US20100242679A1

US20100242679A1 - Method for continuously fabricating silver nanowire

Info

Publication number: US20100242679A1
Application number: US12/413,598
Authority: US
Inventors: Yi-Hsiuan Yu; Bao-Yann Lin; Ming-Hsiung Wei; Cheng-En Shen; Lea-Hwung Leu; Kai-Yai Chang; Chen-Chim Ma
Original assignee: National Chung Shan Institute of Science and Technology NCSIST
Current assignee: National Chung Shan Institute of Science and Technology NCSIST
Priority date: 2009-03-29
Filing date: 2009-03-29
Publication date: 2010-09-30

Abstract

The invention discloses a method for continuously fabricating silver nanowires. The method mixes a glycol solution of a silver salt and a glycol solution of a polyvinyl pyrrolidone, and the mixed solution reacts in a temperature range and a time range to form the silver nanowires. The polyvinyl pyrrolidone has high boiling point and reduction ability so as to reduce the silver salt to the silver nanoparticles, and simultaneously, the polyvinyl pyrrolidone can provide barriers for limiting the particle growth. Besides, the oxygen functional groups on the long chains of the polyvinyl pyrrolidone can keep the stably one-dimensional growth of the silver nanoparticles to form the silver nanowires during the aging process.

Description

BACKGROUND OF THE INVENTION

1. Field of the Invention
The invention relates to a method for continuously fabricating silver nanowires, and more particularly, the invention relates to a method for mass-producing high quality silver nanowires.
2. Description of the Prior Art
One-dimensional nanostructure, such as nanowire, nanotube, nanorod, or nanofiber, has different characteristics from the one-dimensional macro-material. Recently, many businesses and groups continuously research the applications of the one-dimensional nanostructure and earn some important achievements. The applications include ultra-thin full-color LED panel, photocopy apparatus, field emission display, low power-consumption nanowire LED, and sensor for sensing NH₃or H₂.
One-dimensional metal nanostructure, such as gold, tin, silver, or platinum nanowire, has well electrical characteristics so as to be applied to the materials of leading wires. Because the micro-effects, such as the surface effect, quantum effect, and tunneling effect, become obvious in nanoscale, the one-dimensional metal nanostructure suits for various electrical nanodevices utilizing the micro-effects. Furthermore, the one-dimensional metal nanostructure suits for electrode, low temperature conductive paste, superconducting thick-film circuit, and materials for absorbing micro wave and electromagnetic wave. Silver is an optimum metal conductive material, therefore, the applications of silver nanowires are considered as important issues.
However, for the one-dimensional nanostructures, only the nanotubes but not the one-dimensional metal nanomaterials with high conductive effect are commercialized in the business situations. Although several methods for fabricating one-dimensional metal nanowires exist, the methods have some disadvantages to commercialization. The advantages and the disadvantages of the method for fabricating one-dimensional metal nanowires in the prior arts are described in the following.
In the prior arts, the method for fabricating one-dimensional metal nanowires includes Anodic Aluminum Oxide (AAO) method, electron beam spinning method, catalyst growth method, chemical pyrolysis method, and core-shell growth method. The nanowires fabricated by AAO method have small diameters and fine uniformity, but the processes of AAO method are complex and then it does not suit for mass-production. For the reason, AAO method is disadvantageous for commercialization. Electron beam spinning method and catalyst growth method can control the growing position of metal nanowires, however, similarly, the processes of electron beam spinning method and catalyst growth method are complex and the cost of the equipment required is high. For the reason, electron beam spinning method and catalyst growth method are disadvantageous for commercialization. Chemical pyrolysis method just requires general equipments and the processes thereof are simple for commercialization, however, the metal nanowires fabricated by chemical pyrolysis method have larger diameter and a trace of nanoparticles grow at the same time. In other words, the yield of chemical pyrolysis method is lower than the yields of other method. Core-shell growth method treats the carbon nanotubes as cores and coats films on the carbon nanotubes to form multi-functional composite materials, however, the diameters of the multi-functional composite materials are larger and the uniformity thereof is not easy to control, and furthermore, the processes are complex and the cost is quite high.
As described above, chemical pyrolysis method is more suitable for mass-producing silver nanowires than other method, however, the problems of this method are that the larger particles grow according to condensation effect, and the high temperature reaction is suitable for batch fabricating so that the capacity is still not enough.

