Carbon nanotubes and their applications are discussed. There are several allotropes of carbon including diamond, graphite, buckminsterfullerene (C60), and single-walled carbon nanotubes. Carbon nanotubes are cylindrical forms of carbon that can be single-walled or multi-walled. They are composed of hexagonal networks of carbon atoms and have remarkable mechanical, thermal, and electrical properties. Common synthesis methods for carbon nanotubes include arc discharge, laser ablation, and chemical vapor deposition using metal catalysts.

1. CARBON NANOTUBES AND
THEIR APPLICATIONS
DEPARTMENT OF PHYSICS
MALAVIYA NATIONAL INSTITUTE OF TECHNOLOGY JAIPUR
SUBMITTED TO: Dr. Srinivasa Rao Nelamarri
SUBMITTED FROM: YOGESH CHILLAR
I.D. 2017PPH5336
M.SC. PHYSICS 3RD SEMESTER

2. CARBON
group- 14
period-2
 Carbon is the 15th most abundant element in the Earth's crust, and
the fourth most abundant element in the universe by
mass after hydrogen, helium, and oxygen.
 It is reactive non metal.
 Electron configuration [He] 2s2 2p2
 Electrons per shell 2,4 .
 tetravalent—making four electrons available to
form covalent chemical bonds
 Three isotopes occur naturally, 12C and 13C being stable, while 14C is
a radionuclide, decaying with a half-life of about 5,730 years.

3. Allotropes of carbon
Allotropy is the property of some chemical elements to exist in
two or more different forms, or allotropes, when found in nature.
There are several allotropes of carbon.
a) Diamond,
b) Graphite,
c) Lonsdaleite,
d) C60 (Buckminsterfullerene or buckyball),
e) C540,
f) C70,
g) Amorphous carbon, and
h) single-walled carbon nanotube, or buckytube.

4. FULLERENCES
 Fullerenes are a family of
carbon allotropes.
 They are molecules composed
entirely of carbon.
 They are in the form of a hollow
sphere, ellipsoid, or tube.
FULLERENCE

5. Discovery of the first fullerene: C60
 In 1985, Prof. Harold W. Kroto of the University of Sussex joined
Robert F. Curl and Prof. Richard E. Smalley at Rice University to study
the products of carbon vaporization.
 They carried out molecular beam experiments.
 From the result, discrete peaks were observed corresponding to
molecules with the exact mass of sixty or seventy or more carbon
atoms.
 C60 was then discovered, and it was named buckminsterfullerene
which is named after Richard Buckminster Fuller who designed
geodesic domes which is the same structure as C60.
 Shortly after discovery of C60, it came to discover the fullerenes.

6. Structures of some fullerenes
 C60 (Buckminsterfullerene)
 Its like the shape of a football.
*grey ball represents a carbon atom

7. CARBON NANOTUBES
DISCOVERY
 In the late 1950s, Roger Bacon at Union Carbide, found a strange new carbon
fibre while studying carbon under conditions near its triple point. He observed
straight, hollow tubes of carbon that appeared to consist in graphitic layers of
carbon separated by the same spacing as the planar layers of graphite. In the
1970s, Morinobu Endo observed these tubes again, produced by a gas-phase
process. Indeed, he even observed some tubes consisting in only a single layer of
rolled-up graphite.
 In 1991, after the discovery and verification of the fullerenes, Sumio Iijima of NEC
observed multiwall nanotubes formed in a carbon arc discharge, and two years
later, he and Donald Bethune at IBM independently observed single-wall
nanotubes – buckytubes.

8. CARBON NANOTUBES
 Carbon nanotubes are tubular forms of carbon that can be envisaged as
graphene sheets rolled into cylindrical form.
 These nanotubes have diameters of few nanometers and their lengths are
up to several micrometers.
 Each nanotube is made up of a hexagonal network of covalently bonded
carbon atoms.
 Carbon nanotubes are of two types: single-walled and multi-walled.
 A single-walled carbon nanotube (SWNT) consists of a single graphene
cylinder whereas a multi-walled carbon nanotube (MWNT) comprises of
several concentric graphene cylinders.

