New Series of Solid-Solution Semiconductors - American Chemical

Feb 6, 2009 - (NIMS), 1-2-1 Sengen, Tsukuba, Ibaraki 305-0047, Japan, and ... A novel series of solid-solution semiconductors (AgNbO3)1-x(SrTiO3)x (0 ...
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J. Phys. Chem. C 2009, 113, 3785–3792

3785

New Series of Solid-Solution Semiconductors (AgNbO3)1-x(SrTiO3)x with Modulated Band Structure and Enhanced Visible-Light Photocatalytic Activity Defa Wang,† Tetsuya Kako,‡ and Jinhua Ye*,†,‡ International Center for Materials Nanoarchitectonics (MANA), National Institute for Materials Science (NIMS), 1-2-1 Sengen, Tsukuba, Ibaraki 305-0047, Japan, and Photocatalytic Materials Center, National Institute for Materials Science (NIMS), 1-2-1 Sengen, Tsukuba, Ibaraki 305-047, Japan ReceiVed: August 18, 2008; ReVised Manuscript ReceiVed: December 7, 2008

A novel series of solid-solution semiconductors (AgNbO3)1-x(SrTiO3)x (0 e x e 1) have been developed as highly visible-light-active photocatalysts for efficient O2 evolution and decomposition of organic pollutants. Rietveld refinement reveals that the perovskite-type solid solutions (AgNbO3)1-x(SrTiO3)x are crystallized in an orthorhombic system (0 e x < 0.9) or a cubic system (0.9 e x e 1). In the mixed valent perovskites (AgNbO3)1-x(SrTiO3)x, the hybridization behaviors between the Ag 4d and O 2p orbitals and between the Nb 4d and Ti 3d orbitals play a crucial role in tuning the energy band structure (band gap, band edge, and bandwidth, etc.) and, thus, in tailoring the photophysical and photocatalytic properties. As a result of competition between the absorption ability to visible-light and the reductive/oxidative abilities, the highest visible-light activities for both O2 evolution and decomposition of gaseous 2-propanol (IPA) are realized over (AgNbO3)0.75(SrTiO3)0.25. In addition, very fine Ag particles are precipitated on the catalyst surface to construct a nanocomposite structure of Ag/(AgNbO3)1-x(SrTiO3)x, of which the photocatalytic activities are further improved significantly. 1. Introduction Concerning the global energy crisis and environmental issues, photocatalysis using semiconductors and solar light has been attracting tremendous attention.1-3 As an ambient-temperature process for carrying out complete decomposition of pollutants at low levels, such an ideal green chemistry technology is undoubtedly advantageous over the conventional catalytic combustion processes that are usually energy intensive due to elevated operation temperatures (200-1200 °C). To construct a photocatalysis system for efficiently harvesting the visible light that occupies ∼43% of the total sunlight or indoor illumination, many efforts have been expended to develop highly visible-light-active photocatalysts. On the one hand, various methods have been adopted to modify the wide bandgap TiO2 (∼3.2 eV for anatase; ∼3.0 eV for rutile) to be visiblelight-sensitive by photosensitizing with suitable dyes that act as visible-light harvesters but are degraded eventually,4 and doping with transition-metal cations5 or anions (e.g., N, C, and S).6 Unfortunately, the localized states of dopants usually act as recombination centers for photogenerated charge carriers due to the formation of interbands or color centers (e.g., F, F+, F+2, and Ti3+).3 On the other hand, development of new visiblelight photocatalysts has gained more and more attention in recent years.7-9 Among them, solid-solution semiconductors are of particular interest for the tunable energy band structures and the derived advantageous performances that are unattainable by the individual end material.9 Owing to the high susceptibility of partial substitution of a wide range of cations and valences at both A and B sites, ABO3 perovskite-type metal oxides are promising candidates for * To whom correspondence should be addressed. Fax: +81-298-59-2301. E-mail address: [email protected]. † International Center for Materials Nanoarchitectonics (MANA). ‡ Photocatalytic Materials Center.