SUMMARY OF THE INVENTION

A scope of the invention is to provide a method for mass-producing high quality silver nanowires to solve the above-mentioned problems.
According to an embodiment, the method of the invention could be used for fabricating silver nanowires, and the steps of the method are described as below. Firstly, a glycol solution is fed into a reacting tank and an aging tank, and the glycol solution is preheated in the reacting tank and the aging tank. And then, a silver-salt solution and a protecting agent solution are fed into the reacting tank by continuous feeding and are mixed, and the mixed solution proceeds with the reaction in a suitable temperature range in the reacting tank. After a first staying duration and solids are separated out from the mixed solution in the reacting tank, the mixed solution could be fed into the aging tank to proceed with the aging process. After staying in the aging tank for a second staying duration, the mixed solution could be took out and purified to obtain the silver nanowires.
In this embodiment, the silver-salt solution is formed by dissolving a silver nitrate in a glycol solution, and the protecting agent solution is formed by dissolving a polyvinyl pyrrolidone (PVP) in a glycol solution.
Furthermore, the method for purifying the mixed solution for obtaining the silver nanowires can further comprise the following steps. Acetone is mixed with the mixed solution to substantially remove the glycol to obtain the precipitates. The precipitates are dissolved in a hot water to form a solution and the solution proceeds with ion exchange. The solution can proceed with solid-liquid separation for several times in repeatedly heating and stirring states to obtain the solution having high quality silver nanowires. Besides, the water in the solution having the silver nanowires could be removed by spray drying to obtain the powder of the silver nanowires.
The advantage and spirit of the invention may be understood by the following recitations together with the appended drawings.

BRIEF DESCRIPTION OF THE APPENDED DRAWINGS

FIG. 1 is a flow chart illustrating the method for continuously fabricating silver nanowires according to an embodiment of the invention.

FIG. 2 is a flow chart illustrating the purification of the mixed solution for obtaining the silver nanowires according to another embodiment of the invention

FIG. 3 is a schematic diagram illustrating the distribution of the yield to the reactive time according to the embodiments in table 3.

FIG. 4 is a schematic diagram illustrating the distribution of the diameters of the silver nanowires to the feeding concentrations of the silver salt according to the embodiments in table 4.

FIG. 5 is a SEM diagram illustrating the silver nanowires obtained in the embodiment 1.

