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.