Improvement of magnetic property of Fe nanoparticles dispersed in single-walled carbon nanohorns by a vacuum heat treatment

Abstract Carbon nanohorns (CNHs) hybridized with Fe nanoparticles (Fe-CNHs) were synthesized by a gas-injected arc-in-water (GI-AIW) method using a holey graphite anode in which Fe wires were inserted. Because of the ferromagnetic property of the Fe nanoparticles, Fe-CNHs can be manipulated by applying a magnetic field. In this study, a heat treatment on Fe-CNHs under a vacuum condition at 50 Pa up to 800 °C was performed to enhance their magnetic property. An analysis of AC magnetic susceptibility at room temperature indicated that the magnetic susceptibility of Fe-CNHs was enhanced by more than twice using this vacuum heat treatment. This increase in the magnetic susceptibility should enhance the handling performance of the Fe-CNHs in fluid media, and this finding should be useful to develop catalyst supports which can be recovered using magnetic field. Additionally, an X-ray diffraction (XRD) analysis indicated that Fe was oxidized partially by residual air to form ferrimagnetic Fe3O4 (magnetite) in the vacuumed system, contributing to the enhancement in the magnetic susceptibility of Fe-CNHs. Meanwhile, the growth of bcc Fe crystal grain was observed. It was remarkable that the products observed here did not suffer from the transformation of Fe to paramagnetic Fe2O3. Also, there was no serious damage in the carbonaceous part of Fe-CNHs detected by microscopic observation. N2 adsorption analysis suggested that only a moderate increase in BET specific surface area of Fe-CNHs was observed by the vacuum heat treatment. All of these results suggest that while the magnetic property of Fe-CNHs is highly enhanced, most of the carbonaceous structures can be preserved by the present method. Such functionalization of Fe-CNHs cannot be achieved by conventional methods using heat treatment in air or in CO2–N2 mixture.