A method for preparing a flexible printed circuit using copper-nickel composite nanoparticles

Abstract

The application discloses a kind of copper-nickel composite nanoparticles to prepare flexible printed circuit method belonging to flexible printed circuit technical field.The method is first with copper-nickel composite nanoparticles of core-shell structure and is prepared into conductive ink and is printed on the surface of substrate, and line pattern is prepared;Then copper plating layer is prepared on line pattern using chemical plating method, and high-conductivity copper printed circuit is obtained;Copper-nickel composite nanoparticles can realize magnetic separation and have conductive function, simultaneously as catalyst seed of chemical plating process.The application uses magnetic copper-nickel composite nanoparticles, which can be magnetically separated, has excellent oxidation resistance, and can significantly reduce cost, and its chemical plating process is operated under low temperature atmospheric conditions of 20-60 DEG C, and can be used for the preparation of flexible printed circuit on heat-sensitive flexible substrate.

Description

Technical Field

[0001] This invention belongs to the field of flexible printed circuit technology, and particularly relates to a method for preparing flexible printed circuits using core-shell copper-nickel composite nanoparticles. Background Technology

[0002] In recent years, flexible printed electronics technology has flourished, bringing significant progress to advanced electronic product manufacturing, smart wearable devices, and the information industry. Compared with traditional electronic manufacturing technologies, the advantages of flexible printed electronics technology mainly stem from its superior intelligence and integration. Most importantly, its strong flexibility, lightweight nature, and shape designability give flexible printed electronics technology significant and unique advantages—advantages not found in conventional etching techniques.

[0003] Currently, flexible printed electronics technology is mainly used to fabricate electronic devices such as flexible displays, RFID tags, sensors, and capacitors. In these applications, flexible printed circuits (FPCs) are the foundation and core components. FPCs are fabricated by printing conductive materials or precursors onto a flexible substrate and then performing post-processing. Printing methods mainly include inkjet printing, screen printing, and flexographic printing. Recently, FPC fabrication processes have been extensively researched and applied, including methods based on conductive inks and electroless plating. Conductive ink is printed onto a flexible substrate, and conductive patterns are obtained through sintering. Conductive ink consists of metal (Ag or Cu) nanoparticles or metal-organic complexes. In electroless plating, metal ions in the plating bath are reduced to metals and deposited on the substrate to form conductive patterns. Traditionally, Pd, Sn, or Ag are used as catalyst seeds for electroless plating, and formaldehyde is used as a reducing agent. However, both conductive ink processes and electroless plating processes have some drawbacks. Conductive ink processes generally require high-temperature (>200℃) sintering, which is unsuitable for heat-sensitive substrates such as PET and fiber paper. Therefore, many researchers have developed alternative sintering methods, such as plasma sintering and photolithography; however, because the non-conductive materials contained in the ink are not completely removed, the conductive patterns prepared by these sintering methods have relatively high resistance. Furthermore, the limited thickness of the lines printed with conductive ink, due to the limited content of conductive components in the ink, also leads to relatively high resistance in the conductive patterns. For electroless plating processes, formaldehyde, a common reducing agent, is volatile and carcinogenic, and formaldehyde-containing wastewater poses a catastrophic environmental hazard. In addition, precious metal catalysts increase the cost of electroless plating processes.

[0004] Compared to Ag, Cu is much cheaper and has a similar resistance. However, a significant drawback is that Cu is particularly prone to oxidation at the nanoscale, which increases the resistance of conductive copper patterns. In our previous study (Ind. Eng. Chem. Res. 2018, 57, 2508−2516), we synthesized core-shell structured silver-coated copper nanoparticle conductive inks and successfully fabricated flexible printed circuits. The silver-coated copper nanoparticles exhibited good oxidation resistance, but the conductivity of the fabricated circuits was relatively poor compared to bulk copper (resistivity of 25.3 μΩ cm, 14.88 times that of bulk copper). Fu-Tao Zhang et al. (ACS Appl. Mater. Interfaces 2018, 10, 2075−2082) prepared highly conductive Cu-PDMS conductors using silver ions as initiators and formaldehyde as a reducing agent in a chemical plating bath, with the synergistic effect of polydopamine. However, this process is complex and uses volatile formaldehyde as a reducing agent. Jingxuan Cai et al. (ACS Appl. Mater. Interfaces 2018, 10, 28754−28763) used noble metal Pd nanoparticles as catalysts and formaldehyde as a reducing agent in the chemical plating bath to prepare transparent electrodes with a light transmittance greater than 80% and a sheet resistance less than 1 Ω / sq through photolithography, hot pressing printing, and chemical plating. However, the electrodes prepared by this process have a large resistance, and it still uses noble metal Pd as a catalyst seed and formaldehyde as a reducing agent in the chemical plating bath.

