In situ STM and electrochemical investigation of sulfur oxidative underpotential deposition on Ag(111)

The oxidative underpotential deposition of sulfur on Ag(111) from alkaline solutions of Na2S was investigated by in situ scanning tunneling microscopy (STM), cyclic voltammetry, and chronocoulometry. Proceeding toward more positive potentials, the cyclic voltammetric curve shows three partially overlapping peaks A-C and an isolated and more acute peak D. The STM images of the overlayer of adsorbed sulfur over the potential region between peaks C and D reveal a (√3 × √3)R30° structure; those at potentials positive to peak D a (√7 × √7)R19° structure: each lattice site of the latter structure is occupied by a triplet of sulfur atoms. The fractional coverage, 1/3, for the (√3 × √3)R30° structure is in perfect agreement with the maximum surface concentration, Γmax = 7.7 × 10-10 mol cm-2, obtained from a thermodynamic analysis of the chronocoulometric charge vs potential curves; 2FΓmax is about 10% larger than the charge associated with the combination of peaks A-C. On the other hand, the 2FΓ value corresponding to the fractional coverage, 3/7, for the (√7 × √7)R19° structure agrees satisfactorily with the charge associated with the sum of peaks A-D, thus suggesting a total electron transfer from sulfide ions to the metal over the range of stability of the latter structure.