Abstract:
A uniform composite nanofiber includes a tubular first nanofiber, and a second nanofiber formed inside or outside the first nanofiber. The first nanofiber is first formed within a plurality of nano-scale pores of a template placed on a current collector, and then the second nanofiber is formed on inner or outer surface of the first nanofiber, and the template is removed afterwards for obtaining the composite nanofiber.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a continuation-in-part patent application of U.S. application Ser. No. 10/419,167 filed on Apr. 21, 2003, the entire contents of which are hereby incorporated by reference for which priority is claimed under 35 U.S.C. § 120. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The invention generally relates to a synthesis method of composite nanofibers, and particularly relates to a method for synthesizing composite nanofibers by forming a second nanofiber inside a first hollow nanofiber that plays as a secondary template. 
         [0004]    2. Related Art 
         [0005]    Recently, nanotechnology is extremely hot in industries. Many breakthroughs are obtained and undoubtedly cause great impacts to the industry. Among numerous nano-scale materials, nanofibers have excellent characteristics in their energy and photoelectric properties so as to be highly noticed. 
         [0006]    A general method for producing nanofibers is the vapor deposition for fabricating vapor-growth carbon fibers. A carbon fiber is a hollow tubular structure having a diameter of 5˜20 nanometers and having an outer surface on which a porous high surface area layer is formed. The porous surface makes the nanofiber an excellent adsorbent and catalyst support. However, the fabrication process is costly and energy intensive that limits the production and applications. 
         [0007]    In view of this limitation, cost-oriented manufacturing process is an important point of nanofiber fabrication. For example, template synthesis is a method for producing high quality and lower cost nanofibers and taking the place of the expensive vapor deposition. 
         [0008]    Different template synthesis methods have been developed for nanofiber fabrication. For example, sol-gel for SiO 2 , SnO 2 , V 2 O 5 , etc; electroless plating for Nickel; electro-deposition for ZnO, and so on. Specific template synthesis methods are applied in accordance with the materials and applications. However, a single material nanofiber usually cannot meet the application requirements. For example, in the application of lithium-ion secondary batteries, the Martin research group found that SnO 2  nanofibers for negative pole material of a lithium cell, though having a high reversible electric capacity larger than 700 mAh/g and high current discharge rate of 58 C, has a high irreversible electric capacity that limits the applications. The irreversible electric capacity is caused by a solid-electrolyte interphase of Li 2 O formed from deoxidation of SnO 2  and lithium-ions. The high irreversible electric capacity increases the surface impedance and decrease the lifetime of the nanofibers. The reaction mechanism is shown in  FIG. 5 . The reactions are as follows: 
         [0000]      4Li++4e−+SnO 2 →2Li 2 O+Sn   (equation I) 
         [0000]      xLi++xe−+Sn←→Li x Sn,0≦x≦4.4   (equation II) 
         [0000]    Wherein equation I shows the formation of Li 2 O; equation II shows the reversible reaction of Li-Sn alloy, which provides the reversible electric capacity. 
         [0009]    Therefore, as shown in  FIG. 6 , if a suitable material, such as a carbon coating, is applied on surface of a single material nanofiber, such as tin oxide SnO 2 , for acquiring stable solid-electrolyte interphase (SEI) film and decreasing the irreversible electric capacity, then the applicability of nanofibers can be improved. 
         [0010]    The concept of synthesizing composite nanofibers for overcoming the problem of irreversible electric capacity in lithium battery application is thus generated. However, though the fabrication of single material nanofiber is easier, when forming a second material coating on exterior of the first nanofiber through conventional chemical vapor deposition or chemical impregnation, the coating is uneven in thickness and hard to be obtained. Therefore, bi-material nanofiber with even composition is a great difficulty of fabrication with conventional processes. 
       SUMMARY OF THE INVENTION 
       [0011]    The object of the invention is to provide a method for synthesizing bi-material nanofibers. The invention overcomes the difficulties of precise controls to the construction, tube dimensions and chemical composition of the bi-material nanofibers. 
         [0012]    The invention provides a method for fabricating composite nanofibers under a base concept of “secondary template”. A precursor of carbon, metal or metal oxide is first embedded on a template membrane with pores of 50˜800 nm diameters and 6-50 micron thickness, so that first tubular nanofibers are grown up in the pores of the template membrane through controls of process parameters. Next, by using the hollow first nanofibers as a secondary template, second nanofibers are produced in the inner surfaces of the first nanofibers. Finally, the template membrane is removed to obtain the composite nanofibers. The aspect ratio of the composite nanofiber can be controlled within 10 to 1000, and the inner and outer diameters can be within 10 to 700 nm and 50 to 800 nm respectively. 
