Nanoparticles Prep & App

7/31/2019 Nanoparticles Prep & App

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Preparation, Characterization and

Application of Nanoparticles


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Preparation Techniques using Chemical

Approach

Microemulsion Technique

Sol-Gel Technique

Pyrolysis Technique


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Two Basic Processes

Nucleation: stochastic process stronglydependent on supersaturation (bulk

concentration solubility) Growth: deterministic process directly

dependent on supersaturation


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Microemulsion Technique

A microemulsion can be defined as the

thermodynamically stable, optically clear dispersion

of two immiscible liquids (water and oil) consisting ofnano size domain of one or both liquids in the other

that are stabilized by an interfacial film of surfactants

molecules.


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Oil in water (O/W)O/W microemulsion consists

of water as continuous phaseand nanodroplets of oil covered

with surfactants layer (micelle)

Water in oil (W/O).W/O microemulsion consists of

oil as continuous phase and

nanodroplets of water covered

by surfactants layer (reverse

micelle).

Two types of Microemulsions


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Water in Oil Microemulsion or Reverse

Micelles

The main components

a) Surfactant: AOT, CTAB, Triton X 100, SDS

b) Co surfactant: Aliphatic alcohols with chain length of C6 to C8.

c) Organic solvents:Alkanes or cycloalkanes with six to eight carbon.d) Water

The water solubilized in the core, forming a waterpool ischaracterized by R, the water to surfactant molar ratio.

(R= [H2O]/[S]).

The aqueous core of the w/o microemulsion can be used as a nano

reactor to produce nano particles by chemical reactions within thecore.

oil


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Steps in the Formation of Nanoparticles in

O/W Microemulsions

1. Equal volumes of twomicroemulsions containingtwo different reactants intheir aqueous core aremixed.

2. The Brownian motion of thereverse micelles leads tocollision.

3. The surfactant layers openup and coalesce formingtransient dimmers. (Fusion).

4. Mixing of reactants duringfusion.

5. Reaction betweenreactants, giving rise toproducts.

6. Decoalescence to return as

reverse micelles (Fission)


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Parameters affecting the Formation ofNanoparticles in Reverse Micelles.

1.

Average occupancy numberthe number of ions/reactant

molecules present in the microemulsion droplet.

Intermicellar exchange rate.Nature of surfactant molecule

Water to surfactant molar ratio (R).

Nature of oil phase

Additives.Some Nanoparticles synthesized by this method

( CdS, ZnS, Ag, AgCl, TiO2 etc).


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Core- Shell and Composite Nanoparticles

CdS-Ag2S core-shell nanoparticles were prepared by Hota and Khilar in

microemulsions of AOT in heptane. (Colloids and Surfaces A, 232, 2004)

using two mixing methods.

1) Post Core Method:

A microemulsion of AgNO3 solution was added to the micro emulsion

containing CdS nanoparticles, and an excess amount of (NH4)2S, which

act as cores.

2) Partial Micro emulsion method :

AgNO3 solution was added drop wise directly to the microemulsion

containing core CdS nanoparticles with excess (NH4)2S .


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Advantages of using Microemulsions for

Nanoparticle Synthesis

1* The W/O microemulsions can be used as nanoreactors to produce

nanoparticles by carrying out chemical reactions in their aqueous core

and also the surfactant layer of the microemulsion droplets prevents the

aggregation of particles by acting like cage.* This technique does not require any special equipment and extreme

conditions of temperature and pressure.

3* It is possible to control the size and morphology of particles formed

by controlling the initial parameters.

* The nanoparticles can be stored in these W/O microemulsions for a

long time without aggregation.


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Disadvantages of this Method

*The amount of nanoparticles formed is very small

compared to the amount of surfactant/oil phase used.

*Therefore the method may turn out to be expensive

for commercial applications.

*The separation of particles from the microemulsiondroplets is also a challenging task.


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Sol- Gel TechniquesThe sol-gel process is a wet-chemical technique for the

fabrication of materials starting either from a molecular

solution and/or from a colloidal solution(sol) to produce anintegrated network (gel).

Typical precursors - metal alkoxidesmetal chlorides

Some nanoparticles synthesized by this method :TiO2, TnO2, ZrO2, CeO2, SiO2, SnO2, ZnO, Al2O3, Sc2O3,ZnTiO3, SrTiO3, UO2.


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The Steps of Sol-Gel SynthesisHydrolysis

PolycondensationDrying

Thermal decomposition


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Principles of Sol-Gel Synthesis The size of the sol particles and the

crosslinking between the particles dependupon some variable factors such as pH,

solution composition, and temperature etc. Thus by controlling the experimental

conditions, one can obtain the nanostructuredtarget materials in the form of powder or thin

film. Used for prepartion of inorganic oxides such

as glasses and ceramics,andorganic/inorganic hybrid materials.


