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carbon nanotubes

Carbon nanotubes and their applications are discussed. There are several allotropes of carbon including diamond, graphite, buckminsterfullerene (C60), and single-walled carbon nanotubes. Carbon nanotubes are cylindrical forms of carbon that can be single-walled or multi-walled. They are composed of hexagonal networks of carbon atoms and have remarkable mechanical, thermal, and electrical properties. Common synthesis methods for carbon nanotubes include arc discharge, laser ablation, and chemical vapor deposition using metal catalysts.

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CARBON NANOTUBES AND THEIR APPLICATIONS DEPARTMENT OF PHYSICS MALAVIYA NATIONAL INSTITUTE OF TECHNOLOGY JAIPUR SUBMITTED TO: Dr. Srinivasa Rao Nelamarri SUBMITTED FROM: YOGESH CHILLAR I.D. 2017PPH5336 M.SC. PHYSICS 3RD SEMESTER
CARBON group- 14 period-2  Carbon is the 15th most abundant element in the Earth's crust, and the fourth most abundant element in the universe by mass after hydrogen, helium, and oxygen.  It is reactive non metal.  Electron configuration [He] 2s2 2p2  Electrons per shell 2,4 .  tetravalent—making four electrons available to form covalent chemical bonds  Three isotopes occur naturally, 12C and 13C being stable, while 14C is a radionuclide, decaying with a half-life of about 5,730 years.
Allotropes of carbon Allotropy is the property of some chemical elements to exist in two or more different forms, or allotropes, when found in nature. There are several allotropes of carbon. a) Diamond, b) Graphite, c) Lonsdaleite, d) C60 (Buckminsterfullerene or buckyball), e) C540, f) C70, g) Amorphous carbon, and h) single-walled carbon nanotube, or buckytube.
FULLERENCES  Fullerenes are a family of carbon allotropes.  They are molecules composed entirely of carbon.  They are in the form of a hollow sphere, ellipsoid, or tube. FULLERENCE
Discovery of the first fullerene: C60  In 1985, Prof. Harold W. Kroto of the University of Sussex joined Robert F. Curl and Prof. Richard E. Smalley at Rice University to study the products of carbon vaporization.  They carried out molecular beam experiments.  From the result, discrete peaks were observed corresponding to molecules with the exact mass of sixty or seventy or more carbon atoms.  C60 was then discovered, and it was named buckminsterfullerene which is named after Richard Buckminster Fuller who designed geodesic domes which is the same structure as C60.  Shortly after discovery of C60, it came to discover the fullerenes.
Structures of some fullerenes  C60 (Buckminsterfullerene)  Its like the shape of a football. *grey ball represents a carbon atom
CARBON NANOTUBES DISCOVERY  In the late 1950s, Roger Bacon at Union Carbide, found a strange new carbon fibre while studying carbon under conditions near its triple point. He observed straight, hollow tubes of carbon that appeared to consist in graphitic layers of carbon separated by the same spacing as the planar layers of graphite. In the 1970s, Morinobu Endo observed these tubes again, produced by a gas-phase process. Indeed, he even observed some tubes consisting in only a single layer of rolled-up graphite.  In 1991, after the discovery and verification of the fullerenes, Sumio Iijima of NEC observed multiwall nanotubes formed in a carbon arc discharge, and two years later, he and Donald Bethune at IBM independently observed single-wall nanotubes – buckytubes.
CARBON NANOTUBES  Carbon nanotubes are tubular forms of carbon that can be envisaged as graphene sheets rolled into cylindrical form.  These nanotubes have diameters of few nanometers and their lengths are up to several micrometers.  Each nanotube is made up of a hexagonal network of covalently bonded carbon atoms.  Carbon nanotubes are of two types: single-walled and multi-walled.  A single-walled carbon nanotube (SWNT) consists of a single graphene cylinder whereas a multi-walled carbon nanotube (MWNT) comprises of several concentric graphene cylinders.
 Depending on the way of rolling of graphene sheets (as shown in Fig),single- walled nanotubes of different types, viz. armchair, zig-zag and chiral could be produced. They can be represented using the method given by Hamada. For example, to realize an (n, m) nanotube, one has to move n times a1 from the selected origin and then m times a2. On rolling the graphite sheet these points coincide to form the (n, m) nanotube. Thus armchair, zig-zag and chiral nanotubes can be represented as (n, n), (n, 0) and (n, m) respectively  Different wrapping results in different structures and electronic properties.
Schematic representation of the relation between nanotubes and graphene. The three rectangles can be rolled up into seamless nanotubes; the short side, referred to as the roll-up vector R, becomes the circumference. R= na1+ma2, is a graphene 2D lattice vector, where a1 and a2 are unit vectors. Integers n and m uniquely define the tube: diameter, chirality, metal vs. semiconducting nature, and band gap, if semiconducting. In a bulk polydisperse sample consisting of a distribution of diameters, the larger the average diameter, the greater the number of n, m pairs that will satisfy the seamless roll-up condition.
Types of CNTs  Carbon nanotubes are of two types : 1. Single wall carbon nanotube (SWNT) 2. Multiple wall carbon nanotube (MWNT)  Single wall nanotube (SWNT ) consist of one cylinder. It is made of single graphene sheet rolled up into cylinder closed by two caps (semi fullerenes). The SWNTs have diameter in the range of 0.5 -2.0 nm. The length is in the range of 50-150 μm length. The SWNTs are microporous and the specific surface area is in the range of 1300 m2/g (outer surface). SWCNTs are commonly arranged in bundles. SWNTs have less topological defects and have better mechanical and electro physical properties. Electronic properties of SWNTs are governed by two factors, tube diameters and helicity, which further depend on the way graphene layer is rolled up, arm chair or chiral. Armchair SWNTs shows conductivity as similar to metal whereas zigzag SWNTs behave as semiconductors. In catalysis CNTs have high application as support.
 Electrical conductivity, surface curvature and presence of inner cavity in CNTs make the metal –support interaction different compared to that in activated carbon or graphite support. Mechanically bent SWNTs present kink sites that are chemically more active. Metal nanoparticles size depends strongly on metal-CNT interactions with stronger interaction giving rise to smaller nanoparticles. Studies have shown that convex surface of CNTs are more reactive than concave surface and the difference in reactivity increases when the tube diameter decreases.  Multiwall (MWNT) nano tubes consist of many nested concentric SWNTs cylinders with increasing successive radii. The concentric walls are spaced regularly at 0.34 nm similar to inter graphene distance. MWNTs have outer diameter in range of 2 – 100 nm depending on number of coaxial tubes present. MWNTs are usually mesoporous in nature and specific area depends on the number of walls. The length of MWNTs can range from few to hundreds μm. The advantage of MWNT over SWNT is that the multi-shell structures of MWNTs are stiffer than single wall hence stability is higher. Also large scale synthesis of MWNT is possible by various methods. The most common characterization techniques of these materials are electron microscopy, Raman spectroscopy, TGA , IR and UV-Vis.
PHYSICAL PROPERTIES OF CARBON NANOTUBES  MECHANICAL PROPERTIES  THERMAL PROPERTIES  ELECTRONIC PROPERTIES  MAGNETIC PROPERTIES  SUPERCONDUCTING PROPERTIES
PROPERTIES FOUNDATION  Nanotubes are fully described by their chiral vector  R = n â1 + m â2  Important parameters  dt = (3/p)ac-c(m2 + mn + n2)1/2  Q=tan-1(3n/(2m + n))  Grouped according to q  Armchair: n=m, q=30°  Zigzag: n or m=0, q=0°  Chiral: 0°<q < 30° A. Maiti, Caron Nanotubes: Band gap engineering with strain, Nature Materials 2 (2003) 440 V. Popov, Carbon nanotubes: properties and applications, Materials Science and Engineering R 43 (2004) 61-102
