Abstract:
Example embodiments are directed to a nanofiber-nanowire composite includes a polymer nanofiber; and a plurality of nanowires of a metal oxide extending from inside to outside of the polymer nanofiber and covering the polymer nanofiber. According to example embodiments, a method of fabricating a nanofiber-nanowire composite includes forming a nanofiber including a metal seed; and growing nanowires of a metal oxide from the metal seed to the outside of the nanofiber.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims benefit under 35 U.S.C. §119 to Korean Patent Application No. 10-2010-0003933, filed on Jan. 15, 2010, in the Korean Intellectual Property Office, the disclosure of which is incorporated herein in its entirety by reference. 
     BACKGROUND 
     1. Field 
     Example embodiments relate to a nanofiber-nanowire composite fabricated by growing nanowires of a metal oxide from a polymer nanofiber and a method of fabricating the nanofiber-nanowire composite. 
     2. Description of the Related Art 
     A nanofiber is an ultra fine fiber having a very small diameter of about 1 μm or less and can be used in materials for medical purposes, filters, microelectro-mechanical system (MEMS), nano devices, etc. A nanofiber has a large surface area per unit mass, is flexible, and fine spaces. A nanofiber may be easily blended with another material and can distribute external stress exerted thereon. 
     One method of fabricating nanofiber is electrospinning. An electrospinning device includes a spinning tip through which a solution for electrospinning is supplied, a high-voltage device, and a collector for collecting nanofiber. A high voltage is applied to the spinning tip to charge droplets of the solution supplied via the spinning tip, and a stream of the droplets is emitted by electrostatic repulsion to form nanofiber on the collector. 
     In addition, nanofiber may be fabricated using a microfluidic technique. Nanofiber having a core-shell structure may be fabricated using a device including an injection tube and a collection tube by which a middle fluid and an outer fluid, which are different from each other, are emitted by external pressure. 
     Since nanofiber has a very large surface area, the surface of the nanofiber may be used in various ways. For example, nanofiber may be used to prepare functional nano devices. For example, a nanowire may be formed from the nanofiber to prepare an elastic electrode. Functional nano devices may be applied to stretchable electronics, wearable devices, and the like. 
     SUMMARY 
     According to example embodiments, a nanofiber-nanowire composite includes a polymer nanofiber; and a plurality of nanowires of a metal oxide extending from inside to outside of the polymer nanofiber and covering the polymer nanofiber. 
     According to example embodiments, the polymer nanofiber includes a water-insoluble polymer. 
     According to example embodiments, the water-insoluble polymer is prepared by cross-linking a water-soluble polymer. 
     According to example embodiments, the water-insoluble polymer is a multi-functional resin. 
     According to example embodiments, the water-soluble polymer includes at least one selected from a group consisting of polyvinyl alcohol (PVA), polyacrylic acid (PAA), polystyrene sulfonic acid, polyhydroxyethyl methacrylate, polyethylene oxide (PEO), polyvinyl pyrrolidone (PVP), polyacrylamide (PA), PVA-PEO-PVA, PVA-poly propylene oxide (PPO)-PVA, PVA-PA, PVA-PA-PAA, PVA-polystyrene (PS) -PVA, and PVA-PVA, PVA-SbQ prepared by introducing a stilbazolium group to PVA, and polyethylene glycol diacrylate. 
     According to example embodiments, the polymer nanofiber further includes an elastic binder. 
     According to example embodiments, the polymer nanofiber further includes a conductive material. 
     According to example embodiments, the conductive material includes a nano metal particle, an ionic liquid, or an ionomer. 
     According to example embodiments, the metal oxide includes zinc oxide (ZnO), tin-dioxide (SnO2), titanium dioxide (TiO2), or indium oxide (In2O3). 
     According to example embodiments, the polymer nanofiber has a core-shell structure. 
     According to example embodiments, a method of fabricating a nanofiber-nanowire composite includes forming a nanofiber including a metal seed; and growing nanowires of a metal oxide from the metal seed to the outside of the nanofiber. 
     According to example embodiments, forming the nanofiber includes forming a polymer film by spin coating a composition including a photosensitive polymer, a precursor of the metal oxide, and a solvent; and patterning the nanofiber by exposing the polymer film to light using a photomask and developing the polymer film. 
     According to example embodiments, forming the nanofiber includes forming a pre-nanofiber by electrospinning a composition including a water-soluble polymer, a first precursor of the metal oxide, and a solvent; and converting the water-soluble polymer in the pre-nanofiber into a water-insoluble polymer by cross-linking the water-soluble polymer. 
     According to example embodiments, the water-soluble polymer is cross-linked by heat-treatment. 
     According to example embodiments, the water-soluble polymer is cross-linked by UV-radiation. 
     According to example embodiments, growing the nanowires of the metal oxide includes immersing the nanofiber in a hexamethyltetradiamine (HMTA, (CH2)6N4) solution. 
     According to example embodiments, growing the nanowires of the metal oxide further includes adding a second precursor of the metal oxide to the HMTA solution in which the nanofiber is immersed. 
