Efficient Photocatalytic Degradation of Organic Compounds by

Jun 11, 2009 - (15) Other Sb5+-containing photocatalytic oxides investigated for the photolysis of water under UV radiation include RuO2 loaded M2Sb2O...
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J. Phys. Chem. C 2009, 113, 12483–12488

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Efficient Photocatalytic Degradation of Organic Compounds by Ilmenite AgSbO3 under Visible and UV Light Irradiation Jyoti Singh and S. Uma* Materials Chemistry Group, Department of Chemistry, UniVersity of Delhi, Delhi-110007, India ReceiVed: February 25, 2009; ReVised Manuscript ReceiVed: May 11, 2009

Ilmenite AgSbO3 was synthesized by ion-exchange reaction of NaSbO3 with silver nitrate and characterized by powder X-ray diffraction, diffuse reflectance, surface area, and photocatalytic measurements. AgSbO3 exhibited unique optical absorption behavior capable of absorbing UV and visible light with a wavelength λ e 500 nm and with a clear tail extending up to 650 nm. Investigation of photocatalytic degradation of dyes, such as methylene blue (MB) and rhodamine B (Rh B), agreed very well with the optical absorption behavior and showed excellent activity under UV light (λ < 400 nm) radiation. Complete mineralization of MB and Rh B was observed within the first 30-40 min of UV light irradiation. Further, the single phase ilmenite AgSbO3 efficiently decomposed MB, Rh B, and 4-chlorophenol solutions under visible light radiation (400 nm e λ e 800 nm). The present investigation results point toward the possibility of an ilmenite form of silver antimony oxide as a potential visible-light-based photocatalyst for splitting water to produce hydrogen. 1. Introduction Exploration of materials to identify effective photocatalysts for the photolysis of water to produce hydrogen and for the decomposition of harmful organic contaminants has grown into an intense area of research in the past decade. The objective behind the study of these photocatalysts is to achieve efficient utilization of solar radiation to combat the global energy and environment-related concerns.1,2 The research on semiconductorbased photosensitive materials for solar hydrogen production was initiated from the first report of the TiO2-based photoelectrochemical cell by Fujishima and Honda.3 Subsequently, the significance of semiconductor photocatalysis for the degradation of harmful organic compounds was also recognized for environmental remediation.4-8 Highly stable, cost-effective oxide semiconductor TiO2 (Eg ∼ 3.2 eV), capable of mineralizing various organics, has been studied by many research groups.4-9 However, the limitation of TiO2 is that its photoexcitation occurs only with wavelengths near or shorter than 385 nm, and therefore, the need arises for photocatalytic materials which are visible and/or near UV light active to efficiently utilize the solar radiation. Numerous research attempts are currently being employed to modify the band gap of TiO2 by cation or anion doping.5-10 Another approach to discover new light-active catalysts has been to investigate different mixed metal oxides as photocatalysts for the production of hydrogen from water11 and also for the decomposition of several organic pollutants, including organic dyes that are released as effluents from the textile industries.12-21 Although TiO2 has been known to decompose dyes, such as rhodamine B (Rh B) and alizarin red, by photosensitization of the dyes under visible light, alternate catalysts are essential for the complete mineralization of dyes such as methylene blue (MB). MB, which is difficult to decompose under visible light irradiation, has been considered to be the model dye for testing the activities of various new photocatalytic materials.9,10,12-21 Among the several undoped mixed metal oxides that are known so far to decompose * To whom correspondence should be addressed. E-mail: suma@ chemistry.du.ac.in.

