Quantitative analysis of defective sites in titanium(IV) oxide photocatalyst powders

Abstract

The molar amounts of defective sites (M_d) in several titanium(IV) oxide (TiO₂) powders were determined using photoinduced reactions of electron accumulation in deaerated aqueous solutions containing sacrificial hole scavengers and subsequent reduction of methylviologen to its cation radical. Measurements of pH dependence of typical anatase and rutile TiO₂ powders showed that these defective sites were of electronic energy just below the conduction band edge of TiO₂ in ranges of 0–0.35 V for anatase and 0–0.25 V for rutile. A linear relation of M_d with the rate constant of electron-hole recombination determined by femtosecond pump-probe diffuse reflection spectroscopy revealed that M_d could be a quantitative parameter of recombination between a photoexcited electron and a positive hole. The fact that there was no linear relation between M_d and the specific surface area suggests that the surface area was not directly reflected on M_d. A reciprocal correlation between photocatalytic activity for water oxidation in aqueous silver sulfate solution and M_d revealed that the rate of recombination is one of the predominant physical properties governing the activities of TiO₂ powders in this reaction system.