     According to example embodiments, growing the nanowires of the metal oxide includes heating the nanofiber in a mixed solution including zinc acetate, deionized water, and methanol; and adding a solution including potassium hydroxide and methanol into the mixed solution. 
     According to example embodiments, the composition further includes a conductive material. 
     According to example embodiments, the metal oxide includes zinc oxide (ZnO), tin-dioxide (SnO2), titanium dioxide (TiO2), or indium oxide (In2O3). 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above and other features and advantages will become more apparent by describing in detail example embodiments with reference to the attached drawings. The accompanying drawings are intended to depict example embodiments and should not be interpreted to limit the intended scope of the claims. The accompanying drawings are not to be considered as drawn to scale unless explicitly noted. 
         FIG. 1A  is a longitudinal sectional view of a nanofiber-nanowire composite in which nanowires of a metal oxide extend from the inside to the outside of a nanofiber, and  FIG. 1B  is a cross sectional view of the nanofiber-nanowire composite of  FIG. 1A  according to example embodiments. 
         FIG. 2A  is a longitudinal sectional view of a nanofiber-nanowire composite in which nanowires of a metal oxide extend from the inside to the outside of a nanofiber having a core-shell structure, and  FIG. 2B  is a cross sectional view of the nanofiber-nanowire composite of  FIG. 2A  according to example embodiments. 
         FIG. 3  is a flowchart illustrating a method of fabricating a nanofiber-nanowire composite, according to example embodiments; 
         FIG. 4  schematically shows a process of converting a water-soluble polymer into a water-insoluble polymer including a metal seed by cross-linking the water-soluble polymer in the presence of a precursor of a metal oxide according to example embodiments. 
         FIG. 5  is a flowchart illustrating a method of fabricating a nanofiber-nanowire composite, according to example embodiments; and 
         FIG. 6  is an electron microscope image of a nanofiber from which zinc oxide nanowires have grown (a nanofiber-nanowire composite), according to example embodiments. 
     
    
    
     DETAILED DESCRIPTION 
     Detailed example embodiments are disclosed herein. However, specific structural and functional details disclosed herein are merely representative for purposes of describing example embodiments. Example embodiments may, however, be embodied in many alternate forms and should not be construed as limited to only the embodiments set forth herein. 
     Accordingly, while example embodiments are capable of various modifications and alternative forms, embodiments thereof are shown by way of example in the drawings and will herein be described in detail. It should be understood, however, that there is no intent to limit example embodiments to the particular forms disclosed, but to the contrary, example embodiments are to cover all modifications, equivalents, and alternatives falling within the scope of example embodiments. Like numbers refer to like elements throughout the description of the figures. 
     It will be understood that, although the terms first, second, 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. For example, a first element could be termed a second element, and, similarly, a second element could be termed a first element, without departing from the scope of example embodiments. 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. In contrast, when an element is referred to as being “directly connected” or “directly coupled” to another element, there are no intervening elements present. Other words used to describe the relationship between elements should be interpreted in a like fashion (e.g., “between” versus “directly between”, “adjacent” versus “directly adjacent”, etc.). 
     The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of example embodiments. As used herein, the singular forms “a”, “an” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. 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. 
     It should also be noted that in some alternative implementations, the functions/acts noted may occur out of the order noted in the figures. For example, two figures shown in succession may in fact be executed substantially concurrently or may sometimes be executed in the reverse order, depending upon the functionality/acts involved. 
       FIG. 1A  is a longitudinal sectional view of a nanofiber-nanowire composite  10  in which nanowires of a metal oxide  13  extend from the inside to the outside of a nanofiber  11 , and  FIG. 1B  is a cross sectional view of the nanofiber-nanowire composite  10  of  FIG. 1A , according to example embodiments. 
     Referring to  FIGS. 1A and 1B , in the nanofiber-nanowire composite  10 , the nanowires  13  extend radially from the inside to the outside of the nanofiber  11  cover the surface of the nanofiber  11 . The nanowires  13  are not attached to the surface of the nanofiber  11 , but one end of each nanowire  13  is planted in the body of the nanofiber  11 . Thus, the nanowires  13  are rigidly supported by the nanofiber  11 , and durability of the nanofiber-nanowire composite  10  may be improved. 
     The nanofiber  11  of the nanofiber-nanowire composite  10  may be formed of a polymer. The nanofiber  11  may be formed of a water-insoluble polymer that is obtained from a water-soluble polymer by cross-linking the water-soluble polymer by heat-treatment and/or UV-radiation. Alternatively, the nanofiber  11  may be formed of a water-insoluble photosensitive polymer such as a multi-functional resin. 
