Unit IV
PAGE 2
Unit 4-Introduction to Nanomaterials 4.1.Introduction to
nanomaterials: Bulk materials and nanomaterials Changes in bulk and
nanomaterials of silicon, sodium chloride, gold. (8 Periods)
4.2.General methods of preparation of nanomaterials: Physical
and chemical methods
4.3Applications of nanomaterials: Nanodevices- Carbon
nanotubes-preparation, purification and applications in electronic
industries
4.1. Introduction A nanoparticle is an entity with a width of a
few nanometers to a few hundred, containing tens to thousands of
atoms.
Nano size:
One nanometre is a millionth part of the size of the tip of a
needle.
1 nm = 10-6 mm = 10-9 m
Table 1. Some examples of size from macro to molecular
Size (nm) Examples Terminology
0.1-0.5 Individual chemical bonds Molecular/atomic
0.5-1.0 Small molecules, pores in zeolites Molecular
1-1000 Proteins, DNA, inorganic nanoparticles Nano
103-104living cells, human hair Micro
>104Normal bulk matter Macro
From: USDAs roadmap of nanotechnology.
In recent times nanomaterials have been the subject of enormous
interest. These materials, notable for their extremely small size
exhibit unusual mechanical, electrical, optical and magnetic
properties. Due to their special properties, nanomaterials .have
the potential for wide-ranging industrial, biomedical, and
electronic applications. For example, long lasting medical implants
of biocompatible nanostructured ceramic and carbides, biocompatible
coating, drug delivery, protection coatings, composite materials,
anti fogging coatings for spectacles and car windows etc. The
nanoworld lies midway between the scale of atomic and quantum
phenomena, and the scale of bulk materials. At the nanomaterial
level, some material properties are affected by the laws of atomic
physics, rather than behaving as traditional bulk materials do.
Nanomaterials can be metals, ceramics, polymeric materials, or
composite materials. Their defining characteristic is a very small
feature size in the range of 1-100 (nm). 4.1.1. Bulk and
nanomaterialsAlthough widespread interest in nanomaterials is
recent, the concept was raised over 40 years ago. Physicist Richard
Feynman delivered a talk in 1959 entitled "There's Plenty of Room
at the Bottom", in which he commented that there were no
fundamental physical reasons that materials could not be fabricated
by maneuvering individual atoms. Nanomaterials have actually been
produced and used by humans for hundreds of years - the beautiful
ruby red color of some glass is due to gold nanoparticles trapped
in the glass matrix. The decorative glaze known as luster, found on
some medieval pottery, contains metallic spherical nanoparticles
dispersed in a complex way in the glaze, which give rise to its
special optical properties. What makes these nanomaterials so
different and so intriguing? The properties of nanomaterials
deviate from those of single crystals or polycrystals and glasses
with the same average chemical composition. This deviation results
from the reduced size and dimensionality of the nanometer-sized
crystallites, the numerous interfaces between adjacent
crystallites, grain boundaries and surfaces.
Their extremely small feature size is of the same scale as the
critical size for physical phenomena - for example, the radius of
the tip of a crack in a material may be in the range 1-100 nm. The
way a crack grows in a larger-scale, bulk material is likely to be
different from crack propagation in a nanomaterial where crack and
particle size are comparable. Fundamental electronic, magnetic,
optical, chemical, and biological processes are also different at
this level. Where proteins are 10-1000 nm in size, and cell walls
1-100 nm thick, their behavior on encountering a nanomaterial may
be quite different from that seen in relation to larger-scale
materials.
Nanomaterials are assembled from nanometer-sized (crystallites).
These building blocks may differ in their crystallographic
orientation that may lead to incoherent or coherent interfaces
between them that lead to inherent heterogeneous structure on a
nanometer scale. Grain boundaries make up a major portion of the
material at nanoscales, and strongly affect properties and
processing. Surfaces and interfaces are also important in
explaining nanomaterial behavior. In bulk materials, only a
relatively small percentage of atoms will be at or near a surface
or interface (like a crystal grain boundary). In nanomaterials, the
small feature size ensures that many atoms, perhaps half or more in
some cases, will be near interfaces. Surface properties such as
energy levels, electronic structure, and reactivity can be quite
different from interior states, and give rise to quite different
material properties.
Nanocrystallites of bulk inorganic solids have been shown to
exhibit size dependent properties, such as lower melting points,
higher energy gaps etc. In comparison to macro-scale powders,
increased ductility has been observed in nanopowders of metal
alloys. In addition, quantum effects from boundary become
significant leading to phenomena such as quantum dots.
Gold : Bulk Vs NanoProperties of gold in bulk form lustrousthey
have a shiny surface when polished
Malleablethey can be hammered, bent or rolled into any desired
shape
Ductilethey can be drawn out into wires
is metallic, with a yellow colour when in a mass
good conductors of heat and electricity
generally have high densities
have high melting point (~1080 deg C)
are often hard and tough with high tensile strength, meaning
that they offer high resistance to the stresses of being stretched
or drawn out and therefore do not easily break is inert-unaffected
by air and most reagents
4.1.1.1 Gold nanoparticles
Fig 1. Size and shape dependent colors of Au and Ag
nanoparticles At the nano-level, gold acquires a new shine, a new
set of properties and a host of potential new applications.
Nanoparticles often have unexpected visible properties because they
are small enough to scatter visible light rather than absorb it
(Fig.1). Gold nanoparticles appear deep red to black in solution.
