Metal nanoparticles increase the efficiency of materials that use sunlight to split water
By NEIL SAVAGE / AUGUST 2011
10 August 2011—Using metal nanoparticles as antennas for light could make the production of hydrogen more affordable, say researchers at Stanford University.
Hydrogen is viewed as a potential carbon-free replacement fuel for automobiles. On Earth, the primary source of hydrogen is water, but today the processes for splitting water into oxygen and hydrogen, while straightforward, usually require the burning offossil fuels to produce electricity. Solar power can be used to drive the splitting, but to compete with electricity from fossil fuels, it would have to be less expensive and more efficient.
The Stanford group focused on photoelectrochemical (PEC) conversion, in which a semiconductor absorbs sunlight and turns it into electricity. Current passing from the semiconductor through the water breaks the water molecules apart. Most of the materials used in this process must be several micrometers thick to absorb enough incoming photons, but most of the photogenerated charge carriers produced travel only a few tens of nanometers before recombining to make light or heat, and thus do not lead to external current that can be used to split water.
”The carriers may never reach that interface [with the water], and they need to reach that interface to participate in the redox reaction,” says Mark Brongersma, a professor of materials science at Stanford and author of a paper on the work, which appeared in the online version of Nano Letters.
To increase light absorption near the surface and give the carriers a better chance of participating in chemical reactions, the researchers built a PEC cell of iron oxide and laced it with nanoparticles. Each nanoparticle was 50 nanometers in diameter and made of a gold core surrounded by a silica shell. Because the nanoparticles are so much smaller than the wavelengths of the photons hitting them, they interacted with the electromagnetic field of the photons to create surface plasmons—oscillations of electrons on the surface of the metal. The plasmons, in turn, caused a strong concentration of light in the immediate vicinity of the nanoparticle, much the same way a standard antenna concentrates radio waves.
Isabell Thomann, a postdoctoral researcher in Brongersma’s lab who led the work, says the light was concentrated within 20 nm of the nanoparticle. ”So their [charge] carriers don’t recombine and will actually make it to the surface and participate in the chemical reaction,” she explains.
The antennas have two benefits, says Brongersma. One is that they increase the efficiency of the conversion of solar energy into hydrogen. While the work described in Nano Letters doesn’t break any solar-to-hydrogen efficiency records, Brongersma says it could lead to a significant efficiency increase The researchers hope to push iron-oxide-based cells toward 10 percent efficiency, from their high today of about 1 percent. The theoretical maximum for iron oxide is 15 percent.
The more important benefit, Brongersma says, is that enhancing light absorption near the surface of the semiconductor will allow them to build much thinner cells, saving materials cost and production time. It also allows the use of cheap, common materials by enhancing both the electrical and optical properties.
Thomann says the method will work with a variety of materials. Titanium dioxide is also popular for PEC cells. Metals such as silver, aluminum, or copper could be used for the nanoparticles, though because the metals have to be stable in a wet environment, chemists will need to develop suitable coatings. Varying the size of the nanoparticles and the combination of materials will also allow the researchers to optimize the cells for different wavelengths of incoming light.
The method could also be used for other sunlight-driven chemical reactions, such as toxic-chemical breakdown. Brongersma says that within a couple of years the technology may be ready to contribute to the production of commercial cells.