Preparation and Characterization of Bismuth Tungstate Polycrystalline

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J. Phys. Chem. C 2008, 112, 9320–9326

Preparation and Characterization of Bismuth Tungstate Polycrystalline Flake-Ball Particles for Photocatalytic Reactions Fumiaki Amano,*,†,‡ Kohei Nogami,‡ Ryu Abe,†,‡ and Bunsho Ohtani†,‡ Catalysis Research Center, Hokkaido UniVersity, Sapporo 001-0021, Japan, and Graduate School of EnVironmental Science, Hokkaido UniVersity, Sapporo 060-0810, Japan ReceiVed: March 3, 2008; ReVised Manuscript ReceiVed: April 8, 2008

Micrometer-sized spherical polycrystalline particles of bismuth tungstate (Bi2WO6) of a hierarchical “flakeball” shape were prepared by a facile hydrothermal reaction without using any surfactants and polymers as structure-directing agents. The flake-ball particles were assemblies of polycrystalline flakes composed of rectangular platelets with a lateral size of a few hundred nanometers and thickness of 20-35 nm. An excess amount of a tungstate precursor (10%) and an acidic condition (pH 1.2) during the hydrothermal reaction were required to obtain a high yield of uniform particles with the flake-ball architecture. The mechanism by which the flake-ball particles are formed is discussed. The photocatalytic activities under ultraviolet light irradiation were investigated by using oxygen liberation from water, oxidative decomposition of acetic acid in an aqueous solution, and oxidative decomposition of gaseous acetaldehyde. The photocatalytic activity level of the flake-ball particles was higher than the photocatalytic activity levels of other Bi2WO6 samples prepared by conventional solid-state and hydrothermal reactions using a stoichiometric amount of a tungstate precursor. It was revealed that the 10% excess amount of tungsten plays a key role in the high level of photocatalytic activity of flake-ball particles. 1. Introduction There have been many studies on the preparation of hierarchical architectures by assembling nanomateials with anisotropic structures, such as one-dimensional and two-dimensional shapes.1–6 The architectures are expected to exhibit characteristic physical and chemical properties depending on the size, shape, orientation, alignment, and dimensionality. Hydrothermal preparation is one of the suitable methods for anisotropic crystal growth of hierarchical architectures since the preparation conditions, such as temperature, composition of the precursor, and addition of a template, are easily tunable.7–9 Recently, one-pot hydrothermal preparations of micrometersized spherical polycrystalline particles of bismuth tungstate (Bi2WO6) with hierarchical architectures have been reported by several research groups.10–14 The particles are assemblies of polycrystalline flakes composed of rectangular platelets. Wang et al. reported the preparation of such Bi2WO6 particles with a “flake-ball” shape for the first time by a facile hydrothermal reaction without any surfactant and template in an aqueous solution of low pH value.10,11 We have also developed, independently from the work by Wang’s group, a method for preparation of flake-ball particles without using organic compounds.12 In our preparation conditions, an excess amount of a tungstate precursor is needed for the production of flake-ball particles in relatively high yield. Huang et al. and Xie et al. have prepared Bi2WO6 flake-ball particles by a hydrothermal reaction in the presence of polyvinyl pyrrolidone (PVP) as a structure-directing agent.13,14 Since there are some differences in preparation conditions of Bi2WO6 flake-ball particles, it is * Corresponding author. E-mail: [email protected]. Phone: +8111-706-9130. Fax: +81-11-706-9130. † Catalysis Research Center, Hokkaido University. ‡ Graduate School of Environmental Science, Hokkaido University.

