Formation of Surface Complex Leading to Efficient Visible

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Formation of Surface Complex Leading to Efficient Visible Photocatalytic Activity and Improvement of Photostabilty of ZnO Yuanzhi Li,*,† Xi Zhou,† Xuelei Hu,‡ Xiujian Zhao,† and Pengfei Fang§ Key Laboratory of Silicate Materials Science and Engineering, Wuhan UniVersity of Technology, Ministry of Education, 122 Luoshi Road, Wuhan 430070, China, School of Chemical Engineering and Pharmacy, Hubei Key Laboratory of NoVel Chemical Reactor and Green Chemical Technology, Wuhan Institute of Technology, 693 Xiongchu Road, Wuhan 430073, China, and Department of Physics, Wuhan UniVersity, Luojia Hill, Wuhan 430072, China ReceiVed: June 17, 2009; ReVised Manuscript ReceiVed: July 28, 2009

Phenolic compounds (PCs) such as 4-chlorophenol, 2,4-dichlorophenol, and phenol, can form a surface complex with ZnO. The surface complex was characterized with FTIR, PL, DTG, DRUV-vis, and photoelectrochemical measurement. The surface 4-CP/ZnO complex binds to ZnO through phenolate Zn-O-Ph-Cl linkage, which is much stronger than the phenolate Ti-O-Ph-Cl linkage of the surface 4-CP/TiO2 complex. The formation of the surface complex shifts the absorption response of ZnO from the UV to the visible region through ligand-to-metal charge transfer with excitation of visible photon, and induces efficient visible photocatalytic activity. ZnO exhibits 3.7 times higher photocatalytic activity for the photodegradation of 4-CP than TiO2 under visible irradiation. The much higher visible photocataltyic activity of ZnO than TiO2 is attributed to the higher efficiency of the charge transfer in the former than in the latter evidenced by the much larger photocurrent of PC/ZnO than PC/TiO2. Moreover, the formation of the surface complex results in the substantial improvement of the photostability of ZnO as it leads to a considerable decrease of the surface defect sites of ZnO, and the photogenerated holes trapped on the surface of ZnO probably prefer reacting with the surface complex to reacting with the surface oxygen atom under UV-visible irradiation. Introduction ZnO has attracted scientific interest as a potential photocatalyst in recent years. Various kinds of nanostructured ZnO, such as nanoparticle, nanorode, nanobelt, nanoplate, hollow sphere, and micro/nanostructure, have been used for the photodegradation of organic pollutants.1-6 In some cases, ZnO even exhibits a better photocatalytic efficiency than TiO2, which has been extensively studied for decades, and accepted as the most efficient photocatalyst for the detoxification of air and water pollutants. However, it is activated only under UV light irradiation because of its large band gap (3.2 eV). But solar spectra contain only 5% of UV. Thus 95% of the solar photons are useless for ZnO photocatalyst, which greatly limits its practical application in environmental decontamination. Efforts to extend its photocatalytic response from the UV to the visible region have been made, but successful works are quite limited, which include doping ZnO by N and C.7-10 Another drawback for the ZnO photocatalyst is its photoinstability in aqueous solution due to its photocorrosion with UV irradiation, which significantly decreases the photocatalytic activity of ZnO and blocks its practical application in environment purification.11 Recently, several methods have been developed to improve its photostability, including surface organic coating of ZnO12 and surface hybridization of ZnO with carbon and fullerenes C60.13 These works provided the possibility for the practical application of ZnO in the photocatalytic removal of organic pollutants. As * To whom correspondence should be addressed. † Key Laboratory of Silicate Materials Science and Engineering, Wuhan University of Technology. ‡ Hubei Key Laboratory of Novel Chemical Reactor and Green Chemical Technology, Wuhan Institute of Technology. § Department of Physics, Wuhan University.

