE determines the rate of reduction of oxidant,

H2O2, in the case of silicon. In general, the rate of the MACE process for
the noble metal catalyst follows the trend Pd > Pt > Au > Ag. Also, we
should note that Ag is not a stable catalyst in the MACE solution as it
gets easily oxidised into a solution forming Ag+ ions that diffuse a short
distance before getting re-deposited after reduction at the HF/Si interface
through the oxidation of silicon, which does not happen with Pt or Au
catalysts. As the oxidant is reduced at the catalyst, holes are injected
into the valence band. This charge transfer process is heavily affected
by the surface bend banding of the Si at the metal catalyst contact
interface. The calculations of the Schottky barrier height (SBH) and
potential of valence band maximum (PVBM) made by Huang et al. [96]
provided the first quantitative explanation of the localised galvanic
etching taking place in MACE.
3.8 tyPeS of dePoSItIon method
Ag, Au, Pt, and Pd are the most frequently used noble metals in MACE.
Some of the commonly used methods for metal deposition on the Si
substrate are thermal evaporation, sputtering, electron beam (e-beam)

evaporation, electroless deposition, electrode deposition, focused ion
beam (FIB)-assisted deposition, and spin-coating of particles, physical
deposition in vacuum (e.g., thermal evaporation, sputtering, and e-beam
evaporation) is favourable to obtain patterned structures of Si by MACE
because the morphology of the resulting noble metal film can more
easily be controlled in these methods. For electroless deposition, there
are several different plating solutions containing noble metal ions that
can be used to deposit these noble metals onto a Si substrate.
3.9 the ShaPe of the metal and dIStance

betWeen metalS
The shape of the metal catalyst generally defines the morphologies

of the resulting etched structures as the Si under a metal catalyst is
etched much faster than Si without metal coverage. With well-separated
noble metal particles, we usually get well-defined pores, but the
etched structures might evolve from pores into wall-like or wire-like
structures as the distance between noble metal particles decreases.
Discontinuous patches will result in wall-like or wire-like structures
with a broad distribution of cross-sectional shapes and spacing. If the
metal film contains orderly distributed pores with uniform diameters
and cross-sectional shapes, the Si substrate will be etched into an array
of Si nanowires with identical cross-sectional shapes and spacing [97].
Table 3.1 describes the method of Ag deposition, electrolyte composition,
and the substrate used for MACE. MACE methods have also already
been shown to be compatible with both single-and multicrystalline
diamond-wire sawn Si solar cells, which can further reduce PV
costs [98].
Let us consider an instance for the electrochemical etching process
and MACE to know the details of optical and morphological changes
in the silicon. In both cases, oxidation was done in a controlled manner
to study the behaviour.
3.9.1 ece
When the electrochemical etching was carried out by applying a constant
current density (6 mA/cm2) for 10 minutes, 20 minutes, and 30 minutes
using aqueous HF (48 wt.%) and ethanol (1:2) as the electrolyte solution.
After electrochemical etching, samples were rinsed with deionised
water and ethanol sequence to avoid capillarity forces. For passivation,
the prepared samples were oxidised using an in-house low-temperature
Table 3.1 Method of Ag deposition with electrolyte composition and types of Si substrates
74
S.
Metal Catalyst Bath Composition [Oxidant]:[HF] Substrate Types References
No.
Ag thin film by c-Si (p) (100), Si (n) (100), and
1 2%–49% HF: [H2O2] bubbling [97]
PVD mc-Si (p)
Ag dots by ionic precursor
2 [H2O2]:[HF]=1:4.16 c-Si (p) (2.7 Ω cm) (100) [99]
reduction
Ag, dots [H2O2]:[HF]
3 Si (p) (100), (111) Si (n)(100), (111) [100–102]
by ionic precursor reduction = 0.4:4.8
Ag, Pt dots [H2O2]:[HF]
4 Si (p) (100) [103]
by ionic precursor reduction =5.3:1.8
Ag dots and antidots by PVD [H2O2]:[HF]=2.5:10 for (110) Si (p) (100), (110) Also seen in
5 [104]
and electroless in Si/Ge or=1:2 for (100) Si/Ge superlattice

Ag nanoparticles by ionic
6 4.8M HF + 0.15M H2O2 Si (p) (100) [105]
precursor reduction
7 [HF]:[H2O2]:[H2O]=25:10:4 Si (100) [106]
precursor reduction
8 4M HF + 0.10M AgNO3 c-Si (p) (100) mc-Si (p) [107]
precursor reduction
9 [HF]:[H2O2] Si (100), (110), (111) [108]
precursor reduction
10 [HF]:[H2O2]:[H2O]=4:1.3:2.7 Si (p) (100) [109]
precursor reduction
11 [HF]:[H2O2]:[H2O]=10:5:35 Si (p) (100) [110]
precursor reduction
12 [HF]:[H2O2]=4:15.6 Si (p) (100) [111]
precursor reduction
Ag dots by ionic precursor
13 7.3M HF with Ar+O2 bubbling Si (p) (100) [112]
reduction
and high-pressure setup. During the oxidation process, the pressure is

maintained at 20–25 kg/cm2 (30 Psi) for 2 hours. The as-prepared and
oxidised PS samples were investigated to find the spectral changes and
structure of PS after oxidation.
Figure 3.12 Diffused reflectance spectra of (a) as prepared and (b) oxidised PS
made from UMG wafer with different etching times; 10 minutes, 20 minutes, and
30 minutes along with reference silicon.
The reflection measurements for as-prepared samples and oxidised

PS samples were recorded in the wavelength range of (200–900 nm)
illustrated in Figure 3.12(a) and (b) respectively. It can be seen that
the reflectance of PS layers and oxidised PS samples decreased when
compared with the Ref-Si sample. The maximum reduction in light
reflection is around 200 nm to 500 nm and a slight increase in the
reflection occurred from 500 to 900 nm, perhaps because of the random
distribution of the pores and the increasing roughness on the PS surface.
The PS layer reduces light reflection and leads to changes in the optical,
vibrational, and electronic transitions [113]. The surface roughness of
the Si samples is directly related to the percentage of its reflectivity.
The variation in the optical properties of the PS layer appears to be a
good result for the possibility of the use of these PS layers as an ARC
for Si solar cells [114].
Figure 3.13 shows the photoluminescence spectra (PL) from the
porous layers formed by varying the etching time. The presented spectra
show the shift of the maximum emission towards shorter wavelengths
as the porosity increases. The observed “blue shift” is in agreement with
the quantum size effect model proposed by Canham [33]. The spectra
exhibit some additional maxima and shoulders for the 10 minutes
etched sample in Figure 3.13. A broad PL band, a structural character
of the spectrum, and the continuous shift of the maximum emission
by changing the etching conditions suggest the existence of various
optical transition processes, possibly because of a dispersion of the
Si crystallites size. The PL of porous Si prepared by the electrochemical

dissolution suffers from the problems of instability and degradation.
The PL peak centred around 2.1 eV was observed for 20 minutes and
30 minutes etched samples, while a broad PL peak extending up to
1.7 eV was observed for 10 minutes etched samples. The PL can be
deconvoluted into two peaks for 20 minutes and 30 minutes samples
at 2.2 eV and 2.0 eV and an additional peak centred at 1.8 ev for 10
minutes etched sample, respectively. The PL intensity decreases during
oxidation. Studying degradation effects in our samples we have found
that for oxidised samples “a blue shift” of about 40 nm was observed
as a result of oxidation of the Si skeleton in Figure 3.13. The PL occurs
around 2.2 eV for all samples. The PL can be deconvoluted into two
peaks for all samples at 2.25 eV and 2.05 eV, respectively.
Figure 3.13 PL Spectra of (a) as prepared and (b) oxidised PS made from UMG
wafer with different etching times; 10 minutes, 20 minutes, and 30 minutes along
with their respective multiple peak fit.
Figure 3.14 SEM image of (a) as prepared and (b) oxidised PS made from UMG
wafer with 10 minutes etching time.
It was produced by non-uniform etching during the electropolishing

of silicon with an electrolyte containing hydrofluoric acid. In the most
basic sense, PS is a network of air holes within an interconnected silicon

matrix as shown in Figure 3.14. The size of these air holes, called pores,
can vary from a few nanometers to a few microns depending on the
conditions of formation and the characteristics of the silicon. Some of
the works done in ECE are listed in Table 3.2.
3.9.2 mace
Then the silver nanoparticles (AgNPs) were coated on the freshly
cleaned Si wafers by immersing in 3.6 ml of HF with 28 mg of silver
nitrate (AgNO3) aqueous solution (20 ml) for one minute. The excess
Ag+ ions present on the surface were washed with distilled water, then
samples were immersed in an etching solution comprising 3.6 ml of
HF in 20 ml of H2O and 0.6 ml of H2O2 for different etching time, i.e.,
15 minutes, 30 minutes, 45 minutes, 60 minutes, and 75 minutes. The
residual AgNPs on the sample surface and in the pores of the Si were
removed by immersing the samples in diluted nitric acid (HNO3) for
60 minutes. Finally, the prepared samples were washed with distilled
water and dried in nitrogen (N2). The dry oxidation is done by annealing
the samples in an oxygen atmosphere (100 cc/min) at 700oC for 30 minutes.
Figure 3.15 Reflectance spectra of the PS samples of different etching times

(a) 15 minutes, (b) 30 minutes, (c) 45 minutes, (d) 60 minutes, and (e) 75 minutes
including as-cut UMG Si wafer (Reference) without CP5 polishing.
Figure 3.15 shows the reflectance spectra for PS fabricated using

different etching times. The measured reflectance of the PS sample
Table 3.2 Important literature review on the PS by anodisation
78
Author Silicon Electrolyte Porous ARC Materials I-V Measurements References
and Wafer Solution and Silicon and Coating
Voc (mV) Jsc FF%
Published Year Details Etching Details Details Methods
(mA/m 2)
K.A. Solman n-type HF (48%): Porosity — [115]
2012 [100] C2H5OH 91% 595 33 76.65
[111] (98%) 14% 500 20.50 70.24
p-type [1:4] 45% 545 29.05 78.02
[100] 20 min 27% 510 27.15 70.00
[111] 6 mA/cm2
K.A. Solman n-type HF (48%): — ZnO-135 nm [116]
2011 [100] C2H5OH Rf sputtering 627.3 35.50 81.50
306-406 µm (98%) 595 33.90 76.40

[1:4]
20 min
6 mA/cm2
Aisha Gakce n-type HF (18%): Pore depth — [117]
2016 [100] C2H5OH 250 nm 286 8.42 44
500 µm (22%) 52% 303 8.97 50
1 to 30 mA/cm2 390 nm
56%
C.S. Solanki — — 20–30% ZnO as epitaxial [86]
2004 20 µm layer 606 32.9 60
Juc Hyun Kim p-type HF:(CH2)2SO: Aspect ratio AZO-1.8 µm [118]
2007 [100] H2O 35 ALD method 530 19.5 49.9
1-10 Ω-cm [2:5:15] 61 520 25.3 54
80.1 517 26.2 48.7
101 502 18.3 49.2
Nanostructured Silicon for Solar Energy Conversion Applications

Voc (mV) Jsc FF%
(mA/m 2)
W.J. Aziz 2010 p-type HF: Pore size ZnO/TiO2 [119]
[111] C2H5OH 8 nm ZnO-Rf 440 12.4 82
0.75 Ω-cm 60 mA/cm2 Sputtering 370 6.04 79
283 µm 30 minutes TiO2-DC 340 5.1 77
RCA sputtering
Jae-Hong Kwon mc-si (49%)HF: — SiNX [120]
2007 p-type C2H5OH PECVD 604 30.5 72.1
330 µm [3:1] 585 28.9 67.6
0.5–2 Ω-cm 10–270 mA/cm2
RCA I 1–35 sec
RCA II
Asmiet Ramizy n-type HF: C2H5OH — — [121]
2010 283 µm [1:4] 490 12.37 84
[100] 60 mA/cm2 450 15.5 88
15 min
C.L. Clement mc-si HF electrolyte — [122]
2003 p-type 1 mA/cm2 75% 585 29.6 77
1 Ω-cm 80 sec 73% 587 29.0 77
140 sec
Asmiet Ramizy n-type HF: C2H5OH — — 310 6.72 83 [123]
2010 [111] [1:4] 430 8.83 78
283 µm 60 mA/cm2 490 12.37 84
0.17 µm 15 min 450 15.5 88
O. Tobail 2009 p-type HF:H2O: — [124]
[100] [1:1:1] 380 µm 611.3 38.4 70.9
0.01–0.016 47.7 µm 601.2 29.2 69.2
Ώ-cm 38 µm 615 28.9 76.6
79
(Contd.)
Table 3.2 Important literature review on the PS by anodisation (Cont.)
80
Voc (mV) Jsc FF%
(mA/m 2)
Hyukyong Kwon p-type HF electrolyte — ZnO 560 0.13 55.27 [125]
2011 [100] as epitaxial layer 560 0.15 64.33
0.5–3 KOH texture 570 0.16 65.77
Ώ-cm
p-type C2H5OH: HF [1:1] — [126]
B.C. Chakrovarty 300 µm 20 mA/cm2 70 nm 578.4 20.9 74.1
2007 5–10 µm 1–16 min 580.3 26.3 73.7
[100]
[111]

[110]
P.N. Vinod 2009 p-type HF: C2H5OH:H2O — — [127]
[100] [1:1:5] 580.1 24.2 70.6
1 Ώ.cm 25 mA/cm2 582.3 25.5 72.2
5 minutes 589.2 27.4 74.1
C.L. Clement 1997 p-type HF electrolyte — SiO2+TiO2 [128]
[100] 50 mA/cm2 595 7.90 72.5
[111] 3.5 s 583 5.83
varies from 15% in longer wavelengths (800 nm) and gradually decreases
to below 5% at shorter wavelengths (200 nm) for 15-minute, 30-minute,
and 45-minute etched samples. The reflectance increases further up
to 25% for 60-minute and 75-minute samples near longer wavelengths
(800 nm). On the other hand, the reference Si surface has reflectance
from 35% to 25% in the UV and around 20% to 15% visible region.
The average surface reflectance of the Si substrate in the measured
range decreased dramatically from 35% for the reference to less than
5% in the lower wave length region for all samples throughout the UV
wavelength range. In the present chapter, all the etched PS samples
exhibit reflectance <5% in 200–400 nm wavelength.
Figure 3.16 Photoluminescence spectra for the porous Si samples of different

etching times (a) 15 minutes, (b) 30 minutes, (c) 45 minutes, (d) 60 minutes, and
(e) 75 minutes with simulated peak for 15 minutes sample.
The photoluminescence (PL) spectra of the mesoporous silicon with

different etching times are shown in Figure 3.16. The PL spectra can be
deconvoluted into three peaks centred around 550, 590, and 660 nm.
Efficient red luminescence, often referred to as S-band was first reported
from free-standing Si quantum wires and attributed to 2-dimensional
(2-D) quantum confinement of carriers widening the Si band gap. Later,
a weak luminescence band in the blue spectral range (F-band) was
observed in aged porous Si and was attributed to Γ–Γ transitions in
Si nanocrystals. Similar observations were made in Si oxidised at high
temperatures, photoanodised Si, etc. Generally, a PL signature in a
semiconductor consists of near-band-edge emission and defects-related
luminescence. Near-band-edge emission results from recombination

between free electrons (holes) with bound holes (electrons), known as
free-to-bound (FB) transitions. The defects related to PL occur from
recombination between (i) electrons bound in donors and holes bound
in acceptors, known as DAP, (ii) electrons from the conduction band
to holes bound in acceptors, and (iii) electrons from donors to valance
band. From PL studies, they identified their dominant processes, (i) blue
emission from oxygen-related recombination, (ii) green emission from
Γ–Γ transition, and (iii) NIR emission from Γ–X excitonic transition.
However, the hot PL (Γ–Γ transition) was found to red-shift with a
decrease in SiNC size, white the excitonic PL (Γ–X transition) blue shifts
with a decrease in size. Moreover, the hot PL intensity was dependant
on excitation intensity.
Figure 3.17 SEM image of silicon nanostructure made by MACE (a) top view
and (b) cross-section image.
Figure 3.17 shows an SEM image of the silicon nanostructure made

by MACE (a) top view and (b) a cross-section image cross-section of the
mesoporous Si layers. The 11 µm to 30 µm thick mesoporous Si layer can
be easily distinguished. It can be observed that in all the samples the
MACE process has created vertical nanostructures. However, porous/
defect structures were also formed on these vertical nanowires. The
AgNPs did not sink homogenously on the silicon surface. Another
major obstacle in the etching experiment is the formation of gas bubbles,
which interfere with the fabrication of homogenous PS layers. From
the results obtained, we observe that during the MACE process, the
Ag catalyst morphology has a significant influence on the formation
of PS. The formation of porous Si layers was not uniform because
of the non-homogeneous formation of AgNPs on the Si surface. The
extended etched regions were formed, where the AgNPs’ sizes were
larger continued the etching process. However, it should be noted that
the irregular distribution of the pores could enhance photo conversion
few works in MACE are listed in Table 3.3.
Table 3.3 Specific literature review on the PS by MACE
Nanostructured Silicon for Solar Energy Conversion Applications

Device Output
Junction Type and Year Device Structure References
Parameters
Crystalline
h = 9.31%
Voc = 0.5485 V
Jsc = 26.06 mA/cm2
Substrate
Conventional phosphorous diffusion on SiNWs fabricated by wet FF = 65.12%
p-n homojunction [129]
etching on p-type crystalline (100) and polycrystalline substrate. Polycrystalline
(2005)
h = 4.73%
Voc = 0.4756 V
Jsc = 20.9 mA/cm2
FF= 47.4%
h = 11.37%
Voc = 0.58 V
Standard phosphorous diffusion on SiNWs fabricated by wet Jsc = 27.1 mA/cm2
Substrate p-n homojunction
etching on p-type c-Si crystalline (111) wafers. SiNWs length was FF= 72.22% [130]
(2008)
about 3–5 µm Voc = 0.58 V
Jsc = 27.1 mA/cm2
FF= 72.22%
h = 13.7%
Substrate p-n homojunction Conventional phosphorous diffusion on SiNWs fabricated by wet Voc = 0.544 V [131]
(2011) etching on p-type c- Si (100) wafers. Jsc = 37 mA/cm2
FF = 68%
SiNWs were fabricated by MACE on texturised p-type c-Si with h = 17.11%

Substrate p-n homojunction Voc = 0.5485 V
pyramids. After completion of the cell, SiNWs were passivated by [132]
(2013) Jsc = 35.8 mA/cm2
SiO2, SiO2- SiNx, or SiNx only. The area of the cell is 154.83 cm2.
FF = 77.2%
83
(Contd.)
Table 3.3 Specific literature review on the PS by MACE (Cont.)
84
Device Output
Junction Type and Year Device Structure References
Parameters
Black silicon was fabricated via wet etching on a p-type (100) c-Si
wafer. The junction was formed via a conventional phosphorous h = 18.2%
Substrate p-n homojunction Voc = 0.628 V
diffusion process. The completed cell was capped by SiO2 obtained [133]
(2012) Jsc = 36.5 mA/cm2
by thermal oxidation to passivate the nanostructure. The area of
the cell is 0.8081 cm2. FF = 79.6%
Black silicon was fabricated by using cryogenic DRIE. The black h = 22.1%
IBC substrate p-n silicon was passivated by a thin Al2O3 layer. The design of the Voc = 0.665 V [134]
homojunction (2015) structure was an interdigitated back-contact junction (IBC). The Jsc = 42.2 mA/cm2
area of the cell is 9 cm2. FF = 78.7%
MACE on 2.7 µm multicrystalline p+nn+ doped silicon layers

deposited on glass. The multicrystalline layer was obtained η = 1.4–4.4%
Axial p-n homojunction
through the laser crystallisation of a (p) a-Si:H film deposited Voc = 0.45 V [135]
(2009)
by electron beam evaporation (EBE). Doping was realised by a FF = 30%
standard phosphorus diffusion process.
3.10 concluSIon
The efficient conversion of solar radiation to electrical energy is a

challenging problem with many facets and potential solutions. The
people working in this area are approaching the problem from two
angles. On one side, working to reduce the amount of silicon material
required in crystalline silicon photovoltaics to reduce their overall cost
while maintaining high efficiencies. To do this, developing surface
structures designed to trap incident photons in thin silicon films to
overcome the weak absorption of silicon that becomes problematic as the
silicon thickness is reduced. Silicon nanostructures can be an alternative
to the (ARC) silicon nitride coating on solar cells. Si nanostructures
in various forms can be controllably synthesised easily using
electrochemical etching and MACE methods as well. Si nanostructures
(nanowires and quantum dots) exhibit unique and interesting structural,
optical, electronic, and chemical properties, which are being exploited
for myriad exciting applications. For instance, energy devices based on
Si-nanowire arrays or network structures can achieve efficiencies as
high as 12% for solar energy conversion. Additionally, Si nanodots and
hybrid nanostructures can serve as efficient photo-catalysts for the redox
reactions of organics. This chapter has highlighted the recent works in
developing silicon nanostructures for green, high-efficiency, and low-
cost solar energy harvesting and catalysis applications.
RefeRences
[1] Letcher, T.M. 2022. Global warming, greenhouse gases, renewable
energy, and storing energy. pp. 3–12. In: Letcher, T.M. (ed.). Storing
Energy: with Special Reference to Renewable Energy Sources, 2nd Ed.
Elsevier.
[2] Kumar, B., M. Llorente, J. Froehlich, T. Dang, A. Sathrum and C.P.
Kubiak. 2012. Photochemical and photoelectrochemical reduction of
CO2. Annu. Rev. Phys. Chem. 63: 541–569.
[3] Mayer, T.M., L. Yongjing, Y. Guangbi and D. Wang. 2013. Forming
heterojunctions at the nanoscale for improved photoelectrochemical
water splitting by semiconductor materials: case studies on hematite.
Acc. Chem. Res. 46(7): 1558–1566.
[4] Moore, F.G. and W.B. Gary. 2011. Energy conversion in photosynthesis:
a paradigm for solar fuel production. Annu. Rev. Condens. Matter.
Phys. 2(1): 303–327.
[5] https://eu.usatoday.com/story/news/world/2022/03/30clean-energy
-wind-solar-2021/7219298001/published 3:05 pm ET 30 March 2022
[6] Mahmoud, N., Z. Issaabadi, M. Sajjadi, S.M. Sajadi and M. Atarod. 2019.
Types of nanostructures. Interface Sci. Technol. 28: 29–80.
[7] Branker, K., M.J.M. Pathak and J.M. Pearce. 2011. A review of solar
photovoltaic levelized cost of electricity. Renewable Sustainable Energy
Rev. 15(9): 4470–4482.
[8] Hayibo, K.S. and J.M. Pearce. 2021. A review of the value of solar
methodology with a case study of the US VOS. Renewable Sustainable
Energy Rev. 137: 110599.
[9] Pearce, J. and A. Lau. 2002. Net energy analysis for sustainable energy
production from silicon based solar cells. In International Solar Energy
Conference 16893: 181–186.
[10] https://www.unsustainablemagazine.com/solar-is-now-the-cheapest
-energy/Published on March 11, 2022
[11] https://www.iea.org/topics/world-energy-outlook/Published October
2022
[12] Pearce, J.M. 2002. Photovoltaics—a path to sustainable futures. Futures
34(7): 663–674.
[13] Andrea, F. and A. Freundlich. 2008. Material considerations for terawatt
level deployment of photovoltaics. Renewable Energy 33(2): 180–185.
[14] Narendra, S., R. Arya, A. Chaubal, K. Deshmukh, P. Ghosh,
A. Kottantharayil, et al. 2022. Recent developments in solar
manufacturing in India. Solar Compass. 1: 100009.
[15] Zhongliang, G., G. Lin, Y. Zheng, N. Sang, Y. Li, L. Chen and M. Li.
2020. Excellent light-capture capability of trilobal SiNW for ultra-high
JSC in single-nanowire solar cells. Photonics Res. 8(6): 995–1001.
[16] Tom, B., G. Zoppi, L. Bowen, T.P. Shalvey, S. Mariotti, K. Durose, et al.
2018. Incorporation of CdSe layers into CdTe thin film solar cells. Sol.
Energy Mater. Sol. Cells. 180: 196–204.
[17] Chuang, L., G. Hu, X. Hao, C. Li, B. Tan, Y. Wang, et al. 2021.
Performance improvement of CdS/CdTe solar cells by incorporation of
CdSe layers. J. Mater. Sci.: Mater. Electron. 32: 19083–19094.
[18] Isshiki, M. and J. Wang. 2017. II–IV Semiconductors for Optoelectronics:
CdS, CdSe, CdTe. pp. 853–863. In: S. Kasap and P Capper (eds). Springer
Handbook of Electronic and Photonic Materials. Springer Handbooks.
Springer, Cham.
[19] Smida, A., Z. Zaaboub, N. Bel Haj Mohamed, M. Hassen, F. Laatar,
H. Maaref, et al. 2018. Photoluminescence behavior in the synthesized
CdSe thin films deposited on ITO substrates. J. Lumin. 194: 686–691.
[20] Samuel, A. and K. Dellinger. 2022. ZnO and TiO2 nanostructures for
surface-enhanced Raman scattering-based biosensing: a review. Sens.
Bio-Sens. Res. 100499.
[21] Ammar Mahmoud Al-Husseini and Bashar Lahlouh. 2017. Silicon
pyramid structure as a reflectivity reduction mechanism. J. Appl. Sci.
17(8): 374–383.
[22] Ali Samet, S., N. Ekren and Ş. Sağlam. 2020. A review of anti-reflection
and self-cleaning coatings on photovoltaic panels. Sol. Energy 199:
63–73.
[23] Zhou, Z., Z. Zhang, J. Jing, R. Gao, Z. Liao, W. Zhang, et al. 2023. Black
silicon for near-infrared and ultraviolet photodetection: a review. APL
Mater. 11(2): 021107.
[24] Jicheng, Z., Y. Tan, W. Liu, X. Cai, H. Huang and Y. Cao. 2020. Effect
of front surface light trapping structures on the PERC solar cell. SN
Appl. Sci. 2: 1–10.
[25] Kuan, W.A.C., Z. Tang, H. Lu and F. Huang. 2018. Anti-reflective
structures for photovoltaics: Numerical and experimental design.
Energy Rep. 4: 266–273.
[26] Lucia, V.M. and Paola D. Veneri. 2020. Silicon solar cells: materials,
technologies, architectures. Solar Cells and Light Management 35–57.
[27] Chai, J.Y.-H., B.T. Wong and S. Juodkazis. 2020. Black-silicon-assisted
photovoltaic cells for better conversion efficiencies: a review on recent
research and development efforts. Mater. Today Energy 18: 100539.
[28] Hele, S., P. Repo, G.V. Gastrow, P. Ortega, E. Calle, M. Garín, et al. 2015.
Black silicon solar cells with interdigitated back-contacts achieve 22.1%
efficiency. Nat. Nanotechnol. 10(7): 624–628.
[29] Chiara, M., H.S. Laine, T.P. Pasanen, H. Savin and J.M. Pearce. 2018.
Economic advantages of dry-etched black silicon in passivated emitter
rear cell (PERC) photovoltaic manufacturing. Energies 11(9): 2337.
[30] Uhlir, J.A. 1956. Electrolytic shaping of germanium and silicon. The
BSTJ 35(2): 333–347.
[31] Watanabe, Y., Y. Arita, T. Yokoyama and Y. Igarashi. 1975. Formation
and properties of porous silicon and its application. J. Electrochem.
Soc. 122(10): 1351.
[32] Ito, T., A. Yamama, A. Hiraki and M. Satou. 1990. Silicidation of porous
silicon and its application for the fabrication of a buried metal layer.
Appl. Surf. Sci. 41: 301–305.
[33] Canham, L.T. 1990. Silicon quantum wire array fabrication by
electrochemical and chemical dissolution of wafers. Appl. Phys. Lett.
57(10): 1046–1048.
[34] Violeta, P., V. Rădiţoiu, A. Raditoiu, F.M. Raduly, R. Manea, A. Frone,
et al. 2021. Bilayer coatings based on silica materials and iron (III)
phthalocyanine–Sensitized TiO2 photocatalyst. Mater. Res. Bull. 138:
111222.
[35] Mouafki, Α.Μ., F. Bouaicha, A. Hedibi and A. Gueddim. 2022. Porous
Silicon Antireflective Coatings for Silicon Solar Cells. Eng. Technol.
Appl Sci. Res. 12(2): 8354–8358.
[36] Wei, L., Z. Liu, F. Fontana, Y. Ding, D. Liu, J.T. Hirvonen, et al. 2018.
Tailoring porous silicon for biomedical applications: from drug delivery
to cancer immunotherapy. Adv. Mater. 30(24): 1703740.
[37] Farid, A.H. 2014. Porous silicon chemical sensors and biosensors: a
review. Sens. Actuators, B: Chemical. 202: 897–912.
[38] Noor, S.M. 2014. Understanding quantum confinement in nanowires:
basics, applications and possible laws. J. Phys.: Condens. Matter. 26(42):
423202.
[39] Ma, D.D.D., C.S. Lee, F.C.K. Au, S.Y. Tong and S.T. Lee. 2003. Small-
diameter silicon nanowire surfaces. Science. 299(5614): 1874–1877.
[40] Renkun, C., A.I. Hochbaum, P. Murphy, J. Moore, P. Yang and
A. Majumdar. 2008. Thermal conductance of thin silicon nanowires.
Phys. Rev. Lett. 101(10): 105501.
[41] Alan, C., A. Fasoli, P. Beecher, P. Servati, S. Pisana, Y. Fu, et al. 2007.
Thermal and chemical vapor deposition of Si nanowires: Shape control,
dispersion, and electrical properties. J. Appl. Phys. 102(3): 034302.
[42] Hofmann, S., C. Ducati, R.J. Neill, S. Piscanec, A.C. Ferrari, J. Geng,
et al. 2003. Gold catalyzed growth of silicon nanowires by plasma
enhanced chemical vapor deposition. J. Appl. Phys. 94(9): 6005–6012.
[43] Madan, S., G. Monika and D. Kamal. 2018. Size and shape effects on
the band gap of semiconductor compound nanomaterials. J. Taibah
Univ. Sci. 12(4): 470–475.
[44] Gilles, L., O. Guillois, D. Porterat, C. Reynaud, F. Huisken, B. Kohn,
et al. 2000. Photoluminescence properties of silicon nanocrystals as a
function of their size. Phys. Rev. B 62(23): 15942.
[45] Ruoxue, Y., D. Gargas and P. Yang. 2009. Nanowire photonics. Nat.
Photonics 3(10): 569–576.
[46] Jawdat, A.B. 2022. Size effect of band gap in semiconductor nanocrystals
and nanostructures from density functional theory within HSE06. Mater.
Sci. Semicond. Process 137: 106214.
[47] Zheng, Yun, Cristian Rivas, Roger Lake, Khairul Alam, T.B. Boykin
and Gerhard Klimeck. 2005. Electronic properties of silicon nanowires.
IEEE Trans. Electron Devices 52(6): 1097–1103.
[48] Sacconi, F., M.P. Persson, M. Povolotskyi, L. Latessa, A. Pecchia,
A. Gagliardi, et al. 2007. Electronic and transport properties of silicon
nanowires. J. Comput. Electron. 6: 329–333.
[49] Jiansheng, J., W. Zhang, K. Peng, G. Yuan, C.S. Lee and S.-T. Lee. 2008.
Surface-dominated transport properties of silicon nanowires. Adv.
Funct. Mater. 18(20): 3251–3257.
[50] Mehedhi, H., M.F. Huq and Z.H. Mahmood. 2013. A review on
electronic and optical properties of silicon nanowire and its different
growth techniques. Springer Plus 2(1): 1–9.
[51] Tianzhuo, Z., R. Yamato, S. Hashimoto, M. Tomita, S. Oba, Y. Himeda,
et al. 2018. Miniaturized planar Si-nanowire micro-thermoelectric
generator using exuded thermal field for power generation. Sci.
Technol. Adv. Mater. 19(1): 443–453.
[52] Stefan, W., E. Bertagnolli, S. Kawase, Y. Isono and A. Lugstein. 2014.
Electrostatic actuated strain engineering in monolithically integrated
VLS grown silicon nanowires. Nanotechnology 25(45): 455705.
[53] Schneider, B.W., N.N. Lal, S.B. Finch and T.P. White. 2014. Pyramidal
surface textures for light trapping and antireflection in perovskite-on-
silicon tandem solar cells. Optics Express 22(106): A1422–A1430.
[54] Basore, P.A. 1990. Numerical modelling of textured silicon solar cells
using PC-1D. IEEE Transactions on Electron Devices 37(2): 337–343.
[55] Biao, S., B. Liu, J. Luo, Y. Li, C. Zheng, X. Yao, et al. 2017. Enhanced light
absorption of thin perovskite solar cells using textured substrates. Sol.
Energy Mater. Sol. Cells 168: 214–220.
[56] Yubo, C., Q. Wang and J. Xie. 2011. Online social interactions: a natural
experiment on word of mouth versus observational learning. J. Market.
Reser. 48(2): 238–254.
[57] Ibrahim, K., K. Saeed and I. Khan. 2019. Nanoparticles: properties,
applications and toxicities. Arabian J. Chem. 12(7): 908–931.
[58] Oktaviani, O. 2021. Nanoparticles: properties, applications and
toxicities. Jurnal Latihan 1(2): 11–20.
[59] Wagner, A.R.S. and S.W.C. Ellis. 1964. Vapor-liquid-solid mechanism
of single crystal growth. Appl. Phys. Lett. 4(5): 89–90.
[60] Volker, S., S. Senz and U. Gösele. 2005. Diameter-dependent growth
direction of epitaxial silicon nanowires. Nano Lett. 5(5): 931–935.
[61] Marolop, S., K. Usami, T. Kodera, K. Uchida and S. Oda. 2011. Growth
of narrow and straight germanium nanowires by vapor–liquid–solid
chemical vapor deposition. Jpn. J. Appl. Phys. 50(10R): 105002.
[62] Cui, Y.L., J. Lauhon, M.S. Gudiksen, J. Wang and C.M. Lieber. 2001.
Diameter-controlled synthesis of single-crystal silicon nanowires. Appl.
Phys. Lett. 78(15): 2214–2216.
[63] Lu, W. and C.M. Lieber. 2006. Semiconductor nanowires. J. Phys. D:
Appl. Phys. 39(21): R387.
[64] Hsu, C.M., S.T. Connor, M.X. Tang and Y. Cui. 2008. Wafer-scale
silicon nanopillars and nanocones by Langmuir–Blodgett assembly
and etching. Appl. Phys. Lett. 93(13): 133109.
[65] Huang, Z., N. Geyer, P. Werner, J. De Boor and U. Gösele. 2011. Metal-
assisted chemical etching of silicon: a review: in memory of Prof.
Ulrich Gösele. Adv. Mater. 23(2): 285–308.
[66] Peng, K.Q., X. Wang, L. Li, Y. Hu and S.T. Lee. 2013. Silicon nanowires
for advanced energy conversion and storage. Nano Today 8(1): 75–97.
[67] Sandulova, A.V., P.S. Bogoyavlenskii and M.I. Dronyuk. 1964. The
obtaining of, and certain properties of filiform and acicular single
crystals of germanium and silicon, and their solid solutions. Sov. Phys.
Sol. State 5(9): 1883.
[68] Zhang, Y.F., Y.H. Tang, N. Wang, D.P. Yu, C.S. Lee, I. Bello, et al. 1998.
Silicon nanowires prepared by laser ablation at high temperature. Appl.
Phys. Lett. 72(15): 1835–1837.
[69] Werner, P., N.D. Zakharov, G. Gerth, L. Schubert and U. Gösele. 2022.
On the formation of Si nanowires by molecular beam epitaxy. Int. J.
Mater. Res. 97(7): 1008–1015.
[70] Sivakov, V., G. Andra, A. Gawlik, A. Berger, J. Plentz, F. Falk, et al.
2009. Silicon nanowire-based solar cells on glass: synthesis, optical
properties, and cell parameters. Nano Lett. 9(4): 1549–1554.
[71] Uhlir, J.A. 1956. Electrolytic shaping of germanium and silicon. The
Bell System Technical Journal 35(2): 333–347.
[72] Cullis, A.G. and L. T. Canham, Visible light emission due to quantum size
effects in highly porous crystalline silicon. Nature 353(6342): 335–338.
[73] Lehmann, V. and U. Gösele. 1991. Porous silicon formation: a quantum
wire effect. Appl. Phys. Lett. 58(8): 856–858.
[74] Sailor, M.J. 2012. Porous Silicon in Practice: Preparation, Characterization
and Applications. John Wiley and Sons.
[75] Lee, J.H., G.A. Galli and J.C. Grossman. 2008. Nanoporous Si as an
efficient thermoelectric material. Nano Lett. 8(11): 3750–3754.
[76] Anglin, E.J., L. Cheng, W.R. Freeman and M.J. Sailor. 2008. Porous
silicon in drug delivery devices and materials. Adv. Drug Delivery
Rev. 60(11): 1266–1277.
[77] Bisi, O., S. Ossicini and L. Pavesi. 2000. Porous silicon: a quantum
sponge structure for silicon-based optoelectronics. Surf. Sci. Rep.
38(1–3): 1–126.
[78] Foll, H., M. Christophersen, J. Carstensen and G. Hasse. 2002. Formation
and application of porous silicon. Mater. Sci. Eng.: R: Rep. 39(4): 93–141.
[79] Kim, H., B. Han, J. Choo and J. Cho. 2008. Three-dimensional porous
silicon particles for use in high-performance lithium secondary
batteries. Angew. Chem. 120(52): 10305–10308.
[80] Trucks, G.W., K. Raghavachari, G.S. Higashi and Y.J. Chabal. 1990.
Mechanism of HF etching of silicon surfaces: a theoretical understanding
of hydrogen passivation. Phys. Rev. Lett. 65(4): 504.
[81] Cullis, A.G., L..T. Canham and P.D.J. Calcott. 1997. The structural and
luminescence properties of porous silicon. J. Appl. Phys. 82(3): 909–965.
[82] Smith, R.L. and S.D. Collins. 1992. Porous silicon formation mechanisms.
J. Appl. Phys. 71(8): R1–R22.
[83] Kim, J., H. Han, Y.H. Kim, S.H. Choi, J.C. Kim and W. Lee. 2011. Au/Ag
bilayered metal mesh as a Si etching catalyst for controlled fabrication
of Si nanowires. ACS Nano 5(4): 3222–3229.
[84] Zhang, X.G. 2003. Morphology and formation mechanisms of porous
silicon. J. Electrochem. Soc. 151(1): C69.
[85] Zhang, X.G., S.D. Collins and R.L. Smith. 1989. Porous silicon formation
and electropolishing of silicon by anodic polarization in HF solution.
J. Electrochem. Soc. 136(5): 1561.
[86] Solanki, C.S., R.R. Bilyalov, J. Poortmans, J.P. Celis, J. Nijs and
R. Mertens. 2004. Self-standing porous silicon films by one-step
anodizing. J. Electrochem. Soc. 151(5): C307.
[87] Bocking, T., K.A. Kilian, P.J. Reece, K. Gaus, M. Gal and J.J. Gooding.
2012. Biofunctionalization of free-standing porous silicon films for self-
assembly of photonic devices. Soft. Matter. 8(2): 360–366.
[88] Ghulinyan, M., C.J. Oton, G. Bonetti, Z. Gaburro and L. Pavesi. 2003.
Free-standing porous silicon single and multiple optical cavities.
J. Appl. Phys. 93(12): 9724–9729.
[89] Von Behren, J., L. Tsybeskov and P.M. Fauchet. 1995. Preparation
and characterization of ultrathin porous silicon films. Appl. Phys.
Lett. 66(13): 1662–1664.
[90] Xiao, Y., X. Li, H.D. Um, X. Gao, Z. Guo and J.H. Lee. 2012. Controlled
exfoliation of a heavily n-doped porous silicon double layer
electrochemically etched for layer-transfer photovoltaics. Electrochim.
Acta. 74: 93–97.
[91] Archer, R.J. 1960. Stain films on silicon. J. Phys. Chem. Solids. 14: 104–110.
[92] Li, X. and P.W. Bohn. 2000. Metal-assisted chemical etching in HF/H2O2
produces porous silicon. Appl. Phys. Lett. 77(16): 2572–2574.
[93] Chartier, C., S. Bastide and L.C. Clement. 2008. Metal-assisted chemical
etching of silicon in HF/H2O2. Electrochim. Acta 53(17): 5509–5516.
[94] Ragavendran, V., A. Muthu Kumar, V. Vishnukanthan, J.M. Pearce
and J. Mayandi. 2018. Effects of silver catalyst concentration in metal
assisted chemical etching of silicon. Mater. Lett. 221: 206–210.
[95] Ragavendran, V., J. Mayandi, J.M. Pearce and V. Vishnukanthan. 2019.
Influence of metal assisted chemical etching time period on mesoporous
structure in as-cut upgraded metallurgical grade silicon for solar cell
application. J. Mater. Sci.: Mater. Electron. 30: 8676–8685.
[96] Huang, Z.P., N. Geyer, L.F. Liu, M.Y. Li and P. Zhong. 2010. Metal-assisted
electrochemical etching of silicon. Nanotechnology 21(46): 465301.
[97] Toor, F., J.B. Miller, L.M. Davidson, L. Nichols, W. Duan, M.P. Jura, et al.
2016. Nanostructured silicon via metal assisted catalysed etch (MACE):
chemistry fundamentals and pattern engineering. Nanotechnology
27(41): 412003.
[98] Kexun, C., J. Zha, F. Hu, X. Ye, S. Zou, V. Vahanissi, et al. 2019. MACE
nano-texture process applicable for both single-and multi-crystalline
diamond-wire sawn Si solar cells. Sol. Energy Mater. Sol. Cells 191: 1–8.
[99] Oh, J., H.C. Yuan and H.M. Branz. 2012. An 18.2%-efficient black-
silicon solar cell achieved through control of carrier recombination in
nanostructures. Nat. Nanotechnol. 7(11): 743–748.
[100] Jung, J.Y., Z. Guo, S.W. Jee, H.D. Um, K.T. Park and J.H. Lee. 2010. A
strong antireflective solar cell prepared by tapering silicon nanowires.
Optics Express 18(103): A286–A292.
[101] Peng, K.Q., X. Wang, L. Li, Y. Hu and S.T. Lee. 2013. Silicon nanowires
for advanced energy conversion and storage. Nano Today 8(1): 75–97.
[102] Chartier, C., S. Bastide and C.L. Clement. 2008. Metal-assisted chemical
etching of silicon in HF/H2O2. Electrochim. Acta 53(17): 5509–5516.
[103] Tsujino, K. and M. Matsumura. 2005. Helical nanoholes bored in
silicon by wet chemical etching using platinum nanoparticles as
catalyst. Electrochem. Solid-State Lett. 8(12): C193.
[104] Korotcenkov, G. (ed.). 2016. Porous silicon: from formation to application:
formation and properties. CRC Press.
[105] Zhang, M.L., K.Q. Peng, X. Fan, J.S. Jie, R.Q. Zhang, S.T. Lee, et al. 2008.
Preparation of large-area uniform silicon nanowires arrays through
metal-assisted chemical etching. J. Phys. Chem. C 112(12): 4444–4450.
[106] Abdullah, C.A.C., D.F.A. Razak, M.B.M. Yunus, M. Zaki and M. Yusoff,
2019. Structural and optical properties of N-Type and P-Type porous
silicon produced at different etching time. Int. J. Electroactive Mater.

7: 28–37.
[107] Yaoping, L., T. Lai, H. Li, Y. Wang, Z. Mei, H. Liang, et al. 2012.
Nanostructure formation and passivation of large-area black silicon
for solar cell applications. Small 8(9): 1392–1397.
[108] Han, H., Z. Huang and W. Lee. 2014. Metal-assisted chemical etching
of silicon and nanotechnology applications. Nano Today 9(3): 271–304.
[109] Hildreth, O.J. and D.R. Schmidt. 2014. Vapor phase metal-assisted
chemical etching of silicon. Adv. Funct. Mater. 24(24): 3827–3833.
[110] Huang, Z., X. Zhang, M. Reiche, L. Liu, W. Lee, T. Shimizu, et al.
2008. Extended arrays of vertically aligned sub-10 nm diameter [100]
Si nanowires by metal-assisted chemical etching. Nano Lett. 8(9):
3046–3051.
[111] Hildreth, O.J., W. Lin and C.P. Wong. 2009. Effect of catalyst shape and
etchant composition on etching direction in metal-assisted chemical
etching of silicon to fabricate 3D nanostructures. ACS Nano 3(12):
4033–4042.
[112] Yae, S., Y. Morii, M. Enomoto, N. Fukumuro and H. Matsuda. 2013.
MACE of silicon using oxygen as an oxidizing agent: influence of HF
concentration on etching rate and pore morphology. ECS Transactions
50(37): 31.
[113] Naderi, N. and M.R. Hashim. 2012. A combination of electroless and
electrochemical etching methods for enhancing the uniformity of
porous silicon substrate for light detection application. Appl. Surf.
Sci. 258(17): 6436–6440.
[114] Ragavendran, V., P. Jayabal and J. Mayandi. 2014. Investigation on low
temperature high pressure oxidation of porous silicon J. NanoScience
and NanoTechnology 2: 711.
[115] Salman, K.A., Z. Hassan and K. Omar. 2012. Effect of silicon porosity
on solar cell efficiency. Int. J. Electrochem. Sci. 7(9): 376–386.
[116] Salman, K.A., K. Omar and Z. Hassan. 2011. The effect of etching time
of porous silicon on solar cell performance. Superlattices Microstruct.
50(6): 647–658.
[117] Gokce, A.G. and E. Akturk. 2015. A first-principles study of n-type
and p-type doping of germanium carbide sheet. Appl. Surf. Sci. 332:
147–151.
[118] Kim, J. 2007. Formation of a porous silicon anti-reflection layer for a
silicon solar cell. J. Korean Phys. Soc. 50(4): 1168–1171.
[119] Aziz, W.J., A. Ramizy, K. Ibrahim, Z. Hassan and K. Omar. 2011. The
effect of anti-reflection coating of porous silicon on solar cells efficiency.
Optik 122(16): 1462–1465.
[120] Kwon, J.H., S.H. Lee and B.K. Ju. 2009. Thin film silicon substrate
formation using electrochemical anodic etching method. Surf. Eng.
25(8): 603–605.
[121] Ramizy, A., W.J. Aziz, Z. Hassan, K. Omar and K. Ibrahim. 2010. The
effect of porosity on the properties of silicon solar cell. Microelectron.
Int. 27(2): 117–120.
[122] Clement, C.L., S. Lust, M. Mamor, J. Rappich and T. Dittrich. 2005.
Investigation of p-type macroporous silicon formation. Physica. Status
Solidi (a) 202(8): 1390–1395.
[123] Aziz, W.J., A. Ramizy, K. Ibrahim, Z. Hassan and K. Omar. 2011.
The effect of anti-reflection coating of porous silicon on solar cells
efficiency. Optik 122(16): 1462–1465.
[124] Tobail, O., Z. Yan, M. Reuter and J.H. Werner. 2008. Lateral homogeneity
of porous silicon for large area transfers solar cells. Thin Solid Films
516(20): 6959–6962.
[125] Kwon, H., J. Lee, M. Kim and S. Lee. 2011. Investigation of antireflective
porous silicon coating for solar cells. International Scholarly Research
Notices. 2011: Article ID 716409.
[126] Chakraborty, B.R., D. Kabiraj, K. Diva, J.C. Pivin and D.K. Avasthi,
2006. Mixing behaviour of buried transition metal layer in silicon due
to swift heavy ion irradiation. Nucl. Instrum. Methods Phys. Res.,
Sect. B. 244(1): 209–212.
[127] Vinod, P.N. 2007. Application of power loss calculation to estimate the
specific contact resistance of the screen-printed silver ohmic contacts of
the large area silicon solar cells. J. Mater. Sci.: Mater. Electron. 18: 805–810.
[128] Clement, C.L., A. Lagoubi, R. Tenne and M.N. Spallart. 1992.
Photoelectrochemical etching of silicon. Electrochim. Acta. 37(5): 877–888.
[129] Coakley, K.M. and M.D. McGehee. 2003. Photovoltaic cells made from
conjugated polymers infiltrated into mesoporous titania. Appl. Phys.
Lett. 83(16): 3380–3382.
[130] Fang, H., X. Li, S. Song, Y. Xu and J. Zhu. 2008. Fabrication of
slantingly-aligned silicon nanowire arrays for solar cell applications.
Nanotechnology 19(25): 255703.
[131] Kumar, D., S.K. Srivastava, P.K. Singh, M. Husain and V. Kumar. 2011.
Fabrication of silicon nanowire arrays based solar cell with improved
performance. Sol. Energy Mater. Sol. Cells 95(1): 215–218.
[132] Lin, X.X., X. Hua, Z.G. Huang and W.Z. Shen. 2013. Realization
of high-performance silicon nanowire based solar cells with large
size. Nanotechnology 24(23): 235402.
[133] Oh, J., H.C. Yuan and H.M. Branz. 2012. An 18.2%-efficient black-
silicon solar cell achieved through control of carrier recombination in
nanostructures. Nat. Nanotechnol. 7(11): 743–748.
[134] Savin, H., P. Repo, G.V. Gastrow, P. Ortega, E. Calle, M. Garin, et al.
2015. Black silicon solar cells with interdigitated back-contacts achieve
22.1% efficiency. Nat. Nanotechnol. 10(7): 624–628.
[135] Sivakov, V., G. Andra, A. Gawlik, A. Berger, J. Plentz, F. Falk, et al.
2009. Silicon nanowire-based solar cells on glass: synthesis, optical
properties, and cell parameters. Nano Lett. 9(4): 1549–1554.`
Chapter 4
Selenium-Based Metal
Chalcogenides Thin Films on
Flexible Metal Foils for PEC
Water-Splitting Applications
Bheem Singh1,2, Sudhanshu Gautam1,2,

Vishnu Aggarwal1,2, Rahul Kumar1,2,
Vidya Nand Singh1,2 and Sunil Singh Kushvaha1,2*
1
CSIR-National Physical Laboratory, Dr. K.S. Krishnan Marg,
New Delhi, India 110012.
2
Academy of Scientific and Innovative Research (AcSIR),
Ghaziabad, India 201002.
4.1 IntroductIon
The search for renewable and environment-friendly (non-polluting)

energy resources has become indispensable for a sustainable society to
overcome global energy and environmental depletion over the last few
decades. Hydrogen gas (H2) is widely considered a sophisticated future
fuel. It is probable to substitute traditional fossil fuels with sustainable H2
fuel [1–3]. Nonrenewable energy sources, such as coal, oil, gasoline, etc.,
*For Correspondence: Sunil Singh Kushvaha (kushvahas@nplindia.org)

