RU2644176C1 - Method of producing stable high-concentrated organosols based on silver nanoparticles to produce conductive films - Google Patents

Abstract

FIELD: chemistry.

SUBSTANCE: invention relates to the field of colloid chemistry, namely to the methods of producing stable organosols of the metal nanoparticles, particularly silver nanoparticles, that are promising as ink-paints to produce conductive films, electronic inks for electrophoretic displays, medicines for external use with an antimicrobial activity, heat liquids, active amplifying media for "random" lasers. The method of producing a stable high-concentrated organosol based on silver nanoparticles to produce conductive films, which includes a stage of solubilization and a stage of recovery of silver ions by hydrazine in the dispersion medium of 0.25 M of the solution of bis (2-ethylhexyl) sulfosuccinate of sodium in n-decane, in which the source reagent is silver nitrate, and hydrazine is injected into the dispersion medium in the mode of dynamic inverse emulsion, and at first a solution of silver nitrate is introduced. The stages of solubilization and recovery of silver ions are combined in one stage, with the volume ratio of the phases of aqueous solutions of reagents: dispersion medium is 1:2.5 with the subsequent separation of the phases, the organic phase is dehydrated, 1.0 to 2.5. % of the total volume of organosol and water is introduced into dehydrated organosol, and the electrophoretic concentration is carried out at an intensity of a permanent field of 300-600 V/cm for 1.5-2 hours.

EFFECT: method is proposed to provide a stable concentrate with a concentration of silver nanoparticles sufficient to produce conductive films.

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Description

The invention relates to the field of colloid chemistry, the chemistry section of ultrafine systems, and in particular to methods for producing stable organosols of metal nanoparticles. Stable organosols of silver nanoparticles are promising as ink-inks for producing electrically conductive films, electronic paints for electrophoretic displays, external drugs with antimicrobial effects, heat-transferring liquids, active amplifying media for "random" lasers.

Known methods for producing organosols of silver and other metal nanoparticles by micellar (microemulsion) synthesis [1. Capek I. Preparation of metal nanoparticles in water-in-oil (w / o) microemulsions // Adv. Colloid Interface Sci. - 2004. - V. 110. - N 1-2. - P. 49-74. 2. Eastoe J., Hollamby M. J., Hudson L. Recent advances in nanoparticle synthesis with reversed micelles // Adv. Colloid Interface Sci. - 2006. - V. 128-130. - P. 5-15].

The methods make it possible to adjust the size of the resulting nanoparticles and to obtain organosol particles with a narrow size distribution. Solutions of the starting reagents, most often water-soluble metal salts and reducing agents, are injected (solubilized) by injection into two different parts of a solution of micelle-forming surfactant with a concentration of 0.1 to 10% in organic solvents, most often in saturated hydrocarbons, which are then mixed with the formation of optically transparent solutions.

A disadvantage of the known microemulsion injection methods is the low concentration of metal nanoparticles in dispersions, insufficient for the formation of conductive films. The indicated drawback is due to the extremely low solubilization capacity of micellar surfactant solutions, usually not more than 1-5 vol.%, And the solubilization capacity decreases sharply with increasing concentration of the introduced salts. Therefore, micellar synthesis is usually carried out with solubilization capacities of not more than 1 vol%. In addition, low solubilization capacities provide a small nanoparticle size (2-10 nm) and good monodispersity. However, as a result, already at the stage of synthesis, the concentrations of the reagents are reduced 100 times, compared with the "water" synthesis.

Known methods for increasing the concentration of metal nanoparticles in an organosol by introducing reagents (silver nitrate and reducing agents) into the initial micellar solutions by liquid or solid phase extraction [3. Noritomi N., Umezawa Y., Miyagawa S., Kato S. Preparation of Highly Concentrated Silver Nanoparticles in Reverse Micelles of Sucrose Fatty Acid Esters through Solid-Liquid Extraction Method // Advances in Chemical Engineering and Science-2011. - V. 1. - N 4. - P. 299-304]. As a result, the concentration of nanoparticles was increased by an order of magnitude. However, the obtained concentration of nanoparticles in the organosol was not enough to obtain conductive films.

