Download III B.Sc. Int.nanomaterials - E

NEHRU ARTS AND SCIENCE COLLEGE
DEPARTMENT OF MICROBIOLOGY WITH NANOTECHNOLOGY
E-LEARNING
CLASS
SUBJECT
: III B.Sc.
: INTRODUCTION TO NANOMATERIALS
UNIT I
Introduction and Classification: What is nanotechnology – Classification of Nanostructures - 1D,
2D and 3 D nanomaterials – Nanoscale Architecture.
--------------------------------------------------------------------------------------------------------------------UNIT I
Part A
1. Define NANO TECHNOLOGY
The term nano technology has been used at least as early as 1974 by Taniguchi.
Nanotechnology is defined as a technology where dimensions and tolerances are in the range
of 0.1-100 nm (from size of the atom to about the wavelength of light) play a critical role.
Another popular definition for Nano technology is: “Nano technology relates to the
ability to build functional devices based on the controlled assembly of nano scale objects for
specific technological applications”.
2. What is the difference between NANO SCIENCE & NANO TECHNOLOGY?
Study on fundamental relationships between physical properties and phenomena
and material dimensions in the nanometer scale referred to as Nano science.
But Nano technology is the application of these nano structures and principles
behind them to make nano scale devices and to produce new materials.
Part B
1. “One nanometer is a magical point on the dimensional scale”. Why?
One nanometer is a magical point on the dimensional scale. Because there is a
sudden shift of all properties of material when they just enters into the nanoscale.
As material size reduces from centimeter (bulk) to nanometer scale, properties
mostly decreases as much as six orders of magnitude to that at macro level. The reason
for this change is due to the nature of interactions among the atoms that are averaged out
of existence in the bulk material. The same can be explained in another way i.e., surface
energy increases with the overall surface area which in turn strongly dependent on the
dimension of material. As nanostructures are having reduced dimensions, it leads to
increase in surface energy via increase in surface area.
The change in properties from macro scale to nano scale can be observed by taking a
simple example as given below.
Let us take an imaginary cube of gold 3 feet on each side. It is sliced in half along
its length, width and height to produce eight little cubes, each 18 inches on a side. If we
continue cutting the gold in this way from inches to centimeters, from centimeters to
millimeters, and from millimeters to microns; we still notice no change in properties of
gold between each stage except cash value and weight. All gold cubes are soft, shiny
yellow and having same melting point.
But when these µm size gold particles are further sliced into nano size particles,
every thing will be changed including gold’s color, melting point and chemical
properties.
Melting point of nano gold is less than that of bulk gold melting point. Similarly
instead of yellow color, nano gold particles appear in different color. This color depends
on the size of the particle.
Not only for gold, all the materials will show the peculiar behavior and change in
their properties when they enter into the nano scale. That is why one nanometer is called
as a magical point on the dimensional scale.
2. When and where FEYNMAN delivered his lecture on nanotechnology and what is
the name of his classical lecture?
One of the first to advocate a future for nanotechnology was Richard Feynman, a
Physics Nobel laureate who died in 1988. In late 1959 at the California Institute of
Technology, he presented what has become one of 20th century science’s classic lectures
entitled “There is Plenty of Room at the Bottom”. This classic lecture has become part of
the nanotechnology community’s founding liturgy.
Feynman got his motivation from biology since biological systems can be
exceedingly small. He said, “Many of the cells are very tiny, but they are active; they
manufacture substances; they walk around; they wiggle; and they do all kind of
marvelous things–all on a very small scale. Also, they store information. Consider the
possibility that we too can make a thing very small which does what we want—that we
can manufacture an object that manoeuvres at that level!” Feynman talked about
nanotechnology before the word existed. Feynman dreamed with a technological vision
of extreme miniaturization in 1959, several years before the word “chip” became part of
our every day life. Extrapolating from known physical laws, Feynman argued it was
possible (with, say, an electron beam that could form lines in materials) to write all
25,000 pages of the 1959 edition of the Encyclopedia Britannica in an area the size of a
pin head! He calculated that a million such pinheads would amount to an area of about a
35 page pamphlet.
Feynman further added “All of the information which all of mankind has ever
recorded in books can be carried in a pamphlet in your hand–and not written in code, but
a simple reproduction of the original pictures, engravings and everything else on a small
scale with-out loss of resolution.” And that’s just how his talk began. He outlined how,
with proper coding, all the world’s books at the time actually could be stored in
something the size of a dust speck, with each of the billions of bits in those books
requiring a mere 100 atoms to store. How about building computers using wires,
transistors, and other components that were that small? “They could make judgments,”
Feynman predicted. He discussed about using big tools to make smaller tools suitable for
making yet smaller tools, and so on, until researchers had tools sized just right for
directly manipulating atoms and molecules.
Feynman further predicted that we will be able to literally place atoms one by one
in exactly the arrangement that we want. “Up to now,” he added, “we have been content
to dig in the ground to find minerals. We heat them and we do things on a large scale
with them, and we hope to get a pure substance with just so much impurity, and so on.
