Carbon nanotube purification

Carbon nanotubes were first thought of as perfeet seamless eylindrieal graphene sheets —a defeet-free strueture. However, with time and as more studies have been undertaken, it is elear that nanotubes are not neeessarily that perfeet this issue is not simple bc-eause of a variety of seemingly eontradictory observations. The issue is further eomplicated by the faet that the quality of a nanotube sample depends very mueh on the type of maehine used to prepare it[l]. Although nanotubes have been available in large quantities sinee 1992[2], it is only recently that a purification method was found[3]. So, it is now possible to undertake various accurate property measurements of nanotubes. However, for those measurements to be meaningful, the presence and role of defeets must be elearly understood. 

Synthesis and Purification of Multi-Walled and Single-Walled Carbon Nanotubes... 

Purification, Opening, and Size Reduction of Carbon Nanotubes by Oxidative Treatments... 

Chiang, I.W., Brinson, B.E., Smalley, R.E., Margrave, J.L., and Hauge, R.H. (2001) Purification and characterization of single-wall carbon nanotubes./. Phys. Chem. B105, 1157-1161. 

Many chromatographic methods such as permeation chromatography, column chromatography, and size exclusion chromatography have been used to purify CNTs. The size exclusion chromatography (SEC) is the only carbon nanotube purification method in the literature that is not subjected to the acid treatments which tend to create the carboxylic functionality on CNTs. 

F. Valentini, A. Amine, S. Orlanducci, M.L. Terranova, and G. Palleschi, Carbon nanotube purification preparation and characterization of carbon nanotube paste electrodes. Anal. Chem. 75, 5413-5421 (2003). 

K.B. Shelimov, R.O. Esenaliev, A.G. Rinzler, C.B. Huffman, and R.E. Smalley, Purification of singlewall carbon nanotubes by ultrasonically assisted filtration. Chem. Phys. Lett. 282, 429—434 (1998). 

K. Shen, S. Curran, H. Xu, S. Rogelj, Y. Jiang, J. Dewald, and T. Pietrass, Single-walled carbon nanotube purification, pelletization, and surfactant-assisted dispersion a combined term and resonant micro-Raman spectroscopy study. J. Phys. Chem. B 109, 4455 1463 (2005). 

Depending on the synthesis procedure (see Section 1.4) and purification methods (Section 1.6.1), the structure of synthesized carbon nanotubes may include a range of defects (see Chapter 4). 

Carbon nanotubes comprise a very promising material for various applications and especially as an active component in composites and hybrids as will be documented in the other chapters of this book. Harnessing these nanoscopic assets in a macroscopic material would maximize CNTs potential and applicability. The choice of synthesis technique and purification method, which define size, type, properties, quality and purity of CNTs as well as their processability, is crucial for their implementation into composites and hybrids. 

Y. Chen, S. Mitra, Fast microwave-assisted purification functionalization and dispersion of multi-walled carbon nanotubes, Journal of nanosciences and Nanotechnology, vol. 8, pp. 5770-5775, 2008. 

Apart from the promising electrochemical properties that will be exhaustively discussed through this chapter, carbon nanotubes have become a hot research topic due to their outstanding electronic, mechanical, thermal, optical and chemical properties and their biocompatibility. Near- and long-term innovative applications can be foreseen including nanoelectronic and nanoelectromechanical devices, held emitters, probes, sensors and actuators as well as novel materials for mechanical reinforcement, fuel cells, batteries, energy storage, (bio)chemical separation, purification and catalysis [20]. 

In order to guarantee an efficient performance of the CNT-based electrochemical devices, attention has to be paid not only to CNT synthesis and purification but also to the way that the CNT electrode is built up. There have been many studies in the literature dealing with CNT dispersions either on conducting substrates or forming composites. In this subsection we will address the different carbon-nanotube deposition techniques and carbon-nanotube arrangements on different electrode surfaces. 

Pillai SK, Ray SS, Moodley M (2007) Purification of single-walled carbon nanotubes. J Nanosci Nanotechnol 7 3011-3047... 

Matlhoko L, Pillai SK, Moodley M et al (2009) A comparison of purification procedures for multi-walled carbon nanotubes produced by chemical vapor deposition. J Nanosci Nanotechnol 9 5431-5435... 

Huang X, Mclean RS, Zheng M (2005) High-resolution length sorting and purification of DNA-wrapped carbon nanotubes by size-exclusion chromatography. Anal Chem 77 6225-6228... 

This is mainly due to their laborious purification procedures and their required chemical modification for solubilization. Only recently, Prato et al. reported the electrochemistry of carbon nanotubes functionalized using the 1,3-dipolar cycloaddition reaction.120 The cyclic voltammogram obtained is shown in Fig. 8.9. 

Figure 4. Raman spectrum of multi-walled carbon nanotubes after purification recorded for kexc. = 676.4 nm [22], The low frequency modes have been calculated as explained in the text.

Noncovalent functional strategies to modify the outer surface of CNTs in order to preserve the sp2 network of carbon nanotubes are attractive and represent an effective alternative for sidewall functionalization. Some molecules, including small gas molecules [195], anthracene derivatives [196-198] and polymer molecules [118, 199], have been found liable to absorb to or wrap around CNTs. Nanotubes can be transferred to the aqueous phase through noncovalent functionalization of surface-active molecules such as SDS or benzylalkonium chloride for purification [200-202]. With the surfactant Triton X-100 [203], the surfaces of the CNTs were changed from hydrophobic to hydrophilic, thus allowing the hydrophilic surface of the conjugate to interact with the hydrophilic surface of biliverdin reductase to create a water-soluble complex of the immobilized enzyme [203]. 

These examples of functionalization of carbon nanotubes demonstrate that the chemistry of this new class of molecules represents a promising field within nanochemistry. Functionalization provides for the potential for the manipulation of their unique properties, which can be tuned and coupled with those of other classes of materials. The surface chemistry of SWCNTs allows for dispersibility, purification, solubilization, biocompatibility and separation of these nanostructures. Additionally, derivatization allows for site-selective nanochemistry applications such as self-assembly, shows potential as catalytic supports, biological transport vesicles, demonstrates novel charge-transfer properties and allows the construction of functional nanoarchitectures, nanocomposites and nanocircuits. 

In 2007, a report on single-walled carbon nanotubes (SWCNTs) described the reaction with singlet oxygen [66]. Purification was simplified due to the contrasting solubility properties of unfunctionalized and oxyfunctionalized SWCNTs, although it was uncertain whether the SWCNT peroxides were formed from a [2 + 2]-cycload-dition, a [2 + 4]-cydoaddition, or both [66]. Subsequent reports found SWCNTs to be of very low chemical reactivity with 102 [67, 68], Rather, SWCNTs were proposed to physically quench singlet oxygen (to convert 02 to 302) with rate constants on the order of 108 M 1 s 3. 

Lobach A.S., Spicyna N.G., Terhov S.V., Obrazthova E.D. Comparative study of different purification methods of single-walled carbon nanotubes. Sol. St. Phys. 2002 44 457-459. 

The carbon nanotubes up to 10-15 nm in diameter have been produced by the graphite evaporation in water. The resulting structures produced in water do not contain catalysts. This simplifies the process of their purification and reduces the net cost (Fig. 5). Varying the regime of synthesis one can produce both tubular and ribbon structures. 

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