Synthesis and characterization of Co2+: MgAl2O4 nanocrystal

Nanocrystalline Co²⁺-doped magnesium aluminate spinel (MgAl₂O₄) has been synthesized for the first time from aqueous solution of metal nitrates containing citric acid as chelating agent by a sol-gel method. The gel was heat-treated at temperatures ranging from 710°C to 1000°C. The heated powder samples were characterized by X-ray diffraction analysis (XRD), transmission electron microscope (TEM), infrared (IR) and absorption spectroscopy. The results showed that the homogeneous nanocrystalline Co²⁺: MgAl₂O₄ could be obtained at the low temperature of 710°C. The optimal temperature is about 900°C and the average size of the powder grains is 50 nm or so. In the absorption spectrum, a broad absorption band from 1200 nm to 1600 nm was found, which indicated the existence of Co²⁺ in the tetrahedral sites because of the ⁴A₂(⁴F) → ⁴T(⁴F) transition of Co²⁺.