Improvement of Photocatalytic Degradation Activity of Visible-Light

Mar 18, 2009 - Improvement of Photocatalytic Degradation Activity of Visible-Light-Responsive TiO2 by Aid of Ultraviolet-Light Pretreatment. Taizo San...
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J. Phys. Chem. C 2009, 113, 5535–5540

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Improvement of Photocatalytic Degradation Activity of Visible-Light-Responsive TiO2 by Aid of Ultraviolet-Light Pretreatment Taizo Sano,*,† Eric Puzenat,‡ Chantal Guillard,‡ Christaphe Geantet,‡ Sadao Matsuzawa,† and Nobuaki Negishi† Research Institute for EnVironmental Management Technology, National Institute of AdVanced Industrial Science and Technology (AIST), 16-1 Onogawa, Tsukuba, Ibaraki 305-8569, Japan, and Institut de recherches sur la catalyse et l’enVironnement de Lyon (IRCELYON), 2 AVenue Albert Einstein, Villeurbanne Cedex 69626, France ReceiVed: September 10, 2008; ReVised Manuscript ReceiVed: January 30, 2009

The photocatalytic degradation activity of visible-light-responsive TiO2, which was developed as oxygendeficient TiO2, was improved by pretreatment with UV-light irradiation in the presence of oxygen and adsorbed water. The C2H2 degradation rate under visible light was increased up to twice by the UV pretreatment. During the UV pretreatment, peroxo species was produced on the surface, and the visible-light absorbance of the photocatalyst between 400 and 500 nm was increased. The modification of the surface accompanied by the formation of peroxo species seems to enhance the photocatalytic oxidation of C2H2. The increased activity was gradually diminished within 24 h under dark condition, suggesting that the modified surface is metastable and is reverted to the original surface. Introduction Titanium dioxide (TiO2) has been getting attention as a photocatalyst that can be utilized for the oxidative removal of environmental pollutants, such as NOx and VOC, under ultraviolet (UV) irradiation (λ < 380 nm).1-3 However, the activity of TiO2 in a closed environment is often low since UV light, which is essential for activating pure TiO2, is insufficient. Therefore, many researchers are making efforts to develop visible-light-responsive photocatalysts. The major part of indoor light is visible light, and therefore an efficient utilization of visible light will improve the degradation rate of the pollutants. Because TiO2 has many advantages against other photocatalysts, such as high photostability, high resistance for acid and base, and low price, the modification of TiO2 is an attractive method to develop a visible-light-responsive photocatalyst. Recently, the replacement of the oxide ion of the TiO2 lattice with a nitrogen atom or an oxygen defect has been studied to improve the visible-light response.4-9 Nitrogen-doped TiO2 (NTiO2) is prepared by reduction with gaseous NH3, spattering, or hydrolysis of titanium salt by ammonia solution.4,5 We also succeeded in the preparation of different types of N-doped TiO2 from the titanium-bipyridine complex (TBC).6 The newly prepared TiO2 oxidized NO under visible light with longer wavelengths (λ < 650 nm) and the oxidation rate at wavelengths above 450 nm was more than 10 times that by conventional N-TiO2. However, the photocatalyst derived from TBC did not degrade acetaldehyde, while conventional N-TiO2 decomposed acetaldehyde to CO2 under visible light. The presence of a carbon atom in the anatase lattice is proposed to explain the different response to visible light from the conventional N-TiO2. The H2-plasma treatment on an Ultrafine particle of TiO2 caused the creation of oxygen defects in the anatase lattice, and * To whom correspondence should be addressed. E-mail: sano-t@ aist.go.jp. † National Institute of Advanced Industrial Science and Technology (AIST). ‡ Institut de recherches sur la catalyse et l’environnement de Lyon (IRCELYON).

