Preparation method of silver-coated copper nanowire

Abstract

The invention discloses a preparation method of a silver-coated copper nanowire, and belongs to the technical field of nano material preparation. According to the silver-coated copper nanowire, a copper nanowire serves as an inner core, the outer side of the inner core is coated with a compact and continuous silver shell layer, and the preparation method comprises the steps that firstly, under the protection of inert atmosphere, long-chain alkylamine serves as a reducing agent and an end-capping reagent, and the copper nanowire with the high length-diameter ratio is synthesized through a thermal reduction method; and then, a chemical reduction deposition method is adopted, ascorbic acid serves as a reducing agent, a silver salt solution is slowly dropwise added into the copper nanowire dispersion liquid, and in-situ reduction and deposition are conducted on the surface of the copper nanowire to form a uniform silver shell layer. The preparation method effectively solves the problems that the copper core is easy to oxidize and the silver shell layer is discontinuous, and the prepared silver-coated copper nanowire has the characteristics of low cost, high conductivity, high oxidation resistance and excellent dispersity, and is very suitable for preparing a high-performance and flexible transparent conductive film.

Description

Technical Field

[0001] This invention belongs to the field of nanomaterial preparation technology, and particularly relates to a method for preparing silver-coated copper nanowires. Background Technology

[0002] Transparent conductive films are key materials for touch screens, liquid crystal displays, solar cells, and flexible electronic devices. Currently, indium tin oxide (ITO) is the most widely used transparent conductive material, but it suffers from drawbacks such as scarce indium resources, brittleness, and high manufacturing costs.

[0003] Silver nanowires, as an ideal alternative to ITO, possess high conductivity, high light transmittance, and good flexibility. However, silver, as a precious metal, is expensive, and its performance deteriorates easily due to reactions with sulfides in the air. Copper nanowires have conductivity close to that of silver and are much cheaper, but they are easily oxidized in air, leading to a rapid decrease in conductivity, which severely limits their practical applications. To balance cost and stability, researchers have proposed the concept of silver-coated copper nanowires, which involve wrapping a copper core with a thin layer of silver. The chemical stability of silver protects the internal copper, while its high conductivity ensures overall performance. Existing methods for preparing silver-coated copper nanowires, such as the galvanic displacement method, are simple, but the reaction is difficult to control, easily leading to discontinuous and porous silver shells, or even corrosion of the copper core, forming defects and accelerating the oxidation of the copper core. Therefore, developing a method to prepare silver-coated copper nanowires with a dense and continuous silver shell is a pressing technical problem to be solved in this field. Summary of the Invention

[0004] The purpose of this invention is to provide a method for preparing silver-coated copper nanowires, aiming to solve the problems in the prior art where the electrocouple displacement method is difficult to control, easily leading to discontinuous and porous silver shells, and even corrosion of the copper core, forming defects and accelerating copper core oxidation. To achieve the above objective, the technical solution adopted by this invention is: a method for preparing silver-coated copper nanowires, comprising the following steps performed sequentially.

[0005] Synthesis of S1 and copper nanowires

[0006] The copper salt precursor was mixed with a long-chain alkylamine and reacted at 50-150℃ for 15-60 hours. After the reaction was completed, the mixture was cooled, centrifuged, and washed to obtain purified copper nanowires.

[0007] S2, Silver shell layer coating

[0008] The copper nanowires are redispersed in an organic solvent to form a dispersion; the silver salt and reducing agent are prepared into solutions respectively, and under stirring conditions, the silver salt solution and reducing agent solution are slowly and synchronously added dropwise to the copper nanowire dispersion to carry out the coating reaction for 0.5-2 hours;

[0009] S3, Post-processing

[0010] After the reaction was completed, the product was centrifuged, washed, and redispersed in a suitable solvent for storage.

[0011] Further, in step S1, the copper salt precursor is one of copper acetate, copper chloride, or copper nitrate; the long-chain alkylamine is one of oleylamine, hexadecylamine, or octadecylamine.

[0012] Preferably, the volume / mass ratio of the copper salt precursor to the long-chain alkylamine satisfies the following: 0.1-0.5 g of copper salt precursor per 100 mL of long-chain alkylamine.

[0013] Furthermore, in step S1, a halide additive is added simultaneously during mixing, wherein the halide additive is one of sodium chloride, potassium bromide or potassium iodide.

[0014] Preferably, the molar ratio of the halide additive to the copper salt precursor is 1:50 to 1:200.

