Applied Surface Science 426 (2017) 833–843

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Applied Surface Science

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Full length article

Fabrication of A/R-TiO2 composite for enhanced photoelectrochemical

performance: Solar hydrogen generation and dye degradation
Mahadeo A. Mahadik a , Gil Woo An a , Selvaraj David a , Sun Hee Choi b , Min Cho a,∗ ,
Jum Suk Jang a,∗
a
Division of Biotechnology, Safety, Environment and Life Science Institute, College of Environmental and Bioresource Sciences, Chonbuk National
University, Iksan 570-752, Republic of Korea
b
Pohang Accelerator Laboratory, Pohang University of Science and Technology (POSTECH), Pohang 790-784, Republic of Korea

a r t i c l e i n f o a b s t r a c t

Article history: Anatase/rutile TiO2 nanorods composites were prepared by a facile hydrothermal method followed by
Received 13 June 2017 dip coating method using titanium isopropoxide in acetic acid and ethanol solvent. The effects of the tita-
Received in revised form 15 July 2017 nium isopropoxide precursor concentration, on the formation of dip coated anatase/rutile TiO2 nanorods
Accepted 20 July 2017
composite were systematically explored. The growth of anatase on rutile TiO2 nanorods can be controlled
Available online 29 July 2017
by varying the titanium isopropoxide concentration. The morphological study reveals that anatase TiO2
nanograins formed on the surface of rutile TiO2 nanorod arrays through dip coating method. Photoelec-
Keywords:
trochemical analyses showed that the enhancement of the photocatalytic activities of the samples is
Anatase nanograin
Rutile TiO2 nanorod
affected by the anatase nanograins present on the rutile TiO2 nanorods, which can induce the separation
Composite of electrons and holes. To interpret the photoelectrochemical behaviors, the prepared photoelectrodes
Degradation were applied in photoelectrochemical solar hydrogen generation and orange II dye degradation. The
Solar hydrogen generation optimized photocurrent density of 1.8 mA cm−2 and the 625 ␮mol hydrogen generation was observed for
10 mM anatase/rutile TiO2 NRs composites. Additionally, 96% removal of the orange II dye was achieved
within 5 h during oxidative degradation under solar light irradiation. One of the benefits of high specific
surface area and the efficient photogenerated charge transport in the anatase/rutile TiO2 nanorod com-
posite improves the photoelectrochemical hydrogen generation and orange dye degradation compared
to the rutile TiO2 . Thus, our strategy provides a promising, stable, and low cost alternative to existing
photocatalysts and is expected to attract considerable attention for industrial applications.
© 2017 Elsevier B.V. All rights reserved.

1. Introduction the phase mixture of different polymorphs is known to substan-

tially improve the PC performance because of the presence of a
Due to the intrinsic properties of nanostructured semiconduc- heterojunction photocatalyst [10–14]. The enhancement in perfor-
tors which are generally phase, shape, and size-dependent, the mance is explained by the facilitated charge separation from the
selective synthesis of integrated nanomaterials with controllable conduction band (CB) of one polymorph of TiO2 which migrates
morphology and composition represents an emerging research across the phase boundary to the CB of the other polymorph, thus
area in nanoscience and nanotechnology [1]. Nanostructured semi- prolonging the carrier lifetime due to the higher valence band
conductors have recently attracted considerable interest because of energy levels relative to the redox potentials of electrolyte species
their possible applicability to solar-energy conversion and detoxi- [15,16]. However, in these studies, a clear understanding of the
fication of environmental pollutants [2–6]. Amongst these, titania direction of electron transfer has not been reached. Hence, to guide
(TiO2 ) is one of the most promising photocatalytic (PC) and photo- the future development of more efficient photocatalysts, in recent
electrocatalytic (PEC) materials owing to its favorable conduction years, due to excellent incident light scattering and the effective
band edge, good stability, and low cost [7–9]. However, it has been interfacial charge transfer capacity compared with nanoparticle
reported that, compared with pure single anatase or rutile-TiO2 , photocatalysts, the synthesis of hierarchical architectures and sur-
face modified nanostructures on conducting substrates is worth
further study [17–19]. Although some methods have recently been
adopted for the preparation of mixed phase rutile/anatase or
∗ Corresponding authors.
anatase/rutile TiO2 [20,21], during the photocatalyst synthesis, the
E-mail addresses: cho317@jbnu.ac.kr (M. Cho), jangjs75@jbnu.ac.kr (J.S. Jang).

http://dx.doi.org/10.1016/j.apsusc.2017.07.179
0169-4332/© 2017 Elsevier B.V. All rights reserved.
834 M.A. Mahadik et al. / Applied Surface Science 426 (2017) 833–843

Scheme 1. Synthesis of anatase modified R-TiO2 nanorod arrays (A/R-TiO2 ) composite electrodes.