9.  Depending on the way of rolling of graphene sheets (as shown in
Fig),single- walled nanotubes of different types, viz. armchair, zig-zag and
chiral could be produced.
They can be represented using the method given by Hamada.
For example, to realize an (n, m) nanotube, one has to move n times a1
from the selected origin and then m times a2.
On rolling the graphite sheet these points coincide to form the (n, m)
nanotube.
Thus armchair, zig-zag and chiral nanotubes can be represented as (n, n),
(n, 0) and (n, m) respectively
 Different wrapping results in different structures and electronic properties.

10. Schematic representation of the relation between nanotubes and graphene. The three
rectangles can be rolled up into seamless nanotubes; the short side, referred to as the
roll-up vector R, becomes the circumference. R= na1+ma2, is a graphene 2D lattice
vector, where a1 and a2 are unit vectors. Integers n and m uniquely define the tube:
diameter, chirality, metal vs. semiconducting nature, and band gap, if
semiconducting. In a bulk polydisperse sample consisting of a distribution of diameters,
the larger the average diameter, the greater the number of n, m pairs that will satisfy
the seamless roll-up condition.

11. Types of CNTs
 Carbon nanotubes are of two types :
1. Single wall carbon nanotube (SWNT)
2. Multiple wall carbon nanotube (MWNT)
 Single wall nanotube (SWNT ) consist of one cylinder. It is made of
single graphene sheet rolled up into cylinder closed by two caps
(semi fullerenes). The SWNTs have diameter in the range of 0.5 -2.0
nm. The length is in the range of 50-150 μm length. The SWNTs are
microporous and the specific surface area is in the range of 1300
m2/g (outer surface). SWCNTs are commonly arranged in bundles.
SWNTs have less topological defects and have better mechanical
and electro physical properties. Electronic properties of SWNTs are
governed by two factors, tube diameters and helicity, which further
depend on the way graphene layer is rolled up, arm chair or chiral.
Armchair SWNTs shows conductivity as similar to metal whereas
zigzag SWNTs behave as semiconductors. In catalysis CNTs have
high application as support.

12.  Electrical conductivity, surface curvature and presence of inner cavity in
CNTs make the metal –support interaction different compared to that in
activated carbon or graphite support. Mechanically bent SWNTs present
kink sites that are chemically more active. Metal nanoparticles size
depends strongly on metal-CNT interactions with stronger interaction giving
rise to smaller nanoparticles. Studies have shown that convex surface of
CNTs are more reactive than concave surface and the difference in
reactivity increases when the tube diameter decreases.
 Multiwall (MWNT) nano tubes consist of many nested concentric SWNTs
cylinders with increasing successive radii. The concentric walls are spaced
regularly at 0.34 nm similar to inter graphene distance. MWNTs have outer
diameter in range of 2 – 100 nm depending on number of coaxial tubes
present. MWNTs are usually mesoporous in nature and specific area
depends on the number of walls. The length of MWNTs can range from few
to hundreds μm. The advantage of MWNT over SWNT is that the multi-shell
structures of MWNTs are stiffer than single wall hence stability is higher. Also
large scale synthesis of MWNT is possible by various methods. The most
common characterization techniques of these materials are electron
microscopy, Raman spectroscopy, TGA , IR and UV-Vis.

13. PHYSICAL PROPERTIES OF CARBON
NANOTUBES
 MECHANICAL PROPERTIES
 THERMAL PROPERTIES
 ELECTRONIC PROPERTIES
 MAGNETIC PROPERTIES
 SUPERCONDUCTING PROPERTIES

14. PROPERTIES FOUNDATION
 Nanotubes are fully described by their chiral vector
 R = n â1 + m â2
 Important parameters
 dt = (3/p)ac-c(m2 + mn + n2)1/2
 Q=tan-1(3n/(2m + n))
 Grouped according to q
 Armchair: n=m, q=30°
 Zigzag: n or m=0, q=0°
 Chiral: 0°<q < 30°
A. Maiti, Caron Nanotubes: Band gap engineering
with strain, Nature Materials 2 (2003) 440
V. Popov, Carbon nanotubes: properties and applications,
Materials Science and Engineering R 43 (2004) 61-102

15. MECHANICAL PROPERTIES
The strength of the carbon–carbon bond gives rise to the extreme
interest in the mechanical properties of nanotubes.
 Theoretically, they should be stiffer and stronger than any known
substance.
 Simulations and experiments demonstrate a remarkable “bend, don’t
break” response of individual SWNT to large transverse deformations; an
example from Yakobson’s simulation is shown in Figure 4.9.

