Each carbon atoms bonds to 3 other carbon atoms forming hexagons. Each hexagon shares each side with another hexagon. So this lattice of hexagons extends indefinitely, but it is only 1 carbon atom high! For this reason graphene is referred to as a two-dimensioal lattice or array.
Graphene is strong, light, nearly transparent and an excellent conductor of heat and electricity. Graphene is also hydrophobic, it repels water. However, narrow capillaries made out of graphene actually suck in water, if the water layer is only one atom thick.
Two years ago, researchers at the University of Manchester found that graphene capillaries could be made by stacking layers of graphene oxide on top of each to form a laminate. One-atom wide graphene capillaries are produced between these layers. These laminates are impermeable to all gases and vapours, except for water. This means that no gas or vapour, except for water, can pass through the laminate. Even helium, the smallest of the Nobel Gases cannot pass through the laminate.
These results suggest that graphene capillaries could be used to filter water. Researchers at the University of Manchester having been studying the use of graphene for water filtration and have found that ions less than 9 angstroms can quickly flow through with the water, but larger ions are blocked. With further research it is hoped to control the graphene mesh size to reduce it below 9 angstroms so that even the smallest ions like those found in seawater could be filtered out of water.
R. K. Joshi, P. Carbone, F. C. Wang, V. G. Kravets, Y. Su, I. V. Grigorieva, H. A. Wu, A. K. Geim, R. R. Nair. Precise and Ultrafast Molecular Sieving Through Graphene Oxide Membranes. Science, February 14, 2014 DOI: 10.1126/science.1245711
Suggested Study Questions:
- What is meant by the term "graphene is an allotrope of carbon?"
- Name, and describe the structure of, two other allotropes of carbon.
- Explain how the structure of graphene is different to the structure of graphite.
- Why is graphene considered to be a good electrical conductor?
- Which other allotrope of carbon is considered to be good electrical conductor? Explain how this allotrope conducts electricity.
- 1 angstrom = 1 Å = 10-10 metres. Convert 9 Å to:
- microns (micrometres)
- 1 picometre = 1 pm = 10-12 metres. Convert 900 pm to :
- microns (micrometres)
- Consider the crystal ionic radius for each of the following ions commonly found in seawater:
- Na+ : 116 pm
- Mg2+: 86 pm
- Cl- : 167 pm
- F- : 119 pm
- What is the diameter of each of these ions in angstroms?
- Which, if any, of these ions would pass through a graphene capillary? Explain your answer.
- A nitrate ion has a diameter of about 0.33nm and a sulfate ion has a diameter of about 0.49 nm. Which of these ions, if any, could pass through a graphene capillary? Explain your answer.