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J. Phys. Chem. C 2009, 113, 9950–9955
The Doping Effect of Bi on TiO2 for Photocatalytic Hydrogen Generation and Photodecolorization of Rhodamine B Yuqi Wu,†,‡ Gongxuan Lu,*,† and Shuben Li† State Key Laboratory for Oxo Synthesis and SelectiVe Oxidation, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, 730000, Lanzhou, China, and Graduate UniVersity of Chinese Academy of Sciences, Beijing, 100049, China ReceiVed: February 2, 2009; ReVised Manuscript ReceiVed: March 23, 2009
The dependence of photoactivity on Bi doping on TiO2 was studied for photocatalytic hydrogen generation and rhodamine B (RhB) decolorization. After Bi doping, the hydrogen photogeneration rate increased by a factor of ca. 10, and the decolorization rate for RhB increased two times under visible light radiation (>420 nm). The doping also led to the remarkable photocurrent enhancement. The clear dependence between photocurrent intensity and photoactivity was observed. The optimal dosage of Bi in TiO2 was about 1.0 mol %. The Bi species existed both in Bi2O3 and Bi0 forms in TiO2, and the ratio of Bi2O3 and Bi0 was dependent on the treatment processes. Results implied that Bi doping increased photoinduced charge separation and resulted in a relatively lower overpotential for H2 photoevolution. 1. Introduction Hydrogen is expected to be the main energy carrier in the future because of its high-energy capacity and environmental friendliness. Sunlight-driven hydrogen production from water may become more competitive when the technological cost decreases. TiO2, one of the most stable photocatalysts, seems to be one of the candidates for this purpose because of its lowcost and environmentally benign properties. However, because of the large band gap, TiO2 can only utilize ultraviolet irradiation of the solar radiation. In addition, the photocatalytic activity of TiO2 itself is quite low for many reactions, for example, photocatalytic hydrogen generation. Many efforts of the doping or modification of TiO2 in bulk or in surface have been carried out to enhance absorbing ability in the visible region and lift time of separated electron/hole charges.1-27 The clear enhancement of activities was observed after doping or modification of metals and 2-8 nonmetal elements, 9-21such as N,9-14,25-27 C,15-17 S,18-20,22 P,21 B, 23-26and F 23,27 on TiO2. The undoped and modified/doped bismuth oxides are good photocatalysts28-30 especially in photoinduced degradation of pollution and evolution of O2. Bi doped TiO2 photocatalysts were also active for photodegradation of pollution in aqueous solution.31-43 Yao and co-workers and Thanabodeekij et al. reported that bismuth titanate compounds BixTiyOz31-34 were photocatalytically active for photodecolorization of methyl orange and photodegradation of 4-nitrophenol (4-NP). They found that the Bi-O polyhedra in bismuth titanate compounds were photocatalytic centers. It was also suggested that the Bi3+ species in doped TiO2 photocatalysts were active centers in the photoinduced degradation, reduction, or decolorization of pollutions,35-44 such as benzene,35 methyl parathion,36 aqueous nitrate solution,37 p-nitroaniline,38 methyl orange,39 p-chlorophenol,40 orange II, 4-hydroxybenzoic and benzamide,41 acetaldehyde,42 azo dyes X-3B,43 RhB, and p-nitrobenzonic acid.44 * To whom correspondence should be addressed. Phone: +86-9314968178. E-mail:
[email protected]. † Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences. ‡ Graduate University of Chinese Academy of Sciences.
