Nano materials

For example, the fundamental properties like electronic, magnetic, optical, chemical and biological Surface properties: energy levels, electronic structure, and reactivity are different

Nano materials

Module2

Nanoscale

Cross section of human hair

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

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)

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.

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 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

• 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

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

Size increase Size increase

Fig 1 Size and shape

dependent colors of Au & Ag

nanoparticles

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 catalysis

Silver nanocubesSilver nanowires

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

• 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

SiO2 - NanotubesSiO2 - Nanospheres

Si - SiO2 - Nanotubes

Nanomaterials synthesis approach

1.Top down approach: Breaking of bulk material

2.Bottom approach: Build up of material

Atom→molecule→cluster

Electrochemical method Reverse micelles

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.

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

*Hardened steel or tungsten carbide (WC) coated balls→ the basic process of mechanical attrition (rubbing away) (Fig.3)

Preparation – Physical method

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 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:

Nanoparticles deposits (2-50nm)

phase

Cooling (Rotating cylinder) Liquid N2 (-80 oC)

Collection of the nanoparticles

scrapping

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

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.

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

particle sizes

•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

1 Laser vaporization cluster beam is used→ preparing nanoparticle web-like structure

2 High energy pulsed laser (107 W/cm3, under

plasma) focused on the analyst

substrate→ can generate substrate vapor

(1014-1015 atoms/0.01cm2/10-8 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

(C) Laser ablation (Bottom-up approach):

After Ablation

Scanning electron microscope (SEM)’s picture of metal - Ablation

Fig 5 A schematic of a typical CVC reactor

•Involves pyrolysis (heat treatment) of vapors of metal

organic precursors (starting materials) like

Hexamethyldisilazane (CH3)3Si-NH-Si-(CH3)3 to produce

SiCxNyOz

•Evaporate source in the GPC is replaced by a hot wall

reactor in the CVC process (Fig.5)

•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

(D) Chemical Vapour Condensation (CVC) (Bottom-up approach):

Preparation

Tubular furnace for synthesis of nanomaterials, nanowires by Chemical vapour depositionThe precursor materials were converted to desired product and deposited on the substrateThe precursor material, temperature, the carrier gas are important in product formationWhen catalyst is used it is known as catalytic chemical vapour deposition (CCVD)

Preparation - Chemical Methods (Bottom-up approaches): Wet Chemical Synthesis of nanomaterials (Sol-gel Process)

1 Very popular & widely employed to prepare oxide materials (SiOx).

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).

Sol – Gel synthesis

Classic sol-gel reaction scheme

• 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:

• Over all Steps:

• Step 1: Formation of different stable solutions of the alkoxide (the sol).

• Step 2: Gelation resulting from the formation of an oxide- or alcohol- bridged network (the gel) by a polycondensation or polyesterification reaction

• 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

• 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

Wet chemical synthesis

• Use of chemical stabilizing agents

• Preparation of different types of nanostructures

• Stabilizing agents - eg., Citrate, Thiols, Amines, Carboxylates, Phosphine, Phosphine oxide, 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

• Shape and size depends on starting materials and reaction conditions

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

• Citrate method of Au nanoparticle preparation

HAuCl4 + Trisodium citrate + NaBH4 Au nanoparticles

Ratio of gold to citrate is important in size control

Wet chemical synthesis of Au NP

• Stabilization with thiols involves two phase synthesis ( Thiols bind strongly with gold due to soft character of Au and S)

• This method give smaller particle size and reduced dispersity

• The concentration of Au/thiol ratio determines the particle size

• The stability of particles depends on the chain length of thiols

Brust-Schiffrin method – two phase synthesis

Wet chemical synthesis of Au NP

• 1 AuCl4- is transferred into toluene from aqueous phase using tetraoctylammonium salts as phase transfer agent

• 2 it is reduced in presence of thiol to give the NPs

• Alloy nanoparticles with large control in their composition can be prepared by this method

1

2

Wet chemical synthesis

• Silver nanoparticles also can be prepared using citrate, thiol methods

• Silver nanocubes synthesis using AgNO3 and Ethylene glycol at 160 oC

0.05 M Zn2+/

70oC/pH2 TAA

• 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)

• Precursor zinc salts were chloride, nitric acid solutions, or zinc salts with ligands (i.e., acetylacetonate, trifluorocarbonsulfonate, and dithiocarbamate)

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

Precipitation method

Electrochemical method

• Large control on the rate of electro-deposition

• Substrate dependent

• Product depends on the electrolytic composition

• The size and shapes can be controlled by the method of deposition

• Methods of electrochemical preparation (Galvanostatic, potentiostatic, potentiodynamic, pulse method) Method have control on the composition, size and shape.

• Nucleation and growth of nanostructured can be studied by electrochemical method

• Different types of metal, alloy, metaloxiide nanomaterials can be prepared by this method.

Electrodeposited nanostructures

• Au nanoclusters electrodeposited on glassy carbon electrode by electroreduction of Au3+ solution

• Deposition was done using cyclic voltammetric method

• Nanowires of Au were deposited using polycarbonates template

Template synthesis - CNT

• Template technique - Catalyst free formation of CNT

Characterisation techniques

• Microscopy – SEM( scanning electron microscopy), TEM (transmisssion

electron), AFM(Atomic force), STM (scanning tunneling )

• Spectroscopic (UV-Vis, IR, NMR, Raman, XPS)

• Structural characterisation X-Ray Diffraction XRD, Synchrotron measurement, neutron diffraction

• Other surface techniques

Applications of nanomaterials

In major view nanomaterials has found their

applications in many major areas

• Energy production , storage and conversion devices

• Nanomedicine, drug delivery systems, disease diagnosis

• Chemical , bio, pressure, thermal sensors

• Magnetic materials

Carbon nanotubes (CNT)

Carbon nanotubes

Carbon Nanotubes (CNTs)/Basics

Fig 6 Prof Iijima (Japan) with a CNT model.

~1 mm

nm

Fig.7 Shape and structure of Carbon nanotube (SWNT)

* 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.