Multielectron Photoprocesses

This section has thus far considered redox electrolytes comprising one electron oxidizing or reducing agents. Multielectron redox processes, however, are important in a variety of scenarios. Consider the reduction of protons to H2 (HER) - a 

Photoelectrolytic processes such as HER can be carried out on semiconductor electrodes. One can envision a HER mechanism on a p-type semiconductor of 

In this above scheme, S denotes a surface state and both direct (Reactions 30c and 30d) and indirect (i.e. surface state-mediated) (Reactions 30b and 30e) radical and H2-generating pathways are shown. Reactions 30f and 30g represent recombination routes involving the reaction intermediates. 

Admittedly, this scheme is daunting in its complexity, and the kinetics implications are as yet unclear. Early studies on p-GaP, p-GaAs, and other Group III-V (13-15) semiconductors reported onset of cathodic photocurrents (attributable to HER) only at potentials far removed (ca. 0.6 V) from Vft (174). This was attributed to Steps 30b and 30g in the preceding scheme. More recent work [175] has shown that the HER at illuminated p-InP-electrolyte contacts is accompanied by a photocorrosion reaction, leading to indium formation on the semiconductor surface. 

Another interesting characteristic of many multiequivalent redox systems is the phenomenon of photocurrent multiplication. This phenomenon may be illustrated for two systems utilizing illuminated n-type and p-type semiconductors respectively n-type 

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B electrostatic modification C catalysis of multielectron photoprocesses (refer to text). 

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