Abstract:
Disclosed is a method of manufacturing a nano metal wire, including: putting a metal precursor solution in a core pipe of a needle; putting a polymer solution in a shell pipe of the needle, wherein the shell pipe surrounds the core pipe; applying a voltage to the needle while simultaneously jetting the metal precursor solution and the polymer solution to form a nano line on a collector, wherein the nano line includes a metal precursor wire surrounded by a polymer tube; chemically reducing the metal precursor wire of the nano line to form a nano line of metal wire surrounded by the polymer tube; and washing out the polymer tube by a solvent.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     The present application is based on, and claims priority from Taiwan Application Serial Number 102125685, filed on Jul. 18, 2013, and claims the benefit of U.S. Provisional Application No. 61/813,445, filed on Apr. 18, 2013, the entirety of which are incorporated by reference herein. 
    
    
     TECHNICAL FIELD 
     The technical field relates to nano metal wire, and in particular, relates to a method for manufacturing the same. 
     BACKGROUND 
     Recently, nano technology is widely used in information technology, material technology, biotechnology, and the likes. When the size of a material is scaled down to nano scale, its properties will change according to its shape and size. For example, a silver nanorod or nanowire may have absorption peaks of longitudinal mode and traverse mode under surface plasmon resonance. The nanorod or nanowire with a larger aspect (length-diameter) ratio has a red-shifted absorption peak of longitudinal mode. 
     A silver nanowire or silver wire with a high aspect ratio has been disclosed by some research teams. However, the conventional silver nanowires have a length of several nanometers (nm) to several micrometers (μm), an aspect ratio of less than 1000 (or even less than 100), and low conductivity. 
     Accordingly, a novel method for preparing silver nanowires with high conductivity and a high aspect ratio is called-for. 
     SUMMARY 
     One embodiment of the disclosure provides a method of manufacturing a nano metal wire, comprising: putting a metal precursor solution in a core pipe of a needle; putting a polymer solution in a shell pipe of the needle, wherein the shell pipe surrounds the core pipe; applying a voltage to the needle while simultaneously jetting the metal precursor solution and the polymer solution to form a nano line on a collector, wherein the nano line includes a metal precursor wire surrounded by a polymer tube; chemically reducing the metal precursor wire of the nano line to form a nano line of a nano metal wire surrounded by the polymer tube; and washing out the polymer tube by a solvent. 
     One embodiment of the disclosure provides a nano line, comprising: a metal precursor wire; and a polymer tube surrounding the metal precursor wire, wherein the metal precursor wire comprises a metal compound and a chemically reducing agent. 
     One embodiment of the disclosure provides a nano metal wire, having an aspect ratio of greater than 1000, and a conductivity of between 10 4  S/m to 10 7  S/m. 
     A detailed description is given in the following embodiments with reference to the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The disclosure can be more fully understood by reading the subsequent detailed description and examples with references made to the accompanying drawings, wherein: 
         FIG. 1  shows an electrostatic spinning apparatus for manufacturing nano metal wires in one embodiment of the disclosure; 
         FIG. 2  illustrates a cross-sectional view of a shell pipe and a core pipe of a needle in one embodiment of the disclosure; 
         FIG. 3  shows a nano line in one embodiment of the disclosure; 
         FIG. 4  shows a nano metal wire in one embodiment of the disclosure; 
         FIG. 5  shows absorption spectra of nano silver wires without annealing or after annealing for different periods of time in some embodiments of the disclosure; 
         FIG. 6  shows absorption spectra of nano silver wires left to stand at room temperature for different periods of time or annealing for different periods of time in some embodiments of the disclosure; and 
         FIG. 7  shows an XRD spectrum of nano silver wires in one embodiment of the disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     In the following detailed description, for purposes of explanation, numerous specific details are set forth in order to provide a thorough understanding of the disclosed embodiments. It will be apparent, however, that one or more embodiments may be practiced without these specific details. In other instances, well-known structures and devices are schematically shown in order to simplify the drawing. 
