Nano Materials

DEFINITIONS Nanochemistry is the science of

tools, technologies and methodologies for chemical synthesis, analysis and biochemical diagnostics, performed in nanolitre to femtolitre domains.

Nanoparticles are the particles within the siqe ranging from 1-50 nm.

Nanomaterials are the materials having components within size less than 100 nm atleast in one dimension.

Nanomaterials in one dimension are layers like thin films or surface coatings.

Nanomaterials in two dimensions are tubes like nanotubes and nanowires.

Nanomaterials in three dimensions are particles are particles like precipitates, colloids and quantum dots.

Nanowires are considered as building blocks for the next generation of electronics, photonics, sensors and energy applications.

A quantum dot is a particle having an approximate size of one nanometer which has the display properties of a semiconductor.

The size of a quantum dot, one-billionth of a meter, can cause it to exhibit unusual properties that are not present in larger samples of a semiconductor material.

The key to the quantum dot is in the electrons.

Electrons occupy one of tow bands in a material’s crystal.

As it moves from one band to other it creates a hole which is a positive charge, together the hole and the electron are called exciton.

The electron and hole in the exciton keep their distance from each other. This is called the Exciton Bohr Radius.

The quantum dot has a number of different applications.

Much like fibre optics, quantum dots may also be used to transmit data.

TYPES OF CARBON NANOTUBES

Carbon nanotubes are lattice of carbon atoms, having each carbon is covalently bonded to there other carbon atoms.

Depending upon the way in which they are arranged, there are two types of CNT’s.

A single walled nanotubes. (SWNT’s)

A multi walled nanotubes. (MWNT’s)

Single walled nanotubes Most single walled nanotubes have a

diameter of close to 1nm, with a tube length that can be many millions of times longer.

There are three types of SWNT’s based on the way the graphene sheet is wrapped.

If m=0, the nanotubes are called “zigzag”

If n=m, the nanotubes are called “armchair”

Otherwise, they are called “chiral”

Multi walled nanotubes Multi walled nanotubes consist of multiple rolled

layers of graphite. There are two models which can be used to

describe the structures of multi-walled nanotubes:

In the Russian Doll model, sheets of graphite are arranged in concentric circles.

In the Parchment Model, a single sheet of graphite is rolled in around itself, resembling scroll of parchment or a rolled newspaper.

SYNTHESIS OF CARBON TUBES Carbon tubes are generally produced by

three main techniques, arc discharge, laser ablation and chemical vapour deposition.

At the moment, laser ablation produces a small amount of clean nanotubes, whereas arc discharge methods generally produce large quantities of impure material.

In general, chemical vapour deposition (CVD) results in MWNT’s or poor quality SWNT’s.

Arc discharge The arc discharge was the first

available method for the production of both MWNTs and SWNTs .

MWNTs can be produced in a carbon arc apparatus similar to the one depicted below using the method described by Ebbesen and Ajayan. An arc is struck between two graphite electrodes in a gas atmosphere. MWNTs produced by arc discharge are long and straight tubes closed at both ends with graphitic walls running parallel to the tube axis.

Laser ablation In this set-up the flow tube

is heated to ~1200°C by a tube furnace.

Laser pulses enter the tube and strike a target consisting of a mixture of graphite and a metal catalyst such as Co or Ni.

SWNTs condense from the laser vaporization plume and are deposited on a collector outside the furnace zone.

Chemical vapour deposition

For the production of MWNTs acetylene is usually used as source of carbon atoms at temperatures typically between 600 – 800°C.

To grow SWNTs the temperature has to be significantly higher (900 – 1200°C) due to the fact that they have a higher energy of formation.

In this case carbon monoxide or methane must be used because of their increased stability at higher temperatures as compared to acetylene.

PROPERTIES OF CARBON NANOTUBES

Mechanical Properties: Strength: The strength of sp2 carbon-carbon bonds gives

carbon nanotubes amazing mechanical properties. Because of their hollow structure and high aspect ratio the tend to undergo buckling when placed under compressive, torsional or bending stress. These properties coupled with the lightness of carbon , give them great potential in applications such as Aerospace.

Hardness: One study succeeded in the synthesis of a super-hard

material by compressing SWNT’s to above 25 Gpa at room temperature.

Electrical Properties: Carbon nanotubes can be metallic or

semi conducting depending on their structure.

This depends on the symmetry and unique electronic structure of graphene. For a given nanotube, if n=m, the nanotube is metallic; if n-m is a multiple of 3, then the nanotube is semiconducting with a moderate semiconductor. Based on their electrical properties nanotubes are used in flat-panel displays, scanning probe microscopes and sensing devices.

Vibrational Properties: Atoms in CNT are continuously

vibrating back and forth. They have two normal modes of vibrations.

• A1g mode: It involves ‘in’ and ‘out’ oscillation.

• E2g mode: Here squashing of the tube takes place. The frequencies of two modes of vibrations are Raman active.

ENGINEERING APPLICATIONS

The small dimensions, strength and the remarkable physical properties of these structures make them a unique material with a whole range of promising applications.

