Synthesis of Nanoparticles
Author: Dr. Indu Chopra

Nanotechnology represents the design, production and application of materials at atomic, molecular and macromolecular scales, in order to produce new nanosized material (Hahens et al, 2007). Nanoparticles are submicron sized colloidal polymeric systems whose size is above molecular dimensions and below macroscopic ones (generally >1nm and < 100nm). The interest in nanoscale materials is increasing day by day due to the fact that at this level new properties are acquired these materials like optical, magnetic and electronic ones that is not exhibited by bulk solid materials usually. As nanomaterials are finding application in various material technologies like catalytic systems, chemical nanosensors (Liu et al, 2003) therefore it becomes imperative to know about different methods of synthesis of these materials.

There are two approaches to synthesize nanomaterials:

1. Bottom–up Approach includes synthesis of nanomaterials from atoms or molecules ie. Basic building blocks

2. Top- down approach involves self assembly of molecular components.

There are different methods for synthesis of nanoparticles and can be divided into three main groups:

Ø Gas phase methods which include vapor deposition, flame pyrolysis, arc discharge

Ø Liquid phase methods

Ø Solid phase mechanical processes including grinding, milling and alloying.

I) Gas Phase Synthesis

1) Chemical Vapor Condensation: It is the most frequently used method for synthesis of nanoparticles in which the target material is vaporized by heat source and rapidly condensed thereafter. If both the target material and the resultant nanoparticle product have same composition, they are prepared by physical vapor condensation. If the composition of the product is different from the target material due to chemical reaction between the reactant and other system components it is prepared by chemical vapor condensation. By carefully choosing precursor compounds and carrier gases, this method can be utilized for production of nanopowders of metals, oxides, carbides, nitrides, borides with their potential application as semiconductors, superconductors, ferroelectrics, optically active materials etc. This technique can be used for the preparation of nanoparticles with a narrow size distribution. In addition, large amount of nanoparticles with non- agglomerated state can be prepared by this method.

2) Arc Discharge Method: It is an electrical break down of inert gas, which produces an ongoing plasma discharge. In this method, metal precursor is packed inside a cave drilled into a graphite electrode kept at certain distance of few millimeters, which is then subjected to arc vaporization under reduced inert gas atmosphere. The quantity and quality of the nanotubes obtained through this method depend on various parameters that include metal concentration, inert gas pressure, kind of gas, current and system geometry, electrode material etc. by this technique high quality carbon nanotubes, fullerenes, nanowires, nanorods, nanofibers can be produced. The electric arc method produces stiff, near perfect and whisker like multiwalled carbon nanotubes (MWCNT).

3) Laser Pyrolysis: It is the general tool to synthesize nanoparticles of range 2 nm to 20 nm from organometallic precursors at rapid heating and cooling rates. This technique involves heating of flowing mixture of gases with a continuous wave of CO₂ laser, which is used for initiating and continuing the chemical reaction. After crossing certain pressure and laser power, a critical concentration of nuclei reaches in reaction zone leading to homogeneous nucleation of particles. The nucleated particles, transported by inert gas, are collected in a trap. This method is potentially clean which produces nanoparticles with uniform, controllable and narrow size distribution.

4) Sputtered Plasma Processing: this method is like gas- condensation method except that the source material is sputtering target, which is sputtered by rare gases. Synthesis of nanoparticles can be done either by direct current (dc) or radiofrquency (rf) sputtering methods. This method is utilized for preparation of ultrapure and non- agglomerated metal nanoparticles.

II) Liquid Phase Synthesis

1) Hydrothermal Method: This method is defined as any heterogeneous reaction which is carried out in the presence of aqueous solvents or mineralizers under high pressure and temperature conditions. At high temperature and pressure properties of almost all inorganic substances change and thus they dissolve and recrystallize in water. At elevated temperature as vapour pressure is much higher, water plays a significant role in precursor material transformation. Other polar or non polar solvents can also be used for the reaction therefore the method is more appropriately called solvothermal synthesis.

2) Microemulsion Method: It is a technique to synthesize nanoparticles in which two immiscible fluids are mixed together with the help of a surfactant to form a thermodynamically stable dispersion. The microemulsion is said to be oil in water (O/W) if water is the bulk fluid and oil is in less quantity along with small amount of surfactant. In water in oil (W/O) emulsion, oil or organic solvent is present in bulk

3) Chemical Reduction Method: This method carries out chemical reaction of metal ions to their oxidation states with the help of suitable reducing agent. With this method, it is possible to control shape and size of the nanoparticles by changing the reducing agent, dispersing agent, reaction temperature and time to carry out the reaction. The process uses non- complicated instruments and can yield large quantities of nanoparticles at a low cost in a short time.

4) Sonochemical Method: Preparation of nanoparticles using sonochemical method is simple and can be carried out at ambient conditions. In this method, powerful ultrasound radiations (20 KHz to 10 MHz) are applied to molecules to undergo chemical reaction. The chemical changes occur due to acoustic cavitation resulting in formation, growth and collapse of bubbles in liquid. This technique involves passage of sound waves of fixed frequency through a slurry/ solution of carefully selected metal complex precursors. The size of the nanoparticles can be controlled by using precursors with different concentrations in the solution. The method that was initially proposed for the synthesis of iron nanoparticles is used now days for synthesizing different metal oxides.

5) Sol gel Method: This technique is extensively used for the fabrication of metal oxide nanoparticles. The method is based on inorganic polymerization reaction including hydrolysis, polycondensation, drying and thermal decomposition. Various factors affecting rate of hydrolysis and condensations reactions include pH, temperature, molar ratio, concentration of catalyst, process of drying. Sol gel synthesis can be used to prepare materials with a variety of shapes such as porous structures, thin fibres, dense powders and thin films.

III) Solid Phase Synthesis

These methods involve synthesis of nanoparticles by top down method. These strategies involve physical breaking of the source material through high energy processes.

Mechanical Milling: This involves breaking of bulk material (in microdimensions) to nanoscale with strong mechanical shear forces applied by millng technique. The method that is also known as mechanical alloying can be used to produce fine, uniform dispersions of oxide particles (Al₂O ₃, Y₂O₃, ThO₂) in nickel-base super alloys that cannot be achieved by conventional methods. Mechanical alloying of materials is a complex process that depends on different factors including physical and chemical parameters, temperature, nature of the grinding atmosphere, chemical composition of the powder mixtures, chemical nature of the grinding tools etc. Different high-energy mills that are used for milling include Attrition ball mill, Planetary Ball Mill, Vibrating Ball Mill, Low Energy Tumbling Mill, High Energy Ball Mill. This method of synthesis is suitable for producing amorphous or nanocrystalline alloy particles, elemental or compound powders.

So nanoparticles can be synthesized from any of the methods depending upon the size, shape and distribution of these nanoparticles along with the purpose of their use.

References:

1. Hahens WI, Oomen AG, de Jong WH, Cassee FR. Regul. Toxicol & Pharmocol., 2007; 49: 217-229.
2. Liu T, Shaa HY and Li XG. Phys. Condens. Matter, 2003; 15, 2507.

About Author / Additional Info:
Working as Scientist and Faculty in the Division of Agricultural Chemicals, IARI, New Delhi