Methods of Carbon Nanotube and Nanohorn Synthesis: A Review

Namdeo Jadhav

Until 1985, only two forms of pure-carbon structures were known as diamond and graphite.

But, Richard Smalley and Harry Kroto in November 1985, discovered C-60 (buckyball) during a series of experiments on the laser-vaporization of graphite.In 1990, Wolfgang Kratschmer and Donald Huffman used a carbon arc instead of laser to vaporize graphite¹. This produced macroscopic amounts of C-60 marking milestone in the beginning of fullerene research.

Mainly, Buckminster Fuller in 1985 from Rice University discovered carbon allotropes composed entirely of carbon in the form of a hollow sphere, ellipsoid, or tube. Spherical fullerenes were called as buckyball while cylindrical fullerenes as buckytubes or nanotubes². Their structure is composed of graphite sheets having pentagonal, hexagonal and sometimes heptagonal rings that prevent the sheets from being planar².

1) Carbon nanotubes (CNTs)

CNTs were discovered in October 1991 by Sumio Iijima. He studied the material deposited on the cathode during the arc-evaporation of graphite and found the cathodic deposition contained tubular structures called as nanotubes¹. These led to an explosion of research into the physical and chemical properties of CNTs all over the world.

CNTs possess various novel properties that make them useful in the field of nanotechnology and pharmaceuticals. They are tubular in shape, made of graphite and are members of the fullerene family only. They are nanometers in diameter and several millimeters in length. And, have a very broad range of electronic, thermal, and structural properties. These properties vary with kind of nanotubes defined by its diameter, length, chirality or twist and wall nature. Single walled nanotubes (SWNTs) and multiple walled nanotubes (MWNTs) are two types of nanotubes produced so far. Their unique surface area, stiffness, strength and resilience have led to much excitement in the field of pharmacy³.

Structural description of CNTs

CNTs are sp² bonded, with each atom joined to three neighbours, as in graphite. Thus, tubes are considered as rolled-up graphene sheets⁴.

Comparison between SWNT and MWNT^2,5

2) Carbon Nanohorns (CNHs)

Carbon Nanohorns (CNHs) with morphologies similar to those of CNTs were first prepared by Peter Harris, Edman Tsang and colleagues in 1994¹ and have the same graphitic carbon atom structure as that of CNTs. The main characteristic of the CNHs is that it creates an aggregate (a secondary particle) of about 100 nanometers when many of the CNTs group together⁶. CNHs are having high purity and can be easily prepared with very low cost. This are the only single walled nanotubes produced without a metal catalyst⁶, aggregated and possesses wide application in medical, bio-medical, chemical, catalytic processes, heat transfer, electrical conductivity, chemical filters, gas separation, methane and hydrogen storage. There are three adsorption sites on CNHs. The deepest site is the inside surface of the walls at tips and convex parts, the second deepest site is other wall surface regions and the shallowest site is the central region of the hollow space inside the SWNHs⁷.

Properties of Carbon nanotubes  and Carbon Nanohorns^5-10

CNTs and CNHs are one of the stiffest, strongest and toughest fiber that world has ever seen and have huge tensile strength. They are the best conductor of electricity on a nanoscale level. And, have thermal conductivity comparable to diamond along the tube axis. Strong van dar Waals attraction operating in them leads to spontaneous roping of many nanotubes because of self assembly. Being inert, it can be reacted and manipulated with the richness and flexibility of other carbon molecules. They have great molecular perfection and are free of defects.

3) Methods of Production of Carbon nanotubes  

Arc Discharge method^11-14

Arc Discharge method has been reported for producing fullerenes. It is the most common and easiest way to produce nanotubes. In this method, nanotubes are produced through arc-vaporization of two carbon rods placed end to end with a distance of 1mm in an environment of inert gas such as helium, argon at pressure between 50 to 700 mbar. Carbon rods are evaporated by a direct current of 50 to 100 Amps driven by 20V which will create high temperature discharge between two electrodes. Due to this anode will get evaporated and rod shaped tubes will be deposited on cathode. Bulk production of CNTs depends on uniformity of plasma arc and temperature of deposition.

