WO2015146657A1

WO2015146657A1 - Highly oriented metal nanofiber sheet material and method for manufacturing same

Info

Publication number: WO2015146657A1
Application number: PCT/JP2015/057619
Authority: WO
Inventors: 松原　英一郎; 博紀有馬; 亮輔冨岡
Original assignee: 国立大学法人京都大学
Priority date: 2014-03-28
Filing date: 2015-03-16
Publication date: 2015-10-01

Abstract

The problem addressed by the present invention is to provide a metal nanofiber sheet material, which is a structure formed from fibrous nano-metal and has a high degree of orientation wherein the metal nanofibers that constitute the structure are aligned along a fixed direction, and a method for manufacturing the same. The method manufactures a sheet-shaped metal nanofiber structure by reducing and depositing ferromagnetic metal from a reaction solution that includes ions of the ferromagnetic metal and a reducing agent. The metal nanofiber sheet material having a high degree of orientation is obtained by a method of making the reduction reaction progress in a state wherein a magnetic field is applied so as to maximize the magnetic flux density at the bottom of a reaction vessel accommodating the reaction solution.

Description

Highly oriented metal nanofiber sheet and method for producing the same

The present invention relates to a highly oriented metal nanofiber sheet and a method for producing the same.

It is known that metal nanostructures such as metal nanoparticles, metal nanowires, and metal nanorods exhibit different physical properties from metal bulk materials, such as high catalytic activity, depending on the type of metal constituting the metal nanostructure. It has been. For this reason, the metal nanostructure is expected to be used in various industrial fields as a material such as an electronic component, an optical component, and a magnetic material.

As a method for producing metal nanoparticles, a method of reducing metal ions or metal compounds using a reducing agent is known (see Patent Document 1). In this method, silver nanoparticles are obtained by mixing silver oxide, gelatin or a gelatin derivative, and a reducing monosaccharide or disaccharide and heating at 55 to 80 ° C. in an aqueous solvent.

However, in the method of Patent Document 1, only metal nanoparticles are obtained, and a one-dimensional metal nanostructure such as nanofiber cannot be obtained.

On the other hand, as a method for producing metal nanofibers such as nanorods, nanotubes, and nanowires, for example, electrodeposition of metal is performed in the pores of a template such as a porous film having minute pores, thereby forming rods or tubes. Alternatively, a method of growing a wire-like nanostructure is known (referred to as “template method”). However, the template method is not suitable for mass production of metal nanostructures because the preparation of the template and the operation of separating and recovering the metal nanostructure from the template are complicated.

In Patent Document 2 described below, a ferromagnetic metal nanostructure can be obtained by reducing a metal ion and precipitating a ferromagnetic metal in a solution containing a ferromagnetic metal ion while applying a magnetic field. Are listed. However, with this method, the growth direction of the deposited nanometal cannot be controlled, and the structure is in an entangled state in the reaction solution, and a highly oriented sheet-like structure in which metal nanofibers are aligned in a certain direction. It cannot be made into a body.

JP 2009-97082 A JP 2011-58021 A

The present invention has been made in view of the current state of the prior art described above, and its main purpose is a structure made of fibrous nanometal, and the metal nanofibers constituting the structure are in a certain direction. The object is to provide a metal nanofiber sheet having a high orientation and aligned, and a method for producing the same.

The present inventors have conducted intensive research to achieve the above-described purpose. As a result, in the method of reducing and precipitating a ferromagnetic metal in a state where a magnetic field is applied from a reaction solution containing a ferromagnetic metal ion and a reducing agent, by applying the magnetic field so that the magnetic flux density at the bottom of the reaction vessel becomes the highest. In addition, the ferromagnetic metal nanoparticles that have been reduced and precipitated are arranged by magnetic interaction and grow into a fiber shape, and the metal nanofibers are aligned in the direction of the magnetic field lines near the bottom surface of the reaction vessel having the highest magnetic flux density. It has been found that a sheet-like material composed of metal nanofibers having a high orientation, which has not been obtained in the past, is formed as the growth reaction proceeds. Furthermore, it discovered that the metal nanofiber sheet-like material which shows a higher orientation is formed by applying a pressure to the sheet | seat surface of this metal nanofiber sheet-like material. The present invention has been completed as a result of further research based on such knowledge.

That is, the present invention relates to the following highly oriented metal nanofiber sheet and a method for producing the same.
Item 1. A method of producing a sheet-like metal nanofiber structure by reducing and precipitating a ferromagnetic metal from a reaction solution containing a ferromagnetic metal ion and a reducing agent,
A method for producing a highly oriented metal nanofiber sheet, wherein the reduction reaction is allowed to proceed in a state where a magnetic field is applied so that the magnetic flux density at the bottom of the reaction vessel containing the reaction solution is maximized.
Item 2. A method of applying a magnetic field is a method of arranging a reaction vessel containing a reaction solution on a sheet-like magnet, or a method of forming a magnetic field by arranging a magnetic source below or near the bottom of the reaction vessel. The method for producing a metal nanofiber sheet according to Item 1, wherein
Item 3. Item 3. The method for producing a metal nanofiber sheet according to

