Method for Manufacturing Metal Nanoparticles Having a Core-Shell Structure with Good Oxidation Stability

The present disclosure relates to a method of manufacturing metal nanoparticles having a core-shell structure with excellent oxidation stability.

There are various methods of manufacturing metal nanoparticles having a core-shell structure. Methods of manufacturing metal nanoparticles by using a chemical reduction method or by physically separating bulk metal particles have largely been used.

In order to manufacture metal nanoparticles, a chemical reduction method using a chemical reducing agent or an electroless plating method for synthesizing metal nanoparticles by changing a reduction potential of a metal precursor solution may be employed. Here, the chemical reducing agent may include hydrazine, alcohol, a surfactant, citrate acid or the like. Metals from metal ions or organic metal compounds may be reduced with the use of the above-mentioned chemical reducing agent to thereby synthesize metal nanoparticles having a core-shell structure and/or metal nanoparticles having an alloy structure. Such chemical synthesis of metal nanoparticles using the chemical reduction method may allow for production of uniform metal nanoparticles; however, aggregation of metal nanoparticles tends to be extremely strong, and thus a post heat treatment is required. Furthermore, since a large amount of reducing agent harmful to human bodies is used, a process of treating the remaining reducing agent after reaction is additionally required.

Besides the chemical reduction method, the synthesis of metal nanoparticles may include a method of synthesizing metal nanoparticles under high temperature, high pressure or a specific gas atmosphere by controlling a synthesizing atmosphere and a method of physically separating bulk metal particles using physical strength. These methods may facilitate the production of nano-particulates of various metal components; however, impurities may be mixed and expensive equipment may be required.

In order to solve these problems, a metal precursor solution may be irradiated with radiation, and free radicals generated in the solution may be used to reduce metal precursors.

However, as a result of experimentation, the irradiation of radiation is not sufficient to secure the oxidation stability of metal nanoparticles having a core-shell structure. Therefore, research into a new scheme for improving the oxidation stability of metal nanoparticles in addition to the manufacturing of metal nanoparticles using the irradiation of radiation is urgently needed.

An aspect of the present disclosure provides a method of manufacturing metal nanoparticles having a core-shell structure with excellent oxidation stability through the irradiation of radiation without the use of a chemical reducing agent.

According to an aspect of the present disclosure, there is provided a method of manufacturing metal nanoparticles having a core-shell structure with excellent oxidation stability, the method including: heating and agitating a core metal precursor solution; mixing the heated and agitated core metal precursor solution with a shell metal precursor solution, and heating and agitating the mixed metal precursor solutions; and irradiating the heated and agitated metal precursor solutions with radiation.

The core metal precursor solution may be heated at 30° C. to 300° C. and agitated for 10 to 120 minutes.

The mixed metal precursor solutions may be heated at 30° C. to 300° C. and agitated for 10 to 120 minutes.

The radiation may include one or more types of radiation selected from the group consisting of electron beam radiation, X-radiation and gamma radiation, and the radiation may have an absorbed dose of 10 kGy to 500 kGy.

The core metal precursor solution may include one or more metal ions selected from the group consisting of gold, silver, copper, platinum, nickel, zinc, palladium, rhodium, ruthenium, iridium, osmium, tungsten, tantalum, titanium, aluminum, cobalt and iron.

The core metal precursor solution may include capping molecules.

The capping molecules may include one or more compounds selected from the group consisting of a compound having a thiol group, a compound having a carboxyl group, and a compound having an amine group.

The capping molecules may include one or more compounds having an amine group selected from the group consisting of propylamine, butylamine, octylamine, decylamine, dodecylamine, hexadecylamine, and oleylamine.

The shell metal precursor solution may include one or more metal ions selected from the group consisting of gold, silver, copper, platinum, nickel, zinc, palladium, rhodium, ruthenium, iridium, osmium, tungsten, tantalum, titanium, aluminum, cobalt and iron.

A metal included in the shell metal precursor solution may have a lower degree of oxidation than that included in the core metal precursor solution.

According to an aspect of the present disclosure, there is provided a method of manufacturing metal nanoparticles having a core-shell structure, allowing for an increase in manufacturing yield and a reduction in manufacturing costs due to a simplified manufacturing process, that is, an environmentally friendly process without the use of a chemical reducing agent, which does not require a process of removing the remaining reducing agent and a post heat treatment.

In particular, since metal precursor solutions are irradiated with radiation after a heat treatment, oxidation stability of metal nanoparticles may be further improved.

According to an embodiment of the inventive concept, a method of manufacturing metal nanoparticles having a core-shell structure with excellent oxidation stability may include heating and agitating a core metal precursor solution, mixing the heated and agitated core metal precursor solution with a shell metal precursor solution and heating and agitating the mixed metal precursor solutions, and irradiating the heated and agitated metal precursor solutions with radiation.

First of all, according to the embodiment of the inventive concept, the metal nanoparticles having a core-shell structure may be manufactured by irradiating the metal precursor solutions with radiation and reducing the precursors. However, as a result of experimentation, such a radiation irradiating method may provide metal nanoparticles without chemical additives or environmental problems, but is not sufficient to secure the oxidation stability of the metal nanoparticles.

