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What is Nanotechnology?
• The study of the controlling of matter on an atomic and
molecular scale. Generally nanotechnology deals with structures
sized between 1 to 100 nanometer in at least one dimension, and
involves developing or modifying materials or devices within
that size.
Applications
Disruptive Apps: Materials
• Fiber that is stronger than spider web
• Metal 100 x’s stronger than steel, 1/6 weight
• Catalysts that respond more quickly and to more agents
• Plastics that conduct electricity
• Coatings that are nearly frictionless –(Shipping Industry)
• Materials that change color and transparency on demand.
• Materials that are self repairing, self cleaning, and never need repainting.
• Nanoscale powders that are five times as light as plastic but provide the
same radiation protection as metal.
• Fuel cell technology becomes cost effective within 3 years.
• Batteries that store more energy and are much more efficient
• Plastics and paints that will store solar power and convert to energy for $1 per
watt.
Continue…..
• Nanostructured materials are classified as Zero
dimensional, one dimensional, two dimensional, three
dimensional nanostructures.
(a) 0 D spheres and clusters, (b) 1 D nanofibers, wires, and rods (c)2 D films, plates and networks, (d) 3D nanomaterials
Classification of Nanomaterials
Examples of Nanomaterials
Au nanoparticle Buckminsterfullerene FePt nanosphere
Titanium nanoflower Silver nanocubes SnO2 nanoflower
Quantum dots
• Quantum dots (QD) are nanoparticles/structures that
exhibit 3 dimensional quantum confinement, which leads to
many unique optical and transport properties.
Lin-Wang Wang, National Energy Research
Scientific Computing Center at Lawrence Berkeley
National Laboratory. <http://www.nersc.gov>
GaAs Quantum dot containing just 465 atoms.
Quantum dots properties
• Quantum dots are usually regarded as
semiconductors by definition.
• Similar behavior is observed in some metals.
Therefore, in some cases it may be acceptable
to speak about metal quantum dots.
• Typically, quantum dots are composed of groups
II-VI, III-V, and IV-VI materials.
• QDs are bandgap tunable by size which means
their optical and electrical properties can be
engineered to meet specific applications.
Quantum Confinement
Definition:
• Quantum Confinement is the spatial confinement of
electron-hole pairs (excitons) in one or more dimensions
within a material.
– 1D confinement: Quantum Wells
– 2D confinement: Quantum Wire
– 3D confinement: Quantum Dot
• Quantum confinement is more prominent in
semiconductors because they have an energy gap in
their electronic band structure.
• Metals do not have a bandgap, so quantum size effects
are less prevalent. Quantum confinement is only
observed at dimensions below 2 nm.
Quantum Confinement
• Recall that when atoms are brought together in
a bulk material the number of energy states
increases substantially to form nearly continuous
bands of states.
N
Energy
Energy
Quantum Confinement
• The reduction in the number of atoms in a material
results in the confinement of normally delocalized energy
states.
• Electron-hole pairs become spatially confined when the
diameter of a particle approaches the de Broglie
wavelength of electrons in the conduction band.
• As a result the energy difference between energy bands
is increased with decreasing particle size.
Energy
Eg
Eg
Quantum Confinement
• This is very similar to the famous particle-in-a-box scenario
and can be understood by examining the Heisenberg
Uncertainty Principle.
• The Uncertainty Principle states that the more precisely one
knows the position of a particle, the more uncertainty in its
momentum (and vice versa).
• Therefore, the more spatially confined and localized a particle
becomes, the broader the range of its momentum/energy.
• This is manifested as an increase in the average energy of
electrons in the conduction band = increased energy level
spacing = larger bandgap
• The bandgap of a spherical quantum dot is increased from its
bulk value by a factor of 1/R2, where R is the particle radius.*
* Based upon single particle solutions of the schrodinger wave equation valid for R< the exciton bohr ra
Quantum Confinement
• What does this mean?
– Quantum dots are bandgap tunable by size. We can
engineer their optical and electrical properties.
– Smaller QDs have a large bandgap.
– Absorbance and luminescence spectrums are blue
shifted with decreasing particle size.
Energy
650 nm555 nm
Quantum Dots (QD)
• Nanocrystals (2-10 nm) of
semiconductor compounds
• Small size leads to
confinement of excitons
(electron-hole pairs)
• Quantized energy levels
and altered relaxation
dynamics
• Examples: CdSe, PbSe,
PbTe, InP
Eg
Quantum Dots
Absorption and emission occur at specific
wavelengths, which are related to QD size
Eg
QD Synthesis: Colloidal
Methods
Journal of Chemcial Education. Vol. 82 No.11 Nov 2005
• Example: CdSe quantum dots
• 30mg of Elemental Se and 5mL of octadecene are
used to create a stock precursor Se solution.
• 0.4mL of Trioctylphosphine oxide (TOPO) is added to
the Se precursor solution to disassociate and cap the
Se.
• Separately, 13mg of CdO, 0.6mL of oleic acid and
10mL of octadecene were combined and heated to
225oC
• Once the CdO solution reaches 225oC, room-
temperature Se precursor solution was added. Varying
the amount of Se solution added to the CdO solution
will result in different sized QDs.
QD color with size
Journal of Chemcial Education. Vol. 82 No.11 Nov 2005
Applications of QDs: Biological
• Biological Tagging and Labeling
– Biological assays and microarrays
– Labeling of cells and intracellular structures
– in vivo and in vitro imaging
– Pathogen and Toxin detection
Applications of QDs: Biological
• Biological Tagging
– Organic fluorophores such as genetically encoded
fluorescent protein, like GFP, or chemically
synthesized fluorescent dyes have been the most
common way of tagging biological entities.
– Some limitations of organic fluorophores:
• do not continuously fluoresce for extended periods
of time
• Degrade or photo-bleach
• are not optimized for multicolor applications
Applications of QDs: Biological
• The unique optical properties of quantum dots
make them suitable for biological tagging and
labeling applications.
• QDs are excellent fluorophores.
– Fluorescence is a type of luminescence in which the
absorption of an incident photon triggers the emission
of a lower energy or longer wavelength photon.
– Quantum dots absorb over a broad spectrum and
fluoresce over a narrow range of wavelengths. This is
tunable by particle size.
– So, a single excitation source can be used to excite
QDs of different colors making them ideal for imaging
multiple targets simultaneously.
Applications of QDs: Biological
Absorption and emission Spectra of CdSe/ZnS QDs
compared to Rhodamine, a common organic die.
– The absorption spectrum (dashed lines) of the QD (green) is
very broad, whereas that of the organic die (orange) is narrow.
– Conversely, the emission spectrum (solid lines) of the QD is
more narrow than that of the organic die
Jyoti K. Jaiswal and Sanford M. Simon. Potentials and pitfalls of fluorescent quantum
dots for biological imaging. TRENDS in Cell Biology Vol.14 No.9 September 2004
• A broad absorption and narrow emission spectrum means a single
excitation source can be used to excite QDs of different colors making
them ideal for imaging multiple targets simultaneously.
Gao, Xiaohu. "In vivo cancer targeting and imaging
with." Nature Biotechnology 22(2004): 8.
Applications of QDs: Biological
CdSe/ZnS QDs used to image cancer cells in a live mouse.
Applications of QDs: Biological
• Quantum dots are an attractive alternative to traditional
organic dies because of their high quantum yield and
photostability.
• Quantum Yield = # emitted photons / # absorbed
photons.
– Quantum dots have a high quantum yield because they have a
high density of energy states near the bandgap.
– A higher quantum yield means a brighter emission. The quantum
yield of some QDs is 20 times greater than traditional organic
fluorophores.
• Photostability is a fluorophore’s resistance to
photobleaching or photochemical degradation due to
prolonged exposure to the excitation source.
