1

NANOMATERIALS
INTRODUCTION
The fascinating and challenging areas of study in the fields of Basic Sciences and Engineering,
nowadays is the progress and numerous advancements that science has made in the field of
Nanotechnology.
The word “nano” is derived from a Greek word meaning dwarf or extremely small and means
a billionth (10–9) part of a unit. A nanometre or nm is one thousand millionth of a metre, i.e.,
1 nm = 10–9 m = 10–3 μm = 10 Å.
Nanoscience and Nanotechnology shows great potential for providing mankind in the near future
with many discoveries that will revolutionise all the technological advancements in science.
Nanoscience is defined as the study of phenomena and manipulation of materials at atomic,
molecular and macromolecular scales, where properties differ significantly from those at a
larger scale.
To understand this new technology, it is important to get a basic idea what nanoscience is about;
what nanomaterials are; their different physical and chemical properties; how they can be
produced artificially; their applications and impact on society. This chapter on Nanoscience
gives a broad overview and insight into this relatively new field of science covering all the
above-mentioned aspects
Nanotechnology deals with various structures of matter having dimensions of the order
of a billionth of a meter. It refers to the control and manipulation of matter at nanometre
dimensions.
The techniques involved in the preparation, characterisation and use of nanomaterials in
different applications is called Nanotechnology.
It is the study of design, characterisation, production and application of structures, devices
and systems by controlling shape and size at the nanometer scale.

The concept of nanotechnology was first highlighted by Nobel Laureate, Richard

Feynman (1959) in his lecture “There is plenty of room at the bottom”. The term
‘Nanotechnology’ was first coined by Norio Taniguchi in 1974.

NANOMATERIALS
Nanomaterials are defined as a set of substances where at least one dimension is less than
approximately 100 nanometres.

What do you mean by word Nano?

The word “nano” is derived from a Greek word meaning dwarf or extremely small and
means a billionth (10–9) part of a unit. A nanometer or nm is one thousand millionth of a
metre, i.e., 1 nm = 10–9 m = 10–3 μm = 10 Å.

In order to have an idea about nanosize, a human hair thickness accommodates around 50000
nanometers
 2

CLASSIFICATION OF NANO MATERIALS

Nanomaterials can be classified on the basis of origin, dimensions and their structural
configuration.
I. According to their origin, nanomaterials are classified as:
 Natural nanomaterials: Natural nanomaterials examples are Viruses, Protein molecules
etc. Few other examples are minerals such as clays, natural colloids, such as milk and
blood (liquid colloids), fog (aerosol type), gelatin (gel type),mineralized natural materials
such as shells, corals and bones, insect wings and opals, spider silk, lotus leaf ,Gecko’s
feet, volcanic ash, ocean spray etc.
 Artificial nanomaterials: Artificial nanoparticles are those which are prepared by well-
defined mechanical and fabrication process. The examples of such materials are carbon
nanotubes, semiconductor nanoparticles like quantum dots etc.

II. On the basis of the dimensions, nanomaterials also can be divided into zero dimensional,
one dimensional, two dimensional and three- dimensional nano materials as shown in figure

Fig. Different dimensional Nanomaterials (a) 0-D spheres and clusters, (b) 1-D nanofibers, wires and rods, (c) 2-D
films, networks, (d) 3-D nanomaterials.

 Zero dimensional(0-D):In these nanomaterials, all the dimensions are measured within
the nanoscale. Metallic nanoparticles including gold and silver nanoparticles and
semiconductor such as quantum dots are the perfect example of this kind of
nanoparticles. Most of these nanoparticles are spherical in size and the diameter of these
particles will be in the range of 1-50 nm.
 One dimensional(1-D):In these nanostructures, one dimension of the nanostructure will
be outside the nanometer range. These include nanowires, nanorods, and nanotubes.
These materials are long (several micrometer in length), but with diameter of only a few
nanometers.
 Two dimensional(2-D):In this type of nanomaterials, two dimensions are outside the
nanometer range. These include different kinds of nano films such as coatings and thin-
film-multilayers, nano sheets or nano-walls. The area of the nano films can be large
(several square micrometer), but the thickness is always in nano scale range.
 Three Dimensional(3-D): All dimensions of these are outside the nano meter range.
These include bulk materials composed of the individual blocks which are in the
nanometer scale (1-100 nm), dispersions of nanoparticles, bundles of nanowires and
nanotubes as well as multi-nanolayers etc.
 3

III. On the basis of structural configuration, nanomaterials can be classified into four types:
 Carbon Based Nano materials: These nanomaterials are hollow spheres, ellipsoids, or
tubes. Spherical and ellipsoidal carbon nanomaterials are called as fullerenes, while
cylindrical ones are described as nanotubes.
 Metal Based Materials: The main component of these particles is metal. These
nanomaterials include nanogold, nanosilver and metal oxides, such as titanium dioxide
and closely packed semiconductors like quantum dots.
 Dendrimers: Dendrimers are highly branched macromolecules with the dimensions in
nanometer-scale.
 Composites:The most common examples of these materials are colloids, gels and
copolymers.
 4

SYNTHESIS OF NANOMATERIALS
Nanomaterials can be synthesized by two main approaches: -top- down and bottom- up
approach.