Selenium-Based Metal Chalcogenides Thin Films on Flexible Metal Foils... 95
are supplied to the world for a decade to fulfill the demands of energy
needs. However, to produce H2, these processes use a massive number of
natural resources. Meanwhile, with the increments in world population
and improvement in living standards, fossil fuels are also depleting
rapidly. Moreover, nonrenewable energy sources harm human health
and our earth’s climate due to the emission of harmful gases such as
CO2 [4, 5]. To confront this global problem, we need a technology that
can replace fossil fuel or nonrenewable energy sources to save our planet
from ecological deprivation.
Among all available green fuels, hydrogen (H2) has the highest
energy yield per unit mass of 121 KJ/g compared to fossil fuels, liquefied
petroleum gases (LPG gases), and diesel, as these all have specific
energy densities of less than 40 KJ/g. As an energy carrier, H2 is highly
adorable for conveyance, storage, and power replacement in places
where other energy sources are difficult or expensive to access [6, 7].
Even though sustainable energy sources such as solar, wind, tidal,
geothermal, and hydrothermal have been widely used to replace fossil
fuels by supplying sustainable energy for a long time. However, the
most developed renewable energy sources have limitations due to their
enormous electricity production cost and low efficiency, which remains
a challenge [8].
In this way, the abundant water supply and sunlight offer us an
affordable alternate source to produce hydrogen from fossil fuel and
biomass products. The production of usable energy from abundant
solar energy, and water can be an ideal solution to our future energy
needs [9]. H2 production by solar-driven water-splitting process emerged
as an efficient way to solve the scarcity of fossil fuels and overcome
the environmental degradation situation by zero carbon emission. H2
production by water splitting is an environment-friendly technique
without the emission of harmful gases as it is based on water and
sunlight, renewable energy sources [10, 11].
Water splitting is the process whereby the water splits into hydrogen
(H2) and oxygen (O2) molecules. There are various processes of water
splitting, including photocatalytic (PC), electrochemical, thermochemical,
radiolysis, photobiological, and photoelectrochemical (PEC), etc. [12].
Exclusive to PEC, the rest of the techniques are limited due to cost and
efficiency, which are responsible for their limited use. For example, in the
photobiological process, the solar-to-hydrogen conversion performance
is not more than 1%, which is not desired for large-scale implementation
[13]. In the solar thermochemical process, to split water, we need to
heat the water to 2,000 °C. Therefore, designing a robust reactor is also
challenging, and poor H2 efficiency is also the leading cause of their
limited use [14]. The by-product of radiolysis is nuclear waste, which is
also not desired. The electrochemical process is limited by the cost of
electricity and by its production method [15]. The more efficient, most
straightforward, and inexpensive way to split water is by photocatalytic
(PC) and photoelectrochemical (PEC) processes. However, there are
some disadvantages of the PC process over PEC, such as the separation
of generated H2 and O2 is required immediately, which will consume
additional energy, causing low efficiency of the water-splitting process.
Besides this, a gas separator is also required in the PC system to separate
H2, and O2 gases as the water oxidation and reduction process occur at
the same surface of the photocatalyst [16, 17].
Hydrogen production by PEC water splitting has been considered
a promising approach for solar energy conversion to chemical energy
(H2). By PEC water splitting, HER and OER can be readily achieved at a
potential below 1.23 V and above 0 V, compared to many photovoltaic cells
that require appropriate voltage for electrocatalytic water splitting [18].
PEC can willingly achieve high photocatalytic activity on the sensible
selection of photoanode and photocathode. The pure form of H2 and
O2 via PEC water splitting can be attained, which is environmentally
friendly. There is no need for high-temperature treatment and several
steps like solar thermochemical and hydrothermal processes in PEC
water-splitting process [2, 19]. In 1972, Honda and Fujishima first
demonstrated water splitting using a TiO2 single crystal as a photoanode
and Pt as a cathode in an aqueous electrolyte solution [18]. Since then,
researchers worldwide have been working toward cumulative solar-to-
hydrogen conversion (STH) efficiency of photocatalysts through several
methods like doping, ion implantation, dye sensitization, and bilayered
system, etc. [20, 21].
The natural photosynthesis process allows us to think about the direct
conversion of sunlight into chemical fuel (H2), where solar radiation
converts H2O and CO2 into oxygen and carbohydrates. Splitting water
into H2 and O2 molecules via PEC is one of the most suitable approaches
under solar radiation, also called the artificial photosynthesis process.
The water-splitting process is readily be achieved with a net Gibbs free
energy of 238 KJ/mol or 1.23 eV, as shown below [16, 17, 19]:
2hn + photoelectrode → 2e– + 2h+
2h+ + H2O (liquid) → ½O2 + 2H+ water oxidation (1.23 V vs. NHE)
2H+ + 2e– → H2(gas) water reduction (+0.00 V vs. NHE)
2H2O → 2H2 + O2 (Overall water splitting)
The PEC water-splitting process is based on the conversion of solar
energy into electrical energy within a cell equipped with two electrodes:
a photoelectrode to absorb light and another metal electrode (counter
electrode) to collect the carriers; the whole system is immersed in an
aqueous electrolyte solution and the generated electricity used for
water decomposition [22]. The schematic of the PEC water-splitting

working process is shown in Figure 4.1. The main components of a
PEC process are its working electrode and the counter electrode. The
counter electrode is usually inert Pt, while the working electrode is a
semiconductor photoelectrode, wherein the third electrode is termed a
reference electrode (Ag/AgCl). When light is exposed to a semiconductor
photoelectrode, it can generate excitons (electron-hole pair), hole reacts
with water molecules and oxidizes them in O2 molecules and H+ ions.
These H+ ions transport through the electrolyte to the Pt electrode,
electrons will come through the external circuit to the Pt electrode,
and then H+ and e – reduce to H2(gas). Thus, for n-type semiconducting
materials, oxygen evolution occurs at the photoanode and hydrogen
evolution at the counter electrode (Pt cathode). An n-type semiconductor
produces an anodic current.
Figure 4.1 Schematic diagram of the PEC water spitting process in which three
electrodes, namely the photoelectrode, the counter electrode, and the reference
electrode.
Similarly, a p-type semiconductor photoelectrode can generate

excitons and efficiently reduce water (hydrogen evolution reaction; HER)
and is referred to as a photocathode, and OER takes place at the cathode
and a cathodic current flow by a p-type semiconductor. Moreover, n-type
and p-type materials are used as the photoanode and photocathode in
the bi-photoelectrode PEC system. The advantage of such a system is
that the photovoltages are developed on both electrodes, resulting in
the formation of an overall voltage sufficient for water decomposition
without applying a bias [23, 24]. The comparison of these processes has
been shown in Figure 4.2. The conventional three-electrode PEC cells
are commonly used to measure PEC performance. A three-electrode cell

offers subsidiary information about the semiconductor photoelectrodes,
such as the relationship between an applied bias on the photoelectrodes
and the capability of photoelectrodes to convert solar energy into
valuable fuels such as H2 and O2 at this applied bias. This conversion
efficiency is referred to as applied bias photon-to-current conversion
efficiency (ABPE) [25–27].
Figure 4.2 Schematic diagram of PEC water spitting process for (a) n-type
semiconductor, (b) p-type semiconductor, (c) bi-photoelectrode PEC system.
Since the first demonstration of water photoelectrolysis by Fujishima

and Honda in the early 1970s, various kinds of material have been
explored that fulfill the essential requirement to convert solar energy
into helpful H2 fuel, as discussed following:
1. The Band gap of the semiconductors should be more than or
equivalent to 1.23 eV or DG0 = 237.1 kJ·mol−1 to split water in H2 and O2
gas. The overpotential loss at the electrode and ionic conductivity
loss in the electrolyte also have to be taken into consideration.
2. Semiconductors should have the capability to create photo
excitons (electron-hole pairs) from the absorbed solar light
(effective charge transport).
3. For the spontaneous water-splitting process, semiconducting
materials should have proper conduction band and valence band
edge positions. For the water redox process, the C.B. position of
the semiconductor should be above the water redox potential
0 V NHE [E (H2O/H2)] and for water oxidation, the V.B. position
should be below the water oxidation potential 1.23 V NHE
[E (H2O/H2)]. The conduction and valance band edge position
of various semiconducting materials has been presented in
Figure 4.3 [28–33].
Figure 4.3 Band gap and band edge positions (conduction and valence band)
of various semiconducting materials in the aqueous electrolyte at pH=0 for PEC
water applications.
4. To achieve efficient charge separation and collection,

semiconductors should be more crystalline for easy charge
carrier transport and a large surface area to produce more HER
and OER activities.
5. Stability is of the utmost importance to the solar water-splitting
application. Selected materials should be stable in an aqueous
electrolyte solution of a specific pH range and possess corrosion-
resistance properties (long-term stability).
6. Semiconducting material also should be cost-effective and earth-
abundant elements to enable large-scale implementation; for
instance, a-Fe2O3, ZnSiN2, etc.
The semiconductor materials that meet all these criteria can be
considered for the commercial production of solar fuels. The efficiency
and performance of semiconductors are mainly determined by their
morphology, tidiness, and homogeneity. Various key factors affect
the PEC performance, including electrolyte pH, electrolyte resistance,
particle size and defects, shape and size of nanoclusters, band gap
energy and intensity of light, etc. All these factors should be considered
to achieve better PEC performance of photoelectrode materials.
The hunt for an efficient semiconductor photocatalyst is a major
driving force in artificial photosynthesis. After the discovery of TiO2 in
the water-splitting process, several semiconducting materials have been
explored for the same purpose. Various p-type materials, such as CuO
(1.2–1.8 eV), Cu2O (1.9–2.2 eV), CuFeO2 (1.5–1.6 eV), NiO (3.6–4.0 eV),
CaFe2O4 (1.9 eV), etc., [2, 29, 30, 34] has been widely investigated due to
their significant photocathode performance and excellent photovoltaic
properties. At the same time, excellent photoconversion efficiency has
been achieved for compound semiconductors, such as GaAs, GaInP,
AlGaAs, etc. Furthermore, various n-type semiconducting materials,
such as TiO2 (3.1–3.2 eV), ZnO (3.2 eV), WO3 (2.5–2.7 eV), a-Fe2O3
(2.1–2.3 eV), BiVO4 (2.4–2.5 eV), Ta3N5 (2.1 eV), ZnS (3.6 eV), TaON (2.4 eV),
CdS (2.4 eV), etc., have been used as a photoanode for the PEC water-
splitting process [35–37]. Currently, transition metal dichalcogenides
(TMDCs) materials such as MoS2 (1.2–1.8 eV), MoSe2 (0.9–1.5 eV), WSe2
(1–1.7 eV), etc., have been explored for artificial photosynthesis due to
their excellent electronic, optical, and photocatalytic activities [38–41].
However, each semiconductor has its pros and cons concerning earth
abundance, toxicity, stability, and fabrication cost. Thus, there is still a
need for an efficient, stable, and robust semiconductor photocatalyst that
can help to provide energy carriers for future generations. Therefore,
there is still demand for the search for high-performance photocatalysts
to hunt artificial photosynthesis by fabricating various kinds of
compositions, nanostructures, heterostructures, hybrid structures, etc.
Another aspect is to couple the low band gap material, such as binary
chalcogenides materials: Sb2S3, Bi2S3, and Sb2S3, Sb2Se3, Bi2Se3, Bi2Te3, etc.,
with large band gap materials (e.g., stacked dual-electrode PEC cell),
which can absorb a wide range of solar spectrum and can enhance the
photocatalyst performance in water-splitting process [31, 42].
Recently, V–VI group binary chalcogenides semiconducting
materials (such as Bi2Se3, Bi2Te3, Sb2Se3, Sb2Te3, etc.) emerged as efficient
photocatalysts for water-splitting applications due to their favorable
conduction band edge position for facile H2 production [42–44]. Although
binary chalcogenides are a vast family of compound materials, many
elements belong to chalcogen, i.e., S, Se, and Te. However, out of several
compounds, Bi and Sb-based chalcogenides have excellent properties
and tremendous application in energy devices. These materials have
been extensively used in the field of thermoelectric devices, infrared
photodetectors, spintronic devices, terahertz detection, optical recording,
electrical transport devices, etc., due to their high electron mobility,
good electrical conductivity, photoresponsivity, and electrochemical
properties as shown in Figure 4.4 [45, 46]. These materials also come
in a particular class of material, i.e., topological insulators (TIs). TIs
materials are the materials that exhibit insulating properties in the
interior but have conducting surface states. These materials have robust
metallic surface states property against nonmagnetic impurities and
disorders protected by time-reversible symmetry. In addition, elastic
backscattering is forbidden and electron transport is spin momentum
locked, where a super current (resistance less) flows through the surface
edges of these materials. These unique properties of TIs give birth to

various fundamental quantum aspects such as opto-spintronic devices,
quantum Hall effect, quantum computers, etc. [47–51].
Figure 4.4 Schematic diagram of binary chalcogenides materials in various

applications.
The crystal structure representation of Bi2Se3, Bi2Te3, Sb2Se3, and

Sb2Te3 materials is shown in Figures 4.5 and 4.6. Figure 4.5(a) portrays
the rhombohedral crystal structure of Bi2Se3, with space group D53d
(R3m) with lattice parameters: a = b = 4.14 Å and c = 28.64 Å. Usually, it
is presented in quintuple layers with two equivalent Se and Bi atoms:
Se (1)–Bi-Se (2)–Bi-Se (1) arrangement along the z-direction, where one
quintuple layer is 0.955 nm thick linked with another quintuple layer
with weak van der Waals (vdW) forces. One single layer of Bi2Se3 is
composed of hexagonal and trigonal lattice planes. The most stable
phase of Bi2Se3 is hexagonal at room temperature [52]. Figure 4.5(b)
also shows the rhombohedral crystal structure for Bi2Te3 with quintuple
layer arrangement with lattice parameters: a = b = 4.38 Å, c = 30.5 Å. It
consists of one QLs layer thickness ~1 nm bonded with weak van der
Waals forces. The crystal structure of antimony selenide (Sb2Te3) also
has a rhombohedral crystal structure similar to that of Bi2Te3 and Bi2Se3
with lattice constant a = 4.32 Å and c = 31.5 Å belonging to the R3m (166)
space group.
Figure 4.5 The Rhombohedral crystal structure of (a) Bi2Se3 and (b) Bi2Te3
compounds.
Figure 4.6 (a) Rhombohedral structure of Sb2Te3 compound (b) Orthorhombic

crystal structure of Sb2Se3 compound.
The crystal structure of Sb2Te3 also contains five atomic layers

structure in the primitive cell leading to the formation of a rhombohedral
structure, where Sb and Te atoms are covalently stacked within the QLs
along the c-axis as shown in Figure 4.6(a) [53, 54]. Figure 4.6(b) shows
the orthorhombic Pnma crystal structure of Sb2Se3 made up of AB6 and
AB6+1 polyhedra with constant lattice values of a = 11.78 Å, b = 3.97 Å,
c = 11.63 Å, where strong covalent Sb-Se bond keeps the cells together
in the (001) direction at room temperature. The physical and electronic
properties of these materials have been mentioned in Table 4.1 [51–55].
Table 4.1 Physical, electronic, and optical properties of V–VI group binary
chalcogenides materials
Materials Band gap Mobility Melting point Thermal Density r
(eV) (cm 2V–1s –1) (K) conductivity (g cm−3)
(W m−1 K−1)
Bi2Se3 0.3 1407 979 1.70 7.51
Bi2Te3 0.1 481 858 1.37 7.85
Sb2Te3 0.21 675 893 2.2 6.5
Sb2Se3 1.1 15 (for electron), 884 — 5.81
42 (for hole)
Out of these materials, Bi2Se3 and Sb2Se3 semiconductors have been

widely used as a photocatalyst for PEC water splitting due to their
massive electronic and optical properties with suitable conduction
band edge position for hydrogen evolution. Bi2Se3 is an n-type direct
band gap semiconducting material with fascinating electronic and optical
properties. This small and optimum band gap (0.3–2.0 eV) semiconductor
has the potential to absorb visible and near-infra-red radiation, which
can be beneficial to achieve high solar-to-chemical energy conversion
efficiency [31, 43]. The high electron mobility (1,407 cm 2V−1 s−1) of
Bi2Se3 is beneficial for better device performance. The high thermal
conductivity of Bi2Se3 (1.70 W m−1 K−1) can more effectively transfer heat
and readily take up heat from the environment. The high melting point
(979 K) gives the advantage to the growth of Bi2Se3 thin film at elevated
temperatures to use in relatively high-temperature applications [56–61].
Bi2Se3 also comes from the topological insulator family, where the strong
metallic surface states of Bi2Se3 can play an essential role by strongly
ceasing the elastic backscattering of electrons. As the surface electronic
structure of the semiconductors has a significant role in PEC water-
splitting performance, the fast and low dissipation electron transport
can enhance the electrical conductivity of photoelectrode in the water-
splitting process [42, 44]. Even though Bi2Se3 was not explored more as
a photocatalyst in PEC water splitting due to the rapid recombination of
charge carriers. However, some recent reports of Bi2Se3 semiconductor-
based photocatalysts draw great attention by getting excellent solar-to-
hydrogen conversion efficiency as summarized in Table 4.2 [31, 62–65].
Table 4.2 Literature survey on PEC water splitting of Bi2Se3 and Sb2Se3-
based photoelectrodes
Photoelectrode Photocurrent Density Electrolyte Year References
Bi2Se3/TiO2/FTO (NFs) 1.76 mA/cm2 at 1.23 VRHE 0.5 M Na2SO3 2021 [31]
2
Bi2Se3/TiO2/FTO (QDs) 1 mA/cm at 1.6 VRHE 0.1 M Na2SO4 2022 [62]
Bi2Se3/TiO2/ITO 6 µA/cm2 0.5 VAg/AgCl 0.5, 1 M Na2SO4 2021 [63]
MoSe2/Bi2Se3/FTO 85 mA/cm2 at −0.6 VRHE 0.5 M H2SO4 2017 [64]
(Hybrids)
Bi2Te3/TiO2/FTO (QDs) 0.86 mA/cm2 at 1.6 VRHE 0.1 M Na2SO4 2021 [62]
Sb2Se3/CuSbS2/FTO 18.0 mA/cm2 at 0 VRHE 1 M H2SO4 2022 [71]
Sb2Se3 (Se-annealed)/ −8.6 mA/cm2 at 0 VRHE 0.5 Na2SO4, 2017 [70]
CdS/TiO2/Pt on soda 0.25 M
lime glass Na2HPO4
Au/Sb2Se3/MoSx/FTO 16 mA/cm2 at 0 VRHE 1 M H2SO4 2018 [72]
Sb2Se3/MoS2/FTO 10 mA/cm2 at 0 VRHE 0.5 M Na2SO4 2019 [73]
Bi2Se3/FTO thin film 10 mA /cm2 at 385 mV 0.5 M Na2SO4 2021 [65]
(4 nm) overpotential
Bi2Te3@CoNiMo/Ni –60 mA cm2 at –0.1 VRHE 0.9 M KOH 2015 [78]
foam
MoS2/Bi2Te3/SrTiO3 10 mA/cm2 at -0.4 VRHE 0.5 M H2SO4 2021 [79]
Sb2Se3 is also a member of the binary chalcogenide family that

has been investigated as an efficient material for thermoelectric and
photovoltaic areas; it is also used to fabricate PEC photocathodes, which
profit from its low toxicity, appropriate band gap, and exceptional photo
corrosion stability [66, 67]. Sb2Se3 shows p-type conductivity, which is
suitable for constructing a suitable photocathode for hydrogen evolution
reaction. Sb2Se3 is a low band gap semiconducting material that can be
a promising candidate for a stacked bilayered system because of the
following advantage:
1. Sb2Se3 has a proper conduction band edge position, i.e. −0.29 V
NHE at pH = 0, which makes it suitable for hydrogen production.
2. It has an optical band gap (1.1−1.4 eV) and also contains a high
optical absorption coefficient (~105 cm−1), which permits to
absorption of visible near-infrared sunlight within a thickness
of ~500–600 nm.
3. The constituent elements Sb and Se are relatively earth-abundant
and low-toxic, which also have the advantage of fabricating PEC
energy devices at a large scale.
4. A low melting point (884 K) also permits us to prepare high-
quality Sb2Se3 thin films at a relatively low temperature using
thermal sublimation of a powder source.
5. It consists of good electron and hole mobility with high relative

permittivity of 15. The high relative permittivity of Sb2Se3 can be
beneficial to lower exciton binding energy that infers an instant
separation of electrons and holes upon light irradiation.
6. Sb2Se3 consists of only a single phase, orthorhombic Sb2Se3, which
allows us to control the phase and defects in Sb2Se3 much more
manageable than other chalcogenides materials [68–73], although
Sb2Se3 has a small band gap in the range of 1.0–1.3 eV. However,
in many pieces of literature, the optical band gap of Sb2Se3
has been reported as 1.4 to 1.8 eV depending on the annealing
temperature after film growth [74–77]. In various kinds of
literature, Sb2Se3 and also Bi2Te3-based heterostructures have
been showing excellent photocurrent density, as summarized
in Table 4.2 [70–73, 78, 79]. However, these hard substrate-based
photoelectrodes (e.g., Si, glass, quartz, etc.) have limitations in the
use of flexible energy device applications due to their brittleness,
and unbending nature, etc.
Nowadays, flexible electronics draw great attention due to their
excellent transportability, lightweight, stretchability, bendability, and
human-friendly interfaces to provide new avenues for next-generation
electronics. Flexible electronics have been developed in flexible
transistors, supercapacitors, displays, batteries, nanogenerators, light-
emitting diodes (LEDs), solar cells, thermoelectric devices, etc. [80, 81].
To open new prospects in the advancement of solar cells or fuel
applications, the search for innovative, flexible PEC devices is highly
in demand. For a long time, FTO or ITO doped glass, silicon (Si) based
rigid and heavyweight substrates have been widely used to construct
photoelectrode, which has limitations in flexible PEC devices due to
their hardness and unbinding nature. Selecting a flexible substrate is
challenging to design flexible electrodes since it depends on several
parameters such as excellent flexibility, robustness, pliability, thermal
and electric conductivity, chemical and thermal stability at elevated
temperatures, etc. [82]. The various organic and polymer-based substrates
such as polyethylene terephthalate (PET), polyethylene naphthalate (PEN),
and polyimide carbon cloth are not suitable for the growth of high-
quality photocatalysts at high temperatures as well as high penetrability
of oxygen and humidity also unfavorable to fabricate energy devices [83].
This way, flexible metal foils like Mo, Ti, Al, stainless steel, etc., have the
potential advantage due to their excellent thermal and chemical stability
at elevated temperatures. Flexible metal foils have been widely used
in flexible solar cells due to their robustness, lightweight, outstanding
flexibility, etc. Flexible metal foils are bearable at high-temperature
sulfurization and selenization processes, whereas polymer substrates
have their limitation at high temperatures [84]. There are some potential
advantages of flexible metal foils. (1) They can be used to fabricate large-
scale roll-to-roll devices; (2) the total weight of devices will be lowered,
and easy for transportation; (3) the excellent flexibility of substrate can
be beneficial for integrating irregular surfaces, such as an electric vehicle,
ship, drone, etc., and this feature could help enhance the photocatalytic
performance; (4) on the requirement of the specific application, metal
foil-based thin-film cells can be easily molded/twisted into various
design and shape; (5) there is no need for metallic back contact for
photoelectrode if we select metal foils which also has a particular
advantage over polymer substrates [85, 86]. The usefulness of flexible
metal foil in various flexible energy device applications has been shown
in Figure 4.7. Shiyani et al. prepared a flexible zinc oxide photoelectrode
for PEC energy conservation have generated photocurrents of about
1.89 µA/cm2 [87]. Quynh et al. prepared Fe2O3/ZnO heterostructure on
mica for PEC water splitting and obtained a photocurrent density of
0.38 mA/cm2 [88]. In another literature, 1.37 mA/cm2 photocurrent density
was achieved for flexible PVDF/Cu/PVDF-NaNbO3 photoanode [89]. These
results offer a fundamental understanding of flexible photoelectrode,
which can be used to develop hybrid solar-based devices to generate
solar fuels. However, there is a limited report for flexible metal foil-based
photoelectrode for PEC water-splitting applications.
Figure 4.7 Role of flexible metal foils as futuristics: (a) flexible and wearable
thermoelectric devices, (b) flexible photodetectors, (c) flexible PEC devices on
the irregular surface such as electric vehicles, airplanes, ships, drones, etc., and
(d) flexible solar cell.
Various growth technique has been used to grow Bi2Se3 thin films,
such as magnetron sputtering, thermal evaporation, molecular beam
epitaxy (MBE), pulse laser deposition (PLD), etc. For instance, Park et al.
adopted the MBE technique to grow Bi2Se3 thin film, which showed
high crystallinity, epitaxially oriented Bi2Se3 film on an h-BN substrate
with atomically sharp interfaces [90]. Tabor et al. used this technique
and obtained good crystallinity and optimal stoichiometric Bi2Se3 thin
film on Al2O3 (110) substrate with epitaxial layer-by-layer growth in the
c-axis oriented [91]. Among these techniques, the magnetron sputtering
deposition technique offers large-area deposition thin films, low cost,
good film-forming uniformity, and a relatively simple process. Using
magnetron sputtering techniques, large-area Bi2Se3 thin films can be
achieved on various substrates with good crystalline quality [45, 92–96].
Wei et al. group adopted the magnetron sputtering technique to grow
Bi2Se3 thin films on Si (111) and found the highly c-axis-oriented Bi2Se3
thin films after post-annealing under a Se-rich environment [97]. Tang
et al. adopted this technique to produce stoichiometric Sb2Se3 thin films
with desired crystallinity and orientation for solar cell application [98].
In another report, Chen et al. showed that r.f. Magnetron sputtered
Sb2Se3 thin film has a highly crystalline order with large crystal grains
after in situ heat treatment [99]. These works of literature disclose
the importance of magnetron sputtering systems for depositing high-
quality and large-area Bi2Se3 thin films for various applications. We
have adopted this technique to grow Bi2Se3 thin film on Ti foil. The
post-selenization process in the Se-rich environment promotes obtaining
a suitable stoichiometry Bi2Se3 thin film. The structural and crystalline
properties and PEC performances of prepared Bi2Se3 thin film on metal
foil-based substrates were investigated.
4.2 ExpErImEntal SEctIon
The Bi2Se3 thin film was deposited on Ti foil using r.f. Magnetron
sputtering technique. A magnetron sputtering system is ornamented
with a high-temperature sustainable substrate heater (<1,000°C), high
vacuum (base pressure < 2 × 10 –7 Torr), confocal sputtering targets, and
argon (Ar) gas mass flow controller. It consists of two isolated chambers
connected by a manual gate valve; the main deposition chamber (growth
chamber) is equipped with a turbo molecular pump to provide a high
vacuum and a load lock chamber connected with a rotary pump. The
load lock chamber has a transfer arm for transferring the samples
inside the main chamber. First, Ti foil was cleaned with acetone and
IPA and dried with nitrogen gas. A commercially high pure Bi 2Se3
(purity 99.99%; ACI Alloys) material target was used as the sputtering
source. The growth process was carried out in a pure Ar (99.9999%)

environment with an Ar gas flow rate of 20 sccm and working pressure of
~2.3 × 10 –3 torr. The radio frequency (rf) of 10 W and substrate temperature
of 400°C were kept for the growth of the thin film. Moreover, the
post-selenization process was carried out in Se-rich environment to
obtain suitable stoichiometry Bi2Se3 thin film, as there is Se-deficiency
in sputtered Bi2Se3 thin film due to the momentum mismatch between
Bi and Se atoms.
Raman spectroscopy was used to disclose the formation of thin
films and structural quality by using an excitation laser source with
a wavelength of 514 nm in the backscattering mode. To study the
crystalline structure of prepared thin films, an X-ray diffraction (XRD)
pattern was performed with CuKa1 x-ray source having a wavelength of
0.15406 nm. The surface morphology of Bi2Se3 thin film was tested by
field emission scanning electron microscopy (FESEM) at an operating
voltage of 15 kV. X-ray photoelectron spectroscopy (XPS) technique with
an AlKa x-ray source having an energy of 1,486.7 eV has confirmed the
chemical composition of Bi2Se3 compounds.
To investigate the photocatalyst performance of prepared samples,
PEC measurement was carried out in sodium sulfate Na2SO4 (0.5 M)
aqueous electrolyte solution under 100 mW/cm2 (1.5AM) light intensity.
A PEC system equipped with three standard electrodes, where Bi2Se3/
Ti (working electrode) is used as a photoanode to absorb light, where
oxygen evolution takes place, second a platinum (Pt) sheet as a cathode
(counter electrode) where hydrogen evolution takes place and Ag/AgCl
as a reference electrode for potential measurement. The samples were
excited with the light source AM 1.5 G simulated sunlight using a
300 W Xenon lamp. Transient photocurrent (I-t) curves were measured
at intervals of 30 s according to the light ON–OFF cycling process
at a given bias of 0.3 and 0.6 V vs Ag/AgCl. EIS measurements were
performed in the frequency range of 0.01 Hz to 1 M Hz at an applied
ac voltage of 5 mV for the sample. All the measurements were recorded
on the CHI6054E electrochemical workstation.
4.3 rESult and dIScuSSIon
Figure 4.8(a) depicts the schematic diagram for the relative motion
of Bi and Se atoms for three A11g, A21g, and E2g observed phonon
vibrational mode. The A11g and A21g vibrational modes correspond to
out-of-the-plane, and E2g corresponds to the in-plane vibrational motion
of Bi and Se atoms. Another low-frequency vibrational mode E1g at low
wavenumber has not been observed generally due to high Rayleigh
scattering. Figure 4.8(b) shows the Raman spectrum for Bi2Se3 thin
film on polished Ti metal foil. It has shown three fingerprint Raman
peaks of Bi2Se3 thin film, which were located at 71.3, 131.4, and 173.7
cm–1 peak positions corresponding to A11g (out-of-plane), E2g (In-plane),
A21g (Out-of-plane) mode, respectively, which confirmed the formation
of crystalline Bi2Se3 thin film on Ti foil [100–102]. Figure 4.9 shows
the XRD pattern in which all signature peaks of Bi2Se3 correspond
to rhombohedral crystal structure. Diffraction XRD peaks of Bi 2Se3
thin film are found at 9.1°, 18.5°, 29.4°, 35.05°, 38.2°, 43.6°, 47.7°, 57.5°,
60.8°, and 68.8° peak positions which are indexed to (0003), (0006),
(015), (018), (00012), (0111), (00015), (00018), and (00021) lattice planes,
respectively [95, 103, 104]. The observed peaks were found oriented in
various planes due to the polycrystalline nature of Ti metal foil. The
remaining XRD peaks at position 40.3°, 53.2°, 63.1°, 70.7°, and 76.1° can
be indexed respectively to (101), (102), (110), (103), and (112) lattice planes
of Ti foil [95, 105, 106]. The surface morphology of the deposited thin
film has been characterized by FESEM characterization, as shown in
Figure 4.10. The FESEM image of pure Ti foil is presented in Figure
4.10(a), which clearly shows the large grains having cracks. Figure 10(b)
revealed the layered hexagonal nanoflakes morphology of Bi2Se3 with
the calculated grain size of 250–300 nm. It also has been seen that grains
were found randomly oriented in different directions, which was also
confirmed by oriented XRD peaks in Figure 4.9.
Figure 4.8 (a) Schematic diagram of vibration Raman mode of Bi2Se3 and
(b) lorentzian fitted Raman spectrum of Bi2Se3 thin film on flexible Ti foil.
Figure 4.9 XRD pattern of Bi2Se3 thin films on Ti metal foil.
Figure 4.10 FESEM images of (a) bare Ti foil (b) sputtered Bi2Se3 thin film on
Ti foil.
The XPS technique was executed to reveal the chemical and

electronic composition of the Bi2Se3 thin film on Ti foil. The core level
scan for Bi 4f spin states in the binding energy range of 153–168 eV is
shown in Figure 4.11(a). It shows the two dominant peaks for Bi 4f states,
located at binding energy 163.2 and 158.0 eV, corresponding to 4f5/2 and
4f7/2 valence states, respectively. These two spin-orbit coupled peaks are
found to shift slightly to a high-energy region compared to elemental
bulk Bi 4f5/2 and 4f7/2 peaks at 161.9 and 156.6 eV, respectively. The other
four peaks correspond to the Bi-Se oxidation state situated at 165.1, and
159.3 eV peaks position correspond to Bi+5 states as well as 157.2 and
162.4 eV peaks related to Bi+3 oxidation states, which may occur due to
air exposure after the deposition of the film. Figure 4.11(b) shows the
deconvoluted Se 3d spectra in the binding energy range of 57–51 eV. The
two highly core level peaks at binding energies 54.5 and 53.2 eV were
assigned to Se 3d3/2 and Se 3d5/2 peak positions attributed to Se –2 states
in Bi2Se3. One single peak at peak position 55.3 eV is attributed to the
Se-Se bond. Bi and Se’s binding energies shifted opposite, which infers
the formation of the Bi2Se3 thin film [93, 107].
Figure 4.11 XPS core level spectra for Bi2Se3 thin film on Ti foil: (a) Bi 4f and
(b) Se 3d core level.
After the successfully deposited of Bi2Se3 thin film on Ti foil, we

investigated the performance of Bi2Se3/Ti foil-based photocatalyst in
0.5 M Na2SO4 electrolyte solution under 100 mW/cm2 light intensity
toward PEC water splitting. Figure 4.12 shows the I-t curve of
Bi2Se3/Ti foil for each 30-second ON and OFF operation for Bi2Se3/Ti foil
under 0.3 and 0.6 V vs Ag/AgCl bias voltages. The I-t curve shows the
excellent stability of photoanode in an electrolyte solution and resultant
photocurrent density of 2.2 and 4.3 µA/cm2 for Bi2Se3/Ti foil at 0.3 and
0.6 V vs Ag/AgCl, respectively.
Figure 4.12 The I-t curve for Bi2Se3/Ti photoanode at: (a) 0.3 and (b) 0.6 V vs.
Ag/AgCl bias v.
The EIS measurement was performed on Bi2Se3/Ti photoanode in

a frequency range from 0.01 Hz to 1 M Hz at an applied ac voltage of
5 mV to understand the charge transport kinetics of photoanodes. EIS is
the method of investigating the properties of an electrochemical system
through the lens of impedance. In Figure 4.13, the resistance between the
surface of the semiconducting photoelectrode and the electrolyte interface
is followed by the semicircle (radius of arc), as shown in EIS spectra.
Generally, the radius arc corresponds to the charge carrier’s origin and
transportation [108–110]. The EIS data of Bi2Se3/Ti foil is fitted with an
equivalent circuit shown as the inset of Figure 4.13. The constant phase
elements (CPE) describe the frequency-dependent impedance caused by
surface roughness or nonuniformly properties of the electrode surface.
Rs is the electrolyte resistance, and Rct corresponds to charge transfer
interfacial resistance across the semiconductor electrolyte interface [111,
112]. The EIS spectrum shows charge transfer resistance (Rct) ~238 kΩ
for Bi2Se3/Ti foil. It indicates that Bi2Se3/Ti photoanode is facing high

interfacial resistance. In Figure 4.14, the Bode plot reveals the electron
lifetime for Bi2Se3/Ti foil. The lifetime (t) value can be evaluated using
the equation t = 1/(2pfmax), where fmax is the maximum frequency [113].
The t values calculated for Bi2Se3/Ti foil is ~2.5 µs.
Figure 4.13 EIS Nyquist plot of Bi2Se3/Ti and photoanodes under applied ac
voltage of 5 mV.
Figuer 4.14 The Bode plot of Bi2Se3/Ti photoanode.
Our study revealed that metal foil-based photoelectrode might

benefit futuristic, flexible energy device applications, even though
the obtained photocurrent density was not enough. To enhance the
performance of Bi2Se3 photoelectrode, it needs to couple with various

kinds of photocatalysts, which can enhance the photocurrent density by
enabling the absorption of more sunlight and suppressing the charge
carriers’ recombination.
4.4 concluSIon and FuturE challEngES
We have discussed the importance of binary metal dichalcogenides

semiconducting materials for PEC water-splitting applications. The
advantage of thin metal foils for the flexible energy device application
has been discussed thoroughly. We deposited Bi 2Se3 thin film on
functionalized Ti foil using a magnetron sputtering technique. The
PEC measurements show that the obtained photocurrent density
of Bi2Se3/Ti foil was 4.2 µA/cm 2. Future work will be dedicated to
increasing the efficiency of Bi 2Se3 material by coupling it with
various photocatalysts, such as TiO2, Fe2O3, etc. In various literature,
Bi2Se3 functionalized TiO2/Ti foil shows excellent solar-to-hydrogen
conversion efficiency; for instance, [31] fabricated Bi 2 Se 3/TiO 2
heterostructure and obtained 1.7 mA/cm 2 photocurrent density. In
another report, [62] designed Bi2Se3/TiO2 quantum dot and obtained
0.8 mA/cm2 photocurrent density. These reports infer us the conductivity
of Bi2Se3/Ti photoanode can be increased by coupling it with TiO2. We
will fabricate heterostructure with Bi2Se3 such as MoS2/Bi2Se3, MoSe2/
Bi2Se3, WSe2/Bi2Se3, etc. to enhance the conductivity of photoelectrodes
by absorbing the maximum portion of the solar spectrum, as TMDC
materials are suitable photocatalyst for hydrogen evolution process. Our
focus will also be on earth-abundant and nontoxic materials, such as
Sb2Se3, Fe2O3, etc., that can be beneficial for large-scale implementation.
Photobiological-inspired methods are a new tool for designing novel
photoelectrochemically active materials, focusing on PEC technology’s
advantages. Solar water splitting using earth-abundant materials is one
of the most lucrative approaches to finding an alternative to fossil fuels.
Lastly, the search for cost-effective and high-performance materials is
the primary driving parameter for the commercialization of PEC energy
technology.
acknowlEdgmEntS
The authors would like to thank the Director of CSIR-NPL for

providing constant encouragement and support. They would also like to
acknowledge J.S. Tawale, S. Sharma of CSIR-NPL, Prof. Somnath C. Roy
from IIT Madras, and Dr. R. Ganesan from BITS Pilani Hyderabad for
their help with different sample characterization. This work is supported

under the early career research award scheme (ECR/2017/001852) from
the Science and Engineering Research Board (SERB-DST), Government
of India.
RefeRences
[1] Bak, T., J. Nowotny, M. Rekas and C.C. Sorrell. 2022 . Photoelectrochemical
hydrogen generation from water using solar energy: materials-related
aspects. Int. J. Hydrogen Energy 27: 991–1022.
[2] Bandar, Y.A., H. Ullah, S. Alfaifi, A.A. Tahir and T.K. Mallick. 2018.
Photoelectrochemical solar water splitting: from basic principles to
advanced devices. Veruscript Functional Nanomaterials 2(12):BDJOC3.
[3] Gratzelm, M. 2001. Photoelectrochemical cells. Nature 414: 338.
[4] Jacobsson, T.J. 2018. Photoelectrochemical water splitting: an idea
heading towards obsolescence. Energy and Env. Sci. 11: 1977.
[5] Li X., Li. Zhao, J. Yu, X. Liu, X. Zhang, H. Liu, et al. 2020. Water splitting:
from electrode to green energy system. Nano-Micro Lett. 12: 131.
[6] Vanka, S., G. Zeng, T.G. Deutsch, F.M. Toma and Z. Mi. 2022. Long-Term
stability metrics of photoelectrochemical water splitting. Front. Energy
Res. 10: 840140.
[7] Maeda, K. and K. Domen. 2010. Photocatalytic water splitting: recent
progress and future challenges. J. Phys. Chem. Lett. 1: 2655–2661.
[8] Landman, A., H. Dotan, G.E. Shter, M. Wullenkord, A. Houaijia,
A. Maljusch, et al. 2017. Photoelectrochemical water splitting in separate
oxygen and hydrogen. Nat. Mater. 16: 646–651.
[9] Walter, M.G., E.L. Warren, J.R. McKone, S.W. Boettcher, Q. Mi,
E.A. Santori, et al. 2010. . Solar water splitting cells. Chemi. Rev. 110:
6446–6473.
[10] Brown, L.F. 2001. A comparative study of fuels for on-board hydrogen
production for fuel-cell-powered automobiles. Int. J. Hydrogen Energy
26: 381–397.
[11] Wu, H., H.L. Tan, C.Y. Toe, J. Scott, L.Z. Wang, R. Amal, et al. 2019.
Photocatalytic and photoelectrochemical systems: similarities and
differences. Adv. Mat. 32: 1904717.
[12] Akkerman, I., M. Janssen, J. Rocha and R.H. Wijffels. 2002. Photobiological
hydrogen production: photochemical efficiency and bioreactor design.
Int. J. Hydrogen Energy 27: 1195–208.
[13] Lede, J., F. Lapicque and J. Villermaux. 1983. Production of hydrogen
by direct thermal decomposition of water. Int. J. Hydrogen Energy
8: 675–679.
[14] Cecal, A., A. Paraschivescu, K. Popa, D. Colisnic, G. Timco and
L. Singerean. 2003. Radiolytic splitting of water molecules in the presence
of some supramolecular compounds. J. Serbian Chemi. Soc. 68: 593–598.
[15] Yu, J.M., J. Lee, Y.S. Kim, J. Song, J. Oh, S.M. Lee, et al. 2020. High-
performance and stable photoelectrochemical water splitting cell with
organic-photoactive-layer-based photoanode. Nat. Comm. 11: 5509.
[16] Fujishima, A. and K. Honda. 1972. Electrochemical photolysis of water at
a semiconductor electrode. Nature 238: 37–38.
[17] Marwat, M.A., M. Humayun, M.W. Afridi, H. Zhang, M.R.A. Karim,
M. Ashtar, et al. 2021. Advanced catalysts for photoelectrochemical water
splitting. ACS Appl. Energy Mat. 4: 12007–12031.
[18] Gai, Y., J. Li, S.S. Li, J.B. Xia and S.H. Wei. 2009. Design of
narrow-gap tio2: a passivated codoping approach for enhanced
photoelectrochemical activity. Phys. Rev. Lett. 102: 036402.
[19] Shi, X., K. Zhang, K. Shina, M. Maa, J. Kwona, I.T. Choib, et al. 2015.
Unassisted photoelectrochemical water splitting beyond 5.7% solar-to-
hydrogen conversion efficiency by a wireless monolithic photoanode/
dye-sensitized solar cell tandem device. Nano Energy 13: 182–191.
[20] Li, X., Z. Wang and L. Wang. 2021. Metal-organic framework-based
materials for solar water splitting. Small Sci. 1:2000074.
[21] Pengtao, X., N.S. McCool and T.E. Mallouk. 2017. Water-splitting dye-
sensitized solar cells. Nano Today 14: 1–17.
[22] Parihar, N.S. and S.K. Saraswat. 2020. Photoelectrochemical water
splitting: an ideal technique for pure hydrogen production. J. Indian
Chemi. Soc. 97: 1099–1103.
[23] Chen, S., D. Huang, P. Xu, W. Xue, L. Lei, M. Cheng, et al. 2020.
Semiconductor-based photocatalysts for photocatalytic and photo-
electrochemical water splitting: will we stop with photo corrosion?
J. Mat. Chem. A 8: 2286–2322.
[24] Dotan, H., N. Mathews, T. Hisatomi, M. Gratzel and A. Rothschild. 2014.
On the solar to hydrogen conversion efficiency of photoelectrodes for
water splitting. J. Phys. Chem. Lett. 5: 3330−3334.
[25] Joy, J., J. Mathew and S.C. George. 2018. Nanomaterials for PEC water
splitting: a review. Int. J. Hydrogen Energy 43: 4804–48017.
[26] Wilke, T., D. Schricker, J. Rolf and K. Kleinermanns. 2012. Solar water
splitting by semiconductor nanocomposites and hydrogen storage with
quinoid systems. Open J. Phys. Chem. 2:195–203.
[27] Tamirat, A.G., J. Rick, A.A. Dubale, W.N. Sub and B.J. Hwang. 2016.
Using hematite for photoelectrochemical water splitting: a review of
current progress and challenges. Nanoscale Horiz. 1: 243–267.
[28] Lu, Q., Y. Yu, Q. Ma, B. Chen and H. Zhang. 2015. 2D Transition-metal-
dichalcogenide-nanosheet-based composites for photocatalytic and
electrocatalytic hydrogen evolution reactions. Adv. Mat. 28: 1917–1933.
[29] Li, Y., J. Li, W. Yang and X. Wang. 2020. Implementation of ferroelectric
materials in photocatalytic and photoelectrochemical water splitting.
Nanoscale Horiz. 5: 1174.
[30] Martín, S.S., M.J. Rivero and I. Ortiz. 2020. Unravelling the mechanisms
that drive the performance of photocatalytic hydrogen production.
Catalysts 10: 901.
[31] Subramanyam, P., B. Meena, D. Suryakala and C. Subrahmanyam. 2021.

TiO2 photoanodes sensitized with Bi2Se3 nanoflowers for visible near-
Infrared photoelectrochemical water splitting. ACS Appl. Nano Mat.
4: 739−745.
[32] Subramaniam, M.N., P.S. Goh, D. Kanakaraju, J.W. Lim, W.J. Lau and
A.F. Ismail. 2022. Photocatalytic membranes: a new perspective for
persistent organic pollutants removal. Env. Sci. and Poll. Res. 29: 12506–
12530.
[33] Wang, J., T.V Ree., Y. Wu, P. Zhang and L. Gao. 2018. Metal oxide
semiconductors for solar water splitting. Metal Oxides in Energy Techn.
205–249.
[34] Cho, S., J.W. Jang, K.H. Lee and J.S. Lee. 2014. Research update: strategies
for efficient photoelectrochemical water splitting using metal oxide
photoanodes. APL Materials 2: 010703.
[35] Szymanski, P. and M.A. El-Sayed. 2012. Some recent developments in
photoelectrochemical water splitting using nanostructured TiO2: a short
review. Theor. Chem. Acc. 131: 1202.
[36] Concina, I. and Z.H. Ibupoto, A. Vomiero. 2017. Semiconducting metal
oxide nanostructures for water splitting and photovoltaics. Adv. Energy
Mat. 7: 1700706.
[37] Xia, X., L. Wang, N. Sui, V.L. Colvin and W.W. Yu. 2020. Recent progress
in transition metal selenide electrocatalysts for water splitting. Nanoscale
12: 12249–12262.
[38] Hou, X., Y. Liu, H. Ju, B. Pan, J. Zhu, T. Ding, et al. 2016. Design and
epitaxial growth of MoSe2-NiSe vertical hetero nanostructures with
electronic modulation for enhanced hydrogen evolution reaction. Chem.
Mat. 28: 1838–1846.
[39] Haque, F., T. Daeneke, K.K. Zadeh and J.Z. Ou. 2018. Two-dimensional
transition metal oxide and chalcogenide-based photocatalysts. Nano-
Micro Lett. 10: 4–27.
[40] Wu, L., S. Shi, Q. Li, X. Zhang and X. Cui. 2019. TiO2 nanoparticles
modified with 2D MoSe2 for enhanced photocatalytic activity on
hydrogen evolution. Int. J. Hydrogen Energy 44: 720–728.
[41] Zhang, J.Z. 2011. Metal oxide nanomaterials for solar hydrogen generation
from photoelectrochemical water splitting. MRS Bulletin 36: 48–55.
[42] Rajamathi, C.R., U. Gupta, K. Pal, N. Kumar, H. Yang, Y. Sun, et al.
2017. Photochemical water splitting by bismuth chalcogenide topological
insulators. Chem. Phys. Chem. 18: 2322–2327.
[43] Jayachitra, S., P. Ravi, P. Murugan and M. Sathish. 2022. Super critically
exfoliated Bi2Se3 nanosheets for enhanced photocatalytic hydrogen
production by topological surface states over TiO2. J. Colloid and Inter.
Sci. 605: 871–880.
[44] Ranjbar, A., H. Mirhosseini and T.D. Kühne. 2021. On topological
materials as photocatalysts for water splitting by visible light. J. Phys.
Mater. 5: 1–12.
[45] Gautam, S., A.K. Verma, A. Balapure, B. Singh, R. Ganesan, M.S. Kumar.
2022. Structural, electronic, and thermoelectric properties of Bi2Se3 thin
films deposited by RF magnetron sputtering. J. Elect. Mat. 51: 2500–2509.
[46] Yang, X.X., Wang and Z. Zhang. 2005. Synthesis and optical properties
of single crystalline bismuth selenide nanorods via a convenient route.
J. Crys. Growth. 276: 566–570.
[47] Yang, S.D., L. Yang, Y.X. Zheng, W.J. Zhou, M.Y. Gao, S.Y. Wang, et
al. 2017. Structure-dependent optical properties of self-organized Bi2Se3
Nanostructures: From Nanocrystals to Nanoflakes, ACS Appl. Mat. Inter.
9: 29295–29301.
[48] Fu, L., C.L. Kane and E.J. Mele. 2007. Topological insulators in three
dimensions. Phys. Rev. Lett. 98: 106803.
[49] Zhang, H., C.X. Liu, X.L. Qi, X. Dai, Z. Fang and S.C. Zhang. 2009.
Topological insulators in Bi2Se3, Bi2Te3, and Sb2Te3 with a single dirac
cone on the surface. Nat. Phy. 5: 438–442.
[50] Kane, C.L. and E.J. Mele. 2005. Z2 topological order and the quantum
spin hall effect. Phys. Rev. Lett. 95: 146802.
[51] Tokura, Y., K. Yasuda and A. Tsukazaki. 2019. Magnetic topological
insulators. Nat. Rev. Phy. 1: 126–143.
[52] Chege, S., P. Ning, J. Sifuna and G.O. Amolo. 2020. Origin of band
inversion in topological insulators. AIP Adv. 10: 095018.
[53] Park, K.H., M. Mohammad, Z. Aksamija and U. Ravaioli. 2008. Phonon
scattering due to van der Waals forces in the lattice thermal conductivity
of Bi2Te3 thin films. J. Appl. Phy. 103: 024314.
[54] Singh, M.P., M. Mandal, K. Sethupathi, M.S. Ramachandra Rao and P.K.
Nayak. 2021. Study of thermometry in two-dimensional Sb2Te3 from
temperature-dependent Raman Spectroscopy. Nanoscale Res. Lett. 16: 22.
[55] Ko, T.Y., M. Shellaiah and K.W. Sun. 2016. Thermal and thermoelectric
transport in highly resistive single Sb2Se3 nanowires and nanowire
bundles. Sci. Rep. 6: 35086.
[56] Garcia, V.M., M.T.S. Nair, P.K. Nair and R.A. Zingaro. 1997. Chemical
deposition of bismuth selenide thin films using N,N -dimethylselenourea.
Semi. Sci. Tech. 12: 645–653.
[57] Adam, A.M., E. Lilov, E.M.M. Ibrahim, P. Petkov, L.V. Panina and M.A.
Darwish. 2019. Correlation of structural and optical properties in as-
prepared and annealed Bi2Se3 thin films. J. Mat. Proc. Tech. 264: 76–83.
[58] Wang, F.K., S.J. Yang and T.Y. Zhai. 2021. 2D Bi2Se3 materials for
optoelectronics. iScience. 24: 103291.
[59] Lai, H.D., S.R. Jian, L.T.C. Tuyen, P.H. Le, C.W. Luo and J.Y. Juang. 2018.
Nanoindentation of Bi2Se3 thin films. Micromachines 9: 518.
[60] Chen, X., H.D. Zhou, A. Kiswandhi, I. Miotkowski, Y.P. Chen,
P.A. Sharma, et al. 2011. Thermal expansion coefficients of Bi2Se3 and
Sb2Te3 crystals from 10 K to 270 K. Appl. Phy. Lett. 99: 261912.
[61] Q. Chen, J. Chen, X. Xu, Z. Wang, Y. Ding, L. Xiong, et al. 2021.
Morphology optimization of Bi2Se3 thin films for enhanced thermoelectric
performance. Cryst. Growth Des. 21: 6737–6743.
[62] G. Zhou, T. Zhao, O. Wang, X. Xia and J.H. Pan. 2021. Bi2Se3, Bi2Te3
quantum dots-sensitized rutile TiO2 nanorod arrays for enhanced solar
photo electrocatalysis in azo dye degradation. J. Phy. Energy 3: 014003.
[63] Guo, C., R. Hu, H. Qiao, C. Dua and X. Qi. 2021. TiO2 nanoparticles
anchoring on two-dimensional Bi2Se3 nanosheet as an enhanced visible
light catalyst. J. Mat. Sci. Mat. Elect. 32: 19424–19433.
[64] Yang, J., C. Wang, H. Ju, Y. Sun, S. Xing, J. Zhu, et al. 2017. Integrated
quasi plane hetero nanostructures of MoSe2/ Bi2Se3 hexagonal nanosheets:
synergetic electrocatalytic water splitting and enhanced supercapacitor
performance. Adv. Func. Mat. 1703864: 1–10.
[65] Razzaque, Sh., M.D. Khan, M. Aamir, M. Sohail, S. Bhoyate, R.K. Gupta,
et al. 2021. Selective synthesis of bismuth or bismuth selenide nanosheets
from a metal-organic precursor: investigation of their catalytic
performance for water splitting. Inorganic Chem. 60:1449−1461.
[66] Liu, X., J. Chen, M. Luo, M. Leng, Z. Xia, Y. Zhou, et al. 2014. Thermal
evaporation and characterization of Sb2Se3 thin film for substrate Sb2Se3/
CdS solar cells. ACS Appl. Mat. Inter. 6: 10687−10695.
[67] Yang, W., J. Ahn, Y. Oh, J. Tan, H. Lee, J. Park, et al. 2018. Adjusting
the Anisotropy of 1D Sb2Se3 Nanostructures for Highly Efficient
Photoelectrochemical Water Splitting. Adv. Energy Mat. 8:1702888.
[68] Chen, S., T. Liu, Z. Zheng, M. Ishaq, G. Liang, P. Fan, et al. 2021. Recent
progress and perspectives on Sb2Se3 -based photocathodes for solar
hydrogen production via photoelectrochemical water splitting. J. Energy
Chem. pp. 1–49.
[69] Yang, W., J.H. Kim, O.S. Hutter, L.J. Phillips, J. Tan, J. Park, et al. 2020.
Benchmark performance of low-cost Sb2Se3 photocathodes for unassisted
solar overall water splitting. Nat. Comm. 11: 861.
[70] Zhang, L., Y. Li, C. Li, Q. Chen, Z. Zhen, X. Jiang, et al. 2017. Scalable
low-band-gap Sb2Se3 thin-film photocathodes for efficient visible−near
infrared solar hydrogen evolution. ACS Nano 11: 12753–12763.
[71] Ran, F., P. Li, X. Yuan, J. Zhang, D. Zhang and S. Chen. 2022. Fabrication
of a Sb2Se3/CuSbS2 heterojunction photocathode for photoelectrochemical
water splitting. J Phy. Chem. C. 126: 8581−8587.
[72] Prabhakar, R.R., W. Septina, S. Siol, T. Moehl, R. Wick-Joliat and
S.D. Tilley. 2017. Photocorrosion-resistant Sb2Se3 photocathodes with
earth-abundant MoSx hydrogen evolution catalyst. J. Mat. Chem.
5: 23139–23145.
[73] Guo, L., P.S. Shinde, Y. Ma, L. Li, S. Pan and F. Yan. 2019. Scalable core-
shell MoS2/Sb2Se3 nanorod array photocathode for enhanced PEC water
splitting. Solar RRL. 4: 1900442.
[74] Jain, A.K., C. Gopalakrishnan and P. Malar. 2022. Study of pulsed laser
deposited antimony selenide thin films. J. Mat. Sci.: Mat. Elect. 33: 10430–
10438.
[75] Liu, C., Y. Yuan, L. Cheng, J. Sua, X. Zhang, X. Lia, et al. 2019. A study
on optical properties of Sb2Se3 thin films and resistive switching behavior
in Ag/Sb2Se3/W heterojunctions. Results in Phy. 13: 102228.
[76] Hamrouni, R., N.E.H. Segmane, D. Abdelkader, A. Amara, A. Drici,

