Presentation1.pptx cnt

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

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.

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.

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

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, alkalitreatment and annealing in inert gases are some examples of chemicalpurification methods.

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.

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. 

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.

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

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

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..

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.

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

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.

Ultra Sonication Sonication has been identified as one of the effective

processes to getrid of the amorphous impurities when the CNTs were

treated withhigh-energy ultrasound in the presence of the suitable

solvents suchAs dichloromethane and dichlorobenzene. During

sonication, thesolvent molecules are able to interact with CNTs and hence

lead tosolubilisation. Ultrasonic treatment normally causes an

increasein the isolation of the MWCNTs, while nearly no carbon

nanoparticle agglomerations were observed.

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.

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.

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.

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.

FUNCTIONALIZATION OF CNTs

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.

TYPES OF FUNCTIONALIZATION

Endohedral functionalization

Exohedral functionalization

Endohedral functionalization;

Here CNTs are treated by filling their inner empty cavity with different molecules or nano particles

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.

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

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

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

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

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

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

Interaction of nanotubes with pyrene derivatives

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

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