Preparation of Nanomaterials

PREPARATION OF NANOMATERIALS

The following two approaches are used for the synthesis of nanomaterials.

1. Top-down process (or) Physical (or) Hard methods.

2. Bottom-up process (or) Chemical (or) Soft methods.

1. Top-down process

Top-down process involves the conversion of bulk materials into smaller particles of nano-scale structure.

Fig. 2.6 Top-down process

2. Bottom-up process

Bottom-up process involves building-up of materials from the bottom by atom by atoms, molecule by molecule or cluster to the nanomaterials.

Fig. 2.7 Bottom up process

3. Important Preparations

1. Sol-Gel process

The sol-gel process is a wet chemical technique also known as chemical solution deposition. It is the method for producing solid materials from small molecules. This method is used for the fabrication of metal oxides. It involves conversion of monomers into a colloidal solution (sol), that acts as the precursor. This colloidal solution gradually evolves towards the formation of a gel-like system.

It involves the following steps.

1. Hydrolysis and polycondensation

2. Gelation

3. Aging

4. Drying

5. Densification

6. Crystallization

The volume fraction of particles (particle density) may be slow that a significant amount of fluid need to be removed for the gel-like properties to be recognized. It is done by two ways,

(i) Sedimentation

The solution is allowed to keep for some time for sedimentation to occur and then pour off the remaining liquid.

(ii) Centrifugation

Centrifugation can also be used to accelerate the process of phase separation.

Drying and densification

Removal of the remaining liquid (solvent) is done by drying process, which accompanied by shrinkage and densification.

Firing (or) crystallization

A thermal treatment (firing) is necessary to enhance mechanical properties and structural stability via sintering, densification.

Fig. 2.8 Various steps of Sol-Gel process

2. Solvothermal Synthesis

Solvothermal synthesis involves the use of solvent under high temperature (between 100°C to 1000°C) and moderate to high pressure (1 atm to 10,000 atm) that facilitate the interaction of precursors during synthesis.

Method

A solvent like ethanol, methanol, 2-propanol is mixed with certain metal precursors and the solution mixture is placed in an autoclave kept at relatively high temperature and pressure in an oven to carry out the crystal growth. The pressure generated in the vessel, due to the solvent vapour, elevates the boiling point of the solvent.

Example : Solvothermal synthesis of zinc oxide

Solvothermal synthesis of zinc oxide

Zinc acetate dihydrate is dissolved in 2-propanol at 50°C. Subsequently, the solution is cooled to 0°C and NaOH is added to precipitate ZnO. The solution is then heated to 65°C to allow ZnO growth for some period of time. Then a capping agent (1-dodecanethiol) is injected into the suspension to arrest the growth. The rod shaped ZnO nano-crystal is obtained.

Fig. 2.9 Solvothermal synthesis

3. Laser ablation

In laser ablation technique, high-power laser pulse is used to evaporate the material from the target. The stoichiometry of the material is protected in the interaction.

The total mass ablated from the target per laser pulse is referred to as the ablation rate.

Fig. 2.10 Laser ablation chamber

This method involves vapourisation of target material containing small amount of catalyst (nickel or cobalt) by passing an intense pulsed laser beam at a higher temperature to about 120°C in a quartz tube reactor. Simultaneously, an inert gas such as argon, helium is allowed to pass into the reactor to sweep the evaporated particles from the furnace to the colder collector.

Uses

1. Nanotubes having a diameter of 10 to 20 nm and 100 um can be produced by this method.

2. Ceramic particles and coating can be produced.

3. Other materials like silicon, carbon can also be converted into nanoparticles by this method.

Advantages of laser ablation.

1. It is very easy to operate.

2. The amount of heat required is less.

3. It is eco-friendly method because no solvent is used.

4. The product, obtained by this method, is stable.

5. This process is economical.

4. Chemical Vapour Deposition (CVD)

This process involves conversion of gaseous molecules into Solid nanomaterials in the form of tubes, wires or thin films. First the solid materials are converted into gaseous molecules and then deposited as nanomaterials.

Example : CNT preparation.

The CVD reactor consists of a higher temperature vacuum furnace maintained at inert atmosphere. The solid substrate containing catalyst like nickel, cobalt, iron supported on a substrate material like, silica, quarts is kept inside the furnace. The hydrocarbons such as ethylene, acetylene and nitrogen cylinders are connected to the furnace.

