Carbolex

Carbon nanotubes (single-wall and multiwall) were purchased from Helix Material (Richardson, Texas), Carbolex (Broomall, PA), and Carbon Nanotechnologies Incorporation (CNI, Houston, Texas, now Unidym Inc). The carbon nanombes were used as received without any further purification or functionalization. Therefore, there are some impurities such as carbon black and metal in the samples. 

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... 

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. 

The general trend of the UV-Vis absorbance versus total energy curves obtained for aqueous SDS-Carbolex solutions (not shown) is very similar to the HiPCO curve, given in Figure 3.5 after an increase at the beginning of the sonicating process, the value of the absorbance at 250 nm reaches a plateau value of about 0.65... 

Figure 3.6 SEM micrograph of SDS-dispersed Carbolex SWCNTs after sonication for 15 min at 20 W (corresponding to a total energy input of ca. 17,000 J]. Note the "spider-web-like structure of the SWCNTs, which are interconnected by catalyst particles, certainly embedded in capsules of amorphous carbon. (Reproduced with permission from ref. 70. Copyright 2007 Elsevier Ltd.]...

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. ... 

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... 

For the range of SWCNT concentrations studied by Grossiord et al. (below 0.5 wt% for HiPCO CNTs and below 1 wt% for Carbolex... 

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... 

A different batch of SWCNTs, namely Carbolex SWCNTs, were used in the preparation of SWCNT/PS/PEDOT PSS composites.Carbolex SWCNTs are produced by arc-discharge technology. This technique is used to produce SWCNTs on a large scale but with the disadvantage... 

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. 

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 average SWCNT length for the Carbolex SWCNTs was examined using TEM. A TEM micrograph of SDS-stabilized Carbolex SWCNTs is shown in Figure 6.14. The metallic catalyst residues are easily seen. 

Composites with varying loadings of SDS- and PEDOT PSS-stabilized Carbolex SWCNTs were prepared. Percolation thresholds, along with the control system (PS/PEDOT PSS blend), were constructed and are given in Figure 6.15. 

The ultimate conductivity of the SWCNT/PEDOT PSS/PS composites is similar to that of the control blends (PEDOT PSS/ PS). It is important to note that the ultimate conductivity of HiPCO SWCNT/PS/PEDOT PSS composites is less than one order of magnitude higher than the corresponding conductivity of poor quality Carbolex SWCNT/PS/PEDOT PSS composites. The difference between HiPCO SWCNT/PS and Carbolex SWCNT/PS composites without the presence of PEDOT PSS, however, is approximately two... 

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