By Jovin Ho, VI Form

A Study of Fullerenes

A fullerene is an allotrope of carbon which denotes a series of carbon molecules that form a multitude of different shapes including hollow spheres (Buckminsterfullerenes) or cylindrical tubes (carbon nanotubes). Buckminsterfullerenes (or Buckyballs) were the first type of Fullerene whose structure was determined. In 1980, Sumio Iijima examined an electron microscope image, and found a cluster of carbon molecules which formed the core of a “bucky onion.”^[1]Buckyballs are a chemical compound with the formula C60 and comprises of carbon atoms arranged in a hollow cage-like ball shape (truncated icosahedron) – not dissimilar to a soccer ball – and made up of twelve pentagons and twenty hexagons. The existence of the Buckyball had originally been predicted in 1970 by Eji Osawa after observing the structure of corannulene (C20H10) molecule which appeared to be a fragment of the full soccer ball shape that Buckyballs take.

It took until 1985, however, before Buckyballs were synthesized for the first time. British astronomer Harold Kroto had independently come to discover the existence of Fullerenes by observing “Red Giants” – stars that have exhausted the supply of hydrogen in their cores and have begun undergoing thermonuclear fusion of hydrogen in an outer shell – which emitted long chains of carbon atoms. Kroto alongside American scientists Richard Smalley and Robert Curl, simulated the heat and pressure of Red Giants in order to manufacture these long carbon chains. They put carbon in a helium-filled chamber and vapourized it with a laser, resulting in the creation of carbon molecules with exactly 60 carbon atoms each which were roughly arranged in the soccer ball shape. These newly manufactured molecules of carbon were named Buckyballs after their resemblance to the “geodesic” domes invented by architect Buckminster Fuller. Kroto alongside his partners received the 1996 Nobel Prize for Chemistry for their accomplishment.

Following the first synthesis of Buckyballs, the manufacturing of other forms of fullerene followed, with carbon nanotubes being discovered and synthesized in 1991. It was later observed that fullerenes were naturally occurring in very small quantities, found in soot and formed by lightning discharges in the atmosphere as well as being found in a family of minerals called Shungites. Buckyballs were also discovered in a cloud of cosmic dust surrounding a star 6500 light years away after scientists spotted the molecule’s infrared signature through NASA’s Spitzer telescope providing evidence that Buckyballs have been in existence for incredibly long periods of time. This prompted astronomer Letizia Stanghellini to thereoticize that “It’s possible that buckyballs from outer space provided seeds for life on Earth.^[2]

Structure

Fullerenes are allotropes of carbon. Other examples of carbon allotropes include diamonds, in which carbon atoms are arranged in a tetrahedral structure and held together by strong electrostatic forces of attraction, and graphite, in which carbon atoms are bonded to three other carbon atoms in a hexagonal tessellation and these layers of carbon atoms are stacked upon each other held by van der waals forces. The 60 carbon atoms in Buckyballs are arranged in closed shells, in the structure of a truncated icosahedron – a solid with 12 regular pentagon faces, 20 regular hexagonal surfaces, containing 60 vertices and 90 edges.

Each carbon atom in a Buckyball is covalently bonded to three other carbon atoms (similar to graphite) and has an orbital hybridization of sp2. There are two distinct bond lengths found in Buckyballs, those found between two hexagonal rings – which can be considered double bonds – as well as those found between a hexagonal ring and a pentagonal ring – which are longer than the aforementioned bonds. The presence of double bonds are only located within hexagonal rings rather than in pentagonal rings as Buckyballs are not superaromatic (due to its localized pi-electron system), meaning it has poor electron delocalisation^[3]. Due to the way that the atoms are bonded within the Buckyball, the allotrope is not very reactive and are fairly insoluble. Buckyballs have a distinct infrared signature, with four IR active vibrational bands as a result of its icosahedral (a polygon with 60 rotational symmetries and a symmetry order of 120) symmetry. Scientists have used this infrared signature in order to identify the presence of naturally-occuring Buckyballs in soot as well as in the clouds of cosmic dust of stars.

