Simple Surfactant Concentration-Dependent Shape Control of Polyhedral Fe₃O₄ Nanoparticles and Their Magnetic Properties

The shape and size of monodisperse Fe₃O₄ nanoparticles (NPs) are controlled using a chemical solution synthesis in the presence of the surfactant cetylpyridinium chloride (CPC). Cubic Fe₃O₄ NPs surrounded by six {100} planes are obtained in the absence of CPC. Increasing the CPC content during synthesis causes the shape of the resulting Fe₃O₄ NPs to change from cubic to truncated cubic, cuboctahedral, truncated octahedral, and finally octahedral. During this evolution, the predominantly exposed planes of the Fe₃O₄ NPs vary from {100} to {111}. The shape control results from the synergistic effect of the pyridinium cations, chloride anions, and long-chain alkyl groups of CPC, which is confirmed by comparison with NPs synthesized in the presence of various related cationic surfactants. The size of the cubic Fe₃O₄ NPs can be tuned from 50 to 200nm, by changing the concentration of oleic acid in the reaction solution. The Fe₃O₄ NPs exhibit shape-dependent saturation magnetization, remanent magnetization, and coercivity.