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Harvesting the Sun's Energy
updated: Feb 25, 2013, 3:00 PM
Source: UCSB
A new method of harvesting the Sun's energy is emerging, thanks to scientists at
UC Santa Barbara's Departments of Chemistry, Chemical Engineering, and
Materials. Though still in its infancy, the research promises to convert
sunlight into energy using a process based on metals that are more robust than
many of the semiconductors used in conventional methods. The researchers'
findings are 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.
In conventional photoprocesses, a technology developed and used over the last
century, sunlight hits the surface of semiconductor material, one side of which
is electron-rich, while the other side is not. The photon, or light particle,
excites the electrons, causing them to leave their postions, and create
positively-charged "holes." The result is a current of charged particles that
can be captured and delivered for various uses, including powering lightbulbs,
charging batteries, or facilitating chemical reactions.
"For example, the electrons might cause hydrogen ions in water to be converted
into hydrogen, a fuel, while the holes produce oxygen," said Moskovits.
In the technology developed by Moskovits and his team, it is not semiconductor
materials that provide the electrons and venue for the conversion of solar
energy, but nanostructured metals -- a "forest" of gold nanorods, to be
specific.
For this experiment, gold nanorods were capped with a layer of crystalline
titanium dioxide decorated with platinum nanoparticles, and set in water. A
cobalt-based oxidation catalyst was deposited on the lower portion of the array.
"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."
As the "hot" electrons in these plasmonic waves are excited by light particles,
some travel up the nanorod, through a filter layer of crystalline titanium
dioxide, and are captured by platinum particles. This causes the reaction that
splits hydrogen ions from the bond that forms water. Meanwhile, the holes left
behind by the excited electrons head toward the cobalt-based catalyst on the
lower part of the rod to form oxygen.
According to the study, hydrogen production was clearly observable after about
two hours. Additionally, the nanorods were not subject to the photocorrosion
that often causes traditional semiconductor material to fail in minutes.
"The device operated with no hint of failure for many weeks," Moskovits said.
The plasmonic method of splitting water is currently less efficient and more
costly than conventional photoprocesses, but if the last century of photovoltaic
technology has shown anything, it is that continued research will improve on the
cost and efficiency of this new method -- and likely in far less time than it
took for the semiconductor-based technology, said Moskovits.
"Despite the recentness of the discovery, we have already attained ‘respectable'
efficiencies. More importantly, we can imagine achievable strategies for
improving the efficiencies radically," he said.
Research in this study was also performed by postdoctoral researchers Syed
Mubeen and Joun Lee; grad student Nirala Singh; materials engineer Stephan
Kraemer; and chemistry professor Galen Stucky.
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