Transcript nano - SNS Courseware

Chapter VII
INTRODUCTION
The prefix (nano) in the word nanochemistry means a
billionth (1 x 10-9 m). Atoms are very small and the
diameter of
a single atom can vary from 0.1 to 0.5 nm. It deals with various
structures of matter having
billionth of meter.
dimensions of the order of a
BASICS OF NANOCHEMISTRY
1. Nanoparticles





Nanoparticles are the particles, the size of which
ranges from 1-50 nm.
Generally they are obtained as colloids.
The colloidal particles have a tendency to
remain single crystal and hence are called as
nanocrystals.
A large percentage of atoms in nanocrystals are
present on the surface Nanocrystals possess
electronic, magnetic and opticalproperties.
Since the nanoparticles exhibit an electronic
behavior, governed by the quantum physics,
they are also called as quantum dots.
2. Nanomaterials

Nanomaterials are the materials having components with
size less than 100 nm at least in one dimension.

Nanomaterials, in one dimension, are layers such as a
thin films or surface coatings.

Nanomaterials, in two dimension, are tubes such as
nanotubes and nanowires.

Nanomaterials, in three dimension, are particles like
precipitates, colloids and quantum dots.
3. Nanochemistry (or) Nanoscience
 Nanoscience is defined as the study of phenomena and
manipulation of materials at atomic, molecular and
macromolecular scales.
4. Nanotechnology
 Nanotechnology
is
defined
as
the
design,
characterization, production and applications of
structures, systems and devices by controlling size and
shape at 10-9 m scale or the single-atomic level.
DISTINCTION BETWEEN NANO PARTICLES,
MOLECULES AND BULCK MATERIALS

The size of nano particles are less than 100 nm in
diameter, molecules are in the range of picometers, but
bulk materials are larger in micron size.

Molecule is a collection of atoms, nano particles are
collection of few molecules that is less than 100 nm but
bulk materials contains thousands of molecules.

Surface area of nano particles is more than the bulk
materials.
 Strength of nano materials is 3 - 10 times higher than
 Nano particles possesses size dependent properties, but bulk
materials possess constant physical properties.
 Corrosion resistance is more than the bulk materials, hence
localised corrosion in nano materials is stopped.
 Behavior of bulk materials can be changed, but cannot enter
inside the nano particles.
 Nano particles, due to its size, possess unexpected optical
(visible) properties.
I.
Gold nano particles appear deep red to black colour in
solution compared to yellow colour with Gold.
II. ZnO nano particles possesses superior UV blocking property
compared to bulk material.
III. Absorption of solar radiation in photovoltaic cell containing
nano particles are higher than the film (bulk material).
10.
Nano particles possesses lower melting point than the
bulk materials.
Gold nanoparticles melt at lower temperature
2.5 nm, but Gold slab melts at 1064OC.
(3000C)
for
11. Sinter ing of nano particles takes place at lower temper
ature and in shor t time than the bulk mater ials.
12. Electr ical proper ties, resistivity of nano par ticles ar
eincreased by 3 times.
13. Suspension of nano par ticles is possible, because nano
particles possess high sur face area, but bulk mater ials
cannot.
14. The wear resistance of nano particles are 170 times higher
than the bulk mater ials.
Table 7.1 Comparison of atom/molecule,
particles/cluster, bulk materials
nano
PROPERTIES OF NANO-MATERIALS
1. Melting Points
Nano-materials have a significantly lower melting
point and appreciable reduced lattice constants. This is due
to huge fraction of surface atoms in the total amount of
atoms.
2. Optical Properties
Reduction of material dimensions has pronounced
effects on the optical properties. Optical properties of nanomaterials are different from bulk forms. The change in optical
properties is caused by two factors
The quantum confinement of electrons within the nanoparticles increases the energy level spacing.
The optical absorption peak of a semiconductor nanoparticles shifts to a short wavelength, due to an increased band
gap.
(ii) Surface plasma resonance, which is due to smaller size of
nano-particles than the wavelength of incident radiation.
The colour of metallic nano-particles may change with their
sizes due to surface plasma resonance.
3. Magnetic Properties
Magnetic properties of nano materials are different from
that of bulk materials. Ferro-magnetic behaviour of bulk
materials disappear, when the particle size is reduced and
transfers to super-paramagnetics. This is due to the huge
surface area.
4. Mechanical Properties
The nano-materials have less defects compared to bulk
materials, which increases the mechanical strength.
(i)
Mechanical properties of polymeric materials
can be increased by the addition of nano-fillers.
(ii)
As nano-materials are stronger, harder and
more wear resistant and corrosion resistant,
they are used in spark plugs.
Nano-crystalline carbides are much stronger, harder
and wear resistant and are used in micro drills.
5. Electrical Properties
(i) Electrical conductivity decreases with a reduced
dimension due to increased surface scattering. However, it
can be increased, due to better ordering in micro-structure.
Polymeric fibres
(ii)
Nanocrystalline materials are used as very good
separator plates in batteries, because they can hold
more energy than the bulk materials.
Nickel-metal hydride batteries made of
nanocrystalline nickel and metal hydride,
require far less frequent recharging and last
much longer.
6. Chemical Properties
Any heat treatment increases the diffusion of
impurities, structural defects and dislocations and can be
easily push them to the nearby surface. Increased
perfection will have increased chemical properties.
SIZE DEPENDENT PROPERTIES