Top- down approach:

 A top-down approach where very small structures are produced from larger(Bulk)
pieces of material.
 It includes slicing or successive cutting of a bulk material to get nanosized particles.
 Synthesis of nanomaterials on large scale production can be easily done.
 Chemical purification is not required.
 It is an expensive method where deposition parameters cannot be controlled. Particles
of different sizes are obtained and impurities can be introduced during synthesis.
 For example,Ball milling method, Electrochemical etching and Photo-Lithography.
Bottom- up approach
 A bottom-up approach can be viewed as a synthesis approach where the building
blocks (atoms or molecules) are added onto the substrate to form the nanostructures.
 Bottom-up approach is building up of a material from the bottom i.e., atom by atom,
molecule by molecule or cluster by cluster.
 In this approach of synthesis of nanomaterials large scale production is difficult.
 Chemical purification is required.
 It is comparatively less expensive method where deposition parameters can be
controlled. Ultra-fine Particles are obtained and impurities are not introduced during
synthesis.
 For example, Chemical Vapor deposition method, Physical Vapor Deposition and Sol
Gel method.
Both the approaches play an important role in synthesis of nanomaterials.

Fig.6.2: Synthesis of nanomaterials by top down and bottom up approach

 5

Synthesis of Nanomaterials
Bottom- up Approach Top- down Approach

1. Building up of nanomaterials 1. Process of miniaturizing

from the bottom: atom–by- or breaking down bulk
atom, molecule-by-molecule materials into nanosized
or cluster-by-cluster. structures by mechanical
2. Method used are Sol-gel deformation produced by
method, Hydrothermal ball milling.
synthesis. 2. Methods used are ball-
3. There are less chances of milling, lithography.
contamination. 3. The nanomaterials
prepared by this process
may be contaminated by
tools and atmosphere.

BALL MILLING METHOD OF SYNTHESIS OF NANOMATERIALS: (Learn only

this method)
Principle:Tiny rigid balls, usually, ceramic, flint pebbles and stainless steel,in a concealed
container collide with each other to generate high pressure. The bulk material placed in this
container is then crushed into nano crystal.
Construction and Working: Hardened steel or tungsten carbide balls are put in a container along
with powder of particles (50µm) of a desired material as shown in figure 6.6. The container is
closed with tight lids. When the container is rotating around the central axis, the material is
forced to press against the walls. The milling balls impart energy on collision and produce
smaller grains of nano-meter sizes.This process is used in producing metallic and ceramic nano
materials.These mills are equipped with grinding media composed of wolfram carbide or steel.

Fig.6.6: Ball milling Process

Advantages
 The significant advantage of this method is that it can be readily implemented
commercially.
 Ball milling can be used to make carbon nanotubes and boron nitride nanotubes.
 It is a preferred method for preparing metal oxide nano crystals like Cerium(CeO2) and
Zinc Oxide (ZnO).
 6

SOL-GEL PROCESS OF SYNTHESIS OF NANOMATERIALS: (Learn only this

method):
Sol Gel processis a wet chemical technique that uses sol to produce integrated network
called gel. ‘Sol’ is a colloidal or molecular suspension of solid particle of ions in a solvent while
‘Gel’ is the semi rigid mass that is formed when solvent from sol evaporates and the particles or
ions left behind join together in a continuous network.On drying the liquid, it is possible to
obtain powders, thin films etc.
Sol can be obtained by following four steps:
 Hydrolysis of metal alkoxides and metal chlorides which act as precursors.An alkoxide is
the conjugate base of an alcohol and therefore consists of an organic group bonded to a
negatively charged oxygen atom. They are written as RO−, where R is the organic
substituent.
 Condensation
 (Growth of particles) Polymerization of monomers to form colloids (Sol).
 Agglomeration of Colloids to form Gel.
Precursors (starting chemicals) are to be chosen such that they have a tendency to form
gels. Rate of hydrolysis and condensation reactions are governed by various factors such as
pH,temperature,Molar ratio, nature of precursors, concentration of catalyst and process of
drying.In proper conditions spherical nanoparticles are produced.An ‘aerogel’ is obtained when
the liquid phase of a gel is replaced by a gas (CO2) in such a way that its solid network is
retained, with only a slight or no shrinkage in the gel. A ‘Xerogel’is obtained when the liquid
phase of a gel is removed by evaporation.
Advantages
1. Sol–gel process produces very pure and homogeneous nanostructures, with relatively large
quantities of final product at low cost at relatively low temperatures.
2. Reagents are very simple compounds.
3. Special equipment is not required (borosil/pyrex containers are used).
 7

