10 Synthesis and Purification of Carbon Nanotubes

Dr. Anchal Srivastava

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Contents of this Unit

 

1   Synthesis of CNTs

1.1 Arc Discharge

1.2 Laser Ablation

1.3 Chemical Vapor Deposition (CVD)

2  Purification of CNTs 2.1 Oxidation

2.2 Acid treatment

2.3 Annealing and thermal treatment

2.4 Ultrasonication

2.5 Magnetic purification

2.6 Micro-filtration

2.7 Cutting

2.8 Functionalisation

2.9 Chromatography

3 Summary

 

Learning Outcomes

  • After studying this module, you shall be able to
  • Learn about different techniques for carbon nanotubes synthesis o Arc discharge,
  • Laser ablation and
  • Chemical vapour deposition (CVD).
  • Learn about the growth mechanism of carbon nanotubes. Learn the purification techniques for nanotube
  • purification
  • Oxidation
  • Acid treatment
  • Annealing and thermal treatment o Ultra-sonication
  • Magnetic purification o Micro-filtration
  • Cutting
  • Functionalisation o Chromatography

 

1 Synthesis of CNTs

 

The way in which CNTs are formed is not exactly known. The growth mechanism is still a subject of study, and more than one mechanism might be operative during the formation of CNTs. One of the mechanisms consists out of three steps. First a precursor to the formation of CNTs and fullerenes, C2, is formed on the surface of the metal catalyst particle. From this metastable carbide particle, a rod-like carbon is formed rapidly. Secondly there is a slow graphitisation of its wall. This mechanism is based on in situ TEM observations. The exact atmospheric conditions depend onthe technique used. The actual growth of the CNT seems to be the same for all techniques mentioned. The most accepted growth mechanisms are two models: tip growth and root growth. In the former, a tubule tip is open so that carbon atoms can be added to its circumference, and the metal catalyst promotes the growth reaction and also prevents the tubule tip closure. One study proposed that fullerene molecules would act as the growth nuclei such that the diameter of the tubule will determine CNT size. The latter model is based on the phase diagram of carbon and a metal. The SWCNTs grow as carbon precipitates when the molten metal dissolving carbon is cooled and solidified. More convincing experimental evidence is needed for better understanding of the growth mechanism. There are several theories on the exact growth mechanism for CNTs. One of the most accepted theories postulates that metal catalyst particles are floating or are supported on graphite or another substrate. It presumes that the catalyst particles are spherical or pear shaped, in which case the deposition will take place on only one half of the surface (this is the lower curvature side for the pear shaped particles). The carbon diffuses along the concentration gradient and precipitates on the opposite half, around and below the bisecting diameter. However, it does not precipitate from the apex of the hemisphere, which accounts for the hollow core that is characteristic of these filaments. For supported metals, filaments can form either by extrusion, in which the CNT grows upwards from the metal particles that remain attached to the substrate, or by tip-growth, in which the particles detach and move at the head of the growing CNT. Depending on the size of the catalyst particles, SWCNT or MWCNT is grown (Fig. 1). Carbon nanotubes are generally produced by three main techniques: arc discharge, laser ablation, and chemical vapour deposition. Though scientists are researching more economic ways to produce these structures. In arc discharge technique, a vapour is created by an arc discharge between two carbon electrodes with or without catalyst. CNTs self assemble from the resulting carbon vapour. In the laser ablation technique, a high power laser beam impinges on a volume of carbon containing feedstock gas (such as methane or carbon monoxide). At the moment, laser ablation produces a small amount of clean CNTs, whereas arc discharge methods generally produce large quantities of impure material. In general, chemical vapour deposition (CVD) results in MWCNTs or poor quality SWCNTs. The SWCNTs produced with CVD have a large diameter range, which can be poorly controlled. But on the other hand, this method is very easy to scale up, what favours commercial production. Laser ablation was the first technique used to generate fullerenes in clusters. In this process, a piece of graphite is vaporised by laser irradiation under an inert atmosphere. This results in soot containing CNTs which are cooled at the walls of a quartz tube. The CVD and the most current fluidised bed (FB) chemical vapour deposition method has shown the most promise in terms of its price/unit ratio due to excellent heat and mass transfer ensuing a homogeneous product, inherent scalability and comparatively low cost. CVD generally involves reacting a carbon containing gas (such as acetylene, ethylene, and ethanol) with a metal catalyst particle (usually cobalt, nickel, iron or a combination of these such as cobalt/iron or cobalt/molybdenium) at temperatures above 600ºC. Unfortunately, although these methods can produce large quantities of CNTs, their cost is still high to make any large-scale applications.