the photogenerated active oxygen species have very strong oxidative ability, the organic compound or carbon used could be gradually oxidized during the long photocatalytic process, thus probably resulting in the deterioration of their photostable role. Therefore, it is desirable and of great challenge to explore the high photostable and efficient visible-light-induced photocatalyst properties of ZnO. It has been reported that the formation of surface complex between pure titania and substrates such as H2O2, a nonionic surfactant having polyoxyethylene groups (Brij), salicylate, and phenolic compounds (e.g., 4-chlorophenol (4-CP), 2, 4-dichlorophenol (2,4-DCP), 2,4,6-trichlorophenol, catechol, etc.), induces visible photocatalytic activity although both titania and the corresponding substrates could not absorb the visible light at all.14-19 Among them, the visible photodegradation of phenolic compounds (PCs) is especially significant as the widely used PCs produce a serious environmental problem because of their toxicity, unacceptable odor, persistence in the environment, and difficulty to be removed by conventional methods such as biological treatment. The visible adsorption of the surface complex is attributed to the ligand-to-metal charge transfer (LMCT) between the substrate (ligand) and the Ti(IV) site on the surface with excitation of visible photons. Recently, Orlov et al.20 studied the interaction between 4-CP and TiO2 under well-defined UHV conditions by means of NEXAFS, timedependent XPS, and UPS and concluded that the adsorbed molecule of 4-CP is attached to the surface of TiO2 via a phenolate link with a coverage-dependent tilted geometry to form a surface 4-CP/TiO2 complex. The complex formation leads to a decrease in the TiO2 work function pointing to adsorbate-substrate charge transfer, and induces occupied states in the TiO2 band gap, resulting in direct photoexcitation of the

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Improvement of Photostabilty of ZnO complex for photon energies of less than 3 eV and hence visiblelight photoactivity. It has been reported that surface modification of ZnO can greatly change its photoluminescent and photocatalytic properties.21 As there are abundant hydroxyl groups as well as defects on the surface of ZnO, and ZnO has a band gap structure similar to that of TiO2, it is expected that a surface complex between some substrate such as PC and ZnO should be formed, and may have substantial influence on its physichemical properties. But, up to now, there has been no report about the surface complex between PC and ZnO, and its modification to the photocatalytic activity of ZnO. Herein, it was found for the first time that PCs such as 4-CP, 2,4-DCP, and phenol can form a surface complex with ZnO. The formation of the surface complex not only induces visible photocatalytic activity due to its absorption response shifting from the UV to the visible region, but it also improves its photostability.

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Figure 1. XRD pattern of ZnO.

Experimental Section Materials. ZnO was obtained from the Wuhan Zhongbei Chemical Reagent Co. Titania P25 (TiO2 ca. 80% anatase, 20% rutile) was obtained from the Deggussa Co. 4-CP, 2,4-DCP, phenol, crystal violet, poly(ethylene glycol), and ethanol were purchased from Sinopharm Chemical Reagent Co. All of these chemicals were used without further purification. Characterization. The X-ray diffraction (XRD) pattern was obtained on a Rigaku D/Max-IIIA X-ray diffractometer. Fourier transform infrared (FTIR) spectra were recorded on a Thermo Nicolet spectrometer. Photoluminescence (PL) spectra were recorded at room temperature on a Shimadzu RF-5301 PC spectrometer, using 320 nm excitation light. To record the PL spectra of samples, a thin film of the samples was coated on a glass substrate. Their PL spectra were obtained by subtracting the noise of the glass substrate. Thermal analysis was taken on a Simultaneous Thermal Analyzer (STA 449C) in the flow of pure Ar at a rate of 10 deg min-1. Diffusive reflectance UV-vis (DRUV-vis) absorption spectra were recorded on a Shimadzu UV-2550 spectrophotometer. The Brunauer-Emmett-Teller (BET) surface area was measured on an Autosorb-1, using N2 adsorption at -196 °C for the sample predegassed at 200 °C in a vacuum for 2 h. For the characterization of PCs adsorbed on ZnO or TiO2, the sample was prepared as follows: 0.5 g of ZnO or TiO2 was dispersed in 50 mL of 1.0 × 10-3 mol L-1 PC solution in ethanol for 1 h to ensure the establishment of an adsorption/desorption equilibrium, filtered, and dried at 80 °C for 2 h in the dark. Photoelectrochemical measurement was carried out by using a homemade three-electrode quartz cell, Pt wire as the counter electrode, a saturated Ag/AgCl electrode as the reference electrode, and the thin film of ZnO or TiO2 on ITO as the working electrode on an electrochemical analyzer (CHI750). The electrolyte used was an aqueous solution of 0.2 mol L-1 Na2SO4 and 4.0 × 10-6 mol L-1 PC. A 125 W high-pressure Hg Lamp (Shanghai Yaming Lighting Appliance Co. Ltd.) was used as the light source. Before the measurement, the electrolyte was purged by pure N2 to remove the dissolved oxygen. To measure the photocurrent under visible irradiation (λ > 420 nm), a UV cut-filter, which can filter out UV light with wavelengths below 420 nm, was placed between the lamp and the cell. The working photoelectrode was prepared according to the following procedure. A mixture of ZnO or TiO2 powder and poly(ethylene glycol) with a weight ratio of 5% was added to 15 mL of ethanol, then the solution was ultrasonicated for 30 min. The obtained mixture was uniformly spread on an ITO glass substrate

