Diameter selection of single-walled carbon nanotubes through programmable solvation in binary sulfonic acid mixtures

Pristine carbon nanotubes (CNTs) dissolve as polycarbocations in superacids through direct protonation. The solvating power of a superacid is determined by the stability of the conjugate base anion that competes with the CNTs for the dissociated proton. We have demonstrated that this equilibrium can be controlled in a predictable fashion, thus rendering the solvating power of a superacid system tunable. In this article, we show that the solvating power of chlorosulfonic acid can be changed in a desired fashion by forming binary mixtures in different proportions with a non-superacid such as methane sulfonic acid. Thus, the successive extraction of carbon nanotubes with binary acid mixtures of increasing solvating power leads to the differentiation of CNTs by their molecular geometry. We show that solvation by direct protonation is sensitive to the geometric strain at the carbon atom and, hence, to the nanotube diameter. In this respect, the direct protonation method is distinct from surfactant-based or electrical-field-based methods that distinguish metallic CNTs from semiconducting types mainly on the existence of finite density of states or not at the Fermi level. We have employed solid-state Raman spectroscopic analysis of the CNT radial breathing modes and UV-vis absorption spectroscopy and a systematic mapping method to support our conclusions. We believe the concept demonstrated in this paper holds the potential to be developed into a chemical tool kit useful in the scaleable separation of CNTs by their (n, m) types, thus paving the way for molecular carbon nanotechnology.