Direct Microwave Production of Graphene

Abstract

Provided is a method of producing graphene directly from a non-intercalated and non-oxidized graphitic material, comprising: (a) dispersing the graphitic material in a liquid solution to form a suspension, wherein the graphitic material has never been previously exposed to chemical intercalation or oxidation; and (b) subjecting the suspension to microwave or radio frequency irradiation with a frequency and an intensity for a length of time sufficient for producing graphene; wherein the liquid solution contains a metal salt dissolved in water, organic solvent, ionic liquid solvent, or a combination thereof. The method is fast (minutes as opposed to hours or days of conventional processes), environmentally benign, and highly scalable.

Claims

1. A method of producing graphene directly from a non-intercalated and non-oxidized graphitic material, said method comprising: (a) dispersing said graphitic material in a liquid solution to form a suspension, wherein said graphitic material has never been previously exposed to chemical intercalation or oxidation; and (b) subjecting said suspension to microwave or radio frequency irradiation with a frequency and an intensity for a length of time sufficient for producing said graphene; wherein said liquid solution contains a metal salt dissolved in water, organic solvent, ionic liquid solvent, or a combination thereof.

2. The method of claim 1, wherein said metal salt is a metal halide selected from the group consisting of MCl (M=Li, Na, K, Cs), MCl.sub.2 (M=Zn, Ni, Cu, Mn), MCl.sub.3 (M=Al, Fe, Ga), MCl.sub.4 (M=Zr, Pt), MF.sub.2 (M=Zn, Ni, Cu, Mn), MF.sub.3 (M=Al, Fe, Ga), MF.sub.4 (M=Zr, Pt), and combinations thereof.

3. The method of claim 1, wherein said metal salt includes an alkali metal salt selected from lithium perchlorate (LiClO.sub.4), sodium perchlorate (NaClO.sub.4), potassium perchlorate (KClO.sub.4), sodium hexafluorophosphate (NaPF.sub.6), potassium hexafluorophosphate (KPF.sub.6), sodium borofluoride (NaBF.sub.4), potassium borofluoride (KBF.sub.4), sodium hexafluoroarsenide, potassium hexafluoroarsenide, sodium trifluoro-metasulfonate (NaCF.sub.3SO.sub.3), potassium trifluoro-metasulfonate (KCF.sub.3SO.sub.3), bis-trifluoromethyl sulfonylimide sodium (NaN(CF.sub.3SO.sub.2).sub.2), sodium trifluoromethanesulfonimide (NaTFSI), bis-trifluoromethyl sulfonylimide potassium (KN(CF.sub.3SO.sub.2).sub.2), a sodium ionic liquid salt, lithium hexafluorophosphate (LiPF.sub.6), lithium borofluoride (LiBF.sub.4), lithium hexafluoroarsenide (LiAsF.sub.6), lithium trifluoro-metasulfonate (LiCF.sub.3SO.sub.3), bis-trifluoromethyl sulfonylimide lithium (LiN(CF.sub.3SO.sub.2).sub.2), lithium bis(oxalato)borate (LiBOB), lithium oxalyldifluoroborate (LiBF.sub.2C.sub.2O.sub.4), lithium oxalyldifluoroborate (LiBF.sub.2C.sub.2O.sub.4), lithium nitrate (LiNO.sub.3), Li-Fluoroalkyl-Phosphates (LiPF.sub.3(CF.sub.2CF.sub.3).sub.3), lithium bisperfluoro-ethysulfonylimide (LiBETI), lithium bis(trifluoromethanesulphonyl)imide, lithium bis(fluorosulphonyl)imide, lithium trifluoromethanesulfonimide (LiTFSI), an ionic liquid lithium salt, or a combination thereof.

