Limits on gas impermeability of graphene

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.

• The paper demonstrates graphene’s near-complete impermeability to most gases, with helium detection limits improved by eight to nine orders of magnitude.
• The study employs micrometer-sized containers sealed with monolayer graphene, achieving a detection limit of only a few helium atoms per hour.
• The findings show that hydrogen uniquely permeates due to catalytic activity at graphene ripples, with an activation energy of approximately 1.0 eV.

Gas Impermeability of Graphene: Experimental Investigation

The paper, "Limits on gas impermeability of graphene," presents a detailed exploration of the gas impermeability properties of defect-free graphene at a significantly improved level of accuracy compared to previous studies. The research expands our understanding of graphene's resistance to gas permeation, deploying micrometer-sized containers sealed with monolayer graphene to quantify permeability with unprecedented precision.

The authors utilize monocrystalline graphite or hexagonal boron nitride containers with tightly sealed graphene membranes, overcoming limitations seen in traditional SiO$_2$-based setups. The experimental design achieves a detection limit for helium atoms on the order of a few atoms per hour, representing an improvement of eight to nine orders of magnitude over prior methodologies.

Notably, the study confirms graphene's impermeability to a comprehensive range of gases, including helium, neon, nitrogen, oxygen, argon, krypton, and xenon. However, the research outlines a peculiar anomaly with hydrogen, which exhibits distinct permeation behavior despite theoretical predictions suggesting otherwise. The paper attributes this to the catalytic activity at graphene ripples, facilitating hydrogen dissociation and subsequent atom flipping across the graphene sheet. The reported activation energy of 1.0 eV for this mechanism is aligned with values noted for proton transport through similar membranes.

Theoretical predictions, rooted in density functional theory (DFT) calculations, propose significant energy barriers for gas penetration, reinforcing the empirical observations. The estimated energy barriers (E ≥ 1.2 eV for helium) support the experimental findings of graphene's impermeability at ambient conditions and highlight an arrangement where helium permeability would necessitate a monolayer thickness comparable to one-kilometer-thick quartz glass.

This study's examination of hydrogen permeability has broader implications, especially considering the observed isotope effects. Absent permeation for deuterium and the measurable passage of hydrogen imply a nuanced interaction between hydrogen isotopes and graphene. These findings not only challenge existing theoretical frameworks but also highlight the importance of intrinsic and extrinsic structural graphene features, such as ripples and defects, in facilitating selective permeability.

The results from this study critically reshape the understanding of permeation dynamics in two-dimensional materials. They emphasize the potential applications of graphene in industries where molecular filtration and separation are crucial. The elucidation of hydrogen's interaction pathways opens avenues for graphene's application in proton exchange membranes and similar technologies. Furthermore, the study anticipates the broader catalytic implications of non-flat graphene, fostering advancements in fields ranging from energy conversion to materials chemistry.

For future developments, the experimental framework established herein invites further exploration of alternative two-dimensional materials such as bilayer graphene and molybdenum disulfide, potentially catalyzing the development of ultrathin materials capable of selective molecular transport. This research represents a foundational reference point for ongoing investigations into the scalability and functional versatility of two-dimensional impermeable barriers.