JP2020111831A - Production method of metal fine particle and production apparatus of metal fine particle - Google Patents

Abstract

To provide a production method of a metal nanoparticle material, by which metal nanoparticles having a narrow particle size distribution can be continuously synthesized in a short time, at a high yield and at high energy efficiency.SOLUTION: A production method of metal fine particles is provided for synthesizing metal fine particles by circulating a reaction liquid 8 containing a metal precursor into a reaction pipe 7 disposed in a microwave irradiation space, and irradiating the reaction pipe 7 in the microwave irradiation space with microwaves from outside the reaction pipe 7 so as to heat the reaction liquid 8 in the reaction pipe 7 by the microwave irradiation. An apparatus having a frequency band from 2.4 to 2.5 GHz allocated to the apparatus or a standard frequency band as an industrial product is used as a microwave generator. The reaction pipe 7 used has an average inner diameter of less than 2.84 mm in a portion of the reaction pipe 7 disposed in the microwave irradiation space.SELECTED DRAWING: Figure 1

Description

The present invention relates to a method for producing metal fine particles and an apparatus for producing metal fine particles, more specifically, by uniformly irradiating a reaction liquid with electromagnetic waves to heat the metal nanoparticles in a short time and continuously. The present invention relates to a method for producing fine metal particles and an apparatus for producing fine metal particles that enable production.

Metal nanoparticles (metal nanometer order particles) are used for various applications such as catalysts, electrode materials, and antibacterial agents. Moreover, in particular, recently, it has been required to realize high performance and high accuracy in the fields of food allergy test kits such as labeling materials for immunochromatography and immunodiagnostic kits.
Examples of such a material include transition metals such as Ag, Au, Ir, Pt, Pd, Rh, Re, Ru, Cu, Ni, and Os, and Ce, Pr, Nd, Pm, Sm, Eu, Gd, and Tb. , Dy, Ho, Er, Tm and other rare earth elements.
As methods for producing these metal nanoparticles, there are a top-down method of physically miniaturizing a raw material metal such as crushing, and a bottom-up method of growing fine particles from a metal precursor by a chemical reaction. In particular, the bottom-up method is widely used for producing fine particles having a uniform particle size distribution of 100 nm or less. In particular, the wet reduction method, which is one of the bottom-up methods, has many research examples because various types of metal nanoparticles can be produced with good dispersibility. Since the wet reduction method is a chemical reaction in a solution, precise reaction temperature control and reaction time control are important for producing nanoparticles with uniform particle diameters. Therefore, in order to maintain a uniform temperature field and reaction field, batch type reactors are often used. However, the batch method has a problem that it is not suitable for industrial mass production because it requires time and effort such as solution exchange and cleaning. Moreover, continuous synthesis using a microreactor is also performed. However, the method using the microreactor has some problems to be solved such as initial cost, operation cost, and problem of clogging of reaction tube.

Further, a method for producing metal particles by heating a reaction mixture containing a metal precursor and an alcohol solvent with localized microwave or millimeter wave energy has been proposed (see Patent Documents 1 and 2). However, this method cannot be continuously operated, and there is a problem in applying it to industrial production.

Moreover, in a chemical reaction utilizing microwaves, conventional electromagnetic wave irradiation causes unevenness in the irradiation intensity of the electromagnetic waves into the reaction tube, which causes a problem in reproducibility, and it is necessary to stir the reaction solution. Most were carried out by batch-type reactions. On the other hand, in recent years, there has been a demand for industrial development of a method capable of efficiently and continuously producing fine metal particles, particularly high-quality metal nanoparticles.
Non-Patent Document 1 discloses an attempt to continuously synthesize nanoparticles by microwave irradiation. A reaction tube is placed in these cooking microwave ovens, and it is difficult to maintain a uniform temperature field when irradiating the reaction tube with electromagnetic waves due to spatial unevenness of electromagnetic wave intensity and temporal fluctuations. Therefore, there is a possibility that the particle size distribution and the particle properties will be inhomogeneous. There is also an attempt to continuously synthesize nanoparticles according to Patent Document 3. This is to arrange the reaction tube in the waveguide. In this case, the microwave intensity at the waveguide inlet is higher than the microwave intensity at the waveguide outlet, and the microwave is not present in the reaction product. There was a problem that energy efficiency was poor because it was not completely absorbed. Patent Document 3 also proposes a microwave irradiation method using a standing wave (cavity resonator), but there is a possibility that microwave irradiation unevenness may occur depending on the microwave distribution in the cavity resonator. there were.

Japanese Patent No. 3005683 Japanese Patent No. 3612546 Japanese Patent Publication No. 2006-517260

Y. Groisman and A. Gedanken, J.; Phys. Chem. C, 112, 8802-8808 (2008)

Therefore, the present invention provides a method for producing a metal nanoparticle material and a metal nanoparticle capable of continuously synthesizing metal nanoparticles having a narrow particle size distribution in a short time at a high yield and with high energy efficiency. It is an object of the present invention to provide an apparatus for producing a particulate material.

