A method for purifying silver nanowires prepared by a polyol method

Abstract

The present application belongs to the technical field of material engineering, and specifically relates to a purification method for silver nanowires prepared by a polyol method. The purification method comprises the following steps: uniformly mixing a silver nanowire stock solution prepared by a polyol method through oscillation, and then centrifuging to obtain supernatant and precipitate I; centrifuging the supernatant again to obtain precipitate II; dispersing the precipitate I and the precipitate II in a polyvinylpyrrolidone solution to obtain a silver nanowire mixed solution; adding acetone dropwise to the silver nanowire mixed solution, and stopping the addition and standing when flocculation appears, to obtain precipitate III; repeating the operations of dispersing in the polyvinylpyrrolidone solution and purifying in acetone, and finally obtaining purified silver nanowires. The present application is based on the defects of the prior art, and adopts a centrifugation method combined with a sedimentation method, with the purpose of shortening the purification time of silver nanowires and improving the purification quality of the silver nanowires.

Description

Technical Field

[0001] This invention belongs to the field of materials engineering technology, specifically a purification method for silver nanowires prepared by the polyol method. Background Technology

[0002] One-dimensional metallic nanowires hold significant promise for applications in various fields such as conductivity, electrochemistry, and optics. Silver nanowires, with their excellent electrical conductivity, high optical transmittance, and good mechanical flexibility, are widely considered promising candidate materials for preparing transparent conductive films and conductive pastes. Among various methods for preparing silver nanowires, the polyol chemical liquid-phase synthesis method currently offers a large-scale, low-cost, simple, and controllable way to produce silver nanowires with good liquid-phase dispersion. Silver nanowires prepared by the polyol method exhibit uniform diameter, high aspect ratio, and controllable size, and are already capable of large-scale industrial production.

[0003] The silver nanowire stock solution prepared using the polyol method requires cleaning to remove various chemically reacted components from the solution. This prevents chloride, sodium, and nitrate ions in the solution from corroding the silver nanowires and affecting their subsequent use. Furthermore, due to the inherent characteristics of the polyol synthesis method, the solid components in the obtained silver nanowire stock solution, besides silver nanowires, include large nanoparticles, small nanoparticles, and short nanorods, among other impurities. Large nanoparticles are unreacted AgCl or AgBr, small nanoparticles are silver seeds that did not grow into nanowires, and there are also some relatively short silver nanorods. These impurities account for 10% to 20% of the volume of the solid silver nanowire material prepared by the polyol method. These impurities have a significant impact on the subsequent performance of the silver nanowires. For example, when silver nanowires are used to prepare transparent conductive films, even a small amount of nanoparticles can significantly reduce the light transmittance and haze of the transparent conductive film.

[0004] Purification of silver nanowires prepared by the polyol method has always been a challenge in the field of silver nanowire application technology. There are three main technical approaches: centrifugation, filtration, and sedimentation. Centrifugation is a conventional method for cleaning and purifying nanomaterials in the liquid phase. However, in the purification of silver nanowires, centrifugation has two drawbacks: firstly, it can cause nanowires to agglomerate, making them difficult to disperse; secondly, the nanowires can entangle particles in the centrifuged precipitate, leading to incomplete purification. Filtration includes positive pressure filtration and vacuum filtration. Regardless of the method, it suffers from low efficiency, clogging, and difficulty in dispersing the silver nanowires in the filter cake when the pressure is too high. Sedimentation utilizes the poor dispersion of silver nanowires in acetone, using acetone to separate them by sedimentation. However, it has problems such as requiring large amounts of acetone, long processing time, and the presence of small amounts of particles and short rods in the precipitate, resulting in incomplete purification.

[0005] To overcome the problems encountered in purifying silver nanowires using methods such as centrifugation, filtration, and sedimentation, various techniques have been developed. These include shaking filtration, which incorporates shaking to prevent pore clogging and nanowire agglomeration; and shaking sedimentation, which uses shaking in the acetone sedimentation system to prevent silver nanowires from becoming entangled in the sediment. While these methods have addressed some of the problems in silver nanowire purification, issues remain such as long purification times, incomplete purification, and difficulty in dispersing the purified silver nanowires. Summary of the Invention

[0006] To address the shortcomings of the existing technology, the purpose of this invention is to provide a purification method for silver nanowires prepared by the polyol method. Based on the technical defects of the existing technology, this invention adopts a combination of centrifugation and sedimentation methods, aiming to shorten the purification time of silver nanowires and improve the purification quality of silver nanowires.

[0007] To achieve the above objectives, the technical solution adopted in this application is as follows:

[0008] A method for purifying silver nanowires prepared by the polyol method includes the following steps:

[0009] The silver nanowire stock solution prepared by the polyol method was shaken and mixed evenly, and then centrifuged at 3500-4500 rpm for 10-20 min to obtain the supernatant and precipitate I.

[0010] Take the supernatant and mix it thoroughly by shaking. Then centrifuge at 3700-4500 rpm for 10-20 min. After centrifugation, remove the supernatant to obtain precipitate II.

