A wide variety of metal oxide manganite and ferrite core-shell nanoparticles (NPs) with adjustable composition were characterized by means of Transmission Electron Microscopy related tools in order to precisely understand their composition, the arrangement of the different chemical species and cation valence state variation along the nanoparticles.

The nanoparticles were produced at low temperature and ambient atmosphere using a one-pot two-step synthesis protocol involving the cation exchange of Mn or Fe by Co or Ni in preformed Mn₃O₄ or Fe₃O₄ NPs, allowing the formation of a core shell structure. These nanoparticle systems present a complex 3D structure, which has been modelled with Rhodius software [1] and simulated with the STEM-CELL packages [2], giving rise to a fine determination of the studied systems.

Special attention has been paid on manganite – cobalt systems. By selecting the proper cobalt precursor, CoO crystallites could be simultaneously nucleated on the NP surface to form Mn₃O₄@CoMn₂O₄–CoO (Figure 1). In this latest case,  heterostructured NPs exhibited improved performance and durability as bifunctional catalysts for the oxygen reduction and evolution reactions (ORR, OER) over commercial Pt and IrO2-based catalysts and over previously reported spinel electrocatalysts in alkaline solution.

Acknowledgements:

SM acknowledges funding from “Programa Internacional de Becas “la Caixa”-Severo Ochoa”, from Generalitat de Catalunya 2014 SGR 1638, the Spanish MINECO MAT2014-51480-ERC (e-ATOM), Severo Ochoa Excellence Program and coordinated projects between IREC and ICN2, TNT-FUELS and e-TNT (MAT2014-59961-C2-2-R). Part of the present work has been performed in the framework of Universitat Autònoma de Barcelona Materials Science PhD program.

[1] S. Bernal, et al., Ultramicroscopy 72, 135−164 (1998)

[2] V. Grillo, E. Rotunno, Ultramicroscopy 125, 97–111 (2013)

Figures:

Figure 1. (a)-(c) HRTEM micrographs of NPs grown using Co(ClO4)·6H2Oas cobalt precursor with different molar ratio between Mn and Co: (a) [Co]/[Mn]=0.5 (b) [Co]/[Mn]=1 and (c) [Co]/[Mn]=2. (d) HRTEM micrograph of a [Co]/[Mn]=1 NP with details on the Mn3O4 and CoO phases (squared regions) and filtered image showing the spatial distribution of the phases. (e) ADF STEM image, EELS elemental mapping and compositional profile on [Co]/[Mn]=2 NPs. (f) Mn oxidation state/valence mapping obtained by EELS.

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EMC Abstracts - https://emc-proceedings.com/abstract/structural-and-chemical-characterization-and-3d-modelling-of-metal-oxide-core-shell-nanoparticles-with-complex-morphology/