JP2010077472A - Method for producing metal nanoparticles - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for easily producing metal nanoparticles which are made uniform in size and shape. <P>SOLUTION: In the method for producing metal nanoparticles by reducing metal ions with polyhydric alcohol, oxygen is fed to a polyhydric alcohol solution containing the above metal ions at a fixed flow rate so as to produce metal nanoparticles with a fixed size and a fixed shape. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to a method for producing metal nanoparticles used for a plasmon sensor using localized surface plasmons, an electric field enhancement probe for surface-enhanced Raman scattering spectroscopy, and the like.

Metal nanoparticles having a particle size of several tens to several hundreds of nanometers exhibit localized surface plasmon resonance when irradiated with light, and exhibit unique optical characteristics such as local electric field enhancement and color development. Nano-area optical measurement methods such as surface-enhanced Raman spectroscopy, plasmon sensors, optoelectronic devices, etc. are based on the unique optical properties of metal nanoparticles, and applied research using such metal nanoparticles in various fields. Has been deployed.
Since electric field enhancement and light absorption due to localized surface plasmon resonance vary depending on the size and shape of the metal nanoparticles, it is very important to control the size and shape of the metal nanoparticles in applied research of metal nanoparticles.

  The method for producing metal nanoparticles is roughly divided into a top-down method and a bottom-up method. The top-down manufacturing method is a method of manufacturing metal nanoparticles by pulverizing a bulk material, and examples thereof include a focused ion beam (FIB) method and an electron beam lithography method. In the top-down method, the processing resolution is limited to about several tens of nanometers, and it is difficult to select and control the metal crystal plane.

The bottom-up manufacturing method is further classified into three types: a solid phase method, a gas phase method, and a liquid phase method. Among these, the liquid phase method is a method of self-growth of metal crystals using the reduction reaction of metal ions in solution, and the shape and crystallinity of metal nanoparticles can be controlled with nanometer order precision ( (See Non-Patent Document 1 and Non-Patent Document 2). However, it takes time to synthesize, and it is necessary to optimize many parameters in order to control the structure of the nanoparticles.
Y. Sun and Y. Xia, Science, 298 (2002) 2176-2179 BJ Wiley, Y. Xiong, Zhi-Yuan Li, Y. Yin, and Y. Xia, Nano Lett., Vol. 6, No. 4 (2006) 765-768

  The problem to be solved by the present invention is to provide a method capable of easily producing metal nanoparticles having a uniform size and shape.

  The present invention made to solve the above-mentioned problems is a method for producing metal nanoparticles by reducing metal ions with a polyhydric alcohol, wherein oxygen is added to the polyhydric alcohol solution containing the metal ions at a constant flow rate. To produce metal nanoparticles having a certain size and a certain shape.

Any metal can be used as long as it is reduced by a polyhydric alcohol, and examples thereof include Ag, Au, Cu, Co, Ir, Ni, Pd, Pt, Ru, CoNi, and FeNi.
In particular, when the metal ions are silver ions, the spherical multi-twinned silver nanoparticles are controlled by controlling the amount of oxygen supplied to the polyhydric alcohol solution at a volume ratio of 0 to 0.25 per minute with respect to the polyhydric alcohol solution. Can be obtained. That is, even if oxygen is not supplied, spherical multi-twinned silver nanoparticles are obtained by the reducing action of polyhydric alcohol, but can be obtained if the oxygen supply amount is 0 to 0.25 per minute in volume ratio. Most of the silver nanoparticles become spherical multiple twins.

  When the metal ions are silver ions, the amount of oxygen supplied to the polyhydric alcohol solution is controlled to 0.25 to 0.45 per minute by the volume ratio with respect to the polyhydric alcohol solution. Bipyramidal silver nanoparticles can be obtained by controlling to 0.45 to 0.65 per minute, and cubic silver can be obtained by controlling to 0.65 to 0.80 per minute. Nanoparticles can be obtained.

  According to the present invention, the shape and size of the obtained metal nanoparticles can be controlled by adjusting the amount of oxygen supplied to the polyhydric alcohol solution containing metal ions, and the shape and size are uniform. Metal nanoparticles can be easily produced.

