The Discovery, Development, and Applications of Fullerenes
Kristen Leskow and Laura Weaver
March 2007
Figure 1. C60 buckyball (Haymet).
History and Discovery:
Until the discovery of fullerenes in 1985, the only known forms of pure carbon were
graphite and diamond. Graphite consists of sheets of carbon atoms bonded hexagonally,
and diamond is a tetrahedral crystal lattice structure, making diamond very hard
(“Buckyballs”). In 1985, British chemist Harry Kroto studied combinations of carbon
atoms that appeared to be primarily molecules with exactly sixty carbons. Kroto
collaborated with Richard Smalley and Robert Curl to recreate in the laboratory the
conditions near the red giant stars where Kroto had detected the carbon molecules. The
scientists could not quickly determine the structure of the C60 molecules, however. They
hypothesized that the molecules must be spherical in nature, due to its high stability
(“Buckyballs”).
Since these carbon-60 molecules could be produced by laser vaporization of graphite,
they believed that individual hexagonal six-carbon rings were blasted apart from the
graphite structure, and these rings collide with each other and form an equilibrium
structure. The most common equilibrium structure combined sixty carbons in a unique
arrangement, although seventy-carbon and other structures were also found. Kroto and
his colleagues searched for a geometric arrangement of the sixty carbons based upon
combinations of six-membered rings, and reasoned that the molecule must be spherical in
order for all of the sp2 valences to be satisfied (Kroto).
Eventually, they found that the geometric shape must be a series of interconnected
hexagons and pentagons, such as in the soccer ball (football), with the hexagons in white
leather and the pentagons in black leather. Thinking that the shape might be a spherical
version of the hexagonal sheets of graphite, they tried to make spheres of hexagons.
Finally, they determined that the combination of twelve pentagons and twenty hexagons
formed a perfect ball with sixty vertices. The pentagonal rings sit at the vertices of an
icosahedron.
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The architect R. Buckminster Fuller had designed a geodesic dome for the 1967 Montreal
World Exhibition with the same combination of hexagons and pentagons. Kroto,
Smalley, and their colleagues named the C60 molecule buckminsterfullerene in his honor
(“Buckyballs”). When other spherical carbon molecules were found, the name was
shortened to fullerene to refer to the family of molecules.
Curl, Kroto, and Smalley received the Nobel Prize in 1996 for this discovery.
Independently, near the same time, Tony Haymet of the University of California at
Berkeley published a paper predicting the existence of a compound of this kind
(“Buckyballs”). Based upon the molecule corannulene, a similar nonplanar assembly of
carbons in pentagonal and hexagonal rings, he estimated that such a molecule existed,
which he named “footballene,” with sixty carbons in twelve pentagons and twenty
hexagons (Haymet).
Figure 2. Corannulene molecule (Haymet).
Chemistry:
All sp2 valences are satisfied with the buckyball structure. The hexagonal rings consist of
three single bonds and three double bonds, and the pentagonal rings have only single
bonds.
Figure 3. C60 buckyball bond structure. The red bonds indicate single bonds and the yellow
bonds indicate double bonds (http://www.godunov.com/Bucky/buckyball-3.gif)
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The stability of the molecule is largely determined by the location of the double bonds.
A double bond in a pentagonal ring further shortens the bond length, straining the ring.
The structure shown in Fig. 3 is the only structure for C60 without double bonds in
pentagonal rings. This leads to poor delocalization of electrons, since the limitations on
the location of double bonds limit the movement of electrons, causing the molecule to be
less stable than originally predicted and thus more reactive. Thus C60 is not an aromatic
molecule, but in fact more like a large closed-cage electron-deficient alkene which can
react in similar ways to chain alkenes (Taylor).
Up to six electrons can be added to C60 and C70 fullerenes, creating aromaticity in one of
the pentagonal rings in each paracyclene unit, comprised of two pentagonal rings
separated by a double bond. Thus the anions of C60 and C70 are more stable than the
neutral molecules, which can be used as oxidizing agents in this way. Anions of C60 can
be reacted with methyl iodide to add methyl groups. Adding larger groups to fullerenes
is affected by the bulkiness of the group because of the steric constraints of the molecule.
Additions are possible in the form of cycloadditions, bridging (such as forming epoxides,
carbon bridges, and metallic bridges), and group additions (such as halogenation,
hydrogenation, anionic reaction with electrophiles, addition of neutral and charged
nucleophiles, and radical addition). Up to twenty-four small molecules can be added to
C60 without any two being adjacent, and this number decreases for more bulky additions.
