Environmentally benign production of graphene suspensions

Abstract

A method of producing a graphene suspension, comprising: (a) mixing multiple particles of a graphitic material and multiple particles of a solid carrier material to form a mixture in an impacting chamber of an energy impacting apparatus; (b) operating the energy impacting apparatus with a frequency and an intensity for a length of time sufficient for peeling off graphene sheets from the graphitic material and transferring the graphene sheets to surfaces of the carrier material particles to produce graphene-coated carrier particles inside the impacting chamber; and (c) dispersing the graphene-coated carrier particles in a liquid medium and separating the graphene sheets from the carrier material particles using ultrasonication or mechanical shearing means and removing the carrier material from the liquid medium to produce the graphene suspension. The process is fast (1-4 hours as opposed to 5-120 hours of conventional processes), environmentally benign, cost effective, and highly scalable.

Claims

1. A method of producing a graphene suspension comprising isolated graphene sheets dispersed in a liquid medium, said method comprising: (a) mixing multiple particles of a graphitic material and multiple particles of a solid carrier material to form a mixture in an impacting chamber of an energy impacting apparatus; (b) operating said energy impacting apparatus with a frequency and an intensity for a length of time sufficient for peeling off graphene sheets from said graphitic material and transferring said graphene sheets to surfaces of said solid carrier material particles to produce graphene-coated solid carrier particles inside said impacting chamber; and (c) dispersing said graphene-coated solid carrier particles in said liquid medium in a solvent and separating said graphene sheets from said solid carrier material particles using mechanical shearing means and removing said solid carrier material from said liquid medium to produce said graphene suspension, wherein said mechanical shearing means comprises operating a rotatory blade mixer or reverse-blade mixer.

2. The method of claim 1, wherein said step (a) further comprises adding an oxidizing liquid in said mixture so that said oxidizing liquid acts to partially oxidize said graphene sheets to produce graphene oxide during step (b).

3. The method of claim 2, wherein said oxidizing liquid is selected from a liquid containing an oxidizer selected from H.sub.2O.sub.2, nitric acid, potassium permanganate, sodium permanganate, transition metal permanganate, sodium chlorate, potassium chlorate, or a combination thereof.

4. The method of claim 1, wherein said step (c) comprises exposing said graphene sheets to an oxidizing medium, before, during or after the graphene sheets are separated from said solid carrier material particle surfaces, wherein said oxidizing medium is selected from an oxidizing gas or vapor, an oxidizing plasma, or an oxidizing liquid.

5. The method of claim 1, wherein said solid carrier material is selected from solid particles of an organic, polymeric, metal, glass, ceramic, or inorganic material.

6. The method of claim 1, wherein said solid carrier material includes plastic beads, plastic pellets, wax pellets, polymer powder or polymer reactor spheres, glass beads or fibers, metal particles or wires, metal oxide particles, ceramic particles, or a combination thereof.

7. The method of claim 1, wherein said step (c) comprises oxidizing said graphene sheets on said solid carrier material particle surfaces in an oxidizing liquid medium while being submitted to ultrasonication or mechanical shearing.

8. The method of claim 1 wherein said graphitic material is selected from natural graphite, synthetic graphite, highly oriented pyrolytic graphite, graphite fiber, graphitic nanofiber, graphite fluoride, oxidized graphite, chemically modified graphite, exfoliated graphite, recompressed exfoliated graphite, expanded graphite, mesocarbon microbead, or a combination thereof.

9. The method of claim 1, wherein the energy impacting apparatus is a vibratory ball mill, planetary ball mill, high energy mill, basket mill, agitator ball mill, continuous ball mill, stirred ball mill, pressurized ball mill, freezer mill, vibratory sieve, ultrasonic homogenizer mill, or resonant acoustic mixer.

10. The method of claim 1, wherein said graphitic material contains a non-intercalated and non-oxidized graphitic material that has never been previously exposed to a chemical or oxidation treatment prior to said mixing step.

11. The method of claim 1, wherein said graphitic material contains previously fluorinated, chlorinated, brominated, iodized, nitrogenated, or hydrogenated graphite or carbon material and the graphene suspension contains graphene fluoride, graphene chloride, graphene bromide, graphene iodide, nitrogenated graphene, or hydrogenated graphene.