The present invention for solving the above-mentioned problems comprises the following technical means.
A reaction liquid containing a metal precursor is circulated in a distribution pipe arranged in an electromagnetic wave irradiation field, and an electromagnetic wave is irradiated from the outside of the distribution pipe toward the distribution pipe, and the reaction liquid in the distribution pipe is irradiated by the electromagnetic wave. A method for producing metal fine particles, which comprises heating and synthesizing metal fine particles, wherein a frequency band of 2.4 to 2.5 GHz is assigned as a frequency band of the electromagnetic wave, or a standard frequency band of the product is obtained. When an electromagnetic wave of 2.45 GHz is radiated toward the flow pipe, it has been found out that it is important to set the average inner diameter (diameter) of the portion of the flow pipe arranged in the electromagnetic wave irradiation field to 2.9 mm or less. When irradiating the electromagnetic wave of any frequency in the frequency band 2.4 to 2.5 GHz as the flow pipe, the average inner diameter of the portion of the flow pipe arranged in the electromagnetic wave irradiation field is 2.9×( 2.45 GHz/2.5 GHz)=2.84 mm, which is characterized by using a flow pipe thinner than that.
Then, for example, in the method for producing metal fine particles described above, the reaction liquid irradiated with the electromagnetic wave is guided to a temperature controller.

According to the present invention, it is possible to continuously synthesize metal nanoparticles having a narrow particle size distribution in a short time with a high yield and high energy efficiency.

It is explanatory drawing which shows a preferable mode of the chemical reaction device using a microwave used in Example 1 which concerns on this invention with a partial cross section figure. It is explanatory drawing which shows the electric field strength distribution in the cylindrical cavity resonator of a present Example. It is a figure which shows the particle size distribution of the silver nanoparticle synthesize|combined in Examples 1-3. It is a figure which shows the particle size distribution of the silver nanoparticle synthesize|combined in Examples 4-8. It is a figure which shows the particle size distribution of the silver nanoparticle synthesize|combined in Examples 9-14. 16 is a transmission electron micrograph of silver nanoparticles synthesized under the conditions of Example 14. FIG. 16 is a diagram showing a time change of a solution temperature when performing continuous synthesis for a long time in Example 15. It is a figure which shows the microwave irradiation power in Example 15 (incident wave to _TM010 cavity-reflected wave). FIG. 8 is a diagram showing the relationship between the particle size distribution of silver nanoparticles and the reaction time in Example 15. FIG. 8 is a diagram showing a transition of an absorption spectrum of a product at each time during continuous synthesis of silver nanoparticles in Example 15. 9 is a diagram showing the relationship between the liquid feed rate of the reaction liquid and the particle size distribution of silver nanoparticles in Example 7. FIG. It is a figure which shows the particle size distribution of the silver nanoparticle synthesize|combined using glycerin as a liquid solvent. It is a figure which shows the particle size distribution of the silver nanoparticle synthesize|combined using benzyl alcohol as a liquid solvent. FIG. 8 is a diagram showing a particle size distribution of gold nanoparticles synthesized under the conditions of Example 17. It is a figure which shows the particle diameter distribution of the platinum nanoparticle and the palladium nanoparticle which were synthesize|combined on the conditions of Examples 18 and 19. FIG. 5 is a diagram showing particle size distributions of silver nanoparticles and platinum nanoparticles synthesized under the conditions of Examples 20 and 21. 9 is a transmission electron microscope image of silver nanoparticles synthesized under the conditions of Example 22. 9 is a transmission electron microscope image of platinum nanoparticles synthesized under the conditions of Example 23. 9 is a transmission electron microscope image of platinum nanoparticles synthesized under the conditions of Example 24. 9 is a diagram showing a particle size distribution of silver nanoparticles synthesized under the conditions of Example 25. FIG. In Example 26, it is a figure which shows the particle size distribution of the silver nanoparticle synthesize|combined by the structural example shown by FIG. 1, and the structural example shown by FIG. 28 is a transmission electron microscope image of the silver nanoparticles synthesized in the configuration example shown in FIG. 24 in Example 27. It is explanatory drawing of the modification of the microwave chemical reaction apparatus which supplies several raw material solutions individually. It is explanatory drawing of the modification of a microwave chemical reaction apparatus. It is explanatory drawing of another modification of a microwave chemical reactor.

The metal fine particles that can be produced by the method of the present invention are most preferably transition metals such as Ag, Au, Ir, Pt, Pd, Rh, Re, Ru, and Os in the case of transition metal and typical metal complexes. Is a transition element such as Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Y, Zr, Nb, Mo, Tc, W, Al, In, Ga, Zn, Cd, Sb, Sn, It may be a typical element such as Ge, Be or Mg.

