and Pb2+-Doped SrTiO3 Photocatalysts. A Correlation Between Band

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J. Phys. Chem. C 2007, 111, 1847-1852

1847

Ag+- and Pb2+-Doped SrTiO3 Photocatalysts. A Correlation Between Band Structure and Photocatalytic Activity Hiroshi Irie,*,† Yoshihiko Maruyama,† and Kazuhito Hashimoto*,†,‡ Department of Applied Chemistry, The UniVersity of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan and Research Center for AdVanced Science and Technology (RCAST), The UniVersity of Tokyo, 4-6-1 Komaba, Meguro-ku, Tokyo 153-8904, Japan ReceiVed: October 7, 2006; In Final Form: NoVember 21, 2006

We have prepared the two series of yellowish (PbxSr1-x)TiO3 (x ) 0.01-0.3) and (AgySr1-y)(Ti1-yNby)O3 (y ) 0.03-0.1) powders. The UV-vis absorption spectra of these powders indicate that the bandgaps of (PbxSr1-x)TiO3 were narrowed but those of (AgySr1-y)(Ti1-yNby)O3 were kept constant in comparison with that of SrTiO3. Combining the results of ab initio DOS calculations, we were convinced that the valence band of (PbxSr1-x)TiO3 consisted of Pb 6s and O 2p and that the top of the valence band shifted to a higher energy, resulting in the visible light absorption. In contrast, (AgySr1-y)(Ti1-yNby)O3 formed the isolated miniband composed of Ag 4d in the forbidden band above the valence band composed of O 2p, resulting in the visible light absorption. We investigated the band structure dependence on photocatalytic oxidative activity evaluated by gaseous 2-propanol decomposition under UV (300-400 nm) and visible (400-530 nm) light irradiations. The quantum efficiencies of the photocatalytic reaction with the (PbxSr1-x)TiO3 (x ) 0.1) were approximately the same between under UV and visible light irradiations. In contrast, the quantum efficiency under visible light irradiation was approximately one-third of that under UV light for the (AgySr1-y)(Ti1-yNby)O3 (y ) 0.03). Each result in the oxidative activity coincided with respective band structures obtained from the DOS calculations and UV-vis spectra.

Introduction Because TiO2 is an effective photocatalyst, it has been used for various industrial applications, such as water or air purification, antibacterial agents, and self-cleaning surfaces.1-10 In addition to TiO2, other metal oxides such as ZnO, Ta2O5, and SrTiO3 have been reported as efficient photocatalysts.11-13 However, all these efficient photocatalysts are generally widegap semiconductors and require UV light to generate the photocatalytic activity. Numerous studies have attempted to extend the photosensitivity of these semiconductors toward the visible light region to use solar or indoor light effectively. Typically, they are modified by anion doping,14-18 metal doping,19-21 and generating oxygen deficiency.22 Another common method is to produce photocatalysts with smaller band gap energies without dopants by forming the maximums of their valence bands in the higher energy region. These photocatalysts have been intensely investigated, especially in the field of photocatalytic water splitting with specific d10/d10s2 metal ions (Ag+,23,24 In3+,25-27 Bi3+, 28 etc.), but there are limited reports in the field of photooxidation of organic compounds.29-32 In the present study, we adopted the doping technique and chose SrTiO3 as a mother structure. Regarding the SrTiO3-based photocatalyst, Sb5+- and Cr3+-codoped SrTiO3 was reported as a water-splitting material under visible light irradiation with the * Corresponding authors. E-mail: (H.I.) [email protected]; (K.H.) [email protected]. Tel: +81-3-5452-5080. Fax: +813-5452-6593. † Department of Applied Chemistry. ‡ Research Center for Advanced Science and Technology (RCAST).

