Carbon nanotubes have gained attention in recent times due to their extraordinary physicochemical properties like strength, flexibility, sensors, conducting etc. Several preparation methods of Carbon nanotubes are discussed in this chapter. The properties and applications of nanotubes are also discussed.
Keywords:carbon nanotubes, single walled nanotubes, multiwalled nanotubes, Functionalization, nanoelectronics
ICarbon nanotubes (CNT) are the basis of nanotechnology. Carbon with an atomic number of 6 plays a pivotal role in nanotechnology. They were discovered by Iijima  accidentally while studying the surface of graphite electrode used in electric arc discharge. This accidental observation laid a foundation of exciting field of nanotechnology and started a new direction in the carbon research.
A carbon nanotube (CNT) is a hexagonal array of carbon atoms rolled up into a long, thin, hollow cylinder  and are known for their size, shape, and remarkable physical properties. They can be manipulated chemically and physically for their application in material science, electronics, energy management, biomedical application and many more..
A carbon nanotube is a tube shaped material, made of carbon and has diameter on the nanometer scale. Two types of carbon nanotubes are observed-Single walled nanotubes (SWNT) and Multiwalled nanotubes (MWNT).
SWNT: They have diameter close to 1 nanometer and tube length may vary millions of times longer. They can be formed in three different forms – archair, zigzag and chiral forms on basis of rolling of grapheme sheet in to a seamless cylinder (Figure 1). Each form has a special effect on electrical property of nanotube.
MWNT: The discovery of C60 motivated the researchers to search for other carbon compounds containing curved graphenes. This led to the discovery of multiwalled nanotubes were made of concentric cylinders of rolled-up graphene sheets, capped with semi-fullerenes. The length of a tube was in the range of a few μm, the diameter was 10-20 nm (Figure 2). As the size increases, these structures exhibit properties between fullerenes and graphite.
Two models can be used to describe the structure of
multiwalled nanotubes (Figure 3). In Russian doll model,
sheets of graphite are arranged in concentric cylinders within
a larger single walled nanotube. In the parchment model, single
sheet of graphite is rolled in around itself, resembling a scroll
of parchment or rolled newspaper. The interlayer distance in
multiwalled nanotubes is close to distance between graphene
layers approximately 3.3Å. They have greater tensile strength
than single walled nanotubes.
There are different preparation methods for carbon
nanotubes like arc discharge (AD), laser ablation, chemical
vapour deposition (CVD) as well as some of the more recent
methods working with high pressure of the carbon monoxide
or some unique catalytic mixture. Carbon nanostructures like
fullerenes, graphene and nanotubes are of great interest for the
current research as well as for future industrial applications.
The reason for this is that the band gap of single-walled carbon
nanotubes (SWNTs) can vary from zero to about 2 eV and hence
their electrical conductivity can be the one of a metal or the
Arc Discharge: In 1990, Kratschmer and Huffman developed
an arc discharge technique and reported the synthesis of new
form of solid, consisting of somewhat disordered hexagonal
packing of soccer-ball shaped C60 molecule. It produces the
best quality nanotubes [3-5]. During arc discharge method, a
current of about 50 amps is passed between two graphite rods
placed in an enclosure filled with some inert gas (like helium or
argon) at low pressure (between 50 and 700 mbar). The carbon
rods act as electrodes which are kept at different potentials. The
anode is moved close to the cathode until an arc appears and
the electrodes are kept at the distance of 1 mm for the whole
duration of the process that takes about one minute. After the
de-pressurisation and cooling of the chamber the nanotubes
together with the by-products, can be collected. Most nanotubes
deposit on the cathode. Single walled nanotubes are produced
when Co and Ni or some other metal is added to the anode
One can tune the process to produce primarily multiwalled
CNTs using two standard graphite electrodes to a process that
produces single-walled CNTs using metal catalysts such as Ni, Fe,
Mo, or Co doped into the electrode. These metals catalyze the
breakdown of gaseous molecule into carbon anf then tube starts
to grow with a metal particle at the tip [6,7]. Arc discharge is the
most straightforward approach to synthesize carbon nanotubes,
but its application as a large-scale production technique suffers
due to the moderate yield of CNTs. Ebbensen Ajayan reported
large scale of MWNT by variant of standard arc discharge
Laser Abalation: In 1995 Richard E. Smalley and his
group used for the first time laser ablation to grow high quality
nanotubes. Intense laser pulses ablate a carbon target which is
placed in a tube-furnace heated to 1200°C. During the process
some inert gas like helium or argon flows through the chamber
to carry the grown nanotubes to the copper collector. After the
cooling of the chamber the nanotubes and the by-products, like
fullerenes and amorphous carbon over-coating on the sidewalls
of nanotubes can be collected (Figure 5). The SWNT formed
bundle together by van der waals forces.
