Abstract:
Existing methods of producing high quality graphene/graphite oxides are generally accomplished by exfoliating graphite into flakes and oxidizing the graphite flakes with strong oxidizers under extreme conditions and require careful purification. The oxidizers are typically strong acids used in high concentrations at elevated temperatures requiring complicated purification processes to yield oxidized graphene/sheets. The existing processes can cost up to $12,000/gram. This invention uses a mild oxidant combined with mechanical processing where the sole products are oxidized graphite/graphene and water without the need for further purification.

Description:
TECHNICAL FIELD OF THE INVENTION 
     The present invention relates in general to the field of graphene manufacturing, and more particularly, to large scale oxidized graphene production for industrial applications. 
     BACKGROUND OF THE INVENTION 
     Without limiting the scope of the invention, its background is described in connection with composite materials. 
     U.S. Pat. No. 8,580,132, issued to Lin, et al., is directed to a method for making strip shaped graphene layer. Briefly, these inventors describe a method for making a strip shaped graphene layer that includes the steps of: providing a graphene film on a surface of a substrate, drawing a carbon nanotube film composite is disposed on the graphene film, partly removing the polymer material to expose the plurality of carbon nanotube segments, etching the plurality of carbon nanotube segments and the graphene film covered by the plurality of carbon nanotube segments, and removing the remained polymer material to obtain the strip shaped graphene layer. 
     U.S. Pat. No. 8,216,541, issued to Jang, et al. is directed to a process for producing dispersible and conductive nano-graphene platelets from non-oxidized graphitic materials. Briefly, these inventors are said to teach a process for producing nano-graphene platelets (NGPs) that are both dispersible and electrically conducting. The process is said to includes: (a) preparing a pristine NGP material from a graphitic material; and (b) subjecting the pristine NGP material to an oxidation treatment to obtain the dispersible NGP material, wherein the NGP material has an oxygen content no greater than 25% by weight. The conductive NGPs are said to find applications in transparent electrodes for solar cells or flat panel displays, additives for battery and supercapacitor electrodes, conductive nanocomposite for electromagnetic wave interference (EMI) shielding and static charge dissipation. 
     United States Patent Publication No. 20120298620, filed by Jiang, et al., is directed to a method for making graphene composite structure. Briefly the method is said to include providing a metal substrate including a first surface and a second surface opposite to the first surface, growing a graphene film on the first surface of the metal substrate by a CVD method, providing a polymer layer on the graphene film and combining the polymer layer with the graphene film, and forming a plurality of stripped electrodes by etching the metal substrate from the second surface. 
     Finally, United States Patent Publication No. 20120228555, filed by Cheng, et al., is directed to a method of making graphene. Briefly, the application is said to disclose a method for making graphene by providing a starting material and heating the starting material for a time and to a temperature effective to produce graphene. In certain embodiments the applicants are said to use starting materials that include carbonaceous materials used in conjunction with, or comprising, sulfur, and essentially free of a transition metal. The graphene produced by the current method is said to be used to coat graphene-coatable materials. 
     SUMMARY OF THE INVENTION 
     In one embodiment, the present invention includes a method of making graphene oxide, comprising: placing graphite into water containing and an oxidizing agent, wherein said oxidizing agent has an oxidation potential less than 2V; mechanically exfoliating graphite into graphene flakes; and separating the graphene flakes from the water, wherein the graphene flakes are substantially flat. In one aspect, the step of mechanically exfoliating graphite into graphene flakes in done in a stirred media mill. In another aspect, the step of mechanically exfoliating graphite into graphene flakes in done in a stirred media mill, and the stirred media mill is an Attrition mill, a dry grind Attritor, a wet grind Attritor, a regular speed Attritor, and a high speed Attritor or a Attritor. In another aspect, the method produces an output that is substantially limited to substantially flat graphene flakes and water. In another aspect, the oxidizing agent is selected from at least one of CO 2 , O 2 , steam, N 2 O, NO, NO 2 , O 3 , ClO 2 , or H 2 O 2 , at a concentration wherein the oxidation potential is less than 2V. In another aspect, the oxidizing agent is aqueous or non-aqueous. In another aspect, the pH of the water containing the oxidized graphene flakes is from pH 2 to pH 9. In another aspect, the pH of the resulting water graphene flakes mixture is about 7. In another aspect, the graphene flakes are oxidized from 1% to 15%. In another aspect, the method further comprises the step of precipitating any remaining graphite by increasing the pH of the mixture above pH 9, or below a pH of 3. 
