Eco-friendly combustion-based synthesis of metal aluminates MAl2O4 (M = Ni, Co)

Nanosized metal aluminates, MAl2O4 (M = Ni, Co), have been prepared following a nonpolluting, low temperature, and self-sustaining starch single-fuel combustion synthesis. The mixed fuel-coordinating actions of starch have given rise to an intermediary precursor which afforded monodisperse metal aluminate nanoparticles. The thermal analysis of the [M(II), Al(III)]-starch precursors indicates a similar thermochemical reactivity for the two compounds, displaying a sequence of well-defined decomposition stages associated with three endothermic effects and three/four (nickel/cobalt) exothermic ones. The XRD data confirm the formation of spinelic phase and a continuous growth of particle sizes with the increase of calcination temperatures. The mechanisms proposed for the formation of metal aluminates essentially consist in a combination of solid-state reactions of amorphous NiO/Co3O4 and Al2O3 simple oxides. The evaluation criterion of Ni(II) cations into the spinelic lattice is original and is based on the distinct occupancy degree of tetrahedral and octahedral sites in NiAl2O4 and γ-Al2O3. TEM/HRTEM investigations performed on the cobalt(II) and nickel(II) aluminate oxide powders resulted after calcination at 800 and 900 °C, respectively, for 1 h show the formation of irregular and isolated plate-like particles for Co(II)-based spinelic oxides (the average particle size is 16.6 nm) and submicron aggregates of small, bimodal, and almost uniform (as shape and size) of NiAl2O4 mixed oxide (the mean particle size is 33.6 nm). The NIR–UV–Vis spectra for the resulted MAl2O4 (M = Co, Ni) mixed oxides reveal a massive presence of tetrahedral divalent cations both for short- and long-time calcined samples. NiO impurities are detected using FTIR and electronic spectra for all NiAl2O4 samples.