In Situ Atomic Force Microscopy of Underpotential Deposition of Ag on Au(111)

Atom-resolved atomic force microscope (AFM) images of underpotentially deposited (upd) monolayers of Ag on Au(111) are presented. These images show that the structure of this monolayer is dependent on the nature of the electrolyte in which the upd process occurs. Specifically, Ag shows a 3 × 3 overlayer in sulfuric acid and a 4 × 4 overlayer in nitrate- and carbonate-containing electrolytes. In perchloric acid a more complex structure is observed which, like the sulfate and nitrate systems, is also not close packed. In acetate, a close-packed monolayer is formed. The different monolayer structures give rise to packing densities which correlate with electrolyte size, except in the case of perchloric acid. However, there is little agreement between packing densities predicted from coulometric measurements and those actually observed with the AFM. The discrepancy between the two measurements is ascribed to differing amounts of charge transfer between electrolyte and adatom which alters the amount of charge needed for upd lattice formation.