Two‐Step Redox Processes with Quinones at Semiconductor Electrodes

Charge transfer processes with quinones at semiconductor electrodes were studied. It is demonstrated that with semiconductor electrodes (GaP, Ge, Si and SnO2) detailed information about the charge transfer mechanism in redox reactions with quinones can be obtained. Quinones, those as benzoquinone and duroquinone, are reduced by two subsequent electron transfer reactions. In some cases the first reduction step occurs via the conduction band followed by a second step via the valence band, in others only one energy band of the semiconductor electrode is involved. This behavior of quinones depends on the band gap of the semiconductor, on the pH‐value of the electrolyte and on the potential drop across the Helmholtz layer. The whole charge transfer mechanism is essentially determined by two normal potentials, one for each step, and by a certain electron energy distribution in the redox system, characterized by the rearrangement energy. Values of the normal potentials and of the rearrangement energy were determined quantitatively. The pH‐dependence could be related to differently protonated semiquinones. The charge transfer processes are discussed in detail.