Figure 2. FT-IR spectra for 4-CP, ZnO, and 4-CP/ZnO.

(1.0 cm × 1.2 cm). After the evaporation of ethanol, the sample was heated to 480 °C in a muffle furnace at a rate of 2 deg min-1, and remained at this temperature for 2 h to completely remove poly(ethylene glycol) . Photocatalytic Activity. The photocatalytic activity of the photocatalysts was evaluated by the photodegradation of PC. The light source was a 125 W high-pressure Hg lamp. The reaction was maintained at ambient temperature. In a typical experiment, aqueous suspensions of PC (50 mL, 1 × 10-4 mol · L-1) and 0.2000 g of the photocatalyst powder were placed in the beaker. Prior to irradiation, the suspension was magnetically stirred in the dark to ensure the establishment of an adsorption/desorption equilibrium. The suspension was kept under constant air-equilibrated conditions. At given irradiation time intervals, 1 mL of the suspension was collected and centrifuged to remove the particles. The PC concentration was determined by measuring the UV-vis absorbance of the PC aqueous solution. To measure the photocatalytic activity under visible irradiation (λ > 420 nm), a UV cut-filter was used to filter out UV light with wavelengths below 420 nm. Results and Discussion XRD. Figure 1 shows the XRD pattern of the ZnO photocatalyst. The observed diffraction peaks of ZnO can be indexed to those of hexagonal wurtzite ZnO (J4-CPDS 89-0511). No impurity phases were detected. Its average crystal size is determined to be 353 nm according to the Scherrer formula (L ) 0.89λ/(β cosθ)). Its BET surface area is 3.0 m2 g-1. FTIR. Figure 2 shows the FT-IR spectra for 4-CP, ZnO, and 4-CP/ZnO. For 4-CP, the sharp peaks at 1589 and 1495 cm-1 are attributed to the CdC stretching from the conjugated aryl, while the absorption peaks at 1090 and 823 cm-1 are assigned to C-H in-plane bending and out-of-plane bending, respectively. The peak at 698 cm-1 is originated from the stretching vibration of C-Cl. The broad strong peak around 3311 cm-1 is due to stretching vibrations of O-H, while the absorption peak that represents O-H in-plane bending occurs at 1443 cm-1. The strong bands at 1234 cm-1 are the stretching vibration of

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Figure 3. The room temperature photoluminescence spectra with excitation at 320 nm for ZnO and 4-CP/ZnO.

Li et al.

Figure 4. DTG traces of 4-CP/TiO2 and 4-CP/ZnO under flowing pure Ar.

C-O.22-24 After adsorption of 4-CP on ZnO, the peaks at 1554, 1512, 1097, 831, and 688 cm-1, which are characteristic of a Cl-substituted aromatic ring, are still observed except for shifts in the peaks. However, the adsorption of 4-CP on ZnO leads to a substantial change in the spectra related to the -OH group. The strong peaks of O-H in-plane bending and C-O stretching become weak shoulder bands. The O-H stretching vibration of 4-CP seems to disappear, while the intensity of the stretching vibration of hydroxyl groups in ZnO decreases. On the basis of these results, it is concluded that a surface complex between 4-CP and ZnO is formed on the surface of ZnO through a condensation reaction as follows:

-Zn-OH + HO-Ph-Cl ) -Zn-O-Ph-Cl + H2O Photoluminescence. Figure 3 gives the photoluminescence of ZnO and 4-CP/ZnO with an excitation wavelength of 320 nm. A strong UV emission at 390 nm and a relatively weak visible emission in the range of 400-520 nm is observed for ZnO as shown in Figure 3. The UV emission is attributed to free excitonic emission near the band edge. The visible emission is due to transition in defect states, in particular the oxygen vacancies, that is, photogenerated electrons transfer from the conduction band of ZnO to defect states through a nonradiactive pathway, then recombine with holes in the valence band accompanied by visible emissions.25 Adsorption of 4-CP on ZnO leads to a significant increase of the UV emission but a slight decrease of visible emission. This observation indicates that more photoexcited electrons in the conduction band recombine directly with holes in the valence band in ZnO rather than transition in defect states, and the surface defects in ZnO are reduced after the adsorption of 4-CP on ZnO. These results suggest that some of the surface complex between 4-CP and ZnO is formed on the defect sites, most probably on oxygen vacancies. DTG. Figure 4 shows the DTG traces of 4-CP/TiO2 and 4-CP/ ZnO under flowing pure Ar. For 4-CP/TiO2, the strong desorption maximum at 80.3 °C is due to the loss of water, and the desorption maxima at 118.6 and 147.8 °C reflect the dehydroxylation from deferent binding sites of TiO2. The peak at 174.2 °C is assigned to the desorption of the surface complex of 4-CP/TiO2. In the case of 4-CP/ZnO, the strong maximum at 74.4 °C is due to desorption of water, and the weaker peaks at 123.9 and 161.9 °C are attributed to the dehydroxylation from deferent binding sites of ZnO. The strong peak at 271.2 °C is assigned to the desorption of the surface complex of 4-CP/ZnO. These results indicate that the phenolate Zn-O-Ph-Cl linkage of the surface 4-CP/ZnO complex is much stronger than the phenolate Ti-O-Ph-Cl linkage of the surface 4-CP/ TiO2 complex.

Figure 5. DRUV-vis spectra of pure ZnO, TiO2, and phenolic compound adsorbed on ZnO and TiO2.

DRUV-Vis. Figure 5 shows the DRUV-vis spectra of pure ZnO, TiO2, and phenolic compounds adsorbed on ZnO and TiO2. No absorption in the visible region was observed for pure ZnO and TiO2. It is reported that the formation of surface complex between TiO2 and phenolic compounds results in the visible light absorption through ligand-to-metal charge transfer (LMCT) between the substrate (ligand) and the Ti(IV) site on the surface.16,17 In our case, 4-CP adsorbed on TiO2 shows adsorption in the visible region of 400-600 nm, which is in agreement with the literature. As shown in Figure 5A, 4-CP/ZnO exhibits absorption in the visible region of 400-700 nm, suggesting that the formation of surface 4-CP/ZnO shifts the absorption of ZnO from the UV to the visible region through LMCT between 4-CP (ligand) and the Zn(Π) site. As shown in Figure 5B, adsorption of other phenolic compounds such phenol and 2,4-DCP also exhibits absorption in the visible region, suggesting the formation of a similar surface complex between ZnO and phenol or 2,4-DCP, which shifts the absorption of ZnO from the UV to the visible region through LMCT between the substrates and the Zn(Π) site. Photocatlytic Activity. Figure 6A shows the time course of the decrease in the concentration of 4-CP. Under visible irradiation (λ > 420 nm), 4-CP adsorbed on TiO2 is gradually photodegraded. After visible irradiation for 360 min, 45.0% of 4-CP is photodegraded. The photodegradation of 4-CP follows first-order kinetics as confirmed in Figure 6B. Its rate constant

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Figure 7. The photoelectrochemical response of ZnO/ITO and TiO2/ ITO electrode in a solution of 0.2 mol L-1 Na2SO4 and 4.0 × 10-6 mol L-1 phenolic compounds vs. bias potential under visible irradiation.

Figure 6. The time course of the decrease in the concentration (A) and ln(C0/C) (B) for the photodegradation of phenolic compounds under visible irradiation.