Laser ablation processes can also be tuned to yield either
single-walled CNTs or multi-walled CNTs based on the variation
of parameters in the growth process such as the laser wavelength,
laser power, laser pulse duration, furnace temperature, and
graphite target composition. In particular, a low metal : graphite
ratio in the target and a high furnace temperature (typically ≈
1200 ◦C) is generally associated with good quality crystalline
single-walled CNTs, whereas greater amounts of metal yields
multi-walled CNTs, and lower furnace temperatures compromise
crystallinity of the CNT tube walls.
Chemical Vapour Deposition: During CVD, a substrate
covered with metal catalysts, like nickel, cobalt, iron, or a
combination is heated to approximately 700°C and promotes the
growth of carbon nanotubes is by exciting carbon atoms that are
in contact with metallic catalyst particles (Figure 6).
The CVD method extends this idea by embedding these
metallic particles (iron, in the case of the seminal paper) in
properly aligned holes in a substrate (silicon, in this case).
The growth starts after two gases are passed through the
chamber, a carrier gas like nitrogen, hydrogen or argon, and
some hydrocarbon gas like acetylene (C2H2) or methane (CH4).
The synthesis production yield, which indicates the amount of
carbon nanotubes in the converted carbon, reaches 90%. CVD is
commonly used for the industrial purposes because the method
is already well investigated and offers acceptable results on the
The size of the nanoparticles is a very important parameter
as there is an optimal size for each application. For example, for
in vivo experiments, it must be taken into account that to cross
the blood brain barrier, the nanoparticles have to be in a range of
15-50 nm whereas to pass through the endothelium, they must
be smaller than 150 nm. Thus, particles between 30 to 150 nm
are retained in the heart, stomach and kidney whereas particles
between 150-300 usually stay in liver and spleen. Another
example is when magnetic particles are used as carriers for the
purification of biomolecules. In this case, sizes above 40 nm are
necessary in order to have a good migration toward the magnet
Carbon nanotubes do not disperse in organic matrices due
their inert nature and forms bundles with each other. The poor
solubility of carbon nanotubes in organic solvents restricts them
to be used as drug delivery agents into living systems in drug
therapy. Hence many modification approaches like physical,
chemical or combined have been exploited for their homogeneous
dispersion in common solvents to improve their solubility. The
surface of nanotubes can be modified by various ways to enhance
their dispersion in organic media. Many applications require
covalent modification to meet specific requirements i.e. in case
of biosensors the biomolecules require electron mediators to
promote electron transfer [9,10].
Similarly electrochemical metal ion sensors require specific
functional groups which show potential affinity towards
particular metal ion. The modification protocol was generally
achieved by attaching specific molecule or entity which imparts
chemical specificity to the substrate material. These chemical
modifications can be easily achieved in many ways.
Covalent Interactions: Covalent modification involves
attachment of a functional group onto the carbon nanotube. The
functional groups can be attached onto the side wall or ends of
the carbon nanotube. The end caps of the carbon nanotubes have
the highest reactivity due to its higher pyrimidization angle and
the walls of the carbon nanotubes have lower pyrimidization
angles which has lower reactivity (Figure 7). Although covalent
modifications are very stable, the bonding process disrupts
the sp2 hybridization of the carbon atoms because a σ-bond
is formed. The disruption of the extended sp2 hybridization
typically decreases the conductance of the carbon nanotubes
Oxidation: Oxidation of carbon nanotubes also functionalizes
the surface by breaking the carbon-carbon bonded network of the
nano layers under acidic conditions. It allows the introduction of
oxygen units in the form of carboxyl, phenolic and lactone groups.
In liquid-phase reactions, carbon nanotubes are treated with
oxidizing solutions of nitric acid or a combination of nitric and
sulphuric acid to the same effect. However, over oxidation may
occur causing the carbon nanotube to break up into fragments,
which are known as carbonaceous fragments. Bifeng Pan et al.
 reported the MWCNT nano hybrids prepared initially by the
oxidative pretreatment of CNTs with 3:1 H2SO4/HNO3 mixture
then they were activated using SOCl2 and finally acyl chloride
was coupled with ethylene diamine. The resulted MWCNTs were
again modified with mercaptoacetic acid coated QDs.
