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 includes three main steps: (i) growth of ultraporous nanofibrous Al2O3 (UPA) monoliths and their preliminary thermal treatment to remove the adsorbed and structural water, (ii) liquid-phase M-nitrate impregnation, and (iii) heat treatment allowing to form desirable nanocrystallites. Small crystallites with a spinel structure and mean size below 10 nm were obtained in Mg, Ni, and Zn aluminates after heat treatment at 500 °C. The results suggest formation of fully inverse MgAl2O4 spinels (inversion degree I ≈ 1) after the heat treatment at temperatures below 700 °C, followed by defect healing and formation of a normal spinel (I ∼ 0.3) at higher temperatures above the onset of the UPA bulk mass transport of ∼850 °C. This behavior is due to the interplay between cation diffusion and phase nucleation processes, in agreement with Ostwald's rule. The formation of metastable spinel phases in agreement with Ostwald's rule was confirmed after insertion of Co2+, Zn2+ (inverse spinel), and Ni2+ (normal spinel) cations. The insertion of Ba2+ cations into the UPA precursor resulted in the stuffed tridymite phase. The nanopowder synthesis route after upgrading to a large scale can contribute to the fabrication of functional materials for radiation-tolerant optics (M = Mg, Zn), catalysis (M = Ni, Co), nanophosphors (M = Ba), etc.