Distinct optical and magnetic properties of ionic liquid tuned hematite nanocrystals having different exposed (001) facets

In this work, we report a new synthesis system to prepare hematite nanostructures with different exposed crystal facets. Ionic liquid, 1-n-butyl-3-methylimidazolium tetrafluoroborate ([bmim]BF₄), was used as an important structural directing agent in the mixed solvent containing water and N,N-dimethylformamide. By tuning the experimental conditions, samples were prepared with distinct morphologies: hexagonal nanodiscs, polyhedra, and bipyramids. The products were thoroughly characterized by scanning electron microscopy, transmission electron microscopy, X-ray diffraction, Raman spectroscopy, and UV-Vis spectroscopy. We discovered that these samples possessed a decreasing percentage of exposed (001) facets, which leads to their contrasting optical properties and the change of colour from maroon to dark grey. The magnetism measurement data suggested that such structural differences are also responsible for the distinct magnetic properties of these samples.