aid of sacrificial agents.33 For organic compounds-decomposing materials under visible light irradiation, La3+- and N3--codoped SrTiO3 was reported.34 La3+ was introduced into N-doped SrTiO3 to keep electrical neutrality and La affected neither its valence band top, conduction band bottom, nor band gap. The origin of visible light sensitivity was the isolated miniband composed of N 2p above valence band. La3+- and N3--codoped SrTiO3 had rather strong oxidation power and could decompose 2-propanol (IPA) into CO2 via acetone. In addition to the doping anionic nitrogen into SrTiO3, the doping either Ag+ or Pb2+ cation might be one of the candidate methods for generating the visible light sensitivity because Ag+ (Ag 4d10) or Pb2+ (Pb 6s2) possibly contributes to the electronic structure of the valence band of SrTiO3. This is because the Ag 4d and Pb 6s in addition to O2p contributed to the valence bands of AgNbO3 and PbTiO3, respectively.23,35 In contrast, neither Ag 4d nor Pb 6s affects the electronic structures of their conduction bands. In this study, we doped Ag+ and Pb2+ at the Sr2+ site substitutionally in SrTiO3, so that they could decompose IPA into CO2 under the visible light regions, similar to La- and N-codoped SrTiO3. To keep electrical neutrality when doping Ag+ at the Sr2+ site, Nb5+ (or Ta5+) also was substituted at the Ti4+ site as a counterdopant. Here, we report fabrications of (PbxSr1-x)TiO3 and (AgySr1-y)(Ti1-yNby)O3 and evaluations of the photocatalytic oxidation activity measured by the decomposition of gaseous 2-propanol (IPA) under UV and visible light irradiations. In addition, we discuss the features of the electronic band structures of (PbxSr1-x)TiO3 and (AgySr1-y) (Ti1-yNby)O3, and the correlations of the band structures with the UV and visible light-induced photocatalytic activities.

10.1021/jp066591i CCC: $37.00 © 2007 American Chemical Society Published on Web 01/06/2007

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Experimental Polycrystalline powder samples of (PbxSr1-x)TiO3 (x ) 0.05, 0.1, and 0.2) were synthesized using a two-step solid-state reaction method. Commercial TiO2 (Wako Chemicals, 99.9%), PbO (Wako Chemicals, 99.9%), and SrCO3 (High Purity Chemicals, 99.9%) powders were used as starting materials. The stoichiometric amounts of TiO2 and SrCO3 powders for SrTiO3 and 1 mol % excess amount of PbO to the stoichiometric PbO and TiO2 for PbTiO3 were wet ball-milled for 20 h using ZrO2 balls as the milling medium in polyethylene bottles. The resulting mixture for SrTiO3 was calcined twice at 1100 °C for 5 h, and the mixture for PbTiO3 was calcined at 1000 °C for 5 h in air. Then, the calcined SrTiO3 and PbTiO3 powders were again wet ball-milled for 20 h in ratios corresponding to the compositions of x in (PbxSr1-x)TiO3. The respective powders were heated twice at 1100 °C for 5 h in air and then thoroughly ground. Polycrystalline powders of (AgySr1-y)(Ti1-yNby)O3 (y ) 0.03, 0.1) were synthesized as follows. Commercial Ag2O5 (Wako Chemicals, 99.9%) and Nb2O5 (Wako Chemicals, 99.9%) powders as starting materials were mixed stoichiometrically. Mixtures were wet ball-milled for 20 h, using ZrO2 balls as the milling medium in polyethylene bottles. The resulting mixture was calcined at 970 °C for 5 h under oxygen flow. Subsequently, the obtained AgNbO3 powders were washed with HNO3, followed by washing with distilled water and were dried. Then, the obtained AgNbO3 and commercial SrTiO3 (Aldrich, 99.5%) powders were again wet ball-milled for 20 h in ratios corresponding to the compositions of y in (AgySr1-y)(Ti1-yNby)O3. The powders were heated twice at 970 °C for 5 h under oxygen flow. Similarly, HNO3, distilled water, and dehydration treatments were performed. The crystal phases of the prepared samples were identified by an X-ray diffractometer (XRD) (Rigaku; RINT-2100). UVvis absorption spectra using the diffuse reflection method were obtained by a spectrometer (Shimadzu; UV-3100). BrunauerEmmett-Teller (BET) surface areas of the prepared powders were determined using a nitrogen adsorption apparatus (Micromeritics; TriStar 3000). The compositions of the samples were determined by X-ray photoelectron spectroscopy (XPS) (Perkin-Elmer; 5600). The plane-wave-based density functional method calculation was carried out within the generalized gradient approximation TABLE 1: Experimental Conditions for Gaseous IPA Decomposition under UV Light (a) and Visible Light (b) Irradiations (a) UV light