Esterification/Amidification: The functionlization of
nanotubes are carried out by using carboxylic groups, which acts
the precursor for most esterificationa nd amidation reactions.
The carboxylic group is converted into an acyl chloride with the
use of thionyl or oxalyl chloride which is then reacted with the
desired amide, amine, or alcohol. . Carbon nanotubes modified
with acyl chloride react readily with highly branched molecules
such as poly(amindoamine), which acts as a template for silver
ion and later being reduced by formaldehyde. Amino-modified
carbon nanotubes can be prepared by reacting ethylenediamine
with an acyl chloride functionalized carbon nanotubes. In a
similar way, thiol stabilized ZnS capped CdSe QDs were protected
with 2-aminoethanethiol and linked to the acid terminated
carbon nanotubes in presence of a coupling agent like EDC
(Figure 8) .
Strength: Carbon nanotubes have a higher tensile strength
than steel and Kevlar. This strength originates from sp2 bonds
between the individual carbon atoms. Carbon nanotubes are
not only strong, they are also elastic. Upon application of force,
nanotube can bend and returns to its original shape when the
force is removed. A nanotube’s elasticity does have a limit,
and under very strong forces, it is possible to permanently
deform to shape of a nanotube. A nanotube’s strength can be
weakened by defects in the structure of the nanotube. Defects
occur from atomic vacancies or a rearrangement of the carbon
bonds. Defects in the structure can cause a small segment of the
nanotube to become weaker, which in turn causes the tensile
strength of the entire nanotube to weaken. The tensile strength
of a nanotube depends on the strength of the weakest segment in
the tube similar to the way the strength of a chain.
Electrical Properties: The sp2 bonds between carbon
atoms results in conducting nature of carbon nanotubes. They
can also withstand strong electric currents because of the strong
nature of bonds. Single walled nanotubes can route electrical
signals at speeds up to 10 GHz when used as interconnects on
semi-conducting devices. Their electronic properties can be
manipulatedby application of external magnetic field, mechanical
Thermal Stability: Carbon nanotubes are able to withstand
high temperatures, thus acting as very good thermal conductors.
The temperature stability of carbon nanotubes is estimated to be
upto 28000oC and about 750oC in air. The carbon nanotubes are
shown to transmit over 15 times the amount of watt per minute
as compared to copper wires .
There are many potential applications of carbon nanotubes
owing to its remarkable properties. They have potential to be
used in electronics, textile industry as water proof and tear proof
fabric, sensor based on the property of thermal conductivity
and many more. They possess extraordinary heat and electrical
conductivity behaviour, making it a suitable candidate for
numerous applications. . Some of the important applications of
carbon nanotubes are discussed below.
Sensors: Carbon nanotubes have been reported to a good
gas sensor, because of its elongated shape. They have reported to
an excellent oxygen gas sensor by Zettl et al. . They measured
DC electrical resistance and the thermoelectric power of bundles
and thin films of SWNTs. The sensitivity of carbon nanotubes
to NO2 and NH3 is also reported by Dai and co-workers , by
measuring measuring conductivity of MWNT upon exposure
to NO2 and NH3 at variable time periods and temperature.
Functionalised MWNT have also been used for gas sensing.
Robert Haddon and colleagues found that functionalized SWNTs
experienced a much greater change of resistance upon exposure
to NH3 than did pristine tubes, giving them greater sensitivity
as sensors. Alexander Star and colleagues from the University
of Pittsburgh described a sensor for nitric oxide that employed
SWNTs functionalizedwith poly (ethylene imine) 
Nanoelectronics: One of the potential application of
nanotubes is the field of electronics, due to its highly conducting
nature. Out of two types, single walled nanotubes are the most
conducting type. Twisting and bending of nanotube makes
it highly conducting. with high conductivity and small size,
nanotubes may be an alternate option to copper which is
generally used, but has limitation of ineffective at size less than
40 nm .
Carbon nanotubes have been extensively studied making
way for basic understanding and potential for various
applications. This article has discussed several applications of
carbon nanotubes. The remarkable physical properties of carbon
nanotubes have created a host of application possibilities, based
on electronic and mechanical behaviour of nanotubes.