In Situ Vibrational Study of the Reductive Desorption of Alkanethiol Monolayers on Gold by Sum Frequency Generation Spectroscopy

In situ sum frequency generation vibrational spectroscopy (SFG) was used to monitor the reductive desorption of decanethiol (DT) and octadecanethiol (ODT) self-assembled monolayers (SAMs) grown on evaporated gold on silica. At negative potentials, the alkyl chains of both monolayers became disordered as monitored by the appearance of methylene symmetric stretching modes in the collected spectra. The increased tilting of the terminal methyl groups on the chains of the DT monolayer further support this observation. The disappearance of the methyl C–H stretching vibrational modes at the reductive potential suggests that DT molecules diffused away from the surface after reduction. ODT molecules, on the other hand, retained their two-dimensional structure near the gold surface, implied by the strong methyl vibrational modes at the reductive potential. After four reductive cycles, a large portion of the DT SAM was fully desorbed, while the ODT monolayer existed as a combination of physisorbed and chemisorbed molecules on the gold surface, held in place by the van der Waals interactions between the alkyl chains