Patent Publication Number: US-2020277194-A1

Title: Nanocarbon separation method and nanocarbon separation apparatus

Description:
TECHNICAL FIELD 
     The present invention relates to a nanocarbon separation method and a nanocarbon separation apparatus. 
     BACKGROUND ART 
     In recent years, carbon materials having a size in a nanometer range (hereinafter referred to as “nanocarbons”) are expected to be applied to various fields due to their mechanical properties, electrical properties, chemical properties, and the like. 
     In the case of nanocarbons, nanocarbons with different properties are simultaneously produced in a manufacturing stage in some cases. When nanocarbons with different electrical characteristics are used in an electronic material when mixed together, a problem such as the deterioration in the characteristics thereof is likely to be caused. Thus, it is necessary to separate nanocarbons with different properties. 
     In order to separate nanocarbons, Patent Document 1 describes a nanocarbon material separation method which includes: a step of introducing and arranging a dispersion solution including nanocarbon materials dispersed in nanocarbon micelle groups having a plurality of different charges and introducing a holding solution having a specific gravity different from that of the nanocarbon materials into an electrophoresis tank by laminating the solutions by disposing them in a predetermined direction; and a step of separating the nanocarbon micelle groups into two or more nanocarbon micelle groups by applying a voltage in a serial direction to the introduced, arranged, and laminated dispersion liquid and holding solution. 
     PRIOR ART DOCUMENTS 
     Patent Documents 
     [Patent Document 1] PCT International Publication No. WO 2010/150808 
     SUMMARY OF THE INVENTION 
     Problem to be Solved by the Invention 
     However, although the separation method described in Patent Document 1 can separate nanocarbons, there is a problem in the efficiency of separation. That is to say, the separation method described in Patent Document 1 requires time for separation. 
     An object of the present invention is to provide a nanocarbon separation method and a nanocarbon separation apparatus which reduce the time required for separation when nanocarbons having different properties are separated. 
     Means for Solving the Problem 
     A nanocarbon separation method of the present invention includes: a step of preparing a nanocarbon dispersion liquid in which nanocarbons and a non-ionic surfactant are dispersed in a solvent; a step of injecting the nanocarbon dispersion liquid into a separation tank; a step of applying a direct current voltage to a first electrode provided at an upper part of the interior of the separation tank and a second electrode provided at a lower part of the interior of the separation tank and generating a pH gradient in the nanocarbon dispersion liquid inside the separation tank; and a step of separating metallic nanocarbons and semiconductor nanocarbons included in the nanocarbon dispersion liquid. 
     A nanocarbon separation apparatus of the present invention is a nanocarbon separation apparatus used for separation of metallic nanocarbons and semiconductor nanocarbons included in a nanocarbon dispersion liquid in which nanocarbons and a non-ionic surfactant are dispersed in a solvent, the nanocarbon separation apparatus including: a separation tank which is configured to accommodate the nanocarbon dispersion liquid; and electrode members disposed inside the separation tank, wherein the separation tank has a lower end having an injection/recovery port communicating with an outer bottom surface of the separation tank, the electrode member includes a cylindrical member, a columnar member inserted into the cylindrical member, a first electrode provided on an outer circumferential surface of an upper end portion of the cylindrical member and a second electrode provided on a lower end portion of the columnar member, the columnar member is capable of moving in a height direction of the separation tank in a state being inserted into the cylindrical member, and a lower end of the columnar member includes a fitting member configured to inscribe in a lower end portion of the cylindrical member when the columnar member moves upward in the height direction of the separation tank. 
     Effect of the Invention 
     According to the present invention, it is possible to improve separation efficiency when nanocarbons having different properties are separated. Alternatively, according to the present invention, it is possible to shorten the time required for separation when nanocarbons having different properties are separated. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic diagram showing a nanocarbon separation apparatus used in a nanocarbon separation method according to a first example embodiment. 
         FIG. 2  is a flowchart showing the nanocarbon separation method according to the first example embodiment. 
         FIG. 3  is a schematic diagram showing a nanocarbon separation apparatus used in a nanocarbon separation method according to a second example embodiment. 
         FIG. 4  is a diagram showing a pH distribution of a single-walled carbon nanotube dispersion liquid which has been subjected to a separation operation in working example 1. 
         FIG. 5  is a diagram showing an absorption spectrum of a single-walled carbon nanotube dispersion liquid which has been subjected to a separation operation in working example 1. 
         FIG. 6  is a diagram showing the absorbance of a single-walled carbon nanotube dispersion liquid which has been subjected to a separation operation in working example 1. 
         FIG. 7  is a diagram showing a pH and a density of a single-walled carbon nanotube dispersion liquid which has been subjected to a separation operation in working example 2. 
         FIG. 8  is a diagram showing a pH after an aqueous surfactant solution and water have been subjected to an electrophoretic treatment in working example 3. 
         FIG. 9  is a diagram showing a layer structure of a solution in a separation tank in working example 4. 
         FIG. 10  is a diagram showing a pH distribution of an aqueous solution which has been subjected to a separation operation in working example 4. 
         FIG. 11  is a diagram showing a layer structure of a solution which has not been subjected to a separation operation in a separation tank in working example 5. 
         FIG. 12  is a diagram showing a layer structure of a solution which has been subjected to a separation operation in a separation tank in working example 5. 
         FIG. 13  is a diagram showing a pH of a solution which has been subjected to a separation operation in working example 5. 
         FIG. 14  is a diagram showing a Raman spectrum of a solution which has been subjected to a separation operation in working example 5. 
     
    
    
     EMBODIMENTS FOR CARRYING OUT THE INVENTION 
     Example embodiments of a nanocarbon separation method and a nanocarbon separation apparatus according to the present invention will be described. 
     These example embodiments are specifically described to make the gist of the invention better understood and are not intended to limit the present invention unless otherwise stated. 
     First Example Embodiment 
     (Nanocarbon Separation Apparatus) 
       FIG. 1  is a schematic diagram showing a nanocarbon separation apparatus used in a nanocarbon separation method in this example embodiment. 
     A nanocarbon separation apparatus  10  in this example embodiment includes a separation tank (an electrophoresis tank)  11  having an I-shaped structure (a vertical type structure), a first electrode  12  provided at an upper part of the interior of the separation tank  11 , a second electrode  13  provided at a lower part of the interior of the separation tank  11 , an injection port  14  through which a nanocarbon dispersion liquid  30  is injected into the separation tank  11 , a first recovery port  15  which recovers the nanocarbon dispersion liquid  30  from the separation tank  11 , and a second recovery port  16  which recovers the nanocarbon dispersion liquid  30  from the separation tank  11 . 
     The first electrode  12  is disposed at an upper part of the separation tank  11  in a height direction thereof inside the separation tank  11  (a region above half a height of the separation tank  11  inside the separation tank  11 ). 
     The second electrode  13  is disposed at lower part of the separation tank  11  in the height direction thereof inside the separation tank  11  (a region below half a height of the separation tank  11  inside the separation tank  11 ). 
     In the nanocarbon separation apparatus  10  in this example embodiment, the first electrode  12  is a negative electrode and the second electrode  13  is the positive electrode. In this case, if a direct current (DC) voltage is applied to the first electrode  12  and the second electrode  13 , an electric field E is directed from the bottom to the top of the separation tank  11  as shown in  FIG. 1 . 
     The first recovery port  15  is provided in the vicinity of the first electrode  12 . That is to say, the first recovery port  15  is provided at an upper part of the separation tank  11  in the height direction thereof in the separation tank  11  (a region above half the height of the separation tank  11  in the separation tank  11 ). 
     The second recovery port  16  is provided in the vicinity of the second electrode  13 . That is to say, the second recovery port  16  is provided at a lower part of the separation tank  11  in the height direction thereof in the separation tank  11  (a region below half the height of the separation tank  11  in the separation tank  11 ). 
     The injection port  14  is provided at an upper part of the separation tank  11  in the height direction thereof in the separation tank  11  (a region above half the height of the separation tank  11  in the separation tank  11 ) and below the first recovery port  15 . 
