TiO2 Heterojunction under

Jul 21, 2009 - Yongfei Cui , Joe Briscoe , Yaqiong Wang , Nadezda V. Tarakina , and ... CaIn2O4/Fe-TiO2 Composite Photocatalysts with Enhanced Visible...
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Efficient Photocatalytic Activity of PZT/TiO2 Heterojunction under Visible Light Irradiation Hanjie Huang, Danzhen Li,* Qiang Lin, Yu Shao, Wei Chen, Yin Hu, Yibin Chen, and Xianzhi Fu Research Institute of Photocatalysis, State Key Laboratory Breeding Base of Photocatalysis, Fuzhou UniVersity, Fuzhou, 350002, P. R. China ReceiVed: March 16, 2009; ReVised Manuscript ReceiVed: June 9, 2009

A Pb(Zr0.52Ti0.48)O3/TiO2 composite photocatalyst with nanostructured heterojunction was prepared by a simple sol-gel method. The catalyst was characterized by powder X-ray diffraction, nitrogen adsorption-desorption, transmission electron microscopy, UV-vis diffuse reflectance spectroscopy, electrochemistry technology, and spin-trapping electron paramagnetic resonance. The visible light-induced photocatalytic activities were evaluated by decomposing ethylene in gas phase. The result showed that the prepared composite catalyst exhibited efficient photocatalytic activities with high photochemical stability under visible light irradiation. Moreover, it also showed excellent photocatalytic performance under UV light or simulated sunlight irradiation. On the basis of the measurement of flatband potentials of the samples and the detection of active oxygen species (such as O2•- and OH•), a visible light-induced photocatalytic degradation mechanism of ethylene on PZT/TiO2 was proposed. 1. Introduction Semiconductor photocatalytic oxidation technology has been established to be one of the most promising methods for environment remediation.1 Among various semiconductors, TiO2 has been considered as an excellent photocatalyst for degradation of organic pollutants in air and water for its nontoxicity, cheapness, and chemical stability. However, due to its wide band gap (3.2 eV for anatase), TiO2 is known to be active only under UV light, which largely inhibits its overall efficiency under natural sunlight.2 Meanwhile, the relatively high rate of electron-hole recombination often results in a low quantum yield and poor efficiency of photocatalytic reactions.3 Currently, there are many reports on metal-ion and anion-doped TiO2 catalysts, which can broaden the spectral response of TiO2 in photocatalysis schemes and show activities under visible light.4-8 However, few of them gave prospective satisfactory results because of the increase of carrier-recombination centers, thermal instability,4 or the requirement of expensive ion-implantation equipments.9 For example, nitrogen-doped TiO2 (TiO2-xNx), a typical anion-doped TiO2 visible photocatalyst and has been investigated in many researches. It was reported that the quantum yield was lower than TiO2 alone under UV light irradiation because the doping sites also served as recombination sites.7 Because of photocorrosion or rapid recombination of photogenerated electron-hole pair, traditional visible light photocatalysts with narrow band gap are either unstable (CdS, CdSe, etc.) or have low activity (Fe2O3, WO3, etc.).10 Hence, it is of vital interest to develop a photocatalyst which can work effectively under visible light irradiation with photochemical stability in photocatalysis research field. One of the potential solutions to achieve this objective is to construct a heterojunction structure between a narrow band gap semiconductor and TiO2 with matching band potentials.11,12 In this way, the electrons or holes photogenerated from the narrow band gap semiconductor can be transferred to TiO2 via hetero* To whom correspondence should be addressed. Phone and Fax: (+86)591-83779256. E-mail: [email protected].

