Coal as a carbon source for carbon nanotube synthesis

Abstract

This article reviews the recent advances on the various processes used in the synthesis of carbon nanotubes (CNTs) from different types of coal (anthracite, bituminous, etc.) and on the role played by coal as carbon source in the production of CNTs. The molecular solid coal is inexpensive and widely available in comparison to the most widely used solid carbon precursor, graphite (a lattice solid) and high purity hydrocarbon gas sources. An account is given on the different processes involved in the synthesis of various CNTs (single and multi-walled, bamboo-shaped, branched, etc.) from different types of coal (anthracite, bituminous, etc.). Both arc-discharge and thermal plasma jet produce high quality CNTs but fundamental disadvantages limit their use as large-scale synthesis routes. Chemical vapour deposition appears to be promising but further experimental work is necessary in order to develop an understanding of the complex factors governing the formation of different carbon nanomaterials from coal. Successful utilization of CNTs in various applications is strongly dependent on the development of simple, efficient and inexpensive technology for mass production and coal as a carbon source has the potential to meet the needs.

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.