However, there are less reports on the photocatalytic hydrogen production over Bi modified TiO2 up to now. One example is Bi2S3/TiO2 heterosystem as reducing agent for hydrogen evolution by Brahimi et al.45 In this work, we investigate the influence of Bi doping on TiO2 photocatalysts for hydrogen evolution and photoinduced decolorization of RhB to understand the Bi species modification role in the surface microstructure of TiO2 photocatalyst. On the basis of the characterization, we found that Bi species exists in different forms and that suitable doping of Bi can enhance the photoactivity and photocurrent very significantly. Furthermore, the significant surface chemical state of Bi species was observed in higher post-treatment temperature, which has important effects on the photoactivity for hydrogen evolution, decolorization of RhB, and unbiased photocurrent of TiO2 photocatalyst. 2. Experimental Section 2.1. Photocatalyst Preparation. Tetrabutyl titanate, ethanol, acetic acid, bismuth nitrate, and RhB were of analytical grade and were used without further purification. The content of Bi was expressed as mol %. The Preparation of TiO2 Photocatalyst by Sol-Gel Method. The calculated amount of tetrabutyl titanate was dissolved in ethanol in a volume ratio of 1:1, and the mixture was added dropwise to the solution of ethanol-water-acetic acid solution (1:1:1) under vigorous stirring. The resulting transparent colloidal suspension was heated slowly under an infrared light for drying, and then the obtained powders were calcined in air at different temperatures at 673, 773, 873, and 973 K for 120 min. The obtained photocatalysts were denoted as P-673, P-773, P-873, and P-973, respectively. The Preparation of Bi Doped TiO2 Photocatalysts by Sol-Gel Method. The calculated amount of tetrabutyl titanate was dissolved in ethanol in a volume ratio of 1:1; the mixture was then added dropwise to ethanol-water-acetic acid solution (1:1:1) containing a calculated amount of bismuth nitrate under vigorous stirring. The drying and treatment processes were similar to the procedure of TiO2 photocatalyst preparation, and the samples were denoted as B-673, B-773, B-873, and B-973.
10.1021/jp9009433 CCC: $40.75 2009 American Chemical Society Published on Web 05/08/2009
Doping Effect of Bi on TiO2 The Preparation of Bi Doped Photocatalysts by Surface Impregnation Method. After TiO2 was synthesized, the obtained powders were dispersed again into the solution containing a calculated amount of bismuth nitrate (with a drop of nitric acid to enhance the dissolvability of bismuth nitrate in water) under vigorous stirring. The post-treatment processes were similar to the above procedure. The obtained photocatalysts were denoted as C-673, C-773, C-873, and C-973. 2.2. Photocatalyst Characterization. The X-ray photoelectron spectroscopy (XPS) was carried out on a photospectrometer (VG Scientific Escalab210-XPS) equipped with Mg (KR) X-ray resource (the binding energy data was referenced to the carbon C1s signal at 284.6 eV). Crystallographic structure and particle size were determined by X-ray diffraction (XRD) using a D/max RB X-ray diffractometer with Cu KR radiation (λ ) 1.5406 Å, the accelerating voltage, applied current, scan rate, step pace, and scan rang were 40 kV, 30 mA, 3.5° min-1, 0.0167°, and 20∼80°, respectively). UV-visible DRS spectra were obtained on a HITACHI U-3310 spectrophotometer equipped with an integrating sphere accessory (BaSO4 was used as a reference). 2.3. Photocatalytic Activity Test for Hydrogen Production. The experiment of the photocatalytic activity for hydrogen production was carried out in a 105 mL quartz flask with a septum for sampling. The reactant mixture (20 mg of powder photocatalyst used in 70 mL of aqueous ethanol solution with a 20 vol % ethanol concentration in each run) was placed in the flask. The dispersion was purged with argon gas for 40 min to remove the dissolved oxygen and to ensure that the reaction operated in an anaerobic condition. The reactant mixture was stirred with a magnetic stirrer and was illuminated. The amount of hydrogen generated in 120 min was analyzed with a GC8800 gas chromatographer equipped with a thermal-conductor detector (TCD) for the calculation of average hydrogen production rate. For long-term photocatalytic stability of Bi doping powders, sample C-773 was tested for the H2 production from aqueous ethanol solution; the hydrogen amount was sampled with 120 min time intervals. 