Electrochemical Reductive Stability of Self-Assembled Monolayers on Transition Metal Electrodes

Abstract

Alkanethiol based self-assembled monolayers (SAMs) on Au surfaces are an attractive platform for studying tethered reductive electrocatalysts, but the reductive potential stability window of SAMs prohibits their usefulness in studying CO2 and NO3- electrocatalysts which activate at low potentials where the Au-S bond is cleaved via a one-electron reduction. Previous work suggests that SAMs formed on other metals such as Cu and Ni may be reductively stable for tethering low potential electrocatalysts, but the methods used to assess the stability window are indirect and qualitative. In this work, the use of ferrocene-capped (Fc) SAMs as a quantitative redox probe is explored in the classic Au-SAM system to inform the usefulness of a tethered redox probe for coverage measurements as a function of potential on other metals to establish their stability limits. The voltammograms of the Fc-SAMs after the application of reductive potentials used for quantitative analysis were found to be challenging due to their non-ideal baselines which effect peak area measurements. The bias of traditional baselining techniques was explored, and new methods using empirical baselines in conjunction with machine learning provided useful estimates of the SAM coverage after the application of destabilizing reductive potentials, which were compared with secondary measurements using ICP-MS. After establishing the bias from baselining, the effect of electrolyte composition and holding time at a given potential were explored to explain the differences observed in the stability window measure by traditional reductive desorption cyclic voltammetry, and the potential dependent coverage measurements using Fc-SAMs. Looking towards direct measurement of SAMs on other metals, the d-band model is proposed for predicting and explaining the reductive stability on other metals. The first results from operando probing of the Au-S bond using x-ray absorbance spectroscopy are presented, paving the way forward for direct measurement of metal-SAM surface interactions as function of potential.