A cleaning method for silver nanoparticles

Abstract

The present invention relates to a method for cleaning nano silver, comprising the steps of: cleaning the nano silver solution; adding hydroxyethyl cellulose to the nano silver solution after cleaning; subjecting the nano silver solution containing hydroxyethyl cellulose to ultrasonic dispersion, and gradually adding a water-soluble organic solvent to the nano silver solution during ultrasonic dispersion until nano silver precipitates; removing the supernatant to obtain nano silver particles. The cleaned nano silver particles can be placed in a freeze dryer for drying and then stored as a solid phase for later use. Cleaning nano silver according to the method of the present invention overcomes the difficulty of purifying and separating PVP from nano silver. It does not require long-term ceramic membrane cleaning treatment, reduces the energy consumption during the cleaning process, and saves costs. The impurity content of the cleaned nano silver is very small, with a loss on ignition at 300 °C ≤ 5% and a loss on ignition at 600 °C ≤ 6%. Moreover, the nano silver prepared after cleaning is stored as a solid phase, which is convenient for subsequent use and broadens the application scenarios of nano silver.

Description

Technical Field

[0001] The present invention belongs to the technical field of nano - silver preparation, and particularly relates to a cleaning method for nano - silver. Background Art

[0002] Due to its high surface activity, surface energy, catalytic performance, electrical and thermal conductivity, nano - silver powder has broad application prospects in fields such as catalyst materials, low - temperature thermal conductive materials, and conductive coatings. Currently, there are many methods for preparing nano - silver, mainly including liquid - phase reduction method, electrochemical method, micro - emulsion method, etc. These methods can all prepare nano - silver with an average particle size below 100 nm. Among them, the liquid - phase reduction method is the most widely used due to its simple process conditions and easy operation. The nano - silver particles prepared by the liquid - phase reduction method have a high specific surface area and can be melted and solidified at low temperature (below 300 °C), which is suitable for applications in fields such as radio - frequency identification (RFID) electronic tags and transparent conductive films.

[0003] In order to maintain the dispersibility of nano - silver and control the particle size of silver particles, a large amount of PVP (polyvinylpyrrolidone) is used during the preparation of nano - silver. Since the decomposition temperature of PVP is 450 - 500 °C, the residual PVP during the low - temperature curing process cannot be decomposed and removed. When the residual amount of PVP on the surface of nano - silver is too high, it will affect the electrical conductivity of the nano - silver coating, increase the resistivity, and affect the application performance. The purification and separation of PVP and silver are the difficulties in the current method for preparing nano - silver. Currently, the commonly used purification and separation methods include ceramic membrane separation, plasma cleaning, and centrifugal separation, etc. Among them, plasma cleaning has a high energy consumption, and using only ceramic membrane cleaning not only requires a long cleaning time but also results in high energy consumption, and the cleaning effect is difficult to meet the requirements. Generally speaking, the cleaning processes of ceramic membrane separation and plasma cleaning are usually cumbersome, time - consuming, inefficient, and extremely difficult to reduce the PVP content on the surface of nano - silver to a satisfactory level; during high - speed centrifugal separation, the smaller the particle size of the nano - particles, the greater the centrifugal force required to effectively separate and purify the nano - particles, which easily leads to the agglomeration of nano - silver particles and inability to redisperse, ultimately affecting the use effect of nano - silver. Therefore, it is necessary to propose a cleaning method for nano - silver with low energy consumption and good cleaning effect. Summary of the Invention

[0004] To overcome the defects in the prior art, the present invention provides a cleaning method for nano - silver, which overcomes the difficulty of purifying and separating PVP from nano - silver, significantly reduces the cleaning energy consumption, and saves costs; the obtained nano - silver powder can be redispersed in a water - soluble solvent system, broadening the application scenarios of nano - silver.

[0005] To achieve the above object, the technical solution adopted by the present invention is a cleaning method for nano - silver, which is characterized by comprising the following steps:

[0006] S1. Perform membrane cleaning on the silver nanosol.

[0007] S2. Add hydroxyethyl cellulose to the silver nanosol after cleaning.

[0008] S3. Ultrasonically disperse the silver nanosol containing hydroxyethyl cellulose, and gradually add a water-soluble organic solvent to the silver nanosol during ultrasonic dispersion until silver nanoparticles precipitate; remove the supernatant, and obtain silver nanoparticles after washing and drying. Among them, freeze-drying is preferably used for drying, and the dried silver nanoparticles are in a solid phase and can be stored at low temperature.

