JP5514770B2 - Method for producing silver nanoparticles - Google Patents

Description

  Embodiments of the present invention relate to a method for producing silver nanoparticles.

  As a method for producing metal nanoparticles such as silver nanoparticles, a gas phase method and a liquid phase method are known. In the vapor phase method, a metal evaporation device and a vacuum device are required to cool the metal vapor in a vacuum in the presence of an inert gas. The production volume of metal nanoparticles is small and the gas phase method is not suitable for mass production. In the liquid phase method, since a metal compound solution such as a metal salt is reduced, a reducing agent or a surfactant for suppressing particle growth is required. The dispersibility of the resulting particles is not always good.

  In recent years, methods for obtaining metal nanoparticles without using a reducing agent, a surfactant, a solvent, and the like have been proposed, but the dispersion stability of the obtained metal nanoparticles is not sufficient.

Japanese Patent No. 4352258

  The problem to be solved by the present invention is to provide a method for producing silver nanoparticles having excellent dispersion stability in a dispersion medium and exhibiting high conductivity by firing at a relatively low temperature.

  In the method for producing silver nanoparticles according to the embodiment, a mixture of a primary amine having a hydrocarbon group having 4 to 12 carbon atoms and fatty acid silver is subjected to a temperature T exceeding the boiling point of the primary amine in an inert gas atmosphere. The mixture is melted to produce silver nanoparticles in the melt. The temperature T satisfies the following conditions.

T _L ≦ T ≦ T _H
(T _L and T _H are a lower limit temperature and an upper limit temperature at which silver nanoparticles having a filtration yield of 10% or more are obtained, respectively, and the filtration yield is obtained by dispersing the obtained silver nanoparticles in 5% by mass of toluene. The ratio of the solid content before and after filtration of the filtrate that passed through a 5 μm filter as the liquid, and the solid content concentration is calculated from the reduced weight when heated to 150 ° C. by thermogravimetric analysis.

The graph which shows the relationship between heating temperature and the filtration yield of silver nanoparticles. The graph which shows the relationship between heating temperature and the metal content rate in a silver nanoparticle.

  Embodiments of the present invention will be described below.

  Silver nanoparticles can be used to form a conductive film such as a wiring. For example, a target wiring pattern is formed on a substrate using ink containing silver nanoparticles. By firing the wiring pattern, the silver nanoparticles are sintered and a wiring as a patterned conductive film is formed.

  If it is a silver nanoparticle provided with the predetermined | prescribed characteristic, it can disperse | distribute to a dispersion medium and can be used as an inkjet ink. For example, silver nanoparticles having a particle size suitable for ejection from a nozzle of an ink jet head and capable of being stably dispersed in a dispersion medium. The inventors have found a suitable method for obtaining silver nanoparticles with such properties.

  In the method of this embodiment, fatty acid silver is heated with the primary amine which has a C4-C12 hydrocarbon group. The primary amine acts as a catalyst, and the fatty acid silver is decomposed by heating to produce silver particles. The fatty acid decomposed and liberated from the fatty acid silver is adsorbed on the surface of the silver particles thus produced and acts as a dispersant. Primary amines also adsorb on the surface of the silver particles and act as dispersants as well. Thus, since the silver nanoparticle manufactured by the method of this embodiment has a fatty acid and a primary amine as a dispersing agent component on the surface, the dispersion stability in a dispersion medium is excellent.

  When fatty acid silver is heated together with a primary amine having a hydrocarbon group having 4 to 12 carbon atoms, silver nanoparticles are produced in the resulting melt. In addition, a nanoparticle means the particle | grains whose primary particle diameter calculated | required with the transmission electron microscope (TEM) is 100 nm or less.

