Origin of the multiple voltammetric desorption waves of long-chain alkanethiolate monolayers chemisorbed on annealed gold electrodes

This paper examines possible factors responsible for the voltammetric fine structure exhibited from the reductive desorption and oxidative redeposition of long-chain alkanethiolate monolayers on smooth gold electrodes. Results reveal that the fine structure is connected to the local order of the adlayer. This conclusion is based on a series of characterizations of monolayers formed from hexadecanethiol (HDT) and octadecanethiol (ODT) on annealed mica-supported gold (Au/mica) and on template-stripped gold (TSG). For annealed Au/mica, a pair of overlapping waves separated by 20–100 mV are found for both the desorption and the redeposition process. In contrast, the voltammetry of the TSG system exhibits a single wave for the two processes. Findings from morphological assessments (oxidative stripping of iodine, scanning tunneling microscopy, and underpotential metal deposition of lead) of the underlying substrates reveal that the differences in the voltammetry are connected to a subtle difference in the microscopic roughness of the two substrates. The morphological data indicate that the surfaces of annealed Au/mica and of TSG are strongly (111)-terraced, but that the sizes of the (111) terraces at TSG are significantly smaller. As a consequence, the TSG surface has a higher density of steps. Since the adlayer domain size is ultimately limited by the microscopic topography of the substrate, the voltammetric differences observed for the two types of substrates are attributed to differences in the ordering of the adlayer. A description of the differences in the processes for the desorption and redeposition is proposed and is examined within the context of recent interpretations.