is 1.54 × 10-3 min-1. The photodegradation of 4-CP is due to the photosensitization of TiO2 by the 4-CP/TiO2 surface complex as follows: The 4-CP/TiO2 surface complex is excited by visible light through ligand-to-metal charge transfer with direct transfer of an electron from 4-CP to the conduction band (CB) of TiO2. Once an electron is injected into CB, it is transferred to electron acceptors such as O2 to form active oxygen species (e.g., O2-), which is subsequently followed by a series of oxidative degradation reactions of phenolic compounds.17,26 As shown in Figure 6A, the photodegradation of 4-CP on ZnO is much faster than that on TiO2 under visible irradiation. Thus 83.5% of 4-CP is photodegraded on ZnO in 360 min. The rate constant on ZnO (5.13 × 10-3 min-1) is 3.3 times higher than that on TiO2 (Figure 6B). Obviously, the visible photocatalytic activity of ZnO is attributed to the photosensitization of ZnO by the 4-CP/ZnO surface complex similar to that of the 4-CP/TiO2 system as a similar surface complex is formed on both TiO2 and ZnO. The photodegradation of other phenolic compound such as phenol and 2,4-DCP on ZnO under visible irradiation was also examined. As can be seen from Figure 6A, the ZnO photocatalyst shows efficient visible photocatalytic activity for the photodegradation of both phenol and 2,4-DCP. These results demonstrate that the visible adsorption of the surface complex between ZnO and phenol or 2,4-DCP also induces visible photocatalytic activity. Photoelectrochemical Property. The photoelectrochemical performance of ZnO and TiO2 was studied to evaluate the separation efficiency of photogenerated electrons and holes. Under visible irradiation, the photocurrent of the TiO2/ITO electrode in the solution of 4-CP is quite low as shown in Figure 7. In contrast, the ZnO/ITO electrode exhibits much larger photocurrent than TiO2/ITO in the bias potential range of -0.10 to 0.6 V. As ZnO and TiO2 could not be excited by visible irradiation due to their large band gap, the observed photocurrent results from the ligand-to-metal charge transfer in the surface complex formed on ZnO or TiO2.17 With visible excitation, photogenerated electron transfers from the ligand to the metal site (e.g., Zn(Π) or Ti(IV)) of the surface complex through

Figure 8. The durability of the ZnO photocatalyst for the photodegradation of CV and 4-CP under UV-visible irradiation for 2 h.

LMCT, then delocalizes to other metal sites farther away from the complex, finally moving to the ITO electrode, and thus photocurrent is generated. The photocurrent depends on the back electron transfer, which is nonsingle-exponential with a fast component and multiple slower components. The former is attributed to electrons near the initial metal center, while the latter are attributed to the ability of electrons in metal oxide to delocalize to other metal sites farther away from the complex.27 The faster the back electron transfer is, the lower the photocurrent is. As the mobility of charge carriers (e.g., electron and hole) in ZnO (100-205 cm2 V-1 s-1)28 is much higher than that in TiO2 (0.1-1.0 cm2 V-1 s-1),29 the delocalization of electron from the metal site of the surface complex to other metal sites farther away from the complex in the 4-CP/ZnO system is much faster than that in the 4-CP/TiO2 system, leading to the much larger photocurrent of ZnO than that of TiO2. The much larger photocurrent of ZnO than TiO2 indicates the higher separation efficiency of the photogenerated electrons and holes in 4-CP/ZnO than in 4-CP/TiO2, which results in the much higher visible photocataltyic activity of ZnO than TiO2. Similar results that the photocurrent of the ZnO/ITO electrode is much larger than that of the TiO2/ITO electrode in the solution of other phenolic compounds such as phenol and 2,4-DCP under visible irradiation were obtained, which can account for the efficient visible photocatalytic activity for the photodegradation of phenol and 2,4-DCP on ZnO. Photostability. The photostability of ZnO for the photodegradation of 4-CP under UV-visible irradiation was evaluated by recycling the photocatalyst. It was reported that ZnO suffers from a considerable decrease of its photocatalytic activity due to the photocorrosion when it was recycled for the photodegradation of various dyes.13 A similar phenomenon was observed in our case. As shown in Figure 8, when the ZnO photocatalyst is recycled for the photodegradation of crystal violet under UV-visible irradiation, its photocatalytic conversion is 99.8% within 2 h when it is used for the first time. Recycling the

