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Graphene displays unexpected permeability

30 March 2020

In a two-step process, diatomic hydrogen flips through the monolayer’s intrinsic pores.

The angstrom-scale hexagonal voids in a sheet of graphene resemble the holes in chicken wire. But despite the material’s appearance, studies have shown that defect-free graphene is nearly impermeable. It blocks all liquids and gases; only protons have been found to pass through (see Physics Today, February 2015, page 11). Quantum calculations indicate that any atom or molecule attempting to cross faces an all but insurmountable energy barrier of a few electron volts or more.

Now Pengzhan Sun, Andre Geim (both at the University of Manchester in the UK), and their collaborators have found another exception to graphene’s impermeability: hydrogen gas.

Diagram of graphene membrane setup
Credit: Adapted from P. Z. Sun et al., Nature 579, 229, 2020

The researchers’ experimental setup, depicted in the first image, resembled that used in previous experiments to test graphene’s permeability. Micron-scale wells of trapped air were sealed with graphene membranes and surrounded by the gas being investigated. If any gas molecules got into a well, its membrane would balloon outward because of the increased pressure inside the well, and that deflection could be measured using atomic force microscopy. The new setup’s main advantage was an improved seal between the graphene and the well. Previous experiments with oxidized silicon wells had leaky seals, so Sun, Geim, and coworkers instead used graphite and hexagonal boron nitride wells. The graphene membranes formed leak-proof seals with the atomically flat ring-shaped surfaces surrounding each well.

The researchers tested a few dozen wells with helium—the smallest and potentially leakiest gas—and saw no membrane deflection over 30 days. Neon, nitrogen, oxygen, argon, krypton, and xenon also showed no discernable penetration. The precise measurements set an upper bound on graphene’s permeability, which is around that of a 1-km-thick piece of glass. However, when they tested hydrogen, the researchers were surprised to see a slow but steady upward membrane deflection of a few nanometers.

Diagram of graphene with chemisorbed hydrogen
Credit: Adapted from P. Z. Sun et al., Nature 579, 229, 2020

Although the researchers couldn’t directly observe the hydrogen transport, theoretical predictions for one route agreed with the observed leak rates. Hydrogen molecules first chemisorb on the membrane. That process is facilitated by the graphene’s local curvature; numerous ripples in the sheet make it more reactive. A chemisorbed hydrogen atom is effectively a proton—it shares its electron with the graphene—so it can flip through the membrane and desorb inside the well, as illustrated in the second image. The proposed mechanism could explain observations of hydrogen intercalation into supposedly defect-free multilayer graphene. (P. Z. Sun et al., Nature 579, 229, 2020.)

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