Molecular Orientation and Ordered Structure of Benzenethiol Adsorbed on Gold(111)

Abstract

The adsorption of benzenethiol on Au(111) from aqueous solution has been investigated by using surface-enhanced infrared absorption spectroscopy (SEIRAS), scanning tunneling microscopy (STM), and cyclic voltammetry (CV). This molecule is dissociatively adsorbed on the surface as a Au benzenethiolate and forms a well-ordered monolayer with a commensurate (√13×√13)R13.9° symmetry. The ordered phase is formed within a few minutes in 0.1 mM aqueous solution, and further immersion of the substrate results in the deposition of multilayers. The phenyl ring is tilted about 30° from the surface normal in the monolayer. The coverage of this molecule is calculated to be 0.31 (or 0.71 nmol cm-2) from the (√13×√13)R13.9° symmetry, but it was estimated to be 0.23 (or 0.53 nmol cm-2) from the reductive desorption of the monolayer in alkaline solution. The discrepancy between the two measurements may be ascribed to the coadsorption of sulfur atoms and the presence of vacancies.