Another useful fullerene are carbon nanotubes. The structure of carbon nanotubes is akin to a sheet of graphene rolled into a tube. Graphene is a single atom layer of graphite, with carbon atoms covalently bonded in a hexagonal or honeycomb lattice with each atom having a hybridization of sp2.^[4]The carbon-carbon bonds found in these nanotubes can be parallel or perpendicular to the axis of the nanotube. Nanotubes that has carbon-carbon bonds that are parallel to the tube axis are named “zig-zag” tubes. Those with carbon-carbon bonds perpendicular to the tube axis are denoted “armchair” tubes. At the same time, the carbon-carbon can also not be either parallel or perpendicular which changes the orientation of the hexagons as they spiral around the nanotube.^[5]These tubes are referred to as chiral (meaning they have a distinct mirror image) with left and right-handed variants.

Properties

Buckminsterfullerenes, as a result of not being “superaromatic” (extra stable nature of cyclic or planar molecules), behave similarly to an electron deficient alkene and reacts readily with electron rich species such as halogens. In general, BuckyBalls are very stable molecules, which are able to withstand high temperatures and pressures. The exposed surface of the truncated icosahedron-structure of BuckyBalls is able to react with the aforementioned electron rich species while still retaining the spherical geometry of the original BuckyBall. The hollow interior enables the entrapment of most if not all elements within it without reacting with the fullerene molecule. BuckyBalls are held together through Van der Waals forces.^[6]C60 can be doped to insulate, conduct, semiconduct or superconduct electricity through the addition of alkali or alkaline-earth metals. Electrons are donated to the fullerene molecules to fill the molecular orbital derived bands. When these bands are half filled, the BuckyBalls gain the properties of  superconductors. ^[7]

Carbon nanotubes by default are good conductors of electricity but the extent of this conductivity is determined by the manner of which the nanotube is structured. The sp2 hybridization of carbon atoms and the strength of the covalent bonds between each atom and three neighbouring atoms results in nanotubes exhibiting very high levels of tensile strength and flexibility. Carbon nanotubes can exhibit superconductivity below 20 K due to the strong in-plane carbon-carbon bonds of graphene, while also being a good conductor of heat.^[8]

Manufacturing Process

The original process utilised by Kroto and his team entailed the usage of lasers to vaporize carbon in an inert atmosphere, this method produced very minimal amounts of fullerenes which were unsuitable for commercial or industrial purposes. A new apparatus to vaporize carbon was invented in 1990 by German scientists Kratschmer and Huffmann.^[9]Helium gas is introduced into a bell-jar and purged repeatedly, before finally filling the bell-jar with 100 Torr of helium which is then vaporized in an electric arc between two carbon electrodes in a process that takes between 10 to 15 seconds. The energy from this arc is dissipated by breaking carbon from the surface, which then cools in the helium to form buckyballs. What remains in the bell-jar is soot which contains fullerenes and is comprised of about 10% buckminsterfullerene. Carbon nanotubes are deposited on the electrode. To separate the fullerenes from the unwanted soot, the fullerenes are dissolves in a small amount of toluene. Obtaining pure C60 (BuckyBalls) is achieved through liquid chromatography with solvent being activated charcoal mixed with silica gel. C60 has a magenta pigmentation versus the red colouring of C70.

Applications of Fullerenes

Fullerenes are currently the most efficient acceptor component in solar cells due to their high electron affinity and their ability to transport charge. This high electron affinity is due to Fullerenes having triply-degenerate (contains two or more different measurable states of a quantum system) low-lying lowest unoccupied molecular orbitals. BuckyBalls have demonstrated the ability to stabilize negative charge as the molecule is able to be reduced with up to six electrons (reversible).^[10]The efficiency of solar cells which are composed of a blend of conjugated polymer and fullerene has exceeded 9%, making it viable for commercial usage.^[11]

Fullerenes are the only allotrope of carbon that can be hydrogenated and de-hydrogenated reversibly as a result of its molecular structure. The carbon double bonds become single carbon bonds and form carbon-hydrogen bonds when hydrogenated. The bond strength of carbon-hydrogen bonds are weaker than those of carbon-carbon bonds. When heated, the carbon-hydrogen bonds will break first, successfully de-hydrogenating the molecule and at the same time, preserving the structure of the fullerene. Furthermore, hydrogenation of C60 is thermodynamically favoured, as the heat of formation for the hydrogenated molecule is lower than that of the unhydrogenated BuckyBall. Hence, BuckyBalls can potentially be used as a hydrogen gas storage device. The benefit over current storage devices such as compressed gas or metal hydrides would be that fullerenes would be less volatile and have greater storage capacity.^[12]

The small size and high reactivity of fullerenes allow them to strengthen metals and alloys without severely compromising their ambient temperature ductility. There also has been recent developments utilising fullerenes in sensors. The films of fullerene accept electrons and changes when the film surface interacts with planar molecules. The high electron affinity allows molecular conduction (electron transport) through a single molecule under specific experimental conditions. When ultraviolet light is applied to fullerenes acting as molecular wires, the fullerene molecules become excited and electrons move from one end of the wire towards the excited fullerene molecules.

