Abstract:
A method of manufacturing a graphene oxide composite fiber includes forming a supporting fiber having positive charges thereon; forming a solution containing graphene oxide having a negative charge thereon to provide a graphene oxide-containing solution; coating the supporting fiber with the graphene oxide-containing solution; and permitting self-assembly of the graphene oxide of the graphene oxide -containing solution and the supporting fiber by an attraction between the positive and negative charges to form the graphene oxide composite fiber. The method may include coating the supporting fiber with a solution containing a material having an amine group to provide a positive surface charge on the supporting fiber. The method may include reducing the graphene oxide composite fiber to a graphene composite fiber, such as by one of a thermal reduction method, an optical reduction method, and a chemical reduction method. Large-area graphene fiber structures having high strength, flexibility, and porosity are enabled.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This U.S. non-provisional patent application claims priority under 35 U.S.C. §119 to Korean Patent Application No. 10-2012-0039129, filed on Apr. 16, 2012, the entirety of which is incorporated by reference herein. 
     BACKGROUND 
     The inventive concept relates to methods of manufacturing a fiber and, more particularly, to methods of manufacturing a graphene fiber. 
     Fibers have been increasingly demanded with an increase of population and development of an industry. New fibers having more excellent function than a natural fiber have been increasingly demanded. Dupont Co. (U.S.A) announced a new synthetic fiber so-called ‘Nylon’ in 1938. Thereafter, a polyester fiber, an acrylic fiber, and a polyurethane fiber have been developed. Recently, various researches have been conducted for high performance and high functional fibers and nano fibers using new materials overcoming performance limitation of existing materials. 
     Graphene includes carbon atoms constituting hexagonal shapes. Each of hexagonal shapes may consist of six carbon atoms. The graphene has a single-layered structure where the hexagonal shapes are two-dimensionally arranged. The structure of the graphene may be similar to that of graphite consisting of plates three-dimensionally stacked. The plates of the graphite may be divided to form the graphene. For example, the plates of the graphite may be divided using a scotch tape. The graphene may have excellent properties such as a surface area (e.g., about 2650 m 2 /g) two times or more than active carbon, a high elasticity force (e.g., about 1 TPa), and chemical stability as well as electric properties such as high conductivity (e.g., 1×10 −6  Ωcm) and high electron mobility. Recently, it has been announced that the graphene also has antibiosis removing bacteria. Thus, the graphene have been developed in various fields such as a display, a cathode material of a lithium-ion battery, an electrode material of an electric double-layered capacitor, environmental filters, and biomaterials. 
     SUMMARY 
     Embodiments of the inventive concept may provide methods of easily manufacturing a graphene fiber having a large area. 
     According to embodiments of the inventive concept, a method of manufacturing a graphene fiber includes: forming a supporting fiber; forming a graphene oxide-containing solution; coating the supporting fiber with the graphene oxide-containing solution to form a graphene oxide composite fiber; and separating the supporting fiber from the graphene oxide composite fiber. 
     In some embodiments, the supporting fiber may be a polymer fiber. A polymer solution may be electro-spun on a collector to form the polymer fiber. The polymer solution may be formed by dissolving a polymer material in a solvent. The polymer fiber may have insolubility to by using an ammonia solution or a sodium hydroxide solution. The polymer fiber may be coated with an amine group-containing solution so that the polymer fiber may have a high reactivity with respect to graphene oxide. The amine group-containing solution may include at least one of bovine serum albumin (BSA), amyloid beta, poly-D-lysine, poly-L-lysine, and chitosan. 
     In other embodiments, forming the graphene oxide composite fiber may include: self-assembling graphene oxide of the graphene oxide-containing solution and the supporting fiber. 