  The fatty acid in the fatty acid silver may be either a saturated fatty acid or an unsaturated fatty acid, but the molecular weight is desirably 500 or less. If the fatty acid has a molecular weight of 500 or less, it is less likely to remain in the film and adversely affect the electrical characteristics during the firing process of the wiring pattern. Examples of the fatty acid silver include silver laurate, silver myristate, silver palmitate, silver stearate, silver caprate, silver caproate, silver caprylate, silver valerate, and silver oleate. As described above, the fatty acid is decomposed and released from the fatty acid silver by heating and adsorbs on the surface of the particles to protect the silver nanoparticles, and also acts as a dispersant that enhances dispersibility.

  The particle diameter of the silver nanoparticles to be generated and the ratio of the dispersant component on the particle surface vary depending on the length of the carbon chain in the fatty acid. Fatty acids having too few carbon atoms are thermally unstable and easily desorbed from the surface of silver nanoparticles. For this reason, the particle diameter of the silver nanoparticles obtained will become large. On the other hand, when the number of carbon atoms is too large, the proportion of the dispersant component on the particle surface becomes relatively large, and the metal content in the silver nanoparticles becomes small. Considering these, the number of carbon atoms in the fatty acid (here, the number of carbon atoms including C in COOH) is preferably 5 to 20, and the most preferable fatty acid silver is silver laurate.

  Silver laurate can be synthesized from, for example, equimolar sodium laurate and silver nitrate. When sodium laurate is dissolved in pure water and an aqueous silver nitrate solution is dropped into the resulting solution, powdered silver laurate is produced. This is filtered and washed with water to remove excess sodium laurate and by-product sodium nitrate to obtain silver laurate.

  Generally, fatty acid silver can be synthesized using a silver compound and a fatty acid as raw materials. Specific examples of the silver compound include silver nitrate, silver benzoate, silver citrate, silver carbonate, silver sulfate, silver nitrate, silver tetrafluoroborate, silver hexafluorophosphate, silver hexafluoroantimonate, and trifluoromethanesulfonic acid. Examples thereof include silver and silver trifluoroacetate.

  The fatty acid may contain any of a saturated hydrocarbon group, an unsaturated hydrocarbon group, or a cyclic hydrocarbon group. As described above, any fatty acid can be used as long as the molecular weight is 500 or less. Specific examples include lauric acid, myristic acid, palmitic acid, stearic acid, capric acid, caproic acid, caprylic acid, valeric acid, and oleic acid. The fatty acid can be used in an amount corresponding to 0.5 to 1.5 equivalents of the silver compound.

  In the method according to this embodiment, the fatty acid silver is heated and melted together with the primary amine. The primary amine acts as a catalyst to promote the decomposition of fatty acid silver and reduction to metallic silver. As will be described later, the primary amine is adsorbed on the surface of the silver nanoparticles during growth. Thus, the primary amine suppresses excessive growth of silver nanoparticles and also functions as a dispersant by coordinating firmly on the surface of the silver nanoparticles. As a result, silver nanoparticles having good dispersibility in the dispersion medium can be obtained. Primary amines have a higher binding capacity to the surface of silver nanoparticles compared to secondary and tertiary amines. A secondary amine and / or a tertiary amine may be used in combination as long as the effect of the primary amine is not impaired.

  The primary amine has a hydrocarbon group having 4 to 12 carbon atoms. If the number of carbon atoms in the hydrocarbon group is within the range of 4 to 12, the dispersion stability and the firing temperature of the obtained silver nanoparticles can be set within the optimum range. When a conductive film is formed by applying to a substrate or the like and firing, the silver nanoparticles are required to exhibit high conductivity after sintering. When silver nanoparticles are sintered, if the dispersant component does not remain and is detached from the surface of the particles, the conductivity is improved. In consideration of the influence on the substrate and the electronic component, the firing temperature is preferably a relatively low temperature of about 250 ° C. or less, which is equivalent to the conventional solder reflow temperature.

  When a primary amine having a hydrocarbon group having 3 or less carbon atoms is used, silver nanoparticles having good dispersion stability cannot be obtained. On the other hand, a primary amine having a hydrocarbon group having 13 or more carbon atoms does not desorb from the surface of silver nanoparticles unless fired at a high temperature exceeding 260 ° C. Therefore, firing at a high temperature is necessary to ensure high conductivity. Is needed. Moreover, the number of carbon atoms of the hydrocarbon group contained in the primary amine also affects the particle size of the resulting silver nanoparticles. For these reasons, the hydrocarbon group contained in the primary amine used in this embodiment is defined as having 4 to 12 carbon atoms.

