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Carbon nanotubes are among the amazing objects that science sometimes creates by accident, without meaning to, but that will likely revolutionize the technological landscape of the century ahead. Our society stands to be significantly influenced by carbon nanotubes, shaped by nanotube applications in every aspect, just as silicon-based technology still shapes society today.
The world already dreams of spaceelevators tethered by the strongest of cables, hydrogen-powered vehicles, artificial muscles, and so on – feasts that would be made possible by the emerging carbon nanotube science.
Some of the first evidence that the nanofilaments thus produced were actually nanotubes – i. e., exhibiting an inner cavity – can be found in the transmission electron microscope micrographs published by Hillert et al. in 1958.
The worldwide enthusiasm came unexpectedly in 1991, after the catalyst-free formation of nearly perfect concentric multiwall carbon nanotubes (c-MWNTs) was reported as by-products of the formation of fullerenes by the electric-arc technique.
single-wall carbon nanotubes (SWNTs) simultaneously by Iijima et al. and Bethune et al.
There are three significant reasons for the wide range of properties of materials containing carbon:
Carbon atoms can bond together with many types of atoms, using a process called covalent bonding. When carbon atoms bond with different types of atoms, they form molecules with properties that vary according to the atoms they’ve bonded with.
Each carbon atom can form these covalent bonds with four other atoms at a time. This four-bond capability allows carbon atoms to bond to other carbon atoms to make chains of atoms — and to bond with other kinds of atoms at various points along such chains. This wide range of potential combinations of atoms in a molecule allows for a correspondingly wide range of potential properties.
There’s no other element in the periodic table that bonds as strongly to itself and in as many ways as the carbon atom. Carbon atoms can bond together in short chains, in which case they may have the properties of a gas. They may bond together as long chains, which might give you a solid, like a plastic. Or, they can bond together in 2- or 3-dimensional lattices, which can make for some very hard materials, such as a diamond.
graphite The structure of carbon atoms connected by covalent bonds in a sheet of graphite.
Buckyball Source: Nanotechnology For Dummies Sixty carbon atoms in the shape of a sphere — a buckyball.
A buckyball (short for buckminsterfullerene) is a molecule containing 60 carbon atoms. Each carbon atom is bonded to three adjacent carbon atoms, just as in graphite. However the carbon atoms in a buckyball form a teensy-weensy sphere that’s about 1 nanometer in diameter, as shown in Figure. Because one of the properties of carbon atoms is that they can bond to many other types of atoms, researchers can use them to create customized molecules, useful in various applications.
Such nanotubes are generally formed either by the electric-arc technique (without need of any catalyst) or by catalyst-enhanced thermal cracking of gaseous hydrocarbons or CO disproportionation.
The number of walls (or number of coaxial tubes) can be anything, starting from two, with no upper limit. The intertube distance is approximately that of the intergraphene distance in turbostratic, polyaromatic solids, i. e. 0 .34 nm .
c-MWNT concentric type (c-MWNT), in which SWNTs with regularly increasing diameters are coaxially
high-pressure carbon monoxide deposition, or HiPCO
chemical-vapor deposition, or CVD
high-pressure carbon monoxide deposition, or HiPCO
This method involves a heated chamber through which carbon monoxide gas and small clusters of iron atoms flow. When carbon monoxide molecules land on the iron clusters, the iron acts as a catalyst and helps a carbon monoxide molecule break up into a carbon atom and an oxygen atom. The carbon atom bonds with other carbon atoms to start the nanotube lattice; the oxygen atom joins with another carbon monoxide molecule to form carbon dioxide gas, which then floats off into the air.
In this method, a hydrocarbon — say, methane gas (one carbon atom and four hydrogen atoms) — flows into a heated chamber containing a substrate coated with a catalyst, such as iron particles. The temperature in the chamber is high enough to break the bonds between the carbon atoms and the hydrogen atoms in the methane molecules — resulting in carbon atoms with no hydrogen atoms attached. Those carbon atoms attach to the catalyst particles, where they bond to other carbon atoms — forming a nanotube.
A brand-new method uses a plasma process to produce nanotubes. Methane gas, used as the source of carbon, is passed through a plasma torch. Nobody’s revealed the details of this process yet, such as what, if any, catalyst is used. One of the initial claims is that this process is 25 times more efficient at producing nanotubes than the other two methods.
In the absence of catalysts in the target, the soot collected mainly contains multiwall nanotubes (c-MWNTs). Their lengths can reach 300 nm.
Their quantity and structure quality are dependent on the oven temperature. The best quality is obtained for an oven temperature set at 1 ,200 ◦C. At lower oven temperatures, the structure quality decreases, and the nanotubes start presenting many defects.
As soon as small quantities (few percents or less) of transition metals (Ni, Co) playing the role of catalysts are incorporated into the graphite pellet, products yielded undergo significant modifications, and SWNTs are formed instead of MWNTs.
