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[BEIJING] Scientists have used nanotechnology to develop a more efficient way of using light to purify water — even in the dark.

Light is often used as a water purifier and existing methods rely on processes stimulated by ultraviolet (UV) light.

But UV accounts for just five per cent of daylight so a method using visible light — which accounts for almost half — is more desirable.

Now researchers from the Shenyang National Laboratory for Materials Science in China and the University of Illinois have developed a photocatalyst that uses visible light to kill bacteria.

The catalyst is made from a grid of titanium oxide fibres impregnated with nitrogen. When light photons hit the grid a positive charge is created which splits water molecules, producing a substance  deadly to microbes.

The photocatalyst becomes more efficient when nanoparticles of the metal palladium are added as these hold the positive charge for longer.

The researchers tested the photocatalyst by placing it in water, containing a high level of the bacterium Escherichia coli, under a lamp. After one hour the concentration of bacteria had been reduced to below the safe level for drinking water.

After ten hours under the lamp — to simulate daytime — the solution was placed in the dark. The researchers found the catalyst continued to kill bacteria for up to 24 hours with no light source. This is because the palladium nanoparticles continued to release trapped electrons.

Shang Jian-Ku, associate professor of materials science and engineering at the University of Illinois and lead author of the paper, told SciDev.Net that the new catalysts are both more energy efficient and more effective than previous photocatalysts used with UV light.

"Unlike UV or other disinfection techniques [such as chlorination] they can also kill some of the toughest microbes such as spores," he said.

The researchers believe this purification technique could find a broad range of environmental applications, from water treatment plants to devices used to disinfect water in homes. And because it works in the dark, it could be used overnight or during power cuts.

Alexander Orlov, assistant professor of materials science and engineering at Stony Brook University, United States, described Shang's study as interesting but said there are still uncertainties about whether it could be used in developing countries.

He said precious metals such as palladium are expensive. Also the study does not address how well the method works in the long term.

Orlov suggested researchers compare their method with simpler and more traditional water disinfection techniques.

Link to full paper in the Journal of Materials Chemistry