Measurement of the “wettability” of graphene and other 2D materials

(left) VSFG spectra of interfacial water show that a suspended OH peak (3600 cm-1) appears when graphene is more than 4 layers thick. (right) The calculated wettability of VSFG is plotted as a function of adhesion energy from macroscopic observation. The two values ​​coincide closely, indicating increasing hydrophobicity as the number of graphene layers increases. Credit: Institute of Basic Sciences

The wettability of a material is the ability of a liquid to maintain contact with a solid surface, and it is proportional to hydrophilicity and inversely proportional to hydrophobicity. It is one of the most important properties of a solid, and understanding the wettability of different substrates is essential for various industrial uses, such as desalination, coating agents, and water electrolytes.

Until now, studies on the wettability of substrates have been mainly measured at the macroscopic level. The macroscopic measure of wettability is usually determined by measuring the water contact angle (WCA), which is the angle that a drop of water makes with respect to the surface of the substrate. However, it is currently very difficult to accurately measure what is happening at the interface between a substrate and water at the molecular level.

Currently used microscopic measurement techniques, such as reflection infrared spectroscopy or Raman spectroscopy, are unable to selectively observe interfacial water molecules. Since the number of water molecules in the whole mass of the liquid is much larger than the molecules that come into contact with the surface, the signal from the interfacial water molecules is obscured by the signal from the water molecules in the mass of the liquid.

To overcome this limitation, a research team from the Center for Molecular Spectroscopy and Dynamics (CMSD) at the Institute for Basic Science (IBS) in Seoul, South Korea, and Korea University found that the Sum frequency generation vibrational spectroscopy (VSFG) could be used to measure the wettability of 2D materials. The team succeeded in measuring the vibrational mode of water molecules in the interfaces between graphene and water using VSFG spectroscopy.

VSFG is a useful technique that can relate macroscopic measurement results to properties at the molecular level. It is a surface selective tool to study interface molecules using its own surface selection rule, and it has very good surface resolution with few molecular layers.

Measurements of the water contact angle of graphene provide information on the macroscopic wettability. On the other hand, the VSFG experiment can provide information about the microscopic structure of interfacial water and the wettability of graphene. Credit: Institute of Basic Sciences

The group identified graphene’s unique ability to project the wettability of the substrate onto its surface, called ‘wetting transparency’. They observed that the wetting transparency of graphene decreases as the number of graphene layers increases, disappearing when the graphene is more than four layers thick. This is the first observation to describe that the surface of graphene becomes hydrophobic above a number of layers at the molecular level.

In addition, the researchers defined the new concept of VSFG wettability, which is the ratio of water molecules forming strong hydrogen bonds to water molecules with weak or no hydrogen bond formation. The wettability of VSFG is strongly correlated with the adhesion energy, which is calculated from the observed macroscopic measurements of WCA. This proved that the VSFG is an effective tool for defining the surface wettability of a material.

Using the wettability of VSFG, the researchers measured the wettability of graphene in real time, when an electric field was applied to it to form graphene oxide. It is impossible to observe real-time wettability with traditional WCA experiments. Therefore, this suggests that VSFG could be a decisive technique to measure the adhesion energy of water on any spatially confined interface where water contact angle measurement cannot be applied. In addition to graphene, VSFG spectroscopy should shed light on the wettability of other low-dimensional materials.

First author Eunchan Kim notes, “This study confirmed that VSFG spectroscopy can be used as a versatile tool for measuring wettability” and “We demonstrate the potential to measure the wettability of previously unobservable complex systems through VSFG spectroscopy. .

Professor CHO Minhaeng, Director of CMSD, notes, “With VSFG spectroscopy, we study the microscopic properties of graphene as well as other two-dimensional functional materials such as graphene oxide and hexagonal boron nitride,” and “Thanks to this, it will be possible to solve various problems that hinder the commercialization of two-dimensional functional materials.

This research has been published in the online edition of Chemistry April 26.


Identification of the wettability of graphene layers at the molecular level


More information:

Eunchan Kim et al, Graphene wettability, water contact angle and water interfacial structure, Chemistry (2022). DOI: 10.1016/j.chempr.2022.04.002

Journal information:
Chemistry

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Institute of Basic Sciences

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