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photocatalyst leads to a gradual decrease of its photocatalytic activity. After seven recycles, its photocatalytic conversion decreases to 58.0%. To make a comparison, we performed an experiment to evaluate the photostability of ZnO for the photodegradation of 4-CP under UV-visible irradiation. As shown in Figure 8, 92.8% of 4-CP is photodegraded within 2 h when the ZnO photocatalyst was used for the first time. With recycling of the photocatalyst, no obvious decrease of its photocatalytic activity was observed. After seven recycles, its photocatalytic conversion remains almost unchanged (91.8%). This interesting result shows that ZnO remains photostable for the photodegradation of 4-CP, which is in striking contrast to the photoinstability of ZnO for the photodegradation of various dyes. The formation of surface 4-CP/ZnO complex not only induces a new function of visible photodegradation, but also inhibits photocorrosion of ZnO and thus improves its photostability. The photocorrosion of ZnO proceeds as follows. When ZnO is subjected to intrinsic excitation, electrons are promoted from the valence band to the conduction band generating electron-hole pair separation. The holes are transported to the solid-solution interface and are prone to undergo a reaction with the surface oxygen atom, resulting in the escape of oxygen from the surface. The overall reaction may be represented as follows: ZnO + 2h+ f Zn2+ + 0.5O2.13,30 In this process, the photocorrosion of ZnO consumes some photoinduced holes, which damage the photocatalytic activity of ZnO. Furthermore, the crystal structure of ZnO collapses, and loses the activity for the degradation ultimately.13 Very recently, it was found by Kislov et al.31 that the photocorrosion mainly occurs in the surface defect sites of ZnO. Zhu et al. achieved a photostable hybrid photocatalyst of C60/ZnO by anchoring fullerenes C60 molecule on the surface defect sites of ZnO, which substantially reduced the activation of the surface oxygen atom, and effectively inhibited the photocorrosion of ZnO.13 In our case, the formation of the surface complex between 4-CP and ZnO results in a considerable decrease of the surface defect sites of ZnO, which inhibits the photocorrosion of ZnO and improves its photostability. On the other hand, under UV irradiation the photogenerated holes trapped on the surface of ZnO probably prefer reacting with the surface complex to reacting with the surface oxygen atom, and thus effectively inhibit the photocorrosion of ZnO. Conclusion In summary, phenolic compounds such as 4-CP, 2,4-DCP, and phenol can form a surface complex with ZnO through phenolate linkage. The phenolate Zn-O-Ph-Cl linkage of the surface 4-CP/ZnO complex is much stronger than the phenolate Ti-O-Ph-Cl linkage of the surface 4-CP/ TiO2 complex. The formation of the surface complex shifts the absorption response of ZnO from the UV to the visible region through ligand-tometal charge transfer with excitation of the visible photon, and induces efficient visible photocatalytic activity. The much larger photocurrent of PC/ZnO than PC/TiO2, indicating the higher efficiency of the charge transfer in the former than in the latter, results in the much higher visible photocataltyic activity of ZnO than TiO2. Moreover, the formation of the surface complex greatly improves the photostability of ZnO because it results in a considerable decrease of the surface defect sites of ZnO, and the photogenerated holes trapped on the surface of ZnO probably

