Water- and Oxygen-Induced Decay Kinetics of Photogenerated Electrons in TiO₂ and Pt/TiO₂:  A Time-Resolved Infrared Absorption Study

Electrons photogenerated in TiO2 and Pt/TiO2 catalysts were observed by time-resolved IR absorption spectroscopy. Transient IR absorption of an identical spectrum appeared on TiO2 and Pt/TiO2 irradiated by a 355 nm pump pulse. The absorption was attributed to optical transition of photogenerated electrons trapped in shallow midgap states. Electron- and hole-consuming reactions of adsorbates controlled the decay kinetics of the electrons competing with the electron−hole recombination. On TiO2, O2 from the gas-phase captured the electrons and accelerated the decay rate at a delay time of 10−100 μs. In water vapor, holes reacted with surface hydroxyls within 2 μs, and the recombination decay of the electrons was obstructed. When Pt/TiO2 was exposed to water, the oxidation and reduction steps of the water splitting reaction affected the decay kinetics in different time domains. Photogenerated holes oxidized water within 2 μs, whereas electrons reduced water at 10−900 μs.