[0005] Patent CN 108866521 B developed a formaldehyde / sodium hypophosphite electroless copper plating solution for copper plating on PCB boards. The advantage of this method is that it does not contain cyanide stabilizers, but the reducing agent still contains formaldehyde. Patent CN 105821396 A developed a palladium-free electroless plating process that uses dopamine to reduce silver ions as silver particles as a catalyst seed layer to prepare conductive copper layers on various substrates. However, this process is complex and requires the use of precious metal silver as an activator.

[0006] Based on existing research and reports, copper printed circuits prepared by direct sintering of conductive copper ink generally have high resistance and cannot be used on heat-sensitive flexible substrates. Conductive patterns prepared using electroless copper plating generally have lower resistance; however, most current electroless plating research uses noble metals such as Pd, Pt, and Ag as catalyst seeds and formaldehyde as a reducing agent in the electroless plating bath. This increases the cost of the process, and the large-scale use of volatile, carcinogenic formaldehyde poses a threat to human health and the environment. Summary of the Invention

[0007] To address the above problems, this invention proposes a method for fabricating flexible printed circuits using copper-nickel composite nanoparticles, comprising the following steps:

[0008] First, copper-nickel composite nanoparticles with a core-shell structure and copper-coated nickel are formulated into conductive ink and printed on the surface of a substrate to create circuit patterns.

[0009] Then, a copper plating layer is prepared on the circuit pattern using chemical plating to obtain a highly conductive copper printed circuit.

[0010] Copper-nickel composite nanoparticles can achieve magnetic separation and have electrical conductivity, while also serving as catalyst seeds for electroless plating. Copper-nickel composite nanoparticles can utilize the electrical and magnetic properties of nickel metal, making them easy to recycle; the copper-encapsulated structure also overcomes the defect of pure nickel being difficult to disperse uniformly.

[0011] The preparation method of copper-nickel composite nanoparticles is as follows:

[0012] Copper-nickel composite nanoparticles with a copper-coated nickel core-shell structure were prepared by thermal decomposition of nickel acetate-oleylamine complex and copper formate-oleylamine complex in liquid paraffin.

[0013] Furthermore, the specific preparation method of copper-nickel composite nanoparticles is as follows: nickel acetate and oleylamine are mixed and added to liquid paraffin, and complexed at 40°C~60°C under nitrogen atmosphere for 1h~2h. Then, the temperature is raised to 220°C and reacted for 30min. After natural cooling to 80°C, the complex of copper formate and oleylamine is added and reacted for 15h. After the reaction, the particles are separated from the solution by magnetic force and washed with an equal volume of n-hexane and isopropanol mixed solvent to obtain copper-nickel composite nanoparticles with a copper-coated nickel core-shell structure, with a particle size of 30nm~60nm and the surface is coated with oleylamine.

[0014] The method for preparing conductive ink is as follows: copper-nickel composite nanoparticles are dispersed in a solvent to prepare copper-nickel composite nanoparticle conductive ink with a solid content of 0.1wt%~10wt%; the solvent is n-hexane, n-octane, petroleum ether, turpentine or a mixture of the above solvents.

[0015] The substrates include PET, PI, or PEN;

[0016] The substrate undergoes a pretreatment process before use: after washing with deionized water and ethanol, it is dried at 50°C~60°C, immersed in an ethanol solution containing 1wt%~10wt% silane for 1h~6h, then washed with ethanol multiple times to remove residues, and dried at 50°C~60°C for 6h.

[0017] The silane is 3-mercaptotriethoxysilane or 3-aminotriethoxysilane.

[0018] The printing methods for conductive inks include inkjet printing, screen printing, gravure printing, letterpress printing, drop coating, scraping coating, or coating to prepare circuit patterns.

