Abstract:
This thesis deals with the electrochemistry of the bismuth electrode in aqueous alkaline sulphide solutions. Cyclic voltammetry, potential step and current step experiments were used to study the growth and structure of anodic films. The composition of these films was investigated by radiochemical (35S) experiments, and their defect structure was investigated by a galvanostatic photoflash method. An impedance study of the hydrogen evolution region in these media is also described. A small peak observed in cyclic voltammetry is attributed to deposition of a monolayer of bismuth sulphide film. A number of monolayer deposition mechanisms with the rate controlled by adsorption kinetics are discussed, including cases where the film grows as two-dimensional patches but the rate is determined by adsorption of intermediates on the uncovered surface. Computer simulations are used to compare the models with the experimental data. Evidence is presented that the process occurs at different rates on different parts of the surface, and that the rate determining step is after the first electron transfer. Other anodic peaks are attributed to bismuth sulphide film growth, and photo-assisted anodic oxidation of sulphide occurring at the surface of the bismuth sulphide semiconductor film. Passivation by an anodically formed sulphur film also occurs. Potential step transients show a peak. They are interpreted as the simultaneous growth of a sulphur film and a bismuth sulphide film. The latter film, which gives rise to the peak, forms by instantaneous nucleation and growth of patches with incorporation of material at the periphery as the rate determining step. Radiochemical experiments in which 35S is used as a label are described. The presence of sulphur in the anodic films is confirmed and the deficiency of sulphur relative to the amount expected from the anodic charge is attributed to the presence of bismuth oxide. The film thickening which occurs on galvanostatic anodisation in solutions of low sulphide concentration was studied and the results are interpreted in terms of the high field ionic migration mechanism. At higher sulphide concentrations, galvanostatic anodisation gives potential oscillations, and these are ascribed to periodic formation and dissolution of sulphur plugs in the pores of the bismuth sulphide film. A sweep-step method for studying the processes occurring in voltammetry peaks is proposed, and theory for it is presented. The hydrogen evolution reaction was studied by the a.c. impedance method. The experimental impedance loci are compared with the theoretical impedance for a two-electrochemical-step mechanism with a single adsorbed species by a non-linear least squares procedure. The potential dependence of the parameters so obtained is compared with theory and values of some rate constants are deduced. Galvanostatic photoflash experiments were used to obtain information about electron trapping within the film. A brief flash of light during galvanostatic film thickening causes a sharp decrease in potential followed by a slower relaxation back to the pre-flash value. Such experiments were used to obtain information about electron trapping within the film. A method of analysis is presented and used to estimate the concentration of traps as 10 19cm.-3.