making solid-solution photocatalysts. Previously, SrTiO310 and AgNbO311 have been reported respectively as photocatalysts for H2 evolution under UV and for O2 evolution with a low visiblelight activity due to their different band gap energies and band edge potentials. It is thus expected that, through modulation of electronic structure, a solid-solution material can be created from SrTiO3 and AgNbO3 to exhibit an advantageous photocatalytic property over either of these two end compounds. Recently, we have succeeded in developing a new series of solid solutions between SrTiO3 and AgNbO3. Efficient photocatalytic decomposition of acetaldehyde has been realized over (Ag0.75Sr0.25)(Nb0.75Ti0.25)O3 under visible light.12 In this study, the crystal structure, electronic structure, photophysical, and photocatalytic properties of the mixed valent solid solutions (AgNbO3)1-x(SrTiO3)x (0 e x e 1) are systematically investigated. Efficient O2 evolution from silver nitrate solution and decomposition of 2-propanol (IPA) over (AgNbO3)1-x(SrTiO3)x under visible-light irradiation are reported. The compositional dependence of photocatalytic activities in the solid solutions (AgNbO3)1-x(SrTiO3)x is discussed in terms of electronic structure modulation. 2. Experimental Procedures 2.1. Materials Preparation. Powder samples of the nominal (AgNbO3)1-x(SrTiO3)x (0 e x e 1) and Agy(NbO3)0.75(SrTiO3)0.25 (0.5 e y e 1.5) solid solutions were synthesized from reagentgrade chemicals Ag2O, SrCO3, Nb2O5, and TiO2 (99.9%, Wako) through solid-state reaction. The reactants in desired ratios were thoroughly mixed with addition of methanol, dried, pressed into pellets, and preliminarily heated at 900 °C for 12 h. After remilling, the powders were finally calcined at 1000 °C for 24 h. 2.2. Materials Characterization. The solid solution powder samples were analyzed by an X-ray diffractometer (XRD, JEOLJDX 3500) using Cu KR radiation (λ ) 1.54178 Å) at room temperature. The data were collected from 2θ ) 10-90° in a

10.1021/jp807393a CCC: $40.75  2009 American Chemical Society Published on Web 02/06/2009

3786 J. Phys. Chem. C, Vol. 113, No. 9, 2009 step-scan mode (step, 0.02°; counting time, 5 s). Crystal structure and lattice parameters were determined by Rietveld refinement (RIETAN2000) based on the powder XRD data.13 UV-vis diffuse reflectance spectrum was measured on a UV-vis spectrometer (UV-2500, Shimadzu) at room temperature and was converted to absorbance spectrum by the Kubelka-Munk method. A field emission scanning electron microscope (FESEM, JEOL-JSM 6500F) and a high-resolution transmission electron microscope (HRTEM, JEOL-JEM 2010, operated at 200 kV), both of which were equipped with an energy-dispersive spectrometer (EDS), were employed for morphology observation and composition analysis. Surface area was measured on a Gemini-2360 analyzer (Micromeritics, Shimadzu, Japan) by nitrogen absorption at -196 °C using the Brunauer-EmmettTeller (BET) method. 2.3. Evaluation of Photocatalytic Properties. O2 evolution was carried out in a Pyrex glass cell with a flat side window for light irradiation. The catalyst powders (0.5 g) were suspended by a magnetic stirrer in an aqueous AgNO3 solution (5 mmol AgNO3 + 270 mL H2O), where AgNO3 was used as an electron acceptor. Before reaction, the cell connected to a closed gas circulation system was well evacuated and then introduced into ∼2.5 kPa of argon carrier gas. Upon irradiation, the oxygen gas evolved was mixed with argon gas by a glass piston pump and was analyzed in situ with an online gas chromatograph (GC-8AIT, Shimadzu) equipped with a thermal conductive detector (TCD). Photocatalytic decomposition of IPA was carried out in a cylindrical air-filled static Pyrex glass vessel (500 mL of total volume). The catalyst powders (∼0.4 g) were evenly dispersed on the bottom of a circular glass dish to have a uniform area of ∼8 cm2 and the dish was mounted in the vessel. After sealing with a quartz cover and a rubber O-ring, the vessel was carefully washed by artificial pure air (O2/N2 ) 1:9) to replace the assealed CO2-containing natural air. The pressure inside the vessel was ∼1 atm. A certain amount of gaseous IPA was injected into the vessel. Prior to light irradiation, the vessel was kept in the dark for sufficient time to get adsorption saturation of IPA on the sample surface, that is there was no apparent change of the measured concentration of reactants. As we know, the photocatalytic oxidation of IPA proceeds via formation of acetone as an intermediate, followed by further oxidation of acetone to the final products, CO2, and H2O.14 Therefore, the activity for photocatalytic decomposition of IPA was evaluated by monitoring the concentrations of IPA, acetone, and CO2. Upon light irradiation, gaseous sample (5 µL) was periodically extracted from the reaction vessel and measured on a gas chromatograph (GC-14B, Shimadzu) equipped with a flame ionization detector (FID) for analysis of IPA/acetone and a methanizer-FID for analysis of CO2, respectively. The light source was a 300 W xenon arc lamp (ILC Technology, CERMAX LX-300). A cutoff filter (L42, Hoya Optics) was used to obtain visible light (λ g 410 nm). A circulating water filter was used particularly for filtering the infrared heat from light irradiation so that the possible thermal effect was excluded and the photoreaction was ensured to perform at ambient temperature. Interference filters (Optical Coatings) were used to get the respective monowavelength light irradiations for measurement of quantum efficiency. The light intensity under each irradiation condition was measured by a spectroradiometer (USR-40D, Ushio). 2.4. Calculation of Band Structure. Using the single crystal parameters, the plane-wave-density function (DFT) calculation for SrTiO3, AgNbO3, and (AgNbO3)0.75(SrTiO3)0.25 was per-

