Tiny Porphyrin Tubes Developed By Sandia May Lead To New Nanodevices

Science Daily ^ | 04-13-2005 | None listed

[PeaceBeWithYou]

ALBUQUERQUE, N.M. - Sunlight splitting water molecules to produce hydrogen using devices too small to be seen in a standard microscope. That's a goal of a research team from the National Nuclear Security Administration's Sandia National Laboratories. The research has captured the interest of chemists around the world pursuing methods of producing hydrogen from water.

Sandia researchers John Shelnutt and
Zhongchun Wang gaze upon the glow of
porphryin nanotubes caused by the
nanotubes’ intense resonance light
scattering activity.
(Photo by Chris Burroughs)

"The broad objective of the research is to design and fabricate new types of nanoscale devices," says John Shelnutt, Sandia research team leader. "This investigation is exciting because it promises to provide fundamental scientific breakthroughs in chemical synthesis, self-assembly, electron and energy transfer processes, and photocatalysis. Controlling these processes is necessary to build nanodevices for efficient water splitting, potentially enabling a solar hydrogen-based economy."

The prospect of using sunlight to split water at the nanoscale grew out of Shelnutt's research into the development of hollow porphyrin nanotubes. (See "Porphyrin nanotubes versus carbon nanotubes" below.) These light-active nanotubes can be engineered to have minute deposits of platinum and other metals and semiconductors on the outside or inside of the tube.

The key to making water-splitting nanodevices is the discovery by Zhongchun Wang of nanotubes composed entirely of porphyrins. Wang is a postdoctoral fellow at the University of Georgia working in Shelnutt's Sandia research group. The porphyrin nanotubes are micrometers in length and have diameters in the range of 50-70 nm with approximately 20 nm thick walls. They are prepared by ionic self-assembly of two oppositely charged porphyrins - molecules that are closely related to chlorophyll, the active parts of photosynthetic proteins. These hollow structures are one member of a new class of nanostructures made of porphyrins that Shelnutt and his team are developing. The porphyrin building blocks (tectons) can be altered to control their structural and functional properties.

Shelnutt says these porphyrin nanotubes have "interesting electronic and optical properties such as an intense resonance light scattering ability and photocatalytic activity." When exposed to light, some porphyrin nanotubes can photocatalytically grow metal structures onto tube surfaces to create a functional nanodevice. For example, when the nanotubes are put into a solution with gold or platinum ions and exposed to sunlight, their photocatalytic activity causes the reduction of the ions to the metal. Using this method the researchers have deposited platinum outside the nanotube and grown a nanowire of gold inside the tube.

The nanotube with the gold inside and platinum outside is the heart 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. To complete the nanodevice that splits water, a nanoparticle of an inorganic photocatalyst that produces oxygen must be attached to the gold contact ball that naturally forms at the end of the tube. The gold nanowire and ball serve as a conductor of electrons between the oxygen- and hydrogen- producing components of the nanodevice. The gold conductor also keeps the oxygen and hydrogen parts separate to prevent damage during operation.

"Laboratory-scale devices of this type have already been built by others," Shelnutt says. "What we are doing is reducing the size of the device to reap the benefits of the nanoscale architecture."

Shelnutt says the nanodevice could efficiently use the entire visible and ultraviolet parts of the solar spectrum absorbed by the tubes to produce hydrogen, one of the Holy Grails of chemistry.

These nanotube devices could be suspended in a solution and used for photocatalytic solar hydrogen production.

"Once we have functional nanodevices that operate with reasonable efficiency in solution, we will turn our attention to the development of nanodevice-based solar light-harvesting cells and the systems integration issues involved in their production," Shelnutt says. "There are many possible routes to the construction of functional solar cells based on the porphyrin nanodevices. For example, we may fabricate nanodevices in arrays on transparent surfaces, perhaps on a masked free-standing film. However, we have a lot of issues to resolve before we get to that point."

Water-splitting is just one of the possible applications of the nanodevices based on porphyrin nanostructures. Shelnutt expects the tubes to have uses as conductors, semiconductors, and photoconductors, and to have other properties that permit them to be used in electronic and photonic devices and as chemical sensors.

