Probing the Meta-Stability of Oxide Core/Shell Nanoparticle Systems at Atomic Resolution

TL;DR

This study investigates the atomic-scale stability and interfacial dynamics of oxide core/shell nanoparticles under reducing conditions using in-situ electron microscopy, revealing how different configurations influence transformation pathways and defect formation.

Contribution

It provides detailed insights into the atomic-level stability and transformation mechanisms of magnetic oxide core/shell nanoparticles under in-situ conditions, highlighting the importance of local dynamics.

Findings

Smooth transformation in Fe3O4/Mn3O4 core/shell without defects

Stage-wise transformation with defect formation in Mn3O4/Fe3O4 core/shell

Understanding local dynamics aids in controlling nanoparticle stability

Abstract

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.