CNT Synthesis and Applications in 40 Characters

Carbon Nanotube: Synthesis,
Classification, and
Applications
5/3/2020 2:53 PM Nanomaterials for Energy and
Environmental Applications
1

Discovery of Fullerenes
• “C” 6 th atom in the periodic table. It has been found to be atleast a partial constituent in
over 90 per cent of all chemicals known to man.
• The structure of truncated icosahedron was already known in about more than 500 years.
• Archimedes is credited for discovering the structure and Leonardo da Vinci included it in
one of his drawings.
• In 1970, Eiji Osawa realized that a molecule made up of sp2 hybridized carbons could have
the soccer structure.
• Then, a group of Russian scientists independently proposed the C60 structure, the paper
published by Bochvar and Gal’pern in 1973 not only predicted some properties of C60, but
also of C20 (the smallest fullerene) as well.
• The first spectroscopic evidence for C60 and other fullerenes was published in 1984 by
Rohlfing and coworkers.
2

Carbon nanotubes: Background
C60: Fullerene or C60 is soccer-ball-shaped
C60 is a molecule that consists of 60 carbon atoms, arranged as 12
pentagons and 20 hexagons. The shape is the same as that of a soccer
ball.
The most striking property of the C60 molecule is its high symmetry
They vaporised graphite with a powerful laser in an atmosphere of helium gas. When they
analysed the resulting carbon clusters, they found many previously unknown carbon
molecules. These varied in size, but the most common molecule contained 60 carbon atoms.
Because of this work, Kroto, Smalley, and Curl were awarded the Nobel Prize in Chemistry
in 1996.
R. Buckminster Fuller
3

Properties
Unlike graphite or diamond, fullerenes are closed–cage carbon molecules,
consisting of a number of five-membered rings and six-membered rings.
In order to make a closed cage, all the fullerene molecules should have the
formula of C20+m, where m is a integer number. For example: the structure of C60
is a truncated icosahedron, which looks like a soccer ball with 12 pentagons and
20 hexagons.
The bonds in C60 are having two different kinds. As a result, the length of a bond
in a pentagon is 1.45Å, that of a bond between pentagons is 1.40Å. 1.54Å for SP3
Applications:
Lubricant,
Rocket fuel
Drug delivery
Promising preliminary biological activities, such as DNA photocleavage, HIV-
Protease (HIV-P) inhibition, neuroprotection and apoptosis.
4

Carbon Nanotube (CNT)
Discovery: S. Iijima in 1991
The tube like structure was produced during arc-
discharge evaporation method similar to that
used in fullerene synthesis, the needles grow at
the negative end of the electrode (Ar at 100 torr).
Each tube comprises coaxial tubes of graphite
sheets, ranging in number from 2 up to 50. Each
tube carbon atom hexagons are arranged in a
helical fashion about the needle axis.
The smallest dia (2.2 nm) corresponds roughly to
a ring of 30 carbon hexagons; this small dia
imposes strain on the planer bonds of the
hexagons and this causes two neighbouring
hexagons on the ring to meet at an angle of ~ 6º,
that for C60 is 42º.
C-C bond energy of C60 is lower than graphite,
Suggested that bending lower the BE.
(a) Five sheet, 6.7 nm dia, (b) Two
sheet, 5.5 nm dia, (c) Seven sheet, 6.5
nm dia, 2.2 ID
5

Diamond: The four valence electrons are thus equally distributed among the sp3orbitals, while
each orbital points to one of the four corners of a tetrahedron. The tetrahedral structure, together
with the highly directed charge density, give strength and stability to the  bonds. Consequently,
all the bonds in diamond are of the same length (1.54 Å) and bond angle (109.47 o).
Diamond is the hardest known material due to  bonds and no delocalized  bonds and also shows
electrically insulating. The electrons within diamond are tightly held within the bonds among the
carbon atoms. These electrons absorb light in the ultraviolet region but not in the visible or
infrared region, so pure diamond appears clear to human eyes. Diamond has unusually high
thermal conductivity.
Bonding in Carbon
Bonding of Carbon atoms
Six electrons present in the orbital
In ground state: 1S2 2S2 2Px
1 2Py
1
In excited state: 1S2 2S1 2Px
1 2Py
1 2Pz
1
yx PPSS 2221

zyx PPPSS 22221

6

Graphite: Three outer-shell electrons of each carbon atom occupy the planar sp2 hybrid
orbital to form three in-plane σ bonds with an out-of-plane π orbital (bond). This makes a
planar hexagonal network. The bond angle is 120°
van der Waals force holds sheets of hexagonal networks parallel with each other with a
spacing of 0.34 nm. The σ bond is 1.4 Å nm long. An out-of-plane orbital or electron is
distributed over a graphite plane and makes it more thermally and electrically conductive.
The interaction of the loose electron with light causes graphite to appear black. The weak van
der Waals interaction among graphite sheets makes graphite soft and hence ideal as a
lubricant because the sheets are easy to glide relative to each other.
Bonding in Carbon
7