Substitutions and polymerization are also possible (Taylor).
Figure 4. Summary of possible reactions that C60 and C70 can undergo (Taylor).
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Other fullerenes:
Although the C60 buckyball is the most famous fullerene, many other molecules of the
family of interlocking hexagons and pentagons have been discovered. The most
important groups of fullerenes include exohedral fullerenes (with molecules attached to
the external cage), endohedral fullerenes (encaging some other molecule), and
nanopeapods (fullerene molecules enclosed in carbon nanotubes).
Conventional Synthesis:
Kroto, Smalley, and Curl were only able to produce very small quantities of buckyballs.
In 1990, both German and American scientists independently developed a procedure for
making larger quantities of buckyballs. They sent a large current between two graphite
electrodes, resulting in a carbon plasma arc which cooled into soot, in which the
fullerenes could be isolated. The fullerenes in the soot were comprised of about 75% C60,
23% C70, and the rest larger molecules (“Buckyballs”).
The method published by Krätschmer et al for mass production of C60 uses pure carbon
soot produced by evaporating graphite electrodes in helium at about 100 torr. The soot is
gently scraped from the surfaces of the evaporation chamber and dispersed in benzene.
The dissolved material gives rise to C60 and is a wine-red to brown liquid, varying with
concentration. The liquid is separated from the soot and dried with gentle heat,
producing a residue of dark brown to black crystalline material. The material can also be
dissolved in other nonpolar solvents, such as carbon disulfide and carbon tetrachloride.
Krätschmer’s paper also mentions concentrating the material by heating the soot to 400ºC
in a vacuum or inert atmosphere, subliming the C60 out of the soot. The paper
recommends purifying by washing the initial soot with ether before the concentration
procedure is applied. These techniques produce a thin film or powder, which can be
handled without special precautions and which are somewhat stable in air, with some
deterioration after several weeks. The material can be sublimed repeatedly without
decomposition, and at least 100mg of purified C60 material can be produced by one
person per day. The solid produced after the benzene has been evaporated forms a
variety of crystals of packed fullerenes, all with hexagonal symmetry. However, the
diffraction patterns of larger crystals show no clear pattern, indicating that the crystals do
not have long-range periodicity in all directions due to the disorder within the crystal
(Krätschmer).
Subsequent articles provide further details about the Krätschmer-Huffman contact-arc
technique. Carbon electrodes are arced in an atmosphere of helium, typically with an
alternating current. This causes the deposition of soot, from which can be extracted the
fullerenes. The use of a flow-through reactor assembly is helpful for removing the soot
from the arcing zone; the soot generated by arcing is flushed by the helium carrier gas
(Weston). The graphite rod electrode is kept in gentle contact with a graphite disk by an
adjustable spring. The graphite rod and disk are connected to an external alternatingcurrent power source at 60 Hz. The graphite is vaporized by driving current between the
electrodes between 100 and 200 A, with an rms voltage of 10-20 V. The spring tension is
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varied to optimize the contact between the graphite rod and disk in order to ensure that
the bulk of the power is dissipated in the arc as opposed to Ohmic heating of the rod
(Haufler).
Weston and Murthy discuss the optimization of the contact arc method for fullerene
synthesis. Since UV radiation results from the current applied for arcing, removal of the
soot helps to prevent UV decomposition of the fullerenes. Although UV radiation is
proportional to reactor current, lower currents give lower vaporization rates and require
longer arcing times, so the reactor current must be optimized in order to minimize the UV
effect. The helium pressure should be optimized and other gases such as hydrogen and
water vapor should be eliminated. Weston and Murthy report an increased yield with
application of a black coating to the inner reactor surface and with an increased residence
time of the carbon vapor. They recommend using cyclohexane as the solvent for
chromatographic separation of C60 and C70 products for reasons of time, volume, and
toxicity of solvent; C70 is more difficult to elute out than C60 and requires more solvent.
Weston and Murthy report fullerene yields of 15 wt%, and yields of up to 40% with
direct laser vaporization in the single-walled nanotube experiments of Guo et al.
(Weston). Despite these optimization methods for contact arc fullerene synthesis, the
process variables are relatively rigid and can be changed only so much in order to
maximize the product yield. However, the procedure is relatively simple and can be
carried out in many laboratories with minimal requirements.
