Photo decomposition ability of ultra-fine rutile TiO2 powder was investigated using the photo-catalytic reaction in
aqueous 1.0 mmol 4-chlorophenol (4CP) solutions with pH-controlled conditions. Its photo-catalytic characteristics were
then compared with those of commercial P-25 powder having mainly anatase phase. When 4CP was completely
decomposed by the photo-catalytic reaction, HPPLTed TiO2 powder was more effective than the P-25 powder regardless
of the crystalline structures. As the photo-catalytic reaction time increased, the decomposition of 4CP in the aqueous
solution was accompanied with much consumption of OH- ions. However, in the case of the aqueous solution at pH=4
naturally obtained by mixing of water and 4CP, the photo-catalytic reaction of the HPPLTed TiO2 powder occurred more
actively, compared with in the cases of the more acidic and caustic aqueous solutions. Therefore, it is thought that the
decomposition of non-degradable 4CP would take place well at a certain amount of OH- ion concentration in the aqueous
solution, considering to show no difference in the adsorption of 4CP on the surface of TiO2 particle with various pHs of
the solution, when the HPPLTed TiO2 powder with high surface areas more than 180 m2/g was used.
Formation behavior and photo-oxidation abilities of nanostructured TiO2 powders were investigated through a direct
crystallization from aqueous TiOCl2 solutions containing various metal-chlorides at 100°C. The obtained TiO2 powders without any additives and those added with Ni2+, Fe3+ and Nb5+ ions, which have a similar positive ionic radius to Ti4+, were mainly crystallized with rutile phase, whereas those added with Al3+ and Zr4+ ions, which have a quite different positive ionic radius, were mainly crystallized with anatase phase. On the other hand, the secondary particles in the TiO2 powder consisted of acicular and spherical primary particles corresponding to rutile and anatase phases, respectively. From these results, it seems that the positive ionic radius of the additives would affect phase formation as well as morphology of TiO2 precipitates. Among the TiO2 powders prepared, Ni-added powder, which consisted mainly of rutile phase with a small amount of anatase phase, showed excellent photocatalytic ability in decomposition of 4-chlorophenol.