Harvesting More Solar Energy With Supercrystals

electron microscopic image depicts a monolayer

Photocatalysis Record: A newly developed bimetallic supercrystal produces hydrogen using sunlight, setting a new world record for solar hydrogen production. The combination of gold and platinum nanoparticles in a monolayer composite crystal has made this achievement possible. This crystal absorbs sunlight and utilizes this energy to efficiently split formic acid into hydrogen and CO2. The researchers report that this approach also optimizes other photocatalytic reactions.

“Green” hydrogen is a crucial component of the energy transition. To produce it, renewable electricity can be used for the electrolysis of water. However, a more direct method involves solar water splitting. Following the model of plant photosynthesis, catalysts are employed to directly absorb the energy of sunlight and utilize it for the chemical reaction. The range of previous approaches includes floating leaves, solar nano factories, and photoreactors for household rooftops.

A Combination Crystal Made of Gold and Platinum

Testing the supercrystal in the laboratory.
Testing the supercrystal in the laboratory.

Now another approach has been added: Matias Herran from Ludwig-Maximilians-Universität München (LMU) and his team have combined catalytic nanoparticles in such a way that they can produce solar hydrogen with high efficiency. The starting point for this is gold nanoparticles 100 to 200 nanometers in size, which are already known to absorb sunlight highly efficiently. “At this size, the visible light interacts very strongly with the gold electrons and causes them to vibrate resonantly,” explains Herran.

For their novel “Supercrystal,” the researchers created a layer of gold nanoparticles that is only one atom thick for the most part. In this two-dimensional crystal, the gold atoms are arranged in a regular hexagonal pattern. When this gold layer is irradiated with light, the absorbed energy is concentrated at certain points: “Strong local electric fields, the hotspots, are created between the gold particles,” says Herrán. In these hotspots, the team placed platinum nanoparticles, which are known for their catalytic effect in splitting hydrogen-carbon bonds.

Division of Labor in the Super Crystal

The outcome is a “super crystal” composed of two metallic nanoparticles working together in an almost perfect division of labor: gold nanoparticles absorb the energy from sunlight and convert it into electric fields. Platinum nanoparticles utilize these electrons to drive a chemical reaction; in initial tests, the team chose the cleavage of formic acid (CH2O2) into hydrogen and CO2.

It was observed that under irradiation equivalent to normal sunlight exposure and without additional additives, the supercrystal began producing hydrogen. The solar hydrogen production rate reached 139 millimoles per gram of catalyst per hour—a new world record in photocatalytic hydrogen production, as reported by the team. They attribute this high efficiency to the combination of plasmonic, light-absorbing gold particles with catalytically active platinum particles.

Additionally Applicable to Other Photocatalytic Reactions

According to the research team, their approach could be adapted for other photocatalytic reactions, such as the conversion of CO2 into synthesis gas, carbon monoxide, or methanol. “By combining plasmonic and catalytic metals, we are advancing the development of potent photocatalysts for the industry,” explains the researchers. They have already patented their “super crystal.”

Source:  (Nature Catalysis, 2023; doi: 10.1038/s41929-023-01053-9)

Featured Image: This electron microscopic image depicts a monolayer “super crystal” composed of gold nanoparticles with interspersed smaller platinum particles. Together, they form a “factory” for solar hydrogen. © Herran et al./ Nature Catalysis, CC-by 4.0