[0009] Further, before step S1, there is also step S0: Dilute the silver nanosol and adjust the pH value of the solution.

[0010] Diluting the original silver nanosol with deionized water in a certain proportion can improve the permeation flux of the liquid through the ceramic membrane, thereby improving the efficiency of ceramic membrane cleaning; adjusting the pH value of the silver nanosol to 6.5 - 7.5 in advance to make the solution neutral can protect the cleaning equipment from acid-base erosion.

[0011] Further, in step S0, the concentration of the non-solvent component in the diluted silver nanosol is 0 - 20 wt%.

[0012] Further, in step S0, the silver nanosol is a silver nanodispersion prepared by a liquid phase reduction method using polyvinylpyrrolidone as a dispersant.

[0013] Further, in step S1, a ceramic membrane tube is used to clean the silver nanosol. Centrifugal separation or plasma cleaning can also be used, but centrifugal separation will cause silver nanoparticles to agglomerate, increasing the subsequent dispersion time, and plasma cleaning has high energy consumption. Therefore, ceramic membrane cleaning is preferred.

[0014] Preferably, the silver nanoparticles have an average particle size of 10 - 100 nm and can be used for low-temperature curable aqueous silver nanoinks. At the same time, a ceramic membrane tube with a pore size similar to that of the silver nanoparticles is selected, which can prevent the waste caused by the silver nanoparticles being filtered out together with the filtered waste liquid due to the too large pore size of the ceramic membrane, and at the same time avoid the low cleaning efficiency and poor cleaning effect caused by the too small pore size of the ceramic membrane. Therefore, in step S1, the pore size of the ceramic membrane tube is preferably 20 - 150 nm.

[0015] Further, in step S2, the weight ratio of hydroxyethyl cellulose to silver nanoparticles is 1:(5 - 20), preferably 1:(15 - 10).

[0016] Further, in step S3, the addition amount of the water-soluble organic solvent is 0.5 times - 10 times the mass of the silver nanosol.

[0017] Further, the time for ultrasonic dispersion is preferably 30 - 90 minutes.

[0018] Further, in the step S3, the water-soluble organic solvent is selected from alcohols and ketone organic solvents that can be miscible with water, in which PVP is easily soluble and hydroxyethyl cellulose is insoluble.

[0019] Compared with the prior art, the present invention has the following beneficial effects:

[0020] In the process for cleaning nano silver of the present invention, most of the organic components can be removed by ceramic membrane cleaning. Adding hydroxyethyl cellulose to re-treat the surface of nano silver, ultrasonic dispersion and gradually adding an appropriate amount of water-soluble organic solvent can dissolve PVP, and hydroxyethyl cellulose cannot be dissolved in the mixed solution and precipitates out of the solvent with nano silver particles. The decomposition temperature of hydroxyethyl cellulose is 200 - 210 °C, which does not affect the use performance of nano silver and can maintain the dispersibility of nano silver at the same time.

[0021] Cleaning nano silver according to the method of the present invention overcomes the difficulty of purifying and separating PVP from nano silver. It does not require long-time ceramic membrane cleaning treatment, reduces the energy consumption during the cleaning process, and saves costs. The impurity content of the cleaned nano silver is very low, the loss on ignition at 300 °C ≤ 5%, and the loss on ignition at 600 °C ≤ 6%. Moreover, the prepared nano silver after cleaning is stored as a solid phase, which is convenient for subsequent use. At the same time, it broadens the application scenarios of nano silver and provides the possibility for the industrial development of nano silver in fields such as conductive inks, electronic pastes, catalyst materials, and antibacterial materials. Description of the Drawings

[0022] Figure 1 is a schematic diagram of a ceramic membrane separation device;

[0023] Figure 2 is a schematic diagram of the corresponding permeation flux relationship of the nano silver mixed solution used in Example 1 at different dilution concentrations;

[0024] Figure 3 is a comparison chart of filtering waste liquid using ceramic membrane tubes with different pore sizes;

[0025] Figure 4 is a transmission electron microscope picture (TEM image) of different samples in Example 1 and Comparative Example 1;

[0026] Figure 5 is a TG chart of different samples in Example 1 and Comparative Example 2.

[0027] Symbol Explanation: 1 - nano silver mixed solution, 2 - ceramic membrane, 3 - nano silver concentrate, 4 - flowmeter, 5 - pump, 6 - waste liquid. Detailed Embodiments

[0028] The present invention will be further described in detail below in conjunction with specific embodiments.

