CONTINUOUS PRODUCTION OF PRISTINE GRAPHENE AND GRAPHENE OXIDE

Abstract

Provided is a process for manufacturing a graphene material, the process comprising (a) injecting a rust stock into a first end of a continuous reactor having a toroidal vortex flow, wherein the first stock comprises graphite and a non-oxidizing liquid (or, alternatively, graphite, an acid, and an optional oxidizer) and the continuous flow reactor is configured to produce the toroidal vortex flow, enabling the formation of a reaction product suspension or slurry at the second end, downstream from the first end, of the continuous reactor; and (b) introducing the reaction product suspension/slurry from the second end back to enter the continuous reactor at or near the first end, allowing the reaction product suspension/slurry to form a toroidal vortex flow and move down to or near the second end to produce a graphene suspension or graphene oxide slurry. The process may further comprise repeating step (b) for at least one time.

Claims

1. A process for manufacturing a graphene material, the process comprising: a) injecting a first stock of reactants into a first inlet near a first end of a continuous or semi-continuous reactor, the first stock comprising a graphitic material and an acid or an intercalating agent; b) allowing said reactants to react with one another while being driven to flow from said first end to a second end, downstream from said first inlet, of said reactor to form a first reaction product slurry, wherein said graphitic material is intercalated with bisulfate downstream of the first inlet to form an intercalated graphite; c) driving said first reactant product slurry to flow through a conduit, disposed in working relation to said reactor, back to said first end and re-enter said reactor through a second inlet near said first end, allowing said first product slurry to flow from said first end to said second end of said reactor to form a second reaction product slurry; and d) collecting the second reaction product slurry or the finishing product slurry from an outlet of the reactor, disposed downstream from said first inlet, to produce said graphene material.

2. The process of claim 1, wherein the continuous reactor has a flow from the first inlet to the outlet, the flow being a vortex flow.

3. The process of claim 2, wherein the vortex flow is a toroidal vortex flow.

4. The process of claim 1, wherein the continuous reactor is configured to produce a toroidal vortex flow.

5. The process of claim 4, wherein the toroidal vortex flow comprises a plurality of non-axisymmetric toroidal vortices.

6. The process of claim 1, wherein the continuous reactor is a continuous Couette-Taylor reactor.

7. The process of claim 1, wherein the continuous reactor comprises a reactor chamber into which the first stocks is injected; the reactor chamber being configured between an outer wall of a first cylindrical body and the inner wall of a second cylindrical body or bore, one or both of the first and second cylindrical bodies rotating around the axis thereof.

8. The process of claim 7, wherein the first and second cylindrical bodies are rotating in opposite directions.

9. The process of claim 1, wherein the acid is selected from sulfuric acid, nitric acid, phosphoric acid, or a combination thereof.

10. The process of claim 1, further comprising a step of introducing a quenching agent into the second reaction product slurry after step (c) or into the finishing product slurry after step (d) to terminate a chemical reaction.

11. (canceled)

12. The process of claim 1, wherein the intercalated graphite comprises a stage-1 graphite intercalation compound (GIC).

13. The process of claim 1, wherein the graphitic material is converted to graphite oxide near or at the second end of the reactor.

14. The process of claim 1, wherein the intercalated graphite is converted to graphite oxide near or at the second end of the reactor.

15. The process of claim 13, wherein covalent sulfates of the graphite oxide are hydrolyzed and interlayer registry of the graphite oxide is lost near or at the second end of the reactor.

16. The process of claim 1, wherein the graphene material comprises single-layer graphene oxide sheets.

17. The process of claim 1, wherein the graphene material comprises few-layer graphene oxide sheets having 2-10 graphene planes.

18. The process of claim 1, wherein the graphene oxide contains an oxygen content from 20 to 50% by weight of the total graphene oxide weight.

19. The process of claim 1, wherein the first stock comprises the graphitic material in a concentration of about 0.1 wt. % to about 50 wt. %.

20. The process of claim 1, wherein the graphitic material is selected from natural graphite, artificial graphite, highly oriented pyrolytic graphite (HOPG), meso-carbon micro-bead (MCMB), carbon fiber, graphite fiber, carbon nano-fiber, graphite nano-fiber, multi-walled carbon nano-tube, soft carbon, hard carbon, activated carbon, meso-phase pitch, needle coke, coal, graphite oxide, graphite halogenide, hydrogenated graphite, nitrogenated graphite, or a combination thereof.