     In still other embodiments, separating the supporting fiber from the graphene oxide composite fiber may include: thermally treating or chemically melting the graphene oxide composite fiber. The thermal treating may be performed at a temperature within a range of about 25 degrees Celsius to about 3000 degrees Celsius (particularly, a range of about 100 degrees Celsius to about 3000 degrees Celsius) for a thermal treating time within a range of about 1 minute to about 24 hours. The chemical melting may be performed using an acid-based solvent. The acid-based solvent may include at least one of acetic acid (C 2 H 4 O 2 ), formic acid (HCOOH), citric acid (C 6 H 8 O 7 ), hydrochloric acid (HCl), sulfuric acid (H 2 SO 4 ), nitric acid (HNO 3 ), perchloric acid (HClO 4 ), fluoric acid (HF), phosphoric acid (H 3 PO 4 ), chromic acid (HCrO 4 ), CH 3 CH 2 COOH, oxalic acid, glycol acid, tartaric acid (C 4 H 5 O 6 ), acetone, and toluene. 
     In even other embodiments, the method may further include: reducing the graphene oxide composite fiber to a graphene composite fiber. The graphene composite fiber may be reduced from the graphene oxide composite fiber by a thermal reduction method, an optical reduction method, or a chemical reduction method. The thermal reduction method may be performed at a temperature within a range of about 40 degrees Celsius to about 3000 degrees Celsius. The optical reduction method may use light of a wavelength within a range of about 200 nm to about 1500 nm. The chemical reduction method may use a chemical reagent including at least one of hydriodic acid with acetic acid (HI—AcOH), hydrazine (N 2 H 4 ), dimethyl hydrazine (C 2 H 8 N 2 ), sodium borohydride (NaBH 4 ), sodium hydroxide (NaOH), ascorbic acid, glucose, hydrogen sulfide (H 2 S), hydroquinone (C 6 H 4 (OH) 2 ), and sulfuric acid (H 2 SO 4 ). 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The inventive concept will become more apparent in view of the attached drawings and accompanying detailed description. 
         FIG. 1  is a flow chart illustrating a method of manufacturing a graphene fiber according to some embodiments of the inventive concept; 
         FIG. 2  is a flow chart illustrating a method of manufacturing a graphene fiber according to other embodiments of the inventive concept; 
         FIG. 3  is a schematic diagram illustrating an electro spinning apparatus according to embodiments of the inventive concept; 
         FIG. 4 , a schematic diagram illustrating a process of a method of manufacturing a graphene fiber according to embodiments of the inventive concept; 
         FIG. 5  is an enlarged view of a portion ‘A’ of  FIG. 4 ; 
         FIG. 6  is a schematic diagram illustrating a graphene oxide composite fiber according to embodiments of the inventive concept; 
         FIG. 7  is an enlarged view of a cross section taken along a long axis of a portion “B” of  FIG. 6 ; 
         FIG. 8  is a scanning electron microscope (SEM) photograph of a chitosan fiber formed by a first experimental example; and 
         FIG. 9  is a SEM photograph of a graphene oxide composite fiber formed by a first experimental example. 
     
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     The inventive concept will now be described more fully hereinafter with reference to the accompanying drawings, in which exemplary embodiments of the inventive concept are shown. The advantages and features of the inventive concept and methods of achieving them will be apparent from the following exemplary embodiments that will be described in more detail with reference to the accompanying drawings. It should be noted, however, that the inventive concept is not limited to the following exemplary embodiments, and may be implemented in various forms. Accordingly, the exemplary embodiments are provided only to disclose the inventive concept and let those skilled in the art know the category of the inventive concept. In the drawings, embodiments of the inventive concept are not limited to the specific examples provided herein and are exaggerated for clarity. 
     The terminology used herein is for the purpose of describing particular embodiments only and is not intended to limit the invention. As used herein, the singular terms “a,” “an” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. As used herein, the term “and/or” includes any and all combinations of one or more of the associated listed items. It will be understood that when an element is referred to as being “connected” or “coupled” to another element, it may be directly connected or coupled to the other element or intervening elements may be present. 
     Similarly, it will be understood that when an element such as a layer, region or substrate is referred to as being “on” another element, it can be directly on the other element or intervening elements may be present. In contrast, the term “directly” means that there are no intervening elements. It will be further understood that the terms “comprises”, “comprising,”, “includes” and/or “including”, when used herein, specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof. 
     Additionally, the embodiment in the detailed description will be described with sectional views as ideal exemplary views of the inventive concept. Accordingly, shapes of the exemplary views may be modified according to manufacturing techniques and/or allowable errors. Therefore, the embodiments of the inventive concept are not limited to the specific shape illustrated in the exemplary views, but may include other shapes that may be created according to manufacturing processes. Areas exemplified in the drawings have general properties, and are used to illustrate specific shapes of elements. Thus, this should not be construed as limited to the scope of the inventive concept. 