Figure 1: Visualisation of a possible CNT growth mechanism.

 

 

Carbon nanotubes were observed in 1991 in the carbon soot of graphite electrodes during an arc discharge, by using a cur- rent of 100 amps, which was intended to produce fullerenes. However, the first macroscopic production of CNTs was made in 1992 by two researchers at NEC’s Funda- mental Research Laboratory. The method used was the same as in 1991. During this process, the carbon contained in the negative electrode sublimates because of the high temperatures caused by the discharge. Because CNTs were initially discovered using this technique, it has been the most widely used method of CNT synthesis. Commonly used gaseous carbon sources include: methane, ethylene, ethanol, carbon monoxide and acetylene. If both electrodes are graphite (pure graphite electrodes), the main product will be MWCNTs. But if SWCNTs are preferable, the anode has to be doped with metal catalyst, such as: Fe, Co, Ni, Y or Mo (use a mixture of graphite with metal catalysts). In addition to previous methods, high pressure CO dispro- portionation process, flame synthesis, plasma torch method, electrolysis, and solar energy methods have also been proposed to the synthesis of CNTs and specially to the synthesis of SWCNTs. Fullerenes and CNTs are not necessarily products of high-tech. laboratories; they are commonly formed in such mun- dane places as ordinary flames (Singer and Grumer, 1959), produced by burning methane, ethylene, and benzene, and they have been found in soot from both indoor and outdoor air. However, these naturally occurring varieties can be highly irregular in size and quality because the environment in which they are produced is often highly uncontrolled. Thus, although they can be used in some applications, they can lack in the high degree of uniformity necessary to meet many needs of both research and industry. Table 1 summarises the three most common techniques used. Currently, a CVD is the most widely used method to produce the CNTs, other methods had been recently developed for the preparation of CNTs and improving the existing methods, promising results have been achieved for reaching the better degree of purification and effectively reducing the treatment time. Some of the CNTs preparation methods are more effective than others but a problem that all methods face is the ability of the CNTs to self align. Many applications of CNTs require controlled growth of aligned CNTs with surface modification. Controlled synthesis of well-aligned CNTs in predetermined patterns is particularly important in terms of fundamental studies and applications (Fig. 2A and B).

 

1.1 Arc discharge

 