Item

1 or 2, wherein the reaction solution containing the ferromagnetic metal ion and the reducing agent further contains a complexing agent.
Item 4. Item 4. The method for producing a metal nanofiber sheet according to any one of Items 1 to 3, wherein the reaction solution containing the ferromagnetic metal ion and the reducing agent further contains a nucleating agent.
Item 5. Item 5. The method for producing a metal nanofiber sheet according to any one of Items 1 to 4, wherein the reaction solution has a pH of 12 or more and a liquid temperature of 55 to 85 ° C.
Item 6. Item 6. The method for producing a metal nanofiber sheet according to any one of Items 1 to 5, wherein the ferromagnetic metal is Fe, Co, Ni, or an alloy thereof.
Item 7. A sheet-like structure made of a ferromagnetic metal fiber having a nanowire diameter,
A highly oriented metal nanofiber sheet-like material having an orientation degree of 1.65 or more determined by the following methods (1) to (5):
(1) Binarize the micrograph of the sheet,
(2) The binarized image is Fourier transformed to obtain a power spectrum,
(3) Obtain the angular distribution of the average amplitude of the power spectrum,
(4) Create a fiber orientation distribution diagram with the average amplitude displayed in polar coordinates for the angle θ,
(5) Ellipse approximation of the plot of average amplitude in the fiber orientation distribution diagram is taken, and the major axis / minor axis ratio of the obtained ellipse is taken as the degree of orientation.
Item 8. Item 8. The highly oriented metal nanofiber sheet according to Item 7, wherein the degree of orientation is 1.8 or more.
Item 9. After obtaining a highly oriented nanofiber sheet by the method according to any one of items 1 to 6 above, further comprising the step of applying pressure to the sheet surface of the sheet, to produce a metal nanofiber sheet Method.
Item 10. Item 10. The method for producing a metal nanofiber sheet according to Item 9, wherein the method of applying pressure to the sheet surface is by rolling.
Item 11. Item 10. The method for producing a metal nanofiber sheet according to Item 9, wherein the method of applying pressure to the sheet surface is by pressing.
Item 12. 12. A metal nanofiber sheet with improved orientation obtained by the method of any one of Items 9 to 11 above.

As described above, the metal nanofiber sheet-like material of the present invention is a sheet-like material in which metal fibers with nano-sized wire diameters are aligned in a certain direction, and compared with a sheet made of conventionally known metal nanofibers. And a sheet-like structure having high orientation. The metal nanofiber sheet-like material of the present invention having such a feature realizes both homogenization of the thickness distribution of the metal nanofiber sheet-like material and thinning, for example, an electrode substrate, It can be effectively used for various applications such as solar cell base materials, capacitor electrode base materials, catalyst carriers, gas filters, and sensor element base materials.

Further, according to the production method of the present invention, a novel metal nanofiber sheet having such excellent performance can be easily produced by a simple method of reduction precipitation from a solution.

Drawing which shows the process in which a metal nanofiber is formed typically. Drawing which shows an example of the arrangement | positioning method of the magnetic force source at the time of producing a metal nanofiber sheet-like thing. The photograph of the state which has arrange | positioned the reaction container on the sheet | seat which has the magnetism used in Example 1. FIG. Drawing which shows an example of a magnetic sheet typically. The figure which shows typically the state which advances the reductive reaction of a ferromagnetic metal ion in the state which has arrange | positioned the reaction container on the magnetic sheet. The photograph which shows the metal nanofiber sheet-like material obtained in Example 1. The scanning electron microscope (SEM) photograph of the glossy part of the metal nanofiber sheet shown in the left figure of FIG. The scanning electron microscope (SEM) photograph of the non-glossy part of the metal nanofiber sheet shown in the left figure of FIG. FIG. 7 is a scanning electron microscope (SEM) photograph of the metal nanofiber sheet shown in the right diagram of FIG. 6. Drawing which shows an example of the fiber orientation distribution figure used for the orientation degree measurement of the metal nanofiber sheet-like material of this invention. In the manufacturing method of the Ni nanofiber sheet in Example 1, the graph which shows the relationship between reaction time and deposited Ni mass. The photograph which shows the metal nanofiber sheet-like material obtained in Example 2. The scanning electron microscope (SEM) photograph of the glossy part of the metal nanofiber sheet shown in the left figure of FIG. The scanning electron microscope (SEM) photograph of the metal nanofiber sheet-like material shown in the right figure of FIG. The scanning electron microscope (SEM) photograph of the Ni nanofiber sheet lower surface obtained in Example 3. The scanning electron microscope (SEM) photograph of the Ni nanofiber sheet upper surface obtained in Example 3.

First, a method for producing a highly oriented metal nanofiber sheet according to the present invention will be described.

Method for producing highly oriented metal nanofiber sheet In the method for producing highly oriented metal nanofiber sheet of the present invention, the reduction reaction proceeds while a magnetic field is applied to a reaction solution containing ferromagnetic metal ions and a reducing agent. Let At this time, it is necessary to apply a magnetic field to the reaction solution so that the magnetic flux density at the bottom of the reaction vessel containing the reaction solution, that is, the lower part of the reaction vessel in the direction of gravity is maximized, so that the reduction reaction proceeds. is there.

According to this method, by proceeding the reduction reaction with a magnetic field applied, the ferromagnetic metal nanoparticles formed by the reduction reaction are arranged by magnetic interaction along the magnetic field direction, and the reduction reaction proceeds. As a result, the metal nanoparticles are bonded to form a nanofiber made of a ferromagnetic metal. At this time, by applying a magnetic field so that the magnetic flux density at the bottom of the reaction vessel containing the reaction solution is maximized, the metal nanoparticles formed by the reduction reaction are attracted to the bottom of the reaction vessel having a high magnetic flux density. In addition, the entanglement of the metal nanofibers due to the bubbles generated by the reduction reaction or the convection of the reaction solution is suppressed, and it grows into a fiber shape in the state of being aligned in the direction of the magnetic force lines in the vicinity of the bottom surface of the reaction vessel, and has a high orientation. The metal nanofiber structure is obtained. Here, the vicinity of the bottom surface means the vicinity of about 10 mm on the upper side to 10 mm on the lower side of the bottom surface of the reaction vessel.

Hereinafter, the method for producing the highly oriented metal nanofiber sheet of the present invention will be specifically described.

(I) Reaction solution The reaction solution used to obtain the highly oriented metal nanofiber sheet of the present invention is a solution containing ferromagnetic metal ions and a reducing agent.

