What Determines the Lifetime of Photoexcited Carriers on TiO₂ Surfaces?

Pump-probe time-resolved X-ray photoelectron spectroscopy measurements have been carried out to comparatively assess the relaxation process of the photoexcited states on pristine and Ar⁺-sputtered TiO₂(110) surfaces and a TiO₂(011)-2 × 1 surface, on which the accumulation-type space charge layers are developed. Ultraviolet laser irradiation induces a surface photovoltage (SPV) of around 0.1 eV. The SPV relaxation time on pristine TiO₂(110) is determined to be approximately 100 ns and is doubled on the sputtered surface. In contrast, a much shorter time of 1 ns is observed on TiO₂(011)-2 × 1. The difference in the relaxation time on the two TiO₂(110) surfaces is explained by differences in the O vacancy density on the surface as well as the barrier height of the surface potential for the photoexcited holes. A large hole capture cross section of a state characteristic of TiO₂(011)-2 × 1 is, on the other hand, responsible for the fast SPV relaxation on this surface.