Electron dynamics in nanocrystalline ZnO and TiO2 films probed by potential step chronoamperometry and transient absorption spectroscopy

Potential step chronoamperometry is employed to compare the capacitances of nanocrystalline ZnO and TiO2 electrodes, These capacitance data are complemented by transient optical absorption studies of charge recombination following adsorption of molecular sensitizer dyes to these metal oxide electrodes. Both measurements are conducted as a function of electrochemical bias applied to the metal oxide film in a three-electrode photoelectrochemical cell. For both metal oxides, a power law dependence was observed between the half times for charge recombination (t(50%)) and the metal oxide electron density n determined from integration of the capacitance data, t(50%) proportional to n(-1/alpha), where alpha = 0.27 and 0.30 +/- 0.05 for ZnO and TiO2, respectively. A numerical model for the recombination dynamics based upon a random walk of electrons between localized sub-bandgap states is found to be in good agreement with experimental observations for both metal oxides. At negative applied potentials, the film capacitance, and therefore electron density, is observed to increase more rapidly with increasingly negative applied potential for the ZnO film compared to the TiO2 film. This observation is quantitatively correlated with a more rapid acceleration of the recombination dynamics observed for dye sensitized ZnO films under negative biases. It is suggested that the faster recombination dynamics observed under negative bias may be the origin of the lower open circuit voltages reported previously for dye sensitized photoelectrochemical cells employing ZnO electrodes relative to comparable devices employing TiO2.