M. Bououdina, et al. 2018. Linear and nonlinear optical properties
of Sb2Se3 thin films elaborated from nano-crystalline mechanically
alloyed powder. App. Phy. A 124: 861.
[77] Singh, Y., M. Kumar, R. Yadav, A. Kumar, S. Rani, Shashi, et al. 2022.
Enhanced photoconductivity performance of microrod-based Sb2Se3
device. Solar Energy Mat. Solar Cells 243: 111765.
[78] Yin, K., Z.D. Cui, X.R. Zheng, X.J. Yang, S.L. Zhu, Z.Y. Li, et al. 2015.
Bi2Te3@CoNiMo composite as a high-performance bifunctional catalyst
for the hydrogen and oxygen evolution reactions. J. Mat. Chem. A
3: 22770–22780.
[79] Li, G., J. Huang, Q. Yang, L. Zhang, Q. Mu, Y. Sun, et al. 2021. MoS2 on
topological insulator Bi2Te3 thin films: activation of the basal plane for
hydrogen reduction. J. Energy Chem. 62: 516–522.
[80] Bahk, J.H., H. Fang, K. Yazawa and A. Shakouri. 2015. Flexible
thermoelectric materials and device optimization for wearable energy
harvesting. J. Mat. Chem. C 3: 10362.
[81] Corzo, D., G.T. Blázquez and D. Baran. 2020. Flexible electronics
challenges and opportunities. Front. Elec. 1: 594003.
[82] Sircar, A. and H. Kumar. 2022. An introduction to flexible electronics:
manufacturing techniques, types, and future. J. Phys. Conf. Ser. 1913:
012047.
[83] Lee, Y., B. Gupta, H.H. Tan, C. Jagadish, J. Oh and S. Karuturi. 2021. Thin
silicon via crack-assisted layer exfoliation for photoelectrochemical water
splitting. iScience 24: 102921.
[84] Cordill, M.J., P. Kreiml and C. Mitterer. 2022. Materials engineering for
flexible metallic thin film applications. Materials 15: 926.
[85] Hassan, M., G. Abbas, N. Li, A. Afzal, Z. Haider, and . Ahmed, et al.
2021. Significance of flexible substrates for wearable and implantable
devices: recent advances and perspectives. Adv. Mat. Tech. 7: 2100773.
[86] Khalil, M.I., R. Bernasconi, A. Lucotti, A.L. Donne, R.A. Mereu, S. Binetti,
et al. 2021. CZTS thin-film solar cells on flexible molybdenum foil by the
electrodeposition-annealing route. J. Appl. Electrochem. 51: 209–218.
[87] Shiyani, T., S.K. Mahapatra and A.K. Ray. 2021. Flexible zinc oxide
photoelectrode for photoelectrochemical energy conversion. Journal of
Materials Science: Mater. Electron. 32: 15386–15392.
[88] Quynh, L.T., C.N. Van, W.Y. Tzeng, C.W. Huang, Y.H. Lai and J.W. Chen.
2018. Flexible heteroepitaxy photoelectrode for photoelectrochemical
water splitting. ACS App. Energy Mat. 1: 3900–3907.
[89] Singh, S. and N. Khare. 2017. Flexible PVDF/Cu/PVDF-NaNbO3
Photoanode with ferroelectric properties: an efficient tuning of
photoelectrochemical water splitting with electric field polarization and
piezophototronic effect. Nano Energy 42: 173–180.
[90] Park, J.Y., G.H. Lee, J. Jo, A.K Cheng, H. Yoon, K. Watanabe, et al. 2016.
Molecular beam epitaxial growth and electronic transport properties
of the high-quality topological insulator Bi2Se3 thin films on hexagonal

boron nitride. 2d Mater. 3: 035029.
[91] Tabor, Ph., C. Keenan, S. Urazhdin and D. Lederman. 2011. Molecular
beam epitaxy and characterization of thin Bi2Se3 films on Al2O3 (110).
App. Phy. Lett. 99: 013111.
[92] Gautam, S., B. Sin,gh, V. Aggarwal, M. Senthil Kumar, V.N. Singh
and S.P. Singh, et al. 2022. Thickness-dependent optical properties of
sputtered Bi2Se3 films on mica. Mater. Today 64: 1725–1731.
[93] Gautam, S., V. Aggarwal, B. Singh, V.P.S. Awana, R. Ganesan and
S.S. Kushvaha. 2022. Signature of weakantilocalization in sputtered
topological insulator Bi2Se3 thin films with varying thickness. Sci. Rep.
12: 9770.
[94] Gautam, S.V. Aggarwal, B. Singh, R. Kumar, J.S. Tawale, B.S. Yadav,
et al. 2022. Structural and optical properties of sputtered Bi2Se3 thin films
on sapphire (0001), quartz, and GaN/sapphire (0001). J. Mat. Res. 1–12.
[95] Singh, B., S. Gautam, V. Aggarwal, J.S. Tawale and S.S. Kushvaha. 2022.
Growth and characterization of crystalline Bi2Se3 thin films on flexible
metal foils by magnetron sputtering system. Mater. Today 64: 1701–1706.
[96] Gautam, V., S. Gautam, G.K. Maurya, K. Kandpal, B. Singh, R. Ganesan,
et al. 2022. Investigation of RF sputtered, n-Bi2Se3 heterojunction on p-Si
for enhanced NIR optoelectronic applications. Solar Energy Mat. and
Solar Cells. 248: 112028.
[97] Wei, Z.T., M. Zhang, Y. Yan, X. Kan, Z. Yu, Y.L. Chen, et al. 2015.
Transport properties of Bi2Se3 thin films grown by magnetron sputtering.
Funct. Mate. Lett. 8: 1550020.
[98] Tang, R., X. Chen, Y. Luo, Z. Chen, Y. Liu, Y. Li, et al. 2020. Controlled
sputtering pressure on high-quality Sb2Se3 thin film for substrate
configurated solar cells. Nanomater. 10: 574.
[99] Chen, Sh., Z. Zheng, M. Cathelinaud, H. Ma, X. Qiao, Z. Su, et al. 2019.
Magnetron sputtered Sb2Se3-based thin films towards high-performance
quasi-homojunction thin film solar cells. Sol. Energy Mater. Sol. Cells
203: 110154.
[100] Wang, S., Y. Li, A. Ng, Q. Hu, Q. Zhou, X. Li, et al. 2020. 2D Bi2Se3 van der
Waals epitaxy on mica for optoelectronics applications. J. Nano 10: 1653.
[101] Yin, C., C. Gong, S. Tian, Y. Cui, X. Wang, Y. Wang, et al. 2021. Low-
energy oxygen plasma injection of 2D Bi2Se3 realizes highly controllable
resistive random-access memory. Adv. Func. Mat. 32: 2108455.
[102] SchÖenherr, P., A.A. Baker, P. Kusch, S. Reich and T. Hesjedal. 2014.
Engineering of Bi2Se3 nanowires by laser cutting. European Phys. J. Appl.
Phy. 66: 10401.
[103] Zang, C., X. Qi, L. Ren, G. Hao, Y. Liu, J. Li, et al. 2014. Photoresponse
properties of ultrathin Bi2Se3 nanosheets synthesized by hydrothermal
intercalation and exfoliation route. Appl. Sur. Sci. 316: 341–347.
[104] Kim, S., S. Lee, J. Woo and G Lee. 2018. Growth of Bi2Se3 topological
insulator thin film on Ge (1 1 1) substrate. Appl. Sur. Sci. B 432: 152–155.
[105] Ramesh, C., P. Tyagi, G. Gupta, M.S. Kumar and S.S. Kushvaha. 2019.
Influence of surface nitridation and an AlN buffer layer on the growth
of GaN nanostructures on a flexible Ti metal foil using laser molecular
beam epitaxy. Jpn. J. Appl. Phy. 58: 079301.
[106] Ramesh, C., P. Tyagi, J. Kaswan, B.S. Yadav, A.K. Shukla, M. Senthil
Kumar, et al. 2020. Effect of surface modification and laser repetition rate
on growth, structural, electronic, and optical properties of GaN nanorods
on flexible Ti metal foil. RSC Adv. 10: 2113–2122.
[107] Nascimento, V.B., V.E. Carvalho, R. Paniago, E.A. Soares, L.O. Ladeira,
H.D. Pfannes. 1999. XPS and EELS study of the bismuth selenide.
J. Electron. Spect. Related Phen. 104: 99–107.
[108] Xue, D., J. Luo, Z. Li, Y. Yin and J. Shen. 2020. Enhanced
photoelectrochemical properties from mo‐doped TiO2 nanotube arrays
film. Coating 10: 75.
[109] Ali, A., X. Li, J. Song, S. Yang, W. Zhang, Z. Zhang, et al. 2017.
Nature‐mimic ZnO nanoflowers architecture: chalcogenide quantum
dots coupling with ZnO/ZnTiO3 nano heterostructures for efficient
photoelectrochemical water splitting. J. Phys. Chem. C 121: 21096–21104.
[110] Ali, A., F.A. Mangrio, X. Chen, Y. Dai, K. Chen, X. Xu, et al. 2019.
Ultrathin MoS2 nanosheets for high‐performance photoelectrochemical
applications via plasmonic coupling with Au nanocrystals. Nanoscale
11: 7813.
[111] Shahrezaei, M., S.M.H. Hejazi, Y. Rambabu, M. Vavrecka, A. Bakandritsos,
S. Oezkan, et al. 2020. Multi‐leg TiO2 nanotube photoelectrodes modified
by platinized cyanographene with enhanced photoelectrochemical
performance. Catalysts 10: 717.
[112] Khatun, N., S. Dey, G.C. Behera and S.C. Roy. 2022. Ti3C2Tx MXene
functionalization induced enhancement of photoelectrochemical
performance of TiO2 nanotube arrays. Mat. Chem. Phy. 278: 125651.
[113] Hu, L., Y. Li, W. Chen, X. Liu, Sh. Liang, Z. Cheng, et al. 2022. Controlled
synthesis and photoelectrochemical performance enhancement of Cu2−xSe
decorated porous Au/Bi2Se3 Z‐scheme plasmonic photoelectrocatalyst.
Catalysts 12: 359.
Chapter 5
Quantum-Cutting Phosphors
for Thermal Sensor
Applications
Abhijit Jadhav
Hyderabad Laboratories Private Limited, Hyderabad, India.
Email: ajadhav@hyderabadlaboratories.com
5.1 IntroductIon
Luminescence is a phenomenon of spontaneous light emission by a

substance that is not due to heat or cold light. It involves the promotion
of ground-state electrons to higher energy states/levels, eventually
emitting light during the relaxation process. Luminescence induced
by light energy is termed photoluminescence and is formally divided
into two categories: fluorescence and phosphorescence. Phosphorescence
has longer excited state lifetimes than fluorescence; it is usually used
to determine the temperature in a thermographic phosphor system [1].
Phosphors are inorganic oxides that are usually white and show
luminescence upon excitation. Down-conversion phosphors absorb
high-energy UV photons and convert them to lower-energy visible
photons. Upconversion phosphors absorb lower-energy IR or near IR
(NIR) photons and convert them to higher-energy visible photons. At
the same time, quantum-cutting (QC) phosphors absorb high-energy UV
photons and convert them into lower-energy visible and NIR photons.
That means that with QC phosphors, the obtained quantum efficiency (QE)
is more than unity. These phosphors find various applications in various
technologies, such as CRT tubing, plasma display, light bulbs, and X-ray
conversion screens.
Temperature is the most essential thermodynamic parameter to
describe any physical, chemical, and biological process. In various
applications, knowledge of temperature plays an essential role in
understanding exact process conditions and maintaining proper reaction
conditions. The accurate temperature measurement during combustion
helps to understand the heat transfer phenomenon. An infrared camera
is used during thermometry to identify abnormally hot or cold areas
on a component operating under normal conditions. The phosphors
are also known to be thermographic if they show emission-changing
characteristics with varying temperature conditions. The present chapter
will overview QC phosphors used for thermal sensor applications.
5.2 Quantum-cuttIng Phenomenon
Quantum cutting is a type of luminescence observed in different types

of dopant ions host materials. The phosphors with singly, doubly, and
sometimes triply doped ions show a quantum-cutting phenomenon,
which is a conversion of photons from high to low energy and vice versa
during the process of down-conversion and upconversion process. The
process has been explained through the energy level diagram shown
in Figure 5.1.
Figure 5.1 Energy level diagrams of quantum-cutting mechanisms for single

(a) and double (b) ions [2].
The phosphor doped with a single rare-earth ion is excited with

hυ energy, and the ground-state electrons get transferred to a higher
Quantum-Cutting Phosphors for Thermal Sensor Applications 125
(excited state energy level). During the relaxation process, they emit two
photons of equal energy. Thus, one high-energy photon is converted
successfully into two lower-energy photons during this process, i.e.,
2hυ′ = hυ. This process is reported with singly doped Pr3+, Eu3+, and
Tm3+ host materials [3, 4]. Yu et al. showed that the cross-relaxation
between the same ions in Gd2O2S:Tm3+ samples led to the generation of
QC emissions of the two, three, and four NIR photons, respectively [5].
Meanwhile, in the two ions’ dopant phosphor material, both ions
can be excited using the same source or light sources with different
wavelengths. Upon excitation, the ground-state electrons get promoted
to a higher energy level 3, and during the relaxation process, they arrive
at level 2 non-radiatively. From intermediate level 2, the electrons emit
a radiative transition of energy lower than the excitation energy, i.e.,
hυ′ < hυ. Successful energy transfer through dipole-dipole interaction
between donor ions in level 2 and acceptor ions in level 1. This results
in the emission of lower-energy photons and subsequently confirms the
conversion of one high-energy photon into two lower-energy photons,
i.e., hυ = 2 hυ′, making the QC efficiency 200%. This QC mechanism is
reported for Ca2Al14O33 doped with Eu3+ and Yb3+ with a QC efficiency
of 199% upon excitation at 394 nm wavelength [6].
5.2.1 Visible Quantum cutting

Quantum cutting in phosphors occurs when every absorbed photon
provides two or more emitted photons. During visible QC, the phosphors
utilise vacuum UV (VUV) light and generate visible light with conversion
efficiency higher than unity. This help to provide improved energy
efficiency in many lighting devices [7]. The energy of the absorbed VUV
photon is much higher than the emitted visible photon and thus makes
it capable of emitting two visible photons upon every absorbed photon.
Wegh et al. explained the energy level diagram for the two types of rare-
earth ions (I and II), which shows the down-conversion phenomenon [8].
The concept of obtaining more than 100% quantum efficiencies is based
on a combination of two lanthanide dopants. A high QE (close to 200%)
can be obtained through partial energy transfer between the co-dopant
ions [3].
The process of obtaining higher QE (near 200%) is also known as
upconversion [9, 10]. The quantum cutting through energy transfer, i.e.,
down-conversion, has been shown in Figure 5.2. The concept of down-
conversion has been explained with two types of ions (I and II) with
a hypothetical energy level scheme. Efficient visible QC obtained from
two-photon emission from a high-energy level for a single lanthanide
ion is possible theoretically, as shown in Figure 5.2(a) with red lines.
The resulting emissions in the IR and UV regions are shown with black
lines. This mechanism is based on one luminescent centre with three
energy levels [11].
Figure 5.2 Energy level diagram for ions (type I and II) illustrates visible quan-
tum cutting via down conversion. Type I is an ion showing emission from a high-
energy level, and Type II emission is dedicated to an activator ion to which energy
transfer takes place. (a) Quantum cutting on single ion I by the sequential emis-
sion of two visible photons. (b) Quantum cutting by two-step energy transfer. The
(c) and (d) involve only one energy transfer step from ion I to ion II. These are the
possible mechanisms of obtaining visible quantum cutting if one of two visible
photons can be emitted by ion I [8].
5.2.2 near Ir (nIr) Quantum cutting

Various mechanisms that are possible for NIR quantum cutting have
been shown in Figure 5.3. Figure 5.3(a) shows the mechanism based on
one luminescent centre with three energy levels. The emission of two
consecutive NIR photons is possible, with the optical centre showing
a transition to the highest energy level by absorbing one high-energy
UV or visible photon. The sequential emission of two NIR photons is
possible when the optical centre absorbs one UV or visible photon and
shows transition at the highest energy level. The populated intermediate
state was also observed when the optical centre returned to its ground
state. This phenomenon has been reported for single ions like Ho3+,
Tm3+, or Er3+ [12–17]. The NIR quantum-cutting mechanism through
two luminescent centres has been shown in Figure 5.3(b–e). Here we
can see a plausible mechanism via a two-step electron transfer process
through ion pairs of physically interacting lanthanide ions, such as
Pr3+–Yb3+ [18] and Er3+–Yb3+ [19], accompanied by the emission of two NIR
photons [11]. Figures 5.3(c) and (d) show emitted NIR photons, resulting
from quantum cutting due to a one-step energy transfer between two

optical centres. This phenomenon was successfully demonstrated
by the ion pairs Tm3+–Yb3+ [20] and Ho3+–Yb3+ [21]. Energy splitting
depends on the donor species’ population of an intermediate energy
level. The relaxation process is dominated by second-order cooperative
sensitisation, resulting in the simultaneous excitation of two acceptors
with the emission of two NIR photons, as shown in Figure 5.3(e).
Figure 5.3 Typical mechanisms of NIR quantum cutting for PV applications.

NIR quantum cutting is shown through simplified energy level diagrams for ions
(type I and II). (a) NIR quantum cutting on a single ion by the successive emission
of two NIR photons. (b–d) NIR quantum cutting due to resonant energy transfer
from ion I to ion II, and (e) NIR quantum cutting due to cooperative energy trans-
fer from ion I to ion II [11].
5.3 PhosPhor thermograPhy
Thermography uses an infrared camera to look for abnormally hot or

cold areas on a given component operating under normal conditions.
Phosphor thermometry is an optical method for surface temperature
measurement using the phosphor materials’ luminescence emitted.
Upon stimulation with particular light sources, phosphors emit light
of different colours and wavelengths. It emits light change with
temperature, including its brightness, colour and afterglow duration,
which is most commonly used for temperature measurement. After
illumination, the time required for the brightness to decrease to 1/e of
its original value is known as the decay time indicated by t. This decay
time is a function of temperature T. The intensity, I, of the luminescence
decay exponentially, as shown in Equation 5.1:
-t
I = I0 e t (5.1)
I0 is the initial intensity, the t is the time, and t is the temperature-

dependent parameter.
Phosphors show the thermographic property when they display
temperature-dependent emission-changing characteristics. During the
development of the fluorescent lamp, Nuebert et al. [22] discovered the
idea of using phosphors for temperature measurement after they observed
the loss of luminescence with increasing temperature. Wickersheim
and his team applied a phosphor at the tip of an optical fibre and
investigated many phosphors with various applications leading to the
commercialisation of fluorescence-based thermometry systems [23].
Cates [24, 25] and co-workers developed remote measurement systems
by adhering a phosphor layer over the interested surface instead at the
tip of a fibre. It helped the system to have greater flexibility, and remote
measurements of moving surfaces were also possible.
Figure 5.4 Luminescence transitions and mechanism for different rare-earth-

doped phosphors used for optical thermometry.
Phosphor thermometry shows a non-contact technique based

on the intensity of luminescence signals for the remote temperature
measurement. Phosphors are usually composed of inorganic oxides or
ceramic materials resistant to high temperatures and harsh chemicals.
As shown in Figure 5.4, trivalent lanthanide ions are used as a doping
material in inorganic oxides, which are now working as a phosphor
material. Trivalent lanthanide ions such as Eu3+, Tm3+, Er3+, Yb3+, Ho3+,
Nd3+, and Dy3+ are used as luminescence centres or activators for the
study of temperature change during thermometric measurements.
Different lanthanide ions [26–31] have different luminescence transitions
during the emission process, as shown in Figure 5.4. The effect of
various shapes of the host material, such as nanorods [29], spherical
and tetragonal [32], core-shell [33], hollow nanostructures [34], fibres [35],
glass ceramics with fluoride nanocrystals [36], and bulk oxides [37].
Upon excitation with a suitable light source, these phosphors show
emitted luminescence in UV, visible, infrared, and NIR regions.
Host matrix doped with rare-earth ions plays a vital role in deciding
whether the phosphor material satisfies the thermally coupled energy
level required to be used as an optical temperature sensor. Table 5.1
represents host dependent ΔEf, ΔEm, and d in Er3+ doped and Er3+–Yb3+
co-doped phosphors. The term d calculates the fluorescence intensity
ratio between ΔEf and ΔEm. From Table 5.1, we can see that the small
d value is the result of successive excited state absorption, which has
overcome the energy transfer process between thermally coupled levels
and other energy levels due to irregular ligand fields around the dopant
lanthanide ion sites inside the host crystals. The value of d varies with
different host crystals and shows a maximum value of 90.44% for Er3+,
Yb3+ co-doped b – NaLuF4, which is ascribed to the energy of the excited
state absorption due to Er3+ ion.
Table 5.1 Host dependent ΔEf, ΔEm, and d in Er3+ doped and Er3+–Yb3+
co-doped phosphors
Samples ΔEf (cm–1) ΔEm (cm–1) δ (%) References
3+
Er doped In – Zn – Sr – Ba glass 861.0 771.8 11.55 [38]
Er3+ doped Sr – Ba – Nb – B glass 872.3 748.0 16.62 [39]
Er3+ doped BaTiO3 nanocrystals 662.4 729.9 09.25 [40]
Er3+ doped Si – B – Ba – Na glass 236.0 511.7 53.89 [41]
Er3+, Yb3+ co-doped b – NaLuF4 270.6 2830.6 90.44 [42]
Er3+, Yb3+ co-doped Y2SiO5 781.0 686.1 13.83 [43]
Er3+, Yb3+ co-doped BaMoO4 607.0 716.0 15.22 [44]
Er3+, Yb3+ co-doped CaWO4 1455.0 1530.0 04.90 [45]
Er3+, Yb3+ co-doped Yb2Ti2O7 478.6 482.0 0.71 [46]
Er3+, Yb3+ co-doped LiNbO3 860.0 686.2 25.33 [47]
In applications with high temperatures, phosphors are chosen

according to their high thermal stability and adaptability of dopant
lanthanides with the host crystal structure. Yttrium oxide (Y2O3) is a
popular oxide material for a host crystal due to its wide optical band
gap of 5.6 eV [48], high melting point, and high possible adaptation
of dopant ions, such as Er3+ in the host crystal along with good
transparency from UV to IR. Lojpur et al. [42] studied Er3+–Yb3+co-doped
Y2O3. Nanoparticles for optical temperature sensing properties in the
temperature range of 93 K to 613 K. They analysed the temperature-
dependent fluorescence intensity ratio of the two green emissions
and obtained maximum sensitivity of 528 × 10 –4 K–1 for 150 K for

Y1.97 Yb0.02Er0.01O3 nano phosphors. Another case study was done by
Dong et al. [49], where they studied the fluorescence intensity ratio of
the green upconversion emission at 523 nm and 545 nm in Er3+–Yb3+
co-doped Al2O3. The study was conducted at 495 K, and the maximum
sensitivity was 0.0051 K–1. Furthermore, the study was continued with
other oxide materials co-doped with Er3+–Yb3+ [45, 50–52].
5.4 thermal stabIlIty
Thermal stability is one of the critical quality a phosphor that involves

the durability, longevity, and consistency of its performance in light
emitting devices in the required working temperature at the very least,
and ideally at very high temperatures [53]. Figure 5.5 explains the
essential parameters that define the thermal stability of a phosphor.
Figure 5.5 Factors affecting thermal stability of phosphors.
The host crystals in the phosphor may have similar structures, such as
tetragonal, hexagonal, cubic, etc., but the characteristic emission intensity
at an increasing temperature can be assigned to its electronic structure.
Thus, the thermal stability of a phosphor is defined by the effective
band gap, chemical tunability, and the degree of condensation [53].
Qin et al. [54] mentioned the relation of emission intensity of the 5d – 4f
transition and temperature by an equation:
I(0) (5.2)
I (T ) =
[1 + (t 0 /t V ) exp( - DE /kBT )
where t V is the radiative decay rate corresponding to the state of the
lanthanide ion, t 0 is the attempt rate of the thermal quenching process,
k is Boltzmann’s constant, and ΔE is the energy barrier for the thermal
quenching process.
Figure 5.6 Various modes of the thermal-assisted quenching process [53].
Various modes of the thermal-assisted quenching process have been

shown in Figure 5.6. The autoionisation model described by Dorenbos
et al. [55] shows that the 5d orbital of Eu2+ is near the conduction band
of the host lattice before the crossing point of the 4f and 5d orbitals. At
high temperatures, the electrons at the 5d orbital get thermally activated
and transferred to the conduction band. The activated electron in the 4f
energy level releases its energy through non-radiative processes such as
heat loss, traps in host lattice, and defects which can turn into quenching
centres and by lattice vibration process. Thus, it was observed that the
more significant the gap between the 5d orbital to the conduction band,
the phosphor shows higher thermal stability [56, 57]; mode (B) shows
that different valance states of activators can coexist at similar energy
levels positions [58, 59]. During the study of [60] red emitting Eu-doped
SrLiAl3N4 and Ce-doped SrMg2Al2N4 with green emission, Leaño Jr.
observed that valance states of Eu and Ce show coexistence. Mode (C)
and mode (D) are respectively based on the 4f–5d crossing model and
electron holes transfer from the activator’s ground state to the valance
band [61–63].
5.5 conclusIon
It is a well–a known phenomenon that the emission properties of the

phosphors are highly affected by temperature. These emission properties
include emission intensity, emission and excitation peak wavelength,
spectrum shape, decay, and rise time. The temperature of a phosphor
can be calculated by measuring one or more of these properties.
The phosphors used for temperature measurements are known as
thermographic phosphors and are helpful for various applications where
temperature measurement is a critical parameter of the system.
In quantum-cutting phosphors, a pair of lanthanide activators are

usually used. So, the two different activators with their emission peak
show a response to the increasing temperature. The emission intensity
of the activator with a higher energy excitation state will show a
considerable increase in peak intensity compared to the peak intensity of
the lower-energy excitation state. These QC phosphors show applications
in many engineering fields where temperature measurement is critical,
such as thermal barrier coatings, surface temperature monitoring,
fluid flow analysis, and circuit boards, etc. QC phosphors also show
their use in various biomedical applications (cellular level temperature
measurements) such as tumour monitoring, diagnosing ischaemia, etc.
RefeRences
[1] Khalid, A.H. and K. Kontis. 2008. Thermographic phosphors for high
temperature measurements: principles, current states of the art and
recent applications. Sensors 8: 5673–5744.
[2] Yadav, R.S. and R.S. Ningthoujam. 2021. Synthesis and characterization
of quantum cutting phosphor materials. In: Handbook on Synthesis
Strategies for Advanced Materials. Indian Institute of Metal Series.
Springer, Singapore.
[3] Wegh, R.T., H. Donker, K.D. Oskam and A. Meijerink. 1999. Visible
quantum cutting in LiGdF4:Eu3+ through downconversion. Science 283:
663–666.
[4] Piper, W.W., J.A. DeLuca and F.S. Ham. 1974. Cascade fluorescence
decay in Pr3+ – doped fluorides: achievement of a quantum yield greater
than unity for emission of visible light. J. Lumin. 8: 344–348.
[5] Yu, D.C., R. Martin–Rodriguez, Q.Y. Zhang, A. Meijerink and
F.T. Rabouw. 2015. Multi-photon quantum cutting in Gd2O2S:Tm3+ to
enhance the photo response of solar cells. Light Sci. Appl. 4: e344.
[6] Yadav, R.V., R.S. Yadav, A. Bahadur and S.B. Rai. 2016. Down shifting
and quantum cutting from Eu3+, Yb3+ co-doped Ca12Al14O33 phosphor: a
dual mode emitting material. RSC Adv. 6: 9049–9056.
[7] Shionoya, S., H. Yamamoto amd W.M. Yen. 2008. Phosphor Handbook,
2nd Ed. CRC. Taylor & Francis Distributor, London.
[8] Wegh, R.T., H. Donker, K.D. Oskam and A. Meijerink. 1999.
Visible quantum cutting in Eu3+ doped gadolinium fluorides via
downconversion. J. Lumin. 82: 93–104.
[9] Blasse, G. and B.C. Grabmaier. 1994. Luminescent Materials. Springer–
Verlag, Berlin.
[10] Henderson, B. and G.F. Imbusch. 1989. Optical Spectroscopy in Inorganic
Solids. Clarendon, Oxford.
[11] Huang, X., S. Han, W. Huang and X. Liu. 2013. Enhancing solar cell
efficiency: the search for luminescent materials as spectral converters,
Chem. Soc. Rev. 42: 173–201.
[12] Yu, D.C., X.Y. Huang, S. Ye, M.Y. Peng, Q.Y. Zhang and L. Wondraczek.
2011. Three – photon near – infrared quantum splitting in b–NaYF4:Ho3+.
Appl. Phys. Lett. 99: 161904.
[13] Yu, D.C., X.Y. Huang, S. Ye, Q.Y. Zhang and J. Wang. 2011. A sequential
two – step near – infrared quantum splitting in Ho3+ singly doped
NaYF4. AIP Adv. 1: 042161.
[14] Chen, X.B., J.G. Wu, X.L. Xu, Z.Y. Zhang, N. Sawanobori, C.L. Zhang,
et al. 2009. Three photon infrared quantum cutting from single species
of rare–earth Er3+ ions in Er0.3Gd0.7VO4 crystalline. Opt. Lett. 34(7): 887–
889.
[15] Miritello, M., R. Lo Savio., P. Cardile and F. Priolo. 2010. Enhanced
down conversion of photons emitted by photoexcited ErxY2-xSi2O7 films
grown on silicon. Phys. Rev. B: Condens. Matter. 81: 041411.
[16] Yu, D.C., S. Ye, M.Y. Peng, Q.Y. Zhang and L. Wondraczek. 2012.
Sequential three – step three photon near infrared quantum splitting
in β–NaYF4:Tm3+. Appl. Phys. Lett. 100: 191911.
[17] Zhang, W.J., D.C. Yu, J.P. Zhang, Q. Qian, S.H. Xu, Z.M. Yang, et al. 2012.
Near infrared quantum splitting in Ho3+:LaF3 nanocrystals embedded
germanate glass ceramic. Opt. Mater. Express 2: 636–643.
[18] van der Ende, B.M., L. Aarts and A. Meijerink. 2009. Near infrared
quantum cutting for photovoltaics. Adv. Mater. 21: 3073–3077.
[19] Eilers, J.J., D. Biner, J.T. van Wijngaarden, K. Kraemer, H.U. Güedel and
A. Meijerink. 2010. Efficient visible to infrared quantum cutting through
downconversion with the Er3+–Yb3+ couple in Cs3Y2Br9. Appl. Phys.
Lett. 96: 151106.
[20] Zheng, W., H. Zhu, R. Li, D. Tu, Y. Liu, W. Luo, et al. 2012. Visible
to infrared quantum cutting by phonon assisted energy transfer in
YPO4:Tm3+, Yb3+ phosphors, Phys. Chem. Chem. Phys. 14: 6974–6980.
[21] Yu, D.C., X.Y. Huang, S. Ye. and Q.Y. Zhang. 2011. Efficient first order
resonant near infrared quantum cutting in b–NaYF4:Ho3+, Yb3+. J. Alloys
Compd. 509: 9919–9923.
[22] Nuebert, P. 1937. Device for indicating the temperature distribution of
hot bodies–US Patent No. 2,071,471.
[23] Wickersheim, K. amd R. Alves. 1979. Recent advances in optical
temperature measurement. Ind. Res. Dev. 21: 82
[24] Cates, M.R. 1984. Applications of pulsed laser techniques and
thermographic phosphors to dynamic thermometry of rotating surfaces.
ICALEO 84: Proceedings: Inspection, Measurement and Control 45:
4–10.
[25] Alaruri, S., D. McFarland, A. Brewington, M. Thomas and N. Sallee.
1995. Development of a Fiber-Optic Probe for Thermographic Phosphor
Measurements in Turbine Engines. Opt. Lasers Eng. 22: 17–31.
[26] Jadhav, A.P., J.H. Oh, S.W. Park, H.Y. Choi, B.K. Moon, B.C. Choi,
et al. 2016. Enhanced down and upconversion emission for Li+ co–doped
Gd2O3:Er3+ nanostructures. Curr. Appl. Phys. 16: 1374–1381.
[27] Jadhav, A.P., T.D. Thi Dinh, S. Khan, S.Y. Lee, J.K. Park, S.W. Park, et al.
2016. Enhanced photoluminescence due to Bi3+–Eu3+ energy transfer and
re-precipitation of RE doped homogeneous sized Y2O3 nanophosphors.
Mater. Res. Bull. 83: 186–192.
[28] Li, D.Y., Y.X. Wang, X.R. Zhang, K. Yang, L. Liu and Y.L. Song. 2012.
Optical temperature sensor through infrared excited blue upconversion
emission in Tm3+/Yb3+ co-doped Y2O3. Opt. Commun. 285: 1925–1928.
[29] Zheng, K.Z., Z.Y. Liu, C.J. Lv and W.P. Qin. 2013. Temperature sensor
based on the UV upconversion luminescence of Gd3+ in Yb3+–Tm3+–
Gd3+ co-doped NaLuF4 microcrystals. J. Mater. Chem. C 1: 5502–5507.
[30] Xu, W., H. Zhao, Z.G. Zhang and W.W. Cao. 2013. Highly sensitive
optical thermometry through thermally enhanced near infrared
emissions from Nd3+/Yb3+ co-doped oxyfluoride glass ceramic. Sens.
Actuators B: Chem. 178: 520–524.
[31] Cao, Z.M., S.S. Zhou, G.C. Jiang, Y.H. Chen, C.K. Duan and M. Yin.
2014. Temperature dependent luminescence of Dy3+ doped BaYF5
nanoparticles for optical thermometry. Curr. Appl. Phys. 14: 1067–1071.
[32] Lojpur, V., M. Nikolic, L. Mancic, O. Milosevic and M.D. Dramicanin.
2013. Y2O3: Yb, Tm and Y2O3: Yb, Ho powders for low–temperature
thermometry based on up-conversion fluorescence. Ceram. Int. 39:
1129–1134.
[33] Wang, X.F., J. Zheng, Y. Xuan and X.H. Yan. 2013. Optical temperature
sensing of NaYbF4:Tm3+ @SiO2 core–shell micro-particles induced by
infrared excitation. Opt. Express 21(18): 21596–21606.
[34] Luis, S.F.L., U.R.R. Mendoza, P.H. González, I.R. Martín and V.
Lavín. 2012. Role of the host matrix on the thermal sensitivity of Er3+
luminescence in optical temperature sensors, 2012. Sens. Actuators B:
Chem. 174: 176–186.
[35] Wade, S.A., S.F. Collins and G.W. Baxter. 2003. Fluorescence intensity
ratio technique for optical fiber point temperature sensing. J. Appl.
Phys. 94: 4743–4756.
[36] Xu, W., X.Y. Gao, L.J. Zheng, Z.G. Zhang and W.W. Cao. 2012. Short
wavelength upconversion emissions in Ho3+/Yb3+ co-doped glass
ceramic and the optical thermometry behavior. Opt. Express 20(16):
18127–18137.
[37] Dong, B., B.S. Cao, Y.Y. He, Z. Liu, Z.P. Li and Z.Q. Feng. 2012.
Temperature sensing and in vivo imaging by molybdenum sensitized
visible upconversion luminescence of rare earth oxides. Adv. Mater.
24(15): 1987–1993.
[38] González, P.H., S.F.L. Luis, S.G. Pérez and I.R. Martín. 2011. Analysis of
Er3+ and Ho3+ codoped fluoroindate glasses as wide range temperature
sensor. Mater. Res. Bull. 46(7): 1051–1054.
[39] González, P.H., I.R. Martín, L.L. Martín, S.F.L. Luis, C.P. Rodríguez
and V. Lavín. 2011. Characterization of Er3+ and Nd3+ doped strontium
barium niobate glass ceramic as temperature sensors. Opt. Mater. 33:
742–745.
[40] Alencar, M.A.R.C., G.S. Maciel and C.B. de Araújo. 2004. Er3+ doped
BaTiO3 nanocrystals for thermometry: influence of nano environment
on the sensitivity of a fluorescence–based temperature sensor. Appl.
Phys. Lett. 84: 4753–4755.
[41] Li, C.R., B. Dong, C.G. Ming and M.K. Lei. 2007. Application to
temperature sensor based on green upconversion of Er3+ doped silicate
glass, Sensors 7(11): 2652–2659.
[42] Zheng, K.Z., W.Y. Song, G.H. He, Z. Yuan and W.P. Qin. 2015. Five
photon UV upconversion emissions of Er3+ for temperature sensing.
Opt. Express 23(6): 7653–7658.
[43] Rakov, N. and G.S. Maciel. 2012. Three photon upconversion and optical
thermometry characterization of Er3+: Yb3+ co-doped yttrium silicate
powders. Sens. Actuators B: Chem. 164: 96–100.
[44] Soni, A.K., A. Kumari and V.K. Rai. 2015. Optical investigation in shuttle
like BaMoO4:Er3+ – Yb3+ phosphor in display and temperature sensing.
Sens. Actuators B: Chem. 216: 64–71.
[45] Xu, W., Z.G. Zhang and W.W. Cao. 2012. Excellent optical thermometry
based on short wavelength upconversion emissions in Er3+/Yb3+ co-
doped CaWO4. Opt. Lett. 37: 4865–4867.
[46] Cao, B.S., Y.Y. He, Z.Q. Feng, Y.S. Li and B. Dong. 2011. Optical
temperature sensing behavior of enhanced green upconversion emission
from Er–Mo: Yb2Ti2O7 nanophosphor. Sens. Actuators B: Chem. 159:
8–11.
[47] Quintanilla, M., E. Cantelar, F. Cussó, M. Villegas and A.C. Caballero.
2011. Temperature sensing with upconverting submicron sized
LiNbO3:Er3+/Yb3+ particles. Appl. Phys. Express 4: 022601.
[48] Wang, W.C., M. Badylevich, V.V. Afanas’ev, A. Stesmans, C. Adelmann,
S. Van Elshocht, et al. 2009. Band alignment and electron traps in Y2O3
layers on (100) Si. Appl. Phys. Lett. 95: 132903.
[49] Dong, B. and D.P. Liu. 2007. Optical thermometry through infrared
excited green upconversion emissions in Er3+–Yb3+ co-doped Al2O3.
Appl. Phys. Lett. 90, 181117.
[50] Rakov, N. and G.S. Maciel. 2014. Nd3+–Yb3+ doped powder for near-
infrared optical temperature sensing. Opt. Lett. 39: 3767–3769.
[51] Mahata, M.K., K. Kumar and V.K. Rai. 2015. Er3+–Yb3+ doped vanadate
nanocrystals: a highly sensitive thermographic phosphor and its optical
nano heater behavior. Sens. Actuators B: Chem. 209: 775–780.
[52] Zou, H., J. Li, X.S. Wang, Y.S. Li and X. Yao. 2014. Color tunable
upconversion emission and optical temperature sensing behavior in
Er – Yb – Mo co-doped Bi7Ti4NbO21 multifunctional ferroelectric oxide.
Opt. Mater. Express 4: 1545–1554.
[53] Leaño Jr., J.L., M.H. Fang and R.S. Liu. 2018. Review—narrow band
emission of nitride phosphors for light emitting diodes: perspectives
and opportunities. ESC J. Solid State Sci. Technol. 7(1): R3111–R3133.
[54] Qin, X., X. Liu, W. Huang, M. Bertinelli and X. Liu. 2017. Lanthanide
activated phosphors based on 4f–5d optical transitions: theoretical and
experimental aspects. Chem. Rev. 117: 4488–4527.
[55] Dorenbos, P. 2005. Thermal quenching of Eu2+ 5d–4f luminescence in
inorganic compounds. J. Phys.: Condens. Matter. 17: 8103–8111.
[56] Liu, C., Z. Qi, C.C. Ma, P. Dorenbos, D. Hou, S. Zhan, et al. 2014.
High light yield of Sr8(Si4O12) Cl8: Eu2+ under X–ray excitation and its
temperature dependent luminescence characteristics. Chem. Mater. 26:
3709–3715.
[57] Luo, H., L. Ning, Y. Dong, A.J.J. Bos and P. Dorenbos. 2016. Electronic
structure and site occupancy of lanthanide doped (Sr, Ca)3(Y, Lu)2Ge3O12
garnets: a spectroscopic and first principles study. J. Phys. Chem. C 120:
28743–28752.
[58] Lin, C.C., Z.R. Xia, G.Y. Guo, T.S. Chan and R.S. Liu. 2010. Versatile
phosphate phosphors ABPO4 in white light emitting diodes: collocated
characteristics analysis and theoretical calculations. J. Am. Chem. Soc.
132: 3020–3028.
[59] Sontakke, A.D., J. Ueda, J. Xu, K. Asami, M. Katayama, Y. Inada,
et al. 2016. A comparison on Ce3+ luminescence in borate glass and YAG
ceramics: understanding the role of host’s characteristics. J. Phys. Chem. C
120(31): 17683–17691.
[60] Leaño Jr., J.L., S.Y. Lin, A. Lazarowska, S. Mahlik, M. Grinberg, C. Liang,
et al. 2016. Green light excitable Ce-doped nitrodomagnesoalumiate
Sr[Mg0Al2N4] Phosphor for white light emitting diodes, Chem. Mater.
28(19): 6822–6825.
[61] Blasse, G. and B.C. Grabmaier. 1994. A general introduction to
luminescent materials. pp. 1–9. In: Luminescent Materials. Springer,
Berlin, Heidelberg.
[62] Ivanovski, K.V., J.M. Ogieglo, A. Zych, C.R. Ronda and A. Meijerink.
2013. Luminescence temperature quenching for Ce3+ and Pr3+ d–f
emission in YAG and LuAG. ECS Solid State Sci. Technol. 2: R3148.
[63] Kramers, M., G.O. Mueller, R.B. Mueller–Mach, H. Bechtel and
P.J. Schmidt. 2010. PCT Pat. International Applications. A1. WO201013
1133.
Chapter 6
A Review of Flexible Sensors
Surendra Maharjan and Ahalapitiya H. Jayatissa*

Nanotechnology and MEMS Laboratory,
Department of Mechanical, Industrial, and
Manufacturing Engineering (MIME),
The University of Toledo, OH 43606, USA.
Email: smaharj7@rockets.utoledo.edu
6.1 IntroductIon
A sensor is a device or a unit of an electronic system that perceives

external information or stimulus and converts them into electrical signals.
The signals are then transferred to the system or other devices. They are
built to enable them to interact with the system and surroundings. Thus,
it can be seen as a vital component of the perception system in electronic
devices [1]. Flexible sensors have been intensively used in numerous
applications, including those fields where conventional sensors cannot
be imagined due to their flexibility. Advancements in flexible sensing
technology can be observed due to the emerging Internet of Things
(IoT) and intelligent systems and the massive demand for such systems
or devices in human-machine interaction [2].
Recent research and development of flexible sensors are heading
towards the innovation of novel materials, ease of mass fabrication,
integration into systems, and imparting quality features. The spotlight
*For Correspondence: Ahalapitiya H. Jayatissa (ahalapitiya.jayatissa@utoledo.edu)

of flexible sensors lies in their seamless applications to soft and

irregular surfaces, such as textile fabrics, artificial skins, smart tattoos,
soft robotics, and wearable devices. Advances in material science and
device architecture have enabled large-scale manufacturing of flexible
sensors to become a realistic option, simultaneously enhancing their
conductivity and deformability [3]. They can be rolled, folded, or twisted
without losing functionality [4, 5]. For instance, Samsung and Huawei
developed foldable phones using flexible technology [4, 6–8]. Flexible
sensors can address conventional sensor issues, such as poor signal
transduction, low sensitivity, and rigid nature. The primary importance
of flexible sensors can also be visualised in Figure 6.1.
Figure 6.1 Importance of flexible sensors in the real world.
The flexible sensors are made by coupling conductive material with

a flexible substrate. Recent advances in semiconductors, such as metal
oxides, silicon nanomembranes, and organic materials have enabled high-
performance circuits capable of on-site sensor conditioning. Graphene,
nanowires, black phosphorus [9], transition metal dichalcogenides
(TMDs) [10], and non-transition metal oxides ZnO, SnO2, In2O3, and
Ga2O3 have been studied for their application in gas and strain sensors.
Perovskites have been in the limelight for fabricating ultra-sensitive light
sensors. Conductive or active materials are processed as nanoparticles,
nanotubes, nanowires, or thin films.
Substrate materials play another significant role along with
conductive materials to make feasible for flexible sensors. While selecting
substrates, specific properties should be carefully considered, such as
chemical resistance, thermal resistance, transparency, and flexibility. In
consumer electronics, polyimide (PI), polyethylene (PE), polyethylene
terephthalate (PET), polyetheretherketone (PEEK), polyethersulfone (PES),
polycarbonate (PC), and polyethylene naphthalene (PEN) are popular
substrate materials. Nowadays, flexible sensors are ubiquitous, and the
market value is expected to rise from $3.6 billion to $7.6 billion by 2027,
which is more than double within a few years [11].
A Review of Flexible Sensors 139
6.2 WorkIng MechanIsMs of flexIble sensors
Flexible sensors are categorised into different groups based on their

working mechanism. However, the most common types are piezoresistive,
capacitive, and piezoelectric, as shown in Figure 6.2.
Figure 6.2 Schematics illustration of the different working mechanisms of a

flexible sensor [12].
6.2.1 Piezoresistive type

6.2.1.1 Geometrical Effect
The working mechanism can be explained as the change in resistance
of a material caused by mechanical stimulus. The change of resistance
can be converted into other types of electrical signals. The factors that
result in resistance change depend on the property of the material
and its structures, including geometrical effect, structural effect, and
disconnection mechanism.
The geometrical effect illustrates the change in resistance due to a
change in the geometrical dimensions of a sensor, such as a length (L)
and area (A) when pressure or strain is applied. The geometrical changes
appear in the form of elongation or contraction. The resistance of a
conductor is given by R = rL/A, where r is the electrical resistivity. This
effect is minimal compared to other effects for such kinds of sensors.
6.2.1.2 Structural Effect

When pressure or strain is applied to the sensor having semiconductors
as an active material, the structural deformation causes a change in
interatomic space, which changes the band gap, resulting in the change
in resistivity of the material [13]. It illustrates the structural effect.
For instance, carbon nanotube (CNT) shows extremely high resistivity
change due to their chirality and change in barrier height.
6.2.1.3 Disconnection Mechanism

Three situations cause the disconnection between the adjacent nanoflakes:
contact area change, tunnelling effect, and crack propagation. The
change in the contact area between adjacent nanoflakes is dominant
when the applied pressure or strain is slight, and the electrons travel
through the overlapped nanoflakes within the percolation conductive
network. As the applied pressure increases, the adjacent nanoflakes
pull apart, creating a tunnel. However, electrons can pass through the
tunnel due to exceedingly small separation. It is called the tunnelling
effect, and the separation space is called the tunnelling distance. As
the distance grows, so does the tunnelling resistance. The distance
no electrons pass through by tunnelling is called cut-off tunnelling
distance. The tunnelling resistance between two adjacent particles can
be estimated by Simmons’s theory [14].
h2 d Ê 4p d ˆ
Rtunnel = exp Á 2ml ˜ (1.1)
Ae 2
2ml Ë h ¯
Here A, e, h, d, m, and l represent the cross-sectional area of the

tunnelling junction, single-electron charge, Plank’s constant, the distance
between adjacent nanoflakes, the mass of an electron, and the height
of energy barrier for insulators, respectively. The third one is crack
propagation which occurs when the applied pressure or strain is even
higher. Initially, the crack initiates and propagates along with time
and pressure conditions. The separation of crack edges critically limits
electrical conduction.
The two essential parameters of piezoresistive sensors are gauge
factor (GF) and sensitivity, which can be calculated by equations (1.2)
[15] and (1.3). The change in resistance relative to the strain determines
GF. The sensitivity is defined as the ratio of resistance change to the
change in pressure.
DR /R0
GF = (1.2)
Strain ( DL /L0 )
( DR /R0 )
S= (1.3)
DP
An example of a piezoresistive-based flexible sensor is an airflow
sensor made by coating graphene/PVDF nanocomposite on PE material
having a sensitivity of 1.21% kPa–1 in the pressure range of 0–2.7 kPa
[29], as shown in Figure 6.3.
Figure 6.3 A piezoresistive airflow sensor made of graphene/PVDF

nanoparticles coated on PE substrate.
6.2.2 Piezoelectric type

The sensing mechanism of piezoelectric-type sensors is a piezoelectric
effect, which means electric charge accumulates in piezoelectric
materials when a mechanical stimulus is applied. The change in
electrical polarisation inside the material results in a change in surface
charge called voltage which can be measured by connecting two
surfaces. Piezoelectric materials can be crystals, certain ceramics, and
even biological matter. Quartz is the most common natural piezoelectric
material, but synthesised piezoelectric materials are more efficient and
are primarily ceramics based. Some examples include lead zirconate
titanate (PZT), barium titanate (BaTiO3), and lead titanate (PbTiO3).
Potassium niobate (KNbO3), lithium niobate (LiNbO3), lithium tantalate
(LiTaO3), gallium nitride (GaN), and zinc oxide (ZnO).
6.2.3 capacitive type

This type of sensor is based on a change in the capacitance of a capacitor
under the mechanical stimulus. The most common type of parallel
plate capacitor due to simple configuration and fabrication is presented
by C = kA/d, where k, A, and d represent the medium’s dielectric
constant, the electrode’s overlap area, and the distance between two
plates, respectively. When the parallel plate electrode is stretched, the
overlap area will change. When the pressure is applied, the distance
between electrodes changes. As any of the above parameters change,
the capacitance will change.
Sensitivity is one of the essential parameters for capacitive sensors
and parallel plate capacitors. It is expressed by equation (1.4).
( DC /C0 )
S= (1.4)
DP
Here ΔC, C0, and ΔP represent the relative change of capacitance,

initial capacitance, and the change in capacitance when pressure is
applied. The main factor that controls sensitivity is the relative change
of capacitance under pressure. Many studies have shown that improving
the deformation behaviour of the dielectric increases sensitivity, for
which they have adopted porous structures such as sponges or foams [16]
and some microstructures [17, 18].
6.3 basIc ParaMeters of a flexIble sensor
6.3.1 sensitivity
The slope of the sensor’s response curve determines the sensitivity of
a sensor. It is sometimes used as a GF. Sensitivity is a critical factor
in determining sensor performance. For weak signals, such as human
body signals, to be detected, sensitivity should be higher. The GF
demonstrated by the flexible strain sensor based on ZnSnO3 nanowires
is up to 3,740, which is 19 times higher than the silicon-based strain
sensor [1]. The sensitivity of flexible pressure sensors could be achieved
up to 133.1 kPa−1 by designing hollow-sphere structured conducting
polymers [19]. Another approach to increase sensitivity is using unique
electrical designs such as a charge-coupled device (CCD). The pH sensor
based on CCD could have sensitivity up to 240 mV pH−1, which is four
times higher than the value obtained by the Nernst equation at room
temperature (59 mV pH−1) [20].
6.3.2 linearity
Linearity is one of the essential parameters the sensor should display
the response while detecting the external stimulus to integrate with the
system quickly. If the response curve is not linear, then extra signal
processing circuits are needed for calibration, dramatically increasing
design complexity and compensation [21]. An example of a strain sensor
made by CNT thin film and PDMS as active material and dielectric,
respectively, the sensor could respond near-perfect linear under strain.
6.3.3 selectivity
Sensors should be capable of detecting selective things and giving an
accurate signal. For instance, the SiO2 sensing layer in the pH sensor
interacts with hydrogen ions (H+), which are present in hydroxyl groups
(OH) when it is in contact with a certain kind of solution. Sodium
(Na+) and potassium (K+) ions in the solution would not react with the
OH groups; thus, the measured signals from the pH sensor would not
be affected by the presence of those ions. Selective detection of other
biomarkers, such as glucose, would be beneficial.
6.3.4 resolution
The sensor’s capability to detect very subtle changes depends upon
its resolution. Many physiological indexes vary over a limited
range, and a slight change may contain much information about the
physiological status. For instance, the average human body temperature
is typically between 36.5 and 37.5 °C, and changes occur due to physical
activity, fatigue, hormone level, etc. Minimising interference from the
measurement environment and suppressing noise level could promote
the sensor’s resolution. For example, graphene having low thermal noise
as an active material could achieve high resolution. The temperature
sensor made up of a mixture of SnO2 nanoparticles and single-walled
carbon nanotubes and a graphene-based sensor (low thermal noise)
could achieve higher resolution [22].
6.3.5 detection limit

The detection limit can be the lowest quantity of a substance the sensor
can detect to meet the target requirements. For example, a flexible
glucose sensor fabricated based on In2O3 nanoribbon FETs can achieve
an ultralow detection limit of 10 nmol of glucose in biofluids on a human
body surface, such as tears and sweat.
6.3.6 durability
Durability shows a sensor’s ability to perform typically for a long time
without excessive repair or maintenance. It is usually measured by
cyclic stability (endurance to periodic loading and unloading cycles).
The sensing material or substrates are prone to buckle, fracture, and
even strip after enough cycles, resulting in the cyclic unstable problem.
One of the ways to promote durability is to prepare a sensor enabling
self-healing ability.
6.3.7 hysteresis and response time

Hysteresis is the dependence of performance on the history of the sensor,
which should be reduced or avoided. The interfacial bonding between
sensing material and substrate significantly affects the optimisation of
hysteresis. Capacitive sensors have lower hysteresis than piezoresistive
sensors as they respond immediately to the variation of overlapped

areas. Response time means the speed of achieving a steady response
to the external stimulus. Piezoresistive sensors display a significant
response time than others due to the re-establishment of the percolation
network in resistive composites.
6.4 MaterIals and fabrIcatIon technIques
Materials used in flexible sensors are usually divided into four groups
depending upon their typical roles: conductors, semiconductors,
insulators, and substrates. Conductive materials are discussed based
on their conductivity and transparency. Semiconductors play a role in
field-effect mobility and stability while bending, insulators, or dielectrics
are selected based on their dielectric constant and breakdown voltage.
In contrast, substrates are highlighted based on flexibility, stretchability,
surface quality, transparency, and thermal and chemical stability [2].
Figure 6.4 presents the overview of everyday materials and respective
fabrication methods used in flexible sensing technology.
Figure 6.4 Standard materials and respective fabrication methods are used to
fabricate flexible sensors [3].
6.4.1 conductors
Usually, conductors are used as contact materials in flexible sensors.
However, nanoparticles and nanowires are best suited for conductors.
Some conductors provide transparency such as AgNWs. The general
overview of conductors used in flexible sensors is presented in Figure 6.5.
Figure 6.5 Classification of conductors used in flexible sensors.
6.4.2 semiconductors
Semiconductors are used as sensitive active areas or sensing material. The
conductivity of semiconductors lies between metal and insulators, and
the transport of charge carriers depends upon covalent bonds. Figure 6.6
depicts diverse types of semiconductors used in flexible sensors. Silicon (Si)
and germanium (Ge) are the most used semiconductors. However, they
are doped with other elements to obtain desirable properties for sensor
applications. Typically, semiconductors are compared in terms of carrier
mobility, compatibility with large-area fabrication, and processing
temperature. Due to their rigid nature, they are mostly preferred as
thin-film transistor (TFT) channels or as sensitive layers.
Figure 6.6 Classification of semiconductors used in flexible sensors.
6.4.3 Insulators/dielectrics
Dielectrics used in flexible sensors are either organic materials such
as polyvinyl phenol (PVP), polyvinylpyrrolidone, poly(perfluorobut
enylvinylether), PDMS, PLA, polyvinylidene fluoride (PVDF), PVDF-

