# A Review of Double-Walled and Triple-Walled Carbon Nanotube Synthesis and Applications
## 5 Applications Involving DWNTs and TWNTs
There is plenty of room, as well, for the application of few-walled carbon nanotubes in devices for energy harvesting/storage (solar cells, for example) and sensors [179; 180; 181; 182; 183; 184; 185; 186; 187; 188; 189; 190; 191; 192; 193; 194; 195; 196; 197]. Indeed, we currently feel that finally the synthesis methods have advanced significantly to be technologically useful for producing high quality materials that are appropriate for testing several CNT applications that have been proposed in the literature or have been poorly tested. In 2010, [PERSON] and co-authors [182] demonstrated that DWNTs can serve as new channels for the establishment of an emerging class of delivery systems for the transport and translocation of DNA molecules and other types of biomolecules. Their method depends on the fact that DWNTs are more robust than SWNTs as regards  
Figure 13: (**a**) The design of a nanomotor fabricated with one DWNT and one SWNT, which are mechanically coupled to each other. Reproduced with permission from [176], Copyright AIP Publishing LLC, 2008. (**b**) The side and top views of the nanomotor, respectively. Reproduced with permission from [176], Copyright AIP Publishing LLC, 2008. (**c**) SEM image of a nanomotor fabricated with a plate attached to the outer SWNT constituting a DWNT. Reproduced with permission from [177], Copyright Nature Publishing Group, 2003. (**d**) The plate at various angular positions (from 0–340 \({}^{\circ}\)). Reproduced with permission from [177], Copyright Nature Publishing Group, 2003.  
voltage applications, and by following this line of reasoning, we suggest that TWNTs may provide another convenient option for the further development of such delivery systems. In the field of solar cells, CNTs have been mostly used as base materials for transparent conductive films as an attempt to replace ITO (indium tin oxide) substrates, which are very expensive [183; 184; 185; 186; 187; 188]. In some cases, the nanotube systems were not involved in the photo-generation process [183; 184; 185]. However, the nanotube structures can also be combined with silicon to produce heterojunction solar cells [186; 187; 188]. In these cases, the CNTs play an important role in the photo-generation process. [PERSON] _et al._[186] proposed a design in which the manufacturing process is simple and scalable, involving solution transfer of a porous, single-layer film of double-walled nanotubes to a silicon surface to form heterojunctions with silicon. The method does not require the separation of metallic and semiconducting nanotubes, and their solar cell devices achieved efficiencies of about 7% compared to the 10% usually observed in the commonly-used polymer-based cells. [PERSON] _et al._[187] also demonstrated that DWNTs could be potentially used for both photogeneration sites and charge carrier collecting/transport layer in DWNT-based solar cells. However, in this design, the efficiency, which lays around 1%, is not as good as the one obtained by the nanotube-silicon heterojunctions solar cells. In the same line, [PERSON] _et al._[188] designed and measured a hybrid photoelectrochemical cell (PEC) formed by a heterojunction made of a thin film of DWNTs and silicon nanowires, as show in Figure 14. In their design, the DWNTs also serve as the transparent counter electrode of the PEC cell. Again, the efficiency obtained by their solar cell device was lower than 1.3% (see Figure 14b). Although DWNTs can be used for photogeneration and electrodes simultaneously, the efficiencies obtained from devices are still farfrom reaching those observed in polymer-based cells. As base materials for sensors, these multi-walled carbon nanotube (MWNT) systems are promising in a variety of ways: mass sensors, bio-sensors, strain sensors, temperature sensors and  
Figure 14: (**a**) Schematics and side view of a hybrid heterojunction/photoelectrochemical cell (PEC) solar cell fabricated with silicon nanowire (SiNW) arrays and DWNT films. Reproduced with permission from [188], Copyright American Chemical Society, 2009.