Anodic Passivation of Tin by Alkanethiol Self-Assembled Monolayers Examined by Cyclic Voltammetry and Coulometry

Abstract

The self-assembly of medium chain length alkanethiol monolayers on polycrystalline Sn electrodes has been investigated by cyclic voltammetry and coulometry. These studies have been performed in order to ascertain the conditions under which their oxidative deposition can be achieved directly on the oxide-free Sn surface, and the extent to which these electrochemically prepared self-assembled monolayers (SAMs) act as barriers to surface oxide growth. This work has shown that the potentials for their oxidative deposition are more cathodic (by 100-200 mV) than those for Sn surface oxidation and that the passivating abilities of these SAMs improve with increasing film thickness (or chain length). Oxidative desorption potentials for these films have been observed to shift more positively, and in a highly linear fashion, with increasing film thickness (~75 mV/CH2). Although reductive desorption potentials for the SAMs are in close proximity to those for reduction of the surface oxide (SnOx), little or no SnOx formation occurs unless the potential is made sufficiently anodic that the monolayers start to be removed oxidatively. Our coulometric data indicate that the charge involved with alkanethiol reductive desorption or oxidative deposition is consistent with the formation of a close-packed monolayer, given uncertainties attributable to surface roughness and heterogeneity phenomena. These experiments also reveal that the quantity of charge passed during oxidative desorption is significantly larger than what would be predicted for simple alkylsulfinate or alkylsulfonate formation, suggesting that oxidative removal involves a more complex oxidation mechanism. Analogous chronocoulometric experiments for short-chain alkanethiols on polycrystalline Au electrodes have evidenced similar oxidative charge densities. This implies that the mechanism for oxidative desorption on both surfaces may be very similar, despite the significant differences in the inherent dissolution characteristics of the two materials at the anodic potentials employed.

Layered Sn–Au Thin Films for Increased Electrochemical ATR-SEIRAS Enhancement

“…The EC ATR-SEIRAS performance of the electrodes made by each fabrication technique was experimentally examined next. To probe SEIRAS signals of adsorbates, we deposited C 10 alkane SAMs onto the electrode surfaces (procedure described in the SI), as they can organize to have similar dense coverages on all of the materials. − Decanethiol was used for Au- and Sn-based electrodes (including OD-ED-Sn), − while on the as-synthesized SnO 2 electrode, decylphosphonic acid was used to form the SAM (Figure S19). Having a hydrocarbon chain, these SAMs are characterized by strong vibrational signals at frequencies in the 2850–3000 cm –1 range, attributed to various CH 2 and CH 3 stretching modes .…”

“…But, following previous reports in the literature, on Au-based electrocatalysts, decanethiols drop below 75% of a monolayer above the theoretical HER onset potential, 54 while in the case of Sn-based electrocatalysts, decanethiols were proposed to desorb reductively around the SnO x reduction potential. 46 As an attempt to compare penetration and propagation lengths, while considering a simplified set of equations (Supporting Information), we calculated a ratio of ∼1.41 for Sn/Au penetration depth (normal to the surface) and a ratio of ∼0.07 for Sn/Au lateral propagation depth (parallel to the surface). This is in line with the proposed evanescent wave profile in Figure 4.…”

An electrochemical sensing approach for scouting microbial chemolithotrophic metabolisms

“…In general, all these results are well correlated with those obtained by the corrosion rates obtained previously. The changes in the Q c values are related to changes in the electrode area by the formation of electroactive and/or passive compounds [52].…”