Factors affecting oxygen evolution through water oxidation on polycrystalline titanium dioxide

Abstract

The effect of physicochemical properties such as specific surface area, crystalline phase, crystallite size, and crystallinity of TiO₂ on photocatalytic water oxidation was investigated. Two types of polycrystalline TiO₂ samples which have different physicochemical properties were prepared by a sol–gel method. The photocatalytic activities of the TiO₂ samples for water oxidation were evaluated by the O₂ evolution rate from an aqueous silver nitrate solution under ultraviolet light irradiation. Comparison of the two types of TiO₂ samples revealed that the specific surface area and crystalline phase were not related to the O₂ evolution rate. In contrast, a linear relationship between the crystallite sizes and the O₂ evolution rates was observed for most of the TiO₂ samples. Crystal face selective Pt and PbO₂ depositions on the TiO₂ samples were observed by photocatalytic reduction of PtCl₆²⁻ and oxidation of Pb²⁺ ions, respectively, indicating that the oxidation and reduction sites are separated on the surface. An increase in the crystallite size of the TiO₂ promotes a spatial separation of the redox sites and suppresses the electron–hole recombination, leading to the enhancement of the photocatalytic activity.