     The separation tank  11  has a space in which the nanocarbon dispersion liquid  30  can be accommodated. The separation tank  11  accommodates the nanocarbon dispersion liquid  30  to be separated and separates a nanocarbon mixture using carrier-free electrophoresis. A shape and a size of the separation tank  11  are not particularly limited as long as the separation tank  11  can accommodate the nanocarbon dispersion liquid  30 . 
     A material of the separation tank  11  is not particularly limited as long as the material is stable with respect to the nanocarbon dispersion liquid  30  and is an insulating material. Examples for the separation tank  11  include glass, quartz, an acrylic resin, and the like. 
     The first electrode  12  and the second electrode  13  are not particularly limited as long as the electrodes can be used for carrier-free electrophoresis and are stable with respect to the nanocarbon dispersion liquid  30 . Examples of the first electrode  12  and the second electrode  13  include a platinum electrode and the like. 
     Although a constitution having the injection port  14  and the first recovery port  15  has been provided as an exemplary example of the case of the nanocarbon separation apparatus  10  in this example embodiment, the nanocarbon separation apparatus  10  in this example embodiment is not limited thereto. In the nanocarbon separation apparatus  10  in this example embodiment, the injection port  14  may also serve as the first recovery port  15 . Furthermore, the injection port  14 , the first recovery port  15 , and the second recovery port  16  are not provided and the nanocarbon dispersion liquid may be injected and recovered using another method, for example, a Pasteur pipette. 
     Also, although a case in which the first electrode  12  is a negative electrode and the second electrode  13  is a positive electrode has been provided as an exemplary example of the nanocarbon separation apparatus  10  in this example embodiment, the nanocarbon separation apparatus  10  in this example embodiment is not limited thereto. In the nanocarbon separation apparatus  10  in this example embodiment, the first electrode  12  may be a positive electrode and the second electrode  13  may be a negative electrode. In this case, a direction of a pH gradient of the nanocarbon dispersion liquid  30  formed inside the separation tank  11  through carrier-free electrophoresis is opposite to that of an example embodiment which will be described later. 
     Furthermore, in the nanocarbon separation method using the nanocarbon separation apparatus  10  which will be described later, if a temperature gradient occurs inside the separation tank  11 , a convection phenomenon of the nanocarbon dispersion liquid  30  occurs inside the separation tank  11  in some cases. As a result, it is not possible to stably separate metallic nanocarbons and semiconductor nanocarbons contained in the nanocarbon dispersion liquid  30 . Thus, it is desirable that the nanocarbon separation apparatus  10  include a temperature adjusting means for keeping a temperature of the nanocarbon dispersion liquid  30  inside the separation tank  11  constant. The temperature adjusting means is not particularly limited and can be any means, for example, installation or the like of a water-cooling jacket can be used as long as a temperature of a liquid accommodated inside a container can be kept constant. 
     According to the nanocarbon separation apparatus  10  for the nanocarbons in this example embodiment, for example, it is possible to perform the nanocarbon separation method which will be described later and it is possible to generate a pH gradient in the nanocarbon dispersion liquid  30  in which the nanocarbons and the non-ionic surfactant are dispersed in the solvent and which has been accommodated inside the separation tank  11 . Furthermore, according to the nanocarbon separation apparatus  10  for the nanocarbons in this example embodiment, for example, it is possible to perform the nanocarbon separation method which will be described later and it is possible to increase the amount of metallic nanocarbons in the vicinity of the first electrode  12  which is a positive electrode and to increase the amount of semiconductor nanocarbons in the vicinity of the second electrode  13  which is a negative electrode. Thus, it is possible stably separate the metallic nanocarbons and the semiconductor nanocarbons. As a result, it is possible to obtain metallic nanocarbons and semiconductor nanocarbons having a high purity. Furthermore, according to the nanocarbon separation apparatus  10  in this example embodiment, since the nanocarbon dispersion liquid  30  contains the non-ionic surfactant, in carrier-free electrophoresis, it is possible to reduce an amount of current flowing through the nanocarbon dispersion liquid  30  and it is possible to minimize the amount of heat generated in the nanocarbon dispersion liquid  30 . 
     (Nanocarbon Separation Method) 
     The nanocarbon separation method using the nanocarbon separation apparatus  10  will be described and an action of the nanocarbon separation apparatus  10  will be described with reference to  FIGS. 1 and 2 .  FIG. 2  is a flowchart showing the separation method in this example embodiment. 
     The nanocarbon separation method in this example embodiment includes a step of preparing the nanocarbon dispersion liquid  30  in which the nanocarbons and the non-ionic surfactant are dispersed in the solvent (hereinafter referred to as a “preparation step”), a step of injecting the nanocarbon dispersion liquid  30  into the separation tank  11  (hereinafter referred to as an “injection step”), a step of applying the DC voltage to the first electrode  12  and the second electrode  13  and generating the pH gradient in the nanocarbon dispersion liquid  30  inside the separation tank  11  (hereinafter referred to as a “pH gradient formation step”), and a step of separating the metallic nanocarbons and the semiconductor nanocarbons contained in the nanocarbon dispersion liquid  30  (hereinafter referred to as a “separation step”). 
     In the nanocarbon separation method in this example embodiment, nanocarbons refer to carbon materials mainly composed of carbon such as single-walled carbon nanotubes, double-walled carbon nanotubes, multi-walled carbon nanotubes, carbon nanohorns, carbon nanotwists, graphenes, and fullerenes. In the nanocarbon separation method in this example embodiment, a case in which a semiconductor type single-walled carbon nanotube and a metallic type single-walled carbon nanotube are separated from the nanocarbon dispersion liquid  30  containing single-walled carbon nanotubes as nanocarbons will be described in detail. 
     It is known that single-walled carbon nanotubes are divided into single-walled carbon nanotubes having two different properties, i.e., metallic type single-walled carbon nanotubes and semiconductor type single-walled carbon nanotubes in accordance with diameters of tubes and a winding method. If single-walled carbon nanotubes are synthesized using conventional manufacturing methods, single-walled carbon nanotube mixtures containing metallic type single-walled carbon nanotubes having metallic properties and semiconductor type single-walled carbon nanotubes having semiconductive properties in a statistical ratio of 1:2 are obtained. 
     The single-walled carbon nanotube mixtures are not particularly limited as long as the single-walled carbon nanotube mixtures contain a metallic type single-walled carbon nanotube and a semiconductor type single-walled carbon nanotube. Furthermore, the single-walled carbon nanotubes in this example embodiment may be single-walled carbon nanotubes alone and may be single-walled carbon nanotubes in which carbon is partially substituted with an arbitrary functional group or single-walled carbon nanotubes modified with an arbitrary functional group. 
     In the nanocarbon separation method in this example embodiment, the nanocarbon mixture contains metallic nanocarbons and semiconductor nanocarbons. 
     First, in the preparation step, the nanocarbon dispersion liquid  30  in which the nanocarbon mixture is dispersed in a dispersion medium together with a non-ionic surfactant is prepared. 
     The dispersion medium is not particularly limited as long as the dispersion medium can disperse a nanocarbon mixture. Examples of the dispersion medium include water, heavy water, organic solvents, ionic solutions, and the like. Among these dispersion mediums, it is desirable that water or heavy water be used because in this case the quality of nanocarbons does not change. 
     As the non-ionic surfactant, non-ionic surfactants having a hydrophilic site in which ionization does not occur and a hydrophobic site such as an alkyl chain are used. Examples of such non-ionic surfactants include non-ionic surfactants having a polyethylene glycol structure represented by a polyoxyethylene alkyl ether type. 
     As such non-ionic surfactants, it is desirable that a polyoxyethylene alkyl ether represented by the following Expression (1) be used: 
       C n H 2n (OCH 2 CH 2 ) m OH  (1)
 
     (where, n=12 to 18 and m=20 to 100). 