[0019] Before electroless plating, the substrate for printing circuit patterns is placed in a 1wt%~5wt% formic acid solution and washed for 1~5 minutes to remove the oleamine coating on the surface of copper nanoparticles. Then it is taken out, washed with deionized water and dried. Further, the solvent of the formic acid solution is ethanol or methanol.

[0020] The chemical plating method is as follows: the substrate with printed circuit patterns and treated with de-oiling amine is placed in the chemical plating solution and chemically plating copper at 20°C~60°C for 1h~6h. After removal, it is washed multiple times with deionized water and dried at 50°C~60°C for 6h to obtain a highly conductive copper printed circuit.

[0021] The composition and concentration of the chemical plating solution are as follows: dimethylaminoborane (DMAB, 0.01 mol / L~0.5 mol / L), ethylenediaminetetraacetic acid (EDTA, 0.001 mol / L~0.1 mol / L), triethanolamine (TEA, 0.05 mol / L~1.2 mol / L), copper sulfate pentahydrate (0.001 mol / L~0.1 mol / L), and deionized water.

[0022] The electroless plating solution is recycled after being replenished with copper salt and reducing agent; preferably, the replenished copper salt is copper sulfate pentahydrate and the reducing agent is dimethylaminoborane.

[0023] The beneficial effects of this invention are as follows:

[0024] 1. This invention addresses the shortcomings of existing conductive inks and electroless plating methods by providing a method for fabricating highly conductive copper printed circuits. This method utilizes inexpensive and magnetically separable magnetic copper-coated nickel core-shell copper-nickel composite nanoparticles as the catalyst seed for electroless plating, replacing traditional precious metals. Combined with a non-formaldehyde electroless plating process, this provides a method for fabricating highly conductive copper printed circuits. The copper-nickel composite nanoparticles are relatively inexpensive compared to precious metals, possess oxidation resistance, are magnetic, and can achieve magnetic separation instead of centrifugal separation.

[0025] 2. This invention uses dimethylaminoborane instead of the traditional highly toxic formaldehyde as a reducing agent, enabling electroless copper plating under weakly alkaline and mild conditions. The electroless plating solution is formaldehyde-free, achieving long-term stability and allowing for recycling by replenishing reactants.

[0026] 3. The prepared copper printed circuit has high conductivity, strong adhesion, and good oxidation resistance;

[0027] 4. The preparation process is simple and easy to implement, environmentally friendly, and suitable for large-scale production;

[0028] 5. The preparation process of this invention is carried out under medium and low temperature (20~60°C) and atmospheric conditions. The equipment is simple and avoids the influence of high temperature sintering and other operations on the thermosensitive flexible substrate. It can be used for the preparation of printed circuits on thermosensitive flexible substrates. Attached Figure Description

[0029] Figure 1 This is a transmission electron microscope image of copper-coated nickel nanoparticles;

[0030] Figure 2 This is the XRD pattern of copper-coated nickel nanoparticles;

[0031] Figure 3 It is an optical photograph of the printed circuit being fabricated;

[0032] Figure 4 These are conductivity test images of the fabricated printed circuit.

[0033] Figure 5 These are scanning electron microscope images of the copper film of the fabricated printed circuit.

[0034] Figure 6 This is the relationship between the sheet resistance of the conductive copper film in Case 1 and time.

[0035] Figure 7 This is the relationship between the sheet resistance of the conductive copper film and time in Case 2;

[0036] Figure 8 The amount of copper sulfate pentahydrate and DMAB replenished during the cyclic experiment using the conductive copper printed circuit obtained in Example 3;

[0037] Figure 9 It is a sheet resistor used in the cyclic experiment process of the conductive copper printed circuit obtained in Example 3. Detailed Implementation

[0038] The present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments:

[0039] 1) Core-shell copper-coated nickel composite nanoparticles with a particle size of 30 nm to 60 nm were prepared by thermal decomposition of nickel acetate-oleylamine complex and copper formate-oleylamine complex in liquid paraffin.

[0040] 2) Disperse oleylamine-coated copper-nickel composite nanoparticles in a solvent to prepare a conductive ink with a solid content of 0.1wt%~10wt%;

[0041] 3) Wash the heat-sensitive flexible substrate multiple times with deionized water and ethanol, dry it at 50°C~60°C, immerse it in an ethanol solution containing 1wt%~10wt% silane for 1h~6h, then wash it multiple times with ethanol to remove residues, and dry it at 50°C~60°C for 6h.