The present invention selectively removes metal particles having low crystallinity such as multiple twins by oxidative etching action with oxygen, and grows only metal particles having high crystallinity to form metal nanoparticles having a uniform shape and size. Is the way to get.
That is, the present invention relates to a method for producing metal nanoparticles called a polyol method, in which metal ions are produced by reducing metal ions with a polyhydric alcohol, and the polyhydric alcohol solution containing the metal ions at a constant flow rate. By supplying oxygen, metal nanoparticles having a certain size and a certain shape are manufactured.

  In the method of the present invention, metal ions are reduced to deposit metal in the polyhydric alcohol solution. On the other hand, the metal deposited in the polyhydric alcohol solution is reversely oxidized due to the presence of oxygen, and metal atoms are ionized and dissolved from the metal surface. The growth rate of the fine particles due to the reduction action and the dissolution rate of the fine particles due to the oxidation action depend on the crystal structure of the fine particles. For this reason, the crystal structure of the particles finally obtained is selected by suppressing or promoting the growth of a specific crystal structure by the balance between the reducing action of the polyhydric alcohol and the oxidizing action of oxygen. Therefore, the structure (shape and size) of the metal nanoparticles obtained by adjusting the oxygen supply amount can be controlled.

Hereinafter, the present invention will be described by taking as an example a method for producing silver nanoparticles by reducing silver ions with ethylene glycol, which is one of polyhydric alcohols.
The following formula (1) shows an equilibrium formula of ethylene glycol, and formula (2) shows a redox reaction formula of an ethylene glycol solution containing silver ions.
As shown in the formula (2), the silver ions are reduced in the ethylene glycol solution and precipitated as silver particles.

  In the initial stage of the reaction, crystals of various shapes are mixed in the ethylene glycol solution. These seed crystals grow into crystals of a specific shape by the reducing action of ethylene glycol. For example, as shown in FIG. 1, single crystals grow into cubes, single twins grow into bipyramids, ten-faced multiple twins grow into wires, and multiple twins grow into spherical crystals.

On the other hand, when oxygen gas is continuously supplied to the ethylene glycol solution during the reaction, a part of silver deposited by the reduction reaction is oxidized, and silver atoms are ionized and dissolved. This action is called an oxidative etching action.
The activity of metal against oxidative etching depends on the number of lattice defects. In other words, a crystal structure with a larger number of lattice defects is more likely to adsorb oxygen on the metal surface and is more likely to be oxidatively etched, so that oxidative etching is induced even with a small amount of oxygen.

Looking at the silver crystals as seeds contained in the ethylene glycol solution, the number of lattice defects is large in the order of single crystal, single twin, decahedral multiple twin, and multiple twin. For this reason, single twins are easier to oxidatively etch than single twins, decahedral multiple twins than single twins, and multiple twins rather than decahedral multiple twins.
On the other hand, because of the entropic stability of the crystal structure, a crystal structure with many lattice defects is easily formed, and thus precipitates in a quantitative permutation of multiple twins, decahedral multiple twins, single twins, and single crystals. Therefore, by adjusting the amount of oxygen supplied to the ethylene glycol solution, all fine particles having a crystal structure having a lattice defect larger than the corresponding amount are removed by etching. As a result, silver nanoparticles having a desired shape can be synthesized. It becomes possible.

  In the present invention, it is preferable that polyvinyl pyrrolidone is present in the reaction solution from the viewpoint of efficiently producing metal nanoparticles having high crystallinity and a uniform shape. As shown in FIG. 2, when polyvinyl pyrrolidone is present in the reaction solution in the case of a single crystal of silver, polyvinyl pyrrolidone is adsorbed on the (100) plane and growth is restricted. For this reason, the (111) plane grows and cube-shaped silver nanoparticles can be stably generated. Moreover, polyvinylpyrrolidone has the effect | action which prevents that microparticles | fine-particles adsorb | suck and stabilizes reaction.

  Next, the present invention will be described in more detail by showing specific examples. In addition, this invention is not limited to a following example, A suitable change is possible.

