NANO MATERIALS
IRFANA B
NANOMATERIALS
The word “nano” derives from the Latin word “nanus”
indicating a person of very low height, i.e. a dwarf. The
International System (SI) of units considers nano as a prefix
to indicate 10−9 part of a unit.
Nanomaterials are conventionally defined as materials
having at least a dimension between 1 and 100 nm.
Nanomaterials are interesting because at the small scale,
materials have fundamentally different properties than at the
bulk due to increased surface area to volume ratios.
"There's Plenty of Room at the Bottom"
Richard Feynman, an American physicist, introduced the
concepts of nanotechnology in his talk "There's Plenty of
Room at the Bottom". He described a process where
scientists would be able to control and manipulate individual
atoms and molecules.
The term nanotechnology was first used in 1974 by
Japanese professor Norio Taniguchi as a contribution to
semiconductor processes and possible applications.
This interest led to the development of the scanning
tunnelling microscope in 1981 and physicists Binnig and
Rohrer were awarded the Nobel Prize in 1986.
Lycurgus Cup
The mysterious Lycurgus Cup is convincing evidence that ancient
Romans used nanotechnology, or at least knew how to get the
desired effects, long before the availability of modern technology.
The cup is made of a special type of glass known as dichroic,
meaning “two-colored” in Greek, which changes hue when held
up to the light. It is opaque green but turns to glowing translucent
red when light shines through it.
The Lycurgus Cup owes its unusual properties to the use of tiny
quantities of colloidal gold and silver. The rim of the cup is
mounted with a silver-gilt band of leaf ornament. Its type is known
as a “cage cup,” as it consists of a cage around the glass.
• The creators of these cups used gold nanoparticles to
produce the ruby glass and silver nanoparticles for the
green.
Types of Nanomaterials
Most current nanomaterials could be organized into four
types:
• Carbon Based Materials
• Metal Based Materials
• Dendrimers
• Composites
(i) Carbon based materials:
These are composed of carbon, taking the form of hollow
spheres, ellipsoids or tubes. The spherical and ellipsoidal
forms are referred as fullerenes, while cylindrical forms are
called nanotubes.
(ii) Metal based materials:
These include quantum dots, nanogold, nanosilver and metal
oxides like TiO2. A quantum dot is a closely packed
semiconductor crystal comprised of hundreds or thousands
of atoms, whose size is on the order of a few nanometers to
a few hundred nanometers.
(iii) Dendrimers:
Dendrimers are repetitively branched
molecules. The name comes from the Greek
word ‘dendron’ (tree). These nanomaterials are
nanosized polymers built from branched units.
The surface of a dendrimer has numerous chain
ends, which can perform specific chemical
functions. Dendrimers are used in molecular
recognition, nanosensing, light harvesting, and
opto-electrochemical devices. They may be
useful for drug delivery.
(iv) Composites:
Composites are combination of nanoparticles with other
nanoparticles or with larger, bulk-type materials.
Nanoparticles like nanosized clays are added to products
(auto parts, packaging materials, etc.) to enhance
mechanical, thermal, and flame-retardant properties.
CLASSIFICATION OF NANOMATERIALS
According to Siegel, nanostructured materials are classified
as: zerodimensional (0D), one-dimensional (1D), two-
dimensional (2D) and threedimensional (3D) nanomaterials.
(i) Zero-dimensional nanomaterials: Here, all dimensions (x,
y, z) are at nanoscale, i.e., no dimensions are greater than
100 nm. It includes nanospheres and nanoclusters. Metallic
nanoparticles including gold and silver nanoparticles and
semiconductor such as quantam 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
the1-50 nm range.
(ii) One-dimensional nanomaterials: Here, two dimensions (x,
y) are at nanoscale and the other is outside the nanoscale.
This leads to needle shaped nanomaterials. It includes
nanofibres, nanotubes, nanorods, and nanowires
(iii) Two-dimensional nanomaterials: Here, one dimension (x)
is at nanoscale and the other two are outside the nanoscale.
The 2D nanomaterials exhibit platelike shapes. It includes
nanofilms, nanolayers and nanocoatings with nanometre
thickness.
(iv) Three-dimensional nanomaterials: These are the
nanomaterials that are not confined to the nanoscale in any
dimension. These materials have three arbitrary dimensions
above 100 nm. The bulk (3D) nanomaterials are composed of
a multiple arrangement of nanosize crystals in different
orientations. It includes dispersions of nanoparticles, bundles
of nanowires and nanotubes as well as multinanolayers
(polycrystals) in which the 0D, 1D and 2D structural elements
are in close contact with each other and form interfaces.