Nearly all the properties as shown in following figure 7.1
like hardness, strength, ductility, melting point and
density, change for nano materials. These behaviors
vary so significantly by a mere reduction in grain size.

Nanomaterials are composed of grains and grain
boundaries.

Nanometre sized grains contains only
thousands of atoms with in each grain.
a
few

A large number
boundaries.

As the grain size decreases, there is a significant
increase in the volume fraction of grain boundaries or
interfaces.

The properties of the materials are bound to be
governed to a large extent by defect configurations.
Hence the mechanical and chemical properties of
nanomaterials are significantly altered due to defect
dynamics.

The elastic property of nanomaterials are different from
that of bulk alloys due to the presence of increased
fraction of defects
of
atoms
reside
at
the
grain
Fig 7.1 Shows how different properties change in
the nano-materials
1.
Nanocrystalline ceramics are tougher and
stronger than those with coarse grains.
2.
Nano-sized
metals
exhibit
significant
decrease in toughness and yield strength
increases.
SYNTHESIS OF NANO - MATERIALS
Nano-materials are synthesised in two methods.
Top-down (or) Physical (or) Hard methods
It involves conversion of larger particles into
smaller particles of nano-scale structure. This methods is
carried out by the following process.
1. Laser ablation
2. Chemical Vapour Deposition (CVD)
3. Electro-deposition
1. Laser Ablation
In laser ablation, high-power laser pulse is used to
evaporate the matter from the target. The stoichiometry of the
material is preserved in the interaction. The total mass ablated
from the target per laser pulse is referred to as the ablation rate.
Reaction Setup
A typical laser ablation setup in shown in the following
figure.7.2
Fig 7.2 Laser ablation chamber equipped with a rotating target holder

When a beam of laser is allowed to irradiate the
target, a supersonic jet of particles is evaporated
from the target surface. Simultaneously, an inert gas
such as argon, helium is allowed into the reactor to
sweep the evaporated particles from the furnace zone
to the colder collector.

The ablated species condense on the substrate
placed opposite to the target.