М.Е, Saade Н. and

R. G. High-Yield Synthesis of Silver Nanoparticles by Precipitation in a High-Aqueous Phase Content Reverse Microemulsion // Hindawi Publishing Corporation. Journal of Nanomaterials V. 2010, Article ID 392572, 6 pages doi:10.1155/2010/392572]. Его недостатками являются сложный состав микроэмульсии, использование смешанных мицелл AOT+DDSNa (додецилсульфат натрия), синтез при повышенной температуре (70°C) и сложность подбора растворителя для редиспергирования.A known method of producing organosols by synthesis in microemulsions with a high content of aqueous pseudophase with the release of solid phase precipitate and its subsequent redispersion in a suitable solvent [4. Sosa Y. D, Rabelero M.,

M.E., Saade N. and

RG High-Yield Synthesis of Silver Nanoparticles by Precipitation in a High-Aqueous Phase Content Reverse Microemulsion // Hindawi Publishing Corporation. Journal of Nanomaterials V. 2010, Article ID 392572, 6 pages doi: 10.1155 / 2010/392572]. Its disadvantages are the complex composition of the microemulsion, the use of mixed AOT + DDSNa micelles (sodium dodecyl sulfate), synthesis at elevated temperatures (70 ° C), and the difficulty in choosing a solvent for redispersion.

The known method [5. Bulavchenko A.I., Pletnev D.N. Electrophoretic concentration of nanoparticles of gold in reversed micellar solutions of AOT // J. Phys. Chem. C - 2008. - V. 112. - No. 42. - P. 16365-16369] obtaining concentrated organosols of gold nanoparticles, in which the injection of reagents and subsequent synthesis were supplemented by the introduction of an additional stage of electrophoretic concentration in an electrophoretic cell of a capacitor type. The concentration of gold in the organosol increased to 2 mol / L, however, the volume of the obtained concentrate was only 0.1 vol.% Of the volume of the microemulsion (for example, no more than 20 μl of a liquid concentrate with a concentration of 2 M can be extracted from 20 ml of micellar solution after synthesis).

The known method [6. Bulavchenko A.I. Popovetsky P.S. The electrokinetic potential of nanoparticles in reverse AOT micelles: photometric determination and role in the processes of heterocoagulation, separation, and concentration // Langmuir. - 2010. - V. 26. - No. 2. - P. 736-742] obtaining concentrated organosols of silver by electrophoretic concentration. The silver concentration in the organosol increased to 1 M, which was enough to obtain conductive films. However, the volume of emitted concentrate was also insignificant, not more than 0.1 vol. % of the volume of microemulsions.

The known method [7. Popovetsky P.S., Bulavchenko A.I., Manakov A.Yu. Obtaining and physicochemical properties of a hydrophobic concentrate of silver nanoparticles // Optical journal. - 2011. - T. 78, No. 7. - S. 67-72], in which the preparation of organosol was carried out as follows. At the synthesis stage, in 10 ml portions of a 0.25 M solution of AOT (sodium bis- (2-ethylhexyl) sodium sulfosuccinate) in decane are injected: in the first 0.1 ml of silver nitrate (AgNO ₃ ) with a concentration of 0.25 M; in the second 0.1 ml of 20 M hydrazine (N ₂ H ₄ ). Solutions after administration of reagents should be transparent. Then the solutions with the reagents are mixed. The reaction mixture is left overnight. The concentration of silver in the intensively yellow organozole was 0.00125 M. When a solution of AgNO ₃ with a higher concentration or large volumes was introduced into the first portion, a stable organosol with silver nanoparticles was not formed: a black precipitate formed already during the reduction. Further, electrophoretic concentration was carried out similarly [5, 6]. The silver concentration in the organosol increased to 1 M, which was enough to obtain conductive films. However, the allocated volume of the concentrate was also insignificant (not more than 0.1 vol.% Of the volume of the microemulsion). This method is a prototype of the proposed method.

