Download Nanomaterials Module2

Nanomaterials
Module2
Nanoscale
Nanoscale
Nano - Dwarf
Nano size: 1 nm = 10-6 millimeter (mm) = 10-9 meter (m) nm
Cross section of human hair
Scale
•Nano-materials: Used by humans for 100 of years, the beautiful ruby red color
of some glass is due to gold Nano particles trapped in the glass (ceramic)
matrix.
•The decorative glaze known as
luster. Ruby Red glass pot
(entrapped with gold
nanoparticles)
What’s special with Nano?
The properties of nanomaterials deviate from those of single crystals or polycrystals
(bulk). For example, the fundamental properties like electronic, magnetic, optical,
chemical and biological Surface properties: energy levels, electronic structure, and
reactivity are different for nano materials.
Exhibit size dependent properties, such as lower melting points, higher energy
gaps etc. On the Surfaces and interfaces basics:
Bulk. In bulk materials, only a
relatively small percentage of
atoms will be at or near a
surface or interface (like a
crystal grain boundary).
Nano. In nanomaterials, large no.
of atomic features near the
interface.
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Nanostructured materials
Nanoparticles
Nanowires
Nanotubes
Nanorods
Nanoporous materials
Bulk and Nanoscale
Density of states for 3D, 2D, 1D, 0D showing discretization of energy and discontinuity of DOS
Size variation
Various size of CdSe nanoparticles and their solution. The bulk CdSe is black
Effect of Nano size
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Nano size increase the surface
Change in surface energy
Change in the electronic properties
Change in optical band gap
Change in electrical conductivity
Higher and specific catalytic activity
Bulk and Nanoscale
Bulk (eg. Gold)
Nano (eg. Gold)
1. Lustrous–Shiny surface when
polished.
2. Malleable–Can be hammered,
bent or rolledany desired shape.
3. Ductile–Can be drawn out into
wires
4. Yellow colour when in a mass
5. Heat & electricity conductor
6. High densities
7. High melting point (1080oC)
8. Tough with high tensile strength
9. Inert-unaffected by air and most
reagents
1. Vary in appearance depending on
size & shape of cluster.
2. Are never gold in colour!.
3. Are found in a range of colours.
4. Are very good catalysts.
5. Are not “metals” but are
semiconductors.
6. Melts at relatively low temperature
(~940º C).
7. Size & Shape of the nanoparticles
determines the color.
8. For example; Gold particles in
glass:
25 nm —
Red reflected
50 nm —
Green reflected
(Unexpected visible properties & they
are small enough to scatter visible
light rather than absorb)
Optical properties of Gold NPs
Size, shape, change the optical
properties of nano sized gold
§4.1.1.1 Gold nanoparticles
Red
Yellow
Size
increase
Size increase
Fig 1. Size and
shape dependent
colors of Au & Ag
nanoparticles
Green
Blue
Orange
Brown
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nanotubes
Au - Nanotubes
Silver : Bulk - Nano
• In nano size not only the surface area increased the electronic
properties are modified. This influence the optical, electrical,
catalytic properties
• It improve the selectivity in catalysus
Silver nanocubes
Silicon
SiO2 - Nanotubes
Nanomaterials synthesis approach
1.Top down approach: Breaking of bulk material
2.Bottom approach: Build up of material
Atommoleculecluster
Preparation
Nanomaterials preparation
Physical Methods
Ball milling
Laser ablation
Gas condensation processing (GPC)
Chemical Methods
Sol-gel synthesis
Solution phase ( stabilizing Ligands)
Precipitation method
Chemical vapour condensation
Catalytic chemical vapour deposition
Template assisted CVD
Electrochemical method
Preparation
Any Preparation technique should provide:
1. Identical size of all particles (mono sized or uniform size
distribution).
2. Identical shape or morphology.
3 Identical chemical composition and crystal structure.
4 Individually dispersed or mono dispersed i.e., no
agglomeration.
