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Instructor: Instructor:
Dr. Marinella SandrosDr. Marinella Sandros
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NanochemistrNanochemistryy
NAN 601NAN 601
Lecture 14: Carbon Nanotubes
Allotropes of carbon have different covalent bonding arrangements.
Image: Mstroeck @ Wikipedia
diamond graphite buckyball nanotube• Carbon atoms form covalent bonds by sharing outer shell electrons
with each other• Diamond, graphite, buckyballs and carbon nanotubes all have different
covalent arrangements of carbon atoms • The differing covalent arrangements of carbon atoms lead to the
different properties of carbon allotropes.
Image: Google, © NDT Education Resource Centre
A covalent bond is a form of chemical bonding that is characterised by the sharing of pairs of electrons between atoms
Valence electrons are the electrons in the outer shell or energy level of an atom that form covalent bonds
A carbon atom has 6 electrons, 4 of which are Valence electronsTherefore, carbon atoms can form up to 4 Covalent Bonds
protonneutronelectron
6 protons + 6 neutrons
Why do Carbon Nanotubes form?
Carbon Graphite (Ambient conditions)sp2 hybridization: planar
Diamond (High temperature and pressure)sp3 hybridization: cubic
Nanotube/Fullerene (certain growth conditions)sp2 + sp3 character: cylindrical
Finite size of graphene layer has dangling bonds. These dangling bonds correspond to high energy states.
Eliminates dangling bonds Nanotube formation + Total Energy
Increases Strain Energy decreases
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CNT can be described as a sheet of graphite rolled into a cylinder
Constructed from hexagonal rings of carbon
Can have one layer or multiple layers
Can have caps at the ends making them look like pills
Information retrieved from: http://www.photon.t.u-tokyo.ac.jp/~maruyama/agallery/agallery.html
1970: Morinobu Endo-- First carbon filaments of nanometer dimensions, as part of his PhD studies at the University of Orleans in France. He grew carbon fibers about 7 nm in diameter using a vapor-growth technique. Filaments were not recognized as nanotubes and were not studied.
1991:Sumio Iijima-- NEC Laboratory in Tsukuba-- used high-resolution transmission electron microscopy to observe carbon nanotubes.
Single Wall CNT (SWCNT) Multiple Wall CNT (MWCNT) Can be metallic or semiconducting
depending on their geometry.
Nanotubes form different types, which can be described by the chiral vector
Armchair NT Zigzag NT
Chiral Tube
SWNTs with different chiral vectors have dissimilar properties such as optical activity, mechanical strength and electrical conductivity.
http://www.youtube.com/watch?v=l3lRDG1HAmA&feature=related
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Chirality - twist of the nanotube
Described as the vector R (n, m)
Armchair vector, R vector, angle
= 0º, armchair nanotube
0º < < 30º, chiral nanotube
> 30º, zigzag nanotube
Information and image retrieved from: http://www.pa.msu.edu/cmp/csc/ntproperties/
(a) Armchair (metallic)
(b) zig-zag (metallic)
(c) chiral chirality (semiconducting)
Image: Wikipedia
carbonatoms
covalentbonds
• Diamond is formed by a 3D box-like network of carbon atoms
• The continuous nature of thecovalent arrangements forms
a giant molecule
• Electrons are fixed.
Image: Wikipedia
• Graphite is formed by hexagonally-arranged carbon molecules forming 2D layers of sheets
• Electrons are free to move between each carbon sheet.
Image: Mstroeck @ Wikipedia
• Carbon atoms in buckyballs are arranged in a soccer ball shape
• C60 Buckyballs have 20 regular hexagon faces and 12 regular pentagon faces- these faces come together at 60 carbon atom vertices
• Electrons are localised internally due to the curvature of the structure.
Image: Wikipedia
• Carbon nanotubes are formed by a layer of hexagonally-arranged carbon atoms rolled into a cylinder- usually have half buckyballs on one or both ends
• Electrons are localised internally, and some can move along the length of the tube by ballistic transport
• Carbon nanotube diameter ~ 1nm• Carbon nanotube length can be a
million times greater than its width• Nanotubes can be
- single-walled (d = 1-2 nm), or - multi-walled (d = 5-80 nm).
++++++
+
no
+++++
no
Conducts electricity
++++++++++Buckyballs
++++++++++++++++Carbon Nanotubes
+++Not known+++++Diamond
+++++++++Graphite
+++Coal
Conducts heat
Tensile strengthHardnessAllotrope
200x stronger than steel of the same diameter The first synthetic material to have greater strength
than spider silk Excellent conductors of electricity and heat Have huge potential for product development.
Image: wafonso@flickr, digidreamgraphix@flickr
Given their unique properties, what can carbon nanotubes be used for?
Image: Schwarzm, Wikipedia
Image: [email protected]
• Scientists have developed the ‘blackest black’ colour using carbon nanotubes
• The carbon nanotubes are arranged like blades of grass in a lawn- they absorb nearly all light
• Use of carbon nanotubes in solar cells could vastly improve their efficiency.
Image: [email protected]
• Badminton racquet manufacturer Yonex incorporates carbon nanotubes into their cup stack carbon nanotubes racquets (www.yonex.com)
• American baseball bat manufacturer Easton Sports has formed an alliance with a nanotechnology company Zyvex to develop baseball bats incorporating carbon nanotubes
• Tennis racquets also incorporate carbon nanotubes (www.babolat.com).