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

13. The method of claim 1 wherein said graphene sheets contain at least 80% single-layer graphene oxide or at least 80% few-layer graphene having no greater than 10 graphene planes.

14. The method of claim 1, wherein said step (a) further comprises adding a chemical functionalizing agent to said mixture to functionalize said graphene sheets.

15. The method of claim 14 wherein said functionalizing agent contains a chemical functional group selected from alkyl or aryl silane, alkyl or aralkyl group, hydroxyl group, carboxyl group, amine group, sulfonate group (—SO.sub.3H), aldehydic group, quinoidal, fluorocarbon, or a combination thereof.

16. The method of claim 14, wherein said functionalizing agent contains a chemical functional group, wherein said chemical functional group is selected from the group consisting of 2-azidoethanol, 3-azidopropan-1-amine, 4-(2-azidoethoxy) -4-oxobutanoic acid, 2-azidoethyl-2-bromo-2-methylpropanoate, chlorocarbonate, azidocarbonate, dichlorocarbene, carbene, aryne, nitrene, (R—)-oxycarbonyl nitrenes, where R =any one of the following groups, ##STR00002## and combinations thereof.

17. The method of claim 14, wherein said functionalizing agent contains an oxygenated group selected from the group consisting of hydroxyl, peroxide, ether, keto, and aldehyde.

18. The method of claim 14, wherein said functionalizing agent contains a functional group selected from the group consisting of SO.sub.3H, COOH, NH.sub.2, OH, R′CHOH, CHO, CN, COCl, halide, COSH, SH, COOR′, SR′, SiR′.sub.3, Si(—OR′—).sub.yR′.sub.3-y, Si(—O—SiR′.sub.2—)OR′, R″, Li, AlR′.sub.2, Hg—X, TIZ.sub.2 and Mg—X; wherein y is an integer equal to or less than 3, R′ is hydrogen, alkyl, aryl, cycloalkyl, or aralkyl, cycloaryl, or poly(alkylether), R″ is fluoroalkyl, fluoroaryl, fluorocycloalkyl, fluoroaralkyl or cycloaryl, X is halide, and Z is carboxylate or trifluoroacetate, and combinations thereof.

19. The method of claim 14, wherein said functionalizing agent contains a functional group is selected from the group consisting of amidoamines, polyamides, aliphatic amines, modified aliphatic amines, cycloaliphatic amines, aromatic amines, anhydrides, ketimines, diethylenetriamine (DETA), triethylene-tetramine (TETA), tetraethylene-pentamine (TEPA), polyethylene polyamine, polyamine epoxy adduct, phenolic hardener, non-brominated curing agent, non-amine curatives, and combinations thereof.

20. The method of claim 14, wherein said functionalizing agent contains a functional group selected from OY, NHY, O═C—OY, P═C—NR′Y, O═C—SY, O═C—Y, —CR′1OY, N′Y or C′Y, and Y is a functional group of a protein, a peptide, an amino acid, an enzyme, an antibody, a nucleotide, an oligonucleotide, an antigen, or an enzyme substrate, enzyme inhibitor or the transition state analog of an enzyme substrate or is selected from R′—OH, R′—NR′.sub.2, R′SH, R′CHO, R′CN, R′X, R′N.sup.+(R′).sub.3X.sup.−, R′SiR′.sub.3, R′Si(—OR′—).sub.yR′.sub.3−y , R′Si(—O—SiR′.sub.2—)OR′, R′—R″, R′—N—CO, (C.sub.2H.sub.4O—).sub.wH, (—C.sub.3H.sub.6O—).sub.wH, (—C.sub.2H.sub.4O).sub.w—R′, (C.sub.3H.sub.6O).sub.w—R′, R′, and w is an integer greater than one and less than 200.

21. The method of claim 1, wherein said procedure of operating said energy impacting apparatus is conducted in a continuous manner using a continuous energy impacting device.

