Measuring the ‘wettability’ of graphene and other 2D materials
- Microscopic understanding of wettability can be achieved at the molecular level using 'vibrational
sum-frequency generation spectroscopy' (VSFG) -
Wettability of the 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.
So far, studies on the wettability of substrates have mainly been measured at the macroscopic level. The
macroscopic measurement of wettability is typically determined by measuring the water contact angle (WCA), which
is the angle a water droplet makes with respect to the surface of the substrate. However, it is currently very
difficult to accurately measure what happens at the interface between a substrate and water at the molecular
level.
Currently used microscopic measurement techniques, such as reflection-based infrared spectroscopy or Raman
spectroscopy, are incapable of selectively observing the interfacial water molecules. Since the number of water
molecules in the entire bulk of the liquid is much larger than the molecules that are making contact with the
surface, the signal of interfacial water molecules is obscured by the signal of water molecules in the bulk
liquid.
To overcome this limitation, a research team at the Center for Molecular Spectroscopy and Dynamics (CMSD) within
the Institute for Basic Science (IBS) in Seoul, South Korea, and the Korea University revealed that vibrational
sum-frequency generation spectroscopy (VSFG) could be used for measuring the wettability of 2D-materials. The
team succeeded in measuring the vibrational mode of water molecules in interfaces between graphene and water
using VSFG spectroscopy.
VSFG is a useful technique that can connect the macroscopic measurement results with molecular-level properties.
It is a surface-selective tool for investigating interfacial molecules using its own surface selection rule, and
it has a very good surface resolution with a few molecular layers.
The group identified the unique ability of the graphene to project the wettability of the substrate onto its
surface, which is called ‘wetting transparency’. They observed that the wetting transparency of graphene
diminish as the number of graphene layers increased, disappearing when the graphene is more than 4 layers thick.
This is the first observation to describe that graphene surface becomes hydrophobic above a certain number of
layers at the molecular level.
Also, the researchers defined the new concept of VSFG wettability, which is the ratio of water molecules forming
strong hydrogen bonds against water molecules with weak or no hydrogen bond formation. The VSFG wettability
correlated strongly with the adhesion energy, which is calculated from the observed macroscopic WCA
measurements. This proved that VSFG is an effective tool for defining the wettability of a material’s surface.
Using VSFG wettability, the researchers measured the wettability of the graphene in real-time, as an electric
field was applied for it to form graphene oxide. It is impossible to observe wettability in real-time with the
traditional WCA experiments. Therefore, this suggests that VSFG could be a decisive technique for measuring the
water adhesion energy on any spatially confined interface where the water contact angle measurement cannot be
applied. In addition to graphene, VSFG spectroscopy is expected to shed light on the wettability of other
low-dimensional materials.
First author Eunchan Kim notes: "This study confirmed that VSFG spectroscopy could be used as a versatile tool
for measuring the wettability.”, and “We demonstrate the potential to measure the wettability of previously
unobservable complex systems through VSFG spectroscopy.”
Professor CHO Minhaeng, the Director of CMSD notes: "With VSFG spectroscopy, we are studying the microscopic
properties of graphene as well as other two-dimensional functional materials such as graphene oxide and
hexagonal boron nitride.", and “Through this, it will be possible to solve various problems that hinder the
commercialization of two-dimensional functional materials.”
This research was published in the online edition of Chem (IF 22.804) on April 26th.
Figure 1. VSFG spectra of interfacial water at graphene/water interface and comparison of VSFG
wettability and adhesion energy
(left) VSFG spectra of interfacial water show that a dangling OH peak
(3600 cm-1) appears when the graphene is more than 4 layers thick. (right) The calculated VSFG wettability is
plotted against adhesion energy from macroscopic observation. Both values coincide closely, which indicates
increasing hydrophobicity as the number of graphene layers increases.
Figure 2. Graphical illustration of VSFG experiment and water contact angle measurements
Water
contact angle measurements of graphene give information about macroscopic wettability. On the other hand, the
VSFG experiment can provide information about the microscopic structure of interfacial water and the wettability
of graphene.
Notes for editors
- References
Eunchan Kim, Donghwan Kim, Kyungwon Kwak, Yuki Nagata, Mischa Bonn, and Minhaeng Cho “Wettability of
graphene, water contact angle, and interfacial water structure,” Chem, (2022).
- Media Contact
For further information or to request media assistance, please contact Minhaeng Cho, Center for
Molecular Spectroscopy and Dynamics, Institute for Basic Science (IBS) (+82-2-3290-3133; mcho@korea.ac.kr
Eunchan Kim (eunchan9@korea.ac.kr), or William I. Suh at the IBS
Communications Team (willisuh@ibs.re.kr).
- About the Institute for Basic Science (IBS)
IBS was founded in 2011 by the government of the Republic of Korea with the sole purpose of driving
forward the development of basic science in South Korea. IBS has 1 research institute and 34 research
centers as of April 2022. There are eleven physics, three mathematics, seven chemistry, seven life science,
one earth science, and five interdisciplinary research centers.