Preparations and Photocatalytic Properties of ... - ACS Publications

J. Phys. Chem. C 2009, 113, 2463–2467

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Preparations and Photocatalytic Properties of Visible-Light-Active Zinc Ferrite-Doped TiO2 Photocatalyst Shihong Xu,*,† Daolun Feng,‡ and Wenfeng Shangguan§ School of EnVironmental Science and Engineering, Donghua UniVersity, Shanghai 201620, People’s Republic of China, College of Marine EnVironment and Security Engineering, Shanghai Maritime UniVersity, Shanghai 200135, People’s Republic of China, and Research Center for Combustion and EnVironment Technology, Shanghai Jiao Tong UniVersity, Shanghai 200240, People’s Republic of China ReceiVed: December 26, 2007; ReVised Manuscript ReceiVed: NoVember 27, 2008

A visible-light-active TiO2/ZnFe2O4 photocatalyst was prepared by liquid catalytic phase transformation and the sol-gel method. The diffuse reflection spectra results show that TiO2/ZnFe2O4 photocatalyst can absorb visible light. The photocatalytic experimental result demonstrates that TiO2/ZnFe2O4 powder can effectively photodegrade methyl orange under visible light irradiation. The analysis of XRD indicated that the highly dispersed ZnFe2O4 nanoparticles prevented the formation of the rutile phase to some extent. A transmission electron microscope was used to characterize the structure of the photocatalyst, indicating that the ZnFe2O4 nanoparticles are highly dispersed among TiO2 nanoparticles. In TiO2/ZnFe2O4 photocatalyst, the relatively narrow bandgap ZnFe2O4 as the light-absorbing semiconductor and the wide bandgap titanium oxide formed a coupled semiconductor system resulting in an efficient primary charge separation and photocatalytic activity under visible light irradiation. The effect of calcination temperature and the amount of ZnFe2O4 on photocatalytic activity of TiO2/ZnFe2O4 was also investigated. 1. Introduction Semiconductor photocatalysts have attracted much attention in the past decade because of their potential application in the removal of all kinds of pollutants in air or water.1-3 Most of the investigations have focused on TiO2 (titania), which shows relatively high reactivity and chemical stability under ultraviolet (UV) light. However, titania is a wide bandgap semiconductor (3.03 eV for rutile and 3.18 eV for anatase form) and can only absorb about 5% of sunlight in the ultraviolet region, which greatly limits its practical applications. Many studies have been made in the development of titania photocatalysts that can efficiently utilize solar or indoor light. The approaches include the incorporation of transition metals into TiO2 by ion implantation or chemical doping;4-9 the introduction of oxygen deficiencies by treating TiO2 with H2 plasma or X-ray irradiation;10,11 the doping of nonmetal ions (e.g., N, C, F, S) into titania crystal lattice12-26 and the coupling of titania and vis-active semiconductors such as CdS, WO3, and SnO2 by sol-gel process, coprecipitation, or simply physical mixing.27-29 Spinel zinc ferrite, ZnFe2O4, is a narrow bandgap semiconductor that has a potential application in the conversion of sunlight, because of its sensitivity to visible light and no photochemical corrosion.30,31 However, zinc ferrite cannot be used directly in the photocatalytic degradation of organic pollutants due to the lower valence band potential and poor property in photoelectric conversion.32 Titania has high photoactivity under UV-light irradiation, while zinc ferrite is sensitive to visible light. So the coupling of these two semiconductors may become a new type of composite having high utility of sunlight and high photoactivity. Some publications in the past * Corresponding author. Phone: +86 21 67792541. Fax: +86 21 67792522. E-mail: [email protected]. † Donghua University. ‡ Shanghai Maritime University. § Shanghai Jiao Tong University.

