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Since their discovery in 1991 by Iijima, carbon nanotubes have been of great interest, both from a fundamental point of view and for future applications. The most eye-catching features of these structures are their electronic, mechanical, optical and chemical characteristics, which open a way tofuture applications. These properties can even be measured on single nanotubes. For commercial application, large quantities of purified nanotubes are needed.Different types of carbon nanotubes can be produced in various ways. The most common techniques used nowadays are: arc discharge, laser ablation, chemical vapour deposition and flame synthesis.Purification of the tubes can be divided into a couple of main techniques: oxidation, acid treatment,annealing, sonication, filtering and functionalisation techniques. Economically feasible large-scale production and purification techniques still have to be developed.Fundamental and practical nanotube researches have shown possible applications in the fields of energy storage, molecular electronics, nanomechanic devices, and composite materials. Real applications are still under development.This report provides an overview of current nanotube technology, with a special focus on synthesis and purification, energy storage in nanotubes and molecular electronics. Four types of energy storage known in carbon nanotubes are: electrochemical hydrogen storage, gas phase intercalation,electrochemical lithium storage and charge storage in supercapacitors.
The special nature of carbon combines with the molecular perfection of buckytubes (single-wall carbon nanotubes) to endow them with exceptionally high material properties such as electrical and thermal conductivity, strength, stiffness, and toughness. No other element in the periodic table bonds to itself in an extended network with the strength of the carbon-carbon bond. The delocalised pi-electron donated by each atom is free to move about the entire structure, rather than stay home with its donor atom, giving rise to the first molecule with metallic-type electrical conductivity. The high-frequency carbon-carbon bond vibrations provide an intrinsic thermal conductivity higher than even diamond.
In most materials, however, the actual observed material properties - strength, electrical conductivity, etc. - are degraded very substantially by the occurrence of defects in their structure. For example, high strength steel typically fails at about 1% of its theoretical breaking strength. Buckytubes, however, achieve values very close to their theoretical limits because of their perfection of structure - theirÂ molecular perfection. This aspect is part of the unique story of buckytubes.
Buckytubes are an example of true nanotechnology: only a nanometer in diameter, but molecules that can be manipulated chemically and physically. They open incredible applications in materials, electronics, chemical processing and energy management.
History of carbon Nanotubes
The current huge interest in carbon nanotubes is a direct consequence of the synthesis of buckminsterfullerene, C60 , and other fullerenes, in 1985. The discovery that carbon could form stable, ordered structures other than graphite and diamond stimulated researchers worldwide to search for other new forms of carbon. The search was given new impetus when it was shown in 1990 that C60 could be produced in a simple arc-evaporation apparatus readily available in all laboratories. It was using such an evaporator that the Japanese scientist Sumio Iijima discovered fullerene-related carbon nanotubes in 1991. The tubes contained at least two layers, often many more, and ranged in outer diameter from about 3 nm to 30 nm. They were invariably closed at both ends.
A transmission electron micrograph of some multiwalled nanotubes is shown in the figure (left). In 1993, a new class of carbon nanotube was discovered, with just a single layer. These single-walled nanotubes are generally narrower than the multiwalled tubes, with diameters typically in the range 1-2 nm, and tend to be curved rather than straight. The image on the right shows some typical single-walled tubes It was soon established that these new fibres had a range of exceptional properties (see below), and this sparked off an explosion of research into carbon nanotubes. It is important to note, however, that nanoscale tubes of carbon, produced catalytically, had been known for many years before Iijima's discovery. The main reason why these early tubes did not excite wide interest is that they were structurally rather imperfect, so did not have particularly interesting properties. Recent research has focused on improving the quality of catalytically-produced nanotubes.
On reading articles in newspapers and science and technology magazines one gets that impression that Suomo Iijima, a scientist at NEC Japan, is the unique discoverer of carbon nanotubes. While it is certainly obvious that S. Iijima made at least two key contributions to this field, careful analysis of the literature shows that he is most certainly not the first who reported about carbon nanotubes.
On reading articles in newspapers and science and technology magazines one gets that impression that Suomo Iijima, a scientist at NEC Japan, is the unique discoverer of carbon nanotubes. While it is certainly obvious that S. Iijima made at least two key contributions to this field, careful analysis of the literature shows that he is most certainly not the first who reported about carbon nanotubes. Already in the seventies, there have been reports on the existence of carbon tubes with nanometer diameter from scientists working on different forms of graphite in France and Japan.
