Electrodeposition of silver sulfide adlayers at silver(111) and electrodes of nanometer dimensions

This dissertation describes electrochemical investigations of the formation and characterization of Ag₂S adlayers at Ag(111) and the formation and voltammetric behavior of electrodes of submicrometer dimensions (i.e., nanometer electrodes). Chapter 1 illustrates the fundamental aspects of adlayer formation and presents an overview of the first part of the thesis. Chapter 2 explores the stepwise electrochemical formation of the first two monolayers of Ag₂S at Ag(111) electrodes. Cyclic voltammetry and coulometric data suggest that the first monolayer, formed through the initial oxidative adsorption of HS−, is deposited in a reversible two-step fashion where a Ag-SH monolayer precedes Ag₂S formation. The second monolayer is deposited in a single irreversible step at potentials slightly positive of the thermodynamic value, Eo, for bulk Ag₂S formation. Chapter 3 examines the role of pH in the deposition of the first monolayer of Ag₂S. Adlayer formation becomes more energetically favorable as the solution alkalinity is increased, although the transition from a Ag-SH to a Ag₂S surface phase becomes more kinetically hindered.; Chapter 4 serves to introduce the area of microelectrode theory and fabrication and presents a synopsis of the second part of the thesis. Chapter 5 provides a detailed procedure for the construction of electrodes of submicrometer dimensions. Hemispherical shaped electrodes ranging in size (i.e., apparent radius, r_app) from several micrometers to several nanometers are fabricated by varying the amount of insulation deposited on etched Pt wires. The behavior of these electrodes is shown to conform to classical electrochemical theory and the diffusive and migrational fluxes are accurately described when r_app > 10 nm. Chapter 6 investigates more fully the electrochemical behavior of nanometer size electrodes using steady-state cyclic voltammetry. The contribution of migration for several electroactive species of varying charge near the vicinity of electrodes with r_app < 10 nm is shown to deviate from classical behavior derived under the premise that the Nernst-Planck equation is adequate to describe mass transport. The results are discussed in terms of the breakdown of the electroneutrality assumption (i.e., Σz_iC_i = 0, where z_i and C_i are the charge and concentration of species i, respectively) in the region of solution adjacent to the electrode surface.