Effect of Annealing Temperature on the Hydrogen Production of TiO2

ARTICLE pubs.acs.org/JPCC

Effect of Annealing Temperature on the Hydrogen Production of TiO2 Nanotube Arrays in a Two-Compartment Photoelectrochemical Cell Yan Sun,† Kangping Yan,*,† Guixin Wang,† Wei Guo,‡ and Tingli Ma*,‡ † ‡

School of Chemical Engineering, Sichuan University, Chengdu 610065, Sichuan, China State Key Laboratory of Fine Chemicals, Dalian University of Technology, Dalian 116024, Liaoning, China ABSTRACT: Due to the energy crisis, it is necessary to develop clean and renewable energy sources. In this study, we report an efficient and economical technology to produce hydrogen from solar energy by splitting water in a twocompartment photoelectrochemical (PEC) cell without any external applied voltage. To enhance the solar conversion efficiency, highly ordered TiO2 nanotube arrays with 4 μm in length were synthesized by a rapid anodization process in ethylene glycol electrolyte. Crystal phase and morphology of the TiO2 nanotubes (NTs) samples annealed at various temperatures were characterized by XRD and FESEM. Transient photocurrent response and linear sweep voltammetry curves were measured using electrochemical working station under solar light illumination. The photocatalytic activity was evaluated by the hydrogen production in the PEC cell. The results indicated that the crystal phase and morphology of TiO2 NTs had no great changes at low annealing temperatures. Anatase phase and tubular structure of TiO2 NTs were stable up to 450 °C. With further increase in temperature, the crystallization transformation from anatase to rutile phase appeared, accompanied by the destruction of tubular structures. Due to the excellent crystallization and the maintenance of tubular structures, TiO2 NTs annealed at 450 °C exhibited the highest photoconversion efficiency of 4.49% and maximum hydrogen production rate of 122 μmol/(h 3 cm2), which is superior to most of those reported so far.

1. INTRODUCTION Due to the depletion and pollution of fossil fuel, extensive studies have been carried out on clean and renewable energy. Therefore, hydrogen obtained from renewable sources such as sunlight has attracted many researchers’ attention. Since Fujishima and Honda1 have demonstrated that water could be decomposed into H2 and O2 on a TiO2 electrode in a photoelectrochemical (PEC) cell, photocatalytic splitting of water using semiconductor has been regarded as an ideal method for converting solar energy into hydrogen energy. TiO2 has become the most promising material for photocatalytic applications due to its low cost, nontoxicity, and photostability. In fact, the powder suspension system has been employed intensively for water splitting.2
4 However, several problems have limited the practical application of powder photocatalysts. (1) It is difficult for the separation and recovery of photocatalysts in the aqueous solution. (2) The photogenerated electrons (e
) and holes (hþ) easily recombined in the suspension. The rate of recombination could be restrained by loading Pt on the catalyst surface or adding various sacrificial agents such as methanol, ethanol, and formaldehyde,5 which apparently raise the cost of hydrogen production. (3) The system yields a mixture of H2 and O2 for the proximity of the redox sites.6 Thus, the immobilization of TiO2 photocatalysts in the form of films has been widely investigated for hydrogen production utilizing solar energy in photoelectrochemical cells.7
10 r 2011 American Chemical Society

Compared with other TiO2 films prepared by chemical vapor deposition (CVD), liquid-phase deposition (LPD), sol
gel, and magnetron sputtering deposition methods,11
14 nanotubular structure shows large surface area without increasing the geometric area due to the external and internal surfaces. Moreover, the one-dimensional highly ordered nanotube architecture offers an excellent electrical channel for vectorial electron transfer.15 Therefore, TiO2 nanotubes (NTs) have been intensively studied and fabricated with various methods,16
19 including electrochemical anodization, hydrothermal treatment, and template synthesis. Among these methods, electrochemical anodization is considered as a relative simple technique to synthesize self-organized TiO2 NTs discovered by Grimes group.20 The pore size, wall thickness, and tube length can be precisely controlled by varying the electrochemical conditions consisting of anodization voltage, anodization time, the concentration of electrolyte, and heat treatment, etc.21
23 As we know, the heat treatment of TiO2 has a great effect on its grain size, surface morphology, crystalline phase composition, and photoelectrochemical properties.24
26 Therefore, it is necessary to further study the effect of annealing temperature on TiO2 NTs.27
30 To the best of our knowledge, Received: December 7, 2010 Revised: March 24, 2011 Published: June 13, 2011 12844

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there has been no report on the effect of annealing temperature on the hydrogen production of TiO2 NTs in a two-compartment PEC cell (as described below in detail) without any external applied voltage. For this purpose, a series of TiO2 NTs annealed at different temperatures has been synthesized in this work. The effects of annealing temperature on the crystallization, morphology structures, photoelectrochemical properties, and photocatalytic activity measured by producing hydrogen are discussed in order to obtain TiO2 NTs with higher photocatalytic activity and then enhance the solar conversion efficiency.

