Tiny porphyrin tubes may lead to new nanodevices
Sunlight splitting water molecules to produce hydrogen by devices too small to be seen in a standard microscope. That’s a goal of a research team led by Sandian John Shelnutt (1116) that has captured the interest of chemists around the world who pursue this “Holy Grail of chemistry."
“The broad objective of the research is to design and fabricate new types of nanoscale devices,” John says. “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 John’s research into the development of hollow porphyrin nano-tubes (see “Porphyrin nanotubes versus carbon” on page 4). 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 (1116) of nanotubes composed entirely of porphyrins. Wang is a postdoctoral fellow at the University of Georgia working in John’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 John and his team are developing. The porphyrin building blocks (tectons) can be altered to control their structural and functional properties.
John 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 nano device. 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,” John says. “All we are doing is reducing the size of the device to reap the benefits of the nanoscale architecture.”
John 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,” John 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. John 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.