[0015] Furthermore, in step S1, a reducing agent is added simultaneously during mixing. The reducing agent is one of glucose, ascorbic acid, or hydrazine hydrate, and its molar ratio with the copper salt precursor is 1:1 to 1:100.

[0016] Furthermore, the organic solvent is one of hexane, toluene, or chlorobenzene.

[0017] Furthermore, the silver salt is silver nitrate; the reducing agent is one of ascorbic acid, glucose, or sodium borohydride.

[0018] Furthermore, in step S2, the molar ratio of silver element in the silver salt solution to copper nanowire core in step S1 is 1:1 to 3:1.

[0019] Furthermore, in step S2, the silver salt solution and reducing agent solution are added dropwise using a dual-channel syringe pump or a separatory funnel, with the dropping rate controlled at 1-5 mL / min.

[0020] Preferably, the molar ratio of the reducing agent solution to the silver salt solution is 1.5:1 to 2.5:1 to ensure that the silver ions are fully reduced.

[0021] Furthermore, the coating reaction is carried out with the assistance of ultrasound, with an ultrasound power of 50-200W and a frequency of 40kHz.

[0022] Furthermore, in step S3, the washing process involves alternating washing with ethanol and acetone at least three times;

[0023] Preferably, the preservation solvent is ethanol, isopropanol, or water (a polymeric dispersant such as hydroxypropyl methylcellulose needs to be added).

[0024] Compared with the prior art, the present invention has the following beneficial effects:

[0025] The silver-coated copper nanowires prepared by this invention have a dense and continuous silver shell layer, which can effectively isolate the copper core from air contact, significantly improving the oxidation resistance of the nanowires and solving the core problem of easy oxidation of pure copper nanowires. This structure combines the low cost advantage of copper with the high conductivity and surface stability of silver. The transparent conductive film prepared has performance close to that of pure silver nanowire films, but the cost is significantly reduced. By precisely controlling the reaction droplet acceleration rate, the uncontrollability of the galvanocoupler method is avoided, achieving uniform and controllable coating of the silver shell layer. The process has good repeatability and is easy to scale up. Attached Figure Description

[0026] Figure 1 The image shown is a scanning electron microscope (SEM) image of the silver-coated copper nanowires prepared in Example 1 of the present invention, which clearly shows the smoothness of the surface of the silver-coated copper nanowires.

[0027] Figure 2 This is an EDS point scan elemental analysis diagram of the silver-coated copper nanowires prepared in Example 1 of this invention;

[0028] Figure 3 The image shown is a transmission electron microscope (TEM) image of the silver-coated copper nanowires prepared in Example 1 of this invention, which clearly shows the core-shell structure of the copper core and the silver shell.

[0029] Figure 4 This is a scanning electron microscope (SEM) image of the silver-coated copper nanowires prepared in Example 2 of the present invention;

[0030] Figure 5 This is a scanning electron microscope (SEM) image of the silver-coated copper nanowires prepared in Example 3 of the present invention. Detailed Implementation

[0031] To facilitate understanding of the present invention, a more complete description will be given below with reference to the accompanying drawings. Preferred embodiments of the invention are shown in the drawings. However, the invention can be implemented in many different forms and is not limited to the embodiments described herein. Rather, these embodiments are provided to provide a thorough and complete understanding of the disclosure of the invention.

[0032] Example 1

[0033] A method for preparing silver-coated copper nanowires includes the following steps performed sequentially:

[0034] Synthesis of S1 and copper nanowires

[0035] Add 5.4 g of hexadecylamine, 0.99 g of glucose, and 400 mL of deionized water sequentially to a 250 mL three-necked round-bottom flask. Remove the flask from the glove box and connect it to a synthesis apparatus equipped with a magnetic stirrer and a condenser. Stir at 200 rpm and heat the reaction system to 70 °C at a heating rate of 5 °C / min, maintaining this temperature for 12 hours. The solution color gradually changes from blue to a turbid brownish-red, and finally to a dark red, indicating the formation of copper nanowires. After the reaction is complete, allow it to cool naturally to room temperature. Transfer the reaction mixture to a centrifuge tube, add an equal volume of n-hexane as a diluent, and centrifuge at 8000 rpm for 5 minutes. Remove the supernatant and wash the precipitate three times with anhydrous ethanol (centrifuging at 8000 rpm for 5 minutes each time) to thoroughly remove excess oleylamine and reaction byproducts. The resulting copper nanowire precipitate is redispersed in 50 mL of n-hexane to form dispersion A with a concentration of approximately 2 mg / mL. Seal and store in a sample vial for later use.

