Nickel nanoparticles reveal secrets of charge collection

Using an atomic-force microscope fitted with a tiny electrode tip, researchers have identified in real-time how catalysts made from nickel nanoparticles collect charges that are excited by light in semiconductors.

nickel nanoparticles
Illustration depicts a specially fitted electrode tip on an atomic force microscope providing a nanoscale view of activity occurring at the interface of a single-crystal silicon wafer coated with metallic nickel nanoparticles (Image: Shannon Boettcher)

The findings could lead to improved systems that use light to make chemicals and fuels, such as by splitting water to make hydrogen or by combining carbon dioxide and water to make carbon-based fuels or chemicals.

The team from the University of Oregon found that as the size of the catalytic particles shrinks below 100nm the collection of excited positive charges (holes) becomes much more efficient than the collection of excited negative charges. This phenomenon prevents the excited positive and negative charges from recombining and is said to increase the system efficiency. The research is described in Nature Materials.

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"We found a design principle that points to making catalytic particles really small because of the physics at the interface, which allows one to increase efficiency," said Shannon Boettcher, a professor in Oregon's Department of Chemistry and Biochemistry and member of the university's Materials Science Institute.

"Our technique allowed us to watch the flow of excited charges with nanometre-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 used a model system consisting of a well-defined single-crystal silicon wafer coated with metallic nickel nanoparticles of different sizes. The silicon absorbs sunlight and creates excited positive and negative charges. The nickel nanoparticles selectively collect the positive charges and speed up the reaction of those positive charges with electrons in water molecules, pulling them apart.

Previously, researchers could only measure the average current moving across such a surface and the average voltage generated by the light hitting the semiconductor, Boettcher said. To look closer, the team collaborated with Bruker Nano Surfaces, the manufacturer of the UO's atomic force microscope that images the topography of surfaces by tapping a sharp tip over it, to develop the techniques needed to measure voltage at the nanoscale.

As the electrode tip touched each of the nickel nanoparticles, the researchers were able to record the build-up of holes by measuring a voltage.

According to UO, the voltage measured as the device was operating depended strongly on the size of the nickel nanoparticle. Small particles were able to better select for the collection of excited positive charges over negative charges, reducing the rate of charge recombination and generating higher voltages that better split apart water molecules.

A key, Boettcher said in a statement, is that oxidation at the nickel nanoparticle surface leads to a barrier that prevents the negatively charged electrons from flowing to the catalyst and annihilating the positively charged holes. This effect has been termed "pinch-off" and was hypothesised to occur in solid-state devices for decades but never before observed in fuel-forming photoelectrochemical systems.

"This new technique is a general means to investigate the state of nanoscale features in electrochemical environments," said the study's lead author Forrest Laskowski. "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."