The objective of the invention is to significantly increase the volume of allocated stable concentrate with a concentration of nanoparticles sufficient to obtain conductive films. The problem is solved in that in the method for producing stable highly concentrated organosols based on silver nanoparticles to obtain electrically conductive films, the starting reagents — silver nitrate and hydrazine — are introduced into the dispersion medium in the dynamic inverse emulsion mode, and silver nitrate solution is first introduced, at the same time, the solubilization and reduction of silver ions are combined in one stage, with a ratio of phase volumes, aqueous solutions of reagents: dispersion medium equal to 1: 2.5, followed by by separation of phases, the organic phase is dehydrated, 1.0 to 2.5 vol.%, of the volume of organosol, water is introduced into the dehydrated organosol and electrophoretic concentration is carried out at a constant field strength of 300-600 V / cm for 1.5-2 hours while restoring using 0.3 M silver nitrate solution and 10 M hydrazine solution, the resulting organosol concentrate is stable, has a density of 1.1-1.3 g / cm ³ , contains 3-4.5 M silver in the form of nanoparticles of size 3-6 nm.

Distinctive features of the invention:

- initial reagents: silver nitrate and hydrazine, introduced into the dispersion medium in the dynamic reverse emulsion mode;

- sequence of introduction of reagents;

- combining in one stage the stage of solubilization and recovery of silver ions;

- the ratio of the volume of phases to aqueous solutions of reagents: dispersion medium is 1: 2.5;

- the introduction of dehydrated organosol from 1.0 to 2.5 vol.% water, based on the volume of organosol;

- the concentration of the starting reagents and the characteristics of the obtained organosol concentrate.

In the prototype method, the classical micellar (microemulsion) synthesis scheme is used: the initial reagents are introduced into the AOT micellar solution by injection solubilization, i.e. successive dropping of aqueous solutions of reagents to the organic phase (AOT solution in saturated hydrocarbons); the resulting microemulsions should be transparent. An essential and difficult to overcome disadvantage of micellar synthesis is the low mass concentration of the resulting organosols. The problem is caused by the small solubilization capacity of the reverse microemulsions with respect to aqueous solutions of the starting reagents (1-5 vol.%). Thus, in comparison with the “water” synthesis, the reagents are diluted by injection solubilization immediately 20-100 times! Moreover, it is most often low solubilization capacities (0-1 vol.%) That are used, since only small nanoparticles are formed at low contents of the aqueous pseudophase.

This problem is solved in the proposed method in that the starting reagents — silver nitrate and hydrazine — are introduced into the dispersion medium in the dynamic reverse emulsion mode, the solubilization and reduction stages of silver ions are combined into one, which allows the introduction of much larger volumes of silver nitrate (for example, 4 ml versus 0.1 ml in 20 ml of the organic phase). And the recovery is carried out not in micellar mode, but in the dynamic reverse emulsion mode. In the emulsion, the reagents are initially in micron size droplets; while in micellar systems - in micelles of nanometer size. After synthesis, excess water is removed and an organosol with a silver concentration of 0.04-0.05 M is sent to electrophoresis, which is ~ 30 times higher than the concentration in the organosol obtained by micellar synthesis.

Justification of the features of the formula. In order to find the optimal conditions for the emulsion synthesis of silver nanoparticles in a dispersion medium (0.25 M solution of AOT in n-decane) with a fixed ratio of the volumes of the phases of the dispersion medium and aqueous solutions of the starting reagents equal to 2.5, we varied the sequence of the introduction of the reagents, the concentration of hydrazine and silver nitrate in an aqueous solution (phase of the starting reagents). To calculate the yield of the product, the silver content in the dispersion medium (organic phase) was controlled by spectrophotometry by its plasmon absorption. It was shown that silver organosol can be obtained only by first introducing a solution of AgNO ₃ into the organic phase. The effect of hydrazine concentration in the range of 1–20 M was studied. Table 1 shows the effect of hydrazine concentration on the yield of silver nanoparticles. Conditions: the volume of the organic phase is 20 ml; the volume of the starting reagents is 8 ml (4 ml of 1 M AgNO ₃ + 4 ml of N ₂ H ₄ ). Based on the obtained data, the optimum value of the concentration of hydrazine was chosen - 10 M (silver yield - 25%). To increase the yield of silver nanoparticles in the organosol and eliminate its losses, the effect of the concentration of silver nitrate in an aqueous solution was studied. Table 2 shows the effect of silver concentration (0.3-1 M) on the yield of silver nanoparticles. From the data obtained it follows that under conditions: the volume of the organic phase is 20 ml; the total volume of the starting reagents is 8 ml (4 ml each); the concentration of AgNO ₃ - 0.3 M; the concentration of N ₂ H ₄ - 10 M achieved a high yield of silver nanoparticles - 78%. These synthesis conditions are selected as optimal.