In the production of the metal fine particles of the present invention, the reaction liquid containing the metal precursor substance is circulated in the flow pipe, while the electromagnetic wave is uniformly and intensively radiated toward the flow pipe in the length direction of the flow pipe. Then, the electromagnetic wave irradiation space in the flow tube is heated in the flow direction to produce fine metal particles, which can be carried out by a continuous method for producing metal nanoparticles.
By the production method of the present invention, preferably, metal fine particles having an average particle diameter of about 100 nm or less can be produced. In the present specification, the average particle size of the particle size of the metal fine particles refers to a value measured by dynamic light scattering of a nanoparticle suspension, as shown in Examples described later.
The flow tube used in the present invention is preferably one that easily transmits microwaves, and examples of the material of the container include glass, quartz, alumina, fluororesin Teflon (registered trademark), plastic, polyethyl ether ketone resin. And so on. However, the present invention is not limited to these, and the same materials as those can be used.
The thickness of the wall of the distribution pipe is preferably 0.05 to 10 mm, more preferably 0.1 to 2 mm. Stable metal nanoparticles can be continuously synthesized by setting the thickness of the tube wall to the above range, and it is difficult to maintain the shape of the flow tube as the reaction tube if the thickness of the tube wall is too thin. In addition, there is a possibility of damage due to pressure fluctuations in the reaction tube, and if it is too thick, there is a problem that the heating efficiency is reduced due to microwave transmission loss.
The irradiation intensity of the electromagnetic wave (microwave) on the metal precursor-containing liquid in the flow pipe is preferably 0.1 mW to 20 kW, and more preferably 1 mW to 100 W. When the irradiation intensity of electromagnetic waves is within this range, reduction of the metal precursor occurs and metal fine particles are generated. If the electromagnetic wave intensity is too low, metal fine particles are not produced, and if it is too high, the circulating solution boils. The liquid feed rate of the metal precursor-containing liquid in the flow pipe is preferably 0.1 ml/h to 5 l/h, more preferably 5 to 200 ml/h. By adjusting the liquid feeding speed within the above range, fine metal particles having a desired size are produced. If the liquid feeding speed is too high, the liquid will not boil, but it will be difficult to obtain metal fine particles having a particle size of nm order. If it is too small, the liquid will boil and it will be difficult to obtain fine metal particles having a desired particle diameter.

In the present invention, the metal precursor is dispersed or dissolved in a solvent having a reducing action on the metal and irradiated with electromagnetic waves. Such solvents include alcohols (methanol, ethanol, ethylene glycol, diethylene glycol, propylene glycol, tetraethylene glycol, glycerol, benzyl alcohol, dipropylene glycol, etc.), inorganic acids (boron hydroxide salt, dimethylaminoborane, phosphorous acid). Acid, hypophosphorous acid, sulfurous acid, sodium thiosulfate, Fe ion complex, hydrazine, etc.), organic acids (citric acid, malic acid, oxalic acid, formic acid, etc.), sugars and the like. Further, in the present invention, it is preferable to add a dispersant to the liquid containing the metal precursor to prevent the generated metal fine particles from aggregating and improve the dispersion stability. Examples of such dispersants include polyvinylpyrrolidone, polyethylene glycol, polyvinyl alcohol, and polymer dispersants. The polymer dispersant is a high molecular weight polymer having a functional group having a high affinity for the surface of the pigment introduced therein, and is an amphipathic one having a structure including a solvating portion. As a polymer dispersant, it is soluble in a solvent that absorbs microwaves, and it is possible to capture and disperse in a solvent the fine particles that are instantly generated by an extremely fast reduction reaction without aggregating. It is preferable that the copolymer is effective for long-term stability, and the number average molecular weight of the polymer dispersant is preferably 1,000 to 1,000,000. Examples of the polymer dispersant include those exemplified in JP-A No. 11-80647, and preferable polymer dispersants (commercially available products) include DISPERBYK102, 108, 116, 145 manufactured by BYK Chemie. 180, 2096, 2155, BYK9076, 9077, Kyoeisha Chemical Co., Ltd. Floren G700, G900 etc. can be mentioned. The number average molecular weight of 10,000 to 50,000 is particularly preferable.
As the solvent, a solvent having a high boiling point is preferably used, and these solvents may be mixed and used.

In the present invention, the concentration of the metal precursor substance in the reaction solution is preferably 0.01 mM to 10 M, more preferably 0.1 mM to 3 M.
In the above, by optimizing the reaction liquid containing the metal precursor substance which is circulated by liquid feeding as described above, nanometer-sized metal fine particles can be produced.
INDUSTRIAL APPLICABILITY According to the present invention, by irradiating the above-mentioned reaction liquid continuously supplied with electromagnetic waves, the above-mentioned metal fine particles can be continuously synthesized in a short time with a high yield and high energy efficiency. It provides a method. The electromagnetic wave irradiation method preferable for the present invention will be described below.

