Near-spherical silver powder formed by self-assembly of silver nanosheets as well as preparation method and application of near-spherical silver powder

Abstract

The invention discloses nearly-spherical silver powder formed by self-assembly of silver nanosheets and a preparation method and application of the nearly-spherical silver powder, and belongs to the technical field of micro-nano material preparation. The preparation method comprises the following steps: firstly, adding a dispersing agent solution into a silver precursor compound solution, stirring, then adding a morphology control agent solution, stirring, and finally adding a reducing agent solution for reaction; and after the reaction is completed, the prepared silver powder is washed and dried, and the nearly-spherical silver powder formed by self-assembly of the silver nanosheets is obtained. The nearly-spherical silver powder is formed through self-assembly of the silver nanosheets, the particle size range of the nearly-spherical silver powder is 0.5-2.0 m, the thickness range of the self-assembled silver nanosheets is 10-50 nm, the overall size is uniform, a large number of stacking faults exist on the surfaces of the silver nanosheets, and the silver nanosheets show an excellent catalytic effect in conversion of p-nitrophenol into p-aminophenol.

Description

Technical Field

[0001] The present invention belongs to the technical field of micro-nano material preparation, and in particular relates to a nearly spherical silver powder formed by self-assembly of silver nanosheets, a preparation method and an application thereof. Background Art

[0002] p-Nitrophenol is an organic pollutant that is highly harmful to living things and is difficult to degrade naturally. However, by adding a catalyst to lower its reaction energy barrier, it can be easily converted into p-aminophenol, an important pharmaceutical intermediate.

[0003] In the traditional catalytic reaction of p-nitrophenol, people mostly use precious metals such as platinum (Pt) and palladium (Pd). However, the high price of Pt and Pd greatly limits their widespread application; therefore, there is an urgent need to find effective alternatives to Pt and Pd. Compared with Pt and Pd, silver (Ag) is not only cheaper but also has certain catalytic activity for p-nitrophenol. For example, Yuan et al. [Polymers 2023, 15 (10), 2366] deposited silver nanoparticles on the surface of micron-sized silica spheres to prepare spherical SiO2@Ag particles for the catalytic reduction of p-nitrophenol. Minisy et al. [Applied surface science 283 (2013): 389-395] prepared polyaniline, polypyrrole and poly (3,4-ethylenedioxythiophene) / silver composites by oxidation / reduction method. The composite material can achieve a conversion rate of 98.5% of p-nitrophenol within 12 minutes. However, compared with Pt and Pd, the catalytic performance of Ag still needs to be improved.

[0004] It's well known that the catalytic activity of a catalyst is closely related to its structural properties. For example, invention patent CN107838435A discloses a method for preparing silver nanoflowers with surface-enhanced Raman effect. This method uses ethylene glycol as a solvent and a reducing agent, and is prepared by adding FeCl3 and PVP and reacting them in a 160°C oven for 2.5 hours. Invention patent CN115464148B utilizes the complexation of carboxylates and a secondary ultrasound method to produce spherical silver powder with uniform morphology and size, good dispersion, and stability. Invention patent CN115971506B utilizes a mixed reducing agent to self-assemble silver nanoparticles by controlling the pH and temperature of the reaction to produce spherical silver powder. While all of these methods successfully produce spherical silver powder, they also have numerous drawbacks, including a complex preparation process; many of the reaction reagents are difficult to dissolve in water, which can cause environmental pollution and harm the health of workers; and the need for high temperatures, ultrasound, or other conditions during the preparation process, resulting in high overall energy consumption. Therefore, it is necessary to develop a preparation method that is fast, simple, green and environmentally friendly and easy to achieve large-scale industrial production to obtain spherical silver powder with uniform morphology. Summary of the Invention

[0005] To solve the above problems, the present invention provides a nearly spherical silver powder formed by self-assembly of silver nanosheets, as well as a preparation method and application. The method can achieve rapid, mild, green and environmentally friendly large-scale preparation of nearly spherical silver powder in an aqueous phase under room temperature conditions.

