Quaternary Wurtzitic Nitrides in the System ZnGeN₂–GaN: Powder Synthesis, Characterization, and Potentiality as a Photocatalyst

We developed a new quaternary wurtzitic nitride system by formation of the solid solution between ZnGeN 2 and GaN. Near stoichiometric and monophasic powder samples in the composition Zn 1– x Ge 1– x Ga 2 x N 2 ( x ≤ 0.50) were obtained by the reduction–nitridation synthesis conducted at 900 °C. The results of crystal structure refinement clearly revealed that the cation ordering in the structure of ZnGeN 2 ( Pna 2 1 ) tends to disappear by introducing Ga into the lattice, and the structure transforms to a simple wurtzite phase ( P 6 3 mc ) with the composition of x ≥ 0.33. The observed structural evolution was further confirmed by the results of 71 Ga solid-state nuclear magnetic resonance (NMR) spectroscopy, showing an unsplit single peak observed for x ≥ 0.33. The dissolution of GaN into ZnGeN 2 also resulted in a marked narrowing of the band gap, from the ultraviolet region of 3.42 eV to the visible-light range of 3.02–3.05 eV, depending scarcely on the value of x . The results of photocatalytic test reactions for water splitting showed that the synthesized Zn 1– x Ge 1– x Ga 2 x N 2 solid solution possessed the H 2 evolution rate of 2.8–3.6 μmol/h and the relatively high O 2 evolution rate of 100.4–126.6 μmol/h, as well as the capability for overall water splitting under the visible-light irradiation of λ > 400 nm.