JP6112508B2 - Method for producing metal nanoparticles - Google Patents

Description

  The present invention relates to a method for producing metal nanoparticles. More specifically, the present invention relates to a method for producing metal nanoparticles by plasma generated in an aqueous solution.

  Nanoparticles with a particle size in the nanometer range may differ from atoms, molecules, or bulk materials in terms of optical properties, electrical properties, mechanical properties, etc. due to their shape specificity. Utilization in various fields as a material or a catalyst material has been studied and its application has been promoted.

Metal nanoparticles made of metal can be produced relatively easily by the recent development of mass synthesis technology and high-speed synthesis technology of nanoparticles. The present inventors also proposed a completely new metal nanoparticle generator capable of generating a large amount and rapidly of metal nanoparticles having a particle size of 500 nm or less by the action of submerged plasma generated in the solution. (For example, refer to Patent Document 1).
According to this metal nanoparticle generator, glow discharge plasma can be generated in an aqueous solution. This glow discharge plasma contains at least one of hydrogen radicals, hydroxide radicals, and electrons. Therefore, by generating glow discharge in an aqueous solution containing a metal salt, the metal ions dissociated in the aqueous solution are reduced by the action of these radicals and the like, and metal nanoparticles constituting the metal salt are generated. It is.

JP 2008-13810 A

According to the above metal nanoparticle generator, stable plasma can be generated in an aqueous solution. Therefore, the synthesis of metal nanoparticles can also be performed at a sufficiently high speed.
However, to generate stable plasma in an aqueous solution, it is necessary to control the plasma generation conditions at a very high level, and the plasma generation conditions that can realize a stable reduction reaction must be limited. I didn't get it. In particular, when manufacturing metal nanoparticles having a particle size of 100 nm or less (for example, 10 nm or less), more precise control of plasma generation conditions is required. For example, in order to stably generate plasma under stricter conditions, control of the form and arrangement of electrodes that generate plasma, the type of plasma to be generated, and voltage application conditions for maintaining such plasma, etc. Is required. Therefore, from the viewpoint of producing metal nanoparticles at higher speed, the room for improvement has been limited to a very narrow range.

  The present invention has been created in view of the above problems, and provides a method for producing metal nanoparticles, which can form metal nanoparticles at a higher rate using plasma generated in an aqueous solution. For the purpose.

  In order to solve the above problems, the method for producing metal nanoparticles provided by the present invention provides a solution in which a salt containing a metal ion is dissolved in a mixed solvent of a lower alcohol and water, Generating a glow discharge plasma to reduce the metal ions to form nanoparticles of the metal. And the molar ratio of the lower alcohol which occupies for the whole said mixed solvent shall be 0.05-0.2, It is characterized by the above-mentioned.

The present inventors have found that metal nanoparticles can be produced by utilizing the action of glow discharge plasma generated in an aqueous solution (hereinafter sometimes simply referred to as “solution plasma”), Improvements have been made to apply this technique to a form more suitable for actual practical application. At the same time, we have conducted intensive research to elucidate the basic characteristics of the solution plasma and to apply this solution plasma technology to a wide range of technical fields, not just the production of metal nanoparticles. From these experiences, the present inventors have found the following findings and have completed the present invention.
That is, (1) The dielectric breakdown voltage can be increased by including a lower alcohol in the aqueous solution, and a larger amount of radical species acting on the reduction can be generated. (2) However, this effect strongly depends on the content ratio of the lower alcohol, and a particularly suitable effect is exhibited when the molar ratio of the lower alcohol is within the above range.
Thereby, the production rate of the metal nanoparticles can be increased by a very simple method of adjusting the composition of the solvent with a lower alcohol. For example, production at an extremely high speed of about 18 times as compared with the case where a lower alcohol is desired to be included in an aqueous solution becomes possible.

In a preferred embodiment of the method for producing metal nanoparticles disclosed herein, the concentration of the metal ion in the solution is adjusted to 0.05 mM to 0.5 mM.
By adjusting the concentration of the metal ions to such a range, the produced metal nanoparticles can be prevented from agglomerating, and more efficient metal nanoparticles can be produced.

In a preferred embodiment of the method for producing metal nanoparticles disclosed herein, the metal ion is a noble metal ion.
The method of the present invention can be preferably applied to the production of noble metal nanoparticles produced by reducing noble metal ions.

In a preferred embodiment of the method for producing metal nanoparticles disclosed herein, a solution further containing a protective agent is prepared in the aqueous solution.
It is also a preferred embodiment that a protective agent for preventing aggregation of the formed metal nanoparticles is added to the aqueous solution. Various materials are known as such a protective agent. For example, it is preferable to use a cationic surfactant from the viewpoint that finer and more stable metal nanoparticles can be formed.