[0006] The present invention provides a method for preparing nearly spherical silver powder formed by self-assembly of silver nanosheets, the specific steps of which are as follows: (1) adding a dispersant solution to a silver precursor compound solution and stirring, then adding a morphology control agent solution and stirring, and finally adding a reducing agent solution to carry out an oxidation-reduction reaction; (2) After the reaction is completed, the prepared silver powder is washed and dried to obtain nearly spherical silver powder formed by self-assembly of silver nanosheets.

[0007] Preferably, the dispersant includes PVP and dispersant B; the dispersant B is at least one of gelatin, xanthan gum, and gum arabic.

[0008] Preferably, the precursor compound is one of silver nitrate, silver acetate, and silver acetylacetonate.

[0009] Preferably, the morphology control agent is one of disodium succinate, disodium glutarate, and disodium adipate.

[0010] Preferably, the reducing agent is one of sodium hypophosphite, polyethyleneimine, gallic acid and ascorbic acid.

[0011] Preferably, the concentration of polyvinyl pyrrolidone in the dispersant solution is 0.01~0.08 mM; the concentration of dispersant B is 0.05~0.10 mol / L; the concentration of the silver precursor compound in the silver precursor compound solution is 0.1~0.2 mol / L; the concentration of the morphology control agent in the morphology control agent solution is 0.05~0.10 mol / L; the concentration of the reducing agent in the reducing agent solution is 0.1~0.5 mol / L; and the volume ratio of the dispersant solution, the silver precursor compound solution, the morphology control agent solution and the reducing agent solution is 0.8~1.2:0.8~1.2:0.8~1.2:0.8~1.2.

[0012] The present invention also claims protection for the preparation method of the nearly spherical silver powder self-assembled by silver nanosheets, and the nearly spherical silver powder self-assembled by silver nanosheets; the nearly spherical silver powder is assembled by nanosheets, the particle size of the nearly spherical silver powder is 0.5~2.0µm, the thickness of the nanosheets is 10~50 nm, and the surface of the silver nanosheets has a large number of stacking faults.

[0013] The present invention also claims to protect the use of the nearly spherical silver powder self-assembled by the silver nanosheets in catalyzing p-nitrophenol.

[0014] The mechanism involved in the present invention is: (1) Initial self-assembly of silver atoms At room temperature, the hydroxyl groups on C2 and C3 in the ascorbic acid molecule form a stable enediol structure under the conjugation of double bonds. The synergistic effect of the conjugated double bonds and the adjacent hydroxyl groups gives the structure a high electron density, making it easy to lose electrons and undergo oxidation reactions. After the addition of ascorbic acid, the silver ions in the silver precursor compound solution receive electrons and are quickly reduced to silver atoms. Then, since the adsorption capacity of the carboxylate (COOH) in the morphology control agent for different silver crystal planes is Ag (111) > Ag (110) > Ag (100), COOH preferentially adsorbs to the Ag (111) crystal plane, thereby inhibiting the deposition of silver atoms on the Ag (111) crystal plane and promoting their deposition and growth along the Ag (110) and Ag (111) crystal planes. In view of this, silver planar twin particles are finally formed. In addition, according to the Ostwald ripening mechanism, under the driving force provided by the reduction of the total interfacial energy, the small silver particles dissolve and self-assemble and grow on the planar twin particles to form silver nanosheets.