     It will be also understood that although the terms first, second, third etc. may be used herein to describe various elements, these elements should not be limited by these terms. These terms are only used to distinguish one element from another element. Thus, a first element in some embodiments could be termed a second element in other embodiments without departing from the teachings of the present invention. Exemplary embodiments of aspects of the present inventive concept explained and illustrated herein include their complementary counterparts. The same reference numerals or the same reference designators denote the same elements throughout the specification. 
       FIG. 1  is a flow chart illustrating a method of manufacturing a graphene fiber according to some embodiments of the inventive concept. 
     Referring to  FIG. 1 , first, a supporting fiber may be formed (S 10 ). The supporting fiber may be a polymer fiber. A polymer solution may be electro-spun on a collector to form the polymer fiber. The polymer solution may be formed by dissolving a polymer material in a solvent. 
     The polymer material may include at least one of polyamide-6, polyamide-6,6, polyurehthanes, polybenzimidazole, polyacrylonitrile, polyaniline (PANI), polyvinylcarbazole, polyacrylamide (PAAm), polyimide, poly-metaphenylene isophtalamides, polylactic-co-glycolic acid, poly caprolactone, polyglycolide, poly lactic acid, poly-3-hydroxylbutyrate, betaamyloid, collagen, fibrin, chitosan, and gelatin. 
     The solvent may include at least one of water, ethanol, methanol, acetone, phosphate buffered saline (PBS) buffer, acetic acid (C 2 H 4 O 2 ), formic acid (CH 2 O 2 ), hexafluoro-2-propanol ((CF 3 ) 2 CHOH), trifluoroaceticacid (C 2 HF 3 O 2 ), dichloromethane (CH 2 Cl 2 ), acetonitrile (C 2 H 3 N), benzene (C 6 H 6 ), 1-butanol (C 4 H 10 O), 2-butanol (C 4 H 10 O), 2-butanone (C 4 H 8 O), t-butyl alcohol (C 4 H 10 O), carbon tetrachloride (CCl 4 ), chlorobenzene (C 6 H 5 Cl), chloroform (CHCl 3 ), cyclohexane (C 6 H 12 ), 1,2-dichloroethane (C 2 H 4 Cl 2 ), dichlorobenzene, diethyl ether (C 4 H 10 O), diethylene glycol (C 4 H 10 O 3 ), diglyme (diethylene glycol, dimethyl ether) (C 6 H 14 O 3 ), 1,2-dimethoxy-ethane (glyme, DME, C 4 H 10 O 2 ), dimethylether (C 2 H 6 O), dimethyl-formamide (DMF, C 3 H 7 NO), dimethyl sulfoxide (DMSO, C 2 H 6 OS), dioxane (C 4 H 8 O 2 ), ethyl acetate (C 4 H 8 O 2 ), ethylene glycol (C 2 H 6 O 2 ), glycerin (C 3 H 8 O 3 ), heptanes (C 7 H 16 ), hexamethylphosphoramide (HMPA, C 6 H 18 N 3 OP), hexamethylphosphoroustriamide (HMPT, C 6 H 18 N 3 P), hexane (C 6 H 14 ), methyl t-butyl ether (MTBE, C 5 H 12 O), methylene chloride (CH 2 Cl 2 ), N-methyl-2-pyrrolidinone (NMP, CH 5 H 9 NO), nitromethane (CH 3 NO 2 ), pentane (C 5 H 12 ), petroleum ether (ligroine), 1-propanol (C 3 H 8 O), 2-propanol (C 3 H 8 O), pyridine (C 5 H 5 N), tetrahydrofuran (THF, C 4 H 8 O), toluene (C 7 H 8 ), triethyl amine (C 6 H 15 N), o-xylene (C 8 H 10 ), m-xylene (C 8 H 10 ), and p-xylene (C 8 H 10 ). 
     A concentration of the polymer material in the polymer solution may have a range of about 0.1 wt % (weight percent) to about 50 wt %. 