Production of SWNTs:

In the synthesis SWNTs anode is dipped with a metal catalyst such as Fe, Co, Ni, Y, or Mo. It produces SWNTs with a diameter of 1.2 to 1.4nm. Efficiency of SWNT production by arc discharge method is improved with,

a) Inert Gas: Argon with a lower diffusion coefficient and thermal conductivity has given nanotube with smaller diameter (1.2nm) and 0.2nm diameter decrease with 10% increase in argon: helium ratio ,when Nickel and Yttrium is used as a catalyst (4.2: 1).

b)Optical Plasma Control: As the distance between anode and cathode is increases, anode vaporization increases, due to which strong visible vortices around cathode is occurred. With a nickel and yttrium catalyst (C/Ni/Y is 94.8:4.2:1) the optimum nanotubes were produced at a pressure of 660 mbar for pure helium and 100 mbar for pure argon. The nanotubes diameter ranges from 1.27 to 1.37 nanometer.

c) Catalyst: By changing metal catalyst, the nanotubes with a diameter of 0.6 to 1.2nm are produced. Catalysts used are Co and Mo.

d)Open Air Synthesis with Welding Arc Torch: This method is specifically used for SWNTs with graphite rod containing metal catalyst. Ni and Y (3.6: 0.8 at %) is fixed at side wall of water cooled, steel based electrode, torch arc aimed at the edge of target and soot is deposited on substrate behind the target. The arch is operated at 100amp current and shielding Ar gas flowed through torch to enhance arc jet formation. This method is very convenient and inexpensive with Ni: Y (3.6: 0.8).Nanotubes produced by this method is of diameter of 1.32nm.

Production of MWNTs

MWNTs produced with the use of pure graphite are with an inner diameter 1-3nm and outer diameter10nm (approx.). Since catalyst is not used in this process there is no need for a heavy acidic purification. So, MWNTs can be formed with a less number of defects. Different methods used to synthesize are,

a) Synthesis in Liquid Nitrogen¹²: MWNTs are formed by generating arc- discharge in liquid nitrogen. For which low pressure and expensive inert gas are not needed. Yield is about 70% of reaction product.

b)Magnetic Field Synthesis¹³: MWNTs formed by this method are defect free and having high purity. In this arc- discharge is controlled by a magnetic field around the arc plasma. Extremely pure graphite rods (purity > 99.999 %) are used as electrodes. Highly pure MWNTs (purity > 95 %) are obtained without further purification, which disorders walls of MWNTs.

c) Plasma Rotating Arc Discharge¹⁴: The centrifugal force caused by the rotation generates turbulence and accelerates the carbon vapor perpendicular to the anode and the rotation distributes the micro discharges uniformly and generates stable plasma. Consequently, it increases the plasma volume and raises the plasma temperature. At the rotation speed of 5000 rpm, a yield of 60 % was found at a temperature 1025 °c without the use of a catalyst. The yield can be increased up to 90% after purification if the rotation speed is increased and the temperature is enlarged.

Laser Ablation method¹⁴

A pulsed or continuous laser is used which will vaporize a graphite target in an oven at 1200 °c. The oven is filled with helium or argon gas in order to keep the pressure at 500 torr. Since the optimum background gas and catalyst mixture is the same as in the arc discharge process, is almost similar to arc discharge. This might be due to very similar reaction conditions and mechanisms. This method is very expensive so it is mainly used for SWNTs. Laser vaporization results higher yield of SWNTs with narrower size distribution than those produced in arc discharge process. Catalyst used for SWNTs is Ni: Y (4.2: 1 at %). MWNTs can be also produced with pure graphite.

Production of SWNTs

a) Ultra Fast Pulses from a Free Electron Laser (FEL) Method: In this method the pulse width is ~ 400 fs. The repetition rate of the pulse is increased from 10 Hz to 75 MHz The intensity of the laser bundle ~5 x 10¹¹ w/cm².A jet of preheated argon gas is located near the rotating graphite target. In that argon gas deflects the ablation 90away from incident beam direction, clearing carbon soot from front of target. If this system is upgraded with full power a yield of 45gm/ hr can be achieved. Catalyst used is NiCo or NiY.

b) Continuous Wave Laser-Powder Method: In this method instead of Nd: YAG laser, CO₂ laser is used in an argon stream. Laser ablation of mixture of graphite and catalyst powder is carried out, due to which thermal conductivity losses are significantly reduced. It is more economical in comparison with Nd: YAG laser system and yield is 5gm/hr.Catalyst used is Ni: Co 1: 1.

Chemical Vapors Deposition method^14-23

It is carried out in two step process:-

1. Catalyst is deposited on substrate and then nucleation of catalyst s carried via chemical etching or thermal annealing. Ammonia is used as an etchant. Metal catalysts used are Ni, Fe or Co.

