Pal. Jour. V.16, I.2, 2017, 149-158

Copyright © 2017 by Palma Journal, All Rights Reserved Available online at: http://palmajournal.org/

Palma Journal

Applied Analysis of Methods of Synthesizing Carbon Nanotubes in

Order to Identify Defects and Advantages of these Methods

Ramin Kamali MS in Organic Chemistry Center of Payam Noor University of Hamedan

Hamedan

Abstract Nowadays, researches in field of carbon nanotubes are being conducted in wide range and

because of having unique features, it has gained attention of scholars significantly. Nanotubes

are divided to two groups of single-walled and multi-walled nanotubes. Experiments have

shown that tensile strength of these materials is 40 times more than high quality steel and

these materials have various properties such as electrical properties, field emission (FED) and

a one-dimensional and hallowed structure. The most important feature playing key role in

determining properties of nanotubes is known as Chirality or torsion. The purpose of this

study is to investigate methods of producing carbon nanotubes and among the synthesis

methods of these materials, the most important methods include evaporation or laser ablation,

electric arc, chemical vapor deposition (CVD) using heating plasma. In this study, field and

applied methods are used to study types of synthesis methods of carbon nanotubes and the

main purpose is investigation and use of properties and synthesis methods of carbon

nanotubes and explanation of defects and benefits of these methods and investigation of

purification methods and functionalize carbon nanotubes and factors affecting oxidation of

nanotubes based on the results presented in reliable academic sources.

Key words: carbon nanotubes, synthesis, laser ablation, electric arc (CVD), chemical vapor

deposition

Introduction

Carbon could be considered as the most complex element of the periodic table with the capability of

formation of lots of Allotropes. Some of these allotropes like diamond and graphite have been discovered

since long ago and some others like fullerenes and carbon nanotubes have been discovered over the two

decades. Carbon nanotubes are made of carbon sheets with just one atom thickness and in hallowed

cylindrical form and have been discovered in 1991 by Sumio Iijima (NEC Corporation, Japan) [2].

Although discovering carbon nanotubes (Carbon Nanotubes, CNT) has been incidental, these products have

caused a revolution in technological future of the upcoming century. It is expected that similar to silicon-

based technologies that have covered the current society, the future society could be also affected by carbon

nanotubes and CNTs could be changed into a key element of nanotechnology. Almost every weak, new

applied potential of CNTs is being discovered and this issue has gained attention of scientists to these

nanotubes and has stimulated their curiosity. Common commercial uses of these materials include fuel

system components, motor vehicles and special sport equipment. It is expected that in short time, the

international demand for nanotubes is increased significantly and reaches to more than 200million dollars.

However, some problems such as high costs, inadequate purity and low production output are still existed

and should be considered by the officials. If the problems are solved, it is expected that increased

international demand for nanotubes is accelerated and could reach to more than 9billion dollars in 2020.

There are different methods to produce SNTs and each method has some advantages and restrictions.

Choosing the optimal method is not simple, since there are many qualitative and quantitative criteria for

this choice that makes it hard to compare them [8]. Carbon nanotubes have very high special level, high

permeability and high mechanical and thermal strength. Although porosities of CNTs are small

considerably, nanotube membranes have shown that they have higher or same flow intensity compared to

larger porosities because of smooth internal surface of nanotubes. These materials are durable and resistant

against heat and could be cleaned and reused in the refinery processes such as water and wastewater easily.

150 R.Kamali

Nanotube membranes can relatively omit all water contaminations such as bacteria, viruses, organic

compounds and opacity, special and unique properties of nanotubes such as high Young's modulus and

good tensile strength on one hand and carbonic nature of nanotubes (as carbon is light, strength and simple

substance, it is cost-effective for the processes of experiments compared to metals) on the other hand have

caused important investigations in field of efficiency of methods of development of nanotubes. Lots of

theoretical and practical works are conducted on atomic structure and electron structures of nanotube. Also,

many efforts are taken to investigate mechanical properties such as Young's modulus and tensile strength

and the mechanism of defects and effect of deformation of nanotubes on electrical properties [4]. It could

be mentioned that such special interest in nanotubes is rooted in their unique structure and properties.

