Graphene, fullerenes and carbon nanotubes

How did two scientists win a Nobel Prize with sticky tape? In two thousand and four, in Manchester, they pressed tape onto graphite and peeled it off, over and over again. Some of the flakes left on the tape were one atom thick. Nothing was added and nothing reacted, yet that sheet, graphene, does things the lump of graphite cannot. The sheet itself comes first, then two more shapes made from the same carbon.

Graphene is a single layer of graphite, one atom thick. Each carbon atom is joined by covalent bonds to three others, in hexagonal rings, just as inside a layer of graphite. Each atom uses three of its four outer electrons in those bonds. The fourth is delocalised, so it is free to move across the sheet. Graphene matters at Foundation and Higher alike. Every property in this video comes from those two facts, the bonds and the spare electron. What property follows from graphene's bonds, and which one from its spare electron? From the bonds: graphene is very strong for its thickness, because every atom is held in the sheet by strong covalent bonds. From the electron: it conducts, because delocalised electrons can move and carry the charge. Each property gives a use. Because graphene conducts, it is used in electronics. Because it is so strong for its thickness, it is added to composite materials to make them stronger. When a question asks for structure and bonding, it wants two answers. Graphene's structure is a single layer of hexagonal rings, a giant covalent structure. Its bonding is three covalent bonds per carbon atom, with one delocalised electron each. Giant covalent on its own is only a label, and a label is half an answer. The habit that makes it whole is two lines every time: one for the structure, one for the bonding.

Fullerenes are molecules made only of carbon atoms, with hollow shapes. They are built mainly from hexagonal rings of carbon atoms, though rings of five or seven carbon atoms also occur. The first fullerene discovered was buckminsterfullerene, C sixty. It is sixty carbon atoms joined into a hollow ball, patterned rather like a football. A fullerene is a molecule, with a fixed number of atoms. It is not a giant structure that goes on and on: C sixty stops at sixty, and the next ball is a separate molecule. Solid C sixty turns to a vapour at a far lower temperature than graphite does, although both are pure carbon. Name what has to be overcome or broken in each, and the difference explains itself. Why does C sixty change state so much more readily than graphite? C sixty is made of separate molecules, so only the weak intermolecular forces between the balls are overcome. Graphite is a giant covalent structure, so a huge number of strong covalent bonds would have to break. Those weak forces also let the balls roll and slide past one another, so C sixty is slippery. That is a property of separate molecules, nothing to do with graphite. One examiner's report, on a fullerene question, says students needed to explain that the forces between the molecules are weak, so the fullerene molecules can slide over one another. Some mixed fullerenes up with graphite, and wrongly wrote about layers sliding over each other. Graphite's answer was borrowed for a different structure. The fix is to say molecules every time you talk about a fullerene, because a ball has no sheets to slide.

A carbon nanotube is a fullerene shaped as a cylinder, like a sheet of hexagonal carbon rings rolled into a tube. It is very long compared with its width, so its length to diameter ratio is very high. Along the tube, every carbon atom is held by strong covalent bonds, so a nanotube is very strong along its length. Each atom also has a delocalised electron, so a nanotube conducts electricity. A student writes: carbon nanotubes conduct electricity because carbon is a metal. The conclusion is right, and the reason given for it is not. What is wrong with the student's reason, and what should it say instead? Carbon is not a metal. A nanotube conducts because its delocalised electrons are free to move through the structure and carry the charge. One examiner's report on nanotubes records wrong reasons of its own: "Explanations involving delocalised electrons were rarely seen, with many writing about space for electricity to flow in nanotubes rather than about the electrons." The hole down the middle carries nothing. The fix is a sentence with two verbs: the delocalised electrons move through the structure, and they carry the charge. Now a use. A tennis racket frame needs to be stiffer without getting heavier, and carbon nanotubes are added to the frame material. Name the property first, then the use it leads to. Strength or conduction: which does the racket use, and what use does the other give? Strength along the length, from the strong covalent bonds, makes the frame stiffer for very little extra mass. A different property, conduction, gives a different use: electronics. On a racket question like that, one examiner's report says: "Almost all students made an attempt at comparing the properties of the three materials. Most achieved a Level 2 mark by giving a consequence of one of the stated differences in properties." The same report notes where the electrons part went: "Only a small proportion scored the mark for explaining the role of these electrons in the conduction of electricity." So the shape of the answer is usually there, and the two verbs are what to add.

A short spin through sheet, ball and tube, to see what has stayed with you. In one sentence, what is the structure of graphene? A single layer of graphite, one atom thick, with each carbon atom bonded to three others in hexagonal rings. Now an unseen fullerene: C seventy is another hollow ball. Does it change state at a low or a high temperature, and why? Low, compared with a giant structure. C seventy is made of separate molecules, so only weak intermolecular forces between them are overcome. Here's a different one: why does a nanotube conduct? Use both verbs. Its delocalised electrons move through the structure and carry the charge. One more: what did the sticky tape peel off the graphite, and why does that sheet conduct? Graphene, a single layer of carbon atoms. Each atom has one delocalised electron, free to move across the sheet and carry the charge.

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