Classification (or) Types of Nano Materials

CLASSIFICATION (OR) TYPES OF NANO MATERIALS

Based on dimensions, nano materials are classified in to four types.

1. Nanoparticles

2. Nanoclusters

3. Nanowire

4. Nanorods

5. Nanotubes

1. Nanoparticles

Nanoparticles are the particles, the size of which ranges from 1 to 100 nm. These are tiny aggregates of atoms but smaller than their crystals but bigger than molecules.

Examples

TiO₂, gold, silver, ZnO, etc.,

Properties of nanoparticles

1. Nanoparticles have anti‒reflective properties, so they can be used as a hard coatings on eye glasses.

2. Hard nanoparticles increase the reinforcement of soft polymers to stronger plastics.

3. Nanoparticles are manipulated by the use of magnetic field, so self‒agglomeration can be prevented.

4. Absorption of solar radiation in photovoltaic cell is much higher in nanoparticles than in thin film of bulk materials.

5. Nanoparticles have enhanced diffusion at elevated temperatures. Due to this property, sintering occurs at lower temperature than the larger particles.

Applications of Nanoparticles

1. TiO₂ is used in cosmetics as they are very good UV absorber.

2. Nano silver particles are used as a catalyst in industries.

3. Nanoparticles are used in medicine.

4. Nano silver particles are used in making bone cement, surgical instruments, etc.,

2. Nanoclusters

Nanoclusters are fine aggregates of atoms (or) molecules. The size of which ranges from 0.1 to 10 nm. Of all the nano materials, nanoclusters are the smallest sized nano materials because of their close packing arrangement of atoms.

Examples: CdS, ZnO, etc.,

All the atoms, in nanocluster, are bound by forces like metallic, covalent, ionic, hydrogen bond (or) Vander Waals forces of attraction. Clusters of certain critical size are more stable than others. Nanoclusters consisting of up to a couple of hundred atoms, but larger aggregates, containing 10³ (or) more atoms, are called nanoparticles.

Magic number

Magic number is the number of atoms present in the clusters of criticle sizes with higher stability.

Different types of nanoclusters can be distinguished from the nature of forces present between atoms. Generally clusters containing transition metal atom have unique chemical, electronic and magnetic properties. These properties vary with the number of constituent atoms, the type of element and the net charge on the cluster.

Properties of nanoclusters

1. Atomic clusters (or) molecular clusters are formed by the nucleation of atoms (or) molecules respectively.

2. The reactivity of nanoclusters are decreased due to their decrease in size.

3. The melting point of nanoclusters are lower than the bulk materials due to high surface to volume ratio.

4. The electronic structure of the nanocluster is more confined than the bulk materials.

Applications of nanocluster

1. Nanoclusters are used as catalysts in many reactions.

2. It is used in nano based chemical sensors.

3. It is also used as a light emitting diode in quantum computers.

3. Nanorods

Nanorod is one dimensional cylindrical solid material having an aspect ratio i.e., length to width ratio less than 20.

Examples

Zinc oxide, Cadmium sulphide, Gallium nitride nano rods.

Synthesis of nanorods

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.

This allows different nanorods to grow at different rates producing an elongated objects. Many of the above nanorods are not manufactured due to lack of commercial demand.

Properties of nanorods

1. Nanorods are one‒dimensional materials.

2. It exhibits optical and electrical properties.

Applications of nanorods

1. Nanorods find application in display technologies.

2. It is also used in the manufacturing of micro mechanical switches.

3. Nanorods are used in an applied electric field, micro electro mechanical systems, etc.,

4. Nanorods along with noble metal nanoparticles function as theragnostic agents.

5. They are used in energy harvesting and light emitting devices.

6. Nanorods have used as cancer therapeutics.

4. Nanowires

Nanowire is one dimensional cylindrical solid material having an aspect ratio ie., length to width ratio greater than 20. Diameter of the nanowire ranges from 10 ‒ 100 nm.

Examples : Different types of nanowires

Types of nanowires

1. Metallic nanowires‒ Examples: Au, Ni, Pt

2. Nanowires of semiconductors ‒ Examples: InP, Si, GaN

3. Nanowires of insulators ‒ Examples: SiO₂, TiO₂

4. Molecular nanowires ‒ Examples: DNA

Synthesis of Nanowires

1. Template‒assisted synthesis

Template assisted synthesis of nanowires is a 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.

2. VLS (Vapour Liquid ‒ Solid) method

It involves the absorption of the source material from the gas phase into a liquid phase 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.

Properties of nanowires

1. Nanowires are one‒dimensional material.

2. Conductivity of a nanowire 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 nanowires show strong photoluminescence characteristics.

Uses of nanowires

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) as patterned storage media.

5. Nanowires replace conventional copper wires used in computers, televisions.

6. It is also used to link tiny components into very small circuits.

5. Nanotubes

Nanotubes are tube like structures with diameter of 1‒100 nm and a length of few nm to microns. Nanotubes consist of tiny cylinders of carbon and other materials like boron nitride. Nanotubes may be organic (or) inorganic.

Examples

1. Carbon nanotube

2. Silicon nanotube

3. DNA nanotube

4. Boron nitride nanotube

1. Carbon nanotubes (CNTs)

Carbon nanotube is a tubular form of carbon with 1‒3 nm diameter and a length of few nm to microns.

Generally carbon in the solid phase exits in different allotropic forms like graphite, diamond, fullerence and nano tubes.

Carbon nanotubes are tubular forms of carbon. When graphite sheets are rolled into a cylinder, their edges join to each other form carbon nanotubes. Each carbon atom in the carbon nanotubes is linked by covalent bonds. But the number of nanotubes align into ropes and are held together by weak Vander Walls forces.

Structures (or) types of carbon nanotubes

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).

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 μm. 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.

Synthesis (or) Fabrication of Carbon Nanotubes

Carbon nanotubes can be synthesized by the following methods.

1. Pyrolysis of hydrocarbons.

2. Laser evaporation.

1. Pyrolysis

Carbon nanotubes are synthesized by the pyrolysis of hydrocarbons such as acetylene at about 700°C 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 (1200°C) in a quartz tube reactor. An inert gas such as argon (or) helium 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.

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.

5. It also acts as an efficient catalysts for some chemical reactions.

6. It acts as a very good biosensor. Due to its chemical inertness carbon nanotubes are used to detect many molecules present in the blood.

7. It is also used in water softening process as a filter.