Li et al. prefer reacting with the surface complex to reacting with the surface oxygen atom under UV irradiation. Acknowledgment. This work was supported by National Basic Research Program of China (2009CB939704), Important Project of Ministry of Education of China (309021), Scientific Research Foundation for the Returned Overseas Chinese Scholars (2008 890), the Program for New Teacher from Ministry of Education (20070497003), and Nippon Sheet Glass Foundation. References and Notes (1) Jang, E. S.; Won, J. H.; Hwang, S. J.; Choy, J. H. AdV. Mater. 2006, 18, 3309. (2) Sun, T. J.; Qiu, J. S.; Liang, C. H. J. Phys. Chem. C 2008, 112, 715. (3) Ye, C. H.; Bando, Y.; Shen, G. Z.; Golberg, D. J. Phys. Chem. B 2006, 110, 15146. (4) Yu, J. G.; Yu, X. X. EnViron. Sci. Technol. 2008, 42, 4902. (5) Deng, Z. W.; Chen, M.; Gu, G. X.; Wu, L. M. J. Phys. Chem. B 2008, 112, 16. (6) Lu, F.; Cai, W. P.; Zhang, Y. G. AdV. Funct. Mater. 2008, 18, 1047. (7) Lin, H. F.; Liao, S. C.; Hung, S. W. J. Photochem. Photobiol. A 2005, 174, 82. (8) Wei, H. Y.; Wu, Y. S.; Wu, L. L.; Hu, C. X. Mater. Lett. 2005, 59, 271. (9) Chen, S. F.; Zhao, W.; Zhang, S. J.; Liu, W. Chem. Eng. J. 2009, 148, 263. (10) Zhou, X.; Li, Y. Z.; Peng, T.; Xie, W.; Zhao, X. J. Mater. Lett. 2009, 63, 1747. (11) (a) Meulenkamp, E. A. J. Phys. Chem. B 1998, 102, 7764. (b) van Dijken, A.; Janssen, A. H.; Smitsmans, M. H. P.; Vanmaekelbergh, D.; Meijerink, A. Chem. Mater. 1998, 10, 3513. (c) Jongh, P. E.; Meulenkamp, E. A.; Vanmaekelbergh, D.; Kelly, J. J. J. Phys. Chem. B 2000, 104, 7686. (12) Comparelli, R.; Fanizza, E.; Curri, M. L.; Cozzi, P. D.; Mascolo, G.; Agostiano, A. Appl. Catal., B 2005, 60, 1. (13) (a) Zhang, L. W.; Cheng, H. Y.; Zong, R. L.; Zhu, Y. F. J. Phys. Chem. C 2009, 113, 2368. (b) Fu, H. B.; Xu, T. G.; Zhu, S. B.; Zhu, Y. F. EnViron. Sci. Technol. 2008, 42, 8064. (14) Li, X.; Chen, C.; Zhao, J. C. Langmuir 2001, 17, 4118. (15) Cho, Y.; Kyung, H.; Choi, W. Appl. Catal., B 2004, 52, 23. (16) (a) Agrios, A. G.; Gray, K. A.; Weitz, E. Langmuir 2003, 19, 1402. (b) Agrios, A. G.; Gray, K. A.; Weitz, E. Langmuir 2004, 20, 5911. (17) Kim, S. H.; Choi, W. Y. J. Phys. Chem. B 2005, 109, 5143. (18) Li, M.; Tang, P. S.; Hong, Z. L.; Wang, M. Q. Colloids Surf., A 2008, 318, 285. (19) Moser, J.; Punchihewa, S.; Infelta, P. P.; Gratzel, M. Langmuir 1991, 7, 3012. (20) Orlov, A.; Watson, D. J.; Williams, F. J.; Tikhov, M.; Lambert, R. M. Langmuir 2007, 23, 9551. (21) Bohle, D. S.; Spina, C. J. J. Am. Chem. Soc. 2009, 131, 4397. (22) Eaton, D. C. Laboratory InVestigation in Organic Chemistry; McGraw-Hill, Inc.: New York, 1989. (23) Alderman, S. L.; Dellinger, B. Phys. Chem. A 2005, 109, 7725. (24) Huang, G. L.; Zhang, S. C.; Xu, T. G.; Zhu, Y. F. EnViron. Sci. Technol. 2008, 42, 8516. (25) (a) Sagar, P.; Shishodia, P. K.; Mehra, R. M.; Okada, H.; Wakahara, A.; Yoshida, A. J. Lumin. 2007, 126, 800. (b) Vanheusden, K.; Seager, C. H.; Warren, W. L.; Tallant, D. R.; Voigt, J. A. Appl. Phys. Lett. 1996, 68, 403. (c) Fujihara, S.; Ogawa, Y.; Kasai, A. Chem. Mater. 2004, 16, 2965. (26) Hurum, D. C.; Gray, K. A.; Rajh, T.; Thurnauer, M. C. J. Phys. Chem. B 2004, 108, 16483. (27) Wang, Y. H.; Hang, K.; Anderson, N. A.; Lian, T. Q. J. Phys. Chem. B 2003, 107, 9434. (28) Look, D. C.; Reynolds, D. C.; Sizelove, J. R.; Jones, R. L.; Litton, C. W.; Cantwell, G.; Harsch, W. C. Solid State Commun. 1998, 105, 399. (29) Frederikse, H. P. R. J. Appl. Phys. 1961, 32, 2211. (30) (a) Rudd, A. L.; Bresli, C. B. Electrochim. Acta 2000, 45, 1571. (b) Spathis, P.; Poulio, I. Corros. Sci. 1995, 51, 673. (31) Kislov, N.; Lahiri, J.; Verma, H.; Goswami, D. Y.; Stefanakos, E.; Batzill, M. Langmuir 2009, 25, 3310.

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