In the present invention, in order to arrange metal nanoparticles by a reduction reaction in a magnetic field, the metal nanoparticles need to be a ferromagnetic metal. Therefore, a solution containing ferromagnetic metal ions is used as the reaction solution.

Examples of ferromagnetic metals include iron group metals such as Fe, Co, and Ni, and alloys thereof (eg, Fe-Co, Fe-Ni, Co-Ni, and the like). As the ferromagnetic metal, Co, Ni, and alloys thereof are preferable.

A solution containing ferromagnetic metal ions can be obtained by dissolving the above-mentioned ferromagnetic metal salt in a solvent. As the metal salt, a known metal salt that is soluble in the solvent to be used and can form a metal ion that can be easily reduced can be widely used. For example, the above-described ferromagnetic metal chlorides, sulfates, nitrates, acetates, and the like can be used. These salts may be hydrates or anhydrides. Specific examples of the metal salt of the ferromagnetic metal include, for example, cobalt acetate (II) tetrahydrate, cobalt acetate (II) anhydride, cobalt sulfate (II) heptahydrate, cobalt sulfate (II) anhydride, Cobalt (II) chloride hexahydrate, cobalt (II) chloride anhydride, cobalt nitrate (II) hexahydrate, cobalt nitrate (II) anhydride, nickel acetate (II) tetrahydrate, nickel acetate (II) ) Anhydride, nickel (II) chloride hexahydrate, nickel chloride (II) anhydride, nickel sulfate (II) hexahydrate, nickel sulfate (II) anhydride, nickel nitrate (II) hexahydrate, Examples thereof include nickel (II) nitrate anhydride, iron (II) sulfate heptahydrate, and iron (II) sulfate anhydride. When the ferromagnetic metal is an alloy, two kinds of metal salts constituting the alloy are used. When the alloy is, for example, Co—Ni, a cobalt salt such as cobalt acetate (II) tetrahydrate and a nickel salt such as nickel acetate (II) tetrahydrate are used.

The concentration of ions of the ferromagnetic metal is not particularly limited, and can be, for example, about 0.001 to 1 mol / dm ³ , preferably about 0.01 to 1 mol / dm ³ .

Any reducing agent can be used without particular limitation as long as it can form a ferromagnetic metal by reducing ions of the ferromagnetic metal. Examples thereof include hydrazine, ferrous chloride (FeCl ₂ ), hypophosphorous acid, borohydride, salts thereof, dimethylamine borane (DMAB), and the like.

In the present invention, in particular, hydrazine is used as the reducing agent from the viewpoint of maintaining the ferromagnetic properties of the metal, fully exhibiting the effect of applying a strong magnetic field, and obtaining high-purity ferromagnetic metal nanofibers. preferable.

The concentration of the reducing agent in the reaction solution is not particularly limited, and may be a concentration at which the reduction reaction proceeds favorably in the combination of the ferromagnetic metal ion and the reducing agent to be used. Usually, the concentration of the reducing agent is about 0.1 to 10 mol / dm ³ , and preferably about equimolar to 10 times the molar concentration of the ferromagnetic metal ions in the reaction solution.

By adjusting the concentration of the reducing agent within the above-described concentration range, the form of the nanofiber to be generated, for example, the wire diameter, the aspect ratio, etc. can be controlled. For example, by increasing the concentration of the reducing agent in the reaction solution, it is possible to control the nanofiber to have a small wire diameter and a large aspect ratio. Further, by reducing the concentration of the reducing agent, it is possible to control the nanofiber to have a large wire diameter and a small aspect ratio.

It is preferable to further add a complexing agent to the reaction solution for forming the metal nanofiber sheet. By adding a complexing agent, the reduction reaction rate of the ferromagnetic metal ion can be controlled. In particular, for metal ions with relatively weak magnetization, such as Ni, if the reduction reaction is too fast, the reduction reaction proceeds without the generated metal nanoparticles being aligned along the magnetic field, and metal nanoparticles are likely to be formed. However, by adding a complexing agent, the reduction reaction rate can be reduced and the formation of metal nanofibers can be facilitated.

The complexing agent is not particularly limited, and it is preferable to use a complexing agent having a high complex formation constant with a ferromagnetic metal ion. Examples of such a complexing agent include citrate, tartrate, ethylenediaminetetraacetic acid, ammonia, cyano complex and the like.

The amount of the complexing agent to be used is not particularly limited, and is a molar concentration of 1/10 times mol or more with respect to the transition metal ion to be used, and the concentration in the reaction solution is 0.01 to 10 mol / dm. Preferably it is about ³ .

It is preferable that a noble metal salt is further added to the reaction solution described above as a nucleating agent that provides nuclei for forming ferromagnetic metal nanoparticles. In general, noble metal salts are easy to reduce and are liable to be liquid phase reduced as fine particles. For this reason, by adding a noble metal salt to the reaction solution, it is possible to provide fine nuclei and control the particle size of the formed particles. Thereby, the wire diameter of the metal nanofiber can be controlled. For example, by increasing the concentration of the nucleating agent in the liquid, the wire diameter of the metal nanofiber can be controlled to be small and the aspect ratio to be large. By reducing the concentration of the nucleating agent in the liquid, it is possible to control the metal nanofiber to have a large wire diameter and a small aspect ratio.

For example, when the concentration of the nucleating agent in the reaction solution is high, the amount of ferromagnetic metal in contact with one nanoparticle nucleus formed by this nucleating agent is relatively reduced, and the particle size of the formed metal nanoparticles Is considered to be smaller. When the said density | concentration is low, it is thought that the quantity of the ferromagnetic metal which contacts one nanoparticle nucleus formed with this nucleating agent increases relatively, and the particle size of the metal nanoparticle formed becomes large.