[0042] 4) The copper-nickel composite nanoparticle conductive ink is printed onto the surface of the modified substrate using an inkjet printer or by coating to prepare a printed pattern.

[0043] 5) Place the printed pattern in a methanol or ethanol solution containing 1wt%~5wt% formic acid and wash for 1~5 minutes to remove the oleamine coating on the surface of the nanoparticles. Then take it out, wash it several times with deionized water, and blow it dry.

[0044] 6) Place the pattern treated in 5) in a chemical plating bath and perform chemical copper plating at 20°C~60°C for 1h~6h. Remove and wash several times with deionized water, and dry at 50°C~60°C for 6h to obtain a flexible, highly conductive copper printed circuit.

[0045] The chemical plating solution in step 6) can be recycled after being replenished with copper salt and reducing agent.

[0046] Example 1

[0047] 1) 0.707 g of nickel acetate and 4.76 g of oleylamine were mixed and added to 70 mL of liquid paraffin. The mixture was complexed at 60°C under nitrogen purging for 2 h. The temperature was then raised to 220°C and reacted for 30 min. After natural cooling to 80°C, a complex of copper formate and oleylamine (0.154 g copper formate, 1.2 g oleylamine, 10 mL liquid paraffin, complexed at 60°C for 1 h) was added, and the mixture was reacted for 15 h. After the reaction, the particles were magnetically separated and washed once with a mixture of 15 mL of n-hexane and 15 mL of isopropanol to obtain copper-coated nickel core-shell copper-nickel composite nanoparticles.

[0048] 2) Weigh 0.2g of the copper-nickel composite nanoparticles with a particle size of 39nm obtained in step 1), add them to a mixture of 2mL n-hexane and 2mL n-octane, and prepare a copper-nickel composite nanoparticle conductive ink with a volume concentration of 0.05g / mL.

[0049] 3) Wash the PET substrate with deionized water and ethanol multiple times, dry it at 50°C~60°C, immerse it in an ethanol solution containing 5wt% 3-mercaptotriethoxysilane for 3 hours, then wash it with ethanol multiple times to remove residues, and dry it at 50°C~60°C for 6 hours.

[0050] 4) Take 0.1 mL of conductive ink and drop it onto the surface of a 5 cm × 5 cm PET substrate modified with 3-mercaptotriethoxysilane to prepare the printed pattern;

[0051] 5) Cut the substrate obtained in step 4) into 2cm×2cm thin slices, then soak them in an ethanol solution containing 1wt% formic acid for 2min to remove the oleamine on the surface of the copper-nickel composite nanoparticles, then take them out, clean them with deionized water and blow them dry.

[0052] 6) Add the substrate from step 5) to 10 mL of plating solution (plating bath) (copper sulfate pentahydrate: 0.004 mol / L, ethylenediaminetetraacetic acid 0.028 mol / L, triethanolamine 0.3 mol / L, dimethylamine borane 0.08 mol / L), chemically plating at 20°C for 1 h to 6 h, then remove and wash with deionized water, and treat in an oven at 60°C for 3 h to obtain conductive copper printed circuit.

[0053] like Figure 1 The transmission electron microscope (TEM) image of the copper-nickel composite nanoparticles shows that the prepared copper-nickel composite nanoparticles are of the copper-coated nickel core-shell type, are magnetic, can achieve magnetic separation, and have an average particle size of 39 nm. Figure 2 The image shows the XRD pattern of copper-nickel composite nanoparticles, with diffraction peaks for Ni, Cu, and Cu₂O. The small Cu₂O diffraction peak indicates only weak oxidation on the particle surface; no NiO peak is visible. Optical images of copper printed circuits were obtained after 6 hours of electroless copper plating on a PET surface following 3-mercaptotriethoxysilane modification. Figure 3 As shown, the copper film has a smooth surface and good adhesion. The conductivity test of the copper printed circuit is as follows: Figure 4 As shown, the circuit has good conductivity and can light a light bulb. A scanning electron microscope image of the copper layer is shown below. Figure 5 As shown, the copper film surface is dense and the structure is complete. The relationship between the sheet resistance of the copper layer and time is as follows: Figure 6 As shown, the sheet resistance of the copper film decreases significantly with increasing electroless plating time, until it tends to stabilize. The minimum sheet resistance of the copper film is 43.6 mΩ / sq, the maximum film thickness is 1.12 μm, and the minimum resistivity is 4.88 μΩ·cm (2.87 times the resistivity of bulk copper).