Applications of QDs: Biological
X. Michalet, et al. Quantum Dots for Live Cells, in Vivo Imaging,
and Diagnostics Science 307, 538 (2005)
Common QD Materials, their size and emitted wavelengths
Applications of QDs: Biological
Bioconjugated QDs:
• The surface of QDs can be functionalized with affinity ligands: antibodies,
peptides, or small-molecule drug inhibitors, to target specific types of cells for in
vivo or in vitro imaging.
• The affinity ligands are not bound directly to the surface of the quantum
dots. They are usually connected to a linker molecule referred to as a capping
ligand or coordinating ligand.
• Polymers such as poly ethylene glycol (PEG) may be introduced to reduce
nonspecific binding of the affinity ligands.
Gao, Xiaohu. "In vivo cancer targeting and imaging
with." Nature Biotechnology 22(2004): 8.
Applications of QDs: Biological
Bioconjugated QDs:
• The surface of QDs can be functionalized with affinity ligands: antibodies,
peptides, or small-molecule drug inhibitors, to target specific types of cells for in
vivo or in vitro imaging.
• The affinity ligands are not bound directly to the surface of the quantum
dots. They are usually connected to a linker molecule referred to as a capping
ligand or coordinating ligand.
• Polymers such as poly ethylene glycol (PEG) may be introduced to reduce
nonspecific binding of the affinity ligands.
Gao, Xiaohu. "In vivo cancer targeting and imaging
with." Nature Biotechnology 22(2004): 8.
Applications of QDs: Biological
Bioconjugated QDs:
• The coordinating ligands serve a dual purpose:
1. To bind the affinity ligands to the surface of the QD.
2. To encapsulate the quantum dot in a protective layer that
prevents enzymatic degradation and aggregation.
• The coordinating ligands dictate the hydrodynamic
behavior of the QD and are chosen according to the
desired biocompatibility.
• Common coordinating ligands:
• Avidin-biotin complex
• Protein A or protein G
• Simple polymers and amphiphilic lipids
Applications of QDs
Jyoti K. Jaiswal and Sanford M. Simon. Potentials and pitfalls of fluorescent
quantum
dots for biological imaging. TRENDS in Cell Biology Vol.14 No.9 September
2004
QDs conjugated with antibody molecules (blue) by using avidin (purple) or
protein A (green) as linkers. Between 10 and 15 linker molecules can be
attached covalently or electrostatically to a single QD, which facilitates the
binding of many or a few antibody molecules on each QD.
Applications of QDs: Biological
DNA assays and microarrays
Each pixel contains a different DNA sequence
Fluorescence observed if sample binds
QD-functionalized DNA
Image source: Wikipedia: Gene Expression Profiling
BioMems Applications Overview
SCME: www.scme-nm.org
Applications of QDs: Light
Emitters
• The discovery of quantum dots has led to the
development of an entirely new gamut of materials for
the active regions in LEDs and laser diodes.
• Indirect gap semiconductors that don’t luminesce in their
bulk form such as Si become efficient light emitters at
the nanoscale due quantum confinement effects.
• The study of QDs has advanced our understanding of
the emission mechanisms in conventional LED materials
such as InGaN, the active region of blue LEDs.
• The high radiative-recombination efficiency of epitaxial
InGaN is due to self-assembled, localized, In rich
clusters that behave like QDs.
Additional Applications of QDs
• New applications for QDs are continuously
being discovered.
• For example: Solar cells that incorporate
QDs may lead to more efficient light
harvesting and energy conversion.
Quantum Dot Solar Cells
Possible benefits of using quantum dots (QD):
• “Hot carrier” collection: increased voltage due
to reduced thermalization
• Multiple exciton generation: more than one
electron-hole pair per photon absorbed
• Intermediate bands: QDs allow for absorption
of light below the band gap, without
sacrificing voltage
MRS Bulletin 2007, 32(3), 236.
QDs: Collect Hot Carriers
Eg
Conduction
Band
Valence
Band
1. Tune QD absorption (band gap)
to match incident light.
2. Extract carriers without loss of
voltage due to thermalization.
Band structure of bulk semi-
conductors absorbs light having
energy > Eg. However, photo-
generated carriers thermalize to
band edges.
QDs: Multiple Exciton Generation
In bulk semiconductors:
1 photon = 1 exciton
Eg
In QDs:
1 photon = multiple excitons
Impact ionization
The thermalization of the
original electron-hole pair
creates another pair.
Absorption of one photon of light
creates one electron-hole pair,
which then relaxes to the band
edges.
QDs: Multiple Exciton Generation
1 2 3 4 5
Photon Energy (Ehv/Eg)
QuantumEff(%)
100
150
200
250
300
Quantum efficiency for
exciton generation: The
ratio of excitons produced
to photons absorbed
>100% means multiple
exciton generation
Occurs at photon energies
(Ehv) much greater than the
band gap (Eg)
QDs: Intermediate Bands
Eg
Intermediate
band formed
by an array of
QDs
Conventional band structure does
not absorb light with energy < Eg
Intermediate bands in the band gap
allow for absorption of low energy light
P3HT:CdSe Solar Cells
J. Am. Chem. Soc., 2004, 126 (21), 6550.
CdSe Sensitizers/Nano TiO2
“Rainbow Cell”
2.3 2.6 3.0 3.7 nm
J. Am. Chem. Soc., 2008, 130 (12), 4007.
Carbon nanostructures
from informaworld.com
Carbon nanotubes
Rolled up
sheet of sp2
bonded
carbon atoms
1-D Electronic Structures: Carbon Nanotubes
Wrapping vector:
2211 anann


Diameter: 0.0783nm21
2
2
2
1 nnnnd 
The folding of the sheet controls the electronic properties of the nanotubes.
D = diameter of the nanotube
The component of the wave vector perpendicular to the CNT long
axis is quantized
D
n
k
2

Metallic behavior: the allowed values of intersect the k points at which the
conduction and valence bands meet.
CNTs can behave either like metal or semiconductors depending on their
chirality.
k
Development of electronic properties
as a function of cluster size
Each band has a width that reflects the interaction between atoms,
with a bandgap between the conduction and the valence bands that
reflects the original separation of the bonding ad antibonding states.
pz electrons hybridize to form π e π* valence and conduction bands
that are separated by an energy gap of about 1V (semiconductor).
For certain high simmetry directions (the K points in the reciprocal
lattice) the material behaves like a metal.
Conduction in CNTs
Apex: at this point CB
meets VB for graphene
sheets (metal-like
behavior)
Allowed statesK
k wave vector perpendicular to the
CNT long axis
Field effect transistor made from
a single semiconducting CNT
connecting source and drain
connectors.
Semiconductor Nanowires
• Ga-P/Ga-As p/n nanojunctions
Copyright Stuart Lindsay 2009
(IOP)
TEM images
Line profiles of the
composition through
the junction region
from rice.edu
Single walled
carbon nanotube
(single sheet of
carbon atoms)
Multiwalled
carbon nanotube
(several sheets
of carbon atoms)
Carbon nanotubes can be formed from a
single sheet of C atoms or several sheets
Carbon sheets can also be rolled up in different
directions to give different types of nanotubes.
from phycomp.technion.ac.il
Electrical conductor
Electrical insulator
The properties of nanotubes depend on how they
are rolled up
Nanotube sizes also depend on how they are rolled
up
Carbon Nanotubes Applications
Carbon nanotubes are strong and light at the same time. It makes
them a perfect choice for using as reinforcing additive in composite
fibers, to make them extremely strong and electrically conductive
simultaneously.
There is enormous potential for the nanotubes to used in the design
and development of space elevators, bulletproof wear, clothing and
others.