DETAILED DESCRIPTION OF THE INVENTION

Please refer to FIG. 1. FIG. 1 is a flow chart illustrating the method for continuously fabricating silver nanowires according to an embodiment of the invention. As shown in FIG. 1, the method in the embodiment comprises the following steps of: in step S10, feeding a first solution into a reacting tank and a aging tank, and preheating the first solution to a first temperature range; in step S12, feeding a silver-salt solution and a protecting agent solution into the reacting tank, and the silver-salt solution and the protecting agent solution mixing with each other in a second temperature range and staying for a first staying duration to form a second solution; in step S14, feeding the second solution into the aging tank and making the second solution to stay in the aging tank for a second staying duration to form a third solution; in step S16, purifying the third solution to obtain the silver nanowires.
In this embodiment, the first solution could be a glycol solution, but in practice, it is not limited to the glycol solution. Besides, the silver-salt solution could be formed by dissolving a silver nitrate in a glycol solution in this embodiment. However, it could be formed by dissolving water-soluble silver salt in other solvent in practice but not limited to the silver nitrate and the glycol. For example, the silver-salt solution could be formed by dissolving silver acetate or silver nitrite in an organic solvent. Similarly, the protecting agent solution could be formed by dissolving a polyvinyl pyrrolidone in a glycol solution, but it is not a limitation.
In step S12, the silver-salt solution and the protecting agent solution could be fed into the reacting tank at the same time according to a ratio, and in this embodiment, the ratio could be equality. It should be noted that because the silver-salt solution starts to contact the protecting agent solution in the reacting tank, it could be prevented from early contact and reacting in non-predetermined conditions, which may result in generation of silver nanoparticles or other unanticipated effects.
The silver-salt solution and the protecting agent solution could be fed into the reacting tank by continuous feeding. Therefore, the reaction can continuously proceed to mass-produce the silver nanowires. For example, the reacting tank can include two different entrances for continuously feeding the silver-salt solution and the protecting agent solution respectively according to the ratio of equality. It should be noted that the continuous feeding process decreases the temperature of the mixed solution, so the preheating in step S10 is required to ensure that the reaction of the silver-salt solution and the protecting agent solution proceeds at the reactive temperature. If the preheating is absent, the reaction proceeds at lower temperature after feeding the silver-salt solution and the protecting agent solution that results in more nanoparticles growing.
In this embodiment, the protecting agent solution is formed by the dissolving polyvinyl pyrrolidone in the glycol solution. The polyvinyl pyrrolidone is a water-soluble macromolecular compound capable of dissolving in the glycol. The molecules of the polyvinyl pyrrolidone can provide barriers for limiting the growth of the silver particles to fit the nanoscale when the silver nanoparticles are separated from the mixed solution of the silver-salt solution and the protecting agent solution. On the other hand, the oxygen functional groups on the long chains of the polyvinyl pyrrolidone can lead the silver nanoparticles to assemble and stably grow along a one-dimensional direction in the reacting tank, and then the one-dimensional silver nanowires are formed during the aging process in the aging tank.
To well mix the silver-salt solution and the protecting agent solution, a homogenizer, a magnetic mixer, or a motor mixer could be disposed in the reacting tank to assist the mixing. It should be noted that if a motor mixer is used, the first solution (the glycol solution in this embodiment) for preheating should be fed into the reacting tank to the quantity which the vanes of the motor mixer can stir. In this embodiment, the concentration of the silver salt of the silver-salt solution could be larger than 0.5 wt %. Besides, the range of molecular weights of the polyvinyl pyrrolidone of the protecting agent solution could be 5000˜360000. The weight ratio of the polyvinyl pyrrolidone to the silver nitrate when the silver-salt solution and the protecting agent solution are fed into the reacting tank could be 0.5˜6. Furthermore, the second temperature range kept in the reacting tank could be 140° C. to 180° C., and the first temperature range for preheating could be 170° C. to 180° C. It should be noted that the temperature range in the reacting tank and the aging tank and the preheating could be provided by micro-wave heating, but it is not a limitation.