Selective-Size Synthesis:
The first fullerene discovered was C60, the buckyball, and it is known to be the most
stable of all neutral carbon fullerenes. Since that discovery, research has progressed
toward investigating various sizes of fullerenes to learn more about the compound and
explore more possible applications. Large fullerenes could possibly fit needs that fall in
the gap between classic fullerenes and carbon nanotubes because of their small band
gaps. Small fullerenes may prove to be important for astrophysical implications
(Umeda). The need for varying cage sizes arises with endohedral fullerenes. This family
of fullerenes capture other atoms or molecules within the carbon cage. An important
property that determines the ability of the cage to enclose another molecule is the relative
sizes of the cage and molecules. For example, the larger the molecule encaged, the larger
the cage must be. This has led to an entire area of research and synthesis. There is a
great challenge in the control of the size of the carbon fullerene that is synthesized. In the
conventional arc-discharge method, the final product is a function of the final product’s
stability. C60 and C70 are the most stable fullerenes and therefore will be synthesized
before other fullerenes, which may be the preferred product.
The first successful synthesis of C60 by size-selective flash vacuum pyrolysis (FVP) at
1100°C was accomplished in 2002 by Scott and co-workers. They explain the entire
twelve-step synthesis to form the cholorhydrocarbon, C60H27Cl3, precursor, and the
ensuing pyrolysis to form the carbon 60 fullerene (Scott). This process only provided a
product with a maximum yield of 1%, but showed promise that C60 could be selectively
formed (Umeda).
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Figure 5. First known precursor to produce C60 by FVP.
Where X=Cl. (Umeda)
This result motivated other researchers to look for precursors to size-selectively
synthesize C60. The reason these precursors are so hard to determine and use is the lack
of knowledge of how the fullerene formation mechanism proceeds. Many will agree that
cyclic polyynes are one of the most probable precursors to fullerenes. Two different
research groups, Tobe and Rubin, spent substantial effort attempting to synthesize
carbon-60 molecules through three-dimensional cyclic polyynes. Both groups are trying
to create the closed fullerene molecule by the FVP process. The following synthesis is
the same for both groups where they differ is in the way by which they create the
precursor (Umeda).
Both groups report proudly that one should be able to achieve C60 by their method of
synthesis. However, neither ever produced any feasible amount of carbon 60 product.
They both have formed C60+ or C60- and report this as a breakthrough (Rubin, Tobe).
This may be some proof of the ability to have control over the size of the fullerene by the
precursors, but the ion formation is still very different than the classical fullerene cage.
Since charge transfer can be a very important characteristic of an endo- or exohedral
fullerene, a charge on the carbon-60 cage cannot be ignored. More work must be done on
this synthesis to prove the synthesis actually proceeds as suggested and to synthesize the
desired product, not an ion corresponding to the molecule, due to the importance of
charges in this area of science.
Exohedral Fullerenes Application:
Soon after buckyballs were first discovered, researchers began thinking about how the
cage could be changed or adjusted. A popular use of a fullerene cage is the addition of
molecules to the external cage. A recent application of this group of fullerenes has been
in hydrogen storage.
Zhao et al report that “the U.S. Department of Energy has determined that a hydrogen
storage density of 9 wt% will be required for fuel-cell powered vehicles to able to replace
petroleum-fueled vehicles on a large scale.” They suggest that by using organometallic
molecules based upon C60 to store hydrogen, nearly 9 wt% can be retrieved reversibly at
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room temperature and near ambient pressure. Complexes of transition metals with
hydrogen on pentadiene rings can store up to six dihydrogen species, but the complexes
may polymerize when the hydrogen is removed, rendering the process irreversible.
Arranging the complexes on buckyballs, such as C60[ScH2]12 and C48B12[ScH]12, leads to
stable species which can reversibly absorb additional hydrogen. Doping the buckyballs
with boron has been experimentally observed. This reduces the fullerene weight and
enhances the complex’s stability by increasing the binding energy. The boron-doped
buckyball also allows the binding of an additional H2 molecule per Sc, increasing the
amount of retrievable H2 to 8.77 wt% (Zhao).
Figure 6. Optimized Molecular Structures of C60[ScH2(H2)4]12 (a) and
C48B12[ScH(H2)5]12 (b) (Zhao).