4. The method of claim 3, wherein said organic solvent is selected from 1,3-dioxolane (DOL), 1,2-dimethoxyethane (DME), tetraethylene glycol dimethylether (TEGDME), poly(ethylene glycol) dimethyl ether (PEGDME), diethylene glycol dibutyl ether (DEGDBE), 2-ethoxyethyl ether (EEE), sulfone, sulfolane, ethylene carbonate (EC), propylene carbonate (PC), dimethyl carbonate (DMC), methylethyl carbonate (MEC), diethyl carbonate (DEC), ethyl propionate, methyl propionate, gamma-butyrolactone (-BL), acetonitrile (AN), ethyl acetate (EA), propyl formate (PF), methyl formate (MF), toluene, xylene, methyl acetate (MA), fluoroethylene carbonate (FEC), vinylene carbonate (VC), allyl ethyl carbonate (AEC), a hydrofluoroether, or a combination thereof.

5. The method of claim 1 wherein said graphitic material is selected from natural graphite, synthetic graphite, amorphous graphite, highly oriented pyrolytic graphite, meso-carbon micro-bead, graphitized meso-phase carbon, needle coke, carbon fiber, graphite fiber, carbon nano-fiber, graphitic nano-fiber, graphite fluoride, chemically modified graphite, expanded graphite, or a combination thereof.

6. The method of claim 1 wherein said microwave or radio frequency irradiation time is from 1 minute to 60 minutes.

7. The method of claim 1 wherein said microwave or radio frequency irradiation time is less than 30 minutes.

8. The method of claim 1 wherein said microwave or radio frequency irradiation time is less than 10 minutes.

9. The method of claim 1, wherein said microwave or radio frequency irradiation is followed by a mechanical shearing treatment to produce a thinner or smaller graphene material.

10. The method of claim 9, wherein said mechanical shearing treatment comprises using air milling, air jet milling, ball milling, rotating-blade mechanical shearing, ultrasonication, cavitation, or a combination thereof.

11. The method of claim 1, wherein said liquid solution further comprises a chemical functionalizing agent and said microwave or radio frequency irradiation activates a chemical reaction between said agent and said graphene to produce a chemically functionalized graphene material.

12. The method of claim 1 wherein said graphene contains single-layer graphene sheets.

13. The method of claim 1 wherein said graphene contains at least 80% single-layer graphene sheets.

14. The method of claim 1 wherein said graphene contains pristine graphene, oxidized graphene with less than 5% oxygen content by weight, graphene fluoride, graphene fluoride with less than 5% fluorine by weight, graphene with a carbon content no less than 95% by weight, or functionalized graphene.

15. A slurry containing said metal salt solution and graphene sheets created by the process of claim 1.

16. A supercapacitor, sodium battery, or lithium battery containing the slurry of claim 15.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0037] FIG. 1 A flow chart showing the most commonly used prior art process of producing highly oxidized NGPs that entails tedious chemical oxidation/intercalation, rinsing, and high-temperature exfoliation procedures.

[0038] FIG. 2(A) Flowchart to illustrate presently invented one-step process;

[0039] FIG. 2(B) Flow chart for the conventional, multi-step process of producing graphene materials.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

[0040] Carbon materials can assume an essentially amorphous structure (glassy carbon), a highly organized crystal (graphite), or a whole range of intermediate structures that are characterized in that various proportions and sizes of graphite crystallites and defects are dispersed in an amorphous matrix. Typically, a graphite crystallite is composed of a number of graphene sheets 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 graphite flake, carbon/graphite fiber segment, carbon/graphite whisker, or carbon/graphite nano-fiber, etc.

[0041] One preferred specific embodiment of the present invention is a method of producing a graphene material (also referred to as nano graphene platelet, NGP) that is essentially composed of a sheet of graphene plane or multiple sheets of graphene plane stacked and bonded together (typically, on an average, up to five sheets 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 graphite plane. By definition, the thickness of an NGP is 100 nanometers (nm) or smaller, with a single-sheet NGP being as thin as 0.34 nm. However, the presently invented method produces graphene sheets that are typically from 1 to 5 layers, or from 0.34 nm to 1.7 nm. The length and width of a NGP are typically between 200 nm and 20 m, but could be longer or shorter.

