Chemistry 1.38 - 1.39 - Carbon sheets, cages and chains

Graphene sheets and fullerene cages

Graphene and $\ce{C60}$ are also forms of carbon, but they must not be treated as the same kind of particle. The specification studies them together under fullerenes and graphene; structurally, graphene is an extended sheet, whereas $\ce{C60}$ is a separate molecule.

Graphene is one layer of graphite, only one carbon atom thick. Each carbon is covalently bonded to three others in a continuous hexagonal network. The strong covalent bonds across the sheet make graphene very strong for its tiny mass and make it difficult to break apart. Because it is a single layer, it is extremely thin, light and almost transparent; the sheet can also bend. One electron per carbon is delocalised and can move across the sheet, so graphene conducts electricity.

Fullerenes are carbon molecules with hollow shapes. $\ce{C60}$, called buckminsterfullerene, is one spherical example: a cage of 60 carbon atoms arranged in pentagons and hexagons, with each carbon bonded to three others. Strong covalent bonds hold the atoms together within each $\ce{C60}$ molecule, but weak intermolecular forces act between $\ce{C60}$ molecules in a sample. These weak forces are much easier to overcome than the covalent bonds in a giant structure, so a molecular $\ce{C60}$ solid is relatively soft and separating its cages requires far less energy than breaking apart a giant covalent network.

$\ce{C60}$ also has electrons delocalised over each cage. Some $\ce{C60}$-based materials can show electrical conduction, but the electrons do not have the same continuous giant sheet through which to move as they do in graphene. Pure $\ce{C60}$ is a poor electrical conductor compared with graphene: electrons confined mainly to individual cages do not move freely through the entire sample.

[DIAGRAM: asset_slug: che_b_graphene_cages_v2; description: A single extended hexagonal graphene sheet contrasted with three separate hollow C60 cage projections; solid bonds are within the sheet and cages, and the empty gap between cages is labelled weak attractions.]

The $\ce{C60}$ cage is a schematic projection: the formula gives the exact count of 60 carbon atoms, while the drawing emphasises the closed cage of pentagons and hexagons rather than inviting an atom-by-atom count from one viewpoint.