Magnetism for understanding catalyst analysis of purified carbon nanotubes

• Cl-gas treatment of Ni catalyst of carbon nanotubes leads to NiCl2 residue.
• Magnetic measurements show the transformation of Ni0 in Ni2+through a purification process.
• High temperature Cl treatment removes 75% of metallic impurities.
• Cl-purification yields to an amount of metal of 1.5% in arc-discharge CNT samples.

The precise quantification of catalyst residues in purified carbon nanotubes is often a major issue in view of any fundamental and/or applicative studies. More importantly, since the best CNTs are successfully grown with magnetic catalysts, their quantification becomes strictly necessary to better understand intrinsic properties of CNT. For these reasons, we have deeply analyzed the catalyst content remained in nickel–yttrium arc-discharge single walled carbon nanotubes purified by both a chlorine-gas phase and a standard acid-based treatment. The study focuses on Ni analysis which has been investigated by transmission electron microscopy, X-ray diffraction, thermogravimetry analysis, and magnetic measurements. In the case of the acid-based treatment, all quantifications result in a decrease of the nanocrystallized Ni by a factor of two. In the case of the halogen gas treatment, analysis and quantification of Ni content is less straightforward: a huge difference appears between X-ray diffraction and thermogravimetry results. Thanks to magnetic measurements, this disagreement is explained by the presence of Ni2+ ions, belonging to NiCl2 formed during the Cl-based purification process. In particular, NiCl2 compound appears under different magnetic/crystalline phases: paramagnetic or diamagnetic, or well intercalated in between carbon sheets with an ordered magnetic phase at low temperature.