Tuning The Photoactivity of Zirconia Nanotubes-Based Photoanodes via Ultrathin Layers of ZrN: An Effective Approach toward Visible-Light Water Splitting

We present, herein, visible-light water splitting using earth-abundant zirconium-based nanostructured photoanodes. ZrO2/ZrON core/shell arrays were fabricated via the atomic layer deposition (ALD) of various ZrN layers on anodically synthesized hexagonal ZrO2 nanotubes. Compositional analysis of the composite photoanodes showed the transformation of the nitride layers to oxynitride phases, with the sample made of 95 ALD cycles having a structure nearest to stoichiometry. Optical analysis showed visible light absorption within the oxynitride layers with an estimated band gap of 2.6 eV, as compared to 3.8 eV for the bare oxide nanotubes. This decrease in band gap was attributed to the cathodic shift of the valence band maximum (VBM), as confirmed by X-ray photoelectron spectroscopy valence band and photoluminescence spectra. The core/shell photoanodes made of 10–95 cycles of ZrN showed photocurrent enhancements over the bare nanotubes, with samples having 95 ALD cycles exhibiting a photocurrent density of 1.2 mA/cm2 at an applied potential of 1 V versus Ag/AgCl reference electrode under AM 1.5 illumination. Further increase in deposition cycles resulted in photocurrent deterioration, which was attributed to the increased surface states. The electrochemical impedance spectra (EIS) revealed electron lifetimes in the core/shell electrodes that are 2 orders of magnitude longer than those in the bare oxide nanotube samples. Finally, Mott–Schottky analysis confirmed the cathodic shift of the valence band maximum, as evidenced by a very small anodic shift in the conduction band minimum. The results attained in this study compose a step toward earth-abundant, visible-light absorbing photoanodes for solar water splitting.