RU2821522C1 - Method of producing highly concentrated organosol of silver nanoparticles - Google Patents

Abstract

FIELD: various technological processes.

SUBSTANCE: invention can be used in production of optical devices, chemical and biological sensors, catalysts, antibacterial materials, inks for 2D- and 3D-printing, electroconductive and heat-conducting materials, plasma and liquid crystal displays, solar cells. Method of producing highly concentrated organosol of silver nanoparticles involves reduction of an aqueous solution of silver nitrate. Concentration of silver nitrate in the initial solution is 0.6 M. Reducing agent used is a mixture of solutions of iron (II) sulphate and trisodium citrate. Sodium citrate and silver nitrate solutions are alternately added to the FeSO₄ solution until a molar ratio of 1:1:(1.5÷3.2) is achieved. Obtained precipitate is separated by centrifugation and washed with a solution of trisodium citrate, again centrifuged, the operation is repeated 3-4 times. Coagulate of nanoparticles is peptized in distilled water with formation of hydrosols of silver nanoparticles. Extraction of silver metal nanoparticles is carried out from aqueous solutions into the phase of o-xylene or butyl acetate in the presence of ethyl alcohol and cetyltrimethylammonium bromide or cetyltrimethylammonium nitrate. Obtained silver organosol is separated by decantation and concentrated by distillation of the solvent to a metal concentration of up to 1,800 g/l.

EFFECT: invention simplifies synthesis of silver nanoparticles of spherical morphology with low polydispersity and increases efficiency thereof.

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Description

The invention relates to a method for producing highly concentrated organosols of metallic silver nanoparticles, which can be used in the production of optical devices, chemical and biological sensors, catalysts, antibacterial materials, inks for 2D and 3D printing, electrically conductive and thermally conductive materials (films), in the production plasma and liquid crystal displays, for creating solar cells, etc.

In most known patents and scientific works, metallic silver organosols are obtained by thermal decomposition or reduction of organic silver compounds in the non-polar phase.

There is a known method for producing silver nanoparticles or silver nanocolloid, as well as compositions of silver ink containing silver nanoparticles [KR100918231B1, publ. 09/21/2009], in which by reacting benzoic acid and NaOH in methanol, sodium benzoate is first obtained, and then a precipitate of silver (I) benzoate is obtained during the further addition of an aqueous solution of silver nitrate. Silver (I) benzoate, after separation and drying, is dissolved in triethanolamine and used as “ink”, which decomposes at a temperature of 150 C° to form conductive patterns of metallic silver.

The disadvantages of this method include the formation of photosensitive compounds and the use of toxic reagents.

In another example [KR100587404B1, publ. 06/08/2006], decanoic acid was dissolved in methanol, NaOH was added until sodium decanoate was formed, a solution of silver nitrate was introduced, resulting in the formation of a white precipitate of silver (I) decanoate, which was separated, thoroughly washed and dried at room temperature. Silver decanoate was dissolved in toluene in the presence of octanethiol, this suspension was applied to the target surface, dried at room temperature, and successively annealed for 10 minutes at 150°C and 10 minutes at 500°C. The resulting films showed high electrical conductivity.

The disadvantages of the method include the use of toxic reagents, the multi-stage process and high annealing temperature.

In a patent describing ink with nanosized silver particles and a method for their production [US2017/0215279A1, publ. 07/27/2017], a stabilizer (hexylamine, dodecylamine or oleic acid) was added to a suspension of silver oxalate in an organic medium (decane, methanol or alkyldiamines) with stirring. The mixture was heated to 110°C, resulting in the formation of silver nanoparticles with a change in the color of the suspension from cream to black and the release of gas. The resulting nanoparticles were separated, shaken with methanol, and centrifuged at least twice to remove interfering reaction products. Purified nanoparticles were dispersed in a mixture of octane and butanol (4:1) and used as “ink,” which was spin-coated onto a substrate and annealed at 100°C to form conductive patterns.

Among the disadvantages of the method, one can note the use of toxic methanol and the low aggregative stability of the resulting “ink”.

