Carbon nanotubes

CARBON NANOTUBES

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CONTENTS

• Introduction• Classification– SWNT– MWNT– Other structures

• Synthesis• Properties• Applications

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INTRODUCTIONIMPORTANCE

• Diamond– High stiffness– Poor electrical conductivity– High thermal conductivity

• Graphite– Electrical conductor– Less stiff

• Fullerene– Superconductors

• CNT– High elastic stiffness– High strength– High thermal conductivity 3

INTRODUCTION

• Allotrope of carbon.• Cylindrical nanostructure.• Hexagonal shaped arrangements of carbon.• Rolled in tubes.• Walls made of one atom thick sheets of carbon

called graphene.• sp2 hybridised carbon tubule rolled up into

graphene sheets giving chiral and non chiral arrangements.

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CLASSIFICATION

• Single Walled Nano Tubes(SWNTs).

• Multi Walled Nano Tubes(MWNTs).

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SWNTs

• Diameter close to 1 nm.• No restrictions on tube length.• Exhibit electrical properties that are not shared by

MWNTs.• Conducting properties are sensitive to degree of

graphitization, chirality and diameter.

Structure:Wrapping one atom thick layer of carbon in a seamless cylinder.

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SWNT (contd.)

• n,m are the chiral vectors. They tell about the way graphene sheet is wrapped.

• n and m are integers. They denote number of unit vectors in a direction in the honeycomb crystal lattice of graphene.

• Zigzag if m=0. C-C bonds parallel to tube axis.• Armchair if n=m. C-C bonds on opposite side of

each hexagon are perpendicular to tube axis.• Otherwise Chiral.

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These three arrangements can be metals or semiconductors.

General rule for metallicity of SWNT:• (n,n) tubes are metals.• (n,m) tubes with n–m=3j are tiny gap semiconductors (j≠0, integer).• Others, large gap semiconductors.

• Phil. Trans. R. Soc. Lond. A (2004)

SWNTs (contd..)

MWNTs

• Consists of multiple rolled layers of graphite (concentric circles).

• Telescopic motion ability.• Mechanical strength : mechanically stronger than

conventional carbon fibres.• Movable arms in nano-mechanical devices.

Two models1. Russian Doll Model2. Parchment Model

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OTHER STRUCTURES

TORUS• Called nanotori.• Have many unique properties like magnetic

moments.• Have magnetic moments 1000 times larger

than previously expected for any radii.• Properties like magnetic moment and thermal

stability depend upon the radius of the torus.

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NANOBUD• Combination of carbon nanotubes and

fullerenes.• Fullerene like buds are covalently bonded

to outer side walls of underlying carbon.• In composite materials, attached fullerene

molecules may function as molecular anchors preventing slipping of nanotubes, improving composite’s mechanical properties.

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GRAPHENATED CARBON NANOTUBES• Combines graphitic foliates grown along the

sidewalls of multi-walled or bamboo styled CNTs.

• Use in super capacitor applications.PEAPOD

• Novel hybrid carbon material which traps fullerene in CNT.CUP-STACKED CNTs

• Exhibit semiconducting behaviour due to stacking microstructure of graphene layers.

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SYNTHESIS

1. Arc Discharge

2. Laser Ablation

3. Chemical Vapour Deposition(CVD)

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ARC Discharge• Three main elements are required– Carbon feed– Metal catalyst– Heat

• Two graphite electrodes inserted vertically. Maintained at a constant distance of 1-2mm.

• Chamber evacuated with vacuum pump.• Chamber filled with ambient gas (usually inert gases like He or

Ar).• When DC current is passed, anode is consumed and material

forms on cathode (MWNT).• Mixed metal catalyst (Fe or Co) is inserted in anode to get

SWNT as soot in the chamber.16

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ARC DISCHARGE(contd..)• Samples on cathode were observed under

TEM & SEM and were found out to be MWNTs.

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ARC DISCHARGE (contd..)

• 100 mg/min SWNIT is produced.

DrawbackHigh amount of undesired by-products (fullerene, graphite & amorphous carbon).

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•New Diamond and Frontier Carbon Technology, Vol 16, No.3 (2006)

LASER ABLATION

• Pulsed laser made to strike a graphite target (containing small amount of Ni & Co) in a high temperature reactor.

• Inert gas is bled into the chamber.• Graphite target vaporised.• Nanotubes develop on cooler surfaces of the reactor as

the vaporised carbon condenses.• Tubes grow on catalyst atoms.• Yield is 70%.• SWNT are prepared.• Diameter is controlled by reaction temperature.• More expensive than the other two. 21

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Chemical Vapour deposition (CVD)

• A hydrocarbon vapour is passed through a tubular reactor in which catalyst material is present at high temperature (600-1200˚C) to decompose hydrocarbon.

• CNTs grow on the catalyst.• Collected upon cooling the system to room temperature.• For liquid hydrocarbons (benzene, alcohol etc.) , liquid is

heated in a flask and inert gas is purged through it to carry hydrocarbon vapour in reaction zone.

• If solid hydrocarbon used, it can be directly kept in the low temperature zone of reaction zone.

• Catalyst precursors can be used in any form: solid, liquid or gas.

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Chemical Vapour deposition (CVD)• Diameter of nano tube depends upon the size of metal

particle.CNT Growth mechanism:

• When hydrocarbon vapour comes in contact with hot metal nanoparticles, it decomposes into hydrogen and carbon species. Hydrogen is evolved and carbon dissolves in the metal.

• After reaching carbon solubility limit at that temperature, carbon precipitates out and crystallizes in the form of cylindrical tubes having no dangling bonds. Hence, they are energetically stable.

• Hydrocarbon decomposition (exothermic process) releases some heat and carbon crystallization (endothermic process) absorbs heat. It keeps the process on. 25

Chemical Vapour deposition (CVD)

Advantages • Simple and economic.• CNT synthesized at low temperature and

ambient pressure.• Better yield and purity.• Control on structure and growth parameters.

26•J. Nanosci. Nanotechnol. 10, 3739–3758, 2010

PROPERTIES

1. Strength• Strong and flexible.• Strength due to covalent sp2 bonds (stronger

than sp3 found in diamond).• Not strong under compressive, torsional or

bending stress.2. Hardness (152 GPa)

Greater than diamond (150 GPa).

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PROPERTIES (contd.)

3. Electrical propertyElectrical properties are sensitive to their structure. Graphene is a zero gap semiconductor and CNTs can be metals or semiconductors with varying energy gaps depending on the diameter and helicity of the tubes.

4. Thermal conductivityThermal conductivity of SWNT more than copper at room temperature.

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APPLICATIONS• Desalination– Water can be made to pass through a network of carbon

nanotubes which requires less pressure than the conventional osmosis methods.

• Solar cells– Nanotubes can act as transparent film in solar cells to allow

light to pass through active layers and generate current.• Ultracapacitors– Can be used to charge capacitors which will increase the

surface area and hence energy storage ability.• Light bulb filament– Alternative to tungsten filament.

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THANK YOU

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