     A water-soluble polymer that is thermally curable may be polyvinyl alcohol (PVA), polyacrylic acid (PAA), polystyrene sulfonic acid, polyhydroxyethyl methacrylate, polyethylene oxide (PEO), polyvinyl pyrrolidone (PVP), polyacrylamide (PA), and/or the like. The water-soluble polymer that is thermally curable may also be a block copolymer such as PVA-PEO-PVA, PVA-poly propylene oxide (PPO)-PVA, PVA-PA, PVA-PA-PAA, PVA-polystyrene (PS) -PVA, and/or PVA-PVA. Meanwhile, the water-soluble polymer that is thermally cured to become a water-insoluble polymer may be carboxy methyl cellulose, methyl cellulose, hydroxy ethyl cellulose, dextrine, and/or the like. A polymer that is photo curable by UV-radiation may be PVA-SbQ prepared by introducing a stilbazolium group to PVA, polyethylene glycol diacrylate, and/or the like. That is, the nanofiber  11  may be formed of a water-insoluble polymer that is obtained by thermally curing or photo curing the water-soluble polymer. 
     The nanofiber  11  may further include an elastic binder. Due to the elastic binder, the elasticity of the nanofiber-nanowire composite  10  may be further improved. The elastic binder may include natural rubber, styrene butadiene rubber, polybutadiene rubber, chloroprene rubber, acrylonitrile butadiene rubber (NBR), isobutylene isoprene rubber, ethylene propylene diene rubber (DPDM), chlorosulphonated polyethylene, acrylic elastomer, fluoro elastomer, silicone elastomer, and/or polyurethane. 
     In addition, the nanofiber  11  may include a conductive material such as a nano metal particle, an ionic liquid, and/or an ionomer. The nano metal particle may be a nano particle of one selected from the group consisting of gold (Au), silver (Ag), copper (Cu), palladium (Pd), platinum (Pt), Ag/Pd, and aluminum (Al). The nano metal particle may have a particle size in the range of about 5 to about 10 nm. The amount of the nano metal particle may be in the range of about 10 to about 70 parts by weight, for example, about 20 to about 60 parts by weight, based on 100 parts by weight of the nanofiber polymer. 
     The ionic liquid refers to a salt having liquid properties at room temperature, and may also refer to a molten salt. The ionic liquid includes organic cations and/or inorganic or organic anions and has high evaporation temperature, high ionic conductivity, excellent heat resistance, and excellent flame retardant properties. 
     A cation of the ionic liquid may be substituted or unsubstituted imidazolium, pyrazolium, triazolium, thiazolium, oxazolium, pyridazinium, pyrimidinium, pyrazinium, ammonium, phosphonium, guanidinium, euronium, thioeuronium, pyridinium, and/or pyrrollium. 
     The anion may be any organic or inorganic anion that binds to the cation to form the ionic liquid. For example, the anion may include at least one selected from the group consisting of a halide anion, a borate anion, a phosphate anion, a phosphinate anion, an imide anion, a sulphonate anion, an acetate anion, a sulfate anion, a cyanate anion, a thiocyanate anion, a carbonaceous anion, a complex anion, and ClO 4 —. 
     Examples of the anion include at least one selected from the group consisting of PF 6 —, BF 4 —, B(C 2 O 4 )—, CH 3 (C 6 H 5 )SO 3 —, (CF 3 CF 2 ) 2 PO 2 —, CF 3 SO 3 —, CH 3 SO 4 —, CH 3 (CH 2 ) 7 SO 4 —, N(CF 3 SO 2 ) 2 —, N(C 2 F 5 SO 2 ) 2 —, C(CF 2 SO 2 ) 3 —, AsF 6 —, SbF 6 —, AlCl 4 —, NbF 6 —, HSO 4 —, ClO 4 —, CH 3 SO 3 —, and CF 3 CO 2 —. 
     The ionomer may include an ethylene acrylic acid copolymer, a polyurethane ionomer having a polytrimethylene oxide bond, and an α-olefin copolymer ionomer having ethylene, propylene, 1-butene, 1-pentene, 1-hexene, 1-heptene, 3-methyl-1-butene, or 4-methyl-1-pentene as α-olefin. 
     The amount of the ionic liquid or ionomer may be in the range of about 10 to about 70 parts by weight, for example, about 20 to about 60 parts by weight, based on 100 parts by weight of the nanofiber polymer. 
     The nanowires  13  of the nanofiber-nanowire composite  10  may be formed of a conductive metal oxide. For example, the nanowires  13  may be formed of zinc oxide (ZnO), tin-dioxide (SnO 2 ), titanium dioxide (TiO 2 ), and/or indium oxide (In 2 O 3 ). The zinc oxide (ZnO) is an n-type semiconductor having a direct bandgap of about 3.3 eV and is used in electronic devices. The tin-dioxide (SnO 2 ), titanium dioxide (TiO 2 ), and/or indium oxide (In 2 O 3 ) are used in electronic optoelectronic fields due to their electrical and optical properties. 
     The nanofiber  11  may have a diameter in the range of about 100 nm to about 5 μm, and a length of equal to or greater than 1 μm. The nanowire  13  may have a diameter in the range of about 100 nm to about 5 μm, and a length in the range of about 0.1 to about 5 μm. 
