Role of electrons and electron holes

Not only must we consider the ionic defects produced by the interfacial reactions, but equally important in a description of metal oxidation are the electronic defects produced by the interfacial reactions. This is illustrated schematically in Fig. 5. The parent metal constitutes a ready source 

Any electrons which reach the oxide—oxygen interface will be quickly utilized in the formation of 0 ions, and similarly, any positive holes which manage to reach the metal—oxide interface will be quickly annihilated by electrons from the parent metal. The asymmetrical surface reactions thus lead us to expect widely different electron concentrations in the oxide at the two interfaces of the oxide. This difference in electron concentration is equivalent to a difference in the chemical potential for the electronic species at the two interfaces. As in the case of the ionic defect species, such differences in concentration (and chemical potential) can be expected to produce particle currents of the defect species in question. If the primary electronic defects are excess electrons, then we can expect an electron particle current from metal to oxygen if the primary electron defects are the positive holes, then we can expect a positive-hole particle current from oxygen to metal. Of course, an intermediate situation is also possible in which electrons flow from the metal towards the oxygen while simultaneously a positive-hole current flows from the oxygen towards the metal, with recombination [8] (partial or total) occurring within the oxide film. 

There are other cases of electron motion which must also be considered, as for example, when the oxide is an impurity semiconductor. In that case, any electrons which enter the oxide may be able to migrate through it with relative ease. The question hinges on how easily the electrons can enter and exit from the oxide at the oxide interfaces. One possibility is the thermal excitation of electrons into the oxide from the metal interface, but that in itself may prove to be rate limiting. 

In all cases of electron transport, whether it be hopping, thermal emission, or quantum tunneling, the effect of the electric field in the oxide film is extremely important. In fact, the electric field effect on ion motion is the primary reason the electronic species must be considered at all in most real metal oxidation reactions. This can be understood better when we discuss the coupled-currents approach [10,11] in Sect. 1.15. 

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