formed oxygen-deficient TiO2 (TiO2-δ) oxidized NO under visible light up to ca. 600 nm.7 Ihara et al.8 reported the synthesis of oxygen-deficient TiO2 in the aqueous phase. They concluded that oxygen-deficient sites formed in grain boundaries are important to develop the visible-light responsibility and that the nitrogen atom doped in a portion of the oxygen defects worked as a blocker for reoxidation. BA-PW25 (Ecodevice Co.) is an anatase-type photocatalyst commercialized as an oxygen-deficient TiO2. BA-PW25 is prepared by the calcinations of titanium hydroxide precursor made from titanium(IV) sulfate with ammonia solution.9 This preparation method is quite similar to that described in the report by Ihara et.al.8 BA-PW25 degrades 2-propanol and acetone into CO2 under blue LED light, and the visible-light response was attributed to the oxygen deficiency of TiO2. We have studied the photocatalytic degradation of C2H2,10,11 since the oxidation of C2H2 is much more difficult than that of acetaldehyde and therefore it reveals the oxidative power of each photocatalyst. Under visible light (λ > 400 nm), BA-PW25 decomposed C2H2 into CO2 while N-TiO2 and conventional TiO2 (P25, Degssa) did not degrade C2H2.10 The formation of formic acid on the surface of BA-PW25 was transiently observed, and the formic acid was decomposed into CO2 by further visiblelight irradiation (eq 1).

C2H2 f (HCOOH) f CO2 + H2O

(1)

It is inferred that the active species effective for C2H2 degradation is formed on the midgap level of BA-PW25 between the top of the valence band of conventional TiO2 and the N-induced midgap level. The UV-vis absorption edge of BA-PW25 located at a slightly longer wavelength (400 nm) than that of N-TiO2 prepared with gaseous NH3 treatment of TiO2, although XRD, XPS, FT-IR, and TG did not show any clear evidence for the presence of oxygen deficiency or any special structures in the bulk. Therefore, it is inferred that the novel degradation activity of BA-PW25 is due to the anomalous surface morphology as mentioned in the previous report.8

10.1021/jp808032y CCC: $40.75  2009 American Chemical Society Published on Web 03/18/2009

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Figure 1. Experimental procedure for C2H2 degradation with visible light.

BA-PW25 showed a novel phenomenon in our recent experiments. It was found that the photocatalytic degradation rate of C2H2 or acetaldehyde with visible light was improved by UV irradiation (UV pretreatment) to the BA-PW25 before the degradation experiment. If the conventional TiO2 was used, this phenomenon can be hardly observed since the conventional TiO2 does not degrade C2H2 or acetaldehyde under visible light in the first place. Thus, BA-PW25 showed the novel photocatalytic activities; however, the mechanisms of visible-light responsibility of BA-PW25 have not been well-understood. In this research, we studied the effect of UV pretreatment on the C2H2 degradation activity of visible-light-responsive TiO2 (BA-PW25), and developed an understanding of its novel characteristics. We also analyzed the roles of the adsorbed water on the photocatalyst, the atmosphere, and peroxo species formed on the surface during the UV pretreatment. Experimental Section The degradation of C2H2 was analyzed with a batch reactor.10 The typical experimental procedure with UV pretreatment was simply shown in Figure 1. A 40 mg portion of visible-lightresponsive photocatalyst (BA-PW25 (Ecodevice Co.)) was suspended in 1 cm3 of distilled water and the suspension was dispersed on a glass plate (9.0 cm2), followed by drying in air at 50 or 110 °C for 24 h. The samples dried at 50 and 110 °C were denoted as S50 and S110, respectively. The glass plate with photocatalyst was placed in a reactor (320 cm3) with a cover window of Pyrex glass. The temperature of the reactor was maintained at 15 °C by cooling water. A light source (highpressure Hg lamp (Philips HPK-125 W)) was placed on the upper side of the reactor. A water cell was placed between the reactor and the light source to absorb infrared light. When only visible light is used for irradiation, a UV-cut filter (Sumitomo kagaku, LF-41) was placed under the water cell. The transmittance of LF-41 at 380, 400, and >430 nm was 0.05%, 1%, and 90%, respectively. In the UV preatment of photocatalyst, UV light was irradiated to the photocatalyst sample for 30 min to 3 h while synthetic air was passed through the reactor. The UV intensity was 3.4 mW/cm2 (2.1 × 10-8 mol/cm2/s (300 < λ < 380 nm)). When the activity without UV pretreatment was evaluated, synthetic air was passed for 30 min in darkness. After the pretreatment, the reactor was closed and then the required amount of C2H2 gas (Air Liquide) was introduced. The reaction atmosphere was vigorously stirred by a magnetic stirrer to make the atmosphere homogeneous. Any decreases in the C2H2 concentration by adsorption were not observed in all the experiments in this study. The degradation of C2H2 was performed by irradiating visible light. The photon flux at the sample surface was 3.1 × 10-8 mol/cm2/s (380 < λ < 600 nm). The UV intensity was less than the detection limit (1 µW/cm2) of the UV analyzer (Vilber Lourmat, VLX-3W with CX365). The concentrations of C2H2 and CO2 were analyzed by gas chromatographs with a flame