     In the disclosure, a nano metal wire having a high aspect ratio (e.g. greater than 1000) is formed by an electrostatic spinning apparatus. As shown in  FIG. 1 , a polymer solution is put into a syringe  11 , and a metal precursor solution is put into a syringe  13 . The syringe  11  connects to a shell pipe  15 O of a needle  15 , and the syringe  13  connects to a core pipe  15 I of the needle  15 , respectively. As shown in  FIG. 2 , the shell pipe  15 O and the core pipe  15 I are concentric cylinders. A voltage is then applied to the needle  15  while simultaneously jetting the metal precursor solution and the polymer solution from the needle  15 , thereby forming a nano line  17  on a collector  19 . As shown in  FIG. 3 , the nano line  17  includes a metal precursor wire  17 A surrounded by a polymer tube  17 B. The described process of forming the nano line  17  is the so-called electrostatic spinning method. 
     In one embodiment, a solvent of the polymer solution is an organic solvent with high-polarity such as methanol or acetone, and the corresponding polymer is polyvinylpyrrolidone (PVP). In addition, a salt such as tetrabutyl ammonium phosphate (TBAP) or cetyltrimethylammonium bromide (CTAB) can be optionally added into the polymer solution. The salt may enhance the polarization degree of the electrostatic spinning, thereby reducing the polymer amount. 
     In one embodiment, the additive amount of the salt is of about 1 mg/mL to 100 mg/mL. Alternatively, a solvent of the polymer solution can be an organic solvent with low-polarity such as tetrahydrofuran (THF), toluene, or chloroform. In this case, the corresponding polymer can be polyacrylonitrile (PAN), polyvinyl alcohol (PVA), or ethylene vinyl alcohol (EVA). If the solvent of the polymer solution is an organic solvent with high-polarity, it can be washed out by water to meet environmentally friendly requirements after the forming of a nano metal wire. If the solvent of the polymer solution is an organic solvent with low-polarity, the polymer solution and the metal precursor solution will be immiscible when forming the nano metal wire having a high quality. In one embodiment, the polymer in the polymer solution has a concentration of about 100 mg/mL to 200 mg/mL. 
     In one embodiment, the metal precursor solution includes a metal compound and chemically reducing agent. The metal compound can be a silver compound (e.g. silver nitrate or silver oxide), platinum compound (e.g. platinum chloride or platinous oxide), gold compound (e.g. gold chloride or auric acid), or combinations thereof. The selection of the chemically reducing agent depends on the metal compound type. For example, when the metal compound is silver nitrate, the chemically reducing agent can be ethylene glycol. When the metal compound is silver oxide, the chemically reducing agent can be ammonium hydroxide. When the metal compound is platinum chloride, the chemically reducing agent can be hydrazine, sodium hydroborate, hydrogen, or alcohol. When the metal compound is gold chloride, the chemically reducing agent can be an aqueous solution of sodium citrate or Vitamin C. The metal compound concentration depends on the metal compound type. For example, the silver nitrate has a concentration of about 1 mg/mL to 100 mg/mL, and the silver oxide has a concentration of about 1 mg/mL to 100 mg/mL. The chemically reducing agent concentration depends on the chemically reducing agent type. For example, the ethylene glycol may directly serve as an organic solvent with high-polarity, and the ammonium hydroxide may have a concentration of about 1 wt % to 50 wt %. 
     In one embodiment, the core pipe  15 I of the needle  15  has a diameter of about 0.5 m to 2 mm, which is determined by the desired diameter of the nano metal wire. In one embodiment, the shell pipe  15 O and the core pipe  15 I of the needle  15  have a difference of about 0.01 mm to 5 mm. 
     In one embodiment, the voltage applied to the needle  15  is about 10 kV to 12 kV. In one embodiment, a tip of the needle  15  and the collector  19  have a distance therebetween of about 5 cm to 50 cm. If the collector  19  is a common plate, random arranged nano lines  17  will be easily formed. If the collector  19  is parallel electrode plate, parallel arranged nano lines  17  will be formed. 
     In one embodiment, the syringes  11  and  13  are controlled by syringe pumps  12  and  14 , respectively, to tune flow rates of the polymer solution and the metal precursor solution. For example, the polymer solution is jetted out of the needle  15  with a flow rate of about 0.1 mL/hr to 5 mL/hr, and the metal precursor solution is jetted out of the needle  15  with a flow rate of about 0.01 mL/hr to 1 mL/hr. 