Because the temperature of the reaction influences the rate for separating out the silver nanowires from the mixed solution, the first staying duration which the second solution stays for in the reacting tank could be a range of 10 minutes to 30 minutes, but it is not a limitation. The second staying duration which the second solution stays for in the aging tank could be, but not limited to 30 minutes. Practically, under a temperature condition and a concentration condition suitable for reacting, the first staying duration could be reasonably set as a range of 10 minutes to 45 minutes. Besides, a stable state is reached after feeding the silver-salt solution and the protecting for 30 minutes, and because of continuous feeding, the yield for the silver ions transforming into the silver nanoparticles is a stable value roughly in the reacting tank, and in practice, the stable value is 90% roughly. The yield could be raised to about 100% after feeding the mixed solution into the aging tank for 30 minutes. Therefore, the staying duration in the aging tank could be reasonably set as 30 minutes.
Please refer to FIG. 2. FIG. 2 is a flow chart illustrating the purification of the mixed solution for obtaining the silver nanowires according to another embodiment of the invention. As shown in FIG. 2, in this embodiment, step S16 in the last embodiment can further comprise the following step of: in step S160, feeding an excess of acetone into the third solution to mix with glycol to make the third solution to form a clear liquid portion and a precipitate portion; in step S162, removing the clear liquid portion and dissolving the precipitate portion in the hot water to form a fourth solution; in step S164, processing the fourth solution by ion exchange process to form a fifth solution; in step S166, repeatedly heating, stirring, and solid-liquid separating the fifth solution for a plurality times to obtain a sixth solution; and, in step S168, spray drying the sixth solution to remove the water of the sixth solution to obtain the powder of the silver nanowires.
The polyvinyl pyrrolidone can mix with the glycol but not the acetone, therefore, in this embodiment, the excess of acetone fed in step S160 can mix well with the glycol of the third solution to make the silver nanowires and the polyvinyl pyrrolidone packing the silver nanowires to precipitate, and then the third solution forms the clear liquid portion and the precipitate portion, wherein the clear liquid portion is formed by dissolving the glycol in the acetone. After precipitating for a while, the clear liquid portion is removed in step S162. The hot water can dissolve the polyvinyl pyrrolidone of the precipitate portion, and additionally, the hot water can assist the residual acetone in the precipitate portion to evaporate.
In this embodiment, the fourth solution could be processed by ion exchange process (as described in step S164) to remove the positive ions and the negative ions so as to form the neutral fifth solution. It should be noted that the fourth solution could be cooled before the ion exchange process in step S164. The fifth solution obtained in step S164 has well mixed and high quality silver nanowires covered by the polyvinyl pyrrolidone.
Practically, the polyvinyl pyrrolidone covering the silver nanowires could be removed so that the silver nanowires could be used directly. The fifth solution could be repeatedly heated and stirred to raise the solubility for polyvinyl pyrrolidone to the water, and simultaneously, the fifth solution could be solid-liquid separated for several times, as described in step S166. The polyvinyl pyrrolidone covering the silver nanowires in the fifth nanowires is removed in the above-mentioned step to form the sixth solution. Therefore, the sixth solution has the silver nanowires without covering. Finally, the powder of the high quality silver nanowires could be obtained by removing the water of the sixth solution in step S168.
Furthermore, in the method for continuously fabricating silver nanowires of the invention, the molecular weight of the macromolecular compounds of the protecting agent solution, the weight ratio of the macromolecular compounds to the silver nitrate, the reactive temperature, and the feeding concentrations of silver-salt solution and the protecting agent solution influence the quantity of output and the quality of the silver nanowires obtained.
Please refer to table 1. Table 1 shows the appearances of silver nanowires influenced by the polyvinyl pyrrolidone with different molecular weights in the following embodiments. It should be noted that the embodiment 1 in table 1 is provided with a silver-salt solution formed by dissolving 20 g silver nitrate in 1200 g glycol solution (the concentration of silver is 1.66 wt %) and the protecting agent solution formed by dissolving 80 g polyvinyl pyrrolidone in 1200 g glycol solution. The reacting tank and the aging tank are fed with 100 g glycol solutions respectively. The glycol solutions are preheated to 170 ° C., and the reacting tank and the aging tank are kept at 150° C. during the reaction. Besides, the mixed solution stays in the reacting tank for 30 minutes and stays in the aging tank for 30 minutes. After all, the mixed solution is took out from the aging tank and then purified to obtain the silver nanowires. The reacting conditions of the embodiments 2 and 3 of table 1 are the same as those of the embodiment 1 of table 1 except the molecular weight of the polyvinyl pyrrolidone.

