Direct Observation of Surface-Mediated Electron−Hole Pair Recombination in TiO₂(110)

Abstract

The kinetics of surface and bulk electron−hole pair recombination have been measured separately on TiO2(110) in ultrahigh vacuum under well-controlled surface conditions for the first time. Using 100 eV incident electrons, excitation of electron−hole pairs occurs within ∼6 Å of the surface, and the rate of stimulated desorption of O2 was measured as a means of determining the charge carrier recombination kinetics, which are found to be mediated by the surface and to be first-order in charge carrier concentration. Comparison with a previous O2 photodesorption experiment, where excitation by 3.4 eV photons was used for electron−hole pair generation in a deeper region below the surface (∼100 Å), shows that bulk recombination in TiO2 occurs by second-order kinetics in charge carrier concentration.

A surface science perspective on TiO2 photocatalysis

“…These results are consistent with what one would expect for the carrier concentration dependence of the recombination process. Several studies [528,531,533,599,603] observed 2nd order recombination kinetics, as one would expect. However, there is some disagreement on this issue.…”

A review on plasmonic metal⿿TiO2 composite for generation, trapping, storing and dynamic vectorial transfer of photogenerated electrons across the Schottky junction in a photocatalytic system

“…Surface-science measurements have implied half-order dependency on light intensity for the TiO 2 bulk holes and linear dependency on light intensity for surface generated holes [171]. In those measurements, the surface-specific formation of charge carriers (holes) was accomplished by using a flux of energetic electrons with penetration depths significantly smaller than those of photons [172]. The observed intensity dependence difference was attributed to the charge carriers formed in the bulk are lost mainly through the process of charge carrier recombination (exhibiting a half-order dependence on the intensity), while the charge carriers formed close to the surface of the semiconductor mainly decayed in their reaction with surface trap states (exhibiting a first-order dependence on the light intensity).…”

Effect of Adsorbed Donor and Acceptor Molecules on Electron Stimulated Desorption: O2/TiO2(110)

“…15,16 An O 2 molecule, adsorbed on oxygen-vacancy defect sites, is desorbed by photons with energy above the bandgap (∼3.1 eV). 17 Theoretical considerations 18 support the involvement of holes as do experiments where a hole-trapping molecule was shown to suppress the rate of O 2 photodesorption.…”