Removal of entrapped iron compounds from isothermally treated catalytic chemical vapor deposition derived multi-walled carbon nanotubes

Compositional changes of the residual iron compounds in isothermally treated catalytic chemical vapor deposition derived multi-walled carbon nanotubes have been monitored using 57Fe transmission Mössbauer spectroscopy and X-ray fluorescence. The iron phases entrapped in the as-synthesized carbon nanotubes consist of γ-iron, α-iron, Fe3C and Fe1−xS. The Fe1−xS phase decomposes completely around 1500 °C while the iron carbide phase decomposes in the temperature range of 1500–2400 °C. The obtained apparent activation energy of ca. 76 kcal/mol suggests that the entrapped iron was removed via a diffusion process during thermal treatment.