     The nanofiber-nanowire composite  10  may be used as a functional nano device such as an elastic electrode since flexibility, elasticity, and large surface area of the nanofiber  11  and electrical properties of the nanowire  13  are combined. Meanwhile, since the nanowires  13  are rigidly fixed from the inside of the nanofiber  11 , the nanofiber-nanowire composite  10  has excellent durability. 
       FIG. 2A  is a longitudinal sectional view of a nanofiber-nanowire composite  20  in which nanowires of a metal oxide  23  extend from the inside to the outside of nanofiber shell  21 . The nanofiber  25  has a nanofiber core  21  and a nanofiber shell  22  surrounding the nanofiber core  21  having a core-shell structure, and  FIG. 2B  is a cross sectional view of the nanofiber-nanowire composite  20  of  FIG. 2A , according to example embodiments. 
     Referring to  FIGS. 2A and 2B , in the nanofiber-nanowire composite  20 , like the nanofiber-nanowire composite  10  of  FIGS. 1A and 1B , the nanowires  23  extend radially from the inside to the outside of the nanofiber shell  21  that covers the surface of the nanofiber shell  21 . 
     The nanofiber core  21  of the nanofiber  25  may be formed of a first polymer, and the nanofiber shell  22  may be formed of a second polymer. A nanofiber having a core-shell structure may have a hydrophobic surface if desired or may be used to form a p-n junction in which a bandgap is adjusted. 
     In  FIGS. 2A and 2B , the nanowires  23  extend from the nanofiber shell  22 . However, the nanowires  23  may also extend from the nanofiber core  21 . 
     The nanofiber core  21  and the nanofiber shell  22  may each be formed of a water-insoluble polymer formed as a result of cross-linking of a water-soluble polymer. In particular, the nanofiber core  21  and the nanofiber shell  22  may be formed of the water-insoluble polymer formed as a result of cross-linking of the water-soluble polymer by heat-treatment or UV-radiation. Alternatively, the nanofiber core  21  and the nanofiber shell  22  may be formed of a water-insoluble photosensitive polymer such as a multi-functional resin. 
     A water-soluble polymer that is thermally curable may be polyvinyl alcohol (PVA), polyacrylic acid (PAA), polystyrene sulfonic acid, polyhydroxyethyl methacrylate, polyethylene oxide (PEO), polyvinyl pyrrolidone (PVP), polyacrylamide (PA), and/or the like. The water-soluble polymer that is thermally curable may also be a block copolymer such as PVA-PEO-PVA, PVA-poly propylene oxide (PPO)-PVA, PVA-PA, PVA-PA-PAA, PVA-polystyrene (PS) -PVA, and/or PVA-PVA. Meanwhile, the water-soluble polymer that is thermally cured to become a water-insoluble polymer may be carboxy methyl cellulose, methyl cellulose, hydroxy ethyl cellulose, dextrine, and/or the like. A polymer that is photo curable by UV-radiation may be PVA-SbQ prepared by introducing a stilbazolium group to PVA, polyethylene glycol diacrylate, and/or the like. That is, the nanofiber  21  and  22  may be formed of a water-insoluble polymer that is obtained by thermally curing or photo curing the water-soluble polymer. 
     The nanofiber core  21  may further include an elastic binder. The elastic binder may include natural rubber, styrene butadiene rubber, polybutadiene rubber, chloroprene rubber, acrylonitrile butadiene rubber (NBR), isobutylene isoprene rubber, ethylene propylene diene rubber (DPDM), chlorosulphonated polyethylene, acrylic elastomer, fluoro elastomer, silicone elastomer, and/or polyurethane. 
     The nanofiber core  21  and the nanofiber shell  22  may include a conductive material such as a nano metal particle, an ionic liquid, and/or an ionomer, respectively. 
     The nano metal particle may be a nano particle of one selected from the group consisting of gold (Au), silver (Ag), copper (Cu), palladium (Pd), platinum (Pt), Ag/Pd, and aluminum (Al). The nano metal particle may have a particle size in the range of about 5 to about 10 nm. 
     A cation of the ionic liquid may be substituted or unsubstituted imidazolium, pyrazolium, triazolium, thiazolium, oxazolium, pyridazinium, pyrimidinium, pyrazinium, ammonium, phosphonium, guanidinium, euronium, thioeuronium, pyridinium, or pyrrollium. 
     The anion may be any organic or inorganic anion that binds to the cation to form the ionic liquid. For example, the anion may include at least one selected from the group consisting of a halide anion, a borate anion, a phosphate anion, a phosphinate anion, an imide anion, a sulphonate anion, an acetate anion, a sulfate anion, a cyanate anion, a thiocyanate anion, a carbonaceous anion, a complex anion, and ClO 4 —. 
     Examples of the anion include at least one selected from the group consisting of PF 6 —, BF 4 —, B(C 2 O 4 )—, CH 3 (C 6 H 5 )SO 3 —, (CF 3 CF 2 ) 2 PO 2 —, CF 3 SO 3 —, CH 3 SO 4 —, CH 3 (CH 2 ) 7 SO 4 —, N(CF 3 SO 2 ) 2 —, N(C 2 F 5 SO 2 ) 2 —, C(CF 2 SO 2 ) 3 —, AsF 6 —, SbF 6 —, AlCl 4 —, NbF 6 —, HSO 4 —, ClO 4 —, CH 3 SO 3 —, and CF 3 CO 2 —. 
