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1460308024-synthesisofnanoparticles130703041157phpapp02.pptx (Size: 2.33 MB / Downloads: 6)
Materials having unique properties arising from their nanoscale dimensions
Nanomaterials with fast ion transport are related also to nanoionics and nanoelectronics
Nanoscale materials can also be used for bulk applications
Nanomaterials are sometimes used in solar cells which combats the cost of traditional solar silicon cells
BOTTOM UP APPROACH
These seek to arrange smaller components into more complex assemblies
Use chemical or physical forces operating at the nanoscale to assemble basic units into larger structures
examples :
1. Indiun gallium arsenide(InGaAs) quantum dots can be formed by growing thin layers of InGaAs on GaAs
2. Formation of carbon nanotubes
TOP DOWN APPROACH
These seek to create smaller devices by using larger ones to direct their assembly
The most common top-down approach to fabrication involves lithographic patterning techniques using short wavelength optical sources
METHODS
3 methods of synthesis
Physical
Chemical
Biological
PHYSICAL METHODS OF SYNTHESIS
2 methods
Mechanical
High energy ball milling
Melt mixing
Vapour
Physical vapour deposition
Laser ablation
Sputter deposition
Electric arc deposition
Ion implantation
HIGH ENERGY BALL MILLING
Simplest method of making nanoparticle in the form of powder
Various types of mills
Planetary
Vibratory
Rod
Tumbler
Consists of a container filled with hardened steel or tungsten carbide balls
Material of interest is fed as flakes
2:1 mass ratio of balls to materials
Container may be filled with air or inert gas
Containers are rotated at high speed around a central axis
Material is forced to the walls and pressed against the walls
Control the speed of rotation and duration of milling- grind material to fine powder( few nm to few tens of nm)
Some materials like Co, Cr, W, Al-Fe, Ag-Fe etc are made nanocrystalline using ball mill.
MELT MIXING
To form or arrest nanoparticles in glass
Glass – amorphous solid, lacking symmetric arrangement of atoms/molecules
Metals , when cooled at very high cooling rates (10⁵-10⁶ K/s) can form amorphous solids- metallic glasses
Mixing molten streams of metals at high velocity with turbulence- form nanoparticles
Ex: a molten stream of Cu-B and molten stream of Ti form nanoparticles of TiB₂
EVAPORATION BASED METHODPHYSICAL VAPOUR DEPOSITION
Material of interest as source of evaporation
An inert or reactive gas
A cold finger(water or liquid N₂ cooled)
Scraper
All processes are carried out in a vacuum chamber so that the desired purity of end product can be obtained
Materials to be evaporated are held in filaments or boats of refractory metals like W, Mo etc
Density of the evaporated material is quite high and particle size is small (< 5 nm)
Acquire a stable low surface energy state
Cluster-cluster interaction- big particles are formed
Removed by forcing an inert gas near the source(cold finger)
If reactive gases such as O₂, N₂,H₂, NH₃ are used, evaporated material will react with these gases forming oxide, nitride or hydride particles.
Nanoparticles formed on the cold finger are scraped off
Process can be repeated several times
LASER VAPOURIZATION (ABLATION)
Vaporization of the material is effected by using pulses of laser beam of high power
The set up is an Ultra High Vacuum (UHV) or high vacuum system
Inert or reactive gas introduction facility, laser beam, target and cooled substrate
Laser giving UV wavelength such as Excimer laser is necessary
Powerful laser beam evaporates atoms from a solid source
Atoms collide with inert or reactive gases
Condense on cooled substrate
Gas pressure- particle size and distribution
Single Wall Carbon Nanotubes (SWNT) are mostly synthesized by this method
LASER PYROLYSIS
Thin film synthesis using lasers
Mixture of reactant gases is deposited on a powerful laser beam in the presence of some inert gas like helium or argon
Atoms or molecules of decomposed reactant gases collide with inert gas atoms and interact with each other, grow and are then deposited on cooled substrate
Many nanoparticles of materials like Al₂O₃, WC, Si₃N₄ are synthesized by this method
Gas pressure- particle sizes and their distribution
SPUTTER DEPOSITION
Widely used thin film technique, specially to obtain stoichiometric thin films from target material (alloy, ceramic or compound)
Non porous compact films
Very good technique to deposit multi layer films
1. DC sputtering
2.RF sputtering
3. Magnetron sputtering
DC SPUTTERING
Target is held at high negative voltage
Substrate may be at positive, ground or floating potential
Argon gas is introduced at a pressure <10 Pa
High voltage (100 to 3000 V) is applied between anode and cathode
Visible glow is observed when current flows between anode and cathode
Glow discharge is set up with different regions such as
- cathode glow
- Crooke’s dark space
-negative glow
-Faraday dark space
-positive column
- anode dark space
- anode glow
These regions are a result of plasma- a mixture of electrons, ions, neutrals and photos
Density of particles depends on gas pressure
RF SPUTTERING
If the target to be spluttered is insulating
High frequency voltage is applied between the anode and cathode
Alternatively keep on changing the polarity
Oscillating electrons cause ionization
5 to 30 MHz frequency can be used
13.56 MHz frequency is commonly used
MAGNETRON SPUTTERING
RF/DC sputtering rates can be increased by using magnetic field
Magnetron sputtering use powerful magnets to confine the plasma to the region closest to the ‘target’.
This condenses the ion-space ratio, increases the collision rate, and thus improves deposition rate
When both electric and magnetic field act simultaneously on a charged particle , the force on it is given by Lorentz force.