The ablation process takes
place
in
vacuum
chamber, either in vacuum or in the presence of some
background gas.
2. Chemical Vapour Deposition (CVD)
It is a process of chemically reacting a volatile
compound of a material with other gases, to produce a nonvolatile solid that deposits automatically on a suitably placed
substrate.
CVD reaction requires activation energy to proceed.
This energy can be provided by several methods.
(a) Thermal CVD
In thermal CVD, the reaction is activated by high
temperature above 9000C. Typical apparatus comprises of gas
supply system, deposition chamber and an exhaust system.
(b) Plasma CVD
In plasma CVD, the reaction is activated by plasma at
temperature between 300 - 7000C.
(c) Laser CVD
In laser CVD, pyrolysis occurs when laser thermal
energy of laser heats falls on an absorbing substrate.
(d) Photo-laser CVD
In photo-laser CVD, the chemical reaction is induced by
ultra violet radiation, which has sufficient photon energy, to
break the chemical bond in the reactant molecules.
Various steps involved in synthesis of CVD
The various steps involved in synthesis of CVD are
summarized as follows.
1. Transport of gaseous reactants to the surface.
2. Adsorption of gaseous reactant on the surface.
3. Catalysed reaction occurs on the surface.
4. Product diffuses to the growth sites.
5. Nucleation and growth occurs on the growth site.
6. Desorption of reaction products away from the
surface.
CVD Reactor
The CVD reactors are of generally two types
1. Hot-wall CVD
2. Cold-wall CVD
1.
Hot-wall CVD reactors are usually tubular in
form, and heating is accomplished by
surrounding the reactor with resistance
elements.
2.
But in cold-wall CVD reactors, substrates are
directly heated inductively by graphite
susceptors, while chamber walls are air (or)
water-cooled
Fig 7.3 CVD Reactors
3. ELECTRO - DEPOSITION
Template
assisted
electro-deposition
is
an
important technique for synthesizing metallic nanomaterials with controlled shape and size. Arrays of nano-
structured materials with specific arrangements can be
prepared by this method, using an active template as a
cathode in an electrochemical cell.
Fig 7.4 Electro-deposition

The electro-deposition method consists of an
electrochemical cell. The cell usually contains a
reference electrode, a specially designed cathodes
and an anode.

The cathode, substrate on which electro-deposition
of the nano-structure takes place, can be made of
either non-metallic or metallic materials.

