Abstract
Defect-free monolayers of graphene and hexagonal boron nitride are surprisingly permeable to thermal protons, despite being completely impenetrable to all gases. It remains untested whether small ions can permeate through the two-dimensional crystals. Here we show that mechanically exfoliated graphene and hexagonal boron nitride exhibit perfect Nernst selectivity such that only protons can permeate through, with no detectable flow of counterions. In the experiments, we use suspended monolayers that have few, if any, atomic-scale defects, as shown by gas permeation tests, and place them to separate reservoirs filled with hydrochloric acid solutions. Protons account for all the electrical current and chloride ions are blocked. This result corroborates the previous conclusion that thermal protons can pierce defect-free two-dimensional crystals. Besides the importance for theoretical developments, our results are also of interest for research on various separation technologies based on two-dimensional materials.
Highlights
Defect-free monolayers of graphene and hexagonal boron nitride are surprisingly permeable to thermal protons, despite being completely impenetrable to all gases
As for the experiment, it was found that proton permeation through mechanically exfoliated crystals is thermally activated with energy barriers of ≈0.8 eV for graphene and ≈0.3 eV for monolayer hexagonal boron nitride[1]
The investigated devices were fabricated using monolayer graphene and mono- and bilayer hexagonal boron nitride (hBN) crystals that were isolated by micromechanical cleavage[18]
Summary
The investigated devices were fabricated using monolayer graphene and mono- and bilayer hBN crystals that were isolated by micromechanical cleavage[18] (see Methods and Supplementary Fig. 1). 15,16 and previously used in our experiments[1] To this end, we made hBN and graphene membranes to cover micrometer-sized cavities etched in an oxidized Si wafer and tested the enclosures for possible gas leaks (see inset Fig. 1b and Supplementary Section ‘Leak tests using nanoballoons’). (tH and tCl for protons and chloride, respectively) and, by definition, they satisfy tH + tCl ≡ 1 and the inequality: 0 ≤ both tH and tCl ≤ 1 To find their values for our 2D membranes, we used the same setup as in the measurements discussed in Fig. 1 but with different HCl concentrations in the two compartments (inset of Fig. 2b). The force pushing ions across the membrane, due to the concentration gradient, can be counteracted by applying voltage a 90 b
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