In a preferred embodiment of the method for producing metal nanoparticles disclosed herein, a pair of linear electrodes are arranged in the aqueous solution, the pulse width is 0.1 μs to 5 μs, and the frequency is 10 between the linear electrodes. is characterized in that plasma is generated by applying a DC pulse voltage of ³ Hz to 10 5 ^Hz.
According to such a configuration, most of the bubbles generated in the aqueous solution by the Joule heat generated between the linear electrodes can be maintained in a stable state in the aqueous solution without being floated toward the water surface. Plasma can be generated in a stable state. Thereby, metal nanoparticles can be produced in a more efficient and stable state.

It is a schematic diagram which shows an example of a structure of the solution plasma generator used for manufacture of a metal nanoparticle. It is a conceptual diagram explaining the reaction field by the solution plasma used for manufacture of a metal nanoparticle. It is an observation image at the time of generating solution plasma in (A) water and (B) ethanol. It is the figure which illustrated the emission spectrum when the solution plasma was generated in (A) water and (B) ethanol. It is the figure which illustrated the relationship between the dielectric breakdown voltage at the time of generating solution plasma, and the ethanol concentration in a solution. It is the figure which showed the time change of the UV-vis absorption spectrum when the ethanol ratio in an ethanol-water mixed solution is set to 0.14 ( _Xalc = 0.14). It is the figure which illustrated the relationship between the ethanol ratio and reaction constant in an ethanol-water mixed solution.

  Hereinafter, the manufacturing method of the metal nanoparticle of this invention is demonstrated. In addition, matters other than matters particularly mentioned in this specification (preparation conditions of mixed solvent, plasma generation conditions, etc.) and matters necessary for carrying out the present invention (configuration of plasma generation device, etc.) It can be grasped as a design matter of a person skilled in the art based on the prior art in the field. The present invention can be carried out based on the contents disclosed in this specification and the drawings and common general technical knowledge in the field.

  The method for producing metal nanoparticles disclosed herein essentially prepares a solution in which a salt containing metal ions is dissolved in a mixed solvent of lower alcohol and water, and generates a solution plasma in this solution. By reducing the metal ions, metal nanoparticles are formed in the solution. In such a method, the molar ratio of the lower alcohol in the mixed solvent is preferably adjusted to 0.05 to 0.2.

  Here, the solution plasma serving as the reaction field will be described. A solution plasma is a glow formed by partially or completely ionizing molecules that constitute a gas in a gas (gas phase) generated by applying a predetermined pulse voltage between electrodes in the liquid. Discharge plasma. In general, in a plasma generated in a solution, the gas phase surrounding the plasma phase is further surrounded by a liquid phase, and active species such as ions, electrons, and radicals constituting the plasma can be freely set in the limited gas phase. You can exercise. Therefore, it exhibits physical and chemical properties that are different from gas phase plasma (typically atmospheric pressure plasma, low pressure plasma, etc.) generated in the released gas phase.

  For example, gas phase plasma is generated when neutral molecules constituting the gas are ionized and turned into plasma when the temperature of the gas is raised. At this time, unlike the phase transition between solid, liquid, and gas, the transition from gas to plasma occurs gradually, so that even a very small amount of ionization of constituent molecules can be sufficiently plasma. . On the other hand, plasma generated in a solution typically has a liquid phase that is first vaporized by Joule heating by discharge in the liquid, and further plasma is generated in this gas phase. Formed with. That is, in the plasma in liquid, a high energy state called plasma is confined in the liquid (that is, the condensed phase), so that a closed system physics is realized and a high-density plasma reaction field that is not released is formed.

  The plasma generated in the solution is classified into spark discharge such as lightning, corona discharge, glow discharge, arc discharge, etc., depending on the difference in potential applied between the electrodes. When the spark discharge continuously flows, it becomes glow discharge or arc discharge. Here, the glow discharge plasma (that is, solution plasma) generated in the liquid has further different characteristics from other liquid plasmas. For example, the arc discharge plasma is a thermal plasma having a high particle density and a local thermal equilibrium state in which the temperature of ions and neutral particles is substantially equal to the electron temperature. In contrast, glow discharge plasma is a low-temperature plasma in a non-equilibrium state in which the electron temperature is high but the temperature of ions and neutral particles is low. In addition, it is difficult to generate a continuous plasma by corona discharge, and there is a feature that a relatively large amount of oxidizing hydroxy radicals are formed together with hydrogen radicals by decomposition of water. On the other hand, glow discharge plasma has high plasma energy, and oxidizing hydroxy radicals are further decomposed to generate many reducing hydrogen radicals. That is, according to the solution plasma (glow discharge plasma), a more reducing reaction field is realized. For this reason, in the present invention, solution plasma is used as plasma used for reduction of metal ions.