MECHANICAL PROPERTIES The strength of the carbon–carbon bond gives rise to the extreme interest in the mechanical properties of nanotubes.  Theoretically, they should be stiffer and stronger than any known substance.  Simulations and experiments demonstrate a remarkable “bend, don’t break” response of individual SWNT to large transverse deformations; an example from Yakobson’s simulation is shown in Figure 4.9.
 The two segments on either side of the buckled region can be bent into an acute angle without breaking bonds; simulations and experiments show the full recovery of a straight perfect tube once the force is removed.  Young’s modulus of a cantilevered individual MWNT was measured as 1.0 to 1.8 TPa from the amplitude of thermally driven vibrations observed in the TEM. At the low end, this is only ~20% better than the best high-modulus graphite fibers  Multiwall nanotubes and SWNT bundles may be stiffer in bending but are expected to be weaker in tension due to “pullout” of individual tubes. Comparison of mechanical properties
Thermal Properties  All nanotubes are expected to be very good thermal conductors along the tube, but good insulators laterally to the tube axis.  It is predicted that carbon nanotubes will be able to transmit up to 6000 watts per meter per Kelvin at room temperature; compare this to copper, a metal well-known for its good thermal conductivity, which transmits 385 watts per meter per K.  The temperature stability of carbon nanotubes is estimated to be up to 2800oC in vacuum and about 750oC in air.
ELECTRONIC PROPERTIES  1-D band structure calculated from 2-D graphene band structure using “zone folding” scheme  Ekμ= E2D(k*K2/|K2|+μK1)  K1=(-t2b1+ t1b2)/ N  K2=(mb1- nb2)/ N V. Popov, Carbon nanotubes: properties and applications, Materials Science and Engineering R 43 (2004) 61-102
 Theory predicts nanotubes exhibit both metallic and semi-conducting behavior  |n-m| evenly divisible by 3- metallic  All others semi-conducting with a band gap inversely proportional to the tube diameter T.W. Odomet al, Atomic Structure and Electronic Properties of Single-Walled Nanotubes, Nature (London) 391 (1998) 62
 If the nanotube structure is armchair then the electrical properties are metallic  If the nanotube structure is chiral then the electrical properties can be either semiconducting with a very small band gap, otherwise the nanotube is a moderate semiconductor  In theory, metallic nanotubes can carry an electrical current density of 4×109 A/cm2 which is more than 1,000 times greater than metals such as copper
MAGNETIC PROPERTY  It has been observed that there is no bulk magnetism in a clean multiwall nanotube sample.  On the other hand, when these are placed on a flat ferromagnetic substrate, fringing fields can be observed by magnetic force microscopy. Therefore the prospect for “contact-induced magnetism” and the application of carbon nanotubes to nanoscale spintronic devices remain open.  Magnetic contrast is observed for carbon nanotubes placed on cobalt or magnetite substrates, but is absent on silicon, copper, or gold. Spin transfer of about 0.1 µB per contact carbon atom is obtained
Superconducting properties  The dialog about nanotube superconductivity began with a very simple argument: if one could tune the chemical potential to the peak of a van Hove singularity, either chemically or electrostatically, one should have a high N(EF) value, a prerequisite for BCS superconductivity.  Super-current flow through an SWNT with low-resistance contacts was reported in 1999; analogous to contact-induced magnetism, no claims were made for bulk nanotube superconductivity. Evidence for a bulk anisotropic Meissner effect below 20 K in aligned 0.4-nm-diameter SWNT was claimed and justified by the argument that small-diameter tubes will be the stiffest, and thus, the average phonon energy in the BCS equation will be favorable. This dramatic result has not been reproduced by other researchers. As with bulk magnetism, it would appear that the jury is still out concerning bulk superconductivity in carbon nanotubes and other nanostructures.
Synthesis of CNTs  There are two main methods for preparation of CNT: 1. Sublimation of graphite with subsequent desublimation - Arc Discharge Method - Laser ablation 2. Decomposition of carbon containing compounds - Chemical Vapor Deposition ( CVD )
1. Sublimation of graphite with subsequent desublimation  This method involves condensation of carbon atoms generated from evaporation of solid carbon sources of graphite. The sublimation of the solid can be done using electric arc or laser ablation where the temperature reaches to 2500 -3500°C. Fig. 2. Schematics for CNT formation by sublimation of graphite with subsequent desublimation.
 The electric arc discharge method is one of the efficient techniques for synthesis of CNT. Typically, about 60 to70 wt% of the arc-synthesized soot is CNT. The rest of the soot comprises of fullerenes, amorphous carbon and catalyst nanoparticles. In electric arc discharge production of CNT two graphite rods are used and a current is passed continuously between the electrodes. The anode is drilled and filled with catalysts. The metal oxides (Ni, Co, Fe) are used as catalyst. In some cases the catalyst/graphite composite is used as electrode. The synthesis is performed in cooled chamber in presence of helium, argon or methane environment. During the arcing, the catalyst/graphite anode is evaporated and consumed with simultaneous carbon deposition around the cathode. The quality of CNT samples depends upon arc stability, current density and cooling of cathode.  In laser ablation method the graphite target is subjected to laser and sublimated carbon is recollected. Inert gas atmosphere is maintained within the chamber.
2. Decomposition of carbon containing compounds  The most used method to prepare CNT is pyrolysis of hydrocarbon gases or vapors such as propane, butane, hexane, benzene, toluene etc. The method is also known as chemical vapor deposition (CVD) process. Chemical Vapor Deposition ( CVD ) By chemical vapor deposition CNTs can be produced in large quantities. The process temperature can vary from 500 – 1300°C. The hydrocarbon precursors include CH4, C2H2, C6H6, alcohols etc. Fig. 3 . Schematics for carbon vapor deposition method
 In CVD method there is initial dissociation of hydrocarbons followed by dissolution and saturation of C atoms in metal nanoparticles. Thereafter there is precipitation of carbon. Vapor-grown CNTs generally use metal catalyst particles. Fe, Co and Ni catalysts are mostly used for the catalytic growth of CNT. More recently, CNTs have also been grown from metal such as Au, Ag and Cu. Catalyst serves as nucleation sites and also promotes pyrolysis of hydrocarbons. Growth Mechanisms : The growth mechanism of CNT on metal based catalysts and nonmetal based catalysts are discussed below. 1. Metal based catalysts 2. Non metal based catalysts
1. Metal based catalysts  The growth mechanism of CNTs is yet to be fully established. In general it is proposed that hydrocarbons adsorb on metal particles and are catalytically decomposed. This results in carbon dissolving into particle.  Upon super saturation, carbon precipitates in tubular crystalline form. However, various alternative models have been proposed and appropriate description of growth depends on synthesis route and conditions used.  Two most described models are root growth and tip growth . The growth mechanisms are schematically explained in Fig. 4. For formation of CNTs the metal catalysts have to be dispersed well on the substrate forming nano clusters on the substrate surface. When there is a strong interaction between the catalyst clusters and substrate, the CNT grows by the root-growth mechanism .  The pyrolysis of hydrocarbons produces the carbon atoms which are extremely mobile on metal surfaces and rapidly diffuse over and through the metal particles. The graphite precipitates around the catalyst particles and cylindrical structures are formed in a nested fashion from the catalyst particle, with the catalyst particle at the root.
