# 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  
For CNT growth, transition metal nanoparticles, including Fe, Ni, Co and their binary alloys, have been frequently used as catalyst metal nanoparticles. In the case of the supported catalyst method, metal nanoparticle catalysts are prepared either by a chemical technique (chemical synthesis, impregnation, combustion, _etc._) or a physical deposition technique (sputtering, e-beam/thermal deposition, _etc._). The diameter of the metal nanoparticles is controlled by the conditions used for the catalyst preparation, and the prepared metal nanoparticles are supported on an oxide substrate (_ex._ SiO\({}_{2}\), MgO, Al\({}_{2}\)O\({}_{3}\)) to prevent aggregation during the heating process prior to the synthesis reaction. Both porous powder and planar substrates are used as supporting materials for CNTs' synthesis. Within this method, DWNTs have been selectively prepared by optimizing the diameter of the metal nanoparticles, as well as the parameters of the synthesis reaction. In the case of metal nanoparticles on a porous support, a chemical controlling method is frequently used. Chemical synthesis is the one of chemical control methods, and the metal nanoparticles are prepared by chemical reaction and using a metal catalyst precursor and a capping agent to obtain a finite growth time for the nanoparticles [100; 114]. The diameter selected for the metal nanoparticle is related to the desired CNT diameter and the number of layers of the CNT. [PERSON] _et al_. [125] investigated the interaction of metal (Fe) nanoparticles and the supporting material (MgO) for the growth. In this method, Fe nanoparticles are prepared by thermal decomposition using iron pentacarbonyl (Fe(CO)\({}_{5}\)) as a precursor and oleic acid as a capping agent. It was found that MgO and Fe nanoparticles interact strongly, and thus, the Fe nanoparticles keep a smaller diameter at the CNT synthesis temperature. Because of the strong interaction between FeOx and MgO, the agglomeration of catalyst particles was highly suppressed. Diffusion of Mg and Fe occurs in the temperature range between 500 \({}^{\circ}\)C and 900 \({}^{\circ}\)C, and then, this phenomena reduces the iron oxide particle and, thereby, moderates the size of the Fe nanoparticle to be suitable for the thinner CNT growth (SWNTs and DWNTs). Smaller Fe nanoparticles (1.6 or 2.8 nm) resulted in SWNTs' growth, and a slightly larger Fe nanoparticle (4.5 nm) exhibited DWNTs' growth, whereas a 6.4-nm or larger Fe nanoparticle did not grow any CNTs. [PERSON] _et al_. [100] further investigated the effect of the diameter of Fe nanoparticles on CNTs' growth. Both the inner and outer tube diameter distribution of the DWNTs was controlled by the diameter of the Fe nanoparticle. Fe nanoparticles are prepared by means of the thermal decomposition of Fe(CO)\({}_{5}\), and oleic acid was used as the surface capping agent. The size of the nanoparticles was controlled to be 4 nm (Figure 5a) and 10 nm (Figure 5b) by the molar ratio of Fe(CO)\({}_{5}\) and oleic acid. The prepared Fe nanoparticles are here supported on MgO. Both catalysts yielded DWNTs with high selectivity up to 80%, but the observed catalyst nanoparticle after the CVD reaction exhibited a smaller size than its original size (Figure 5c,d). The Fe nanoparticles with a diameter of 4 nm and 10 nm produced DWNTs with an inner diameter of 0.91 and 1.42 nm and an outer diameter of 1.68 and 2.19 nm, respectively. A simple impregnation/precipitation technique was also used for the preparation of the catalyst. It is possible to control the density of the metal nanoparticle by changing the ratio of the amount of metal nanoparticles and the supporting material. Various binary catalysts were investigated by [PERSON] _et al_. [98] for DWNT growth over Co-Fe, Co-Ni, Co-V, Co-Cr and Co-Mn supported on a zeolite substrate. The Co-Fe binary catalyst yielded a high selectivity of DWNTs, whereas other catalysts exhibit a low yield of DWNTs.