       FIG. 3  schematically illustrates an electro spinning apparatus used for forming the supporting fiber in the step S 10  of  FIG. 1 . 
     Referring to  FIG. 3 , a collector  25  may include a support part  24  and a form part  23 . The form part  23  may have a plate-shape or a circular shape. The collector  25  may include a conductive material or a non-conductive material. The conductive material may include at least one of gold, silver, aluminum, copper, stainless, palladium, platinum, silicon, poly-silicon, a conductive polymer, carbon nanotube, graphene, and indium tin oxide (ITO). The non-conductive material may include at least one of glass, quartz, acryl, a OHP film, polyethylene terephthalate (PET), polyethylene naphthalate (PEN), PES, PEEK, polyimide (PI), polynorbonene, polyarylate, polycarbonate (PC), PAR, PDMS, and non-woven fabric. 
     The method of forming the polymer fiber (i.e., the supporting fiber) will be described with reference to  FIG. 3 . The polymer solution may be provided into a cylinder  21  and then the polymer solution may be electro-spun on a surface of the form part  23  of the collector  25  through a nozzle  22  of the cylinder  21 . At this time, a specific voltage may be applied between the cylinder  21  and the collector  25 . Thus, the polymer fiber  26  may be formed on the surface of the form part  23  of the collector  25 . The polymer fiber  26  may have a diameter within a range of about 1 nm to about 100 μm. The nozzle  22  may be single-mode. If the nozzle  22  is the single mode, the nozzle  22  may have one hole. Alternatively, the nozzle  22  may be multi-mode. If the nozzle  22  is the multi-mode, the nozzle  22  may have two or more holes. For example, if the nozzle  22  is dual-mode, the nozzle  22  may have two holes having a first hole and a second hole surrounding the first hole. 
     The polymer fiber may have insolubility by using an ammonia solution or a sodium hydroxide (NaOH) solution. The polymer fiber may be coated with an amine group-containing solution, so that the polymer fiber may have a high reactivity with respect to a graphene oxide. The amine group-containing solution may include at least one of bovine serum albumin (BSA), amyloid beta, poly-D-lysine, poly-L-lysine, and chitosan. The polymer fiber may be separated from the collector  25  by a vacuum dry process. 
     Referring to  FIG. 1  again, a graphene oxide-containing solution may be formed (S 20 ). Graphene oxide particles may be dispersed in a solvent, thereby forming the graphene oxide-containing solution. 
     The solvent of the graphene oxide-containing solution may include at least one of water, acetic acid (C 2 H 4 O 2 ), acetone (C 3 H 6 O), acetonitrile (C 2 H 3 N), benzene (C 6 H 6 ), 1-butanol (C 4 H 10 O), 2-butanol (C 4 H 10 O), 2-butanone (C 4 H 8 O), t-butyl alcohol (C 4 H 10 O), carbon tetrachloride (CCl 4 ), chlorobenzene (C 6 H 5 Cl), chloroform (CHCl 3 ), cyclohexane (C 6 H 12 ), 1,2-dichloroethane (C 2 H 4 Cl 2 ), dichlorobenzene, dichloromethane (CH 2 Cl 2 ), diethyl ether (C 4 H 10 O), diethylene glycol (C 4 H 10 O 3 ), diglyme (diethylene glycol, dimethyl ether) (C 6 H 14 O 3 ), 1,2-dimethoxy-ethane (glyme, DME, C 4 H 10 O 2 ), dimethylether (C 2 H 6 O), dimethyl-formamide (DMF, C 3 H 7 NO), dimethyl sulfoxide (DMSO, C 2 H 6 OS), dioxane (C 4 H 8 O2), ethanol (C 2 H 6 O), ethyl acetate (C 4 H 8 O 2 ), ethylene glycol (C 2 H 6 O 2 ), glycerin (C 3 H 8 O 3 ), heptanes (C 7 H 16 ), hexamethylphosphoramide (HMPA, C 6 H 18 N 3 OP), hexamethylphosphoroustriamide (HMPT, C 6 H 18 N 3 P), hexane (C 6 H 14 ), methanol (CH 4 O), methyl t-butyl ether (MTBE, C 5 H 12 O), methylene chloride (CH 2 Cl 2 ), N-methyl-2-pyrrolidinone (NMP, CH 5 H 9 NO), nitromethane (CH 3 NO 2 ), pentane (C 5 H 12 ), petroleum ether (ligroine), 1-propanol (C 3 H 8 O), 2-propanol (C 3 H 8 O), pyridine (C 5 H 5 N), tetrahydrofuran (THF, C 4 H 8 O), toluene (C 7 H 8 ), triethyl amine (C 6 H 15 N), o-xylene (C 8 H 10 ), m-xylene (C 8 H 10 ), and p-xylene (C 8 H 10 ). 
