Charge transfer and recombination at the single crystalline ZnO photoanode were studied by a combination of dc and ac techniques. The potential dependence of the spectrally resolved photoluminescence gives information on the competition between bulk recombination and electron−hole pair separation. The kinetics of surface recombination were studied by electrical and optoelectrical admittance spectroscopy. These results provide insight into the mechanisms of recombination and charge transfer at ZnO photoanodes.

The mechanism of photocurrent-doubling reactions at ZnO single-crystal electrodes in aqueous formate, tartrate and methanol solutions has been reinvestigated. On the basis of steady-state photocurrent measurements in a wide range of light intensity it is concluded that, in the case of formate and tartrate, the electron-injecting species arises from a reaction between the currentdoubling agent and oxygen, formed in the photoanodic decomposition of ZnO. This is not the case for methanol. These conclusions are supported by the results of electrical impedance and intensity-modulated photocurrent spectroscopy measurements.

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The mechanism of the two‐step reduction of the methylviologen cation radical (MV2+) was studied at CdS, GaAs and InP single crystal electrodes. In aqueous solution the second step (MV+• → MV0) gives rise to layer formation which is not observed in mixed ethanol/water solutions. This layer has interesting electrical properties. The reoxidation of MV0 occurs by electron injection into the conduction band of all three semiconductors. The reoxidation of MV+• at InP is also a conduction band reaction and electroluminescence is observed with the p‐type semiconductor. Electron injection from MV+• into the conduction band of GaAs is not observed while the kinetics of the reaction at CdS depend critically on the state of the electrode surface and on the nature of the electrolyte solution.

Spectrally resolved electroluminescence (EL) has been studied during anodic dissolution of p-lnP, p-GaAs, and p-GaP electrodes in indifferent aqueous electrolyte solutions. The most efficient EL was found for p-lnP whereas p-GaP showed no emission. The EL is attributed to radiative band-band recombination of holes with electrons excited from dissolution intermediates.