Late-Transition Metal Oxynitrides: Overcoming Challenges by Leveraging Their Earlier Counterparts

Coupled with low-cost renewable electricity sources, electrochemistry has the potential to decarbonize multiple industrial processes, including energy storage, production of organic chemicals, and water treatment. To advance these electrochemical technologies toward wide-spread adoption, we must broaden our understanding of their underlying mechanisms. On this note, there is an expanding body of literature identifying reaction mechanisms in which <i>anions</i> act as the active site for electrocatalysis, contradicting the traditionally-held notion that catalysis occurs at metal cation sites. Heteroanionic materials, particularly oxynitrides, are uniquely poised to investigate such mechanisms in O and N-based reactions due to their composition and tunable anion band structures.<br/>Early transition metal (TM) oxynitrides are commonly synthesized via ammonolysis, whereby an oxide precursor is heated under flowing ammonia. Following this approach, we attempted to synthesize oxynitrides from monoclinic SrIrO₃powders and pulsed laser deposition (PLD)-grown thin films of perovskite SrIrO₃, the latter being an active catalyst for electrochemical water oxidation. X-ray photoelectron spectroscopy (XPS) of the products reveal no N 1s signal, and powder x-ray diffraction (PXRD) indicates decomposition into Ir metal and SrOH. These results, as well as a lack of relevant literature, underscore the difficulty in synthesizing late-TM oxynitrides: the conditions of ammonolysis reduce the reactants to their base metals. Thus, we find that investigating the influence of N on the electronic structure of SrIrO₃ requires alternative synthesis methods.<br/>While iridate perovskites (ABX₃, B = Ir) are unlikely to form an oxynitride, literature reveals that numerous B = Mo or W perovskites do form such materials.¹ Thus, we exploited the stability of these early-TM oxynitrides to form mixed B-site perovskite oxynitrides including Ir (AB_yB’_1-yX₃, B = Ir, B’ = Mo or W). Following this approach, we have successfully synthesized perovskite SrW_xIr_1-xO_yN_3-y via a combination of hydrothermal and solid state methods. The PXRD pattern matches literature SrWN_2.05O_0.95 with slight peak shifts, which are expected due to the large radius of Ir species. This result is supported by XPS, transmission electron microscopy, and energy dispersive x-ray spectroscopy, which confirm the uniform distribution of Ir and N within the perovskite crystals. We believe that this material is the first reported iridium-containing oxynitride; ongoing x-ray absorption analysis will provide further support for this claim.<br/>In addition to altering B-site composition, we can also utilize early-TM oxynitrides to tune anion band structure through the deposition of thin film heterostructures. Specifically, epitaxy of SrIrO₃ on a lattice-matched oxynitride is expected to influence its anion band structure. Towards this end, we have synthesized multiple perovskite oxynitrides with a pseudo-cubic lattice parameter within 5% of SrIrO₃.^2,3 These materials may be used as ablation targets for PLD, alternating laser pulses between the oxynitride and SrIrO₃ to form thin film heterostructures that can be compared with their isolated components to elucidate the influence of the interface on SrIrO₃. Finally, the use of PLD for film growth affords us the added capability of N plasma-assisted deposition, a proven method for successfully forming oxynitride films.<br/>With this work, we address challenges with nitridation of late-TM oxides by identifying early-TM’s as a tool for investigating the impact of N on iridate catalyst materials. Following this notion, we demonstrate the successful formation an Ir-containing perovskite oxynitride and highlight additional synthesis methods for investigating the role of heteroanions on the electronic structure of perovskite SrIrO₃.<br/>1. Fawcett <i>et al., Mater Res Bull </i><b>32</b>, 1565-1570 (1997). 2. Saal <i>et al., JOM </i><b>65</b>, 1501-1509 (2013). 3. Kirklin <i>et al., npj Comput Mater </i><b>1</b>, 15010 (2015).