organic dyes, including MB, under visible radiation are Bi2WO6,12 BiVO4,13 CaBi2O4,14 NaBiO3,15 BaBiO3,16 CaIn2O4,17 Bi2GaTaO7,18 CaBiVMO8 (M ) W and Mo),19 and NiTiO3.20,21 Mixed metal oxides, such as PbSb2O6,22 Bi2Ti2O7,23 La2Sn2O7,24 ZnWO4,25 MxMoxTi1-xO6 (M ) Ni, Cu, Zn),26 LiBi4M3O14 (M ) Nb, Ta),27 BaBi2Mo4-xWxO16 (0.25 e x e 0.1),28 LnMo0.15V0.85O4 (Ln ) Ce, Pr, Nd),29 and GdCoO3,30 were investigated for the photodegradation of organic substances, including dyes, phenol, and cholorophenols under the irradiation of UV light. The criteria to develop new photocatalytic materials involve many controlling factors, such as the band gap, carrier transport, catalytic activity, surface-related adsorption properties, and chemical stability.31 Efficient carrier transport is required to prevent electron-hole recombination. Our aim was to investigate mixed metal oxides containing p-block elements, such as Sb5+ and Bi5+, with (n - 1)d10ns0 electronic configuration mainly because of the possibility of formation of a wide delocalized conduction band. In addition, the presence of an active Ag+ cation in the above-mentioned oxides might further reduce the band gap.32,33 Jinhua Ye and co-workers have shown that the pyrochlore silver antimony oxide with a band gap of 2.6 eV decomposed water to produce O2 in the presence of silver nitrate solution and also have carried out the successful decomposition of gaseous propanol under visible light (400 nm < λ < 530 nm) radiation.34 Ilmenite NaBiO3 containing Bi5+ with a band gap of 2.5 eV has been shown to decompose gaseous 2-propanol and aqueous MB solution under visible light.15 Other Sb5+-containing photocatalytic oxides investigated for the photolysis of water under UV radiation include RuO2 loaded M2Sb2O7 (M ) Ca, Sr), CaSb2O6, and ilmenite NaSbO3.35 In our search for visible-light-active mixed metal oxides, we investigated ilmenite AgSbO3 synthesized by low-temperature ion-exchange reaction from ilmenite NaSbO3. For the first time, we have demonstrated that ilmenite AgSbO3, considered to be the metastable form36 of pyrochlore AgSbO3, is a novel photocatalyst that can decompose MB under UV light and is also found to be an efficient visible light photocatalyst with activities equivalent to that of photocatalysts such as NaBiO3 under similar experimental conditions.

10.1021/jp901729v CCC: $40.75  2009 American Chemical Society Published on Web 06/11/2009

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2. Experimental Section The parent NaSbO3 was prepared by mixing and heating stoichiometric amounts of Sb2O3 (99%, Aldrich), analytical grade NaNO3 (Thomas Baker), and Na2CO3 (Merck), initially at 550 °C for 3 h, then it was reground and calcined at 850 °C for 12 h. The powder X-ray diffraction indicated that the product was essentially NaSbO3 in ilmenite form. The white solid was then ion-exchanged with 10% excess of AgNO3 at 300 °C for 2 h. AgSbO3 obtained after repeated washings with water was yellowish green in color.37 A QUANTA 200 FEG (FEI Netherlands) scanning electron microscope and Zeiss EVO-40 scanning electron microscope with EDX attachment were used to obtain the cation stoichiometry. As-received NaBiO3 · H2O (AR, Qualigens), which turned brown after drying at 200 °C, was used to carry out the photocatalytic experiments. The powder X-ray diffraction patterns were recorded using a Bruker D8 high-resolution diffractometer and PANalytical X’pert Pro diffractometer employing Cu KR radiation. The BrunauerEmmett-Teller (BET) surface area of the different samples was obtained from physical adsorption of N2 at 77K, using a Quantachrome Autosorb-1 analyzer. About 100 mg of a sample was utilized for this purpose, which was degassed under vacuum (106 Torr) at 300 °C, prior to N2 adsorption. The SEM micrographs of the samples were recorded in a JEOL 200 keV instrument. UV-visible diffuse reflectance data were collected over the spectral range of 200-1000 nm using a PerkinElmer Lamda 35 scanning double-beam spectrometer equipped with a 50 mm integrating sphere. BaSO4 was used as a reference. The data were transformed into absorbance with the KubelkaMunk function. Photocatalytic studies were carried out using a 450 W xenon arc lamp (Oriel, Newport, USA) along with a water filter to cut down IR radiation and glass cut off filters, Melles Griot03SWP602 to permit only UV light (λ < 400 nm) radiation and Melles Griot-03FCG057 to permit only visible light (400 nm e λ e 800 nm) radiation as desired. Irradiation was carried out over an external Pyrex container with a volume of 250 mL (9.5 cm height and 6 cm diameter), and water circulation was carried out to avoid any thermal effects. Light fluxes were established using ferrioxalate actinometer,38 and the incident photon rate for UV irradiation (λ < 400 nm) experiments was I0 ) 2.85 × 10-7 einstein s-1 and I0 ) 2.31 × 10-7 einstein s-1 for visible irradiation (400 nm e λ e 800 nm) experiments. The appropriate dye solution to be decomposed was taken along with the required amount of the catalyst in the Pyrex container and was constantly stirred to maintain a homogeneous suspension. The dyes and the organic compounds were dissolved in doubly distilled water. All the experiments were carried out at room temperature. A typical experiment of degradation was carried out as follows: The catalyst (0.5 g) was added to 150 mL of aqueous MB (pH ∼ 10) solution with an initial concentration of 3.5 × 10-7 mol/L for UV irradiation experiments and 1.75 × 10-7 mol/L for visible irradiation experiments. The initial concentration of Rh B was 1.5 × 10-7 mol/L along with 0.5 g of the catalyst for both UV and visible irradiation experiments. Prior to irradiation, the suspension of the catalyst and dye solution was stirred in the dark for 30-60 min so as to reach the equilibrium adsorption. Five milliliter aliquots were pipetted out periodically from the reaction mixture. The solutions were centrifuged, and the concentration of the solutions was determined by measuring the maximum absorbance (λmax ) 665 nm). For the experiments involving the decomposition of Rh B and 4-chlorophenol, the corresponding changes in the absorbance maximum around 552 and 280 nm, respectively, were

