Preparation of (Ga_1−<i>x</i>Zn_<i>x</i>)(N_1−<i>x</i>O_<i>x</i>) Photocatalysts from the Reaction of NH₃ with Ga₂O₃/ZnO and ZnGa₂O₄: In Situ Time-Resolved XRD and XAFS Studies

Gallium zinc oxynitrides (Ga_1−<i>x</i>Zn_<i>x</i>)(N_1−<i>x</i>O_<i>x</i>) 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−<i>x</i>Zn_<i>x</i>)(N_1−<i>x</i>O_<i>x</i>) compounds during the solid-state reaction of NH₃ with Ga₂O₃/ZnO mixtures or a ZnGa₂O₄ spinel. The ZnGa₂O₄ spinel was found to be a key intermediate in the formation of (Ga_1−<i>x</i>Zn_<i>x</i>)(N_1−<i>x</i>O_<i>x</i>) 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 NH₃. 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−<i>x</i>Zn_<i>x</i>)(N_1−<i>x</i>O_<i>x</i>) 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 <i>c</i>-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−<i>x</i>Zn_<i>x</i>)(N_1−<i>x</i>O_<i>x</i>) 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−<i>x</i>Zn_<i>x</i>)(N_1−<i>x</i>O_<i>x</i>). N K-edge NEXAFS data show that the bonding geometry and electronic properties of the nitrogen atoms in (Ga_1−<i>x</i>Zn_<i>x</i>)(N_1−<i>x</i>O_<i>x</i>) are similar to those in GaN. However, the O K-edge spectra exhibit a pre-edge feature not seen for ZnO or Ga₂O₃. This unique property of the oxygen atoms in (Ga_1−<i>x</i>Zn_<i>x</i>)(N_1−<i>x</i>O_<i>x</i>) may be related to the existence of holes and affect visible light absorption and surface chemistry.