     The ionomer may include an ethylene acrylic acid copolymer, a polyurethane ionomer having a polytrimethylene oxide bond, and an α-olefin copolymer ionomer having ethylene, propylene, 1-butene, 1-pentene, 1-hexene, 1-heptene, 3-methyl-1-butene, or 4-methyl-1-pentene as α-olefin. 
     The amount of the ionic liquid or ionomer may be in the range of about 10 to about 70 parts by weight, for example, about 20 to about 60 parts by weight, based on 100 parts by weight of the nanofiber polymer. 
     The nanowires  23  of the nanofiber-nanowire composite  20  may be formed of a conductive metal oxide. For example, the nanowires  23  may be formed of zinc oxide (ZnO), tin-dioxide (SnO 2 ), titanium dioxide (TiO 2 ), and/or indium oxide (In 2 O 3 ). 
     The nanofiber  25  may have a diameter in the range of about 100 nm to about 5 μm, and a length of equal to or greater than 1 μm. The nanofiber core  21  may have a diameter in the range of about 0.1 nm to about 4 μm, and the nanofiber shell  22  may have a thickness in the range of about 0.1 nm to 3 5 μm. 
     The nanowire  23  may have a diameter in the range of about 100 nm to about 5 μm, and a length in the range of about 0.1 to about 5 μm. 
     Since the nanofiber-nanowire composite  20  has a core-shell structure, hydrophilicity or bandgaps may be efficiently adjusted by reforming the surface of the nanofiber shell  22  and/or selecting materials of nanofiber core  21  and nanofiber shell  23 . 
     In  FIGS. 2A and 2B , the nanofiber  25  has a core-shell structure. However, the nanofiber core  21  and shell  22  may be a gradient nanofiber having a composition gradient in the lengthwise direction or thicknesswise direction of the nanofiber core  21  and shell  22 . 
       FIG. 3  is a flowchart illustrating a method of fabricating a nanofiber-nanowire composite, according to example embodiments. 
     First, a composition including a water-soluble polymer, a metal oxide precursor, and a solvent is prepared in operation S 110 . A pre-nanofiber including the metal oxide precursor is formed by electrospinning the composition in operation S 120 . 
     The pre-nanofiber may have a core-shell structure using an electrospinning device including a spinning tip having a double nozzle, for example, a concentric nozzle, in the electrospinning. In other words, for example, the pre-nanofiber (now shown) having a core-shell structure may be prepared by emitting the first polymer via an inner nozzle and the second polymer via an outer nozzle. 
     The water-soluble polymer may be any polymer that may be cross-linked by heat-treatment or UV-radiation and converted into a water-insoluble polymer. 
     A water-soluble polymer that is thermally curable may be polyvinyl alcohol (PVA), polyacrylic acid (PAA), polystyrene sulfonic acid, polyhydroxyethyl methacrylate, polyethylene oxide (PEO), polyvinyl pyrrolidone (PVP), polyacrylamide (PA), and/or the like. The water-soluble polymer that is thermally curable may also be a block copolymer such as PVA-PEO-PVA, PVA-poly propylene oxide (PPO)-PVA, PVA-PA, PVA-PA-PAA, PVA-polystyrene (PS) -PVA, and/or PVA-PVA. 
     Additionally, the water-soluble polymer that is thermally cured to become a water-insoluble polymer may be carboxy methyl cellulose, methyl cellulose, hydroxy ethyl cellulose, dextrine, and/or the like. 
     A polymer that is photo curable by UV-radiation may be PVA-SbQ prepared by introducing a stilbazolium group to PVA, polyethylene glycol diacrylate, and/or the like. 
     The solvent may be water. Water may be used with a hydrophilic polymer or a polar solvent such as alcohol and dimethyl formaldehyde, which have high affinity to water. 
     The metal oxide precursor contained in the composition for electrospinning may be a nitride, chloride, sulfide, acetate, acetylacetonate, cyanate, isopropyl oxide, or butoxide of zinc (Zn), tin (Sn), titanium (Ti), and/or indium (In). 
     The composition for electrospinning may further include a cross-linking agent such as formaldehyde, glyoxal, glutadialdehyde, and/or the like. 
     Meanwhile, the composition for electrospinning may further include an elastic binder. The elastic binder may include at least one selected from the group consisting of a polymer elastomer such as natural rubber, styrene butadiene rubber, polybutadiene rubber, chioroprene rubber, acrylonitrile butadiene rubber (NBR), isobutylene isoprene rubber, ethylene propylene diene rubber (DPDM), chlorosulphonated polyethylene, acrylic elastomer, fluoro elastomer, silicone elastomer or polyurethane, and foam. 
     In addition, the composition for electrospinning may include a conductive material such as a nano metal particle, an ionic liquid, and/or an ionomer. The conductive material may function as a seed of the metal oxide with the metal oxide precursor. 