A method for producing hydrophobic metal nanoparticles was proposed [US8591624B2, publ. 11/26/2013] mix aqueous solutions of carboxylic acids with NaOH to obtain the corresponding sodium salts, after which a solution of silver nitrate was added dropwise until the corresponding carboxylic salts of silver (I) were formed. The resulting precipitate was collected, washed and dried under reduced pressure and a temperature of 40°C for 24 hours. The resulting powder was dissolved in heptane, placed in a stainless steel reactor and at 40°C filled with H ₂ to 7 bar, followed by the addition of CO ₂ until the total pressure was reached 55 bar. During the reduction, yellow silver organosols were obtained.

The disadvantages of this method include the complexity of the process and low product yield.

There is a known method for producing silver nanoparticles in toluene by the two-phase transfer method [Sarathy KV, Raina G., Yadav RT, Kulkarni GU, Rao CNR Thiol-derivatized nanocrystalline arrays of gold, silver, and platinum. J. Phys. Chem. B., 1997, 101, 9876], in which an aqueous solution of AgNO ₃ was reduced with sodium borohydride, after which dodecanethiol and HCl were added and the nanoparticles were extracted into the toluene phase.

The disadvantage of this method is the use of toxic toluene and low concentration of silver, which is associated with the low sedimentation stability of the original silver hydrosol (0.03 g/l).

Another approach to the preparation of organosols is also known, which consists in carrying out liquid extraction of silver nanoparticles using surfactants (“transfer catalysts”) [Sarkar A., Kapoor S., Mukherjee T. Oleic acid-assisted phase transfer of nanosized silver colloids. Res Chem Intermed (2010) 36:403–410]. A mixture of formamide as a reducing agent and oleic acid as a stabilizer was added to a 0.01 M solution of AgNO ₃ until metallic silver nanoparticles were formed, which were concentrated in the organic phase. Next, silver was extracted into CHCl ₃ . The particles showed high aggregative stability. However, the process involved toxic and expensive reagents, and the metal concentration in the resulting organosol was below 1 g/l, which requires additional costs for disposal and processing of large quantities of waste solutions.

There is a known method for producing metallic silver nanoparticles in reverse micelles [RU No. 2644176C1, publ. 02.08.2018], where in a solution of n-decane in the presence of the micelle-forming agent sodium bis(2-ethylhexyl)sulfosuccinate (AOT), an aqueous solution of AgNO ₃ was introduced dropwise with intense stirring, after which a reducing agent, hydrazine, was added in a similar manner. The ratio of the volumes of the n-decane phases and aqueous solutions of the reagents was 2.5, and the silver concentration was 3.5 g per liter of the reaction medium. In this case, in the n-decane environment, the formation of reverse micelles containing aqueous solutions of silver and a reducing agent occurred, which, during coalescence, interacted with the formation of metallic silver nanoparticles. The emulsion was stirred for 60 minutes, after which it was left for a day and additionally centrifuged at 3000 rpm until complete phase separation. Next, the organic phase was stirred on a magnetic stirrer (500 rpm) in an open beaker for 3 hours until the water completely evaporated. The resulting dehydrated organosol of nanoparticles was filtered through a paper filter. Next, 500 μl of distilled water was gradually added to the organosol with manual shaking in a test tube. The microemulsion was left overnight and then subjected to electrophoresis for 1.5 hours at a field strength of 600 V/cm. As a result of electrophoresis, a concentrate of nanoparticles with a silver concentration of up to 464 g/l accumulated on the anode surface.

The disadvantages of this method include the low initial concentration of silver nanoparticles (less than 3.5 g/l), the yield of the reduction reaction for silver is less than 78%, the high consumption of organic solvents (300 ml of n-heptane to produce 1 g of silver nanoparticles) and the duration of the process.

The technical solution closest in purpose to the proposed method is the method for producing hydrosol and organosol of silver [Wei W., Gu B. Preparation and Characterization of Silver Nanoparticles at High Concentrations. Washington, ACS, V. 878, 2004. pp. 1-14], which consists in the reduction of aqueous solutions of AgNO ₃ with sodium borohydride in the presence of the stabilizer cetyltrimethylammonium bromide (CTAB) at the molar ratios [BH ₄ ^- ]/[Ag ⁺ ] = 2 and [Ag ⁺ ]/[CTAB] = 4. At This results in the formation of aqueous solutions of metallic silver nanoparticles with a size of 5-30 nm with a concentration of up to 10 g/l. To transfer nanoparticles into the organic phase, 25 ml of chloroform is added to 25 ml of hydrosol and 0.1 g of solid NaCl is added with stirring. The extraction efficiency (the proportion of silver that has passed into the chloroform phase) reaches 95%. The disadvantage of this method is the low productivity of silver nanoparticles (up to 0.5 g/l), since when using more concentrated solutions, large nanoparticles of irregular shape were formed, characterized by significant polydispersity.

