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 pote ntials below -100 mV versus a saturated Ag/AgCl electrode was related to th e reversible intercalation of H+ and/or Na+. The photocurrent onset was at similar to 100 mV, and the saturation photocurrent was at potentials >800 m V. In the range 300-1000 mV, photocurrent increased linearly with the incre asing light intensity, indicating that charge carrier generation dominates the photoelectrochemical cell. Under illumination, linear log\i\ versus pot ential (Tafel) behavior was registered in the range 300-650 mV. Tafel slope s and exchange current densities are reported. The incident photon-to-curre nt efficiency (IPCE) and the quantum yield (Phi) were high, regardless of t he incidence of the light (front side, EE, or backside, SE, illumination). Both IPCEEE and IPCESE increased with film thickness. The low wavelength ed ge of the action spectra was red-shifted and moved toward the absorption ba nd edge. Both Phi(EE) and Phi(SE) reached a plateau region at shorter wavel ength. 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 inc reasing 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 obtaine d 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 existe nce of a distribution of electron-trapping states in an energy range around 0.6 V below the conduction band.