Coal as a carbon source for carbon nanotube synthesis

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Available at www.sciencedirect.com

journal homepage: www.elsevier.com/locate/carbon

Review

Coal as a carbon source for carbon nanotube synthesis Kapil Moothi

a,b

, Sunny E. Iyuke

a,b,* ,

M. Meyyappan

c,d

, Rosemary Falcon

a,*

a

School of Chemical and Metallurgical Engineering, University of the Witwatersrand, Johannesburg, Private Bag 3, WITS 2050, South Africa DST/NRF Centre of Excellence in Strong Materials, University of the Witwatersrand, South Africa c NASA Ames Research Center, Moffett Field, CA 94035, USA d Division of IT-Convergence Engineering, POSTECH, Pohang, South Korea b

A R T I C L E I N F O

A B S T R A C T

Article history:

This article reviews the recent advances on the various processes used in the synthesis of

Received 12 September 2011

carbon nanotubes (CNTs) from different types of coal (anthracite, bituminous, etc.) and on

Accepted 16 February 2012

the role played by coal as carbon source in the production of CNTs. The molecular solid coal

Available online 28 February 2012

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. Ó 2012 Elsevier Ltd. All rights reserved.

Contents 1. 2. 3.

4.

5.

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Different carbon nanotube production techniques . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Processes used to synthesise carbon nanotubes using coal as a carbon source. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1. Arc discharge method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2. Thermal plasma jet . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3. Chemical vapour deposition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.4. Summary of production techniques using coal . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . The role played by coal in carbon nanotube production . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.1. Composition of coal . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2. Differences between the carbon nanotube formation mechanism between coal and graphite. . . . . . . . . . . . . Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

2680 2680 2681 2681 2681 2683 2685 2685 2685 2687 2687

* Corresponding authors at: School of Chemical and Metallurgical Engineering, University of the Witwatersrand, Johannesburg, Private Bag 3, WITS 2050, South Africa. (S.E. Iyuke). E-mail addresses: [email protected] (S.E. Iyuke), [email protected] (R. Falcon). 0008-6223/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.carbon.2012.02.048

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Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2688 Appendix A. Supplementary data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2688 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2688

1.

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 [3–12]. 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.

2. Different techniques

carbon

nanotube

production

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 led to the unearthing of various kinds of CNTs (e.g. single and multi-walled, bambooshaped, branched). The arc discharge method is the most practical for synthesizing single-wall CNTs (SWCNTs) according to Keidar [16]. This technique involves the use of two high-purity graphite rods as electrodes that are placed in a noble gas atmosphere [17–19]. An electric discharge is

established between the electrodes and the resulting vapour allows the assembly of carbon nanostructures including CNTs and fullerenes. The reaction takes place at high temperatures of 2400–6000 K [20–23]. A disadvantage of this method is that various nanostructures of carbon are produced simultaneously in most cases, and therefore, separation and purification are required to obtain specific species. Besides, it is a non-continuous process requiring periodic shutdowns for replacement of the graphite electrode and product collection [24]. The energy consumption involved in generating and maintaining the discharge is a factor in the cost of the CNTs. As for laser ablation, the laboratory setup consists of a quartz tube placed in a temperature-controlled oven with a solid graphite target mounted inside the tube. After sealing and evacuating the tube, a flow of argon or helium buffer gas is maintained. The graphite target is vaporised by a pulsed Nd:YAG laser or other appropriate lasers, producing a carbon based soot that is swept out of the furnace zone by the carrier gas. This soot, containing SWCNTs, multiwalled carbon nanotube (MWCNTs), and fullerenes, is deposited on the water-cooled collector [25–27]. The reaction temperature is approximately 1200 °C, and Arepalli [28] and Kingston and Simard [29] have presented comprehensive reviews of the synthesis of SWCNTs using laser vaporisation. The complexity of this technique hinders its development for large-scale production and laser based processing in general is not suitable for scaleup due to high costs of equipment and energy. Chemical vapour deposition (CVD) is a method first presented in 1960 for the synthesis of carbon nanomaterials. This method appears to present the best hope for large-scale and continuous production of CNTs owing to its relatively low cost and high yield potential [30–35]. The basic principle behind this approach involves the decomposition of gas-phase carbon-rich source gases (methane, acetylene or carbon monoxide, etc.) with the aid of a catalyst at elevated temperatures of 700–1000 °C [36–39]. The catalyst particle remains in a molten state at the temperatures prevailing in the reactor. The reactive radical carbon molecules generated from the decomposition of source gases then diffuse into the catalyst particles, and when supersaturation within the particle is reached, carbon begins to precipitate out of the particle and elongates into a nanotube. The precursor diffusion into the particle is often thought to be the rate-controlling step [40]. Hydrogen, nitrogen and argon are commonly used as carrier gases in CNT–CVD. The catalysts employed are transition metals such as iron, cobalt or nickel which can be prepared by a variety of physical or chemical processes [3,41]. Combinatorial approaches have been used to study the effectiveness of various catalyst and substrate combinations and growth conditions [42–45]. A review by Dupuis [41] details the role played by the catalyst in the CVD of CNTs. However, Chen et al. [46] and others [47–49] reported that it is possible to synthesize SWCNTs and MWCNTs using CVD without metal catalysts. Besides hydrocarbons, carbon dioxide has also been used as a carbon source for CNT production [50]. Thermal

