Atomic-Scale Determination of Cation Inversion in Spinel-Based Oxide Nanoparticles

The atomic structure of nanoparticles can be easily determined by transmission electron microscopy. However, obtaining atomic-resolution chemical information about the individual atomic columns is a rather challenging endeavor. Here, crystalline monodispersed spinel Fe3O4/Mn3O4 core–shell nanoparticles have been thoroughly characterized in a high-resolution scanning transmission electron microscope. Electron energy-loss spectroscopy (EELS) measurements performed with atomic resolution allow the direct mapping of the Mn2+/Mn3+ ions in the shell and the Fe2+/Fe3+ in the core structure. This enables a precise understanding of the core–shell interface and of the cation distribution in the crystalline lattice of the nanoparticles. Considering how the different oxidation states of transition metals are reflected in EELS, two methods of performing a local evaluation of the cation inversion in spinel lattices are introduced. Both methods allow the determination of the inversion parameter in the iron oxide core and manganese oxide shell, as well as detecting spatial variations in this parameter, with atomic resolution. X-ray absorption measurements on the whole sample confirm the presence of cation inversion. These results present a significant advance toward a better correlation of the structural and functional properties of nanostructured spinel oxides.
The authors acknowledge funding from the Spanish Ministerio de Economía y Competitividad (MINECO) through the projects MAT2016-77391-R and MAT2016-79455-P/FEDER (UE). This work has also been carried out in the framework of the projects 2017-SGR-292 and 2017-SGR-776 from the Generalitat de Catalunya. A.L.O. acknowledges the MINECO through the Juan de la Cierva Program (IJCI-2014-21530). M.W. acknowledges the European Union Seventh Framework Programme under grant agreement no. 312483-ESTEEM2. ICN2 is funded by the CERCA Programme/Generalitat de Catalunya. ICN2 also acknowledges support from the Severo Ochoa Program (MINECO, grant SEV-2013-0295).