Spontaneously formed sulfur adlayers on gold in electrolyte solutions: Adsorbed sulfur or gold sulfide?

High coverage S phases (surface coverage >= 0.33), spontaneously formed by immersion of Au(111) in Na2S aqueous solutions at room temperature, have been studied by scanning tunneling microscopy (STM), X-ray photoelectron spectroscopy (XPS), surface enhanced Raman spectroscopy (SERS), electrochemistry, and density functional theory (DFT) calculations. XPS data show no evidence of a AuS phase, as no oxidized gold is detected. Voltammetric data are also inconsistent with the formation of a AuS phase with 0.5 stoichiometry. In situ and ex situ SERS measurements of S-covered nanostructured gold substrates demonstrate that the surface species present at the gold surface consist of a mixture of chemisorbed S and polysulfide species, as already proposed based on in situ STM images. A DFT surface model that is energetically feasible and reproduces well the experimental STM images is presented. The proposed model involves only a small rearrangement of the upper An layer and coexistence of monomeric and polymeric S. Therefore, the high coverage S phase should be described as a mixture of monomeric and polymeric chemisorbed sulfur rather than as an extended 2D AuS phase.