FRUMKIN CORRECTIONS FOR HETEROGENEOUS RATE CONSTANTS AT SEMICONDUCTING ELECTRODES

Frumkin corrections for semiconductor electrodes have been evaluated i n both depletion and accumulation conditions. In conjunction with the Gouy-Chapman-Stern model, a finite difference approach was used to cal culate the potential drop in a depleted semiconductor and in the compa ct and diffuse layers of the contacting solution as a function of the potential applied to the solid/liquid interface. At potentials greater than 30 mV positive of the flat-band potential E-fb the potential dro p across the solution accounts for less than 3% of the total potential drop across an n-type semiconductor of dopant density 1 X 10(15) cm(- 3) in a methanolic solution of 1.0 M LiCl. Under these conditions, the concentration of a non-adsorbing, dipositively-charged redox species at the outer Helmholtz plane does not vary from its concentration in t he bulk of the solution by more than 2%. This relatively small concent ration gradient and potential drop across the Helmholtz layer combine to produce negligible Frumkin correction terms for kinetic data at dep leted semiconductor electrodes compared to those for metallic electrod es at the same applied potential relative to the potential of zero cha rge. Under accumulation conditions, the potential drop across the solu tion is more significant, and the concentration of redox species at th e surface can be as much as twice as great as that in the bulk of the solution. However, these conditions require an applied potential of -1 V relative to E-fb. Additionally, under all conditions that were simu lated, the correction to the driving force used to evaluate the hetero geneous rate constant does not exceed 2% of the uncorrected heterogene ous rate constant. (C) 1997 Elsevier Science S.A.