Pure hydrogen fuel is non-polluting. Current methods of extracting hydrogen, however, use energy derived from sources that pollute. Finding ways to use the sun's energy to split water to extract hydrogen would make for a truly clean energy source.

Several research efforts are using materials engineered at the molecular scale to tap the sun as an energy source to extract hydrogen from water.

Researchers from Pennsylvania State University have constructed a material made from titanium dioxide nanotubes that is 97 percent efficient at harvesting the ultraviolet portion of the sun's light and 6.8 percent efficient at extracting hydrogen from water.

The material is easy to make, inexpensive, and photochemically stable, according to the researchers. The 97 percent efficiency is the highest reported, according to the researchers. There is one catch -- only five percent of the sun's energy is ultraviolet light.

The researchers are working to find a way to shift the response of the nanotube arrays into the visible spectrum.

The key to making titanium dioxide nanotubes that efficiently harvest the energy from light is controlling the thickness of the nanotube walls, according to the researchers. Nanotubes 224 nanometers long with 34-nanometer-thick walls are three times more efficient than those that are 120 nanometers long with 9-nanometer-thick walls.

The researchers made the titanium dioxide nanotube material by mixing titanium with acid and electrifying the mixture, which caused the tiny tubes to grow, then heating them to cause the material to crystallize.

The material could be ready for practical use in two to five years, according to the researchers. The work appeared in the January 12, 2005 issue of Nano Letters.

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