ReviewCoal as a carbon source for carbon nanotube synthesis
Introduction
The synthesis of carbon nanotubes (CNTs) using an arc discharge and characterisation with a transmission electron microscope (TEM) was reported in the early 1990s [1], [2] and since then, research in this field has grown exponentially. A CNT can be described as a sheet of graphene rolled into a seamless cylinder, with a very high length-to-diameter ratio. It exhibits extraordinary mechanical properties and unique electronic properties. Electrical conductivity, thermal conductivity, field emission characteristics and several other properties of CNTs are also impressive [3]. Consequently, application development in electronics, high strength composites, chemical and biosensors, interconnects and chip cooling in integrated circuit manufacturing, field emission devices, catalyst support, fuel cells, batteries, shielding of electromagnetic interference and many other fields has been pursued vigorously [3], [4], [5], [6], [7], [8], [9], [10], [11], [12]. Whereas applications in electronics, photonics and sensors may need direct growth of CNTs on patterned or un-patterned substrates with exquisite control on size and positioning, all other applications require production in bulk quantities, for example tons a day, to realize cost advantages.
Several methods have been used in the production of CNTs where hydrocarbons such as methane and acetylene are commonly used as precursors. However, there has been promising research into the use of coal as a source material. Coal is cheap and abundant naturally occurring material compared to other derivative source materials, such as graphite, methane or other hydrocarbons [13]. The feasibility of preparing carbon nanomaterials from coal was first established by Pang et al. [14] with the synthesis of C60 and C70. Since then, there have been several studies on the feasibility of CNT synthesis from coal and exploring the potential for large scale production. This article reviews the current CNT production techniques that are based on coal as the carbon source.
Section snippets
Different carbon nanotube production techniques
A review of various synthesis techniques, properties and characterization of CNTs can be found in [3]. The predominant methods currently used for the synthesis of CNTs are arc discharge, laser ablation and chemical vapor deposition (CVD). A reasonable amount of debate still encompasses which researcher/s should be acknowledged for the first observation of the nanometer-sized carbon tubes [15]. However, it is clear that their “re-discovery” by Iijima in 1991 [1] started energetic research that
Arc discharge method
Pure carbon electrodes [62], [63], [64] have been the norm in arc discharge production of CNTs and other nanostructured carbon materials. Coal has garnered interest as an electrode material because this would reduce raw material costs by approximately ten-fold as estimated by Williams et al. [65]. Wilson and co-workers first explored the feasibility of preparing CNTs from coal using arc discharge [66], [67] and obtained nanotubes smaller than 5 nm in diameter from Bacchus Marsh brown coal.
Composition of coal
As coal is an inexpensive and readily available source of carbon, it is interesting to investigate the formation of CNTs directly from coal using CVD. Regardless of CVD or arc methods, there are a number of critical issues which need to be addressed before realising the full potential of coal as a carbon source to reduce the product cost. Systematic investigations to increase yield and purity are needed and this obviously depends on the origin of coal since the content and quality varies from
Conclusions
This review has built upon previous report by Yu et al. [13] through expanding knowledge concerning the properties, composition and structure of coal, and relating these to CNT synthesising techniques. More work is needed to optimise processes that can selectively yield one type of nanostructure (SWCNT or MWCNT or fullerene) and to develop processes that take advantage of catalytic metal impurity already present in coal instead of using external catalysts sources, since derivative sources such
Acknowledgements
The authors acknowledge the financial support from the National Research Foundation (NRF) under South Africa NRF Focus Area, NRF Nanotechnology flagship programme, DST-funded Chair of Clean Coal Technology grant and DST/NRF Centre of Excellence. The student bursaries provided by the University of the Witwatersrand are also much appreciated. Support from the World Class University program through the National Research Foundation of Korea funded by the Ministry of Education, Science and Technology
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