As the nucleating agent, a salt having a redox potential more noble than a pig iron group metal ion is preferable. Specific examples of the nucleating agent include noble metal salts such as chloroplatinic acid, chloroauric acid, palladium chloride, ruthenium chloride, and silver nitrate.

The concentration of the nucleating agent in the reaction solution is not particularly limited, and can be usually 0.01 to 10 mmol / dm ³ , preferably 0.1 to 1 mol / dm ³ . This concentration is a concentration in terms of metal ions.

As a solvent used in the reaction solution, water, a polar organic solvent, or the like can be used. The polar organic solvent is not particularly limited, and examples thereof include alcohols having 1 to 6 carbon atoms, alkylene glycols having 2 to 4 carbon atoms, ketones having 3 to 6 carbon atoms, and alkylene glycol alkyl ethers having 3 to 6 carbon atoms. Can be used.

More specifically, examples of the alcohol include methanol, ethanol, isopropyl alcohol, propyl alcohol, butanol, pentanol, and hexanol. Examples of the alkylene glycol include ethylene glycol and propylene glycol. Examples of the ketone include acetone, methyl ethyl ketone, ethyl isobutyl ketone, and methyl isobutyl ketone. Examples of the alkylene glycol alkyl ether include ethylene glycol methyl ether, ethylene glycol mono-n-propyl ether, propylene glycol methyl ether, propylene glycol ethyl ether, propylene glycol butyl ether, propylene glycol propyl ether and the like. Among these polar organic solvents, alkylene glycols having 2 to 4 carbon atoms such as ethylene glycol and propylene glycol are preferred from the viewpoint of availability and excellent dispersibility of metal particles in the liquid phase.

The pH of the reaction solution may be a pH at which metal ions can be reduced according to the type of ferromagnetic metal ions and the type of reducing agent in the reaction solution. In general, the higher the pH, the higher the reducing power of the reducing agent. Therefore, by increasing the pH, it is possible to promote the generation of metal particles and reduce the diameter of the formed metal nanofibers. it can.

Usually, the pH is preferably about 12 or more, more preferably 12.5 or more, from the viewpoint of favoring the reduction reaction. About the upper limit of pH, it can be set as about 14, for example.

The liquid temperature of the reaction solution is not particularly limited, and the liquid temperature at which the metal nanofibers are formed at an appropriate rate according to the type and concentration of the ferromagnetic metal ion, the reducing agent, and the strength of the magnetic field applied. And it is sufficient. For example, the liquid temperature can usually be about 25 to 90 ° C., preferably about 55 to 85 ° C.

The reaction time may be appropriately determined according to the ferromagnetic metal ion concentration in the reaction solution, the thickness of the target metal nanofiber sheet, and is usually in the range of about 1 to 10 hours.

(Ii) Method for producing highly oriented metal nanofiber sheet By preparing a solution containing a ferromagnetic metal ion and a reducing agent that satisfies the above conditions, the reduction reaction of the ferromagnetic metal ion proceeds. Thus, ferromagnetic metal nanoparticles are formed.

At this time, by applying a magnetic field to the reaction solution, the reduced and precipitated metal nanoparticles are arranged along the magnetic field direction, and self-organization proceeds to form metal nanofibers. FIG. 1 is a drawing schematically showing a process of forming metal nanofibers.

In the production method of the present invention, a magnetic field is applied to the reaction solution so that the magnetic flux density at the bottom of the reaction vessel is maximized in the above-described method of proceeding the reduction reaction of the ferromagnetic metal ions in the presence of the magnetic field. .

The specific magnetic field application method is not particularly limited. For example, the magnetic flux density at the bottom of the reaction vessel can be maximized by arranging the reaction vessel with the reaction solution on a magnetic sheet. .

In addition, for example, a magnetic field such as a permanent magnet or a superconducting magnet may be arranged on both sides of the reaction vessel to form a magnetic field in the reaction solution. For example, a magnetic field may be formed using two magnetic sources, and a reaction vessel containing a reaction solution may be disposed in the magnetic field. At this time, the magnetic flux density at the bottom of the reaction vessel can be maximized by arranging the magnetic force source below the bottom surface of the reaction vessel or near the bottom surface of the reaction vessel. FIG. 2 is a drawing showing an example of a method of arranging magnetic sources in this method.

In addition, for example, by placing a soft magnetic material plate such as an iron plate under a magnetic force source arranged in parallel as described above and combining the magnetic force lines formed near the surface of the plate, By arranging the reaction vessel in the vicinity thereof, the magnetic flux density at the bottom of the reaction vessel can be maximized.

The strength of the magnetic field applied to the reaction solution varies depending on the magnetic strength of the metal nanoparticles to be formed, the particle size of the target metal nanofibers, etc., and thus cannot be specified unconditionally. Usually, depending on the type of ferromagnetic metal, the deposition rate, etc., the ferromagnetic metal does not precipitate as metal nanoparticles, the metal nanoparticles are arranged by magnetic interaction, and the metal particles are produced by the progress of the reduction precipitation reaction. A magnetic field having a strength necessary to cause the coupling is applied. For example, the magnetic field is applied so that the magnetic flux density at the bottom of the solution is about 0.01 to 1 Tesla, preferably about 0.05 to 1 Tesla.

The shape of the bottom surface of the reaction vessel is not particularly limited. For example, when a reaction vessel having a flat bottom surface is used, the reaction vessel may be formed by using the above-described magnetic sheet and arranging the reaction vessel thereon, or arranging magnetic force sources on both sides of the reaction vessel. The magnetic field can be applied so that the magnetic flux density at the bottom of the substrate is maximized.

The bottom surface of the reaction vessel may have an inclination. In this case, the magnetic flux density on the bottom surface of the reaction vessel can be maximized by arranging the magnetic sheet along the inclination.

