ANODIC GROWTH AND INTERPHASIAL PHOTOELECTROCHEMISTRY OF CADMIUM-SULFIDE THIN-FILMS AS PROBED BY LASER RAMAN-SPECTROSCOPY

The anodic growth and interphasial photoelectrochemistry of CdS films were studied by cyclic voltammetry and laser Raman spectroscopy in aqueous sulfide electrolytes. The potential regimes and sulfide levels in the electrolyte were carefully chosen to avoid interference from oxide and hydroxide growth at the cadmium electrode surface. The use of the 488-nm Ar+ laser line was effective in generating Raman signals in the resonance scattering mode via absorption of the excitation light by the incipient CdS semiconductor layer. At 0.1 M sulfide, sulfur (predominantly S8) was detected via its Raman signature as a CdS photocorrosion product. The evolution of the Raman bands (attributable to these species and to CdS) as a function of time and potential was seen to reflect a complex interplay of several concurrent processes including photocorrosion, film regeneration, and desorption of the photogenerated sulfur from the CdS surface. On the other hand, an increase of the sulfide concentration to 0.5 M resulted in the absence of Raman signals due to sulfur at laser outputs ranging from 20 to 200 mW. The experiments described herein also serve to underline the utility of laser resonance Raman spectroscopy as an in situ tool for molecular-level tailoring of the variables in a photoelectrochemical system such that photogenerated carrier (electron or hole) transfer to an electrolyte species may be promoted at the expense of the electrode corrosion pathway.