**Abstract**

Microemulsions are self-aggregated colloidal systems that provide a controllable system with a promising application as nanoreactors: they can act as pools within which the properties of the nanoparticles can be controlled without difficulty. So in this chapter, I will deal with the metal NPs synthesized by the microemulsion method. This method allows in some cases to control the properties of size, shape, and crystal structure of the metallic NPs, thus generating with the same reagents a series of seeds of different shapes and sizes. The control of the reaction time, the temperature, and the reaction conditions will give us a production of different geometries that will find different applications in large range of research fields.

**Keywords:** microemulsion method, nanoparticles, colloidal suspensions, self-assembly, surfactant

## **1. Introduction**

During the last decades, self-assembly of micro- and nano-materials attracted a widespread mindfulness because of special properties when comparing with bulk counterparts, because of mechanical and physicochemical property changes at micro- and nanoscale. I will focus on a particular three-phase diagram proportion that is known as the microemulsion system. It is thermodynamically stable and isotropic dispersion of two immiscible liquids, where a surfactant stabilizes micro-domains [1]. It includes different structures and configurations corresponding to different performances of self-assembled colloidal systems. Then I will focus on the assembled structure of nanoparticles. To be able to obtain these structures by microemulsion method, it must be known making a good use of the properties of the microemulsion systems, in which the composition of these structures will determine properties as important as the size of the microdroplets. That ultimately will determine the reaction system of our synthesis of nanoparticles.

The microdroplet environment offers a controllable medium to obtain NPs with tailored size, shape, and structural properties.

#### **2. Metal nanoparticles (MNPs)**

#### **2.1 Anionic surfactants**

Here I focus on MNPs synthesized by microemulsion method using the anionic surfactant, reviewing literature that employed this surfactant type I found a very noticeable research.

**Figure 1.** *AOT chemical structure.*

Microemulsion method was used by Kon-No et al. [2]; they prepared two microemulsions, one them solubilizing FeCl3 aqueous solution in AOT/cyclohexane and the other aqueous NH3 in AOT/cyclohexane; both were mixed joint (see AOT chemical structure in **Figure 1**). To obtain the formation of Fe3O4 in the micelle, authors added a FeCl2 solution to the mixture of two microemulsions. The solution showed stability during 2 years. A diluted solution of the colloid obtained was characterized by TEM to determine the crystal structure and size distribution by electron micrography. Then the authors obtained magnetic measurements together with an extended analysis to determine detailed surface magnetic properties that allowed them to conclude that (i) distribution function of colloidal particles is a log-normal type, coincidentally with results obtained by gas evaporation technique, (ii) the magnetic interaction between the colloidal particles is negligible, (iii) the colloidal system showed superparamagnetism about 50K, (iv) paramagnetic Fe ions potentially are present on a particle surface, and (v) the temperature dependence of the spontaneous magnetization linearly decreases with the increase of temperature.

López-Quintela and Rivas [3] reported a simple and powerful method to obtain ultrafine particles by means of chemical reactions in microemulsions. The reaction happened within the nanodroplets of the microemulsion. Varying droplets' radii authors were able to control particle dimension. They reported that the proper selection of microemulsion components would ease the exchange of reagents between the "transition dimers" formed by two droplets of the microemulsions; for this to happen, an important alteration in the local curvature is necessary, so the selection of adequate surfactants that shows a radius of curvature close to their natural radius will facilitate the opening of channels [4–9], particle formation, and growth. Once the particles are formed, the surfactant particles act as surface agents, limiting the future growth of the particles. The sizes of the microemulsion droplets can be tuned between 5 and 50 nm by changing the relation of the components of the microemulsion (e.g., *W* = [H2O]/[AOT]) or varying the microemulsion itself. The results for Fe particles obtained in an AOT microemulsion system (*W* = 22.2; [AOT] = 0.05 M; reactant A: [FeCl2] = 1.9 × 10<sup>−</sup><sup>4</sup> M; reactant B: [NaBH4] = 8.8 × 10<sup>−</sup><sup>4</sup> M) were discriminated using XRD. Nucleation process was confirmed by enhancing the number of scattering centers and hence the scattering intensity. Conversely, growth of the particles is associated with a decrease on the scattering intensity because of the "disappearing" of the smaller particles during their growth. The authors concluded that this general method produces ultrafine particles with tunable size by means of chemical reactions within microemulsions. This simple and reproducible method allowed them to produce quasi-monodisperse particles, within thermodynamically stable system (microemulsion) that was used to control the growth of the particles.

Mann et al. reported [10] that interfacial activity could be used to combine nanoparticle synthesis and self-assembly to produce complex organized materials rising from the interaction of tensioactive molecules attached to specific nanoparticle crystal faces. Authors demonstrated this principle by controlling the [Ba2+]:[CrO4 <sup>2</sup><sup>−</sup>] inside of droplets of the microemulsion that tunes the fusing

**79**

*Synthesis of NPs by Microemulsion Method DOI: http://dx.doi.org/10.5772/intechopen.80633*

Ba(AOT)2 by direct precipitation [11].

solution (1:1) to obtain magnetite NPs.

