Photoelectrochemistry with the optical rotating disc electrode. Part 1. The theoretical analysis for photophysical–electrochemical processes

Abstract

Exact and asymptotic analytical expressions are obtained for the diffusion-controlled light-on transient photocurrents exhibited at an optical disc electrode (ODE) by photophysical–electrochemical (PE) systems wherein the decay of the photogenerated excited species, S*, obeys first-order kinetics with a rate parameter k₀. Exact descriptions of the concentration of S* as a function of distance from the electrode surface at the steady-state are given for a range of values of k₀. Pre-existing approximate expressions for the rotation-speed dependence of the diffusion-controlled steady-state photocurrent recorded at an optical rotating disc electrode (ORDE) are extended and applied to the PE process. The asymptotic expressions for the transient and steady-state photocurrent behaviour at stationary and rotating electrodes are combined, producing seven composite cases that completely describe the time and rotation-speed dependence of the photocurrents generated by PE systems at ODEs. Each case is discussed and procedures are given for extracting values of k₀ and ϕ (the quantum efficiency for the photogeneration of S*) from experimental data. The relationship between the cases is illustrated by a series of case diagrams.