Oxidation states of submonolayer copper islands on a Pd(111) surface exposed to oxygen

The adsorption of O-2 on a monoatomic copper layer on Pd(111) leads to the formation of two distinctive copper oxide features which, in scanning tunne ling microscopy (STM) images, appear in dark hexagonal form (A) and bright triangular form (B). By providing either the dark hexagonal or the bright t riangular patches at a time, the chemical (oxidation) states of copper were studied by X-ray photoelectron specroscopy (XPS). The Cu LMM Auger peaks f rom these copper oxide islands exhibited distinct chemical shifts relative to the metallic copper islands on Pd(lll): the kinetic energy was 0.6 eV lo wer for the copper oxide islands of type A and 0.3 eV lower for B. No shake -up satellite accompanying the Cu 2p photoelectron peaks was observed, indi cating that the chemical state of copper atoms was not the Cu2+ but the Cu oxidation state (Cu2O). By combining photoelectron spectra of the surfaces with STM results, it was deduced that the dark hexagonal patches (A) are a Cu2O monolayer lying on a Pd(111) surface and the bright triangular patche s (B) are a Cu2O monolayer grown on a copper layer formed on Pd(111). (C) 1 999 Elsevier Science B.V. All rights reserved.