In fact, M Endo from Japan was interested in carbon fibers and was collaborating with A Oberlin in France, reported on the observation of carbon nanotubes by electron microscopy in 1976. In Russia, LV Radushkevich and collaborators reported about carbon nanotubes as early as 1952. Hence, Carbon nanotubes were shown to exist but no fabrication process was known that would lead to the synthesis of macroscopic amounts of carbon nanotubes. As a result, not much interest sparked from these early reports on carbon nanotubes. Unrelated to this first work on carbon nanotubes, S. Iijima was working on diamond like carbon at NEC. Diamond like carbon, a disordered form of diamond, was and still is of much interest for industrial applications. Interestingly, in one of his older publications on diamond like carbon, rod or tube like objects can be seen in the electron microscopy images but they again did not attract much interest.
It was at this point in time that Donald Huffman in Arizona, US, and Wolfgang Krätschmer in Heidelberg, Germany discovered the arc evaporation method to produce macroscopic amounts of C60, a carbon molecule in the shape of a soccer ball. S. Iijima studied the arc evaporation process that efficiently produced C60 for a certain He gas pressure. When analyzing the content of a cylindrical electrode deposit produced during arc evaporation for different He pressures, Iijima found large amounts of multiwalled carbon nanotubes mixed with faceted graphitic particles. This was a great breakthrough and the discovery was unexpected in that the same method that produced macroscopic amounts of C60 did also produce macroscopic amounts of multiwalled carbon nanotubes (MWNTs). Even more surprising is that at the time this breakthrough was published in the journal Nature, a small startup company in Boston (Hyperion Catalysis) was already producing carbon fibrils. Carbon fibrils are produced in a chemical vapor deposition process. This process is now used commonly to produce carbon nanotubes. The fibrils are indeed carbon nanotubes with only a few walls that are not straight but are bent and contain characteristic defects. S. Iijima after having discovered that large amounts of multiwalled carbon nanotubes can be produced by the arc method aimed at filling the tubes with metals. Transition metals were mixed into the electrode. The arc produced this time was not the expected metal filled carbon nanotubes but a new form of carbon nanotubes: single shell carbon nanotubes (SWNTs) with a diameter in the 1.1-1.3nm range. Nearly simultaneously, D Bethune a scientist at IBM research laboratory working on C60 made the same discovery.
So, who discovered carbon nanotubes? While this must really be the group of Radushkevich and Endo, only later discoveries by Iijima led to the spate of activity in this field resulting in significant breakthroughs in structure-property correlations. We realize that it is not enough to know that carbon nanotubes exist but we need to be able to produce macroscopic amounts to make use of them. S. Iijima's key contributions and contributions of many devoted scientists, who came subsequently on the scene, show that there is a long way from discovery to taking full advantage of the unique physical and chemical properties of carbon nanotubes.
TYPES OF CARBON NANOTUBES
Several types of nanotubes exist; but they can be divided in two main categories: single-walled (SWNT) and multi-walled (MWNT). The form of nanotubes is identified by a sequence of two numbers, the first one of which represents the number of carbon atoms around the tube, while the second identifies an offset of where the nanotube wraps around to.
SINGLE-WALLED CARBON NANOTUBES
A single-wall nanotube (SWNT) is a rolled-up sheet of graphene that can be metallic or semiconducting depending on its chiral vector (N, M), where N and M are two integers. The rule is that a metallic or a semiconducting nanotube is obtained when the difference N-M is not a multiple of 3, respectively. The resulting tubes have a perfectly straight shape, but in reality demonstrate to us that according to the synthesis methods used, SWNTs also consistently display structural defects. Theory states that the most common defects consist of pentagon and heptagon inclusions in the perfect hexagonal lattice. This kind of defects mostly affect the local electronic density of states (LDOS), near the Fermi level of the system in a relatively large neighbourhood. Such non-hexagonal inclusions observed by scanning tunneling microscopy (STM) images will be presented. Considering the perturbation created in the electronic structure of those quasi one-dimensional heterojunctions, we shall propose several possible applications based on these interesting properties that may be foreseen.
MULTI-WALLED CARBON NANOTUBES
Multi-walled nanotubes (MWNT) consist of multiple rolled layers (concentric tubes) of graphite. There are two models which can be used to describe the structures of multi-walled nanotubes. In the Russian Doll model, sheets of graphite are arranged in concentric cylinders, e.g. a (0,8) single-walled nanotube (SWNT) within a larger (0,17) single-walled nanotube. In the Parchment model, a single sheet of graphite is rolled in around itself, resembling a scroll of parchment or a rolled newspaper. The interlayer distance in multi-walled nanotubes is close to the distance between graphene layers in graphite, approximately 3.4 Å.
