MX2009011436A - Method for preparing silver nanoparticles by precipitation in inverse microemulsions. - Google Patents

Abstract

Described is a method for preparing silver nanoparticles including the following steps: a) preparing an inverse microemulsion having an aqueous-phase micelle structure, the inverse microemulsion is formed by water in large concentrations, one or more water-soluble metallic precursors in large amounts, one or more non-water miscible organic solvents of small molecular volume, and one or more surfactants: b) adding one or more reducing agents in low concentrations and dosing flows over the microemulsion for producing a reduction reaction of the metallic precursors in the aqueous-phase micelles in order to form silver nanoparticles; and c) separating the silver nanoparticles from the inverse microemulsion.

Description

HIGH PRODUCTIVITY METHOD TO PREPARE SILVER NANOPARTICLES FOR PRECIPITATION IN REVERSE MICROEMULSIONS DESCRIPTION OBJECT OF THE INVENTION The present invention relates to a method for producing nanoparticles by the use of microemulsions. More specifically, it refers to a high productivity method for producing silver nanoparticles by precipitation in inverse microemulsions.

BACKGROUND Studies related to the synthesis and application of materials in the presentation of particles with characteristic lengths less than 100 nm are becoming increasingly important, among which silver nanoparticles are particularly interesting due to their use in applications such as improved surface Raman spectroscopy [ZQ Tian et al., J. Phys. Chem. B, 2002, 106, 9463; A. Champion and P. Kambhampati in Chem. Soc. Rev., 1998, 27, 241], catalysis [N. Pradhan y cois, in Colloids Surf, 2002, 196, 247; H. Nakatsuji and cois, in Surf. Sci, 1997, 384, 315] and bactericidal agent [Q.L. Feng and cois, in Biomed. Maters. Res., 2000, 52, 662, J.W.

Kim and cois, in Polymer, 2004, 45, 4741]. Inverse microemulsions have been widely used in the last two decades as nano-reactors for the preparation of well-defined small spherical particles and a wide variety of various inorganic materials [K. Osseo- Asare in the Cap. 18 of the Hanbook of Micromeulsion Science and Technology, Marcel Dekker, New York, 1999]. With reference to the method of synthesis of nanoparticles from precipitation in reverse microemulsions, it is convenient to first define the term microemulsion, with which isotropic, microstructured and thermodynamically stable dispersion of two immiscible liquids is known (for example, water and oil), stabilized by one or more surfactants. According to its microstructure, microemulsions are classified as: or Normal microemulsions: those whose structure consists of swollen micelles of organic phase or oil dispersed in the aqueous phase or water, or inverse microemulsions: those whose structure consists of swollen micelles of water or aqueous phase dispersed in the organic phase or oil, or microemulsions bicontinuous: those whose structure consists of interconnected swollen nanotubes of aqueous phase or water dispersed in the organic phase or oil.

Although several methods of preparation of silver nanoparticles have been reported, the one of precipitation in inverse microemulsions is perhaps the only one that allows to obtain particles with diameters smaller than 10 nm and low polidispersity, that is, narrow distributions of size [M. Anderson and cois, in Langmüir, 2005, 21, 11387; P. Barnickel and A. Wokaun in Molecular Physics, 1990, 69, 1; C. Petit and Cois, in J. Phys. Chem., 1993, 97, 12974; THE. Pavlyukhina et al, in Inorganic Materials, 1998, 34, 109; R.P. Bagwe and K.C. Khilar in Langmüir, 2000, 16, 905; Z. Zhang et al, in J. Phys. 2000, 104, 1176].

As far as is known, Barnickel and Woakun were the first to obtain silver nanoparticles by precipitation in inverse microemulsions [Molecular Physics, 1990, 69, 1]. These authors prepared a reverse microemulsion with a solution of silver nitrate (AgNOa) as an aqueous phase to which they added an aqueous solution of the reducing agent sodium borohydride (NaBFLj). They obtained silver particles with a diameter close to 5 nm and with a relatively wide size distribution. With respect to the subsequent works, the following aspects can be mentioned.