Fig.6.7: Sol-Gel process

Industrial application:

FULLERENE

The first fullerene was discovered by Harold Kroto, Richard Smalley and Robert Curl in 1985 by
using a laser to vaporise graphite rods in an atmosphere of helium gas. The fullerenes (allotropes of
carbon) are graphene sheets rolled into tubes or spheres. It is a cage like molecule composed of 60
carbon atoms (C60) joined together by single and double bonds to form a hollow sphere with 20
hexagonal and 12 pentagonal faces (a design that resembles a football). It was named as
buckminsterfullerene or buckyball after the name of American architect Buckminster Fuller. The
structure of fullerene (C60)
 8

Properties of Fullerenes:
 They are insoluble in water, but soluble in organic solvent. C-60 is soluble in benzene.
 They exhibit excellent tensile strength and very high packing density.
 C-60 is non-toxic and a powerful antioxidant. In living cells, it react with free radicals
responsible for the damage.
 They possesses cage like structure.
 C-60 crystal is an insulator but when dopped with an alkali atom, it become conducting.
 C-60 crystal doped with potassium becomes superconductor at 15K.

Applications of Fullerenes:
 Magnetic nanoparticles show great potential for high density magnetic storage media.
 C-60 dispersed into ferromagnetic materials such as iron, cobalt can form thin films with
promising magnetic properties.
 A number of organometallic fullerene compounds have recently been synthesized which are
being used in industry.
 Fullerenes are used as small ball bearings for lubrication.
 They can be used as drug deliverers by attachment of fundamental ligands to their carbon
cage.
 They can be used as tracers molecules by trapping lanthanide metals inside their cage.
 They are impervious to lasers and can be used as defensive measures in war.
 Preparation of superconductors.

Carbon Nanotubes (CNT)

The carbon nanotubes (elongated form of fullerenes) were identified in 1991 by Iijima Sumio of
Japan. A carbon nanotube is a tube-shaped material, made up of carbon, having a diameter ranging
from < 1 nm to 50 nm. Simply we can say, carbon nanotubes (CNTs) are cylinders of one or more
layers of graphene (lattice). Carbon nanotubes show a unique combination of stiffness, strength, and
tenacity compared to other fibre materials. Thermal and electrical conductivity are also very high as
comparable to other conductive materials. Carbon nanotubes may be categorized as follows:
 9

 Single-wall nanotubes (SWNT):

Single-wall nanotubes (SWNT) are tubes of graphite that are normally capped at the ends. They have
a single cylindrical wall. The structure of a SWNT can be visualized as a layer of graphite, a single
atom thick, called graphene, which is rolled into a seamless cylinder

 Multi wall carbon nanotubes (MWCNT):

There are two structural models of multi wall nanotubes. In the Russian Doll model, a carbon
nanotube contains another nanotube inside it (the inner nanotube has a smaller diameter than
the outer nanotube). In the Parchment model, a single graphene sheet is rolled around itself
multiple times, resembling a rolled up scroll of paper.

Properties of CNT :

i) CNT are extremely strong, about 100 times stronger than steel.
ii) CNT can also act as either conductors of semiconductors depending on their chirality.
iii) They can act as superconductors and also behave as field emitters.
 10

iv) They possess very low density about half of density of aluminium metal
v) They possess high elasticity and high thermal conductivity almost twice as that of diamond.
vi) They have high current density and high aspect ratio i.e. length to diameter ratio.

Applicantions of CNTs :

i) CNTs are now the top candidates to replace silicon based semiconductors as they have variable
semiconducting properties with energy gaps ranging from a few milli electron volt(meV) to few tenth
of an electron volt(eV).
ii) Nanotubes have led to a new generation of electronic devices.
iii) Their use as ultra sensitive electromechanical sensors has also been explored.
iv) They are used in designing of field-effect transistors and integrated electronics devices.
v) They are used in designing chemical and radiation sensors.
vi) They are used in composites and manufacture of fuel cells.