The special place of double-walled carbon nanotubes (DWNT) must be emphasized here because their morphology and properties are similar to SWNT but their resistance to chemicals is significantly improved. This is especially important when functionalization is required (this means grafting of chemical functions at the surface of the nanotubes) to add new properties to the CNT. In the case of SWNT, covalent functionalization will break some C=C double bonds, leaving "holes" in the structure on the nanotube and thus modifying both its mechanical and electrical properties. In the case of DWNT, only the outer wall is modified. DWNT synthesis on the gram-scale was first proposed in 2003 by the CCVD technique, from the selective reduction of oxide solutions in methane and hydrogen.
STRUCTURE OF CARBON NANOTUBES
In order to visualize how nanotubes are built up, we start with graphite, which is the most stable form of crystalline carbon.
Graphite consists of layers of carbon atoms. Within the layers the atoms are arranged at the corners of hexagons which fill the whole plane. The carbon atoms are strongly (covalently) bound to each other (carbon-carbon distance âˆ¼ 0.14 nm ). The layers themselves are rather weakly bound to each other (weak longrange Van der Waals type interaction, interlayer distance of âˆ¼ 0.34 nm). The weak interlayer coupling gives graphite the property of a seemingly very soft material. The property us to use allows to use graphite in a pencil.
Carbon Nanotubes are considered to be a curved graphene sheet. Graphene sheets are seamless cylinders derived from a honeycomb lattice, representing a single atomic layer of crystalline graphite.
The structure of a Single-Wall Carbon Nanotube (SWCT) is expressed in terms of one-dimensional unit cell, defined by the vector
where a1 and a2 are unit vectors, and n and m are integers. A nanotube constructed in this way is called an (n,m) nanotube.
Rolling up the sheet along one of the symmetry axis gives either a zig-zag (m=0) tube or an armchair (n=m) tube. It is also possible to roll up the sheet in a direction that differs from a symmetry axis to obtain a chiral nanotube. As a well as the chiral angle, the circumference of the cylinder can also be varied.
Here is an example of a carbon nanotube (8,8), an armchair nanotube of a radius 5.42Å and length of 24.6Å, consists of 320 carbon atoms, generated by the visualization program - AViz2,
PROPERTIES OF CARBON NANOTUBES
Carbon nanotubes, long, thin cylinders of carbon, were discovered in 1991 by S. Iijima.These are large macromolecules that are unique for their size, shape, and remarkable physical properties. They can be thought of as a sheet of graphite (a hexagonal lattice of carbon) rolled into a cylinder. These intriguing structures have sparked much excitement in the recent years and a large amount of research has been dedicated to their understanding. Currently, the physical properties are still being discovered and disputed. What makes it so difficult is that nanotubes have a very broad range of electonic, thermal, and structural properties that change depending on the different kinds of nanotube (defined by its diameter, length, and chirality, or twist). To make things more interesting, besides having a single cylindrical wall (SWNTs), nanotubes can have multiple walls (MWNTs)--cylinders inside the other cylinders. This web site is an ongoing effort to provide researchers, students, and other interested scientists with a central location for the exchange of current knowledge and information.
OPTICAL PROPERTIES OF CARBON NANOTUBES
Within materials science, the optical properties of carbon nanotubes refer specifically to the absorption, photoluminescence, and Raman spectroscopy of carbon nanotubes. Spectroscopic methods offer the possibility of quick and non-destructive characterization of relatively large amounts of carbon nanotubes. There is a strong demand for such characterization from the industrial point of view: numerous parameters of the nanotube synthesis can be changed, intentionally or unintentionally, to alter the nanotube quality. As shown below, optical absorption, photoluminescence and Raman spectroscopies allow quick and reliable characterization of this "nanotube quality" in terms of non-tubular carbon content, structure (chirality) of the produced nanotubes, and structural defects. Those features determine nearly any other properties such as optical, mechanical, and electrical properties.
Carbon nanotubes are unique "one dimensional systems" which can be envisioned as rolled single sheets of graphite (or more precisely graphene). This rolling can be done at different angles and curvatures resulting in different nanotube properties. The diameter typically varies in the range 0.4-40Â nm (i.e. "only" ~100 times), but the length can vary ~10,000 times reaching 4Â cm. Thus the nanotube aspect ratio, or the length-to-diameter ratio, can be as high as 132,000,000:1, which is unequalled by any other material. Consequently, all the properties of the carbon nanotubes relative to those of typical semiconductors are extremely anisotropic (directionally dependent) and tunable.