2. Carbon source is then placed in gas phase in reaction chamber. Then carbon molecule is converted to atomic level by using energy source like plasma or heated coil. This carbon will get diffuse towards substrate, which is coated with catalyst and nanotubes grow over this metal catalyst. Carbon source used is methane, carbon monoxide, acetylene. Temperature used for synthesis of nanotube is 650 – 900 C range. The typical yield is 30%. Production of nanotubes by different chemical vapor deposition can be summarized as:

Flame Synthesis method

SWNTs are formed in controlled flame environment from hydrocarbon fuels and small aerosol metal catalyst islands. These Catalyst Island was prepared in three ways-

1. Coated mesh with catalyst

2. Burning filter paper rinsed with metal solution.

3. Metal powder was inserted in trough and evaporated.

Silane Solution Method²⁴

Carbon nanotubes were produced using a silane solution method, in which a substrate such as carbon paper or stainless steel mesh was immersed in a silane solution of a metal catalyst, preferably Co: Ni in a 1:1 ratio; and a feedstock gas containing a carbon source such as ethylene was fed through the substrate and the catalyst deposited thereon while the substrate was heated by applying an electrical current thereto. Thus, a reaction occurs between the catalyst and the gas to yield CNTssupported on the conductive substrate.

Purification of CNTs^25-26

Nanotubes usually contain a large amount of impurities such as metal particles, amorphous carbon, and multishell.There are different steps in purifications of nanotube.

·Air Oxidation:The carbon nanotubes are having less purity; the average purity is about 5-10%. So purification is needed before attachment of drugs onto CNTS. Air oxidation is useful in reducing the amount of amorphous carbon and metal catalyst particles (Ni, Y).Optimal oxidation condition is found to be at 673 k for 40 min.

·Acid Refluxing:Refluxing the sample in strong acid is effective in reducing the amount of metal particles and amorphous carbon. Different acids used were HCl, HNO₃ and H₂SO_4, but HCl was identified to be the ideal refluxing acid.

·Surfactant aided sonication, filtration and annealing

After acid refluxing, the CNT were purer but, tubes were entangled together, trapping most of the impurities, such as carbon particles and catalyst particles, which were difficult with filtration. So surfactant-aided sonication was carried out.

Sodium dodecyl benzene sulphonate (SDBS) aided sonication with ethanol (or methanol) as organic solvent were preferred because it took the longest time for CNTS to settle down, indicating an even suspension state was achieved. The sample was then filtered with an ultra filtration unit and annealed at 1273 k in N₂ for 4 h. Annealing is effective in optimizing the CNT structures. It was proved the surfactant-aided sonication is effective to untangle CNT, thus to free the particulate impurities embedded in the entanglement. Nanotube can also be purified by multistep purification method²⁶.

4) Methods of Production for Carbon Nanohorns (CNHs)^27-31

A method of synthesizing bulk amounts of single-walled carbon nanohorns (SWNHs) on a significant low-cost basis was proposed based on the 'arc in water' method with N₂ injection into the arc plasma. It was elucidated that rapid quenching of the carbon vapors in an inert gas environment was necessary to form the delicate SWNHs structures. To realize this reaction field, the arc plasma between the graphite electrodes was isolated from the surrounding water by a thin graphite wall with an N₂ flow that excluded reactive gas species (H₂O, CO and H₂) from the arc zone. High concentrations of SWNHs were found as fine powders floating on the water surface. The particle size distribution of the SWNHs produced by this method was evaluated by dynamic light scattering, resulting in an approximately 70 nm peak diameter^27-30.

SWNHs aggregates can be produced by CO₂ laser vaporization of carbon³¹, and a single aggregate can take either a "dahlia-like" or "bud-like" form. It was found that "dahlia-like SWNHs aggregates were produced with a yield of 95% when Ar at 760 torr was used as the buffer gas, while "bud-like" SWNHs aggregates were produced with a yield of 70 or 80% when either He or N₂ at 760 torr was used.

Purification of CNHs

Single wall carbon nanohorns were produced with high purity and yield than SWNTS by CO₂ laser ablation of pure graphite without metal catalyst and arc in water method. But SWNHS thus produced were always mixed with micrometer order graphite based particles viz., Giant Graphite (GG) balls, which were the obstacle for various applications of SWNHS.

·Gravitational Sedimentation Method³²

Prepared SWNHS were dispersed in ethanol with concentration of 1mg/30ml, by sonication using ultrasonic bath for 30min. The suspension liquid prepared in this way spunned at 9000 rpm for 30min.Then supernatant liquid was decanted and stocked. SWNH were present in this liquid obtained by evaporation of ethanol at 40c_.

