Fullerene

Fullerene is nothing but an allotrope of carbon wherein its molecules consist of carbon atoms that are connected by single and double bonds. This results in the formation of a closed or partially closed cage-like structure (a mesh consisting of fused rings) that further contain several atoms. The fullerene molecule in this form can either have a hollow sphere, be it an ellipsoid, a tube, or it can also have many other different shapes and sizes. When the carbon molecules are arranged in a cylindrical form they usually form a tube-like structure known as carbon nanotubes.

Structure of Fullerene

Fullerenes in their natural form tend to be highly symmetrical. Their structure is quite similar to that of graphite and is made up of a sheet of connected hexagonal rings (cage structure). However, they have pentagonal and sometimes heptagonal rings which do not allow the sheet to become planar. They are often referred to as buckyballs and buckytubes depending on their shape. Cylindrical fullerenes are referred to as nanotubes. 

Model of the C60 fullerene (buckminsterfullerene)

In any case, there is an infinite number of fullerenes that can exist. Fullerenes can be C₆₀, C₇₀, C8₀, and C9₀ existing in various forms. This depends largely on the number of carbon atoms present in the molecule. Nonetheless, their structure is primarily based on pentagonal and hexagonal rings that are constructed as per the rules for making icosahedra.

Properties of Fullerene

Let us look at some of the physical and chemical properties of fullerene.

Physical Properties of Fullerene

• Its behaviour and structure depend on the temperature. As the temperature is increased fullerene gets converted into the C₇₀. 
• The structure of fullerenes can change under different pressures.
• Fullerene has an ionization enthalpy of 7.61 electron volts.
• Its electron affinity is 2.6 to 2.8 electrons volts.

Chemical Properties of Fullerene

• Fullerenes are stable, but not totally unreactive.
• In chemical reactions, fullerene can act as an electrophile.
• It acts as an electron-accepting group and is characterised as an oxidizing agent.
• Fullerenes when doped or crystallized with alkali or alkaline earth metals showcases superconductivity properties.
• Fullerene is ferromagnetic.
• Some fullerenes are inherently chiral.
• It is soluble in organic solvents such as toluene, chlorobenzene, and 1,2,3-trichloropropane.

Types of Fullerene

Fullerenes exist in two major families depending on some distinct properties and applications. The two families include the closed buckyballs and the open-ended cylindrical carbon nanotubes. However, there are also hybrid structures that can exist between these two classes. They can be carbon nanobuds which are basically nanotubes capped by hemispherical meshes or larger “buckybuds”.

Buckyball Clusters

These fullerenes are basically unsaturated versions of dodecahedra. These are also some of the smallest members of the fullerene group. Its structural formula is C₂₀.

Buckminsterfullerene

Buckminster-fullerene is the smallest fullerene molecule. It contains pentagonal and hexagonal rings and no two pentagons will share an edge. Buckminster-fullerene exists in C₆₀ form is the most common in terms of natural occurrence. The structure of buckminsterfullerene is a truncated icosahedron similar to that of a football.

Linked Ball and Chain Dimer: In this type of fullerene, two buckyballs are mainly linked by a carbon chain. There are also other fullerenes such as heterofullerenes, which have heteroatoms substituting carbons in a cage or tube-shaped structures. Then there is metallofullerene whose molecule is composed of a metal atom trapped inside a fullerene cage.

Carbon Nanotubes

This type of fullerene has cylindrical or hollow tubes of very small dimensions; they are mostly a few nanometers wide. However, they can also be micrometre to several millimetres in length. Talking about some of the properties of carbon nanotubes, they can be both closed and open-ended. As a result of their unique molecular structure, they have some unique macroscopic properties. Some of them include high tensile strength and electrical conductivity, high ductility and heat conductivity including relative chemical inactivity. Another form that we find is the megatubes. These are slightly bigger in diameter than nanotubes and are generally found with walls of different thicknesses. 

Production of Fullerene

Fullerene preparation starts with the production of fullerene-rich soot. The method that was used to produce fullerene involved sending a large electric current between two nearby graphite electrodes in an inert atmosphere. The electric arc that was created vaporized the carbon into a plasma that then cooled into the sooty residue. Alternatively, the soot can also be produced by laser ablation of graphite or pyrolysis of aromatic hydrocarbons. Combustion is the most efficient process that was used.

These methods usually resulted in the production of different fullerenes in various mixtures and other forms of carbon. So the fullerenes are then basically extracted from the soot using appropriate organic solvents. Usually, the mixture is separated by chromatography.

Uses of Fullerene

Buckyballs and carbon nanotubes have been used as building blocks for a great variety of derivatives and larger structures.

• Fullerenes are used in the medical field as light-activated antimicrobial agents.
• It is also used in several biomedical applications including the design of high-performance MRI contrast agents, X-ray imaging contrast agents, photodynamic therapy and drug and gene delivery.
• Buckminsterfullerene is used in drug delivery systems, in lubricants and as a catalyst.
• It is also used as a conductor.
• Some types of fullerene can be used as an absorbent for gases.
• It is used in making cosmetic products.
• C₆₀  based films are used for photovoltaic applications.
• Fullerenes are used in making carbon nanotubes-based fabrics and fibres.

Fullerenes Video Lesson