Graphene so smooth it sheds water

VANDERBILT (US) — Getting graphene to cast off water is no easy task, but researchers have discovered a way to make water either bead up and run off or spread out in a thin layer on the surface.

They did it by using a wet technique that combines an electric field within a liquid medium to create a freestanding film of graphene oxide. They then were able to alter the film’s surface roughness by varying the pH of the liquid medium and the voltage used in the process. The approach is documented in the journal ACSNano.

“Graphene films are transparent and, because they are made of carbon, they are very inexpensive to make,” says James Dickerson, assistant professor of physics at Vanderbilt University. “The technique that we use can be rapidly scaled up to produce it in commercial quantities.”

Graphene is made up of sheets of carbon atoms arranged in rings—something like molecular chicken wire. Not only is this one of the thinnest materials possible, but it is 10 times stronger than steel and conducts electricity better at room temperature than any other known material.

Graphene’s exotic properties have attracted widespread scientific interest, but Dickerson is one of the first to investigate how it interacts with water.

Many scientists studying graphene make it using a dry method, called mechanical cleavage, that involves rubbing or scraping graphite against a hard surface. The technique produces sheets that are both extremely thin and extremely fragile.

Dickerson’s wet method, known as electrophoretic deposition, can produce sheets equally as thin but considerably stronger than those made by other techniques. By varying the pH of the liquid and the voltage, they can change the manner in which the graphene oxide particles assemble into a film.

One pair of settings lay down the particles in a “rug” arrangement that creates a nearly atomically smooth surface. A different pair of settings causes the particles to clump into tiny “bricks” forming a bumpy and uneven surface. The researchers determined that the rug surface causes water to spread out in a thin layer, while the brick surface causes water to bead up and run off.

Dickerson is pursuing an approach that could create film that enhances these water-associated properties, making them even more effective at either spreading out water or causing it to bead up and run off.

There is considerable academic and commercial interest in the development of coatings with these enhanced properties, called super-hydrophobic and super-hydrophilic. Potential applications range from self-cleaning glasses and clothes to antifogging surfaces to corrosion protection and snow-load protection on buildings.

Effective, low-cost, and durable coatings have yet to make it out of the laboratory.

Researchers can tweak the process for creating films of graphene oxide so they are formed by “rug” process, above, that is extreme smooth and “water loving” or by a “brick” process, below, that is rough and “water hating.” (Credit: James Dickerson)

Dickerson’s idea is to apply his basic procedure to “fluorographene”—a fluorinated version of graphene that is a two-dimensional version of Teflon—recently produced by Kostya Novoselov and Andre Geim at the University of Manchester, who received the 2010 Nobel Prize for the discovery of graphene.

Normal fluorographene under tension should be considerably more effective in repelling water than graphene oxide. So there is a good chance a “brick” version and a “rug” version would have extreme water-associated effects, Dickerson figures.

Dickerson’s work was funded by a Vanderbilt Discovery grant and by the National Science Foundation.

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