IN-SITU X-RAY-ABSORPTION NEAR-EDGE STRUCTURE - A PROBE OF THE OXIDATION-STATE OF UNDERPOTENTIALLY ADSORBED METALS

Underpotential deposited Ni and Cu monolayers on Au(100) single crystals were investigated in situ by X-ray absorption near-edge structure (XANES) in grazing incidence and in fluorescence detection mode at the Ni and Cu K edges. Comparison of XANES data for monolayers and model compounds allows direct determination of the oxidation state of the adsorbate and the partial charge transfer to the substrate. Our results show that Ni is completely discharged while Cu is close to the Cu1+ oxidation state, which reveals a large charge transfer from adsorbed Cu to the Au substrate. Thus the XANES technique is established as a unique method for unambiguously obtaining the actual charge of the adsorbed layers. This brings new insight to the description of the electrode/electrolyte interface and to the quantitative evaluation of the interactions between the interfacial species.