CARBON NANOTUBES: STRUCTURE, PROPERTIES, APPLICATIONS, TOXICITY - CHEMISTRY - 2024

The carbon nanotubes are tubes or cylinders very small and very thin formed only by carbon atoms (C). Its tubular structure is visible only through electron microscopes. It is a solid black material, made up of very small bundles or bundles of several dozen nanotubes, entangled with each other to form a complicated network.

The prefix "nano" means "very small." The word "nano" used in measurement means that it is one billionth of a measurement. For example, a nanometer (nm) is one billionth of a meter, that is, 1 nm = 10 ^-9 m.

Carbon nanotube sample. It can be seen that it is a black solid with a carbon-like appearance. Shaddack. Source: Wikimedia Commons.

Each tiny carbon nanotube is made up of one or more sheets of graphite wrapped around themselves. They are classified into single-walled nanotubes (a single rolled sheet) and multi-walled nanotubes (two or more cylinders one inside the other).

Carbon nanotubes are very strong, have a high resistance to breaking and are very flexible. They conduct heat and electricity very well. They also make up a very light material.

These properties make them useful in various fields of application, such as the automotive, aerospace and electronics industries, among others. They have also been used in medicine, for example to transport and deliver anticancer drugs, vaccines, proteins, etc.

However, its handling must be done with protective equipment as when inhaled they can cause damage to the lungs.

Discovery of carbon nanotubes

There are different opinions in the scientific community about who discovered carbon nanotubes. Although there are many research papers on these materials, only a few important dates are mentioned below.

- In 1903, the French scientist Pélabon observed carbon filaments in a sample (electron microscopes were not yet available at this date).

- In 1950, physicist Roger Bacon of the Union Carbide company was studying certain samples of carbon fibers and observed images of straight and hollowed nano-fluff or nanobigots (nanowhiskers).

- In 1952, the Russian scientists Radushkevich and Lukyanovich published photos of images of carbon nanotubes synthesized by themselves and obtained with an electron microscope, where it is clearly observed that they are hollow.

- In 1973, Russian scientists Bochvar and Gal'pern completed a series of calculations of the energy levels of molecular orbitals showing that graphite sheets can twist on themselves to form "hollow molecules".

- In 1976, Morinobu Endo observed carbon fibers with a hollowed center produced by the pyrolysis of benzene and ferrocene at 1000 ° C (pyrolysis is a type of decomposition that occurs with heating to very high temperatures in the absence of oxygen).

- In 1991, enthusiasm for carbon nanotubes was sparked after Sumio Iijima synthesized carbon needles made from hollow tubes using the electric arc technique.

- In 1993, Sumio Iijima and Donald Bethune (working independently of each other) simultaneously discovered single-walled carbon nanotubes.

Interpretations of some of the sources consulted

According to some sources of information, perhaps the credit for the discovery of carbon nanotubes should go to the Russian scientists Radushkevich and Lukyanovich in 1952.

It is thought that they were not given their deserved credit because at that time the so-called “cold war” existed and Western scientists did not have access to Russian articles. In addition, not many could translate from Russian, which further delayed their research from being analyzed abroad.

In many articles it is said that Iijima was the one who discovered carbon nanotubes in 1991. However, some researchers estimate that the impact of Iijima's work is due to the fact that science had already reached a sufficient degree of maturity to appreciate the importance of carbon nanotubes. nanomaterials.

Some say that in those decades physicists generally did not read articles in chemistry magazines, where carbon nanotubes were already being discussed, and that for this reason they were "surprised" by Iijima's article.

But all this does not diminish the high quality of Iijima's work from 1991. And the difference of opinion remains.

Nomenclature

- Carbon nanotubes, or CNTs (Carbon NanoTubes).

- Single-walled carbon nanotubes, or SWCNTs (Single-Walled Carbon NanoTubes).

- Multi-walled carbon nanotubes, or MWCNTs (Multi-Walled Carbon NanoTubes).

Structure

Physic structure

Carbon nanotubes are very fine and small tubes or cylinders whose structure can be seen only with an electron microscope. They consist of a sheet of graphite (graphene) rolled into a tube.

A carbon nanotube is a rolled sheet of graphite or graphene: (a) theoretical image of sheet of graphite, (b) theoretical image of rolled sheet or nanotube of carbon. OpenStax. Source: Wikimedia Commons.

