Surface structural transitions induced by repetitive underpotential deposition of Ag on Au(111)

The underpotential deposition (upd) of Ag on a Au(111) electrode in aqueous sulphuric acid has been studied by electrochemical scanning tunneling micr oscopy (ECSTM). The Au(111) surface morphology is substantially altered dur ing repeated cycles in the Ag upd region. Prior to Ag upd, the surface cons ists of large, flat terraces (50-200 nm in width) with monatomic and diatom ic steps. After several cycles of Ag deposition and dissolution, the surfac e is characterised by the appearance and growth of a high density of monola yer deep 'pits.' As cycling is continued, the pits expand rapidly to leave isolated Au islands accompanied by receding terrace edges. This process is followed by a slow lateral growth of Au islands which apparently fuse with terrace edges or aggregate with each other to form extended, atomically Bat terraces. The surface topography is dominated by straight-edged, narrow te rraces (15-50 Mm in width) running parallel to or crossing each other to fo rm a mesh, in which the terrace edges meet at an angle of 120 or 60 degrees . characteristic of a (111) close-packed plane. The slow transformation of a high quality Au(111) surface in comparison to the faster transformation o f a defect rich surface suggests that the kinetics of the transition proces s are related to the initial number of surface defects. The surface morphol ogical changes accompanying Ag upd cycling are reflected in the cyclic volt ammograms via dramatic changes in both peak shape and potential. The effect of variation of the supporting anion on this transition has also been inve stigated. Repetitive cycling of Ag upd on the Au(111) electrode in the pres ence of perchlorate anions results in similar alterations in voltammetric p eak shape and potential. This weakly adsorbing anion appears to enhance the rate of structural transformation of the Au(111) surface. (C) 1999 Elsevie r Science S.A. All rights reserved.