TABLE 1

the appearances of silver nanowires influenced by the polyvinyl
pyrrolidone with different molecular weights

	molecular weight
	of polyvinyl	appearance of silver
embodiment	pyrrolidone	nanowires

1	58000	diameter 80~100 nm
		length 6~10 μm
2	8000	diameter 60~80 nm
		length 2~5 μm
3	360000	diameter 90~120 nm
		length 8~12 μm

As shown in table 1, the higher molecular weight of polyvinyl pyrrolidone causes the longer and thicker silver nanowires.
Please refer to table 2. Table 2 shows the appearances of silver nanowires influenced by different weight ratios of polyvinyl pyrrolidone to silver nitrate in the following embodiments. The conditions of the embodiment 1 in table 2 are the same as those of the embodiment 1 in table 1. The conditions of the embodiments 4, 5, and 6 are the same as those of the embodiment 1 in table 2 expect different weights of polyvinyl pyrrolidone used to dissolve in 1200 g glycol solution to form the protecting agent solutions.

TABLE 2

the appearances of silver nanowires influenced by different weight
ratios of polyvinyl pyrrolidone to silver nitrate

	weight of polyvinyl	appearance of silver
embodiment	pyrrolidone (g)	nanowires

	1	80	diameter 80~100 nm
			length 6~10 μm
	4	20	diameter ~230 nm
			length 12~25 μm
	5	30	diameter 90~120 nm
			length
5~15 μm
	6	160	a great quantity of
			nanoparticles and a few
			nanowires

As shown in table 2, high weight ratio of polyvinyl pyrrolidone to silver nitrate causes the generation of the silver nanoparticles and then it is harmful to the silver nanowires. Oppositely, the lower weight ratio causes the longer and thicker silver nanowires.
Please refer to table 3. Table 3 shows the appearances of silver nanowires influenced by different reactive temperatures in the following embodiments. The conditions of the embodiment 1 in table 3 are the same as those of the embodiment 1 in table 1. The conditions of the embodiments 7, 8, and 9 are the same as those of the embodiment 1 in table 3 expect different reactive temperature.

TABLE 3

the appearances of silver nanowires influenced by different reactive
temperatures

	reactive	appearance of silver
embodiment	temperatures (° C.)	nanowires

1	150	diameter 80~100 nm
		length 6~10 μm
7	140	all nanoparticles but no
		nanowires
8	160	diameter 100~110 nm
		length 7~10 μm
9	170	diameter 130~150 nm
		length 7~11 μm

As shown in table 3, the silver nanoparticles but not the silver nanowires are generated at the condition that the reactive temperature is lower than 150° C. Oppositely, the higher reactive temperature causes the longer and thicker silver nanowires.
Please refer to FIG. 3. FIG. 3 is a schematic diagram illustrating the distribution of the yield X to the reactive time T according to the embodiments in table 3. As shown in FIG. 3, the curves 20, 22, 24, and 26 represent the distributions of distribution of the yield to the reactive time according to the embodiments 1, 7, 8, and 9 respectively. If the reactive temperature is lower than 150° C., the reactive rate is too low and then only the silver nanoparticles but not the silver nanowires could be separated out. Oppositely, if the reactive temperature is higher than 150° C., a great quantity of the silver nanowires is separated out in 30 minutes. Therefore, the staying duration which the mixed solution stays for in the reacting tank could be reasonable set as 30 minutes.
Please refer to table 4. Table 4 shows the appearances of silver nanowires influenced by different feeding concentrations in the following embodiments. The conditions of the embodiment 1 in table 4 are the same as those of the embodiment 1 in table 1. The conditions of the embodiments 10, 11, 12, 13, and 14 are the same as those of the embodiment 1 in table 4 expect different weights of the silver nitrate and the polyvinyl pyrrolidone.

TABLE 4

the appearances of silver nanowires influenced by different feeding
concentrations

		weight of
	weight of silver	polyvinyl	appearance of silver
embodiment	nitrate (g)	pyrrolidone (g)	nanowires

1	20	80	diameter 80~100 nm
			length 6~10 μm
10	10	160	all nanoparticles but no
			nanowires
11	40	160	diameter ~160 nm
			length
5~8 μm
12	60	240	diameter ~210 nm
			length ~5 μm
13	100	400	diameter ~290 nm
			length ~5 μm
14	265	400	diameter ~540 nm
			length ~5 μm