Figure 1. Powder X-ray diffraction patterns of (a) NaSbO3 and (b) AgSbO3.

followed. For comparison, Degussa P 25 TiO2 (0.1 g with a surface area of 55m2/g) and NaBiO3 (0.5 g) with ilmenite structure were also investigated under similar experimental conditions. The stability of the ilmenite AgSbO3 with respect to the MB dye solution under visible light irradiation was also determined by multiple cycles of MB degradation. The concentration of the MB solution was measured with time. Fresh MB solution was added after each cycle of MB decomposition in such a way to adjust the concentration to the initial concentration (1.75 × 10-7 mol/L). Multiple (up to six) cycles of photodecomposition have also been carried out under continuous O2 circulation because of the increased rates observed for successive cycles of decomposition. The photocatalyst thus obtained has been characterized by powder X-ray diffraction, scanning electron micrographs (SEM), and energy-dispersive X-ray analysis (EDAX). 3. Results and Discussion 3.1. Crystal Structure and Physical Properties. Figure 1 shows the powder X-ray diffraction patterns of NaSbO3 and the ion-exchanged AgSbO3. Both crystallize in an ilmenite structure (space group, R3j) with lattice parameters a ) 5.292(9), c ) 15.930(1) Å for NaSbO3 and a ) 5.325(7), c ) 16.694(1) Å for AgSbO3. The powder X-ray patterns were sharp, indicating the formation of completely Ag+-exchanged product.37 The absence of sodium ions in the ion-exchanged product along with the 1:1 stoichiometric ratio between silver and antimony were confirmed from the EDAX analysis (Figure S1a in the Supporting Information). The ilmenite structure of AgSbO3 was described as a layer structure wherein SbO6 and AgO6 octahedra are alternately stacked along the c-axis (Figure 2). The efficiency of a photocatalyst is proportional to the surface adsorption of the reactant on the catalyst surface, and so we have measured the surface areas of the synthesized materials. BET surface areas of NaSbO3 and AgSbO3 were found to be 1.9 and 10.4 m2/g, respectively. The increased surface area of the latter has been an added advantage of synthesizing the silver antimony oxide by ion-exchange process to carry out the photocatalytic reactions. The SEM image of the ilmenite AgSbO3 (Figure 3) showed particle agglomeration of the order of 1 µm, obtained as a result of the low-temperature method of synthesis. We also prepared the thermodynamically stable form of the defect pyrochlore oxide AgSbO3 by heating the ilmenite AgSbO3 in air around 780 °C for 2 h.36,37 The powder X-ray diffraction pattern of the product obtained after heating (Figure S2 in the Supporting Information) confirmed the formation of cubic pyrochlore AgSbO3 (lattice parameter, a ) 10.244(4) Å).

Degradation of Organic Compounds by Ilmenite AgSbO3

Figure 2. Crystal structure of ilmenite AgSbO3.

Figure 3. SEM image of AgSbO3.

Figure 4. Diffuse reflectance spectra of (a) NaSbO3, (b) ilmenite AgSbO3, (c) pyrochlore AgSbO3, and (d) NaBiO3. (Inset) Corresponding absorbance versus energy in eV.

