Electron- and Hole-transfer from TiO2 Particles to Adsorbates Studied by Time-Resolved Infrared Absorption Spectroscopy.

Abstract

Behavior of the electrons and holes photogenerated in TiO2 particles was observed by time-resolved infrared absorption spectroscopy in the presence of oxygen, water, and methanol vapor. The electrons photogenerated by a band-gap excitation displayed a structureless, broad absorption of IR light from 3000 to 900 cm−1, which was assigned to the intra-conduction-band transition and/or excitation from mid-gap traps to the conduction band. This electron-induced absorption was probed as a function of time delay after the photoexcitation. Electron decay caused by the recombination with holes and by the charge-transfer reactions with adsorbates were kinetically analyzed. The electron decay was accelerated in the presence of oxygen gas due to an electron-capture reaction at the interface, whereas was decelerated in methanol vapor due to an effective hole-capture by methoxy groups. On platinized TiO2 particles exposed to water vapor, a hole-capture reaction completed within 2 μs after band-gap excitation, whereas the electron-capture reaction occurred in 10 μs or later. These results demonstrate the effectiveness of this method to identify individual steps of photo-induced reactions at interfaces.