Probing the meta-stability of oxide core/shell nanoparticle systems at atomic resolution

Hybrid nanoparticles allow exploiting the interplay of confinement, proximity between different materials and interfacial effects. However, to harness their properties an in-depth understanding of their (meta)stability and interfacial characteristics is crucial. This is especially the case of nanosystems based on functional oxides working under reducing conditions, which may severely impact their properties. In this work, the in-situ electron-induced selective reduction of Mn3O4 to MnO is studied in magnetic Fe3O4/Mn3O4 and Mn3O4/Fe3O4 core/shell nanoparticles by means of high-resolution scanning transmission electron microscopy combined with electron energy-loss spectroscopy. Such in-situ transformation allows mimicking the actual processes in operando environments. A multi-stage image analysis using geometric phase analysis combined with particle image velocity enables direct monitoring of the relationship between structure, chemical composition and strain relaxation during the Mn3O4 reduction. In the case of Fe3O4/Mn3O4 core/shell the transformation occurs smoothly without the formation of defects. However, for the inverse Mn3O4/Fe3O4 core/shell configuration the electron beam-induced transformation occurs in different stages that include redox reactions and void formation followed by strain field relaxation via formation of defects. This study highlights the relevance of understanding the local dynamics responsible for changes in the particle composition in order to control stability and, ultimately, macroscopic functionality.
Research supported by the European Research Council Starting Investigator Award STEMOX # 239739 (M.R. and J. S.), JSPS Postdoctoral Fellowship for Research Abroad (R.I.) and by Spanish MAT2015-066888-C3-3-R and RTI2018-097895-B-C43 (MINECO/FEDER). Electron microscopy observations at Oak Ridge National Laboratory (ORNL) supported by the U.S. Department of Energy (DOE), Basic Energy Sciences (BES), Materials Sciences and Engineering Division and through a user project supported by ORNL’s Center for Nanophase Materials Sciences (CNMS), which is sponsored by the Scientific User Facilities Division, Office of Basic Energy Sciences, U.S. Department of Energy. A.M. and G.S.A. thank the financial support of the Knut and Alice Wallenberg Foundation through the project 3DEM-NATUR. The work at INC2 has been supported by the 2017-SGR-292 project of the Generalitat de Catalunya and by the MAT2016-77391-R project of the Spanish MINECO. ALO acknowledges the Spanish Ministerio de Economía y Competitividad through the Juan de la Cierva Program (IJCI-2014-21530). ICN2 is funded by the CERCA Programme/Generalitat de Catalunya. ICN2 also acknowledges support from the Severo Ochoa Centres of Excellence programme, funded by the Spanish Research Agency (AEI, grant no. SEV-2017-0706). S. J. P. thanks the National University of Singapore for funding. M. E. Acknowledges the Spanish MINECO for her Ramón y Cajal Fellowship (RYC2018-024396-I).