The work was partially funded by a grant to the University of Georgia from the Department of Energy, Basic Energy Sciences, Division of Chemical Sciences, Geosciences, and Biosciences.

Porphyrin nanotubes versus carbon nanotubes

Porphyrins are light-absorbing molecules related to chlorophyll, the active part of photosynthetic proteins and light-harvesting nanostructures (chlorosomal rods). They are the active molecules in many other proteins such as hemoglobin, which gets its intense red color from a porphyrin.

Porphyrin nanotubes are made entirely of oppositely charged porphyrin molecules that self-assemble in water at room temperature. The more well-known carbon nanotubes are formed at high temperatures and have covalent bonds between carbon atoms. Porphyrin nanotubes lack the high mechanical strength of the carbon tubes but possess a wider range of optical and electronic properties that can be exploited in making nanodevices. In fact, carbon nanotubes are often modified by attaching porphyrins to increase their utility. This is unnecessary for the porphyrin nanotubes, which can be tailored to specific purposes like water-splitting by varying the type of porphyrin incorporated into the nanotube itself to obtain the desired properties.

Other porphyrin nanostructures such as nanofibers and rectangular cross-section nanotubes have been made and can also be used in the fabrication of nanodevices.

This story has been adapted from a news release issued by Sandia National Laboratories.

[PeaceBeWithYou]

Could easily be adapted to produce any hydrocarbon fuel using carbon dioxide from the air like plants already do.

[ProudVet77]

The PhDs that can't, teach, in MA
The PhDs that can, do, in NM

[Samurai_Jack]

This is the kind of breakthrough that could make Hydrogen Fuel Cells a viable alternative to 'fossil' fuels.

[Termite_Commander]

They ain't got nothing on me. I was able to set the clock on my VCR.

[cabojoe]

No way.

[Tangaray]

Thanks for the interesting post. Also, more evidence that foreign students are in the forefront of scientific developments, while our American kids don't like math and science, but prefer MTV and rap.

[Paul_Denton]

very cool stuff. Would be a nice cheap, energy efficient way to produce hydrogen fuel.

[willgolfforfood]

Oh, I'm SUUUUUUUUUUUUUURRRRRRRRRRRRREEEEEEEEEEEE you can.

But can you program it to record a show while you're away?

Didn't think so!!

[PeaceBeWithYou]

Nanotech Ping.

[Ernest_at_the_Beach]

Thanks for the ping.

Sandia is always doing some interesting stuff....

[okie01]

Sandia is always doing some interesting stuff....

Interesting? Interesting?

Using two raw materials -- water and light -- to produce hydrogen ought to rate a "moderately impressive", at least.

[Ernest_at_the_Beach]

Their mission was suppose to be electronics in the beginning....Los Alamos was the pure science place.....but they do a lot of interesting stuff ....that isn't electronics.

And lots of it is very impressive.

[okie01]

And lots of it is very impressive.

More than "moderately impressive"?

For some reason, the layman in me found this development potentially stunning.

So many catalytic reactions seem to rely on platinum. Is that because of its valence properties? (it's been a long time since chemistry).

[Billthedrill]

This incredible development and all you can talk about is your VCR?

Uh...

Listen, could you drop into my place? Mine is still flashing 12:00 at me...

[SunkenCiv]

Solar cells aiming for full spectrum efficiency
by Jenny Hogan
10:15 08 December 02

Today's best cells have layers of two different semiconductors stacked together to absorb light at different energies but they still only manage to use 30 per cent of the Sun's energy. Theorists have calculated which two bandgaps would give a maximum efficiency of 50 per cent, but until now they have not had the semiconductors to do the job. Now Wladek Walukiewicz and his team at the Lawrence Berkeley National Laboratory in California have found a material that fits the bill - a semiconductor called indium gallium nitride (InGaN)... There is one catch. Scientists had previously overlooked InGaN because its bandgap range was thought to be much smaller - data books quote the lower limit to be twice as high as Walukiewicz claims. The difference may be due to the purity of the semiconductor. The samples Walukiewicz tested were made using a painstaking, and prohibitively expensive, method to grow very pure crystals of InGaN one atomic layer at a time. The team now hopes to collaborate with the National Renewable Energy Laboratory in Colorado to try to build cheap InGaN solar cells. Chlamydomonas reinhardtii that has been genetically modified can produce hydrogen as a byproduct of photosynthesis. Links are probably dead, check the Web Archive.