  Examples of the hydrocarbon group include a saturated hydrocarbon alkyl group, an unsaturated hydrocarbon group, an alicyclic hydrocarbon, and an aromatic hydrocarbon. Alkyl groups are preferred because silver nanoparticles with good dispersibility can be obtained. Examples of the alkylamine include propylamine, butylamine, hexylamine, octylamine, nonylamine, decylamine, and dodecylamine.

  The boiling point of the alkylamine is preferably 50 ° C or higher and 250 ° C or lower. Alkylamine having a boiling point that is too low tends to be detached when coordinated on the surface of the silver nanoparticles. For this reason, silver nanoparticles that can be sufficiently stably dispersed in the dispersion medium cannot be obtained. On the other hand, an alkylamine having a too high boiling point may remain without being detached from the surface of the silver nanoparticles even when sintered at about 250 ° C. As a result, the conductivity after firing is reduced. Hexylamine is most preferred because it can be fired at a low temperature of about 250 ° C. or lower and desired conductivity can be obtained after firing.

  The hydrocarbon group in the primary amine may contain a hydrocarbon group other than a saturated hydrocarbon group. For example, an unsaturated hydrocarbon group, an alicyclic hydrocarbon group, an aromatic hydrocarbon group and the like can be mentioned, and can be appropriately selected within a range where the carbon number in the hydrocarbon group is 12 or less. In addition to the terminal amino group, a hydrophilic terminal group may be present. For example, a hydroxylamine having a hydroxyl group can be used in combination.

  In producing silver nanoparticles by the method according to this embodiment, first, fatty acid silver and a primary amine corresponding to 0.5 to 1.5 equivalents of fatty acid silver are placed in a reaction vessel and heated at a predetermined temperature. To do. By heating, a melt containing the fatty acid silver and the primary amine is obtained. The heating temperature here is higher than the boiling point of the primary amine and satisfies the conditions described later. Thermal energy becomes the driving force, and reduction to metallic silver is induced simultaneously with the thermal decomposition of fatty acid silver. For this reason, conventional reducing agents such as ascorbic acid, sodium borohydride, glucose, formic acid, and hydrazine are not used. In the conventional reducing agent, components other than the organic protective group of the silver nanoparticles remain, and when the conductive film is produced, an adverse effect is exerted. Since the primary amine itself becomes an organic protecting group, such a problem does not occur.

  A small amount of a solvent may be added to make the thermal decomposition reaction of fatty acid silver uniform, but it is desirable that no solvent is present in order to efficiently produce a large amount of silver nanoparticles. When a solvent is used, a saturated hydrocarbon solvent having a boiling point higher than that of the primary amine and not participating in the silver nanoparticle formation reaction is preferable. Specifically, decane, undecane, and the like. It is desirable that the amount of the solvent be limited to about 50% by mass or less of the total amount of fatty acid silver and primary amine.

  Heating of the fatty acid silver and the primary amine is performed in an inert gas. The inert gas can be selected from, for example, nitrogen, argon, helium and carbon dioxide. Usually, heating is performed while stirring a mixture of fatty acid silver and a primary amine. This mixture only needs to be melted by heating to be liquid, and fatty acid silver and primary amine are not necessarily liquid at room temperature (25 ° C.).

  The fatty acid silver and the primary amine can be heated from the outside of the reaction vessel using an oil bath or a heater. An oil bath is desirable to uniformly heat the reaction vessel, but a mantle heater, a ribbon heater, or the like may be used. The heating temperature at this time is a temperature exceeding the boiling point of the primary amine. The heating temperature further satisfies a specific condition, which will be described later. The heating temperature can be controlled by the temperature of the reaction vessel wall. In some cases, the temperature may be controlled by the oil bath or heater temperature.