 The growth of the nanotubes stops when the catalyst particles are completely covered with layers of carbon. The tip growth mechanism is observed when the interaction between metal catalyst clusters and substrate is weak.  Due to the weak interaction, the catalyst particles are lifted off the substrate by the cylindrical structures formed below it. While, the carbon nanotube base remains anchored to the substrate, the tip carrying the catalyst particle grows toward the region of higher feed gas concentration. The tip growth stops when the catalyst particle is covered with carbon layers or when the supply of feed gas is cut off. Fig. 4. Schmatics showing root growth and tip growth mechanism of CNTs
 Synthesis of carbon nanotubes over Fe catalyst on aluminium is described by Emmenegger as follows. Initially iron nitrate layer on support transforms to crystalline Fe2O3 film during heating under nitrogen. After acetylene is introduced, Fe2O3 film is fragmented and smaller particles are formed. Fe2O3 crystals are reduced to inter-mediate oxides such as Fe3O4, FeO by hydrogen released from pyrolysis of acetylene. Finally metastable Fe3C is formed and growth of CNT occurs by tip growth mechanism. 2. Non-metal based catalysts (Ceramic and semiconductor catalysts) Among non-metallic catalysts SiC is most widely used. The Fig.5 shows the carbon nanostructure formation over silicon carbide by carbothermal reduction of silica. At first stage SiO2 is reduced to SiC via a carbothermal reaction. In next step SiC nano particle coalesce and finally carbon nanotube growth occurs on surface of SiC particles through carbon precipitation and/or SiC decomposition. The carbon generation reaction is given below.
Fig. 5. Schematic representation of CNT formation over silica MgO, Al2O3, zirconia, magnesium borates etc. are also reported to be used to grow carbon nanotubes.
Characterization of CNT X-ray diffraction pattern of CNTs are close to graphite. A graphite-like peak (002) is present and measurements of interlayer spacing can be obtained from its position using the Bragg law . Carbon nanotubes are also active in Raman spectroscopy. Most characteristic features are : peak <200 cm−1 which is characteristic of SWNT, frequency depend on tube diameter. The 1340 cm−1is assigned to residual ill-organized graphite. The 1500 - 1600 cm−1 peak also characteristic of nanotubes. The TEM images are absolute necessary for studying CNTs. Application of CNT and CNT based catalysts: CNTs are used in several catalytic reactions as catalyst or catalyst supports. In particular liquid phase reactions were studied extensively with MWNT. Higher surface area and mesoporous nature resulted in significant decrease in mass transfer limitations compared to activated carbon.
1. Hydrogenation reactions This is one of the most studied catalytic reactions both in liquid and gaseous phases. Ni, Rh, Ru supported on CNT were reported to be more active for hydrogenation reactions compared to when supported on activated carbon. Hydrogenation reactions such as hydrogenation of alkenes and α,β – unsaturated aldehyde have been reported for CNT supported catalysts. Ruthenium nanoparticles supported on MWCNTs showed excellent catalytic activity for hydrogenation of aromatic hydrocarbon . The 5wt% Pt/CNT catalyst was reported to be significantly more active than 5wt% Pt/AC for hydrogenation of trans –diphenylethene and trans β-methylstyrene . Rhodium complex grafted on MWCNTs was reported to be very active for cyclohexene hydrogenation while Pd/CNT catalyst was found to be active for benzene hydrogenation . Pt supported on SWCNTs has been found to be active and selective in hydrogenation of prenal (3-methyl-2-butenal) to prenol (3-methyl-2- butenol) .
2. Reaction involving CO/H2  CNTs have been investigated for Fischer –Tropsch reactions, methanol and higher alcohol synthesis and hydroformylation reactions. Copper promoted Fe/MWCNT catalyst are active for Fischer-Tropsch synthesis with olefins . Co- Re/Al2O3 deposited on MWCNT by dip coating exhibited an enhancement in Fischer-Tropsch activity than observed with a similar system without CNT arrays. MWCNTs also have been used as promoter for Cu-ZnO- Al2O3 catalysts for methanol synthesis using H2/CO/CO2 The complex [HRh(CO)(PPh3)3] has been grafted onto MCWNTs and used for hydroformylation of propene. Higher conversion and higher regioselectivity toward n-butaldehyde have been reported for CNT supported catalysts compared to that activated carbon or carbon molecular sieve supported catalysts.
3. Ammonia synthesis and decomposition  The Ru/C catalyst is studied as an alternative to conventional iron based catalyst for ammonia synthesis at high pressure and temperature. However, Ru/C catalyst is prone to deactivation due to metal sintering, metal leaching or methanation of support. The stability of the catalyst are reported to increase on using CNT as support. Ru-K/MWCNT catalyst has been found to be significantly more active than Ru supported on other carbon supports . The catalytic decomposition of ammonia to generate CO- free hydrogen for fuel cells is receiving increasing attention since the process is more economical than using methanol as hydrogen source. The MWCNT supported ruthenium was found to be more active than MgO, TiO2 or alumina supported Ru .
4. Polymerization The CNTs have excellent thermal and electrical conductivities and reported to be used as fillers in polymer based advanced composites. However due to poor solubility of CNTs, homogeneous dispersion is difficult task. The polymer functionalized CNTs are prepared following three approaches:  A non-covalent functionalization method in which polymers are produced by ring opening metathesis polymerization. The coating of hyper branched polymers on MWCNTs has been obtained via cationic ring opening polymerization of 3-ethyl-3-(hydroxymethyl)oxetane with a BF3 .Et2O catalyst  A covalent functionalization performed by, first grafting polymerization initiators onto the tubes through covalent bond and then exposing these CNT based macro-initiators to the monomer. The polymer is obtained by atom transfer radical polymerization. The polyethylene-MWCNT composite has been produced using catalyst grafting procedure by polymerization of ethylene on [ZrCl2Cp2] MAO/MWCNT where Cp = C5H5 and MAO = methylaluminoxane .
5. Fuel cell electrocatalysts  Olefin polymerization via anchored metallocenes catalysts. It has been proposed that MWCNT play a key role in increasing the molecular mass. Supporting the catalytic system on MWCNTs increases the polymerization rate of ethylene. Syndiotactic polypropylene-MWCNT composites have been prepared by propylene polymerization on zirconocene-MAO catalysts. CNTs are used as catalysts supports for anode or cathode catalysis in direct methanol fuel cells or proton exchange membrane fuel cells. The structure and properties of carbon supports which constitute the electrode material have a direct impact on performance of fuel cells. The most studied reactions are methanol oxidation (anode catalyst), oxygen reduction (cathode catalyst) and hydrogen oxidation (anode catalyst). The Pt is most used metal followed by Pt- Ru system. The general observation is that CNT based catalysts are more active and better resistant to poisoning compared to conventional carbon black support. The advantage of CNT supports for fuel cell applications is attributed to:
1. Higher metal dispersion and higher electroactive surface area 2. Higher mesopores 3D network facilitating mass transport 3. Excellent conducting properties which improve electron transfer. 6. Other applications CNT is also being investigated as support for biocatalysts. CNTs have been used for enzyme immobilization which increases enzyme stability, control of pore size, multiple active sites and reduced mass transfer limitations. CNTs have also been used as direct catalyst for some specific reactions such as methanation to produce CO and CO2 free hydrogen, oxidative dehydrogenation of ethyl benzene to styrene and oxidative dehydrogenation of propane to propene, selective oxidation of H2S, oxidation of aniline, esterification and hydroxylations.
References  https://nptel.ac.in/courses/103103026/  Nanomaterials Handbook edited by Yuri Gogotsi  Lecture on Carbon nanotubes and their applications SRM University  Carbon Nanotubes and their applications edited by Qing Zhang.

