Niobium-Zirconium Oxynitride Nanotube Arrays for Photoelectrochemical Water Splitting

We report on the first fabrication of vertically oriented niobium-zirconium oxynitride nanotube arrays and their use as an attractive and robust material for visible-light-driven water oxidation. These nanotube arrays with an average diameter of similar to 120 nm and very short length similar to 90 nm were synthesized via one-step anodization of Nb-Zr alloy sheet in NH4F-containing electrolytes. Ammonolysis of the nanotubes resulted in narrowing the bandgap energy from 3.23 to similar to 2.67 eV. The Nb-Zr oxynitride nanotube arrays showed approximately an enhancement of about 1900% over that reported for thin film electrodes made of niobium oxynitride and 3700% greater than that recorded for nitrogen-doped mesoporous Nb2O5. Mott-Schottky and the valence band XPS analyses revealed the favorable band positions of the fabricated oxynitride nanotubes with respect to the water redox potentials with very high charge carriers density. The photocurrent transient measurements revealed the remarkable stability of the fabricated oxynitride nanotubes.