Effects of Annealing Temperature and SiO2 Matrix on the Structure and Optical Properties of Co-Doped ZnAl2O4/SiO2 Nanoglass-Ceramic Composites

Co-doped glass ceramics containing spinel nanocrystals attach much attention as optical materials. The composition and preparation conditions of the material will influence its structure and optical properties. In this study, the samples (100-x)Co0.1Zn0.9Al2O4-xSiO(2) (x = 0-95) were prepared by the sol-gel method and characterized by X-ray powder diffraction and transmission electron microscopy. X-ray photoelectron spectroscopy and Al-27 solid-state NMR spectroscopy were used to study the chemical environments of cations and oxygen in the nanocomposites as a function of annealing temperature and SiO2 concentration. The results show that all the samples are composed of a silica matrix and enclosed ZnAl2O4 nanocrystalline particles. Besides the tetrahedral, octahedral, and second octahedral coordination sites present in Co0.1Zn0.9Al2O4 nanocrystals, the five-coordinate sites of Al3+ ions are observed in the nanocomposites. The inversion parameter (two times the fraction of Al3+ ions in the tetrahedral sites) increases with the decrease of SiO2 concentration and annealing temperature. The absorption spectra indicate that Co2+ ions are located in the tetrahedral sites as well as in the octahedral sites in the nanocomposites. The intensity of the absorption peak corresponding to octahedral Co2+ ions decreases with the increase of annealing temperature and SiO2 concentration.