Anion and electrode surface structure effects on the deposition of metal monolayers: electrochemical and time-resolved surface diffraction studies

Abstract

The effects of anions and electrode surface structure on the UPD of metal monolayers are illustrated with three different examples. In the first, we show that for Cu UPD on Pt[n(111)×(110)] (n=2, 3, 5, 9 and 19) stepped surfaces in sulfuric acid medium, submonolayer amounts of underpotentially deposited copper induce the adsorption of (bi)sulfate in the vicinity of copper adatoms deposited on (110) step sites. The induced anion adsorption increases with terrace width up to a three Pt atoms wide terrace, suggesting that this is the minimum width to accommodate the copper adatom and the coadsorbed anion. In the second case we present data from simultaneous time-resolved surface X-ray scattering and chronoamperometric measurements of Cu UPD on Pt(111) electrodes in the presence of chloride anions. These studies demonstrate that the kinetics of formation of the incommensurate CuCl adlayer from the commensurate (1×1) Cu layer takes place in a much longer time scale than the current response. This is a clear indication of the temporal separation (resolution) between electrochemical events and processes associated with surface reorganization to achieve long-range periodic ordering. Finally, we consider the UPD of Hg on Au(111) electrodes with emphasis on the relationship between the partial charge retained by the mercury and anion adsorption. At the early stages of Hg UPD, when mercury is still partially charged, an ordered mercurous–sulfate bilayer structure is formed on the electrode surface. At more negative potentials, where mercury is almost fully discharged, two additional ordered hexagonal mercury adlayers are formed with little, if any, interactions with the anions, suggesting that the interactions between them are largely electrostatic in nature.