The Effect of CNTs Purification on their Parameters, Dispersion, and Phase Behavior in Superacid Solutions

On the molecular level, carbon nanotubes (CNTs) exhibit excellent mechanical strength, thermal and electrical conductivity properties, combined with low density. This unique feature combination makes CNTs ideal candidates for processing multifunctional macroscopic fibers. However, translating the unique characteristics of a single molecule into macroscopic material is challenging.

The most significant limitation that prevents wide-scale use of CNTs in applications is the difficulty of dispersing CNTs to form thermodynamically stable solution, free of aggregates. Not that long ago, it was discovered that CNTs spontaneously dissolve in chlorosulfonic acid (CSA), forming a molecular solution, namely, the nanotubes are dispersed as individual molecules, and at higher concentrations form a lyotropic liquid crystalline nematic phase. Alignment of CNTs in a solution is essential to achieve the best properties in a CNT-based macroscopic material through liquid-phase processing.

The purification process of CNTs is the first essential step to remove metal catalyst impurities and non-nanotube carbon materials from raw CNTs, prior to CNTs dissolution in CSA. The purification is done by liquid-phase oxidation using H₂O₂, followed by HCl treatment. H₂O₂ attacks the amorphous carbon, and exposes the metal catalysts, and HCl dissolves the metal nanoparticles. We show here that the process determines the CNTs aspect ratio, diameter, and the degree of purity. These parameters dictate CNTs solubility in acid, their ability to form liquid crystalline phase, and processing into high quality fibers. We analyzed purified CNTs vs. raw CNTs with high-resolution SEM, cryogenic electron microscopy, scanning transmission electron microscopy, and high-resolution TEM.