  In the present invention, the lower alcohol is a linear or branched alcohol having 1 to 5 carbon atoms, and may be a saturated or unsaturated alcohol. Specific examples of such lower alcohols include monohydric alcohols such as methanol, ethanol, propanol, butanol and pentanol, dihydric alcohols such as ethylene glycol and propylene glycol, and polyhydric alcohols such as glycerin. Illustrated. Among them, the lower alcohol in the present invention is a monovalent saturated alcohol such as methanol, ethanol, n-propanol and i-propanol, n-butanol, i-butanol and sec-butanol, and various pentanols. More preferably, it is a linear saturated alcohol. These may be used individually by 1 type, and may mix and use 2 or more types. When the number of carbon atoms is too large, the molecular weight and the molecular diameter become too large, which is not preferable because it may hinder the state of radicals in the solution plasma, particularly the generation and action of hydrogen radicals described later.

  A mixed solvent is prepared by mixing the above lower alcohol with water. As water, pure water such as distilled water or ion exchange water, tap water, or the like can be used. Here, the molar ratio of the lower alcohol in the entire mixed solvent is 0.05 to 0.2. By mixing the lower alcohol with water as much as possible, the amount of hydrogen radicals contained in the solution plasma can be increased, and the reduction reaction can be promoted. That is, it becomes possible to produce metal nanoparticles at a faster reaction rate. In particular, the molar ratio of the lower alcohol is preferably about 0.04 or more, for example, 0.05 or more, because the amount of hydrogen radicals is remarkably increased. However, the amount of such hydrogen radicals becomes maximum when the molar ratio of the lower alcohol is about 0.14, and tends to decrease when the molar ratio is higher. Therefore, the molar ratio of the lower alcohol is preferably adjusted to about 0.6 or less, for example, 0.4 or less, and more preferably 0.2 or less. For example, the lower limit of the molar ratio of the lower alcohol can be suitably 0.08 or more, for example 0.1 or more, more preferably 0.12 or more. One upper limit can be suitably 0.18 or less, for example 0.16 or less, more preferably 0.15 or less.

As the salt containing a metal ion, any salt of various metals that can constitute the target metal nanoparticles can be used without any particular limitation. In other words, what is necessary is just to prepare so that the metal ion which can comprise the target metal nanoparticle is contained in the said solution.
There is no strict limitation on the metal that can constitute the nanoparticles that can be produced in the present invention. For example, a metal composed of a single element belonging to Group 1 to Group 14 of the periodic table of elements, or two or more of these metals Alloys composed of elements (including intermetallic compounds and multiphase alloys) can be considered. As a particularly preferred embodiment, nanoparticles of a noble metal element or an alloy containing a noble metal element can be exemplified. Specifically, for example, as the metal element, potassium (K), iron (Fe), cobalt (Co), nickel (Ni), copper (Cu), zinc (Zn), gallium (Ga), ruthenium (Ru) ), Rhodium (Rh), palladium (Pd), silver (Ag), cadmium (Cd), indium (In), tungsten (W), osmium (Os), iridium (Ir), platinum (Pt) and gold (Au) And the like. Of these, noble metal elements such as gold, silver, platinum, palladium, rhodium, iridium, ruthenium, and osmium are preferable. Thus, unlike the process using the conventional chemical reduction method, the production of metal nanoparticles by solution plasma can be widely applied to various metal elements as described above.

  Examples of salts containing these metal ions include halides such as chlorides, bromides and iodides, borides, hydroxides and sulfides of desired metals; carbonates, sulfates, nitrates and oxalic acids. Salts, perchlorates and the like can be preferably used. In particular, salts containing noble metal ions include various noble metal chlorides, bromides, iodides, hydroxides, sulfides, sulfates, nitrates, potassium composite oxides, ammonium A complex oxide such as a complex oxide or a sodium complex oxide can be used. In addition, as a complex of a noble metal element, an ammine complex, a cyano complex, a halogeno complex, a hydroxy complex, or the like of a desired noble metal can be used.

Specifically, for example, in the case where the noble metal element is platinum (Pt), a salt that forms a simple hydrated ion such as Pt ²⁺ may be used, but a stable complex is more preferably used. Various ammine complex salts, cyano complex salts, halogeno complex salts, hydroxy complex salts and the like that can be formed can be considered. Typically, for example, platinum chloride hexahydrate (H ₂ (PtCl ₆ ) · 6H ₂ O), platinum (IV) chloride, platinum (II) bromide, platinum (IV) iodide, platinum (IV) Sulfide, potassium tetrachloroplatinate (II), ammonium tetrachloroplatinate (II), sodium hexachloroplatinate (IV) hexahydrate, platinum (II) hexafluoroacetylacetonate complex, platinum (II) acetylacetate Examples include nato complex salts.