[0011] Precipitates I and II were dispersed together in a polyvinylpyrrolidone solution to obtain a silver nanowire mixed solution. Acetone was added dropwise to the silver nanowire mixed solution. When flocculent matter began to appear, the addition was stopped and the solution was allowed to stand to obtain precipitate III.

[0012] Redisperse precipitate III in polyvinylpyrrolidone solution, then add acetone dropwise. When flocculent matter begins to appear, stop adding acetone and let stand. The precipitate obtained is the purified silver nanowire. Repeat the above acetone precipitation step once, otherwise the purification effect will not be good.

[0013] Preferably, in the silver nanowire stock solution prepared by the polyol method, the diameter of the silver nanowires is 15-80 nm, and there is an optimal range of centrifugation speed and time for nanowires of different diameters.

[0014] Preferably, for silver nanowires with a diameter of 50-80 nm, the centrifugation speed of the silver nanowire stock solution is 3500-3800 rpm and the time is 10-15 min;

[0015] For silver nanowires with a diameter of 30-50 nm, the centrifugation speed of the silver nanowire stock solution is 3700-4000 rpm and the time is 12-17 min.

[0016] For silver nanowires with a diameter of 15-30 nm, the centrifugation speed of the silver nanowire stock solution is 3900-4500 rpm, and the time is 15-20 min.

[0017] Preferably, for silver nanowires with a diameter of 50-80 nm, the centrifugation speed of the supernatant is 3700-4100 rpm and the time is 10-15 min;

[0018] For silver nanowires with a diameter of 30-50 nm, the supernatant is centrifuged at 3900-4200 rpm for 12-17 min.

[0019] For silver nanowires with a diameter of 15-30 nm, the supernatant is centrifuged at 4200-4500 rpm for 15-20 min.

[0020] Preferably, the oscillation mixing speed is 100-200 rpm.

[0021] Preferably, the polyvinylpyrrolidone solution has a mass percentage of 0.3-0.8 wt% and a molecular weight of 30,000-80,000.

[0022] Preferably, the dispersion of precipitates I, II, and III in the polyvinylpyrrolidone solution is carried out by shaking in a shaker for 10-30 minutes.

[0023] Preferably, the silver nanowires are allowed to settle completely, and the settling time is 5-15 minutes.

[0024] Preferably, the purified silver nanowires are dispersed in water or alcohol.

[0025] Compared with the prior art, the beneficial effects of this application are as follows:

[0026] 1. The silver nanowire stock solution prepared by the polyol method contains reaction precursors, surfactants, large particles, small particles, short rods, and other impurities. Removing these impurities to obtain pure silver nanowires has always been a challenge for silver nanowire applications. In this invention, a portion of the silver nanowires is first separated by centrifugation at a relatively low speed. Due to the low centrifugation speed, the silver nanowires do not aggregate excessively, preventing them from becoming difficult to redisperse. Then, a slightly higher centrifugation speed is used to separate the remaining silver nanowires from the supernatant. During centrifugation, the aggregation of silver nanowires will encapsulate some large and small particles into the precipitate. Therefore, the silver nanowires obtained by centrifugation still contain some nanoparticles, although the content of nanoparticles is significantly reduced.

[0027] The silver nanowires obtained by centrifugation were dispersed in an aqueous solution of polyvinylpyrrolidone (PVP). The surface of the silver nanowires is the (100) crystal plane of silver, while the silver nanoparticles and short rods are mainly the (111) crystal plane of silver. PVP tends to be densely adsorbed on the silver (100) surface, while it is rarely adsorbed on the surfaces of silver nanoparticles, short rods, and large AgBr or AgCl.

[0028] PVP has very low solubility in acetone. When the centrifuged and purified silver nanowires are dispersed in an aqueous PVP solution, the surface of the silver nanowires will be coated with dense PVP molecules. When acetone is added dropwise to the solution, the silver nanowires coated with PVP molecules will aggregate in the solution and then settle. However, impurities such as nanoparticles and nanorods with less PVP content on the surface will not aggregate or settle, but will remain stably dispersed in the supernatant. After removing the supernatant, the content of nanoparticles and short rods in the remaining silver nanowires will be significantly reduced.

[0029] Because silver nanowires will also encapsulate a small number of nanoparticles into the sediment during the aggregation process, the silver nanowires need to be purified twice by sedimentation.

[0030] 2. Existing technologies often use only centrifugation or sedimentation. However, with only centrifugation, repeated centrifugation can lead to nanowire agglomeration, making dispersion difficult. This patent overcomes this problem by: a) combining centrifugation with acetone sedimentation, significantly reducing the number of centrifugation cycles and lowering the likelihood of nanowire agglomeration; b) using low-speed centrifugation of the supernatant to avoid agglomeration of easily separable nanowires caused by high-speed centrifugation; c) adding PVP surfactant before acetone sedimentation to replenish the PVP coating on the silver nanowires, effectively preventing agglomeration during sedimentation.