Generally, the heat generation when a substance is heated by microwaves is represented by the following equation.

Among them, |E|[V/m] and |H|[A/m] are electric field strength and magnetic field strength of microwave, respectively, and σ[S/m] is electric conductivity and f[1/ sec] is the microwave frequency, ε ₀ [F/m] is the dielectric constant in vacuum, ε″ is the dielectric loss factor, μ ₀ [H/m] is the magnetic permeability of the vacuum, and μ″ is the magnetic loss factor. Is.

Of these, the heat generated by the electric field expressed by the second term and the heat generated by the magnetic field expressed by the third term on the right-hand side of the above expression 1 often greatly affect the microwave heating. Here, the heat generation by the electric field of the second term will be described as an example, but the same applies to the first term and the third term of the formula 1.

It can be seen from the above formula 1 that it is effective to select a substance having a large dielectric loss factor ε″ or to increase the electric field strength in order to increase the amount of heat generated by microwave heating. Therefore, microwave heating of a substance having a small dielectric loss factor (nonpolar substance) is difficult. It can be seen that in order to heat such a substance, it is effective to increase the electric field strength E or to apparently increase the dielectric loss factor of the substance.

In order to increase the electric field strength E, irradiation is performed so that microwaves are concentrated on a specific part, and in order to increase the apparent dielectric loss factor, the substance to be heated and the flow pipe wall (that is, the reaction It is necessary to realize the effective use of the interaction with the tube wall.

Generally, in order to increase the electric field strength, it is necessary to use a large-sized microwave generator, but there are problems such as an increase in the size of the device and an increase in price. There is a problem that device design becomes difficult, such as abnormal heating. By constructing a microwave irradiation method that concentrates microwaves and maximizes the electric field intensity at a specific portion, it is possible to increase the electric field intensity while solving the above problems.

In the present invention, a method of using a single mode cavity that forms a standing wave is used as a mechanism for irradiating a specific portion with microwaves in a concentrated manner. In the single mode cavity, there is no time change between the place where the electromagnetic field strength is strong and the place where the electromagnetic field strength is weak. Therefore, by arranging the microwave heating target material in the strong place, effective microwave heating becomes possible.
Here, the single mode cavity refers to the inside of the microwave irradiation space in which a specific standing wave can be stably formed. A standing wave is a wave in which the waveform seems to stop and vibrate without proceeding, and a state where the position of the electric field strength is 0 and a position where the electric field strength is strong does not change. In particular, when a standing wave of TM _nm0 mode (n is 0 or more and m is an integer of 1 or more) is formed by a cylindrical resonance cavity (cavity), in the reaction tube flow path portion arranged on the central axis of the cylinder, In addition to being able to irradiate microwaves in a concentrated manner, the distribution of the electric field strength becomes uniform in the axial direction of the central axis, and the reaction liquid can always be irradiated with microwaves having a controlled electric field strength. At this time, by configuring the reaction tube flow path with a material that easily transmits microwaves, the microwaves directly reach the internal solution, and the reaction solution can be directly dielectrically heated. Dielectric heating can generate heat in an extremely short time as compared with conventional heat transfer, so that the solution can be rapidly heated to a predetermined reaction temperature. Further, the chemical reaction of the metal precursor substance in the reaction solution may be accelerated by microwave irradiation. In this case, not only the time required for heating but also the reduction of reaction time can be expected.
The electromagnetic wave irradiating means used in the present invention is preferably one capable of irradiating the reaction liquid with concentrated electromagnetic waves.
For example, as one of the electromagnetic wave irradiation spaces having a structure capable of concentrating an electric field, there is a method of using a space called a cavity resonator and using a container capable of stably forming a specific standing wave. FIG. 2 shows the electric field intensity distribution of a standing wave called TM ₀₁₀ formed in a cylindrical cavity resonator. In the figure, (A) is an overall view of the cavity resonator 11, (B) is a graph corresponding to (A) and showing the relationship between the cylindrical inner diameter direction of the cavity resonator and the electric field strength, and (C) is the above. It is a graph which shows the relationship between the axial direction of the cylinder of a cavity resonator corresponding to (A), and electric field strength. In FIG. 2A, 11 is a cavity resonator, 12 is a microwave irradiation port, and the graph of FIG. 2B shows the electric field strength in the radial direction of the cavity resonator 11 (the horizontal axis represents the cavity). (It corresponds to the radius of the resonator 11). It can be seen that the electric field can be concentrated at the center of the cylinder by using the standing wave of TM ₀₁₀ . The graph in FIG. 2C shows the electric field strength in the axial direction on the central axis of the cavity resonator 11 (the vertical axis corresponds to the position on the central axis of the cylindrical cavity). It can be seen that when the standing wave of TM ₀₁₀ is used, the electric field strength on the central axis of the cylinder is uniform regardless of the position. That is, a tubular resonator having a standing wave of TM ₀₁₀ formed inside a cylinder and arranged along the central axis of the cylinder is always used as a microwave reactor having a strong and uniform electric field. Can be irradiated. Although TM ₀₁₀ was described in FIG. 2, a standing wave of TM _mn0 (m is an integer of 0 or more and n is an integer of 1 or more) also has a location where the electric field is concentrated in the radial direction of the cylinder and is parallel to the central axis. Since the electric field strength is uniform at the site, it can be used similarly. In addition, although the explanation has been given with respect to the electric field in FIG. 2, since the electromagnetic wave also has a heating effect by the magnetic field, the same effect can be obtained even by utilizing the portion where the magnetic field becomes strong.