The objective of the invention is to develop a simple and high-performance method for producing silver organosols with a metal concentration of up to 1800 g/l, containing nanoparticles of spherical morphology with a size of about 10 nm and low polydispersity.

The technical result of the claimed invention is to simplify the method for the synthesis of silver nanoparticles by simply mixing aqueous solutions of iron (II) sulfate, sodium citrate and silver nitrate; increasing the productivity of the method through the use of high concentrations of initial solutions; simplifying the method of purifying nanoparticles from reaction products using successive stages of coagulation-peptization; improving the quality of the target product by obtaining nanoparticles similar in morphology and size; increasing extraction productivity by carrying out the process from high-concentration hydrosols.

The technical result is achieved by the fact that in the method for producing a highly concentrated organosol of silver nanoparticles, including the reduction of an aqueous solution of silver nitrate, what is new is that the concentration of silver nitrate in the initial solution is 0.6 M; a mixture of solutions of iron (II) sulfate and trisodium citrate, while solutions of sodium and silver citrate are alternately added to the FeSO4 solution until a molar ratio of 1:1:(1.5÷3.2) is achieved, the resulting precipitate is separated by centrifugation and washed with a solution of sodium citrate, centrifuged again, operation repeat 3-4 times, the coagulum of nanoparticles is peptized in distilled water to form hydrosols of silver nanoparticles, extraction of metallic silver nanoparticles is carried out from aqueous solutions into the o-xylene or butyl acetate phase in the presence of ethyl alcohol and cetyltrimethylammonium bromide or cetyltrimethylammonium nitrate, the resulting silver organosol is separated by decantation and concentrated by distilling off the solvent to a metal concentration of up to 1800 g/l.

The invention is illustrated by drawings. In fig. Figure 1 shows a diagram of the synthesis and washing of silver nanoparticles. In fig. Figure 2 shows the dependence of the degree of extraction of silver nanoparticles into the o-xylene phase on the concentration of ethyl alcohol at a silver hydrosol concentration = 0.4 M; concentrations of CTAB = 0.015 M and Vorg: Vaq = 2.0. In fig. Figure 3 shows the dependence of the degree of extraction of silver nanoparticles into the o-xylene phase on the concentration of CTAB (a) at Vorg: Vaq = 2.0 and various concentrations of silver in the original hydrosol, mol/l: 1 - 0.2; 2 - 0.3; 3 - 0.4; and 4 - 0.5, and also (b) at an initial silver concentration of 0.4 M and Vorg: Vaq: 5 - 0.25; 6 - 0.5; 7 - 1; 8 - 2. In FIG. Figure 4 shows the dependence of the extraction of silver nanoparticles into the organic phase on the concentration of cetyltrimethylammonium nitrate (CTAN) (a) at Vorg: Vaq = 2.0 and different concentrations of silver in the original hydrosol, mol/l: 1 - 0.2; 2 - 0.3 and 3 - 0.4; and also (b) at an initial silver concentration of 0.4 M and the ratio Vorg: Vaq: 4 - 2; 5 - 1; 6 - 0.5; 7 - 0.25; 8 - 2. In FIG. Figure 5 shows XRD of nanoparticles in the original hydrosol “black curve” and nanoparticles extracted into the o-xylene phase “red curve”. In fig. Figure 6 shows micrographs (TEM) of nanoparticles in the original hydrosols (Figure 6a) and after extraction into the o-xylene phase (Figure 6b). In fig. Figure 7 shows the hydrodynamic diameter of particles according to DLS data (a) and (b) optical absorption spectra of nanoparticles in: 1 - initial hydrosols, 2 - extracted into the o-xylene phase and 3 - extracted into the butyl acetate phase. In fig. Figure 8 shows thermogravimetric analysis (TG) and differential scanning calorimetry (DSC) curves of nanoparticles extracted into the o-xylene phase using CTAB (a) and CTAN (b) in an argon atmosphere. In fig. Figure 9 shows a photograph of a drop of a solution of metallic silver nanoparticles with a metal concentration of 1800 g/l in the o-xylene phase.

