The channels between graphene sheets are horizontal, which is not great for applications like water filtration. But researchers from Brown University have shown a way to flip those channels to make them vertical in relation to the sheets, which is an ideal filtration orientation. Credit: Hurt Lab/Brown University.Stacks of layered two-dimensional materials like graphene oxide (GO) can be used as highly selective membranes. When sheets of these materials are laid on top of each other, the gaps in between act as nanoscale channels. But liquids passing through these membranes have to follow highly circuitous routes, so flux is very low. Now researchers have found a novel way of aligning sheets of GO so the nanochannels are better aligned, improving flux while retaining excellent selectivity [Liu et al., Nature Communications 12 (2021) 507, https://doi.org/10.1038/s41467–020–20837–2].
In graphene-based membranes, sheets are stacked on top of each other, like pages in a book. This means that the nanochannels are oriented horizontally compared with the sheet stack. For liquids flowing through the membrane, this implies a relatively long pathway to travel across the membrane. To get around this limitation, Robert H. Hurt and his colleagues at Brown University and Massachusetts Institute of Technology fabricated GO nanosheets on a stretched polymer substrate. When the tension on the substrate is released, the polymer contracts and the graphene nanosheets are compressed, wrinkling up into a zigzag pattern of steep mountains and valleys. The orientation of the graphene nanosheets is effectively rotated so they are now almost vertical with respect to the sheet stack, significantly reducing the pathway for liquids passing through.
“When you start wrinkling the graphene, you’re tilting the sheets and the channels out of plane. If you wrinkle it a lot, the channels end up being aligned almost vertically,” explains Muchun Liu, now a researcher at Massachusetts Institute of Technology, who devised the approach.
“This allows the tiny channels between the sheets to be used more effectively as selective pores that pass directly from top to bottom in a thin and mechanically stable membrane,” adds Hurt.
To create an actual membrane, the vertically aligned nanosheets are encased in epoxy resin with the top and bottom trimmed away to create open channels. The resulting vertically aligned graphene membranes (VAGMEs) only allow transport through the now-vertical nanochannels. This means small molecules like water can pass through easily while larger organic molecules, such as hexane as the researchers demonstrate, are filtered out.
“What we end up with is a membrane with short and very narrow channels through which only very small molecules can pass,” points out Hurt. “For example, water can pass through but organic contaminants or some metal ions would be too large to go through, so you could filter those out.”
The simple flipping of graphene sheet orientation leads to a 300-fold increase in active area in the resulting membranes. While the proof-of-principle wrinkling approach outlined in the work produces a relatively modest tilt angle of 74°, the researchers are confident that honing the process could yield even better tilt angles approaching 90°.
“It has been known for some while that this vertical alignment would be advantageous, but it was difficult to fabricate,” says Hurt. “It is much easier to transport fluid, for example liquid water, in a straight path across the membrane … thus back pressure is reduced and/or throughput is increased to more practical values.”
While he cautions that, at the moment, the approach is more suited to the lab than large-scale manufacturing, it does hold promise for small-molecule separations, including removing contaminants from water. The composite membranes are also robust, thermally stable and less prone to swelling.
“The structure is promising, and the trends are clear,” Hurt told Nano Today, “but the full quantitative implications need to be characterized for specific applications such as water treatment.”
The researchers now anticipate developing the approach to produce membranes for specific technological applications such as molecular sieves for liquid phase separations including ultrafiltration and reverse osmosis, which are in demand for water purification.
“One unique feature of the two-dimensional nanofluidic material is that it enables two independent transport directions, either vertical or horizontal, in the substrate membrane,” comments Wei Guo of the Technical Institute of Physics and Chemistry, Chinese Academy of Sciences in Beijing. “The coupling between the two orthogonal transport directions provides a facile, yet highly efficient way to modulate the overall transport properties. This work provides a facile and efficient way to fabricate such materials, [which will] attract great interest in this field,” he adds.
This article was originally published in Nano Today 37 (2021) 101116.