Another method for concentrating an electric field is to use a mirror that reflects electromagnetic waves. A method using an elliptical electromagnetic wave reflection space as described in JP-A 2006-173069 and JP-A 2006-286588 can be used. If the electromagnetic wave is supplied from one focus of the ellipse, the electromagnetic wave can be concentrated at the other focus. The same effect can be obtained by arranging the reaction tube so that the reaction solution passes through this focal point.

In the present invention, as the cavity resonator of the single mode cavity, for example, in addition to the TM ₀₁₀ single mode cavity, a TM ₁₁₀ mode cavity, a TM ₂₁₀ mode cavity, a TM ₀₂₀ mode cavity, a TE ₀₁ mode cavity, etc. are used. Further, as the flow pipe, a millimeter-size flow pipe having an inner diameter of 2.9 mm or less when the microwave frequency to be irradiated is 2.4 to 2.5 GHz, for example, 1.5 mm or more and 2 mm or less, 1 mm or more and 1.5 mm or less, 0 A distribution pipe of 0.5 mm or more and 1 mm or less is used. When the microwave frequency for irradiation is lower than 2.4 GHz, the thickness of the flow pipe may be larger, but when the microwave frequency is higher than 2.5 GHz, a thinner flow pipe is desirable.

In the present invention, it is important to use the above-mentioned flow pipe having an inner diameter of 2.9 mm when the microwave frequency for irradiation is 2.4 to 2.5 GHz. The outer diameter and the length of the flow pipe are not particularly limited, and the shape and structure of the flow pipe arranged in the cavity can be appropriately designed.

In the present invention, the number of distribution pipes arranged in the cavity is not limited to a single, it is also possible to appropriately arrange a plurality, and by connecting and arranging a plurality of distribution pipes by a suitable connection method, It is possible to improve the microwave heating efficiency for the circulating solution. Not limited to the method of arranging a single distribution pipe, for example, a method of arranging 2 to 8 distribution pipes, or even a single method of arranging a spiral distribution pipe, an appropriate method may be adopted. You can

In the present invention, the desired effect can be obtained by reducing the inner diameter of the flow pipe to a millimeter size of 2.9 mm or less. The effect can be further improved by performing appropriate processing that can increase the contact area with the solution.

One of the features of the present invention is to use an interaction generated between a substance to be heated and the wall surface of a container (flow pipe) holding the substance to be heated as a method for increasing the apparent dielectric loss factor. For example, the molecules of the substance to be heated near the charged wall surface are subjected to dielectric polarization due to the charge on the wall surface. Dielectric polarization is a phenomenon in which a bias in charge occurs, and the bias in charge increases microwave absorption.

In the present invention, in order to increase the dielectric polarization due to the surface of the flow pipe, a means for increasing the contact area of the flow pipe is adopted. For example, a method of increasing the surface area per volume of the reaction solution by reducing the size of the container, a method of making the flow pipe elongated as shown in the figure, a method of flattening it, and the like are adopted.

In the present invention, the non-polar solvent can be microwave-heated by disposing the reaction solution held in the flow tube at the site where the electric field is concentrated as described above. In the present invention, a flow pipe for circulating the fluid of the reaction liquid is used.

The electromagnetic wave is uniformly and intensively irradiated (microwave heating) in the length direction of the flow pipe. As a result, the reaction liquid in the electromagnetic wave irradiation space in the flow pipe can be uniformly and intensively heated in the flow direction. In the present specification, to uniformly irradiate electromagnetic waves means to form a space of high energy density in the reaction tube portion in the microwave irradiation space, and to uniformly irradiate at any position of this reaction tube portion, It has a uniform energy density. As this means, as described above, for example, it is preferable to provide a resonant cavity that forms a standing wave in the irradiation space of the flow pipe. A metal fine particle synthesizer used for this purpose will be described with reference to the drawings.

