Photophysical, Photoelectrochemical, and Photocatalytic Properties of

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J. Phys. Chem. C 2009, 113, 10647–10653

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Photophysical, Photoelectrochemical, and Photocatalytic Properties of Novel SnWO4 Oxide Semiconductors with Narrow Band Gaps In-Sun Cho, Chae Hyun Kwak, Dong Wook Kim, Sangwook Lee, and Kug Sun Hong* Department of Materials Science & Engineering, Seoul National UniVersity, Shillim-dong San 56-1, Gwanak-gu Seoul, 151-744, Korea ReceiVed: February 20, 2009; ReVised Manuscript ReceiVed: May 6, 2009

Novel SnWO4 visible-light active photocatalysts with two polymorphs (orthorhombic R and cubic β phases) were prepared by a conventional solid-state reaction method, and their optical properties, electronic band structure, and photocatalytic activities were investigated. It was found that the low-temperature phase, R-SnWO4 with corner-shared WO6 octahedra, exhibited a dark-red color and indirect band gap of 1.64 eV, whereas the high-temperature phase, β-SnWO4 with unshared WO4 tetrahedra, exhibited a light-yellow color and direct band gap of 2.68 eV. The Mott-Schottky plots obtained using a thick film electrode in 1 M NaCl electrolyte revealed the n-type semiconductive properties of the SnWO4 polymorphs; i.e., the flat-band potential values of R- and β-SnWO4 were -0.61 and -0.66 V (SCE), respectively. From the electronic band structure calculations performed using density functional theory, the Sn 5p and O 2p orbitals were hybridized to construct the valence band in both SnWO4 polymorphs. However, the constructions of the conduction band were quite different. β-SnWO4 with its shorter W-O bond lengths in the WO4 tetrahedra has a higher conduction-band potential than R-SnWO4 phase, which has larger W-O bond lengths in the WO6 octahedra and, thus, was able to produce H2 from an aqueous methanol solution under visible-light irradiation (>400 nm). Both SnWO4 polymorphs also exhibited good photocatalytic activity for the degradation of rhodamine B dye solution under visible-light irradiation (>420 nm). The photocatalytic activity of these SnWO4 polymorphs was higher than that of other visible-light active photocatalysts with much smaller particle sizes, such as nanosized WO3 (9.72 m2/g) and TiONx (112.13 m2/g). This higher photocatalytic activity of the SnWO4 polymorphs is mainly attributed to their smaller band gaps and unique band structures, resulting from their different bonding nature. 1. Introduction In the past few years, global environmental deterioration has become more serious year by year, and, thus, scientific interest in environmental technology has grown exponentially.1 Among these technologies, photocatalysis using solar energy is an area of particular interest, because it is considered a promising “green” technology for solving these environmental problems as well as providing renewable energy sources such as hydrogen.2-4 The removal of organic contaminants with solar energy using photocatalysts, which mainly involves the oxidative decomposition of volatile organic compounds (VOCs) and the purification of wastewater, has many advantages over other treatment methods; for instance, it involves the use of the environmentally friendly oxidant, O2, the reaction is performed at room temperature, and it allows for the oxidation of organic compounds, even at low concentrations.5,6 To date, TiO2 has undoubtedly proven to be the most efficient photocatalyst for the oxidative decomposition of many organic compounds under UV-light irradiation.2,3,6-8 However, its relatively wide band gap of 3.2 eV limits its further application in the visible-light region (λ > 400 nm). Furthermore, in order to enhance the utilization of solar light and indoor illumination, it is indispensable to develop visible-light-active photocatalysts that are active enough for practical applications. There are two main ways to exploit photocatalysts responsive to visible-light irradiation through band engineering: The first involves the ion doping of wide band gap materials using * Corresponding author. Fax: +82-2-886-4156. Tel: +82-2-880-8024. E-mail: [email protected].