The optimal conditions for electrophoretic concentration were dependent on the content of the aqueous pseudophase in the microemulsion. Especially for this, after synthesis, the microemulsion was dehydrated by evaporation of water with stirring in an open glass. Then, from 0.5 to 10 vol.% Distilled water was injected into the microemulsion. This technique allows you to control the water content in the microemulsion and to find the optimal water content for electrophoresis. Electrophoresis was performed the day after injection solubilization of the aqueous pseudophase. The results of the electrophoresis concentration of six parallel experiments with an aqueous pseudophase content of 2.5 vol% and four parallel experiments with an aqueous pseudophase content of 1.5 vol% are shown in Tables 3 and 4. The synthesis conditions are as follows: in 20 ml of dispersion medium (0.25 M AOT in decane) was introduced 4 ml of 0.3 M AgNO ₃ and 4 ml of 10 M hydrazine. In the first case, electrophoresis was carried out for 1.5 hours at a field strength of 600 V / cm; in the second - 2 hours at a field strength of 300 V / cm. The dependence of the silver concentration in the isolated concentrate on the content of the aqueous pseudophase is shown in FIG. 1. From the data obtained it follows that silver concentrations above 3 M are obtained with an aqueous pseudophase content in a rather narrow range: from 1 to 2.5 vol.%. As a result, after electrophoresis, a significantly larger (about an order of magnitude) volume of concentrate (0.21 and 0.27 ml versus 0.025 ml) with a higher silver concentration (4.3 and 3.34 M versus 1 M in the prototype method) is obtained. The sizes of nanoparticles and other parameters of the organosol are not significantly different, the obtained organosol concentrates under the conditions of the proposed synthesis and concentration contain 3-4.5 M silver in the form of nanoparticles with a size of 3-6 nm. In FIG. Figure 2 shows the spectra of plasmon resonance absorption of silver nanoparticles. The wavelength of the absorption maximum (405 nm) is characteristic in accordance with the theory of M for small nanoparticles. According to photon correlation spectroscopy, the effective hydrodynamic diameter of the obtained nanoparticles is 6 nm (n-averaging). The obtained value is close to the diameter determined by transmission electron microscopy (3 nm) (Fig. 3, 4). The difference of 3 nm should be attributed to the doubled length of the AOT molecule adsorbed by the nanoparticle. The dependence of the density of the obtained organosols on the silver concentration is shown in FIG. 5.

The mechanism of the proposed emulsion synthesis has not been studied in detail. Apparently, the reagents from micron droplets of the dynamic inverse emulsion pass into the nanometer cavities of the reverse micelles of the dispersion medium, where they interact. The adsorption layer of micelles limits the growth of silver nanoparticles and stabilizes nanoparticles with respect to coagulation.

The synthesis is carried out at room temperature. In a micellar solution of the surface-active substance AOT (sodium bis (2-ethylhexyl) sulfosuccinate) in n-decane, with strong stirring, an aqueous solution of silver nitrate salt (AgNO ₃ ) is first introduced (20% of the volume of the micellar solution), and then immediately the same volume of concentrated reducing agent is hydrazine. In total, the volume of reagents is 40 vol.% Of the initial volume of the AOT micellar solution. As a result, the reaction mixture is a dynamic inverse emulsion with a water / oil phase ratio of 1: 2.5. The synthesis is carried out for 60 minutes with vigorous stirring; the emulsion changes color from colorless to dark brown (ink color). The emulsion is left overnight until the phases are completely separated and centrifuged the next day. The phases are separated, the organic is stirred on a magnetic stirrer in an open glass until the water evaporates completely. The resulting organosol of the nanoparticles is filtered; an aliquot of the organosol is diluted and analyzed spectrophotometrically for silver.

