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CHAPTER 1

INTRODUCTION

1.1 NANOTECHNOLOGY

The goal of nanotechnology is to control individual atoms and

molecules to create computer chips and other devices that are thousand times

smaller than that are authorized in current technologies. Beyond being used in

computers and communication, nanotechnology could lead to advances in

biosensors, biotechnology and medical devices to designer materials for use

in numerous industries including aerospace, textiles, architecture, pollution

control, efficient lighting and energy production.

The term nano refers to the length scales involved, the nanometer

or 10-9 meters length scale, where systems consist of only a few hundreds of

atoms. Nanotechnology is a fundamentally new and different way of thinking

about the creation of devices and systems. It is really a building of

functionality from the most fundamental level of matter upward to the

macroscopic system. More specifically, nanotechnology is the creation and

utilization of materials, devices and systems through the control of matter on

the nanometer - length scale - the ability to obtain matter at the level of atoms,

molecules and supramolecular structures and the generation of larger

structures with basically new molecular organizations exhibiting novel

physical, chemical and biological properties and phenomena.

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In the year 1959, Caltech physics professor and Nobel Laureate

(1965), Richard P. Feynman delivers a stunning lecture on the possibility of

science research from the bottom up approach. The lecture, cleverly titled,

“There is plenty of room at the bottom”, suggests that there are no limits on

producing things from the atomic level. He has quoted as saying, the

principles of physics, as far as I can see; do not speak against the possibility

of manoeuvring things atom by atom. Feynman suggested in his lecture that

we build better microscopes to accomplish the task of seeing things on the

atomic level. "Nanotechnology" was defined by Tokyo Science University

Professor Norio Taniguchi in 1974 as a term that mainly consists of the

processing, separation, consolidation and deformation of materials by one

atom or by one molecule. In the 1980s the basic idea of this definition was

explored in much more depth by Dr. K. Eric Drexler, who promoted the

technological significance of nano-scale phenomena and devices through

speeches and the books. The coming era of nanotechnology is considered as

the first book on the topic of nanotechnology. Nanotechnology and

nanoscience got started in the early 1980s with two major developments; the

birth of Cluster Science and the invention of the Scanning Tunneling

Microscope (STM). This development led to the discovery of fullerenes in

1985 (Kroto et al) and carbon nanotubes a few years later. This device has

been widely used in industrial and fundamental research to obtain images of

metal surfaces at the atomic scale. It allows us to produce a 3-D profile of the

surface, which we can use to characterize roughness, defects, size (micro to

nano level) and confirmation of molecules.

There are two main approaches used in nanotechnology. One is

bottom-up and the other one is top-down approaches which is shown in

Figure 1.1. In the "bottom-up" approach, materials and devices are built from

molecular components which assemble themselves chemically by principles

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of molecular recognition. In the "top-down" approach, nano-objects are

constructed from larger entities without atomic-level control.

Figure 1.1 Schematic representation of bottom-up and top-down

approaches

The fundamental physics and chemistry changes when the

dimensions of a solid become comparable to one or more of these

characteristic lengths, many of which are in the nanometer (nm) range. One of

the most important examples of this happens when the size of a semi-

conducting material is in the order of the wavelength of the electrons or holes

that carry current. The electronic structure of the system completely changes.

This is the basis of the quantum dot, which is a relatively established

application of nanotechnology, resulting in the quantum dot laser presently

used to read Compact Disks (CDs). If only one length of a three-dimensional

nanostructure is of a nanodimension, the structure is known as a quantum well

and the electronic structure is quite different from the arrangement where two

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sides are of nanometer length, constituting quantum wire. A quantum dot has

all three dimensions in the nanorange. These nanoscale phenomena include

quantum effect and short range forces like van der Waals forces.

1.2 CARBON

Carbon is the most versatile element in the periodic table, owing to

the type, strength and number of bonds it can form with many different

elements. The diversity of bonds and their corresponding geometries enable

the existence of structural isomers, geometric isomers and enantiomers. These

are found in large, complex and diverse structures and allow for an endless

variety of organic molecules.

1.2.1 Allotropes of Carbon

Carbon's ability to exist in various forms in the same physical state

is known as ‘Allotropy’. Carbon in the solid phase can exist predominantly in

five allotropic forms: diamond, graphite, amorphous carbon,

buckminsterfullerene and carbon nanotubes (Figure 1.2). The properties of

various allotropic forms of carbon are shown in Table 1.1.

Diamond: Diamond is one of the best known allotropes of carbon, whose

hardness and high dispersion of light make it useful for industrial applications

and jewellery. Diamond is the hardest known natural mineral, making it an

excellent abrasive and also means a diamond holds its polish extremely well

and retains cluster. With the continuing advances being made in the

production of synthetic diamond, future applications are beginning to become

feasible. Garnering much excitement is the possible use of diamond as a

semiconductor suitable to build microchips from, or the use of diamond as a

heat sink in electronics. Significant research efforts in Japan, Europe and the

United States are under way to capitalize on the potential offered by

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diamond's unique material properties, combined with increased quality and

quantity of supply starting to become available from synthetic diamond

manufacturers. Each carbon atom in diamond is covalently bonded to four

other carbons in a tetrahedron. These tetrahedrons together form a

3-dimensional (3D) network of puckered six-membered rings of atoms.

Because of the stable network of covalent bonds and the three dimensional

arrangement of bonds, diamond is so strong.

Figure 1.2 Allotropes of Carbon

Graphite: Graphite is one of the most common allotropes of carbon. Unlike

diamond, graphite is a conductor and can be used, for instance, as the material

in the electrodes of an electrical arc lamp. Graphite holds the distinction of

being the most stable form of solid carbon ever discovered. Graphite is able to

conduct electricity due to the unpaired fourth electron in each carbon atom.

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This unpaired 4th electron forms delocalized planes above and below the

planes of the carbon atoms. These electrons are free to move, so are able to

conduct electricity. However, the electricity is only conducted within the

plane of the layers. Natural and crystalline graphite is not often used in pure

form as structural materials due to their shear-planes, brittleness and

inconsistent mechanical properties. In its pure glassy (isotropic) synthetic

forms, pyrolytic graphite and carbon fibre graphite is an extremely strong,

heat-resistant (to 3000 °C) material, used in re-entry shields for missile

nosecones, solid rocket engines, high temperature reactors, brake shoes and

electric motor brushes.

Amorphous carbon: Amorphous carbon is the name used for carbon that

does not have any crystalline structure. As with all glassy materials, some

short-range order can be observed but there is no long-range pattern of atomic

positions. While entirely amorphous carbon can be made, most of the material

described as “amorphous” actually contains crystallites of graphite or

diamond with varying amounts of amorphous carbon holding them together,

making them technically polycrystalline or nanocrystalline materials.

Commercial carbon also usually contains significant quantities of other

elements, which may form crystalline impurities. Coal and soot are both

informally called amorphous carbon. However, both are products of pyrolysis,

which does not produce true amorphous carbon under normal conditions.

Fullerenes: The fullerenes are fourth allotropes of carbon. They are

molecules composed entirely of carbon, which take the form of a hollow

sphere, ellipsoid or tube. Spherical fullerenes are sometimes called

buckyballs, while cylindrical fullerenes are called buckytubes or nanotubes.

As of the early twenty-first century, the chemical and physical properties of

fullerenes are still under heavy study, in both pure and applied research in

medicinal uses. Fullerenes are similar in structure to graphite, which is

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composed of a sheet of linked hexagonal rings, but they contain pentagonal

(or heptagonal) rings that prevent the sheet from being planar.

Carbon nanotubes: Carbon nanotubes (CNTs) are cylindrical carbon

molecules with novel properties that make them potentially useful in a wide

variety of applications (e.g., nano-electronics, supercapacitors, solar cells, fuel

cells and targeted drug delivery). They exhibit extra-ordinary strength and

unique electrical properties and are efficient conductors of heat. The nanotube

is a member of the fullerene structural family, which also includes buckyballs.

Whereas buckyballs are spherical in shape, a nanotube is cylindrical, with

atleast one end typically capped with a hemisphere of the buckyball structure

(Iijima 1991). The name is derived from their size, since the diameter of a

nanotube is on the order of a few nanometers (approximately 50,000 times

smaller than the width of a human hair), while they can be up to several

centimeters in length. There are two main types of carbon nanotubes: single-

walled carbon nanotubes (SWCNTs) and multi-walled carbon nanotubes

(MWCNTs).

Table 1.1 Isomers made of carbon

Dimensions 0-D 1-D 2-D 3-D

Isomers C60

Fullerenes

Nanotubes

Carbine

Graphite

Fibre

Diamond

Amorphous

Hybridization sp2 sp2 (sp) sp2 sp3

Density

(g/cm3)

1.72 1.2-2.0

2.68-3.13

2.26

~2

3.515

~3

Bond length

(Å)

1.40 (C=C)

1.46 (C-C)

1.44 (C=C) 1.42 (C=C)

1.44 (C=C)

1.54 (C-C)

Electronic

properties

Semiconductor Metal or

Semiconductor

Semimetal Insulating

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1.2.2 Bonding of Carbon Atoms

To understand the structure and properties of nanotubes (NTs), the

bonding of carbon atoms were to be discussed. A carbon atom has six

electrons with two of them filling the 1s orbital. The remaining four electrons

fill the sp3 or sp2 as well as the sp hybrid orbital, responsible for bonding

structures of diamond, graphite, NTs or fullerenes, as shown in Figure 1.3.

Figure 1.3 Bonding structures of diamond, graphite, CNTs and

fullerenes

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In diamond (Prelas et al 1998), the four valence electrons of each

carbon occupy the sp3 hybrid orbital and create four equivalent covalent

bonds to connect four other carbons in the four tetrahedral directions. This

three-dimensional interlocking structure makes diamond the hardest known

material. Because the electrons in diamond form covalent bonds and no

delocalized bonds, diamond is electrically insulating. The electrons within

diamond are tightly held within the bonds among the carbon atoms. These

electrons absorb light in the ultraviolet region but not in the visible or infrared

region, so pure diamond appears clear to human eyes. Diamond also has a

high index of refraction, which makes large diamond single crystals gems.

Diamond has unusually high thermal conductivity.

In graphite (Kelly 1981 ), three outer shell electrons of each carbon

atom occupy the planar sp2 hybrid orbital to form three in-plane bonds with

an out-of-plane orbital (bond). This makes a planar hexagonal network.

Van der Waals force holds sheets of hexagonal networks parallel with each

other with a spacing of 0.34 nm. The bond is 0.14 nm long and

420 kcal/mol in sp2 orbital, and is 0.15 nm and 360 kcal/mol in sp3

configuration. Therefore, graphite is stronger in-plane than diamond. In

addition, an out of plane orbital or electron is distributed over a graphite

plane and makes it more thermally and electrically conductive. The

interaction of the loose electron with light causes graphite to appear black.

The weak van der Waals interaction among graphite sheets makes graphite

soft and hence ideal as a lubricant because the sheets are easy to glide relative

to each other.

CNTs bonding structure has been sp2 hybridization. However, the

circular curvature will cause quantum confinement and - re-hybridization

in which three bonds are slightly out of plane; for compensation, the

-orbital is more delocalized outside the tube. This makes CNTs mechanically

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stronger, electrically and thermally more conductive and chemically and

biologically more active than graphite. In addition, they allow topological

defects such as pentagons and heptagons to be incorporated into the

hexagonal network to form capped, bent, toroidal and helical CNTs whereas

electrons will be localized in pentagons and heptagons because of

redistribution of electrons.

