ADSORPTION AND ELECTROOXIDATIVE PATHWAYS FOR SULFIDE ON GOLD AS PROBED BY REAL-TIME SURFACE-ENHANCED RAMAN-SPECTROSCOPY

The adsorption and electrooxidation of sulfide on gold in aqueous alkaline and acidic media have been examined by means of cyclic voltammetry coupled with sequences of surface-enhanced Raman (SER) spectra, the latter providing molecular-level information on the adsorbed species involved. The required real-time acquisition of SER spectra was facilitated by the use of charge-coupled device (CCD) detection. At the most negative potentials, a single SER band at ca. 270 cm-1 is obtained, ascribed to the surface-monoatomic sulfur stretch. Sweeping the potential positive, in the region below solution-phase sulfide electrooxidation, yielded the onset of a band at significantly higher frequencies, ca. 310 cm-1, coinciding with a reversible voltammetric prepeak. The spectra are consistent with an earlier suggestion that the voltammetric feature arises from adlayer deprotonation. The voltammetric onset of solution sulfide electrooxidation yields several new spectral features. For sulfide concentrations above ca. 1 mM in alkaline solution, bands at 150, 220, and about 450 cm-1 appear; these are diagnostic of the formation of S8 rings. The additional formation of open-chain polymers and polysulfides is indicated by the breadth of the 450-cm-1 feature; these adsorbates appear to dominate for reactant concentrations below 1 mM from the observed absence of the 150- and 220-cm-1 bands. The monomeric sulfur layer is re-formed during the cathodic portion of the voltammogram. Some mechanistic implications of these findings are discussed.