[0015] (2) Secondary self-assembly of silver nanosheets After the silver nanosheets formed by the initial self-assembly grow to a certain size, they stop growing due to factors such as insufficient supply of silver atoms in the reaction solution, equilibrium between surface free energy and volume free energy, and surface passivation caused by organic adsorption. At room temperature, the carboxyl (-COOH) and amino (-NH2) groups in dispersant B will adsorb on the surface of the silver nanosheets, and the -OH, -NH2, and -COOH groups in dispersant B can act as H donors to react with other silver nanosheets adsorbed on the surface of PVP and the COO in dispersant B. - , C=O, and the O provided by CO form interactions (H···O). On the other hand, compared to other structures, the spherical structure has the lowest surface free energy and the most stable structure. Therefore, driven by the numerous chemical bonds (H···O) between the silver nanosheets, the nanosheets ultimately self-assemble into a nearly spherical structure. It is worth noting that the addition of PVP can significantly enhance the interaction between silver nanosheets. This is because, on the one hand, the oxygen atom in the carbonyl group (C=O) in the molecular structure of PVP has a lone pair of electrons and can act as a hydrogen bond acceptor. On the other hand, the polar side chain of PVP can form a hydrogen bond network with the hydroxyl and amino groups in dispersant B, thereby significantly enhancing hydrogen bond interactions. In addition, due to the rapid deposition of silver atoms during the preparation process and the adsorption of organic matter such as dispersants and morphology control agents, local stress is generated during the growth of the silver nanosheets, leading to disordered atomic stacking and ultimately the formation of a large number of stacking faults on the surface of the silver nanosheets.

[0016] (3) Catalytic mechanism The nearly spherical silver powders formed by self-assembly possess numerous stacking faults on their surfaces. The presence of these stacking faults disrupts the crystal periodicity and reduces the coordination number of surrounding atoms. According to crystal field theory, in transition metals, d orbitals undergo energy level splitting under the influence of the ligand field (surrounding atoms). This reduced coordination number indicates a loss of ligand field symmetry, leading to a shift in the d orbital splitting pattern and an increase in some energy levels. This ultimately causes d orbital electrons to occupy higher energy levels, forming highly active d-band centers, thereby enhancing the adsorption of H in NaBH4 and N and O in nitrophenol. Furthermore, stacking faults can act as electron scattering centers, forming localized electron accumulation or depletion zones, which promote electron transfer from the catalyst to the reactants and accelerate the catalytic activation of the nitro group in p-nitrophenol. Therefore, the nearly spherical silver powders formed by self-assembly of silver nanosheets exhibit excellent catalytic performance in the conversion of nitrophenol.

[0017] The technical solution provided by the present invention has the following beneficial effects: The present invention utilizes a combination of dispersants, supplemented by a morphology control agent, to achieve the aqueous synthesis of nearly spherical silver powder at room temperature, with a short overall reaction time. Furthermore, the nearly spherical silver powder formed by self-assembly of silver nanosheets has a particle size range of 0.5 to 2.0 µm and is uniform overall. The silver nanosheets have a thickness range of 10 to 50 nm and possess numerous stacking faults on their surfaces, demonstrating excellent catalytic performance in the conversion of p-nitrophenol to p-aminophenol. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] Figure 1 This is a scanning electron microscope (SEM) image of the silver powder prepared in Example 1 of the present invention.

[0019] Figure 2 This is a spherical aberration transmission electron microscopy (STEM) image of silver nanosheets in the silver powder prepared in Example 1 of the present invention.

[0020] Figure 3 This is a scanning electron microscope (SEM) image of the silver powder prepared in Example 2 of the present invention.

[0021] Figure 4 This is a scanning electron microscope (SEM) image of the silver powder prepared in Example 3 of the present invention.

[0022] Figure 5 This is a scanning electron microscope (SEM) image of the silver powder prepared in Comparative Example 1 of the present invention.

[0023] Figure 6 This is a scanning electron microscope (SEM) image of the silver powder prepared in Comparative Example 2 of the present invention.

[0024] Figure 7 This is a scanning electron microscope (SEM) image of the silver powder prepared in Comparative Example 3 of the present invention.

[0025] Figure 8 This is a scanning electron microscope (SEM) image of the silver powder prepared in Comparative Example 4 of the present invention. DETAILED DESCRIPTION

[0026] Example 1 A method for preparing nearly spherical silver powder formed by self-assembly of silver nanosheets, comprising the following steps: (1) Weigh 1.0 g of silver nitrate and dissolve it in 50 mL of deionized water under ultrasonication at room temperature for 1 min to prepare a 0.1176 mol / L silver nitrate solution.