  The heating temperature can be appropriately selected according to the type of primary amine, but is at least higher than the boiling point of the primary amine. By heating, a melt containing fatty acid silver and a primary amine is produced. In the melt, as described above, the primary amine acts as a catalyst, and the fatty acid silver is decomposed and simultaneously reduced to metallic silver. Although the produced metallic silver grows as particles, the increase in particle size is suppressed by the adsorption of fatty acids and primary amines decomposed from fatty acid silver onto the surface of the particles. As a result, the particle size of the particles is limited to about several nm to several tens of nm, and nanoparticles can be obtained.

  The particle diameter of the silver nanoparticles obtained can be controlled by the number of carbon atoms of the hydrocarbon group contained in the primary amine and the number of carbon atoms in the fatty acid of the fatty acid silver. In order to obtain silver nanoparticles having an appropriate particle size, the carbon number of the hydrocarbon group contained in the primary amine is defined as 4 to 12, and the carbon number of the fatty acid silver in the fatty acid is preferably 5 to 20.

  When the heating temperature is too low, decomposition of fatty acid silver and reduction to metallic silver do not proceed sufficiently, and fatty acid silver as a silver raw material remains. The silver nanoparticles thus obtained cannot be stably dispersed in the dispersion medium due to the influence of the residual fatty acid silver. On the other hand, when the heating temperature is too high, organic by-products derived from primary amines, fatty acids in fatty acid silver and decomposition products thereof are generated, and silver nanoparticles are aggregated. Aggregated silver nanoparticles are also not stably dispersible in the dispersion medium. It is desired to select an appropriate heating temperature according to the type of primary amine or the like so that the generation of silver nanoparticles proceeds sufficiently while avoiding the generation of organic by-products as much as possible.

  As described above, since the heating of the mixture containing the fatty acid silver and the primary amine is performed at a temperature exceeding the boiling point of the primary amine, the primary amine is refluxed by a refluxer during the heating. The temperature of the mixture during heating does not necessarily match the set heating temperature, but may be set to an optimal range as appropriate according to the type of primary amine. Usually, silver nanoparticles are produced in the melt in about 3 to 10 hours. For example, when silver laurate and hexylamine are used, silver nanoparticles are produced by heating at 135 to 145 ° C. for about 4 to 7 hours.

  Thereafter, the temperature of the melt in which the silver nanoparticles are generated can be lowered, and if necessary, an organic solvent can be added to cause aggregation and precipitation to recover the silver nanoparticles. For example, by cooling with water using a water bath or the like, the temperature of the melt can be lowered below the boiling point of the organic solvent to which the melt is added. The temperature of the melt can be determined by measuring the liquid temperature in the flask with a thermocouple or a rod thermometer.

  As the organic solvent, for example, methanol or acetone can be used. In some cases, the silver nanoparticles may then be washed by filtration, centrifugation, decantation, or the like. Alternatively, silver nanoparticles can be recovered from the melt by solvent extraction.

  On the surface of the obtained silver nanoparticles, primary amines and fatty acids as dispersant components are present. The presence of such a dispersant component can be confirmed, for example, by the following method. First, reflux in a polar solvent is performed to extract the dispersant components (primary amine and fatty acid) on the surface of the silver nanoparticles. Examples of the polar solvent include acetone and acetonitrile. The presence of the primary amine can be confirmed by performing IR analysis or NMR analysis on the obtained dispersant component. The presence of fatty acid can be confirmed, for example, by direct IR measurement of silver nanoparticles.

  The primary particle diameter of the produced silver nanoparticles is 60 nm or less as an average particle diameter. The primary particle diameter of the silver nanoparticles can be determined as an average value by observation using a scanning electron microscope (SEM) or a transmission electron microscope (TEM). When silver nanoparticles having an average particle size of 60 nm or less are dispersed in a dispersion medium such as an organic solvent, an ink having good dispersion stability can be obtained. This ink can be stably ejected from the nozzles of a general inkjet head, and can be applied as an inkjet ink.

