Is graphene hydrophilic or hydrophobic?
As many researchers believe, graphene floating on the water does not repel water, but attracts it.
Graphene is made of the same material as graphite-for example, found in pencils-which is why graphene has long been thought to be as waterproof as graphite. A key difference from graphite is that graphene is no more than one atom thick.
The study of graphene has given rise to different views on its wetting properties. Researchers at Leiden have now found that graphene on the surface of the water is hydrophilic as long as it is clean and smooth. In other words, graphene in water attracts water.
This view is very important for the future application of graphene. Its thin layers of carbon atoms are arranged in a honeycomb structure, making the material suitable for biosensors to decode DNA. The ultra-thin material can also be used in sensors, water filtration and fuel cell membranes. In all these cases, both sides of the graphene layer are exposed to water.
Graphene has long been considered hydrophobic or waterproof. The researchers hypothesized that this is because graphene is usually produced on metal surfaces and then transferred to solid carriers, such as silicon wafers. However, during this transfer process, graphene may be damaged or contaminated, which can affect the wetting behavior.
In order to determine whether the surface of the complete graphene layer is hydrophilic, a drop of water must be introduced into the layer. The curvature of the droplet can then be used to determine whether the surface is waterproof, like a raincoat, or whether it absorbs water. There is so much theory, because in practice such droplets will immediately tear a thin layer of graphene, causing cracks, and then there is not much that can be measured.
The researchers solved this problem by using ice or hydrogel instead of water as a support layer. Ice and hydrogels are stronger and are good imitators of water. The researchers were then able to introduce tiny water droplets into graphene and measure the contact angle between the graphene surface and the outside of the water droplets. They also repeated measurements using other liquids with different polarities. They were then able to determine that all molecular interactions in the underlying water were felt by the water molecules in the droplets above. This complete transparency is the result of the extremely thin graphene, which explains why graphene is hydrophilic in water.
This discovery has a significant impact on the application of graphene in sensors, water filtration and membrane-based fuel cells, all of which are characterized by graphene exposure to water. Developers will have to consider a completely different starting point, that is, the graphene they use is not hydrophobic, but hydrophilic.
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