Sulfur−Substrate Interactions in Spontaneously Formed Sulfur Adlayers on Au(111)

Abstract

The electroadsorption of S on Au(111) from 0.1 M NaOH + 3 × 10-3 M Na2S solutions has been studied by in situ scanning tunneling microscopy (STM), electrochemical methods, and ex situ X-ray photoemission spectrocopy (XPS). By analyzing STM images, we have observed that S adsorbs on Au(111) forming a √3×√3R30° superstructure. Under potential control this lattice slowly and continuously transforms into S octomers (S8) in the range −0.7/−0.5 V (i.e., at typical potentials observed under open circuit conditions). In this potential range, mixtures of both structures are present on the Au(111) surface. An XPS study of the S 2p peak from the adlayers reveals the presence of three components that can be assigned to S forming a √3×√3R30° structure, S8, and bulk S at surface defects. The most important component is that corresponding to S8, in good agreement with the STM images. Furthermore, XPS spectra recorded for √3×√3R30° thiol adlayers on Au(111), characterized by STM and atomic force microscopy, lead to similar S 2p XPS spectra. A comparison between these cases allows us to conclude that S in spontaneously formed S8 on Au(111) exhibits the same binding energy of the core electronic levels (i.e., same chemical state) as S in √3×√3R30° spontaneously formed thiol lattices, although the adsorption sites are different.