HiPCO SWCNT dispersions

The same Carbolex and HiPCO SWCNT dispersions as those previously studied in Section 3.1.2 were examined with UV-Vis spectroscopy. Samples were taken regularly during the sonicating process, diluted and UV-Vis spectra were recorded. Since desorption processes are typically quite slow, it was assumed that the amount of SDS molecules adsorbed on the CNT walls was not significantly influenced by dilution, and UV-Vis spectra were typically immediately recorded after dilution. " Please note that the dilution factor — in other words, the CNT concentration after dilution [i.e., 6.7 x 10 wt% for the standard SWCNT dispersions) — was chosen in such a way that all the UV-Vis absorbance values remained below 1 so that the error inherently present in the measurement itself is reduced. At this dilution, the contribution of scattering can be ignored. " ... 

The UV-Vis spectra recorded for aqueous HiPCO SWCNT dispersions, obtained after different energy-inputs and sonication times, are given in Figure 3.4. The corresponding spectra for Carbolex SWCNTs show a similar development, but exhibit one maximum instead of two around 250-300 nm. This difference indicates that the UV-Vis spectra obtained are specific for the CNT type studied. This is not surprising since the Carbolex and HiPCO CNTs studied... 

The nature and the quantity of impurities removed are strongly connected to the surfactant type, as well as to the CNT type and batch (notably their density and the homogeneity of the batch). In order to Illustrate this last point, SDS-Carbolex SWCNT and SDS-HiPCO SWCNT dispersions were exfoliated and subsequently centrifuged at... 

Addition of carbanions - syntheses of tert-butyl-H-SWCNTs and of tert-butyl-SWCNTs In a nitrogen-purged flask, equipped with a gas inlet and a pressure compensator, 20 mg of HiPCO tubes (1.7 mmol of carbon) was dispersed in 50 mL of anhydrous benzene. To this dispersion 2.5 mL of a 1.7 M solution of tert-butyllithium (4.25 mmol) in hexane were added dropwise over a period of 10 min. Subsequently, the suspension was additionally stirred for 1 h at room temperature and the SWCNT dispersion turned into a black homogeneous solution. The solution was stirred for a further 1 h and subsequently quenched by the addition of... 

Surface resistivity performance tests were done for the HiPco SWCNT that was dispersed with blocky VA/AA material at 1 1 VA/AA/HiPco wt/wt ratio. Results indicate that proper dispersion maintained electrical properties of HiPco, because surface resistivities range from 2.17 to 3.50 k Ohms/sq from HiPco loadings of 0.059-0.557 mg/cm (Caneba and Axland, 2004). Note that electrically conductive surfaces require surface resistivities below 100 k-Ohm/sq. 

It appears that all the CNT dispersions [of SDS-HiPCO SWCNTs, SDS-Carbolex SWCNTs, and SDS-MWCNTs (thin MWCNTs of the batches MWA P041206 and 060213 provided by Nanocyl SA]] presented in this chapter are stable for at least several months, if not years, since the value of the UV-Vis absorbance of these dispersions remained virtually constant during this period. ... 

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.

For Carbolex SWCNTs, it was found that the critical SDS concentration is about 0.06 wt% for a 0.1 wt% Carbolex SWCNT dispersion. For a similar 0.1 wt% HiPCO dispersion, the critical SDS concentration is around 0.17 wt% (see Figure 3.8]. The somewhat higher critical SDS concentration for HiPCO is most probably related to the higher purity of these tubes. It might also be due to the... 

Figure 3.13 Evolution of the value of the absorbance of the peak located at 272 nm for an aqueous 0.1 wt% HiPCO CNT dispersion, containing 1 wt% of SDS. The SWCNTs for which the exfoliation behavior is monitored here come from a lower quality batch, and are exfoliated either before ( ) or after purification (O).

Figure 3.15 UV-Vis spectra of SDS-HiPCO SWCNT aqueous dispersions before and after ultracentrifugation. Notice the broadening of the absorption features due to the presence of small CNT bundles in the non-centrifuged sample.

