Photolelectrochemistry of nanostructured WO3 thin film electrodes for water oxidation:: Mechanism of electron transport

Nanostructured WO3 thin films were prepared, and photooxidation of water at such films was studied in a pH 4.68 solution. The cathodic current at potentials below -100 mV versus a saturated Ag/AgCl electrode was related to the reversible intercalation of H+ and/or Na+. The photocurrent onset was at similar to 100 mV, and the saturation photocurrent was at potentials >800 mV. In the range 300-1000 mV, photocurrent increased linearly with the increasing light intensity, indicating that charge carrier generation dominates the photoelectrochemical cell. Under illumination, linear log\i\ versus potential (Tafel) behavior was registered in the range 300-650 mV. Tafel slopes and exchange current densities are reported. The incident photon-to-current efficiency (IPCE) and the quantum yield (Phi) were high, regardless of the incidence of the light (front side, EE, or backside, SE, illumination). Both IPCEEE and IPCESE increased with film thickness. The low wavelength edge of the action spectra was red-shifted and moved toward the absorption band edge. Both Phi(EE) and Phi(SE) reached a plateau region at shorter wavelength. In the plateau, (Phi(SE) was close to I and independent of the film thickness, whereas Phi(EE) was similar to 20% lower and decreasing with increasing film thickness. Adopting a simple diffusion model for the electron transport, the diffusion length of electrons (L) was estimated to be 6.7 mu m for a 5.0-mu m thick film. Higher activation energies, E-A, were obtained at lower potentials (e.g., 0.60 eV at 200 mV and 0.32 eV at 300 mV). The E-A was <0.21 eV in the range 400-700 mV, and decreased further to 0.06 eV at 1000 mV. The variation of E-A with potential was explained by the existence of a distribution of electron-trapping states in an energy range around 0.6 V below the conduction band.