[0042] The present invention provides a strikingly simple, fast, scalable, environmentally benign, and cost-effective process that avoids essentially all of the drawbacks associated with prior art processes. As schematically illustrated in FIG. 2(A), one preferred embodiment of this method entails subjecting a suspension (containing a graphitic material dispersed in a metal salt solution) to microwave or radio frequency irradiation for a length of time sufficient for producing the exfoliated graphite or graphene. This is essentially a single-step process. After graphite powder is dispersed in a select solution, the resulting suspension is immediately exposed to microwave irradiation. In less than another 50 minutes (typically less than 30 minutes, and often less than 10 minutes), graphite is fully exfoliated, forming mostly single-layer graphene and, in some cases, some few-layer graphene (mostly no more than 5 layers). This process is stunningly short and simple. The metal salt preferably contains a metal halide salt or a lithium salt, sodium salt, potassium salt that contains a halogen element (e.g. F, Cl, Br, or I), B, N, or a combination thereof. Preferred metal salts and solvents that can be used are summarized in later paragraphs of this section.

[0043] In contrast, as shown in FIG. 1 and FIG. 2(B), the prior art chemical processes typically involve immersing graphite powder in a mixture of concentrated sulfuric acid, nitric acid, and an oxidizer, such as potassium permanganate or sodium perchlorate, forming a reacting mass that requires typically 5-120 hours to complete the chemical intercalation/oxidation reaction. Once the reaction is completed, the slurry is subjected to repeated steps of rinsing and washing with water and then subjected to drying treatments to remove water. The dried powder, referred to as graphite intercalation compound (GIC) or graphite oxide (GO), is then subjected to a thermal shock treatment. This can be accomplished in two ways. One is to expose the GIC to a furnace pre-set at a temperature of typically 800-1100 C. (more typically 950-1050 C.). The other is to expose the GIC to microwave irradiation, as disclosed in the patent applications of L. Drzal, et al [Expanded Graphite and Products Produced Therefrom, US Pub. No. 20040127621 (Jul. 1, 2004 and US 20060148965 (Jul. 6, 2006)].

[0044] It is again critically important to recognize that Drzal et al use microwave to produce exfoliated graphite nanoplatelets from GIC (not from the starting graphite, as in our invention) after natural graphite has been fully intercalated and oxidized with the conventional strong acid treatments, and after the intercalated powder had been dried and recovered from the liquid. This Drzal approach still suffers from the seven (7) major problems described in the Background section, despite the notion that microwave might be more energy efficient as compared to tube furnace heating for the purpose of exfoliating pre-intercalated graphite. In contrast, in the presently invented process, microwave energy is switched on as soon as graphite is poured into and uniformly dispersed in a solution. Since no pre-intercalation or pre-oxidation is involved, we can save 5-120 hours of graphite intercalation/oxidation time.

[0045] It is also significant to understand that Drzal's GICs are identical to the GICs obtained by all prior art processes and necessarily contain sulfuric acid and nitric acid in the inter-graphene spaces and, hence, necessarily involve the decomposition of H.sub.2SO.sub.4 and HNO.sub.3 to produce volatile gases (e.g. NO.sub.x and SO.sub.x) during their subsequent microwave energy-assisted, thermal exfoliation process. The NO.sub.x and SO.sub.x are highly regulated species that can potentially pose some environmental issues. In contrast, our new process does not involve exposing GIC to a high temperature and, hence, does not generate any of these volatile species. Clearly, the presently invented process is not an obvious variant of the microwave energy-assisted thermal exfoliation of GIC. The GIC and exfoliated graphite have a long history (>50 years) and over such a long period of time, the prior art workers have always believed that thermal exfoliation of graphite must go through a tedious chemical intercalation/oxidation of graphite. The need to use combined strong acids and oxidizers to intercalate and oxidize graphite for an extended period of time to produce the so-called GIC or expandable graphite is now completely avoided.

[0046] Additionally, our new process does not require a mixture of concentrated sulfuric acid and fuming nitric acid and/or potassium permanganate. These are very surprising and have defied the expectations of those who work in exfoliated graphite or graphene industry. Furthermore, no subsequent high temperature exposure for exfoliation is required since exfoliated graphite or graphene is directly produced with a simple microwave irradiation of graphite in the presence of a select solution containing a certain type of metal salt (particularly lithium salt or sodium salt) dissolved in certain type of liquid medium.