By using the surface of the cathode, as a template,
various desired nano-structures can be synthesized
for specific applications
Bottom-up (or) Chemical (or) Soft methods
(or) Small to Big methods
It involves building-up of materials from the bottom by
atom by atom (≈ 0.1 nm ), molecule by molecule or cluster by
cluster. This method is carried out by the following process
1. Precipitation
2. Thermolysis
(a) Solvothermal method
(b) Hydrothermal method
1. PRECIPITION
Generally nano-particles are synthesised by the
precipitation reaction between the reactants in presence
of water soluble inorganic stabilizing agent.
(i) Precipitation of BaSO4 Nano-particles
10 gm of sodium hexameta-phosphate (stabilizing
agent) was dissolved in 80 ml of distilled water in 250 ml
beaker with constant stirring. Then 10 ml of 1M sodium
sulphate solution was added followed by 10 ml of 1M
Ba( NO3 )2 solution. The resulting solution was stirred for 1
hr. Precipitation occurs slowly. The resulting precipitate was
then centrifuged, washed with distilled water and vacuum
dried.
In the absence of stabilizing agent, Bulk BaSO4 is
obtained.
(ii) Precipitation by reduction
Reduction of metal salt to the corresponding metal
atoms. These atoms act as nucleation centres leading to
formation of atomic clusters. These clusters are surrounded
by stabilizing molecule that prevent the atoms
agglomerating.
2. THERMOLYSIS
Thermolysis is characterized by subjecting the
metal precursors (usually organometallic compounds in
oxidation state zero) at high temperatures together with a
stabilizing agent. Nano-particles show an increase in size
relating to the temperature rise. This is due to the
elimination of stabilizing molecule, generating a greater
aggregation of the particles.
(a) Hydrothermal synthesis
It involves crystalisation of substances from high
temperature aqueous solutions at high vapour pressure.
Hydrothermal synthesis is usually performed below the super
critical temperature of water (3740C ).
Method
Hydrothermal synthesis is performed in an apparatus
consisting of a steel pressure vessel called autoclave in which
Fig 7.5 Hydrothermal synthesis
a nutrient is supplied along with water. A gradient of
temperature is maintained at the opposite ends of the growth
chamber, so that the hotter end dissolves the nutrient and
the cooler end causes seeds to take additional growth.
(b) Solvothermal Synthesis
Solvothermal synthesis involves the use of solvent
under high temperature (between 1000C to 10000C) and
moderate to high pressure (1 atm to 10,000 atm) that facilitate
the interaction of precursors during synthesis.
Method
A solvent is mixed with certain metal precursors and
the solution mixture is placed in an autoclave kept at
relatively high temperature and pressure in an oven to carry
out the crystal growth. The pressure generated in the vessel,
due to the solvent vapour, elevates the boiling point of the
solvent.
Fig 7.6 Solvothermal synthesis
Example for solvent
Ethanol, methanol, toluene, cyclohexane, etc.,
Solvothermal synthesis of zinc oxide
Zinc acetate dihydrate is dissolved in 2-propanol at
500C. Subsequently, the solution is cooled to 00C and NaOH is
added to precipitate ZnO. The solution is then heated to 650C
to allow ZnO growth for some period of time before a capping
agent (1-dodecanethiol) is injected into the suspension to
arrest the growth. The rod shaped ZnO nano-crystal is
obtained.
Uses
1.
Many geometries including thin film, bulk powder,
single crystals can be prepared.
2.
Thermodynamically stable novel materials can also be
prepared easily.
NANO-WIRES
Nano-wire is a material having an aspect ratio ie.,
length to width ratio greater than 20. Nano-wires are also
referred to as “quantum wires”.
1. Nano-wires of metals
:
Au, Ni, Pt.
2. Nano-wires of semiconductors
:
InP, Si, GaN
3. Nano-wires of Insulators
:
SiO2, TiO2
4. Molecular nanowires
:
DNA
Characteristics of Nano-wires
1.
Nano-wires are one-dimensional material.
2.
Conductivity of a nano-wire is less than that of
the corresponding bulk materials.
3.
It exhibits distinct optical, chemical, thermal and
electrical properties due to this large surface area.
4.
Silicon nano-wires show strong photo luminescence
characteristics.
Synthesis of Nano-wires
Nanowires can be synthesised by any one of the
following methods.
1. Template-assisted synthesis
Template assisted synthesis of nanowires is simple way
to fabricate nanostructures. These templates contain very small
cylindrical pores or voids within the host material and the empty
spaces are filled with the chosen material to form nanowires.
Examples for templates
Alumina (Al2O3), nano-channel glass, mica films, ion
track-edged polymers.
2. VLS method
It involves the absorption of the source material from the
gas phase into a liquid droplet of catalyst. Upon supersaturation
of the liquid alloy, a nucleation event generates a solid
precipitate of the source material. This seed serves as a
preferred site for further deposition of material at the interface of
the liquid droplet, promoting the elongation of the seed into a
nanowire.
Applications of Nano-wires
1. Nanowires are used for enhancing mechanical
of composites.
properties
2. It is also used to prepare active electronic components such
as p n junction and logic gates.
3. Semiconductor nanowire crossings are expected to play a
important role in future of digital computing.
