Two years ago, we covered research out of the University of Manchester that demonstrated that graphene-based membranes could serve as a filter for cleaning up nuclear waste at nuclear power plants.
While it’s not clear that this particular application for the graphene membranes ever made much headway in nuclear waste cleanup, researchers did discover an interesting phenomenon about these graphene membranes in the ensuing two years: Protons can transport through graphene.
Based on that knowledge, Andre Geim’s team at the University of Manchester began to investigate whether light could be used to enhance proton transport through graphene by the addition of other light-sensitive materials, such as titanium dioxide (TiO2). Turns out that graphene did the job quite effectively on its own.
“We were not expecting that graphene on its own—without the addition of these light sensitive ingredients—would show any response,” said Marcelo Lozada-Hidalgo of the University of Manchester and coauthor of this research and the work from two years ago. “We were very surprised by our results.”
In research published in the journal Nature Nanotechnology, Lozada-Hidalgo and his colleagues fabricated devices made from monolayer graphene decorated with platinum (Pt) nanoparticles. Essentially, photons excite the electrons in graphene around the nanoparticles. As a result, the electrons become highly reactive to protons. This, in turn, induces the electrons to recombine with protons to form hydrogen molecules at the Pt nanoparticles.
“In a way, this mechanism is not too dissimilar to electron-hole recombination in semiconductor photodetectors,” explained Lozada-Hidalgo.
While the mechanism might not be too different than semiconductor photodetectors, these devices are based on proton transport as opposed to all current photodetectors today, which are based on electron transport.
“Because our devices work with protons instead of electrons, they may lead to novel photodetector architectures, perhaps even with additional functionalities,” said Lozada-Hidalgo.
One of the most striking features of this work, according to Lozada-Hidalgo, is that these devices not only match most commercial photodetectors but also, on certain key figures of merit, already greatly exceed their performance.
Three parameters are usually used to evaluate photodetectors: photoresponsivity, noise equivalent power (a measure of minimum radiant power measurable, taking into account the finite electric noise in the dark), and response time.
“Our devices exhibit 100,000 times higher photoresponsivity than most commercial silicon photodiodes,” said Lozada-Hidalgo.
Although Lozada-Hidalgo concedes that the response time is currently limited by the measurement setup, it is still comparable to nonspecialized silicon photodiodes.
The third parameter, noise equivalent power, is comparable or higher to that of commercial silicon photodetectors.
“This comparison suggests that the proton-based devices may be suitable for some applications even in their current nonoptimized design, especially if all figures of merit are considered together,” he added.
The researchers also found that every photon incident on the membrane induces the transport of 10,000 protons. “Most photodetectors based on electron transport, such as silicon, would struggle to produce one electron for every photon incident, let alone 10,000,” added Lozada-Hidalgo.
There is also room for improvement for these devices, according to Lozada-Hidalgo. “One interesting research avenue is to add other photosensitive materials, like quantum dots, that could further enhance the photoresponse of the devices,” he added.
While photodetectors look to be the most promising applications for this technology, they could also be another solution to artificial photosynthesis, otherwise known as photoelectrochemical reduction. In traditional artificial photosynthesis, the bandgap of a semiconductor generates an electron-and-hole pair when it is struck by a photon with an energy level that is higher than the bandgap of the semiconductor.
The photon-proton effect—the effect responsible for the operation of these devices—would represent a big departure from the semiconductor approach. Lozada-Hidlago acknowledges that it is such an exotic phenomenon that it is still not fully known what its implications in other technologies could be.
Lozada-Hidalgo added: “Our devices not only detect light, but they also produce hydrogen gas in the process. Only time will tell if this can be exploited in novel technologies.”