CNT debundling

At first, an overview of the experimental methods which are suitable to characterize the CNT aggregation state in general, and thus are suitable to monitor CNT debundling in aqueous medium and in presence of surfactant, will be reviewed. Three main streams will be presented (i) "direct imaging of the CNT dispersions by m icroscopic techniques (ii] spectroscopic techniques, such as Raman spectroscopy, which exploit the difference in electronic properties between bundled and individualized CNTs and (iii] depolarized dynamic light scattering which is commonly used to characterize colloidal systems. 

Monitoring the debundling of the CNTs by UV-Vis spectroscopy enables one to determine when the CNT debundling process is finished, and thus, when the sonication can be stopped. 

It is worth mentioning that, independently of the CNT debundling state, it is not even clear whether removing all CNT impurities is necessarily beneficial with respectto their electrical properties. Grimes et al. reported that the presence of residual Fe catalyst inclusions in the core of the SWCNT produced by chemical vapor deposition enhances the average electrical conductivity by a factor 3.5, and that the purification of these CNTs results in a lowering of their conductivity. 

As for the covalent type, modification of CNTs by this approach has as its first goal to lead to debundling of the tubes, thus increasing their solubility and facilitating their manipulation. However, while the covalent method destroys the extended aromatic framework, noncovalent interactions preserve the original regular carbon network. This is important in those applications requiring use of the nanotubes without alteration of their electronic and optical properties, a process that normally occurs when the aromatic periodicity is disrupted. 

One of the potential ways how to improve CNT dispersion in polymer matrixes is in-situ polymerization of monomers in presence of nanotubes. Monomers have very small shear viscosity in orders of about lO -lO"3 Pa.s, compared to relatively high viscosity of polymer melts, 103-106 Pa.s. This low viscosity helps to better impregnation and wetting of CNT material, which leads to more efficient dispersion and debundling of the nanotubes aggregates, especially when ultrasound is used as a dispersing agent. 

CNTs in polymer-CNT composites are efficiently debundled and isotropically dispersed in polymer matrices, the efficient interaction between CNT and polymer provides good dispersion and a low percolation threshold, but only relatively low conductivity near and above percolation, frequently around 10 s cm is achieved at close to 2wt% CNT loading [70, 71], The polymer layer in the intemanotube connections is supposed to be the highest resistance section in the electrical pathway. This polymer layer is a barrier to efficient carrier transport between CNTs, and models for conductivity based on fluctuation-induced tunneling have been proposed [72]. A power law related to percolation theory can be used to model conductivity in the following form ... 

Use of surfactants is an effective way for dispersing CNTs [39]. Reports show that the outer most nanotubes in a bundle are treated more than the innermost tubes and the nanotube remains predominantly btmdled even after surfactant treatment. But mechanical methods like ultrasonication can debundle the nanotubes by steric or electrostatic repulsions [40]. On sonication the high local shear will unravel the outer carbon nanotubes in a bundle and expose other sites for additional surfactant adsorption, thus the surfactant molecules gradually exfoliate the bundle in an unzippering mechanism [41]. Some of the common surfactants used for the dispersion of carbon nanotubes are sodium dodecyl benzene sulfonate (SDBS) [42], dodecyl trimethyl ammonium bromide (DTAB) [43], hexadecyl trimethyl ammonium bromide (CTAB) [44], octylphenol ethoxylate (Triton X-100) [45] and sodium dodecyl sulfate (SDS) [46]. Covalent modification is another way to solubilize the CNTs in different solvents and to improve the interaction with the matrix in composites [47]. 

M. Soehn, S. Zils, N. Nicoloso, C. Roth, Effective debundling of CNTs and simultaneous symthesis of Pt nanopartides by Nation induced emulsions, J. Power Sources 196 (2011) 6079-6084. 

Since simple stirring of CNTs in a liquid is generally not sufficient to achieve the individualization of CNTs, bath or tip/horn sonicatlon is frequently applied in order to achieve the debundling of CNTs... 

Consequently, it allows a quick and simple determination of the optimal exposure time to ultrasound. Obtaining the optimum debundling of the CNTs, while minimizing as much as possible damage caused by sonication, by reducing the exposure time, are crucial issues — the sine qua non conditions to produce technically interesting C NT/polymer nanocomposites, among others. I n the same order of ideas, UV-Vis spectroscopy can also be used to quantitatively characterize the colloidal stability in time of CNT dispersions. ... 

Quasi one-dimensional confinement ofthe electronic and phonon states of the SWCNT 7i-electron system causes theCNTdensityof state to show very sharp and characteristic van Hove maxima at energies depending on the CNT diameters and chiralities. As a result, the UV-Vis spectrum of the SDS-CNT dispersions is a superposition of distinct electronic transitions, generated by a variety of SWCNTs with different diameters and chiralities. This is reflected in the separated absorption features (like little humps ) observed in the UV-Vis spectra of the SDS-SWCNT dispersions. These observed features become narrower and narrower as the debundling proceeds. This constitutes an additional indication that CNT individualization from the CNT ropes occurs. It has, indeed, been observed and proven that CNT bundling leads to broadening of spectral features of CNT dispersions. ... 

Nevertheless, these features could not be observed in the UV-Vis spectra of Carbolex CNTs, not even at the end of the debundling, whereas these van Hove singularities could be observed for the debundling of the same type of CNTs in sulfuric acid. Vaccarini et al. showed that the absorption peak width does not necessarily exclusively correlate with the level of debundling, and the interaction between the CNT and the dispersion medium can have a noticeable effect on the peak width. It can thus be conjectured that Carbolex and H iPCO CNTs might not have the same interaction with SDS molecules adsorbed on their walls. 

It is worth mentioning that the described concept used to monitor the debundling of CNTs, i.e., taking the absorbance value of the UV-Vis spectrum of a CNT dispersion as a direct measure for the concentration of individual exfoliated CNTs, can also be applied... 

With the simple and efficient UV-Vis tool to monitor the sonication-driven debundling of SWCNTs and MWCNTs, the optimum debundling conditions can be determined to produce stable surfactant-CNT dispersions that can be further stored and employed to make conductive nanocomposites. Finding trends in the general debundling behavior of several types of CNTs, as well as selecting the right power of sonication and the optimum CNT and surfactant concentrations, are dealt with in this section. 

Several aqueous SDS-CNT solutions/dispersions containing a fixed amount of bundled CNTs and different SDS concentrations were exfoliated until the maximum degree of debundling was observed with UV-Vis spectroscopy.i ... 

The surfaces of the potentially exfoliable CNTs are presumably completely covered with surfactant molecules. If that is the case, then one would expect a linear relation between the plateau of the UV absorbance and the SDS concentration till all CNTs are exfoliated, after which the absorbance should remain constant. Since SDS is inactive in the UV-Vis region, CNT dispersions having a different excess of surfactant should all exhibit the same absorbance, as soon as the maximum debundling is reached. This seems to be roughly the case, as shown in Figure 3.9. It is clear that the minimum SDS concentration necessary to stabilize all the CNTs that can potentially be exfoliated is about 0.17 wt% [or 5.88 mol SDS/m ater) for 0-1 wt% of CNTs. Consequently, the wt% ratio surfactant over CNT concentration at full coverage is about 1.7. 

Figure 3.9 UV-Vis absorbance at 272 nm, at the end of ultrasonication-induced debundling of HiPCO CNT dispersions containing different CNT concentrations as a function of the ratio of the SDS concentration and the SWCNT concentration.

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