Behavior of illuminated n-type gallium arsenide and p-type silicon rotating disk electrodes in non-aqueous solutions

The behavior of n-GaAs rotating disk electrodes in CH₃CN/0.1 M [n-Bu₄N]ClO₄ containing 1-5 m M [Fe(η⁵-C₅H₅)₂] has been examined as a function of rotation velocity, ω, and light intensity. For electrode potentials positive of ca. 0.0 V vs. SCE and at sufficiently high light intensity the observed photoanodic current, i_obsv, is directly proportional to ω^1/2, but depending on potential, the slope is less than expected for an oxidation process limited only by mass transport and the intercept is non-zero. The value of i_obsv is the sum of decomposition current, i_d, and desired current corresponding to ferrocene oxidation, i_et, giving rise to a non-zero intercept and less than expected slopes for plots of i_obsv vs. ω^1/2. For a small potential regime i_obsv=i_et (i_d=0) and is limited only by the mass transport of [Fe(η⁵-C₅H₅)₂] to the surface. For more positive potentials i_d becomes significant and i_obsv increases. Even when t_d is significant it has been established that i_et is still limited only by the mass transport of [Fe(η⁵-C₅H₅)₂] to the electrode. At a fixed potential where i_d≠0 we find that i_d can be linearly suppressed with increasing ω^1/2 when i_et is limited only by mass transport.