Popcorn-Shaped FexO (Wustite) 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 wustite FexO, however, obtaining phase-pure NPs with homogeneous morphologies have remained challenging. Herein, we report the colloidal synthesis of phase-pure FexO (x approximate to 0.94) popcorn-shaped NPs by decomposition of a single-source precursor, [Fe-3(mu(3)-O)(CF3COO)(mu-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. [Fe-3(mu(3)-O)(CF3COO)(mu-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 wustite 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.