21. A process for manufacturing graphene oxide, the process comprising: (A) injecting a first stock into a continuous reactor through a first inlet at a first end of said continuous reactor having a toroidal vortex flow, wherein the first stock comprises a graphitic material and an acid and wherein the continuous flow reactor is configured to produce said toroidal vortex flow, enabling the formation of a reaction product slurry at the second end, downstream from the first end, of said continuous reactor, wherein said graphitic material is intercalated with bisulfate downstream of the first inlet to form an intercalated graphite; and (B) introducing said reaction product slurry from said second end back to enter said continuous reactor through an inlet at or near said first end, allowing said reaction product slurry to form a toroidal vortex flow and move down to or near said second end to produce a graphene oxide slurry.

22. The process of claim 20, wherein said acid is selected from sulfuric acid, nitric acid, phosphoric acid, or a combination thereof.

23. The process of claim 20, further comprising repeating said step (B) for at least one time.

24. The process of claim 20, further comprising a step of collecting said graphene oxide slurry from an outlet of the continuous flow reactor.

25. A process for manufacturing pristine graphene, the process comprising: A) injecting a first stock into a continuous reactor through a first inlet at a first end of said continuous reactor having a toroidal vortex flow, wherein the first stock comprises a graphitic material and a non-oxidizing liquid medium, and the continuous flow reactor is configured to produce said toroidal vortex flow, enabling the formation of a reaction product suspension at a second end, downstream from the first end, of said continuous reactor, wherein said non-oxidizing liquid medium comprises water, alcohol, an organic solvent, or a combination thereof, wherein said organic solvent is selected from Dimethylformamyde (DMF), Benzyl benzoate, -Butyrolactone (GBL), or a combination thereof; and B) introducing said reaction product suspension from said second end back to enter said continuous reactor at or near said first end, allowing said reaction product slurry to form a toroidal vortex flow and move down to or near said second end to produce a pristine graphene suspension.

26. The process of claim 25, further comprising repeating said step (B) for at least one time.

27. The process of claim 25, further comprising a step of collecting said pristine graphene suspension from an outlet of the continuous flow reactor.

28. (canceled)

29. (canceled)

30. The process of claim 25, wherein said non-oxidizing liquid medium further comprises a surfactant.

31. The process of claim 1, wherein the first stock also includes an oxidizing agent.

32. The process of claim 1, further including repeating said step (c) for at least one time by driving said second reaction product slurry to flow through said conduit back to said first end and re-enter said reactor through said second inlet, allowing said second product slurry to flow from said first end to said second end of said reactor to form a finishing product slurry.

33. The process of claim 21, wherein the first stock also includes an oxidizer.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0078] FIG. 1(A) A flow chart showing the most commonly used prior art process of producing highly oxidized graphite and graphene oxide sheets that entails tedious chemical oxidation/intercalation, rinsing, and high-temperature exfoliation procedures.

[0079] FIG. 1(B) A conventional chemical reactor used for producing highly oxidized graphite and graphene oxide.

[0080] FIG. 2(A) A diagram to schematically illustrate the working principle of a Couette-Taylor reactor, without a flow return conduit.

[0081] FIG. 2(B) Schematic of a new continuous reactor having a flow return conduit according to some embodiments of the instant disclosure.

[0082] FIG. 3(A) X-ray diffraction data of original graphite and the graphene oxide powder produced by the new Couette-Taylor reactor, indicating fully separated graphene oxide sheets.

[0083] FIG. 3(B) Raman spectra of original graphite and the graphene oxide powder produced by the new Couette-Taylor reactor.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

[0084] Carbon materials can assume an essentially amorphous structure (glassy carbon), a highly organized crystal (graphite), or a wide range of intermediate structures that are characterized in that various proportions and sizes of graphite crystallites and defects are dispersed in an amorphous carbon matrix. Typically, a graphite crystallite is composed of multiple graphene planes (planes of hexagonal structured carbon atoms or basal planes) that are bonded together through van der Waals forces in the c-axis direction, the direction perpendicular to the basal plane. These graphite crystallites are typically micron- or nanometer-sized. The graphite crystallites are dispersed in or connected by crystal defects or an amorphous phase in a graphite particle, which can be a natural graphite flake, artificial graphite bead, carbon/graphite fiber segment, carbon/graphite whisker, or carbon/graphite nano-fiber.