2. EXPERIMENTAL SECTION Preparation of TiO2 Nanotube Arrays. Titanium foil (0.2 mm thick, 99.7% purity) was mechanically polished with abrasive paper, successively, degreased ultrasonically in acetone and ethanol, and finally rinsed with deionized water and dried in air before anodization. All the anodization was carried out in a two-electrode electrochemical cell with titanium foil as the anode and platinum foil as the cathode under a constant voltage 60 V using a direct current power supply for 10 min at room temperature. All the electrolytes consisted of 0.25 wt % NH4F in an ethylene glycol solution with 2 vol % H2O. After electrochemical anodization, the as-anodized TiO2 NTs were rinsed with deionized water and dried at 80 °C. Crystallized samples were obtained by annealing asanodized amorphous TiO2 nanotubes in oxygen ambient at different temperatures (from 250 to 750 °C) for 1 h. Characterization. The morphology of TiO2 nanotubes was observed by a field emission scanning electron microscope (FESEM, FEI Inspect F, USA) with an acceleration voltage 20 kV. The crystal phases were analyzed by an X-ray diffractometer (X’Pert MPD, Phillips) with Cu KR radiation at 35 kV and 25 mA, employing a scan rate of 0.04°/s in the range of 20°
60°. Photoelectrochemical Measurement. The photoelectrochemical characterization was carried out using an electrochemical working station (Model PAR273A, Princeton Applied Research) in a standard three-electrode configuration with TiO2 nanotubes on Ti foil as the working electrode, platinum electrode as a counter electrode, and saturated calomel electrode (SCE) as a reference electrode, respectively. The working electrode (area 0.5 cm 2.0 cm) was illuminated by a 350 W xenon lamp (110 mW/cm2) as the light source. Photocatalytic Activity. The photocatalytic activity of TiO2 nanotubes was investigated by the evolution of H2 in the PEC cell without any external applied voltage, schematically shown in Figure 1. The PEC cell used in this study was a chemically biased two-compartment cell which was separated by the Nafion membrane. Anodic and cathodic compartments were filled with 1 M KOH and 0.5 M H2SO4 electrolytes, respectively. The synthesized TiO2 film was used as photoanode, and a platinum electrode was served as cathode. The anode and cathode were connected with a copper wire. The H2 gas generated at Pt electrode was collected in an inverted buret by displacing electrolyte in the buret column. Materials. The following chemicals were used in this study: acetone, ethanol, ammonium fluoride, ethylene glycol, sulfuric acid, and potassium hydroxide. All chemicals of analytical grade were purchased from Chengdu Kelong Chemical Reagent Factory (China). Deionized water was used in all cases for making solutions. Titanium foil (0.2 mm thick, 99.7% purity) was purchased from Baoji Ronghao Titanium Industry Co., China.

Figure 1. Scheme of the two-compartment photoelectrochemical cell.

Figure 2. X-ray diffraction patterns of TiO2 NTs annealed at various temperatures: A, anatase; R, rutile; T, titanium.

The platinum foil (0.1 mm thick, 99.99% purity) was purchased from Beijing General Research Institute for Nonferrous Metals.

