Anodic formation of thin CdS films. II. Dependence of the kinetics on S2?concentration

Abstract

The kinetics of anodic formation of CdS films is examined in 1.0 mol dm−3 NaHCO3 solutions with different concentrations of Na2S. It is confirmed that films formed initially under galvanostatic conditions grow according to the high field assisted migration of ions. The exchange current density is firstorder with respect to the concentration of Na2S. This dependence can be accounted for with a process at the anodic film/solution interface as the rate determining step. Such a step is usually not considered to be rate determining in the formation of anodic films according to the high field model of growth. The surface density of the species participating in the rate determining step is calculated and compared to the concentration of various species in the bulk of the solutions. This calculation shows that S2− is the species participating in the rate determining step across the inner Helmholtz layer. The concentration of HS−, which may be thought to participate in the rate determining step, is higher, by six orders of magnitude, than the concentration of the species participating in the rate determining step. A rate equation for the process in the inner Helmholtz layer as the rate determining step is developed and discussed.