Machine Design

Generating hydrogen with nanotubes

Researchers at Sandia National Laboratory have fashioned nanotubes that one day may use the entire visible and UV spectrum of sunlight to produce hydrogen.

An electron microscope photo of a porphyrin nanotube. They measure about 50 to 70 nm in diameter with walls approximately 20 nm thick.

Sandia scientist Zhongchun Wang examines the glow emitted by porphyrin nanotubes. It is caused by the tubes' resonance light scattering.

The scientists build the tubes out of porphyrins which are molecules closely resembling chlorophyll, the active ingredient in photosynthetic proteins. The hollow tubes measure a few micrometers long with outer diameters of 50 to 70 nm and walls approximately 20 nm thick. The tubes are built up through ionic selfassembly of two oppositely charged porphyrins. Most importantly, the tubes exhibit intense light scattering and are photocatalytic. Using the latter quality, researchers have built tubes with platinum deposits scattered on the outside and a gold wire running down the inside. This type of device could be used to separate water into oxygen and hydrogen, which could prove key to jumpstarting a hydrogen economy. And similar devices could be engineered into conductors, semiconductors and photoconductors for electronic and photonic circuits.

Porhyrin tubes lack the strength and uniformity of carbon nanotubes, but they possess a wider range of electrical and optical properties. Porphyrin tubes are also self-assembling at room temperature, while carbon tubes require high temperatures.

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