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Photoelectrocatalytic Activity of Highly Ordered TiO2 Nanotube Arrays Electrode for Azo Dye Degradation ZHONGHAI ZHANG, YUAN YUAN, GUOYUE SHI, YANJU FANG, LINHONG LIANG, HONGCHUN DING, AND LITONG JIN* Department of Chemistry, East China Normal University, Shanghai 200062, China
A highly ordered titanium dioxide nanotube arrays (HOTDNA) electrode was prepared in hydrofluoric acid solution by electrochemical anodization technique on a pure titanium sheet. The HOTDNA electrode was characterized by FE-SEM microscopy, X-ray diffraction, and UV-vis spectra. The linear-sweep photovoltammetry response on the HOTDNA electrode is presented in this work. The photogenerated current of 0.3 mA/cm2 was observed significantly upon illumination with applied potential of 0.5 V in the UV regions at the central wavelength of 253.7 nm. Photoelectrocatalytic (PEC) and photocatalytic (PC) activities of the HOTDNA electrode were evaluated in the degradation of methyl orange (MeO) in aqueous solution. A set of optimized conditions such as anodic potential, calcinations temperature, and MeO concentration on the PEC activity was investigated. The PEC and PC activities of HOTDNA electrode were compared. We concluded that the HOTDNA electrode was an effective photoelectrode for achieving an enhanced MeO degradation.
Introduction Over the past decades, heterogeneous photocatalysis has attracted increasing attention in the field of environmental protection for its photodegradation of undesirable organics in aqueous phase (1-3). A variety of semiconducting metal oxides have been used in this area (4-9). Among them, TiO2 is widely used as the photocatalyst during the photocatalytic process, because it is nonphoto corrosive, nontoxic, and capable of photooxidative destruction of most organic pollutants (10-13). While there are still some shortcomings, including relatively low specific surface area and weak photocatalytic efficiency of bulk TiO2 in the TiO2 film, the large surface area of the TiO2 film could lead to more efficient light harvesting and photogenerated charge. Electron transport is another crucial factor in the performance of these TiO2 films. Faster transferring can achieve higher efficiencies (14). The specific TiO2 materials with nanostructures, such as tubes, wires, dots, pillars, and fibers, have become a focus of considerable interest as they possess unique properties relevant to applications in photocatalysis (15-18). Among them, the material with tubular structure has been considered the most suitable way to achieve larger enhancement of surface area without an increase in the geometric area. Several fabrication methods including anodic oxidation, sol-gel, * Corresponding author tel: +86-21-62232627; fax: +86-2162232627; e-mail:
[email protected]. 10.1021/es070212x CCC: $37.00 Published on Web 07/26/2007
2007 American Chemical Society
electrodeposition, sonochemical deposition, hydrothermal synthesis, and template synthesis have been used to prepare TiO2 nanotubes (19-26). Among these methods, electrochemical anodization of titanium in fluorinated electrolytes is a relatively simple process for the fabrication of highly ordered TiO2 nanotube arrays (HOTDNA). The HOTDNA structure reduces the scattering of free electrons and enhances electron mobility, which offers the potential for improved electron transport leading to higher photocatalytic efficiency. TiO2 nanotubes fabrication with anodization was first reported by Zwilling et al. (27), and the highly ordered TiO2 nanotube array film was first applied by Grimes et al. to fabricate the solar cell (28). Quan et al. (29) and Xie et al. (30) also utilized the TiO2 nanotube electrode for the degradation of pentachlorophenol and the photoelectronchemical study in aqueous solution. However, to the best of our knowledge, there has been no report regarding application of the TiO2 nanotube electrode in the degradation of azo dye in a photoelectrocatalytic process. In this paper, we used HOTDNA electrodes to study the photocatalytic (PC) and photoelectrocatalytic (PEC) degradation of azo dye, such as methyl orange (MeO). MeO is selected as a model dyeing pollutant because it is one of the most important commercial dyes, has a very short excited-state life time, and is stable to visible and near UV light.
