Electrochemical studies of anodic films on silver
Abstract (Summary)Restricted Item. Print thesis available in the University of Auckland Library or available through Inter-Library Loan. The formation and reduction of anodic films on silver was studied in aqueous solutions at room temperature. Also, the effect of light on AgBr films was investigated. There were four main topics of study. PHASE SILVER SULFIDE FILMS Phase Ag2S films, formed by reaction (1), were studied by cyclic voltammetry with a rotating disc electrode (rde), and by scanning electron microscopy (SEM). 2Ag + HS- + OH- ? Ag2S + H2O + 2e- (1) On rough silver surfaces, the rate of phase Ag2S film formation is controlled by the solution resistance and by the diffusion of HS- to the electrode surface. On smooth silver surfaces, these physical processes also control the rate of film growth under most conditions. When the rate of transport of HS- from solution becomes sufficiently high, the rate of film growth is controlled by the electromigration of interstitial silver ions through the Ag2S film. On the basis of the low-field approximation for ionic solid state transport, the conductivity of the Ag2S film is estimated as 5.9 x 10-4 S/cm. SEM studies show that the Ag2S film forms initially as a compact basal layer and becomes granular with increasing film thickness. Ag2S film reduction occurs by the injection of electrons through the film to the film/electrolyte interface. The reduction of the film proceeds in the direction of the silver base. SILVER SULFIDE MONOLAYER The growth of phase Ag2S film is preceded by the formation of a monolayer of Ag2S. It is characterized by deposition at an underpotential (~-120 mV), by a linear dependence of peak current density on the potential sweep rate, and by a charge density of 0.2 mC/cm2. The predicted value for a Ag2S monolayer is 0.18 mC/cm2. The polarisation curves for the formation and reduction of the Ag2S monolayer were simulated by a computer best-fit technique on the basis of the following mechanism. HS- adsorbs on the silver surface in a fast equilibrium step (K = 0.17 1/mmol) to form the Ag(HS-)ads species. This is followed by a slow, electron transfer step which is activation controlled to form the surface intermediate, AgHS. The AgHS species rapidly diffuse on the Ag surface to a growing two-dimensional Ag2S monolayer nucleus. ANODIC SILVER BROMIDE AND SILVER IODIDE FILMS The formation and reduction of anodic AgBr and AgI films by reactions (2) and (3) was studied potentiodynamically with a rde. Ag + Br- ? AgBr + e- (2) Ag + I- ? AgI + e- (3) AgBr film growth was also studied by the potentiostatic step technique revealing a monolayer stage of AgBr film growth. The anodic potentiodynamic polarisation curves for AgBr formation were simulated by computer on the basis of the following mechanism. Film initiation occurs by the nucleation of microscopic islands of film to a critical thickness. These thicken and spread until only small pores exist between them. This mechanism of film growth is substantiated by SEM observations. As the pores in the film become smaller, the resistance of the electrolyte in the pores becomes rate limiting (at the anodic current peak). After the peak, the concentration of bromide ion at the pore base falls to zero and current becomes controlled by the diffusion of bromide ions within the pores. This mechanism of film growth is also likely to apply to most cases of AgI film formation. When the solubility of the AgI film is high, the pores of the film seal and film growth occurs by the low-field electromigration of interstitial silver ions through the AgI film. The ionic conductivity of the AgI film is obtained as 8.7 x 10-5 S/cm. AgBr and AgI film reduction occurs initially at the film/electrolyte interface and continues in the direction of the silver base by electron injection into the film. PHOTOEFFECTS OF SILVER BROMIDE FILMS The effect of light on anodic AgBr films, formed at constant cd by reaction (2) and on AgBr films formed by the reaction of silver with bromine vapor was studied. The films formed by the reaction of silver with bromine vapor were obtained at room temperature and at temperatures of 215-225°C. Both open-circuit and closed-circuit photopotentials were measured and two main photoeffects were observed. AgBr films composed of large grains (those formed at slow rates or those which are comparatively thick) showed an initially fast, negative photopotential, with a time constant less than 0.1 s. This associated with the motion of electrons into the AgBr films as anticipated on the basis of the positive space charge region and negative surface charge of AgBr. AgBr films composed of small grains (those formed at fast rates or those that are thin) yielded a similar initial, fast, negative phototransient followed by a positive steady-state photopotential with a time constant of about 0.5 s. This is equivalent to electron motion towards the surface of the film to surface electron trap sites. These are likely to be interstitial silver ions and the trapping of an electron at these sites results in the formation of a nucleus of silver metal similar to the photographic latent image at the film surface. The AgBr surface and space charge regions develop three layers of charge of alternating sign; these regions are represented as two capacitors in series.
School Location:New Zealand
Source Type:Master's Thesis
Date of Publication:01/01/1978