15818
J. Phys. Chem. C 2008, 112, 15818–15823
Selective Production of Superoxide Ions and Hydrogen Peroxide over Nitrogen- and Sulfur-Doped TiO2 Photocatalysts with Visible Light in Aqueous Suspension Systems Tsutomu Hirakawa*,† and Yoshio Nosaka*,‡ National Institute of AdVanced Industrial Science and Technology (AIST), Tsukuba-west 16-1, Onogawa, Tsukuba, Ibaraki 305-8569, Japan, and Department of Chemistry, Nagaoka UniVersity of Technology, Kamitomioka, Nagaoka, Niigata 940-2188, Japan ReceiVed: June 22, 2008; ReVised Manuscript ReceiVed: July 31, 2008
Superoxide ions (O2•-) and hydrogen peroxide (H2O2) produced by visible light irradiation were analyzed to investigate the photocatalytic reaction mechanism for commercially available nitrogen- and sulfur-doped TiO2 (BAPW25 and SJC). O2•- was detected by the chemiluminescence (CL) probe method with luminol, while the amount of H2O2 was measured as the CL intensity by the reaction with the luminol oxidized upon the addition of hemoglobin. By irradiation with 442-nm light, the S-doped TiO2 surpassed the N-doped TiO2 in the ability to produce O2•-, while the N-doped TiO2 surpassed the S-doped TiO2 in accumulating H2O2. The difference in the photocatalytic processes was attributed to the multivalence property of sulfur atoms which may promote the decomposition of H2O2. 1. Introduction Photocatalytic reactions at the surface of titanium dioxide (TiO2) have been attracting much attention and practically used for environmental cleaning such as the self-cleaning of houses, buildings, outside walls, and windows.1-9 However, the photocatalytic activity of TiO2 is effective only in the UV range of the irradiating light. The current study in the research field of utilizing TiO2 has then been shifted to the modification of TiO2 for a visible response. Various transition-metal dopants have been studied for producing a visible response TiO2.10-13 Carbonand fluorine-doped TiO2 powders have also been synthesized to study the visible response characterization of TiO2.14-16 Most reported articles of visibly responsive TiO2 involved the nitrogen (N)-doped TiO2. N-doped TiO2 was first reported by Sato in 1986 and many extensive studies from basic research to practical applications have been reported recently.17-29 Sulfur-doped TiO2 has also been synthesized to study the visible-response characterization of TiO2.24b,30-33 Two synthesis methods have recently been reported: calcining TiS31,32b and hydrolysis/ calcining the mixture of titanium alkoxide and thiourea.32 For the S-doped TiO2 photocatalysts, the oxidation kinetics24b and photocatalytic decomposition of methylene blue and acetone31,32 by visible light have been reported. The principle of visible-light response is to form a midgap energy state by the dopant substituting for O or Ti in the TiO2 crystal.18-24,33 For the N-doped TiO2, the midgap energy level consists of the localized N 2p states above the valence band.18,22,24,25,28,29 In the reaction mechanism, the doped atoms may play the role of an oxidation site. The N site of the N-doped TiO2 is reported to have a photocatalytic activity to decompose an organic compound such as isopropanol and acetone.18,19,25-27 On the other hand, the lack of the ability to oxidize an organic compound was reported for the hole produced at the N site on N-doped TiO2.28 N-doped TiO2 photocatalysts may oxidize * To whom correspondence should be addressed. † National Institute of Advanced Industrial Science and Technology (AIST). E-mail:
[email protected]. Fax: +81-29-861-8051. ‡ Nagaoka University of Technology. E-mail:
[email protected]. Fax: +81-258-47-9315.