Therefore, in order to secure the oxidation stability of the metal nanoparticles, the heating and agitating of the core metal precursor solution may be performed beforehand, and then the core metal precursor solution and the shell metal precursor solution may be mixed with one another and the mixture thereof may be heated and agitated again.

In a case in which the core metal precursor solution and the shell metal precursor solution are heated and agitated after being mixed with one another, a metal included in the core metal precursor solution is alloyed with a metal included in the shell metal precursor solution, resulting in a failure to manufacture metal nanoparticles having a core-shell structure.

In a case in which the heat treatment is not performed, nanoparticles in a shell may have pores such that they may contact air through the pores, whereby a core may be easily oxidized. When the metal precursor solutions are subjected to the heat treatment to increase a temperature thereof to a melting point of the shell, the nanoparticles in the shell may be melted and completely enclose the core, and thus completely prevent the core that may be easily oxidized from contacting the air, whereby oxidation stability may be improved.

Therefore, when the metal precursor solutions are heated and agitated and then irradiated with radiation, the metal nanoparticles having a core-shell structure may achieve improved oxidation stability.

When the core metal precursor solution is heated and agitated, a heating temperature may be controlled to be 30° C. to 300° C. In a case in which the heating temperature is less than 30° C., the effect of securing the oxidation stability through the heat treatment may be insignificant. In a case in which the heating temperature exceeds 300° C., alloying may occur, resulting in a reduction in manufacturing yield.

In order to manufacture uniform core-shell nanoparticles, the core metal precursor solution needs to be smoothly agitated. To enable this, an agitation process needs to be performed for a predetermined period of time. The agitation time may be controlled to be 10 to 120 minutes. In a case in which the agitation time is less than 10 minutes, it may be difficult to obtain sufficient uniformity. In a case in which the agitation time exceeds 120 minutes, it may disadvantageously affect manufacturing yield.

Then, the heated and agitated core metal precursor solution may be mixed with the shell metal precursor solution. Thereafter, the mixture of the core metal precursor solution and the shell metal precursor solution may be heated and agitated again. Here, when a temperature of the mixture is increased to a melting point of the shell, the nanoparticles in the shell may be melted and completely enclose the core, and thus completely prevent the core that may be easily oxidized from contacting the air, whereby oxidation stability may be improved.

In the heating and agitating of the mixture after the core metal precursor solution and the shell metal precursor solution are mixed with one another, a heating temperature may be controlled to be 30° C. to 300° C. In a case in which the heating temperature is less than 30° C., the effect of securing the oxidation stability through the heat treatment may be insignificant. In a case in which the heating temperature exceeds 300° C., alloying may occur, resulting in a reduction in manufacturing yield.

In order to manufacture uniform core-shell nanoparticles, the mixed metal precursor solutions need to be smoothly agitated. To enable this, the agitation process needs to be performed for a predetermined period of time. The agitation time may be controlled to be 10 to 120 minutes. In a case in which the agitation time is less than 10 minutes, it may be difficult to obtain sufficient uniformity. In a case in which the agitation time exceeds 120 minutes, it may disadvantageously affect a manufacturing yield.

Thereafter, the heated and agitated metal precursor solutions may be irradiated with radiation. Here, the radiation may include one or more types of radiation selected from the group consisting of electron beam radiation, X-radiation and gamma radiation. In addition, the irradiation of the radiation may be performed by controlling absorbed dose of the radiation to be 10 kGy to 500 kGy. The irradiation of the radiation is intended to reduce the precursor solutions. In a case in which the absorbed dose is less than 10 kGy, the reduction process may not be sufficient to properly form metal nanoparticles. In a case in which the absorbed dose exceeds 500 kGy, the size of nanoparticles to be manufactured may be increased and the core and the shell may be separately formed, whereby the performance of the nanoparticles may be deteriorated. Therefore, energy of the radiation and the absorbed dose thereof may be appropriately controlled in consideration of the size of the nanoparticles.

Here, the core metal precursor solution may include one or more metal ions selected from the group consisting of gold, silver, copper, platinum, nickel, zinc, palladium, rhodium, ruthenium, iridium, osmium, tungsten, tantalum, titanium, aluminum, cobalt and iron.

In addition, the shell metal precursor solution may include one or more metal ions selected from the group consisting of gold, silver, copper, platinum, nickel, zinc, palladium, rhodium, ruthenium, iridium, osmium, tungsten, tantalum, titanium, aluminum, cobalt and iron.

The metal included in the shell metal precursor solution may have a lower degree of oxidation than that included in the core metal precursor solution. The metal included in the shell metal precursor solution forming the shell serving to coat the core may be relatively difficult to be oxidized as compared to the metal included in the core metal precursor solution, such that oxidation of the core metal or aggregation between the metal nanoparticles may be prevented, whereby stability of the metal nanoparticles may be further secured.