     Yet another embodiment of the present invention includes a method of making graphene flakes, comprising: placing graphite into water containing and an oxidizing agent, wherein said oxidizing agent is at a concentration at which it has an oxidation potential less than 2V; mechanically exfoliating graphite into graphene flakes; and separating the graphene flakes from the water, wherein the graphene flakes are substantially flat. In one aspect, the step of mechanically exfoliating graphite into graphene flakes in done in a stirred media mill. In another aspect, the step of mechanically exfoliating graphite into graphene flakes in done in a stirred media mill, and the stirred media mill is an Attrition mill, a dry grind Attritor, a wet grind Attritor, a regular speed Attritor, and a high speed Attritor or a Attritor. In another aspect, the method produces an output that is substantially limited to substantially flat graphene flakes and water. In another aspect, the oxidizing agent is selected from at least one of CO 2 , O 2 , steam, N 2 O, NO, NO 2 , O 3 , ClO 2 , or H 2 O 2 , at a concentration wherein the oxidation potential is less than 2V. In another aspect, the oxidizing agent is aqueous or non-aqueous. In another aspect, the pH of the water containing the oxidized graphene flakes is from pH 2 to pH 9. In another aspect, the pH of the resulting water graphene flakes mixture is about 7. In another aspect, the graphene flakes are oxidized from 1% to 15%. In another aspect, the method further comprises the step of precipitating any remaining graphite by increasing the pH of the mixture above pH 9, or below a pH of 3. 
     Yet another embodiment of the present invention includes a graphene oxide made by a method comprising: placing graphite into water containing and an oxidizing agent, wherein said oxidizing agent has an oxidation potential less than 2V; mechanically exfoliating graphite into graphene flakes; and separating the graphene flakes from the water, wherein the graphene flakes are substantially flat. 
     Yet another embodiment of the present invention includes a graphene flakes made by a method comprising: placing graphite into water containing and an oxidizing agent, wherein said oxidizing agent is at a concentration at which it has an oxidation potential less than 2V; mechanically exfoliating graphite into graphene flakes; and separating the graphene flakes from the water, wherein the graphene flakes are substantially flat. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     While the making and using of various embodiments of the present invention are discussed in detail below, it should be appreciated that the present invention provides many applicable inventive concepts that can be embodied in a wide variety of specific contexts. The specific embodiments discussed herein are merely illustrative of specific ways to make and use the invention and do not delimit the scope of the invention. 
     To facilitate the understanding of this invention, a number of terms are defined below. Terms defined herein have meanings as commonly understood by a person of ordinary skill in the areas relevant to the present invention. Terms such as “a”, “an” and “the” are not intended to refer to only a singular entity, but include the general class of which a specific example may be used for illustration. The terminology herein is used to describe specific embodiments of the invention, but their usage does not delimit the invention, except as outlined in the claims. 
     Graphene is one of the strongest materials ever tested. Measurements have shown that graphene has a breaking strength 200 times greater than steel, with a tensile modulus (stiffness) of 1 TPa (150,000,000 psi). An atomic Force Microscope (AFM) has been used to measure the mechanical properties of a suspended graphene sheet. Graphene sheets, held together by van der Waals forces, were suspended over SiO 2  cavities where an AFM tip was probed to test its mechanical properties. Its spring constant was in the range 1-5 N/m and the Young&#39;s modulus was 0.5 TPa (500 GPa) thereby demonstrating that graphene can be mechanically very strong and rigid. Despite these nanoscale mechanical properties, Graphene has not been able to transition to a macro-scale mechanical structure. Various research institutes have loaded plastic/polymer/epoxy with carbon nanotubes (CNT), graphene flakes (GF), and graphene oxide (GO) and seen up to a 200% increase in tensile strength in the loaded plastic/polymer/epoxy. The process of producing a loaded plastic/polymer/epoxy does not translate to a commercially viable composite structure. 
     The inability to translate the technology to a viable composite structure is a combination of technical issues and cost factors. The technical limitation includes stochastic processes in the curing of the plastic/polymer/epoxy that results in random shrinkage phenomena that is exacerbated in larger composite structures/devices. The distribution of the laded mechanical structural materials (CNT, GF, and GO) is non-uniform creating weak regions and failure points in the loaded plastic/polymer/epoxy material. The superior properties of graphene compared to polymers are also reflected in polymer/graphene nanocomposites. Polymer/graphene nanocomposites show superior mechanical, thermal, gas barrier, electrical and flame retardant properties compared to the neat polymer. Improvement in the physicochemical properties of the nanocomposites depends on the distribution of graphene layers in the polymer matrix as well as interfacial bonding between the graphene layers and polymer matrix. The combined low yield and high cost of the CNT, GF, and GO materials makes the approach not viable. Interfacial bonding between graphene and the host polymer dictates the final properties of the graphene reinforced polymer nanocomposite. 