2.4. Working Electrode Preparation and Photoelectrochemical Measurement. The preparation of working electrode was carried out according to ref 46. The suspension of Bi doped or undoped TiO2 powder with a concentration of ca. 1.0 g · L-1 was dispersed ultrasonically before use. Ten drops of mixture (ca. 1.0 mL of the suspension) was loaded onto a piece of transparent ITO conducting glass with area of 2.0 cm × 5.0 cm. Then, the ITO glass was heated slowly under an infrared lamp (200 W) until the suspension was dried and a layer of porous film was formed on the conducting glass. The surface of the circular film semiconductor working electrode was exposed to the electrolyte with a geometrical surface area of ca. 1.6 cm2. All photoelectrochemical measurements were carried out using a standard three-electrode system equipped with a quartz window, a saturated calomel reference electrode, and a platinum wire counter electrode. A 0.1 mol · L-1 Na2SO4 ethanol-water solution (20 vol % ethanol, pH 6.5) was used as the supporting electrolyte. A CHI-660 photoelectrochemical workstation was used for potentiostat controlling, bias voltage supply, and the photocurrent or photopotential recording. All potentials reported were measured against a saturated calomel electrode (SCE). The film electrode was illuminated from the side of the cell. The unbiased anodic photocurrent and the photoreduction potential for hydrogen production or photo-oxidization potential of ethanol were investigated with current-time and cyclicvoltammetry model.
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Figure 1. XRD patterns of 1.0 mol % Bi doped TiO2 and undoped TiO2 photocatalysts.
2.5. Photodecolorization of the Aqueous RhB Solution. The photocatalytic activities of synthesized samples for the photodecolorization of RhB in the aqueous solution were determined by monitoring the concentration of RhB in solution. Doped and undoped TiO2 powders were used. A 420 nm cutoff filter (Toshiba, S Y44.2) was used to ensure the visible light irradiation. The experiments were carried out in a quartz flask of ca. 140 mL with a flat window (with an efficient irradiation area of 10 cm2) containing 80 mg of catalyst and 110 mL of 1.0 × 10-5 mol · L-1 aqueous RhB solutions. The distance between the liquid surface and the light source was about 20 cm. The photodecolorization ratios of RhB over synthesized powders were expressed as η ) [(A - At)/A0] × 100% (η, photodecolorization ratio of RhB; A0, absorbency before irradiation; and At, absorbency after irradiation). 3. Results and Discussion 3.1. Analysis of Crystal Structure and Chemical Composition of Photocatalysts. Figure 1 shows the XRD patterns of undoped TiO2 and 1.0 mol % Bi doped TiO2 photocatalysts prepared by sol-gel and surface impregnation methods. Only the anatase phase was found in all samples. Clearly, the doping of Bi does not affect the crystal structure of photocatalysts. In addition, no significant diffraction peak of Bi species was observed regardless of the calcination temperature because of the higher dispersion of Bi and the lower content of Bi. The average crystal size of particle is estimated from the widths of anatase (101) reflection by the Scherrer formula, L ) 0.89λ/β cos θ, where λ (1.5406 Å) is the wavelength, θ is the Bragg angle (deg), L is the average crystallite size (nm), and β is the full width at half-maximum. The calculated data indicated that the average crystal sizes for samples became larger after calcination, for example, 9.5 nm for P-673, 11.5 nm for P-773, 23.7 nm for P-873, and 39.4 nm for P-973. Similar variation was observed in doped samples. However, the average crystal sizes for the samples calcined at the same temperature were almost the same (e.g., 10.5 nm for sample B-773, 11.3 nm for C-773) and were independent of modification methods. To determine the surface chemical state of Bi species, the XPS was used to study the samples. The Bi 2p7/2 XPS profiles and the contents of various Bi species are shown in Figure 2 and Table 1. No obvious variation of Ti 2p (binding energy at ca. 458.30 eV) and O 1s (binding energy at ca. 529.60 eV) was observed after Bi doping. However, there were two Bi species
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Figure 3. Diffuse reflectance UV-vis spectra of pure (P, P-773) and BiOx-modified TiO2 (C, C-773 and B, B-773). Figure 2. The Bi 2p7/2 XPS profiles of Bi doped TiO2 photocatalyst.