[0054] Example 2

[0055] 1) 0.707 g of nickel acetate and 4.76 g of oleylamine were mixed and added to 70 mL of liquid paraffin. The mixture was complexed at 60°C under nitrogen purging for 2 h. The temperature was then raised to 220°C and reacted for 30 min. After natural cooling to 80°C, a complex of copper formate and oleylamine (0.154 g copper formate, 1.2 g oleylamine, 10 mL liquid paraffin, complexed at 60°C for 1 h) was added, and the mixture was reacted for 15 h. After the reaction, the particles were magnetically separated and washed once with a mixture of 15 mL of n-hexane and 15 mL of isopropanol to obtain copper-coated nickel core-shell copper-nickel composite nanoparticles.

[0056] 2) Weigh 0.2g (particle size 39nm) of the copper-nickel composite nanoparticles obtained in 1) and add them to a mixture of 2mL n-hexane and 2mL n-octane to prepare a copper-nickel composite nanoparticle conductive ink with a volume concentration of 0.05g / mL.

[0057] 3) Wash the PET substrate with deionized water and ethanol multiple times, dry it at 50°C~60°C, immerse it in an ethanol solution containing 10wt% 3-mercaptotriethoxysilane for 1 hour, then wash it with ethanol multiple times to remove residues, and dry it at 50°C~60°C for 6 hours.

[0058] 4) Take 0.1 mL of conductive ink and drop it onto the surface of a 5 cm × 5 cm PET substrate modified with 3-mercaptotriethoxysilane to prepare the printed pattern;

[0059] 5) Cut the substrate obtained in step 4) into 2cm×2cm thin slices, then treat them in an ethanol solution containing 1wt% formic acid for 2min to remove the oleamine on the surface of the copper-nickel composite nanoparticles, then take them out, clean them with deionized water and blow them dry.

[0060] 6) Add the substrate from step 5) to a 10 mL plating bath (copper sulfate pentahydrate: 0.024 mol / L, ethylenediaminetetraacetic acid: 0.028 mol / L, triethanolamine: 0.3 mol / L, dimethylamine borane: 0.08 mol / L), and chemically plating at 45°C for 1 to 6 hours. Then remove it, wash it with deionized water, and dry it in a 60°C oven for 3 hours to obtain a conductive copper printed circuit.

[0061] Figure 7 The diagram shows the relationship between the sheet resistance of the copper film prepared in step 5) of Example 3 and time. The sheet resistance of the copper film gradually decreases with increasing electroless plating time. The copper film obtained after 6 hours of electroless plating has a sheet resistance of 5.25 mΩ / sq, a film thickness of 3.62 μm, and a resistivity of [missing value]. (It has 1.12 times the resistivity of bulk copper) and has excellent electrical conductivity.

[0062] Example 3

[0063] Following the method of Example 2, and using the plating bath of step 6), chemical plating was performed for 1 hour. Then, the plated copper film was taken out, washed with deionized water, and treated in a 60°C oven for 3 hours to obtain a conductive copper film. The sheet resistance of the copper film was measured using a four-probe resistance meter.

[0064] The concentrations of copper ions and dimethylamine borane in the plating bath after chemical plating in step 6) of Example 2 were determined by flame atomic absorption and potentiometric titration, respectively. Based on the measured concentrations of copper ions and dimethylamine, copper sulfate pentahydrate and dimethylamine borane were added in appropriate amounts to make the concentration of the plating bath equal to the concentration at the beginning of step 1). The chemical plating process was repeated, and the resistance of the copper film was measured after drying.

[0065] After replenishing the copper ion concentration and reducing agent, the chemical plating bath was cycled 5 times.

[0066] Figure 8 The figure shows the amounts of copper sulfate pentahydrate and DMAB replenished after each cycle in Example 3. As can be seen from the figure, the replenishment amounts fluctuate very little. Figure 9 This refers to the sheet resistance of the copper film obtained in each cycle. As the number of cycles increases, the sheet resistance of the copper film remains essentially constant. By continuously replenishing the reactants, the plating bath can be effectively recycled, thereby reducing waste liquid discharge.

