Probing non-equilibrium dynamics of photoexcited polarons on a metal oxide surface with atomic precision

Understanding the non-equilibrium dynamics of photoexcited polarons at atomic scale is of great importance for improving the performance of photocatalytic and solar-energy materials, but remains a grand challenge in experiment so far. Using a pulsed-laser-combined scanning tunneling microscopy and spectroscopy, we succeeded to resolve the photoexcitation and recovery dynamics of single polarons bound to oxygen vacancies on a prototypical photocatalyst, rutile TiO₂(110). The visible-light excitation of the defect-derived polarons leads to depletion of the polaron states and delocalized free electrons in conduction band. We found that the formation time of polarons becomes considerably shorter when the polaron is bound to two surface oxygen vacancies than that to one. In contrast, the lifetime of photogenerated free electrons is insensitive to the atomic-scale distribution of the defects but correlated with the averaged defect density within a nanometer-sized area. The results shed new lights on the photocatalytically active sites at the metal oxides surface.