Ambient Pressure X-ray Photoelectron Spectroscopy Study of Oxidation Phase Transitions on Cu(111) and Cu(110).

The surface structure effect on the oxidation of Cu has been investigated by performing ambient-pressure X-ray photoelectron spectroscopy (APXPS) on Cu(111) and Cu(110) surfaces under oxygen pressures ranging from10-8 to 1 mbar and temperatures from 300 to 750 K. The APXPS results show a subsequential phase transition from chemisorbed O/Cu overlayer to Cu2O and then to CuO on both surfaces. For a given temperature, the oxygen pressure needed to induce initial formation of Cu2O on Cu(110) is about two order of magnitude greater than that on Cu(111), which is in contrast with the facile formation of O/Cu overlayer on clean Cu(110). The depth profile measurements during the initial stage of Cu2O formation indicate the distinct growth modes of Cu2O on the two surface orientations. We attribute these prominent effects of surface structure to the disparities in the kinetic processes, such as the dissociation and surface/bulk diffusion over O/Cu overlayers. Our findings provide new insights into the kinetics-controlled process of Cu oxidation by oxygen.