Preparation – Physical method
High-Energy ball milling (Top down approach) :
*Interest in the mineral, ceramic processing, and
powder metallurgy industry.
* Involves milling process include particle size
reduction .
* Restricted to relatively hard, brittle materials which
fracture and/or deform during the milling operation.
* Different purposes including; tumbler mills, attrition
mills, shaker mills, vibratory mills, planetary mills, etc.
Violent or agitation,
~50 m  nm
Schematic representation of
the principle of mechanical
milling.
*Hardened steel or tungsten carbide (WC) coated balls the basic process of
mechanical attrition (rubbing away) .
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Preparation – physical method
• Limitation of Ball milling: (Even though high
production rates)
1. Severe plastic deformation associated with mechanical
attrition due to generation of high temperature in the
interphase, 100 to 200º C.
2. Difficulty in broken down to the required particle size.
3. Contamination by the milling tools (Fe) and atmosphere
(trace elements of O2, N2, in rare gases) can be a problem.
(inert condition necessary)
Preparation – Physical method
(B) Gas Condensation Processing (GPC)-Bottom-up approach:
Thermal or electric or
e- beam evaporation
(like PVD)
Metal in
crucible
Cooling
(Rotating cylinder)
Liquid N2 (-80oC)
Metal cluster
(gaseous state)
Homogenous
nucleation in gas
phase
Nanoparticles
deposits
(2-50nm)
scrapping
Collection of the
nanoparticles
General Scheme for GPC for
the nanoparticle synthesis
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•Major advantage over conventional gas
flow is the improved control of the particle
sizes.
Schematic representation of typical set-up for gas
condensation synthesis of nanomaterials followed by
consolidation in a mechanical press or collection in
an appropriate solvent media.
•These methods allow for the continuous
operation of the collection device and are
better suited for larger scale synthesis of
nanopowders.
•However, these methods can only be used
in a system designed for gas flow, i.e. a
dynamic vacuum is generated by means of
both continuous pumping and gas inlet via
mass flow controller.
Limitation:1.Control of the composition of the
elements has been difficult and
reproducibility is poor.
2.Oxide impurities are often formed.
The method is extremely slow.
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Preparation
(C) Chemical Vapour Condensation (CVC) (Bottom-up approach):
•Involves pyrolysis (heat
treatment) of vapors of metal
organic precursors (starting
materials) like
Hexamethyldisilazane (CH3)3Si-NHSi-(CH3)3 to produce SiCxNyOz.
•Evaporate source in the GPC is
replaced by a hot wall reactor in
the CVC process (Fig.5).
Fig. 5 A schematic of a
typical CVC reactor
•Precursor residence time is the
key parameter to control the size
of nanoparticle here (gas flow
rate, pressure, heating
temperature can be controlled).
•Other procedure similar to GPC.
Production capabilities are much larger than in the GPC processing.
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Preparation
Tubular furnace for synthesis of nanomaterials, nanowires by Chemical vapour deposition
Preparation - Chemical Methods (Bottom-up approachs): Wet
Chemical Synthesis of nanomaterials (Sol-gel Process)
1.
Very popular & widely employed to prepare oxide materials
(SiOx).
2.
The sol-gel process: formation of a colloidal suspension (sol)
gelation of the sol to form a network in a continuous liquid
phase (gel) solid.
3.
Metal or metalloid element surrounded by various reactive
ligands (Si(OCH3)4, tetramethoxy silane, TMOS, alkoxide) is the
reactant.
4.
The starting material is processed to form a sol in contact with
water or dilute acid. Removal of the liquid from the sol yields
the gel, and the sol/gel transition controls the particle size and
shape. Calcination of the gel produces the product (eg. Oxide).
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Sol – Gel synthesis
Sol-Gel
• Sol-gel processing refers to the hydrolysis and condensation of alkoxide-based
precursors such as Si(OEt)4 (tetraethyl orthosilicate, or TEOS).
