Direct measurements of flat-band potential shifts under illumination of the semiconductor-electrolyte interface by electrolyte electroreflectance

Abstract

Direct measurements of flat-band potential shifts produced by illumination of the semiconductor-electrolyte interface can be performed by means of the electrolyte electroreflectance technique. The method has been applied for the first time to monitor the flat-band potential shifts occurring during photooxidation of water at an n-TiO 2 single crystal. The influence of electrolyte pH, applied bias, light intensity and time of illumination has been studied. Two main mechanisms seem to be responsible for the band-edge shifts measured: on the one hand the accumulation of photogenerated holes at OH 0 surface radicals, on the other hand the changes in the local pH at the TiO 2-electrolyte interface. Experimental evidence is given that in a basic electrolyte holes are mainly trapped by chemisorbed OH − groups of basic character, while in acid medium photogenerated holes mainly react with adsorbed water molecules. We estimate the OH 0 surface radical density under steady state working illumination conditions to be of the order of 10 13 cm −2.