     After the described steps, the nano lines  17  can be left at room temperature under the regular atmosphere, such that the metal compound is slowly chemically reduced by the chemically reducing agent in the metal precursor wires  17 A. As a result, nano metal wires  21  are obtained. In one embodiment, the nano lines  17  can be annealed under the atmosphere to accelerate chemical reduction. For example, the anneal step can be performed at a temperature of about 100° C. to 200° C. A suitable solvent can be adopted to wash out the polymer tube  17 B surrounding around the nano metal wire  21 . For example, when the polymer tube  17 B is PVP, it can be washed out by water, and the nano metal wires  21  in  FIG. 4  are left. When the polymer tube  17 B is PAN, it can be washed out by THF. The nano metal wire  21  prepared by the described steps has a diameter of 50 nm to 500 nm, an aspect ratio of greater than 1000, and a conductivity of about 10 4 S/m to 10 7 S/m. Note that the nano metal wire  21  has an unlimited maximum length. In other words, the nano metal wire has an unlimited maximum aspect ratio. In one embodiment, the nano metal wire  21  may have a centimeter-scaled length, e.g. at least 1 cm or even at least 10 cm. The nano metal wire  21  can be applied to an anti-EMI paint, an RFID device, a solar cell conductive paste, a long-lasting and anti-bacterial peelable spray, and a transparent conductive film, and the likes. 
     Below, exemplary embodiments will be described in detail with reference to the accompanying drawings so as to be easily realized by a person having ordinary knowledge in the art. The inventive concept may be embodied in various forms without being limited to the exemplary embodiments set forth herein. Descriptions of well-known parts are omitted for clarity, and like reference numerals refer to like elements throughout. 
     EXAMPLES 
     In following examples, the needle had a shell pipe with a diameter of 1.25 mm and a core pipe with a diameter of 0.95 mm. The needle and the parallel electrode collector plate had a distance of 13 cm therebetween. The voltage applied to the needle was 10 kV. One electrode plate of the parallel electrode collector plate was electrically connected to ground, and another electrode plate was electrically connected to a voltage of 1 kV. Diameters of the nano lines and the nano metal wires were all measured by transmission electron microscopy (TEM, JEOL JEM-2100F). 
     Example 1 
     An ethylene glycol solution of silver nitrate (30 mg/mL) was put into a syringe connected to a core pipe of a needle. A methanol solution of PVP (200 mg/mL) was put into another syringe connected to a shell pipe of the needle. The silver precursor solution in the core pipe was controlled by a syringe pump to have a flow rate of 0.1 mL/hr, and the polymer solution in the shell pipe was controlled by another syringe pump to have a flow rate of 1 mL/hr. A nano line having a diameter of about 2.2 μm was electrostatically spun. 
     The nano line was annealed at 150° C. under the atmosphere for about 8 minutes, and then washed by water to remove the polymer tube. As such, a nano silver wire with a diameter of about 500 nm, a length of about 10 cm, and an aspect ratio of 200000 was obtained. The nano silver wire was measured by a spectrometer to obtain its absorption spectrum as shown in  FIG. 5 . 
     Example 2 
     Similar to Example 1, the difference in Example 2 was the annealing period being changed to about 20 minutes. After annealing, the nano line was washed by water to remove the polymer tube. As such, a nano silver wire with a diameter of about 500 nm, a length of about 10 cm, and an aspect ratio of 200000 was obtained. The nano silver wire was measured by a spectrometer to obtain its absorption spectrum as shown in  FIG. 5 . 
     Example 3 
     Similar to Example 1, the difference in Example 3 was the annealing period being changed to about 10 hours. After annealing, the nano line was washed by water to remove the polymer tube. As such, a nano silver wire with a diameter of about 500 nm, a length of about 10 cm, and an aspect ratio of 200000 was obtained. The nano silver wire was measured by a spectrometer to obtain its absorption spectrum as shown in  FIG. 5 . 
     Comparative Example 1 
     Similar to Example 1, the difference in Comparative Example 1 was the nano line having a diameter of 2.2 μm being directly washed by water to remove the polymer tube (without any annealing). The silver precursor wire was measured by a spectrometer to obtain its absorption spectrum as shown in  FIG. 5 . 
     