SrTiO3 (prepared) (Pb0.1,Sr0.9)TiO3 SrTiO3 (Aldrich) (Ag0.03,Sr0.97) (Ti0.97,Nb0.03)O3

light intensity/ mW cm-2

Absorbed photon number/ quanta cm-2 s-1

0.13 0.13 0.39 0.37

1.7×1014 2.0×1014 4.5×1014 4.5×1014

(b) visible light light intensity/ mW cm-2 SrTiO3 (prepared) (Pb0.1,Sr0.9)TiO3 SrTiO3 (Aldrich) (Ag0.03,Sr0.97) (Ti0.97,Nb0.03)O3

1.0 1.0 1.7 1.1

absorbed photon number/ quanta cm-2 s-1 2.2×1014 4.5×1014

Figure 1. X-ray powder diffraction profiles of SrTiO3 (a), (PbxSr1-x)TiO3 (x ) 0.05 (b), 0.1 (c), 0.2 (d)), and (AgySr1-y)(Ti1-yNby)O3 (y ) 0.03 (e), 0.1 (f)).

using the ab initio total-energy and molecular dynamics program VASP (Vienna Ab initio Simulation Program).36,37 The core orbitals were replaced by ultrasoft pseudopotentials and the kinetic energy cutoff was 500 eV. The compositions used for the calculation were SrTiO3, (Pb0.25Sr0.25)TiO3, and (Ag0.25Sr0.75)(Ti0.75Nb0.25)O3. The photocatalytic oxidation activity was evaluated by the decomposition of gaseous IPA under light irradiations with wavelengths between 300-400 nm and 400-530 nm. Wavelengths of 300-400 nm and 400-530 nm were achieved using a Xe lamp (Hayashi Tokei; Luminar Ace 210) with a glass filter (Asahi-technoglass; UV-D36B) and a combination of glass filters (Asahi-technoglass; B-47, L-42, and C-40C), respectively.38 Light conditions are shown in Tables 1(a) and (b). A 300-mg sample was spread evenly over the irradiation area (approximately 9.1 cm2) in a 500 mL vessel. After sealing the sample in the vessel, the atmosphere in it was replaced by the synthetic air. The 300 ppm (300 ppmv, i.e., 6.7 µmol) of reactant gas (IPA) was injected into the vessel and then the samples were stored in the dark. After confirming that the IPA concentration was constant, which implied that the reactant gas finished adsorbing onto the powder surface, light irradiation commenced. The photocatalytic oxidation of IPA proceeds via acetone as an intermediate, followed by the slow oxidation of acetone to the final products, CO2 and H2O. To evaluate the photocatalytic activity, the amounts of IPA, acetone, and CO2 were monitored using gas chromatography (Shimadzu; GC-8A). Results and Discussion 1. Sample Characterizations. (PbxSr1-x)TiO3 and (AgySr1-y) (Ti1-yNby)O3 were obtained as yellow powders. The yellow color became deeper with increasing both x and y values. Figure 1 shows the XRD patterns of the obtained samples. The structure of them was kept cubic crystal system with a homogeneous SrTiO3 crystalline phase at least up to x ) 0.2 and y ) 0.1. Figures 2a and 3a show the UV-visible absorption spectra of (PbxSr1-x)TiO3 and (AgySr1-y)(Ti1-yNby)O3, respectively. It is obvious from Figure 2a that doping Pb2+ at the Sr2+ site narrowed the band gap of SrTiO3 and that the absorption edges shifted to a longer wavelength region with increasing x. SrTiO3 is an indirect gap semiconductor39 and thus the bandgaps of the (PbxSr1-x)TiO3 and SrTiO3 can be estimated from the tangent lines in the plots of the square root of the Kubelka-Munk functions against the photon energy, as shown in Figure 2b.40 The tangent lines, which are extrapolated to R1/2 ) 0, indicate the band gaps of 3.18, 3.10, 3.03, and 2.98 eV for x ) 0, 0.05, 0.1, and 0.2 in (PbxSr1-x)TiO3, respectively. Thus, (PbxSr1-x)TiO3 can absorb light in a longer wavelength region up to visible light.

Ag- and Pb-Doped SrTiO3

J. Phys. Chem. C, Vol. 111, No. 4, 2007 1849 TABLE 2: BET Surface Areas and Ratios of Constituent Elements surface area/m2 g-1 SrTiO3 (prepared) (Pb0.1,Sr0.9)TiO3 SrTiO3 (Aldrich) (Ag0.03,Sr0.97) (Ti0.97,Nb0.03)O3

Figure 2. UV-vis absorption spectra of (PbxSr1-x)TiO3 (a) and plots of the square root of Kubelka-Munk function against photon energy (b). The inset in (b) is the enlargement.

