Electrochemical method for studying the kinetics of electron recombination and transfer reactions in heterogeneous photocatalysis:: The effect of fluorination on TiO2 nanoporous layers

An electrochemical method for evaluating the rate constants of recombination and transfer to solution for electrons generated upon illumination of TiO2 photocatalysts is presented. It is based on the combination of voltammetric measurements in the dark and open circuit photopotential relaxation measurements done with nanoporous thin film electrodes. Not only the average first-order rate constants for electron consumption are obtained in such a way but also the values of such constants as a function of the electrode potential (microcanonical rate constants). This method is applied to different titanium dioxide samples as to elucidate the effect of fluorination on the rate of both electron recombination (with surface trapped holes) and electron transfer to dissolved oxygen. In both cases, but especially for recombination, there is, upon fluorination, a significant retardation of the electron consumption process in agreement with several photocatalytic studies found in the literature. Finally, we take advantage of the obtainment of microcanonical constants to fit the theoretical Gerischer expression for the rate constant in the case of oxygen reduction on TiO2 fluorinated surfaces.