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Figure 1. Crystal structures of (a) AgNbO3 (projected down [010]) and (b) SrTiO3 perovskites at room temperature. The octahedra represent [NbO6] or [TiO6] units, and spheres represent Ag+ or Sr2+ ions. Solid lines outline the unit cell, and dashed lines in (a) outline the pseudocubic subunit cell.

formed using the CASTEP program (Materials Studio, Accelrys Inc.).15 The core electrons were replaced with ultrasoft pseudopotentials. The generalized gradient approximation (GGARPBE) was applied. The kinetic energy cutoff was 340 eV for all calculations. For (AgNbO3)0.75(SrTiO3)0.25, in which all the cations (Ag+, Sr2+, Nb5+, and Ti4+) are randomly distributed, the calculation was actually performed by placing the atoms in trans positions to each other so as to simplify the whole calculation process. 3. Results and Discussion 3.1. Materials Characterization. 3.1.1. Crystal Structure and Lattice Parameters. As the typical ABO3 perovskite, AgNbO3 and SrTiO3 comprise a 3D framework of cornersharing [BO6] (B ) Nb or Ti) octahedra with A cations (A ) Ag or Sr) occupying its cavities (Figure 1). At room temperature, SrTiO3 possesses an ideal cubic perovskite structure (space group Pm3jm) in which the bond angle Ti-O-Ti ) 180° (part a of Figure 1).16 Upon cooling, it transforms to a tetragonal phase (space group P42/nbm) at -163 °C.17 In contrast, AgNbO3 at room temperature has an orthorhombic structure (space group Pbcm) (part b of Figure 1).18 Upon heating, AgNbO3 undergoes a sequence of displacive transitions from orthorhombic to neartetragonal at ∼325 °C and then to cubic at ∼560 °C.19 These symmetric transitions occur mainly via tilting of [NbO6]/[TiO6] octahedra about the cubic or pseudocubic 〈100〉 axes. Considering the transition sequences in SrTiO3 upon cooling and in AgNbO3 upon heating, one expects that the (AgNbO3)1-x(SrTiO3)x solid solutions may undergo a similar sequence of transitions that mimic the effect of temperature on the change of AgNbO3 perovskite framework from the prototypic cubic phase to orthorhombic phase upon cooling from calcination temperature to room temperature. Figure 2 shows the powder XRD patterns of (AgNbO3)1-x(SrTiO3)x (0 e x e 1) at room temperature. We can see that, with the (Sr2+ + Ti4+) f (Ag+ + Nb5+) substitution, certain

(AgNbO3)1-x(SrTiO3)x Semiconductors

Figure 2. (a) X-ray diffraction patterns of the (AgNbO3)1-x(SrTiO3)x perovskite-type solid solutions at room temperature. (b) Enlarged profile of the (114) diffraction for 0 e x e 0.9 and the (110) diffraction for 0.9 < x e 1.

sets of split diffraction peaks in AgNbO3 were gradually converged into single peaks, suggesting a systematic increase in crystal symmetry. Rietveld refinement clarified that the solid solutions (AgNbO3)1-x(SrTiO3)x crystallized in an orthorhombic system (0 e x < 0.9) or a cubic system (0.9 e x e 1). These compositionally induced phase transitions are analogous to the thermally induced transformations in AgNbO3 on heating (orthorhombic f tetragonal f cubic) and in SrTiO3 on cooling (cubic f tetragonal). The refined lattice parameters (Table S1 in the Supporting Information) did not change linearly with the (Sr2+ + Ti4+) f (Ag+ + Nb5+) substitution. This phenomenon was also observed clearly from the (114) diffraction for 0 e x < 0.9 and the (110) diffraction for 0.9 e x e 1 in an enlarged XRD profile as shown in Figure 2 (b): the peak shift due to (Sr2+ + Ti4+) f (Ag+ + Nb5+) substitution was not continuous around x ) 0.7. We think that the nonlinear change of lattice parameters was probably related to the precipitation of metallic silver. Part a of Figure 2 clearly shows that, besides the main diffraction from the perovskite phase, additional diffractions from the face-centered cubic (fcc) metallic silver were also indexed. The precipitation of metallic silver indicated the deficiency of Ag, that is nonstoichiometry, in the nominal (AgNbO3)1-x(SrTiO3)x. We ascribe this behavior to a result of structural accommodation upon cooling. A similar phenomenon was also observed in AgNbO3 previously,20 which was supposed to be accompanied by reduction of oxygen, or release of oxygen, or formation of some Nb atoms with valences lower than +5. However, the X-ray photoelectron spectroscopy (XPS) analysis results revealed that, in our solid solution (AgNbO3)1-x(SrTiO3)x samples, the valence states of Nb5+, Sr2+, Ti4+, and O2- were not changed but probably accompanied by the formation of oxygen vacancies due to the precipitation of metallic silver. As will be demonstrated in the section 3.2.3, the Ag nanoparticles precipitated