3. RESULTS AND DISCUSSION Crystal Properties. Figure 2 shows the XRD patterns of asanodized and the annealed samples at different temperatures from 250 to 750 °C, in which amorphous regions were gradually crystallized to form anatase/rutile phases. The diffraction pattern of as-anodized sample only shows Ti substrate. It indicates that as-anodized TiO2 is amorphous. The sample annealed at 250 °C is also amorphous. However, with increasing the annealing temperature to 350 °C, two obvious peaks at 25.3° and 48.1° of 2θ angle can be identified, corresponding to anatase TiO2 (101) and (200). It can be seen that there are only anatase diffraction peaks that can be observed for samples annealed at 450 °C. A signal diffraction peak at 27.4° of rutile (110) begins to appear when the temperature is raised to 550 °C, indicating that the anatase phase starts to transform into rutile phase during heat 12845

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Table 1. Effect of Annealing Temperature on Crystal Phase Composition and Mean Crystallite Sizes of TiO2 NTs crystallite size (nm) temp (°C)

anatase (101)

mass fraction (%)

rutile (110)

anatase

rutile

350

20.4




100




450

27.6




100




550 650

28.7 33.0

30.5 39.2

89.8 40.7

9.2 50.3

750

35.6

46.5

9.4

90.6

treatment at a high temperature. With further increase in temperature up to 750 °C, the intensity of rutile increases sharply while the intensity of anatase almost diminishes. The corresponding result is listed in Table 1. It can be seen that 90.6% anatase TiO2 was transformed into rutile phase. The relative amounts of anatase and rutile were calculated by eq 131 χ¼

1 1 þ 0:8

IA IR

ð1Þ

where χ is the mass fraction of rutile phase, IA is the diffraction peak intensity of anatase phase (101), and IR is the diffraction peak intensity of rutile phase (110). Hence, the crystal phase composition as a function of annealing temperature was determined. Table 1 also lists the mean crystallite sizes calculated by the Scherrer equation.32 It can be found that the average crystallite sizes of both anatase and rutile phase increase with increase of temperature after the nucleation, which is explained that the heating temperature offers a higher energy to accelerate the growth of crystal grains.33 It is noticeable that the diffraction peaks of titanium substrate begin to decrease at 550 °C accompanied by an increase in the intensity of rutile. Especially at 750 °C, the diffraction peaks of titanium almost disappear. Previous studies have also reported similar phenomenon.34,35 They suggest that titanium is directly oxidized and transformed into rutile TiO2 at high temperature. To investigate the possibility, bare titanium foils were annealed at 550, 650, and 750 °C for 1 h. As shown in Figure 3, the XRD of Ti substrate is almost similar to the TiO2 NTs annealed at the same temperature except that there is no any anatase diffraction peak observed. Therefore, it is reasonable to conclude that part of rutile phase of TiO2 NTs is directly transformed from Ti substrate, which is attributed to the atmospheric O2 diffusing through the oxide layer and oxidizing the Ti substrate at high temperature.36 Morphological Characterization. The morphologies of TiO2 NTs samples annealed from 450 to 750 °C are shown in Figure 4. A compound structure—a top nanoporous layer with the average pore diameter of 40 nm and an underneath highly regular nanotube layer with the length of approximately 4 μm can be observed from Figure 4, a and e. According to the theory,37 the nanotubular morphology results from chemical etching of the tube walls from both sides leading to wall thinning and tube separation. It can be explained that the nanoporous layer is not completely dissolved during the anodization process for 10 min, which leads to the nanoporous structure on the top surface. Extending anodization time, the top nanoporous layer is dissolved completely. Thereby, individual nanotube arrays separated and unconnected can be obtained. In addition, it is clear

Figure 3. X-ray diffraction patterns of bare Ti substrate annealed at various temperatures: T, titanium; R, rutile.

that the length of the nanotubes mainly depends on the anodization time. As reported by Grimes et al.,15 a maximum nanotube growth rate is observed at 60 V with ethylene glycol electrolyte. The present work shows that anodization of Ti foil leads to TiO2 nanotubes of 4 μm in length only after anodization for 10 min under the same condition. The result also shows clearly that the morphology of the TiO2 NTs structure is significantly affected by the annealing temperature. No great changes in the pore diameter and tube length were observed after annealing below 450 °C (not shown here). The sample annealed at 550 °C still remains as tubular structures (Figure 4f). However, at 650 °C, the nanopores on the surface seem to have grown together and become aggregates of TiO2 nanoparticles (Figure 4c). Moreover, the tubular structure has been destroyed but not collapsed completely and the length of nanotube is shortened obviously (Figure 4g). When the temperature increases to 750 °C, tubular structures completely disappear (Figure 4h) instead of following the formation of a dense rutile film (Figure 4d), which is attributed to the large percent transformation from anatase phase to rutile phase (90.6%) and the rapid growth of rutile grains (600 nm) at a high temperature. These results are consistent with the model of the transformation from anatase to rutile phase and the morphology architecture evolution of TiO2 nanotubes.35 The nucleation of rutile phase occurs first at the interface between Ti substrates and TiO2 NTs, then on the top of nanotubes, and finally on the bulk of nanotubes. Photoelectrochemical Characterization. To investigate the photoresponse of TiO2 NTs samples annealed at different temperatures, the photoelectrochemical measurement was carried out in a two-compartment PEC cell under illumination with several 30 s light on/off cycles at 0 V (vs SCE). The plot of the transient photocurrent response vs time is shown in Figure 5. All samples have good photoresponses under conditions of light on
off cycles. Without illumination, the current value is almost zero while the photocurrent rapidly rises to a steady-state value upon illumination, which is reproducible for several on
off cycles. The observed photocurrent was due to the photoinduced electrons of TiO2 NTs photoanode. This result indicates that the charge transportation process from the wall of TiO2 NTs to Ti substrate is very rapid. As shown in 12846