In fact a whole range of colours can be observed depending on the
size of the gold nanoparticles. The distance between particles also
effects colour. Surface plasmon resonance (The excitation of
surface plasmons by light is denoted as a surface plasmon
resonance) is the term used by nanotechnologists to describe this
effect. Gold nanoparticles. Vary in appearance depending on size
and shape of cluster
Are never gold in colour
Are found in a range of colours
Are very good catalysts
Are not metals but are semiconductors. Melts at relatively low
temperature (~940 C)
Size & Shape of the nanoparticles determines the colourFor
example, Gold particles in glass
25 nm
red reflected
50 nm
green reflected
100 nmorange reflected4.2.General methods of preparation of
nanomaterials
There are two approaches for the synthesis of nanomaterials and
the fabrication of nanostructures, viz., top-down and bottom-up
(Fig.2). Top down approach involves the breaking down of the bulk
material into nano sized structures or particles. These techniques
are an extension of those that have been used for producing micron-
sized particles. An example of such a technique is high-energy wet
ball milling. The alternative approach, which has the potential of
creating less waste and hence the more economical, is the bottom-
up. Bottom up approach refers to the build up of a material from
the bottom: atom-by-atom, molecule-by-molecule, or
cluster-by-cluster. Many of these techniques are still under
development or are just beginning to be used for commercial
production of nano powders. Any fabrication technique should
provide,(i) Identical size of all particles (also called mono sized
or with uniform size
distribution.
(ii) Identical shape or morphology.
(iii) Identical chemical composition and crystal structure(iv)
Individually dispersed or mono dispersed i.e., no
agglomeration.
4.2.1. Physical Method : (A) High-Energy ball milling (Top down
approach) :The milling of materials is of prime interest in the
mineral, ceramic processing, and powder metallurgy industry.
Typical objectives of the milling process include particle size
reduction, solid-state alloying, mixing or blending, and particle
shape changes (Fig.3). These industrial processes are mostly
restricted to relatively hard, brittle materials which fracture,
deform, and cold weld during the milling operation. This technique
has been extended to produce a variety of nonequilibrium structures
including nanocrystalline, amorphous and quasicrystalline
materials. A variety of ball mills has been developed for different
purposes including tumbler mills, attrition mills, shaker mills,
vibratory mills, planetary mills, etc. The basic process of
mechanical attrition is illustrated in fig below.
Powders with typical particle diameters of about 50 m are placed
together with a number of hardened steel or tungsten carbide (WC)
coated balls in a sealed container which is shaken or violently
agitated. The most effective ratio for the ball to powder mass is 5
: 10.
High-energy milling forces can be obtained using high
frequencies and small amplitudes of vibration. Shaker mills (e.g.
SPEX model 8000) which are preferable for small batches of powder
(approximately 10 cm3 is sufficient for research purposes) are
highly energetic and reactions can take place one order of
magnitude faster than with other types of mill. Since the kinetic
energy of the balls is a function of their mass and velocity, dense
materials (steel or tungsten carbide) are preferable to ceramic
balls. During the continuous severe plastic deformation associated
with mechanical attrition, a continuous refinement of the internal
structure of the powder particles to nanometer scales occurs during
high energy mechanical attrition. The temperature rise during this
process is modest and is estimated to be less than or equal to 100
to 200 C. The difficulty with top-down approaches is ensuring all
the particles are broken down to the required particle size. During
mechanical attrition, contamination by the milling tools (Fe) and
atmosphere (trace elements of O2, N2, in rare gases) can be a
problem. By minimizing the milling time and using the purest, most
ductile metal powders available, a thin coating of the milling
tools by the respective powder material can be obtained which
reduces Fe contamination tremendously. Atmospheric contamination
can be minimized or eliminated by sealing the vial with a flexible
O ring after the powder has been loaded in an inert gas glove box.
Small experimental ball mills can also be enclosed completely in an
inert gas glove box. As a consequence, the contamination with Fe
based wear debris can generally be reduced to less than 1-2 % and
oxygen and nitrogen contamination to less than 300 ppm. However,
milling of refractory metals in a shaker or planetary mill for
extended periods of time (>30 h) can result in levels of Fe
contamination of more than 10% if high vibrational or rotational
frequencies are employed. The main advantage of top-down approach
is high production rates of nanopowders.(B) Gas Condensation
Processing (GCP) (Bottom-up approach): Gas condensation was the
first technique used to synthesize nanocrystalline metals and
alloys. In this technique which was pioneered by Gleiter and
co-workers a metallic or inorganic material, e.g. a suboxide, is
vaporised using thermal evaporation sources such as Joule heated
refractory crucibles (Joule heating is a process by which the
passage of an electric current through a conductor releases heat),
electron beam evaporation devices or sputtering sources in an
atmosphere of 1-50 mbar He (or other inert gases such as Ar, Ne,
Kr) (Fig.4). Cluster form in the vicinity of the source by
homogenous nucleation in the gas phase and grow by coalescence and
incorporation of atoms from the gas phase. The cluster or particle
size depends critically on the residence time of the particles in
the growth regime and can be influenced by the gas pressure, the
kind of inert gas, i.e. He, Ar or Kr, and on the evaporation
rate/vapour pressure of the evaporating material. With increasing
gas pressure, vapour pressure and mass of the inert gas; the
average particle size of the nanoparticles increases. A rotating
cylindrical device cooled with liquid nitrogen was employed for the
particle collection: the nanoparticles in the size range from 2-50
nm are extracted from the gas flow by thermophoretic forces and
deposited loosely on the surface of the collection device as a
powder of low density and no agglomeration. Subsequenly, the
nanoparticles are removed from the surface of the cylinder by means
of a scraper in the form of a metallic plate. In addition to this
cold finger device several techniques known from aerosol science
have now been implemented for the use in gas condensation systems
such as corona discharge, etc. These methods allow for the
continuous operation of the collection device and are better suited
for larger scale synthesis of nanopowders. However, these methods
can only be used in a system designed for gas flow, i.e. a dynamic
vacuum is generated by means of both continuous pumping and gas
inlet via mass flow controller. A major advantage over conventional
gas flow is the improved control of the particle sizes.
Evaporation can be carried out using refractory metal crucibles
(W, Ta or Mo). If metals with high melting points or metals which
react with the crucibles, are to be prepared, sputtering, i.e. for
W and Zr, or laser or electron beam evaporation has to be used.