[0036] S2, Silver shell layer coating

[0037] Take approximately 32 mL of dispersion A, equivalent to 0.1 mmol of copper nanowires (approximately 6.4 mg), and place it in a 100 mL three-necked flask. Stir gently to allow the solvent to evaporate slightly to a total volume of approximately 25 mL, increasing the concentration of copper nanowires. Prepare the coating solutions: Solution B is 0.2 mmol of silver nitrate dissolved in 5 mL of deionized water; Solution C is 0.4 mmol of ascorbic acid dissolved in 5 mL of deionized water. Add solutions B and C, respectively, simultaneously and slowly dropwise to the vigorously stirred copper nanowire dispersion A at a rate of 2 mL / min. The entire dropwise addition process should last approximately 5 minutes. After the addition is complete, continue the reaction for 1 hour. The solution color will gradually change from dark red to grayish-black, indicating that the silver has been reduced and coated onto the surface of the copper nanowires.

[0038] S3, Post-processing

[0039] After the reaction was completed, the mixture was allowed to cool naturally. The reaction solution was then transferred to a centrifuge tube and centrifuged at 6000 rpm for 10 minutes. The supernatant was removed, and the precipitate was washed twice with ethanol and once with acetone (under the same centrifugation conditions each time) to remove aqueous residues and unreacted ions. The resulting silver-coated copper nanowire precipitate was dispersed in 20 mL of anhydrous ethanol to form a uniform dispersion with a concentration of approximately 0.5 mg / mL.

[0040] The system consists of a copper nanowire core and a silver shell covering the core. The silver shell is dense and continuous, with a face-centered cubic crystal structure, and exhibits epitaxial growth with the copper nanowire core. The copper nanowire core has a diameter of 20-100 nm, a length of 30-150 μm, and an aspect ratio greater than 300. The silver shell has a thickness of 5-20 nm, preferably 8-15 nm, and the ratio of the silver shell thickness to the diameter of the copper nanowire core is 1:4 to 1:5. The overall diameter of the silver-coated copper nanowire is 30-140 nm. A transition layer, a copper-silver alloy with a thickness of 1-3 atomic layers, exists between the copper nanowire core and the silver shell.

[0041] Example 2

[0042] Adjusting the amount of silver halide used to control the thickness of the silver shell layer;

[0043] This embodiment aims to investigate the effect of silver salt dosage on the thickness of the silver shell and the performance of the final product. Its preparation process is basically the same as that of Example 1, with the core difference being the amount of silver salt used in step S2.

[0044] Synthesis of S1 and copper nanowires

[0045] The steps and parameters are exactly the same as in Example 1, and dispersion A is obtained.

[0046] S2, Silver shell layer coating

[0047] Take a dispersion A (approximately 32 mL) equivalent to 0.1 mmol of copper and place it in a 100 mL three-necked flask. In this experiment, the amount of silver salt (silver nitrate) was significantly increased. Solution B consisted of 0.4 mmol of silver nitrate (68 mg) dissolved in 5 mL of deionized water (i.e., silver / copper molar ratio = 4:1), and solution C consisted of 0.8 mmol of ascorbic acid (140.8 mg) dissolved in 5 mL of deionized water. The molar ratio of reducing agent to silver was kept at 2:1. The dropping and reaction process was exactly the same as in Example 1 (dropping rate 2 mL / min, reaction time 1 hour).

[0048] S3, Post-processing

[0049] After the reaction was completed, the reaction solution was transferred to a centrifuge tube and centrifuged at 6000 rpm for 10 minutes. The supernatant was removed, and the precipitate was washed twice with ethanol and once with acetone (under the same centrifugation conditions each time) to remove aqueous residues and unreacted ions. The final silver-coated copper nanowire precipitate was dispersed in 20 mL of anhydrous ethanol to form a uniform dispersion with a concentration of approximately 0.5 mg / mL.

[0050] The resulting silver-coated copper nanowires had a large number of silver particles adsorbed on their surface and continued to grow, with the silver layer reaching a thickness of 15 nm.

[0051] Example 3

[0052] Silver-coated copper nanowires were prepared using different reducing agents;

[0053] This embodiment aims to illustrate that the method described in this invention has a certain degree of universality in terms of the types of reducing agents, and is not limited to ascorbic acid.