junction interface, leading to efficient charge separation by minimizing the electron-hole recombination. One common method for preparing these visible light photocatalysts is the simple physical mixing of them. Obviously, the efficiency of interparticle electron transfer is poor due to the momentary random collision of heterophase particles. Hence, it would be desirable to prepare heterojunction composite with intimate contact between heterophase nanoparticles to make interparticle electron transfer more efficient13,14 and then to achieve a higher photocatalytic performance. Recently, perovskite-type multimetallic oxides have been considered as candidates for stable photocatalyst materials.15-18 Lead zirconate titanate (PZT) is a ferroelectric material with perovskite structure and has been reported as visible photocatalyt for decomposition of Rhodamine B in liquid phase.19,20 Herein, a PZT/TiO2 composite with nanostructure heterojunction was prepared by a simple method and the visible light-induced photocatalytic behaviors of the sample were investigated in detail. The photocatalytic activities were evaluated by decomposing ethylene in the gas phase. Results showed that the prepared PZT/TiO2 heterojunction photocatalyst exhibited efficient activity on the photocatalytic oxidation of ethylene with high photochemical stability under visible light irradiation. Moreover, it also showed excellent photocatalytic performance under UV light or simulated sunlight irradiation. The relative band positions of PZT and TiO2 were measured by an electrochemical method, and the generated intermediate active oxygen species were investigated by electron paramagnetic resonance (EPR) spin-trapping technique. The mechanism related to the photocatalytic process under visible light irradiation was proposed and discussed. 2. Experiment Section Preparation of Photocatalysts. All chemicals were analytical grade without further purification. The PZT/TiO2 heterojunction photocatalyst was prepared using a simple sol-gel method. Mixture was formed by adding PZT nanocrystal powder to TiO2 sol, and then gelatinized by heating treatment. In a typical

10.1021/jp902330w CCC: $40.75  2009 American Chemical Society Published on Web 07/21/2009

Efficient Photocatalytic Activity of PZT/TiO2 synthesis procedure, titanium isopropoxide (Ti(OC3H7)4, 0.0144 mol) in 25 mL of isopropanol was mixed with zirconium acetylacetonate (Zr(CH3COCHCOCH3)4, 0.0156 mol) dissolved in 8 mL of acetic acid under constant stirring. Lead acetate trihydrate (Pb(CH3COO)2 · 3H2O, 0.0309 mol) dissolved in 10 mL of acetic acid was then added to the above precursor solution under stirring to form PZT precursor solution. The precursor solution was dried at 120 °C to produce dried gels then ground and calcined at 600 °C for 3 h to obtain PZT (Pb(Zr0.52Ti0.48)O3) nanocrystal powder. A mixture of 1.3 mL of HNO3, 180 mL of H2O, and 15 mL of Ti(OC3H7)4 was peptized, dialyzed, and concentrated at room temperature to form a highly dispersed TiO2 colloidal solution. Finally, the PZT powder was impregnated by self-made TiO2 colloid (the initial ratio of PZT to TiO2 was fixed at 5 mol %) with ultrasonic dispersed for 1 h and sustained stirring for 24 h and then was heated and stirred to remove solvents. The resultant gel was calcined at 400 °C for 3 h to obtain PZT/TiO2 heterojunction photocatalyst. As a comparison, self-made pure TiO2 (T400), TiO2 (P25, Degussa Co.), N-doped TiO2 (TiO2-xNx) and the physical mixture of TiO2 and PZT (PZT/TiO2(PM)) were also prepared. Self-made TiO2 colloid was treated with the same heating process of PZT/TiO2 to synthesize T400. TiO2-xNx was synthesized by a traditional method5,7 through calcination of P25 at 550 °C under dry NH3 flow for 3 h. The PZT/TiO2(PM) was prepared by mechanically mixing of PZT and T400 (or P25) powders in the agate mortar without further treatment. Characterization. X-ray diffraction (XRD) analysis was performed on a Bruker D8 Advance X-ray diffractometer with Cu KR radiation. Transmission electron microscopy (TEM) images were collected by using a JEOL JEM 2010F microscope working at 200 kV. A Varian Cary 500 UV-vis spectrophotometer was used to record the UV-vis diffuse reflectance spectra (DRS) of various samples. Nitrogen adsorption-desorption isothermals for the specific surface area and pore size distribution of the samples were collected at 77 K using Micromeritics ASAP2010 equipment. The flat-band potentials (Vfb) of TiO2 and PZT were determined from Mott-Schottky plots by an electrochemical method, which was carried out in conventional three electrode cells using a PAR VMP3 Multi Potentiotat apparatus. (The experimental details of Vfb measurement by the electrochemical method are present in the Supporting Information.) EPR signals of the radicals spin-trapped by 5,5-dimethyll-pyrroline-N-oxide (DMPO) were recorded with a Bruker EMX A300 spectrometer. The irradiation source was a 500 W Xearc lamp equipped with an IR-cutoff filter and an UV-cutoff filter, which was the same with that used in visible photocatalytic reaction. Photocatalytic Activity Measurement. The photocatalytic activity was evaluated by the photocatalytic oxidation of ethylene (C2H4) into carbon dioxide (CO2) in the gas phase, which was carried out in a 5 × 2 × 0.1 cm3 fixed-bed quartz reactor and operated in a single pass mode. All catalysts were sieved to obtain particles of the 0.21-0.25 mm size. A 500 W Xe-arc lamp equipped with an IR-cutoff filter and an UV-cutoff filter (450 nm < λ < 900 nm) was used as visible light source, while with a special cutoff filter (280 nm < λ < 400 nm) was used as ultraviolet light source and with the IR-cutoff filter (320 nm < λ < 900 nm) was used as simulated sunlight (the comparative light intensity distribution of Xe-arc lamp and sunlight was shown in Figure S1, and the pass-band of these filters was shown in Figure S2). As a reactant stream, ethylene was diluted with a zero air (21% oxygen and 79% nitrogen) stream and fed to 1.2 g of catalyst at a total flow-rate of 15 cm3

