Carbon Nanotube Synthesis

Carbon nanotube synthesis mainly classified into two types. They are Physical process and Chemical process. In the physical process the source target i.e graphite is evaporated and allowing to form Carbon nanotubes on substrate. In the chemical process instead of evaporating solid target, source gas of carbon is used. In this source gas is allowed form carbon nanotubes on the substrate with the help of catalyst.

2.1 Physical Process

2.1.1 Arc Discharge

This is a general method to synthesize fullerenes. It is the most common and easiest method to synthesize carbon nanotubes. Ijima synthesized Carbon Nanotubes using arc discharge in 1991 [1]. In the arc discharge the carbon source target is vaporized by arc vaporization. Carbon electrodes are used as cathode and anode. The vapors are cooled down and collected on the cathode.

Figure 2.1: Arc discharge setup for Carbon Nanotube synthesis.

The arc discharge process is carried out in inert gas (He, Ar, N₂) atmosphere. The typical voltage and currents for the arc generation are ~18V and 100 A. Because of the applied voltage and current, high temperature will produces, which produces arc. The arc will vaporize the carbon from the anode and the vapor is collected on the cathode and cooled down. The carbon nanotubes will form on the cathode. The carbon nanotube yield will mainly depend on the inert gas pressure. Ajayan group did the optimization studies of carbon nanotube synthesis by varying the inert gas pressure. The optimized pressure for He gas is 500 torr, which gives the best results [2].

Figure 2.2: Carbon Nanotubes synthesized by i) Ijima tubes of a) 5 graphene sheets b) 2 graphene sheets c) 7 graphene sheets ii) Ajayan tubes of a) 2 graphene sheets b) 5 graphene sheets c) 7 graphene sheets.

Pure graphite anode gives the multi-walled carbon nanotubes. For the synthesis of single-walled carbon nanotubes we have to dope the graphite target with fine metal particles. The typical metal particles are Fe, Co, Ni, Pd, Pt, Ru. The diameter of Single-walled Carbon Nanotubes (SWCNT) is depends on temperature and metal particle size and density. The growth parameters of SWCNTs are

• Electrode size
• Gap between electrodes
• Metal particles type
• Metal particles size
• Metal particles density
• Inert gas type and pressure
• Temperature

Table 2.1: SWCNT synthesis based on Arc discharge with different catalysts.

The variation of metal particles type and corresponding SWCNT diameter was shown in the Table 2.1.

2.1.2 Laser Ablation

Smalley group from Rice University were synthesized Carbon Nanotubes by laser ablation in 1995 [8]. The block diagram of laser ablation process was shown in Figure 3.3. The laser ablation process is similar to arc discharge. The difference is instead of arc, a laser is used to evaporate the graphite. A laser is made to strike at graphite target in the presence of inert gas atmosphere, which vaporizes graphite in an oven kept at 1200 ⁰C. The vapors are cooled down and carbon nanotubes are collected on collector.

The laser source can be pulsed or continuous laser. The main difference between the pulsed and continuous laser is the light intensity of pulsed laser (100 kW/cm²) is to be more as compared to continuous laser (12 kW/cm²). The inert gas pressure in the chamber is typically 500 torr. When laser strikes the graphite target hot vapor plume forms and it is super cooled. As a result small carbon atoms quickly condense to form larger clusters. The Multi-walled Carbon Nanotubes will form from pure graphite target.

Figure 2.3: Schematic diagram of Laser ablation

In order to get Single walled Carbon Nanotubes we have to dope the graphite target with fine metal particles same as the arc discharge process for SWCNT. The metal particles are also called as catalysts. The catalyst atoms will condense slower than carbon atoms and attach to the carbon clusters. Thus the SWCNTs will form and these catalyst particles will stops closing into cage structures. The carbon atoms may condense over the metal particles forming single graphite wrapped sheet i.e SWCNTs. These catalysts will helps to form long length carbon nanotubes.

The growth parameters of CNTs by Laser ablation process are

• Laser properties: Energy, peak power, continuous or pulsed laser
• Structural and chemical compositions of target materials
• Inert gas type and pressure
• Temperature
• Distance between target and substrate

Even though the synthesis of CNTs is easy but there are some disadvantages with physical process. The graphite targets should be prepared carefully for controllable synthesis of CNTs. The production quantity is dependent on the size of target and scale up is not easy. Both processes will produce CNTs in highly tangled form with fullerenes and graphite particles. So, further purification is required for CNT based device fabrication

2.2 Chemical Process

There are mainly three methods are there in chemical process. They are Chemical Vapor Deposition (CVD), High Pressure Carbon Monoxide Reaction (HiPco) and Cobalt Molybdenum CATalysis (CoMoCAT).

2.2.1 Chemical Vapor Deposition (CVD)

Figure 2.4: Schematic apparatus of Chemical Vapor Deposition

Chemical Vapor Deposition is done by using carbon source in gas phase and energy source as plasma, hot filament, microwave assisted to transfer the energy to gaseous carbon source. The energy supplied is used to crack the source molecules into reactive carbon atoms. These reactive carbon atoms will diffuse to the substrate and forms carbon nanotubes.

