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.

Research highlights▶ The temperature dependence of the distribution of Al3+ and Co2+ ions in the octahedral and tetrahedral sites in nanocrystals has been investigated by X-ray photoelectron spectroscopy (XPS). It has been observed that the inversion parameter decreases with increasing annealing temperature. The optical properties of the nanocrystals are discussed on the basis of the structural results. The 300–500 nm absorption band, responsible for the green color, is related to the octahedrally coordinated Co2+ ions of the (Co1−xAlx)[CoxAl2−x]O4 crystalline phase.