[0027] (2) Weigh 1.5 g of xanthan gum and dissolve it in 50 mL of deionized water at 60°C with magnetic stirring at 200 rpm for 30 min to prepare a 0.1020 mol / L xanthan gum colloidal solution.

[0028] (3) Weigh 0.10 g of polyvinylpyrrolidone (PVP) (average molecular weight: 1,300,000) and dissolve it under magnetic stirring at 200 rpm at 60°C for 30 min to obtain a mixed solution of xanthan gum and PVP in the xanthan gum colloidal solution prepared in step (2).

[0029] (4) Weigh 0.5 g of disodium succinate and dissolve it in 50 mL of deionized water under ultrasonication at room temperature for 2 min to prepare a 0.0617 mol / L disodium succinate solution.

[0030] (5) Weigh 1.0 g of ascorbic acid and dissolve it in 50 mL of deionized water under ultrasonication at room temperature for 5 min to prepare a 0.1136 mol / L ascorbic acid solution.

[0031] (6) At room temperature, the mixed solution of xanthan gum and PVP (average molecular weight of 1,300,000) prepared in step (3) was added to the silver nitrate solution prepared in step (1), and magnetic stirring was performed for 5 minutes. Then, the disodium succinate solution prepared in step (4) was added to the mixed solution, and magnetic stirring was performed for 5 minutes. Finally, the ascorbic acid solution prepared in step (5) was added to the mixed solution, and the mixture was reacted at a constant temperature under magnetic stirring for 10 minutes. (7) After the reaction is completed, the prepared silver powder solution is washed with deionized water three times and dried in an oven at 60°C to obtain nearly spherical silver powder.

[0032] like Figure 1 As shown in FIG, the particle size of the nearly spherical silver powder synthesized in this embodiment ranges from 0.5 to 2.0 μm, the thickness of the silver nanosheets ranges from 10 to 50 nm, the overall size is uniform, and the dispersion is good. Figure 2 As shown, there are a large number of stacking faults on the surface of silver nanosheets.

[0033] Example 2 A method for preparing nearly spherical silver powder formed by self-assembly of silver nanosheets, comprising the following steps: (1) Weigh 0.85 g of silver nitrate and dissolve it in 50 mL of deionized water under ultrasonication at room temperature for 1 min to prepare a 0.10 mol / L silver nitrate solution.

[0034] (2) Weigh 0.735 g of xanthan gum and dissolve it in 50 mL of deionized water at 40 °C with magnetic stirring at 200 rpm for 30 min to prepare a 0.05 mol / L xanthan gum colloidal solution.

[0035] (3) Weigh 0.10 g of PVP (average molecular weight of 58,000) and dissolve it under magnetic stirring at 200 rpm at 40°C for 30 min to obtain a mixed solution of xanthan gum and PVP (average molecular weight of 58,000) in the xanthan gum colloidal solution prepared in step (2).

[0036] (4) Weigh 0.44 g of disodium glutarate and dissolve it in 50 mL of deionized water under ultrasonication at room temperature for 2 min to prepare a 0.05 mol / L disodium glutarate solution.

[0037] (5) Weigh 0.88 g of ascorbic acid and dissolve it in 50 mL of deionized water under ultrasonication at room temperature for 5 min to prepare a 0.10 mol / L ascorbic acid solution.

[0038] (6) At room temperature, the mixed solution of xanthan gum and PVP (average molecular weight 58,000) prepared in step (3) was added to the silver nitrate solution prepared in step (1) and magnetically stirred for 5 minutes. Then, the disodium glutarate solution prepared in step (4) was added to the mixed solution and magnetically stirred for 5 minutes. Finally, the ascorbic acid solution prepared in step (5) was added to the mixed solution and the mixture was reacted at a constant temperature under magnetic stirring for 10 minutes.

[0039] (7) After the reaction is completed, the prepared silver powder solution is washed with deionized water three times and dried in an oven at 60 °C to obtain nearly spherical silver powder.