Experimental Section Materials. Pieces of titamium sheets (purity >99.7% and dimension 15 mm × 7mm × 0.5 mm) were purchased from Tite Inc., Shanghai. Methyl orange, hydrofluoric acid, nitric acid sodium sulfate, and ethanol of analytical grade were obtained from Shanghai Chemical Reagent Company (China) without further purification. All solutions were prepared with doubly distilled deionized water. Preparation of HOTDNA Electrode. The electrochemical anodic oxidation technique was used to fabricate the HOTDNA electrode. Prior to anodization, the titanium sheets were first mechanically polished with different abrasive papers and rinsed in an ultrasonic bath of cold distilled water for 10 min. Then the cleaned titanium sheets were soaked in a mixture of HF and HNO3 acids for 1 min (the mixing ratio of HF/HNO/H2O was 1:4:5 in volume). After the substrate sheets were rinsed with acetone and deionized water for 10 min, they were dried in air at room temperature. The TiO2 nanotube electrode was fabricated in a cylindrical electrochemical reactor (radius 30 mm and height about 70 mm). 0.5 wt % hydrofluoric acid was used as the electrolyte, and a platinum electrode served as the cathode during the entire process. A potential of 20 V was used in this study. Anodized titanium sheets were annealed in dry oxygen environment at 500 °C for 1 h; heating and cooling rates were kept at 2.5 °C min-1. Characterization of HOTDNA Electrode. X-ray diffraction (XRD) patterns of the HOTDNA electrode were performed using a diffractometer (model D/max 2550V, Rigaku Co., Japan) with radiation of a Cu target (KR, λ ) 0.15406 nm) to determine the crystal structure. The electrode morphology was observed by field-emission scanning electron microscopy (FE-SEM, Hitachi S-4800) under voltage of 15 KV. Light absorption of TiO2 nanotube arrays was measured using a UV-vis spectrophotometer (Varian Corp., model Cary 50) with a wavelength range of 200-600 nm. PEC Degradation Experiments. PEC degradation experiments were carried out in a single photoreactor, as shown in Figure 1. The HOTDNA electrode with an active area of VOL. 41, NO. 17, 2007 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 1. Photoelectrocatalytic reactor: (1) working electrode (HOTDNA electrode); (2) inlet of cooling water; (3) magnetic stirrer; (4) stirrer; (5) reference electrode (SCE); (6) counter electrode (Pt); (7) outlet of cooling water; (8) UV light; (9) electrochemical workstation; (10) computer.
FIGURE 3. XRD patterns of TiO2 nanotube arrays electrode (a) without calcinations and (b) calcinations at 500 °C.
FIGURE 4. UV-vis absorption spectra of (a) TiO2 nanotube prepared with anodization technique and (b) TiO2 film prepared with sol-gel method.
FIGURE 2. FE-SEM images of TiO2 nanotubes: (a) low-magnification top view, (b) high magnification of top view, and (c) cross-sectional view. 1.0 cm2 was placed in the photoreactor as the working electrode, and a saturated calomel electrode (SCE) and a platinum electrode served as the reference and counter electrode, respectively. All the potentials were referred to SCE unless otherwise stated in this paper. Photopotential and photocurrent were measured using a CH Instruments 1232 electrochemical workstation (CH Instruments Inc.). An 11 W UV lamp with central wavelength of 253.7 nm and maximal light intensity of 10 mW/cm2 was used as light source to measure the degradation efficiency of PEC. A 0.5 M K2SO4 solution (pH 6.2) was used as supporting electrolyte. MeO aqueous solution (5 × 10-5 M) was used in the PEC reaction and the pH value of the solution was not controlled during the reaction. Analytical Methods. The UV-vis absorbance of MeO solution showed two maxim: the first one was observed at 270 nm and the second one, more intensive, was observed at 470 nm. There existed an excellent linear relationship between the absorbance at 470 nm and the concentration of MeO solution.