[0031] 3. Existing technologies that solely employ centrifugation or sedimentation for silver nanowire purification often fail to achieve satisfactory purification results and are time-consuming. Centrifugation alone is insufficient to completely remove nanoparticles after one or two centrifugations; further centrifugation leads to nanowire aggregation and entanglement, making dispersion difficult. Sedimentation alone, due to the high nanoparticle content, removes only a small number of nanoparticles per sedimentation cycle; even five sedimentation cycles are insufficient for optimal purification, and each sedimentation cycle results in the loss of some silver nanoparticles, consumes large amounts of acetone, and is extremely time-consuming. Compared to existing technologies, this application combines centrifugation and sedimentation, resulting in the highest quality purified silver nanowires. Attached Figure Description

[0032] Figure 1 The image shows the purification characterization of the silver nanowire stock solution by centrifugation speed. Among them, (a) is the actual image of silver nanowires before centrifugation (left) and after centrifugation (right), (b) is the UV spectrum of silver nanowires before and after centrifugation, (c) is the UV spectrum of silver nanowires at different centrifugation speeds, and (d) is a local magnified image of the UV spectrum of silver nanowires at different centrifugation speeds at 400-500 nm.

[0033] Figure 2 The purification characterization of the silver nanowire stock solution by centrifugation time; where (a) is the UV spectrum of silver nanowires at different centrifugation times, and (b) is a local magnified view of the UV spectrum of silver nanowires at different centrifugation times at 400-500 nm.

[0034] Figure 3 The number of centrifugation cycles is used to characterize the silver nanowire stock solution. Among them, (a) is the UV spectrum of silver nanowires under different centrifugation cycles, (b) is a magnified view of the UV spectrum of silver nanowires under different centrifugation cycles at 400-500 nm, (c) is a visual diagram of the purification effect of silver nanowires under different centrifugation cycles, and (d) is a picture of the silver nanowire stock solution and the silver nanowire stock solution after four centrifugation cycles.

[0035] Figure 4 For the purification effect characterization; (a) is the UV spectrum of the silver nanowire stock solution, the silver nanowire stock solution after centrifugation, and the silver nanowires after centrifugation and acetone precipitation in Example 1; (b) is the scanning electron microscope image of the silver nanowire stock solution; (c) is the scanning electron microscope image of the silver nanowire stock solution after centrifugation; (d) is the scanning electron microscope image of the silver nanowires after centrifugation and acetone precipitation in Example 1.

[0036] Figure 5 Schematic diagram of the principle of purifying silver nanowires with acetone;

[0037] Figure 6Images of the silver nanowire stock solution after acetone sedimentation at different times are shown; (a) is 2 min, (b) is 6 min, and (c) is 120 min.

[0038] Figure 7 The purification process of silver nanowire stock solution under different acetone dosages is shown in the graph; (a) shows the addition of 5 mL, 10 mL, and 15 mL of acetone and standing for 6 min, and (b) shows the addition of 5 mL, 10 mL, and 15 mL of acetone and standing for 120 min.

[0039] Figure 8 The images show the characterization effect of acetone purification; (a) is the SEM image of the original silver nanowire solution, (b) is the SEM image of the silver nanowires in Example 1, (c) is the dark field optical microscope image of the silver nanowires before purification with acetone, and (d) is the dark field optical microscope image of the silver nanowires after purification with acetone. Detailed Implementation

[0040] The specific embodiments of the present invention will now be described in detail with reference to the accompanying drawings. However, it should be understood that the scope of protection of the present invention is not limited to the specific embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention. Unless otherwise specified, the raw materials used in the present invention are all conventional products that can be purchased commercially.

[0041] To make the content of this invention easier to understand, the technical solution of this invention will be further described below in conjunction with specific embodiments, but this invention is not limited thereto.

[0042] This application studies the purification of silver nanowires, and optimizes the purification of silver nanowires using different methods. The specific conclusions are as follows:

[0043] Centrifugation method for purifying fine silver nanowires:

[0044] The effects of centrifugation speed, time, and number of centrifugation cycles on the purification of 20 nm diameter ultrafine silver nanowires were investigated. After gradual optimization of centrifugation speed, time, and number of cycles, the optimal purification effect was achieved at 4400 rpm, 18 min, and two centrifugation cycles. When the centrifugation speed exceeded 4800 rpm, the silver nanowires agglomerated, exhibiting adhesion to the centrifugation wall and becoming difficult to disperse. When the centrifugation speed was less than 4200 rpm, some silver nanowires could be centrifuged, but a significant amount was lost. A centrifugation time of 10 min was relatively short and could remove some particulate impurities compared to the original silver nanowire solution, but the effect was not as good as centrifugation at 18 min. Centrifugation at 26 min was relatively long, but the effect was only moderate. Centrifugation at 18 min effectively removed particles from the original silver nanowire solution, reducing the silver particle content. Centrifugation at 10 min and 26 min showed inaccurate time control; although some particles were removed, agglomeration of silver nanowires was observed after centrifugation to 470 nm. The purification effect of silver nanowires is best when centrifuged twice. Centrifugation once will result in the loss of a large number of silver nanowires, and centrifugation more than three times will cause the silver nanowires to aggregate.

