Changes in Surface Crystallinity and Morphology of CdS and CdSe Photoelectrodes upon Use in Polysulfide Electrolyte

Surface changes that occur when CdS and CdSe photoelectrodes are used in photoelectrochemical ceils containing polysulfide redox couples are investigated using low angle x-ray diffraction, reflection electron diffraction, and scanning electron microscopy. Both single crystal CdS and CdSe, and thin layer polycrystalline CdSe samples are used and in all cases restructuring of the surface, accompanied by some decrease in crystallinity of the surface layer, exposed to the electrolyte, is observed. For CdSe samples, crystallographic evidence for a predominant CdS top layer was seen after deactivation. Impurities in thin film CdSe samples are not affected by deactivation and they themselves do not seem to influence electrode activity or stability. These results support the occurrence of a dynamic restructuring process of the electrode surface in this system, which can cause electrode deactivation, by the formation of a layer that impedes charge transfer, if high current densities are used. Evidence is presented that surface recrystallization can occur in the dark, when such a process occurs probably very slowly.