Results and Discussion Characteristics of the HOTDNA electrode. Figure 2a, b, and c are the FE-SEM images of the HOTDNA electrode. It is observed that high-density, well-ordered, and uniform TiO2 nanotubes are fabricated on the pure titanium sheet with the electrochemical anodic oxidation technique. The tops of the tubes were open, similar to those of porous alumina obtained by anodic oxidation on aluminum (31). The diameters of these nanotubes range from 30 to 90 nm with wall thickness of about 10 nm and length of 200-300 nm. 6260
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The HOTDNA electrodes are analyzed by XRD, which is shown in Figure 3. Figure 3a is the XRD pattern of the TiO2 nanotube electrode prepared without calcinations. Obviously, an amorphous phase is dominant. After the TiO2 nanotube electrode was calcined at 500 °C, an anatase phase corresponding to 25.3 °C appeared in Figure 3b and no rutile phase and other phase appeared. The XRD patterns are similar to those of titania films prepared by sol-gel method (32). Figure 4 shows the UV-vis absorption spectra of the HOTDNA electrode prepared with anodization technique and the TiO2 film prepared with sol-gel method, which indicates that the maximum absorption wavelengths are around 260 and 290 nm. The UV-vis absorption band edge is a strong function of the crystallite size of nanosize TiO2 catalyst. Usually the band gap between the valance band and the conduction band of semiconductor increases with the decrease of the particle size. In Figure 4, the band gap absorption edges of TiO2 nanotube and TiO2 film are around 372 and 385 nm. The band gap edge of TiO2 is well-known to be λ ) 385 nm and Eg ) 3.23 eV for anatase phase and λ ) 415 nm and Eg ) 3.0 eV for rutile phase (33, 34), respectively. The band gap energy of TiO2 nanotubes is measured to be 3.34 eV corresponding to 372 nm (35). The 0.11 eV blue shift from the TiO2 film band gap energy (3.2 eV) indicated that the small TiO2 particles are formed. Photoelectrochemical Properties of MeO on HOTDNA Electrode. To study the photoelectrochemical response of HOTDNA electrode, linear sweep photovoltammetry was used in Na2SO4 supporting electrolyte with or without MeO solution under 0.1 Hz chopped UV irradiation. The photovoltammetry can be used to evaluate both of electrochemical behavior with light off and the photoelectrochemical behavior with light on under the same experimental conditions (36). The results of photoelectrochemical response of HOTDNA electrode are shown in Figure 5. It can be seen that the rise and fall of the photocurrent corresponded well to the
FIGURE 6. Degradation of MeO on TiO2 electrodes with different anodic potential. Calcination temperature 500 °C, concentration of MeO 5 × 10-5 M, applied potential 0.5 V, illumination tim: 90 min. FIGURE 5. Linear-sweep photovoltammograms with 0.1 Hz chopped irradiation of HOTDNA electrode in (a) 0.5 M Na2SO4 supporting electrolyte and (b) 0.5 M Na2SO4 + 5 × 10-5 M MeO. Photovoltammograms were obtained with a slow scan rate of 2 mV/s. illumination being switched on and off. The generation of photocurrent consisted of two steps. The first step of the photocurrent appears promptly after the illumination, and the second step of the photocurrent reaches a steady state. This pattern of photocurrent is highly reproducible for numerous on-off cycles of illumination. The current response on the HOTDNA electrode in dark is insignificant even at a potential of up to 1.4 V, which means that no electrochemical oxidation occurred. Under illumination, a significant increase in the photocurrent is observed throughout the potential window. Moreover, the photocurrent is potential dependent. It increases as the applied potential is scanned toward more positive potential, particularly at potentials >0.1 V and this trend gradually reaches a maximum photocurrent at a positive potential of 0.4 V. This indicates that the photogenerated electrons on the HOTDNA electrode could be effectively driven to the counter electrode by this positive potential, which would be beneficial to charge separation. A small cathodic current appeared at potentials more negative than -0.3 V under both dark and illumination condition, which might be associated with the electrochemical reduction of dissolved oxygen on the HOTDNA electrode (37). In Na2SO4 electrolyte, the photogenerated holes in HOTDNA electrode oxidize either adsorbed water molecules or hydroxyl groups, while the presence of MeO provides a much more facile pathway for the transfer of holes across the