amount of anatase and rutile phase seems to be difficult to control; After 2 h stirring, the R-TiO2 NRs/FTO were dipped into the resulting
therefore, the performance and stability is still unsatisfactory. Thus, solution for 30 min as shown in Scheme 1. Finally, the films were
the issue of the need for improvement of the photoelectrochemical dried in air and were then annealed at 450 ◦ C for 1 h in air.
performance and stability of anatase nanograins modified TiO2 NRs
photocatalyst for photoelectrocatalytic hydrogen generation and 2.2. Characterization
dye degradation is seldom discussed in the previous literatures. To
the best of our knowledge, the dip coated anatase nanograins mod- The prepared A/R-TiO2 composite photoanodes were char-
ified hydrothermal TiO2 nanorods (TiO2 NRs) on fluorine doped tin acterized further according to the nature of phase(s) of titania
oxide (FTO) has not yet been reported. Also, the dip coated anatase and crystallinity studies using Raman spectroscopy and grazing
nanograins modified TiO2 NRs have not been used for bifunctional incidence X-ray diffraction (GIXRD) pattern (under CuK␣ radia-
application (i.e. hydrogen generation and dye degradation). tion, respectively. The morphologies of the deposited films were
Here, we prepared anatase nanograins modified rutile TiO2 examined using a field emission scanning electron microscope
nanorods (A/R-TiO2 ) composite on FTO via a facile one-step (FESEM) (SUPRA 40VP, Carl Zeiss, Germany) equipped with an X-ray
hydrothermal followed by dip coating method, respectively. A energy dispersive spectrometer (EDS). UV−Vis−diffuse reflectance
series of 5 mM, 10 mM, and 20 mM titanium isopropoxide solu- (DRS) spectra were measured using a dual-beam spectropho-
tion were prepared and used as anatase source to modify the tometer (Shimadzu, UV−2600 series) in the wavelength range
R-TiO2 photocatalysts using a simple environmentally friendly dip of 300−800 nm. The electronic structure of titanium was inves-
coating method. The formation of the anatase on rutile TiO2 NRs tigated using synchrotron X-ray absorption near-edge structure
is differentiated by field emission scanning electron microscopy (XANES) spectroscopy. The spectra for the Ti K-edges were taken
(FESEM), Raman spectroscopy, and UV–vis spectroscopy. The pho- on a 7D beamline of Pohang Accelerator Laboratory, Korea (3.0 GeV,
tocatalytic activity of A/R-TiO2 nanorod composite is found to 360 mA). The incident beam was detuned by 30% and its intensity
be directly related to its surface phase modification of R-TiO2 was monitored with a He-filled IC Spec ion chamber. The fluores-
nanorods. The photoelectrochemical hydrogen production and dye cence signal from the sample was measured using a passivated
degradation activity of optimized A/R-TiO2 composite can be sig- implanted planar silicon detector under a helium atmosphere. The
nificantly enhanced when anatase nanoparticles are deposited on obtained data were analyzed using Athena in the IFEFFIT suite of
the surface of rutile TiO2 nanorods (R-TiO2 NRs). In addition, based programs.
on the obtained experimental results, a detailed mechanism of the
charge separation in the anatase/R-TiO2 NRs (A/R-TiO2 ) composite 2.3. Photoelectrochemical (PEC) measurements
was systematically studied.
Photoelectrochemical (PEC) measurements of A/R TiO2 com-
2. Experimental posite electrodes (1 × 1 cm2 area) were performed in a 0.5 M
NaOH electrolyte solution under 100 mW cm−2 simulated sun-
2.1. Synthesis of anatase/rutile-TiO2 (A/R-TiO2 ) composite light irradiation. The photocurrent measurements (photocurrent
photoelectrodes density–voltage (J–V) curves, electrochemical impedance spec-
troscopy (EIS), and Mott–Schottky) were carried out with a
The A/R-TiO2 composite was prepared using hydrothermal conventional three-electrode electrochemical cell; Pt wire and
method followed by dip coating. In the first step, R-TiO2 NRs were Ag/AgCl (saturated KCl) were used as the counter and reference
synthesized by a facile hydrothermal method [22]; in a typical syn- electrodes, respectively. All the potentials mentioned in present
thesis process, one milliliter of titanium butoxide was mixed with work were originally measured with reference to a Ag/AgCl elec-
30 ml of water and HCl at a ratio of 1:1. The homogeneous solution trode (sat. KCl) and were converted to the reversible hydrogen
was then added to a Teflon lined stainless steel cylinder and the electrode (RHE) scale using the Nernst Eq. (1) [23].
reaction was kept at 150 ◦ C for 4 h. After cooling to room temper- VRHE = VAg/AgCl + 0.059pH + V0 Ag/AgCl (1)
ature, the R-TiO2 NRs were deposited on FTO substrates, washed
with deionized (DI) water, and then calcined in air at 500 ◦ C for 1 h. where VRHE is converted potential vs. RHE in V vs. RHE (VRHE),
Anatase nanograins were deposited on R-TiO2 NRs through the dip VAg/AgCl is the experimental potential measured against the Ag/AgCl
coating method. A dip coating solution was prepared by mixing the reference electrode in V vs. Ag/AgCl (VAg/AgCl ), and V0 Ag/AgCl is
titanium isopropoxide solution in 36 ml ethanol. The 0.4 ml acetic the standard potential of Ag/AgCl (sat. KCl) at 25 ◦ C (i.e.0.1976 V).
acid solution was added drop-wise under a vigorous stirring condi- The photocurrent density–time (J–t) curves were measured at
tion for 2 h. During the addition of acetic acid, white nanoparticles 1.09 V vs RHE. A portable potentiostat (COMPACTSTAT.e, Ivium,
of TiO2 were obtained as indicated by the appearance of turbidity. Netherlands) equipped with an electrochemical interface and
 M.A. Mahadik et al. / Applied Surface Science 426 (2017) 833–843 835