But we must always accept some atomic arrangement that nature gives us...I can hardly
doubt that when we have some control of the arrangement of things on a small scale we
will get an enormously greater range of possible properties that substances can have, and
of different things that we can do.” Repeatedly, during this famous lecture, Feynman
reminded his audience that he wasn’t joking. “I am not inventing anti-gravity, which is
possible someday only if the laws are not what we think,” he said. “I am telling you what
could be done if the laws are what we think; we are not doing it simply because we
haven’t yet gotten around to it.”
3. Give Moore’s I law & II law
Gordon Moore, one of the founders of the Intel corporation, came up with two empirical
laws to describe the amazing advances in integrated circuit electronics.
Moore’s first law (usually referred to simply Moore’s law) says that the amount of space
required to install a transistor on a chip shrinks by roughly half every 18 months. This means that
the spot that could hold one transistor 15 years ago can hold 1000 transistors today. Moore’s first
law is good news.
The bad news is Moore’s second law. It is really a corollary to the first, which gloomily
predicts that the cost of building a chip manufacturing plant (also called a fabrication line or just
fab) doubles with every other chip generation, or roughly every 36 months. The following figure
shows Moore’s laws in a graphical way.
4. Write about Nanoscale architecture
Nanotechnology is the design, fabrication and use of nanostructured systems, and the
growing, shaping or assembling of such systems either mechanically, chemically or biologically
to form nanoscale architectures, systems and devices. The original vision of Richard Feynman1
was of the ‘bottom-up’ approach of fabricating materials and devices at the atomic or molecular
scale, possibly using methods of self-organization and selfassembly of the individual building
blocks.
An alternative ‘top-down’ approach is the ultraminiaturization or etching/milling of
smaller structures from larger ones. Both approaches require a means of visualizing, measuring
andmanipulating the properties of nanostructures; computer-based simulations of the behaviour
of materials at these length scales are also necessary.
PART C
1. Classify nanomaterials and give examples for them?
Classification of nanostructured materials and systems essentially depends on the number
of dimensions which lie within the nanometre range, as shown in Figure below (a) systems
confined in three dimensions, (b) systems confined in two dimensions, (c) systems confined in
one dimension.
Classification of nanostructures. (a) Nanoparticles and nanopores (nanosized in three
dimensions): (i) high-resolution TEM image of magnetic iron oxide nanoparticle, (ii) TEM
image of ferritin nanoparticles in a liver biopsy specimen, and (iii) high-resolution TEM image
of nanoporosity in an activated carbon). (b) Nanotubes and nanofilaments (nanosized in two
dimensions): (i) TEM image of single-walled carbon nanotubes prepared by chemical vapour
deposition, (ii) TEM image of ordered block copolymer film, and (iii) SEM image of silica
nanotube formed via templating on a tartaric acid crystal. (c) Nanolayers and nanofilms
(nanosized in one dimension): (i) TEM image of a ferroelectric thin film on an electrode, (ii)
TEM image of cementite (carbide) layers in a carbon steel, and (iii) high-resolution TEM image
of glassy grain boundary film in an alumina polycrystal. Images courtesy of Andy Brown,
Zabeada Aslam, Sarah Pan, Manoch Naksata and John Harrington, IMR, Leeds
Nanoparticles and nanopores exhibit three-dimensional confinement (note that
historically pores below about 100 nm in dimension are often sometimes confusingly referred to
as micropores). In semiconductor terminology such systems are often called quasi-zero
dimensional, as the structure does not permit free particle motion in any dimension.
Nanoparticles may have a random arrangement of the constituent atoms or molecules
(e.g., an amorphous or glassy material) or the individual atomic or molecular units may be
ordered into a regular, periodic crystalline structure which may not necessarily be the same as
that which is observed in a much larger system (Section 1.3.1). If crystalline, each nanoparticle
may be either a single crystal or itself composed of a number of different crystalline regions or
grains of differing crystallographic orientations (i.e., polycrystalline) giving rise to the presence
of associated grain boundaries within the nanoparticle.
Nanoparticles may also be quasi-crystalline, the atoms being packed together in an
icosahedral arrangement and showing non-crystalline symmetry characteristics. Such
quasicrystals are generally only stable at the nanometre or, at most, the micrometre scale.
Nanoparticles may be present within another medium, such as nanometre-sized precipitates in a
surrounding matrix material. These nanoprecipitates will have a specific morphology (e.g.,
spherical, needle-shaped or plate-shaped) and may possess certain crystallographic orientation
relationships with the atomic arrangement of the matrix depending on the nature (coherency) of
the interfacewhich may lead to coherency strains in the particle and the matrix. One such
example is the case of self-assembled semiconductor quantum dots, which form due to lattice
mismatch strain relative to the surrounding layers and whose geometry is determined by the
details of the strain field (Chapter 3). Another feature which may be of importance for the overall
transport properties of the composite system is the connectivity of such nanometre-sized regions
or, in the case of a nanoporous material, nanopore connectivity.
In three dimensions we also have to consider collections of consolidated nanoparticles;
e.g., a nanocrystalline solid consisting of nanometre-sized crystalline grains each in a specific
crystallographic orientation. As the grain size d of the solid decreases the proportion of atoms
located at or near grain boundaries, relative to those within the interior of a crystalline grain,
scales as 1/d. This has important implications for properties in ultrafine-grained materials which
will be principally controlled by interfacial properties rather than those of the bulk.
