Literature Review on the Synthesis of Carbon Nanotubes
Carter F. JacksonCarbon nanotubes (CNTs) were first found in 1991 by S. Iijima.9 They consist of covalently bonded carbon atoms rolled into tubes of varying length and structure. There are single walled variants (SWNTs) and multiwalled variants (MWNTs). An alteration in certain significant variables such as number of walls, length, width, or structure of atoms can completely change the properties of the nanotube, allowing for a wide range of possible effects. CNTs present the possibility of many applications in the near future such as, according to Baughman et al., extremely strong composites, electrochemical devices, hydrogen storage, field emission devices, electronic nano-devices, sensors and probes.6 As stated by Wang et al., MWCNTs are very effective in the sorption of gasses using stable complexes.7 The main issue facing these applications is the synthesis, or means of creation, of the nanotubes. Currently, most methods of synthesis result in a high amount of impurities or are not viable resource-wise.6 As time passes and more research is done, more feasible CNT synthesis processes are being found and hypothetical applications are inching closer to reality.
One of the earliest methods of synthesis to be discovered was the electric arc discharge process.9 It was used in the 1960s by R. Bacon to create carbon fibers, but was later adapted to synthesize CNTs.9 In this method, two electrodes are used with carbon atoms present to create a variety of carbon nanostructures.8 According to Journet et al., this method is useful for creating a wide variety of nanostructures, but yields small amounts of the intended carbon nanotubes in comparison to the product.8 Since it was one of the first methods used for CNT synthesis, this process was popular when they were not largely understood. Due to this, the low yield in CNTs is expected, though this process opened the door for the initial understanding of CNT synthesis.
A synthesis method using laser vaporization at high temperatures was discovered by Guo et al. at Rice University.9 Smalley et al. effectively applied this method to mass production in 1996.9 In this process, carbon containing molecules are ablated into a plume, which then interacts with a background gas to form a ring.3 After several seconds, the vaporized materials in this ring begin to congregate into clusters of nanoparticles containing CNTs on cool surfaces of the reactor.3 This whole process can be monitored closely by transmission electron microscopy, allowing for significant research to be done leading to insight regarding CNT synthesis.3 The discovery of the laser vaporization method was a vital step in the history of CNT synthesis because it allowed for structured creation of high purity SWNTs much more efficiently than by the arc process, though still with issues regarding commercial viability.9
The chemical vapor deposition method of CNT synthesis was discovered by Yacamàn et al. This method had previously been used in the creation of carbon filaments, but Yacamàn et al. applied the process to CNTs and succeeded tremendously.9 According to Szabó et al., the catalytic chemical vapor deposition method is currently the “only economically viable process for large-scale CNT production” since it provides accurate control of significant properties, mass production, and high purity.9 A numerous amount of variations to the vapor deposition process have been discovered, allowing for the augmentation of many aspects of the method. Most means of CNT synthesis require very high temperatures, raising one of the issues that scientists face when creating CNTs. However, plasma-enhanced vapor deposition can be used with nickel coated glass for synthesis of aligned CNTs of controllable length at a significantly lower temperature.1 Two gasses are used, one as a carbon source and one as a catalyst and dilution gas.1 The discovery of this addition to the process marked a huge step forward in CNT synthesis since it contributed to the address of the temperature issue. The vapor deposition process can also be enhanced significantly through the use of water to control and boost various desirable properties of the results such as nanotube size/length.2 Through the use of n-hexane with the vapor deposition process “with an enhanced vertical floating technique,” the method can yield nanotubes significantly greater in length than the product of any other currently known form of synthesis.4 Since longer nanotubes are easier to study, this variation of the process allows for the conduction of more informative research.4
For a long time, CNTs could only be assembled into a continuous fiber through postprocessing means. It was discovered by Li et al. that using a liquid source of carbon with an iron nanocatalyst allowed for the spinning of CNT fibers directly following chemical vapor deposition at the same site.5 This was achieved using carefully controlled reaction conditions, reactants, and rotating spindle.5 This method was the first to be discovered that allowed for such direct, efficient assembly of fibers, and the physics recorded from the process also apply to several other materials, making the discovery of this method significant to the manufacturing of CNTs and providing a one-step technique for creation of CNT fibers.5
The synthesis of carbon nanotubes is a highly researched topic, and it is one that will continue to be studied until the most efficient method of creation is found. The problems posed by current synthesis methods are some of the only obstacles between modern science and the potential applications of CNTs, and, once they are solved, CNTs will likely be utilized more widely in countless areas of science and engineering. Some companies, such as Nanocyl, Hyperion, and Nanopart are producing nanotubes for commercial use, and many small companies, mostly located in the US, are attempting to synthesize SWNTs.9 As commercial demand for CNTs increases and more efficient synthesis methods are discovered, many companies will likely begin to mass produce significantly more pure CNTs.9
Works Cited
1 Ren, Z. F., Z. P. Huang, J. W. Xu, J. H. Wang, P. Bush, M. P. Siegal, and P. N. Provencio. “Synthesis of Large Arrays of Well-Aligned Carbon Nanotubes on Glass.” Science 282 (n.d.): 1105-107. Science. Web. 4 Sept. 2014.
2 Futaba, Don N., Kenji Hata, Takeo Yamada, Kohei Mizuno, Motoo Yumura, and Sumio Iijima. “Kinetics of Water-Assisted Single-Walled Carbon Nanotube Synthesis Revealed by a Time-Evolution Analysis.” Physical Review Letters (2005): n. pag. Physical Review Letters. Web. 15 Sept. 2014.
3 Puretzky, A.A., D.B. Geohegan, X. Fan, and S.J. Pennycook. “Dynamics of Single-wall Carbon Nanotube Synthesis by Laser Vaporization.” Applied Physics A 70.2 (2000): 153-60. Springer Link. Web. 15 Sept. 2014.
4 Zhu, H. W., C. L. Xu, D. H. Wu, B. Q. Wei, R. Vajtai, and P. M. Ajayan. “Direct Synthesis of Long Single-Walled Carbon Nanotube Strands.” Science 296 (2002): 884-86. ScienceMag. Web. 15 Sept. 2014.
5 Li, Ya-Li, Ian A. Kinloch, and Alan H. Windle. “Direct Spinning of Carbon Nanotube Fibers from Chemical Vapor Deposition Synthesis.” Science 304 (2004): 276-78. ScienceMag. Web. 15 Sept. 2014.
6 Baughman, Ray H., Anvar A. Zakhidov, and Walt A. De Heer. “Carbon Nanotubes–the Route Toward Applications.” Science 297 (2002): 787-92. ScienceMag. Web. 4 Sept. 2014.
7 Wang, Xiangke, Changlun Chen, Wenping Hu, Aiping Ding, Di Xu, and Xiang Zhou. “Sorption of 243Am(III) to Multiwall Carbon Nanotubes.” Environmental Science Technology 39 (2005): 2856-860. ACS Publications. Web. 15 Sept. 2014.
8 Journet, C., W. K. Maser, P. Bernier, A. Loiseau, M. Lamy De La Chapelle, S. Lefrant, P. Deniard, R. Lee, and J. E. Fischer. “Large-scale Production of Single-walled Carbon Nanotubes by the Electric-arc Technique.” Nature (1997): 756-58. Nature. Web. 15 Sept. 2014.
9 Szabó, Andrea, Caterina Perri, Anita Csató, Girolamo Giordano, Danilo Vuono, and János B. Nagy. “Synthesis Methods of Carbon Nanotubes and Related Materials.” Materials (2010): 3092-140. MDPI. Web. 24 Sept. 2014.