Electrochemical studies of the effects of pH and the surface structure of gold substrates on the underpotential deposition of sulfur

The kinetics of the electrochemical deposition and desorption of sulfur monolayers on highly oriented Au(111) and polycrystalline Au electrodes in aqueous solutions containing sodium sulfide was studied by cyclic voltammetry, chronocoulometry, and chronoamperometry. Cyclic voltarnmetry experiments reveal that underpotential deposition (upd) and stripping of sulfur takes place at two different potentials at polycrystalline Au substrates leading to two oxidative and two reductive peaks. However, sulfur upd consists of only one oxidative and reductive peak at single crystalline Au(111) substrates. Electrosorption valancy measurements and pH dependency showed that the upd of sulfur involved two-electron and one-step mechanism. The charge corresponding to one monolayer of sulfur was determined by cyclic voltammetry and chronocoulometry, indicating a 0.33 coverage. The chronoamperometric results at polycrystalline Au electrodes indicates that deposition takes place in a Langmuir-type mechanism, whereas the stripping of sulfur follows a two-dimensional nucleation and growth mechanism which is accompanied by Langmuir adsorption. Although the appearance of current transients, observed for the deposition of sulfur on Au(111), was similar to what seen for deposition on polycrystalline Au substrates, the stripping process was totally different from deposition, involving only a two-dimensional nucleation and growth mechanism, at pH 12. From an analysis of the desorption current transient, we describe the stripping mechanism as instantaneous. Chronoamperometry experiments suggest C, cc that sulfur is first adsorbed on to a Au(111) surface randomly, then reorganizes itself to form a well-ordered structure, and finally strips off in a two-dimensional nucleation and growth mechanism.