Pyrochlore AgSbO3 was obtained as a pale green powder with a light yellowish tinge. Optical absorption behavior of AgSbO3 was investigated by UV-visible diffuse reflectance spectra. In Figure 4, we have shown the absorption spectra of ilmenite AgSbO3 along with that of NaSbO3, NaBiO3, and the pyrochlore AgSbO3 for comparison. The absorption spectra of white colored NaSbO3 agreed very well with the reported band gap of 4.9 eV.32 On the other hand, the absorption spectra of the ilmenite silver

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Figure 5. Photodegradation of methylene blue as indicated by the concentration (C0 is the initial concentration, and C is the concentration at any time, t) of MB with time under UV radiation: (a) MB blank, (b) MB on pyrochlore AgSbO3, (c) MB on NaBiO3, (d) MB on Degussa P25 TiO2, (e) MB on ilmenite AgSbO3, and (f) MB on ilmenite NaSbO3. (Inset) Absorption changes at 665 nm of methylene blue solutions in the presence of ilmenite AgSbO3.

antimony oxide began in the UV region and clearly extended into the visible region up to 520 nm, accompanied by a tapering tail caused mainly by the presence of lattice defects, whose precise origin is currently unclear. The influence of the cations, such as Zn, Cd, and Ag, on the electronic structure of the ilmenites has been explained in the case of oxides, such as ZnSnO3, CdSnO3, and AgBiO3.33 A similar cation effect has been known for the reduction in the band gap of the pyrochlore polymorph of AgSbO3 with an experimental band gap of 2.6-2.7 eV.32,34 The band structure calculations attributed the reduction in the band gap to the valence band formation consisting of Ag 4d and O 2p orbitals and a conduction band formation by the overlap of Ag 5s and Sb 5s orbitals.32,34 The optical absorption spectra of the ilmenite AgSbO3 agreed well with the Ag+ inductive effect, and the band gap estimated from the onset of the absorption edge was 2.5 eV and considered suitable as a visible-light-active photocatalyst. In addition, the ilmenite AgSbO3 oxide might be a potential catalyst for UV irradiation experiments because of its ability to absorb light radiation of λ g 300 nm (Figure 4). Whereas similar absorption edges were observed for ilmenite AgSbO3 and NaBiO3, that of pyrochlore AgSbO3 indicated a slight increase in the band gap (Figure 4). 3.2. Photocatalytic Properties. MB degradation over ilmenite AgSbO3 was investigated under UV (λ < 400 nm) radiation. The inset in Figure 5 shows the absorbance variations of MB solutions during the photodegradation experiments. The aqueous MB solution has the maximum absorbance around 660 nm, and in the presence of AgSbO3 catalyst, the absorbance decreased, suggesting the adsorption of MB occurs in the dark. Further decrease in the absorbance of MB occurred after UV light irradiation, and the solution turned colorless within 30 min of irradiation. Gradual decrease in the intensity at 665 nm confirmed the decomposition of the conjugated π system of the MB dye molecule. Similar experiments were carried out for the reference P-25 TiO2 and NaBiO3, and the concentrations of the MB solutions were plotted with time (Figure 5). The results clearly demonstrated that ilmenite AgSbO3 mineralized MB faster than NaBiO3 under similar experimental conditions. We further examined the activities of the parent ilmenite NaSbO3 oxide along with that of the pyrochlore AgSbO3, and both the oxides did not show any appreciable photodecomposition for MB under UV light. We also observed efficient degradation of aqueous Rh B in the presence of ilmenite AgSbO3 within 40

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Figure 6. Photodegradation of Rh B with time under UV radiation: (a) Rh B blank, (b) Rh B on pyrochlore AgSbO3, (c) Rh B on NaBiO3, (d) Rh B on Degussa P25 TiO2, (e) Rh B on ilmenite AgSbO3, and (f) Rh B on ilmenite NaSbO3. (Inset) Absorption changes at 552 nm of Rh B solutions in the presence of ilmenite AgSbO3.

Figure 8. Repeated decomposition of MB under visible radiation by ilmenite AgSbO3 under (top) ambient conditions and (bottom) under continuous O2 circulation. Figure 7. Photocatalytic decomposition of MB with time under visible radiation: (a) MB blank, (b) MB on pyrochlore AgSbO3, (c) MB on NaBiO3, (d) MB on Degussa P25 TiO2, (e) MB on ilmenite AgSbO3, and (f) MB on ilmenite NaSbO3. (Inset) Absorption changes at 665 nm of methylene blue solutions in the presence of ilmenite AgSbO3.