  The above metal ions are preferably contained in the solution before the solution plasma is generated at a concentration that does not cause the formed metal nanoparticles to aggregate. Such a concentration is not strictly limited because it depends on, for example, the form of generation of plasma in the liquid, but can be set, for example, at a concentration of about 0.05 mM to 0.5 mM. When a plurality of types of metal ions are contained in the solution, the total concentration thereof is preferably within the above range. If the concentration of metal ions is lower than 0.05 mM, the amount of metal nanoparticles formed in a unit volume decreases, which is not preferable in terms of reducing the production efficiency of metal nanoparticles. On the other hand, if the concentration exceeds 0.5 mM, the possibility that the metal nanoparticles partially aggregate in the solution is not preferable. Therefore, the concentration of metal ions is preferably in the range of about 0.08 mM to 0.3 mM, and more preferably in the range of about 0.1 mM to 0.3 mM, for example.

In addition, although not essential, in order to stably maintain the dispersed state of the formed metal nanoparticles within a range that does not impair the object of the present invention, it is also referred to as various protective agents (also referred to as surface modifiers, etc.) in the solution. ) Is also preferably included. As such a protective agent, for example, polymers that can be dispersed or dissolved in water, saccharides, dendritic polymer compounds, thiol compounds, metal coordination compounds, surfactants, and the like can be used. More specifically, for example, polymers such as gelatin, polyvinylpyrrolidone, polyvinyl alcohol, polyethylene glycol, gum arabic, polymer cyclodextrin and acrylonitrile, saccharides represented by oligosaccharides, cyclic oligosaccharides, Dendrimer compounds represented by dendrimers, etc., mercaptocarboxylic acids such as mercaptoacetic acid and mercaptopropionic acid, mercaptoalkylamines, alkylthiols, halogenated thiocholines, thiophenols, thiol compounds represented by benzenethiols, Metal coordination compounds represented by isocyanides, nitriles, amines, taurine, cetyltrimethylammonium bromide (CTAB), dodecyltrimethylammonium bromide (DTAB), octyltrimethyl Cationic surfactants such as quaternary ammonium salt types such as ammonium bromide (OTAB), benzalkonium salts such as sodium α-sulfo fatty acid ester, alkylamine salt types such as alkylamine oxide, sodium dodecyl sulfate (SDS) ), Anionic surfactant such as α-sulfo fatty acid ester sodium salt, amphoteric surfactant represented by N, N-dimethyldodecylamine = N = oxide, nonionic interface represented by aerosol OT, etc. The use of surfactants such as activators is exemplified. Among these, a cationic surfactant is preferable as a protective agent used when producing metal nanoparticles by generating plasma in liquid because metal nanoparticles can be formed with better form and dispersibility.
Any one of these protective agents may be used alone, or two or more thereof may be mixed and used. Such a protective agent may be used at a concentration that does not cause the formed metal nanoparticles to aggregate.

In the present invention, solution plasma is generated in the solution prepared above. Specifically, the solution plasma can be generated relatively stably in a solution by applying a voltage having a sub-microsecond pulse width between electrodes at a high repetition frequency. Moreover, the glow discharge constituting the solution plasma can be generated with a relatively small amount of energy. By applying such a voltage, a stable state can be maintained for a long time (for example, 2 hours or more) while the expansion / compression motion of the liquid surrounding the plasma phase and the plasma phase are interlocked. Therefore, for example, in the solution plasma, a part of the gas phase generated between the electrodes may rise from the electrodes due to buoyancy and reach the liquid surface, but most of the gas phase is constant between the electrodes. It is constantly maintained as a large gas phase. Therefore, in the solution plasma, the plasma generation state can be constantly controlled. That is, the method for producing metal nanoparticles of the present invention makes it possible to produce metal nanoparticles efficiently by using such controlled plasma.
Whether or not the generated plasma is glow discharge plasma can be confirmed, for example, when the Townsend second coefficient obtained by plasma emission spectroscopic analysis or the like is in the range of 0.0005 to 0.005.