water to produce OH radicals,27,28 but a lack of the oxidation potential to produce OH radicals by the midgap state has thoroughly been reported.17,18,28 The S-doped TiO2 cannot oxidize water even by UV-irradiation.32c Hence the decomposition and oxidation of organic compounds by the N- and S-doped TiO2′s may be carried out via a reactive oxygen species produced by reduction, not via the trapped holes localized on the N and S atoms at the midgap potential.24,28 During photocatalysis with undoped TiO2 such as P25, the photocatalytic reduction of O2 is an important step when decomposing an organic compound in order to effectively separate the photoinduced holes from the associated electrons. The energy levels of the conduction band of the N- and S-doped TiO2′s are essentially the same as that of anatase TiO2,31b,32b,c,34,35 having a sufficient reduction potential for O2 to produce O2•-. In fact, the production of O2•- at the surface of the N-doped TiO2 was observed in the ESR measurements under visible light irradiation.36 Similar to the undoped TiO2, the further reduction of O2•- by the conduction band electrons of the doped TiO2 may produce H2O2, and then OH radicals, or surface active species such as Ti-•OH (or Ti-O•) that have a strong oxidation power.37,38a,39 As for the N- and S-doped TiO2 photocatalysts, however, the detailed behavior of O2•- or the detection of H2O2 has not yet been reported. We have been studying active oxygen species such as the O2•- and OH radicals in TiO2 photocatalysis during the UV irradiation, and recently proposed the importance of H2O2 as the reaction intermediate.38 The analysis of the behavior of O2•and H2O2 by visible light irradiation for doped TiO2 photocatalysts provides essential information for understanding the detailed reaction mechanism for improving the activity. To detect O2•-, the luminol chemiluminescence probe method has proved to be useful for the investigation of undoped TiO2 photocatalysts.38 In the present study, the luminol CL method was extended to the detection of H2O2, as well as the detection of O2•-, to investigate the reaction mechanism of doped TiO2 photocatalysts during visible light irradiation.
10.1021/jp8055015 CCC: $40.75 2008 American Chemical Society Published on Web 09/11/2008
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J. Phys. Chem. C, Vol. 112, No. 40, 2008 15819
2. Experimental Section Reagents. In the present study, the photocatalysts were commercially available doped TiO2 powders for visible response: N-doped TiO2 (labeled BAPW25) as supplied by EcoDevice and S-doped TiO2 (labeled SJC) as supplied by Sekisui Jyushi Chemical. These catalysts were prepared as anatase powders by using specific compounds containing each objective atom: ammonia was used for doping N19 and thiourea for doping S.32 The characteristics of these doped TiO2′s are listed in the Supporting Information as Table S1. The photoabsorption of the N- and S-doped TiO2′s was characterized by using a UV-vis-NIR spectrophotometer (Shimadzu, UV-3150) equipped with an integrating sphere assembly (Shimadzu, ISR3100). The adsorption spectra are consistent with the reported spectra for the N- and S-doped TiO2′s as shown in Figure S1a,b in the Supporting Information. The absorbance at the excitation wavelength (442 nm) for the S-doped TiO2 was not significantly different from that of the N-doped TiO2, assuming the same scattering factor in the Kubelka-Munk analysis. P25 TiO2 (Degussa, Japan Aerosil) was also used as a sample of the undoped TiO2 photocatalyst for the UV response. All TiO2 photocatalysts were generous gifts from the corresponding manufacturers. Luminol (Nacalai Tesque, Ltd.) was used as received without further purification. Hemoglobin (Hb) from bovine blood (Wako, assay 80%) was used to initiate the luminol chemiluminescence for detecting H2O2. O2•- Measurements. The formation of O2•- was observed by using the luminol CL probe method with a photon-counting system by the following procedure. Fifteen milligrams of photocatalyst powder was added to 3.5 mL of a 0.01 M NaOH solution of pH 11.5. This suspension was vigorously stirred for 10 min before the visible-light irradiation and this stirring was continued during the experiment. A He-Cd laser (KIMMON, IK5652R-G) of 14 mW specific incident power at the wavelength of 442 nm was used as the excitation light source. The laser beam was guided to the side face of the cell. Immediately after stopping the laser irradiation, 50 µL of a 7 mM luminol solution (0.01 M NaOH) was injected by a microsyringe into the irradiated TiO2 suspension. The CL intensity was measured by a photomultiplier tube (PMT) that was mounted in a Peltier cooling box. The CL intensity integrated over 20 s and the integrated CL intensity was used to calculate the amount of O2•produced during the irradiation.38c Other details about the apparatus and the CL reaction have been described previously.38,41a,b,42a,b H2O2 Measurements. The experimental setup for the selective detection of H2O2 is basically the same as that for the O2•measurement. The suspension was mounted in the dark box at the same place for the O2•- measurement, but the injection solution and the timing were different. After stopping the irradiation, the photocatalyst suspension was kept in the dark for 30 min, and then 50 µL of a 7 mM luminol solution (0.01 M NaOH) was added. The suspension containing luminol was again stored for 10 min in the dark. Fifty microliters of the Hb solution (in 0.01 M NaOH) was then injected by a microsyringe into the suspension and the time profile of the CL intensity was measured for 100 s. The integrated CL intensity corresponds to the amount of H2O2 produced from the irradiation. This procedure is based on the fact that the O2•- disappears within 30 min after the formation, while H2O2 does not decrease in the dark. Hb was added to oxidize the luminol in order to participate in the CL reaction with H2O2.43 Calibration for the CL intensity. To convert the observed CL intensities to the concentrations of O2•- and H2O2, the
Figure 1. The concentration of O2•- produced in the suspension (15 mg/3.5 mL) of the N- and S-doped TiO2’s as a function of the irradiation time. The irradiation was performed by a He-Cd laser (14 mW/cm2 at 442 nm).
