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(1 1 1) 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- right arrow over left arrow Ag - SH + e(-)) followed by a one-electron oxidative phase transition to yield a complete Ag2S monolayer (Ag - SH + Ag + OH- right arrow over left arrow 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- right arrow over left arrow Ag2S + H2O + 2e(-)) is voltammetrically resolved at slow scan rates ( < 25 mV/s). Coulometric analysis of the voltammetric data demonstrates that the quantity of electricity (similar to 164 mu C/cm(2)) consumed during the formation of the first and second monolayers is identical. X-ray photoelecton spectroscopy is used to monitor the surface coverage of sulfur as a function of the electrode potential during the successive deposition of the Ag-SH adlayer, the first Ag2S monolayer, and the second Ag2S monolayer.