Electroreductive Desorption of Alkanethiols on Metal Surfaces Using In-situ Second Harmonic Generation
Surfaces are continually exposed to different environments such as gases, liquids, or even other solids. At these interfaces, many chemical reactions occur. The corrosion of metals is a widely studied phenomena in both academia and industry. At the metal-solution interface, processes such as self-assembled monolayers (SAMs) and underpotential deposition (UPD) can be used to inhibit corrosion. In-situ second harmonic generation (SHG) allows these processes to be monitored in real-time. Coupling with cyclic voltammetry (CV), the electrodesorption of three alkanethiols was observed for a gold and copperUPD-gold electrode. The SH response, as a function of potential, was sensitive to changes at the electrode surfaces as alkanethiols were reductively desorbed. After 30 CV sweeps, decanethiol (DT) was shown to reductively desorb from both the gold and copperUPD-gold electrodes. Long chain alkanethiol, octadecanethiol (ODT), remained near the surface for both electrodes. However, for dodecanethiol (DDT), after 30 CV sweeps, DDT was reductively desorbed from the gold electrode, but remained near the surface of the copperUPD-gold electrode. These results were confirmed with ex-situ sum frequency generation (SFG), X-ray photoelectron spectroscopy (XPS), and contact angle (CA) measurements. Compared to a gold electrode, the copperUPD-gold electrode showed differing responses in both the SH and cyclic voltammograms. The addition of the copper UPD layer stabilized the alkanethiols, which was evident not only by the presence of DDT after 30 CVs, but also by the increase in reductive potentials to more negative potentials than that for a gold electrode. These effects were visualized by the variations in the SH response between the two electrode surfaces. By adding a copper UPD layer onto a gold surface, there is a change in capacitance in the system. This change is magnified by the SH response as alkanethiols are reductively desorbed from a gold and copperUPD-gold electrode surface.