[0040] This embodiment ( Figure 3 The particle size of the synthesized nearly spherical silver powder ranges from 0.5 to 2.0 μm, the thickness of the silver nanosheets ranges from 10 to 50 nm, and the overall morphology is uniform and presents a nearly spherical shape with openings. Figure 3 It can be clearly seen that its morphology is composed of multilayer self-assembled silver nanosheets. Compared with Example 1, the morphology of Example 2 samples is slightly different. On the one hand, this is because the average molecular weight of PVP is reduced to 58,000, which reduces the hydrogen bonding interaction between gel molecules. On the other hand, as the carbon chain length of the dicarboxylic acid increases, the adsorption energy of dicarboxylate ions on different silver crystal faces varies, which in turn leads to differences in the initially formed self-assembled structures, ultimately resulting in slightly different morphologies of the nearly spherical silver powder.

[0041] Example 3 A method for preparing nearly spherical silver powder formed by self-assembly of silver nanosheets, comprising the following steps: (1) Weigh 1.76 g of silver nitrate and dissolve it in 50 mL of deionized water under ultrasonication at room temperature for 1 min to prepare a 0.20 mol / L silver nitrate solution; (2) Weigh 1.47 g of xanthan gum and dissolve it in 50 mL of deionized water at 80 °C with magnetic stirring at 200 rpm for 30 min to prepare a 0.10 mol / L xanthan gum colloidal solution. (3) Weigh 0.10 g of PVP (average molecular weight 30,000) and dissolve it in the xanthan gum colloidal solution prepared in step (2) by magnetic stirring at 200 rpm at 80°C for 30 min. (4) Weigh 0.95 g of disodium adipate and dissolve it in 50 mL of deionized water under ultrasonication at room temperature for 2 min to prepare a 0.10 mol / L disodium adipate solution; (5) Weigh 4.40 g of ascorbic acid as a reducing agent and dissolve it in 50 mL of deionized water under ultrasonication at room temperature for 5 min to prepare a 0.50 mol / L ascorbic acid solution; (6) At room temperature, the mixed solution of xanthan gum and PVP (average molecular weight of 30,000) prepared in step (3) was added to the silver nitrate solution prepared in step (1), and magnetic stirring was performed for 10 minutes. Then, the disodium adipate solution prepared in step (3) was added to the mixed solution, and magnetic stirring was performed for 10 minutes. Finally, the ascorbic acid solution prepared in step (4) was added to the mixed solution, and the mixture was reacted at a constant temperature under magnetic stirring for 30 minutes. (7) After the reaction is completed, the prepared silver powder solution is washed with deionized water three times and dried in an oven at 60 °C to obtain nearly spherical silver powder.

[0042] This embodiment ( Figure 4 ) and Example 1 ( Figure 1 ) and Example 2 ( Figure 3 ). When the morphology control agent is replaced with disodium adipate and the average molecular weight of PVP is reduced to 30,000, the sphericity decreases slightly and the overall size decreases. This is because as the carbon chain of the morphology control agent continues to grow, the overall adsorption capacity of the carboxylic acid molecules on the silver crystal surface increases, thereby inhibiting the growth of its overall size. In addition, the reduction in the average molecular weight of PVP weakens the hydrogen bonding interactions within the gel network, resulting in a slight decrease in the sphericity of the nearly spherical silver powder.

[0043] Comparative Example 1 This comparative example adopts the same method as Example 1 to prepare nearly spherical silver powder, except that PVP is not added. The specific steps are as follows: (1) Weigh 1.0 g of silver nitrate and dissolve it in 50 mL of deionized water under ultrasonication at room temperature for 1 min to prepare a 0.1176 mol / L silver nitrate solution.

[0044] (2) Weigh 1.5 g of xanthan gum and dissolve it in 50 mL of deionized water at 60 °C with magnetic stirring at 200 rpm for 30 min to prepare a 0.1020 mol / L xanthan gum colloidal solution.

[0045] (3) Weigh 0.5 g of disodium succinate and dissolve it in 50 mL of deionized water under ultrasonication at room temperature for 2 min to prepare a 0.0617 mol / L disodium succinate solution.