As shown in table 4, the feeding concentrations have a non-linear direct ratio relationship with the diameters of the silver nanowires, in other words, the higher feeding concentration causes the larger diameter. The lengths of the silver nanowires do not change obviously. It should be noted that because the insufficient concentration of silver ions results in the low probability of collisions in the embodiment 10, only the silver nanoparticles but not the silver nanowires are obtained.
Please refer to FIG. 4. FIG. 4 is a schematic diagram illustrating the distribution of the diameters of the silver nanowires D to the feeding concentrations of the silver salt C according to the embodiments in table 4. As shown in FIG. 4, the feeding concentrations of the silver salt C (wt %) have a non-linear direct ratio relationship with the diameters of the silver nanowires D (nm).
Furthermore, please refer to FIG. 5. FIG. 5 is a SEM diagram illustrating the silver nanowires obtained in the above-mentioned embodiment 1. As shown in FIG. 5, the diameters of the silver nanowires obtained in the embodiment 1 are about 80 nm to 100 nm.
Compared to the prior art, the method for continuously fabricating silver nanowires of the invention is to provide a water-soluble silver-salt solution for obtaining the silver nanoparticles at a suitable temperature and simultaneously mixes the protecting agent solution to provide barriers for limiting the growth of silver nanoparticles. On the other hand, the functional groups in the protecting agent solution can keep the stably one-dimensional growth of the silver nanoparticles to form the silver nanowires during the aging process. The method of the invention can utilize continuous feeding to mass-produce the silver nanowires.
With the example and explanations above, the features and spirits of the invention will be hopefully well described. Those skilled in the art will readily observe that numerous modifications and alterations of the device may be made while retaining the teaching of the invention. Accordingly, the above disclosure should be construed as limited only by the metes and bounds of the appended claims.

Claims

1. A method for continuously fabricating silver nanowires, the method comprising the following steps of:

feeding a first solution into a reacting tank and an aging tank, and heating the first solution to a first temperature range in the reacting tank and the aging tank;

continuously feeding a silver-salt solution and a protecting agent solution into the reacting tank according to a ratio, the silver-salt solution and the protecting agent solution mixing with each other in a second temperature range and staying in the reacting tank for a first staying duration to form a second solution;

feeding the second solution into the aging tank and making the second solution to stay in the aging tank for a second staying duration to form a third solution; and

purifying the third solution to obtain at least one silver nanowire.

2. The method of claim 1, wherein the first solution is a glycol solution.

3. The method of claim 1, wherein the silver-salt solution is formed by dissolving a water-soluble silver salt in a first solvent.

4. The method of claim 3, wherein the silver-salt solution is formed by dissolving a silver nitrate in the first solvent.

5. The method of claim 3, wherein the silver-salt solution is formed by dissolving a silver acetate or a silver nitrite in the first solvent.

6. The method of claim 3, wherein the concentration of the water-soluble silver salt of the silver-salt solution is larger than 0.5 wt %.

7. The method of claim 4, wherein the protecting agent solution is formed by dissolving a polyvinyl pyrrolidone in a second solvent.

8. The method of claim 7, wherein the first solvent and the second solvent are glycol.

9. The method of claim 8, wherein the step for purifying the third solution comprises the following steps of:

feeding an excess of acetone into the third solution to mix with the glycol to make the third solution to form a clear liquid portion and a precipitate portion;

removing the clear liquid portion and dissolving the precipitate portion in a hot water to form a fourth solution;

processing the fourth solution by ion exchange process to form a fifth solution;

repeatedly heating, stirring, and solid-liquid separating the fifth solution for a plurality times to obtain a sixth solution; and

spray drying the sixth solution to remove the water of the sixth solution to obtain a powder of the at least one silver nanowire.

10. The method of claim 9, wherein the volume ratio of the acetone to the third solution comprises 2˜6.

11. The method of claim 7, wherein the weight ratio of the polyvinyl pyrrolidone to the silver nitrate comprises 0.5˜6 when the silver-salt solution and the protecting agent solution are fed into the reacting tank.

12. The method of claim 7, wherein the molecular weight of the polyvinyl pyrrolidone of the protecting agent solution comprises 5000˜360000.

13. The method of claim 1, wherein the silver-salt solution and the protecting agent solution are fed into the reacting tank by continuous feeding.

14. The method of claim 1, wherein the first temperature range comprises 170° C. to 180° C.

15. The method of claim 1, wherein the second temperature range comprises 140° C. to 180° C.

16. The method of claim 15, wherein the second temperature range comprises 150° C. to 170° C.

17. The method of claim 1, wherein the first staying duration comprises a range of 10 minutes to 30 minutes.

18. The method of claim 1, wherein the second staying duration comprises 30 minutes.

19. The method of claim 1, wherein the first temperature range is provided by micro-wave heating.

20. The method of claim 1, wherein the second temperature range is provided by micro-wave heating.