Also, the sufficient electrical and thermal conductivity of carbon
nanotubes makes them potentially useful in revolutionizing the industry
with use in solar cells, as sensors, in batteries, and in transistors.
Carbon Nanotubes
• 4 nm width (smaller diameter than DNA)
• 100x’s stronger than steel 1/6 weight
• Thermal/electrically conductive
• Metallic and Semi-Conductive
Graphene
Graphene is classified as a semi-metal and is an allotrope of carbon.
It is a 2-dimensional, single layer of graphite, made of 6 carbon
molecules join with others to form a perfect hexagonal lattice-shaped
structure. Incidentally, graphene is the fundamental structural element
of other carbon allotropes, including carbon nanotubes.
Graphene is extremely malleable and elastic. Since it is only one-
atom-thick, graphene is nearly transparent however it is also an
excellent electrical and thermal conductor.
Graphene is very thin and flexible yet is highly conductive. Hence, it
finds potential application as a transparent conductor for use in
photovoltaic cells and other types of flexible electronic devices.
Also, its larger ratio of surface area to mass makes it excellent
potential for use in energy storage or chemical sensing.
BuckyBalls – C60
• Roundest and most symmetrical molecule known to man
• Compressed – becomes stronger than diamond
• Third major form of pure carbon
• Heat resistance and electrical conductivity
Top - Down
method
High energy
Ball Milling
Lithography
Gas
condensation
Severe plastic
deformation
Synthesis of Nanomaterials
Synthesis of
Nanomaterials
Top - down
method
(Destruction)
Bottom-up
method
(Construction)
Synthesis of Nanomaterials
To synthesize nanomaterials, either to assemble atoms together or to dis-assemble (Break or
dissociate) bulk solids into finer pieces until they are constituted of only few atoms.
SYNTHESIS
OF NANOMATERIALS
Bottom
Up
method
Physical
Vapor
Deposition
Chemical
Vapor
Deposition
Plasma
Processes
Sol-gel
Processing
Soft-
Lithography
Self-
Assembly
Mechanical Grinding (Physical Method)
Mechanical milling is achieved using high energy shaker, planetary ball, or tumbler mills
Energy transferred to the powder from refractory depends on the
• Rotational speed, size and number of the balls
• Ratio of the ball to powder mass
• The time of milling
• Milling atmosphere
Nanoparticle are produced by the shear action during grinding.
Wet Chemical Synthesis (Chemical Method)
Wet Chemical Synthesis
Top down method
(Electrochemical etching)
Bottom up method
(Sol-gel)
• In Top down method single crystals are etched in an aqueous solution for producing
nanomaterials, e.g., synthesis of porous silicon by electrochemical etching.
• In Bottom up method (Sol-gel, precipitation etc.) materials containing the desired
precursors are mixed in a controlled fashion to form a colloidal solution.
Sol-gel process
The sol-gel process may be described as:
”Formation of network (oxide) through poly-
condensation reactions of a molecular precursor in a
liquid.”
One of the highly attractive features of the sol-gel process
is the possibility to shape the material into any desired form
such as monoliths, films, fibers, and monosized powders,
and subsequently to convert it into a ceramic material by
heat treatment.
What is a Sol?
A sol is a stable dispersion of colloidal particles or polymers in a solvent.
A sol consists of a liquid with solid particles which are not dissolved, but do
not agglomerate or sediment. The particles may be amorphous or crystalline.
(An aerosol is particles in a gas phase, while a sol is particles in a liquid).
Agglomeration of small particles are due to van der Waals forces and a tendency
to decrease the total surface energy. Van der Waals forces are weak, and extend
only for a few nanometers.
In order to counter the van der Waals interactions, repulsive forces must be
established.
May be accomplished by:
Electrostatic repulsion. By adsorption of charged species onto the surface
of the particles, repulsion between the particles will increase and agglomeration
will be prevented. Most important for colloidal systems.
Steric hindrance. By adsorbing a thick layer of organic molecules, the
particles are prevented from approaching each other reducing the role of the van
der Waals forces. Works best in concentrated dispersions. Branched adsorbates
works best. Usual for nanomaterials.
What is a gel?
A gel consists of a three dimensional continuous
network, which encloses a liquid phase.
In a colloidal gel, the network is built from agglomeration
of colloidal particles. In a polymer gel the particles have a
polymeric sub-structure made by aggregates of sub-
colloidal particles.
Generally, the sol particles may interact by van der Waals
forces or hydrogen bonds. A gel may also be formed from
linking polymer chains.
In most gel systems used for materials synthesis, the
interactions are of a covalent nature and the gel process is
irreversible. The
gelation process may be reversible if other interactions are
involved.
Why sol-gel synthesis?
Sol-gel synthesis may be used to prepare materials with a
variety of shapes, such as porous structures, thin fibers,
dense powders and thin films.
Sol gel synthesis
•The idea behind sol-gel synthesis is to “dissolve” the
compound in a liquid in order to bring it back as a solid in a
controlled manner.
•Multi component compounds may be prepared with a
controlled stoichiometry by mixing sols of different
compounds.
•The sol-gel method prevents the problems with co-
precipitation, which may be inhomogeneous, be a gelation
reaction.
•Enables mixing at an atomic level.
•Results in small particles, which are easily sinterable.
Sol-gel processing options
Depending on the nature of the precursors sol-gel
process can be of two type:
a) the precursor is an aqueous solution of an
inorganic salt or,
b) a metal organic compound
Aqueous sol-gel Chemistry
Conversion of a precursor solution into an inorganic solid via inorganic
polymerization reactions induced by water.
In general, precursor (or starting compound) is an inorganic metal salt
(chloride, nitrate, sulfate etc) or a metal organic compound such as an
alkoxide.
(Alkoxides of Si, Ti, Al, Zr, (or, Fe, Ni, Co, Mn, Cu, Y, Nb, Ta… )
The sol-gel process consists of the following steps:
(i) Preparation of a homogeneous solution either by dissolution of metal
organic precursors in an organic solvent that is miscible with water, or
by dissolution of inorganic salts in water;
ii) conversion of the homogeneous solution into a sol by treatment with
a suitable reagent (generally water with or without any acid/base);
iii) aging;
iv) shaping;
and v) thermal treatment/sintering.
Aqueous sol-gel Chemistry p.2
The first step in a sol-gel reaction is the formation of an inorganic
polymer by hydrolysis and condensation reactions, i.e., the
transformation of the molecular precursor into a highly crosslinked
solid. Hydrolysis leads to a sol, a dispersion of colloidal particles in a
liquid, and further condensation results in a gel, an interconnected, rigid
and porous inorganic network enclosing a continuous liquid phase.
This transformation is called the sol-gel transition.
There are two possibilities to dry the gels. Upon removal of the pore
liquid under hypercritical conditions, the network does not collapse and
aerogels are produced.
When the gel is dried under ambient conditions, shrinkage of the pores
occurs, yielding a xerogel.
Xerogel and Aerogel
• If the gel is dried by evaporation, then the capillary
forces will result in shrinkage, the gel network will
collapse, and a xerogel is formed.
• If drying is performed under supercritical conditions,
the network structure may be retained and a gel
with large pores may be formed. This is called an
aerogel, and the density will be very low. A record
is < 0.005 g/cm3.
Aqueous sol-gel Chemistry p.3
The sol-gel conversion of metal alkoxides involves two
main reaction types: hydrolysis and condensation.
During hydrolysis, the alkoxide groups (-OR) are replaced
via the nucleophilic attack of the oxygen atom of a water
molecule under release of alcohol and the formation of a
metal hydroxide.
Condensation reactions between two hydroxylated metal
species leads to M-O-M bonds under release of water
(oxolation), whereas the reaction between a hydroxide and
an alkoxide leads to M-O-M bonds under release of an
alcohol (alkoxolation).
