# A Review of Double-Walled and Triple-Walled Carbon Nanotube Synthesis and Applications
## 3 Synthesis and Characterization of TWNTs  
Unlike the synthesis of DWNTs, the synthesis of TWNTs remains a more challenging topic, and there are only a few reports approaching this theme. In the case of DWNT synthesis, the number of layers is controlled by either the precise control of the metal nanoparticles diameter, synthesis conditions or the addition of a growth promoter, such as Mo, S and O. However, these tuning approaches introduce instability into the synthesis system while increasing the degree of tuning. Therefore, controllability tends to become fuzzier when the focus is shifted from DWNT to TWNT synthesis. Moreover, in the case of TWNT synthesis, it is difficult to remove DWNTs mixed with TWNTs, since the difference in physicochemical properties between them is smaller than that between SWNTs and DWNTs. To the best of our knowledge, [PERSON] _et al_. [144] were the first to observe a large amount of TWNT growth (Figure 8a-c). The TWNTs were found in a bundle structure with a ring shape where each TWNT was closely packed into a triangular configuration (Figure 8b). Only 10% of all of the ring structure was composed of only TWNTs. The CNT growth is quite sensitive to the size of the metal catalyst and/or the synthesis conditions, suggesting that the reaction window for the selective growth of TWNTs could be narrow. To advance this research direction, the growth kinetics of long TWNTs was investigated by [PERSON] _et al_. [145], and 100 mm-long TWNTs were grown on a Si substrate by a gas flow-directed CVD method [145]. The reaction window for the preferential TWNT growth was narrow, as well. When the synthesis temperature was increased by 20 \({}^{\circ}\)C or 40 \({}^{\circ}\)C, the yield of SWNT and DWNT was dramatically increased in the intended TWNT growth, and then, the selectivity of the TWNT growth was decreased down to 30% within typical sample growth runs. It was demonstrated when the TWNT growth condition was in a narrow reaction window, the growth of TWNTs was stable [145]. The CNTs with chiral angles close to 30\({}^{\circ}\) (close to the armchair CNT) showed a stable structure over 60 mm of length, and the growth rate of the CNT was about 16% higher than for the other lengths and for the SWNTs species. Notoriously, TWNTs with lengths less than 30 mm presented smaller chiral angles (ranging from 10-20\({}^{\circ}\)) and four dominant flavors: S@S@S, M@S@M, S@S@M, S@M@S. For TWNTs with lengths more than 30 mm, chiral angles ranging from 20-30\({}^{\circ}\) were preferred, and only one flavor was observed: S@S@S[145]. For all of the lengths, the differences between the chiral angles for the nanotubes composing the TWNTs were around 6.5\({}^{\circ}\). [PERSON] _et al_. [116] achieved selective growth of SWNTs and DWNTs by controlling the thickness of the metal catalyst film. This suggested that it could be possible to synthesize TWNTs selectively by simultaneously optimizing the metal catalyst film thickness, as well as the reaction conditions. To increase TWNT synthesis yields, [PERSON] _et al_. [146] synthesized vertically-aligned TWNT films from thickness-controlled Fe film deposited on a Si substrate. It was found that a precisely controlled 3 nm-thick Fe film yielded \(\sim\)8 nm-diameter TWNTs with a TWNT selectivity as high as \(\sim\)60%. Chemical control of catalytic metal nanoparticles was also achieved to benefit selectivity and TWNT quality. In this context, [PERSON] _et al_. [147] used a microwave plasma-enhanced CVD (PECVD) system and engineered mono-dispersed hollow nanoparticles (HNPs) of Fe\({}_{3}\)O\({}_{4}\) (Figure 8d) as a catalyst for studying TWNTs' growth. Because the shrinkage of the HNPs during the heating process avoided the aggregation of the catalyst nanoparticles, then dense, vertically-aligned CNTs with a uniform internal diameter of 4.86 nm were grown on the Si substrate (Figure 8e-g). With this approach, 83% of grown CNTs have three concentric walls (TWNTs), and the area density of the CNTs was 0.6 \(\times\) 10\({}^{12}\) cm\({}^{-2}\). [PERSON] _et al_.