NANOMATERIALS

All materials are composed of grains, which in turn comprise many atoms. These grains can be visible or invisible to the naked eye, depending on their size. Conventional materials have grains varying in size anywhere from hundreds of microns to centimeters. Nanomaterials, sometimes called nanopowders when not compressed, have grain sizes in the order of 1-100 nm in at least one coordinate and normally in three.

Size of nano

1 nm = 10-3 μm = 10-6 mm = 10-9 m = 10-9 yard

Classification:-

Nanomaterials can be classified in several ways, based on

(1) Their origin

(2) Based on phase composition

(3) Based on dimensions

Based on their origin, nanomaterials are broadly classified as

(a) Natural nanomaterials and (b) Artificial nanomaterials

Natural nanomaterials are those which are obtained naturally.

Examples:- Carbon-nanotubes and fibers

Artificial nanomaterials are those which are synthesized in laboratories

Examples:- Au/Ag np system and Gold nanoparticles, Polymeric nanocomposites.

Table :- Classification of nanomaterials with regard to different parameters

Dimension / Examples
3 dimensions<100 nm
2 dimensions< 100 nm
1 dimension< 100 nm / Particles, quantum dots, hollo spheres, etc.
Tubes, fibers, wires, platelets, etc.
Films, coatings, multilayer, etc.
Phase composition
Single-Phase solids
Multi-phase solids
Multi-phase system / Crystalline, amorphous particles and layers, etc.
Matrix composites, coated particles, etc.
Colloids, aerogels, ferrofluids, etc.

3 dimension (< 100nm Quantum dots):-

Semiconductor nanoparticles or quantum dots are normally prepared chemically via solution-based routes, often at elevated temperatures and sometimes at elevated pressures (hydro-or solvo-thermal methods). The most commonly studies quantum dots include metal sulfide or metal selenide compounds such as CdS, CdSe, InSe, PbS, ZnS, etc.

2 dimension (< 100nm Quantum wires):-

Surface melting assisted oxidization can be used to directly grow metal oxide nanostructures without the presence of solution or vapour. Nanostructures of metal oxides such as ZnO, MgO, TiO2 and SnO2 etc are examples for 2 dimension quantum wires.

1 dimension (< 100nm Multilayer):-

Nanolayers
Nanolayers are one of the most important topics within the range of nanotechnology. Through nanoscale engineering of surfaces and layers, a vast range of functionalities and new physical effects (e.g. magnetoelectronic or optical) can be achieved. Furthermore, a nanoscale design of surfaces and layers is often necessary to optimise the interfaces between different material classes (e.g. compound semiconductors on silicon wafers), and to obtain the desired special properties. Some application ranges of nanolayers and coatings are summarised in table.

Tunable properties by nanoscale surface design and their application potentials.

Mechanical properties (e.g. tribology, hardness, scratch-resistance). / Wear protection of machinery and equipment, mechanical protection of soft materials (polymers, wood, textiles, etc.).
Wetting properties (e.g. anti-adhesive, hydrophobic, hydrophilic). / Anti-graffiti, anti-fouling, Lotus-effect, self-cleaning surface for textiles and ceramics, etc.
Thermal and chemical properties (e.g. heat resistance and insulation, corrosion resistance). / Corrosion protection for machinery and equipment, heat resistance for turbines and engines, thermal insulation equipment and building materials, etc.
Biological properties (biocompatibility, anti-infective). / Biocompatible implants, a bacterial medical tools and wound dressings, etc.
Electronical and magnetic properties (e.g. magnetoresistance, dielectric). / Ultra-thin dielectrics for field-effect transistors, magneto-resistive sensors and data memory, etc.
Optical properties (e. anti-reflection, photo- and electro-chromatic). / Photo- and electro-chromic windows, anti-reflective screens and solar cells, etc.

Fig 6.7 Page no 119.

Single-phase solids:-

CuS amorphous nanoparticles are example for single-phase solids, and its inhibit the proliferation of cancer cells rather than normal cells.

Multi-phase solids:-

A method for coating magnetic nanoparticles with a very thin layer of gold. Because many biological markers and linkers have been adapted to attach to gold surfaces, a functional coating of gold allows nanoparticles of other materials to be used with the established markers and linkers. Magnetic nanoparticles are of particular interest for in vivo imaging and treatment operations.

Applications:-

(1)  Image enhancement in magnetic based diagnostics (such as MRI or other proprietary techniques).

(2)  Cancer imaging and treatment.

