August 15th, 2016
"Black rectangle" uses the sun to make water safe to drink
If you're trying to kill bacteria in tainted water, one of the best ways is simply to leave that water in a clear bottle in direct sunlight, letting the ultraviolet rays do the work. That said, this method can take six to 48 hours to be fully effective. Instead, scientists at the US Department of Energy's SLAC National Accelerator Laboratory and Stanford University have developed a quicker alternative. It's a sun-activated device that kills 99.999 percent of bacteria in just 20 minutes.
The rectangular device is about half the size of a postage stamp, and looks like nothing more than a piece of black glass that's placed in a clear container of water. In fact, it consists of a glass substrate coated with "nanoflakes" of molybdenum disulfide. These flakes are stacked sideways, so that their edges are facing up. Each of those edges is in turn coated with a thin layer of copper.
When exposed to the visible part of the solar spectrum, both the molybdenum disulfide and the copper act as photocatalysts, triggering the formation of hydrogen peroxide and other disinfectant chemicals within the water. Once they've finished killing the bacteria, all of the chemicals dissipate, leaving nothing but pure water behind.
Part of the reason that it works so much more quickly than the ultraviolet method lies in the fact that while UV rays carry just 4 percent of the sun's energy, the visible light carries 50 percent.
That said, it should be noted that the technology only works on bacterial contamination, not chemical. Also, it has so far only been tested on three strains of bacteria, although there is reason to believe that it should work on others not yet tested.
A paper on the research was recently published in the journal Nature Nanotechnology.