Adsorption of gold on stoichiometric and reduced rutile TiO2 (110) surfaces

We present a density functional study of reduced and stoichiometric rutile TiO2 (110) surfaces, and of binding of gold monomers and dimers to them. On the stoichiometric TiO2 surface, a Au atom binds to either a five-coordinated Ti atom on the basal plane, or atop a bridging oxygen atom. The two sites have nearly the same binding energy, suggesting diffusion of Au across unreduced regions of TiO2 (110) will be fast. The reduction of the rutile surface, by removal of bridging oxygen atoms, causes a charge redistribution in the system, which extends far from the vacancy site. A Au atom binds strongly to the reduced surfaces: the greater the degree of reduction the stronger the binding. On all reduced surfaces, the preferred binding sites are the vacant bridging oxygen sites. Less stable is the binding to a nearby five-coordinated Ti atom. The binding of Au2 on the reduced surfaces follows a similar pattern. Specifically, if two adjacent vacant sites are available, the optimal structure involves the dimer “dissociating” to occupy them. Several other stable binding situations for gold dimers on the reduced surfaces are discussed in the text. The binding of Au to the reduced TiO2 surface involves a substantial charge transfer to gold. We argue that this will affect the chemistry in which Au will engage.