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heating can be replaced by a low temperature plasma as energy source and the resulting plasma enhanced CVD has been extensively studied for the growth of various carbon nanostructures [51]. Growth of CNTs by CVD falls into two categories based on how the catalyst is deployed in the process: supported catalyst [30,52,53] or floating catalyst growth [54,55]. In the former, the prepared catalyst is deposited on a support medium such as a substrate which is placed inside the reactor. The source gases flow through the reactor at the set temperature over the substrate. For floating catalyst growth, the basic difference lies in that the catalyst and gas are injected into the system concurrently in the gas phase [56–58]. The CVD reactors are designed to be either horizontal or vertical [59], and vertically-orientated CVD reactors as described by Philippe et al. [60] are configured to produce CNFs and MWCNTs under fluidized bed conditions (FB-CVD) [61]. In recent years, there has been promising research into the use of coal as a source material. Since the early 1990s, coal has been documented as a carbon source in the synthesis of carbon nanomaterials when fullerenes were produced from coke [14]. The advances made in using coal for CNT production are discussed below.

3. Processes used to synthesise carbon nanotubes using coal as a carbon source 3.1.

Arc discharge method

Pure carbon electrodes [62–64] 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. Williams et al. [65] achieved a moderate yield of SWCNTs using coal-based electrodes (bituminous coal from eastern Kentucky, USA) which were loaded with catalyst Ni-Y. The diameter distribution in the product was 1.2–1.7 nm with individual bundle diameters of about 10 nm. They recognized that the presence of catalyst metal impurities in coal may have a synergistic effect in CNT production. Indeed, they were able to produce CNTs without any catalyst from pyrite-rich coal (5%). Mathur et al. [68] also demonstrated CNT production from coal without the aid of any catalyst using the arc discharge technique. Wang et al. [69] reported the synthesis of branched carbon nanotubes (BCNTs) with purity of ca. 70% from anthracite coal by arc discharge with copper as a catalyst. A typical HRTEM image of a BCNT is shown in Fig. 1(a), which is different from straight CNTs that have perfect cylindrical structures. It is deduced that coal must play a critical role in this process since only a few BCNTs were obtained in similar tests using high-purity graphite powder as carbon source instead of coal powder. The presence of sulphur in the coal has also been speculated to be responsible for producing the branched structures [69]. The results presented by Li et al. [70] show

