Preparation of (Ga1-xZnx)(N1-xOx) Photocatalysts from the Reaction of NH3 with Ga2O3/ZnO and ZnGa2O4: In Situ Time-Resolved XRD and XAFS Studies

Gallium zinc oxynitrides (Ga1-xZnx)(N1-xOx) 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 (Ga1-xZnx)(N1-xOx) 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 (Ga1-xZnx)(N1-xOx) and imposes a limit on the zinc content in the gallium zinc oxynitrides. Furthermore, after its formation, a wurtzite (Ga2/3Zn1/3)(N2/3O1/3) phase evolves to (Ga0.9Zn0.1)(N0.9O0.1) with increasing nitridation reaction time as a result of the removal of Zn and O atoms from the system. Once (Ga2/3Zn1/3)(N2/3O1/3) 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 (Ga1-xZnx)(N1-xOx) 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 (Ga1-xZnx)(N1-xOx) display a position red-shifted toward lower energies by similar to 0.5 eV with respect to that of ZnO, indicating a lower oxidation state of Zn in (Ga1-xZnx)(N1-xOx). N K-edge NEXAFS data show that the bonding geometry and electronic properties of the nitrogen atoms in (Ga1-xZnx)(N1-xOx) 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 (Ga1-xZnx)(N1-xOx) may be related to the existence of holes and affect visible light absorption and surface chemistry.