J. Phys. Chem. C, Vol. 113, No. 9, 2009 3787 on the catalyst surface are supposed to play a very significant role in improving the photocatalytic activity. 3.1.2. Morphology, BET Surface Area, and UV-Wis Diffuse Reflectance Spectrum. SEM observation showed that the mean particle size of (AgNbO3)1-x(SrTiO3)x solid solution powders decreased with increasing the (Sr + Ti) content. This change can be clearly seen from three typical SEM micrographs of AgNbO3, (AgNbO3)0.75(SrTiO3)0.25, and SrTiO3 powder samples as shown in Figure 3. In consistence with the SEM observation, the BET surface area of (AgNbO3)1-x(SrTiO3)x increased with increasing the (Sr + Ti) content (Figure S1 in the Supporting Information). Figure 4 shows the UV-vis diffuse reflectance spectra of the (AgNbO3)1-x(SrTiO3)x (0 e x e 1) powder samples at room temperature. We can see that the absorption edges of solid solutions (AgNbO3)1-x(SrTiO3)x were located between those of SrTiO3 (∼380 nm) and AgNbO3 (∼450 nm). With increasing the (Sr + Ti) content, the absorption edge was gradually shifted to the longer wavelength range. In the (AgNbO3)1-x(SrTiO3)x samples with a small amount of AgNbO3 (e.g., x ) 0.95), the background was very high and the absorption edge was not so sharp. Similar phenomenon was also observed in (Ag+ + Nb5+)codoped SrTiO3,21 which showed a fairly low visible-light activity for decomposition of IPA because only separate doping levels within the forbidden band of SrTiO3 were formed by the small amount of dopants (e3 mol%). In contrast, no additional absorption edge was observed in the UV-vis spectra of (AgNbO3)1-x(SrTiO3)x solid solutions with higher AgNbO3, suggesting the formation of a continuous valence band by the hybridized (Ag 4d + O 2p) orbitals and a continuous conduction band by the hybridized (Ti 3d + Nb 4d) orbitals. The details will be discussed in section 3.3. We also noticed that even for the well hybridized samples, the backgrounds in the long wavelength range of UV-vis spectra were increased in comparison with the two end compounds AgNbO3 and SrTiO3. This could be ascribed to the precipitation of metallic silver as detected in the XRD patterns. It should be pointed out that a solid solution sample was absolutely different from a simple mixture of the same composition. This could be clarified clearly from the UV-vis spectrum of a mixture (3/4 AgNbO3 + 1 /4 SrTiO3) that had the same compositions of (AgNbO3)0.75(SrTiO3)0.25. As shown in Figure 5 (inset), the solid solution had a sharp absorption edge at ∼440 nm, whereas the mixture (dot line) showed two separate absorption edges that correspond to SrTiO3 and AgNbO3, respectively. 3.2. Photocatalytic Activities. 3.2.1. Photocatalytic O2 eWolution. Figure 5 shows O2 evolution from an aqueous AgNO3 solution suspended with the (AgNbO3)1-x(SrTiO3)x powders under visible-light irradiation. Compared to the two end compounds AgNbO3 and SrTiO3, the solid solutions (AgNbO3)1-x(SrTiO3)x showed significantly increased activities. The highest activity was obtained over (AgNbO3)0.75(SrTiO3)0.25: in the initial 2 h of reaction, the average evolution rate was 162 µmol/h; using an interference filter (λ0 ) 420.4 nm, Tmax ) 44.8%, ∆λ/2 ) 14.7 nm; optical coatings), the apparent quantum efficiency (Q.E.) at λ ) 420.4 nm was measured to be ∼16.4%.30 A dark test with (AgNbO3)0.75(SrTiO3)0.25 showed that no O2 evolution was observed when the irradiation light was turned off. Using a series of cutoff filters, the wavelength dependence of O2 evolution over (AgNbO3)0.75(SrTiO3)0.25 was measured to be well consistent with the UV-vis diffuse reflectance spectrum (Figure S2 in the Supporting Information). All of these results confirmed that the O2 evolution was inherently the result of photocatalytic reaction. The gradual decrease of O2 evolution

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Figure 3. Representative SEM micrographs of (a) AgNbO3, (b) (AgNbO3)0.75(SrTiO3)0.25, and (c) SrTiO3. The scale bar is 2 µm.

Figure 4. UV-vis diffuse reflectance spectra of the (AgNbO3)1-x (SrTiO3)x powder samples at room temperature. The inset shows the UV-vis absorption spectra of (a) SrTiO3, (b) the mixture of (0.75 AgNbO3 + 0.25 SrTiO3), (c) the solid solution (AgNbO3)0.75(SrTiO3)0.25, and (d) AgNbO3.