film/electrolyte interface, which results in the higher photocurrent density. The photocurrent onset shifts to negative potentials in the presence of MeO due to the ability of MeO to efficiently capture the photogenerated holes. The photocurrent spike is observed at low potential especially at negative potential, while it is not observed at high positive potential. This phenomenon can presumably be explained as the following: prior to illumination, the MeO molecules are in equilibrium with the HOTDNA electrode, and a higher concentration may be found at the electrode surface because of adsorption. Under illumination, MeO molecules are rapidly oxidized by photogenerated holes at the electrode surface, and the photogenerated electrons are collected by the electrode, giving an initial surge of the photocurrent. At low potential, the transfer rate of electrons is slow, which caused the photocurrent decay, so a photocurrent spike is observed. At high positive potential, the electrons can be transported quickly, and the photocurrent quickly reaches a steady state.
FIGURE 7. Degradation of MeO on TiO2 electrodes with different calcinations temperature. The anodic potential is 20 V, and the other conditions are the same as those for Figure 6. PEC Degradation of MeO on HOTDNA electrode. The PEC activity of the HOTDNA electrode was evaluated by studying the degradation of MeO in aqueous solution. Since the anodization voltage and the annealing temperature are two critical conditions that affect the morphology and the crystallization (38), which can cause the different PEC activity of TiO2 electrode, two sets of PEC degradation experiments were carried out in the MeO solution with an initial concentration of 5 × 10-5 M under UV illumination using different TiO2 electrodes fabricated at different anodization voltage and annealing temperatures. All tests lasted for 60 min and the experimental results are shown in Figures 6 and 7 . In Figure 6, with the increase the anodization voltage, the removal efficiencies of MeO on those electrodes increases and the highest PEC activity is achieved at 20 V. As references 20, 30, and 38 reported, an anodization voltage of less than 10 V yielded a nanoporous structure. Well-defined nanotube arrays were obtained at a potential of 10 V and above. Spongetype nanotubes were achieved with the potential up than 30 V. These results indicated that the highly orderly nanotube arrays were produced at 20 V leading to higher PEC activity. The experimental results in Figure 7 demonstrate that the calcinations temperature also affected the PEC activity of TiO2 electrode significantly. It can be seen that the PEC activity increased with the increase of calcinations temperature. The electrodes calcined at 400 and 500 °C achieved the highest PEC activity in the MeO degradation due to good crystallization. These results had good agreement with the XRD results. According to the XRD analysis shown in Figure 3, the TiO2 electrode appeared an amorphous phase without VOL. 41, NO. 17, 2007 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 1. Values of Rate Constant k and Regression Coefficients of MeO Photoelectrocatalytic with Different Processes
FIGURE 8. Degradation of MeO with different concentrations. Same experimental conditions as those in Figure 6.
processes
rate constant (min-1)
R2
PEC PC direct photolysis electrochemical oxidation
0.0515 0.0025 0.0005 0.00003
0.982 0.9961 0.9854 0.9916
stable with UV illumination. The result with electrochemical oxidation was also in good agreement with the data in Figure 5, that is, the electrochemical oxidation did not occur evidently in this process. The experimental data of Figure 9 were found to fit approximately a pseudo-first-order kinetic model by the linear transforms ln(C0/C) ) f(t) ) kt (k is rate constant). The values of the rate constant, k, and regression coefficient are listed in Table 1. The reaction rate of PEC process (0.0515 min-1) was about 20 times higher than that in PC process (0.0025). Durability of HOTDNA Electrode. The PEC activity and the durability or lifetime of the electrode are very important for the practical application of such a PEC degradation process. The HOTDNA electrodes were tested (in the PEC process) for the degradation of MeO (5 × 10-5 M) in aqueous solution for a series of 10 identical tests. The performance of the HOTDNA electrode showed a good reproducibility and retained the degradation rate of 99% after 10 times experiments for PEC degradation of MeO in 90 min. The result is within the limits of experimental error.
