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New Coating Seen With ‘Invisible’ Graphene

Thursday, January 26, 2012

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Graphene, already a wonder material for its strength and thinness, now may hold the key to a new generation of non-invasive coatings, researchers say.

Largely transparent to the eye, graphene is apparently also largely transparent to water, scientists at Rice University and Rensselaer Polytechnic Institute (RPI) have found.

Rahul Rao/Rensselaer Polytechnic Institute

Drops of water on a piece of silicon and on silicon with a layer of graphene show minimal change in the contact angle between the water and the substrate. When applied to most metals and silicon, a single layer of graphene is transparent to water.

Their new study has determined that gold, copper and silicon get just as wet when clad by a single continuous layer of graphene as they would without the layer.

That could be a boon for scientists who are fine-tuning surface coatings for a variety of applications, says the team, whose research was published this week in the online edition of Nature Materials.

‘Totally Non-Invasive’

"The extreme thinness of graphene makes it a totally non-invasive coating," said Pulickel Ajayan, Rice professor of mechanical engineering, materials science and chemistry.

"A drop of water sitting on a surface 'sees through' the graphene layers and conforms to the wetting forces dictated by the surface beneath. It's quite an interesting phenomenon unseen in any other coatings and once again proves that graphene is really unique in many different ways."

Ajayan is co-principal investigator of the study with Nikhil Koratkar, a professor of mechanical, aerospace and nuclear engineering at RPI.

Changing Surface Properties

A typical surface of graphite—the form of carbon most commonly known as pencil lead—should be hydrophobic, and repel water, Ajayan said.

But his team found, to its surprise, that a single-atom-thick layer of the carbon lattice presents a negligible barrier between water and a hydrophilic (water-loving) surface. Piling on more layers reduces wetting; at about six layers, graphene essentially becomes graphite.

The findings may even allow scientists to change conductivity and other surface properties while retaining wetting characteristics. Because pure graphene is highly conductive, the discovery could lead to a new class of conductive, yet impermeable, surface coatings, Ajayan said.

‘One of a Kind’

One caveat: The team observed wetting transparency only on most metals, silicon and other surfaces where interaction with water is dominated by weak van der Waals http://en.wikipedia.org/wiki/Van_der_Waals_force forces, and not for glass and other materials where wettability is dominated by strong chemical bonding.

“But such applications as condensation heat transfer—integral to heating, cooling, dehumidifying, water harvesting and many industrial processes—may benefit greatly from the discovery,” Rice reported.

“Copper is commonly used for its high thermal conductivity, but it corrodes easily. The team coated a copper sample with a single layer of graphene and found the subnanometer barrier protected the copper from oxidation with no impact on its interaction with water; in fact, it enhanced the copper's thermal effectiveness by 30 to 40 percent.”

The research was funded by the Advanced Energy Consortium, National Science Foundation and the Office of Naval Research graphene MURI program.

"The finding is interesting from a fundamental point of view as well as for practical uses," Ajayan said. "Graphene could be one of a kind as a coating, allowing the intrinsic physical nature of surfaces, such as wetting and optical properties, to be retained while altering other specific functionalities like conductivity."