Perfect proton selectivity in ion transport through two-dimensional crystals

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.

Stone–Wales Defects Cause High Proton Permeability and Isotope Selectivity of Single‐Layer Graphene

“…For Raman spectroscopy, a typical quality control experiment, the detectable concentration limit of topological SW 55–77 defects in a graphene monolayer is about 2 × 10 9 cm −2 , [ 26–28 ] well above the ppm‐scale concentration of 7MR that is needed to govern the proton transfer. At the same time, our conclusions are in agreement with a recent experiment by Lozada‐Hidalgo and co‐workers, [ 15 ] which showed that the membranes employed in earlier experiments are free of vacancy defects. Vacancy defects and their energy barriers have been discussed in the literature by others.…”

“…The underlying question is whether or not thermal protons can permeate through perfect 2D membranes. While the original experiments by Lozada‐Hidalgo, Geim, and co‐workers report high permeability and isotope selectivity in the 2D defect‐free materials, [ 6,7,15 ] other experimental works either report improved results using chemical vapor deposited (CVD) 2D systems [ 13 ] or suggest that the proton flow penetrates through local defects rather than the defect‐free graphene membrane. [ 10,14 ] Recent nanoballoon tests unambiguously showed that there are no atomic‐vacancy defects in the graphene used in the previous experiments.…”

“…[ 10,14 ] Recent nanoballoon tests unambiguously showed that there are no atomic‐vacancy defects in the graphene used in the previous experiments. [ 15 ] However, other structural defect types without involving atomic vacancies are well‐known for sp 2 carbon systems, such as grain boundaries, topological Stone–Wales (SW) 55–77 defects, [ 16 ] and local hydrogenation ( Figure ). In particular, the SW 55–77 defect has been observed in mechanically exfoliated CVD graphene [ 17 ] and in graphene obtained by liquid phase exfoliation of graphite.…”

“…[ 11 ] While there is a general agreement in the literature on the graphene penetration barriers for hydrogen and the nuclear quantum effects, only a few articles address the hydrogen isotope selectivity, [ 8,9,22 ] and none of them offers a consistent picture explaining the experimental results that are available to date. [ 7,15,23 ]…”

Novel Nanofluidic Cells Based on Nanowires and Nanotubes for Advanced Chemical and Bio-Sensing Applications

“…has greatly enriched and promoted the field of nanofluid ion transport [ 5 ]. Compared with nanopores or other nanofluidic channels, 2D nanofluidic channel systems with high flow rates are relatively easy and expandable to manufacture, which can be used for applications in bionic transmission and manipulation of ions, molecular screening, and energy conversion [ 6 , 7 , 8 ]. However, the currently reported research on the asymmetric ion transmission characteristics of the 2D nanofluidic homogeneous structure is still in its infancy, due to their problems of insufficient charge density and low rectification ratio [ 9 ].…”