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The voltage dependence of the green luminescence of ZnO crystals used as electrodes in an electrochemical cell was investigated and compared with the voltage dependence of the photocurrent. When the exciting light is of shorter wavelength than the band edge of ZnO, this luminescence can be varied from its maximal value to complete extinction by applying small positive voltages. The luminescence resulting from excitation within the long wavelength tail of absorption, however, cannot be influenced. The potential dependence of the luminescence has been investigated with differently doped crystals under various conditions. The addition of formic acid to the electrolyte causes an increase of luminescence; this is found to be a consequence of photoelectrochemically induced radical reaction by which electrons are injected into the conduction band of the electrode. The potential dependence of the green luminescence is explained by a mechanism in which the trapping of the holes in recombination centers competes with their extraction, which leads to the generation of the photocurrent. The average concentration of photogenerated holes in the crystal is therefore rate limiting for the intensity of luminescence, provided that an excess of electrons is available for recombination. At sufficiently large positive potentials, however, when electrons are being depleted in the space charge layer, their concentration also becomes limiting, and electron injection can stimulate luminescence. The experimentally observed potential dependence of luminescence and photocurrent, as well as their interrelation, is shown to be consistent with results which were derived from a theoretical treatment of these effects. It is suggested that this green luminescence may be used as a probe for the study of the space charge layer and of electrochemical reactions.

In order to explore the feasibility of microwave absorption techniques as a tool for the study of interfaces and electrode reactions we have measured the electrochemical and photochemical properties of ZnO in contact with an aqueous electrolyte by conventional electrochemical procedures and simultaneously by electrodeless detection of microwave absorption at 9.5 GHz in a resonant cavity. Pronounced potential dependent microwave absorption signals were found which proved to be influenced by illumination and electrochemical surface reactions. The comparison of microwave absorption with the electrochemically measured electrode behavior indicates, that microwave absorption data can provide interesting complementary information on the nature and reactivity of charge carriers and dipoles in the electrode/electrolyte interface. The proposed method is so sensitive that signals arising from fractions of monolayers will be detectable. A comprehensive interpretation of the results is not yet attempted. Further experimental studies on simple electrode systems are needed to clarify the mechanism. Because of its nondestructive nature, the microwave method could develop into a valuable new technique in surface chemistry.