With almost three quarters of the Earth covered in water, it’s hard to imagine that there are places in the world where safe drinking water is scarce. Yet more than 1 billion people worldwide lack access to freshwater. That’s because nearly 97 percent of Earth’s water is locked away in salty ocean water. The process of desalinating this seawater is both energy intensive and expensive, as the chemical bonds formed by salt with water are extremely difficult to manipulate.
Penn researchers are working on a technology that could potentially offer a new method of desalinating water that would be both fast and scalable. Although the research is only in its fundamental stages, the researchers hope that, down the line, the general method using thin membranes could lead to effective desalination techniques and increase access to clean drinking water.
The research, which was published in Nano Letters, was led by physics professor Marija Drndić and graduate students Jothi Priyanka Thiruraman, Gopinath Danda, and Paul Masih Das. They worked in collaboration with researchers at Penn State, led by professor Mauricio Terrones and postdoc Kazunori Fujisawa, and researchers from the Laboratory Interdisciplinaire Carnot de Bourgogne, led by professors Adrien Nicolai and Patrick Senet.
The researchers were working with two-dimensional materials, a class of nanomaterials that is only one or two atoms thick. These materials are created in a chemical vapor deposition chamber, where the researchers flow gases onto a substrate to grow the material in a 2-D fashion. They then transfer the single-atom thick material over a larger hole to form a membrane and expose it to a focused ion beam to kick out single atoms, creating tiny atomic vacancies, or holes, in the material.
Previously, the researchers had been investigating creating similar but larger holes called nanopores in two-dimensional materials such as graphene. These nanopores are typically just a few nanometers across, one nanometer being 100,000 times smaller than the width of a single strand human hair.
This research pushes that concept even further, utilizing what the researchers call “angstrom-size” pores, 1 million times smaller than the width of a single strand of human hair. Here, the holes are smaller than one nanometer and consist of only one or a few missing atoms. Because of the size of these pores, when the researchers separate salt solutions with the membrane, the salt ions are left behind as the water molecules pass through.
“It’s like a tollgate where we’re engineering a pathway for only small cars to pass through and block big vehicles, the salts ions,” says Thiruraman. “In addition, we would like the small cars to have a swift transport while encountering the tollgate. This is what we’re working towards.”
Using a form of imaging called transmission electron microscopy, in which a beam of electrons is transmitted through a specimen to form a higher resolution image, the researchers could see these tiny pores up close and confirm that they truly were single atom defects by counting the missing atoms.
“The details about particle-matter interactions that can lead to the removal of atoms with ions is a complex subject,” Drndić says. “Depending on the particle type, the particle energy and the material, different processes can dominate. … The study is contributing to a lot of different areas of science. Making these connections better and with more examples really helps inform and move all these fields forward.”
One of the challenges the researchers expect to face when it comes to applying this method for water desalination is that many of these two-dimensional materials are difficult to grow in large areas.
“Right now, we’re limited to the size of the crystal, which is only a few microns,” says Masih Das. “If you compare that to your Brita filter at home it's clearly not the same size.”
They are currently collaborating with other researchers to investigate how to scale this method, as well as to determine how different atomic configurations of the pores affect the amount of water that passes through. They are also investigating what happens when they try different salt solutions and ion sizes.
“The interactions between molecules and the lattice matter a lot,” Drndić says. “Theorists are noticing a very large difference between different configurations. We're working with them to map and make a catalogue of possible holes to determine the optimal size and shape of the hole.”
In addition to its potential applications in water desalination, Masih Das says he is most excited about the fundamental aspects of this research.
“We’re working with two-dimensional materials,” he says, “which are essentially sheets of atoms, and we’re literally removing single atoms, making the smallest possible holes that exist and pushing water through them. It’s a pretty novel feat in and of itself.”
Photo at top: Penn researchers are working on a technology that could potentially offer a new method of desalinating water, increasing access to clean drinking water around the world.