     Graphene is an allotrope of carbon. Graphene&#39;s structure is a one-atom-thick planar sheet of sp 2 -bonded carbon atoms that are densely packed in a honeycomb or hexagonal crystal lattice. The carbon-carbon bond length in graphene is about 1.42Å. Graphene sheets stack to form graphite with an inter-planar spacing of 3.35Å. Multiple graphene sheets/flakes are bonded together by van der Waals forces. 
     Graphene can be oxidized by a number of processes including thermal, chemical or chemical-mechanical. Reduction of graphite oxide monolayer films by hydrazine, annealing in argon/hydrogen was reported to yield graphene films of low quality. Graphene can be produced in significant quantities from microcrystalline graphite that is treated with a mixture of acids such as sulfuric, nitric, and other oxidizing chemical in combination mechanical and/or thermal energy elements. This processing will produce graphene flakes ranging from a few nanometers to tens of microns depending and the specific processing environment. If one uses a mill in conjunction with an oxidizing agent the dwell time in the mill will determine the size of the flake of graphene. In general, the longer the processing time in the mill the smaller the graphene flake. The oxidizing process produces a carboxyl group on the perimeter of the graphene flake. The resulting graphene flakes can be on the order of 5 Å in thickness and can be suspended in a number of solvents with a dielectric constant greater than 32. These include, but are not limited to N-methylpyrrolidone, acetonitrile, dimethyl sulfoxide, propylene carbonate, water, and formamide. 
     Using strong oxidizers such as sulfuric and nitric acids result in a graphene oxide with texturing and folding of the individual sheets/flakes and the loss of carboxylic group functionality. The products of this synthesis technique are graphite/graphene oxide. Separating the strong oxidizer reactants from the products is a time consuming and expensive process. The graphene/graphite oxide, without the strong oxidizers, has an oxygen content ranging between 1 and 50 wt %. Graphite/Graphene oxide contains oxygen attached to the layers as both epoxy bridges and hydroxyl groups (—COOH). The oxidized graphene/graphite is hydrophilic. Analyses show that the graphene particle/flake is fully oxidized with —COOH groups along the edges. 
     Production of graphene oxides requires the use of oxidizing agent in conjunction with mechanical energy (milling). Mechanical processing can be energy efficient and prevents the decomposition of the chemical functionalities that can occur during thermal processing. Oxidizing agents can either be aqueous or non-aqueous graphene/graphite. Published literature had identified urea hydrogen peroxide adduct (UHPA) for use in solvent free and non-aqueous chemical reactions as a solid source of hydrogen peroxide. UHPA provides a basis for anhydrous, solid delivery system for H 2 O 2 . 
     Oxidized graphene can be produced in a wide number of mechanical milling apparatus that create the necessary mechanical energy. The current device being used is an attrition mill or Attritor. The Attritor is a grinding mill containing internally agitated media such as balls, cylinders or pebbles. It has been generically referred to as a “stirred ball mill.” There are quite a few types of Attritors; which can be categorized as dry grind Attritors, wet grind Attritors, regular speed Attritors, and high speed Attritors. 
     A useful and simple equation describing the grinding momentum is M×V (mass×velocity), which enables a determination of how the Attritor fits into the family of mills. For example, ball mills use large media, normally ½″ or larger, and run at a low (10-50) rpm. The other mills, such as sand, bead, and horizontal, use smaller media from 0.3 mm to 2 mm, but run at a very high rpm (roughly 800-1200). High-speed dispersers with no media run even faster rpm (1200-1800). 
     The most important aspect for the Attritor is that the power input is directly used for agitating the media to achieve grinding and is generally not used for rotating or vibrating a large, heavy tank in addition to the media. 
     The present inventors have found that for efficient, fine grinding, both impact action and shearing force are required. When wet grinding in the Attritor as the balls (media) in their random movement are spinning in different rotation and, therefore, exerting shearing forces on the adjacent slurry. As a result, both liquid shearing force and media impact force are present. Such combined shearing and impact results in size reduction as well as good dispersion. 
