Postage Stamp-sized Device Traps More Solar Energy to Purify Water Faster

Scientists have found a way to make water as clean as this, thanks to a tiny gadget. Atomic Imagery/Getty Images

Water purification is a big-time problem in the developing world, where clean running water cannot be taken for granted. Current methods involve either boiling water (which uses precious resources) or relying on the sun, which is free but takes several hours to work.

However, this issue could someday be fixable thanks to an itsy-bitsy device. Although it's still a long way off from the market, the nanostructured device, created by the bright minds at the Department of Energy's SLAC National Accelerator Laboratory and Stanford University, hopes to make much slower purification techniques obsolete.

A researcher holds a small, nanostructured device that uses sunlight to disinfect water
Xie/Stanford University

At half the size of a postage stamp, the device uses visible light to get the job done. Visible light, a type of electromagnetic radiation, is pretty much exactly as it sounds – light that most human eyes can pick up on. Other types of electromagnetic radiation include infrared radiation, ultraviolet (UV) radiation, radio waves, microwaves and X-rays.

Current purification methods rely on UV rays to make water safe, but such rays comprise a much smaller percentage of the sun's energy (4 percent), and require six to 48 hours of direct sunlight to work. The new gadget is able to harness visible light, which carries roughly 50 percent of the sun's energy.

The device features "nanoflakes" composed of molybdenum disulfide stacked just so on a tiny glass rectangle. The layers of molybdenum turn into a photocatalyst when affected by incoming light, so they can absorb the full range of visible sunlight. They're also capped with a very thin layer of copper, which helps to trigger the production of disinfectants like hydrogen peroxide and other reactive oxygen species, which effectively rid the water of 99.999 percent of bacteria in a measly 20 minutes. Bonus: molybdenum disulfide is inexpensive and easy to make. 

The device consists of thin flakes of molybdenum disulfide arranged like walls on a glass surface and topped with a thin layer of copper. Light falling on the walls triggers formation of hydrogen peroxide (H2O2) and other reactive oxygen species ...
C. Liu et al., Nature Nanotechnology

"Our device looks like a little rectangle of black glass. We just dropped it into the water and put everything under the sun, and the sun did all the work," says lead author Chong Liu in a press release. The study results were published in Nature Nanotechnology.

So far, the device has only been challenged by three lab-based strains of bacteria, rather than the complex collection of contaminants found in the real world. Researchers say the next steps are to test it on other bacteria as well as viruses in the lab. Then they will try out the device in naturally polluted water such as that found in developing countries. The scientists estimate it will take three to five more years to finish development.