TABLE 1: Analysis of Surface Bi Atomic Content of the As-Prepared Samples surface atomic ratio of Bi to Ti (%) samples B-773 C-773 B-973 C-973
Bib/Ti 0.75 0.95 2.40 2.45
Bia/Ti 0.21 0.29 0.47 0.49
Bia/Bib 28.33 30.82 19.74 19.96
(Bib + Bia)/Ti 2p3/2 0.96 1.24 2.87 2.94
a Bi atomic content for Bi0 species. b Bi atomic content for Bi2O3 species.
found in the Bi doped samples. The XPS results indicated that the Bi species existed mainly in the form of Bi2O3 in all samples (Bi4f7/2b ) 158.70 ( 0.10 eV), but a weak signal of Bi0 (Bi 4f7/2a )156.70 ( 0.10 eV) was also observed. After calcination at higher temperature, the significant surface chemical state of Bi species (i.e., the Bi species would segregate from the shallow surface and move onto the photocatalyst surface, and some Bi0 species oxidized into Bi3+ species) was observed as shown in Table 1. The Bi species concentration on B-773 and B-973 samples calcined at 973 K increased from 0.96% to 2.87%. The data increased from 1.24% to 2.94% for C-773 and C-973 samples. However, the ratio of Bi0/Bi2O3 became smaller after calcination, for example, from 28.33% to 19.74% for B-773 and B-973 samples. This means that calcination may lead to more Bi2O3 species formed and to segregation on photocatalyst surface. The reason for Bi0 species formation is uncertain although the samples were prepared and calcined in air. The remaining carbonaceous species might have reduced partial Bi3+ to Bi0 during the calcination. The UV-vis absorption spectra of undoped and Bi doped TiO2 samples are given in Figure 3. Bi doping led to absorption increase in the visible region, for example, 400-600 nm. However, the doped samples prepared either by sol-gel or by surface impregnation method showed the weak absorption intensity in the 400-600 nm regions, although the Bi concentration on C-sample surface was a little higher than that of B-sample. The important reason could be due to the lower content of Bi species, and Bi species only existed on the outside and shallow surface of TiO2 samples rather than in the bulk crystal lattice.
TABLE 2: Dependence of Photocatalytic H2 Production Rate (mL · h-1) on Bi Content (mol %) and Calcined Temperature (K) over Bi Doped TiO2 Prepared by Sol-Gel Method photocatalytic H2 production rate (mL · h-1) on Bi content (mol %) temperature (K)
0.0 mol %
0.5 mol %
1.0 mol %
2.0 mol %
B-673 B-773 B-873 B-973
0.031 0.031 0.030 0.030
0.205 0.305 0.236 0.161
0.278 0.320 0.268 0.172
0.170 0.195 0.183 0.162
TABLE 3: Photocatalytic Hydrogen Production Rates (mL · h-1) over Samples with 1.0 mol % Bi Modifying Process and Calcined Temperature (K) under UV Light Radiation photocatalytic H2 production rates (mL · h-1) temperature (K)
sol-gel method sample B
surface impregnation sample C
673 773 873 973
0.278 0.320 0.268 0.172
0.331 0.362 0.299 0.268
3.2. Photocatalytic Activities for Hydrogen Production and Photodecolorization of RhB. The properties of doped and undoped samples were tested on photocatalytic hydrogen generation and RhB decolorization. In the dark condition, the hydrogen production was not observed. The main results for photocatalytic hydrogen generation are listed in Table 2. The clear dependences of photocatalytic H2 production rate on Bi concentration were observed. For example, the photoactivity increased by a factor of 6 and 9 after doping 0.5% of Bi and 1.0% of Bi on TiO2 prepared by sol-gel method. Doping much more Bi led to the decrease of photocatalytic activity. In addition, it was found that calcination temperature affected the activity significantly. The activity for hydrogen generation of B-973 was lower than that of B-773 which indicated that calcination at higher temperature resulted in the lower activity. The results in Table 1 indicate that the atomic ratio of Bi species to Ti species increased by a factor of 3.0 for B sample and 2.4 for C sample when the treatment temperature increased (from 773 K to 973 K). The decreased activity might mainly be due to the lower dispersion or serious segregation of Bi on the TiO2 surface. Of course, the increase in crystal sizes of TiO2 particle is also one notable factor because the crystal sizes of TiO2 particle increased remarkably with the variety of treatment
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Figure 4. The practice photographs for photodecolorization of RhB. S0 corresponds to the original solution (1.0 × 10-5 mol · L-1); P0, B0, and C0 correspond to the samples after adsorption-desorption equilibrium; P0∼P9, B0∼B9, and C0∼C9 correspond to the samples P-773, B-773, and C-773 for various photoreaction times, respectively.
