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

Abstract

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 Ag 2 S formation. Voltammetric and electrochemical quartz crystal measurements demonstrate that the resulting AgSH adlayer undergoes a second one-electron oxidation (AgSH + Ag + OH - → Ag 2 S + H 2 O + e - ) at the (111) and (110) surfaces prior to bulk Ag 2 S formation, yielding an underpotential deposited Ag 2 S adlayer (surface coverages (S/Ag): θ Ag(111) = 0.46 ± 0.02 and θ Ag(110) = 0.54 ± 0.03). In contrast, the AgSH adlayer on Ag(100) is chemically inert prior to bulk Ag 2 S formation. Structural models indicate that the formation of a nearly stoichiometric Ag 2 S adlayer (i.e., θ ∼ 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 Ag 2 S monolayer is allowed only when the number density of S and Ag atoms at the interface is nearly coincident with the reaction stoichiometry.