Single-walled carbon nanotubes in superacid: X-ray and calorimetric evidence for partly ordered ${\mathrm{H}}_2{\mathrm{SO}}_4$

Liquid anhydrous sulfuric acid forms a partly ordered structure in the presence of single-walled carbon nanotubes (SWNTs). X-ray scattering from aligned fibers immersed in acid shows the formation of molecular shells wrapped around SWNTs. Differential scanning calorimetry of SWNT-acid suspensions exhibits concentration-dependent supercooling/melting behavior, confirming that the partly ordered molecules are a new phase. We propose that charge transfer between nanotube $\pi$ electrons and highly oxidizing superacid is responsible for the unique partly ordered structure.

Figure 1

(Color online) X-ray scattering data. (a) Integrated scattering profile (empty circles) of dry annealed SWNT fibers. Model calculations (dotted line) assuming a mean diameter of 11.2 Å reproduce the Bragg peaks very well. Inset is the 2D detector false-color image. (b) Same for swollen fiber in acid (solid squares) and pure acid (empty circles). Scattering from acid is enhanced and anisotropic in the presence of fibers, and SWNT ropes are no longer detectable. (c) Azimuthal intensity variations of the (10) rope Bragg peak at $Q=0.5{\mathrm{\AA }}^{-1}$ from dry fiber (top), and of the first order diffuse scattering at $Q=1.5{\mathrm{\AA }}^{-1}$ from acid in which the swollen fiber is immersed (bottom); the mosaic FWHM’s are the same $(\sim 32°)$ within experimental error.

Figure 2

Schematic showing the cross section of a SWNT or a rope wrapped in cylindrical shells of sulfuric acid.

Figure 3

Differential scanning calorimetry data. Left, SWNT dispersion in sulfuric acid. Right, benzene (top) or water (bottom) with and without SWNTs. Enthalpies of the phase transition are normalized to “$\mathrm{J}∕\text{gram}$ of solvent.” SWNTs dramatically affect the phase transition of sulfuric acid, suggesting the presence of a acid phase associated with SWNT. In contrast, SWNT have no significant effect on the phase transitions of benzene or water.

Figure 4

(Color online) X-ray scattering data from SWNT fibers in benzene (a) or water (b). Insets are 2D detector false-color images. Azimuthal integrations of the 2D data 60° wide in $\chi$ centered at 90° (I, empty squares) and 180° (II, empty circles) are shown. The difference (I–II, solid circles), which would expose an ordered structure of solvent wrapped around SWNT, shows only Bragg peaks from SWNT ropes. The absence of excess solvent scattering proves the absence of ordered solvent phases associated with SWNT.