Fig. 1. FE-SEM micrographs of (a) R-TiO2 , (b) 5 mM, (c) 10 mM, (d) 20 mM A/R-TiO2 composites and (e,f) TEM images of optimized 10 mM A/R-TiO2 composites.

impedance analyzer was employed for the EIS measurements. The one sun illumination. The hydrogen was measured at 0.5 h intervals
EIS data were measured in the range from 0.1 Hz to 100 kHz, with and analyzed using a gas chromatograph (GC) equipped with a ther-
an AC amplitude of 10 mV, out under 1 sun illumination at 1.09 vs. mal conductivity detector (TCD) detector and a molecular sieve 5 Å
RHE (VRHE). The experimental EIS (real vs. imaginary impedance) packed column. The photocatalytic activity of the anatase/R-TiO2
data was validated using the Kramers–Kronig transform test and NR composite photoelectrode was evaluated by the PEC oxidation
then fitted using a suitable equivalent circuit model by the ZView of orange (II) dye. The reaction was carried out under solar light
(Scribner Associates Inc.) program. MS (C−2 sc vs. V) measurements irradiation in a conventional three-electrode electrochemical cell
were performed under dark conditions with an applied DC potential used in the PEC measurements. Prior to light irradiation, the dye
window of −1.2 to 1.0 V vs. Ag/AgCl at 0.5 kHz AC frequency. solution in the reactor was stirred for 30 min in the dark to obtain
the adsorption–desorption equilibrium for model pollutant on the
2.4. PEC hydrogen generation and dye degradation surface of the photoelectrode. During the degradation experiments,
a 1.2 ml of aliquot was withdrawn, and the concentrations of orange
PEC hydrogen generation was performed using a typical three- II dye in the solutions after photo-irradiation were measured from
electrode electrochemical cell with aqueous solution containing a the peak intensity of the UV visible absorbance of the solutions at
2:8 mixture of methanol and 0.5 M NaOH. (pH ≈ 13.5) as a elec- 454 nm [24]. The changes in the concentrations of orange II dye
trolyte. The 10 mM A/R-TiO2 composite was used as a working solution with reaction time for the samples were also investigated
electrode, saturated Ag/AgCl was used as a reference electrode, and for up to 5 h. To demonstrate the stability of the photocatalysts, we
platinum was used as counter electrodes, respectively. The hydro- measured the photocurrent during the PEC experiments.
gen evolution test was performed in a sealed PEC cell for 3 h under
836 M.A. Mahadik et al. / Applied Surface Science 426 (2017) 833–843

Fig. 2. (A) XRD and (B) Ti K-edge XANES spectra of (a) R-TiO2 , (b) 5 mM, (c) 10 mM, and (d) 20 mM A/R-TiO2 composite prepared on FTO. The reference spectra for anatase
and rutile structures are also shown in (B).