The inventive method for producing a highly concentrated organosol of silver nanoparticles is carried out as follows.

Reaction solutions of iron (II) sulfate, sodium citrate and silver nitrate are prepared. While stirring, solutions of sodium and silver citrate are alternately added to the FeSO ₄ solution until a molar ratio of 1:1:(1.5÷3.2) is achieved. The reduction process is carried out on a magnetic stirrer with stirring at a speed of 200 min ^-1 and a temperature of 25 ° C for 5 minutes. The resulting nanoparticles are separated from the solution by centrifugation for 3-5 minutes at a speed of 1000 rpm. Distilled water is added to the precipitate to the original volume, stirred intensively until the precipitate is completely dissolved, after which an equal volume solution of sodium citrate is added to a concentration of 0.1 M - 0.3 M to coagulate the particles and centrifuged again for 3-5 minutes at a speed of 1000 rpm /min (the operation is repeated 3-4 times). The coagulum of nanoparticles is peptized in distilled water to the original volume of solution to form hydrosols of silver nanoparticles. Water and other reagents are successively added to the resulting hydrosol of silver nanoparticles until the final concentration in the solution for silver is from 0.2 M to 0.5 M, for ethyl alcohol from 0 M to 6 M, and for CTAB or CTAN from 0.005 M to 0.02 M, add o-xylene or butyl acetate in a volume ratio Vorg:Vaq = 0.5 and mix gently. Next, centrifuge for 5 minutes at a speed of 2000 rpm. After centrifugation, phase separation was observed. The organic phase, containing silver nanoparticles and colored dark red, was separated from the colorless aqueous phase, which contained a small amount of insoluble precipitate, by decantation.

Cetyltrimethylammonium nitrate (CTAN), due to its lack of commercial availability, is prepared independently by replacing the Br- anion in cetyltrimethylammonium bromide (CTAB) with NO ₃ ^- by the ion exchange method using a strong basic anion exchanger AV-17-8 in NO ₃ ^- form. To do this, the AV-17-8 anion exchanger is converted to NO ₃ ^- form, treating it several times with an excess of 1 M HNO ₃ and washing with deionized water to a neutral environment. The ion exchanger is dried at room temperature. The capacity of the ion exchanger is determined by the change in the concentration of the NaOH solution before and after contact with the ion exchanger and its titration with 0.1 M HCl in the presence of phenolphthalein. A 1.5-fold excess of pre-swollen anion exchanger AV-17-8 in NO ₃ ^- form is added to a 0.1 M aqueous solution of CTAB and mixed on a shaker for 24 hours. After which the solution is separated, the ion exchanger is thoroughly washed with deionized water, the main volume of the solution and the washing water are collected and dried under vacuum at room temperature, the dry precipitate of CTAN is collected and dissolved in ethyl alcohol for further use.

To determine the concentration of silver nanoparticles in the aqueous or organic phase, an aliquot of the solution is dried at 150 °C, the silver precipitate is dissolved in 1 ml of concentrated nitric acid and dried at 150 °C until white crystals of AgNO ₃ are obtained, which are dissolved in water and titrated with 0.1 N solution of KSCN in the presence of Fe(NO ₃ ) ₃ until a non-disappearing pink color appears. Next, the degree of silver extraction into the organic phase is calculated as the ratio of the amount of silver in the organic phase to the initial amount of silver in the hydrosol.

