Device disinfects water faster with solar energy

In experiments reported today 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% 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.

She is a postdoctoral researcher in the laboratory of Yi Cui, a SLAC/Stanford associate professor and investigator with SIMES, the Stanford Institute for Materials and Energy Sciences at SLAC.

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.

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.

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; for instance, 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 bacteria mixed with water in the lab, not on the complex stews of contaminants found in the real world.