More specifically, the method for producing metal nanoparticles of the present invention will be described in detail based on the following examples of suitable solution plasma generation modes.
FIG. 1 is a diagram showing an outline of a solution plasma generator 10 for generating a solution plasma 4 in an aqueous solution 2. In this embodiment, a lower alcohol-water mixed solution 2 containing metal ions is placed in a container 5 such as a glass beaker. In addition, electrodes 6 for generating plasma 4 are disposed in the aqueous solution 2 at a predetermined interval. In the present embodiment, the electrode 6 is held in the container 5 via the insulating member 9 and is connected to the external power source 8, and a pulse voltage of a predetermined condition is applied from the external power source 8. As a result, the solution plasma 4 can be constantly generated between the pair of electrodes 6.

The electrode 6 may have various forms such as a flat electrode, a rod electrode, and a combination thereof. For example, it is preferable to use a linear (wire-like or needle-like) electrode because the electric field can be locally concentrated. Moreover, there is no restriction | limiting in particular about a material, For example, it can be an electrode which consists of electroconductive materials, such as iron (Fe), gold | metal | money (Au), tungsten (W), platinum (Pt). In this embodiment, a pair of tungsten linear electrodes 6 are used, and only the tip portion (for example, about 0.1 to 2 mm) is exposed to suppress an excess current that hinders electric field concentration, and the rest. This part is insulated by an insulating member 9 or the like. For example, the insulating member 9 is made of rubber or resin (for example, silicon resin or fluororesin). In this embodiment, the insulating member 9 has a configuration in which the electrode 6 is fixed to the container 5 and also serves as a stopper for keeping the electrode 6 and the container 5 watertight. In such an apparatus 10, the pulse voltage application condition for generating the solution plasma 4 depends on conditions such as the type and concentration of the raw material compound contained in the aqueous solution 2, and further on the configuration conditions of the apparatus 10, For example, the voltage is about 1000 to 2000 V, the pulse width is about 0.1 μs to ⁵ μs (more preferably about 1 μs to ⁵ μs), and the repetition frequency is about 10 ³ Hz to 10 ⁵ Hz (more preferably about 10 to ³⁰ kHz). ) Is applied within the range.

Although not an essential requirement, the electrical conductivity of the aqueous solution 2 may be increased in order to enable stable generation of solution plasma. Examples of the standard for adjusting the electrical conductivity include a range of about 100 μS · cm ^{−1 to} 2500 μS · cm ⁻¹ . When adjusting the electrical conductivity, for example, an electrolyte such as potassium bromide (BrK), potassium chloride (KCl), potassium hydroxide (KOH), or sodium hydroxide (NaOH) is dissolved in the aqueous solution 2. It is good to do. If the electrical conductivity is less than 100 μS · cm ⁻¹ , a relatively large amount of power is required to generate the solution plasma, which is not preferable. In addition, when the electric conductivity exceeds 2500 μS · cm ⁻¹ , the electric power supplied between the electrodes for plasma generation is consumed as an ionic current, which makes it difficult to generate plasma constantly. It is not preferable. Electrical conductivity, for example, be a 300μS ^{· cm} -1 ~2300μS ^· cm approximately ^-1, further, it is exemplified that the 1000μS ^{· cm} -1 ~2000μS ^· cm approximately ^-1.

  In this way, the solution plasma 4 is formed in the aqueous solution 2 by the solution plasma generator 10. The plasma reaction field generated by the solution plasma generator 10 has a configuration as shown in FIG. 2, for example. That is, a gas phase 3 is formed in the aqueous solution (liquid phase) 2, and a solution plasma (plasma phase) 4 is formed in the gas phase 3. This plasma reaction field is typically maintained constantly between the pair of electrodes 6. In such a plasma reaction field, active species such as electrons, ions and radicals having high energy are supplied from the plasma phase 4 toward the liquid phase 2. On the other hand, water, alcohol and metal ions constituting the liquid phase 2 are supplied from the liquid phase 2 to the gas phase 3 and the plasma phase 4. These contact (collision) mainly at the interface between the liquid phase 2 and the gas phase 3. For the sake of easy understanding, FIG. 2 shows a state in which each interface between the liquid phase 2 and the gas phase 3 and between the gas phase 3 and the plasma phase 4 is clearly formed in a substantially spherical shape. The interface is not necessarily limited to being clearly formed. For example, there is no critical interface between the gas phase 3 and the plasma phase 4, and this interface may have a spatial extent.

