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 redepositio n of long-chain alkanethiolate monolayers on smooth gold electrodes. Result s reveal that the fine structure is connected to the local order of the adl ayer. This conclusion is based on a series of characterizations of monolaye rs formed from hexadecanethiol (HDT) and octadecanethiol (ODT) on annealed mica-supported gold (Au mica) and on template-stripped gold (TSG). For anne aled 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 volt ammetry of the TSG system exhibits a single wave for the two processes. Fin dings from morphological assessments (oxidative stripping of iodine, scanni ng tunneling microscopy, and underpotential metal deposition of lead) of th e underlying substrates reveal that the differences in the voltammetry are connected to a subtle difference in the microscopic roughness of the two su bstrates. 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 surf ace has a higher density of steps. Since the adlayer domain size is ultimat ely limited by the microscopic topography of the substrate, the voltammetri c differences observed for the two types of substrates are attributed to di fferences in the ordering of the adlayer. A description of the differences in the processes for the desorption and redeposition is proposed and is exa mined within the context of recent interpretations. (C) 2000 Elsevier Scien ce S.A. All rights reserved.