One-step redox reaction

The same type of arguments prove that the anodic decomposition reaction occurs via the valence band. Here we see that the corresponding anodic current at p-GaAs increases steeply with increasing anodic polarization whereas a very small anodic current is found with n-type electrodes. The latter could be increased by light excitation. Accordingly, holes from the valence band are required for the anodic decomposition of the semiconductor. It should be emphasized here that not holes but electrons are actually transferred across the interface, but an injection of electrons into the valence band is only possible if holes are present at the semiconductor surface. 

These are typical phenomena which are found in principle with all semiconductor electrodes. These rules are also valid for redox processes. However, redox reactions may occur either via the conduction or the valence band whereas the anodic decomposition occurs always via the valence band and the H2 formation always via the conduction band. Frequently, investigations of redox processes are limited in aqueous solutions because of interference with reactions of H2O at the electrode. This can only be avoided by using non-aqueous electrolytes. More details concerning charge transfer reactions between a semiconductor electrode and a redox system will be given in the following section. 

Many of the basic processes have also been treated in review articles and some books [12-20]. 

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