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that a large amount of well-defined bamboo-shaped carbon tubes could be synthesized by arc evaporation of coal-derived carbon rods with iron as catalyst. Typical outer diameters of their nanotubes are in the range of 40–60 nm. A perfect bamboo-shaped carbon nanotube (BCT) with a length of about 5 lm is shown in Fig. 1(b). Detailed studies on the production of CNTs from Chinese coals using arc discharge has been carried out by Qiu and co-workers [71–75]. They showed [75] that high quality double-walled carbon nanotubes (DWCNTs) can be produced in large scale from anthracite coal-based electrodes by arc discharge method with Fe as catalyst. Typical images of DWCNTs are shown in Fig. 1(c) and on average, the interlayer spacing between the inner and outer walls is ca. 0.4 nm. The arc discharge process using graphite electrodes depends on breaking of the graphite lattice structure and releasing of C1, C2 species etc. for the formation of CNTs. Qiu et al. [75] believe that this breakup scheme is completely different in the case of coal due to its striking difference in texture compared to graphite (macromolecular structure vs. lattice structure) which can contribute to the production of DWCNTs. The presence of sulfur is also thought to play a role in producing double-walled nanotubes. Their group also demonstrated the synthesis of high-purity SWCNTs from coal-based carbon rods by arc discharge with iron powder as catalyst [73]. Typical high magnification image of seven SWCNT bundles are shown in Fig. 1(d). The use of a wire cage around the carbon electrodes for collecting the nanotubes is credited with improving the purity of the SWCNT bundles. Qiu et al. [72] successfully prepared MWCNTs from 10 typical Chinese caking-coals and one New Zealand coal using the arc discharge technique. The yield of CNTs from the 10 bituminous Chinese coals was between 23.5% (Tonghua coal) and 60.4% (Xiangyuan coal). The New Zealand coal had the highest CNT yield among all coal samples i.e. 62.2%. It was found that the CNT yield increases as the fixed carbon content in coal increases (Fig. 2a) or as the volatile matter content in coal decreases (Fig. 2b). They also produced large quantities of MWCNTs from three kinds of Chinese coals by a DC arc-discharge method [74]. The yield of CNTs obtained from the three coals was compared at a pressure of 0.06 Mpa. The anthracite coal gives the highest CNT yield of 8.17 wt.% while the CNT yield for the long-flame coal is 2.72 wt.% and that of the natural coal is 1.44 wt.%. Higher carbon content in the coals means that more ion species or carbon clusters in the gas phase of the plasma zone are available for CNT formation [72]. Therefore, carbon rods with high carbon content would be good starting material for producing CNTs. A typical HRTEM image of the coal-derived MWCNTs is shown in Fig. 1(e).

3.2.

Thermal plasma jet

The thermal plasma jet (TPJ) process is a relatively new technique in the synthesis of CNTs. This technique has grown in popularity because of advantages such as continuous operation, high growth rate of CNTs and easy collection of products [76]. However, the drawback of this method inhibiting process scale-up is mostly related to the high consumption of electrical energy [77]. Thermal plasmas can be described as partially

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Fig. 1 – (a) HRTEM image of one Y-junction CNT Reprinted with permission from [69]; (b) TEM image of a long CNTwith perfect bamboo-shaped structure Reprinted with permission from [70]; (c) HRTEM images of DWCNTs prepared from coal Reprinted with permission from [75]; (d) HRTEM image of coal-derived film-like SWCNTs, Scale bar = 20 nm Reprinted with permission from [73]; (e) Typical HRTEM image of MWCNT with a cone-like end, Scale bar = 4 nm Reprinted with permission from [74].

or fully ionized gases consisting of electrons, ions, neutral atoms, and radicals, in which the negatively and positively charged particles balance each other (a property known as quasi-neutrality). The set-up for TPJ mainly consists of an arc plasmatron, a feed motor, a reactor (usually graphite-lined steel) and an accumulator [78]. An arc is ignited between the water-cooled metal anode and cathode [79–82]. A reaction mixture of Ar and H2 (or He) [76,83] passes the arc and is heated immediately to high temperatures (3700 K) leading to the generation

of a plasma jet containing various active species [84]. Coal, with or without a catalyst, is then successively and directly injected into the arc plasma instead of arcing graphite or coal-based electrodes [85,86]. Accompanying reactions lead to deposition of products on the reactor wall containing various carbon nanostructures such as CNTs and onion-like fullerenes (OLFs) [78,87–89]. Tian et al. [78] describe the production of MWCNTs from Baode coal with a size distribution of 5–25 microns using a direct current TPJ. Copper as catalyst was generated by the