They are hollowed-out cylindrical molecules composed solely of carbon atoms. The carbon atoms are arranged in the form of small hexagons (6-sided polygons) similar to benzene and linked together (condensed benzene rings).

Drawing of a carbon nanotube where you can see the small hexagons of 6 carbon atoms. User: Gmdm. Source: Wikimedia Commons.

The tubes may or may not be plugged at their openings and can be extremely long compared to their diameters. They are equivalent to sheets of graphite (graphene) rolled into seamless tubes.

Chemical structure

CNTs are polyaromatic structures. The bonds between carbon atoms are covalent (that is, they are not ionic). These links are within the same plane and are very strong.

The strength of the C = C bonds makes CNTs very rigid and strong. In other words, the walls of these tubes are very strong.

Out-of-plane joints are very weak, which means that there are no strong joints between one tube and another. However, they are attractive forces that allow the formation of bundles or bundles of nanotubes.

Classification according to the number of tubes

Carbon nanotubes are divided into two groups: single-wall nanotubes, or SWCNT (Single-Wall Carbon NanoTube), and multi-wall nanotubes, or MWCNT (Multi-Wall Carbon NanoTube).

Nanotube types: (1) multi-walled nanotube real image, (2) single-walled nanotube drawing, (3) graphite or graphene sheet drawing. W2raphael. Source: Wikimedia Commons.

Single-walled carbon nanotubes (SWCNTs) are made up of a single graphene sheet rolled into a cylinder, where the vertices of the hexagons fit together perfectly to form a seamless tube.

Multi-walled carbon nanotubes (MWCNTs) are made up of concentric cylinders placed around a common hollow center, that is, two or more hollow cylinders placed inside each other.

Multi-walled nanotubes are made up of two or more cylinders one inside the other. Eric Wieser. Source: Wikimedia Commons.

Real image of a multi-walled carbon nanotube obtained with an electron microscope. Oxirane. Source: Wikimedia Commons.

Classification according to the form of winding

Depending on the way the graphene sheet is rolled, the pattern formed by the hexagons in the CNTs can be: chair-shaped, zigzag-shaped, and helical or chiral. And this influences its properties.

Real image of chiral or helical carbon nanotube. Taner Yildirim (The National Institute of Standards and Technology - NIST). Source: Wikimedia Commons.

Physical properties

Carbon nanotubes are solid. They come together to form bouquets, bundles, bundles or "strings" of several dozen nanotubes, entangled together to form a very dense and complicated network.

Real image of carbon nanotubes obtained with an electron microscope. It can be seen that they form bundles that become entangled with each other. Materialscientist at English Wikipedia. Source: Wikimedia Commons.

They have a tensile strength greater than that of steel. This means that they have a high resistance to breaking when subjected to stress. In theory they can be hundreds of times stronger than steel.

They are very elastic, they can be bent, twisted and folded without damage and then return to their initial shape. They are very light.

They are good conductors of heat and electricity. They are said to have very versatile electronic behavior or to have high electronic conductivity.

CNTs tubes whose hexagons are arranged in the shape of an armchair have metallic behavior or similar to that of metals.

Those arranged in a zigzag and helical pattern can be metallic and semiconductor.

Chemical properties

Due to the strength of the bonds between their carbon atoms, CNTs can withstand very high temperatures (750 ° C at atmospheric pressure and 2800 ° C under vacuum).

The ends of the nanotubes are chemically more reactive than the cylindrical part. If they are subjected to oxidation, the ends are oxidized first. If the tubes are closed the ends open.

When treated with nitric acid HNO ₃ or sulfuric acid H ₂ SO ₄ under certain conditions CNTs can form carboxylic-type groups -COOH or quinone-type groups O = CC ₄ H ₄ -C = O.

CNTs with smaller diameters are more reactive. Carbon nanotubes can contain atoms or molecules of other species in their internal channels.

Solubility

Due to the fact that CNTs do not have any functional group on their surface, it is very hydrophobic, that is, it is extremely poorly compatible with water and is not soluble in it or in nonpolar organic solvents.

However, if they are reacted with some compounds the CNTs can become soluble. For example with nitric acid HNO ₃ can be solubilized in some amide type solvents under certain conditions.

Biochemical properties

Pure carbon nanotubes are bioincompatible, meaning that they are not compatible or related to life or living tissues. They generate an immune response from the body, as they are considered aggressive elements.