[0045] Purification of silver nanowires by acetone precipitation:

[0046] The purification process, dosage, and number of acetone purification cycles using acetone sedimentation were investigated. After adding acetone to the silver nanowire stock solution, a large number of silver nanowires flocculated, agglomerated, and then selectively settled to the bottom of the container. Some nanoparticles and short rods in the silver nanowire stock solution, due to their smaller quantity and lighter mass, remained suspended in the acetone-containing supernatant. The silver nanowires and impurities were separated by removing the supernatant using a pipette. Adding twice the amount of acetone to the silver nanowire stock solution was sufficient to achieve natural sedimentation of the silver nanowires. Furthermore, the supernatant from this sedimentation was relatively pure, allowing for the removal of the supernatant after sedimentation and subsequent high-speed centrifugation for recycling, thus reducing acetone usage costs. Two acetone sedimentation cycles resulted in better purification. After only one sedimentation, most particles remained, leading to only moderate purification. Excessive sedimentation cycles resulted in excessive acetone usage, increasing costs.

[0047] Combining centrifugation with acetone precipitation can further optimize the purification effect of silver nanowires, resulting in a silver nanowire content of 95% after purification, making them even purer.

[0048] Example 1

[0049] A method for purifying silver nanowires prepared by the polyol method includes the following steps:

[0050] S1. Take 50 mL of the silver nanowire stock solution with a diameter of 20 nm prepared by the polyol method and place it in a centrifuge tube. Shake and mix it evenly in a shaker at a speed of 150 rpm to obtain a well dispersed silver nanowire stock solution.

[0051] The dispersed silver nanowire stock solution was centrifuged in a centrifuge tube at a speed of 4200 rpm for 18 min. After centrifugation, the supernatant was aspirated and placed in another centrifuge tube to obtain the supernatant and precipitate I.

[0052] S2. Place the supernatant from step S1 into a centrifuge tube and mix it thoroughly by shaking at 150 rpm in a shaker. Then centrifuge the dispersed supernatant at 4400 rpm for 18 min to obtain precipitate II.

[0053] S3. Mix precipitate I from step S1 and precipitate II from step S2, and disperse them in a 0.5 wt% PVP (molecular weight 50000) aqueous solution. Mix them evenly on a shaker for 20 min to obtain a silver nanowire mixed solution.

[0054] S4. Slowly add acetone dropwise to the silver nanowire mixed solution in step S3. When flocculent matter begins to appear, stop adding acetone and let it stand for 10 minutes. After the silver nanowires settle to the bottom, aspirate the supernatant to obtain precipitate III.

[0055] S5. The silver nanowires precipitated at the bottom (precipitate III in step S4) are redispersed in the PVP aqueous solution and mixed evenly by shaking for 20 minutes. Then, acetone is slowly added dropwise. When flocculent matter begins to appear, the addition is stopped and the mixture is allowed to stand for 10 minutes. After the silver nanowires settle to the bottom, the supernatant is aspirated to obtain high-purity silver nanowires.

[0056] Example 2

[0057] A method for purifying silver nanowires prepared by the polyol method includes the following steps:

[0058] S1. Take 50 mL of the silver nanowire stock solution with a diameter of 20 nm prepared by the polyol method and place it in a centrifuge tube. Shake and mix it evenly in a shaker at a speed of 100 rpm to obtain a well dispersed silver nanowire stock solution.

[0059] The dispersed silver nanowire stock solution was centrifuged in a centrifuge tube at a speed of 4300 rpm for 15 min. After centrifugation, the supernatant was aspirated and placed in another centrifuge tube to obtain the supernatant and precipitate I.

[0060] S2. Place the supernatant from step S1 into a centrifuge tube and mix it thoroughly by shaking at 150 rpm in a shaker. Then centrifuge the dispersed supernatant at 4500 rpm for 15 min to obtain precipitate II.

[0061] S3. Mix precipitate I from step S1 and precipitate II from step S2, and disperse them in a 0.3 wt% PVP (molecular weight 80000) aqueous solution. Mix them evenly on a shaker for 10 min to obtain a silver nanowire mixed solution.

[0062] S4. Slowly add acetone dropwise to the silver nanowire mixed solution in step S3. When flocculent matter begins to appear, stop adding acetone and let it stand for 15 minutes. After the silver nanowires settle to the bottom, aspirate the supernatant to obtain precipitate III.

[0063] S5. The silver nanowires precipitated at the bottom (precipitate III in step S4) are redispersed in the PVP aqueous solution and mixed evenly by shaking for 10 minutes. Then, acetone is slowly added dropwise. When flocculent matter begins to appear, the addition is stopped and the mixture is allowed to stand for 15 minutes. After the silver nanowires settle to the bottom, the supernatant is aspirated to obtain high-purity silver nanowires.

