R-TiO2 composite for enhanced photoelectrochemical performance: Solar hydrogen generation and dye degradation

Accepted Manuscript Title: Fabrication of A/R-TiO2 composite for enhanced photoelectrochemical performance: solar hydrogen generation and dye degradation† Authors: Mahadeo A. Mahadik, An Gil Woo, David Selvaraj, Sun Hee Choi, Min Cho, Jum Suk Jang PII: DOI: Reference:

S0169-4332(17)32180-3 http://dx.doi.org/doi:10.1016/j.apsusc.2017.07.179 APSUSC 36713

To appear in:

APSUSC

Received date: Revised date: Accepted date:

13-6-2017 15-7-2017 20-7-2017

Please cite this article as: Mahadeo A.Mahadik, An Gil Woo, David Selvaraj, Sun Hee Choi, Min Cho, Jum Suk Jang, Fabrication of A/R-TiO2 composite for enhanced photoelectrochemical performance: solar hydrogen generation and dye degradation†, Applied Surface Sciencehttp://dx.doi.org/10.1016/j.apsusc.2017.07.179 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Fabrication of A/R-TiO2 composite for enhanced photoelectrochemical performance: solar hydrogen generation and dye degradation†

Mahadeo A. Mahadika, An Gil Wooa, David Selvaraja, Sun Hee Choib, Min Choa,* and Jum Suk Janga,*

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.

*Corresponding authors, Tel.: +82 63 850 0846; fax: +82 63 850 0834. E-mail addresses: [email protected] (J.S. Jang); [email protected] (Min Cho).

Graphical abstract

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Highlights 

Dip coated A/R-TiO2 composite strategy using titanium isopropoxide is proposed.



Effective light scattering and improved charge transport improves the PEC performance.



Composite enhances the photocurrent density of A/R-TiO2 electrods.



A/R-TiO2 composite achieves hydrogen generation activity of 208.3 μmol/h.



A/R-TiO2 composite exhibits excellent performance to remove orange (II) dye.

Abstract Anatase/rutile TiO2 nanorods composites were prepared by a facile hydrothermal method followed by dip coating method using titanium isopropoxide in acetic acid and ethanol solvent. The effects of the titanium isopropoxide precursor concentration, on the formation of dip coated anatase/rutile TiO2 nanorods composite were systematically explored. The growth of anatase on rutile TiO2 nanorods can be controlled 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. Photoelectrochemical analyses showed that the enhancement of the photocatalytic activities of the samples is affected by the anatase nanograins present on the rutile TiO2 nanorods, which can induce the separation of electrons and holes. To interpret the photoelectrochemical behaviors, the prepared photoelectrodes were applied in photoelectrochemical solar hydrogen generation and orange II dye degradation. The optimized photocurrent density of 1.8 mA.cm-2 and the 625 μmol 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 2

anatase/rutile TiO2 nanorod composite 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.

Keywords: Anatase nanograin; Rutile TiO2 nanorod; composite; Degradation; Solar hydrogen generation

1. Introduction Due to the intrinsic properties of nanostructured semiconductors which are generally phase, shape, and size-dependent, the selective synthesis of integrated nanomaterials with controllable morphology and composition represents an emerging research area in nanoscience and nanotechnology [1]. Nanostructured semiconductors have recently attracted considerable interest because of their possible applicability to solar-energy conversion and detoxification of environmental pollutants [2-6]. Amongst these, titania (TiO2) is one of the most promising photocatalytic (PC) and photoelectrocatalytic (PEC) materials owing to its favorable conduction band edge, good stability, and low cost [7-9]. However, it has been reported that, compared with pure single anatase or rutile-TiO2, the phase mixture of different polymorphs is known to substantially improve the PC performance because of the presence of a heterojunction photocatalyst [10-14]. The enhancement in performance is explained by the facilitated charge separation from the conduction band (CB) of one polymorph of TiO2 which migrates across the phase boundary to the CB of the other polymorph, thus prolonging the carrier lifetime due to the higher valence band energy levels relative to the redox potentials of electrolyte species [15,16]. However, in these studies, a clear understanding of the direction of electron transfer has not been

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reached. Hence, to guide the future development of more efficient photocatalysts, in recent years, due to excellent incident light scattering and the effective interfacial charge transfer capacity compared with nanoparticle photocatalysts, the synthesis of hierarchical architectures and surface 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 anatase/rutile TiO2 [20, 21], during the photocatalyst synthesis, the amount of anatase and rutile phase seems to be difficult to control; therefore, the performance and stability is still unsatisfactory. Thus, the issue of the need for improvement of the photoelectrochemical performance and stability of anatase nanograins modified TiO2 NRs photocatalyst for photoelectrocatalytic hydrogen generation and dye degradation is seldom discussed in the previous literatures. To the best of our knowledge, the dip coated anatase nanograins modified hydrothermal TiO2 nanorods (TiO2 NRs) on fluorine doped tin oxide (FTO) has not yet been reported. Also, the dip coated anatase nanograins modified TiO2 NRs have not been used for bifunctional application (i.e. hydrogen generation and dye degradation). Here, we prepared anatase nanograins modified rutile TiO2 nanorods (A/R-TiO2) composite on FTO via a facile one-step hydrothermal followed by dip coating method, respectively. A series of 5 mM, 10 mM, and 20 mM titanium isopropoxide solution were prepared and used as anatase source to modify the R-TiO2 photocatalysts using a simple environmentally friendly dip coating method. The formation of the anatase on rutile TiO2 NRs is differentiated by field emission scanning electron microscopy (FESEM), Raman spectroscopy, and UV-Vis spectroscopy. The photocatalytic activity of A/R-TiO2 nanorod composite is found to be directly related to its surface phase modification of R-TiO2 nanorods. The photoelectrochemical hydrogen production and dye degradation activity of optimized A/R-TiO2 composite can be

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significantly enhanced when anatase nanoparticles are deposited on the surface of rutile TiO2 nanorods (R-TiO2 NRs). In addition, based on the obtained experimental results, a detailed mechanism of the charge separation in the anatase/R-TiO2 NRs (A/R-TiO2) composite was systematically studied. 2. Experimental 2.1 Synthesis of anatase/Rutile-TiO2 (A/R-TiO2) composite photoelectrodes The A/R-TiO2 composite was prepared using hydrothermal method followed by dip coating. In the first step, R-TiO2 NRs were synthesized by a facile hydrothermal method [22]; in a typical synthesis process, one milliliter of titanium butoxide was mixed with 30 mL of water and HCl at a ratio of 1:1. The homogeneous solution was then added to a Teflon lined stainless steel cylinder and the reaction was kept at 150 0C for 4 h. After cooling to room temperature, the RTiO2 NRs were deposited on FTO substrates, washed with deionized (DI) water, and then calcined in air at 500 oC for 1 h. Anatase nanograins were deposited on R-TiO2 NRs through the dip coating method. A dip coating solution was prepared by mixing the titanium isopropoxide solution in 36 ml ethanol. The 0.4 ml acetic acid solution was added drop-wise under a vigorous stirring condition for 2h. During the addition of acetic acid, white nanoparticles of TiO2 were obtained as indicated by the appearance of turbidity. After 2h stirring, the R-TiO2 NRs/FTO were dipped into the resulting solution for 30 min as shown in Scheme 1. Finally, the films were dried in air and were then annealed at 450°C for 1 h in air. 2.2 Characterization The prepared A/R-TiO2 composite photoanodes were characterized further according to the nature of phase(s) of titania and crystallinity studies using Raman spectroscopy and grazing incidence X-ray diffraction (GIXRD) pattern (under CuKα radiation, respectively. The

