Electrochemistry of Sulfur Adlayers on the Low-Index Faces of Silver.

The formation and reactivity of sulfur adlayers on single-crystal Ag electrodes 111, 110, and 100 orientations in aqueous solutions pH 13 containing HS- is reported. Oxidative adsorption of HS- Ag HS- right arrow AgSH e- occurs on all three low-index surfaces at potentials ranging between -0.5 and -0.7V of the thermodynamic value for bulk Ag2S formation. Voltammetric and electrochemical quartz crystal measurements demonstrate that the resulting AgSH adlayer undergoes a second one-electron oxidation AgSH Ag OH- right arrow Ag2S H2O e- at the 111 and 110 surfaces prior to bulk Ag2S formation, yielding an underpotential deposited Ag2S adlayer surface coverages SAg theta Ag111 0.46 - 0.02 and theta Ag110 0.54 - 0.03. In contrast, the AgSH adlayer on Ag100 is chemically inert prior to bulk Ag2S formation. Structural models indicate that the formation of a nearly stoichiometric Ag2S adlayer i.e., theta approx. 0.5 is feasible on the 111 and 110 surfaces without significant reconstruction of the outermost atomic layers of the substrate, but not on the 100 surface. The results suggest that the formation of a Ag2S monolayer is allowed only when the number density of S and Ag atoms at the interface is nearly coincident with the reaction stoichiometry.