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 (Bi 2 WO 6 ) of a hierarchical “flake- ball” 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 Bi 2 WO 6 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 hierar- chical 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 prepa- ration 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 micrometer- sized spherical polycrystalline particles of bismuth tungstate (Bi 2 WO 6 ) 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 Bi 2 WO 6 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 com- pounds. 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 Bi 2 WO 6 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 Bi 2 WO 6 flake-ball particles, it is required to elucidate the crystal growth mechanism to recognize the general concept for preparation of hierarchical architectures. Crystallites of Bi 2 WO 6 and bismuth molybdate (Bi 2 MoO 6 ), 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 Bi 2 WO 6 is composed of accumulated layers of corner-sharing WO 6 octahedral sheets and bismuth oxide sheets. It has been reported that a hydrothermal condition promotes anisotropic crystal growth of Bi 2 WO 6 and formation of rectangle-shaped nanoplates. 19–22 Hydrothermally prepared Bi 2 WO 6 photocatalysts with a flake-ball shape or rectangular-plate shape have been shown to induce photocata- lytic 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 Bi 2 WO 6 . 12 In the present study, we prepared Bi 2 WO 6 flake-ball particles and other Bi 2 WO 6 morphologies by slightly different hydro- thermal conditions without organic agents. The mechanism of flake-ball particle formation was examined. The photocatalytic activities were evaluated by using several photocatalytic reac- tions, 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 Bi 2 WO 6 samples with different morphologies. The level of photocatalytic activity under ultra- violet 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. * Corresponding author. E-mail: amano@cat.hokudai.ac.jp. Phone: +81- 11-706-9130. Fax: +81-11-706-9130. † Catalysis Research Center, Hokkaido University. ‡ Graduate School of Environmental Science, Hokkaido University. J. Phys. Chem. C 2008, 112, 9320–9326 9320 10.1021/jp801861r CCC: $40.75  2008 American Chemical Society Published on Web 05/29/2008