Figure 3. UV-vis absorption spectra of (AgySr1-y)(Ti1-yNby)O3 (a) and plots of the square root of Kubelka-Munk function against photon energy (b). The inset in (a) also shows the spectrum after UV light irradiation of (AgySr1-y)(Ti1-yNby)O3 (y ) 0.03).

In contrast, for (AgySr1-y)(Ti1-yNby)O3 generations of the absorption shoulders into the visible light region up to approximately 450-500 nm were observed and the negligible shifts of the absorption edges were observed, as shown in Figure

1.16 1.04 17.3 14.4

ratios of constituent elements Sr/Ti ) 0.98 Pb/Ti ) 0.10, Sr/Ti ) 0.89 Sr/Ti ) 0.97 (before UV) Ag/Ti ) 0.032, Sr/Ti ) 0.95, Nb/Ti ) 0.033 (after UV) Ag/Ti ) 0.030, Sr/Ti ) 0.95, Nb/Ti ) 0.032

3a. In fact, the plots of the square root of the Kubelka-Munk functions against the photon energy indicated almost the same band gap energy of 3.18 eV for (AgySr1-y)(Ti1-yNby)O3 as that for SrTiO3, as shown in Figure 3b. Obtained BET surface areas and ratios of constituent elements are shown in Table 2.41 The surface area of the commercial SrTiO3 was one digit larger than that of the prepared one. However, the surface areas of (Pb0.25Sr0.25)TiO3 and (Ag0.03Sr0.97)(Ti0.97Nb0.03)O3 powders were similar to those of the prepared and commercial SrTiO3, respectively, corresponding to raw materials. The practical compositions of all powders coincided with the weighed elements within experimental errors. 2. Density of States (DOS) Calculations. Figure 4a-c shows the DOS of SrTiO3, (Pb0.25Sr0.25)TiO3 and (Ag0.25Sr0.75)(Ti0.75Nb0.25)O3, respectively. The conduction and valence bands of SrTiO3 mainly consisted of Ti 3d and O 2p, respectively, as shown in Figure 4a. It was confirmed from Figure 4b that the Pb 6s did not contribute to the conduction band of (Pb0.25Sr0.25)TiO3, but only to the valence band. Thus, the valence band in (Pb0.25Sr0.25)TiO3 consisted of the Pb 6s and O 2p hybrid orbitals and the top of the valence band shifted to the high-energy side, resulting in the decrease in the band gap energy, which is consistent with UV-visible absorption spectra in Figure 2a. As for the DOS of (Ag0.25Sr0.75)(Ti0.75Nb0.25)O3 in Figure 4c, Nb 4d scarcely contributed to the conduction band but did not contribute to the valence band. Ag 4d did not contribute to the conduction band but contributed only to the valence band. It seemed that the band gap narrowed by mixing Ag 4d with O 2p, which is inconsistent with the actual UV-visible absorption spectra in Figure 3a. This is probably because of the small concentration with y ) 0.03 and 0.06 in (AgySr1-y)(Ti1-yNby)O3 for the prepared composition, compared to that with y ) 0.25 for the calculated composition. In the present study, we could only prepare single phases of (AgySr1-y)(Ti1-yNby)O3 up to y ) 0.06 due to a soluble limitation between SrTiO3 and AgNbO3. Thus, it can be considered that the Ag 4d did not mix with the valence band composed of O 2p, but formed the isolated band in the forbidden band above the valence band. Schematic band structuresofthepreparedSrTiO3,(PbxSr1-x)TiO3,and(AgySr1-y)(Ti1-yNby)O3 were shown in Figures 5 a, b, and c, respectively. 3. Photocatalytic Activity. Figure 6a,b show the changes of acetone and CO2 concentrations, respectively, as a function of time in the presence of SrTiO3 (x ) 0) and (PbxSr1-x)TiO3 (x ) 0.1) powders while irradiating with UV and visible lights. Not shown here, however, are that the IPA concentrations in the presence of SrTiO3 were 105 and 85 ppmv when the UV and visible irradiations started, respectively, (i.e., when IPA gas finished adsorbing onto the powder surfaces). Those of (PbxSr1-x)TiO3 (x ) 0.1) were 157 and 83 ppmv. So, the adsorption capabilities of SrTiO3 and (PbxSr1-x)TiO3 (x ) 0.1) against IPA were quite similar. Both acetone and CO2 were generated when irradiating with UV and visible lights in the presence of all these photocatalysts. Even in the presence of SrTiO3, they were generated under visible light irradiation. It

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Figure 4. Density of states for SrTiO3 (a), (Pb0.25Sr0.75)TiO3 (b), and (Ag0.25Sr0.75)(Ti0.75Nb0.25)O3 (c).