transition metals9-12 and anions13-15 such as Co, Fe, N, and C. The second is the formation of a valence band with elements other than oxygen, in which the new valence band is formed above the O 2p orbitals through hybridization between the p orbitals of oxygen and s and/or p orbitals of the other element.16,17 The first method has been widely investigated through the modification of TiO2, and the recent work by Asahi et al. is a representative example.14 On the other hand, there have only been a few reports on the second method.18 Stannous tungstate (SnWO4) was reported by Jeitschko and Sleight in 1972 to have two polymorphs:19,20 the low-temperature R phase with an orthorhombic structure which is stable below 670 °C and the high-temperature β phase with a cubic structure which is stable above 670 °C. The β phase can be obtained by the rapid quenching in water after calcination above 670 °C. The use of both R- and β-SnWO4 as gas-sensitive oxides was studied by Solis et al.21-23 However, although these two polymorphs of SnWO4 exhibit interesting semiconductive properties, to the best of our knowledge, there have been no other reports on their other optical properties and applications. In the present study, the two polymorphs of SnWO4 were prepared as the pure phases by a conventional solid-state reaction method, and their optical properties and electronic band structure were investigated. Moreover, their novel photocatalytic activities for the degradation of rhodamine B dye, as well as water splitting, under visible-light irradiation were also investigated, thus confirming their ability to act as highly efficient visiblelight-active photocatalysts.

10.1021/jp901557z CCC: $40.75  2009 American Chemical Society Published on Web 05/26/2009

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2. Experimental Section 2.1. Preparation of SnWO4 Polymorph Powders. SnWO4 powders (R and β phase) were prepared by a conventional solidstate reaction method. SnO and WO3 (99.9%, High Purity Chemicals, Japan) were used as the raw materials. Stoichiometric mixtures of the starting materials were homogenized by ball milling with zirconia media for 24 h. The R phase was obtained by calcination at 600 °C for 2 h in an argon atmosphere. The β phase was obtained by rapid quenching at 800 °C after soaking for 2 h in an argon atmosphere. For the comparison of their photocatalytic activity, TiONx powder was prepared by heat treatment at 400 °C in an NH3 flow using amorphous TiO2 powder, which was synthesized by the sol-gel method using 5 M TiCl4 aqueous stock solution (TiCl4 (99%, Junsei Chemicals, Japan) + ice water) and NH4OH (28%).24 The powder slurry used for spin coating on the ITO electrodes was prepared by a balling-milling method (powder: ethanol:dispersant ) 2 g:20 mL:0.1 g) for 6 h, and the thick film electrodes were prepared using this powder slurry via spin coating on the ITO electrodes. After coating twice, the film electrodes were heat treated at 350 °C for 1 h in an air atmosphere. 2.2. Characterizations and Measurements. The crystal structures of the prepared powders were determined using an X-ray powder diffractometer (XRD; M18XHF, Mac-science, Japan). The lattice parameters and atomic coordinates were obtained using GSAS Rietveld refinement software25 and package26 to refine the XRD patterns, which were measured by step scanning (0.02°) using Si (99.999%) as an external standard. The band structure calculation based on plane wave density functional theory (DFT) was performed using the CASTEP program.27 The powder morphologies and microstructures were investigated using a field-emission scanning electron microscope (FESEM; JSM-6330F, JEOL, Japan). The UV-vis diffuse reflectance spectra were obtained using a UV-vis-NIR spectrophotometer (U-4001, Hitachi, Japan) at room temperature and were converted to the absorbance spectra by the Kubelka-Munk method. The specific surface areas of the powder samples were measuredbyN2 adsorptionat77KusingaBrunauer-Emmett-Teller (BET) surface area analyzer (BELSORP-mini II, BEL, Japan). The X-ray photoelectron spectroscopy spectra were collected using an ESCA spectrometer (XPS; Al KR X-ray source, SIGMA PROBE, U.K.). The obtained binding energy (BE) was calibrated with respect to the C 1s value of contaminated carbon of 284.5 eV. The photoelectrochemical measurements were performed using a potentiostat (CHI 608A, CH Instruments, U.S.) in a conventional three-electrode cell with a platinum counter electrode and a saturated calomel reference electrode (SCE). 2.3. Photocatalytic Reactions. Degradation of Rhodamine B Dye. The photocatalytic degradation of RhB dye solution was performed under visible-light irradiation (>420 nm) using 100 W tungsten halogen lamps (Ushio) with a cutoff filter (λ > 420 nm). The reaction bath (Pyrex glass) was placed at the center of a black acryl box (200 mm × 150 mm × 200 mm), on either side of which was positioned a 100 W halogen lamp (distance between lamps ) 200 mm). The reaction suspensions were

Figure 1. XRD patterns of SnWO4 polymorph powders prepared by calcination in an argon atmosphere: (a) R-SnWO4, furnace cooling (10 °C/min) after calcination at 600 °C for 2 h; (b) β-SnWO4, rapid cooling (300 °C/min) after calcination at 800 °C for 2 h.

