Materials Science in Semiconductor Processing 31 (2015) 380–385

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Materials Science in Semiconductor Processing

journal homepage: www.elsevier.com/locate/mssp

Effect of potential on the early stages of nucleation

and properties of the electrochemically synthesized
ZnO nanorods
A. Henni a,n, A. Merrouche a, L. Telli a, A. Azizi b, R. Nechache c
a
Laboratoire des Matériaux inorganiques, Université de M’sila, M’sila 28000, Algeria
b
Laboratoire de Chimie, Ingénierie Moléculaire et Nanostructures, Université Ferhat Abbas-Sétif 1, Sétif 19000, Algeria
c
Institut national de la recherche scientifique, Énergie, Matériaux et Télécommunications, INRS, 1650, Boulevard Lionel-Boulet, Varennes,
QC, Canada J3X 1S2

a r t i c l e in f o abstract

The zinc oxide nanostructured were synthesized on Indium doped tin oxide substrate
Keywords: using cathodic reduction of H2O2 and ZnCl2 from chloride aqueous electrolyte under
Zinc oxide differents applied potentials. The effects of this potential on the nucleation of ZnO seeds
Chronoamperometry were investigated by performing transient current measurements and using model based
Photocurrent on Scharifker–Hills equations. The results suggest that the nucleation mechanism of ZnO
Nucleation is progressive with a three-dimensional growth of the hemispherical nuclei. The XRD
Nanorods patterns show that the ZnO crystallizes in a hexagonal Würtzite-type structure with a
phase preferentially orientated along c-axis. The structural analysis evidences a strong
relationship between the directional growth along (0 0 2) crystallographic plane and the
applied potentials. ZnO crystalizes in hexagonal nanorods (NRs) with diameters in the
ranges of 160–250 nm. Photoelectrochemical study indicates that the obtained films have
n-type semiconducting behaviour, and generate high photocurrents up to 40 μA/cm2 at

1.0 and
1.1 V. The transmittance spectra indicate that the films exhibit a good optical
quality with low defects density with an averaged band gap of  3.31 eV.
& 2014 Elsevier Ltd. All rights reserved.

1. Introduction layer in nanogenerators [8] and humidity sensors [9]. To

date, various methods are used to obtain ZnO films with
Zinc oxide (ZnO) has recently attracted considerable different morphologies [10–13] such as the sol–gel [14],
attention because it exhibits a large band gap of 3.3 eV [1], hydrothermal [15], spray pyrolysis [16], template method
a large exciton binding energy of 60 meV at room tempera- [17], chemical vapor deposition (CVD) [18] and electrodepo-
ture, excellent catalytic, optical, electrical, optoelectronic, sition methods [19–21]. Preceded the electrodeposition of
gas-sensing, piezoelectric, and photoelectrochemical proper- thin films is considered one of the promising ways due to the
ties [2,3]. ZnO is currently implemented in many applications ability to control the thickness, a large-area deposition, a
including solar cells [4], light emitting diodes [5], waveguides simple process, good adhesion and a low cost fabrication
[6] and lasers [7]. In addition, ZnO is piezoelectric and process. There have been many reports on the electrodeposi-
moisture sensitive material and has been explored as active tion of ZnO using aqueous [22,23] and non-aqueous routes
[24]. Most of the reported work on electrodeposition was
done using the nitrate ions [25–27] or molecular oxygen [28]
n
Corresponding author. as precursors for electrodeposition of ZnO.

http://dx.doi.org/10.1016/j.mssp.2014.12.011
1369-8001/& 2014 Elsevier Ltd. All rights reserved.
 A. Henni et al. / Materials Science in Semiconductor Processing 31 (2015) 380–385 381