For convention, we call a CNT defect free if it is of only hexagonal

network and defective if it also contains topological defects such as pentagon

and heptagon or other chemical and structural defects. Fullerenes (C60) are

made of 20 hexagons and 12 pentagons (Kroto et al 1985). The bonding is

also sp2, although once again mixed with sp3 character because of high

curvature. The special bonded structures in fullerene molecules have provided

several surprises such as metal-insulator transition, unusual magnetic

correlations, very rich electronic and optical band structural properties,

chemical functionalizations and molecular packing. Because of these

properties, fullerenes have been widely exploited for electronic, magnetic,

optical, chemical, biological and medical applications.

1.2.3 Classification of Carbon Nanostructures

CNTs and carbon nanofibres (CNFs) are graphitic filaments with

diameters ranging from 0.4 to 500 nm and lengths in the range of several

micrometers to millimeters. Three distinct structural types of filaments have

been identified (Figure 1.4) based on the angle of the graphene layers ( ) with

respect to the filament axis (Bessel et al 2001), namely

Stacked

Herringbone (or cup-stacked)

Nanotubular

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In the literature today, the common categorization method is to

refer graphitic filaments with the stacked or herringbone form as fibers and

those with a nanotubular structure as CNTs.

Figure 1.4 Types of carbon filaments (a) Stacked, (b) Herringbone and

(c) Tubular structure

1.3 CARBON NANOTUBES

CNTs were discovered accidentally by Sumio Iijima in 1991 at the

NEC research laboratory in Japan and by Bethune in 1993 at the IBM

Almaden laboratory (Dresselhaus et al 1996) in California while studying the

surface of graphite electrodes used in an electric arc discharge. Ijima

observation and analysis of the CNT structure started a new direction in

carbon research and attracted many scientists worldwide. The carbon

molecules have novel properties which make them potentially useful in many

applications in nanotechnology, electronics, optics, and other fields of

materials science, as well as potential uses in architectural fields. They may

also have applications in the construction of body armour. They exhibit

extraordinary strength and unique electrical properties and are efficient

thermal conductors.

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1.3.1 Types of CNTs and Related Structures

The CNTs has received enormous attention from researchers over

the past 15 years and promises, along with close relatives such as the

nanohorn, a host of interesting applications. There are many other types of

NT, from various inorganic kinds, such as those made from boron nitride, to

organic ones, such as those made from self-assembling cyclic peptides

(protein components) or from naturally-occurring heat-shock proteins

(extracted from bacteria that thrive in extreme environments). However,

CNTs excite the most interest and promise in the greatest variety of

applications, and currently appear to have by far the highest commercial

potential. CNTs are often referred to in the press, including the scientific

press, as if these were one consistent item. These are in fact a hugely varied

range of structures, with similar huge variations in properties and ease of

production. The structure of NTs remains distinctly different from traditional

carbon fibres that have been industrially used for several decades (e.g.

reinforcements in tennis rackets, aeroplanes frame parts and batteries, etc.)

(Yakobson et al 1997). Most importantly, NTs, for the first time, represent the

ideal, most perfect and ordered, but carbon fibre structure, which is entirely

known at the atomic level. It is this predictability that distinguishes NTs from

other carbon fibres and puts these along with molecular fullerene species in a

special category of prototype materials. Among the NTs, two varieties, which

differ in the arrangement of their graphene cylinders, share the limelight.

MWCNTs are collections of several concentric graphene cylinders and are

larger structures compared to SWCNTs which are individual cylinders with

range of 1-3 nm in diameter (Figure 1.5).

The former can be considered as a mesoscale graphite system,

whereas the latter is truly a single large molecule. However, SWCNT also

shows a strong tendency to bundle up into ropes, consisting of aggregates to

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several tens of individual tubes organised into a one-dimensional triangular

lattice (Ajayan 1999, Liu et al 1998, Collins and Avouris 2000). SWCNT is a

hollow cylinder of a graphite sheet whereas a MWCNT is a group of coaxial

SWCNTs. SWCNT was discovered in 1993 (Iijima and Ichihashi), 2 years

after the discovery of MWCNTs (Iijima 1991). They are often seen as straight

or elastic bending structures individually or in ropes (Thess et al 1996) by

Transmission Electron Microscopy (TEM), Scanning Electron Microscopy

(SEM), Atomic Force Microscopy (AFM) and Scanning Tunneling

Microscopy (STM). In addition, Electron Diffraction (EDR), X-ray

Diffraction (XRD), Raman and other optical spectroscopy can be also used to

study structural features of NTs.

Figure 1.5 Types of CNTs

A SWCNT can be visualized as a hollow cylinder, formed by

rolling over a graphite sheet. It can be uniquely characterized by a vector C in

terms of a set of two integers (n, m) corresponding to graphite vectors a1 and

a2 (Figure 1.6) (Dresselhaus et al 1996).

C = na1+ ma2 (1.1)

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Thus, the SWCNT is constructed by rolling up the sheet such that

the two end-points of the vector C are superimposed. This tube is denoted as

(n, m) tube with diameter given by equation 1.2. By rolling a graphite sheet in

different directions, three typical NTs can be obtained: zig-zag (n, 0),

armchair (m, m or n=m) and chiral (n, m) where n>m>0 by definition. In the

specific example, they are (10, 0), (6, 6) and (8, 4) NTs.

D = |C|/ = a (n2 + nm + m2)1/2/ (1.2)

where a = |a1| = |a2| is lattice constant of graphite. The tubes with m = n are

commonly referred to as armchair tubes and m = 0 as zig-zag tubes. Others

are called chiral tubes in general with the chiral angle, defined as that

between the vector C and the zig-zag direction a1,

= tan–1 [31/2m/ (m + 2n)] (1.3)

ranges from 0 for zig-zag (m = 0) and 30° for armchair (m = n)

tubes. Note that n m is used for convention.

Figure 1.6 Grid of graphene sheet with the lattice vector a1, a2 and the

angle , which determine the type of NTs

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Figure 1.7 SWCNT with different chirality

A nanotube (n, m) is formed by rolling a graphite sheet along the

chiral vector C = na1 + ma2 on the graphite where a1 and a2 are graphite lattice

vector. The NT can also be characterized by the diameter |C| and the chiral

angle, is with respect to the zig-zag axis, = 0°. The diagram is constructed

for a (8, 4) NT.

The lattice constant and inter tube spacing are required to generate

a SWCNT, SWCNT bundle and MWCNT. These two parameters vary with

tube diameter or in radial direction. Most experimental measurements and

theoretical calculations agree that, on average, the C–C bond length

dcc = 0.142 nm or a = |a1| = |a2|= 0.246 nm and inter tube spacing dtt = 0.34 nm.

Thus, equations (1.1) to (1.3) can be used to model various tube structures and

interpret experimental observations. Figure 1.7 illustrates examples of NT

models. Now we consider the energetic or stability of NTs. Strain energy

caused by forming a SWCNT from a graphite sheet is proportional to 1/D per

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tube or 1/D2 per atom (where D is diameter) (Roberson et al 1992). It is

suggested that a SWCNT should be at least 0.4 nm large to afford strain

energy and at most about 3 nm large to maintain tubular structure and prevent

collapsing (Lucas et al 1993). Typical experimentally observed SWCNT

diameter is between 0.6 to 2 nm while smaller (0.4 nm) or larger (3 nm)

SWCNTs have also been reported (Wang et al 2000). A larger SWCNT tends

to collapse unless it is supported by other force or surrounded by

neighbouring tubes, for example, as in a MWCNT. The smallest innermost

tube in a MWCNT was found to be as small as 0.4 nm whereas the outermost

tube in a MWCNT can be as large as hundreds of nm. But typically, MWCNT

diameter is larger than 2 nm inside and smaller than 100 nm outside. A

SWCNT rope is formed usually through a self organization process in which

van der Waals force holds individual SWCNT together to form a triangle

lattice with lattice constant of 0.34 nm.

The structural model is of special interest to derive the tube

chirality (n, m) from simple structural relation or experimentally measurable

geometry (D, ). This is because important properties of NTs are function of

tube chirality, as will be discussed. For example, we may exclude the

presence of all zig-zag tubes in a MWCNT from the structural relations. The

spacing between any two coaxial neighbouring zig-zag tubes (n, 0) and (m, 0)

is D/2 = (0.123/ (n-m)) from equation (1.2) and a = 0.246 nm. This,

however, cannot be close to 0.34 nm spacing required to form a MWCNT

regardless of values of integers n and m. However, a MWCNT can be made

of all armchair tubes (5 m, 5 m) where m = 1, 2, 3, etc. The interspacing for

all armchair MWCNTs is D/2 = (0.123/ ) (3)1/2 (5) = 0.334 nm, very close

to 0.34 nm. An experimentally observed MWCNT can be interpreted with

other models as well. For example, a MWCNT can also be viewed as a

scrolled graphite sheet or a spiral graphite sheet, or mixture of scrolled

structure and concentric shells (Zhou et al 1994), rather than coaxial

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SWCNTs. These models however, have not been accepted in general. But it is

still likely that they present some of experimentally observed carbon

nanostructures or even reported MWCNTs because graphite does show

diverse structures such as graphite whiskers and carbon fibres (Kelly 1981).

1.3.2 Speciality of CNTs

CNTs are one of the most commonly mentioned building blocks of

nanotechnology. With one hundred time the tensile strength of steel, thermal

conductivity better than all but the purest diamond and electrical conductivity

similar to copper, but with the ability to carry much higher current, these

seem to be a wonder material. However, when one hears of some companies

planning to produce hundreds of tons per year, while others seem to have

extreme difficulty in producing grams, there is clearly more to this material

than meets the eye. In fact, NTs come in a variety of forms: long, short,

single-walled, multi-walled, opened and closed, with different types of spiral

structure. Each type has specific production costs and applications. Some

have been produced in large quantities for years while others are only recently

being produced commercially with sophisticated purity and in quantities

greater than a few grams (Collins and Avouris 2000).

1.4 SYNTHESIS OF CNTS

CNTs were synthesized by various techniques. Some of the

important techniques have been discussed. They include:

Arc discharge method

Laser ablation method

Chemical Vapour Deposition (CVD) method

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1.4.1 Arc Discharge Method

This method creates CNTs through arc-vaporization of two carbon

rods placed end to end, separated by approximately 1 mm, in an enclosure

that is usually filled with inert gas at low pressure (Figure 1.8). A direct

current of 50 to 100 A, driven by a potential difference of approximately

20 V, creates a high temperature arc discharge between the two electrodes.

The arc provides high temperatures which are needed to vaporize carbon

atoms into a plasma (>3000 °C) (Ebbesen and Ajayan 1992). The arc

discharge method is the one by which CNTs were first produced and

recognized. The history of CNTs is closely related to the mass production of

fullerenes developed by Kratschmer in 1990. When pure graphite rods are

used, the anode evaporates to form fullerenes, which are deposited in the form

of soot in the chamber (Saito et al 1992). However, a small part of the

evaporated anode is deposited on the cathode, which includes CNTs. When a

graphite rod containing metal catalyst (Fe, Co, Ni, etc.) is used as the anode

with a pure graphite cathode, SWCNTs are generated in the form of soot. The

CNTs made of coaxial graphene sheets and called MWCNTs, are found not

only on the top surface of the cathode deposit but also deep inside the deposit

(Ando and Iijima 1993). Soon after, the studies with a dc arc voltage (Ando

and Iijima 1993, Ando 1994) between two separated graphite rods were

carried out. Among the various inert gases, Helium (He) gives the best results,

probably due to its high ionization potential (54.416 eV). Large scale

synthesis of MWCNTs by arc discharge has been achieved in Helium gas

(Colbert et al 1994). MWCNTs with high crystallinity and few coexisting

carbon nanoparticles were formed with CH4. On the contrary, fullerenes

cannot be produced in gas including Hydrogen atoms (Tai et al 1994). This is

the essential difference between CNT and fullerene production. The role of

gas including Hydrogen atoms in MWCNT production is clarified based on

analysis of ambient CH4 gas before and after arc discharge by mass

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spectroscopy (Wang et al 1996), which revealed that the thermal

decomposition of CH4 gas generates H2 gas in-situ (equation 1.4).

