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.