Trifluoroethylene (PVDF-TrFE), or inorganic materials such as Al2O3,
SiO2, HfO2, and TiO2. Such dielectric materials are just part of field-effect
transistors or capacitive sensors. Al2O3 and TiO2 have higher dielectric
constants, 9.7 and 27, respectively, when spin coating, followed by an
annealing step, obtains thin films. SiO2 presents a dielectric constant of
3.9, which is lower than some organic materials. Due to well-established
properties and fabrication techniques, inorganic materials are more
widely used as dielectrics than organic materials. PDMS is used as
stretchable dielectrics. PVDF-TrFE presents a higher dielectric constant
than Al2O3 when fabricated by spin coating and processed at 140°C.
PVDF possesses good dielectric and piezoelectric properties.
6.4.4 substrates
Substrates are desired to conform to soft and irregular surfaces and
hold properties such as thermal and chemical resistance, extensive
area compatibility, transparency, low surface roughness, and process-
compatible glass transition temperature to be compatible with flexible
sensor applications. Polyimide (PI), polyethylene terephthalate (PET),
and polyethylene naphtholate (PEN) substrates dominate in
flexible applications due to their excellent properties, whereas
polydimethylsiloxane (PDMS) is widely used in the stretchable sensor.
Other substrates include polyethylene (PE), polycarbonate (PC),
polyvinyl alcohol (PVA), polylactic acid (PLA), polysulfide (PSU), and
polyetheretherketone (PEEK).
6.5 tyPes of flexIble sensors and theIr

aPPlIcatIons
Flexible sensors are categorised into strain, pressure, temperature,

humidity, magnetic, chemical, electromagnetic radiation, multi-modal,
and orientation.
6.5.1 strain sensors

Strain sensors convert a mechanical stimulus into an electrical signal.
A strain sensor is characterised by its GF (sensitivity), stretchability,
hysteresis, and response time. Strain sensors are categorised into
resistive, capacitive, piezoelectric, and triboelectric based on their
working mechanism. Materials such as ZnONWs, ZnSnO3, liquid metals,
metal nanowires/nanostructures, carbon black, CNT, and graphene have
been used to fabricate strain sensors. These sensors are used to detect
the physical dimension changes in the body or structures.
6.5.2 Pressure sensors

Pressure sensors are among the most used sensors that detect the pressure
of the human body, machinery device, or surrounding environment.
The critical parameters to the performance of pressure sensors include
sensitivity, detection range, and response time. Different approaches
have been adopted to fabricate pressure sensors, such as the resistive,
capacitive, piezoelectric, field-effect transistor, and peizocapacitive.
Some examples of pressure sensors include ZnO-microparticle pressure
sensors, laser-scribed graphene (LSG) pressure sensors with foam-like
structures, multi-functional P(VDF-TrFE) field-effect organic transistors
using a microstructured dielectric layer, and AgNWs/PDMS pressure
sensor array using a stacked structure.
6.5.3 shear stress sensors

Shear stress sensors are essential for monitoring and characterising the
dynamic properties of the fluid. In real life, many things continuously
interact with a fluid, such as an aircraft, automobiles, and fluid flow
in a pipe. By determining the shear stress, the design of an object can
be optimised for better performance. The critical parameters for the
wall shear stress are fluid viscosity and the velocity gradient. Rigid
shear stress sensors are found to be challenging to apply on uneven or
curved surfaces, which the flexible type can mitigate. Hasegawa et al.
[23] developed a smooth-surfaced flexible thermal shear stress sensor
from thin parylene film on a micromachined PDMS structure. It was
tested by placing it on the curved streamlined surface of a vehicle to
measure shear stress with high accuracy.
6.5.4 temperature sensors

Flexible temperature sensors belong to the electrical contact sensors
whose electrical properties change to temperature. They are categorised
into resistive, capacitive, thermoelectric, pyroelectric, transistors, or
diodes. The critical factors of a temperature sensor are temperature
sensitivity, temperature range, hysteresis, and response time. Resistive
thin-film temperature sensors, developed by integrating gold resistance
temperature detectors (RTDs) on flexible PI substrates, are used to detect
the temperature of the metal. Other materials fabricating these types
of sensors include lead zirconate titanate (PZT), ZnO, PVDF, graphene,
CNTs, etc.
6.5.5 humidity sensors

Humidity sensors measure relative humidity (RH) levels in the air. RH
is the ratio of water vapour in the air to the air needed for saturation
at a given temperature and pressure. The most critical parameters are
sensitivity to RH, humidity range, response time, and hysteresis, and
bendability, stretchability, and transparency are further investigated.
Humidity sensors are categorised into resistive, piezoelectric, capacitive,
TFT, and fibre optic. Some examples of humidity sensors include a
humidity-reactive PTFE sensor applied on a curved surface and
performance comparison before and after NaOH treatment and a GO
hydrophilic quartz crystal microbalance humidity sensor.
6.5.6 Magnetic sensors

The performance of a flexible magnetic sensor is assessed with
sensitivity, magnetic field direction, and bendability. These sensors
are developed by depositing stacked thin films on a flexible substrate.
The approaches that are adopted while developing magnetic sensors
include giant magnetoresistance (GMR), anisotropic magnetoresistance
(AMR), tunnelling magnetoresistance (TMR), magnetoimpedance (MI),
and hall sensor. For example, an AMR sensor, fabricated by sequentially
depositing permalloy, Au, and Pt layers on a PI substrate [24] is used to
detect geomagnetic fields (40–60 μT).
6.5.7 chemical sensors

The most essential parameters for chemical sensors are sensitivity, chemical
concentration, detection range, and response time. Chemical sensors have
taken multiple approaches for development including resistive chemical,
electrochemical, FET chemical, and optical. Some examples of chemical
sensors are a photo-induced room temperature gas sensor using an
IGZO thin film transistor [25], SnO2 flexible sensor, and CNTs field-effect
transistor for enzymatic acetylcholinesterase detection [26].
6.5.8 electromagnetic radiation sensors

These sensors use the photoelectric properties of semiconductors to
detect electromagnetic radiation. When light hits on these materials,
electrical properties will change. The critical parameters required for
these sensors are sensitivity and electromagnetic response. ZnO and
perovskites have dominated this field. An X-ray sensor is an example
of this type of sensor.
6.5.9 Multi-modal sensors

Multi-modal sensors can sense independent phenomena with an
individual sensing component. A sensor made by PVDF with attached
tetrapod ZnO (T-ZnO) nanostructures on a PET substrate [27] displays
this type of sensing behaviour. This sensor can detect bending.
Meantime, it is suitable for O2 gas sensing and humidity detection.
6.5.10 electropotential sensors

Electropotential sensors consist of electrode and signal acquisition
electronics that measure electric potentials and fields. Signal acquisition
electrodes are the critical factor for the performance of electropotential
sensors. These sensors are categorised into resistively coupled electrodes
(direct conductive contact with the skin) and capacitively coupled
electrodes (not direct conductive contact with the skin). Flexible resistive
coupled electrodes are fabricated by Ag/AgCl conductive inks, Au, or
graphene. Copper electrodes fabricate a capacitively coupled electrode
on a PI substrate, and the textile is used as the dielectric [28]; for
example, a washable textile electrode for ECG monitoring.
6.5.11 orientation sensors

Orientation sensors measure the tilt angle of an object in 3D space.
The critical parameters for the better performance of these sensors are
determined by resolution and hysteresis. The conductive microspheres
create an electrical connection between the contact and ground pads
based on the sensor orientation. Such sensors are used in gaming
platforms.
6.5.12 ultrasonic sensors

These sensors perform non-destructive imaging of 3D objects in the
free space by detecting reflected pulse-echo ultrasound waves. Flexible
electronics contribute to solidly adhering to irregular and non-planar
surfaces, improving test results’ reliability. The resonant frequency is
the critical parameter to determine the performance of such sensors. A
piezoelectric, stretchable ultrasonic array of anisotropic PZT elements
and flexible capacitive micromachined ultrasonic transducers (CMUTs)
are some materials used for these applications.
6.6 suMMary
Flexible sensors are widely used in various sectors, such as artificial

intelligence, soft robotics, biomedical devices, human-machine
interface, etc., due to their advantages over conventional sensors. The
implementation on soft and irregular surfaces and the capability to
twist, turn and roll without change in functionality made humans
invent new things that seemed impossible before. Most of the sensors
work on piezoresistive, piezoelectric, and capacitive principles. While
developing the sensors, the basic parameters of the sensors, such as
sensitivity, linearity, selectivity, resolution, durability, detection limit,
hysteresis, and response time, should be analysed appropriately to
make them viable for commercial use. The four main types of materials
play a significant role in designing and developing a sound sensor. For
instance, conductors are used as contact material. Semiconductors are
used as sensitive active areas. Dielectrics are used to isolate substrate
with conductive materials, whereas substrates provide flexibility to
the sensor. Flexible substrates are conductive by pasting nanoparticles,
nanowires, or nanotubes either by coating or 3D printing. Some flexible
sensors work based on physical parameter change, some on chemical
parameters, and some on optical. Extensive research is going on in this
field to reduce cost, size, and weight and find new possibilities.
RefeRences
[1] Wu, J.M., C.Y. Chen, Y. Zhang, K.H. Chen, Y. Yang, Y. Hu, et al. 2012.
Ultrahigh sensitive piezotronic strain sensors Based on a ZnSnO3
nanowire/microwire. ACS Nano 6: 4369–4374.
[2] Li, X. and Y. Chai. 2021. Design and applications of graphene-based
flexible and wearable physical sensing devices. 2D Mater. 8: 022001.
[3] Costa, J.C., F. Spina, P. Lugoda, L. Garcia-Garcia, D. Roggen and
N. Münzenrieder. 2019. Flexible sensors—from materials to applications.
Technologies 7: 35.
[4] Weiser, M. 1999. The computer for the 21st century. ACM SIGMOBILE
Mobile Computing and Communications Review 3: 3–11. Accessed:
(2022).
[5] Bauer, S., S. Bauer-Gogonea, I. Graz, M. Kaltenbrunner, C. Keplinger
and R. Schwödiauer. 2014. 25th anniversary article: a soft future: from
robots and sensor skin to energy harvesters. Adv. Mater. 26: 149–162.
[6] Myny, K. 2018. The development of flexible integrated circuits based on
thin-film transistors. Nat. Electron. 1: 30–39.
[7] Nathan, A., A. Ahnood, M.T. Cole, S. Lee, Y. Suzuki, P. Hiralal, et al.
2012. Flexible electronics: the next ubiquitous platform. Proc. IEEE, Vol.
100, no. Special Centennial Issue: 1486–1517.
[8] Featherstone, D.J., R.J. Werner, C.A. Camarce and S.E. Cullen. 2014.
Flexible display patent landscape and implications from the america
invents act. Nanotech. L. & Bus. 11: 181–194.
[9] Coleman, J.N., M. Lotya, A. O’Neill, S.D. Bergin, P.J. King, U. Khan, et al.
2011. Two-dimensional nanosheets produced by liquid exfoliation of
layered materials. Science. 331: (80) 568–571.
[10] Zhang, X., H. Xie, Z. Liu, C. Tan, Z. Luo, H. Li, et al. 2015. Black
phosphorus quantum dots. Angew. Chemie. Int. Ed. 54: 3653–3657.
[11] Das, R. and X. He. 2018. Flexible, Printed and organic electronics 2019-
2029: forecasts, players & opportunities. IDTechEx.
[12] Liu, D. and G. Hong. 2019. Wearable electromechanical sensors and its
applications, in wearable devices—the big wave of innovation. Intech.
Open 13.
[13] Yin, B., X. Liu, H. Gao, T. Fu and J. Yao. 2018. Bioinspired and bristled
microparticles for ultrasensitive pressure and strain sensors. Nat.
Commun. 9: 5161.
[14] Amjadi, M., A. Pichitpajongkit, S. Lee, S. Ryu and I. Park. 2014. Highly
stretchable and sensitive strain sensor based on silver nanowire–
elastomer nanocomposite. ACS Nano 8: 5154–5163.
[15] Boland, C.S. 2019. Stumbling through the research wilderness, standard
methods to shine light on electrically conductive nanocomposites for
future healthcare monitoring. ACS Nano 13: 13627–13636.
[16] Yang, Y.F., L.Q. Tao, Y. Pang, H. Tian, Z.Y. Ju, X.M. Wu, et al. 2018. An
ultrasensitive strain sensor with a wide strain range based on graphene
armour scales. Nanoscale 10: 11524–11530.
[17] Cai, Y., J. Shen, G. Ge, Y. Zhang, W. Jin, W. Huang, et al. 2018. Stretchable
Ti3C2Tx MXene/Carbon nanotube composite based strain sensor with
ultrahigh sensitivity and tunable sensing range. ACS Nano 12: 56–62.
[18] Ma, Y., Y. Yue, H. Zhang, F. Cheng, W. Zhao, J. Rao, et al. 2018.
3D synergistical mxene/reduced graphene oxide aerogel for a
piezoresistive sensor. ACS Nano 12: 3209–3216.
[19] Pan, L., A. Chortos, G. Yu, Y. Wang, S. Isaacson, R. Allen, et al. 2014.
An ultra-sensitive resistive pressure sensor based on hollow-sphere
microstructure induced elasticity in conducting polymer film. Nat.
Commun. 5: 3002.
[20] Nakata, S., M. Shiomi, Y. Fujita, T. Arie, S. Akita and K. Takei. 2018. A
wearable pH sensor with high sensitivity based on a flexible charge-
coupled device. Nat. Electron. 1: 596–603.
[21] Hille, P., R. Höhler and H. Strack. 1994. A linearisation and compensation
method for integrated sensors. Sens. Actuators, A: Phys. 44: 95–102.
[22] Cheng, Z., Q. Li, Z. Li, Q. Zhou and Y. Fang. 2010. Suspended graphene
sensors with improved signal and reduced noise. Nano Lett. 10: 1864–1868.
[23] Hasegawa, Y., C. Okihara, T. Yamada, K. Komatsubara, K. Iwano,
Y. Sakai, et al. 2017. Smooth-surfaced flexible wall shear stress sensor
fabricated by film transfer technology. Sens. Actuators, A: Phys. 265: 86–93.
[24] Yu, J., X. Tang, H. Su and Z. Zhong. 2020. A self-biased linear anisotropic
magnetoresistance sensor realised by exchange biased bilayers. J. Magn.
Magn. Mater. 493: 165695.
[25] Knobelspies, S., B. Bierer, A. Daus, A. Takabayashi, G.A. Salvatore,
G. Cantarella, et al. 2018. Photo-induced room-temperature gas sensing
with a-IGZO based thin-film transistors fabricated on flexible plastic
foil. Sensors 18: 358.
[26] Bhatt, V., S. Joshi, M. Becherer and P. Lugli. 2017. Flexible, low-cost
sensor based on electrolyte gated carbon nanotube field effect transistor
for organo-phosphate detection. Sensors 17: 1147.
[27] He, H., Y. Fu, W. Zang, Q. Wang, L. Xing, Y. Zhang, et al. 2017. A
flexible self-powered T-ZnO/PVDF/fabric electronic-skin with multi-
functions of tactile-perception, atmosphere-detection and self-clean.
Nano Energy 31: 37–48.
[28] Lee, S.M., K.S. Sim, K.K. Kim, Y.G. Lim and K.S. Park. 2010. Thin and
flexible active electrodes with shield for capacitive electrocardiogram
measurement. Med. Biol. Eng. Comput. 48: 447–457.
Chapter 7
The Transition from

Pb- to Pb-Free Halide-Based
Perovskite Inks for
Optoelectronic Applications
Sonali Mehra1,2, A.K. Srivastava1,3 and

Shailesh Narain Sharma1,2*
1
CSIR- National Physical Laboratory,
Dr. KS Krishnan Marg, New Delhi - 110012, India.
2
Academy of Scientific and Innovative Research (AcSIR),
Ghaziabad - 201002, India.
3
CSIR- Advanced Materials and Processes Research Institute,
Bhopal - 462026, India.
7.1 IntroductIon
OIMH perovskites are highly emphasized instead of their characteristic
properties of wide absorption-coefficient, band gap tunability, high
carrier mobility, longer carrier diffusion length, high charge transport
properties, and weak exciton binding. OIMH perovskites are composed
of the general formula ABX3, which is composed of three different
species, where A is an organic and inorganic monovalent cation [for
*For Correspondence: Shailesh Narain Sharma (shailesh@nplindia.org)

The Transition from Pb- to Pb-Free Halide-Based Perovskite Inks for . . . 155
instance, methylammonium (CH3NH3+), formamidinium (CH3(NH3)2+)

and cesium (Cs+)], B is a divalent cation [for instance lead (Pb2+), tin (Sn2+),
germanium (Ge2+), and bismuth (Bi2+)], and X is a monovalent halide
anion (for instance, Cl–, Br–, I–) [1]. These halide perovskites are known
for their diverse applications in solar harvesting, lasing, quantum
dots, light emission, water splitting, and thin-film electronics [2].
The properties of these perovskite materials can vary by modifying
A, B, or X site in the ABX3 perovskite structural lattice. Generally,
perovskites possess a cubic structure consisting of a closely-packed
sub-lattice of AX3 with divalent B-site cations in the six-fold coordinated
cavity. B-X bonding of perovskite rules over the electronic behavior of
perovskite semiconductors, while A cation has no direct rule for the
electronic properties and its size leads to symmetry disruption of the
material [3]. Perovskites also possess cubic, tetragonal, and orthorhombic
phases. Even organic-inorganic perovskite materials show different
optical energy gaps than their bulk counterpart because of the quantum-
confinement effect [4].
Other than the properties mentioned above, perovskite materials
are easily solution-processable, making them a potential candidate for
optoelectronic applications with enhanced power conversion efficiency.
Perovskites emit light very strongly that can be tuned in visible to IR
region, which makes them ideal for making LEDs and other optoelectronic
applications. Perovskite inks have a significant role in displays and solid-
state lightning with improved efficiency and advantages, such as color
gamut and low material cost [5].
Despite the high performance, OMH perovskites are facing stability
issues. These OMH perovskites possess very low thermal decomposition
temperatures due to unstable organic monovalent cations, such as
MAPbI3 materials that undergo poor water, oxygen, and thermal stability
and decompose rapidly within 30 minutes at 150 °C in the air. At the
same time, α-FAPbI3 is comparatively more stable, and its black phase
is stable only at a temperature of more than 160 °C, while it forms a
yellow non-perovskite d phase at room temperature or below the phase
transition temperature [6].
The mixed-halide (Br– and I–) perovskites show a high degree of
segregation under the illumination of the full sun, which leads to poor
light stability, but this issue can be minimized by incorporating small
inorganic cesium (Cs) ions. Other than decomposition temperature,
moisture [7], light-induced [8] trap-state formation, and halide
segregation are also considered essential degradation factors for the
OMH perovskites. While inorganic materials possess higher stability than
organic materials, specifically at higher temperatures, thus substituting
the inorganic cations in place of organic cations in the perovskite lattice
is the best way to increase the stability of perovskites.
However, due to high toxicity and low stability, the inorganic lead
(Pb) cation is getting replaced by another cation of equivalent electronic
configuration. Here, the main disadvantage of Pb perovskites is their
degradation in the presence of water leading to the formation of soluble
PbI2. Soluble PbI2 is highly toxic to humans and the natural environment [9]
because it gets consumed into the human blood, causing harmful
impacts on the central nervous system, cardiovascular system, kidneys,
and immune system. Due to high levels of Pb-toxicity, nowadays, Pb-free
perovskites are an emerging field in optoelectronics [10–12]. Thus, the
need to develop alternate Pb-free perovskites arises [13, 14], such as
CsSnI3, MASnI3, FABiI3, FABiICl2, FABiClBr2, etc. [15]. The non-toxic
and environmental-friendly perovskites can be synthesized by replacing
Pb with non-toxic elements [16, 17], which can be achieved using the
following two techniques:
1. Homovalent substitution
2. Heterovalent substitution
Homovalent Substitution
Homovalent elements with +2 stable oxidation-states are the best
alternative for Pb-free perovskite materials. For instance, Sn2+ and Ge2+
also belong to group 14 as of Pb2+, and they are the best alternate
for Pb-substitution. Besides, transition metals, such as Cd 2+, Mn 2+,
Fe2+, Cu2+, and Zn2+, alkaline-earth metals, such as Ba2+, Sr2+, and Ca2+,
and rare-earth elements, such as Eu2+ and Yb2+, could be advised for
Pb-free perovskites [16], while following tolerance factor calculations
Pb-substituents were perfect for perovskite structure. However, some
elements such as Ba2+, Sr2+, and Ca2+ have a larger band gap, which makes
them unsuitable for semiconducting materials. In contrast, working with
Cu2+ and Zn2+ in ambient conditions per the perovskite requirement
is not easy. Thus, Sn2+ and Mn2+ are the most favorable candidate for
homovalent substitution in perovskite quantum dots.
Heterovalent Substitution
This is a second feasible substitution method for Pb-free perovskite
materials. Here, the Pb2+ can be replaced with a cation in different
valence states, e.g., mono-, tri, or tetra-valent cation as well as the direct
substitution with heterovalent-cations is viable due to their existence
in different valence states. Thus, two approaches for heterovalent
substitution can be given:
(i) Mixed-valence approach, where the equal number of mono- and
trivalent cations give an average valence state of +2; for instance,
Pb2+, thallium, and gold-halide perovskites.
(ii) Heterovalent substitution of Pb2+ with trivalent cations like Sb3+

and Bi3+. It is also accompanied by the appreciable variation in
perovskite structure from ABX3-type to A3B2X9-type to maintain
the charge-neutrality [14, 18].
In spite, organic halide perovskites (OHP) also faced photo and
thermal instability issues due to the presence of organic groups, which
were sensitive to oxygen and moisture and sustained the particular
environment restrictions for storage, fabrication, and device operation
using these perovskite inks. Thus, the environmental stability and the
stability of all these OHP in electrical and optical properties can be
improved by substituting the organic methylammonium (MA) group
with inorganic cations, like cesium (Cs+), and formamidinium (FA+) by
surface modification with moisture tolerant molecules, by integrating
OHP with polymers, and by substituting organic carrier-transport
layers with inorganic oxides. However, replacing these organic cations
with inorganic cations leads to the formation of inorganic halide
perovskites (IHP) [19, 20]. However, various methods were used to
improve efficiency. However, still, the low stability of organic groups
in metal-halide perovskites (MHP) hinders the commercialization of
perovskite LED devices, as shown in Figure 7.1. So, the low stability of
perovskite LEDs is attributed to the intrinsic instability of the operation
of PeLEDs as well as of MHP materials. Thus, the inorganic materials in
these IHP have a higher melting point (>500 °C) and improved photo-
stability, which makes them a promising candidate for optoelectronic
applications in LEDs. According to previous literature, these perovskite
inks possess enhanced photoluminescence quantum yield (PLQY),
narrow line width, wide color gamut, and PL emission in the broad
spectral region. However, as in the case of CsSnI3, Sn2+ can be easily
oxidized into Sn4+ [21] and its maximum reported PLQY is below 1%, but
Cs2SnX6 perovskite possesses good stability compared with other IHP
but still suffers from high PLQY. However, for IHP, Mn2+ can be doped
into Pb-based perovskites, but until Pb2+ is replaced entirely, it can only
attain half PLQY in comparison to other Pb-based perovskites [22–25].
Similarly, bismuth (Bi2+) and lead (Pb2+) are adjacent periodic elements of
period-6, and Bi is not as toxic as Pb. Here, Bi (6s2 and 6p3) has a similar
state of lone pair 6s2 as that of Pb (6s2 and 6p2) and exhibits the same
electronic properties and comparable energy levels with Pb. Bi-based
perovskites show a different configuration than Pb-based as A3Bi2X9
configuration to balance the charge, and Bi3+ produces a layered form
of vacancy-ordered perovskite unit cell with 2/3rd of octahedral position
fully occupied. It results in the variation of 3D to 2D crystal dimensions,
while OIH Bi-based perovskites MA3Bi2X9 QDs have a 2D structure
and exhibit good ethanol stability and PLQY of 12% with emission at
423 nm in the blue region. On the other hand, air-stable inorganic
Bi-based Cs3Bi2Br9 perovskites exhibit the emission in blue region at

410 nm with PLQY of 19.4%, but still, this PLQY value in Bi-based
perovskites is lower as compared to Pb-based perovskites [25]. Thus,
it is observed that substituting lead with Sn 2+, Mn 2+, Zn 2+, and Bi2+
homovalent cations reduces the toxicity of perovskite material and helps
enhance the optical performance of LEDs. Pb-free perovskite quantum
dots (PeQDs) based LEDs were useful for environment-friendly and
highly performing Pe ink-based LEDs. Table 7.1 gives the comparison
between inorganic metal-halide perovskites in respect of their shape
and emission spectra.
It was observed that these hybrid Pb-free OIMH perovskite inks

are a far better option than conventional Pb-based perovskite quantum
nanocrystals [26]. Due to these unique optical properties, these perovskite
inks are observed to be a suitable candidate for advanced PL-based
light-emitting devices with high efficiency, high color purity, and broad
wavelength tunability. Generally, all compounds with ABX3 geometry
cannot be defined as a perovskite; the criteria for the perovskite structure
are as follows:
1. Charge-neutrality: Here, cations and anions charge would be
equal. For ABX3 structure, A and B are monovalent (+1) and
divalent (+2) cations, respectively (total= +3), while anion-X has a
–3 charge, which implies that both the cations and anions have
equal charge [27].
2. Goldschmidt tolerance factor, t and Octahedral factor, m [28].
Here, t and µ are important factors that decide the existence and
structure of perovskite material [29, 30]. t and µ are defined according
to the given equation:
R + RX R
t= 1 A and m = B
RX
2 2 ( RX + RB )
R A, RB, and RX denote the ionic radii of A, B, and X ions in the above
equation [28, 31, 32].
To maintain the symmetry of the perovskite structure, the tolerance
factor and octahedral factor values should lie between the given limit:
0.8 < t < 1.1 and 0.442 < m < 0.895 [33, 34]. Or else, the cubic structure
of perovskite will be exhausted, and as the Cs+ ion complies well with
the requisites of t; thus, it is the most suitable alternate for replacing
MA+ (inorganic metal halide) with the inorganic monovalent cation
[35]. Therefore, the size of A, B, and X ions play a significant part in
the perovskites material, and the t factor is also essential to determine
the Pb-free perovskites material depending on the ionic radius of the
implicated ions [36–39].
7.2 SyntheSIS MethodS
Generally, the hybrid OIMH perovskites were synthesized using a

colloidal or solution-reaction process that exhibits high crystallinity and
quantum-confinement effect [40]. These colloidal processes generally
used for the synthesis of perovskites include:
7.2.1 Ligand-Assisted reprecipitation Method (LArP)

In the LARP technique, two precursor solutions were mixed, which
induces supersaturated precipitation at room temperature and is poured
into a non-polar or wrong solvent, such as toluene (as shown in Fig. 7.2); the
resulting precipitate will not get dispersed into the non-polar solvent [41].
For instance, to synthesize MAPbBr3 QDs [42], the precursors lead
bromide, methylammonium bromide, n-octylamine (OM), and oleic
acid (OA) were mixed and dispersed in dimethylformamide (DMF) to
obtain a clear precursor solution; an optimized quantity of this solution
was further added to toluene with vigorous stirring, thus obtaining
a yellowish-green colloidal solution which indicated the formation of
nanoparticles. However, as-synthesized MAPbBr3 perovskites have a
longer recombination lifetime [43], which is associated with surface-
passivation using octyl ammonium bromide capping-ligand and reduced
defect density in the nanoscale single crystals. Even MAPbBr3 perovskites
became resistant to moisture using long-chain capping ligands, which

helps reduce the probability of water-induced degradation. We can also
synthesize colloidal MAPbX3, CsPbX3, and FAPbX3 using the LARP
technique [26, 44] but after certain modifications in a process. Here, we
work to avoid using organic solvents, like oleic acid [45] and octadecene
(ODE), because excessive use of these organic solvents will cause
difficulties in charge transport mechanism during device fabrication.
Thus, the LARP method is suitable for introducing good solvents into
the wrong solvent and obtaining clear solutions. This LARP technique
is easily process-able, producing QDs with high luminescence and QY
at room temperature. But, the QDs synthesized by the LARP method
were not easy to separate from the colloidal solution because of their
non-stability in various non-polar solvents like DMF, ethanol, methanol,
etc., that show lower QY and also causes hindrance in the performance
of the photoelectric devices [45].
Figure 7.2 Ligand-assisted reprecipitation method (LARP).
7.2.2 hot-Injection Method with centrifugation or

Solvothermal Synthesis
In the hot-injection method, Pb-based and Pb-free perovskite QDs were
synthesized with a high degree of compositional band gap engineering.
Figure 7.3 shows the schematic for Hot-injection method, where the
Cs-oleate precursor was injected into the precursor solution of PbX2
containing the hot, high boiling point solvents under an optimized
temperature and inert-gas atmosphere using OA and OM as the capping
ligands to dissolve the Pb-halide sources and to stabilize the QDs [3, 46, 47].
Then, the colloidal inorganic perovskite QDs can be synthesized by
cooling the solution in an ice-water bath. In this rapid process, the
reaction temperature and halide elements are major controlling factors

for tuning the emissive color and size of QDs. These QDs crystallize
in the cubic phase rather than the tetragonal or orthorhombic phase
at high temperatures. After synthesizing perovskite QDs, these QDs
were purified by centrifugation at 8,000 rpm for 10 minutes to remove
the unreacted precursors. Further, these precipitates were re-dispersed
in hexane and again centrifuged at a speed of 3,000 rpm to eliminate
larger particles [48]. These QDs show bright PL with higher QY and
long-term stability.
Figure 7.3 Hot-Injection Method.
Similarly, in a solvothermal reaction, cesium carbonate, lead halide,

octadecene, oleic acid and octyl-amine were added in an autoclave
and put in the oven for 30 minutes to complete the reaction process.
The obtained product was washed with hexane and collected by
centrifugation for further characterization. This method is known for
controlling the size of QDs because this method is characterized by long
reaction times [49]. So, to obtain the homogeneous QDs, the particles
were purified and segregated to remove the large particles, which
generated a tremendous amount of waste-product in this technique. Due
to various characteristics, such as easy-setup, controlled morphology
and composition, high uniformity, and high crystallinity of products
over other methods, this method is widely used for the preparation of
perovskites NCs [50].
It can be concluded that these techniques can synthesize perovskite
QDs to increase mass production quickly as LARP and emulsion routes
operate at room temperature owing to the easy synthesis technique
and small reaction-time. Although in the LARP method, due to low

temperature, QDs synthesized by this method are of low crystallinity
and stability. On the other hand, QDs synthesized using the hot-
injection method possess comparatively high crystallinity because of
high reaction temperature and uniform particle size. Therefore, ignoring
the complicated sequence, the hot-injection technique is known as the
most specific and reliable method for synthesizing perovskite QDs.
Table 7.1 Comparison between different inorganic metal-halide perovskites
to their shapes and emission spectra [4]
S.No. Materials Shapes PL Peaks PLQY (%) References
(nm)
1. CsPbX3 Nanocrystals 410–700 90 [51]
2. CsPbX3 Quantum dots 410–700 72 [34]
3. CsPbBr3 –CsPb(Br/I)3 Nanocrystals 520–580 75 [52]
4. CsPbX3 Quantum dots Blue-red 70–95 [53]
5. CsSnX3 Nanocrystals 607–696 ~ [54]
6. CsPbBr3/CdS Quantum dots 514 88 [55]
7. CsPbX3 Nanorods 505 34 [56]
Entire visible
8. CsPbX3 Nanowires 20–80 [49]
spectrum
9. CsPbBr3 Nanowires 442 30 [34]
10. CsPbBr3 Nanoplates 452 33 [57]
11. CsPbBr3 Nanocrystals 390–660 1–78 [58]
12. CsPbI3 Nanocrystals 648–692 ~ [59]
13. CsPbCl3 Nanowires 410–460 ~ [60]
7.3 A BrIef revIew of the work ALreAdy

BeIng done
Pal et al. [61] reported the colloidal synthesis of Pb-free Cs 3Sb2I9
nanoplatelets (NPLs) and Rb3Sb2I9 nanorods (NRs) for the first time
along with the characterization of their crystal structure, morphological
characteristics, capping ligands, and thermal stability for the optimization
of their properties. He observed that these Pb-free perovskites have
similar optical and optoelectronic properties as that of colloidal CsPbX3
nanocrystals (NCs). Nevertheless, Sb-halide perovskite NCs are more
prone to defects than Pb-halide-based perovskite NCs (PeNCs) within
the band gap. Thus, the defect chemistry of Sb-halide-based PeNCs
should be controlled for comparison with Pb-halide-based PeNCs.
Protesescu et al. [51] studied the synthesis of monodisperse CsPbX3
nanocrystals in the size range of 4–15 nm possessing a cubic shape
of the perovskite crystal structure. These nanocrystals exhibit size-
tunability of band gap in the visible spectral range from 410–700 nm
because the exciton Bohr diameter ranges up to 12 nm. Here, the

size of these nanocrystals can be varied from 4 nm to 15 nm using
a reaction temperature of 140–200°C instead of the growth-time. As
(Cl/Br)3 and (Br/I)3 mixed-halide perovskites were easily synthesized
using the optimized amount of PbX2 salts, while (Cl/I)3 perovskites
cannot synthesize with much ease due to the significant difference
in the ionic radius of Cl– and I– ions that also correlates with phase
diagram of bulk materials. CsPbX3 materials are ionic, stoichiometric
and highly-arranged due to the difference in the size and charge of Cs+
and Pb2+ ions. These obtained chalcogenide QDs are bright (50–90%),
stable, spectrally-narrow, and broadly-tunable photoluminescent for
optoelectronic applications.
Jellicoe et al. [54] stated the synthesis of perovskite CsSnX3 (X = I, Br,
Cl, Br/I, and Cl/Br) NCs and studied the tunability of optical band
gap in NIR and visible spectral region using quantum-confinement
effect and varying the halide composition in perovskite lattice. This
research elaborated on the various techniques to produce mixed-halide
perovskites, either by synthesizing them using different halide sources
or by using a post-synthesis anion exchange approach through mixing
as-synthesized pure halide perovskite particles. However, the band gap
of Sn-containing PeNCs shows the red-shift compared to lead-based
perovskites due to higher electronegativity of Sn-ion in place of Pb-ion
at B-site in ABX3 perovskite structure.
Shen et al. [62] studied the synthesis of Pb-free FA3Bi2Br9 PeQDs
via the ligand-assisted reprecipitation (LARP) technique. These PeQDs
exhibit bright blue-emission at 437 nm with PLQY of 52%, while Pb-free
Sn and Bi-halide-based (Cs3Bi2Br9 and MA3Bi2Br9) PeQDs suffer from
low PLQY. This configuration of Bi-based perovskites maintains charge
balance because Bi3+ produces a layered form of vacancy-ordered
perovskite unit cell with 2/3rd of octahedral position fully occupied,
which leads to the reduction from 3D to 2D crystal dimensions. Due
to the Br-rich component, these QDs possess good air stability and
exceptional ethanol stability, although the PLQY value of FA3Bi2Br9 PeQDs
is highest compared to the other Pb-free (Cs3Bi2Br9 and MA3Bi2Br9) and
Pb-based perovskites in the blue-emitting region. The primary reasons
for the high PLQY are the very low defect density of FA3Bi2Br9 QDs
because of ligand surface passivation that leads to the suppression of
non-radiative-recombination channels. Moreover, high exciton binding
energy (BE) of materials enable efficient radiative recombination of
excitons leading to enhanced PLQY. These QDs show a PL emission in
the 399 to 526 nm range and a direct band gap of 2.84 eV that coincides
nicely with the experimental data. These FA3Bi2Br9 QDs with bright blue
color lead to the fabrication of LED with promising applications in blue
light emission.
Song et al. [63] reported the synthesis of all-inorganic CsPbX3 (X =

I, Br, Cl) PeQDs with sharp emission wavelength (FWHM < 30 nm). In
these high-quality PeQDs, the luminescence wavelength and color of
QDs from blue to orange can be tuned with different sized QDs and
varying the halide composition (I, Br, Cl). They can easily dissolve in
different non-polar solvents (toluene, octane, and hexane) that serve
as ink for solution-based optoelectronic devices. The inorganic PeQDs
can be stored for a more extended period of >60 days, which exhibits
higher stability than OI MAPbX3 PeQDs. These all-inorganic PeQDs
exhibit single-crystal structure, good dispersity, and high-ink stability,
making them suitable for low-cost, solution-processed, and flexible
optoelectronics. Thus, due to high PL efficiency and the whole visible
wavelength range, unlike organic-inorganic QDs, all-inorganic PeQDs
are a potential candidate for LED devices.
In summary, Table 7.2 gives a detailed view about the metal-halide
perovskites along with their synthesis method, principal solvent and
conclusive findings.
7.4 gLIMPSe on devIce fABrIcAtIon

The high performance of halide perovskite (HP) LEDs is assigned to
the intrinsic properties of HP inks, such as low defect density, high
crystallinity, high absorption, high PLQY, and efficient charge transport.
In device fabrication using these perovskite inks, the critical parameters
that determine the performance of LEDs are external quantum efficiency
(EQE), current efficiency (CE), turn-on voltage (Von), maximum luminance
(Lmax), and stability. Here, these parameters can be calculated as follows:
EQE = IQE.h
where, IQE is the internal quantum efficiency.
Η is the fraction of photons emitted to free space.
P
PE =
IV
where, P = Power emitted into free space
L
CE =
J
where, L = Luminance of LEDs
J = Current density
It also helps to feature the effect of dimensional evolution on the
optical and electrical properties of HP materials and their performance
in LEDs. The robustness of highly luminescent properties and effective
The Transition from Pb- to Pb-Free Halide-Based Perovskite Inks for . . .
Table 7.2 Metal-halide perovskites along with their synthesis method, principal solvent, and conclusive findings
S.No. Pb-Based Pb-Free Technique Principal Solvent Results/Conclusions References
Perovskite Perovskite Used
1. — Cs3Sb2I9 Hot-Injection Octadecene, • Have potential for optical and optoelec- [61]
Rb3Sb2I9 Oleylamine, and tronic perovskites.
Octanoic acid • Improved defect chemistry is required
for Sb-halide PeNCs in comparison to
Pb-halide PeNCs.
2. CsPbX3 — Hot-Injection Oleic acid, • The combination of optical properties [51]
(X = I–, Br–, Cl–, Octadecene, and and chemical robustness makes CsPbX3
Cl–/Br– or Br–/I–) tri-octyl-phosphine NCs the best candidate for optoelectronic
applications in the green and blue spectral
region (410–530 nm).
3. — CsSnX3 Hot-Injection Oleic acid, • Here, the replacement of Pb with Sn [54]
(X = I–, Br–, Cl–, Octadecene, and reveals their spectral tenability via the
Cl0.5Br0.5, Br0.5I0.5) Tri-octyl-phosphine quantum-confinement effect and varying
the halide ions.
4. — FA3Bi2Br9 Ligand-Assisted Toluene, N, N-DMF • As-synthesized FA3Bi2Br9 QDs exhibit high [62]
Reprecipitation PLQY of over 50% in the blue-emitting
(LARP) region, and these quantum-confined QDs
exhibit high exciton BE leading to the
generation of excitons and their high-rate
recombination.
5. MAPbI3/ — Hot-Injection Oleic acid and • Phase transitions lead to the variation [64]
MAPbBr3 Oleylamine in the band-width and peak positions of
the MA-cage vibrations and some bands
associated with the NH3+ group.
(Contd.)
165
Table 7.2 Metal-halide perovskites along with their synthesis method, principal solvent, and conclusive findings (Contd.)
166
S.No. Pb-Based Pb-Free Technique Principal Solvent Results/Conclusions References
Perovskite Perovskite Used
6. CsPbX3 — Hot-Injection Oleic acid, • These all-inorganic PeQDs can be stored [63]
(X = Cl–, Br–, I–) Oleylamine, and for more than two months and exhibit
Octadecene higher stability than MAPbX3PeQDs, and
when they disperse in various non-polar
solvents can serve as ink for solution-
based optoelectronic devices.
7. MAPb(IxBr1-x)3 — Thin-film N, N-Dimethyl- • Mixed-halide perovskites are more critical [65]
growth method formamide than pure-phase perovskites because band
gaps of mixed lead-halide perovskites
are tuned by halide composition, which
is the most optimum characteristic for

optoelectronic applications.
8. MAPbX3 (X= Br–, — Aqueous Phenylalanine • MAPbX3 Perovskite nanocrystals have been [66]
Cl–/Br–) method (PLLA) and synthesized using an aqueous method,
Di-dodecyl- which identifies the influence of the
dimethyl- Pb-halide complex, pH-value, and ligands
ammonium on the formation of these nanocrystals.
bromide (DDAB)
9. — Cs3Bi2Br9/Cs3Sb2Br9/ Ligand-Assisted Dimethylformamide • Cs3Sb2Br9 exhibits a relatively high [13, 67, 68]
(CH3NH3)3Bi2X9 Reprecipitation or Dimethyl- PLQY of 46% where it was shelled with
(LARP) method sulfoxide and a Br-rich surface, and high exciton BE
octane with oleic contributes to enhanced quantum yield of
acid material. While PLQY of (CH3NH3)3Bi2X9
nanocrystals improved and reported
values of 0.03–15% and 0.018–26.4%.
10. — (CH3NH3)2MnCl4 Single-Crystal Dimethyl- • (CH3NH3)2MnCl4 single crystal shows [69]
Growth Method formamide and enhanced luminescence properties leading
Dimethyl-sulfoxide, to red light emission upon excitation at 417
Hydrochloric acid nm based on the purple light absorption
for white light illumination.
electrical injection and transportation features of QD films were assumed

to achieve high exciton recombination efficiency in devices. Here, the
device fabrication consists of multiple layers arranged in the following
sequence: ITO, PEDOT-PSS (40 nm), PTAA (40 nm), perovskite QDs ink
(40 nm), TPBi (40 nm), and LiF/Al (1/100 nm). The HTL layer of PTAA
used exhibits high hole mobility [70, 71], which helps to achieve high
efficiency. The electron transport mechanism and device structure in
perovskite-based devices is shown in Fig. 7.4
ITO-coated glass substrates were cleaned following a sequence using
ultra-sonication in acetone and IPA for 10 minutes each. Firstly, the HTL
layer of PEDOT: PSS is deposited on a glass substrate via spin-coating at
5,000 rpm for 40 seconds. It was followed by annealing the substrate for
15 minutes @ 130°C. Then, the active layer of perovskite NCs dispersed
in toluene (20–40 mg/ml) was deposited by spin-coating on PEDOT: PSS
at 1,000 rpm for 30 seconds in the glovebox and annealed at 50°C for
2 minutes. Further, the ETL SPB-02T dispersed in chlorobenzene (0.4 wt.%)
was spin-coated at 2,000 rpm for 45 seconds. Lastly, thermal evaporation
deposited LiF (1 nm) and silver (100 nm).
HTL
Anode e-
Energy
e-
e-
h+ QD
Cathode
ETL
h+
h+
Figure 7.4 Electron transport mechanism in Perovskite-based devices.
For instance, in a device fabrication of CsPbBr3 and FA-doped CsPbBr3,

the current-voltage (I–V) curve of the device was measured where the
controlled devices (without FA cation) showed higher current [72].
After introducing the FA+ dopant, the devices’ current reduces slightly,
leading to the suppression of charge in balance. The voltage-dependent
luminance of QLEDs shows a minor increase in turn-on-voltage after
incorporation of FA+, which ascribes to the deeper valence band of
FA-doped CsPbBr3 (5.96 eV) compared to CsPbBr3 (5.67 eV). However,
the QLEDs based on FA-doped CsPbBr3 QDs possess high luminance
compared to pure CsPbBr3 [34, 73]. For the device, internal quantum
efficiency (IQE) can be calculated as follows:
IQE = 2n2EQE
Here, EQE is external quantum efficiency, and n is the refractive
index of the glass substrate.
An excess amount of FA+ cation mixed in equal stoichiometry with

CsPbBr3 shows decreased LED performance which attributes to the poor
morphology of QD films originating from poor inks, and these devices
exhibit high reproducibility for future practical applications [74].
Thus, these CsPbBr3 QD inks can be referred to as ideal emitters
with single radiative decay in the intrinsic channel synthesized through
synergistic ligands [75, 76] at room temperature in the open air.
Although, QD films with few non-radiative-recombination centers show
improved PL efficiency after introducing a small amount of FA+ dopant.
It also demonstrates that the CsPbBr3 QDs-based electroluminescent
LEDs show a peak EQE of 11.6% and peak CE of 45.4 Cd/A, which is
the most significant value for perovskite QLEDs [77, 78]. The proposed
simple and easily accessible synthesis method is used to scale up the
QDs production without any noticeable change in material properties
on the device’s performance, which holds excellent commercialized
industrial applications [74].
7.5 ProPertIeS of PerovSkIte MAterIALS

Swarnkar et al. stated [79] the synthesis of stable cubic phase QDs
of CsPbI3 QDs by transforming their synthesized cubic phase to
orthorhombic. Core-shell structures are the best alternative to enhance
the optical properties and stability of all-inorganic MHP nanomaterials
(NMs) [13]. Luo et al. synthesized [80] the CsPbBr3/Cs4PbBr6 core-
shell structure nanocrystals using a microchannel reactor and these
nanocrystals retain more than 90% of PL intensity after 84 days. While
Tang et al. stated the synthesis of CsPbBr3/CdS core-shell structured QDs
by hot-injection method [55]. Chen et al. adopted an ion-doping strategy
for improving the stability and efficiency of as-synthesized CsPbI2Br and
related semiconductor devices [81]. Zhao et al. [59] synthesized CsPbI3
nanocrystals with variation in size distribution and also studied the
perovskite structure and confinement property of these PeNCs. Thus,
it is summarized that the perovskite NC structure can more effectively
improve stability as compared to thin-film perovskite materials. It is
also important to discuss the excellent optical properties of AIMHP
concerning bulk materials, including the following three characteristics:
7.5.1 Strong Quantum-Confinement Effect

The principal property of nanomaterials is that quantum conversion
efficiency (QCE) was taken under consideration among other optical
properties of materials when the size of an AIMHP is tiny as compared
with Bohr radius of excitons where excitons were confined in three
spatial dimensions leading to a transition from continuous to discrete

energy levels. For instance, the exciton Bohr radius of inorganic CsPbBr3
perovskite nanocrystals (PeNCs) is approx. 7 nm theoretically, and the
QCE is an essential factor in these PeNCs when their size is compared
with the exciton Bohr radius [82]. So, if the emission band gap of CsPbBr3
perovskite NCs is tuned from 2.7 to 2.4 eV with a change in particle
size from 4 to 12 nm, theoretically, thus it shows well-agreement with
the quantum-confinement effect [58, 82, 83].
7.5.2 A wider range of optical Properties

Here, the band gap and optical properties can be tuned to the size
of AIMHP nanomaterials. The characteristic property of AIMHP is
its visible range emitting tunable wavelength [73] by controlling the
material composition, dimensions, and structure. As the emission range
of all-inorganic perovskite (CsPbX3 and other mixtures) QDs, quantum
wells (QWs), and nanoplatelets (NPLs) lies in the whole visible range
of 400–700 nm (blue to red) that can also be tuned to NIR or UV
range [84–86] via particle replacement method and new device structure
for display applications [87].
7.5.3 High Quantum Efficiency

Quantum efficiency can be defined as the ratio of the number of
converted photons to absorbed photons, and it is a well-known
characteristic property of light emitters [80, 88–96]. High quantum
efficiency denotes that more absorbed photons were converted via
the radiative-recombination process instead of the non-radiative-
recombination process. Perovskites are also known as exceptional light
emitters because of their high absorption coefficient and quantum
efficiency [93, 95]. The high quantum efficiency in perovskites is due to
a clear band gap with negligible charge-trapping states promoting the
exciton radiative-recombination efficiency [97, 98], although these unique
properties of AIMHP nanomaterials still require some essential aspects
for improving the performance of the perovskite-based devices:
(i) Understanding physics in low-dimensional perovskites:
Generally, low-dimensional perovskites are not very popular,
and there is minimal research on these perovskites. However,
this low-dimensionality of perovskites could bring restricted
charge transport in an appropriate direction, which forms the
basis for field-effect devices showing various unique electrical
properties, such as 2D electron gas, etc. However, these studies
are lagging for various reasons and will be under discussion in
the upcoming years [99–101].
(ii) Precisely control in size and dimensions: To date, the optimistic

control on size and dimension for band gaps is not attracting much
attention but still, some synthesis methods of perovskites with
low dimensions were tried for the development of perovskites.
At the same time, size control and low-dimensionality are
critical factors in inducing the morphology and applications of
as-synthesized perovskites [102, 103].
(iii) Lead-free HP nanocrystals: The toxicity problem of lead (Pb)
arises from the need to replace Pb with other earth-abundant
germanium (Ge), tin (Sn), bismuth (Bi), and antimony (Sb) metal
ions. Even the development of these environment-friendly Pb-free
halide perovskites with improved optical and electronic properties
as well as excellent environmental stability makes these Pb-free
perovskites (for instance, CsSnX3, Cs3Bi2X9, FA3Bi2X9, MASnX3,
etc.) promising candidates for display applications [104, 105].
7.6 APPLIcAtIonS of MetAL-hALIde

PerovSkIteS
In general, inorganic-organic metal-halide (IOMH) perovskite

nanomaterials are a topic of discussion among researchers because
of their enhanced device performance and higher thermal stability
compared with organic metal-halide perovskite (OMHP). These NCs
and NWs utilized for nanoscale photonic, electronic, and optoelectronic
devices, such as semiconductors, LEDs, lasers, photodetectors, single-
quantum-photon sources (SPQS), etc., some of the significant applications
are discussed below [4].
7.6.1 Solar cells

High-efficiency perovskite semiconductors (PSCs) were realized
by tuning the band gap and stabilizing the black perovskite phase
of CsPbI2Br at lower temperatures. However, the power conversion
efficiency (PCE) of CsPbX3-based PSCs has enhanced rapidly from 2.9%
to 23.7% with a tremendous increase in stability [106–108], and here the
device also realized 9.8% PCE and over 5% stabilized power output [83].
Based on the slow-photon effect of carbon-QD sensitized CsPbBr3, Zhou
et al. [109] synthesized inverse opal-perovskite semiconductors (SCs),
but as compared to planar CsPbBr3, the perovskite SCs exhibit power
conversion efficiency (PCE) up to 8.29% and incident-photon-to-electron
conversion efficiency up to 76.9%. Further, researchers have adapted
the various device optimization techniques like interface engineering
(self-passivation and electron-transporting layer) and defect engineering

to improve the stability as well as QCE of PSCs [107, 110–115].
Nanocrystalline-controlled synthesis methods were mainly utilized
for improving the interfacial energy, optoelectronic and recombination
properties of devices and materials [116–118].
7.6.2 Light-emitting diodes (Leds)

The external quantum efficiencies (EQE) of the blue, orange and green
LEDs were 0.07%, 0.09%, and 0.12%, respectively. Thus, inorganic
perovskites can be used as LED-device emitters instead of low
EQE [119–122]. According to the literature, the highest EQE of the
inorganic LEDs is greater than 20% compared with organic LEDs [123].
At the same time, the operational stability of CsPbBr3 QD-based LED
devices was found to be 36 times higher due to the lower surface-ligand
density of corresponding QDs [20, 124–126]. Thus, it is advisable to
utilize the materials and devices optimization techniques for improving
the characteristics of LEDs [74, 89, 127–129]. Further, Shan et al. [130]
synthesized perovskite CsPbX3 (X = Br, Cl) QDs with halide-ion pair
ligands and using this perovskite, green and blue LEDs were fabricated,
which exhibits the higher EQE in comparison with untreated QDs. In
contrast, Song et al. [74] used a tetra butyl-ammonium bromide to reduce
non-radiative defects of CsPbBr3, promoting the electroluminescence
performance of LEDs. However, various device engineering
technology was utilized to improve the efficiency and performance of
LEDs [127–130]. Therefore, various approaches like improved hole
injection, enhanced electron injection, increase in charge balance
and decrease in charge leakage, etc., were adopted to enhance the
efficiency [131–133] and stability of perovskite LEDs.
7.6.3 Lasing
The emission-wavelength tunability illustrated in perovskite materials
was an essential property for nanoscale lasers. Generally, the lasing
can be maintained for over 1 hour on persisting on exposure to the
surrounding atmosphere signifying their improved performance
compared to hybrid perovskite NW lasers [134, 135]. For instance,
Wang et al. [136] stated the multi-photon pumped lasing from CsPbX3
nanorods with a threshold value of ~0.6 and 1.7 mJ cm–2 under an
excitation wavelength of 800 nm and 1,200 nm with 80 fs and 1 kHz as
shown in Fig. 7.5. AILHP-based temperature-dependent nanostructured
lasers were studied due to their temperature sensitivity [55, 137–139].
Thus researchers have also adopted the new material structure or device
engineering to improve the detector properties [140–145]. Pushkarev

et al. [143] reported a new method for synthesizing high-quality CsPbBr3
nano-lasers using the rapid precipitation from DMSO solution sprayed
onto hydrophobic substrates at ambient conditions. However, in-plane
self-assembly CsPbBr3 NWs and CsPbBr3 QDs, when incorporated in
a silica sphere, was the best way to improve the performance and
stability of CsPbBr3 nano-lasers [146, 147]. On the other hand, mainly
inorganic and hybrid perovskites were utilized to enhance the external
luminescence efficiency of lasers [140, 142, 145].
Figure 7.5 Lasing in single-crystal CsPbBr3 NWs were (a) dark-field images of
CsPbBr3 NWs (b-d) under excitation of NWs from femtosecond pulsed laser with
increasing excitation fluence [60].
7.6.4 Photodetectors
An ideal photodetector (PD) must have high speed, spectral selectivity,
signal-to-noise ratio, and sensitivity. PDs have diversified applications
such as imaging, optical communication, remote control, chemical/
biological sensing, etc. [139]. However, Inorganic scattered CsPbBr3
nanoplatelets-based low-voltage PDs with high-responsivity were
fabricated [148, 149]. In addition to 0D and 1D CsPbBr3 nanomaterials,

1D AILHP were also used to fabricate optical detectors with high
stability and detection rate [139, 150–155]. Thus, PDs based on 1D CsPbI3
nanorods were developed, exhibiting 2.92 × 103 AW–1 responsivity,
0.05 ms ultrafast response time, and detectivity approaching up to
5.17 × 10 3 jones [155]. Then, Sn-doped CsPbI 3 nanobelts were
synthesized, which holds stability for 15 days under air conditions.
These as-synthesized PDs of CsPb0.922Sn0.078I3 nanobelts show ultrahigh
selectivity of up to 6.43 × 1013 jones [151]. Indeed, SPPDs with no external
power source, high sensitivity, and fast-response speed an essential type
of PDs [156–160]. Various researchers have reported the synthesis of PDs
with characteristic properties [157]. However, they have still accepted
the material and its device optimization techniques like the core-shell
method, vertical Schottky junction, inverse structure, and fast interfacial
charge-transfer layer for improving the PD properties [150, 161–163].
Thus, the improved performance of PDs is assigned to improved
crystalline and fewer surface defects of AIMHP NMs.
7.6.5 In opto-electronic device

