Modulation of the rate of electron injection in dye-sensitized nanocrystalline TiO2 films by externally applied bias

We present a study of the kinetics of electron injection in ruthenium(II) c is-(2,2 ' -bipyridyl-4,4 ' -dicarboxylate)(2)-(-NCS)(2)-sensitized nanocrys talline TiO2 films as a function of electrical potential applied to the TiO 2 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 i n 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 retar ded by either the omission of potential determining ions or the application of more negative potentials. Omission of Li+ ions from the electrolyte res ulted in a 7-fold retardation of the injections kinetics. The application o f -0.7 V to the TiO2 electrode resulted in a 25-fold retardation of the inj ection kinetics. These observations are discussed in terms of nonadiabatic interfacial electron transfer theory. The retardation of the injection kine tics in the absence of potential determining ions is attributed to the infl uence of these ions upon the electronic density of states of the TiO2 elect rode. The retardation of the injection kinetics at negative applied potenti als is attributed to the increased occupancy of this density of states. Fit s to the potential dependence of the injection kinetics following nonadiaba tic theory yield a reorganizational energy for the electron injection proce ss of 0.25 +/- 0.05 eV.