In the present work, we report the electrochemical The zinc hydroxide formed in this reaction is trans-
synthesis of ZnO nanorods on Indium tin oxide (ITO) con- formed at 65 1C into zinc oxide as following:
ducting glass in chloride medium and using a hydrogen
peroxide as a precursor. More efforts have been focused to Zn(OH)2-ZnOþH2O (2)
investigate the effect of applied potential on microstructural
and functional properties of ZnO. The chronoamperometry
techniques were employed to study the initial growth stages Scharifker and Hills [32] have developed theoretical
of ZnO during the electrodeposition. Models based on models to describe the nucleation process during the
Scharifker-Hills calculations were established to determine initial few seconds using chronoamperometric curves.
the nucleation and growth mechanism. The nucleation rate According to the three-dimensional (3D) nucleation
and the diffusion coefficient of ZnO were also investigated. model, two limiting cases referring to instantaneous and
The effects of the applied potential on electrochemical, progressive nucleation modes have been considered.
structures, morphologies, photoelectrochemical and optical Instantaneous nucleation corresponds to fast growth of
properties of ZnO nanostructures were studied in detail to nuclei on many active sites, all activated during the
evaluate the potential of use of such fabrication method to electro-reduction step [33] according to the following
achieve ZnO nanorods with a crystal quality comparable to relationship (Eq. (3)):
transparent conducting oxide films prepared by other tech-
 2    2
niques [29–31]. i t max t
¼ 1:9542 1
exp
1:2564 ð3Þ
imax t t max

2. Experimental whereas, progressive nucleation describe a slower growth

of nuclei on a small number of active sites, all activated at
Films were electrodeposited onto ITO coated glass
substrates (Solems Paris, 1 cm  2 cm, 25 Ω/sq). The sub-
strates were first ultrasonically cleaned in acetone, then in
ethanol for 10 min, and finally well rinsed with distilled
water. The electrodeposition procedure was carried out in
a three-electrode geometry system, in which a Pt electrode
behave, a saturated calomel electrode (SCE) as and the ITO
was used as counter, reference and working electrodes,
respectively. The films were cathodically electrodeposited
in a bath containing 5  10
3 mol/L ZnCl2, 5  10
3 mol/L
H2O2, where 0.1 mol/L KCl was used as supporting electro-
lyte. The above chemical products were dissolved in the
deionized water. The bath temperature was controlled at
65 1C with a LabTech LCB-11D water circulating system.
The electrochemical growth, light transmission, photocur-
rent generated, structure, and morphology of the obtained
films were systematically compared.
Electrodeposition, Current-time transients and photocur-
rent measurements were performed using an Autolab
PGSTAT302N Galvanostat–Potentiostat equipped with a Fre-
quency Analyzer Module. The UV–vis transmittance spectra
have been recorded with a Shimadzu UV-1800 UV/visible
Scanning Spectrophotometer. X-ray diffraction (XRD) mea-
surements were conducted using a Bruker D8 Advance
diffractometer with Cu Kα radiation (1.5406 Å). The surface
morphology of these ZnO films was investigated by using a
JSM-7001F scanning electron microscope (SEM).

3. Results and discussion

3.1. Nucleation and growth mechanism study

A fundamental reaction characterizing the electrode-

position of ZnO is based on reduction of a hydrogen
peroxide precursor (H2O2) to OH
, followed by reaction
Fig. 1. (a) Experimental current transients for ZnO electrodeposition on
with Zn2 þ ions, according to the equation: ITO electrodes from, the electrode potentials were imposed from
0.8 to

1.2 V vs. SCE. (b) Non-dimensional ((i/imax)2 vs. t/tmax) plots of the data
Zn2 þ þ2OH
-Zn(OH)2 (1) of current transients.
382 A. Henni et al. / Materials Science in Semiconductor Processing 31 (2015) 380–385

the same time [33] following this relationship (Eq. (4)):