Figure 5. Photocatalytic O2 evolution from aqueous AgNO3 solution (5 mmol AgNO3 + 270 mL H2O) suspended with the (AgNbO3)1-x(SrTiO3)x powder catalysts (0.5 g) under visible-light irradiation (λ g 410 nm). Light intensity: ∼30 mW/cm2. The inset shows the O2 evolution rate as a function of x value in (AgNbO3)1-x(SrTiO3)x.

rate with increase of irradiation time was probably due to the shielding effect by more and more metallic silver reduced on the catalyst surface from the AgNO3 solution. 3.2.2. Photocatalytic Decomposition of IPA. As described previously, a strong oxidative ability of (AgNbO3)1-x(SrTiO3)x has been demonstrated by the efficient photocatalytic O2 evolution, implying the possibility of efficient decomposition of some organic substances over these solid solutions. Gaseous 2-proponal (IPA) is a model organic pollutant that is commonly used for photocatalytic decomposition to evaluate the activity of a semiconductor photocatalyst. With O2 being the electron acceptor, 2-proponal ((CH3)2CHOH) is first decomposed into acetone (CH3COCH3), and then acetone is further decomposed into the final product CO2. Similar to the case of O2 evolution, the evolution rates of acetone and CO2 from decomposition of

Figure 6. Photocatalytic IPA decomposition into acetone and CO2 over the (AgNbO3)0.75(SrTiO3)0.25 powder catalyst (0.4 g) under visiblelight irradiation (λ g 410 nm). Light intensity: ∼30 mW/cm2. The inset shows evolution rates of acetone and CO2 as a function of x value in (AgNbO3)1-x(SrTiO3)x.

IPA were also closely dependent on the x value in (AgNbO3)1-x(SrTiO3)x, and the highest activity was obtained over (AgNbO3)0.75(SrTiO3)0.25 (insert in Figure 6). Figure 6 shows a typical process of the decomposition of IPA into acetone and CO2 over (AgNbO3)0.75(SrTiO3)0.25 under visiblelight irradiation. Essentially, the photocatalytic process of (CH3)2CHOH f CH3COCH3 f CO2 can be divided into two stages. In the first stage, the concentration of (CH3)2CHOH decreased to zero very quickly and the concentration of CH3COCH3 reached the maximum value simultaneously. However, only a small amount of CO2 was observed in this stage. This means that the first stage was mainly involved the decomposition of (CH3)2CHOH f CH3COCH3. In the second stage, the reaction was mainly involved in the decomposition of CH3COCH3 f CO2, which could be characterized as a second-order reaction. The reaction rate became lower in the late period; nevertheless, the final concentration of CO2 (∼24.2 µmol) after 1320 min of reaction was nearly three times that of the initially injected IPA (∼8.2 µmol), that is almost all the injected IPA was mineralized, reaching a carbon balance eventually. Assuming that the reaction from IPA to CO2 is proceeded as: C3H8O + 5H2O + 18 h+ f 3CO2 + 18H+,21 the quantum efficiency (Q.E.) was calculated using the following equation:

Q . E . (%) ) Npr ⁄ Npi ) 6NCO2 ⁄ Npi

(1)

where, NCO2 is the number of CO2 molecules, Npr is the number of photons involved in the reaction of CO2 generation, and Npi is the number of incident photons. Using an interference filter (λ0 ) 420.4 nm, Tmax ) 44.8%, ∆λ/2 ) 14.7 nm), the apparent quantum efficiency (Q.E.) of CO2 evolution over (AgNbO3)0.75-

(AgNbO3)1-x(SrTiO3)x Semiconductors

Figure 7. Photocatalytic IPA decomposition into acetone and CO2 over the (AgNbO3)0.75(SrTiO3)0.25 powder catalyst (0.4 g) under visiblelight irradiation. Light source: BLEDs (410 nm e λ e 530 nm). Light intensity: ∼0.01 mW/cm2. The inset shows the photon spectrum of BLEDs.

(SrTiO3)0.25 at λ ) 420.4 nm was measured to be ∼3.4%. We expect that the quantum efficiency of our solid solution catalyst could be further improved by increasing its surface area. It is worthy to note that, as shown in Figure 7, IPA decomposition into acetone and CO2 over (AgNbO3)0.75(SrTiO3)0.25 could be proceeded under very weak visible light (∼0.01 mW/cm2) emitted from the blue-light-emitting diodes (BLEDs). A dark test showed that, when the light was turned off, almost no evolution of both acetone and CO2 could be observed (Figure S3 in the Supporting Information). This result suggests that the reported solid solution material is a very promising visible-light photocatalyst for practical application in indoor air purification, where only weak visible light is available. 3.2.3. Role of Ag Nanoparticles Precipitated on the Catalyst Surface. It is well-known that the activity of a noble-metalmediated semiconductor photocatalyst for either water splitting or oxidization of organic compounds can be improved significantly due to the enhanced interfacial charge carrier kinetics.22 Numerous investigations have reported that the addition of noble metals such as gold,23 platinum,24 silver,25 or palladium26 may enhance the overall photoefficiency of TiO2. This effect is almost conclusively attributed to a reduction in the recombination rate of photoinduced electron/hole pairs in such a way that the electrons accumulate on the noble metal, whereas the holes remain on the photocatalyst surface.27 As mentioned previously, the metallic silver (fcc) was detected in the XRD patterns of (AgNbO3)1-x(SrTiO3)x. From a typical HRTEM micrograph of the nominal (AgNbO3)0.75(SrTiO3)0.25 powder sample as shown in Figure 8, we can see clearly that nanosized Ag particles (∼10 nm) were precipitated on the sample surface. Similarly, the nanosized Ag particles also play a very important role in improving the activity of the (AgNbO3)1-x(SrTiO3)x system. To verify this effect, we purposely synthesized a series of nominal compounds Agy(NbO3)0.75(SrTiO3)0.25 (y ) 0.5, 0.65, 0.75, 0.85, 1.0, 1.25, and 1.5) with different amounts of silver. XRD analysis confirmed that the amount of precipitated metallic silver varied with the y value in Agy(NbO3)0.75(SrTiO3)0.25 (Figure S4 in the Supporting Information). Correspondingly, the background of UV-vis absorption spectrum increases with increasing the y value (Figure S5 in the Supporting Information). Figure 9 shows that the activity for O2 evolution or for IPA decomposition is closely dependent on the deficiency or excess of metallic silver nanoparticles, and the best performance was obtained over (AgNbO3)0.75(SrTiO3)0.25, in which the appropriate amount of Ag nanoparticles enhanced the separation and migration abilities