Systems confined in two dimensions, or quasi-1D systems, include nanowires, nanorods,
nanofilaments and nanotubes: again these could either be amorphous, singlecrystalline or
polycrystalline (with nanometre-sized grains). The term ‘nanoropes’ is often employed to
describe bundles of nanowires or nanotubes. Systems confined in one dimension, or quasi-2D
systems, include discs or platelets, ultrathin films on a surface and multilayered materials; the
films themselves could be amorphous, single-crystalline or nanocrystalline.
Table below gives examples of nanostructured systems which fall into each of the three
categories described above. It can be argued that self-assembled monolayers and multi layered
Langmuir–Blodgett films (Section 1.4.3.1) represent a special case in that they represent a quasi2D system with a further nanodimensional scale within the surface film caused by the molecular
self-organization.
Examples of reduced-dimensionality systems
UNIT II
Synthesis of Nanomaterials: Top down – ball milerling; Bottom up – co-precipitaion – sol-gel –
electrodeposition – using natural nanoparticles – chemical vapor deposition.
--------------------------------------------------------------------------------------------------------------------Part A
1. Define top down and bottom up approach?
There are two approaches for synthesis of nano materials and the fabrication of
nano structures.
Top down approach refers to slicing or successive cutting of a bulk material to get nano
sized particle.
Bottom up approach refers to the build up of a material from the bottom: atom by atom,
molecule by molecule or cluster by cluster.
2. List any four processes to produce nanopowders?
1. Ball milerling, 2. Liquid solid reactions (co-precipitaion), 3. Wet Chemical Synthesis
of nanomaterials (Sol-gel process) and 4. Chemical vapor deposition or Chemical Vapour
Condensation (CVC).
Part B
1. Explain about the Top down – ball milerling
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 (comminution), solid-state alloying, mixing or blending, and particle shape changes.
These industrial processes are mostly restricted to relatively hard, brittle materials which
fracture, deform, and cold weld during the milling operation. While oxide-dispersion
strengthened super alloys have been the primary application of mechanical attrition, the
technique has been extended to produce a variety of non equilibrium structures including
nanocrystalline, amorphous and quasi crystalline 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.
Schematic representation of the principle of mechanical milling
Powders with typical particle diameters of about 50 µm 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 masses is five to 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 2000 C.
The difficulty with top-down approaches is ensuring all the particles are broken down to
the required particle size. Furthermore, for all nanocrystalline materials prepared by a variety of
different synthesis routes, surface and interface contamination is a major concern. In particular,
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. On
the other hand, contamination through the milling atmosphere can have a positive impact on the
milling conditions if one wants to prepare metal or ceramic nanocomposites with one of the
metallic elements being chemically highly reactive with the gas (or fluid) environment. On the
other side, main advantage of top-down approach is high production rates of nano powders.
2. Write about the co-precipitaion
Ultrafine particles are produced by precipitation from a solution, the process being
dependent on the presence of the desired nuclei. For example, TiO2 powders have been produced
with particle sizes in the range 70-300 nm from titanium tetraisopropoxide. 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 noncommon associated ligands
(i.e., acetylacetonate, trifluorocarbonsulfonate, and dithiocarbamate). The 0.05 M cation
solutions were heated in a thermal bath maintained at 70° or 80 °C in batches of 100 or 250 ml.
Acid was added drop wise 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. In intervals, aliquots were collected from the reacting solution.
3. Describe about the Sol-gel process
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 dispersible oxide and forms 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 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+ROM→M-O-M+ROH (condensation)
Sol-gel method of synthesizing nanomaterials is very popular amongst chemists and is
widely employed to prepare oxide materials.
The sol-gel process can be characterized by a series of distinct 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 (Syneresis), 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 with syneresis.
The aging process of gels can exceed 7 days and is critical to the prevention of 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. This process is complicated due to fundamental changes in the structure of the gel. The
drying process has itself been broken into four distinct steps: (i) the constant rate period, (ii) the
critical point, (iii) the falling rate period, (iv)the second falling rate period.
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.
Schematic representation of sol-gel process of synthesis of nanomaterials
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.
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 nano powders are very very low
by this process. The main advantage is one can get monosized nanoparticles by any bottom up
approach.
4. Discuss in detail about the Chemical vapor deposition
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
Figure, the evaporative source used in GPC is replaced by a hot wall reactor in the Chemical
Vapour Condensation or the CVC 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. In the
simplest form a metalorganic precursor is introduced into the hot zone of the reactor using mass
flow controller. For instance, hexamethyldisilazane (CH3)3 Si NHSi (CH3)3 was used to produce
SiCxNyOz powder by CVC technique. Besides the increased quantities in this Continuous
process compared to GPC 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-Al2 O3 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.
A schematic of a typical CVC reactor
Because CVC processing is continuous, the production capabilities are much larger than
in GPC 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 GPC prepared
powders.
Part C
1. Explain the role of bottom up & top down approaches in nanotechnology?
There are two approaches for synthesis of nano materials and the fabrication of
nano structures.
Top down approach refers to slicing or successive cutting of a bulk material to get nano
sized particle.
Bottom up approach refers to the build up of a material from the bottom: atom by atom,
molecule by molecule or cluster by cluster.
