Synthesis, structure and optical properties of CoAl2O4 spinel nanocrystals

Abstract CoAl 2 O 4 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 CoAl 2 O 4 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 Al 3+ and Co 2+ 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 Co 2+ ions are located in the tetrahedral sites as well as in the octahedral sites in the CoAl 2 O 4 nanocrystals. The origin of the green color (300–500 nm absorption band) should be due to the octahedrally coordinated Co 2+ ions.