Unit IV

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Unit 4-Introduction to Nanomaterials 4.1.Introduction to nanomaterials: Bulk materials and nanomaterials Changes in bulk and nanomaterials of silicon, sodium chloride, gold. (8 Periods)

4.2.General methods of preparation of nanomaterials: Physical and chemical methods

4.3Applications of nanomaterials: Nanodevices- Carbon nanotubes-preparation, purification and applications in electronic industries

4.1. Introduction A nanoparticle is an entity with a width of a few nanometers to a few hundred, containing tens to thousands of atoms.

Nano size:

One nanometre is a millionth part of the size of the tip of a needle.

1 nm = 10-6 mm = 10-9 m

Table 1. Some examples of size from macro to molecular

Size (nm) Examples Terminology

0.1-0.5 Individual chemical bonds Molecular/atomic

0.5-1.0 Small molecules, pores in zeolites Molecular

1-1000 Proteins, DNA, inorganic nanoparticles Nano

103-104living cells, human hair Micro

>104Normal bulk matter Macro

From: USDAs roadmap of nanotechnology.

In recent times nanomaterials have been the subject of enormous interest. These materials, notable for their extremely small size exhibit unusual mechanical, electrical, optical and magnetic properties. Due to their special properties, nanomaterials .have the potential for wide-ranging industrial, biomedical, and electronic applications. For example, long lasting medical implants of biocompatible nanostructured ceramic and carbides, biocompatible coating, drug delivery, protection coatings, composite materials, anti fogging coatings for spectacles and car windows etc. The nanoworld lies midway between the scale of atomic and quantum phenomena, and the scale of bulk materials. At the nanomaterial level, some material properties are affected by the laws of atomic physics, rather than behaving as traditional bulk materials do. Nanomaterials can be metals, ceramics, polymeric materials, or composite materials. Their defining characteristic is a very small feature size in the range of 1-100 (nm). 4.1.1. Bulk and nanomaterialsAlthough widespread interest in nanomaterials is recent, the concept was raised over 40 years ago. Physicist Richard Feynman delivered a talk in 1959 entitled "There's Plenty of Room at the Bottom", in which he commented that there were no fundamental physical reasons that materials could not be fabricated by maneuvering individual atoms. Nanomaterials have actually been produced and used by humans for hundreds of years - the beautiful ruby red color of some glass is due to gold nanoparticles trapped in the glass matrix. The decorative glaze known as luster, found on some medieval pottery, contains metallic spherical nanoparticles dispersed in a complex way in the glaze, which give rise to its special optical properties. What makes these nanomaterials so different and so intriguing? The properties of nanomaterials deviate from those of single crystals or polycrystals and glasses with the same average chemical composition. This deviation results from the reduced size and dimensionality of the nanometer-sized crystallites, the numerous interfaces between adjacent crystallites, grain boundaries and surfaces.

Their extremely small feature size is of the same scale as the critical size for physical phenomena - for example, the radius of the tip of a crack in a material may be in the range 1-100 nm. The way a crack grows in a larger-scale, bulk material is likely to be different from crack propagation in a nanomaterial where crack and particle size are comparable. Fundamental electronic, magnetic, optical, chemical, and biological processes are also different at this level. Where proteins are 10-1000 nm in size, and cell walls 1-100 nm thick, their behavior on encountering a nanomaterial may be quite different from that seen in relation to larger-scale materials.

Nanomaterials are assembled from nanometer-sized (crystallites). These building blocks may differ in their crystallographic orientation that may lead to incoherent or coherent interfaces between them that lead to inherent heterogeneous structure on a nanometer scale. Grain boundaries make up a major portion of the material at nanoscales, and strongly affect properties and processing. Surfaces and interfaces are also important in explaining nanomaterial behavior. In bulk materials, only a relatively small percentage of atoms will be at or near a surface or interface (like a crystal grain boundary). In nanomaterials, the small feature size ensures that many atoms, perhaps half or more in some cases, will be near interfaces. Surface properties such as energy levels, electronic structure, and reactivity can be quite different from interior states, and give rise to quite different material properties.

Nanocrystallites of bulk inorganic solids have been shown to exhibit size dependent properties, such as lower melting points, higher energy gaps etc. In comparison to macro-scale powders, increased ductility has been observed in nanopowders of metal alloys. In addition, quantum effects from boundary become significant leading to phenomena such as quantum dots.

Gold : Bulk Vs NanoProperties of gold in bulk form lustrousthey have a shiny surface when polished

Malleablethey can be hammered, bent or rolled into any desired shape

Ductilethey can be drawn out into wires

is metallic, with a yellow colour when in a mass

good conductors of heat and electricity

generally have high densities

have high melting point (~1080 deg C)

are often hard and tough with high tensile strength, meaning that they offer high resistance to the stresses of being stretched or drawn out and therefore do not easily break is inert-unaffected by air and most reagents

4.1.1.1 Gold nanoparticles

Fig 1. Size and shape dependent colors of Au and Ag nanoparticles At the nano-level, gold acquires a new shine, a new set of properties and a host of potential new applications. Nanoparticles often have unexpected visible properties because they are small enough to scatter visible light rather than absorb it (Fig.1). Gold nanoparticles appear deep red to black in solution. In fact a whole range of colours can be observed depending on the size of the gold nanoparticles. The distance between particles also effects colour. Surface plasmon resonance (The excitation of surface plasmons by light is denoted as a surface plasmon resonance) is the term used by nanotechnologists to describe this effect. Gold nanoparticles. Vary in appearance depending on size and shape of cluster

Are never gold in colour

Are found in a range of colours

Are very good catalysts

Are not metals but are semiconductors. Melts at relatively low temperature (~940 C)