22. A method of producing a graphene suspension comprising isolated graphene sheets dispersed in a liquid medium, said method comprising: (a) mixing multiple particles of a graphitic material and multiple particles of a solid carrier material to form a mixture in an impacting chamber of an energy impacting apparatus; (b) operating said energy impacting apparatus with a frequency and an intensity for a length of time sufficient for peeling off graphene sheets from said graphitic material and transferring said graphene sheets to surfaces of said solid carrier material particles to produce graphene-coated solid carrier particles inside said impacting chamber; and (c) dispersing said graphene-coated solid carrier particles in said liquid medium and separating said graphene sheets from said solid carrier material particles using mechanical shearing means and removing said solid carrier material from said liquid medium to produce said graphene suspension, wherein said mechanical shearing means comprises operating a rotatory blade mixer or reverse-blade mixer, wherein step (a) further comprises adding a plurality of impacting balls or media to the impacting chamber of said energy impacting apparatus and said step (c) further comprises removing said impacting balls or media from said liquid medium.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) 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.

(2) FIG. 2 A flow chart showing the presently invented two-step process for producing graphene suspensions.

(3) FIG. 3 A flow chart showing the presently invented process for producing isolated graphene sheets via a continuous ball mill.

(4) FIG. 4(A) Transmission electron micrograph of graphene sheets produced by conventional Hummer's route (much smaller graphene sheets, but comparable thickness).

(5) FIG. 4(B) Transmission electron micrograph of graphene sheets produced by the presently invented impact energy method.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

(6) 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 nanofiber.

(7) One preferred specific embodiment of the present invention is a method of producing oxidized versions of graphene sheets or nano graphene platelet (NGP) material. The NGP is essentially composed of a sheet of graphene plane (hexagonal lattice of carbon atoms) or multiple sheets of graphene plane stacked and bonded together (typically, on an average, less than five sheets per multi-layer platelet). Each graphene plane, also referred to as a graphene sheet or basal plane comprises a two-dimensional hexagonal structure of carbon atoms. Each platelet has a length and a width parallel to the graphite 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 NGPs produced with the instant methods are mostly single-layer graphene with some few-layer graphene sheets (2-10 layers, but mostly <5 layers). The length and width of a NGP are typically between 200 nm and 20 μm, but could be longer or shorter, depending upon the sizes of source graphite material particles.

(8) 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, one preferred embodiment of this method entails placing source graphitic material particles, an optional oxidizing agent and/or chemical functionalization agent, and carrier material particles (plus optional impacting balls, if so desired) in an impacting chamber. After loading, the resulting mixture is immediately exposed to impacting energy, which is accomplished by rotating the chamber to enable the impacting of the carrier particles (and optional impacting balls) against graphite particles. These repeated impacting events (occurring in high frequencies and high intensity) act to peel off graphene sheets from the surfaces of graphitic material particles and directly transfer these graphene sheets to the surfaces of carrier particles. This is a “direct transfer” process.

(9) If an oxidizing agent and/or chemical functionalization agent is present in the impacting chamber, oxidation and/or chemical functionalization of the produced graphene sheets also occurs substantially concurrently.

(10) In certain embodiments, in the impacting chambers containing impacting balls (e.g. stainless steel or zirconia beads), graphene sheets are peeled off by the impacting balls and tentatively transferred to the surfaces of impacting balls first. When the graphene-coated impacting balls impinge upon the carrier material particles, the graphene sheets are transferred to surfaces of the carrier material particles. This is an “indirect transfer” process. Again, if an oxidizing agent and/or chemical functionalization agent is present in the impacting chamber, oxidation and/or chemical functionalization of the produced graphene sheets also occurs substantially concurrently.

(11) In less than 1-4 hours, most of the constituent graphene sheets of source graphite particles are peeled off, forming mostly single-layer graphene and few-layer graphene (mostly less than 5 layers). Following the direct or indirect transfer process (coating of graphene sheets on carrier material particles), the graphene sheets can be separated from the carrier material particles and directly dispersed in a liquid medium if the graphene-coated carrier particles are dispersed in the liquid medium, which is submitted to ultrasonication or mechanical shearing.

