The electrochemical behavior of a nonanethiol layer adsorbed on 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 peak, 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 pattern that is related to the different crystallographic domains present on the polycrystalline electrode.

The electrochemistry of a hexadecanethiol monolayer deposited on a Au(111) single crystal electrode has been examined in electrolyte solutions of different pH values, and it has been found that a significant fraction of the hexadecanethiol monolayer can be electrochemically removed and redeposited repetitively from the Au(111) surface. We suggest that this behavior is caused by the low solubility of the reduced molecules which prevents their diffusion into the bulk of the solution. The solubility of the thiol is confirmed to be a most important factor in the oxidative redeposition process by identical experiments performed on butanethiol and nonanethiol layers which show a decrease in the oxidative redeposition with the increasing solubility of the thiol. Preliminary work suggests that surface roughness may also play a part in influencing the extent of oxidative redeposition, with smoother surfaces giving more redeposition. The reductive desorption/oxidative redeposition of hexadecanethiols in an aqueous solution of high pH (0.5 M KOH) consists of two distinct processes. This is suggested to arise from the presence of domains of thiols of different ionic permeabilities.

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We present in-situ vibrational measurements of the reductive and the oxidative removals of a self-assembled nonanethiol monolayer from a Au(111) single crystal electrode in an alkaline aqueous solution. Immersion in an alkaline solution causes a disordering of the thiol layer which involves a significant tilt of the aliphatic chain toward the surface. The combined electrochemical/vibrational data show that the nonanethiols are reductively removed as thiolates via a one-electron process. The reductively desorbed thiolates display intense CH stretching bands after their desorption which, we suggest, is due to the formation of micelles of nonanethiolates. The oxidative removal of the nonanethiol layer is found to be a slow multiple-step process in which the carbon−sulfur bond can be broken and up to 11 electrons can be involved in the oxidation of a single chemisorbed thiol. In contrast to the reductive process, the oxidatively desorbed molecules have very weak CH stretching bands. We believe this is due to the slow oxidation of the thiols that leads to the desorption of individual molecules from the surface.

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Adsorption of 2,2'-bipyridine (22BPY) at the Au(1 1 1)-solution interface has been studied using cyclic voltammetry, ac voltammetry, chronocoulommetry, and second harmonic generation spectroscopy (SHG). The thermodynamic quantities describing the energetics of 22BPY adsorption at the Au( 11 1) surface such as the Gibbs surface excess, Gibbs energy of adsorption, and the electrosorption valency were determined from electrochemical measurements. The effect of 22BPY on the crystallographic and electronic structure of the Au(ll1) surface was assessed with the help of SHG experiments. It was found that 22BPY adsorbs at the Au( 11 1) surface in a number of states corresponding to different orientations and/or conformations of this molecule. The 22BPY molecule assumes a flat orientation in which the two aromatic rings are parallel to the gold surface at the negatively charged interface. At the positively charged interface, the molecule assumes a vertical orientation with a coplanar cis configuration in which the two nitrogen atoms of the molecule face the metal surface. The transition from the flat to the vertical orientation is gradual and goes through a series of intermediate states. Adsorption of 22BPY has little effect on the crystallographic structure of the Au (ll1) surface. A significant change of the phase angle between the real and imaginary components of the isotropic term of the SHG signal indicates, however, that mixing of the molecular orbitals of the 22BPY with the electronic states of the metal affects the electronic structure of the surface.

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