Size & Shape of the nanoparticles determines the colourFor example, Gold particles in glass

25 nm

red reflected

50 nm

green reflected

100 nmorange reflected4.2.General methods of preparation of nanomaterials

There are two approaches for the synthesis of nanomaterials and the fabrication of nanostructures, viz., top-down and bottom-up (Fig.2). Top down approach involves the breaking down of the bulk material into nano sized structures or particles. These techniques are an extension of those that have been used for producing micron- sized particles. An example of such a technique is high-energy wet ball milling. The alternative approach, which has the potential of creating less waste and hence the more economical, is the bottom- up. Bottom up approach refers to the build up of a material from the bottom: atom-by-atom, molecule-by-molecule, or cluster-by-cluster. Many of these techniques are still under development or are just beginning to be used for commercial production of nano powders. Any fabrication technique should provide,(i) Identical size of all particles (also called mono sized or with uniform size

distribution.

(ii) Identical shape or morphology.

(iii) Identical chemical composition and crystal structure(iv) Individually dispersed or mono dispersed i.e., no agglomeration.

4.2.1. Physical Method : (A) High-Energy ball milling (Top down approach) :The milling of materials is of prime interest in the mineral, ceramic processing, and powder metallurgy industry. Typical objectives of the milling process include particle size reduction, solid-state alloying, mixing or blending, and particle shape changes (Fig.3). These industrial processes are mostly restricted to relatively hard, brittle materials which fracture, deform, and cold weld during the milling operation. This technique has been extended to produce a variety of nonequilibrium structures including nanocrystalline, amorphous and quasicrystalline materials. A variety of ball mills has been developed for different purposes including tumbler mills, attrition mills, shaker mills, vibratory mills, planetary mills, etc. The basic process of mechanical attrition is illustrated in fig below.

Powders with typical particle diameters of about 50 m are placed together with a number of hardened steel or tungsten carbide (WC) coated balls in a sealed container which is shaken or violently agitated. The most effective ratio for the ball to powder mass is 5 : 10.

High-energy milling forces can be obtained using high frequencies and small amplitudes of vibration. Shaker mills (e.g. SPEX model 8000) which are preferable for small batches of powder (approximately 10 cm3 is sufficient for research purposes) are highly energetic and reactions can take place one order of magnitude faster than with other types of mill. Since the kinetic energy of the balls is a function of their mass and velocity, dense materials (steel or tungsten carbide) are preferable to ceramic balls. During the continuous severe plastic deformation associated with mechanical attrition, a continuous refinement of the internal structure of the powder particles to nanometer scales occurs during high energy mechanical attrition. The temperature rise during this process is modest and is estimated to be less than or equal to 100 to 200 C. The difficulty with top-down approaches is ensuring all the particles are broken down to the required particle size. During mechanical attrition, contamination by the milling tools (Fe) and atmosphere (trace elements of O2, N2, in rare gases) can be a problem. By minimizing the milling time and using the purest, most ductile metal powders available, a thin coating of the milling tools by the respective powder material can be obtained which reduces Fe contamination tremendously. Atmospheric contamination can be minimized or eliminated by sealing the vial with a flexible O ring after the powder has been loaded in an inert gas glove box. Small experimental ball mills can also be enclosed completely in an inert gas glove box. As a consequence, the contamination with Fe based wear debris can generally be reduced to less than 1-2 % and oxygen and nitrogen contamination to less than 300 ppm. However, milling of refractory metals in a shaker or planetary mill for extended periods of time (>30 h) can result in levels of Fe contamination of more than 10% if high vibrational or rotational frequencies are employed. The main advantage of top-down approach is high production rates of nanopowders.(B) Gas Condensation Processing (GCP) (Bottom-up approach): Gas condensation was the first technique used to synthesize nanocrystalline metals and alloys. In this technique which was pioneered by Gleiter and co-workers a metallic or inorganic material, e.g. a suboxide, is vaporised using thermal evaporation sources such as Joule heated refractory crucibles (Joule heating is a process by which the passage of an electric current through a conductor releases heat), electron beam evaporation devices or sputtering sources in an atmosphere of 1-50 mbar He (or other inert gases such as Ar, Ne, Kr) (Fig.4). Cluster form in the vicinity of the source by homogenous nucleation in the gas phase and grow by coalescence and incorporation of atoms from the gas phase. The cluster or particle size depends critically on the residence time of the particles in the growth regime and can be influenced by the gas pressure, the kind of inert gas, i.e. He, Ar or Kr, and on the evaporation rate/vapour pressure of the evaporating material. With increasing gas pressure, vapour pressure and mass of the inert gas; the average particle size of the nanoparticles increases. A rotating cylindrical device cooled with liquid nitrogen was employed for the particle collection: the nanoparticles in the size range from 2-50 nm are extracted from the gas flow by thermophoretic forces and deposited loosely on the surface of the collection device as a powder of low density and no agglomeration. Subsequenly, the nanoparticles are removed from the surface of the cylinder by means of a scraper in the form of a metallic plate. In addition to this cold finger device several techniques known from aerosol science have now been implemented for the use in gas condensation systems such as corona discharge, etc. These methods allow for the continuous operation of the collection device and are better suited for larger scale synthesis of nanopowders. However, these methods can only be used in a system designed for gas flow, i.e. a dynamic vacuum is generated by means of both continuous pumping and gas inlet via mass flow controller. A major advantage over conventional gas flow is the improved control of the particle sizes.

