Electron transfer at semiconductor electrodes in the dark

In general, Csc is orders of magnitude smaller than the Helmholtz capacitance, reflecting the fact that the major apart of any change in electrode potential appears across the space charge region. 

Before considering the kinetics of photoelectrochemical reactions, it is useful to compare electron transfer at metal electrodes with electron trans- 

It is important to note that the description of electron transfer kinetics is different in the case of semiconductor electrodes. For an n-type semiconductor electrode in the dark, the rate of electron transfer depends not only on the concentration of redox species in the solution but also on the potential dependent density of electrons in the semiconductor. Under depletion conditions, most of the potential drop is located in the solid, so that to a good approximation the activation energy for electron transfer is independent of potential. Electron transfer at semiconductor electrodes is therefore characterised in terms of a second order heterogeneous rate constant with units cm4 s-1. 

Electron transfer rate constants of outer sphere redox reactions can be measured relatively easily at n-type semiconductor electrodes. This is because electrons are withdrawn from the surface under depletion conditions, so that their concentration is lower than in the bulk. Under ideal 

---