Light boosts hydrogen generation on graphene

Shining a light on graphene decorated with Pt nanoparticles massively speeds up the transit of protons through the material and boosts hydrogen generation, according to researchers from the University of Manchester and National Graphene Institute [Lozada-Hidalgo et al., Nature Nanotechnology (2018), doi: 10.1038/s41565-017-0051-5].

Only relatively recently has it been found that graphene is permeable to protons, making it potentially attractive for technologies using proton-conducting membranes such as solar energy harvesting devices and fuel cells. But it turns out that graphene’s proton-conducting properties could have an added benefit.

“We were interested in finding ways of using graphene to harvest solar energy to produce renewable fuels, which is a scientifically and technologically relevant challenge,” explains Marcelo Lozada-Hidalgo, first author of the study. “We found that graphene can use sunlight to produce hydrogen and, in doing so, produces large electrical currents from tiny amounts of light.”

The researchers fabricated devices by suspending mechanically exfoliated graphene membranes over etched porous silicon nitride films. On one side, the membranes are decorated with Pt nanoparticles, while a proton-conducting polymer (Nafion) is deposited onto the other. A proton-injecting electrode is then contacted to the device and a voltage applied.

“A voltage bias between the graphene membrane and the electrolyte pushes protons through the membrane, which evolve as hydrogen gas (H₂) on the metal nanoparticles,” explains LozadaHidalgo. “We show that shining light on these membranes hugely enhances the process.”

According to the researchers’ electrical and mass spectrometry measurements, every photon that hits the Pt-decorated graphene membrane induces the transport of 10,000 protons. This figure of merit for graphene outperforms all the but the most specialized state-of-the-art photodetectors based on electron transport in silicon and other two-dimensional materials.

The process is very fast, taking only microseconds for the device to respond to light. The researchers dub the phenomenon, which has not been observed before in any other material including graphene, the ‘photo-proton effect’.

“This giant photo-effect was completely unexpected and is a result of the combination of several unique properties in graphene,” says Lozada-Hidalgo.

The same process also leads to the formation of 5000 H₂ molecules, which heralds the possibility of generating green fuels in a photosynthesis-like manner.

“This is a huge number, since usually millions of photons are needed to produce just a single H₂ molecule in such photovoltaic membranes,” Lozada-Hidalgo points out.

When light is shone on graphene, highly energetic electrons are created. In other materials, these electrons collide with the lattice or neighbouring materials and lose their energy. In graphene, however, the electrons collide with each other, producing more energetic or ‘hot’ electrons in the process. This process is well known in graphene, but the hot electrons are very well insulated and hard to use or ‘harvest’.

“This is where the protons and the Pt nanoparticles in our devices come in,” explains Lozada-Hidalgo. “The Pt nanoparticles essentially create tiny p-n junctions around them, which pull in photogenerated hot electrons. This creates a local photovoltage, which acts just like an external applied voltage, funnelling protons toward the nanoparticles.”

Facilitated by the presence of the nanoparticles, the protons react with the hot electrons to produce H2 molecules. Many renewable energy technologies could benefit from the findings, believe the researchers.

“The production of ‘green’ fuels such as H₂ from sunlight has the potential to contribute to the storage of solar energy in a scalable and on-demand way,” says Lozada-Hidalgo. “Fuels are also necessary for 40% of global transportation. Wouldn’t it be great if, just like plants, we could extract all the energy we need from the sun?”

Graphene has all the properties required for artificial photosynthesis membranes, he believes, as well as providing a new way of harvesting sunlight. According to Lozada-Hidalgo, there are no obvious drawbacks to the new approach.

“Only time will tell if this is correct,” he says, “but one cannot help speculating that this could enable new technologies we have not even thought of yet.”

Mikhail I. Katsnelson of Radboud University in the Netherlands agrees that the findings could open up new ways of using graphene in energy-related technologies.

“Graphene still remains amazing material, with a lot of surprises and huge potential for applications,” he comments. “Anomalous proton permeation through single-layer graphene is mysterious, but hopefully the giant photo-proton effect discovered in this work will help to elucidate the mechanism.”

Frank Koppens of The Institute of Photonic Sciences (ICFO) agrees, saying:

“This work is highly novel and impressive as it is the first time that proton permeation has been used for photodetection. Many applications can be envisioned, far beyond applications we know in our every day life. Examples might include light-induced water splitting, photocatalysis, and photodetectors, but I am sure many more will be possible once industries grasp the enormous potential of this system.”

This article was originally published in Nano Today 19 (2018) 4-5.