[0046] (4) Weigh 1.0 g of ascorbic acid and dissolve it in 50 mL of deionized water under ultrasonication at room temperature for 5 min to prepare a 0.1136 mol / L ascorbic acid solution.

[0047] (5) At room temperature, the xanthan gum colloidal solution prepared in step (2) was added to the silver nitrate solution prepared in step (1) and magnetically stirred for 5 minutes. Then, the disodium succinate solution prepared in step (3) was added to the mixed solution and magnetically stirred for 5 minutes. Finally, the ascorbic acid solution prepared in step (4) was added to the mixed solution and the mixture was reacted at a constant temperature for 10 minutes under magnetic stirring.

[0048] (6) After the reaction is completed, the prepared silver powder solution is washed with deionized water three times and dried in an oven at 60 °C to obtain nearly spherical silver powder.

[0049] pass Figure 1 and Figure 5 By comparison, the sphericity of the self-assembled spherical silver powder in Comparative Example 1 decreased significantly after removing PVP, and the packing density of the silver nanosheets decreased. This is because the removal of PVP weakened the interaction between the gelatin gel network and the driving force for self-assembly.

[0050] Comparative Example 2 This comparative example adopts the same method as Example 1 to prepare nearly spherical silver powder, except that the dispersants xanthan gum and PVP are not added. The specific steps are as follows: (1) Weigh 1.0 g of silver nitrate and dissolve it in 50 mL of deionized water under ultrasonication at room temperature for 1 min to prepare a 0.1176 mol / L silver nitrate solution.

[0051] (2) Weigh 0.5 g of disodium succinate and dissolve it in 50 mL of deionized water under ultrasonication at room temperature for 2 min to prepare a 0.0617 mol / L disodium succinate solution.

[0052] (3) Weigh 1.0 g of ascorbic acid and dissolve it in 50 mL of deionized water under ultrasonication at room temperature for 5 min to prepare a 0.1136 mol / L ascorbic acid solution.

[0053] (4) At room temperature, the disodium succinate solution prepared in step (2) was added to the silver nitrate solution prepared in step (1), and the mixture was stirred magnetically for 5 minutes. Then, the ascorbic acid solution prepared in step (3) was added to the above solution, and the mixture was reacted at a constant temperature for 10 minutes under magnetic stirring.

[0054] (5) After the reaction is completed, the prepared silver powder solution is washed with deionized water three times and dried in an oven at 60 °C to obtain nearly spherical silver powder.

[0055] pass Figure 1 and Figure 6 By comparison, after removing PVP and xanthan gum, the reaction only produces polyhedral twin particles of uneven size. This is because the lack of a combined dispersant weakens the electrostatic interaction on the particle surface and reduces the steric hindrance effect, making the synthesized silver particles more likely to agglomerate.

[0056] Comparative Example 3 This comparative example adopts the same method as Example 1 to prepare nearly spherical silver powder, except that the shape control agent disodium succinate is not added. The specific steps are as follows: (1) Weigh 1.0 g of silver nitrate and dissolve it in 50 mL of deionized water under ultrasonication at room temperature for 1 min to prepare a 0.1176 mol / L silver nitrate solution.

[0057] (2) Weigh 1.5 g of xanthan gum and dissolve it in 50 mL of deionized water at 60 °C with magnetic stirring at 200 rpm for 30 min to prepare a 0.1020 mol / L xanthan gum colloidal solution.

[0058] (3) Weigh 0.10 g of PVP (average molecular weight: 1,300,000) and dissolve it in the xanthan gum colloidal solution prepared in step (2) by magnetic stirring at 200 rpm at 60 °C for 30 min.

[0059] (4) Weigh 1.0 g of ascorbic acid and dissolve it in 50 mL of deionized water under ultrasonication at room temperature for 5 min to prepare a 0.1136 mol / L ascorbic acid solution.

