The important component of nanoscience and nanotechnology is the synthesis of nanomaterials and nanostructures. New physical properties and applications of nanomaterials are only achievable when nanostructured materials of the necessary size, shape, morphology, crystal structure, and chemical composition are made available. Innovative advances in science and engineering have accelerated the production of nanomaterials to attain unique qualities that differ from bulk material properties.
The particle exhibits fascinating features at dimensions less than 100 nm, owing mostly to two physical processes. The quantization of electronic states visible leads to particularly sensitive size-dependent effects such as optical and magnetic characteristics, and the high surface-to-volume ratio alters the thermal, mechanical, and chemical properties of materials. The unique physical and chemical properties of nanoparticles make them ideal for a variety of specialized applications.
Techniques for Synthesis of Nanomaterials Classification
There are two general techniques to the synthesis of nanomaterials:
a) Top-down Approach
b) Bottom-up Approach
- Top-down approach involves the breaking down of the bulk material into nanosized structures or particles.
- Top-down synthesis techniques are an extension of those that have been used for producing micron sized particles.
- Top-down approaches are inherently simpler and depend either on removal or division of the bulk material or on miniaturization of bulk fabrication processes to produce the desired structure with appropriate properties.
- The biggest problem with the top-down approach is the imperfection of surface structure.
- For example, nanowires made by lithography are not smooth and may contain a lot of impurities and structural defects on their surface. Examples of such techniques are high-energy wet ball-milling, electron beam lithography, atomic force manipulation, gas-phase condensation, aerosol spray, etc.
- This method involves building nanostructures out of smaller building blocks like atoms, molecules, or clusters.
- These atoms or molecules coalesce into nanometer-sized particles as a result of various interatomic or intermolecular forces, van der Waals forces, electrostatic forces, and other short-range forces.
- The chemical production of nanoparticles is where the bottom up method is most frequently applied. The creation of a wide variety of nanoparticles with extremely small to big scale sizes as well as a more uniform particle size distribution are the main benefits of the bottom-up technique.
- Due to the controllability of materials’ size and qualities through careful control of the reaction conditions, bottom-up methods are typically the most alluring for both laboratory and industrial-scale nanomaterial synthesis.
Methods of Synthesis of Nanomaterials
Nanostructure materials have garnered a lot of attention since their physical, chemical, electrical, and magnetic properties differ dramatically from greater dimensional counterparts and depend on their form and size.
- Many approaches have been developed to manufacture and construct nanostructure materials with controlled shape, size, dimensionality, and structure.
- The properties of materials are determined by the atomic structure, composition, microstructure, defects, and interfaces, which are governed by the thermodynamics and kinetics of the synthesis.
The Mechanical Method / Ball Milling
Ball milling is a mechanical technique for creating nanoparticles.
- The ingredients are ground in a closed container during this procedure. Shear force is created during grinding by small glass, ceramic, and stainless steel pebbles. Bulk materials are loaded into a closed container. The grinding process converts bulk materials to fine-tuned nanoparticles.
- We can make metallic hydrides and nitrides using this procedure. Their hardness and stability are employed in microelectronic applications to cut tools and coat tooling (e.g., titanium nitride (TiN) alloy).
- It is created through the reactive ball milling method. In this method, the bulk metallic powder is deposited in a closed container with a nitrogen gas atmosphere that is then exposed to high-energy ball milling.
- Metallic powders are degraded to generate tiny particles, and oxygen-free active surfaces on nanomaterials are formed.
- By including a functional moiety on the surface of the nanotube in the container, the quality of the nanotubes is improved.
- The size of the pebbles, rotational speed, milling time, and amount of nanotube injected are all parameters that influence nanotube dispersion.
The advantages of the ball-milling method include:
- producing fine powder
- being suitable for milling hazardous materials; and
- milling abrasive materials.
The disadvantages of the ball-milling approach include:
- an increase in contamination caused by wear and tear in ball collisions;
- an increase in machine noise when the concealed cylinder is made of metal;
- a time-consuming operation.