Figure 5. Schematic band structures of SrTiO3 (a), (PbxSr1-x)TiO3 (b) and (AgySr1-y)(Ti1-yNby)O3 (c) in the present study.

is possibly because of a slight quantity of UV light which cannot be cut off completely by the glass filters. More importantly, (PbxSr1-x)TiO3 (x ) 0.1) could decompose IPA into acetone and CO2 by absorbing visible light. A similar experiment was conducted with (AgySr1-y)(Ti1-yNby)O3 (y ) 0.3). When IPA finished adsorbing onto the powder surfaces, the IPA concentrations in the presence of SrTiO3 were 0 and 12 ppmv and those of (AgySr1-y)(Ti1-yNby)O3 (y ) 0.3) were 18 and 2.5 ppmv. So,theadsorptioncapabilitiesofSrTiO3 and(AgySr1-y)(Ti1-yNby)O3 (y ) 0.3) against IPA were quite similar. However, the capabilities of SrTiO3 and (AgySr1-y)(Ti1-yNby)O3 (y ) 0.3) were much larger than those of SrTiO3 and (PbxSr1-x)TiO3 (x ) 0.1) due to larger surface areas. In the presence of (AgySr1-y)(Ti1-yNby)O3 (y ) 0.3), both acetone and CO2 also

were generated by absorbing either UV or visible light. After photocatalytic reaction under either UV or visible light irradiation, the color of (AgySr1-y)(Ti1-yNby)O3 (y ) 0.3) did not change at all, indicating its stability under light irradiation. Additionally, the stability of (AgySr1-y)(Ti1-yNby)O3 (y ) 0.3) under UV light irradiation was confirmed using the XPS and UV-visible spectrometer. The UV light condition was controlled to be the same as the IPA decomposition test, as shown in Table 1a. Even after 100 h irradiation, the UV-visible absorption spectrum did not change within experimental errors as shown in the inset of Figure 3a. Data not shown here, however, neither XPS spectra of Ag 3d, Sr 3p, Ti 2p, Nb 3d, nor O 1s were changed after UV light irradiation. The ratios of Ag/Ti, Sr/Ti, and Nb/Ti after UV irradiation also were unchanged compared to those before UV irradiation, as shown in Table 2. Because no changes in the absorption spectrum and XPS were observed, the present (AgySr1-y)(Ti1-yNby)O3 (y ) 0.3) was stable under UV light. When using photocatalysts for decomposition of organic stains in air or water, they work effectively if photoexcited electrons are consumed in the reduction of oxygen. In the present study, either Pb2+ or Ag+ doping into SrTiO3 was preferable to visible light sensitivity in that point, because either dopant attibuted to the electronic structure of its valence band and did not to that of its conduction band. If either dopant attibuted to the electronic structure of its conduction band (i.e., either the

Ag- and Pb-Doped SrTiO3

J. Phys. Chem. C, Vol. 111, No. 4, 2007 1851

Figure 6. Changes of acetone (a) and CO2 (b) concentrations as a function of time in the presences of SrTiO3 and (Pb, Sr)TiO3 under visible light irradiation.

