Buckyballs a new sphere of science
This topic is sponsored by the Australian Government's National Innovation Awareness Strategy.
When buckyballs bounced onto the scene in 1985, they became an overnight sensation. More than a decade later, scientists are still trying to score goals with these extraordinary molecules.
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The buckyball story is mostly a story about carbon. Carbon is an amazing element: more than 90 per cent of all known chemical substances are built around it, including those that form the basis of life, such as DNA and proteins.
Carbon is well-studied: an entire scientific discipline, organic chemistry, is based on it. Yet despite all the research, it was only in 1985 that one of its most extraordinary features was discovered.
Up until then, scientists knew of only two forms in which pure carbon occurred: diamond and graphite. Both these substances consist entirely of carbon atoms, but differ greatly in their structure and physical properties. In diamond, each carbon atom is bound to four other carbon atoms in a pattern of tetrahedrons. This structure makes diamond extremely hard.
In graphite, the carbon atoms form sheets of linked hexagons, giving the appearance of chicken wire. Each carbon atom within a sheet forms strong bonds to three other carbon atoms, but the stacked sheets are only held together by weak bonds. This means that the sheets can slide past each other, giving graphite its soft and greasy feel.
In 1985, British chemist Harry Kroto was puzzling over strange chains of carbon atoms that could be detected billions of kilometres away in space by radiotelescopes. He thought that these chains might form in conditions that are found near red giant stars.
Kroto visited the US laboratory of Richard Smalley and Robert Curl, who were studying 'clusters' aggregates of atoms that only exist briefly. Together they attempted to create high-temperature conditions in the laboratory, conditions similar to those near red giants. They vaporised graphite with a powerful laser in an atmosphere of helium gas.
When they analysed the resulting carbon clusters, they found many previously unknown carbon molecules. These varied in size, but the most common molecule contained 60 carbon atoms.
The structure of this molecule was not immediately apparent, although Curl, Kroto and Smalley knew that it was extremely stable. Only a spherical molecule, they reasoned, could produce this stability. After considerable debate, they worked out that the only geometric shape that could combine 60 carbon atoms into a spherical structure was a set of interlocking hexagons and pentagons (Box 1: Finding the molecular structure of buckyballs). Incidentally, an Australian theoretician at the University of California at Berkeley, Tony Haymet, published a paper at about this time predicting the existence of such a compound (he called it 'footballene').
Kroto and his colleagues then discovered that the combination of hexagons and pentagons also formed the basis of a geodesic dome designed by the architect and engineer, R. Buckminster Fuller, for the 1967 Montreal World Exhibition. So they decided to name the new molecule buckminsterfullerene (these days shortened to fullerene or buckyball). Chemists write it as C60 .
The announcement of the discovery in the prestigious scientific journal Nature created quite a stir in the scientific community people quickly realised that buckyballs could be very useful substances (Box 2). In 1996 Curl, Kroto and Smalley were awarded the Nobel Prize for their discovery.
The soccerball effect
You could go to Montreal to get an idea of what a buckyball looks like. But perhaps an easier way is to look at a soccer ball: you will see that it consists of 20 hexagons (the white patches of leather) and 12 pentagons (the black patches), exactly the same pattern as that of the new molecule.
Buckyballs have more in common with soccerballs than just their looks. They spin (much faster than a soccerball at more than 100 million times per second). If they are squeezed and then released they spring back to their original shape. And they bounce if they are hurled against a hard surface such as steel.
The C60 buckyball is the most famous of the fullerenes but by no means the only one. In fact, scientists have now discovered hundreds of different combinations of these interlocking pentagon/hexagon formations. Examples include
- ‘buckybabies’ spheroid carbon molecules containing fewer than 60 carbon atoms,
- ‘fuzzyballs’ C60 buckyballs with 60 hydrogen atoms attached,
- ‘giant fullerenes’ fullerenes containing hundreds of carbon atoms, and
- C70 molecules with 70 carbon atoms, shaped a bit like a rugby ball or an Australian Rules football.
Buckyballs in bulk
Curl, Kroto and Smalley were the first to make and identify buckyballs in the laboratory but they were only able to produce tiny quantities. The race was on to manufacture buckyballs in large enough quantities for detailed investigation of their properties and potential.
In 1990, five years after the first synthesis of buckyballs, German and American scientists independently made larger quantities of buckyballs. They heated graphite rods to a high temperature by passing an electric current between them, then separated the fullerenes from other carbon compounds in the resulting soot (fine carbon particles). About 75 per cent of the crystals were C60 molecules, 23 per cent were C70 and the rest were larger molecules. Soon after this manufacturing breakthrough, dozens of groups around the world were making fullerenes. It wasn't long before research papers began to appear at the rate of almost one a day.
The not-so-new form of carbon
But it turns out that we've actually been making fullerenes unknowingly for thousands of years whenever we burn a candle or an oil lamp. The candle's flickering flame vaporises wax molecules containing carbon, hydrogen and oxygen. Some of these molecules burn instantly in the blue heart of the flame. Others move upwards into the yellow tip where the temperature is great enough to split them apart. The result is carbon-rich soot particles that glow, giving off gentle yellow light. Amid this soot are buckyballs.
Buckyballs also exist in interstellar dust and in geological formations on Earth. So while they are new to science they are reasonably common in nature.
Chemical and physical properties of buckyballs
Buckyballs and other fullerenes intrigue scientists because of their chemistry and their unusual hollow, cage-like shape. Buckyballs are extremely stable and can withstand very high temperatures and pressures. The carbon atoms of buckyballs can react with other atoms and molecules, leaving the stable, spherical structure intact. Researchers are interested in creating new molecules by adding other molecules to the outside of a buckyball and also in the possibility of trapping smaller molecules inside a buckyball.
As well as carbon spheres in many sizes, scientists have discovered tubes of carbon. These nanotubes, or buckytubes, are created in a similar way to buckyballs: by passing an electric current between graphite rods. Nanotubes formed in this way are a series of tubes packed inside each other. When a tiny dose of cobalt, nickel or iron catalyst is added during manufacture, the result is an empty nanotube with a wall just one atom thick.
You can imagine an empty nanotube as being formed from a flat sheet of graphite. The sheet, like a length of chicken wire, is rolled into a cylinder with the opposite edges forming a perfect join. Nanotubes can be extremely long (eg, a nanotube might contain 1,000,000 carbon atoms).
Nanotubes exhibit some peculiar characteristics. For example, experiments suggest that they are incredibly tough. Other properties such as electrical conductivity seem to vary with the particular geometry of the tube. This means it could be possible to have two concentric nanotubes, one inside the other, the outer one acting as an insulator and the inner one conducting a current.
Scientific fun and games
The emergence of the buckyball and its cousins has been a stimulus to both scientific research and the human imagination, although we are yet to see any practical applications (Box 2: The many potential uses of fullerenes). One day, perhaps, they will have a major impact on our lives. In the meantime, hundreds or even thousands of chemists, physicists and molecular biologists in laboratories around the world continue to play molecular football with these most intriguing of structures.
Posted November 1999.