Electrochemical oxidative formation of ordered monolayers of thiol molecules on Au(111) surface.

Electrochemical oxidative formation of thiolate monolayers on a Au(111) surface in KOH ethanol solutions of various thiol concentrations is described. The formation process was investigated by electrochemistry, in situ scanning tunneling microscopy (STM), and surface X-ray diffraction (SXRD). The reductive charge in the linear sweep voltammogram after keeping the potential at +0.1 V increased with holding time and reached the saturated value of 103 microC cm(-2), corresponding to the full monolayer coverage of the (square root 3 x square root 3) structure. The desorption peak shifted negatively with holding time even after the monolayer was formed, suggesting that ordering of the monolayer requires a much longer time than full coverage adsorption. The herringbone structure, corresponding to the (square root 3 x 23) structure, was observed on the Au(111) surface in KOH ethanol solution by in situ STM, which shows that a clean surface was exposed. When hexanethiol ethanol solution was added into the ethanol solution at -450 mV so that the final thiol concentration was higher than ca. 5 microM, generation of vacancy islands (VIs) was observed, which shows the potentiostatic monolayer formation. When the potential was scanned positively from -950 mV where a clean reconstructed Au(111) surface was exposed, generation of VIs was observed accompanied by anodic current flow. During both oxidative adsorption and reductive desorption of the monolayer, the shape of the steps of the gold surface changed drastically, which suggests that the gold atoms on the surface are extremely mobile during the monolayer formation. SXRD measurement confirmed the surface reconstruction lifting upon monolayer formation.