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Figure 1. XRD patterns of T400 (A), PZT/TiO2 (B), and PZT (C).

Figure 2. N2-sorption isotherm (A) and the pore size distribution plot (B) of PZT/TiO2.

min-1. The respective initial concentrations of ethylene and carbon dioxide were 203 and 0 ppm. The temperature of the reactions was controlled at 30 ( 1 °C by an air-cooling system. Analysis of the reactor effluent was conducted by a gas chromatograph (HP6890). The concentrations of ethylene and carbon dioxide were determined by using the flame ionization detector (FID) and thermal conductivity detector (TCD) detectors, respectively. The reactor effluent was also analyzed online with QGS-08B infrared analyzers to detect CO yield. The adsorption/desorption equilibrium of ethylene on sample was obtained after 3 h in the dark before an activity measurement. 3. Results and Discussions Figure 1 gives the XRD patterns of the samples. The results demonstrated that the prepared PZT was a single perovskitetype phase, and both anatase (the main peak at 2θ ) 25.4°) and rutile phase (the main peak at 2θ ) 27.4°) of TiO2 were presented on T400 sample. We could observe the distinct peaks corresponding to PZT and TiO2 in PZT/TiO2 composite. Moreover, the rutile phase of TiO2 can hardly be detected in PZT/TiO2 composite, while it can be obviously seen in T400. The result indicated that the addition of PZT remarkably impeded the formation of rutile phase of TiO2. It maybe caused by the interactions between PZT and TiO2 nanocrystals in PZT/ TiO2 composite, which resulted from PZT being embedded in TiO2 during the preparation procedure. Figure 2 shows the N2-sorption and the pore size distribution plot of PZT/TiO2 sample. The stepwise adsorption and desorption isotherm exhibited a type IV isotherms and a narrow pore size distribution with average pore diameter of 4 nm was also observed. These results indicated the PZT/TiO2 sample was a mesoporous solid. Furthermore, the Brunauer-Emmett-Teller (BET) specific surface area of the PZT/TiO2 sample was found

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Figure 3. TEM images (A, B) and HRTEM image (C) of the synthesized PZT/TiO2 sample.

Figure 5. Mott-Schottky plots of PZT, T400, and P25 electrodes in 0.3 M LiClO4 at pH 3. Figure 4. UV-vis DRS spectra of T400, PZT/TiO2 and PZT (A) and the band gap of PZT estimated from the plot of transformed Kubelka-Munk function versus photoenergy (B).