Figure 2. Degradation of C2H2 by visible-light-responsive photocatalyst (BA-PW25) under visible light (λ > 400 nm). Before the first, second, and sixth run, UV light was irradiated to the photocatalyst for 1 h.

ionization detector (Intersmat: IGC120FL) and a thermal conductivity detector. The solid phases were mainly analyzed with UV-vis diffuse reflectance spectroscopy (Perkin-Elmer, Lambda 45). H2O2 was analyzed with a colorimetric method, using N,N-diethyl-pphenylenediamine (DPD) and horseradish peroxidaze (POD).12,13 A 3 cm3 sample solution was fed into the quartz cell, and solutions of DPD (10 g/dm3, 0.030 cm3) and POD (1 g/dm3, 0.030 cm3) were added in sequence with vigorous mixing. The absorbance at 551 nm was measured with a UV-vis-NIR spectrometer (Shimadzu UV-3600) after 30 s of addition of POD. Results and Discussion Figure 2 shows the time courses of C2H2 concentration in the reactor, in which the visible-light-responsible photocatalyst (S50) was placed. The C2H2 concentration decreased while the photocatalyst was irradiated with visible light. When the C2H2 concentration became lower than ca. 30 ppm, the reaction atmosphere was replaced with synthetic air, and then the degradation of C2H2 was repeatedly performed. Before the first, second, and sixth runs, the photocatalyst was irradiated with UV light for 1 h (UV pretreatment), and the degradation with visible light was observed. In the third-fifth runs, the C2H2 degradation was performed without UV pretreatment. It is obvious that the C2H2 concentration decreased rapidly after UV pretreatment. This suggests that the characteristics of the sample were changed by absorbing UV light and that the sample became more active for C2H2 degradation. Meanwhile, the UV pretreatment was not essential for the degradation of C2H2 since the sample repeatedly degraded C2H2 at a constant rate without UV pretreatment. The effect of UV pretreatment was further analyzed. Figure 3a shows the decrease in the C2H2 concentration under visible light with the photocatalyst dried at 50 °C (S50). S50 was irradiated with UV light or visible light before C2H2 degradation, and the degradation rates were compared. The degradation rate by the samples after UV pretreatment for 1 h was significantly larger than that by the sample without light pretreatment. Furthermore, the UV pretreatment for 17 h provided a higher degradation rate than the treatment for 1 h. On the other hand, the pretreatment with visible light did not change the degradation rate. Thus, the C2H2 degradation activity of S50 was improved by the UV pretreatment. Next, the effect of UV pretreatment on the photocatalyst dried at 110 °C (S110) was analyzed

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Figure 5. Decay curve of C2H2 degradation rate with keeping time in darkness. The degradation rate was obtained under visible light.

Figure 3. Degradation of C2H2 under visible light by the samples dried at 50 (a) and 110 °C (b).

Figure 4. Increase in degradation rate of C2H2 with UV-pretreatment time.

