Analysis of Ga coordination environment in novel spinel zinc gallium oxy-nitride photocatalysts

Abstract

Novel zinc gallium oxy-nitrides, with the spinel structure and visible light band gaps, have been synthesized by nitridation of zinc gallate produced by sol–gel synthesis. These spinel oxy-nitrides have band gaps of 2.5 to 2.7 eV, surface areas of 16 to 36 m² g⁻¹, and nitrogen content less than 1.5%. They degrade methylene blue dye in visible light. The reduction in band gap is associated with the incorporation of nitrogen in the zinc gallate structure and corresponding changes in the anion position parameter as well as the presence of a small fraction of gallium tetrahedral centers and anion vacancies. While spinel oxy-nitrides are produced under nitridation at 550 °C, at higher temperatures they are consumed to form wurzitic oxy-nitrides. The wurtzite materials also have band gaps less than 3 eV but their surface areas are 2 to 5 m² g⁻¹. The changes associated with the gallium coordination as the spinel zinc gallate precursor transforms into the spinel oxy-nitride at 550 °C, and further changes into the wurtzite oxy-nitride at 850 °C are studied through X-ray diffraction, ultraviolet-visible diffuse reflectance spectroscopy, neutron powder diffraction, X-ray absorption spectroscopy and other techniques. The protocol developed in this report opens an avenue for the synthesis of semiconductors having the spinel crystal structure and band gaps engineered to the visible region with potential applications for opto-electronic devices and photocatalytic processes.