|A scheme of an M13 virus and its cloning vector for genetic engineering. Click to enlarge.|
Researchers at MIT led by Drs. Angela Belcher and Paula Hammond have synthesized single-walled carbon nanotube–TiO2 nanocrystal core–shell nanocomposites using a genetically engineered M13 virus as a template. Using the nanocomposites as photoanodes in dye-sensitized solar cells, they achieved a power conversion efficiency in the solar cells of 10.6%, up from 8%—an increase of almost one-third. Their paper is published in the journal Nature Nanotechnology.
Belcher and her colleagues have previously used differently engineered versions of the same virus to enhance the performance of batteries (earlier post). The technology is also the basis for start-up Siluria Technologies’ bio-templated catalysts for the oxidative coupling of methane (OCM) reaction to produce ethylene directly from methane (earlier post). This new method used to enhance solar cell performance is quite different, Belcher says.
The viruses perform two different functions in this new process. First, they possess short proteins (peptides) that can bind tightly to the carbon nanotubes, holding them in place and keeping them separated from each other. Each virus can hold five to 10 nanotubes, each of which is held firmly in place by about 300 of the virus’s peptide molecules. In addition, the virus was engineered to produce a coating of titanium dioxide (TiO2), a key ingredient for dye-sensitized solar cells, over each of the nanotubes, putting the titanium dioxide in close proximity to the wire-like nanotubes that carry the electrons.
The two functions are carried out in succession by the same virus, whose activity is “switched” from one function to the next by changing the acidity of its environment. This switching feature is an important new capability that has been demonstrated for the first time in this research, Belcher says.
In addition, the viruses make the nanotubes soluble in water, which makes it possible to incorporate the nanotubes into the solar cell using a water-based process that works at room temperature.
Previous attempts to use nanotubes to enhance the efficiency of electron collection from a solar cell’s surface have been thwarted by two problems. First, the making of carbon nanotubes generally produces a mix of two types, some of which act as semiconductors (sometimes allowing an electric current to flow, sometimes not) or metals (which act like wires, allowing current to flow easily). The new research, for the first time, showed that the effects of these two types tend to be different, because the semiconducting nanotubes can enhance the performance of solar cells, but the metallic ones have the opposite effect. Second, nanotubes tend to clump together, which reduces their effectiveness.
The MIT team found that a genetically engineered version of the M13 bacteriophage virus can be used to control the arrangement of the nanotubes on a surface, keeping the tubes separate so they can’t short out the circuits, and keeping the tubes apart so they don’t clump.
|“A little biology goes a long way.”|
In their tests, adding the virus-built structures enhanced the power conversion efficiency to 10.6%. This improvement takes place even though the viruses and the nanotubes make up only 0.1% by weight of the finished cell. With further work, the researchers think they can ramp up the efficiency even further.
The viruses are used to help improve one particular step in the process of converting sunlight to electricity. In a solar cell, the first step is for the energy of the light to knock electrons loose from the solar-cell material (usually silicon); then, those electrons need to be funneled toward a collector, from which they can form a current that flows to charge a battery or power a device. After that, they return to the original material, where the cycle can start again. The new system is intended to enhance the efficiency of the second step. Adding the carbon nanotubes to the cell provides a more direct path to the current collector, Belcher says.
Prashant Kamat, a professor of chemistry and biochemistry at Notre Dame University who has done extensive work on dye-sensitized solar cells, says that while others have attempted to use carbon nanotubes to improve solar cell efficiency, “the improvements observed in earlier studies were marginal,” while the improvements by the MIT team using the virus assembly method are “impressive.”
It is likely that the virus template assembly has enabled the researchers to establish a better contact between the TiO2 nanoparticles and carbon nanotubes. Such close contact with TiO2 nanoparticles is essential to drive away the photo-generated electrons quickly and transport it efficiently to the collecting electrode surface.—Prashant Kamat
Because the process would just add one simple step to a standard solar-cell manufacturing process, it should be quite easy to adapt existing production facilities and thus should be possible to implement relatively rapidly, Belcher says.
The work was funded by Eni, through the MIT Energy Initiative’s Solar Futures Program.
Xiangnan Dang, Hyunjung Yi, Moon-Ho Ham, Jifa Qi, Dong Soo Yun, Rebecca Ladewski, Michael S. Strano, Paula T. Hammond & Angela M. Belcher (2011) Virus-templated self-assembled single-walled carbon nanotubes for highly efficient electron collection in photovoltaic devices. Nature Nanotechnology (2011) doi: 10.1038/nnano.2011.50