F = q(E+ v X B)
By introducing gases like O₂, N₂,H₂, NH₃ , CH₄ while metal targets are sputtered, one can obtain metal oxides like Al₂O₃, nitrides like TiN, carbides like WC etc- “Reactive sputtering”
ELECTRIC ARC DEPOSITION
Simplest and most useful methods
Mass scale production of Fullerenes, carbon nanotubes etc
Consists of a water cooled vacuum chamber and electrodes to strike an arc in between them
Gap between the electrodes is 1mm
High current- 50 to 100 amperes
Low voltage power supply- 12 to 15 volts
Inert or reactive gas introduction is necessary- gas pressure is maintained in the vacuum system
When an arc is set up ,anode material evaporates.
This is possible as long as the discharge can be maintained
CHEMICAL METHODS OF SYNTHESIS
ADVANTAGES
Simple techniques
Inexpensive instrumentation
Low temperature (<350ºC) synthesis
Doping of foreign atoms (ions) is possible during synthesis
Large quantities of material can be obtained
Variety of sizes and shapes are possible
Self assembly or patterning is possible
COLLOIDS AND COLLOIDS IN SOLUTION
Nanoparticles synthesized by chemical methods form “colloids”
Two or more phases (solid, liquid or gas) of same or different materials co-exist with the dimensions of at least one of the phases less than a micrometre
May be particles, plates or fibres
Nanomaterials are a subclass of colloids, in which the dimensions of colloids is in the nanometre range
SYNTHESIS OF METAL NANOPARTICLES BY COLLOIDAL ROUTE
Reduction of some metal salt or acid
Highly stable gold particles can be obtained by reducing chloroauric acid (HAuCl₄)with tri sodium citrate(Na₃C₆H₅O₇)
HAuCl₄+ Na₃C₆H₅O₇ Au ⁺+ C₆H₅O₇⁻+ HCl+3 NaCl
Metal gold nanoparticles exhibit intense red, magenta etc., colours depending upon the particle size
Gold nanoparticles can be stabilised by repulsive Coloumbic interactions
Also stabilised by thiol or some other capping molecules
In a similar manner, silver, palladium, copper and few other metal nanoparticles can be synthesized.
SYNTHESIS OF SEMU-CONDUCTOR NANOPARTICLES BY COLLOIDAL ROUTE
Wet chemical route using appropriate salts
Sulphide semiconductors like CdS and ZnS can be synthesized by coprecipitation
To obtain Zns nanoparticles, any Zn salt is dissolved in aqueous( or non aqueous) medium and H₂S is added
ZnCl₂+ H₂S ZnS + 2 HCl
Steric hindrance created by “chemical capping”
Chemical capping- high or low temperature depending on the reactants
High temp reactions- cold organometallic reactants are injected in solvent like trioctylphosphineoxide(TOPO) held at > 300ºC
Although it Is a very good method of synthesis, most organometallic compounds are expensive.
SOL GEL METHOD
2types of materials or components- “sol” and “gel”
M. Ebelman synthesized them in 1845
Low temperature process- less energy consumption and less pollution
Generates highly pure, well controlled ceramics
Economical route, provided precursors are not expensive
Possible to synthesize nanoparticles, nanorods, nanotubes etc.,
Sols are solid particles in a liquid- subclass of colloids
Gels – polymers containing liquid
The process involves formation of ‘sols’ in a liquid and then connecting the sol particles to form a network
Liquid is dried- powders, thin films or even monolithic solid
Particularly useful to synthesize ceramics or metal oxides
Hydrolysis of precursors condensation
polycondensation
Precursors-tendency to form gels
Alkoxides or metal salts
Oxide ceramics are best synthesized by sol gel route
For ex: in SiO₄, Si is at the centre
and 4 oxygen atoms at the apexes
of tetrahedron
Very ideal for forming sols
By polycondensation process
sols are nucleated and sol-gel is formed
BIOLOGICAL METHODS
Green synthesis
3 types:
1. Use of microorganisms like fungi, yeats(eukaryotes)
or bacteria, actinomycetes(prokaryotes)
2. Use of plant extracts or enzymes
3. Use of templates like DNA, membranes, viruses and diatoms
SYNTHESIS USING MICROORGANISMS
Microorganisms are capable of interacting with metals coming in contact with hem through their cells and form nanoparticles.
The cell- metal interactions are quite complex
Certain microorganisms are capable of separating metal ions.
Pseudomonas stuzeri Ag259 bacteria are commonly found in silver mines.
Capable of accumulating silver inside or outside their cell walls
Numerous types of silver nanoparticles of different shapes can be produced having size <200nm intracellularly
Low concentrations of metal ions (Au⁺,Ag⁺ etc) can be converted to metal nanoparticles by Lactobacillus strain present in butter milk.
Fungi – Fusarium oxysporum challenged with gold or silver salt for app. 3 days produces gold or silver nanoparticles extracellularly.
Extremophilic actinomycete Thermomonospora sp. Produces gold nanoparticles extracellularly.
Semiconductor nanoparticles like CdS, ZnS, PbS etc., can be produced using different microbial routes.
Sulphate reducing bateria of the family Desulfobacteriaceae can form 2-5nm ZnS nanoparticle.
Klebsiella pneumoniae can be used to synthesize CdS nanoparticles.
when [Cd(NO₃)₂] salt is mixed in a solution containing bacteria and solution is shaken for about1 day at ~38ºC ,CdS nanoparticle in the size range ~5 to 200 nm can be formed.
SYNTHESIS USING PLANT EXTRACTS
Leaves of geranium plant ( Pelargonium graveolens) have been used to synthesize gold nanoparticles
Plant associated fungus- produce compounds such as taxol and gibberellins
Exchange of intergenic genetics between
fungus and plant.
Nanoparticles produced by fungus and
leaves have different shapes and sizes.
Nanoparticles obtained using Colletotrichum sp., fungus is mostly spherical while thoe obtained from geranium leaves are rod and disk shaped.