3. Results and discussion

The morphology of bare TiO2 and A/R-TiO2 composite was

examined by FESEM. Fig. 1a and Fig. S1a reveals that the R-TiO2
NR’s with average length of about 1.7 ␮m are uniformly and per-
pendicularly grown to the entire surface of the FTO. However, from
Fig. 1(a) and (b), it can be clearly seen that the TiO2 nanograins starts
to assemble on the surface of the R-TiO2 NRs. Also, as the concentra-
tion of the Titanium isopropoxide precursor increased from 5 mM
to 10 mM, a thick layer of TiO2 nanograins is observed growing on
the top of the TiO2 NRs (Fig. 1(c)). The average diameter of TiO2
nanograins grown on R-TiO2 NRs is less than 60 nm, as shown in
Fig. S1c. Further increases in the titanium isopropoxide precursor
concentration to 20 mM, the TiO2 NRs were coated with numerous
nanoparticles and the average thickness of the nanograin layer is
260 nm. Fig. S1 shows the cross-sectional view of the bare and A/R-
TiO2 composite, showing that the morphology of pure TiO2 NRs is
distinctly changed after the anatase TiO2 layer. The morphology of
the 10 mM A/R-TiO2 composite is further investigated by TEM char-
acterizations. Fig. 1(e and f) is the low-magnification TEM images
of optimized 10 mM A/R TiO2 composites. It is revealed that TiO2
nanoparticles have been deposited on the tip of TiO2 nanorods. Fig. Fig. 3. Raman spectra of (a) R-TiO2 , (b) 5 mM, (c) 10 mM, and (d) 20 mM A/R-TiO2
S2a (Supporting Information), also shows that the showing that a composites prepared on FTO.
large amount of TiO2 nanoparticles have been compactly deposited
on the top surface of nanorods TiO2 nanorods. Such a composite
nanostructure is able to maximize the contact between the photo- ing octahedron around the first target [25]. The post-edge feature
electrode and electrolyte and facilitates interfacial charge transfer. (E > 4987 eV) and the pre-edge characteristics of the prepared sam-
In order to identify the phase structure of the TiO2 NR based sam- ples, regardless of dip coating and the precursor concentration,
ples, the prepared samples were characterized by XRD, the results are almost the same as those of the reference rutile samples. The
of which are shown in Fig. 2A. The appearance of diffraction peaks at anatase phase is not detectable because XANES explores the local
27. 4◦ , 36.14◦ , 39.3◦ , 41.2◦ , and 54.29◦ is attributed to the tetragonal bulk structure and a surface-residing species might be present
structure of rutile TiO2 (reference code, 98-002-4277). However, with the extremely diluted concentration. Raman spectroscopy has
after dip coating in various concentrations of titanium isopropox- also been employed to differentiate the anatase and rutile [26,27].
ide precursor solution, apart from the FTO substrate and R-TiO2 , the Raman band positions depend on the type of TiO2 phase; there-
there is no peak corresponding to the anatase phase of TiO2 , this fore, these peak positions act as signs of a particular phase [28].
is due to the non-uniform and lower amount of loading of Anatase As anatase and rutile belong to different space groups (anatase:
on the Rutile nanorods (See FESEM image of 10 mM A/R TiO2 NR I41/amd with Z = 4, rutile: P42/mnm with Z = 2), their characteristic
composite photoanode). In addition to this, the Ti K-edge XANES Raman spectra somewhat differ. Anatase exhibited characteris-
was applied to examine the electronic structure around titanium tic scatterings at 146 (Eg ), 396 (B 1 g), 515 (A 1 g), and 641 cm−1
in A/R TiO2 NR composite samples, as shown in Fig. 2B. The refer- (Eg ), while rutile typically exhibited characteristic scatterings at
ence spectrum for a rutile structure represents three weak peaks 235 (two-phonon scattering), 447 (Eg ), and 612 cm−1 (A 1 g) [29].
at 4967–4976 eV in the pre-edge region, of which the first peak is Fig. 3 shows the Raman spectra of the R-TiO2 and A/R-TiO2 com-
attributable to the quadrupole transition from Ti 1 s to t2g levels of posites. Comparing the Raman spectra of anatase modified R-TiO2
the TiO6 octahedron and the other peaks are assigned as 1 s to 3d with the reference spectra of R-TiO2 and anatase TiO2 (Fig. S2b),
dipolar transitions to the t2g and the eg orbitals of the neighbor- it is clearly seen that the characteristic peaks of the of mixture of
anatase and rutile peaks are present in the anatase modified R-TiO2
 M.A. Mahadik et al. / Applied Surface Science 426 (2017) 833–843 837

Fig. 4. (A) UV–vis–DRS absorbance spectra, and (B) Kubelka–Munk plots for (a) R-TiO2 , (b) 5 mM, (c) 10 mM, and (d) 20 mM A/R-TiO2 composite.