Preparation of Nanoparticles
• Sol- Gel Method
• Hydrothermal Synthesis
• Combustion Flame-chemical Vapour Condensation Process
• Colloidal Precipitation
• Top-down approach
• Bottom-up approach
SOL- GEL METHOD
The sol-gel process, 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.
Hydrothermal synthesis
Hydrothermal synthesis includes the various techniques of
crystallizing substances from high-temperature aqueous solutions
at high vapor pressures; also termed "hydrothermal method". The
term "hydrothermal" is of geologic origin. Hydrothermal synthesis
can be defined as a method of synthesis of single crystals that
depends on the solubility of minerals in hot water under high
pressure. The crystal growth is performed in an apparatus
consisting of a steel pressure vessel called an autoclave, in which
a nutrient is supplied along with water. A temperature gradient is
maintained between the opposite ends of the growth chamber. At
the hotter end the nutrient solute dissolves, while at the cooler
end it is deposited on a seed crystal, growing the desired crystal.
Chemical vapor deposition (CVD)
Chemical vapor deposition (CVD) is a method for synthesizing
nanoparticles by using a chemical reaction to deposit a solid
material from a vapor onto a heated substrate.
The Chemical Vapour Deposition (CVD) technique is one of the
most suitable methods for the synthesis of carbon nanomaterials
(CNMs) in terms of product purity and large scale production. The
method is simple and economic for synthesizing at low
temperature and ambient pressure. CVD allows the experimenter
to avoid the process of separating nanotubes/ Nanofibres/
Nanobeads etc. from the carbonaceous particulate. CVD
reactions are strongly affected by the experimental parameters,
such as reactor temperature, pressure, precursor composition
catalyst and concentration.
Chemical vapor
deposition (CVD)
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 occurs.
Then the temperature of the hot wall reactor
results in the fertile production of nanosized
particles of metals and ceramics instead of thin
films as in CVD processing. In the simplest form
a metal organic precursor is introduced into the
hot zone of the reactor using mass flow
controller.
Combustion technique
A simple combustion technique is used for the precipitation of
nanoparticle sized LiBiO2 powder with urea as the igniter and
glycerol as the binding material.
Lithium nitrate and bismuth nitrate are mixed together to form
a uniform mixture. Required quantities of urea and glycerol
are added to this mixture to form a homogeneous paste. This
paste is preheated at 150-2000C to form dried mass which is
further calcined at 4600C for 5 hours to obtain LiBiO2
nanoparticles
Colloidal method
The colloidal method is one of the primary synthetic protocols
of nanomaterials, including metals and metal chalcogenides.
It is capable of controlling the size, composition, and
morphology of nanoparticles, which have been critical
parameters in the advancement of materials science.
Chemical reduction method
Chemical reduction is the most frequently applied method for
the preparation of silver nanoparticles as stable, colloidal
dispersions in water or organic solvents. Commonly used
reducing agents are borohydride, citrate, and elemental
hydrogen. The reduction of silver ions (Ag+) in aqueous
solution generally yields colloidal silver with particle
diameters of several nanometers. Initially, the reduction of
various complexes with Ag+ ions leads to the formation of
silver atoms, which is followed by agglomeration into
oligomeric clusters. These clusters eventually lead to the
formation of colloidal silver particles.
Top-down approach
The principle behind the top-down approach is to take a bulk
piece of the material and then modify it into the wanted
nanostructure and subsequent stabilization of the resulting
nanosized metal nanoparticles by the addition of colloidal
protecting agents. Cutting, grinding and etching are typical
fabrication techniques, which have been developed to work
on the nano scale. The sizes of the nanostructures which can
be produced with top-down techniques are between 10 to
100 nm.
Bottom-up approach
Bottom-up self assembly refers to construction of a structure
atom by atom, molecule-by-molecule or cluster-by-cluster.
Colloidal dispersion used in the synthesis of nanoparticles is
a good example of a bottom-up approach. An advantage of
the bottom-up approach is the better possibilities to obtain
nanostructures with less defects and more homogeneous
chemical compositions.
Fullerenes
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 inventor of the geodesic
dome.
The fullerene family includes a number of atomic Cn
clusters(n>20), composed of carbon atoms on a spherical
surface. Carbon atoms are usually located on the surface of
the sphere at the vertices of pentagons and hexagons. In
fullerenes, carbon atoms are usually present in the sp2-
hybrid form and linked together by covalent bonds. Fullerene
C60 is the most common and best-investigated fullerene.
