Fyzikální ústav Akademie věd ČR

Existence of magnetic alignment in Carbon nanotubes (CNTs) has been theoretically predicted [1, 2], but the existence of localized magnetic moment on carbon atoms has not been experimentally observed yet. The main complication is the presence of residual metal catalyst, which is used in the preparation of CNTs, thus the removal of catalyst and subsequent approval of CNT purity is essential [3].

We have tested several methods of purification processes and focused on the analysis of the properties of residual metal catalyst. It has been demonstrated that commercially prepared (HiPco_raw) and purified (HiPco_SP) CNTs contain ~2 nm Fe₃C nanoparticles (NPs) with magnetically aligned core and paramagnetic shell [4].

We have demonstrated that efficiency of the purification of the SWCNTs can be evaluated by inspecting distribution of the NP magnetic moments, µ obtained from measurements of magnetization isotherms. We have shown that magnetic filtration (MF) of the SWCNTs sonicated in different solvents results in partial removal of the free NPs [5]. However, due to the NPs strongly attached to the SWCNTs, significant amount of the SWCNTs has been washed away together with the catalyst. MF hence does not seem as the best approach for complete removal of metal catalyst.

Annealing of the HiPco_raw sample at 400 °C and subsequent reflux in mild acid resulted into creation of ~10-18 nm α-Fe₂O₃ NPs, where only the largest ones persisted after final filtration of the sample. Annealing of the HiPco_raw at 1000 °C led to 10 % minimization of the catalyst content, remaining NPs have transformed to γ-Fe₂O₃ phase and their diameters decreased with respect to the original sample by minimization of the paramagnetic shell. CNTs annealed at 2200 °C exhibited diamagnetic response and undetectable amount of the metal catalyst on synchrotron powder diffraction measurement and thermogravimetry data, respectively. Measurement of the EXAFS (Extended X-ray absorption fine structure) spectra at the Fe-K_α edge unfortunately confirmed presence of little amount of catalyst in the form of α-Fe₂O₃ phase.

It has been demonstrated that the diamagnetic response of purified CNTs supported by other macroscopic methods is insufficient for confirmation of the sample purity and the presence of metal catalyst should be elided by the local sensitive probes such as EXAFS [5].

Fig. 1: (a) Illustration of CNTs with magnetic NPs before and after purification. (b) Left: Normalized EXAFS spectra with the depicted absorption of embedded Fe atom (black line) and Right: Magnitude of Fourier transform k_χ(k) with the lines representing the Fe-O (red dash dotted line), Fe-C (green long dashed line) and Fe-Fe first paths (blue short dashed line). for the sample annealed at 2200 °C. (c) Illustration of the purification processes described in the text.

References:
[1] R. Moradian, A. Fathalian, Nanotechnology 17 (2006) 183.
[2] W. Orelleana, P. Fuenthealba, Surf. Sci. 600 (2006) 4305.
[3] A. C. Dillon, T. Gennett, K. M. Jones, J. L. Alleman, P. A. Parilla, M. J. Heben, Adv. Matter. 11 (1999) 1354.
[4] B. Bittova, J. Poltierova Vejpravova, M. Kalbac, S. Burianova, A. Mantlikova, S. Danis, S. Doyle, Magnetic Properties of Iron Catalyst Particles in HiPco Single Wall Carbon Nanotubes, J. Phys. Chem. C 115 (2011) 17303 .
[5] B.Pacakova, Z.Kominkova, J. Vejpravova, A. Mantlikova, M. Kalbac, Analysis of metal catalyst content in magnetically filtered SWCNTs by SQUID magnetometry J. Mater. Sci. 50 (2015) 2544.
[6] B. Pacakova Bittova, M. Kalbac, S. Kubickova, A. Mantlikova, S. Mangold and J. Vejpravova, Structure and magnetic response of a residual metal catalyst in highly purified single walled carbon nanotubes , Phys. Chem. Chem. Phys. 15 (2013) 5992.