Properties of carbon nanotubes. Purification of carbon nanotubes

Ministry of Education and Science of the Russian Federation

Federal State Institution of Higher Professional Education

Russian University of Chemical Technology D. I. Mendeleev

Faculty of Petroleum Chemistry and Polymeric Materials

Department of Chemical Technology of Carbon Materials

PRACTICE REPORT

on the topic CARBON NANOTUBES AND NANOVOLKS

Completed by: Marinin S. D.

Checked by: Doctor of Chemical Sciences, Bukharkina T.V.

Moscow, 2013

Introduction

The field of nanotechnology is considered worldwide as a key topic for the technologies of the 21st century. The possibilities of their versatile application in such areas of the economy as the production of semiconductors, medicine, sensor technology, ecology, automotive, building materials, biotechnology, chemistry, aviation and aerospace, mechanical engineering and the textile industry, carry a huge potential for growth. The use of nanotechnology products will save on raw materials and energy consumption, reduce emissions into the atmosphere and thus contribute to the sustainable development of the economy.

Developments in the field of nanotechnologies are carried out by a new interdisciplinary field - nanoscience, one of the areas of which is nanochemistry. Nanochemistry arose at the turn of the century, when it seemed that everything in chemistry was already open, everything was clear, and all that remained was to use the acquired knowledge for the benefit of society.

Chemists have always known and well understood the importance of atoms and molecules as the basic building blocks of a vast chemical foundation. At the same time, the development of new research methods, such as electron microscopy, highly selective mass spectroscopy, in combination with special sample preparation methods, made it possible to obtain information on particles containing a small, less than a hundred, number of atoms.

These particles, about 1 nm in size (10-9 m is just a millimeter divided by a million), have unusual, hard-to-predict chemical properties.

The most famous and understandable for most people are the following nanostructures such as fullerenes, graphene, carbon nanotubes and nanofibers. They all consist of carbon atoms bonded to each other, but their shape varies significantly. Graphene is a plane, monolayer, "veil" of carbon atoms in SP 2 hybridization. Fullerenes are closed polygons, somewhat reminiscent of a soccer ball. Nanotubes are cylindrical hollow volumetric bodies. Nanofibers can be cones, cylinders, bowls. In my work, I will try to highlight exactly nanotubes and nanofibers.

Structure of nanotubes and nanofibers

What are carbon nanotubes? Carbon nanotubes are a carbon material that is a cylindrical structure with a diameter of the order of several nanometers, consisting of graphite planes rolled into a tube. The graphite plane is a continuous hexagonal grid with carbon atoms at the vertices of the hexagons. Carbon nanotubes can vary in length, diameter, chirality (symmetries of the rolled graphite plane), and number of layers. Chirality<#"280" src="/wimg/13/doc_zip1.jpg" />

Single-walled nanotubes. Single-walled carbon nanotubes (SWCNTs) are a subspecies of carbon nanofibers with a structure formed by folding graphene into a cylinder with its sides joined without a seam. Rolling graphene into a cylinder without a seam is only possible in a finite number of ways, differing in the direction of the two-dimensional vector that connects two equivalent points on graphene that coincide when it is rolled into a cylinder. This vector is called the chirality vector single-layer carbon nanotube. Thus, single-walled carbon nanotubes differ in diameter and chirality. The diameter of single-walled nanotubes, according to experimental data, varies from ~ 0.7 nm to ~ 3-4 nm. The length of a single-walled nanotube can reach 4 cm. There are three forms of SWCNTs: achiral "chair" type (two sides of each hexagon are oriented perpendicular to the CNT axis), achiral "zigzag" type (two sides of each hexagon are oriented parallel to the CNT axis), and chiral or helical (each the side of the hexagon is located to the CNT axis at an angle other than 0 and 90 º ). Thus, achiral CNTs of the “armchair” type are characterized by indices (n, n), of the “zigzag” type - (n, 0), chiral - (n, m).

The number of layers in an MWCNT is most often no more than 10, but in some cases it reaches several tens.

Sometimes, among multilayer nanotubes, two-layer nanotubes are singled out as a special type. The “Russian dolls” type structure is a set of coaxially nested cylindrical tubes. Another type of this structure is a set of nested coaxial prisms. Finally, the last of these structures resembles a scroll (scroll). For all structures in Fig. characteristic value of the distance between adjacent graphene layers, close to the value of 0.34 nm, inherent in the distance between adjacent planes of crystalline graphite<#"128" src="/wimg/13/doc_zip3.jpg" />

Russian Matryoshka Roll Papier-mache

Carbon nanofibers (CNFs) are a class of materials in which curved graphene layers or nanocones are folded into a one-dimensional filament whose internal structure can be characterized by the angle α between the graphene layers and the fiber axis. One common distinction is between the two main fiber types: Herringbone, with densely packed conical graphene layers and large α, and Bamboo, with cylindrical cup-like graphene layers and small α, which are more like multiwalled carbon nanotubes.<#"228" src="/wimg/13/doc_zip4.jpg" />

a - nanofiber "coin column";

b - "Christmas tree structure" nanofiber (stack of cones, "fish bone");

c - nanofiber "stack of cups" ("lamp shades");

d - nanotube "Russian matryoshka";

e - bamboo-shaped nanofiber;

e - nanofiber with spherical sections;

g - nanofiber with polyhedral sections

The isolation of carbon nanotubes as a separate subspecies is due to the fact that their properties differ markedly for the better from the properties of other types of carbon nanofibers. This is explained by the fact that the graphene layer, which forms the nanotube wall along its entire length, has high tensile strength, thermal and electrical conductivity. In contrast to this, transitions from one graphene layer to another occur in carbon nanofibers moving along the wall. The presence of interlayer contacts and high defectiveness of the structure of nanofibers significantly impairs their physical characteristics.

History

It is difficult to talk about the history of nanotubes and nanofibers separately, because these products often accompany each other during synthesis. One of the first data on the production of carbon nanofibers is probably an 1889 patent for the production of tubular forms of carbon formed during the pyrolysis of a mixture of CH4 and H2 in an iron crucible by Hughes and Chambers. They used a mixture of methane and hydrogen to grow carbon filaments by pyrolysis of the gas, followed by carbon precipitation. It became possible to talk about obtaining these fibers for sure much later, when it became possible to study their structure using an electron microscope. The first observation of carbon nanofibers using electron microscopy was made in the early 1950s by Soviet scientists Radushkevich and Lukyanovich, who published an article in the Soviet Journal of Physical Chemistry showing hollow graphite fibers of carbon that were 50 nanometers in diameter. In the early 1970s, Japanese researchers Koyama and Endo succeeded in producing carbon fibers by vapor deposition (VGCF) with a diameter of 1 µm and a length of more than 1 mm. Later, in the early 1980s, Tibbets in the USA and Benissad in France continued to improve the carbon fiber (VGCF) process. In the US, more in-depth research on the synthesis and properties of these materials for practical application, were conducted by R. Terry K. Baker and were motivated by the need to suppress the growth of carbon nanofibers due to persistent problems caused by material accumulation in various commercial processes, especially in the field of oil refining. The first attempt to commercialize gas-grown carbon fibers was made by the Japanese company Nikosso in 1991 under the brand name Grasker, in the same year Ijima published his famous article announcing the discovery of carbon nanotubes<#"justify">Receipt

Currently, syntheses based on pyrolysis of hydrocarbons and sublimation and desublimation of graphite are mainly used.

arc method,
radiant heating (use of solar concentrators or laser radiation),
laser-thermal,
heating with an electron or ion beam,
plasma sublimation,
resistive heating.

Many of these options have their own variations. The hierarchy of some variants of the electric arc method is shown in the diagram:

At present, the most common method is thermal sputtering of graphite electrodes in arc discharge plasma. The synthesis process is carried out in a chamber filled with helium at a pressure of about 500 mm Hg. Art. During plasma combustion, intense thermal evaporation of the anode occurs, while a deposit is formed on the end surface of the cathode, in which carbon nanotubes are formed. The maximum number of nanotubes is formed when the plasma current is minimal and its density is about 100 A/cm2. In experimental setups, the voltage between the electrodes is about 15–25 V, the discharge current is several tens of amperes, and the distance between the ends of the graphite electrodes is 1–2 mm. During the synthesis process, about 90% of the mass of the anode is deposited on the cathode. The resulting numerous nanotubes have a length of about 40 μm. They grow on the cathode perpendicular to the flat surface of its end and are collected into cylindrical beams about 50 μm in diameter.

