Electrochemical Growth of Ag2S on Ag(111) Electrodes. Coulometric and X-ray Photoelectron Spectroscopic Analysis of the Stepwise Formation of the First and Second Monolayers of Ag2S

Stepwise electrochemical growth of the first and second monolayers of Ag2S at a Ag(111) electrode in aqueous hydrosulfide (HS-) solutions is reported. The deposition of the first and second monolayers occurs at different electrode potentials and can be resolved in voltammetric experiments prior to the formation of a bulk Ag2S layer. Oxidative formation of the first Ag2S monolayer occurs by a kinetically facile two-electron, two-step mechanism, involving the one-electron oxidative adsorption of HS- (Ag + HS- ⇌ Ag−SH + e -) followed by a one-electron oxidative phase transition to yield a complete Ag2S monolayer (Ag−SH + Ag + OH- ⇌ Ag2S + H2O + e -). An irreversible wave corresponding to the formation of a second monolayer of Ag2S in a single two-electron step (2Ag + HS- + OH- ⇌ Ag2S + H2O + 2e -) is voltammetrically resolved at slow scan rates (