Electron Transfer Reactions on Extremely Small Semiconductor Colloids Studied by Pulse Radiolysis

Abstract

Optical effects due to size quantization have been observed for CdS, HgSe and PbSe colloids with particle diameters 20–100 Å in water and acetonitrile. For HgSe and PbSe semiconductors with diameter size less than 50 Å, the optical absorption edge is blue shifted ∼2.8 eV. With decreasing particle size, the conduction band edges in bulk colloids are shifted to negative values and cathodic corrosion is the dominant process during illumination of the colloids in the presence of hole scavengers or during injection of electrons by redox couples created by radiation processes. Electron transfer reactions from different electron donors to TiO₂, α-Fe₂O₃, WO₃, p- Cu₂O, HgSe and PbSe colloidal particles were studied by pulse radiolysis. Equilibrium concentration of the reactants can be exploited to derive electron energy levels in the semiconductor colloids. The flat-band potential for TiO₂, α-Fe₂O₃ and WO₃, colloids becomes slightly more negative (0.1 to 0.2 V) than the corresponding single crystal electrodes, which can be explained by the corresponding change in the surface charge on the colloids.