year have used sol-gel techniques33 and coprecipitation/ hydrolysis methods34 to prepare visible-light-active TiO2/ ZnFe2O4 photocatalysts. Because the ZnFe2O4 crystal does not form before calcination, the substitutional presence of Fe3+ ions may play a role of carrier-recombination centers after calcination.35 Srinivasan et al.36 prepared TiO2/ZnFe2O4 nanocomposites by a mechanochemical synthesis approach using high-energy milling. However, the synthetic processes need to expend lots of energy. To obtain visible-light-active TiO2/ZnFe2O4 photocatalysts which can be prepared easily and improve the physicochemical properties, the development of preparation methods is indispensable. In this paper, ZnFe2O4 nanoparticles were prepared by the liquid catalytic phase transformation method at a low temperature, and visible-light-active TiO2/ZnFe2O4 were synthesized by the sol-gel process. Their phase and photocatalytic activity for degradation of methyl orange in water were investigated. 2. Experimental Section 2.1. Preparation of Magnetic ZnFe2O4 Nanoparticles. ZnFe2O4 nanoparticles were prepared by the following procedures. Fe(NO3)3 solution (2.5 M, 60 mL) and Zn(NO3)2 solution (1.25 M, 60 mL) were mixed together with the mole ratio of Zn2+/Fe3+ 1:2 in the solution. NaOH solution (6 M) was added slowly into the solution until pH 9.5-10 was reached, followed by FeCl2 solution (1 M) added slowly into the mixture under vigorous stirring until the mole ratio of Fe2+/Fe3+ reached about 0.02. The total concentration of Zn2+ and Fe3+ in the mixture was adjusted to about 0.9 M by the addition of deionized water. The pH value of the above mixture was adjusted to ca. 9.5 by the dropwise addition of NaOH solution (6 M). ZnFe2O4 nanoparticles were synthesized after the resulting mixture was kept boiling (about 100 °C) and refluxing for 2 h under vigorous stirring.37,38 Then, the as-prepared ZnFe2O4 nanoparticles were washed by centrifugation and redispersion four times with

10.1021/jp806704y CCC: $40.75  2009 American Chemical Society Published on Web 01/21/2009

2464 J. Phys. Chem. C, Vol. 113, No. 6, 2009 deionized water. Finally, the resulting ZnFe2O4 nanoparticles were washed by centrifugation and redispersion with anhydrous ethanol until there was no water in the ethanol dispersion of ZnFe2O4 nanoparticles. To prevent the ZnFe2O4 nanoparticles from agglomeration, they were redispersed in anhydrous ethanol with the solid phase content of 72.3 g L-1. 2.2. Preparation of Composite Photocatalysts. Composite photocatalysts were prepared by the sol-gel process. Tetrabutyl titante (Ti(OBu)4, 10.2 mL), diethanolamine (DEA, 2.9 mL), and the ZnFe2O4 dispersion (1.0 mL) were added into anhydrous ethanol (18.5 mL). After 15 min of sonification, the mixture of deionized water (0.5 mL) and anhydrous ethanol (18.5 mL) was added dropwise into the solution under stirring. The mole ratio of Ti:H2O:DEA is about 1:1:1. The concentration of Ti4+ in the sol of Ti(OBu)4 was about 0.6 M. The weight ratio of zinc ferrite to titania was ca. 3%. The sol of Ti(OBu)4 was kept stirring until the gel formed, followed by drying at 70 °C, and calcination at 400, 500, and 600 °C for 2 h, respectively. Thus, the target photocatalyst TiO2/ZnFe2O4 (TZ) was obtained. For comparison, pure titania and pure zinc ferrite nanopowders were also prepared with the procedures described above. 2.3. Characterization of the As Prepared Samples. The XRD patterns were obtained with a Bruker D8 advance diffractometer using Cu KR radiation. The micrographs were taken with a transmission electron microscope (TEM, JEOL JEM-2010 electron microscope). UV-vis absorption spectra and the maximum of methyl orange adsorption wavelength were obtained by employing a TU-1901 spectrophotometer (Beijing Purkinje General Instrument Co., Ltd.). 2.4. Photocatalytic Activities. The photoactivity of the as prepared samples was measured to degrading methyl orange (C14H14N3NaO3S) in a water solution. A 300 W xenon lamp was used as the light source, which emits both UV and vis light over a wide wavelength. To limit the irradiation wavelength, the light beam was passed through a filter to cutoff wavelengths shorter than 400 nm. The photocatalytic tests were carried out in a quartz glass reactor of 200 mL capacity (40 mm height, 85 mm i.d.) under stirring. In our experiments, the photocatalyst dosage was constant (100 mg of photocatalyst in 100 mL of solution) for easy comparison of all the photocatalysts: 1.0 g L-1 of ZnFe2O4, 1.0 g L-1 of TZ, 1.0 g L-1 of pure titania, respectively, and the initial concentration of methyl orange solution was 10.0 mg L-1. In the photocatalytic experiments, an aqueous solution of methyl orange containing photocatalyst was stirred in the dark to establish the adsorption/desorption equilibrium until the concentration of methyl orange solution was constant. After defined irradiation times, 5.0 mL of dispersion was centrifuged to separate the photocatalyst. The supernatant solution was analyzed by a UV-vis spectrophotometer at a wavelength of 461.0 nm (the maximum of the methyl orange adsorption wavelength). 3. Results and Discussion 3.1. XRD Results. The powder X-ray diffraction patterns of ZnFe2O4 calcined at different temperatures are depicted in Figure 1. Only the ZnFe2O4 crystalline phase is detected, while the peak intensity increases with increasing the calcination temperatures from 105 to 500 °C. For the ZnFe2O4 sample just prepared by the liquid catalytic phase transformation method at 100 °C, the ZnFe2O4 crystalline phase is evident, although the intensity of the peak is very weak due to the nanocrystal size and the poor crystal structure of the ZnFe2O4, indicating that zinc ferrite was synthesized by this method at a low temperature.

