Low temperature synthesis of Co2SiO4/SiO2 nanocomposite using a modified sol–gel method

Abstract

The paper presents a study on the preparation of Co2SiO4/SiO2 nanocomposites by a new modified sol–gel method. We have prepared gels starting from tetraethylorthosilicate (Si(OC2H5)4), cobalt nitrate Co(NO3)2·6H2O and some diols: ethylene glycol (C2H6O2), 1,2propanediol (C3H8O2) and 1,3propanediol (C3H8O2), for a final composition: 30% CoO/70% SiO2. During the heating of the gels at 140 °C, a redox reaction takes place between NO3− ions and diol with formation of some carboxylate anions. These carboxylate anions react with the Co(II) ions to form coordination compounds embedded in silica matrix, as evidenced by FT-IR spectrometry and thermal analysis. These Co(II) coordinative compounds thermally decompose in the range 250–300 °C to the corresponding oxides: CoO and/or Co3O4 inside the matrices pores. When CoO results, it reacts with SiO2 at low temperature leading to Co2SiO4, which crystallizes at 700 °C. XRD patterns of the samples annealed at temperatures lower than 700 °C were characteristic to amorphous phases. The samples annealed at temperatures ≥700 °C, contain Co2SiO4 (olivine) as unique crystalline phase inside the amorphous silica matrix, according to XRD patterns. As evidenced by TEM images, Co2SiO4 nanoparticles are homogenously dispersed inside the silica matrix.