Evaporation can be carried out using refractory metal crucibles (W, Ta or Mo). If metals with high melting points or metals which react with the crucibles, are to be prepared, sputtering, i.e. for W and Zr, or laser or electron beam evaporation has to be used. Sputtering is a non-thermal process in which surface atoms are physically ejected from the surface by momentum transfer from an energetic bombarding species of atomic/molecular size. Synthesis of alloys or intermetallic compounds by thermal evaporation can only be done in the exceptional cases that the vapour pressures of the constituents elements are similar. As an alternative, sputtering from an alloy or mixed target can be employed. Composite materials such as Cu/Bi or W/Ga have been synthesised by simultaneous evaporation from two separate crucibles onto a rotating collection device. It has been found that excellent intermixing on the scale of the particle size can be obtained.

However, control of the composition of the elements has been difficult and reproducibility is poor. Nanocrystalline oxide powders are formed by controlled postoxidation of primary nanoparticles of a pure metal (e.g. Ti to TiO2) or a suboxide (e.g. ZrO to ZrO2). Although the gas condensation method including the variations has been widely employed to prepare a variety of metallic and ceramic materials, quantities have so far been limited to a laboratory scale. The method is extremely slow. The quantities of metals are below 1 g/day, while quantities of oxides can be as high as 20 g/day for simple oxides such as CeO2 or ZrO2. These quantities are sufficient for materials testing but not for industrial production. However, it should be mentioned that the scale-up of the gas condensation method for industrial production of nanocrystalline oxides by a company called nanophase technologies has been successful.

(C) Chemical Vapour Condensation (CVC) (Bottom-up approach): Chemical vapor condensation (CVC) was developed in Germany in 1994. It involves pyrolysis of vapors of metal organic precursors in a reduced pressure atmosphere. Particles of ZrO2, Y2O3 and nanowhiskers have been produced by CVC method. As shown schematically in Fig.6, the evaporative source used in GPC is replaced by a hot wall reactor in the Chemical Vapour Condensation process. The original idea of the novel CVC process which is schematically shown below where it was intended to adjust the parameter field during the synthesis in order to suppress film formation and enhance homogeneous nucleation of particles in the gas flow. It is readily found that the residence time of the precursor in the reactor determines if films or particles are formed. In a certain range of residence time both particle and film formation can be obtained. Adjusting the residence time of the precursor molecules by changing the gas flow rate, the pressure difference between the precursor delivery system and the main chamber and the temperature of the hot wall reactor results in the prolific production of nanosized particles of metals and ceramics instead of thin films as in CVD processing (Fig.5). In the simplest form a metalorganic precursor is introduced into the hot zone of the reactor using mass flow controller. For instance, hexamethyldisilazane (CH3)3SiNHSi(CH3)3 was used to produce SiCxNyOz powder by CVC technique. Besides the increased quantities in this Continuous process compared to GCP it has been demonstrated that a wider range of ceramics including nitrides and carbides can be synthesised. Additionally, more complex oxides such as BaTiO3 or composite structures can be formed as well. In addition to the formation of single phase nanoparticles by CVC of a single precursor the reactor allows the synthesis of

1. Mixtures of nanoparticles of two phases or doped nanoparticles by supplying

two precursors at the front end of the reactor, and

2. Coated nanoparticles, i.e., n-ZrO2 coated with n-Al2O3 or vice versa, by

Supplying a second precursor at a second stage of the reactor. In this case

nanoparticles which have been formed by homogeneous nucleation are coated

by heterogeneous nucleation in a second stage of the reactor.

Because CVC processing is continuous, the production capabilities are much larger than in GCP processing. Quantities in excess of 20 g/hr have been readily produced with a small scale laboratory reactor. A further expansion can be envisaged by simply enlarging the diameter of the hot wall reactor and the mass flow through the reactor. The microstructure of nanoparticles as well as the properties of materials obtained by CVC has been identical to GCP prepared powders.(D) Laser ablation (Bottom-up approach): Laser ablation has been extensively used for the preparation of nanoparticles and particulate films. In this process a laser beam is used as the primary excitation source of ablation for generating clusters directly from a solid sample in a wide variety of applications. The possibility for preparing nanoparticulate web-like structures over large sample area is of particular interest in view of their novel properties that can be applied to new technological applications.

Laser vaporization cluster beams were introduced by Smalley and coworkers to overcome the limitations of oven sources. In this method, a high energy pulsed laser with an intensity flux exceeding 107 W/cm3 is focused on a target containing the material to be made into clusters. The resulting plasma causes highly efficient vaporization since with current, pulsed lasers one can easily generate temperatures at the target material greater than 104 K. This high temperature vaporizes all known substances so quickly that the rest of the source can operate at room temperature. Typical yields are 1014-1015 atoms from a surface area of 0.01 cm2 in a 10-8 s pulse. The local atomic vapor density can exceed 1018 atom/cm3 (equivalent to 100 Torr pressure) in the microseconds following the laser pulse. The hot metal vapor is entrained in a pulsed flow of carrier gas (typically He) and expanded through a nozzle into a vacuum. The cool, high-density He flowing over the target serves as a buffer gas in which clusters of the target material form, thermalize to near room temperature and then cool to a few K in the subsequent supersonic expansion.

In a recent investigation utilizing a novel atomization system, (LINA-SPARK), LSA, based on laser spark atomization of solids has been developed that seems to be very versatile for different materials. Briefly, the LSA is capable of evaporating material at a rate of about 20 g/s from a solid target under argon atmosphere. The small dimensions of the particles and the possibility to form thick films make the LSA quite an efficient tool for the production of ceramic particles and coatings and also an ablation source for analytical applications such as the coupling to induced coupled plasma emission spectrometry, ICP, the formation of the nanoparticles has been explained following a liquefaction process which generates an aerosol, followed by the cooling/solidification of the droplets which results in the formation of fog. The general dynamics of both the aerosol and the fog favours the aggregation process and micrometer-sized fractal-like particles are formed. The laser spark atomizer can be used to produce highly mesoporous thick films and the porosity can be modified by the carrier gas flow rate thus enabling for a control of the microstructure of the coatings which make these nanoparticulate thick films suitable candidates for application in membrane technology, catalysis and lithium ion batteries. ZrO2 and SnO2 nanoparticulate thick films were also synthesized successfully using this process with quite identical microstructure. Synthesis of other materials such as lithium manganate, silicon and carbon has also been carried out by this technique.