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Figure 1. XRD patterns of ZnFe2O4 dried at 105 °C (A) and calcined at 500 °C (B) for 2 h.

Figure 2. XRD patterns for photocatalysts of pure titania (A) calcined at 500 °C for 2 h and TZ calcined at 400 (B), 500 (C) and 600 °C (D) for 2 h, respectively.

Figure 2 shows the XRD patterns of pure titania, TZ calcined at different temperatures. Zinc ferrite peaks can be detected in the TZ samples calcined at different temperatures, although they are very weak. The XRD patterns of TZ samples calcined at different temperatures show that all peaks are clearly assigned to either the anatase phase of titania or the rutile phase of titania besides ZnFe2O4. Comparing XRD patterns A and C in Figure 2, the peaks from the rutile phase of titania in Figure 2A are stronger than that in Figure 2C, indicating that ZnFe2O4 restrains the transformation of anatase to rutile. Nobile and co-worker39 reported that iron ions could catalyze the anatase-to-rutile transformation. However, the result in Figure 2 is not in agreement with what they reported. Obviously, the highly dispersed ZnFe2O4 nanoparticles prevent the formation of the rutile phase. Actually, the ease of the anatase-to-rutile transformation depends on the nature of the TiO2 precursor. The phase transformation anatase-rutile is catalyzed by transition ions, only when they are effectively dissolved inside the support phase.40 To prove the interaction among the components in the composite photocatalyst, the micrographs of ZnFe2O4, TiO2, and TZ were investigated by TEM measurement. Figure 3a shows the diameter of the ZnFe2O4 nanoparticles less than 5 nm, and the crystallization is imperfect. This is in agreement with the XRD result in Figure 1A, where the peaks of ZnFe2O4 are very broad and weak. The TEM micrographs show clearly the ZnFe2O4 nanoparticles are adhered uniformly to TiO2 to form a coupled semiconductor (refer to Figure 3, panels b and c). The electron diffraction pattern obtained from a region in panel c also shows the existence of the ZnFe2O4 phase. However, the

Visible-Light-Active TiO2/ZnFe2O4 Photocatalysts

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Figure 4. Spectra of UV-vis absorption of the prepared samples: (A) ZnFe2O4 calcined at 500 °C for 2 h; (B) TZ calcined at 500 °C for 2 h; and (C) pure titania calcined at 500 °C for 2 h.