  Below, the specific example of the manufacturing method of a silver nanoparticle is shown.

  First, equimolar amounts of sodium laurate and silver nitrate were mixed, and silver laurate as fatty acid silver was synthesized by ion exchange in pure water. The obtained silver laurate (20 g) and hexylamine (9.9 g) as a primary amine were placed in a four-necked flask and heated with an oil bath while stirring in a nitrogen stream. In addition, the boiling point of hexylamine is about 131 ° C., and the amount of hexylamine corresponds to 1.5 times equivalent of silver laurate.

  When the oil bath was heated to a predetermined temperature, the temperature was maintained, and heating was performed while refluxing hexylamine. The temperature was changed in the range of 135 to 143 ° C. while monitoring with a thermocouple installed in the vicinity of the reaction vessel in the oil bath. Here, the temperature of the oil bath is defined as the heating temperature. The mixture of silver laurate and hexylamine was melted by heating. When held for 5 hours, the formation of silver nanoparticles in the melt was confirmed when the heating temperature was 131 ° C. or higher.

  About what the silver nanoparticle produced | generated, the reaction container was cooled with the water bath, the temperature of the melt was reduced to 60 degreeC, and methanol (300 mL) was added and the particle | grains were aggregated. Aggregated silver nanoparticles were collected by suction filtration.

  At any heating temperature, the average primary particle size obtained by TEM observation of the silver nanoparticles was about 20 nm. The maximum primary particle size was 60 nm, and the minimum primary particle size was 12 nm.

  Silver nanoparticles obtained at each temperature were dispersed in toluene to prepare a 5 mass% dispersion. This was filtered through a 5 μm polytetrafluoroethylene (PTFE) filter, and the solid content concentration in the filtrate was measured to calculate the filtration yield of the filter. The solid content concentration was calculated from the decrease in weight when heated to 150 ° C. by thermogravimetric analysis (TG). The relationship between the heating temperature and the filtration yield is shown in FIG.

  As shown in FIG. 1, the filtration yield changes as the heating temperature increases. The filtration yield is 0% when the heating temperature is 131 ° C. or lower, but increases with the temperature when it exceeds 131 ° C., and reaches about 20% when the heating temperature is about 138 ° C. As the heating temperature increases further, the filtration yield decreases. It can be seen that when the heating is performed at a temperature of 135 to 145 ° C., a filtration yield of 10% or more is obtained. If the filtration yield determined here is 10% or more, the silver nanoparticles can be stably dispersed in the dispersion medium, and the dispersion stability is good.

  As described above, when heated at a temperature of less than 131 ° C., the filtration yield is 0%, and the production of dispersible silver nanoparticles is not confirmed. This temperature of 131 ° C. is the boiling point of hexylamine as a primary amine. Therefore, it can be seen that heating at a temperature higher than the boiling point of the primary amine is required to obtain silver nanoparticles by the method according to the present embodiment.

About the silver nanoparticle obtained at each heating temperature, the amount of contained inorganic substance (metallic silver) was determined. Specifically, the metal content 100 (w ₁ / w ₀ ) was determined from the weight loss by TG measurement. w ₀ is the weight before analysis of the whole silver nanoparticles obtained at each heating temperature, and w ₁ is the weight after heating the silver nanoparticles to 350 ° C. by thermogravimetric analysis. The relationship between the heating temperature and the metal content is shown in FIG.

  As shown in FIG. 2, the content of metallic silver in the silver nanoparticles increases as the heating temperature increases. If the heating temperature is 135 ° C. or higher, the content is 60 wt% or higher. If content of metallic silver is 60 wt% or more, desired electroconductivity can be ensured after baking. This temperature coincides with the lower limit of the heating temperature described with reference to FIG. 1 and capable of obtaining silver nanoparticles having a filtration yield of 10% or more.

