# A Review of Double-Walled and Triple-Walled Carbon Nanotube Synthesis and Applications
## 2 Synthesis and Characterization of DWNTs
### The Arc Discharge Method  
The arc discharge method was developed for the synthesis of fullerenes. However, in 1991, [PERSON] [1] discovered multi-walled carbon nanotubes (MWNTs) in the carbon soot at the end of an electrode (graphite rod), which was in a different place from where the fullerenes he was preparing were found. The synthetic apparatus used for the arc discharge method is simple, and the experimental parameters are not as numerous as for the CCVD method; this method therefore exhibits high reproducibility. In this method, a pair of carbon rods (usually made as a composite of graphite and metal nanoparticles) is placed in a vacuum chamber with a small gap, and then, a high voltage is applied over the gap in order to generate a plasma. By using a high temperature plasma, the carbon rod is partially vaporized, and CNTs are formed by the catalytic effect of the metal nanoparticles. In this growth method, the CNT growth occurs at a higher temperature than by the other methods for the preparation of DWNTs, such as the CCVD method. By the annealing of molecules encapsulated with the SWNT, it is widely accepted that the arc discharge-produced CNTs exhibit high crystallinity and low defect density [21; 73; 74; 75; 76; 77; 78; 79; 80; 81; 82; 83]. While arc discharge-produced DWNTs exhibit a high oxidation temperature close to 800 \({}^{\circ}\)C, the defect healing at high temperature was proposed as a justification for the high quality product outcomes [73; 74]. In contrast to the high crystallinity of DWNTs, the high temperature also leads to the formation of highly graphitized non-CNT carbon impurities, which are difficult to remove by a simple purification process, such as oxidation and/or acid treatments, rather than the formation of amorphous carbons. Several carbon materials other than graphite can be used as carbon sources for CNTs. For example, [PERSON] _et al_. [75] used carbon black as a carbon source for arc discharge-based DWNTs' growth. It was found that the structural features of the carbon black, which correspond to the presence of the curvature induced by the heptagon and pentagon, are favored to increase the concentration of carbon species at the time of the arc discharge. The preparation of DWNTs from MWNT/carbon nanofiber (CNF) and fullerene waste has also been reported [76; 77]. The use of MWNT/carbon nanofibers as carbon feedstock in the arc-discharge method converts about 80% of the carbon feedstock in high quality DWNTs with outer diameters ranging from 1.75-4.78 nm and inner diameters ranging from 1.06-3.93 nm [76]. On the other hand, the DWNTs produced from fullerene waste present outer tube diameters ranging from 1.50-5.00 nm and inner tube diameters ranging from 2.00-6.00 nm [77]. Although the DWNTs produced are also of high quality, there is no information on how much fullerene waste was actually converted in DWNTs. However, the authors reported that the purification process of the samples produced is easier when fullerene waste is used as a carbon source [77]. The addition of a promoter can increase the yield and the selectivity of DWNTs relative to other carbon forms, and even until now, these effects have not been well understood. Sulfur has been used as a promoter for the growth of carbon fibers (CF) [78; 79]. By introducing sulfur as a promoter, [PERSON] _et al_. [80] reported for the first time the selective growth of DWNTs, using the arc discharge method. The effect of sulfur on the growth of DWNTs has been further investigated by [PERSON] _et al_. [81] with various metal catalysts under different gas environments (H\({}_{2}\), He and their mixtures). It was found that sulfur is effective only for iron-group metal catalysts, and the presence of H\({}_{2}\) is found to be important. When non-iron-group metal catalysts, such as yttrium or lanthanum, were used, no DWNT growth was observed [81].