Much has been made of graphene's exceptional qualities, from its
ability to conduct heat and electricity better than any other material
to its unparalleled strength: Worked into a composite material,
graphene can repel bullets better than Kevlar. Previous research has
also shown that pristine graphene - a microscopic sheet of carbon atoms
arranged in a honeycomb pattern - is among the most impermeable
materials ever discovered, making the substance ideal as a barrier film.
A high resolution STEM image of a large hole in graphene. The hole is about 10nm in diameter. Taken at Oak Ridge National Laboratory. Image credit: Juan-Carlos Idrobo
But the material may not be as impenetrable as scientists have
thought. By engineering relatively large membranes from single sheets
of graphene grown by chemical vapor deposition, researchers from MIT,
Oak Ridge National Laboratory (ORNL) and elsewhere have found that the
material bears intrinsic defects, or holes in its atom-sized armor. In
experiments, the researchers found that small molecules like salts
passed easily through a graphene membrane's tiny pores, while larger
molecules were unable to penetrate.
The results, the researchers say, point not to a flaw in graphene,
but to the possibility of promising applications, such as membranes
that filter microscopic contaminants from water, or that separate
specific types of molecules from biological samples.
"No one has looked for holes in graphene before," says Rohit Karnik,
associate professor of mechanical engineering at MIT. "There's a lot of
chemical methods that can be used to modify these pores, so it's a
platform technology for a new class of membranes."
Karnik and his colleagues, including researchers from the Indian
Institute of Technology and King Fahd University of Petroleum and
Minerals, have published their results in the journal ACS Nano.
Karnik worked with MIT graduate student Sean O'Hern to look for
materials "that could lead to not just incremental changes, but
substantial leaps in terms of the way membranes perform." In
particular, the team cast around for materials with two key attributes,
high flux and tunability: that is, membranes that quickly filter
fluids, but are also easily tailored to let certain molecules through
while trapping others. The group settled on graphene, in part because
of its extremely thin structure and its strength: A sheet of graphene
is as thin as a single atom, but strong enough to let high volumes of
fluids through without shredding apart.
The team set out to engineer a membrane spanning 25 square
millimeters - a surface area that is large by graphene standards,
holding about a quadrillion carbon atoms. They used graphene
synthesized by chemical vapor deposition, borrowing on expertise from
the research group of Jing Kong, the ITT Career Development Associate
Professor of Electrical Engineering at MIT. The team then developed
techniques to transfer the graphene sheet to a polycarbonate substrate
dotted with holes.
Once the researchers successfully transferred the graphene, they
began to experiment with the resulting membrane, exposing it to flowing
water containing molecules of varying sizes. They theorized that if
graphene were indeed impermeable, the molecules would be blocked from
flowing across. However, experiments showed otherwise, as researchers
observed salts flowing through the membrane.
As another test, the team exposed a copper foil with graphene grown
on it to a chemical agent that dissolves copper. Instead of protecting
the metal, graphene let the agent through, corroding the underlying
copper. To test the size of the pores within graphene, the group
attempted to filter water with larger molecules. It appeared that there
was a limit to the size of the pores, as larger molecules were unable
to pass through the membrane.
As a final experiment, Karnik and O'Hern observed the actual holes
in the graphene membrane, looking at the material through a
high-powered electron microscope at ORNL in collaboration with
Juan-Carlos Idrobo. They found that pores ranged in size from about 1
to 12 nanometers - just wide enough to selectively let some small
molecules through.
"Right now we know from this characterization how the graphene
behaves, and what kind of intrinsic pores it has," Karnik says. "In
some sense it's the first step to practically realizing graphene-based
membranes."
Karnik adds that a near-term application for such membranes may
include a portable sensor in which a layer of graphene "could shield
the sensor from the environment," letting through only a molecule or
contaminant of interest. Another use may be in drug delivery, with
graphene, dotted with pores of a determined size, delivering therapies
in a controlled release.
"We're right now in the process of transferring more graphene to
different substrates and making holes of our own, making a viable
membrane for water filtration," O'Hern says.
Scott Bunch, an assistant professor of mechanical engineering at the
University of Colorado, says the group's results are the first
demonstration that graphene bears defects. The membrane developed by
the group "has the potential to be a revolutionary membrane" that
separates particles at the molecular scale.
"The issue that now needs to be addressed is whether one can
discriminate between smaller molecules," Bunch says. "Once this
happens, graphene membranes will eventually live up to the truly
remarkable properties that they promise."
Other researchers involved in the work are Cameron Stewart, Michael
Boutilier, Sreekar Bhaviripudi, Sarit Das, Tahar Laoui and Muataz
Atieh. This work was funded by the King Fahd University of Petroleum
and Minerals through the Center for Clean Water and Clean Energy at MIT
and KFUPM, and was also supported by the ORNL ShaRE program.
Source: MIT