Geometrical Structure of Adlayers on Immersed Electrodes

Abstract

Lead and copper monolayers (produced by underpotential deposition) on silver(111) and gold(111) surfaces, respectively, have been characterized by x-ray absorption spectroscopy and x-ray diffraction. The experiments were performed in situ, with the electrode immersed in aqueous electrolyte, and at a controlled potential. A combination of grazing incidence excitation and fluorescence detection (for x-ray absorption) provided the required sensitivity and surface selectivity. The metal adlayers are fully reduced (zero valent). Lead forms a well ordered two dimensional solid with a close packed hexagonal structure. Its lattice is slightly compressed relative to bulk lead and is incommensurate with the underlying silver lattice. The copper EXAFS is consistent with a commensurate overlayer with the copper atoms in the three fold sites. Oxygen atoms of the first layer of water or adsorbed electrolyte lie at a well defined distance from the metal adlayers.