Modulation of the Rate of Electron Injection in Dye-Sensitized Nanocrystalline TiO2 Films by Externally Applied Bias

Abstract

We present a study of the kinetics of electron injection in ruthenium(II) cis-(2,2‘-bipyridyl-4,4‘-dicarboxylate)2(NCS)2-sensitized nanocrystalline TiO2 films as a function of electrical potential applied to the TiO2 film and as a function of the composition of the electrolyte in which the film is immersed. At moderate applied potentials (−0.2 V vs Ag/AgCl), and in the presence of potential determining ions (0.1 M Li+) in the electrolyte, the electron injection kinetics were found to be multiphasic, with a half time for electron injection of 500 fs. These injection kinetics were retarded by either the omission of potential determining ions or the application of more negative potentials. Omission of Li+ ions from the electrolyte resulted in a 7-fold retardation of the injections kinetics. The application of −0.7 V to the TiO2 electrode resulted in a 25-fold retardation of the injection kinetics. These observations are discussed in terms of nonadiabatic interfacial electron transfer theory. The retardation of the injection kinetics in the absence of potential determining ions is attributed to the influence of these ions upon the electronic density of states of the TiO2 electrode. The retardation of the injection kinetics at negative applied potentials is attributed to the increased occupancy of this density of states. Fits to the potential dependence of the injection kinetics following nonadiabatic theory yield a reorganizational energy for the electron injection process of 0.25 ± 0.05 eV.