Direct XPS evidence for charge transfer from a reduced rutile TiO2(110) surface to Au clusters

The electronic structures of Au clusters deposited on the stoichiometric and reduced rutile TiO2(110) surfaces were investigated by means of X-ray photoelectron spectroscopy. With the increasing Au thickness, the Au 4f binding energy shifts monotonically toward lower binding energy on the stoichiometric surface, essentially demonstrating a particle size effect. On the reduced surface, the Au 4f binding energy exhibits a monotonically downward shift with the increasing Au thickness below 1.5 A, then unexpectedly shifts to higher binding energy when the Au thickness increases from 1.5 A to 2 A. The Au 4f binding energy shift observed on the reduced surface can be reasonably explained by a collective contribution from the particle size effect and the charge transfer from the reduced surface to Au clusters. Our results experimentally present direct XPS evidence for the charge transfer from the reduced TiO2(110) surface to Au clusters.