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Figure 5. Transient photocurrent response of TiO2 NTs samples annealed at temperatures ranging from 350 to 750 °C.

Figure 4. FESEM images of TiO2 NTs samples annealed at various temperatures from 450 to 750 °C: (a
d) the top surface views; (e
h) the cross-section views.

Figure 5, the annealing temperature has a significant effect on the photocurrent. The photocurrent of samples annealed at 350 and 450 °C increases with increasing temperature due to the formation of anatase phase and the enhancement of crystallization. TiO2 NTs annealed at 450 °C exhibits the highest photocurrent density of 5.8 mA/cm2. Beyond this temperature, a decrease of photocurrent is observed which could be explained that sample annealed at 450 °C has good crystallization and still remains tubular structures. With further increase in temperature, the decrease of anatase fraction and the destruction of tubular structures result in the decrease of photocurrent. Especially at 750 °C, the photocurrent dropped rapidly owing to the large percent of rutile phase (nearly 90%). Previous investigation38 has indicated that rutile has a greater recombination rate than anatase and shows neglectable photocurrent.

Figure 6. Photocurrent density and corresponding photoconversion efficiency of TiO2 NTs annealed at 450 °C.

Figure 6 shows the linear sweep voltammetry curves and corresponding photoconversion efficiency of TiO2 NTs annealed at 450 °C. The potential was swept linearly at a scan rate of 10 mV/s from
1.0 to 1.0 V (vs SCE). In dark condition, the current value is around 10
3 mA/cm2, almost 3 orders of magnitude less than the photocurrent magnitude. The photoconversion efficiency η was calculated via the following equation39 " # E0rev
jEapp j η ð%Þ ¼ jp  100 ð2Þ I0 where jp is the photocurrent density (mA/cm2), jpE0rev is the total power output, jp|Eapp| is the electrical power input, and I0 is the power density of incident light. E0rev is the standard reversible potential of 1.23 V/NHE, and the applied potential is Eapp = Emeas
Eaoc, where Emeas is the electrode potential (vs SCE) of 12847

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the excellent crystallization and the maintenance of tubular structures which are beneficial for vectorial charge transfer between interfaces. The maximum rate of hydrogen production is 122 μmol/(h 3 cm2), which is higher than most of those reported so far.

Figure 7. Comparison of the rates of hydrogen production of TiO2 NTs samples annealed at different temperatures.

the working electrode at which photocurrent was measured under illumination and Eaoc is the electrode potential (vs SCE) of the same working electrode under open circuit condition under the same illumination and in the same electrolyte. The voltage at which the photocurrent becomes zero was taken as Eaoc. TiO2 NTs annealed at 450 °C showed maximum photoconversion efficiency of 4.49% at
0.46 V vs SCE. Hydrogen Production in PEC Cell. The photocatalytic activity of TiO2 NTs was evaluated by measuring the rate of hydrogen production. In principle, the hydrogen generation by TiO2 photoanode under solar light irradiation in KOH electrolyte proceeds as follows:40 TiO2 þ hν f e
þ hþ