Nanotube bundles regularly coat the cathode surface, forming a honeycomb structure. The content of nanotubes in the carbon deposit is about 60%. To separate the components, the resulting precipitate is placed in methanol and sonicated. The result is a suspension which, after the addition of water, is subjected to separation in a centrifuge. Large particles adhere to the walls of the centrifuge, while the nanotubes remain floating in suspension. Then the nanotubes are washed in nitric acid and dried in a gaseous flow of oxygen and hydrogen in a ratio of 1:4 at a temperature of 750 0C for 5 minutes. As a result of such processing, a light porous material is obtained, consisting of numerous nanotubes with an average diameter of 20 nm and a length of 10 μm. So far, the maximum nanofiber length achieved is 1 cm.

Pyrolysis of hydrocarbons

In terms of the choice of initial reagents and methods of conducting processes, this group has a significantly larger number of options than the methods of sublimation and desublimation of graphite. It provides more precise control over the process of CNT formation, is more suitable for large-scale production and allows the production of not only carbon nanomaterials themselves, but also certain structures on substrates, macroscopic fibers consisting of nanotubes, as well as composite materials, in particular, modified with carbon CNTs. carbon fibers and carbon paper, ceramic composites. Using the recently developed nanospheric lithography, it was possible to obtain photonic crystals from CNTs. In this way, it is possible to isolate CNTs of a certain diameter and length.

The advantages of the pyrolytic method, in addition, include the possibility of its implementation for matrix synthesis, for example, using porous alumina membranes or molecular sieves. Using aluminum oxide, it is possible to obtain branched CNTs and CNT membranes. The main disadvantages of the matrix method are the high cost of many matrices, their small size, and the need to use active reagents and harsh conditions for dissolving the matrices.

The pyrolysis of three hydrocarbons, methane, acetylene, and benzene, as well as the thermal decomposition (disproportionation) of CO are most often used for the synthesis of CNTs and CNFs. Methane, like carbon monoxide, is not prone to decomposition at low temperatures (non-catalytic decomposition of methane begins at ~900 about C), which makes it possible to synthesize SWCNTs with a relatively small amount of amorphous carbon impurities. Carbon monoxide does not decompose at low temperatures for another reason: kinetic. The difference in the behavior of various substances is visible in Fig. 94.

The advantages of methane over other hydrocarbons and carbon monoxide include the fact that its pyrolysis with the formation of CNTs or CNFs is combined with the release of H 2and can be used in existing productions N 2.

Catalysts

The catalysts for the formation of CNTs and CNFs are Fe, Co, and Ni; promoters, which are introduced in smaller amounts, are mainly Mo, W or Cr (less often - V, Mn, Pt and Pd), catalyst carriers are non-volatile oxides and hydroxides of metals (Mg, Ca, Al, La, Si, Ti, Zr) , solid solutions, some salts and minerals (carbonates, spinels, perovskites, hydrotalcite, natural clays, diatomites), molecular sieves (in particular, zeolites), silica gel, airgel, aluminum gel, porous Si and amorphous C. At the same time, V, Cr, Mo, W, Mn and, probably, some other metals under pyrolysis conditions are in the form of compounds - oxides, carbides, metallates, etc.

Noble metals (Pd, Ru, PdSe), alloys (mischmetal, permalloy, nichrome, monel, stainless steel, Co-V, Fe-Cr, Fe-Sn, Fe-Ni-Cr, Fe-Ni-C, Co-Fe-Ni, hard alloy Co-WC, etc.), CoSi 2and CoGe 2, LaNi 5, MmNi 5(Mm - mischmetal), alloys of Zr and other hydride-forming metals. On the contrary, Au and Ag inhibit the formation of CNTs.

Catalysts can be deposited on silicon coated with a thin oxide film, on germanium, some types of glass, and substrates made of other materials.

Porous silicon obtained from electrochemical etching monocrystalline silicon in a solution of a certain composition. Porous silicon may contain micropores (< 2 нм), мезопоры и макропоры (>100 nm). To obtain catalysts, traditional methods are used:

mixing (rarely sintering) of powders;
deposition or electrochemical deposition of metals on a substrate, followed by the transformation of a continuous thin film into nanosized islands (layer-by-layer deposition of several metals is also used;
chemical vapor deposition;
dipping the substrate into the solution;
applying a suspension of catalyst particles to a substrate;
applying the solution to a rotating substrate;
impregnation of inert powders with salts;
coprecipitation of oxides or hydroxides;
ion exchange;
colloidal methods (sol-gel process, reverse micelles method);
thermal decomposition of salts;
combustion of metal nitrates.

In addition to the two groups described above, a large number of other methods for obtaining CNTs have been developed. They can be classified according to the carbon sources used. The starting compounds are: graphite and other forms of solid carbon, organic compounds, inorganic compounds, organometallic compounds. Graphite can be converted into CNTs in several ways: by intense ball milling followed by high-temperature annealing; electrolysis of molten salts; splitting into separate graphene sheets and subsequent spontaneous twisting of these sheets. Amorphous carbon can be converted into CNTs when processed under hydrothermal conditions. From carbon black (soot), CNTs were obtained by high-temperature transformation with or without catalysts, as well as by interaction with pressurized water vapor. Nanotubular structures are contained in products of vacuum annealing (1000 about C) films of diamond-like carbon in the presence of a catalyst. Finally, the catalytic high-temperature transformation of fullerite C 60or its treatment under hydrothermal conditions also leads to the formation of CNTs.

Carbon nanotubes exist in nature. A group of Mexican researchers found them in oil samples taken from a depth of 5.6 km (Velasco-Santos, 2003). The CNT diameter ranged from several nanometers to tens of nanometers, and the length reached 2 μm. Some of them were filled with various nanoparticles.

Purification of carbon nanotubes

None of the common methods for obtaining CNTs allows them to be isolated in their pure form. Impurities to NT can be fullerenes, amorphous carbon, graphitized particles, catalyst particles.

destructive,
non-destructive,
combined.

Destructive methods use chemical reactions, which can be oxidative or reductive, and are based on differences in the reactivity of different carbon forms. For oxidation, either solutions of oxidizing agents or gaseous reagents are used; for reduction, hydrogen is used. The methods make it possible to isolate high-purity CNTs, but are associated with the loss of tubes.

Non-destructive methods include extraction, flocculation and selective precipitation, cross-flow microfiltration, exclusion chromatography, electrophoresis, selective reaction with organic polymers. As a rule, these methods are inefficient and inefficient.

Properties of carbon nanotubes

Mechanical. Nanotubes, as was said, are an extremely strong material, both in tension and in bending. Moreover, under the action of mechanical stresses exceeding the critical ones, nanotubes do not "break", but are rearranged. Based on the high strength properties of nanotubes, it can be argued that they are the best material for a space elevator tether at the moment. As the results of experiments and numerical simulation show, the Young's modulus of a single-layer nanotube reaches values of the order of 1-5 TPa, which is an order of magnitude greater than that of steel. The graph below shows a comparison between a single-walled nanotube and high-strength steel.

1 2

The cable of the space elevator is estimated to withstand a mechanical stress of 62.5 GPa

Tensile diagram (dependence of mechanical stress σ from elongation ε)

To demonstrate the significant difference between currently the strongest materials and carbon nanotubes, let's do the following thought experiment. Imagine that, as it was assumed earlier, a certain wedge-shaped homogeneous structure consisting of the most durable materials to date will serve as a cable for a space elevator, then the diameter of the cable at GEO (geostationary Earth orbit) will be about 2 km and will narrow to 1 mm at the surface Earth. In this case, the total mass will be 60 * 1010 tons. If carbon nanotubes were used as the material, then the diameter of the cable at GEO was 0.26 mm and 0.15 mm at the Earth's surface, and therefore the total mass was 9.2 tons. As can be seen from the above facts, carbon nanofiber is exactly the material that is needed to build a cable, the actual diameter of which will be about 0.75 m, in order to withstand also the electromagnetic system used to propel the space elevator car.

Electrical. Due to the small size of carbon nanotubes, only in 1996 was it possible to directly measure their electrical resistivity using a four-prong method.

Gold stripes were deposited on a polished silicon oxide surface in a vacuum. Nanotubes 2–3 µm long were deposited between them. Then, four tungsten conductors 80 nm thick were deposited on one of the nanotubes chosen for measurement. Each of the tungsten conductors had contact with one of the gold strips. The distance between contacts on the nanotube was from 0.3 to 1 μm. The results of direct measurement showed that the resistivity of nanotubes can vary within a significant range - from 5.1 * 10 -6up to 0.8 ohm/cm. The minimum resistivity is an order of magnitude lower than that of graphite. Most of the nanotubes have metallic conductivity, while the smaller part exhibits the properties of a semiconductor with a band gap of 0.1 to 0.3 eV.