• The reactions involved in the sol-gel chemistry based on the hydrolysis and
condensation of metal alkoxides can be described as follows:
Classic sol-gel reaction scheme
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Over all Steps:
Step 1: Formation of different stable solutions of the alkoxide (the sol).
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Step 2: Gelation resulting from the formation of an oxide- or alcohol- bridged
network (the gel) by a polycondensation or polyesterification reaction
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Step 3: Aging of the gel, during which the polycondensation reactions continue until
the gel transforms into a solid mass, accompanied by contraction of the gel network
and expulsion of solvent from gel pores.
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Step 4: Drying of the gel, when water and other volatile liquids are removed from the
gel network.
– If isolated by thermal evaporation, the resulting monolith is termed a xerogel.
– If the solvent (such as water) is extracted under supercritical or near super
critical conditions, the product is an aerogel.
•
Step 5: Dehydration, during which surface- bound M-OH groups are removed, there
by stabilizing the gel against rehydration. This is normally achieved by calcining the
monolith at temperatures up to 8000C.
•
Step 6: Densification and decomposition of the gels at high temperatures (T>8000C).
The pores of the gel network are collapsed, and remaining organic species are
volatilized. The typical steps that are involved in sol-gel processing are shown in the
schematic diagram below.
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Wet chemical synthesis
• Use of chemical stabilizing agents
• Preparation of different types of nanostructures
• Stabilizing agents - eg., Citrate, Thiols, Surfactants,
coordinating polymer
• Use of different reducing agents
• Coordination of stabilizing agents with the nanostructures
• Stability of the nanostructures depends on the chemical
nature of the stabilizing agents too.
• Control on the composition, size, shape of the
nanostructures
• Larger control on the reaction rates during preparation
Wet chemical synthesis
• Preparation nanoparticles
• HAuCl4 + Stabilizing agent + NaBH4
Au nanoparticles
AgNO3 + Stabilizing agent + NaBH4
Ag nanoparticles
HAuCl4 + AgNO3 + Stabilizing agent + NaBH4
AuAg alloy NPs
Stabilizing agents – Sodium citrate, Alkanethiols, alkylammonium
salts, R- amines, R-COOH, surfactants etc
Precipitation method
•
Nanomaterials are produced by precipitation from a
solution.
•
The method involves high degree of
homogenization and low processing temperature.
•
ZnS powders were produced by reaction of aqueous
zinc salt solutions with thioacetamide (TAA).
TAA
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Precursor zinc salts were chloride, nitric acid
solutions, or zinc salts with ligands (i.e.,
acetylacetonate, trifluorocarbonsulfonate, and
dithiocarbamate).
0.05 M Zn2+/
70oC/pH2
Eg.1: The 0.05 M cation solution was heated in a thermal bath maintained at 70° or 80 °C
in batches of 100 or 250 ml. Acid was added dropwise to bring it to a pH of 2. The
reaction was started by adding the TAA to the zinc salt solution, with the molar ratio of
TAA and zinc ions being set to an initial value of either 4 or 8.
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Applications of nanomaterials
• In major view nanomaterials has found their
application in many major areas
• Electronics
• Medicine
• Industries
• Environment
• Sensing
Applications of nanomaterials
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Catalysis
Nanotransitors, Field effect transistors
Field emission
Scanning probes in STM
MEMS devices
Hydrogen storage
Energy conversion devices
Nanomedicine
Chemical , bio, pressure, thermal sensors
Magnetic materials
Applications of nanomaterials
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Gaint magnetic resistance materials
Nanomachining
Nanodevices
Nanolithography
Magnetic storage disk materials
Thermoelectric materials
Piezoelectric materials
Nanoelectrodes
Applications of nanomaterials
Carbon Nanotubes (CNTs)/Basics
* Discovered accidently during bulk preparation
of C60 by the arc method.
*Graphite carbon needles grew on the -ve
side carbon electrode (arc method)
*CNT also member of Fullerene structural
family.
Fig. 6 Prof. Iijuma (Japan)
with a CNT model.
