Synthesis, structure and optical properties of Co-doped MgGa2O4/SiO2 nano-glass-ceramic composites

Co-doped MgGa2O4/SiO2 nano-glass-ceramic composites with the composition of (1 − y) CoxMg1−xGa2O4-ySiO2 (0 ≤ x, y ≤ 1) were synthesized by the sol-gel method and characterized by X-ray powder diffraction and transmission electron microscopy. X-ray photoelectron spectroscopy was used to study the chemical environments of cations and oxygen in the nanocomposites as a function of annealing temperature, cobalt content and SiO2 content. The results show that Ga3+ and Mg2+ ions occupy both the tetrahedral and octahedral sites of spinel structure in the samples. The inversion parameter (two times the fraction of Ga3+ ions in the tetrahedral sites) of the nanocomposites is relatively larger in comparison with that of ZnGa2O4, and the value decreases with the increase of annealing temperature and silica concentration, while increases with cobalt-enrichment. All the Co-doped samples exhibit the intense absorption peak at ~600 nm, which is characteristic of tetrahedral Co2+. But only in the composites of Co0.3Mg0.7Ga2O4 and 0.6Co0.6Mg0.4Ga2O4-0.4SiO2, the absorption band at 300-500 nm characteristic of octahedral Co2+ ions is observed. The intensity of the absorption peak corresponding to octahedral Co2+ ions increases with decreasing annealing temperature for the Co0.3Mg0.7Ga2O4 nanocrystals.