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 dimensions of the order of a

billionth of meter.

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 (3000C) for

2.5 nm, but Gold slab melts at 1064OC.

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, nano

particles/cluster, bulk materials

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 nano-

materials are different from bulk forms. The change in optical

properties is caused by two factors

The quantum confinement of electrons within the nano-

particles increases the energy level spacing.

The optical absorption peak of a semiconductor nano-

particles 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 a few

thousands of atoms with in each grain.

A large number of atoms reside at the grain

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

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 non-

volatile 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 nano-

materials 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.

1. Many geometries including thin film, bulk powder,

single crystals can be prepared.

2. Thermodynamically stable novel materials can also be

prepared easily.

Uses

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 properties

of composites.

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-to-

medium 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 (OR)

NANO PARTICLES

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 nano-

particles have an appreciable fraction of their atom at the surface,

its catalytic activity is good.

Bulk gold is chemically inert, where as gold nano-

particles 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.