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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 ∼120 nm and very short length ∼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 ∼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.

Herein, the ability to convert waste stainless steel (SS) 316L meshes into highly efficient and durable oxygen evolution reaction (OER) catalysts is demonstrated. The process involves surface treatment of previously anodized SS meshes in different gaseous atmospheres. The activity of the resulted electrocatalysts varies as-anodized SS annealed in oxygen (ASS-O2) > anodized SS annealed in hydrogen (ASS-H2) > anodized SS annealed in air (ASS-Air). The ASS-O2 showed an impressive low overpotential of 280 mV at the benchmark current density of 10 mA/cm2, which is 120 mV less than that of the as-received SS (SS-AR), and a low Tafel slope of 63 mV dec–1 in 1 M KOH. These findings have also been asserted by the estimated electrochemical active surface area, electrochemical impedance spectroscopy analysis, Mott–Schottky analysis, and the calculated turnover frequency, affirming the superiority of the ASS-O2 electrocatalyst over the ASS-H2 and ASS-Air counterparts. The high activity of the ASS-O2 electrocatalyst can be ascribed to the surface composition that is rich in Fe3+ and Ni2+ as revealed by the X-ray photoelectron spectroscopy analysis. The simple method of anodization and thermal annealing in O2 at moderate conditions (450 °C for 1 h) lead to the formation of a SS mesh-based OER electrocatalyst with activity exceeding that of the state-of-the-art IrO2/RuO2 and other complex modified SS catalysts. These results were also confirmed via density functional theory calculations, which unveiled the OER reaction mechanism and elucidated the d-band center in different SS samples with different oxygen content. The presence of oxygen moved the d-band center closer to the Fermi level in the case of ASS-O2, explaining its superior activity.

Designing efficient and stable water splitting photocatalysts is an intriguing challenge for energy conversion systems. We report on the optimal fabrication of perfectly aligned nanotubes on trimetallic Ti–Mo–Fe alloy with different compositions prepared via the combination of metallurgical control and facile electrochemical anodization in organic media. The X-ray diffraction (XRD) patterns revealed the presence of composite oxides of anatase TiO2 and magnetite Fe3O4 with better stability and crystallinity. With the optimal alloy composition Ti–(5.0 atom %) Mo–(5.0 atom %) Fe anodized for 16 h, enhanced conductivity, improved photocatalytic performance, and remarkable stability were achieved in comparison with Ti–(3.0 atom %) Mo–(1.0 atom %) Fe samples. Such optimized nanotube films attained an enhanced photocatalytic activity of ∼0.272 mA/cm2 at 0.9 VSCE, which is approximately 4 times compared to the bare TiO2 nanotubes fabricated under the same conditions (∼0.041 mA/cm2 at 0.9 VSCE). That was mainly correlated with the emergence of Mo and Fe impurities within the lattice, providing excess charge carriers. Meanwhile, the nanotubes showed outstanding stability with a longer electron lifetime. Moreover, carrier density variations, lower charge transfer resistance, and charge carriers dynamics features were demonstrated via the Mott–Schottky and electrochemical impedance analyses.

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