This Tiny Black Rectangle Disinfects Water in Minutes

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©. Jin Xie/Stanford University

One of the major crises facing the world is access to clean drinking water. Because of that, we've seen a variety of water purification gadgets and materials over the years that could make it easier for people to attain clean water. One approach is using UV rays to disinfect water, but since UV rays only carry about 4 percent of the sun's energy, that method can take up to 48 hours, which limits the amount of water people can treat at a time.

Stanford University researchers and the SLAC National Accelerator Laboratory decided there had to be a faster way. What it you could use the visible part of the solar spectrum, not just UV rays, harnessing 50 percent of the sun's energy? With that in mind, the researchers created a small device that when dropped in water uses solar energy to disinfect it in just minutes.

“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,” said Chong Liu, lead author of the report published in Nature Nanotechnology.

The nanostructured device is about half the size of a postage stamp. When sunlight hits it, it forms hydrogen peroxide and other bacteria-killing chemicals that are able to eliminate 99.999 percent of the bacteria present in the water sample in only 20 minutes. The chemicals then dissipate and leave pure water behind.

The small glass is topped with what the researchers call "nanoflakes" of molybdenum disulfide. The thin flakes are stacked on their edges, creating a labyrinth like shape that resembles a fingerprint when viewed with a microscope.

water disinfect stanford

© C. Liu et al., Nature Nanotechnology

Molybdenum disulfide is an industrial lubricant, but in very thin layers like what is used in this device, it becomes a photocatalyst, releasing electrons that take part in chemical reactions. The researchers were able to create layers that absorb the full range of visible sunlight and trigger reactions with oxygen, like hydrogen peroxide, which kills the bacteria present in the water.

While the device does disinfect the water, it's not able to remove any chemical pollutants, so it's best suited for areas that are mostly concerned with microbes in the water, not industrial pollution. The researchers need to do further testing to make sure that it can eliminate more strains of bacteria and work in water that hosts a complex mixture of them like what would occur in the real world.