Corrosion competing with redox

FIGURE 1. The Photocatalytic Semiconductor Particle "Corrosion" Cell Illustrated by TiOs. Note that the Redox Couples may need to be Adsorbed if Reaction is to Compete with Recombination. 

We saw above that the study of the competition between Fe3+ and H + reduction on illuminated p-GaP led to an increased understanding of the nature of surface electrochemical processes on that material. For many n-type materials, however, the most serious competing reaction with the oxidation of some redox couple in solution is the oxidative corrosion of the semiconductor itself. This has considerable practical consequencies a photoelectrochemical device for the conversion of solar energy must be one in which the desired electrochemical route is overwhelmingly probable compared with semiconductor dissolution. So essential is this requirement, and so difficult has it proved to find satisfactory solutions for n-type semiconductors, that a substantial fraction of the recent literature on semiconductor electrochemistry has been devoted to both practical and theoretical considerations of the problem. 

Dark current-potential curves representing a majority carrier transfer to a redox system have been measured by many research groups. Mostly cathodic currents at n-type electrodes have been studied rather than anodic currents at p-type semiconductors. This is because anodic hole consumption from p-type electrodes usually results in corrosion of the material. At least it is difficult to find a redox system where the oxidation of the redox couple competes sufficiently quickly with the corrosion. 

Among the approaches taken to solve this problem, the most successful concern the matching of an electolyte to the semiconductor. The rate of corrosion is reduced if the semiconductor is in equilibrium with the corrosion products. The rate of corrosion can also be reduced by using a redox couple which oxidizes easily. The oxidation of the redox couple SeJ /Se2- for example, has been shown to compete successfully with photocorrosion reactions for holes in n-type GaAs electrodes.28,113-116... 

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