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Figure 8. Typical HRTEM micrograph of (AgNbO3)0.75(SrTiO3)0.25 powder sample. Metallic Ag nanoparticles of ∼10 nm were precipitated on the sample surface to construct a nanocomposite structure Ag/ (AgNbO3)0.75(SrTiO3)0.25.

Figure 9. (a) O2 evolution rates from aqueous AgNO3 solution (5 mM AgNO3 + 270 mL H2O), (b) acetone and CO2 evolution rates from photocatalytic decomposition of IPA as a function of the y value in Agy(NbO3)0.75(SrTiO3)0.25 under visible-light irradiation; (λ g 410 nm); light intensity, ∼30 mW/cm2.

of photoinduced charge carriers. The decreased activity in the materials with too small amount of precipitated Ag could be ascribed to the deficient charge carrier separation effect, whereas the decreased activity in the materials with too much precipitated Ag was due to the shielding effect of metallic silver on the sample surface. In this sense, our solid solution material can be regarded as a nanocomposite structure Ag/(AgNbO3)1-x(SrTiO3)x. 3.3. Electronic Structure. The electronic structure (band gap energy, band edge potentials) of a semiconductor material plays a crucial role in its functional properties. For example, photocatalytic decomposition of organic compounds in the presence of O2 generally involves: (1) photoexcitation of electrons from the valence band (VB) to the conduction band (CB), (2) oxidation of organic compound directly via the VB holes or indirectly with the surface-bound hydroxyl radical (•OH), and (3) reduction of O2 by the CB electrons. Dynamically, increased

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Figure 10. Calculated band structures and projected total density of states (DOS) for (a)-(a’) AgNbO3, (b)-(b’) (AgNbO3)0.75(SrTiO3)0.25, and (c)-(c’) SrTiO3.

overpotentials, that is the difference between VB top potential (Hvb) and oxidation potential (Hox) and the difference between CB bottom potential (Hcb) and reduction potential (Hred), are clearly favorable for the redox reactions. In particular, the presence of O2, which acts as the primary electron acceptor, also plays a significant role in preventing the recombination of photoexcited electrons and holes so as to promote the subsequent redox reactions.28 In terms of the kinetics of surface redox reactions, a small bandgap is equally important to the visiblelight activity of a semiconductor photocatalyst. With the lattice parameters and the atomic coordinates and occupation factors,18 the electronic structures of SrTiO3, AgNbO3, and (AgNbO3)0.75(SrTiO3)0.25 were calculated by the plane-wave-density function theory (DFT) using the CASTEP program package. Parts a-a’, b-b’, and c-c’ of Figure 10 show the calculated band structures and projected total density of states (DOS) of AgNbO3, (AgNbO3)0.75(SrTiO3)0.25, and SrTiO3, respectively. For SrTiO3, the conduction band (CB) is comprised of the empty Ti 3d orbitals and the valence band (VB) is composed of O 2p orbitals. For AgNbO3, the CB is composed of the empty Nb 4d, whereas the VB is composed of the hybridized (O 2p + Ag 4d) orbitals. For (AgNbO3)0.75(SrTiO3)0.25, the CB is constructed by hybridized (Ti 3d + Nb