  5 ml of ethylene glycol was put in a beaker, oxygen gas was supplied at a predetermined flow rate while stirring, and the water in the ethylene glycol was removed by heating in an oil bath set at 160 ° C. for 1 to 2 hours. Then, while continuing heating in the oil bath and supplying oxygen gas, 3 ml each of 2 kinds of ethylene glycol solutions containing 94 mM silver nitrate, 144 mM polyvinylpyrrolidone (PVP) and 0.11 mM sodium bromide, flow rate 45 ml It was simultaneously injected into a beaker containing ethylene glycol at / h. Although the action of sodium bromide is unknown, the addition of sodium bromide allows stable growth of highly crystalline cubes and bipyramids. After the injection of the two kinds of ethylene glycol solutions was completed, the solution was left for about 2 hours to grow silver nanoparticles. The reaction was performed using the apparatus shown in FIG. During the reaction, the beaker was covered with an aluminum lid to prevent evaporation of ethylene glycol. During the reaction, oxygen gas was continuously supplied.

In this example, the reaction time was set to 2 hours, but the present inventor has confirmed that silver nanoparticles are produced if the reaction time is between 30 minutes and 8 hours.
Moreover, when dissolved oxygen exists in ethylene glycol, it is preferable to add ethylene glycol in advance for a small amount of iron ions. The amount of iron ion added depends on the amount of dissolved oxygen in ethylene glycol, but is usually about 100 μM. By adding iron ions, the iron ions are combined with dissolved oxygen in ethylene glycol, so that it is possible to prevent the oxidative etching action from proceeding with the dissolved oxygen. Therefore, silver nanoparticles having a shape corresponding to the oxygen gas flow rate supplied to the ethylene glycol solution can be stably obtained.

4-8 show electron microscope (SEM) photographs of silver nanoparticles produced when the oxygen flow rate is adjusted to 2 ml / min, 4 ml / min, 6 ml / min, 8 ml / min, 10 ml / min or more. ing.
As shown in FIG. 4, spherical silver nanoparticles were obtained when the oxygen flow rate was 2 ml / min (yield: ~ 90%).
As shown in FIG. 5, when the oxygen flow rate was 4 ml / min, wire-like silver nanoparticles were obtained (yield: ˜80%).
As shown in FIG. 6, when the oxygen flow rate was 6 ml / min, bipyramidal silver nanoparticles were obtained (yield: ˜60%).
As shown in FIG. 7, when the oxygen flow rate was 8 ml / min, cube-shaped (cubic) silver nanoparticles were obtained (yield: ˜100%).
Also, the silver nanoparticles of any shape were uniform in size and shape.
On the other hand, as shown in FIG. 8, when the oxygen flow rate was 10 ml / min or more, although mainly cube-shaped silver nanoparticles were obtained, their sizes were not uniform. In addition, silver nanoparticles having a shape other than the cube shape were included. This is probably because oxidative etching acted on some single crystal cubes due to excessive supply of oxygen.

FIG. 9 is a graph showing the relationship between the oxygen flow rate and the volume ratio of the oxygen flow rate to the amount of ethylene glycol solution and the yield of the obtained silver nanoparticles, and FIG. 10 shows the volume ratio of the oxygen flow rate and oxygen flow rate to the amount of ethylene glycol solution. It is a figure which shows the relationship of the crystal structure obtained mainly.
As shown in these figures, it can be seen that as the oxygen flow rate and the volume ratio of the ethylene glycol solution increase, the generated crystal structure changes from a spherical multiple twin to a wire, bipyramid, or cube.
Therefore, according to the present Example, the shape of the silver nanoparticles obtained by adjusting the oxygen flow rate can be controlled.