apparatus factor of the CL intensity to the concentration of the CL molecule was measured by using the following luminal standard emission method (LSEM) procedure. A buffer solution of pH 11.5 was prepared using Na2HPO4 and NaOH. Fifteen milligrams of the photocatalyst powder was added to 3.5-mL of luminol solution (pH 11.5) of various concentrations (0.01-1 µM) and the suspension was vigorously stirred for 10 min. After opening the shutter of the photomultiplier (PMT) for 5 s, 50 µL of a 0.35 M H2O2 solution was injected into the suspension by a microsyringe and then after a 10-s observation, 50 µL of a 6.2 µM Hb solution (neutral water) was injected into the powder suspension. Upon injection of the Hb solution, a strong luminol CL was observed and the CL intensity was recorded for 15 min. Following the recording, the injection of Hb solution and the CL observation were repeated 5 times to ensure the complete consumption of the luminal molecules in the suspension. The total CL intensity observed for the LSEM is then plotted as the function of the initial luminol concentration, and the slope provides the conversion factor. Thus the CL intensity can be converted to the concentrations of O2•- and H2O2 produced by the photocatalytic reaction using the CL quantum yields.40-43 3. Results Production of O2•- with Visible Light and Its Decay Process. In Figure 1, the concentrations of O2•- observed for the N- and S-doped TiO2 photocatalysts are plotted as a function of the duration of the visible-light irradiation. In the measurements, the probing reagent, luminol, does not affect the observed TiO2 photocatalytic reaction, since the luminol solution can be added to the TiO2 suspension after stopping the light irradiation. As the irradiation period increased, the amount of O2•- increased and reached a steady value in 30 s for the S-doped TiO2, while it gradually increased for the N-doped TiO2 up to about 180 s. The concentrations of O2•- produced during the 180-s irradiation were 0.07 and 0.2 µM for the N- and S-doped TiO2’s, respectively. In a control experiment, P25 TiO2 did not produce O2•- by the visible-light irradiation at 442 nm. Upon UVirradiation at 325 nm (7 mW) for 180 s, 0.02 µM of O2•- was produced by the P25, indicating that the steady state concentration of O2•- for the N- and S-doped TiO2 is higher than that for the undoped TiO2 (P25). To investigate the decay process, the amount of O2•- was measured as a function of the time after stopping the irradiation,
15820 J. Phys. Chem. C, Vol. 112, No. 40, 2008
Figure 2. Time dependence on the concentration of O2•- after stopping the 180-s irradiation on the N- and S-doped TiO2’s in the suspension. The inset shows the reciprocal plot of the concentration of O2•produced.