         [0013]    The method of “secondary template” of the invention is capable of producing high quality composite nanofibers and providing precise controls to the constructions, dimensions and chemical compositions of the nanofibers. The process reduces the cost, and provides nanofibers of small size, high weight energy density and high recharge and discharge efficiencies that meet the requirements of minimization of future products. The composite nanofibers can be applied to extensive scopes of micro electromechanical devices, micro integrated circuits and biochips, etc. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS  
         [0014]    The invention will become more fully understood from the detailed description given hereinbelow. However, this description is for purposes of illustration only, and thus is not limitative of the invention, wherein: 
           [0015]      FIGS. 1 to 4  are explanatory views of fabrication processes for producing composite nanofibers of the invention; 
           [0016]      FIG. 5  is a functional view of charge reaction of a metal oxide nanofiber applied to a lithium battery in prior art; 
           [0017]      FIG. 6  is a functional view of charge reaction of a composite nanofiber of the invention applied to a lithium battery in prior art; 
           [0018]      FIG. 7(   a ) is a scanning electron microscopy (SEM) photo of a hollow nanofiber generated through electron cyclotron resonance-chemical vapor deposition (ECR-CVD) process in a polycarbonate membrane having pore diameter of 400 nm, thickness of 6-10 microns and pore density of 10 7 /cm 2 ; 
           [0019]      FIG. 7(   b ) is a SEM photo of a hollow epoxy-based carbon nanofiber produced through sol-gel process; 
           [0020]      FIG. 7(   c ) is a SEM photo of a hollow silicon dioxide carbon nanofiber produced through sol-gel process; 
           [0021]      FIG. 8  is a thickness of pore wall to concentration curve diagram of a hollow epoxy-based carbon nanofiber produced through sol-gel process; 
           [0022]      FIG. 9(   a ) is a SEM photo of a tin dioxide and carbon composite nanofiber; 
           [0023]      FIG. 9(   b ) is a transmission electron microscopy (TEM) photo of a hollow carbon nanofiber before embedding the tin dioxide; 
           [0024]      FIG. 9(   c ) is a TEM photo of a tin dioxide and carbon composite nanofiber after embedding the tin dioxide; 
           [0025]      FIG. 10  is a diagram of 0.2 C charge/discharge curves of a tin dioxide nanofiber and a tin dioxide/carbon composite nanofiber; and 
           [0026]      FIG. 11  is a diagram of electrochemical performance of a tin dioxide nanofiber and a tin dioxide/carbon composite nanofiber under different C-rates. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0027]    A process for fabricating composite nanofibers according to the invention is shown in  FIGS. 1 to 4 . 
         [0028]    a) First, preparing a first tubular nanofiber. The first nanofiber is formed through a template  100  made of thin membrane of polycarbonate or anodic alumina and embedded with a first precursor (macromolecule, inorganic matter, metal oxide or carbon, etc) in the pores  110  of the template  100  through a method of sol-gel, chemical impregnation, electroless plating, electro-deposition or electron cyclotron resonance-chemical vapor deposition (ECR-CVD). The thickness of the hollow tubular nanofiber is controlled in accordance with the method and the parameters. For example, in sol-gel, the concentration, pH scale and soakage time are attended. In ECR-CVD, the vapor volume, deposition time and the kind of catalyst are attended. In electroless plating, the concentration, reaction time, pH scale and temperature are noticed. In electro-deposition, the voltage, current, time and pH scale are monitored. At last, a hollow tubular first carbon nanofiber  200  is obtained. Under suitable conditions, the pore wall thickness of the nanofiber is easy to be controlled. For example,  FIG. 8  shows the relationship between pore wall thickness and concentration of epoxy-based hollow nanofibers made with sol-gel and with a same soakage time. The pore wall thickness is controllable through the concentration of the first precursor. The experiments show that the aspect ratio of the composite nanofiber can be controlled within 10 to 1000, and the inner and outer diameters can be controlled within 10 to 700 nm and 50 to 800 nm respectively. 
         [0029]    b) Then, placing the template  100  on a current collector  300  and using the first carbon nanofiber  200  embedded on the template as a secondary template for embedding a second precursor (macromolecule, inorganic matter, metal oxide or carbon, etc) to obtain a second nanofiber  400 . A current collector is a conductive material placed between the electrodes to secure the electric conduction and to reduce the internal resistance of the battery. The embedding methods include sol-gel, ECR-CVD, chemical impregnation, electro-deposition, electroless plating and so on. Some heat treatments may also be applied in accordance with the embedding method. 