[0029] In the following embodiments, the structure of the ceramic membrane separation equipment is as Figure 1 shown. The nano-silver mixed solution (stock solution) is pumped to the ceramic membrane for separation through a pump. The nano-silver concentrate obtained after separation is recycled back to the loading hopper of the stock solution through cleaning, and water or alcohol washing solution is added to the loading hopper for cyclic cleaning; the original solution pumped by the pump can also be directly returned to the stock solution loading hopper as needed. A flow meter is provided on the waste liquid discharge pipeline to observe the permeation flux of the solution.

[0030] Example 1

[0031] A cleaning method for nano-silver of the present invention includes the following steps:

[0032] Prepare a nano-silver mixed solution with a particle size of 20 - 100 nm in an aqueous phase by a reduction method, where the silver content in the mixed solution is 3.4 wt%, and the contents of impurities such as PVP, Na + , NO3 — are 24.3%. Dilute it with deionized water until the non-solvent component accounts for 10 wt% of the total amount, and adjust the pH value of the solution to 7; take 2 L of the diluted mixed solution and add it to the feed tank of the ceramic membrane separation equipment. Select a ceramic membrane tube with an outer diameter of 30 mm, an inner diameter of the membrane tube channel of 4 mm, a membrane tube length of 1178 mm, and a pore diameter of 50 nm for filtration and cleaning. After cleaning for 2 h, take 2 ml of the waste liquid, add 2 ml of 1 mol / L HCl solution, and add 2 ml of 0.02 mol / L potassium dichromate as an indicator. No precipitation is formed in the waste liquid. Stop cleaning, and label the nano-silver concentrate sample after cleaning as A;

[0033] Take a part of sample A and place it in a glass beaker for ultrasonic oscillation. Add hydroxyethyl cellulose, and the weight ratio of the added hydroxyethyl cellulose to the nano-silver in the solution is 1:10. Wait until the hydroxyethyl cellulose is completely dissolved, and gradually add an ethanol solution twice the weight of the original solution (the solution in the beaker at this time). Keep ultrasonic for 30 minutes, remove the supernatant, rinse the separated nano-silver with the ethanol solution twice, and place it in a freeze dryer for drying. Label the sample as B.

[0034] Example 2

[0035] A cleaning method for nano-silver of the present invention includes the following steps:

[0036] Prepare a nano-silver mixed solution with a particle size of 20 - 100 nm in an aqueous phase by a reduction method, where the silver content in the mixed solution is 3.4 wt%, and the contents of impurities such as PVP, Na + , NO3 —The content of impurities such as

[0037] Take the cleaned nano - silver concentrated solution and place it in a glass beaker for ultrasonic oscillation. Add hydroxyethyl cellulose, and the weight ratio of the added hydroxyethyl cellulose to the nano - silver in the solution is 1:5. After all the hydroxyethyl cellulose is dissolved, gradually add an ethanol solution 4 times the weight of the original solution (the solution in the beaker at this time), keep ultrasonic oscillation for 60 minutes, remove the supernatant, rinse the separated nano - silver with the ethanol solution twice, and place it in a freeze - dryer for drying to obtain solid - phase nano - silver.

[0038] Example 3

[0039] A cleaning method for the nano - silver of the present invention includes the following steps:

[0040] Prepare a nano - silver mixed solution with a particle size of 20 - 100 nm in an aqueous phase by a reduction method. The silver content in the mixed solution is 3.4 wt%, and the content of impurities such as PVP, Na + , NO3 — is 24.3%. Dilute it with deionized water until the non - solvent component accounts for 20 wt% of the total amount, and adjust the pH value of the solution to 7. Take 2 L of the diluted mixed solution and add it to the feed tank of the ceramic membrane separation equipment. Select a ceramic membrane tube with an outer diameter of 30 mm, an inner diameter of the membrane tube channel of 4 mm, a membrane tube length of 1178 mm, and a pore diameter of 140 nm for filtration and cleaning. After cleaning for 2 h, take 2 ml of the waste liquid, add 2 ml of 1 mol / L HCl solution, and add 2 ml of 0.02 mol / L potassium dichromate as an indicator. No precipitate is formed in the waste liquid. Stop cleaning;

[0041] Take the cleaned nano - silver concentrated solution and place it in a glass beaker for ultrasonic oscillation. Add hydroxyethyl cellulose, and the weight ratio of the added hydroxyethyl cellulose to the nano - silver in the solution is 1:15. After all the hydroxyethyl cellulose is dissolved, gradually add an acetone solution equal to the weight of the original solution (the solution in the beaker at this time), keep ultrasonic oscillation for 50 minutes, remove the supernatant, rinse the separated nano - silver with the ethanol solution twice, and place it in a freeze - dryer for drying to obtain solid - phase nano - silver.