Acknowledgments The work is supported by the fund of Natural Science Foundation of P.R. China (20327001 and 20675032) and Shanghai Rising-Star program (06QH14004). FIGURE 9. PEC, PC, photolysis, and electrochemical oxidation processes of MeO degradation. calcinations, and appeared an anatase phase with calcinations above 400 °C, which caused the higher PEC activity. The influence of the initial concentration of MeO on the photoelectrocatalytic degradation was investigated. The characteristic dyes concentrations in wastewater were in the range of 3.0 × 10-5 to 1.5 × 10-4 M; the PEC degradation of MeO was studied by varying the initial concentrations from 1.0 × 10-5 to 2 × 10-4 M. The quantitative description of initial concentration versus time profiles is presented in Figure 8. It seemed that the degradation of MeO followed the model of Langmuir-Hinshelwood satisfactorily. On the basis of the range of dye concentration, the degradation efficiency increased with the decreasing concentration of MeO. At the dye concentration of 1 × 10-5 M, which is very low for the photoelectrocatalytic degradation on HOTDNA electrode, 94.19% of the dye was degraded in only 10 min. Whereas at the dye concentration of 2 × 10-4 M, the degradation efficiency was nearly 56.3% for total irradiation time of 90 min. The MeO removal in the various degradation processes, that is, PEC, PC, electrochemical oxidation, and direct photolysis processes, is summarized in Figure 9. Applied bias potential in PEC and electrochemical oxidation processes was 0.5 V. It was easily observed from Figure 9 that the photoelectrocatalytic process provided the most powerful way to degrade the MeO in aqueous solution. The complete removal of MeO (99.06%) was observed after 90 min, while only 21.52% of the MeO removal was obtained in PC process with the same illumination time. The removal with direct photolysis was insignificant, which proved that the MeO was 6262
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Supporting Information Available Photocurrent action spectra of HOTDNA electrode (Figure S1) and effect of the applied potential on degradation of MeO (Figure S2). This material is available free of charge via the Internet at http://pubs.acs.org.
Literature Cited (1) Hagfeld, A.; Gratzel, M. Light-induced redox reactions in nanocrystalline systems. Chem. Rev. 1995, 95, 49-68. (2) Fujishima, A.; Rao, T. N.; Tryk, D. A. Titanium dioxide photocatalysis. J. Photochem. Photobiol. C 2000, 1, 1-21. (3) Hoffmann, M. R.; Martin, S. T.; Choi, W. Y.; Bahnemann, D. W. Environmental applications of semiconductor photocatalysis. Chem. Rev. 1995, 95, 69-96. (4) Zen, J. M.; Chung, H. H.; Yang, H. H.; Chiu, M. H.; Sue, J. W. Photoelectrocatalytic oxidation of o-phenols on copper-plated screen-printed electrodes. Anal. Chem. 2003, 75, 7020-7025. (5) Hepel, M.; Luo, J. Photoelectrochemical mineralization of textile diazo dye pollutants using nanocrystalline WO3 electrodes. Electrochim. Acta. 2001, 47, 729-740. (6) Luo, J.; Hepel, M. Photoelectrochemical degradation of naphthol blue black diazo dye on WO3 film electrode. Electrochim. Acta. 2001, 46, 