Structure of Chemisorbed Sulfur on a Pt(111) Electrode

Electron spectroscopic and diffraction results obtained in ultra-high-vacuum, combined with cyclic voltammetric data, are reported for sulfur adlayers deposited from aqueous sulfide and bisulfide media on Pt(111). The highest coverage obtained by Auger electron spectroscopy, 0.94 ± 0.05 monolayer, is very close to the coverage obtained from coulometry, and is associated with a (1×1) surface phase. This coverage is much higher than that obtained in previous electrochemical studies but is the same as found by other investigators using S2 beam dosing in vacuum. The near complete sulfur monolayer is characterized by a rapid and incomplete oxidation in a narrow potential range near 0.70 V vs a Ag/AgCl reference. Neither full sulfur monolayer coverage nor a sharp voltammetric transition could be obtained when traces of oxygen were present in the electrochemical cell. Oxidation of the (1×1) adlayer (at ≈1 monolayer) gave rise to a previously unreported (2×2) structure, at 1/2 monolayer. Further voltammetric stripping resulted in two more adlattices: (√3×√3)R30° at 1/3 monolayer and p(2×2) at 1/4 monolayer, as reported in previous gas phase studies. The selective stripping procedure provides unique electrochemical control at room temperature of surface structure and coverage, without any change in the long-range surface order of the substrate. When dosing was carried out from bisulfide solution, a (√3×√7) phase at 3/5 monolayer was formed, which once again was not reported previously. The results of the core-level electron energy loss spectroscopy studies suggest that sulfur adatoms retain some of the negative charge and that this charge plays a major role in controlling hydrogen adsorption coverage in the presence of coadsorbed sulfur on Pt(111).