In fig. Figure 2 shows that the degree of silver extraction into the organic phase of silver nanoparticles increases almost linearly over the entire studied range with increasing concentration of ethyl alcohol. Ethyl alcohol molecules act as a “transfer catalyst,” allowing the water-o-xylene boundary to “blur” and thereby increase extraction efficiency. Based on the data in Fig. 3, when using CTAB, the recovery degree reaches a maximum value of 62.5% at concentrations of silver nanoparticles of 0.4 M, CTAB 0.01 M and V _org : V _aq ≥ 0.5. According to Fig. 4, the degree of silver extraction into the organic phase in the presence of CTAN reaches a value of ~ 90% at silver concentrations of 0.3 M and 0.4 M at CTAN concentrations of 0.01 M and 0.012 M, respectively, and at V _org : V _aq ≥0.5. According to Fig. 5 silver nanoparticles, both in the original hydrosols and after their extraction into o-xylene, represent a phase of pure metallic silver without any impurities. According to FIG. 6 shows that the shape and size of the particles changed slightly during the extraction process. Based on the data in Fig. 7 the hydrodynamic diameter of the obtained nanoparticles is established.

Example 1.

In a heat-resistant glass with a volume of 500 ml, mix 70 ml of a 1.38 M sodium citrate solution and 50 ml of a 1.08 M FeSO ₄ solution, then add 50 ml of a 0.6 M AgNO ₃ solution. The synthesis of nanoparticles was carried out at room temperature on a magnetic stirrer at a stirring speed of 200 rpm for 15 minutes. Next, the solution with nanoparticles is centrifuged for 3 minutes at a speed of 1000 rpm, the centrifuge is drained, and 20 ml of distilled water is added to the precipitate, mixed, 20 ml (0.4 M) sodium citrate is added and centrifuged again. The operation is repeated 3 times. The resulting sediment of coagulated nanoparticles is dissolved in 50 ml of deionized water. The concentration of silver in the resulting hydrosol is determined by titration according to the method described above. The silver concentration is about 0.6 M. Next, about 9.5 ml of water, 6.5 ml of 95% ethyl alcohol, 2.8 ml of a 0.25 M solution of CTAB in 95% ethyl alcohol are added to the hydrosol so that the final concentration of nanoparticles silver is 0.4 M; 2 M ethyl alcohol and 0.01 M CTAB. Add twice the volume of o-xylene to the mixture and mix gently. The mixture was centrifuged for 5 minutes at 1000 rpm, resulting in separation into a colored organic layer, a clear aqueous part, and a certain amount of aggregated particles. The recovery rate of silver nanoparticles was 62.5% (Fig. 3a). According to X-ray diffraction data, the nanoparticles both in the initial hydrosol phase and after their transition to the o-xylene phase were pure metallic silver. According to Fig. 6, the initial nanoparticles had a round shape and an average size of about 8.4 nm, which increased to 9.1 nm after their transfer to o-xylene. According to Fig. 7a, the average hydrodynamic size of nanoparticles before and after extraction into o-xylene changed from 8.3 nm to 10 nm, respectively. In the optical absorption spectra presented in Fig. 7b, for both hydrosol and organosol, an absorption band is observed in the region of 400-430 nm, characteristic of metallic silver nanoparticles. According to the thermal analysis carried out in an Ar atmosphere in FIG. 8a, silver samples obtained using CTAB contain up to 11% wt. impurities (solvent, stabilizer), which are almost completely removed when heated to a temperature of 300°C. The organosols obtained on the basis of o-xylene are subjected to vacuum evaporation in a desiccator using a water-jet pump at a temperature of 35°C, as a result of which low-viscosity, homogeneous in texture and stable to aggregation for more than 60 days concentrates of nanoparticles in o-xylene were obtained. The appearance of an organosol with a concentration of metallic silver nanoparticles of 1800 g/l is shown in Fig. 9.

Example 2.