CNT: When a graphite sheet is rolled over to form a nanotube, the sp2 hybrid orbital is
deformed for rehybridization of sp2 toward sp3 orbital or σ−π bond mixing. The circular
curvature will cause quantum confinement and σ−π rehybridization in which three σ bonds
are slightly out of plane; for compensation, the π orbital is more delocalized outside the tube.
This makes nanotubes mechanically stronger, electrically and thermally more conductive,
and chemically and biologically more active than graphite.
In addition, they allow topological defects such as pentagons and heptagons to be
incorporated into the hexagonal network to form capped, bent, toroidal, and helical
nanotubes whereas electrons will be localized in pentagons and heptagons because of
redistribution of electrons.
For convention, we call a nanotube defect free if it is of only hexagonal network and defective
if it also contains topological defects such as pentagon and heptagon or other chemical and
structural defects.
Bonding in Carbon
8

Applications
Applications:
• Materials
• Chemical and biological separation, purification, and catalysis
• Energy storage such as hydrogen storage, fuel cells, and the lithium battery
• Composites for coating, filling, and structural materials
• Devices
• Probes, sensors, and actuators for molecular imaging, sensing, and
manipulation
• Transistors, memories, logic devices, and other nanoelectronic devices
• Field emission devices for x-ray instruments, flat panel display, and other
vacuum nanoelectronic applications
9

CNT synthesis methods
•Arc discharge synthesis
•Laser ablation synthesis
•Thermal synthesis
Chemical vapor deposition (CVD)
High-pressure carbon monoxide synthesis
Flame synthesis
Plasma-enhanced chemical vapor deposition
(PECVD)
10

Arc discharge synthesis
•Arc discharge synthesis uses a low-voltage (~12 to 25 V),
high-current (50 to 120 amps).
•An arc is produced across a 1-mm gap between two
graphite electrodes 5 to 20 mm in diameter. An inert gas such
as He or Ar is used as the atmosphere for the reaction, at a
pressure of 100 to 1000 torr.
Iijima produced the first MWCNTs by this method. He found
that nanotubes formed on the cathode, along with soot and
fullerenes.
Both Iijima and Bethune found that SWCNTs could only form by adding metal catalyst
to the anode; specifically, Iijima used an Fe:C anode in a methane:argon environment,
while Bethune utilized a Co:C anode with a He environment.
By tailoring the Ar:He gas ratio, the diameter of the SWCNTs formed can be controlled,
with greater Ar yielding smaller diameters.
+
-
11

Laser ablation synthesis
The first large-scale (gram quantities) production
of SWCNTs was achieved in 1996 by the
Smalley’s group at Rice University.
The laser ablation technique uses a 1.2 wt. % of cobalt/nickel with 98.8 wt.% of graphite
composite target that is placed in a 1200°C quartz tube furnace with an inert atmosphere of
~500 Torr of Ar or He and vaporized with a laser pulse. A pulsed- or continuous-wave laser
can be used. Nanometer-size metal catalyst particles are formed in the plume of vaporized
graphite. The metal particles catalyze the growth of SWCNTs in the plasma plume, but
many by-products are formed at the same time. The nanotubes and by-products are
collected via condensation on a cold finger downstream from the target.
12

SWNT and MWNT
A SWNT is a hollow cylinder of a graphite sheet whereas a MWNT is a group of coaxial
SWNTs. SWNT was discovered in 1993.
A carbon nanotube is based on a two-dimensional graphite sheet. The chiral vector is OA
defined on the hexagonal lattice as
Ch = na1 + ma2; a1, a2 = unit vectors
Lattice vector OB = T
 = chiral angle = the angle between chiral and zigzag direction
Chiral (n, m)
(n, 0)
(m, m)
Ch
Ch
A
O
13

