# A Review of Double-Walled and Triple-Walled Carbon Nanotube Synthesis and Applications
## 3 Synthesis and Characterization of TWNTs
### Annealing of Confined Molecules inside DWNTs  
[PERSON] _et al_. prepared high quality TWNTs by introducing C\({}_{60}\) into DWNTs followed by high temperature annealing [149; 150] and starting from CCVD-prepared high quality DWNTs [104]. In order to enlarge the DWNT diameters, high temperature annealing was carried out prior to C\({}_{60}\) encapsulation [149]. At a temperature of 2400 \({}^{\circ}\)C, the diameter of both the inner tube and the outer tube of the DWNT increased via the coalescence mechanism [149]. After the annealing process, the inner tube diameter of the DWNT was in the range of 1.0-1.8 nm, which is wide enough for C\({}_{60}\) encapsulation. The DWNT and C\({}_{60}\) were sealed inside a glass ampoule under vacuum, and  
Figure 8: Synthesis of triple-walled carbon nanotubes (TWNTs) using various methods. Catalytic chemical vapor deposition (CCVD)-synthesized TWNT ring. Reproduced with permission from [144], Copyright AIP Publishing LLC, 2006. Low (**a**) and high (**b**) magnification TEM image of a TWNT ring. TWNTs are packed into a triangular configuration (**b**) and faceted TWNTs within the ring (**c**). Vertically-aligned TWNT grown from engineered hollow nanoparticles (HNPs) by plasma-enhanced CVD (PECVD); (**d**) hollow nanoparticles used as a catalyst; the scale bar represents 5 nm ; (**e**) SEM image of vertically-aligned TWNTs; the scale bar represents 10 nm; (**f**) low and (**g**) high magnification TEM images of TWNTs; the scale bars represent 50 and 5 nm, respectively. Reproduced with permission from [147], Copyright American Chemical Society, 2014. DWNT pea pod-derived TWNTs; TEM images (**h**)–(**k**) and Raman spectrum (**l**)–(**m**). Reproduced with permission from [149], Copyright John Wiley and Sons, 2011. High magnification image of C\({}_{60}\)-encapsulated DWNTs (**h**) and TWNTs formed by high temperature annealing at 2000 \({}^{\circ}\)C (**i**). The generation of peaks at 380 cm\({}^{-1}\) in (**l**) confirms the formation of the inner most tube of TWNTs by annealing over 1500 \({}^{\circ}\)C, especially between 1700 \({}^{\circ}\)C and 2000 \({}^{\circ}\)C (m) [149; 150]. then, the glass ampoule was heated at 600 \({}^{\circ}\)C for the C\({}_{60}\) encapsulation. Finally, a secondary high temperature heat treatment process (1500 \({}^{\circ}\)C-2000 \({}^{\circ}\)C) transformed the DWNT-pea pod (Figure 8h) into TWNTs (Figure 8i-k) [149]. The diameter of the newly-generated inner-most tube was in the range of 0.4-1.0 nm, and the generation of an inner-most tube was confirmed by Raman spectroscopy (Figure 8l-m). Recently, [PERSON] _et al_. [57; 58; 59] used resonant Raman spectroscopy (RRS) to study in greater detail these fullerene-derived TWNTs in both bundles and isolated versions. In these studies, the authors confirmed the high-quality TWNT throughput of the fullerene-derived method reported by [PERSON] and co-authors [149]. The works by [PERSON] and co-authors also reported the first comprehensive studies on the vibrational properties of these high-quality TWNTs, connecting such properties with their SWNT and DWNT counterparts [57; 58; 59; 151; 152; 153]. Table 2 summarizes the different synthesis conditions that produce TWNTs.