Electrochemistry of sulfur adlayers on Ag(111). Evidence for a concentration- and potential-dependent surface-phase transition

The electrochemical deposition of sulfur adlayers on highly oriented, thin-film Ag(111) electrodes in aqueous solutions (pH = 13) containing sodium sulfide, Na2S, is reported. Three voltammetric waves, corresponding to the stepwise formation of a sulfur adlayer, are observed at potentials negative of that necessary to induce bulk oxidation of Ag. The total charge obtained by coulometric integration of the three voltammetric surface waves (206 +/- 8 mu C/cm(2)) is equivalent to a sulfur adatom coverage of (1.14 +/- 0.04) x 10(-9) mol/cm(2), in agreement with expectations based on a complete Ag2Sads layer (theta similar to 0.5). The dependencies of the voltammetric response on scan rate and Na2S concentration are used to support a multistep mechanism in which adsorption and oxidation of HS- yields a nearly complete layer of AgSHads prior to a separate and kinetically slow surface-phase transition to a Ag2Sads layer. The voltammetric wave corresponding to the transition of the AgSHads layer to the Ag2Sads layer is greatly diminished or absent in dilute Na2S solutions, suggesting that a critical sulfur surface concentration is required for the transition to occur. Electrochemical quartz crystal microbalance (EQCM) measurements support the hypothesis that the three-wave voltammetric response reflects a surface-phase transition of the AgSHads layer to the Ag2Sads layer. A 2.15 +/- 0.30 Hz (2 sigma) frequency shift equivalent to (1.19 +/- 0.16) x 10(-9) mol/cm(2) of HS- is observed during the first and second voltammetric wave (corresponding to oxidative adsorption), with no frequency change observed for the third voltammetric wave (corresponding to an oxidative phase transition).