Nanoparticles: Two-faced materials boost hydrogen production

Hydrogen is crucial for the oil-refining industry and the production of essential chemicals such as the ammonia used in fertilizers. Since producing hydrogen is costly, scientists have long searched for alternative, energy-efficient methods to separate hydrogen atoms from abundant sources such as water.

Nanometer-scale structures consisting of cheap metal and oxide spheres were recently demonstrated as an excellent catalyst for a hydrogen-production reaction powered only by sunlight. The study was completed by Ming-Yong Han and his colleagues of the A*STAR Institute of Materials Research and Engineering, Singapore, working in collaboration with a team of researchers from Singapore and France1.

Han and his team mixed 50-nanometer diameter spheres of gold into a titanium dioxide precursor such that a sphere of titanium dioxide formed on the side of each gold nanoparticle. Structures with this two-sphere arrangement are known as Janus particles, named after the two-headed god from Roman mythology. While the Janus particles were suspended in a mixture of water and isopropyl alcohol, Han and co-workers shone visible light on them and measured hydrogen production, which proceeded at a rate as fast as 2 milliliters per minute.

The researchers then used theoretical models to show that this production rate was caused by so-called plasmonics effects: that is, the electrons on the surface of the gold nanoparticle at the junction with the titanium dioxide coupled to the incoming light and formed light-matter hybrid particles called plasmon polaritons. The energy absorbed by these particles then passed into the surrounding liquid, and this drove the hydrogen-releasing chemical reaction.

"Our work provides insight into mechanisms that will be useful for the future development of high-performance photocatalysts," says Han. Indeed, Han and his co-workers were able to improve the efficiency of the hydrogen production even further: they increased the area of the metal-oxide interface by using larger gold nanoparticles.

The Janus particles were 100 times more efficient as a catalyst for hydrogen production than bare gold nanoparticles. Moreover, they were over one-and-a-half times better than another common type of plasmonic nanoparticle, core-shell particles, in which the oxide material forms a coating around the metal nanoparticle.

"We next hope to develop a better understanding of the processes that occur at the metal-titanium-dioxide interface using a combination of experimental observations and theoretical simulations," says Han. "This will get us closer to our ultimate goal of using solar illumination as an abundant source of renewable energy."

The A*STAR-affiliated researchers contributing to this research are from the Institute of Materials Research and Engineering.