(Figure 3b). The degradation rate by S110 was higher than that by S50 without UV pretreatment. This suggests that the dried surface is favorable for the degradation of C2H2. In the case of S110, the rates for decrease in C2H2 concentration were almost the same regardless of the pretreatments, and the activity was not improved by the UV pretreatment. The difference between the effects of UV pretreatment on the S50 and S110 suggests that adsorbed water has an important role for improving photocatalytic activity by UV pretreatment. Further discussions are given later. Figure 4 shows the relation between the initial rate of C2H2 degradation by S50 and UV pretreatment time. The degradation rate increased significantly with the UV pretreatment time between 0 and 3 h, and the effect of UV pretreatment was saturated. The maximum degradation rate was approximately

twice that without UV pretreatment. Also, the samples that were UV irradiated for more than 1 h exhibited higher activities than S110. The improved activity of S50 by UV pretreatment was gradually diminished by keeping the sample without light in dry air (Figure 5). The degradation rate of the sample after 24 h from the UV pretreatment was equal to that before UV pretreatment. This suggests that the condition of photocatalyst was reverted to the original condition and that the improved activity is due to a reversible reaction. The lifetime of the improved activity was much longer than those of radicals formed by TiO2 (e.g., O2-• ∼200 s, OH• ∼1 s),13 and the time scale is relatively close to the superhydrophilicity.14 The photocatalyst (S50) after 3 min from UV pretreatment for 2 h was contacted with C2H2 in a dark condition. The concentration of C2H2 did not change. This indicates that an active species that can directly oxidize C2H2 was not present on the photocatalyst after 3 min from the UV pretreatment. It is inferred that the surface of photocatalyst was modified by UV-light absorption and that a metastable surface structure, whose lifetime is several hours, was formed. The metastable structure possibly enhances the production of active species for C2H2 degradation with visible light. In the UV-vis diffuse reflectance spectra of S50 and S110, the absorption shoulder was observed in the visible light region between 400 and 550 nm neighboring the UV absorption (Figure 6). The absorbance (∝ Kubelka-Munk function, if the scattering coefficient is identical) of S50 between 400 and 500 nm was increased significantly by the UV irradiation in air for 1 h. The visible-light irradiation on S50 also increased the absorbance in the similar wavelength range. The irradiations increased the absorbance in the visible-light region by approximately 1.5 times. Since the increment did not coincide with the increase in the degradation rate (ca. 2 times for UV pretreatment and 0 times for visible pretreatment), the increase in the degradation rate did not directly depend on the increase in the number of photons absorbed. Remarkably, the spectrum of S110 was not changed by the UV irradiation. This is similar to the result that the activity of S110 was not changed by UV pretreatment while that of S50 was improved. Since the effect of UV pretreatment on the samples dried at 50 °C (S50) and 110 °C (S110) was different, the characteristics of the BA-PW25 photocatalyst should be changed during heating the sample from 50 to 110 °C. Figure 7 shows the TG-DTA curves of S50. The rapid weight loss with the endothermic process was observed between 50 and 100 °C. Then, the weight of sample decreased slowly above 100 °C. Neither significant

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Figure 8. Effect of rehydration to S110 on C2H2 degradation. The photocatalyst dried at 110 °C (S110) was rehydrated with a small amount of water, followed by drying at 50 °C and UV irradiation for 3 h. Degradation was performed under visible light.

Figure 6. UV-vis diffuse reflectance spectra of photocatalyst powder dried at 50 °C (S50) or 110 °C (S110) before and after UV (or visible light) irradiation for 1 h. The Kubelka-Munk function is proportional to the absorption coefficient if the scattering coefficient is identical.

Figure 7. Thermogravimetry with differential thermal analysis (TGDTA) on BA-PW25 (S50) recorded in an air stream at a heating rate of 2 deg/min.

increase in the weight nor exothermic process was observed. The weight loss between 50 and 100 °C is due to the release of adsorbed water in layer II, which was proposed by Nosaka et al.15 The adsorbed water in layer II is less mobile and harder to volatilize than the water in the outermost layer (layer III), and is weakly stabilized compared with the water in the closest layer to the solid surface (layer I). Water in layers III and II is released below 50 and 100 °C, respectively, and that in layer I is not released at around 100 °C. Therefore, S50 contained water in layers I and II, and S110 contained only water in layer I. As

