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.
researchers' findings are published in the latest issue of the journal Nature
"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
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 light bulbs, charging batteries, or facilitating chemical
"For example, the electrons might cause hydrogen ions
in water to be converted into hydrogen, a fuel, while the holes produce
oxygen," said Moskovits.
How it will work
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.
More costly - for now
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,
"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.