to be 122.21 m2 g-1, which is much larger than T400 (81.01 m2 g-1) and pure PZT (0.78 m2 g-1). The porous nature of PZT/TiO2 was also characterized by TEM, as shown in Figure 3A and B. The porous structure observed in the TEM images indicates that the porosity of the PZT/TiO2 is originated from interparticle porosity. In the experiment, when the sample was calcined at 400 °C, the organic part from the precursor used to prepare TiO2 in PZT/TiO2 composite was removed, leaving interparticle pores in the sample,3 and therefore, the PZT/TiO2 composite showed mesoporousity. In order to investigate the nanocrystalline nature of PZT/TiO2, the sample was characterized by high-resolution transmission electron microscopy (HRTEM). As shown in Figure 3C, many different lattice fringes can be found which allowed for identification of the crystallographic spacing of TiO2 and PZT. The fringes of d ) 0.35 and 0.29 nm match the (101) crystallographic planes of anatase TiO2 and PZT nanoparticles, respectively. Furthermore, interconnected heterophase nanoparticulate morphology was observed, indicating that the nanostructured heterojunction was really formed in PZT/TiO2 composite. The intimate contact between PZT nanocrystal and TiO2 precursor and the calcination of the composite xerogel at high temperature (400 °C) may favor the formation of PZT/ TiO2 nanostructured heterojunction.13,14 The UV-vis DRS of TiO2, PZT/TiO2, and PZT samples and the estimated band gap of PZT are presented in Figure 4. We can find that the visible light absorption (λ > 400 nm) of PZT/ TiO2 composite is clearly stronger than that of TiO2. Obviously, it was due to the contribution of PZT, and the band gap of which was about 2.4 eV estimated from the plot of transformed Kubelka-Munk function versus the energy of light.21 In order to examine the direction of charge transfer in PZT/ TiO2 composite, the relative flat-band potentials (Vfb) of PZT

and TiO2 were measured using an electrochemical method.22 As shown in Figure 5, we can deduce from Mott-Schottky plots that the Vfb of T400 and P25 were almost the same at -0.40 V vs SCE at pH 3 (equivalent to -0.40 V vs NHE at pH 7), which is compatible with previous reports of TiO2.23,24 The Mott-Schottky plots also demonstrate that both PZT and TiO2 were n-type semiconductor25 and the Vfb of PZT was about -0.55 V, which was more negative than that of TiO2 by about 0.15 V under the same condition. It is generally known that the conduction band potentials (ECB) of n-type semiconductor was very close to (about 0.1 V more negative) the flat-band potentials,25 so it can be deduced that the conduction band (CB) position of PZT was more negative than that of TiO2. Obviously, the difference of ECB between PZT and TiO2 allowed the transfer of electron from the CB of PZT to that of TiO2. Figure 6 displays the photocatalytic activities of the prepared samples for degradation of ethylene in air under visible light irradiation. The result demonstrates that PZT/TiO2 composite catalyst exhibits high visible light photocatalytic performance compared with other samples. The photocatalytic activities of T400, P25, and PZT were very low under visible light irradiation. By contrast, the conversion of ethylene on PZT/ TiO2 was about 15% and the produced CO2 was about 37 ppm under visible light irradiation when the reaction was steady. Beside CO2, a few CO (about 2 ppm) were also detected. The experiment was also carried out for five runs to examine the stability of the catalyst under visible light irradiation as shown in Figure 7. The result demonstrates the high photochemical stability of PZT/TiO2 for the ethylene oxidation after 50 h of testing. In addition, there is also no photocatalytic activity of physical mixture of pure TiO2 (T400 or P25) and PZT powders under the same condition, even if the mixture was calcinated at 400 °C. It is therefore believed that the intimate contact and the strong interactions at the heterojunction interface between PZT and TiO2 nanocrystal contribute to the efficient activity and high photochemical stability of PZT/TiO2 in the process of

Efficient Photocatalytic Activity of PZT/TiO2

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Figure 6. Conversion of ethylene (A) and the amount of produced CO2 (B) on PZT/TiO2, T400, P25, and PZT under visible light irradiation and on PZT/TiO2 in the dark.

Figure 7. Photodegradation of ethylene on PZT/TiO2 photocatalyst during repetition operation upon visible light irradiation.

Figure 8. Comparative catalytic activity of photodegradation of ethylene on TiO2-xNx and PZT/TiO2 under visible light irradiation.

degradation of ethylene. To further understand the visible photocatalytic performance of the PZT/TiO2 composite, the nitrogen-doped TiO2 (TiO2-xNx) photocatalyst was chosen for comparison. As shown in Figure 8, the conversion of ethylene and the amount of produced CO2 on PZT/TiO2 were also higher than those on TiO2-xNx under visible light irradiation. Moreover, the photocatalytic activities of PZT/TiO2 catalyst under UV light or simulated sunlight irradiation were also