ð3Þ

2H2 O þ 2hþ f O2 þ 4Hþ

ð4Þ

2Hþ þ 2e
f H2

ð5Þ

In this work, high-efficiency production of hydrogen was performed in a two-compartment PEC cell. As described above, anodic and cathodic compartments were filled with potassium hydroxide and sulfuric acid aqueous electrolytes with the same concentration, respectively. Therefore, a small chemical bias can be produced by the different pH to assist the electron transfer from the TiO2 into the Pt side through the metal substrate rapidly, as well as reduce the recombination of electrons and holes. Furthermore, efficient separation of H2 and O2 has already been achieved by producing H2 on the Pt side in the cathodic compartment and O2 on the TiO2 NTs side in the anodic compartment, respectively. The rate of hydrogen production shown in Figure 7 indicates that the amount of evolved H2 is linear with the irradiation time approximately. It indicates that TiO2 NTs can be effectively applied for hydrogen production in the PEC cell, which utilizes light from a xenon lamp converting solar energy to hydrogen energy without any external applied voltage. The result also demonstrates that the effect of annealing temperature on hydrogen production of TiO2 NTs is consistent with the transient photocurrent response, which mostly relies on the charge carrier generation and their combination with hydrogen ions. The sample annealed at 450 °C shows the highest photocatalytic activity evaluated by hydrogen production rate. It can be due to

4. CONCLUSIONS Highly ordered TiO2 nanotube arrays 4 μm in length have been synthesized by a rapid anodization process in ethylene glycol electrolyte under a constant voltage of 60 V dc for 10 min at room temperature. Utilizing the prepared TiO2 NTs as photoanode, efficient and economical conversion of solar energy to hydrogen has already been successfully achieved by splitting water in the two-compartment PEC cell without any external applied voltage. The annealing temperature has a significant effect on the crystal phase, morphology, and photoelectrochemical properties of TiO2 NTs. The crystal phase and morphology of TiO2 NTs samples annealed at low temperatures have no great changes. Anatase phase and tubular structure of TiO2 NTs are stable up to 450 °C. With further increase in temperature, the crystallization transformation from anatase to rutile phase appears, accompanied by the destruction of tubular structures for vectorial charge transfer, which result in the sharp decrease of photocurrent. The experiment results show that TiO2 NTs annealed at 450 °C exhibit the highest photoconversion efficiency of 4.49% and maximum hydrogen production rate of 122 μmol/(h 3 cm2) due to the excellent crystallization and the remaining tubular structures. ’ REFERENCES (1) Fujishima, A.; Honda, K. Nature 1972, 238, 37–38. (2) Nada, A. A.; Barakat, M. H.; Hamed, H. A.; Mohamed, N. R.; Veziroglu, T. N. Int. J. Hydrogen Energy 2005, 30, 687–691. (3) Ahmmad, B.; Kusumoto, Y.; Somekawa, S.; Ikeda, M. Catal. Commun. 2008, 9, 1410–1413. (4) Yi, H. B.; Peng, T. Y.; Ke, D. N.; Ke, D.; Zan, L.; Yan, C. H. Int. J. Hydrogen Energy 2008, 33, 672–678. (5) Patsoura, A.; Kondarides, D. I.; Verykios, X. E. Catal. Today 2007, 124, 94–102. (6) Kitano, M.; Takeuchi, M.; Matsuoka, M.; Thomas, J. M.; Anpo, M. Catal. Today 2007, 120, 133–138. (7) Srivastava, O. N.; Karn, R. K.; Misra, M. Int. J. Hydrogen Energy 2000, 25, 495–503. (8) Arakelyan, V. M.; Aroutiounian, V. M.; Shahnazaryan, G. E.; Khachaturyan, E. A. Renewable Energy 2008, 33, 299–303. (9) Mahajan, V. K.; Misra1, M.; Raja, K. S.; Mohapatra, S. K. J. Phys. D 2008, 41, 125307–125315. (10) Allam, N. K.; Shankar, K.; Grimes, C. A. J. Mater. Chem. 2008, 18, 2341–2348. (11) Yates, H. M.; Nolan, M. G.; Sheel, D. W.; Pemble, M. E. J. Photochem. Photobiol. A 2006, 179, 213–223. (12) Deki, S.; Nakata, A.; Sakakibara, Y.; Mizuhata, M. J. Phys. Chem. C 2008, 112, 13535–13539. (13) Nakajima, A.; Nakamura, A.; Arimitsu, N.; Kameshima, Y.; Okada, K. Thin Solid Films 2008, 516, 6392–6397. (14) Eufinger, K.; Poelman, D.; Poelman, H.; Gryse, R. D.; Marin, G. B. Appl. Surf. Sci. 2007, 254, 148–152. (15) Rani, S.; Roy, S. C.; Paulose, M.; Varghese, O. K.; Mor, G. K.; Kim, S.; Yoriya, S.; LaTempa, T. J.; Grimes, C. A. Phys. Chem. Chem. Phys. 2010, 12, 2780–2800. (16) Mor, G. K.; Shankar, K.; Paulose, M.; Varghese, O. K.; Grimes, C. A. Nano Lett. 2005, 5, 191–195. (17) Yu, J. G.; Dai, G. P.; Cheng, B. J. Phys. Chem. C 2010, 114, 19378–19385. 12848