[0085] One embodiment of the present disclosure is a method of producing isolated/separated graphene sheets or nano graphene platelet (NGP). A NGP is essentially composed of a graphene plane (hexagonal lattice of carbon atoms) or multiple graphene planes stacked and bonded together (typically up to 10 graphene planes per multi-layer platelet). Each graphene plane, also referred to as a graphene sheet, comprises a two-dimensional hexagonal structure of carbon atoms. Each platelet has a length and a width parallel to the graphene plane and a thickness orthogonal to the graphene plane. By definition, the thickness of an NGP can be 100 nanometers (nm) or smaller (preferably containing no greater than 10 hexagonal planes), with a single-sheet graphene being as thin as 0.34 nm. Few-layer graphene refers to the graphene platelet containing 2-10 hexagonal planes of carbon atoms (2-10 graphene planes) bonded together mainly through van der Waals forces).

[0086] Currently, the most commonly used graphene production method is the so-called chemical method, referred to in the Background section as Approach 1: Chemical Formation and Reduction. This method, commonly referred to as the Hummer's method, entails chemical intercalation or oxidation of natural graphite or synthetic graphite particles. These particles are essentially already in the fully graphitized state. Prior to intercalation or oxidation, the graphite particle has an inter-graphene plane spacing as small as approximately 0.335 nm (L.sub.d= d.sub.002=0.335 nm). Due to the short-range force nature of van der Waals forces, the bonding between closely spaced graphene planes is very strong, making it difficult for any chemical species to intercalate into the inter-graphene spaces.

[0087] Hence, it normally takes a combination of a strong acid (e.g. sulfuric acid) and a strong oxidant (e.g. potassium permanganate or nitic acid) and a long reaction time (4-120 hours) to achieve full chemical intercalation or oxidation of graphite to obtain the graphite intercalation compound (GIC) or graphite oxide (GO). With an intercalation and oxidation treatment, the inter-graphene spacing is increased to a value typically greater than 0.6 nm. This is the first expansion stage experienced by the graphite material during this chemical route. The obtained GIC or GO is then subjected to further expansion (often referred to as exfoliation) using either a thermal shock exposure or a solution-based, ultrasonication-assisted graphene layer exfoliation approach.

[0088] When the graphene planes contain a sufficient amount of non-carbon atoms or functional groups (e.g. OH, COOH, O, etc.), such as 20% by weight or greater of oxygen in GO, ultrasonication may be used to separate graphene oxide sheets/molecules.

[0089] In the thermal shock exposure approach, the GIC or GO is exposed to a high temperature (typically 800-1,050 C.) for a short period of time (typically 15 to 60 seconds) to exfoliate or expand the GIC or GO for the formation of exfoliated or further expanded graphite, which is typically in the form of a graphite worm composed of graphite flakes that are still interconnected with one another. This thermal shock procedure can produce some separated graphite flakes or graphene sheets, but normally the majority of graphite flakes remain interconnected. Typically, the exfoliated graphite or graphite worm is then subjected to a flake separation treatment using air milling, mechanical shearing, or ultrasonication in water to produce graphene sheets.

