ANION AND ELECTRODE SURFACE-STRUCTURE EFFECTS ON THE DEPOSITION OF METAL MONOLAYERS - ELECTROCHEMICAL AND TIME-RESOLVED SURFACE DIFFRACTIONSTUDIES

The effects of anions and electrode surface structure on the UPD of me tal monolayers are illustrated with three different examples. In the f irst, we show that for Cu UPD on Pt[n(111) x (110)] (n = 2, 3, 5, 9 an d 19) stepped surfaces in sulfuric acid medium, submonolayer amounts o f underpotentially deposited copper induce the adsorption of (bi)sulfa te in the vicinity of copper adatoms deposited on (110) step sites. Th e induced anion adsorption increases with terrace width up to a three Pt atoms wide terrace, suggesting that this is the minimum width to ac commodate the copper adatom and the coadsorbed anion. In the second ca se we present data from simultaneous time-resolved surface X-ray scatt ering and chronoamperometric measurements of Cu UPD on Pt(111) electro des in the presence of chloride anions. These studies demonstrate that the kinetics of formation of the incommensurate CuCl adlayer from the commensurate (1 x 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 wit h 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 bilay er structure is formed on the electrode surface. At more negative pote ntials, where mercury is almost fully discharged, two additional order ed hexagonal mercury adlayers are formed with little, if any, interact ions with the anions, suggesting that the interactions between them ar e largely electrostatic in nature. (C) 1998 Published by Elsevier Scie nce Ltd. All rights reserved.