excellent, as shown in Figure 9. It exhibited obviously better photocatalytic performance than T400, P25, and TiO2-xNx under UV light or simulated sunlight irradiation, except slightly inferior to commercial P25 under UV light irradiation. Like under visible light irradiation (see Figure 6), there was an induction period (several hours) to obtain steady C2H4 conversion and CO2 yield for all these photocatalytic reactions, and this phenomenon may be related to the sorption equilibrium of reactor effluent on photocatalyst when part of ethylene was converted to CO2. Furthermore, there was also no photocatalytic activity of PZT under UV light or simulated light irradiation. The negligible activity of PZT may be caused by its weak adsorption of reactants molecules, which resulted from its small surface area. The better photocatalytic performance of PZT/TiO2 composite than TiO2 (T400) and PZT alone under UV light or simulated sunlight irradiation indicates that the electron transfer from PZT to TiO2 also occurred efficiently in such condition. It is known that the photocatalytic reactions proceed mainly by the contributions of active oxygen species, such as superoxide radical anion O2•-, hydroxyl radical OH•, and hydrogen peroxide H2O2.1,26 Therefore, in order to probe the nature of the reactive oxygen species generated during the visible light irradiation of PZT/TiO2 system, the EPR spectra of DMPO trapped O2•- in methanol medium and DMPO trapped OH• in aqueous medium were investigated according to the previous reports.27 As shown in Figure 10, six characteristic peaks of the DMPO-O2•adducts and four characteristic peaks of DMPO-OH• adducts were observed on PZT/TiO2 system under visible light irradiation, respectively. By contrast, there are no signals detected in the dark or blank test. These results elucidated that O2•- and OH• were really generated on PZT/TiO2 composite under visible light irradiation. On the basis of literature reports1,27-30 and experimental results, especially the measurement of flatband of the samples and the detection of some intermediate active oxygen species,

Figure 9. Photodegradation of ethylene on T400, P25, TiO2-xNx and PZT/TiO2 under UV light (A) and simulated sunlight (B) irradiation.

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Figure 10. EPR signals of DMPO trapped O2•- in methanol dispersions (A) and OH• in aqueous dispersions (B): (a) system with PZT/TiO2 under visible light irradiation, (b) system with PZT/TiO2 in dark, and (c) system without PZT/TiO2 under visible light irradiation.

SCHEME 1: Proposed Mechanism for the Visible Light Photodegradation of Ethylene on PZT/TiO2 Composite

4. Conclusion In summary, a PZT/TiO2 composite with nanostructured heterojunction was prepared by a simple coupled method, and the photocatalytic behavior of the sample was investigated in detail. The prepared PZT/TiO2 photocatalyst with large special surface area showed enhanced visible light absorption and exhibited efficient photocatalytic activity for decomposition of ethylene under visible light irradiation with high photochemical stability. Moreover, it also showed excellent photocatalytic performance under UV light or simulated sunlight irradiation. A visible light-induced photocatalytic degradation mechanism of ethylene on PZT/TiO2 is proposed and discussed. Acknowledgment. This work was financially supported by the National Natural Science Foundation of China (20537010, 20677010, and 20873023), An “863” Project From the MOST of China (2006AA03Z340), National Basic Research Program of China (973 Program, 2007CB613306), and the Science Foundation of Fujian, China (2007F5066, JA07001).

a possible photocatalysis process for the degradation of ethylene under visible light irradiation could be concluded as follows (Scheme 1). (1) Due to the high visible absorbance of PZT, electrons and holes generated in PZT were separated when visible light supplied to the PZT/TiO2 heterojunction. (2) Some electrons were injected into TiO2 nanoparticles via heterojunction interface since the conduction band of PZT is more negative than that of TiO2. Furthermore, the formed nanostructured heterojunction on PZT/TiO2 composite also led to a more efficient interelectron transfer between the two components.13,14 (3) The transferred electrons were then scavenged by O2 to yield O2•- and H2O2, and then the OH• could be formed when O2•reacted with H2O2. Since OH• is a powerful oxidizing agent capable of degrading most organic molecule, the ethylene adsorbed on PZT/TiO2 substrate was degraded into CO2 and H2O. (4) The photogenerated hole in PZT may also activate some unsaturated organic molecule (e.g., ethylene), leading to a subsequent decomposition.12 Furthermore, the large specific surface area of PZT/TiO2 composite was also favorable for photocatalytic reaction. In a word, the improvement of visible light adsorption, the vectorial transfer of photogenerated electrons, the formed heterojunction interface, the produced OH•, and large specific surface area were supposed to be responsible for the high efficient photocatalytic activity of the PZT/TiO2 composite. Further research of the explicit mechanism for the photocatalytic behavior of the PZT/TiO2 heterojunction is now under investigation in our laboratory.