     2
i 2 t max t
¼ 1:2254 1
exp
1:3367 ð4Þ
imax t t max
To determine the nucleation and growth mechanism of
our electrodeposited ZnO, a series of current-time transi-
ents was performed at different applied potentials during
initial time period of the growth (Fig. 1a).
From the current-transit time (Fig. 1a) three distinct
steps are observed. The first step, absolute value of the
initial current abruptly decreases which is related to
double layer charging. Once Nersnt’s conditions satisfied,
the current increases with time and passes through a
maximum value (imax) at time (tmax). In the last step the
current reduces and stabilizes is transient mode following
the Cottrell equation. Maximum current (imax) and time
(tmax) values are affected the applied potential.
The fitting of the experimental curves using the above
theoretical models for ZnO obtained at different potentials
illustrated in Fig. 1b. (i/imax)2 vs. t/tmax were plotted and
compared to the theoretical curves describing the instan-
taneous and progressive 3D nucleation modes. The results
Fig. 2. X-ray diffractograms of ZnO nanostructures at various deposition
suggests that, for all films, ZnO growth is characterized by potentials. Asterisk for ITO diffraction peaks.
a progressive nucleation mechanism. Furthermore detailed
analysis of the kinetics mechanism (cf. Fig. 1b) is per-
formed using Scharifker’s general equations to estimate
the typical kinetic parameters (Table 1). The nucleation substrates (JCPDS no. 41-1445) are observed. The films crystal-
rate ðAN0 Þ and the diffusion coefficient ðDprog Þ were esti- lizes in single wurtzite phase, and no additional lines corre-
mated from the following equations, respectively (Eqs. (5) sponding to other phases or impurities were visible. The
and (6)): peaks marked with an asterisk (n) are attributed to ITO-coated
glass substrates.
AN 0 ¼ 0:2898ð8πCM=ρÞ
1=2 ðzFCÞ2 =imax 2 t 3max ð5Þ
High intensity of ZnO (0 0 2) reflection at 34.41 suggests
that the crystalline grains are mainly oriented along the c-axis
i2max t max ¼ 0:2598ðnFC Þ2 Dprog ð6Þ
normal to the ITO substrate surface. The intensity of these
where zF is the molar charge transferred during reflection which is related to crystallinity degree of the film,
electrodeposition, D is the diffusion coefficient, C is fluctuates with the potential applied during the deposition. A
the bulk concentration of the electroactive species, t is similar behaviour has been reported for ZnO grown using
time, M and ρ are the atomic weight and the density of nitrates electrolytes [34]. Fig. 2 shows that the intensity of
the deposit, respectively. (0 0 2) reflection is high when ZnO is deposited at applied
potentials of
1.0 and
1.1 V vs. SCE and decrease for
3.2. Structural and morphological studies potentials exceeding
1.1 V. This is attributed to the zinc
deposition reaction in competition with the ZnO deposition.
To investigate the microstructural evolution of ZnO thin ZnO obtained at different potential, forms aligned hexago-
films with different the applied potential deposition, XRD nal nanowires with diameters ranging from 160 to 250 nm
measurements were performed. XRD spectra of the nanocrys- (cf. Fig. 3). The sample prepared at
0.8 V exhibits an
talline ZnO films obtained directly on ITO substrates after inhomogeneous layer in the opposite of those synthetized at
40 min electrodeposition at
0.8 V,
0.9 V,
1.0 V,
1.1 V higher potential (4
0.8 V), which exhibit more packed and
and
1.2 V are shown in Fig. 2. Only peaks corresponding to dense hexagonal grains. The used potential affects the mor-
reflections of ZnO thin layers (JCPDS no. 36-1451). and ITO phological characteristics of nanowires, including diameter,
height and density. This is due to the influence of such the
formation of nucleation sites, the direction and the rate of
Table 1 growth nanowires. The crystallites have a columnar-like
Electrochemical parameters resulting from the current transients for ZnO structure perpendicular to the substrate and preferentially
deposited at different potential imposed. oriented along c-axis. The measured diameters are summar-
E (V vs. SCE) 10
6 AN0 (cm
2/s) 10 þ 6 Dprog (cm
2/s)
ized in Table 2. The diameters of fabricated ZnO nanorods are
similar with results previously reported [35].

0.8 1.38 0.60 Relative potential used for electrochemical deposition

0.9 2.84 1.16 ranges from
0.8 V to
1.2 V vs. SCE the higher the potential

1.0 3.31 1.65
is the faster the growth rate will be. And the corresponding

1.1 3.65 1.85

1.2 4.98 2.10 current is from
0.4 mA to
0.8 mA, respectively. These
structural and morphological studies reveal that the optimal
 A. Henni et al. / Materials Science in Semiconductor Processing 31 (2015) 380–385 383

Fig. 3. SEM micrographs showing the surface morphology of the ZnO nanowires deposited at different potential: (a)
0.8 V, (b)
0.9 V, (c)
1.0 V,
(d)
1.1 V and (e)
1.2 V vs. SCE.

