Abstract:
A method of creating graphene comprising the steps of dispersing graphene oxide into water to form a dispersion. Where the method further comprises adding a solvent to the dispersion to form a solution, and controlling a temperature of the solution to form graphene.

Description:
TECHNICAL FIELD 
       [0001]    This invention relates generally to a method of reducing graphene oxide to graphene and, more particularly, to a method of reducing graphene oxide to graphene in high boiling point solvents. 
       BACKGROUND 
       [0002]    Graphene is an emerging material with potential use in electronics and material sciences. Researchers are always trying to find easier and more efficient ways to create graphene. One method of creating graphene involves reducing graphene oxide to graphene. Graphene oxide is a layered material derived from the oxidation of graphite and is dispersible in water as independent sheets. These sheets may be reduced to graphene by deoxygenating the graphene oxide. As the reduction occurs, it is typical for the graphene sheets to agglomerate with each other, thus crashing as a solid precipitate. A goal is to maintain the uniform dispersibility of these sheets. In other words, agglomeration or clumping in the final product should be minimized as much as possible. 
         [0003]    One chemical method of deoxygenating graphene oxide uses hydrazine. Exposing a graphene oxide solution to hydrazine typically causes graphene platelets to precipitate out of the solution. Recent literature reports indicate that careful control of pH and hydrazine concentration can alleviate the tendency of the graphene sheets to agglomerate upon reduction. The requirements of carefully adjusted pH and the removal of hydrazine using dialysis, render this method commercially unattractive. Hydrazine is also a hazardous material that is explosive and highly toxic. The use of hydrazine in this process means that special handling is needed. Also, when using hydrazine, platelets of the graphene tend to agglomerate or clump over prolonged periods. A method of producing graphene without dangerous chemicals would be desirable. A dispersion of individual sheets of graphene that does not contain clumps is more desirable and allows for the use of the resultant product in applications, such as transparent conductors, filler materials for composites or polymeric films. 
         [0004]    Thermal deoxygenating is another method of deoxygenating sheets of graphene oxide. This method, however, requires exposing the graphene oxide to temperatures in excess of 1000° C. This process is also slow and requires the use of an inert gas atmosphere, such as Argon. The requirement for an inert gas atmosphere and exceedingly high temperatures makes the creation of graphene more cumbersome. The product of thermal decomposition tends to occur as crumpled sheets instead of flat sheets which also limit the commercial usefulness of the sheets. 
       SUMMARY 
       [0005]    One embodiment of a method and composition is a method of creating graphene comprising the steps of dispersing graphene oxide into water to form a dispersion. The method further comprises adding a solvent to the dispersion to form a solution, and controlling a temperature of the solution to form dispersible graphene. 
         [0006]    Another embodiment of the method and composition is a composition comprising reduced graphitic carbon and a solvent where the solvent is at least one of n-methylpyrrolidone, ethylene glycol, glycerin and dimethylpyrrolidone. 
     
    
     
       DESCRIPTION OF THE DRAWINGS 
         [0007]    The features of the embodiments of the present method and composition are set forth with particularity in the appended claims. These embodiments may best be understood by reference to the following description taken in conjunction with the accompanying drawings, in the several figures of which like reference numerals identify like elements, and in which: 
           [0008]      FIG. 1  is a flow chart depicting steps comprising a first embodiment of a method of reducing graphene oxide to graphene. 
           [0009]      FIG. 2  is a flow chart depicting steps comprising a second embodiment of the method of reducing graphene oxide to graphene. 
       
    
    
