Popcorn-Shaped Fe x O (Wüstite) Nanoparticles from a Single-Source Precursor: Colloidal Synthesis and Magnetic Properties

Christoph P Guntlin; Stefan T Ochsenbein; Michael Wörle; Rolf Erni; Kostiantyn V Kravchyk; Maksym V Kovalenko

doi:10.1021/acs.chemmater.7b04382

Popcorn-Shaped Fe _x O (Wüstite) Nanoparticles from a Single-Source Precursor: Colloidal Synthesis and Magnetic Properties

Chem Mater. 2018 Feb 27;30(4):1249-1256. doi: 10.1021/acs.chemmater.7b04382. Epub 2018 Jan 25.

Authors

Christoph P Guntlin^{1

2}, Stefan T Ochsenbein^{1

2}, Michael Wörle¹, Rolf Erni², Kostiantyn V Kravchyk^{1

2}, Maksym V Kovalenko^{1

2}

Affiliations

¹ Laboratory for Inorganic Chemistry, Department of Chemistry and Applied Biosciences, ETH Zürich, Vladimir Prelog Weg 1, CH-8093 Zürich, Switzerland.
² Laboratory for Thin Films and Photovoltaics and Electron Microscopy Center, EMPA-Swiss Federal Laboratories for Materials Science and Technology, Überlandstrasse 129, CH-8600 Dübendorf, Switzerland.

Abstract

Colloidal nanoparticles (NPs) with myriads of compositions and morphologies have been synthesized and characterized in recent years. For wüstite Fe _x O, however, obtaining phase-pure NPs with homogeneous morphologies have remained challenging. Herein, we report the colloidal synthesis of phase-pure Fe _x O (x ≈ 0.94) popcorn-shaped NPs by decomposition of a single-source precursor, [Fe₃(μ₃-O)(CF₃COO)(μ-CF₃COO)₆(H₂O)₂]·CF₃COOH. The popcorn shape and multigrain structure had been reconstructed using high-angle annular dark-field scanning transmission electron micrograph (HAADF-STEM) tomography. This morphology offers a large surface area and internal channels and prevents further agglomeration and thermal tumbling of the subparticles. [Fe₃(μ₃-O)(CF₃COO)(μ-CF₃COO)₆(H₂O)₂]·CF₃COOH behaves as an antiferromagnetic triangle whose magnetic frustration is mitigated by the low symmetry of the complex. The popcorn-shaped Fe _x O NPs show the typical wüstite antiferromagnetic transition at approximately 200 K, but behave very differently to their bulk counterpart below 200 K. The magnetization curves show a clear, unsymmetrical hysteresis, which arises from a combined effect of the superparamagnetic behavior and exchange bias.