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

Nanostructured Ta 3 N 5 photoanodes (band gap of ∼2.0 eV) were synthesized via a two-step process: first, nanocolumnar Ta 2 O 5 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 Ta 2 O 5 into orthorhombic Ta 3 N 5 . Under Xe lamp irradiation (∼73 mW/cm 2 ), a 100 nm nanoporous Ta 3 N 5 electrode achieved an anodic photocurrent of ∼1.4 mA/cm 2 at +0.5 V versus Ag/AgCl in 1 M KOH solution. By comparison, a dense film achieved ∼0.4 mA/cm 2 clearly illustrating the importance of nanostructuring for improving the performance of Ta 3 N 5 photoanodes. However, Ta 3 N 5 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 Ta 3 N 5 films.