Sulfur-substrate interactions in spontaneously formed sulfur adlayers on Au(111)

The electroadsorption of S on Au(1 1 1) from 0.1 M NaOH + 3 x 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(1 1 1) forming a root3 x root 3R30 degrees superstructure. Under potential control this lattice slowly and continuously transforms into S octomers (S-8) 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(1 1 1) 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 root3 x root 3R30 degrees structure, S-8, 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 root3 x root 3R30 degrees thiol adlayers on Au(1 1 1), 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(1 1 1) exhibits the same binding energy of the core electronic levels (i.e., same chemical state) as S in root3 x root 3R30 degrees spontaneously formed thiol lattices, although the adsorption sites are different.