Sunlight makes a great disinfectant: Leave a clear bottle of water in the sun for a day, and its UV light will have zapped all the bacteria inside, making it safe to drink (unless it’s contaminated with chemicals, in which case you’re on your own).
But UV is inefficient. It carries only a small fraction of the sun’s total energy output. This little black chip, on the other hand, can harness the much more of the sun’s energy. You just have to toss it onto the water, wait 20 minutes, and you’re done. Your water is clean.
The device, from a team at Stanford University, is covered in nanoflakes of molybdenum disulfide, a cheap and easy-to-make material. Because the flakes are so thin, they act differently, becoming a photocatalyst. That means that when they are hit by sunlight, they help along a chemical reaction in the water itself. In this case, the researchers added a little copper to the mix, fine-tuning the recipe to make "reactive oxygen species" like hydrogen peroxide, a common ingredient in bleach.
The nano-flakes on the little chip’s surface are arranged so they stand on edge, maximizing their surface area. The catalyst is so efficient that, if tossed into water, it can kill 99.999% of bacteria in just 20 minutes. The corresponding paper, published in Nature Nanotechnology, says that the device has been tested on only three kinds of bacteria and on water in the lab. But the disinfectants it produces should work fine on other bacteria, as well as on viruses. Like plain old UV light, though, the device can’t do anything for chemical contamination.
The device is both ingenious and simple, and because it is so cheap to make, there’s a real possibility that it could be deployed in numbers big enough to make a difference, making it practical to clean large amounts of drinking water in places where it would normally be scarce. I guess the biggest problem is that the slivers are so small that they might end up being swallowed.
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