Whereas mechanical, electrical and electrochemical (supercapacitor) properties of the carbon nanotubes are well established and have immediate applications, the practical use of optical properties is yet unclear. The aforementioned tunability of properties is potentially useful in optics and photonics. In particular, light-emitting diodes (LEDs)and photo-detectors based on a single nanotube have been produced in the lab. Their unique feature is not the efficiency, which is yet relatively low, but the narrow selectivity in the wavelength of emission and detection of light and the possibility of its fine tuning through the nanotube structure. In addition, bolometer and optoelectronic memory devices have been realised on ensembles of single-walled carbon nanotube.
APPLICATION OF CARBON NANOTUBES
Potential Application of CNTs
in Vacuum Microelectronics
Field emission is an attractive source for electrons compared to thermionic emission. It is a quantum effect. When subject to a sufficiently high electric field, electrons near the Fermi level can overcome the energy barrier to escape to the vacuum level. The basic physics of electron emission is well developed. The emission current from a metal surface is determined by the
Fowler-Nordheim equation: I = aV 2 exp (âˆ’bÏ†3/2/Î²V) where I, V , Ï†, Î², are Applications of Carbon Nanotubes 395 the current, applied voltage, work function, and field enhancement factor,
respectively. Electron field emission materials have been investigated extensively for technological applications, such as flat panel displays, electron guns in electron microscopes, microwave amplifiers. For technological applications,electron emissive materials should have low threshold emission fields and should be stable at high current density. A current density of 1-10mA/cm2 is required for displays and > 500mA/cm2 for a microwave amplifier. In order to minimize the electron emission threshold field, it is desirable to have emitters with a low work function and a large field enhancement factor. The work function is an intrinsic materials property. The field enhancement factor depends mostly on the geometry of the emitter and can be approximated as: Î² = 1/5r where r is the radius of the emitter tip. Processing techniques have been developed to fabricate emitters such as Spindt-type emitters, with a sub-micron tip radius. However, the process is costly and the emitters have only limited lifetime. Failure is often caused by ion bombardment from the residual gas species that blunt the emission tips. Table 1 lists the threshold electrical field values for a 10mA/cm2 current density for some typical materials Threshold electrical field values for different materials for a 10mA/cm2 current density. Material Threshold electrical field (V/_m) Mo tips 50-100 Si tips 50-100 p-type semiconducting diamond 130 Undoped, defective CVD diamond 30-120 Amorphous diamond 20-40 Cs-coated diamond 20-30 Graphite powder (<1mm size) 17 Nano structured diamond a 3-5 Carbon nanotubes b 1-3 a Heat-treated in H plasma.
Carbon nanotubes are being considered for energy production and storage. Graphite, carbonaceous materials and carbon fiber electrodes have been used for decades in fuel cells, battery and several other electrochemical applications.Nanotubes are special because they have small dimensions, a smooth surface topology, and perfect surface specificity, since only the basal
graphite planes are exposed in their structure. The rate of electron transfer at carbon electrodes ultimately determines the efficiency of fuel cells and this depends on various factors, such as the structure and morphology of the carbon material used in the electrodes. Several experiments have pointed out that compared to conventional carbon electrodes, the electron transfer
kinetics take place fastest on nanotubes, following ideal Nernstian behavior Nanotube microelectrodes have been constructed using a binder and Pulickel M. Ajayan and Otto Z. Zhou have been successfully used in bioelectrochemical reactions (e.g., oxidation of dopamine). Their performance has been found to be superior to other carbon electrodes in terms of reaction rates and reversibility. Pure MWNTs and MWNTs deposited with metal catalysts (Pd, Pt, Ag) have been used to electro-catalyze an oxygen reduction reaction, which is important for fuel
cells. It is seen from several studies that nanotubes could be excellent replacements for conventional carbon-based electrodes. Similarly, the improved selectivity of nanotube-based catalysts have been demonstrated in heterogeneous catalysis. Ru-supported nanotubes were found to be superior to the same metal on graphite and on other carbons in the liquid phase hydrogenation reaction of cinnamaldehyde . The properties of catalytically
grown carbon nanofibers (which are basically defective nanotubes) have been
found to be desirable for high power electrochemical capacitors .