Exact and asymptotic analytical expressions are obtained for the diffusion-controlled light-on transient photocurrents exhibited at an optical disc electrode (ODE) by two types of photoelectrochemical systems: (i) the photophysical-reversible–chemical–electrochemical
(PrCE) system wherein the decay of the photogenerated excited species, S*, and the reversible chemical reaction of S* with a solution phase charge scavenger, A, obey pseudo-first-order
kinetics with rate parameters k0, k1 (forward chemical) and k2 (reverse chemical) respectively;
and (ii) the photophysical-irreversible–chemical–electrochemical (PiCE) system wherein the chemical reaction
between S* and A is considered to be irreversible i.e. k2=0. Exact descriptions of [S*] as a function of distance from
the electrode surface at t→∞ are given for both systems for a range of values of k0, k1 and, where appropriate,
k2. The photoelectrochemical collection efficiency of the ODE is also presented as a function of t
and the rate parameters. 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 PiCE process allowing ORDE diffusion layer profiles for [S*] to be calculated for the first time.
The asymptotic expressions for the transient and steady-state photocurrent behaviour at stationary
and rotating electrodes are combined, producing fourteen composite cases that completely describe the behaviour
of PrCE and PiCE systems at ODEs. Each case is discussed and procedures are given for extracting values of
k0, k1, k2 and (the quantum efficiency for the photogeneration of S*) from experimental data. The relationship
between the cases is illustrated by a series of case diagrams. A case assignment flow diagram is also
presented. For the purposes of illustration, the colloidal CdS/Fe(CN)63− system is case-assigned
and its associated
values are calculated.
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Physical Chemistry Chemical Physics
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