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Graphene could enable faster, more durable water filters

08 May 2015

Ultra-thin graphene membranes could have a role in water filtration, removing contaminants to quickly purify high volumes of water.

Illustration courtesy of the researchers.  For an explanation, see base of article.

Graphene’s unique properties make it a potentially ideal membrane for water filtration or desalination. But there’s been one main drawback to its wider use: making membranes in one-atom-thick layers of graphene is a meticulous process that can tear the thin material — creating defects through which contaminants are able to pass.

Now, engineers at MIT, Oak Ridge National Laboratory, and King Fahd University of Petroleum and Minerals have devised a process to repair these defects, filling cracks and plugging holes using a combination of chemical deposition, polymerisation and etching techniques.

The researchers were able to engineer a relatively large defect-free graphene membrane — about the size of a US penny coin. The membrane’s size is significant; to be exploited as a filtration membrane, graphene would have to be manufactured at a scale of centimetres, or larger.

In experiments, the researchers pumped water through a graphene membrane treated with both defect-sealing and pore-producing processes, and found that water flowed through at rates comparable to current desalination membranes. The graphene was able to filter out most large-molecule contaminants, such as magnesium sulphate and dextran.

“We’ve been able to seal defects, at least on the lab scale, to realise molecular filtration across a macroscopic area of graphene, which has not been possible before,” says Rohit Karnik, an associate professor of mechanical engineering at MIT. “If we have better process control, maybe in the future we don’t even need defect sealing. But I think it’s very unlikely that we’ll ever have perfect graphene — there will always be some need to control leakages.”

To plug graphene’s leaks, the team came up with a technique to first tackle the smaller intrinsic defects, then the larger transfer-induced defects. For the intrinsic defects, the researchers used atomic layer deposition involving a hafnium-containing chemical that does not normally interact with graphene. However, if the chemical comes in contact with a small opening in graphene, it will tend to stick to that opening, attracted by the area’s higher surface energy.

The team applied several rounds of atomic layer deposition, finding that the deposited hafnium oxide successfully filled in graphene’s nanometre-scale intrinsic defects.

A second technique, to fill in larger defects, uses a process called 'interfacial polymerisation' that is often employed in membrane synthesis. After they filled in graphene’s intrinsic defects, the researchers submerged the membrane at the interface of two solutions: a water bath and a water-immiscible organic solvent.

In the two solutions, the researchers dissolved two different molecules that can react to form nylon. Once the graphene membrane is placed at the interface of the two solutions, the nylon plugs formed only in tears and holes — regions where the two molecules could come in contact because of tears in the otherwise impermeable graphene — effectively sealing the remaining defects.

Using a technique they had previously developed, the researchers then etched tiny, uniform holes in the graphene — small enough to let water molecules through, but not larger contaminants. In experiments, the group tested the membrane with water containing several different molecules, including salt, and found that the membrane rejected up to 90 percent of larger molecules. However, it let salt through at a faster rate than water.

The preliminary tests suggest that graphene may be a viable alternative to existing filtration membranes, although Karnik says techniques to seal its defects and control its permeability will need further improvements.

Key to illustration
In a two-step process, engineers have successfully sealed leaks in graphene. First, the team fabricated graphene on a copper surface (top left) — a process that can create intrinsic defects in graphene, shown as cracks on the surface. After lifting the graphene and depositing it on a porous surface (top right), the transfer creates further holes and tears. In a first step (bottom left), the team used atomic layer deposition to deposit hafnium (shown in grey) to seal intrinsic cracks, then plugged the remaining holes (bottom left) with nylon (shown in red), via interfacial polymerisation.

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