“Blue energy” makes use of the osmotic pressure difference between fresh water and seawater, and is a good, renewable and clean way to generate power. Another electrokinetic phenomenon, known as the streaming potential, occurs when an electrolyte is made to pass through narrow pores, either by a pressure gradient or by an osmotic potential resulting from a salt concentration gradient. Here, membranes made from 2D materials should be efficient because the amount of water passing through a membrane increases as the membrane thickness decreases.

Now, a team of researchers led by Aleksandra Radenovic of the Laboratory of Nanoscale Biology at EPFL in Lausanne has made such a membrane from a single layer of MoS2 containing nanopores. The membrane generates a large, osmotically induced current produced from a salt gradient and a high power density.

Generating osmotic power

Water passes freely though MoS2 nanopores thanks to the enriched hydrophilic surface sites provided by the molybdenum that are produced by irradiating the material in a transmission electron microscope or by electrochemically oxidizing it. In this work, the researchers generated osmotic power by separating two reservoirs containing potassium chloride solutions of different concentrations with a freestanding MoS2 membrane that contains a single nanopore. A chemical potential gradient arises at the interface of these two liquids at a nanopore in a 0.65 nm-thick single-layer MoS2 membrane and drives ions across the nanopore, producing an osmotic ion flux, explains Radenovic.

Surface charges on the pore screen the passing ions according to their charge polarity and this produces a net measurable osmotic current, called reverse electrodialysis.

"We made our device by suspending monolayer MoS2 on a pre-etched square-shaped opening on 20 nm-thick supporting silicon nitride (SiNx) membranes on a silicon chip," she says. "We grew the MoS2 film by chemical vapour deposition. The SiNx provides structural stability and was fabricated in the cleanroom facilities at EPFL using an easy, inexpensive and scalable method that we developed to produce nanopores with sub-nanometre precision.

As for possible applications, it is still early to say and will certainly depend on industrial interest and the economic benefits that our work brings, she tells nanotechweb.org.

The team, reporting its work in Nature doi:10.1038/nature18593, is now busy looking for materials, and processing techniques, to fabricate uniform pores over larger surface areas.