4. Nanowires find applications in high-density data storage
either as magnetic read heads or aspatterned storage media.
NANO-RODS
Nano-rod is a material having an aspect ratio
in the range 1 to 20 with short dimension of the
material being 10-100 nm.
Characteristics of Nano-rods
1. Nano-rods are one-dimensional materials.
2. It also exhibits optical and electrical properties
Synthesis
Nano-rods are produced by direct chemical synthesis. A
combination of ligands act as shape control agents and bond to
different facets of the nano-rods with different strength.
Applications
It finds applications in display technologies and micro
mechanical switches
NANO CLUSTER
Nano clusters constitute an intermediate state of
matter between molecules and bulk materials. These are
fine aggregates of atoms or molecules. They are bound by
forces, which may be metallic, covalent, ionic, hydrogen
bond or vander waals force in character. The size of
nanocluster ranges from sub-nanometer to 10 nm in
diameter. It has been found that clusters of certain critical
size (clusters with a certain number of atoms in the group)
are more stable than others. Nanoclusters consisting of
upto a couple of hundred atoms, but larger aggregates
containing 103 or more atoms are called nanoparticles.
Magic number
It is the number of atoms in the clusters of criticle
sizes with higher stability. Different types of clusters can be
distinguished by the nature of the force between the atoms.
Clusters containing a transition metal atoms have unique
chemical, electronic and magnetic properties, which vary with
the number of constituent atoms, the type of element and the
charge on the cluster.
Production of Nano Cluster
Fig 7.7 Production of nano clusters from atoms or
molecules or from bulk materials
Clusters can be produced from atomic or molecular
constituents or from the bulk materials as shown in the figure.
Atomic clusters or molecular clusters are formed by nucleation
of atoms or molecules respectively. Clusters of the same type
may be obtained by top down process also.
Sources of Clusters
There are many kinds of cluster sources. Two of them are
1. Supersonic nozzle source
2. Gas-aggregation source
1. Supersonic Nozzle Source
Here metal is vapourized in an oven and the vapour is
mixed with an inert carrier gas (seeded) at a pressure of
several atmosphere at a temperature of 75 - 1500 K. The
metal/carrier gas mixture is then allowed through a nozzle in
to high vacuum, which creates supersonic beam. Seeding
produces large clusters while in the absence of a carrier gas
smaller clusters are formed.
2. Gas-aggregation source
The source utilizes the property of aggregation of
atoms in an inert media. The vapours generated by any method
are introduced in to a cold inert gas at a higher pressure. The
species at high temperature are thermalized. The gas phase is
super saturated with the species and they aggregate. These
sources produce continuous beams of clusters of low-tomedium boiling metals
NANO TUBES
Nano-tubes are one of the most widespread studied
and used materials, consists of tiny cylinders of carbon and
other materials like boron nitride. Nano-tubes of carbon and
inorganic compounds with structures comparable to the
layered structure of graphite have been prepared. Studies
on carbon nano-tubes are quite extensive.
Carbon Nanotubes (CNT)
Carbon nanotubes are allotropes of carbon with a
nanostructure having a length-to-diameter ratio greater
than 1,000,000. When graphite sheets are rolled into a
cylinder, their
Single walled carbon nanotubes
Fig 7.8 Single walled carbon nano tubes
edges joined and form carbon nanotubes i.e., carbon
nanotubes are extended tubes of rolled graphite sheets.
Nanotubes naturally align themselves into “ropes” and held
together by vanderwaals forces. But each carbon atoms in
the carbon nanotubes are linked by the covalent bond.
STRUCTURE
(OR)
TYPES
OF
CARBON
NANOTUBES
Carbon nanotubes are lattice of carbon atoms, in
which each carbon is covalently bonded to three other carbon
atoms. Depending upon the way in which graphite sheets are
rolled, two types of CNTs are formed.
1. Single - walled nanotubes (SWNTs).
2.Multi – Walled nanotubes (MWNTs)
Fig 7.9 Structure of Single walled carbon nanotubes
1. Single - walled nanotubes (SWNTs)
SWNTs consist of one tube of graphite. It is one-atom
thick having a diameter of 2 nm and a length of 100 m.
SWNTs are very important, because they exhibit important
electrical properties. It is an excellent conductor. Three kinds
of nanotubes are resulted, based on the orientation of the
hexagon lattice.
(a) Arm-chair structures: The lines of hexagons are
parallel to the axis of the nanotube.
(b) Zig-zag structures: The lines of carbon bonds are
down the centre.
(c) Chiral nanotubes:
It exhibits twist or spiral around
the nanotubes.
It has been confirmed that arm-chair carbon
nanotubes are metallic while zig-zag and chiral nanotubes
are semiconducting.
2. Multi - walled nanotubes (MWNTs)
MWNTs (nested nanotubes) consist of multiple layers
of graphite rolled in on themselves to form a tube shape. It
exhibits both metallic and semiconducting properties. It is
used for storing fuels such as hydrogen and methane.
Fig 7.10 Multiwalled Carbon Nanotubes
SYNTHESIS OF CARBON NANOTUBES
Carbon nanotubes can be synthesized by any one of the
following methods.
1. Pyrolysis of hydrocarbons.
2. Laser evaporation.
3. Carbon arc method.
4. Chemical vapour deposition.