Schematic representation of the ‘bottom up’ and top down’ synthesis processes of
nanomaterials
Top down – ball milerling
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 (comminution), solid-state alloying, mixing or blending, and particle shape changes.
These industrial processes are mostly restricted to relatively hard, brittle materials which
fracture, deform, and cold weld during the milling operation. While oxide-dispersion
strengthened super alloys have been the primary application of mechanical attrition, the
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.
Schematic representation of the principle of mechanical milling
Powders with typical particle diameters of about 50 µm 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 masses is five to 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 2000 C.
The difficulty with top-down approaches is ensuring all the particles are broken down to
the required particle size. Furthermore, for all nanocrystalline materials prepared by a variety of
different synthesis routes, surface and interface contamination is a major concern. In particular,
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. On
the other hand, contamination through the milling atmosphere can have a positive impact on the
milling conditions if one wants to prepare metal or ceramic nanocomposites with one of the
metallic elements being chemically highly reactive with the gas (or fluid) environment. On the
other side, main advantage of top-down approach is high production rates of nano powders.
Liquid solid reactions (co-precipitaion)
Ultrafine particles are produced by precipitation from a solution, the process being
dependent on the presence of the desired nuclei. For example, TiO2 powders have been produced
with particle sizes in the range 70-300 nm from titanium tetraisopropoxide. 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 noncommon associated ligands
(i.e., acetylacetonate, trifluorocarbonsulfonate, and dithiocarbamate). The 0.05 M cation
solutions were 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. In intervals, aliquots were collected from the reacting solution.
Wet Chemical Synthesis of nanomaterials (Sol-gel process)
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 dispersible oxide and forms 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 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+ROM→M-O-M+ROH (condensation)
Sol-gel method of synthesizing nanomaterials is very popular amongst chemists and is
widely employed to prepare oxide materials.
The sol-gel process can be characterized by a series of distinct 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 (Syneresis), 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 with syneresis.
The aging process of gels can exceed 7 days and is critical to the prevention of 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. This process is complicated due to fundamental changes in the structure of the gel. The
drying process has itself been broken into four distinct steps: (i) the constant rate period, (ii) the
critical point, (iii) the falling rate period, (iv)the second falling rate period.
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.
Schematic representation of sol-gel process of synthesis of nanomaterials
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.
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 nano powders are very very low
by this process. The main advantage is one can get monosized nano particles by any bottom up
approach.
Chemical vapor deposition or Chemical Vapour Condensation (CVC)
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
Figure, the evaporative source used in GPC is replaced by a hot wall reactor in the Chemical
Vapour Condensation or the CVC 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. In the
simplest form a metalorganic precursor is introduced into the hot zone of the reactor using mass
flow controller. For instance, hexamethyldisilazane (CH3)3 Si NHSi (CH3)3 was used to produce
SiCxNyOz powder by CVC technique. Besides the increased quantities in this Continuous
process compared to GPC 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-Al2 O3 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.
A schematic of a typical CVC reactor
Because CVC processing is continuous, the production capabilities are much larger than
in GPC 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 GPC prepared
powders.
UNIT III
Characterization: X-ray diffraction – Scherrer”s formula – Scanning Electron Microscopy –
Transmission Electron Microscopy – Fluorescence Microscopy.
--------------------------------------------------------------------------------------------------------------------Part A
1. What do you mean by characterization in relation with materials?
Characterization, when used in materials science, refers to the use of external techniques
to probe into the internal structure and properties of a material. Characterization can take the
form of actual materials testing, or analysis, for example in some form of microscope.
Analysis techniques are used simply to magnify the specimen, to visualize its internal structure,
and to gain knowledge as to the distribution of elements within the specimen and their
interactions.
2. What is the difference between SEM & TEM?
SEM is one of the most widely used techniques used in characterization of nanomaterials
and nanostructures. The resolution of the SEM approaches a few nanometers, and the
instruments can operate at magnifications that are easily adjusted from ~10 to over 300,000. Not
only topographical information SEM also provides chemical composition information near the
surface.
The transmission electron microscope (TEM) forms an image by accelerating a beam of
electrons that pass through the specimen. In TEM, electrons are accelerated to 100 KeV or
higher (up to 1MeV), projected onto a thin specimen (less than 200 nm) by means of the
condenser lens system, and penetrate the sample thickness either undeflected or deflected. The
greatest advantages that TEM offers are the high magnification ranging from 50 to 10 6 and its
ability to provide both image and diffraction information from a single sample.
Part B
1. What is X-ray diffraction? Explain
XRD is a very important experimental technique that has long been used
to address all issues related to the crystal structure of solids, including lattice constants and
geometry, identification of unknown materials, orientation of single crystals, preferred
orientation of polycrystals, defects, stresses, etc. in XRD, a collimated beam of X-rays, with a
wavelength typically ranging from 0.7 to 2 A0, is incident on a specimen and is diffracted by the
crystalline phases in the specimen according to Bragg’s law:
λ = 2d sinө
Where d is the spacing between atomic planes in the crystalline phase and
λ is the
X-ray wave length. The intensity of the diffracted X-rays is measured as a function of the
diffraction angle 2ө and the specimen’s orientation. This diffraction pattern is used to identify
the specimen’s crystalline phases and to measure its structural properties. XRD is nondestructive
and does not require elaborate sample preparation, which partly explains the wide usage of XRD
method in materials characterization.
