Electron-exchange reactions barrier height

When immersed in an electrolyte, a semiconductor undergoes an exchange of electrons with the liquid at the interface to equalize the work functions of the two phases. The result is often a rectifying barrier between the semiconductor and liquid that has properties similar to a Schottky barrier. Like semiconductor-metal contacts, the barrier height can be fixed by the semiconductor surface state distribution, or, in the case of a low density of surface states, by the difference between the work functions of the bulk semiconductor and the liquid. Similarly, a reaction between the semiconductor surface and the eiectrolyte can produce a surface layer that fixes the barrier height. 

The systems of the first class afford the closest approach to a simple barrier penetration process, and perhaps they more readily respond to a theoretical analysis. It can reasonably be supposed that for these systems orbital overlap for the two ions is small, so that the frequency of the electronic transition is small, and there is no substantial binding between the two exchanging centers. A model of this kind presumably corresponds to the weak overlap cases as defined and discussed by Marcus (8 ). In attempting to calculate the rates of these reactions, besides the problem of the shape and height of the barrier for the electron transfer, electrostatic interaction of the reactants must be dealt with and the energy necessary to distort the solvent and ionic atmosphere about each ion to make the enei of the electron equal at the two sites. Different workers have emphasized different ones of these factors, and serious differences of opinion are recorded. 

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