4.2.2. Chemical Methods (Bottom-up approachs):

(A) Wet Chemical Synthesis of nanomaterials (Sol-gel process): Sol-gel method of synthesizing nanomaterials is very popular amongst chemists and is widely employed to prepare oxide materials. The sol-gel process, as the name implies, involves the evolution of inorganic networks through the formation of a colloidal suspension (sol) and gelation of the sol to form a network in a continuous liquid phase (gel). The precursors for synthesizing these colloids consist usually of a metal or metalloid element surrounded by various reactive ligands. The starting material is processed to form a sol in contact with water or dilute acid. Removal of the liquid from the sol yields the gel, and the sol/gel transition controls the particle size and shape. Calcination of the gel produces the product (eg. Oxide).

Sol-gel processing refers to the hydrolysis and condensation of alkoxide-based precursors such as Si(OEt) 4 (tetraethyl orthosilicate, or TEOS). The reactions involved in the sol-gel chemistry based on the hydrolysis and condensation of metal alkoxides M(OR)z can be described as follows:

MOR + H2O MOH + ROH (hydrolysis)

MOH+ROMM-O-M+ROH (condensation)

Steps:Step 1: Formation of different stable solutions of the alkoxide or solvated metal precursor (the sol).

Step 2: Gelation resulting from the formation of an oxide- or alcohol- bridged network (the gel) by a polycondensation or polyesterification reaction that results in a dramatic increase in the viscocity of the solution.

Step 3: Aging of the gel, during which the polycondensation reactions continue until the gel transforms into a solid mass, accompanied by contraction of the gel network and expulsion of solvent from gel pores. Ostwald ripening (also referred to as coarsening, is the phenomenon by which smaller particles are consumed by larger particles during the growth process) and phase transformations may occur concurrently. The aging process of gels can exceed 7 days and is critical to prevent the cracks in gels that have been cast.

Step 4: Drying of the gel, when water and other volatile liquids are removed from the gel network. If isolated by thermal evaporation, the resulting monolith is termed a xerogel. If the solvent (such as water) is extracted under supercritical or near super critical conditions, the product is an aerogel.

Step 5: Dehydration, during which surface- bound M-OH groups are removed, there by stabilizing the gel against rehydration. This is normally achieved by calcining the monolith at temperatures up to 8000C.

Step 6: Densification and decomposition of the gels at high temperatures (T>8000C). The pores of the gel network are collapsed, and remaining organic species are volatilized. The typical steps that are involved in sol-gel processing are shown in the schematic diagram below.

The interest in this synthesis method arises due to the possibility of synthesizing nonmetallic inorganic materials like glasses, glass ceramics or ceramic materials at very low temperatures compared to the high temperature process required by melting glass or firing ceramics (Fig.6). In addition, one can get monosized nanoparticles by this bottom up approach.The major technical difficulties to overcome in developing a successful bottom-up approach is controlling the growth of the particles and then stopping the newly formed particles from agglomerating. Other technical issues are ensuring the reactions are complete so that no unwanted reactant is left on the product and completely removing any growth aids that may have been used in the process. Also production rates of nanopowders are very low by this process.

(B) Precipitation method: Nanomaterials are produced by precipitation from a solution. The method involves high degree of homogenization and low processing temperature. The ZnS powders were produced by reaction of aqueous zinc salt solutions with thioacetamide (TAA). Precursor zinc salts were chloride, nitric acid solutions, or zinc salts with ligands (i.e., acetylacetonate, trifluorocarbonsulfonate, and dithiocarbamate). The 0.05 M cation solution was heated in a thermal bath maintained at 70 or 80 C in batches of 100 or 250 ml. Acid was added dropwise to bring it to a pH of 2. The reaction was started by adding the TAA to the zinc salt solution, with the molar ratio of TAA and zinc ions being set to an initial value of either 4 or 8.

SnO2 nanopowder was prepared by precipitation method using stannic chloride, ammonium hydroxide. The precipitate obtained by adding ammonia in drops to ice cold solution of stannic chloride was heated at around 100C. The white mass was collected after cooling the mixture and dried.

4.3. Applications of nanomaterials

4.3.1 Carbon Nanotubes (CNTs)/Basics

Carbon nanotubes are molecular-scale tubes of graphitic carbon with outstanding properties. They are among the stiffest and strongest fibres known, and have remarkable electronic properties and many other unique characteristics. The current huge interest in carbon nanotubes is a direct consequence of the synthesis of buckminsterfullerene, C60, and other fullerenes, in 1985. In 1991, scientists at NEC Corporation in Japan discovered that graphitic carbon needles grew on the negative carbon electrode of the arc-discharge apparatus used for the mass production of C60 (Iijima, 1991, Fig.6). The needles ranged up to 1 mm in length and consisted of nested tubes (concentric cylinders) of rolled graphite sheets (see Fig.7). The smallest tube observed was 2.2 nm in diameter, which corresponds roughly to a ring of 30 carbon hexagons. Some of the needles consisted of only two nested tubes (Fig.8), while others contained as many as 50. The separation between the tubes was 0.34 nm (3.4 angstroms), which matches the separation of the sheets in bulk graphite. The tips of the needles were generally closed by caps that were curved or cone-shaped (Fig.6). Subsequent work at NEC optimized the synthetic procedure, allowing gram quantities of carbon needles.