French and Russian researchers (from IPTM RAS, Chernogolovka) discovered another property of nanotubes, which is superconductivity. They measured the current-voltage characteristics of an individual single-walled nanotube with a diameter of ~1 nm, rolled into a bundle of a large number of single-walled nanotubes, as well as individual multilayer nanotubes. A superconducting current at a temperature close to 4K was observed between two superconducting metal contacts. The features of charge transfer in a nanotube essentially differ from those that are inherent in ordinary, three-dimensional conductors and, apparently, are explained by the one-dimensional nature of the transfer.

Also, de Girom from the University of Lausanne (Switzerland) discovered an interesting property: a sharp (about two orders of magnitude) change in conductivity with a small, by 5-10o, bending of a single-layer nanotube. This property can expand the scope of nanotubes. On the one hand, the nanotube turns out to be a ready-made highly sensitive converter of mechanical vibrations into an electrical signal and vice versa (in fact, it is a telephone receiver a few microns long and about a nanometer in diameter), and, on the other hand, it is a practically ready-made sensor of the smallest deformations. Such a sensor could be used in devices that monitor the state of mechanical components and parts on which the safety of people depends, for example, passengers of trains and aircraft, personnel of nuclear and thermal power plants, etc.

Capillary. Experiments have shown that an open nanotube has capillary properties. To open a nanotube, it is necessary to remove the upper part - the cap. One way to remove is to anneal nanotubes at a temperature of 850 0C for several hours in a stream of carbon dioxide. As a result of oxidation, about 10% of all nanotubes are open. Another way to destroy the closed ends of nanotubes is exposure to concentrated nitric acid for 4.5 hours at a temperature of 2400 C. As a result of this treatment, 80% of the nanotubes become open.

The first studies of capillary phenomena showed that a liquid penetrates into the nanotube channel if its surface tension is not higher than 200 mN/m. Therefore, to introduce any substances into nanotubes, solvents with a low surface tension are used. For example, concentrated nitric acid, the surface tension of which is low (43 mN/m), is used to introduce certain metals into the nanotube channel. Then annealing is carried out at 4000 C for 4 hours in a hydrogen atmosphere, which leads to the reduction of the metal. In this way, nanotubes containing nickel, cobalt, and iron were obtained.

Along with metals, carbon nanotubes can be filled with gaseous substances, such as molecular hydrogen. This ability is of practical importance, because it opens up the possibility of safe storage of hydrogen, which can be used as an environmentally friendly fuel in internal combustion engines. Also, scientists were able to place inside the nanotube a whole chain of fullerenes with gadolinium atoms already embedded in them. (see Fig.5).

Rice. 5. Inside C60 inside a single-walled nanotube

Capillary effects and filling of nanotubes

nanotube carbon pyrolysis electric arc

Capillary phenomena in carbon nanotubes were first experimentally carried out in a work where the effect of capillary retraction of molten lead into nanotubes was observed. In this experiment, an electric arc intended for the synthesis of nanotubes was ignited between electrodes with a diameter of 0.8 and a length of 15 cm at a voltage of 30 V and a current of 180–200 A. A layer of material 3–4 cm high formed on the cathode surface as a result of thermal destruction of the anode surface was removed from the chamber and kept for 5 h at T = 850°C in a flow of carbon dioxide. This operation, as a result of which the sample lost about 10% of the mass, contributed to the purification of the sample from particles of amorphous graphite and the discovery of nanotubes in the precipitate. The central part of the precipitate containing nanotubes was placed in ethanol and sonicated. The oxidation product dispersed in chloroform was applied to a carbon tape with holes for observation with an electron microscope. As observations showed, the tubes that were not subjected to processing had a seamless structure, heads of the correct shape and a diameter of 0.8 to 10 nm. As a result of oxidation, about 10% of the nanotubes turned out to have damaged caps, and some of the layers near the top were torn off. A sample containing nanotubes intended for observation was filled in vacuum with drops of molten lead, which were obtained by irradiating a metal surface with an electron beam. In this case, lead droplets 1 to 15 nm in size were observed on the outer surface of the nanotubes. The nanotubes were annealed in air at Т = 400°С (above the melting point of lead) for 30 min. As the results of observations made with the help of an electron microscope show, after annealing some of the nanotubes turned out to be filled with a solid material. A similar effect of filling nanotubes was observed upon irradiation of the heads of tubes opened as a result of annealing with a powerful electron beam. With a sufficiently strong irradiation, the material near the open end of the tube melts and penetrates inside. The presence of lead inside the tubes was established by X-ray diffraction and electron spectroscopy. The diameter of the thinnest lead wire was 1.5 nm. According to the results of observations, the number of filled nanotubes did not exceed 1%.

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1 TECHNICAL INNOVATIONS UDC BBK 30.6 FILTER BASED ON CARBON NANOTUBES FOR CLEANING ALCOHOL-CONTAINING LIQUIDS N.P. Polikarpova, I.V. Zaporotskova, T.A. Ermakova, P.A. Zaporotskov Experiments were carried out on the purification of alcohol-containing liquids by filtration and transmission methods, it was established mass fraction carbon nanotubes, leading to the best result. A model of a filter based on a nanomaterial enclosed in a space between layers of porous glass has been created, and its design features have been determined. Polikarpova N.P., Zaporotskova I.V., Ermakova T.A., Zaporotskov P.A., 2012 Key words: carbon nanotubes, alcohol-containing liquid, adsorption, filter, porous glass, porous ceramics. Introduction Purification of alcohol-containing liquids, which include vodka food industry products, plays an important role in the process of their production. Each manufacturer tries to make the most of effective methods purification of alcohol-containing liquid from impurities and fusel oils. Fusel oils, aldehydes, mineral salts and other impurities are removed from the product by filtration using charcoal, quartz sand, silver dust, platinum filters, milk powder, egg white. Many of the producers of expensive varieties of vodka repeat the purification many times, combining different options. Each subsequent cleaning further removes fusel oils and other impurities from the product. A double or triple degree of purification significantly improves the taste, but also significantly increases the cost of the manufacturing process. Currently, distilleries use various methods for cleaning alcohol-containing products. The most common of these are cleaning with carbon filters, cleaning with milk and egg whites, "silver filtration" and cleaning with gold and precious stones. In the works of I.V. Zaporotskova and N.P. Zaporotskova presents the results of theoretical calculations of the adsorption interaction of carbon nanotubes (CNTs) with molecules of heavy organic alcohols, which are part of alcohol-containing liquids in the form of undesirable impurities, and the possibility of their sorption on the surface of nanotubes is proved. This made it possible to propose an innovative method for purifying water-ethanol mixtures, which include vodka, using a carbon nanomaterial. As you know, graphite sorbents and charcoal purify the product from harmful impurities by 60%, milk by 70%, precious metals (silver, gold) by 75%. The use of carbon nanotubes as a sorbent material will make it possible to purify the alcohol-containing liquid from impurities by 98%. Also, the advantages of the claimed filters based on CNTs include: 1) high performance of the process at low cost; 2) dozens of times smaller volume of the adsorbing substance; 3) the absence of side effects from the use of graphite nature adsorbents with the preservation and multiple increase in the activity of the process; Vestnik VolGU. Series 10. Issue

2 4) the possibility of selective adsorption. It should be noted that the introduction of a filter based on nanomaterials into a complete production cycle at the final stage without a fundamental change technological process provides almost 100% purification of the product of water-ethanol mixtures without a significant increase in production costs. 1. Determining the optimal amount of carbon nanomaterial for liquid purification Before proceeding directly to laboratory experiments on the purification of alcohol-containing liquids (domestic vodka), it was necessary to determine the optimal amount of nanomaterial that would lead to the desired effect of a high degree of purification. As an object of research, vodka "Let's drink for" was chosen, which belongs to the class of ordinary low-cost vodkas. The liquid was studied by the titrimetric method until the minimum mass of nanotubes required for effective purification of 50 ml of vodka was revealed. The selection was carried out by the “from largest to smallest” method, the initial number of carbon nanotubes was 1 g. The accuracy of CNT weighing was determined by the accuracy of the analytical balance used and was 0.0001 g. The number of nanotubes was reduced until the moment when the alkalinity of vodka ceased to decrease. According to the GOST R standards “Special vodkas and vodkas. General technical conditions”, the alkalinity of vodka should not exceed 2.5-3.0 ml. Prior to purification, the alkalinity of the selected vodka was 2.5 ml. The results of the performed titrimetric studies are presented in the table. An analysis of the results showed that passing an alcohol-containing liquid through a filter with carbon nanotubes reduces the alkalinity index by an average of 98% (by 2.45 ml). The minimum amount of the required nanomaterial is 0.001 g, since with a decrease in this amount, alkalinity increases sharply, and with a larger amount, its decrease is insignificant. 2. Selection of material for creating a filter shell based on carbon nanotubes In the production of vodka, both filters with porous glass, such as Schott filters, and ceramic filters can be used as filters. These porous materials can also be used as materials for creating filter shells based on carbon nanotubes. Consider the features of these materials. Porous glass is a glassy porous material with a spongy structure and a content of silicon oxide SiO 2 of about 96% (mass.). Porous glass is the result of thermal and chemical treatment of glass of a special composition. Porous glasses can only be obtained from glasses with a sufficiently high content of Na 2 O , in which the coexisting phases form interpenetrating frameworks after prolonged heat treatment. A necessary condition for obtaining porous glasses is also the content of at least 40% (wt.) silicon dioxide in the initial glasses, which ensures the formation of a continuous spatial SiO 2 network in the glass. Glass filters are plates of crushed and fused glass. For their manufacture, glass is ground in ball mills and sieved using a set of sieves. Glass powder is sintered by heating in a furnace in metal or ceramic molds. The resulting plates are soldered into tubes, glasses, funnels, crucibles and other glassware of the same composition. Hot solutions, concentrated acids and dilute alkalis can be filtered through such plates, as such filters are resistant to aggressive media. Filter plates are distinguished by porosity. Depending on the pore size, several classes of filters are made. Glass filters, or the so-called Schott filters, are available in the following types: 1 pore size is µm, used to work with coarse-grained precipitates; 7 6 N.P. Polikarpova et al. Filter based on carbon nanotubes