0.25 M NaOH solution was produced.

**Figure 2**) [14, 15].

**Figure 2.**

*SDS chemical structure.*

**2.2 Cationic surfactants**

outstanding research.

of microdroplets and reverse micelles. It allowed them to produce linear chains, rectangular superlattices, and long filaments, as a function of reactant ratio. They carried out the synthesis and self-assembly of NPs by converting sodium AOT to

Wang et al. [12] reported by the first time a new microemulsion method to synthesize magnetite NPs. The novelty of their method founded on the costeffective use of a single microemulsion. The authors prepared the microemulsion by solubilizing NaOH solution into DBS/ethanol/toluene system (DBS, sodium dodecylbenzene sulfonate), and then ferric and ferrous salts with molar ratio (2:1) were made into a mixed solution. A volume of this mixture was drop-wise added to microemulsion under vigorous stirring. The mixed system turned black immediately. After the mixed system was stirred during ½ h and then left undisturbed during 5 h, a two-layer system appeared. The upper layer was black, which contained the NPs, and lower layer was discarded. Following a magnetic separation technique, the magnetite NPs were extracted and then washed several times in ethanol/water

He et al. reported [13] the chemical synthesis of a nonviral gene transferor, calcium carbonate NP using SDS microemulsions (see SDS chemical structure in

Here I focus on MNPs synthesized by microemulsion method using the cationic surfactant, examining literature that employed this surfactant type I found very

Chin and Yaacob [16] reported the synthesis of magnetic iron oxide nanoparticles by preparing water-in-oil microemulsion system. Authors investigated two different volumetric ratios of Fe2+ and OH<sup>−</sup> (1:1 and 2:1). Crystallized, physical, and magnetic sizes of magnetic iron oxide nanoparticles were in the superparamagnetic size range (XRD, TEM, and AGM). Superparamagnetic behavior was warranted by hysteresis loop nonappearance in magnetization curve at room temperature. MIONPs were obtained by Massart's method too. The saturation magnetization of MIONPs obtained by *w/o* microemulsion system was larger though the crystallization size was lower, which demonstrated that MIONPs could be custom-made by distinct methods. The preparation of iron oxide NPs was carried out according to the following route. Ferrous chloride salt solution was prepared (0.2 M); it was steady by adding a few drops of hydrochloric acid (0.5 M). Afterward, the microemulsion was set by melting HTAB in n-octane; subsequently 1-butanol was added followed by FeCl2 aqueous salt solution. The system was gradually shaken till a clear microemulsion suspension was attained. By repeating above method a microemulsion containing

These microemulsion systems were next fused at 1:1 vol. Instantaneously, a dark green solid was produced before it turned to black. The NPs were gathered and later cleaned repeatedly with acetone and de-ionized water and then they were dried at room temperature which is called S1. The same route was followed to yield a second sample with primary [Fe2+] = 0.1 M. [NaOH] was kept at 0.25 M. The volumetric

*Synthesis of NPs by Microemulsion Method DOI: http://dx.doi.org/10.5772/intechopen.80633*

**Figure 2.** *SDS chemical structure.*

*Microemulsion - A Chemical Nanoreactor*

**Figure 1.**

*AOT chemical structure.*

Microemulsion method was used by Kon-No et al. [2]; they prepared two microemulsions, one them solubilizing FeCl3 aqueous solution in AOT/cyclohexane and the other aqueous NH3 in AOT/cyclohexane; both were mixed joint (see AOT chemical structure in **Figure 1**). To obtain the formation of Fe3O4 in the micelle, authors added a FeCl2 solution to the mixture of two microemulsions. The solution showed stability during 2 years. A diluted solution of the colloid obtained was characterized by TEM to determine the crystal structure and size distribution by electron micrography. Then the authors obtained magnetic measurements together with an extended analysis to determine detailed surface magnetic properties that allowed them to conclude that (i) distribution function of colloidal particles is a log-normal type, coincidentally with results obtained by gas evaporation technique, (ii) the magnetic interaction between the colloidal particles is negligible, (iii) the colloidal system showed superparamagnetism about 50K, (iv) paramagnetic Fe ions potentially are present on a particle surface, and (v) the temperature dependence of the spontaneous magnetization linearly decreases with the increase of temperature. López-Quintela and Rivas [3] reported a simple and powerful method to obtain ultrafine particles by means of chemical reactions in microemulsions. The reaction happened within the nanodroplets of the microemulsion. Varying droplets' radii authors were able to control particle dimension. They reported that the proper selection of microemulsion components would ease the exchange of reagents between the "transition dimers" formed by two droplets of the microemulsions; for this to happen, an important alteration in the local curvature is necessary, so the selection of adequate surfactants that shows a radius of curvature close to their natural radius will facilitate the opening of channels [4–9], particle formation, and growth. Once the particles are formed, the surfactant particles act as surface agents, limiting the future growth of the particles. The sizes of the microemulsion droplets can be tuned between 5 and 50 nm by changing the relation of the components of the microemulsion (e.g., *W* = [H2O]/[AOT]) or varying the microemulsion itself. The results for Fe particles obtained in an AOT microemulsion

