Shape evolution and tunable properties of monodisperse magnetite crystals synthesized by a facile surfactant-free hydrothermal method

Monodisperse Fe₃O₄ crystals have been synthesized by a simple hydrothermal method. Reaction conditions are performed in an ethylene glycol (EG)/NaOH system using FeCl3 as the iron source without adding any surfactants. By adjusting the initial molar ratio of NaOH to Fe³⁺, several morphologies including solid spheres, hollow spheres, spherical chains, tetrakaidecahedrons, octahedrons, and some flowerlike structures can be obtained. Further investigation reveals that all of the spherical structures are formed by the highly oriented assembly of Fe₃O₄ nanoparticles. The evolution of the morphology from spheres to octahedrons can be attributed to the competition effect between ethylene glycol and NaOH under the reaction conditions. The saturation magnetization, coercivity, and remanent magnetization of the assynthesized samples vary as the morphology and grain size changes.