The perfect atomic-scale sieve

September 18, 2019 , University of Manchester
Proton transport through 2D crystals studied using aqueous solutions. a Examples of IV characteristics for 1 M HCl. Bottom inset: zoom-in. Top inset: electron micrograph of a suspended hBN membrane (aperture diameter, 2 µm). b Concentration dependence of the areal conductivity σ for monolayer hBN. Gray area indicates our detection limit given by parasitic leakage currents. Error bars: SD from different measurements. Dashed line: best linear fit to the data. Top inset: atomic force microscopy (AFM) height profile of an ‘inflated nanoballoon’. Here, graphene monolayer seals a micrometer-sized cavity containing pressurized Ar. The pressure difference across the membrane makes it to bulge up. Lateral scale bar, 1 μm; color scale, 130 nm. Bottom inset: AFM line trace taken along the blue dotted line in the top inset. DOI: 10.1038/s41467-019-12314-2

Graphene is perfectly selective to protons and blocks even smallest ions like chlorine, University of Manchester research shows. This result will be important for the development of graphene-based membranes for applications ranging from fuel cells to desalination.

Writing in Nature Communications, a team led by Dr. Marcelo Lozada-Hidalgo and Professor Andre Geim shows that 2-D materials like graphene and , also known as 'white graphene," are impenetrable to all ions. Only protons can transport through these crystals, which allows membranes with perfect proton selectivity.

The researchers had previously found that protons easily permeate through graphene's . However, it remained unknown whether other small ions could pierce the dense crystal lattice in these materials. Now the researchers have found only protons can.

Besides its relevance for the development of 2-D crystal membranes, the work further supports the previous conclusion that holes in the crystal lattice are not necessary for proton transport through 2-D crystals. The 2-D crystal itself is highly permeable for protons.

Lucas Mogg, a Ph.D. student on the project and the first author of the paper said: "In our experiments, the 2-D crystal membranes separate reservoirs that contain both protons and chlorine ions. These reservoirs are practically infinite compared to our 2-D crystal 's size. We were very surprised to see that a one-atom-thick barrier was enough to stop all chlorine ions from crossing. Even thick polymer membranes especially designed to separate ions sometimes fail to achieve such perfect selectivity."

These findings are relevant for theory developments in the field of 2-D ionic conductors, adds Dr. Lozada-Hidalgo. "Our results conclusively show that proton transport through the two-dimensional crystals occurs through their bulk and does not require atomic-scale defects. This is an important development in our understanding of the interaction between ions and atomically-thin crystals with implications that extend well beyond the present study."

The results are also thought to be important in the development of a wide range of applications that use graphene as membrane materials. "Our results have implications for technologies that use graphene as a membrane material. The fast permeation of protons through the pristine 2-D crystal bulk is typically not taken into consideration. However, it could be important for designing and optimizing these membranes, especially when operating in ," explains Marcelo Lozada-Hidalgo.

The researchers are enthusiastic about the prospects opened by this work. They believe that many more crystals could be studied using a similar approach. Most 2-D remain unexplored from this perspective. The researchers think more unexpected phenomena and new applications could be found in these new materials.

More information: L. Mogg et al. Perfect proton selectivity in ion transport through two-dimensional crystals, Nature Communications (2019). DOI: 10.1038/s41467-019-12314-2

Journal information: Nature Communications

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