     This invention includes a method for low cost, mass-production of a partially oxidized to fully oxidized graphite/graphene material using mechanical processing (Attritor Mill) in conjunction with a oxidizing agent with an oxidation potential less than 2V, that produces oxidized graphite/graphene and water as its sole product or output requiring no additional post processing for purification to create a suspension, concentration, or dried powder. Directly milling of graphite powder in an aqueous oxidizing agent with an oxidation potential less than 2V, without the need for concentrated acid, exfoliation, or filtration/purification process to produce high quality oxidized graphene/graphite was demonstrated. The use of an aqueous or non-aqueous oxidizing agent with an oxidation potential less than 2V produces oxidized graphene with no distortion or texturing. Prior to this technological advancement the use of a strong oxidizing agent such as manganic or nitric acids with an oxidation potential &gt;2V produced oxidized graphite/graphite that is both expensive and highly textured. Textured graphene oxide produces significant problems when depositing the graphene oxide, using the graphene oxide in a suspension or as an additive to other either aqueous or non-aqueous media. 
     Larger scale milling experiments were performed in an Attritor with 6 lbs (2.7 Kg) (or ˜2,600 stainless steel balls) of 0.25″ diameter stainless steel balls weighing 1 g each. Typically, graphite (TC306, 30 g) was milled with an aqueous or non-aqueous oxidizing agent with an oxidation potential less than 2V such as 35% hydrogen peroxide (H 2 O 2 ) (96 ml or 107 g) or aqueous or non-aqueous media caring ozone O 3 . Milling was carried out for 120 minutes at 350 RPM. The mechanical agitation supplied by the Attritor is sufficient to prevent agglomeration of the graphite or the graphite from adhering to the milling balls or tank during the oxidation process. Mills where the tank is agitated (such as a shaker mill, planetary mill, or pebble mill), in general, do not effectively agitate liquid/solid mixtures with high solid content. These mills cannot prevent agglomeration or the graphite from adhering to the milling balls and tank. Once the milling balls are entrained in the graphite/oxidant mixture the process must be stopped and the mechanical system cleaned. No additional cooling, processing, or purification is required to produce the oxidized graphite/graphene and water slurry. The pH of the water containing the oxidized graphite/graphene can range from 2 to 9 while maintaining the suspension of the media the pH of the resulting water/graphene mixture is typically is 7. The resulting graphite/graphene is oxidized from 1% to 15% depending on and a function of the quantity of oxidizing agent with an oxidation potential less than 2V used in the process. The optimal amount of oxidizing agent with an oxidation potential less than 2V, is 4 to 1 molar equivalent weight of graphite to oxidizing agent with an oxidation potential less than 2V, to produce 15% oxidation of the graphite. Larger portions of oxidizing agent with an oxidation potential less than 2V, may be used but it does not increase the percentage of oxidation. Lower molar equivalent weights of oxidizing agent with an oxidation potential less than 2V, may be used resulting in a lower percentage of graphite a 10 to 1 or 15 to 1 molar equivalent weight of graphite to oxidizing agent with an oxidation potential less than 2V will result in a graphite oxidation percentage ranging 8% to 5%. In addition, graphene with oxidation percentages less than 5% are not dispersible in a polar solvent and/or depositable. 
     The chemical reactions (—COOH) of the edge carbons would preserve the graphene structure. Oxidized graphite produced by this method is very hydrophilic and is easily suspended in an aqueous solution. The oxidized graphite can be kept in solution until varying the pH of the solution. If the solutions pH is increased above 9 or decreased below 3 the oxidized graphite suspension will precipitate out of solution. The acidic precipitation process is reversible and the oxidized graphite will return to a suspension as the pH is increased above 3. As the pH is increased the smaller oxidized graphite flakes return into the suspension first. This phenomenon can be used as a method to both purify the oxidized graphite flakes and separate the flakes by size. Basic precipitation does not return to a suspension as the pH is decreased to a pH of 7. 
     The resulting suspension had the pH decreased to precipitate the suspension and dried. The precipitate can then be washed with deionized (DI) water. The DI water also raises the pH of the material and re-suspends the oxidized graphite/graphene. The suspended oxidized graphite/graphene can easily be placed in almost any solution as long as the pH is between 3 and 9. The resulting suspension can be deposited by any number of processes including electroreduction, spin, spray pyrolysis, dip, Langmuir-Blodgett or other coating processes plus used as an additive for loaded materials such as plastics, epoxies, metals ceramics and paints. The oxidation of the graphite/graphene was validated by the aqueous suspension and acid based precipitation then resuspended. The purified graphite/graphene oxide without texturing was then deposited on a surface and evaluated for its electrical, optical, and mechanical properties. This is a 100 fold improvement over prior developments and over a 100,000 fold improvement over commercially available oxidized graphite/graphene technology enabling graphene to be viable for industrial applications. Where the resulting oxidized graphene/graphite has been oxidization percentage between 5 and 25 percent at costs less than significantly less than current methods. 