     Examples of the polyoxyethylene alkyl ether represented by the foregoing Expression (1) include polyoxyethylene (23) lauryl ether (trade name: Brij L23; manufactured by Sigma-Aldrich), polyoxyethylene (20) cetyl ether (trade name: Brij C20; manufactured by Sigma-Aldrich), polyoxyethylene (20) stearyl ether (trade name: Brij S20; manufactured by Sigma-Aldrich), polyoxyethylene (20) oleyl ether (trade name: Brij 020; manufactured by Sigma-Aldrich), polyoxyethylene (100) stearyl ether (trade name: Brij S100; manufactured by Sigma-Aldrich), and the like. 
     As the non-ionic surfactant, polyoxyethylene sorbitan monostearate (molecular expression: C 64 H 126 O 26 ; trade name: Tween 60; manufactured by Sigma-Aldrich), polyoxyethylene sorbitan trioleate (molecular expression: C 24 H 44 O 6 ; trade name: Tween 85; manufactured by Sigma-Aldrich), octylphenol ethoxylate (molecular expression: C 14 H 22 O(C 2 H 4 O) n ; n=1 to 10; trade name: Triton X-100; manufactured by Sigma-Aldrich), polyoxyethylene (40) isooctyl phenyl ether (molecular expression: C 8 H 17 C 6 H 4 O(CH 2 CH 2 O) 40 H; trade name: Triton X-405; manufactured by Sigma-Aldrich), poloxamer (molecular expression: C 5 H 10 O 2 ; trade name: Pluronic; manufactured by Sigma-Aldrich), polyvinylpyrrolidone (molecular expression: (C 6 H 9 NO) n ; n=5 to 100; manufactured by Sigma-Aldrich), and the like can also be used. 
     The amount of the non-ionic surfactant in the nanocarbon dispersion liquid  30  is preferably 0.1 wt % or more and 5 wt % or less, and more preferably 0.5 wt % or more and 2 wt % or less. 
     If the amount of the non-ionic surfactant is 0.1 wt % or more, it is possible to form a pH gradient of the nanocarbon dispersion liquid  30  inside the separation tank  11  through carrier-free electrophoresis. On the other hand, if the amount of non-ionic surfactants is 5 wt % or less, a viscosity of the nanocarbon dispersion liquid  30  does not excessively increase and it is possible to easily separate the metallic nanocarbons and the semiconductor nanocarbons contained in the nanocarbon dispersion liquid  30  through carrier-free electrophoresis. 
     The amount of nanocarbons in the nanocarbon dispersion liquid  30  is preferably 1 μg/mL or more and 100 μg/mL or less, and more preferably 5 μg/mL or more and 40 μg/mL or less. 
     If the amount of nanocarbons is in the above range, it is possible to easily separate the metallic nanocarbons and the semiconductor nanocarbons contained in the nanocarbon dispersion liquid  30  through carrier-free electrophoresis. 
     A method for preparing the nanocarbon dispersion liquid  30  is not particularly limited as long as a known method is used as the method. Examples of the method include a method for subjecting a mixed solution of a nanocarbon mixture and a dispersion medium containing a non-ionic surfactant to ultrasonic treatment to disperse the nanocarbon mixture in the dispersion medium. This ultrasonic treatment sufficiently separates aggregated mixtures of metallic nanocarbons and semiconductor nanocarbons and the nanocarbon dispersion liquid  30  is obtained by uniformly dispersing metallic nanocarbons and semiconductor nanocarbons in the dispersion medium. Therefore, the metallic nanocarbons and the semiconductor nanocarbons are easily separated through a carrier-free electrophoresis method which will be described later. It is desirable that metallic nanocarbons and semiconductor nanocarbons which have not been dispersed through ultrasonic treatment be separated and removed through ultracentrifugation. 
     Subsequently, in the injection step, the nanocarbon dispersion liquid  30  prepared in the preparation step is injected into the separation tank  11  through the injection port  14 . 
     Subsequently, in the pH gradient formation step, a pH gradient is generated in the nanocarbon dispersion liquid  30  inside the separation tank  11  by applying a DC voltage to the first electrode  12  and the second electrode  13 . 
     By applying a DC voltage to the first electrode  12  and the second electrode  13  for a predetermined time (for example, 1 to 24 hours), a pH gradient of the nanocarbon dispersion liquid  30  is formed inside the separation tank  11 . 
     Subsequently, in the separation step, the metallic nanocarbons and the semiconductor nanocarbons contained in the nanocarbon dispersion liquid  30  are separated through the carrier-free electrophoresis method. 
     The nanocarbon mixture contained in the nanocarbon dispersion liquid  30  is separated into the metallic nanocarbons and the semiconductor nanocarbons in the separation step due to a resultant force of a moving force generated due to a pH gradient (a pH difference) formed in the pH gradient formation step in the nanocarbon dispersion liquid  30  inside the separation tank  11  and an electrophoretic force generated due to an electric field and charges. 
     If a DC voltage is applied to the first electrode  12  and the second electrode  13  in a state in which the nanocarbon dispersion liquid  30  has been accommodated inside the separation tank  11 , in the nanocarbon dispersion liquid  30 , a region having a pH of 6 to 7 is formed on the first electrode  12  (the negative electrode) side and a region having a pH of 4 to 5 is formed on the second electrode  13  (the positive electrode) side. A pH gradient in a direction of an electric field E generated in the separation tank  11  is formed in the nanocarbon dispersion liquid  30  by applying a DC voltage to the first electrode  12  and the second electrode  13 . That is to say, the pH gradient in the nanocarbon dispersion liquid  30  is formed so that the pH increases from a lower side of the separation tank  11  toward an upper side thereof. 
     In the nanocarbon dispersion liquid  30  containing a non-ionic surfactant, the metallic nanocarbons have a positive charge and the semiconductor nanocarbons have a very weak negative charge. 
     Therefore, if a DC voltage is applied to the first electrode  12  and the second electrode  13 , in the nanocarbon mixture contained in the nanocarbon dispersion liquid  30 , the metallic nanocarbons move toward the first electrode  12  (the negative electrode) side and the semiconductor nanocarbons move toward the second electrode  13  (the positive electrode) side. As a result, the nanocarbon dispersion liquid  30  is phase-separated into three phases, i.e., a dispersion phase having a relatively high amount of metallic nanocarbons (hereinafter referred to as a “dispersion phase A”), a dispersion phase having a relatively high amount of semiconductor nanocarbons (hereinafter referred to as a “dispersion phase B”), and a dispersion phase formed between the dispersion phase A and the dispersion phase B and having a relatively low amount of metallic nanocarbons and semiconductor nanocarbons (hereinafter referred to as a “dispersion phase C”). 
     In this example embodiment, the dispersion phase A is formed on the first electrode  12  side and the dispersion phase B is formed on the second electrode  13  side. 
     A DC voltage applied to the first electrode  12  and the second electrode  13  is not particularly limited and is appropriately adjusted in accordance with a distance between the first electrode  12  and the second electrode  13 , the amount of nanocarbon mixtures in the nanocarbon dispersion liquid  30 , and the like. 
     When water or heavy water is used as the dispersion medium of the nanocarbon dispersion liquid  30 , a DC voltage applied to the first electrode  12  and the second electrode  13  is set to an arbitrary value higher than 0 V and 1000 V or lower. 
     For example, when a distance between the first electrode  12  and the second electrode  13  (a distance between electrodes) is 30 cm, a DC voltage applied to the first electrode  12  and the second electrode  13  is preferably 15 V or higher and 450 V or lower, and more preferably 30 V or higher and 300 V or lower. 
     If a DC voltage applied to the first electrode  12  and the second electrode  13  is 15 V or higher, it is possible to form a pH gradient in the nanocarbon dispersion liquid  30  inside the separation tank  11  and separate the metallic nanocarbons and the semiconductor nanocarbons contained in the nanocarbon dispersion liquid  30 . On the other hand, if a DC voltage applied to the first electrode  12  and the second electrode  13  is 450 V or lower, an influence of water or heavy water electrolysis can be minimized. 
     Also, when a DC voltage is applied to the first electrode  12  and the second electrode  13 , an electric field between the first electrode  12  and the second electrode  13  is preferably 0.5 V/cm or higher and 15 V/cm or lower, and more preferably 1 V/cm or higher and 10 V/cm or lower. 