In general, the concentrations of the aqueous phase of the inverse microemulsion containing AgN03 are low (< 10%). In addition, the concentration of Ag > 3 in the aqueous phase is also low (<0.2 M). Only some reports indicate the temperature at which the precipitation was made and in all of them it is indicated that it was at room temperature [M. Anderson and cois, in Langmuir, 2005, 21, 11387; J.W. Kim and cois, in Polymer, 2004, 45, 4741; Bagwe and .C. Khilar in Langmuir, 2000, 16, 905], Three methods were used to contact the aqueous solution of AgN03 and the aqueous solution of the reducing agent (NaBHt or hydrazine): i) mixture of a microemulsion of the AgNC solution > 3 with a microemulsion of the reducing agent solution; ii) adding the solution of the reducing agent to a microemulsion of the AgNÜ3 solution; and iii) adding the AgN03 solution to a microemulsion of the reducing agent solution. According to Zhang and cois, the procedure is the most appropriate to obtain particles of uniform size at room temperature. There is little information about the appearance of the mixture after the reaction.

Regarding the productivity of the silver precipitation technique in reverse microemulsions, there is no mention in these reports. However, a rapid calculation based on the maximum concentrations of the aqueous phase of AgN03 in the microemulsion and the molar concentration of this salt in said phase, indicates that the maximum productivity would be close to 0.4 g of AgNÜ3 per 100 g of mixture of reaction [LA Pavlyukhina et al, in Inorganic Materials, 1998, 34, 109].

Finally, regarding the characterization of the product obtained from precipitation, one of the most used techniques is UV-visible absorption spectroscopy, since colloidally dispersed silver shows a typical absorption peak at 400 nm. Another commonly used technique is transmission electron microscopy (TEM) and, to a lesser extent, X-ray scattering, since the silver nanoparticles crystallize in a centered cubic phase (fcc).

The method described above allows the obtaining of silver nanoparticles of low dispersion in the distribution of their sizes, with a disadvantage: the low productivity per batch of reaction, due to the low concentrations of aqueous phase (less than 10% by weight) that normally they are used in reverse microemulsions to precipitate silver nanoparticles. According to the specialized literature, the use of large concentrations of aqueous phase is avoided due to the fact that under these conditions the surfactant layer of the nuceles loses rigidity, which favors micellar exchange and leads to larger and more polydispersity particles in their sizes [W. Zhang et al. in Materials Science and Engineering B, 2007, 142, 1 and M. M. Husein and N. N. Nassar in Current Nanoscience, 2008, 4, 370]. Furthermore, according to Husein and Nassar, this decrease in the stiffness of the surfactant layer also leads to an increase in the fraction of inter-particle collisions that end in aggregation and, therefore, in larger particles.

According to the above, until now it has not been possible to obtain silver nanoparticles, with diameters less than 10 nm and low polydispersity, by precipitation in inverse microemulsions with large concentrations of aqueous phase. However, this could be possible if an appropriate selection of the compound that would constitute the organic phase of the microemulsion is made. As a main characteristic, this compound should have a small molecular volume, which would allow its molecules to enter the hydrocarbon chains of the surfactant and impart rigidity, even at high concentrations of aqueous phase.

In addition to the use of a constituent compound of the organic phase with a small molecular volume, it is necessary that the concentration of the aqueous solution of the reducing agent and the dosage flow are small enough not to cause destabilization in the system, which would lead to the formation of very large particles, in the order of tens of nanometers. If the dosage flow is too large, the added aqueous solution is not stabilized in the swollen micelles. If the concentration of the reducing agent is very large, the rate of reduction is too fast. In both cases the system is momentarily destabilized, which leads to the formation of silver particles with undesirable sizes.

From the foregoing, the inventors see the need to offer a method for preparing silver nanoparticles by using reverse microemulsions with high concentrations of aqueous phase, with which it is possible to achieve a higher yield and a narrow distribution of the size of the nanoparticles. .