Advantages:-

(1)  Avoids direct contact between biological tissue and the core nanoparticle material

(2)  Permits a wide range of magnetic materials to be used in biological tissue

(3)  Simple, rapid, and relatively inexpensive chemical process

Multi-phase system:-

Aerogel is a manufactured material with the lowest bulk density of any known porous solid.[1] It is derived from a gel in which the liquid component of the gel has been replaced with a gas. The result is an extremely low-density solid with several remarkable properties, most notably its effectiveness as a thermal insulator. It is nicknamed frozen smoke, solid smoke, solid air or blue smoke due to its translucent nature and the way light scatters in the material; however, it feels like expanded polystyrene to the touch.

Eq:- Carbon aerogels are composed of particles with sizes in the nanometer range, covalently bonded together. They have high porosity over 50%, with pore diameter under 100 nm and surface areas ranging between 400-1000 m2/g.

Nanotube (carbon):- 1D fullerene (a convex cage of atoms with only hexagonal and/or pentagonal faces) with a cylindrical shape. Sheets of graphite rolled up to make a tube. Graphitic layers seamlessly wrapped to cylinders. A new class of carbon materials consists of closed (SP2 hybridized) carbon chains, organized on the basis of 12 pentagons and any number of hexagons. More generally, any tube with nanoscale dimensions, e.g., a boron-nitride-based tube.

Nanowires:- Nanoscale rods of some length made of semiconducting materials. Long-chain molecule capable of carrying a current. Microscopic wires from layers of different materials. Wires that are structured like “regular wires” but are at the nanoscale.

Quantum dot (QD):- Nanometer-scale “boxex” for selectively or releasing electrons; the size of the box can be from 30 to 1000 nm, but more advanced only 1-100 nm across.

Nanofoam:- The new structure was created when physicists bombarded a carbon target with a laser capable of firing 10000 pulses a second. As the carbon reached temperatures of around 10000oC, it formed an interesting web of carbon tubes, each just 1 nm in diameter.

Nanoclusters:- Nanoscale metal and semiconductor particles are of interest because they mark a material transition range between quantum and bulk properties. With decreasing particle size, bulk properties are lost as the continuum of electronic states becomes discrete and as the fraction of surface atoms becomes large. The electronic and magnetic properties of metallic nanoparticles and nanoclusters have new characteristics that can be utilized in novel applications.

Manufacturing Process
Gas phase reaction
Liquid phase reaction
Mechanical procedures / Flame synthesis, condensation, CVD etc.
Sol-gel, precipitation, hydrothermal processing, etc.
Ball milling, plastic deformation, etc.

Vapor condensation:-

This approach is used to make metallic or metal oxide ceramic nanoparticles. It involves evaporation of solid metal followed by rapid condensation to form nanosized clusters that settle in the form of a powder. Various approaches to vaporize the metal can be used and variation of the medium into which the vapor is released affects the nature and size of the particles. Inert gases are used to prevent oxidation when creating metal nanoparticles, whereas a reactive oxygen atmosphere is used to produce metal oxide ceramic nanoparticles. The main advantage of this approach is low contamination levels. Final particle size is controlled by variation of parameters such as temperature, gas environment and evaporation rate.

Another variation on the vapor condensation technique is the vacuum evaporation on running liquids (VERL) method. This uses a thin film of a relatively viscous material, an oil, or a polymer, for instance, on a rotating drum. A vacuum is maintained in the apparatus and the desired metal is evaporated or sputtered into the vacuum. Particles form in suspension in the liquid and can be grown to a variety of sizes.

Chemical vapour deposition:-

Among other growth methods, chemical-vapor deposition (CVD) technology is particularly interesting not only because it gives rise to high-quality films but also because it is applicable to large-scale production. This technique is widely used in the fabrication of epitaxial films toward various GaN-based optoelectronic devices, and similar trend might be expected for future applications of ZnO. There are several modifications of this method depending on precursors used. When metal-organic precursors are used, the technique is called MOCVD, metal-organic vapor-phase epitaxy (MOVPE), or organomettallic vapor-phase epitaxy (OMVPE). In the case of hydride or halide precursors, the technique is named hydride or halide CVD or VPE.

Pulsed Laser Deposition:-

In the pulsed-laser deposition (PLD) method, high power laser pulses are used to evaporate from a target surface such that the stoichiometry of the material is preserved in the interaction. As a result, a supersonic jet of particles (plume) is directed normal to the target surface. The plume expands away from the target with a strong forward directed velocity distribution of different particles. The ablated species condense on the substrate placed opposite to the target.

The main advantages of PLD are its ability to create high-energy source particles, permitting high quality film growth at low substrate temperatures, typically ranging from 200 to 800oC, its simple experimental setup, and operation in high ambient gas pressures in the 10-5-10-1 Torr range.

