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Science matters: Carbon: versatility exemplified, Richard Harwood
Carbon: versatility exemplified
Richard Harwood looks at a chemical element that is essential to everyday life
We are all familiar with several of the different structures of carbon. As we draw with pencil or charcoal we slide layers of graphite onto our paper. At other times we may aspire to give, or indeed receive, diamond in some setting or other! These two structural forms of this pre-eminently important element depend on the versatility in which carbon atoms can chemically bond with each other, forming two- or threedimensional networks. Our very existence is dependent on the capacity of carbon atoms to form chains or rings; combining together to make the myriad of molecules that function in reproducing and sustaining life. Life is carbonbased.
However, the significance of carbon has expanded even more dramatically in recent years. The discovery of the fullerenes (such as C60) and carbon nanotubes fostered to a large extent the development of nanotechnology.
Graphite has a structure in which layers of hexagonally arranged carbon atoms are stacked on each other, and it was the manipulation of such individual layers that led to the development of carbon nanotubes.
Even more recently, a further new form of carbon – graphene – has been isolated at the University of Manchester in the UK. It is essentially a single-layered material made up of individual sheets of graphite. The first samples of graphene were isolated in experiments aimed at seeing how thin a piece of graphite could be made by polishing it down. However, thinner material was obtained by cleaning graphite with ‘sticky tape’ – ‘peeling off’ the layers of graphite for surface science experiments. This developed into an investigation by Andre Geim and Konstantin Novoselov aimed at determining just how thin the layers ‘stripped’ from a piece of graphite could be. The first isolation of graphene flakes was achieved in 2004.
Subsequent studies were carried out on these thin graphite layers and methods devised for reproducibly forming graphene monolayers. Geim and Novoselov were awarded the Nobel Prize for Physics in 2010 and the potential for this novel material has generated immense excitement.
Graphene is also, in fact, the structural unit of fullerenes and nanotubes. A sheet can be viewed as a very large aromatic molecule formed from many fused benzene molecules. High-quality graphene is strong, light, and almost transparent. It is an excellent conductor of heat and electricity (300 times better than copper). Its interactions with other materials, with light, and its two-dimensional nature give rise to unique properties.
Graphene can be prepared, as described above, by removing monolayers from a sample of graphite or, alternatively, by heating a sample of silicon carbide (SiC) to remove the silicon. Recently, a method has been devised of treating a suspension of graphite powder in a blender. This offers the potential for producing graphene on a large scale for use in industry.
Graphene also has many interesting properties. For example: its tensile strength is 200 times greater than steel and is incredibly flexible; it behaves as a semi-metal, making it very suitable for electronic devices; and the introduction of about 1% content of graphene into plastics could make those plastics electrically conducting.
(a) A computer model of the C60 molecule
A piece of graphite (top left), a graphene transistor, and a tape dispenser used in the experiments that led to the first layers of graphene isolated in 2004. (Items donated to the Nobel Museum in Stockholm)
Of the different allotropes (structural forms) of carbon, graphene is the most chemically reactive. This is because of the reactive edges of the structure where there are carbon atoms with unoccupied (‘dangling’) bonds. This reactive form has potentially important uses too. For example, membranes of graphene oxide have been shown to be preferentially permeable to water, suggesting possible uses in desalination and water purification. This application is highly significant as current methods of large scale desalination by distillation or reverse osmosis are economically expensive.
In the UK the National Graphene Institute (NGI) has already been established in Manchester, and that university’s second multi-million pound centre – the Graphene Engineering Innovation Centre (GEIC) – will open in 2018. Further information on this exciting, revolutionary material can be found on the Nobel Prize website: www.nobelprize. org/nobel_prizes/physics/laureates/2010/popularphysicsprize2010.pdf and on the dedicated graphene research website set up by the NGI: www.graphene.manchester.ac.uk which outlines the history of the discovery and the projected uses of this novel form of carbon.
