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

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 k(0). 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 k(0). 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 k(0) and phi (the quantum efficiency for the photogeneration of S*) from experimental data. The relationship between the cases is illustrated by a series of case diagrams.