samples (Fig. 1(a)), this confirms the presence of anatase particles TiO2 are consistent with the literature results [36]. Interestingly,
on the surface of rutile TiO2 nanorods [30]. As the concentration when increasing the thicknesses of anatase on the R-TiO2 films, a
of titanium isopropoxide solution was increased for the prepara- decreased absorption was observed. This could be explained by the
tion of A/R-TiO2 composite, the Raman peak intensity corresponds enhanced scattering of light with crystallites of anatase on the R-
to anatase of TiO2 start to increase. The Raman results confirm TiO2 NRs [37–39]. As the light absorption decreases with increasing
the formation of anatase TiO2 nanograins on the surface of the amount of anatase, it can be assumed that abundant surface oxy-
R-TiO2 NRs. Thus, it could be suggest that the titanium isopropox- gen vacancies or defects exist in anatase and R-TiO2 could promote
ide could tailor the morphologies of the composite without any the separation of electron–hole pairs under irradiation, which may
change to the crystal structure of the TiO2 NRs. Similar results were play a central role in the PEC performance of the electrodes [40].
reported by Han et al. for a Bi2 S3 /TiO2 cross-linked heterostructure To assess the PEC performance for solar hydrogen genera-
[31]. In order to quantify the anatase/rutile ratio, the calibration tion and dye degradation, the linear sweep voltammetry (LSV) of
curve was used by measuring the intensities of anatase and Rutile curves were performed for bare R-TiO2 NRs, and A/R-TiO2 com-
phases in A/R TiO2 composite photoelectrodes. The bare TiO2 NRs posite photoanodes under simulated solar illumination (Fig. 5A).
are considered as 0 wt.% of anatase and 100 wt.% rutile. Accord- Under the dark condition, the current density is almost zero for all
ing to Clegg et al. [32], as the peak at 144 cm−1 is used because it the photoanodes. However, upon illumination, all of the anatase
is very sensitive to anatase even at lower content of anatase and modified/R-TiO2 composite exhibits a higher photocurrent value
613 cm−1 , which correspond to rutile phase. For Anatase/Rutile than the R-TiO2 and reaches its maximum of 1.6 mA cm−2 at 1.029 V
photoanodes, the intensities of the Raman peaks located at 144 vs. RHE. This potential was chosen since it is a representative value
and 613 cm−1 , which belong to the anatase and rutile phase, are after the photocurrent saturation for all R-TiO2 NR based samples.
denoted as IA144 and IR613 respectively. The relative intensities were However, this trend is not followed for the 20 mM coated sam-
calculated each samples (bare R-TiO2 , 5, 10 and 20 mM A/R TiO2 ples, which show poor activity (1.32 mA cm−2 ). This is probably
photoanode) as, Irelative = IA144 /IR613 , and then the relative intensity due to the increased film thickness in 20 mM compared to pristine
is plotted as a function of amount of anatase precursor on Rutile and other anatase coated R-TiO2 leads to higher charge recom-
nanorods (WA /WR ), where, WA is represents the amount of anatase bination [41]. Fig. 5B shows the chronoamperometric response
modified on rutile (WR ) respectively. This calibration curve (Fig. of A/R-TiO2 composite at 1.029 V vs RHE under simulated solar
S2c) is used to determine the anatase/rutile ratio and the content light illumination using the J–t method. It is worth noting that the
of anatase present on the surface of Rutile nanorods were deter- photoresponse obtained from the A/R-TiO2 composite electrodes
mined from the value of WA /WR and the relative intensities [33]. are very high among the previously studied anatase/rutile TiO2
The anatase/rutile ratio for the (0, 5, 10 and 20 mM Anatase/Rutile powder heterostructure and photoanodes [42]. The photocurrent
photoanode is 0.3%, 0.4% and 0.87% respectively. Fig. 4A and B show density can remain at a stable value after the first 5 min, reveal-
the UV–vis absorption spectra of pure R-TiO2 and A/R-TiO2 com- ing a good ability to resist photo-corrosion. However, the transient
posite, respectively, converted from the corresponding diffusion photocurrent responses of all the measured samples show that the
reflectance spectra by the Kubelka–Munk relation. When anatase photocurrent rapidly decreases to zero after the light is switched
was coated on the R-TiO2 NRs, UV light absorption in the range off. The improved value of photocurrent in the A/R-TiO2 composite
of 300–360 nm decreased because anatase also possesses a similar is attributed to a more efficient separation and transport of charge
large band gap to that of rutile TiO2 . This behavior is very similar to carriers. Thus, further, to investigate the underlying reason for the
that reported in the previous results of the TiO2 –BiOCl double-layer enhanced PEC performance, the EIS Nyquist plots of A/R-TiO2 com-
nanostructure arrays and phenyl-C61-butyric acid methyl ester posite electrodes were measured. Fig. 5(C) shows the Nyquist plots
(PCBM) coated TiO2 electrode [34,35]. To obtain the exact band fitted with the equivalent circuit consisting of the series resistance
gap, the incident photon energy and the absorption coefficient are (R1 ) of the conductive substrate, the external electrical contacts,
plotted in Fig. 4B, indicating the band gap of rutile TiO2 calculated the liquid electrolyte, and the charge transfer resistance (R2 ) and
as 3.16 eV. However, the value of the band gap increases slightly capacitance (CCPE ) at the electrode/semiconductor interface. The
with the increasing amount of anatase on the R-TiO2 NRs (3.17, smaller resistance-circle obtained for the 10 mM A/R-TiO2 com-
3.189, and 3.19 eV for 5 mM, 10 mM, and 20 mM A/R-TiO2 com- posite suggests a quick separation of electrons and holes in the
posite, respectively). The calculated band gap of anatase and rutile interface of the electrode/electrolyte [43]. The EIS parameters were
838 M.A. Mahadik et al. / Applied Surface Science 426 (2017) 833–843