The active species generated here are typically derived from alcohol in addition to hydrogen ions, hydroxide ions, oxygen ions, hydrogen radicals, oxygen radicals and hydroxy radicals, which are generated by the decomposition of water molecules in aqueous solutions. It may be a decomposition product, an intermediate chemical species such as C ₂ , C ₃ , or CH ₂ . In the solution plasma, in particular, since the decomposition of hydroxy radicals proceeds and a lot of hydrogen radicals are formed, it exhibits a strong reducing action on metal ions contained in the liquid phase. In the present invention, the mixed solvent of the lower alcohol and water as described above is used as the solvent, so that the generation amount of hydrogen radicals is remarkably increased and a further reducing action is exhibited.
This phenomenon is caused by the fact that when hydrogen, which is a hydrogen-bonded liquid, is added to lower hydrogen alcohol, the ability to form hydrogen bonds in water is changed, resulting in changes in the structure and density of water. It is understood that this is because the plasma excited state is also affected. That is, in a mixed solvent of lower alcohol and water, changes occur in ion mobility and water stabilization energy. Therefore, the conditions such as the composition, concentration, pressure, etc. of the gas phase formed by heating such a solvent are changed, the dielectric breakdown voltage is increased in a predetermined alcohol concentration region, and the active species formed are also changed. It is thought that it was brought about.

  According to the above configuration, for example, the metal ions are reduced by the action of the solution plasma, and the metal nanoparticles in the solution are formed. The nanoparticles are typically generated without being agglomerated, for example, in the state of being highly dispersed in an aqueous solution as very fine particles, whereby metal nanoparticles are formed. Such metal nanoparticles can be in a state close to monodispersed with a uniform particle size. For example, when observed with an electron microscope (for example, a transmission electron microscope (SEM)), the average particle size is typically 20 nm or less. Can be formed with an average particle size of 3 μm or less, typically about 1 μm to 2 μm. As an example, such metal nanoparticles can be produced with almost all (for example, 70% by number or more, typically 90% by number or more) particles having a particle size of 3 nm or less.

Such monodispersity of metal nanoparticles is also characteristic in the production of metal nanoparticles by solution plasma. That is, the metal ions contained in the solution are supplied to the reaction field at high density via the liquid phase. The metal ions are reduced by the reducing action of the active species, thereby promoting the nucleation of the metal nanoparticles. On the other hand, the plasma in liquid is a low-temperature plasma, and the grain growth of the generated particle nuclei is suppressed. Thus, in the method for producing metal nanoparticles of the present invention, fine metal nanoparticles can be easily and efficiently formed in a state close to monodispersion. The metal nanoparticles produced in this manner can exhibit various characteristics (for example, catalytic action, heat conduction medium, etc.) depending on the type of constituent elements. For example, the alloy nanoparticles obtained by the above production method can be suitably used as catalysts such as battery electrode materials, semiconductor materials, and other functional powders.
And the reaction rate of such metal nanoparticles is about a reaction rate constant as compared with the case of using water (that is, not containing lower alcohol) as a solvent, for example, with the increase of hydrogen radicals in the plasma reaction field. It can be as much as 20 times. This makes it possible to produce high-quality metal nanoparticles with the same particle size as described above at high speed.

  As mentioned above, although the manufacturing method of the metal nanoparticle was demonstrated based on suitable embodiment, this manufacturing method is not limited to this example, It can change and change an aspect suitably. For example, when generating the solution plasma, it is not always necessary to use a needle-like electrode made of tungsten. For example, the solution plasma may be generated by a low inductance induction coil. Furthermore, it goes without saying that the conditions for generating the solution plasma can also be adjusted as appropriate according to the metal species of the target nanoparticles, the conditions of the apparatus, and the like. In addition, the metal nanoparticles may be composed of a single metal composed of a single metal element, a solid solution composed of a plurality of metal elements, an intermetallic compound, or a mixture of these metal phases or alloy phases. It may be composed of a mixture. The metal strings of such metal nanoparticles are not necessarily all contained in the solution. For example, the electrode is used as a part of the raw material of the metal nanoparticles, and the metal ions in the solution and the electrode material are used. The alloy nanoparticles may be produced by a reduction action using solution plasma.

As described above, the method for producing metal nanoparticles of the present invention significantly increases the reaction rate in a known plasma reaction field by a very simple operation such as changing the composition of the solvent. It may reflect the possibility of deployment.
Next, although the Example regarding this invention is described, it is not intending limiting this invention to what is shown to this Example.

[Embodiment]
An alcohol-water mixed solvent was prepared using (a) methanol and (b) ethanol as the lower alcohol and distilled water as the water. The molar ratio (X _alc ) of the lower alcohol in the mixed solvent is 8 types of 0, 0.04, 0.08, 0.14, 0.2, 0.4, 0.6 and 1, and 16 types in total. A mixed solvent was prepared.
Gold (III) chloride trihydrate was used as a metal salt and dissolved in the above mixed solvent to prepare a 0.3 mM HAuCl ₄ solution.