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Fig. 2 – (a) Variation of the yield of CNTs with the fixed carbon content in coal; (b) Variation of the yield of CNTs with the volatile matter content in coal Reprinted with permission from [72]. direct arcing of the copper anode. The results showed that using the appropriate kind of catalyst, increasing the homogeneity of the mixture of coal and catalyst and extending the reaction time could further develop this as a viable technique. Well-graphitised MWCNTs were also produced using this process from Co and Fe catalysts [86]. The yield of CNTs with Cu as catalyst was found to be higher than that with Co and Fe. A novel method using a radio frequency (RF) plasma reactor to synthesize high purity OLFs in large quantities

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from coal was reported by Du et al. [87]. Plasma processing of solid carbon sources other than graphite or coal [88,89] also result in a variety of carbon nanomaterials. For example, Okuno et al. [90] report the high-yield synthesis of ‘‘stackedcup’’ CNTs and nano-necklaces from carbon black using 3-phase alternating current plasma technology. It was found that ‘‘stacked-cup’’ CNTs form at temperatures of between 1000 and 1300 °C where the metal catalyst is at a solid state, while the nano-necklaces grow at temperatures of between 1700 and 2400 °C when the metal catalyst is in a molten state. Experimental data presented by Dubrovsky et al. [91] demonstrate a possibility for mass production of fullerenes in a TPJ from finely powdered low cost carbon black produced from waste hydrocarbons. Cota-Sanchez et al. [77] demonstrated a continuous synthesis route of OLFs and MWCNTs using RF induction plasma to activate carbon-bearing reactants.

3.3.

Chemical vapour deposition

There have not been many reports on the use of CVD with coal as carbon source, compared to the relatively large number of studies using arc discharge and thermal plasma jet described in the previous sections. Qiu et al. [92] describe the

Fig. 3 – (a) and (b) Two typical TEM images of SWCNTs prepared from coal gas by catalytic CVD Reprinted with permission from [92]; (c) TEM image of iron carbide-oxide filled CNTs; (d) HRTEM image showing the graphitic carbon layers of CNT and the lattice fringes of the filled metal-like material Reprinted with permission from [93].

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Table 1 – Different types of CNTs produced from coal (solid and gas) using arc discharge, TPJ and CVD techniques (arranged chronologically). ID, OD: inner and outer diameters. Carbon product

Catalyst

Synthesis technique

Carbon source

Reference

Bituminous (Kentucky, USA) Anthracite (Sinkiang Uighur Autonomous Region, China) Bituminous ((Taiji, Fushun, Tonghua, Sanbao, Gujiao, Xiaoyi, Kailuan, Xiongtai, Pangzhuang, Xiangyuan), China), New Zealand Anthracite (China) Natural Coke (Anhui Province, China), Long-Flame coal and Anthracite (Ningxia Hui Autonomous Region, China) Anthracite (Yunnan Province, China)

Williams et al. [65]

Single-walled CNTs (OD: 1.2–1.7 nm) Bamboo-shaped CNTs (OD: 40–60 nm; length of several microns) Multi-walled CNTs (OD: 2–15 nm; length from 5 to 60 microns)

Ni-Y

DC arc discharge

Fe

DC arc discharge

–

DC arc discharge

Single-walled CNTs (OD: 1.24–2.19 nm) Multi-walled CNTs (OD: 2–15 nm; length from 4 to 70 microns)

Fe

DC arc discharge

–

Arc discharge

Branched CNTs (ID: 40–50 nm, OD: 50–60 nm) Copper-filled CNTs (OD: 30–80 nm; length is over several tens of micrometers) Double-walled CNTs (OD: 1.0–5.0 nm) Single-walled CNTs and Multi-walled CNTs (OD: 20–30 nm) Multi-walled CNTs (length 7 micons) Multi-walled CNTs (OD: 50–70 nm) Single-walled CNTs (OD: 0.84–1.29 nm; length of several micrometers) Iron carbide-oxide filled CNTs (OD: 30–50 nm)

CuO

DC arc discharge

CuO

DC arc discharge

Anthracite (Yunnan Province, China)

Wang et al. [96]

Fe

DC arc discharge

Qiu et al. [75]

