| Abstract
| - Potential step chronoamperometry is employed to compare the capacitances of nanocrystalline ZnO and TiO2electrodes. These capacitance data are complemented by transient optical absorption studies of chargerecombination following adsorption of molecular sensitizer dyes to these metal oxide electrodes. Bothmeasurements 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 betweenthe half times for charge recombination (t50%) and the metal oxide electron density n determined from integrationof the capacitance data, t50% ∝ n-1/α, where α = 0.27 and 0.30 ± 0.05 for ZnO and TiO2, respectively. Anumerical model for the recombination dynamics based upon a random walk of electrons between localizedsub-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 increasemore rapidly with increasingly negative applied potential for the ZnO film compared to the TiO2 film. Thisobservation is quantitatively correlated with a more rapid acceleration of the recombination dynamics observedfor dye sensitized ZnO films under negative biases. It is suggested that the faster recombination dynamicsobserved under negative bias may be the origin of the lower open circuit voltages reported previously for dyesensitized photoelectrochemical cells employing ZnO electrodes relative to comparable devices employingTiO2.
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