Phase selective preparations and surface modifications of spherical hollow nanomagnets

Abstract

We carried out the phase selective preparations of hollow spheres of ccp- and hcp-Co, Co₃O₄, α-Fe, Fe₃O₄ and α-Fe₂O₃ with a diameter of ca. 600 nm and a shell thickness of 40 nm, using polystyrene (PS)-bead templates. The 600 nm PS beads were uniformly coated with cobalt or iron hydroxides by means of a homogeneous precipitation method. These cobalt-salt/PS hybrid particles were calcined in air at 400 °C and were transformed into Co₃O₄ hollow spheres. The hcp-Co hollow spheres were obtained by a reduction reaction of the Co₃O₄ particles under a stream of a 1 ∶ 1 mixed gas of H₂ and N₂ at 400 °C; in contrast, the ccp-Co hollow spheres were obtained by the calcination of the parent cobalt-salt/PS particles under H₂–N₂ at 400 °C. Hollow spheres of Fe₃O₄ and α-Fe₂O₃ were selectively prepared by calcination of iron-salt/PS particles at 400–500 °C in two different atmospheres, namely under H₂–N₂ and in air, respectively. Furthermore, the α-Fe₂O₃ particles were reduced into α-Fe under H₂–N₂ at 350 °C without any changes in morphology. These spheres were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), and X-ray diffraction (XRD) analyses. Magnetic measurements revealed a significant temperature dependence of the coercive field for the Fe₃O₄ (magnetite) particles, whereas the magnetic properties of the other particles were generally similar to those of the corresponding bulk samples. Surface chemical modifications were carried out on the Fe₃O₄ hollow spheres; the surfaces were grafted with poly(4-N-methylvinylpyridinium) brushes. This treatment was found to induce good dispersibility in water.