None

DC arc discharge

Anthracite (Shanxi Province, China) Bituminous (Raniganj, India)

Cu

DC TPJ

Tian et al. [78]

Fe, Co, Cu

DC TPJ

Ferrocene

CVD

Gas-fat coal (Baode, China) Gas-fat coal (Baode, China) Coal gas (Dalian Company, China)

Ferrocene

CVD

Coal gas (Dalian Company, China)

Qiu et al. [93]

Li et al. [70] Qiu et al. [72]

Qiu et al. (2004) [74]

Mathur et al. [68]

Tian et al. [86] Qiu et al. [92]

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Wang et al. [69]

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Qiu et al. [73]

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production of SWCNTs from coal gas using the CVD technique. The coal gas, consisting mainly of components such as CH4, CO, H2, N2 and CO2 with the remaining being C2–C3 hydrocarbons, was fed into a horizontal quartz reactor with a two stage furnace. Ferrocene was used as the iron-bearing catalyst source. Fig. 3 shows SWCNT bundles or ropes with a length of several microns formed by this process. The formation rate of SWCNTs was found to increase with the flow rate of coal gas. Iron carbide-oxide filled CNTs have also been synthesized by catalytic CVD using coal gas at 950 °C with ferrocene as catalyst [93]. TEM images of MWCNTs that are partly filled with the metal-like particles are shown in Fig. 3(c and d). However, the possible synergic effect of gaseous species in the coal gas in the formation of the filled-carbon nanostructures still needs to be clarified.

4. The role played by coal in carbon nanotube production

3.4.

4.1.

Summary of production techniques using coal

It is seen from Table 1 that different types of CNTs can be produced from arc discharge, TPJ and CVD methods using coal as carbon source. The feasibility and practicality of using coal as an electrode material for arc discharge synthesis of CNTs is at present unclear. Both the arc discharge and TPJ techniques are energy intensive and uneconomical methods of production of CNTs on large-scale despite offering high yield and quality of CNTs [94]. Even though coal is cheap, the cost saving in all probability is insignificant compared with the costs of labour and energy associated with these two methods [95]. CVD appears to be the best-suited, economically viable synthesis route for large scale CNT production using coal; however, there is very little work establishing the full scale potential at present. CVD is a dynamic technique with significant promise in terms of quality and quantity for the production of CNTs using hydrocarbons until now [94]. CNTs produced using this method are considered high quality (minimal surface defects) and the technique has been scaled up to synthesize large quantities of CNTs in a continuous operating mode [61]. It is expected that CVD will be a viable technique for CNT production using coal as the carbon source. Some specific details related to using coal, regardless of CVD or arc technique, are discussed in Section 4. Although there are several production methods and carbon sources to synthesize various kinds of CNTs and carbon nanomaterials, large quantities of high purity and high quality CNTs are still too expensive for the realisation of industrial applications [61]. The market for CNTs, carbon nanofibers, fullerenes and graphene has been growing rapidly in recent years. The current market prices for various types of carbon nanotubes worldwide are compiled in Tables S1 and S2 (see Supplementary data). Table S1 shows the current prices for commercially available SWCNTs where a direct correlation is seen between the purity and the cost, even though the commercial SWCNT purification rates barely reach 80% in most cases. The trends in Table S1 indicate higher prices for higher purity for arc discharge products, with a price of $2500/g for greater than 90% purity. The prices for SWCNTs produced by CVD methods in Table S2 ranges from $150/g to $510/g for purities of at least 90% [97]. Table S2 shows the current prices for commercially available MWCNTs, DWCNTs and bamboo-shaped CNTs. The

CVD technique is used to produce relatively cheaper MWCNTs, with prices starting from $5 per gram. High purity (>95%) DWCNTs have a price of $350/g with the price decreasing as the purity decreases. As large quantities of various types of CNTs are needed in market-driven applications such as composites, catalyst support, semiconductors, electronics, etc., the prohibitive costs could drastically diminish the benefits of using CNTs in these applications. Hence, there is a strong need for establishing practical techniques for the selective large-scale synthesis of high purity carbon-based nanomaterials.

