# 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
In order to achieve a high selectivity of DWNT growth, the physicochemical properties of the supporting material is also important, since the interaction between the nanoparticles and the supporting material plays an important role during the heating process. The catalyst preparation method affects the diameter distribution of the metal nanoparticles, and it therefore also affects the selectivity of the chirality and metallicity of the DWNTs. [PERSON] _et al_. [96] reported the selective growth of DWNTs (in comparison to other CNT versions) over the Mg\({}_{1-x-y}\)Co\({}_{x}\)Mo\({}_{y}\)O catalyst. The catalyst for DWNTs' growth was prepared by a simple combustion method using urea and citric acid as fuel. Compared to the urea-based combustion process, the citric acid-based combustion process led to the formation of a homogeneous catalyst. Thus, the selectivity of DWNTs was high for the citric acid-based catalyst. They also mentioned that by changing the catalyst preparation conditions, they can prepare a mixture of DWNTs and TWNTs. [PERSON] _et al_. improved the yield and the purity of the DWNTs by introducing small amounts of Al into the Fe/MgO system [85]. As a result of the phase separation that occurred due to the Al addition, the crystalline size of the MgO decreased as the Al content increased, and therefore, the dispersity of the Fe nanoparticles was improved. The improved dispersion of Fe nanoparticle resulted in a higher  
Figure 5: Various methods for DWNT synthesis and layer number control. (**a**)–(**d**) DWNTs from diameter-controlled Fe nanoparticles supported on MgO. Reproduced with permission from [100], Copyright Elsevier, 2004. Transmission electron microscopy (TEM) images of (**a**) 4-nm and (**b**) 10-nm Fe nanoparticles on MgO and (**c**) small and (**d**) large diameter DWNTs from diameter-controlled Fe nanoparticles (**a**) and (**b**), respectively. Highly bundled DWNTs grown from Fe/MgO with Mo/Al\({}_{2}\)O\({}_{3}\) conditioning catalysts: (**e**) Scanning Electron Microscopy (SEM) image, (**f**) TEM image and (**g**) optical image of flat and origami structured buckypaper. Reproduced with permission from [104], Copyright Nature Publishing Group, 2005.  
yield and higher purity for DWNTs. Both the physical properties and the chemical properties of the silica-based supporting material were investigated [85]. [PERSON] _et al_. [99] have used the Fe/Co nanoparticles in connection with different silica-based porous supporting substrates and they have investigated the relationship between the pore structure/surface property of the supporting substrate material and the selectivity of the DWNT growth process. A sol-gel-based microporous silica and a hydrothermal synthesis-based mesoporous silica were used. It was found that the number of layers of CNTs varied depending sensitively on the synthesis temperature. When the synthesis was carried out at 800 \({}^{\circ}\)C, 90% of the product was SWNTs. In the case of mesoporous silica, the yield of DWNTs increased up to 80% with increasing synthesis temperature, whereas the yield of DWNTs saturated and decreased when microporous silica was used. Moreover, it was found that the difference in the acidity of the supporting material also affects the selectivity of DWNTs relative to other carbon nanostructures when the pore structure was the same, and weakly acidic mesoporous silica yielded DWNTs preferentially. [PERSON] _et al_. [102] found that the commercially-available FeSi\({}_{2}\) powder worked as a catalyst for the selective growth of DWNTs. After proper oxidation of the FeSi\({}_{2}\) powder prior to CNT growth, oxidation induced Fe\({}_{2}\)O\({}_{3}\) formation occurred on top of the FeSi\({}_{2}\) particle, sitting on the SiO\({}_{2}\) substrates. Thus, the treatment of the Fe nanoparticles produced by the reduction to form uniformly nucleated Fe\({}_{2}\)O\({}_{3}\) under proper environmental conditions yielded a uniform DWNT structure.