A fundamental discovery made with a specially equipped microscope in a University of Oregon laboratory is pointing toward a new design strategy for devices that can produce hydrogen from sunlight.
The key is shrinking the size of particles used as catalysts to well below 100 nanometers, researchers in Shannon Boettcher's lab reported Oct. 7 in the journal Nature Materials.
As smaller particles are applied to semiconductors, the particles are better able to selectively collect positive charges, known as holes, that are created when light hits the semiconductor, over negatively charged electrons. That, Boettcher said, increases efficiency by keeping the two charges from recombining.
The discovery is useful for devices that use light to make chemicals and fuels, for example by splitting water to make hydrogen gas or by combining carbon dioxide and water to make carbon-based fuels or chemicals, said Boettcher, an associate professor in the UO's Department of Chemistry and Biochemistry and member of the university's Materials Science Institute.
"We found a design principle that points to making catalytic particles really small because of the physics at the interface," he said. "Our technique allowed us to watch the flow of excited charges with nanometer-scale resolution, which is relevant for devices that use catalytic and semiconductor components to make hydrogen that we can store for use when the sun is not shining."
Boettcher's team created a test model using a well-defined, single-crystal silicon wafer coated with metallic nickel nanoparticles of different sizes. Silicon absorbs sunlight and creates excited positive and negative charges. The nickel nanoparticles then selectively collect positive charges and speed up the reaction of positive charges with electrons in water molecules, pulling them apart.
Previously, Boettcher said, researchers could only measure the average current moving across such a surface and the average voltage generated by the light hitting the semiconductor.
To get a closer look, his team collaborated with Bruker Nano Surfaces, the manufacturer of the UO's atomic force microscope, which images surfaces by tapping a sharp tip over it, much like a blind person tapping their cane, to develop the techniques needed to measure voltage at the nanoscale.
When a specially engineered electrode tip - 1,000 times smaller than a human hair - on the microscope touched the nickel nanoparticles, the researchers recorded the buildup of holes by measuring a voltage, similar to how one tests a battery's output.
Surprisingly, the voltage measured, as it was operating, depended on the size of the specific particle tested. Oxidation at the nickel particle surface creates a barrier, much like overlapping ridges in a mountain valley, Boettcher said. That, in turn, prevents negatively charged electrons from flowing to the catalyst and destroying the positively charged holes.
This "pinch-off" effect had been hypothesized to occur in solid-state devices for decades but had never been directly observed in fuel-forming photoelectrochemical systems.
"While our results are useful for understanding photoelectrochemical energy storage, the technique could more broadly be applied to study electrochemical processes in actively operating systems such as fuels cells, batteries or even biological membranes," said Forrest Laskowski, the study's lead author.
Laskowski was a National Science Foundation graduate research fellow in Boettcher's lab during the project. He is now a postdoctoral researcher at the California Institute of Technology in Pasadena.
The U.S. Department of Energy primarily funded the research. The microscope was purchased with a National Science Foundation grant. Instrumentation used in the project is based in the Center for Advanced Materials Characterization in Oregon, the UO's high-tech extension service available to researchers worldwide, and the Oregon Rapid Materials Prototyping Facility, which was funded by the Murdock Trust.