     If an electric field between the first electrode  12  and the second electrode  13  is 0.5 V/cm or higher, it is possible to form a pH gradient in the nanocarbon dispersion liquid  30  inside the separation tank  11  and separate the metallic nanocarbons and the semiconductor nanocarbons contained in the nanocarbon dispersion liquid  30 . On the other hand, when an electric field between the first electrode  12  and the second electrode  13  is 15 V/cm or lower, an influence of water or heavy water electrolysis can be minimized. 
     Subsequently, the separated dispersion phase A and dispersion phase B are recovered (fractionated). 
     A recovery method is not particularly limited and any method may be used as long as the dispersion phase A and the dispersion phase B are not mixed in a diffusion manner. 
     Examples of the recovery method include a method in which application of a DC voltage to the first electrode  12  and the second electrode  13  is stopped and a small amount is gently suctioned out from each phase using a pipette. 
     Also, examples of the recovery method include a method in which the nanocarbon dispersion liquid  30  with the dispersion phase A is continuously suctioned out from the first recovery port  15  and the nanocarbon dispersion liquid  30  with the dispersion phase B is continuously suctioned out from the second recovery port  16  while a DC voltage is being applied to the first electrode  12  and the second electrode  13 . This method uses the fact that the dispersion phase A having a high amount of metallic nanocarbons is formed in the vicinity of the first electrode  12  and the dispersion phase B having a high amount of semiconductor nanocarbons is formed in the vicinity of the second electrode  13 . 
     When the recovered dispersion liquid is accommodated in the separation tank  11  again and an operation of separating the metallic nanocarbons and the semiconductor nanocarbons contained in the nanocarbon dispersion liquid  30  is repeatedly performed using the carrier-free electrophoresis method in the same manner as described above, it is possible to obtain metallic nanocarbons and semiconductor nanocarbons having higher purity. 
     It is possible to evaluate the separation efficiency for the recovered dispersion liquid using a technique such as micro-Raman spectroscopy (a change in Raman spectrum of a radial breathing mode (RBM) region and a change in Raman spectrum shape of a Breit-Wigner-Fano (BWF) region) and ultraviolet (UV)-visible near-absorptive spectrophotometry (a change in peak shape of an absorption spectrum). Furthermore, it is also possible to evaluate the separation efficiency for the dispersion liquid by evaluating the electrical characteristics of the nanocarbon. For example, it is possible to evaluate the separation efficiency for the dispersion liquid by preparing a field effect transistor and measuring the transistor characteristics thereof. 
     According to the nanocarbon separation method in this example embodiment, the amount of metallic nanocarbons increases in the vicinity of the first electrode  12  which is a positive electrode and the amount of semiconductor nanocarbons increases in the vicinity of the second electrode  13  which is a negative electrode. Thus, it is possible to stably separate the metallic nanocarbons and the semiconductor nanocarbons. As a result, it is possible to obtain metallic nanocarbons and semiconductor nanocarbons having high purity. Furthermore, according to the nanocarbon separation method in this example embodiment, the nanocarbon dispersion liquid  30  contains an non-ionic surfactant. Thus, in carrier-free electrophoresis, it is possible to reduce the amount of current flowing through the nanocarbon dispersion liquid  30  and it is possible to minimize the amount of heat generated in the nanocarbon dispersion liquid  30 . 
     Although a case in which the nanocarbon mixture is separated into metallic nanocarbons and semiconductor nanocarbons has been provided as an exemplary example of the nanocarbon separation method in this example embodiment, the nanocarbon separation method in this example embodiment is not limited thereto. The nanocarbon separation method in this example embodiment may be performed as, for example, a nanocarbon purification method in which separation into metallic nanocarbons and semiconductor nanocarbons has been performed inside the separation tank  11  and only nanocarbons having target properties are recovered. 
     Second Example Embodiment 
     (Nanocarbon Separation Apparatus) 
       FIG. 3  is a schematic diagram showing a nanocarbon separation apparatus used in a nanocarbon separation method in this example embodiment. 
     A nanocarbon separation apparatus  100  in this example embodiment includes a separation tank (an electrophoresis tank)  101  having an I-shaped structure (a vertical type structure) and an electrode member  104  which has a first electrode  102  provided at an upper part of the interior of a separation tank  101  and a second electrode  103  provided at a lower part of the interior of the separation tank  101 . 
     The separation tank  101  has an upper end having an opening section  105 . Furthermore, the separation tank  101  has a lower end having an injection/recovery port  106  communicating with an outer bottom surface  101   a  of the separation tank  101 . The injection/recovery port  106  is used for injecting a nanocarbon dispersion liquid  30  into the separation tank  101  and recovering the nanocarbon dispersion liquid  30  from the separation tank  101 . In addition, the injection/recovery port  106  has a sealing structure (not shown) such as a rotary cock having a ground glass joint. 
     When the nanocarbon dispersion liquid  30  is gently injected and recovered through the injection/recovery port  106  in a bottom portion of the separation tank  101  using, for example, a peristaltic pump or the like, it is possible to perform an injection/recovery operation without moving the injection/recovery port in accordance with a change in liquid level and disturbing a liquid phase interface inside the separation tank  101  at the time of injection/recovery. Furthermore, when the capacity of the separation tank  101  is increased, there is no need to prepare a long injection/recovery nozzle, which is very reasonable. 
     The electrode member  104  includes an insulating cylindrical member  107 , an insulating columnar member  108  inserted into the cylindrical member  107 , the first electrode  102  provided on an outer circumferential surface  107   a  of the cylindrical member  107 , and the second electrode  103  provided on a lower end portion of the columnar member  108 . Furthermore, the first electrode  102  is provided at an upper end portion of the cylindrical member  107 . In the nanocarbon separation apparatus  100  in this example embodiment, the first electrode  102  is a negative electrode and the second electrode  103  is a positive electrode. 
     The cylindrical member  107  extends over substantially the entire area of the separation tank  101  in a height direction thereof. 
     The columnar member  108  extends over substantially the entire area of the separation tank  101  in the height direction thereof in a state of being inserted into the cylindrical member  107 . Furthermore, the columnar member  108  is capable of moving in the height direction of the separation tank  101  in a state of being inserted into the cylindrical member  107 . In addition, a plate-like fitting member  109  inscribed in a lower end portion of the cylindrical member  107  when the columnar member  108  moves upward in the height direction of the separation tank  101  is provided at a lower end of the columnar member  108 . A side surface  109   a  of the fitting member  109  has a shape similar to that of an inner side surface  107   b  of the lower end portion of the cylindrical member  107 . Thus, when the columnar member  108  moves upward in the height direction of the separation tank  101 , the fitting member  109  can be inscribed and fitted to the lower end portion of the cylindrical member  107 . On the other hand, when the columnar member  108  moves downward in the height direction of the separation tank  101 , a gap can be provided between a lower end of the cylindrical member  107  and the fitting member  109 . When metallic nanocarbons and semiconductor nanocarbons contained in the nanocarbon dispersion liquid  30  are separated through carrier-free electrophoresis, the gap is provided between the lower end of the cylindrical member  107  and the fitting member  109 . In this state, if a DC voltage is applied to the first electrode  102  and the second electrode  103 , a pH gradient is formed in the nanocarbon dispersion liquid  30  inside the separation tank  101 . Even when the gap is provided between the lower end of the cylindrical member  107  and the fitting member  109 , it is desirable that most of the second electrode  103  be arranged inside the cylindrical member  107 . 
     Examples of a material of the separation tank  101  include the same as the material of the separation tank  11 . 
     Examples of the first electrode  102  and the second electrode  103  include the same as the first electrode  12  and the second electrode  13 . 
     Materials of the cylindrical member  107 , the columnar member  108 , and the fitting member  109  are not particularly limited as long as the materials are stable to the nanocarbon dispersion liquid  30  and insulating materials and examples thereof include glass, quartz, acrylic resins, and the like. 