Fullerene has potential applications in the medical field. The cage-like structure of BuckyBalls in particular alongside with the molecule’s versatility make them a possible therapeutic agent. The size of fullerenes enable them to fit inside the cavities of HIV proteases, inhibiting substrates from reaching the catalytic site of the enzyme. Fullerenes can be used to cleave DNA. When fullerenes are exposed to light, they can produce singlet oxygen in high yields which in conjunction with its ability to transfer electrons from excited fullerene molecules to DNA bases enable it to cleave DNA. Another way the cavity in BuckyBalls is taken advantage of is through utilizing it to carry drugs and other medicine through the human body.^[13]

Carbon nanotubes are usually used for their very high electrical conductivity, heat conductivity and its mechanical properties. Current applications of carbon nanotubes include its usage as field emitters, due to its great electrical conductivity (and high, but stable, current density) and the sharpness of its tips which creates concentrated electric fields and emits at low voltages. By combining carbon nanotubes with plastics, it overcomes the electrical conductivity deficiency of standard plastics, making them suitable replacement for metals. Nanotubes are also used in fuel cells, due to having large surface area, their conductivity, and its perfect linear geometry which enables its surface to be easily accessed be the electrolyte. There is also a place for carbon nanotubes in the medical fields, which cells being shown to grow on the tubes while while sticking to the surface, which may allow nanotubes to be used as a coating for prosthetics and ships. In addition, versatility of carbon nanotubes allow it to be modified to fit a wider variety of purposes.^[14]

Future of Fullerenes

Despite the great progress that has been made in the field of fullerenes in the past decades, fullerenes remain a relatively untapped field and while its potential is remarkable, it will take significantly more headway before fullerenes become integrated in day-to-day services. Currently, a barrier to the adoption of fullerene is partly due to the manufacturing process. While significant amounts can be produced for experimentation and study, large-scale commercial production still proves a significant issue. However, as the methods continue to mature, it is possible that fullerenes will be mass produced in the near future. The applications of fullerenes will also continue to develop, with solar cell efficiency continuing to increase and further advances in the medical field inevitable, it seems like fullerenes are primed to take a significant role in transforming the way many of current services work.

^[1]Iijima, S (1980). “Direct observation of the tetrahedral bonding in graphitized carbon black by high resolution electron microscopy”. Journal of Crystal Growth. 50 (3): 675–683. Bibcode:1980JCrGr..50..675I. doi:10.1016/0022-0248(80)90013-5.

^[2] Atkinson, Nancy (27 October 2010). “Buckyballs Could Be Plentiful in the Universe”. Universe Today. Retrieved 10 January 2018.

^[3]http://www.ch.ic.ac.uk/local/projects/unwin/Fullerenes.htmlRetrieved 18 January 2018

^[4]http://phycomp.technion.ac.il/~talimu/structure.htmlRetrieved 18 January 2018

^[5]https://www.azonano.com/article.aspx?ArticleID=983Retrieved 18 January 2018

^[6]https://www.azom.com/article.aspx?ArticleID=3499Retrieved 18 January 2018

^[7]https://www.sciencedirect.com/science/article/pii/002236979290160FRetrieved 18 January 2018

^[8]Carbon Nanotube: Properties and Applications, Springer Retrieved 18 January 2018

^[9]https://link.springer.com/chapter/10.1007/0-306-47621-5_4Retrieved 18 January 2018

^[10]Fullerenes and their Applications in Science and Technology; Introduction to Nanotechnology, Spring 2013, Eulises Ulloa; Retrieved 19 January 2018

^[11]Polymer:fullerene bulk heterojunction solar cells; materialstoday, Volume 14, Issue 10, October 2011; Retrieved 19 January 2018

^[12]Molecular hydrogen storage in fullerenes – A dispersion-corrected density functional theory study; International Journal of Hydrogen Energy, Volume 41, Issue 30, 10 August 2016, Retrieved 19 January 2018

^[13]Medicinal applications of fullerenes; International Journal of Nanomedicine, December 2007, Retrieved 19 January 2018

^[14]https://www.azonano.com/article.aspx?ArticleID=980, Retrieved 19 January