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 region to region. In comparing coal to graphite, Patney et al. [98] state that, unlike graphite which is an allotropic form of carbon, coal is a molecular solid consisting of a wide range of macromolecular organic and inorganic compounds. Multiple variations of composition in naturally arising coals therefore exist. Furthermore, a range of secondary products in the form of light and heavy hydrocarbons, tars, chars and coke may be derived from coal. In the literature, both coal and its derived products (coke and hydrocarbons), have been used in the synthesis and production of CNTs [13,39,72]. In simple terms, it is believed that yield of CNTs increases as the fixed carbon content in a coal and derived product increases or as volatile matter and ash content decrease [13,72]. Qiu et al. [72] offer an explanation that such correlations are attributable to the fact that hydrogen and other hetero-atoms which occur in high volatile matter may inhibit the formation of CNTs whereas the higher carbon content in the source coals (or cokes, chars and tars) means that more carbon ion species or carbon clusters are available for CNT production. However, coal is far more complex in terms of its constituents than simply volatile matter, carbon and ash content. In several recent papers [13,72,98], authors have considered the role of the microscopic organic components in coal termed macerals, each varying in chemical and physical properties according to the precursor source of plant matter and its geochemical history during the course of coal formation. The most successful components in terms of yield and purity of CNT production were found to be the vitrinite and liptinite group macerals [13,98]. This is attributable to the fact that these components possess much higher proportions of aliphatic chains and bridges which, in specific ranges within the bituminous rank of coal, emit light and heavy hydrocarbons, tars and chars as well as volatile matter on heating.

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This is in contrast to the third maceral group, inertinite, which is characterised by high aromaticity, large, tightly bonded aromatic carbon rings and higher concentrations of oxygen-functional groups which generally do not yield hydrocarbon products at all and are not favourable for nanotube formation [13,99,100]. Qiu et al. [72] found that, in a range of caking coals used for the metallurgical industry, the highest yields of CNTs were provided by some of the best caking coals and that these yields were comparable to that obtained using graphite. These coals contained relatively high vitrinite contents. It is conceivable that the reason for the high yields of CNTs was due to the nature and quality of the tars and cokes that were derived from the vitrinite macerals in those coking coals. In support of this, Patney et al. [98] reported that fullerenes and a range of CNTs and pyrolytic carbon forms were produced from a plasma arc process using tar (mesophase) formed during the coking of coal and from the heavy hydrocarbons (kerogen-like material) produced from heating (pyrolysing) liptinite. These experiences indicate that the characteristics of both the natural coal and the secondary products derived from coal (volatiles, light and heavy hydrocarbons, tars, chars) would need to be considered when evaluating the use of coals for CNT production and selecting the most suitable process. It is also important to note that the temperature, time and atmosphere to which coals are exposed during the course of electrode preparation will dictate the form of char or coke that will be used as the electrode to produce the CNTs. In addition to maceral composition in coal, the rank or degree of maturity of coal has become an issue of importance. Researchers have found that the higher rank coal (more mature, lean coals and anthracites) with high carbon contents and reduced volatile matter provide better CNT yields than coals in the lower ranks (less mature, higher volatiles) [13,72,98]. However, mid-ranking coals in the bituminous range have also been shown to provide good yields of CNTs, including the caking and coking coals of China [72]. The latter situation arises because the prime coking coals and some caking coals which occur in this range of rank undergo severe molecular changes: once reaching this level of rank, resulting in maximum fluidity, minimum density and major molecular re-arrangement, all of which take place in the vitrinite maceral. On heating, devolatilisation is followed by the release of heavy hydrocarbons and the production of tars and mesophase (a liquid crystal structure formed during the coking of prime coal). A solid carbon-rich coke remains. Both the tars and mesophase can be condensed into secondary carbon-rich solid coke forms as well as the naturally-derived coke, all of which provide a good source for CNTs. Similarly, on producing electrodes using natural coal as the feedstock for arc discharge, the coal will pass through physical molecular changes during the carbonisation process, arriving at different structural products in the electrode subject to temperature, time and atmosphere. For this reason, it is important when selecting the best coal for a specific process that the full chain reaction during preparation of the coal-to-electrode be considered. Based upon the observations noted to date, and subject to the processes to be used in the manufacture of CNTs, it may

