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.

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.

3. Important Preparations

1. Sol‒Gel process

Definition

Sol‒gel process is the conversion of colloidal solution (sol) to "gel" like structure.

Principles of Sol‒gel process

It is a method of producing solid materials from small molecules. In this method, the precursor is dissolved in water (or) alcohol to form "sol" and then converted to gel like structure by heating.

Sol‒gel process is a wet chemical technique. Generally, it is used to produce metal oxides.

Preparation: Preparation of metal oxide sol‒gel

Preparation of sol‒gel involves the following steps.

Step 1: Preparation of monomers (precursor)

Metal alkoxide is dissolved in alcohol and then diluted with water. Metal alkoxide gets hydrolysed to form reactive monomers.

M‒OR (metal alkoxide) + H₂O (catalyst) → M‒OH (monomer) + ROH

Step II: Formation of "Sol"

Condensation of these monomers to form colloid like solution (sol).

M‒OH + ROM → M‒O‒M + ROH

M‒OH + HO‒M → M‒O‒M + H₂O

Step III: Formation of "Gel"

"Sol" gets converted to "gel" via polycondensation.

Step IV: Aging process

It is the process, where condensation occurs with the gel network that can cause expulsion of solvent.

Step V: Drying

It removes liquid phases and M‒OH groups

Step VI: Tempering

It is the process of densification of the gel to remove the pores of the gel network.

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.

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.

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 μm 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. 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

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

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, Au⁺ 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 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.)

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.