4d) orbitals and the VB is constructed by the hybridized (O 2p + Ag 4d) orbitals. As mentioned previously, some metallic silver nanoparticles were precipitated on the sample surface, that is the ionic silver in the solid solution lattice was not exactly the nominal amount. As a consequence, the calculated bandgap of (AgNbO3)0.75(SrTiO3)0.25 was only a qualitative value rather than a quantitative one. Nevertheless, our calculation result has clearly shown the hybridization effect of (Ti 3d and Nb 4d) and (O 2p + Ag 4d) on modulation of the energy band structures of (AgNbO3)1-x(SrTiO3)x. On the basis of the experimental and theoretically calculated results, the energy band structures of SrTiO3, AgNbO3, (AgNbO3)0.75(SrTiO3)0.25, and (Ag+ + Nb5+)-codoped SrTiO3 are schematically illustrated in Figure 11. Apparently, the involvement of Ag 4d orbitals moves the VB top toward the more negative position, whereas the potential of Nb 4d orbitals is more positive than that of Ti 3d orbitals. As a consequence, the band gaps of (AgNbO3)1-x(SrTiO3)x are smaller than that of SrTiO3 but larger than that of AgNbO3. On the other hand, the oxidative ability of (AgNbO3)1-x(SrTiO3)x is higher than that of AgNbO3 but lower than that of SrTiO3, whereas its reductive ability is higher than AgNbO3 but lower than SrTiO3. Owing to its large band gap, SrTiO3 shows hardly visible-light activity

(AgNbO3)1-x(SrTiO3)x Semiconductors

J. Phys. Chem. C, Vol. 113, No. 9, 2009 3791 structure, the Ag nanoparticles precipitated on the catalyst surface also play an important role in enhancing the separation and migration ability of photoinduced charge carriers and thus in improving the photocatalytic activities. The present study proves that making solid-solution oxide semiconductors with tunable electronic structures is a feasible band engineering approach for the development of highly visible-light-active photocatalysts. Acknowledgment. This work was partially supported by the World Premier International Research Center Initiative (WPI Initiative) on Materials Nanoarchitectonics and by the Global Environment Research Fund, MEXT, Japan.

Figure 11. Band structures of AgNbO3, SrTiO3, and (AgNbO3)1-x(SrTiO3)x with x ) 0.25 and 0.95, respectively. The hybridization of the Ag 4d with O 2p lifts the valence band top. In the sample of (AgNbO3)0.75(SrTiO3)0.25, both the conduction band and valence band are continuous. In the case of (AgNbO3)0.05(SrTiO3)0.95, discontinuous interbands (or doping levels) are formed by the small amount of dopants.

although its CB bottom potential is sufficiently negative (-0.8 eV vs SHE).29 On the contrary, the CB bottom of AgNbO3 is very close to H+/H2 (0 eV vs SHE) or O2/O2- (-0.046 eV vs SHE), thus the over potential is too small to reduce O2 by the photoexcited electrons, resulting in a very low visible-light activity although its band gap is small. For (Ag0.75Sr0.25)(Nb0.75Ti0.25)O3, it appears that a good compatibility between the absorption ability to visible-light and the reductive/oxidative ability has been obtained in (Ag0.75Sr0.25)(Nb0.75Ti0.25)O3 with a modulated band structure. In addition, the VB and CB respectively formed by the hybridized (Ag 4d + O 2p) orbitals and the hybridized (Ti 3d + Nb 4d) orbitals are supposed to disperse continuously in a relatively wide energy range, which is undoubtedly favorable for the charge carrier transportation. Thus, the photocatalytic activities for both O2 evolution and decomposition of organic compounds (IPA, acetone, acetaldehyde, etc.) under visible-light irradiation are significantly enhanced. It is worthy to note that when the x value in (AgNbO3)1-x(SrTiO3)x is less than 0.25 (e.g., (AgNbO3)0.85(SrTiO3)0.15), the activities of the solid solutions reduced significantly in comparison with (AgNbO3)0.75(SrTiO3)0.25 (Figures 6 and 7). Probably, the CB bottom of (AgNbO3)0.85(SrTiO3)0.15 is not negative sufficiently, that is the over potential is not larger enough for the reduction of O2 by the CB electrons, although their band gaps are smaller than that of (AgNbO3)0.75(SrTiO3)0.25. 4. Conclusions A novel series of solid solution photocatalysts (AgNbO3)1-x(SrTiO3)x crystallized in an orthorhombic system (0 e x < 0.9) or a cubic system (0.9 e x e 1) have been found to show high activities for O2 evolution and decomposition of organic pollutants under visible-light irradiation. The mixed valent perovskites (AgNbO3)1-x(SrTiO3)x possess a modulated energy band structure, in which the conduction band is composed of the hybrid (Ti 3d + Nb 4d) orbitals and the valence band is constructed by the hybrid (O 2p + Ag 4d) orbitals. The modulation of band structure (band gap energy, band edge positions, etc.) depends on the extent of orbital hybridization, giving rise to an optimal composition of (AgNbO3)0.75(SrTiO3)0.25 that shows the highest photocatalytic activity under visible-light irradiation. In addition to the modulated band