(0,0) (6,0)
(6,0) zigzag
(0,0)
(2,2)
(2,2) armchair
Different CNTs
(0,5) (1,5) (2,5) (3,5) (4,5) (5,5) (6,5) (7,5) )
(0,4) (1,4) (2,4) (3,4) (4,4) (5,4) (6,4) (7,4) (8,4)
(0,3) (1,3) (2,3) (3,3) (4,3) (5,3) (6,3) (7,3) (8,3)
(0,2) (1,2) (2,2) (3,2) (4,2) (5,2) (6,2) (7,2) (8,2) (9,2)
(0,1) (1,1) (2,1) (3,1) (4,1) (5,1) (6,1) (7,1) (8,1) (9,1)
(0,0) (1,0) (2,0) (3,0) (4,0) (5,0) (6,0) (7,0) (8,0) (9,0) (10,0)
a
b
x
y
Zigzag
Arm chair
(1,1)
(0,0)
14

Different nanotubes
By rolling a graphite sheet in different directions, two typical nanotubes can be obtained:
zigzag (n, 0), armchair (m, m) and chiral (n,m) where n>m>0 by definition. In the specific
example, they are (10,0), (6,6), and (8,4) nanotubes.
“Armchair” nanotubes correspond to the configuration with no 'twist' in
the rolling. Thus, if you were to follow one of the bonds and make
alternating left and right turns at the intersections, you would eventually
come back to your starting position.
Arm chair
15

Zigzag
16

Zigzag and chiral nanotubes are metallic when (n – m) = 3q (where q is an
integer) or semiconductors in all other cases. This includes armchair tube
where n = m.
17

Mechanical properties of CNTs
Material Young's modulus
(GPa)
Tensile Strength
(GPa)
Density (g/cm3)
Multi wall
nanotube
1200 150 2.6
Single wall
nanotube
1054 75 1.3
SWNT Bundle 563 150 1.3
Graphite (in plane) 350 2.5 2.6
Steel 208 0.4 7.8
Epoxy 3.5 0.005 1.25
Wood 16 0.008 0.6
18

Young’s modulus of SWNT and MWNT
Young’s modulus (Tensile stress/tensile strain) is independent of tube chirality,
but dependent on tube diameter. The highest value is from tube diameter
between 1 and 2 nm, about 1 TPa (1012 Pa).
Large tube is approaching graphite and smaller one is less mechanically
stable. When different diameters of SWNTs consist in a coaxial MWNT, the
Young’s modulus will take the highest value of a SWNT plus contributions
from coaxial intertube coupling or van der Waals force. Thus, the Young’s
modulus for MWNT is higher than a SWNT, typically 1.1 to 1.3 Tpa.
When many SWNTs are held together in a bundle or a rope, the weak van der
Waal force induces a strong shearing among the packed SWNTs. This does not
increase but decreases the Young’s modulus. It is shown experimentally that
the Young’s modulus decreases from 1 TPa to 100 GPa when the diameter of a
SWNT bundle increases from 3 nm (about 7 (10,10) SWNTs) to 20 nm.
19

The elastic response of a nanotube to deformation is also very remarkable.
Most hard materials fail with a strain of 1% or less due to propagation of
dislocations and defects. Both theory and experiment show that CNTs can
sustain up to 15% tensile strain before fracture. Thus the tensile strength of
individual nanotube can be as high as 150 GPa, assuming 1 TPa for Young’s
modulus (E = Tensile Stress/Tensile strain = /; Tensile strength is the ultimate
tensile stress before breaking) .
20

Improved Battery Life
Low-Power Switching of Phase-Change Materials with Carbon Nanotube Electrodes
Feng Xiong, Albert Liao, David Estrada, and Eric Pop
Science 323, 568-570 , 2011.
High-power lithium batteries from functionalized carbon-nanotube electrodes
Nature Nanotechnology, 5, 531–537 (2010)
21

FET (Field Effect Transistor)
A basic transistor has three terminals. A FET's three terminals are usually called
“source, “drain” and “gate”. Applying a threshold voltage to the gate allows a current to
flow from the source to the drain. A FET can be used as a switch simply by raising and
lowering the gate voltage around its threshold value. FETs also operate as current
amplifiers by increasing the gate voltage (above the threshold)
Gate voltage (VG)
Au/PtA MOSFET (metal oxide semiconductor field effect
transistor) is the fundamental transistor behind
most of todays electronics including computers. It
has a metal contact for the gate and is separated
from the bulk transistor (substrate) by an oxide
layer, typically SiO2 (silicon dioxide). This makes
sure no current flows through the gate.
S - type SWNT, m - n  3 q, are used. Where the
band gap Eg scales with diameter d as Eg ~ 1/d (Eg 
0.5 eV for d ~ 1.4 nm).
S-SWNT gives a metal/S-SWNT/metal gives a p-type
transistor characteristics with several order of
magnitude change in conductance under various
gate voltage.
22