The CVD synthesis of carbon nanotubes is two steps. One step is substrate preparation followed by actual CVD process. Initially substrate is deposited with catalyst (metal). Then the substrate is placed in CVD chamber at high temperature (720 ⁰C). The reactive carbon atoms will diffuse towards the catalyst coated substrate and carbon nanotubes will grow over the catalyst. The catalyst is generally prepared by sputtering or e-beam evaporation. After that metal will be patterned for selective growth of carbon nanotubes [9,10]. The carbon source gases are

• Methane (CH₄)
• Ethane (C₂H₆)
• Ethylene (C₂H₄)
• Acetylene (C₂H₂)
• Isobutane (C₄H₁₀)

Figure 2.5: Tip growth model

Figure 2.6: Base growth model

The growth of carbon nanotubes can be explained by Tip growth model and Base growth model. These two models are experimentally verified with in-situ models. In the Tip growth model as shown in Figure 3.5 the source gas molecule will crack and reactive carbon atoms will form carbon nanotubes around the metal particle and metal particle will moved upwards. In the base growth model as shown in the Figure 3.6 the carbon nanotube will forms around the catalyst particle and CNT will grow upwards and catalyst will be on substrate. Thus the vertical growth of carbon nanotubes will possible by chemical vapor deposition [11].

(a) (b)

Figure 2.7: TEM image of a) MWCNT b) SWCNT

In order to produce SWCNT the size of catalyst particle should be less than 3nm. General catalysts are Fe, Co, Ni, Au. Typical substrates used to grow carbon nanotubes by CVD are Si, SiO2, Cu, stainless steel etc. The choice of catalyst used is one of the most important parameters effecting the CNT growth. Therefor substrate preparation is most crucial step in the chemical vapor deposition [12-14].

2.2.2 High Pressure CO disproportionation process (HiPco)

The High Pressure CO disproportionation process (HiPco) was developed by Rice University in 1999 [15]. Unlike other CVD techniques in HiPco process the catalyst is in gas phase. The carbon source gas is Carbon Monoxide (CO) and catalyst gas is Fe(CO)₅. The source gas CO mixed with catalysis gas through a heated reactor results formation of carbon nanotubes because of catalytic reaction. The schematic diagram of HiPco process is shown in the Figure 3.8 (a). The typical temperature of reactor is 1200 ⁰C. The grown carbon nanotubes are of high quality with few structural defects.

(a) (b)

Figure 2.8: a) Schematic diagram of HiPco b) TEM image of SWCNT

The size and diameter distribution of carbon nanotubes can be roughly selected by controlling the source gas CO pressure. The average diameter of SWCNT from HiPco process is approximately 1.1nm. The highest yields and smallest diameter SWCNTs can be produced at higher temperature and pressure.

2.2.3 CoMoCAT Process

CoMoCAT process was developed by Oklahoma University [16]. In CoMoCAT method SWCNTs are grown by CO disproportionation at 700 ⁰C − 950 ⁰C. The schematic diagram of the process was shown in the Figure 3.9 a) and its growth mechanism was shown in the Figure 3.9 b). The growth of SWCNT mainly depends on the Co-Mo catalyst formation. In the SWCNT formation Co becomes metallic form oxidic state and Mo is converted into carbide form (Mo₂C). When the Co clusters are very small typically few atoms cluster, SWCNTs of small diameter forms.

(a) (b)

Figure 2.9: a) Schematic diagram of CoMoCAT process b) CNT growth mechanism

Figure 2.10: TEM image of SWCNT

The TEM image of SWCNTs produced from CoMoCAT process was shown in the Figure 2.10. Even though Co acts as active species, the role of Mo is dual. It would stabilize the Co, so that it won’t act as carbon sink. This method can be easily scaled up to produce high quantity of SWCNTs without effecting quality. By varying the operation conditions SWCNTs can by synthesized with different diameter range.

Chemical process reactions are simpler, easily controllable and can manipulate the reactions. The raw materials used are abundant and readily available in nature. The cost of production is low because of absence of expensive targets and huge amount of energy. Vertical growth of CNTs is possible with chemical processes. These processes are capable of growing CNTs directly on the substrate. So, further collection and separation steps are eliminated.

2.3 Other Methods

NASA’s Goddard Space Flight Center reported in 2006 that they developed a simple, safe and highly economical process of SWCNT synthesis by Helium Arc Discharge. They used helium arc welding process to vaporize an amorphous carbon rod and then forms carbon nanotubes by depositing the vapor onto water cooled carbon cathode. This process yields bundles of SWCNTs at a rate of 2g/hr using single setup [17].

Flame synthesis is another method to synthesize SWCNTs. In this method carbon nanotubes are produced by using controlled flame, which produces high temperature. This result vaporization of carbon atoms and nanotubes forms on metal mesh. The carbon source is hydrocarbon fuels, which are less expensive [18, 19].

Table 2.2: Carbon Nanotube synthesis processes overview

References

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[16] National Aeronautics and Space Administration (NASA), NASA’s Goddard Space Flight Center, Report, 2005.

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