Figure 9. Effect of atmosphere during UV pretreatment on degradation of C2H2. The degradation of C2H2 was observed under visible light in air.

described above, the activity and the visible-light absorbance of S50 are changed with UV pretreatment while those of S110 were not changed. Therefore, the adsorbed water in layer II seems to be responsible for the improvement of the C2H2 degradation activity and the visible-light absorbance with UV pretreatment. The effect of adsorbed water was further confirmed by the rehydration experiment. A small amount of distilled water was added to S110 powder at room temperature and then the powder was dried at 50 °C. The C2H2 degradation rate by the rehydrated powder was almost the same as that by S50. Additionally, the increments in the degradation rate by UV pretreatment on the rehydrated powder and S50 were similar (Figure 8). These results support the speculation that the water in layer II is essential for the improvement of the activity with UV pretreatment. Furthermore, it is inferred that the drying process between 50 and 110 °C is not an irreversible process like a change in the bulk structure but a reversible process around the neighborhood of the surface. Oxygen was essential for the improvement of C2H2 degradation activity by UV pretreatment. When the UV pretreatment on S50 was performed in a nitrogen gas stream instead of an air stream, the degradation rate was almost the same as that without UV pretreatment (Figure 9). Also, the absorption spectrum of S50 was not changed by UV irradiation in nitrogen (Figure 10). Generally, an oxygen molecule enhances photocatalytic processes by accepting a conduction band electron

Improvement of Photocatalytic Degradation Activity

J. Phys. Chem. C, Vol. 113, No. 14, 2009 5539 and Ti-O-OH, is reported.18 The formation mechanisms were estimated as follows:

TisOsTi · + H2O + h+ f TisO · + TisOH + H+

(9) 2[TisO · + TisOH] f TisOsOsTi + TisOsTi + H2O (10) TisOsOsTi + H2O f TisOsOH + TisOH (11)

Figure 10. UV-vis diffuse reflectance spectra after UV treatment in air and nitrogen gas.

formed by UV-light absorption. It is considered that the lack of oxygen prohibited the photocatalytic process that improves the C2H2 degradation activity and that increases the visiblelight absorbance. The improvement of degradation activity is probably related to the increase in the visible light absorbance by UV pretreatment, since the factors necessary for improving the degradation activity were very similar to the factors for increasing visible light absorbance: oxygen, adsorbed water in layer II, and UV light were necessary. And these results indicate that the observed behaviors are due to the photoinduced reaction with adsorbed water and oxygen. However, there is one pretreatment condition that increased the visible-light absorbance but did not change the activity. The visible-light pretreatment increased the visiblelight absorbance, whereas it did not change the degradation activity. Therefore, it is considered that the change in the visiblelight absorbance is partially related to the improvement of the degradation activity. The participation of peroxo species was analyzed, since the production of hydrogen peroxide species is one of the important reactions with photocatalyst, water, and oxygen.2,16-18 It is proposed that H2O2 is produced photocatalytically as shown by the following equations.

TiO2 + hν f e- + h+

(2)

e- + O2 f O2- ·

(3)

h+ + H2O f OH · + H+

(4)

O2- · + H+ + e- f HO2

(5)

2O2- · + H+ f HO2 + O2

(6)

HO2- + H+ f H2O2

(7)

OH · + OH · f H2O2

(8)

where adsorbed species and free species were not distinguished. Also, the formation of surface peroxo species, Ti-O-O-Ti