For this reason, scientists chemically modify them in such a way that they are accepted by the body's tissues and can be used in medical applications.

They can interact with macromolecules such as proteins and DNA, which is the protein that makes up the genes of living beings.

Obtaining

Carbon nanotubes are made from graphite by various techniques such as laser pulse vaporization, electric arc discharges, and chemical vapor deposition.

They have also been obtained from a high pressure stream of carbon monoxide (CO) by catalytic growth in the gas phase.

The presence of metallic catalysts in some production methods helps the alignment of the multi-walled nanotubes.

However, a carbon nanotube is not a molecule that always turns out the same. According to the preparation method and the conditions, they are obtained with different length, diameter, structure, weight, and as a result they present different properties.

Applications of carbon nanotubes

The properties of CNTs make them suitable for a wide variety of uses.

They have been used in structural materials for electronics, optics, plastics and other products in the fields of nanotechnology, aerospace and automotive production.

Carbon nanotubes have many different uses. This is a real image of carbon nanotubes obtained with an electron microscope. Ilmar Kink. Source: Wikimedia Commons.

Compositions or mixtures of materials with CNTs

CNTs have been combined with polymers to make high performance reinforced polymer fibers and fabrics. For example they have been used to reinforce polyacrylonitrile fibers for defense purposes.

Blends of CNTs with polymers can also be designed to possess different electrically conductive properties. They improve not only the strength and stiffness of the polymer but also add properties of electrical conductivity.

CNTs fibers and fabrics are also manufactured with strengths similar to aluminum and carbon steel, but which are much lighter than these. Body armor has been designed with such fibers.

They have also been used to obtain more resistant ceramics.

Electronics devices

Carbon nanotubes have great potential in vacuum electronics, nanodevices, and energy storage.

CNTs can function as diodes, transistors and relays (electromagnetic devices that allow opening and closing electrical circuits).

They can also emit electrons when subjected to an electric field or if a voltage is applied.

Gas sensors

The use of CNTs in gas sensors allows these to be small, compact and light and that they can be combined with electronic applications.

The electronic configuration of CNTs makes the sensors very sensitive to extremely small amounts of gases and, in addition, CNTs can be chemically adapted to detect specific gases.

Medical applications

Due to their high surface area, excellent chemical stability and electron-rich polyaromatic structure, CNTs can adsorb or conjugate with a wide variety of therapeutic molecules, such as drugs, proteins, antibodies, enzymes, vaccines, etc.

They have proven to be excellent vehicles for the transport and delivery of drugs, penetrating directly into cells and keeping the drug intact during its transport through the body.

The latter makes it possible to reduce the dose of the medicine and its toxicity, especially anticancer drugs.

CNTs have proven useful in therapies against cancer, infections, tissue regeneration, neurodegenerative diseases, and as antioxidants.

They are also used in the diagnosis of diseases, in certain analyzes, such as biosensors, drug separation and extraction of biochemical compounds.

They are also being used in orthopedic prostheses and as a support material for the growth of bone tissue.

Other apps

They have also been suggested as materials for battery and fuel cell membranes, anodes for lithium ion batteries, supercapacitors, and chemical filters.

Their high electrical conductivity and relative chemical inertness make them useful as electrodes in electrochemical reactions.

They can also adhere to reactant particles and, due to their large surface area, can function as catalyst supports.

They also have the capacity to store hydrogen, which is very useful in vehicles that run on said gas, since with CNTs it could be transported safely.

Carbon nanotube toxicity

Studies have revealed difficulties in evaluating the toxicity of CNTs. This appears to depend on characteristics such as length, stiffness, concentration and duration of exposure to CNTs. It also depends on the production method and purity of the CNTs.

However, it is recommended to use protective equipment when handling CNTs as there are studies that indicate their similarity to asbestos fibers and that inhalation of CNT dust can cause damage to the lungs.

Technician weighing samples of carbon nanotubes. You can see the protective implements it uses. US National Institute for Occupational Safety and Health. Source: Wikimedia Commons.

Real image of how a carbon nanotube passes through a cell in a lung. Robert R. Mercer, Ann F. Hubbs, James F. Scabilloni, Liying Wang, Lori A. Battelli, Diane Schwegler-Berry, Vincent Castranova and Dale W. Porter / NIOSH. Source: Wikimedia Commons.

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