[0047] Although the mechanisms remain poorly understood, this revolutionary process of the present invention appears to essentially combine the required functions of graphene plane expansion, intercalant penetration, exfoliation, and separation of graphene sheets from one another into one single step. The whole process can take less than 10 minutes. This is absolutely stunning, a shocking surprise to even those top scientists and engineers or those of extraordinary ability in the art.

[0048] The frequency of microwave or radio frequency irradiation that can be used for practicing the instant invention does not have to be 2.45 GHz, which is used in a domestic microwave oven. Preferably, the frequency is between 1 and 20 GHz and more preferably between 2 and 10 GHz.

[0049] In summary, an embodiment of the present invention is a method of producing graphene directly from a non-intercalated and non-oxidized graphitic material. The method comprises: (a) dispersing a graphitic material in a liquid solution to form a suspension, wherein the graphitic material has never been previously exposed to chemical intercalation or oxidation; and (b) subjecting the suspension to microwave or radio frequency irradiation with a frequency and an intensity for a length of time sufficient for producing graphene; wherein the liquid solution contains a metal salt dissolved in water, organic solvent, ionic liquid solvent, or a combination thereof.

[0050] The metal salt preferably contains a metal halide or an alkali metal salt containing F, Cl, Br, I, B, or N element. In certain preferred embodiments, the metal salt may contain a metal halide selected from the group consisting of MCl (M=Li, Na, K, Cs), MCl, (M=Zn, Ni, Cu, Mn), MCl.sub.3 (M=Al, Fe, Ga), MCl.sub.4 (M=Zr, Pt), MF.sub.2 (M=Zn, Ni, Cu, Mn), MF.sub.3 (M=Al, Fe, Ga), MF.sub.4 (M=Zr, Pt), and combinations thereof. We have unexpectedly discovered that the presence of a halogen ion (e.g. F.sup.+, Cl.sup.+, Br.sup.+, and I.sup.+) seems to be effective in promoting intercalation of chemical species into spaces between graphene planes and exfoliation, and separation of graphene planes or graphene sheets. The reason behind the ability of this particular group of salt in a liquid medium to promote graphene production is not yet clear.

[0051] In certain preferred embodiments, the metal salt includes an alkali metal salt selected from lithium perchlorate (LiClO.sub.4), sodium perchlorate (NaClO.sub.4), potassium perchlorate (KClO.sub.4), sodium hexafluorophosphate (NaPF.sub.6), potassium hexafluorophosphate (KPF.sub.6), sodium borofluoride (NaBF.sub.4), potassium borofluoride (KBF.sub.4), sodium hexafluoroarsenide, potassium hexafluoroarsenide, sodium trifluoro-metasulfonate (NaCF.sub.3SO.sub.3), potassium trifluoro-metasulfonate (KCF.sub.3SO.sub.3), bis-trifluoromethyl sulfonylimide sodium (NaN(CF.sub.3SO.sub.2).sub.2), sodium trifluoromethanesulfonimide (NaTFSI), bis-trifluoromethyl sulfonylimide potassium (KN(CF.sub.3SO.sub.2).sub.2), a sodium ionic liquid salt, lithium hexafluorophosphate (LiPF.sub.6), lithium borofluoride (LiBF.sub.4), lithium hexafluoroarsenide (LiAsF.sub.6), lithium trifluoro-metasulfonate (LiCF.sub.3SO.sub.3), bis-trifluoromethyl sulfonylimide lithium (LiN(CF.sub.3SO.sub.2).sub.2), lithium bis(oxalato)borate (LiBOB), lithium oxalyldifluoroborate (LiBF.sub.2C.sub.2O.sub.4), lithium oxalyldifluoroborate (LiBF.sub.2C.sub.2O.sub.4), lithium nitrate (LiNO.sub.3), Li-Fluoroalkyl-Phosphates (LiPF.sub.3(CF.sub.2CF.sub.3).sub.3), lithium bisperfluoro-ethysulfonylimide (LiBETI), lithium bis(trifluoromethanesulphonyl)imide, lithium bis(fluorosulphonyl)imide, lithium trifluoromethanesulfonimide (LiTFSI), an ionic liquid lithium salt, or a combination thereof.

