Effects of Co content and annealing temperature on the structure and optical properties of CoxMg1−xAl2O4 nanoparticles

Co_xMg_1−xAl₂O₄ (x = 0–0.8) nanoparticles were synthesized by sol–gel method, and characterized by X-ray powder diffraction and transmission electron microscopy. X-ray photoelectron spectroscopy and ²⁷Al solid-state NMR spectroscopy were performed to study the chemical environments of cations in the nanoparticles as a function of cobalt content and annealing temperature. The results show that the crystallite size of the particles is about 20–40 nm. Besides the tetrahedral and octahedral coordinations, the second octahedrally coordinated Al³⁺ ions are observed in the samples. The inversion parameter (two times the fraction of Al³⁺ ions in tetrahedral sites) decreases with the increase of annealing temperature and cobalt content. The fraction of octahedral Mg²⁺ decreases with the increase of Co concentration. The absorption spectra indicate that Co²⁺ ions are located in the tetrahedral sites as well as in the octahedral sites in the nanoparticles. The intensity of the absorption peak corresponding to octahedral Co²⁺ ions (300–500 nm) decreases with increasing annealing temperature.