In addition, the core metal precursor solution may further include capping molecules. In a case in which the capping molecules are mixed with the core metal precursor solution to enclose the nanoparticles, as compared with a case in which the core metal precursor solution is merely heat-treated to form the core, the particles may be further stably grown on a nanoscale, advantageous to the stability of the metal nanoparticles.

Here, the capping molecules may include one or more compounds selected from the group consisting of a compound having a thiol group, a compound having a carboxyl group, and a compound having an amine group.

The capping molecules may include at least one selected from propylamine, butylamine, octylamine, decylamine, dodecylamine, hexadecylamine, and oleylamine. A compound having an amine group may be used as the most appropriate capping molecule. In particular, dodecylamine, hexadecylamine, and oleylamine may be preferably used to form uniform particles, considering that as lengths of carbocycles are increased, the formation of uniform particles is effectively facilitated.

Hereinafter, embodiments of the inventive concept will be described in detail with reference to the accompanying drawings. The inventive concept may, however, be exemplified in many different forms and should not be construed as being limited to the specific embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the inventive concept to those skilled in the art.

Copper acetylacetonate (C₅H₇CuO₂) was used as a core metal precursor solution, and the core metal precursor solution was heated to 100° C. and agitated for 30 minutes. Then, a silver precursor solution as a shell metal precursor solution was mixed therewith, and the mixture was heated to 50° C. and agitated for one hour. Thereafter, the mixture was irradiated with electron beams under conditions of 0.1 MeV to 20 MeV, 0.001 mA to 50 mA, and 10 kGy to 500 kGy, whereby copper-silver core-shell nanoparticles were manufactured.

FIGS. 1A and 1B illustrate images of the manufactured copper-silver core-shell nanoparticles analyzed by a high resolution transmission electron microscopy (HR-TEM). As illustrated, a surface of a copper nanoparticle having a particle size of 150 nm±50 nm is enclosed with silver nanoparticles to a thickness of 60 nm±10 nm.

In addition, FIGS. 2A through 2E illustrate elemental mapping images of the manufactured copper-silver core-shell nanoparticles. As illustrated, the core and the shell do not form an alloy; rather, the copper nanoparticle as the core is positioned inside and the silver nanoparticles as the shell are positioned to enclose the copper nanoparticle, whereby a core-shell structure is formed.

Furthermore, FIG. 3 illustrates energy dispersive spectroscopy (EDS) spectrum analysis results of the manufactured copper-silver core-shell nanoparticles. As illustrated, the manufactured copper and silver nanoparticles are not oxidized, exhibiting excellent oxidation stability.

In addition, FIGS. 4 through 7 illustrate elemental distribution analysis results of the manufactured copper-silver core-shell nanoparticles by the use of high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM). As illustrated, the silver nanoparticles completely enclose the copper nanoparticles, thereby forming uniform core-shell nanoparticles.

Lastly, FIG. 8 illustrates X-ray diffraction (XRD) analysis results of the manufactured copper-silver core-shell nanoparticles. As the results of the XRD analysis, the manufactured copper-silver nanoparticles are identified as non-oxidized copper-silver nanoparticles having a face centered cubic (FCC) lattice structure, and no oxidation peak occurs for a measuring time of 70 weeks. Through the irradiation of radiation after the heat treatment of the precursor solutions, the non-oxidized copper-silver nanoparticles achieve superior oxidation stability.

Copper acetylacetonate (C₅H₇CuO₂) was used as a core metal precursor solution, and the core metal precursor solution was heated to 250° C. and agitated for 30 minutes. Then, a silver precursor solution as a shell metal precursor solution was mixed therewith, and the mixture was heated to 25° C. and agitated for one hour. Thereafter, the mixture was irradiated with electron beams under conditions of 0.1 MeV to 20 MeV, 0.001 mA to 50 mA, and 10 kGy to 500 kGy.

FIGS. 9A through 9E illustrate elemental mapping images of the manufactured copper-silver nanoparticles. As illustrated, a precise shape of a copper nanoparticle is not clearly identified. That is, a core-shell structure is not formed.

In addition, FIG. 10 illustrates EDS spectrum analysis results of the manufactured copper-silver nanoparticles. FIG. 10 supports the copper shape illustrated in FIG. 9.

Copper acetylacetonate (C₅H₇CuO₂) was used as a core metal precursor solution, and the core metal precursor solution was heated to 350° C. and agitated for 30 minutes. Then, a silver precursor solution as a shell metal precursor solution was mixed therewith, and the mixture was heated to 350° C. and agitated for one hour. Thereafter, the mixture was irradiated with electron beams under conditions of 0.1 MeV to 20 MeV, 0.001 mA to 50 mA, and 10 kGy to 500 kGy.

FIG. 11 illustrates an image of the manufactured copper-silver nanoparticles, analyzed by an HR-TEM. FIG. 11 illustrates the copper-silver nanoparticles having an alloy structure, not a core-shell structure.

In addition, FIG. 12 illustrates EDS spectrum analysis results of the manufactured copper-silver nanoparticles. FIG. 12 supports the shape of the copper-silver alloy illustrated in FIG. 11.