TABLE 4: Photodecolorization Ratios of RhB (%) over Pure and BiOx-Modified TiO2 Photocatalysts Obtained with a Variety of Photoreaction Times (h) η (photodecolorization ratios of RhB, %) photoreaction time (h)
P-773
B-773
C-773
0 3 6 9
12.3 36.4 42.7 47.8
18.0 68.6 76.2 94.4
26.8 77.5 91.3 96.0
temperature (i.e., 9.5 nm for P-673, 11.5 nm for P-773, 23.7 nm for P-873, and 39.4 nm for P-973). Moreover, the Bi doped TiO2 prepared by surface impregnation showed higher hydrogen production rates than that prepared by a sol-gel method under the same conditions as listed in Table 3. For decolorization of RhB, the mixture of photocatalyst and RhB solution was stirred over 12 h for the adsorption-desorption equilibrium before light irradiation. For the samples P-773, B-773, and C-773, the obvious adsorption was observed after 12 h both for pure and Bi doped TiO2 photocatalysts. For example, about 12.3%, 18.0%, and 26.8% of RhB were absorbed on P-773, B-773, and C-773, respectively, which indicated that B and P samples exhibited strong adsorption interaction with RhB. Under light irradiation, significant photocatalytic decolorization was observed. The results are shown in Figure 4 and Table 4. The color of suspension changed from initially pink-
Figure 6. Unbiased photocurrent vs time curves for Bi doped and undoped electrodes (P curve, sample P-773; B curve, B-773; C curve, C-773) recorded during on-off cycles under UV light illumination. Solution: 0.1 mol · L-1 Na2SO4, 20 vol % ethanol-water.
red to colorless. For the samples B-773 and C-773, the decolorization reaction was almost finished after 9 h (with photodecolorization ratios of 94.4 and 96.0%, respectively). C sample showed comparatively higher activity than B sample. However, the decolorization of P-773 sample was only 47.8% in the same reaction period. It is obvious that Bi doping has enhanced the decolorization rate significantly. 3.3. Photoelectrochemical Behaviors and Photocurrent Dependence on Bi Doping. The photoelectrochemical performance of bare and Bi doped TiO2 has been investigated with the cyclic-voltammetry and the time-current methods. The experiments were carried out in the self-designed photoelectrochemical experiment apparatus as shown in Figure 5.46 A septum inserted with two tubules was used for the inlet and outlet of argon gas. Before measurement, the electrolyte solution was carefully purged with argon gas to give an anaerobic reaction condition. Figure 6 shows the unbiased photocurrent-time response curves for samples P, B, and C. Bi doped TiO2 prepared either by sol-gel method or by surface impregnation exhibited remarkable enhanced photocurrent compared to the sample P. The photocurrents of sample P (curve P), sample B with 1% Bi doping (curve B), and sample C with 1% Bi doping (curve C) were 2.5, 5.0, and 7.2 µA, respectively. Those results indicated that Bi doping led to significant inhibition of photoinduced electron-hole pairs’ recombination. The suitable doping Bi, such as sample C, might result in high-dispersed Bi species, which in turn results in enhanced photoactivity. The Cyclic-Voltammograms (CV) curves to photoinduce hydrogen evolving and ethanol oxidation over photocatalyst
Figure 5. The schematic diagram of the self-designed photoelectrochemical experiment setup.
