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- are reported. Oxidative adsorption of HS- (Ag + HS- → AgSH + e-) occurs on all three low-index surfaces at potentials ranging between −0.5 and −0.7 V 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- → Ag2S + H2O + e-) at the (111) and (110) surfaces prior to bulk Ag2S formation, yielding an underpotential deposited Ag2S adlayer (surface coverages (S/Ag):  θAg(111) = 0.46 ± 0.02 and θAg(110) = 0.54 ± 0.03). In contrast, the AgSH adlayer on Ag(100) is chemically inert prior to bulk Ag2S formation. Structural models indicate that the formation of a nearly stoichiometric Ag2S adlayer (i.e., θ ∼ 0.5) is feasible on the (111) and (110) surfaces without significant reconstruction of the...