Supporting Information Available: Experimental details. This material is available free of charge via the Internet at http:// pubs.acs.org. References and Notes (1) Hoffmann, M. R.; Martin, S. T.; Choi, W.; Bahnemann, D. W. Chem. ReV. 1995, 95, 69. (2) Karvinen, S.; Lamminma¨ ki, R.-J. Solid State Sci. 2003, 5, 1159. (3) Carp, O.; Huisman, C. L.; Reller, A. Prog. Solid State Chem. 2004, 32, 33. (4) Choi, W. Y.; Termin, A.; Hoffmann, M. R. J. Phys. Chem. 1994, 98, 13669. (5) Asahi, R.; Morikawa, T.; Ohwaki, T.; Aoki, K.; Taga, Y. Science 2001, 293, 269. (6) Ohno, T.; Akiyoshi, M.; Umebayashi, T.; Asai, K.; Mitsui, T.; Matsumura, M. Appl. Catal. A 2004, 265, 115. (7) Irie, H.; Watanabe, Y.; Hashimoto, K. J. Phys. Chem. B 2003, 107, 5483. (8) Ohno, T.; Mitsui, T.; Matsumura, M. Chem. Lett. 2003, 32 (4), 364. (9) Anpo, M. Catal. SurV. Jpn. 1997, 1, 169. (10) Kim, H. G.; Hwang, D. W.; Lee, J. S. J. Am. Chem. Soc. 2004, 126, 8912. (11) Yu, J. C.; Wu, L.; Lin, J.; Li, P.; Li, Q. Chem. Commun. 2003, 1552. (12) Xiao, G.; Wang, X.; Li, D.; Fu, X. J. Photochem. Photobiol. A 2008, 193, 213. (13) Zhang, J.; Xu, Q.; Feng, Z. C.; Li, M. J.; Li, C. Angew. Chem., Int. Ed. 2008, 47, 1766. (14) Zong, X.; Yan, H. J.; Wu, G. P.; Ma, G. J.; Wen, F. Y.; Wang, L.; Li, C. J. Am. Chem. Soc. 2008, 130, 7176. (15) Kim, H. G.; Hwang, D. W.; Lee, J. S. J. Am. Chem. Soc. 2004, 126, 8912. (16) Yin, J.; Zou, Z.; Ye, J. J. Phys. Chem. B 2004, 108, 12790.

Efficient Photocatalytic Activity of PZT/TiO2 (17) Yin, J.; Zou, Z.; Ye, J. J. Phys. Chem. B 2003, 107, 4936. (18) Machida, M.; Yabunaka, J.; Kijima, T. Chem. Mater. 2000, 12, 812. (19) Liu, C.; Zou, B.; Rondinone, A. J.; Zhang, Z. J. J. Am. Chem. Soc. 2001, 123, 4344. (20) Wu, Q.; Li, D.; Wu, L.; Wang, J.; Fu, X.; Wang, X. J. Mater. Chem. 2006, 16, 1116. (21) Zhao, W.; Ma, W.; Chen, C.; Zhao, J.; Shuai, Z. J. Am. Chem. Soc. 2004, 126, 4782. (22) Ramesham, R. Thin Solid Films 1998, 322, 158. (23) Butler, M. A.; Ginley, D. S. J. Electrochem. Soc. 1978, 125, 228. (24) Xu, Y.; Schoonen, M. A. A. Am. Mineral. 2000, 85, 543.

J. Phys. Chem. C, Vol. 113, No. 32, 2009 14269 (25) Morrison, S. R. Electrochemistry at Semiconductor and Oxidized Metal Electrodes; Plenum Press: New York, 1980; Chapter 4. (26) Hirakawa, T.; Nosaka, Y. Langmuir 2002, 18, 3247. (27) Wu, T.; Lin, T.; Zhao, J.; Hidaka, H.; Serpone, N. EnViron. Sci. Technol. 1999, 33, 1379. (28) Kaneko, M.; Okura, I., Eds. Photocatalysis, Science and Technology; Springer: New York, 2002. (29) Fujishima, A.; Rao, T. N.; Tryk, D. A. J. Photochem. Photobiol. C 2000, 1, 1. (30) Sakthivel, S.; Kisch, H. Angew. Chem., Int. Ed. 2003, 42, 4908.

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