[0090] As illustrated in FIG. 1(B), the industrial-scale production of GIC or GO and, subsequently, isolated/separated graphene sheets typically requires the use of a massive reactor, from 100 gallons (pilot-scaled or prototyping-scaled) to 10,000 gallons in reactant volume. There are several previously un-recognized problems associated with the use of these massive chemical reactors for chemical intercalation/oxidation of graphite/carbon materials: [0091] 1) When a stoichiometric balance amount of graphite/carbon particles and intercalant/oxidizer is introduced into the reactor, the solid content (amount of graphite or carbon particles) is too high and the reacting mixture is too viscous to mechanically stir and disperse properly. In order to overcome this stirring difficulty issue, the operator typically has to add excessively large quantities of liquid intercalants/oxidizers (e.g. sulfuric acid/nitric acid), which are not desirable chemicals and their use must be minimized where possible. [0092] 2) Even with vigorous stirring, the graphite/carbon particles cannot be homogeneously mixed and dispersed in such a huge reactor. There are always spots in the reactor where either the graphite/carbon powder is in excess (hence, resulting in incomplete reaction) or the strong acid/oxidizer is in excess (hence, simply eating away graphite/carbon, producing CO, CO.sub.2 and other volatile species, instead of just intercalating/oxidizing graphite/carbon, resulting in low production yield). [0093] 3) It is very difficult to control reaction rates uniformly throughout the entire reacting mass and, thus, reactions are allowed to proceed in a non-optimized manner. One consequence is the notion that the chemical treatment process requires a long intercalation and oxidation time. [0094] 4) Some of the reaction steps in chemical intercalation/oxidation are highly exothermic, generating large amounts of heat in a short period of time (high heat production rates). Such high amounts of heat generated in the interior of a chemical reactor, away from the reactor walls (where cooling jacket is wound around), cannot be dissipated fast enough. The internal heat build-up, in turn, generates more heat, leading to auto-acceleration of reactions that could result in material over-heating and even explosion. [0095] 5) Oxidation of graphite using the conventional Hummers' method yielded graphite oxide along with a significant amount of non- or under-oxidized graphitic particles under a certain reaction time. To increase yields of graphite oxides that can be exfoliated into single- or few-layer graphene, it often requires either pre-oxidation steps, sequential addition of excess KMnO.sub.4, or extended reaction time to several hours. [0096] 6) The use of an excessive amount of chemicals also implies the need to repeatedly wash and rinse the reaction products, generating more waste water. The process can exert negative environmental impact.

[0097] To achieve large scale production of single-layer or few-layer GO sheets, it is important to oxidize graphite with the intercalating and oxidizing agents fully penetrating between stacked graphene layers and overcome the strong interlayer van der Waals forces. Efficient oxidation or complete intercalation process would give high yields of homogeneously exfoliated and separated GO sheets in a reduced reaction time.

[0098] Disclosed herein is a new process and apparatus to produce single-layer or few-layer graphene oxide from bulk graphite with high yields in a significantly shortened reaction time. This process involves the use of a Couette-Taylor flow reactor, in which the oxidation of graphite with an acid (e.g. H.sub.2SO.sub.4) and an oxidizing agent (e.g. KMnO.sub.4 and/or NaNO.sub.3) is accelerated by the turbulent Couette-Taylor vortex flow. The disclosed process and apparatus are applicable to not just Hummer's method, but also other methods such as the well-known Brodie's and Staudinger methods.

[0099] As illustrated in FIG. 2(A) for a Couette-Taylor reactor and FIG. 2(B) for a presently disclosed new Couette-Taylor reactor, the reactor consists of two coaxial cylinders with the inner one rotating. At a critical rotating speed, toroidal vortices are created and regularly spaced along the cylinder axis. This toroidal motion of fluids can lead to highly efficient radial mixing of graphite, acid, and oxidizing agents in the system, thereby enhancing the oxidation efficiency.

[0100] As discussed in the Background section, the current design of Couette-Taylor reactor requires a long inner cylindrical rod (referred to herein as the first body), having a high aspect ratio; the rod length/diameter ratio typically greater than 20, often greater than 50. The rotation of such a long inner rod relative to the outer annular body (the second body) means the inner rod can experience an excessively large resistance force imparted by a large amount of viscous fluid being configured to create large amounts of toroidal vortex zones. This would require the implementation of a significantly larger and more powerful electric motor, which would be heavier, more difficult to handle, and much more expensive. The operation of such a long Couette-Taylor reactor normally requires the use of a larger amount of un-desirable liquid medium (e.g. sulfuric acid, sodium nitrate, and potassium permanganate) relative to the proportion of solid graphitic material in order for the motor to drive the inner rod at a sufficiently high speed to create the needed Taylor vortex flow.