3 2 pore size is µm, used to work with medium crystalline sediments; 3 pore size is µm, applicable to work with fine crystalline deposits; 4 pore size is 4-10 µm, used for very fine crystalline deposits. Ceramic membranes are porous ceramic fine filters made by sintering metal-ceramic materials such as aluminum oxide, titanium dioxide or zirconium (Fig. 1) at ultrahigh temperatures. Ceramic membranes usually have an asymmetric structure that supports the active membrane layer (Fig. 2). Porous ceramics are composed of bonded particles of approximately the same size, which creates a uniform, permeable material that provides tortuous channels for fluid flow. Silica and alumina are the most commonly used filters, although the choice of material, size and shape is virtually unlimited. Ceramic filters are usually classified by average pore diameter or/and permeability. The average pore diameter is the average minimum pore diameter measured in microns. Dimensions of ceramic filter membranes: - microfiltration: 1.2 µm 0.5 µm 0.2 µm 0.1 µm; - ultrafiltration: 50 nm 20 nm. Macroporous materials provide mechanical stability, while the active membrane layer provides separation: microfiltration, ultrafiltration, nanofiltration. Ceramic membrane filters always operate in tangential filtration mode with optimal hydrodynamic conditions. Turbid liquid passes through the membrane layer inside the single or multi-channel membrane at high speed. Under the action of transmembrane pressure (TMP), micromolecules and water pass vertically through the membrane layer, forming a permeate flow. Suspended substances and macromolecular compounds are retained inside the membrane, forming a concentrate flow. Thus, the contaminated liquids are purified. Ceramic membranes allow a physical method to separate mixtures of components without the use of additives. The introduction of carbon nanotube material into these systems can additionally increase the efficiency of such a filter. Fig. 3. Model of a filter based on carbon nanotubes in a shell of porous glass To create a model of a filter through which an alcohol-containing liquid was passed vertically (Fig. 3), we used glass Schott filters made of porous glass with a carbon nanomaterial placed inside, carbon nanotubes obtained on installation of CVDomna according to the method described in the work of I. V. Zaporotskova. The filtering part of the used filters is a porous glass substance. Fig. 1. Porous ceramics. 2. Ceramic filter Vestnik VolGU. Series 10. Issue

4 with a membrane size of 4 10 μm. For the preliminary layout, two Schott filters of different diameters were used, which were docked together, forming a closed filtering system. A layer of carbon nanotubes was placed between glass plates with pore sizes of 4–10 μm. An enlarged image of the porous glass is shown in Figure 4. To ensure closure, carbon nanotubes were additionally placed between layers of filter paper. The studied product, vodka "Let's drink for", freely flowed vertically through the filter created in this way under the action of gravity. The amount of filtering carbon nanomaterial and the volume of alcohol-containing liquid flowing through the manufactured filter were chosen in accordance with the results obtained earlier: 0.001 g of CNTs for cleaning 50 ml of vodka. These types of filters turned out to be sufficiently effective to ensure the free flow of a water-ethanol mixture through them without penetrating the carbon nanomaterial through the glass, which can be explained by the random arrangement of pores in the shell. Further studies of the quality of the purified product using the methods of molecular spectroscopy and liquid chromatography (Fig. 5, 6) confirmed the high degree of purification of vodka from impurities of high molecular weight alcohols of fusel oils: there are no peaks related to these alcohols in the spectra. Results of titration of vodka "Let's drink for" with different amounts of carbon nanotubes Fig. 3. Model of a filter with porous glass plates. 4. View of a glass plate with a pore size of 4 10 μm with an increase in x N.P. Polikarpova et al. Filter based on carbon nanotubes

5 Transmission, % Wave number, cm -1 Pic. Fig. 5. IR spectra of Let's Drink for vodka: red spectrum before purification; violet spectrum after purification by passing through a filter with carbon nanotubes a Conclusion 6. Chromatograms of vodka "Let's drink for": a) before purification; b) after cleaning by passing through a filter with carbon nanotubes, while the content of the main useful component of the product is ethyl alcohol. The created and tested model of a filter based on carbon nanotubes enclosed in a porous glass shell can be used as a basis for creating an industrial filter. Further research Vestnik VolGU. Series 10. Issue

6 will be aimed at creating a filter model with a ceramic shell, the smaller pore sizes of which (compared to the pores of a glass shell) can provide better protection of the product to be purified from the ingress of carbon nanoparticles into it. REFERENCES 1. Berkman, AS Berkman, porous permeable ceramics. Moscow: Gosstroyizdat, p. 2. V. P. Vasiliev, Analytical Chemistry. Titrimetric and gravimetric methods of analysis: textbook / V. P. Vasiliev. M. : Bustard, p. 3. Garmash, E. P. Ceramic membranes for ultra- and microfiltration / E. P. Garmash, Yu. N. Kryuchkov, V. P. Pavlikov. General specifications. State standard of the Russian Federation. M. : Gosstandart of Russia, p. 5. Zaporotskova, I.V., Kozhitov, L.V., Kozlov, V.V., Promising nanomaterials based on carbon, Vestn. Volgograd state university Ser. 10, Innovative activity S Zaporotskova, IV Sorption activity of carbon nanotubes as the basis of innovative technology for purification of water-ethanol mixtures / IV Zaporotskova, NP Zaporotskova, TA Ermakova // Vestn. Volgograd state university Ser. 10, Innovative activity S Zaporotskova, IV Carbon and non-carbon nanomaterials and composite structures based on them: structure and electronic properties / IV Zaporotskova. Volgograd: From VolGU, p. 8. Study of the effect of carbon nanotubes on the process of purification of alcohol-containing liquids / I. V. Zaporotskova [et al.] // Vestn. Volgograd state university Ser. 10, Innovative activity S Kazitsyna, L. A. Application of UV, IR, NMR spectroscopy in organic chemistry: textbook. manual for universities / L. A. Kazitsyna, N. B. Kupletskaya. M. : Vyssh. school, s. 10. Sychev, S. N., Gavrilina, V. A., and Murzalevskaya, R. S., High performance liquid chromatography as a method for determining product falsification and safety. M. : DeLi print, p. 11. Chemical encyclopedia / ed. I. L. Knunyants. M. : Soviet Encyclopedia, Dresselhaus, M. S. / M. S. Dresselhaus, G. Dresselhaus, P. Avouris // Carbon nanotubes: synthesis, structure, properties, and application. Springer-Verlag, p. 13. Zaporotskova, I. V. Active properties of nanotubular carbon structures with respect to heavy organic molecules / I. V. Zaporotskova // Nanoscience & nanotechnology-2011: Book of abstract. Frascati National Laboratories INFN. Frascati, Sept , Frascati: INFN, P Zaporotskova, N. P. Investigation of carbon nanotube activity to heavy organic molecules / N. P. Zaporotskova, I. V. Zaporotskova, and T. A. Ermakova // Fullerenes and Atomic clusters. Abstracts of invited lectures & contributed papers. St.-Peterburg, July 4 8, St.-Peterb., P THE FILTER ON THE BASIS OF CARBON NANOTUBES FOR PURIFICATION OF ALCOHOL-CONTAINING LIQUIDS N.P. Polikarpova, I.V. Zaporotskova, T.A. Ermakova, P.A. Zaporotskov Experiments on purification of alcohol-containing liquids by filtration and transmission methods are made, the mass fraction of carbon nanotubes leading to the best result is established. The filter model on the basis of a nanomaterial concluded in space between layers of porous glass is created, and its constructional features are defined. Key words: carbon nanotubes, alcohol-containing liquids, adsorption, filter, porous glass, porous ceramics. 8 0 N.P. Polikarpova et al. Filter based on carbon nanotubes

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Purification of carbon nanotubes

None of the common methods for obtaining CNTs allows them to be isolated in their pure form. Impurities to NT can be fullerenes, amorphous carbon, graphitized particles, catalyst particles.

