|TEM images of the virus-templated IrO2 nanowires. Scale = 100 nm (a) and 10 nm (b). Source: Nam et al., Supplementary materials. Click to enlarge.|
A team of MIT researchers, led by Dr. Angela Belcher, has engineered a common bacteriophage virus (M13) to function as a scaffold to mediate the co-assembly of zinc porphyrins (photosensitizer) and iridium oxide hydrosol clusters (catalyst) for visible light-driven water oxidation. The viruses become wire-like devices that can very efficiently split the oxygen from water molecules using solar energy.
To prevent the virus-wires from clumping together and lose their effectiveness over time, the researchers encapsulated them in a microgel matrix, so they maintained their uniform arrangement and kept their stability and efficiency. The advance is described in a paper published on 11 April in the journal Nature Nanotechnology.
|“We use components people have used before, but we use biology to organize them for us, so you get better efficiency.”|
|—Dr. Angela Belcher|
Splitting water into hydrogen and oxygen comprises two half reactions: water oxidation and water reduction. Water oxidation is the more technically challenging half-reaction in the water splitting process, Belcher says, so her team focused on this part. Plants and cyanobacteria have evolved highly organized photosynthetic systems for the efficient oxidation of water, she noted. Other researchers have tried to use the photosynthetic parts of plants directly for harnessing sunlight, but these materials can have structural stability issues.
Belcher decided that instead of borrowing plants’ components, she would borrow their methods. In plant cells, natural pigments are used to absorb sunlight, while catalysts then promote the water-splitting reaction. That’s the process Belcher and her team, including doctoral student Yoon Sung Nam, the lead author of the new paper, decided to imitate.
In the team’s system, the viruses simply act as a kind of scaffolding, causing the pigments and catalysts to line up with the right kind of spacing to trigger the water-splitting reaction. The role of the pigments is to act as an antenna to capture the light, and then transfer the energy down the length of the virus, like a wire, Belcher says. The virus is a very efficient harvester of light, with these porphyrins attached.
Our results suggest that the biotemplated nanoscale assembly of functional components is a promising route to significantly improved photocatalytic water-splitting systems.
—Nam et al.
Using the virus to make the system assemble itself improves the efficiency of the oxygen production fourfold, Nam says. The researchers hope to find a similar biologically based system to perform the other half of the process, the production of hydrogen. Currently, the hydrogen atoms from the water get split into their component protons and electrons; a second part of the system, now being developed, would combine these back into hydrogen atoms and molecules. The team is also working to find a more commonplace, less-expensive material for the catalyst, to replace the relatively rare and costly iridium used in this proof-of-concept study.
Thomas Mallouk, the DuPont Professor of Materials Chemistry and Physics at Pennsylvania State University, who was not involved in this work, says:
This is an extremely clever piece of work that addresses one of the most difficult problems in artificial photosynthesis, namely, the nanoscale organization of the components in order to control electron transfer rates. There is a daunting combination of problems to be solved before this or any other artificial photosynthetic system could actually be useful for energy conversion.
To be cost-competitive with other approaches to solar power, he adds, the system would need to be at least 10 times more efficient than natural photosynthesis, be able to repeat the reaction a billion times, and use less expensive materials.
Belcher will not speculate about how long it might take to develop this into a commercial product, but says that within two years she expects to have a prototype device that can carry out the whole process of splitting water into oxygen and hydrogen, using a self-sustaining and durable system.
The Italian energy company Eni provided funding for this work through the MIT Energy Initiative (MITEI).
Yoon Sung Nam, Andrew P. Magyar, Daeyeon Lee, Jin-Woong Kim, Dong Soo Yun, Heechul Park, Thomas S. Pollom Jr, David A. Weitz and Angela M. Belcher (2010) Biologically templated photocatalytic nanostructures for sustained light-driven water oxidation. Nature Nanotechnology doi: 10.1038/nnano.2010.57