Chip-sized Device Disinfects Water in Minutes

In many parts of the world, the only way to make germy water safe is by boiling, which consumes fuel, or by putting it in the sun in a bottle so ultraviolet (UV) rays will kill the microbes. But because these rays carry only 4% of the sun’s total energy, the UV method takes up to 48 hours, limiting the amount of water that can be disinfected.

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 faster than the UV method by making use of the visible part of the solar spectrum, which contains 50% of the sun’s energy.

The device works faster than those that use only ultraviolet rays. Image credit: Jin Xie/Stanford University

In experiments carried out by the researchers, sunlight falling on the device triggered the formation of hydrogen peroxide and other disinfecting chemicals that killed more than 99.999% of bacteria in 20 minutes. The chemicals quickly dissipated, leaving drinkable 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,” says Chong Liu, postdoctoral researcher in the laboratory of Yi Cui, a SLAC/Stanford associate professor.

Under an electron microscope, the surface of the device looks like a fingerprint, with many closely spaced lines. Those lines are thin films of molybdenum disulfide that are stacked on edge atop a rectangle of glass.

Molybdenum disulfide is an industrial lubricant. But it takes on different properties when made in layers 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 take part in chemical reactions.

The researchers succeeded in making the molybdenum disulfide walls the right thickness 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 reactions that produce “reactive oxygen species” such as hydrogen peroxide, a commonly used disinfectant, to kill bacteria in the surrounding water.

Molybdenum disulfide is relatively inexpensive and easy to make Cui says. It also absorbs a broader range of solar wavelengths than traditional photocatalysts.

The method does not remove chemical pollutants from water. So far, it has been tested on three strains of bacteria, although—according to the researchers—there is no reason to think it would not kill other bacterial strains and other types of microbes, such as viruses. Additionally, it has been tested only on specific concentrations of bacteria mixed with less than an ounce of water in the lab.