Endohedral Fullerene Synthesis:
Endohedral fullerenes are a class of chemicals that are of great interest, especially in the
biochemical world. Endohedral fullerenes consist of a carbon cage with another
molecule enclosed within the cage. For possible applications, some imagine the ability to
capture a drug in this very stable carbon cage in order to control the delivery time of the
drug to the body. Another possibility is to place a radioactive “tracer” in the fullerene
and then safely inject it into the blood stream. Also, endohedral fullerenes are the most
likely type of fullerene to be used studied for use in nanoscale electronic devices because
of their ability to have a specific band gap (0.2-1.0 eV) as a function of chemical
structure (Shinohara).
The possibility of what fullerenes can hold inside their cage is always growing.
Conventional endohedral fullerenes were monometallofullerenes like La@C82. The
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formation of these first endohedral fullerenes was achieved by a high-temperature laservaporization process in 1991. Knowledge of these molecules was limited to the fact that
the metal was truly enclosed in the cage and that there exists a charge transfer from the
caged metal to the carbon cage. For example, La@C82 is better described by La3+@C823-.
There was great difficulty in applying these products to any application or even obtaining
accurate analysis of them, because the synthesis produced such a small amount of product
(less than 1% yield). Research of endohedral fullerenes was then extended to encaging
nonmetals and noble gases. Nonmetals and noble gases were confined primarily in C60
fullerenes, and they do not carry a charge. This family of endohedral fullerenes was
produced by placing the already-made C60 under high pressure in the presence of the gas
to be encaged (Dunsch 2006).
Multimetallofullerenes were the next group to be investigated. These endohedral
fullerenes were the first to break the well-accepted isolated pentagon rule (abbreviated as
IPR). The violation of this rule means that the carbon cage can have pentagons fused
together, creating a fullerene that isn’t perfectly spherical, as can be seen in figure 7.
Only endohedral fullerenes are known to be able to violate this stability rule. These
endohedral fullerenes were originally synthesized by the Krätschmer-Huffman arc
method. This conventional process produced only a maximum of 2% yield of
metallofullerenes in the soot (Dunsch 2004).
Figure 7. Calculated Sc2@C66 structure: top view at left, side view at right.
Violation of IPR. (Dunsch 2)
The production of each new endohedral fullerene suggests new possibilities for atoms to
be enclosed and new applications. However, all of the preceding endohedral fullerenes
and their respective formation processes have one major disadvantage. They produce
extremely low yields (less than 2%) of the endohedral fullerene as final product. No
application would be possible if only such a small amount of material could be produced.
The first noticeable improvement in yield of endohedral fullerenes came in 1999 when a
small amount of nitrogen gas (from air) was introduced into the Krätschmer-Huffman
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generator during the vaporization of the metal oxide containing graphite rods. This new
process was named trimetallic nitride template (TNT) and noticeably improved
endohedral fullerene yields. This synthesis first created Sc3N@C80. The empty cage of
C80 has never been isolated and the trimetallic nitride is not stable as a single molecule.
Therefore the endohedral fullerene must stabilize itself. After the success of the first
trimetallic nitride fullerene various others were investigated. The general formula for
these molecules is M3N@C80 where M=Sc, Y, Tb, Ho, and Er (Dunsch 2006).
The TNT method had been proven to be successful up to about 5% yield, which is still
not reasonable for any realistic applications. The last adjustment to the process came in
2004 when NH3 was introduced as the reactive gas in the TNT process instead of the
small amount of nitrogen gas (Dunsch 2006). For the first time, these endohedral
fullerenes were the main product. With an increased production of trimetal nitride
fullerenes (M3N@C80), more extensive characterization could be achieved. It was
determined that this family of endohedral fullerenes are among the most stable fullerene
structures known; only C60 and C70 are more stable (Dunsch 2004).
The TNT method has proved to be successful in producing extremely stable trimetal
nitride fullerenes in reasonable quantities. This breakthrough has shown another
significant property of fullerenes; they have the ability to stabilize a molecule that is not
stable on its own. However, this synthesis is very restricting and limiting. For example,
if one of the ultimate applications of this compound is to encage various drugs inside the
fullerene cage, this method provides no benefit. This synthesis is successful because of
nitrogen, and therefore it only can apply to trimetal nitride fullerenes.
Synthesizing Nanopeapods—Single-Walled Nanotubes With C60:
After the breakthrough of buckyballs, the discovery of carbon nanotubes soon followed.