Next, the results of examining the optical properties of the obtained silver nanoparticles are shown below.
11 and 12 show the results of measuring the extinction spectrum by dispersing the silver nanoparticles shown in FIG. 6 and the silver nanoparticles shown in FIG. 7 in pure water. As shown in these figures, a characteristic peak (indicated by an arrow in FIG. 11) was observed in silver nanoparticles having a uniform size, but the extinction wavelength was averaged in non-uniform silver nanoparticles. No peak was observed.
Moreover, when the silver nanoparticle shown in FIG. 6 and the silver nanoparticle shown in FIG. 7 were dispersed on the cover glass and the dark field observation was performed, the results shown in FIGS. 13 (a) and (b) were obtained. As shown in FIG. 13 (a), the cube-shaped silver nanoparticles having a uniform size showed a uniform scattered color (scattered light spectrum). However, as shown in FIG. In both non-uniform silver nanoparticles, the color of the scattered light was also non-uniform.
Furthermore, when surface-enhanced Raman spectroscopy (SERS) was measured using a single particle in which a monomolecular film of 4-aminothiophenol was formed on the surface of the silver nanoparticle shown in FIG. 6, the result shown in FIG. 13 was obtained. From this result, it was confirmed that the cube-shaped silver nanoparticles having a uniform size obtained by the production method of the present invention have an effect of enhancing Raman scattered light.

The figure which shows the relationship between the silver crystal used as the seed | species in the reduction | restoration reaction of the silver ion by polyhydric alcohol, and a final crystal. The figure for demonstrating the effect | action of polyvinylpyrrolidone. The figure of the experimental apparatus used in the Example of this invention. SEM photograph of silver nanoparticles produced when the oxygen flow rate is adjusted to 2 ml / min. SEM photograph of silver nanoparticles produced when the oxygen flow rate is adjusted to 4 ml / min. SEM photograph of silver nanoparticles produced when the oxygen flow rate is adjusted to 6 ml / min. SEM photograph of silver nanoparticles produced when the oxygen flow rate is adjusted to 8 ml / min. SEM photograph of silver nanoparticles produced when the oxygen flow rate is adjusted to 10 ml / min. The figure which shows the relationship between the volume ratio with respect to the ethylene glycol solution amount of oxygen flow rate and oxygen flow rate, and the yield of the obtained silver nanoparticle. The figure which shows the relationship between the volume ratio with respect to the amount of ethylene glycol solutions of oxygen flow rate and oxygen flow rate, and the crystal structure obtained mainly. An extinction spectrum measured by dispersing silver nanoparticles shown in FIG. 6 in pure water. An extinction spectrum measured by dispersing silver nanoparticles shown in FIG. 7 in pure water. The dark field observation photograph which disperse | distributed the silver nanoparticle shown in FIG.6 and FIG.7 on a cover glass. FIG. 7 is a surface-enhanced Raman spectroscopy (SERS) spectrum of a single particle in which a monomolecular film of 4-aminothiophenol is formed on the surface of the silver nanoparticle shown in FIG. 6.

Claims (7)

In a method for producing metal nanoparticles by reducing metal ions with a polyhydric alcohol,
A method for producing metal nanoparticles, characterized by producing metal nanoparticles having a constant size and shape by supplying oxygen to the polyhydric alcohol solution containing metal ions at a constant flow rate.   When the metal ion is silver ion, the amount of oxygen supplied to the polyhydric alcohol solution is controlled to 0 to 0.25 per minute by the volume ratio with respect to the polyhydric alcohol solution to obtain spherical multiple twinned silver nanoparticles. The method for producing metal nanoparticles according to claim 1.   When the metal ion is silver ion, wire-like silver nanoparticles are obtained by controlling the oxygen supply amount to the polyhydric alcohol solution at a volume ratio of 0.25 to 0.45 per minute with respect to the polyhydric alcohol solution. The method for producing metal nanoparticles according to claim 1.   When metal ions are silver ions, bipyramidal silver nanoparticles are obtained by controlling the oxygen supply rate to the polyhydric alcohol solution at a volume ratio of 0.45 to 0.65 per minute with respect to the polyhydric alcohol solution. The method for producing metal nanoparticles according to claim 1.   When the metal ions are silver ions, cube-shaped silver nanoparticles are obtained by controlling the oxygen supply amount to the polyhydric alcohol solution at a volume ratio of 0.65 to 0.80 per minute with respect to the polyhydric alcohol solution. The method for producing metal nanoparticles according to claim 1.   The method for producing metal nanoparticles according to claim 1, wherein metal ions are reduced in the presence of polyvinylpyrrolidone.   The method for producing metal nanoparticles according to claim 1, wherein metal ions are reduced in the presence of sodium bromide.