Electrophoretic concentration. 1-2.5 vol% distilled water (aqueous pseudophase) is introduced into the dehydrated organosol. The resulting microemulsion is poured into an electrophoretic cell with plane-parallel electrodes of a capacitor type and subjected to electrophoresis in order to further concentrate and separate the nanoparticles from reaction by-products. The resulting liquid concentrate is analyzed for metal content, the size is determined and the absorption spectra of the nanoparticles are recorded; measure density, viscosity and surface tension - parameters that determine the technological characteristics of the resulting final ink concentrate.

Characterization. Surface plasmon resonance spectra of nanoparticles were recorded with a Shimadzu 1700 spectrophotometer. The size of the nanoparticles directly in the organosol was determined by photon correlation spectroscopy (PCF, NanoBrook Omni spectrometer, United States), and dried samples by transmission electron microscopy (TEM). We used a JEM-2010 electron microscope with a maximum resolution of 0.2 nm per point. The electrokinetic potential (zeta potential) of silver nanoparticles in an organosol after synthesis was determined on a NanoBrook Omni spectrometer in the PALS option, manufactured by Brookhaven, USA. Organosol viscosity was determined using a MicroVISC viscometer (ReoSense Inc, USA). Surface tension was determined by analyzing the shape of a hanging drop on an OCA 15Pro tensiometer (Dataphysics, Germany). The tension was calculated by the Young-Laplace equation. Density was measured by weighing known volumes of organosols on a ViBRA HTR-220CE analytical balance (Sinko Denshi, Japan).

Obtaining conductive films from the final concentrate. Electrically conductive films were obtained in two ways: thermal and chemical metallization [8. Bulavchenko A.I., Popovetsky P.S., Maksimovsky E.A. Properties of conductive films from electrophoretic concentrate of silver and gold nanoparticles in AOT // Journal of Physical Chemistry. - 2013, T. 87. No. 10. - S. 1779-1784], the application of the concentrate by spin coating.

Thermal metallization is the thermolysis of films of dried concentrates on glass substrates at temperatures of 200-250 ° C, and chemical metallization is the aging of films of dried concentrates in aqueous-alcoholic solutions. The electrical resistance of the obtained films was measured using a UT70A digital multimeter with needle probes located at a distance of 2 cm from each other. The film was recognized as conducting if the resistance between the probes did not exceed 10 ohms.

Example 1. In 20 ml of a 0.25 M solution of AOT in n-decane with vigorous stirring (500 rpm) for 4 minutes, 4 ml of an AgNO ₃ solution with a concentration of 0.3 mol / l is added dropwise, and then similarly added 4 ml of hydrazine monohydrate (10 mol / L). The ratio of phase volumes to aqueous reagent solutions: dispersion medium was 1: 2.5.

Note that the volumes of the reagents are 40 times higher than the volumes introduced by the prototype method. Stirring is continued for 60 minutes. The emulsion is left overnight until the phases are completely separated and further centrifuged the next day for 10 minutes at a rotation speed of 3000 rpm. The phases are separated, organic (20 ml) is stirred on a magnetic stirrer (500 rpm) in an open glass for 3 hours until the water evaporates completely. The obtained dehydrated organosol of nanoparticles is filtered through a yellow ribbon paper filter. An aliquot of the organosol is diluted 1000 times, analyzed spectrophotometrically for the silver content and the hydrodynamic diameter is determined. The wavelength of the maximum absorption of nanoparticles was 405 nm, the molar absorption coefficient was 1.5 × 10 ⁴ l / (cm × mol), the effective hydrodynamic diameter was 6 nm (by TEM 3 nm), the silver concentration was 0.048 mol / l (Fig. 6), and the yield silver after synthesis was 80%.

Further, 500 μl (2.5 vol.%) Of distilled water is gradually added to the organosol with manual shaking in a test tube. The microemulsion is left overnight, then the electrokinetic potential of the nanoparticles (21 mV) is measured and poured into the electrophoretic cell. The cell is a cubic glass cuvette with a facet size of 4.5 cm, in which two copper plane-parallel electrodes 4 * 4 cm in size with an interelectrode gap of 1 cm are horizontally located. Electrophoresis was performed for 1.5 hours at a constant field strength of 600 V / cm. After the liquid concentrate accumulated on the lower electrode (anode), the electric field was turned off, and the cell was tilted to one of the corners, where the whole concentrate is stacked. The concentrate was taken with a pipette. Its volume was 0.210 ml, the silver concentration was 4.3 mol / l, the density of the concentrate was 1.30 g / cm ³ , the surface tension was 23.6 ± 0.4 mN / m, and the viscosity was 1.265 ± 0.0004 cP. The effective hydrodynamic diameter of the nanoparticles in the concentrate was 6 nm (by TEM 3 nm); the wavelength of the maximum absorption after electrophoresis 405 nm. As a result, the volume of the concentrate increased by an order of magnitude compared with the prototype method.