Sputtering is a non-thermal process in which surface atoms are
physically ejected from the surface by momentum transfer from an
energetic bombarding species of atomic/molecular size. Synthesis of
alloys or intermetallic compounds by thermal evaporation can only
be done in the exceptional cases that the vapour pressures of the
constituents elements are similar. As an alternative, sputtering
from an alloy or mixed target can be employed. Composite materials
such as Cu/Bi or W/Ga have been synthesised by simultaneous
evaporation from two separate crucibles onto a rotating collection
device. It has been found that excellent intermixing on the scale
of the particle size can be obtained.
However, control of the composition of the elements has been
difficult and reproducibility is poor. Nanocrystalline oxide
powders are formed by controlled postoxidation of primary
nanoparticles of a pure metal (e.g. Ti to TiO2) or a suboxide (e.g.
ZrO to ZrO2). Although the gas condensation method including the
variations has been widely employed to prepare a variety of
metallic and ceramic materials, quantities have so far been limited
to a laboratory scale. The method is extremely slow. The quantities
of metals are below 1 g/day, while quantities of oxides can be as
high as 20 g/day for simple oxides such as CeO2 or ZrO2. These
quantities are sufficient for materials testing but not for
industrial production. However, it should be mentioned that the
scale-up of the gas condensation method for industrial production
of nanocrystalline oxides by a company called nanophase
technologies has been successful.
(C) Chemical Vapour Condensation (CVC) (Bottom-up approach):
Chemical vapor condensation (CVC) was developed in Germany in 1994.
It involves pyrolysis of vapors of metal organic precursors in a
reduced pressure atmosphere. Particles of ZrO2, Y2O3 and
nanowhiskers have been produced by CVC method. As shown
schematically in Fig.6, the evaporative source used in GPC is
replaced by a hot wall reactor in the Chemical Vapour Condensation
process. The original idea of the novel CVC process which is
schematically shown below where it was intended to adjust the
parameter field during the synthesis in order to suppress film
formation and enhance homogeneous nucleation of particles in the
gas flow. It is readily found that the residence time of the
precursor in the reactor determines if films or particles are
formed. In a certain range of residence time both particle and film
formation can be obtained. Adjusting the residence time of the
precursor molecules by changing the gas flow rate, the pressure
difference between the precursor delivery system and the main
chamber and the temperature of the hot wall reactor results in the
prolific production of nanosized particles of metals and ceramics
instead of thin films as in CVD processing (Fig.5). In the simplest
form a metalorganic precursor is introduced into the hot zone of
the reactor using mass flow controller. For instance,
hexamethyldisilazane (CH3)3SiNHSi(CH3)3 was used to produce
SiCxNyOz powder by CVC technique. Besides the increased quantities
in this Continuous process compared to GCP it has been demonstrated
that a wider range of ceramics including nitrides and carbides can
be synthesised. Additionally, more complex oxides such as BaTiO3 or
composite structures can be formed as well. In addition to the
formation of single phase nanoparticles by CVC of a single
precursor the reactor allows the synthesis of
1. Mixtures of nanoparticles of two phases or doped
nanoparticles by supplying
two precursors at the front end of the reactor, and
2. Coated nanoparticles, i.e., n-ZrO2 coated with n-Al2O3 or
vice versa, by
Supplying a second precursor at a second stage of the reactor.
In this case
nanoparticles which have been formed by homogeneous nucleation
are coated
by heterogeneous nucleation in a second stage of the
reactor.
Because CVC processing is continuous, the production
capabilities are much larger than in GCP processing. Quantities in
excess of 20 g/hr have been readily produced with a small scale
laboratory reactor. A further expansion can be envisaged by simply
enlarging the diameter of the hot wall reactor and the mass flow
through the reactor. The microstructure of nanoparticles as well as
the properties of materials obtained by CVC has been identical to
GCP prepared powders.(D) Laser ablation (Bottom-up approach): Laser
ablation has been extensively used for the preparation of
nanoparticles and particulate films. In this process a laser beam
is used as the primary excitation source of ablation for generating
clusters directly from a solid sample in a wide variety of
applications. The possibility for preparing nanoparticulate
web-like structures over large sample area is of particular
interest in view of their novel properties that can be applied to
new technological applications.
Laser vaporization cluster beams were introduced by Smalley and
coworkers to overcome the limitations of oven sources. In this
method, a high energy pulsed laser with an intensity flux exceeding
107 W/cm3 is focused on a target containing the material to be made
into clusters. The resulting plasma causes highly efficient
vaporization since with current, pulsed lasers one can easily
generate temperatures at the target material greater than 104 K.
This high temperature vaporizes all known substances so quickly
that the rest of the source can operate at room temperature.
Typical yields are 1014-1015 atoms from a surface area of 0.01 cm2
in a 10-8 s pulse. The local atomic vapor density can exceed 1018
atom/cm3 (equivalent to 100 Torr pressure) in the microseconds
following the laser pulse. The hot metal vapor is entrained in a
pulsed flow of carrier gas (typically He) and expanded through a
nozzle into a vacuum. The cool, high-density He flowing over the
target serves as a buffer gas in which clusters of the target
material form, thermalize to near room temperature and then cool to
a few K in the subsequent supersonic expansion.
In a recent investigation utilizing a novel atomization system,
(LINA-SPARK), LSA, based on laser spark atomization of solids has
been developed that seems to be very versatile for different
materials. Briefly, the LSA is capable of evaporating material at a
rate of about 20 g/s from a solid target under argon atmosphere.
