Interfacial Electron-Transfer Reactions in Colloidal Semiconductor Dispersions. Kinetic Analysis

A kinetic model is presented describing interfacial electron-transfer reactions in colloidal semiconductor dispersions induced by an ultrashort light pulse. The three elementary steps considered are (i) diffusion of charge carriers from the particle interior to the interphase, (ii) encounter-complex formation, and (iii) interfacial electron transfer. The first process is shown to occur so rapidly in colloidal particles that it does not contribute to the overall kinetics of the charge-transfer event. Steps ii and iii can be treated by solving the reaction-diffusion equation, thus obtaining an expression relating the observed bimolecular rate constant to the rate constant for heterogeneous electron transfer at the particle surface. The model is used to analyze the previously observed reduction of methylviologen (MV²⁺) by conduction-band electrons produced via laser excitation of TiO₂ colloids. The heterogeneous rate constant for electron transfer k_et follows a Tafel relation at pH ≥ 5. Its value is k_et⁰ = 4 × 10^-3 cm/s at pH 5.4 where the conduction-band potential of the colloidal particle is equal to the standard potential of the MV^2+/+ couple. The transfer coefficient obtained is 0.52.