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How to directly measure the surface energy of pure graphene

(Nanowerk Spotlight) In the past decade, graphene has received significant scientific interest as a new material and its potential to impact a wide range of technologies. Huge efforts have gone into studying its synthesis, fundamental properties, and the development of potential applications – and some applications based on bulk graphene are slowly entering the market.
Surface energy is defined as the energy cost to increase a materials' surface per unit area. Therefore it indicates how likely molecules are to adsorb (/desorb) onto the surface or how strong it forms (non-covalent) bonds with other materials. Access to accurate surface energy values of graphene is thus not only of fundamental interest, but provides a useful reference for anyone involved in research on graphene properties, (surface) modifications, and the implementation of graphene in devices.
New research by scientists in the UK demonstrates the successful application of the graphene surface force balance (g-SFB), which they developed earlier. In this new work, reported in Nano Letters ("Direct Measurement of the Surface Energy of Graphene"), they used it to directly measure the surface energy of pure graphene.
Direct measurement of the surface energy of graphene
Direct measurement of the surface energy of graphene. Crossed cylinder geometry: The use of crossed cylinders (equal to sphere-on-flat) allows for facile conversion of measured values to that of parallel plates. Radius: The radius of curvature can be measured in situ though the analysis of secondary (not shown) and primary interference fringes resulting from the reflections between the semi-transparent gold mirrors. Distance: Can be directly measured by studying the number of fringes between spectral reference lines (not shown). FN; FL: Normal and lateral forces can be directly measured. (Image: Perkin Group, University of Oxford)
Whilst a limited number of studies have reported surface energies for graphene already, all previous studies relied only on estimates based on indirect measurements, for instance, by pushing down a graphene beam onto an underlying graphite substrate, studying the intercalation of noble gas atoms between the graphene and a graphite substrate, contact angle, or gas chromatography measurements.
The drawback of these approaches is that the measurements may be influenced by the adsorption of airborne contaminants and, in case of contact angle measurements, both the method of measurement as such and the underlying substrate.
"Our work is of fundamental interest to a broad community and will aid the advancement of fundamental measurements of 2D and other nanomaterials," Nicole Grobert, a professor in the Department of Materials at the University of Oxford, tells Nanowerk. She led the work together with Susan Perkin, an Associate Professor of Physical Chemistry at Oxford University.
"The fact that we were able to measure two pristine graphenes also demonstrates that the setup of the g-SFB is robust and that the graphene surfaces are contamination free," explains Christian van Engers, a PhD Student in his final year working on 2D nanomaterials synthesis and the development of g-SFB, and first author of the paper. "Such measurements will pave the way to further the development of tailored nanomaterials device design for industrial applications."
Previous measurements generally used graphite rather than graphene, or relied on the use of complicated models. A large fraction of previous work by Grobert's group, that lead to the successful creation of the graphene lenses for surface forces measurements, was devoted to the development of the production of high-quality large area graphene and the transfer of these large area atomically flat graphenes.
Both the controlled production and transfer of high-quality large-area graphene are still major bottlenecks in the application of graphene in general. Equally important, in this work, was the careful design and accurate description of the contact mechanics for reliable interpretation of the measured data.
"Having achieved all of the above, we are establishing the g-SFB as a robust research tool," says Grobert. "For example, a large portion of energy storage technology relies on the use of porous carbon as anode material. The graphene surfaces used in the g-SFB can serve as a model of a two-dimensional (2D) carbon pore allowing detailed studies of pore/electrolyte interactions and various other processes that occur within these technologies and have yet to be understood. Ultimately, the g-SFB could lead to tailored design of highly efficient storage devices."
The SFB-technique relies on the use of atomically smooth surfaces and the use of white light interferometry. Traditionally, this has been done using muscovite mica – an insulator (Physical Chemistry Chemical Physics, "Direct measurements of ionic liquid layering at a single mica–liquid interface and in nano-films between two mica–liquid interfaces").
As the researchers point out, the strengths of the SFB lie in the ability to measure very small displacements and forces between surfaces, across liquids or air. For example, the short-range layering of liquid molecules near surfaces, but also forces extending over a much longer range, such as those resulting from electrostatic repulsion between electric double layers on the surfaces have been measured recently (Chemical Communications, "Long range electrostatic forces in ionic liquids").
However, it has so far not been possible to perform studies in with control of the potential of both surfaces. The g-SFB is unique in that allows the detailed study of structure, dynamics, and physical properties of electrolytes at electrified interfaces.
"We wanted to develop a SFB in which we could apply a potential to both surfaces," Perkin notes. "In 2014, we showed that graphene could be used to meet all criteria to replace mica and described the g-SFB for the first time." (We reported about this in a previous Nanowerk Spotlight: "Ultraflat transfer method for graphene surface force balance").
"The creation of the g-SFB was only possible through the long-term collaboration – and the access to blue-sky research funds – between internationally leading experts in theory and experiment of surface force balances and in nanomaterials synthesis, processing, and application," she adds.
Direct measurement of the surface energy of graphene
(a) Experimental interferogram. The closely spaced curved lines correspond to the primary fringes. The thin dark vertical line is produced by a speck of dust in the light path. The two large parabolic bands correspond to the secondary fringes used to determine RS⊥. The horizontal band at 550-560 nm is the secondary fringe corresponding to R. (b) Schematic showing the geometry used for the calculated interferogram. (c) Interferogram calculated using the value for RS⊥ measured from the experimental interferogram shown in (a). (Reprinted with permission by American Chemical Society) (click on image to enlarge)
The team hopes that their work will improve understanding of chemical and physical processes near graphitic interfaces. This has started with their direct measurement of the surface energy of graphene in dry nitrogen and liquids, which impacts the scientists' fundamental understanding and may impact the design of devices incorporating graphene.
Now, they are applying the g-SFB to study processes in energy storage devices, such as double layer capacitors.
"We expect to be able to perform a lot of exciting studies, starting with the study of the potential dependent behavior of ionic liquids and other electrolytes in confinement," Perkin explains the team's next steps. "For example, we know that when an electric field is applied between electrodes, ions migrate to the electrode surface to screen the electric field. However, our knowledge of the exact structure of the ions near the surface, the dynamics of this process and how this affects the physical properties of the layer is limited. The g-SFB would be uniquely suited to study these phenomena."
One challenge is the stability of the graphene surface. When the surfaces are brought into/moved out of contact, this process is often not reversible, indicating the graphene gets damaged. The researchers believe this is due to strong adhesion between the graphenes in combination with defects in the lattice.
"We are now trying to improve this, for example by using graphene with mm-sized grains that have also been developed within our team (see our previous Nanowerk Spotlight on this: "Innovative substrate engineering for high quality 2D nanomaterials"), to avoid the influence of any grain boundaries in our area of contact," concludes Grobert. "With the wide range of 2D materials that are emerging, other 2D systems are potential candidates to serve as model systems."
By Michael is author of two books by the Royal Society of Chemistry: Nano-Society: Pushing the Boundaries of Technology and Nanotechnology: The Future is Tiny. Copyright © Nanowerk
 

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