TREATMENT AND FUNCTIONALIZATION OF CNT

1. CARBON NANO TUBE
TREATMENT AND
FUNCTIONALIZATION
ASWATHY N R
2ND MSC BPS

2. WHY WE HAVE TO TREAT
CNTs
A variety of techniques have been used to synthesize CNTs including
electric-arc discharge, laser vaporization, and catalytic chemical vapour
deposition (CVD).
Most approaches, produce powders containing not only CNTs but also other
carbonaceous particles such as
amorphous carbon, fullerenes, nanocrystalline graphite,
and metals that were introduced as catalysts during the synthesis.

3. To a large extent, impurities embedded in CNTs
influence the physical and chemical
characteristics of the CNTs.
Different purification methods yield different
CNT characteristics and may be suitable for
the production of different type of CNTs.
In order to fully exploit the properties of CNTs,
effective purification is therefore needed
to remove all by-products, at the same time maintaining
the original structure and length of CNTs
as far as possible.

4.  A number of purification procedures have been reported in the literature,
both chemical and physical methods, depending on nanotube morphologies
(single-walled or multi-walled), growth processes, and metal catalysts.
 The chemical methods separate the synthesis products as a function of their
reactivity, generally resulting in CNTs of higher purity but causing remarkable
damages to the nanotube morphology and wide loss of products.
In fact, the unavoidable defects along the tubes and the pentagonal structures
at the tube ends are sites of high reactivity, and can be attacked by the
chemical treatments

5. On the other hand, the physical methods separate the synthesis products as
a function of their size and are not damaging the tubes, but it is more
complex and less effective compared to the chemical methods, thus
leading to a lower purity of CNTs.
Oxidation by heating, acids and oxidizing agents, alkali
treatment and annealing in inert gases are some examples of chemical
purification methods.

6. Purification of CNTs generally refers to the
separation of CNTs from other entities, such as
carbon nanoparticles, amorphous carbon, residual
catalyst, and other unwanted species.
Three basic chemical methods
Gas-phase
Liquid-phase
Intercalation methods.

7. Gas Phase
The first successful technique for purification of nanotubes was
developed by Thomas Ebbesen and co-workers.
These workers realized that nanoparticles, with their defect rich
structures might be oxidised more readily than the relatively perfect
nanotubes. They found that a significant relative enrichment of
nanotubes could be achieved this way.

8. Gas phase oxidation is a milder purification technique. In this treatment, carbon
impurities are selectively burnt out during heating at elevated temperature (250–500
C, depending on the material and oxidation conditions) in the presence of gaseous
oxidants such as oxygen, carbon dioxide, water vapour or ozone.
The major drawback of gas phase oxidation is the damage and high sample loss
under non-optimized purification conditions.
Gas phase oxidation is based on the assumption that SWCNTs are more resistant to
oxidation than C-impurities. Thus, C-impurities (mainly amorphous carbon) can be
selectively removed by purifying the sample at a temperature below the oxidation
temperature of SWCNTs, without damaging or destroying valuable nanotubes.

9. It has a drawback that metal particles cannot be directly removed, and further
acid treatment is needed.
In order to overcome this limitation, liquid phase purification that always
simultaneously removes both amorphous carbon and metal catalyst was
developed.
amorphous carbon and carbon particles can be eliminated more easily than
CNTs due to their higher oxidation reaction rate than CNTs.
The high oxidative activity demonstrated by the amorphous carbon is due to
the presence of hanging bonds with high energy, which can be easily
Oxidized; meanwhile the higher reactivity of the carbon nanoparticles can be
attributed to their big curvature and pentagonal carbon ring.
Liquid Phase

10. Single- walled carbon, such as C 60 can be oxidized more easily, and
its reactivity is increased with decreasing diameter.
The commonly used oxidants for liquid phase oxidation include HNO3,
H2O2 and KMnO4 or a mixture of H2O2 and HCl, or a mixture of H2SO4
and HNO3, or a KMnO4 and NaOH.
The shortcomings of this method are that it causes reaction products on the
surface of CNTs, adds functional groups, and destroys CNT structures
(including cutting and opening CNTs)
same time, the structures of different CNTs have an
indisputable effect on their oxidation rate