  Even when the heating temperature is increased to 140 ° C. or higher, the content of metallic silver in the silver nanoparticles does not increase remarkably, so that the formation reaction of metallic silver is assumed to have proceeded sufficiently. However, when the heating temperature exceeds 140 ° C., the range of weight loss in the TG measurement is different (shifted). It is considered that a side reaction occurred and the primary amine as the dispersant component was changed. As described above, it is shown in FIG. 1 that the dispersion stability of the obtained silver nanoparticles deteriorates when the heating temperature exceeds a predetermined range.

  Based on the above result, in this embodiment, the range of the temperature T which heats fatty acid silver with a primary amine was determined as follows.

T _L ≦ T ≦ T _H
(T _L and T _H are a lower limit temperature and an upper limit temperature at which silver nanoparticles having a filtration yield of 10% or more are obtained, respectively, and the filtration yield is obtained by dispersing the obtained silver nanoparticles in 5% by mass of toluene. The ratio of the solid content before and after filtration of the filtrate that passed through a 5 μm filter as the liquid, and the solid content concentration is calculated from the reduced weight when heated to 150 ° C. by thermogravimetric analysis, where T _L Is higher than the boiling point of the primary amine.)
In particular, when silver nanoparticles are produced by the method according to this embodiment using silver laurate as fatty acid silver and hexylamine as primary amine, the heating temperature is 135 ° C. or higher and 145 ° C. or lower. It is preferable.

  As described above, the present inventors have found that there is an appropriate range in the conditions during production in order to obtain silver nanoparticles that can be stably dispersed in a dispersion medium. According to the method of this embodiment, by heating a low molecular weight primary amine together with fatty acid silver at a predetermined temperature, silver nanoparticles having excellent dispersion stability and high conductivity by firing at a relatively low temperature are obtained. It became possible to manufacture efficiently.

  Although several embodiments of the present invention have been described, these embodiments are presented by way of example and are not intended to limit the scope of the invention. These novel embodiments can be implemented in various other forms, and various omissions, replacements, and changes can be made without departing from the spirit of the invention. These embodiments and modifications thereof are included in the scope and gist of the invention, and are included in the invention described in the claims and the equivalents thereof.

Claims (6)

The mixture of a primary amine having a hydrocarbon group having 4 to 12 carbon atoms and fatty acid silver is subjected to a temperature T exceeding the boiling point of the primary amine in an inert gas atmosphere (where T satisfies the following conditions). Melting silver and producing silver nanoparticles in the melt
T _L ≦ T ≦ T _H
(T _L and T _H are a lower limit temperature and an upper limit temperature at which silver nanoparticles having a filtration yield of 10% or more are obtained, respectively, and the filtration yield is obtained by dispersing the obtained silver nanoparticles in 5% by mass of toluene. The ratio of the solid content before and after filtration of the filtrate that passed through a 5 μm filter as the liquid, and the solid content concentration is calculated from the reduced weight when heated to 150 ° C. by thermogravimetric analysis.
The manufacturing method of the silver nanoparticle characterized by the above-mentioned. The lower limit temperature T _L is a metal content represented by 100 (w ₁ / w ₀ ) (w ₀ is the total weight of the obtained silver nanoparticles, and w ₁ is 350 by thermogravimetric analysis of the silver nanoparticles. 2. The method for producing silver nanoparticles according to claim 1, wherein the temperature is a temperature at which 60 wt% or more of silver nanoparticles are obtained.   The silver nanoparticles according to claim 1, further comprising a step of lowering the temperature of the melt to a temperature lower than the boiling point of the organic solvent to which the melt is added, and adding the organic solvent to aggregate the silver nanoparticles. Manufacturing method.   The method for producing silver nanoparticles according to any one of claims 1 to 3, wherein the primary amine is used in an amount corresponding to 0.5 to 1.5 equivalents of the fatty acid silver.   The method for producing silver nanoparticles according to any one of claims 1 to 4, wherein the heating is performed for 3 to 10 hours.   The method for producing silver nanoparticles according to claim 1, wherein the primary amine is hexylamine, the fatty acid silver is silver laurate, and the heating temperature is 135 ° C or higher and 145 ° C or lower.

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