[0064] Example 3

[0065] A method for purifying silver nanowires prepared by the polyol method includes the following steps:

[0066] S1. Take 50 mL of the silver nanowire stock solution with a diameter of 20 nm prepared by the polyol method and place it in a centrifuge tube. Shake and mix it evenly in a shaker at a speed of 200 rpm to obtain a well dispersed silver nanowire stock solution.

[0067] The dispersed silver nanowire stock solution was centrifuged in a centrifuge tube at a speed of 4200 rpm for 20 min. After centrifugation, the supernatant was aspirated and placed in another centrifuge tube to obtain the supernatant and precipitate I.

[0068] S2. Place the supernatant from step S1 into a centrifuge tube and mix it thoroughly by shaking at 200 rpm in a shaker. Then centrifuge the dispersed supernatant at 4500 rpm for 20 min to obtain precipitate II.

[0069] S3. Mix precipitate I from step S1 and precipitate II from step S2, and disperse them in a 0.8 wt% PVP (molecular weight 30000) aqueous solution. Mix them evenly on a shaker for 30 min to obtain a silver nanowire mixed solution.

[0070] S4. Slowly add acetone dropwise to the silver nanowire mixed solution in step S3. When flocculent matter begins to appear, stop adding acetone and let it stand for 5 minutes. After the silver nanowires settle to the bottom, aspirate the supernatant to obtain precipitate III.

[0071] S5. The silver nanowires precipitated at the bottom (precipitate III in step S4) are redispersed in the PVP aqueous solution and mixed evenly by shaking for 30 minutes. Then, acetone is slowly added dropwise. When flocculent matter begins to appear, the addition is stopped and the mixture is allowed to stand for 5 minutes. After the silver nanowires settle to the bottom, the supernatant is aspirated to obtain high-purity silver nanowires.

[0072] For silver nanowires of other diameters, the experimental conditions described in the claims of this application can be used for screening.

[0073] To address the issue that while current synthesized ultrafine silver nanowires with a diameter of approximately 20 nm achieve high yields, they often contain impurities such as silver nanoparticles and short rods. This application combines the advantages of various existing purification methods. Based on traditional centrifugation, the speed, time, and number of centrifugation cycles are modified to find the optimal centrifugation conditions for silver nanowire purification. Furthermore, centrifugation and acetone precipitation are combined, with silver nanowires subjected to centrifugation followed by acetone precipitation, and the purification effect is investigated. Finally, the purification effect of acetone precipitation on silver nanowires is explored, and the acetone purification process and parameters affecting the purification effect, such as acetone dosage and the number of purification cycles, are optimized. The optimized conditions are integrated to obtain the technical solutions of Examples 1-3 of this application. The conditions for centrifugation purification and acetone purification of silver nanowires are screened as follows:

[0074] 1. Discussion of the results of silver nanowire purification by centrifugation

[0075] 1.1 Effect of centrifugation speed on the purification of silver nanowires:

[0076] The silver nanowire solution synthesized by the polyol method contains a large number of byproducts such as nanoparticles and short nanorods, which need to be removed. Centrifugation uses a centrifuge to generate strong centrifugal force at high speed, utilizing the density gradient between the silver nanowires and impurity particles in the original silver nanowire slurry to cause the silver nanowires to settle, while the nanoparticles and short rods are separated. Figure 1 (b) shows that the UV curve at 400-500 nm decreased before and after centrifugation, which means that some small-diameter nanoparticles can be removed after centrifugation of the silver nanowire stock solution.

[0077] Based on the principle of centrifugation, the weight difference between silver nanowires and impurity silver nanorods and nanoparticles is utilized. By adjusting the centrifugation speed and time, the weight of the precipitate is made uniform, thus achieving separation of silver nanowires and particles. Furthermore, the PVP coating on the surface of the silver nanowires thins during repeated centrifugation, allowing some to be removed. Different centrifugation speeds result in varying degrees of dispersion of the silver nanowires. Too low a speed leads to significant loss of the original silver nanowire slurry, failing to achieve the desired centrifugation effect; too high a speed causes irreversible changes in the silver nanowires, resulting in them adhering to the centrifuge tube wall and hindering dispersion.