Claims

1. A method for fabricating flexible printed circuits using copper-nickel composite nanoparticles, characterized in that, Includes the following steps: First, conductive ink is prepared from core-shell structured copper-nickel composite nanoparticles and printed onto the surface of a substrate to create a circuit pattern. The conductive ink is prepared by dispersing copper-nickel composite nanoparticles in a solvent to prepare a copper-nickel composite nanoparticle conductive ink with a solid content of 0.1wt% to 10wt%. The solvent is n-hexane, n-octane, petroleum ether, turpentine, or a mixture of the above solvents. Then, a copper plating layer is prepared on the circuit pattern using chemical plating to obtain a highly conductive copper printed circuit. Copper-nickel composite nanoparticles can achieve magnetic separation and have electrical conductivity, while also serving as catalyst seeds in the electroless plating process; The preparation method of copper-nickel composite nanoparticles is as follows: Nickel acetate and oleylamine were mixed and added to liquid paraffin. The mixture was complexed at 40℃~60℃ under nitrogen atmosphere for 1h~2h. Then the temperature was raised to 220℃ and reacted for 30min. The mixture was then naturally cooled to 80℃, and the complex of copper formate and oleylamine was added. The reaction was allowed to proceed for 15h. After the reaction was completed, the particles were separated from the solution by magnetic force and washed with an equal volume of n-hexane and isopropanol mixed solvent to obtain copper-nickel composite nanoparticles with a copper-coated nickel core-shell structure and a particle size of 30nm~60nm.

2. The method for preparing flexible printed circuits using copper-nickel composite nanoparticles according to claim 1, characterized in that, The substrates include PET, PI, or PEN; The substrate undergoes a pretreatment process before use: after washing with deionized water and ethanol, it is dried at 50℃~60℃, immersed in an ethanol solution containing 1wt%~10wt% silane for 1h~6h, then washed with ethanol multiple times to remove residues, and dried at 50℃~60℃ for 6h. The silane is 3-mercaptotriethoxysilane or 3-aminotriethoxysilane.

3. The method for preparing flexible printed circuits using copper-nickel composite nanoparticles according to claim 1, characterized in that, The printing methods for conductive inks include inkjet printing, screen printing, gravure printing, letterpress printing, drop coating, scraping coating, or coating to prepare circuit patterns.

4. The method for preparing flexible printed circuits using copper-nickel composite nanoparticles according to claim 1, characterized in that, Before electroless plating, the substrate for printing circuit patterns is placed in a 1wt% to 5wt% formic acid solution and washed for 1 to 5 minutes to remove the oleamine coating on the surface of copper nanoparticles. Then it is taken out, washed with deionized water and dried.

5. The method for preparing flexible printed circuits using copper-nickel composite nanoparticles according to claim 4, characterized in that, The solvent for the formic acid solution is ethanol or methanol.

6. A method for fabricating flexible printed circuits using copper-nickel composite nanoparticles according to any one of claims 1, 4, or 5, characterized in that, The chemical plating method is as follows: the substrate with printed circuit patterns treated with degreasing amine is placed in the chemical plating solution and chemically plating copper at 20℃~60℃ for 1h~6h. After removal, it is washed multiple times with deionized water and dried at 50℃~60℃ for 6h to obtain a highly conductive copper printed circuit.

7. The method for preparing flexible printed circuits using copper-nickel composite nanoparticles according to claim 1, characterized in that, The electroless plating solution consists of dimethylaminoborane, ethylenediaminetetraacetic acid, triethanolamine, copper sulfate pentahydrate, and deionized water; the concentrations of the components are 0.01 mol / L to 0.5 mol / L for dimethylaminoborane, 0.001 mol / L to 0.1 mol / L for ethylenediaminetetraacetic acid, 0.05 mol / L to 1.2 mol / L for triethanolamine, and 0.001 mol / L to 0.1 mol / L for copper sulfate pentahydrate.

8. The method for preparing flexible printed circuits using copper-nickel composite nanoparticles according to claim 6, characterized in that, The electroless plating solution is recycled after being replenished with copper salt and reducing agent.

9. The method for preparing flexible printed circuits using copper-nickel composite nanoparticles according to claim 8, characterized in that, The copper salt is copper sulfate pentahydrate, and the reducing agent is dimethylaminoborane.

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