2913-2922. (7) Zhao, X.; Xu, T. G.; Yao, W. Q.; Zhang, C.; Zhu, Y. F. Photoelectrocatalytic degradation of 4-chlorophenol at Bi2WO6 nanoflake film electrode under visible light irradiation. Appl. Catal. B 2007, 72, 92-97. (8) Hepel, M.; Hazelton, S. Photoelectrocatalytic degradation of diazo dyes on nanostructured WO3 electrodes. Electrochim. Acta 2005, 50, 5278-5291. (9) Luo, J.; Yartym, J.; Hepel, M. Photoelectrochemical degradation of orange II textile dye on nanostructured WO3 film electrodes. J. New Mater. Electrochem. Syst. 2002, 5, 315-322. (10) Navio, J. A.; Cerrillos, C.; Pradera, M. A.; Morales, E.; GomezAriza, J. L. Photoassisted degradation (in the UV) of phenyltin
(11)
(12) (13) (14)
(15)
(16)
(17)
(18)
(19)
(20) (21) (22) (23) (24) (25)
(IV) chlorides in the presence of titanium dioxide. Langmuir 1998, 14, 388-395. Ranjit, K. T.; Willner, I.; Bossmann, S.; Braun, A. Iron(III) phthalocyanine-modified titanium dioxide: a novel photocatalyst for the enhanced photodegradation of organic pollutants. J. Phys. Chem. B 1998, 102, 9397-9403. Hepel, M.; Kumarihamy, I.; Zhong, C. J. Nanoporous TiO2supported bimetallic catalysts for methanol oxidation in acidic media. Electrochem. Commun. 2006, 8, 1439-1444. Poulios, I.; Kositzi, M.; Kouras, A. Photocatalytic decomposition of triclopyr over aqueous semiconductors suspensions. J. Photochem. Photobiol., A 1998, 115, 175-183. Peng, T. Y.; Hasegawa, A.; Qiu, J. R.; Hirao, K. Fabrication of titania tubules with high surface area and well-developed mesostructural walls by surfactant-mediated templating method. Chem. Mater. 2003, 15, 2011-2016. Yang, S. G.; Liu, Y. Z.; Sun, C. Preparation of anatase TiO2/Ti nanotube-like electrodes and their high photoelectrocatalytic activity for the degradation of PCP in aqueous solution. Appl. Catal., A 2006, 301, 284-291. Valverde, J. L.; de Lucas, A.; Dorado, F.; Romero, A.; Garcia, P. B. Influence of the operating parameters on the selective catalytic reduction of NO with hydrocarbons using Cu-ion-exchanged titanium-pillared interlayer clays (Ti-PILCs). Ind. Eng. Chem. Res. 2005, 44, 2955-2965. Zhan, S. H.; Chen, D. R.; Jiao, X. L.; Tao, C. H. Long TiO2 hollow fibers with mesoporous walls: sol-gel combined electrospun fabrication and photocatalytic properties. J. Phys. Chem. B 2006, 110, 11199-11204. Damin, A.; Llabres, i Xamena, F. X.; Lamberti, C.; Civalleri, B.; Zicovich-Wilson, C. M.; Zecchina, A. Sturctral, electronic, and vibrational properties of the Ti-O-Ti quantum wires in the titanosilicate ETS-10. J. Phys. Chem. B 2004, 108, 1328-1336. Varghese, O. K.; Gong, D. W.; Paulose, M.; Grimes, C. A.; Dickey, E. C. Crystallization and high-temperature structural stability of titanium oxide nanotube arrays. J. Mater. Res. 2003, 18, 156165. Mor. G. K.; Varghese, O. K.; Paulose, M.; Grimes, C. A. Transparent highly ordered TiO2 nanotube arrays via anodization of titanium thin films. Adv. Funct. Mater. 2005, 15, 1297-1296. Hoyer, P. Formation of a titanium dioxide nanotube array. Langmuir 1996, 12, 1411-1413. Adachi, M.; Murata, Y.; Harada, M.; Yoshikawa, S. Formation of titania nanotubes with high photo-catalytic activity. Chem. Lett. 2000, 8, 942-946. Zhu, Y.; Li, H.; Koltypin, Y.; Hacohen, Y. R.; Gedanken, A. Sonochemical synthesis of titania whiskers and nanotubes. Chem. Commun. 2001, 24, 2616-2617. Imai, H.; Takei, Y.; Shimizu, K.; Matsuda, M.; Hirashima, H. Direct preparation of anatase TiO2 nanotubes in porous alumina memberanes. J. Mater. Chem. 1999, 9, 2971-2973. Li, W. J.; Fu, T.; Xie, F.; Yu, S. F.; He, S. L. The multi-staged formation process of titanium oxide nanotubes and its thermal