Figure 3. Superposition of luminol chemiluminescence (CL) intensity, where the 0.1 mM of luminol was previously added to the suspension of the N-doped TiO2 after 10 min from the end of the 442-nm irradiation. Hemoglobin (Hb) was added to the suspension at 5 s to start CL observation.
and the experimental results are plotted in Figure 2 on a logarithmic scale. Since the data do not fit a linear line, the decay process does not obey the first-order or pseudo-first-order kinetics. Fitting to the fractal-like kinetics, which could be adopted for the undoped TiO2,38b also failed. The decay of O2•for the doped TiO2 can be analyzed as a second-order reaction as shown in the inset of Figure 2. This observation indicates that O2•- decays by the disproportionation after at least stopping the irradiation. The disproportionation rate constants for the Nand S-doped TiO2′s were calculated to be 6.8 × 105 and 5.1 × 106 M-1 s-1, respectively. The disproportionation rate constant for O2•- in a bulk solution44 is 19 M-1 s-1, therefore, the surface of the TiO2 might stabilize the O2•- and serve as a suitable environment to accelerate the disproportionation. Since the product of the disproportionation of O2•- is H2O2, the amount of H2O2 was measured as follows. Production of H2O2 by Visible Light. The concentration of H2O2 formed by the irradiation of various durations was measured as the CL intensity recorded with oxidized luminol. For the N-doped TiO2, as shown in Figure 3, the CL intensity was changed by the addition of Hb: it first increased, reached a maximum, then slowly decreased within 100 s.45 For the S-doped TiO2 photocatalyst, the observed CL intensity was
Hirakawa and Nosaka
Figure 4. The concentration of H2O2 produced in the suspensions of the N- and S-doped TiO2′s as a function of the irradiation time. The experimental conditions are the same as in Figure 1.
almost unchanged by the visible-light excitation (Figure S2 in the Supporting Information). By integrating the CL intensity, the concentration of H2O2 was calculated for each irradiation time as described in the Experimental Section. The concentrations of the produced H2O2 were plotted in Figure 4 as a function of the irradiation time. For the N-doped TiO2, the H2O2 concentration increased with the increasing irradiation time and reached about 0.04 µM. On the other hand, for the S-doped TiO2, the concentration of the produced H2O2 was less than 0.002 µM. The non-negligible formation of H2O2 before the irradiation observed for the N-doped TiO2 (Figure 4) can be explained by the photocatalytic reaction due to the room light. These observations indicate that the surface condition of the N-doped TiO2 is suitable to produce or accumulate H2O2. If all of the produced O2•- in Figure 1 decays by disproportionation after stopping the irradiation, the concentrations of H2O2 produced will become half the O2•- concentration, which are calculated to be 0.035 and 0.10 µM on the N- and S-doped TiO2, respectively. For the N-doped TiO2, 0.04 µM of H2O2 was actually produced, indicating that the O2•- disproportionated to produce H2O2 without decay of the product. On the other hand, the concentration of H2O2 produced on the S-doped TiO2 was far less than 0.09 µM, indicating that the produced H2O2 reacts with some species that is absent in the N-doped TiO2 and remains after stopping the irradiation. Effect of H2O2 Addition on the O2 · - Production. To verify the reaction of H2O2, the O2•- concentration for a 10-s irradiation was measured in the presence of various amounts of H2O2 and plotted in Figure 5 as a function of the H2O2 concentration. For the N-doped TiO2, the O2•- concentration gradually decreased with the addition of H2O2, while the decrease was significant for the S-doped TiO2. Though the H2O2 was produced upon disproportionation of the O2•- for both of the doped TiO2′s, the H2O2 produced by the S-doped TiO2 rapidly decayed and resulted in a significantly small amount of the H2O2 formation as observed in Figure 4. 4. Discussion The midgap energy edge of the N-doped TiO2 was reported by many research groups to range from +1.81 to +2.1 V vs NHE.18,25,27,28,31a,32b The midgap energy edge of the S-doped TiO2 was also similar to the N-doped TiO2 and estimated to be 2.0 eV (vs NHE) by the F-LAPW calculation.31a,32b,c As
Selective Production of O2•- and H2O2
J. Phys. Chem. C, Vol. 112, No. 40, 2008 15821 migrate to the surface of TiO2, were not produced upon the visible light excitation. Therefore, reactions 4 and 4′ may be ruled out in the case of visible excitation for the doped TiO2. Alternatively, the deactivation of trapped electron Ti3+(e-) by the oxidized nitrogen, i.e., the direct recombination between trapped electron-hole pairs (reactions 5 and 5′) takes place.