16.  The two segments on either side of the buckled region can be bent into an
acute angle without breaking bonds; simulations and experiments show the
full recovery of a straight perfect tube once the force is removed.
 Young’s modulus of a cantilevered individual MWNT was measured as 1.0
to 1.8 TPa from the amplitude of thermally driven vibrations observed in the
TEM. At the low end, this is only ~20% better than the best high-modulus
graphite fibers
 Multiwall nanotubes and SWNT bundles may be stiffer in bending but are
expected to be weaker in tension due to “pullout” of individual tubes.
Comparison of mechanical properties

17. Thermal Properties
 All nanotubes are expected to be very good thermal conductors along the
tube, but good insulators laterally to the tube axis.
 It is predicted that carbon nanotubes will be able to transmit up to 6000
watts per meter per Kelvin at room temperature; compare this to copper, a
metal well-known for its good thermal conductivity, which transmits 385
watts per meter per K.
 The temperature stability of carbon nanotubes is estimated to be up to
2800oC in vacuum and about 750oC in air.

18. ELECTRONIC PROPERTIES
 1-D band structure calculated from 2-D
graphene band structure using
“zone folding” scheme
 Ekμ= E2D(k*K2/|K2|+μK1)
 K1=(-t2b1+ t1b2)/ N
 K2=(mb1- nb2)/ N
V. Popov, Carbon nanotubes: properties and applications,
Materials Science and Engineering R 43 (2004) 61-102

19.  Theory predicts nanotubes exhibit both metallic and
semi-conducting behavior
 |n-m| evenly divisible by 3- metallic
 All others semi-conducting with a band gap inversely proportional to
the tube diameter
T.W. Odomet al, Atomic Structure and Electronic Properties of Single-Walled
Nanotubes, Nature (London) 391 (1998) 62

20.  If the nanotube structure is armchair then the
electrical properties are metallic
 If the nanotube structure is chiral then the
electrical properties can be either
semiconducting with a very small band gap,
otherwise the nanotube is a moderate
semiconductor
 In theory, metallic nanotubes can carry an
electrical current density of 4×109 A/cm2 which is
more than 1,000 times greater than metals such
as copper

21. MAGNETIC PROPERTY
 It has been observed that there is no bulk magnetism in a clean multiwall
nanotube sample.
 On the other hand, when these are placed on a flat ferromagnetic
substrate, fringing fields can be observed by magnetic force microscopy.
Therefore the prospect for “contact-induced magnetism” and the
application of carbon nanotubes to nanoscale spintronic devices remain
open.
 Magnetic contrast is observed for carbon nanotubes placed on cobalt or
magnetite substrates, but is
absent on silicon, copper, or gold. Spin transfer of about 0.1 µB per contact
carbon atom is obtained

22. Superconducting properties
 The dialog about nanotube superconductivity began with a very
simple argument: if one could tune the chemical potential to the
peak of a van Hove singularity, either chemically or electrostatically,
one should have a high N(EF) value, a prerequisite for BCS
superconductivity.
 Super-current flow through an SWNT with low-resistance contacts
was reported in 1999; analogous to contact-induced magnetism,
no claims were made for bulk nanotube superconductivity.
Evidence for a bulk anisotropic Meissner effect below 20 K in
aligned 0.4-nm-diameter SWNT was claimed and justified by the
argument that small-diameter tubes will be the stiffest, and thus, the
average phonon energy in the BCS equation will be favorable. This
dramatic result has not been reproduced by other researchers. As
with bulk magnetism, it would appear that the jury is still out
concerning bulk superconductivity in carbon nanotubes and other
nanostructures.