required to elucidate the crystal growth mechanism to recognize the general concept for preparation of hierarchical architectures. Crystallites of Bi2WO6 and bismuth molybdate (Bi2MoO6), which are members of cation-deficient Aurivillius phases, have the potential for photocatalytic oxygen liberation from water and oxidative decomposition of organic pollutants under visible light irradiation.15–18 The crystal of Bi2WO6 is composed of accumulated layers of corner-sharing WO6 octahedral sheets and bismuth oxide sheets. It has been reported that a hydrothermal condition promotes anisotropic crystal growth of Bi2WO6 and formation of rectangle-shaped nanoplates.19–22 Hydrothermally prepared Bi2WO6 photocatalysts with a flake-ball shape or rectangular-plate shape have been shown to induce photocatalytic decolorization of organic dies, such as rhodamine B, under visible-light irradiation.10,11,13,20–22 However, there have been few reports of other photocatalytic reactions, such as oxidative decomposition of organic compounds to carbon dioxide, and photocatalytic activity under ultraviolet light irradiation over hydrothermally prepared Bi2WO6.12 In the present study, we prepared Bi2WO6 flake-ball particles and other Bi2WO6 morphologies by slightly different hydrothermal conditions without organic agents. The mechanism of flake-ball particle formation was examined. The photocatalytic activities were evaluated by using several photocatalytic reactions, including oxygen liberation from water in the presence of silver sulfate, oxidative decomposition of acetic acid in an aqueous suspension, and oxidative decomposition of gaseous acetaldehyde in air under ultraviolet light irradiation. It was found that the flake-ball particles show the highest level of photocatalytic activity among Bi2WO6 samples with different morphologies. The level of photocatalytic activity under ultraviolet light irradiation was as high as that of a commercial anatase titania photocatalyst. Careful characterizations were performed to elucidate the physical properties influencing the photocatalytic activity.

10.1021/jp801861r CCC: $40.75  2008 American Chemical Society Published on Web 05/29/2008

Bismuth Tungstate Flake-Ball Particles 2. Experimental Section 2.1. Preparation. Laboratory-grade water was prepared by using a Milli-Q water system (Yamato-Millipore WQ501). By modifying the method reported for preparation of Bi2WO6 rectangular plates,20–22 a hydrothermal reaction of sodium tungstate (Na2WO4) and bismuth nitrate (Bi(NO3)3) was performed at 443 K in an aqueous medium without addition of organic compounds. Na2WO4 (dihydrate, >99%, 2.50 or 2.75 mmol) was dissolved in 30 mL of water. The aqueous solution was added dropwise to a mixture of Bi(NO3)3 (pentahydrate, 99.9%, 5.0 mmol) and water (20 mL) with vigorous magnetic stirring. The white slurry was stirred for an additional 10 min and then sonicated for 20 min at ambient temperature under ambient air, followed by pouring into a 100-mL Teflon-lined autoclave. The suspension was strongly acidic (pH 1.2) due to hydrolysis of Bi(NO3)3. When necessary, pH of the suspension was adjusted by the addition of an aqueous solution of sodium hydroxide (NaOH, 1.0 mol L-1). After addition of water to ca. 80% of the capacity of the autoclave, the autoclave was sealed and heated in an oven at 433 K for 20 h. After reaction under self-generated pressure and cooling to room temperature, the yellowish white precipitate was collected by centrifugation, washed with 50 mL of water three times, and dried in an oven at 393 K. Conventional solid-state reaction of a stoichiometric powder mixture of bismuth oxide (Bi2O3, 99.9%) and tungsten oxide (WO3, 99.99%) was carried in air at 1073 K for 1 h and at 1173 K for an additional 12 h with grinding in an agate mortar after the first heating. The sample prepared by solid-state reaction was denoted by SSR. 2.2. Characterization. Morphology of the samples was observed by a field-emission-type scanning electron microscope (FE-SEM; JEOL JSM-7400F). The chemical composition was analyzed by a JEOL JED-2300 energy dispersive X-ray spectrometer (EDS). Powder X-ray diffraction (XRD) patterns were recorded on a Rigaku RINT ULTIMA diffractometer with Cu KR radiation. Laser Raman spectra were obtained by using a Perkin-Elmer System 2000R Fourier-transform Raman spectrometer with Nd:YAG laser (1064 nm). The incident laser power was 200 mW and the spectra were recorded with a resolution of 4 cm-1. Diffuse reflection photoabsorption spectra were recorded with barium sulfate as a standard material on a JASCO V-670 spectrometer and a PIN-757 integrating sphere equipped with a horizontal-type sample holder. X-ray photoelectron spectroscopy (XPS) measurements were carried out with JEOL JPS-9010MC with Mg KR radiation. The binding energies were calibrated to C 1s peak at 284.7 eV of surface adventitious carbon of the sample pellet surface. BrunauerEmmett-Teller (BET) specific surface area was determined from nitrogen adsorption at 77 K on a Yuasa Ionics Autosorb6-AG surface area and pore size analyzer. Pore volume distribution was estimated from the desorption isotherm recorded on the analyzer with the Barret-Joyner-Halenda (BJH) method. Before the surface area and pore volume distribution analyses, the samples were heated at 393 K under vacuum for 2 h. Particle size distributions were measured by the laser diffusion/scattering method, using a Shimadzu SALD-7000 particle size analyzer and sodium hexametaphosphate ((NaPO3)6) as a dispersant for preparing a highly dispersed aqueous suspension of samples. 2.3. Photocatalytic Reaction. Photocatalytic oxygen liberation from water in the presence of silver ions as a sacrificial electron acceptor was performed under an argon atmosphere. Photocatalyst powder (50 mg) was suspended in an aqueous solution (5.0 mL) of silver sulfate (Ag2SO4, 25 mmol L-1) in

