Method for obtaining graphene oxide

Abstract

A method for obtaining graphene oxide is provided comprising the steps of a) adding an acid and a salt to graphite for obtaining a graphite oxide, and b) exfoliating the graphite oxide by mixing it, wherein the steps a) and b) are carried out simultaneously in a high shear mixer.

Claims

1. A method for obtaining graphene oxide comprising: adding an acid and a salt to graphite for obtaining a graphite oxide; and exfoliating the graphite oxide by mixing it; wherein the adding and exfoliating are carried out simultaneously in a high shear mixer.

2. The method of claim 1, wherein the acid is sulphuric acid and the salt is potassium permanganate.

3. The method of claim 1, wherein no other components are added in the adding or exfoliating.

4. The method of claim 1, further comprising: carrying out a second mixing in the high shear mixer once a purification has been completed.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) To complete the description and in order to provide for a better understanding of the invention, a set of drawings is provided. Said drawings form an integral part of the description and illustrate an embodiment of the invention, which should not be interpreted as restricting the scope of the invention, but just as an example of how the invention can be carried out. The drawings comprise the following figures:

(2) FIG. 1 is a XPS spectrum of graphene oxide from batch 2.

(3) FIG. 2 shows the results after ultrasonication treatment for 15 minutes of the purified graphite oxide.

(4) FIG. 3 shows the monolayer flakes produced using a process according to the disclosure and 15 minutes of high shear mixing treatment on the graphene oxide material of the present invention.

(5) FIGS. 4A and 4B show the graphene oxide flakes obtained using the process of the present invention. Monolayer content is higher than 98%.

(6) FIG. 5 shows the graphene oxide flakes obtained without using the high shear mixing process during the oxidation reaction or exfoliation. Monolayer content is lower than 50%.

(7) FIGS. 6 and 7 show the AFM thickness profile for a monolayer graphene oxide flake produced using the process covered in this patent.

(8) FIGS. 8 and 9 are SEM images of the graphene oxide paper before and after the 1000° C. treatment respectively.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

(9) The present invention relates to a method for obtaining graphene oxide comprising the steps of:

(10) a) adding an acid and a salt to graphite for obtaining a graphite oxide;

(11) b) exfoliating the graphite oxide by mixing it;

(12) characterised in that the steps a) and b) are carried out simultaneously in a high shear mixer.

(13) For implementing the present invention, the inventors used a Dispermat CN30F2 high shear mixer, provided with a mixer disc.

(14) Before the present invention, the most common scenario was not to use any mixing at all during this type of reactions and occasionally magnetic stirrers or mechanical overhead stirrers were used. Magnetic stirrers and mechanical overhead stirrers both have a limited mixing power, specially the magnetic stirrers while the maximum velocity of mechanical overhead stirrers is usually around 1500 rpm. In contrast, for the case of the high shear mixer the velocity can be increased up to 5000-10000 rpm. This makes the mixture much more homogeneous, thus increasing the yield. On the other hand, due to the shear force the material gets exfoliated at the same time that the oxidation reaction is taking place.

(15) When comparing both methods it can be concluded that the high shear mixer is clearly more effective from the point of view of time and yield, even for similar rotation speeds.

(16) TABLE-US-00001 TABLE 1 Optimised conditions for mixing the reactants. Parameters Conditions Tool rpm/energy t (min) Product yield (%) 1 High shear mixer 1000 80 92 2 High shear mixer 3000 60 89 3 High shear mixer 5000 40 94 4 High shear mixer 8000 30 90 5 Mechanical stirrer 1000 180 70 6 Mechanical stirrer 1800 100 75

(17) Process Reproducibility

(18) From an industrial perspective, a production process not only needs to be cost effective but the reproducibility is also an extremely crucial factor. In this context the inventors have assessed the reproducibility of the inventive manufacturing process by analyzing the produced graphene oxide material from different batches. The elemental analysis was used to determine the oxidation level of the produced graphene oxide flakes. As it can be observed, the oxidation level is almost identical for all the manufactured batches.

(19) In the following table the percentage of oxidation measured by elemental analysis is described. The achieved functionalisation or oxidation level is between 43 to 45% thus showing a very good reproducibility.

(20) TABLE-US-00002 TABLE 2 Oxygen content determined using elemental analysis. Batch number % O 1 44.83 2 44.95 3 45.02 4 43.05 5 43.84 6 44.95 7 45.01 8 43.00

(21) In order to further demonstrate the incorporation of the oxygen functionality and prove that graphene oxide has been produced the XPS (X-Ray photoelectron spectroscopy) spectra were recorded, as shown in FIG. 1. From the XPS it can be concluded that the starting graphite material has been oxidised and that oxygen functionalities have been covalently bonded onto the sp.sup.2 honeycomb structure of graphene.

