Ligand-Assisted Solid-State Transformation of Nanoparticles

Thermal treatment is generally a desirable process to improve the properties of nanomaterials, which however often leads to undesirable problems such as aggregation and shape deformation. Here, we overcome this challenge by developing a ligand-assisted calcination strategy for shape-preserved chemical transformation of nanostructures. While capping ligands are often thought to be effective in solution phase synthesis, we show that their presence during high-temperature calcination not only maintains the overall particle morphology but also offers the possibility of effective creation of controllable porosity in metal oxide nanostructures. We demonstrate a particularly elegant example of this strategy, which involves the chemical conversion of β-FeOOH ellipsoids into porous α-Fe₂O₃ and magnetic Fe₃O₄ ellipsoids with morphological preservation and excellent solution dispersity via stabilization with strong coordinating capping ligands. The ligand-assisted solid-state conversion strategy represents a general self-templating method for creating nanomaterials, as confirmed by its successful application to a wide range of morphologies (ellipsoids, rods, cubes, and plates) and compositions (hydroxides and metal–organic frameworks).