prepared by adding the powder (0.3 g) to 100 mL of an RhB solution (10-5 M). The suspensions were sonicated for 10 min and then stirred in the dark for 60 min to ensure adsorption/ desorption equilibrium prior to irradiation. During irradiation, 2 mL of the suspension was removed at given time intervals for subsequent RhB concentration analysis following centrifugation. The RhB concentration was analyzed using a UV-vis spectrophotometer (Lambda 35, Perkin-Elmer, U.S.). For comparison, the photocatalytic activities of bulk-WO3 powder (99.9%, High Purity Chemicals) with a BET surface area of 2.93 m2/g, nanosized-WO3 powder (99.9%, Aldrich Chemicals, 9.72 m2/g), and TiONx powder (112.13 m2/g) were also evaluated under the same conditions. H2 or O2 EWolution from Water Splitting. The photocatalytic reactions for H2 or O2 evolution were conducted at room temperature in a closed gas circulation system with an outerirradiation-type quartz reactor (200 mL). A 300 W xenon lamp system (MAX302, Asahi spectra, Japan) was used as the visiblelight source. The distance between the reactor and the lamp was 10 cm. The photocatalyst powder (0.3 g) was dispersed in an aqueous solution of CH3OH/H2O (130 mL of distilled water + 20 mL of methanol) or AgNO3/H2O (1 mmol + 150 mL of distilled water) by magnetic stirring after sonication (10 min). The amount of H2 or O2 evolved was determined using gas chromatography (Donam, DS6200, Korea). The Pt cocatalyst was loaded on the surface of the powder samples by an in situ photodeposition method using an aqueous H2PtCl6 · 6H2O solution. 3. Results and Discussion 3.1. Crystal Structures and Powder Morphologies. Figure 1 shows the XRD patterns of the SnWO4 polymorph powders prepared by the solid-state reaction method. The R-SnWO4 (RSW) powder was prepared by calcination at 600 °C for 2 h in an argon flow (500 cm3/min). To obtain the β-SnWO4 (β-SW) powder, the rapid quenching method was employed after calcination at 800 °C for 2 h in an argon flow (500 cm3/min), because the β phase reversibly transforms into the R phase.19 Both powders exhibited single phases, and all of the reflection

TABLE 1: Crystal Structure Parameters and BET Surface Areas of SnWO4 Polymorphs lattice parameters (Å) phases

crystal system

a

b

c

unit cell volume (Å3)

surface area (m2/g)

R-SnWO4 β-SnWO4

orthorhombic cubic

5.6274 ( 0.0002 7.2977 ( 0.0002

11.6489 ( 0.0004 -

4.9975 ( 0.0002 -

327.6 388.6

2.77 0.56

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Figure 2. FESEM images of SnWO4 polymorph powders: (a) R-SnWO4; (b) β-SnWO4.

peaks were clearly indexed as the orthorhombic phase (JCPDS No. 29-354) for R-SW and cubic phase (JCPDS No. 70-1497) for β-SW. The crystal structure parameters of these SnWO4 polymorphs calculated by the Rietveld refinement method are summarized in Table 1. The typical FESEM images of the SnWO4 polymorph powders are shown in Figure 2. As can be seen in the figure, the R-SW powder exhibited a spherical morphology with an average size of 1 µm, whereas the β-SW powder exhibited a large, irregular morphology with a size range of 1-3 µm, as well as broken pieces resulting from the rapid quenching process. The BET surface area measurements showed that the R-SW had a higher surface area (2.77 m2/g) than the β-SW (0.56 m2/g), which were consistent with the FESEM results. After the photocatalytic reactions, these SnWO4 powders were checked again by XRD and FESEM (Figure S1 and S2). No changes in either their crystal structures or morphologies were detected, thus indicating the stability of these powders against visible-light irradiation. 3.2. Optical Absorption Properties and Electronic Band Structures. Figure 3 shows the diffuse reflectance spectra of the SnWO4 polymorph powders. Both powders exhibited a strong absorption band in the visible-light region. The absorption edges were 692 nm for R-SW and 537 nm for β-SW, suggesting that R-SW could be responsive to incident photons with less energy (smaller band gap). From the electronic band structure calculation (to be discussed later), R-SW and β-SW were found to have indirect and direct band gaps, respectively, and, thus, the optical band gaps could be estimated from the tangent lines in the (Rhν)1/2 versus hν plots for R-SW and (Rhν)2 versus hν plots for β-SW, respectively (Figure 3b).28 The estimated band gap of R-SW (1.64 eV) was much smaller than that of β-SW (2.68 eV). To further understand these different band gap natures in the SnWO4 polymorphs, we examined the relationship between the crystal structure and electronic band structure. With the data of the atomic positions reported in the literature,19,20 the crystal structures of R-SW and β-SW were constructed (Figure 4). In the case of R-SW, the tungsten atom is octahedrally coordinated with the oxygen atoms to form corner-shared WO6 octahedra, whereas β-SW is composed of unshared WO4 tetrahedra. The SnO6 octahedra were asymmetric and distorted in both polymorphs, due to the lone pair effect of Sn2+.23,29,30 The effects of these different crystal structure environments in the SnWO4 polymorphs on their electronic band structures were investigated by the DFT method. Figure 5 shows the band structures and density of states (DOS) of the monoclinic WO3 and SnWO4 polymorphs. The band structure of WO3 with distorted corner-sharing WO6 octahedra is well established,31,32 with its conduction band being mainly composed of the empty W 5d orbitals and valence band mainly composed of the filled O 2p orbitals. The calculated band structure and DOS of WO3