[0054] Synthesis of S1 and copper nanowires

[0055] The steps and parameters are exactly the same as in Example 1, and dispersion A is obtained.

[0056] S2, Silver shell layer coating

[0057] Silver coating: Take dispersion A (approximately 32 mL) equivalent to 0.1 mmol of copper and process it as before. In this experiment, glucose was used as the reducing agent. Solution B consisted of 0.2 mmol of silver nitrate (34 mg) dissolved in 5 mL of deionized water. Solution C consisted of 0.6 mmol of glucose (108 mg) dissolved in 5 mL of deionized water (glucose / silver molar ratio = 3:1). Since the reducing power of glucose is weaker than that of ascorbic acid, its dosage and reaction temperature need to be increased. Solutions B and C were added dropwise simultaneously at a rate of 2 mL / min. After the addition was complete, the reaction continued for 1.5 hours.

[0058] S3, Post-processing

[0059] After the reaction was completed, the reaction solution was transferred to a centrifuge tube and centrifuged at 6000 rpm for 10 minutes. The supernatant was removed, and the precipitate was washed twice with ethanol and once with acetone (under the same centrifugation conditions each time) to remove aqueous residues and unreacted ions. The final silver-coated copper nanowire precipitate was dispersed in 20 mL of anhydrous ethanol to form a uniform dispersion with a concentration of approximately 0.5 mg / mL.

[0060] The final silver-coated copper nanowires had many silver particles adsorbed on their surface. The results of the three examples are summarized in the table below.

[0061] Example reducing agent Ag⁺ concentration Silver layer morphology Key control factors 1 ascorbic acid 0.1 mM Continuous density Matching reduction potential, slow addition 2 ascorbic acid 1.0 mM Silver particle adhesion Local supersaturation of silver ions 3 glucose 0.1 mM Sparse small particles Insufficient reduction kinetics

[0062] In summary, this invention systematically demonstrates the key technical parameters and their effects on the silver-coated copper nanowire preparation method through Examples 1 to 3. The results show that by precisely controlling the molar ratio, type, and reaction conditions of silver salt and reducing agent, silver-coated copper nanowires with continuous and dense silver shells, excellent core oxidation resistance, and stable electrical properties can be successfully prepared. Examples 2 and 3 further confirm the significant advantages of the method of this invention over the traditional galvanic displacement method in solving the problems of discontinuous silver shells and easy corrosion of copper cores.

[0063] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains. The terminology used herein in the description of this invention is for the purpose of describing particular embodiments only and is not intended to limit the invention. The term "and / or" as used herein includes any and all combinations of one or more of the associated listed items.

[0064] The following is in conjunction with the appendix Figure 1-5 This invention will be described in detail.

[0065] Specific reference Figure 1 Under weakly alkaline conditions, ascorbic acid (AA) has a moderate reduction potential, which can gently reduce silver ions. Through a substitution reaction and surface diffusion, a uniform silver layer is formed on the surface of copper nanowires. Ag⁺ forms crystal nuclei through a substitution reaction (Cu + 2Ag⁺ → Cu²⁺ + 2Ag). Ascorbic acid further reduces silver ions, causing epitaxial growth on the crystal nuclei. The silver layer is very well-coated. (See reference...) Figure 2 EDS point scan element analysis diagram.

[0066] Specific reference Figure 2 The proportion of Cu is 70.9% and the proportion of Ag is 29.1%, indicating that a core-shell structure with copper as the core and silver as the shell has been formed, and the silver layer is very uniformly coated.

[0067] Specific reference Figure 3 The morphology of silver-coated copper nanowires at 50 nm clearly shows a dense silver layer on the surface of the copper nanowires, with a thickness of approximately 8 nm. Figure 1-3 All images are of the samples from Example 1.

[0068] Specific reference Figure 4 In Example 2, increasing the amount of silver salt caused particle precipitation. Under high concentration of silver nitrate, the silver particles were unevenly deposited. Excessive silver ions caused a violent replacement reaction, and the local silver nuclei were generated too quickly. Ascorbic acid could not guide the epitaxial growth in time, thus forming silver particles instead of a continuous layer. Local silver atoms aggregated to form particles instead of layered growth.