     Referring to  FIG. 1 , a graphene oxide composite fiber may be formed (S 30 ). The graphene oxide composite fiber may be formed using the supporting fiber formed in the step S 10  and the graphene oxide-containing solution formed in the step S 20 . 
       FIG. 4  is a schematic diagram illustrating a process of a method of manufacturing a graphene fiber according to embodiments of the inventive concept, and  FIG. 5  is an enlarged view of a portion ‘A’ of  FIG. 4 .  FIG. 6  is a schematic diagram illustrating a graphene oxide composite fiber according to embodiments of the inventive concept, and  FIG. 7  is an enlarged view of a cross section taken along a long axis of a portion ‘B’ of  FIG. 6 . 
     The method of forming the graphene oxide composite fiber will be described in detail with reference to  FIGS. 4 to 7 . The supporting fiber  32  formed in the step S 10  may be provided into the graphene oxide-containing solution  31  formed in the step S 20 . As described above, the supporting fiber  32  may be the polymer fiber. And then a hydrogen chloride solution may be added to the graphene oxide-containing solution  31  to control a hydrogen ion concentration (pH) of the graphene oxide-containing solution  31  at or below about 5. Thus, the supporting fiber  32  having positive charges on a surface thereof and a graphene oxide  33  having negative charges may be self-assembled to form the graphene oxide composite fiber  34  of  FIGS. 5 and 6 . In a cross section of the graphene oxide composite fiber  34  as illustrated in  FIG. 7 , the graphene oxide composite fiber  34  may include the supporting fiber  32  and the graphene oxide  33  surrounding the supporting fiber  32 . 
     Referring to  FIG. 1 , the supporting fiber  32  may be separated from the graphene oxide composite fiber  34  formed in the step S 30  (S 40 ). As described above, the supporting fiber  32  may be the polymer fiber. The graphene oxide composite fiber  34  may be thermally treated or chemically melted so that the supporting fiber  32  may be separated from the graphene oxide composite fiber  34 . For example, the graphene oxide composite fiber  34  may be thermally treated at a temperature within a range of about 25 degrees Celsius to about 3000 degrees Celsius (particularly, a range of about 100 degrees Celsius to about 3000 degrees Celsius) for a thermal treating time within a range of about 1 minute to about 24 hours. The chemical melting of the supporting fiber  32  in the graphene oxide composite fiber  34  may be performed using an acid-based solvent. The acid-based solvent may include at least one of acetic acid (C 2 H 4 O 2 ), formic acid (HCOOH), citric acid (C 6 H 8 O 7 ), hydrochloric acid (HCl), sulfuric acid (H 2 SO 4 ), nitric acid (HNO 3 ), perchloric acid (HClO 4 ), fluoric acid (HF), phosphoric acid (H 3 PO 4 ), chromic acid (HCrO 4 ), CH 3 CH 2 COOH, oxalic acid, glycol acid, tartaric acid (C 4 H 5 O 6 ), acetone, and toluene. 
     The graphene fiber may be manufactured by the method described above. The graphene fiber may have a structure including an outer surface and an inner surface surrounded by the outer surface. And the graphene fiber may include graphene oxide particles between the outer surface and the inner surface. An inner diameter of the graphene fiber may have a range of about 1 nm to 100 μm. A length of the graphene fiber may be several nanometers or more. 
       FIG. 2  is a flow chart illustrating a method of manufacturing a graphene fiber according to other embodiments of the inventive concept. 
