Vibrational study of the fast reductive and the slow oxidative desorptions of a nonanethiol self-assembled monolayer from a Au(111) single crystal electrode

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.