1. Pyrolysis
Carbon nanotubes are synthesized by the pyrolysis of
hydrocarbons such as acetylene at about 7000C in the
presence of Fe-silica or Fe-graphite catalyst under inert
conditions.
2. Laser evaporation
It involves vapourization of graphite target, containing
small amount of cobalt and nickel, by exposing it to an intense
pulsed laser beam at higher temperature 12000C in a quartz
tube reactor. An inert gas such as argon is simultaneously
allowed to pass into the reactor to sweep the evaporated
carbon atoms from the furnace to the colder copper collector,
on which they condense as carbon nanotubes.
3. Carbon arc method
It is carried out by applying direct current (60 - 100 A
and 20 - 25 V) arc between graphite electrodes of 10 - 20 m
diameter.
4. Chemical vapour deposition
It involves decomposition of vapour of hydrocarbons
such as methane, acetylene, ethylene, etc., at high
temperatures
11000C
in presence of metal nanoparticle
catalysts like nickel, cobalt, iron supported on MgO or Al2O3.
Carbon atoms produced by the decomposition condense on a
cooler surface of the catalyst.
Properties of CNTs
1. CNTs are very strong, withstand extreme strain in
tension and posses elastic flexibility.
2. The atoms in a nano-tube are continuously vibrating
back and forth.
3. It is highly conducting and behaves like metallic or
semiconducting materials.
4. It has very high thermal conductivity and kinetic
properties.
Uses of CNTs
1. It is used in battery technology and in industries as
catalyst.
2. It is also used as light weight shielding materials for
protecting electronic equipments.
3. CNTs are used effectively inside the body for drug
delivery.
4. It is used in composites, ICs.
APPLICAITONS OF NANO MATERIALS
NANO PARTICLES
(OR)
Nano-technology finds significant impact on all most all
the industries and all areas of society. Since nano-materials
possess unique beneficial chemical, physical and mechanical
properties, they can be used for a wide variety of applications
I.Medicine
1. Nano drugs
Nano materials are used as nano drugs for the
cancer and TB therapy,
2. Laboratories on a chip
Nano technology is used in the production of
laboratories on a chip.
3. Nano-medibots
Nano particles function as nano-medibots that release
anti-cancer drug and treat cancer.
4. Gold-coated nanoshells
It converts light into heat, enabling the destruction of
tumours.
5. Gold nano particles as sensors
Gold nano particles undergo colour change during the
transition of nano particles.
6. Protein analysis
Protein analysis can also be done using nanomaterials.
7. Gold nanoshells for blood immuno assay
Gold nano shells are used for blood immuno assay.
8. Gold nano shells in imaging
Optical properties of the gold nano shells are utilized
for both imaging and therapy.
9. Targeted drug delivery using gold nano particles
It involves slow and selective release of drugs to the
targeted organs.
10. Repairing work
Nano technology is used to partially repair neurological
damage.
II.Industries
1. As Catalyst
It depends on the surface area of the material. As nanoparticles have an appreciable fraction of their atom at the surface,
its catalytic activity is good.
Bulk gold is chemically inert, where as gold nanoparticles have excellent catalytic property.
2. In water purification
Nano-filtration makes use of nano-porous membranes
having pores smaller than 10 nm. Dissolved solids and colour
producing organic compounds can be filtered very easily
from water. Magnetic nano-particles are effective in removing
heavy metal contamination from waste water.
3. In fabric industry
The production of smart-clothing is possible by
putting a nano-coating on the fabric.
(i)
Embedding of nano-particles on fabric makes
them stain repellent.
(ii)
Socks with embedded silver nano-particles
fills all the bacteria and makes it odour free.
4. In Automobiles
(i)
Incorporation of small amount of nano-
particles in car bumpers can make them
stronger than steel.
(ii)
Specially designed nano-particles are used as
fuel additive to lower consumption in
vehicles.
5. In food industry
The inclusion of nano-particles in food contact
materials can be used to generate novel type of packing
materials and containers.
6. In energy sector
In solar power, nano-technology reduces the cost of
photovoltaic cells by 10 to 100 times.
III.Electronics
 Quantum wires are found to have high electrical
conductivity.
 The integrated memory circuits have been found to be
effective devices.
 A transistor, called NOMFET, (Nano particle organic
memory field effect transistor) is created by combining
gold nano particles with organic molecules.
 Nano wires are used to build transistors without p - n
junctions.
 Nano radios are the other important devices, using
carbon nanotubes.
 MOSFET (Metal Oxide Semi conductor Field Effect
Transistor), performs both as switches and as amplifiers.
IV.Bio-materials (Biology)
 Nano materials are used as bone cement and bone plates in
hospitals.
 It is also used as a material for joint replacements.
 Nano technology is being used to develop miniature video
camera attached to a blind person’s glasses.
 Nano materials are also used in the manufacture of some
components like heart valves and contact lenses.
 Nano materials are also used in dental implants and breast
implants.
 CNTs are used as light weight shielding materials for
protecting electronic equipments against electromagnetic
radiation.