[0078] To ensure the silver nanowires were centrifuged precisely without aggregation, this study investigated the centrifugation speeds of equal volumes of the silver nanowire stock solution. Centrifugation speeds were set at 4000 rpm, 4200 rpm, 4400 rpm, and 4800 rpm, with a duration of 18 minutes for each. After centrifugation, the supernatant was removed, and the silver nanowires were then dispersed in water. The dispersion of the silver nanowires after centrifugation was observed. It was found that at speeds of 4400 rpm and below, the centrifuged silver nanowires could be uniformly dispersed in water. At speeds of 4800 rpm and above, although some silver nanowires could be dispersed in water, significant adhesion to the water walls was observed, indicating irreversible aggregation and difficulty in achieving complete dispersion. Figure 1 The illustration in (c) shows a physical sample where some silver nanowires agglomerated and could not be dispersed in water due to centrifugation at 4800 rpm, and some silver nanowires adhered to the wall. Figure 1 The "large bulge" in the UV spectrum in (c) further confirms that centrifugation at 4800 rpm causes some silver nanowires to agglomerate, resulting in a loss in silver nanowire production. Figure 1 In (d), the lower the UV curve of the silver nanowire spectrum in the 400-500 nm range, the fewer particulate impurities in the silver nanowire solution, indicating better quality and purity of the silver nanowires. Figure 1 (d) As can be seen from the magnified portion of the figure, in the wavelength range of 400-500nm, the curve shifts the most downward after centrifugation at 4800rpm compared to the original silver nanowire solution, indicating that more particles are removed and the purification effect of silver nanowires is the best. However, due to the adhesion phenomenon, the silver nanowire yield will be lost. Considering both the purification effect and the silver nanowire yield, the centrifugation speed of 4400rpm is adopted to purify silver nanowires.

[0079] 1.2 Effect of centrifugation time on the purification of silver nanowires:

[0080] When purifying silver nanowires using centrifugation, controlling the centrifugation time is crucial for achieving high-quality and efficient extraction and separation. Excessive centrifugation time can cause irreversible aggregation of the silver nanowires, while insufficient centrifugation time will result in low yields and significant waste of the silver nanowire concentrate.

[0081] To explore the optimal centrifugation time for purifying silver nanowires, a controlled variable method was adopted. The silver nanowire stock solution was centrifuged at a uniform speed of 4400 rpm for 10 min, 18 min, and 26 min, respectively, and the ultraviolet-visible light spectrum of the centrifuged silver nanowires was observed. Figure 2 As shown in (a), when centrifuged at 4400 rpm for 18 minutes, the UV curve of the silver nanowires showed the lowest value at 400-500 nm, indicating the best particle removal effect. Centrifugation for 10 minutes, while shorter, removed some particulate impurities compared to the original silver nanowire solution, but the removal effect was not as good as with centrifugation for 18 minutes. Centrifugation for 26 minutes, while relatively longer, did not follow the rule that longer centrifugation leads to more thorough centrifugation and better results; instead, the particle removal effect was similar to that of centrifugation for 10 minutes. This demonstrates that in the centrifugal purification method, finding the optimal centrifugation time and optimizing this parameter is necessary for purifying silver nanowires. Figure 2 In (b), it is clearly visible that the silver nanowire curve at 18 min shows the largest decrease compared to the original silver nanowire solution, indicating that centrifugation at 4400 rpm for 18 min can effectively remove particles from the original silver nanowire solution and reduce the silver particle content. Although some particles were removed at 10 min and 26 min, silver nanowire aggregation was observed after centrifugation to 470 nm. Therefore, centrifugation at 4400 rpm for 18 min is the optimal time for removing impurities such as silver nanowire particles.

[0082] 1.3 Effect of centrifugation times on the purification of silver nanowires:

[0083] The silver nanowire stock solution showed significant silver nanowire loss after a single centrifugation, necessitating an investigation into the effect of multiple centrifugations on purification. The silver nanowire stock solution was centrifuged multiple times, and the effects of different centrifugation cycles on the removal of particles and impurities were observed.

[0084] In this application, the stock solution was subjected to four centrifugal cycles. After the first centrifugation, the silver nanowires separated from the centrifuged solution were dispersed in water. The remaining stock solution was then subjected to a second centrifugation, and this cycle was repeated until the fourth centrifugation. Figure 3 (d) The comparison of the silver nanowire stock solution before and after centrifugation shows that after four centrifugations, the silver nanowire stock solution is almost transparent green, indicating that most of the silver nanowires have been centrifuged out. Figure 3(b) The UV spectrum in the 400-500 nm wavelength range shows that the curve after two centrifugations of the silver nanowire stock solution is furthest from the stock solution, indicating that centrifugation at this point is most effective in removing particles and short rods. Compared to the second centrifugation, the curve shifts to the right after the third centrifugation, i.e., a red shift occurs. This is because the silver nanowires aggregated during the third centrifugation. In the 430-500 nm wavelength range, the curve after the fourth centrifugation shifts to the right compared to both the third and the stock solution, indicating that irreversible aggregation of the silver nanowires occurred after four centrifugation cycles. Therefore, the best purification effect is achieved when the silver nanowire stock solution is centrifuged twice at 4400 rpm for 18 min, effectively removing impurities such as the original solvent, particles, and short rods, thus purifying the silver nanowires and increasing their yield.