         [0030]    c) Finally, removing the template  100  with chemical etching or plasma etching in order to obtain composite nanofibers  500  composed of the first nanofibers  200  and the second nanofibers  400 . 
         [0031]    A detailed embodiment of the invention is further described hereinafter. The embodiment relates to fabrication of tin dioxide and carbon (SnO 2 /C) composite nanofibers serving as negative pole materials of lithium batteries. The SnO 2 /C composite nanofiber uses a polycarbonate membrane as a template and applies ECR-CVD or sol-gel process. The process is as follows. 
         [0032]    a) Using palladium catalyst to prepare 1 M PdCl 2 . Applying the 1 M PdCl 2  to the polycarbonate film. The film has pores with inner diameters of 100 to 800 nanometers and thickness of 6 to 10 microns; 
         [0033]    b) Forming hollow carbon nanofibers by ECR-CVD. The wall thickness of the hollow carbon nanofiber is controlled through suitable voltage and operational time during using C 2 H 2  as reaction gas, using inert gases (nitrogen, argon) as form-carrier under room temperature reaction and preventing deformation of the template; 
         [0034]    c) Using the finished hollow carbon nanofiber as a secondary template and embedding SnO 2  precursor with sol-gel. The mole ratio of a Sn-based solution is SnCl 2  C 2 H 5 OH:H 2 O:HCl=3:20:6:0.6. After a 24-hour sol-gel process, the prior template of polycarbonate membrane with carbon is soaked in the Sn-based solution for several hours, then taken out and placed on a clean stainless steel or nickel foil; 
         [0035]    d) Placing the work piece in a furnace for heat treatment. With air atmosphere, increasing the air temperature up to 440 centigrade degrees at a rate of 10 degrees per minute. Maintaining the high temperature for one hour till the whole polycarbonate membrane being burned out and the SnO 2 /C composite nanofibers being obtained. 
         [0036]    Some microscopy photos of SnO 2 /C composite nanofibers fabricated with afore the processes are shown in  FIGS. 9(   a ) to  9 ( c ).  FIG. 9(   a ) is a scanning electron microscopy (SEM) photo of a SnO 2 /C composite nanofiber;  FIG. 9(   b ) is a transmission electron microscopy (TEM) photo of a hollow carbon nanofiber before embedding the tin dioxide; and  FIG. 9(   c ) is a TEM photo of a SnO 2 /C composite nanofiber after embedding the tin dioxide. 
         [0037]    When being applied as negative pole materials of a lithium-ion secondary battery, experimental test results of 0.2 C charge/discharge curves of SnO 2  and SnO 2 /C composite nanofibers are shown in  FIG. 10 . It shows the SnO 2  nanofiber has irreversible capacity of 338 mAh/g and reversible capacity of 591 mAh/g; while the SnO 2 /C nanofiber has irreversible capacity of 131 mAh/g and reversible capacity of 741 mAh/g. It proves that the composite nanofiber has a lower irreversible capacity (decreasing from 338 mAh/g to 131 mAh/g). 
         [0038]      FIG. 11  is a diagram of electrochemical performance of a SnO 2  and a SnO 2 /C composite nanofiber under different C-rates. It proves that the composite nanofiber has a higher current discharge rate. 
         [0039]    In conclusion, the composite nanofibers, such as SnO 2 /C, fabricated through process of the invention have advantages of higher weight energy density (740 mAh/g), lower irreversible capacity and higher current discharge rate (14.5 C). Moreover, the total thickness of the current collector (negative pole) and the nanofiber is only 20 to 35 microns that is a breakthrough for extremely thin lithium batteries and suitable for applications of power supplies for future micro-electromechanical products. 
         [0040]    Though the above embodiment explains composite nanofiber applications for lithium-ion batteries, there is no limitation for other applications such as for thin-film batteries, hydrogen storage, molecular sieving, bio-sensors, gas sensors, catalysts, lithium battery, super capacitor and so on. 
         [0041]    Also, according to experiments, the materials for the outer layer of a composite nanofiber can be chosen from silicon and carbon. The materials for the inner layer can be Si, Sn, Ni, Cu, Pt, Ru,; metal oxide AO x  (A=Si, Sn, Sb, Co, Cu, Fe, Ni, Zn, Ru; 0≦x≦2); tin alloys SnM y  (M=Sb, Cu, Mg, Si, Fe, Ti; 0≦y≦2) and others. 
         [0042]    The invention being thus 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 spirit and scope of the invention, 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.