Ti3+(e-) + N+ (or N •) f Ti4++N • (or N-) -
+
Ti (e ) + S (TiO2) f Ti +S(TiO2) 3+
4+
(5) (5′)
•-
Figure 5. Concentration of O2•- (in unit of µM) produced during irradiation for the N- and S-doped TiO2′s in the suspension (15 mg/ 3.5 mL) containing various concentrations of H2O2. The luminol solution for the detection was injected immediately after the 442nm-light irradiation for 10 s.
for its reduction ability, the flat band potential of the N-doped TiO2 was estimated to range from -0.48 to -0.50 V (vs NHE) and a similar flat band potential on S-doped TiO2 was suggested.31b,32b,c,33-35 These doped TiO2 photocatalysts can then reduce O2 to produce O2•-. Thus, the visible-light excitation causes the trapped holes at the midgap states as the oxidized doped atoms and trapped electrons as Ti3+(e-) near the conduction band which produces O2•-. For the N-doped TiO2, the initial events during the irradiation are represented by reactions 1 and 2.
N-+Ti4+ + hν (442 nm) f N •+Ti3+(e-)
(1)
Ti3+(e-)+O2 f Ti4++O2•-
(2)
where N- and N• denote the electronic states of the nitrogen atom doped in the TiO2 lattice, which were treated by the DFT method and assigned by the ESR study.22,36 According to the ESR study, the N• state could absorb visible light to excite electrons to the conduction band forming another oxidized state, N+, as shown by reaction 3.36
N •+Ti4+ + hν (442 nm) f N++Ti3+(e-)
(3)
N+,
N•
These photoinduced oxidized states, and cannot react with water.28 In the absence of reactants, O2•- may then be oxidized by these states produced in the same particle during the next excitation event back to the initial O2 molecules as reaction 4.
O2•-+N+
•
•
-
(or N ) f O2+N (or N )
(4)
Processes similar to reactions 1 and 4 for the N-doped TiO2 are expected for the S-doped TiO2 as represented by reactions 1′ and 4′.
S(TiO2) + Ti4+ + hν (442 nm) f S+(TiO2) + Ti3+(e-) (1′) O2•-+S+(TiO2) f O2 + S(TiO2) N+
N•)
S+(TiO2)
(4′)
However, the (or and states are mainly formed inside the particles because almost all of the dopant locate in the bulk TiO2 particle. The valence band holes, which can
As shown in Figure 1, the increase in O2 with the irradiation time for the S-doped TiO2 was very rapid, while it was slow for the N-doped TiO2, indicating that reaction 5 rapidly decreases the rate of the O2•- formation (reaction 2) in comparison to the S-doped TiO2 (reaction 5′). Although the detailed chemical structures of the S+(TiO2) states are not presently known, some S+ states are expected to be more stable than N+ or N• in N-doped TiO2 because sulfur atom can take many valences, such as -2, +4, and +6. Actually, the long life of the e- trapped at Ti3+ created by S doping and the small contribution of the h+ produced at the S+ site to charge recombination has been suggested.24b As shown by the second-order decay in Figure 2, O2•- ions decay by disproportionation in alkaline solution (reaction 6). This observation is consistent with the report that O2•- ions migrate at the surface of a metal oxide and TiO2.38b,46-48
O2•-+O2•- +H+ f HO2- + O2
(6)
O2•-
Under light irradiation, the reduction of (eq 7) with trapped electrons may also contribute to the production of H2O2, since the redox potential of O2•-/HO2- is +1.0 V. 44
O2•-+Ti3+(e-)+H+ f HO2- + Ti4+
(7)
The contribution of reaction 7 may become significant when photoinduced holes are consumed for the oxidation of some molecules, where Ti3+(e-) remains before the recombination. Although both catalysts produce O2•-, a significant amount of H2O2 was detected for the N-doped TiO2 photocatalyst, while almost no H2O2 was produced with the S-doped TiO2 (Figure 4). This difference in the formation of H2O2 indicates that the photocatalytic oxidation and reduction of H2O2 might take place via different mechanisms for the two kinds of doped photocatalysts. Since nine commercially available undoped TiO2 photocatalysts showed significant increases in O2•- by addition of H2O2,38a,b a similar increase in O2•- for the doped TiO2 as reaction 8 or 8′ was expected.