Fig.7 Shape and structure of Carbon nanotube (SWNT).

Note that a nanotube (also known as a buckytube) is a member of the fullerene structural family, which also includes buckyballs. Whereas buckyballs are spherical in shape (seen in the Unit-I), a nanotube is cylindrical, with at least one end typically capped with a hemisphere of the buckyball structure. Their name is derived from their size, since the diameter of a nanotube is on the order of a few nanometers (approximately 50,000 times smaller than the width of a human hair), while they can be up to several centimeters in length. There are two main types of nanotubes: single-walled nanotubes (SWNTs) and multi-walled nanotubes (MWNTs, combination of various CNTs).

4.3.2 Chemical bonding Structure

(A) Graphite (layered)

(B) Rolled graphite (a CNT)

The bonding in carbon nanotubes is sp2, with each atom joined to three neighbours, as in graphite (Fig.9A). The tubes can therefore be considered as rolled-up graphene sheets (graphene is an individual graphite layer) (Fig.9B). Note that sp2 bonds are stronger than the sp bonds found in diamond, this bonding structure provides them with their unique strength and amazing in mechanical properties. Nanotubes naturally align themselves into "ropes" held together by Van der Waals forces. Under high pressure, nanotubes can merge together, trading some sp bonds for sp bonds, giving great possibility for producing strong, unlimited-length wires through high-pressure nanotube linking. 4.3.3 Preparation of CNTs

Multiwalled carbon nanotubes (MWNTs). Carbon nanotubes are readily prepared by striking an arc between graphite electrodes in ~0.7 atm (~500 torr) of helium, considerably larger than the helium pressure used for the production of fullerene soot. The schematic diagram of the apparatus is shown in Fig.10. A current of 60-100 A across a potential drop of about 25 V gives high yields of carbon nanotubes. The arcing process can be optimized such that the major portion of the carbon anode gets deposited on the cathode in the form of carbon nanotubes and graphitic nanoparticles. The arc evaporation of graphite has been carried out in various kinds of ambient gases (He, Ar, and CH4). Hydrogen appears to be effective in producing MWNTs of high crystallinity. Arc-produced MWNTs in hydrogen also contain very few carbon nanoparticles. Carbon nanotubes have been produced in large quantities by using plasma arc-jets by optimizing the quenching process in an arc between a graphite anode and a cooled copper electrode. If both the electrodes are of graphite, MWNTs are the main products, although side products such as fullerenes, amorphous carbon, and graphite sheets are also formed.

A route to highly crystalline MWNTs is the arc-discharge method in liquid nitrogen. In this method, vacuum is replaced with liquid nitrogen in the arc discharge chamber. In a typical experiment direct current was supplied to the apparatus using a power supply. The anode is a pure carbon rod of 8-mm diameter and the cathode is a pure carbon rod of 10-mm diameter. The Dewar flask is filled with liquid nitrogen and the electrode assembly immersed in nitrogen. Arc discharge occurs as the distance between the electrodes became less than 1 mm, and a current of ~80 A flows between the electrodes. When the arc discharge is over, the carbon deposits near the cathode are recovered for analysis. Liquid nitrogen prevents the electrodes from contamination with unwanted gases and also lowers the temperature of the electrodes. Furthermore, CNTs do not stick to the wall of the chamber. The content of the MWNTs can be as high as 70% of the reaction product. Analysis with Auger-spectroscopy revealed that no nitrogen was incorporated in the MWNTs. This method considered to be economical and its does not require expensive components.

Chemical vapour deposition (CVD) based MWNT preparation. CVD method uses a carbon source in the gas phase and plasma or a resistively heated coil, to transfer the energy to the gaseous carbon molecule. The energy source cracks the molecule into atomic carbon. The carbon then diffuses towards the substrate, which is heated and coated with a catalyst (usually a first row transition metal such as Ni, Fe or Co) and binds to it. Carbon nanotubes are formed in this procedure if the proper parameters are maintained. Good alignments as well as positional control on a nanometric scale are achieved by using CVD. Use of an appropriate metal catalyst permits the preferential growth of single-walled rather than multi-walled nanotubes.

CVD synthesis of nanotubes is essentially a two-step process, consisting of a catalyst preparation (step-1) step followed by the actual synthesis of the nanotubes (step-2). The catalyst is generally prepared by sputtering a transition metal onto a substrate and then using etching by chemicals such as ammonia or thermal annealing to induce the nucleation of catalyst particles. Thermal annealing results in metal cluster formation on the substrate, from which the nanotubes grow. The temperature for the synthesis of nanotubes by CVD is generally in the 650-900oC range. Typical nanotube yields from CVD are around 30%. A variety of CVD processes have been used for carbon nanotubes synthesis, which include plasma-enhanced CVD, thermal chemical CVD, alcohol catalytic CVD, aero gel-supported CVD and laser-assisted CVD. In the thermal CVD process, Fe, Ni, Co or an alloy of these metals is initially deposited on a substrate. After the substrate is etched (skin removed) by a dilute HF solution, the substrate is placed in a quartz boat, positioned in a CVD reaction furnace. Nanometer-sized catalytic metal particles get formed after an additional etching of the catalytic metal film using NH3 gas at 750-1050oC. The nanotubes grow on the fine catalytic metal particles by the CVD process.