     The nano metal particle may be a nano particle of one metal selected from the group consisting of gold (Au), silver (Ag), copper (Cu), palladium (Pd), platinum (Pt), Ag/Pd, and aluminum (Al). The nano metal particle may have a particle size in the range of about 5 to about 10 nm and may be introduced into the composition for electrospinning in a colloidal solution or a nanoparticlulate form. The amount of the nano metal particle may be in the range of about 10 to about 70 parts by weight, for example, about 20 to about 60 parts by weight, based on 100 parts by weight of the nanofiber polymer. 
     A cation of the ionic liquid may be substituted or unsubstituted imidazolium, pyrazolium, triazolium, thiazolium, oxazolium, pyridazinium, pyrimidinium, pyrazinium, ammonium, phosphonium, guanidinium, euronium, thioeuronium, pyridinium, and/or pyrrollium. 
     The anion may be any organic or inorganic anion that binds to the cation to form the ionic liquid. For example, the anion may include at least one selected from the group consisting of a halide anion, a borate anion, a phosphate anion, a phosphinate anion, an imide anion, a sulphonate anion, an acetate anion, a sulfate anion, a cyanate anion, a thiocyanate anion, a carbonaceous anion, a complex anion, and ClO 4 —. 
     Examples of the anion include at least one selected from the group consisting of PF 6 —, BF 4 —, B(C 2 O 4 )—, CH 3 (C 6 H 5 )SO 3 —, (CF 3 CF 2 ) 2 PO 2 —, CF 3 SO 3 —, CH 3 SO 4 —, CH 3 (CH 2 ) 7 SO 4 —, N(CF 3 SO 2 ) 2 —, N(C 2 F 5 SO 2 ) 2 —, C(CF 2 SO 2 ) 3 —, AsF 6 —, SbF 6 —, AlCl 4 —, NbF 6 —, HSO 4 —, ClO 4 —, CH 3 SO 3 —, and CF 3 CO 2 —. 
     The ionomer may include an ethylene acrylic acid copolymer, a polyurethane ionomer having a polytrimethylene oxide bond, and an α-olefin copolymer ionomer having ethylene, propylene, 1-butene, 1-pentene, 1-hexene, 1-heptene, 3-methyl-1-butene, or 4-methyl-1-pentene as α-olefin. 
     The amount of the ionic liquid or ionomer may be in the range of about 10 to about 70 parts by weight, for example, about 20 to about 60 parts by weight, based on 100 parts by weight of the nanofiber polymer. 
     When the composition is electrospun, a pre-nanofiber including the water-soluble polymer and the metal oxide precursor or a pre-nanofiber including the water-soluble polymer, the metal oxide precursor, and the elastic binder or the conductive material may be formed. 
     The electrospun pre-nanofiber is thermally cured to form a nanofiber including cross-linked water-soluble polymer in operation S 130 . The pre-nanofiber may be thermally cured by hot-air heating at a temperature in the range of about 100 to about 140° C . The water-soluble polymer of the nanofiber  11  is cross-linked by thermal curing and converted into a water-insoluble polymer, and the nanofiber  11  includes a metal seed that grows to form the nanowire of the metal oxide. 
     Alternatively, the electrospun pre-nanofiber may be prepared by UV-radiation using PVA-SbQ, polyethylene glycol diacrylate, and/or the like. 
       FIG. 4  schematically shows a process of converting a water-soluble polymer (PVA) into a water-insoluble polymer including a metal seed by cross-linking the water-soluble polymer in the presence of a cross-linking agent and a metal oxide precursor, according to example embodiments. Referring to  FIG. 4 , in the cross-linked polymer, a metal atom is bound to portions that are not cross-linked via hydrogen bonds to form a metal seed. 
     The nanofiber including the metal seed is immersed in a solution including a compound that may form a nucleus of a metal oxide, and the solution is maintained at about 90° C. to grow the nucleus of the metal oxide from the metal seed in the nanofiber in operation S 140 . 
     A metal oxide precursor solution is added to the solution including the nanofiber in which the nucleus of the metal oxide is formed to grow nanowires of the metal oxide from the nucleus of the metal oxide of the nanofiber in operation S 150 . 
       FIG. 5  is a flowchart illustrating a method of fabricating a nanofiber-nanowire composite, according to example embodiments. The nanofiber is prepared by photo-patterning instead of electrospinning, and the nanowires are prepared in the same manner as in the preparation of the nanofiber-nanowire composite described above with reference to  FIG. 4 . 
     First, a composition including a photosensitive resin, a metal oxide precursor, and a solvent is prepared in operation S 210 . The photosensitive resin may be, for example, a multi-functional epoxy resin. The composition may further include any material that may be a seed of the metal oxide in addition to the metal oxide precursor. 
     The solvent may be an organic solvent. Then, the composition is mixed and filtered in operation S 220 . Elements of the composition may be sufficiently mixed by an ultrasonic treatment. After mixing the composition, materials insoluble in the composition are filtered using a micron syringe filter. 