min of UV light irradiation, as seen from the decrease in the maximum absorbance at λ ) 552 nm (Figure 6). The decrease in the concentration of the dye solutions in the presence of the catalysts in the dark and also under direct irradiation of the solutions without the catalysts was negligible, supporting that the decompositions of MB and Rh B solutions were, indeed, due to the photocatalysis (Figures 5 and 6). The behavior of the different catalysts toward Rh B decomposition was also similar to that observed for MB decomposition. Photocatalytic decomposition of MB over ilmenite AgSbO3 was also investigated under visible light (400 nm e λ e 800 nm) radiation. The adsorption characteristics of the photocatalyst play a greater role, as evident from the decrease in the concentration of MB over ilmenite AgSbO3 (surface area, 10.4 m2/g) in the dark after 60 min (Figure 7). Accordingly, the catalyst attained a bluish tinge after adsorption in the dark. Subsequent irradiation of light decomposed the MB solution, and the catalyst regained its original color. Aqueous solutions of MB have been known to undergo self-photolysis under visible light radiation.14-16 To differentiate between the photocatalysis and just the photolysis of MB, experiments were carried out in the presence and in the absence of catalyst AgSbO3 under visible light irradiation. The rate of photolysis of the MB solution was definitely much lower than the photocatalytic decomposition of the MB solution by ilmenite AgSbO3. The catalyst efficiency has been found to be comparable with that of the already known visible-light-active NaBiO3 photocatalyst (Figure 7).

To study the stability of the photocatalyst, ilmenite AgSbO3, repeated experiments of MB decomposition were performed under visible radiation (Figure 8). The rate of decomposition decreased after three cycles of decomposition under ambient conditions (Figure 8 top). Previous photocatalytic studies have stressed the importance of a continuous supply of O2 for the decomposition of MB under visible radiation.39 Oxygen acts as an electron scavenger and has been known to prevent the recombination of holes and the electrons. In the present study, we could improve considerably our photocatalytic cycling efficiency when the experiments were carried out under continuous oxygen circulation (Figure 8 bottom). After six cycles of decomposition, the catalyst was characterized by powder X-ray diffraction and was similar to that of the as-prepared catalyst (Figure S3 in the Supporting Information). The refined lattice parameters obtained after photocatalysis (a ) 5.3272(6), c ) 16.695(2) Å) did not differ much from those obtained before photocatalysis. EDAX of AgSbO3 after photocatalysis did not show any variation in the ratio of Ag to Sb (Figure S1b in the Supporting Information). Particle agglomeration was observed in the SEM of the used catalyst (Figure S4 in the Supporting Information). These results clearly showed that the photocatalyst (ilmenite AgSbO3) was stable after six cycles of decomposition of MB and that there was no reaction between the organic dye (MB) and the photocatalyst. The efficiency of the photocatalyst was also investigated for the decomposition of Rh B and 4-chlorophenol solutions under visible radiation. Here again, ilmenite AgSbO3 showed the maximum decomposition activity (Figures 9 and 10). The observed decomposition of 4-chlorophenol under visible light irradiation is significant because of the fact that 4-chlorophenol by itself does not absorb in the visible region, thereby excluding

Degradation of Organic Compounds by Ilmenite AgSbO3

J. Phys. Chem. C, Vol. 113, No. 28, 2009 12487 though, in the present study, we could obtain the pyrochlore polymorph at temperatures as low as 780 °C by heating ilmenite AgSbO3,we did not observe any degradation of the organic compounds by the pyrochlore oxide. These observations suggest that probably the photocatalytic activities were very sensitive to the amount of silver and might change drastically even to a small variation in the amount of silver. On the other hand, the low-temperature ion-exchanged ilmenite AgSbO3 was expected to have a Ag/Sb ratio of 1.00 as per the stoichiometry of the parent NaSbO3 and is probably one of the reasons for its efficient photocatalytic behavior. 4. Conclusions

Figure 9. Photocatalytic decomposition of Rh B with time under visible radiation: (a) Rh B blank, (b) Rh B on pyrochlore AgSbO3, (c) Rh B on NaBiO3, (d) Rh B on Degussa P25 TiO2, (e) Rh B on ilmenite AgSbO3, and (f) Rh B on ilmenite NaSbO3. (Inset) Absorption changes at 552 nm of Rh B solutions in the presence of ilmenite AgSbO3.

