Synthesis, structure and optical properties of CoAl2O4 spinel nanocrystals

CoAl₂O₄ nanocrystals were synthesized by sol-gel method using citric acid as a chelating agent at low temperature. The as-synthesized samples were characterized by thermal analysis, X-ray powder diffraction, infrared spectroscopy and transmission electron microscopy. The results show that CoAl₂O₄ spinel is the only crystalline phase with a size of 10-30 nm in the temperature range 500-1000 °C. The temperature dependence of the distribution of Al³⁺ and Co²⁺ ions in the octahedral and tetrahedral sites in nanocrystals was investigated by X-ray photoelectron spectroscopy (XPS). It is observed that the inversion parameter decreases with increasing annealing temperature. Analysis of the absorption properties indicates that Co²⁺ ions are located in the tetrahedral sites as well as in the octahedral sites in the CoAl₂O₄ nanocrystals. The origin of the green color (300-500 nm absorption band) should be due to the octahedrally coordinated Co²⁺ ions.