Image: [email protected]
• Branching and switching of signals at electronic junctions is similar to what happens in nerves
• A carbon nanotube ‘neural tree’ can be trained to perform complex switching and computing functions
• Could be used to detect/respond to electronic, acoustic, chemical or thermal signals.
Image: [email protected]
• Carbon nanotubes are being used to develop flat screen televisions with higher resolution than the human eye can detect
• Your next TV screen could be thin, ultra-light and foldable…
Molecular Engineering
• Carbon nanotubes can be made using molecular engineering
• Molecular templates are created- under the right chemical conditions carbon atoms arrange themselves into nanotubes on the template
• This process is also known as chemical synthesis or self-assembly, and is an example of the ‘bottom-up’ approach to molecular engineering.
2 Approaches
• ‘Bottom-up’ approach: structures are built atom by atom- can use self-assembly or sophisticated tools (eg scanning tunnelling microscope, atomic force microscope) which can pick up, slide or drag atoms or molecules around to build simple nanostructures
• ‘Top-down’ approach: traditional engineering techniques such as machining and etching are used at very small scales- products tend to be refinements of existing products, such as electronic chips with more and more components crammed onto them.
Image: [email protected]
a) Arc Discharge b) Laser Abalation Involve condensation of C-atoms generated from evaporation of
solid carbon sources. Temperature ~ 3000-4000K, close to melting point of graphite.
Both produce high-quality SWNTs and MWNTs. MWNT: 10’s of m long, very straight & have 5-30nm diameter. SWNT: needs metal catalyst (Ni,Co etc.).
Produced in form of ropes consisting of 10’s of individual nanotubes close packed in hexagonal crystals.
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1. Gas enters chamber at room temperature (cooler than the reaction temperature)
2. Gas is heated as it approaches the substrate
3. Gases then react with the substrate or undergo chemical reaction in the “Reaction Zone” before reacting with the substrate forming the deposited material
4. Gaseous products are then removed from the reaction chamber
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Source of carbon atoms usually comes from an organic compoundMixed with a metal catalyst and inert gasAtomized and sprayed into reactor with temperatures ranging from 600ºC to 1200ºCPyrolysis of organic compound deposits carbon (as soot) and carbon nanotubes on reactor wall (usually a tube constructed from quartz)
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Typical Organic/Catalyst MixturesXylene/ferroceneToluene, benzene, xylene, mesitylene, and n-hexane/ferrocene Ethylene and ethanol/Fe, Co, and Mo alloys (K. Mizuno et al.)
Typical Carrier GasesArgonHydrogen
c) Chemical Vapor Deposition:
Hydrocarbon + Fe/Co/Ni catalyst 550-750°C CNT
Steps:• Dissociation of hydrocarbon.• Dissolution and saturation
of C atoms in metal nanoparticle.• Precipitation of Carbon.
Choice of catalyst material?
Base Growth Mode or Tip Growth Mode?• Metal support interactions
Electronic and Mechanical Properties are closely related to the atomic structure of the tube.
Essential to understand what controls the size, number of shells, helicity & structure during synthesis.
Mechanism should account for the experimental facts: metal catalyst necessary for SWNT growth, size dependent on the composition of catalyst, growth temperature etc.
Is uncatalyzed growth possible?
Simulations & Observations No! Spontaneous closure at experimental temperatures of
2000K to 3000K. Closure reduces reactivity.
Transition metal surface decorated fullerene nucleates SWNT growth
around periphery.
Catalyst atom chemisorbed onto the open edge. Catalyst keeps the tube open by scooting around the open edge, ensuring and pentagons and heptagons do not form.
Schematic illustration of a ‘sweep’ and ‘rotate’ brush that can be used to clean nanoparticles and narrow trenches, paint the inside of capillaries, and adsorb liquid chemicals trapped in small area. b)A dump of nanoparticles formed by a sweep brush.
c) 10-μm-wide trenches cleaned by sweeping the brush over the surface. Inset: Dispersed nanoparticles inside trenches before brushing.
d) A rotate brush attached to an electrical motor.
e) Use of a rotate brush first to clean the inside of a contaminated capillary (inner diameter of 300 μm), and then paint the inner wall red
Cao et al., Nature Materials,2005, 4, 540.
Illustration showing the dipping of a pyrene-functionalized nanotube brush to pick up silver ions in solution. d, XPS spectrum of Ag adsorption by as-grown (black) and pyrene-functionalized (red) brushes. Inset: Ag 3d peaks from pyrene-brushes.
Prototype built as a “backpack”
US Air Force is currently testing the device
Can filter large volumes of water from dirty sources
Even URINE!!!!
Their phenomenal mechanical properties, and unique electronic properties make them both interestingas well as potentially useful in future technologies.
Significant improvement over current state of electronics can be achieved if controllable growth is achieved.
Growth conditions play a significant role in deciding the electronic and mechanical properties of CNTs.
Growth Mechanisms yet to be fully established.
http://www.youtube.com/watch?v=19nzPt62UPg&feature=related
http://www.youtube.com/watch?v=ZwiXUAY2LuE&feature=related
http://www.youtube.com/watch?v=ikYhyjPjKBs&feature=related