FIG. 1 shows a configuration example of a microwave-assisted chemical reaction device. The apparatus of FIG. 1 includes a microwave oscillator/controller 6, a TM ₀₁₀ cavity 2, and a liquid feed pump 3. The cavity is configured as a metallic cavity resonator having a cylindrical space inside. The inner size of this space can be appropriately set so that a standing wave called TM ₀₁₀ can be formed.

The standing wave of TM ₀₁₀ has an electric field concentrated in the central portion as shown in the lower graph in FIG. 2, and has a uniform electric field intensity distribution along the axis. A reaction tube 7 composed of a quartz glass tube or the like is installed so as to penetrate along the central axis. The liquid feed pump 3 was attached to one side so that the reaction liquid 8 could flow through the quartz glass tube. A thermocouple as a thermometer 5 was attached to the opposite side of the quartz glass tube so that the temperature of the fluid could be measured. Further, an electric field monitor 4 was attached to measure the electric field strength inside.

The microwave generated from the microwave oscillator/controller 6 is applied to the cylindrical TM ₀₁₀ cavity 2 through the microwave irradiation port 1. The oscillation frequency of the microwave at this time or the inner diameter of the cylindrical cavity 2 can be adjusted so that a standing wave of TM ₀₁₀ can be formed inside the cavity 2. At this time, based on the signal from the electric field monitor 4, it is possible to know whether the standing wave of TM ₀₁₀ is formed. If the standing wave is not formed, the standing wave is formed by changing the microwave oscillation frequency oscillated from the microwave oscillator/controller 6 or adjusting the inner diameter of the cavity. Feedback control may be performed.
A matching device may be incorporated in the microwave oscillator/controller 6 in order to reduce the reflected wave from the cavity. Further, in order to prevent damage to the microwave oscillator and the controller due to the reflected wave, an isolator that absorbs the reflected wave may be incorporated.

As the reaction tube, a Teflon (registered trademark) tube having an inner diameter of 1 mm, an outer diameter of 3 mm and a length of 200 mm was used. Of these, a 100 mm portion in the length direction was placed in the cavity, and this portion was irradiated with microwaves. However, the length of the reaction tube and the length of the microwave irradiation part may be longer or shorter than this. The microwave output may be automatically adjusted via the microwave oscillator/controller 6 so that the value of the thermometer 5 installed at the outlet of the reaction tube can be kept constant. As a method of measuring the temperature of the reaction tube, there is also a method of measuring the temperature of the outer wall of the reaction tube in a non-contact manner with a radiation thermometer.

A second configuration example for realizing the present invention is shown in FIG. As a method of supplying the reaction solution, two or more kinds of separately prepared raw material solutions are supplied using individual pumps, and the mixture is mixed by the mixer 13 immediately before flowing into the microwave cavity 2 to obtain the reaction solution. To do. For example, when the reactivity of the metal precursor and the additive is high, unintended reaction may proceed before microwave irradiation and metal nanoparticles having unintended particle size distribution may be synthesized. However, with this configuration, even in such a case, it is possible to produce metal nanoparticles having a uniform particle size.

A third configuration example for realizing the present invention is shown in FIG. This introduces the reaction liquid after microwave irradiation into another temperature controller 14. When the reaction time is slow, the growth of the metal nanoparticles can be promoted by heating to an appropriate temperature with a temperature controller. Further, in the case of a reaction solution having a short reaction time, it is possible to stop the growth of the metal nanoparticles by cooling to produce metal nanoparticles having a uniform particle size.

FIG. 25 shows a fourth configuration example for realizing the embodiment of the present invention. In this method, the reaction liquid after microwave irradiation is recirculated and microwaves are irradiated again. This configuration is suitable when it is necessary to irradiate the microwave for a long time.

Next, the present invention will be described in more detail based on examples, but the present invention is not limited thereto.
Using the microwave-based chemical reaction device shown in FIG. 1, a Teflon (registered trademark) reaction tube having an inner diameter of 1 mm, an outer diameter of 3 mm and a length of 200 mm was attached as a reaction tube along the central axis of the TM ₀₁₀ cavity. The reaction raw material dissolved in ethylene glycol was supplied from one side of the Teflon (registered trademark) reaction tube by a syringe pump. The reaction raw material is irradiated with microwaves having a uniform energy distribution in a section of 100 mm. This raises the solution temperature. The temperature of the solution is measured by a thermocouple inserted in a part 10 mm away from the outlet of the microwave irradiation space of the TM ₀₁₀ cavity, and the microwave power is adjusted by feedback control so that the temperature of this part becomes constant. Is done by.
In Example 25, which will be described later, microwave frequencies of 2.45 GHz and 5.8 GHz were used, and other frequencies were 2.45 GHz.

