Hot electrons generated on gold nanoparticles by irradiation with light can be used to split water. Illustration: I. Thomann/Rice University.
Hot electrons generated on gold nanoparticles by irradiation with light can be used to split water. Illustration: I. Thomann/Rice University.

Rice University researchers have demonstrated an efficient new way to capture the energy from sunlight and convert it into clean, renewable energy by splitting water molecules. The technology, which is described in Nano Letters, relies on a configuration of light-activated gold nanoparticles that harvest sunlight and transfer solar energy to highly excited electrons, which scientists sometimes refer to as ‘hot electrons’.

"Hot electrons have the potential to drive very useful chemical reactions, but they decay very rapidly, and people have struggled to harness their energy," says lead researcher Isabell Thomann, assistant professor of electrical and computer engineering and of chemistry and materials science and nanoengineering at Rice. "For example, most of the energy losses in today's best photovoltaic solar panels are the result of hot electrons that cool within a few trillionths of a second and release their energy as wasted heat." Capturing these high-energy electrons before they cool could allow a significant increase in solar-to-electric power conversion efficiencies.

The gold nanoparticles studied by Thomann and colleagues at Rice's Laboratory for Nanophotonics (LANP) are able to capture light and convert it into plasmons, waves of electrons that flow like a fluid across the metal surface of the nanoparticles. Plasmons are short-lived high-energy states, but researchers at Rice and elsewhere have found ways to capture plasmonic energy and convert it into useful heat or light. These plasmonic gold nanoparticles also offer one of the most promising means of harnessing the power of hot electrons, and LANP researchers have made progress toward that goal in several recent studies.

Building on this work, Thomann and her team, graduate students Hossein Robatjazi, Shah Mohammad Bahauddin and Chloe Doiron, have now harnessed the energy from hot electrons on plasmonic nanoparticles to split molecules of water into oxygen and hydrogen. That's important because oxygen and hydrogen are the feedstocks for fuel cells, electrochemical devices that produce electricity cleanly and efficiently.

To use the hot electrons, Thomann's team first had to find a way to separate them from their corresponding ‘electron holes’, the low-energy states vacated by the hot electrons when they receive their plasmonic jolt of energy. One reason why hot electrons are so short-lived is because they have a strong tendency to release their newfound energy and revert to the low-energy state. The only way to avoid this is to engineer a system where the hot electrons and electron holes are rapidly separated from one another. The standard way for electrical engineers to do this is to drive the hot electrons over an energy barrier that acts like a one-way valve. According to Thomann, this approach has inherent inefficiencies, but is attractive to engineers because it uses a well-understood technology called Schottky barriers, which are a tried-and-true component of electrical engineering.

"Because of the inherent inefficiencies, we wanted to find a new approach to the problem," Thomann explains. "We took an unconventional approach: Rather than driving off the hot electrons, we designed a system to carry away the electron holes. In effect, our setup acts like a sieve or a membrane. The holes can pass through, but the hot electrons cannot, so they are left available on the surface of the plasmonic nanoparticles."

The setup employs three layers of materials. The bottom layer is a thin sheet of shiny aluminium, which is covered with a thin coating of transparent nickel-oxide. Finally, a collection of plasmonic gold nanoparticles – puck-shaped disks 10–30nm in diameter – is scattered on top.

When sunlight hits the gold nanoparticles, either directly or via a reflection from the aluminum, they convert the light energy into hot electrons. The aluminum attracts the resulting electron holes and the nickel oxide allows these to pass through while also acting as an impervious barrier to the hot electrons, which stay on the gold nanoparticles.

When the researchers covered the material in water, the hot electrons on the gold nanoparticles were able to catalyze water splitting. In the current round of experiments, the researchers measured the photocurrent available for water splitting rather than directly measuring the evolved hydrogen and oxygen gases produced by splitting, but Thomann said the results warrant further study.

"Utilizing hot electron solar water-splitting technologies, we measured photocurrent efficiencies that were on par with considerably more complicated structures that also use more expensive components," Thomann said. "We are confident that we can optimize our system to significantly improve upon the results we have already seen."

This story is adapted from material from Rice University, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.