(12) In contrast, as shown in FIG. 1, 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 by placing GIC in a furnace pre-set at a temperature of typically 800-1100° C. (more typically 950-1050° C.). The resulting products are typically highly oxidized graphene (i.e. graphene oxide with a high oxygen content), which must be chemically or thermal reduced to obtain reduced graphene oxide (RGO). RGO is found to contain a high defect population and, hence, is not as conducting as pristine graphene. We have observed that that the pristine graphene paper (prepared by vacuum-assisted filtration of pristine graphene sheets) exhibit electrical conductivity values in the range of 1,500-4,500 S/cm. In contrast, the RGO paper prepared by the same paper-making procedure typically exhibits electrical conductivity values in the range of 100-1,000 S/cm.

(13) It is again critically important to recognize that the impacting process not only avoids significant chemical usage, but also produces a higher quality final product—pristine graphene as opposes to thermally reduced graphene oxide, as produced by the prior art process. Pristine graphene enables the creation of GO materials in a controlled manner.

(14) Although the mechanisms remain incompletely understood, this revolutionary process of the present invention appears to essentially eliminate the required functions of graphene plane expansion, intercalant penetration, exfoliation, and separation of graphene sheets and replace it with an entirely mechanical exfoliation process. The whole process can take less than 1-4 hours, and can be done with no added undesirable chemicals. This is absolutely stunning, a shocking surprise to even those top scientists and engineers or those of extraordinary ability in the art.

(15) Another surprising result of the present study is the observation that a wide variety of carbonaceous and graphitic materials can be directly processed without any particle size reduction or pre-treatment. This material may be selected from natural graphite, synthetic graphite, highly oriented pyrolytic graphite, mesocarbon microbead, graphite fiber, graphitic nanofiber, graphite oxide, graphite fluoride, chemically modified graphite, exfoliated graphite, or a combination thereof. By contrast, graphitic material for used for the prior art chemical formation and reduction of graphene oxide requires size reduction to 75 um or less average particle size. This process requires size reduction equipment (for example hammer mills or screening mills), energy input, and dust mitigation. By contrast, the energy impacting device method can accept almost any size of graphitic material. Starting material of mm or cm in size or larger has been successfully processed to create graphene. The only size limitation is the chamber capacity of the energy impacting device.

(16) The presently invented process is capable of producing single-layer graphene or graphene oxide sheets dispersed in a liquid medium. In many examples, the graphene material produced contains at least 80% (can be higher than 90%) single-layer graphene oxide sheets.

(17) The presently invented process does not involve the production of GIC and, hence, does not require the exfoliation of GIC at a high exfoliation temperature (e.g. 800-1,100° C.). This is another major advantage from environmental protection perspective. The prior art processes require the preparation of dried GICs containing sulfuric acid and nitric acid intentionally implemented 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) that are highly regulated environmental hazards. The presently invented process completely obviates the need to decompose H.sub.2SO.sub.4 and HNO.sub.3 and, hence, is environmentally benign. No undesirable gases are released into the atmosphere during the combined graphite expansion/exfoliation/separation process of the present invention.

(18) One preferred embodiment of the present invention is the inclusion of impacting balls or media to the impacting chamber, as illustrated in FIG. 2. The impact media may contain balls of metal, glass, ceramic, or organic materials. The size of the impacting media may be in the range of 1 mm to 20 mm, or it may be larger or smaller. The shape of the impacting media may be spherical, needle like, cylindrical, conical, pyramidal, rectilinear, or other shapes. A combination of shapes and sizes may be selected. The size distribution may be unimodal Gaussian, bimodal or tri-modal.

(19) After direct transfer or indirect transfer of graphene sheets to carrier particle surfaces, the graphene-coated carrier particles may be dispersed in a desired liquid medium and the method further comprises operating ultrasonication or mechanical shearing means to separate graphene sheets from the supporting carrier particles. The resulting suspension now contains both separated graphene sheets and solid carrier particles. The carrier particles may be removed by means of filtration, centrifugation, etc.

(20) One significant advantage of the present invention as compared to prior art is flexibility of selecting carrier materials. There are many opportunities to use pre-consumer or post-consumer waste material as the carrier, diverting this material from disposal by landfill or incineration. Nut shells, rice husks, shredded tires, and ground co-mingled recycled plastic are all possible cost-effective carrier materials for the production of graphene.