AIMHP nanomaterials were also used in various optoelectronic devices,
unlike SCs, LEDs, lasers, and PDs. For instance, the development
of polarization-sensitive and CsPbBr3 and CsPbCl3 nanocrystals-
based light sources [164]. Similarly, CsPbBr3 thin-film-based hybrid
phototransistors were reported with a responsivity of 4.9 × 106 AW–1,
fast-response of 0.45 s/0.55 s, and long-term stability of 200 h in ambient
conditions [165]. On the other hand, field-effect transistors were
developed using the CsPbBr3 nanocrystals [166–168] that exhibit unipolar
transport characteristics in a p-type mode featured well-aligned linear
and saturation regimes.
7.7 concLuSIon
This chapter gives a very conceptual summary of the transition from
Pb-based to Pb-free halide perovskite inks for optoelectronic applications.
Moreover, this chapter discusses the various synthesis methods,
characteristic properties, and applications of the Pb-based and Pb-free
perovskite inks. It also gives an insight into the organic and inorganic
constituting ions of the perovskite inks and the advantages of the hot-
injection synthesis technique over other methods for the perovskite inks.
This chapter gives a brief idea about the device fabrication using the
specific HTL and ETL for the Pb-based/Pb-free perovskite active layer
depending on the thickness of each layer required for the optoelectronic
devices using these perovskite inks. However, here we have also discussed
various important comparable properties and properties required to
enhance the performance of IOMH perovskite inks-based devices. Thus,
this chapter generally gives a detailed description of the Pb-based and
Pb-free halide perovskite inks, along with the advantages and challenges
ahead in the transition from Pb-based to Pb-free perovskite inks for
commercial and industrial applications.
7.8 chALLengeS And future ScoPe
Researchers are struggling with the stability issues of metal-halide

perovskite inks. Here, incorporating large organic cations was suggested
to improve the stability of these perovskite inks. However, by tuning
the dimensionality and crystal structure, some halide perovskite inks
have to pertain to the stability of optoelectronic applications. Moreover,
concerning the issue of lead toxicity, the development of Pb-free halide-
based perovskite inks will be the priority for researchers. It raises
the possibility of wholly or partly substituting the Pb with Sn in the
perovskite lattice because of several parameters, such as suitable ionic
radii and existence in the unstable +2 state. This substitution has
various advantages of band gap narrowing on the perovskite inks-
based materials and devices. Despite its advantages, there are still
pertinent issues that need to be addressed regarding the fabrication of
stable and high-performance Sn-based devices compared to the stability
characteristics of Pb-based devices, although working on the stability
of Sn-halide perovskite inks is a more troublesome task than Pb-halide-
based perovskite inks due to the conversion of Sn from +2 to +4 state
in ambient conditions. Indeed, there are various disadvantages of using
Sn over Pb for perovskite device fabrication due to the higher cost
and restricted production of Sn than that of Pb. In the case of Sn,
many optimizations are required for the fabrication process of Sn-based
devices. Pb-based analogues have shown remarkable properties with
optimum PCE but still work for the devices’ stability. Therefore, along
with PCE, we will work further on the stability of Pb-based and Sn-based
halide perovskite inks-based devices for optoelectronic applications.
AcknowLedgMent
The authors sincerely thank the Director, CSIR-NPL, for his constant
support and encouragement. Author SM gratefully acknowledges the
Department of Science and Technology for providing a WOS-A project
grant (WOS-A/CS-132/2018).
RefeRences
[1] Li, J., J. Duan, X. Yang, Y. Duan, P. Yang and Q Tang. 2020. Review
on recent progress of lead-free halide perovskites in optoelectronic
applications. Nano Energy 80: 105526. https://doi.org/10.1016/j.nanoen.
2020.105526
[2] Jian, W., R. Jia, H.-X. Zhang and F.-Q Bai. 2020. Arranging strategies for
A-site cations: impact on the stability and carrier migration of hybrid
perovskite materials. Inorg. Chem. Front. 7: 1741–1749. https://doi.org/
10.1039/D0QI00102C
[3] Jesper Jacobsson, T., J.-P. Correa-Baena, M. Pazoki, M. Saliba, K. Schenk,
M. Grätzel, et al. 2016. Exploration of the compositional space for mixed
lead halogen perovskites for high efficiency solar cells. Energy Environ.
Sci. 9: 1706–1724.. https://doi.org/10.1039/C6EE00030D
[4] Cao, L., X. Liu, Y. Li, X. Li, L. Du, S. Chen, et al. 2021. Recent progress in
all-inorganic metal halide nanostructured perovskites: materials design,
optical properties, and application. Front. Phys. 16: 33201. https://doi.
org/10.1007/s11467-020-1026-9
[5] Chen, P., Y. Bai, M. Lyu, J.-H. Yun, M. Hao and L. Wang. 2018. Progress
and perspective in low-dimensional metal halide perovskites for
optoelectronic applications. Sol. RRL 2: 1700186. https://doi.org/10.1002/
solr.201700186
[6] Saidaminov, M.I., J. Kim, A. Jain, R. Quintero-Bermudez, H. Tan,
G. Long, et al. 2018. Suppression of atomic vacancies via incorporation
of isovalent small ions to increase the stability of halide perovskite
solar cells in ambient air. Nat Energy 3: 648–654. https://doi.org/10.1038/
s41560-018-0192-2
[7] Yang, J., B.D. Siempelkamp, D. Liu and T.L. Kelly. 2015. Investigation of
CH3NH3PbI3 degradation rates and mechanisms in controlled humidity
environments using in situ techniques. ACS Nano 9: 1955–1963. https://
doi.org/10.1021/nn506864k
[8] Conings, B., J. Drijkoningen, N. Gauquelin, A. Babayigit, J. D’Haen,
L. D’Olieslaeger, et al. 2015. Intrinsic thermal instability of
methylammonium lead trihalide perovskite. Adv. Energy Mater 5:
1500477. https://doi.org/10.1002/aenm.201500477
[9] Lyu, M., J. Yun, P. Chen, M. Hao and L. Wang. 2017. Addressing toxicity
of lead: progress and applications of low-toxic metal halide perovskites
and their derivatives. Adv. Energy Mater. 7: 1602512. https://doi.org/
10.1002/aenm.201602512
[10] Chen, Q., N. De Marco, Y. Yang, (Michael), T.-B. Song, C.-C. Chen,
H. Zhao, et al. 2015. Under the spotlight: the organic–inorganic hybrid
halide perovskite for optoelectronic applications. Nano Today 10: 355–
396. https://doi.org/10.1016/j.nantod.2015.04.009
[11] Marshall, K.P., R.I. Walton and R.A. Hatton. 2015. Tin perovskite/
fullerene planar layer photovoltaics: improving the efficiency and
stability of lead-free devices. J. Mater. Chem. A 3: 11631–11640. https://
doi.org/10.1039/C5TA02950C
[12] Park, B.-W., B. Philippe, X., Zhang, H. Rensmo, G. Boschloo and

E.M.J. Johansson. 2015. Bismuth based hybrid perovskites A3Bi2I9
(A: Methylammonium or Cesium) for solar cell application. Adv. Mater.
27: 6806–6813. https://doi.org/10.1002/adma.201501978
[13] Sun, J., J. Yang, J.I. Lee, J.H. Cho and M.S. Kang. 2018. Lead-free
perovskite nanocrystals for light-emitting devices. J. Phys. Chem. Lett.
9: 1573–1583. https://doi.org/10.1021/acs.jpclett.8b00301
[14] Fan, Q., G.V. Biesold-McGee, J. Ma, Q. Xu, S. Pan, J. Peng, et al. 2020.
Lead-free halide perovskite nanocrystals: crystal structures, synthesis,
stabilities, and optical properties. Angewandte Chemie Intl. Edit. 59:
1030–1046. https://doi.org/10.1002/anie.201904862
[15] Dimesso, L., C. Das, M. Stöhr, T. Mayer and W. Jaegermann. 2017.
Properties of cesium tin iodide (Cs-Sn-I) systems after annealing under
different atmospheres. Mater. Chem. Phy. 197: 27–35. https://doi.
org/10.1016/j.matchemphys.2017.05.018
[16] Ning, W. and F. Gao. 2019. Structural and functional diversity in lead-
free halide perovskite materials. Adv. Mater. 31: 1900326. https://doi.
org/10.1002/adma.201900326
[17] Xiao, Z., Z. Song and Y. Yan. 2019. From lead halide perovskites to lead-
free metal halide perovskites and perovskite derivatives. Adv. Mater.
31: 1803792. https://doi.org/10.1002/adma.201803792
[18] Hoefler, S.F., G. Trimmel and T. Rath. 2017. Progress on lead-free metal
halide perovskites for photovoltaic applications: a review. Monatsh
Chem. 148: 795–826. https://doi.org/10.1007/s00706-017-1933-9
[19] Chang, S., Z. Bai and H. Zhong. 2018. In Situ Fabricated perovskite
nanocrystals: a revolution in optical materials. Adv. Opt. Mater. 6:
1800380 https://doi.org/10.1002/adom.201800380
[20] Wang, Y., S. Chang, X. Chen, Y. Ren, L. Shi, Y. Liu, et al. 2019.
Rapid growth of halide perovskite single crystals: from methods to
optimization control. Chin. J. Chem. 37: 616–629. https://doi.org/10.1002/
cjoc.201900071
[21] Kwoka, M., L. Ottaviano, M. Passacantando, S. Santucci, G. Czempik
and J. Szuber. 2005. XPS study of the surface chemistry of L-CVD
SnO2 thin films after oxidation. Thin Solid Films 490: 36–42. https://doi.
org/10.1016/j.tsf.2005.04.014
[22] Takahashi, Y., R. Obara, Z.-Z. Lin, Y. Takahashi, T. Naito, T. Inabe,
et al. 2011. Charge-transport in tin-iodide perovskite CH3NH3SnI3: origin
of high conductivity. Dalton Trans. 40: 5563. https://doi.org/10.1039/
c0dt01601b
[23] Takahashi, Y., H. Hasegawa, Y. Takahashi and T. Inabe. 2013. Hall
mobility in tin iodide perovskite CH3NH3SnI3: evidence for a doped
semiconductor. J. Solid State Chem. 205: 39–43. https://doi.org/10.1016/j.
jssc.2013.07.008
[24] Kumar, M.H., S. Dharani, W.L. Leong, P.P. Boix, R.R. Prabhakar,
T. Baikie, et al. 2014. Lead-free halide perovskite solar cells with high
photocurrents realized through vacancy modulation. Adv. Mater. 26:
7122–7127. https://doi.org/10.1002/adma.201401991
[25] Noel, N.K., S.D. Stranks, A. Abate, C. Wehrenfennig, S. Guarnera,

A.-A. Haghighirad, et al. 2014. Lead-free organic–inorganic tin halide
perovskites for photovoltaic applications. Energy Environ. Sci. 7: 3061–
3068. https://doi.org/10.1039/C4EE01076K
[26] Li, Q., H. Li, H. Shen, F. Wang, F. Zhao, F. Li, et al. 2017. Solid ligand-
assisted storage of air-stable formamidinium lead halide quantum
dots via restraining the highly dynamic surface toward brightly
luminescent light-emitting diodes. ACS Photonics 4: 2504–2512. https://
doi.org/10.1021/acsphotonics.7b00743
[27] Qian, J., B. Xu and W. Tian. 2016. A comprehensive theoretical study
of halide perovskites ABX3. Org. Electron. 37: 61–73. https://doi.org/10.
1016/j.orgel.2016.05.046
[28] Goldschmidt, V.M. 1926. Die gesetze der krystallochemie.
Naturwissenschaften 14: 477–485. https://doi.org/10.1007/BF01507527
[29] Zhou, L., J. Liao, Z. Huang, J. Wei, X. Wang, W. Li, et al. 2019.
A highly red-emissive lead-free indium-based perovskite single crystal
for sensitive water detection. Angew. Chem. 131: 5331–5335. https://doi.
org/10.1002/ange.201814564
[30] Zhou, C., Y. Tian, M. Wang, A. Rose, T. Besara, N.K. Doyle, et al.
2017. Low-dimensional organic tin bromide perovskites and their
photoinduced structural transformation. Angew. Chem. Int. Ed. 56:
[31] Huang, T.J., Z.X. Thiang, X. Yin, C. Tang, G. Qi and H. Gong. 2016.
(CH3NH3)2 PdCl4: A compound with two-dimensional organic-inorganic
layered perovskite structure. Chem. Eur. J. 22: 2146–2152. https://doi.
org/10.1002/chem.201503680
[32] Travis, W., E.N.K. Glover, H. Bronstein, D.O. Scanlon and R.G. Palgrave.
2016. On the application of the tolerance factor to inorganic and hybrid
halide perovskites: a revised system. Chem. Sci. 7: 4548–4556. https://
doi.org/10.1039/C5SC04845A
[33] Li, C., X. Lu, W. Ding, L. Feng, Y. Gao and Z. Guo. 2008. Formability of
ABX3 (X = F, Cl, Br, I) halide perovskites. Acta Crystallogr B Struct Sci.
64: 702–707. https://doi.org/10.1107/S0108768108032734
[34] Li, Z., M. Yang, J.-S. Park, S.-H. Wei, J.J. Berry and K. Zhu. 2016.
Stabilizing perovskite structures by tuning tolerance factor: formation of
formamidinium and cesium lead iodide solid-state alloys. Chem. Mater.
28: 284–292. https://doi.org/10.1021/acs.chemmater.5b04107
[35] Kieslich, G., S. Sun and A.K. Cheetham. 2015. An extended tolerance
factor approach for organic-inorganic perovskites. Chem. Sci. 6: 3430–
3433. https://doi.org/10.1039/C5SC00961H
[36] Uribe, J.I., D. Ramirez, J.M. Osorio-Guillén, J. Osorio and F. Jaramillo.
2016. CH3NH3CaI3 Perovskite: synthesis, characterization, and first-
principles studies. J. Phys. Chem. C 120: 16393–16398. https://doi.org/10.
1021/acs.jpcc.6b04207
[37] Filip, M.R. and F. Giustino. 2016. Computational screening of
homovalent lead substitution in organic–inorganic halide perovskites.
J. Phys. Chem. C 120: 166–173. https://doi.org/10.1021/acs.jpcc.5b11845
[38] Becker, M., T. Klüner and M. Wark. 2017. Formation of hybrid ABX3
perovskite compounds for solar cell application: first-principles
calculations of effective ionic radii and determination of tolerance factors.
Dalton Trans. 46: 3500–3509. https://doi.org/10.1039/C6DT04796C
[39] Kieslich, G., S. Sun and A.K. Cheetham. 2014. Solid-state principles
applied to organic–inorganic perovskites: new tricks for an old dog.
Chem. Sci. 5: 4712–4715. https://doi.org/10.1039/C4SC02211D
[40] Deng, W., H. Fang, X. Jin, X. Zhang, X. Zhang and J. Jie. 2018. Organic–
inorganic hybrid perovskite quantum dots for light-emitting diodes.
J. Mater. Chem. C. 6: 4831–4841. https://doi.org/10.1039/C8TC01214H
[41] Zhang, F., H. Zhong, C. Chen, X. Wu, X. Hu, H. Huang, et al. 2015.
Brightly luminescent and color-tunable colloidal CH3NH3PbX3
(X = Br, I, Cl) Quantum dots: potential alternatives for display technology.
ACS Nano 9: 4533–4542. https://doi.org/10.1021/acsnano.5b01154
[42] Dong, H. and W. Hu. 2013. Organic nanomaterials. pp. 905–940. In:
R. Vajtai (ed.). Springer Handbook of Nanomaterials. Springer Berlin
Heidelberg, Berlin, Heidelberg.
[43] Huang, H., A.S. Susha, S.V. Kershaw, T.F. Hung and A.L. Rogach.
2015. Control of emission color of high quantum yield CH3NH3PbBr3
perovskite quantum dots by precipitation temperature. Adv. Sci. 2:
1500194. https://doi.org/10.1002/advs.201500194
[44] Levchuk, I., A. Osvet, X. Tang, M. Brandl, J.D. Perea, F. Hoegl, et al.
2017. Brightly luminescent and color-tunable formamidinium lead
halide perovskite FAPbX3 (X = Cl, Br, I) colloidal nanocrystals. Nano
Lett. 17: 2765–2770. https://doi.org/10.1021/acs.nanolett.6b04781
[45] Kumar, S., J. Jagielski, S. Yakunin, P. Rice, Y.-C. Chiu, M. Wang, et al.
2016. Efficient blue electroluminescence using quantum-confined two-
dimensional perovskites. ACS Nano 10: 9720–9729. https://doi.org/10.
1021/acsnano.6b05775
[46] Gonzalez-Carrero, S., R.E. Galian and J. Pérez-Prieto. 2015. Maximizing
the emissive properties of CH3 NH3 PbBr3 perovskite nanoparticles. J.
Mater. Chem. A 3: 9187–9193. https://doi.org/10.1039/C4TA05878J
[47] Meng, L., E. Yao, Z. Hong, H. Chen, P. Sun, Z. Yang, et al. 2017.
Pure formamidinium-based perovskite light-emitting diodes with high
efficiency and low driving voltage. Adv. Mater. 29: 1603826. https://doi.
org/10.1002/adma.201603826
[48] Protesescu, L., S. Yakunin, M.I. Bodnarchuk, F. Bertolotti, N. Masciocchi,
A. Guagliardi, et al. 2016. Monodisperse formamidinium lead bromide
nanocrystals with bright and stable green photoluminescence. J. Am.
Chem. Soc. 138: 14202–14205. https://doi.org/10.1021/jacs.6b08900
[49] Zhang, D., Y. Yu, Y. Bekenstein, A.B. Wong, A.P. Alivisatos and P. Yang.
2016. Ultrathin colloidal cesium lead halide perovskite nanowires.
J. Am. Chem. Soc. 138: 13155–13158. https://doi.org/10.1021/jacs.6b08373
[50] Chen, M., Y. Zou, L. Wu, Q. Pan, D. Yang, H. Hu, et al. 2017. Solvothermal
synthesis of high-quality all-inorganic cesium lead halide perovskite
nanocrystals: from nanocube to ultrathin nanowire. Adv. Funct. Mater.
27: 1701121. https://doi.org/10.1002/adfm.201701121
[51] Protesescu, L., S. Yakunin, M.I. Bodnarchuk, F. Krieg, R. Caputo,

C.H. Hendon, et al. 2015. Nanocrystals of cesium lead halide perovskites
(CsPbX3, X = Cl, Br, and I): novel optoelectronic materials showing bright
emission with wide color gamut. Nano Lett. 15: 3692–3696. https://doi.
org/10.1021/nl5048779
[52] Swarnkar, A., V.K. Ravi and A. Nag. 2017. Beyond colloidal cesium lead
halide perovskite nanocrystals: analogous metal halides and doping. ACS
Energy Lett. 2: 1089–1098. https://doi.org/10.1021/acsenergylett.7b00191
[53] Li, J., X. Shan, S.G.R. Bade, T. Geske, Q. Jiang, X. Yang, et al. 2016.
Single-layer halide perovskite light-emitting diodes with sub-band gap
turn-on voltage and high brightness. J. Phys. Chem. Lett. 7: 4059–4066.
https://doi.org/10.1021/acs.jpclett.6b01942
[54] Jellicoe, T.C., J.M. Richter, H.F.J. Glass, M. Tabachnyk, R. Brady,
S.E. Dutton, et al. 2016. Synthesis and optical properties of lead-free
cesium tin halide perovskite nanocrystals. J. Am. Chem. Soc. 138: 2941–
2944. https://doi.org/10.1021/jacs.5b13470
[55] Tang, X., Y. Bian, Z. Liu, J. Du, M. Li, Z. Hu, et al. 2019. Room-temperature
up-conversion random lasing from CsPbBr3 quantum dots with TiO2
nanotubes. Opt. Lett. 44: 4706. https://doi.org/10.1364/OL.44.004706
[56] Seth, S. and A. Samanta. 2016. A facile methodology for engineering
the morphology of CsPbX3 perovskite nanocrystals under ambient
condition. Sci. Rep. 6: 37693. https://doi.org/10.1038/srep37693
[57] Shamsi, J., Z. Dang, P. Bianchini, C. Canale, F. Di Stasio, R. Brescia,
et al. 2016. Colloidal Synthesis of quantum confined single crystal
CsPbBr3 nanosheets with lateral size control up to the micrometer range.
J. Am. Chem. Soc. 138: 7240–7243. https://doi.org/10.1021/jacs.6b03166
[58] Akkerman, Q.A., V. D’Innocenzo, S. Accornero, A. Scarpellini,
A. Petrozza, M. Prato, et al. 2015. Tuning the optical properties of cesium
lead halide perovskite nanocrystals by anion exchange reactions. J. Am.
Chem. Soc. 137: 10276–10281. https://doi.org/10.1021/jacs.5b05602
[59] Zhao, Q., A. Hazarika, L.T. Schelhas, J. Liu, E.A. Gaulding, G. Li, et al.
2020. Size-dependent lattice structure and confinement properties
in CsPbi3 perovskite nanocrystals: negative surface energy for
stabilization. ACS Energy Lett. 5: 238–247. https://doi.org/10.1021/
acsenergylett.9b02395
[60] Eaton, S.W., M. Lai, N.A. Gibson, A.B. Wong, L. Dou, J. Ma, et al. 2016.
Lasing in robust cesium lead halide perovskite nanowires. Proc. Natl.
Acad. Sci. 113: 1993–1998. https://doi.org/10.1073/pnas.1600789113
[61] Pal, J., S. Manna, A. Mondal, S. Das, K.V. Adarsh and A. Nag. 2017.
Colloidal synthesis and photophysics of M 3Sb2I 9(M=Cs and Rb)
nanocrystals: lead-free perovskites. Angew. Chem. Int. Ed. 56: 14187–
14191. https://doi.org/10.1002/anie.201709040
[62] Shen, Y., J. Yin, B. Cai, Z. Wang, Y. Dong, X. Xu, et al. 2020. Lead-
free, stable, high-efficiency (52%) blue luminescent FA3Bi2Br9 perovskite
quantum dots. Nanoscale Horiz. 5: 580–585. https://doi.org/10.1039/
C9NH00685K
[63] Song, J., J. Li, X. Li, L. Xu, Y. Dong and H. Zeng. 2015. Quantum dot
light-emitting diodes based on inorganic perovskite cesium lead halides
(CsPbX3). Adv. Mater. 27: 7162–7167. https://doi.org/10.1002/adma.
201502567
[64] Nakada, K., Y. Matsumoto, Y. Shimoi, K. Yamada and Y. Furukawa. 2019.
Temperature-dependent evolution of raman spectra of methylammonium
lead halide perovskites, CH3NH3PbX3(X = I, Br). Molecules. 24: 626.
https://doi.org/10.3390/molecules24030626
[65] Gan, Z., Z. Yu, M. Meng, W. Xia and X. Zhang. 2019. Hydration of
mixed halide perovskites investigated by Fourier transform infrared
spectroscopy. APL Materials 7: 031107. https://doi.org/10.1063/1.5087914
[66] Geng, C., S. Xu, H. Zhong, A.L. Rogach and W. Bi. 2018. Aqueous
synthesis of methylammonium lead halide perovskite nanocrystals.
Angew. Chem. Int. Ed. 57: 9650–9654. https://doi.org/10.1002/anie.
201802670
[67] Leng, M., Z. Chen, Y. Yang, Z. Li, K. Zeng, K. Li, et al. 2016. Lead-free,
blue emitting bismuth halide perovskite quantum dots. Angew. Chem.
Int. Ed. 55: 15012–15016. https://doi.org/10.1002/anie.201608160
[68] Leng, M., Y. Yang, K. Zeng, Z. Chen, Z. Tan, S. Li, et al. 2018. All-
inorganic bismuth-based perovskite quantum dots with bright blue
photoluminescence and excellent stability. Adv. Funct. Mater. 28:
1704446. https://doi.org/10.1002/adfm.201704446
[69] Cheng, X., L. Jing, Y. Yuan, S. Du, Q. Yao, J. Zhang, et al. 2019.
Centimeter-size square 2D layered Pb-free hybrid perovskite single
crystal (CH3NH3)2 MnCl4 for red photoluminescence. Cryst. Eng.
Comm. 21: 4085–4091. https://doi.org/10.1039/C9CE00591A
[70] Zhang, X., Y. Zhang, Y. Wang, S. Kalytchuk, S.V. Kershaw, Y. Wang,
et al. 2013. Color-switchable electroluminescence of carbon dot light-
emitting diodes. ACS Nano. 7: 11234–11241. https://doi.org/10.1021/
nn405017q
[71] Zielke, D., A.C. Hübler, U. Hahn, N. Brandt, M. Bartzsch, U. Fügmann,
et al. 2005. Polymer-based organic field-effect transistor using offset
printed source/drain structures. Appl. Phys. Lett. 87: 123508. https://doi.
org/10.1063/1.2056579
[72] Lee, J.-W., D.-H. Kim, H.-S. Kim, S.-W. Seo, S.M. Cho and N.-G. Park.
2015. Formamidinium and cesium hybridization for photo- and
moisture-stable perovskite solar cell. Adv. Energy Mater. 5: 1501310.
https://doi.org/10.1002/aenm.201501310
[73] Schelhas, L.T., Z. Li, J.A. Christians, A. Goyal, P. Kairys, S.P. Harvey,
et al. 2019. Insights into operational stability and processing of halide
perovskite active layers. Energy Environ. Sci. 12: 1341–1348. https://doi.
org/10.1039/C8EE03051K
[74] Song, L., X. Guo, Y. Hu, Y. Lv, J. Lin, Y. Fan, et al. 2018. Improved
performance of CsPbBr3 perovskite light-emitting devices by both
boundary and interface defects passivation. Nanoscale. 10: 18315–18322.
https://doi.org/10.1039/C8NR06311G
[75] Yassitepe, E., Z. Yang, O. Voznyy, Y. Kim, G. Walters, J.A. Castañeda,

et al. 2016. Amine-free synthesis of cesium lead halide perovskite
quantum dots for efficient light-emitting diodes. Adv. Funct. Mater. 26:
8757–8763. https://doi.org/10.1002/adfm.201604580
[76] Huang, H., F. Zhao, L. Liu, F. Zhang, X. Wu, L. Shi, et al. 2015. Emulsion
synthesis of size-tunable CH3NH3PbBr3 quantum dots: an alternative
route toward efficient light-emitting diodes. ACS Appl. Mater. Interfaces
7: 28128–28133. https://doi.org/10.1021/acsami.5b10373
[77] Wei, S., Y. Yang, X. Kang, L. Wang, L. Huang and D. Pan. 2017.
Homogeneous synthesis and electroluminescence device of highly
luminescent CsPbBr3 perovskite nanocrystals. Inorg. Chem. 56: 2596–
2601. https://doi.org/10.1021/acs.inorgchem.6b02763
[78] Di Stasio, F., S. Christodoulou, N. Huo and G. Konstantatos. 2017. Near-
unity photoluminescence quantum yield in CsPbBr3 nanocrystal solid-
state films via postsynthesis treatment with lead bromide. Chem. Mater.
29: 7663–7667. https://doi.org/10.1021/acs.chemmater.7b02834
[79] Swarnkar, A., A.R. Marshall, E.M. Sanehira, B.D. Chernomordik,
D.T. Moore, J.A. Christians, et al. 2016. Quantum dot–induced phase
stabilization of a-CsPbI3 perovskite for high-efficiency photovoltaics.
Science 354: 92–95. https://doi.org/10.1126/science.aag2700
[80] Luo, S.-Q., J.-F. Wang, B. Yang and Y.-B. Yuan. 2019. Recent advances
in controlling the crystallization of two-dimensional perovskites for
optoelectronic device. Front. Phys. 14: 53401. https://doi.org/10.1007/
s11467-019-0901-8
[81] Chen, C., L. Zhang, T. Shi, G. Liao and Z. Tang. 2019. Controllable
synthesis of all inorganic lead halide perovskite nanocrystals with
various appearances in multiligand reaction system. Nanomater. 9: 1751.
https://doi.org/10.3390/nano9121751
[82] Pan, A., B. He, X. Fan, Z. Liu, J.J. Urban, A.P. Alivisatos, et al. 2016. Insight
into the ligand-mediated synthesis of colloidal CsPbBr3 perovskite
nanocrystals: the role of organic acid, base, and cesium precursors. ACS
Nano 10: 7943–7954. https://doi.org/10.1021/acsnano.6b03863
[83] Li, X., F. Cao, D. Yu, J. Chen, Z. Sun, Y. Shen, et al. 2017. All inorganic
halide perovskites nanosystem: synthesis, structural features, optical
properties and optoelectronic applications. Small 13: 1603996. https://
doi.org/10.1002/smll.201603996
[84] Lao, X., X. Li, H. Ågren and G. Chen. 2019. Highly controllable
synthesis and DFT calculations of double/triple-halide CsPbX3(X = Cl,
Br, I) perovskite quantum dots: application to light-emitting diodes.
Nanomater. 9: 172. https://doi.org/10.3390/nano9020172
[85] Ha, S.-T., R. Su, J. Xing, Q. Zhang and Q. Xiong. 2017. Metal halide
perovskite nanomaterials: synthesis and applications. Chem. Sci. 8:
2522–2536. https://doi.org/10.1039/C6SC04474C
[86] Kovalenko, M.V., L. Protesescu and M.I. Bodnarchuk. 2017. Properties
and potential optoelectronic applications of lead halide perovskite nano-
crystals. Science 358: 745–750. https://doi.org/10.1126/science.aam7093
[87] Oksenberg, E., A. Merdasa, L. Houben, I. Kaplan-Ashiri, A. Rothman,

I.G. Scheblykin, et al. 2020. Large lattice distortions and size-dependent
bandgap modulation in epitaxial halide perovskite nanowires. Nat.
Commun. 11: 489. https://doi.org/10.1038/s41467-020-14365-2
[88] Dasgupta, N.P. and P. Yang. 2014. Semiconductor nanowires for
photovoltaic and photoelectrochemical energy conversion. Front. Phys.
9: 289–302. https://doi.org/10.1007/s11467-013-0305-0
[89] Shan, G.-C., Z.-Q. Yin, C.H. Shek and W. Huang. 2014. Single photon
sources with single semiconductor quantum dots. Front. Phys. 9: 170–
193. https://doi.org/10.1007/s11467-013-0360-6
[90] Lei, J.-C., X. Zhang and Z. Zhou. 2015. Recent advances in MXene:
preparation, properties, and applications. Front. Phys. 10: 276–286.
https://doi.org/10.1007/s11467-015-0493-x
[91] Yan, Z.-Z., Z.-H. Jiang, J.-P. Lu and Z.-H. Ni. 2018. Interfacial charge
transfer in WS2 monolayer/CsPbBr3 microplate heterostructure. Front.
Phys. 13: 138115. https://doi.org/10.1007/s11467-018-0785-z
[92] Novoselov, K.S., D.V. Andreeva, W. Ren and G. Shan. 2019. Graphene
and other two-dimensional materials. Front. Phys. 14: 13301. https://doi.
org/10.1007/s11467-018-0835-6
[93] Zhang, B., Y. Wang, S. Chou, H. Liu and S. Dou. 2019. Fabrication of
superior single-atom catalysts toward diverse electrochemical reactions.
Small Methods 3: 1800497. https://doi.org/10.1002/smtd. 201800497
[94] Zhang, B., T. Sheng, Y. Wang, S. Chou, K. Davey, S. Dou, et al. 2019.
Long-life room-temperature sodium–sulfur batteries by virtue of
transition-metal-nanocluster–sulfur interactions. Angew. Chem. Int. Ed.
58: 1484–1488. https://doi.org/10.1002/anie.201811080
[95] Chen, Z., Y. Zhang, S. Chu, R. Sun, J. Wang, J. Chen, et al. 2020.
Grain boundary induced ultralow threshold random laser in a single
GaTe flake. ACS Appl. Mater. Interfaces 12: 23323–23329. https://doi.
org/10.1021/acsami.0c03419
[96] Wang, J., X. Li, B. Wei, R. Sun, W. Yu, H.Y. Hoh, et al. 2020. Activating
Basal planes of NiPS3 for hydrogen evolution by nonmetal heteroatom
doping. Adv. Funct. Mater. 30: 1908708. https://doi.org/10.1002/adfm.
201908708
[97] Xin, B., Y. Pak, S. Mitra, D. Almalawi, N. Alwadai, Y. Zhang, et al.
2019. Self-patterned CsPbBr3 nanocrystals for high-performance
optoelectronics. ACS Appl. Mater. Interfaces 11: 5223–5231. https://doi.
org/10.1021/acsami.8b17249
[98] Liu, J., K. Chen, S.A. Khan, B. Shabbir, Y. Zhang, Q. Khan, et al. 2020.
Synthesis and optical applications of low dimensional metal-halide
perovskites. Nanotechnol. 31: 152002. https://doi.org/10.1088/1361-6528/
ab5a19
[99] Akkerman, Q.A., G. Rainò, M.V. Kovalenko and L. Manna. 2018.
Genesis, challenges and opportunities for colloidal lead halide perovskite
nanocrystals. Nature Mater. 17: 394–405. https://doi.org/10.1038/s41563-
018-0018-4
[100] Hassanabadi, E., M. Latifi, Andrés. F. Gualdrón-Reyes, S. Masi, S.J. Yoon,

M. Poyatos, et al. 2020: Ligand and band gap engineering: tailoring the
protocol synthesis for achieving high-quality CsPbI3 quantum dots.
Nanoscale 12: 14194–14203. https://doi.org/10.1039/D0NR03180A
[101] Zhang, R., Y. Yuan, J. Li, Z. Qin, Q. Zhang, B. Xiong, et al. 2020. Ni and
K ion doped CsPbX3 NCs for the improvement of luminescence
properties by a facile synthesis method in ambient air. J. Lumin. 221:
117044. https://doi.org/10.1016/j.jlumin.2020.117044
[102] Wang, R., Y. Muhammad, X. Xu, M. Ran, Q. Zhang, J. Zhong, et al. 2020.
Facilitating all-inorganic halide perovskites fabrication in confined-
space deposition. Small Methods 4: 2000102. https://doi.org/10.1002/
smtd.202000102
[103] Thesika, K. and A. Vadivel Murugan. 2020. Microwave-enhanced
chemistry at solid–liquid interfaces: synthesis of all-inorganic CsPbX3
nanocrystals and unveiling the anion-induced evolution of structural
and optical properties. Inorg. Chem. 59: 6161–6175. https://doi.
org/10.1021/acs.inorgchem.0c00294
[104] Bekenstein, Y., J.C. Dahl, J. Huang, W.T. Osowiecki, J.K. Swabeck,
E.M. Chan, et al. 2018. The making and breaking of lead-free double
perovskite nanocrystals of cesium silver–bismuth halide compositions.
Nano Lett. 18: 3502–3508. https://doi.org/10.1021/acs.nanolett.8b00560
[105] Huang, J., T. Lei, M. Siron, Y. Zhang, S. Yu, F. Seeler, et al. 2020. Lead-
free cesium europium halide perovskite nanocrystals. Nano Lett. 20:
3734–3739. https://doi.org/10.1021/acs.nanolett.0c00692
[106] Sutton, R.J., G.E. Eperon, L. Miranda, E.S. Parrott, B.A. Kamino, J.B.
Patel, et al. 2016. Bandgap-tunable cesium lead halide perovskites with
high thermal stability for efficient solar cells. Adv. Energy Mater. 6:
1502458. https://doi.org/10.1002/aenm.201502458
[107] Liang, J., C. Wang, Y. Wang, Z. Xu, Z. Lu, Y. Ma, et al. 2016. all-inorganic
perovskite solar cells. J. Am. Chem. Soc. 138: 15829–15832. https://doi.
org/10.1021/jacs.6b10227
[108] Kulbak, M., D. Cahen and G. Hodes. 2015. How important is the organic
part of lead halide perovskite photovoltaic cells? Efficient CsPbBr3
Cells. J. Phys. Chem. Lett. 6: 2452–2456. https://doi.org/10.1021/acs.
jpclett.5b00968
[109] Zhou, C., Y. Tian, M. Wang, A. Rose, T. Besara, N.K. Doyle, et al.
2017. Low-dimensional organic tin bromide perovskites and their
photoinduced structural transformation. Angew. Chem. Int. Ed. 56:
[110] Li, H., G. Tong, T. Chen, H. Zhu, G. Li, Y. Chang, et al. 2018. Interface
engineering using a perovskite derivative phase for efficient and stable
CsPbBr3 solar cells. J. Mater. Chem. A 6: 14255–14261. https://doi.org/10.
1039/C8TA03811B
[111] Jiang, Y., J. Yuan, Y. Ni, J. Yang, Y. Wang, T. Jiu, et al. 2018. Reduced-
dimensional a-CsPbX3 perovskites for efficient and stable photovoltaics.
Joule 2: 1356–1368. https://doi.org/10.1016/j.joule.2018.05.004
[112] Qian, C.-X., Z.-Y. Deng, K. Yang, J. Feng, M.-Z. Wang, Z. Yang, et al.
2018. Interface engineering of CsPbBr3/TiO2 heterostructure with
enhanced optoelectronic properties for all-inorganic perovskite solar
cells. Appl. Phys. Lett. 112: 093901. https://doi.org/10.1063/1.5019608
[113] Yan, L., Q. Xue, M. Liu, Z. Zhu, J. Tian, Z. Li, et al. 2018. Interface
engineering for all-inorganic CsPbI2Br perovskite solar cells with
efficiency over 14%. Adv. Mater. 30: 1802509. https://doi.org/10.1002/
adma.201802509
[114] Li, J., J. Duan, X. Yang, Y. Duan, P. Yang and Q. Tang. 2021. Review
on recent progress of lead-free halide perovskites in optoelectronic
applications. Nano Energy 80: 105526. https://doi.org/10.1016/j.nanoen.
2020.105526
[115] Bian, H., H. Wang, Z. Li, F. Zhou, Y. Xu, H. Zhang, et al. 2020. Unveiling
the effects of hydrolysis-derived DMAI/DMAPbIx intermediate
compound on the performance of CsPbI3 solar cells. Adv. Sci. 7: 1902868.
https://doi.org/10.1002/advs.201902868
[116] Wang, L., B. Fan, B. Zheng, Z. Yang, P. Yin and L. Huo. 2020. Organic
functional materials: recent advances in all-inorganic perovskite solar
cells. Sustainable Energy Fuels 4: 2134–2148. https://doi.org/10.1039/
D0SE00214C
[117] Zeng, Q., X. Zhang, C. Liu, T. Feng, Z. Chen, W. Zhang, et al. 2019.
inorganic CsPbI2Br perovskite solar cells: The progress and perspective.
Sol. RRL 3: 1800239. https://doi.org/10.1002/solr.201800239
[118] Li, J., J. Xia, Y. Liu, S. Zhang, C. Teng, X. Zhang, et al. 2020. Ultrasensitive
organic-modulated CsPbBr3 quantum dot photodetectors via fast
interfacial charge transfer. Adv. Mater. Interfaces 7: 1901741. https://doi.
org/10.1002/admi.201901741
[119] Swarnkar, A., R. Chulliyil, V.K. Ravi, M. Irfanullah, A. Chowdhury and
A. Nag. 2015. Colloidal CsPbBr3 perovskite nanocrystals: luminescence
beyond traditional quantum dots. Angew. Chem. 127: 15644–15648.
https://doi.org/10.1002/ange.201508276
[120] Yettapu, G.R., D. Talukdar, S. Sarkar, A. Swarnkar, A. Nag, P. Ghosh,
et al. 2016. Terahertz conductivity within colloidal CsPbBr3 perovskite
nanocrystals: remarkably high carrier mobilities and large diffusion
lengths. Nano Lett. 16: 4838–4848. https://doi.org/10.1021/acs.nanolett.
6b01168
[121] Wei, S., Y. Yang, X. Kang, L. Wang, L. Huang and D. Pan, et al. 2017.
Homogeneous synthesis and electroluminescence device of highly
luminescent CsPbBr3 perovskite nanocrystals. Inorg. Chem. 56: 2596–
2601. https://doi.org/10.1021/acs.inorgchem.6b02763
[122] Zhihai, W., W. Jiao, S. Yanni, W. Jun, H. Yafei, W. Pan. 2019. Air-stable
all-inorganic perovskite quantum dot inks for multicolor patterns and
white LEDs. J. Mater. Sci. 54: 6917–6929. https://doi.org/10.1007/s10853-
019-03382-2
[123] Chiba, T., Y. Hayashi, H. Ebe, K. Hoshi, J. Sato, S. Sato, et al. 2018.
Anion-exchange red perovskite quantum dots with ammonium iodine
salts for highly efficient light-emitting devices. Nature Photon. 12: 681–
687. https://doi.org/10.1038/s41566-018-0260-y
[124] Wu, S., S. Zhao, Z. Xu, D. Song, B. Qiao, H. Yue, et al. 2018. Highly
bright and stable all-inorganic perovskite light-emitting diodes with
methoxypolyethylene glycols modified CsPbBr3 emission layer. Appl.
Phys. Lett. 113: 213501. https://doi.org/10.1063/1.5054367
[125] Sasaki, H., N. Kamata, Z. Honda and T. Yasuda. 2019. Improved
thermal stability of CsPbBr3 quantum dots by ligand exchange and their
application to light-emitting diodes. Appl. Phys. Express 12: 035004.
https://doi.org/10.7567/1882-0786/ab0019
[126] Wang, K.-H., B.-S. Zhu, J.-S. Yao and H.-B. Yao. 2018. Chemical
regulation of metal halide perovskite nanomaterials for efficient light-
emitting diodes. Sci. China Chem. 61: 1047–1061. https://doi.org/10.1007/
s11426-018-9325-7
[127] Xu, L., J. Li, B. Cai, J. Song, F. Zhang, T. Fang, et al. 2020. A bilateral
interfacial passivation strategy promoting efficiency and stability of
perovskite quantum dot light-emitting diodes. Nat. Commun. 11: 3902.
https://doi.org/10.1038/s41467-020-17633-3
[128] Lozano, G. 2018. The role of metal halide perovskites in next-Generation
lighting devices. J. Phys. Chem. Lett. 9: 3987–3997. https://doi.org/10.1021/
acs.jpclett.8b01417
[129] Luo, D., Q. Chen, Y. Qiu, M. Zhang and B. Liu. 2019. Device engineering
for all-inorganic perovskite light-emitting diodes. Nanomaterials
9: 1007. https://doi.org/10.3390/nano9071007
[130] Shan, Q., J. Song, Y. Zou, J. Li, L. Xu, J. Xue, et al. 2017. High performance
metal halide perovskite light-emitting diode: from material design to
device optimization. Small 13: 1701770. https://doi.org/10.1002/smll.
201701770
[131] Wang, X., H. Zhou, S. Yuan, W. Zheng, Y. Jiang, X. Zhuang, et al.
2017. Cesium lead halide perovskite triangular nanorods as high-gain
medium and effective cavities for multiphoton-pumped lasing. Nano
Res. 10: 3385–3395. https://doi.org/10.1007/s12274-017-1551-1
[132] Park, J.H., A. Lee, J.C. Yu, Y.S. Nam, Y. Choi, J. Park, et al. 2019. Surface
ligand engineering for efficient perovskite nanocrystal-based light-
emitting diodes. ACS Appl. Mater. Interfaces 11: 8428–8435. https://doi.
org/10.1021/acsami.8b20808
[133] Wu, H., Y. Zhang, M. Lu, X. Zhang, C. Sun, T. Zhang, et al. 2018.
Surface ligand modification of cesium lead bromide nanocrystals for
improved light-emitting performance. Nanoscale 10: 4173–4178. https://
doi.org/10.1039/C7NR09126E
[134] Fu, Y., H. Zhu, C.C. Stoumpos, Q. Ding, J. Wang, M.G. Kanatzidis,
et al. 2016. Broad wavelength tunable robust lasing from single-crystal
nanowires of cesium lead halide perovskites (CsPbX3, X = Cl, Br, I). ACS
Nano 10: 7963–7972. https://doi.org/10.1021/acsnano.6b03916
[135] Evans, T.J.S., A. Schlaus, Y. Fu, X. Zhong, T.L. Atallah, M.S. Spencer,
et al. 2018. Continuous-wave lasing in cesium lead bromide perovskite
nanowires. Adv. Opt. Mater. 6: 1700982. https://doi.org/10.1002/
adom.201700982
[136] Wang, Z., Z. Luo, C. Zhao, Q. Guo, Y. Wang, F. Wang, et al. 2017.
Efficient and stable pure green all-inorganic perovskite CsPbBr3 light-
emitting diodes with a solution-processed NiOx interlayer. J. Phys.
Chem. C. 121: 28132–28138. https://doi.org/10.1021/acs.jpcc.7b11518
[137] Liu, Z., J. Yang, J. Du, Z. Hu, T. Shi, Z. Zhang, et al. 2018. Robust
Subwavelength single-mode perovskite nanocuboid laser. ACS Nano
12: 5923–5931. https://doi.org/10.1021/acsnano.8b02143
[138] Liu, Z., Q. Shang, C. Li, L. Zhao, Y. Gao, Q. Li, et al. 2019. Temperature-
dependent photoluminescence and lasing properties of CsPbBr3
nanowires. Appl. Phys. Lett. 114: 101902. https://doi.org/10.1063/1.5082759
[139] Yang, L., Z. Li, C. Liu, X. Yao, H. Li, X. Liu, et al. 2019. Temperature-
dependent lasing of CsPbI3 triangular pyramid. J. Phys. Chem. Lett. 10:
7056–7061. https://doi.org/10.1021/acs.jpclett.9b02703
[140] Bao, C., J. Yang, S. Bai, W. Xu, Z. Yan, Q. Xu, et al. 2018. High
performance and stable all-inorganic metal halide perovskite-based
photodetectors for optical communication applications. Adv. Mater. 30:
1803422. https://doi.org/10.1002/adma.201803422
[141] Wang, Y., X. Li, V. Nalla, H. Zeng and H. Sun. 2017. Solution-processed
low threshold vertical cavity surface emitting lasers from all-inorganic
perovskite nanocrystals. Adv. Funct. Mater. 27: 1605088. https://doi.
org/10.1002/adfm.201605088
[142] Stylianakis, M., T. Maksudov, A. Panagiotopoulos, G. Kakavelakis and
K. Petridis. 2019. Inorganic and hybrid perovskite based laser devices: a
review. Materials 12: 859. https://doi.org/10.3390/ma12060859
[143] Pushkarev, A.P., V.I. Korolev, D.I. Markina, F.E. Komissarenko, A.
Naujokaitis, A. Drabavičius, et al. 2019. A few-minute synthesis of
CsPbBr3 Nanolasers with a high quality factor by spraying at ambient
conditions. ACS Appl. Mater. Interfaces 11: 1040–1048 (2019). https://
doi.org/10.1021/acsami.8b17396
[144] Liu, Z., Z. Hu, T. Shi, J. Du, J. Yang, Z. Zhang, et al. 2019. Stable and
enhanced frequency up-converted lasing from CsPbBr3 quantum dots
embedded in silica sphere. Opt. Express 27: 9459. https://doi.org/10.
1364/OE.27.009459
[145] Liu, Z., S. Huang, J. Du, C. Wang and Y. Leng. 2020. Advances in
inorganic and hybrid perovskites for miniaturized lasers. Nanophotonics
9: 2251–2272. https://doi.org/10.1515/nanoph-2019-0572
[146] Xu, G., Y. Li, J. Yan, X. Lv, Y. Liu and B. Cai. 2019. In-plane self-assembly
and lasing performance of cesium lead halide perovskite nanowires.
Mater. Res. Lett. 7: 203–209. https://doi.org/10.1080/21663831.2019.1576
797
[147] Yan, F., S.T. Tan, X. Li and H.V. Demir. 2019. Light generation in lead
halide perovskite nanocrystals: LEDs, color converters, lasers, and other
applications. Small 15: 1902079. https://doi.org/10.1002/smll.201902079
[148] Li, X., D. Yu, J. Chen, Y. Wang, F. Cao, Y. Wei, et al. 2017. Constructing
fast carrier tracks into flexible perovskite photodetectors to greatly
improve responsivity. ACS Nano 11: 2015–2023. https://doi.org/10.1021/
acsnano.6b08194
[149] Liu, X., D. Yu, F. Cao, X. Li, J. Ji, J. Chen, et al. 2017. Low-voltage
photodetectors with high responsivity based on solution-processed
micrometer-scale all-inorganic perovskite nanoplatelets. Small 13:
1700364. https://doi.org/10.1002/smll.201700364
[150] Gui, P., Z. Chen, B. Li, F. Yao, X. Zheng, Q. Lin, et al. 2018. High-
performance photodetectors based on single all-inorganic CsPbBr3
perovskite microwire. ACS Photonics 5: 2113–2119. https://doi.org/10.
1021/acsphotonics.7b01567
[151] Du, Z., D. Fu, T. Yang, Z. Fang, W. Liu, F. Gao, et al. 2018. Photodetectors
with ultra-high detectivity based on stabilized all-inorganic perovskite
CsPb0.922 Sn0.078 I3 nanobelts. J. Mater. Chem. C 6: 6287–6296. https://doi.
org/10.1039/C8TC01837E
[152] Zhai, W., J. Lin, C. Li, S. Hu, Y. Huang, C. Yu, et al. 2018. Solvothermal
synthesis of cesium lead halide perovskite nanowires with ultra-high
aspect ratios for high-performance photodetectors. Nanoscale 10: 21451–
21458. https://doi.org/10.1039/C8NR05683H
[153] Han, M., J. Sun, M. Peng, N. Han, Z. Chen, D. Liu, et al. 2019. Controllable
growth of lead-free all-inorganic perovskite nanowire array with fast
and stable near-infrared photodetection. J. Phys. Chem. C 123: 17566–
17573. https://doi.org/10.1021/acs.jpcc.9b03289
[154] Zhang, Z.-X., C. Li, Y. Lu, X.-W. Tong, F.-X. Liang, X.-Y. Zhao, et al. 2019.
Sensitive deep ultraviolet photodetector and image sensor composed of
inorganic lead-free Cs3Cu2I5 perovskite with wide bandgap. J. Phys.
Chem. Lett. 10: 5343–5350. https://doi.org/10.1021/acs.jpclett.9b02390
[155] Yang, T., Y. Zheng, Z. Du, W. Liu, Z. Yang, F. Gao, et al. 2018. Superior
photodetectors based on all-inorganic perovskite CsPbI3 nanorods with
ultrafast response and high stability. ACS Nano 12: 1611–1617. https://
doi.org/10.1021/acsnano.7b08201
[156] Xue, M., H. Zhou, G. Ma, L. Yang, Z. Song, J. Zhang, et al. 2018.
Investigation of the stability for self-powered CsPbBr3 perovskite
photodetector with an all-inorganic structure. Sol. Energy Mater. Sol.
Cells. 187: 69–75. https://doi.org/10.1016/j.solmat.2018.07.023
[157] Cen, G., Y. Liu, C. Zhao, G. Wang, Y. Fu, G. Yan, et al. 2019. Atomic-
layer deposition-assisted double-side interfacial engineering for high-
performance flexible and stable CsPbBr3 perovskite photodetectors
toward visible light communication applications. Small. 15: 1902135.
https://doi.org/10.1002/smll.201902135
[158] Yang, Y., H. Dai, F. Yang, Y. Zhang, D. Luo, X. Zhang, et al. 2019. All-
perovskite photodetector with fast response. Nanoscale Res. Lett. 14:
291. https://doi.org/10.1186/s11671-019-3082-z
[159] Tian, C., F. Wang, Y. Wang, Z. Yang, X. Chen, J. Mei, et al. 2019. Chemical
vapor deposition method grown all-inorganic perovskite microcrystals
for self-powered photodetectors. ACS Appl. Mater. Interfaces 11: 15804–
15812. https://doi.org/10.1021/acsami.9b03551
[160] Du, Z., D. Fu, J. Teng, L. Wang, F. Gao, W. Yang, et al. 2019. CsPbI3
Nanotube photodetectors with high detectivity. Small 15: 1905253.
https://doi.org/10.1002/smll.201905253
[161] Ji, Z., Y. Liu, W. Li, C. Zhao and W. Mai. 2020. Reducing current
fluctuation of Cs3Bi2Br9 perovskite photodetectors for diffuse reflection
imaging with wide dynamic range. Sci.e Bull. 65: 1371–1379. (2020).
https://doi.org/10.1016/j.scib.2020.04.018
[162] Saleem, M.I., S. Yang, R. Zhi, M. Sulaman, P.V. Chandrasekar, Y. Jiang,
et al. 2020. Surface engineering of all-inorganic perovskite quantum dots
with quasi core−shell technique for high-performance photodetectors.
Adv. Mater. Interfaces 7: 2000360. https://doi.org/10.1002/admi.202000360
[163] Tong, G., M. Jiang, D.-Y. Son, L. Qiu, Z. Liu, L.K. Ono, et al. 2020.
Inverse growth of large-grain-size and stable inorganic perovskite
micronanowire photodetectors. ACS Appl. Mater. Interfaces 12: 14185–
14194. https://doi.org/10.1021/acsami.0c01056
[164] Gao, Y., L. Zhao, Q. Shang, Y. Zhong, Z. Liu, J. Chen, et al. 2018. Ultrathin
CsPbX3 nanowire arrays with strong emission anisotropy. Adv. Mater.
30: 1801805. https://doi.org/10.1002/adma.201801805
[165] Hou, Y., L. Wang, X. Zou, D. Wan, C. Liu, G. Li, et al. 2020. Substantially
improving device performance of all-inorganic perovskite-based
phototransistors via indium tin oxide nanowire incorporation. Small
16: 1905609. https://doi.org/10.1002/smll.201905609
[166] Kim, D.-K., D. Choi, M. Park, K.S. Jeong and J.-H. Choi. 2020. Cesium
lead bromide quantum dot light-emitting field-effect transistors. ACS
Appl. Mater. Interfaces 12: 21944–21951. https://doi.org/10.1021/acsami.
0c06904
[167] Rainò, G., G. Nedelcu, L. Protesescu, M.I. Bodnarchuk, M.V. Kovalenko,
R.F. Mahrt, et al. 2016. Single cesium lead halide perovskite nanocrystals
at low temperature: fast single-photon emission, reduced blinking, and
exciton fine structure. ACS Nano 10: 2485–2490. https://doi.org/10.1021/
acsnano.5b07328
[168] Zhou, S., G. Zhou, Y. Li, X. Xu, Y.-J. Hsu, J. Xu, et al. 2020. Understanding
charge transport in all-inorganic halide perovskite nanocrystal thin-
film field effect transistors. ACS Energy Lett. 5: 2614–2623. https://doi.
org/10.1021/acsenergylett.0c01295
Chapter 8
Impacts of Working Electrode