Aqueous sol-gel Chemistry p.4
• The chemical reactivity of metal alkoxides towards hydrolysis and
condensation depends mainly on the electronegativity of the metal
atom, its ability to increase the coordination number, the steric
hindrance of the alkoxy group, and on the molecular structure of the
metal alkoxides (monomeric or oligomeric). The amount of added
water in the hydrolysis step and how the water is added, determines,
whether the alkoxides are completely hydrolyzed or not and which
oligomeric intermediate species are formed. Additional parameters
are the polarity, the dipole moment, and the acidity of the solvent.
Non –aqueous sol-gel Chemistry
Nonaqueous processes can be divided into two general methodologies:
Surfactant directed and Solvent controlled.
• Surfactant-Directed vs. Solvent-Controlled
Surfactant-controlled synthesis routes involve the transformation of the
precursor species into the oxidic compound in the presence of
stabilizing ligands in a typical temperature range of 250 to 350 ◦C.
Alternative to surfactants is the use of common organic solvents, which
act as reactant as well as control agent for particle growth, enabling the
synthesis of high-purity nanomaterials in surfactant-free medium.
In comparison to the synthesis of metal oxides in the presence of
surfactants, the solvent-controlled approaches are simpler, because the
initial reaction mixture just consists of two components, the metal oxide
precursor(s) and a common organic solvent.
Non –aqueous sol-gel Chemistry p.2
Non –aqueous sol-gel Chemistry p.3
Equation 1 displays the condensation between metal halides and metal
alkoxides (formed upon the reaction of metal halides with alcohols) under
release of an alkyl halide. One of the early examples was the preparation of
anatase nanocrystals from titanium isopropoxide and titanium chloride.
Eq 2 (Ether elimination) leads to the formation of a M–O–M bond upon
condensation of two metal alkoxides under elimination of an organic ether. This
mechanism was reported for the formation of hafnium oxide nanoparticles.
The ester elimination process involves the reaction between metal
carboxylates and metal alkoxides (eq 3), as reported for zinc oxide, titania,and
ndium oxide.
Analogous to ester eliminations are amide eliminations. Reacting metal oleates
with amines, for example, enabled the controlled growth of titania nanorods.
In the case of ketones as solvents, the release of xygen usually involves aldol
condensation, where two carbonyl compounds react with each other with
elimination of water. The water molecules act as the oxygen-supplying agent
for metal oxide formation (eq 4).
Literature examples include the synthesis of ZnO and TiO2 in acetone.
Liquid solid reactions (precipitation)
• In this method, ultrafine particles of the desired materials are produced by
precipitation from a solution. The presence of desired nuclei in the solution is
a necessary condition to initiate the process. TiO2 powders with particle sizes
in the range 70-300 nm have been prepared from titanium tetraisopropoxide
by this method. Similarly ZnS powders may be produced by reaction of
aqueous zinc salt solutions with thioacetamide.
Hydrothermal synthesis
• It is based on the ability of water an aqueous solutions to dilute the substances
at high temperatures and pressures, which are practically insoluble under
normal condition. These include oxides, silicates and sulphides. Autoclaves are
used for this purposes because they can withstand high temperature and
pressure for long period of time. The possible advantages of the process over
the other are the ability to create crystalline phase that are not stable at the
melting point. This method is used for the synthesis of materials which have
high vapor pressure near their melting point.
Gas phase synthesis of nanomaterials
• This method allow elegant way to control process parameter in order to be
able to produce size, shape, and chemical composition controlled
nanostructures.
• In conventional chemical vapor deposition (CVD) synthesis, gaseous
products either are allowed to react homogeneously or heterogeneously
depending on a particular application.
1. In homogeneous CVD, particles form in the gas phase and diffuse towards
a cold surface due to thermophoretic forces, and can either be scrapped of
from the cold surface to give nano powders or deposited on to a substrate
to yield what is called ‘particulate films’.
2. In heterogeneous CVD, the solid is formed on the solid surface, which
catalyses the reaction and a dense film is formed.
Advantages of gas phase processes
• An excellent control of size, shape, crystallinity and chemical
composition .
• Highly pure materials can be obtained
• Multicomponent systems are relatively easy to form
• Easy control of the reaction mechanisms
Most of the synthesis routes are based on the production of
small clusters that can aggregate to form nano particles.
Schematic representation of gas phase process of
single phase nanomaterials from a heated crucible
Flame assisted ultrasonic spray pyrolysis
• In this process precursors are nebulized and then unwanted components are burnt in
a flame to get required materials. ZrO2 has been obtained by this method from a
precursor of Zr(CH3 CH2 CH2O)4.
• This process is mainly used for synthesis of fumed silica.
• Silicon tetrachloride is heated in an oxy-hydrogen flame to give highly dispersed
silica. The resulting amorphous powder consists of spherical particles with sizes in
the range 7-40 nm.
• In combustion flame synthesis mixture of acetylene and oxygen or hydrogen and
oxygen supplies the energy to initiate the pyrolysis of precursor. This process is
widely used for industrial production of powders in large quantities, such as carbon
black, fumed silica, and titanium dioxide.
• At high pressure, highly agglomerated powders are produced which is
disadvantageous for subsequent processing. The basic ides for low pressure flame
synthesis is to reduce agglomeration. Aerosol scientists widely use this process.
• Exact control of the flame produces a flat flame front. Due to oxidative atmosphere
of the flame, this synthesis process is limited to the formation of the oxides.
Schematic diagram of flame assisted ultrasonic
spray pyrolysis
Chemical vapour condensation (CVC)
• This technique involves pyrolysis of vapors of metal organic precursors in a
reduced pressure atmosphere.
• Nanoparticles of ZrO2, Y2O3, have been produced by this method.
• The evaporative source used in GPC is replaced by a hot reactor in the
Chemical vapor condensation method.
• Nanosized particles metals and ceramics can be obtained in good amounts
by adjusting the residence time of the precursor molecules, changing the gas
flow rate, pressure difference between the precursor delivery system and
the main chamber and the temperature of the reactor.
• The production capacities in CVC process are much higher than GPC
system.
• In addition to the formation of nanoparticles of a single precursor The
CVC reactor can also be used for the synthesis of mixture of nanoparticles
of two phases by supplying two precursors at the front end of the reactor,
and
• Coated nanoparticles like n-ZrO2 coated with n-Al2O3 or vice versa, by
supplying a second precursor at the second stage of the reactor.
Schematic diagram of set up for GPC system followed by consolidation in a
mechnical press or collection in an appropriate solvent media
Laser ablation
• This method is extensively used for preparation of nanoparticles and
particulate films.
• In this process a laser beam is used as the primary excitation source of
ablation for generating clusters directly from solid sample.
• The small dimension of the particles and the possibility to form thick films
make this process an efficient tool for the production of ceramic particles
and coatings.
• Synthesis of ZrO2, SnO2 nanoparticls thick films are synthesized using this
process.
• Synthesis of other materials such as lithium manganate, silicon and carbon
has also been carried out by this technique.
Thermolysis
• This method involves solvent free pyrolysis of a suitable precursor
material. One of the oldest nanomaterials, activated charcoal is
prepared using this method. Precursor materials are pyrolyzed at
600-900 °C in the absence of air to obtain charcoal of specific
surface area.
• In the process the materials are carbonized in oxidizing atmosphere
of CO2,O2 or steam at 600-1200 °C and activation is carried using
chemicals such as KOH, NaOH, ZnCl2 or phosphoric acid.
Solvothermal synthesis
• This is a versatile method for synthesizing nanomaterials at temperatures
below 200 C. This technique enables synthesis of crystalline products at
low temperature and also helps in control the size and morphology of the
resultant products.