Single-walled carbon nanotubes (SWNT). The carbon nanotubes generally obtained by the arc method or hydrocarbon pyrolysis are multi-walled, having several graphitic sheets or layers. Depending on the exact technique, it is possible to selectively grow SWNTs or MWNTs. Two distinct methods of synthesis can be performed with the same arc discharge apparatus. If SWNTs are required, the anode has to be doped with a metal catalyst based on Fe, Co, Ni, or Mo. Several elements and mixtures of elements have been tested and the results vary considerably.

Single-walled nanotubes were first prepared by the metal-catalyzed dc arcing of graphite rods in a Helium gas atmosphere. The graphite anode was filled with metal powders (Fe, Co or Ni) and the cathode was of pure graphite. SWNTs generally occur in the web-like material deposited behind the cathode (Fig.11B). Various metal catalysts have been used to make SWNTs by this route. For examples;

1. Molybdenum (Mo) particles of few nanometer diameters dispersed in a fumed alumina matrix catalyst.

2. Co (or) a Fe/Ni bimetallic catalyst 3. Various oxides catalysts: Y2O3, La2O3, CeO2 as catalysts. 4. 1% Yttrium and 4.2 at.% Ni as catalyst.

5. Graphite rods filled with Ni and Y2O3 catalyst in a He atmosphere (660 Torr) gives rise to web-like deposits on the chamber walls near the cathode, consisting of SWNT bundles.

6. Nanoparticles of Fe catalyst

A common problem with SWNT synthesis is that the product contains metal catalyst particles and defects, rendering purification difficult. On the other hand, an advantage is that the diameter can be controlled by changing the thermal transfer and diffusion, and hence the condensation of atomic carbon. This has been demonstrated in an experiment where different mixtures of inert gases were used. Argon, with a lower thermal conductivity and diffusion coefficient, gives SWNTs with a diameter of ~1.2 nm. A linear fit of the average nanotube diameter showed a 0.2 nm diameter decrease per 10 % increase in argon helium ratio, when nickel/yttrium was used (C/Ni/Y was 94.8:4.2:1) as catalyst.

4.3.4 Purification Techniques of CNTs

A large problem with nanotube application is next to large-scale synthesis also the purification. The as-produced SWNT soot contains a lot of impurities. The main impurities in the soot are graphite (wrapped up) sheets, amorphous carbon, metal catalyst and the smaller fullerenes. These impurities will interfere with most of the desired properties of the SWNTs.

In this chapter several purification techniques of the SWNT will be discussed. Basically, these techniques can be divided into two mainstreams, structure selective and size selective separations. The first one will separate the SWNTs from the impurities; the second one will give a more homogeneous diameter or size distribution. The techniques that will be discussed are oxidation, acid treatment, annealing, ultrasonication, micro filtration, ferromagnetic separation, cutting, functionalisation and chromatography techniques.

(A) By chemical oxidation:

Oxidative treatment of the SWNTs is a good way to remove carbonaceous impurities (or) to clear the metal surface. The main disadvantages of oxidation are that not only the impurities are oxidized, but also the SWNTs. Luckily the damage to SWNTs is less than the damage to the impurities. Another reason why impurity oxidation is preferred is that these impurities are most commonly attached to the metal catalyst, which also acts as oxidizing catalyst. Efficiency of the purification procedure dependable on lot of factors; such as metal content, oxidation time, environment, oxidizing agent and temperature. For example, when the temperature is raised above 600C, SWNTs will also oxidize, even without catalyst.

Well know method is mild oxidizing in a wet environment with soluble oxidizing agents, such as H2O2 and H2SO4. These will only oxidize the defects and will clear the surface of the metal. However, if oxygen impurities present in the solution, that leads to oxidation of metal( metal oxide and in turn to rupture the CNT structure.

(B) By acid treatment:

In general the acid treatment will remove the metal catalyst as soluble metal salts. In that treatment, the surface of the metal must be first exposed by oxidation or sonication. The metal catalyst is then exposed to acid and solvated. The SWNTs remain in suspended form. Note, when using a treatment in HNO3, the acid only has an effect on the metal catalyst. It has no effect on the SWNTs and other carbon particles. If a treatment in HCl is used, the acid has also a little effect on the SWNTs and other carbon particles. The mild acid treatment (4 M HCl reflux) is basically the same as the HNO3 reflux, but here the metal has to be totally exposed to the acid to solvate it.

(C) By annealing (heat treatment):

Due to high temperatures (873 1873 K) the nanotubes will be rearranged and defects will be consumed. The high temperature also causes the graphitic carbon and the short fullerenes to pyrolyse. When using high temperature vacuum treatment (1873 K) the metal will be melted and can also be removed.

(D) By ultrasonication:

In this technique particles are separated due to ultrasonic vibrations. Agglomerates of different nanoparticles will be forced to vibrate and will become more dispersed. The separation of the particles is highly dependable on the surfactant, solvent and reagent used. In poor solvents the SWNTs are more stable if they are still attached to the metal. But in some solvents, such as alcohols, monodispersed particles are relatively stable. When an acid is used, the purity of the SWNTs depends on the exposure time. When the tubes are exposed to the acid for a short time, only the metal solvates, but for a longer exposure time, the tubes will also be chemically cut .

(E) By magnetic purification:

In this method ferromagnetic (catalytic) particles are mechanically removed from their graphitic shells. The SWNTs suspension is mixed with inorganic nanoparticles (mainly ZrO2 or CaCO3) in an ultrasonic bath to remove the ferromagnetic particles. Then, the particles are trapped with permanent magnetic poles. After a subsequent chemical treatment, a high purity SWNT material will be obtained. This process does not require large equipment and enables the production of laboratory-sized quantities of SWNTs containing no magnetic impurities.

