Graphene’s ‘Love of Water’ Depends on Its Thickness

Professor Cho Min-haeng’s group revealed that at a molecular level the wettability of graphene depends on the number of layers.

The same material can have different properties depending on its thickness. Consider how difficult it is to tear many sheets of paper, while a single sheet of paper is easily torn up. The same is true of graphene, a ‘new dream material.’

The research group of Cho Min-haeng, the Director of the Center for Molecular Spectroscopy and Dynamics of the Institute for Basic Science (IBS, President Noh Do-young) and a professor in the Department of Chemistry, Korea University, revealed for the first time that at the molecular level the wettability of graphene depends its thickness.

The research results were published in the April 10 (KST) online edition of Chem (IF 19.735), a sister journal of Cell in the field of chemistry.

- Title of Article: Wettability of Graphene and Interfacial Water Structure

Wettability is the property of materials that represents how easily a surface may be wetted by water. Wettability is proportional to hydrophilicity and inversely proportional to hydrophobicity. In contrast to other deposited materials, the wettability of graphene is dependent upon the substrate that it is used with. It was known that the wettability of the substrate penetrates thin graphene and is transferred to the surface, but the mechanism was not clearly understood. Therefore, whether graphene was a hydrophilic material or a hydrophobic material was also unknown.

Studies on the wettability of graphene used to be based on the observation of microscopic phenomena. For example, studies used to be conducted by dropping a single drop of water on graphene and investigating wettability based on the shape the water drop formed on the surface. Such an approach has allowed a rough understanding of the characteristics of graphene surfaces but failed to enable researchers to thoroughly measure what happens at the graphene-water interface at a molecular level.

*Interface: The boundary surface between two different phases is referred to as an interface. A surface simply means the edge of an object, while an interface includes a material placed upon the surface of another object.

Graphene is in contact with water in some application environments, including its use as electrodes or filters. Hence, understanding the wettability of graphene on the interface in contact with water rather than the wettability of graphene itself is important. Experiments used to be performed using Raman spectroscopy. However, since signals from the water molecules adjacent to the interface are also measured by this method, selectively investigating phenomena at only the interface was impossible.

*Raman spectroscopy: Raman spectroscopy is a spectroscopic methodology in which a laser is shone on a molecule and the resulting scattered light, which possesses a frequency shift from the original laser frequency corresponding to the frequency of the molecule itself, is analyzed to acquire the vibrational spectrum of the molecule.

The IBS research group found that the technology called, ‘sum frequency generation spectroscopy,’ may be employed to selectively observe the hydrogen bonding structure of only the water molecules placed on the graphene-water interface. Water molecules, which are usually randomly oriented, have a constant orientation on the graphene-water interface, and the technology can selectively detect the signals from these molecules.

The research group deposited graphene on calcium fluoride (CaF2) layer by layer, while observing the vibration of the interfacial water molecules through sum frequency generation spectroscopy. The group found changes in the hydrogen bonding structure of water molecules depending on the number of graphene layers. The penetration of graphene wettability through the substrate decreased with the increase of the number of accumulated graphene layers. In particular, water molecules that do not form hydrogen bonds, which are observed only on hydrophobic interfaces, were found on graphene of 4 or more layers.

Kim Eun-chan, the first author, commented that ‘the adhesion energy, representing the hydrophilicity of the substrate, was acquired by sum frequency generation spectroscopy, and the results acquired by the method were consistent with the adhesion energy obtained conventionally through water contact angle measurement.’ Kim also explained, ‘This shows that sum frequency generation spectroscopy may serve as an important tool for the investigation of the properties of two-dimensional functional materials such as graphene.’
Cho Min-haeng who directed the research said, ‘This is the first molecular-level evidence that increasing the graphene layers increases the hydrophobicity of the interface. When graphene is used in water, interfacial hydrophobicity is critical for efficiency of graphene. Hence, our study will provide ideas for optimal graphene design.’

[Figure 1] Hydrogen-bonded structure of the water molecules on the graphene-water interface, as acquired by sum frequency generation spectroscopy.
A single graphene layer (left) consists of water molecules that form 4 hydrogen bonds (red) and those that form 2 hydrogen bonds (green). As the number of graphene layers is increased, the number of water molecules forming more hydrogen bonds decreases, and the number of water molecules forming less hydrogen bonds increases. The IBS research group discovered that, when the graphene is stacked to 4 layers or more, some water molecules do not form hydrogen bonds, a phenomenon which is found only on hydrophobic interfaces.