(21) In a desired embodiment, the presently invented method is carried out in an automated and/or continuous manner. For instance, as illustrated in FIG. 3, the mixture of graphite particles and solid carrier particles (plus optional impacting balls) is delivered by a conveyer belt 12 and fed into a continuous ball mill 14. After ball milling to form graphene-coated solid carrier particles, the product mixture (possibly also containing some residual graphite particles and optional impacting balls) is discharged from the ball mill apparatus 14 into a screening device (e.g. a rotary drum 16) to separate graphene-coated solid carrier particles from residual graphite particles (if any) and impacting balls (if any). This separation operation may be assisted by a magnetic separator 18 if the impacting balls are ferromagnetic (e.g. balls of Fe, Co. Ni, or Mn-based metal). The graphene-coated carrier particles may be delivered into a combustion chamber 20, if the solid carrier can be burned off (e.g. plastic beads, rubber particles, and wax particles, etc.). Alternatively, these particles can be discharged into a dissolving chamber for dissolving the carrier particles (e.g. plastic beads). The product mass can be further screened in another (optional) screening device 22, a powder classifier or cyclone 24, and/or an electrostatic separator 26. These procedures can be fully automated.

(22) FIG. 4(A) shows a transmission electron micrograph of graphene sheets produced by conventional Hummer's route (much smaller graphene sheets, but comparable thickness). FIG. 4(B) shows a transmission electron micrograph of graphene sheets produced by the presently invented impact energy method, which are larger in length and width.

(23) Graphene sheets transferred to carrier solid particle surfaces, before or after separation, have a significant proportion of surfaces that correspond to the edge planes of graphite crystals. The carbon atoms at the edge planes are reactive and must contain some heteroatom or group to satisfy carbon valency. There are many types of functional groups (e.g. hydroxyl and carboxylic) that are naturally present at the edge or surface of graphene nano platelets produced through transfer to a solid carrier particle. The impact-induced kinetic energy experienced by the carrier particles are of sufficient energy and intensity to chemically activate the edges and even surfaces of graphene sheets coated on carrier particle surfaces (e.g. creating highly active sites or free radicals). These conditions enable ready oxidation and/or chemical functionalization of the graphene sheets (free-standing or being supported on a solid carrier surface) in the presence of an oxidizing agent/functionalization agent in the impacting chamber.

(24) If certain chemical species containing desired chemical function groups (e.g. —NH.sub.2, Br—, etc.) are added in the impacting chamber (preferably after oxidation of graphene occurs), these functional groups can be imparted to graphene edges and/or surfaces. In other words, production and chemical functionalization of graphene oxide sheets can be accomplished concurrently by including appropriate chemical compounds in the impacting chamber. In summary, a major advantage of the present invention over other processes is the simplicity of simultaneous production and modification of surface chemistry.

(25) In one preferred group of chemical agents, the resulting functionalized NGP may broadly have the following formula(e): [NGP]—R.sub.m, wherein m is the number of different functional group types (typically between 1 and 5), R is selected from SO.sub.3H, COOH, NH.sub.2, OH, R′CHOH, CHO, CN, COCl, halide, COSH, SH, COOR′, SR′, SiR′.sub.3, Si(—OR′—).sub.yR′.sub.3-y, Si(—O—SiR′.sub.2—)OR′, R″, Li, AlR′.sub.2, Hg—X, TlZ.sub.2 and Mg—X, wherein y is an integer equal to or less than 3, R′ is hydrogen, alkyl, aryl, cycloalkyl, or aralkyl, cycloaryl, or poly(alkylether), R″ is fluoroalkyl, fluoroaryl, fluorocycloalkyl, fluoroaralkyl or cycloaryl, X is halide, and Z is carboxylate or trifluoroacetate.

(26) For NGPs to be effective reinforcement fillers in epoxy resin, the function group —NH.sub.2 is of particular interest. For example, a commonly used curing agent for epoxy resin is diethylenetriamine (DETA), which has three —NH.sub.2 groups. If DETA is included in the impacting chamber, one of the three —NH.sub.2 groups may be bonded to the edge or surface of a graphene sheet and the remaining two un-reacted —NH.sub.2 groups will be available for reacting with epoxy resin later. Such an arrangement provides a good interfacial bonding between the NGP (graphene sheets) and the matrix resin of a composite material.

