ALLOTROPES OF CARBON: AMORPHOUS CARBON, GRAPHITE, GRAPHENES, NANOTUBES - CHEMISTRY - 2025

The allotropes of carbon are different physical forms sortable and bind their atoms. Each one corresponds to a solid with its own special characteristics. Molecularly and structurally they are distinguished from each other. There are two main types of these allotropes: crystalline and amorphous.

Crystalline allotropes are those that have a repeating pattern of their atoms in space. Meanwhile, in amorphous allotropes, the atoms are arranged disorderly, without there being two identical regions in the solid. So the former are ordered, and the latter are disordered.

Main allotropes of carbon. Source: Jozef Sivek

Among the crystalline ones are diamond (a) and graphite (e) par excellence. It is observed in the upper image different structures which have a common aspect: they are composed only of carbon atoms (black spheres).

And among the amorphous allotropes, we have the amorphous carbon (b), which, as can be seen, its structure is disorderly. However, there are many types of amorphous carbons, so it is a family of solids.

Also, carbon atoms can form supramolecules, such as fullerenes (c) and nanotubes (d). These supramolecules can vary in size and shape, but they retain the same geometries; spherical and tubular for fullerenes and nanotubes, respectively.

Covalent bonds of carbon

Before addressing some of the known allotropes of carbon, it is necessary to review how carbon atoms are bonded.

According to the valence bond theory, carbon has four electrons in its valence shell, with which they form covalent bonds. Thanks to electronic promotion and hybridization, the four electrons can be placed in four separate orbitals, be they pure or hybrid.

Therefore, carbon has the ability to form up to a maximum of four bonds.

DC. With four CC bonds, the atoms reach the valence octet, and they become very stable. However, this is not to say that there cannot be only three of these links, such as those seen in hexagons.

Hexagons

Depending on the hybridizations of the carbon atom, double or triple bonds can be found in the structure of their respective allotropes. But, even more evident than the existence of such bonds, is the geometry that the carbons adopt.

For example, if a hexagon is observed, it means that the carbons have sp ² hybridization and therefore have a pure p orbital with a lone electron. Can you see perfect hexagons in the first image? Those allotropes that contain them imply that their carbons are sp ², whether or not there are double bonds (such as those of the benzene ring).

A mesh, plane or hexagonal layer then consists of sp ² carbons that have an electronic “roof” or “cloud”, a product of the unpaired electron of the p orbital. This electron can form covalent bonds with other molecules, or attract the positive charges of metal ions; like those of K ⁺ and Na ⁺.

Likewise, these electrons allow these shells to stack on top of each other, without bonding (due to a geometric and spatial impediment to the overlap of the two p orbitals). This means that allotropes with hexagonal geometries may or may not be ordered to build a crystal.

Tetrahedra

If a tetrahedron is observed, as will be explained in the last section, it means that the carbons have sp ³ hybridization. In them there are four simple CC bonds, and they form a tetrahedral crystal lattice. In such tetrahedra there are no free electrons as there are in hexagons.

Amorphous carbon

Chunks of coal, representative of amorphous carbon. Source: Pxhere.

Amorphous carbon can be imagined as a kind of porous sponge, with lots of arbitrarily arranged hexagonal and tetrahedral networks. In this mineral matrix they can trap other elements, which can compact or expand said sponge; and in the same way, its structural nuclei can be larger or smaller.

Thus, depending on the% carbon, various types of amorphous carbons are derived; such as soot, charcoal, anthracite, carbon black, peat, coke, and activated carbon.

At first glance, they all look remotely similar (top image), with gradations to the edge of black, dull, or metallic and grayish overtones.

Not all amorphous carbons have the same origin. Vegetable carbon, as its name indicates, is the product of the combustion of vegetable masses and wood. While carbon black and coke are products of different stages and conditions of petroleum processes.

Although they do not seem very attractive and it can be believed that they only serve as fuels, the porosities of their solids attract the attention in technological purification applications, as absorbents and substances storage, and also as catalytic supports.

Politypism

The structures of amorphous carbons are complex and disordered; However, crystallographic studies have shown that they are actually tetrahedral (diamond) and hexagonal (graphite) polytypes, arranged arbitrarily in layers.

For example, if T and H are the tetrahedral and hexagonal layers, respectively, then an amorphous carbon can be structurally described as: THTHHTH; or HTHTTHTHHHT, etc. Certain T and H layer sequences define a type of amorphous carbon; but within them, there is no repetitive trend or pattern.

It is for this reason that it is structurally difficult to characterize these carbon allotropes; and instead of that, its% of carbon is preferred, which is a variable that facilitates its differences, as well as its physical properties and its tendency to burn or burn.

Functional groups

It was mentioned that hexagonal planes have an unpaired electron with which it can form a bond with other molecules or atoms. If, say, the surrounding molecules are H ₂ O and CO ₂, OH and COOH groups can be expected to form, respectively. They can also bind to hydrogen atoms, forming CH bonds.

The possibilities are very varied, but in summary amorphous carbons can host oxygenated functional groups. When these heteroatoms are present, they are not only located at the edges of the planes, but also and even inside them.

Graphite

Crystal structure of hexagonal layers of graphite. Source: MartinThoma.

The upper image shows a model with spheres and strings of the crystalline structure of graphite. The shadows of the spheres, fortunately, help to visualize the π clouds product of the delocalization of their unpaired electrons. This was mentioned in the first section, without so many details.