There are three groups of CNT cleaning methods:

destructive,

non-destructive,

combined.

Properties of carbon nanotubes

1 - The cable of the space elevator, according to calculations, must withstand a mechanical stress of 62.5 GPa

2 - Diagram of tension (dependence of mechanical stress y on relative elongation e)

Electrical. Due to the small size of carbon nanotubes, only in 1996 was it possible to directly measure their electrical resistivity using a four-prong method.

Gold stripes were deposited on a polished silicon oxide surface in a vacuum. Nanotubes 2–3 µm long were deposited between them. Then, four tungsten conductors 80 nm thick were deposited on one of the nanotubes chosen for measurement. Each of the tungsten conductors had contact with one of the gold strips. The distance between contacts on the nanotube was from 0.3 to 1 μm. The results of direct measurement showed that the resistivity of nanotubes can vary within a significant range - from 5.1*10 -6 to 0.8 ohm/cm. The minimum resistivity is an order of magnitude lower than that of graphite. Most of the nanotubes have metallic conductivity, while the smaller part exhibits the properties of a semiconductor with a band gap of 0.1 to 0.3 eV.

Capillary. Experiments have shown that an open nanotube has capillary properties. To open a nanotube, it is necessary to remove the upper part - the cap. One way to remove is to anneal nanotubes at a temperature of 850 0 C for several hours in a flow of carbon dioxide. As a result of oxidation, about 10% of all nanotubes are open. Another way to destroy the closed ends of nanotubes is exposure to concentrated nitric acid for 4.5 hours at a temperature of 2400 C. As a result of this treatment, 80% of the nanotubes become open.

Along with metals, carbon nanotubes can be filled with gaseous substances, such as molecular hydrogen. This ability is of practical importance, because it opens up the possibility of safe storage of hydrogen, which can be used as an environmentally friendly fuel in internal combustion engines. Also, scientists were able to place a whole chain of fullerenes with gadolinium atoms already embedded in them inside the nanotube (see Fig. 5).

Rice. five. Inside C60 inside a single-walled nanotube

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Introduction

Nanotechnology - the science of the manufacture and properties of elements of technology at the atomic and molecular level - is now "on everyone's lips". Nanodevices and nanomachines made of such elements from the realm of fantasy are already moving into modern life. And part of this science is the rapidly growing branch of nanotube and fullerene research, which has attracted hundreds of research groups of physicists, chemists and materials scientists.

The problem of creating nanostructures with desired properties and controlled sizes is one of the most important problems of the 21st century. Its solution will revolutionize electronics, materials science, mechanics, chemistry, medicine and biology.

Carbon nanotubes (CNTs) are unique macromolecular systems. Their very small nanometer diameter and large micron length indicate that they are closest in structure to ideal one-dimensional (ID) systems. Therefore, CNTs are ideal objects for testing the theory of quantum phenomena, in particular, quantum transport in low-dimensional solid-state systems. They are chemically and thermally stable up to at least 2000 K, have excellent thermal conductivity, unique strength and mechanical characteristics.

The simplicity of the structure of nanotubes makes it possible to develop theoretical models of their structures. Therefore, new unexpected applications await CNTs in the future, especially for applications in biology (manipulation of molecules inside the cell, artificial neural networks, nanomechanical memory, etc.).

1. Single-layer nanotubes

1.1 Discovery

In early 1993, several groups of scientists stated that foreign materials could be introduced into carbon nanoparticles or nanotubes by using modified electrodes in the arc evaporation process. Rodney Ruoff's group in California and Yahachi Saito's group in Japan obtained encapsulated LaC2 crystals using lanthanum-stuffed electrodes, while Suppapan Serafin and colleagues reported that YC2 could be incorporated into nanotubes using yttrium-containing electrodes. This work opened up a whole new field based on nanoparticles and nanotubes as "molecular containers", but it also indirectly led to a completely different discovery with equally important applications.

Donald Bethune and his colleagues at IBM's California Almaden Research Center in San Luis were very interested in the papers by Ruoff and others. This group was working on magnetic materials in their memory applications and believed that carbon-encapsulated ferromagnetic transition metal crystallites could be of great value in this area. In such materials, encapsulated metal particles must retain their magnetic moments and simultaneously be chemically and magnetically isolated from their neighbors. For several years this IBM group has been working on "eschuedral fullerenes"; fullerenes containing a small number of metal atoms inside. But large clusters or crystals inside fullerene-like cells could be of the greatest practical interest. Therefore, Bethune decided to attempt some arc evaporation experiments using electrodes impregnated with the ferromagnetic transition metals iron, cobalt, and nickel. However, the result of this experiment was not at all what was expected. First of all, the soot obtained by arc evaporation was not similar to the usual material produced by arc evaporation of pure graphite. Layers of soot hung like cobwebs from the walls of the chamber, while the material deposited on the walls had the texture of rubber and could be stripped off. When Bethune and his colleague Robert Beyers tested this strange new material, using high resolution electron microscopy, they were astounded to find that it contained many nanotubes with single atomic layer walls. These beautiful pipes were confused with amorphous soot and particles of metal or metal carbide, holding the material in place to match its strange texture. This paper was accepted for publication by Nature and appeared in June 1993. Micrographs from this paper are shown in Figure 1.1.

Figure 1.1 - Images from Bethune et al. showing single-walled carbon nanotubes produced by co-evaporation of graphite and cobalt. The pipes have diameters of about 1.2 nm.

Independently of the American group, Sumio Iijima and Toshinari Ichihashi of NEC Laboratories in Japan have also experimented with arc evaporation using modified electrodes. In addition, they were interested in the effect of changing the atmosphere inside the arc evaporation chamber. Like Bethune and his colleagues, they discovered that under certain conditions, a very different type of soot is produced, different from that usually formed by arc evaporation. For this study, Japanese scientists implanted iron in their electrodes and used a mixture of methane and argon instead of helium as the atmosphere. When checked by high-resolution electron microscopy, it was found that the material of such arc evaporation contained very - remarkable nanotubes, stretching like threads between clusters of amorphous material or metal particles. Single-walled nanotubes differ from those obtained in continuous arc evaporation by a very narrow diameter distribution. In the case of "conventional" pipes, the inner diameter ranges from 1.5 to 15 nm and the outer diameter ranges from 2.5 to 30 nm. On the other hand, single-walled nanotubes all have very similar diameters. In the material of Bethune and colleagues, the nanotubes had diameters of 1.2 (± 0.1) nm, while Iijimai Ichihashi found that the tube diameters ranged from 0.7 to 1.6 nm, centered at about 1.05 nm. Like the tubes produced by conventional arc evaporation, all single-walled nanotubes were capped, and there was no evidence of the presence of metal catalyst particles at the ends of these tubes. Nevertheless, it is believed that the growth of single-walled nanotubes is essentially catalytic.

1.2 Follow-up work on single-walled nanotubes

Following the original fundamental research, Donald Bethune and his colleagues at IBM in San Jose, in collaboration with scientists from the California Institute of Technology, Tech, and Virginia State University, conducted a series of studies on the preparation of single-walled nanotubes using a mass of "catalysts." In one of the first series, they showed that the addition of sulfur and cobalt to the anode (either as pure S or CoS) resulted in nanotubes with a wider range of diameters than when prepared with cobalt alone. Thus, single-layer nanotubes with diameters from 1 to 6 nm were obtained when sulfur was detected in the cathode, compared with 1–2 nm in the case of pure cobalt. Subsequently, it was shown that bismuth and lead could similarly promote the formation of large diameter pipes.

In 1997, a French group showed that a high yield of nanotubes can also be achieved with arc evaporation. Their method was similar to the original technique by Bethune and colleagues, but they used a slightly different reactor geometry. Also, a nickel/yttrium mixture was used as a catalyst, rather than cobalt, which was preferred by the Bethune group. It was found that the largest number of nanotubes formed in the "collar" around the cathode deposit, which was approximately 20% of the total mass of the evaporated material. The total yield of pipes was estimated at 70-90%. The study of the "collar" material by high-resolution electron microscopy showed the presence of many bundles from tubes with a diameter of about 1.4 nm. This output and the appearance of the resulting tubes are similar to the "tow" samples of Smalley's group using laser evaporation.