Diffraction peak positions are accurately measured with XRD, which makes it the
best method for characterizing homogeneous and inhomogeneous strains. Homogeneous or
uniform elastic strain shifts the diffraction peak positions. From the shift in peak positions, one
can calculate the change in d-spacing, which is the result of the change of lattice constants under
a strain. Inhomogeneous strains vary from crystallite to crystallite or within a single crystallite
and this causes a broadening of the diffraction peaks that increase with sinө. Peak broadening is
also caused by the finite size of crystallites, but here the broadening is independent of sinө.
When both crystallite size and inhomogeneous strain contribute to the peak width, these can be
separately determined by careful analysis of peak shapes.
If there is no inhomogeneous strain, the crystallite size, D, can be estimated from
the peak width with the Scherrer’s formula:
D
K
B cos  B
Where λ is the X-ray wave length, B is the full width of height maximum of a diffraction
peak,  B is the diffraction angle, and K is the Scherrer’s constant of the order of unity for usual
crystal. However, one should be altered to the fact that nanoparticles often form twinned
structures: therefore, Scherrer’s formula may produce results different from the true particle
sizes. In addition, X-ray diffraction only provides the collective information of the particle sizes
and usually requires a sizeable amount of powder. It should be noted that since the estimation
would work only for very small particles, this technique is very useful in characterizing
nanoparticles. Similarly, the film thickness of epitaxial and highly textured thin films can also be
estimated with XRD.
One of the disadvantages of XRD, compared to electron diffraction, is the low
intensity of diffracted X-rays, particularly for low-Z materials. XRD is more sensitive to high –
Z materials, and for low Z-materials, neutron or electron diffraction is more suitable. Typical
intensities for electron diffraction are ~108 times larger than for XRD. Because of small
diffraction intensities, XRD generally requires large specimens and the information acquired is
an average over a large amount of material. The following figure shows the powder XRD spectra
of a series of nanoparticles with different sizes.
2. Explain the working of scanning electron microscopy (SEM) with a neat sketch?
SEM is one of the most widely used techniques used in characterization of
nanomaterials and nanostructures. The resolution of the SEM approaches a few nanometers, and
the instruments can operate at magnifications that are easily adjusted from ~10 to over 300,000.
Not only topographical information SEM also provides chemical composition information near
the surface.
Schematic diagram of Scanning Electron Microscope
In a typical SEM, a source of electrons is focused into a beam, with very fine spot size of
~5 nm and having energy ranging from a few hundred eV to 50 KeV, which is rastered over the
surface of the specimen by deflection coils. As the electrons strikes and penetrate the surface, a
number of interactions occur that result in the emission of electrons and photons from the
sample, and SEM images are produced by collecting the emitted electrons on a cathode ray tube
(CRT). Various SEM techniques are differentiated on the basis of what is subsequently detected
and imaged, and the principle images produced in the SEM are of three types: secondary electron
images, back scattered electron images and elemental X-ray maps.
When a high energy primary electron interacts with an atom, it undergoes either inelastic
scattering (deflecting the electrons with loss of energy) with atomic electrons or elastic scattering
(deflecting the electrons with no loss of energy) with the atomic nucleus.
In an inelastic collision with an electron, the primary electron transfers part of its energy
to the other electron. When the energy transferred is large enough, the other electron will emit
from the sample. If the emitted electron has the energy of less than 50 eV, it is referred to as
secondary electron. Back scattered electrons are the high energy electrons that are elastically
scattered and essentially possess the same energy as the incident or primary electrons. The
probability of backscattering increases with the atomic number of the sample material. Although
backscattering images can not be used for elemental identification, useful contrast can develop
between regions of the specimen that differ widely in atomic number, Z.
An additional electron interaction in the SEM is that the primary electron collides with
and ejects a core electron from an atom in the sample. The excited atom will decay to its ground
state by emitting either a characteristic X-ray photon or an Auger electron, both of which have
been used for chemical characterization.
Combining with chemical analytical capabilities, SEM not only provides the image of the
morphology and microstructures of bulk and nanostructured materials and devices, but can also
provide detailed information of chemical composition and distribution.
3. How do you characterize a material with transmission electron microscope (TEM) with
a neat sketch?
The transmission electron microscope (TEM) forms an image by accelerating a
beam of electrons that pass through the specimen. In TEM, electrons are accelerated to 100 KeV
or higher (up to 1MeV), projected onto a thin specimen (less than 200 nm) by means of the
condenser lens system, and penetrate the sample thickness either undeflected or deflected. The
greatest advantages that TEM offers are the high magnification ranging from 50 to 10 6 and its
ability to provide both image and diffraction information from a single sample.
The scattering processes experienced by electrons during their passage through
the specimen determine the kind of information obtained. Elastic scattering involves no energy
loss and gives rise to diffraction patterns. Inelastic interactions between primary electrons and
sample electrons at heterogeneities such as grain boundaries, dislocations, second phase
particles, defects, density variations, etc., cause complex absorption and scattering effects,
leading to a spatial variation in the intensity of the transmitted electrons. In TEM one can switch
between imaging the sample and viewing its diffraction pattern by changing the strength of the
intermediate lens.
