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Morphological and magnetic anisotropy can be combined in colloidal assembly to create unconventional secondary structures. We show here that magnetite nanorods interact along a critical angle, depending on their aspect ratios and assemble into body-centered tetragonal colloidal crystals. Under a magnetic field, size-dependent attractive and repulsive domains develop on the ends and center of the nanorods, respectively. Our joint experiment-computational multiscale study demonstrates the presence of a critical angle in the attractive domain, which defines the equilibrium bonding states of interacting rods and leads to the formation of non–close-packed yet hard-contact tetragonal crystals. Small-angle x-ray scattering measurement attributes the perfect tetragonal phase to the slow assembly kinetics. The crystals exhibit brilliant structural colors, which can be actively tuned by changing the magnetic field direction. These highly ordered frameworks and well-defined three-dimensional nanochannels may offer new opportunities for manipulating nanoscale chemical transformation, mass transportation, and wave propagation. 

We report here that dissolution and regrowth of resorcinol formaldehyde (RF) colloidal particles can occur spontaneously when they are subjected to etching in solvents such as ethanol and tetrahydrofuran, resulting in the formation of hollow nanostructures with controllable shell thickness. The hollowing process of the RF particles is attributed to their structural inhomogeneity, which results from the successive deposition of oligomers with different chain lengths during their initial growth. As the near-surface layer of RF colloids mainly consists of long-chain oligomers while the inner part consists of short-chain oligomers, selective etching removes the latter and produces the hollow structures. By revealing the important effects of the condensation degree of RF, the etching time and temperature, and the composition of solvents, we demonstrate that the morphology and structure of the resulting RF nanostructures can be conveniently and precisely controlled. This study not only improves our understanding of the structural heterogeneity of colloidal polymer particles, but also provides a practical and universal self-templated approach for the synthesis of hollow nanostructures.