Charge Carrier Dynamics in TiO2 Mesocrystals with Oxygen Vacancies for Photocatalytic Hydrogen Generation under Solar Light Irradiation

There is a great concern about black TiO2 prepared by H-2 treatment because of its ability to enhance light harvesting of TiO2. Black TiO2 shows different photocatalytic activities compared with white TiO2. However, the mechanism of photocatalytic reaction has not been clearly understood. Here, femtosecond time -resolved diffuse reflectance (fs-TRDR) spectroscopy and single-particle photoluminescence measurements were applied to gain better understanding about the relation between oxygen vacancy, charge transfer, lifetime of photogenerated charge, and photocatalytic activity. We prepared reduced TiO2 mesocrystals (R-TMC) through simple solid-state chemical reduction at moderate temperature 350 degrees C. R-TMC has nearly two times higher photocatalytic activity for H-2 production under solar light irradiation. The presence of oxygen vacancies and Ti3+ was studied by electro paramagnetic resonance and Xray photoelectron spectroscopy. This work confirms the findings of previous studies that enhancement of light absorption by formation of surface defects does not always lead to high photocatalytic activity. Femtosecond time -resolved diffuse reflectance (fs-TRDR) spectra reveal that proper concentration of oxygen vacancies enhances the charge separation of the photogenerated carriers, leading to high photocatalytic activity.