Parameters on Dye-Sensitised
Solar Cell Performance
V. Sasirekha1*, J. Mayandi2, J. Vinodhini1, R. Selvapriya1,

P. Jayabal3, V. Ragavendran2 and J.M. Pearce4
1
Department of Physics, Avinashilingam Institute for Home Science and Higher
Education for Women, Coimbatore – 641043, Tamil Nadu, India.
2
Department of Materials Science, School of Chemistry, Madurai Kamaraj
University, Madurai – 625 021, Tamil Nadu, India.
3
Department of Physics, Gobi Arts and Science College, Gobichettipalayam –
638453, Tamil Nadu, India.
4
Department of Electrical and Computer Engineering, Western University,
London, ON, Canada.
8.1 IntroductIon
In the world of booming artificial intelligence (AI) and indoor Internet of

Things (IoTs) applications, there is a need for uninterrupted low-power
supplies for sensors and transducers. To prevent climate destabilisation
demands, sustainable renewable energy sources must be opted for
powering them. Although solar photovoltaic (PV) power generation
* For Correspondence: V. Sasirekha (sasirekha_phy@avinuty.ac.in)

is the least expensive globally, the capital costs of transitioning the

entire global energy system to sustainable energy are challenging. One
approach directly provides electricity for low-power appliances with
small-scale PV systems [1]. Historically, the PV industry was dominated
by first-generation silicon-based single-crystalline and polycrystalline
wafers. The second generation thin films were developed that included
amorphous silicon, chalcogenide [CdTe, Cu(In,Ga)(S,Se)2, (CIGSSe),
Cu2ZnSn(S,Se)4, and (CZTSSe)], semiconductors, and multijunction III–V
solar cells [2–4]. Emerging third-generation solar cells have the potential
to be even more accessible, which include dye-sensitised solar cells
(DSSCs), quantum dots, perovskite, organic, inorganic, and tandem
or multijunction low-cost PV material-based solar cells. Of these new
types of solar cells, DSSCs show particular promise to utilise indoor
light sources, compared with other conventional solar cells, with added
advantages of flexibility and, if needed, some degrees of transparency [5].
The history of DSSCs started with zinc oxide (ZnO), using chlorophyll
and rose bengal dyes as sensitisers, in 1971 with an efficiency of 1% [6].
By 1991, Michael Grätzel’s research group had attained an efficiency
of 7.12% and 7.9% in AM 1.5 solar simulator (750 W/m2) and with low
light levels (83 W/m2). They achieved this with a 10-micrometre titanium
dioxide (TiO2) cubic particle active layer made from a colloidal solution
and enabled charge transfer with ruthenium (Ru) complex as a sensitiser.
The same DSSC device showed an increase in efficiency of 12% in diffuse
daylight and proved the ability of DSSCs better performance under
shaded conditions with current densities in the order of 13 mA cm–2 [7].
This discovery caused a flurry of activity in the PV community because
DSSCs could be researched and manufactured for minimal capital
investments. It led to rapid technological progress. The U.S. National
Renewable Energy Lab (NREL) maintains a chart to the maximum
certified efficiency of 13.0% in DSSC, as shown in Figure 8.1(a) [8]. On
the laboratory scale, 14.7% efficiency has been reported [9].
The architecture of DSSC, as shown in Figure 8.1(b), includes a
conductive substrate (FTO) to transport charge carriers to the load,
a compact layer, a mesoporous active layer (TiO2), a scattering layer,
sensitiser, electrolyte, and counter electrode with electrocatalyst
(FTO/Pt). The photoanode should possess a high surface area for
anchoring sensitiser to harvest the maximum number of photons per
unit area to enhance photon-to-electron conversion. The generated
carriers should be efficiently collected from the sensitiser to reduce
back-electron transfer. The collected electrons should be transported
to the load to avoid recombination, and the overall power conversion
efficiency can be increased by introducing a compact layer with high
electron mobility.
Impacts of Working Electrode Parameters on Dye-Sensitised Solar . . . 191
(a)
(b)
Figure 8.1 (a) Classification of solar cells and the highest efficiency reported
in each category from 1976 to till date. The emerging solar cells are highlighted
(b) Architecture of DSSC device.
8.2 WorkIng PrIncIPle
In a working DSSC, the incident photons pass through the transparent

conductive substrate and sensitisers (S), which are adsorbed on the
active layer. Sensitisers trap the photons and utilise the energy to move
electrons into an excited state (S*), represented as (1) excited energy

carriers are injected into the conduction band (CB) of active layers and
(2) return to the ground state with a loss of energy (S+).
When the electrons are injected into the CB band of the semiconductor
(3), the oxidised dye molecules must regenerate to start the cycle again
so that oxidised dye molecules get an electron from the redox couple
(4). On the other hand, the excited electron may combine with a redox
couple via back-electron transfer, and the energy is lost which is not
desired. Higher energy electron injected into the semiconductor oxide
materials reaches the load via the current collector and reaches the
counter electrode (5). Then, the oxidised redox couple gets reduced (6)
and completes the cycle (Figure 8.2).
Pt
D Electrolyte
ye
TiO2

FTO FTO
3I-
Figure 8.2 Schematic representation of sensitiser excitation (1) and step-by-

step charge transfer processes (2–5).
8.3 SubStrate
One crucial parameter that determines DSSC’s performance is the

nature of the conductive substrate and its resistivity. In general, n-type
semiconductor materials are usually used as substrates due to their
wide band gap, high free electron concentration, and high photons
transmittance. Usually, DSSCs are constructed with transparent
conducting oxides (TCO) that act as a substrate for preparing the
working and counter electrodes and as current collectors. Fluorine-
doped tin oxide (FTO) and indium-doped tin oxide (ITO) are the two
commonly used TCO substrates. In addition, aluminium-doped zinc
oxide glass substrates are also used to prepare photoanodes. The

research community mostly chooses FTO or three material properties:
(i) high thermal stability (up to 600°C);
(ii) good conductivity;
(iii) good transparency for visible light [10].
Instead of using FTO and ITO, polymer substrates have recently
surged in popularity due to their flexibility and lightweight. Flexible
substrates can be made from common thermoplastics like polyethylene
terephthalate (PET), polyethylene naphthalate (PEN), nickel-polyethylene,
and polyether sulfone (PES) [11, 12]. Even though there is an increase
in sheet resistance, which is in the order of 40–100 Ohm/cm2 compared
to 7 Ohm/cm2 in FTO, flexible substrates allow DSSCs to be formed
into various shapes and enable applications that demand malleabilities
like clothes and bags as wearable solar cells [13, 14]. However, the
high-temperature instability of polymer substrates restricts the use of
polymer-based substrates in DSSC. Generally, the TCO substrate used
in DSSC fabrication should possess more than 90% transparency to
allow the maximum photons to enter the cell and offer higher electrical
conductivity for efficient charge transport [15]. In addition, metal and
metal wire-based substrates were also investigated as working and
counter electrodes. Class I (Ti, stainless steel, W, and Zn) metal substrates
were widely used instead of class II (Al, Ni, Co, and Pt) metals because
the latter becomes an insulating layer during annealing, which prevents
electron transfer from TiO2 to the conductive metal substrate. Class I
metal-based working electrodes are sandwiched against glass-FTO-Pt
based counter electrodes or PET/PEN/ITO/Pt (PEDOT/SWCNTs). In these
configurations, illumination was done at the counter electrode side (i.e.
back illumination). Similarly, stainless steel (Ni, carbon steel)/Pt, and
Ti metal-based counter electrodes against the glass (PET)/FTO/TiO2
working electrodes were used for DSSCs [16].
8.4 comPact layer and blockIng layer
The primary function of the compact layer is to avoid direct contact

between TCO and electrolyte to prevent the recombination of charges.
This compact layer should also offer good adhesion for the photoactive
mesoporous layer on the TCO substrate. Typically, TiO2 compact layers
made from the hydrolysis of the different molar concentration of TiCl4
(pre-treatment with annealing) is known to improve the efficiency of
DSSCs. The thickness of the compact layer also plays a vital role in the
performance of DSSC. Generally, thicknesses of the compact layers at
or below 20 nm offer higher power conversion efficiency, while a thick
compact layer film causes a reduction in the optical transmittance, which

in turn decreases the power conversion efficiency. If the layer is too thin,
it does not shield it from recombination and the loss of carriers. For
instance, in a recent study, TiO2 compact layers with different thicknesses
of 13, 19, 25, 38, and 50 nm were prepared using hydrolysis of 40 mM
TiCl4 as a function of dipping time, and the maximum efficiency was
reported for 25 nm thickness compact layer. Higher efficiency was noted
with the addition of this compact layer when compared to bare cells
due to the reduction in recombination at the FTO-electrolyte interface
[17]. FESEM images of untreated and pretreated FTO plates are shown
in Figure 8.3 [18].
Figure 8.3 Field-emission scanning electron microscopy images at high magni-

fication for (a) untreated and (b) treated with TiCl4 FTO substrates [18].
Researchers were also interested in a SnO2-based compact layer

for more electron mobility than a conventional TiO2 compact layer
made from TiCl4 pre-treatment. From the Hall effect measurements, the
maximum value of carrier concentration of 9.166 × 1016 cm–3 was obtained
for SnO2 compact layer prepared from 40 mM precursor solution, and
it exhibited the lowest resistivity value of 1.967 × 10 –2 Ω-cm and the
mobility of the sample was also high compared to TiCl4 pretreated
sample as shown in Figure 8.4.
It is well established that, in addition to the compact layer, a blocking
layer formed between mesoporous and sensitiser are known as TiCl4
post-treated photoanode, which further boosts the DSSC performance.
For instance, a ZnO blocking layer with different thicknesses from
55–310 nm was investigated; out of this, 120 nm offered more efficiency
with an increase in short-circuit current and open-circuit voltage [19].
TiO2 blocking layers with a high charge carrier density of 1.25 × 1,019 cm–3
have also been prepared using spray pyrolysis instead of the conventional
dip coating method [20]. Other than TiCl4 -based blocking layers,
ZnO, SnO2, MgO, Al2O3, and CaCo3-based higher CB edge materials
have also been investigated [21–24]. The compact and blocking layers
reduce recombination and offer a high surface area for anchoring more
dye molecules. Compact and blocking layers can also be made of a
combination of nanomaterials (e.g. ZnO as a compact layer material
and TiO2 from TiCl4 used as a blocking layer; similarly, TiO2/ZnO,
SnO2/TiCl4, TiCl4/SnO2 as a compact and blocking layer were used,
respectively) [25].
Figure 8.4 Electrical Parameters carrier concentration, mobility, and resistiv-

ity were measured using Hall effect SnO2 pretreated and TiCl4 pretreated FTO
substrate.
8.5 meSoPorouS actIve layer
There are many metal oxides materials (i.e. ZnO, SnO2, TiO2, Fe2O3, V2O5,
and Nb2O5) and perovskite materials (i.e. ZnTiO3, SrTiO3, CaTiO3, and
BaTiO3), which have been utilised for the preparation of active layers in
working electrodes [26, 27]. They were further used as doped materials
with some elements, i.e. as N, S, or as hetero/core-shell structures such
as CdS/TiO2, SrTiO3/ TiO2, ZnO/TiO2, SnO2/TiO2, TiO2/MgO, TiO2/Fe2O3
[28–31]. While considering pure semiconductor oxide materials, TiO2 is
the most investigated due to its physiochemical stability against both
temperature and environmental extremes and most importantly it is
less toxic. The function of active layer materials is to collect the excited
charge carriers from the sensitiser and transport them to the TCO. It
can also act as a charge conversion agent by trapping photons directly
from different regions of the electromagnetic spectrum (e.g. N-doping
harvests visible photons, and Y- and Er-doping trap IR photons). Further,

TiO2 optical excitation energy is also reduced by various doping such as
tin (Sn), iron (Fe), chromium (Cr), vanadium (V), zinc (Zn), silver (Ag),
copper–nitrogen (CuN), magnesium (Mg), sulphur (S), barium (Ba), and
cobalt (Co) and the resultant DSSC performances have been reported [32].
Plasmonic photoanodes have also been investigated by incorporating
noble metal nanoparticles or metal oxides in the TiO2 matrix. For instance,
localised surface plasmonic effects influence DSSC performance when
incorporating Ag and Au nanoparticles in the TiO2 matrix as a single
or bimetallic material. The reported efficiencies depend on the sizes
and shapes of Ag or Au nanoparticles such as spheres, rods, prisms,
or combinations of multi-shaped nanoparticles [see Figure 8.5(a)]. The
plasmonic effect is realised either by immobilising Ag or Au on the
TiO2 matrix or preparating as a composite Au/Ag-TiO2, Ag2O-TiO2, and
Fe2O3-TiO2. Other reports are also based on Cu, Al, and CuO used in
plasmonic photoanodes. The increase in short- and open-circuit voltages
is reported up to specific concentrations of plasmonic particles. The
plasmonic metal or metal oxide nanoparticles help increase the local
electric field near the sensitiser to harvest more photons and/or act as
co-sensitiser by directly harvesting photons in the window region of
the used sensitiser and helping carrier transportation by decreasing
the back-electron transfer. In addition, altering the band position of the
semiconductor oxide (i.e. TiO2) by the charging effect leads to increases
in open-circuit voltage [33–36]. The entire process is represented in
Figure 8.5(b).
Figure 8.5 (a) HRTEM image of multi-shaped Ag nanoparticles. (b). Schematic

representation of charge transfer and local field enhancement due to the presence
of metal nanoparticles in the TiO2 matrix [34].
In addition, composites of TiO2 with carbon/graphene/rGO carbon-

based materials have been investigated and resulted in an increase
in conductivity compared with pure semiconductor oxide materials.
Recently reports show that graphitic carbon materials (e.g. C3N4 and
C3N5) can make a cascading band alignment with TiO2, which decreases
the charge transport resistance. This heterojunction minimises the back-
electron transfer [37, 38].
8.6 morPhology
The morphology of the metal oxide material plays an essential role in the
overall performance of DSSCs. The commonly reported morphological
features in TiO2 are nanoparticles, rods/wires/tubes/fibres, 2D sheets,
belts, and 3D nano/microstructures. Each morphology has unique
impacts on the DSSC performance. The TiO2 particle [Figure 8.6(a)]
morphology at the nanoscale offers a higher surface area, which enables
it to adsorb a more significant number of sensitisers, but it also offers
higher charge transport resistance as they have more grain boundaries
with dead ends for charge transport. The one-dimensional structure
[Figure 8.6(b)] offers a unidirectional path for charge transport, but the
surface area is less when compared with particles. In the case of three-
dimensional structures, several aspects need to be considered. Three-
dimensional structures reported include solid spheres, hollow spheres,
core-shell structures, and most importantly three-dimension structures
made up of grains [Figure 8.6(c)], either small spherical particles, rod-
like structures, or sheets [Figure 8.6(d)]. 3D spheres possess unique
characteristics of offering optimum surface area, charge transport via its
grains, and the ability to optimise size. They also serve as a scattering
centre by increasing the optical path length of the incident photons
inside the photoanode active layer. There were also investigations based
on mixed morphologies (e.g. rod/particles, rod/sphere, sphere/particles,
etc.). This mixed morphology [Figures 8.6(e) and 8.6(f)] can be obtained
by mixing different weight ratios of different morphologies or by direct
single-step synthesis. In addition to the high surface area and good
charge transport, mesoporous photo anode material is equally essential
for the diffusion of the electrolyte to rejuvenate the sensitiser molecules.
The effect of the morphology of particles, tubes, and rods has been
investigated, and it is expected that nanotubes offer advantages for
light scattering, electron transportation, and fewer trap sites [39]. For
instance, TiO2 as nanoparticles, nanofibers, hierarchical spheres, and
ellipsoid spheres are successfully obtained via titanium n-butoxide and
acetic acid solvothermal reactions. DSSC consisting of nanoparticles,
nanoparticles/nanowire, nanoparticles/nanotubes TiO2 were sensitised
with the sensitisers N719 and 3,7′-bis(2-cyano-1-acrylicacid)-10-ethyl-
phenothiazine with chenodeoxycholic acid as co-adsorbent. Nanotube-
nanoparticle used to produce photoanodes exhibited better performance
among the investigations due to high active surface area confirmed by

dye loading and facile charge transfer [40]. Nanoparticle (NP), nanofiber
(NF), hierarchical TiO2 sphere (HTS), and ellipsoid TiO2 sphere (ETS)
were investigated, and the HTS morphology achieved higher efficiency
due to large surface area and superior light scattering ability [41].
(a) (b) (c)
(e) (f)
(d)
Figure 8.6 (a) particle morphology, (b) rod-like morphology, (c) sphere-like mor-
phology, (d) 3D morphology with rod granules, (e) mixed morphology of rods and
particles, and (f) mixed morphology of spheres and particles.
8.7 actIve layer PreParatIon
There are two approaches in preparing active layers for DSSCs:

1. direct growth;
2. making thin film layers using appropriate coating techniques
from semiconductor oxide paste.
8.7.1 direct growth

Direct mesoporous layers can be obtained using the hydrothermal
method. Within a Teflon-coated vessel autoclave, FTO plates were
directly immersed in the precursor solution and subjected to various
temperatures ranging from 110°C to 240°C with different time duration
starting from two to several hours. Vertically grown rods were usually
obtained in the natural growth process. Dip coating, spray pyrolysis,

liquid jet via electrospinning, and electrochemical anodisation are other
possible approaches to obtaining TiO2 layers directly on the conductive
substrate.
8.7.2 tio2 nanostructures Powder Preparation

For the thin film TiO2 coating, TiO2 is required in powder form. There
are numerous well-reported methods, including sol-gel, hydrothermal,
solvothermal, reflex, and microwave techniques to obtain TiO2 powders
from different precursors, mostly from titanium isopropoxide, titanium
butoxide, titanium nitrate, and titanium oxalate. Depending on the
initial concentration of precursor and solvents (water, ethanol, acetone,
propanol, glycerol, ethylene glycol, ionic solvents, and acetic acid) with
and without surfactant/morphology tuning agents, one can obtain
different TiO2 morphology and phases as individuals or mixtures.
Sometimes commercially available TiO2 powders (P25) were also used for
the active layer paste preparation, and the performance of DSSCs made
from user-prepared TiO2 powders have been compared. Microwave-
based synthesis techniques have emerged as one of the most efficient,
simple, and fastest ways to create TiO2 nanostructures, which provide
high yields in short reaction times using domestic microwave ovens
[Figure 8.7(a)], user-modified domestic microwave ovens and commercial
microwave ovens [42–48]. In the case of the domestic microwave oven,
synthesis can be done at different microwave powers, but temperature
cannot be controlled as in a commercial microwave reactor. Using low-
cost and readily available domestic microwave ovens is appealing, but
there are restrictions on the choice of solvents.
8.7.2.1 TiO2 Paste Preparation

Alpha terpinol, ethyl cellulose, acetic acid, water, ethanol, triton-X,
tween-80, acetylacetone, polyethylene glycol, nitric acid and PVA, PVP,
polystyrene, and natural polymers are primarily used to prepare paste
either as individuals or mixtures by using a ball milling process, paint
shaker or mortar and pistol [49–53]. Paste quality is essential to ensure that
the final cell has a crack-free active layer. The solvent/dispersing agent
choice depends on the nature of the substrate on which the TiO2 paste
will be coated as a film. Low-temperature curing is a critical parameter
for flexible substrates like PET and PEN, while ITO-based DSSCs offer
better performance if the curing temperature is less than 350 °C, unlike
conventional FTO plates. It is because the sheet resistance increases for
ITO at curing temperatures of 450 °C. The viscosity of the paste is also
an important consideration in obtaining uniform layers without cracks,
as it is one of the factors that decide the interparticle connection. Cracks

in the TiO2-coated thin film are shown in Figure 8.7(b).
(b)
(a)
Figure 8.7 (a) TiO2 nanopowders were obtained using the domestic microwave
oven (b) Cracks in the TiO2-coated film [55].
Moreover, different printing techniques require different viscosity

(e.g. screen printing requires highly viscous pastes compared to spin
coating) [54]. In addition, the choice of solvents decides the adhesion
of TiO2 film with the substrate. Further interconnection between TiO2
particles can be enhanced by mechanical or cold pressing of the TiO2
layer, which reduces the dead ends for the charge carrier transport [55].
8.7.2.2 Thin Film Active Layer Preparation

Thin film active layers deposited from the prepared pastes usually are
obtained via spin coating, inkjet printing, digital printing, doctor blade
method, screen printing, and roll-to-roll printing methods [56]. Uniform
layers can be obtained with the spin coating technique with user-
defined rotation per minute and a restricted active area. High viscous
pastes cannot be used, however, for spin coating. Viscosity also plays
a significant role in determining the thickness of the coated layer. The
thickness can also be tuned for thicker films by repeating the coating
process. Among the methods mentioned above, screen printing and roll-
to-roll printing are utilised in the commercial sectors to obtain larger
uniform and highly reproducible active areas from the prepared pastes.
Similarly, the thickness can also be varied by multiple coating and
floating processes. Roll-to-roll printing is suited for flexible commercial
substrates. G24 power, a leading DSSC module manufacturer, has adopted
roll-to-roll printing to prepare flexible DSSC modules [57]. Small active
areas (0.25 cm2 to 1 cm2) of working electrodes are prepared for laboratory
investigation, but the output power from these active areas cannot be used
for most practical/commercial applications. Hence, the active area has to
be increased to power the most devices of interest. When increasing the
active area, it has to be patterned for effective charge carriers collection,
like conductive finger and bus bars in silicon-based solar cells. For the
current collection, conductive fingers have to be patterned depending on

the chosen area and design of the active layer. Usually, silver conductive
fingers are deposited either by screen printing [58, 59]. Silver conductive
fingers are efficient in charge transport, but stability is an issue when
exposed to iodine-based liquid electrolytes. Hence, it needs a proper
mask in such a way as to avoid contact between the electrolyte to
silver conductive fingers. There are also other investigations based on
carbon-based conductive fingers for charge collection. This direction
of investigation may help to reduce the cost of DSSC production. The
corrosion of metal conductive fingers can be reduced using quasi-
solid electrolytes or solid electrolytes. Recently, most liquid electrolytes
have been replaced by gel polymer electrolytes because of their higher
ionic conductivity, and the possibility of tuning ionic conductivity
by adding a couple of precursors compared to solid electrolytes [60].
The Japanese electronics company Ricoh, which has launched a
commercial DSSC for indoor applications, found a solution to rectify the
shortcoming of solid-state DSSC by the hole transport layer, consisting
of an organic p-type semiconductor and a solid additive. Their DSSC
module of 5.2 cm × 8.4 cm produced 230 microwatts of power, and they
have extended the application of solid-state DSSC modules to charge
IoT sensing devices and LEDs even under shadow conditions [61, 62].
8.7.3 Small lab-Scale dSSc towards large areas for

Practical applications
Protype DSSC is fabricated in an open and closed configuration so
that working and counter electrodes are sandwiched using a binder
clip with a separator. The liquid electrolyte is poured in between the
layers of the sandwich. In the case of a closed configuration, the holes
are drilled in the counter electrode to fill the electrolyte. Then polymer
films (surlyn), available in different thicknesses, are utilised as separators
to avoid shorts between the working and counter electrodes and
blockers from the liquid electrolyte leakage in small active area DSSC
devices. For large surface area DSSCs, the following steps must be
followed both in the working electrode, counter electrode, and device
assembly. The steps for the counter electrode include: (i) holes are drilled
for electrolyte injection, (ii) cleaning the substrate, (iii) platinisation either
using paste or drop-casting of hexachloroplatinic, (iv) acid solution, (v)
paste curing/ reduction at 450 °C, (vi) conductive finer patterning, and
(vii) masking.
Similarly for working electrode steps needed are: (i) substrate
cleaning, (ii) making a compact layer, (iii) active layer coating in the
predefined pattern, (iv) paste curing, (v) post-treatment and annealing,
(vi) coating of silver conductive fingers, and (vii) masking. The working
electrode preparation steps are represented in Figure 8.8. Then both
working and counter electrodes are sandwiched together using a hot
melt press, and the electrolytes are injected through counter electrode
holes. Finally, the holes are sealed. The schematic representation for the
assembled DSSC device is given in Figure 8.8(g).
Figure 8.8 (a) FTO plate, (b) screen printer used for the preparation, (c) active
layer obtained, (d) active layer with a silver conductive finger, (e) sensitisation
process, (f) sensitised working electrode, and (g) fabricated DSSC device powering
digital watch.
8.7.4 dSSc cell to module towards commercialisation

The assembled DSSC large-area devices (cells) can be integrated for
the efficient utilisation of available light dependent on the practical
requirements of the application (e.g. whether the device needs more
current or more voltage to enable the working of the devices). Based

on the required criteria to power the devices with DSSCs, the module
is assembled using cells in parallel or series in combination. It can be
done as monolithic, Z, and W configurations, as shown in Figure 8.9 [63].
Figure 8.9 Schematic configuration to integrate DSSC cells into modules [63].
8.8 concluSIon
This chapter discusses how DSSCs can provide sustainable solutions for
an uninterrupted power supply for low-power-consuming devices, even
under diffused room ambient light conditions. The architecture of DSSCs
and details of working electrodes, including conductive substrates,
compact layers, and mesoporous active layers, were also discussed.
The preparation of TiO2 powders, doping, composite structures, junction
formations, and unique and mixed morphological influences on the
DSSC device performance was also reviewed. Paste preparation and
coating methods were also discussed, as well as considerations for
scaling up from small-area lab-scale DSSC preparation to large-area
DSSC devices and DSSC devices integrated into modules for real-world
applications.
RefeRences
[1] Grafman, L. and J.M. Pearce. 2021. To Catch The Sun. Arcata. CA:
Humbodlt State University Press.
[2] Dhruba, B.K., X. Chen, T.R. Rana and S. Kim. 2020. Chalcogenide
semiconductors based thin-film solar cell. Int. J. Photoenergy. Special
Issue.
[3] Heriche, H., I. Bouchama, N. Bouarissa, Z. Rouabah and A. Dilmi. 2017.

Enhanced efficiency of Cu (In, Ga) Se2 solar cells by adding Cu2ZnSn
(S, Se)4 absorber layer. Optik 144: 378–386.
[4] Nakamura, M., K. Yamaguchi, Y. Kimoto, Y. Yasaki, T. Kato and
H. Sugimoto. 2019. Cd-free Cu (In, Ga)(Se, S)2 thin-film solar cell with
record efficiency of 23.35%. IEEE J. Photovoltaics 9: 1863–1867.
[5] Wu, C., B. Chen, X. Zheng and S. Priya. 2016. Scaling of the flexible dye
sensitised solar cell module. Sol. Energy Mater. Sol. Cells 157: 438–446.
[6] Tributsch, H. and M. Calvin. 1971. Electrochemistry of excited molecules:
photo-electrochemical reactions of chlorophylls. Photochem. Photobiol.
14: 95–112.
[7] O’regan, B. and M. Gratzel. 1991. A low-cost, high-efficiency solar cell
based on dye-sensitised colloidal TiO2 films. Nature 353: 737–740.
[8] https://www.nrel.gov/pv/cell-efficiency.html
[9] Kakiage, K., Y. Aoyama, T. Yano, K. Oya, J.-I. Fujisawa, M. Hanaya.
2015. Highly-efficient dye-sensitised solar cells with collaborative
sensitisation by silyl-anchor and carboxy-anchor dyes. Chem. Commun.
51: 15894–15897.
[10] Patni, N., P. Sharma, M. Parikh, P. Joshi and S.G. Pillai. 2018. Cost
effective approach of using substrates for electrodes of enhanced
efficient dye sensitised solar cell. Mater. Res. Express 5: 095509.
[11] Su, H., M. Zhang, Y.-H. Chang, P. Zhai, N.Y. Hau, Y.-T. Huang, et al.
2014. Highly conductive and low cost Ni-PET flexible substrate for
efficient dye-sensitised solar cells. ACS Appl. Mater. Interfaces 6:
5577–5584.
[12] 천. 김. 안성훈. 2010. Flexible dye-sensitised solar cell and preparation
method thereof. South Korea Patent KR100994902B1, 16.11.2010.
[13] Zhao, B., Z. He, X. Cheng, D. Qin, M. Yun, M. Wang, et al. 2014. Flexible
polymer solar cells with power conversion efficiency of 8.7%. J. Mater.
Chem. C 2: 5077–5082.
[14] Chowdhury, M.S., K.S. Rahman, V. Selvanathan, A.K.M. Hasan, M.S.
Jamal, N.A. Samsudin, et al. 2021. Recovery of FTO coated glass
substrate via environment-friendly facile recycling perovskite solar
cells. RSC Adv. 11: 14534–14541.
[15] Hu, Z., J. Zhang, Z. Hao, Q. Hao, X. Geng and Y. Zhao. 2011. Highly
efficient organic photovoltaic devices using F-doped SnO2 anodes.
Appl. Phys. Lett. 98: 66.
[16] Balasingam, S.K., M.G. Kang and Y. Jun. 2013. Metal substrate based
electrodes for flexible dye-sensitised solar cells: fabrication methods,
progress and challenges. Chem. Commun. 49: 11457–11475.
[17] Choi, H., C. Nahm, J. Kim, J. Moon, S. Nam, D.-R. Jung, et al. 2012.
The effect of TiCl4-treated TiO2 compact layer on the performance of
dye-sensitised solar cell. Curr. Appl Phys. 12: 737–741.
[18] Lokman, M.Q., S. Shaban, S. Shafie, F. Ahmad, H. Yahaya, R.M. Rosnan,
et al. 2021. Improving Ag-TiO2 nanocomposites’ current density by
TiCl4 pretreated on FTO glass for dye-sensitised solar cells. Micro Nano
Lett. 16: 381–386.
[19] Yeoh, M.-E. and K.-Y. Chan. 2019. Efficiency enhancement in dye-
sensitised solar cells with ZnO and TiO2 blocking layers. J. Electron.
Mater. 48: 4342–4350.
[20] Musila, N., M. Munji, J. Simiyu, E. Masika, R. Nyenge and M. Kineene.
2018. Characteristics of TiO2 compact layer prepared for DSSC
application. Traektoria Nauki= Path of Science 4: 3006–3012.
[21] Law, M., L.E. Greene, A. Radenovic, T. Kuykendall, J. Liphardt and
P. Yang. 2006. ZnO – Al2O3 and ZnO – TiO2 core-shell nanowire dye-
sensitised solar cells. J. Phys. Chem. B 110: 22652–22663.
[22] Kay, A. and M. Grätzel. 2002. Dye-sensitised core-shell nanocrystals:
improved efficiency of mesoporous tin oxide electrodes coated with a
thin layer of an insulating oxide. Chem. Mater. 14: 2930–2935.
[23] Kaur, M. and N.K. Verma. 2014. CaCO3/TiO2 nanoparticles based dye
sensitised solar cell. J. Mater. Sci. Technol. 30: 328–334.
[24] Jayabal, P., S. Gayathri, V. Sasirekha, J. Mayandi and V. Ramakrishnan.
2015. Effect of electronic-insulating oxides overlayer on the performance
of zinc oxide based dye sensitised solar cells. J. Photochem. Photobiol.
A: Chemistry 305: 37–44.
[25] Bhuiyan, MMH., F. Kabir, M.S. Manir, M.S. Rahaman, M.R. Hossain,
P. Barua, et al. 2021. Effect of combination of natural dyes and the
blocking layer on the performance of DSSC. Solar Cells: Theory, Mater.
Recent Adv. 313.
[26] Okamoto, Y. and Y. Suzuki. 2014. Perovskite-type SrTiO3, CaTiO3 and
BaTiO3 porous film electrodes for dye-sensitised solar cells. J. Ceram.
Soc. Jpn. 122: 728–731.
[27] Sasikala, R., M. Kandasamy, V. Ragavendran, S. Suresh, V. Sasirekha,
S. Murugesan, et al. 2022. Perovskite zinc titanate-reduced graphene
oxide nanocomposite photoanode for improved photovoltaic
performance in dye-sensitised solar cell. Physica B: Condensed Matter.
646: 414300.
[28] Guo, E. and L. Yin. 2015. Tailored SrTiO3/TiO2 heterostructures for
dye-sensitised solar cells with enhanced photoelectric conversion
performance. J. Mater. Chem. A 3: 13390–13401.
[29] Taguchi, T., X.-T. Zhang, I. Sutanto, K.-I. Tokuhiro, T.N. Rao,
H. Watanabe, et al. 2003. Improving the performance of solid-state dye-
sensitised solar cell using MgO-coated TiO2 nanoporous film. Chem.
Commun. 19: 2480–2481.
[30] Qureshi, A.A., S. Javed, H.M.A. Javed, A. Akram, MS. Mustafa,
U. Ali, et al. 2021. Facile formation of SnO2–TiO2 based photoanode and
Fe3O4@rGO based counter electrode for efficient dye-sensitised solar
cells. Mater. Sci. Semicond. Process. 123: 105545.
[31] Hussein, A.M., A.V. Iefanova, R.T. Koodali, B.A. Logue and R.V. Shende.
2018. Interconnected ZrO2 doped ZnO/TiO2 network photoanode for
dye-sensitised solar cells. Energy Rep. 4: 56–64.
[32] Dubey, R.S., S.R. Jadkar and A.B. Bhorde. 2021. Synthesis and
characterisation of various doped TiO2 nanocrystals for dye-sensitised
solar cells. ACS Omega 6: 3470–3482.
[33] Song, D.H., H.-S. Kim, J.S. Suh, B.-H. Jun and W.-Y. Rho. 2017. Multi-
shaped Ag nanoparticles in the plasmonic layer of dye-sensitised solar
cells for increased power conversion efficiency. Nanomaterials 7: 136.
[34] Selvapriya, R., T. Abhijith, V. Ragavendran, V. Sasirekha, V.S. Reddy,
J.M. Pearce, et al. 2022. Impact of coupled plasmonic effect with
multishaped silver nanoparticles on efficiency of dye sensitised solar
cells. J. Alloys Compd. 894: 162339.
[35] Kaur, N., V. Bhullar, D.P. Singh and A. Mahajan. 2020. Bimetallic
implanted plasmonic photoanodes for TiO2 sensitised third generation
solar cells. Sci. Rep. 10: 7657.
[36] Zarick, H.F., W.R. Erwin, A. Boulesbaa, O.K. Hurd, J.A. Webb,
A.A. Puretzky, et al. 2016. Improving light harvesting in dye-sensitised
solar cells using hybrid bimetallic nanostructures. ACS Photonics 3:
385–394.
[37] Xu, J., G. Wang, J. Fan, B. Liu, S. Cao and J. Yu. 2015. g-C3N4 modified
TiO2 nanosheets with enhanced photoelectric conversion efficiency in
dye-sensitised solar cells. J. Power Sources 274: 77–84.
[38] Nien, Y.-H., Z.-R. Yong, J.-C. Chou, C.-H. Lai, P.-Y. Kuo, Y.-C. Lin,
et al. 2021. Improving photovoltaic performance of dye-sensitized solar
cell by modification of photoanode with gC3N4/TiO2 nanofibers. IEEE
Trans. Electron Devices 68: 4982–4988.
[39] Arla, S.K., N.S.S. Konidena, S.S. Sana, S.K. Godlaveeti and V.K.N. Boya.
2022. Structural and morphological effect of multidimensional TiO2
nanostructures on the dye-sensitised solar cells performance. Sādhanā
47: 129.
[40] Gnida, P., P. Jarka, P. Chulkin, A. Drygała, M. Libera, T. Tański, et al.
2021. Impact of TiO2 nanostructures on dye-sensitised solar cells
performance. Materials 14: 1633.
[41] Liao, J.-Y., J.-W. He, H. Xu, D.-B. Kuang and C.-Y. Su. 2012. Effect of TiO2
morphology on photovoltaic performance of dye-sensitised solar cells:
nanoparticles, nanofibers, hierarchical spheres and ellipsoid spheres.
J. Mater. Chem. 22: 7910–7918.
[42] Vinodhini, J., J. Mayandi, R. Atchudan, P. Jayabal, V. Sasirekha and
J.M. Pearce. 2019. Effect of microwave power irradiation on TiO2
nanostructures and binder free paste screen printed dye sensitised
solar cells. Ceram. Int. 45: 4667–4673.
[43] Ramakrishnan, V.M., S. Pitchaiya, N. Muthukumarasamy, K. Kvamme,
G. Rajesh, S. Agilan, et al. 2020. Performance of TiO2 nanoparticles
synthesised by microwave and solvothermal methods as photoanode
in dye-sensitised solar cells. Int. J. Hydrogen Energy 45: 27036–27046.
[44] Ramakrishnan, V.M., S. Sandberg, N. Muthukumarasamy, K. Kvamme,
P. Balraju, S. Agilan, et al. 2019. Microwave-assisted solvothermal
synthesis of worms-like TiO2 nanostructures in submicron regime as
light scattering layers for dye-sensitised solar cells. Mater. Lett. 236:
747–751.
[45] Huang, C.-H., Y.-T. Yang and R.-A. Doong. 2011. Microwave-assisted
hydrothermal synthesis of mesoporous anatase TiO2 via sol–gel
process for dye-sensitised solar cells. Microporous Mesoporous Mater.
142: 473–480.
[46] Hart, J.N., R. Cervini, Y.-B. Cheng, G.P. Simon and L. Spiccia. 2004.
Formation of anatase TiO2 by microwave processing. Sol. Energy Mater.
Sol. Cells 84: 135–143.
[47] Hart, J.N., D. Menzies, Y.-B. Cheng, G.P. Simon and L. Spiccia. 2017.
A comparison of microwave and conventional heat treatments of
nanocrystalline TiO2. Sol. Energy Mater. Sol. Cells 91: 6–16.
[48] Sahu, K., M. Dhonde and V.V.S. Murty. 2021. Microwave-assisted
hydrothermal synthesis of Cu-doped TiO2 nanoparticles for efficient
dye-sensitised solar cell with improved open-circuit voltage. Int. J.
Energy Res. 45: 5423–5432.
[49] Gemeiner, P. and M. Mikula. 2013. Efficiency of dye sensitised solar cells
with various compositions of TiO2 based screen printed photoactive
electrodes. Acta Chimica Slovaca 6: 29–34.
[50] Mori, R., T. Ueta, K. Sakai, Y. Niida, Y. Koshiba, L. Lei, et al. 2011.
Organic solvent based TiO2 dispersion paste for dye-sensitised solar
cells prepared by industrial production level procedur. J. Mater. Sci.
46: 1341–135.
[51] Karthick, S.N., K.V. Hemalatha, C.J. Raj, A. Subramania and H.-J. Kim.
2012. Preparation of TiO2 paste using poly (vinylpyrrolidone) for dye
sensitised solar cells. Thin Solid Films 520: 7018–7021.
[52] Arla, S.K., S.S. Sana, V. Badineni and V.K.N. Boya. 2020. Effect of nature
of polymer as a binder in making photoanode layer and its influence
on the efficiency of dye-sensitised solar cells. Bull. Mater. Sci. 43: 1–7.
[53] Alwin, S., X.S. Shajan, K. Karuppasamy and K.G.K. Warrier. 2017.
Microwave assisted synthesis of high surface area TiO2 aerogels: a
competent photoanode material for quasi-solid dye-sensitised solar
cells. Mater. Chem. Phys. 196: 37–44.
[54] Weerasinghe, H.C., F. Huang and Y.-B. Cheng. 2014. Fabrication of
flexible dye sensitised solar cells on plastic substrates. Nano Energy
2: 17–189.
[55] Meen, T.H., J.K. Tsai, Y.S. Tu, T.C. Wu, W.D. Hsu and S.-J. Chang,
2014. Optimization of the dye-sensitized solar cell performance by
mechanical compression. Nanoscale Res. Lett. 9: 523.
[56] Mariani, P. and L. Vesce and A. Di Carlo. 2015. The role of printing
techniques for large-area dye sensitised solar cells. Semicond. Sci.
Technol. 30: 104003.
[57] GCell by G24 Power, https://gcell.com/about-g24-power/manufacturing
-process.
[58] Chandrakala, K.R.M.V., K.R. Teja, N.S. Kumar, P.V.P. Raghavendra and
L.N.V.S.B. Majji. 2017. Fabrication of high efficient dye sensitized solar
cell using eosin blue sensitizer. International Journal on Electrical
Engineering and Informatics 9: 185–183.
[59] Gonzalez-Flores, C.A., D. Pourjafari, R. Escalante, E.J. Canto-Aguilar,
A.V. Poot, J.M. Andres Castán, et al. 2021. Influence of redox couple
on the performance of ZnO dye solar cells and minimodules with
benzothiadiazole-based photosensitizers. ACS Appl. Energy Mater. 5:
14092–14106.
[60] Raut, P., V. Kishnani, K. Mondal, A. Gupta and S.C. Jana. 2022. A
review on gel polymer electrolytes for dye-sensitised solar cells.
Micromachines 13: 680.
[61] Ricoh (The Japanese electronics company). 2020. https://www.
pv-magazine.com/2020/02/05/ricoh-launches-solar-cell-for-indoor-
applications/. pv magazine - Photovoltaics Mark. Technol. (online).
[62] Ricoh (online). 2014. https://www.ricoh.com/technology/rd/f_runner/fr15.
[63] Aslam, A., U. Mehmood, M.H. Arshad, A. Ishfaq, J. Zaheer,
A.U.l.H. Khan, et al. 2020. Dye-sensitised solar cells (DSSCs) as a
potential photovoltaic technology for the self-powered internet of
things (IoTs) applications. Sol. Energy 207: 874–892.
Chapter 9
Nanostructured Metal Oxides

for Photocataly tic Water
Split ting
Anu Kumari#, Shaswati Jyoti# and Sonalika Vaidya*

Institute of Nano Science and Technology, Knowledge City, Sector-81, Sahibzada
Ajit Singh Nagar, Punjab –140306, India
9.1 IntroductIon
At a global level, the issues about energy and the environment can be
solved by constructing a clean energy system. Hydrogen is considered
as one of the clean energy source and is expected to become one of
the significant source of energy due to its high energy capacity and
environmentally friendly byproducts. Hydrogen is used in fuel cells
and chemical industries [1]. It is primarily produced from fossil fuels
(e.g. natural gas) by steam reforming. The main disadvantage of these
methods is that fossil fuels are a non-renewable source, and there is
an emission of CO2 during the production of hydrogen from fossil
fuels. Thus, to make the production of hydrogen more environment
friendly, it is desired that hydrogen be produced from a renewable
and abundant source, i.e. water. For producing hydrogen from water,
*For Correspondence: Sonalika Vaidya (svaidya@inst.ac.in)

#
The authors have an equal contribution
a catalyst is required. The term “catalyst” is a derivative of the Greek

word καταλύειν, katalúō, which means “loosen” or “untie”. Catalysts are
chemical species that enhance the reaction rate by forming intermediates
without participating in the reaction. Catalysis is the process of speeding
up a reaction. There are mainly two ways for producing hydrogen/
oxygen from water using catalysis, viz. (i) electrolysis of water and
(ii) photocatalytic or photoelectrochemical water splitting. In this chapter,
we focus on water-splitting using photocatalysis.
9.2 MechanIsM of PhotocatalytIc Water

sPlIttIng
Photocatalytic water-splitting reaction (artificial photosynthesis) is

an uphill reaction that requires Gibb’s energy more significant than
237 kJ to split water into hydrogen and oxygen, which is overcome by
a catalyst. The first demonstration of overall water splitting through
UV-light-induced electrocatalysis (photoelectro-catalysis) was introduced
by Fujishima and Honda on TiO2 electrodes in 1972 [2]. After that,
various other metal oxides, like titanates, niobates, etc., were reported,
which have been used for photocatalytic hydrogen evolution. The most
common feature of a heterogeneous catalyst is that the semiconducting
metal oxide and other kinds of semiconductors should have a suitable
band gap. Figure 9.1 shows schematic for the photocatalytic process.
In the photocatalytic process, when the light of a suitable wavelength
is made to fall on semiconductors having an appropriate band gap,
electrons move from the valence band to the conduction band, creating
holes in the valence band. These electrons and holes drift to the surface
and thus participate in the reaction. The electrons cause a reduction
of the water molecules leading to the formation of hydrogen, while
the holes cause oxidation of the water molecules to form oxygen [3].
The entire process takes place in three main steps:
(i) Generation of photo-induced charge carriers upon light
irradiation.
(ii) Separation of charge carriers and migration to their respective
reaction site.
(iii) Photo-generated charge carriers participated in the reactions on
the surface of the catalyst.
The catalyst’s size, shape, crystal structure, and crystallinity
influence the separation of photo-generated electrons and holes and their
migration to the reaction sites. With higher crystallinity, the amount of
defects present is less. The photo-generated electrons and holes are
trapped at the defect site. These sites may also act as recombination
Nanostructured Metal Oxides for Photocatalytic Water Splitting 211
centers. Both these processes affect photocatalytic activity. When the

size of the photocatalyst is decreased, the migration of photo-generated
electrons and holes to the surface is facilitated. The distance between
their generation site and the surface, i.e. the reaction site decreases [3].
Figure 9.1 Schematic showcasing water splitting using a photocatalyst.
The presence of active sites at the surface is another important

criterion for improving the photocatalytic performance of the catalyst.
The electrons and holes generated by shining light are likely to recombine
with each other, even if they possess suitable thermodynamic potentials
for water splitting if no active sites are present over the surface for the
reactions. Thus, the photocatalytic performance of the catalyst can also
be promoted by co-catalysts, usually metal or metal oxide deposited on
the surface of the semiconductor. The co-catalysts provide reaction sites
and also help in the collection of charge carriers [3].
1
Oxidation : H2O  2H+ + O2 + 2e–
2
Reduction : 2H+ + 2e–  H2
1
Overall reaction : H2O  H2 + O2
2
9.3 essentIal condItIons for the MaterIal

for the Photocatalyst
The standard Gibbs free energy change for the water-splitting reaction
is 237 kJ/mol or 1.23 eV. The essential criteria for a semiconductor
photocatalyst for water splitting are a suitable band gap and the
band edge positions (conduction and valence band edge levels) vs.
NHE. Semiconductors having a band gap energy 3.0 eV > Eg > 1.23 eV
show photocatalytic activity under UV and visible range of the solar
spectrum. For hydrogen evolution, the position of the conduction band
edges minimum should be more negative than 0 V vs NHE (pH = 0);

whereas, for oxygen evolution, the position of the valence band maxima
should be more positive than 1.23 V vs NHE (pH = 0). It suggests that a
semiconductor’s minimum band gap requirement or minimum photon
energy required to act as a photocatalyst for water splitting is 1.23 eV.
9.4 hIstory of PhotocatalysIs
In 1901, chemist Giacomo Ciamician while investigating whether chemical

reactions could be carried out by “light and light alone”, observed that
certain chemical reactions responded to specific wavelengths only. The
term “photocatalysis” first appeared in scientific journals as a keyword in
1911 [4]. In 1972, Fujishima and Honda reported photo-assisted production
of H2 from water using TiO2 electrodes under UV light irradiation. The
authors fabricated an electrochemical cell, wherein the TiO2 electrode
was connected to a black platinum electrode. Upon irradiation, the
current flows from the Pt to the TiO2 electrode, along with the evolution
of O2 at the TiO2 electrode and hydrogen at the Pt electrode [2].
With this breakthrough in the 1970s, the application of metal oxides
in photocatalysis became an important area of research. Several other
photoanodes were also studied, like SrTiO3 [5], SnO2 [6], KTaO3 [7],
WO3 [8], etc. The band gap of most of these semiconducting oxides is
more significant than 3.2 eV, which means that the UV light was suitable
for exciting electrons in these oxides. Thus, most semiconductors could
not use sunlight due to their large band gaps. In 1977, Fujishima and
Honda observed that CdS could be used as a visible light photocatalyst
as it showed strong absorption at less than 520 nm. However, it was
observed that the anodic reaction occurring using CdS caused its
dissolution, which was prevented by using appropriate reducing agents,
such as I–, S2– etc., into the electrolyte solution [9].
Thus, in general, certain elements have been used in heterogeneous
photocatalysis [3], which were categorized into four classes:
(a) Metal sulfide, oxides, and nitrides-based catalysts with d0 and
d10 metal ions.
(b) Alkali, alkaline earth metals, and lanthanides-based catalysts.
(c) To introduce impurity levels as dopants using transition metal
cations having partially filled d-orbitals such as (Rh3+, Ni2+, and
Cr3+)
(d) To use co-catalysts, e.g., some transition metals and oxides (Pt,
Rh, Au), NiO, and RuO2.
We briefly discuss various kinds of oxides reported as photocatalysts
for water-splitting reactions.
9.5 BInary Metal oxIdes
9.5.1 titanates
Researchers have worked extensively with titanates since TiO2 was
first reported to be used as a photocatalyst in 1972 [2]. Since the 1970s,
much work has been done with TiO2 by applying various modifications
like different crystal structures, dopants, co-catalysts, etc. In the last
seven years, many review articles have been published which are based
on TiO2 [10–12]. TiO2 crystallizes in three structures: rutile, anatase,
and brookite, containing TiO6 octahedra. They are connected through
distinct edges and shared corners. The typical edges are two in rutile,
three in brookite, and four in anatase. The band gap of rutile is 3.0 eV,
and anatase is 3.15 eV. Due to the large band gap in TiO2, it is unsuitable
as a visible light photocatalyst. In general, titanates have a band gap
greater than 3 eV. These exhibit good photostability and corrosion
resistance and are n-type semiconductors, thus serving as photoanodes.
Doping is one of the strategies to improve the photocatalytic activity
of TiO2 and other titanates, as well as to shift their band gap in the
visible light region. Single-element, double-element, and tri-element
doping are reported. Doping results in the creation of donors or acceptor
levels in the forbidden region. It also decreases the band gap. Transition
metals, alkaline, and rare earth metals are commonly used as dopants.
Chemical doping of TiO2 with transition metal ions having partially
filled d-orbitals induces a visible light response in TiO2 [13]. These ions
do not show any significant reactivity for water splitting. In 2002, Kato
and Krudo reported [14] that simultaneous co-doping of Sb5+ and Cr3+ in
TiO2 resulted in its use as a photocatalyst for oxygen evolution in visible
light. Pt or Au loading on conducting TiO2 has been shown to improve
the performance of the modified TiO2 in visible light toward hydrogen
production from a water-ethanol solution [15]. Doping with nitrogen in
TiO2 [16, 17] is found to be most effective for photocatalysis, which has
been attributed to the comparable atomic sizes of nitrogen and oxygen,
small ionization energy, and stability. Preethi, L.K. et al. [18] synthesized
biphasic TiO2 containing anatase and rutile and triphasic TiO2 nanotubes
containing all three phases, viz. anatase, rutile, and brookite. They
evaluated the efficiency of the catalysts toward hydrogen evolution,
wherein they observed that the triphasic system of TiO2 was more efficient
than the biphasic system. Zhang Yan et al. [19] worked with blue TiO2(B)
single-crystalline nanorods and showed that the photocatalytic activity
is enhanced due to their rod shape and oxygen vacancies in TiO2, which
prevented the recombination of electrons and holes. Diaz L. et al. [20]
reported using non-noble metals as co-catalysts instead of costly noble
metals like Pt, Au, Ag, or Pd to prepare low-cost M/TiO2 using the
impregnation method. In the presence of both UV and visible light, M/
TiO2 was observed to give higher rates of hydrogen evolution than bare
TiO2, wherein it was observed that Cu/TiO2 achieved almost 80% of
the H2 production rate of that of Pt/TiO2. In 2020, Yang Libin et al. [21]
reported the photo-assisted deposition of clusters of Ru or Au as a
co-catalyst on anatase TiO2 to enhance the catalyst’s performance toward
water splitting. This enhancement was attributed to the chemical bonds,
i.e. Ru-O-Ti and Au-O-Ti bonds, which acted as traps for the electron
and showed a surface plasmon resonance (SPR) effect.
There are numerous reports on titanates [22–24]. We take SrTiO3,
one of the most studied titanates, as an example to showcase
their importance and strategies adopted in titanates in improving
photocatalytic performance. Doping generates cation vacancies, which
promote hydrogen evolution in the photocatalytic processes. In one of
the studies, it has been reported [25] that doping by nitrogen, which
replaces oxygen, in SrTiO3 increases its visible light activity toward
photocatalysis. Cr3+ and Ta5+ doped at Ti4+ sites in SrTiO3 have been
shown to increase the hydrogen evolution rate to almost twice that
observed for SrTiO3 doped with only Cr under visible light. Yu et al. [26]
reported similar results showing that simultaneous doping of Cr and B
in SrTiO3 increased the hydrogen production rate to almost twice that
of Cr-doped SrTiO3. Co-doping of Cr/Ta ions in SrTiO3 [27] resulted in
the formation of a donor level due to the 3d orbital of Cr3+ above the
O-2p valence band. Efforts to use co-catalysts by loading them with a
catalyst and forming a heterojunction without changing the perovskite
structure were also used to enhance the photocatalytic activity in visible
light. It is shown by citing one example wherein a composite formed
from N-doped SrTiO3 and TiO2 resulted in improving the photocatalytic
performance due to the creation of the unique energy band structure [28].
In another example, a multijunction formed from SrTiO3/TiO2 nanotube/
N-doped TiO2 composite resulted in forming a particular band in the
depletion region that was formed due to TiO2 nanotubes [29]. It resulted
in improved photocatalytic performance. Manchala et al. reported a
SrTiO3/CdS/carbon nanospheres-based photocatalytic system that
showed hydrogen evolution exhibiting a rate of 3085 μmol h−1 gcat−1
in the sunlight [30]. Tuning the nature of exposed surfaces is another
strategic approach to improve the photocatalytic performance of the
oxide. We have showcased using SrTiO3 wherein the nature of exposed
surfaces were tuned by changing the solvent used during synthesis [5].
We discuss this example in detail in subsequent section.
9.5.2 tantalates and niobates

Kato and Kudo have reported [31, 32] that alkali metal-based tantalates
having a general formula, ATaO3 (A: Li, Na, and K), showed increased
photocatalytic activity, having order Li> Na>>K, for water splitting under
UV irradiation. They further doped various lanthanides and alkaline
earth metals in NaTaO3, wherein they observed enhanced activity which
they attributed to the decreased particle size and ordered surface area.
Among all the different doping studied by them, doping of La resulted in
the highest activity having a quantum yield of about 56% at 270 nm [32].
Shimizu and co-workers first developed hydrated perovskite having
a layered structure and general formula as A2A’Ta2O7 wherein A was
taken as H, K, Rb, and A’ were taken as Sr and La. These showed
increased photocatalytic activity without even using co-catalysts [33, 34].
Many transition metal oxides-based tantalates containing metals, such
as Cd, Pb, Ag, and Bi, have been studied for photocatalytic water-
splitting [35, 36].
Pure Nb2O5 with a band gap of 3.4 eV is inactive under the threshold
of the UV region, whereas Ta-based oxides with a band gap under 4 eV,
are active under UV irradiation. Use of co-catalysts and intercalation
in Nb2O5 has increased the system’s efficiency in participating in
HER [37]. Domen and co-workers, in 1986, first developed K4Nb6O17
that efficiently produced hydrogen from an aqueous methanol solution
without any aid under a Xe light source [38, 39]. On using co-catalysts
like NiO [40], Au [41], Pt [42], and Cs [43], this niobate was able to
evolve hydrogen and oxygen from water. Similarly, ANbO3 where A was
taken as Na, K [44, 45], ANbWO6 wherein A was taken as Rb, Cs [46],
and nickel oxide [47] loaded catalyst showed photocatalytic activity
toward overall splitting of water under UV irradiation. Under UV light,
strontium niobates, viz. SrNb2O6 [48], Sr2Nb2O7 [48–50], and Sr5Nb4O15 [51]
are observed to be efficient photocatalysts for the evolution of hydrogen
and oxygen. Ca2Nb2O7 [51] and Ba5Nb4O15 [50, 51] are other examples
of niobates that were shown to act as photocatalysts for water splitting.
However, the quantum yields for water-splitting reactions were found
to be less than their strontium counterparts. Layered perovskite-based
niobates that were capable of exchanging and having a general formula
as A(Mn–1NbnO3n+1) wherein A was taken as Na, K, Rb, Cs, and M was
taken as La, Ca, and Sr have been shown to give photocatalytic activity for
H2 evolution under Hg lamp from a mixture of water and methanol [52, 53].
Reports about changes in the photocatalytic activity due to varying
crystal structures are also made. Chen and co-workers reported the
activity order of a series of perovskites, synthesized using the solid-
state method and having general formula as ABi2Nb2O9 (A: Ca, Sr, Ba)
to be varying as Sr > Ba > Ca by conventional solid-state method [54].
Though there are plethora of examples of titanates, niobates and

tantalates being used as photocatalysts [55–61]; we, by giving a few
examples in this chapter, have tried to highlight the importance of these
oxides in the field of material science, especially in the application of
photocatalysis.
9.5.3 other Metal oxides

Under UV irradiation, various d10 metal oxides like In3+ [62], Ga3+ [63],
Sn4+ [64], Sb5+ [65], and a group of p-block metal oxides with co-catalysts
like RuO2 or Pt show effective photochemical water splitting [66].
Tungstates and molybdates-based heterogeneous photocatalysts have
shown activity only in the UV region [67]. Under UV irradiation and in
the presence of sacrificial reagents, Na2W4O13 [68] and Bi2W2O9 [69] were
observed to evolve H2 and O2. f-block elements, in combination with
other metals, behave as promising photocatalysts. For instance, CeO2
was reported to produce O2 from aqueous solutions which contained Fe3+
and Ce4+ as electron acceptors [70]. BaCeO3 in the presence of CH3OH
and AgNO3 as sacrificial agents resulted in H2 and O2, respectively [71].
Crystal structure and electronic states at the surface play an active role
in modulating the catalytic activity of the material. Zhang Xiandi et al.
reported the synthesis of WO3 via a wet chemical approach, wherein
introducing oxygen vacancy in the WO3 band structure helped to tune
the photocatalytic efficiency of the system [72]. Jun Wang et al. used
monoclinic and hexagonal crystalline structures of WO3 to check the
variation in the catalytic properties and found that monoclinic WO3
shows higher catalytic activity than hexagonal WO3 [73]. They also
used graphene oxide to increase the charge transfer efficiency. These
systems show more excellent stability in the durability test, wherein
the monoclinic system was more efficient than the hexagonal one. Wen-
Cheng Ke et al. reported [74] that p-n heterojunction of SiO2/Ag2O/
Zn(O, S) formed from p-type Ag2O and n-type Zn (O, S) loaded on
mesoporous silica resulted in the evolution of hydrogen. There was
a significant improvement in the catalytic activity in this case, which
was attributed to the synergetic effects of various factors such as band
structure, active surface oxygen, increased photo absorption, increased
electron-hole separation, and low charge transfer resistance. Another p-n
heterojunction system reported by Gang Zhou et al. [75], with multiple
components (CuO, ZnO, and Au) wherein large active surface area and
synergistic effects of CuO, ZnO and formed heterojunctions extended
the light absorption range and photocatalytic activity of the material.
9.6 role of structural ParaMeters In

enhancIng PhotocatalytIc effIcIency
There has been development of many novel catalysts for photocatalysis.

