Penternary Wurtzitic Nitrides Li1-xZnxGe2-xGaxN3: Powder Synthesis, Crystal Structure, and Potentiality As a Solar-Active Photocatalyst.

We developed a new penternary wurtzitic nitride system Li1-xZnxGe2-xGaxN3 (0 <= x <= 1) by hybridizing LiGe2N3 and ZnGeGaN3. Fairly stoichiometric fine powder samples were synthesized by the reduction-nitridation process at 900 degrees C. While the end member LiGe2N3 possessed a relatively large band gap of 4.16 eV, the band gap of the developed penternary system varied in a broad range of 3.81 to 3.10 eV, showing promising responsivity to the solar spectrum. The crystal structure of LiGe2N3 was precisely determined by time-of-flight neutron powder diffraction for the first time, revealing the complete ordering of Li and Ge in the Cmc2(1) structure. The structural evolution from completely ordered LiGe2N3 to fully disordered ZnGeGaN3 was quantitatively analyzed by Rietveld refinement based on a partially disordered Cmc2(1) model, and the obtained results were also supported by Ga-71 solid-state NMR spectroscopy. The synthesized Li1-xZnxGe2-xGaxN3 powder samples exhibited photocatalytic activities for the water reduction and oxidation reactions under solar light irradiation, with the H-2 evolution rate of 0.3-59.0 mu mol/h and the O-2 evolution rate of 3.1-296.2 mu mol/h, depending on the composition. Stable solar hydrogen generation of up to 48 h was demonstrated by the x = 0.80 sample, with the total amount of H-2 evolved over 1.6 mmol and an external quantum efficiency of 2.1%.