Synthesis of CNTs

Even though an overwhelming number of CNT applications is currently being pursued, there is a long list of unsolved fundamental issues in the carbon-nanotube 

The growth direction of nanotubes can be controlled by the gas flow or by applying electric fields (plasma-enhanced CVD) [67]. Controlling CNTgrowth with CVD yields more organized CNT that can be readily integrated into addressable structures for fundamental characterization and potential applications. 

As a result of CNT synthesis, catalyst metal nanoparticles (iron, cobalt, nickel) together with amorphous carbon and fullerenes are unavoidably present in the CNT soot. 

An important innovation in the synthesis of CNTs is the use of organometalhc precursors which provide the source of metal as well as of carbon. 

Due to their unique properties, a large number of groups all over the world are working in the area of CNTs for diverse applications and the most important are development and fabrication of sensors and thermal protection for atmospheric re-entry of space vehicles. Gas and glucose sensors based on CNTs have been developed. Some American researchers have recently claimed that they have developed CNT-based sensors for the detection of chemical warfare agents [80]. 

Until recently, the exorbitant prices of CNTs meant that they were only used in limited quantities mainly in applications in high-tech fields such as the optics/electronics fields. We have reported in our previous paper (12) that CNTs cost between 5 to 800 /g depending on their type (SWNT/MWNT), synthesis process (Arc/CVD), diameter, purity and defect density and stressed that to continue to encourage wider applications of CNTs, efficient novel processing routes, which could be scaled up for commercial production, should be explored. In fact, compared to three years ago, new CNT suppliers have appeared in the market offering CNTs at much lower prices than was previously the case. Table 15.1 lists some suppliers of MWNTs and SWNTs and the 2009 prices and specifications of their products (13-18). Prices given are based on kg quantities, 

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Laser-ablation method shown in Fig. 3 was used when Cgo was first discovered in 1985 [15]. This method has also been applied for the synthesis of CNT, but length of MWCNT is much shorter than that by arc-discharge method [17]. Therefore, this method does not seem adequate to the synthesis of MWCNT. However, in the synthesis of SWCNT described later (Sec. 3.1.2), marvelously high yield has been obtained by this method. Hence, laser-ablation method has become another important technology in this respect. 

Covalent linkages through the carboxylic groups introduced during oxidation can anchor metal complexes (Fig. 3.17) to enable the synthesis of CNT-inorganic hybrids for applications in nanoelectronics [96]. 

Li, X., et al., Atomic layer deposition ofZnO on multi-walled carbon nanotubes and its use for synthesis of CNT-ZnO heterostructures. Nanoscale Research Letters, 2010. 5(11) p.1836-1840. 

The third way to prepare CNT-ceramic composite powders is via the synthesis of CNT by a CCVD process, in situ in the ceramic powder. A ceramic powder which contains catalytic metal particles at a nanometric size, appropriate to the formation of CNTs, is treated at a high temperature (600-1100°C), in an atmosphere containing a hydrocarbon or CO. In the method reported in 1997 by the present authors,27 iron nanoparticles are generated in the reactor itself, at a high temperature (>800°C), by the selective reduction in H2/CH4 (18% CH4) of an a-Al203 based oxide solid solution ... 

CVD is an established method for the synthesis of CNTs [59, 60]. Supported transition metals that catalyze the growth of CNTs, such as iron, nickel, or cobalt, are situated in a tubular reactor, and CNTs are grown at elevated temperature on the surface of the catalyst particles by decomposition of a carbon-containing precursor. The catalyst particles have to be removed by chemical treatment /washing in order to obtain a metal-free final product. 

Figure 2.2. Schematic of synthesis of CNT polycarbonate nanocomposites by solution mixing approach. Reproduced from reference 19 with permission from American Chemical Society.

The synthesis of CNTs is reeeiving considerable interest and the main goal is to obtain large scale produetion of highly pure CNTs. There are three basic methods for synthesis of SWCNTs and MWCNTs eleetrical arch discharge, laser ablation (laser vaporization) and ehemical vapor deposition (CVD) (or catalytic decomposition of hydrocarbons) [1,7, 9,10, 25,26],... 

Consequently, CVD is now the method-of-choice for the synthesis of CNTs. As discussed in Chapter 4, these methods consist of the decomposition (typically thermal) of a hydrocarbon precursor on the surface of catalytic metal nanostructures. Methane and acetylene have been used most extensively as precursors other alternatives now include CO, C2H4, and methanol/ethanol. As with any CVD approach, this method is easily scaleable, and is used to generate kilogram quantities of CNTs for an ever-increasing laundry list of applications. 

The synthesis of CNTs was realized in the tubular type quartz reactor [13] on the surface of Si/Si02 substrates at 870°C. Argon flow rate was 100 cmVmin. After 1 min period of the process the reactor was cooled up to room temperature. A series of experiments was carried out with the variation of the ferrocene percentage in the feeding solution (1.0% and 10%) injected into the Ar flow. 

Details of the synthesis of cnt-clividine [ent-52 shown in Scheme 7 have now been published (L. V. White, B. D. Schwartz, M. G. Banwell and A. C. Willis,... 

Synthesis of CNTs Arc Discharge, Laser Ablation, Chemicai Vapor Deposition... 

Other procedures for the synthesis of CNTs use a gas phase for introducing the catalyst, in which both the catalyst and the hydrocarbon gas are fed into a furnace, followed by a catalytic reaction in the gas phase. The method is suitable for large-scale synthesis, because nanotubes are free from catalytic supports and the reaction can be operated continuously. A high-pressure carbon monoxide reaction method, in which the CO gas reacts with iron pentacarbonyl to form SWNTs, has been developed [38]. SWNTs have been synthesized from a mixture of benzene and ferrocene in a hydrogen gas flow [55]. In both methods, catalyst nanoparticles are formed through thermal decomposition of organometallic compounds, such as iron pentacarbonyl and ferrocene. 

Three additional approaches to the synthesis of CNT are shown in Fig. 9. The first [77, 78] involves transformation of 3 -ketonucleoside 44 into cyanohydrin 45, followed by deoxygenation at 3 and then at 2 to afford 47, which is easily epimerized to 31 by treatment with base (pH = 9). Treatment of 2 -0-acyl derivatives of 46 with bases produced elimination to give the 2, 3 -unsaturated nucleoside 48. An important factor in the chemistry of these cyanonucleosides is the acidity of the H-3 hydrogen atom in a-position to the nitrile group. This acidity is responsible for the facile epimerization 47 31 and for the elimination... 

The second approach to the synthesis of CNT 31 [81] involves opening of the 2, 3 -anhydro-P-D-lyxonucleoside 50 with lithium cyanide to give stereo-selectively 51 (R = Trityl). Attempts to deoxygenate the 2 -OH group by reaction with Ar,AT-thiocarbonyldiimidazole gave the a,P-unsaturated nitrile 48 (R = trityl). Reduction of the double bond of 48 (R = trityl) with triethylsilane and tris(triphenylphosphine)rhodium (I) chloride gave a mixture of the two... 

The third approach involves a short, elegant, free radical reaction of the 3 -deoxy-3 -iodo-5 -0-tritylthymidine 49 with hexamethylditin, t-butylisonitrile and azobisisobutyronitrile to give stereoselectively CNT 31. This synthetic pathway allows the stereocontrolled synthesis of CNT 31 in four steps and 22% overall yield from thymidine [79]. 

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