stability. Mater. Lett. 2007, 61, 730-735. (26) Mohapatra, S. K.; Misra, M.; Mahajan, V. K.; Raja, K. S. A novel method for the synthesis of titania nanotubes using sonoelectrochemical method and its application for photoelectrochemical splitting of water. J. Catal. 2007, 246, 362-369. (27) Zwilling, V.; Darque-Ceretti, E.; Boutry-Forveille, A.; David, D.; Perrin, M. Y.; Aucouturier, M. Structure and physicochemistry of anodic oxide films on titanium and TA6V alloy. Surf. Interface Anal. 1999, 27, 629-637. (28) Mor, G. K.; Shankar, K.; Paulose, M.; Varghese, O. K.; Grimes, C. A. Use of highly-ordered TiO2 nanotube arrays in dyesensitized solar cells. Nano Lett. 2006, 6, 215-218. (29) Quan, X.; Yang, S. G.; Ruan, X. L.; Zhao, H. M. Preparation of titania nanotubes and their environmental applications as electrode. Environ. Sci. Technol. 2005, 39, 3770-3775. (30) Xie, Y. B.; Zhou, L. M.; Huang, H. T. Enhanced photoelectrochemical current response of titania nanotube array. Mater. Lett. 2006, 60, 3558-3560. (31) Cai, A. L.; Zhang, H. Y.; Hua, H.; Zhang, Z. B. Direct formation of self-assembled nanoporous aluminium oxide on SiO2 and Si substrates. Nanotechnology 2002, 13, 627-630. (32) Qiu, J. J.; Yu, W. D.; Gao, X. D.; Li, X. M. Sol-gel assisted ZnO nanorod array template to synthesize TiO2 nanotube arrays. Nanotechnology 2006, 17, 4695-4698. (33) Kormann, C.; Bahnemann, D. W.; Hoffmann, M. R. Preparation and characterization of quantum-size titanium dioxide. J. Phys. Chem. 1988, 92, 5196-5201. (34) Kandori, K.; Kon-no, K.; Kitahara, A. Formation of ionic water/ oil microemulsions and their application in the preparation of CaCO3 particles. J. Colloid Interface Sci. 1988, 122, 78-82. (35) Choi, H.; Stathatos, E.; Dionysiou, D. D. Sol-gel preparation of mesoporous photocatalytic TiO2 films and TiO2/Al2O3 composite membranes for environmental applications. Appl. Catal., B 2006, 63, 60-67. (36) de Tacconi, N. R.; Chenthamarakshan, C. R.; Yogeeswaran, G.; Watcharenwong, A.; de Zoysa, R. S.; Basit, N. A.; Rajeshwar, K. Nanoporous TiO2 and WO3 films by anodization of titanium and tungsten substrates: influence of process variables on morphology and photoelectrochemical response. J. Phys. Chem. B 2006, 110, 25347-25355. (37) Tansil, N. C.; Xie, H.; Gao, Z. Q. Photoelectrochemical behavior of oxalate at an indium tin oxide electrode. J. Phys. Chem. B 2004, 108, 16850-16854. (38) Xie, Y. B. Photoelectrochemical application of nanotubular titania photoanode. Electrochim. Acta 2006, 51, 3399-3406.
Received for review January 26, 2007. Revised manuscript received April 17, 2007. Accepted June 19, 2007. ES070212X
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