Proton transport through one-atom-thick crystals

Abstract

Graphene is impermeable to all gases and liquids 1-3 , and even such a small atom as hydrogen is not expected to penetrate through graphene's dense electronic cloud within billions of years 3-6 . Here we show that monolayers of graphene and hexagonal boron nitride (hBN) are unexpectedly permeable to thermal protons, hydrogen ions under ambient conditions. As a reference, no proton transport could be detected for a monolayer of molybdenum disulfide, bilayer graphene or multilayer hBN. At room temperature, monolayer hBN exhibits the highest proton conductivity with a low activation energy of 0.3 eV but graphene becomes a better conductor at elevated temperatures such that its resistivity to proton flow is estimated to fall below 10 -3 Ohm per cm 2 above 250°C. The proton barriers can be further reduced by decorating monolayers with catalytic nanoparticles. These atomically thin proton conductors could be of interest for many hydrogen-based technologies.Graphene has recently attracted renewed attention as an ultimately thin membrane that can be used for development of novel separation technologies (for review, see refs. 7,8). If perforated with atomic or nanometer accuracy, graphene may provide ultrafast and highly selective sieving of gases, liquids, ions, etc. 2,9-19 However, in its pristine state, graphene is absolutely impermeable for all atoms and molecules moving at thermal energies [1][2][3][4][5][6][7] . Theoretical estimates for the kinetic energy E required for an atom to penetrate through monolayer graphene vary significantly, depending on the employed model, but even the smallest literature value of 2.4 eV for atomic hydrogen 3-6 is 100 times larger than typical k B T which ensures essentially an impenetrable barrier (k B is the Boltzmann constant and T the temperature). Therefore, only accelerated atoms are capable of penetrating through the one atom thick crystal 20,21 . The same is likely to be valid for other two dimensional (2D) crystals 22,23 , although only graphene has so far been considered in this context. Protons can be considered as an intermediate case between electrons that tunnel relatively easily through atomically thin barriers 24 and small atoms. It has been calculated that E decreases by a factor of up to 2 if hydrogen is stripped of its electron 4,5 . Unfortunately,

Understanding the interaction between energetic ions and freestanding graphene towards practical 2D perforation

“…(c) Binary collision model illustrated in a centerof-mass reference frame moving at a speed of vc, where collision parameters p and θ are the shortest projected distance and the scattering angle between two colliding particles, respectively (following reference 19 (Figure 2b). Reaffirming the mechanism of HIM imaging, 30,31 our finding sheds a renewed light on the possibility that, not only to proton, 34,35 graphene can be nearly transparent to energetic He + ions as shown theoretically. 17 For example, at a kinetic energy of 30 keV approximately 99% of He + ions can penetrate through the monolayer graphene with statistically sputtering little or no carbon atom from the lattice, reminiscent of the photon and proton transmission.…”

Electrochemical Applications of Two-Dimensional Nanosheets: The Effect of Nanosheet Length and Thickness

“…16 A range of 2D materials including graphene and MoS2 have been used in electrochemical energy storage applications, 9,17 typically as electrodes in batteries [18][19][20] and supercapacitors 21,22 .…”

Graphene opens up to new applications

“…A number of theoretical studies have suggested that the selectivity and permeability of such membranes could be vastly superior to the polymerbased filtration membranes that are typically used today [1][2][3] . And in the past few years, experimental demonstrations of the potential of nanoporous graphene membranes have begun to emerge [4][5][6][7][8][9] .…”

“…Recently, it has been reported that protons can, in fact, be transported through pristine monolayers of graphene 9 . (Other two-dimensional materials were also examined, and it was found that protons can pass through monolayers of hexagonal boron nitride, but not through monolayers of molybdenum disulphide.)…”