[0101] As schematically illustrated in FIG. 2(B), in some embodiments, the presently disclosed continuous reactor 10 or reactor system for producing sheets of graphene or thin platelets of an inorganic 2-D compound typically comprise the following components: (a) a first body 16 comprising an outer wall 18 and a second body 12 comprising an inner wall 14, wherein the inner wall defines a bore and the first body is configured within the bore and wherein a motor (not shown) is configured to rotate, through a shaft 32, the first and/or second body; (b) a reaction chamber 15 being configured between the outer wall 18 of the first body and the inner wall 14 of the second body; (c) a first inlet 34 and a second inlet 30 disposed at or near a first end of the reactor and being configured to be in fluid communication with the reaction chamber 15; (d) a first outlet 20 and a second outlet 24 disposed downstream from the first and second inlet at or near a second end of the reactor, the first outlet and the second outlet being configured to be in fluid communication with the reaction chamber 15; and (e) a flow return conduit 28 (e.g. piping means) having a first conduit inlet in fluid communication with the second outlet 24 of the reactor and having a first conduit outlet in fluid communication with the second inlet 30 of the reactor. A control valve 26 is implemented to turn on and off the second outlet 24 of the reactor on demand. Another control valve 22 is implemented to open the first outlet 20 of the reactor (for discharging the reaction product slurry out of the reactor upon completion of the needed degree of reaction) and close the first outlet (to direct the first reaction product to flow through the second outlet 24 into the flow return conduit 28 for additional processing) on demand.

[0102] In such a design, the reactor or reactor system is operated to receive a first stock of reactants, containing a graphitic material or an inorganic layered compound dispersed in a liquid medium, through the first inlet 34 into the reaction chamber 15, driving the reactants downstream toward the first 20 and second outlet 24 and facilitating reactions between the reactants to produce a first product slurry, driving the first product slurry through the flow return conduit 28 and the second inlet 30 of the reactor to re-enter the reaction chamber 15, and further driving the first product slurry downstream toward the first 20 and second outlet 24 to form a second product slurry, which is either discharged out of the reaction chamber through the first outlet 20 of the reactor (where the control valve 22 being open) or driven to flow through the conduit 28 to return to the reaction chamber 15 through the second inlet 30 for at least another time. Such a design enables the use of a much shorter reactor since the reaction product slurry, if containing un-reacted or incompletely reacted species, may be returned to the reaction chamber for a desired number of times (e.g. repeated for 1-10 times) until a desired final product is obtained. Even for a long reactor, this return flow conduit strategy enables the completion of desired reactions (and/or complete exfoliation and separation of graphene sheets or inorganic compound platelets) in a convenient and cost-effective manner.

[0103] As compared to the Hummers method executed in a conventional reactor, the presently disclosed reactor system allows for the production of mostly single-layer or few-layer GO sheets/molecules or pristine graphene sheets at a high yield within an hour of reaction time. This method is also capable of concurrently performing continuous production of graphene sheets and the chemical functionalization of separated graphene sheets.

[0104] According to the modified Hummers method, graphite oxide is produced by the oxidation of bulk graphite in the acidic oxidizing medium. During the course of the reaction, the oxidation begins by the oxidizing agent attacking graphene layer, opening up the inter-planar spacing to allow for easier entry of an intercalating agent (e.g. sulfuric acid) and additional oxidizing agent species. The oxidizing agent species penetrating into inter-graphene spacing cause more oxidation and help to further open up the spacing, and so on. To enable the efficient oxidation of graphite, it would be important to enhance the rate of diffusion of the oxidizing agent and intercalating agent into the graphite interlayer and the subsequent oxidation reaction. For this purpose, a new Couette-Taylor flow reactor is used for the efficient mixing and mass transfer of graphite and oxidizing/intercalating agents, thereby enhancing the efficiency and rate of graphite oxidation reaction and separation of graphene oxide sheets.

[0105] The intercalating agent or oxidizing agent may be selected from an acid, a lithium salt, a sodium salt, a potassium salt, lithium perchlorate, sodium perchlorate, potassium perchlorate, potassium manganese, lithium manganese, sodium manganese, hydrogen peroxide, a metal halide, or a combination thereof. The metal halide is preferably selected from the group consisting of MCl.sub.2 (M=Zn, Ni, Cu, Mn), MCl.sub.3 (M=Al, Fe, Ga), MCl.sub.4 (M=Zr, Pt), and combinations thereof. The acid may be selected from sulfuric acid, nitric acid, carboxylic acid, phosphoric acid, sorbic acid, acetic acid, or a combination thereof.

[0106] As shown in FIG. 2(A) and FIG. 2(B), the Couette-Taylor reactor consists of two concentric cylinders and the inner cylinder rotates at a controlled speed while the outer cylinder is maintained stationary. A mixture of graphite flakes (or other types of graphitic material), the concentrated acids and oxidizing agents were fed into the reactor. As the rotation speed of the inner cylinder reaches a critical value, it develops the counter-rotating toroidal vortices in a periodic arrangement along the cylinder axis. This Couette-Taylor vortex induces highly effective radial mixing within each vortex cell and uniform fluidic motion, enabling enhanced mass transfer of the reactants. This toroidal motion also causes a high wall shear stress that improves the dispersion of graphite in an acidic oxidizing medium and enhances the rate of diffusion of the oxidizing agent into the inter-graphene spacing of a graphite structure.

