Chloride adsorption on Cu(111) electrodes in dilute HCl solutions

The adsorption of Cl- ions on a Cu(111) electrode was studied by UHV-based spectroscopies and by cyclic voltammetry. After standard UHV preparation the sample was transferred to an electrochemical cell. The shape of the voltammograms can be explained by a superposition of Cl- adsorption/desorption and a hydrogen reduction current, if an enhancement of the hydrogen reduction by adsorbed Cl- is assumed. After backtransfer to the UHV an additional peak was observed in the ultraviolet photoemission spectrum. In combination with X-ray photoemission and low-energy ion scattering measurements this additional feature is unambiguously assigned to adsorbed CI. It is also evident from the spectra that the electrochemical double-layer does not remain intact in some cases during the transfer. A more detailed study of the surface structure as a function of the electrode potential was performed by in situ scanning tunneling microscopy. In situ STM images show a clear-cut transition from the (1 x 1) structure of the substrate to a (root 3 x root 3)R30 degrees adsorbate structure if the potential is scanned through the Cl- adsorption and vice versa if scanned through the desorption region.