[0085] 1.4 Investigation into the effect of centrifugation combined with acetone precipitation on the purification of silver nanowires:

[0086] The silver nanowire stock solution contains not only a certain number of particulate impurities, but also reaction solvents such as ethylene glycol and PVP in the untreated stock solution. These substances can be removed by centrifugation to remove interference with the purification effect, and then acetone precipitation purification can be performed to improve the purification quality of silver nanowires. Figure 4 The UV spectrum in (a) shows that centrifugation removes the original solvent used in the silver nanowire preparation process, as well as particulate impurities from the solution. Further centrifugation followed by acetone precipitation can further remove particulate matter and other impurities, improving the quality of the silver nanowires and facilitating their application in high-end products. Figure 4 Images (b), (c), and (d) show the scan images of the silver nanoparticle stock solution, the silver nanowires after centrifugation, and the silver nanowires after centrifugation and acetone precipitation in Example 1, respectively. It is clearly visible that in Example 1, purification using these two methods removes 95% of the silver nanoparticles. After centrifugation, most of the silver nanoparticles have been removed. Further precipitation with acetone after centrifugation can also remove PVP adhering to the surface of the silver nanowires to a certain extent, relatively reducing the diameter of the silver nanowires, achieving a double purification effect. Figure 4 (d) shows that the silver nanowires are purified more effectively after centrifugation and acetone precipitation. Therefore, to meet the requirements of high-end applications, the silver nanowire stock solution needs to be purified further using acetone precipitation after centrifugation. The combination of centrifugation and acetone precipitation results in the best quality purified silver nanowires.

[0087] 2. Purification of silver nanowires by acetone precipitation method

[0088] 2.1 Purification mechanism of acetone precipitation method:

[0089] The principle of the acetone sedimentation method is based on the immiscibility of PVP in acetone. For example... Figure 5 The mechanism of purifying silver nanowires with acetone was demonstrated. Since the silver nanowires are coated with a large amount of PVP during preparation, after acetone is added to the silver nanowire stock solution, a large number of silver nanowires flocculate and then agglomerate, and then selectively settle to the bottom of the container. Some nanoparticles and short rods in the silver nanowire stock solution, due to their small number and light weight, will be suspended in the supernatant mixed with acetone. After removing the supernatant with a pipette, the silver nanowires and impurities can be separated. Then, the silver nanowires that have settled to the bottom of the bottle are evenly dispersed into an aqueous solution containing PVP. This is the principle of purifying silver nanowires using the acetone sedimentation method.

[0090] To obtain high-quality silver nanowires, the silver nanowire stock solution was purified using the acetone precipitation method, such as... Figure 6 As shown in (a) to (b), after adding acetone to the silver nanowire stock solution for 2 minutes, the silver nanowires exhibited flocculation and sedimentation, with slight settling beginning. The solution color changed from dark green to light green and then to light yellow. After adding acetone for 6 minutes, clear stratification of the solution was observed, with significant sedimentation and aggregation of the silver nanowires. The solution color changed from light green to light yellow, and the silver nanowires continued to settle and aggregate over this period. When left to stand for a longer time, such as 120 minutes, ... Figure 6 As shown in (c), the upper layer of solution in the bottle becomes transparent, and the silver nanowires in the lower layer have completely settled to the bottom of the bottle. Flocculent material, consisting of a small amount of nanoparticles and short silver nanowire rods, can be faintly seen in the upper transparent solution. After removing the supernatant with a pipette, the silver nanowires are dispersed in PVP water, completing one purification process. This yields a silver nanowire solution purified once with acetone. This process constitutes the entire process of acetone purification of silver nanowires. After the selective sedimentation process with acetone, the particulate impurities in the silver nanowires are significantly reduced. Repeating the above process several times yields relatively high-purity ultrafine silver nanowires.

[0091] 2.2 Investigation into the purification effect of acetone dosage:

[0092] Since the acetone sedimentation method is a complicated and time-consuming process, and the acetone itself is toxic and harmful to the human body, this application optimizes the dosage of acetone to reduce the harm and the amount of acetone used.

[0093] like Figure 7 (a) Take three equal volumes of silver nanowire stock solution and slowly add acetone to it, mixing the acetone and stock solution at volume ratios of 1:1, 1:2, and 1:3, respectively. During the dropwise addition process, if... Figure 7(a) Observing from left to right, it can be seen that after adding 5 mL of acetone and letting it stand for 6 minutes, there was no obvious phenomenon; the silver nanowires showed a tendency to aggregate but did not show obvious macroscopic sedimentation. After adding 10 mL of acetone and letting it stand for 6 minutes, the silver nanowires showed aggregation and sedimentation and gradually aggregated and sank to the bottom. After adding 15 mL of acetone and letting it stand for 6 minutes, a more obvious natural sedimentation phenomenon appeared in the bottle, and the suspended matter in the solution gradually changed from micro-flocculation to flocculent sedimentation as the standing time increased. This indicates that at least twice the amount of acetone as the original silver nanowire solution was added to achieve sedimentation of the silver nanowires. After the silver nanowires sank to the bottom, the supernatant containing short silver nanowires and silver nanoparticles was removed with a pipette, and the settled silver nanowires were evenly dispersed in water.

