Visible Light-Responsive Bismuth Tungstate Photocatalysts: Effects of Hierarchical Architecture on Photocatalytic Activity

Bismuth tungstate (Bi2WO6) 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 Bi2WO6 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 Bi2WO6 samples of similar W/Bi ratio, the photocatalytic activity increased with an increase in their specific surface area. Spherical assemblies of Bi2WO6 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 Bi2WO6 and that the photocatalytic activity monotonically increased with an increase in specific surface area for samples having similar densities of electron-hole recombination centers.