11. Intercalation
An alternative approach to purifying multi walled nanotubes was introduced
in 1994 by a Japanese research group.
This technique made use of the fact that nanoparticles and other graphitic
contaminants have relatively “open” structures and can therefore be more
readily intercalated with a variety of materials that can close nanotubes.
By intercalating with copper chloride, and then reducing this to metallic
copper, we are able to preferentially oxidize the nanoparticles away, using
copper as an oxidation catalyst.
“The first stage is to immerse the crude cathodic deposit in a molten copper
chloride and potassium chloride mixture at 400°C and leave it for one week.
The product of this treatment, which contains intercalated nanoparticles and
graphitic fragments, is then washed in ion exchanged water to remove excess
copper chloride and potassium chloride..

12. In order to reduce the intercalated copper chloride-potassium chloride metal,
the washed product is slowly heated to 500°C in a mixture of Helium and
hydrogen and held at this temperature for 1 hour. Finally, the material is
oxidized in flowing air at a rate of 10°C/min to a temperature of 555°C.
Samples of cathodic soot which have been treated this way consist almost
entirely of nanotubes.
A disadvantage of this method is that some amount of nanotubes are
inevitably lost in the oxidation stage, and the final material may be
contaminated with residues of intercalates.

13. Physical methods
 Ultra sonication
 Filtration
 Size-exclusive chromatography
• separate the impurities based on their size.
• These processes are relatively mild and do not cause severe
damages to the tubes, but they are normally more complex and
less effective.
• In general, physical methods are applied to separate and
remove the undesirable impurities such as Nano capsules,
aggregate and amorphous carbon

14. FILTRATION
general, all large aggregates were retained by the
larger pore size membranes, whereas CNTs were
retained on the smaller pore size membranes. In
this manner, the CNTs, polyhedral nanoparticles,
and large aggregates were separated from each
other during the filtration
Filtration with membranes of narrow pore size distribution has been developed
to separate the CNTs from impurities and also to fractionate the nanotubes by length.

15. Ultra Sonication
Sonication has been identified as one of the effective processes to get
rid of the amorphous impurities when the CNTs were treated with
high-energy ultrasound in the presence of the suitable solvents such
As dichloromethane and dichlorobenzene. During sonication, the
solvent molecules are able to interact with CNTs and hence lead to
solubilisation. Ultrasonic treatment normally causes an increase
in the isolation of the MWCNTs, while nearly no carbon nanoparticle
agglomerations were observed.

16. Chromatography
Purification and length separation of CNTs can
be achieved through chromatography. High perfor-
mance liquid chromatography (HPLC) and size
exclusion chromatography (SEC) are the most
commonly used techniques which are successful in
length separation.
SEC separation can be carried out without or with
the assistance of reagents to
improve the CNT dispersion in common solvent.
The results obtained showed that the shortened and
oxidized CNTs can be efficiently purified and sepa-
rated according to their length.

17. The purification of CNTs through the physical
methods have not attracted great attention compared
to that of chemical methods due to its mild condition
which normally leads to ineffective purification.
However, the advantages of these physical
separations are the impurities such as nanocapsules,
and amorphous carbon can be removed
simultaneously, and the CNTs are not chemically
modified.

21. Multi-Step Purification
Multistep purification of CNTs can be carried out by combining the chemical
treatment and physical separation in a multi-step procedure in order to effectively
remove the amorphous carbon, metal particles, and multi shell carbon nano capsules
Multi-step purification is necessary particularly when a single treatment is not
sufficient to simultaneously remove all the impurities that are present in the CNTs.

22. A multi-step purification which consists of four procedures has been
studied.
• Sohxlet extraction with toluene was first carried out to remove the
fullerene and soluble impurities
• Liquid phase oxidation with H2O2 to get rid of the amorphous carbon.
• The metallic particles in the sample were eliminated by carrying out acid
treatment with the presence of SDS[sodium dodecyl sulphate] surfactant.
• Finally, physical separation in SDS solution was conducted to remove
the graphite and protected metallic particles.
• It was found that these procedures are efficient and appropriate to obtain
high purity CNTs with minimal wall damage.

