Using the sun to disinfect water is nothing new; UV rays start killing bacteria after six hours, although the process can take up to 48 hours. It’s time-consuming, but often the only option in developing countries. Researchers at the Department of Energy's SLAC National Accelerator Laboratory and Stanford University's Institute for Materials and Energy Science created a device to speed up that process. Their findings were published in Nature Nanotechnology earlier this week. The device, which is smaller than a postage stamp, not only uses UV rays but also the visible part of the solar spectrum to disinfect water.

The device is made of glass and invisible layers of molybdenum disulfide. That compound becomes a photocatalyst that produces hydrogen peroxide (a disinfectant) when hit with visible light. In this case, the device killed 99.99 percent of bacteria after 20 minutes of use in 25 milliliters of water.

While this device is incredibly cheap and easy to manufacture, the researchers stipulate that they only tested it with lab-produced E. coli and lactic acid bacteria. It isn’t proven to effectively destroy viruses or chemicals.