Formation of Sulfur Atomic Layers on Gold from Aqueous Solutions of Sulfide and Thiosulfate:  Studies Using EC-STM, UHV-EC, and TLEC

The formation of S atomic layers on Au from alkaline solutions of sulfide and thiosulfate is described. These studies are relevant to the formation of self-assembled monolayers (SAMs), as sulfide can be thought of as the simplest thiol and, thus, S atomic layers as the shortest chain SAM. S atomic layers are also of interest as precursors for the electrochemical formation of compound semiconductor thin films such as ZnS, CdS, and PbS. The deposition of S was investigated using electrochemical scanning tunneling microscopy (EC-STM), thin layer electrochemistry (TLEC), and ultrahigh vacuum electrochemical techniques (UHV-EC). UHV-EC studies included analysis with Auger electron spectroscopy (AES), low energy electron diffraction (LEED), and X-ray photoelectron spectroscopy (XPS). EC-STM and LEED revealed a 1 / 3 coverage (√3 × √3) R 30°-S structure, previously observed by a number of workers, formed from both sulfide and thiosulfate solutions. From sulfide solutions, EC-STM showed that the (√3 × √3) R 30°-S structure converted to a more complex (3√3 × 3√3) R 30°-S structure at higher potentials, with an accompanying surface roughening transition. LEED indicated the formation of a structure with a (2 × 2) unit cell near bulk deposition potentials in sulfide and thiosulfate solutions. EC-STM studies in thiosulfate solutions evinced this (2 × 2) structure, apparently composed of S dimers. Initial immersion into the thiosulfate solution resulted in a structure with a c(4 × 2√3) unit cell, believed to contain adsorbed thiosulfate species. The c(4 × 2√3) structure was replaced by the (√3 × √3) R 30°-S structure via reduction, as thiosulfate can be reductively decomposed to produce sulfide. In general, sulfide can be oxidized at fairly low potentials to form an adsorbed sulfur atomic layer, initially the (√3 × √3) R 30°-S structure on Au(111), a process referred to as oxidative underpotential deposition (upd). Bulk deposits of sulfur and possibly polysulfides are formed at more positive potentials.