Carbon nanotubes have a cylindrical structure and are made of carbon atoms. The nanotubes have thickness

to nano size; although their height reaches to a few micrometers. CNTs could be in form of metal or semi-

conductor in terms of thickness and chirality; although it is important to say that all of these properties are

related to ideal CNTs and the produced CNTs may lack such properties [1]. Although there are lots of

advancements in researches of nanotubes, scientists have not been successful to produce nanotubes with

high properties and in large amount with cost-effective methods and techniques. This problem is rooted in

lack of appropriate recognition of growth mechanism of CNT's [8].

Properties and uses of CNTs

Wide research activities have been allocated to the discussion of carbon nanostructures and their uses. The

main reason for this issue is the expected structural evolution, small size, low density, high stiffness, high

strength (tensile strength of the most outer wall of a multi-walled CNT is about 100 times more than

aluminum) and their excellent electrical properties. As a result, CNTs may be used widely in strengthening

the materials, flat screen with field emission, chemical sensors, complex drug therapies and nano-

electronics Science. In tables 1 and 2, properties and uses of CNTs are respectively presented.

Table 1: Properties of CNTs 1 Conductor or semi-conductor based on their geometry: nanotubes are changed into conductor or semi-conductor form based on the

rolling mode and graphite plates of their formation. In other words, as nanotubes seem as intertwined strip of wire in molecular

level, carbon atoms are attached in 6-dimensional frame and the 6-D patterns form cylindrical walls with size of just a few

nanometers. The chirality angle of nanotube defined as the angle between 6-D pattern axis and tube axis could determine conducting or semi-conducting nature [18].

2 Having unique property of missile transportation [18]

3 Very high thermal conductivity [18]

4 Smooth wall surface or high separation ability of wall surface of CNTs could increase the gas passage throughout them more than

conventional micro-hole membranes used to isolate gases. Hence, gases such as hydrogen and carbon dioxide could be isolated with conducting them in nanotube. The issue that whether the nanotube can pass the gases out of lab optionally or not has caused

scientists hope to produce hydrogen and nitrogen from the air [14].

5 Expression of unique electrical and mechanical properties along them [18]

6 High Young's Modulus

7 Sensitive to slight changes: the loads applied on a nanotube can change its electrical properties and its conductivity could be

increased or decreased depending on type of traction of a nanotube. This happens because of change in quantum structure of

electrons. Hence, the physicians are allowed to make transformers based on nanotubes with high sensitivity to applying slight loads. Moreover, ability of nanotubes in feeling slight changes in pressure and transformation of the pressure as an electrical sign could

give the ability to make nanotube switches sensitive to slight changes of pressure to scientists in future [18].

8 Emission and absorption of light: nanotubes can adsorb and dispose the infrared light. Moreover, simultaneous injection of electron

from one side and injection of hole from other side of CNT can lead to emission of light with wavelength of 1.5µm of nanotube

[18].

9 High electricity stimulation coefficient of nanotubes: in ambient temperature, nanotubes have highest electricity stimulation

coefficient compared to other materials [13].

Synthesizing Carbon Nanotubes 151

Table 2: Uses of CNTs 1 Use Notes

2 As amplifier in composites

Nanotubes could be one of the most resistant materials. Tis issue can clear use of CNTs as a filler substance to produce nanocomposites properly. Composites with the base of CNT have high strength to high weight

and have wide uses in the industry [19].

3 Use in field

emission displays (FED)

One problem with the current field emission devices is instability of production fields in long time intervals.

The problem could solved using CNTs. More than 700 research papers have been published in field of FED uses of CNTs. The statistics shows the importance of this issue [19].

4 Using single-

walled CNTs in electronic

industry

Nanotubes are resistant and strong considerably and can conduct the electricity and heat. These properties

have led to use these materials in electronic industry. CNTs are large molecular wires, in which electrons can move freely and they have complicated behavior. In this field, the behavior of multi-walled nanotubes is more

complicated than single-walled nanotubes, since adjacent layers can affect each other. Modeling such effects

could be the subject of current researches [18].

