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 tunneling microscopy (STM) images, appear in dark hexagonal form (A) and bright triangular form (B). By providing either the dark hexagonal or the bright triangular patches at a time, the chemical (oxidation) states of copper were studied by X-ray photoelectron specroscopy (XPS). The Cu LMM Auger peaks from these copper oxide islands exhibited distinct chemical shifts relative to the metallic copper islands on Pd(111): the kinetic energy was 0.6 eV lower 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, indicating that the chemical state of copper atoms was not the Cu 2+ but the Cu + oxidation state (Cu 2O). By combining photoelectron spectra of the surfaces with STM results, it was deduced that the dark hexagonal patches (A) are a Cu 2O monolayer lying on a Pd(111) surface and the bright triangular patches (B) are a Cu 2O monolayer grown on a copper layer formed on Pd(111).