In addition, if the shape of the bottom surface of the reaction vessel is elongated, a ribbon-like metal nanofiber structure can be obtained. At this time, by adjusting the direction in which the magnetic field is applied, the ribbon-like structure in which the metal nanofibers are aligned in the length direction of the ribbon-like metal nanofiber structure, or the metal nanofibers are aligned in the direction perpendicular to the length direction. The obtained ribbon-like structure can be obtained.

Alternatively, the magnetic field may be applied only to a part of the bottom surface without applying the magnetic field to the entire bottom surface of the reaction vessel. For example, if two or more bar magnets each having N and S poles at both ends are used, and a reaction vessel is arranged thereon, magnetic lines of force are formed between both poles, and a bar magnet is arranged at the bottom portion of the reaction vessel. A portion having the maximum magnetic flux density is formed along the position.

FIG. 3 is a photograph showing a state in which the reaction vessel is arranged on the magnetic sheet used in Example 1 described later. The magnetic sheet used includes a plurality of bar magnets, one side of which has N poles and the other side has S poles, so that the N pole surfaces and S pole surfaces of each bar magnet are alternately adjacent to each other. Are arranged in parallel, and this is a sheet-like material covered with synthetic rubber. FIG. 4 is a drawing schematically showing this magnetic sheet, and a curve with an arrow shown in the drawing indicates the direction of the lines of magnetic force. In this magnetic sheet, magnetic field lines are formed from the N pole surface side to the S curved surface side in the direction perpendicular to the length direction between adjacent bar magnets, and the magnetic flux density of the surface portion of the magnetic sheet is maximized. Therefore, by arranging the reaction vessel on the magnetic sheet, a magnetic field is formed in the reaction solution in the same direction as the magnetic force lines of the magnetic sheet at the bottom of the reaction vessel, and the magnetic flux density at the bottom of the reaction vessel is maximized. .

FIG. 5 is a drawing schematically showing a state in which the reduction reaction of the ferromagnetic metal ions proceeds while the reaction vessel is arranged on the magnetic sheet shown in FIG. In this drawing, in a reaction solution, ferromagnetic metal nanoparticles are formed by a reduction reaction, which is attracted to the bottom of the reaction vessel, grows in a fiber shape along the direction of magnetic force, and has a highly oriented metal. The state where a nanofiber sheet is obtained is shown.

FIG. 6 is a photograph showing the metal nanofiber sheet obtained in Example 1 described later as an example of the metal nanofiber sheet obtained by this method. The left figure in FIG. 6 is a photograph of the surface (lower surface) in contact with the bottom surface of the reaction vessel of the formed metal nanofiber sheet, the right figure is the opposite surface of the metal nanofiber sheet shown in the left figure, That is, it is a photograph of the surface (upper surface) on the reaction solution side of the metal nanofiber sheet. In the left figure, the surface of the formed metal nanofiber sheet is striped with alternating glossy and matte parts. Among these, the glossy portion is a portion corresponding to the gap portion between the bar magnets arranged in parallel in the magnetic sheet arranged under the reaction vessel, and the bar magnet in the sheet surface is perpendicular to the adjacent bar magnets. This is a portion where magnetic field lines having a high magnetic flux density are formed. FIG. 7 is a scanning electron microscope (SEM) photograph of this glossy portion. From FIG. 7, in the glossy part of the surface of the metal nanofiber sheet that is in contact with the bottom surface of the reaction vessel, a highly oriented metal nanofiber sheet is formed in which the metal nanofibers are grown in a certain direction. I understand.

The matte part in the left figure of FIG. 6 is a part directly above the bar magnet in the magnetic sheet placed under the reaction vessel, and is a part where the magnetic flux density in the vertical direction of the bar magnet in the sheet is weak. FIG. 8 is an SEM photograph of the metal nanofiber sheet (matte portion) formed in this portion, and it can be seen that the metal nanoparticles are in a disordered state.

On the other hand, the right side of FIG. 6, that is, the surface on the reaction solution side of the formed metal nanofiber sheet is less glossy than the glossy surface of the surface in contact with the bottom surface of the reaction vessel of the metal nanofiber sheet. Surface. FIG. 9 is an SEM photograph of this surface. Although the fiber which remained the shape of the metal nanoparticle can be confirmed, the orientation is largely inferior to the glossy part. This is thought to be due to the fact that the influence of the magnetic field was reduced on the reaction solution side surface due to the increase in the film thickness of the metal nanofiber sheet.

As described above, in the method of the present invention, by maximizing the magnetic flux density at the bottom of the reaction vessel, a highly oriented metal nanofiber sheet is formed near the bottom of the reaction vessel to which the strongest magnetic field is applied. Is done. In this case, in order to further improve the overall orientation of the obtained sheet-like material, the reaction vessel is strongly influenced by a magnetic field by reducing the reduction precipitation amount of the metal nanoparticles to reduce the thickness of the sheet-like material. A sheet-like structure of metal nanofibers is formed only in the vicinity of the bottom surface. For example, the amount of deposited metal nanoparticles can be reduced by a method of reducing the ferromagnetic metal ion concentration in the reaction solution, a method of shortening the reaction time, or the like.