miniband below conduction band was formed or the conduction band bottom was shifted to the positive direction), photoexcited electrons could not be consumed in the reduction of oxygen because the potential for the bottom edge of conduction band in SrTiO3 is approximately only 0.1 eV negative, compared to that for the oxygen reduction by electrons (-0.046 V vs SHE).42 This phenomenon is quite similar to nitrogen dopings into TiO2, ZnO, Ta2O5, and so on. To discuss the correlation between band structures and photocatalytic activities, we calculated the quantum efficiencies (QEs). Only one photon participates in the decomposition process of IPA to acetone.43 Therefore, the QE values for acetone generation (QEacetone) were calculated using the following equation: QEAcetone ) initial acetone generation rate/ absorption rate of incident photon. Compared to the mechanism for acetone generation, the mechanism for CO2 generation is complex. Thus, it is difficult to determine the number of photons participating in the reaction because there are two reaction paths. Assuming the following formula, C3H8O + 5H2O + 18h+ f 3CO2 + 18H+ (i.e., six photons are required to produce one CO2 molecule), the QE values for CO2 generation (QECO2) were calculated using the following equation, QECO2 ) 6 × initial CO2 generation rate/absorption rate of incident photon.15,30,32 The initial acetone and CO2 generation rates were calculated using the conventional least-squares method. The apparent total QE values, QE ) QEAcetone + QECO2, are shown in Table 3. QE values were quite different between two kinds of SrTiO3 powders, our prepared SrTiO3 used to prepare (PbxSr1-x)TiO3 and the commercially available one to prepare (AgySr1-y)(Ti1-yNby)O3. Up to now, we do not know the reasons for it. When irradiating with UV light, QE values of x ) 0.1 and y )

0.03 decreased, compared to x ) 0 and y ) 0, respectively. Most importantly, the QE values of (PbxSr1-x)TiO3 (x ) 0.1) under UV and visible light irradiations were quite similar within experimental errors. In contrast, the QE value of (AgySr1-y) (Ti1-yNby)O3 (y ) 0.3) under UV light was 3 times larger than that under visible light. These will be discussed later. Not shown here, however, we performed exactly the same experiments in the presence of (AgySr1-y)(Ti1-yTay)O3 (y ) 0.3) under both visible and UV light irradiations and obtained the same results as (AgySr1-y)(Ti1-yNby)O3 (y ) 0.3). That is, it had the absorption shoulders into the visible light region up to approximately 450-500 nm and the QE value of it under UV light was larger than that under visible light. Thus, it can be considered that the effect of Ta substitution is the same as that of Nb substitution. Irradiating SrTiO3 with UV light resulted in a higher QE value than irradiating (PbxSr1-x)TiO3 (x ) 0.1) with UV light. This trend is plausible, considering below. The top of the valence band in (PbxSr1-x)TiO3 (x ) 0.1) shifted to the high-energy side compared to SrTiO3 due to the hybridization of the Pb 6s and O 2p orbitals, as confirmed by the calculated DOS and observed UV-vis spectra of (PbxSr1-x)TiO3. The position of the top of the valence band determines the oxidation power of a photogenerated hole and its activity increases as the top of the valence band shifts to the low-energy side. Therefore, the oxidation power of the hole generated in (PbxSr1-x)TiO3 (x ) 0.1) under UV light irradiation was weaker than that in SrTiO3, resulting in the lower QE value of (PbxSr1-x)TiO3 (x ) 0.1) than that of SrTiO3. The similar QE values of (PbxSr1-x)TiO3 (x ) 0.1) under UV and visible light irradiations are plausible considering that the Pb 6s and O 2p orbitals completely mix to form the valence band of (PbxSr1-x)TiO3. The reasons are as follows. As soon as the free charge carriers are photogenerated in a semiconductor, such as TiO2 and there is rapid communication between the valence band and conduction band, then the photocarriers will occupy the lowest states in their corresponding bands (i.e., they become thermalized). The carrier temperature becomes equal to ambient temperature, and the carriers are indistinguishable regardless of the initial state immediately occupied upon photoexcitation. That is, the excess photoenergy is dissipated as heat rather than as chemical potential.44 The similar QE values under UV and visible light irradiations also were observed in the presence of Ag-inserted NbO2F30 and R-AgGaO2.32 Next, let us compare the UV light-induced photocatalytic activity in the presence of SrTiO3 and (AgySr1-y)(Ti1-yNby)O3 (y ) 0.3) and also compare the UV light- and visible lightinduced activity in the presence of (AgySr1-y)(Ti1-yNby)O3 (y ) 0.3). Irradiating SrTiO3 with UV light resulted in a higher QE value than irradiating (AgySr1-y)(Ti1-yNby)O3 (y ) 0.3) with UV light. This is because Ag 4d formed the isolated band above the valence band, as was confirmed by UV-vis spectrum. Under UV light irradiation, the isolated band in (AgySr1-y) (Ti1-yNby)O3 (y ) 0.3) should act as recombination centers for holes and electrons, resulting in the low QE value compared to SrTiO3. Irradiating with UV light resulted in a higher QE value