(22) Comparison of Ultrasonic Treatment and High Shear Mixing

(23) The most established procedure in the prior art to produce monolayer graphene oxide flakes is the ultrasonic treatment of the purified graphite oxide material. The ultrasonic treatment exfoliates the graphite oxide into individual monolayer graphene oxide flakes. However using high concentration dispersions (2-4 g/L) very long exfoliation times are needed in order to get a high monolayer graphene oxide content. If we compare the ultrasonic treatment with the one undertaken in this invention, it can be clearly concluded that the proposed method is much faster, therefore more cost effective and produces a much higher yield of monolayer flakes.

(24) FIG. 2 shows a SEM (scanning electron microscope) image of the graphene oxide flakes obtained using a modified Hummers's method (sulphuric acid, sodium nitrate and potassium permanganate) followed by purification of the produced graphite oxide and subsequent ultrasonic treatment. On the other hand FIG. 3 shows a SEM image of the graphene oxide flakes obtained using the current invention. Sulphuric acid and potassium permanganate, without sodium nitrate) were added onto graphite under high shear mixing followed by purification of the graphene oxide material and a second high shear mixing in order to obtain a higher yield of monolayer flakes. 15 minutes of exfoliation time are sufficient to obtain much better results in terms of monolayer content and flake size distribution.

(25) On the contrary, with 15 minutes of ultra sonication treatment there are still huge agglomerates present and the percentage of monolayer graphene oxide flakes is low, as shown in FIG. 2. An ultrasonic treatment of up to 5 h is required in order to obtain comparable results to our process. In addition the extended ultrasonic treatment tends to damage the flake size due to the high energy that is required to delaminate the flakes.

(26) Monolayer Content in Graphene Oxide Dispersions

(27) In FIGS. 4A and 4B the high monolayer content of the dispersions after the high shear mixing treatment can be seen. It is clear that the 98% of the graphene oxide flakes produced using the inventive process are monolayer. Actually, the darker areas correspond to flakes that are in contact with each other and not bilayer flakes.

(28) For comparison, an SME image (FIG. 5) of the flake size distribution where the high shear mixer was not used during the reaction or exfoliation process can be observed. The monolayer % is clearly much lower.

(29) Monolayer Graphene Oxide Flake

(30) An AFM (atomic force microscopy) is used in order to prove the thickness of the produced monolayer graphene oxide flakes. It can be observed in FIGS. 6 and 7 that the flake thickness is a few nanometers, exactly the thickness reported in the literature for graphene oxide monolayer flakes.

(31) Preparation of Graphene Oxide Films—Thermal Conductivity Measurements

(32) With the graphene oxide obtained by the method of this invention, films can be prepared by casting high concentrated dispersions into a mould and eliminating the water by evaporation. After drying, this graphene oxide papers were easily detached from the mould and small pieces were treated at high temperatures (300° C., 600° C., 1000° C.) in order to measure the thermal conductivity using the laser flash technique. As it can be observed in the table 3, the thermal conductivity increased 15 times when the paper was treated to 1000° C.

(33) This material can be used as a filler in composites for enhancing the thermal conductivity of different matrix materials. The anisotropic behaviour of the material is an added advantage for this kind of applications where the heat needs to be transported in just one of the directions.

(34) TABLE-US-00003 TABLE 3 Thermal conductivity of thermally treated graphene oxide papers. Material κ (W/mK) In-plane κ (W/mK) cross-plane GO paper no treatment 3.09 0.165 GO paper 300° C. 13.6 0.032 GO paper 600° C. 25.6 0.039 GO paper 1000° C. 62.6 0.022

(35) In summary, the present invention covers the production of monolayer graphene oxide flakes in a cost effective manner, with a high yield of monolayer flakes and excellent oxidation percentages.

(36) It was surprising to obtain monolayer graphene oxide flakes already during the oxidizing reaction step by using high shear mixing during the reaction process. On the other hand, once the purification is completed a second high shear mixing step was performed for a few minutes in order to obtain more than 99.8% of monolayer graphene oxide flakes.

(37) Furthermore, the proposed process produces a graphene oxide material in a reproducible and more cost effective manner suitable for industrial applications.

(38) In this text, the term “comprises” and its derivations (such as “comprising”, etc.) should not be understood in an excluding sense, that is, these terms should not be interpreted as excluding the possibility that what is described and defined may include further elements.

(39) The invention is obviously not limited to the specific embodiments described herein, but also encompasses any variations that may be considered by any person skilled in the art within the general scope of the invention as defined in the claims.