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In this document
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Overview of carbon nanotubes and their applications, along with institutional details.

Carbon's abundance, properties, isotopes, electron configuration, and reactivity.

Types of carbon allotropes such as Diamond, Graphite, Fullerenes, and Carbon Nanotubes.

Introduction to fullerenes, a family of carbon allotropes with unique hollow structures.

Historical context and discovery of C60 fullerene by Kroto, Curl, and Smalley.

Detailed description of the C60 fullerene structure resembling a football.

Historical discovery of carbon nanotubes by Roger Bacon and Sumio Iijima.

Definition, structure, types (single-walled and multi-walled), and properties of carbon nanotubes.

Detailing single-walled and multi-walled nanotubes, properties, structures, and applications.

Examining mechanical, thermal, electronic, magnetic, and superconducting properties of CNTs.

Highlights of mechanical strength and excellent thermal conductivity of carbon nanotubes.

Insights into the electronic properties of CNTs, including conductivity based on structure.

Discussion of magnetic behaviors and the potential for superconductivity in CNTs.

Overview of synthesis techniques including arc discharge, laser ablation, and CVD methods.

Explanation of metal-based growth mechanisms, root growth, and tip growth processes.

Characterization techniques and various applications of CNTs in catalysis and fuel cells.

List of references and resources for further reading on carbon nanotubes.

