Graphene balloons to identify noble gases

New research led by scientists from Delft University of Technology and the University of Duisburg-Essen uses the movement of atomically thin graphene to identify noble gases. These gases are chemically passive and do not react with other materials, which makes them difficult to detect. The results are reported in the journal Nature Communications.

Graphene is an ultimately thin material consisting of a single layer of carbon atoms. Its atomic thickness makes it a perfect filter material for gases and liquids: graphene itself is not permeable, but small perforations make it very permeable. Furthermore, the material is among the strongest known and withstands severe stress. Together, these two features provide the perfect foundation for new types of gas sensors.

Nano balloons

Scientists use microscopic balloons made of double-layered graphene (0.7 nm thick), with very small nanopore perforations with diameters up to 25 nm, to detect gases. They use a laser to heat the gas inside the balloon and make it expand. The pressurized gas then escapes through the perforation. “Imagine a balloon deflating when you let the air run out,” says TU Delft researcher Irek Rosłoń, “We measure the time it takes for the balloon to deflate. On such a small scale, this happens very quickly, within about 1 / 100,000th of a second, and it is interesting to note that the length of time strongly depends on the type of gas and the size of the pores. For example, helium, a light gas with high molecular speed, escapes five times faster than krypton, a heavy gas and that slowly moves gas. ” The method makes it possible to distinguish gases on the basis of their mass and molecular velocity, which normally requires large mass spectrometers.

Gas pumping

The graphene balloons are continuously propelled by a high-frequency octothermic force of 100 kHz, causing the gas to be pumped in and out through the nanopores very quickly. Gas permeation can be studied by observing the mechanical motion of graphene. At low pumping frequencies, the gas takes a long time to escape and does not significantly affect the movement of graphene. However, the membrane suffers a great deal of resistance to increasing pumping frequencies, particularly when the pumping period is the typical time it takes for the gas to leave the balloon. “By measuring at various frequencies, we can find that peak in resistance. The frequency at which a peak is observed corresponds to the gas permeation rate.”

The researchers extended this idea to the study of gas flow through nanochannels. Connecting the balloon to a long channel makes it much more difficult for the gas to escape. Increasing the deflation time provides an experimental insight into the mechanics of gas flow within the nanochannels. Overall, this work shows how graphene’s extraordinary properties can be used to study gas dynamics at the nanoscale, as well as to design new types of sensors and devices. In the future, this may allow small, inexpensive and versatile sensor devices to determine the composition of gas mixtures in industrial applications or for air quality monitoring.