ZrTaN₃—A New Visible Light Absorbing Ternary Nitride Semiconductor Photoanode

Solar water splitting provides a promising approach to harvest and store energy from sunlight in carbon-neutral, energy-dense fuel, thereby providing a route to mitigating global climate change. An ongoing challenge in the field of photoelectrochemical (PEC) energy conversion remains the lack of semiconductor compounds that possess moderate bandgaps with appropriate band edge positions, high chemical stability, and favorable charge transport characteristics. While material discovery efforts have largely targeted oxide compounds, transition metal oxynitride and nitride semiconductors are comparatively underexplored. In this combined experimental and computational study, we report a new visible light absorbing ternary nitride semiconductor, bixbyite-type ZrTaN₃, as a promising photoanode material. Calculations based on density functional theory using the HSE functional predict ZrTaN₃ to be a semiconductor with a direct bandgap of approximately 1.9 eV and large electronic band dispersion, which could imply high electron and hole mobilities. To compare this prediction with experiment, we are using reactive co-sputtering to realize the first thin films of this material on a wide range of substrates. Our measurements confirm that these polycrystalline layers exhibit an optical bandgap of ~2.4 eV and non-degenerate n-type conductivity. The semiconducting properties can be rationalized by comparison to bixbyite Ta₂N₃, which is a degenerate semiconductor in which each Ta cation donates an average of 0.5 free e^-[1]. In contrast, replacement of half the Ta cations with Zr provides an appropriate charge balance, incorporating Ta⁵⁺ and Zr⁴⁺, which reduces the free electron concentration below the metal-semiconductor transition. Photoelectrochemical characterization reveals an appreciable anodic photocurrent. Benefiting from the tunability of both the cation (Ta,Zr) and anion (N,O) ratios of reactive sputtered and post-annealed films, stable solid solutions of Zr_xTa_2-xN₃(O) offer a large parameter space to control and optimize material properties, not just for photoelectrochemical energy conversion but also for broader electronic applications.<br/>[1] C.-M. Jiang, L. I. Wagner, M. K. Horton, J. Eichhorn, T. Rieth, V. F. Kunzelmann, Y. Li, K. A.<br/>Persson, I. D. Sharp. <b>Mater. </b><b>Horiz</b>., 2021, <b>8</b>, 1744-1755