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Popcorn-Shaped Fe O (Wüstite) Nanoparticles from a Single-Source Precursor: Colloidal Synthesis and Magnetic
Christoph P Guntlin1,2, Stefan T Ochsenbein1,2, Michael Wörle1
1Laboratory for Inorganic Chemistry, Department of Chemistry and Applied Biosciences, ETH Zürich, Vladimir Prelog Weg 1, CH-8093 Zürich, Switzerland.
Abstract:
Colloidal nanoparticles (NPs) with myriads of compositions and morphologies have been synthesized and characterized in recent years. For wüstite Fe O, however, obtaining phase-pure NPs with homogeneous morphologies have remained challenging. Herein, we report the colloidal synthesis of phase-pure Fe 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 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.
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