125 Stanford Stories

NO. 75

Nanoengineering for good

SLAC and Stanford researchers devised this small, nanostructured filter that uses sunlight to disinfect water. By harnessing a broad spectrum of sunlight, it works faster than devices that use only ultraviolet rays.
Jin Xie
An electron micrograph shows the pattern of nanostructured walls on the surface of the device. When the device was dropped into a sample of contaminated water and placed in sunlight, it killed more than 99.999 percent of bacteria in just 20 minutes.
C. Liu et al., Nature Nanotechnology
nano air filter
Chong Liu, Stanford postdoctoral researcher in materials science and engineering, is lead author of the report that a nanostructured Stanford/SLAC filter disinfected water many times faster than other solar methods. Here, Liu demonstrates a nanoengineered air filter devised in Yi Cui's lab in 2015.
Kurt Hickman/Stanford Video
The device consists of thin flakes of molybdenum disulfide arranged like walls on a glass surface and topped with a thin layer of copper. Light falling on the walls triggers formation of hydrogen peroxide (H2O2) and other “reactive oxygen species” that kill bacteria in water.
C. Liu et al., Nature Nanotechnology
cui nano
Yi Cui, associate professor of materials science and engineering at Stanford, and members of his lab publish more than 30 papers annually on projects that apply nanotechnology to environmental problems such as battery storage, renewable energy and pollution.
Linda A. Cicero/Stanford News Service

Faster water filter is Stanford, SLAC team’s latest creation

Providing clean water is one of the most pressing challenges in the developing world. It takes expensive infrastructure to purify a municipal water supply, hours of household labor to boil or chemically treat impure water, or even longer to put water in a plastic bottle and wait for the sun’s ultraviolet rays to disinfect it.

Now, researchers at Stanford and the SLAC National Accelerator Laboratory have developed a nanostructured water filter half the size of a postage stamp that can disinfect water in minutes by harnessing more of the sun’s power to trigger microbe-killing chemical reactions.

In lab tests, the little filter dropped into about an ounce of water killed more than 99.999 percent of bacteria in 20 minutes without further human effort.

“We just dropped it into the water and put everything under the sun, and the sun did all the work,” Chong Liu, lead author of the report in in Nature Nanotechnology, told SLAC. Liu is a postdoctoral researcher in the laboratory of Stanford associate professor of materials science and engineering Yi Cui, an investigator with the Stanford Institute for Materials and Energy Sciences at SLAC (SIMES).

The device exploits how the properties of its material change when milled at nanoscale. It’s made with molybdenum disulfide, a common industrial lubricant. When fabricated into ridges just a few atoms thick, topped with copper and exposed to light, molybdenum disulfide triggers the formation of hydrogen peroxide and other chemicals that kill bacteria.

By reacting to visible light — 50 percent of the sun’s energy — rather than just the 4 percent of solar energy that is ultraviolet rays, the new filter works many times faster than other solar purification methods. Because it is made of inexpensive materials, its creators believe it holds promise for the developing world.

As a graduate student in Cui’s lab in 2015, Liu worked on a similar project to create a low-cost, highly efficient air filter from nanospun polyacrylonitrile (PAN), a material commonly used to make surgical gloves. Spun at nanoscale, PAN develops properties that attract and trap particles of a size most dangerous to human lungs.

These are only two of the many innovations Cui’s lab has developed to apply nanotechnology to environmental challenges.  Since 2010, members of the group have published more than 30 papers each year on concepts ranging from pollution filters to renewable energy to ultra-efficient batteries.

Next steps for the water filter are to lab-test it on more bacteria and on viruses, and then to field-test it in naturally polluted water, including in developing countries.

“Nanotechnology is bringing very exciting opportunities – new ideas, new materials, new devices, new mechanisms for treating water,” Cui told Stanford Engineering. “This has become some of the key research guiding my group.”