Researchers Look To New Process To Harvest Solar Energy
February 25, 2013

Researchers Look To New Process To Harvest Solar Energy

Peter Suciu for — Your Universe Online

Since the seventh century B.C., mankind has looked to the power of the sun to provide an energy source. In the earliest days, humans used glass and mirrors to concentrate the sun's rays to make fire. Later, around the first century A.D., the Romans discovered large south facing windows could help let in the sun´s warmth. There's even one infamous tale of Greek scientist Archimedes using bronze shields to reflect and focus sunlight to set fire to invading wooden ships.

More recently, there have been great strides in actually converting sunlight into electricity — either by photovoltaics (PV) or indirectly using concentrated solar power (CSP). Now a new method of harvesting the sun´s energy could be emerging.

Scientists at UC Santa Barbara's (UCSB) Departments of Chemistry, Chemical Engineering and Materials  have delved into new research to convert sunlight into energy using a process based on metals. This line of research promises to be more robust than many of the semiconductors used in conventional methods.

The findings were published in the latest issue of the journal Nature Nanotechnology.

“It is the first radically new and potentially workable alternative to semiconductor-based solar conversion devices to be developed in the past 70 years or so,” said Martin Moskovits, professor of chemistry at UCSB.

This new process involves sunlight hitting the surface of a semiconductor material, with one side being electron-rich while the other side is not. When the photon, or light particle, excites the electrons this causes them to leave their positions, whereby positively-charged “holes” are created. The result is a current of these charged particles that can then be captured and utilized to power light bulbs, charge batteries or even facilitate chemical reactions.

“For example, the electrons might cause hydrogen ions in water to be converted into hydrogen, a fuel, while the holes produce oxygen,” Moskovits added.

The technology developed by Moskovits and his team as UCSB — which included postdoctoral researchers Syed Mubeen and Joun Lee; grad student Nirala Singh; materials engineer Stephan Kraemer; and chemistry professor Galen Stucky — does not use a semiconductor material to provide the electronics and venue with a conversion to solar energy. Instead, it utilizes nanostructured metals — what the team referred to as a “forest” of gold nanorods — which were capped with a layer of crystalline titanium dioxide decorated with platinum nanoparticles and set in water.

A cobalt-based oxidation catalyst was then deposited in the lower portion of the array. When the “hot” electronics in the plasmonic waves were excited by light particles some traveled up to the nanorod, and through a filter layer of crystalline titanium dioxide, which were then captured by the platinum particles.

This caused the reaction that splits hydrogen ions from the bond that forms water, and the holes left behind by the excited electronics headed toward the cobalt-based catalyst on the lower part of the rod to form oxygen.

“When nanostructures, such as nanorods, of certain metals are exposed to visible light, the conduction electrons of the metal can be caused to oscillate collectively, absorbing a great deal of the light,” said Moskovits. This excitation is called a surface plasmon.”

The study showed hydrogen production was observable after about two hours, and nanorods were not subject to the photocorrosion that often caused traditional semiconductor materials to fail within minutes. The study found the device could operate with no hint of failure for many weeks.

This plasmonic method of splitting water won´t likely replace the photoprocess anytime soon, however.

For one, it is still less efficient and more costly than conventional semiconductor based methods. But Moskovits noted this is just a first step and continued research will likely improve on the cost and efficiency. “Despite the recentness of the discovery, we have already attained ℠respectable´ efficiencies. More importantly, we can imagine achievable strategies for improving the efficiencies radically.”