Evaluation of electron-hole recombination properties of titanium (IV) oxide particles with high photocatalytic activity

Electron-hole recombination in nano-sized titanium(IV) oxide (TiO₂) particles with various physical properties, which have been shown to be highly active photocatalysts, was evaluated by quantitative analysis of reduced titanium species (Ti³⁺), which might be formed at crystalline defective sites in TiO₂ particles through photo-irradiation in the presence of a hole scavenger under deaerated conditions. These highly active photocatalyst samples were synthesized by hydrothermal crystallization in organic media (HyCOM method) and post-calcination. The Ti³⁺ density decreased with increasing calcination temperature (T _c), and a linear correlation was observed between the Ti³⁺ density and rate constant for electron-hole recombination evaluated by femtosecond pump-probe diffuse reflection spectroscopy. Reaction rate (R _Ag) and the amount of silver ions (Ag⁺) adsorbed on TiO₂ particles ([Ag⁺]_ads) were measured for photocatalytic silver metal deposition along with oxygen formation from an aqueous Ag⁺ solution under deaerated conditions, and the slope of the R _Ag versus [Ag⁺]_ads plot was determined. Kinetic investigation of this reaction showed that the reciprocal of the slope was approximately related to the ratio of the rates for electron-hole recombination and electron trapping (k _r/k _e ratio). The k _r/k _e ratio decreased as T _c increased, and the logarithm of the k _r/k _e ratio was linearly related with Ti³⁺ density. These two parameters were used as a measure for the recombination properties of TiO₂ photocatalysts with various physical properties.