Origin Of The Thickness-Dependent Oxidation Of Ultrathin Cu Films On Au(111)

Ultrathin Cu films deposited on a metal substrate have been used as a model system to understand the structure function relationship in electro-catalysis, heterogeneous catalysis, and microelectronics. The stability of ultrathin Cu films against oxidation has been of particular interest, but there is a lack of microscopic understanding. We report here an atomic-level study on the thickness dependent oxidation kinetics of Cu layers on Au(111), from ultrahigh vacuum to near ambient conditions. Ultrathin Cu films on Au(111) were found to exhibit a superior oxidation resistance over Cu(111), and their oxidation resistances increase in the order of Cu(111) < 2.4 ML Cu < 0.4 ML Cu. For 0.4 ML Cu, the spontaneous subsurface diffusion of Cu at 300 K and the formation of a Au-rich surface alloy inhibit the formation of copper oxides at the O-2 pressure below 10(-4) mbar. However, at near ambient conditions, 0.4 ML Cu would be partially oxidized to the CuO phase directly. In contrast, multilayer Cu or bulk Cu(111), though oxidized more rapidly, forms only Cu2O surface layers under the same oxidation conditions. We analyzed further the atomic process of alloying at elevated temperatures. An intermediate Au3Cu alloy phase was suggested at the subsurface at 400 K. The diffusion of Cu into bulk Au(111) at 600 K prevents the formation of copper oxides at 300 K even under near-ambient conditions. Our study could thus provide insight for the rational design of a highly efficient Cu-based oxidation catalyst.