Utilization of solar energy is a necessary path forward to deal with the looming energy crisis and worldwide effects of fossil fuel usage. A key component of this utilization is how to store excess solar energy for use on demand. One such method is splitting water into hydrogen and oxygen, both of which can be stored and either burned as fuel or used as feedstock components in industrial processes to synthesize high-value materials. Metal oxynitrides have shown great potential as photocatalysts for overall water splitting applications. Incorporation of nitrogen into metal oxides narrows the band gap to enable absorption of light in the visible spectrum. The typical synthesis of a metal oxynitride, however, requires annealing temperatures at or above 1000 ^oC and nitridation by flowing pure ammonia gas for anywhere from 1 to 48 hours. Such harsh synthesis conditions can limit the discovery of new candidate materials.

In our work, we have prepared an array of gallium zinc oxynitrides (Ga_xZn_1-xO_yN_1-y) as a model system using a combustion synthesis method and demonstrate that gallium zinc oxynitrides can be synthesized in 30 minutes at 500 ^oC using this method. X-ray diffraction (XRD), UV-visible spectroscopy, scanning electron microscopy (SEM), and X-ray photoelectron spectroscopy (XPS) are used to characterize the physical properties of the as-synthesized materials. XPS analysis reveals that as much as 50% nitrogen substitution in the metal oxide lattice can be achieved, dependent on the ratio of gallium and zinc nitrates used as precursors. XPS also reveals that the nitrogen valency is that of a nitride material, as would be expected for a substitution into the crystal lattice. Relative atomic concentrations from XPS also reveals that the complex formed is essentially GaN:ZnO. UV-visible spectroscopy reveals that the band gap of the Ga_xZn_1-xO_yN_1-ymaterials can be controlled simply by altering the precursor ratio. X-ray diffraction measurements reveal shifts in the crystalline peaks from ZnO toward GaN, confirming the incorporation of nitrogen into the lattice. The as-synthesized products are capable of generating photocurrent from visible light irradiation and are able to carry out water splitting in pure water without any co-catalysts, such as Pt, IrO₂, or Rh/Cr₂O₃.

Finally, we discuss how to take the results gained from this work and expound upon it to begin the search for new candidate oxynitride materials, including potential high-throughput methods for rapid screening of such candidates. We also discuss the apparent conditions necessary for successful formation of an oxynitride material based on initial experiments with new candidates.

Figure caption: Picture of as-synthesized powders from an initial Ga:Zn:urea molar ratio of (A) 1:1:0 and (B) 1:1:10.