Photocatalytic activities of amorphous and crystal bismuth tungstate (Bi(2)WO(6)) were investigated using oxidative decomposition of gaseous acetaldehyde under visible light irradiation (>400 nm). Here, for the first time, negligible photocatalytic activity of amorphous Bi(2)WO(6) owing to the fast recombination of electron-hole pairs and the high quantum efficiency of Bi(2)WO(6) crystallites under visible light were demonstrated by action spectrum analysis and time-resolved infrared absorption measurements. Crystallization of the amorphous phase provided a red shift of the photoabsorption edge and marked increase in the lifetime of photoexcited electrons, resulting in an increase of photocatalytic activity.

Micrometer-sized spherical polycrystalline particles of bismuth tungstate (Bi 2 WO 6 ) 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 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.

The photocatalytic oxidative decomposition of gaseous acetaldehyde (AcH) in air under visible light irradiation (wavelength >400 nm) was driven by bismuth tungstate (Bi(2)WO(6)) polycrystalline particles with a hierarchical structure, flake-ball particles, prepared by hydrothermal reaction at various temperatures. Complete decomposition of AcH into CO(2) was proven for all of the flake-ball particle photocatalysts. The rate of CO(2) liberation was increased in proportional to the specific surface area for flake-ball particles with similar high degrees of crystallinity. Kinetic analysis assuming Langmuirian adsorption of AcH revealed that the initial rate of photocatalytic decomposition could be reproduced by first-order kinetics with respect to the amount of surface-adsorbed AcH. A linear relationship between the photocatalytic activity and surface area of photocatalysts under conditions in which other physical properties such as the photoabsorption property, crystalline content, exposed crystal facets, and secondary particle size are almost the same was experimentally revealed.

Bismuth tungstate (Bi 2 WO 6 ) particles as a visible light-responsive photocatalyst were prepared by a facile hydrothermal reaction method in a wide range of pH's (1.5-8.0) in a feed solution, and the effects of calcination on photocatalytic activity were investigated. Change in pH affected their crystallite size, shape, assembling morphology, and specific surface area. The tungsten-to-bismuth ratio (W/Bi ratio) of Bi 2 WO 6 gradually decreased with an increase in pH of the feed solution because of dissolution of tungstate in alkaline solution. The photocatalytic activities were evaluated with oxidative decomposition of acetic acid into carbon dioxide under ultraviolet and visible light irradiation. For Bi 2 WO 6 samples of similar W/Bi ratio, the photocatalytic activity increased with an increase in their specific surface area. Spherical assemblies of Bi 2 WO 6 polycrystalline flakes with a hierarchical architecture (flake-ball particles), which were prepared under the conditions of pH 1.2 and W/Bi ratio of 0.55, exhibited the highest level of photocatalytic activity among the hydrothermally prepared samples. Although a light-harvesting effect, which is an effect of photoabsorption enhancement, on the flake-ball particles was confirmed, the high level of photocatalytic activity did not depend on the assembling morphology of flakes. Photocatalytic activity of the flake-ball particles was improved by calcination at 873 K in air. The calcined particles exhibited a markedly high level of visible-light-induced photocatalytic activity. Analyses of the photocatalytic activities and physical properties indicated that the density of electron-hole recombination centers is a primary factor governing the photocatalytic activity of Bi 2 WO 6 and that the photocatalytic activity monotonically increased with an increase in specific surface area for samples having similar densities of electron-hole recombination centers.

Micrometer-sized spherical particles of bismuth tungstate (Bi2WO6) with hierarchical architecture were prepared by facile hydrothermal reaction without using any surfactants and polymers as structure-directing agents. The particles were assemblies of polycrystalline flakes composed of rectangular platelets. The hierarchical polycrystalline particles of “flake-ball” shape exhibited relatively high photocatalytic activity for oxidative decomposition of acetic acid in aqueous suspensions

Photoexcited carrier dynamics of bismuth tungstate (Bi2WO6) photocatalysts was investigated by time-resolved infrared (IR) absorption spectroscopy. Monotonic absorption at the mid-IR region, which is attributable to absorption by photoexcited electrons, was monitored as a function of time delay from the microsecond to millisecond range after photoexcitation. Bi2WO6 particles with different crystalline content were prepared by hydrothermal reaction at several temperatures and used to elucidate the relation between density of photoexcited carriers and steadystate photocatalytic efficiency. Photocatalytic efficiency was tested using two reactions: oxidative decomposition of acetic acid in an aqueous solution (reaction 1) and oxidative decomposition of acetaldehyde in air (reaction 2). Crystallization of Bi2WO6 particles suppressed the fast recombination of photoexcited electrons and holes within 1 μs. In the case of crystallized particles, the density of the photoexcited electron increased with an increase in the crystalline content, and the photocatalytic efficiency for reaction 1 strongly depended on the crystalline content, indicating that photoexcited electrons remaining in the submillisecond time range significantly affect the reaction rate. On the other hand, photocatalytic efficiency for reaction 2 showed a proportional relation with specific surface area rather than crystalline content. The difference in a decisive factor depending on reaction condition is considered to be the slower rate of reaction of photoexcited electrons with molecular oxygen, which might occur within a time range between 200 μs and 3 ms over Bi2WO6

Russelite bismuth tungstate (Bi 2 WO 6 ) flake-ball particles prepared by a hydrothermal reaction method have been reported to be visible-light-responsive photocatalysts for mineralization of organic compounds. Here, we report new bismuth-tungsten mixed oxide particles with a tungsten-to-bismuth ratio (W/Bi ratio) in feed of 1.0, which is two-times higher than the stoichiometric ratio of Bi 2 WO 6 (W/Bi = 0.50). Compared to the conventional Bi 2 WO 6 flake-ball particles, the tungsten-rich particles exhibited a narrow band gap (2.70 eV), large specific surface area due to the rectangular platelet nanostructure, and high level of photocatalytic activity for oxidative decomposition of acetaldehyde.

No abstract