Electrodeposition
• This technique involves creation of solid materials from electrochemical
reactions in liquid. A conducting substance is placed in liquid containing
electrolytes. When a potential is applied, redox reaction takes place and
the material is deposited as thin film at the cathode. Semiconductor ZnO
nanotube arrays are synthesized by direct electrochemical deposition from
aqueous solutions into the pore channels of anodically-formed alumina .

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Nanomaterials

  • 1. What is Nanotechnology? • The study of the controlling of matter on an atomic and molecular scale. Generally nanotechnology deals with structures sized between 1 to 100 nanometer in at least one dimension, and involves developing or modifying materials or devices within that size.
  • 3. Disruptive Apps: Materials • Fiber that is stronger than spider web • Metal 100 x’s stronger than steel, 1/6 weight • Catalysts that respond more quickly and to more agents • Plastics that conduct electricity • Coatings that are nearly frictionless –(Shipping Industry) • Materials that change color and transparency on demand. • Materials that are self repairing, self cleaning, and never need repainting. • Nanoscale powders that are five times as light as plastic but provide the same radiation protection as metal.
  • 4. • Fuel cell technology becomes cost effective within 3 years. • Batteries that store more energy and are much more efficient • Plastics and paints that will store solar power and convert to energy for $1 per watt. Continue…..
  • 5. • Nanostructured materials are classified as Zero dimensional, one dimensional, two dimensional, three dimensional nanostructures. (a) 0 D spheres and clusters, (b) 1 D nanofibers, wires, and rods (c)2 D films, plates and networks, (d) 3D nanomaterials Classification of Nanomaterials
  • 6. Examples of Nanomaterials Au nanoparticle Buckminsterfullerene FePt nanosphere Titanium nanoflower Silver nanocubes SnO2 nanoflower
  • 7. Quantum dots • Quantum dots (QD) are nanoparticles/structures that exhibit 3 dimensional quantum confinement, which leads to many unique optical and transport properties. Lin-Wang Wang, National Energy Research Scientific Computing Center at Lawrence Berkeley National Laboratory. <http://www.nersc.gov> GaAs Quantum dot containing just 465 atoms.
  • 8. Quantum dots properties • Quantum dots are usually regarded as semiconductors by definition. • Similar behavior is observed in some metals. Therefore, in some cases it may be acceptable to speak about metal quantum dots. • Typically, quantum dots are composed of groups II-VI, III-V, and IV-VI materials. • QDs are bandgap tunable by size which means their optical and electrical properties can be engineered to meet specific applications.
  • 9. Quantum Confinement Definition: • Quantum Confinement is the spatial confinement of electron-hole pairs (excitons) in one or more dimensions within a material. – 1D confinement: Quantum Wells – 2D confinement: Quantum Wire – 3D confinement: Quantum Dot • Quantum confinement is more prominent in semiconductors because they have an energy gap in their electronic band structure. • Metals do not have a bandgap, so quantum size effects are less prevalent. Quantum confinement is only observed at dimensions below 2 nm.
  • 10. Quantum Confinement • Recall that when atoms are brought together in a bulk material the number of energy states increases substantially to form nearly continuous bands of states. N Energy Energy
  • 11. Quantum Confinement • The reduction in the number of atoms in a material results in the confinement of normally delocalized energy states. • Electron-hole pairs become spatially confined when the diameter of a particle approaches the de Broglie wavelength of electrons in the conduction band. • As a result the energy difference between energy bands is increased with decreasing particle size. Energy Eg Eg
  • 12. Quantum Confinement • This is very similar to the famous particle-in-a-box scenario and can be understood by examining the Heisenberg Uncertainty Principle. • The Uncertainty Principle states that the more precisely one knows the position of a particle, the more uncertainty in its momentum (and vice versa). • Therefore, the more spatially confined and localized a particle becomes, the broader the range of its momentum/energy. • This is manifested as an increase in the average energy of electrons in the conduction band = increased energy level spacing = larger bandgap • The bandgap of a spherical quantum dot is increased from its bulk value by a factor of 1/R2, where R is the particle radius.* * Based upon single particle solutions of the schrodinger wave equation valid for R< the exciton bohr ra
  • 13. Quantum Confinement • What does this mean? – Quantum dots are bandgap tunable by size. We can engineer their optical and electrical properties. – Smaller QDs have a large bandgap. – Absorbance and luminescence spectrums are blue shifted with decreasing particle size. Energy 650 nm555 nm
  • 14. Quantum Dots (QD) • Nanocrystals (2-10 nm) of semiconductor compounds • Small size leads to confinement of excitons (electron-hole pairs) • Quantized energy levels and altered relaxation dynamics • Examples: CdSe, PbSe, PbTe, InP Eg
  • 15. Quantum Dots Absorption and emission occur at specific wavelengths, which are related to QD size Eg
  • 16. QD Synthesis: Colloidal Methods Journal of Chemcial Education. Vol. 82 No.11 Nov 2005 • Example: CdSe quantum dots • 30mg of Elemental Se and 5mL of octadecene are used to create a stock precursor Se solution. • 0.4mL of Trioctylphosphine oxide (TOPO) is added to the Se precursor solution to disassociate and cap the Se. • Separately, 13mg of CdO, 0.6mL of oleic acid and 10mL of octadecene were combined and heated to 225oC • Once the CdO solution reaches 225oC, room- temperature Se precursor solution was added. Varying the amount of Se solution added to the CdO solution will result in different sized QDs.
  • 17. QD color with size Journal of Chemcial Education. Vol. 82 No.11 Nov 2005
  • 18. Applications of QDs: Biological • Biological Tagging and Labeling – Biological assays and microarrays – Labeling of cells and intracellular structures – in vivo and in vitro imaging – Pathogen and Toxin detection
  • 19. Applications of QDs: Biological • Biological Tagging – Organic fluorophores such as genetically encoded fluorescent protein, like GFP, or chemically synthesized fluorescent dyes have been the most common way of tagging biological entities. – Some limitations of organic fluorophores: • do not continuously fluoresce for extended periods of time • Degrade or photo-bleach • are not optimized for multicolor applications
  • 20. Applications of QDs: Biological • The unique optical properties of quantum dots make them suitable for biological tagging and labeling applications. • QDs are excellent fluorophores. – Fluorescence is a type of luminescence in which the absorption of an incident photon triggers the emission of a lower energy or longer wavelength photon. – Quantum dots absorb over a broad spectrum and fluoresce over a narrow range of wavelengths. This is tunable by particle size. – So, a single excitation source can be used to excite QDs of different colors making them ideal for imaging multiple targets simultaneously.
  • 21. Applications of QDs: Biological Absorption and emission Spectra of CdSe/ZnS QDs compared to Rhodamine, a common organic die. – The absorption spectrum (dashed lines) of the QD (green) is very broad, whereas that of the organic die (orange) is narrow. – Conversely, the emission spectrum (solid lines) of the QD is more narrow than that of the organic die Jyoti K. Jaiswal and Sanford M. Simon. Potentials and pitfalls of fluorescent quantum dots for biological imaging. TRENDS in Cell Biology Vol.14 No.9 September 2004
  • 22. • A broad absorption and narrow emission spectrum means a single excitation source can be used to excite QDs of different colors making them ideal for imaging multiple targets simultaneously. Gao, Xiaohu. "In vivo cancer targeting and imaging with." Nature Biotechnology 22(2004): 8. Applications of QDs: Biological CdSe/ZnS QDs used to image cancer cells in a live mouse.