Carbon nanotubes are made of the same sp2 hybridization of fullerenes, but they have a
cylindrical structure, as opposed to the spherical structure of fullerenes. Some potential
applications using carbon nanotubes are in the areas of nanoelectronics, biomedical, and
organometallics. Because carbon nanotubes have the ability to host other molecules,
researchers believe they could be promising in those applications. The factor which is
holding these applications back is the difficulty of placing guest molecules in the carbon
nanotube at mild conditions (Khlobystov).
Nanopeapods are the most popular form of molecular filled carbon nanotubes.
Nanopeapods are the name given to single-walled carbon nanotubes (SWNTs) which host
fullerenes. The reason for nanopeapods’ popularity is the favorable interaction between
the pure carbon nanotube and pure carbon fullerene. Conventionally, these nanotubes
were synthesized under extremely tough conditions. The reactions had temperatures
exceeding 350°C and needed to proceed in very high vacuum to vaporize the fullerenes.
Numerous theoretical studies were done that determined the activation barrier was zero or
small (0.37eV). This energy is on the order of molecules at room temperature; therefore,
according to Khlobystov, if an appropriate mechanism was determined to encapsulate the
fullerenes in the nanotubes, it could be accomplished at room temperature.
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Figure 8. Nanopeapod structure. (Khlobystov)
Many attempts at low temperature encapsulating fullerenes were attempted by
Khlobystov’s group. Methods investigated included mechanical mixing of the solid
fullerenes and nanotubes for 10-15 minutes, addition of SWNTs to concentrated solution
of fullerenes in aromatic organic solvents (high fullerene solubility), and addition of
fullerenes to solution of nanotubes with polar non-aromatic solvents (low fullerene
solubility). All of these methods failed and produced nanopeapods in less than 1% yield.
The mechanical method fails because mixing is very limited in the solid states. The
solution mechanisms failed due to the solvent molecules’ interaction with the SWNTs
and fullerenes. The solvent molecules surrounding the fullerene become trapped in the
nanotube and must be small enough to diffuse between the inner wall of the SWNT and
C60 fullerene. None of the organic molecules attempted had this property (Khlobystov).
The next effort focused on supercritical fluids (SCF), due to their high diffusivity, almost
that of a gas. SWNTs were mixed with a solution of fullerene and CS2 solvent and the
CS2 was quickly removed. The mixture was then placed in supercritical carbon dioxide
under high pressure for 10 days. This was the only synthesis that proved to have
reasonable yields (around 30%). The most important properties of the supercritical CO2
which make it such a viable option are its critical diameter and its ability to interact with
the π electrons of the C60 molecule. Supercritical carbon dioxide is able to attract a
carbon-60 molecule, carry it into the SWNT, and then because of its small critical
diameter of 2.8 Ǻ, it is able to diffuse out of the nanotube, leaving just the nanopeapod
(Khlobystov). Because this synthesis is performed with supercritical fluids it can be done
at moderate temperatures, which is a vast improvement to conventional methods.
a)
b)
Figure 9. a) Solubility of C60 and critical diameter of solvents. b) Dependence of C60CO2 on molar density of CO2. (Khlobystov)
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Another improvement to the process came from examining the interaction between CO2
and C60. By varying the temperature or pressure, the potential between the two molecules
can change from zero to a maximum attraction and then to a repulsion. When applied
correctly, this property can assist in transferring the fullerene into the SWNT (maximum
attraction) to aiding diffusion of CO2 out of the SWNT (repulsion) (Khlobystov). This
relationship can be seen in Fig. 9b.
Conclusion:
The various types of fullerenes give rise to potential applications in a diverse group of
fields. Many different types of molecules can be attached to the exterior of the fullerene
cage to form exohedral fullerene for a wide variety of applications, including hydrogen
storage. Endohedral fullerenes are considered to have numerous probable uses in
biochemical research, such as safe transport of radioactive tracer molecules. Fullerenes
can be enclosed in carbon nanotubes to form nanopeapods. These nanopeapods
potentially could be used to aid in drug delivery. These applications discussed here are
only the tip of iceberg of possibilities of novel uses of fullerenes. Although the discovery
of buckyballs is relatively recent and they have very cutting edge potential applications,
these molecules have long existed in nature both in geological formations on Earth and in
interstellar dust.
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