       
         
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                 Annealing 
                 Nano silver 
                   
                 Nano silver 
               
               
                   
                 period at 
                 wire 
                 Nano silver 
                 wire aspect 
               
               
                   
                 150° C. 
                 diameter 
                 wire length 
                 ratio 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                 Example 1 
                  8 minutes 
                 ~500 nm 
                 10 cm 
                 2 × 10 5   
               
               
                 Example 2 
                 20 minutes 
                 ~500 nm 
                 10 cm 
                 2 × 10 5   
               
               
                 Example 3 
                 10 hours 
                 ~500 nm 
                 10 cm 
                 2 × 10 5   
               
               
                 Comparative 
                 Without 
                 none 
                 none 
                 none 
               
               
                 Example 1 
                 annealing 
               
               
                   
               
             
          
         
       
     
     As shown in  FIG. 5  and Table 1, the absorption peaks at about 420 nm of the nano silver wires were higher and red-shifted as the length of the annealing periods were increased. Accordingly, the annealing step was beneficial for chemically reducing the silver nitrate to silver. 
     Example 4 
     An ammonium hydroxide solution of silver oxide (with a silver oxide concentration of 5 mg/mL and an ammonium hydroxide concentration of 33%) was put into a syringe connected to a core pipe of a needle. A methanol solution of PVP (200 mg/mL) was put into another syringe connected to a shell pipe of the needle. The silver precursor solution in the core pipe was controlled by a syringe pump to have a flow rate of 0.01 mL/hr, and the polymer solution in the shell pipe was controlled by another syringe pump to have a flow rate of 1 mL/hr. A nano line having a diameter of about 1 μm was electrostatically spun. The nano line was left to stand at room temperature under the atmosphere for 4 hours, and then washed by water to remove the polymer tube. As such, a nano silver wire with a diameter of about 300 nm and a length of 10 cm was obtained. The nano silver wire was measured by a spectrometer to obtain its absorption spectrum as shown in  FIG. 6 . 
     Example 5 
     Similar to Example 4, the difference in Example 5 was the nano line being left to stand at room temperature under the atmosphere for 4 days. Thereafter, the nano line was washed by water to remove the polymer tube. As such, the nano silver wire with a diameter of about 300 nm and a length of 10 cm was obtained. The nano silver wire was measured by a spectrometer to obtain its absorption spectrum as shown in  FIG. 6 . 
     Example 6 
     Similar to Example 4, the difference in Example 6 was the nano line having a diameter of about 1 μm being annealed at 200° C. under the atmosphere for 10 minutes. Thereafter, the nano line was washed by water to remove the polymer tube. As such, the nano silver wire with a diameter of about 300 nm and a length of 10 cm was obtained. The nano silver wire was measured by a spectrometer to obtain its absorption spectrum as shown in  FIG. 6 . 
     Example 7 
     Similar to Example 6, the difference in Example 7 was the nano line being annealed at 200° C. for 20 minutes. Thereafter, the nano line was washed by water to remove the polymer tube. As such, the nano silver wire with a diameter of about 300 nm and a length of 10 cm was obtained. The nano silver wire was measured by a spectrometer to obtain its absorption spectrum as shown in  FIG. 6 . 
     Example 8 
     Similar to Example 6, the difference in Example 8 was the nano line being annealed at 200° C. for 30 minutes. Thereafter, the nano line was washed by water to remove the polymer tube. As such, the nano silver wire with a diameter of about 300 nm and a length of 10 cm was obtained. The nano silver wire was measured by a spectrometer to obtain its absorption spectrum as shown in  FIG. 6 . 
     