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morphologies of the deposited films were examined using a field emission scanning electron microscope (FESEM) (SUPRA 40VP, Carl Zeiss, Germany) equipped with an X-ray energy dispersive spectrometer (EDS). UV−Vis−diffuse reflectance (DRS) spectra were measured using a dual-beam spectrophotometer (Shimadzu, UV−2600 series) in the wavelength range of 300−800 nm. The electronic structure of titanium was investigated using synchrotron X-ray absorption near-edge structure (XANES) spectroscopy. The spectra for the Ti K-edges were taken on a 7D beamline of Pohang Accelerator Laboratory, Korea (3.0 GeV, 360mA). The incident beam was detuned by 30% and its intensity was monitored with a He-filled IC Spec ion chamber. The fluorescence signal from the sample was measured using a passivated implanted planar silicon detector under a helium atmosphere. The obtained data were analyzed using Athena in the IFEFFIT suite of programs.

2.3 Photoelectrochemical (PEC) measurements Photoelectrochemical (PEC) measurements of A/R TiO2 composite electrodes (1 × 1 cm2 area) were performed in a 0.5 M NaOH electrolyte solution under 100 mW cm−2 simulated sunlight irradiation. The photocurrent measurements (photocurrent density–voltage (J–V) curves, electrochemical impedance spectroscopy (EIS), and Mott–Schottky) were carried out with a conventional three-electrode electrochemical cell; Pt wire and Ag/AgCl (saturated KCl) were used as the counter and reference electrodes, respectively. All the potentials mentioned in present work were originally measured with reference to a Ag/AgCl electrode (sat. KCl) and were converted to the reversible hydrogen electrode (RHE) scale using the Nernst eqn. (1).[23] VRHE = VAg/AgCl + 0.059pH + V0Ag/AgCl

(1)

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where VRHE is converted potential vs. RHE in V vs. RHE (VRHE), VAg/AgCl is the experimental potential measured against the Ag/AgCl reference electrode in V vs. Ag/AgCl (V Ag/AgCl), and V0Ag/AgCl is the standard potential of Ag/AgCl (sat. KCl) at 25 °C (i.e.0.1976 V). The photocurrent density–time (J-t) curves were measured at 1.09 V vs RHE. A portable potentiostat (COMPACTSTAT.e, Ivium, Netherlands) equipped with an electrochemical interface and impedance analyzer was employed for the EIS measurements. The EIS data were measured in the range from 0.1 Hz to 100 kHz, with an AC amplitude of 10 mV, out under 1 sun illumination at 1.09 vs. RHE (VRHE). The experimental EIS (real vs. imaginary impedance) data was validated using the Kramers–Kronig transform test and then fitted using a suitable equivalent circuit model by the ZView (Scribner Associates Inc.) program. MS (C−2sc vs. V) measurements were performed under dark conditions with an applied DC potential window of −1.2 to 1.0 V vs. Ag/AgCl at 0.5 kHz AC frequency.

2.4 PEC hydrogen generation and dye degradation PEC hydrogen generation was performed using a typical three-electrode electrochemical cell with aqueous solution containing a 2:8 mixture of methanol and 0.5 M NaOH. (pH ≈ 13.5) as a electrolyte. The 10 mM A/R-TiO2 composite was used as a working electrode, saturated Ag/AgCl was used as a reference electrode, and platinum was used as counter electrodes, respectively. The hydrogen evolution test was performed in a sealed PEC cell for 3 h under one sun illumination. The hydrogen was measured at 0.5 h intervals and analyzed using a gas chromatograph (GC) equipped with a thermal conductivity detector (TCD) detector and a molecular sieve 5 Å packed column. The photocatalytic activity of the anatase/R-TiO2 NR composite photoelectrode was evaluated by the PEC oxidation of orange (II) dye. The reaction

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was carried out under solar light irradiation in a conventional three-electrode electrochemical cell used in the PEC measurements. Prior to light irradiation, the dye solution in the reactor was stirred for 30 min in the dark to obtain the adsorption–desorption equilibrium for model pollutant on the surface of the photoelectrode. During the degradation experiments, a 1.2 mL of aliquot was withdrawn, and the concentrations of orange II dye in the solutions after photo-irradiation were measured from the peak intensity of the UV visible absorbance of the solutions at 454 nm [24]. The changes in the concentrations of orange II dye solution with reaction time for the samples were also investigated for up to 5hs. To demonstrate the stability of the photocatalysts, we measured the photocurrent during the PEC experiments.

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 μm are uniformly and perpendicularly grown to the entire surface of the FTO. However, from Figs. 1(a) and 1(b), it can be clearly seen that the TiO2 nanograins starts to assemble on the surface of the R-TiO2 NRs. Also, as the concentration 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 8

characterizations. 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. S2a (Supporting Information), also shows that the showing that a 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 photoelectrode and electrolyte and facilitates interfacial charge transfer. In order to identify the phase structure of the TiO2 NR based samples, the prepared samples were characterized by XRD, the results of which are shown in Fig. 2A. The appearance of diffraction peaks at 27. 4°, 36.14°, 39.3°, 41.2°, and 54.29° is attributed to the tetragonal structure of rutile TiO2 (reference code, 98-002-4277). However, after dip coating in various concentrations of titanium isopropoxide precursor solution, apart from the FTO substrate and R-TiO2, the there is no peak corresponding to the anatase phase of TiO2, this is due to the non-uniform and lower amount of loading of Anatase on the Rutile nanorods (See FESEM image of 10 mM A/R TiO2 NR composite photoanode). In addition to this, the Ti K-edge XANES was applied to examine the electronic structure around titanium in A/R TiO2 NR composite samples, as shown in Fig. 2B. The reference spectrum for a rutile structure represents three weak peaks at 4967-4976 eV in the preedge region, of which the first peak is attributable to the quadrupole transition from Ti 1s to t2g levels of the TiO6 octahedron and the other peaks are assigned as 1s to 3d dipolar transitions to the t2g and the eg orbitals of the neighboring octahedron around the first target [25]. The postedge feature (E >4987 eV) and the pre-edge characteristics of the prepared samples, regardless of dip coating and the precursor concentration, are almost the same as those of the reference rutile samples . The anatase phase is not detectable because XANES explores the local bulk structure and a surface-residing species might be present with the extremely diluted concentration. Raman

