Interfacial electron-transfer equilibria and flat-band potentials of α-Fe2O3 and TiO2 colloids studied by pulse radiolysis

The kinetics and equilibria of electron transfer between methylviologen cation radicals and α-Fe2O3 or TiO2 colloidal particles were studied with the pulse-radiolysis technique. The rates of electron transfer to both colloids are lower than those predicted for a diffusion-controlled reaction. For higher pHs (TiO2, pH > 2; α-Fe2O3, pH > 9) the established equilibrium MV+ in equilibrium MV2+ + (e-)/sub coll/ is strongly influenced by the MV2+ concentration and pH. The MV+ equilibrium concentration can be exploited to derive the flat-band potential of the semiconductor colloids. The method for determining the flat-band potential of the particles is independent of whether the injected electrons are free or trapped, and whether the electrons raise the bulk Fermi level toward the conduction band or just produce a space charge. The flat-band potentials for both colloids appear to be somewhat more negative (-0.1 to -0.2 V) than the corresponding single-crystal electrodes. Also, the flat-band potentials become slightly more negative with increasing radiation dose (initial MV+ concentration). The effect of absorbed radiation dose is explained by the corresponding changes in the ratio of oxidized to reduced forms of the redox couple, which in turn changes the adsorbed ionic charge on the semiconductor surface. For colloidal particles of TiO2 stabilized by poly(vinyl alcohol) (PVA), the flat-band potentials were almost the same as those for PVA-free TiO2 sols. The decrease of particle diameter from 800 to 70 A does not affect the value of the flat-band potentials for TiO2 and α-Fe2O3 colloids. 28 references, 9 figures