     The mixed and filtered composition is spin-coated on a substrate to form a film in operation S 230 . The substrate may be a glass substrate, a plastic substrate, a semiconductor substrate, and/or the like. After the spin coating, the solvent may be removed by drying and/or baking. The film formed by spin coating is exposed to light using a photomask having a pattern and developed to prepare a nanofiber in operation S 240 . Thus, a nanofiber corresponding to the pattern of the photomask may be formed. The nanofiber includes a metal seed that is formed from a metal oxide precursor or other seed materials. 
     The nanofiber including the metal seed is immersed in a solution including a compound that may form a nucleus of a metal oxide, and the solution is maintained at about 90° C. to grow the nucleus of the metal oxide from the metal seed in the nanofiber. 
     A metal oxide precursor solution is added to the solution including the nanofiber in which the nucleus of the metal oxide is formed to grow nanowires of the metal oxide from the nucleus of the metal oxide of the nanofiber in operation S 250 . 
     As described above, by using the wet process by which nanowires grow in an aqueous solution, a nanofiber-nanowire composite may be mass-produced at a low temperature. Furthermore, since the nanowires grow from the inside of the nanofiber, the nanofiber-nanowire composite may have improved durability. 
     EXAMPLE 1 
     A composition including 2 wt % of zinc acetate dissolved in a 7 wt % polyvinyl alcohol (PVA) aqueous solution and 3 wt % of glyoxal, as a cross-linking agent, was electrospun at a voltage of 15 kV, at a radiation distance of 5 cm, at room temperature. A nanofiber obtained by electrospinning the composition was thermally cured at 120° C. for 5 minutes. 
     The thermally cured nanofiber was subjected to hydrothermal reaction in an aqueous solution including 0.01 M hexamethyltetradiamine (HMTA, ((CH 2 ) 6 N 4 ) and 0.01 M zinc nitrate hexa hydrate (Zn(NO 3 ) 2 .6H 2 O) at 90° C. to grow zinc oxide (ZnO) nanowires on the nanofiber. 
     A nucleus of zinc oxide (ZnO) is formed according to the following reaction schemes.
 
(CH 2 ) 6 N 4 +6H 2 O→4NH 3 +6HCHO   (1)
 
NH 3 +H 2 O→NH 4+ +OH—   (2)
 
Zn+2NH 4+ →Zn 2+ +2NH 3 +H 2    (3)
 
Zn 2+ +2OH—→ZnO( s )+H2O   (4)
 
     An ammonia aqueous solution (NH 3 ) is formed from HMTA as in reaction scheme (1), and an ammonium ion (NH 4+ ) is formed as in reaction scheme (2). The ammonium ion (NH 4+ ) reacts with Zn contained in the nanofiber to form Zn 2+  (3), and Zn 2+  reacts with OH— of scheme ( 2 ) to form ZnO(s). 
       FIG. 6  is an electron microscopic image of a nanofiber from which zinc oxide nanowires extend (a nanofiber-nanowire composite), according to example embodiments. Referring to  FIG. 6 , it is seen that the nanowires grow from the nanofiber. However, as will be understood, the nanowires may not always grow from the nanofiber at regular intervals and may not always be uniform in all dimensions as illustrated in the example embodiments of  FIGS. 1A-2B . The nanowires may either grow in a uniform, symmetric fashion as illustrated in  FIGS. 1A-2B , in a random, asymmetric fashion as shown in  FIG. 6  or a nanofiber may include portion(s) wherein the nanowires grow in a uniform, symmetric fashion ( FIGS. 1A-2B ) and portion(s) wherein the nanowires grow in a random, asymmetric fashion ( FIGS. 6 ). 
     EXAMPLE 2 
     A composition including 2 wt % of zinc acetate dissolved in a 7 wt % polyvinyl alcohol (PVA) aqueous solution and 3 wt % of glyoxal, as a cross-linking agent, was electrospun at a voltage of 15 kV, at a radiation distance of 5 cm, at room temperature. A nanofiber obtained by electrospinning the composition was thermally cured by hot-air heating at 120° C. for 5 minutes. 
     The thermally cured nanofiber was maintained at 60° C. for 30 minutes in a solution prepared by dissolving 3 g of zinc acetate in a solution including 10 g of deionized water and 10 g of methanol. Then, the solution was maintained at the same temperature for 3 hours while a solution prepared by dissolving 1 g of potassium hydroxide in 10 g of methanol was added into the solution, and further stirred for 5 hours while maintaining at the same temperature. An electron microscope illustrated that the nanofiber-nanowire composite obtained was somewhat similar to that obtained in Example 1. 
     EXAMPLE 3 
     A composition including 2 wt % of zinc acetate dissolved in a 10 wt % PVA-SbQ aqueous solution and 3 wt % of glyoxal, as a cross-linking agent, was electrospun at a voltage of 15 kV, at a radiation distance of 5 cm, at room temperature. A nanofiber obtained by electrospinning the composition was photo cured by UV radiation. 
