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The atomic and electronic structures of Au/ TiO 2 ͑110͒ systems have been investigated theoretically based on the density-functional theory. We have examined Au adsorption on three types of TiO 2 ͑110͒ surfaces with different stoichiometry and defects; the stoichiometric surface, the surface formed by removing bridgingoxygen atoms (Ti-rich surface), and the surface formed by removing sixfold Ti and bridging-oxygen atoms (O-rich surface). For the stoichiometric surface, the stable site for the Au adatom is the hollow site surrounded by one bridging-oxygen and two in-plane oxygen atoms or the on-top site above the fivefold Ti atom. For the Ti-rich surface, the bridging site between fourfold Ti atoms along the [001] direction is the most stable. For the O-rich surface, the vacant site formed by removing a sixfold Ti atom is the most stable. The adhesive energies between the Au layer and the TiO 2 surface for the nonstoichiometric surfaces are much larger than that for the stoichiometric surface. The charge transfer between the Au adatom and the substrate is negligible for the stoichiometric surface, in accordance with its smaller adhesive energy. The electron transfer occurs from the sixfold Ti atom to the Au atom for the Ti-rich surface, while from the Au atom to the in-plane and inner oxygen atoms for the O-rich surface. Significant orbital hybridization between Au and O or Ti is also observed for the nonstoichiometric surfaces. It can be said that the TiO 2 surface conditions such as defects or nonstoichiometry strongly affect the adsorption energy and the electronic structure of the Au adlayer. This point should be closely related to the catalytic property of the Au/ TiO 2 system.

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