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.

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

Mesoscopic crystalline anatase particles of titanium(IV) oxide (titania) with decahedral morphology and with octahedral morphology were synthesized by gas-phase reaction of titanium(IV) chloride with oxygen and hydrothermal reaction of titanate nanowires in an alkaline medium, respectively, and their photocatalytic activities in relation with their crystal morphology were investigated.

Nanowire-structured titanate with titanium(IV) oxide (titania) was obtained by calcination of potassium ion-containing titanate nanowires prepared through alkali treatment of titania nanoparticles.The presence of potassium ions in the as-synthesized titanate nanowires was required for maintenance of the nanowire structure under the conditions of post heat treatment. The crystallite structure, composition, morphology, specific surface area, pore volume distribution, and optical properties were found to be dependent on the temperature at which titanate nanowires were calcined. Photocatalytic activity was examined using three probe reactions: oxidative decomposition of acetic acid in an aqueous solution, oxygen liberation from an aqueous silver sulfate solution, and hydrogen liberation from an aqueous methanol solution in the presence of hexachloroplatinic acid as a precursor of photodeposition of platinum particles. Detailed characterization and results of photocatalytic activity tests revealed that titanate crystallites greatly contributed to the photocatalytic activities of the calcined nanowires except for photocatalytic hydrogen liberation. It was found that platinum was preferentially photodeposited on anatase crystallites rather than on titanate crystallites for hydrogen liberation.2