A novel method for the dissolution of carbon nanotubes in organic solvents

Olga Kleinerman, Chemical Engineering, Technion, Haifa, Israel
Lucy Liberman, Chemical Engineering, Technion, Haifa, Israel
Orr Zohar, Chemical Engineering, Technion, Haifa, Israel
Liron Issman, Tortech Nano Fibers, Ma'alot Tarshiha, Israel
Yeshayahu Talmon, Chemical Engineering, Technion, Haifa, Israel

In applications of carbon nanotubes (CNTs) such as fibers and films, the goal is to retain in the final product the electronic and mechanical properties of individual CNTs. This could be accomplished by different separation techniques, such as direct growth of CNTs on a substrate with large distances between the nanotubes to decrease van der Waals interactions, or use of liquid solvents, including chemical functionalization of the CNT sidewalls. However, aqueous dispersants with surfactants or other exfoliation agents and organic solvents are limited to very low CNT concentration. The true solvents for nanotubes are superacids, in particular chlorosulfonic acid (CSA), which dissolves long CNTs at relatively high concentrations (>10 wt. %). CNT arrangement in these thermodynamically stable solutions could be tuned by CNT concentration only. CNT network is formed in dilute solutions, while an aligned nematic phase is formed at higher CNT content. Although CSA is an excellent CNT solvent, it presents processing difficulties, as it reacts with water forming hydrochloric and sulfuric acid. Therefore, certain applications would benefit from less reactive solvents. Organic solvent can reduce processing issues related to corrosion and to the need to work in a water-free environment. In this work we developed a procedure for CSA replacement with an organic solvent, by reacting CNT/CSA solutions with 1,2 dichlorobenzene (DCB) to obtain CNT dispersions of up to 50 ppm, which are stable for several months. This method is applicable to ultra long CNTs, it is based on DCB chlorosulfonation by the superacid, and does not use destructive steps, such as ultrasonic sonication, which break up CNTs and even may reduce number of walls. Moreover, we examined several techniques to prepare homogeneous, thin CNT films from those organic dispersions, with tunable opto-electrical properties.