Measuring the 'wettability' of graphene and other 2D materials

Date:
April 26, 2022
Source:
Institute for Basic Science
Summary:
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).



FULL STORY ========================================================================== 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.


========================================================================== Story Source: Materials provided by Institute_for_Basic_Science. Note:
Content may be edited for style and length.


========================================================================== Journal Reference:
1. Donghwan Kim, Eunchan Kim, Sohyun Park, Seungah Kim, Byoung Koun
Min, Hyo
Jae Yoon, Kyungwon Kwak, Minhaeng Cho. Wettability of graphene
and interfacial water structure. Chem, 2021; 7 (6): 1602 DOI:
10.1016/ j.chempr.2021.03.006 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2022/04/220426153656.htm

--- up 8 weeks, 1 day, 10 hours, 51 minutes
* Origin: -=> Castle Rock BBS <=- Now Husky HPT Powered! (1:317/3)