     Referring to  FIG. 2 , a supporting fiber may be formed (S 10 ) and then a graphene oxide-containing solution may be formed (S 20 ). Subsequently, a graphene oxide composite fiber may be formed using the supporting fiber and the graphene oxide-containing solution (S 30 ). The method of forming the supporting fiber, the graphene oxide-containing solution and the graphene oxide composite fiber may be substantially the same as the method described with reference to  FIG. 1 . 
     Thereafter, the graphene oxide composite fiber formed in the step S 30  may be reduced to form a graphene composite fiber (S 50 ). The reduction method may include at least one of a thermal reduction method, an optical reduction method, and a chemical reduction method. The thermal reduction method may be performed at a temperature within a range of about 40 degrees Celsius to about 3000 degrees Celsius. The optical reduction method may be performed using light having a wavelength within a range of about 200 nm to about 1500 nm. The chemical reduction method may be performed using a chemical reagent including at least one of hydriodic acid with acetic acid (HI—AcOH), hydrazine (N 2 H 4 ), dimethyl hydrazine (C 2 H 8 N 2 ), sodium borohydride (NaBH 4 ), sodium hydroxide (NaOH), ascorbic acid, glucose, hydrogen sulfide (H 2 S), hydroquinone (C 6 H 4 (OH) 2 ), and sulfuric acid (H 2 SO 4 ). 
     The supporting fiber may be separated from the graphene composite fiber formed in the step S 50  (S 60 ). The supporting fiber may be the polymer fiber. The graphene composite fiber may be thermally treated or chemically melted so that the supporting fiber may be separated from the graphene composite fiber. The thermal treating and the chemical melting of the graphene composite fiber may be performed by the same method as described in the step S 40  of  FIG. 1 . 
     The graphene fiber may be manufactured by the method described above. The graphene fiber may include an outer surface, an inner surface surrounded by the outer surface, and graphene particles between the outer surface and the inner surface. An inner diameter of the graphene fiber may have a range of about 1 nm to 100 μm. A length of the graphene fiber may be several nanometers or more. 
     EXPERIMENTAL EXAMPLE 1 
     A chitosan fiber was formed by the step S 10 . A chitosan powder (Sigma Aldrich Co. LLC.) was supplied into a trifluoroacetic acid solution and then the trifluoroacetic acid solution including the chitosan powder was mixed by a stirrer at 50 degrees Celsius for 6 hours, thereby forming a chitosan solution. 
     After the formed chitosan solution was supplied into the cylinder  21  of  FIG. 3 , the chitosan solution was electro-spun to the collector  25  under the condition that the applied voltage was 15 kV, a distance between the cylinder  21  and the collector  25  was 10 cm, and a solution injection speed was within a range of about 1 ml/h to about 5 ml/h. A chitosan fiber having a diameter within a range of several nanometers to several micrometers may be formed according to a spinning condition of a concentration of the chitosan solution and the applied voltage. The chitosan fiber was neutralized in an ammonia solution for about 4 hours and then was cleaned three times by distilled water so that the chitosan fiber had insolubility. After the chitosan fiber was dried in a vacuum oven for about 24 hours, the chitosan fiber was separated from the collector  25 . A scanning electron microscope (SEM) photograph of the chitosan fiber formed at this time is illustrated in  FIG. 8 . Referring to  FIG. 8 , a diameter of the chitosan fiber was within a range of about 251 nm to about 331 nm. 
     The graphene oxide-containing solution was formed by the step S 20 . Modified Hummers and offenmans methods were performed on SP-1 graphite powder (Bay carbon Co.) so that graphene oxide powder was formed. After the graphene oxide powder was added to distilled water in a weight ratio of about 0.05 wt % to about 0.5 wt %, the graphene oxide powder was dispersed in the distilled water by an ultrasonic method for about 4 hours. Thus, a graphene oxide-containing solution was formed. Sizes of particles of the graphene oxide were within a range of about 0.5 nm to about 2.9 nm. 
