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Turning Over a New Leaf

When you look at a leaf, the first thing that comes to mind may not be its ability to harness solar energy. In fact, the surface area of a leaf, with its inherent curves and waves, makes it perfect for the absorption of sunlight, which a plant then uses for growth. Probably the last thing that comes to mind is its ability to “dance.”

A team of WVU researchers is taking both ideas a step further in an effort to create an artificial leaf that can be used to harvest solar energy. Nianqiang (Nick) Wu, an associate professor of mechanical and aerospace engineering, and Alan Bristow, an assistant professor of physics and astronomy, are using the latest advances in nanotechnology and optics to mimic and improve on the leaf. The team’s research is being funded through a grant from the National Science Foundation.

Turning a new leaf on solar energy.

The abundance and renewability of sunlight make it a primary source for meeting the world’s growing energy needs, without further harm to the environment. However, previous attempts to create artificial leaves, which basically mimic photosynthesis, have been mixed.

“To date, only low conversion efficiencies have been obtained, primarily because most of the materials used to create artificial leaves have been able to absorb less than five percent of the available solar spectrum,” Bristow explained. “The current goal of our research is to extend that range.” The team’s approach to extending the conversion range involves using light to stimulate a collective movement—or dance—of electrons, known as a localized surface plasmon resonance.

“When sunlight hits on very tiny gold nanoparticles, which are 10,000 times smaller than the diameter of a human hair, the electrons on the gold nanoparticle surface can dance hand-in-hand,” Wu said. “This united rhythm can then store the solar energy inside the gold nanoparticle as if it were hundreds of times its actual size.

“We can also choose what ‘music’ the electrons will dance to by changing the particles’ shape, allowing them to pick what part of the solar spectrum is converted to energy,” Wu continued. “Normally, it is difficult to extract the energy stored in the dancing electrons to create the all-important chemical reaction. This difficulty has prevented localized surface plasmon resonances from effectively being used in solar light harvesting technology.”

“In our studies, we are able to grow great materials to test our ideas,” Bristow said. “We are also able to use extremely short pulses of laser light—similar to flash photography—to actually capture the electrons’ dance.”

The research team’s discovery of this “dance” has been well received in the scientific community, having been published in the Journal of the American Chemical Society as well as highlighted at the annual meeting of the American Physical Society.