Patent Application
20100242679
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 NH3 or 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.
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.
Please refer to
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
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.
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.
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.
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
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.
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.
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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.