Figure 10. Temporal changes of 4-chlorophenol (0.5 × 10-7 mol/L) decomposition over 0.5 g of ilmenite AgSbO3 as monitored by the UV-visible absorption spectra at 280 nm under visible radiation. (Inset) Corresponding absorption spectra over pyrochlore AgSbO3.

the possible indirect semiconductor photocatalysis, unlike the MB and Rh B dye solutions that absorb in the visible region.40 The decrease in the concentrations of 4-chlorophenol as determined from its absorbance at 280 nm has been shown in Figure 10. In addition, we were interested in the photocatalytic decomposition of the dye solutions over the thermodynamically stable pyrochlore polymorph of AgSbO3. The pyrochlore silver antimony oxide synthesized by the solid-state high-temperature method has been identified as a visible photocatalyst possessing a strong oxidizing potential and has been shown to decompose gaseous propanol and acetone and also to evolve O2 from silver nitrate solution.34 However, under the present experimental conditions, the pyrochlore oxide synthesized from the corresponding ilmenite silver antimony oxide did not efficiently decompose MB solution neither under UV (Figure 5) nor under visible radiation (Figure 7). Similar behavior was observed for the other organic probes, such as Rh B, under UV (Figure 6) and visible light (Figure 9). Furthermore, experiments under visible irradiation for the decomposition of 4-chlorophenol over pyrochlore AgSbO3 also showed no significant activity (Figure 10). Formation of AgxSbO3 with a Ag/Sb ratio ranging from 0.99 to 1.02 has been reported during the photocatalytic testing of pyrochlore AgSbO3.34 The extent of photocatalytic activities was found to vary in the order Ag1.00SbO3 > Ag1.02SbO3 > Ag0.99SbO3. Partial volatilization of silver during high-temperature (900 °C) synthesis from Ag2O and Sb2O3 has been attributed to this observed difference in the silver content. Even

We investigated the photophysical and photocatalytic properties of ilmenite AgSbO3. The electronic structure is comparable to that of the pyrochlore AgSbO3 wherein the top of the valence band originated from Ag 4d and O 2p, whereas the bottom of the conduction band originated from the hybridized Ag 5s and Sb 5s orbitals. Accordingly, the optical absorption of AgSbO3 extends into the visible region. The photooxidation activities were carried out using organic molecules, such as MB and Rh B, under UV radiation and using MB, Rh B, and 4-chlorophenol under visible radiation. The catalyst showed excellent decomposition rates for the organic compounds under UV and visible light irradiation. The visible light activity has been attributed to the electronic structure of the Sb5+ ion along with Ag+ ion. The results point toward the possibility of ilmenite AgSbO3 as a potential photocatalyst to split water under visible irradiation. Acknowledgment. The present research is supported by the Department of Science and Technology, Government of India. The authors also thank the University of Delhi for the financial support in terms of chemicals and laboratory supplies. Also, thanks are due to Professors A. K. Ganguli and A. Ramanan of the Indian Institute of Technology (IIT-Delhi) for the use of the XRD facility. We thank Dr. A. K. Tyagi, Baba Atomic Research Center, Mumbai, India, for surface area measurements and Dr. R. Nagarajan for extending some of the experimental facilities. We also thank Ms. Mamta Kharkwal for the assistance in some experiments. Supporting Information Available: The EDAX analysis data for the ilmenite AgSbO3 before and after photocatalysis (Figure S1), powder XRD pattern of the pyrochlore AgSbO3 (Figure S2), powder XRD patterns of the catalyst AgSbO3 before and after adsorption of MB and after photodecomposition of MB under visible or UV light (Figure S3), SEM images of the ilmenite AgSbO3 before and after photocatalysis (Figure S4). This material is available free of charge via the Internet at http:// pubs.acs.org. References and Notes (1) Lewis, N. S.; Nocera, D. G. Proc. Natl. Acad. Sci. U.S.A. 2006, 103, 15729–15735. (2) Nowotny, J.; Sorrell, C. C.; Bak, T.; Sheppard, L. R. Sol. Energy 2005, 78, 593–602. (3) Fujishima, A.; Honda, K. Nature 1972, 238, 37–38. (4) Pruden, A. L.; Ollis, D. F. J. Catal. 1983, 82, 404–417. (a) Hsiao, C. Y.; Lee, C. L.; Ollis, D. F. J. Catal. 1983, 82, 418–423. (5) Hoffman, M. R.; Martin, S. T.; Choi, W.; Bahnemann, D. W. Chem. ReV. 1995, 95, 69–96. (6) Yates, J. Chem. ReV. 1995, 95, 735–758. (7) Fox, M. A.; Dulay, M. T. Chem. ReV. 1993, 93, 341–357. (8) Mills, A.; Hunte, S. L. J. Photochem. Photobiol., A 1997, 108, 1– 35. (9) Carp, O.; Huisman, C. L.; Reller, A. Prog. Solid State Chem. 2004, 32, 33–177.

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