Using the apparatus shown in FIG. 1, solution plasma was generated in the HAuCl ₄ solution prepared above. In this embodiment, a tungsten wire (manufactured by Nilaco) having a diameter of 0.8 mm was used as the electrode 6 and the distance between the facing electrodes was set to 0.5 mm. As the external power source 8, a bipolar pulse power source (manufactured by Kurita Manufacturing Co., Ltd., MPS-R06K02C-WP1F) was used. The solution plasma generation conditions were such that the temperature of the solution was 25 ° C., the pulse voltage was 1000 V, the pulse width was constant at 1.5 μs, and the pulse frequency was changed under three conditions of 15 Hz, 20 Hz, and 25 Hz. In addition, the voltage waveform and electric current waveform during discharge were measured using the digital oscilloscope. Further, the emission spectrum of plasma emission during discharge was measured. Further, the solution was collected every predetermined time from the start of discharge, and the electronic spectrum (absorption spectrum) of the solution was measured by visible / ultraviolet spectroscopy.

[Observation of solution plasma]
The form of the solution plasma during discharge varied greatly depending on the composition of the alcohol-water mixed solution. In FIG. 3, (A) 100% solvent (molar ratio: X _alk = 0) was used, and (B) 100% ethanol solvent (molar ratio: X _alk = 1) was used. An image of the solution plasma was observed. (A) In a 100% water solvent, the plasma emits red light, and the bubbles surrounding the plasma discharge part are gathered into almost one large ellipsoid, and small bubbles are occasionally formed on the surface and gradually grow and separated. Then, it was observed that it went up in the water as small bubbles. (B) In the case of a 100% ethanol solvent, it was observed that the plasma emitted blue light and small bubbles were generated in a tuft and then separated and floated. In addition, in the case of (B) ethanol 100% solvent, it was observed that the plasma spread widely in the lateral direction inside the bubbles.

[IV waveform]
The voltage and current waveforms associated with the discharge of the solution plasma were measured using an oscilloscope. This voltage and current waveform may represent the resistance behavior of the alcohol-water mixture.
In the obtained voltage waveform, generally, when a pulse voltage is applied, a high voltage is suddenly applied between the electrodes, and then a sudden drop in the voltage due to dielectric breakdown occurs, and thereafter a substantially constant voltage value is maintained. Met. On the other hand, it was confirmed that the discharge current has a mountain-shaped waveform that increases linearly with the application of the pulse voltage and then gradually decreases after a large current flows during dielectric breakdown. Under the condition that the pulse width was constant, no large difference was observed in the current-voltage waveform even when the pulse frequency was changed. From these voltage waveforms, the dielectric breakdown voltage in each alcohol-water mixed solution system was examined.

[Emission spectroscopic analysis]
Optical emission spectroscopy (OES) of solution plasma generated in each alcohol-water mixed solution system was performed. FIG. 4 shows emission line spectra obtained when (A) 100% water solvent was used and (B) 100% ethanol solvent was used. In FIG. 4A, the spectrum of hydrogen radicals and atomic oxygen accompanying water decomposition is clear, while in FIG. 4B, the spectrum of carbon dimer molecules derived from ethanol together with hydrogen radicals (C2 swan) Band) etc. are observed, and it can be seen that different reaction fields are formed. It was confirmed that such a spectral difference was dependent on the alcohol concentration in the solution.

[Dielectric breakdown voltage]
The relationship between the dielectric breakdown voltage examined from the above voltage waveform and the molar ratio of alcohol in the mixed solvent was examined. FIG. 5 shows the dielectric breakdown voltage and the ethanol concentration dependency. As shown in FIG. 5, it can be seen that a lower pulse frequency requires a higher voltage for dielectric breakdown. Although not specifically shown, the dependence of the dielectric breakdown voltage on the methanol concentration showed almost the same tendency as in the case of ethanol, but the dielectric breakdown voltage was slightly lower (about 20%) than in the case of ethanol.
In any pulse frequency, in a region where the alcohol concentration is low (for example, X _alk ≦ 0.14), the dielectric breakdown voltage increases rapidly as the alcohol concentration increases, and a region where the alcohol concentration is high (for example, X _alc It has been found that the dielectric breakdown voltage gradually decreases when _alc > 0.14).

It is known that the breakdown voltage and the electron mean free path are inversely proportional.
Here, in a region where the alcohol concentration in the solvent is sufficiently low, the proportion of alcohol in the gas phase in the solution plasma increases in proportion to the alcohol concentration according to Henry's law. And when the average molecular diameter of water molecules (0.27 nm) and the average molecular diameter of alcohols (for example, methanol: 0.35 nm, ethanol: 0.38 nm) are compared, alcohol is more preferable than water. The ethanol having a higher alcohol molecular weight has a larger average molecular diameter. That is, in the region where the alcohol concentration is sufficiently low, the electron mean free path decreases as the alcohol concentration and molecular weight increase, and the breakdown voltage increases.