HO2-+N+ (or N•) f O2•-+N • (or N-)+H+
(8)
HO2- + S+(TiO2) f O2•- + S(TiO2) + H+
(8′)
Figure 5 shows the concentration of O2•- produced during a 10-s irradiation in the presence of various concentrations of H2O2. Contrary to the expected increase, the experimental results showed gradual and steep decreases in O2•- with H2O2 for the N- and S-doped TiO2′s, respectively. It may then be concluded that the doped TiO2 cannot oxidize H2O2 to O2•-. Namely, reactions 8 and 8′ do not take place in contrast to the undoped TiO2. In the case of the S-doped TiO2, the decreased O2•concentration by the addition of H2O2 was more apparent than that observed for the N-doped TiO2. This characteristic effect of the S-doped TiO2 could be attributable to the characteristic doping states of sulfur. Ohno et al.32d described
15822 J. Phys. Chem. C, Vol. 112, No. 40, 2008
Hirakawa and Nosaka
Figure 6. Proposed processes of the photocatalytic reactions of O2•and H2O2 on the N- and S-doped TiO2′s in the absence of organic substrates. The N-doped TiO2 selectively produces H2O2, while the S-doped TiO2 produces O2•- and reduces H2O2 to water.
the electronic structure of the S-doped TiO2 based on the band calculation using the F-LAPW method. According to this report, the doped S site above the valence band consists of S 3s states and S 3p orbitals distributed in the conduction band causing some expansion of the energy band. Since the midgap energy level for the N-doped TiO2 consists of N 2p states, the electronic structure of the S-doped TiO2 may be different from that of the N-doped TiO2. The two-electron oxidation of H2O2 may take place without O2•- formation. However, if this were the case, the addition of H2O2 must accelerate the photocatalytic reduction, which increases the formation of O2•-. The possible explanation of the decrease in O2•- with H2O2 is that the reduction of H2O2 (reaction 9) takes place faster than the reduction of O2 (reaction 2). Since H2O2 is adsorbed as peroxo species, it can be reduced to an intermediate state28 (formally trapped OH radical, Ti-•OH). This intermediate state may be unstable and easily reduced to water as represented by reaction 10 at least in the absence of an organic reactant.
H2O2+Ti3+(e-) f Ti-•OH + OH•
-
-
Ti- OH + Ti (e ) f OH 3+
(9) (10)
Since the consumption of Ti3+(e-) in reactions 9 and 10 suppresses the formation of O2•- (reaction 2), if both the oxidation (reaction 8) and reduction (reaction 9) simultaneously take place, the amount of O2•- may be unchanged with the addition of H2O2. This may not be the present case for the N-doped TiO2 because the formation of O2•- was not as high as that observed for the S-doped TiO2. When the possible oxidation processes of organic reactants are compared between the N- and S-doped TiO2′s, since the S-doped TiO2 has a high O2•- formation ability and the formed O2•- can be easily reduced, the reaction may proceed with the activated oxygen such as trapped OH radicals. On the other hand, such an oxidation ability for the N-doped TiO2 is lower than that for the S-doped TiO2. 5. Conclusions The ability to produce O2•- and H2O2 on N- and S-doped TiO2′s was studied in an alkaline aqueous suspension by using the chemiluminescence probe method with luminol. When