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spectroscopy has also been employed to differentiate the anatase and rutile [26, 27]. Raman band positions depend on the type of TiO2 phase; therefore, these peak positions act as signs of a particular phase [28]. As anatase and rutile belong to different space groups (anatase : I41/amd with Z = 4, rutile : P42/mnm with Z = 2), their characteristic Raman spectra somewhat differ. Anatase exhibited characteristic scatterings at 146 (Eg), 396 (B 1g), 515 (A 1g), and 641 cm-1 (Eg), while rutile typically exhibited characteristic scatterings at 235 (two-phonon scattering), 447 (Eg), and 612 cm-1(A 1g) [29]. Fig. 3 shows the Raman spectra of the R-TiO2 and A/R-TiO2 composites. Comparing the Raman spectra of anatase modified R-TiO2 with the reference spectra of R-TiO2 and anatase TiO2 (Fig. S2b), 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 samples (Fig. 1(a)), this confirms the presence of anatase particles on the surface of rutile TiO2 nanorods [30]. As the concentration of titanium isopropoxide solution was increased for the preparation of A/R-TiO2 composite, the Raman peak intensity corresponds to anatase of TiO2 start to increase. The Raman results confirm the formation of anatase TiO2 nanograins on the surface of the RTiO2 NRs. Thus, it could be suggest that the titanium isopropoxide could tailor the morphologies of the composite without any change to the crystal structure of the TiO2 NRs. Similar results were reported by Han et al. for a Bi2S3/TiO2 cross-linked heterostructure [31]. In order to quantify the anatase/rutile ratio, the calibration curve was used by measuring the intensities of anatase and Rutile phases in A/R TiO2 composite photoelectrodes. The bare TiO2 NRs are considered as 0 wt. % of anatase and and 100 wt. % rutile. According to Clegg et al [32], as the peak at 144 cm-1 is used because it is very sensitive to anatase even at lower content of anatase and 613 cm-1, which correspond to rutile phase. For Anatase/Rutile photoanodes, the intensities of the Raman peaks located at 144 and 613 cm-1, which belong to the anatase and rutile phase,

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are denoted as IA144 and IR613 respectively. The relative intensities were calculated each samples (bare R-TiO2, 5, 10 and 20 mM A/R TiO2 photoanode) as, Irelative= IA144/IR613, and then the relative intensity is plotted as a function of amount of anatase precursor on Rutile nanorods (W A /WR), where, WA is represents the amount of anatase modified on rutile (WR) respectively. This calibration curve (Fig. S2c) is used to determine the anatase/rutile ratio and the content of anatase present on the surface of Rutile nanorods were determined from the value of W A/WR and the relative intensities [33]. The anatase/rutile ratio for the (0, 5, 10 and 20 mM Anatase/Rutile photoanode is 0.3%, 0.4% and 0.87% respectively. Figs. 4A and 4B show the UV-Vis absorption spectra of pure R-TiO2 and A/R-TiO2 composite, respectively, converted from the corresponding diffusion reflectance spectra by the Kubelka–Munk relation. When anatase was coated on the RTiO2 NRs, UV light absorption in the range of 300 to 360 nm decreased because anatase also possesses a similar large band gap to that of rutile TiO2. This behavior is very similar to that reported in the previous results of the TiO2–BiOCl double-layer nanostructure arrays and phenylC61-butyric acid methyl ester (PCBM) coated TiO2 electrode [34, 35]. To obtain the exact band gap, the incident photon energy and the absorption coefficient are plotted in Fig. 4B, indicating the band gap of rutile TiO2 calculated as 3.16 eV. However, the value of the band gap increases slightly with the increasing amount of anatase on the R-TiO2 NRs (3.17, 3.189, and 3.19 eV for 5 mM, 10 mM, and 20 mM A/R-TiO2 composite, respectively). The calculated band gap of anatase and rutile TiO2 are consistent with the literature results [36]. Interestingly, when increasing the thicknesses of anatase on the R-TiO2 films, a decreased absorption was observed. This could be explained by the enhanced scattering of light with crystallites of anatase on the R-TiO2 NRs [3739]. As the light absorption decreases with increasing amount of anatase, it can be assumed that abundant surface oxygen vacancies or defects exist in anatase and R-TiO2 could promote the

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separation of electron–hole pairs under irradiation, which may play a central role in the PEC performance of the electrodes [40]. To assess the PEC performance for solar hydrogen generation and dye degradation, the linear sweep voltammetry (LSV) of curves were performed for bare R-TiO2 NRs, and A/RTiO2 composite photoanodes under simulated solar illumination (Fig. 5A). Under the dark condition, the current density is almost zero for all the photoanodes. However, upon illumination, all of the anatase modified/R-TiO2 composite exhibits a higher photocurrent value than the R-TiO2 and reaches its maximum of 1.6 mAcm−2 at 1.029 V vs. RHE. This potential was chosen since it is a representative value after the photocurrent saturation for all R-TiO2 NR based samples. However, this trend is not followed for the 20 mM coated samples, which show poor activity (1.32 mA.cm-2). This is probably due to the increased film thickness in 20 mM compared to pristine and other anatase coated R-TiO2 leads to higher charge recombination [41]. Fig. 5B shows the chronoamperometric response of A/R-TiO2 composite at 1.029 V vs RHE under simulated solar light illumination using the J–t method. It is worth noting that the photoresponse obtained from the A/R-TiO2 composite electrodes are very high among the previously studied anatase/rutile TiO2 powder heterostructure and photoanodes [42]. The photocurrent density can remain at a stable value after the first 5 min, revealing a good ability to resist photo-corrosion. However, the transient photocurrent responses of all the measured samples show that the photocurrent rapidly decreases to zero after the light is switched off. The improved value of photocurrent in the A/R-TiO2 composite is attributed to a more efficient separation and transport of charge carriers. Thus, further, to investigate the underlying reason for the enhanced PEC performance, the EIS Nyquist plots of A/R-TiO2 composite electrodes were measured. Fig. 5(C) shows the Nyquist plots fitted with the equivalent circuit consisting