Fig. 2.11. Chemical vapour deposition and

Carbon atoms, produced by the decomposition at 1000°C, condense on the cooler surface of the catalyst.

As this process is continuous, CNT is produced continuously.

Types of CVD Reactor

Generally the CVD reactors are of two types

Fig. 2.12 Types of CVD Reactors

1. Hot-wall CVD

Hot wall CVD reactors are usually tubular in form. Heating is done by surrounding the reactor with resistance elements.

2. Cold-wall CVD

In cold-wall CVD reactors, substrates are directly heated inductively while chamber walls are air (or) water cooled.

Advantages of CVD

1. Nanomaterials, produced by this method, are highly pure.

2. It is economical.

3. Nanomaterials, produced by this method, are defect free.

4. As it is simple experiment, mass production in industry can be done without major difficulties.

5. Electro-deposition (or) Electrochemical deposition

Electro-deposition is an electrochemical method in which ions from the solution are deposited at the surface of cathode. Template assisted electro-deposition is an important technique for synthesizing metallic nanomaterials with controlled shape and size. Array of nano-structured materials with specific arrangements can be prepared by this method using an active template as a cathode.

Process of electro-deposition

Fig. 2.13 Electrodeposition method

The cell consists of a reference electrode, specially designed cathode and anode. All these electrodes are connected with the battery through an, voltmeter and dipped in an electrolytic solution of a soluble metal as shown in figure. When the current is passed through the electrodes of template, the metal ions from the solution enter into the pores and gets reduced at the cathode, resulting in the growth of nanowire inside the pores of the template.

Example : Electrodeposition of Gold on Silver

Nanostructured gold can be prepared by the electrodeposition technique using gold sheets as an anode and silver plate as a cathode. An array of alumina template is kept over the cathode as shown in the figure 2.13 and AuCl₃ is used as an electrolyte.

When the current of required strength is applied through the electrodes, Aut ions diffuse into the pores of alumina templates and gets reduced at the cathode resulting in the growth of nanowires (or) nanorods inside the pores of the alumina templates.

Advantages of Electro-deposition

1. This method is relatively cheap and fast.

2. Complex shaped objects can be coated.

3. The film or wire obtained is uniform.

4. Metal nanowires including Ni, Co, Cu and Au can be fabricated by this method.

6. Electrospinning

Definition

Electrospinning is a method of producing ultrafine (in nanometers) fibres by charging and ejecting a polymer solution through a spinneret under a high-voltage electric field and to solidify (or) coagulate it to form a filament.

Components  

1. A high voltage power supply.

2. A polymer reservoir that can maintain a constant flow rate of solution.

3. A conductive needle, as polymer source, connected to the high voltage power supply.

4. A conductive collector (plate, drum, etc.)

Fig. 2.14 Electrospinning

Process

A polymer is dissolved in a suitable solvent and is filled in the capillary reservoir. When sufficiently high voltage is applied to create an electric field between the needle tip and the collector, a charge accumulates at the liquid surface. When the electrostatic repulsion is higher than the surface tension the liquid meniscus is deformed into conically shaped structure known as a Taylor cone.

Once the Taylor cone is formed, the charged liquid jet is ejected towards the collector. Depending upon the viscosity of the solution, solid fibre will be formed as the solvent evaporates.

Applications

1. Electrospinning is used in diagnosis and treatment of diabetes.

2. Electrospun fibres are used in energy storage devices such as, solar cell, fuel cell, super capacitors.

3. It is also used in textiles for smart clothing, protecting clothing and fire retardant fibres.

4. It is used in sensors like gas sensors, chemical sensors and fluorescence sensors.

5. In biomedical, it is used in drug delivery, artificial blood vessel and wound dressing.

6. e-spun fibres employed in a variety of applications such as filtration and thermal insulation.

 

4. Applications of Nanomaterials

Nano-technology finds significant impact on all most all the industries and all areas of society. Since nano-materials possess unique beneficial chemical, physical and mechanical properties, they can be used for a wide variety of applications.

I. Medicine

1. Nano drugs

Nano materials are used as nano drugs for the cancer and TB therapy,

2. Laboratories on a chip

Nano technology is used in the production of laboratories on a chip.

3. Nano-medibots

Nano particles function as nano-medibots that release anti-cancer drug and treat cancer.