5 Hallowed structure of

nanotubes and

using them as storage and fuel

cell

Nanotubes are hallowed carbon structures. Hence, it is possible to place materials in them. Also, CNTs could be also used to store alkane fuels and hydrogen and to create fuel cells. Storage of hydrogen inside the single-

walled CNTs is possible. Adsorption capacity of hydrogen by single-walled CNTs is about 3-5% weighted

percent of nanotubes [13].

3 Making nanomachines

using CNTs

CNTs have been also offered to make nanomachines. Nanotubes have been replaced adequately by different structures that can act as axes in nanomachines. Different nanotubes may form gears with each other to

transfer different rotational movement. This could be done through making Gear Tooth (substituent) on

nanotubes [13].

Types of CNTs

Nanotubes are divided to two groups of single-walled nanotubes (SWNT) and multi-walled nanotubes

(MWNT). Based on array of carbon atoms of tube section, SWNT are also divided to three important groups

including Armchair and Chiral with metal properties and Zigzag with semi-conductivity property. SWNT

are just formed of carbon and a simple structure (a sheet of regular hexagons) [9]. Some predictions show

that SWNTs can be conductor or semi-conductor. The high electrical conductivity is depended on exact

geometry of carbon atoms. Since the beginning of working on SWNTs, they are called as a one-dimensional

phenomenon and the theory has been advanced step by step. The cause of interest in these SWNTs and

attempting to replace them in industry, based on theoretical and empirical studies, has been its high

mechanical properties and their electrical conductivity like metals. However, production of SWNTs is

expensive and it is hard to produce them, along with strengthening their properties while processing

polymer-nanotube. However, nanotubes produced using Lunghuri-Blajet technique including horizontal

and vertical movements like traditional painting of Japan are remained fixed in addition to be controlled in

terms of structure and are uniform and homogenous in terms of optic. Despite to this, availability and

commercial nature of MWNTs has led to more advancement in this field to an extent that some products

are produced in threshold of commercialization. For example, MWNTs (replacement for Carbon-Black)

are used in paint powders [5]. One defect of MWNT compared to SWNT is that they are less resistant, since

bonds of internal plates are weak. However, as uses of nanotubes in reinforcement of polymers is currently

leading to improvement of thermal and electrical properties more than mechanical properties, use of

MWNTs is very high. On the other hand, the existing techniques to produce SWNTs are not efficient

enough and can't also provide required purity. Purification of these materials is time-consuming and may

also damage the structure of nanotube [12].

Figure 1: a) Multi-walled nanotube b) Single-walled nanotube [12]

152 R.Kamali

Processes of synthesis of CNTs

There are various synthesis methods for CNTs and some of them are referred as follows [1]:

1. Arc discharge

2. Laser ablation

3. Chemical vapor deposition (CVD)

4. Electrolyze method

5. Solar production

In this regard, the first 3 methods are more important than others and are mostly used to produce CNTs.

Arc discharge

In this method, two graphite bars are used as electrode (cathode and anode). Along the anode axis, a hole

is created and is filled by a mixture of graphite powder and catalyst. One of the electrodes with diameter of

16 and length of 40mm is used as anode and the other electrode with diameter of 16 and length of 100mm

is used as cathode. Inside the cathode electrode, a hole is embedded with length of 40mm. inside the hole

is filled by graphite materials and metal catalysts such as nickel. Vertical direction of nanotubes or parallel

direction of cathode and anode could have no significant effect on synthesis. To implement electric arc, the

surrounding area of the device is firstly vacuumed and then, it is filled with low pressure of helium or argon

as inert gases [3]. Electric arc is created between two electrodes and as a result of electric discharge caused

by it; a crude carbon black is formed around the electrodes. The products of the carbon black could be CNT

and large amount of carbon amorphous carbon. One of the most important factors in synthesis of nanotubes

using electrical arc is stability of electric arc applied and the intensity of flow and voltage, which could

affect the amount of obtained product. If the desired product is multi-walled nanotube, there is no necessity