The small dimensions of the particles and the possibility to form
thick films make the LSA quite an efficient tool for the production
of ceramic particles and coatings and also an ablation source for
analytical applications such as the coupling to induced coupled
plasma emission spectrometry, ICP, the formation of the
nanoparticles has been explained following a liquefaction process
which generates an aerosol, followed by the cooling/solidification
of the droplets which results in the formation of fog. The general
dynamics of both the aerosol and the fog favours the aggregation
process and micrometer-sized fractal-like particles are formed. The
laser spark atomizer can be used to produce highly mesoporous thick
films and the porosity can be modified by the carrier gas flow rate
thus enabling for a control of the microstructure of the coatings
which make these nanoparticulate thick films suitable candidates
for application in membrane technology, catalysis and lithium ion
batteries. ZrO2 and SnO2 nanoparticulate thick films were also
synthesized successfully using this process with quite identical
microstructure. Synthesis of other materials such as lithium
manganate, silicon and carbon has also been carried out by this
technique.
4.2.2. Chemical Methods (Bottom-up approachs):
(A) Wet Chemical Synthesis of nanomaterials (Sol-gel process):
Sol-gel method of synthesizing nanomaterials is very popular
amongst chemists and is widely employed to prepare oxide materials.
The sol-gel process, as the name implies, involves the evolution of
inorganic networks through the formation of a colloidal suspension
(sol) and gelation of the sol to form a network in a continuous
liquid phase (gel). The precursors for synthesizing these colloids
consist usually of a metal or metalloid element surrounded by
various reactive ligands. The starting material is processed to
form a sol in contact with water or dilute acid. Removal of the
liquid from the sol yields the gel, and the sol/gel transition
controls the particle size and shape. Calcination of the gel
produces the product (eg. Oxide).
Sol-gel processing refers to the hydrolysis and condensation of
alkoxide-based precursors such as Si(OEt) 4 (tetraethyl
orthosilicate, or TEOS). The reactions involved in the sol-gel
chemistry based on the hydrolysis and condensation of metal
alkoxides M(OR)z can be described as follows:
MOR + H2O MOH + ROH (hydrolysis)
MOH+ROMM-O-M+ROH (condensation)
Steps:Step 1: Formation of different stable solutions of the
alkoxide or solvated metal precursor (the sol).
Step 2: Gelation resulting from the formation of an oxide- or
alcohol- bridged network (the gel) by a polycondensation or
polyesterification reaction that results in a dramatic increase in
the viscocity of the solution.
Step 3: Aging of the gel, during which the polycondensation
reactions continue until the gel transforms into a solid mass,
accompanied by contraction of the gel network and expulsion of
solvent from gel pores. Ostwald ripening (also referred to as
coarsening, is the phenomenon by which smaller particles are
consumed by larger particles during the growth process) and phase
transformations may occur concurrently. The aging process of gels
can exceed 7 days and is critical to prevent the cracks in gels
that have been cast.
Step 4: Drying of the gel, when water and other volatile liquids
are removed from the gel network. If isolated by thermal
evaporation, the resulting monolith is termed a xerogel. If the
solvent (such as water) is extracted under supercritical or near
super critical conditions, the product is an aerogel.
Step 5: Dehydration, during which surface- bound M-OH groups are
removed, there by stabilizing the gel against rehydration. This is
normally achieved by calcining the monolith at temperatures up to
8000C.
Step 6: Densification and decomposition of the gels at high
temperatures (T>8000C). The pores of the gel network are
collapsed, and remaining organic species are volatilized. The
typical steps that are involved in sol-gel processing are shown in
the schematic diagram below.
The interest in this synthesis method arises due to the
possibility of synthesizing nonmetallic inorganic materials like
glasses, glass ceramics or ceramic materials at very low
temperatures compared to the high temperature process required by
melting glass or firing ceramics (Fig.6). In addition, one can get
monosized nanoparticles by this bottom up approach.The major
technical difficulties to overcome in developing a successful
bottom-up approach is controlling the growth of the particles and
then stopping the newly formed particles from agglomerating. Other
technical issues are ensuring the reactions are complete so that no
unwanted reactant is left on the product and completely removing
any growth aids that may have been used in the process. Also
production rates of nanopowders are very low by this process.
(B) Precipitation method: Nanomaterials are produced by
precipitation from a solution. The method involves high degree of
homogenization and low processing temperature. The ZnS powders were
produced by reaction of aqueous zinc salt solutions with
thioacetamide (TAA). Precursor zinc salts were chloride, nitric
acid solutions, or zinc salts with ligands (i.e., acetylacetonate,
trifluorocarbonsulfonate, and dithiocarbamate). The 0.05 M cation
solution was heated in a thermal bath maintained at 70 or 80 C in
batches of 100 or 250 ml. Acid was added dropwise to bring it to a
pH of 2. The reaction was started by adding the TAA to the zinc
salt solution, with the molar ratio of TAA and zinc ions being set
to an initial value of either 4 or 8.
SnO2 nanopowder was prepared by precipitation method using
stannic chloride, ammonium hydroxide. The precipitate obtained by
adding ammonia in drops to ice cold solution of stannic chloride
was heated at around 100C. The white mass was collected after
cooling the mixture and dried.
4.3. Applications of nanomaterials
4.3.1 Carbon Nanotubes (CNTs)/Basics
Carbon nanotubes are molecular-scale tubes of graphitic carbon
with outstanding properties. They are among the stiffest and
strongest fibres known, and have remarkable electronic properties
and many other unique characteristics. The current huge interest in
carbon nanotubes is a direct consequence of the synthesis of
buckminsterfullerene, C60, and other fullerenes, in 1985. In 1991,
scientists at NEC Corporation in Japan discovered that graphitic
carbon needles grew on the negative carbon electrode of the
arc-discharge apparatus used for the mass production of C60
(Iijima, 1991, Fig.6). The needles ranged up to 1 mm in length and
consisted of nested tubes (concentric cylinders) of rolled graphite
sheets (see Fig.7). The smallest tube observed was 2.2 nm in
diameter, which corresponds roughly to a ring of 30 carbon
hexagons. Some of the needles consisted of only two nested tubes
(Fig.8), while others contained as many as 50. The separation
between the tubes was 0.34 nm (3.4 angstroms), which matches the
separation of the sheets in bulk graphite. The tips of the needles
were generally closed by caps that were curved or cone-shaped
(Fig.6). Subsequent work at NEC optimized the synthetic procedure,
allowing gram quantities of carbon needles.