Based on what has been described and with the purpose of solving the constraints found, it is the object of the present invention to prepare silver nanoparticles using a method that has the steps of: a) preparing a reverse microemulsion by mixing water, one or more water-soluble metal precursors, one or more organic solvents immiscible with water and, one or more surfactants, to obtain a system whose structure consists of micelles swollen with the aqueous phase dispersed in an oil phase; b) adding one or more reducing agents either in their natural presentation or in an aqueous solution so that a reduction reaction of the metallic precursors occurs in the micelles swollen with the aqueous phase, in order to form the silver nanoparticles; and c) separating the silver nanoparticles from the inverse microemulsion.

DETAILED DESCRIPTION OF THE INVENTION The method starts with the preparation of a reverse microemulsion by mixing a range of 15% by weight to 50% by weight of the aqueous solution of the metal precursor, with a range of 20% by weight to 50% by weight of at least one solvent non-miscible organic water, and a range of 25% by weight to 45% by weight of at least one surfactant agent to stabilize the microemulsion. In this way, an inverse microemulsion is obtained where the reverse micelles are swollen with a mixture of water with at least one metal precursor soluble in water. Alternatively and for the purpose of contributing to the stabilization of the microemulsion, co-surfactants may be employed. Once the inverse microemulsion is stabilized, it is brought to a temperature of approximately 20 ° C to 80 ° C and at least one reducing agent is added in order to produce a reduction reaction of the metallic precursors contained in the aqueous phase of the micelles, promoting the formation of silver nanoparticles, generally spherical. Finally, the silver nanoparticles are separated from the inverse microemulsion, preferably by a precipitation process with the addition of acetone, followed by a washing process with a water-acetone mixture, centrifugation and drying.

COMPOSITION OF THE REVERSE MICROEMULSION Metal precursor soluble in water The water-soluble metal precursor may be one or more silver salts, one or more compounds based on silver salts, one or more silver halides, one or more compounds based on silver halides and mixtures thereof.

Among the silver salts, any of their types consisting of acetates, carbonates, chlorates, phosphates, nitrates, nitrites, derivative compounds and mixtures thereof can be used.

Among the silver haiogenides, any of their types consisting of fluorides, chlorides, bromides, iodides, atates, derivative compounds and mixtures thereof can be used.

Organic solvents The organic solvent can be any non-miscible with water, such as methyl methacrylate, vinyl acetate, styrene or toluene, among others, or mixtures thereof.

Surfactants The surfactant may be of the cationic type such as dodecyltrimethylammonium bromide, didodecyldimethylammonium bromide and hexadecyltrimethylammonium bromide, of the anionic type such as sodium dodecyl sulfate and sodium bis (2-ethylhexyl) sulfosuccinate, or of non-ionic type such as nonylphenol. ethoxylated with "n" moles of ethylene oxide, where "n" must be greater than 6. Mixtures of two surfactants in different ratios can also be used. Also, the surfactant can be mixed with a co-surfactant. As an example of co-surfactant are the chain alcohols cuts like n-pentanol, n-butanol and n-propanol. The weight ratios of surfactant / co-surfactant can be from 5/95 to 95/5.

Reducing agents The reducing agent consists of an alkali borohydride or hydrazine and mixtures thereof.

The invention having been described in a general manner, an exemplary embodiment is described below that serves only as an illustration of the procedure and is not intended to be limiting in any way.

EXAMPLE An inverted microemulsion of the swollen nuceles structure was prepared with the aqueous phase. First an aqueous solution obtained from the water mixture was prepared with the metal precursor, which in this case was silver nitrate in a concentration of 0.5 M. Then, a range of 30% by weight to 40% by weight of the aqueous solution of the metallic precursor was mixed with a range of 20% by weight to 40% by weight of the organic solvent immiscible in water, which in this case was toluene; the solution was stabilized with a range of 30% by weight to 40% by weight of a surfactant agent which in this case consisted of a mixture of sodium bis (2-ethylhexyl) sulfosuccinate and sodium dodecyl sulfate in a weight ratio of 2/1.

