Water prefers graphene to gold

The authors of the following reference reported that Graphene flakes were transparent for water wetting properties of the supporting surface.

J. Rafiee, Xi Mi, Hemtej Gullapalli, Abbay Thomas, Fazel Yavari, Yunfeng Shi, Pulickl M. Ajayan and Nikhil A. Koratkar , Wetting transparency of graphene, Nature Materials, on line 2012, jan. 22.

See digest in French: Pour la Science, 01/02/2012, Le graphène : imperméable mais transparent au mouillage !, Maurice Mashaal

The following video sequence was obtained with monolayer graphen oxide flakes deposited on gold. In the conditions of the experiment, the ECOM technique clearly shows that (these) graphen flakes are actually more hydrophilic than the gold surface. The receding liquid is water.

image 1

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image 5

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The graphene flakes are G+ samples from DirectaPlus SpA (Lomazzo, Italia) which were kindly provided by Laura Giorgia Rizzi, R&D Manager.

ECOM reports that graphene oxide monolayers do not dissolve in water

Several days ago, I introduced the existence of a new optical technique named ECOM. My first concern was to demonstrate its sensitivity by testing the technique in air. It corresponds to the conditions in which graphene platelet manufacturing and deposits are characterized. However, major graphene applications rely on electrochemistry, which requires to use graphene layers in (salted) water.

Here I will demonstrate the sensitivity of the ECOM technique when observing in water graphene monolayers supported by a solid surface.

graphene in water - image 1

graphene in water – image 1

The profile displayed in image 1 shows the image intensity variation along the colored arrow. Since each step corresponds to one graphene layer, one can check that this intensity is a linear function of the layer number. It is therefore possible to identify the number of layers covering a given region by measuring this intensity, or by counting steps when starting from a known situation. Notice that the height z of one step is 0.335 nm, while the total length L of the yellow line is about 120 µm. The z/L ratio is therefore 0.3/120 000 = 2,5 10-6 . By comparison, it would be equivalent to consider variations in height of 2,5 cm along a 10 km route. What is actually amazing is the flatness of each plateau between two steps in the absence of any image treatment other than overall contrast and lighting adjustments. For the above mentioned linear relationship between thickness and intensity, it is possible to convert the same image into topographic representation, as done in image 2. The z dimension (height) is considerably enlarged compared to x and y. Otherwise, the steps would not be seen in the image.

graphene wing topography

graphene in water – image 2

The next pictures, image  3 and upper image 4, present three series of side by side images of a same region of the sample. Graphene oxide platelets were deposited on the solid from evaporation of a first water drop, then covered again with water after deposition and observed and then observed again after the second drying. The left and right images were captured during, respectively, the second and the third steps.

graphene in water - image 3

graphene in water – image 3

graphene in water - image 4

graphene in water – image 4

Graphene sheets observed in water much resemble those observed in air. One can check that graphene monolayers do not dissolve in water. One could still doubt of it, however, when looking at the lower picture in image 4, where a graphene platelet seems to disappear. This is obviously not due to graphene dissolving, but to graphene taking off from the surface, probably related to some non identified invasive molecule creeping in between the two materials. When enlarging this zone, one may distinguish some kind of short noodles in the upper part. I remember to have seen such noodles in some experiment with highly confined water, where the authors were explaining that water was breaking down in filaments. I have no idea of what are the noodles. I just notice that graphene is no more visible when such structures appear.

The main point is that graphene sheets can be viewed in water, which corresponds to the conditions where they could be used as electrodes.



I have seldom encountered totally defect-free graphene sheets, new born ECOM technology says

Electrochemical reactions mainly occur at the interface between a solid electrode and a liquid electrolyte, because they require a source (or sink) of electrons (the solid) and a charge carrier (the electrolyte). A new instrument emerged two months ago which allows highly sensitive optical probing of such an electrode while it is working. By highly sensitive we mean a z-resolution which is routinely better than 0.1 nm. We name the instrument ECOM, meaning ElectroChemical Optical Microscope. Although amazingly simple, it results from long developments to which the following people contributed at some step: Refahi Abou Khachfe, Ludovic Roussille, Guillaume Brotons, Myriam Zerrad, Fabien Lemarchand, Claude Amra and Sylvie Ricard-Blum.

In order to demonstrate its outstanding sensitivity, we applied this instrument to imaging graphene monolayers, the thinnest self-sustaining solid materials ever realized. Their thickness is only 0.335 nm. These materials make everywhere the subject of huge research programs because they open promising possibilities for batteries, fuel cells, solar cells, light emitters and molecular microelectronic components. On this way, researchers need to produce, characterize, manipulate, and modify graphene platelets, and finally to understand how they interact with their environment. ECOM will help them at all steps.

The current technique employed for characterizing graphene platelets is Scanning Electron Microscopy (SEM). The expected information is about the lateral size and relative arrangement of the platelets after deposition on a solid surface. Searchers need to identify the number of monolayers in a given platelet related to their manufacturing process or in a given surface region related to platelet superimposition. It is also important to qualify folds and folding of the molecular sheets. In these practices, supported platelets are observed in air.

ECOM can deliver all this information. It is demonstrated here by giving image examples of supported graphene oxide single sheets deposited on a solid from a dilute water suspension, and observed in air. The graphene platelets are of outstanding quality. They were produced by S. Campidelli in the CEA/IRAMIS/Licsen Lab. headed by V. Derycke. Images 1 and 2 show folds in isolated graphene layers; image 3 shows residual defects is a giant and regular graphene plate; images 4 to 7 illustrate the diversity of graphene sheet conformations that can be encountered in a single deposition process. A special attention may be given to image 5 because its format is the one suited for direct comparison with SEM images.

The next step will be to demonstrate that ECOM imaging of graphene platelets may be achieved in water as well.

Graphene - Image 1

Graphene – Image 1

Graphene - Image 2

Graphene – Image 2

Graphene - Image 3

Graphene – Image 3

Graphene - Image 4

Graphene – Image 4

Graphene - Image 5

Graphene – Image 5

Graphene - Image 6

Graphene – Image 6

Graphene - Image 7

Graphene – Image 7