Arc discharge belongs to the methods that use higher temperatures (above 1700°C) for CNT synthesis which usually causes the growth of CNTs with fewer structural defects in comparison with other techniques. The arc discharge synthesis of MWNTs is very simple in the case when all growth conditions are ensured. The most utilized methods use DC arc discharge between two graphite usually water-cooled electrodes with diameters between 6 and 12 mm in a chamber filled with helium at sub atmospheric pressure. Nevertheless, some other works with the use of hydrogen or methane atmosphere have been also reported. For example, Ebbesen and Ajayan use a variant of the standard arc discharge technique also used by Iijima for fullerene synthesis under He atmosphere to obtain first large-scale synthesis of CNTs. Under certain conditions, a pure nanotube and nanoscale particles in high yield were obtained. The purity and yield depended sensitively on the gas pressure in the reaction vessel. Wang et al. showed that different atmospheres markedly influence the final morphology of CNTs. They used DC arc discharge of graphite electrodes in He and methane. By evaporation under high pressured CH4 gas and high arc current, thick nanotubes embellished with many carbon nanoparticles were obtained. On the other hand, thin and long MWNTs were obtained under a CH4 gas pressure of 50 Torr and an arc current of 20 A for the anode with a diameter of 6 mm.21 Moreover, Zhao et al. found that the variation of carbon nanotube morphology was more marked for the case of evaporation in CH4 gas than that in He gas. In different work, Zhao et al. used hydrogen gas atmosphere for preparation of fine and long MWNTs. By comparing with He and methane gases, a very big difference was found. Namely, little carbon smoke occurred in H2 gas, but much more carbon smoke was observed for the evaporation in CH4 and He gases. Later they showed that evaporation of graphite electrodes in H2 gas by DC arc discharge forms not only fine and long MWNTs but also graphene sheets deposited on the cathode. Shimotani et al. reported synthesis of MWNTs using an arc discharge technique under He, ethanol, acetone and hexane atmosphere at various pressures (from 150 to 500 Torr). They concluded that arc discharges in the three organic atmospheres (ethanol, acetone and hexane) produce more MWNTs, by two times at least, than those in the He atmosphere. This can be explained as follows: contrary to helium, the acetone, ethanol and hexane can be ionized and the molecules can be decomposed into hydrogen and carbon atoms. These ionized species may contribute the synthesis of MWNTs, so the higher yield of CNTs is produced. They showed that in all the cases of organic molecular atmospheres, yields of MWNTs increase as the pressure increases up to 400 Torr. Jiang et al. studied the influence of NH3 atmosphere on the arc-discharge growth of CNTs and demonstrating that the arc-discharge method in NH3 atmosphere is one highly efficient method for CNTs preparation. They concluded that there is no significant difference of the shapes and the structures between NH3 atmosphere and other atmospheres such as He, H2, etc.The consumption of anode during the process is faster than the growth of MWNTs layer on the cathode. Therefore the gap between the electrodes of 30 to 110 mm2 surface area has to be held in the desired distance during the growth process (usually between 1 and 4 mm). This is ensured by one electrode constant feed that leads to a high yield and stable arc discharge growth process. The arc discharge deposition is usually done as a DC arc discharge, but pulsed techniques were also reported. For example Parkansky et al. reported single-pulse arc production of near vertically oriented MWNTs deposited on the Ni/glass samples using a graphite counter-electrode in ambient air. MWNTs (typically 5–15 walls) with a diameter of about 10 nm and lengths of up to 3 mm were produced on the samples with a single 0.2 mspulse. Tsai et al. also used single-pulse discharge in air. They obtained MWNTs with the outer diameter of 17 nm and an inner diameter of 5 nm using a peak current of 2.5 A and a discharging time of 1000 ms.28 Arc discharge is usually used for some non-standard CNTs deposition. Contrary to standard MWNTs deposition using a gas atmosphere there were reported several works involving arc discharge in liquid solutions. Jung et al. reported high yield synthesis of MWNTs by arc discharge in liquid nitrogen. They concluded that this technique can be a practical option for the large-scale synthesis of MWNTs with high purity. A similar method was also used for MWNTs deposition by Sornsuwit and Maaithong Montoro et al. reported the synthesis of highquality SWNTs and MWNTs through arc discharge in H3VO4 aqueous solution from pure graphite electrodes. DC arc discharge was generated between two high purity graphite electrodes. The high-resolution TEM images clearly showed that MWNTs are highly crystalline, with a well-ordered structure and free of defects. They obtain MWNTs with an outer diameter of 10–20 nm and an interlayer distance of approximately 0.35 nm between graphene layers. MWNTs were also synthesized in high yield by arc discharge in water between pure graphite electrodes by Guo et al. The production of carbon nanomaterials by arc discharge under water or liquid nitrogen was also reported by Xing et al.

 

1.2 Laser ablation

 