to use catalysts anymore. Although the obtained product using electric arc method is low in amount because

of limitation of experimental instruments, this method has been implemented by many scholars, since the

amount of product is not important for a research project on nanotube and the important issue is purity of

product and perfection of its structure and this problem is solved to wide extent by the electrical arc

technique. However, the other problem with arc discharge method is the vacuum technique that is

impossible in lots of low level labs and also, using helium and argon gases as expensive gases is not also

reasonable. Although hydrogen gas is used in some methods, this has been inefficient and hydrogen may

be exploited and have some complications. Stability of electric arc is an important factor in synthesis;

although using a DC feed source could have significant and positive effect on synthesis [6]. The

experiments have shown that the more the intensity of flow is compared to potential difference, the better

conditions should be; although it is hard to reach such flows [16].

Products of arc discharge method

Grading and productivity in this method is depended on empirical parameters and especially type of catalyst

used. In this method, in addition to CNTs, some impurities are also created such as C60 nanoparticles,

carbons with polyaromatic weak structure, amorphous nano-fibers, multi-walled layers and carbon nano-

capsules. Moreover, through changing the parameters, various products could be created; the parameters

include pressure, flow, space between the two electrodes and using catalysts. Different experiments and

comparing homogenous and heterogeneous anodes and the phenomena happened during the experiments

show that electrical charge and thermal transition in anode while creating electric arc are very important.

In the arc discharge method, double-walled nanotubes are produced like SWNTs with high diameter and

using a SWNT flow, even more DCs could be produced. In general, growing parallel CNTs (SWNTs,

DWNTs, and MWNTs) using electric arc is hard to do. However, partial alignments could be created using

convection methods or direct electric arc technique using plasma. On the other hand, temperature of growth

in arc discharge method is higher than other methods of CNT production and as a result, the obtained

products have perfect structure and the only defect in their structure is low alignment [12].

Synthesizing Carbon Nanotubes 153

A B Figure 2: a) A view of electric arc device b) SEM image of nanotubes produced in 0.3molar sodium chloride

solution with composite catalyst of Ni-Co and applied voltage of 20v [10]

Laser ablation method

In this method, catalytic growth process of gases containing carbon in high temperature and in presence of

metal nanoparticles as catalyst is used. In this method, a pulsed or continuous laser beam is radiated to

graphite sample containing 0.5% of nickel and cobalt atom as catalyst to evaporate it and isolate carbon

cluster from it. The main difference of pulsed and continuous laser is that the pulsed laser has higher optical

intensity (100kw/cm2) compared to (12kw/cm2). The oven is filled by helium or argon gases and its

pressure is fixed on 500torr. The argon or helium flow in reactor heated by the oven to 1200ºC carries the

vapor and creates nanocarbon cores that continue their growth. Nanotubes are deposited on colder walls of

the quartz tube in lower part of the oven [11]. In this process, high percentage of SWNTs (about 70%) are

produced and other particles are catalyst and carbon black. Moreover, while the vapors are cooled, tiny

molecules and atoms of carbon may be agglomerated with each other and change into larger clusters. Hence,

fullerene compounds may be also observed. This happens when the graphite sample lacks catalyst, since

catalyst is bonded to carbon clusters and prevents closing cage structures. Similar to arc discharge method,

pure graphite electrode can lead to synthesis of MWNTs; although for the SWNTs synthesis, graphite

should be mixed with metals such as Co, Ni, Fe and Y. The nanotubes produced by laser ablation are purer

than nanotubes produced by electrical arc discharge [13].

