Material transparency: Water cannot see graphene

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  • Published: Feb 1, 2012
  • Author: David Bradley
  • Channels: Raman
thumbnail image: Material transparency: Water cannot see graphene

Seeing through graphene

Raman spectroscopy has been used to investigate graphene, which might be considered the thinnest material known. The nanomaterial is not transparent to light, but water behaves as if it were not there.

Essentially, graphene is an isolated single layer of the carbon allotrope graphite. Graphene is of increasing interest to technologists and materials scientists keen to exploit its unique properties as a strong, flexible and optoelectronically fascinating material. One area in which thin coatings of graphene might be used are in light-weight, flexible, and paper-thin devices and displays. However, the issue of how graphene responds to water or humidity remains to be addressed completely.

To materials scientists, the issue of wettability of a solid surface is usually determined by placing a drop of water on the surface being tested and measuring the angle at which the droplet meets the surface, the contact angle. Water repellent, or hydrophobic surfaces, are not readily wetted and so a droplet will form a tight sphere sitting on the surface and have a high contact angle. Conversely, a low contact angle is seen for a droplet as it spreads out on a hydrophilic surface.

Monolayer protection

Engineers at Rensselaer Polytechnic Institute led by Nikhil Koratkar and Rice University led by Pulickel Ajayan have coated pieces of gold, copper, and silicon with a single layer of graphene, and then placed a drop of water on these coated surfaces to measure contact angles and assess graphene's wettability . To their astonishment, the team found that the presence of a layer of graphene on metal and silicon surfaces had almost no effect on the contact angle.

"We coated several different surfaces with graphene, and then put a drop of water on them to see what would happen. What we saw was a big surprise - nothing changed. The graphene was completely transparent to the water," explains Koratkar.

The water contact angle for gold is approximately 77 degrees. The graphene-coated gold tested by Koratkar and Ajayan had a contact angle of 78 degrees. For silicon, the angle also increased only marginally from about 32 to 33 degrees. Copper too saw only a fractional change in contact angle, increasing from 85 to about 86 degrees with just a single layer of graphene to coat the metal.

"The single layer of graphene was so thin that it did not significantly disrupt the non-bonding van der Waals forces that control the interaction of water with the solid surface. It's an exciting discovery, and is another example of the unique and extraordinary characteristics of graphene." This is rather surprising given that even a single layer of graphene is normally thought of as entirely impermeable to water and even helium atoms. The research clearly demonstrated that water molecules behave as if the graphene is not present even though physically the purportedly impermeable layer separates them from the metal atoms beneath.

The explanation lies, as it so often does in explaining water's anomalous behaviour, in the perplexing bonding network that forms within water. The attraction of water molecules to other surfaces is directed by van der Waals forces which have a range of several nanometres. These forces are not disrupted by the presence of a single layer of graphene, which is just one atom thick. In other words, the van der Waals forces are able to "look through" ultra-thin graphene coatings, Koratkar said. When more graphene layers are added to the metals or the silicon, the van der Waals forces are disrupted and the water behaves as if it were sitting on graphite rather than the underlying material. "Just six layers of graphene stacked together is sufficient to ensure that the van der Waals forces between the water and the substrate are dictated by the graphene layers rather than the underlying bulk copper substrate,"  Koratkar.

No disruption

"We found that van der Waals forces are not disrupted by graphene. This effect is an artefact of the extreme thinness of graphene - which is only about 0.3 nm," Koratkar explains. "Nothing can rival the thinness of graphene. Because of this, graphene is the ideal material for wetting angle transparency."

One practical application of this discovery might be to coat the copper surfaces used in dehumidifiers to prevent oxidation. Writing in the January issue of the journal Nature Materials, the team explains how another potential spinoff of this discovery might be to coat copper heat pipes used in the cooling systems for microprocessors.

"It's an interesting idea. The graphene doesn't cause any significant change to the wettability of copper, and at the same time it passivates the copper surface and prevents it from oxidizing," he said.


The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.

Credit: Rensselaer/Koratkar Raman spectroscopy has been used to investigate graphene, which might be considered the thinnest material known. The nanomaterial is not transparent to light, but water behaves as if it were not there.
Wetting transparent graphene 

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