Fig. 5. (A) Linear sweep voltammograms (J–V curve). (B) Photocurrent versus time tests (J−t curves) under chopped light illumination. (C) EIS of (a) R-TiO2 nanorods, (b)
5 mM (c) 10 mM, (d) 20 mM Titanium isopropoxide for A/R-TiO2 composites, and (D) Stability curve for 10 mM, Titanium isopropoxide for A/R-TiO2 composites, inset shows
the PEC reactor used to measure the stability curve (WE: working electrode, CE: counter electrode and RE: reference electrode).

Table 1 between position of the conduction band edge and flat band poten-
Parameters determined from EIS fitting of A/R-TiO2 composites.
tial (Vfb ) is very small, the determination of the conduction band
Sample R1 () R2 () CCPE (␮F) edge of TiO2 films often translates into the measurement of the flat
0 mM 43 5684 6.72 band potential. The R-TiO2 NR has a flat band potential (Vfb ) value
5 mM 26 3966 7.6 of 0.06 V vs RHE, whereas the VFB value of A/R-TiO2 composite (the
10 mM 74 3766 5.9 intercept on X-axis) shifts more negatively than the R-TiO2 (0.03 V
20 mm 45 12483 1.0 vs. RHE). Thus, using these values of flat band potentials the posi-
tions of conduction band edges were determined. Also the band gap
values were estimated the valence-band edges.
determined by fitting the impedance spectra and are listed in Fig. 6 shows the plots of hydrogen generation with simultane-
Table 1. The lower value of R2 in the 10 mM A/R TiO2 indicates the ously recorded J–t profiles for pristine and A/R-TiO2 composite. A
fastening of the charge-transfer at the semiconductor–electrolyte steady photocurrent was observed for both pristine and A/R-TiO2
interphase, which consequently helps to enhance the photocur- composite over a period of four hours. The PEC solar hydrogen gen-
rent density. Thus, based on the above analysis, the imperative eration experiments were carried out in 2:8 mixtures of methanol
role of the anatase modification was ascertained for prolonging and 0.5 M NaOH as electrolyte. To eliminate the external losses such
the lifetime of the photogenerated charge carriers [44]. In order to as the resistive loss of the system, a bias of 1.04 V vs. RHE was
assess the long-term performance of the 10 mM A/R TiO2 photoan- applied in this experiment. The use of sacrificial reagents that can
odes, the photostability test was measured for 180 min under the be oxidized more easily than water and/or capture photoproduced
continuous illumination (Fig. 5(D)). The current transient under a h+ VB carriers has been proven to enhance H2 S splitting by favoring
potentiostatic bias of 1.029 VRHE ; less than 0.04 mA cm−2 decrease the charge separation between photogenerated carriers, increas-
in the photocurrent is observed over 180 min. This proves the sat- ing their lifetime [47]. However, due to the addition of methanol
isfactory performance of the 10 mM A/R TiO2 photoanodes for its in the NaOH electrolyte during hydrogen generation, the increase
practical applications (H2 generation and dye degradation). of photocurrent was observed compared to the PEC measurements.
Since the alignment of the energy levels of the composites is The PEC hydrogen generation is accompanied with the flow of pho-
closely related to its PEC performance, it is established on the togenerated electrons in the photoanode materials to counter the
basis of the Mott Schottky and band gap results. Fig. S3 shows the electrode, and thus is directly proportional to the photocurrent
Mott Schottky plots of R-TiO2 nanorods and Anatase (5, 10, 20 mM) density [48]. The bare TiO2 NRs films showed a photocurrent of
modified R-TiO2 composite. The determination of the position of 1.5 mA cm−2 (line b, Fig. 6), and a hydrogen generation of 580 ␮mol
conduction band in the semiconductor is explained by R. Beranek was detected at 1.04 V vs. RHE (line d, Fig. 6). However, for the
[45,46] .In the n-type semiconductor’s assuming that the difference 10 mM A/R-TiO2 composite, the PEC performance is significantly
 M.A. Mahadik et al. / Applied Surface Science 426 (2017) 833–843 839