J. Phys. Chem. C, Vol. 112, No. 25, 2008 9321 TABLE 1: Tungsten to Bismuth Ratio, pH in the Feed Solution, Crystallite Size, and BET Specific Surface Area of Bi2WO6 Samples Prepared by Solid-State and Hydrothermal Reactions sample

W/Bi ratioa

pHa

D131b/ nm

SBETc/ m2 g-1

W/Bi (EDS)d

W/Bi (XPS)d

SSR HTR-A HTR-B HTR-C

0.50 0.50 0.50 0.55

4.5 1.3 1.2

350e 31 17 17e

0.2e 8.6 22.6 21.2e

0.56 0.60 0.58 0.67

0.47 0.46 0.44 0.53

a

Molar ratio of tungsten to bismuth and pH value in the feed solution of the hydrothermal reaction. b Crystallite size calculated from the fwhm of the 131 X-ray diffraction peak. c BET specific surface area. d Molar ratio of tungsten to bismuth in the sample determined by EDS and XPS analyses. e Data reported previously.12

a glass tube. The suspension was magnetically stirred at 1000 rpm during photoirradiation. Photoirradiation at >300 nm was performed by using a 400-W high-pressure mercury lamp (Eikosha). The reaction temperature was controlled at 298 K, using a water-cooling bath. The amount of oxygen liberated in the gas phase was measured with a TCD-gas chromatograph (Shimadzu GC-8A) equipped with an MS-5A column. Photocatalytic oxidative decomposition of acetic acid in an aqueous suspension was performed as follows. Photocatalyst powder (50 mg) was suspended in a 5 vol % aqueous solution (5.0 mL) of acetic acid under ambient air in a glass tube. The other reaction conditions were similar to those of the photocatalytic oxygen liberation. The amount of carbon dioxide (CO2) liberated in the gas phase was measured by using a TCD-gas chromatograph (Shimadzu GC-8A) equipped with a Porapak-Q column. Photocatalytic oxidative decomposition of gaseous acetaldehyde in air was carried out in a cylindrical glass vessel with a volume of 330 mL. Photocatalyst powder (50 mg except for SSR (150 mg)) was uniformly spread on a glass plate (1.5 cm × 1.5 cm). The plate was placed on the bottom of the cylindrical vessel. Gaseous acetaldehyde (0.33 mL) was injected into the vessel filled with ambient air to adjust the initial concentration to 1000 ppm. Consumption of acetaldehyde was measured by an FIDgas chromatograph (Shimadzu GC-14B) equipped with a DB-1 column. After adsorption of acetaldehyde had reached an equilibrium in the dark, photoirradiation (>300 nm) through a top window was performed with use of a 300-W xenon lamp (ILC Technology CERMAX-LX300F). The amount of liberated CO2 was measured by an FID-gas chromatograph equipped with a Porapak-Q column and a methanizer (Shimadzu MTN-1). 3. Results and Discussion 3.1. Preparation of Flake-Ball Particles. Samples of Bi2WO6 (HTR-A, HTR-B, and HTR-C) were prepared by hydrothermal reaction under three kinds of conditions at 433 K for 20 h. Table 1 shows the molar ratio of tungsten to bismuth (W/Bi ratio) and pH value in the feed solution. The difference between the three conditions was the amount of starting materials (sodium tungstate, bismuth nitrate, and sodium hydroxide). HTR-A was prepared by adding 10 mmol of sodium hydroxide to increase the pH value of the solution. The pH values of HTR-B and HTR-C were ca. 1.2. In the case of HTR-C, the W/Bi ratio in the preparation was adjusted to 0.55, which corresponds to 10% excess (W) to the stoichiometric ratio (W:Bi ) 1:2). Figure 1 shows FE-SEM images of HTR-A, HTR-B, and HTR-C. The particle size and assembling morphologies were