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of the series resistance (R1) of the conductive substrate, the external electrical contacts, the liquid electrolyte, and the charge transfer resistance (R2) and capacitance (CCPE) at the electrode/semiconductor interface. The smaller resistance-circle obtained for the 10 mM A/RTiO2 composite suggests a quick separation of electrons and holes in the interface of the electrode/electrolyte [42]. The EIS parameters were determined by fitting the impedance spectra and are listed in Table 1. The lower value of R2 in the 10 mM A/R TiO2 indicates the fastening of the charge-transfer at the semiconductor–electrolyte interphase, which consequently helps to enhance the photocurrent density. Thus, based on the above analysis, the imperative role of the anatase modification was ascertained for prolonging the lifetime of the photogenerated charge carriers [44]. In order to assess the long-term performance of the 10 mM A/R TiO2 photoanodes, the photostability test was measured for 180 min under the continuous illumination (Fig. 5(D)). The current transient under a potentiostatic bias of 1.029 VRHE; less than 0.04 mA cm−2 decrease in the photocurrent is observed over 180 min. This proves the satisfactory performance of the 10 mM A/R TiO2 photoanodes for its practical applications (H2 generation and dye degradation). Since the alignment of the energy levels of the composites is closely related to its PEC performance, it is established on the basis of the Mott Schottky and band gap results. Fig. S3 shows the Mott Schottky plots of R-TiO2 nanorods and Anatase (5, 10, 20 mM) modified RTiO2 composite. The determination of the position of conduction band in the semiconductor is explained by R. Beranek [45, 46] .In the n-type semiconductor’s assuming that the difference between position of the conduction band edge and flat band potential (Vfb) is very small, the determination of the conduction band edge of TiO2 films often translates into the measurement of the flat band potential. The R-TiO2 NR has a flat band potential (Vfb) value of 0.06 V vs

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RHE, whereas the VFB value of A/R-TiO2 composite (the intercept on X-axis) shifts more negatively than the R-TiO2 (0.03 V vs. RHE). Thus, using these values of flat band potentials the positions of conduction band edges were determined. Also the band gap values were estimated the valence-band edges. Fig. 6 shows the plots of hydrogen generation with simultaneously recorded J-t profiles for pristine and A/R-TiO2 composite. A steady photocurrent was observed for both pristine and A/R-TiO2 composite over a period of four hours. The PEC solar hydrogen generation experiments were carried out in 2:8 mixtures of methanol and 0.5 M NaOH as electrolyte. To eliminate the external losses such as the resistive loss of the system, a bias of 1.04 V vs. RHE was applied in this experiment. The use of sacrificial reagents that can be oxidized more easily than water and/or capture photoproduced h+VB

carriers has been proven to enhance H2S

splitting by favoring the charge separation between photogenerated carriers, increasing their lifetime [47]. However, due to the addition of methanol in the NaOH electrolyte during hydrogen generation, the increase of photocurrent was observed compared to the PEC measurements. The PEC hydrogen generation is accompanied with the flow of photogenerated electrons in the photoanode materials to counter the electrode, and thus is directly proportional to the photocurrent density [48]. The bare TiO2 NRs films showed a photocurrent of 1.5 mA.cm-2 (line b, Fig. 6), and a hydrogen generation of 580 μmol was detected at 1.04 V vs. RHE (line d, Fig. 6). However, for the 10 mM A/R-TiO2 composite, the PEC performance is significantly enhanced. The highest photocurrent density of 1.8 mA.cm-2 (line a, Fig. 6) and the corresponding hydrogen generation of 625 μmol (line c, Fig. 6A) were obtained after 4 hs. This enhancement in PEC activity is probably due to the effective light scatterings which help to

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generate more energy harvesting and reduce the bulk and surface electron–hole recombination’s [49]. Besides the PEC hydrogen generation application, the PEC activity 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. Figs. 7(A) and 7(B) shows the photoelectrocatalytic 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 μmol orange II dye dissolved 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 spectra 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 illumination. 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 II at λ = 453 nm with the solution concentration, as it is below 0.8 [50, 51]. It is generally accepted that adsorption is critical in heterogeneous photocatalytic oxidation processes [52]. However, in this study, dark adsorption experiment shows that no obvious dye adsorption could be observed on the photocatalyst surface. However, most of the photocatalytic reactions take place with hydroxyl radicals generated after the adsorption of hydroxyl ions onto the catalyst surface, followed by reactions with holes in the excited, semiconductor catalyst [53]. Fig. S4a shows that the 10 mM A/R-TiO2 composite sample showed poorer activities in the presence of applied bias only. However, under solar light, orange II could be degraded up to 71% (photodegradation) within

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the 5 hr (Fig. S4b), by the degradation mechanism [54]. Interestingly, the orange II dye is rapidly decolorized and Fig. 7(C) shows that the PEC degradation of orange II dye (extinction taken at λ = 455 nm) follows a pseudo first order reaction and its kinetics can be expressed using equation ln(C/C0) = −kt, where C is the concentration of orange II dye at time t and 0 min respectively. The 96% degradation was achieved within 5 h for the 10 mM A/R-TiO2 composite under applied bias and solar light illumination conditions. The actual photographs of the samples after the >3 h photocatalytic decolorization experiments are shown in Fig. 7(D). In Addition to this, the stability of the photocatalyst during the PEC degradation reactions was measured by varying the photocurrent as a function of time and is shown in Fig. S5. The slight increase in photocurrent over time is due to the oxidation of organic species. The average photocurrent of 1.5 mA.cm -2 is drawn from the degradation of orange II dye using 10 mM A/R-TiO2 composite thin film. In the PEC reactions, when the semiconductor is illuminated with light having energy greater than its band gap, the photogenerated electrons and holes are formed. Maximum number of the photogenerated holes can react with water at the surface of the semiconductor 

photocatalyst to produce OH radical ( OH) radicals [55]. Similarly, photogenerated electrons appear on the surface of the counter electrode, where they are trapped by molecular oxygen, to 

form superoxide radical anion (O2 ). These superoxide radical ions further react with H+ ions 

and finally produce the H2O2 and O2. In the overall photoelectrochemical process, the OH and 

O2 are very reactive species that react with the orange II dye [56, 57]. The possible reactions during the PEC dye degradation are explained in the S1-S9 (supporting information) [58, 59]. During the PEC decolorization, the orange dye molecules are actually broken down and new products are formed which causes a reduction of absorbance. These products can then further break down as mineralized products such as CO2 and H2O, as reported in various cases of dye 16

degradation [60, 61]. Thus, the photocatalytic process is highly dependent on the hydroxyl group, 

and the OH radical present on the surface, which attacks the contaminant present in the water [62]. To confirm the presence of main reactive species (*OH and