     It is contemplated that any embodiment discussed in this specification can be implemented with respect to any method, kit, reagent, or composition of the invention, and vice versa. Furthermore, compositions of the invention can be used to achieve methods of the invention. 
     It will be understood that particular embodiments described herein are shown by way of illustration and not as limitations of the invention. The principal features of this invention can be employed in various embodiments without departing from the scope of the invention. Those skilled in the art will recognize, or be able to ascertain using no more than routine experimentation, numerous equivalents to the specific procedures described herein. Such equivalents are considered to be within the scope of this invention and are covered by the claims. 
     All publications and patent applications mentioned in the specification are indicative of the level of skill of those skilled in the art to which this invention pertains. All publications and patent applications are herein incorporated by reference to the same extent as if each individual publication or patent application was specifically and individually indicated to be incorporated by reference. 
     The use of the word “a” or “an” when used in conjunction with the term “comprising” in the claims and/or the specification may mean “one,” but it is also consistent with the meaning of “one or more,” “at least one,” and “one or more than one.” The use of the term “or” in the claims is used to mean “and/or” unless explicitly indicated to refer to alternatives only or the alternatives are mutually exclusive, although the disclosure supports a definition that refers to only alternatives and “and/or.” Throughout this application, the term “about” is used to indicate that a value includes the inherent variation of error for the device, the method being employed to determine the value, or the variation that exists among the study subjects. 
     As used in this specification and claim(s), the words “comprising” (and any form of comprising, such as “comprise” and “comprises”), “having” (and any form of having, such as “have” and “has”), “including” (and any form of including, such as “includes” and “include”) or “containing” (and any form of containing, such as “contains” and “contain”) are inclusive or open-ended and do not exclude additional, unrecited elements or method steps. In embodiments of any of the compositions and methods provided herein, “comprising” may be replaced with “consisting essentially of” or “consisting of”. As used herein, the phrase “consisting essentially of” requires the specified integer(s) or steps as well as those that do not materially affect the character or function of the claimed invention. As used herein, the term “consisting” is used to indicate the presence of the recited integer (e.g., a feature, an element, a characteristic, a property, a method/process step or a limitation) or group of integers (e.g., feature(s), element(s), characteristic(s), propertie(s), method/process steps or limitation(s)) only. 
     The term “or combinations thereof” as used herein refers to all permutations and combinations of the listed items preceding the term. For example, “A, B, C, or combinations thereof” is intended to include at least one of: A, B, C, AB, AC, BC, or ABC, and if order is important in a particular context, also BA, CA, CB, CBA, BCA, ACB, BAC, or CAB. Continuing with this example, expressly included are combinations that contain repeats of one or more item or term, such as BB, AAA, AB, BBC, AAABCCCC, CBBAAA, CABABB, and so forth. The skilled artisan will understand that typically there is no limit on the number of items or terms in any combination, unless otherwise apparent from the context. 
     As used herein, words of approximation such as, without limitation, “about”, “substantial” or “substantially” refers to a condition that when so modified is understood to not necessarily be absolute or perfect but would be considered close enough to those of ordinary skill in the art to warrant designating the condition as being present. The extent to which the description may vary will depend on how great a change can be instituted and still have one of ordinary skilled in the art recognize the modified feature as still having the required characteristics and capabilities of the unmodified feature. In general, but subject to the preceding discussion, a numerical value herein that is modified by a word of approximation such as “about” may vary from the stated value by at least ±1, 2, 3, 4, 5, 6, 7, 10, 12 or 15%. 
     All of the compositions and/or methods disclosed and claimed herein can be made and executed without undue experimentation in light of the present disclosure. While the compositions and methods of this invention have been described in terms of preferred embodiments, it will be apparent to those of skill in the art that variations may be applied to the compositions and/or methods and in the steps or in the sequence of steps of the method described herein without departing from the concept, spirit and scope of the invention. All such similar substitutes and modifications apparent to those skilled in the art are deemed to be within the spirit, scope and concept of the invention as defined by the appended claims.