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Amano et al.

Figure 2. Diffuse reflection photoabsorption spectra of Bi2WO6 samples: (a) SSR, (b) HTR-A, (c) HTR-B, and (d) HTR-C.

Figure 1. FE-SEM images of Bi2WO6 samples: (a, b) HTR-A, (c, d) HTR-B, and (e, f) HTR-C.

quite different. HTR-A exhibited a plate-like rectangular shape with a lateral size of several hundred nanometers. The morphology of HTR-B, which was prepared without addition of sodium hydroxide, was aggregates of flakes with irregular shape and size. From a high magnification image (Figure 1d), it was found that the flakes were composed of small-sized platelets. On the other hand, spherical ball-like particles with an average diameter of 3-4 µm were observed in HTR-C. Each particle was an assembly of a number of flakes (“flake-ball”). A high magnification image (Figure 1f) shows that the flakes are composed of well-developed rectangular platelets with a lateral size of a few hundred nanometers and thickness of 20-35 nm. The hierarchical structure of the flake aggregates (HTR-B and HTRC) resembled that of Bi2WO6 hierarchical particles reported recently by Wang’s group.10,11 Since the pH in their method is similar to that in the present study, it is thought that the condition of a strong acidic aqueous solution promotes the aggregation of flakes. On the other hand, it has been reported that flake-ball particles with a concave structure were prepared in the presence of PVP as a structure-directing agent.13,14 In our preparation conditions, PVP is not needed and the use of an excess amount of a tungstate precursor is indispensable for the production of spherical flake-ball particles in relatively high yield. 3.2. Physical Properties of Flake-Ball Particles. Figure S1 in the Supporting Information shows XRD patterns and Raman spectra of Bi2WO6 samples. The XRD patterns of samples prepared by hydrothermal reaction gave peaks broader than those of SSR (Bi2WO6, JCPDS Card no. 39-0256), indicating their smaller crystallite sizes. The crystallite sizes of Bi2WO6 samples were calculated from fwhm of the most intense 131 diffraction peak at 28.8° using Scherrer’s equation as shown in Table 1 (Data for SSR and HTR-C were reproduced from ref 12 for comparison.) The Raman spectra of samples prepared by hydrothermal reaction gave peaks slightly broader than those of SSR. The Raman peak intensities of HTR-series samples were about 30× lower than those of SSR. Raman bands observed at 500-1000 cm-1 are assigned to W-O stretches of the WO6