The properties of CNTs prepared by the pulsed laser deposition process (PLD) are strongly dependent on many parameters such as: the laser properties (energy fluence, peak power, cw versus pulse, repetition rate and oscillation wavelength), the structural and chemical composition of the target material, the chamber pressure and the chemical composition, flow and pressure of the buffer gas, the substrate and ambient temperature and the distance between the target and the substrates. Laser ablation, as crucial step of PLD, is one of the superior methods to grow SWNTs with high-quality and high-purity. In this method, which was first demonstrated by Smalley’s group in 1995, the principles and mechanisms are similar to the arc discharge with the difference that the energy is provided by a laser hitting a graphite pellet containing catalyst materials (usually nickel or cobalt). Almost all the lasers used for the ablation have been Nd:YAG and CO2. For example, Zhang et al. prepared SWNTs by continuous wave CO2 laser ablation without applying additional heat to the target. They observed that the average diameter of SWNTs produced by CO2 laser increased with increasing laser power. Until now, the relationship between the excitation wavelength and the growth mechanisms of SWNTs has not been clarified. It may be expected that a UV laser creates a new species of nanoparticles and suggests a new generation mechanism of CNTs because the UV laser is superior in the photochemical ablation to the infrared laser which is effective for photothermal ablation. Lebel et al. synthesized SWNTs using the UV-laser (KrF excimer) ablation of a graphite target appropriately doped with Co/ Ni metal catalyst. In their work, they tested as-prepared SWNTs as a reinforcing agent of polyurethane. Kusaba and Tsunawaki used XeCl excimer laser with the oscillation wavelength of 308 nm to irradiate a graphite containing Co and Ni at various temperatures and they found that laser ablation at 1623 K produced the highest yield of SWNTs with the diameter between 1.2 and 1.7 nm and the length of 2 mm or above.64 Recently, Stramel et al. have successfully applied commercial MWNTS and MWNTs–polystyrene targets (PSNTs) for deposition of composite thin films onto silicon substrates using PLD with a pulsed, diode pumped, Tm:Ho:LuLF laser (a laser host material LuLF (LuLiF4) is doped with holmium and thulium in order to reach a laser light production in the vicinity of 2 mm). They found that usage of pure MWNTs targets gives rise to a thin film containing much higher quality MWNTs compared to PSNTs targets. Similarly, Bonaccorso et al. prepared MWNTs thin films deposited by PLD techniques (with Nd:YAG laser) ablating commercially polystyrene-nanotubes pellets on alumina substrates.

 

1.3 Chemical Vapor Deposition (CVD)

 

