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.