Preparation of Mn, Ni, Co ferrite highly porous silica nanocomposite aerogels by an urea-assisted sol-gel procedure

The preparation of highly porous MnFe 2O 4-SiO 2 and NiFe 2O 4-SiO 2 nanocomposite aerogels with high purity and homogeneity was successfully achieved by a sol-gel procedure involving urea- assisted co-gelation of the precursor phases firstly applied for the synthesis of CoFe 2O 4-SiO 2. This method allows fast gelation, giving rise to aerogels with 97% porosity. The structural, morphological and textural characterization as a function of thermal treatments was carried out by a multitechnique approach confirming that, as in the case of CoFe 2O 4-SiO 2, the formation of single nanocrystals of manganese ferrite and nickel ferrite with spinel structure occurs after heating at 750 °C and is complete at 900 °C when the high porosity typical of aerogels is still retained. Thermogravimetric analysis (TG), differential thermal analysis (DTA), N 2-physisorption at 77 K, powder X-ray diffraction (XRD), and transmission electron microscopy (TEM) indicate that the compositional homogeneity, crystallite size, thermal stability, and porosity are controlled by the sol-gel parameters of the preparation.