Fig.7 Shape and structure of Carbon nanotube (SWNT).
Note that a nanotube (also known as a buckytube) is a member of
the fullerene structural family, which also includes buckyballs.
Whereas buckyballs are spherical in shape (seen in the Unit-I), a
nanotube is cylindrical, with at least one end typically capped
with a hemisphere of the buckyball structure. Their 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 nanotubes: single-walled
nanotubes (SWNTs) and multi-walled nanotubes (MWNTs, combination of
various CNTs).
4.3.2 Chemical bonding Structure
(A) Graphite (layered)
(B) Rolled graphite (a CNT)
The bonding in carbon nanotubes is sp2, with each atom joined to
three neighbours, as in graphite (Fig.9A). The tubes can therefore
be considered as rolled-up graphene sheets (graphene is an
individual graphite layer) (Fig.9B). Note that sp2 bonds are
stronger than the sp bonds found in diamond, this bonding structure
provides them with their unique strength and amazing in mechanical
properties. Nanotubes naturally align themselves into "ropes" held
together by Van der Waals forces. Under high pressure, nanotubes
can merge together, trading some sp bonds for sp bonds, giving
great possibility for producing strong, unlimited-length wires
through high-pressure nanotube linking. 4.3.3 Preparation of
CNTs
Multiwalled carbon nanotubes (MWNTs). Carbon nanotubes are
readily prepared by striking an arc between graphite electrodes in
~0.7 atm (~500 torr) of helium, considerably larger than the helium
pressure used for the production of fullerene soot. The schematic
diagram of the apparatus is shown in Fig.10. A current of 60-100 A
across a potential drop of about 25 V gives high yields of carbon
nanotubes. The arcing process can be optimized such that the major
portion of the carbon anode gets deposited on the cathode in the
form of carbon nanotubes and graphitic nanoparticles. The arc
evaporation of graphite has been carried out in various kinds of
ambient gases (He, Ar, and CH4). Hydrogen appears to be effective
in producing MWNTs of high crystallinity. Arc-produced MWNTs in
hydrogen also contain very few carbon nanoparticles. Carbon
nanotubes have been produced in large quantities by using plasma
arc-jets by optimizing the quenching process in an arc between a
graphite anode and a cooled copper electrode. If both the
electrodes are of graphite, MWNTs are the main products, although
side products such as fullerenes, amorphous carbon, and graphite
sheets are also formed.
A route to highly crystalline MWNTs is the arc-discharge method
in liquid nitrogen. In this method, vacuum is replaced with liquid
nitrogen in the arc discharge chamber. In a typical experiment
direct current was supplied to the apparatus using a power supply.
The anode is a pure carbon rod of 8-mm diameter and the cathode is
a pure carbon rod of 10-mm diameter. The Dewar flask is filled with
liquid nitrogen and the electrode assembly immersed in nitrogen.
Arc discharge occurs as the distance between the electrodes became
less than 1 mm, and a current of ~80 A flows between the
electrodes. When the arc discharge is over, the carbon deposits
near the cathode are recovered for analysis. Liquid nitrogen
prevents the electrodes from contamination with unwanted gases and
also lowers the temperature of the electrodes. Furthermore, CNTs do
not stick to the wall of the chamber. The content of the MWNTs can
be as high as 70% of the reaction product. Analysis with
Auger-spectroscopy revealed that no nitrogen was incorporated in
the MWNTs. This method considered to be economical and its does not
require expensive components.
Chemical vapour deposition (CVD) based MWNT preparation. CVD
method uses a carbon source in the gas phase and plasma or a
resistively heated coil, to transfer the energy to the gaseous
carbon molecule. The energy source cracks the molecule into atomic
carbon. The carbon then diffuses towards the substrate, which is
heated and coated with a catalyst (usually a first row transition
metal such as Ni, Fe or Co) and binds to it. Carbon nanotubes are
formed in this procedure if the proper parameters are maintained.
Good alignments as well as positional control on a nanometric scale
are achieved by using CVD. Use of an appropriate metal catalyst
permits the preferential growth of single-walled rather than
multi-walled nanotubes.
CVD synthesis of nanotubes is essentially a two-step process,
consisting of a catalyst preparation (step-1) step followed by the
actual synthesis of the nanotubes (step-2). The catalyst is
generally prepared by sputtering a transition metal onto a
substrate and then using etching by chemicals such as ammonia or
thermal annealing to induce the nucleation of catalyst particles.
Thermal annealing results in metal cluster formation on the
substrate, from which the nanotubes grow. The temperature for the
synthesis of nanotubes by CVD is generally in the 650-900oC range.
Typical nanotube yields from CVD are around 30%. A variety of CVD
processes have been used for carbon nanotubes synthesis, which
include plasma-enhanced CVD, thermal chemical CVD, alcohol
catalytic CVD, aero gel-supported CVD and laser-assisted CVD. In
the thermal CVD process, Fe, Ni, Co or an alloy of these metals is
initially deposited on a substrate. After the substrate is etched
(skin removed) by a dilute HF solution, the substrate is placed in
a quartz boat, positioned in a CVD reaction furnace.
Nanometer-sized catalytic metal particles get formed after an
additional etching of the catalytic metal film using NH3 gas at
750-1050oC. The nanotubes grow on the fine catalytic metal
particles by the CVD process.
Single-walled carbon nanotubes (SWNT). The carbon nanotubes
generally obtained by the arc method or hydrocarbon pyrolysis are
multi-walled, having several graphitic sheets or layers. Depending
on the exact technique, it is possible to selectively grow SWNTs or
MWNTs. Two distinct methods of synthesis can be performed with the
same arc discharge apparatus. If SWNTs are required, the anode has
to be doped with a metal catalyst based on Fe, Co, Ni, or Mo.