[0107] In a Couette-Taylor reactor, the hydrodynamic condition of the fluids is dependent on the rotating speed of the inner cylinder relative to the outer cylinder. The Couette-Taylor vortex can be formed when the Taylor number proportional to the angular velocity of the inner cylinder exceeds a critical value. Our studies indicate that the threshold rotation speed is typically from 300 to 500 rpm for the formation of Couette-Taylor vortex that can lead to the efficient oxidation reaction of graphite. In a shearing stress reactor, including the Couette-Taylor reactor, the hydrodynamic condition of the fluids depends on the rotating speed of the inner cylinder. The shearing stress flow is formed when the Taylor number (Ta) proportional to the angular velocity of the inner cylinder exceeds a critical value, which is determined by the following relation:

[00001]$\begin{array}{cc}{T}_a={\left(\frac{d}{R_1}\right)}^{1/2}.Math.\frac{{}_1.Math.R_1.Math.d}{v}& \left(\mathrm{Equation}.Math..Math.1\right)\end{array}$

where R.sub.1 is the radius of inner cylinder, w is the angular velocity of the inner cylinder, d is the width of the annular gap (herein also referred to as the reaction chamber), and v is the kinematic viscosity. The viscosity of the reaction mass was found to be typically from 200 to 1,000 cP. The shearing stress increases with rotational speed, resulting in more efficient exfoliation of graphite or layered inorganic compounds at higher shear force. We have further observed that a stoichiometric ratio between the graphite/carbon powder and the intercalant/oxidizer can be maintained (no excess chemicals are required, nor desired) if these reactants are forced to flow through the reaction chamber defined between the inner cylinder and outer cylinder. Further surprisingly, there is no problem of pumping the reaction product slurry back to the reaction chamber through the flow return conduit.

[0108] The present disclosure provides a method of producing graphene sheets (single-layer or few layer graphene having 2-10 layers) from particles of a graphite or graphitic carbon material in a significantly shorter period of time, using lesser amounts of intercalant and oxidizer, and achieving a higher production yield (without consuming any significant amount of starting material).

[0109] The disclosed process and apparatus are also capable of exfoliating graphitic materials and layered inorganic compounds in a non-oxidizing liquid medium to produce pristine graphene sheets and 2D inorganic compound platelets. In principle, one can divide the reactants in the reaction chamber into a large number of small Taylor vortex zones. These small Taylor vortices of reactants are each a minute high-shear a reactor, capable of exfoliating substantially any type of layered materials.

[0110] Thus, the present disclosure also provides a process for manufacturing pristine graphene, the process comprising: (A) injecting a first stock into a continuous reactor through a first inlet at a first end of the continuous reactor having a toroidal vortex flow, wherein the first stock comprises a graphitic material and a non-oxidizing liquid medium, and the continuous flow reactor is configured to produce said toroidal vortex flow, enabling the formation of a reaction product suspension at the second end, downstream from the first end, of said continuous reactor; and (B) introducing said reaction product suspension from said second end back to enter said continuous reactor at or near said first end, allowing said reaction product slurry to form a toroidal vortex flow and move down to or near said second end to produce a pristine graphene suspension.

[0111] The graphitic material may be selected from natural graphite, artificial graphite, highly oriented pyrolytic graphite (HOPG), meso-carbon micro-bead (MCMB), carbon fiber, graphite fiber, carbon nano-fiber, graphite nano-fiber, multi-walled carbon nano-tube, soft carbon, hard carbon, activated carbon, meso-phase pitch, needle coke, coal, graphite oxide, graphite halogenide (e.g. graphite fluoride), hydrogenated graphite, nitrogenated graphite, or a combination thereof.

[0112] The process may further comprise repeating step (B) for at least one time (e.g. another 1-10 times). The process may further comprise a step of collecting the pristine graphene suspension from an outlet of the continuous flow reactor and then drying the suspension to obtain pristine graphene sheets.

