Structure, Dynamics, and Electrochemistry of Covalently Modified Gold Surfaces

Abstract

The electrochemical properties of an electrode can be modified by the chemical adsorption of species onto its surface. Future molecular and hybrid electronics, therefore, depend on the mastery of chemical binding between dissimilar materials down to the individual bond. Only with a clear understanding of the adsorption and bonding of both inorganic and carbonaceous materials can new materials and devices be developed, with wide-reaching applications such as electrocatalysis, energy storage, sensors, and corrosion protection. Although covalent grafting using diazonium chemistry on mostly carbonaceous surfaces is widely reported in the literature, direct covalent carbon-metal bonds have been relatively less studied. In this dissertation, we present a detailed study of the covalent grafting of aryl radicals on single- and polycrystalline gold surfaces and investigate their electrochemical stability and oxidative desorption. Voltammetry in a redox electrolyte is used to explore the barrier properties of the resulting layers and to inquire under which conditions ultramicroelectrode array behavior can be observed. Copper underpotential deposition and Gaussian deconvolution of its current-potential trace were used to map the thermodynamic and kinetic landscape of the modified Au surface and to check for signs of metal intercalation and other nanoscale effects. Electrochemical impedance measurements, principally requiring a time-invariant system, are integrated with an intermittent grafting/open circuit potential protocol to allow operando monitoring of the grafting process. Scanning tunneling microscopy in a high-boiling point organic solvent suggests that grafting is accompanied by the formation of vacancy islands on the Au surface, while iodide adsorption heals these vacancies, enabling high-contrast imaging of individual covalent grafts. Here, we investigate the structure and dynamics of structural changes in the iodine adlayer adsorbed on Au(111) at varying potentials over time using electrochemical scanning tunneling microscopy (EC-STM). While the structure of iodine on the Au(111) surface has been extensively studied, we focus our EC-STM study on the bright chains on the surface previously identified as polyiodide. In this study, the chains’ identity is revisited, and the dynamic changes of these polymers at the surface are explored.