Formation of a more noble underpotentially deposited silver layer on gold by the adsorption of chloride

The repetitive electrochemical underpotential deposition (upd) and stripping of Ag on Au(111) in sulfuric acid in the presence of small amounts of chloride produce a substantially more noble Ag upd adlayer than that formed during initial depositions. The evidence for this increase in nobility is a gradual and direct replacement of the primary upd and stripping peaks in initial cyclic voltammograms by peaks at anodic potentials higher by roughly 80 mV. The transformation is quantitative and reversible and occurs with a smoothing of the gold substrates, as observed by scanning probe microscopies. The amount of transferred charge increases by about 20% from the initial depositions (92 +/- 2 mu C/cm(2)) to the more noble state (110 +/- 2 mu C/cm(2)), indicating a change in the Ag adlattice from its initial 3 x 3 structure to a more densely packed structure. X-ray photoelectron spectroscopy (XPS) shows that the more noble Ag adlayer includes a stoichiometric amount of chloride and that its amount varies directly with the amount of silver that deposits at the higher oxidation potential. The combined results from electrochemistry and XPS reveal that chloride adsorbs onto the surface during the underpotential deposition of Ag, produces a weakly adsorbed chloride layer on gold when the silver layer is removed, and is responsible for producing a more noble Ag upd layer on gold.