       
         
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                   
                 TABLE 2 
               
               
                   
                   
               
               
                   
                   
                 Nano 
                 Nano 
                   
               
               
                   
                   
                 silver 
                 silver 
                 Nano silver 
               
               
                   
                 Anneal 
                 wire 
                 wire 
                 wire aspect 
               
               
                   
                 temperature/period 
                 diameter 
                 length 
                 ratio 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                 Example 4 
                 Room temperature/ 
                 ~300 nm 
                 10 cm 
                 3.3 × 10 5   
               
               
                   
                 4 hours 
               
               
                 Example 5 
                 Room temperature/ 
                 ~300 nm 
                 10 cm 
                 3.3 × 10 5   
               
               
                   
                 4 days 
               
               
                 Example 6 
                 200° C./10 minutes 
                 ~300 nm 
                 10 cm 
                 3.3 × 10 5   
               
               
                 Example 7 
                 200° C./20 minutes 
                 ~300 nm 
                 10 cm 
                 3.3 × 10 5   
               
               
                 Example 8 
                 200° C./30 minutes 
                 ~300 nm 
                 10 cm 
                 3.3 × 10 5   
               
               
                   
               
             
          
         
       
     
     As shown in  FIG. 6  and Table 2, the nano silver wires were formed by only being left to stand at room temperature for a long period without annealing. However, the anneal step may accelerate the forming of the nano silver wires. The nano silver wire having a diameter of 300 nm and a length of 10 cm was formed by annealing at a temperature of 200° C. for a period of 10 minutes (longer annealing period was not needed). The nano silver wire had a conductivity of 6.9×10 4 S/m. 
     Example 9 
     An ammonium hydroxide solution of silver oxide (with a silver oxide concentration of 1 mg/mL and an ammonium hydroxide concentration of 33%) was put into a syringe connected to a core pipe of a needle. A methanol solution of PVP and TBAP (with a PVP concentration of 100 mg/mL and a TBAP concentration of 10 mg/mL) was put into another syringe connected to a shell pipe of the needle. The silver precursor solution in the core pipe was controlled by a syringe pump to have a flow rate of 0.01 mL/hr, and the polymer solution in the shell pipe was controlled by another syringe pump to have a flow rate of 1 mL/hr. A nano line having a diameter of about 0.6 μm and a length of 10 cm was electrostatically spun. The nano line was annealed at 200° C. under the atmosphere for 20 minutes, and then washed by water to remove the polymer tube. As such, a nano silver wire with a diameter of about 357 nm was obtained. 
     Example 10 
     An ammonium hydroxide solution of silver oxide (with a silver oxide concentration of 5 mg/mL and an ammonium hydroxide concentration of 33%) was put into a syringe connected to a core pipe of a needle. A methanol solution of PVP and TBAP (with a PVP concentration of 100 mg/mL and a TBAP concentration of 10 mg/mL) was put into another syringe connected to a shell pipe of the needle. The silver precursor solution in the core pipe was controlled by a syringe pump to have a flow rate of 0.01 mL/hr, and the polymer solution in the shell pipe was controlled by another syringe pump to have a flow rate of 1 mL/hr. A nano line having a diameter of about 0.7 μm and a length of 10 cm was electrostatically spun. The nano line was annealed at 200° C. under the atmosphere for 20 minutes, and then washed by water to remove the polymer tube. As such, a nano silver wire with a diameter of about 464 nm was obtained. As known by comparison with Example 9, a nano silver wire having a larger diameter can be obtained through a higher silver oxide concentration. 
     Example 11 
     An ammonium hydroxide solution of silver oxide (with a silver oxide concentration of 1 mg/mL and an ammonium hydroxide concentration of 33%) was put into a syringe connected to a core pipe of a needle. A methanol solution of PVP and TBAP (with a PVP concentration of 100 mg/mL and a TBAP concentration of 30 mg/mL) was put into another syringe connected to a shell pipe of the needle. The silver precursor solution in the core pipe was controlled by a syringe pump to have a flow rate of 0.01 mL/hr, and the polymer solution in the shell pipe was controlled by another syringe pump to have a flow rate of 1 mL/hr. A nano line having a diameter of about 0.4 μm and a length of 10 cm was electrostatically spun. The nano line was annealed at 200° C. under the atmosphere for 20 minutes, and then washed by water to remove the polymer tube. As such, a nano silver wire with a diameter of about 285 nm was obtained. As known by comparison with Example 9, a nano silver wire having a smaller diameter can be obtained through a higher TBAP concentration. 
     The nano silver wire in Example 11 had a resistivity of 4.3×10 −4 Ω·cm. A bulk silver had a resistivity of 1.6×10 −6 Ω·cm (See Applied Physics Letters 95, 103112, 2009). A single crystalline nano silver wire had a resistivity of 2.19×10 −4 Ω·cm (See Applied Physics Letters 95, 103112, 2009). A poly crystalline nano silver wire had a resistivity of 8.29×10 −4 Ω·cm (See Nano letter, Vol. 2, No. 2, 2002). Accordingly, the nano silver wire prepared in Example 11 of the disclosure should be a single crystalline nano silver wire. An XRD spectrum of the nano silver wire is shown in  FIG. 7 . The nano silver wire had a single crystalline face-centered cubic structure, as determined by TEM and XRD. Also, the nano silver wire had high uniformity and a high conductivity. 
     Example 12 
     An ammonium hydroxide solution of silver oxide (with a silver oxide concentration of 5 mg/mL and an ammonium hydroxide concentration of 33%) was put into a syringe connected to a core pipe of a needle. A methanol solution of PVP and TBAP (with a PVP concentration of 100 mg/mL and a TBAP concentration of 30 mg/mL) was put into another syringe connected to a shell pipe of the needle. The silver precursor solution in the core pipe was controlled by a syringe pump to have a flow rate of 0.01 mL/hr, and the polymer solution in the shell pipe was controlled by another syringe pump to have a flow rate of 1 mL/hr. A nano line having a diameter of about 0.6 μm and a length of 10 cm was electrostatically spun. The nano line was annealed at 200° C. under the atmosphere for 20 minutes, and then washed by water to remove the polymer tube. As such, a nano silver wire with a diameter of about 375 nm was obtained. As known by comparison with Example 11, a nano silver wire having a larger diameter can be obtained through a higher silver oxide concentration. As known by comparison with Example 10, a nano silver wire having a smaller diameter can be obtained through a higher TBAP concentration. 
     