[0042] Example 4

[0043] A cleaning method for the silver nanoparticles of the present invention, comprising the following steps:

[0044] Prepare a silver nanoparticle mixture with a particle size of 20 - 100 nm in an aqueous phase by a reduction method, wherein the silver content in the mixture is 3.4 wt%, and the contents of impurities such as PVP, Na + , NO3 — are 24.3%. Dilute it with deionized water until the non-solvent component accounts for 5 wt% of the total amount, and adjust the pH value of the solution to 7. Take 2 L of the diluted mixture and add it to the feed tank of a ceramic membrane separation device. Select a ceramic membrane tube with an outer diameter of 30 mm, an inner diameter of the membrane tube channel of 4 mm, a membrane tube length of 1178 mm, and a pore diameter of 30 nm for filtration and cleaning. After cleaning for 2 h, take 2 ml of the waste liquid, add 2 ml of 1 mol / L HCl solution, and add 2 ml of 0.02 mol / L potassium dichromate as an indicator. No precipitate is formed in the waste liquid. Stop cleaning. Take the concentrated silver nanoparticle solution after cleaning and place it in a glass beaker for ultrasonic oscillation. Add hydroxyethyl cellulose, and the weight ratio of the added hydroxyethyl cellulose to the silver nanoparticles in the solution is 1:8. Wait until the hydroxyethyl cellulose is completely dissolved, and gradually add an acetone solution 6 times the weight of the original solution (the solution in the beaker at this time). Keep ultrasonic for 40 minutes, remove the supernatant, rinse the separated silver nanoparticles with an ethanol solution 2 times, and place them in a freeze dryer for drying to obtain solid-phase silver nanoparticles.

[0045] Example 5

[0046] A cleaning method for the silver nanoparticles of the present invention, comprising the following steps:

[0047] This example is basically the same as Example 1, except that a ceramic membrane tube with an outer diameter of 30 mm, an inner diameter of the membrane tube channel of 4 mm, a membrane tube length of 1178 mm, and a pore diameter of 100 nm is selected.

[0048] Comparative Example 1

[0049] A cleaning method for silver nanoparticles, comprising the following steps:

[0050] Dilute a silver nanoparticle mixture with a particle size of 20 - 100 nm prepared by a reduction method in an aqueous phase with deionized water until the non-solvent component accounts for 10 wt% of the total amount, wherein the silver content in the original solution is 3.4 wt%, and the contents of impurities such as PVP, Na + , NO3 - are 24.3%. Take the diluted solution and place it in a centrifuge for centrifugal separation. Set the centrifugal speed to 4200 rpm and the centrifugal time to 30 min. After centrifugation, label the sample as C.

[0051] Comparative Example 2

[0052] A method for cleaning nano silver, comprising the following steps:

[0053] The nano silver mixture solution with a particle size of 20 - 100 nm prepared by a reduction method in an aqueous phase is diluted with deionized water until the non-solvent component accounts for 10 wt% of the total amount. The silver content in the original solution is 3.4 wt%, and the impurity contents of PVP, Na + , NO3 - etc. are 24.3%. Take this diluted solution and place it in a centrifuge for centrifugal separation. The centrifugal speed is set at 8000 rpm, and the centrifugal time is set at 30 min. After centrifugation, the sample is labeled as D.

[0054] Analyze the samples in the above examples and comparative examples as follows:

[0055] The dilution concentrations of the nano silver mixture solutions in Examples 1 - 4 are different. A ceramic membrane tube with a pore size of 50 nm is used to clean the nano silver solutions with different dilution concentrations. Figure 2 It is a schematic diagram of the corresponding permeation flux relationship of the nano silver mixture solution at different dilution concentrations (the abscissa is the concentration of the non-solvent component after dilution, and the ordinate is the permeation flux).

[0056] Through Figure 2 It can be seen that the permeation flux decreases as the content of the non-solvent component in the diluted solution increases. When the content of the non-solvent component in the diluted solution is 15 wt% - 20 wt%, the decreasing trend of the permeation flux is obvious. When the content of the non-solvent component in the diluted solution is less than 15 wt%, the waste liquid will quickly permeate through the ceramic membrane, improving the cleaning rate of the ceramic membrane. When the content of the non-solvent component in the diluted solution decreases from 10 wt% to 5 wt%, the increase in the permeation flux becomes not obvious, a large amount of diluted solution needs to be added, the total amount of substances to be cleaned increases, and the cleaning efficiency decreases. Therefore, it is preferably diluted until the concentration of the non-liquid component is 5 - 15 wt%.

