Controllable Synthesis and Magnetic Properties of Monodisperse Fe_3O_4 Nanoparticles

Magnetite (Fe_3O_4) nanoparticles with different sizes and shapes are synthesized by the thermal decomposition method. Two approaches, non-injection one-pot and hot-injection methods, are designed to investigate the growth mechanism in detail. It is found that the size and shape of nanoparticles are determined by adjusting the precursor concentration and duration time, which can be well explained by the mechanism based on the LaMer model in our synthetic system. The monodisperse Fe_3O_4 nanoparticles have a mean diameter from 5 nm to 16 nm, and shape evolution from spherical to triangular and cubic. The magnetic properties are size-dependent, and Fe_3O_4 nanoparticles in small size about 5 nm exhibit superparamagnetic properties at room temperature and maximum saturation magnetization approaches to 78 emu/g, whereas Fe_3O_4 nanoparticles develop ferromagnetic properties when the diameter increases to about 16 nm.