Synthesis of various magnetite nanoparticles through simple phase transformation and their shape-dependent magnetic properties

Abstract

Shape-controlled synthesis of magnetite nanoparticles has been successfully carried out through simple phase transformation via a modified polyol process using akaganeite (β-FeOOH) as the precursor. Different shapes of magnetite nanoparticles, including solid nanospheres and solid and hollow nanoellipsoids, could be obtained by either adding the appropriate amount of sodium acetate (NaOAc) or using the anion exchange of β-FeOOH. NaOAc makes it possible to retain the rod-like shape of the precursor material during the phase transformation by coordinating the surface and controlling the basic pH to slow down dissolution–recrystallization mechanism. It is also proposed that chloride ions in the β-FeOOH structure play a key role in the formation of hollow structure. The shape-dependent magnetic properties were investigated using a magnetic property measurement system at 300 K. All magnetite nanoparticles exhibited ferromagnetic behaviour with different values of saturation magnetization (M_s) and coercivity (H_c), and these values were highly depend on the shape of the nanoparticles due to their different grain size, spin disorder, shape, and surface anisotropy.