Preparation of (Ga _1-xZn _x)(N _1-xO _x) photocatalysts from the Reaction of NH ₃ with Ga ₂O ₃/ZnO and ZnGa ₂O ₄: In situ time-resolved XRD and XAFS studies

Gallium zinc oxynitrides (Ga _1-xZnx)(N _1-xO _x) are important due to their visible-light photocatalytic activity. Using in situ time-resolved X-ray diffraction (XRD), we have monitored the formation of wurtzite (Ga _1-xZn _x)(N _1-xO _x) compounds during the solid-state reaction of NH ₃ with Ga ₂O ₃/ZnO mixtures or a ZnGa ₂O ₄ spinel. The ZnGa ₂O ₄ spinel was found to be a key intermediate in the formation of (Ga _1-xZn _x)(N _1-xO _x) and imposes a limit on the zinc content in the gallium zinc oxynitrides. Furthermore, after its formation, a wurtzite (Ga _2/3Zn _1/3)(N _2/3O _1/3) phase evolves to (Ga _0.9Zn _0.1)(N _0.9O _0.1) with increasing nitridation reaction time as a result of the removal of Zn and O atoms from the system. Once (Ga _2/3Zn _1/3)(N _2/3O _1/3) is formed, one must minimize exposure of the compound to NH ₃. Zinc and gallium K-edge X-ray absorption fine structure (XAFS) data revealed that the local structures around gallium and zinc atoms in the (Ga _1-xZn _x)(N _1-xO _x) systems are similar to those of GaN and ZnO, respectively, with relatively minor distortions in the Ga-N and Zn-O bond lengths. The Zn-O/N bonds prefer to align along the c-axis of the lattice, in agreement with the findings of DFT calculations reported in the literature. The corresponding Zn K-edge XANES spectra of (Ga _1-xZn _x)(N _1-xO _x) display a position red-shifted toward lower energies by ̃ 0.5 eV with respect to that of ZnO, indicating a lower oxidation state of Zn in (Ga _1-xZn _x)(N _1-xO _x). N K-edge NEXAFS data show that the bonding geometry and electronic properties of the nitrogen atoms in (Ga _1-xZn _x)(N _1-xO _x) are similar to those in GaN. However, the O K-edge spectra exhibit a pre-edge feature not seen for ZnO or Ga ₂O ₃. This unique property of the oxygen atoms in (Ga _1-xZn _x)(N _1-xO _x) may be related to the existence of holes and affect visible light absorption and surface chemistry.