Catalytic chemical vapor deposition (CCVD)—either thermal or plasma enhanced (PE)—is now the standard method for the CNTs production. Moreover, there are trends to use other CVD techniques, like water assisted CVD, oxygen assisted CVD,hot-filament (HFCVD), microwave plasma (MPECVD) or radiofrequency CVD (RF-CVD). CCVD is considered to be an economically viable process for large scale and quite pure CNTs production compared with laser ablation. The main advantages of CVD are easy control of the reaction course and high purity of the obtained material, etc. The CNT growth model is still under discussion. Recently, Fotopoulos and Xanthakis discussed the traditionally accepted models, which are base growth and tip growth. In addition, they mentioned a hypothesis that SWNTs are produced by base growth only, i.e. the cap is formed first and then by a lift off process the CNT is created by addition of carbon atoms at the base. They refer to recent in situ video rate TEM studies which have revealed that the base growth of SWNT in thermal CVD is accompanied by a considerable deformation of the Ni catalyst nanoparticle and the creation of a subsurface carbon layer. These effects may be produced by the adsorption on the catalyst nanoparticle during pyrolysis. In order to produce SWNTs, the size of the nanoparticle catalyst must be smaller than about 3 nm. The function of the catalyst in the CVD process is the decomposition of carbon source via either plasma irradiation (plasma-enhanced CVD, PECVD) or heat (thermal CVD) and its new nucleation to form CNTs. The most frequently used catalysts are transition metals, primarily Fe, Co, or Ni. Sometimes, the traditionally used catalysts are further doped with other metals, e.g. with Au. Concerning the carbon source, the most preferred in CVD are hydrocarbons such as methane, ethane, ethylene, acetylene, xylene, eventually their mixture, isobutane or ethanol. In the case of gaseous carbon source, the CNTs growth efficiency strongly depends on the reactivity and concentration of gas phase intermediates produced together with reactive species and free radicals as a result of hydrocarbon decomposition. Thus, it can be expected that the most efficient intermediates, which have the potential of chemisorption or physisorption on the catalyst surface to initiate CNT growth, should be produced in the gas phase. Commonly used substrates are Ni, Si, SiO2, Cu, Cu/Ti/Si, stainless steel or glass, rarely CaCO3; graphite and tungsten foil or other substrates were also tested. A special type of substrate, mesoporous silica, was also tested since it might play a templating role in guiding the initial nanotube growth. For example, Zhu et al. reported a CCVD synthesis of DWNTs over supported metal catalysts decomposed from Fe and Co on mesoporous silica. They obtained bundles of tubes with a relatively high percentage of DWNTs in areas where tubular layered structures could be clearly resolved. Moreover, the crystal-like alignment of very uniform DWNTs was observed. Similarly, Ramesh et al. succeeded in high-yield selective CVD synthesis of DWNTs over Fe/Co loaded high-temperature stable mesoporous silica. Another substrate, eolites, was studied by Hiraoka et al. They used CCVD of acetylene over well-dispersed metal particles (typically Co/Fe binary system) embedded in heat-resistant zeolites at temperatures above 900 C for selective synthesis of DWNTs. The choice of catalyst is one of the most important parameters affecting the CNTs growth. Therefore, its preparation is also a crucial step in CNTs synthesis. The influence of the composition and the morphology of the catalyst nanoparticles on CNTs growth by CVD are summarized in a review paper. Flahaut et al. reported the influence of catalyst preparation conditions for the synthesis of CNTs by CCVD. In their work, the catalysts were prepared by the combustion route using either urea or citric acid as the fuel. They found that the milder combustion conditions obtained in the case of citric acid can either limit the formation of carbon nanofibers or increase the selectivity of the CCVD synthesis towards CNTs with fewer walls, depending on the catalyst composition.98 Xiang et al. prepared CNTs via CCVD of acetylene on a series of catalysts derived from Co/Fe/ Al layered double hydroxides (LDHs). They observed that the content of Co in the precursors had a distinct effect on the growth of CNTs. Increasing Co content enhanced the carbon yield, due to good dispersion of a large number of active Co species. Higher Co content led to the formation of CNTs with smaller diameters and less structural disorder. Lyu et al. produced high-quality and high-purity DWNTs by catalytic decomposition of benzene as an ideal carbon source and Fe–Mo/Al2O3 as a catalyst at 900 C. They obtained DWNTs bundles free of amorphous carbon covering on the surface and of a low defect level in the atomic carbon structure.Zhang et al. prepared MWNTs with diameters of 40–60 nm by the catalytic decomposition of methane at 680 C for 120 min, using nickel oxide–silica binary aerogels as the catalyst. Sano and colleagues evaluated two systems of metallic catalyst/carbon sources for CNTs growth: ethanol/Co and benzene/Fe. Moreover, they investigated the effects of two different reactors (gas flow reactor and a submerged-in-liquid reactor) on the quality of CNTs. Jiang et al. studied the growth of CNTs in situ on the pretreated graphite electrode (GE) via CCVD using Ni(NO3)2 as the catalyst. The prepared CNTs had 80 and 20 nm in outer and inner diameter, respectively. Moreover, the CNTs were not very long (compared with data reported elsewhere): their length was from about 200 to 1000 nm as a result of shorter growing time. Scheibe et al. tested Fe and Co for MWNTs fabrication. Additionally, the authors were interested in concentrations of the carboxyl and hydroxyl groups on the carbon nanotube surface, which are essential features for applications in many science branches such as nanomedicine, biosensors or polymer nanocomposites.

 

2 Purification of CNTs

 

A large problem with CNT application is next to large-scale synthesis and also the purification. In all the CNT preparation methods, the CNTs come with a number of impurities whose type and amount depend on the technique used. The most common impurities are carbonaceous materials, whereas metals are the other types of impurities generally observed. The as-produced CNT soot contains a lot of impurities. The main impurities in the soot are graphite (wrapped up) sheets, amorphous carbon, metal catalyst and the smaller fullerenes. These impurities will interfere with most of the desired properties of the CNTs. Also in the fundamental research, it is preferred to obtain CNTs, as pure as possible. In order to understand the measurements better, the CNT samples also have to be as homogeneous as possible. The common industrial techniques use strong oxidation and acid-refluxing techniques, which have an effect on the structure of the tubes. Purification difficulties are considerable because CNTs are insoluble and, hence, liquid chromatography is limited.

Figure 2: (A) Scanning electron micrographs of CNTs at 10,000 and 20,000 magnifications.

 

(B) Micrographs showing aligned CNTs.

