Tiny Solar Device Rapidly Disinfects Water With Nanoflakes & “Eager Electrons”

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A tiny device, half the size of a postage stamp, which can rapidly disinfect water with solar energy, has been developed by researchers at Stanford University and the SLAC National Accelerator Laboratory. While the nanostructured device has only been tested with small amounts of water, and on three strains of bacteria, the little gadget shows promise in the quest for affordable and effective low-power water purification solutions.

We already know how to use the sun’s energy to disinfect and purify water, such as with solar stills or through the simpler method of passive solar UV disinfection (exposing the water to the sun’s rays for up to 48 hours), but considering the severe health threats that contaminated water still poses to many people around the world, there’s always room for more options. And this gadget, because it’s small and made from a “cheap and easy to make” material, molybdenum disulfide, could eventually be a viable solution for point-of-use water disinfection, either in the developing world or for emergency situations.

According to the findings of the researchers, which were published in Nature Nanotechnology under the title “Rapid water disinfection using vertically aligned MoS2 nanofilms and visible light,” the tiny device was able to kill “more than 99.999 percent of bacteria in just 20 minutes” through the actions of its molybdenum disulfide nanoflakes (very thin films stacked on edge on a glass substrate).

“In ordinary life, molybdenum disulfide is an industrial lubricant. But like many materials, it takes on entirely different properties when made in layers just a few atoms thick. In this case it becomes a photocatalyst: When hit by incoming light, many of its electrons leave their usual places, and both the electrons and the “holes” they leave behind are eager to take part in chemical reactions.

“By making their molybdenum disulfide walls in just the right thickness, the scientists got them to absorb the full range of visible sunlight. And by topping each tiny wall with a thin layer of copper, which also acts as a catalyst, they were able to use that sunlight to trigger exactly the reactions they wanted – reactions that produce “reactive oxygen species” like hydrogen peroxide, a commonly used disinfectant, which kill bacteria in the surrounding water.”

“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.

According to the researchers, “the killer chemicals quickly dissipated,” leaving behind just potable drinking water. However, because the device hasn’t been tested with other bacterial strains, or under real world conditions with the “complex stews of contaminants” found there, and doesn’t remove chemical pollutants from water, it’s not a one-size-fits-all purification device, but could lead to further innovations in materials design and/or effective water disinfectant technology.