03/13/2012 - Engineers at University of California, San Diego are creating a  “nanoforest” that can capture solar energy and use it generate fuel from hydrogen.

Running cars, homes and buildings on hydrogen has long thought to be the ultimate clean solution, but getting there has been tough.

A nanowire forest is a small forest but full of energy.

They say the tiny nanowire trees, made from abundant sources like silicon and zinc oxide, mimic the way trees naturally absorb energy from the sun through photosynthesis.

Using these inexpensive materials could be a key to mainstreaming hydrogen fuels. Until now, hydrogen fuel cells have depended on platinum, an expensive catalyst.

And they’ve also depended on electricity to separate hydrogen from water, usually from fossil fuels.

3D vertical nanotrees capture much more sunlight than do flat surfaces, which simply reflect it.

The structure also maximizes hydrogen gas output, using a process for separating water into oxygen and hydrogen called photoelectrochemical water-splitting.

“With this structure, we have enhanced, by at least 400,000 times, the surface area for chemical reactions,” says Ke Sun, an  electrical engineering Ph.D. student who is leading the project.

If hydrogen fuel cells were cheap enough they would be ideal for energy storage for buildings, the grid, and in fuel cell vehicles.

In the long run, the team is aiming for a holy grail of energy production: artificial photosynthesis. It would also capture carbon from the atmosphere, reducing emissions and converting it to hydrocarbon fuel.

The research is published in Nanoscale, and will be presented at the annual Research Expo, in April at the Jacobs School of Engineering.

Learn more:

Website: http://news.discovery.com/tech/nanotrees-harvest-sunlight-120312.html

18 October 2007 – A new type of solar cell made from a nanometer-scale wire might one day provide an on-chip power source for nanoelectronic devices or run microscopic robots, say scientists at Harvard University.

The solar cell is a coaxial silicon nanowire, approximately 300 nanometers thick. In experiments, it produced 200 picowatts of electricity, a tiny amount to be sure, but enough that it was used to operate a nanowire-based pH sensor. Chemistry professor Charles Lieber and members of his research group at Harvard described the device in the 18 October issue of the journal Nature .

A new type of solar cell made from a nanometer-scale wire

The nanowire consists of three layers of silicon: a positively charged core; a thin intrinsic, or neutrally charged, inner shell; and a negatively charged outer shell. This p-i-n structure is common in flat photovoltaic devices, but Lieber says that this is the first time it has been applied to a coaxial wire. When a photon hits the nanowire, it generates a pair of charges: an electron and a hole. The charges then move radially—electrons outward from the center to a contact in the shell, holes inward to a contact in the core. The advantage of the circular cross section is that the electrons and holes must move across a much shorter distance than they would in a flat cell to reach the contacts that collect them and send electricity flowing out of the device. ”They only have to travel on the order of 100 nm or less, so they’re less likely to recombine before they are collected,” says Lieber.

Recombination, when an electron and a hole rejoin instead of exiting the device, is a big problem for silicon-based solar cells. Silicon does a poor job of absorbing light, particularly at infrared wavelengths. To increase the likelihood a photon will be absorbed and converted into electricity, engineers make the silicon relatively thick. But that thickness means the carriers have to travel farther and are more likely to recombine with each other and produce heat rather than electricity. Engineers use high-quality silicon with few crystal defects to decrease the recombination rate, but growing those crystals is expensive. Lieber’s method works with less pure, and therefore less costly, silicon.

Other researchers have made nanoscale solar cells, but those have generally consisted of nanoparticles or rods of inorganic materials combined with an organic polymer or dye, both of which degrade with use. In contrast, Lieber says his device has been operating nearly a year with no decrease in function. He got more power out of his nanowire solar cell by using a lens to concentrate more light onto it; the same approach would quickly destroy an organic cell.

Lieber’s solar cell converted about 3.4 percent of the light shone on it to electricity—too low an efficiency for a practical device, but not bad for a prototype. ”While 3.4 percent is low compared with commercial solar cells, it is really remarkable for a first try,” says Eray Aydil, a chemical engineer at the University of Minnesota. ”Efficiencies tend to get better as people improve on these pioneering studies.”” – more at ieee Spectrum