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The Steps of Sol-Gel Synthesis

1.Precursors of the metal or nonmetal alkoxides hydrolyze with water oralcohols

M (OR)x+ mH2O M (OR) x-m (OH) m+ m ROH

2 Followed by eithera) water condensation

2M(OR) x-m (OH) m (OH) m-1 (OR) x-m -M-O-M (OR) x-m (OH) m-1 +H2O

Or b) alcohol condensation

2M (OR) x-m (OH) mOH) m-1 (OR) x-m-M-O-M (OR) x-m-1 + ROH3. The total reaction can be expressed as

M (OR) +x/2H2O MOx/2+xHOR

The size of the sol particles depends on : the solution composition, pHand temperature.


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The Steps of Sol-Gel Synthesis Drying

Solvent removal Porous and homogeneous aerogel Cracking Shrinkage

Thermal Decomposition Decomposition of organic precursors Removal of organic substances

Sintering Collapse the gel structure Solidify the gel Enhance further crystallization


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The Steps of Sol-Gel Synthesis Thin films

Dipping factors include viscosity, removal

rate of the substrate Spin coating factors include viscosity,

surface tension, time to gel, spin speed

Creation of pores in the gel must be rapid


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Sol-Gel Synthesis Advantages:

Better control of particle size

Homogeneity in particle distribution Potentially higher purity

Lower processing temperatures

Disadvantages:

High cost of raw materials

Low yield and density of the products

Residual carbon


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Pyrolysis

Pyrolysis is a chemical process in which chemical precursorsdecompose under suitable thermal treatment into a solid compoundand the waste evaporates away.

Common precursors used : MCO3

, MC2

O4

, M (CO)x

, MNO3

, glycolate,

citrate and alkoxides.

Commonly used protecting agents : Polyvinyl alcohol (PVA) and

polyethylene glycol (PEG)

Nanoparticles produced : metals, metal oxides, semiconductors,

composite materials and carbon nanotubes.

The conditions of the reaction which need to be controlled In order toget a uniform nanosized material:

* slowing of the reaction rate,

* decomposition of precursor in an inert solvent.


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Synthesis of Cobalt Nanoclusters

Cobalt nanoclusters weresynthesized by the thermaldecomposition of anorganometallic precursor,

octacarbonyl dicobalt in an inertatmosphere

Stabilizer used polystyrenesolvent used - toluene.

The reaction condition: Thereaction was carried out at 90

C with constant stirring for aperiod of ten to twelve hours.

Kinetics of growth of thenanoclusters was studied inorder to understand the role ofpolymers in the nucleation and

growth process.


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Characterization Techniques forNanoparticles

Absorption Spectroscopy

The spectrophotometer records theintensity of absorption (A) or optical

density (O.D.) as a function ofwavelength.

The technique gives the preliminaryconcept of particle size and sizedistribution.

Usually a blue shift (decrease inwavelength ) is associated with adecrease in particle size.

This technique has been used tocharacterize Ag, AgCl , CdS , ZnS ,CdS/Ag2 S core shell nano particles.

UV/ Visible spectra of silver nano

particles


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Transmission Electron Microscopy (TEM)

TEM is a microscopy technique where animage is formed from the interaction of theelectrons transmitted through the specimen,which is magnified and focused onto animaging device.

TEM is a powerful analytical technique usedfor size and morphology charecterization ofdifferent materials like metals, semicoductorsto polymers and biological specimens.

With the addition of energy dispersive x-rayanalysis (EDAX) , the TEM can also be usedas a tool for identifying the elements in the

nano samples. This technique has been used to characterize

different types of nanostructured materialslike nanoparticles, nanowires, nanotubes aswell as core-shell and composite structures.

TEM micrographs of Ag2S

coated CdS nanoparticles

(Hota etal, colloids and surfaces A,

232,119-127, 2003. )


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Scanning Electron Microscope

The scanning electron microscope(SEM)is atype of electron microscope that images thesample surface by scanning it with a high-energy beam of electrons across arectangular area of the specimen.

The signals result from the interactions ofthe electron beam with atoms at or near thesurface of the sample.

Generally, the image resolution of an SEM isabout an order of magnitude poorer thanthat of a TEM.

SEM image relies on surface processes ratherthan transmission , so it has a much greater

depth of view, and so can produce imagesthat are a good representation of the 3Dstructure of the sample.