     Although a case in which the first electrode  102  is the negative electrode and the second electrode  103  is the positive electrode has been provided as an exemplary example of the nanocarbon separation apparatus  100  in this example embodiment, the nanocarbon separation apparatus  100  in this example embodiment is not limited thereto. In the nanocarbon separation apparatus  100  in this example embodiment, the first electrode  102  may be a positive electrode and the second electrode  103  may be a negative electrode. In this case, the direction of a pH gradient of the nanocarbon dispersion liquid  30  formed inside the separation tank  101  through carrier-free electrophoresis is opposite to that of an example embodiment which will be described later. 
     Also, if a temperature gradient is generated inside the separation tank  101  in the nanocarbon separation method using the nanocarbon separation apparatus  100  which will be described later, a convection phenomenon in the nanocarbon dispersion liquid  30  occurs inside the separation tank  101  in some cases. As a result, the metallic nanocarbons and the semiconductor nanocarbons contained in the nanocarbon dispersion liquid  30  cannot be stably separated. Thus, it is desirable that the nanocarbon separation apparatus  100  include a temperature adjusting means for keeping a temperature of the nanocarbon dispersion liquid  30  inside the separation tank  101  constant. The temperature adjusting means is not particularly limited and can be any means, for example, installation of a water-cooling jacket or the like as long as the temperature adjusting means can keep a temperature of a liquid accommodated inside a container constant. 
     According to the nanocarbon separation apparatus  100  for the nanocarbons in this example embodiment, for example, it is possible to perform the nanocarbon separation method which will be described later and it is possible to generate a pH gradient in the nanocarbon dispersion liquid  30  in which the nanocarbons and the non-ionic surfactant are dispersed in the solvent and which is accommodated inside the separation tank  101 . Furthermore, according to the nanocarbon separation apparatus  100  for the nanocarbons in this example embodiment, for example, it is possible to perform the nanocarbon separation method which will be described later and it is possible to increase the amount of metallic nanocarbons in the vicinity of the first electrode  102  which is a positive electrode and increase the amount of semiconductor nanocarbons in the vicinity of the second electrode  103  which is a negative electrode. Thus, it is possible to stably separate the metallic nanocarbons and the semiconductor nanocarbons. As a result, it is possible to obtain the metallic nanocarbons and the semiconductor nanocarbons having high purity. Furthermore, according to the nanocarbon separation apparatus  100  for the nanocarbons in this example embodiment, the nanocarbon dispersion liquid  30  contains the non-ionic surfactant. Thus, it is possible to reduce the amount of current flowing through the nanocarbon dispersion liquid  30  in carrier-free electrophoresis and it is possible to minimize the amount of heat generated in the nanocarbon dispersion liquid  30 . 
     (Nanocarbon Separation Method) 
     The nanocarbon separation method using the nanocarbon separation apparatus  100  will be described and an action of the nanocarbon separation apparatus  100  will be described with reference to  FIG. 3 . 
     The nanocarbon separation method in this example embodiment includes a step of preparing the nanocarbon dispersion liquid  30  (the preparation step), a step of injecting the nanocarbon dispersion liquid  30  into the separation tank  101  (the injection step), a step of generating a pH gradient in the nanocarbon dispersion liquid  30  inside the separation tank  11  by applying a DC voltage to the first electrode  102  and the second electrode  103  (the pH gradient formation step), and a step of separating the metallic nanocarbons and the semiconductor nanocarbons contained in the nanocarbon dispersion liquid  30  (the separation step). 
     The preparation step and the injection step are performed in the same manner as in the first example embodiment. Furthermore, as described above, in the injection step in this example embodiment, it is also possible to inject the nanocarbon dispersion liquid  30  gently through the injection/recovery port  106  provided in a bottom portion of the separation tank  101  using, for example, a peristaltic pump or the like. 
     Subsequently, in the pH gradient formation step, a pH gradient is generated in the nanocarbon dispersion liquid  30  inside the separation tank  101  by applying a DC voltage to the first electrode  12  and the second electrode  13 . 
     In the pH gradient formation step, when the columnar member  108  moves downward in the height direction of the separation tank  101 , a DC voltage is applied to the first electrode  12  and the second electrode  13  in a state in which a gap is provided between the lower end of the cylindrical member  107  and the fitting member  109 . 
     Subsequently, in the separation step, the metallic nanocarbons and the semiconductor nanocarbons contained in the nanocarbon dispersion liquid  30  are separated through the carrier-free electrophoresis method. 
     In the pH gradient formation step, the metallic nanocarbons and the semiconductor nanocarbons contained in the nanocarbon dispersion liquid  30  are separated through carrier-free electrophoresis in the same manner as in the first example embodiment in a state in which a pH gradient is formed in the nanocarbon dispersion liquid  30  inside the separation tank  101 . As described above, even when a gap is provided between the lower end of the cylindrical member  107  and the fitting member  109 , it is desirable that most of the second electrode  103  be arranged inside the cylindrical member  107 . 
     If a DC voltage is applied to the first electrode  102  as a negative electrode and the second electrode  103  as a positive electrode, water or heavy water is subjected to electrolysis, hydrogen (deuterium) is generated in the first electrode  102  and oxygen is generated in the second electrode  103 . If the oxygen generated in the second electrode  103  forms air bubbles and moves to an upper part of the separation tank  101 , a convection phenomenon of the nanocarbon dispersion liquid  30  being separated occurs in some cases. 
     In the nanocarbon separation apparatus  100  in this example embodiment, a voltage for carrier-free electrophoresis is applied in a state in which a gap is provided between the lower end of the cylindrical member  107  and the fitting member  109 . Oxygen generated through electrolysis in the second electrode  103  moves up inside the cylindrical member  107  and goes outside of the separation tank  101 . In the nanocarbon separation method in this example embodiment, separation of the metallic nanocarbons and the semiconductor nanocarbons contained in the nanocarbon dispersion liquid  30  is mainly performed between the separation tank  101  and the cylindrical member  107 . Therefore, according to the nanocarbon separation method in this example embodiment, it is possible to prevent a convection phenomenon of the nanocarbon dispersion liquid  30  being separated due to air bubbles formed by oxygen generated in the second electrode  103 . Accordingly, disturbance of air bubbles generated due to electrolysis of water or heavy water is reduced, stable separation is performed, and accuracy of separation is improved compared to when the nanocarbon separation apparatus  10  used in the first example embodiment is used. Furthermore, it is possible to apply a higher voltage and perform separation faster and more stably. 
     Subsequently, the separated dispersion phase A and dispersion phase B are recovered (fractionated). 
     A recovery method is not particularly limited and any method may be used as long as the dispersion phase A and the dispersion phase B are not mixed in a diffusion manner. 
     Examples of the recovery method include a method in which application of a DC voltage to the first electrode  102  and the second electrode  103  is stopped, a rotary cock provided in the injection/recovery port  106  opens, a dispersion liquid with the dispersion phase A is slowly discharged from the bottom of the separation tank  101  using a peristaltic pump or the like, and this dispersion liquid is recovered. Subsequently, a dispersion liquid with the dispersion phase B is slowly discharged from the bottom of the separation tank  101  and this dispersion liquid is recovered. 
     It is possible to obtain the metallic nanocarbons and the semiconductor nanocarbons having high purity by accommodating the recovered dispersion liquids in the separation tank  101  again and repeatedly performing an operation in which the metallic nanocarbons and the semiconductor nanocarbons contained in the nanocarbon dispersion liquid  30  are separated through the carrier-free electrophoresis method in the same manner as described above. 
     The separation efficiency of the recovered dispersion liquid can be evaluated in the same manner as in the first example embodiment. 
     According to the nanocarbon separation method in this example embodiment, the amount of metallic nanocarbons increases in the vicinity of the first electrode  102  which is a negative electrode and the amount of semiconductor nanocarbons increases in the vicinity of the second electrode  103  which is a positive electrode. Thus, it is possible to stably separate the metallic nanocarbons and the semiconductor nanocarbons. As a result, it is possible to obtain the metallic nanocarbons and the semiconductor nanocarbons having high purity. Furthermore, according to the nanocarbon separation method in this example embodiment, the nanocarbon dispersion liquid  30  contains the non-ionic surfactant. Thus, in carrier-free electrophoresis, it is possible to reduce an amount of current flowing through the nanocarbon dispersion liquid  30  and it is possible to minimize the amount of heat generated in the nanocarbon dispersion liquid  30 . 