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(18) Qu, J.; Gao, X. P.; Li, G. R.; Jiang, Q. W.; Yan, T. Y. J. Phys. Chem. C 2009, 113, 3359–3363. (19) Chu, S. Z.; Wada, K.; Inoue, S.; Todoroki, S. Surf. Coat. Technol. 2003, 169, 190–194. (20) Gong, D.; Grimes, C. A.; Varghese, O. K.; Hu, W. C.; Singh, R. S; Chen, Z.; Dickey, E. C. J. Mater. Res. 2001, 16, 3331–3334. (21) Prakasam, H. E.; Shankar, K.; Paulose, M.; Varghese, O. K.; Grimes, C. A. J. Phys. Chem. C 2007, 111, 7235–7241. (22) Macak, J. M.; Hildebrand, H.; Marten-Jahns, U.; Schmuki, P. J. Electroanal. Chem. 2008, 621, 254–266. (23) Mura, F.; Masci, A.; Pasquali, M.; Pozio, A. Electrochim. Acta 2010, 55, 2246–2251. (24) Hsiang, H. I.; Lin, S. C. Mater. Sci. Eng., A 2004, 380, 67–72. (25) Hsiang, H. I.; Lin, S. C. Ceram. Int. 2008, 34, 557–561. (26) Qamar, M.; Yoon, C. R.; Oh, H. J.; Lee, N. H.; Park, K.; Kim, D. H.; Lee, K. S.; Lee, W. J.; Kim, S. J. Catal. Today 2008, 131, 3–14. (27) Kim, E. Y.; Park, J. H.; Han, G. Y. J. Power Sources 2008, 184, 284–287. (28) Liang, H. C.; Li, X. Z. J. Hazard. Mater. 2009, 162, 1415–1422. (29) Zhu, K.; Neale, N. R.; Halverson, A. F.; Kim, J. Y.; Frank, A. J. J. Phys. Chem. C 2010, 114, 13433–13441. (30) Allam, N. K.; El-Sayed, M. A. J. Phys. Chem. C 2010, 114, 12024–12029. (31) Kim, C.; Kwon, I.; Moon, B. K.; Jeong, J. H.; Choi, B.; Kim, J. H.; Choi, H.; Yi, S. S.; Yoo, D.; Hong, K.; Park, J.; Lee, H. S. Mater. Sci. Eng. 2007, 27, 1343–1346. (32) Uvarov, V.; Popov, I. Mater. Charact. 2007, 58, 883–891. (33) Liao, Y. L.; Que, W. X. J. Alloys Compd. 2010, 505, 243–248. (34) Varghese, O. K.; Gong, D. W.; Paulose, M.; Grimes, C. A.; Dickey, E. C. J. Mater. Res. 2003, 18, 156–165. (35) Yu, J. G.; Wang, B. Appl. Catal., B: Environ. 2010, 94, 295–302. (36) Schulte, K. L.; DeSario, P. A.; Gray, K. A. Appl. Catal., B: Environ. 2010, 97, 354–360. (37) Tsuchiya, H.; Macak, J. M.; Ghicov, A.; Tang, Y. C.; Fujimoto, S.; Niinomi, M.; Noda, T.; Schmuki, P. Electrochim. Acta 2006, 52, 94–101. (38) Yu, J. G.; Zhang, L. J.; Cheng, B.; Su, Y. R. J. Phys. Chem. C 2007, 111, 10582–10589. (39) Paulose, M.; Shankar, K.; Yoriya, S.; Prakasam, H. E.; Varghese, O. K.; Mor, G. K.; Latempa, T. A.; Fitzgerald, A.; Grimes, C. A. J. Phys. Chem. B 2006, 110, 16179–16184. (40) Mohapatra, S. K.; Raja, K. S.; Mahajan, V. K.; Misra, M. J. Phys. Chem. C 2008, 112, 11007–11012.

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