Dissolution of Pristine Single Walled Carbon Nanotubes in Superacids by Direct Protonation

Abstract

Strong tube-tube van der Waal attractions that hinder the dissolution of single walled carbon nanotubes (SWNTs) as individuals have been overcome by the direct protonation of the tubes by superacids. The pristine nanotubes disperse as individuals in 100% H 2 SO 4 , oleum, trifluromethanesulfonic acid, and chlorosulfonic acid to varying solubility levels. Optically homogeneous solutions with concentration as high as 45 g/L (2.5 wt %) at room temperature have been observed in the case of chlorosulfonic acid, the strongest among the series of superacids investigated. A dissolution model is proposed wherein the solubilized tubes exist as protonated polycarbocations, charge balanced by corresponding conjugate base anions. The removal of electrons from the SWNT, leading to a fractional positive charge on the carbons in the protonated state, is reversible upon deprotonation. With increasing concentration of the solubilized nanotubes, anion-mediated attractions at lower concentrations and spatial constraints at high concentrations guide the transformation of the SWNTacid system into a nematic mesophase. Highly anisotropic, nematic mesophases formed in the dissolution process are promising precursors for the fabrication of macroscopic forms of SWNTs with high alignment of the carbon nanotubes.

Single-walled carbon nanotubes in superacid: X-ray and calorimetric evidence for partly orderedH2SO4

“…% excess SO 3 ͒ and the solute is single-walled carbon nanotubes ͑SWNTs͒. 3 We show that H 2 SO 4 molecules readily intercalate SWNT ropes, 4 forming a partly ordered solvent structure wrapped around the tubes and tube ropes. Moreover, the solvent molecules associated with the nanotube solute exhibit different phase behavior compared to pure acid.…”

“…Swollen fibers were prepared as above by adding anhydrous sulfuric acid to the capillary. The density of dry fiber is ϳ1.1 g / cm 3 and thus contains ϳ30 vol. % voids 5 since the ideal density of an ͑8, 8͒ nanotube crystal is ϳ1.5 g / cm 3 ͓we take ͑8, 8͒ as representative of the ϳ11 Å mean diameter of HiPco tubes͔.…”

“…The density of dry fiber is ϳ1.1 g / cm 3 and thus contains ϳ30 vol. % voids 5 since the ideal density of an ͑8, 8͒ nanotube crystal is ϳ1.5 g / cm 3 ͓we take ͑8, 8͒ as representative of the ϳ11 Å mean diameter of HiPco tubes͔. These dry fibers immersed in sulfuric acid swell by 30%-60% in diameter upon reimmersion.…”

Liquid‐Phase Exfoliation of Nanotubes and Graphene

“…Various methods to provide this repulsive potential have been explored. Nanotubes have been dispersed and stabilised with the aid of specific solvents [16][17][18][19][20][21][22][23][24] , acids [25,26] , macromolecules [27][28][29][30] and surfactants [31][32][33][34] as well as through covalent functionalisation strategies [35,36] . Such systems have been characterised by a range of techniques such as atomic force microscopy [37] , infra-red photoluminescence and absorbance spectroscopy [34] , viscometry [25] , small angle neutron scattering [32] to name but a few.…”

Raman studies of suspensions and solutions of singlewall carbon nanotubes

“…An alternative and popular route is the preparation of aqueous supensions with the help of surfactants [2]. On the other hand, a new route was recently explored to prepare solutions of nanotubes, via chemical protonation [3] or chemical reduction in polar organic solvents [4]. Raman spectroscopy is the most popular technique to characterize samples of single wall carbon nanotubes (SWNT), because it provides reliable informations not only on their vibrational properties, but also on their structural and electronic properties [5,6].…”