(27) Other useful chemical functional groups or reactive molecules may be selected from the group consisting of amidoamines, polyamides, aliphatic amines, modified aliphatic amines, cycloaliphatic amines, aromatic amines, anhydrides, ketimines, diethylenetriamine (DETA), triethylene-tetramine (TETA), tetraethylene-pentamine (TEPA), hexamethylenetetramine, polyethylene polyamine, polyamine epoxy adduct, phenolic hardener, non-brominated curing agent, non-amine curatives, and combinations thereof. These functional groups are multi-functional, with the capability of reacting with at least two chemical species from at least two ends. Most importantly, they are capable of bonding to the edge or surface of graphene using one of their ends and, during subsequent epoxy curing stage, are able to react with epoxide or epoxy resin at one or two other ends.

(28) The above-described [NGP]—R.sub.m may be further functionalized. This can be conducted by opening up the lid of an impacting chamber after the —R.sub.m groups have been attached to graphene sheets and then adding the new functionalizing agents to the impacting chamber and resuming the impacting operation. The resulting graphene sheets or platelets include compositions of the formula: [NGP]-A.sub.m, where A is selected from OY, NHY, O═C—OY, P═C—NR′Y, O═C—SY, O═C—Y, —CR′1-OY, N′Y or C′Y, and Y is an appropriate functional group of a protein, a peptide, an amino acid, an enzyme, an antibody, a nucleotide, an oligonucleotide, an antigen, or an enzyme substrate, enzyme inhibitor or the transition state analog of an enzyme substrate or is selected from R′—OH, R′—NR′.sub.2, R′SH, R′CHO, R′CN, R′X, R′N.sup.+(R′).sub.3X.sup.−, R′SiR′.sub.3, R′Si(—OR′—).sub.y R′.sub.3-y, R′Si(—O—SiR′.sub.2—)OR′, R′—R″, R′—N—CO, (C.sub.2H.sub.4O—).sub.wH, (—C.sub.3H.sub.6O—).sub.wH, (—C.sub.2H.sub.4O).sub.w—R′, (C.sub.3H.sub.6O).sub.w—R′, R′, and w is an integer greater than one and less than 200.

(29) The NGPs may also be functionalized to produce compositions having the formula: [NGP]—[R′-A].sub.m, where m, R′ and A are as defined above. The compositions of the invention also include NGPs upon which certain cyclic compounds are adsorbed. These include compositions of matter of the formula: [NGP]—[X—R.sub.a].sub.m, where a is zero or a number less than 10, X is a polynuclear aromatic, polyheteronuclear aromatic or metallopolyheteronuclear aromatic moiety and R is as defined above. Preferred cyclic compounds are planar. More preferred cyclic compounds for adsorption are porphyrins and phthalocyanines. The adsorbed cyclic compounds may be functionalized. Such compositions include compounds of the formula, [NGP]—[X-A.sub.a].sub.m, where m, a, X and A are as defined above.

(30) The functionalized NGPs of the instant invention can be prepared by sulfonation, electrophilic addition to deoxygenated platelet surfaces, or metallation. The graphitic platelets can be processed prior to being contacted with a functionalizing agent. Such processing may include dispersing the platelets in a solvent. In some instances the platelets may then be filtered and dried prior to contact. One particularly useful type of functional group is the carboxylic acid moieties, which naturally exist on the surfaces of NGPs if they are prepared from the acid intercalation route discussed earlier. If carboxylic acid functionalization is needed, the NGPs may be subjected to chlorate, nitric acid, or ammonium persulfate oxidation.

(31) Carboxylic acid functionalized graphitic platelets are particularly useful because they can serve as the starting point for preparing other types of functionalized NGPs. For example, alcohols or amides can be easily linked to the acid to give stable esters or amides. If the alcohol or amine is part of a di- or poly-functional molecule, then linkage through the O- or NH-leaves the other functionalities as pendant groups. These reactions can be carried out using any of the methods developed for esterifying or aminating carboxylic acids with alcohols or amines as known in the art. Examples of these methods can be found in G. W. Anderson, et al., J. Amer. Chem. Soc. 96, 1839 (1965), which is hereby incorporated by reference in its entirety. Amino groups can be introduced directly onto graphitic platelets by treating the platelets with nitric acid and sulfuric acid to obtain nitrated platelets, then chemically reducing the nitrated form with a reducing agent, such as sodium dithionite, to obtain amino-functionalized platelets.

