February 26, 2013 | Leave a Comment
Researchers at the University of California Santa Barbara (UCSB) have developed an efficient, autonomous solar water-splitting device. The new idea is based on a gold nanorod array where essentially all charge carriers involved in the oxidation and reduction steps arise from the hot electrons resulting from the excitation of surface plasmons in the nanostructured gold (plasmonic) water-splitter.
To do the experiment gold nanorods were capped with a layer of crystalline titanium dioxide (TiO2) with platinum nanoparticles that functions as the hydrogen evolution catalyst, which is positioned in water. The titanium dioxide would be low cost, but the gold and platinum are quite expensive.
A cobalt-based oxidation catalyst (Co-OEC) was deposited on the exposed portions of the gold nanorods to enhance oxygen gas evolution. The TiO2 acts as an electron filter and as support for the platinum nanoparticles that serve as the hydrogen evolution catalyst.
In comparison to conventional methods the UCSB research offers the promise to convert sunlight into hydrogen fuel using a process based on metals that are more robust than many of the semiconductors in use today.
Martin Moskovits, professor of chemistry at UCSB said, “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.”
In the conventional semiconductor photo process 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 facilitating the chemical reactions and other familiar uses, including powering light bulbs and charging batteries.
Moskovits explains, “For example, the electrons might cause hydrogen ions in water to be converted into hydrogen, a fuel, while the holes produce oxygen.”
Moskivits and his team have a completely new process made from nanostructured metals.
“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 only minutes.
Perhaps the most interesting and worthwhile news is, “The device operated with no hint of failure for many weeks,” Moskovits said.
Looking ahead Moskovits explained 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; graduate student Nirala Singh; materials engineer Stephan Kraemer; and chemistry professor Galen Stucky.
The new process is a grand idea that works exceptionally well right at the first step. The prime alarm is the materials cost and the unexplained operations involved to build a module. Time and the incentive for a free energy route to a store of hydrogen fuel should drive to understanding the costs and the inevitable goal of lower material costs. Low enough and the idea could have real market potential.