*-

O2) involved in the

degradation process the similar degradation experiment were carried out by adding methanol as reactive oxygen species scavenger [63]. In order to study the comparative experiments, the 10 µM of Orange dye and various concentrations of methanol as reactive oxygen species scavenger were used as electrolyte during degradation experiments. The photocatalytic activity was observed for five hours under simulated solar irradiation (AM 1.5) at applied potential of 1.09 V vs RHE. The maximum degradation performance was observed for without methanol (0 ml methanol) as shown in Fig. S6. A decreased photocatalytic activity was observed with the increase in of the methanol concentration ( 5 and 10 ml) in the electrolyte solution. These results indicated the generation of main reactive species (*OH and *-O2) involved in the degradation process and are affected due to methanol during the photocatalytic reaction [64, 65]. In addition to this the mechanism of photosensitization of orange II dye/10 mM A/R-TiO2 composite, a monochromatic light filter with the pass wavelength of 420 nm was used for the visible light illumination. The orange II dye sensitization process involves the excitation of dye molecules by absorbing visible light photons and subsequent electron injection from excitation state dye to A/R TiO2. It was observed that the Orange II dye was still degraded (Fig. S7a, b), although A/RTiO2 composite cannot absorb light with a wavelength of ≥420 nm. The decrease in absorption band, indicating that the complex aromatic structures in Orange II dye is still attacked by the active species leading to the decomposition of dye. However slight shift in the absorption band of oranbge II dye reflects a typical chemical process [66, 67]. Thus, degradation of orange II dye

17

using 10 mM A/R-TiO2 composite is the combined effect of photosensitization and oxidative degradation over the surface of A/R-TiO2 composite catalyst. To demonstrate the high specific area of the A/R-TiO2 nanorod as compared to bare Rutile nanorods, the cyclic voltammetry (CV) measurements were performed in a three electrode configuration, in which Pt wire acted as counter electrode, Ag/AgCl as reference electrode, and R-TiO2 NRs based composites as the working electrode. Fig. S8 compares the normalized CV curves recorded in 0.5 M NaOH electrolyte from scan range of -0.4 to 1.0 V Vs RHE with 10 a mV s-1 scan rate using a portable potentiostat (COMPACTSTAT.e, Ivium, Netherlands). The obtained surface areasfor the 0, 5 mM, 10 mM and 20 mM are 5.83, 6.71,10.48, and 9.03 cm2 respectively. The photoelectrode prepared at optimal condition 10 mM A/R TiO2 composite yielding the highest surface area amongst the studied samples. The cyclic voltammetry measurements revealed significantly high electrochemically-active surface areas that depended only upon the amount of amount of Anatase modified on the Rutile. Therefore, these results indicate that formation of A/R TiO2 composites leads to an increase of electrochemically accessible surface areas, thereby increasing the photoelectrochemical performance [68-70]. 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 analysis, a tentative charge separation mechanism in A/R-TiO2 composite prepared on FTO substrate is illustrated in Fig. 8. Compared to bare R-TiO2 nanorods, A/R-TiO2 offers more surface area (Figs. 8(a) and 8(b)). Using UV visible spectra (Fig. 4A) and Mott Schottky plots (Fig.S3), the conduction band potentials of anatase and anatase modified TiO2 were determined. The schematic of the band diagram and charge transfer mechanism in the A/R-TiO2 composite photoanode is shown in Figs. 8(c) and 8(d). It is well known that, when anatase is coupled with

18

R-TiO2, their Fermi levels are aligned, leading to the formation of a composite at the interface and a built-in electric field appears across the interface of the anatase and rutile. As a result, upon illumination of photoelectordes with solar light, the photogenerated electrons will be injected from the CB of anatase into the CB of rutile due to the energy difference, and will separate from the holes, which results in efficient inhibition of the recombination. These electrons further become counter electrodes where they interact with molecular oxygen to produce the superoxide 

radical anion radical (O2 ) (in dye degradation experiments) or scavenged by hydrogen ions forming H2 gas (in hydrogen generation experiments). At the same time, the photogenerated holes are trapped by the reduced species in the electrolyte and further photo-oxide H2O to form



OH radicals [71]. These radicals will participate in the oxidation reaction and oxidize the Orange II dye molecules at the surface of photoelectrodes (in dye degradation experiments). Thus, through this redox reaction, the recombination is efficiently inhibited and promoted to a high photocurrent in the A/R-TiO2 composite photoanodes. Therefore, in addition to light scattering, the nanograin anatase and rutile TiO2 NRs favor the effective separation and transport of the photo-generated charge carriers, subsequently aiding in the PEC performance of the A/R-TiO2 composite. 4. Conclusions A/R-TiO2 composite photoanodes were successfully synthesized by dip coating of hydrothermal R-TiO2 NRs in a titanium isopropoxide solution followed by annealing at 400 °C. The composite photocatalysts showed higher photoelecrochemical activity than pure R-TiO2 under solar light irradiation, due to the improved separation of photogenerated electrons and holes. The stability and increased PEC performance of the A/R-TiO2 composite catalyst was studied by hydrogen generation and dye degradation approaches. The high performance of the A/R-TiO2 composite 19

could be attributed to the larger surface area of the anatase and longer direct electron path in TiO2 NR. Meanwhile, UV-Visible and Mott Schottky analyses confirm that the internal cascade process is advantageous to the separation of electrons and holes. The possible growth mechanism and charge transfer mechanism are also proposed. The present study provides the important step towards the formation of an A/R-TiO2 composite, while the further optimizations of thin and compact anatase layer will offer new strategies for the fabrication of efficient photoelectrode with improved PEC degradation as well as PEC hydrogen generation.

5. Acknowledgements This research was supported by the BK21 Plus program, the Korean National Research Foundation

(NRF)

(Nano-Material

Fundamental

Technology

Development,

2016M3A7B4909370) and the Korean Ministry of the Environment (MOE) as part of the Public Technology Program based on Environmental Policy (2014000160001).

20

References [1] A. C. Genevrier, T. Puig, X. Obradors, N. Mestres, Ferromagnetic 1D oxide nanostructures grown from chemical solutions in confined geometries, Chem. Soc. Rev. 43 (2014) 2042-2054. [2] M. D. Regulacio, M. Y. Han, Multinary I-III-VI2 and I2-II-IV-VI4 Semiconductor Nanostructures for Photocatalytic Applications, Acc. Chem. Res. 49 (2016) 511–519. [3] M. A. Mahadik, P. S. Shinde, M. Cho, J. S. Jang, Fabrication of a ternary CdS/ZnIn 2S4/TiO2 heterojunction for enhancing photoelectrochemical performance: effect of cascading electron– hole transfer, J. Mater. Chem. A 3 (2015) 23597-23606. [4] J. Low, J. Yu, M. Jaroniec, S. Wageh, A. A. Al-Ghamdi, Heterojunction Photocatalysts, Adv. Mater. 29 (2017) 1601694. [5] C. Lai, M. M. Wang, G. M. Zeng, Y.G. Liu, D. L. Huang, C. Zhang, R.Z. Wang, P.Xu, M. Cheng, C. Huang,Synthesis of surface molecular imprinted TiO2/graphene photocatalyst and its highly efficient photocatalytic degradation of target pollutant under visible light irradiation, Appl. Surf. Sci. 390 (2016) 368-376. [6] Y. Yang, Z.Ma, L. Xu, H. Wang, N. Fu, Preparation of reduced graphene oxide/mesoTiO2/AuNPs ternary composites and their visible-light-induced photocatalytic degradation n of methylene blue, Appl. Surf. Sci. 369 (2016) 576-583. [7] A. Fujishima, X. Zhang, D. A. Tryk, TiO2 photocatalysis and related surface phenomena, Surf. Sci. Rep. 63 (2008) 515–582. [8] J. Wu, H.Xu, W. Yan, Photoelectrocatalytic degradation Rhodamine B over highly ordered TiO2 nanotube arrays photoelectrode, Appl. Surf. Sci. 386 (2016) 1-13. [9] M. Nischk, P. Mazierski, Z. Wei, K. Siuzdak, N. A. Kouame, E. Kowalska, H. Remita, A. Z. Medynska, Enhanced photocatalytic electrochemical and photoelectrochemical properties of 21