Besides the PEC hydrogen generation application, the PEC activ-

ity of 10 mM A/R-TiO2 composite was further examined from the
oxidative degradation of the non-biodegradable orange II dye under
one sun illumination. Fig. 7(A) and (B) shows the photoelectro-
catalytic degradation of orange II dye solution at different time
intervals over the 10 mM A/R-TiO2 composite and bare TiO2 NRs.
In a typical degradation experiment, 10 ␮mol orange II dye dis-
solved in double distilled water (65 ml) was used as a model organic
species. During the PEC degradation experiments, the required
amount of aliquots withdrawn from the PEC electrolyte solution at
specific intervals of times. Further the concentration of orange II dye
in the solutions was determined by measuring the extinction spec-
tra using a dual-beam spectrophotometer (Shimadzu, UV−2600
series) in the wavelength range of 300−800 nm. The photoelectrode
(1 cm2 ) was illuminated from the front side using one sun illumina-
tion. The external bias voltage of about 1.029 V vs. RHE was applied
in order to increase the rate of reaction.
The Beer’s law is used to co-relate the absorbance peak of orange
Fig. 6. Amperometric (J–t) curves and hydrogen generation over the TiO2 NRs and II at ␭ = 453 nm with the solution concentration, as it is below 0.8
10 mM A/R-TiO2 composite electrodes under illumination (100 mW cm−2 ) in a 2:8 [50,51]. It is generally accepted that adsorption is critical in het-
mixture of methanol and 0.5 M NaOH. erogeneous photocatalytic oxidation processes [52]. However, in
this study, dark adsorption experiment shows that no obvious dye
adsorption could be observed on the photocatalyst surface. How-
ever, most of the photocatalytic reactions take place with hydroxyl
enhanced. The highest photocurrent density of 1.8 mA cm−2 (line radicals generated after the adsorption of hydroxyl ions onto the
a, Fig. 6) and the corresponding hydrogen generation of 625 ␮mol catalyst surface, followed by reactions with holes in the excited,
(line c, Fig. 6A) were obtained after 4 h. This enhancement in PEC semiconductor catalyst [53]. Fig. S4a shows that the 10 mM A/R-
activity is probably due to the effective light scatterings which help TiO2 composite sample showed poorer activities in the presence
to generate more energy harvesting and reduce the bulk and surface of applied bias only. However, under solar light, orange II could be
electron–hole recombination’s [49].

Fig. 7. UV–vis absorption spectra of Orange II dye solution at different time intervals under the solar light illumination with applied bias for (A) the 10 mM A/R-TiO2
composite and (B) Bare TiO2 sample (C) Comparison of photodegradation efficiency at various experimental conditions, The error bars in photodegradation of the 10 mM
A/R-TiO2 composite represent the standard deviations of the degradation values of the independently carried out experiments. (d) Actual photographs of dye solutions after
the PEC degradation by 10 mM A/R-TiO2 composite.
840 M.A. Mahadik et al. / Applied Surface Science 426 (2017) 833–843

Fig. 8. (a–d) Schematics of effective charge separation in A/R-TiO2 composite photoelectrodes.

degraded up to 71% (photodegradation) within the 5 h (Fig. S4b), by absorbance. These products can then further break down as miner-
the degradation mechanism [54]. Interestingly, the orange II dye is alized products such as CO2 and H2 O, as reported in various cases of
rapidly decolorized and Fig. 7(C) shows that the PEC degradation dye degradation [60,61]. Thus, the photocatalytic process is highly
of orange II dye (extinction taken at ␭ = 455 nm) follows a pseudo dependent on the hydroxyl group, and the OH radical present on
first order reaction and its kinetics can be expressed using equation the surface, which attacks the contaminant present in the water
ln(C/C0 ) = −kt, where C is the concentration of orange II dye at time [62]. To confirm the presence of main reactive species (* OH and
t and 0 min respectively. The 96% degradation was achieved within *− O ) involved in the degradation process the similar degrada-
2
5 h for the 10 mM A/R-TiO2 composite under applied bias and solar tion experiment were carried out by adding methanol as reactive
light illumination conditions. The actual photographs of the sam- oxygen species scavenger [63]. In order to study the comparative
ples after the >3 h photocatalytic decolorization experiments are experiments, the 10 ␮M of Orange dye and various concentra-
shown in Fig. 7(D). In Addition to this, the stability of the photo- tions of methanol as reactive oxygen species scavenger were used
catalyst during the PEC degradation reactions was measured by as electrolyte during degradation experiments. The photocatalytic
varying the photocurrent as a function of time and is shown in Fig. activity was observed for five hours under simulated solar irradia-
S5. The slight increase in photocurrent over time is due to the oxida- tion (AM 1.5) at applied potential of 1.09 V vs RHE. The maximum
tion of organic species. The average photocurrent of 1.5 mA cm−2 is degradation performance was observed for without methanol (0 ml
drawn from the degradation of orange II dye using 10 mM A/R-TiO2 methanol) as shown in Fig. S6. A decreased photocatalytic activity
composite thin film. was observed with the increase in of the methanol concentration
In the PEC reactions, when the semiconductor is illuminated (5 and 10 ml) in the electrolyte solution. These results indicated the
with light having energy greater than its band gap, the photogen- generation of main reactive species (* OH and *− O2 ) involved in the
erated electrons and holes are formed. Maximum number of the degradation process and are affected due to methanol during the
photogenerated holes can react with water at the surface of the photocatalytic reaction [64,65]. In addition to this the mechanism
semiconductor photocatalyst to produce OH radical (• OH) radicals of photosensitization of orange II dye/10 mM A/R-TiO2 composite, a
[55]. Similarly, photogenerated electrons appear on the surface of monochromatic light filter with the pass wavelength of 420 nm was
the counter electrode, where they are trapped by molecular oxygen, used for the visible light illumination. The orange II dye sensitiza-
to form superoxide radical anion (O2 − ). These superoxide radi- tion process involves the excitation of dye molecules by absorbing
cal ions further react with H+ ions and finally produce the H2 O2 visible light photons and subsequent electron injection from exci-
and O2 . In the overall photoelectrochemical process, the OH and tation state dye to A/R TiO2 . It was observed that the Orange II dye
O2 •− are very reactive species that react with the orange II dye was still degraded (Fig. S7a, b), although A/R-TiO2 composite can-
[56,57]. The possible reactions during the PEC dye degradation are not absorb light with a wavelength of ≥420 nm. The decrease in
explained in the S1-S9 (supporting information) [58,59]. During the absorption band, indicating that the complex aromatic structures
PEC decolorization, the orange dye molecules are actually broken in Orange II dye is still attacked by the active species leading to the
down and new products are formed which causes a reduction of decomposition of dye. However slight shift in the absorption band
 M.A. Mahadik et al. / Applied Surface Science 426 (2017) 833–843 841