In the method for producing a metal nanofiber sheet according to the present invention, after obtaining a highly oriented metal nanofiber sheet by the above-described method, further, if necessary, on the sheet surface of the obtained sheet. By applying pressure to the metal nanofiber sheet, it is possible to further improve the degree of orientation of the metal nanofibers and reduce variations in sheet thickness, thereby smoothing the metal nanofiber sheet. The method for applying pressure to the sheet surface of the sheet-like material is not particularly limited, and examples thereof include a method that can apply pressure uniformly to the sheet surface from a substantially vertical direction. For example, a method of rolling the sheet material, a method of pressing the sheet material, and the like can be applied. Since the strength of the pressure varies depending on the thickness of the sheet-like material, the type of fiber constituting the sheet-like material, the diameter, and the like, it cannot be defined unconditionally. Examples of the pressure include a pressure within a range in which the sheet-like material does not collapse, and a pressure that can uniformly smooth the whole. For example, in the case of a sheet made of nickel nanofibers, it is preferable to press at a pressure of about 4% to 40% of the yield stress of nickel metal in a pressure range of about 2 MPa to 20 MPa, and a pressure range of 4 MPa to 10 MPa. It is more preferable to pressurize at a pressure of 7% to 20% of the yield stress of nickel metal.

Highly Oriented Metal Nanofiber Sheet Material The metal nanofiber sheet material obtained by the production method of the present invention has a nano-sized metal fiber, for example, a metal nano-fiber having a wire diameter of about 50 nm to 500 nm, having a constant directivity. It is a sheet-like structure that is aligned in the direction, and is a sheet-like material having a higher orientation as compared with a sheet made of conventionally known metal nanofibers.

The metal nanofiber sheet material of the present invention is a sheet-like structure exhibiting high orientation, and has a high degree of orientation of 1.65 or more determined by a method using Fourier transform image analysis described later. It is.

Hereinafter, a method for measuring the degree of orientation of the metal nanofiber sheet according to the present invention will be described.

First, for a portion for obtaining the degree of orientation of the metal nanofiber sheet of the present invention, a microphotograph at a magnification of 500 times is taken, and this image is binarized in order to correct the influence of uneven illumination, 0.2 Take out the area of millimeter square or more.

Next, the binarized image is subjected to Fourier transform processing to obtain a power spectrum. An average value of the amplitude is calculated for each angle θ of the obtained power spectrum, and a fiber orientation distribution diagram in which the average amplitude is displayed in polar coordinates with respect to the angle θ is created. FIG. 10 is a drawing showing an example of a fiber orientation distribution diagram obtained by this method. In this figure, the long axis indicates that the undulations of the shading are large in that direction, and is in the direction crossing many fibers. On the other hand, the minor axis direction is the direction with the least shading, indicating that the minor axis direction is the direction of the main orientation of the fiber. In the present invention, this fiber orientation distribution diagram is approximated to an ellipse, and the major / short axis ratio of the approximated ellipse is defined as the degree of orientation.

According to this method, when the orientation direction of the fiber is completely disordered, the lengths of the major axis and the minor axis are the same, the degree of orientation is 1, and the higher the degree of orientation, the more the metal nanofibers are in the same direction. It becomes a highly oriented sheet-like structure that is aligned and aligned. This method is based on, for example, a method for obtaining fiber orientation described in “Method for analyzing physical properties of paper using image processing (Paper Pulp Technology Times, 48 (11) 1-5 (2005))”. For example, it can be analyzed using a non-destructive paper surface fiber orientation analysis program (http://www.enomae.com/FiberOri/index.htm).

The metal nanofiber sheet obtained by the method of the present invention exhibits a high degree of orientation of 1.55 or more, and the metal nanofibers having a high degree of orientation of 1.70 or more. A sheet-like material can be obtained. Furthermore, a metal nanofiber sheet having a very high degree of orientation of 1.8 or more can be obtained by applying a force from a direction perpendicular to the sheet surface and pressing it for smoothing.

The metal nanofiber sheet-like material of the present invention can be formed by a reduction reaction using the above-described reaction solution, and the kind or addition amount of components contained in the reaction solution, reaction conditions, etc. are adjusted according to the aforementioned conditions. Thus, it is possible to control the wire diameter, orientation degree, film thickness, and the like of the metal nanofibers.

In the method of the present invention, since the formed metal nanofibers are aligned in a certain direction, the thickness distribution of the metal nanofibers is uniform, and the thickness variation can be about 10% or less. . Furthermore, since the orientation of the metal nanofibers is high, it is possible to obtain a sheet-like structure with a thinner film thickness by reducing the amount of deposited metal nanoparticles. For example, the film thickness is about 10 μm. It can be a sheet.

Further, by pressing the sheet-like material, the thickness variation of the sheet-like material can be reduced to about 5% or less.

The metal nanofiber sheet of the present invention is a two-dimensional metal sheet material that includes the characteristics of a one-dimensional nanofiber exhibiting a large specific surface area or curvature peculiar to nanomaterials. It is a substance.

The metal nanofiber sheet-like material of the present invention having such characteristics can control the sheet thickness on the order of submicrons, which corresponds to the diameter of the nanofiber, by aligning the nanofibers in a certain direction. It can also be integrated by stacking, and as a metal nanosheet with a uniform thickness of micron order and capable of high integration, for example, it can be used for electrode manufacturing, making it compact and lightweight, and fast charge and discharge etc. Applicable electrodes can be obtained. Furthermore, by making use of the characteristics of the large specific surface area inherent to nanofibers, a supported catalyst having a high specific surface area and excellent reactivity can be obtained by supporting various catalysts as a catalyst carrier. In addition, from the point that it becomes a fine and high-density electrode material, it can be developed in fields or applications such as a next-generation multilayer capacitor, a fine high-capacity on-board storage battery, and a capacitor.

Specifically, the metal nanofiber sheet material of the present invention can be effectively used as an electrode of a secondary battery, for example, by forming an electrode active material layer on the surface of the sheet material.

The method for forming the electrode active material layer on the surface of the metal nanofiber sheet of the present invention is not particularly limited, and known methods can be applied as means for applying the electrode active material to various substrates.

For example, an active material layer made of a metal oxide can be formed by a simple method by oxidizing the surface of the metal nanofiber sheet to form a metal oxide film. When the active material layer is formed by an oxidation method, the metal nanofiber sheet is usually heated in an oxidizing atmosphere. As a result, the surface of the metal nanofibers constituting the metal nanofiber sheet is oxidized to form an oxide film.

