HREELS, XPS, and electrochemical study of benzenethiol adsorption on Au(111)

Benzenethiol adsorption from the liquid phase on Au(111)has been studied using high-resolution electron energy loss spectroscopy (HREELS), X-ray photoelectron spectroscopy (XPS), and electrochemistry. The absence of a S-H stretching vibration and the presence of benzene-like bands indicate that the sulfhydryl hydrogen is lost during adsorption, consistent with the formation of a thiolate intermediate. On the basis of the intensity of the out-of-plane C-H deformation, an adsorption geometry with a strong inclination of the plane of the phenyl ring toward an upright orientation on the Au(111) surface is favored. XPS provides further evidence that the primary adsorbate species is bonded to Au through the sulfur atom. The monolayer film thickness, absolute sulfur surface coverage and molecular packing density confirm a bonding arrangement in which the phenyl ring is aligned largely perpendicular to the substrate surface. Attenuation of a voltammetric peak, associated with reversible removal of the Au(111)-(root 3x22) reconstruction, and inhibition of chloride anion adsorption, suggest that benzenethiol chemisorption gradually deconstructs the Au(111) surface to the (1x1) bulk truncated phase and self-assembles through a mechanism involving growth of two-dimensional islands. Specifically adsorbing anions leads to formation of a (1x1) surface structure, causing reorganization of the organic adlayer, reducing the ability of the Au(111) surface to undergo reversible reconstruction. The influence of benzenethiol on the underpotential deposition (UPD) of Cu on Au(111) is investigated. At sub-monolayer thiol coverages, a series of broad adsorption features are observed. In contrast, desorption peaks are readily identified from sub-monolayer to high thiol coverages. It appears that saturation coverage of benzenethiol passivates the surface toward Cu adsorption and desorption processes. A benzenethiol coverage-dependent study of Cu UPD supported by XPS uptake data suggests a multistep adsorption mechanism comprising an initial rapid adsorption step with benzenethiol adopting a flat-lying bonding geometry, followed by a slow orientational phase transition to form islands of densely packed upright adsorbate.