2CH4 C2H2 + 3H2 (1.4)

Therefore, pure graphite rods were arc evaporated in pure

Hydrogen gas. The effectiveness of Hydrogen arc discharge in producing

MWCNTs with high crystallinity was confirmed and a new morphology of

carbon, the ‘carbon rose’, was obsereved (Ando et al 1997 and Zhao et al

1997). A similar effect of ambient Hydrogen gas was also reported by another

group. In fact, for MWCNTs production, a gas that includes Hydrogen atoms

is more effective than an inert gas, such as Helium or Argon. The reason

might be the high temperature and high activity of the Hydrogen arc. The

MWCNTs produced by Hydrogen arc discharge (H2-arc MWCNTs) contain

very few coexisting carbon nanoparticles (Ando et al 1998). These

nanoparticles are easily removed by infrared irradiation or heating in air at

500 °C. High-resolution TEM (HR-TEM) observation of the purified H2-arc

MWCNTs reveals that their crystallinity is very high (having regular

graphene sheets at an interlayer spacing of 0.34 nm) and an inner diameter

typically as low as 0.7 nm, which is equal to the diameter of C60. A tube of

0.3 nm diameter has been found exist inside H2-arc MWCNTs. Moreover, a

linear carbon chain has also been observed inserted into MWCNTs with an

innermost tube diameter of 0.7 nm. Surprisingly, both the 0.3 nm tube and the

carbon chain exist in the same H2-arc MWCNTs (Zhao et al 2003a).

Generally, it is hard to grow aligned CNTs (SWCNTs, DWCNTs,

or MWCNTs) by arc discharge, although partial alignment of SWCNTs can

be achieved by convection (Zhao et al 2003) or directed arc plasma (Huang et

al 2001). On the other hand, the growth temperature of the arc discharge

method is higher than that of other CNT production methods.

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Figure 1.8 Schematic diagram of Arc discharge

1.4.2 Laser Ablation Method

The laser ablation method had been originally used as a source of

clusters and ultrafine particles (Keesee and Castleman 1990, Thess et al

1996). It is evident from the report that vaporization of carbon is essential for

the production of CNTs. Among various vaporization devices the laser is

suitable for materials with high boiling temperature elements like carbon

because of its high energy density. In general, when applied to carbon,

formation of fullerene was discovered by mass spectroscopy. But the yield

was too low for structural identification at that time. To produce large

quantities of fullerenes and other nanomaterials, Smalley's group further

developed the laser ablation method also known as the laser-furnace method

together with an annealing system in 1992. Fullerenes with a soccer ball

structure are produced only at higher furnace temperatures, underlining the

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importance of annealing for nanostructures. These discoveries were applied to

produce CNTs in 1996, especially SWCNTs.

Figure 1.9 Schematic drawing of a Laser ablation apparatus

The laser furnace (Figure 1.9), which consists of a furnace, a quartz

tube with a window, a target carbon composite doped with catalytic metals, a

water cooled trap and flow systems for the buffer gas to maintain constant

pressures and flow rates. A laser beam (typically a YAG or CO2 laser) is

introduced through the window and focused onto the target located in the

centre of the furnace. The target is vaporized in high temperature Ar buffer

gas and forms SWCNTs. The Ar flow rate and pressure are typically 1 cm s 1

and 500 torr, respectively. The SWCNTs produced are conveyed by the buffer

gas to the trap, where they are collected. The vaporization surface is kept as

fresh as possible by changing the focus point or moving the target.

The method has several advantages, such as high quality SWCNT

production, diameter control, investigation of growth dynamics and the

production of new materials. High crystallinity has been known to originate in

high power laser vaporization, homogeneous annealing conditions and target

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materials without Hydrogen (Shinohara 2000). The laser has sufficiently high

energy density not to cleave the target into graphite particles but to vaporize it

at the molecular level. The graphite vapour is converted into amorphous

carbon as the starting material of SWCNTs (Kokai et al 2000). The annealing

conditions of the amorphous carbon are more homogeneous than those of the

arc discharge method, in which the electrodes and the convection flow disturb

the homogeneity of the temperature and flow rate (Kanai et al 2001).

SWCNTs with minimal defects and contaminants, such as amorphous carbon

and catalytic metals, have been produced using the laser furnace method

together with purification processes (Ishii et al 2003). To achieve high quality

DWCNTs and SWCNTs, a method called high temperature pulsed

arc discharge has been developed, which uses a dc pulsed arc discharge to

maintain homogeneous condition in arc discharge inside a furnace (Shimada

et al 2004).

Flow rate affects the diameter distribution, which suggests that the

growth process is fairly slow (on a timescale of seconds) compared with

vaporization processes (nanosecond scale). SWCNT growth is initiated by a

short laser shot, so that the start time of the growth is defined. The growth

processes have been traced by high speed video imaging of light emission or

scattering, from which the growth time is estimated to be more than several

milliseconds. A longer pulse duration, which leads to a lower energy density

and lower vaporization temperature, is essential for the production process.

The tunable duration of laser vaporization is another advantage in producing

and exploring these nanoscale carbon materials. That is why the laser furnace

method is being used for growing various nanomaterials and is expected to

play a powerful role in nanotechnology. However, the laser technique is not

economically advantageous because the process involves high purity graphite

rods, high power lasers and low yield of CNTs.

23

1.4.3 Chemical Vapour Deposition Method

Chemical vapour deposition (CVD) is a simple and economic

technique for synthesizing CNTs at relatively low temperature and ambient

pressure, at the cost of crystallinity. It is versatile in that it harnesses a variety

of hydrocarbons in any state (solid, liquid or gas), enables the use of various

substrates, and allows CNT growth in a variety of forms, such as powder, thin

or thick films, aligned or entangled, straight or coiled, or even a desired

architecture of NTs at predefined sites on a patterned substrate. It also offers

better control over growth parameters. In fact, CVD has been used for

producing carbon filaments and fibres by Walker et al 1959. CVD is popular

method for producing CNTs in which a hydrocarbon vapour is thermally

decomposed in the presence of a metal catalyst. This method is also known as

thermal or catalytic CVD to distinguish it from the many other kinds of CVD

used for various purposes and also compared with high temperature

arc discharge and laser ablation methods.

Using the same technique, soon after the discovery of CNTs by

Iijima (1991) and Endo et al (2004) reported CNT growth from pyrolysis of

benzene at 1100 °C, while Yacaman et al (1993), formed clear helical

MWCNTs at 700 °C from acetylene. In both cases, Fe nanoparticles were

used as the catalyst. Later, MWCNTs were grown from methane (Hernadi et

al 1996), ethylene (Satishkumar et al 1999) and many other hydrocarbons.

SWCNTs were first produced by Dai et al (1996) from disproportionation of

CO at 1200 °C, catalyzed by Mo particles. SWCNTs were also produced from

benzene (Cheng et al 1998), acetylene (Satishkumar et al 1998), ethylene

(Hefner et al 1998) and methane (Kong et al 1998), using various catalysts.

Figure 1.10 shows a schematic diagram of the setup used for CNT growth by

CVD in its simplest form. The process involves passing a hydrocarbon vapour

(typically for 15 60 minutes) through a tube furnace in which a catalyst

24

material is present at sufficiently high temperature (600 1200 °C) to

decompose the hydrocarbon. CNTs grow over the catalyst and are collected

upon cooling the system to room temperature. In the case of a liquid

hydrocarbon (benzene, alcohol, etc.), the liquid is heated in a flask and an

inert gas purged through it to carry the vapour into the reaction furnace. The

vaporization of a solid hydrocarbon (camphor, naphthalene, etc.) can be

conveniently achieved in another furnace at low temperature compared to

high temperature reaction furnace. The catalyst material may also be solid,

liquid, or gas and can be placed inside the furnace or fed in from outside.

Pyrolysis of the catalyst vapour at a suitable temperature liberates metal

nanoparticles in-situ (the process is known as the floating catalyst method).

Alternatively, the catalyst plated substrates can be placed in the hot zone of

the furnace to catalyze CNT growth.

Formation of SWCNTs or MWCNTs is governed by the size of the

catalyst particle. Broadly speaking, when the particle size is a few

nanometers, SWCNTs form, whereas particles a few tens of nanometers wide

favour MWCNT formation. The three main parameters for CNT growth in

CVD are the hydrocarbon, catalyst, and growth temperature. General

experience is that low temperature CVD (600–900 °C) yields MWCNTs,

whereas a higher temperature (900–1200 °C) reaction favours SWCNT

growth, indicating that SWCNTs have a higher energy of formation

(presumably owing to their small diameters, which results in high curvature

and high strain energy). This could explain why MWCNTs are easier to grow

from most hydrocarbons than SWCNTs, which can only be grown from

selected hydrocarbons (e.g. CO, CH4, etc., that have a reasonable stability in

the temperature range of 900 1200 °C). Common efficient precursors of

MWCNTs (e.g. acetylene, benzene, etc.) are unstable at higher temperatures

and lead to the deposition of large amounts of carbonaceous compounds other

than CNTs.

25

Figure 1.10 Schematic diagram of CVD setup

The growth of CNTs is catalyzed by transition metals. The metals

such as Fe, Co, Ni are most commonly used for CNT growth, since the phase

diagram of carbon and these metals suggests finite solubility of carbon in

these transition metals at high temperatures. On saturation the carbon

precipitates out. This leads to the formation of CNTs under the growth

mechanism outlined above. It is remarkable that the transition metals have

proven to be efficient catalysts not only in CVD but also in arc discharge and

laser ablation methods. This indicates that these apparently different methods

might have a common growth mechanism for CNTs, which is not yet clear.

The catalyst particle size has been found to state the tube diameter. Hence,

metal nanoparticles of controlled size can be used to grow CNTs of controlled

diameter (Ago et al 2000). Therefore, the catalyst materials such as solid

organometallocenes (ferrocene, cobaltocene, nickelocene) which liberate

nanometal particles in-situ are used to catalyze CNTs growth. Thin films of

catalyst coated onto various substrates have also proved successful in

achieving uniform CNTs deposits (Fan et al 1999).

26

In addition, the material, morphology and textural properties of the

substrate greatly affect the yield and quality of the resulting CNTs. Substrates

such as silica, quartz, alumina and zeolite are mostly used. Zeolite support

with catalysts in their nanopores has resulted in significantly higher yields of

CNTs with a narrow diameter distribution (Hernadi et al 1996a). Alumina

materials are reported to be better catalyst supports than silica owing to their

strong metal support interaction, which allows high metal dispersion and

results in high density of catalytic sites (Nagaraju et al 2002). Such

interactions prevent metal species from aggregating and forming unwanted

large clusters that lead to a graphite particles or defective MWCNTs. The key

to obtaining high yields of pure CNTs is achieving hydrocarbon

decomposition on catalyst sites alone and avoiding spontaneous pyrolysis.

The CNTs have been successfully synthesized using organometallic

compounds such as nickel phthalocyanine (Yudasaka et al 1997) and

ferrocene (Rao et al 1998) as carbon-cum-catalyst precursors, though the

as-grown CNTs were mostly metal encapsulated. The use of ethanol has

drawn attention for synthesizing SWCNTs at relatively low temperatures

(850 °C) on Fe-Co impregnated zeolite supports and Mo-Co coated quartz

substrates (Murakami et al 2004).