Several elements and mixtures of elements have been tested and the
results vary considerably.
Single-walled nanotubes were first prepared by the
metal-catalyzed dc arcing of graphite rods in a Helium gas
atmosphere. The graphite anode was filled with metal powders (Fe,
Co or Ni) and the cathode was of pure graphite. SWNTs generally
occur in the web-like material deposited behind the cathode
(Fig.11B). Various metal catalysts have been used to make SWNTs by
this route. For examples;
1. Molybdenum (Mo) particles of few nanometer diameters
dispersed in a fumed alumina matrix catalyst.
2. Co (or) a Fe/Ni bimetallic catalyst 3. Various oxides
catalysts: Y2O3, La2O3, CeO2 as catalysts. 4. 1% Yttrium and 4.2
at.% Ni as catalyst.
5. Graphite rods filled with Ni and Y2O3 catalyst in a He
atmosphere (660 Torr) gives rise to web-like deposits on the
chamber walls near the cathode, consisting of SWNT bundles.
6. Nanoparticles of Fe catalyst
A common problem with SWNT synthesis is that the product
contains metal catalyst particles and defects, rendering
purification difficult. On the other hand, an advantage is that the
diameter can be controlled by changing the thermal transfer and
diffusion, and hence the condensation of atomic carbon. This has
been demonstrated in an experiment where different mixtures of
inert gases were used. Argon, with a lower thermal conductivity and
diffusion coefficient, gives SWNTs with a diameter of ~1.2 nm. A
linear fit of the average nanotube diameter showed a 0.2 nm
diameter decrease per 10 % increase in argon helium ratio, when
nickel/yttrium was used (C/Ni/Y was 94.8:4.2:1) as catalyst.
4.3.4 Purification Techniques of CNTs
A large problem with nanotube application is next to large-scale
synthesis also the purification. The as-produced SWNT soot contains
a lot of impurities. The main impurities in the soot are graphite
(wrapped up) sheets, amorphous carbon, metal catalyst and the
smaller fullerenes. These impurities will interfere with most of
the desired properties of the SWNTs.
In this chapter several purification techniques of the SWNT will
be discussed. Basically, these techniques can be divided into two
mainstreams, structure selective and size selective separations.
The first one will separate the SWNTs from the impurities; the
second one will give a more homogeneous diameter or size
distribution. The techniques that will be discussed are oxidation,
acid treatment, annealing, ultrasonication, micro filtration,
ferromagnetic separation, cutting, functionalisation and
chromatography techniques.
(A) By chemical oxidation:
Oxidative treatment of the SWNTs is a good way to remove
carbonaceous impurities (or) to clear the metal surface. The main
disadvantages of oxidation are that not only the impurities are
oxidized, but also the SWNTs. Luckily the damage to SWNTs is less
than the damage to the impurities. Another reason why impurity
oxidation is preferred is that these impurities are most commonly
attached to the metal catalyst, which also acts as oxidizing
catalyst. Efficiency of the purification procedure dependable on
lot of factors; such as metal content, oxidation time, environment,
oxidizing agent and temperature. For example, when the temperature
is raised above 600C, SWNTs will also oxidize, even without
catalyst.
Well know method is mild oxidizing in a wet environment with
soluble oxidizing agents, such as H2O2 and H2SO4. These will only
oxidize the defects and will clear the surface of the metal.
However, if oxygen impurities present in the solution, that leads
to oxidation of metal( metal oxide and in turn to rupture the CNT
structure.
(B) By acid treatment:
In general the acid treatment will remove the metal catalyst as
soluble metal salts. In that treatment, the surface of the metal
must be first exposed by oxidation or sonication. The metal
catalyst is then exposed to acid and solvated. The SWNTs remain in
suspended form. Note, when using a treatment in HNO3, the acid only
has an effect on the metal catalyst. It has no effect on the SWNTs
and other carbon particles. If a treatment in HCl is used, the acid
has also a little effect on the SWNTs and other carbon particles.
The mild acid treatment (4 M HCl reflux) is basically the same as
the HNO3 reflux, but here the metal has to be totally exposed to
the acid to solvate it.
(C) By annealing (heat treatment):
Due to high temperatures (873 1873 K) the nanotubes will be
rearranged and defects will be consumed. The high temperature also
causes the graphitic carbon and the short fullerenes to pyrolyse.
When using high temperature vacuum treatment (1873 K) the metal
will be melted and can also be removed.
(D) By ultrasonication:
In this technique particles are separated due to ultrasonic
vibrations. Agglomerates of different nanoparticles will be forced
to vibrate and will become more dispersed. The separation of the
particles is highly dependable on the surfactant, solvent and
reagent used. In poor solvents the SWNTs are more stable if they
are still attached to the metal. But in some solvents, such as
alcohols, monodispersed particles are relatively stable. When an
acid is used, the purity of the SWNTs depends on the exposure time.
When the tubes are exposed to the acid for a short time, only the
metal solvates, but for a longer exposure time, the tubes will also
be chemically cut .
(E) By magnetic purification:
In this method ferromagnetic (catalytic) particles are
mechanically removed from their graphitic shells. The SWNTs
suspension is mixed with inorganic nanoparticles (mainly ZrO2 or
CaCO3) in an ultrasonic bath to remove the ferromagnetic particles.
Then, the particles are trapped with permanent magnetic poles.
After a subsequent chemical treatment, a high purity SWNT material
will be obtained. This process does not require large equipment and
enables the production of laboratory-sized quantities of SWNTs
containing no magnetic impurities.
(F) By micro filtration:
Micro filtration is based on size or particle separation. SWNTs
and a small amount of carbon nanoparticles are trapped in a filter.
