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.