(32) 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:

Example 1: Suspensions Containing Isolated Graphene and Graphene Oxide Sheets from Flake Graphite Via Polypropylene Powder-Based Carrier

(33) In an experiment, 1 kg of polypropylene pellets, 50 grams of flake graphite, 50 mesh (average particle size 0.18 mm; Asbury Carbons, Asbury N.J.) and 250 grams of magnetic stainless steel pins (Raytech Industries. Middletown Conn.) were placed in a ball mill container. The ball mill was operated at 300 rpm for 4 hours. The container lid was removed and stainless steel pins were removed via a magnet. The polymer carrier material was found to be coated with a dark carbon layer. Carrier material was placed over a 50 mesh sieve and a small amount of unprocessed flake graphite was removed.

(34) Subsequently, two separate procedures were conducted to produce suspensions containing pristine graphene sheets or graphene oxide sheets dispersed in a liquid medium. One procedure involved dispersing the coated carrier material in NMP and subjecting the suspension to mechanical shearing for obtaining a suspension containing isolated graphene sheets dispersed in NMP.

(35) The other procedure entailed immersing graphene-coated PP particles in a hydrogen peroxide-water solution (30% H.sub.2O.sub.2 in water) and implementing an ultrasonication tip in the solution to concurrently oxidize and separate graphene sheets from the carrier particles for 15 minutes to 2 hours. The products were graphene oxide sheets, having an oxygen content from 5 to 35% by weight, dispersed in a water solution.

(36) Although polypropylene (PP) is herein used as an example, the carrier material for making isolated graphene oxide sheets is not limited to PP or any polymer (thermoplastic, thermoset, rubber, etc.). The carrier material can be a glass, ceramic, metal, or other organic material, provided the carrier material is hard enough to peel off graphene sheets from the graphitic material (if the optional impacting balls are not present).

Example 2: Graphene Oxide from Expanded Graphite Via ABS Polymer

(37) In an experiment, 100 grams of ABS pellets, as solid carrier material particles, were placed in a 16 oz plastic container along with 5 grains of natural graphite. This container was placed in an acoustic mixing unit (Resodyn Acoustic mixer), and processed for 30 minutes. After processing, carrier material was found to be coated with a thin layer of graphene.

(38) Graphene-coated carrier particles were then immersed in a mixture of sulfuric acid and potassium permanganate (graphene/sulfuric acid/potassium permanganate ratio of 1.0/1.0/0.5) and subjected to ultrasound energy to speed oxidation of graphene and separation of resulting GO sheets from the ABS particles. The solution was filtered using an appropriate filter and washed with distilled water. Subsequent to washing, filtrate was dried in a vacuum oven set at 60° C. for 2 hours.

(39) It may be noted that the conventional Hummer's method typically requires the use of 24 mL (44 grams) of sulfuric acid and 5 grams of potassium permanganate to oxidize 1 grain of natural graphite. The required ratio was 1.0/44/5; i.e. significantly higher amounts of chemicals were used. Additionally, 15-30% of the natural graphite was “eaten away” during the GO production process using the Hummer's method.

Example 3: Functionalized Graphene from Mesocarbon Microbeads (MCMBs) Via PLA

(40) In one example, 100 grams of PLA pellets (carrier material), 2 grams of MCMBs (China Steel Chemical Co., Taiwan), and 2 grams of carboxylic acid (an oxidizing agent) were placed in a vibratory ball mill, which also contains particles of magnetic stainless steel impactor and processed for 1 hour to obtain graphene oxide-coated PLA particles. Subsequently. DETA was added and the material mixture was processed for an additional 1 hour. The vibratory mill was then opened and the carrier material was found to be coated with a brown-color coating of graphene oxide. The magnetic steel particles were removed with a magnet. The graphene oxide-coated carrier material particles were rinsed with isopropyl alcohol and then water. The slurry containing graphene-coated PLA particles was subjected to mechanical shearing using a rotary-blade mixer for 0.5 hours to separate GO sheets from PLA particles. Then, PLA particles were removed via centrifugation. The remaining suspension contained GO dispersed in water.