In late 1993, Shekhar Subramoni of DuPont in Wilmington, Delaware, in collaboration with researchers at SPI International, described the production of single-walled nanotubes in a different way. These scientists applied arc evaporation using gadolinium-filled electrodes and collected soot from the walls of the reactor. Together with large amounts of amorphous carbon, the soot contained structures of the "sea urchin" type, which contained single-layer nanotubes growing on relatively large gadolinium carbide particles (with typical sizes of tens of nanometers). Such pipes were shorter than those obtained with iron-group metals, but had the same range of diameters. Subsequent research showed that radial single-walled nanotubes could form on a variety of other metals, including lanthanum and yttrium. Figure 1.2, taken from the work of Saito et al., shows a typical image of single-walled nanotubes growing radially from a particle containing lanthanum. Unlike the iron group metals, rare earth elements are not known to be catalysts for producing multilayer nanotubes, so it is rather surprising to form tubes on them. The fact that tubes grow on relatively large particles suggests that such a growth mechanism is different. It has been suggested that the growth of tubes on particle surfaces may involve the release of supersaturated carbon atoms from the interior of the carbide particles. Note that the radial growth of multilayer tubes of catalytic particles was observed many years ago by Baker and others.

The methods discussed so far for producing single-walled nanotubes have included arc evaporation using modified electrodes. The work of Smalley and colleagues showed that single-walled nanotubes can also be synthesized using a purely catalytic method. The catalyst, using molybdenum particles of several nanometers in diameter, was located on aluminum. All this was placed inside a tube-like furnace, through which carbon monoxide was passed at a temperature of 1200 °C. This temperature is much higher than that commonly used in the catalytic production of nanotubes, which may explain why single-walled rather than multi-layered nanotubes are formed.

Catalytically prepared single-layer pipes had a number interesting features, which distinguished them from tubes synthesized by arc evaporation. First, catalytic tubes usually had small metal particles attached to the end, just like multilayer tubes produced by catalysis. There was also a wide range of particle diameters (about 1-5 nm) and it seemed that the diameter of each tube was determined by the diameter of the respective catalyst particle. Finally, catalytically formed single-layer tubes have typically been insulated rather than bundled, as is the case with tubes synthesized by arc evaporation.

These observations enabled Smalley and colleagues to propose a growth mechanism for catalytically formed tubes that includes the initial formation of a single-layer cap (which they called yarmolka, the Hebrew name for skullcap), followed by the growth of this cap with detachment from the catalytic particles that subsequently leave the tube. This mechanism is completely different from that proposed by them for the growth of single-layer tubes during laser evaporation.

Figure 1.2 - Single-layer nanotubes growing on a lanthanum particle

Figure 1.3 - TEM images of samples from "bundles" of single-layer nanotubes (a)

Low resolution image showing a large number of bundles, (b) High resolution micrograph of an individual bundle shown along its axis.

1.3 Nanotube bundles

Since the discovery in 1985 at Rice C60, Smalley's group has concentrated on the use of lasers in the synthesis of fullerene-like materials. In 1995, they reported on the development of laser fusion technology, which allowed them to obtain high yield single-walled nanotubes. Subsequent improvements to this method have led to the production of single-walled nanotubes with unusually uniform diameters. The best yield of homogeneous single-walled nanotubes was obtained with a catalyst mixture composed of equal parts of Co and Ni, and a double pulse was used to ensure a more even evaporation of such a target.

Several micrographs of the material obtained by this technology are shown in Figure 1.3. In general appearance, it is very similar to the material obtained by arc evaporation. However, individual pipes tend to form "bundles" or extended bundles, which consist of individual pipes of the same diameter. Sometimes it was possible to detect tourniquets passing through close range from the direction of the electron beam, so that one could see them "end-to-end", as in Figure 1.3(b). In addition to electron microscopy, Smalley and colleagues performed X-ray diffraction measurements on rope samples in collaboration with John Fisher and co-authors at Pennsylvania State University. Well-defined reflections from the 2D grating were obtained, confirming that the tubes had the same diameters. Good agreement was found with the experimental data under the assumption that the diameter of the nanotubes is 1.38 nm with an error of ±0.02 nm. The van der Waals gap between the tubes was found to be 0.315 nm, similar to that in crystalline C 60 . It was concluded from XRD studies that these bundles are predominantly composed of (10,10) chair nanotubes. This was clearly confirmed by measurements of electron nanodiffraction of the electron beam, so that one could see them "on the end", as in Figure 1.3(b).

2. Theories of nanotube growth

2.1 General remarks

It is important to first consider the effect on growth of the pipe structure. In his 1991 Nature paper, Iijima pointed out that a helicoidal structure seems to be preferable, since such tubes have a repeating pitch at the growing end. This assumption, illustrated in Figure 2, is very similar to the appearance of a screw dislocation on the crystal surface. Armchair and zigzag nanotubes do not have this growth-preferential structure and must require the regeneration of a new ring of hexagons. This suggests that helical nanotubes should be more frequently observed than armchair or zigzag ones, although there is currently insufficient experimental evidence to support this.

Figure 2 - Drawing of two concentric spiral pipes showing the presence of steps at the growing ends (5)

Next, there is a very important question for the growth mechanism - do growing tubes have closed or open ends? An early nanotube growth model, first proposed by Endo and Kroto, favored a closed-end mechanism. They assumed that carbon atoms could be inserted into the closed fullerene surface in place in the vicinity of the pentagonal rings, followed by a transition to an equilibrium state, as a result of which the original fullerene would be continuously drawn out. In support of this idea, Endo and Kroto cited the demonstration by Ulmer et al. that C 60 and C 70 can clearly grow into large fullerenes upon the addition of small carbon moieties.

While the Endo-Kroto mechanism provides a plausible explanation for the growth of single-walled nanotubes, it remains a major problem for explaining multilayer growth. In their consideration of the Endo and Kroto models, they suggest that multilayer growth can be carried out "epitaxially". If this is the case, there seems to be no obvious reason why the second layer should not start growing immediately after the initial fullerene has formed, and once the second layer is closed, any further extension of the inner tube should be impossible. But this is at odds with the observation that most pipes are multi-layered throughout their length. Such a model also has difficulties in explaining structures with multiple branches. For these reasons, the Endo-Kroto closed-end growth mechanism has not been widely accepted.

The conclusion that the growth mechanism must occur with an open end of the tube is somewhat preferable. As Richard Smalley said, "If we've learned anything since 1984-1985 about how carbon condenses, it's that open sheets should readily connect pentagons to eliminate dangling bonds." The problem of pipes remaining with an open end under conditions favorable for its closure is one of those problems that have been considered by a number of authors.

2.2 Why nanotubes remain open during growth

Some authors, notably Smalley and colleagues, have suggested that the electric field in the arc may play an important role in keeping the tubes open during growth. More correctly, it should have helped explain why nanotubes are never found in soot condensed on the walls of an arc evaporation chamber. However, calculations have shown that the field-induced reduction in open-end energy is not sufficient to stabilize the open configuration, except for unrealistically high fields. Therefore, an elegant model has been developed in which the atom is "spot welded" between the layers, helping to stabilize the formation of the open end, rather than closing it.

This idea was confirmed by experiments on closing individual multilayer nanotubes with and without applying a voltage difference. Such a model can help understand the growth of nanotubes in an arc, but cannot be applied to the case of tube growth where strong electric fields are not present. This has led some authors to suggest that the interactions between the combined concentric tubes alone may be essential for the stabilization of open tubes.

A detailed analysis of the interaction of two combined tubes was performed by Jean-Christophe Charliere and colleagues using molecular dynamics methods. They looked at a (10,0) pipe inside a (18,0) pipe and found that bridges formed between the ends of the two pipes. It was found that at high temperatures (3000 K) the configuration of sticky bonding structures continuously fluctuates. It was assumed that the fluctuating structure should create active sites for adsorption and introduction of new carbon atoms, thus contributing to the growth of the tube.

The problem with this theory is that it cannot explain the growth of large-diameter single-wall pipes under thermal action on fullerene soot. In general, at present, there does not seem to be a complete explanation for the growth of open nanotubes.

2.3 Arc plasma properties

Most of the nanotube growth models discussed earlier assume that the tubes are nucleated and grow in the arc plasma. However, some authors considered the physical state of the plasma itself and its role in the formation of nanotubes. The most detailed discussion of this problem has been carried out by Evgeny Gamalei, an expert in plasma physics, and Thomas Ebbesen (30, 31). This is a complex issue and only a brief summary is possible here.