4. Gold-coated nanoshells

It converts light into heat, enabling the destruction of tumours.

5. Gold nano particles as sensors

Gold nano particles undergo colour change during the transition of nano particles. .

6. Protein analysis

Protein analysis can also be done using nanomaterials.

7. Gold nanoshells for blood immuno assay

Gold nano shells are used for blood immuno assay.

8. Gold nano shells in imaging

Optical properties of the gold nano shells are utilized for both imaging and therapy.

9. Targeted drug delivery using gold nano particles

It involves slow and selective release of drugs to the targeted organs.

10. Repairing work

Nano technology is used to partially repair neurological damage.

II. In Agriculture

1. Nanomaterials prepared by eco-friendly and green method with plant extracts (Nano formulations) could increase agriculture potential for improving fertilization process, plant growth regulators.

2. They also minimize the amount of harmful chemicals that pollute the environment.

3. Nanosensors are used in crop protection for the identification of diseases and residues of agrochemicals.

4. Nanodevises are used for the genetic engineering of plants.

5. Nanomaterials are used in plant disease diagnostics.

6. It is also used in postharvest management.

7. Precision farming techniques might be used to further improve the crop yields but not damage soil and water.

8. Some nanomaterials are used as antimicrobial agents in food packing especially silver nanoparticles are in great interest.

9. Nano particle - based pesticides and herbicides are being explored for the application of antimicrobial agents to protect crops from various diseases.

III. In Energy

Nanomaterials are used in several applications to improve the efficiency of energy generation (or) develop new methods to generate energy.

1. Power generation

Sun light, concentrated on nanoparticles, can produce steam with high energy efficiency, which can even be used in running power plants.

2. Generating hydrogen from sea water

The use of a nanostructured thin film of nickel selenide as a catalyst for the electrolysis of hydrogen from sea water.

3. Producing high efficiency light bulbs

Nano-engineered polymer matrix is used for the production of high efficiency light bulbs.

4. Increasing the electricity generated by wind mills

Stronger and lower weight blades, made from nanotubes-filled epoxy, in wind mills increases the amount of electricity.

5. Generating electricity from waste heat

Sheets of nanotubes have been used to build thermocells that generates electricity, when the sides of the cell are at different temperature

6. Storing hydrogen for fuel cell powered cars

Graphene layers are used in fuel tank, resulting in a higher amount of hydrogen storage and therefore lighter weight fuel tank.

7. Reducing power loss in electric transmission wires

The wires containing carbon nanotubes lowers resistance than the wires currently used in the electric transmission grid.

8. Reducing the cost of solar cell

Nanotech solar cells are manufactured at significantly lower cost than the conventional solar cells.

9. Nano battery and fuel cell

Nanomaterials, used in batteries and fuel cell, increases their efficiency.

IV. Electronics

1. Quantum wires are found to have high electrical conductivity.

2. The integrated memory circuits have been found to be effective devices.

3. A transistor, called NOMFET, (Nanoparticle Organic Memory Field Effect Transistor) is created by combining gold nanoparticles with organic molecules.

4. Nano wires are used to build transistors without p-n junctions.

5. Nano radios are the other important devices, using carbon nanotubes.

6. MOSFET (Metal Oxide Semiconductor Field Effect Transistor), performs both as switches and as amplifiers.

V. In Catalysis

Nanoparticle catalysts are highly effective because of the following two reasons

(i) huge surface area

(ii) enhanced reactivity

1. Water purification

Nanosilver catalyst is highly efficient in controlling microbes in water.

2. Bio-diesel production

Solid base nanocatalyst KF/CaO can be used for biodiesel production with yield more than 96%.

3. Fuel cell application

Carbon supported electro-catalysts play an important role in fuel cell.

4. In drug delivery

Carbon nanomaterials find more applications in biological fields.

CNTs may be suitable for bio-applications in bio recognition and drug delivery systems.

5. Gold nanoparticles

It is an important catalyst in co-oxidation, epoxidation of propylene, hydrogenation of unsaturated hydrocarbons.

6. Nanopowder silica catalyst (or) platinum nanoparticles exhibit very strong catalytic activity for hydrolysation reactions.

7. Titania-based nanocatalysts are being increasingly used in photocatalysis.

8. Nanocrystalline MgO particles act as an effective catalyst for dehydrogeneration.