     DETAILED DESCRIPTION 
       [0010]    Embodiments of the present method and composition are a description of reducing graphene oxide to graphene in high boiling point solvents. 
         [0011]    As one of ordinary skill in the art will readily appreciate, graphene oxide decomposes to graphene when heated to temperatures around 200° C. When graphene oxide decomposes to graphene, however, it is desirable to keep the graphene as a dispersion so that it can be more easily used in commercial products. One way to reduce graphene oxide to graphene is to deoxygenate the graphene oxide. 
         [0012]    Graphene oxide typically appears as water dispersible sheets. The graphene oxide may be reduced to graphene by deoxygenating the graphene oxide sheets to obtain sheets of graphene. When reducing the graphene oxide to graphene, graphene platelets tend to clump up or agglomerate. As mentioned, it is desirable to keep the graphene oxide as a dispersion as the graphene oxide is reduced to graphene. 
         [0013]    A method that may lead to the production of dispersible sheets of graphene involves dispersing graphene oxide in water to achieve a dispersion of single graphene oxide sheets and then adding a high boiling point solvent to the dispersion to form a solution. The high boiling point solvent may be a solvent with a boiling point of approximately 200° C. or higher. Because the solvent has a high boiling point, the solution may be heated to approximately 200° C. without boiling off the solvent while deoxygenating the graphene oxide and ultimately to arriving at dispersible graphene. A more detailed description of this method follows. 
         [0014]    Turning to  FIG. 1 , which is a flow chart that depicts a first embodiment  100  of a method of reducing graphene oxide to graphene. In step  110 , a dispersion is created. The dispersion may be comprised of graphene oxide dispersed into water by sonication. Sonication as described herein may comprise inducing cavitation through the use of ultrasound for the purpose of achieving a dispersion. The graphene oxide may be in the form of water dispersible sheets. Dispersing the graphene oxide by sonication may result in a dispersion comprised of single platelets of graphene oxide. The single platelets of graphene oxide may form a more stable dispersion. A stable dispersion of graphene oxide may be amenable to forming a dispersion of graphene. A ratio of water to graphene oxide in the dispersion may be approximately one milligram of graphene oxide to approximately one milliliter of water 
         [0015]    A solvent may be added to the dispersion  120  to form a solution. The solvent may be a water miscible solvent, such as, for example n-methylpyrrolidone, ethylene glycol, glycerin, dimethylpyrrolidone, acetone, tetrahydrofuran, acetonitrile, dimethylformamide, an amine or an alcohol. The amount of solvent added to the dispersion may be approximately equivalent to the amount of the dispersion. Thus if the dispersion is comprised of one milliliter of water and one milligram of graphene oxide, a volume or amount of solvent that is approximately equivalent to one milliliter of water and one milligram of graphene oxide may be added to the dispersion. At this point the solution may be comprised of a mixture with a value that is approximately half graphene oxide/water dispersion and half high boiling point solvent. 
         [0016]    The solution may be gradually heated to approximately 200° C.  130 . In some embodiments, the solution may be heated in an autoclave or high pressure chamber. As one of ordinary skill in the art will readily appreciate, heating the solution in a pressurized environment may raise the boiling point of the solution, including the solvent. Thus, the boiling point of the solution may reach or exceed 200° C. If the solution is heated in a pressurized environment, a solvent with a boiling point that is slightly below 200° C. may be used. 
         [0017]    As the solution is heated the solution may be stirred. Water may be removed via evaporation from the solution as the solution is heated. As water is removed, the temperature of the solution is expected to rise. As the temperature rises the graphene oxide deoxygenates. When the temperature of the solution reaches approximately 200° C. a reduction may be formed. As the solution is heated, the surface of the graphene oxide may be functionalized, which may result in less clumping of the platelets in the final product. In an embodiment, the temperature may be held at approximately 200° C. for a period of time  140  to aid in functionalization of the reduction. In some embodiments the temperature may be held for as little as one hour. In other embodiments the temperature may be held as long as twenty-four hours. In still other embodiments the solution temperature may be held only a moment once the temperature reaches approximately 200° C. to form a reduction. 
         [0018]    The reduction may be removed from the heat to allow cooling. Because the reduction may still comprise solvent, the reduction may be purified to remove as much of the remaining solvent as possible  150 . Purifying the reduction may comprise filtrating the reduction. The reduction may also be re-disbursed in acetone and may be centrifuged as part of the purification process. The end result of the purification process may be a solid. The solid may be graphene comprising trace amounts of the solvent. 
         [0019]    Turning to  FIG. 2 , which is a flow-chart that depicts a second embodiment  200  of the method of reducing graphene oxide to graphene. In step  210  of the method a dispersion is created. The dispersion may be comprised of water dispersible sheets of graphene oxide dispersed into water by sonication. The ratio of water to graphene oxide may be approximately two milligrams of graphene oxide to approximately one milligram of water. 
         [0020]    A solvent may be added to the dispersion  220  to form a solution. The solvent may be a water miscible solvent, such as, for example n-methlypyrrolidone, ethylene glycol, glycerin, dimethlypyrrolidone, acetone, tetrahydrofuran, acetonitrile, dimethylformamide, an amine or an alcohol. The amount of solvent added to the dispersion may be approximately equivalent to one half the amount of the dispersion. The if the dispersion is comprised of approximately two milligrams of graphene oxide and approximately one milligram of water, the amount of solvent added to the dispersion may be approximately one half the volume or amount of approximately two milligrams of graphene and approximately one milligram of water. 
         [0021]    The solution may be gradually heated  230 . In some embodiments, the solution may be heated in an autoclave or high pressure chamber. As one of ordinary skill in the art will readily appreciate, heating the solution in a pressurized environment may raise the boiling point of the solution, including the solvent. Thus, the boiling point of the solution may reach or exceed 200° C. If the solution is heated in a pressurized environment, a solvent with a boiling point that is slightly below 200° C. may be used. 
         [0022]    As the solution is heated the solution may be stirred. As the solution is heated and stirred water may evaporate from the solution. As water evaporates from the solution, an amount of solvent approximately equivalent to an amount of evaporated water may be added to the dispersion. The steps of gradually heating the solution, stirring the solution and adding solvent to replace evaporated water may continue until the temperature of the solution reaches approximately 200° C. When the temperature reaches approximately 200° C. a reduction may be formed. As the solution is heated, the surface of the graphene oxide may be functionalized, which may result in less clumping of the platelets in the final product. In an embodiment, the temperature may be held at 200° C. for a period of time  240  to aid in functionalization of the reduction. In some embodiments the temperature may be held for as little as one hour. In other embodiments the temperature may be held as long as twenty-four hours. In still other embodiments the temperature may be held only a moment once the temperature of the solution reaches approximately 200° C. to form a reduction. 
         [0023]    The reduction may be removed from the heat to allow cooling. The cooled reduction may be purified  260 . Purifying the reduction may comprise filtrating the reduction in an effort to remove solvent remaining in the reduction. The reduction may be re-disbursed in acetone and may be centrifuged to recover a solid. The solid may be graphene comprising trace amounts of the solvent. 
         [0024]    The present method and composition are not limited to the particular details of the depicted embodiments and other modifications and applications are contemplated. Certain other changes may be made in the above-described embodiments without departing from the true spirit and scope of the present method and composition herein involved. It is intended, therefore, that the subject matter in the above depiction shall be interpreted as illustrative and not in a limiting sense.