[0052] Preferably, the organic solvent is selected from 1,3-dioxolane (DOL), 1,2-dimethoxyethane (DME), tetraethylene glycol dimethylether (TEGDME), poly(ethylene glycol) dimethyl ether (PEGDME), diethylene glycol dibutyl ether (DEGDBE), 2-ethoxyethyl ether (EEE), sulfone, sulfolane, ethylene carbonate (EC), propylene carbonate (PC), dimethyl carbonate (DMC), methylethyl carbonate (MEC), diethyl carbonate (DEC), ethyl propionate, methyl propionate, gamma-butyrolactone (-BL), acetonitrile (AN), ethyl acetate (EA), propyl formate (PF), methyl formate (MF), toluene, xylene, methyl acetate (MA), fluoroethylene carbonate (FEC), vinylene carbonate (VC), allyl ethyl carbonate (AEC), a hydrofluoroether, or a combination thereof.

[0053] The preferred salt concentration in the liquid medium is from 0.01M to 10 M, more preferably from 0.1 M to 3.5 M, and most preferably from 0.5 M to 2.0 M.

[0054] The starting graphitic material may be selected from natural graphite, amorphous graphite (graphite materials containing micro-scaled graphite crystallites, typically 0.1-1.0 m), synthetic graphite, highly oriented pyrolytic graphite, meso-carbon micro-bead, graphitized meso-phase carbon, needle coke, carbon fiber, graphite fiber, carbon nano-fiber, graphitic nano-fiber, graphite fluoride, chemically modified graphite, expanded graphite, or a combination thereof.

[0055] It has been commonly believed that a combination of an intercalant and an oxidizer is required to obtain a proper graphite intercalation compound (GIC) or graphite oxide (GO). This belief has been based on the notion that sulfuric acid serves as an intercalant that penetrates into inter-graphene spaces and stays therein when the intercalation/oxidation procedure is completed and the powder is recovered by drying. In the meantime, the oxidizing agent serves to oxidize the edge and then the interior of graphene planes, effectively increasing the inter-graphene space or opening up the space to facilitate the entry of sulfuric acid. The interstitial sulfuric acid keeps the space open, allowing the oxidizing agent to further oxidize the graphene planes. Upon removal of excess chemicals, the recovered or dried powder is graphite intercalation compound (GIC) or graphite oxide (GO). The residual chemicals (sulfuric acid and nitric acid) in the dried GIC powder get thermally decomposed during the subsequent high-temperature treatment (heat shock exposure), resulting in the formation of volatile gases in the inter-graphene spaces. These gases, under a high temperature, produce very high pressures that push open graphene planes (expansion and exfoliation). In other words, sulfuric acid and the oxidizer are believed to work in a concerted manner to enable the intercalation and oxidation, both the intercalant and the oxidizer being indispensable. The present invention defies this expectation in many aspects: (a) Strong acids, such as sulfuric acid, are not required; (b) Strong oxidizing liquids are not required; and (c) Select metal salts can work well.

[0056] From the environmental protection perspective, the practice of mixing two or three undesirable chemicals together (e.g. mixing sulfuric acid, nitric acid, and/or potassium permanganate) can be troublesome since it would make the recovery, separation, and re-use of chemicals so much more difficult. This is more than just a cost issue, but a larger environmental and societal issue and an industrial scalability issue. The significance of our surprising discovery to use more environmentally benign chemicals should not be underestimated or ignored.

[0057] Quite significantly, the microwave exposure time can be less than 50 minutes, often less than 30 minutes, or even less than 10 minutes. The microwave exposure step may be followed by a step of subjecting the exfoliated graphite to a mechanical shearing treatment to produce a graphene material. The mechanical shearing treatment comprises using air milling, air jet milling, ball milling, rotating-blade mechanical shearing, ultrasonication, cavitation, or a combination thereof. The graphene may be compressed to form a flexible graphene paper product or a metal-doped graphene product.

[0058] In certain embodiments, the microwave exposure step may be carried out simultaneously with a step of subjecting the irradiated graphite to a mechanical shearing treatment to produce a graphene material. The mechanical shearing treatment comprises using ball milling, rotating-blade mechanical shearing, ultrasonication, cavitation, or a combination thereof.