     The photo cured nanofiber was subjected to hydrothermal reaction in an aqueous solution including 0.01 M hexamethyltetradiamine (HMTA) ((CH 2 ) 6 N 4 ) and 0.01 M zinc nitrate hexa hydrate (Zn(NO 3 ) 2 .6H 2 O) at 90° C. to grow zinc oxide (ZnO) nanowires on the nanofiber. An electron microscope illustrated that the nanofiber-nanowire composite obtained was somewhat similar to that obtained in Example 1. 
     EXAMPLE 4 
     A composition including 2 wt % of zinc acetate dissolved in a 10 wt % polyethylene glycol diacrylate (PEG-DA, Aldrich) aqueous solution and 3 wt % of glyoxal, as a cross-linking agent, was electrospun at a voltage of 15 kV, at a radiation distance of 5 cm, at room temperature. A nanofiber obtained by electrospinning the composition was photo cured by UV radiation. 
     The photo cured nanofiber was subjected to hydrothermal reaction in an aqueous solution including 0.01 M hexamethyltetradiamine (HMTA) ((CH 2 ) 6 N 4 ) and 0.01 M zinc nitrate hexa hydrate (Zn(NO 3 ) 2 .6H 2 O) at 90° C. to grow zinc oxide (ZnO) nanowires on the nanofiber. An electron microscope illustrated that the nanofiber-nanowire composite obtained was somewhat similar to that obtained in Example 1. 
     EXAMPLE 5 
     A colloidal solution including 10 wt % polyethylene glycol diacrylate (PEG-DA, Aldrich) and 3 wt % Ag/Pd solution having a diameter in the range of 5 to 10 nm was sonicated for 30 minutes, 2 wt % of zinc acetate was dissolved therein, and 3 wt % of glyoxal, as a cross-linking agent, was added thereto to prepare a composition. The composition was electrospun at a voltage of 15 kV, at a radiation distance of 5 cm, at room temperature. A nanofiber obtained by electrospinning the composition was photo cured by UV radiation. 
     The photo cured nanofiber was subjected to hydrothermal reaction in an aqueous solution including 0.01 M hexamethyltetradiamine (HMTA) ((CH 2 ) 6 N 4 ) and 0.01 M zinc nitrate hexa hydrate (Zn(NO 3 ) 2 .6H 2 O) at 90° C. to grow zinc oxide (ZnO) nanowires on the nanofiber. An electron microscope illustrated that the nanofiber-nanowire composite obtained was somewhat similar to that obtained in Example 1. 
     EXAMPLE 6 
     A composition for forming a photosensitive polymer complex including 3 g of a multi-functional epoxy resin (SU-8, Hexion specialty Co.), 1 g of cyclopentanone, as an organic solvent, and 0.05 g of silvertrifluoro acetate (Aldrich) was prepared. The composition was sonicated for 1 hour to sufficiently mix the elements, and filtered using a 0.5 micron syringe filter. The filtered composition was spin coated on a glass substrate surface-treated with CF 4  Plasma at a rate of 2000 rpm and dried at 100° C. for 1 minute to remove the organic solvent remaining on the surface of the coating. 
     A film prepared by the spin coating was exposed to UV rays at 100 mJ/cm 2  for 12 seconds using a photomask with a pattern having a size of 3 um, dried at 100° C. for 1 minute, and immersed in 2-methoxyethanol for 20 seconds to be developed. The developed film was baked at 200° C. for 1 minute to align a photosensitive nanofiber having a negative pattern with nanoparticles. 
     The photo cured nanofiber was subjected to hydrothermal reaction in an aqueous solution including 0.01 M hexamethyltetradiamine (HMTA) ((CH 2 ) 6 N 4 ) and 0.01 M zinc nitrate hexa hydrate (Zn(NO 3 ) 2 .6H 2 O) at 90° C. to grow zinc oxide (ZnO) nanowires on the nanofiber. An electron microscope illustrated that the nanofiber-nanowire composite obtained was somewhat similar to that obtained in Example 1. 
     The nanofiber-nanowire composite may be used in an elastic electrode, a wearable electrode, an elastic thin film transistor, an elastic display/device/sensor, and/or the like. Since the nanowires are rigidly fixed to the inside of the nanofiber, the nanofiber-nanowire composite may have improved durability and high reliability. 
     As described above, according to example embodiments, since the nanowires of a metal oxide are formed on the elastic nanofiber, the nanofiber-nanowire composite may have a wider specific surface area. 
     In addition, since the nanowires grow from the inside to the outside of the nanofiber, the nanowires are rigidly fixed to the nanofiber and improve durability of the nanofiber-nanowire composite. 
     Furthermore, since the nanowires grow from the nanofiber by using the wet process, a nanofiber-nanowire composite having a large area may be prepared at a low temperature. 
     Example embodiments having thus been described, it will be obvious that the same may be varied in many ways. Such variations are not to be regarded as a departure from the intended spirit and scope of example embodiments, and all such modifications as would be obvious to one skilled in the art are intended to be included within the scope of the following claims.