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 ZnGeN2 and GaN. Near stoichiometric and monophasic powder samples in the composition Zn1–xGe1–xGa2xN2 (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 ZnGeN2 (Pna21) tends to disappear by introducing Ga into the lattice, and the structure transforms to a simple wurtzite phase (P63mc) with the composition of x ≥ 0.33. The observed structural evolution was further confirmed by the results of 71Ga solid-state nuclear magnetic resonance (NMR) spectroscopy, showing an unsplit single peak observed for x ≥ 0.33. The dissolution of GaN into ZnGeN2 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 Zn1–xGe1–xGa2xN2 solid solution possessed the H2 evolution rate of 2.8–3.6 μmol/h and the relatively high O2 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.