(F) By micro filtration:

Micro filtration is based on size or particle separation. SWNTs and a small amount of carbon nanoparticles are trapped in a filter. The other nanoparticles (catalyst metal, fullerenes and carbon nanoparticles) are passing through the filter. One way of separating fullerenes from the SWNTs by micro filtration is to soak the as-produced SWNTs first in a CS2 solution. The CS2 insolubles are then trapped in a filter. The fullerenes which are solvated in the CS2, pass through the filter 65.

A special form of filtration is cross flow filtration. In cross flow filtration the membrane is a hollow fibre. The membrane is permeable to the solution. The filtrate is pumped down the bore of the fibre at some head pressure from a reservoir and the major fraction of the fast flowing solution which does not permeate out the sides of the fibre is fed back into the same reservoir to be cycled through the fibre repeatedly (Fig.12). A fast hydrodynamic flow down the fibre bore (cross flow) sweeps the membrane surface preventing the build-up of a filter cake.

(G) By cutting:

Cutting of the SWNTs can either be induced chemically (Fig.13), mechanically (or) as a combination of these. SWNTs can be chemically cut by partially functionalizing the tubes, for example with fluorine. Then, the fluorated carbon will be driven off the sidewall with pyrolisation in the form of CF4 or COF2. This will leave behind the chemically cut nanotubes. Mechanical cutting of the nanotubes can be induced by ball-milling. Here, the bonds will break due to the high friction between the nanoparticles and the nanotubes will be disordered. A combination of mechanical and chemical cutting of the nanotubes is by ultrasonication induced cutting in an acid solution. In this way the ultrasonic vibration will give the nanotubes sufficient energy to leave the catalyst surface. Then, in combination with acid the nanotubes will rupture at the defect sites.

(H) By functionalisation:

Functionalisation is based on making SWNTs more soluble than the impurities by attaching other groups to the tubes. Now it is easy to separate them from insoluble impurities, such as metal, with filtration. Another functionalisation technique also leaves the SWNT structure intact and makes them soluble for chromatographic size separation.

For recovery of the purified SWNTs, the functional groups can be simply removed by thermal treatment, such as annealing.

(I) By chromatography:

This technique is mainly used to separate small quantities of SWNTs into fractions with small length and diameter distribution. The SWNTs are run over a column with a porous material, through which the SWNTs will flow. The columns used are GPC (Gel Permeation Chromatography) and HPLC-SEC (High Performance Liquid Chromatography - Size Exclusion Chromatography) columns. The number of pores the SWNTs will flow through depends on their size. This means that, the smaller the molecule, the longer the pathway to the end of the column will be and that the larger molecules will come off first. The pore size will control what size distribution can be separated. However, a problem is that the SWNTs have to be either dispersed or solvated. This can be done by ultrasonication or functionalisation with soluble groups.

4.3.5 Applications of CNTs (electronic industries related):

Buckytubes have extraordinary electrical conductivity, heat conductivity and mechanical properties.Very significantly, buckytubes are molecularly perfect, which means that they are free of property-degrading flaws in the nanotube structure. Their material properties can therefore approach closely the very high levels intrinsic to them. These extraordinary characteristics give buckytubes potential in numerous applications. Some of the examples as follows:

(A) In field emission:

Buckytubes are the best known field emitters of any material. This is understandable, given their high electrical conductivity, and the unbeatable sharpness of their tip (Fig.15(I)) (the sharper the tip, the more concentrated will be an electric field, leading to field emission; this is the same reason lightening rods are sharp) (Fig.10(II), as molecular electronic system). The sharpness of the tip also means that they emit at especially low voltage, an important fact for building electrical devices that utilize this feature. Buckytubes can carry an astonishingly high current density, possibly as high as 1013 A/cm2. Furthermore, the current is extremely stable.

An immediate application of this behaviour receiving considerable interest is in field-emission flat-panel displays. Instead of a single electron gun, as in a traditional cathode ray tube display, here there is a separate electron gun (or many) for each pixel in the display. The high current density, low turn-on and operating voltage, and steady, long-lived behaviour make buckytubes attract field emitters to enable this application.

Other applications utilizing the field-emission characteristics of buckytubes include: general cold-cathode lighting sources, lightning arrestors, and electron microscope sources.

(B) In conductive plastics:

Much of the history of plastics over the last half century has been as a replacement for metal. For structural applications, plastics have made tremendous headway, but not where electrical conductivity is required, plastics being famously good electrical insulators.

This deficiency is overcome by loading plastics up with conductive fillers, such as carbon black and graphite fibres (the larger ones used to make golf clubs and tennis racquets). The loading required to provide the necessary conductivity is typically high, however, resulting in heavy parts, and more importantly, plastic parts whose structural properties are highly degraded.

It is well established that the higher aspect ratio of filler, the lower loading required achieving a given level of conductivity. Buckytubes are ideal in this sense, since they have the highest aspect ratio of any carbon fibre. In addition, their natural tendency to form ropes provides inherently very long conductive pathways even at ultra-low loadings. Applications that exploit this behaviour of buckytubes include EMI/RFI shielding composites and coatings for enclosures, gaskets, and other uses; electrostatic dissipation (ESD), and antistatic materials and (even transparent!) coatings; and radar-absorbing materials.

(C) In energy storage:

Buckytubes have the intrinsic characteristics desired in material used as electrodes in batteries and capacitors, two technologies of rapidly increasing importance. Buckytubes have a tremendously high surface area (~1000 m2/g), good electrical conductivity, and very importantly, their linear geometry makes their surface highly accessible to the electrolyte.

Research has shown that buckytubes have the highest reversible capacity of any carbon material for use in lithium-ion batteries. In addition, buckytubes are outstanding materials for super capacitor electrodes (suitable to sudden release of high-voltage in fraction time) and are now being marketed.