Microwave irradiation accelerates particle formation in the solution containing the reaction raw material, and is recovered as a nanoparticle suspension at the outlet of the reaction tube. The absorption spectrum of the nanoparticle suspension was measured with an ultraviolet-visible absorption spectrometer (Hitachi U-3310). The particle diameter was measured by dynamic light scattering (Zetasizer Nano-S manufactured by MALVERN INSTRUMENTS). The dried nanoparticles were observed by TEM (TECNAI G ² manufactured by FEI). The solution after the reaction was filtered with a filter having a pore size of 0.02 μm to separate the synthesized metal particles and the unreacted liquid. Of these, the concentration of the raw material contained in the unreacted liquid was analyzed by an inductively coupled atomic absorption spectrometer (SPS1500, manufactured by Seiko Instruments), and the reaction rate was calculated from the following formula.
Reaction rate = (1-concentration of raw material contained in unreacted liquid) / (concentration of raw material contained in pre-reaction solution) x 100%

Tables 1 and 2 show the compositions of the reaction raw materials used as examples. In Example 25, which will be described later, microwave frequencies of 2.45 GHz and 5.8 GHz were used, and other frequencies were 2.45 GHz.
In Examples 1 to 16, silver nanoparticles were synthesized using silver nitrate as a metal precursor and ethylene glycol as a liquid medium. By adding polyvinylpyrrolidone as an additive, the particle size is adjusted and the synthesized silver nanoparticles are stabilized. Unless otherwise specified, the reaction raw materials were fed at a liquid feeding rate of 10 ml/h. The irradiation microwave power was 15 W, and the solution temperature at that time was 160°C.

In Examples 1 to 3, the effect on the synthesized silver nanoparticles when the number average molecular weight of polyvinylpyrrolidone as an additive was changed was examined. The supply rate of the reaction raw material at this time was 10 ml/h, and the microwave of 15 W was continuously irradiated. The temperature of the reaction solution was 160°C. FIG. 3 shows the particle size distribution of the synthesized silver nanoparticles. It can be seen that the particle size of the intended silver nanoparticles can be adjusted by changing the composition of the additive.

In Examples 4 to 8, the effect on the synthesized silver nanoparticles when the addition amount of the polyvinylpyrrolidone additive was changed was examined. The supply rate of the reaction raw material at this time was 10 ml/h, and the microwave of 15 W was continuously irradiated. The temperature of the reaction solution was 160°C. FIG. 4 shows the particle size distribution of the synthesized silver nanoparticles. It can be seen that the target particle size of the silver nanoparticles can be adjusted by changing the addition amount of the additive.

In Examples 9 to 14, the effect of changing the concentration of silver nitrate, which is a metal precursor, on the synthesized silver nanoparticles was investigated. The supply rate of the reaction raw material at this time was 10 ml/h, and the microwave of 15 W was continuously irradiated. The temperature of the reaction solution was 160°C. FIG. 5 shows the particle size distribution of the synthesized silver nanoparticles. It can be seen that the particle size of the intended silver nanoparticles can be adjusted by changing the addition amount of silver nitrate. FIG. 6 is a transmission electron microscope image of silver nanoparticles synthesized in Example 14 under the conditions of a solution supply rate of 100 ml/h and a reaction temperature of 160° C. Silver nanoparticles with a uniform particle size having an average particle size of 9.8 nm, a standard deviation of 0.9 and a CV value distribution of 9.2% (result of 50 particles analysis) were synthesized.

Example 15 is a continuous synthesis for a long time. The time change of the solution temperature at that time is shown in FIG. 7, and the microwave irradiation power (incident wave to TM ₀₁₀ cavity—reflected wave) is shown in FIG. The supply rate of the reaction raw material at this time was 10 ml/h, and silver nanoparticles were continuously synthesized for 5 hours or more. FIG. 9 shows the particle distribution of the post-reaction solution sampled every hour. It can be seen that very small and uniform particles of about 15 nm are synthesized. Further, FIG. 10 shows an absorption spectrum. In the absorption spectrum, an absorption spectrum peculiar to silver nanoparticles of 395 nm is confirmed. As is clear from FIG. 7, FIG. 8, FIG. 9, and FIG. 10, it can be seen that stable silver nanoparticles can be continuously synthesized over a long period of time. At this time, when the reaction addition rate in which the metal precursor was converted into metal nanoparticles was analyzed, it was 95% or more in any time period, and the metal nanoparticles could be produced with high efficiency.

The results obtained by changing the liquid feed rate of the reaction solution under the conditions of Example 7 are shown in FIG. It was carried out at a liquid transfer rate of 10 ml/h to 200 ml/h. At 10 ml/h to 100 ml/h, there is no significant difference in the particle size distribution of the silver nanoparticles synthesized, but at 200 ml/h, it can be seen that the uniformity of the particles is significantly impaired. The particles being uniform means that the particle size distribution is within a narrow distribution range around a predetermined value. It is considered that this is because the microwave irradiation space is 100 mm and the microwave required for silver nanoparticle synthesis is not sufficiently irradiated when the liquid feeding speed is high. In such a case, it is considered that the TM ₀₁₀ cavity, which is the microwave irradiation space, should be replaced with a longer one. A feature of the TM ₀₁₀ cavity is that even if the cylinder length is increased, a uniform microwave irradiation space can be created.

