Kinetics and mechanism of electrocrystallization of bismuth in oxide matrix

The kinetics and mechanism of cathodic decomposition of potentiodynamically formed Bi2O3 layer in a berate buffer solution, pH 9.2, were studied in-situ using cyclic voltammetry, potentiostatic transient and electrochemical impedance spectroscopy (EIS) techniques. The electrocrystallization of bismuth in the matrix of the oxide film occurred by injection of electrons from the underlying metal into the film/electrolyte interface. impedance spectra demonstrated that the film/electrolyte interface had a close-to-perfect RC feature, because of intrinsically good electronic conductivity of bismuth oxide. Potentiostatic current-time transients revealed more details of the reduction process, giving further insight into the mechanism of reductive decomposition of Bi2O3 up to metallic Bi. The initial stage of reductive decomposition of Bi2O3 at relatively low cathodic potentials (close to the reversible potential of the Bi2O3/Bi couple) was described by equations valid for progressive nucleation and 3D growth mechanism under charge transfer control. At higher negative potentials, where the metal phase segregation took place, the electroreduction was described by a complex equation which included progressive nucleation and 3D growth mechanism under charge transfer control, as well as instantaneous nucleation and 2D growth mechanism controlled by diffusion of redissolving OH- ions away from the reacting interface. A local increase in the pH at the interface from pH 9.2 up to pH 11.14, during the diffusion stage of OH- ions resulting from the electroreduction process, the density of nuclei instantaneously formed (N-0), the rate constant of crystal growth parallel to the electrode surface (k(1)) and the roughness factor (sigma), have been determined. (C) 1999 Elsevier Science Ltd. All rights reserved.