Researchers at Sandia National Laboratories are investigating the use of a new type of nanodevice for photocatalytic solar hydrogen production from water.
The new devices are porphyrin nanotubes—nanotubes made entirely of oppositely charged porphyrin molecules that self-assemble in water at room temperature. (Electron microscope image of porphyrin nanotube at right.) The better-known carbon nanotubes are formed at high temperatures and have covalent bonds between carbon atoms.
Porphyrins are light-absorbing molecules related to chlorophyll, the active part of photosynthetic proteins and light-harvesting nanostructures (chlorosomal rods).
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.
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 the photolytic 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 photocatalytic nanodevice, 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. What we are doing is reducing the size of the device to reap the benefits of the nanoscale architecture.
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.
There are many possible routes to the construction of functional solar cells based on the porphyrin nanodevices. However, we have a lot of issues to resolve before we get to that point.—John Shelnutt, Sandia research team leader
The research, partially funded by a grant to the University of Georgia from the Department of Energy, is exploring the use of porphyrin nanotubes in a range of applications.