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 France.
Han
and his team mixed 50-nm 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.”
Source: A*STAR