(41) In separate experiments, the following functional group-containing species were introduced to the graphene oxide sheets produced: an amino acid, sulfonate group (—SO.sub.3H), 2-azidoethanol, polyamide (caprolactam), and aldehydic group. In general, these functional groups were found to impart significantly improved interfacial bonding between resulting graphene oxide sheets and epoxy, polyester, polyimide, and vinyl ester matrix materials to make stronger polymer matrix composites. The interfacial bonding strength was semi-quantitatively determined by using a combination of short beam shear test and fracture surface examination via scanning electronic microscopy (SEM). Non-functionalized graphene sheets tend to protrude out of the fractured surface without any residual matrix resin being attached to graphene sheet surfaces. In contrast, the fractured surface of composite samples containing functionalized graphene oxide sheets do not exhibit any bare graphene sheets; any what appears to be graphene sheets were completely embedded in a resin matrix.

Example 4. Graphene Oxide from Highly Oriented Pyrolytic Graphite (HOPG) Via Glass Beads and SPEX Mill

(42) In an experiment, 5 grams of glass beads were placed in a SPEX mill sample holder (SPEX Sample Prep, Metuchen, N.J.) along with 0.25 grams of HOPG derived from graphitized polyimide. The SPEX mill was operated for 10 minutes. After operation, the contents of the sample holder were transferred to a water bath and subjected to ultrasonication, which helped to separate graphene sheets from glass bead surfaces. The glass bead particles were then manually removed from the suspension and, creating a water suspension of graphene. This suspension was determined to be a mixture of single layer graphene (86%) and few layer graphene dispersed in water. The sample was divided into two parts. In one part, hydrogen peroxide-water solution was added into the suspension to obtain a concentration of 30% H.sub.2O.sub.2 in water and the graphene sheets were oxidized for 1 hour to produce graphene oxide suspension.

Example 5: Metal Particles as the Carrier Material

(43) In one example, 100 grams of tin (45 micron, 99.9% purity, Goodfellow Inc.; Coraopolis, Pa.) was mixed with 10 grams of vein graphite (40 mesh size, Asbury Carbons, Asbury N.J.) and loaded into a vibratory mill. The material was processed for 2 hours, and the vibratory mill was opened. The tin powder was found to be coated with few-layer graphene sheets. These pellets were removed from the mill and poured into a hydrogen peroxide-water solution (30% H.sub.2O.sub.2 in water), subjected to ultrasonication. Tin particles were easily removed from the suspension that contains pristine graphene sheets dispersed in the water solution. A small amount of the produced suspension was subjected to filtration using a Teflon membrane as a filter. The specific surface area of the resulting graphene oxide sheets was measured via nitrogen adsorption BET. The specific surface area was found to be approximately 540 m.sup.2/g, indicating that the material was mostly few-layer graphene. A similar procedure was conducted using zinc particles as the solid carrier material.

Example 6: Graphene and Graphene Oxide from Natural Graphite Particles Via Polyethylene (PE) Beads and Ceramic Impacting Balls

(44) In an experiment, 0.5 kg of PE beads (as a solid carrier material), 50 grams of natural graphite (source of graphene sheets) and 250 grams of zirconia powder (impacting balls) were placed in containers of a planetary ball mill. The ball mill was operated at 300 rpm for 4 hours. The container lid was removed and zirconia beads (different sizes and weights than graphene-coated PE beads) were removed through a vibratory screen. The polymer carrier material was found to be coated with a dark carbon layer. Carrier material was placed over a 50 mesh sieve and a small amount of unprocessed flake graphite was removed. A coated carrier material sample was then dispersed into an organic solvent, DMF. The suspension was then subjected to ultrasonication for 0.5 hours to separate graphene sheets from PE particles. The PE particles were removed via centrifugation, resulting in a graphene suspension. A small sample of the suspension was removed and dried to obtain isolated graphene sheets (>95% single-layer graphene).