Decahedral single-crystalline anatase particles prepared by a newly developed gas-phase process showed photocatalytic activities superior to those of P25 photocatalyst despite their relatively smaller specific surface area compared with that of P25, probably because of the low density of particle boundaries and crystalline defects.

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.

Although TiO 2 is generally considered to be an oxygen deficient n-type compound, the role of oxygen vacancies and Ti 3+ ions on its photocatalytic activity is not fully understood. In this study, we investigated the effects of high-temperature calcination and H 2 reduction treatment on the water oxidation activity of rutile TiO 2 under ultraviolet irradiation. Calcination above 900 °C decreased the photocatalytic activity of the TiO 2 owing to strong oxidation, but its initial activity was restored by H 2 treatment at above 500 °C. Electron spin resonance (ESR) spectra showed that the hightemperature calcination created O •− radicals (trapped hole in oxygen lattice site), while the H 2 reduction treatment created Ti 3+ ions (trapped electron in titanium lattice site) with oxygen vacancies. Diffuse reflectance ultraviolet−visible−near-infrared (UV− vis−NIR) spectroscopy indicated an increase in the amount of electrons in shallow traps and the conduction band with H 2 treatment temperature. Measurements of the sheet resistance and space charge layer capacitance of the thermally oxidized TiO 2 films indicated that the H 2 treatment improved the electrical conductivity owing to an increase in donor density (electron density). Thus, the increase in the photocatalytic and photoelectrochemical activities of the rutile TiO 2 was attributed to donor doping by H 2 reduction.

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.

Octahedral titanium(IV) oxide (TiO 2 ) crystallites exposing anatase {101} facets exhibited relatively high photocatalytic activity for oxidative decomposition of organic compounds and low activity for hydrogen liberation in the absence of molecular oxygen probably due to the characteristics of anatase {101} surface.Anatase has been the most studied polymorph of TiO 2 owing to its higher photocatalytic activity for oxidative decomposition of organic compounds in the presence of molecular oxygen, 1,2 hydrogen (H 2 ) liberation, and so on. Surface free energy of anatase {101} lattice plane is reported to be the smallest among the other planes of anatase. 3,4In agreement with natural minerals, a truncated octahedral bipyramid, exposing eight {101} facets as well as two {001} facets, has been shown as a thermodynamically most stable shape of anatase crystallites based on Wulff construction. 3,4 In spite of the important applications of anatase TiO 2 , experimental studies on the photoactive property of singlecrystalline surface are still limited due to the difficulty to obtain high quality anatase samples. 5A few studies suggested the significant role of facets of TiO 2 polyhedral particles for photocatalytic reactions. 6,7 Recent development of hydrothermal reaction method has enabled to prepare anatase crystalline particles with well-developed facets in a relatively high yield. [8][9][10] The aim of the present study is to investigate the roles of {101} facets, which is the most likely observed on anatase mesoparticles, 11 for practical photocatalytic reaction system such as powder suspensions. In order to compare the