A simple and straightforward mechanochemical synthesis of the far-from-equilibrium zinc aluminate, ZnAl2O4, and its response to thermal treatment

Abstract

Zinc aluminate (ZnAl2O4) nanoparticles with an average size of about 10 nm are synthesized via one-step mechanochemical processing of the ZnO : γ-Al2O3 stoichiometric mixture at ambient temperature. The mechanochemically induced formation of the phase is followed by XRD and 27Al MAS NMR. High-resolution TEM studies reveal a non-uniform nanostructure of mechanosynthesized aluminate consisting of ordered grains surrounded or separated by disordered surface and interfacial regions. Due to the capability of 27Al MAS NMR to probe the local environment of the Al cations, valuable insights into the short-range structure of ZnAl2O4 on the Ångström length scale are provided. It is demonstrated that the as-prepared aluminate possesses a partly inverse spinel structure with a far-from equilibrium arrangement of cations and distorted polyhedra, which are spatially confined to the surface and interfacial regions with a volume fraction of ca. 50% and a thickness of ca. 1 nm. The response of the nanostructured ZnAl2O4 to subsequent thermal treatment is further investigated. It turned out that the thermally induced grain growth is accompanied by a release of microstrain, by a shrinkage of the lattice parameter, as well as by a variation in the oxygen parameter and metal-oxygen bond lengths. Evidence is given of the thermally induced redistribution of cations approaching their equilibrium positions. Upon heating above 1100 K, mechanosynthesized ZnAl2O4 relaxes towards a structural state that is similar to the bulk one.