Detailed characterization of potentiostatic current transients with 2D-2D and 2D-3D nucleation transitions

Cyclic voltammetry, chronoamperometry and atomic force microscopy were used to study the kinetics of nucleation and crystal growth in the initial stages of silver overpotential deposition onto a vitreous carbon electrode from an aqueous solution of 10(-3)M Ag(I) in 1.6M NH4OH and 1M KNO3 (pH 11). The silver deposition process was found to be complex and occurred via two different pathways as a function of the applied deposition potential. The identification and characterization of each pathway was based on a quantitative analysis of the current transient curves, which show that within a single potentiostatic current transient, initial progressive nucleation and two-dimensional (2D) growth was followed either by other 2D nucleation and growth processes (lower overpotentials) or by three-dimensional (3D) growth controlled by incorporation of adatoms into the lattice (higher overpotentials). In order to properly identify and interpret each step during the deposition process, the characteristic features of the current transient curve were treated separately using existing mathematical formalisms. After proper identification of all processes within the recorded transients, a general equation for the whole process including contributions from adequate 2D-2D or 2D-3D nucleation transitions and double-layer charging effects was derived. A comparison of theoretically calculated and experimentally obtained current transients via a non-linear fitting method showed that the different consecutive nucleation and growth processes actually overlap. AFM analysis, which was used to characterize the surface morphology of the silver deposit, confirmed the electrochemical findings. AFM images revealed a different morphology of the silver deposit in relation to the existence of 2D or 3D silver deposits, depending on the applied electrode potential. The enhanced growth of 3D silver clusters around surface imperfections on the vitreous carbon electrode surface observed during the early stages of deposition provides an explanation for the origin of the overlap of the nucleation processes. (C) 1998 Elsevier Science B.V.