octahedra and the band position is assumed to reflect W-O bond length.23,24 The Raman stretching at >800 cm-1 is assigned to the shortest W-O bonds. Bands in the low wavenumber region are bending/external modes. It is thought that the lengths of W-O bonds of HTR-series samples were similar to each other and slightly different from those of SSR. The BET specific surface areas of the samples are summarized in Table 1 (Data for SSR and HTR-C were reproduced from ref 12 for comparison.) While SSR had a very small surface area, the HTR samples had relatively large surface areas: surface areas of HTR-B and HTR-C were almost the same and larger than that of HTR-A. Figure S2 in the Supporting Information shows pore size distributions of Bi2WO6 samples. HTR-B and HTR-C exhibited meso and macro pores, which are attributable to the spaces between platelets and between flakes, respectively. The pore volume of HTR-C was almost the same as that of HTR-B in the meso-pore region (2-50 nm), indicating that HTR-B and HTR-C are composed of similar crystalline platelets in size and shape and different alignment of flakes. Figure 2 shows diffuse reflection photoabsorption spectra of the samples. Sharp onset of absorption at ca. 460 and 480 nm was observed for the HTR samples and SSR, respectively. Calculation by the density functional theory method revealed that the valence band of Bi2WO6 consisted of O 2p and Bi 6s orbitals, and the bottom of the conduction band consisted mainly of W 5d orbitals.21 The photoabsorption onset wavelengths of HTR-series samples were similar and shifted to a shorter wavelength compared with that of SSR. Assuming indirect allowed transition, their band gap energy (Eg) was estimated from the intercept of a straight line fitting to a plot of [F(R∞)hν]0.5 against hν, where F(R∞) is the Kubelka-Munk function and hν is the incident photon energy.25 SSR and HTRseries samples exhibited Eg of 2.6 and 2.8 eV, respectively. The smaller Eg of HTR samples indicates a difference in the electronic band structure from that of SSR. Figure S3 in the Supporting Information shows energy dispersive X-ray spectra (EDS) of HTR-C and SSR. The analysis revealed the presence of C, O, W, and Bi atoms. The C atoms might come from carbon paste for fixing the sample powders. The W/Bi ratios estimated by EDS are summarized in Table 1. HTR-C gave a W/Bi ratio ca. 15% larger than those of the other samples. The results are almost consistent with 10% excess of stoichiometric W/Bi ratio in the amount of starting materials for hydrothermal preparation. Results of analyses by XPS (Supporting Information, Figure S4) also supported this. Since

Bismuth Tungstate Flake-Ball Particles

J. Phys. Chem. C, Vol. 112, No. 25, 2008 9323

Figure 4. Effects of hydrothermal reaction time on BET specific surface area (left) and crystallite sizes (right) of Bi2WO6 samples. Figure 3. Particle size distributions measured by a laser diffraction/ scattering method of HTR-C (flake-ball particles). The inset shows an FE-SEM image.

the XPS peaks of O, W, and Bi of HTR and SSR samples appeared to be almost the same, valences of these elements were the same for HTR and SSR samples. The W/Bi ratios estimated by XPS (Table 1) also indicate that HTR-C contains ca. 15% excess W. Therefore, it is concluded that the chemical composition of samples reflects that of the starting materials in preparation, and the flake-ball particles (HTR-C) contain an excess amount of tungsten from the stoichiometry. The difference between the W/Bi ratios estimated by EDS and XPS might be due to the difference in the detection depth; XPS tends to show the composition of the outermost surface of samples. These results indicate that the excess tungstens are dispersed in the bulk, not accumulated on the outer surface. It has been reported that Aurivillius phase Bi2W2O9, with layered crystal structure consisting of bismuth oxide sheets and perovskitelike slabs, can be converted into the corresponding protonated form, H2W2O9, by acid leaching of bismuth oxide layers.26,27 If bismuth oxide sheets in Bi2WO6 are partly exchanged by proton, the chemical formula would be Bi2-x/3HxWO6 (for example, when the W/Bi ratio is 0.55, the composition is determined to be Bi1.82H0.54WO6). Figure 3 shows particle size distributions measured by a laser diffraction/scattering method of HTR-C suspended in water. The flake-ball particles exhibited a relatively uniform diameter of 3-4 µm as shown in an inset FE-SEM image. The mean diameter was calculated to be 3.9 µm, which is consistent with the estimation from the above-mentioned FE-SEM analysis. Thus, the flake-ball particles show uniform particle size and high dispersion. 3.3. Mechanism of Production of Flake-Ball Particles. To analyze the crystal-growth mechanism, samples taken during hydrothermal reaction were characterized by FE-SEM (Figure S5 in the Supporting Information). Nanoparticles were observed in the white precipitates formed by mixing of bismuth nitrate and sodium tungstate aqueous solutions at room temperature. Hydrothermal reaction for 1 h slightly reduced the particle size (Figure S5b in the Supporting Information). At 2-h reaction, irregular-shaped (not spherical) small flake aggregates with diameters of less than 2 µm were formed (Figure S5c,f in the Supporting Information). Further reaction caused the formation of large (3-4 µm) flake-ball particles. The particle size was not greatly changed after 5 h of reaction. From high magnification images, rectangular platelets were not clearly observed by 2 h of reaction (Figure S5g in the Supporting Information) but

