"Graphene Repels Water"... IBS Resolves a Decade-Long Debate [Reading Science]

Cause Identified Through Machine Learning Simulations

Water Molecules Hidden Beneath Graphene Create an "Optical Illusion"

Researchers at the Institute for Basic Science (IBS) have identified that graphene is fundamentally a hydrophobic material, meaning it repels water. They analyzed that the conflicting experimental results to date—some showing that graphene attracts water and others that it repels it—were due to a thin layer of water hidden beneath the graphene.

On May 11, IBS announced that the team led by Minhong Cho, Director of the Center for Molecular Spectroscopy and Dynamics, and Professor Stephan Lingner from the Department of Chemistry at Korea University, analyzed the graphene-water interface at the atomic level using machine learning-based molecular dynamics simulations. Their findings confirmed that defect-free, pure graphene is inherently hydrophobic and does not exhibit "wetting transparency."

Difference in Water Penetration Between Single-Layer and Multi-Layer Graphene. a) Comparison of the energy required for water molecules to penetrate below graphene. Single-layer graphene has a low energy barrier, allowing water to penetrate easily, whereas four-layer graphene requires high energy, making penetration difficult. b) Schematic showing that in single-layer graphene, water molecules are more stable when entering the gap between the graphene and the hydrophilic substrate than remaining above the graphene. c) In four-layer graphene, water penetration into the space below is blocked, causing water molecules to remain on the graphene surface. Provided by the research team.

Graphene is a representative two-dimensional material consisting of a single layer of carbon atoms. It has been highlighted as a promising candidate for next-generation semiconductors and energy materials due to its outstanding strength and electrical properties. However, its interaction with water has been a subject of debate for over a decade.

Some experiments have observed hydrophobic properties, with water forming droplets on graphene. In contrast, other experiments have found hydrophilic results, where water spreads out. Notably, the "wetting transparency" hypothesis was proposed, suggesting that because graphene is only an atom thick, the properties of the underlying substrate would be fully revealed.

"Water Trapped Beneath Graphene Is the Cause"

The research team identified "dangling O-H bonds," a structure where bonds between water molecules near the surface of pure graphene are broken. They also observed that this characteristic becomes more pronounced as the number of graphene layers increases.

The team determined that the reason for the contradictory experimental results was due to water molecules trapped between the graphene and the substrate. In the case of single-layer graphene on a hydrophilic substrate, water vapor from the air can penetrate beneath the graphene, forming a thin water layer. During measurement, the signals from water above and below the graphene are recorded simultaneously and partially offset each other, which made the inherently hydrophobic graphene appear hydrophilic.

In contrast, as the number of graphene layers increases, it becomes much more difficult for water to penetrate below, and the influence of the hidden water layer disappears. As a result, the intrinsic hydrophobicity of graphene is clearly exhibited in multi-layer graphene.

The researchers expect that these findings will impact the development of graphene-based technologies such as nanofluidic devices, desalination membranes, hydrogen fuel cells, and energy storage devices. This is because they confirmed that even an extremely thin water layer can significantly alter interfacial properties.

Minhong Cho, the research director, said, "By revealing the role of water existing beneath graphene, we have been able to understand the intrinsic wettability of graphene."

This research was published online in the international journal Nature Communications on April 2.

This content was produced with the assistance of AI translation services.