~1 mm
nm
Fig.7 Shape and structure of Carbon nanotube (SWNT).
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CNT
• Tubular structure
MWNT & SWNT
High resolution scanning tunneling
micrograph of two single walled nanotubes
High resolution Transmission electron
micrograph of two multi walled nanotubes
§4.3.2 Chemical bonding Structure
Fig.8 Two or more nested tubes of
CNTs. Comparative sheet live
structures are also given.
sp2 boned carbon
sp2 boned carbon
Fig. 9. Chemical structure of graphite (A)
and CNT (B). Both having sp2 bonded
carbons.
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CNT Synthesis
•Preparation by Arc Method (as like C60, but
experimental conditions are different) by graphite
electrodes.
•Conditions:
•Larger amount of He gas (0.7 atm
pressure/500 torr)
•Distance between the graphite electrode
~1mm.
•Arc evaporation of graphite with He or Ar or
CH4 or H2 (effective)
•Maintaining Plasma condition
•Carbon fibre like deposit on the –ve graphite electrode.
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CNT synthesis – Electric arc method
CNT synthesis
Use of double furnace – catalytic chemical vapour deposition (CCVD).
Organometallic/hydrocarbon copyrolysis
a – sublimation of precursor
b – decompostion of precursor and growth on the substrate at
high temperature furnace
c – densly packed and aligned MWNT grown by CCVD
• Highly crytalline Multiwalled carbon nanotube (MWNT) by the arc method
with liquid N2 (Arc submerged in):
– Vacumm is replaced with liq. N2 in the chamber
– After the arc discharge, carbon deposits near the –ve electrodesnot
sticked
– Reaction product ~70% MWNT
• Chemical vapour deposition (CVD):
– Carbon source in the gas phase and plasma with resistively heated coil 
transfer to carbon molecule
– Common carbon sources: CO, Methane and Acetylene.
– Energy source cracks the molecule into atomic carbon  diffuse
towards heated coil (Ni, Fe or Co) and bind on it.
– Two steps involved:
• (1) Catalyst Preparation (Fe, Ni, Co or alloys)
• (2) Actual CVD (Yeild ~30%)
• Other CVD methods: Plasma-enhanced CVD, alcohol catalytic CVD, Aerogel supported CVD and Laser-assisted CVD.
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CNT synthesis
• Template technique - Catalyst free formation
of CNT
SWNT Purification
• Arc method synthesized SWNT always have impurity of the metal
catalyst particles
– Other impurities are; soot, amorphous carbon & smaller fullerenes
• Strong oxidation & Acid Refluxing techniques are commonly used in the
Industry for the cleaning
• Methods:
1. Structure selective
2. Size selective
• Specific Techniques:
• 1. Oxidation: Eg. treatment with H2O2 and H2SO4
Good Way to remove carbonaceous impurities or to clear
metal on the surface.
the
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• Limitations:
– Oxidation of both impurities & SWNTs
– Experimental conditions must be controlled
• Damage to SWNT is relatively less than the damage to the impurities.
• The process depend upon:
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–
–
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Metal impurity content
Oxidation timings
Environment
Oxidizing agent and Temperature
• Example: The H2O2 and H2SO4  can clean the metal surface
• If O2 present in the medium  rupture the CNT
Metal (M)
CNT
MOx
+ MOx
oxidized
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• 2. Acid Treatment:
– Will remove the metal catalyst (impure)
– No effect to CNT; H+ only effect to metal
– Egs. HNO3 or 4M HCl
• 3. Annealing (heating):
– High temperature vacuum treatment (1873 K)  the metal will be melted
and removed
• 4. Ultrasonication:
– Metal & Impurities are separated due to the strong vibrations
– Solvent & surfactant having critical role in the process
– Time also a controlling factor
• 5. Magnetic Purification:
– Ferromagnetic (catalytic) particles are mechanically removed
• 6. Micro-filtration:
– Using CS2 as solvent (fullerenes soluble)  filtered
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