     Although a case in which the nanocarbon mixture is separated into the metallic nanocarbons and the semiconductor nanocarbons has been provided as an exemplary example of the nanocarbon separation method in this example embodiment, the nanocarbon separation method in this example embodiment is not limited thereto. The nanocarbon separation method in this example embodiment may be performed, for example, as a nanocarbon purification method in which separation into the metallic nanocarbons and the semiconductor nanocarbons is performed inside the separation tank  101  and then only nanocarbons having target properties are recovered. 
     Also, when the first electrode  102  is a positive electrode and the second electrode  103  is a negative electrode, occurrence of a convection phenomenon of the nanocarbon dispersion liquid  30  being separated due to air bubbles formed by the hydrogen generated in the second electrode  103  can be prevented by moving up hydrogen generated through electrolysis in the second electrode  103  inside the cylindrical member  107  and discharging the hydrogen outside of the separation tank  101 . 
     Although the example embodiment which can be applied to a case in which the nanocarbon mixture is separated into the metallic nanocarbons and the semiconductor nanocarbons has been described above, the present invention can also be applied when a multi-walled carbon nanotube mixture, a double-walled carbon nanotube mixture, a graphene mixture, or the like is separated. 
     WORKING EXAMPLES 
     Although the present invention will be described in more detail below through working examples, the present invention is not limited to the following working examples. 
     Working Example 1 
     “Preparation of Single-Walled Carbon Nanotube Dispersion Liquid” 
     Solution AA obtained by dissolving 1 wt % of polyoxyethylene (100) stearyl ether (trade name: Brij S100; manufactured by Sigma-Aldrich) which is a non-ionic surfactant in heavy water was prepared. 
     A single-walled carbon nanotube mixture (eDIPS (enhanced Direct Injection Pyrolytic Synthesis), single-walled carbon nanotube; average diameter: 1.3 nm) was put into the solution AA. 
     The single-walled carbon nanotube mixture is dispersed by subjecting the solution AA having the single-walled carbon nanotube mixture put therein to ultrasonic dispersion treatment at an output of 40 W for 20 minutes using a horn type ultrasonic crusher (trade name: Digital Sonifier 450; manufactured by Branson Ultrasonics). After that, an ultracentrifugation operation was performed using an ultracentrifuge (trade name: CS100GX; manufactured by Hitachi koki) at 250000×g and 10° C. for 1 hour. Furthermore, a supernatant corresponding to 80% of the single-walled carbon nanotube mixture was fractionated and a single-walled carbon nanotube dispersion liquid in which the amount of a single-walled carbon nanotube was 10 μg/mL and the amount of polyoxyethylene (100) stearyl ether was 1.0 wt % was obtained. 
     “Injection of Single-Walled Carbon Nanotube Dispersion Liquid” 
     The single-walled carbon nanotube dispersion liquid prepared as described above was injected into the separation tank  101  in the nanocarbon separation apparatus  100  shown in  FIG. 3 . A height of the single-walled carbon nanotube dispersion liquid injected into the separation tank  101  (a height from a bottom surface of the separation tank  101  to a liquid surface) was set to 25 cm. 
     “Separation Operation” 
     A DC voltage of 120 V was applied to the first electrode  102  (a negative electrode) and the second electrode  103  (a positive electrode) of the nanocarbon separation apparatus  100 . A voltage application was stopped when a predetermined time had passed and the separation sufficiently advanced. 
     “Recovery Operation” 
     After the voltage application was completed, the single-walled carbon nanotube dispersion liquid was recovered from the upper part of the separation tank  101  so that 15 fractions were formed for each about 6 mL. The fractions was designated as F1, F2, . . . , and F15 from the second electrode  103  side (a lower part) of the separation tank  101 . 
     “Evaluation” 
     (PH Measurement of Single-Walled Carbon Nanotube Dispersion Liquid) 
     A pH of a single-walled carbon nanotube dispersion liquid recovered from each fraction was measured using a pH meter (trade name: desktop type pH meter F-71; manufactured by HORIBA). 
     The results are shown in  FIG. 4 . In  FIG. 4 , a vertical axis indicates a pH of a single-walled carbon nanotube dispersion liquid and a horizontal axis indicates a fraction. 
     It was confirmed from the results of  FIG. 4  that a pH of the single-walled carbon nanotube dispersion liquid increased from fraction F1 to fraction F15 so that the pH of the single-walled carbon nanotube dispersion liquid changed from an acidic dispersion liquid to an alkaline dispersion liquid. 
     Also, in the fractions F2 to F8 in the separation tank  101  on the second electrode  103  side (the lower part), a pH of the single-walled carbon nanotube dispersion liquid was 4 to 5. On the other hand, in the fractions F9 to F12 in the separation tank  101  on the first electrode  102  side (an upper part), a pH of the single-walled carbon nanotube dispersion liquid was 6 to 7. Furthermore, in the fractions F13 to F15 at the upper part, a pH was about 10. 
     The fraction F2 corresponds to a position in which the second electrode  103  is disposed and the fraction F14 corresponds to a position in which the first electrode  102  is disposed. 
     (Absorbance Measurement of Single-Walled Carbon Nanotube Dispersion Liquid) 
     Absorbance of single-walled carbon nanotube dispersion liquids recovered from the fraction F1, the fraction F4, the fraction F8, the fraction F11, and the fraction F14 was measured using a spectrophotometer (trade name: visible near-infrared spectrophotometer UV-3600; manufactured by Shimadzu Corporation). 
     The results are shown in  FIGS. 5 and 6 . 
     In  FIG. 5 , a vertical axis indicates an absorbance of a single-walled carbon nanotube dispersion liquid and a horizontal axis indicates a wavelength. Furthermore, in  FIG. 5 , a peak at a wavelength of 643 nm is caused by a metallic type single-walled carbon nanotube and a peak at a wavelength of 937 nm is caused by a semiconductor type single-walled carbon nanotube. 
     From the results shown in  FIG. 5 , an absorbance spectrum of single-walled carbon nanotube dispersion liquids recovered from the fractions F1 and F14 had very small peaks at a wavelength of 643 nm and a wavelength of 937 nm. Therefore, it was confirmed that the fractions F1 and F14 contained almost no metallic type single-walled carbon nanotubes and semiconductor type single-walled carbon nanotubes. 
     In an absorbance spectrum of a single-walled carbon nanotube dispersion liquid recovered from the fractions F4 and F8, a peak at a wavelength of 643 nm was extremely small and a large peak at a wavelength of 937 nm was observed. Therefore, it was confirmed that the fractions F4 and F8 did not contain metallic type single-walled carbon nanotubes and contained a large amount of semiconductor type single-walled carbon nanotubes. 
     In an absorbance spectrum of a single-walled carbon nanotube dispersion liquid recovered from the fraction F11, a small peak at a wavelength of 937 nm and a large peak at a wavelength of 643 nm were observed. Therefore, it was confirmed that the fraction F11 contained almost no semiconductor type single-walled carbon nanotubes and contained a large amount of metallic type single-walled carbon nanotubes. 
     From the above results, the fractions F2 to F8 in which the pH of the single-walled carbon nanotube dispersion liquid was 4 to 5 contained the semiconductor type single-walled carbon nanotubes and the fractions F9 to F12 in which the pH of the single-walled carbon nanotube dispersion liquid was 6 to 7 contained the metallic type single-walled carbon nanotubes. That is to say, it was confirmed that these correspond to two regions having different pHs in the single-walled carbon nanotube dispersion liquid and were separated into the metallic type single-walled carbon nanotubes and the semiconductor type single-walled carbon nanotubes. 