Gamalei and Ebbesen start with the assumption that nanotubes and nanoparticles form in the arc region near the cathode surface. So they analyze the density and velocity of the carbon vapor in the area, taking into account the temperature and properties of the arc itself, in order to develop their model. They believe that two groups of carbon particles with different velocity distributions will exist in the carbon vapor layer near the cathode surface. This idea is central to their growth model. One group of carbon particles must have a Maxwellian, i.e. isotropic velocity distribution corresponding to the arc temperature (~ 4000 K). The other group consists of ions accelerating in the gap between the positive space charge and the cathode. The speed of these carbon particles must be greater than the speed of the thermal particles, in which case the flow must be directed rather than isotropic. The process of formation of nanotubes (and nanoparticles) is considered as a series of cycles, each of which consists of the following steps:

1. Formation of the embryo. At the beginning of the discharge process, the velocity distribution of carbon in the evaporated layer is predominantly Maxwellian, and this leads to the formation of structures without any symmetry axis, such as nanoparticles. As the current becomes more directed, open structures begin to form, which Gamalei and Ebbesen consider as nuclei for nanotube growth.

2.Tube growth during stable discharge. When the discharge stabilizes, the flow of carbon ions penetrates into the vapor layer in the direction perpendicular to the cathode surface. These carbon particles will contribute to the elongation of single-walled and multi-walled nanotubes. Since the interaction of directed carbon particles with a solid surface should be more intense than that of carbon particles of the vapor layer, the growth of extended structures should be predominant over the formation of isotropic structures. However, condensation of carbon from the vapor layer on the cathode surface will contribute to the thickening of the nanotubes.

3. End of growth and closing. Gamaley and Ebbesen note that nanotubes are often seen growing in bundles and that in the observed bundle for all tubes, growth and termination occur at approximately the same time. This allows them to suggest that instabilities occur in the arc discharge, which can lead to the sudden termination of nanotube growth. Such instabilities can result from the unstable movement of the cathode spot along the cathode surface or from spontaneous interruption and ignition of the arc. Under such circumstances, carbon particles with a Maxwellian velocity distribution will again prevail, and the condensation of such carbon will eventually lead to capping of the tube and termination of growth.

2.4 Alternative models

Scientists have presented a completely different theory of nanotube growth during arc evaporation. In this model, nanotubes and nanoparticles do not grow in the arc plasma, but rather form at the cathode as a result of the transformation of the solid state. Thus, the growth of nanotubes is not a consequence of the action of an electric field, but simply a result of the very rapid heating to high temperatures experienced by the material deposited on the cathode during the action of the arc. This idea was initiated by the observation that nanotubes can be prepared by high-temperature thermal treatment of fullerene soot and involves a two-stage nanotube growth process in which fullerene soot is an intermediate product. The model can be generalized as follows. In the initial stages of arc evaporation, fullerene-like material (plus fullerenes) must condense on the cathode, and then the condensed material must be subjected to high temperatures during the continuation of the arc process, leading to the formation of first single-layer, nanotube-like structures, and then multilayer nanotubes. In this two-stage model, the key action is the annealing of the fullerene soot. Thus, the soot deposited on the walls of the reactor, which undergoes relatively weak annealing, does not transform into pipes. On the other hand, the soot that condenses on the cathode should just experience significant annealing: it will lead to the formation of pipes and nanoparticles in the form of a solid mass. Therefore, such a model allows us to explain the influence of such variables as electrode cooling and helium pressure on the production of nanotubes. It seems that water cooling should be essential to keep the cathode temperature low to the level necessary to avoid tube slagging. Similarly, the role of helium can be explained in terms of its effect on the temperature of the cathode deposit. Since helium is an excellent conductor of heat, high pressures should cause the temperature of the electrode to drop, causing it to drop in regions where nanotube growth can occur without slagging.

2.5 Growth of single-walled nanotubes

Let us first consider the growth of single-layer nanotubes in an arc evaporator. This process raises no less questions than the growth of multilayer nanotubes in an arc. Among the most obvious are: Why are only single-walled nanotubes observed? Why is there such a narrow distribution of pipe diameters? What is the role of metal? Why do pipes grow most often in bunches? Again, we only have a few definitive answers to these questions.

One thing that seems clear is that the growth of single-walled nanotubes should be largely determined by kinetics rather than thermodynamics, since tubes with a very small diameter are expected to be less stable than those with a large one. The absence of many layers is presumably also constrained by kinetic factors. Regarding the role of the metal, both Bethune and colleagues and Iijima and Ichihashi have suggested that single metal atoms or small clusters of them can act as catalysts for vapor phase growth in a manner similar to the way that small metal particles catalyze the growth of multilayer pipes. The involvement of individual atoms or well-defined clusters should help explain narrow-dimensional distributions. Surprisingly, however, catalytic particles are apparently never observed at the tops of single-walled nanotubes. Even if the catalytic particles were individual atoms, they could be detected by high resolution electron microscopy or scanning transmission electron microscopy (STEM). It is possible that catalytic atoms or particles will become detached during the closing of the pipes. As noted above, Bethune and colleagues have shown that the addition of elements such as sulfur to the metal can greatly disturb the distribution of pipe diameters. Further investigation of this phenomenon may provide useful insights into the growth mechanism.

One of several attempts to develop a detailed model for the growth of single-walled nanotubes was undertaken by Ching-Hwa Kiang and William Goddard. These researchers suggest that plenary polyene rings can serve as nuclei for the formation of single-walled nanotubes. It has been shown that such ring structures should be the dominant particles in carbon pairs, while closed framework structures dominate at large sizes. It has been postulated that carbon rings may be precursors in the formation of fullerenes, although this remains controversial. Kiang and Goddard believe that the initial materials for the formation of single-layer nanotubes are monocyclic carbon rings and gas-phase cobalt carbide clusters, possibly charged. Cobalt carbide clusters act as catalysts when attached to C 2 rings or other particles. These authors suggest that a specific conformation should influence the structure of the emerging nanotube.

Smalley and colleagues, following their syntesis of nanotube bundles, have proposed a growth mechanism that bears some similarities to the Kiang and Goddard mechanism. This model is based on the assumption that all pipes have the same (10,10) chair structure. This structure is unique in allowing the open hexagonal rings to be "overlapped" by triple bonds, although they must be significantly strained compared to their original linear arrangement. Smalley's group then proposes that a single nickel atom will chemically adsorb to the end of the pipe and "run" around the periphery (Figure 2.1), helping the incoming carbon atoms settle on the hexagonal rings. Any locally sub-optimal structures, including pentagons, will be reflected, so that such a tube will continue to grow indefinitely.

Here, as well as for other mechanisms proposed for the growth of single-walled nanotubes, there is no direct experimental evidence.

Figure 2.1 - Illustration of the "scooter" mechanism during the growth of (10,10) chair nanotubes.

A number of groups of scientists around the world have attempted to purify nanotube samples using methods such as centrifugation, filtration, and chromatography. Some of these methods involve the initial preparation of colloidal suspensions of nanotube-containing material using surface active agents. For example, Jean-Marc Bonard and colleagues used an anionic surfactant sodium dodecacyclosulfate (SDS) to achieve a stable suspension of nanotubes and nanoparticles in water. Initially, a filtration method was used to separate the nanotubes from the nanoparticles, but a more successful separation was achieved simply by allowing the nanotubes to flocculate, leaving the nanoparticles in suspension. The precipitate can be removed and then continue with further precipitation procedures. This not only made it possible to extract the nanoparticles, but also led to some separation of the tubes along the lengths.

Another method for achieving nanotube size separation is described by Duisberg and colleagues at the Max-Planck Institute in Stuttgart and Trinity College Dublin. Separation of pipes and other material was again obtained in SDN acid. Separation was then carried out using size exclusion chromatography (SEC). This technology has been widely used to separate biological macromolecules, and the authors have demonstrated that it is possible to successfully separate nanotube samples into fractions with tubes of various lengths. One possible disadvantage of using surfactants such as SDN in nanotube purification is that traces of the surfactant may remain in the final product. However, Bonard and colleagues have shown that it is possible to achieve reductions in SDS below 0.1% by washing.

3. Cleaning Single Layer Pipes

Techniques for cleaning single-walled tubes have also been developed, although this process requires more effort than for multilayer nanotubes. In addition to a large amount of amorphous carbon, carbon black containing nanotubes also contains metal particles, which themselves are often coated with carbon. Moreover, the severe oxidation methods used to purify multilayer nanotubes are also destructive for single-layer pipes.

Japanese scientists have described step by step the process of sequential exclusion of various impurities. The first step involved washing the untreated soot with distilled water for 12 hours, followed by filtration and drying. This procedure made it possible to remove some of the graphite particles and amorphous carbon. Fullerenes were washed out with toluene in a Soxhlet apparatus. Then the soot was heated to 470°C in air for 20 minutes in order to get rid of metal particles. Finally, the remaining soot was exposed to perchloric acid in order to dissolve the metal particles. Inspection of the final product by electron microscopy and X-ray diffraction showed that most of the contaminants were removed, although some filled and empty nanoparticles remained in it.