Schematic diagram of TEM
One short coming of TEM is its limited depth resolution. Electron scattering information
in a TEM image originates from a three-dimensional sample, but is projected onto a two
dimensional detector. Therefore, structure information along the electron beam direction is
superimposed at the image plane. Although the most difficult aspect of the TEM technique is the
preparation of samples, it is less so for nanomaterials.
In addition to the capability of structural characterization and chemical analyses,
TEM also has been explored for other applications in nanotechnology. Examples include the
determination of melting points of nanocrystals, in which, an electron beam is used to heat up the
nanocrystals and the melting points are determined by the disappearance of electron diffraction.
Another example is the measurement of mechanical and electrical properties of individual
nanowires and nanotubes. This technique allows a one-to-one correlation between the structure
and properties of the nanowires.
Part C
1. Explain the working of atomic force microscopy (AFM) with a neat sketch?
In spite of atomic resolution and other advantages, STM is limited to an electrically
conductive surface since it is dependent on monitoring the tunneling current between the sample
surface and the tip. AFM was developed as a modification of STM for dielectric materials. A
variety of tip-sample interactions may be measured by an AFM, depending on the separation. At
short distances, the vander walls interactions are predominant. Van der walls force consists of
interactions of three components: permanent dipoles, induced dipoles and electronic polarization.
Long range forces act in addition to short-range forces between the tip and sample, and become
significant when the tip-sample distance increases such that the van der walls forces become
negligible, examples of such forces include electrostatic attraction or repulsion, current induced
or static-magnetic interactions, and capillary forces due to the condensation of water between the
sample and tip.
In AFM, the motion of a cantilever beam with an ultra small mass is measured, and the
force required to move this beam through measurable distance (10-4 A0) can be as small as 1018
N. The following figure shows a schematic diagram of AFM.
Figure: schematic diagram of AFM.
The instrument consists of a cantilever with a nanoscale tip, a laser pointing at the end of
a cantilever, a mirror and a photodiode collecting the reflected laser beam, and a three
dimensional positioning sample stage which is made of an array of piezoelectrics. Similar to
STM, the images are also generated by scanning the tip across the surface. However, instead of
adjusting the height of the tip to maintain a constant distance between the tip and the surface, and
thus a constant tunneling current as in STM, the AFM measures the minute upward and
downward deflections of the tip cantilever while maintaining a constant force of contact.
UNIT IV
The Carbon Nanotube – New Forms of Cabon – Types of Nanotubes – Formation of Nanotubes
– Uses for nanotubes – Biological Applications.
--------------------------------------------------------------------------------------------------------------------Part A
1. What is carbon nanotubes?
Carbon nanotubes are a new form of carbon made by rolling up a single graphite
sheet to a narrow but long tube closed at both sides by two hemispheres (1/2 section of fullerene
carbon) like end caps.
2. List methods for producing carbon nanotubes?
Three main methods are the laser ablation, electric arc discharge and the chemical
vapor deposition. Chemical vapor deposition is becoming very popular because of its potential
for scale up production.
3. List any two applications of bucky balls and carbon nanotubes?
Currently, carbon Nanotubes are extending our ability to fabricate devices such as
Molecular probes, Pipes, Wires, Bearings, springs, Gears, Pumps, Molecular transistors. In
future we can find some more applications such as Field emitters, Building blocks for bottom-up
electronics, Smaller, lighter weight components for next generation spacecraft and also enable
large quantities of hydrogen to be stored in small low pressure tanks.
Part B
1. Define carbon nanotube? What are the types of carbon nanotubes?
Carbon nanotubes are a new form of carbon made by rolling up a single graphite
sheet to a narrow but long tube closed at both sides by two hemispheres (1/2 section of fullerene
carbon) like end caps.
In 1991, while experimenting on fullerene and looking into soot residues sumio lijima
invented two types of nanotubes namely single walled carbon nanotubes (SWNTs) and multi
walled carbon nanotubes (MWNTs). SWNT consists only of a single graphene sheet with one
atomic layer in thickness, while MWNT is formed from 2 to several tens of graphene sheets
arranged concentrically into tube structures. They are promising one-dimensional periodic
structure along the axis of the tube with high aspect ratio (length/diameter).
2. Highlight the properties of carbon nanotubes?
Properties:
The following table shows selected electrical and mechanical properties of carbon
nanotubes.
Characteristics
Measure
Electrical conductivity
Metallic or semi conducting
Electrical transport
Ballistic, no scattering
Energy gap (semi conducting)
•
E g (ev)=1/d (nm)
Maximum current density
1010 A/cm2
Maximum strain
0.11% at 1 V
Thermal conductivity
6 KW/Km
Diameter
1-100 nm
Length
Up to millimeters
Gravimetric surface
>1500 m2/g
E-modulus
1000 Gpa, harder than steel
Nanotubes can be either electrically conductive or semi conductive, depending on their
helicity.
•
These one-dimensional fibers exhibit electrical conductivity as high as copper, thermal
conductivity as high as diamond.
•
Strength 100 times greater than steel at one sixth the weight, and high strain to failure.
•
Current length limits are about one millimeter.
Part C
1. List the methods for producing carbon nanotubes and explain any of the method with a
neat sketch?
