Underpotential deposition of zinc ions at single crystal electrodes and the effect of the adsorbed anions

Underpotential deposition (upd) of Zn2+ ions on Pt(111) and Au(111) is focussed in view of anion adsorption/desorption process, since Zn upd processes receive various effects of anions during the course of the processes, of which characteristics are different between Au and Pt. The difference is clearly demonstrated, being discussed by means of work function, point of zero charge (pzc) and the presence/absence of preadsorbed anion in the region of upd. The coincidence of kinetics of preadsorbed anion and upd Zn for the case of Pt(111) in phosphate solution at pH 4.6 is discussed by random adsorption/desorption of the Langmuir type. In the case of Zn upd on Au(111) in the same solution, two dimensional phase formation/desorption by nucleation-growth type was concluded, where the preadsorbed anion is not present at onset potential of Zn upd, and therefore the facilitation by the adsorbed anion is not present. However, we can expect that the position of pzc is located at more positive potential than the upd onset potential; the electrostatic interaction toward Zn2+ is an attractive force to the electrode, which is discussed to assist in nucleation-growth mechanism for two-dimensional upd phase formation by lateral interaction. We have tried to understand the upd phenomenon in general by means of the Zn upd process. (C) 1998 Elsevier Science Ltd. All rights reserved.