Transistors
23

Nanotube Molecular Wires as Chemical Sensors
Sensing gas molecules is critical to environmental monitoring, control of
chemical processes, space missions, and agricultural and medical applications.
Upon exposure to gaseous molecules such as NO2 or NH3, the electrical
resistance of a semiconducting SWNT is found to dramatically increase or
decrease. This serves as the basis for nanotube molecular sensors.
The nanotube sensors exhibit a fast response and a substantially higher
sensitivity than that of existing solid-state sensors at room temperature. Sensor
reversibility is achieved by slow recovery under ambient conditions or by
heating to high temperatures.
Science, 287, 622-625, 2000
CNT + Gas  CNTe Gas + or CNT + Gas e
24

Changes of electrical characteristics of a semiconducting
SWNT in chemical environments. (A) Atomic force
microscopy image of a metal/S-SWNT/metal sample used
for the experiments. Nanotube diameter is ~ 1.8 nm. The
metal electrodes consist of 20 nm thick Ni, with 60-nm-
thick Au on top. (B) Current versus voltage curves
recorded before and after exposure to NH3. (C) Current
versus voltage curves recorded under Vg = +4 V, before
and after NO2 exposure.
25

Comparision
NO2: For comparison, a high-performance metal oxide sensor (Cd-doped
SnO2) operates at 250°C for detecting 100 ppm of NO2 with a response time of
50 s, a recovery time of ~8 min, and a sensitivity of ~300. A polypyrole-
conducting polymer sensor can detect 0.1% NO2 by an ~10% resistance change
in ~5 to 10 min at room temperature. Thus, the S-SWNT sensors have the
advantage of room temperature operation with sensitivity up to 103 over these
materials.
NH3: NH3-sensing with the same SWNT sample after recovery from NO2
detection. The conductance of the SWNT decrease ~100-fold after exposure to a
1% NH3 flow. The response times to 1% NH3 for five S-SWNT samples were ~1
to 2 min, and the sensitivity was ~10 to 100. For comparison, metal oxide NH3
sensors typically operate at 300° to 500°C, with a response time of ~1 min and a
sensitivity of ~1 to 100 toward 200 ppm to 1% NH3. Conducting polymer
sensors can detect 1% NH3 with a response time of ~5 to 10 min by an ~30%
resistance change at room temperature
26

Limitations to conventional water
purification systems
27

Nanotechnology-based Water Treatment
• Photocatalytic water treatment
• Adsorption with nanomaterials
• Incorporation of NPs in RO membrane to enhance efficiency
• Nanomaterials based membrane for filtration/desalination
• Although nanmoaterials are appreciated in water desalination, they are not
free from limitations.
• Drawbacks include thermal instability, requirement of high pressure, fouling,
pollutant precipitation, pore blocking, low influx, slow reaction, formation of
toxic intermediates, formation of freshly synthesized ion particles and
aggregation on storage. Low chances of reusability and unknown risks to eco-
systems are also major concerns.
28

CNT-based Membrane
• CNT membranes provide near frictionless water flowthrough themwith the
retention of a broad spectrum of water pollutants.
• The inner hollow cavity of CNTs provides a great possibility for desalinating
water.
• The high aspect ratios, smooth hydrophobic walls and inner pore diameter of
CNTs allow ultra efficient transport of water molecules.
• The smooth and hydrophobic inner core of the hollow CNTs can allow the
uniterrupted and spontaneous passage of water molecules with very little
absorption.
• Frictionless movement of water molecules with high velocities from 9.5 to 43.0 cm
s−1/bar speed through a 7 nm diameter membrane pore (conventional flow:
0.00015 and 0.00057 cm s−1/bar).
• CNT types and conformation play significant roles in water passage and
permeability.
• The water conductance of the (7, 7) and (8, 8) tubes are roughly double and
quadruple that of the (6, 6) tube, respectively.
Desalination 336 (2014) 97–109
29

