Synthesis and Purification of Carbon Nanotubes

Crude MWCNTs and SWCNTs were produced by catalytic decomposition of acetylene on Co-Fe/Al(OH)3 catalysts. The CNTs contained used catalysts and other by-products. The catalysts and support contents of the CNTs samples are shown in Table 7.3. Crude MWCNTs contain Co, Fe, and alumina. Purified MWCNTs were synthesized from crude MWCNTs by dissolution of the catalyst support in concentrated NaOH, dissolution of the metal catalyst in concentrated HCl, drying at 120°C in an air oven and additional drying at 500°C under vacuum. Crude SWCNTs contain Co and MgO. Purified SWCNTs were synthesized from crude SWCNTs by dissolution of the catalyst support in concentrated HCl, purification by air oxidation at 300°C and then drying at 120°C in an air oven. 

---

Selective heating has been used in the synthesis and purification of carbon nanotubes (CNT). For example, Lee [31a] described the preparation of carbon nanotubes in a microwave oven (Fig. 5.10). The support material (carbon black, a microwave absorber, or Si02, a microwave insulator) was placed in a quartz reactor with a flow of acetylene (C2H2) gas without a catalyst and was irradiated by microwaves. Identical experiments were repeated with carbon black loaded with Co, Ni, and Fe catalysts. 

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

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. 

In the section devoted to CNTs, some details concerning synthesis and purification methods, as well as separation techniques for metalfic and semiconducting nanotubes will be reviewed. Some aspects concerning the interactions of CNTs with reactants used in the synthesis of different composites based on conducting polymers, such as PANI, PPy, PEDOT, PBTh, PNVK, PPV, and polyfluorene (PF) will be discussed in the section devoted to the synthesis of the CP/CNT composites. Preparing a composite with the desired properties requires knowledge of the interaction between the host matrix and the guest carbon nanoparticles. 

The outstanding electrical, thermal, mechanical and chemical properties of carbon nanotubes (CNTs), especially single-walled carbon nanotubes (SWNT), have interested researchers aU over the world. One of the major problems concerning the large scale nse of CNT was the high cost and low efficiency of synthesis and purification processes. Several different synthesis methods have been already developed in the last decade (Fig. 1.6). 

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. 

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. 

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]. 

Stobinski L, Lesiak B, Kover L, Toth J, Biniak S, Trykowski G, Judek J (2010) Multiwall carbon nanotubes purification and oxidation by nitric acid studied by the FTIR and electron spectroscopy methods. J Alloy Compd 501 77-84 Sudesh K, Abe H, Doi Y (2000) Synthesis, structure and properties of polyhydroxyalkanoates biological polyesters. Prog Polym Sci 25 1503-1555 Tang W, Santare MH, Advani SG (2003) Melt processing and mechanical property characterization of multi-walled carbon nanotube/high density polyethylene (MWNT/HDPE) composite films. Carbon 41 2779-2785... 

---