Other research groups have demonstrated macroscopic devices that harness the same mechanism used by the Sandia group, but like simple electrolysis, these laboratory setups are too inefficient to enable a hydrogen-based economy that would render fossil fuels obsolete. The Sandia researchers hope that by downsizing the mechanism to the nanoscale, efficient solar-cell-like devices can be mass-produced.
The device's hydrogen and oxygen generators use slightly different parts of the light spectrum but should be able to derive all their input-energy needs from the sun.
"If you want to use solar [efficiently], you have to capture light throughout the visible spectrum and the UV, and our porphyrin tubes do that. They absorb out to about 700 nanometers [wavelength], so they get most of the visible and the UV, and then the tungsta uses UV and blue light to oxidize water to oxygen," said Shelnutt.
The next step for the group is to create a device that marries the hydrogen- and oxygen-producing parts at the nanoscale. The Sandia prototype cell will be designed to harvest light to split water molecules, while keeping the hydrogen and oxygen separate so they can be stored as two parts of a hydrogen fuel.
"The thing we have in mind is an array of these things sticking up from a sheet, where we can actually separate the tubes all on one side of a thin film that separates two sides of a cell," said Shelnutt.
Porphyrin nanotubes are similar in size to carbon nanotubes, measuring up to 1 micron long but only 50 to 70 nanometers in diameter, with walls just 20 nm thick. They self-assemble under the right conditions and can easily be coated with platinum on the outside along with the gold wire inside. Both are necessary to build a working solar cell.
Porphyrins are just one member of a class of nanostructures, called tectons, that Sandia is investigating. Other structures in addition to tubes can be constructed from tectons, potentially enabling the light-harvesting abilities of nanotubes to be transferred to other architectures.
The researchers are looking into further uses for the nanotubes as conductors, semiconductors and photoconductors. They predict that both electronic and photonic devices will eventually use the organic materials, especially as chemical sensors.