A new method of harvesting the Sun's energy has been uncovered by scientists at the University of California, Santa Barbara.
Though still in its infancy, the research promises to convert sunlight into energy in the form of hydrogen fuel by using a process based on metals that are more robust than many of the semiconductors used in conventional methods.
The work has been carried out by researchers at the university’s Departments of Chemistry, Chemical Engineering, and Materials with the findings published in the latest issue of the journal Nature Nanotechnology.
Martin Moskovits, professor of chemistry at UCSB, has devised a method for using nanostructured metals to harvest the energy from sunlight rather than the traditional semiconductors.
"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 Prof 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 positions 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 Prof Moskovits.
In the technology developed by Prof Moskovits and his team, it is not semiconductor materials that provide the electrons and venue for the conversion of solar energy, a "forest" of gold nanorods. 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 Prof 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, while 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 and 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," Prof Moskovits said.
The plasmonic method of splitting water is currently less efficient and more costly than conventional photoprocesses, but Prof Moskovits believes the rapid development of other photovoltaic technologies in the last century points to the possibility of swift progress.
"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.