Figure 3: A view of first laser device to produce CNT b) a view of reactor for production of CNT using CO2 laser

device c) TEM image of SWNTs produced using laser evaporation technique (black particles are residual catalysts)

Chemical vapor deposition (CVD)

Chemical vapor deposition method is the phase of vapor for deposition of substance containing

nanoparticles of gas phase. The substance is heated to a degree that is converted to gas and is deposited as

a solid matter on the surface usually under the vacuum. Direct deposition or deposition through chemical

reaction may produce a new product through chemical reactions, which is different from the vapored matter

154 R.Kamali

significantly. The process can create some nanopowders of oxides and metal carbides easily, provided that

the carbon vapors or oxygen are existed, along with metal in the environment [12]. The process includes

passage of a hydrocarbon from tube reactor, at which a catalyst matter is existed in high temperature (600-

1200ºC) to catalyze hydrocarbon [11]. Nanotubes are grown on catalyst and then, the system is collected

to the ambient room through cooling. In cases that liquid hydrocarbon is used (like benzene, alcohol, etc.),

the liquid is heated in flask and an inert gas is cleaned by it, which can carry hydrocarbon vapor to the

reaction area. If the solid hydrocarbon is used in this process, it could be directly maintained in a section of

tube with low temperature. Volatiles such as naphthalene and ferrocene are converted from solid to vapor

directly and the deposition process of chemical vapor is done through passing from high temperature.

Similar to CNT materials, catalyst precursors in CVD may be used in any form. Thermal decomposition of

catalyst vapor in appropriate temperature can release metal particles in place (the process is known as

floated catalyst method) [9]. Synthesis of nanotubes is depended on different parameters such as

hydrocarbon, catalyst, temperature, pressure, gas flow velocity, deposition time and form of the reactor.

The most common and general precursors used to the date include methane, ethylene, acetylene, benzene,

xylene and carbon monoxide. Among the early reports of using CVD technique, MWNTs were developed

from thermal decomposition of benzene under temperature of 100ºC and acetylene under temperature of

700ºC. In these cases, steel nanoparticles had been used as catalyst. Later, MWNTs were produced using

different precursors like cyclohexane and fullerenes. On the other hand, SWNTs were produced for the first

time from heterogeneous carbon monoxide under temperature of 1200ºC and in presence of Molybdenum

nanoparticles. Later, the SWNTs were produces from benzene, acetylene, ethylene, methane, cyclohexane

and fullerene. Chemical deposition of vapor phase could be also used for growth of the surfaces. The

substance that is going to be covered is exposed to adjacency of chemical vapor [14]. The first layer of

molecules or atoms may show reaction with the surface or may not. In both cases, the depositing species

created for the first time act as a bed, on which the substance is grown. The created structures of these

materials are usually lined in a row, since the path, in which molecules and atoms are deposited, is affected

by adjacent molecules or atoms. If the bed or host base surface of deposition is flat, surface growth is done

in best manner. While deposition, a section may be formed for crystallization along the deposition axis, so

that the ordered structure is grown vertically. This issue is illustrated in form of a project in figure (4-a) and

is compared with a real structure composed of CNTs in figure (4-b). According to figure 4 (a-b), it could

be observed that surface properties in axis Z are significantly different from X and Y plates. This could

make surface properties unique. Moreover, table 3 has presented general information of the most common

methods of chemical vapor.

A B Figure 4: a) Surface growth of vertical and ordered structure along the deposition b) Comparing types of CNTs

formed by chemical deposition [15]

Table 3: General information of the most common chemical vapor deposition methods * APCVD LPCVD MOCVD PECVD

Advantages Simple, high accumulation

rhythm, cost-effective

Excellent homogeneity,

high purity

usable for metals, semi-

conductors and dielectrics

Low temperature for layering

High coherence

Defects Low homogeneity, lower

purity

Low accumulation

rhythm

High toxicity, expensive Plasma can sometimes lead to

destruction of layer and even he

sample

Main use Thick oxidized layers Layering dielectrics, poly-silicon

Making LED, laser diode, semi-conductors

Dielectric layering

Synthesizing Carbon Nanotubes 155

Advantages of chemical vapor methods [7]

It is possible to create complete and very pure epitaxial layers in this method.

According to nature of reactions, layering could be done properly on porous sub-layers.

Thickness of layer is homogeneous and independent of bed form.

Layering speed is relatively high (10-100nm/s)

Its coherence is better than physical methods.

This method is appropriate for multilayer layering.

The pressure process in conventional pressure could be controlled.

