Carbon Allotropes: Introduction
Carbon is one of the most versatile elements known to man. It has the ability to form a wide range of compounds, and it is also capable of existing in a variety of allotropes. An allotrope is a different structural form of an element in which the atoms are arranged in a different way. In the case of carbon, there are several allotropes that exist, each with its own unique properties and uses.
In this article, we will explore the various allotropes of carbon, including their structures and properties.
Diamond
Diamond is one of the most popular and valuable allotropes of carbon. Its unique properties, such as extreme hardness and high refractive index, make it useful for industrial applications such as cutting, drilling, and grinding, as well as for jewelry. The crystal structure of diamond is a face-centered cubic lattice, with each carbon atom covalently bonded to four others in a tetrahedral geometry. This structure results in a 3-dimensional network of six-membered carbon rings that form a zero bond angle strain, making diamond extremely strong.
Most mined diamonds are not suitable for use as gemstones because of their industrial quality. Nevertheless, they are valuable because of their hardness and thermal conductivity. Specialized uses of industrial diamonds include high-pressure experiments, high-performance bearings, and engineering tools. Research is being conducted into new uses for diamonds, such as semiconductors for microchips and heat sinks in electronics. Synthetic diamonds are also used in industry.
Despite its high cost, diamond remains a valuable and useful allotrope of carbon, with numerous industrial applications and ongoing research to explore its potential uses.
Graphite
Graphite is a common allotrope of carbon, named after its use in pencils. Unlike diamond, it is an electrical conductor due to the delocalization of pi bond electrons above and below the carbon atom planes. This makes it useful in electrical arc lamp electrodes and thermochemistry as a standard state for defining the heat of formation of carbon compounds. Graphite conducts electricity only along the planes of carbon atoms and not in a direction at right angles to the plane.
Graphite powder is used as a dry lubricant due to adsorbed air and water between the layers, but in a vacuum environment, it is a poor lubricant. When crystallographic defects bind the planes together, graphite loses its lubrication properties and becomes pyrolytic carbon, a useful material in blood-contacting implants such as prosthetic heart valves. Graphite is the most stable allotrope of carbon and is used in nuclear reactors, high-temperature crucibles, and as reentry shields for missile nosecones.
Pyrolytic graphite and carbon fiber graphite are extremely strong, heat-resistant materials used in solid rocket engines, high-temperature reactors, brake shoes, and electric motor brushes. Intumescent or expandable graphites are used in fire seals, and during a fire, the graphite intumesces (expands and chars) to resist fire penetration and prevent the spread of fumes.
Graphite’s specific gravity is 2.3, making it lighter than diamond, and it is slightly more reactive than diamond due to the reactants being able to penetrate between the hexagonal layers of carbon atoms. Graphite is unaffected by ordinary solvents, dilute acids, or fused alkalis, but chromic acid oxidizes it to carbon dioxide.
Graphene
Graphite is an allotrope of carbon, which is a relatively soft material composed of layers of carbon atoms arranged in a hexagonal lattice structure. Graphene, on the other hand, is a single layer of graphite and has extraordinary electrical, thermal, and physical properties. It can be produced by epitaxy on an insulating or conducting substrate or by mechanical exfoliation (repeated peeling) from graphite.
Graphene has the potential to replace silicon in high-performance electronic devices due to its unique properties. It has excellent electrical conductivity, making it ideal for use in electronics and optoelectronics. It also has excellent thermal conductivity, which makes it useful in thermal management applications. Additionally, it is extremely strong, yet lightweight, making it ideal for use in structural materials.
Bilayer graphene, which consists of two graphene layers stacked on top of each other, has different properties than single-layer graphene. It exhibits different electronic properties and has an energy band gap that can be tuned by applying an external electric field. Bilayer graphene also has potential applications in areas such as spintronics, energy storage, and sensing.
Lonsdaleite
Lonsdaleite, also known as “hexagonal diamond,” is a rare carbon allotrope that forms from graphite in meteorites upon impact with Earth. It retains the hexagonal crystal lattice of graphite, but has a denser structure similar to diamond due to the heat and pressure of the impact. This allotrope can also be synthesized in the laboratory by compressing and heating graphite or by thermally decomposing a polymer at atmospheric pressure under an inert gas atmosphere.
Graphenylene
Graphenylene, also known as biphenylene-carbon, is a single layer carbon material with biphenylene-like subunits forming its hexagonal lattice structure.
Carbophene
Carbophene is a two-dimensional covalent organic framework composed of 4-carbon and 6-carbon rings in a 1:1 ratio. It was synthesized from 1-3-5 trihydroxybenzene and has bond angles of about 120°, 90° and 150° between its three σ-bonds.
Diamane
Diamane is a 2D form of diamond that can be made with high pressure or by adding hydrogen atoms, but these bonds are weak. F-diamane, created using fluorine, brings the layers closer together, making the bonds stronger.
Amorphous carbon
Amorphous carbon lacks any crystalline structure, and while some short-range order is present, there is no long-range pattern of atomic positions. It may contain small crystals of graphite-like or diamond-like carbon. Coal, soot, and carbon black are not true amorphous carbon, but rather the products of pyrolysis. This process does not typically produce true amorphous carbon under normal conditions.
