SLAC, Stanford gadget
grabs more solar energy to disinfect water faster
DOE/SLAC
NATIONAL ACCELERATOR LABORATORY
Now 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 much faster than the UV method by also
making use of the visible part of the solar spectrum, which contains 50 percent
of the sun's energy.
In experiments reported today in Nature Nanotechnology, sunlight falling on the little device triggered the formation of hydrogen peroxide and other disinfecting chemicals that killed more than 99.999 percent of bacteria in just 20 minutes. When their work was done the killer chemicals quickly dissipated, leaving 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.
Nanoflake Walls and Eager Electrons
Under an electron
microscope the surface of the device looks like a fingerprint, with many
closely spaced lines. Those lines are very thin films - the researchers call
them "nanoflakes" - of molybdenum disulfide that are stacked on edge,
like the walls of a labyrinth, atop a rectangle of glass.
In ordinary life,
molybdenum disulfide is an industrial lubricant. But like many materials, it
takes on entirely different properties when made in layers just 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 are eager to take part in chemical
reactions.
By making their
molybdenum disulfide walls in just the right thickness, the scientists got 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 exactly the reactions they wanted - reactions that
produce "reactive oxygen species" like hydrogen peroxide, a commonly
used disinfectant, which kill bacteria in the surrounding water.
Molybdenum disulfide
is cheap and easy to make - an important consideration when making devices for
widespread use in developing countries, Cui said. It also absorbs a much
broader range of solar wavelengths than traditional photocatalysts.
Solving Pollution Problems
The method is not a
cure-all; for instance, it doesn't remove chemical pollutants from water. So
far it's been tested on only three strains of bacteria, although there's no
reason to think it would not kill other bacterial strains and other types of
microbes, such as viruses. And it's only been tested on bacteria mixed with
water in the lab, not on the complex stews of contaminants found in the real
world.
Still, "It's very
exciting to see that by just designing a material you can achieve a good
performance. It really works," said Liu, who has gone on to work 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."
###
The work was funded by
the Department of Energy Office of Science through SIMES, and carried out in
collaboration with Professor Alexandria Boehm's group in the Stanford
department of civil and environmental engineering.
SLAC is a
multi-program laboratory exploring frontier questions in photon science,
astrophysics, particle physics and accelerator research. Located in Menlo Park,
California, SLAC is operated by Stanford University for the U.S. Department of
Energy Office of Science. To learn more, please visit http://www.slac.stanford.edu.
SLAC National
Accelerator Laboratory is supported by the Office of Science of the U.S.
Department of Energy. The Office of Science is the single largest supporter of
basic research in the physical sciences in the United States, and is working to
address some of the most pressing challenges of our time. For more information,
please visit science.energy.gov.