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 [1][2][3][4][5][6][7][8][9][10] . This conclusion is based on theory 3-8 and supported by experiments 1,9,10 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 11,12 . 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.From a theory standpoint, monolayer graphene imposes a very high energy barrier for penetration of atoms and molecules. Density-functional-theory calculations predict that the barrier E is at least several eV 2-6 , which should prohibit any gas permeation under ambient conditions. Indeed, one can estimate that at room temperature T it would take longer than the lifetime of the universe to find an atom energetic enough to pierce a defect-free membrane of any realistic size. These expectations agree with experiments that reported no detectable gas permeation through mechanically-exfoliated graphene. The highest sensitivity was achieved using micrometersize wells etched in oxidized silicon wafers, which were sealed with graphene 1,9,10 . In those measurements, a pressurized gas (e.g., helium) could permeate along the SiO 2 layer and gradually fill the microcontainers making so-called 'nanoballoons'. Their consecutive deflation in air could be monitored using atomic force microscopy (AFM), and it was shown that the leakage occurred only along the SiO 2 surface, within several minutes but independently of the number of graphene layers used for the sealing 1 . These studies allowed a conclusion that graphene membranes were impermeable to all gases, at least with the achieved accuracy of 10 5 -10 6 atoms s -1 . This was further corroborated by creating individual atomic-scale defects in graphene nanoballoons, which resulted in their relatively fast deflation/inflation and confirmed the high sensitivity of the method 9,10 .

Graphene and Polyethylene: A Strong Combination Towards Multifunctional Nanocomposites

“…In order to solve the problem of high permeability to gases, the incorporation of laminar nanoparticles such as graphene has been intensely investigated [10][11][12][14][15][16][29][30][31][32]. Graphene nanosheets are intrinsically impermeable to almost all gas molecules [33] and also present high aspect ratio structures, which make them ideal to engineer tortuous pathways to decrease the molecular diffusion rates through polymeric materials [34,35]. Therefore, the efficient, aggregate-free dispersion of graphene is fundamental to achieve maximum tortuosity for gas molecules.…”

Graphene Derivatives in Biopolymer-Based Composites for Food Packaging Applications

“…Graphene is one of the most promising nanomaterials which is constituted by a flat monolayer of carbon atoms in a two-dimensional (2D) hexagonal lattice, held together by a backbone of overlapping sp 2 hybrids bonds [ 151 ]. This structure confers to graphene many remarkable properties, such as the strongest mechanical robustness with a modulus of over 1060 GPa [ 152 ], without losing its molding properties, large surface area [ 153 ], impermeability to gases [ 154 ], and optical transparency [ 155 ], among others. Graphene was considered by the scientists A.K.…”

“…Interestingly, graphene derivatives have been widely used as polymer reinforcement and are well known to impact on several properties of the final nanocomposite, namely on its mechanical, thermal, electrical, conductive, and fire retardancy properties, to name just a few [ 150 , 151 , 152 , 153 , 154 , 155 , 156 ]. Due to the already described variety of graphene derivatives, the term “graphene” is often used in a generic manner to describe indifferently each of these nanostructures, thus creating misinterpretations about its properties [ 165 ].…”

Perspectives in the design and application of composites based on graphene derivatives and bio‐based polymers

“…Potentially, 2DMs have excellent barrier properties for ions and molecules. 89 However, the permeability and selectivity of 2DM membranes are difficult to control due to the presence of defects and poor mechanical stability. The introduction of low (surfactants) and high (polymers) molecular weight organic molecules allows enhancement of the mechanical characteristics of membranes and regulation of their permeability and selectivity on demand.…”