COMPARISON OF PROPERTIES OF NANOMATERIALS WITH BULK

MATERIALS
Nanomaterials have numerous potential applications because at this scale they have unique
optical, magnetic, electrical, and other propertieswhich are quite different from that of bulk
materials. These properties are:
 Colour: Some materials show the different colors when they are converted to nanometer
scales. For e.g., when the gold materials are converted to nanomaterials they turn into red
color. Gold nanoparticles absorb the blue-green portion of the spectrum (~450 nm), while
red light (~700 nm) is reflected, producing a rich red color.
 Melting point: The melting point drastically falls when the particle size of the material
approaches to the nanoscale ranges. Decrease in the melting point is mostly clear in
nanowires, nanotubes and nanoparticles, which all melt at lower temperatures than bulk
form of the same material. Changes in melting point occur because nanoscale materials
have a much larger surface to volume ratio than bulk materials, significantly changing
their thermodynamic and thermal properties.
 Mechanical strength: The mechanical strength of nanomaterials may be one or two
times higher in magnitude than that of single crystals in the bulk form. Conversion of
materials into nanoscale increases crystal perfection or reduction of defects, which would
result the increase in mechanical strength. Cutting tools which should be harder than the
material which is to be cut are made of nano materials, such as tungsten carbide, tantalum
carbide, and titanium carbide. These cutting tools are much more erosion-resistant and
durable than their conventional large, grained bulk materials.
 Electrical properties of the nanomaterials: Surface scattering phenomenon is highly
increased due to reduction of particle size hence there is increase of the total resistivity in
nanomaterials compared to bulk. In addition, a reduction in particle size below a critical
dimension, i.e. (electron de Broglie wavelength), results in a modified electronic structure
with wide and discrete band gap. The reduction of particle size into this range results in
 11

an increased electrical resistivity.

 Optical Properties.
The properties like colour and transparency are considered as optical properties. These
properties are observed to change at nanoscale level. For example, bulk gold appears
yellow in colour while in the nanosize, gold appears red in colour.Bulk silicon appears
grey in colour while nanosized silicon appears red in colour. Zinc oxide, at bulk scale
blocks ultraviolet light and scatters visible light and gives white appearance. While on the
other hand, nanoscale zinc oxide is very small in particle size compared with wavelength
of visible light and it does not scatter it. Thus, it appears transparent.
 Magnetic properties
Magnetic properties of nanoparticles are used for drug delivery, therapeutic treatment,
contrast agents for MRI imaging, bioseparation, and in-vitro diagnostics. These nanometer-
sized particles are superparamagnetic, a property resulting from their tiny size—only a few
nanometers—a fraction of the width of a human hair (nanoparticles are approximately
1/1,000 thinner than human hair).
Superparamagnetic nanoparticles are not magnetic when located in a zero magnetic field,
but they quickly become magnetized when an external magnetic field is applied. When
returned to a zero magnetic field they quickly revert to a non-magnetized state.
Superparamagnetism is one of the most important properties of nanoparticles used for
biomagnetic separation.Magnetic nanoparticles are usually based on magnetite and/or
maghemite, two different forms of iron oxide.When they are below the superparamagnetic
diameter the nanoparticles are able to return quickly to a non-magnetized state after an
external magnet is removed. Larger ferro- and ferrimagnetic materials have remnant
magnetism after the applied magnetic field returns to zero.

The reasons for the change in their properties are:

 Large Surface Area to volume ratio:
Nanomaterials have a relatively larger surface area when compared to the same
mass of material produced in a bulk. When a bulk material is reduced to the particle of
smaller size, then the surface to volume ratio becomes very high. As the surface of the
particle is involved in chemical reactions, the larger surface area can make materials
more reactive. For e.g. grains of salt dissolve in water more quickly than a rock of salt.
Why are the properties of Nano-materials different
from bulk-materials?
Reasons:1. Large Surface Area to volume ratio
• When a bulk material is reduced to the particle of smaller size, then the
surface to volume ratio becomes very high.
• As the surface of the particle is involved in chemical reactions, the
larger surface area can make materials more reactive.
 12

• For example, grains of salt dissolve in water more quickly than a rock
of salt.

8/9/2021

 Quantum confinement effect

In the bulk matter, the bands are actually formed by the merger of adjacent energy levels
of a large number of atoms and molecules. As the particle size reaches the nano scale, the
number of overlapping orbitals or energy levels decreases and the width of the band
gets narrower. This will cause an increase in energy gap between the valence band
and the conduction band. Therefore, there is a high energy gap in nanoparticles than the
corresponding bulk material. The band gap is the region forbidden for the electrons. The
larger the forbidden region, the greater will be restriction on the movement of electrons.
The electrons are thus confined in space and are not free to move. This effect is thus
called quantum confinement. This affects the optical, electrical and magnetic behavior
of materials at nano-scale.
Quantum confinement effect

11