Ferromagnetic behaviour of polyaniline-coated multi-wall carbon nanotubes containing nickel nanoparticles

Multi-wall carbon nanotubes (CNT) containing the residual nickel catalyst nanoparticles have been coated with a conducting polymer, polyaniline (PANI), directly during the oxidation of aniline in ethanol (50 vol%)-water mixture. The microscopy reveals that, at 20 wt% of PANI, the polymer is deposited on CNT, at 50 wt% of PANI, free PANI is found to accompany the nanotubes. The latter observation has been confirmed by the Raman spectroscopic mapping. The conductivity of composites of protonated PANI and CNT is practically independent of composition, 0-80 wt% CNT, and is 1-2 S cm−1. The conductivity of similar composites with a non-conducting PANI base increased from 10−7 S cm-1 to 100 S cm−1 as the CNT content increased, without any percolation transition. The content of nickel catalyst, responsible for the ferromagnetic behaviour of CNT, has been determined by thermogravimetric analysis as 28.5 wt% in the original CNT. Nickel, which is incorporated in the CNT, does not dissolve in the acid medium used for the preparation of PANI. Its presence manifests itself in the field dependence of magnetization of neat CNT and by the frequency dispersion of complex permeability of PANI-CNT composites with CNT concentration above 40 wt%. The high value of the coercivity of nickel-incorporated CNT as well as the character of magnetic spectra of CNT-rich composites with PANI can be explained by the cooperative effects in the magnetic system of strongly coupled ferromagnetic nanoparticles encapsulated in thin carbon shells.