Wednesday, 22 May 2013

Convertion of sunlight into energy by fake Forest

Arrays of tree-like nanowires facilitate solar water-splitting in a fully integrated artificial photosynthesis system. One hour of global sunlight contains enough energy to meet the demands of every human on the planet for an entire year. And a recent breakthrough by scientists at the Lawrence Berkeley National Laboratory could make harnessing this energy for human consumption a reality. Researchers have developed an "artificial forest" that can convert solar energy into chemical fuels. In a process that mimics photosynthesis, this artificial forest soaks up light and uses it to generate oxygen and hydrogen, two gases that can be used to power fuel cells.

"To facilitate solar water-splitting in our system, we synthesized tree-like nanowire heterostructures, consisting of silicon trunks and titanium oxide branches," said Peidong Yang, a chemist at Berkeley Lab's Materials Science Division and lead scientist for the study. "Visually, arrays of these nanostructures very much resemble an artificial forest." Much like trees in a real forest, the nanowire trees are densely packed to help suppress sunlight reflection and produce more surface area for fuel-producing reactions. They also do a good job of mimicking natural photosynthesis, the process by which sunlight is absorbed by the chloroplast of green plants.

When sunlight is absorbed into a plant, it triggers a chain reaction of electrons, which move from one molecule to the next, helping the plant convert carbon dioxide into carbohydrate sugars and oxygen. This movement of electrons is called an electron transport train, or "Z-scheme" because the pattern of movement resembles the letter Z. The Berkeley researchers borrowed this Z-scheme for their artificial forest, but instead of relying on the pigment in chloroplast to trigger electron movement they used semiconductors.

One of the light-absorbing semiconductors used was silicon, which generates hydrogen. The other is titanium oxide, which generates oxygen. Together, hydrogen and oxygen can be stored in a fuel cell and used to produce renewable energy. Previous models for artificial photosynthesis, including artificial leaves, have also been successful at generating hydrogen and oxygen, but Yang and his team believe that their nanoscale system is more efficient- and less expensive.

Yang said his team's system can serve as a "conceptual blueprint for better solar-to-fuel conversion efficiencies in the future." The artificial forest only operates at 0.12 percent solar-to-fuel efficiency, a rate that will have to be improved if Yang and his team wish to commercialize the system's use.

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