23. Synthesis of CNTs
 There are two main methods for preparation of CNT:
1. Sublimation of graphite with subsequent desublimation
- Arc Discharge Method
- Laser ablation
2. Decomposition of carbon containing compounds
- Chemical Vapor Deposition ( CVD )

24. 1. Sublimation of graphite with subsequent
desublimation
 This method involves condensation of carbon atoms generated from
evaporation of solid carbon sources of graphite. The sublimation of the solid can
be done using electric arc or laser ablation where the temperature reaches to
2500 -3500°C.
Fig. 2. Schematics for CNT formation by sublimation of graphite
with subsequent desublimation.

25.  The electric arc discharge method is one of the efficient techniques for
synthesis of CNT. Typically, about 60 to70 wt% of the arc-synthesized soot is
CNT. The rest of the soot comprises of fullerenes, amorphous carbon and
catalyst nanoparticles. In electric arc discharge production of CNT two
graphite rods are used and a current is passed continuously between the
electrodes. The anode is drilled and filled with catalysts. The metal oxides
(Ni, Co, Fe) are used as catalyst. In some cases the catalyst/graphite
composite is used as electrode. The synthesis is performed in cooled
chamber in presence of helium, argon or methane environment. During the
arcing, the catalyst/graphite anode is evaporated and consumed with
simultaneous carbon deposition around the cathode. The quality of CNT
samples depends upon arc stability, current density and cooling of
cathode.
 In laser ablation method the graphite target is subjected to laser and
sublimated carbon is recollected. Inert gas atmosphere is maintained
within the chamber.

26. 2. Decomposition of carbon containing
compounds
 The most used method to prepare CNT is pyrolysis of hydrocarbon
gases or vapors such as propane, butane, hexane, benzene,
toluene etc. The method is also known as chemical vapor
deposition (CVD) process.
Chemical Vapor Deposition ( CVD )
By chemical vapor deposition CNTs can be produced in large
quantities. The process temperature can vary from 500 – 1300°C. The
hydrocarbon precursors include CH4, C2H2, C6H6, alcohols etc.
Fig. 3 . Schematics for carbon vapor deposition method

27.  In CVD method there is initial dissociation of hydrocarbons followed by
dissolution and saturation of C atoms in metal nanoparticles. Thereafter there
is precipitation of carbon. Vapor-grown CNTs generally use metal catalyst
particles. Fe, Co and Ni catalysts are mostly used for the catalytic growth of
CNT. More recently, CNTs have also been grown from metal such as Au, Ag
and Cu. Catalyst serves as nucleation sites and also promotes pyrolysis of
hydrocarbons.
Growth Mechanisms :
The growth mechanism of CNT on metal based catalysts and nonmetal based
catalysts are discussed below.
1. Metal based catalysts
2. Non metal based catalysts

28. 1. Metal based catalysts
 The growth mechanism of CNTs is yet to be fully established. In general it is
proposed that hydrocarbons adsorb on metal particles and are catalytically
decomposed. This results in carbon dissolving into particle.
 Upon super saturation, carbon precipitates in tubular crystalline form.
However, various alternative models have been proposed and appropriate
description of growth depends on synthesis route and conditions used.
 Two most described models are root growth and tip growth . The growth
mechanisms are schematically explained in Fig. 4. For formation of CNTs the
metal catalysts have to be dispersed well on the substrate forming nano
clusters on the substrate surface. When there is a strong interaction between
the catalyst clusters and substrate, the CNT grows by the root-growth
mechanism .
 The pyrolysis of hydrocarbons produces the carbon atoms which are
extremely mobile on metal surfaces and rapidly diffuse over and through the
metal particles. The graphite precipitates around the catalyst particles and
cylindrical structures are formed in a nested fashion from the catalyst
particle, with the catalyst particle at the root.