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Figure 8. The long-term photocatalytic stability of Bi doping photocatalyst (sample C-773) for the H2 production from aqueous ethanol solution.
Figure 7. The potential for hydrogen photoevolving (I) and ethanol photo-oxidation (II) over pure and Bi3+ modified TiO2 electrodes obtained by the cyclic-voltammograms under UV light illumination. Solution: 0.1 mol · L-1 Na2SO4, 20 vol % ethanol-water. Scan rate, 50 mV · s-1. (P curve, P-773K; B curve, B-773K; C curve, C-773K).
TABLE 5: Potential for Hydrogen Photo-Evolving and Ethanol Photo-Oxidation Obtained by the Cyclic-Voltammograms under Light Illuminationa samples
potentials for hydrogen photoevolving (V)
potentials for ethanol photo-oxidation (V)
P-773K B-773K C-773K
-0.60 -0.55 -0.53
1.18 1.06 1.03
a Solution: 0.1 mol · L-1 Na2SO4, 20 vol % ethanol-water. Scan rate, 50 mV · s-1.
TABLE 6: Relation between Photocurrent Intensity and the Hydrogen Photo-Evolution Rate of Pure (P-773) and BiOx-Modified TiO2 (C-773 and B-773) samples
photocatalytic hydrogen production rates (mL · h-1)
unbiased photocurrent intensity (µA)
P-773K B-773K C-773K
0.030 0.320 0.362
2.5 5.0 7.2
samples C, B, and P are shown in Figure 7I and II, and the potentials for hydrogen generation and ethanol oxidation under irradiation are given in Table 5. For sample P (pure TiO2), the potentials for hydrogen generation and ethanol photo-oxidation are -0.60 V and 1.18 V versus SCE. For sample B, the corresponding potentials are around -0.55 V and 1.06 V versus SCE. The data for sample C are -0.53 V and 1.03 V versus
SCE. Sample C-773, Bi doped TiO2 prepared by surface impregnation method, exhibited a lower overpotential for hydrogen photoevolving and ethanol photo-oxidation than P-773 and B-773. These results are consistent with the photoactivity differences. Thus, we thought that Bi species has played an important role in capturing photoinduced charges and in decreasing the overpotential of hydrogen production and ethanol oxidation. According to A. J. Bard,47,48 the reaction over the heterogeneous photocatalyst particle can be considered as the reaction in the microelectrochemical cell. From the point of view in electrochemistry, the electric current results from the directional transfer of the charges. For a photocatalyst nanoparticle, the electron-hole pairs induced by light irradiation transfer directionally into particle surface for direct or indirect oxidization/ reduction reaction and results in the formation of a photocurrent. Photocatalytic hydrogen-production ability is mainly dependent on the capture of a surface photoinduced electron. Thus, there is an important relationship between photocurrent and hydrogen production rate as listed in Table 6. That is to say, the photocatalyst sample with higher photocurrent has a higher photocatalytic activity for hydrogen generation. In a certain range of Bi species content, the higher Bi concentration on TiO2 was accorded to the higher photocurrent, which resulted from the formation of Bi species enhancing the electron and hole separation by capturing the photoinduced charges efficiently. It seems that Bi2O3 can promote electron and hole separation,29 therefore, appropriate Bi2O3 leads to much higher activity and photocurrent. The results in Figure 4 and Table 4 indicate that the Bi2O3-doped TiO2 has higher photodecolorization activities for RhB than pure TiO2. In addition, the same tendency was found in sample B-773 and sample C-773 because a higher concentration of Bi2O3 was formed in sample C-773. 