Can we Visualize Graphene Oxide from Suspension?


Graphengitter

Single graphene layers – The phenomenon of restacking. Equipment and chemicals: Snap-top jar (50 ml); squirt bottle; beaker (100ml); suction bottle; filter paper; Nutsche; beaker (25 ml); magnetic stirrer; sodium dithionite; sodium hydroxide; graphene oxide.

Execution

Graphene oxide is prepared as described in Experiment 1 and dissolved from the filter paper with a light stream of water from a spray bottle or the tap and transferred to a beaker. The aqueous, salt-free graphene oxide suspension is sucked off again over a clean suction flask. Approximately 25 ml of the collected clear liquid is then transferred to a snap-top tube. The snap-lid glass is placed on a heatable magnetic stirrer and the contents are heated to approx. 80°C. The liquid is then transferred to a beaker.

In a beaker, 0.45 g of sodium dithionite is mixed with 1.8 g of sodium hydroxide, dissolved in 10 ml of water and added to the graphene oxide suspension in the snap lid glass. The contents of the snap lid tube are observed for about 45 minutes.

Observation

The addition of the sodium dithionite solution slightly clouds the clear liquid. During the first 10 minutes, tiny black particles appear, immersing the solution in a “gray haze”. As the experiment progresses, the particles aggregate more and more into larger black flakes and finally sink to the bottom of the vessel. The solution appears very clear again after 45 minutes.

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Figure 12: Reduction of a clear GO solution by sodium dithionite. Graphene flakes appear out of “nothing”. Figure 1: clear GO solution; Figure 2: GO solution mixed with sodium dithionite; Figure 3 – Figure 8: Observations after 5 minutes, 10 minutes, 15 minutes, 25 minutes, 35 minutes, 45 minutes.

Evaluation

The multilayer graphene oxide particles visible to the naked eye are slipped onto the filter paper, while smaller mono-, di-, tri-layer graphene oxide molecules or aggregates are sucked through the pores of the filter paper and remain dissolved in the solution. By the addition of 15.

Sodium dithionite, the graphene oxide layers are reduced to graphene as described in experiment 2. While the graphene oxide, due to its oxygen-containing functional groups, has a polar character, the graphene molecule behaves like a nonpolar substance. If individual graphene molecules suspended in the solution now meet, there is a possibility that they will form Van der Waals interactions and aggregate, as shown in Figure 10. Depending on time, the microscopic graphene molecules aggregate to form an ever larger macroscopically visible complex or particle [16-19].

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Figure 13: Individual graphene layers aggregate in a polar medium to form a large complex.

 

This process, which is called “restacking”, is responsible for the fact that there are no graphene-based storage systems on the commercial market yet. In graphene batteries, after only a few charge and discharge cycles, the graphene layers aggregate together to form a graphite-like complex.

Many unique properties of graphene, such as its exceptionally high active surface area and good conductivity, are significantly impaired or lost as charge and discharge cycles progress [18].

The smallest graphene oxide particles in experiment 3a are not visible to the naked eye, which is why it appears that after reduction by dithionite the graphene flakes appear out of “nothing”.

Another evidence for the presence of graphene oxide particles could be the Tyndall effect. The Tyndall effect describes the phenomenon of a light beam in a supposedly clear solution, which becomes visible through the reflection of the light beam from the smallest colloidal particles in the solution [14].