Adsorption of telluride ions on cadmium telluride: consequences for photoelectrochemical cells

Abstract

The adsorption of telluride ions on cadmium telluride electrodes is studied in a basic solution (1 M NaOH) by means of capacitance measurements. Negative shifts of the flat-band potential are observed (by up to -0.7 V) when telluride ions are introduced in the solution (the upper concentration was about 10/sup -2/M) showing the adsorption of negative charges at the surface. This process is dependent upon the polarity of the surface: the adsorption is stronger on Cd-terminated (111) surfaces than on Te ones. The results are compared with a model of adsorption based on the Langmuir isotherm. Within this hypothesis, it appears that the process involves more likely the net exchange, at the surface, of an unitary charge, HTe/sup -/ alone or Te/sup 2 -/ with OH/sup -/ substitution. When the interface is illuminated, the adsorption leads to a negative shift of the photocurrent onset as compared with its position in the blank electrolyte. A transition from a one-wave to a two-wave photocurrent/voltage curve appears when the illumination level increases, associated with a large positive shift of the flat-band potential. The first wave is attributed to the oxidation of telluride ions in solution whereas the second one corresponds mainly to themore » corrosion reaction. These results are interpreted by considering that the band edge position under illumination is fixed by the kinetics of the electrode reactions. A simple phenomenological model is presented to discuss the interfacial behavior.« less