Electrode kinetics band model

Although several single-crystal, wide-band gap semiconductors provide electrochemical and optical responses close to those expected from the ideal semiconductor-electrolyte model, most semiconducting electrodes do not behave in this manner. The principal and by far overriding deviation from the behavior described in the previous section is photodecomposition of the electrode. This occurs when the semiconductor thermodynamics are such that thermal or photogenerated valence band holes are sufficiently oxidizing to oxidize the semiconductor lattice [8,9]. In this case, kinetics routinely favor semiconductor oxidation over the oxidation of dissolved redox species. For example, irradiation of n-CdX (X = S, Se, or Te) in an aqueous electrolyte gives rise exclusively to semiconductor decomposition products as indicated by... 

Gerischer(16), Bard and Wrighton(17) have recently discussed a simple model for the thermodynamic stability of a range of photoelectrodes. As has been discussed previously, except for the rare case where the anodic and cathodic decomposition potentials lie outside the band gap, the electrode will be intrinsically unstable anodically, cathodically, or both.(16) It is the relative overpotential of the redox reaction of interest compared to that of the appropriate decomposition potential which determines the relative kinetics and thus stability of the electrode as illustrated in Figure 4. The cathodic and anodic decomposition potentials may be roughly estimated by thermodynamic free-energy calculations but these numbers may not be truly representative due to the mediation of surface effects. 

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