Buckytubes also have applications in a variety of fuel cell components. They have a number of properties including high surface area and thermal conductivity that make them useful as electrode catalyst supports in PEM fuel cells. They may also be used in gas diffusion layers as well as current collectors because of their high electrical conductivity. Buckytubes' high strength and toughness to weight characteristics may also prove valuable as part of composite components in fuel cells that are deployed in transport applications where durability is extremely important. Fig. 16&17 showed photographs of CNT based conductive film in for battery application and super-capacitor device systems respectively.

(D) In conductive adhesives and connectors:

The same issues that make buckytubes attractive as conductive fillers for use in shielding, Electrostatic dissipation (ESD) materials (details how easily an electric charge can travel across a medium) etc., make them attractive for electronics materials, such as adhesives and other connectors (e.g., solders).

(E) In molecular electronics:

The idea of building electronic circuits out of the essential building blocks of materials - molecules - has seen a revival the past five years, and is a key component of nanotechnology. In any electronic circuit, but particularly as dimensions shrink to the nanoscale, the interconnections between switches and other active devices become increasingly important.

Their geometry, electrical conductivity, and ability to be precisely derived, make buckytubes the ideal candidates for the connections in molecular electronics (eg., molecular cables and nanowires, Fig.18). Following are other examples for molecular electronics:

(a) Computer circuits: A nanotube formed by joining nanotubes of two different diameters end to end can act as a diode, suggesting the possibility of constructing electronic computer circuits entirely out of nanotubes (like Fig.13). Because of their good thermal properties, CNTs can also be used to dissipate heat from tiny computer chips. The longest electricity conducting circuit is a fraction of an inch long.. (b) Conductive films: Eikos Inc. of Franklin, Massachusetts is developing transparent, electrically conductive films of carbon nanotubes to replace conducitive glass, indium tin oxide (ITO) in LCDs, touch screens, and photovoltaic devices. Carbon nanotube films are substantially more mechanically robust than ITO films, making them ideal for high reliability touch screens and flexible displays. Nanotube films show promise for use in displays for computers, cell phones, PDAs, and ATMs. (c) Displays: One use for nanotubes that has already been developed is as extremely fine electron guns, which could be used as miniature cathode ray tubes in thin high-brightness low-energy low-weight displays (as in the Fig.10II). This type of display would consist of a group of many tiny CRTs, each providing the electrons to hit the phosphor of one pixel, instead of having one giant CRT whose electrons are aimed using electric and magnetic fields. These displays are known as field emission displays (FEDs).(F) In thermal materials:

The record-setting anisotropic thermal conductivity of buckytubes is enabling applications where heat needs to move from one place to another. Such an application is electronics, particularly advanced computing, where uncooled chips now routinely reach over 100oC. CNT & its composite with buckytubes have been shown to dramatically increase the bulk thermal conductivity at small loading.

(G) In structural composites:

The world-record properties of buckytubes are not limited to electrical and thermal conductivities, but also include mechanical properties, such as stiffness, toughness, and strength. These properties lead to a wealth of applications exploiting them, including advanced composites requiring high values in one or more of these properties. Fibres and Fabrics: Fibres spun of pure buckytubes have recently been demonstrated [R.H. Baughman, Science 290, 1310 (2000)] and are undergoing rapid development, along with buckytube composite fibres. Such super strong fibres will have applications including body and vehicle armour, transmission line cables, woven fabrics and textiles.

(H) In catalyst supports:

Buckytubes have an intrinsically high surface area; in fact, every atom is not just on a surface - each atom is on two surfaces, the inside and outside! Combined with the ability to attach essentially any chemical species to their sidewalls provides an opportunity for unique catalyst supports. Their electrical conductivity may also be exploited in the search for new catalysts and catalytic behaviour.

(I) Other applications:

There is a wealth of other potential applications for buckytubes, such as solar collection; nanoporous filters; catalyst supports; and coatings of all sorts. There are almost certainly many unanticipated applications for this remarkable material that will come to light in the years ahead and which may prove to be the most important and valuable of all.Blue

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Fig. 2. Schematic representation of the bottom up and top down synthesis processes of nanomaterials

Fig. 3 Schematic representation of the principle of mechanical milling.

Fig. 4 Schematic representation of typical set-up for gas condensation synthesis of nanomaterials followed by consolidation in a mechanical press or collection in an appropriate solvent media.

Fig. 5 A schematic of a typical CVC reactor

Fig. 6 Schematic representation of sol-gel process of synthesis of nanomaterials.

Fig. 6 Prof. Iijuma (Japan) with a CNT model.

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Fig.8 Two or more nested tubes of CNTs. Comparative sheet like structures are also given.

sp2 boned carbon

sp2 boned carbon

Fig. 9. Chemical structure of graphite (A) and CNT (B). Both having sp2 bonded carbons.

Fig. 10 CNTs preparation unit.

Fig.11 Transmission electron micrographs (TEM) of multiwall nanotubes (MWNTs)

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B

Fig. 12 SWNT microfiltration unit.

Fig.13 Cutting of a SWNT.

EMBED ChemDraw.Document.6.0

Fig. 14 Functionalized SWNT.

Fig. 15 Field emitting CNTs (I) & its typical device (II).

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(I) (II)

Fig.16 CNT based conductive plastic and a battery assembly (also for charge storage).

Fig.17 Commercial CNT based super capacitor (I) and electrochemical hydrogen charge storage (with NiOH2 as counter electrode).

(I) (II)

Fig. 18 CNT based molecular cables (molecular electronics).

Contact

Contact

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