Tree product, camphor has been used to produce high yields and

high purity of MWCNTs (Kumar et al 2004). The MWCNTs with least

contamination of amorphous carbon and metal particles is achieved, as

oxygen atom present in camphor help oxidize amorphous carbon in-situ and

low catalyst requirement with camphor (Kumar and Ando 2003). The CVD is

ideally suited to growing aligned CNTs on desired substrates for specific

applications, which is not feasible by arc discharge or laser ablation methods.

Li et al (1996) have grown dense MWCNTs arrays on Fe impregnated

mesoporous silica prepared by a sol-gel process. Terrones et al (1997) have

produced CNTs on Co-coated quartz substrates via CVD of a tri-azene

27

compound with nearly no by-products, while Pan et al (1998) have reported

the growth of aligned CNTs of more than 2 mm in length over mesoporous

substrates from acetylene. Highly aligned NTs for electronics have been

grown from acetylene using a Co catalyst impregnated in alumina

nanochannels at 650 °C, while pillars of parallel CNTs have been grown from

ethylene on Fe patterned Si plates at 700 °C for field emission applications

(Li et al 1999). Bearing in mind that pyrolysis of a xylene-ferrocene mixture

leads to the growth of vertical CNTs on quartz tube (Andrews et al 1999),

Ajayan and co-workers have produced organized assemblies of CNTs on

thermally oxidized Si wafers (Cao et al 2004).

Since the CVD is a well known and well established industrial

process, CNTs productions are easy to scale up. MWCNTs with controlled

diameter are being produced in large quantities (100 g/day) from acetylene

using nanoporous materials as the catalyst support (Couteau et al 2003).

Wang et al (2002) have developed a nano agglomerate fluidized-bed reactor

(a quartz cylinder 1 m long and 0.25 m wide) in which the continuous

decomposition of ethylene gas on an Fe/alumina catalyst at 700 °C produces a

few kilograms of MWCNTs per hour with a reported purity of 70%. Dai's

group has scaled up SWCNTs production from methane using a Fe-Mo

bimetallic catalyst supported on a sol-gel derived alumina-silica multi

component material (Alan et al 1999). However, Smalley's lab still leads the

way in the mass production of SWCNTs (10 g/day) by the high pressure

carbon monoxide technique (Bronikowski et al 2001). In this method, a Fe

penta-carbonyl catalyst liberates Fe particles in-situ at high temperatures,

while a high pressure of CO (30 atm) enhances the carbon feedstock

manifolds, which significantly speeds up the disproportionation of CO

molecules into carbon atoms and accelerates SWCNTs growth.

28

Apart from large scale production, the CVD also offers the

possibility of growing single NTs for use as probe tips in AFM or as field

emitters in electron microscopes. Hafner et al (1999) have grown single

SWCNTs and MWCNTs (1-3 nm in diameter) rooted in the pores of Si tips

suitable for AFM imaging. In another approach, SWCNTs are grown directly

onto pyramids of Si cantilever tip assemblies. In this case, SWCNTs grown

on the Si surface (controlled by the catalyst density on the surface) protrudes

from the apex of the pyramid. As-grown CNTs tips are smaller than

mechanically assembled NT tips by a factor of three and enable significantly

improved resolution. The CVD produced CNTs have great promise for the

fabrication of sophisticated instruments and nanodevices.

1.4.3.1 Importance of CVD

The research effort in the field of catalytic CVD has increased

considerably for the past one decade. CVD is an elegant low pressure

deposition technique to deposit functional materials, both inorganic and

organic, based on the decomposition of precursor sources at a heated metallic

surface. An increasing variety of carbon based materials can be obtained with

this method, at high deposition rate and with good feedstock utilization. The

properties of the deposited materials are notably different from those of

materials made with other conventional methods. A number of applications,

such as diamond deposition, functional polymer deposition and passivating

silicon nitride deposition, have already found its way to commercial

manufacturing. In recent years, the CVD method involved for the fabrication

of SWCNTs and MWCNTs with high quality and large quantity. The

technique is attractive in many ways.

29

The most important advantages are:

i. The deposition of SWCNTs and MWCNTs are plasma-free

(i.e. without the risk of a damaging or losing functionality of

precursor molecules).

ii. It is an easily scalable method. Scaling to large areas merely

requires an increase in catalytic surface along with a

proportionally larger supply of source gases.

iii. Substrates (whether rigid or flexible) can be easily handled as

they do not have a role in the decomposition process.

iv. Step coverage is excellent and uniformity can easily be

optimized as substrates can be moved during deposition

without any difficulty.

1.4.4 Growth Mechanism of CNTs

Since the growth of the first confirmed CNTs from CVD (Dai et al

1996), experimentalists and theorists have proposed numerous growth

mechanisms. For a detailed review of the myriad growth mechanisms on the

atomistic scale, it is instructive to look at Charlier and Iijima’s treatment from

2001. To begin with, it should be noted that there will be a few simplifying

assumptions made here that have been borne out by numerous experiments

and appear in most theories of formation of CNTs in CVD synthesis:

(i) An active catalyst nanoparticles (Fe, Ni, Co, etc.) and ready

access to carbon feedstock are essential to the efficient

formation of CNTs

(ii) Ideally, once a NT begins to grow, the diameter is set and will

not change as growth continues

30

(iii) At the onset of growth, the catalyst particle and the resultant

NTs are of similar size which leads to the assumption (Choi et

al 2002).

(iv) One particle leads to only one NT during a single growth step.

There are (sometimes obvious) exceptions to each of these

assumptions. For the first assumption, it should be noted that

NTs can be produced from nanoparticles of graphitic carbon

heated up to high temperature without the need for catalyst,

though the applications for this approach are fairly limited

(Botti et al 2002). Second, there are cases in which long NTs

have been shown to change chirality (though not significant

diameter change) along their length, but this is due to a defect

during growth and should be considered as an (common)

exception rather than a rule (Doorn et al 2005).

Further, though assumptions (iii) and (iv) have been observed

directly, a common alternate situation is where the formation of bundles of

small-diameter NTs (1 to 3 nm) from a 10 20 nm catalyst particle has been

reported and explained (Gavillet et al 2001) and in fact is the primary mode of

production in laser ablation. However, diameter control from the latter growth

mechanism is problematic at best and bundles of NTs have limited uses, so

only the one-to-one formation mechanism from small, discrete nanoparticles

(0.4 to 5 nm) will be discussed.

CNT growth in CVD can be split into two basic types depending on

the location of the catalyst, so-called gas phase growth and substrate growth.

Both of these growth pathways can in turn be split into bulk carbon diffusion

and surface carbon diffusion models. In gas phase growth, the catalyst

formation and NT growth occur literally in mid air. In substrate growth,

catalyst nanoparticles or metal precursors are deposited either on a substrate

31

such as SiO2 or on a high surface area powder before growth. The underlying

chemistry for both methods that leads to the formation of NTs from

nanoparticles is similar and both can be usually classed into surface carbon

diffusion and bulk carbon diffusion.

i. Surface carbon diffusion: The metal particle remains a solid, the

“cracked” carbon diffuses around the surface and CNTs nucleates on the side

of the metal particle. Since carbon continually breaks down on the particle,

the tube continues to grow. This is a common mechanism used to explain low

temperature growth, notably with Ni catalyst nanoparticles (Seidel et al 2004).

ii. Bulk carbon diffusion: The carbon feedstock is “cracked” or broken down

on the surface of the metal particle, similar to above. The metal nanoparticle

dissolves the carbon until it reaches saturation, at which point a CNT with one

or more walls grows from the outer surface. In this situation, the metal can

either remain as a solid or become a liquid nanodroplet. In the case where it

becomes a liquid, it is instructive to imagine the droplet dissolving carbon

until it becomes saturated. At this point, a NT begins to extrude and the

continued dissolution of carbon provides fuel for the process of hydrocarbon

vapour metal–carbon–liquid crystalline carbon solid (vapour–liquid–solid

model).

This model was proposed originally to explain the formation of

silicon and germanium whiskers in the 1960s and was extended to explain NT

formation by Saito et al (1995). Both mechanisms have been indirectly

observed via HR-TEM, where the specific favoured mechanism by a

particular growth method depends on the temperature, the type of metal

catalyst and the carbon feedstock used.

In substrate growth, once the NT begins to grow by either surface

or bulk carbon diffusion, the CNTs will undergo either base growth or tip

32

growth (Figure 1.11). In base growth the catalyst particle remains attached to

the surface and the NT is extruded into the air or along the surface. During tip

growth, the end of the NT remains stuck to the surface and the catalyst

particle shoots into the air at the opposite end of the extruding NT. These two

mechanisms have been proposed and indirectly observed for growth of CNFs,

MWCNTs and SWCNTs, depending on the catalyst type, hydrocarbon source

and growth temperature. Tip growth is considered the dominant mechanism

for MWCNTs growth and base growth is dominant for SWCNTs growth.

Whether the catalyst particles will result in tip or base growth depends largely

on the stiction of the nanoparticle to the substrate or support material. For

instance, Huang et al (2004) utilize a fast heating method that reduces the

stiction of the nanoparticle to the substrate. As a result, they believe the

nanoparticle leaps off the substrate as the CNT grows, leading to a tip growth

situation. If the same sample is heated up slowly, the particle remains stuck to

the surface and the tube grows out from it, an example of base growth.

Further study into how to manipulate growth conditions and catalysts to

favour one or the other continues to be an important topic of research.

Figure 1.11 Schematic diagrams of tip growth and base growth of CNTs

33

For a catalyst particle of unchanging size, the growth of CNTs

should continue until the hydrocarbon is shut off, either by removing the

feedstock from the reaction area or by amorphous or graphitic carbon fully

coating the particle, blocking the gas. Additionally, in the case of base

growth, the growth may slow down or stop due to slow diffusion of

hydrocarbons down to the nanoparticle at the bottom of the CNTs. If nothing

impedes the source of carbon to the nanoparticle and nothing impedes the NT

extrusion, the growth should be continuous. In reality, there are competing

reactions at the nanoparticle site, such as the formation of graphitic shells and

the deposition of amorphous carbon. As a result, in suboptimal growth

conditions, amorphous carbon can coat the nanoparticle, preventing feedstock

from reacting with the particle and cutting off the carbon source, terminating

the growth.

1.5 FACTORS INFLUENCING THE GROWTH OF CARBON

NANOTUBES

1.5.1 Carbon Feedstock

The first SWCNTs synthesis performed with CVD utilized CO as a

feedstock (Dai et al 1996). Since then, methane (Vander Wal et al 2000 and

Harutyunyan et al 2002), ethylene (Kim et al 2002 and Liao et al 2004),

acetylene, ethanol, methanol (Maruyama et al 2002 and Zheng et al 2004),

and benzene (Bai et al 2003), as well as others, have been used successfully to

make SWCNTs at various temperatures and using several catalytic metals.

Since much of the growth dynamics is not well understood, there is no

feedstock that provides a clear advantage over others, though there are many

particular applications in which one or another feedstock excel. Many growth

methods, such as HiPco and CoMoCAT (Kitiyanan, et al 2000), utilize CO as

a feedstock to much success, yielding high quality SWCNTs that are smaller

in diameter (0.7 to 1.5 nm) than those from most other conventional

34

hydrocarbon methods (1 to 5 nm). Recently, the use of C2H4 mixed with a

small and measured amount of H2O has been successfully utilized for highly

efficient and clean growths (Hata et al 2004). The use of etching gases, such

as H2O, OH or O2, to improve growth properties is still in its infancy and is

doubtless to be the next major step toward cheap and reliable production of

SWCNTs.

