Synthesis of MAl2O4 (M = Mg, Co, Ni, Zn, Ba) Nanopowders by Liquid Impregnation of Nanofibrous Alumina

We report on a controlled synthesis of MAl2O4 (M = Mg, Co, Ni, Zn, Ba) nanopowders. The method includesthreemain steps: (i) growth of ultraporous nanofibrous Al2O3 (UPA) monoliths and their preliminary thermal treatment toremove the adsorbed and structural water, (ii) liquid-phase M-nitrateimpregnation, and (iii) heat treatment allowing to form desirablenanocrystallites. Small crystallites with a spinel structure and meansize below 10 nm were obtained in Mg, Ni, and Zn aluminates afterheat treatment at 500 degrees C. The results suggest formation of fullyinverse MgAl2O4 spinels (inversion degree I approximate to 1) after the heat treatment at temperatures below700 degrees C, followed by defect healing and formation of a normalspinel (I similar to 0.3) at higher temperatures abovethe onset of the UPA bulk mass transport of similar to 850 degrees C. Thisbehavior is due to the interplay between cation diffusion and phasenucleation processes, in agreement with Ostwald's rule. Theformation of metastable spinel phases in agreement with Ostwald'srule was confirmed after insertion of Co2+, Zn2+ (inverse spinel), and Ni2+ (normal spinel) cations. Theinsertion of Ba2+ cations into the UPA precursor resultedin the stuffed tridymite phase. The nanopowder synthesis route afterupgrading to a large scale can contribute to the fabrication of functionalmaterials for radiation-tolerant optics (M = Mg, Zn), catalysis (M= Ni, Co), nanophosphors (M = Ba), etc.