Tiny solar device kills bacteria, disinfects water in just 20 minutes

When it comes to disinfecting water, a common method for doing so is boiling. In some parts of the world, though, this method consumes scarce fuel. Another method for disinfection is placing it out in the sun in a plastic bottle so that the ultraviolet rays will kill the microbes. However, UV rays carry only 4% of the sun’s total energy, therefore this method can take anywhere from six hours to two days and limits the amount of water people can disinfect this way.

Now, a new method has been achieved by researchers at the Department of Energy’s SLAC National Accelerator Laboratory and Stanford University. The researchers have created a nanostructured device, about half the size of a postage stamp, that can disinfect water much faster than the UV method, but also makes use of the visible part of the solar spectrum — which contains 50% of the sun’s energy.

In the team’s experiments sunlight that fell on the tiny device actually created hydrogen peroxide and other disinfecting chemicals that killed more than 99.999% of bacteria in just 20 minutes. When the work was done the chemicals quickly dissipated and left nothing but 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.

What is so special about the device?

When you look at the tiny device under an electron microscope, its surface looks like a fingerprint, with many closely spaced lines. Those lines are actually very thin films that the researchers call “nanoflakes” – molybdenum disulfide stacked on edge, like the walls of a labyrinth, atop a rectangle of glass.

In its original state, molybdenum disulfide is an industrial lubricant, but when made in layers that are just a few atoms thick, it takes on different properties, becoming a photocatalyst. This means that when it is 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 creating the molybdenum disulfide walls with just the right amount of thickness, the scientists were able to get 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 the exact reactions they wanted – reactions that produced “reactive oxygen species” like hydrogen peroxide, a commonly used disinfectant, which kill bacteria in the surrounding water.

This nanostructured device, about half the size of a postage stamp, uses sunlight to quickly disinfect water. (C. Liu et al., Nature Nanotechnology)

The good thing about Molybdenum disulfide is that it is cheap and easy to make, which is important when making devices for use in developing countries. It also absorbs a broader range of solar wavelengths than traditional photocatalysts.

“It’s very exciting to see that by just designing a material you can achieve a good performance. It really works,” said Liu, who is now working on a project in Cui’s lab that is developing air filters for combating smog. “Our intention is to solve environmental pollution problems so people can live better.”

Though the device is beneficial, it does not solve all of the issues with water disinfection. For example, it doesn’t remove chemical pollutants from water. So far it’s been tested on only three strains of bacteria and only on specific concentrations of bacteria that are mixed with less than an ounce of water in the lab, not on the complex stews of contaminants found in the real world.