Figure 3. (a) UV-visible diffuse reflectance spectra and (b) optical band gap determination of R-and β-SnWO4 powders.

Figure 4. Crystal structures of SnWO4 polymorphs: (a) R-SnWO4 (orthorhombic); (b) β-SnWO4 (cubic).

shown in Figure 5a were in good agreement with the previously reported values. In the case of the SnWO4 polymorphs, however, the valence band was hybridized by the Sn 5s and O 2p orbitals, due to the strong interaction between them. The Sn 5s orbitals contributed to the upper and lower ends of the valence band, while the O 2p orbitals contributed to the middle of the valence band. These valence band constructions affected the band gap in R-SW; i.e., the calculated band gap of R-SW (1.65 eV) was decreased compared with that of WO3 (1.77 eV), which was attributed to

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Figure 5. Electronic band structures and density of states (DOS) of WO3 and SnWO4 polymorphs: (a) monoclinic WO3, (b) orthorhombic R-SnWO4, and (c) cubic β-SnWO4.

Figure 6. Partial density of states of (a) R-SnWO4 and (b) β-SnWO4.

the broadening of the valence band resulting from the contribution of the Sn 5s orbitals. Similar band gap narrowing in SnM2O6 (M ) Nb, Ta) compounds was recently reported by Hosogi et al.33 Similar to their results, our results also showed the possibility of Sn2+ element for the band gap narrowing, due to the hybridization with the O 2p orbitals. However, it is of interest that the calculated band gap of β-SW (3.45 eV) was increased to a greater extent than those of WO3 and R-SW. Although the Sn 5s orbitals still contributed to the valence band, it is considered that the increase of the crystal field splitting between W and O, due to the shorter W-O bond length (1.751 Å) in the WO4 tetrahedra, led to the shift of the conduction-band position to the upper region, and, thus, the band gap of β-SW was increased compared with that of R-SW with its longer W-O bond length (1.944 Å) in the WO6 octahedra. Figure 6 shows the partial density of states of R-SW and β-SW, which clearly shows the contributions of each element to the valence band and conduction band. It is noteworthy that the construction of the conduction band was quite different from that of the valence band, i.e., the W 5d orbitals mainly contributed to the conduction band in R-SW, whereas both the W 5d and Sn 5p orbitals contributed to the conduction band in β-SW. Consequently, the mobility of the photogenerated electrons in the conduction band of β-SW was higher than that of R-SW, which was attributed to the increase of the hybridization between the W 5d and Sn 5p empty orbitals. As a result, the larger measured band gap of β-SW (2.68 eV) than that of R-SW (1.64 eV) was attributed to the shift of the

Figure 7. Mott-Schottky plots of R-and β-SnWO4 thick film electrodes in 1 M NaCl (pH 7.0) electrolyte measured at a frequency of 3 kHz.

conduction-band position to a higher energy state, resulting from the larger interaction between the W and O orbitals. Furthermore, the contribution of the Sn 5p orbitals to the conduction band increased the mobility of the electrons. 3.3. Surface Properties. The flat-band potential (Vfb) is the potential value which must be applied to the semiconductor electrode, relative to the reference electrode (saturated calomel electrode, SCE), to flatten the band bending due to the difference in the Fermi levels between the electrode and electrolyte.

