There exist a number of methods to synthesize the nanomaterials which are categorized in two techniques “top down and bottom up”. Solid state route, ball milling comes in the category of top down approach, while wet chemical routes like sol-gel, co-precipitation, etc. come in the category of bottom up approach.
Combustion route: Combustion synthesis leads to highly crystalline particles with large surface areas . The process involves a rapid heating of a solution containing redox groups .During combustion, the temperature reaches approximately 650 °C for one or two minutes making the material crystalline.
Hydrothermal method: Hydrothermal synthesis is typically carried out in a pressurized vessel called an autoclave with the reaction in aqueous solution. The temperature in the autoclave can be raised above the boiling point of water, reaching the pressure of vapour saturation. Hydrothermal synthesis is widely used for the preparation of metal oxide nanoparticles which can easily be obtained through hydrothermal treatment of peptized precipitates of a metal precursor with water. The hydrothermal method can be useful to control grain size, particle morphology, crystalline phase and surface chemistry through regulation of the solution composition, reaction temperature, pressure, solvent properties, additives and aging time.
Microwave synthesis: Microwave synthesis is relatively new and an interesting technique for the synthesis of oxide materials. Various nano materials have been synthesized in remarkably short time under microwave irradiation. Microwave techniques eliminate the use of high temperature calcination for extended periods of time and allow for fast, reproducible synthesis of crystalline metal oxide nano materials. Utilizing microwave energy for the thermal treatment generally leads to a very fine particle in the nano crystalline regime because of the shorter synthesis time and a highly focused local heating.
Sol-gel method: It is possible to synthesize complex composition materials, to form higher purity products through the use of high purity reagents. The sol-gel process allows obtaining high quality films up to micron thickness, difficult to obtain using the physical deposition techniques. Moreover, it is possible to synthesize complex composition materials and to provide coatings over complex geometries. The starting materials used in the preparation of the sol are usually inorganic metal salts or metal organic compounds, which by hydrolysis and polycondensation reactions form the sol. Further processing of the sol enables one to make ceramic materials in different forms. Thin films can be produced by spin-coating or dip-coating. When the sol is cast into a mould, a wet gel will form. By drying and heat-treatment, the gel is converted into dense ceramic or glass materials. If the liquid in a wet gel is removed under a supercritical condition, a highly porous and extremely low density aerogel material is obtained. As the viscosity of a sol is adjusted into a suitable viscosity range, ceramic fibres can be drawn from the sol. Ultra-fine and uniform ceramic powders are formed by precipitation, spray pyrolysis, or emulsion techniques.
These were some techniques to manufacture nano particles.