The other nanoparticles (catalyst metal, fullerenes and carbon
nanoparticles) are passing through the filter. One way of
separating fullerenes from the SWNTs by micro filtration is to soak
the as-produced SWNTs first in a CS2 solution. The CS2 insolubles
are then trapped in a filter. The fullerenes which are solvated in
the CS2, pass through the filter 65.
A special form of filtration is cross flow filtration. In cross
flow filtration the membrane is a hollow fibre. The membrane is
permeable to the solution. The filtrate is pumped down the bore of
the fibre at some head pressure from a reservoir and the major
fraction of the fast flowing solution which does not permeate out
the sides of the fibre is fed back into the same reservoir to be
cycled through the fibre repeatedly (Fig.12). A fast hydrodynamic
flow down the fibre bore (cross flow) sweeps the membrane surface
preventing the build-up of a filter cake.
(G) By cutting:
Cutting of the SWNTs can either be induced chemically (Fig.13),
mechanically (or) as a combination of these. SWNTs can be
chemically cut by partially functionalizing the tubes, for example
with fluorine. Then, the fluorated carbon will be driven off the
sidewall with pyrolisation in the form of CF4 or COF2. This will
leave behind the chemically cut nanotubes. Mechanical cutting of
the nanotubes can be induced by ball-milling. Here, the bonds will
break due to the high friction between the nanoparticles and the
nanotubes will be disordered. A combination of mechanical and
chemical cutting of the nanotubes is by ultrasonication induced
cutting in an acid solution. In this way the ultrasonic vibration
will give the nanotubes sufficient energy to leave the catalyst
surface. Then, in combination with acid the nanotubes will rupture
at the defect sites.
(H) By functionalisation:
Functionalisation is based on making SWNTs more soluble than the
impurities by attaching other groups to the tubes. Now it is easy
to separate them from insoluble impurities, such as metal, with
filtration. Another functionalisation technique also leaves the
SWNT structure intact and makes them soluble for chromatographic
size separation.
For recovery of the purified SWNTs, the functional groups can be
simply removed by thermal treatment, such as annealing.
(I) By chromatography:
This technique is mainly used to separate small quantities of
SWNTs into fractions with small length and diameter distribution.
The SWNTs are run over a column with a porous material, through
which the SWNTs will flow. The columns used are GPC (Gel Permeation
Chromatography) and HPLC-SEC (High Performance Liquid
Chromatography - Size Exclusion Chromatography) columns. The number
of pores the SWNTs will flow through depends on their size. This
means that, the smaller the molecule, the longer the pathway to the
end of the column will be and that the larger molecules will come
off first. The pore size will control what size distribution can be
separated. However, a problem is that the SWNTs have to be either
dispersed or solvated. This can be done by ultrasonication or
functionalisation with soluble groups.
4.3.5 Applications of CNTs (electronic industries related):
Buckytubes have extraordinary electrical conductivity, heat
conductivity and mechanical properties.Very significantly,
buckytubes are molecularly perfect, which means that they are free
of property-degrading flaws in the nanotube structure. Their
material properties can therefore approach closely the very high
levels intrinsic to them. These extraordinary characteristics give
buckytubes potential in numerous applications. Some of the examples
as follows:
(A) In field emission:
Buckytubes are the best known field emitters of any material.
This is understandable, given their high electrical conductivity,
and the unbeatable sharpness of their tip (Fig.15(I)) (the sharper
the tip, the more concentrated will be an electric field, leading
to field emission; this is the same reason lightening rods are
sharp) (Fig.10(II), as molecular electronic system). The sharpness
of the tip also means that they emit at especially low voltage, an
important fact for building electrical devices that utilize this
feature. Buckytubes can carry an astonishingly high current
density, possibly as high as 1013 A/cm2. Furthermore, the current
is extremely stable.
An immediate application of this behaviour receiving
considerable interest is in field-emission flat-panel displays.
Instead of a single electron gun, as in a traditional cathode ray
tube display, here there is a separate electron gun (or many) for
each pixel in the display. The high current density, low turn-on
and operating voltage, and steady, long-lived behaviour make
buckytubes attract field emitters to enable this application.
Other applications utilizing the field-emission characteristics
of buckytubes include: general cold-cathode lighting sources,
lightning arrestors, and electron microscope sources.
(B) In conductive plastics:
Much of the history of plastics over the last half century has
been as a replacement for metal. For structural applications,
plastics have made tremendous headway, but not where electrical
conductivity is required, plastics being famously good electrical
insulators.
This deficiency is overcome by loading plastics up with
conductive fillers, such as carbon black and graphite fibres (the
larger ones used to make golf clubs and tennis racquets). The
loading required to provide the necessary conductivity is typically
high, however, resulting in heavy parts, and more importantly,
plastic parts whose structural properties are highly degraded.
It is well established that the higher aspect ratio of filler,
the lower loading required achieving a given level of conductivity.
Buckytubes are ideal in this sense, since they have the highest
aspect ratio of any carbon fibre. In addition, their natural
tendency to form ropes provides inherently very long conductive
pathways even at ultra-low loadings. Applications that exploit this
behaviour of buckytubes include EMI/RFI shielding composites and
coatings for enclosures, gaskets, and other uses; electrostatic
dissipation (ESD), and antistatic materials and (even transparent!)
coatings; and radar-absorbing materials.
(C) In energy storage:
Buckytubes have the intrinsic characteristics desired in
material used as electrodes in batteries and capacitors, two
technologies of rapidly increasing importance. Buckytubes have a
tremendously high surface area (~1000 m2/g), good electrical
conductivity, and very importantly, their linear geometry makes
their surface highly accessible to the electrolyte.
Research has shown that buckytubes have the highest reversible
capacity of any carbon material for use in lithium-ion batteries.
In addition, buckytubes are outstanding materials for super
capacitor electrodes (suitable to sudden release of high-voltage in
fraction time) and are now being marketed.