29.  The growth of the nanotubes stops when the catalyst particles are completely
covered with layers of carbon. The tip growth mechanism is observed when the
interaction between metal catalyst clusters and substrate is weak.
 Due to the weak interaction, the catalyst particles are lifted off the substrate by the
cylindrical structures formed below it. While, the carbon nanotube base remains
anchored to the substrate, the tip carrying the catalyst particle grows toward the
region of higher feed gas concentration. The tip growth stops when the catalyst
particle is covered with carbon layers or when the supply of feed gas is cut off.
Fig. 4. Schmatics showing root growth and tip growth
mechanism of CNTs

30.  Synthesis of carbon nanotubes over Fe catalyst on aluminium is described
by Emmenegger as follows. Initially iron nitrate layer on support transforms
to crystalline Fe2O3 film during heating under nitrogen. After acetylene is
introduced, Fe2O3 film is fragmented and smaller particles are formed.
Fe2O3 crystals are reduced to inter-mediate oxides such as Fe3O4, FeO by
hydrogen released from pyrolysis of acetylene. Finally metastable Fe3C is
formed and growth of CNT occurs by tip growth mechanism.
2. Non-metal based catalysts (Ceramic and
semiconductor catalysts)
Among non-metallic catalysts SiC is most widely used. The Fig.5 shows the
carbon nanostructure formation over silicon carbide by carbothermal
reduction of silica. At first stage SiO2 is reduced to SiC via a carbothermal
reaction. In next step SiC nano particle coalesce and finally carbon
nanotube growth occurs on surface of SiC particles through carbon
precipitation and/or SiC decomposition. The carbon generation reaction is
given below.

31. Fig. 5. Schematic representation of CNT formation over silica
MgO, Al2O3, zirconia, magnesium borates etc. are also reported to be
used to grow carbon nanotubes.

33. Characterization of CNT
X-ray diffraction pattern of CNTs are close to graphite. A graphite-like peak
(002) is present and measurements of interlayer spacing can be obtained
from its position using the Bragg law . Carbon nanotubes are also active in
Raman spectroscopy. Most characteristic features are : peak <200
cm−1 which is characteristic of SWNT, frequency depend on tube
diameter. The 1340 cm−1is assigned to residual ill-organized graphite. The
1500 - 1600 cm−1 peak also characteristic of nanotubes. The TEM images are
absolute necessary for studying CNTs.
Application of CNT and CNT based catalysts:
CNTs are used in several catalytic reactions as catalyst or catalyst supports. In
particular liquid phase reactions were studied extensively with MWNT. Higher
surface area and mesoporous nature resulted in significant decrease in mass
transfer limitations compared to activated carbon.

34. 1. Hydrogenation reactions
This is one of the most studied catalytic reactions both in liquid and gaseous
phases. Ni, Rh, Ru supported on CNT were reported to be more active for
hydrogenation reactions compared to when supported on activated carbon.
Hydrogenation reactions such as hydrogenation of alkenes and α,β –
unsaturated aldehyde have been reported for CNT supported catalysts.
Ruthenium nanoparticles supported on MWCNTs showed excellent catalytic
activity for hydrogenation of aromatic hydrocarbon . The 5wt% Pt/CNT catalyst
was reported to be significantly more active than 5wt% Pt/AC for
hydrogenation of trans –diphenylethene and trans β-methylstyrene . Rhodium
complex grafted on MWCNTs was reported to be very active for cyclohexene
hydrogenation while Pd/CNT catalyst was found to be active for benzene
hydrogenation . Pt supported on SWCNTs has been found to be active and
selective in hydrogenation of prenal (3-methyl-2-butenal) to prenol (3-methyl-2-
butenol) .

35. 2. Reaction involving CO/H2
 CNTs have been investigated for Fischer –Tropsch reactions, methanol and
higher alcohol synthesis and hydroformylation reactions. Copper promoted
Fe/MWCNT catalyst are active for Fischer-Tropsch synthesis with olefins . Co-
Re/Al2O3 deposited on MWCNT by dip coating exhibited an enhancement
in Fischer-Tropsch activity than observed with a similar system without CNT
arrays. MWCNTs also have been used as promoter for Cu-ZnO-
Al2O3 catalysts for methanol synthesis using H2/CO/CO2 The complex
[HRh(CO)(PPh3)3] has been grafted onto MCWNTs and used for
hydroformylation of propene. Higher conversion and higher regioselectivity
toward n-butaldehyde have been reported for CNT supported catalysts
compared to that activated carbon or carbon molecular sieve supported
catalysts.