be possible to select coals with organic matter qualities that would likely provide the highest yield of CNTs. The inorganic content or mineral matter in coal is a further aspect to be considered seriously. Qiu et al. [72] divide these into three groups. The first group is represented by the clay minerals, usually present in the form of kaolin, illite or mixed layers clay. The second group is comprised of carbonates, feldspars and sulphur-bearing compounds. Oxides and other compounds, often combined with heavy metals, make up a third group. Most of the minerals including silica, alumina and clay have been found to reduce the nanotube yields, whereas the iron-containing mineral species are reported to have a catalytic effect during the synthesis process, thereby favouring the formation of nanotubes [13]. The presence of mineral matter (characterised by ash content) in the coal, tends to result in reduced yields of carbon nanomaterials [13,72,98,101]. Qiu et al. [72] and Yu et al. [13] suggest that removing the mineral matter in raw coal is likely to help to further increase the CNT yield. However, the removal of finely entrained minerals such as clays from many ash-rich coals is difficult. Furthermore, the presence of clays may lead to the production of naturally occurring noncarbon-based tubular shapes with nanometre dimensions, as has been reported in Ref. [102]. Most research work reported in literature reviewed here was performed on low ash coals (less than 12%). However, work conducted on medium ash coals up to 24% proved that fullerenes and possibly CNTs could be produced [13,101]. Given that the presence of significant amounts of ash will reduce the yields of CNTs and that low yield level and purity would significantly increase production costs, low ash coal is obviously preferred [13]. Although the role of mineral matter in coal in the production of CNTs is important, the detailed mechanism involved is not fully understood [72,13]. However, the presence of some mineral-derived inorganic elements in coal has been found to be beneficial and to favour the formation of nanotubes [13]. These include the catalytic elements in mineral matter such as the iron containing species. The presence of some suitable catalysts (Fe, Co, and Ni) is necessary to promote SWCNT growth, but at least one of these (Fe) occurs naturally in coal as an oxide and/or in pyritic form. The iron group metals are known for their capability to dissolve or react with carbon to form various metal carbides, of which the surfaces are highly effective at dissociative chemisorption of carbon species and will function as the catalyst for the growth of CNTs if enough carbon species could be supplied continuously [74,103]. Other elements in coal that may have an influence on CNT production include H, N, O, and S. If attached to the CNT structures, they will cause difficulty in controlling the product quality during CNT production [13]. Qui et al. [75] envision that the indigenous sulphur existing in coal and coal-derived electrodes might participate in the formation of DWCNTs [104] and promote the growth of DCWNTs [105–107] as well as generate DWCNTs with a larger diameter distribution owing to its capability of binding and stabilizing the growth ends of large diameter tubes [104,108]. Williams et al. [65] found that under similar conditions the overall diameter distribution for (10, 10) SWCNTs is similar to the SWCNTs grown from

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graphite-derived electrodes. The SWCNT bundle distribution (diameters of 10 nm and smaller) is however wider than that typically observed in graphite-derived SWCNT. Many bundles observed in the coal-derived material consist of only a few individual nanotubes. The presence of sulphur in the coal plays a role in the wider diameter distribution of the SWCNTs [109]. In addition, Qui et al. [73] found that the diameter distribution of the coal-derived SWCNTs was larger than that produced from graphite; however, it is reasoned that the large dispersion could be due to both the Fe catalyst used and the coal-derived electrodes. Sulphur present in coal is also believed by Wang et al. [69] to be accountable for the formation of BCNTs because it could help to form active sites on the catalyst by selectively poisoning the surface of the catalyst particles [106,110] on which the graphite layers could selectively deposit and thus lead to the growth of the CNT branches [111].

