DWCNT

CNTs may consist of just one layer (i.e. single-walled carbon nanotubes, SWCNTs), two layers (DWCNTs) or many layers (MWCNTs) and per definition exhibit diameters in the range of 0.7 < d < 2 nm, 1 < d < 3 nm, and 1. 4 < d < 150 nm, respectively. The length of CNTs depends on the synthesis technique used (Section 1.1.4) and can vary from a few microns to a current world record of a few cm [16]. This amounts to aspect ratios (i.e. length/diameter) of up to 107, which are considerably larger than those of high-performance polyethylene (PE, Dyneema). The aspect ratio is a crucial parameter, since it affects, for example, the electrical and mechanical properties of CNT-containing nanocomposites. 

The obtained CNT-Fe-Al203 powder is composed of clean isolated CNTs and small-diameter bundles of CNTs, surrounding all the oxide grains as a web. SEM and HRTEM studies have shown that most CNTs are SWCNTs or DWCNTs (80%), with only a small proportion of three- to six-walled CNTs, and have external diameters between 0.7 and 5 nm. The method has been extended to MgAl204 (Fig. 12.3) and MgO based powders, using Mg(i A.)MxAl204 or Mg,, M/) (M = Co, Fe or Ni) as starting materials.28,29 The products which were used to prepare dense composites were CNT-Fe-... 

Double-walled carbon nanotubes (DWCNTs) consist of two concentric graphene cylinders. DWCNTs, categorized different from both MWCNTs and SWCNTs, are expected to exhibit mechanical and electronic properties superior to SWCNTs. Endo et al. have suggested that... 

FIGURE 12.22 Discharge/charge profiles of highly pure and bundled SWCNT and DWCNT. (Reprinted from Kim, Y.A., et al., Small, 2, 667, 2006. With permission.)... 

Inner tubes of DWCNTs Catalyst free growth from peapods by coalescence of C bU molecules 0.7 (0.55-1) Well shielded, best quality CNTs. Separation from outer tubes is very challenging... 

The most common method for the production of carbon nanotubes is hydrocarbon-based chemical vapor deposition (CVD) [97] and adaptations of the CVD process [98, 99], where the nanotubes are formed by the dissolution of elemental carbon into metal nanoclusters followed by precipitation into nanotubes [100]. The CVD method is used to produce multiwalled carbon nanotubes (MWCNTs) [101] and double-walled carbon nanotubes (DWCNTs) [102] as well as SWCNTs [103], The biomedical applications of CNTs have been made possible through surface functionalization of CNTs, which has led to drug and vaccine delivery applications [104,105],... 

CNTs are commonly classified into single-waUed (SWCNTs) and multi-walled (MWCNTs) nanotubes [28]. SWCNTs consist of a single graphene layer rolled up into a hollow cylinder and are either metallic or semiconducting, whereas MWCNTs are comprised of two, three, or more concentrically arranged cylinders and exhibit only metallic character. Double-wall carbon nanotubes (DWCNTs) are the most basic members of the MWCNT family. The special role of DWCNTs should be emphasized, as they are the link between SWCNTs and the more complex MWCNTs and, therefore, of great interest for a fundamental understanding of these novel nanostructures. 

SWCNTs, DWCNTs, or MWCNTs with very small inner-tube diameters show another size-dependent Raman feature in the low-frequency range referred to as radial breathing modes (RBMs) [35, 39, 40]. The RBMs are considered as a clear indicator for the presence of CNTs, since this Raman feature is unique to CNTs and is not observed for other carbon materials. As suggested by the name, the RBM is a bond-stretching, out-of-plane mode, where all carbon atoms vibrate simultaneously in the radial direction. The RBM frequencies are between 100 and 400 cm and were found to be inversely proportional to the tube diameter [41 3]. In case of DWCNTs and small-diameter MWCNTs, RBM frequencies higher than 200 cm are ascribed to inner tubes while lower frequencies can be associated with both, inner and outer tubes [44, 45]. SWCNTs typically exhibit... 

Fig. 12.2 Comparison of high- and low-frequency Raman spectra of SWCNTs, DWCNTs, and small-diameter MWCNTs. Spectra were recorded using 633-nm laser excitation wavelength...

The most noticeable effect during the in situ Raman studies was the near complete disappearance of the disorder-induced D band after oxidation (Fig. 12.3c). These results show that for the DWCNT sample, the D band originates mainly from amorphous carbon present in the sample and not from defects in the wall structure of the nanotubes. While the concentration of defects probably increases during the oxidation, disordered carbon and the associated D band disappear completely. However, it is well known that only metallic CNTs contribute to the D band intensity [57]. Therefore, the absence of any Raman signal... 

Figure 12.5 shows the in situ Raman analysis of the low frequency RBM range for DWCNTs, acquired using three different laser excitation wavelengths. The intensity of the peaks between 220 and 280 cm decreases constantly during heating from room temperature to 400°C (Fig. 12.5a). Since there are no observable...

Fig. 12.4 In situ Raman spectra of DWCNTs (a) and SWCNTs (b) during nonisothermal oxidation in air. RBMs of DWCNTSs (c) and SWCNTs (d) before and after oxidation. Spectra were recorded using a 633-run excitation wavelength...

Fig. 12.5 Multiwavelength Raman spectra of DWCNTs recorded in situ using a514-nm (a), 633-nm (b), and 785-nm (c) laser excitation [44]. Different laser wavelengths lead to different RBM spectra due to resonant enhancement effects...

Fig. 12.6 Results of isothermai oxidation of the DWCNTs showing (a) the Id/Iq ratio and (b) refative D band intensity. The curves for 350°C and 365°C in (b) are superimposed, and idem for 370°C, 375°C, and 400°C. (c) Raman spectra of DWCNTs after isothermai oxidation for 5 h at different temperatures, recorded at room temperature, (d) HRTEM images of the DWCNT sample before and after oxidation. All Raman spectra were recorded using a 633-nm excitation wavelength...

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