The "Goldilocks" Platform for Efficient H2 Fuel Generation: Enhanced Solar-Matched Photocatalysis of H2O using GaN Surface States
Jordan M. Gerton, physics, and Michael H. Bartl, chemistry, both associate professors at the University of Utah, foresee a major role for hydrogen fuel in the world’s energy future. The team is working with a $250,000 Scialog grant from RCSA to further that vision. Although hydrogen is the most abundant element in the universe, here on earth hydrogen fuel is currently generated most affordably in large quantities using fossil fuels as feedstock. The two Utah researchers have teamed with P. James Schuck, a staff scientist at the Molecular Foundry at Lawrence-Berkeley National Laboratory, to experiment with gallium nitride “nanowires” for harvesting solar energy as an alternative means to generate hydrogen fuel. A nanowire is an incredibly thin wire, far thinner than the width of a human hair. The scientists intend to fabricate nanowires that are highly efficient at harvesting energy from sunlight. The nanowires will be immersed in water, and the harvested solar energy will be used to catalyze a chemical reaction that splits water molecules into hydrogen and oxygen gas. These gases can then be siphoned off and collected for fuel and other purposes. Galium nitride (GaN) is a chemically robust semiconductor used to generate the laser-light for Blue-ray laser-disc players. In bulk sizes, GaN absorbs and emits high-frequency ultraviolet light, but in nanowire form, the researchers have observed absorption and emission of visible light. Furthermore, this visible light originates from the surface facets of the nanowires, rather than their interior, so solar energy can be harvested directly at the interface between the nanowires and the water medium where the catalysis happens. One important problem that has plagued similar approaches is that the nanowires are very thin and thus do not strongly absorb light; that is, they are mostly transparent. To address this challenge, the research team plans to decorate the surfaces of the nanowires with extremely small metallic particles, called “plasmonic nanoparticles”:upon exposure to light, the electrons within these nanoparticles vibrate in unison leading to very efficient light absorption. This process is similar to the resonant vibrations of a taught string when plucked. Just as a vibrating string connected to a guitar produces a much stronger and clearer tone compared to a string vibrating by itself, the resonant vibrations of electrons in the plasmonic nanoparticles will help pump more solar energy into the nanowires, thereby increasing the overall efficiency of hydrogen gas production. “We expect our innovative approach will significantly advance the solar fuel generation landscape by greatly enhancing the rate and efficiency of direct photon-to-fuel conversion,” the researchers said.