Since the adsorbed water and oxygen were essential for the improvement of the activity by UV pretreatment, there is a high possibility that the peroxo species are involved in the improvement of activity. On the basis of the above consideration, the production of peroxo species by BA-PW25 was analyzed. A 50 mg sample of photocatalyst was dispersed into Millipore water and coated on the Petri dish (26 cm2). The dish was dried at 50 °C for 24 h. The dried photocatalyst was irradiated with UV light (3.0 mW/cm2) or visible light (6 × 103 Lx, ca. 5 mW/ cm2) passed through the UV cut filter for 1 h in air (relative humidity 50%). After the irradiation was finished, 10 cm3 of sodium phosphate buffer (pH 6.0) was added to the Petri dish and the product was extracted with a shaker for 20 min. After filtration, the concentration of H2O2 was analyzed by the colorimetric method with DPD and POD. In this procedure, chemical species that react with water to form H2O2 are counted as H2O2. In other words, surface peroxo species, Ti-O-O-Ti and Ti-O-OH, could be detected as H2O2. For the photocatalyst samples irradiated with UV and visible light, 7.3 × 10-4 and 5.0 × 10-4 µmol/cm2 of H2O2 were detected, respectively. Without light irradiation, H2O2 was not produced. Thus, it was confirmed that BA-PW25 produced peroxo species on the surface by absorbing UV or visible light. Since the production was much smaller than the decomposed C2H2 (ca. 9 × 10-2 µmol/cm2 in 2 h), it seems that the peroxo species produced are not major reactants for the oxidation of C2H2. Alternatively it is possible that peroxo species modify the surface structure (or the band structure) of TiO2 to increase the photocatalytic efficiency with visible light. The following changes can be expected to increase the efficiency: increase in surface defect site, enhancement of charge separation, formation of midgap level, increase in the number of active sites or adsorption sites, etc. As shown in Figure 5, the improved activity diminished within 24 h. This suggests that the modified surface is metastable and is gradually reverted to the original surface. At present, the time course of peroxo species has not been analyzed, and moreover, the extraction efficiency of peroxo species is uncertain. Further analyses of peroxo species are required to conclude the exact role of the peroxo species for the C2H2 degradation. Next, the effect of supplementation of H2O2 to the photocatalyst was analyzed to study influences of peroxo species on the photocatalyst. A 4 cm3 sample of H2O2 solution (5 mmol/ dm3) was added to 1.0 g of S50 powder, and then the sample was dried at 50 °C for 24 h. Figure 11 shows the diffuse reflectance spectra of S50 before and after H2O2 treatment. The visible light absorbance between 400 and 500 nm was increased significantly by the H2O2 supplementation. The change was consistent with the increase in the visible light absorbance with UV pretreatment and visible pretreatment on S50 (Figure 6). This result supports an assumption that the light pretreatment

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Sano et al. in the UV pretreatment is affected by the organic impurities. This will be discussed elsewhere. Conclusion

Figure 11. UV-vis diffuse reflectance spectra of photocatalyst (S50) and S50 treated with H2O2.

Figure 12. Formation of metastable surface structure by UV pretreatment. The visible-light absorbance of the photocatalyst accompanied by peroxo species is higher than that of the original one.

produced H2O2 or other peroxo species which increase the visible-light absorbance as shown in Figure 12. However, the C2H2 degradation rate was not improved by the visible-light pretreatment, although peroxo species were detected and the visible-light absorbance was increased. Therefore, the production of peroxo species is not a sufficient factor for the improvement of C2H2 degradation activity. We speculate that the kind of surface peroxo species formed with visible light is different from that with UV light and that only the latter is effective for the improvement of activity. Additionally, we infer that the peroxo species formed with visible-light pretreatment were also produced during the degradation of C2H2 under visible light even without the pretreatment and therefore the formation of peroxo species with visiblelight pretreatment was not beneficial to improve the activity. In this stage, the peroxo species formed with UV and visible light have not been characterized yet except for their activities. Also, the specific surface structure of BA-PW25 has not been well analyzed yet although it is strongly related to the formation of effective peroxo species since the degradation activity and the visible-light absorbance of conventional TiO2 (P25) were not changed by UV pretreatment. To characterize the effective peroxo species and the specific surface structure of BA-PW25, further analyses of the surface modified by UV pretreatment, such as in situ infrared or near-infrared spectroscopic, and electric microscopic techniques, are necessary. It is known that H2O2 production is enhanced by organic substances as electron donors.2 In this study, organic substances were not fed into the system during UV pretreatment. However, commercialized photocatalysts often contain organic impurities on the surface. It is possible that the formation of peroxo species