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Figure 8. Sn 3d XPS spectra of SnWO4 polymorphs: (a) R-SnWO4 and (b) β-SnWO4.

Generally, the Vfb values are determined by measuring the capacitance (C) of the electrode/electrolyte interface at different electrode potentials (V) using the Mott-Schottky equation34,35

1/C2 ) (2/εoεsqND)[V-Vfb-(kBT/q)]

(1)

where C is the space charge capacitance, εo and εs are the dielectric constants of free space and the film electrode, respectively, q is the electronic charge, Vfb is the flat-band potential, T is the temperature in Kelvin, ND is donor density, V is the applied potential, and kB is Boltzmann’s constant. The Vfb value can be determined from the extrapolation for 1/C2 ) 0. Figure 7 shows the Mott-Schottky plots of the R-SW and β-SW thick film electrodes in 1 M NaCl (pH 7.0) electrolyte. The flat-band potentials obtained from the intercept of the potential axis were negative, viz. -0.61 and -0.66 V vs SCE for R-SW and β-SW, respectively. These negative values of the flat-band potential clearly indicate the n-type semiconducting properties of both SnWO4 polymorphs. Moreover, the more negative Vfb of β-SW means that it has smaller work function, i.e., a higher relative Fermi level in the electrolyte and, thus, higher band bending at the interface and higher diffusion rate

of the charge carriers, which was consistent with the results obtained from the band structure analysis (higher electron mobility). To characterize the chemical and binding state of SnWO4 powders at the surface, the XPS spectra of the SnWO4 polymorphs were also investigated and are shown in Figure 8. The binding energies of Sn2+ (485.9 eV) and Sn4+ (486.6 eV) were taken from the NIST database.36 Interestingly, the presence of Sn4+ was observed in both SnWO4 powders, but the amount of Sn4+ in R-SW was higher than that in β-SW. This result is consistent with the result by J. L. Solis et al.23 They used 119Sn Mo¨ssbauer spectroscopy to confirm the local Sn4+ extra phase. According to Y. Hosogi et al.,33 moreover, some amount of Sn4+ in the source material of SnO was observed and could be substituted for high-valence Nb5+ ions in the SnNb2O6 due to the similar ionic radius. The ionic radius of W6+ (0.6 Å) is similar to that of Sn4+ (0.69 Å), and thus it is considered that the Sn4+ ions could be substitued for W6+ ions in SnWO4. From O 1s XPS spectra (Figure S3), the surface defects such as oxygen vacancies were also observed, which implies the possibility for the substitution of Sn4+ for W6+ ions. The calculated average W-O bond lengths in R-SW and β-SW were 1.944 and 1.751 Å, respectively. The average W-O bond length in R-SW was larger than in β-SW. Therefore, it is considered that larger amount of Sn4+ ions could be substituted for W6+ ions in R-SW than in β-SW, which may led to higher amount of Sn4+ and higher oxygen vacancies in R-SW than those in β-SW. However, further detailed experiments were needed to confirm the partial substitution of Sn4+ for W6+ in SnWO4 polymorphs. 3.4. Photocatalytic Activity. The photocatalytic activities of the SnWO4 polymorph powders were evaluated via the degradation of rhodamine B (RhB) dye under visible-light irradiation (>420 nm), and the results were compared with those obtained from RhB photolysis (without powder). As can be seen in Figure 9, the photolysis of RhB dye under visible-light irradiation was negligible. However, both the R-SW and β-SW powders exhibited obvious degradation of RhB dye solution after the light was turned on, and the degradation rates were quite high. Moreover, the isosbestic points or hypsochromic shifts, which often appear in the absorbance spectra during photosensitization reactions,37-39 were not observed. When using the β-SW powder, the RhB dye solution was almost completely degraded after 4 h irradiation of visible light.

Figure 9. Photocatalytic activity of SnWO4 polymorphs for the degradation of RhB dye solution under visible-light irradiation (>420 nm): (a) and (b) variation of absorbance of RhB dye solution with irradiation time using R-SnWO4 and β-SnWO4 powder, respectively; (c) concentration change profiles with irradiation time.

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Figure 10. Comparison of the photocatalytic activity of the SnWO4 polymorphs with those of various visible-light active photocatalysts.