[0059] In certain embodiments, the microwave exposure step may be carried out in an inert gas atmosphere, in vacuum, or in a pressurized vessel.

[0060] In another embodiment, the dispersion solution can be circulated through the microwave field more than once. Optionally, a size separation method such as hydrocyclone, gravity sedimentation, centrifugation, or filtration can be used to preferentially treat a selected size range of particles in the dispersion.

[0061] Another surprising result of the present study is the observation that a wide variety of carbonaceous and graphitic materials can be directly microwave-irradiated in the presence of an intercalating agent. This material may be selected from natural graphite, synthetic graphite, meso-carbon micro-beads (MCMBs), highly oriented pyrolytic graphite, meso-phase carbon, graphite fiber, graphitic nano-fiber, graphite oxide, graphite fluoride, chemically modified graphite, exfoliated graphite, or a combination thereof. This is surprising based on the observation that several types of graphitic materials (e.g. carbon fibers, graphite fibers, carbon nano-fibers, etc.) have a hard-shell structure enclosing a core structure composed of stacks of graphene sheets. These hard skins are known to be highly impermeable to chemicals. This is in contrast to the natural graphite and some artificial graphite that have graphene edges exposed to chemicals and permeable to chemicals.

[0062] The presently invented process is capable of producing single-layer graphene sheets. In many examples, the graphene material produced contains at least 80% single-layer graphene sheets. The graphene produced can contain pristine graphene, oxidized graphene with less than 5% oxygen content by weight, graphene fluoride, graphene oxide with less than 5% fluorine by weight, graphene with a carbon content no less than 95% by weight, or functionalized graphene.

[0063] The graphene sheets produced remain dispersed in the metal salt solution to form a slurry that can be used in a battery or supercapacitor. The slurry can become part of a supercapacitor or battery (e.g. a lithium battery if the metal salt contains a lithium salt or a sodium battery if the metal salt contains a sodium salt. Since graphene sheets are an electrode active material in a supercapacitor, the slurry can be packed into an electrode.

[0064] The following examples serve to provide the best modes of practice for the present invention and should not be construed as limiting the scope of the invention:

Examples 1-6: Graphene Sheets (NGPs) from Various Graphitic Materials

[0065] As an example, 20 mg of meso-phase pitch-derived artificial graphite of approximately 20 m in size were used in the preparation of Sample 1-A (microwave irradiation) and 1-C (conventional GO/GIC approach), respectively, under the processing conditions and lengths of time as specified in Table 1 below.

[0066] For comparison, Sample 1C was prepared in the following manner: Twenty mg of artificial graphite as used in Example 1A was dispersed in a mixture of sulfuric acid, nitric acid, and potassium permanganate at a weight ratio of 4:1:0.05 (graphite-to-intercalate ratio of 1:3) for 24 hours. Upon completion of the intercalation reaction, the mixture was poured into deionized water and filtered. The sample was then washed with 5% HCl solution to remove most of the sulfate ions and residual salt and then repeatedly rinsed with deionized water until the pH of the filtrate was approximately 5. The dried sample was then exfoliated in a tube furnace at 900 C. for 45 seconds.

[0067] Graphite fibers were chopped into segments with 0.1 mm or smaller in length prior to being immersed in a liquid medium, and the resulting suspension was then exposed to microwave irradiation for 3-30 minutes. The diameter of carbon fibers was approximately 12 m. The process resulted in the formation of ultra-thin NGPs.

[0068] A powder sample of carbon nano-fibers was supplied from Applied Science, Inc. (ASI), Cedarville, Ohio. In one example, approximately 2 grams of CNFs were immersed in a lithium salt solution (Table 1). Portion of the resulting suspension was exposed to microwave irradiation for 2-10 minutes. The process produced ultra-thin NGPs >90% being single-layer graphene.

