Determination of concentration of surface states at the illuminated semiconductor—electrolyte interface

Abstract

Surface states at the semiconductor—electrolyte interface under illumination have been determined. The faradaic reaction involved at the (CdTe and GaP) interface is the photo-electrochemical reduction of carbon dioxide to carbon monoxide in dimethylformamide—water mixtures. A new equivalent circuit has been proposed to account for the impedance data. Surface states act as faradaic mediators for the reduction of carbon dioxide. Surface state density at a given bias potential has been calculated to be of the order of 10 14 cm −2. Adsorbed ions induce surface states. Surface state density induced by tetraalkylammonium ions decrease with increase of alkyl chain length. The number of surface states increases with water concentration. The resistance for the surface states in the Tafel region is larger than the resistance of those in the space—charge region indicating that the rate determining step lies on the solution side of the interface. However, in the limiting current density region, the space charge resistance is larger than the surface state resistance.