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 o xidative removals of a self-assembled nonanethiol monolayer from a Au( 111) single crystal electrode in an alkaline aqueous solution. Immersi on in an alkaline solution causes a disordering of the thiol layer whi ch involves a significant tilt of the aliphatic chain toward the surfa ce. The combined electrochemical/vibrational data show that the nonane thiols are reductively removed as thiolates via a one-electron process . The reductively desorbed thiolates display intense CH stretching ban ds after their desorption which, we suggest, is due to the formation o f micelles of nonanethiolates. The oxidative removal of the nonanethio l layer is found to be a slow multiple-step process in which the carbo n-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 reduct ive process, the oxidatively desorbed molecules have very weak CH stre tching bands. We believe this is due to the slow oxidation of the thio ls that leads to the desorption of individual molecules from the surfa ce.