Figure 3. TEM micrographs for the carrier and the photocatalysts: (a) ZnFe2O4; (b) pure TiO2 calcined at 500 °C for 2 h; (c) TZ calcined at 500 °C for 2 h; and (d) the electron diffraction pattern from the region in panel c.

TABLE 1: A Comparison of Experimental and Standard Interplanar Spacing (d) Values with Their Respective (hkl) Planes in TZ Nanoparticlesa d, nm phase TiO2 (anatase)

TiO2 (rutile) ZnFe2O4 (cubic spinel)

a

experimental

standard41

diffraction plane (hkl)

0.3523 0.2426 0.1897 0.1695 0.1499 0.1376 0.3252 0.2284 0.253 0.2096 0.1472

0.352 0.2431 0.1892 0.16999 0.1493 0.13641 0.3247 0.2297 0.2543 0.2109 0.1491

101 103 200 105 213 116 110 200 311 400 440

The sample is the 3 wt % ZnFe2O4/TiO2.

intensity of the electron diffraction pattern is very weak for the ZnFe2O4 phase, which indicates that the ZnFe2O4 nanoparticles are highly dispersed among the TiO2 nanoparticles. In addition, the electron diffraction pattern in Figure 3d also reveals the existence of the rutile phase, which is in agreement with the XRD result in Figure 2. The d-values obtained from the electron diffraction pattern for titania and zinc ferrite are listed in Table 1. As a comparison, the standard d-values of titania and zinc ferrite are also shown together in Table 1. The symboles A, R, and F relate to anatase, rutile, and zinc ferrite. In Table 1, the d-values from the electron diffraction pattern are basically consistent with the standard d-values for titania and zinc ferrite. 3.2. Photocatalytic Activities. Figure 4 shows the diffuse reflection spectra of the prepared samples. As a comparison, the spectrum of pure titania was also measured and shown together in Figure 4. The UV-vis absorption edge of TZ is similar to that of TiO2, though single ZnFe2O4 exhibits a good absorption for visible light. The result indicates that ZnFe2O4 in TZ samples dose not change the absorption edge of TiO2. However, TZ can absorb visible light, because single ZnFe2O4 is sensitive to visible light, which accounts for expected

Figure 5. Degradation of methyl orange (10.0 mg L-1) with different photocatalysts under visible light irradiation: (A) blank; (B) ZnFe2O4 (1.0 g L-1); (C) pure titania (1.0 g L-1); and (D) TZ (1.0 g L-1).

photoactivity of TZ under visible light irradiation. As shown in Figure 4, the absorption band of ZnFe2O4 has no structure such as shoulders and possesses relatively steep edges, indicating that the absorption in the visible light region should be due not to surface states but to an intrinsic band transition.42 It is wellknown that, in the normal spinel-type compound ZnFe2O4, tetrahedral and octahedral sites are occupied by Zn2+ and Fe3+ cations, respectively.43 The band structure of ZnFe2O4 is generally defined by taking the O-2p orbital as the valence band and the Fe-3d orbital as the conduction band.44 The absorption of ZnFe2O4 in the visible light region may be due to the electron excitation from the O 2p level into the Fe 3d level. To reveal the effect of the ZnFe2O4 nanoparticles on the photocatalytic properties of titania, photocatalytic degradation of methyl orange under visible light irradiation (λ > 400 nm) is examined with ZnFe2O4, TZ, and pure TiO2 (the amount of composite photocatalysts is the same in all experiments, 100 mg in 100 mL of solution). As shown in Figure 5, methyl orange cannot be degraded in the absence of photocatalyst under visible light irradiation. Curve B shows that ZnFe2O4 has a little photocatalytic activity under visible light irradiation, despite single ZnFe2O4 having a good absorption of visible light. Compared with pure TiO2, the photocatalytic activity of TZ is high. This may be the reason that a coupled semiconductor system is formed between ZnFe2O4, as the light-absorbing semiconductor part having a relatively narrow band gap (Eg ) 1.923 eV),32 and titanium oxide, as the wide band gap part.45 In this colloidal ZnFe2O4-TiO2 system, photogenerated electrons can be transferred from zinc ferrite into TiO2 particles, while the

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Figure 6. Effect of calcination temperature on photocatalytic activity of TZ: (A) 600, (B) 400, and (C) 500 °C.