TABLE-US-00001 TABLE 1 Production of graphene from various sources and chemicals. Sample Microwave/ No. Metal salt Liquid medium Graphitic material RF conditions Comments 1-A LiPF.sub.6 PC Natural graphite 5-15 min 1-C H.sub.2SO.sub.4 + HNO.sub.3 + Natural graphite 24 hours Conventional KMnO.sub.4 GO/GIC 2-A NaBF.sub.4 PC + EC CNF 2-10 min 2-B NaBF.sub.4 PC + VC CNF 2-10 min 3-A LiPF.sub.3(CF.sub.2CF.sub.3).sub.3 DEC MCMB 5-25 min 3-B LiPF.sub.3(CF.sub.2CF.sub.3).sub.3 PC + DEC MCMB 3-15 min 4-A KCl + NaF Water Artificial graphite 10-45 min 4-B NaF Water Artificial graphite 12-45 min 5-A AlCl.sub.3 EMIC MCMB 20-50 min Ionic liquid 5-B AlF.sub.3 EMIC MCMB 10-35 min Ionic liquid 6-A NaPF.sub.6 DOL + DME CNF 2-10 min 6-B NaPF.sub.6 DOL + DME Graphite fibers 12-45 min 6-C NaPF.sub.6 DOL + DME MWCNT 4-15 min 6-D NaPF.sub.6 DOL + DME Natural graphite 10-45 min 6-E NaPF.sub.6 DOL + DME Needle coke 5-15 min 6-F H.sub.2SO.sub.4 + Needle coke 24 hours Conventional KMnO.sub.4 GO/GIC * EMIC = 1-ethyl-3-methylimidazolium chloride; PC = propylene carbonate; EC = ethylene carbonate; VC = vinyl acetate; Household microwave ovens used.

[0069] The graphene sheets obtained in each sample were examined using atomic force microscopy (AFM), transmission electron microscopy (TEM), and scanning electron microscopy (SEM) to determine their thickness (number of layers) and lateral dimensions (length and width). The graphene sheets suspended in water were cast onto a glass plate to form a thin film (2-5 m thick) from each sample. The electrical conductivity of the thin film was measured using the four-point probe method. We have also investigated the production yield of each process by comparing the initial graphite material weight with the final graphene material weight. This is significant since we surprisingly observed that conventional Hummer's method of producing graphene involves consumption of graphite, perhaps due to oxidation of carbon in the graphite structure to form carbon dioxide during the chemical oxidation/intercalation process. The reaction of graphite with oxygen to form CO.sub.2 means that a significant proportion of graphite is lost. The experimental data are summarized in Table 2 below.

TABLE-US-00002 TABLE 2 Characteristics of graphene produced by different methods. No. of Conductivity Sample Lateral graphene of thin film Production No. dimensions layers (S/cm) yield 1-A 5-15 m 1-3 5560 98% 1-C 0.3-1.2 m 1-3 2.5 10.sup.3 S/cm 70% (un-reduced); 560 S/cm (reduced); 2-A 0.12-0.5 m Mostly 1 2520 99% 2-B 0.12-0.4 m Mostly 1 2550 99% 3-A 3-10 m 3-10 4342 99% 3-B 3-10 m 1-5 4328 99% 4-A 2-5 m 3-10 5015 95% 4-B 2-5 m 5-10 5008 93% 5-A 3-11 m 3-7 4337 97% 5-B 3-11 m 1-5 4320 97% 6-A 0.12-0.5 m 1-3 2465 99% 6-B 0.6-2.5 m 3-5 3837 95% 6-C 0.1-0.5 m 1-5 3668 96% 6-D 3-11 m 5-10 4546 94% 6-E 2-5 m 1-5 4488 96% 6-F 0.35-0.6 m 3-6 572 (after reduction) 68%

[0070] These data nave clearly demonstrated the superiority of the presently invented direct microwave irradiation method of producing graphene materials over conventional chemical oxidation/intercalation method. Both methods are capable of producing single-layer graphene, but our presently invented method produces graphene sheets that are typically wider, longer, and much more electrically conducting. The conventional Hummer's method and all other chemical oxidation/intercalation method necessarily involve highly oxidizing the graphitic material, creating damage (defects) to the resulting graphene sheets that could never be repaired or recovered (smaller lateral dimensions. Even after heavy chemical reduction with hydrazine, the graphene material (a reduced graphene oxide) still exhibits an electrical conductivity one order of magnitude lower than that of the more pristine graphene produced by the present direct microwave irradiation method.