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David Bradley ISSUE #44
March 2005

Lighting-up Time for Porphyrin Nanotubes

?? Photo by Chris Burroughs

John Shelnutt and Zhongchun Wang illuminated by the glow from porphyrin nanotubes.

Finding an inexpensive and sustainable source of hydrogen will be critical to the success of the so-called hydrogen economy in which fuel cells and other devices are driven by the gas. [See Item?#1 in this month's issue]. Now, researchers at Sandia National Laboratories have constructed nanotubes from nature's light traps, the porphyrins, to make a system that will use sunlight to split water in a process closely related to photosynthesis.

"The broad objective of the research is to design and fabricate new types of nanoscale devices," explains John Shelnutt, "Controlling energy transfer and photocatalytic processes is necessary to build nanodevices for efficient water splitting, potentially enabling a solar hydrogen-based economy."

John Shelnutt and Zhongchun Wang illuminated by the glow from porphyrin nanotubes. ??

Post-doctoral student Zhongchun Wang working in Shelnutt's lab at Sandia has now found that nanotubes composed entirely of porphyrins can be prepared by ionic self-assembly of two oppositely charged porphyrins. By fine-tuning the porphyrin building blocks he can control the structural and functional properties of the nanotubes. The resulting porphyrin nanotubes have "interesting electronic and optical properties such as an intense resonance light scattering ability and photocatalytic activity." Sunlight can then be used to photochemically coat the surface of the nanotubes with platinum or fill them with gold.

The nanotube with the gold inside and a platinum coating forms the core of a nanodevice that may split water into oxygen and hydrogen. The research team has already demonstrated that the nanotubes with platinum particles on the surface can produce hydrogen when illuminated with light. The next step is to attach an inorganic photocatalyst that produces oxygen to the blob of gold present at the end of the nanotube so that water molecules can be split by incident sunlight. "Suspending a multitude of such nanotube devices in a solution and exposing them to sunlight would then photocatalytically generate hydrogen using only water as the feedstock."

"We have not 'split' water yet with the completed nanodevice," Shelnutt told Reactive Reports, "We still have to join the oxygen evolving part to the nanotube with the gold nanowire in the core and platinum particles on the outside. We have shown the nanotube with platinum particles evolves hydrogen, but we still need to attach tungsta (WO3) particles to the gold ball at the end of the nanowire."

?? Click image to magnify
? Click image to magnify

The researchers have a proposal in at the US DOE's Hydrogen Fuel Initiative to investigate these nanodevices for hydrogen production. "Our?most immediate goal is to assemble the full water-splitting device and determine the solar efficiency that we get," adds Shelnutt, "We will then have to start optimizing construction - for example, find the size of the tungsta particle(s) attached to the gold ball that best matches the activity of the hydrogen-evolving tubes."

In the long term, the researchers have ideas about organizing the nanodevices in arrays to make a cell in which the oxygen-evolving part is separated from the hydrogen-evolving part so that gas separations are not necessary. "If we are lucky and the nanodevices are efficient, then I would guess five years at a minimum [to commercialization]", Shelnutt told us, "If we have to do a lot of optimization or switch to one of our other porphyrin nanostructures, it might take significantly longer."