Popcorn-Shaped Fe_<i>x</i>O (Wüstite) Nanoparticles from a Single-Source Precursor: Colloidal Synthesis and Magnetic Properties

Colloidal nanoparticles
(NPs) with myriads of compositions and
morphologies have been synthesized and characterized in recent years.
For wüstite FexO, however, obtaining
phase-pure NPs with homogeneous morphologies have remained challenging.
Herein, we report the colloidal synthesis of phase-pure FexO (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 FexO 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.