Acetylene decomposes more easily compared to other

hydrocarbons and hence, the carbon yield produced during acetylene

decomposition is much higher than the others. Acetylene is widely known for

the large-scale synthesis of MWCNTs and double-walled carbon nanotubes

(DWNTs) using zeolite-CCVD method (Hiraoka et al 2003).

1.5.2 Catalytic Support

A wide variety of catalytic species can be used to produce

SWCNTs in CVD growth. It is important at this point to note that the word

catalyst is used somewhat indiscriminately in NT science, whether or not the

“catalyst” actually remains in its original form after making a NT. In fact,

several catalytic species, such as Fe and Co, in many cases actually form

metal carbides when they produce NTs and remain carbides after growth.

Regardless of feedstock, it has been found that Fe, Co and Ni nanoparticles

are all able to form SWCNTs and MWCNTs. The selection of a metallic

catalyst may affect the growth and morphology of the nanotubes. Nagaraju et

al (2002) compared catalytic activity of Fe, Co and Fe/Co supported on

alumina or silica. They showed that a best yield of MWCNTs resulted at

700 °C on hydrated alumina prepared from aluminium isopropoxide and

containing a mixture of Fe and Co in it. Seo et al (2004) compared the

catalytic activity of Fe, Co or Ni as the catalyst and laser treated vanadium

plates having high surface area as the catalyst support in the decomposition of

acetylene at 720 °C under CVD conditions. Best quality CNTs were obtained

35

over the iron catalyst with high density and small diameter (10–15 nm) carbon

nanotubes. The use of bimetallic or trimetallic mixtures of Fe, Co and Ni with

elements such as Y, Mo, Ru and Pt has led to massive increases in yield under

certain conditions (Lyu et al 2004). These results are empirical and there is

little generally accepted theory for why this is though there are literally

hundreds of experimental and theoretical papers that analyze the effects on

yield of differing concentrations of elements using a particular growth

condition. Most of carbon nanotube synthesis techniques require the

introduction of catalyst in the form of gas particulates or as a solid support.

Specific catalyst mixtures, such as Fe/Mo and Co/Mo, have been

analyzed in depth though usually with only one feedstock gas. Hata et al

(2004) shows that typical Co/Mo-based catalyst is highly active in ethanol

growth while having relatively low SWCNTs yield in CH4 growth, a common

Fe/Mo catalyst has high yield in CH4 growth and low yield in ethanol. This

observation is one of many that make it clear that the catalyst growth

dynamics feedstock picture is not yet complete and there is much more to

learn. Regardless of the catalyst metals chosen for growth, catalyst material

can take any of three basic forms, depending on the type of samples desired:

vapour, unsupported and supported phase (Herrera and Resasco 2004).

1.5.2.1 Vapour phase catalyst

Vapour phase catalysts are utilized in many of the latest CVD

based bulk growth types, such as HiPco (Bronikowski et al 2001, 2001a) or

vertical C2H5OH vapour systems (Li et al 2004). In these systems, a volatile

Fe or Ni containing compound, such as iron pentacarbonyl (Fe(CO)5),

ferrocene (Fe(C5H5)2) or nickelocene (Ni(C5H5)2) is vaporized and released

into a furnace at 900 to 1200 °C along with feedstock gas. The NTs form in

the vapour phase and condense out onto cold surfaces. Gas phase or float zone

growth has been proposed to follow the Boudouard process, wherein catalyst

36

particles are continually aggregating in the hot zone of the furnace, while

simultaneously producing NTs.

1.5.2.2 Unsupported catalyst

Unsupported catalyst is prepared such that discrete catalyst

nanoparticles lie directly on the growth substrates to individually nucleate NT

growth. This can be done in a variety of ways, such as by depositing via high

vacuum thermal evaporation a primary thin film of Fe or Co, possibly

followed by a film of Mo or by soaking the growth substrate in Fe, Co or Mo

nitrate, chloride or acetate salt and then drying the sample. Spin coating and

drop drying of the various solutions or suspensions are also common. Most of

these methods can be easily patterned via photolithography to afford discrete

catalyst islands from which NTs grow. These catalysts range from extremely

simple catalysts that produce a wide range of diameters (such as the

deposition of thin films or the soaking of substrates in salt solutions) to

complex and specialized, such as Fe cored ferritin proteins, dendrimers or Fe

cored di-block copolymer micelles (Fu et al 2004).

1.5.2.3 Supported catalyst

Supported catalysts were originally designed in the late 1990s to

improve the yield per growth of CVD by providing an extremely high surface

area per gram for bulk growth. The catalyst is generally prepared by

impregnating support material such as silica and alumina (Cassell et al 1999),

zeolite (Li et al 2004b) or MgO (Hu et al 2003) via wet chemical reaction or

simply drying a stirred mixture of support and catalyst salt. In addition to bulk

growth, the catalysts can be deposited in photolithographically patterned

islands or via other lithographic techniques, resulting in well-defined

SWCNTs growth sites across a substrate. The surface area of many of these

materials is quite high >300 m2/g, leading to very high yields of SWCNTs in

37

a relatively small sample batch, for example, with yields of >500 wt % of

SWCNTs over the metallic precursors using ethylene feedstock and Fe/Mo on

an MgO support (Lyu et al 2004). The use of three-dimensional

mesostructured supports made of silica and alumina can also lead to diameter

control. In one case, AlPO4 zeolites have been used to produce 0.4 nm in

diameter SWCNTs, which are the smallest possible SWCNTs predicted by

simulation. Also, the use of support can allow the production of tubes on

substrates that are considered aloof to NT growth, such as Si, W, Pt or Au.

1.6 CLASSIFICATION OF POROUS MATERIALS

The templating self-assemblies of inorganic and organic precursors

provide almost unlimited opportunities in the design of porous frameworks,

surface chemistry and pore structures (type, size and shape) of the materials

formed. A true regeneration in zeolite chemistry was realized with the

introduction of multifunctional long alkyl chain quaternary ammonium

cations as structure directing agents for micropore formation in the late 1960s

(Chiola et al 1971). Extending the revolutionary idea of molecular templates

to supramolecular aggregates, a new era was initiated in the design and

synthesis of porous materials with tailored mesoporous structures (Kresge et

al 1992). Ordered mesoporous solids consist of inorganic or inorganic/organic

hybrid units of long range order with amorphous walls, tunable textural and

structural properties with highly controllable pore geometry and narrow pore

size distribution in the 2 50 nm range. The past fifteen years of intense

research in the field of porous materials have led to several important

discoveries providing a fresh incentive for new synthetic methods

accompanied by the development of a vast range of potential applications

(Sears et al 2010). While zeolites are used on a technical scale in catalysis and

separation, ordered mesoporous solids have recently entered the field of

commercialization with applications in incipient technical processes from

38

adsorption, catalysis, separation, energy storage and conversion to

optoelectronics and nanotechnology (Ji and Zhang 2009).

Figure 1.12 Classification of porous materials

In General, the porous materials are classified into three types such

as microporous, mesoporous and macroporous. According to the IUPAC

definition these materials can be microporous (D < 2 nm), mesoporous

(D = 2–50 nm) or macroporous materials (D > 50 nm) depending on the pore

diameter (D) (Sing et al 1985). The classification of porous material is given

in Figure 1.12. The microporous materials are limited pore size with low

surface area and the channel size cannot be varied. Thus, because of their lack

of pore size flexibility, the microporous materials are not an ideal choice for

the synthesis of CNTs. The macroporous materials have some disadvantages

like disordered structure and the CNTs growth over the external surface of the

support. The new family of mesoporous (nanoporous) molecular sieves with

39

exceptionally large uniform pore structures is suitable material with more

application for the synthesis of CNTs.

1.7 MESOPOROUS MATERIALS

Porous materials are solid forms of matter permeated by

interconnected or non-interconnected pores of different kinds: channels,

cavities or interstices; that can be divided into several classes. The internal

structural architecture of the void space potentially controls the physical and

chemical properties; such as reactivity, thermal and electric conductivity, as

well as the kinetics of numerous transport processes. The characterization of

porous materials, therefore, has been of great practical interest in numerous

areas including catalysis, adsorption, purification, separation, etc., where the

essential aspects for such applications are pore accessibility, narrow Pore Size

Distribution (PSD), relatively high specific surface area and easily tunable

pore sizes. Ordered porous solids contain a regularly arranged pore system,

and it is desired to design materials with different cylindrical, window-like,

spherical or slit-like pore shapes and sizes. Synthetic zeolites and related

porous materials belong to the group of ordered microporous solids and have

been commercially utilized on a large scale (Endo et al 2004). However,

zeolites present severe limitations in mass transfer due to their microporosity.

Thus, attempts to increase the zeolite pore size, decrease the crystallite size,

introduce additional mesopore systems and enlarge the pore sizes into the

mesopore range while retaining the sharp PSD and high pore regularity

represent an intense area of research.

Mesoporous materials have been known for long, but their possible

applications were highly restricted as they possessed high heterogeneity in

pore size with irregular arrangements. The first ordered mesoporous material

appeared in a patent in 1969 but in absence of sufficient characterization, the

specific features of this type of material were not recognized. Ordered

40

mesoporous solids were first described, independently by Japanese

researchers (Inagaki et al 1993) and scientists at the Mobil Oil Corporation in

the early 1990´s.

Figure 1.13 Schematic diagram of KIT-6 pore structure

Mesoporous siliceous materials such as MCM-41, MCM-48,

MCM-50, SBA-1, SBA-15, SBA-16 and KIT-6 have been prepared by self-

assembly methods using long-chain ionic and anionic surfactants as templates

(Zhao et al 1998 Kleitz et al 2003). The MCM-41 materials were promising

catalysts and catalyst supports because of their large pore volume (>1.0 cc/g),

high surface area (>1000 m2/g), large pore diameter (>2 nm) and narrow pore

size distribution. The MCM-41 materials offer the opportunity to extend

shape-selective catalysis beyond the micropore domains typical of zeolite

materials, allowing larger molecules to be handled. By introducing the metal

ions, MCM-41 shows catalytic activity in synthesizing various nanostructures.

Among the mesoporous materials, KIT-6 exhibits a three-dimensional cubic

Ia3d symmetric structure with interpenetrating bicontinuous network of

channels (Kim et al 2005). This provides highly opened spaces for direct

access to guest species without pore blockage due to their unique 3D channel

networks (Figure 1.13). Pure siliceous KIT-6 material has an electronically

neutral framework and is consequently devoid of Bronsted and Lewis acid

sites. In order to improve the catalytic activity of this material, one must

41

incorporate heteroatoms into the framework either by grafting or by direct

synthesis.

1.7.1 Inorganic Matrix

Numerous types of metal oxide frameworks are known. Among

them, SiO2 is the most widely used in a variety of applications, but other

porous metal oxides such as titanium, zirconium, aluminium, magnesium,

tungsten and iron oxides are also widely investigated (Wong and Ying 1998).

In this study porous silica matrices were used to investigate both the special

surface characteristics of the silica after metal particle incorporation as well as

it is used as hard template for the fabrication of CNTs and carbon related

materials. The inorganic matrix, using Tetraethylorthosilicate (TEOS) as

silica source was formed via the polymerization of the inorganic species on

the inorganic or polymeric soft templates in aqueous solution including

hydrolysis and condensation steps (Brinker 1988).

1.7.2 Templates

Organic template and in some cases an inorganic template acts as a

spacer that later becomes the void in mesoporous materials. The amphiphiles

are surface active agents (surfactants) consisting of a non-polar hydrophobic

chain attached to a polar or ionic hydrophilic fragment. In general, surfactants

are divided into four classes: a) anionic or b) cationic, with negatively or

positively charged head groups, respectively, c) amphoteric, with zwitterionic

head groups or d) nonionic, with uncharged hydrophilic head groups.

