Review, Pages 9 (2086 words)
Synthesis of Metal Nanoparticles by Microemulsion Systems
The purpose for this article is to see the impact of µ-emulsion framework in the amalgamation of metal nano particles.
µ-emulsion framework is a one of a kind system to acquire the ideal nano molecule and to see the morphology of silver, gold, titanium and Zinc Iron nano particles. Transmission electron microscopy (TEM), IR and XRT systems were utilized to guarantee the arrangement of metal nano particles. Gold and silver nano particles demonstrated the average neighborhood surface plsmon reverberation of secluded nano particles with symmetric assimilation bend. This symmetry did not hold after the expansion of the cross connection. XRD contemplates demonstrates the qualities of collected nano particles.
A thermodynamically stable dispersion of two immiscible liquids is known as Microemulsion. Different surfactans are used to stablize the system. Microemulsion are of various kinds, such as water-in-oil (w/o), oil-in-water (o/w), water-in-sc-CO and intermediate bicontinuous structural types.
Dispersion of water in a hydrocarbon based continuous phase results water-in-oil (W/O) microemulsion, and is normally located towards the oil apex of a water/oil/surfactant triangular phase diagram
Phase diagram for CTAB/1-hexanol/water systems
A thermodynamically driven surfactant self-assembly generates aggregates known as reverse or inverted micelles (L2phase ). Most common form of reverse micelle which minimize the surface energy are spherical reverse micelle. The central cores of these reversed micelles are covered with polar or ionic component, thus corresponds to fine dispersion of inorganic materials in oil.
These systems must be dynamic—— which favours the random collision of micelle due to Brownian motion,resulting coalescene of droplet to form diamer.
Inorganic reagents will become mixed, when encapsulated within micelle. This kind of exchange process is quite similar to fundamental nanoparticle synthesis inside reversed micellar ‘templates’, resulting the mixing of various reactant which are solublized in separate micellar system. Miclle provides a suitable environment for controlled nucleation and growth also called nanoreactor. The surfactant layer provides steric stabilization which prevents the nanoparticles from aggregating.
In macroscale microemulsion are homogeneous and microheterogeneous at nanoscale dispersion of two immiscible liquids consisting of nanosized domains of one or both liquids
An interfacial film of surface active molecules stabilize the system. Normal emulsion and microemulsion differ from eachother due to their particle size and stability. Microemulsions are thermodynamically stable, single optically isotropic and usually form spontaneously. Wherase Normal emulsion age by Ostwald ripening and coalescene.
Microemulsion have capability to solubilize both aqueous and oil-soluble compounds.Large interfacial area and ultralow interfacial tension are other characteristics associated with microemulsion. Nanoparticles forms in microemulsion system exhibit novel and improved biological, physical and chemical properties.Due to nanoscale size in a range of 1-100nm they show significant progress in device development and other materials.
Size of metals nanoparticles can be controlled by concidering different parameters such as water content , water to surfactant mole ratio , continuous state ,amount of precursor and stabilizer. Addition of salts, Temperature, Pressure and concentration of reagents can also influenced the stability of microemulsion.
Preparation of nanoparticles in microemulsion system
In W/O microemulsion ,metal nanoparticles can be prepared by combining two microemulsions consisting of metal salt and reducing agent.
Schematic illustration of nanoparticles preparation using microemulsion techniques:
kchem = rate constant for chemical reaction.
k ex = rate constant for intermicellar exchange dynamics.
kn = rate constant for nucleation.andkg = rate constant for particles.
There is an exchange of reactants between micelle take place due to the mixing of two microemulsions. Brownian motion also play an important role in the intermixing of water droplets . Fusion , coalescene of reactant and their efficient mixing is due to effective collision. Elastic forces, repulsive osmotic and van der Waals force sresults in reverse micelle.
Formation of metal nuclei take place due to the reaction of solubilizates.In the earlier stage; of nucleation process an irreversible seed of stable metal nuclei form due to the reduction of metal salts into zerovalent metal atoms .Intermicellar exchange is responcible for the growth that occurs at the nucleation point, where effective collision take place between reverse micelle,one of them having a nucleus and the other having product monomers.Size and morphology of prepared metal nanoparticles depend on the surfactant type as well as the shape and size of nanodroplets. Surfactant molecules stabilize and protect these particles against further growth by attaching themselves on the surface of particles.This nucleation and growth process takes place within the micelles.
Examples of different metal nanoparticles formation by microemulsion system are given below:
Synthesis of iron (III) oxide, Fe2O3using W/O microemulsion:
Iron(III) oxide Fe2O3 can be prepared by using W/O microemulsion system,which can be establish in a stock solution of AOT in n-heptane by mixing with specific amount of water.Concentrated NH2OH and FeCl3 were dropped in to W/O microemulsion system,after being held overnight. 95% ethanol was used to filtered and washed Fe2O3 suspension.Then dried the system at 300o C for 3 h.As a result of this process spherical, monodispersed nanoparticles of about 50 nm diameter were obtained.By increasing water fraction particle size can be increased.Hence water content is responsible for the particle size in this type of microemulsion system.