5) Characterization Techniques for CNTs and CNHs³³

·Morphological (Size and Length)

TEM-Transmission electron microscope.

HRTEM-High resolution transmission electron microscope.

·Carbon content

TGA- Thermal gravimetric analysis

DSC- Differential scanning colorimetry.

·Co-content

Proton induced x-ray emission with proton beam of 2mev.

·Surface area

By nitrogen adsorption –desorption method

·Oxidised form of carbon on surface of CNTs and CNHs

By x-ray photo electron spectroscopy. (K alpha- 1.486kev)

6) Functionalisation of CNTs and CNHs^34-37:

A wire structure itself was very strong in graphite layer and was not chemically bonded to each other but was held together by van dar Waals force. So, adsorbed molecule tends to slip easily from surface of nanotubes due to their complete graphite structure. As it was also very hydrophobic in nature, it must be modified with certain chemical or their structure must be damage partially to hold the drug on surface of nanotubes and to reduce its hydrophobicity.Chemical oxidation and functionalization process was used for these modifications. Both this process will maintain properties like surface area, tubular shape, wettability and porosity and gave consolidation, compression, and compaction characteristics.

·Oxidation³⁴:

Chemical oxidation was carried out by heating at high temperature with different acids like H₂SO₄, HNO₃ in presence of air. It causes formation of pores and nano windows. Porosity will decide extent of drug loading, smaller molecule will get fitted into small pores.

·Functionalisation^35-37:

The functionalization of single wall carbon nanohorns was carried out by 1, 3-dipolar cycloaddition of azomethinylides and the products was characterized by spectroscopy, microscopy and thermogravimetry³⁵.

Covalent functionalization of multiwalled CNTs(MWNT) with poly (acrylic acid) has been successfully achieved via grafting of poly (acryloyl chloride) on nanotube surface by esterification reaction of acyl chloride-bound polymer with hydroxyl functional groups present on acid-oxidized MWNT and hydrolysis of polymer attached to nanotubes³⁷. Polymer-functionalized MWNT could possess remarkably high solubility in water, and their aqueous solution was very stable without any observable black deposit for a long time.

It should be noted that when functionalization is required to add new properties to the nanotubes. In the case of SWNT, covalent functionalization will break some C = C double bonds, leaving "holes" in the structure on the nanotube and thus modifying both its mechanical and electrical properties².

7) Brief Applications of CNTs and CNHs ³⁸

The exhaustive review of pharmaceutical applications of CNTs and CNHs is given by Pai Pundlik. et al. They are used to deliver protein³⁹by oral route and targeting of amphotericin B⁴⁰ to cells. Cisplatin³³ in oxidized single walled nanohorn (SWNHs) is reported for treatment of lung cancer. Anticancer drug polyphosphazene platinum with SWNTs have showed enhanced permeability, distribution and retention in body because of controlled lipophilicity of tube. CNTs and CNHs are also used as a gene carrier³⁴ and CNTs alone as potassium channel blocker⁴¹etc.

8) Future Prospects

The exponential increase in patent filings and publications on CNTs indicates growing industrial interest that parallels academic interest. Consistent with the demonstrated commercial importance of nanotubes in composites, most of the patent filings (50%) are for nanotube; synthesis, processing, and composites. This reflects the advanced state of carbon nanotube displays, nanotube electronic devices and the attractiveness of related applications. But, a decade or more of additional progress is likely to be required to reliably assess if and when these breakthrough will reach commercial application. Independent of the outcome of the ongoing races to exploit nanotubes in applications,

CNTs have provided possibilities in nanotechnology that were not conceived in the past. Nanotechnologies of the future in many areas will build on the advances that have been made for carbon nanotubes.

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About Authors:

Mr. Malay D. Shah.

M.Pharm (II^nd Year , deptt of Pharmaceutics)

Mr. Karthik Nair

M.S. (1^st Year ,pharmacology)

Mr. Pramod Shirote

M.Pharm., Lecturer in Pharmaceutical Chemistry

Mrs. Neela M. Bhatia

M.Pharm.,Asst. Professor in Pharmaceutical Chemistry.

Mr. Namdeo R. Jadhav

M.Pharm.,Asst. Professor in Pharmaceutics.

Bharati Vidyapeeth College of Pharmacy, Kolhapur-416013, Maharashtra State, India, University of Nottingham, UK.

*Author for correspondence

Email: nrjadhav18@rediffmail.com; Phone: 91-0231-2637286; Fax: 91-0231-2638833