Generally, the most important factors affecting the photocatalytic activity of the photocatalyst are the optical absorption capability (optical band gap), surface area, and separation/ diffusion rate of the photogenerated charge carriers (mobility).2,40 A smaller band gap is effective in generating charge carriers, and a higher surface area provides a larger number of surface active sites, thus allowing more organic molecules to be adsorbed at the surface. Moreover, higher mobilities of the charge carriers indicate a higher probability of transferring the light-generated charge carriers from the bulk to the surface,; i.e., more charge carriers can participate in the photocatalytic reaction. The R-SW has a smaller band gap (1.64 eV) and higher surface area (2.77 m2/g) than the β-SW (2.68 eV, 0.56 m2/g). Nevertheless, the photocatalytic activity of the β-SW powder was slightly higher than that of the R-SW powder. As discussed above, the R-SW has more Sn4+ surface defects. These Sn4+ species at the surface in SnNb2O6 compound usually act as electron trap sites.33 Therefore, the lower photocatalytic activity of R-SW than that of β-SW is affected by their different amounts of surface defects. Moreover, the higher electron mobility in β-SW than that in R-SW, due to the contribution of the Sn 5p orbitals to the conduction band and its higher flat-band potential value, also plays an important role in the higher photocatalytic activity of the former. The photocatalytic activities of these SnWO4 polymorph powders were further compared with those of visible-light active photocatalysts such as WO3 (bulk and nanosized powder) and TiONx (Figure 10). As can be seen in the figure, the photocatalytic activity of both the R-SW and β-SW powders was much higher than those of the WO3 powders. Furthermore, despite the fact that TiONx possesses a much greater surface area (112.13 m2/g) than β-SW (0.56 m2/g), its photocatalytic activity was lower. Therefore, it is expected that the photocatalytic activity of these SnWO4 polymorphs could be further increased by using other synthesis routes to obtain nanostructures with a higher surface area. Figure 11 shows the photocatalytic H2 evolution from an aqueous methanol solution (20 mL of CH3OH + 130 mL of H2O) under visible-light irradiation (>400 nm) using the SnWO4 polymorphs. In the case of R-SW, no evolution of H2 was observed. However, the β-SW had a higher conduction-band

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Figure 11. Photocatalytic activity of SnWO4 polymorphs for the evolution of H2 from aqueous methanol solution (20 mL of methanol + 130 mL of water) under visible-light irradiation (>400 nm).

potential due to the higher crystal field splitting in the W-O bond than that in R-SW and, thus, could produce H2 by water splitting. Moreover, the photocatalytic H2 evolution efficiency was increased from 0.3 to 1.8 µmol/g · h by using Pt as a cocatalyst. Therefore, it is expected that the conduction-band edge potential of tungstate photocatalysts for water splitting could be increased by shortening the W-O bond lengths. 4. Conclusions The SnWO4 oxide semiconductors were found to be novel visible-light active photocatalysts for the degradation of organic compounds. The low-temperature phase, R-SnWO4 with its corner-shared WO6 octahedra, exhibited a dark-red color and indirect band gap of 1.64 eV, while the high-temperature phase, β-SnWO4 with its unshared WO4 tetrahedra, exhibited a lightyellow color and direct band gap of 2.68 eV. From the electronic band structure calculation, the contribution of the Sn 5s filled orbitals to the valence band structure led to a decrease of the band gap in R-SnWO4. However, the band gap of β-SnWO4 was increased, because the conduction band was shifted to higher energy states, due to the higher crystal field splitting in the shorter W-O bonds. Both SnWO4 polymorphs exhibited higher photocatalytic activity for the degradation of RhB dye solution under visible-light irradiation than other visible-light active photocatalysts such as WO3 and TiONx. Moreover, β-SnWO4 with its higher conduction-band edge potential also exhibited photocatalytic H2 evolution from an aqueous methanol solution under visible-light irradiation (>400 nm). This higher photocatalytic activity of the SnWO4 polymorphs was mainly attributed to their smaller band gaps and unique band structures resulting from their different bonding nature. Acknowledgment. This work was supported by the Korea Science and Engineering Foundation (KOSEF) grant funded by the Korean government (MOST) (R01-2007-000-11075-0). Supporting Information Available: XRD and FESEM of SnWO4 powders (Figure S1 and S2), and O 1s XPS spectra (Figure S3). This material is available free of charge via the Internet at http://pubs.acs.org.

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