[0113] In the process, the non-oxidizing liquid medium may comprise water, alcohol, an organic solvent, or a combination thereof. The organic solvent is preferably selected from N-Methyl Pyrrolidone (NMP), Dimethylformamyde (DMF), Benzyl benzoate, 7-Butyrolactone (GBL), or a combination thereof. The non-oxidizing liquid medium may further comprise a surfactant.

[0114] The process entails subjecting reacting mass into numerous vortex flow zones having high shear stresses and shear strains therein, enabling fast, uniform, and complete intercalation/oxidation of graphite/carbon particles. Typically and preferably, the invented method leads to the production of graphene sheets that contain at least 80% single-layer graphene or at least 80% few-layer graphene (defined as graphene sheets having 2-10 graphene planes).

[0115] The following examples serve to provide the best modes of practice for the present disclosure and should not be construed as limiting the scope of the disclosure:

Example 1: Production of GO Sheets Via Hummer's Method Using a Conventional Reactor and a Couette-Taylor Reactor

[0116] Graphite oxide as prepared by oxidation of natural graphite flakes with sulfuric acid, nitrate, and permanganate according to the method of Hummers [U.S. Pat. No. 2,798,878, Jul. 9, 1957] using both a conventional reactor (100 gallons=0.3785 m.sup.3) and the presently disclosed Couette-Taylor reactor (inner cylinder having a diameter of approximately 18 cm and a length of about 85 cm. We found that the instant reactor design enables the production of an equal amount of graphene oxide (after 2 passes) in 45 minutes using approximately 65% of the chemicals (vs. 5 hours of the conventional reactor).

[0117] Upon completion of the reaction, each mixture was separately poured into deionized water and filtered. The graphite oxide was repeatedly washed in a 5% solution of HCl to remove most of the sulfate ions. The sample was then washed repeatedly with deionized water until the pH of the filtrate was 5.0. The slurry was spray-dried and stored in a vacuum oven at 60 C. for 24 hours. The interlayer spacing of the resulting laminar graphite oxide powder was determined by the Debey-Scherrer X-ray technique to be approximately 0.73 nm (from conventional reactor), which requires an additional procedure (e.g. ultrasonication) to fully exfoliate and separate individual graphene oxide sheets. In contrast, as indicated in FIG. 3(A), the graphene oxide powder produced by the instant reactor has completely lost the registry (characteristic peak at 2=26 degrees) of the un-treated graphite. The Raman spectra, FIG. 3(B), further confirm the formation of graphene oxide sheets. Surface area was measured via nitrogen adsorption BET for both samples. The sample from the conventional bulk reactor exhibits a specific surface area of 375 m.sup.2/g (mostly few-layer graphene sheets) after thermal exfoliation and that from presently invented reactor 975 m.sup.2/g (mostly single-layer graphene oxide sheets).

[0118] In addition to natural graphite, we have tested other starting graphitic materials, including synthetic graphite, amorphous graphite (microcrystalline graphite), pieces of highly oriented pyrolytic graphite, meso-carbon micro-bead (MCMB), graphitized meso-phase carbon, needle coke, pitch-based carbon fiber, pitch-based graphite fiber, vapor-grown carbon nano-fiber, graphitic nano-fiber, graphite fluoride, chemically modified graphite, and expanded graphite. The invented process and reactor surprisingly work well for all these starting materials.

[0119] A wide variety of intercalants and oxidizers have also been tested. We have found that the method works well for all these chemicals, requiring different temperatures and reactant flow rates.

Example 2: Pristine Graphene from Natural Graphite Flakes

[0120] Five grams of graphite flakes, ground to approximately 20 m or less in sizes, were dispersed in 1,000 mL of deionized water (containing 0.1% by weight of a dispersing agent, Zonyl FSO from DuPont) to obtain a suspension. The suspension was injected into the first inlet of a Couette-Taylor reactor (same as in Example 1), which was operated at 600 RPM. After three passes, (passing through the reaction chamber three times), graphite was highly exfoliated and separated to produce pristine graphene sheets that are mostly few-layer graphene.

Example 3: Graphene Oxide from Short Carbon Fiber Segments

[0121] The procedure was similar to that used in Example 1, but the starting material was pitch-based graphite fibers chopped into segments with 0.2 mm or smaller in length prior to dispersion in water. The diameter of carbon fibers was approximately 12 m. After three passes, graphite fiber segments are exfoliated and separated into graphene oxide sheets, mostly single-layer GO sheets.