Regulated by the so-called hydrophobic effect in aqueous solution the

surfactant molecules self-assemble into various thermodynamically stable

structures corresponding to classical monolayer and bi-layer structures

(lamellar phase), or the more complex three-dimensional micellar (L),

hexagonal (H) and cubic phases (Q) (Figure 1.14).

42

The Critical Micelle Concentration (CMC) is the lowest

concentration of surfactants in bulk, at which micelles are spontaneously

formed, and this is an important characteristic of the surfactant in a specific

medium. In the case of ionic surfactants the CMC is influenced by the ionic

strength of the solution, while in the case of non-ionic surfactants, it is mainly

temperature dependent. At higher amphiphile concentration, the randomly

disordered micelles spontaneously assemble into liquid-crystal phases of

differing structures such as hexagonal, cubic or lamellar.

Figure 1.14 Structures of 3D-phases: (a) Micellar, (b) Hexagonal and

(c) Cubic

1.7.2.1 Ionic surfactants

The most commonly used cationic surfactants in mesoporous

materials synthesis are the quaternary alkyl-ammonium cations with a general

formula R1R2R3R4N+ (Lin et al 1999). FSM-16 and the mesoporous solids of

the M41S family, MCM-41 (2D hexagonal), MCM-48 (bicontinous cubic)

and MCM-50 (lamellar) are mainly templated by cetyltrimethylammonium

surfactants (CTMA+) accompanied by Cl- or Br- counterions (Takahashi et al

2001). In the MCM-41 preparation, cetyltrimethylammonium bromide was

used for the direction of the inner porosity of the silica formed. The materials

obtained by cationic templating typically have pore sizes between 1.5 and

10 nm and the thickness of pore walls around 1 1.5 nm.

43

1.7.2.2 Non-ionic surfactants

The hydrophilic groups of nonionic surfactants are made from

water-soluble moieties (e.g. water-soluble polymer chains) and most nonionic

surfactants consist of poly ethylene oxide (PEO) chains as the hydrophilic

group with the general formula Cn(EO)m. These materials are widely used in

materials synthesis because of special micro domain morphologies, as well as

their biodegradability and low toxicity. HMS (Hexagonal Mesoporous Silica)

and wormlike MSU-X (Michigan State University; X denotes which PEO is

used) solids are typical examples of nonionic templated mesomaterials

introduced by Tanev and Pinnavaia (1995) using organic amines, nonionic

amphiphiles and block copolymers under neutral conditions proposing an

organic-inorganic Hydrogen bonding interaction. These materials are

mechanically more stable, have thicker walls but similar pore sizes, surface

areas and pore volumes as compared to that of the ionic templated materials.

Zhao et al (1998) used high molecular weight triblock Pluronics as structure

directing agents to design materials with larger mesopores and ordered

structure, denoted as SBA (Santa Barbara Amorphous) and KIT (Korean

Institute of Technology). The most important members are SBA-15

(2D hexagonal), SBA-16 (cubic) and KIT-6 (cubic). Without any additives,

the mesopores of these materials are in the region of 6–15 nm with very thin

3–7 nm walls. This makes them thermally more stable than M41S and similar

silica materials.

1.7.3 Mesostructure Formation Mechanisms of MCM-41

There have been a number of models proposed to explain the

formation of mesoporous materials and to provide a rational basis of the

various synthesis routes. Similar to zeolite synthesis, organic molecules-

surfactants-function as templates forming ordered organic-inorganic

composite materials (Lind et al 2002). Surfactant is removed from the porous

44

silicate network by calcination process. However, in contrast to zeolites, the

templates are not single organic molecules but liquid crystalline self-

assembled surfactant molecules. The formation of the inorganic-organic

composites is based on electrostatic interactions between the positively

charged surfactants and the negatively charged silicate species. Several

studies have investigated the building mechanism of MCM-41 and seem to be

at first inconsistent. However, the “Liquid-Crystal Templating” (LCT)

mechanism was proposed by the Mobil researchers, based on the similarity

between liquid crystalline surfactant assemblies (i.e. lyotrophic phases) and

M41S materials (Beck et al 1992 and Kresge et al 1992). The common traits

were the mesostructure dependence on the hydrocarbon chain length of the

surfactant tail group (Beck et al 1994), the effect of variation of the surfactant

concentrations and the influence of organic swelling agents. With MCM-41

(which has hexagonally packed cylindrical mesopores) as the representative

M41S material, two mechanistic pathways were postulated by the Mobil

researchers (Figure 1.15).

i. The aluminosilicate precursor species occupied the space

between a pre-existing hexagonal lyotropic liquid-crystal (LC)

phases and deposited on the micellar rods of the LC phase.

ii. The inorganic mediated, in some manner, the ordering of the

surfactants into the hexagonal arrangement.

In either case, the inorganic components (which were negatively

charged at the high pH values used) preferentially interacted with the

positively charged ammonium head groups of the surfactants and condensed

into a solid continuous framework. The resulting organic-inorganic

mesostructure could be alternatively viewed as a hexagonal array of surfactant

micellar rods embedded in a silica matrix; removal of the surfactants

produced the open, mesoporous MCM-41 framework. It is now known that

45

pathway (i) did not take place because the surfactant concentrations used were

for below the CMC required for hexagonal LC formation (Vartuli et al 1994).

This mechanistic pathway was shown possible recently under different

synthesis conditions. The second mechanistic pathway (ii) of LCT was

vaguely postulated as a cooperative self-assembly of the ammonium

surfactant and the silicate precursor species below the CMC. It has been

known that the no preformed LC phase was necessary for MCM-41 formation

but, to date, the actual details of MCM-41 formation have not yet been fully

agreed upon. Several mechanistic models have been advanced which share

the basic idea that the silicate species promoted LC phase formation below the

CMC.

Figure 1.15 Two possible pathways for the LCT mechanism

1.7.4 Heteroatom Substituted MCM-41 and KIT-6

The pure siliceous mesoporous molecular sieves possess a neutral

framework, which limits their applications. In order to provide molecular

sieves with potential catalytic applications, it is possible, as in the case of

zeolites, to modify the nature of the framework by introduction of heteroatom

by hydrothermal methods. Besides, other elements can also be incorporated

on the surface of the materials by grafting or impregnation. Several synthesis

46

methods have been proposed and successfully used to synthesize mesoporous

MCM-41 molecular sieves (Schmidt et al 1994, Zhao and Goldfarb 1995).

Kresge et al (1992) first synthesized mesoporous silicates and

aluminosilicates in alkaline condition. Huo et al (1994) reported the first

synthesis of porous silicates in acidic condition while Yanagisawa et al (1990)

pillared layered silica with surfactant cations and after calcination obtained a

mesoporous silica with uniform distribution of pores.

In order to impart catalytic activity to the chemically inert

mesoporous silicate framework, substitution of Si4+ ions by other heteroatoms

in the MCM-41 structure has been attempted. When trivalent cations like

Al3+, B3+, Ga3+ and Fe3+ substitute for silicon in the walls of the mesoporous

silica, the framework possesses negative charges that can be compensated by

protons, providing acid sites (Tuel and Gontier 1996). The incorporation of

Fe to MCM-41 materials by various methods as well as the evaluation of Fe

states in various MCM-41 matrices and its catalytic behaviour were reported

(Decyk et al 2003).

Synthesis of bimetallic substituted mesoporous silica materials has

attracted much attention. Zhou et al (2001) showed that boron is able to

promote aluminium incorporate into the framework of mesoporous MCM-41

molecular sieves. They reported that the crystal structure of B-Al-MCM-41 is

more regular than Al-MCM-41. Hartmann et al (1996) reported the synthesis

of Ni-Al-MCM-41 and carried out ethylene dimerisation at 70 °C. Cu and Zn

containing MCM-41 mesoporous molecular sieves were characterized by N2

and CO adsorption and temperature programmed reduction (Hartmann and Kevan

1999). Velu et al (2002) synthesized Cu-MCM-41 and Cu-Zn-MCM-41 at room

temperature and their catalytic activity was tested for the selective oxidation

of alcohols to aldehydes. It has been reported that bimetallic Al-Zn-MCM-41

47

is an active catalyst for industrially important alkylation reactions

(Selvaraj et al 2004).

Successful incorporation of Cr in the tetrahedral lattice site of

mesoporous silica could be achieved through careful preparation of the

synthesis gel and its subsequent hydrothermal treatment followed by

calcinations to remove the structure directing surfactants. Samanta et al

(2005) have shown that 4.6 wt% Cr could be incorporated successfully

preserving the highly ordered mesoporous MCM-41 framework by using a

simple hydrothermal method and avoiding the co-precipitation of chromium

oxides during synthesis.

The usefulness of mesoporous materials can be improved by

incorporation of heteroatoms such as titanium (Lapisardi et al 2005),

vanadium (Zhang et al 1996, Zhang and Pinnavaia 1996), aluminium (Prabhu

et al 2009 and Li et al 2004a) and cerium (Araujo et al 2003). Although pure

KIT-6 has no catalytic sites, it can be used as a template to fabricate new

bicontinuous arrays of nanotube-type carbon such as CMK-9 as well as

rod-type CMK-8. The large pore mesoporous silica KIT-6 was prepared by

Kim et al (2005). KIT-6 exhibited cubic Ia3d symmetry; its structure consists

of interpenetrating bicontinuous networks of channels such as those in

MCM-48. The previous researchers have focused on the substitution of

catalytically active heteroatoms which could exhibit considerable reactivity

due to easily accessible active sites within the mesoporous network

(Shao et al 2005).

1.7.5 Templates Removal

Removal of template plays a crucial role in the preparation of

molecular sieves. Depending on the preparative method, in general template

can be removed either by calcination in air or by solvent extraction technique

48

(Figure 1.16). Corma et al (1994) used milder activation conditions to remove

occluded organics and carried out calcination at two different conditions

(i) calcination at 540 ºC for 7 h in air and (ii) calcinations at 540 ºC in N2

atmosphere for 1 h and then in air at the same temperature for 6 h and

concluded that the degradation of the structure was reduced when N2 was

used. In 1993, Chen and co-workers suggested that, in order to minimise

structural damages and to possibly reduce synthesis costs, removal of the

organics by solvent extraction methods could be a better option. They used a

mixture of HCl/C2H5OH and extraction of the organics was proven completed

by 13C-NMR, but acid catalysed condensation of adjacent silanol groups

lowered the d100 value. When thermal decomposition is replaced by solvent

extraction, 27Al MAS NMR results indicated that the environment of the Al

atoms is much less disturbed. Chen et al (1993) and Schmidt et al (1994)

concluded that the method of template removal could have significant impact

on the environment of the Al atom and possibly affect the activity of these

mesoporous solid acids.

Figure 1.16 Schematic representation of surfactant removal from

mesoporous silica

1.7.6 Synthesis of CNTs using MCM-41 and KIT-6

The high surface area, well defined regular pore shape, narrow pore

size distribution, large pore volume and tunable pore size, in conjunction with

49

the high thermal, hydrothermal, chemical and mechanical stability of

MCM-41 and KIT-6 are highly conducive for a number of important

applications such as adsorption and separation, ion-exchange, catalysis and

molecular hosts. In the past few years, the scientific community has witnessed

a great deal of work and rapid expansion of the activities pertaining to this

versatile material. Other development include the separation of bulky

molecules conversion of fly ash into M41S type materials, membranes,

chromatography and electron transfer materials as well as the sorption of

methane and of Hydrogen.

