ELECTROCHEMISTRY OF SULFUR ADLAYERS ON AG(111) - EVIDENCE FOR A CONCENTRATION-DEPENDENT 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, correspo nding to the stepwise formation of a sulfur adlayer, are observed at p otentials negative of that necessary to induce bulk oxidation of Ag. T he total charge obtained by coulometric integration of the three volta mmetric 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 w ith expectations based on a complete Ag2Sads layer (theta similar to 0 .5). The dependencies of the voltammetric response on scan rate and Na 2S concentration are used to support a multistep mechanism in which ad sorption and oxidation of HS- yields a nearly complete layer of AgSHad s prior to a separate and kinetically slow surface-phase transition to a Ag2Sads layer. The voltammetric wave corresponding to the transitio n of the AgSHads layer to the Ag2Sads layer is greatly diminished or a bsent in dilute Na2S solutions, suggesting that a critical sulfur surf ace concentration is required for the transition to occur. Electrochem ical quartz crystal microbalance (EQCM) measurements support the hypot hesis that the three-wave voltammetric response reflects a surface-pha se 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 chang e observed for the third voltammetric wave (corresponding to an oxidat ive phase transition).