• They find application in conductive and high strength composites; energy storage and energy conversion devices.

• They are used as nanoprobes in meteorology and biological and chemical investigations and as templates for the creation of other nanostructures.

Applications in Fuel Cells: Hydrogen can be stored in the carbon

nanotube, which is in turn used for the fuel cells.

Carbon nanotubes can replace platinum as the catalyst in fuel cells, which could significantly reduce fuel cell’s overall cost.

The nanotube networks form the fuel cell’s gas diffusion electrode, a layer of a porous material that allows gas and water vapour to pass through to the catalyst area. In the catalyst layer, which typically consists of platinum particles, the protons and electrons of the gaseous reactant material.

Applications in catalysis A catalyst having CNT’s makes a reaction

milder, safer and more selective. CNT’s are increasingly recognized as

materials for catalyst, either as catalyst themselves, as catalysts additives or as catalysts supportive.

The tightly packed, vertically alligned carbon nanotubes doped with nitrogen are used as cathodes in highly alkaline solution, to catalyze the reduction of oxygen more efficiently than platinum.

Applications in medicine Carbon nanotubes are being highly used in the

fields of efficient drug delivery and biosensing methods for disease treatment and health monitoring.

Carbon nanotubes can be used as multifunctional biological transporters and near-infrared agents for selective cancer cell destruction.

CNT’s as drug delivery vehicles gave shown potential in targeting specific cancer cells with a dosage lower than conventional drugs used, does not harm healthy cells and significantly reduces side effects.

FULLERENES

A third newly discovered allotrope of carbon is Buckminster fullerend named in the honour of the American architect Buckminster Fuller, the designer of geodesic dome.

The shape of C60 resembles with the domes designed by Fuller.

Fullerenes were discovered as an unexpected surprise during laser spectroscopy experiments at Rice University in September 1985.

The 1996 Nobel prize in Chemistry was awarded to Professors Robers F. Curl, Jr., Richard E. Smalley, and Sir Harold W. Kroto for their discovery.

TYPES OF FULLERENES Fullerenes are a family of carbon

allotropes, molecules entirely composed of carbon, in the form of hollow, sphere, ellipsoid, tube or plane.

Spherical Fullerene: They look like a soccer ball, and are often called bucky balls.

Cylindrical Fullerene: These are called carbon nanotubes or buckytubes.

Planar Fullerene: Graphene is an example of planar fullerene sheet.

PREPARATION Fullerenes are prepared by

vaporizing a graphite rod in a helium atmosphere.

Mixtures of fullerenes like C60 C70 etc., are formed which are separated by solvent extraction. C60 is isolated from this mixture by column chromatography using alumina/hexane as a solvent system.

STRUCTURE Fullerenes are a class of closed-cage

carbon molecule, Cn , characteristically containing 12 pentagons and a variable number of hexagons.

C60 is a spherical crystal of carbon atoms with an arrangement of even number of sp2 atoms over the surface of a closed hollow cage.

The C60 molecule has a truncated icosahedron structure.

PROPERTIES The bucky ball has certain unique properties: It exists as a discrete molecule. It is moderately soluble in the common organic

solvents, specially aromatic hydrocarbons. It dissolves in benzene forming a deep

magenta solution. It is very tough. It can be accelerated to

1500miles/hour and can be pushed against a tough surface without damaging it.

It can be compressed to lose 30% of its volume without destroying its cage like structure.

DERIVATIVES OF FULLERENES

Fullerenes readily participate in a number of chemical reactions. Its reactivity is very good; therefore a number of fullerene derivatives can be prepared:

Up to 24 methyl groups can be added to C60 polyanions.

Polychlorinated and polybrominated derivatives have been produced.

Charge transfer complexes such as [Fe(C2H5)2]2C60 and exohedral organometallic derivatives (Os, Pt, In) are some of the good examples of fullerene derivatives.

Super conducting fullerides:

C60 is a poor conductor of electricity. When fullerene reacts with good electron donors like alkali metals, its conductivity increases. Alkali-fullerene compounds such as K3C60 are superconducting.

This compound has critical temperature (Tc) 19.2K.

The potassium atoms occupy the octahedral sites in the cubic cell.

MAGNETIC DONORS Organic electron donors like tetrakis

dimethyl amino ethylene (TDAE) reduces C60 and form (TDAE)C60. it is ferromagnetic solid with a acurie temperature of 16K.

ENGINEERING APPLICATIONS

Fullerenes can easily accept electrons, therefore, they may be used as charge carrier in batteries.

Fullerene can be used as organic photovoltaics. Researchers have found that water-oluble

derivatives of fullerenes inhibit the HIV-1 protease and are therefore useful in fighting the HIV that leads to AIDS.

They are used as catalyst as they a have marked ability to accept and to transfer hydrogen atoms; hydrogenation and hydro-dealkylations. They are highly effective in promoting the conversion of methane into higher hydrocarbons. They inhibit coking reactions.

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