Synthesis of pure monodispersed zinc oxide nanoparticls:
For the preparation of pure monodispersed zinc oxide nanoparticles a specific kind of microemulsion was established in which Triton X-100 used as surfactant, hexanol as co-surfactant,cyclohexane and aquous solution of zinc nitrate or ammonium hydroxide/sodium hydroxide complex. TX-100 to hexanol molar ratio was maintained at 1:4. Then microemulsion system of both zinc nitrate and ammonium hydroxide/sodium hydroxide were mix andstirred. Centrifugation at 15,000 rpm for 1 h was used to separate zinc oxide nanoparticles. Nanoparticles then washed with distilled water and alcohol.Finally dried at 50o C for 12 h.
Synthesis of nanometer-sized titania particlesNanometer-sized Titania particles can be prepared by chemical reactions between TiCl4 solution and ammonia in a microemulsion system. A cyclohexane was used as oil phase and a mixture of poly(oxyethylene)9 nonyle phenol ether andpoly(oxyethylene)5 nonyle phenol ether(NP5, chemical purity) with molar ratio 1:1 as non-ionic surfactant(NP9-NP5). 0.5 M Titanium tetrachloride(TiCl4) aquous solution and 2.0 M ammonia as the aqueous phase were prepared as two microemulsion system.In a water bath at 13o C all surfactant,aqueous phase,oil phase were mixed in appropriate proportion.Insoluble titania particles were formed by mixing both TiCl4 and ammonia containing microemulsions. Centrifugation was used to separate these precipitates,which were washed by acetone and were dried in vacuum for 2 hours. About 5nm size Titania particles were obtained with narrow size distribution. RAMAN Spectroscopy studies reveals close transition behaviour with dry gel synthesized by sol-gel method.
Synthesis of Pt, Pd and Rh nanoparticles Pt, Pd and Rh nanoparticles were synthesized by an oil-in-water microemulsion reaction method. By mixing appropriate amounts of oil phase,surfactant,cosurfactant and deionized water with metal precursor (Pd-AAc,Rh-COCl , Pt-COD) Microemulsion was prepared. Three system were used in this procedure that are
(System A); water/Brij 96V/ btyl-S-Lactate
(System B);WATER/Tween 80/Span20/1,2-hexanodiol/ethyl oleate.
(System C) ; WATER/Synperonic 10/5/ isooctane.
Factors affecting formation of metal nanoparticles in microemulsion system 1.Intermicellar Exchange(Kex):
Intermicellar exchange is denoted by the intermicellar exchange rate coefficient (kex).Actually, formation of metal nanoparticles is a strong function of intermicellar exchange in microemulsion system.
Various factors affecting intermicellar exchange rate coefficient (kex) are water content also called molar ratio of water to surfactant (W) , precursor content , type of continuous phase etc.Large number of metal nanoparticles with relatively smaller diameter formed as a result of high exchange rate between micelles. On the other hand, slow exchange between micelle results the formation of a few numbers of nuclei and large final particle size.
As the chain length of the oil increases from 106 M-1s-1 for cyclohexane and 107 for n-heptane to 108 M-1 s-1 for decane , intermicellar exchange rates also increases. While increase in the chain length of the oil from 5.4 nm for cyclohexane and 6nm for decane to 22nm for n-heptane, silver nanoparticles decreases . n-heptane or cyclohexane like less bulky solvent molecules which have lower molecular volumes form additional interfacial rigidity and additional interfacial area due to the penetration in surfactant layer.
Chain length of solvent molecules responsible for the growth change,e.g alkanes molecules become coiled and its also difficult for them to penetrate in the surfactant layer due to increase in chain length.Thats why solvent molecules interaction with surfactant decreases with an increase in the chain length of alkane molecules.On contrary short chain alkane and cyclohexane generates additional interfacial rigidity due to penetration of surfactant layer.
Effect of various surfactant types
Types of surfactant with the addition of cosurfactant is second important factor which can influence metal nanoparticles size and shape. Structure of surfactant comprises of
a hydrophilic headgroup and
a hydrophobic(or lypophilic) tail group.
Van der Waals and ionic forces kept them together which form soft aggregates in solvent.Effectively distributed metal nanoparticles in the solution provides sites for nucleation and prevent aggregation of nanoparticles. Chemical reduction of the metallic precursors and metallic nanoparticles preparation take place in W/O Microemulsions surfactant form reverse micelles causes the dispersion of nano-sized water pools in bulk organic solvent.