Figure 5. Schematic illustration of the process of Bi2WO6 polycrystalline flake-ball particle formation.

appeared as parts of flakes by 5 h of reaction (Figure S5h in the Supporting Information). Figure S6 in the Supporting Information shows XRD patterns of samples taken during hydrothermal reaction. The sample treated for 1 h shows a broad hallo peak, which is characteristic for the amorphous phase. An XRD pattern of poorly crystallized Bi2WO6 was observed by 2 h of reaction. With increase in the reaction time, the intensity of XRD peaks was increased and their full width at half-maxima (fwhm) was decreased. This indicates gradual crystallization of the Bi2WO6 phase and increase in the crystallite size. Figure 4 shows the effect of the hydrothermal reaction time on the BET specific surface area and crystallite size, which was estimated from fwhm of the most intense 131 diffraction peak at 28.8° using Scherrer’s equation. The surface area was markedly increased with increase in hydrothermal reaction time to 48.2 m2 g-1 after 2 h of reaction, in which irregular-shaped small aggregates were formed. Further prolonged reaction up to 10 h caused a decrease in the surface area with the formation of large flake-ball particles, presumably due to crystallization of flakes and coalescence of neighboring crystallized flakes in the balls. After 5 h of reaction, variation of surface area and crystallite size with reaction time was not evident due to termination of the crystallization of Bi2WO6. Figure 5 shows a plausible process of the flake-ball particle formation from fine amorphous particles. FE-SEM observation suggests that flake-ball particles were produced through crystal plane-selective growth starting from cores, not through aggregation of rectangular platelets produced independently. Such anisotropic crystal growth is generally achieved by a selective adsorption of additives on specific sites suppressing isotropic crystal growth. Strongly acidic aqueous media and a 10% excess

9324 J. Phys. Chem. C, Vol. 112, No. 25, 2008

Figure 6. Photocatalytic oxidation of water in the presence of silver sulfate over (a) SSR, (b) HTR-A, (c) HTR-B, (d) HTR-C, and (e) anatase titania ST-01. (a, d) Initial rate ( HTR-C > HTR-B . HTR-A and SSR. The rate of oxygen liberation over ST-01 was reduced after 30 min of photoirradiation. It has been reported that such a decrease in the rate is attributed to a decrease in the amount of adsorbed silver ions due to a decrease in pH caused by protons released in the oxidation of water.29,30 The amount of liberated oxygen from HTR-C was larger than that from anatase titania in the case of prolonged photoirradiation for >1 h. Among the Bi2WO6 samples, HTR-C exhibited the highest level of photocatalytic activity. The low activity level of HTR-B, which has a relatively large BET specific surface area, suggested that the surface area is not important for this type of reaction. It seems that crystal lattice defects work as a recombination center of the photoinduced electrons and holes and decrease the efficiency of the photocatalytic reactions on the surface of particles. Indeed, it has been reported that rutile titania with a small surface area and low defect density exhibits a high level of photocatalytic activity for oxygen liberation.31 Therefore, the high level of photocatalytic activity of HTR-C suggests its high crystallinity, i.e., low lattice defect density, of the flake-ball particles. The photocatalytic activity levels of anatase titania and Bi2WO6 samples for water oxidation were relatively low compared with the activity levels of rutile titania and WO3. It should be noted that oxygen liberation is a reaction requiring four electrons (holes). Therefore, large particle size, in which multiple electron-hole pairs exist at the same time, of rutile TiO2 and WO3 might be beneficial for photocatalytic oxygen liberation. Figure 7 shows the time course of CO2 liberation during photocatalytic oxidative decomposition of acetic acid in the aerated aqueous solution. A portion of the data was reproduced from our previous report for comparison. The amount of liberated CO2 was monotonically increased with irradiation time without induction period, since CO2 was less soluble in an acetic acidic solution. The HTR-C showed a much higher rate of CO2 liberation than those of the other Bi2WO6 samples. The activity level of HTR-C was comparable to that of ST-01 and much higher than that of HTR-B. The flake-ball particles could induce the oxidative decomposition of acetic acid even under visible light irradiation at >400 nm by eliminating ultraviolet light from the light source, using a cutoff filter (Kenko L42).12 The band

Bismuth Tungstate Flake-Ball Particles

Figure 8. Sedimentation of (a) HTR-C (flake-ball particles) and (b) ST-01 in an aqueous suspension for 3 h. Powder (30 mg) was suspended in 10 mL of water by ultrasonication.