carbon nanotubes

  • 1.
    CARBON NANOTUBES AND THEIRAPPLICATIONS DEPARTMENT OF PHYSICS MALAVIYA NATIONAL INSTITUTE OF TECHNOLOGY JAIPUR SUBMITTED TO: Dr. Srinivasa Rao Nelamarri SUBMITTED FROM: YOGESH CHILLAR I.D. 2017PPH5336 M.SC. PHYSICS 3RD SEMESTER
  • 2.
    CARBON group- 14 period-2  Carbonis the 15th most abundant element in the Earth's crust, and the fourth most abundant element in the universe by mass after hydrogen, helium, and oxygen.  It is reactive non metal.  Electron configuration [He] 2s2 2p2  Electrons per shell 2,4 .  tetravalent—making four electrons available to form covalent chemical bonds  Three isotopes occur naturally, 12C and 13C being stable, while 14C is a radionuclide, decaying with a half-life of about 5,730 years.
  • 3.
    Allotropes of carbon Allotropyis the property of some chemical elements to exist in two or more different forms, or allotropes, when found in nature. There are several allotropes of carbon. a) Diamond, b) Graphite, c) Lonsdaleite, d) C60 (Buckminsterfullerene or buckyball), e) C540, f) C70, g) Amorphous carbon, and h) single-walled carbon nanotube, or buckytube.
  • 4.
    FULLERENCES  Fullerenes area family of carbon allotropes.  They are molecules composed entirely of carbon.  They are in the form of a hollow sphere, ellipsoid, or tube. FULLERENCE
  • 5.
    Discovery of thefirst fullerene: C60  In 1985, Prof. Harold W. Kroto of the University of Sussex joined Robert F. Curl and Prof. Richard E. Smalley at Rice University to study the products of carbon vaporization.  They carried out molecular beam experiments.  From the result, discrete peaks were observed corresponding to molecules with the exact mass of sixty or seventy or more carbon atoms.  C60 was then discovered, and it was named buckminsterfullerene which is named after Richard Buckminster Fuller who designed geodesic domes which is the same structure as C60.  Shortly after discovery of C60, it came to discover the fullerenes.
  • 6.
    Structures of somefullerenes  C60 (Buckminsterfullerene)  Its like the shape of a football. *grey ball represents a carbon atom
  • 7.
    CARBON NANOTUBES DISCOVERY  Inthe late 1950s, Roger Bacon at Union Carbide, found a strange new carbon fibre while studying carbon under conditions near its triple point. He observed straight, hollow tubes of carbon that appeared to consist in graphitic layers of carbon separated by the same spacing as the planar layers of graphite. In the 1970s, Morinobu Endo observed these tubes again, produced by a gas-phase process. Indeed, he even observed some tubes consisting in only a single layer of rolled-up graphite.  In 1991, after the discovery and verification of the fullerenes, Sumio Iijima of NEC observed multiwall nanotubes formed in a carbon arc discharge, and two years later, he and Donald Bethune at IBM independently observed single-wall nanotubes – buckytubes.
  • 8.
    CARBON NANOTUBES  Carbonnanotubes are tubular forms of carbon that can be envisaged as graphene sheets rolled into cylindrical form.  These nanotubes have diameters of few nanometers and their lengths are up to several micrometers.  Each nanotube is made up of a hexagonal network of covalently bonded carbon atoms.  Carbon nanotubes are of two types: single-walled and multi-walled.  A single-walled carbon nanotube (SWNT) consists of a single graphene cylinder whereas a multi-walled carbon nanotube (MWNT) comprises of several concentric graphene cylinders.
  • 9.
     Depending onthe way of rolling of graphene sheets (as shown in Fig),single- walled nanotubes of different types, viz. armchair, zig-zag and chiral could be produced. They can be represented using the method given by Hamada. For example, to realize an (n, m) nanotube, one has to move n times a1 from the selected origin and then m times a2. On rolling the graphite sheet these points coincide to form the (n, m) nanotube. Thus armchair, zig-zag and chiral nanotubes can be represented as (n, n), (n, 0) and (n, m) respectively  Different wrapping results in different structures and electronic properties.
  • 10.
    Schematic representation ofthe relation between nanotubes and graphene. The three rectangles can be rolled up into seamless nanotubes; the short side, referred to as the roll-up vector R, becomes the circumference. R= na1+ma2, is a graphene 2D lattice vector, where a1 and a2 are unit vectors. Integers n and m uniquely define the tube: diameter, chirality, metal vs. semiconducting nature, and band gap, if semiconducting. In a bulk polydisperse sample consisting of a distribution of diameters, the larger the average diameter, the greater the number of n, m pairs that will satisfy the seamless roll-up condition.
  • 11.
    Types of CNTs Carbon nanotubes are of two types : 1. Single wall carbon nanotube (SWNT) 2. Multiple wall carbon nanotube (MWNT)  Single wall nanotube (SWNT ) consist of one cylinder. It is made of single graphene sheet rolled up into cylinder closed by two caps (semi fullerenes). The SWNTs have diameter in the range of 0.5 -2.0 nm. The length is in the range of 50-150 μm length. The SWNTs are microporous and the specific surface area is in the range of 1300 m2/g (outer surface). SWCNTs are commonly arranged in bundles. SWNTs have less topological defects and have better mechanical and electro physical properties. Electronic properties of SWNTs are governed by two factors, tube diameters and helicity, which further depend on the way graphene layer is rolled up, arm chair or chiral. Armchair SWNTs shows conductivity as similar to metal whereas zigzag SWNTs behave as semiconductors. In catalysis CNTs have high application as support.
  • 12.
