As-produced CNTs

Variation of the content of impurities in the different CNT preparations [21] offers additional challenges in the accurate and consistent assessment of CNT toxicity. As-produced CNTs generally contain high amounts of catalytic metal particles, such as iron and nickel, used as precursors in their synthesis. The cytotoxicity of high concentrations of these metals is well known [35, 36], mainly due to oxidative stress and induction of inflammatory processes generated by catalytic reactions at the metal particle surface [37]. Another very important contaminant is amorphous carbon, which exhibits comparable biological effects to carbon black or relevant ambient air particles. 

Several alternatives have been proposed to purify the as-produced CNTs. Mixtures of HN03/H2S0.J are most effective in removing amorphous carbon [29], while HCl treatment together with sonication is more useful to remove catalytic NPs [30]. These impurities, particularly the metallic NPs, are probably responsible for the electrocatalysis seen at some CNT-modified electrodes [31]. On the other hand, extensive exposure to oxidant solutions generates cut ends and... 

As-produced CNTs are intrinsically inert and uncharged. In addition, they are often aggregated or entangled, and may contain impurities such as amorphous carbon or catalytic metal particles. A post-synthesis treatment is almost always required to purify and disperse the CNTs in a suitable solvent. 

These two experimental observations point out that the purified CNTs are more bundled at the end of the exfoliation than the as-produced CNTs. This was confirmed by SEM images of the purified HiPCO SWCNTs of the high quality batch, which showed that these... 

Preparation methods for PCNTs have been reviewed in the context of parameters which may lead to large-scale MWCNT synthesis free of by-products. It is noteworthy that the formation of aligned CNTs is currently an active area of research in conjunction with PCNT preparation. The use of SWCNTs and/or MWCNTs in electronic devices are being developed. As yet it has not proved possible to produce CNTs with diameters and helicities to order. The formation of SWCNTs by the PCNT process has not yet been reported and it is of interest to examine whether this process can be used to prepare them. 

The preparation of CNTs is a prerequisite step for the further study and application of CNTs. Considerable efforts have been made to synthesize high quality CNTs since then-discovery in 1991. Numerous methods have been developed for the preparation of CNTs such as arc discharge, laser vaporization, pyrolysis, and plasma-enhanced or thermal chemical vapor deposition (CVD). Among these methods, arc discharge, laser vaporization, and chemical vapor deposition are the main techniques used to produce CNTs. 

The advantages of this method include the simple experimental setup as well as the possibility to produce CNTs in relatively large quantities. However, it typically produces relatively short CNTs with a wide range of diameters as well as low purity, often producing fullerenes, graphite sheets or amorphous carbon as side products [82]. 

Chapelle et al. 1999(54) Stephan et al. 2000 (55) SWCNT Arc- discharge As-produced Spin casting CNT loading levels 1 to20wt% Raman spectroscopy was used to characterize interaction between SWCNT and PMMA The introduction of PMMA into bundles increases the distance between nanotubes and the interactions between themselves decrease and bundles are destroyed for low concentrations (<2.5 wt%). For higher concentrations (10 wt%) the presence of a high quantity of nanotubes does not allow intercalation by PMMA ... 

Although much progress has been made in both synthesis and purification of carbon nanomaterials, commercial samples still contain nanostrucmres of different size, shape, and composition. As-produced carbon nanomaterials are frequently composed of mixtures of CNTs, fullerenes, carbon onions, amorphous carbon and graphite, which are structurally different and possess different reactivity. Since the oxidation kinetics are closely related to structural features, reaction rates and activation energies are expected to differ for the distinct carbon forms, which is an important issue for oxidation-based purification or surface functionalization. In analogy to graphite [3-6], oxidation of a carbon nanostmcture [7-9] can be described by a first-order reaction, with respect to the carbon component. 

Current synthesis techniques are usually unable to provide large quantities of pure CNTs with well-defined physical and chemical properties [28]. The as-produced material is typically a mixture of different types of CNTs, amorphous carbon, catalyst particles, and defective or damaged tubes, all of which may impair potential applications. Another major challenge for a large number of applications rises from the strong tendency of CNTs to agglomerate and arrange in bundles. 

A crucial problem connected to carbon nanotube synthesis on supported catalysts on an industrial scale is the purification step required to remove the support and possibly the catalyst from the final material. To avoid this costly operation, the use of CNT- or CNF-supported catalysts to produce CNTs or CNFs has been investigated. Although most catalytic systems are based on nickel supported on CNFs (see Table 9.4), the use of MWCNTs [305,306] or SWCNTs [307] as supports has also been reported. Nickel, iron [304,308-310], and bimetallic Fe-Mo [305] and Ni-Pd [295] catalysts have been used. Compared to the starting CNTs or CNFs, the hybrid materials produced present higher specific surface area [297,308] or improved field emission characteristics [309]. 

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