 

Carbon nanotube purification step (depending on the type of the purification) removes amorphous carbon from CNTs, improves surface area, increases or decreases mesopore or micropore volume, decomposes functional groups blocking the entrance of the pores or induces additional functional groups. In case of adsorption of bacteria for example, CNT purification steps such as heat treatment, NH3 treatment may be adapted so as to increase surface area and mesopore volume. Basically, these techniques can be divided into two main streams of separation techniques, namely, structure selective and size-selective separations. The first one will separate the CNTs from the impurities; the second one will give a more homogeneous diameter or size distribution. Most of this techniques used, are combined with other techniques to improve the purification and to remove different impurities at the same time. These techniques are:

 

2.1 Oxidation

 

Oxidative treatment of the CNTs is a good way to remove carbonaceous impurities or to clear the metal surface. The main disadvantages of oxidation are that not only the impurities are oxidised, but also the CNTs. Luckily the damage to CNTs is less than the damage to the impurities. These impurities have relatively more defects or a more open structure. Another reason why impurity oxidation is preferred is that these impurities are most commonly attached to the metal catalyst, which also acts as oxidising catalyst. Altogether, the efficiency and yield of the procedure are highly depending on a lot of factors, such as metal content, oxidation time, environment, oxidising agent and temperature.

Figure 3: Scanning electron micrographs of raw MWCNTs (A) and oxidised MWCNTs with concentrated HNO3, (B) The images were amplified 25,000 times.

 

2.2 Acid treatment

 

In general, the acid treatment will remove the metal catalyst. First of all, the surface of the metal must be exposed by oxidation or sonication. The metal catalyst is then exposed to acid and solvated. The CNTs remain in suspended form. When using a treatment in HNO3, the acid only has an effect on the metal catalyst. It has no effect on the CNTs and other carbon particles. If a treatment in HCl is used, the acid has also a little effect on the CNTs and other carbon particles. The mild acid treatment (4 M HCl reflux) is basically the same as the HNO3 reflux, but here the metal has to be totally exposed to the acid to solvate it (Fig. 3). A review of the literature demonstrates that the effect of key variables such as acid type and concentration, temperature, duration and pressure are not well understood and, due to their dependence, must be probed with correct experimental design to elucidate potential interaction effects.

 

2.3 Annealing and thermal treatment

 

Due to high temperatures (873–1873 K), CNTs will be rearranged and defects will be consumed. High temperature also causes the graphitic carbon and the short fullerenes to pyrolyse. When using high temperature vacuum treatment (1873 K) the metal will be melted and can also be removed.

2.4 Ultrasonication

 

This technique is based on the separation of particles due to ultrasonic vibrations. Agglomerates of different nanoparticles will be forced to vibrate and will become more dispersed. The separation of the particles is highly dependable on the surfactant, solvent and reagent used. The solvent influences the stability of the dispersed tubes in the system. In poor solvents the CNTs are more stable if they are still attached to the metal. But in some solvents, such as alcohols, monodispersed particles are relatively stable. When an acid is used, the purity of the CNTs depends on the exposure time. When the tubes are exposed to the acid for a short time, only the metal solvates, but for a longer exposure time, the tubes will also be chemically cut.

 

2.5 Magnetic purification

 

In this method ferromagnetic (catalytic) particles are mechanically removed from their graphitic shells. The CNTs suspension is mixed with inorganic nanoparticles (mainly ZrO2 or CaCO3) in an ultrasonic bath to remove the ferromagnetic particles. Then, the particles are trapped with permanent magnetic poles. After a subsequent chemical treatment, a high purity CNT material will be obtained (Fig. 4). This process does not require large equipment and enables the production of laboratory-sized quantities of CNTs containing no magnetic impurities.

Figure 4: SEM images of MWCNTs synthesised with (A) and without (B) the magnetic field.

 

2.6 Micro-filtration

 

Micro-filtration is based on size or particle separation. CNTs and a small amount of carbon nanoparticles are trapped in a filter. The other nanoparticles (catalyst metal, fullerenes and carbon nanoparticles) are passing through the filter.