36. 3. Ammonia synthesis and
decomposition
 The Ru/C catalyst is studied as an alternative to conventional iron based
catalyst for ammonia synthesis at high pressure and temperature. However,
Ru/C catalyst is prone to deactivation due to metal sintering, metal
leaching or methanation of support. The stability of the catalyst are
reported to increase on using CNT as support. Ru-K/MWCNT catalyst has
been found to be significantly more active than Ru supported on other
carbon supports . The catalytic decomposition of ammonia to generate
CO- free hydrogen for fuel cells is receiving increasing attention since the
process is more economical than using methanol as hydrogen source. The
MWCNT supported ruthenium was found to be more active than MgO,
TiO2 or alumina supported Ru .

37. 4. Polymerization
The CNTs have excellent thermal and electrical conductivities and reported to
be used as fillers in polymer based advanced composites. However due to poor
solubility of CNTs, homogeneous dispersion is difficult task. The polymer
functionalized CNTs are prepared following three approaches:
 A non-covalent functionalization method in which polymers are produced
by ring opening metathesis polymerization. The coating of hyper branched
polymers on MWCNTs has been obtained via cationic ring opening
polymerization of 3-ethyl-3-(hydroxymethyl)oxetane with a BF3 .Et2O catalyst
 A covalent functionalization performed by, first grafting polymerization
initiators onto the tubes through covalent bond and then exposing these CNT
based macro-initiators to the monomer. The polymer is obtained by atom
transfer radical polymerization. The polyethylene-MWCNT composite has
been produced using catalyst grafting procedure by polymerization of
ethylene on [ZrCl2Cp2] MAO/MWCNT where Cp = C5H5 and MAO =
methylaluminoxane .

38. 5. Fuel cell electrocatalysts
 Olefin polymerization via anchored metallocenes catalysts. It has been
proposed that MWCNT play a key role in increasing the molecular mass.
Supporting the catalytic system on MWCNTs increases the polymerization
rate of ethylene. Syndiotactic polypropylene-MWCNT composites have
been prepared by propylene polymerization on zirconocene-MAO
catalysts.
CNTs are used as catalysts supports for anode or cathode catalysis in
direct methanol fuel cells or proton exchange membrane fuel cells.
The structure and properties of carbon supports which constitute the
electrode material have a direct impact on performance of fuel
cells. The most studied reactions are methanol oxidation (anode
catalyst), oxygen reduction (cathode catalyst) and hydrogen
oxidation (anode catalyst). The Pt is most used metal followed by Pt-
Ru system. The general observation is that CNT based catalysts are
more active and better resistant to poisoning compared to
conventional carbon black support. The advantage of CNT supports
for fuel cell applications is attributed to:

39. 1. Higher metal dispersion and higher electroactive surface area
2. Higher mesopores 3D network facilitating mass transport
3. Excellent conducting properties which improve electron transfer.
6. Other applications
CNT is also being investigated as support for biocatalysts. CNTs have been
used for enzyme immobilization which increases enzyme stability, control of
pore size, multiple active sites and reduced mass transfer limitations.
CNTs have also been used as direct catalyst for some specific reactions such
as methanation to produce CO and CO2 free hydrogen, oxidative
dehydrogenation of ethyl benzene to styrene and oxidative
dehydrogenation of propane to propene, selective oxidation of H2S,
oxidation of aniline, esterification and hydroxylations.

40. References
 https://nptel.ac.in/courses/103103026/
 Nanomaterials Handbook edited by Yuri Gogotsi
 Lecture on Carbon nanotubes and their applications SRM University
 Carbon Nanotubes and their applications edited by Qing Zhang.