     Electrical conductivity,surface curvature and presence of inner cavity in CNTs make the metal –support interaction different compared to that in activated carbon or graphite support. Mechanically bent SWNTs present kink sites that are chemically more active. Metal nanoparticles size depends strongly on metal-CNT interactions with stronger interaction giving rise to smaller nanoparticles. Studies have shown that convex surface of CNTs are more reactive than concave surface and the difference in reactivity increases when the tube diameter decreases.  Multiwall (MWNT) nano tubes consist of many nested concentric SWNTs cylinders with increasing successive radii. The concentric walls are spaced regularly at 0.34 nm similar to inter graphene distance. MWNTs have outer diameter in range of 2 – 100 nm depending on number of coaxial tubes present. MWNTs are usually mesoporous in nature and specific area depends on the number of walls. The length of MWNTs can range from few to hundreds μm. The advantage of MWNT over SWNT is that the multi-shell structures of MWNTs are stiffer than single wall hence stability is higher. Also large scale synthesis of MWNT is possible by various methods. The most common characterization techniques of these materials are electron microscopy, Raman spectroscopy, TGA , IR and UV-Vis.
  • 13.
    PHYSICAL PROPERTIES OFCARBON NANOTUBES  MECHANICAL PROPERTIES  THERMAL PROPERTIES  ELECTRONIC PROPERTIES  MAGNETIC PROPERTIES  SUPERCONDUCTING PROPERTIES
  • 14.
    PROPERTIES FOUNDATION  Nanotubesare fully described by their chiral vector  R = n â1 + m â2  Important parameters  dt = (3/p)ac-c(m2 + mn + n2)1/2  Q=tan-1(3n/(2m + n))  Grouped according to q  Armchair: n=m, q=30°  Zigzag: n or m=0, q=0°  Chiral: 0°<q < 30° A. Maiti, Caron Nanotubes: Band gap engineering with strain, Nature Materials 2 (2003) 440 V. Popov, Carbon nanotubes: properties and applications, Materials Science and Engineering R 43 (2004) 61-102
  • 15.
    MECHANICAL PROPERTIES The strengthof the carbon–carbon bond gives rise to the extreme interest in the mechanical properties of nanotubes.  Theoretically, they should be stiffer and stronger than any known substance.  Simulations and experiments demonstrate a remarkable “bend, don’t break” response of individual SWNT to large transverse deformations; an example from Yakobson’s simulation is shown in Figure 4.9.
  • 16.
     The twosegments on either side of the buckled region can be bent into an acute angle without breaking bonds; simulations and experiments show the full recovery of a straight perfect tube once the force is removed.  Young’s modulus of a cantilevered individual MWNT was measured as 1.0 to 1.8 TPa from the amplitude of thermally driven vibrations observed in the TEM. At the low end, this is only ~20% better than the best high-modulus graphite fibers  Multiwall nanotubes and SWNT bundles may be stiffer in bending but are expected to be weaker in tension due to “pullout” of individual tubes. Comparison of mechanical properties
  • 17.
    Thermal Properties  Allnanotubes are expected to be very good thermal conductors along the tube, but good insulators laterally to the tube axis.  It is predicted that carbon nanotubes will be able to transmit up to 6000 watts per meter per Kelvin at room temperature; compare this to copper, a metal well-known for its good thermal conductivity, which transmits 385 watts per meter per K.  The temperature stability of carbon nanotubes is estimated to be up to 2800oC in vacuum and about 750oC in air.
  • 18.
    ELECTRONIC PROPERTIES  1-Dband structure calculated from 2-D graphene band structure using “zone folding” scheme  Ekμ= E2D(k*K2/|K2|+μK1)  K1=(-t2b1+ t1b2)/ N  K2=(mb1- nb2)/ N V. Popov, Carbon nanotubes: properties and applications, Materials Science and Engineering R 43 (2004) 61-102
  • 19.
     Theory predictsnanotubes exhibit both metallic and semi-conducting behavior  |n-m| evenly divisible by 3- metallic  All others semi-conducting with a band gap inversely proportional to the tube diameter T.W. Odomet al, Atomic Structure and Electronic Properties of Single-Walled Nanotubes, Nature (London) 391 (1998) 62
  • 20.
     If thenanotube structure is armchair then the electrical properties are metallic  If the nanotube structure is chiral then the electrical properties can be either semiconducting with a very small band gap, otherwise the nanotube is a moderate semiconductor  In theory, metallic nanotubes can carry an electrical current density of 4×109 A/cm2 which is more than 1,000 times greater than metals such as copper
  • 21.
    MAGNETIC PROPERTY  Ithas been observed that there is no bulk magnetism in a clean multiwall nanotube sample.  On the other hand, when these are placed on a flat ferromagnetic substrate, fringing fields can be observed by magnetic force microscopy. Therefore the prospect for “contact-induced magnetism” and the application of carbon nanotubes to nanoscale spintronic devices remain open.  Magnetic contrast is observed for carbon nanotubes placed on cobalt or magnetite substrates, but is absent on silicon, copper, or gold. Spin transfer of about 0.1 µB per contact carbon atom is obtained
  • 22.
    Superconducting properties  Thedialog about nanotube superconductivity began with a very simple argument: if one could tune the chemical potential to the peak of a van Hove singularity, either chemically or electrostatically, one should have a high N(EF) value, a prerequisite for BCS superconductivity.  Super-current flow through an SWNT with low-resistance contacts was reported in 1999; analogous to contact-induced magnetism, no claims were made for bulk nanotube superconductivity. Evidence for a bulk anisotropic Meissner effect below 20 K in aligned 0.4-nm-diameter SWNT was claimed and justified by the argument that small-diameter tubes will be the stiffest, and thus, the average phonon energy in the BCS equation will be favorable. This dramatic result has not been reproduced by other researchers. As with bulk magnetism, it would appear that the jury is still out concerning bulk superconductivity in carbon nanotubes and other nanostructures.
  • 23.
    Synthesis of CNTs There are two main methods for preparation of CNT: 1. Sublimation of graphite with subsequent desublimation - Arc Discharge Method - Laser ablation 2. Decomposition of carbon containing compounds - Chemical Vapor Deposition ( CVD )
  • 24.