In addition, a metal film can be formed on the surface of the metal nanofiber sheet by an electrodeposition method. For example, a metal nanofiber sheet having a Sn—Ni alloy layer can be obtained by forming a Sn—Ni alloy plating film by an electrodeposition method. The conditions for the electrodeposition method are not particularly limited, and a known method for forming a Sn—Ni plating film can be applied. The thickness of the Sn—Ni film is not particularly limited, and conditions that can coat the surface of the metal nanofibers as uniformly as possible can be appropriately employed. The metal nanofiber sheet-like material on which the Sn—Ni alloy plating film is formed in this way can be effectively used, for example, as a negative electrode for a lithium ion secondary battery, and there is a volume change accompanying the insertion and removal of Li. The negative electrode is relaxed and has excellent cycle characteristics.

In addition, a semi-metal or oxide coating can be formed on the surface of the metal nanofiber sheet by a dry method. For example, a metal nanofiber sheet-like material coated with silicon can be obtained by forming a silicon coating by sputtering. There is no particular limitation on the conditions of the sputtering method, and a known method for forming a silicon coating can be applied. The thickness of the silicon coating is not particularly limited, and a condition that allows the surface of the metal nanofibers to be coated as uniformly as possible may be adopted as appropriate. In this way, the metal nanofiber sheet with the silicon coating formed can be effectively used as, for example, a negative electrode for a lithium ion secondary battery. The negative electrode has excellent characteristics.

Furthermore, it can use for various uses by applying a well-known sol-gel method and forming various oxide films in the metal nanofiber sheet-like material of this invention. For example, it can be used as a photoresponsive electrode by coating TiO ₂ on a metal nanofiber sheet. Furthermore, it can be used as a catalyst electrode by coating a metal nanofiber sheet with a catalyst substance.

Furthermore, it is possible to form a surface film by an electrodeposition method, a method of immersing in a nanoparticle dispersion liquid, or the like. For example, it can be used as a high-efficiency hydrogen generating electrode by forming a Ni-Mo plating film by electrodeposition.

Hereinafter, the present invention will be described in more detail with reference to examples.

Example 1
Nickel (II) chloride hexahydrate (NiCl ₂ · 6H ₂ O) 0.10 mol / L, and trisodium citrate dihydrate (Na ₃ C ₆ H ₅ O ₇ · 2H ₂ O) 37.5 mmol / L It melt | dissolved in ion-exchange water and it adjusted to pH 12.5 with NaOH aqueous solution at room temperature. Hexachloroplatinic acid hexahydrate (H ₂ PtCl ₆ .6H ₂ O) 0.2 mmol / L was added to this solution to prepare 40 cm ³ of a solution containing Ni (II) ions as ferromagnetic metal ions.

On the other hand, 1.00 mol / L of hydrazine (N ₂ H ₄ · H ₂ O) was dissolved in ion-exchanged water and adjusted to pH 12.5 with an aqueous NaOH solution at room temperature to prepare a 40 cm ³ solution containing a reducing agent.

The solution containing Ni (II) ions and the solution containing a reducing agent were each heated to 60 ° C., and these solutions were mixed in a beaker having a capacity of 100 mL to obtain 80 cm ³ of a reaction solution.

Immediately thereafter, as shown in FIG. 3, a beaker containing the reaction solution is placed on a planar sheet-like magnet, and a magnetic field is applied in a state where the magnetic flux density at the bottom of the beaker is highest. The reduction reaction of nickel ions was started. The reaction was terminated by leaving it in this state at 60 ° C. for 240 minutes.

Schematic diagram of the sheet magnet used in this method is as shown in FIG. The size of the sheet magnet is about 10cm square, the bar magnet is 95mm in the longitudinal direction × 5mm in width × 1mm in thickness, the number of the bar magnets is 18, and the strength of the magnetic flux density is about 80mT at the center of the bar magnet just above the magnet. It was about 20mT.

By this method, a sheet of Ni nanofiber having a thickness of about 100 μm was obtained along the shape of the bottom surface of the beaker. A surface photograph of the lower surface of the obtained Ni nanofiber sheet, that is, the surface in contact with the bottom surface of the beaker is shown in the left figure of FIG. 6, and a surface photograph of the upper surface of the Ni nanofiber sheet, that is, the surface in contact with the reaction solution is shown in FIG. The right figure shows. The glossy part on the bottom surface of the Ni nanofiber sheet is formed in the upper part of the gap between the bar magnets arranged in parallel in the sheet magnet installed under the bottom of the beaker, that is, the magnetic field lines with high magnetic flux density are formed. It is a part corresponding to the part. FIG. 7 is a scanning electron microscope (SEM) photograph of the glossy part, and it was confirmed that a highly oriented metal nanofiber sheet was formed in which the metal nanofibers grew in a certain direction.

Using the non-destructive paper surface fiber orientation analysis program (http://www.enomae.com/FiberOri/index.htm), the SEM image of the glossy part obtained by the above method is converted into 2 After obtaining the power spectrum by Fourier transform processing, obtain the angle distribution of the average amplitude of the power spectrum, create a fiber orientation distribution diagram displaying the average amplitude in polar coordinates with respect to the angle θ, and elliptically approximate the plot of the average amplitude The degree of orientation of the Ni nanofibers was determined from the long axis / short axis ratio of the obtained ellipse. As a result, among the Ni nanofiber sheets obtained by the above method, the degree of orientation of the glossy portion of the surface in contact with the bottom surface of the beaker was 1.78, confirming that the orientation was extremely high.

