Synthesis of Magnetite Nanooctahedra and Their Magnetic Field-Induced Two-/Three-Dimensional Superstructure

In this work, we report a robust wet-chemical route to synthesize monosized octahedron-shaped magnetite (Fe 3 O 4 ) nanoparticles with average sizes ranging from 8 to ∼430 nm. In other words, we are able to adjust the magnetic properties of the as-synthesized nanoparticles from superparamagnetic to single-domain to multidomain ferrimagnetic regimes. We also demonstrate a simple solvent-evaporation assembly process to obtain either 2D monolayer or 3D microrod superstructures made of 21 nm-sized nanooctahedra by applying a weak magnetic field (∼0.06 T) in the horizontal or vertical direction, respectively. The as-obtained 2D monolayer assembly not only exhibits a long-range translational order (hexagonal close packing) but also has a high degree of crystallographic orientational order (⟨111⟩ texture normal to the substrate). Large-area assemblies (up to 10 × 10 μm) can be formed on various substrates, for example, silicon substrates and carbon films of transmission electron microscopy copper grids, as demonstrated. By changing the magnetic field to the vertical direction, uniform 3D microrod superstructures with an aspect ratio of approximately 5 are obtained, and they also exhibit the specific crystallographic orientations.