In Example 15 and Example 16, the difference in particle size distribution of silver nanoparticles synthesized by changing the solvent was examined. FIG. 12 shows the particle size distribution of the synthesized silver nanoparticles when a glycerol solvent (reaction temperature 200° C.) and a benzyl alcohol solvent (reaction temperature 180° C.) are used. It can be seen that the metal nanoparticles can be synthesized using a liquid solvent other than ethylene glycol.

As Example 17, gold nanoparticles were synthesized. Chloroauric acid was used as a metal precursor, water was used as a liquid solvent, and citric acid was added as a reducing agent to the reaction liquid. FIG. 14 shows the particle size distribution of the solution after the reaction when the gold nanoparticles were synthesized at a reaction temperature of 90° C. and a liquid feed rate of 5 ml/h. As with silver nanoparticles, it can be seen that continuous synthesis of gold nanoparticles with a uniform particle size distribution is possible.

Examples 18 and 19 are synthetic examples of platinum nanoparticles and palladium nanoparticles. FIG. 15 shows the particle size distribution of platinum particles and palladium particles when synthesized at a reaction temperature of 160° C. and a liquid feed rate of 10 ml/h. It can be seen that it is possible to synthesize particles having a uniform particle size, like silver nanoparticles.
In Example 20 and Example 21, considering the productivity, the effect of the high concentration of the metal precursor on the synthesized particles was examined. FIG. 16 shows the particle size distribution of silver nanoparticles and platinum nanoparticles when synthesized at a reaction temperature of 160° C. and a liquid feed rate of 10 ml/h. It can be seen that even when the metal precursor is synthesized at a high concentration, particles having a uniform particle size distribution are synthesized.
In Examples 22 to 24, the effect on the synthesized particles when a polymer dispersant was used as an additive was investigated. The polymer dispersant used in Examples 22 to 24 is a dispersant having a number average molecular weight of 10,000 to 50,000. 17 is a silver nanoparticle synthesized under the conditions of Example 22, FIG. 18 is a platinum nanoparticle synthesized under the conditions of Example 23, and FIG. 19 is a transmission electron microscope image of platinum nanoparticles synthesized under the conditions of Example 24. is there. It can be seen that spherical nanoparticles of uniform particle size are synthesized.
Example 25 is an examination of the effect on the synthesized particles when the frequency of the irradiated microwave is changed. FIG. 20 shows a particle size distribution of silver nanoparticles synthesized at microwave frequencies of 2.45 GHz and 5.8 GHz. It can be seen that silver nanoparticles having a uniform particle size distribution are synthesized even at a microwave frequency of 5.8 GHz.
Example 26 is a comparison of the effects on the synthesized particles when silver nanoparticles are synthesized in the configuration example shown in FIG. 1 and the second configuration example shown in FIG. 23. FIG. 21 shows the particle size distribution of silver nanoparticles when synthesized at a reaction temperature of 160° C. and a total liquid transfer rate of 10 ml/h. It can be seen that even when the ethylene glycol solution containing silver nitrate and the ethylene glycol solution containing additives are individually supplied, silver nanoparticles having a uniform particle size distribution are synthesized.
Example 27 is an examination of the influence on the synthesized particles when silver nanoparticles were synthesized in the third configuration example shown in FIG. FIG. 22 is a transmission electron microscope image of silver nanoparticles when cooled using a temperature controller. It can be seen that spherical silver nanoparticles of uniform particle size are synthesized.

1 Microwave Irradiation Port 2 TM ₀₁₀ Cavity 3 Liquid Delivery Pump 4 Electric Field Monitor 5 Thermometer 6 Microwave Oscillator/Controller 7 Reaction Tube 8 Reaction Liquid 11 Cavity Resonator 12 Microwave Irradiation Port 13 Mixer 14 Temperature Controller

Claims (2)

A reaction liquid containing a metal precursor is circulated in a flow pipe arranged in the microwave irradiation space, and a microwave is irradiated from the outside of the flow pipe toward the flow pipe in the microwave irradiation space, A method for producing fine metal particles, comprising heating the reaction liquid in the flow pipe by microwave irradiation to synthesize fine metal particles,
As the microwave generation device, a device to which a frequency band of 2.4 to 2.5 GHz is allocated as a frequency band or a standard frequency band of a product is used,
A method for producing metal fine particles, characterized in that a flow tube having an average inner diameter of a portion of the flow tube arranged in the microwave irradiation field is smaller than 2.84 mm. The method for producing metal fine particles according to claim 1, wherein the reaction liquid irradiated with the electromagnetic wave is guided to a temperature controller.

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