Researchers at The University of Manchester have discovered another new and unexpected physical effect in graphene.
The new physical effect – membranes which could be used in devices to artificially mimic photosynthesis – demonstrated an increase in the rate at which the material conducts protons when it is illuminated with sunlight.
The ‘photo-proton’ effect could be applied to design devices able to directly harvest solar energy to produce hydrogen gas, a promising green fuel. It might also be of interest for other applications, such as light-induced water splitting, photo-catalysis, and for making new types of highly efficient photodetectors.
Researchers recently found that it is also permeable to thermal protons (the nuclei of hydrogen atoms), which means that it could be employed as a proton-conducting membrane in various technology applications.
How was the ‘photo-proton’ effect discovered?
To find out how light affects the behaviour of protons permeating through the carbon sheet, a team led by Dr Marcelo Lozada-Hidalgo and Professor Sir Andre Geim fabricated pristine graphene membranes and decorated them on one side with platinum nanoparticles.
The researchers found that the proton conductivity of these membranes was multiplied by 10 when they were illuminated with sunlight.
Scientists around the world are looking into how to use solar energy directly in order to produce renewable fuels by mimicking photosynthesis in plants. These man-made ‘leaves’ will require membranes with very sophisticated properties, including:
- mixed proton-electron conductivity;
- permeability to gases;
- mechanical robustness; and
- optical transparency.
Professor Geim said: “This is essentially a new experimental system in which protons, electrons and photons are all packed together in an atomically thin volume. I am sure that there is a lot of new physics to be unearthed, and new applications will follow.”
Currently, researchers use a mixture of proton and electron-conducting polymers to make such structures, but these require some important trade-offs that could be avoided by using graphene.
5000 hydrogen molecules produced in response to every solar photon
Using electrical measurements and mass spectrometry, the researchers say that they measured a photoresponsivity of around 104 A/W, which translates into around 5000 hydrogen molecules formations, in response to every solar photon incident on the membrane.
This is a huge number compared to existing photovoltaic devices, where many thousands of photons are needed to produce a single hydrogen molecule.
Lozada-Hidalgo added: “We knew that graphene absorbs light of all frequencies and that it is also permeable to protons, but there was no reason for us to expect that the photons absorbed by the material could enhance the permeation rate of protons through it.
“The result is even more surprising when we realised that the membrane was many orders of magnitude more sensitive to light than devices that are specifically designed to be light-sensitive.”
Photodetectors typically harvest light to produce electricity, but graphene membranes produce both electricity and, as a by-product, hydrogen. The speed at which they respond to light in the microsecond range is faster than most commercial photodiodes.