The selected-control synthesis of uniform α-Fe2O3 nanoparticles with morphologies of rhombohedra, rods, and cubes was successfully achieved in large quantities by the capping agent–cationic surfactant–CTAB-mediated hydrolysis of FeCl3. The nanoparticles were obtained when the concentration of FeCl3 ranged from 0.01 to 0.05 M at 120 °C. The growth mechanisms of α-Fe2O3 nanorhombohedra, nanorods, and nanocubes were analysed in detail, based on the topotactic transformation of the precursor β-FeOOH with different morphologies under the capping agent mediation. The concentration of FeCl3, the reaction temperature, and the concentration of CTAB are all responsible for the final crystalline morphologies of the α-Fe2O3 nanoparticles. This result may facilitate not only the exploration of controlled approaches of preparing α-Fe2O3 nanoparticles for potential technical applications but also a deeper understanding of the fundamental physical and chemical processes of hydrothermal methods. X-ray powder diffraction (XRD), transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), selected area electron diffraction (SAED), and field-emission scanning electron microscopy (FE-SEM) were used to characterize the products.

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