Anionic , Cationic and Nonionic Surfactants
Many surfactants can be used to form microemulsion,they are
Cationic surfactants : As cetyltrimethylammonium bromide (CTAB),
Anionic surfactants: As bis(2-ethylhexyl)sulfosuccinate (AOT), sodium dodecyl benzene sulfonate (SDBS) and lauryl sodium sulfate (SDS), and
Nonionic surfactants: As Triton X-100 and sorbitan monooleate Span 80, nonylphenyl ether (NP-5) or polyoxyethylene (9) nonylphenyl ether (NP-9).
AOT Surfactant Molecule:
Sodium bis(2-ethylhexyl) sulfosuccinate is the most commonly used surfactant for the formation of reverse micelles; also known as Aerosol-OT or AOT.
The AOT molecule has proven an effective emulsifier,also has wide range application and numerous intensive studies.AOT is an anionic surfactant with a sulfosuccinate group as a head and is stabilized by a salt of sodium cation.
AOT Microemulsion is made up of three important component, water,alkane(without co-surfactants) and AOT. Water pool characterized by water to surfactant molar ratio(W=[H2O]/[AOT]) and
Model of surfactant molecule bis(2-ethylhexyl)sulfosuccinate (AOT)
Hydrophobic alkyl tails in nonpolar continuous phase solvent. Hydrophilic head group of AOT compartmentalised by hydrophilic core used to formed micelle structure.
Metallic nanoparticles (Pt, Pd, Cu, Ag, Au, Ni, Zn), Metal sulfides (CdS, ZnS) and Metal oxides (TiO2, SiO2 )can be prepared by using AOT microemulsion. Particles of high stability, small particle size, and good monodispersity have been formed by this system. AOT helps to extract metal cations from the aqueous to reverse micellar phase. AOT gives strong protective effect against aggregation , due tointeraction with negatively charged polar head group.
For example Gold nanoparticles formed in water/Triton X-100/hexane microemulsions were much smaller than those obtained in AOT-based microemulsion system.
TX-100 gold nanoparticles were not formed in the water pool but in micelle shell ,for non-ionic surfactant. Oxyethylene groups of TX-100 molecules were used to stabilize the gold particles in core shell. 0.1 M 1-hexanol used as a co-surfactant in water/TX-100/cyclohexane system which results decrease in particle size.
Effect of concentration of Surfactant:
The concentration of surfactants and the hydrophilic group chain length of surfactant affect the physical properties of nanosized TiO2 particles. Non-ionic surfactants – Brij 52 (polyoxyethylene glycol hexadecyl ether polyethylene 2-cetyl ether), Brij 56 (polyoxyethylene glycol hexadecyl ether polyethylene 10-cetyl ether), Brij58 (polyoxyethylene glycol hexadecyl ether polyethylene 20 cetyl ether) were employed in this work.
From Brij 52 to Brij 58 (average number of oxyethylene groups increases from 2 to 20) head group size increases, but tail length remains constant for Brij system.by increasing hydrophilic group chain length photocatalytic activity and size of particles can also increases.
Effect of Reaction Parameters:
Type of continuous oil phase in water/AOT/Cyclohexane and water to surfactant ratio(W) are important reaction parameter which effects microemulsion system.Smaller and narrow in size distribution silver nanoparticles were found at lower water content as compred to higher W value. Particle size increased from 4-9 nm to 50-58 nm, when W value increased from 5 to 8.It can be verified by following example.
Platinum ultrafine particles can be obtained in AOT/Isooctane micro emulsion system. This can be done by the reduction of H2PtCl4 . With increase in W value hydrodynamic diameters of reverse micelles were also increased. Using .1 M H2PtCl6 or 1.0 M hydrazine as an aquous phase observed smaller reverse micellar size as compared to water using as an aquous phase. As the W value increased, the number of nuclei formed in aquous phase also increased with rate of formation.
Ag/Cu modified Titanium dioxide Nanoparticles:
Ag/Cu-TiO2 Nanoparticles were prepared by mixing of two microemulsion systems. The aquous solution of metal ions(Ag+, Cu2+) and N2H4.H2O into the 0.2 M AOT/Cyclohexane. Ag/Cu bimetallic nanoparticles were prepared by dropwise addition.
By mixing the microemulsion containing redusing agent(hydrazine) into the microemulsion containing metal precursor in water cores .By limitizing the molar ratio of water to surfactant (W) at 2 , Water content could contol.Then 0.2 m TIP (Titanium tetraisopropoxide) was added in to microemulsion system. Formation of metal nano particle was further proceed in this way.
lusters in micelle by UV-activation. Bull. Mater. Sci.Vol.25, pp. 581-582
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