TiO2 nanotubes arrays modified with Cu, AgCu and Bi nanoparticles obtained via radiolytic reduction, Appl. Surf. Sci. 387 (2016) 89-102. [10] F. Cao, J. Xiong, F. Wu, Q. Liu, Z. Shi, Y. Yu, X. Wang, L. Li, Enhanced Photoelectrochemical

Performance

from

Rationally

Designed

Anatase/Rutile

TiO2

Heterostructures, ACS Appl. Mater. Interfaces 8 (2016) 12239−12245. [11] L. Kavan, M. Grätzel, S. E. Gilbert, C. Klemenz, H. J. Scheel, Electrochemical and Photoelectrochemical Investigation of Single-Crystal Anatase, J. Am. Chem. Soc. 118 (1996) 6716–6723. [12] Z. Xiong , H. Wu , L. Zhang , Y. Gu, X. S. Zhao, Synthesis of TiO 2 with controllable ratio of anatase to rutile, J. Mater. Chem. A 2 (2014) 9291-9297. [13] S. Girishkumar, K.S.R. Koteswara Rao, Comparison of modification strategies towards enhanced charge carrier separation and photocatalytic degradation activity of metal oxide semiconductors (TiO2, WO3 and ZnO), Appl. Surf. Sci. 391 (2017) 124–148. [14] D. Tsukamoto, Y. Shiraishi, Y. Sugano, S. Ichikawa, S. Tanaka, T. Hirai, Gold Nanoparticles Located at the Interface of Anatase/Rutile TiO2 Particles as Active Plasmonic Photocatalysts for Aerobic Oxidation, J. Am. Chem. Soc. 134 (2012) 6309–6315. [15] H. Yao, W. Fu. L. Liu. X. Li. D. Ding. P. Su. S. Feng. H. Yang, Hierarchical photoanode of rutile TiO2 nanorods coupled with anatase TiO2 nanosheets array for photoelectrochemical application, J. Alloys Compd. 680 (2016) 206-211. [16] D. O. Scanlon, C. W. Dunnill, J. Buckeridge, S. A. Shevlin, A. J. Logsdail, S. M. Woodley, C. R. A. Catlow, M. J. Powell, R. G. Palgrave, I. P. Parkin, G. W. Watson, T. W. Keal, P. Sherwood, A. Walsh, A. A. Sokol, Band alignment of rutile and anatase, Nat. Mater. 12 (2013) 798–801.

22

[17] J. Zhou, G. Tian, Y. Chen, J. Q. Wang, X. Cao, Y. Shi, K. Pan, H. Fu, Synthesis of hierarchical TiO2 nanoflower with anatase–rutile heterojunction as Ag support for efficient visible-light photocatalytic activity, Dalton Trans. 42 (2013) 11242–11251. [18] J. Low, B. Cheng, J.Yu, Surface modification and enhanced photocatalytic CO2 reduction performance of TiO2: a review, Appl. Surf. Sci. 392 (2017) 658–686. [19] X. Li, J. Yu, M. Jaroniec, Hierarchical photocatalysts, Chem. Soc. Rev. 45 (2016) 26032636. [20] A. H. Hochbaum, P. D. Yang, Semiconductor Nanowires for Energy Conversion, Chem. Rev. 110 (2010) 527–546. [21] T. A. Kandiel, R. Dillert, A. Feldhoff, D. W. Bahnemann, Direct Synthesis of Photocatalytically Active Rutile TiO2 Nanorods Partly Decorated with Anatase Nanoparticles, J. Phys. Chem. C 114 (2010) 4909–4915. [22] M. A. Mahadik, P. S. Shinde, H. Lee, M. Cho, J. S. Jang, Highly efficient and stable 3D Ni(OH)2/CdS/ZnIn2S4/TiO2 heterojunction under solar light: Effect of an improved TiO2/FTO interface and cocatalyst, Sol. Energy Mater. & Sol. Cells 159 (2017) 475-487. [23] S. Hernandez, D. Hidalgo, A. Sacco, A. Chiodoni, A. Lamberti, V. Cauda, E. Tresso, G. Saracco, Comparison of photocatalytic and transport properties of TiO2 and ZnO nanostructures for solar-driven water splitting, Phys. Chem. Chem. Phys. 17 (2015) 7775–7786. [24] H.U. Lee, S.C. Lee, S.H. Choi, B. Son, S.J. Lee, H.J. Kim, J. Lee, Highly visible-light active nanoporous TiO2 photocatalysts for efficient solar photocatalytic applications, Appl. Catal. B: Environ. 129 (2013) 106–113.

23

[25] M. Han, J. Jia, 3D Bi2S3/TiO2 cross-linked heterostructure: An efficient strategy to improve charge transport and separation for high photoelectrochemical performance, J. Power Sources 329 (2016) 23–30. [26] D. A. H. Hanaor, C. C. Sorrell, Review of the anatase to rutile phase transformation, J. Mater. Sci. 46 (2011) 855–874. [27] A. Li, Z. Wang, H. Yin, S. Wang, P. Yan, B. Huang, X. Wang, R. Li, X. Zong, H. Han, C. Li, Understanding the anatase–rutile phase junction in charge separation and transfer in a TiO2 electrode for photoelectrochemical water splitting, Chem. Sci. 7 (2016) 6076-6082. [28] N. G. Park, G. Schlichthörl, J. van de Lagemaat, H. M. Cheong, A. Mascarenhas, A. J. Frank, Dye-sensitized TiO2 solar cells: Structural and photoelectrochemical characterization of nanocrystalline electrodes formed from the hydrolysis of TiCl4, J. Phys. Chem. B 103 (1999) 3308 – 3314. [29] C. Wang, X. Zhang, C. Shao, Y. Zhang, J. Yang, P. Sun, X. Liu, H. Liu, Y. Liu, T. Xie, D. Wang, Rutile TiO2 nanowires on anatase TiO2 nanofibers: A branched heterostructured photocatalysts via interface-assisted fabrication approach, J. Colloid Interface Sci. 363

(2011)

157–164. [30] A. Li Bassi, D. Cattaneo, V. Russo, C. E. Bottani, E. Barborini, T. Mazza, P. Piseri, P. Milani, F. O. Ernst, K. Wegner, S. E. Pratsinis, Raman spectroscopy characterization of titania nanoparticles produced by flame pyrolysis: The influence of size and stoichiometry, J. Appl. Phys. 98 (2005) 074305. [31] F. Farges, G. E. Brown, Jr., J. J. Rehr, Ti K-edge XANES studies of Ti coordination and disorder in oxide compounds: Comparison between theory and experimental, Phys. Rev. B 56 (1997) 1809-1819.

