Electrochemical desorption and adsorption of nonyl mercaptan at gold single crystal electrode surfaces

The electrochemical behavior of a nonanethiol layer adsorbed an Au(111), Au(110), and on a Au polycrystal has been examined using cyclic voltammetry. The reductive desorption and the oxidative redeposition of the nonanethiol molecules at the Au(111) surface have been shown to depend strongly on the pH of the electrolyte solutions. While the amount of material reductively desorbed from the surface on the first cathodic cycle is independent of the pH, the amount of material that is oxidatively redeposited increases significantly as the pH is lowered. This behavior is ascribed to a reduction in solubility of the product of desorption (thiolate or thiol) as pH decreases. At high pH the redeposition of the layer seems to occur in one step, but at pH values that are lower than the pK(a) of the nonanethiol the redeposition seems to occur in two steps. In an alkaline solution, the reductive desorption of the nonanethiols from a Au(110) surface is similar to the same process at the Au(111) surface. The double layer charging current, the shape of the reductive current peaks, and its integrated charge are similar to those measured on the Au(111) electrode. Our single crystals study also reveals a correlation between the potential of zero charge of the uncoated gold, single crystal electrodes and the potential at which the reductive desorption of the nonanethiols occurs. The results on a polycrystalline surface indicate a complex stripping patten that is related to the different crystallographic domains present on the polycrystalline electrode.