The growth of carbon nanotubes during synthesis and production is believed to
commence from the recombination of carbon atoms split by heat from its precursor. Although a
number of newer production techniques are being invented, three main methods are the laser
ablation, electric arc discharge and the chemical vapor deposition. Chemical vapor deposition is
becoming very popular because of its potential for scale up production.
Chemical vapor deposition:
In this technique, carbon nano tubes grow from the decomposition of hydrocarbons at
temperature range of 500 to 12000C. They can grow on substrates such as carbon, quartz, silicon,
etc or on floating fine catalyst particles, e.g. Fe, Ni, Co, etc from numerous hydrocarbons e.g.
benzene, xylene, natural gas, acetylene, to mention but few.
The above figure shows the schematic diagram of a typical catalytic chemical vapor deposition
system. It is equipped with a horizontal tubular furnace as the reactor. The tube is made of
quartz, 30 mm in diameter and 1000 mm in length. Ferrocene and Benzene vapors acts as the
catalyst (Fe) and carbon atom precursors respectively were transported either by argon, hydrogen
or mixture of both into the reaction chamber, and decomposed into respective ions of Fe and
carbon atoms, resulting into carbon nanostructures. The growth of the nanostructures occurred in
either the heating zone, before or after the heating zone, which is normally operated between
5000C and 11500C for about 30 min. 200ml/min of hydrogen is used to cool the reactor.
Arc discharge:
The arc discharge method produces a number of carbon nanostructures such as fullerenes,
whiskers, soot and highly graphitized carbon nanotubes from high temperature plasma that
approaches 37000C. The first ever produced nanotube was fabricated with the DC arc discharge
method between two carbon electrodes, anode and the cathode in a noble gas (helium or argon)
environment. Schematic representation of a typical arc discharge unit is presented in figure
below
Figure: Schematic of Arc discharge method.
Relatively large scale yield of carbon nanotubes of about 75% was produced by Ebbesen and
Ajayan with diameter between 2 to 30nm and length 1µm deposited on the cathode at 100 to 500
Torr He and about 18 V DC. It has conveniently been used to produce both SWNTs and
MWNTs as revealed by Transmission Electron Microscope (TEM) analysis. Typical nanotubes
deposition rate is around 1mm/min and the incorporation of transition metals such as Co, Ni or
Fe into the electrodes as catalyst favors nanotubes formation against other nanoparticles, and low
operating temperature. The arc discharge unit must be provided with cooling mechanism whether
catalyst is used or not, because overheating would not only results into safety hazards, but also
into coalescence of the nanotube structure.
Laser ablation:
Laser ablation technique involves the use of laser beam to vaporize a target of a mixture
of graphite and metal catalyst, such as cobalt or nickel at temperature approximately 12000C in a
flow of controlled inert gas (argon) and pressure, where the nanotube deposits are recovered at a
water cooled collector at much lower and convenient temperature. This method was used in early
days to produce ropes of SWNTs with remarkably uniform narrow diameters ranging from 5-20
nm, and high yield with graphite conversion grater than 70-90%.
The bundles entangled into a 2-D triangular lattice via the van der walls bonding to
achieve lattice constant equal to 1.7 nm. The metal atom (catalyst) due to its high
electronegativity, deprived the growth of fullerenes and thus a selective growths of carbon
nanotubes with open ends were obtained. Changing the reaction temperature can control the
tubes diameters, while the growth conditions may be maintained over a higher volume and time,
when two laser pulses are employed.
However, by the virtue of relative operational complexity, the laser ablation method
appears to be economically disadvantageous, which in effect hampers its scale up potentials as
compared to the CVD method. The following figure shows the schematic of laser ablation
method.
2. Discuss in detail about the biological applications of carbon nanotube.
Encapsulation and the controlled release of drugs using nanospheres have proved
extremely successful. The drug to be released is surrounded by a polymeric biodegradable
vehicle that is spherical in shape. As the spheres degrade, the drugs are released into the system.
By immobilizing specific peptide sequences onto the surface of the nanosphere, the permeation
probability of these spheres through cell membranes is enhanced. The process of incorporating
DNA into living cells is referred to as DNA transfection. DNA, being anionic in nature binds to
the outer surface of nanospheres coated with positively charged ammonium groups. The bound
DNA is then introduced into the cells. The process of DNA transfection is also being carried out
using nano liposomes (membrane bound vesicles with a lipid bilayer). Spherical particles, owing
to their ease of manufacture, have been the obvious choice in the above-mentioned applications.
Another alternative is to use nanotubes. It may be possible to load nanotubes with a desired
material. The functionalization of the inner and outer surfaces of these tubes with various
chemicals is also a possible approach. Gold nanoparticles possess an extremely high absorption
coefficient that makes them ideal visual indicating agents.32 These particles exhibit various
colors depending on the size and the shape of the particle. The micro and nano-tubes, which have
evoked interest among researchers are organosilicon polymer tubes, lipid microtubes, carbon
nanotubes, peptide nanotubes and template-synthesized nanotubes. By attaching functional
groups to tube sidewalls, nano- tubes can be used in processes like extraction and catalysis. By
immobilizing antibodies, nanotubes can also be used to separate enantiomers from a racemic
mixture. Enantiomers are usually difficult to separate largely due to their chemical similarity.
