Limits on gas impermeability of graphene

TL;DR

This study demonstrates that defect-free graphene is nearly perfectly impermeable to most gases, with hydrogen being an exception due to a unique dissociation and permeation process, refining our understanding of graphene's barrier properties.

Contribution

The paper provides the most sensitive experimental evidence to date that defect-free graphene is essentially impermeable to gases, except hydrogen, revealing a new permeation mechanism for hydrogen.

Findings

Graphene is impermeable to gases like helium, neon, nitrogen, oxygen, argon, krypton, and xenon.

Hydrogen permeates through graphene via dissociation and atom flipping, not molecular transport.

Permeation detection limit is as low as a few atoms per hour.

Abstract

Despite being only one-atom thick, defect-free graphene is considered to be completely impermeable to all gases and liquids. This conclusion is based on theory and supported by experiments that could not detect gas permeation through micrometre-size membranes within a detection limit of 10^5 to 10^6 atoms per second. Here, using small monocrystalline containers tightly sealed with graphene, we show that defect-free graphene is impermeable with an accuracy of eight to nine orders of magnitude higher than in the previous experiments. We could discern permeation of just a few helium atoms per hour, and this detection limit is also valid for all other tested gases (neon, nitrogen, oxygen, argon, krypton and xenon), except for hydrogen. Hydrogen shows noticeable permeation, even though its molecule is larger than helium and should experience a higher energy barrier. The puzzling observation is attributed to a two-stage process that involves dissociation of molecular hydrogen at catalytically active graphene ripples, followed by adsorbed atoms flipping to the other side of the graphene sheet with a relatively low activation energy of about 1.0 electronvolt, a value close to that previously reported for proton transport. Our work provides a key reference for the impermeability of two-dimensional materials and is important from a fundamental perspective and for their potential applications.