CNT Membrane
30

Fabrication of CNT membrane
31

Functionalized CNT membrane
32

The Mechanism of Water Diffusion in Narrow
Carbon Nanotubes
Fickian (mean square displacement (msqd) scales linearly with time), single file
(msqd scales with the square root of time), and ballistic motions (msqd scales with
the square of time) act as three different isotherms to describe the nature of water
molecules passing through CNTs.
dr2  Ddt (1)
dr2  Fdt1/2 (2)
dr2  Bdt2 (3)
Where, dr2 means square displacement; D, F and B imply proportionality
coefficient; dt represents time.
• Eq. (1): describe water diffusion through CNTs by measuring self-diffusion
coefficient (D) from the mean square displacement (dr2) of water molecules as a
function of time.
• Eq. (2): single file mobility of water molecules through hollow nanotube, and is
dependent on pore size, connectivity and pore–fluid interactions. (anomalous
diffusive behavior in the long time limit).
• Eq. (3): From the ballistic motions, water molecules have small interactions with
CNTs.
Nano Lett., 2006, 6 (4), pp 633–639
33

• Fickian diffusion: In solid-liquid diffusion, a particular stress is created due to
the presence of swelling penetrants. This stress can cause even cracks, which
in other words, morphological changes are induced. Then these swelling and
stress fields can affect the diffusion.
• Single file diffusion: At long times the mean-square displacement (MSD) is
proportional to t1/2, as in the one-dimensional case.
34

Gas-switchable carbon nanotube/polymer hybrid
membrane for separation of oil-in-water
emulsions
Carbon nanotube/poly(N-diethylaminopropyl)
methacrylate (CNT/PDEAEMA)
RSC Adv., 2017, 7, 39465–39470
WCA = 10
WCA = 113
35

Practical example
Unique ability of gecko’s to climb walls and hang from ceilings
How??
Vacuum principle??
36

Van der Waals force
•They are relatively long ranged compared to other atomic or
molecular level forces (range is 0.2 nm to over 10 nm).
37

van der Waals force
Article in Nature: Adhesive force of a single gecko foot-hair. Kellar Autumn, Yiching
A. Liang, S. Tonia Hsieh, Wolfgang Zesch Wai Pang Chan, Thomas W. Kenny, Ronald
Fearing, Robert J. Full, Nature, 405, 681-685, 2000.
Setae
Seta Spatulae
Setae:
•Gecko’s foot has ~500,000 hair or
•Setae of each 30-130m and ~ 5
m dia.
•5000 setae/mm-2
Spatulae:
•Branches of 100-1000 hair
•Length 0.2-0.5 m
•Spatula shaped ending
•10-20 N force/seta
Gecko can produce 10 -100 N/100 mm2 pad area or 0.1 Nmm-2
Each seta produce 0.1/5000 = 20 N force
Spatula radius = 2 m, adhesion distance D ~ 0.3 nm
Spatulae force (S-P) = -AR/6D2
, A = Hamakar constant = 10-19 J
Spatulae force = 0.4 N
Setal force = 0.4  (100-1000) = 40-400 N
38

Force of single seta
(a) After an initial push towards the surface the parallel adhesive force to the surface
increased until the setal began to slide off the edge of the sensor
(b) Setal force parallel to the surface increased with the perpendicular preloading force
Maximum parallel force was observed if the seta was allowed to slide ~ 5 m.
39

Force of single seta
Consider spatula is a curved segment of sphere (R=2 m) and is separated by a
small distance. Flat surface were van der Waals forces become significant
atomic gap distance (D = 0.3 nm)
Spatulae force (S-P) = -AR/6D2 , A = Hamakar constant = 10-19 J
Spatulae force = 0.4 N
Setal force = 0.4  (100-1000) = 40-400 N
40

41

Applications
September 2005 / Volume 2 / Issue 9 / ISSN 1744-1560 / CTHEC2 / www.rsc.org/chemicaltechnology
1. B. Yurdumakan, N. R. Raravikar, P. M. Ajayan, A. Dhinojwala, Synthetic gecko foot-
hairs from multiwalled carbon nanotubes. Chem Comm, 3799-3801, 2005.
2. H. Lee, B. P. Lee, P. B. Messersmith, A reversible wet/dry adhesive inspired by mussels
and geckos. Nature 488, 338-342, 2007.
3. L. Ge, S. Sethi, L. Gi, P. M. Ajayan, A. Dhinojwala, Carbon nanotube-based synthetic
gecko tapes. PNAS 104, 10792-10795, 2007.
42

Dry adhesive tape
Normal viscoelastic tape:
•Rarely hang heavy objects on wall
•Stickiness is time- and rate dependent
•Do not work under vacuum (Space applications, for climbing of robots)
Carbonnanotube based “Gecko tape”
•Comparable shear stress on hydrophilic &
hydrophobic surface
•Shear force supported by the gecko tape is
very stable & time independent
• Visco elastic tape is stronger than gecko
when measured for a short period of time
43

CNT Synthesis and Applications in 40 Characters

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