25. FUNCTIONALIZATION OF
CNTs

26. Functionalization of Carbon Nanotubes
 Pristine nanotubes are unfortunately insoluble in many liquids such as water,
polymer resins, and most solvents. Thus they are difficult to evenly disperse in a
liquid matrix such as epoxies and other polymers. This complicates efforts to utilize
the nanotubes’ outstanding physical properties in the manufacture of composite
materials, as well as in other practical applications which require preparation of
uniform mixtures of CNTs with many different organic, inorganic, and polymeric
materials.
 To make nanotubes more easily dispersible in liquids, it is necessary to physically
or chemically attach certain molecules, or functional groups, to their smooth
sidewalls without significantly changing the nanotubes’ desirable properties. This
process is called functionalization.
 The production of robust composite materials requires strong covalent chemical
bonding between the filler particles and the polymer matrix, rather than the much
weaker vander Waals physical bonds which occur if the CNTs are not properly
functionalized.

27. TYPES OF FUNCTIONALIZATION
 Endohedral functionalization
 Exohedral functionalization

28. Endohedral functionalization;
Here CNTs are treated by filling their inner empty cavity with
different molecules or nano particles

29. Exohedral functionalization
 It involves grafting of molecules on the outer surface of
nanotubes
 Several approaches have been developed and include defect
functionalization covalent functionalization and noncovalent
functionalization with surfactants or polymers
 The different types of exohedral functionalization can be
classified via the nature of the interactions between the surface
of carbon nanotubes and the functional groups or polymer
chains
 These interactions can rely upon covalent or non-covalent
bonds.

30. Functionalization possibilities for CNTs: defect functionalization (A),
covalent sidewall functionalization (B), noncovalent functionalization with
surfactants (C) and polymer wrapping (D)

31. Non-covalent functionalization with
surfactant or polymer
 The noncovalent interaction is based on van der Waals forces
or π-π stacking and it is controlled by thermodynamics
 The great advantage of this type of functionalization relies
upon the possibility of attaching various groups without
disturbing the π electronic system of the rolled graphene
sheets of CNTs
 The formation of non-covalent aggregates with surfactants is a
suitable method for dispersing individual nanotubes in
aqueous or organic solvents

32. The schematic representation of how surfactants may
adsorb onto the schematic representation of how
surfactants may adsorb onto the CNTs surface CNTs
surface

33.  Carbon nanotubes can be also wrapped with polymer chains to form supramolecular
complexes of CNTs
 Different steps in PE coating of nanotubes is given below

35.  Two major groups of chemical functionalization of CNTs via
covalent attachment can be distinguished, the end and “defect-
group” chemistry and the sidewall functionalization
 End and defect-side chemistry
 The functionalization via “end and defect-side” chemistry
consists to graft functional group directly on the already
existing defects in the structure of CNTs
 Indeed, carbon nanotubes are generally described as perfect
graphite sheets rolled into nanocylinders.
 In reality, all CNTs present defects and can be curved
Covalent functionalization

36. Typical defects in a SWNT
Typical defects in a SWNT. a) five-or seven-
membered rings in the carbon framework,
instead of the normal six-membered ring,
leads to a bend in the tube. b) sp 3 -hybrideized
defects (R=H and OH). c) carbon framework
damaged by oxidative conditions, which
leaves a hole lined with –COOH groups. d)
open end of the SWNT, terminated with
COOH groups. Besides carboxyl termini, the
existence of which has been unambiguously
demonstrated, other terminal groups such as -
NO2, -OH, -H, and =O are possible

37. Interaction of nanotubes with pyrene derivatives

39. Sidewall functionalization
 It involves grafting of chemical groups through reactions onto the π-
conjugated skeleton of CNTs
 The reactivity of CNT sidewalls remains low and sidewall-functionalization
is only successful if a highly reactive reagent is used, whereas the nanotube
caps are quite reactive due to their fullerene-like structure
 Another constraint for sidewall functionalization is the tendency of CNTs to
form
 bundles and to limit the available nanotube surface for the grafting of
chemical reagents
 A large majority of covalent sidewall functionalizations is carried out in
organic solvent, which allows the utilization of sonication process to
improve the dispersion of CNTs and, thus, the available surface of carbon
nanotubes

44. THANK YOU…………