Buckytubes also have applications in a variety of fuel cell
components. They have a number of properties including high surface
area and thermal conductivity that make them useful as electrode
catalyst supports in PEM fuel cells. They may also be used in gas
diffusion layers as well as current collectors because of their
high electrical conductivity. Buckytubes' high strength and
toughness to weight characteristics may also prove valuable as part
of composite components in fuel cells that are deployed in
transport applications where durability is extremely important.
Fig. 16&17 showed photographs of CNT based conductive film in
for battery application and super-capacitor device systems
respectively.
(D) In conductive adhesives and connectors:
The same issues that make buckytubes attractive as conductive
fillers for use in shielding, Electrostatic dissipation (ESD)
materials (details how easily an electric charge can travel across
a medium) etc., make them attractive for electronics materials,
such as adhesives and other connectors (e.g., solders).
(E) In molecular electronics:
The idea of building electronic circuits out of the essential
building blocks of materials - molecules - has seen a revival the
past five years, and is a key component of nanotechnology. In any
electronic circuit, but particularly as dimensions shrink to the
nanoscale, the interconnections between switches and other active
devices become increasingly important.
Their geometry, electrical conductivity, and ability to be
precisely derived, make buckytubes the ideal candidates for the
connections in molecular electronics (eg., molecular cables and
nanowires, Fig.18). Following are other examples for molecular
electronics:
(a) Computer circuits: A nanotube formed by joining nanotubes of
two different diameters end to end can act as a diode, suggesting
the possibility of constructing electronic computer circuits
entirely out of nanotubes (like Fig.13). Because of their good
thermal properties, CNTs can also be used to dissipate heat from
tiny computer chips. The longest electricity conducting circuit is
a fraction of an inch long.. (b) Conductive films: Eikos Inc. of
Franklin, Massachusetts is developing transparent, electrically
conductive films of carbon nanotubes to replace conducitive glass,
indium tin oxide (ITO) in LCDs, touch screens, and photovoltaic
devices. Carbon nanotube films are substantially more mechanically
robust than ITO films, making them ideal for high reliability touch
screens and flexible displays. Nanotube films show promise for use
in displays for computers, cell phones, PDAs, and ATMs. (c)
Displays: One use for nanotubes that has already been developed is
as extremely fine electron guns, which could be used as miniature
cathode ray tubes in thin high-brightness low-energy low-weight
displays (as in the Fig.10II). This type of display would consist
of a group of many tiny CRTs, each providing the electrons to hit
the phosphor of one pixel, instead of having one giant CRT whose
electrons are aimed using electric and magnetic fields. These
displays are known as field emission displays (FEDs).(F) In thermal
materials:
The record-setting anisotropic thermal conductivity of
buckytubes is enabling applications where heat needs to move from
one place to another. Such an application is electronics,
particularly advanced computing, where uncooled chips now routinely
reach over 100oC. CNT & its composite with buckytubes have been
shown to dramatically increase the bulk thermal conductivity at
small loading.
(G) In structural composites:
The world-record properties of buckytubes are not limited to
electrical and thermal conductivities, but also include mechanical
properties, such as stiffness, toughness, and strength. These
properties lead to a wealth of applications exploiting them,
including advanced composites requiring high values in one or more
of these properties. Fibres and Fabrics: Fibres spun of pure
buckytubes have recently been demonstrated [R.H. Baughman, Science
290, 1310 (2000)] and are undergoing rapid development, along with
buckytube composite fibres. Such super strong fibres will have
applications including body and vehicle armour, transmission line
cables, woven fabrics and textiles.
(H) In catalyst supports:
Buckytubes have an intrinsically high surface area; in fact,
every atom is not just on a surface - each atom is on two surfaces,
the inside and outside! Combined with the ability to attach
essentially any chemical species to their sidewalls provides an
opportunity for unique catalyst supports. Their electrical
conductivity may also be exploited in the search for new catalysts
and catalytic behaviour.
(I) Other applications:
There is a wealth of other potential applications for
buckytubes, such as solar collection; nanoporous filters; catalyst
supports; and coatings of all sorts. There are almost certainly
many unanticipated applications for this remarkable material that
will come to light in the years ahead and which may prove to be the
most important and valuable of all.Blue
Green
Brown
Orange
Red
Yellow
Fig. 2. Schematic representation of the bottom up and top down
synthesis processes of nanomaterials
Fig. 3 Schematic representation of the principle of mechanical
milling.
Fig. 4 Schematic representation of typical set-up for gas
condensation synthesis of nanomaterials followed by consolidation
in a mechanical press or collection in an appropriate solvent
media.
Fig. 5 A schematic of a typical CVC reactor
Fig. 6 Schematic representation of sol-gel process of synthesis
of nanomaterials.
Fig. 6 Prof. Iijuma (Japan) with a CNT model.
~1 mm
nm
Fig.8 Two or more nested tubes of CNTs. Comparative sheet like
structures are also given.
sp2 boned carbon
sp2 boned carbon
Fig. 9. Chemical structure of graphite (A) and CNT (B). Both
having sp2 bonded carbons.
Fig. 10 CNTs preparation unit.
Fig.11 Transmission electron micrographs (TEM) of multiwall
nanotubes (MWNTs)
A
B
Fig. 12 SWNT microfiltration unit.
Fig.13 Cutting of a SWNT.
EMBED ChemDraw.Document.6.0
Fig. 14 Functionalized SWNT.
Fig. 15 Field emitting CNTs (I) & its typical device
(II).
V
(-)
(+)
CNT
(-)
e-s
At low V
(I) (II)
Fig.16 CNT based conductive plastic and a battery assembly (also
for charge storage).
Fig.17 Commercial CNT based super capacitor (I) and
electrochemical hydrogen charge storage (with NiOH2 as counter
electrode).
(I) (II)
Fig. 18 CNT based molecular cables (molecular electronics).
Contact
Contact
CNT
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