On the other hand, in a region where the alcohol concentration in the solvent is so high that Henry's law is not applied, it is considered that the influence of changes in the formation of bubbles increases as the alcohol concentration increases. Therefore, it is considered that as the alcohol concentration increases, the electron mean free path gradually decreases and the dielectric breakdown voltage also decreases.
The inflection point of the breakdown voltage in the solution plasma discharge depending on the alcohol concentration was confirmed to have an alcohol molar ratio (X _alk ) in the vicinity of 0.14 in the alcohol-water mixed solution. It was also confirmed that the molar ratio (X _alk ) of the alcohol exhibiting such an inflection point is almost constant regardless of the type of alcohol and the generation conditions of the solution plasma if it is a lower alcohol.

[Visible / ultraviolet spectroscopic analysis]
When a solution plasma is generated in an alcohol-water mixed solution to synthesize Au nanoparticles, a solution is collected at a predetermined time from the start of plasma discharge, and the solution is analyzed by visible / ultraviolet (UV-vis) spectroscopy. The time change of the electronic spectrum (UV-vis absorption spectrum) was examined. In addition, to the alcohol-water mixed solution, potassium bromide was added every time the solution was collected to keep the conductivity constant. FIG. 6 shows the time change of the UV-vis absorption spectrum when the molar ratio of ethanol in the ethanol-water mixed solution is 0.14 (X _alk = 0.14).

AuCl 4 _- ^(III) reduction reaction of the ion, the concentration of the hydrogen radicals of the system assuming a constant, regarded as a first-order reaction. Moreover, it is thought that the decrease rate of the peak height of the absorption spectrum of [AuBr ₄ ] ⁻ obtained from the result of UV-vis analysis is equal to the decrease rate of AuCl ₄ ⁻ ions. The primary reaction, since the expressed by the following equation (1), based on the following equation (2), AuCl 4 _- was calculated ^(III) the reaction rate multiplier k of the reduction of the ion. In equations (1) and (2), Co represents the initial concentration of the solution, and c represents the concentration at time t. The results are shown in FIG.

As shown in FIG. 7, the reaction rate constant was maximized when the molar ratio of ethanol in the alcohol-water mixed solution was 0.14 (X _alk = 0.14). The maximum reaction rate constant in was k = 29 × 10 ⁻³ (s ⁻¹ ). This value was as high as about 18 times compared to the value when water (X _alk = 0) was used as the solvent.
When the breakdown voltage is high, more active species including hydrogen radicals can be generated. Therefore, even if the energy application conditions are the same, it can be said that the reaction rate can be increased by _{setting the} ethanol molar ratio in the alcohol-water mixed solution to 0.14 (X _alk = 0.14).

  Specific examples of the present invention have been described in detail above, but these are merely examples and do not limit the scope of the claims. The invention disclosed herein can include various modifications and alterations of the specific examples described above. For example, in the above-described embodiment, in order to clearly confirm the action of the lower alcohol, the protective agent and the electrolyte are not added to the alcohol-water mixed solution. You may make it contain a protective agent and / or electrolyte in a solution.

2 Aqueous solution (liquid phase)
3 Gas phase 4 Solution plasma (plasma phase)
5 Container 6 Electrode 8 External power supply 9 Insulating member 10 Solution plasma generator

Claims (5)

Preparing a solution in which a salt containing a metal ion is dissolved in a mixed solvent of a lower alcohol having 1 to 5 carbon atoms and water;
Reducing the metal ions by generating glow discharge plasma in the solution to form nanoparticles made of the metal,
The manufacturing method of the metal nanoparticle which makes the molar ratio of the said lower alcohol which occupies for the whole of the said mixed solvent 0.05-0.2.   The method for producing metal nanoparticles according to claim 1, wherein the concentration of the metal ions in the solution is adjusted to 0.05 mM to 0.5 mM. The method for producing metal nanoparticles according to claim 1, wherein the metal ion is a noble metal ion. The manufacturing method of the metal nanoparticle of any one of Claims 1-3 which prepares the solution which further contained the protective agent in the said aqueous solution. A pair of linear electrodes are disposed in the aqueous solution,
The plasma is generated by applying a DC pulse voltage having a pulse width of 0.1 μs to ⁵ μs and a frequency of 10 ³ Hz to 10 ⁵ Hz between the linear electrodes. The manufacturing method of the metal nanoparticle of description.