Low-Temperature, Controlled Synthesis of Carbon NanotubesI am grateful to Prof. Richard Smalley for useful discussions, particularly on Ref.[emsp14]14, and to AFRL/ML, Wright Brothers Institute, Dayton Development Colations, and the University of Dayton for a WBI Endowed Chair Professorship in Nanomaterials.

A carbon nanotube may be viewed as a graphite sheet that is rolled up into a nanometer-scale tubular form (that is, a single-walled carbon nanotube, SWNT) or with additional graphene tubes that form around the core of a SWNT (that is, a multi-walled carbon nanotube, MWNT). Because the graphene sheet can be rolled up with varying degrees of twist along its length, carbon nanotubes can have a variety of chiral structures. Depending on their diameter and the helicity of the arrangement of graphitic rings in the walls, carbon nanotubes have been demonstrated to possess unusual electronic, photonic, magnetic, thermal, and mechanical properties. 2] For instance, these tiny elongated carbon nanotubes with a hollow core can be stronger than steel (yet flexible), lighter than aluminum, and more conductive than copper. These peculiar properties have made carbon nanotubes particularly attractive as new materials for a variety of applications, from reinforcement fillers in advanced nanocomposites to wires for nanoelectronics. However, if the price of carbon nanotubes remains as high as it is today (e.g., US$ 500 per gram for SWNT materials— more than 30 times as expensive as gold), any large-scale application of carbon nanotubes will be unrealistic. Consequently, a low-temperature, and hence low-cost, synthesis of carbon nanotubes with a controllable structure and high purity has become absolutely crucial to the development of the whole field of carbon nanoscience and nanotechnology. Since the discovery of carbon nanotubes in 1991, it has been a perpetual endeavor to cost-effectively synthesize carbon nanotubes in both a random and an ordered form (e.g., aligned and patterned) at low temperature so that they can be directly incorporated into composite materials and flexible functional devices, using substrates such as plastics. The past thirteen years of strenuous research around the world has led to the development of several effective synthetic methods, including arc-discharge, laser evaporation, and chemical vapor (pyrolytic) deposition, for the production of carbon nanotubes with an aligned and/or nonaligned structure. However, it is the recent efforts to exploit leading-edge synthetic methods for the low-temperature production of carbon nanotubes, 12] along with the aligned growth of SWNTs, that could open up an avenue for the cost-effective synthesis of carbon nanotubes with controllable structures for practical applications. In this article, a recent and significant advance in the low-temperature, controlled synthesis of carbon nanotubes, which was reported by Vohs, Fahlman, and co-workers, is highlighted. Instead of using the more-conventional hydrocarbon-based precursors, the authors used carbon tetrachloride (CCl4) with weaker carbon–halide (C X) bonds for the pyrolytic growth of MWNTs in a supercritical carbon dioxide medium, in the presence of iron catalysts encapsulated in polypropyleneimine (PPI) dendrimers (designated as Fe@PPI dendrimer) (Figure 1). As demonstrated by Vohs et al., the use of supercritical carbon dioxide as a medium for the decomposition of CCl4 can completely eliminate the co-reactants, such as alkali metals and lithium acetylide, which were required for enhancing the decomposition of chlorinated hydrocarbon precursors. The high pressure exerted by the CO2 (8.3– 27.6 MPa) further assists in the decomposition of CCl4 and facilitates the delivery of the intermediate species to the encapsulated iron surface for the nucleation and growth of carbon nanotubes at a record low temperature (that is, 175 8C). The resultant nanotubes have an average diameter of 20–25 nm with branched and bent nanotube morphologies, which indicates different catalytic rates for nanotube growth from the encapsulated and peripheral iron atoms within the Fe@PPI dendrimer catalysts. Therefore, by controlling the distribution of the catalyst iron nanoparticles within various dendritic structures, it is possible to pro[*] L. Dai Department of Chemical and Materials Engineering School of Engineering University of Dayton 300 College Park, Dayton, OH 45469-0240 (USA) Fax: (+1)937-229-3433 E-mail: [email protected]