The mesoporous molecular sieves are the good carriers for the

catalyst, to synthesis NTs, nanorods, nanoparticles and so on, could be

prepared inside the mesopores and outside the pores (Huang et al 2003).

Apart from catalysis, other application fields seem to be quite promising. It

offers interesting potential for use in membrane separation, adsorption and

electrical/optical applications (Schuth 2001). Synthesis of small size

controllable metallic clusters dispersed on solid oxides is of crucial

importance for catalytic applications. The thermal stability of such uniform

size distribution metallic particles is also highly desirable in commercial

catalysts.

Metal substituted mesoporous molecular sieves have been

investigated as catalysts for oxidation reactions, acid catalysed reactions,

hydroxylation reaction, polymerisation reactions and recently in the field of

nanotechnology. The mesoporosity and the well defined pore structure in

combination with high surface area make MCM-41 and KIT-6 materials while

promising candidates for the synthesis of SWCNTs and MWCNTs. Recently,

these materials have an opportunity for the design of catalytically active sites

inside uniform channels with controllable nanosized order pore diameter,

which extends up to 10 nm.

50

Somanathan and Pandurangan (2008) reported the synthesis of

MWCNTs over Pt impregnated Al-MCM-41 catalyst by decomposition of

acetylene. The optimized reaction condition of CNT synthesis are 800 °C and

60 mL/min flow rate of acetylene and a short reaction period of 10 min.

Subashini and Pandurangan (2009) have reported the synthesis of MWCNTs

over mesoporous Mn-MCM-41 molecular sieves. The reaction parameters

such as temperature of the reaction and the nature of the catalysts on the yield

and quality of the MWCNTs were investigated.

Somanathan et al (2011) reported the synthesis CNTs using highly

ordered three dimensional (3D) mesoporous materials. In this study, the

synthesis of MWCNTs was carried over Fe loaded 3D structured KIT-6

materials through CVD method. The obtained high yield MWCNTs consist of

thick graphene layers of about 10 nm composed of 29 graphene sheets with

inner and outer diameter of ~17 and ~ 37 nm respectively.

1.8 PURIFICATION OF CNTs

Purification of CNTs generally refers to the separation of CNTs

from other entities, such as carbon nanoparticles, amorphous carbon, residual

catalyst and other unwanted species. Three basic methods have been used

with varying degrees of success, namely gas-phase, liquid-phase and

intercalation methods.

A new gas-phase method has been developed at the NASA Glenn

Research Centre to purify gram scale quantities of SWCNTs. This method

uses a combination of high temperature oxidations and repeated extractions

with nitric and hydrochloric acid. This improved procedure significantly

reduces the amount of impurities such as residual catalyst and non-NT forms

of carbon within the CNTs, increasing their stability significantly. One of the

efficient methods is oxidation in air at temperatures around 750 °C

51

(Tsang et al 1993). Oxidation period is highly important to prevent the

existence inedible contaminants to obtain pure NTs. Liquid-phase oxidation

using KMnO4/H2SO4 solution is one of the purification methods (Ajayan et al

1993). Extremely pure CNTs can be obtained by this method, but the final

NTs may be severely damaged.

Another purification method so-called graphite intercalation

showed that the resistance to bromination of CNTs was better than the carbon

nanoparticles (Chen et al 1996). Li et al (2000) developed a procedure for

purifying SWCNTs synthesized by the catalytic decomposition of

hydrocarbons. The characterization results revealed that amorphous carbon,

catalyst particles, vapour-grown CNFs and MWCNTs were removed from the

SWCNTs without any damage. The yield of SWCNTs was 40% and the

purity was about 95% after purification. Hou et al (2002) derived an efficient

technique for purification stage. They improved a multi-step purification

process including bromination mechanism using bromine water, which could

remove undesired impurities with an improved yield.

Chattopadhyay et al (2002) applied a purification process including

a sonication-mediated treatment of obtained CNT soot in a one-to-one

mixture of hydrofluoric and nitric acids. Biro et al (2002) applied wet and dry

chemical purification for the removal of unwanted carbonaceous products and

catalyst particles. Wet oxidation is found to be effective in achieving both

goals and produces a relatively moderate damage of the outer wall of the NTs.

The catalyst particles encapsulated in the central channel of the CNTs cannot

be removed even if repeated treatments are applied.

1.9 FUNCTIONALIZATION OF CNTs

Functionalization of CNTs involves introducing sp3 hybridized

carbon atoms to the graphene sheet. Functionalization occurs at defect sites

52

along the sidewalls and tube ends, which are also easily oxidized to form open

tubes. The addition of functional groups such as fluorine (Mickelson et al

1998), carboxylates (Peng et al 2003 and Zhang et al 2010) and various

organic groups (Dyke and Tour 2004) has allowed for improved solubility of

CNTs in different solvents and processibility in composite materials.

Figure 1.17 Schematic representation of functionalization

1.10 PROPERTIES OF CNTs

There are many useful and unique properties of CNTs. They are

High electrical conductivity

Highly flexible that can be bent considerably without damage

High thermal conductivity

Good field emission of electrons

High mechanical properties

High aspect ratio (length = ~1000 x diameter)

CNTs can be highly conducting and hence said to be metallic. Their

conductivity has shown to be a function of their chirality, the degree of twist

as well as their diameter. CNTs can be either metallic or semi-conducting in

their electrical behaviour. CNTs are tubular carbon molecules with exciting

and fascinating properties compared to the parent planar graphite due to the

53

unique structure, topology and dimensions of the NTs. The topology or the

closed geometry of individual CNT layers also impact significantly on the NT

physical properties. The combination of size, structure and topology endows

CNTs with their unique electronic, mechanical, physical and chemical

properties.

1.10.1 Electrical and Electronic Properties

The electrical and electronic properties of materials are measured in

terms of resistance. The well dispersed electrical fillers create a three-

dimensional network, which provides a conductive path through the

composite. This is a commercial way to turn an insulating material into an

electrical conductor. The loading limit is called percolation threshold and can

be detected as a sharp drop in electrical resistance, because of the peculiar

electronic transport properties of the SWCNTs and the high conducting

properties of the MWCNTs. CNTs arouse great interest in the

microelectronics community for new functional electronic devices.

1.10.2 Mechanical Properties

CNTs are composed entirely of sp2 hybridized –C=C– covalent

bonds, which are stronger than the sp3 bonds found in diamond. This bonding

structure is one of the strongest in nature and endows CNTs with their unique

strength and thus, CNTs are one of the stiffest and most robust synthesized

structures, with high Young’s modulus and high tensile CNTs strength. Early

theoretical calculations predicted a Young’s modulus as high as 1 5 TPa,

while other researcher’s predicated that the CNTs would soften with

decreasing radius and by varying the CNT chirality. The exceptional

mechanical properties and low weight of NTs and nanofibers (NFs) make

them potential filling materials in polymer composites. NTs and NFs can

improve the strength and stiffness of a polymer, as well as add

54

multifunctionality (such as electrical conductivity) to polymer based

composite systems.

1.10.3 Chemical Reactivity

In comparison to a graphene sheet, the chemical reactivity of CNTs

is greatly enhanced by the NT surface curvature and is directly related to the

-orbital mismatch caused by an increased curvature. The sidewall and end

caps of the CNT structure have different chemical reactivity which increases

as the NT diameter decreases, such that the end caps are more reactive than

the sidewalls and a smaller NT results in increased reactivity. For example,

the solubility of CNTs in different solvents can be controlled by the covalent

chemical modification of either the sidewalls or the hemispherical end caps

(Daenen et al 2003). Since CNTs are composed of graphitic carbon, they are

highly resistant to chemical attack and exhibit high thermal stability.

Oxidations studies have shown that, since the end caps are more reactive than

the sidewalls, the CNTs are usually oxidized from their tips, thus, leading to

the possibility of opening CNTs by oxidation techniques.

1.11 APPLICATIONS OF CNTs

The most eye-catching features of these structures are their

electronic, mechanical, physical and chemical characteristics, which open a

way to future applications. Especially, the possibility as an efficient electron

source attracts attention because of the thin needle shape. The technology is

almost in practical use in the flat panel display industry and would gain

importance in the future, because of its crystal completeness and miniature

structure. CNT technology is applied to fuel cells, absorbents, sensors,

lightweight and high strength raw material and medicine etc. CNTs can be

used as catalyst supports due to their high surface area and the ability of

chemical species to attach on their sidewalls. CNTs today are available for

55

industrial applications in bulk quantities up metric ton quantities from Cheap

Tubes Incorporation and CAER, University of Kentucky, USA. Several CNTs

manufacturers have >100 ton per year production capacity for MWCNTs.

CNTs possess many unique properties which make them ideal

AFM probes. Their high aspect ratio provides faithful imaging of deep

trenches, while good resolution is retained due to their nanometer scale

diameter. These geometrical factors also lead to reduced tip-sample adhesion,

which allows gentler imaging. NTs elastically buckle rather than break when

deformed, which results in highly robust probes. Some commercial products

on the market today utilizing CNTs include stain resistant textiles, CNT

reinforced tennis rackets and baseball bats. Companies like Kraft foods are

heavily funding CNT based plastic packaging. Food will stay fresh longer if

the packaging is less permeable to atmosphere. Coors Brewing Company has

developed new plastic beer bottles that stay cold for longer periods of time.

Samsung already has CNT based flat panel displays on the market. It uses

aligned SWCNTs in the transparent conductive layer of their display

manufacturing process.

CNTs have the desired intrinsic characteristics and have been used

as electrodes in batteries and capacitors. CNTs have a tremendously high

surface area, good electrical conductivity and very importantly, their linear

geometry makes their surface highly accessible to the electrolyte. Research

has shown that CNTs have the highest reversible capacity of any carbon

material for use in lithium ion batteries (Gao et al 2000). The exploration of

CNTs in biomedical applications is just underway, but has significant

potential. Since a large part of the human body consists of carbon, it is

generally thought of as a very biocompatible material. The ability to

functionalize the sidewalls of NTs also leads to biomedical applications such

as neuron growth and regeneration (Dwyer et al 2002). The oxidized single-

56

walled nanohorns (SWNHs), nanofoams and entrap cisplastin acts as an

anticancer agent. Cisplastin-incorporated oxidized SWNHs acts as a potential

drug delivery system (Ajima et al 2005).

1.11.1 CNT–Based Nanoelectronics

The possibility of using CNTs in spite of silicon for downsizing

circuit dimensions, based on the metallic and semiconducting behaviour, as

well as the electronic transport properties of CNTs is of considerable interest

in the nanotechnology field. The remarkable electronic properties of CNTs

and its applications as quantum wires (Tans et al 1997) have inspired the

design of several components for nanoelectronics. The pioneering works of

Delft group led to the design and development of the first single molecule

Field Effect Transistor (FET) based on semiconducting SWCNTs (Tans et al

1998). The device comprises of a NT bridging two metal electrodes deposited

on an insulating substrate that serves as a gate electrode (Martel et al 1998).

The design of various tiny devices such as rectifying diodes has also been

proposed by Yao et al in 1999. All the developments in this area have been

really fascinating and provide promising perspectives for NT-based

electronics (Robertson 2004).

CNTs have been recognized as one of the most promising electron

field emitters ever since the first field emission experiments reported by

De Heer et al (1995). Their potential as emitters in various devices has been

demonstrated during the last ten years (Wong et al 2006). A flexible field

emitter was made from MWCNT microwave welded on polycarbonate,

showing excellent electrical conduction and field-emission properties even

under bending (Wang et al 2007). Avouris (2007) discussed about the various

features of electronic with CNTs, which are applicable in switches, transistors

and light-emitting devices with highly useful properties. CNTs due to its high

aspect ratio, mechanical strength and flexibility make them potentially ideal

57

structures for use as tips in Scanning Probes Microscopes (Wong et al 1998).