The principle of this technique is to vaporize carbon in the presence of catalysts (iron, nickel, cobalt, yttrium, boron, gadolinium, and so forth) under reduced atmosphere of inert gas (argon or helium).
After the triggering of the arc between two electrodes, a plasma is formed consisting of the mixture of carbon vapor, the rare gas (helium or argon), and the vapors of catalysts.
The vaporization is the consequence of the energy transfer from the arc to the anode made of graphite doped with catalysts. The anode erosion rate is more or less important depending on the power of the arc and also on the other experimental conditions. It is noteworthy that a high anode erosion does not necessarily lead to a high carbon nanotube production.
Synthesis of Aligned Carbon Nanotubes Several applications (such as field-emission-based display), require that carbon nanotubes grow as highly aligned bunches, in highly ordered arrays, or located at specific positions. In that case, the purpose of the process is not mass production but controlled growth and purity, with subsequent control of nanotube morphology, texture, and structure. Generally speaking, the more promising methods for the synthesis of aligned nanotubes are based on CCVD processes, which involve molecular precursors as carbon source, and method of thermal cracking assisted by the catalytic activity of transition metal (Co, Ni, Fe) nanoparticles deposited onto solid supports.
An interesting feature of SWNTs is their very high surface area, the highest ever due to the fact that a single graphene sheet is probably the unique example of a ma terial energetically stable in normal conditions while consisting of a single layer of atoms. Ideally, i. e. not considering SWNTs in bundles but isolated SWNTs, and provided the SWNTs have one end opened (by oxidation treatment for instance), the real surface area can be equal to that of a single, flat graphene, i. e.∼ 2 ,700m2/g .
The narrow diameter of SWNTs has a strong influence on its electronic excitations due to its small size compared to the characteristic length scale of low energy electronic excitations. Combined with the particular shape of the electronic band structure of graphene, carbon nanotubes are ideal quantum wires.
While tubular nano-morphology is also observed for many two-dimensional solids, carbon nanotubes are unique through the particularly strong threefolded bonding of the curved graphene sheet, which is stronger than in diamond as revealed by their difference in C−C bond length (0.142 vs. 0.154 nm for graphene and diamond respectively).
This makes carbon nanotubes – SWNTs or c-MWNTs – particularly stable against deformations. The tensile strength of SWNTs can be 20 times that of steel and has actually been measured equal to ∼ 45 GPa.
The chemical reactivity of graphite, fullerenes, and carbon nanotubes exhibits some common features. Like any small object, carbon nanotubes have a large surface with which they can interact with their environment.
1998: A group from Northeastern University was the first to report the supposedly successful hydrogen storage in carbon layered nanostructures possessing some crystallinity. The authors claimed that, in platelet nanofibers (3–50 nm in width), hydrogen can be stored up to 75 wt%, meaning a C/H ratio of 1 /9.
1999: Dresselhaus and coworkers reported hydrogen storage in SWNTs (1 .85 nm of average diameter) at room temperature. A hydrogen storage capacity of 4 .2wt% was achieved reproducibly under 100 bar for a SWNT-containing material of about 0 .5 g. 78% of the adsorbed hydrogen could be released under ambient pressure at room temperature, while the release of the residual stored hydrogen required some heating at 200 ◦C.
Gases other than Hydrogen Encouraged by the potential applications related to hydrogen adsorption, several research groups have tried to use carbon nanotubes as a stocking and transporting mean for other gases such as: oxygen, nitrogen, noble gases (argon and xenon), and hydrocarbons (methane, ethane, and ethylene). These studies have shown that carbon nanotubes could become the world’s smallest gas cylinders combining low weight, easy transportability, and safe use with acceptable adsorbed quantities. Thanks to their nano-sizes, nanotubes might also be used in medicine where physically confining special gases (like 133Xe for instance) prior to injection would be extremely useful.
Carbon nanotubes were recently found to be able to adsorb some toxic gases such as dioxins (2001), fluo-ride (2001), lead (2002), or alcohols (2000) better than the materials used so far, such as activated carbon.
These pioneering works opened a new field of applications as cleaning filters for many industrial processes having hazardous by-products.
Adsorption of dioxins, which are very common and persistent carcinogenic by-products of many industrial processes, is a good example of the interest of nanotubes in this field.
Attaching molecules of biological interest to carbon nanotubes is an ideal way to realize nanometer-sized biosensors. Indeed, the electrical conductivity of such functionalized nanotubes would depend on modifications of the interaction of the probe with the studied media, because of chemical changes or as result of their interaction with target species. The science of attaching biomolecules to nanotubes is rather recent and was inspired by similar research in the fullerene area.
The electrical conductance of semiconductor SWNTs was recently demonstrated to be highly sensitive to the change in the chemical composition of the surrounding atmosphere at room temperature, due to the charges transfer between the nanotubes and the molecules from the gases adsorbed onto the SWNT surface.
First tries involved NO 2 or NH 3 and O2. SWNT-based chemical NO 2 and NH 3 sensors are characterized by extremely short response time, thus being different from conventionally used sensors.