FIG. 11 is a graph showing the relationship between the reaction time and the deposited Ni mass in the above-described method for producing the Ni nanofiber sheet. The Ni mass is a value obtained by the quartz crystal microbalance (QCM) method. When the Ni nanofiber sheet was produced under the above-described conditions, it was confirmed that about 60 μg of Ni was precipitated in the reaction time of about 240 minutes and the reaction end point was reached.

Example 2
In the same manner as in Example 1, a solution containing Ni (II) ions and a reducing agent were prepared. 20 cm ³ of each of these solutions was heated to 60 ° C. and mixed in a beaker having a capacity of 100 mL to obtain 40 cm ³ of a reaction solution.

A Ni nanofiber sheet was produced in the same manner as in Example 1 except that the amount of the reaction solution was 40 cm ³ . By this method, a sheet of Ni nanofibers having a thickness of about 100 μm was obtained along the shape of the bottom surface of the beaker.

The surface photograph of the lower surface of the obtained Ni nanofiber sheet, that is, the surface in contact with the bottom surface of the beaker is shown in the left figure of FIG. 12, and the upper surface of the Ni nanofiber sheet, that is, the surface photograph in contact with the reaction solution is shown in FIG. The right figure shows. The glossy part on the bottom surface of the Ni nanofiber sheet is formed in the upper part of the gap between the bar magnets arranged in parallel in the sheet magnet installed under the bottom of the beaker, that is, the magnetic field lines with high magnetic flux density are formed. It is a part corresponding to the part.

FIG. 13 is a scanning electron microscope (SEM) photograph of this glossy portion. Similar to the Ni nanofiber sheet obtained in Example 1, it was confirmed that the lower surface of the Ni nanofiber sheet showed good orientation.

FIG. 14 is an SEM photograph of the upper surface of the Ni nanofiber sheet, that is, the surface in contact with the reaction solution. Compared to Example 1, it was found that the orientation of the Ni nanofibers on the upper surface of the Ni nanofiber sheet was improved. This is considered to be because the formed sheet became thinner and the magnetic field on the upper surface of the Ni nanofiber sheet became stronger by reducing the amount of the reaction solution. In this case, the degree of orientation of the Ni nanofiber sheet was 1.72 on the lower surface and 1.59 on the upper surface.

Example 3
The Ni nanofiber sheet produced in Example 2 was pressed with a press at a pressure of 5 MPa for 1 minute to obtain a Ni nanofiber sheet having a thickness of about 5 μm and a thickness variation of 10%. FIG. 15 is a scanning electron microscope (SEM) photograph of the lower surface (the surface facing the sheet magnet) of the Ni nanofiber sheet. FIG. 16 is a scanning electron microscope (SEM) photograph of the upper surface of the Ni nanofiber sheet (the side opposite to the surface facing the sheet magnet).

The degree of orientation on the lower and upper surfaces of the Ni nanosheet is further improved compared to before the press treatment. By applying pressure to the sheet surface, the alignment of the Ni nanofibers is promoted and the degree of orientation is improved. it is conceivable that. In this case, the degree of orientation of the Ni nanofiber sheet was 1.81 on the lower surface and 1.69 on the upper surface.

Claims

A method of producing a sheet-like metal nanofiber structure by reducing and precipitating a ferromagnetic metal from a reaction solution containing a ferromagnetic metal ion and a reducing agent,
A method for producing a highly oriented metal nanofiber sheet, wherein the reduction reaction is allowed to proceed in a state where a magnetic field is applied so that the magnetic flux density at the bottom of the reaction vessel containing the reaction solution is maximized.
The method of applying a magnetic field is a method of arranging a reaction vessel containing a reaction solution on a sheet-like magnet, or a method of forming a magnetic field by arranging a magnetic source below or near the bottom of the reaction vessel. The method for producing a metal nanofiber sheet according to claim 1, wherein
The method for producing a metal nanofiber sheet according to claim 1 or 2, wherein the reaction solution containing the ferromagnetic metal ion and the reducing agent further contains a complexing agent.
The method for producing a metal nanofiber sheet according to any one of claims 1 to 3, wherein the reaction solution containing the ferromagnetic metal ion and the reducing agent further contains a nucleating agent.
The method for producing a metal nanofiber sheet according to any one of claims 1 to 4, wherein the reaction solution has a pH of 12 or more and a liquid temperature of 55 to 85 ° C.
The method for producing a metal nanofiber sheet according to any one of claims 1 to 5, wherein the ferromagnetic metal is Fe, Co, Ni, or an alloy thereof.
A sheet-like structure made of a ferromagnetic metal fiber having a nanowire diameter,
A highly oriented metal nanofiber sheet-like material having an orientation degree of 1.65 or more determined by the following methods (1) to (5):
(1) Binarize the micrograph of the sheet,
(2) The binarized image is Fourier transformed to obtain a power spectrum,
(3) Obtain the angular distribution of the average amplitude of the power spectrum,
(4) Create a fiber orientation distribution diagram with the average amplitude displayed in polar coordinates for the angle θ,
(5) Ellipse approximation of the plot of average amplitude in the fiber orientation distribution diagram is taken, and the major axis / minor axis ratio of the obtained ellipse is taken as the degree of orientation.
The highly oriented metal nanofiber sheet-like material according to claim 7, wherein the degree of orientation is 1.8 or more.
A process for producing a metal nanofiber sheet comprising the step of applying a pressure to the sheet surface of the sheet-like material after the highly oriented nanofiber sheet-like material is obtained by the method according to any one of claims 1 to 6. Method.
The method for producing a metal nanofiber sheet according to claim 9, wherein the method of applying pressure to the sheet surface is by rolling.
The method for producing a metal nanofiber sheet according to claim 9, wherein the method of applying pressure to the sheet surface is by pressing.
A metal nanofiber sheet material with improved orientation obtained by the method according to any one of claims 9 to 11.