The precipitation reaction in the reverse microemulsion was carried out at a temperature of 70 to 80 ° C in a glass reactor equipped with a condenser for reflux. The reduction reaction started with the dosing at a constant flow of the aqueous solution of sodium borohydride (NaBH_i) at a concentration of 2.9 M; The total dosing time was 110 min, which is equivalent to a flow of 0.06 g / min. The reaction mixture turned black a few minutes after the addition of the reducing agent. At the end of the dosage, the reaction was allowed to proceed for an additional 30 min. At the end of the reaction, acetone was added to promote the precipitation of the silver nanoparticles, which were recovered by a washing process with a water-acetone mixture, centrifugation and a drying process.

The silver nanoparticles obtained were characterized by X-ray diffraction and in all cases the main signals of the X-ray diffraction pattern of the silver crystals were observed, confirming that it is possible to obtain silver particles through precipitation in reverse microemulsions with large concentrations of aqueous phase.

Table 1. Composition of the inverse microemulsions and yield obtained when they were used as a means to obtain silver nanoparticles.

The silver nanoparticles were also characterized by transmission scanning electron microscopy and it was found that the particle diameters obtained, between 8 and 9 nm in diameter, and the polydispersities, expressed as standard deviation, correspond to the typical values of the particles obtained by precipitation in inverse microemulsions. Table 2 shows the results obtained in the characterization of nanoparticles for the determination of their size.

Table 2. Values of the average number diameters and dispersion index of the particles obtained by precipitation in inverse microemulsions.

Standard deviation (nm) R-1 8.6 1.7 R-2 8.8 2.6

Claims (11)

CLAIMS Having sufficiently described the invention, the authors consider as a novelty, and therefore claim as their exclusive property, what is contained in the following clauses:

1. A method for preparing silver nanoparticles that is characterized by comprising the steps of: a) preparing a reverse microemulsion of structure of swollen micelles of aqueous phase, said reverse microemulsion includes: water in large concentrations; one or more metal precursors soluble in water in large concentrations; one or more non-miscible organic solvents in water with small molecular volumes; and one or more surfactants; b) adding one or more reducing agents dosed on the microemulsion at a concentration and dosage flow such that it does not cause destabilization and allows the reduction reaction of said metal precursors to occur within the swollen micelles of the aqueous phase in order to form nanoparticles of silver; Y c) separating said silver nanoparticles from said bicontinuous microemulsion.

2. The method of claim 1, characterized in that said metal precursor is selected from the group consisting of metal salts, compounds with base in metal salts, metal halides, compound based on metal halides and mixtures thereof.

The method of claim 2, characterized in that said metal salts are silver salts consisting of acetates, carbonates, chlorates, phosphates, nitrates, nitrites, compounds derived therefrom and mixtures thereof.

The method of claim 2, characterized in that said metal halides are silver salts selected from the group consisting of fluorides, chlorides, bromides, iodides, iates, compounds derived therefrom and mixtures thereof.

The method of claim 1, characterized in that said organic solvent immiscible with water is selected from the group consisting of methyl methacrylate, vinyl acetate, styrene, toluene, etc., or mixtures thereof.

The method of claim 1, characterized in that said surfactant can be of cationic type such as dodecyltrimethylammonium bromide, didodecyldimethylammonium bromide and hexadecyltrimethylammonium bromide, of the anionic type such as sodium dodecyl sulfate and sodium bis (2-ethylhexyl) sulfosuccinate. , or non-ionic type such as nonylphenol ethoxylated with "n" moles of ethylene oxide, where "n" must be greater than 6. Mixtures of two surfactants in different ratios can also be used.

The method of claim 1, characterized in that the surfactant can be mixed with a co-surfactant. As an example of co-surfactant are the short chain alcohols such as n-pentanol, n-butanol and n-propanol. The weight ratios of surfactant / co-surfactant can be from 5/95 to 95/5.

8. The method of claim 1, characterized in that said reducing agent is selected from the group consisting of an alkaline borohydride, hydrazine and mixtures thereof.

9. The method of claim 1, characterized in that the diameter of said silver nanoparticles is given primarily by the diameter of said swollen micelles of aqueous phase.

10. The method of claim 9, characterized in that the diameter of said silver nanoparticles is of a range of 1 nm to 15 nm.

11. The method of claim 1, characterized in that the step for separating the silver nanoparticles from said inverse microemulsion is performed by destabilizing the inverse microemulsion where said silver nanoparticles were obtained.