Photoelectrochemistry of Nanostructured WO3 Thin Film Electrodes for Water Oxidation: Mechanism of Electron Transport

Nanostructured WO3 thin films were prepd., and photooxidn. of water at such films was studied in a pH 4.68 soln. The cathodic current 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 photoelectrochem. cell. Under illumination, linear log|i| vs. 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 (Φ) 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 ΦEE and ΦSE reached a plateau region at shorter wavelength. In the plateau, ΦSE was ∼1 and independent of the film thickness, whereas ΦEE was ∼20% lower and decreasing with increasing film thickness. Adopting a simple diffusion model for the electron transport, the diffusion length of electrons (L) is 6.7 μm for a 5.0-μm thick film. Higher activation energies, EA, were obtained at lower potentials (e.g., 0.60 eV at 200 mV and 0.32 eV at 300 mV). The EA was