[0060] (5) At room temperature, the mixed solution of xanthan gum and PVP (average molecular weight of 1,300,000) prepared in step (3) was added to the silver nitrate solution prepared in step (1), and magnetic stirring was performed for 5 minutes. Finally, the ascorbic acid solution prepared in step (4) was added to the above solution, and the mixture was reacted at a constant temperature for 10 minutes under magnetic stirring.

[0061] (6) After the reaction is completed, the prepared silver powder solution is washed with deionized water three times and dried in an oven at 60 °C to obtain nearly spherical silver powder.

[0062] pass Figure 1 and Figure 7 By comparison, it can be seen that when no morphology control agent is added, the near-spherical silver powder formed by self-assembly of silver nanosheets cannot be produced, and only smaller, evenly dispersed silver nanoparticles can be obtained. This is because the morphology control agent is indispensable for the formation of silver nanosheet structures. In the absence of the morphology control agent, silver ions are reduced to silver atoms and will only quickly aggregate into silver particles. However, due to the presence of dispersants such as xanthan gum and PVP, these particles will not agglomerate.

[0063] Comparative Example 4 This comparative example adopts the same method as Example 1 to prepare nearly spherical silver powder, except that xanthan gum is not added. The specific steps are as follows: (1) Weigh 1.0 g of silver nitrate and dissolve it in 50 mL of deionized water under ultrasonication at room temperature for 1 min to prepare a 0.1176 mol / L silver nitrate solution.

[0064] (2) 0.10 g of PVP (average molecular weight: 1,300,000) was dissolved in 50 ml of water at 60 °C with magnetic stirring at 200 rpm for 30 min to obtain a PVP solution.

[0065] (3) Weigh 0.5 g of disodium succinate and dissolve it in 50 mL of deionized water under ultrasonication at room temperature for 2 min to prepare a 0.0617 mol / L disodium succinate solution.

[0066] (4) Weigh 1.0 g of ascorbic acid and dissolve it in 50 mL of deionized water under ultrasonication at room temperature for 5 min to prepare a 0.1136 mol / L ascorbic acid solution.

[0067] (5) At room temperature, the PVP solution prepared in step (2) was added to the silver nitrate solution prepared in step (1) and magnetically stirred for 5 min. Then, the disodium succinate solution prepared in step (3) was added to the mixed solution and magnetically stirred for 5 min. Finally, the ascorbic acid solution prepared in step (4) was added to the mixed solution and the mixture was reacted at a constant temperature for 10 min under magnetic stirring.

[0068] (6) After the reaction is completed, the prepared silver powder solution is washed with deionized water three times and dried in an oven at 60 °C to obtain nearly spherical silver powder.

[0069] The morphology of the silver powder prepared in this comparative example is as follows Figure 8 When xanthan gum is not added as a dispersant, the silver powder morphology is uneven, with the majority consisting of small silver nanosheets and only a small amount of nearly spherical silver powder with low sphericity. This is because the lack of xanthan gum in the silver nanosheets inhibits the self-assembly driving force and the formation of nearly spherical silver powder.

[0070] Effect Example 1 The catalytic activity of the silver powders synthesized in Examples 1-3 and Comparative Examples 1-4 on p-nitrophenol was compared, and the specific steps were as follows: (1) Weigh 0.0278 g of 4-nitrophenol (4-NP) and dissolve it in 100 mL of deionized water. Stir magnetically in a water bath at 25°C for 30 min to prepare a 2 mM solution. Then, take 1 mL of 4-NP and add it to 9 mL of deionized water to prepare a 0.2 mM 4-NP solution.

[0071] (2) Weigh 0.0148 g of sodium borohydride (NaBH4) and dissolve it in 20 mL of deionized water to prepare 20 mM NaBH4.

[0072] (3) Add 1.5 mL of 4-NP (0.2 mM) solution to a 5 mL cuvette, then add 1.5 mL of NaBH4 (20 mM), and finally add 20 μL of silver powder sample (1 mg / mL), and then immediately measure using UV-visible absorption spectroscopy.

[0073] The conversion rate and apparent reaction rate constant are shown in Table 1.