 

One way of separating fullerenes from the CNTs by microfiltration is to soak the as-produced CNTs first in a CS2 solution. The CS2 insoluble are then trapped in a filter. The fullerenes which are solvated in the CS2, pass through the filter. A special form of filtration is cross flow filtration. In this method the membrane is a hollow fiber. The membrane is permeable to the solution. The filtrate is pumped down the bore of the fiber at some head pressure from a reservoir and the major fraction of the fast flowing solution which does not permeate out the sides of the fiber is fed back into the same reservoir to be cycled through the fiber repeatedly. A fast hydrodynamic flow down the fiber bore (cross flow) sweeps the membrane surface preventing the build-up of a filter cake.

 

2.7 Cutting

 

Cutting of the CNTs can either be induced chemically, mechanically or as a combination of these. Carbon nanotubes can be chemically cut by partially functionalising the tubes, for example with fluor. Then, the fluorated carbon will be driven off the sidewall with pyrolisation in the form of CF4 or COF2. This will leave behind the chemically cut CNTs. Mechanical cutting of the CNTs can be induced by ball milling. Here, the bonds will break due to the high friction between the nanoparticles and the CNTs will be disordered.A combination of mechanical and chemical cutting of the CNTs is ultrasonically induced cutting in an acid solution. In this way the ultrasonic vibration will give the CNTs sufficient energy to leave the catalyst surface. Then, in combination with acid the CNTs will rupture at the defect sites.

 

2.8 Functionalisation

 

Functionalisation is based on making CNTs more soluble than the impurities by attaching other groups to the tubes. And this will make it easy to separate from insoluble impurities, such as metal, this method is usually doing with filtration. Another functionalisation technique also leaves the CNT structure intact and makes them soluble for chromatographic size separation. For recovery of the purified CNTs, the functional groups can be simply removed by thermal treatment, such as annealing.

 

2.9 Chromatography

 

This technique is mainly used to separate small quantities of CNTs into fractions with small length and diameter distribution. The CNTs are run over a column with a porous material, through which the CNTs will flow. The columns used are Gel Permeation Chromatography (GPC) and High Performance Liquid Chromatography–Size Exclusion Chromatography (HPLC–SEC) columns. The number of pores the CNTs will flow through depends on their size. This means that, the smaller the molecule, the longer the pathway to the end of the column will be and that the larger molecules will come off first. The pore size will control what size distribution can be separated. However, a problem is that the CNTs have to be either dispersed or solvated. This can be done by ultrasonication or functionalisation with soluble groups. In addition to these purification techniques, CNT has been purified by treating with basic compounds such as KOH, and NH3 and gaseous compounds such as air, CO2, and ozone. With the different techniques for purification, there will be different results achieved. Care should be taken when the techniques chosen, as the effect on the entire sample will also depend on the composition and the amount of the sample. What is desired are techniques that only tear down the carbon impurities and the metals, without changing the CNTs. Sometimes extra care should be taken in adjusting the process variables such as temperature, scale and time. Economically feasible large-scale production and purification techniques are still under development. As a concluding remark, the above-mentioned purification methods change the structural surfaces of CNTs. As a result, there may be change in some of purified CNT properties. Therefore, the main thrust of the research should be in the area of producing purified CNTs in a single step process to conserve the fascinating features of CNTs.

 

SUMMARY

 

In this module, you study

 

Carbon nanotubes may have only recently caught the attention of the world but many advances have been made since their discovery. They are unique nanostructures that display the desirable properties of any other known material. The techniques of production have also come a long way but still have some efforts and time to be more efficient and cost effective.

 

Production methods include classical approaches such as the arc method, chemical vapour deposition, and laser ablation, new methods which are being tested such as through solar energy, plasma and microgravity environments. Production and purification methods change types, yields, and structural surfaces of CNTs. As a result, there may be change in the electrical and mechanical properties, and actual structure of CNTs. Therefore, the main thrust of the research should be in the area of producing and purifying CNTs in a single and efficient step process to conserve the fascinating features of CNT.

 

you can view video on Synthesis and Purification of Carbon Nanotubes

Learn More:

  1. G. A. Ozin and A. C. Arsenault, Nanochemistry, RSC Publishing, Cambridge, 2006.
  2. P. Harris, Carbon Nanotube Science, Cambridge University Press, New York, 2009.
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