J. Phys. Chem. C, Vol. 112, No. 25, 2008 9325 eralization, of acetaldehyde into CO2 was proved by the twotimes higher molar yield of CO2 compared with the consumption of acetaldehyde when the flake-ball particles, as well as ST-01, were used as a photocatalyst under ultraviolet light irradiation. Although the rates of acetaldehyde consumption and CO2 liberation by HTR-C (flake-ball particles) were lower than those by ST-01, the superiority of HTR-C to SSR is clearly shown. The level of photocatalytic activity of flake-ball particles was higher than the levels of photocatalytic activity of the other Bi2WO6 samples including SSR. Although the conditions for preparation of HTR-B and HTR-C were the same except for the amount of tungsten in the feed solution and despite the fact that their physical properties, including surface area, crystallite size, W-O bond length, and electronic band structure, were very similar, HTR-C showed a much higher level of photocatalytic activity than that of HTR-B for both photocatalytic oxygen liberation from water and oxidative decomposition of acetic acid. These results clearly indicated that the 10% excess amount of tungsten plays a key role in the high level of photocatalytic activity. 4. Conclusion

Figure 9. Amounts of (a-c) CO2 and (a′-c′) acetaldehyde in the photocatalytic oxidative decomposition of gaseous acetaldehyde over (a, a′) SSR, (b, b′) HTR-C, and (c, c′) ST-01. (a, b) Initial rate (300 nm) induced the consumption of gaseous acetaldehyde and the liberation of CO2 in the presence of photocatalysts. The complete oxidation, i.e., min-

Crystal growth of polycrystalline Bi2WO6 flake-ball particles was promoted in hydrothermal reaction under the conditions of excess amount of a tungsten precursor, low pH value, and supersaturated solutions. The flake-ball particles were produced through crystal plane-selective growth starting from cores, not through aggregation of rectangular platelets produced independently. It was suggested that the mechanism of flake-ball particle formation have two stages, crystal plane-selective growth and Ostwald ripening process. Several kinds of photocatalytic reaction test revealed superior photocatalytic activity of the flake-ball particles, suggesting low lattice defect density of the well-developed rectangular platelets. The low lattice defect density would be related to the 10% excess amount of tungsten in the feed solution for preparation. It was revealed that control of the hierarchical architectures of polycrystalline assemblies resulted in large surface area and large pore volume. The flakeball particles are promising as a photocatalyst for oxidative decomposition of organic pollutants to carbon dioxide, for example, in water purification systems because of the high level of photocatalytic activity, the response to visible light, and feasible separation from suspensions by sedimentation and filtration. Acknowledgment. This work was partially supported by a Grant-in-Aid for Young Scientists (Start-up) (No. 18850002) from the Japan Society for the Promotion of Science (JSPS). F.A. is grateful for financial support by the Association for the Progress of New Chemistry. Supporting Information Available: XRD patterns, Raman spectra, pore size distributions, energy dispersive X-ray spectra, and X-ray photoelectron spectra of Bi2WO6 samples and FESEM images and XRD patterns of Bi2WO6 prepared with different durations of hydrothermal reaction. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Shi, H. T.; Qi, L. M.; Ma, J. M.; Cheng, H. M. J. Am. Chem. Soc. 2003, 125, 3450–3451. (2) Wu, C. Z.; Xie, Y.; Wang, D.; Yang, J.; Li, T. W. J. Phys. Chem. B 2003, 107, 13583–13587. (3) Chen, A. C.; Peng, X. S.; Koczkur, K.; Miller, B. Chem. Commun. 2004, 1964–1965.

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