Supporting Information Available: . BET surface area of (AgNbO3)1-x(SrTiO3)x powders as a function of x ) (Sr + Ti). Wavelength dependence of O2 evolution over the (AgNbO3)0.75(SrTiO3)0.25 powder catalyst (0.5 g) from aqueous AgNO3 solution (5 mM AgNO3 + 270 mL H2O). Powder XRD patterns and UV-vis diffuse reflectance spectra of Agy(NbO3)0.75(SrTiO3)0.25 powders (y ) 0.5, 0.65, 0.75, 0.85, 1.0, 1.25, 1.5). A dark test reaction for decomposition of IPA using BLED light. This material is available free of charge via the Internet at http:// pubs.acs.org. References and Notes (1) (a) Hoffmann, M. R.; Martin, S. T.; Choi, W.; Bahnemann, D. W. Chem. ReV. 1995, 95, 69. (b) Linsebigler, A. L.; Lu, G. Q.; Yates, J. T. Chem. ReV. 1995, 95, 735. (c) Fujishima, A.; Hashimoto, K.; Watanabe, T. TiO2 Photocatalysis: Fundamentals and Applications; BKC Inc.: Tokyo, Japan 1999. (2) (d) Fujishima, A.; Honda, K. Nature 1972, 238, 37. (3) Serpone, N. J. Phys. Chem. B 2006, 110, 24287. (4) (a) O’Regan, B.; Gratzel, M. Nature 1991, 353, 737. (b) Gru¨nwald, R.; Tributsch, H. J. Phys. Chem. B 1997, 101, 2564. (c) He, J.; Benko¨, G.; Korodi, F.; Polı´vka, T.; Lomoth, R.; Åkermark, B.; Sun, L.; Hagfeldt, A.; Sundstro¨m, V. J. Am. Chem. Soc. 2002, 124, 4922. (d) Chen, C.; Zhao, W.; Li, J.; Zhao, J.; Hidaka, H.; Serpone, N. EnViron. Sci. Technol. 2002, 36, 3604. (5) (a) Herrmann, J. M.; Disdier, J.; Pichat, P. Chem. Phys. Lett. 1984, 108, 618. (b) Rodrigues, S.; Ranjit, K. T.; Uma, S.; Martyanov, I. N.; Klabunde, K. J. AdV. Mater. 2005, 17, 2467. (6) (a) Asahi, R.; Morikawa, T.; Ohwaki, T.; Aoki, K.; Taga, Y. Science 2001, 293, 269. (b) Khan, S. U. M.; Al-Shahry, M.; Ingler, W. B. Science 2002, 297, 2243. (c) Sakthivel, S.; Kisch, H. Angew. Chem., Int. Ed. 2003, 42, 4908. (7) (a) Zou, Z.; Ye, J.; Sayama, K.; Arakawa, H. Nature 2001, 414, 625. (b) Tang, J.; Zou, Z.; Ye, J. Angew. Chem., Int. Ed. 2001, 43, 4463. (8) (a) Wang, D.; Zou, Z.; Ye, J. Chem. Mater. 2005, 17, 5177. (b) Wang, D.; Ye, J.; Kitazawa, H.; Kimura, T. J. Phys. Chem. C 2007, 111, 12848. (9) (a) Tsuji, I.; Kato, H.; Kobayashi, H.; Kudo, A. J. Am. Chem. Soc. 2004, 126, 13406. (b) Yao, W.; Ye, J. J. Phys. Chem. B 2006, 110, 11188. (c) Maeda, K.; Teramura, K.; Lu, D.; Takata, T.; Saito, N.; Inoue, Y.; Domen, K. Nature 2006, 440, 295. (10) (a) Wrighton, M. S.; Ellis, A. B.; Wolczanski, P. T.; Morse, D. L.; Abrahamson, H. B.; Ginley, D. S. J. Am. Chem. Soc. 1976, 98, 277. (b) Domen, K.; Kudo, A.; Onishi, T.; Kosugi, N.; Kuroda, K. J. Phys. Chem. 1986, 90, 292. (11) Kato, H.; Kobayashi, H.; Kudo, A. J. Phys. Chem. B 2002, 106, 12441. (12) Wang, D.; Kako, T.; Ye, J. J. Am. Chem. Soc. 2008, 130, 1724. (13) Izumi, F.; Ikeda, T. Mater. Sci. Forum 2000, 198, 321–324. (14) Larson, S. A.; Widegren, J. A.; Falconer, J. L. J. Catal. 1995, 157, 611. (15) Payne, M. C.; Teter, M. P.; Allan, D. C.; Arias, T. A.; Joannopoulos, J. D. ReV. Mod. Phys. 1992, 64, 1045. (16) Abramov,Yu., A.; Tsirel’son, V. G.; Zavodnik, V. E.; Ivanov, S. A.; Brown, I. D. Acta Crystallogr. B 1995, 51, 942. (17) Fleury, P. A.; Scott, J. F.; Worlock, J. M. J. M. Phys. ReV. Lett. 1968, 21, 16. (18) (a) Sakowski-Cowley, A. C.; Lukaszewicz, K.; Megaw, H. D. Acta Crystallogr. 1969, B25, 851. (b) Sciau, P.; Kania, A.; Dkhil, B.; Suard, E.; Ratuszna, A. J. Phys.: Condens. Matter 2004, 16, 2795. (19) Francombe, M. H.; Lewis, B. Acta Crystallogr. 1958, 11, 175.

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