Sol-Gels:-

The sol-gel process is a wet-chemical technique used primarily for the fabrication of materials starting from a chemical solution which acts as the precursor for an integrated network (or gel) of either discrete particles or network polymers. Typical precursors are metal alkoxides and metal chlorides, which undergo various forms of hydrolysis and polycondensation reactions. The formation of a metal oxide involves connecting the metal centers with oxo (M-O-M) or hydroxo (M-OH-M) bridges, therefore generating metal-oxo or metal-hydroxo polymers in solution. Thus, the sol evolves towards the formation of a gel-like diphasic system containing both a liquid phase and solid phase whose morphologies range from discrete particles to continuous polymer networks.

Removal of the remaining liquid (solvent) phase requires a drying process, which is typically accompanied by a significant amount of shrinkage and densification. The rate at which the solvent can be removed is ultimately determined by the distribution of porosity in the gel. The ultimate microstructure of the final component will clearly be strongly influenced by changes imposed upon the structural template during this phase of processing. Afterwards, a thermal treatment, or firing process, is often necessary in order to favor further polycondensation and enhance mechanical properties and structural stability via final sintering, densification and grain growth. One of the distinct advantages of using this methodology as opposed to the more traditional processing techniques is that densification is often achieved at a much lower temperature.

The precursor sol can be either (i) deposited on a substrate to form a film (e.g., by dip coating or spin coating), (ii) cast into a suitable container with the desired shape or (iii) used to synthesize powders (e.g., microspheres, nanospheres). The sol-gel approach is a cheap and low temperature technique that allows for the fine control of the product’s chemical composition. Even small quantities of dopants, such as organic dues and rare earth elements, can be introduced in the sol and end up uniformly dispersed in the final product. It can use used as a means of producing very thin films of metal oxides for various purposes.

Example

1.  Gallium based Nano-Materials

2.  Dye-Doped Gel Glasses

3.  Glass Dispersed Liquid Crystals

4.  Synthesis of Glass-Metal Nano-Composite

5.  Metal-Silica and Metal Oxide-Silica Nanocomposites.

Electrodeposition:-

Electrodeposition has been used for a long time to make electroplated materials. By carefully controlling the number of electrons transferred, the weight of material transferred can be determined in accordance with Faraday’s law of electrolysis. This states that the number of moles of product formed by an electric current is directly proportional to the number of moles of electron supplied. Since the quantity of electricity passed (measured in coulombs) is current (amps) x time (sec) and Faraday’s constant F (96485 coulombs is currently the most accurate estimate) is the charge per mole of electrons (1 mole of electrons = 96485 coulombs), then the number of moles of electron is charged supplied/F.

Specific Advantages of electro-deposition for the synthesis of the nano scale materials:-

The synthesis of naonmaterial’s requires an atmospheric deposition process and extreme control over the deposition. Vapor deposition techniques have been used almost exclusively to produce these materials. The fact that electrochemical deposition, also being at atomic deposition process, can be used to synthesis nanocomposites, has generated a great deal of interest in recent years. The obvious advantage of this century-old process of ED is as follows.

a. Rapidity

b. Low cost

c. Free from porosity

d. High purity

e. Industrial applicability

f. Potential to overcome shape limitations or allows the production of free-standing parts with

complex shapes.

e. Higher deposition rates.

f. Produce coatings on widely differing substrates.

g. Ability to produce structural features with sizes ranging from nm to μm.

h. Easy to control alloy composition.

i. Ability to produce compositions unattainable by other techniques.

j. The possibility of forming of simple low-cost multilayer’s in many different systems, e.g.

Cu/Ni, Ni/Ni-P etc.

k. No Postdeposition treatment.

Ball milling:-

These early nanomaterials were made by a simple method called ball milling, which is better described as mechanical crushing. In this process, small balls are allowed to rotate around the inside of a drum and drop with gravity force on to a solid enclosed in the drum. Ball milling breaks down the structure into nanocrystallites. The significant advantage of this method is that it can be readily implemented commercially. Ball milling can be used to make a variety of new carbon types, including carbon nanotubes. It is useful for preparing other types of nanotubes, such as boron nitride nanotubes and a wide range of elemental and oxide powders. For example, iron with grain sizes of 13-30 nm can be formed. Other crystallites, such as iron nitrides, can be made using ammonia gas. Ball milling is the preferred method for preparing metal oxides. Their used range from pigments to capacitors to coatings to inks. All of these applications rely on the increased surface to bulk ratio, which alters the chemical properties of the metal oxide.