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Figure 8. Effect of the amount of ZnFe2O4 (wt % ZnFe2O4/TiO2) on the rate constant k of TZ: (A) 1.5%, (B) 3%, (C) 4.5%, and (D) 6%.

visible light irradiation, and the high concentration of ZnFe2O4 doped may play the role of electron-hole recombination centers. The photocatalytic degradation of methyl orange is a pseudofirst-order reaction48,49 and its kinetics may be expressed as follows:

( )

ln

Figure 7. Effect of the amount of ZnFe2O4 (wt % ZnFe2O4/TiO2) on photocatalytic activity of TZ: (A) 1.5%, (B) 3%, (C) 4.5%, and (D) 6%.

holes remain in the ZnFe2O4 particles. The difference in energy levels of the two semiconductor systems plays an important role in achieving such a charge separation. The result of this electron transfer is an efficient primary charge separation, which gives rise to photocatalytic degradation of methyl orange in the presence of photocatalyst TZ under visible light irradiation. It is also noted that titania exhibits a little photocatalytic activity under visible light irradiation (λ > 400 nm). This may be ascribed to the glass filter not cutting off completely wavelengths shorter than 400 nm. Figure 6 shows the effect of calcination temperature on the photocatalytic activity of TZ. It can be seen that the photocatalytic activity of TZ, calcined at 500 °C, is the highest under visible light irradiation. Curve A shows that TZ calcined at 600 °C has little photocatalytic activity under visible light irradiation. This may be the reason of the excessive Fe ions doping. At higher temperature, a certain percentage of the Fe3+ ions, present at the surface of the ZnFe2O4 core, diffuses into the TiO2 to produce a substitutional solid solution, in which Fe3+ is dispersed in the lattice of TiO2. In fact, as the radius Fe3+ is similar to that of Ti4+, the substitution of iron in the matrix of TiO2 is a favorable process.46 There is the optimal condition in the number of Fe ions doped to achieve photocatalytic reactivity under visible light irradiation, but the excessive Fe ions doped work as electron-hole recombination centers.47 Figure 7 shows the effect of the amount of ZnFe2O4 on the photocatalytic activity of TZ. It can be seen that the photocatalytic activity of TZ is the highest under visible light irradiation, when the weight ratio of zinc ferrite to titania is ca. 3%. This indicates that there is the optimal condition in the amount of ZnFe2O4 doped to achieve high photocatalytic reactivity under

C0 ) kt C

(1)

where k is the apparent reaction rate constant, and C0 and C are the initial concentration and the reaction concentration of methyl orange, respectively. Figure 8 shows that the apparent first-order photodegradation rate constant k for 3% ZnFe2O4/ TiO2 composition is 1.76 × 10-3 min-1, and the higher or lower amount of ZnFe2O4 results in diminishing the k value of ZnFe2O4/TiO2 composition. This is in agreement with the photodegradation result of the prepared photocatalysts with different ZnFe2O4 amount in Figure 7. 4. Conclusions The conclusions from this study are the following: (1) A visible-light-active TiO2-ZnFe2O4 photocatalyst could be prepared through the sol-gel process, including ZnFe2O4 nanoparticles synthesized by the liquid catalytic phase transformation method at low temperature. (2) In TiO2-ZnFe2O4 photocatalyst, a coupled semiconductor system is formed between ZnFe2O4, as the light-absorbing semiconductor part of a relatively narrow band gap, and titanium oxide, as the wide band gap part, with the results of an efficient primary charge separation and photocatalytic activity under visible light irradiation. (3) The photocatalytic experiment for the degradation of methyl orange indicates that there are optimal preparation conditions for TZ photocatalyst: calcination temperature of 500 °C for 2 h, and 3 wt % ZnFe2O4. Acknowledgment. The work was financially supported in part by the Shanghai Leading Academic Discipline Project, Project No. B604, and the Foundation of Donghua University (No. 113100044017), and Shanghai Key Program of Science and Technology, People’s Republic of China (No. 062312068), and Shanghai Special Research Fund for Outstanding Young Teachers, People’s Republic of China. References and Notes (1) Zhang, Y.; Crittenden, J. C.; Hand, D. W.; Perram, D. L. EnViron. Sci. Technol. 1994, 28, 435. (2) Hoffmann, M. R.; Martin, S. T.; Choi, W.; Bahnemann, D. W. Chem. ReV. 1995, 95, 69.

