Nano materials

Nano materials

Module2


Nanoscale


Nanoscale
Nano - Dwarf
Nano size: 1 nm = 10

-6

millimeter (mm) = 10

-9

meter (m) nm

Combination of atoms or molecules to form objects of nanometer scale

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.

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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

Nano. In nanomaterials, large no. of atomic features near
the interface.

crystal grain boundary).

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Nanostructured materials

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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


Effects of Nano size

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Properties depends on size, composition and structure

Nano size increases the surface area
Change in surface energy (higher)
Change in the electronic properties
Change in optical band gap
Change in electrical conductivity
Higher and specific catalytic activity
Change thermal and mechanical stabilities
Different melting and phase transition temperatures
Change in catalytic and chemical reactivities.


Bulk and Nanoscale
Bulk (eg. Gold)

Nano (eg. Gold)

1.
2.

o
7. High melting point (1080 C)

1.
2.
3.
4.
5.
6.
7.
8.

8. Tough with high tensile strength

25 nm

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Red reflected

9. Inert-unaffected by air and most reagents

50 nm

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Green reflected

Lustrous–Shiny surface when polished.
Malleable–Can be hammered, bent or rolled→any desired
shape.

3.
4.
5.
6.

Ductile–Can be drawn out into wires
Yellow colour when in a mass
Heat & electricity conductor
High densities

Vary in appearance depending on size & shape of cluster.
Are never gold in colour!.
Are found in a range of colours.
Are very good catalysts.
Are not “metals” but are semiconductors.
Melts at relatively low temperature (~940º C).
Size & Shape of the nanoparticles determines the color.
For example; Gold particles in glass:

(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

Green

Blue

Fig 1. Size and shape
dependent colors of Au & Ag

Orange

Brown

nanoparticles

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nanotubes

Au - Nanocrystals

Au - Nanorods

Au - Nanotubes

Different shapes of gold nanostructures were prepared using different methods


Silver : Bulk - Nano
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In nano size not only the surface area increased the electronic properties are modified. This influence the
optical, electrical, catalytic properties

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It improve the selectivity in catalysis

Silver nanowires

Silver nanocubes


Silver nanoparticles

Change in shape and size shows the difference in the
visible spectrum of Ag nanoparticles

The scattering or absorption is due to the localized surface
plasmon resonance


SPR
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Surface plasmon resonance SPR: The collective oscillations of the electron gas at the surface
of nanoparticles ( eg. 6s electrons of the conduction band for Au NPs) that is correlated with
the electromagnetic field of the incoming light, i.e., the excitation of the coherant oscillation
of the conduction band.
SP band provides some information regarding NPs band structure

Silicon

Si - SiO2 - Nanotubes

SiO2 - Nanospheres

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

Chemical Methods

Ball milling

Sol-gel synthesis

Gas condensation processing (GPC)

Wet chemical synthesis

Laser ablation

Precipitation method

Ion beam

Chemical vapour condensation

Electron beam

Catalytic chemical vapour deposition

Nanolithography

Template assisted CVD
Electrochemical method
Reverse micelles


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 (Fig.3).

* 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 µm → 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) (Fig.3).

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Preparation – physical method

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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. Thermal decomposition or
evaporation of materials

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)


(B) Gas Condensation Processing (GPC)-Bottom-up approach:

Thermal or electric or e- beam

Cooling

evaporation (like PVD)

(Rotating cylinder)
o
Liquid N2 (-80 C)
Nanoparticles

Metal in

Metal cluster

crucible

(gaseous state)

deposits
(2-50nm)

Homogenous nucleation in gas
phase
scrapping

Collection of the nanoparticles

Advantages of Gas Phase synthesis
* An excellent control of size, shape, crystallinity and chemical
composition
* Highly pure materials can be obtained
* Multicomonent systems are relatively easy to form
* Easy control of the reaction mechanisms


•Major advantage over conventional gas flow is the improved control of the
particle sizes.

Fig. 4 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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(C) Laser ablation (Bottom-up approach):

1. Laser vaporization cluster beam is used→ preparing nanoparticle web-like structure.
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2. High energy pulsed laser (107 W/cm , under

After Ablation

plasma) focused on the analyst
substrate→ can generate substrate vapor
2 -8
(1014-1015 atoms/0.01cm /10 pulse) and
T = 104K→ liquefaction process → nanoparticles. After Ablation Picture of metal –
Ablation.

3. ZrO2 and SnO2 nanoparticulates thick films were synthesized with quite identical
microstructure.

4. Synthesis of other materials such as lithium manganate, silicon and carbon has also been
carried out by this technique.
Scanning electron microscope (SEM)’s picture of metal Ablation

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