TABLE 3: Quantum Efficiencies for IPA Decomposition in the Presences of SrTiO3, (Pb, Sr)TiO3, and (Ag, Sr)(Ti, Nb)O3 UV light SrTiO3 (prepared) (Pb0.1,Sr0.9)TiO3 SrTiO3 (Aldrich) (Ag0.03,Sr0.97)(Ti0.97,Nb0.03)O3

Visible light

QEacetone/%

QECO2/%

Total QE/%

0.65 ( 0.061 0.31 ( 0.023 0.10 ( 0.0034 0.041 ( 0.0027

0.59 ( 0.13 0.26 ( 0.039 0.14 ( 0.035 0.073 ( 0.0088

1.2 ( 0.19 0.57 ( 0.061 0.24 ( 0.038 0.11 ( 0.012

QEacetone/%

QECO2/%

Total QE/%

0.17 ( 0.012

0.32 ( 0.048

0.49 ( 0.060

0.0012 ( 0.00076

0.027 ( 0.014

0.039 ( 0.014

1852 J. Phys. Chem. C, Vol. 111, No. 4, 2007 than irradiating with visible light in the presence of (AgySr1-y) (Ti1-yNby)O3 (y ) 0.3). Two reasons can be considered for it. Irradiating with UV light to (AgySr1-y)(Ti1-yNby)O3 (y ) 0.3) excites electrons in both the valence band, composed of O 2p, and the isolated band, composed of Ag 4d, but irradiating with visible light only excites electrons in the isolated band of Ag 4d. In addition, the hole mobility in the isolated band should be low. Therefore, the QE values when irradiating with UV light were higher than when irradiating with visible light. These phenomena also were observed in the presence of N-doped TiO2,15,45 La-, N-codoped SrTiO3,34 and N-doped Ta2O5.46 Forming the isolated band in the band gap of a UV light sensitive photocatalyst allows it to absorb visible light, however, its absorption in the visible light region is not so large and its photocatalytic activity under visible light is lower than that under UV light. Narrowing the band gap of a UV light sensitive photocatalyst surpasses in that it can develop the same activity under visible light as that under UV light. Conclusions Pb doping at Sr sites and Ag (and Nb) doping at Sr (and Ti) sites in SrTiO3 that are sensitive to UV light were performed to produce photocatalysts that are sensitive to visible light. The band structures of the doped SrTiO3 depended on the dopants, which was confirmed by UV-vis absorption spectra and photocatalytic oxidative decomposition of IPA under UV light and visible light irradiations. The photocatalytic activities of (Pb,Sr)TiO3 under UV and visible light irradiations showed approximately the same due to the complete mixing of the Pb 6s and O2p orbitals in the valence band of (Pb,Sr)TiO3. In contrast, the activity of (Ag,Sr)(Ti,Nb)O3 under UV light irradiation was superior to that under visible light irradiation due to the formation of the Ag 4d isolated band above in the valence band composed of O 2p. The present study demonstrated that introducing either Pb2+ or Ag+ into oxides that are sensitive to UV light is a potential approach for generating photocatalysts that are sensitive to visible light. The origins of visible light sensitivity (i.e., the band gap narrowing or forming the isolated band) depended on the dopants. Herein, the band gap narrowing is more preferable for obtaining the photocatalytic performance under visible light as high as that under UV light. Acknowledgment. Supported by a Grant-in-Aid for Scientific Research on Priority Areas (417) from the Ministry of Education, Culture, Sports, Science, and Technology (MEXT) of the Japanese Government. References and Notes (1) Fujishima, A.; Honda, K. Nature 1972, 238, 37. (2) Heller, A. Acc. Chem. Res. 1995, 28, 141. (3) Linsebigler, A. L.; Lu, G. Q.; Yates J. T. Chem. ReV. 1995, 95, 735. (4) Nosaka, Y.; Koenuma, K.; Ushida, K.; Kira, A. Langmuir 1996, 12, 736. (5) Fu, X.; Zeltner, W. A.; Anderson, M. A. Appl. Catal., B 1995, 6, 209. (6) Paz, Y.; Luo. Z.; Rabenberg. L.; Heller, A. J. Mater. Res. 1995, 10, 2842. (7) Sopyan, I.; Watanabe, M.; Murasawa, S.; Hashimoto, K.; Fujishima, A. J. Electroanal. Chem. 1996, 415, 183.

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