Nanostructured Ta3N5 Films as Visible-Light Active Photoanodes for Water Oxidation

Nanostructured Ta3N5 photoanodes (band gap of ∼2.0 eV) were synthesized via a two-step process: first, nanocolumnar Ta2O5 films were deposited by evaporation of tantalum metal in a vacuum chamber in a low pressure oxygen ambient followed by heating in an ammonia gas flow to convert Ta2O5 into orthorhombic Ta3N5. Under Xe lamp irradiation (∼73 mW/cm2), a 100 nm nanoporous Ta3N5 electrode achieved an anodic photocurrent of ∼1.4 mA/cm2 at +0.5 V versus Ag/AgCl in 1 M KOH solution. By comparison, a dense film achieved ∼0.4 mA/cm2 clearly illustrating the importance of nanostructuring for improving the performance of Ta3N5 photoanodes. However, Ta3N5 films suffered from inherent self-oxidation under light illumination, and application of a cobalt cocatalyst layer was found to improve the stability as well as photocatalytic activity of the Ta3N5 films.