Patent filed on energy discovery
by Kathleen Scalise

A metabolic switch that triggers algae to turn sunlight into large quantities of hydrogen gas, a valuable fuel, is the subject of a new discovery reported for the first time by University of California, Berkeley, scientists and their Colorado colleagues. UC Berkeley plant and microbial biology professor Tasios Melis and postdoctoral associate Liping Zhang of UC Berkeley made the discovery -- funded by the U.S. Department of Energy (DOE) Hydrogen Program -- with Dr. Michael Seibert, Dr. Maria Ghirardi and postdoctoral associate Marc Forestier of the National Renewable Energy Laboratory (NREL) in Golden, Colorado. Currently, hydrogen fuel is extracted from natural gas, a non-renewable energy source. The new discovery makes it possible to harness nature's own tool, photosynthesis, to produce the promising alternative fuel from sunlight and water. A joint patent on this new technique for capturing solar energy has been taken out by the two institutions. While current production rates are not high enough to make the process immediately viable commercially, the researchers believe that yields could rise by at least 10 fold with further research, someday making the technique an attractive fuel-producing option. Preliminary rough estimates, for instance, suggest it is conceivable that a single, small commercial pond could produce enough hydrogen gas to meet the weekly fuel needs of a dozen or so automobiles, Melis said.

Abstract Number:1027
by Maria L Ghirardi and Michael Seibert

The hydrogen metabolism of photosynthetic bacteria and cyanobacteria involves the coordinated action of three enzymes: nitrogenase, reversible hydrogenase, and uptake hydrogenase. Green algae, on the other hand, contain only the reversible hydrogenase, which is responsible for both hydrogen production and uptake in this organism. The quantum yield for hydrogenase-catalyzed hydrogen production is much higher than that for nitrogenase. Algal hydrogenases, however, are extremely sensitive to oxygen. For this reason, green algae cannot be utilized commercially for hydrogen production. We have investigated two types of selective pressure to isolate mutants of Chlamydomonas reinhardtii that produce hydrogen in the presence of oxygen. The first is based on competition between hydrogenase and metronidazole for electrons from light-reduced ferredoxin. Since reduction of metronidazole results in the release of toxic products that eventually kill the organism, cells with an active oxygen-tolerant hydrogenase will survive a short treatment with the drug in the light in the presence of oxygen. Using this technique, we have isolated a variant of C. reinhardtii that evolves hydrogen with an I50 for oxygen three times higher than the wild type strain. The second selective pressure depends on growth of algal cells under photoreductive conditions. Algal cells must fix carbon dioxide in the presence of oxygen with reductants derived from hydrogen uptake by the reversible hydrogenase. We will describe in detail both selective pressures, as well as the characteristics of the mutants isolated by application of these selective pressures to a population of mutagenized wild type cells. This work was supported by the U.S. DOE Hydrogen Program.

The Department of Energy's Biohydrogen Research Program
by Maria L Ghirardi and Michael Seibert

A recent discovery at ORNL demonstrated that hydrogen production from a green algal Chlamydomonas reinhardtii mutant cannot easily be explained by the Z-scheme, the standard model of photosynthesis. Too much hydrogen was produced to be accounted for by this model. These results may have implications for designing a commercial BioHydrogen organism with improved energetic conversion efficiencies of hydrogen production, especially in the context of the light saturation problem.

Plankton Power
New Scientist

Tiny marine plants and animals can provide limitless power for small electric devices. Plankton in seawater and sediment use different chemical reactions to obtain their energy. This sets up a natural potential difference between the seawater and the sediment a few centimetres beneath. A device called OSCAR (Ocean Sediment Carbon Aerobic Reactor) taps into this tiny voltage. Leonard Tender of the US Naval Research Laboratory believes his system would be ideal for powering oceanographic sensors, whose batteries currently need to be replaced constantly.

Hmm. Interesting. All that's needed now is a seawater version of this critter, and we'll have an unnatural plant that can process seawater into hydrogen for fuel, which when burned can provide freshwater. : )