Researchers at UCSB have found an alternate way to harvest energy from the sun without the use of semiconductors. Although the method is still in its infancy, the research team has already obtained substantial progress in efficiency.
The use of solar energy is becoming viably competitive to fossil fuels. Zero-emission fuel cells deliver a clear advantage over energy generation technology including nearly 50 percent fuel-to-electricity efficiency and zero emissions because they are operating on pure hydrogen. However, this method of splitting water into hydrogen and oxygen is both expensive and energy-intensive.
Plasmonic devices, developed by UCSB chemistry professor Martin Moskovits, offer more resilient and efficient ways to accomplish the same goal of splitting water.
In a conventional solar cell, when sunlight hits the surface, electrons shift positions, leaving positively charged “holes” resulting in an electric current. The positive charges are the agents of oxygen-release out of water, Moskovits said.
Although plasmonic devices also create a shift in electrons, they are made of nanostructures instead of semiconductors. Moskovits and his team created a “forest” of gold nanorods that were capped with titanium titanium dioxide (TiO2) with additional platinum particles.
The gold nanorods serve as a light-harvesting antenna, and the titanium layer serves as an electron filter as well as support for the nanoparticles that are used as a hydrogen evolution catalyst. When the device is placed in water and exposed to visible light, the nanorods oscillate together to create “plasmonic waves.”
The resulting waves contain electrons that are excited by the light particles; some of the excited particles travel up the nanorod through the titanium filter while others are captured by the platinum particles. These “hot” excited electrons are the mediators of hydrogen release.
Such a reaction causes the splitting of the water molecule, forming hydrogen ions. When related to zero-emission energy, the hydrogen molecule is transportable fuel.
“When used, it becomes a ready source of energy,” Moskovits said.
Although semiconductors will likely never be replaced in electronics and photovoltaics, Moskovits said, an all-metal water-splitter is less efficient and more costly. However, some photovoltaics are based on materials that are not abundant in the earth and are therefore expensive or destined to become expensive. This opens a niche for novel technologies to work side-by-side with those based on semiconductors.
“Experience has taught us that such new discoveries, when advanced and perfected by the scientific and engineering communities world-wide, have often developed into useful and economical new technologies,” Moskovits said.
A version of this article appeared on page 4 of March 5th, 2013’s print edition of the Nexus.
Diagram courtesy of Syed Mubeen.