24

[32] I. M. Clegg, N. J. Everall, B. King, H. Melvin, C. Norton, On-Line Analysis Using Raman Spectroscopy for Process Control during the Manufacture of Titanium Dioxide, Appl. Spectrosc. 55 (2001) 1138-1150. [33] V. H. Castrejón-Sánchez, E. Camps, M. Camacho-López, Quantification of phase content in TiO2 thin films by Raman spectroscopy, Superficies y Vacío. 27 (2014) 88-92. [34] W.Q Fan, X. Q. Yu, S. Y. Song, H.Y. Bai, C. Zhang, D. Yan, C. B. Liu, Q. Wang, W. D. Shi, Fabrication of TiO2–BiOCl double-layer nanostructure arrays for photoelectrochemical water splitting, CrystEngComm. 16 (2014) 820-825. [35] S. Hwa Y. J. Min,

K. G. Ali, S. H. Heo, S. O. Cho, Improvement in the

photoelectrochemical responses of PCBM/TiO2 electrode by electron irradiation, Nanoscale Res. Lett. 7 (2012) 1421-1426. [36] A. Kudo, Y. Miseki, Heterogeneous photocatalyst materials for water splitting, Chem. Soc. Rev. 38 (2009) 253-278. [37] F. Cao, J. Xiong, F. Wu, Q. Liu, Z. Shi, Y. Yu, X. Wang, L. Li, Enhanced Photoelectrochemical

Performance

from

Rationally

Designed

Anatase/Rutile

TiO2

Heterostructures, ACS Appl. Mater. Interfaces 8 (2016) 12239−12245. [38] S. Hernández, D. Hidalg, A. Sacco, A. Chiodoni, A. Lamberti, V. Cauda, E. Tresso, G. Saracco, Comparison of photocatalytic and transport properties of TiO2 and ZnO nanostructures for solar-driven water splitting, Phys. Chem. Chem. Phys. 17 (2015) 7775-7786. [39] J. G. Yu, H. G. Yu, B. Cheng, X. J. Zhao, J. C. Yu, W. K. Ho, The Effect of Calcination Temperature on the Surface Microstructure and Photocatalytic Activity of TiO2 Thin Films Prepared by Liquid Phase Deposition, J. Phys. Chem. B 107 (2003) 13871-13879.

25

[40] J. Yan, G. Wu, N. Guan, L. Li, Z. Li, X. Cao, Understanding the effect of surface/bulk defects on the photocatalytic activity of TiO2: anatase versus rutile, Phys. Chem. Chem. Phys. 15 (2013) 10978-10988. [41] B, A. Aragaw, C.J. Pan, W. N. Su, H. M. Chen, J. Rick, B. J. Hwang, Facile one-pot controlled synthesis of Sn and C codoped single crystal TiO2 nanowire arrays for highly efficient photoelectrochemical water splitting, Appl. Catal. B 163 (2015) 478-486. [42] H. Y. Tian, G. H. Zhao, Y. N. Zhang, Y. B. Wang, T. C. Cao, Hierarchical (0 0 1) facet anatase/rutile TiO2 heterojunction photoanode with enhanced photoelectrocatalytic performance, Electrochim. Acta. 96 (2013) 199–205. [43] M. A. Mahadik, P. S. Shinde, M. Cho, J. S. Jang, Metal oxide layer as role of interfacial promoter on ZnIn2S4/TiO2 heterostrcuture photoanode for its enhanced photoelectrochemical performance, Applied. Catal. B: Environ. 184 (2016) 337-346. [44] M. Zeng, X. Zeng, X. Peng, Z. Zhu, J. Liao, K. Liu, G. Wang, S. Lin, Improving photoelectrochemical performance on quantum dots co-sensitized TiO2 nanotube arrays using ZnO energy barrier by atomic layer deposition, Appl. Surf. Sci. 388 (2016) 352-358. [45] R. Beranek, (Photo)electrochemical Methods for the Determination of the Band Edge Positions of TiO2-Based Nanomaterials, Adv. Phys. Chem. 2011 (2011) 786759. [46] R. Beranek, H. Kisch, Surface-modified anodic TiO2 films for visible light photocurrent response, Electrochem. Commun. 9 (2007) 761–766. [47] D. Barreca, G. Carraro, V. Gombac, A. Gasparotto, Chiara Maccato, Paolo Fornasiero, Eugenio Tondello, Supported Metal Oxide Nanosystems for Hydrogen Photogeneration: Quo Vadis, Adv. Funct. Mater. 21 (2011) 2611–2623.

26

[48] J. H. Park, A. Mandal, S. Kang, U. Chatterjee, J. S. Kim, B. G. Park, M. D. Kim, K. U. Jeong, C. R. Lee, Hydrogen Generation using non-polar coaxial InGaN/GaN Multiple Quantum Well Structure Formed on Hollow n-GaN Nanowires, Sci. Rep. 6 (2016) 31996. [49] Q. Wana, N. Aa, Y. Bai, H. Hana, J. La, X.n La, Y. Liu, High photocatalytic hydrogen production from methanol aqueous solution using the photocatalysts CuS/TiO2, Int. J. Hydrogen Energy 38 (2013) 10739–10745. [50] Z. Xie, X. Liu, W. Wang, C. Liu, Z. Li, Z. Zhang, Enhanced photoelectrochemical properties of TiO2 nanorod arrays decorated with CdS nanoparticles, Sci. Technol. Adv. Mater. 15 (2014) 055006. [51] Z. Li, Y. Zhu, Q. Zhou, J. Ni, Z. Zhang, Photocatalytic properties of TiO2 thin films obtained by glancing angle deposition, Appl. Surf. Sci. 258 (2012) 2766–2770. [52] C. C. Wang, C. K. Lee, M. D. Lyu, L. C. Juang, Photocatalytic degradation of C.I. Basic Violet 10 using TiO2 catalysts supported by Y zeolite: An investigation of the effects of operational parameters, Dyes Pigm. 76 (2008) 817–824. [53] B. Zielińnska, J. Grzechulska, R.J. Kaleńczuk, A.W. Morawski, The pH influence on photocatalytic decomposition of organic dyes over A11 and P25 titanium dioxide, Appl. Catal. B 45 (2003) 293–300. [54] R. Malka, P. Sagi, P. Yaron, Using Dyes for Evaluating Photocatalytic Properties: A Critical Review, Molecules 20 (2015) 88-110. [55] T. Tachikawa, M. Fujitsuka, T. Majima, Mechanistic Insight into the TiO2 Photocatalytic Reactions:  Design of New Photocatalysts, J. Phys. Chem. C 111 (2007) 5259–5275.