       
         
               
               
               
               
               
               
             
               
               
               
               
               
               
             
           
               
                   
                 TABLE 3 
               
               
                   
                   
               
               
                   
                   
                   
                   
                   
                 Nano 
               
               
                   
                   
                   
                 Nano 
                 Nano 
                 silver 
               
               
                   
                 Silver oxide 
                 TBAP 
                 silver 
                 silver 
                 wire 
               
               
                   
                 con- 
                 con- 
                 wire 
                 wire 
                 aspect 
               
               
                   
                 centration 
                 centration 
                 diameter 
                 length 
                 ratio 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                 Example 9 
                 1 mg/mL 
                 10 mg/mL 
                 ~357 nm 
                 10 cm 
                 2.8 × 10 5   
               
               
                 Example 10 
                 5 mg/mL 
                 10 mg/mL 
                 ~464 nm 
                 10 cm 
                 2.2 × 10 5   
               
               
                 Example 11 
                 1 mg/mL 
                 30 mg/mL 
                 ~285 nm 
                 10 cm 
                 3.5 × 10 5   
               
               
                 Example 12 
                 5 mg/mL 
                 30 mg/mL 
                 ~375 nm 
                 10 cm 
                 2.7 × 10 5   
               
               
                   
               
             
          
         
       
     
     It will be apparent to those skilled in the art that various modifications and variations can be made to the disclosed methods and materials. It is intended that the specification and examples be considered as exemplary only, with a true scope of the disclosure being indicated by the following claims and their equivalents.