(Physical Vapor Deposition) PVD Method
Physical vapor deposition (PVD) is a technique used to create ultra-thin films and surface coatings. It is utilized to generate metal vapor, which can then be deposited as ultra-thin films and alloy coatings on the conductive layer.
- The entire operation takes place in a vacuum chamber about 106 torr from a cathodic-arc source. Vacuum deposition is performed in a clean environment, and metals are deposited as particles or sputtered in a limited area.
- Reactive PVD is a process of depositing metal on the surface while a reactive gas such as oxygen, nitrogen, or methane is fed through a vacuum chamber.
- Plasma, a high-energy beam, bombards metal surfaces, resulting in a hard and dense covering.
- We can synthesize nanoparticles and build nanocomposites using this method. Metal ions in the vapor phase received from the condensed phase and returned to the condensed phase of thin films characterize thin film formation.
- To create thin films, PVD uses evaporation and a sputtering technique. The PVD approach comprises a sputtering process that is a vapor phase carryover under supersaturation.
- Metal vapors are encouraged to condense in an inert atmosphere before being exposed to thermal treatment to produce nanocomposites.
The advantages of PVD techniques include:
- better characteristics over the substrate material;
- the use of inorganic and few organic ingredients;
- it is a more environmentally friendly approach than electroplating.
The disadvantages of PVD techniques include:
- coating with complex structures;
- it is not cost-effective and generates a low output;
- it is a complex procedure.
Sol-gel is a popular method for producing nanoparticles. Nanoparticles are created by the condensation and hydrolysis processes. Heat treatment is used in the intermediate production to ensure that the nanoparticles are crystallin.
- The alkoxides operate as a precursor to the formation of oxide nanoparticles, which interact via molecular forces (e.g., van der Waals forces or H-bonding) and are dispersed in a sol via evaporation or condensatio.
- In the presence of a basic or acid, the precursor of alkoxide is hydrolyzed, yielding a polymeric gel. The rate of condensation and hydrolysis determines the final product. For example, the lower the hydrolysis rate, the smaller the nanoparticles generated. It is an excellent method for producing composites, oxides, and ceramic nanoparticles with high purity and homogenous dispersion.
- In the presence of hydrogen gas, ferric iron is converted to metallic iron at temperatures ranging from 400 to 700 °C.
- For electrical or magnetic conductivity, xerogel is used by pressing a nanocomposite into a pellet on glass slides.
The sol-gel process has a high purity and generates a uniform nanostructure at low temperature in the presence of ligand as a capping agent.
The high amount of contaminants from reaction by-products, necessitates posttreatment.
Chemical Vapor Deposition (CVD)
Chemical vapor deposition (CVD) is a vacuum-based deposition technology used to create higher-quality, higher-performance solid materials.
- Thin films are deposited over the substrate in this process, which involves chemical reactions between species such as organometallic and other gases.
- CVD is distinguished by its use of a multidirectional deposition approach to cover the substrate, whereas PVD employs a line-of-site impingement method.
- In the microfabrication process, CVD is extensively used to deposit materials in various forms such as crystalline, amorphous, and epitaxial growth.
- A mixture of gases interacts chemically over the surface of bulk materials in CVD, resulting in chemical breakdown and the formation of a thick coating on the material’s base surface.
- Diamond crystals, for example, can be deposited over silicon or molybdenum substrates using the CVD method. The hot filament chemical vapor deposition (HFCVD) technology can be used to create charged diamond nanoparticles.
The CVD technique has the benefit of manufacturing high purity thin films and creating abrupt connections.
Product based on precursor characteristics and poor homogeneity.
Pulsed laser method
The pulsed laser approach is commonly employed in the creation of silver nanoparticles at a high rate of 3 gm/min.
- A blender-like appliance is filled with silver nitrate solution and a reducing agent.
- The gadget is made out of a solid disc that rotates in tandem with the solution.
- The disc is exposed to laser beam pulses, which cause hot spots on the disc’s surface.
- Hot spots are areas where silver nitrate reacts with a reducing agent, resulting in silver particles that may be separated using a centrifuge.
- The particle size is determined by the laser’s energy and the angular velocity of the dispersion.
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