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In order to effectively utilize visible light in photocatalytic reactions, we have developed S-doped TiO2 particles. They show strong absorption for visible light and high activities for degradation of methylene blue in aqueous solution under irradiation at wavelengths longer than 440 nm. The oxidation state of the S atoms incorporated into the TiO2 particles is determined to be S6+ from the XPS spectra.

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Anatase titanium(IV) oxide (TiO2) particles with specific exposed crystal faces were prepared by hydrothermal treatment of peroxo titanic acid (PTA) solution with polyvinyl alcohol as a shape-control reagent. Crystal phase, shape, and size of TiO2 particles were found to be greatly dependent on pH value of PTA solution and time of hydrothermal treatment. TiO2 particles prepared from PTA solution of pH 7 had {101} and {001} exposed crystal faces, and the shape of TiO2 particles changed with the time of hydrothermal treatment. The prepared TiO2 particles with specific exposed crystal faces showed higher photocatalytic activity for acetaldehyde decomposition than commercial spherical TiO2 particles. This result implies that back reaction was prevented by spatial separation of redox sites in the particles because of selective migration of electrons and positive holes to specific exposed crystal faces and/or different reactivity of electrons and positive holes on the specific exposed crystal face. Furthermore, the shape evolution of TiO2 particles showed a relationship with photocatalytic activity, and a stacked structure of octahedral anatase showed the highest photocatalytic activity due to both oxidation and reduction sites with large surface area.

We have found that anatase TiO 2 dissolves into an HF solution more easily than rutile TiO 2 . Using this property, we succeeded in isolating pure rutile particles from TiO 2 powder consisting of rutile and anatase phases by the HF treatment. The isolation of pure rutile particles was confirmed by X-ray diffraction patterns and diffuse reflection spectra. The rutile particle showed much lower photocatalytic activity than the original powder, if compared for the oxidation of 2-propanol in aqueous solution using dissolved oxygen as the electron acceptor. On the other hand, when Fe(III) ions were used as the electron acceptor for oxidation of 2-propanol and water, the activity scarcely changed after the HF treatment. These results indicate that rutile TiO 2 has poor activity for the reduction of oxygen, and the presence of a small amount of an anatase component is crucial for an efficient photocatalytic reaction on TiO 2 particles using oxygen as the electron acceptor.