Using Light to Disinfect Water

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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.