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Shedding Light in the Darkness

Purifying Water With Nanotechnology

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Researchers at the Department of Energy’s SLAC National Accelerator Laboratory and Stanford University have created a nanostructured device, about half the size of a postage stamp, that disinfects water much faster than the typical UV method by also making use of the visible part of the solar spectrum, which contains 50 percent of the sun’s energy.

In experiments reported in Nature Nanotechnology, sunlight falling on the little device triggered the formation of hydrogen peroxide and other disinfecting chemicals that killed more than 99.999 percent of bacteria in just 20 minutes. When their work was done the killer chemicals quickly dissipated, leaving pure water behind.

“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.

Under an electron microscope the surface of the device looks like a fingerprint, with many closely spaced lines. Those lines are very thin films – the researchers call them “nanoflakes” – of molybdenum disulfide that are stacked on edge, like the walls of a labyrinth, atop a rectangle of glass.

Molybdenum disulfide is cheap and easy to make – an important consideration when making devices for widespread use in developing countries, Cui said. It also absorbs a much broader range of solar wavelengths than traditional photocatalysts.

The method is not a cure-all -it doesn’t remove chemical pollutants from water. So far it’s been tested on only three strains of bacteria, although there’s no reason to think it would not kill other bacterial strains and other types of microbes, such as viruses. And it’s only been tested on specific concentrations of bacteria mixed with less than an ounce of water in the lab, not on the complex stews of contaminants found in the real world.

Still, “It’s very exciting to see that by just designing a material you can achieve a good performance. It really works,” said Liu, who has gone on to work on a project in Cui’s lab that is developing air filters for combating smog. “Our intention is to solve environmental pollution problems so people can live better.”

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http://www.nature.com/nnano/journal/vaop/ncurrent/full/nnano.2016.138.html

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