[0074] Table 1 As can be seen from Table 1, the nearly spherical silver powders formed by self-assembly in Examples 1-3 and Comparative Examples 1-4 have different catalytic activities. The silver powders of Examples 1-3 have significantly higher conversion rates and apparent reaction rate constants in catalyzing the conversion of 4-NP to 4-AP than the silver powders of Comparative Examples 1-4. In particular, the nearly spherical silver powder prepared in Example 1 can rapidly catalyze the conversion of p-nitrophenol to p-aminophenol within 5 minutes. This is because the silver nanosheets in the nearly spherical silver powder formed by self-assembly have abundant stacking faults on their surface, which can provide more catalytic active sites and thus have better catalytic performance. In addition, the nearly spherical silver powder prepared by self-assembly of silver nanosheets also has potential application value in the fields of electronic pastes, surface-enhanced Raman spectroscopy, and biological antibacterial.

[0075] The above text has described various embodiments of the present invention. The above description is illustrative, not exhaustive, and is not limited to the disclosed embodiments. Many modifications and variations will be apparent to those skilled in the art without departing from the scope and spirit of the described embodiments. The terminology used herein is selected to best explain the principles of the embodiments, practical applications, or improvements to existing technologies, or to enable others skilled in the art to understand the embodiments disclosed herein.

Claims

1. A method for preparing nearly spherical silver powder formed by self-assembly of silver nanosheets, characterized in that: The steps include: (1) adding a dispersant solution to a silver precursor compound solution and stirring, then adding a morphology control agent solution and stirring, and finally adding a reducing agent solution to carry out an oxidation-reduction reaction; (2) After the reaction is completed, the prepared silver powder is washed and dried to obtain nearly spherical silver powder formed by self-assembly of silver nanosheets; The dispersant includes polyvinyl pyrrolidone and dispersant B, wherein the dispersant B is one of xanthan gum, gelatin and gum arabic.

2. The method for preparing the nearly spherical silver powder formed by self-assembly of silver nanosheets according to claim 1, characterized in that: The silver precursor compound is selected from one of silver nitrate, silver acetate and silver acetylacetonate.

3. The method for preparing the nearly spherical silver powder formed by self-assembly of silver nanosheets according to claim 1, characterized in that: The average molecular weight of the polyvinyl pyrrolidone is 30,000-1,300,000.

4. The method for preparing nearly spherical silver powder formed by self-assembly of silver nanosheets according to claim 1, characterized in that: The morphology control agent is one of disodium succinate, disodium glutarate and disodium adipate.

5. The method for preparing a nearly spherical silver powder formed by self-assembly of silver nanosheets according to claim 1, characterized in that: The reducing agent is one of sodium hypophosphite, polyethyleneimine, gallic acid and ascorbic acid.

6. The method for preparing nearly spherical silver powder formed by self-assembly of silver nanosheets according to claim 1, characterized in that: The concentration of dispersant A in the dispersant solution is 0.01~0.08 mM; the concentration of dispersant B is 0.05~0.10 mol / L; the concentration of the silver precursor compound in the silver precursor compound solution is 0.1~0.2 mol / L; the concentration of the morphology control agent in the morphology control agent solution is 0.05~0.10 mol / L; the concentration of the reducing agent in the reducing agent solution is 0.1~0.5 mol / L; the volume ratio of the dispersant solution, the silver precursor compound solution, the morphology control agent solution and the reducing agent solution is 0.8~1.2:0.8~1.2:0.8~1.2:0.8~1.

2.

7. Nearly spherical silver powder prepared by the method for preparing near-spherical silver powder formed by self-assembly of silver nanosheets according to any one of claims 1 to 6, wherein the near-spherical silver powder is assembled from nanosheets, the particle size of the near-spherical silver powder is 0.5-2.0 µm, the thickness of the nanosheets is 10-50 nm, and the nanosheets have stacking faults on their surfaces.

8. Use of the nearly spherical silver powder self-assembled by silver nanosheets according to claim 7 in catalyzing p-nitrophenol.

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