Simultaneous enhancement in porosity and magnetic property of Fe-dispersing single-walled carbon nanohorns by oxidation using CO2

• Carbon nanohorns dispersed with Fe nanoparticles can be synthesized by one-step.
• Magnetic susceptibility can be preserved by activation in CO2–N2 mixed gas.
• Activation in air cannot preserve the magnetic susceptibility.
• Porosity can be doubled by activation in CO2–N2 mixed gas.

It was discovered that the specific surface area and magnetic susceptibility of Fe-dispersing single-walled carbon nanohorns (Fe-CNHs) were significantly increased without obvious destruction of CNH structures by oxidation in CO2 (10%) diluted with N2 at 1000 °C. As-grown Fe-CNHs synthesized by a submerged arc discharge method had a specific surface area of 184 m2/g, and this surface area was raised to 500 m2/g by opening of pores upon this oxidation treatment. Under this condition, the carbon-based specific surface area are enhanced from 211 m2/g to 863 m2/g. Fe nanoparticles dispersing in Fe-CNHs were transformed into ferrimagnetic Fe3O4 nanoparticles without obvious change in their particle sizes and frames of CNHs. The magnetic susceptibility of the Fe-CNHs was increased by this oxidation, suggesting that their magnetic mobility could be improved. The structure of Fe-CNHs was stable against the extended oxidation duration in the CO2–N2. In contrast to the oxidation in CO2–N2, the oxidation in air resulted in the transformation of Fe nanoparticles into paramagnetic Fe2O3, and the excessive oxidation led to destructive loss of CNHs.