3.4. Long-Term Photocatalytic Stability for H2 Production. To investigate the photocatalytic stability for hydrogen production from aqueous ethanol solution, the Bi doping TiO2 photocatalyst (sample C-773) was used with the reaction condition described in section 2.3. The results in Figure 8 indicate that the Bi doping TiO2 photocatalyst was photocatalytic stable for hydrogen production. After 48 h reaction, the hydrogen volume was 17.28 mL, and the average hydrogen production rate was kept on a level of 0.348 mL · h-1. 4. Conclusions This study focuses on the obviously enhancing role of Bi doping on TiO2 for photocatalytic hydrogen generation (in-
Doping Effect of Bi on TiO2 creased by a factor of ca. 10) and RhB decolorization. The photo-electrochemical results indicated that the appropriate amounts of Bi doping could enhance the photocurrent and could lead to the lower overpotential for photoinduced hydrogen evolution and ethanol oxidization. The optimal Bi content was 1.0 mol %, the optimal heat-treatment temperature was 773 K, and the optimal modification method was surface impregnation. The preparing method and post-treatment condition obviously affected the chemical state and content of Bi species on the surface, for example, segregation of Bi species from the shallow surface of TiO2, which resulted in the difference in the photocatalytic properties for hydrogen evolution and RhB decolorization. Acknowledgment. Financial support from Natural National Science Foundation of China (No. 90210027) and the National Basic Research Program of China (No. 2007CB613305 and 2009CB220003) is gratefully acknowledged. References and Notes (1) 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. (2) Ikeda, S.; Sugiyama, N.; Pal, B.; Marci, G.; Palmisano, L.; Noguchi, H.; Uosaki, K.; Ohtani, B. Phys. Chem. Chem. Phys. 2001, 3, 267. (3) Yamashita, H.; Harada, M.; Misaka, J.; Takeuchi, M.; Ikeue, K.; Anpo, M. J. Photochem. Photobiol., A: Chem. 2002, 148, 257. (4) Umebayashi, T.; Yamaki, T.; Itoh, H.; Asai, K. J. Phys. Chem. Solids 2002, 63, 1909. (5) Dvoranova, D.; Brezova, V.; Mazur, M.; Malati, M. A. Appl. Catal., B: EnViron. 2002, 37, 91. (6) Kemp, T. J.; McIntyre, R. A. Polym. Degrad. Stab. 2006, 91, 165. (7) Peng, S.; Li, Y.; Jiang, F.; Lu, G.; Li, S. Chem. Phys. Lett. 2004, 398, 235. (8) Choi, W. Y.; Termin, A.; Hoffmann, M. R. J. Phys. Chem. 1994, 84, 13669. (9) Lee, D. H.; Cho, Y. S.; Yi, W. I.; Kim, T. S.; Lee, J. K.; Jung, H. J. Appl. Phys. Lett. 1995, 66, 815. (10) Saha, N. C.; Tompkins, H. G. J. Appl. Phys. 1992, 72, 3072. (11) Asahi, R.; Morikawa, T.; Ohwaki, T.; Aoki, K.; Taga, Y. Science 2001, 293, 269. (12) Lindgren, T.; Mwabora, J. M.; Avendano, E.; Jonsson, J.; Granqvist, C. G.; Lindquist, S. E. J. Phys. Chem. B 2003, 107, 5709. (13) Irie, H.; Watanabe, Y.; Hashimoto, K. J. Phys. Chem. B 2003, 107, 5483. (14) Diwald, O.; Thompson, T. L.; Zubkov, T.; Goralski, E. G.; Walck, S. D.; Yates, J. T. J. Phys. Chem. B 2004, 108, 6004. (15) Sakthivel, S.; Kisch, H. Angew. Chem., Int. Ed. 2003, 42, 4908. (16) Khan, S. U. M.; Al-Shahry, M.; Ingler, W. B. Science 2002, 297, 2243. (17) Irie, H.; Watanabe, Y.; Hashimoto, K. Chem. Lett. 2003, 32, 772. (18) Umebayashi, T.; Yamaki, T.; Tanaka, S.; Asai, K. Chem. Lett. 2003, 32, 330.
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