  • 23. Applications of QDs: Biological • Quantum dots are an attractive alternative to traditional organic dies because of their high quantum yield and photostability. • Quantum Yield = # emitted photons / # absorbed photons. – Quantum dots have a high quantum yield because they have a high density of energy states near the bandgap. – A higher quantum yield means a brighter emission. The quantum yield of some QDs is 20 times greater than traditional organic fluorophores. • Photostability is a fluorophore’s resistance to photobleaching or photochemical degradation due to prolonged exposure to the excitation source.
  • 24. Applications of QDs: Biological X. Michalet, et al. Quantum Dots for Live Cells, in Vivo Imaging, and Diagnostics Science 307, 538 (2005) Common QD Materials, their size and emitted wavelengths
  • 25. Applications of QDs: Biological Bioconjugated QDs: • The surface of QDs can be functionalized with affinity ligands: antibodies, peptides, or small-molecule drug inhibitors, to target specific types of cells for in vivo or in vitro imaging. • The affinity ligands are not bound directly to the surface of the quantum dots. They are usually connected to a linker molecule referred to as a capping ligand or coordinating ligand. • Polymers such as poly ethylene glycol (PEG) may be introduced to reduce nonspecific binding of the affinity ligands. Gao, Xiaohu. "In vivo cancer targeting and imaging with." Nature Biotechnology 22(2004): 8.
  • 26. Applications of QDs: Biological Bioconjugated QDs: • The surface of QDs can be functionalized with affinity ligands: antibodies, peptides, or small-molecule drug inhibitors, to target specific types of cells for in vivo or in vitro imaging. • The affinity ligands are not bound directly to the surface of the quantum dots. They are usually connected to a linker molecule referred to as a capping ligand or coordinating ligand. • Polymers such as poly ethylene glycol (PEG) may be introduced to reduce nonspecific binding of the affinity ligands. Gao, Xiaohu. "In vivo cancer targeting and imaging with." Nature Biotechnology 22(2004): 8.
  • 27. Applications of QDs: Biological Bioconjugated QDs: • The coordinating ligands serve a dual purpose: 1. To bind the affinity ligands to the surface of the QD. 2. To encapsulate the quantum dot in a protective layer that prevents enzymatic degradation and aggregation. • The coordinating ligands dictate the hydrodynamic behavior of the QD and are chosen according to the desired biocompatibility. • Common coordinating ligands: • Avidin-biotin complex • Protein A or protein G • Simple polymers and amphiphilic lipids
  • 28. Applications of QDs Jyoti K. Jaiswal and Sanford M. Simon. Potentials and pitfalls of fluorescent quantum dots for biological imaging. TRENDS in Cell Biology Vol.14 No.9 September 2004 QDs conjugated with antibody molecules (blue) by using avidin (purple) or protein A (green) as linkers. Between 10 and 15 linker molecules can be attached covalently or electrostatically to a single QD, which facilitates the binding of many or a few antibody molecules on each QD.
  • 29. Applications of QDs: Biological DNA assays and microarrays Each pixel contains a different DNA sequence Fluorescence observed if sample binds QD-functionalized DNA Image source: Wikipedia: Gene Expression Profiling BioMems Applications Overview SCME: www.scme-nm.org
  • 30. Applications of QDs: Light Emitters • The discovery of quantum dots has led to the development of an entirely new gamut of materials for the active regions in LEDs and laser diodes. • Indirect gap semiconductors that don’t luminesce in their bulk form such as Si become efficient light emitters at the nanoscale due quantum confinement effects. • The study of QDs has advanced our understanding of the emission mechanisms in conventional LED materials such as InGaN, the active region of blue LEDs. • The high radiative-recombination efficiency of epitaxial InGaN is due to self-assembled, localized, In rich clusters that behave like QDs.
  • 31. Additional Applications of QDs • New applications for QDs are continuously being discovered. • For example: Solar cells that incorporate QDs may lead to more efficient light harvesting and energy conversion.
  • 32. Quantum Dot Solar Cells Possible benefits of using quantum dots (QD): • “Hot carrier” collection: increased voltage due to reduced thermalization • Multiple exciton generation: more than one electron-hole pair per photon absorbed • Intermediate bands: QDs allow for absorption of light below the band gap, without sacrificing voltage MRS Bulletin 2007, 32(3), 236.
  • 33. QDs: Collect Hot Carriers Eg Conduction Band Valence Band 1. Tune QD absorption (band gap) to match incident light. 2. Extract carriers without loss of voltage due to thermalization. Band structure of bulk semi- conductors absorbs light having energy > Eg. However, photo- generated carriers thermalize to band edges.
  • 34. QDs: Multiple Exciton Generation In bulk semiconductors: 1 photon = 1 exciton Eg In QDs: 1 photon = multiple excitons Impact ionization The thermalization of the original electron-hole pair creates another pair. Absorption of one photon of light creates one electron-hole pair, which then relaxes to the band edges.
  • 35. QDs: Multiple Exciton Generation 1 2 3 4 5 Photon Energy (Ehv/Eg) QuantumEff(%) 100 150 200 250 300 Quantum efficiency for exciton generation: The ratio of excitons produced to photons absorbed >100% means multiple exciton generation Occurs at photon energies (Ehv) much greater than the band gap (Eg)
  • 36. QDs: Intermediate Bands Eg Intermediate band formed by an array of QDs Conventional band structure does not absorb light with energy < Eg Intermediate bands in the band gap allow for absorption of low energy light
  • 37. P3HT:CdSe Solar Cells J. Am. Chem. Soc., 2004, 126 (21), 6550.
  • 38. CdSe Sensitizers/Nano TiO2 “Rainbow Cell” 2.3 2.6 3.0 3.7 nm J. Am. Chem. Soc., 2008, 130 (12), 4007.
  • 40. from informaworld.com Carbon nanotubes Rolled up sheet of sp2 bonded carbon atoms
  • 41. 1-D Electronic Structures: Carbon Nanotubes Wrapping vector: 2211 anann   Diameter: 0.0783nm21 2 2 2 1 nnnnd  The folding of the sheet controls the electronic properties of the nanotubes.
  • 42. D = diameter of the nanotube The component of the wave vector perpendicular to the CNT long axis is quantized D n k 2  Metallic behavior: the allowed values of intersect the k points at which the conduction and valence bands meet. CNTs can behave either like metal or semiconductors depending on their chirality. k
  • 43. Development of electronic properties as a function of cluster size Each band has a width that reflects the interaction between atoms, with a bandgap between the conduction and the valence bands that reflects the original separation of the bonding ad antibonding states.
  • 44. pz electrons hybridize to form π e π* valence and conduction bands that are separated by an energy gap of about 1V (semiconductor). For certain high simmetry directions (the K points in the reciprocal lattice) the material behaves like a metal. Conduction in CNTs Apex: at this point CB meets VB for graphene sheets (metal-like behavior) Allowed statesK k wave vector perpendicular to the CNT long axis
  • 45. Field effect transistor made from a single semiconducting CNT connecting source and drain connectors.
  • 46. Semiconductor Nanowires • Ga-P/Ga-As p/n nanojunctions Copyright Stuart Lindsay 2009 (IOP) TEM images Line profiles of the composition through the junction region
  • 47. from rice.edu Single walled carbon nanotube (single sheet of carbon atoms) Multiwalled carbon nanotube (several sheets of carbon atoms) Carbon nanotubes can be formed from a single sheet of C atoms or several sheets
  • 48. Carbon sheets can also be rolled up in different directions to give different types of nanotubes.
  • 49. from phycomp.technion.ac.il Electrical conductor Electrical insulator The properties of nanotubes depend on how they are rolled up
  • 50. Nanotube sizes also depend on how they are rolled up
  • 51. Carbon Nanotubes Applications Carbon nanotubes are strong and light at the same time. It makes them a perfect choice for using as reinforcing additive in composite fibers, to make them extremely strong and electrically conductive simultaneously. There is enormous potential for the nanotubes to used in the design and development of space elevators, bulletproof wear, clothing and others. Also, the sufficient electrical and thermal conductivity of carbon nanotubes makes them potentially useful in revolutionizing the industry with use in solar cells, as sensors, in batteries, and in transistors.