     The graphene oxide composite fiber was formed according to the step S  30 . After the hydrogen ion concentration (pH) of the formed graphene oxide solution was adjusted at about 4.3, the formed chitosan fiber was soaked in the graphene oxide solution having the hydrogen ion concentration (pH) of about 4.3 for about 5 hours. Amide groups formed on the surface of the chitosan fiber and hydroxyl groups and carboxyl groups of the graphene oxide were self-assembled to form the composite fiber of a graphene oxide chitosan coaxial type. After the formed graphene oxide composite fiber was taken out, the graphene oxide composite fiber was cleaned three times by distilled water and then was dried in a vacuum oven for about 24 hours. A SEM photograph of the graphene oxide composite fiber formed at this time is illustrated in  FIG. 9 . Referring to  FIG. 9 , a diameter of the graphene oxide composite fiber was a range of about 500 nm to about 800 nm. 
     The graphene oxide composite fiber was reduced to form the graphene composite fiber according to the step S 50 . The graphene oxide composite fiber was reduced to the graphene composite fiber by a room temperature vapor method using a hydriodic acid with acetic acid (HI—AcOH) solution. In more detail, the graphene oxide composite fiber was supplied into an airtight glass container in which a mixture solution of hydriodic acid of about 2 ml and acetic acid of 5 ml was supplied. And then the graphene oxide composite fiber and the mixture solution were reacted with each other at about 40 degrees Celsius for about 24 hours. Thus, the reduced graphene composite fiber was formed. 
     The chitosan fiber being the supporting fiber was separated from the reduced graphene fiber according to the step S 60 . In more detail, the graphene composite fiber may be thermally treated at a temperature within a range of about 100 degrees Celsius to about 200 degrees Celsius for about 24 hours or be soaked in the acid-based solvent for about 30 minutes so that the chitosan fiber supporting the graphene is removed. Thus, the graphene fiber was completed. 
     EXPERIMENTAL EXAMPLE 2 
     A nylon fiber was formed according to the step S 10 . Nylon-6 powder (Sigma-Aldrich Co. LLC.) was supplied into a formic acid solution and then the formic acid solution including the nylon-6 powder was mixed by a stirrer at 50 degrees Celsius for about 6 hours, thereby forming a nylon solution. 
     After the formed nylon solution was supplied into the cylinder  21  of  FIG. 3 , the nylon solution was electro-spun to the collector  25  under the condition that the applied voltage was 15 kV, a distance between the cylinder  21  and the collector  25  was 10 cm, and a solution injection speed was within a range of about 0.5 ml/h to about 5 ml/h. A nylon fiber having a diameter within a range of several nanometers to several micrometers may be formed according to a spinning condition of a concentration of the nylon solution and the applied voltage. The formed nylon fiber was reacted in a bovine serum albumin (BSA) solution of about 10% concentration for about 30 minutes and then was dried in a hood. 
     The graphene oxide-containing solution was formed by the same method as described in the Experimental example according to the step S 20 . 
     The graphene oxide composite fiber was formed according to the step S 30 . After the hydrogen ion concentration (pH) of the formed graphene oxide solution was adjusted at about 4.3, the formed nylon fiber was soaked in the graphene oxide solution having the hydrogen ion concentration (pH) of about 4.3 for about 5 hours. Amide groups formed on the surface of the nylon fiber and hydroxyl groups and carboxyl groups of the graphene oxide were self-assembled to form the composite fiber of a graphene oxide nylon coaxial type. After the formed graphene oxide composite fiber was taken out, the graphene oxide composite fiber was cleaned three times by distilled water and then was dried in a vacuum oven for about 24 hours. 
     The graphene oxide composite fiber was reduced by the same method as described in the experimental example 1 according to the step S 50 , thereby forming the graphene composite fiber. 
     The nylon fiber being the supporting fiber was separated from the reduced graphene composite fiber by the same method as described in the experimental example 1 according to the step S 60 . Thus, the graphene fiber was completed. 
     According to embodiments of the inventive concept, it is possible to easily manufacture the large-area graphene fiber having high strength, high flexibility, and high porosity. 
     While the inventive concept has been described with reference to example embodiments, it will be apparent to those skilled in the art that various changes and modifications may be made without departing from the spirit and scope of the inventive concept. Therefore, it should be understood that the above embodiments are not limiting, but illustrative. Thus, the scope of the inventive concept is to be determined by the broadest permissible interpretation of the following claims and their equivalents, and shall not be restricted or limited by the foregoing description.