Example 4: Pristine Graphene Sheets from Carbon Nano-Fibers (CNFs)

[0122] A powder sample of graphitic nano-fibers was prepared by introducing an ethylene gas through a quartz tube pre-set at a temperature of approximately 800 C. Also contained in the tube was a small amount of nano-scaled CuNi powder supported on a crucible to serve as a catalyst, which promoted the decomposition of the hydrocarbon gas and growth of CNFs. Approximately 2.5 grams of CNFs (diameter of 10 to 80 nm) were dispersed in water (as in Sample 2). The sample was then allowed to pass through a Couette-Taylor reactor at 20 C. twice to effect exfoliation and separation. This procedure was completed in 15 minutes. Fine graphene sheets with an average thickness of 1.05 nm were obtained.

Example 5: Preparation of Graphene Oxide Sheets from Meso-Carbon Micro-Beads (MCMBs) for Bi-Polar Electrodes

[0123] Meso-carbon microbeads (MCMBs) were supplied from China Steel Chemical Co. This material has a density of about 2.24 g/cm.sup.3 with a median particle size of about 16 m. As control samples, MCMB (100 grams) were intercalated with an acid solution (sulfuric acid, nitric acid, and potassium permanganate at a ratio of 4:1:0.05) for 72 hours in a conventional reactor. Upon completion of the reaction, the mixture was poured into deionized water and filtered. The intercalated MCMBs were repeatedly washed in a 5% solution of HCl to remove most of the sulfate ions. The sample was then washed repeatedly with deionized water until the pH of the filtrate was neutral. The slurry was dried and stored in a vacuum oven at 60 C. for 24 hours. The dried powder sample was placed in a quartz tube and inserted into a horizontal tube furnace pre-set at a desired temperature, 1,080 C. for 45 seconds to obtain a graphene material. TEM and atomic force microscopic studies indicate that most of the NGPs were single-layer graphene.

[0124] Separately, MCMB (100 grams) were mixed with an acid solution (sulfuric acid, nitric acid, and potassium permanganate at a ratio of 4:1:0.05) to form a slurry. The slurry was injected into the first inlet and driven to pass through the new Couette-Taylor reactor four times (as short as 65 minutes vs. 72 hours of the conventional reactor). The resulting graphene oxide sheets are mostly single-layer as well, which was achieved in a dramatically reduced period of time.

Example 6: Production of Molybdenum Diselenide Nano Platelets

[0125] The same sequence of steps as in Example 2 can be utilized to form nano platelets from other layered compounds: dispersion of a layered compound in a liquid medium and passing through the Couette-Taylor reactor three times. Dichalcogenides, such as MoS.sub.2, have found applications as electrodes in lithium ion batteries and as hydro-desulfurization catalysts.

[0126] For instance, MoSe.sub.2 consisting of SeMoSe layers held together by weak van der Waals forces can be exfoliated via the presently invented process. Intercalation can be achieved by dispersing MoSe.sub.2 powder in a silicon oil beaker, with the resulting suspension subjected injected into the first inlet of the reactor, which was operated with 650 RPM for 25 minutes per pass. The resulting MoSe.sub.2 platelets were found to have a thickness in the range from approximately 1.4 nm to 13.5 nm with most of the platelets being mono-layers or double layers.

[0127] Other single-layer platelets of the form MX.sub.2 (transition metal dichalcogenide), including MoS.sub.2, TaS.sub.2, and WS.sub.2, were similarly exfoliated. In particular, the inorganic layered compound is selected from boron nitride (h-BN), HfS.sub.2, tungsten disulfide (WS.sub.2), NiTe.sub.2, VSe.sub.2, WSe2, molybdenum disulfide (MoS.sub.2), MoSe.sub.2, MoTe.sub.2, TaS.sub.2, RhTe.sub.2, PdTe.sub.2, NbS.sub.2, NbSe.sub.2, NbTe.sub.2, TaSe.sub.2, Bi.sub.2Se.sub.3, and Bi.sub.2Te.sub.3. Again, most of the platelets were mono-layers or double layers after subjecting the layered inorganic compound through the new Couette-Taylor reactor for 2-5 passes. This observation clearly demonstrates the versatility of the presently invented process and reactor system in terms of producing relatively uniform-thickness platelets that are ultra-thin.