Graphene reveals yet another extraordinary property
Ever since University of Manchester scientists Andre Geim and Konstantin Novoselov first isolated flakes of graphene in 2004 using that most high-tech pieces of equipment - adhesive tape - the one-atom sheet of carbon has continued to astound researchers with its remarkable properties. Now Professor Sir Andre Geim, (he's now not only a Nobel Prize winner but also a Knight Bachelor), has led a team that has added superpermeability with respect to water to graphene's ever lengthening list of extraordinary characteristics.
Graphene has already proven to be the thinnest known material in the universe, strongest material ever measured, the best-known conductor of heat and electricity, and the stiffest known material, while also the most ductile. But it seems the two-dimensional lattice of carbon atoms just can't stop showing off.
Stacking membranes of a chemical derivative of graphene called graphene oxide, which is a graphene sheet randomly covered with other molecules such as hydroxyl groups OH-, scientists at the University of Manchester created laminates that were hundreds of times thinner than a human hair but remained strong, flexible and were easy to handle.
When the team sealed a metal container using this film, they say that even the most sensitive equipment was unable to detect air or any other gas, including helium, leaking through. The team then tried the same thing with water and, to their surprise, found that it evaporated and diffused through the graphene-oxide membranes as if they weren't even there. The evaporation rate was the same whether the container was sealed or completely open.
"Graphene oxide sheets arrange in such a way that between them there is room for exactly one layer of water molecules. They arrange themselves in one molecule thick sheets of ice which slide along the graphene surface with practically no friction, explains Dr Rahul Nair, who was leading the experimental work. "If another atom or molecule tries the same trick, it finds that graphene capillaries either shrink in low humidity or get clogged with water molecules."
Professor Geim added, "Helium gas is hard to stop. It slowly leaks even through a millimetre -thick window glass but our ultra-thin films completely block it. At the same time, water evaporates through them unimpeded. Materials cannot behave any stranger. You cannot help wondering what else graphene has in store for us."
Although graphene's superpermeability to water makes it suitable for situations where water needs to be removed from a mixture without removing the other ingredients, the researchers don't offer ideas for any immediate applications that could take advantage of this property. However, they did seal a bottle of vodka with the membranes and found that the distilled solution did indeed become stronger over time. But they don't foresee graphene being used in distilleries.
However, Professor Geim adds, "the properties are so unusual that it is hard to imagine that they cannot find some use in the design of filtration, separation or barrier membranes and for selective removal of water."
The University of Manchester team's paper, "Unimpeded Permeation of Water Through Helium-Leak-Tight Graphene-Based Membranes," appears in the journal Science.