Studies regarding the formation from metal nitrates and diol of NiM 2 III O4 spinels, inside a silica matrix

Abstract

The present study deals with preparation and characterization of spinel mixed oxide systems NiM2IIIO4, where MIII = FeIII, CrIII. In order to obtain 50% NiFe2O4/50% SiO2 and 50% NiCr2O4/50% SiO2 nanocomposite, we have used a versatile route based on the thermal decomposition inside the SiO2 matrix, of some particular precursors, coordination compounds of the involved MII and MIII cations with dicarboxylate ligands. The ligands form in the redox reaction between metal nitrates mixture and 1,3-propanediol at the heating around 140 °C of the gels (tetraethylorthosilicate–metal nitrates–1,3-propanediol–water). The as-obtained precursors, embedded in silica gels, have been characterized by FT-IR spectrometry and thermal analysis. Both precursors thermally decompose up to 350 °C leading to the formation of the corresponding metal oxides inside the silica matrix. X-ray diffraction of the annealed powders have evidenced the formation of NiFe2O4 starting with 600 °C, and NiCr2O4 starting with 400 °C. This behavior can be explained by the fact that, by thermal decomposition of the Fe(III) carboxylate at 300 °C, the spinelic phase γ-Fe2O3 is formed, which interacts with the NiO, forming the ferrite nuclei. By thermal decomposition of chromium carboxylate, a nonstoichiometric chromium oxide (Cr2O3+x) is formed. In the range 380–400 °C, Cr2O3+x turns into Cr2O3 which immediately interacts with NiO leading to the formation of nickel chromites nuclei inside the pores of silica matrix. Both spinels have been obtained as nanocrystalites homogenously dispersed as resulted from XRD and TEM data.