The process of separation could be made easier by the use of side-wall-functionalized nanotubes.
A single layer of graphite, graphene is composed of a hexagonal array of carbon atoms.
Chiral vector of a CNT and its unit vector.
Image of a single-walled carbon nanotube
UNITV
The Carbon Nanotube – New Forms of Cabon – Types of Nanotubes – Formation of Nanotubes
– Uses for nanotubes – Biological Applications.
--------------------------------------------------------------------------------------------------------------------Part A
1. What are uses of carbon nanotubes?
Currently, carbon Nanotubes are extending our ability to fabricate devices such as
Molecular probes, Pipes, Wires, Bearings, springs, Gears, Pumps, Molecular transistors. In
future we can find some more applications such as Field emitters, Building blocks for bottom-up
electronics, Smaller, lighter weight components for next generation spacecraft and also enable
large quantities of hydrogen to be stored in small low pressure tanks.
Part B
1. Explain about the new forms and types of carbon nanotubes?
In 1991, while experimenting on fullerene and looking into soot residues sumio
lijima invented two types of nanotubes namely single walled carbon nanotubes (SWNTs) and
multi walled carbon nanotubes (MWNTs). SWNT consists only of a single graphene sheet with
one atomic layer in thickness, while MWNT is formed from 2 to several tens of graphene sheets
arranged concentrically into tube structures. They are promising one-dimensional periodic
structure along the axis of the tube with high aspect ratio (length/diameter).
Part C
1. How carbon nanotube is formed? Explain.
The growth of carbon nanotubes during synthesis and production is believed to
commence from the recombination of carbon atoms split by heat from its precursor. Although a
number of newer production techniques are being invented, three main methods are the laser
ablation, electric arc discharge and the chemical vapor deposition. Chemical vapor deposition is
becoming very popular because of its potential for scale up production.
Chemical vapor deposition:
In this technique, carbon nano tubes grow from the decomposition of hydrocarbons at
temperature range of 500 to 12000C. They can grow on substrates such as carbon, quartz, silicon,
etc or on floating fine catalyst particles, e.g. Fe, Ni, Co, etc from numerous hydrocarbons e.g.
benzene, xylene, natural gas, acetylene, to mention but few.
The above figure shows the schematic diagram of a typical catalytic chemical vapor deposition
system. It is equipped with a horizontal tubular furnace as the reactor. The tube is made of
quartz, 30 mm in diameter and 1000 mm in length. Ferrocene and Benzene vapors acts as the
catalyst (Fe) and carbon atom precursors respectively were transported either by argon, hydrogen
or mixture of both into the reaction chamber, and decomposed into respective ions of Fe and
carbon atoms, resulting into carbon nanostructures. The growth of the nanostructures occurred in
either the heating zone, before or after the heating zone, which is normally operated between
5000C and 11500C for about 30 min. 200ml/min of hydrogen is used to cool the reactor.
Arc discharge:
The arc discharge method produces a number of carbon nanostructures such as fullerenes,
whiskers, soot and highly graphitized carbon nanotubes from high temperature plasma that
approaches 37000C. The first ever produced nanotube was fabricated with the DC arc discharge
method between two carbon electrodes, anode and the cathode in a noble gas (helium or argon)
environment. Schematic representation of a typical arc discharge unit is presented in figure
below
Figure: Schematic of Arc discharge method.
Relatively large scale yield of carbon nanotubes of about 75% was produced by Ebbesen and
Ajayan with diameter between 2 to 30nm and length 1µm deposited on the cathode at 100 to 500
Torr He and about 18 V DC. It has conveniently been used to produce both SWNTs and
MWNTs as revealed by Transmission Electron Microscope (TEM) analysis. Typical nanotubes
deposition rate is around 1mm/min and the incorporation of transition metals such as Co, Ni or
Fe into the electrodes as catalyst favors nanotubes formation against other nanoparticles, and low
operating temperature. The arc discharge unit must be provided with cooling mechanism whether
catalyst is used or not, because overheating would not only results into safety hazards, but also
into coalescence of the nanotube structure.
Laser ablation:
Laser ablation technique involves the use of laser beam to vaporize a target of a mixture
of graphite and metal catalyst, such as cobalt or nickel at temperature approximately 1200 0C in a
flow of controlled inert gas (argon) and pressure, where the nanotube deposits are recovered at a
water cooled collector at much lower and convenient temperature. This method was used in early
days to produce ropes of SWNTs with remarkably uniform narrow diameters ranging from 5-20
nm, and high yield with graphite conversion grater than 70-90%.
The bundles entangled into a 2-D triangular lattice via the van der walls bonding to
achieve lattice constant equal to 1.7 nm. The metal atom (catalyst) due to its high
electronegativity, deprived the growth of fullerenes and thus a selective growths of carbon
nanotubes with open ends were obtained. Changing the reaction temperature can control the
tubes diameters, while the growth conditions may be maintained over a higher volume and time,
when two laser pulses are employed.
However, by the virtue of relative operational complexity, the laser ablation method
appears to be economically disadvantageous, which in effect hampers its scale up potentials as
compared to the CVD method. The following figure shows the schematic of laser ablation
method.