The new aspects about UV pretreatment on the visible-lightresponsive TiO2 (BA-PW25) were confirmed. By UV pretreatment, in which UV light was irradiated to the photocatalyst in the presence of oxygen and the adsorbed water in layer II, the C2H2 degradation activity under visible light was improved, and simultaneously, the visible-light absorbance between 400 and 500 nm was increased. These phenomena seem to be due to the metastable surface structure accompanied by the formation of peroxo species induced by UV-light absorption. It is considered that the peroxo species were not the major reactants for C2H2 degradation but the formation of peroxo species improved the photocatalytic efficiency. Unfortunately, the mechanism of C2H2 degradation by using the modified surface has not been clarified yet. Further analyses of the modified surface using spectroscopic and microscopic techniques seem interesting future works to distinguish the effective peroxo species for improving the degradation activity. If a photocatalyst containing a lot of the effective peroxo species can be developed, visible light will be utilized more efficiently for the photoinduced degradation of organic pollutants. Additionally, the UV pretreatment would be an attractive method to improve the degradation activity of visible-light-responsive photocatalyst without chemical reagent. Acknowledgment. This work was partially supported by the ECSAW program (Environmental Catalysis for Sustaining Clean Air and Water) from 2005 to 2006. We greatly appreciate Dr. J. M. Herrmann and all the friends of Institut de recherches sur la catalyse et l’environnement de Lyon (IRCELYON). References and Notes (1) Ibusuki, T.; Takeuchi, K. J. Mol. Catal. 1994, 88, 93–102. (2) Hoffmann, M. R.; Martin, S. T.; Choi, W. Y.; Bahnemann, D. W. Chem. ReV. 1995, 95, 69–96. (3) Sano, T.; Negishi, N.; Takeuchi, K.; Matsuzawa, S. Solar Energy 2004, 77, 543–552. (4) Sato, S. Chem. Phys. Lett. 1986, 123, 126–128. (5) Asahi, R.; Morikawa, T.; Ohwaki, T.; Aoki, K.; Taga, Y. Science 2001, 293, 269–271. (6) Sano, T.; Negishi, N.; Koike, K.; Takeuchi, K.; Matsuzawa, S. J. Mater. Chem. 2004, 14, 380–384. (7) Nakamura, I.; Negishi, N.; Kutsuna, S.; Ihara, T.; Sugihara, S.; Takeuchi, E. J. Mol. Catal. A: Chem. 2000, 161, 205–212. (8) Ihara, T.; Miyoshi, M.; Iriyama, Y.; Matsumoto, O.; Sugihara, S. Appl. Catal. B: EnViron. 2003, 42, 403–409. (9) Sugihara, S. Kogyo Chousakai, Tokyo 2003, 122. (10) Sano, T.; Puzenat, E.; Guillard, C.; Geantet, C.; Matsuzawa, S. J. Mol. Catal. A: Chem. 2008, 284, 127–133. (11) Thevenet, F.; Guaitella, O.; Herrmann, J.; Rousseau, A.; Guillard, C. Appl. Catal. B: EnViron. 2005, 61, 58–68. (12) Bader, H.; Sturzenegger, V.; Hoigne, J. Water Res. 1988, 22, 1109– 1115. (13) Fukushima, M.; Tatsumi, K. Talanta 1998, 47, 899–905. (14) Watanabe, T.; Nakajima, A.; Wang, R.; Minabe, M.; Koizumi, S.; Fujishima, A.; Hashimoto, K. Thin Solid Films 1999, 351, 260–263. (15) Nosaka, A.; Nosaka, Y. Bull. Chem. Soc. Jpn. 2005, 78, 1595– 1607. (16) Nosaka, Y.; Nakamura, M.; Hirakawa, T. Phys. Chem. Chem. Phys. 2002, 4, 1088–1092. (17) Hirakawa, T.; Kominami, H.; Ohtani, B.; Nosaka, Y. J. Phys. Chem. B 2001, 105, 6993–6999. (18) Nakamura, R.; Nakato, Y. J. Am. Chem. Soc. 2004, 126, 1290–1298.

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