Visible-Light-Active TiO2/ZnFe2O4 Photocatalysts (3) Zhao, J. C.; Wu, T. X.; Wu, K. Q.; Oikawa, K.; Hidaka, H.; Serpone, N. EnViron. Sci. Technol. 1998, 32, 2394. (4) Anpo, M.; Takeuchi, M. J. Catal. 2003, 216, 505. (5) Dvoranova´, D.; Brezova´, V.; Mazu´r, M.; Malati, M. A. Appl. Catal., B 2002, 37, 91. (6) Islam, A.; Sugihara, H.; Hara, K.; Singh, L. P.; Katoh, R.; Yanagida, M.; Takahashi, Y.; Murata, S.; Arakawa, H. J. Photochem. Photobiol. A 2001, 145, 135. (7) Xie, Y. B.; Yuan, C. W. Appl. Catal., B 2003, 46, 251. (8) Zakrzewska, K.; Radecka, M.; Kruk, A.; Osuch, W. Solid State Ionics 2003, 157, 349. (9) Pal, M.; Sasaki, T.; Koshizaki, N. Scr. Mater. 2001, 44, 1817. (10) Nakamura, I.; Negishi, N.; Kutsuna, S.; Ihara, T.; Sugihara, S.; Takeuchi, K. J. Mol. Catal. A 2000, 161, 205. (11) Iimura, S.; Teduka, H.; Nakagawa, A.; Yoshihara, S.; Shirakashi, T. Electrochemistry 2001, 69, 324. (12) Asahi, R.; Morikawa, T.; Ohwaki, T.; Aoki, K.; Taga, Y. Science 2001, 293, 269. (13) Khan, S. U. M.; Al-Shahry, M.; Ingler, W. B. Science 2002, 297, 2243. (14) Hattori, A.; Tada, H. J. Sol-Gel Sci. Technol. 2001, 22, 47. (15) Umebayashi, T.; Yamaki, T.; Itoh, H.; Asai, K. Appl. Phys. Lett. 2002, 81, 454. (16) Burda, C.; Lou, Y. B.; Chen, X. B.; Samia, A. C. S.; Stout, J. D.; Gole, J. L. Nano Lett. 2003, 3, 1049. (17) Gole, J. L.; Stout, J. D.; Burda, C.; Lou, Y. B.; Chen, X. B. J. Phys. Chem. B 2004, 108, 1230. (18) Chen, X. B.; Burda, C. J. Phys. Chem. B 2004, 108, 15446. (19) Prokes, S. M.; Gole, J. L.; Chen, X. B.; Burda, C.; Carlos, W. E. AdV. Funct. Mater. 2005, 15, 161. (20) Chen, X. B.; Lou, Y. B.; Samia, A. C. S.; Burda, C.; Gole, J. L. AdV. Funct. Mater. 2005, 15, 41. (21) Chen, X. B.; Lou, Y. B.; Dayal, S.; Qiu, X. F.; Krolicki, R.; Burda, C.; Zhao, C. F.; Becker, J. J. Nanosci. Nanotechnol. 2005, 5, 1408. (22) Qiu, X. F.; Burda, C. Chem. Phys. 2007, 339, 1. (23) Qiu, X. F.; Zhao, Y. X.; Burda, C. AdV. Mater. 2007, 19, 3995. (24) Zhao, Y. X.; Qiu, X. F.; Burda, C. Chem. Mater. 2008, 20, 2629. (25) Chen, X. B.; Burda, C. J. Am. Chem. Soc. 2008, 130, 5018. (26) Chen, X. B.; Glans, P. A.; Qiu, X. F.; Dayal, S.; Jennings, W. D.; Smith, K. E.; Burda, C.; Guo, J. H. J. Electron Spectrosc. Relat. Phenom. 2008, 162, 67. (27) Li, X. Z.; Li, F. B.; Yang, C. L.; Ge, W. K. J. Photochem. Photobiol. A 2001, 141, 209.

