Magnetic-field-assisted synthesis of anisotropic iron oxide particles: Effect of pH

The synthesis of magnetite (Fe₃O₄) nanorods using reverse co-precipitation of Fe³⁺ and Fe²⁺ ions in the presence of a static magnetic field is reported in this work. The phase composition and crystal structure of the synthesized material were investigated using electron diffraction, Raman spectroscopy, and transmission electron microscopy. It was shown that the morphology of the reaction product strongly depends on the amount of OH^- ions in the reaction mixture, varying from Fe₃O₄ nanorods to spherical Fe₃O₄ nanoparticles. Fe₃O₄ nanorods were examined using high-resolution transmission electron microscopy proving that they are single-crystalline and do not have any preferred crystallographic orientation along the axis of the rods. According to the data obtained a growth mechanism was proposed for the rods that consists of the dipole-dipole interaction between their building blocks (small hexagonal faceted magnetite nanocrystals), which are formed during the first step of the reaction. The study suggests a facile, green and controllable method for synthesizing anisotropic magnetic nanoparticles in the absence of stabilizers, which is important for further modification of their surfaces and/or incorporation of the nanoparticles into different media.