Applications
Fullerenes (C60) and their derivatives have potential antiviral
activity, and may be used for the treatment of HIV-infection.
They have potential medicinal applications as they can bind
specific antibiotics and target certain types of cancer cells
such as melanoma.
They are used as biological antioxidants.
They are also used as potential photosensitizers in
photodynamic therapy and catalysts for hydrogenation.
Fullerenes incorporated with sulphides of tungsten and
molybdenum exhibit excellent solid-lubricant properties.
NANOTUBES
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.
According to their structure, CNTs may be classified into two
main groups: single-walled nanotubes (SWCNTs) and multi-
walled nanotubes (MWCNTs).
Generally SWCNTs have a diameter around 1–3 nm and a
length of a few micrometres. Multi-walled CNTs have a
diameter of 5–40 nm and a length around 10 μm. The
structure of CNTs leads to excellent properties with a unique
combination of rigidity, strength and elasticity compared with
other fibrous materials. For instance, CNTs show high
thermal and electrical conductivity compared to other
conductive materials
Electrical properties of SWCNTs depend on their chirality or
hexagon
orientation with respect to the tube axis. So, SWCNTs are
classified into three sub-classes:
(i) Armchair (electrical conductivity > copper).
(ii) Zigzag (semiconductive properties).
(iii) Chiral (semi-conductive properties).
By contrast, MWCNTs consisting of multiple carbon layers,
frequently with variable chirality, can exhibit extraordinary
mechanical properties instead of outstanding electrical
characteristics.
Applications
Carbon nanotube technology can be used for a wide range
of new and existing applications, which are as follows:
Nanotubes can potentially replace indium tin oxide in solar
cells to generate photocurrent.
SWNTs are used in transistors and solar panels. MWNTs
are used in lithium ion batteries to enhance cycle life.
Parallel CNTs have been used to create loudspeakers.
CNTs can serve as a multifunctional coating material.
CNTs can be used to produce nanowires.
Graphene
Graphene is a two-dimensional allotropic form of carbon,
formed by single layers of carbon atoms . In graphene,
carbon atoms exhibit sp2-hybridization connected by σ- and
π-bonds in a two-dimensional hexagonal crystal lattice with a
distance of 0.142 nm between neighbouring atoms of carbon
hexagons. Graphene also represents a structural element of
some other carbon allotropes, such as graphite, carbon
nanotubes and fullerenes.
• Graphene has many unique physical properties, such as
extremely high mechanical rigidity and a high thermal
stability. Also the electric properties of this carbon allotrope
are fundamentally different from the properties of three-
dimensional materials.
QUANTUM DOTS
Quantum dots (QDs) were first discovered by A. Ekimov in glass
matrix and by L. Brus in colloidal solutions .These are the
semiconductor nanoparticles between 10 and 100 atoms in
diameter. Quantum dots are only about 1/10,000th the width of a
human hair, and contain several hundred to several thousand
atoms, depicted as different coloured spheres. The properties of
QDs can vary depending on its shape and size. These are not all
uniform. In spite of having a variety of applications, QDs are a
source of toxic compounds containing in their core. The QDs
toxicity may be due to the leaching of toxic heavy metals from the
colloid form. The toxicity may also be originated from intrinsic
properties of the size and surface chemistry of quantum dots.
Such materials might have potential risks to human health but still
the use of these materials is growing quickly.
Applications
These are used in transistors, solar cells, diode lasers,
LEDs, etc.
These may increase the efficiency of silicon photovoltaic
cells.
These are also significant for optical applications like
amplifiers, biological sensors, etc. These are used as
photocatalysts.
They have potential applications in spectroscopy and
fluorescent biomedical imaging.
MCQ 1
The four types of nanomaterials are
a) Carbon-based, non-metallic, composites and ceramics
b) Carbon-based, metallic, composites and ceramics
c) Carbon-based, non-metallic, composites and dendrimers
d) Carbon-based, metallic, composites and dendrimers
MCQ 2
Solution of pure Buckminster fullerene has a colour of
a) Green b) Purple
c) Pink d) Yellow
MCQ 3
Nano sized polymers built from branched units are called
a) Dendrimers b) Composites
c) Carbon-based materials d) Metal-based materials
MCQ 4
The colour of the nano gold particles is
a) Yellow b) Orange
c) Red d) Variable
MCQ 5
The polymeric dendrimer nanoparticles come under _______________
dimensional nanomaterials?
A) Zero B) One
C) Two D) Three
