(222f) Solution Combustion Synthesis and Photoelectrochemistry of GaxZn1-XOyN1-Y 

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.

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. Gallium zinc oxynitrides can be synthesized in 30 minutes at 500 ^oC using this method. X-ray photoelectron spectroscopic 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. UV-visible spectroscopy also demonstrates 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. Of note is that the amount of gallium in the final product is lower than expected, based on the initial precursor mixture. This suggests that at least some of the gallium vaporizes during the reaction.

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. Ongoing work is focused on improving product yield from the reaction, elucidating the effect of the reaction atmosphere on the final product, and maximizing photocurrent yield.