These π clouds can be compared to two systems: that of benzene rings, and that of "electron seas" in metallic crystals.

The p orbitals join each other to build a track where electrons travel freely; but only between two hexagonal layers; perpendicular to them, there is no flow of electrons or current (the electrons would have to pass through the carbon atoms).

As there is a constant migration of electrons, instantaneous dipoles are constantly formed, which induce other dipoles of carbon atoms that are above or below; that is, the layers or sheets of graphite remain united thanks to the London dispersion forces.

These hexagonal layers, as might be expected, create a hexagonal graphite crystal; or rather, a series of small crystals connected at different angles. The π clouds behave as if they were an "electric butter", allowing the layers to slide before any external disturbance on the crystals.

Physical properties

Graphite's physical properties are easy to understand once its molecular structure has been addressed.

For example, the melting point of graphite is very high (higher than 4400ºC), because the energy supplied in the form of heat has to irreversibly separate the hexagonal layers, and also break their hexagons.

It was just said that their layers can slide over each other; And not only, but they can also end up on other surfaces, such as the cellulose that make up the paper when deposited from the graphite of the pencils. This property allows graphite to act as an excellent lubricant.

And, already mentioned, it is a good conductor of electricity, and also of heat and sound.

Graphenes

Graphene sheet without double bonds. Source: Jynto

Although it was not shown in the first image, this carbon allotrope cannot be left out. Suppose the layers of graphite were grasped and condensed into a single sheet, open and covering a large area. If this were done molecularly, graphenes would be born (top image).

So, graphenes is an individual graphitic sheet, which does not interact with others and which can wave like a flag. Note that it bears a resemblance to the walls of honeycombs.

These graphene sheets preserve and multiply the properties of graphite. Its hexagons are very difficult to separate, so they present an abysmal mechanical resistance; even higher than steel. In addition, they are extremely light and thin, and theoretically one gram of them would be enough to cover an entire soccer field.

If you look at the top image again, you can see that there are no double bonds. Certainly there may be them, as well as triple bonds (graffins). It is here that the chemistry of graphene opens, say.

Like graphite and the other hexagonal layers, other molecules can covalently bind to the surface of graphene, functionalizing its structure for electronic and biological applications.

Carbon nanotubes

The three types of carbon nanotubes. Source: Mstroeck via Wikipedia.

Now suppose we grabbed the graphene sheets and started rolling them into a tube; These are carbon nanotubes. The lengths and radius of these tubes are variable, as are their spatial conformations. Together with graphene and fullerenes, these nanotubes make up the triad of the most amazing carbon allotropes.

Structural conformations

Three carbon nanotubes are shown in the upper image. What is the difference between them? All three have hexagonal patterned walls, and exhibit the same surface properties already discussed. The answer then lies in the relative orientations of these hexagons.

The first conformation corresponds to the zigzag type (upper right corner). If carefully observed, it will be appreciated that it has rows of hexagons positioned perfectly perpendicular to the longitudinal axis of the tube.

In contrast, for the armchair-type conformation (lower right corner), the hexagons are arranged in rows in the same direction as the longitudinal axis of the tube. In the first nanotube, the hexagons run across the surface in the sense of its diameter, and in the second nanotube, they run along the surface, from "end to end".

And finally, there is the chiral nanotube (lower left corner). Compare with a spiral staircase going left or right. The same happens to this carbon nanotube: its hexagons are arranged ascending to the left or right. As there are two spatial versions, it is said then that it exhibits chirality.

Fullerenes

C60 fullerene molecule. Source: Benjah-bmm27.

In the fullerenes, the hexagons are still maintained, but in addition, the pentagons appear, all with sp ² carbons. The sheets or layers are already left behind: now they have been folded in such a way that they form a ball, similar to a soccer ball; and depending on the number of carbons, to a rugby ball.

Fullerenes are molecules that differ in size. The most famous is the C ₆₀ (top image). These carbon allotropes should be treated as balloons, which can squeeze together to form crystals, in which ions and other molecules can be trapped within their interstices.

These balls are special carriers or supports for molecules. How? Through the covalent bonds to its surface, especially, to the adjacent carbons of a hexagon. The fullerene is then said to have been functionalized (an exohedral adduct).

Its walls can be strategically broken to store molecules inside; resembling a spherical capsule. Likewise, these balls can have cracks and be functionalized at the same time; everything will depend on the application to which they are intended.

Cubic crystal structure of diamond. Source: GYassineMrabetTalk✉This structure was created with PyMOL..

And finally, the best known of all the allotropes of carbon: diamond (although not all are carbon).

Structurally, it consists of sp ³ carbon atoms, forming four CC bonds and a three-dimensional network of tetrahedra (upper image) whose crystalline cell is cubic. It is the hardest of the minerals, and its melting point is close to 4000ºC.

Their tetrahedra are able to transfer heat efficiently throughout the crystal lattice; but not so with electricity, because its electrons are very well located in its four covalent bonds and it cannot go anywhere. Therefore, it is a good thermal conductor, but it is an electrical insulator.

Depending on how it is faceted, it can scatter light at many bright and attractive angles, which is why they are coveted as gemstones and jewelry.

The network is very resistant, because it would need a lot of pressure to move its tetrahedra. This property makes it a material with high mechanical resistance and hardness, capable of making precise and clean cuts, as with the diamond-tipped scalpel.

Their colors depend on their crystallographic defects and their impurities.

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