    1. Sublimation ofgraphite with subsequent desublimation  This method involves condensation of carbon atoms generated from evaporation of solid carbon sources of graphite. The sublimation of the solid can be done using electric arc or laser ablation where the temperature reaches to 2500 -3500°C. Fig. 2. Schematics for CNT formation by sublimation of graphite with subsequent desublimation.
  • 25.
     The electricarc discharge method is one of the efficient techniques for synthesis of CNT. Typically, about 60 to70 wt% of the arc-synthesized soot is CNT. The rest of the soot comprises of fullerenes, amorphous carbon and catalyst nanoparticles. In electric arc discharge production of CNT two graphite rods are used and a current is passed continuously between the electrodes. The anode is drilled and filled with catalysts. The metal oxides (Ni, Co, Fe) are used as catalyst. In some cases the catalyst/graphite composite is used as electrode. The synthesis is performed in cooled chamber in presence of helium, argon or methane environment. During the arcing, the catalyst/graphite anode is evaporated and consumed with simultaneous carbon deposition around the cathode. The quality of CNT samples depends upon arc stability, current density and cooling of cathode.  In laser ablation method the graphite target is subjected to laser and sublimated carbon is recollected. Inert gas atmosphere is maintained within the chamber.
  • 26.
    2. Decomposition ofcarbon containing compounds  The most used method to prepare CNT is pyrolysis of hydrocarbon gases or vapors such as propane, butane, hexane, benzene, toluene etc. The method is also known as chemical vapor deposition (CVD) process. Chemical Vapor Deposition ( CVD ) By chemical vapor deposition CNTs can be produced in large quantities. The process temperature can vary from 500 – 1300°C. The hydrocarbon precursors include CH4, C2H2, C6H6, alcohols etc. Fig. 3 . Schematics for carbon vapor deposition method
  • 27.
     In CVDmethod there is initial dissociation of hydrocarbons followed by dissolution and saturation of C atoms in metal nanoparticles. Thereafter there is precipitation of carbon. Vapor-grown CNTs generally use metal catalyst particles. Fe, Co and Ni catalysts are mostly used for the catalytic growth of CNT. More recently, CNTs have also been grown from metal such as Au, Ag and Cu. Catalyst serves as nucleation sites and also promotes pyrolysis of hydrocarbons. Growth Mechanisms : The growth mechanism of CNT on metal based catalysts and nonmetal based catalysts are discussed below. 1. Metal based catalysts 2. Non metal based catalysts
  • 28.
    1. Metal basedcatalysts  The growth mechanism of CNTs is yet to be fully established. In general it is proposed that hydrocarbons adsorb on metal particles and are catalytically decomposed. This results in carbon dissolving into particle.  Upon super saturation, carbon precipitates in tubular crystalline form. However, various alternative models have been proposed and appropriate description of growth depends on synthesis route and conditions used.  Two most described models are root growth and tip growth . The growth mechanisms are schematically explained in Fig. 4. For formation of CNTs the metal catalysts have to be dispersed well on the substrate forming nano clusters on the substrate surface. When there is a strong interaction between the catalyst clusters and substrate, the CNT grows by the root-growth mechanism .  The pyrolysis of hydrocarbons produces the carbon atoms which are extremely mobile on metal surfaces and rapidly diffuse over and through the metal particles. The graphite precipitates around the catalyst particles and cylindrical structures are formed in a nested fashion from the catalyst particle, with the catalyst particle at the root.
  • 29.
     The growthof the nanotubes stops when the catalyst particles are completely covered with layers of carbon. The tip growth mechanism is observed when the interaction between metal catalyst clusters and substrate is weak.  Due to the weak interaction, the catalyst particles are lifted off the substrate by the cylindrical structures formed below it. While, the carbon nanotube base remains anchored to the substrate, the tip carrying the catalyst particle grows toward the region of higher feed gas concentration. The tip growth stops when the catalyst particle is covered with carbon layers or when the supply of feed gas is cut off. Fig. 4. Schmatics showing root growth and tip growth mechanism of CNTs
  • 30.
     Synthesis ofcarbon nanotubes over Fe catalyst on aluminium is described by Emmenegger as follows. Initially iron nitrate layer on support transforms to crystalline Fe2O3 film during heating under nitrogen. After acetylene is introduced, Fe2O3 film is fragmented and smaller particles are formed. Fe2O3 crystals are reduced to inter-mediate oxides such as Fe3O4, FeO by hydrogen released from pyrolysis of acetylene. Finally metastable Fe3C is formed and growth of CNT occurs by tip growth mechanism. 2. Non-metal based catalysts (Ceramic and semiconductor catalysts) Among non-metallic catalysts SiC is most widely used. The Fig.5 shows the carbon nanostructure formation over silicon carbide by carbothermal reduction of silica. At first stage SiO2 is reduced to SiC via a carbothermal reaction. In next step SiC nano particle coalesce and finally carbon nanotube growth occurs on surface of SiC particles through carbon precipitation and/or SiC decomposition. The carbon generation reaction is given below.
  • 31.
    Fig. 5. Schematicrepresentation of CNT formation over silica MgO, Al2O3, zirconia, magnesium borates etc. are also reported to be used to grow carbon nanotubes.
  • 33.
    Characterization of CNT X-raydiffraction pattern of CNTs are close to graphite. A graphite-like peak (002) is present and measurements of interlayer spacing can be obtained from its position using the Bragg law . Carbon nanotubes are also active in Raman spectroscopy. Most characteristic features are : peak <200 cm−1 which is characteristic of SWNT, frequency depend on tube diameter. The 1340 cm−1is assigned to residual ill-organized graphite. The 1500 - 1600 cm−1 peak also characteristic of nanotubes. The TEM images are absolute necessary for studying CNTs. Application of CNT and CNT based catalysts: CNTs are used in several catalytic reactions as catalyst or catalyst supports. In particular liquid phase reactions were studied extensively with MWNT. Higher surface area and mesoporous nature resulted in significant decrease in mass transfer limitations compared to activated carbon.