     In  FIG. 6 , a left vertical axis indicates an absorbance of a single-walled carbon nanotube dispersion liquid at a wavelength of 310 nm, a right vertical axis indicates a value obtained by dividing an absorbance of a single-walled carbon nanotube dispersion liquid at a wavelength of 937 nm by an absorbance of a single-walled carbon nanotube dispersion liquid at a wavelength of 643 nm, and a horizontal axis indicates a fraction. That is to say, the right vertical axis in  FIG. 6  corresponds to a purity of the semiconductor type single-walled carbon nanotubes. Furthermore, the absorbance of the single-walled carbon nanotube dispersion liquid at a wavelength of 310 nm corresponds to a concentration of the single-walled carbon nanotubes. 
     It was seen from the results shown in  FIG. 6  that, since the absorbance of the single-walled carbon nanotube dispersion liquid at a wavelength of 310 nm increased in the fractions F2 to F8 and the fractions F9 to F12, the single-walled carbon nanotubes were separated into two regions inside the separation tank. Furthermore, it was seen that, since a value obtained by dividing an absorbance of a single-walled carbon nanotube dispersion liquid at a wavelength 937 nm by an absorbance of a single-walled carbon nanotube dispersion liquid at a wavelength of 643 nm increased in the fractions F2 to F8, this corresponds to semiconductor type single-walled carbon nanotubes having high purity. It was seen that, since a value obtained by dividing an absorbance of a single-walled carbon nanotube dispersion liquid at a wavelength of 937 nm by an absorbance of a single-walled carbon nanotube dispersion liquid at a wavelength of 643 nm decreased in the fractions F9 to F12, this corresponds to metallic type single-walled carbon nanotubes having high purity. In this way, it was confirmed that the metallic type single-walled carbon nanotubes and the semiconductor type single-walled carbon nanotubes having high purity were separated. 
     Working Example 2 
     In working example 2, instead of the single-walled carbon nanotube dispersion liquid in working example 1, a solution obtained by dissolving 1 wt % of polyoxyethylene (100) stearyl ether (trade name: Brij S100; manufactured by Sigma-Aldrich) which was a non-ionic surfactant in heavy water was prepared. 
     Subsequently, the solution was injected into a separation tank  101  in a nanocarbon separation apparatus  100  in the same manner as in working example 1. 
     Subsequently, a DC voltage of 120 V was applied to a first electrode  102  (a negative electrode) and a second electrode  103  (a positive electrode) of the nanocarbon separation apparatus  100 . When a predetermined time has passed, application of the DC voltage to the first electrode  102  and the second electrode  103  was stopped. 
     After that, a rotary cock provided in an injection/recovery port  106  opened and a solution inside the separation tank  101  was gently recovered using a peristaltic pump. The solution was divided into 17 fractions for each about 6 mL in order of recovery. That is to say, the fractions were designated as F1, F2, . . . , and F17 from the second electrode  103  side (a lower part) of the separation tank  101 . 
     A pH and a density of a heavy water solution of a surfactant recovered from each fraction were measured. The results are shown in  FIG. 7 . In  FIG. 7 , a left vertical axis indicates a pH of the heavy water solution of the recovered surfactant, a right vertical axis indicates a density of the heavy water solution of the recovered surfactant, and a horizontal axis indicates a fraction. 
     It was confirmed from the results shown in  FIG. 7  that the pH of the heavy water solution of the surfactant increased from the fraction F1 to the fraction F17 so that the pH of the heavy water solution changed from an acidic pH to an alkaline pH. On the other hand, the density of the heavy water solution of the surfactant decreased from the fraction F1 to the fraction F17 toward the top of the separation tank  101 . This indicates that, when an electric field was applied to the separation tank, the surfactant moved to the second electrode  103  which was a positive electrode and a concentration gradient of the surfactant is generated. 
     Therefore, it was suggested that, when the surfactant solution was input into the separation tank  101  of the nanocarbon separation apparatus  100  and a DC voltage of 120 V was applied to the first electrode  102  (a negative electrode) and the second electrode  103  (the positive electrode), the surfactant moves to the second electrode  103  which was the positive electrode, a concentration gradient of the surfactant was generated, and a pH gradient was formed. 
     Working Example 3 
     In working example 3, a solution obtained by dissolving 1 wt % of polyoxyethylene (100) stearyl ether (trade name: Brij S100; manufactured by Sigma-Aldrich) which is a non-ionic surfactant in water and water were prepared. 
     Subsequently, each of an aqueous 1% surfactant solution and water was injected into a separation tank  101  of a nanocarbon separation apparatus  100  and a DC voltage of 120 V was applied to a first electrode  102  (a negative electrode) and a second electrode  103  (a positive electrode) of the nanocarbon separation apparatus  100  in the same manner as in working example 1. When a predetermined time has passed, application of the DC voltage to the first electrode  102  and the second electrode  103  was stopped. 
     After that, a rotary cock provided in an injection/recovery port  106  opened and each of the aqueous 1% surfactant solution and water inside the separation tank  101  was gently recovered using a peristaltic pump. 
     Each of the aqueous 1% surfactant solution and water was divided into a plurality of fractions in order of recovery. 
     PHs of an aqueous surfactant solution recovered from each fraction and water were measured. The results are shown in  FIG. 8 . In  FIG. 8 , a vertical axis indicates a pH of the recovered aqueous surfactant solution or water and a horizontal axis indicates a relative Z position (a height) of the recovered fractions in the separation tank  101 . That is to say, Z position 0 is the lowermost fraction and Z position 1 is the uppermost fraction. 
     It was seen from the results shown in  FIG. 8  that the aqueous 1% surfactant solution had a gradient generated so that a pH thereof increased toward the top of the separation tank  101  in the same manner as in the heavy water solution of the surfactant in working example 2. On the other hand, it was seen that, in the case of water which did not contain a surfactant, a pH gradient is not formed. Therefore, it was seen that, in the separation method for the nanocarbons in the present invention, it was important that water or heavy water could be used as a solvent and a surfactant was contained to form a pH gradient. 
     Working Example 4 
     In working example 4, a solution obtained by dissolving 1 wt % of polyoxyethylene (100) stearyl ether (trade name: Brij S100; manufactured by Sigma-Aldrich) in water, a solution obtained by dissolving 2 wt % of polyoxyethylene (100) stearyl ether (trade name: Brij S100; manufactured by Sigma-Aldrich) which is a non-ionic surfactant in water, and water were prepared. 
     A rotary cock provided in an injection/recovery port  106  in a nanocarbon separation apparatus  100  opened and 15 mL of water was gently injected into the separation tank  101  having a volume of 100 mL using a peristaltic pump. 
     Subsequently, similarly, 70 mL of a 1% aqueous solution of polyoxyethylene (100) stearyl ether (trade name: Brij S100; manufactured by Sigma-Aldrich) was injected gently. 
     Also, similarly, 15 mL of a 2% aqueous solution of polyoxyethylene (100) stearyl ether (trade name: Brij S100; manufactured by Sigma-Aldrich) was injected gently. 
     As a result, as shown in  FIG. 9 , a three-layered solution laminated structure, i.e., water in a region close to a first electrode  102  (a negative electrode), a 2% aqueous solution in a region close to a second electrode  103  (a positive electrode), and a 1% aqueous solution in an intermediate region, was formed. 
     Subsequently, a DC voltage of 120 V was applied to the first electrode  102  (the negative electrode) and the second electrode  103  (the positive electrode) in the same manner as in working example 1. When a predetermined time has passed, application of the DC voltage to the first electrode  102  and the second electrode  103  was stopped. 
     After that, the rotary cock provided in the injection/recovery port  106  opened and a solution inside the separation tank  101  was gently recovered using a peristaltic pump. The solution was divided into 20 fractions for each about 5 mL in order of recovery. That is to say, the fractions was designated as F1, F2, . . . , and F20 from the second electrode  103  side (a lower part) of the separation tank  101 . 
     PH of aqueous solution recovered from each of the fractions was measured. The results are shown in  FIG. 10 . In  FIG. 10 , a vertical axis indicates a pH of the recovered aqueous solution and a horizontal axis indicates a fraction. Also in this case, similarly, it was confirmed that a pH of an aqueous surfactant solution increased so that the pH thereof changed from an acidic pH to an alkaline pH from the fraction F1 to the fraction F20, that is, from the bottom to the top of the separation tank  101 . 