  • 52. Carbon Nanotubes • 4 nm width (smaller diameter than DNA) • 100x’s stronger than steel 1/6 weight • Thermal/electrically conductive • Metallic and Semi-Conductive
  • 53. Graphene Graphene is classified as a semi-metal and is an allotrope of carbon. It is a 2-dimensional, single layer of graphite, made of 6 carbon molecules join with others to form a perfect hexagonal lattice-shaped structure. Incidentally, graphene is the fundamental structural element of other carbon allotropes, including carbon nanotubes. Graphene is extremely malleable and elastic. Since it is only one- atom-thick, graphene is nearly transparent however it is also an excellent electrical and thermal conductor. Graphene is very thin and flexible yet is highly conductive. Hence, it finds potential application as a transparent conductor for use in photovoltaic cells and other types of flexible electronic devices. Also, its larger ratio of surface area to mass makes it excellent potential for use in energy storage or chemical sensing.
  • 54. BuckyBalls – C60 • Roundest and most symmetrical molecule known to man • Compressed – becomes stronger than diamond • Third major form of pure carbon • Heat resistance and electrical conductivity
  • 55. Top - Down method High energy Ball Milling Lithography Gas condensation Severe plastic deformation Synthesis of Nanomaterials
  • 56. Synthesis of Nanomaterials Top - down method (Destruction) Bottom-up method (Construction) Synthesis of Nanomaterials To synthesize nanomaterials, either to assemble atoms together or to dis-assemble (Break or dissociate) bulk solids into finer pieces until they are constituted of only few atoms.
  • 58.
  • 59. Mechanical Grinding (Physical Method) Mechanical milling is achieved using high energy shaker, planetary ball, or tumbler mills Energy transferred to the powder from refractory depends on the • Rotational speed, size and number of the balls • Ratio of the ball to powder mass • The time of milling • Milling atmosphere Nanoparticle are produced by the shear action during grinding.
  • 60. Wet Chemical Synthesis (Chemical Method) Wet Chemical Synthesis Top down method (Electrochemical etching) Bottom up method (Sol-gel) • In Top down method single crystals are etched in an aqueous solution for producing nanomaterials, e.g., synthesis of porous silicon by electrochemical etching. • In Bottom up method (Sol-gel, precipitation etc.) materials containing the desired precursors are mixed in a controlled fashion to form a colloidal solution.
  • 61. Sol-gel process The sol-gel process may be described as: ”Formation of network (oxide) through poly- condensation reactions of a molecular precursor in a liquid.” One of the highly attractive features of the sol-gel process is the possibility to shape the material into any desired form such as monoliths, films, fibers, and monosized powders, and subsequently to convert it into a ceramic material by heat treatment.
  • 62. What is a Sol? A sol is a stable dispersion of colloidal particles or polymers in a solvent. A sol consists of a liquid with solid particles which are not dissolved, but do not agglomerate or sediment. The particles may be amorphous or crystalline. (An aerosol is particles in a gas phase, while a sol is particles in a liquid). Agglomeration of small particles are due to van der Waals forces and a tendency to decrease the total surface energy. Van der Waals forces are weak, and extend only for a few nanometers. In order to counter the van der Waals interactions, repulsive forces must be established. May be accomplished by: Electrostatic repulsion. By adsorption of charged species onto the surface of the particles, repulsion between the particles will increase and agglomeration will be prevented. Most important for colloidal systems. Steric hindrance. By adsorbing a thick layer of organic molecules, the particles are prevented from approaching each other reducing the role of the van der Waals forces. Works best in concentrated dispersions. Branched adsorbates works best. Usual for nanomaterials.
  • 63. What is a gel? A gel consists of a three dimensional continuous network, which encloses a liquid phase. In a colloidal gel, the network is built from agglomeration of colloidal particles. In a polymer gel the particles have a polymeric sub-structure made by aggregates of sub- colloidal particles. Generally, the sol particles may interact by van der Waals forces or hydrogen bonds. A gel may also be formed from linking polymer chains. In most gel systems used for materials synthesis, the interactions are of a covalent nature and the gel process is irreversible. The gelation process may be reversible if other interactions are involved.
  • 64. Why sol-gel synthesis? Sol-gel synthesis may be used to prepare materials with a variety of shapes, such as porous structures, thin fibers, dense powders and thin films.
  • 65. Sol gel synthesis •The idea behind sol-gel synthesis is to “dissolve” the compound in a liquid in order to bring it back as a solid in a controlled manner. •Multi component compounds may be prepared with a controlled stoichiometry by mixing sols of different compounds. •The sol-gel method prevents the problems with co- precipitation, which may be inhomogeneous, be a gelation reaction. •Enables mixing at an atomic level. •Results in small particles, which are easily sinterable.
  • 67. Depending on the nature of the precursors sol-gel process can be of two type: a) the precursor is an aqueous solution of an inorganic salt or, b) a metal organic compound
  • 68. Aqueous sol-gel Chemistry Conversion of a precursor solution into an inorganic solid via inorganic polymerization reactions induced by water. In general, precursor (or starting compound) is an inorganic metal salt (chloride, nitrate, sulfate etc) or a metal organic compound such as an alkoxide. (Alkoxides of Si, Ti, Al, Zr, (or, Fe, Ni, Co, Mn, Cu, Y, Nb, Ta… ) The sol-gel process consists of the following steps: (i) Preparation of a homogeneous solution either by dissolution of metal organic precursors in an organic solvent that is miscible with water, or by dissolution of inorganic salts in water; ii) conversion of the homogeneous solution into a sol by treatment with a suitable reagent (generally water with or without any acid/base); iii) aging; iv) shaping; and v) thermal treatment/sintering.
  • 69. Aqueous sol-gel Chemistry p.2 The first step in a sol-gel reaction is the formation of an inorganic polymer by hydrolysis and condensation reactions, i.e., the transformation of the molecular precursor into a highly crosslinked solid. Hydrolysis leads to a sol, a dispersion of colloidal particles in a liquid, and further condensation results in a gel, an interconnected, rigid and porous inorganic network enclosing a continuous liquid phase. This transformation is called the sol-gel transition. There are two possibilities to dry the gels. Upon removal of the pore liquid under hypercritical conditions, the network does not collapse and aerogels are produced. When the gel is dried under ambient conditions, shrinkage of the pores occurs, yielding a xerogel.
  • 70. Xerogel and Aerogel • If the gel is dried by evaporation, then the capillary forces will result in shrinkage, the gel network will collapse, and a xerogel is formed. • If drying is performed under supercritical conditions, the network structure may be retained and a gel with large pores may be formed. This is called an aerogel, and the density will be very low. A record is < 0.005 g/cm3.
  • 71. Aqueous sol-gel Chemistry p.3 The sol-gel conversion of metal alkoxides involves two main reaction types: hydrolysis and condensation. During hydrolysis, the alkoxide groups (-OR) are replaced via the nucleophilic attack of the oxygen atom of a water molecule under release of alcohol and the formation of a metal hydroxide. Condensation reactions between two hydroxylated metal species leads to M-O-M bonds under release of water (oxolation), whereas the reaction between a hydroxide and an alkoxide leads to M-O-M bonds under release of an alcohol (alkoxolation).