The size and morphology of the catalyst, surface area, band gap, crystal
structure, exposed facets, preferred orientation, towards a particular
crystal plane are some critical structural parameters responsible for
affecting the catalytic efficiency of the catalyst. External factors such as
electric polarization, magnetic polarization, metal co-catalyst (discussed
earlier), and type of scavenger or electrolyte used also affect the catalytic
reaction rate. We discuss here the effect of a few structural parameters
on the photocatalytic efficiency of the catalyst.
9.6.1 effect of size

Decreasing the catalyst size in the nano-regime provides a higher
surface-to-volume ratio than the bulk catalyst, thereby allowing more
reaction sites to react simultaneously at the surface. By decreasing the
catalyst size, the transport of charge carriers from bulk to the surface
is increased. Photon absorption is increased in smaller particles, which
is supposed to enhance the photocatalytic activity of the catalyst [76].
However, a further decrease in particle size to quantum dots leads to
photon scattering, thus decreasing the photocatalytic activity.
9.6.2 effect of Morphology

The photocatalytic activity of the catalyst can be increased by tuning
the morphology, which increases the active site and specific surface
area of the catalyst. We explain this by discussing a few examples. Bora
Seo et al. reported that spherical-shaped Ni2P nanoparticles exhibited
higher hydrogen evolution activity when compared with rods shaped
Ni2P [77]. Takeshi Kimijima et al. studied the photocatalytic activity of
SrTiO3 with varying morphology, viz. cube, sphere, and flake-like. They
observed that nanoflakes were bound by {110} surface, which resulted
in their higher photocatalytic activity than other kinds of morphology
for H2 evolution [78].
9.6.3 effect of crystal structure

The crystal structure of the semiconductor plays an essential role in
altering the band gap or band position of the semiconductor. Also, different
surface arrangements of the atoms lead to a difference in the adsorption

of reactant molecules at the surface of the catalyst, influencing the charge
separation and reduction abilities in the surface reactions. Peng Li et al. [79]
synthesized two different crystal structures of perovskite NaNbO3
orthorhombic and cubic. They observed higher photocatalytic activity
of cubic NaNbO3, which they attributed to a more dispersive conduction
band in NaNbO3 with cubic crystal structure than orthorhombic-
NaNbO3 resulting in higher migration of photo-induced electrons. Gian
Luca Chiarello et al. [80] studied the effect of using three different
kinds of crystalline phases of TiO2, viz. anatase, rutile, and brookite, on
photocatalytic activity for hydrogen evolution. The authors observed that
brookite modified by Pt nanoparticles showed increased photocatalytic
activity for hydrogen production from the vapors of a mixture containing
methanol and water under UV-visible irradiation compared to other
crystalline phases. The brookite phase was also showcased to have high
selectivity toward CO2 formation and low CO production. We have
highlighted this influence in Sr-Ti-O system, viz. SrTiO3 (cubic perovskite)
and SrO-(SrTiO3)n (n = 1 and 2) (Ruddlesden-Popper) [81]. We observed
that the order for photocatalytic hydrogen evolution activity of these
three nanostructures was Sr3Ti2O7 > Sr2TiO4 > SrTiO3.
9.6.4 effect of exposed facets

The properties of a catalyst are highly dependent on the nature of
exposed surfaces as it involves the arrangement of atoms on the surface
and their coordination. Adsorption of water molecules and their
reduction/oxidation depends on the surface energy of the particular
facet, which could further improve the reaction rates. Moreover,
oxidation and reduction reactions occurring at different facets improve
charge separation efficiency and thus photocatalytic activity. Moussab
Harb et al. [82] investigated the impact of the exposed facets [including
(010), (110), (001), and (121)] of BiVO4 on photocatalytic water-splitting
reaction. From DFT-based calculations, they predicted that the (001)
surface of BiVO4 is highly active for both hydrogen and oxygen evolution
reactions, whereas the (010) surface is only active for OER reactions.
Jie Meng et al. [83] reported that BaZrO3 with cubic morphology and having
{001}/{011} as exposed facets showed higher activity for photocatalytic
hydrogen evolution in pure water. The authors attributed the increased
activity to modified band structure resulting due to exposed {001}/{011}
facets and also on the larger surface area. The high reduction in the
energy obtained from DFT studies was attributed to the existence
of {001} facets. Jianan Li et al. [84] reported that the development of
facet junction between {001} and {111} facet in ZnFe2O4 nanoparticles
improved the charge separation by decreasing the recombination

rate of the charge carriers generated by light, thereby enhancing the
photocatalytic degradation efficiency over gaseous toluene. In one of our
studies, the size, morphology, and nature of exposed surfaces of SrTiO3
were tuned by varying the synthetic conditions. Herein, the dielectric
constant of the solvent (water and polyols) was varied [5]. The shape
obtained was a cube with water, a flower-like structure formed by
an assembly of hexagonal-shaped particles with ethylene glycol, edge
truncated cuboids with PEG-300 and cuboids with PEG (polyethylene
glycol)-400 as solvent. [001] was observed to be the major exposed facet
of the synthesized SrTiO3, wherein [011] surface was also observed along
with the oxide synthesized using ethylene glycol and PEG-300 as the
solvent. It was observed that the presence of [011] surface decreased the
hydrogen evolution performance of the catalyst.
9.6.5 effect of electrical Polarization

The presence of the internal field within the crystal changes the
bending of the bands at the interfaces. It also provides a driving force
for separating the photo-generated electrons and holes, which helps to
improve the catalytic efficiency of the catalyst. Applying an external
electric field or electrical polarization helps in further transferring of
charge carriers to opposite reaction sites and enhances the adsorption
capacity of the charged ions on the surface. Here we cite examples
from both the photocatalytic and electrocatalytic splitting of water.
Yongfei Cui et al. [85] reported that ferroelectric BaTiO3 showcased better
catalytic activity than non-ferroelectric BaTiO3 for the photocatalytic
dye decolourization, which is due to the separation of photo-induced
charge carriers as a result of the creation of space charge layer. Sangbaek
Park et al. [86] studied the role of ferroelectric K0.5Na0.5NbO3 catalyst
on photocatalytic hydrogen evolution activity. The authors observed
an enhancement in the hydrogen evolution activity by 7-fold when
the catalyst was polarized. Xiaoning Li et al. [87] reported the effect
of ferroelectric polarization in enhancing the electrochemical oxygen
performance of Bi4Ti3O12·(BiCoO3)2. The authors observed a significant
decrease in the overpotential of polarization. H.S. Kushwaha et al. [88]
reported enhanced electrochemical oxygen evolution performance of
Bi0.5Na0.5TiO3 due to polarization.
9.7 conclusIon
Various metal oxides have the potential for being used as photocatalysts
for water splitting. Studies on increasing their efficiency towards
photocatalytic water splitting utilizing doping, forming heterojunctions,

and co-catalysts have been reported widely in the literature. In this
book chapter, we have tried to give a glimpse of the use of metal oxides
for photocatalytic water splitting. We have also tried to highlight the
influence of various structural parameters that affect the photocatalytic
behavior of the oxides.
acknoWledgMents
AK and SJ thank INST for the fellowship. SV thanks CSIR

(01(2943)/18-EMR-II), Govt. of India, for funding.
RefeRences
[1] Dutta, S. 2021. Review on solar hydrogen: its prospects and limitations.
Energy Fuels 35(15): 11613–11639.
[2] Fujishima, A. and K. Honda. 1972. Electrochemical photolysis of water
at a semiconductor electrode. Nature 238: 37. online.
[3] Kudo, A. and Y. Miseki. 2009. Heterogeneous photocatalyst materials
for water splitting. Chem. Soc. Rev. 38(1): 253–278.]
[4] Zhu, S. and D. Wang. 2017. Photocatalysis: basic principles, diverse
forms of implementations and emerging scientific opportunities. Adv.
Energy Mater. 7(23): 1700841.
[5] Vijay, A. and S. Vaidya. 2021. Tuning the morphology and exposed
facets of SrTiO3 nanostructures for photocatalytic dye degradation and
hydrogen evolution. ACS Appl. Nano. Mater. 4(4): 3406–3415.
[6] Sun, C., J. Yang, Y. Cui, W. Ren and J. Zhang. 2022. Recent intensification
strategies of SnO2-based photocatalysts: a review. Chem. Eng. J. 427:
131564.
[7] Ishihara, T., H. Nishiguchi, K. Fukamachi and Y. Takita. 1999. Effects
of acceptor doping to KTaO3 on photocatalytic decomposition of pure
H2O. J. Phys. Chem. B. 103: 1–3.
[8] Zhang, X., G. Jin, D. Wang, Z. Chen, M. Zhao and G. Xi. 2021.
Crystallographic phase and morphology dependent hydrothermal
synthesis of tungsten oxide for robust hydrogen evolution reaction. J.
Alloys. Compd. 875: 160054.
[9] Zhang, F., X. Wang, H. Liu, C. Liu, Y. Wan, Y. Long, et al. 2019. Recent
advances and applications of semiconductor photocatalytic technology.
Appl. Sci. 9(12): 2489.
[10] Ni, M., M.K.H. Leung, D.Y.C. Leung and K. Sumathy. 2007. A review
and recent developments in photocatalytic water-splitting using TiO2
for hydrogen production. Renewable Sustainable Energy Rev. 11(3):
401–425.
[11] Yamakata, A., T.-a. Ishibashi and H. Onishi. 2003. Kinetics of the
photocatalytic water-splitting reaction on TiO2 and Pt/TiO2 studied by
time-resolved infrared absorption spectroscopy. J. Mol. Catal. A: Chem.
199(1): 85–94.
[12] Gupta, N.M. 2017. Factors affecting the efficiency of a water splitting
photocatalyst: A perspective. Renewable Sustainable Energy Rev. 71:
585–601.
[13] Belfaa, K., M.S. Lassoued, S. Ammar and A. Gadri. 2018. Synthesis and
characterization of V-doped TiO2 nanoparticles through polyol method
with enhanced photocatalytic activities. J. Mater. Sci.: Mater. Electron.
29(12): 10269–10276.
[14] Kato, H. and A. Kudo. 2002. Visible-light-response and photocatalytic
activities of TiO2 and SrTiO3 photocatalysts codoped with antimony
and chromium. J. Phys. Chem. B 106(19): 5029–5034.
[15] Bamwenda, G.R., S. Tsubota, T. Nakamura and M. Haruta. 1995. Photo-
assisted hydrogen production from a water-ethanol solution: a
comparison of activities of Au-TiO2 and Pt-TiO2. J. Photochem.
Photobiol., A: Chemistry 89(2): 177–189.
[16] Yang, G., Z. Jiang, H. Shi, T. Xiao and Z. Yan. 2010. Preparation of highly
visible-light active N-doped TiO2 photocatalyst. J. Mater. Chem. 20(25):
5301–5309.
[17] Lu, D., M. Zhang, Z. Zhang, Q. LI, X. Wang and J. Yang. 2014. Self-
organized vanadium and nitrogen co-doped titania nanotube arrays
with enhanced photocatalytic reduction of CO2 into CH4. Nanoscale
Res. Lett. 9(1): 272.
[18] Preethi, L.K., R.P. Antony, T. Mathews, L. Walczak and C.S. Gopinath.
2017. A study on doped heterojunctions in TiO2 nanotubes: an efficient
photocatalyst for solar water splitting. Sci. Rep. 7(1): 14314.
[19] Zhang, Y., Z. Xing, X. Liu, Z. Li, X. Wu, J. Jiang et al. 2016. Ti3+ Self-
doped blue TiO2(B) single-crystalline nanorods for efficient solar-
driven photocatalytic performance. ACS Appl. Mater. Interfaces 8(40):
26851–26859.
[20] Díaz, L., V.D. Rodríguez, M. González-Rodríguez, E. Rodríguez-
Castellón, M. Algarra, P. Núñez, et al.. 2021. M/TiO2 (M = Fe, Co, Ni,
Cu, Zn) catalysts for photocatalytic hydrogen production under UV
and visible light irradiation. Inorg. Chem. Front. 8(14): 3491–3500.
[21] Yang, L., P. Gao, J. Lu, W. Guo, Z. Zhuang, Q. Wang, et al. 2020.
Mechanism analysis of Au, Ru noble metal clusters modified on TiO2
(101) to intensify overall photocatalytic water splitting. RSC Adv. 10(35):
20654–20664.
[22] Kudo, A. 2006. Development of photocatalyst materials for water
splitting. Int. J. Hydrogen Energy 31(2): 197–202.
[23] Kudo, A. 2003. Photocatalyst materials for water splitting. Catal. Surv.
Asia 7(1): 31–38.
[24] Bavykin, D.V., J.M. Friedrich and F.C. Walsh. 2006. Protonated
titanates and TiO2 nanostructured materials: synthesis, properties,
and applications. Adv. Mater. 18(21): 2807–2824.
[25] Ishii, T., H. Kato and A. Kudo. 2004. H2 evolution from an aqueous
methanol solution on SrTiO3 photocatalysts codoped with chromium
and tantalum ions under visible light irradiation. J. Photochem.
Photobiol. A: Chemistry 163(1): 181–186.
[26] Yu, H., J. Wang, S. Yan, T. Yu and Z. Zou. 2014. Elements doping
to expand the light response of SrTiO3. J. Photochem. Photobiol. A:
Chemistry 275: 65–71.
[27] Wang, D., J. Ye, T. Kako and T. Kimura. 2006. Photophysical and
photocatalytic properties of SrTiO3 doped with Cr cations on different
sites. J. Phys. Chem. B 110(32): 15824–15830.
[28] Yan, J.-H., Y.-R. Zhu, Y.-G. Tang and S.-Q. Zheng. 2009. Nitrogen-doped
SrTiO3/TiO2 composite photocatalysts for hydrogen production under
visible light irradiation. J. Alloys. Compd. 472(1): 429–433.
[29] Su, E.-C., B.-S. Huang and M.-Y. Wey. 2016. Enhanced optical and
electronic properties of a solar light-responsive photocatalyst for
efficient hydrogen evolution by SrTiO3/TiO2 nanotube combination.
Sol. Energy 134: 52–63.
[30] Manchala, S., A. Gandamalla, V.N. Rao, S.M. Venkatakrishnan and
V. Shanker. 2021. Solar-light responsive efficient H2 evolution using
a novel ternary hierarchical SrTiO 3/CdS/carbon nanospheres
photocatalytic system. J. Nanostruct. Chem. 12: 179–191.
[31] Kato, H. and A. Kudo. 1998. New tantalate photocatalysts for water
decomposition into H2 and O2. Chem. Phys. Lett. 295(5): 487–492.
[32] Kato, H. and A. Kudo. 2001. Water splitting into H2 and O2 on alkali
tantalate photocatalysts ATaO3 (A = Li, Na, and K). J. Phys. Chem. B
105(19): 4285–4292.
[33] Shimizu, K.-i., Y. Tsuji, T. Hatamachi, K. Toda, T. Kodama, M. Sato, et al.
2004. Photocatalytic water splitting on hydrated layered perovskite
tantalate A2SrTa2O7·nH2O (A = H, K, and Rb). Phys. Chem. Chem. Phys.
6(5): 1064–1069.
[34] Shimizu, K.-i., S. Itoh, T. Hatamachi, T. Kodama, M. Sato and K. Toda.
2005. Photocatalytic water splitting on Ni-intercalated Ruddlesden−
Popper tantalate H2La2/3Ta2O7. Chem. Mater. 17(20): 5161–5166.
[35] Yang, H., X. Liu, Z. Zhou and L. Guo. 2013. Preparation of a novel
Cd2Ta2O7 photocatalyst and its photocatalytic activity in water splitting.
Catal. Commun. 31: 71–75.
[36] Boltersdorf, J., T. Wong and P.A. Maggard. 2013. Synthesis and optical
properties of Ag(I), Pb(II), and Bi(III) tantalate-based photocatalysts.
ACS Catal. 3(12): 2943–2953.
[37] Lin, H.-Y., H.-C. Huang and W.-L. Wang. 2008. Preparation of
mesoporous In–Nb mixed oxides and its application in photocatalytic
water splitting for hydrogen production. Microporous Mesoporous
Mater. 115(3): 568–575.
[38] Domen, K., A. Kudo, M. Shibata, A. Tanaka, K.-I. Maruya and T. Onishi.
1986. Novel photocatalysts, ion-exchanged K4Nb6O17, with a layer
structure. J Chem. Soc. Chem. Commun. 23: 1706–1707.
[39] Domen, K., A. Kudo and A. Shinozaki, A. Tanaka, K. -I. Maruya and
T. Onishi. 1986. Photodecomposition of water and hydrogen evolution
from aqueous methanol solution over novel niobate photocatalysts.
J. Chem. Soc. Chem. Commun. 4: 356–357.
[40] Kudo, A. 2007. Photocatalysis and solar hydrogen production. Pure
Appl. Chem. 79(11): 1917–1927.
[41] Iwase, A., H. Kato and A. Kudo. 2006. Nanosized Au particles as an
efficient co-catalyst for photocatalytic overall water splitting. Catal.
Lett. 108(1): 7–10.
[42] Sayama, K., A. Tanaka, K. Domen, K. Maruya and T. Onishi. 1991.
Photocatalytic decomposition of water over platinum-intercalated
potassium niobate (K4Nb6O17). J. Phys. Chem. 95(3): 1345–1348.
[43] Chung, K.-H. and D.-C. Park. 1998. Photocatalytic decomposition of
water over cesium-loaded potassium niobate photocatalysts. J. Mol.
Catal. A: Chem. 129(1): 53–59.
[44] Li, G., T. Kako, D. Wang, Z. Zou and J. Ye. 2008. Synthesis and enhanced
photocatalytic activity of NaNbO3 prepared by hydrothermal and
polymerized complex methods. J. Phys. Chem. Solids 69(10): 2487–2491.
[45] Kim, S., J.-H. Lee, J. Lee, S.-W. Kim, M.H. Kim, S. Park, et al. 2013.
Synthesis of monoclinic potassium niobate nanowires that are stable
at room temperature. J. Am. Chem. Soc. 135(1): 6–9.
[46] Ikeda, S., T. Itani, K. Nango and M. Matsumura. 2004. Overall water
splitting on tungsten-based photocatalysts with defect pyrochlore
structure. Catal. Lett. 98(4): 229–233.
[47] Sayama, K., A. Tanaka, K. Domen, K. Maruya and T. Onishi. 1990.
Improvement of nickel-loaded K4 Nb 6 O17 photocatalyst for the
decomposition of H2O. Catal. Lett. 4(3): 217–222.
[48] Chen, D. and J. Ye. 2009. Selective-synthesis of high-performance single-
crystalline Sr2Nb2O7 nanoribbon and SrNb2O6 nanorod photocatalysts.
Chem. Mater. 21(11): 2327–2333.
[49] Kudo, A., H. Kato and S. Nakagawa. 2000. Water splitting into H2
and O2 on new Sr2M2O7 (M = Nb and Ta) photocatalysts with layered
perovskite structures: factors affecting the photocatalytic activity.
J. Phys. Chem. B 104(3): 571–575.
[50] Kim G.H., D.W. Hwang, J. Kim, Y.G. Kim and J.S. Lee. 1999. Highly
donor-doped (110) layered perovskite materials as novel photocatalysts
for overall water splitting. Chem. Commun. (12): 1077–1078.
[51] Miseki, Y., H. Kato and A. Kudo. 2009. Water splitting into H2 and O2
over niobate and titanate photocatalysts with (111) plane-type layered
perovskite structure. Energy Environ. Sci. 2(3): 306–314.
[52] Domen, K., J. Yoshimura, T. Sekine, A. Tanaka and T. Onishi. 1990.
A novel series of photocatalysts with an ion-exchangeable layered
structure of niobate. Catal. Lett. 4(4): 339–343.
[53] Kazunari, D., N.J. Kondo, H. Michikazu and T. Tsuyoshi. 2000. Photo-
and mechano-catalytic overall water splitting reactions to form
hydrogen and oxygen on heterogeneous catalysts. Bull. Chem. Soc.
Jpn. 73(6): 1307–1331.
[54] Li, Y., G. Chen, H. Zhang and Z. Lv. 2010. Band structure and
photocatalytic activities for H2 production of ABi2Nb2O9 (A=Ca, Sr, Ba).
Int. J. Hydrogen Energy 35(7): 2652–2656.
[55] Phoon, B.L., C.W. Lai, J.C. Juan, P.-L. Show and G.-T. Pan. 2019. Recent
developments of strontium titanate for photocatalytic water splitting
application. Int. J. Hydrogen Energy 44(28): 14316–14340.
[56] Bin Adnan, M.A., K. Arifin, L.J. Minggu and M.B. Kassim. 2018.
Titanate-based perovskites for photochemical and photoelectrochemical
water splitting applications: a review. Int. J. Hydrogen Energy 43(52):
23209–23220.
[57] Inoue, Y. 2009. Photocatalytic water splitting by RuO2-loaded metal
oxides and nitrides with d0- and d10 -related electronic configurations.
Energy Environ. Sci. 2(4): 364–386.
[58] Tee, S.Y., K.Y. Win, W.S. Teo, L.-D. Koh, S. Liu, C.D. Teng, et al. 2007.
Recent progress in energy-driven water splitting. Adv. Sci. 4(5): 1600337.
[59] Navarro Yerga, R.M., M.C. Álvarez Galván, F. del Valle, A.J. Villoria
de la Mano and J.L.G. Fierro. 2009. Water splitting on semiconductor
catalysts under visible-light irradiation. ChemSusChem. 2(6): 471–485.
[60] Rajeshwar, K. 2007. Hydrogen generation at irradiated oxide
semiconductor–solution interfaces. J. Appl. Electrochem. 37(7): 765–787.
[61] Zhang, P., J. Zhang and J. Gong. 2014. Tantalum-based semiconductors
for solar water splitting. Chem. Soc. Rev. 43(13): 4395–4422.
[62] Yang, X., Z. Guo, X. Zhang, Y. Han, Z. Xue, T. Xie, et al. 2021. The effect
of indium doping on the hydrogen evolution performance of g-C3N4
based photocatalysts. New J. Chem. 45(2): 544–550.
[63] Takashi, Y., S. Yoshihisa and I. Hayao. 2004. Photocatalytic decomposition
of H2O into H2 and O2 over Ga2O3 loaded with NiO. Chem. Lett. 33(6):
726–727.
[64] Manikandan, M., T. Tanabe, P. Li, S. Ueda, G.V. Ramesh, R. Kodiyath,
et al. 2014 Photocatalytic water splitting under visible light by mixed-
valence Sn3O4. ACS Appl. Mater. Interfaces 6(6): 3790–3793.
[65] Annamalai, A., R. Sandström, E. Gracia-Espino, N. Boulanger, J.-F. Boily,
I. Mühlbacher, et al. 2018. Influence of Sb5+ as a double donor on
hematite (Fe3+) photoanodes for surface-enhanced photoelectrochemical
water oxidation. ACS Appl. Mater. Interfaces 10(19): 16467–16473.
[66] Kadowaki, H., N. Saito, H. Nishiyama, H. Kobayashi, Y. Shimodaira
and Y. Inoue. 2007. Overall splitting of water by RuO2-loaded PbWO4
photocatalyst with d10s2-d0 configuration. J. Phys. Chem. C 111(1):
439–444.
[67] Hideki, K., M. Naoko and K. Akihiko. 2004. Photophysical and
photocatalytic properties of molybdates and tungstates with a scheelite
structure. Chem. Lett. 33(9): 1216–1217.
[68] Akihiko, K. and K. Hideki. 1997. Photocatalytic activities of Na2W4O13
with layered structure. Chem. Lett. 26(5): 421–422.
[69] Akihiko, K. and H. Satoshi. 1999. H2 or O2 evolution from aqueous
solutions on layered oxide photocatalysts consisting of Bi3+ with
6s2 configuration and d0 transition metal. Ions. Chem. Lett. 28(10):

1103–1104.
[70] Bamwenda, G.R., T. Uesigi, Y. Abe, K. Sayama and H. Arakawa. 2001.
The photocatalytic oxidation of water to O2 over pure CeO2, WO3, and
TiO2 using Fe3+ and Ce4+ as electron acceptors. Appl. Catal., A: General
205(1): 117–128.
[71] Yuan, Y., J. Zheng, X. Zhang, Z. Li, T. Yu, J. Ye, et al. 2008. BaCeO3 as a
novel photocatalyst with 4f electronic configuration for water splitting.
Solid State Ionics 178(33): 1711–1713.
[72] Zhang, X., W. Hao, C.-S. Tsang, M. Liu, G.S. Hwang and L.Y.S. Lee. 2019.
Psesudocubic phase tungsten oxide as a photocatalyst for hydrogen
evolution reaction. ACS Appl. Energy Mater. 2(12): 8792–8800.
[73] Yang, J., X. Chen, X. Liu, Y. Cao, J. Huang, Y. Li, et al. 2021. From
hexagonal to monoclinic: engineering crystalline phase to boost the
intrinsic catalytic activity of tungsten oxides for the hydrogen evolution
reaction. ACS Sustainable Chem. Eng. 9(16): 5642–5650.
[74] Gultom, N.S., H. Abdullah, D.-H. Kuo and W.-C. Ke. 2019. Oriented
p–n heterojunction Ag2O/Zn(O,S) nanodiodes on mesoporous SiO2 for
photocatalytic hydrogen production. ACS Appl. Energy Mater. 2(5):
3228–3236.
[75] Zhou, G., X. Xu, T. Ding, B. Feng, Z. Bao and J. Hu. 2015. Well-steered
charge–carrier transfer in 3D branched CuxO/ZnO@Au Heterostructures
for efficient photocatalytic hydrogen evolution. ACS Appl. Mater.
Interfaces 7(48): 26819–26827.
[76] Amaechi, I.C., R. Katoch, G. Kolhatkar, S. Sun and A. Ruedieger. 2020.
Particle size effect on the photocatalytic kinetics of barium titanate
powders [10.1039/D0CY01358G]. Catal. Sci. Technol. 10(18): 6274–6284.
[77] Seo, B., D.S. Baek, Y.J. Sa and S.H. Joo. 2016. Shape effects of nickel
phosphide nanocrystals on hydrogen evolution reaction. Cryst. Eng.
Comm. 18(32): 6083–6089.
[78] Kimijima, T., K. Kanie, M. Nakaya and A. Muramatsu. 2014. Solvothermal
synthesis of SrTiO3 nanoparticles precisely controlled in surface crystal
planes and their photocatalytic activity. Appl. Catal. B: Environmental
144: 462–467.
[79] Li, P., S. Ouyang, G. Xi, T. Kako and J. Ye. 2012. The effects of crystal
structure and electronic structure on photocatalytic H2 evolution and
CO2 reduction over two phases of perovskite-structured NaNbO3. J.
Phys. Chem. C. 116(14): 7621–7628.
[80] Chiarello, G.L., A.D. Paola, L. Palmisano and E. Selli. 2011. Effect of
titanium dioxide crystalline structure on the photocatalytic production
of hydrogen. Photochem. Photobiol. Sci. 10(3): 355–360.
[81] Vijay, A., K. Bairagi and S. Vaidya. 2022. Relating the structure,
properties, and activities of nanostructured SrTiO3 and SrO–(SrTiO3)n
(n = 1 and 2) for photocatalytic hydrogen evolution. Mater. Adv. 3:
5055–5063.
[82] Lardhi, S., L. Cavallo, M. Harb. 2020. Significant impact of exposed

facets on the BiVO4 material performance for photocatalytic water
splitting reactions. J. Phys. Chem. Lett. 11(14): 5497–5503.
[83] Meng, J., Z. Lan, Q. Lin, T. Chen, X. Chen, X. Wei, et al. 2019. Cubic-
like BaZrO3 nanocrystals with exposed {001}/{011} facets and tuned
electronic band structure for enhanced photocatalytic hydrogen
production. J. Mater. Sci. 54(3): 1967–1976.
[84] Li, J., X. Li, Z. Yin, X. Wang, H. Ma and L. Wang. 2019. Synergetic effect
of facet junction and specific facet activation of ZnFe2O4 nanoparticles
on photocatalytic activity improvement. ACS Appl. Mater. Interfaces
11(32): 29004–29013.
[85] Cui, Y., J. Briscoe and S. Dunn. 2013. Effect of ferroelectricity on solar-
light driven photocatalytic activity of BaTiO3 - influence on the carrier
separation and stern layer formation. Chem. Mater. 25(21): 4215–4223.
[86] Park, S., C.W. Lee, M.-G. Kang, S. Kim, H.J. Kim, J.E. Kwon, et al. 2014.
A ferroelectric photocatalyst for enhancing hydrogen evolution:
polarized particulate suspension. Phys. Chem. Chem. Phys. 16(22):
10408–10413.
[87] Li, X., H. Liu, Z. Chen, Q. Wu, Z. Yu, M. Yang, et al. 2019. Enhancing
oxygen evolution efficiency of multiferroic oxides by spintronic and
ferroelectric polarization regulation. Nat. Commun. 10(1): Article 1409.
[88] Kushwaha, H.S., A. Halder and R. Vaish. 2018. Ferroelectric electro-
catalysts: a new class of materials for oxygen evolution reaction with
synergistic effect of ferroelectric polarization. J. Mater. Sci. 53(2):
1414–1423.
Chapter 10
Nanof luidics for

Heat Transfer System and
Energy Applications
Nagendra S. Chauhan
Department of Applied Physics, Graduate School of Engineering,
Tohoku University, Sendai, Miyagi, Japan– 980-8579
Email: nagendra599@gmail.com; chauhan.nagendra.singh.b1@tohoku.ac.jp
10.1 IntroductIon
Heat transfer research has a long history of about 30 decades, owing

to its persistent relevance in industrial processing that encompasses
numerous exciting opportunities across a wide spectrum for developing
critical technologies in energy, automobiles, electronics, and the medical
sector. As a widely studied discipline of thermal engineering, any heat
transfer process is a path function governed by three distinctive modes
based on classical heat transfer laws, i.e., Fourier’s law (conduction),
Newton’s law of cooling (convection), and Planck’s law (radiation). In the
context of temperature regimes, the subject of heat transfer is broadly
classified into two sub-categories, i.e., (i) for temperatures well below
room temperature (termed refrigeration or cryogenic technology); (ii) for
temperatures above room temperature. Heat transfer fluids (HTFs), as
a term for heat transfer media, is commonly reserved for heat transfer
occurring above room temperature. While for refrigeration, the heat
transfer media is commonly referred to as refrigerant.
Approaches for improving heat transmission in heat exchangers
commonly include passive and active techniques, such as creating
turbulence, extending the exchange surface (such as fins), or using
a fluid with higher thermophysical properties. Over the years, heat
transfer systems have evolved from geometrical modifications to compact
liquid cooling systems, thus shifting the focus from the geometry of
heat exchangers to HTFs. The engineering data for the process design
targeting heat transfer enhancement and performances indicates heat
transfer media to be the primary determinant. In the wide range of
thermal applications, commonly used HTFs include liquids such as
ethylene glycol, water, silicone oil, propylene glycol, etc., which offer an
advantage over the air in terms of their higher specific heat and thermal
conductivity. Thus, advanced thermal designs are focused on improving
the characteristics of these HTFs for achieving key desirables, such as
low viscosity, higher thermal conductivity and diffusivity, high boiling
and low freezing points, thermal stability, and anti-corrosive behaviour.
Figure 10.1 Nanomaterials for widespread applications of nanofluidics.
In this context, nanofluidics as an emerging thermal science and

engineering domain encompasses the study and manipulation of
fluids confined within nanostructures. Over the years, nanofluidics
has integrated into a wide range of technological areas, where diverse
formulation of nanomaterials is finding growing applicability in emerging
applications, as displayed in Figure 10.1. Remarkably, thermophysical
properties of nanofluids are being widely exploited with a growing
understanding of the micro-nano interface of nanoparticles(NPs)-fluid
Nanofluidics for Heat Transfer System and Energy Applications 229
and motion of NPs for use in applications ranging from lab-on-a-

chip systems for chemical and biochemical analysis to ultra-efficient
reactive processing and catalysis [1]. As a carrier fluid for drug
delivery, nanofluids allow more efficient drug absorption by living cells.
Moreover, the superior photo-thermal properties of nanofluids make
them attractive options for use in direct solar absorption collectors [2, 3].
In tribological applications, lubrication properties are remarkably
enhanced by using nanofluids as they enable the formation of a protective
film with low hardness and elastic modulus of the worn surface [4].
These recent reviews can be referred to for a more detailed account
and references to these emerging applications of nanofluids [1–6]. Here,
the discussion is restricted only to the heat transfer capabilities of
nanofluids.
Improving the heat transfer behaviour of traditional HTFs beyond
the fundamental limits using NPs has attracted significant interest after
the seminal work of Choi et al. [7, 8], who designated the NP’s suspension
in a fluid as “nanofluids”. Tuning the thermophysical properties of
traditional HTFs through control over particle size, shape, composition,
and other parameters, thus motivated several studies in the recent past
involving the formulation, characterisation, evaluation, and application
of promising next-generation HTFs [9–13]. Remarkably, colloidal and
classical thermal science has been established as the basis for these
studies, wherein a comprehensive understanding of the underlying
thermal transport phenomenon is widely sought for optimising the
thermophysical design of nanofluids [14–16]. However, the insufficient
description of the state of the NPs and their aggregates, besides diverse
forces of interaction and fluid dynamics in the base fluid, has often
skewed the overall understanding of nanofluids, impeding a rational
design of such systems.
Although significant efforts were devoted to evaluating the
heat transfer performance and its dependence on varied factors by
investigating the innumerable possible combination of nanofluids,
there remains a significant gap between fundamental research and the
practical applicability of nanofluids for thermal management [11–13, 17].
It is noteworthy that the inherent complexity in the formulation of a
consistent or standardised nanofluid sample necessitates the need for
more benchmark studies in this domain [18–20]. Thus, several factors,
including the lack of agreement between results, poor characterisation
of suspensions, and the vague understanding of the underlying
mechanisms, have remained the major bottlenecks for realising the
potential of nanofluids as advanced, nano-engineered HTFs. In the realm
of thermal science, large and anomalous thermophysical behaviour
measured for several nanofluids has driven the idea of exploiting
nanofluids as HTFs, which is the focus of this chapter with a threefold

purpose. Primarily, the physical mechanisms, theoretical models, and
parametric analysis of the thermal characteristics of nanofluids are
comprehended to understand anomalous thermal transport observed
in nanofluidics. Secondly, the extensive ongoing efforts for synthesis,
stability, and accurate measurements are highlighted to aid researchers
in developing nanofluids with superior thermal properties and enhanced
performance. Finally, the prospects of nanofluids in varied applications
are presented.
10.2 nanofluIdIcs Vs. MIcro-fluIdIcs
Maxwell pioneered the proposition of suspending particles in fluids to

improve their thermal transport in the 19th century as a scheme [21].
In fluidics, at the very beginning, micron or larger scale particles were
blended into the base fluids to make suspensions or slurries that were
mostly unstable with inherent problems, thereby limiting their practical
applicability. Thus, the route of suspending particles in liquid was a
well-known but rejected option for heat transfer applications until the
emergence of the idea of nanofluids in the mid-1990s proposed by Choi
et al. [7, 8], which helped stimulate the re-examination of this option.
Since then, nanofluidics has remained an active field of investigation
in thermal engineering. Compared to microfluidics, nanofluidics offers
advantages:
• The high specific surface area of NPs promotes interactions
between particles and fluids.
• High dispersibility induced Brownian motion of NPs.
• Reduced pumping power to avoid coagulation.
• Higher adaptability to miniaturised heat transfer systems.
Nanofluids formulations aim to embody the Brownian agitation that
overcomes any settling motion due to gravity. It is empirically found that
a stable nanofluid is possible when particles are small enough in size
(usually <100 nm). As the transport phenomena at the nanoscale bring
high particle mobility and a large surface-to-volume ratio in a fluid flow.
However, particle interactions at the nanoscale may lead to crowding
by forming large particle agglomerates that settle out of the suspension.
Thus, to overcome these inherent challenges and to develop nanofluids,
over the years, numerous experimental and theoretical studies on heat
transfer have been undertaken to gain insight into nanoscale thermal
transport and explore the advantages of nanofluids under various
conditions.
10.3 Heat transfer In nanofluIds
Heat is energy in transit whose flow is determined by temperature

gradients and occurs in three modes, i.e. conduction, convection,
and radiation. All heat transfer processes, including boiling and
condensation, combine these modes. The dependence of heat transfer on
temperature gradients was mathematically formulated using Fourier’s
law in both solids and fluids, while the efficiency is quantified using
thermal conductivity (k), which is regarded as a material property for
pure materials. Heat transfer via conduction mode involves three types
of energy carriers, i.e. phonons, electrons, and molecules. In the case
of nanofluids, which constitutes nanoscale solid particles confined in a
fluid, both NPs and based fluid tend to lose their identities, owing to
comparable dimensionality of the nano-sized particles with the mean
free path of the energy carriers, thereby leading to a quantum effect.
Thus, effective thermal conductivity (k NF) in nanofluids depends on
several parameters, and is not a material property, that is assumed only
for quantification. The high k NF exhibited by nanofluids encompasses
related and synergistic influences on specific heat (Cp), viscosity (μ),
convective heat transfer coefficient (h), and agglomeration of NPs, most
of which have been highlighted in several review articles [22–24].
Most studies from the onset has focused on understanding the anomalous
heat transfer enhancement in nanofluids, which is paramount for its
applicability as HTFs. Here, we present only a brief description of
these mechanisms of heat transfer, proposed models, and governing
parameters in nanofluidics to understand heat transfer.
10.3.1 Mechanism of Heat transfer

Till date, several mechanisms have been proposed to explain the
anomalous thermal transport behaviour observed in nanofluids. The
practical mean theory (EMT) [25, 26] based continuum formulations were
the pioneering description, which was modified with newer findings
including Brownian motion of the NPs, molecular-level layering of the
liquid at the liquid/particle interface, the nature of heat transport in
the NPs, and the effects of NPs clustering [27, 28]. These mechanisms
explain the microscopic and macroscopic transport of NPs in the static
and dynamic state of nanofluids, as shown in Figure 10.2. The energy
transport by dynamic motion of NPs is proposed to occur in four modes:
(i) the collision between base fluid molecules; (ii) the thermal interactions
of dynamic NPs with base fluid molecules; (iii) the collision between NPs
due to Brownian motion; and (iv) the thermal diffusion of NPs in the
fluid. These modes result in two kinds of Brownian motion in nanofluids:
collision between Brownian NPs and convection induced by Brownian

NPs at the molecular and nanoscale levels [29–32]. Moreover, as clustering
or aggregation is usual in nanofluids due to van der Waals forces,
clustered NPs may also provide local percolation-like paths for rapid heat
transport, which increases with the increasing volume fraction of NPs.
In such a case, well-dispersed NPs in a fluid matrix result in the lowest
kNF whereas interconnected NPs in the liquid enhance the kNF [33].
Studies to investigate the effects of aggregation on the nanofluids
have shown that the aggregation time constant decreases rapidly
with decreasing NPs size and that the k enhancement increases with
increasing levels of aggregation, levelling off after the optimum level
of aggregation is reached [33, 34]. However, when Brownian motion is
considered, such aggregation of NPs reduces the k NF of nanofluids as the
random motion of aggregates is slower than that of a single nanoparticle.
Moreover, clustering and formation of aggregates often reduces the
efficiency of the energy transport enhancement of the suspended
NPs [31, 32]. It is also observed that liquid molecules near a solid surface
tend to form a layered structure with an intermediate physical state [35].
These layer structures act as a thermal bridge between solid NPs and
bulk liquid in nanofluids and impact its thermophysical transport.
Figure 10.2 Mechanism of heat transfer in nanofluids with classification and

their schematic illustration.
During the natural convection process, where the flow is driven

by buoyancy and temperature, it was observed that when a mixture
of two or more types of motile particles (i.e. particles with moving
capability) is subjected to the force of a temperature gradient [36, 37]
particles commonly travel in the direction of decreasing temperature.

Also in such a case, the process of heat transfer increases with a
decrease in bulk density. This phenomenon called “thermophoresis”
explains the enhancement in k NF at low volume fraction of NPs, with
a subsequent decrease with increasing volume fraction of particles.
Dual-phase-lagging heat conduction is another macroscale phenomenon
developed from first principles that differs from Fourier heat conduction,
as it involves thermal oscillation and resonance originating from a
coupled conduction between the NPs and the base fluids [38, 39]. The
enhancement of the convective heat transfer performances revealed
in several experimental investigations was described by the non-
uniform distribution of thermal conductivity and viscosity field
due to particle re-arrangement, shear-induced thermal conduction
enhancement, reduction of thermal boundary layer thickness due
to the chaotic movements, fluctuations, and interactions of NPs, thereby
enhancing the convective heat transfer coefficient of nanofluids (hNF).
10.3.2 Models of Heat transfer

Several models were developed to predict anomalous thermal conduction
behaviour based on proposed microscopic mechanisms, which can
be classified into two broad categories i.e., static and dynamic. The
static model accounts for the different geometrical, static structures of
an NPs-fluid heterogeneous mixture. While the dynamic models, in
addition, consider nano-convections phenomena. A summary of existing
theoretical models and their significance are presented in Table 10.1 where
k NF is the effective thermal conductivity of nanofluids, k is the thermal
conductivity of base fluids, and f is the phase fraction of nanoparticles.
10.3.2.1 Classical Models

Classical nanofluids’ conduction models were based on diffusive heat
transfer through continuous matrix phase and dispersed phase, where NPs
were considered motionless. Such static models take into consideration
k of the constituents, physical properties of NPs such as volume
fraction, shape, and distribution, and the particle-particle interaction
in dense suspensions. These models were based on (EMT) [25, 26]
and provided a good description of fluidic phenomena occurring
with microns and larger-size particle dispersions but failed to predict
the anomalous thermal transport for nanofluids. Although firmly
established and thoroughly investigated, these correlations considered
no effect of aggregation, interface layer, and Brownian motion, which
supposedly prevailed in nanofluids, thus resulting in conflicts between
the experimental outcomes and theoretical predictions.
Table 10.1 A summary—Analytical models on thermal conductivity of nanofluids
234
Classical Models
Investigator Formula Remarks
Maxwell [21] • Based on the conduction solution through a stationary
k NF k NP + 2k + 2j [k NP -k ]
= random suspension of spheres.
k k NP + 2k - j [k NP - k ] • Spherical Particles.
Bruggeman [25] • Based on the differential effective medium [DEM]

k NF k
= ( 3j - 1) NP + {3(1 - j) - 1} + D theory to estimate the effective thermal conductivity of
k k composites at high particle concentrations.
2 • It consists in building up the composite medium through
È k ˘ k
D = Í(3j - 1) NP + {3(1 - j) - 1} ˙ + 8 NP a process of incremental homogenisation.
Î k ˚ k • Applicable to the high volume fraction of spherical
particles.

Hamilton and k NF k NP + [n - 1]k + [n - 1]j[k NP - k ] • Based on the effective thermal conductivity of a two-
Crosser [26] = component mixture.
k k NP + [n - 1]k - j[k NP - k ] • Applicable to spherical as well as non-spherical particles.
= 4.97j 2 + 2.72j + 1 • n = 3 [Spheres]; n = 6 [Cylinders].
Wasp et al. [40] • Based on the effective thermal conductivity of a two-

k NF k NP + 2k + 2j[k NP - k ]
= component mixture.
k k NP + 2k - j ÈÎk NP - k ˘˚ • Special Case of Hamilton and Crosser Model with n = 3.
Davis [41] • Green’s theorem was applied to the space occupied by

k NF 3[k - 1]
=1+ [j + f (k )j 2 + O(j 3 )] the matrix material [spherical inclusions].
k (k - 2) - j[k - 1] • A decaying temperature field was used.
Lu et al. [42] • The effective conductivity of composites containing

k NF
= 1 + aj + bj 2 aligned spheroids of finite conductivity was modelled
k with the pair interaction.
Nanofluidics for Heat Transfer System and Energy Applications
Jeffery[43] • Based on a conduction solution through a stationary
k NF È 3h 2 9h 3 k 1 + 2 ˘
= 1 + 3hj + j 2 Í 3h 2 + + +º˙ random suspension of spheres.
k ÍÎ 4 16 2k 1 + 3 ˙˚ • High order terms represent pair interactions of randomly
dispersed spherical particles.
k1 - 1 k
h= k = NP
k1 + 2 1 k
Xue et al. [35] • For CNTs-based nanofluids and including the axial ratio
È k NP ˘ k NP + k
1 - j + 2j Í ˙ ln and the space distribution.
k NF Î k NP - k ˚ 2k
=
k È k NP ˘ k NPP + k
1 - j + 2j Í ˙ ln
Î k NP - k ˚ 2k
Bhattacharya n
k NF k NP 1
et al. [29]
k
=
k
j + [1 - j] k NP =
kBT 2
V
Â ÈÎQ(0)Q( jDT )˘˚ DT
j =0
•
Theoretical Models for Nanofluids Effective Thermal Conductivity—Based on Nanolayer Effect
t = nanolayer thickness; – radius of nanoparticles
Xie et al.[44] • Based on Fourier’s Law of Heat Conduction
k NP - k Ê 3j 2q 2 ˆ
= 3qjT + Á T • Low volume fraction
˜
k Ë 1 - qjT ¯ • Nanolayer
4 t
jT = p [r + t]3 N p = j[1 + b]3 , b =
3 p rp
(Contd.)
235
Table 10.1 A summary—Analytical models on thermal conductivity of nanofluids (Contd.)
236
Yu and Choi [45] • Modified Maxwell model
k pe + 2k + 2j (k pe - k )(1 + b)3 • Spherical particles
k NF = k
k pe + 2k - j (k pe - k )(1 + b)3 • Nanolayer
2(1 - g ) + (1 + b )3 [1 + 2g ]g
k pe = k NP
-(1 - g ) + (1 + b)3 [1 + 2g ]
t �layer
b= and g =
rp �NP
Theoretical Models for Nanofluid’s Effective Thermal Conductivity—Based on Brownian Effect

Xuan et al. [34] • Based on the Maxwell model with the theory of Brownian
k NF k NP + 2k + 2j[k NP - k ] rpjC p kBT motion and diffusion-limited aggregation applied to
= +
k k NP + 2k - j[k NP - k ] 2k f 3pm rc simulate random motion and the aggregation process of
the nanoparticles.
• Includes the effect of particle size, random motion,
concentration, and Temperature.
Koo and • Based on Maxwell model.
Kleinstreuer [46, 47] k NF = k NP + 2k + 2j (k NP - k ) + 5 ¥ 10 4 br c kBT
f (T , j ) • Curve fitting of the available experimental data to
p p
k k NP + 2k - j (k NP - k ) rpc p determine the effective conductivity due to Brownian
motion.
T • Considered surrounding liquid travelling with randomly
f (T , j ) = ( -134.63 + 1722.3j) + [0.4705 - 6.04j ]
To moving nanoparticles.
ÔÏ0.0137(100j )
-0.8229
j < 0.01
b =Ì
-0.7272
ÓÔ 0.0011(100j ) j > 0.01
Nanofluidics for Heat Transfer System and Energy Applications
Prasher et al. [31] • Based on Maxwell model and heat transfer in fluidised
k NF k + 2k + 2j ÈÎk NP - k ˘˚
= [1 + A.Re m Pr 0.333 j] NP beds.
k k NP + 2k - j ÈÎk NP - k ˘˚ • Accounts for convection caused by the Brownian motion
from multiple particles.
Chon et al. [48] 0.369 0.747
• Based on curve fitting of experimental data.
k NF Ê df ˆ Ê k NP ˆ • Reynolds number is based on the Brownian motion
= 1 + 64.7j 0.74 Á ˜ ÁË k ˜¯ ¥ Pr 0.9955
Ree 1.2321
velocity.
k ÁË dp ˜¯
• Role of temperature and particle size.
Jang and Choi. [30] df • Based on kinetics, Kapitza resistance, and convection.
k NF
= [1 - j] + j + 3C PrjRe 2 • General expression for thermal conductivity involving
k dp four modes of energy transport in nanofluids: collision
b/w fluid molecules, thermal diffusion of nanoparticles
due to Brownian motion, and the thermal interactions of
dynamic nanoparticles with fluid molecules.
237
10.3.2.2 Dynamic Models

Dynamic models were based on particle dynamics, including particle
geometrical and directional clustering/percolation, matrix-particle
layering effects, and the role of aggregation and interfacial thermal
resistance, all of which were taken into account in addition to the
particle’s conventional static part. Convection phenomena induced
by contribution from thermal Brownian motion, thermophoresis,
diffusiophoresis, and other electromagnetic phenomena, including
near field radiation, thermal waves, dual-phase lagging, and other
unique phenomena, like ballistic phonon transport in NPs, occurring
at mesoscale were thus given due considerations. Empirical correlations
were routinely proposed for Nusselt number (Nu: the ratio of convective
to conductive heat transfer across a boundary), Prandtl number
(Pr: the ratio of momentum diffusivity (kinematic viscosity) to thermal
diffusivity), Reynolds number (Re: a dimensionless quantity that is used
to determine the type of flow pattern), and Peclet number (Pe: the ratio
of the convection rate over the diffusion rate in the convection-diffusion
transport system) in many of these studies based on the measured
flow and convective heat transfer characteristics by regression analysis.
These models and correlations stem from the mechanism, experimental
investigations, and assumptions. However, due to the complexity of
diverse NPs, additives, fluids, and their dynamic interfacial interactions,
including the inter-coupling of many phenomena, these studies on heat
transport in nanofluids requires a more systematic experiments. Well-
dispersed and accurately characterised nanofluids are thus prerequisities
for a better understanding of the physics of fluid flow and heat transfer
at the nanoscale by which more accurate and precise models to predict
the thermal transports in nanofluids can be established.
10.4 nanoMaterIals for nanofluIdIcs In

Heat transfer
Nanofluids can be formulated from any NPs by blending them into a

fluid (liquid or gas), thus offering innumerable combinations. It is now
well recognised that the thermophysical properties of conventional base
fluids can be improved by adding a low concentration of NPs. However,
formulating stable nanofluids with controlled thermophysical properties
for heat transfer applications is a formidable challenge [49, 50]. For this
purpose, several types of NPs based on pure metals (Au, Ag, Cu, Al,
and Fe), metal oxides (Al2O3, CuO, Fe3O4, SiO2, ZrO2, TiO2, and ZnO),
carbides (SiC and TiC), and a variety of carbon materials (diamond,
graphite, and single/multi-wall carbon nanotubes) were studied [4, 51–55].
Interestingly, high k nanomaterials may not continuously improve the

thermal performance of the base fluids, as k NF enhancement depends
on the stability condition and dispersibility of NPs in base fluid [27].
Among studied nanofluids, metals and allotropes of carbon (such
as diamond) were found to be most effective in k NF enhancement.
Also, higher k NF for metallic nanofluids were observed compared to
oxide nanofluids.
Based on the temperature regimes, the HTFs are commonly classified
as low-temperature fluids (i.e. hydrocarbons, silicone oils, and salt brine),
medium-temperature fluids (i.e. glycols like ethylene glycol, Bio-glycol
(1,3 propanediol) and propylene glycol), and high-temperature fluids
(i.e. paraffin oils, synthetic organic oils, molten nitrate salts). Traditional
HTFs, such as water (0.609 Wm–1K–1), ethylene glycol (0.258 Wm–1K–1),
and engine oil were investigated most commonly as base fluids for their
heat transfer aspect in nanofluidics. Also, non-Newtonian fluids such as
carboxy methyl cellulose (CMC) were examined [3, 56, 57]. The inverse
relationship between the k and particle size is experimentally validated
and theoretically supported by mechanisms of Brownian motion of
NPs and liquid layering around NPs for k NF enhancement [30, 48]. The
concentration range of NPs lies typically between 0.01–5 wt.%, while
the mean particle size is usually 10–100 nm. The success of effectively
developing nanofluids depends much on our understanding of the effects
of these interrelated aspects and parameters affecting thermophysical
transport, as shown schematically in Figure 10.3.
Figure 10.3 Parameters affecting heat transfer behaviour in nanofluids.
The concentration of NPs determines the functionality of nanofluids.