J. Phys. Chem. C, Vol. 113, No. 6, 2009 2467 (28) Fuerte, A.; Herna´ndez-Alonso, M. D.; Maira, A. J.; Martı´nez-Arias, A.; Ferna´ndez-Garcı´a, M.; Conesa, J. C.; Soria, J.; Munuera, G. J. Catal. 2002, 212, 1. (29) Wang, C.; Zhao, J. C.; Wang, X. M.; Mai, B. X.; Sheng, G. Y.; Peng, P.; Fu, J. M. Appl. Catal., B 2002, 39, 269. (30) de Haart, L. G. J.; Blasse, G. J. Electrochem. Soc. 1985, 132, 2933. (31) Liu, J. J.; Lu, G. X.; He, H. L.; Tan, H.; Xu, T.; Xu, K. Mater. Res. Bull. 1996, 31, 1049. (32) Valenzuela, M. A.; Bosch, P.; Jime´nez-Becerrill, J.; Quiroz, O.; Pa´ez, A. I. J. Photochem. Photobiol. A 2002, 148, 177. (33) Cheng, P.; Li, W.; Zhou, T. L.; Jin, Y. P.; Gu, M. Y. J. Photochem. Photobiol. A 2004, 168, 97. (34) Srinivasan, S. S.; Wade, J.; Stefanakos, E. K. Mater. Res. Soc. Symp. Proc. 2005, 876, 89. (35) Anpo, M. Catal. SurV. Jpn. 1997, 1, 169. (36) Srinivasan, S. S.; Wade, J.; Kislov, N.; Smith, M.; Stefanakos, E. K.; Goswami, Y. Mater. Res. Soc. Symp. Proc. 2005, 900, 307. (37) Xu, S. H.; Shangguan, W. F.; Yuan, J.; Chen, M. X.; Shi, J. W. Appl. Catal., B 2007, 71, 177. (38) Xu, S. H.; Shangguan, W. F.; Yuan, J.; Chen, M. X.; Shi, J. W. Sci. Technol. AdV. Mater. 2007, 8, 40. (39) Nobile, A., Jr.; Davis, M. W, Jr J. Catal. 1989, 116, 383. (40) Rao, P. M.; Viswanathan, B.; Viswanathan, R. P. J. Mater. Sci. 1995, 30, 4980. (41) Powder Diffraction File, JCPDS. File No. 21-1272, 21-1276, 221012. (42) Xu, S. H.; Shangguan, W. F.; Yuan, J.; Shi, J. W.; Chen, M. X. Mater. Sci. Eng. B 2007, 137, 108. (43) Botta, P. M.; Aglietti, E. F.; Porto Lo´pez, J. M. Mater. Res. Bull. 2006, 41, 714. (44) Wang, D. F.; Zou, Z. G.; Ye, J. H. Chem. Phys. Lett. 2003, 373, 191. (45) Henglein, A. Chem. ReV. 1989, 89, 1861. (46) Gao, Y.; Chen, B. H.; Li, H. L.; Ma, Y. X. Mater. Chem. Phys. 2003, 80, 348. (47) Zhang, Z. B.; Wang, C. C.; Zakaria, R.; Ying, J. Y. J. Phys. Chem. B 1998, 102, 10871. (48) Bekbo¨let, M.; Balcioglu, I. Water Sci. Technol. 1996, 34, 73. (49) Al-Qaradawi, S.; Salman, S. R. J. Photochem. Photobiol. A 2002, 148, 161.

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