Anodic films on Bismuth

Abstract

This thesis deals with the nucleation, growth and defect structure of anodic oxide films formed on bismuth in aqueous electrolytes. The early stages in the formation of continuous anodic layers of bismuth oxide were studied using the technique of cyclic voltammetry. During this period, the oxide, Bi2O3, covered the surface by a process of simultaneous thickening and spreading of patches. The study of oxide nucleation classified the metal surface into two different areas with different kinetics of oxide nucleation. The ratio of the two areas varied according to the history and original preparation of the surface. Film nucleation was also studied by the galvanostatic step and potentiostatic step methods. The rate of spreading of the oxide patches over the surface was shown to be controlled by the magnitude of the surface area still uncovered. Film thickening was studied using both galvanostatic and cyclic voltammetric techniques. The high field growth law i = Ā exp (BĒ) where i is the current density and Ē the field in the oxide layer, was found with parameters Ā = (1 ± 0.5) x 10-5 A cm-2 B = (2.0 ± 0.5) x 10-6 cm V-1 This value of B gives an activation distance for high-field ion transport, a* = 0.2 nm, comparable to the radius of a lattice site. These parameters were determined for very thin films in strongly alkaline electrolyte (pH 13; thickness <30 nm). At greater film thickness, cracking of the film gave a porous layer on top of a continuous barrier layer, and the apparent value of the parameter B increased to 1.7 x 10-5 cm v-1. All previous work on the bismuth anodic film thickening process has been affected to some extent by cracking of the film. In this work, cracking of the film was confirmed by microscopic observation, including scanning electron microscopy. Dissolution of the film, giving breakdown of the oxide layer and pitting of the metal, was an important phenomenon. It became particularly significant for pH <8. The thickness attained by the anodic bismuth oxide layer was limited by this process to only 4 nm at pH 5, increasing to over 200 nm at pH 13. The cathodic reduction of the oxide proceeded from the outer surface inwards, and a rough, porous metal surface resulted. A model involving electron injection from the metal into the oxide, diffusion of electrons through the film and their reaction at the outer surface has been proposed. Microscopic observation confirmed the porous nature of the electro-reduced surface. The transient conductivity of the bismuth anodic film has been investigated, and the effects of standing at open-circuit in the electrolyte or of heating in air studied. The galvanostatic method was used. The observed effects could be adequately explained as being due to the presence ofanon-stoichiometric, n-type (excess metal) layer at the film-solution interface. Cyclic voltammetry was used to investigate the possibility of nucleation of anodic bismuth oxo-halides from solutions containing halide ions. Solutions containing fluoride, chloride, bromide or iodide at pH 4-9, phosphate and phosphate with added methanol at pH 8.5 were used. Consideration of thermodynamic stability would indicate the formation of new phases, but results showed that, in the comparatively short time scale of the experiments, kinetic factors resulted in the formation of anodic Bi2O3, except when the solution contained iodide at low pH, when the oxo-iodide, BiOI, was probably formed. Room lighting had no effect on the cyclic voltammetric or galvanostatic measurements. The transient response of anodic films on bismuth to an intense flash of white light was studied. In the case of a thick anodic oxide film (240 nm), the results were interpreted in terms of photo-excitation of the film, producing electron-hole pairs which were separated by the applied field. The recombination process was best described as rate-limiting thermal excitation of trapped electrons. The lifetime of an electron in the conduction band of anodic Bi2O3 was determined: τc = 23 ms. The photo-response of a very thin anodic oxide film (4 nm) involved internal photo-emission of electrons from the metal into the oxide. Formation of a positive space charge by the injection of extra cations from the metal into the film followed. The transient photo-potential (galvanostatic experiments) or photo-current (potentiostatic experiments) showed the decay of both excess electronic and ionic space charges. The density of electron traps in the anodic oxide film was determined as NT > 3 x 1018 cm-3. When the anodic film was formed in a solution containing bromide or iodide at pH 5, an inversion of the sign of the primary photo-effect from that observed in the absence of additives (4 nm thick film) was seen, and interpreted in terms of the photo-excitation of halide ions incorporated in the film. The lifetime of a conduction electron in anodic BiOI was determined as 1.5 ± 0.2 ms, much less than in anodic Bi2O3 (23 ms). In all the studies of the photo-effect, Gauss' theorem was used to relate the observed photo-potential (galvanostatic experiments) to the photo-induced charge separation. An approximate value of the conduction electron mobility in anodic Bi2O3 was thus obtained: μ ≃ 5 x 10-8 cm2 V-1 s-1. Parameters for the evolution of hydrogen on the bare bismuth metal surface, were obtained: b = 0.11 V, log10(i0/Acm-2) = -10.3 and (∂(log10i0)/∂(PH))η = 0 where η = E – 60 pH mV.