[0069] Specific reference Figure 5 In Example 3, changing the reducing agent resulted in incomplete coating. Insufficient glucose reducing power led to incomplete coating. Glucose has a high reduction potential under alkaline conditions (-0.05 V vs. SHE), indicating a weak ability to reduce silver ions and hindering the growth of continuous silver layers. Replacing ascorbic acid with glucose solution effectively reduced the silver ions generated by the substitution reaction inhibited by Cu²⁺.

[0070] The silver-coated copper nanowires prepared by this invention have a dense and continuous silver shell layer, which can effectively isolate the copper core from air contact, significantly improving the oxidation resistance of the nanowires and solving the core problem of easy oxidation of pure copper nanowires. This structure combines the low cost advantage of copper with the high conductivity and surface stability of silver. The transparent conductive film prepared has performance close to that of pure silver nanowire films, but the cost is significantly reduced. By precisely controlling the reaction droplet acceleration rate, the uncontrollability of the galvanocoupler method is avoided, achieving uniform and controllable coating of the silver shell layer. The process has good repeatability and is easy to scale up.

[0071] The above embodiments are only used to illustrate the present invention and are not intended to limit the present invention. Those skilled in the art can make various changes and modifications without departing from the spirit and scope of the present invention. Therefore, all equivalent technical solutions also fall within the scope of the present invention, and the patent protection scope of the present invention should be defined by the claims.

Claims

1. A method for preparing silver-coated copper nanowires, characterized in that: The steps are as follows, performed sequentially. Synthesis of S1 and copper nanowires The copper salt precursor was mixed with a long-chain alkylamine and reacted at 50-150℃ for 15-60 hours. After the reaction was completed, the mixture was cooled, centrifuged, and washed to obtain purified copper nanowires. S2, Silver shell layer coating The copper nanowires are redispersed in an organic solvent to form a dispersion; the silver salt and reducing agent are prepared into solutions respectively, and under stirring conditions, the silver salt solution and reducing agent solution are slowly and synchronously added dropwise to the copper nanowire dispersion to carry out the coating reaction for 0.5-2 hours; S3, Post-processing After the reaction was completed, the product was centrifuged, washed, and redispersed in a solvent for storage.

2. The method for preparing silver-coated copper nanowires according to claim 1, characterized in that: In step S1, the copper salt precursor is one of copper acetate, copper chloride, or copper nitrate; the long-chain alkylamine is one of oleylamine, hexadecylamine, or octadecylamine; the volume / mass ratio of the copper salt precursor to the long-chain alkylamine satisfies the following: 0.1-0.5 g of copper salt precursor per 100 mL of long-chain alkylamine.

3. The method for preparing silver-coated copper nanowires according to claim 1, characterized in that: In step S1, a halide additive is also added simultaneously during mixing. The halide additive is one of sodium chloride, potassium bromide, or potassium iodide, and its molar ratio with the copper salt precursor is 1:50 to 1:

200.

4. The method for preparing silver-coated copper nanowires according to claim 1, characterized in that: In step S1, a reducing agent is also added simultaneously during mixing. The reducing agent is one of glucose, ascorbic acid, or hydrazine hydrate, and its molar ratio with the copper salt precursor is 1:1 to 1:

100.

5. The method for preparing silver-coated copper nanowires according to claim 1, characterized in that: In step S2, the organic solvent is one of hexane, toluene, or chlorobenzene.

6. The method for preparing silver-coated copper nanowires according to claim 1, characterized in that: In step S2, the silver salt is silver nitrate; the reducing agent is one of ascorbic acid, glucose, or sodium borohydride.

7. The method for preparing silver-coated copper nanowires according to claim 1, characterized in that, In step S2, the molar ratio of silver in the silver salt solution to copper nanowires in step S1 is 1:1 to 3:

1.

8. The method for preparing silver-coated copper nanowires according to claim 1, characterized in that, In step S2, the silver salt solution and the reducing agent solution are added dropwise using a dual-channel syringe pump or a separatory funnel, with the dropping rate controlled at 1-5 mL / min. The molar ratio of the reducing agent solution to the silver salt solution is 1.5:1 to 2.5:1 to ensure that the silver ions are fully reduced.

9. The method for preparing silver-coated copper nanowires according to claim 1, characterized in that, In step S2, the coating reaction is carried out with the assistance of ultrasound, with an ultrasound power of 50-200W and a frequency of 40kHz.

10. The preparation method of silver-coated copper rice noodles according to claim 1, characterized in that, In step S3, the washing process involves alternating washing with ethanol and acetone at least three times, and the solvent is ethanol, isopropanol, or water.

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