Electrochemical Deposition of Hydrosulfide and Ethanethiolate Adlayers on Silver(111). Voltammetric Measurement of Structural Phase Transitions During Adlayer Formation

Reversible, oxidative adsorption of hydrosulfide (HS − ) and ethanethiolate (CH 3 CH 2 S − ) on highly‐ordered, Ag(111) electrodes in aqueous 0.5 M NaOH solutions is reported. Oxidation of HS − and CH 3 CH 2 S − results in adlayer formation, as determined by voltammetry, coulometry, and in‐situ electrochemical quartz‐crystal measurements. The voltammetric response of Ag(111) in HS − solutions displays three characteristic waves. Two waves correspond to the reversible 1‐e − oxidative adsorption of HS”, resulting in the formation of a Ag‐SH adlayer. The third wave corresponds to the kinetically‐slow 1‐e − oxidation of the Ag‐SH adlayer, yielding an underpotential deposited monolayer of Ag 2 S. The voltammetric response observed in CH 3 CH 2 S − solutions is qualitatively similar to that of HS − , displaying two well‐resolved waves corresponding to oxidative adsorption. The observation of two voltammetric waves is suggestive of a mechanism of CH 3 CH 2 S − adlayer formation involving at least two distinct structural phases. However, unlike the situation for HS”, the resulting Ag‐SCH 2 CH 3 adlayer is unable to undergo a second oxidative transition. The free energies of adsorption of HS” and CH 3 CH 2 S − on Ag(111) are measured to be ca. −23 kcal/mol.