The exfoliation of Carbolex SWCNTs was performed with both SDS and PEDOT PSS. The UV-Vis absorption spectra revealed a profile from which it was apparent that the final absorbance level reached was similar for dispersions prepared with SDS and PEDOT PSS, unlike that previously seen for HiPCO SWCNTs.This suggests that the electronic interaction between the Carbolex SWCNTs and PEDOT PSS is different from that observed for HiPCO SWNCTs. Examining their respective absorption spectra, as well as using Raman spectroscopy one can examine the difference in electronic structure of the SWCNTs. 

Basic studies on diazonium-CNT chemistry led to two very efficient techniques for SWCNT derivatization solvent-free functionalization [176] and functionalization of individual (unbundled) nanotubes [175], With the solvent-free functionalization (Scheme 1.26), heavily functionalized and soluble material is obtained and the nanotubes disperse in polymer more efficiently than pristine SWCNTs [176], With the second method, aryldiazonium salts react efficiently with the individual (unbundled) HiPCO produced and sodium dodecyl sulfate (SDS)-coated SWCNTs in water. The resulting functionalized tubes (one addend in nine tube carbons) remained unbundled throughout their entire lengths and were incapable of reroping. [175],... 

Surfactants — either anionic surfactants such as sodium dodecylsulfate [SDS], or sodium dodecyl benzene sulfate [SDBS], or polysaccharide [Gum Arabic GA] — were first used to disperse, and exfoliate as-produced SWCNTs in water by ultrasonication, and to stabilize the resulting aqueous CNT suspension, see Figure 2.12. The SWCNTs were synthesized by either the AD method [about 30 % of impurities], or by the HiPCO process [having a catalyst particle content of about 5 wt%]. Please note that not only short surfactant molecules, but also polymeric surfactants such as polystyrene sulfonate, or even conductive polymers having a surfactant nature, can also be successfully used to disperse CNTs in water. 

Raman spectroscopy is often used to assess the quality of CNTs. -A typical Raman spectrum of a CNT mat or dispersion shows a few characteristic bands. These include the "graphene-like or G-band observed between 1,500 and 1,600 cm , the "disorder-induced or D-band observed at 1,300 cm , and the radial breathing mode [RBM] observed between 100 and 500 cm . A comparison between the G-and D-bands from two samples has been reported to indicate their relative purity.Raman spectra taken for SDS-stabilized dispersions of HiPCO and Carbolex SWCNTs are given in Figure 6.13. 

Figure 6.13 Raman spectra of aqueous dispersions of HiPCO and Carbolex SWCNTs. The inset is an expansion of the G-band. (Reprinted with permission from ACS Publishing).

To further probe the SWCNTs purity, their intrinsic conductivities were measured by preparing buckypapers from the respective dispersions. Conductivities of buckypapers prepared from HiPCO and Carbolex SWCNTs were approximately 1 x 10 and 1 x 10 S m, respectively. The significant difference in intrinsic conductivities indicates that there is certainly a difference in the electronic properties of the SWCNTs. The lower conductivity of the Carbolex SWCNTs could be attributed to more wall defects, a larger fraction of semi-conducting SWCNTs or a carbonaceous coating on the SWCNT wall. To determine the main cause of these lower conductivities would require further investigation. For the application intended here, this difference is of utmost importance. One further prerequisite is that the aspect ratio of the two SWCNT batches is not vastly different. 

The SWCNTs were dispersed in DMF at a concentration of 0.5 g L by bath sonication for 40 min at 40—50 °C. In a typical procedure, 20 mL of this dispersion was filtered and washed on the filter with 10 mL of methanol and 10 mL of water. The solid was transferred into 20 mL of 8 M HNO3 in a glass vial, and the mixture was sonicated in a bath at 40—50 °C for a time ranging firom 30 min to 5 h. The mixture was diluted 1 2 with deionized water and filtered using a 0.45 pm PTFE filter. The solid was washed on the filter with water until the filtrate was neutral, washed with 10 mL of methanol, washed with 10 mL of DMF, and redispersed in 20 mL of DMF by bath sonication for 60 min. The filtrates from CoMoCat and HiPco nanotubes were colorless, but the filtrate from PLV nanotubes was Hght gray. AFM analysis of the PLV filtrate indicated the presence of nanotubes (Tchoul et al., 2007). 

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