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

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.

Neither Sphere nor Cube—Analyzing the Particle Shape Using Small-Angle Scattering and the Superball Model

“…(iii) Electron beam-induced transformation of such particles has been observed, where the local heating results in an increased ordering of vacancies in the nanoparticles and a subsequent lattice contraction, reducing slightly their particle size. 49 Effect (i) is a priori dominant as the full oxidation of wustite to maghemite is accompanied by a volume expansion of about 30%, in line with the obtained results. This is reasonable for the samples presented here, as the reported HR-TEM measurements 44 were typically performed on very fresh samples, whereas the SAXS was measured after several years, with enhanced statistics and resolution as needed for this study.…”

“…(ii) Sampling bias may also be present, as the larger particles tend to agglomerate more and this agglomeration may result in losing the alignment of the zone axis with the electron beam. (iii) Electron beam-induced transformation of such particles has been observed, where the local heating results in an increased ordering of vacancies in the nanoparticles and a subsequent lattice contraction, reducing slightly their particle size …”

Neither Sphere nor Cube - Analyzing the Particle Shape Using Small-Angle Scattering and the Superball Model

“…ii) Sampling bias may also be present, as the larger particles tend to agglomerate more and this agglomeration may result in losing the alignment of the zone axis with the electron beam. iii) Electron beam-induced transformation of such particles has been observed, where the local heating results in an increased ordering of vacancies in the nanoparticles and a subsequent lattice contraction, reducing slightly their particle size 49 .…”

Magnetism of Nanoparticles: Effect of the Organic Coating

“…Since the pioneering works of Louis Néel in 1944 [ 1 ], magnetic nanoparticles have been the subject of continuous interes [ 2 , 3 , 4 , 5 , 6 , 7 , 8 , 9 ], due to the possibilities of designing new magnetic systems with finely tuned properties, thanks to the enormous progress in the synthesis and diagnostic methods, and the increasing number of applications, e.g., biomedicine [ 10 , 11 , 12 , 13 ], ferrofluid technology [ 14 ], catalysis [ 15 , 16 ], sensors [ 17 , 18 , 19 ], magnetic energy storage [ 20 , 21 ], electronics [ 22 ] etc. Therefore, the understanding of the physics of magnetic nanoparticles and the control of their properties still represent a very attractive and challenging topic for both fundamental research and technological applications.…”