of oranbge II dye reflects a typical chemical process [66,67]. Thus, separation of photogenerated electrons and holes. The stability
degradation of orange II dye using 10 mM A/R-TiO2 composite is the and increased PEC performance of the A/R-TiO2 composite cat-
combined effect of photosensitization and oxidative degradation alyst was studied by hydrogen generation and dye degradation
over the surface of A/R-TiO2 composite catalyst. approaches. The high performance of the A/R-TiO2 composite could
To demonstrate the high specific area of the A/R-TiO2 nanorod be attributed to the larger surface area of the anatase and longer
as compared to bare Rutile nanorods, the cyclic voltammetry (CV) direct electron path in TiO2 NR. Meanwhile, UV–vis and Mott
measurements were performed in a three electrode configuration, Schottky analyses confirm that the internal cascade process is
in which Pt wire acted as counter electrode, Ag/AgCl as reference advantageous to the separation of electrons and holes. The possible
electrode, and R-TiO2 NRs based composites as the working elec- growth mechanism and charge transfer mechanism are also pro-
trode. Fig. S8 compares the normalized CV curves recorded in 0.5 M posed. The present study provides the important step towards the
NaOH electrolyte from scan range of −0.4 to 1.0 V vs RHE with 10 formation of an A/R-TiO2 composite, while the further optimiza-
a mV s−1 scan rate using a portable potentiostat (COMPACTSTAT.e, tions of thin and compact anatase layer will offer new strategies
Ivium, Netherlands). The obtained surface areasfor the 0, 5 mM, for the fabrication of efficient photoelectrode with improved PEC
10 mM and 20 mM are 5.83, 6.71,10.48, and 9.03 cm2 respectively. degradation as well as PEC hydrogen generation.
The photoelectrode prepared at optimal condition 10 mM A/R TiO2
composite yielding the highest surface area amongst the studied
Acknowledgements
samples. The cyclic voltammetry measurements revealed signif-
icantly high electrochemically-active surface areas that depended
This research was supported by the BK21 Plus program, the
only upon the amount of amount of Anatase modified on the Rutile.
Korean National Research Foundation (NRF) (Nano-Material Fun-
Therefore, these results indicate that formation of A/R TiO2 com-
damental Technology Development, 2016M3A7B4909370) and
posites leads to an increase of electrochemically accessible surface
the Korean Ministry of the Environment (MOE) as part of
areas, thereby increasing the photoelectrochemical performance
the Public Technology Program based on Environmental Policy
[68–70].
(2014000160001).
The higher photo-electrochemical activity of the 10 mM A/R-
TiO2 composite is related to the role of anatase on the surface of
R-TiO2 NRs. Based on the above-mentioned discussion and analy- Appendix A. Supplementary data
sis, a tentative charge separation mechanism in A/R-TiO2 composite
prepared on FTO substrate is illustrated in Fig. 8. Compared to bare Supplementary data associated with this article can be found, in
R-TiO2 nanorods, A/R-TiO2 offers more surface area (Fig. 8(a) and the online version, at http://dx.doi.org/10.1016/j.apsusc.2017.07.
(b)). Using UV visible spectra (Fig. 4A) and Mott Schottky plots (Fig. 179.
S3), the conduction band potentials of anatase and anatase modi-
fied TiO2 were determined. The schematic of the band diagram and References
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