In most studies, a fascinating increase in the κ with low NPs volume
fraction (<1%) is observed, which is in contrast with the traditional
particle-liquid suspensions that require high concentrations (>10%) of
particles to achieve such dramatic k NF enhancement. As a parameter
investigated in almost all the experimental studies, it was found that
at higher concentrations, there exists a strong tendency among NPs to
form agglomerations in liquid media that affect the stability and NPs
transport in nanofluids. Thus, the dispersibility of NPs in nanofluids is
favoured by their small size, higher aspect ratio, and low volume fraction.
Moreover, dispersants, pH adjustments, or various mixing methods
can be employed to prevent agglomeration at higher volume fractions.
A dynamic behavioural explanation for nanofluids is complex as the
viscosity of the base fluid affects the Brownian motion of NPs which in
turn affects the k NF of the nanofluids. The size and aspect ratio of the
NPs are critical compositional characteristic that affects thermophysical
transport, wherein k NF and hNF increases with decreasing particle
size [48, 58] and increasing aspect ratio of dispersed NPs [56, 59–61].
In nanofluids, change in temperature affects the Brownian motion
of NPs and the clustering of NPs, which results in a dramatic change
in k NF of nanofluids, thereby indicating the temperature dependence of
thermal transport in nanofluids [62–64]. It is also found that the increase
in the difference between the pH value of the nanofluids solution and
the isoelectric point (pH at which a particular molecule or surface
carries no net electrical charge) of NPs resulted in the enhancement
of k NF, as with increasing differential, the mobility of NPs increases,
which improves the micro-convection effect. At the optimum pH value,
the surface charge of NPs increases, creating repulsive forces between
NPs, thus preventing severe clustering of NPs (excessive clustering
may result in sedimentation, which decreases k enhancement). Thus,
to obtain stable nanofluids, the pH value of the suspension must be
far from the isoelectric point of the particles, where the overall charge
on the NPs becomes zero. Thus, it can be concluded that nanofluids’
thermal transport behaviour is strongly correlated to the processing
methodology, compositional characteristics, and external factors, which
can be utilised to interpret conflicting results and inconsistent findings
via parametric analysis.
10.5 forMulatIng nanofluIds
The synthesis methodology regulates the stability and size distribution

of NPs, whereas functionality determines the combinations of
nanomaterials and their concentration in base fluids. In nanofluids,
agglomeration, chemical stability, and homogeneity have remained the
major bottlenecks that advancements have consistently addressed in the
pre-existing methodology for nanofluids synthesis [50, 65, 66]. Extensive
research efforts on nanofluid synthesis were focused on improving
thermal stability, dispensability in diverse media, chemical compatibility,
and ease of chemical manipulation. These methods are broadly classified
into two types: the single-step and the two-step method [27, 50, 67].
10.5.1 two-step Method

The two-step method is the most economical way to formulate the
nanofluids on a large scale, as nano-powder synthesis techniques
have already been scaled up to industrial production levels. Most
studies used a two-step method, including the earliest investigations
of nanofluids. In the first step, nanoparticles, nanofibers, nanotubes,
or other nanomaterials are produced as dry powders employing both
top-down and bottom-up approaches, as shown in Figure 10.4. General
synthetic methods for NPs production includes transition metal salt
reduction, thermal decomposition and photochemical methods, ligand
reduction and displacement from organometallics, metal vapour
synthesis, and electrochemical synthesis. The produced nano-sized
powder is then dispersed in a fluid in the second processing step with
the help of intensive magnetic force agitation, ultrasonic agitation, high-
shear mixing, or ball milling.
Figure 10.4 Length scale indicates bottom-up and top-down approaches for pre-
paring nanomaterials, which are subsequently dispersed in fluids to formulate
nanofluids.
10.5.2 single-step Method

To minimise the agglomeration of NPs, single-step nanofluids processing
methods have also been developed. Few methods exist today for the
preparation of nanofluids by single-step synthesis. In these methods, NPs
are simultaneously produced and dispersed into the base fluid. Thus,
the drying, storage, transportation, and dispersion of NPs are avoided,
minimising the agglomeration of NPs, and increasing nanofluids’
stability. The one-step processes can prepare uniformly dispersed NPs,

and the particles can be stably suspended in the base fluid. The one-
step physical method cannot synthesise nanofluids on a large scale, and
the cost is also high so the one-step chemical method is developing
rapidly. However, there are some disadvantages to the one-step method.
The most important one is that the residual reactants are left in the
nanofluids due to incomplete reaction or stabilisation. It is challenging
to elucidate the NPs effect without eliminating this impurity effect.
Compared to the single-step method, the two-step technique works well
for oxide NPs, while it is less successful with metallic particles.
Thus, the nanofluids preparation method is crucial in achieving
stable suspension. In addition, energetically powerful instruments such
as ultrasonic baths, homogenisers, and the processor can be utilised to
achieve well-dispersed suspensions. Other techniques, such as control of
pH or addition of surface-active agents, are also used to attain stability of
the suspension of the nanofluids against sedimentation. These methods
change the surface properties of the suspended particles and thus
suppress the tendency to form particle clusters. It should be noted that
the selection of surfactants should depend mainly on the properties of
the solutions and particles. In general, the aim of changing the surface
properties of suspended particles and suppressing the formation of
particles cluster is to obtain stable suspensions. However, the addition
of dispersants can affect the heat transfer performance of the nanofluids,
especially at high temperatures, thus requiring careful consideration.
10.6 MeasureMent of tHerMal conductIVIty

of fluIds
Accurate measurement is a prerequisite for developing nanofluids, as
inconsistencies in the evaluation of thermophysical properties made by
separate groups for identical nanofluids raise fundamental doubts and
pose a hindrance to the potential applicability of nanofluids. In general,
there are two methods of measuring the thermal conductivity of liquids:
steady-state methods and transient methods. Steady-state methods are
simple and direct but lack accuracy and require extensive and highly
complex experimental setups and procedures.
The convective heat transfer characteristics in different flows
(turbulent flow; laminar flow in both the developing and fully developed
regions) were investigated for various nanofluids. The experimental
setup in these studies typically consists of a flow loop consisting of
three sections: cooling unit, test section, and measuring units. The
test section comprises a circular tube with specification length, inner
diameter, and outer diameter. A constant wall temperature of pipes
is maintained through constant heat flux conditions as a boundary

condition while measuring the convective heat transfer coefficients.
Besides the characteristics of nanofluids, other determinants also play
a role in the indirect heat transfer process. For instance, average fluid
velocity and geometry of the system, such as tube dimensions and their
arrangement, affect the hNF for forced convection. For this consideration,
the heat transfer performance of nanofluids is measured directly under
flow conditions.
Reviews on techniques for measuring the k NF of nanofluids indicates
the shortcomings of steady-state methods, as the heat lost cannot be
quantified, giving considerable inaccuracy and the possibility of natural
convection, which gives higher conductivity values [68]. Among the
steady-state thermal conductivity measurement methods, i.e. guarded
hot plate, heat flow metre, temperature oscillation method, and the
temperature oscillation method is mainly used. Different techniques
used to measure the thermal conductivity of nanofluids are shown in
Figure 10.5. The critical issues related to the realisation of an appropriate
and comparative experimental protocol among several techniques to
Figure 10.5 Different measurement techniques for thermal properties of

nanofluids.
measure thermal transport in nanofluids and their adaptations, as

discussed in the recent reviews [69, 70] includes following aspects:
• Calibration of the sensors and/or thermocouples.
• Application of uniform heat flux during measurement.
• Accounting dispersion-related changes in nanofluids, including
pH variations, use of surfactants, etc.
• Consideration for the direction of the heat flow (upstream/
downstream)
10.7 nanofluIdIcs for Heat transfer

systeMs
A stable and easily synthesised, inexpensive, and nontoxic fluid with

excellent thermophysical properties (i.e. high thermal conductivity and
low viscosity) and long service life is highly desirable for employability
in heat transfer applications. Most of the experimental and numerical
studies showed that nanofluids exhibit an enhanced hNF compared to
its base fluid that increases with increasing NPs concentration and Re.
Remarkably, enhanced Nu and Re with increasing volume concentration
of ultrafine metallic oxide (Al2O3 and TiO2) particles in water, revealed
in the pioneering experimental investigation by Pak and Cho (1998) [71],
provided a fundamental basis for the idea of improving convective heat
transfer characteristics. Later experimental studies on several nanofluids
with varying flow conditions were conducted, most of which indicated
a considerable augmentation in h NF and k NF [51, 53]. Anomalous
enhancement in k NF of nanofluids is a crucial aspect favouring their
applicability in various types of heat exchangers. Better hNF and k NF
than that of traditional HTFs is experimentally validated in various
heat exchangers, i.e. radiators, circular tube heat exchangers, plate heat
exchangers, shell and tube heat exchangers, and heat sinks [72–74].
Increasing hNF NPs loading was observed for both the laminar and
turbulent flow regimes, wherein the effect of NPs concentration is more
significant in the turbulent flow regimes for the given flow Reynolds
number and particle size [75]. While a marginal change in pressure
drop was observed for low Re and NPs concentrations, a significant
deviation occurring at high Re and NPs concentrations in nanofluids
over base fluids highlights the practical limitations of their use [74]. In
the developing region, the hNF enhancement reduces with increasing
axial distance from the test section entrance, wherein for higher NPs
loading, a longer thermal entrance length is evaluated. With increasing
particle loading, an induced wall shear stress was also evaluated, while
some studies showed a very marginal change in the heat performance
of nanofluids, much lower than that predicted from a conventional

correlation [76, 77].
An optimal particle loading for maximum heat transfer at a minimum
cost of operation is desirable. Interestingly, in several nanofluids, the
optimal concentration of suspended NPs was found to be higher for
increased nanofluid bulk temperature, increased Re of the base fluid,
and the increased diameter-to-length ratio of the pipe. At the same time,
it is practically independent of the nanoparticle diameter [78]. The hNF
and k NF in most studies is found to hold strong relation with the flow
conditions and typically increases with volume flow rate, showing a
nonlinear enhancement with NPs volume fraction. Such enhancements
of nanofluids were directly proportional to the NPs concentration and
Pe. Remarkably, hNF shows a more significant enhancement than the
k NF for nanofluids and has been ascribed to NPs’ chaotic movements,
fluctuations, and interactions that reduce the thermal boundary layer
thickness. Optimisation assessments recommend an optimal NPs loading
of about ≤1% for different NPs and applications. Stable nanofluids with
the optimum NPs selection at an optimal volume concentration can thus
be employed in thermal systems if challenges such as stability, high
prices, and environmental impact are overcome.
10.8 applIcatIons
Nanofluidics is a widely explored domain under the umbrella of

nanotechnology. Over the years, nanofluid research has grown from
measuring and modelling several nanofluids’ thermophysical properties
to exploring the nanofluids’ performance in numerous engineering
applications, such as industrial applications, nuclear reactors,
transportation, electronics, as well as biomedicine and food. In this
section, we discuss prominent areas of applications for which nanofluids
are actively explored and developed.
For decades, research and development activities on improving the heat
transfer capabilities of heat exchangers have played a pre-eminent role in
providing solutions to various thermal problems in various engineering
applications. For instance, radiator (a heat exchanger) is a critical vehicle
component that prevents the engine from overheating. An improved
heating rate by modifications in geometries has enabled a significant
reduction in the size and weight of the radiator, thereby reducing fuel
consumption and improving efficiency. Similarly, miniaturisation and high-
power density of modern electronic devices required thermal solutions
for heat dissipation to realise higher performances, which was effectively
addressed using microchannel heat sinks (heat exchangers) with cooling
fluids. Heat exchangers are critical components in chemical and processing
industries, as they maintain diverse components of machinery, water,

chemical, and oils in a secure operating temperature range. Moreover,
the heat transfer process in solar energy systems is critical for achieving
the better performance of these systems with compact designs. In solar
devices, efficient heat transfer enables more power generation and has
fuel conservation features, by improving the energy utilisation efficiency
and reducing the heat transfer time thus establishing a strong interlink
between energy generation and its conversion.
10.8.1 automotive
Thermal management in automotive engines is a critical issue affecting
their performance and operability in harsh conditions. The heat
exchanger used in vehicles is referred to as a radiator, whose cooling
capabilities are directly related to the engine performance and fuel
consumption. With rising heat generation due to higher engine power
and exhaust gas recirculation, higher cooling needs are considered
critical design challenges in engine cooling. As with existing cooling
technology, it requires larger radiators with increased frontal areas,
thereby increasing fuel consumption and aerodynamic drag. Stable
nanofluids mainly address these limitations by enhancing the thermal
properties of coolants and oils [79]. As demonstrated in many studies, a
mixture of water and ethylene glycol is a standard engine coolant with
poor heat transfer performance that can be significantly improved by
NPs addition. However, problems such as surface erosion, dispersibility
and agglomeration are routinely highlighted as major impediments in
such studies. Thus, adapting nanofluids-based engine cooling, albeit
capable of dissipating heat more efficiently requires optimal design.
Several (TiO2, Al2O3, Cu/graphene, and CeO2) nanofluids as automobile
engine lubricants showed a reduced friction and wear rate coefficient
with enhanced brake thermal efficiency during experimental studies [79].
Also, Cu-based nanofluids showed higher boiling temperatures, viscosity,
and conductivities as brake fluid, which can be developed to prevent the
undesirable thermal degradation of the brake system during braking [80].
Thus, nanofluids’ superior thermophysical, rheological, and tribological
properties have been shown to improve operational capabilities by
enhancing heat dissipation. Automotives with a better thermal design
using nanofluids will offer increased power output, better mileage, and
fewer emissions if associated challenges are overcome soon.
10.8.2 electronics cooling

Commercial liquid cooling kits using (CuO, Al2O3, and water) nanofluids
are emerging products that lower the processor operating temperatures
by convective cooling. A typical cooling system of CPUs contains an

evaporator and condenser sections interconnected by a tube filled
with a nanofluid. The growing acceptability of nanofluids as HTFs
in microchannels is expected to enhance heat dissipation efficiency
further [81]. Interestingly, nanofluids comprising nano-encapsulated
PCM are emerging HTFs for electronic cooling and thermal energy
storage, as they inherit a higher heat capacity and display a high latent
heat of absorption. High particle loading in pre-existing nanofluids
may offer high convective cooling, but it can also reduce hydraulic
performance and cause significant pressure drop issues, which in
turn increases power consumption. Designing heat sinks with flat and
corrugated surfaces will enhance the turbulent intensity and mixing of
fluid flow to provide a higher heat transfer rate. Ongoing research aims
to integrate nanofluids in developing next-generation cooling devices for
dissipating heat in ultrahigh-heat-flux electronic systems [81].
10.8.3 nuclear reactors

The feasibility assessment of nanofluids in nuclear power plant systems,
such as pressurised water reactor (PWR) primary coolants, standby
safety systems, accelerator targets, plasma diverters, etc., for heat
exchange has shown excellent prospects in preliminary experiments [82].
For instance, in PWR, the obstructed critical heat flux (CHF) between the
fuel’s rods and the water due to vapour bubbles can be prevented with
CHF enhancement using alumina-based nanofluids. Thus, the boiling
of nanofluids shows an enhanced surface without altering the pool-
boiling characteristics of water. Also, for faster cooling of overheated
surfaces in critical systems and thermal control of plasma diverters
and accelerator targets, nanofluids display better performances over
traditional HTFs. However, in boiling water reactors, the NPs carried
over to the turbine and condenser may enhance erosion and fouling,
limiting their use. Moreover, fewer studies on nanofluid’s thermophysical
performance, particularly in prototypical reactor conditions with an
obscure understanding of nanofluid’s compatibility with the reactor
materials, require more validation.
10.8.4 solar thermal systems applications

Solar energy conversion into thermal energy involves efficient heat
transfer (convective, conductive, and radiative) between the mediums,
which can be augmented using nanofluids. For solar thermal energy
absorption, water, oil, and ethylene glycol are commonly HTFs. In
several types of solar collectors, such as flat-plate, evacuated tube,
cylindrical, conical, parabolic, and triangular shape solar collectors, the

employability of nanofluids is actively studied [83]. It was found that
for maximum efficiency, a lower concentration of NPs in a base fluid is
optimal for most solar collectors excluding flat-plate collectors, where
exergy efficiency increases with volume concentration. The expanding
applicability of nanofluids holds excellent prospects in solar thermal
systems, such as photovoltaic/thermal systems, solar water heaters, solar-
geothermal combined cooling heating and power system, evaporative
cooling for greenhouses, and water desalination, etc., for improving
energy conversion efficiency.
10.8.5 other emerging applications

Employing nanofluids in closed-loop cooling cycles by replacing the
pre-existing HTFs, i.e. in most cases, water holds excellent prospects in
energy savings and resulting in emissions reductions, particularly in
chemical and process industries. In the extraction of geothermal power
via drilling, the working equipments are often subjected to high friction
and a high-temperature environment, wherein “fluid superconductor”
nanofluids can provide rapid cooling for the machinery [84]. Also, by
improved cooling and lubrication using nanofluids, the operability of
sensors and electronics in harsh and higher-temperature surroundings
can be significantly improved, enabling capabilities to access deeper,
hotter regions in high-grade and economically acceptable temperatures
in lower-grade formations. In machining industries, (SiO2, MoS2, Cu,
Al2O3, and diamond) nanofluids as cutting fluid are widely demonstrated
in turning, milling, drilling, and grinding processes for enhancing
the cooling efficiency and lubricating the tool-workpiece interface [85].
Compared to traditional oil-based lubricants and dry machining,
nanofluid provides better surface finish and cooling efficiency during
machining.
10.9 cHallenges and sustaInabIlIty

assessMent
High thermal conductivity and low viscosity are crucial attributes

for selecting nanofluids in heat pipes. However, thermal conductivity
enhancement is invariably observed at the expense of viscosity. The
major bottlenecks for the applicability of nanofluids as HTFs shown
in Figure 10.6 includes the absence of standardisation resulting in
inconsistent data and conclusions on the nature and effects of particle
dispersion. For instance, different dimensions of a test setup with
varied concentrations and nanofluids type often skew any comparison

of heat transfer and flow properties. The technical specifications, such
as evaluation standards, filling rate, and inclination angle are yet to be
formulated and adapted, impeding scalable application at the industry
level. Although the round-robin test validates several observations,
their adaptation in experimental studies remains critical for the
intercomparison of several nanofluid alternatives and their suitability
for various types of heat pipes under different application scenarios.
Among reported studies on thermal conductivity, investigations
on convective heat transfer of nanofluids are few. Also, agglomeration
tendency and sedimentation of NPs, usually increase during operation
with time. Even the time-dependent evaluation of heat pipes charged
with nanofluids is rarely considered. After use in the heat exchanger
circuits, the manufactured nanofluids will get discharged to air,
soils, and water systems, triggering environmental concerns. It has
impeded the significant scalability of well-established nanofluids, which
necessitates prior studies into nanofluids’ physicochemical properties,
behaviour, and toxicity under realistic conditions such as the natural
aquatic environment. Moreover, the cost-effectiveness of formulating
nanofluids and pumping power requirements further requires evaluation
for assessing nanofluids’ scalability and broader applicability.
Figure 10.6 Challenges and sustainability aspects of nanofluids in heat trans-

fer applications.
10.10 conclusIon and outlook

The advent of nanoscience and nanotechnology and its rapid expansion
over the past two decades have rejuvenated newer solutions to rapidly
emerging heat transfer problems by employing nanofluids. Nanofluids,
an engineered nanoscale colloidal solution, has attracted great attention
from investigators for its superior thermal properties and has found wider
acceptability in many applications, despite the ongoing challenges for
addressing stability problems and inconsistencies within measurement
techniques/reports and ecological implications. NPs of metals and
their oxides, ceramics, and carbon composites were mainly utilised to
formulate homogeneous nanofluids with the desired concentration. The
significance of the complex interaction between NPs and base fluids
in determining thermal transport based on the existing mechanism of
heat transfer with inputs from theoretical and experimental research
is yet to reach conformity in explaining the anomalous enhancement.
However, the synergy within an integrated experimental and theoretical
approach is being actively applied to guide the development of
nanofluids with maximum heat transfer. Nanofluids’ enhanced heat
transfer capabilities make their use in heat exchangers an exciting option,
leading to better system performance and abilities in energy efficiency.
Conversely, nanofluid’s stability, scalability, and high production cost
are detrimental factors hindering widespread commercialisation.
Moreover, the physicochemical properties, behaviour, and toxicity of
nanofluids requires a thorough evaluation as their improper handling
poses potential risks of disrupting the soil environment and aquatic
ecosystem. Thus, a more careful evaluation of technical, economic, and
environmental aspects may provide wider acceptability to nanofluids
for large-scale applications.
RefeRences
[1] Sheikhpour, M., M. Arabi, A. Kasaeian, A. Rokn Rabei and Z. Taherian.
2020. Role of nanofluids in drug delivery and biomedical technology:
methods and applications. NSA 13: 47–59. https://doi.org/10.2147/NSA.
S260374.
[2] Elsheikh, A.H., S.W. Sharshir, M.E. Mostafa, F.A. Essa and M.K.A. Ali.
2018. Applications of nanofluids in solar energy: a review of recent
advances. Renewable Sustainable Energy Rev. 82: 3483–3502. https://
doi.org/10.1016/j.rser.2017.10.108.
[3] Xiong, Q., A. Hajjar, B. Alshuraiaan, M. Izadi, S. Altnji and S.A. Shehzad.
2021. State-of-the-art review of nanofluids in solar collectors: a review
based on the type of the dispersed nanoparticles. J. Cleaner Prod. 310:
127528. https://doi.org/10.1016/j.jclepro.2021.127528.
[4] Rasheed, A.K., M. Khalid, W. Rashmi, T.C.S.M. Gupta and A. Chan.

2016. Graphene based nanofluids and nanolubricants—review of recent
developments. Renewable Sustainable Energy Rev. 63: 346–362. https://
doi.org/10.1016/j.rser.2016.04.072.
[5] Chamkha, A.J., M. Molana, A. Rahnama and F. Ghadami. 2018.
On the nanofluids applications in microchannels: a comprehensive
review. Powder Technol. 332: 287–322. https://doi.org/10.1016/j.powtec.
2018.03.044.
[6] Sheremet, M.A. 2021. Applications of nanofluids. Nanomaterials 11: 1716.
https://doi.org/10.3390/nano11071716.
[7] Choi, S.U.S. 2009. Nanofluids: from vision to reality through research.
J. Heat Transfer. 131: 033106. https://doi.org/10.1115/1.3056479.
[8] Choi, S.U.S. and J. Eastman. 1995. Enhancing thermal conductivity
of fluids with nanoparticles, developments and applications of non-
newtonian flows. ASME, New York. 38.
[9] Devendiran, D.K. and V.A. Amirtham. 2016. A review on preparation,
characterization, properties and applications of nanofluids. Renewable
Sustainable Energy Rev. 60: 21–40. https://doi.org/10.1016/j.rser.2016.01.
055.
[10] Souza, R.R., I.M. Gonçalves, R.O. Rodrigues, G. Minas, J.M. Miranda,
A.L.N. Moreira, et al. 2022. Recent advances on the thermal properties
and applications of nanofluids: from nanomedicine to renewable
energies. Appl. Therm. Eng. 201: 117725. https://doi.org/10.1016/j.
applthermaleng.2021.117725.
[11] Wu, D., H. Zhu, L. Wang and L. Liu. 2009. Critical issues in nanofluids
preparation, characterization and thermal conductivity. Curr. Nanosci.
5: 103–112. https://doi.org/10.2174/157341309787314548.
[12] Okonkwo, E.C., I. Wole-Osho, I.W. Almanassra, Y.M. Abdullatif and
T. Al-Ansari. 2021. An updated review of nanofluids in various heat
transfer devices. J. Therm. Anal. Calorim. 145: 2817–2872. https://doi.
org/10.1007/s10973-020-09760-2.
[13] Adun, H., I. Wole-Osho, E.C. Okonkwo, D. Kavaz and M. Dagbasi.
2021. A critical review of specific heat capacity of hybrid nanofluids
for thermal energy applications. J. Mol. Liq. 340: 116890. https://doi.
org/10.1016/j.molliq.2021.116890.
[14] Awais, M., A.A. Bhuiyan, S. Salehin, M.M. Ehsan, B. Khan and
Md.H. Rahman. 2021. Synthesis, heat transport mechanisms and
thermophysical properties of nanofluids: A critical overview, Int. J.
Thermofluids 10: 100086. https://doi.org/10.1016/j.ijft.2021.100086.
[15] Keblinski, P., S.R. Phillpot, S.U.S. Choi and J.A. Eastman. 2002.
Mechanisms of heat flow in suspensions of nano-sized particles
(nanofluids). Int. J. Heat Mass Transfer. 45: 855–863. https://doi.org/10.
1016/S0017-9310(01)00175-2.
[16] Pinto, R.V. and F.A.S. Fiorelli. 2016. Review of the mechanisms responsible
for heat transfer enhancement using nanofluids. Appl. Therm. Eng. 108:
720–739. https://doi.org/10.1016/j.applthermaleng.2016.07.147.
[17] Wang, L. and J. Fan. 2010. Nanofluids research: key issues. Nanoscale
Res. Lett. 5: 1241–1252. https://doi.org/10.1007/s11671-010-9638-6.
[18] Buongiorno, J., D.C. Venerus, N. Prabhat, T. McKrell, J. Townsend,
R. Christianson, et al. 2009. A benchmark study on the thermal
conductivity of nanofluids. J. Appl. Phys. 106: 094312. https://doi.org/10.
1063/1.3245330.
[19] Lee, J.-H., S.-H. Lee, C. Choi, S. Jang and S. Choi. 2010. A review of
thermal conductivity data, mechanisms and models for nanofluids.
Int. J. Micro-Nano Scale Transp. 1: 269–322. https://doi.org/10.1260/1759-
3093.1.4.269.
[20] Lee, W.-H., C.-K. Rhee, J. Koo, J. Lee, S.P. Jang, S.U. Choi, et al. 2011.
Round-robin test on thermal conductivity measurement of ZnO
nanofluids and comparison of experimental results with theoretical
bounds. Nanoscale Res. Lett. 6: 258. https://doi.org/10.1186/1556-276X
-6-258.
[21] Maxwell, J.C.. 1881. A Treatise on Electricity and Magnetism: Pt. III.
Magnetism. Pt. IV. Electromagnetism. Clarendon Press.
[22] Angayarkanni, S.A. and J. Philip. 2015. Review on thermal properties
of nanofluids: recent developments. Adv. Colloid Interface Sci. 225:
146–176. https://doi.org/10.1016/j.cis.2015.08.014.
[23] Philip, J. and P.D. Shima. 2012. Thermal properties of nanofluids, Adv.
Colloid Interface Sci. 183–184: 30–45. https://doi.org/10.1016/j.cis.2012.
08.001.
[24] Murshed, S.M.S., K.C. Leong and C. Yang. 2008. Thermophysical and
electrokinetic properties of nanofluids—a critical review. Appl. Therm.
Eng. 28: 2109–2125. https://doi.org/10.1016/j.applthermaleng.2008.01.005.
[25] Bruggeman, D.A.G. 1935. Berechnung verschiedener physikalischer
Konstanten von heterogenen Substanzen. I. Dielektrizitätskonstanten
und Leitfähigkeiten der Mischkörper aus isotropen Substanzen. Ann.
Phys. 416: 636–664. https://doi.org/10.1002/andp.19354160705.
[26] Hamilton, R.L. and O.K. Crosser. 1962. Thermal conductivity of
heterogeneous two-component systems. Ind. Eng. Chem. Fund. 1: 187–191.
https://doi.org/10.1021/i160003a005.
[27] Das, S.K., S.U.S. Choi, W. Yu and T. Pradeep. 2007. Nanofluids. John Wiley
& Sons, Inc., Hoboken, NJ, USA. https://doi.org/10.1002/9780470180693.
[28] Xuan, Y. and W. Roetzel. 2000. Conceptions for heat transfer correlation
of nanofluids. Int. J. Heat Mass Transfer. 43: 3701–3707. https://doi.
org/10.1016/S0017-9310(99)00369-5.
[29] Bhattacharya, P., S.K. Saha, A. Yadav, P.E. Phelan and R.S. Prasher.
2004. Brownian dynamics simulation to determine the effective thermal
conductivity of nanofluids. J. Appl. Phys. 95: 6492–6494. https://doi.
org/10.1063/1.1736319.
[30] Jang, S.P. and S.U.S. Choi. 2004. Role of Brownian motion in the
enhanced thermal conductivity of nanofluids. Appl. Phys. Lett. 84:
4316–4318. https://doi.org/10.1063/1.1756684.
[31] Prasher, R., P. Bhattacharya and P.E. Phelan. 2006. Brownian-Motion-

Based Convective-Conductive Model for the Effective Thermal
Conductivity of Nanofluids. J. Heat Transfer 128: 588–595. https://doi.
org/10.1115/1.2188509.
[32] Shima, P.D., J. Philip and B. Raj. 2009. Role of microconvection
induced by Brownian motion of nanoparticles in the enhanced thermal
conductivity of stable nanofluids. Appl. Phys. Lett. 94: 223101. https://
doi.org/10.1063/1.3147855.
[33] Prasher, R., W. Evans, P. Meakin, J. Fish, P. Phelan and P. Keblinski.
2006. Effect of aggregation on thermal conduction in colloidal nanofluids.
Appl. Phys. Lett. 89: 143119. https://doi.org/10.1063/1.2360229.
[34] Xuan, Y., Q. Li and W. Hu. 2003. Aggregation structure and thermal
conductivity of nanofluids. AIChE J. 49: 1038–1043. https://doi.org/10.
1002/aic.690490420.
[35] Xue, L., P. Keblinski, S.R. Phillpot, S.U.-S. Choi and J.A. Eastman. 2004.
Effect of liquid layering at the liquid–solid interface on thermal transport.
Int. J. Heat Mass Transfer. 47: 4277–4284. https://doi.org/10.1016/j.ijheat
masstransfer.2004.05.016.
[36] Talbot, L., R.K. Cheng, R.W. Schefer and D.R. Willis. 1980.
Thermophoresis of particles in a heated boundary layer. J. Fluid Mech.
101: 737–758. https://doi.org/10.1017/S0022112080001905.
[37] Piazza, R. and A. Parola. 2008. Thermophoresis in colloidal suspensions.
J. Phys.: Condens. Matter. 20: 153102. https://doi.org/10.1088/0953-8984/
20/15/153102.
[38] Xu, M. and L. Wang. 2002. Thermal oscillation and resonance in dual-
phase-lagging heat conduction. Int. J. Heat Mass Transfer. 45: 1055–1061.
https://doi.org/10.1016/S0017-9310(01)00199-5.
[39] Xu, M. and L. Wang. 2005. Dual-phase-lagging heat conduction based
on Boltzmann transport equation. Int. J. Heat Mass Transfer. 48: 5616–
5624. https://doi.org/10.1016/j.ijheatmasstransfer.2005.05.040.
[40] Wasp, E.J., J.P. Kenny and R.L. Gandhi. 1977. Solid--liquid flow:
slurry pipeline transportation. [Pumps, valves, mechanical equipment,
economics]. Ser. Bulk Mater. Handl. (United States). 1.
[41] Davis, R.H. 1986. The effective thermal conductivity of a composite
material with spherical inclusions. Int. J. Thermophys. 7: 609–620.
https://doi.org/10.1007/BF00502394.
[42] Lu, S. and H. Lin. 1996. Effective conductivity of composites containing
aligned spheroidal inclusions of finite conductivity. J. Appl. Phys. 79:
6761–6769. https://doi.org/10.1063/1.361498.
[43] Jeffrey, D.J. 1973. Conduction through a random suspension of spheres.
Proc. R. Soc. Lond. A 335: 355–367. http://doi.org/10.1098/rspa.1973.0130.
[44] Xie, H., M. Fujii and X. Zhang. 2005. Effect of interfacial nanolayer
on the effective thermal conductivity of nanoparticle-fluid mixture.
Int. J. Heat Mass Transfer. 48: 2926–2932. https://doi.org/10.1016/j.
ijheatmasstransfer.2004.10.040.
[45] Yu, W. and S.U.S. Choi. 2003. The role of interfacial layers in the enhanced
thermal conductivity of nanofluids: a renovated maxwell model.
J. Nanopart. Res. 5: 167–171. https://doi.org/10.1023/A:1024438603801.
[46] Koo, J. and C. Kleinstreuer. 2004. A new thermal conductivity model for
nanofluids. J. Nanopart Res. 6: 577–588. https://doi.org/10.1007/s11051-
004-3170-5.
[47] Koo, J. and C. Kleinstreuer. 2005. Laminar nanofluid flow in microheat-
sinks. Int. J. Heat Mass Transfer. 48: 2652–2661. https://doi.org/10.1016/j.
[48] Chon, C.H., K.D. Kihm, S.P. Lee and S.U.S. Choi. 2005. Empirical
correlation finding the role of temperature and particle size for nanofluid
(Al2O3) thermal conductivity enhancement. Appl. Phys. Lett. 87: 153107.
https://doi.org/10.1063/1.2093936.
[49] Hwang, Y., J.K. Lee, C.H. Lee, Y.M. Jung, S.I. Cheong, C.G. Lee, et al.
2007. Stability and thermal conductivity characteristics of nanofluids.
Thermochim. Acta 455: 70–74. https://doi.org/10.1016/j.tca.2006.11.036.
[50] Che Sidik, N.A., M. Mahmud Jamil, W.M.A. Aziz Japar and
I. Muhammad Adamu. 2017. A review on preparation methods, stability
and applications of hybrid nanofluids. Renewable Sustainable Energy
Rev. 80: 1112–1122. https://doi.org/10.1016/j.rser.2017.05.221.
[51] Suganthi, K.S. and K.S. Rajan. 2017. Metal oxide nanofluids: review of
formulation, thermo-physical properties, mechanisms, and heat transfer
performance. Renewable Sustainable Energy Rev. 76: 226–255. https://
doi.org/10.1016/j.rser.2017.03.043.
[52] Patel, H.E., T. Sundararajan and S.K. Das. 2010. An experimental
investigation into the thermal conductivity enhancement in oxide and
metallic nanofluids. J. Nanopart Res. 12: 1015–1031. https://doi.org/10.
1007/s11051-009-9658-2.
[53] Das, S.K. and S.U.S. Choi. 2009. A review of heat transfer in nanofluids.
Adv. Heat Transfer 41: 81–197. https://doi.org/10.1016/S0065-2717(08)
41002-X.
[54] Huminic, G., A. Huminic, F. Dumitrache, C. Fleacă and I. Morjan. 2020.
Study of the thermal conductivity of hybrid nanofluids: recent research
and experimental study. Powder Technol. 367: 347–357. https://doi.
org/10.1016/j.powtec.2020.03.052.
[55] Sadeghinezhad, E., M. Mehrali, R. Saidur, M. Mehrali, S. Tahan Latibari,
A.R. Akhiani, et al. 2016. A comprehensive review on graphene
nanofluids: recent research, development and applications. Energy
Convers. Manage. 111: 466–487. https://doi.org/10.1016/j.enconman.2016.
01.004.
[56] Khadangi Mahrood, M.R., S.G. Etemad and R. Bagheri. 2011. Free
convection heat transfer of non-Newtonian nanofluids under constant
heat flux condition. Int. Commun. Heat Mass Transfer 38: 1449–1454.
https://doi.org/10.1016/j.icheatmasstransfer.2011.08.012.
[57] Hojjat, M., S.Gh. Etemad, R. Bagheri and J. Thibault. 2011. Convective
heat transfer of non-Newtonian nanofluids through a uniformly heated
circular tube. Int. J. Therm. Sci. 50: 525–531. https://doi.org/10.1016/j.

ijthermalsci.2010.11.006.
[58] Anoop, K.B., T. Sundararajan and S.K. Das. 2009. Effect of particle size
on the convective heat transfer in nanofluid in the developing region.
Int. J. Heat Mass Transfer 52: 2189–2195. https://doi.org/10.1016/j.
[59] Ding, Y., H. Alias, D. Wen and R.A. Williams. 2006. Heat transfer of
aqueous suspensions of carbon nanotubes (CNT nanofluids). Int. J. Heat
Mass Transfer 49: 240–250. https://doi.org/10.1016/j.ijheatmasstransfer.
2005.07.009.
[60] Ferrouillat, S., A. Bontemps, O. Poncelet, O. Soriano and J.-A. Gruss.
2013. Influence of nanoparticle shape factor on convective heat
transfer and energetic performance of water-based SiO2 and ZnO
nanofluids. Appl. Therm. Eng. 51: 839–851. https://doi.org/10.1016/j.
applthermaleng.2012.10.020.
[61] Timofeeva, E.V., J.L. Routbort and D. Singh. 2009. Particle shape effects
on thermophysical properties of alumina nanofluids. J. Appl. Phys. 106:
014304. https://doi.org/10.1063/1.3155999.
[62] Lee, S., S.U.-S. Choi, S. Li and J.A. Eastman. 1999. Measuring thermal
conductivity of fluids containing oxide nanoparticles. J. Heat Transfer
121: 280–289. https://doi.org/10.1115/1.2825978.
[63] Das, S.K., N. Putra, P. Thiesen and W. Roetzel. 2003. Temperature
dependence of thermal conductivity enhancement for nanofluids.
J. Heat Transfer. 125: 567–574. https://doi.org/10.1115/1.1571080.
[64] Yang, B. and Z.H. Han. 2006. Temperature-dependent thermal
conductivity of nanorod-based nanofluids, Appl. Phys. Lett. 89: 083111.
https://doi.org/10.1063/1.2338424.
[65] Wu, J.M. and J. Zhao. 2013. A review of nanofluid heat transfer and critical
heat flux enhancement—research gap to engineering application, Prog.
Nucl. Energy 66: 13–24. https://doi.org/10.1016/j.pnucene.2013.03.009.
[66] Pordanjani, A.H., S. Aghakhani, M. Afrand, M. Sharifpur, J.P. Meyer,
H. Xu, et al. 2021. Nanofluids: physical phenomena, applications
in thermal systems and the environment effects—a critical review.
J. Cleaner Prod. 320: 128573. https://doi.org/10.1016/j.jclepro.2021.128573.
[67] Keblinski, P., J.A. Eastman and D.G. Cahill. 2005. Nanofluids for
thermal transport, Materials Today 8: 36–44. https://doi.org/10.1016/
S1369-7021(05)70936-6.
[68] Paul, G., M. Chopkar, I. Manna and P.K. Das. 2010. Techniques for
measuring the thermal conductivity of nanofluids: a review. Renewable
Sustainable Energy Rev. 14: 1913–1924. https://doi.org/10.1016/j.rser.
2010.03.017.
[69] Souza, R.R., V. Faustino, I.M. Gonçalves, A.S. Moita, M. Bañobre-López
and R. Lima. 2022. A review of the advances and challenges in measuring
the thermal conductivity of nanofluids. Nanomaterials 12: 2526. https://
doi.org/10.3390/nano12152526.
[70] Pryazhnikov, M.I., A.V. Minakov, V.Ya. Rudyak and D.V. Guzei. 2017.
Thermal conductivity measurements of nanofluids. Int. J. Heat Mass
Transfer. 104 (2017) 1275–1282. https://doi.org/10.1016/j.ijheatmass
transfer.2016.09.080.
[71] Pak, B.C. and Y.I. Cho. 1998. Hydrodynamic and heat transfer study
of dispersed fluids with submicron metallic oxide particles. Exp. Heat
Transfer. 11: 151–170. https://doi.org/10.1080/08916159808946559.
[72] Pandya, N.S., H. Shah, M. Molana and A.K. Tiwari. 2020. Heat transfer
enhancement with nanofluids in plate heat exchangers: a comprehensive
review. Eur. J. Mech. B. Fluids 81: 173–190. https://doi.org/10.1016/j.
euromechflu.2020.02.004.
[73] Bretado-de los Rios, M.S., C.I. Rivera-Solorio and K.D.P. Nigam. 2021.
An overview of sustainability of heat exchangers and solar thermal
applications with nanofluids: a review. Renewable Sustainable Energy
Rev. 142: 110855. https://doi.org/10.1016/j.rser.2021.110855.
[74] Sivashanmugam, P. 2012. Application of nanofluids in heat transfer.
In: S.N. Kazi (ed.). An Overview of Heat Transfer Phenomena. InTech.
https://doi.org/10.5772/52496.
[75] He, Y., Y. Jin, H. Chen, Y. Ding, D. Cang and H. Lu. 2007. Heat
transfer and flow behaviour of aqueous suspensions of TiO2
nanoparticles (nanofluids) flowing upward through a vertical pipe,
Int. J. Heat Mass Transfer. 50: 2272–2281. https://doi.org/10.1016/j.
[76] Fotukian, S.M. and M. Nasr Esfahany. 2010. Experimental investigation
of turbulent convective heat transfer of dilute g-Al2O3/water nanofluid
inside a circular tube. Int. J. Heat Fluid Flow. 31: 606–612. https://doi.
org/10.1016/j.ijheatfluidflow.2010.02.020.
[77] Fotukian, S.M. and M. Nasr Esfahany. 2010. Experimental study of
turbulent convective heat transfer and pressure drop of dilute CuO/
water nanofluid inside a circular tube. Int. Commun. Heat Mass Transfer
37: 214–219. https://doi.org/10.1016/j.icheatmasstransfer.2009.10.003.
[78] Corcione, M., M. Cianfrini and A. Quintino. 2012. Heat transfer of
nanofluids in turbulent pipe flow. Int. J. Therm. Sci. 56: 58–69. https://
doi.org/10.1016/j.ijthermalsci.2012.01.009.
[79] Patel, J., A. Soni, D.P. Barai and B.A. Bhanvase. 2023. A minireview
on nanofluids for automotive applications: current status and future
perspectives. Appl. Therm. Eng. 219: 119428. https://doi.org/10.1016/j.
applthermaleng.2022.119428.
[80] Kao, M.J., C.H. Lo, T.T. Tsung, Y.Y. Wu, C.S. Jwo and H.M. Lin. 2007.
Copper-oxide brake nanofluid manufactured using arc-submerged
nanoparticle synthesis system. J. Alloys Compd. 434–435: 672–674.
https://doi.org/10.1016/j.jallcom.2006.08.305.
[81] Bahiraei, M. and S. Heshmatian. 2018. Electronics cooling with
nanofluids: a critical review, energy convers. Manage. 172: 438–456.
https://doi.org/10.1016/j.enconman.2018.07.047.
[82] Buongiorno, J., L.-W. Hu, S.J. Kim, R. Hannink, B. Truong and E. Forrest.
2008. Nanofluids for enhanced economics and safety of nuclear reactors:
an evaluation of the potential features, issues, and research gaps.
Nuclear Techno. 162: 80–91. https://doi.org/10.13182/NT08-A3934.
[83] Wahab, A., A. Hassan, M.A. Qasim, H.M. Ali, H. Babar and M.U. Sajid.
2019. Solar energy systems—potential of nanofluids. J. Mol. Liq. 289:
111049. https://doi.org/10.1016/j.molliq.2019.111049.
[84] Soltani, M., F. Moradi Kashkooli, M. Alian Fini, D. Gharapetian,
J. Nathwani and M.B. Dusseault. 2022. A review of nanotechnology
fluid applications in geothermal energy systems. Renewable Sustainable
Energy Rev. 167: 112729. https://doi.org/10.1016/j.rser.2022.112729.
[85] Hemmat Esfe, M., M. Bahiraei and A. Mir. 2020. Application of
conventional and hybrid nanofluids in different machining processes:
a critical review. Adv. Colloid Interface Sci. 282: 102199. https://doi.
org/10.1016/j.cis.2020.102199.
Index
A E
Applications 3, 4, 21, 22, 23, 25, 26, Electrospinning 1, 2, 3, 4, 5, 6, 9, 10,
27, 28, 29, 30, 31, 33, 34, 40, 42, 45, 13, 15, 16, 17, 199
46, 54, 56, 58, 59, 61, 68, 85, 94, 99, Energy application 227
100, 101, 103, 105, 106, 107, 113, Energy Harvesters 22, 24, 26, 38, 41,
114, 123, 124, 127, 128, 129, 131, 46
132, 137, 138, 146, 147, 150, 154,
Energy transfer 125, 126, 127, 129
155, 157, 163, 165, 166, 168, 169,
170, 172, 173, 174, 189, 193, 200, Environment-friendly 94, 95, 158,
201, 202, 203, 212, 216, 227, 228, 170, 209
229, 230, 238, 244, 245, 247, 248,
249, 250 F
Flexible metal foils 94, 105, 106
B Flexible sensors 137, 138, 139, 140,
Bi2Se3 thin film 103, 107, 108, 109, 142, 144, 145, 146, 147, 149, 151
110, 111, 114
G
C Graphene 27, 28, 29, 33, 34, 42, 43,
Carbon nanofiber 2, 3, 5, 6, 10, 18, 43 44, 45, 46, 138, 140, 141, 143, 147,
Carbon nanotubes 26, 27, 28, 29, 30, 148, 150, 196, 216, 246
31, 32, 34, 44, 45, 46, 143, 238
Conducting polymers 35, 142 H
Heat transfer 29, 124, 227, 228, 229,
D 230, 231, 232, 233, 237, 238, 239,
242, 243, 244, 245, 246, 247, 249,
Dye-sensitised solar cells (DSSC) 190,
250
191, 192, 193, 194, 196, 197, 198,
199, 200, 201, 202, 203 High efficiency 57, 158, 167
260 Index
L Pb-free 154, 156, 158, 159, 160, 162,

Light trapping 56, 62 163, 165, 166, 170, 173, 174
Perovskite inks 154, 155, 157, 158,
M 164, 173, 174
Magnetron sputtering 107, 114 Phosphors 83, 84, 123, 124, 125, 127,
128, 129, 130, 131, 132, 138
Mechanical properties 28, 29, 30, 31,
33, 61 Photocatalysis 210, 212, 213, 214,
216, 217
Mechanism 2, 8, 23, 29, 30, 62, 69, 71,
124, 125, 126, 127, 128, 139, 140, Photoelectrochemical water
141, 147, 160, 167, 210, 229, 230, splitting 210
231, 232, 233, 238, 239, 250 Photovoltaics 55, 59, 85
Metal oxide 44, 46, 138, 195, 196,
197, 209, 210, 211, 212, 213, 215, Q
216, 219, 220, 238 Quantum cutting 124, 125, 126, 127
Metal-halides 157, 158, 162, 164, 165,
166, 170, 174 R
Module 41, 46, 56, 200, 201, 202, 203 Reduced reflectivity 57
Morphology tuning 199
S
N Screen-printing 200, 201
Nanocomposites 21, 25, 27, 28, 29, Sensor 21, 22, 24, 25, 26, 39, 40, 41,
31, 32, 33, 42, 43, 44, 45, 46, 140 42, 44, 45, 46, 59, 123, 124, 129,
Nanocomposites 21, 25, 27, 28, 29, 137, 138, 139, 140, 141, 142, 143,
31, 32, 33, 42, 43, 44, 45, 46, 140 144, 145, 146, 147, 148, 149, 150,
Nanofluids 25, 26, 227, 228, 229, 230, 151, 189, 244, 248
231, 232, 233, 234, 235, 236, 237, Silicon nanostructures 56, 58, 60, 61,
238, 239, 240, 241, 242, 243, 244, 63, 66, 82, 85
245, 246, 247, 248, 249, 250 Supercapacitor 1, 2, 3, 4, 13, 14, 15,
Nanostructure 3, 4, 5, 11, 18, 25, 28, 18, 26, 41, 43, 105
34, 55, 56, 58, 59, 60, 61, 63, 66, 82,
84, 85, 100, 129, 147, 150, 171, 199,
T
218, 228
Thermal conductivity 23, 29, 34, 61,
Nanotechnology 18, 27, 54, 55, 59,
103, 228, 231, 233, 234, 235, 236,
137, 245, 250
237, 242, 243, 244, 248, 249
O
V
Optoelectronics 59, 67, 68, 154, 155,
V-VI Binary Chalcogenides 100, 111,
156, 157, 162, 163, 164, 165, 166,
103, 104
170, 171, 173, 174
Organic-inorganic 155, 164
W
Water Splitting 55, 95, 96, 103, 104,
P
106, 112, 114, 155, 209, 210, 211,
Paste-preparation 199, 203 212, 213, 214, 215, 216, 219, 220
Pb-based 157, 158, 160, 163, 165, 166, Working Electrode 11, 97, 108, 189,
173, 174 193, 195, 200, 201, 202, 203

Nanomaterials For Energy and Sensor Applications

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Nanomaterials for Energy and Sensor Applications

Nanomaterials for Energy

First edition published 2024

The book, “Nanomaterials for Energy and Sensor Applications,” edited

Chapter 3 explores silicon (Si) based nanostructures for solar energy

The advantage of dye sensitization in solar cell (DSSCs) application

Vidya Nand Singh

First and foremost, we humbly express our gratitude to the Omnipotent

Vidya Nand Singh

Chapter 6. A Review of Flexible Sensors 137

Porous and Hollow Carbon

Kingsford Asare1, Md Faruque Hasan1,

1.1 IntroductIon to SupercapacItor

Electrical energy storage is essential in modern society and spans uses

*For Correspondence: Lifeng Zhang (lzhang@ncat.edu), Abolghasem Shahbazi

1.2 electroSpun carbon nanofIbrouS

N-dimethyl formamide (DMF) to make an electrospinning solution

1.3 porouS/Hollow carbon nanofIbrouS

core solution [18]. In recent years, hollow ECNFs as well as porous

1.4 coMparatIve Study of porouS and

It is noteworthy that all other research about the electrochemical

1.4.1 preparation of electrode Materials

[21], four types of carbon nanofibrous materials including solid carbon

1.4.2 electrode Materials characterization

Figure 1.1 Representative SEM images of electrospun carbon nanofibrous

of these carbon nanofibrous materials. Among all the studied carbon

Raman Shift (cm-1)

BET surface area and porosity analyses of the electrospun carbon

According to BET-specific surface area and porosity analysis results

that of ECNFs, indicating that the PAN/PMMA core-shell structure from

Table 1.1 BET-specific surface area and porosity of electrospun carbon

1.4.3 Electrochemical Evaluation

1.4.3.1 Specific Capacitance

0.4 0.4 0.4

0.2 0.2 0.2

0.0 0.0 0.0

-0.2 -0.2 -0.2

-0.4 -0.4 -0.4

Current density (A/g)

Current density (A/g)

Current density (A/g)

-0.2 ECNF -0.2 -0.2 ECNF

-0.6 -0.6 -0.6

-0.8 -0.8 -0.8

Time (s) Time (s)

H-ECNFs, and HP-ECNFs, electrochemical performance increased with

Specific Capacitance (F/g)

Figure 1.5 Specific capacitances of electrospun carbon nanofibrous materials

Figure 1.6 CV (A) and galvanostatic CD (B) profiles of P-ECNF-50-50 at

1.4.3.2 Electrochemical Impedance Spectroscopy (EIS)

nanofibrous electrodes showed no or much-depressed semi-circles at

1.4.3.3 Cycling Stability

Figure 1.8 Cycling stability of electrospun carbon nanofibrous electrode

Particularly P-ECNF-50-50 and HP-ECNF-50-50 even showed some

caused by continuous diffusion of electrolyte ions into previously

0.30 0.30 0.30

Pore volume (cm3/g)

Pore Volume (cm3/g)

0.15 0.15 0.15