[0057] Figure 3 It is a comparison diagram of filtering waste liquid using ceramic membrane tubes with different pore sizes in Example 1 and Example 5. Among them, a1 is a picture of the waste liquid collected after ceramic membrane separation in Example 1, and b1 is a picture of the waste liquid collected in Example 5 after the pore size of the ceramic membrane is adjusted to 100 nm. The waste liquid of b1 is darker in color. It can be seen that choosing a ceramic tube with a larger pore size has a better cleaning effect, but the nano particles are likely to flow out with the waste liquid. Therefore, it is preferably a ceramic tube with a pore size of the ceramic membrane close to the particle size of the nano particles.

[0058] Figure 4They are transmission electron microscope images (TEM images) of silver nanoparticles collected using different cleaning processes, used to analyze the agglomeration of silver nanoparticles caused by different cleaning processes. A2 is the transmission electron microscope image (TEM image) of the untreated silver nanoparticle mixture sample in Example 1, b2 is the transmission electron microscope image (TEM image) of Sample A in Example 1, c2 is the transmission electron microscope image (TEM image) of Sample C in Comparative Example 1, d2 is the transmission electron microscope image (TEM image) of Sample B in Example 1; it can be seen that cleaning Sample C by centrifugation results in severe agglomeration; while Sample B obtained by the cleaning method of the present invention still has good dispersibility and no agglomeration occurs.

[0059] Figure 5 They are TG diagrams of different samples in Example 1 and Comparative Example 2. a3 is the thermogravimetric analysis of silver nanoparticles in the silver nanoparticle mixture in Example 1, b3 is the thermogravimetric analysis of silver nanoparticles after dilution of the silver nanoparticle mixture, c3 is the thermogravimetric analysis of Sample D in Comparative Example 2, d3 is the thermogravimetric analysis of Sample A in Example 1, e3 is the thermogravimetric analysis of Sample B in Example 1. It can be seen that Sample B obtained by the cleaning method of the present invention has extremely small losses at high temperatures (basically losses caused by the decomposition of hydroxyethyl cellulose), proving that there are very few residual impurities after cleaning, and the effect is significantly better than that of ordinary cleaning methods.

Claims

1. A cleaning method for nano silver, characterized in that, It includes the following steps: S0. Dilute the nano silver solution and adjust the pH value of the solution to 6.5 - 7.5; the nano silver solution is a nano silver dispersion prepared by a liquid phase reduction method using polyvinylpyrrolidone as a dispersant. S1. Use a ceramic membrane tube to clean the nano silver solution. The nano silver solution is pumped to the ceramic membrane tube for separation. The obtained nano silver concentrate after separation flows back to the charging hopper of the original solution through the cleaning cycle. Then, water or an alcohol washing solution is added to the charging hopper, and the cleaning is carried out in a cycle to obtain the nano silver concentrate. S2. Add hydroxyethyl cellulose to the nano silver solution after cleaning. S3. Ultrasonically disperse the nano silver solution containing hydroxyethyl cellulose. While ultrasonically dispersing, gradually add a water-soluble organic solvent to the nano silver solution to precipitate the nano silver; remove the supernatant, and obtain nano silver particles after washing and drying.

2. The cleaning method of silver nanoparticles according to claim 1, characterized in that, In the step S0, the concentration of the non-solvent component in the diluted nano silver solution is 0 - 20 wt%.

3. The cleaning method of silver nanoparticles according to claim 1, characterized in that, The pore size of the ceramic membrane tube is 20 - 150 nm.

4. The cleaning method of silver nanoparticles according to claim 1, wherein, In the step S2, the weight ratio of hydroxyethyl cellulose to nano silver is 1:(5 - 20).

5. The cleaning method of silver nanoparticles according to claim 1, wherein In the step S3, the addition amount of the water-soluble organic solvent is 0.5 times to 10 times the mass of the nano silver solution.

6. The cleaning method of silver nanoparticles according to claim 1, characterized in that, In the step S3, the time of ultrasonic dispersion is 30 - 90 minutes.

7. The cleaning method of silver nanoparticles according to claim 1, wherein In the step S3, the water-soluble organic solvent is an alcohol organic solvent and / or a ketone organic solvent.

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