Modification of (Zn1+xGe)(N2Ox) Solid Solution as a Visible Light Driven Photocatalyst for Overall Water Splitting

Zinc germanium oxynitride, a solid solution between ZnO and ZnGeN2, is prepared by reaction of GeO2 and ZnO under a NH3 flow (20 mL min-1) at 1123 K. Samples nitrided for 5−15 h under these conditions exhibit a single phase of wurtzitic (Zn1+xGe)(N2Ox), and are responsive to visible light with a band gap of ca. 2.7−2.8 eV. The band gap decreases slightly with increasing nitridation time, that is, with increasing zinc content, attributable to p−d repulsion between Zn3d and N2p + O2p electrons in the upper valance band, which raises the top of the valance band. Nitridation for 15 h afforded (Zn1+xGe)(N2Ox) with the highest photocatalytic activity for overall water splitting. The optimized catalyst has high crystallinity due to complete reaction of the starting materials, without volatilization of zinc from the product under prolonged reduction. A range of cocatalysts is also examined, and Rh2-xCrxO3 is identified as the most effective cocatalyst for (Zn1+xGe)(N2Ox), which greatly causes an increase in the activity for hydrogen evolution. Modification of the optimized (Zn1.44Ge)(N2.08O0.38) sample by loading with Rh2-xCrxO3 (3.0 wt % Rh, 0.2 wt % Cr) results in an effective photocatalyst for overall water decomposition with a quantum efficiency of ca. 0.20% at 420 nm.