Defects and hedges to synthesis methods of CNTs

The main defect of arc discharge method is that in these methods, firstly, the evaporation of carbon atoms

is done from the graphite target in very high temperatures and secondly, these methods form ropes or

packages of nanotube and can make problem with purification and use of these nanotubes and amount of

produces nanotube is about a few grams. Moreover, in this method, graphite electrodes with high purity

and metal particles, helium, argon or hydrogen gases with high purity are needed. Moreover, the main

problem with arch discharge method is vacuum technique that is impossible in many labs with low quality

of equipment. Moreover, the products obtained by this method needs purification operations too. Also,

using helium and argon is expensive. Hence, this method is expensive. Although hydrogen is used in some

methods, this has been inefficient and there is risk of explosion and side effects of hydrogen. Sustainability

of electric arc is an important factor in synthesis; although using a DC feed source could be positively

effective in synthesis process. The experiments have shown that the more the intensity of flow is compared

to potential difference, the better conditions should be; although it is hard to reach such flows. In laser

technique, laser with fast pulses could be used to produce large amounts of SWNTs. However, lase ablation

method is not cost-effective economically, since the operations need graphite bars with high purity and

laser with high power. Moreover, speed of production of this method is less than other method. As both

methods of arc and laser ablation are relied on carbon atom vapor inside the small chamber, the production

rate of nanotube in these methods is low. Secondly, nanotubes produced by vapor are intertwined and hence,

their purification would be problematic. Moreover, in electric arc and laser ablation methods, growth of

CNTs is in powder and macaroni form and this can make the CNTs can't be welcomed by the electronic

industries. The chemical vapor deposition method has some problems, since high density of defects is

because of low growth temperature to produce MWNTs, which can't provide required energy for purpose

of annealing of nanotube and completing its structure. Moreover, in this method, nanotubes are optional;

although nanotubes with lower thickness are more appropriate in switching. Hence, in this method,

conducting reactions needs high temperature and this could lead to thermal stresses as a negative factor on

layer and sub-layer. The most important defects of the 3 methods are presented in tables 4, 5 and 6.

Table 4: Defects of arc discharge synthesis Row Defects

1 Production of CNTs in low amount

2 Existence of accessory products such as Fullerenes and amorphous carbon, graphite particles and metal particles

3 Evaporation of carbon atoms of graphite target in very high temperatures

4 Growth of CNTs as powder and macaroni forms

5 Low production speed and high production costs

156 R.Kamali

Table 5: Defects of laser method Row Defects

1 Because of limited size of laser, the Conic section surface is small (about 1cm2). Hence, the size of sub-layer is restricted.

However, as it was mentioned before, this problem could be solved through laser beam scanning on a larger target surface.

2 As the cone has been directed forward intensely, it is hard to control homogeneity of thickness along the sub-layer.

Moreover, thickness control is not easy.

3 As the target is deformed over the time, the laser adsorption efficiency in surface is changed and the accumulation rhythm

is also changed.

4 One of the most important problems with laser layering is droplet of fine and coarse particles (droplet) from the target

surface to sub-layer that can destroy homogeneity of surface. To solve this problem, energy density and laser sport size should be optimized on the target surface.

5 The main mechanism in this method is formation of plasma that has not yet a clear identity. Hence, abundant iterations are

needed to obtain optimal conditions.

6 As the arrangement of laser layering is in such form that fountain is out of the vacuum chamber and the target substance

is inside the vacuum chamber, this could lead to some problems in conducting the layering operation through this method.

This can lead to increased cost of this method. Moreover, the quartz glass passing the laser radiation effectively is blurred usually after numerous layering operations.

7 Choosing a laser with adaptable wavelength with adsorptive properties of target matter is not always possible. Because of

said problems, laser layering is mostly used to test new materials and in research projects and has no significant use in the

industries.

Table 6: Defects of chemical vapor deposition method Row Defects

1 In some layering reactions, the lower layer may be destroyed because of using corrosive vapors

2 Controlling reactions is hardly possible as a result of controlling homogeneity.

3 The thermodynamic details are too complicated in this method and it is sometimes hard to control them.

4 Unwanted reactions may emerge in this method and sometimes, the reactions may result in serious defects in layering or inside the reactor.

