Lab-Scale Synthesis of Hollow Iron Oxide Nanocapsules with a Metastable Magnetic Maghemite Phase

Iron oxide particles are of interest due to their potential to deliver functionality through their magnetic properties and their potential biocompatibility. Similarly, hollow particles are of interest for their potential to provide a chamber for a chemical reaction or delivery. In previous work, we used high-vacuum and high-resolution in situ transmission electron microscopy (TEM) to determine the compositional, structural, and morphological changes accompanying the dehydration of an iron oxyhydroxide nanorod as it formed a series of metastable iron oxide nanocapsule phases. Though in situ TEM of individual particles has the potential (as in this case) to reveal pathways to achieve metastable phases for new materials and applications, a means must be found to translate the findings from individual particle observations to lab-scale synthesis. Such scaling up can be nontrivial due to complexities such as interfacial interactions, diffusion pathways, density, and so on introduced when moving to a bench-scale sample. Here, we report the production of scalable, phase-pure, spinel-structured, hollow iron oxide nanocapsules of the maghemite (γ-Fe₂O₃) phase. The structure and size of the nanocapsules were controlled through the synthesis of the initial iron oxyhydroxide phase (β-FeOOH). The phase isolation and hollowing processes were performed through thermal treatments under specific external conditions. Nanocapsule morphology and size for all iron oxide phases were confirmed through scanning and transmission electron microscopy, while the crystal structure and phases were verified through X-ray diffraction, fast Fourier transform diffraction analysis, and Mössbauer spectroscopy. Energy-dispersive X-ray spectroscopy spectra and elemental mapping were utilized to obtain compositional data, and superconducting quantum interference device (SQUID) magnetometer measurements were used to characterize the paramagnetic, ferrimagnetic, and potential superparamagnetic behavior of the ensemble. We anticipate that the ability to produce anisotropic hollow nanoparticles with spontaneous magnetization and potentially complex magnetic characteristics will greatly increase the versatility and applicability of iron oxide nanoparticles.