Example 2 is carried out analogously to example 1, but the amount of water introduced before electrophoresis is 1.5 vol.%. Electrophoresis was performed for 2 hours at a constant field strength of 300 V / cm. The resulting concentrate had a wavelength of maximum absorption the same as in the first example, the volume of the concentrate was 0.27 ml, a concentration of 3.35 M, a density of 1.23 g / cm ³ . The effective hydrodynamic diameter of the nanoparticles in the concentrate was 5.9 nm.

Example 3 is carried out analogously to example 1, but the amount of water introduced before electrophoresis is 2 vol.%. Electrophoresis was performed for 2 hours at a constant field strength of 400 V / cm. The resulting concentrate had a wavelength of maximum absorption the same as in the first example, the volume of the concentrate was 0.28 ml, a concentration of 3.27 M, a density of 1.20 g / cm ³ . The effective hydrodynamic diameter of the nanoparticles in the concentrate was 6.1 nm.

All obtained electrophoretic concentrates are highly stable with respect to coagulation, which was monitored during the year by periodically measuring the hydrodynamic diameter of the nanoparticles and taking the plasmon absorption spectra. These parameters were constant. Moreover, when the solvent completely dries in a dry paste-like AOT-Ag composite, when stored in an open vessel under room conditions, silver nanoparticles are also not subject to coagulation. The dry composite spontaneously quickly and completely redisperses in saturated hydrocarbons, benzene, toluene, chloroform and hydrogen tetrachloride even after a long (1 year) storage. The silver concentration, nanoparticle size, and absorption spectra do not change. High stability with respect to coagulation allows the use of organosol concentrate to obtain stable ink inks.

Electrically conductive films were obtained from the obtained organosol concentrates. 20 μl of the concentrate is applied to two glass plates; after drying, the first plate is placed in an oven and heated for 2 hours at a temperature of 200 ° C (thermal metallization), the second is lowered for 5-10 seconds in a 70% ethanol solution (chemical metallization). In FIG. Figure 7 shows SEM micrographs of conducting films obtained by chemical metallization (1) and thermolysis (2). It can be seen from them that as a result of the described procedures, coagulation-crystallization conductive contacts formed between the nanoparticles. This work was financially supported by the Russian Science Foundation (RSF) in accordance with Agreement No. 15-13-00080 of 05/20/2015 between the RSF, the project manager A. I. Bulavchenko and the Institute of National Economy of the SB RAS on the provision of a grant for basic research and exploratory research.

Claims (3)

1. A method of obtaining a stable highly concentrated organosol based on silver nanoparticles to obtain electrically conductive films, comprising a solubilization step and a step for reducing silver ions with hydrazine in a dispersion medium of a 0.25 M solution of sodium bis (2-ethylhexyl) sulfosuccinate in n-decane, characterized in that the starting reagents — silver nitrate and hydrazine — are introduced into the dispersion medium in the dynamic inverse emulsion mode, with the silver nitrate solution being introduced first, and the solubilization steps the recovery of silver ions is combined in one stage, with a ratio of phase volumes, aqueous solutions of reagents: dispersion medium equal to 1: 2.5 followed by phase separation, the organic phase is dehydrated, 1.0 to 2.5 vol.% of the volume is introduced into the dehydrated organosol organosol, water and conduct electrophoretic concentration at a constant field strength of 300-600 V / cm for 1.5-2 hours. 2. The method according to p. 1, characterized in that during the restoration use 0.3 M silver nitrate solution and 10 M hydrazine solution. 3. The method according to p. 1, characterized in that the obtained organosol concentrate with a density of 1.1-1.3 g / cm ³ contains 3-4.5 M silver in the form of nanoparticles with a size of 3-6 nm.

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