Nanocarbons
Nanocarbons are a class of carbon-based materials that have at least one dimension in the nanoscale range, typically less than 100 nanometers. This includes fullerenes, carbon nanotubes, and graphene, which are all unique in their properties and potential applications.
Buckminsterfullerenes
Scientists from Rice University and the University of Sussex discovered buckminsterfullerenes, also known as fullerenes or buckyballs, in 1985. These positively curved carbon molecules resemble the geodesic structures of Richard Buckminster “Bucky” Fuller. Fullerenes are being heavily studied for their chemical and physical properties, and have potential for medicinal use, such as binding antibiotics to target resistant bacteria and specific cancer cells like melanoma. Research in both pure and applied labs continues to explore the potential applications of fullerenes.
Carbon nanotubes
Carbon nanotubes are cylindrical carbon molecules with unique properties that make them potentially useful in various fields. They are highly robust, exhibit exceptional electrical properties, and are efficient conductors of heat. These properties make them ideal for applications in nano-electronics, optics, and materials science. The name “nanotube” is derived from their size, with a diameter on the order of a few nanometers, which is about 50,000 times smaller than the width of a human hair. The length of a nanotube can range from a few nanometers to several centimeters.
There are two main types of carbon nanotubes: single-walled nanotubes (SWNTs) and multi-walled nanotubes (MWNTs). SWNTs consist of a single graphene sheet that is rolled into a seamless cylinder, while MWNTs consist of multiple concentric graphene cylinders. Carbon nanotubes are a member of the fullerene family, which also includes buckyballs, and they are synthesized using various techniques. With their unique properties, carbon nanotubes have the potential to revolutionize various fields and advance our understanding of materials science.
Carbon nanobuds
Carbon nanobuds are a recently discovered allotrope of carbon in which fullerene-like “buds” are attached to the outer walls of carbon nanotubes. This hybrid material combines the useful properties of both fullerenes and carbon nanotubes, making it an excellent field emitter. The covalent bonding between the buds and nanotubes results in exceptional mechanical and electrical properties, making carbon nanobuds a promising material for various applications.
Schwarzites
Schwarzites are negatively curved carbon surfaces that are created by introducing ring defects into graphene’s hexagonal lattice. Recent research has suggested that zeolite-templated carbons (ZTCs) may be schwarzites. ZTCs are formed by injecting a vapor of carbon-containing molecules into the pores of a zeolite, where the carbon accumulates on the pores’ walls, creating a negative curve. Dissolving the zeolite leaves the carbon, resulting in structures that resemble schwarzite-like structures. These findings could have significant implications for materials science and the development of new technologies.
Glassy carbon
Glassy carbon, also known as vitreous carbon, is a type of non-graphitizing carbon commonly used as an electrode material in electrochemistry, high-temperature crucibles, and prosthetic devices. It was first produced in the mid-1950s by Bernard Redfern. Vitreous carbon is produced by subjecting organic feedstocks to a series of heat treatments at temperatures up to 3000°C. It is impermeable to gases and chemically inert, making it highly resistant to acids. Concentrated sulfuric and nitric acids do not attack vitreous carbon even after several months.
Carbon nanofoam
In 1997, Andrei V. Rode and colleagues discovered the fifth allotrope of carbon, carbon nanofoam. It is made up of low-density clusters of carbon atoms arranged in a loose three-dimensional web. Each cluster is approximately 6 nanometers wide and contains roughly 4000 carbon atoms linked in graphite-like sheets, which are negatively curved due to the inclusion of heptagons. Carbon nanofoam has a similar structure to aerogels but has a density only a few times greater than that of air at sea level. However, it is a poor electrical conductor unlike carbon aerogels.
Carbide-derived carbon
Carbide-derived carbon (CDC) is a group of carbon materials produced by selectively removing metals from metal carbide precursors through various synthesis methods. This produces different surface geometries and carbon ordering, resulting in a range of carbon allotropes such as carbon nanotubes, onion-like carbon, and graphitic ribbons. These materials have high porosity and specific surface areas, making them promising for various applications including energy storage, water filtration, and catalyst support. CDC structures offer tunable pore diameters and are useful for cytokine removal.
Linear acetylenic carbon
Linear acetylenic carbon is a one-dimensional carbon polymer with a unique structure —(C≡C)n—. It is formed by the repetitive linking of carbon atoms through triple bonds, resulting in a long chain-like structure. This carbon allotrope has potential applications in electronics, optics, and energy storage. Its unique electronic and optical properties make it an interesting material for research and development in these areas. Linear acetylenic carbon is a relatively new discovery and is still being studied to understand its full range of properties and potential applications.
Cyclocarbons
In 2019, scientists successfully synthesized a new carbon allotrope called cyclo[18]carbon (C18), consisting of 18 carbon atoms arranged in a cyclic pattern. This was a significant achievement as previously, the existence of such a carbon allotrope had only been theorized. The successful synthesis of C18 provides new insights into the chemistry of carbon and opens up new possibilities for the development of new materials with unique properties.
There are numerous other allotropes that have been theorised but have not yet been synthesised, including bcc-carbon, bct-carbon, Chaoite, D-carbon, and M-carbon.
Understanding the properties and potential applications of these different carbon allotropes is important for advancing our understanding of materials science and for developing new technologies that can benefit society.