Qualitative Description of Current-Potential Curves at Semiconductor Electrodes

Qualitative Description of Current-Potential Curves at Semiconductor Electrodes 

The same type of argument proves 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 Ught 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 vaUd 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 

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

This equation looks similar to Eq. (7.33) which was derived for the diffusion-controlled case. Eq. (7.42) differs from Eq. (7.33), however, insofar as here the half-wave potential depends on Ldift and therefore on the rotation speed of the electrode. On the other hand, if the current is only diffusion-controlled, Eq. (7.33) determines the current-potential curve. In this case f/i/2 is independent of Ljjff and therefore also independent of the rotation speed (Eq. 7.32). 

According to these differences with respect to L/1/2, an investigation of the rotation dependence yields the best proof either for a kinetically or a diffusion-controlled reaction. This is also true for majority carrier processes at a semiconductor electrode. In the case of a metal electrode, one may be tempted to distinguish between kinetically and diffusion-controlled processes via the slope of ln[(/ im/ ) - 1] vs. because the factor (1 - a) occurs in the equation for the kinetically controlled current (Eq. 7.42) and not in the other (Eq. 7.33). This method can lead to misinterpretations, however [2], In the case of semiconductors, the latter method would even be useless because then a = 0 (see Section 7.3.4). 

In principle any electron transfer at a semiconductor-liquid interface can only occur via the conduction or valence band. Whether then a corresponding current is possible depends on various factors, such as the position of the energy bands and the occupa- 

7 Charge Transfer Processes at the Semiconductor-Liquid Interface 

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