Buckminsterfullerene (or buckyball) is a spherical fullerene molecule with the formula C60. It is a cage-like fused-ring structure which resembles a soccer ball, made of twenty hexagons and twelve pentagons, with a carbon atom at the vertices of each polygon and a bond along each polygon edge.
It was first intentionally prepared in 1985 by Harold Kroto, James R. Heath, Sean O’Brien, Robert Curl and Richard Smalley at Rice University. Kroto, Curl and Smalley were awarded the 1996 Nobel Prize in Chemistry for their roles in the discovery of buckminsterfullerene and the related class of molecules, the fullerenes. The name is an homage to Buckminster Fuller, as C60 resembles his trademark geodesic domes. Buckminsterfullerene is the most common fullerene molecule in terms of natural occurrence, as it can be found in small quantities in soot. Solid and gaseous forms of the molecule have been detected in deep space.
Buckminsterfullerene is the largest matter to have been shown to exhibit wave–particle duality. Its discovery led to the exploration of a new field of chemistry, involving the study of fullerenes.
Buckminsterfullerene’s name derives from the name of the noted futurist and inventor Buckminster Fuller. One of his designs of a geodesic dome structure bore a great resemblance to C60; as a result, the discoverers of the allotrope named the newfound molecule after him. The general public, however, sometimes refers to buckminsterfullerene, and even Mr. Fuller’s dome structure, as buckyballs.
Discovery of C60
C60 was discovered in 1985 by Robert Curl, Harold Kroto and Richard Smalley. Using laser evaporation of graphite they found Cn clusters (where n>20 and even) of which the most common were C60 and C70. For this discovery they were awarded the 1996 Nobel Prize in Chemistry. The discovery of buckyballs was quite surprising, as the scientists aimed the experiment at producing carbon plasmas to replicate and characterize unidentified interstellar matter. Mass spectrometry analysis of the product indicated the formation of spheroidal carbon molecules.
Synthesis of C60
In 1990, W. Krätchmer and D. R. Huffman’s developed a simple and efficient method of producing fullerenes in gram and even kilogram amounts which boosted the fullerene research. In this technique, carbon soot is produced from two high-purity graphite electrodes by igniting an arc discharge between them in an inert atmosphere (helium gas). Alternatively, soot is produced by laser ablation of graphite or pyrolysis of aromatic hydrocarbons. Fullerenes are extracted from the soot using a multistep procedure. First, the soot is dissolved in appropriate organic solvents. This step yields a solution containing up to 75% of C60, as well as other fullerenes. These fractions are separated using chromatography. Generally, the fullerenes are dissolved in hydrocarbon or halogenated hydrocarbon and separated using alumina columns.
Properties of C60
The structure of a buckminsterfullerene is a truncated icosahedron with 60 vertices and 32 faces (20 hexagons and 12 pentagons where no pentagons share a vertex) with a carbon atom at the vertices of each polygon and a bond along each polygon edge. The van der Waals diameter of a C60 molecule is about 1.01 nanometers (nm). The nucleus to nucleus diameter of a C60 molecule is about 0.71 nm. The C60 molecule has two bond lengths. The 6:6 ring bonds (between two hexagons) can be considered “double bonds” and are shorter than the 6:5 bonds (between a hexagon and a pentagon). Its average bond length is 0.14 nm. Each carbon atom in the structure is bonded covalently with 3 others.
The C60 molecule is extremely stable, being able to withstand high temperatures and pressures. The exposed surface of the structure is able to react with other species while maintaining the spherical geometry. The hollow structure is also able to entrap atoms and small molecules, which do not react with the fullerene molecule.
Solution form of C60
Fullerenes are sparingly soluble in many aromatic solvents such as toluene and others like carbon disulfide, but not in water. Solutions of pure C60 have a deep purple color which transforms into brown upon drying off the solvent. The reason for this color change is the relatively narrow energy width of the band of molecular levels responsible for green light absorption by individual C60 molecules. Thus individual molecules transmit some blue and red light light resulting in a purple color. Upon drying, intermolecular interaction results in the overlap and broadening of the energy bands, thereby eliminating the blue light transmittance and causing the purple to brown color change.
Solubility of C60 in some solvents shows unusual behavior due to existence of solvate phases (analogues of crystallohydrates). For example, solubility of C60 in benzene solution shows maximum at about 313 K. Crystallization from benzene solution at temperatures below maximum results in formation of triclinic solid solvate with four benzene molecules C60·4C6H6 which is rather unstable in air. Out of solution, this structure decomposes into usual fcc C60 in a few minutes. At temperatures above solubility maximum the solvate is not stable even when immersed in saturated solution and melts with formation of fcc C60. Crystallization at temperatures above the solubility maximum results in formation of pure fcc C60. Millimeter-sized crystals of C60 and C70 can be grown from solution both for solvates and for pure fullerenes.
Solid form of C60
In a solid, buckminsterfullerene molecules normally stick together via the van der Waals forces; however, exposure to light or oxygen can result in their dimerization and polymerization. At low temperatures they are arranged in a simple cubic structure and locked against rotation. Upon heating, they start rotating at about −20 °C that results in a first-order phase transition to a face-centered cubic (fcc) structure and a small, yet abrupt increase in the lattice constant from 0.1411 to 0.14154 nm.
C60 solid is as soft as graphite, but when compressed to less than 70% of its volume it transforms into a superhard form of diamond (see aggregated diamond nanorod). C60 films and solution have strong non-linear optical properties, particularly, their optical absorption increases with the light intensity (saturable absorption).
Applications of C60
C60 molecules can encage and transport atoms and molecules (e.g. radioactive labels) through the human body. For instance, lanthanum carbide (LaC2), which reacts very strongly with water vapor and oxygen and rapidly degrades in air, has been successfully protected inside C60 molecules for more than six months.
In the medical field, elements such as helium (that can be detected in minute quantities) can be used as chemical tracers in impregnated buckyballs. Buckminsterfullerene could also inhibit the HIV virus. The C60 molecule could block the active site in a key enzyme in the human immunodeficiency virus known as HIV-1 protease; this could inhibit reproduction of the HIV virus in immune cells. Experiments suggest that C60 incorporated with the alkali metals can possess catalytic properties resembling those of platinum.
The C60 molecule can also bind large numbers of hydrogen atoms (up to one hydrogen for each carbon) without disrupting the structure. This property suggests that C60 may be a better storage medium for hydrogen than metal hydrides (currently regarded as the best material for that purpose), and hence a key factor in the development of a new class battery or even non-polluting automobiles based on fuel cells, lighter and more efficient than lead-acid batteries.
The optical absorption properties of C60 match solar spectrum that favors C60-based films for photovoltaic applications. Conversion efficiencies up to 5.7% have been reported in C60-polymer cells.