     In working example 2, it was suggested that the surfactant moved to the second electrode  103  which is a positive electrode, a concentration gradient of the surfactant is generated, and as a result, a pH gradient was formed. However, in working example 4, it was seen that movement of the surfactant for forming a final pH gradient may be small. As shown in working example 1, when the nanocarbons were separated, metallic nanocarbons move in a negative electrode direction and semiconductor nanocarbons move in a positive electrode direction. Therefore, in order to improve separation efficiency, it is necessary that there is little movement of other flows and substances inside the separation tank  101 . In this working example, movement of the surfactant for forming a pH gradient can decrease, which is effective in improving separation efficiency. 
     Working Example 5 
     In working example 5, solution AA obtained by dissolving 1 wt % of polyoxyethylene (100) stearyl ether (trade name: Brij S100; manufactured by Sigma-Aldrich) which is a non-ionic surfactant in heavy water was prepared. 
     A single-walled carbon nanotube mixture (eDIPS (enhanced Direct Injection Pyrolytic Synthesis, single-walled carbon nanotube; average diameter: 1.0 nm) was put into the solution AA. 
     The single-walled carbon nanotube mixture was dispersed by subjecting the solution AA having the single-walled carbon nanotube mixture put therein to ultrasonic dispersion treatment at an output of 40 W for 20 minutes using a horn type ultrasonic crusher (trade name: Digital Sonifier 450; manufactured by Branson Ultrasonics). After that, an ultracentrifugation operation was performed at 250000×g and 10° C. for 1 hour using an ultracentrifuge (trade name: CS100GXII; manufactured by Hitachi koki). Furthermore, a supernatant corresponding to 80% of the single-walled carbon nanotube mixture was fractionated and a single-walled carbon nanotube dispersion liquid in which the amount of a single-walled carbon nanotube was 20 μg/mL and the amount of polyoxyethylene (100) stearyl ether was 1.0 wt % was obtained. 
     Subsequently, solution BB obtained by dissolving 2 wt % of polyoxyethylene (100) stearyl ether (trade name: Brij S100; manufactured by Sigma-Aldrich) which is a non-ionic surfactant in heavy water was prepared. 
     15 mL of heavy water was gently injected into a separation tank having a volume of 100 mL through an injection/recovery port in a bottom portion of a nanocarbon separation apparatus using a peristaltic pump. 
     Subsequently, similarly, 70 mL of the prepared single-walled carbon nanotube dispersion liquid was gently injected. 
     Also, similarly, 10 mL of the solution BB prepared above was gently injected. 
     As a result, as shown in  FIG. 11 , a three-layered solution laminated structure, i.e., heavy water in a region close to a first electrode (a negative electrode), solution BB (2 wt % of a heavy water solution) in a region close to a second electrode (a positive electrode), and a single-walled carbon nanotube dispersion liquid in an intermediate region was formed (the first electrode and the second electrode are unclear and thus difficult to distinguish in  FIG. 11 ). 
     Subsequently, a DC voltage of 120 V was applied to the first electrode (the negative electrode) and the second electrode (the positive electrode) in the same manner as in working example 1. When a predetermined time has passed, as shown in  FIG. 12 , the single-walled carbon nanotube was separated into two upper and lower regions inside the separation tank and an intermediate layer having almost no color was formed between the two regions. 
     After the application of the DC voltage to the first electrode and the second electrode was stopped, the solution inside the separation tank was gently recovered through the injection/recovery port in the bottom portion of the nanocarbon separation apparatus using a peristaltic pump. The solution was divided into 15 fractions for each about 6 mL in order of recovery. That is to say, the fractions were designated as F1, F2, . . . , and F15 in order from the lower part of the separation tank. 
     PH of aqueous solution recovered from each of the fractions was measured. The results were shown in  FIG. 13 . In  FIG. 13 , a vertical axis indicates a pH of the recovered aqueous solution and a horizontal axis indicates a fraction. Also in this case, similarly, a pH gradient was confirmed so that the pH gradient changed from an acidic pH to an alkaline pH from the fraction F1 to the fraction F15, that is, from the bottom to the top of the separation tank. 
     Also,  FIG. 14  shows a Raman spectrum of a radial breathing mode (RBM) region obtained by measuring a single-walled carbon nanotube dispersion liquid of the fractions F4 and F5 having a pH of about 4 and a single-walled carbon nanotube dispersion liquid (which is not separated) which is not subjected to a separation operation using a micro-Raman spectrometer (trade name: HR-800; manufactured by HORIBA). In  FIG. 14 , a vertical axis indicates a normalized intensity, a horizontal axis indicates a wave number, and an excitation wavelength at the time of measurement is 514 nm. In  FIG. 14 , a peak of a region with a wave number of 140 to 220 (cm −1 ) is derived from a semiconductor type single-walled carbon nanotube and a peak of a region with a wave number of 220 to 300 (cm 1 ) is derived from a metallic type single-walled carbon nanotube. It was seen that a single-walled carbon nanotube dispersion liquid (which is not separated) which is not subjected to a separation operation contains a large amount of metallic type single-walled carbon nanotubes, but in the fractions F4 and F5 having a pH of about 4, a peak derived from metallic type single-walled carbon nanotubes was very small and a peak derived from semiconductor type single-walled carbon nanotubes was large. By analyzing these peaks in detail, it was estimated that semiconductor type single-walled carbon nanotubes of the fractions F4 and F5 had a purity of about 98%. 
     The semiconductor type single-walled carbon nanotubes having high purity were obtained using the separation method for the nanocarbons of the present invention. 
     With regard to the working examples of the present invention, the working examples of a case in which, in the nanocarbon separation apparatus  100  shown in  FIG. 3 , the first electrode  102  provided at the upper part is a negative electrode, the second electrode  103  provided at the lower part is a positive electrode, and an upward electric field was provided inside the separation tank (the electrophoresis tank)  101  having an I-shaped structure have been described above. However, the nanocarbon separation method of the present invention is not limited thereto. In the nanocarbon separation apparatus  100 , the first electrode  102  may be a positive electrode and the second electrode  103  may be a negative electrode. In this case, a direction of a pH gradient of the nanocarbon dispersion liquid  30  formed inside the separation tank  101  through carrier-free electrophoresis is opposite to that in the above working examples. At this time, a density gradient was also formed inside the separation tank  101  as shown in  FIG. 7  in working example 2, but a specific gravity difference was about 0.001 (g/cm 3 ). Therefore, nanocarbons can also be separated when an electric field was applied from top to bottom inside the separation tank  101 . 
     However, in order to further maintain separation stability, it is more reasonable to use gravity. Therefore, in the separation method for the nanocarbons of the present invention, the effects of a case in which the first electrode  102  provided at the upper part was a negative electrode and the second electrode  103  provided at the lower part was a positive electrode inside the separation tank  101  and an upward electric field was provided were greater. 
     Also, similarly, in order to further use gravity, it is reasonable that the separation tank  101  has an I (vertical) shape. By providing an I (vertical) shape, it is possible to minimize disturbance due to convection or the like, separation of nanocarbons is stabilized, and it is possible to improve purity of separation. 
     INDUSTRIAL APPLICABILITY 
     The nanocarbon separation method of the present invention can improve separation efficiency when nanocarbons having different properties are separated. Alternatively, it is possible to shorten time required for separation when carbon nanohorns having different properties are separated. 
     REFERENCE SYMBOLS 
     
         
         
           
               10 ,  100  Nanocarbon separation apparatus 
               11 ,  101  Separation tank 
               12 ,  102  First electrode 
               13 ,  103  Second electrode 
               14  Injection port 
               15  First recovery port 
               16  Second recovery port 
               30  Nanocarbon dispersion liquid 
               104  Electrode member 
               105  Opening section 
               106  Injection/recovery port 
               107  Cylindrical member 
               108  Columnar member 
               109  Fitting member