[0094] Figure 7 In (b), after standing for 120 minutes, it was observed that the silver nanowire stock solution with an acetone ratio of 1:2 and 1:3 achieved complete sedimentation, while the 1:1 ratio showed no significant sedimentation due to the small amount of acetone added. Considering both sedimentation time and acetone dosage on the purification effect of silver nanowires, a acetone volume twice that of the silver nanowires is sufficient for rapid and natural sedimentation, and the supernatant from the sedimentation solution with twice the acetone volume is relatively pure. To achieve acetone reuse, the acetone supernatant can be collected after acetone sedimentation and purification of silver nanowires, centrifuged several times at high speed, and then reused, thus enabling multiple uses of acetone and reducing the cost of using acetone.

[0095] 2.3 Investigation into the effect of multiple purification cycles on acetone:

[0096] To further optimize the purification of silver nanowires using acetone precipitation, the number of precipitation cycles was optimized. The effect of purifying silver nanowires multiple times using acetone precipitation was investigated. Figure 8 (a) and Figure 8 (b) shows the scanning electron microscope (SEM) images of the silver nanowire stock solution and the silver nanowires purified twice by centrifugation followed by acetone precipitation in Example 1. It can be clearly seen that some particles, short rods, and other impurities exist in the silver nanowire stock solution; however, after purification by centrifugation followed by two acetone purifications, these impurities are removed, resulting in silver nanowires with a purity of 95%. Figure 8 (c) and Figure 8 (d) Dark-field optical microscopy images of silver nanowires before and after purification show that the original silver nanowire solution contained many particles before purification, while the number of particles was significantly reduced after purification. Acetone precipitation removes impurities such as silver nanoparticles and short rods to a certain extent. Furthermore, through continuous improvements to silver nanowire purification methods, acetone precipitation can be used to purify silver nanowires of different diameters without requiring surface modification or alteration.

[0097] Obviously, those skilled in the art can make various modifications and variations to the invention without departing from the spirit and scope of the invention. Therefore, if these modifications and variations fall within the scope of the claims of the invention and their equivalents, the invention is also intended to include these modifications and variations.

Claims

1. A method for purifying silver nanowires prepared by the polyol method, characterized in that, Includes the following steps: The silver nanowire stock solution prepared by the polyol method was shaken and mixed evenly, and then centrifuged at 3500-4500 rpm for 10-20 min to obtain the supernatant and precipitate I. Take the supernatant and mix it thoroughly by shaking. Then centrifuge at 3700-4500 rpm for 10-20 min. After centrifugation, precipitate II is obtained. Precipitates I and II were dispersed together in a polyvinylpyrrolidone solution to obtain a silver nanowire mixed solution. Acetone was added dropwise to the silver nanowire mixed solution. When flocculent matter began to appear, the addition was stopped and the solution was allowed to stand to obtain precipitate III. Precipitate III was redispersed in a polyvinylpyrrolidone solution, and then acetone was added dropwise. When flocculent matter began to appear, the addition was stopped and the mixture was allowed to stand. The resulting precipitate was purified silver nanowires.

2. The purification method for silver nanowires prepared by the polyol method according to claim 1, characterized in that, In the silver nanowire stock solution prepared by the polyol method, the diameter of the silver nanowires is 15-80 nm.

3. The purification method for silver nanowires prepared by the polyol method according to claim 2, characterized in that, For silver nanowires with a diameter of ≥50nm and ≤80nm, the centrifugation speed of the silver nanowire stock solution is 3500-3800rpm and the time is 10-15min. For silver nanowires with a diameter of ≥30nm and <50nm, the centrifugation speed of the silver nanowire stock solution is 3700-4000rpm and the time is 12-17min. For silver nanowires with a diameter of ≥15nm and <30nm, the centrifugation speed of the silver nanowire stock solution is 3900-4500rpm and the time is 15-20min.

4. The purification method for silver nanowires prepared by the polyol method according to claim 2, characterized in that, For silver nanowires with a diameter of ≥50nm and ≤80nm, the supernatant is centrifuged at 3700-4100rpm for 10-15min. For silver nanowires with a diameter of ≥30nm and <50nm, the supernatant is centrifuged at 3900-4200rpm for 12-17min. For silver nanowires with a diameter of ≥15nm and <30nm, the supernatant is centrifuged at 4200-4500 rpm for 15-20 min.

5. The purification method for silver nanowires prepared by the polyol method according to claim 1, characterized in that, The oscillation mixing speed is 100-200 rpm.

6. The purification method for silver nanowires prepared by the polyol method according to claim 1, characterized in that, The polyvinylpyrrolidone solution has a mass percentage of 0.3-0.8 wt% and a molecular weight of 30,000-80,000.

7. The purification method for silver nanowires prepared by the polyol method according to claim 1, characterized in that, The dispersion of precipitates I, II, and III in the polyvinylpyrrolidone solution was carried out by shaking for 10-30 minutes.

8. The purification method for silver nanowires prepared by the polyol method according to claim 1, characterized in that, Allow to stand until the silver nanowires have completely settled; the standing time is 5-15 minutes.

9. The purification method for silver nanowires prepared by the polyol method according to claim 1, characterized in that, The purified silver nanowires are dispersed in water or alcohol.

Images