the N- and S-doped TiO2 photocatalysts were irradiated at 442 nm, a greater amount of O2•- was produced than that from the undoped TiO2 (P25) irradiated at 325 nm. The S-doped TiO2 surpassed the N-doped TiO2 in the production of O2•- while the N-doped TiO2 surpassed the S-doped TiO2 in the production of H2O2. The dominant reaction processes observed in the absence of an organic substrate are schematically shown in Figure 6. H2O2was mainly produced from the disproportionation of O2•- as proved by the second order decay after stopping the irradiation (Figure 2). Since the O2•concentration was not increased by adding H2O2, H2O2 is not oxidized by both doped TiO2′s, which is in significant contrast to the undoped TiO2 (P25). The H2O2 produced by the S-doped TiO2 might be quickly reduced to H2O via some intermediate states; the reactive oxygen species produced by the reduction of H2O2 may play an important role in the decomposition of organic molecules, and the S-doped TiO2 may surpass N-doped TiO2 in this ability. The analytical method developed to detect H2O2 in the present study may be applicable for investigating the reaction mechanism of doped TiO2 photocatalysts or dye-sensitized photocatalysts. Supporting Information Available: Table showing characteristics of used photocatalysts (Table S1) and figures containing diffuse reflectance absorption spectra (Figure S1) with analysis of the absorption band and time profiles of chemiluminescence intensities for the S-doped TiO2(Figure S2), which corresponds to Figure 2 for the N-doped TiO2. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Fujishima, A.; Hashimoto, K.; Watanabe, T. Photocatalysis; BKC Inc.: Tokyo, Japan, 1999. (2) Fujishima, A.; Rao, T. N.; Tryk, D. A. J. Photochem. Photobiol., C 2000, 1, 1. (3) Photocatalysis; Kaneko, M., Ohkura, I., Eds.; Kodansha-Springer: Tokyo, Japan, 2002. (4) Serpone, N.; Pelizzetti, E.; Hidaka, H. Photocatalytic Purification and Treatment of Water and Ai; Ollis, D. F., Al-Ekabi, H., Eds.; Elsevier: London, UK, 1993; pp 225-250. (5) Fox, M. A.; Dulay, M. T. Chem. ReV. 1993, 93, 341. (6) Kamat, P. V. Chem. ReV. 1993, 93, 267. (7) Hoffmann, M. R.; Martin, S. T.; Choi, W.; Bahnemann, D. W. Chem. ReV. 1995, 95, 69. (8) Handbook of Heterogeneous Catalysis; Ertl, G., Knoezinger, H., Weitkamp, J., Eds.; Wiley-VCH: Weinheim, Germany, 1997. (9) Mills, A.; Hunte, L. S. J. Photochem. Photobiol. A 1997, 108, 1. (10) (a) Choi, W.; Termin, A.; Hoffmann, M. R. J. Phys. Chem. 1994, 98, 13669. (b) Choi, W.; Termin, A.; Hoffmann, M. R. Angew. Chem., Int. Ed. Engl. 1994, 33, 1091. (11) (a) Yamashita, H.; Ichihara, Y.; Takeuchi, M.; Kichiguchi, S.; Anpo, M. J. Synchrotron. Radiat. 1999, 6, 451. (b) Yamashita, H.; Harada, M.; Misaka, J.; Takeuchi, M.; Anpo, M. J. Photochem. Photobiol. A 2002, 148, 257. (12) Kavan, L.; Gratzel, M.; Gilbert, S. E.; Klemenz, C.; Scheel, H. J. J. Am. Chem. Soc. 1996, 118, 6716. (13) Klosek, S.; Raftery, D. J. Phys. Chem. B 2001, 105, 2815. (14) Khan, S. U. M.; Al-Shahry, M.; Ingler, W. B., Jr Science 2002, 297, 2243. (15) Irie, H.; Washizuka, S.; Hashimoto, K. Thin Solid Films 2006, 510, 21. (16) Hattori, A.; Yamamoto, M.; Tada, H.; Ito, S. Chem. Lett. 1998, 707. (17) Sato, S. Chem. Phys. Lett. 1986, 123, 126. (18) Asahi, R.; Morikawa, T.; Ohawaki, T.; Aoki, K.; Taga, Y. Science 2001, 293, 269. (19) Ihara, T.; Miyoshi, M.; Iriyama, Y.; Matsumoto, O.; Sugihara, S. Appl. Catal. B 2003, 42, 403. (20) (a) Nakamura, I.; Negishi, N.; Kutsuna, S.; Ihara, T.; Sugihara, S.; Takeuchi, K. J. Molec. Catal. A: Chem. 2000, 161, 205. (b) Takeuchi, K.; Nakamura, I.; Matsumura, O.; Sugihara, S.; Ando, M.; Ihara, T. Chem. Lett. 2000, 1354. (c) Nakamura, I.; Sugihara, S.; Takeuchi, K. Chem. Lett.
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