5 Probability of hydrogen explosion in reactor

6 Majority of reactor materials are expensive

Purification of CNTs

Filtration, centrifuge and high temperature cooking are the most important methods for physical purification

of CNTs. Using filtration and centrifuge methods, metal and carbon particles could be disposed; although

high-temp cooking method is only applicable for CNTs free from carbon impurities, since only metal

particles could be disposed using this method. Physical purification methods can't provide pure CNTs with

high efficiency by themselves and it would be better to sue them, along with the chemical methods [21]. In

chemical methods, in addition to dispose impurities, functional groups are created on CNT surface. As

CNTs have hydrophobic surface, their surfaces should be modified for better scatter in solutions or

polymers through the covalent and non-covalent functionalization methods with oxygen-containing

functional groups or polymers. Surfactant adsorption on nanotube surface, polymer waving on surface of

nanotubes and functionalization of inside surface of CNTs are the different method for non-covalent

functionalization of CNTs. The problem with non-covalent functionalization is that the power between

nanotube surface and modifier molecules is weak. In covalent functionalization, in addition to create

functional groups, many defects are created in surface of nanotubes and the nanotubes are shortened.

Chemical oxidation as one of the main covalent functionalization methods is divided to two groups of liquid

phase oxidation and gas phase oxidation [19]. To create oxygen-containing functional groups, liquid phase

is more effective than gas phase oxidation; although some defects are also created on nanotubes in liquid

phase oxidation method. Hence, the destructive effects of this method should be decreased while using it

[18]. CNT oxidation is depended on factors such as temperature, time, type and concentration of oxidants

and the oxidation method. Hence, the optimal conditions should be provided for oxidation through further

investigations.

Synthesizing Carbon Nanotubes 157

Table 7: Effect of laboratory parameters on results of CNT oxidation Parameter Effect

Temperature Increased temperature could increase the C-C bond velocity and oxidant, increase contact surface, increase amount of functional groups on nanotube surface and can improve distribution and scatter of nanotubes in solvents; although high

increase in temperature can lead to destruction of nanotubes and drop of their mechanical properties [18]

Time With the increase in time of functionalization, interaction of CNTs and oxidants is increased. Hence, more functional

groups such as carboxyl could be created on nanotube surface. The coarseness of surface is also increased and this shows accumulation of functional groups on nanotube surface [19].

Concentration

and type of oxidant

With the increased in concentration of oxidant, more groups are created on the nanotubes. Also, the type and power of

oxidation of oxidant can affect the results of oxidation [20].

Oxidation method The amount of functional groups and improvement of scatter of functionalized nanotubes using reflex or ultrasonic method

is different [21].

Conclusion

There are three common methods for synthesis and production of nanotubes. One of these methods could

be using electric arc discharge, which was for the first time used by Sumio Iijima in synthesis of nanotubes.

In this method, evaporation of graphite electrodes is used in electric arc with high temperature (4000ºC).

Although nanotubes produced by this method are highly crystalized, they are intensely impure and about

60-70% of products produced by this method contain unformed metal and carbon particles. Laser ablation

technique is very pure graphite evaporation using powerful lasers with high temperature. Although the

nanotubes produced by this method have very high purity, their production performance is too low.

Chemical vapor deposition encompassing thermal decomposition of hydrocarbons by catalyst is one of the

best methods to produce CNTs. This method is cost-effective and is scalable for mass production of

nanotubes. CNTs have trend for accumulation because of their hydrophobic surface. Hence, for better

scatter in solutions and polymers, their surfaces are modified using different functional groups. To omit the

impurities while synthesis, purification of nanotubes is required before functionalization. CNTs could be

purified using two physical and chemical methods. Purification of physical methods is not perfect. Hence,

physical methods are usually used, along with chemical methods. At the same time with chemical

purification, the CNTs are functionalized too. Functionalization of nanotubes could be classified in two

groups of covalent and non-covalent methods. To create oxygen-containing functional groups, chemical

oxidation is the most common method. Factors such as temperature, time, type and concentration of

oxidants and oxidation method could affect the functional groups created.

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