27

[56] B. Yue, Y. Zhou, J. Xu, Z. Wu, X. Zhang, Y. Zou, S. Jin, Photocatalytic Degradation of Aqueous 4-Chlorophenol by Silica-Immobilized Polyoxometalates, Environ. Sci. Technol. 36 (2002) 1325–1329. [57] X. Deng, Q. Ma, Y. Cui, H. Zhang, X. Cheng, X. Li, M. Xie, Q. Cheng, B. Li, Microwaveassisted synthesis of Ag2O/reduced TiO2 nano-tube arrays photoelectrode with enhanced visible photocatalytic activity for degradation of organic pollutants, Sep. Purif. Technol. 182 (2017) 230–237. [58] M. Pelaez, N. T. Nolan, S. C. Pillai, M. K. Seery, P. Falaras, A. G. Kontos, P. S.M. Dunlop, J. W.J. Hamilton, J. A. Byrne, K. O'Shea, M. H. Entezari, D. D. Dionysiou, A review on the visible light active titanium dioxide photocatalysts for environmental applications, Appl. Catal. B 125 (2012) 331–349. [59] C. Baumanis, J.Z. Bloh, R. Dillert, D.W. Bahnemann, Hematite photocatalysis: declorination of 2,6-dichloroindophenol and oxidation of water, J. Phys. Chem. C 115 (2011) 25442–25450. [60] E. Baldev, D. MubarakAli, A. Ilavarasi, D. Pandiaraj, K.A. Sheik Syed Ishack, N. Thajuddin, Degradation of synthetic dye, Rhodamine B to environmentally non-toxic products using microalgae, Colloids Surf. B 105 (2013) 207–214. [61] S. Saraf, B. Thomas, Influence of Feedstock and Process Chemistry on Biodiesel Quality, Process Saf. Environ. 85 (2007) 360-364. [62] R. J. Tayade, P. K. Surolia, R. G. Kulkarni, R. V. Jasra, Photocatalytic degradation of dyes and organic contaminants in water using nanocrystalline anatase and rutile TiO2, Sci. Tech. Adv. Mater. 8 (2007) 455–462.

28

[63] C. Valentin, D. Fittipaldi, Hole Scavenging by Organic Adsorbates on the TiO2 Surface: A DFT Model Study, J. Phys. Chem. Lett. 4 (2013) 1901−1906. [64] Y. Liao, J. Brame, W. Que, Z. Xiu, H. Xie, Q. Li, M. Fabian, P. Alvarez, Photocatalytic generation of multiple ROS types using low-temperature crystallized anodic TiO2 nanotube arrays, J. Hazard. Mater. 260 (2013) 434-441. [65] Q. Guo, C. Zhou, Z. Ma, Z. Ren, H. Fan, X. Yang, Elementary photocatalytic chemistry on TiO2 surfaces, Chem. Soc. Rev. 45 (2016) 3701 -3730. [66] Y. Xie, C. Yuan, X. Li, Photosensitized and photocatalyzed degradation of azo dye using Lnn+-TiO2 sol in aqueous solution under visible light irradiation, Mater. Sci. Eng. B 117 (2005) 325-333. [67] L. Wang, J. Shang, W. Hao, S. Jiang, S. Huang, T. Wang, Z. Sun, Y. Du, S. Dou, T. Xie, D. Wang, J. Wang, A dye-sensitized visible light photocatalyst-Bi24O31Cl10, Sci. Rep. 4 (2014) 73841-73848. [68] S. Khadempir,

A. Ahmadpour,

M. T. Hamed Mosavian,

N. Ashraf,

Fatemeh F.

Bamoharram, S. G. Mitchelld, J. M. de la Fuente, A polyoxometalate-assisted approach for synthesis of Pd nanoparticles on graphene nanosheets: synergistic behaviour for enhanced electrocatalytic activity, RSC Adv. 5 (2015) 24319–24326. [69] A. Morag, T. Golub, J. Becker, R. Jelinek, High surface area electrodes by template-free self-assembled hierarchical porous gold architecture, Adv. Colloid Interface Sci. 472 (2016) 84– 89. [70] Y. Z. Su, Q. Z. Xu, G. F. Chen, H. Cheng, N. Li, Z. Q. Liu, One dimensionally spinel NiCo2O4 nanowire arrays: facile synthesis, water oxidation, and magnetic properties, Electrochim. Acta 174 (2015) 1216-1224.

29

[71] Y. Zhang, N. Qin, J. Li, S. Han, P. Li, G. Zhao,Facet exposure-dependent photoelectrocatalytic oxidation kinetics of bisphenol A on nanocrystalline {001} TiO2/carbon aerogel electrode, Appl. Catal. B: Environ. 216 (2017) 30–40.

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Figure captions 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. 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). Fig. 3 Raman spectra of (a) R-TiO2, (b) 5 mM, (c) 10 mM, and (d) 20 mM A/R-TiO2 composites prepared on FTO. Fig. 4 (A) UV–vis–DRS absorbance spectra, and (B) Kubelka–Munk plots for (a) R-TiO2, (b) 5mM, (c) 10 mM, and (d) 20 mM A/R-TiO2 composite 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) 20mM 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)

Fig. 6 Amperometric (J–t) curves and hydrogen generation over the TiO2 NRs and 10 mM A/RTiO2 composite electrodes under illumination (100 mW cm−2) in a 2:8 mixture of methanol and 0.5 M NaOH. 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

31

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

32

Figures

Fig. 1 33

Fig. 2

34

Fig. 3

35

Fig. 4

36

Fig. 5

37

Fig. 6

38

Fig. 7

39

Fig. 8

40

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

Scheme 1

41

Table captions Table 1 Parameters determined from EIS fitting of A/R-TiO2 composites. Sample 0 mM 5 mM 10 mM 20 mm

R1 (Ω) 43 26 74 45

42

R2 (Ω) 5684 3966 3766 12483

CCPE (μF) 6.72 7.6 5.9 1.0