Smalley and colleagues have developed a method for purifying nanotube samples from bundles using microfiltration. They were the first to describe the technique of using a cationic surfactant to prepare a suspension of nanotubes and an accompanying material in solution, and then depositing the nanotubes on a membrane. However, multiple slurry filtrations were required after each filtration in order to achieve a significant level of purification, making such a procedure very slow and inefficient. An improved method was described in a paper where sonication was used, keeping the material in suspension during filtration and thus allowing a continuous filtration process of large amounts of sample. In this way it was possible to purify up to 150 ml of carbon black within 3-6 hours to obtain a material containing more than 90% SWNT.

Single layer tubes could also be cleaned using chromatography, Duisburg et al. described a method similar to that used for MWNTs and showed it to be effective for SWNTs.

4. Alignment of Nanotube Samples

carbon nanotube fullerene-like plasma

Many of the preparation methods described above produce samples with randomly oriented, nanotubes. Although pipes are often grouped into bundles, the bundles themselves do not align with one another at all. For measuring the properties of nanotubes, it would be very useful to have samples in which all tubes are aligned in the same direction. Although catalytic methods for preparing aligned pipes have already been described, it was also necessary to develop technologies for aligning pipe samples after their synthesis. Thus, one of the first such methods was proposed in 1995 by a group from the École Polytechnique Federale Lausanne in Switzerland. They used a MWNT sample prepared by arc evaporation that had been purified by centrifugation and filtration to remove nanoparticles and other contaminants. Thin films of purified nanotubes were then deposited onto the plastic surface, and SEM images showed that these tubes were aligned perpendicular to the film in this freely deposited state. It was found that the tubes could be aligned parallel to the surface of the sample, previously lightly rubbed with Teflon or aluminum foil. The authors claim that films can be made "arbitrarily large" by this method, and they have used these films to perform field emission experiments.

Another method for aligning nanotubes is to embed these tubes in a matrix and then extrude such a matrix in some way so that the tubes become aligned in the direction of flow.

5. Controlling the Length of Carbon Nanotubes

A technique for cutting individual single-walled nanotubes to controlled lengths was described by researchers at Delft and Rice Universities in late 1997. The nanotubes used were produced by laser evaporation by Smalley's group and deposited on the surface of gold single crystals for examination by scanning tunneling microscopy. When a suitable nanotube was identified, scanning was stopped and the Pt/Ir needle was advanced to the selected point on that tube. Then the feedback was turned off, and a voltage pulse was applied between the tip and the sample for a certain period. When the scan was resumed, a break was visible on the nanotube if the cut was successful. It has been demonstrated that individual pipes can be cut up to four separate positions. It was found that the critical factor in the slicing process is voltage rather than current, the minimum voltage required for the slicing process should be 4 V.

By having individual nanotubes cut into short lengths, the authors were able to show that the electrical properties of the short tubes differed from those of the original nanotubes. These differences were attributed to the manifestation of quantum size effects.

As well as controlling the lengths of individual nanotubes, it is also possible to cut into short lengths bulk samples from single-walled nanotubes. This was demonstrated in 1998 by Smalley's group. The most effective way to obtain samples from short tubes (they were called "fullerene tubes") is the sonication of nanotube material in a solution of sulfuric and nitric acids. During this exposure, loyal sonochemistry produces holes in the tubular surfaces, which are then attacked by acids, forming open "tubules". Smalley and colleagues showed that these tubules could be sorted into fractions of various lengths by a method known as field flow fractionation. They also stuffed the ends of such open nanotubes with various functional groups and showed that gold particles can attach to the funudion tube ends. This work can be considered the beginning of a new organic chemistry based on carbon nanotubes.

6. Research Analysis

The arc evaporation method of Iijima, Ebbesen and Ajayan remains undoubtedly the best technique for the synthesis of high quality nanotubes, but it suffers from a number of disadvantages. First, it is labor intensive and requires some skill to achieve an appropriate level of reproducibility. Secondly, the yield in it is quite low, since more evaporated carbon is deposited on the chamber walls than on the cathode, and the nanotubes become contaminated with nanoparticles and other graphite fragments. Third, it's more of a "bake" than an ongoing process, and it's not easy to scale up. If nanotubes are ever to be used commercially on a large scale, it seems likely that a different preparation method will need to be used. Progress in this direction is hampered by a lack of understanding of the mechanism of tube growth in an arc. Therefore, further studies specifically dedicated to elucidating the mechanism of nanotube growth should be welcomed.

There is another serious weakness of the arc evaporation method and all other current technologies for the preparation of multilayer nanotubes: they produce a wide range of tube sizes and structures. And this can be a problem not only for some applications, but also a disadvantage in areas where specific tubular structures are needed, such as nanoelectronics. Is it possible to predict the way in which pipes with certain structures will be prepared? Perhaps this will be achieved by the creative use of catalysts.

Researchers draw attention to the greater uniformity of single-wall pipes than their multi-wall counterparts, at least in terms of their diameters. However, directly used for the synthesis of single-walled tubes, the methods are more complex than for multi-walled nanotubes. The laser evaporation technique developed by the Smoly group serves to produce the best quality material with the highest yield, but the high energy lasers required for this method are not always available to the average laboratory. As with multilayer pipes, the way forward may include catalytic methods, and current research in this direction is encouraging. Ultimately, it is to be hoped that organic chemists will be able to complete the complete synthesis of nanotubes. However, it should be borne in mind that this may be a distant prospect, since even the complete synthesis of C60 has not yet been carried out.

While currently the best quality nanotubes are obtained using methods that also produce a significant amount of contaminating material, it is important to note that there are methods for removing this material. Fortunately, significant progress has recently been made in this area, and a range of methods are now available to remove unwanted nanoparticles, microporous carbon, and other contaminants from samples of both multilayer and single-walled nanotubes. Procedures have also been developed for aligning pipes and cutting them to controlled lengths. These technologies will allow progress in areas where the lack of pure and well-defined samples is still a serious problem.

Conclusion

The method for preparing nanotubes, described by Injima in 1991, gave a relatively poor yield, making it difficult to further study their structure and properties. A major advance occurred in July 1992, when Thomas Ebessen and Pulikel Ajayan, working in the same Japanese lab as Iijima, described a method for preparing gram quantities of nanotubes. Again, this was an unexpected discovery: while trying to prepare fullerene derivatives, Ebessen and Ajayan found that increasing the helium pressure in the arc chamber dramatically improved the yield of nanotubes formed in cathode soot. The availability of nanotubes in large volumes has led to a tremendous increase in the pace of research around the world.

Another area that attracted early interest was the idea of using carbon nanotubes and nanoparticles as "molecular containers". A milestone in this direction was the demonstration by Ajayan and Iijima that nanotubes could be filled with molten lead and thus be used as templates for "nanowires". Subsequently, more controlled methods of opening and filling nanotubes have been developed, allowing a wide range of materials, including biological ones, to be placed inside. The result of opening and filling nanotubes can be amazing properties that can be applied in catalysis or in biological sensors. Filled carbon nanoparticles may also have important applications in fields as diverse as magnetic recording and nuclear medicine.

Perhaps the largest volume on the study of nanotubes should be devoted to their electronic properties. The theoretical work that preceded the discovery of Iijima has already been noted above. Shortly after Iijima's 1991 Nature letter, two other papers appeared on the electronic properties of carbon nanotubes. An MIT team of scientists and Noriaki Hamada and colleagues at Iijima's lab in Tsukuba performed band structure calculations using a tight-binding model and demonstrated that the electronic properties depend on both the structure of the pipe and its diameter. These remarkable predictions aroused great interest, but the attempt to determine the electronic properties of nanotubes experimentally encountered great difficulties. But it wasn't until 1996 that experimental measurements were made on individual nanotubes that could confirm the theoretical predictions. These results suggested that nanotubes could become components of future nanoelectronic devices.

Determining the mechanical properties of carbon nanotubes presented formidable difficulties, but once again experimenters rose to the challenge. Measurements carried out using transmission electron microscopy and atomic force showed that mechanical characteristics carbon nanotubes can be as exceptional as their electronic properties. As a result, there has been increased interest in the use of nanotubes in composite materials.

Now, a variety of other possible applications of nanotubes are of interest to them. For example, a number of scientists are studying the problem of using nanotubes as tips for scanning probe microscopy. With their elongated shape, pointed tops, and high rigidity, nanotubes should have been ideal for this purpose, and initial experiments in this area have shown extremely impressive results. Nanotubes have also been shown to have useful field emission properties that could lead to their use in flat panel displays. Throughout the world, nanotube research is growing at an astronomical rate, and its commercial applications will certainly not be long in coming.

Bibliography

1. P. Harris, Carbon nanotubes and related structures. New materials of the XXI century - M.: technosphere, 2003.

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Properties of carbon nanotubes. Purification of carbon nanotubes

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Purification of carbon nanotubes

Properties of carbon nanotubes

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