  • 72. Aqueous sol-gel Chemistry p.4 • The chemical reactivity of metal alkoxides towards hydrolysis and condensation depends mainly on the electronegativity of the metal atom, its ability to increase the coordination number, the steric hindrance of the alkoxy group, and on the molecular structure of the metal alkoxides (monomeric or oligomeric). The amount of added water in the hydrolysis step and how the water is added, determines, whether the alkoxides are completely hydrolyzed or not and which oligomeric intermediate species are formed. Additional parameters are the polarity, the dipole moment, and the acidity of the solvent.
  • 73. Non –aqueous sol-gel Chemistry Nonaqueous processes can be divided into two general methodologies: Surfactant directed and Solvent controlled. • Surfactant-Directed vs. Solvent-Controlled Surfactant-controlled synthesis routes involve the transformation of the precursor species into the oxidic compound in the presence of stabilizing ligands in a typical temperature range of 250 to 350 ◦C. Alternative to surfactants is the use of common organic solvents, which act as reactant as well as control agent for particle growth, enabling the synthesis of high-purity nanomaterials in surfactant-free medium. In comparison to the synthesis of metal oxides in the presence of surfactants, the solvent-controlled approaches are simpler, because the initial reaction mixture just consists of two components, the metal oxide precursor(s) and a common organic solvent.
  • 74. Non –aqueous sol-gel Chemistry p.2
  • 75. Non –aqueous sol-gel Chemistry p.3 Equation 1 displays the condensation between metal halides and metal alkoxides (formed upon the reaction of metal halides with alcohols) under release of an alkyl halide. One of the early examples was the preparation of anatase nanocrystals from titanium isopropoxide and titanium chloride. Eq 2 (Ether elimination) leads to the formation of a M–O–M bond upon condensation of two metal alkoxides under elimination of an organic ether. This mechanism was reported for the formation of hafnium oxide nanoparticles. The ester elimination process involves the reaction between metal carboxylates and metal alkoxides (eq 3), as reported for zinc oxide, titania,and ndium oxide. Analogous to ester eliminations are amide eliminations. Reacting metal oleates with amines, for example, enabled the controlled growth of titania nanorods. In the case of ketones as solvents, the release of xygen usually involves aldol condensation, where two carbonyl compounds react with each other with elimination of water. The water molecules act as the oxygen-supplying agent for metal oxide formation (eq 4). Literature examples include the synthesis of ZnO and TiO2 in acetone.
  • 76. Liquid solid reactions (precipitation) • In this method, ultrafine particles of the desired materials are produced by precipitation from a solution. The presence of desired nuclei in the solution is a necessary condition to initiate the process. TiO2 powders with particle sizes in the range 70-300 nm have been prepared from titanium tetraisopropoxide by this method. Similarly ZnS powders may be produced by reaction of aqueous zinc salt solutions with thioacetamide. Hydrothermal synthesis • It is based on the ability of water an aqueous solutions to dilute the substances at high temperatures and pressures, which are practically insoluble under normal condition. These include oxides, silicates and sulphides. Autoclaves are used for this purposes because they can withstand high temperature and pressure for long period of time. The possible advantages of the process over the other are the ability to create crystalline phase that are not stable at the melting point. This method is used for the synthesis of materials which have high vapor pressure near their melting point.
  • 77. Gas phase synthesis of nanomaterials • This method allow elegant way to control process parameter in order to be able to produce size, shape, and chemical composition controlled nanostructures. • In conventional chemical vapor deposition (CVD) synthesis, gaseous products either are allowed to react homogeneously or heterogeneously depending on a particular application. 1. In homogeneous CVD, particles form in the gas phase and diffuse towards a cold surface due to thermophoretic forces, and can either be scrapped of from the cold surface to give nano powders or deposited on to a substrate to yield what is called ‘particulate films’. 2. In heterogeneous CVD, the solid is formed on the solid surface, which catalyses the reaction and a dense film is formed.
  • 78. Advantages of gas phase processes • An excellent control of size, shape, crystallinity and chemical composition . • Highly pure materials can be obtained • Multicomponent systems are relatively easy to form • Easy control of the reaction mechanisms Most of the synthesis routes are based on the production of small clusters that can aggregate to form nano particles.
  • 79. Schematic representation of gas phase process of single phase nanomaterials from a heated crucible
  • 80. Flame assisted ultrasonic spray pyrolysis • In this process precursors are nebulized and then unwanted components are burnt in a flame to get required materials. ZrO2 has been obtained by this method from a precursor of Zr(CH3 CH2 CH2O)4. • This process is mainly used for synthesis of fumed silica. • Silicon tetrachloride is heated in an oxy-hydrogen flame to give highly dispersed silica. The resulting amorphous powder consists of spherical particles with sizes in the range 7-40 nm. • In combustion flame synthesis mixture of acetylene and oxygen or hydrogen and oxygen supplies the energy to initiate the pyrolysis of precursor. This process is widely used for industrial production of powders in large quantities, such as carbon black, fumed silica, and titanium dioxide. • At high pressure, highly agglomerated powders are produced which is disadvantageous for subsequent processing. The basic ides for low pressure flame synthesis is to reduce agglomeration. Aerosol scientists widely use this process. • Exact control of the flame produces a flat flame front. Due to oxidative atmosphere of the flame, this synthesis process is limited to the formation of the oxides.
  • 81. Schematic diagram of flame assisted ultrasonic spray pyrolysis
  • 82. Chemical vapour condensation (CVC) • This technique involves pyrolysis of vapors of metal organic precursors in a reduced pressure atmosphere. • Nanoparticles of ZrO2, Y2O3, have been produced by this method. • The evaporative source used in GPC is replaced by a hot reactor in the Chemical vapor condensation method. • Nanosized particles metals and ceramics can be obtained in good amounts by adjusting the residence time of the precursor molecules, changing the gas flow rate, pressure difference between the precursor delivery system and the main chamber and the temperature of the reactor. • The production capacities in CVC process are much higher than GPC system.
  • 83. • In addition to the formation of nanoparticles of a single precursor The CVC reactor can also be used for the synthesis of mixture of nanoparticles of two phases by supplying two precursors at the front end of the reactor, and • Coated nanoparticles like n-ZrO2 coated with n-Al2O3 or vice versa, by supplying a second precursor at the second stage of the reactor.
  • 84. Schematic diagram of set up for GPC system followed by consolidation in a mechnical press or collection in an appropriate solvent media
  • 85. Laser ablation • This method is extensively used for preparation of nanoparticles and particulate films. • In this process a laser beam is used as the primary excitation source of ablation for generating clusters directly from solid sample. • The small dimension of the particles and the possibility to form thick films make this process an efficient tool for the production of ceramic particles and coatings. • Synthesis of ZrO2, SnO2 nanoparticls thick films are synthesized using this process. • Synthesis of other materials such as lithium manganate, silicon and carbon has also been carried out by this technique.
  • 86. Thermolysis • This method involves solvent free pyrolysis of a suitable precursor material. One of the oldest nanomaterials, activated charcoal is prepared using this method. Precursor materials are pyrolyzed at 600-900 °C in the absence of air to obtain charcoal of specific surface area. • In the process the materials are carbonized in oxidizing atmosphere of CO2,O2 or steam at 600-1200 °C and activation is carried using chemicals such as KOH, NaOH, ZnCl2 or phosphoric acid.
  • 87. Solvothermal synthesis • This is a versatile method for synthesizing nanomaterials at temperatures below 200 C. This technique enables synthesis of crystalline products at low temperature and also helps in control the size and morphology of the resultant products. Electrodeposition • This technique involves creation of solid materials from electrochemical reactions in liquid. A conducting substance is placed in liquid containing electrolytes. When a potential is applied, redox reaction takes place and the material is deposited as thin film at the cathode. Semiconductor ZnO nanotube arrays are synthesized by direct electrochemical deposition from aqueous solutions into the pore channels of anodically-formed alumina .