Study on the formation of MCr2O4/SiO2 nanocomposites from hybrid gels PVA–TEOS–metal nitrates

Nanocomposites of MCr2O4/SiO2 type, for M = Zn, Mg, Ni, have been synthesized using a modified sol–gel method. In the first step, hybrid gels TEOS:PVA:H2O:M(NO3)2:Cr(NO3)3 have been prepared and dried at 80 °C. These gels have been characterized by thermal analysis (TG–DTA) and FT-IR spectroscopy. Both technique have evidenced that around 110 °C a redox reaction takes place between PVA and the mixture of metal nitrates with oxidation of poly(vinyl alcohol), PVA, to carboxylates that coordinate to M(II) and Cr(III) cations. Thus a homogenous mixture of carboxylates formed in the pores of silica gel. By thermal decomposition of these gels at 400 °C the thermal decomposition of the carboxylates takes place leading to a homogenous mixture of oxides (MO and Cr2O3+x) that interacts during the calcinations at higher temperatures with formation of MCr2O4 embedded in silica matrix for M = Zn, Ni. For M = Mg the thermal decomposition of the corresponding carboxylates leads to an intermediary phase (MgCrO4) which further decomposes to MgCr2O4 at higher temperature. Thus Cr2O3 crystallizes well and is stabilised by the silica matrix, resulting in a lower reactivity. Due to this behaviour Cr2O3 is present as secondary crystalline phase in silica matrix beside MgCr2O4, as evidenced by XRD analysis. The obtained chromites nanoparticles with diameter up to 30 nm at 1000 °C, were homogenously distributed in the amorphous silica matrix, as evidenced by TEM and SEM microscopy.