In a heat-resistant glass with a volume of 500 ml, mix 70 ml of 1.38 M sodium citrate and a solution of 50 ml of 1.08 M FeSO ₄ , then add 50 ml of 0.6 M AgNO ₃ . The synthesis of nanoparticles is carried out at room temperature on a magnetic stirrer at a stirring speed of 200 rpm for 15 minutes. Next, the solution with nanoparticles is centrifuged for 3 minutes at a speed of 1000 rpm, the centrifuge is drained, and 20 ml of distilled water is added to the precipitate, mixed, 20 ml (0.4 M) sodium citrate is added and centrifuged again. The operation is repeated 3 times. The resulting sediment of coagulated nanoparticles is dissolved in 50 ml of deionized water. The silver concentration was determined using titration according to the method described above. The silver concentration is about 0.6 M. Next, about 9.5 ml of water, 6.5 ml of 95% ethyl alcohol, 2.8 ml of a 0.25 M solution of CTAB in 95% ethyl alcohol are added to the hydrosol so that the final concentration of nanoparticles silver is 0.4M; ethyl alcohol 2M and CTAB 0.01 M. An equal volume of butyl acetate is added to the mixture and gently mixed. The mixture is centrifuged for 5 minutes at 1000 rpm, resulting in separation into a colored organic layer, a transparent aqueous part and a certain amount of aggregated particles. The degree of extraction of silver nanoparticles is 50%. According to the DLS data in Fig. 7a, the average hydrodynamic size of nanoparticles before and after extraction into o-xylene changed from 8.3 nm to 34.5 nm. According to the optical absorption spectra presented in Fig. 7b, hydrosols and organosols based on butyl acetate had an absorption band in the region of 400-430 nm, characteristic of metal nanoparticles.

Example 3.

In a heat-resistant glass with a volume of 500 ml, mix 70 ml of 1.38 M sodium citrate and a solution of 50 ml of 1.08 M FeSO ₄ , then add 50 ml of 0.6 M AgNO ₃ . The synthesis of nanoparticles is carried out at room temperature on a magnetic stirrer at a stirring speed of 200 rpm for 15 minutes. Next, the solution with nanoparticles is centrifuged for 3 minutes at a speed of 1000 rpm, the centrifuge is drained, and 20 ml of distilled water is added to the sediment, mixed, 20 ml (0.4 M) sodium citrate is added and centrifuged again. The operation is repeated 3 times. The resulting sediment of coagulated nanoparticles is dissolved in 50 ml of deionized water. The silver concentration is determined by titration using the method described above. The silver concentration is about 0.6 M. Next, about 9.5 ml of water, 6.5 ml of 95% ethyl alcohol, 2.8 ml of a 0.25 M solution of CTAN in 95% ethyl alcohol are added to the hydrosol so that the final concentration of nanoparticles silver is 0.4M; 2M ethyl alcohol and 0.01 M CTAB. Add twice the volume of o-xylene to the mixture and mix gently. The mixture was centrifuged for 5 minutes at 1000 rpm, resulting in separation into a colored organic layer, a clear aqueous part, and a certain amount of aggregated particles. The recovery rate of silver nanoparticles was 90% (Fig. 4a). According to the optical absorption spectra presented in Fig. 7b, hydrosols and organosols had an absorption band in the region of 400-430 nm, characteristic of metal nanoparticles. According to TG/DSC data carried out in an Ar atmosphere in Fig. 8b, samples obtained using CTAB contain up to 8% wt. impurities (solvent, stabilizer), which are almost completely removed when heated to a temperature of 300°C.

Thus, the proposed method for producing organosols of metallic silver nanoparticles is simple, easy to use, does not require the use of toxic and hard-to-find reagents, and has high productivity (the concentration of the final product is at least 100 times higher than that of analogues). Thanks to this method, it was possible to obtain aggregation and sedimentation stable organosols containing spherical nanoparticles of metallic silver 5-15 nm in size with a metal concentration of up to 1800 g/l.

Claims (1)

A method for producing a highly concentrated organosol of silver nanoparticles, including the reduction of an aqueous solution of silver nitrate, characterized in that the concentration of silver nitrate in the initial solution is 0.6 M; a mixture of solutions of iron (II) sulfate and trisodium citrate is used as a reducing agent; FeSO ₄ solutions of sodium citrate and silver nitrate are alternately added until a molar ratio of 1:1:(1.5÷3.2) is achieved, the resulting precipitate is separated by centrifugation and washed with a solution of trisodium citrate, centrifuged again, the operation is repeated 3-4 times, the coagulum of nanoparticles is peptized in distilled water to form hydrosols of silver nanoparticles, the extraction of metallic silver nanoparticles is carried out from aqueous solutions into the o-xylene or butyl acetate phase in the presence of ethyl alcohol and cetyltrimethylammonium bromide or cetyltrimethylammonium nitrate, the resulting silver organosol is separated by decantation and concentrated by distilling off the solvent to a concentration for metal up to 1800 g/l.