Coming clean: A micrograph shows the surface of a light-activated catalyst that disinfects water even in the dark. Palladium nanoparticles on the surface of a nitrogen-doped titanium oxide help to extend the catalyst's disinfection power up to 24 hours.
Credit: Shang, et al. University of Illinois at Urbana-Champaign
Getting access to clean drinking water is an ongoing problem for people in developing countries. And even cities that have good water-treatment systems are looking for better ways to deliver safer, cleaner water. Now an international research team has developed a photocatalyst that promises quick, effective water disinfection using sunlight or artificial light. What's more, the photocatalyst keeps working after the light is turned off, disinfecting water even in the dark.

It has long been known that irradiating water with high-intensity ultraviolet light kills bacteria. Some water filters made for campers and hikers, for example, use this technology. Researchers have been working to enhance the method's effectiveness by adding a photocatalyst that gets activated by UV light and generates reactive chemical compounds that break down microbes into carbon dioxide and water.

The new photocatalyst improves on that by using visible, rather than UV, light. Synthesized by Jian-Ku Shang, professor of materials science and engineering at the University of Illinois, Urbana-Champaign, and his colleagues, the photocatalyst works with light in the visible spectrum--wavelengths between 400 and 550 nanometers. It consists of fibers of titanium oxide--a common material used as a white pigment--doped with nitrogen to make it absorb visible light. Alone, the nitrogen-doped titanium oxide kills bacteria, though not efficiently. The researchers added nanoparticles of palladium to the surface of the fibers, greatly increasing the efficiency of the disinfection. He and his colleagues at the Shenyang National Laboratory for Materials Sciences in China published their work online in the Journal of Materials Chemistry.

"It would be very nice to shift activity of the traditional [photocatalyst] materials, which were only activated by ultraviolet radiation, to visible," says Alexander Orlov, assistant professor of materials science and engineering at Stony Brook University in New York. "If you look at the solar spectra, it contains only 5 percent ultraviolet and around 46 of visible." Such photocatalysts would allow solar energy to be used more efficiently as well as used indoors, since fluorescent lighting contains very little ultraviolet light.

Shang and his colleagues tested the photocatalyst by placing it in a solution containing a high concentration of E. coli bacteria and then shining a halogen desk lamp on the solution for varying lengths of time. After an hour, the concentration of bacteria dropped from 10 million cells per liter to just one cell per 10,000 liters.

The researchers also tested the photocatalyst's ability to disinfect in the dark. They shined light on the fibers for 10 hours to simulate exposure to daylight and then stored them in the dark for various times. Even after 24 hours, the photocatalyst still killed bacteria. In fact, just a few minutes of illumination was enough to keep the photocatalyst activated for up to that length of time.

"Typically, when you have a photocatalyst, the activity will stop almost instantaneously when the light is switched off," Shang says. "The chemical species you generate will only last a few nanoseconds. This is an intrinsic drawback of a photocatalytic system, since you require light activation essentially all the time."

The palladium nanoparticles boost the photocatalyst's power in two ways. When photons hit the material, they create pairs of positive and negative charges--holes and electrons. The positively charged holes on the surface of the nitrogen-doped titanium oxide react with water to produce hydroxyl radicals, which then attack bacteria. "What palladium nanoparticles do is they grab electrons away so most of the holes you produce will be able to survive without being neutralized by electrons," says Shang.

As soon as they grab the electrons, the nanoparticles enter a different chemical state and store the negative charges. "When the light is switched off, that charge gets slowly released, and that slow release is what gives us that memory effect," Shang says. "That charge can react with water molecules to produce oxidizing agents again." He says nanoparticles of other transition metals, like silver, also enhance the photocatalyst's effectiveness.

The photocatalyst offers the ability to disinfect at full power during the day and then keep working at night or during power outages. Also, because the disinfection happens quickly, systems could be designed to clean large volumes of water by exposing it to light as the water flows through pipes, Shang says.

Copyright Technology Review 2010.