  • 34.
    1. Hydrogenation reactions Thisis one of the most studied catalytic reactions both in liquid and gaseous phases. Ni, Rh, Ru supported on CNT were reported to be more active for hydrogenation reactions compared to when supported on activated carbon. Hydrogenation reactions such as hydrogenation of alkenes and α,β – unsaturated aldehyde have been reported for CNT supported catalysts. Ruthenium nanoparticles supported on MWCNTs showed excellent catalytic activity for hydrogenation of aromatic hydrocarbon . The 5wt% Pt/CNT catalyst was reported to be significantly more active than 5wt% Pt/AC for hydrogenation of trans –diphenylethene and trans β-methylstyrene . Rhodium complex grafted on MWCNTs was reported to be very active for cyclohexene hydrogenation while Pd/CNT catalyst was found to be active for benzene hydrogenation . Pt supported on SWCNTs has been found to be active and selective in hydrogenation of prenal (3-methyl-2-butenal) to prenol (3-methyl-2- butenol) .
  • 35.
    2. Reaction involvingCO/H2  CNTs have been investigated for Fischer –Tropsch reactions, methanol and higher alcohol synthesis and hydroformylation reactions. Copper promoted Fe/MWCNT catalyst are active for Fischer-Tropsch synthesis with olefins . Co- Re/Al2O3 deposited on MWCNT by dip coating exhibited an enhancement in Fischer-Tropsch activity than observed with a similar system without CNT arrays. MWCNTs also have been used as promoter for Cu-ZnO- Al2O3 catalysts for methanol synthesis using H2/CO/CO2 The complex [HRh(CO)(PPh3)3] has been grafted onto MCWNTs and used for hydroformylation of propene. Higher conversion and higher regioselectivity toward n-butaldehyde have been reported for CNT supported catalysts compared to that activated carbon or carbon molecular sieve supported catalysts.
  • 36.
    3. Ammonia synthesisand decomposition  The Ru/C catalyst is studied as an alternative to conventional iron based catalyst for ammonia synthesis at high pressure and temperature. However, Ru/C catalyst is prone to deactivation due to metal sintering, metal leaching or methanation of support. The stability of the catalyst are reported to increase on using CNT as support. Ru-K/MWCNT catalyst has been found to be significantly more active than Ru supported on other carbon supports . The catalytic decomposition of ammonia to generate CO- free hydrogen for fuel cells is receiving increasing attention since the process is more economical than using methanol as hydrogen source. The MWCNT supported ruthenium was found to be more active than MgO, TiO2 or alumina supported Ru .
  • 37.
    4. Polymerization The CNTshave excellent thermal and electrical conductivities and reported to be used as fillers in polymer based advanced composites. However due to poor solubility of CNTs, homogeneous dispersion is difficult task. The polymer functionalized CNTs are prepared following three approaches:  A non-covalent functionalization method in which polymers are produced by ring opening metathesis polymerization. The coating of hyper branched polymers on MWCNTs has been obtained via cationic ring opening polymerization of 3-ethyl-3-(hydroxymethyl)oxetane with a BF3 .Et2O catalyst  A covalent functionalization performed by, first grafting polymerization initiators onto the tubes through covalent bond and then exposing these CNT based macro-initiators to the monomer. The polymer is obtained by atom transfer radical polymerization. The polyethylene-MWCNT composite has been produced using catalyst grafting procedure by polymerization of ethylene on [ZrCl2Cp2] MAO/MWCNT where Cp = C5H5 and MAO = methylaluminoxane .
  • 38.
    5. Fuel cellelectrocatalysts  Olefin polymerization via anchored metallocenes catalysts. It has been proposed that MWCNT play a key role in increasing the molecular mass. Supporting the catalytic system on MWCNTs increases the polymerization rate of ethylene. Syndiotactic polypropylene-MWCNT composites have been prepared by propylene polymerization on zirconocene-MAO catalysts. CNTs are used as catalysts supports for anode or cathode catalysis in direct methanol fuel cells or proton exchange membrane fuel cells. The structure and properties of carbon supports which constitute the electrode material have a direct impact on performance of fuel cells. The most studied reactions are methanol oxidation (anode catalyst), oxygen reduction (cathode catalyst) and hydrogen oxidation (anode catalyst). The Pt is most used metal followed by Pt- Ru system. The general observation is that CNT based catalysts are more active and better resistant to poisoning compared to conventional carbon black support. The advantage of CNT supports for fuel cell applications is attributed to:
  • 39.
    1. Higher metaldispersion and higher electroactive surface area 2. Higher mesopores 3D network facilitating mass transport 3. Excellent conducting properties which improve electron transfer. 6. Other applications CNT is also being investigated as support for biocatalysts. CNTs have been used for enzyme immobilization which increases enzyme stability, control of pore size, multiple active sites and reduced mass transfer limitations. CNTs have also been used as direct catalyst for some specific reactions such as methanation to produce CO and CO2 free hydrogen, oxidative dehydrogenation of ethyl benzene to styrene and oxidative dehydrogenation of propane to propene, selective oxidation of H2S, oxidation of aniline, esterification and hydroxylations.
  • 40.
    References  https://nptel.ac.in/courses/103103026/  NanomaterialsHandbook edited by Yuri Gogotsi  Lecture on Carbon nanotubes and their applications SRM University  Carbon Nanotubes and their applications edited by Qing Zhang.