# A Review of Double-Walled and Triple-Walled Carbon Nanotube Synthesis and Applications
## 2 Synthesis and Characterization of DWNTs
### The Catalytic Chemical Vapor Deposition Method
#### 2.2.1 The Supported Catalyst Method
As discussed above in the section describing the arc discharge method, promoters are also used to improve both the yield and the selectivity of DWNTs relative to other carbon nanostructures. Mo is frequently used to improve the preference for DWNT production during the growth process [92; 103; 104; 106]. [PERSON] _et al_. [92] reported that the presence of Mo improved the yield and increased the selectivity of DWNTs, but the detailed role of Mo in the growth process remained unclear. Their method increased the DWNT selectivity from less than 50% to over 80% by Mo addition [92]. [PERSON] _et al_. [106] soon thereafter achieved highly selective growth of DWNTs using a Co-Fe-Mo catalyst. The optimized condition Co:Fe:Mo = 2.5:1.0:3.5 delivered essentially a 100% selectivity of DWNTs after the purification process by oxidation at 700 \({}^{\circ}\)C in air. When the Mo content was below the aforementioned optimum condition, larger diameter DWNTs and MWNTs were obtained. The other important factor was the Fe:Co ratio, whereby a lower Co content increased the content of TWNTs from 0 %-45 %, whereas the diameter of the inner-most tube remained unchanged. The effect of Mo addition in the Fe-Mo/MgO system was investigated by [PERSON] _et al_. [105]. Two functions were proposed for Mo addition upon the selective growth of DWNTs: (1) a large particle of Mo on MgO can provide a secondary support for Fe nanoparticle; (2) the breakup the hydrocarbon molecules for efficient carbon precursor conversion prior to CNTs synthesis reaction. The detailed methane conversion behavior over Fe-Mo/MgO was investigated by [PERSON] _et al_. [126], and the addition of a small amount Mo improved the conversion efficiency. For the Mo addition, the alloy formation with Fe, Co or MgO is not necessary to improve the yield and the selectivity of DWNT growth. [PERSON] _et al_. [104] used Mo/Al\({}_{2}\)O\({}_{3}\) as a conditioning catalyst, and it was placed separate from the CNT growth catalyst (Fe/MgO). In the case of the presence of the conditioning catalyst, \(>\)95% of the high DWNT selectivity was achieved after the optimum purification process (Figure 5e-f). Moreover, since the optimization of the conditioning catalyst was separated from the optimization of the CNT growth catalyst, simple oxidation in air and hydrochloric acid (HCI) treatment removed most of the impurities, such as amorphous carbon, Fe nanoparticles and MgO support; this removal of impurities would be useful for commercializing the growth of DWNTs for device applications. The highly bundled structure of DWNTs realized by the narrow diameter distribution and the low amount of impurity enabled DWNTs to form buckypaper by a filtration technique (Figure 5g). The obtained DWNTs buckypaper was tough enough to fold into an origami structure. In 2004, the effect of water as a reaction enhancer, which removes the carbon coating layer on the metal nanoparticle catalyst and thereby prolongs the lifetime of the catalyst, was reported, and the detailed growth mechanism was discussed later [127; 128]. Later research revealed that not only water, but also oxygen molecules act similarly. [PERSON] _et al._[115] investigated the effect of the reaction enhancer on the number of layers of CNTs (Figure 6a-f). Similar to water, it was found that a small amount (hundreds of ppm order of magnitude) of any oxygen-containing molecular form acts as a reaction enhancer. Furthermore, with an enhancer containing aromatic rings, the control over the number of walls in the carbon nanotube structure was observed. Namely, with methyl-benzoate as a reaction enhancer, DWNTs were preferably grown after the synthesis reaction  
Figure 6: (**a**)-(**f**) Layer number control of CNTs by introducing a reaction enhancer. Reproduced with permission from [115], Copyright American Chemical Society, 2009. TEM images and histograms show selective growth of SWNTs (**a**)-(**d**), DWNTs (**b**)-(**e**) and MWNTs (**c**)-(**f**), in the case of water, methyl-benzoate and benzaldehyde used as a reaction enhancer, respectively.