Direct Visualization of Water-Induced Relocation of Au Atoms from Oxygen Vacancies on a TiO2(110) Surface

Abstract

Variable-temperature scanning tunneling microscopy (STM) is used to show that Au1+ deposited onto a TiO2(110)-(1 × 1) surface under soft-landing conditions at 600 K results in a surface decorated with isolated gold atoms bound to oxygen vacancies. This result is in sharp contrast to the large, sintered islands which form from Au1+ deposited onto a hydroxylated TiO2(110)-(1 × 1) surface under soft-landing conditions at 300 K. The position of the isolated Au atoms prepared by deposition at 600 K changes from directly above the bridging oxygen rows to directly above 5c-Ti atoms when the substrate is allowed to cool from 600 to 300 K. The binding site of the Au atoms returns to directly over the bridging oxygen rows when the temperature is returned to 600 K, indicating that this process is reversible. We attribute the change in binding site to a competition between the Au atom and an adsorbed water molecule for an oxygen vacancy on the reduced TiO2 surface. Using density functional theory (DFT), we show that dissociative adsorption of water occurs at an oxygen vacancy occupied by an Au atom, displaces the Au atom, and forms a stable OH−Au−TiO 2 complex on the surface.