Preparation and properties of maleic acid and maleic anhydride functionalized

MWCNTs/Poly (urea urethane) (PUU) nanocomposites showed enhanced

dispersion compared with that of pristine MWCNT and PUU (Wu et al 2007).

CNTs based sensor devices make use of the strong electronic

response of NTs to detect changes in the local environment (Ajayan and Zhou

2001). Significant research is in progress to develop CNT–based chemical,

biological and physical sensors. These efforts can be broadly classified into

two categories: one that utilizes certain properties of the NT, such as a change

in conductivity with gas adsorption and the second that depends on the ability

to modify the CNT tip or sidewall with functional groups that serve as sensing

elements. CNT filters have been recently demonstrated with freestanding

monolithic uniform macroscopic hollow cylinders having radially aligned

CNT walls (Srivastava et al 2004). Zou et al (2007) reported biosensor based

on polyaniline-prussian blue/MWCNTs hybrid nanocomposites which, shows

rapid response, high sensitivity, good reproducibility, long term stability and

freedom of interference from other co-existing electroactive species. The

applications of nanosensors using CNTs include gas sensors, which are used

to monitor leaks in chemical plants, biosensors for cancer diagnostics and

sensitive environmental pressure sensors.

1.11.2 Electrochemical Supercapacitor of CNTs

Supercapacitors are electrochemical capacitors with high

capacitance and power density, which typically comprise of two electrodes

separated by an insulating material that is ionically conducting in

electrochemical devices (Kotz and Carlen 2000). They store the electric

energy in an electrochemical double layer formed at the solid electrolytic

interface. In order to achieve high capacitance, porous electrode materials

with large accessible surface area used. Based on the electrode materials,

58

electrochemical capacitors are classified into three main categories such as

carbon based, metal oxides and polymeric materials (Sarangapani et al 1996).

CNTs with its narrow distribution of mesopore sizes, highly

accessible surface area, low resistivity and high stability, have been attracting

great interest worldwide as electrode materials for supercapacitors (Lee et al

2004). Supercapacitors fabricated using CNTs-nanocomposite electrodes have

shown enhanced performance due to the high usage efficiency of specific

surface area following the disintegrated bundle structure of CNTs (Lee et al

2002). Nanocomposites of Electronically Conducting Polymers (ECPs) and

MWCNTs have been used as electrode materials for supercapacitors and the

specific capacitance of these composite electrodes have been determined

using different cell configurations (Khomenko et al 2005). Supercapacitor

electrodes fabricated using metal oxide/MWCNTs nanocomposite materials

have shown increased specific capacitance with an improvement in cycle life

and power density (Wang et al 2005).

1.12 NANOCOMPOSITES

Nanocomposites have been used commercially since Toyota

introduced the first polymer/clay auto parts in the 1980s. Recently, advances

in the ability to characterize, produce and manipulate nm-scale materials have

led to their increased use as fillers in new types of nanocomposites.

Manufacturers now mix nanoparticulate metals, oxides and additional

materials with polymers and other matrix materials to optimize the

composite’s properties with respect to colour/transparency, conductivity,

flame retardancy, barrier properties, magnetic properties and anticorrosive

properties, in addition to tensile strength, modulus and heat distortion

temperature. These composites offer users significantly enhanced properties

compared to conventional composite and non-composite materials. Global

consumption of nanocomposites has increased rapidly, reaching 23 million

59

pounds in 2005, with an estimated value of $252 million. By 2011, it is

expected to reach almost 95 million pounds worth some $857 million.

1.12.1 CNTs Filled Epoxy Nanocomposite

The research activity has focused mostly on the evaluation of

MWCNT properties themselves, MWCNTs high potential as nano structured

polymer composite filler, and their expected novel material properties. The

unique mechanical properties of MWCNTs, namely their high strength and

stiffness and enormous aspect ratio make them a potential structural element

for the improvement of mechanical properties (Treacy et al 1996 and

Thostenson et al 2001). Further potential advantages of the use of MWCNTs

as ultimate polymer filler improved the electrical and thermal conductivity

properties together with their low density character (Valentini et al 2004).

Mechanical reinforcement of polymers by MWCNTs can be

realized only by solving two main problems,

Dispersion of MWCNTs and

Interfacial adhesion between the MWCNTs and the polymer

matrix.

The first experimental work focusing on the interfacial interaction

in MWCNTs/epoxy nanocomposites was Cooper et al (2002). They

investigated the detachment of MWCNTs from an epoxy matrix. In a special

pull out test of individual MWCNTs, the interfacial shear strength values

were found to be in the 35 376 MPa range. Further investigations on the

NT/polymer interfacial interactions had been performed by Barber et al

(2003). The realization of MWCNTs-reinforced epoxy requires, besides

homogenous dispersion, strong interfacial interaction between the MWCNTs

and the polymer. We believe that MWCNTs effectiveness as reinforcing

60

elements in tough epoxy matrices is hindered by weak interfacial interactions.

As such, stress-induced deformation of composites could lead to failure of the

MWCNTs/epoxy interface and finally to pull out. Further enhancement of

MWCNTs with composite materials possibly is achieved by chemical

functionalization of their surfaces, through physical bonds to the polymeric

matrix. These bonds will enable stress transfer between the polymer and the

MWCNTs, leading to improved interfacial interactions, as qualitatively

determined previously. The nature of the interfacial zone accords with the

microstructural characteristics of the reinforcing MWCNTs, in which any of

the three mechanisms physical interaction, physical-chemical interaction or

mechanical interlock, may be dominant in which the van der Waals forces of

attraction are the primary binding forces at the interface (Pye and Beaudoin

1992). Also having a significant influence on the fracture toughness of the

interface is the surface roughness of the reinforcement.

The part of our study involves the fabrication processes by which

MWCNTs/f-MWCNTs are homogeneously dispersed in an epoxy matrix. In

the present work, the influence of MWCNTs/f-MWCNTs on the thermo-

mechanical properties of MWCNTs/Epoxy and f-MWCNTs/Epoxy

nanocomposites were investigated.

1.13 SCOPE AND MAIN OBJECTIVES OF THE PRESENT

INVESTIGATION

With the revolutionary discovery of so-called fullerenes and CNTs,

different research fields in the domain of carbon experienced an enormous

boom. Until now CNTs could only be synthesized in small quantities or with

complicated methods. The CVD method is believed as the most suitable

CNTs synthesis method in terms of product purity and large scale production.

When the cost of CNTs production comes into account, the first consideration

61

is the synthesis method. As it is mentioned earlier, CVD is the most suitable

low cost mass production method of CNTs. In large-scale production, the cost

of carbon source also plays an important role in the final product cost.

Graphitization of CNTs offers a low-cost and commercially viable

purification process by removing the residual metal catalyst in the CNTs and

reducing the defects in the CNT structure (Andrews et al 2001).

Intensive research activities to improve the synthesis methods and

conditions, quality and productivity of the CNTs reached to rewarding

conclusions because of their high strength, stiffness and electrical

conductivity. The invention of mesoporous molecular sieves by Mobil

researchers in 1992 has given a new direction to the field of porous materials.

MCM-41 has salient features such as hexagonal arrangement with uniform

channel structure. However, the tunable pore size (20 100 Å), large surface

area ( 1000 m2g-1) and moderate acidity are characteristics of MCM-41

silicates. The pore sizes allow catalytic transformations of large molecules to

be carried out, which could not diffuse inside the microporous zeolites. The

mesoporous cubic KIT-6 have large pore and high thermal stability compared

with M41S family due to the construction of thick walls. The KIT-6 provides

highly opened spaces for direct access to guest species (carbon precursor)

without pore blockage due to their unique 3D channel networks. Transition

metal catalysts can be incorporated into the pore walls of the mesoporous

molecular sieves stabilizing dispersed catalytic sites and also exhibit good

structural stability. The catalytic site for CNTs synthesis is provided by

framework substitution with metals. The mesoporosity and the well-defined

pore structure in combination with high surface area make MCM-41 materials

as promising candidates for the synthesis of CNTs. In this thesis, the catalyst

for the CNT deposition, carbon precursor, synthesis temperature and reaction

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time are discussed as the important parameters affecting the quality and

quantity of the produced CNTs. Purification of the CNTs and the synthesis

cost are considered as the important industrial parameters. Also the effects of

pre-treatment of catalyst, reaction temperature, hydrocarbon precursor and

nitrogen (inert gas) are underlined.

The main objectives of the present investigation is to synthesize

various transition metal containing mesoporous MCM-41 and KIT-6 as

catalytic templates for the production of SWCNTs and MWCNTs by CVD

method. This CVD method is simple, efficient and cost effective for the

production of CNTs.

The scope of the present investigation is envisaged below:

Hydrothermal synthesis of Fe-MCM-41, Zn-MCM-41 and

Zn-Fe-MCM-41 molecular sieves with Si/M ratios of 50, 75,

100 and 125, where M = Fe, Zn and Zn-Fe were prepared.

Mesoporous Cr-MCM-41 with Si/Cr ratios of 50, 75, 100 and

125 was hydrothermally synthesized. To this, Fe, Co, Mn, Ni,

Ti, Ru, and Pd were loaded with 0.2 wt. % individually by wet

impregnation method.

Mesoporous Si-MCM-41 was synthesized hydrothermally and

Sb with wt. % of 1.0, 2.0, 3.0, 5.0, 10.0, 15.0 and 20.0 was

loaded on them using wet impregnation method.

Mesoporous 3D cubic Fe-KIT-6 with various Si/Fe ratios

(50, 75 and 100) were synthesized hydrothermally and Zn

with 0.25, 0.50 and 0.75 wt. % was loaded on them by wet

impregnation method.

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Fe and Zn (Fe: Zn = 3:1) with 1.0, 2.0 and 3.0 wt. % was

loaded on MgO by wet impregnation method.

The above synthesized catalytic materials were characterized

using various physico-chemical techniques such as Inductive

Coupled Plasma-Atomic Emission Spectroscopy (ICP-AES),

X-ray Diffraction (XRD), Nitrogen Sorption Isotherm studies,

Thermogravimetric Analysis (TGA), Fourier Transform

Infrared (FT-IR) Spectroscopy, Diffuse Reflectance

Ultraviolet Visible Spectroscopy (DRS-UV), Scanning

Electron Microscopy (SEM) and Transmission Electron

Microscopy (TEM).

The catalytic growth of SWCNTs and/or MWCNTs was

carried out using the above synthesized catalytic materials by

CVD method. The reaction parameters such as metal

concentration over the catalytic template, flow rate of carbon

precursors, growth time and temperature were optimized

individually for the production of well graphitized CNTs over

the above said catalytic materials.

The as-synthesized carbon deposits formed over different

catalytic materials were purified by acid treatment and air

oxidation. The purified CNTs were characterized by various

physico-chemical techniques such as XRD, TGA, SEM,

HR-TEM and Raman spectroscopy.

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The well graphitized MWCNTs were functionalized and

confirmed by SEM and FT-IR spectroscopy. MWCNTs and

f-MWCNTs were compared with one another for the

dispersion behaviour with polymer matrix. MWCNTs/Epoxy

and f-MWCNTs/Epoxy nanocomposites were fabricated with

different wt. % (0.5, 1.0 and 1.5) of MWCNTs and

f-MWCNTs respectively. Thermo-mechanical properties such

as flame retardance, TGA, hardness, tensile strength and

flexural strength were investigated for fabricated

nanocomposites.