4.2. Differences between the carbon nanotube formation mechanism between coal and graphite Currently little work exists that explores the effects of organic and inorganic constituents in coal on the formation and growth of CNTs relative to using graphite as the carbon source in the arc discharge technique. The results from previous studies [70–72,112] have shown that the plasma arcing of coal-based carbon has chemical and process advantages over high purity graphite in the formation of carbon nanomaterials due to coal (a macromolecular solid) consisting of abundant irregular polymerized aromatic hydrocarbon units that are joined together by weak cross-links [13,113]. It is established that the plasma arcing process of graphite electrode involves complete breaking of benzoidal units in the graphite structure, which release C1 and/or C2 carbon species that take part in the formation reactions of CNTs. This means that the growth of graphite-based CNTs have to proceed through a mechanism in which C1 and C2 carbon species are involved [74,112,114]. These bond breaking and reforming processes require intense energy and involve many sterically elementary processes for complex CNT production [72]. In the case of coal-derived carbons, the breaking-up scheme involved in the arc-discharge process significantly differs from that of graphite because of the striking difference in their textures [75]. In addition, due to there being many weak links between carbon polymeric units such as aryl structures in the chemical structure of natural coals, in principle, they can be fragmented easily and incorporated into the CNT structures [72,113,114]. Accordingly, all the bonds in coal-derived electrodes need not be necessarily broken for CNT formation [112,115]. As a result, Qui et al. [72] expect that higher CNT yields would be obtained at lower arcing temperatures when coal-derived electrodes are employed rather than those derived from graphite. Qui et al. [74] and Wilson et al. [112] attribute the different mechanism of CNT formation from coal in comparison to graphite for being partly responsible for the rich morphologies of carbon nanomaterials obtained from coal. Different mechanisms for the growth and formation of BCTS have been proposed [116–120] however, the detailed mechanism involved in the growth process of BCTs using coal as the starting carbon material is not yet well understood. Li

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et al. [70] realise the formation mechanism of BCTs might be different from that with graphite as the starting precursor and as such, a new model needs to be explored to explain the growth of coal-derived BCTs with the peculiar chemical structure and properties of coal taken into account. Given that the mechanism involved in the formation process of coal-based DWCNTs is still not clear, Qui et al. [75] conclude that coal-derived carbon, having unique chemical structures, must play a critical role in the formation of DWCNTs because few DWCNTs were obtained in comparison tests in which high-purity graphite was used as carbon source instead of coal-derived carbon rods. The detailed mechanism involved in the formation of BCNTs is not presently clear. Wang et al. [69] postulate that various reactive hydrocarbon molecules as well as sulphur species generated from fast pyrolysis of coal in arc plasma might be responsible for the formation of BCNTs because few BCNTs were obtained in similar tests using high-purity graphite powder as carbon source instead of coal powder. Wang et al. [96] discovered that coal plays a crucial role in the growth of Cu-filled CNTs by conducting comparative tests under identical conditions with high purity graphite powders as carbon source instead of coal powders resulting in few Cufilled CNTs being observed in the final product. Accordingly, it is resolved that the use of ‘impure’ coal-derived electrodes can be effectively used in the synthesis of highly desirable products such as MWCNTs [72,74], SWCNTs [65,73], DWCNTs [75], BCNTs [69], BCTs [70] etc.

5.

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 (SWCNTor 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 as ferrocene could be expensive. Though these are unique issues arising from the use of coal, advancing CVD technique in general for CNT production has many fundamental unresolved issues including a complete understanding of mechanisms, selective growth and related issues [51]. With regard to coal-related issues, Yu et al. [13] stated that, despite the fact that impurities in coal may cause some difficulty in product control compared to other value-added materials, e.g. graphite and hydrocarbons, coal is likely to have a major role in CNT synthesis due to the following advantages, namely 1. It is inexpensive and is the most abundant carbon source material. 2. Weak bonds in the macromolecular structure of coal may lead to more effective formation of nanotubes. 3. Coal itself can be used as a purifying agent, in particular higher rank coals with mesoporous structures. 4. Catalyst can be easily added into coal during production process.

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Acknowledgements [18]

The authors acknowledge the financial support from the National Research Foundation (NRF) under South Africa NRF Focus Area, NRF Nanotechnology flagship programme, DSTfunded 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 under Project R31-2008-000-10100-0 is acknowledged.

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Appendix A. Supplementary data

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Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.carbon.2012.02.048.

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