An SEM image of trigonal tellurium nano

tubes( B. Mayers, Y. Xia, Adv. Mat. 2002, 14, 279)


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X-ray Photoelectron Spectroscopy (XPS)

The X-ray photoemission technique (XPS) is ananalytical technique generally used for the surfaceanalysis of materials.

XPS is used to obtain information about elementsand their chemical bonds, enabling identificationsof the different chemical compounds on thesurface.

The XPS technique is surface specific due to theshort range of the photoelectrons that are excitedfrom the solid.


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X-ray Photoelectron Spectroscopy (XPS)

The basic process is the interaction of a photon(x-ray) of energy h , with an electron of bidingenergy BE in the material.

BE is the direct measure of the energy requiredto remove an electron from its initial level to

vacuum level. The kinetic energy of the photoelectron in the

vacuum level is given by KE = h BE.

The energy of the photoelectron thus leaving thesurface is determined, and this gives a spectrumof intensity as a function of binding energy.

The binding energies of the peaks arecharacteristics of each element. The peak areascan be used to determine the composition ofmaterial surface.

XPS is not sensitive to hydrogen or helium, butcan detect all other elements.This technique hasbeen used to characterize nanoparticles as wellas nano composites like CdS, Ag2S , CeO2-ZrO2.

XPS spectra of Cd 3d core level (a) CdS nanoparticles;and (b)

CdSAg2S (post-core method) core-shell nanoparticles.(Hota etal, colloids and surfaces A, 293, 5-12, 2007)


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Applications of Nanoparticles

Engineering ApplicationsCatalysis:

The catalytic properties of nanoparticles are enhanced due to

presence of large fraction of reactive atoms that reside on thesurface.

Ni nanoparticles embedded in silica

Paints, pigments, coatings:

Silver nano particles have been used in paints for refrigeratorsand washing machines for their antimicrobial properties.

Zinc nanoparticles dispersed in industrial coatings can be usedto protect wood, plastic and textiles from exposure to UV rays.

Superhydrophobic coatings; lotus spray


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Application of Nanoparticles(contd.)Optical and electronic devices

Semiconductor nanoparticles can be used forpreparation of laser diode, transducer, photo emitterand computer chips.

Polymer materials with high content of inorganicnanoparticles leading to a high dielectric constant areinteresting materials for photonic band gap

structures.


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Applications of Nanoparticles(contd.)

Nanoelectronics

Nanoelectronic devices are likely to use nanoparticles for a variety of

purposes spanning from using it as component device to using it as in

connecting and supporting features.

Nanoparticles can be used in improving display screens on electronics

devices by reducing power consumption while decreasing the weight

and thickness of the screens and increasing the density of memory

chips.

Researchers are also currently developing a type of memory chip with

a projected density of one terabyte of memory per square inch or

greater.


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Healthcare Products Nanocrystalline silver is used as an antimicrobial agent in the

treatment of wounds.

Q-Dots are being developed that identify the location of cancer cells inthe body.

Drug delivery directly to cancer cells to minimize damage to healthycells.

Nanoshells have been developed that concentrate the heat from

infrared light to destroy cancer cells with minimal damage tosurrounding healthy cells.

Nanotubes have been used in broken bones to provide a structure fornew bone material to grow.


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EnergyNanotechnology in fuel cells

Reduce the cost of catalysts used in fuel cells to produce hydrogen ions fromfuel such as methanol.

Improve the efficiency of membranes used in fuel cells to separate hydrogenions from other gases such as oxygen.

Nanotechnology in solar cells Researchers at the University of San Diego have shown how to use nanowires

to improve efficiency of thin film solar cells.

Researchers at Georgia Research Institute demonstrate nanotube based solarcells that absorb much higher percentage of light than ordinary solar cells.

Companies have developed nanotech solar cells that can be manufactured atsignificantly lower cost than conventional solar cells.


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Application of Nanoparticles(contd.)Nanotechnology in Batteries

Battery using nanomaterials will be as good as new

after sitting on the shelf for decades. Another battery can be recharged significantly faster

than conventional batteries.


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Applications of Nanoparticles(contd.)Environment

Nanoparticles are used for reducing the amount ofplatinumused in catalytic converters in automobiles and industries.

Reducing emissions from power plants by converting carbondioxide to carbon nanotubes.

Iron nanoparticles can be used to cleanup carbon tetrachloridepollution in ground water.

Nanoparticles that can absorb radioactive particles pollutingground water are used to remove radioactive waste.

Gold tipped carbon nanotubes have been used to trap oil dropspolluting water.
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Nanoparticles Prep & App