Table 2 constant potential under intermittent illumination of the

Potentials effect on the diameters of ZnO Nanorods. prepared ZnO films. The imposed potential is located at the
depletion zone of the zinc oxide (þ0.2 V). We notice that
Potential (V vs. SCE) Diameter NWs (nm)
when in dark, the current density is close to zero describing

0.8 163 then the fundamental state thermodynamic of the electrode.

0.9 170 The photocurrents in all samples remain constant in the dark.

1.0 174 Under light illumination, the curve has an almost rectan-

1.1 207

1.2 244
gular response with a significant increase of current density.
Meanwhile, we observe a positive photo-generated current
which is a characteristic of a n-type semiconductor, as
reported in previous studies [36]. The observed photocurrent
applied potential for synthesizing well-crystallized ZnO films could be explained by the accumulation of charges at the
is
1.0 V vs. SCE. interface involving electrodes promoted by the injection of the
minor photo carriers (hþ n).
3.3. Photoelectrochemical study From Fig. 4 the ZnO layers obtained at large potential
generate higher photocurrent, in particular for ZnO nanos-
Fig. 4 shown the plot of photocurrent-time response tructured layers obtained at
1.0, and
1.1 V (40 μA/cm2).
obtained by a chronoamperometric method at applied a However photocurrent value is reduced to 33 μA/cm2 for
384 A. Henni et al. / Materials Science in Semiconductor Processing 31 (2015) 380–385

about 90% in visible range. The abrupt drop of the

transmission for wavelengths lower than 380 nm is due
to the ZnO absorption. The transmittance is found to
increase with the applied potential, which is probably
related to the enhancement of scattering of photons by
either crystal defects or grainy surfaces.
The Tauc plots [37] shown in Fig. 5b were performed
using the transmission data obtained from each sample.
The intercepts of these plots with y¼0 axis lead an
estimation of the optical bandgap energy (Eg) of the
corresponding sample. As we know, Eg is related to the
absorption coefficient ðαÞ as following:


ðαhυÞ2 ¼ A hυ
Eg ð7Þ

where A is a constant and hυ is the photon energy. An

average value of 3.31 eV is obtained for electrochemically
synthesized ZnO. On the other hand, the Eg value is close to
those previously reported [38].

4. Conclusion
Fig. 4. Photocurrent response curves of ZnO nanostructures at a constant
potential ( þ 0.2 V).
In summary, highly transparent nanostructured ZnO
films were electrochemically deposited on ITO-glass sub-
strates. The effects of the applied potential on the films
structural, morphology, photocurrent, optical, and electri-
cal properties are investigated. Detailled analysis suggest
that the ZnO electrodeposition is achieved through a 3D
progressive nucleation process. ZnO exhibits aligned
nanorods arrays synthesized directly on ITO substrates
with diameters between 160 and 250 nm. ZnO crystallize
in single phase with Würtzite structure. The microstruc-
tural and functional properties of the obtained ZnO are
strongly affected by the potentials applied during the
electrodeposition. The photoelectrochemical tests demon-
strate clearly the ability of ZnO nanowires to generate
significant photocurrents under UV light. The optical
measurements of all the samples with were found to be
greater 80% in visible region, thus demonstrating that all
films are highly transparent. Band gap measurements
using Tauc plots from UV spectroscopy showed that the
energy gap varies slightly with the potential imposed.

Fig. 5. UV–vis transmission spectra of ZnO thin films electrodeposited

(a) Tauc plots for ZnO samples computed from the UV–visible data. Acknowledgments

We would like to thank Mr. Thomas Cacciaguerra and

Mr. Didier Tichit of team of Advanced Materials for
ZnO layers prepared with applied potential of
1.2 V. ZnO Catalysis and Health (MACS) for their assistance in acquir-
films obtained at potentials of
1.0 and
1.1 V crystallizes ing X-ray diffraction data.
in phase with a strong (0 0 2) peak reflection which might
highlight the interplay relationship between the structural References
and photoinduced properties in ZnO. This is in agreement
with previous reports [36]. Further investigation is under
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