system (*W* = 22.2; [AOT] = 0.05 M; reactant A: [FeCl2] = 1.9 × 10<sup>−</sup><sup>4</sup>

confirmed by enhancing the number of scattering centers and hence the scattering intensity. Conversely, growth of the particles is associated with a decrease on the scattering intensity because of the "disappearing" of the smaller particles during their growth. The authors concluded that this general method produces ultrafine particles with tunable size by means of chemical reactions within microemulsions. This simple and reproducible method allowed them to produce quasi-monodisperse particles, within thermodynamically stable system (microemulsion) that was used

Mann et al. reported [10] that interfacial activity could be used to combine nanoparticle synthesis and self-assembly to produce complex organized materials rising from the interaction of tensioactive molecules attached to specific nanoparticle crystal faces. Authors demonstrated this principle by controlling

M) were discriminated using XRD. Nucleation process was

<sup>2</sup><sup>−</sup>] inside of droplets of the microemulsion that tunes the fusing

M; reactant B:

**78**

the [Ba2+]:[CrO4

[NaBH4] = 8.8 × 10<sup>−</sup><sup>4</sup>

to control the growth of the particles.

of microdroplets and reverse micelles. It allowed them to produce linear chains, rectangular superlattices, and long filaments, as a function of reactant ratio. They carried out the synthesis and self-assembly of NPs by converting sodium AOT to Ba(AOT)2 by direct precipitation [11].

Wang et al. [12] reported by the first time a new microemulsion method to synthesize magnetite NPs. The novelty of their method founded on the costeffective use of a single microemulsion. The authors prepared the microemulsion by solubilizing NaOH solution into DBS/ethanol/toluene system (DBS, sodium dodecylbenzene sulfonate), and then ferric and ferrous salts with molar ratio (2:1) were made into a mixed solution. A volume of this mixture was drop-wise added to microemulsion under vigorous stirring. The mixed system turned black immediately. After the mixed system was stirred during ½ h and then left undisturbed during 5 h, a two-layer system appeared. The upper layer was black, which contained the NPs, and lower layer was discarded. Following a magnetic separation technique, the magnetite NPs were extracted and then washed several times in ethanol/water solution (1:1) to obtain magnetite NPs.

He et al. reported [13] the chemical synthesis of a nonviral gene transferor, calcium carbonate NP using SDS microemulsions (see SDS chemical structure in **Figure 2**) [14, 15].

#### **2.2 Cationic surfactants**

Here I focus on MNPs synthesized by microemulsion method using the cationic surfactant, examining literature that employed this surfactant type I found very outstanding research.

Chin and Yaacob [16] reported the synthesis of magnetic iron oxide nanoparticles by preparing water-in-oil microemulsion system. Authors investigated two different volumetric ratios of Fe2+ and OH<sup>−</sup> (1:1 and 2:1). Crystallized, physical, and magnetic sizes of magnetic iron oxide nanoparticles were in the superparamagnetic size range (XRD, TEM, and AGM). Superparamagnetic behavior was warranted by hysteresis loop nonappearance in magnetization curve at room temperature. MIONPs were obtained by Massart's method too. The saturation magnetization of MIONPs obtained by *w/o* microemulsion system was larger though the crystallization size was lower, which demonstrated that MIONPs could be custom-made by distinct methods.

The preparation of iron oxide NPs was carried out according to the following route. Ferrous chloride salt solution was prepared (0.2 M); it was steady by adding a few drops of hydrochloric acid (0.5 M). Afterward, the microemulsion was set by melting HTAB in n-octane; subsequently 1-butanol was added followed by FeCl2 aqueous salt solution. The system was gradually shaken till a clear microemulsion suspension was attained. By repeating above method a microemulsion containing 0.25 M NaOH solution was produced.

These microemulsion systems were next fused at 1:1 vol. Instantaneously, a dark green solid was produced before it turned to black. The NPs were gathered and later cleaned repeatedly with acetone and de-ionized water and then they were dried at room temperature which is called S1. The same route was followed to yield a second sample with primary [Fe2+] = 0.1 M. [NaOH] was kept at 0.25 M. The volumetric

proportion of microemulsion containing Fe2+ to microemulsion containing OH<sup>−</sup> was 2:1. The sample was called S2 [17]. To compare, by Massart's procedure, S3 has been obtained [18]. Magnetic iron oxide NPs (MIONPs) were effectively formed at r.t. with microemulsion method. The MIONPs showed spherical shape. From the results of TEM, AGM, and XRD, the crystallite, physical, and magnetic sizes of the MIONPs were <10 nm. The mean physical size for S1, S2, and S3 is 6.5, 4.2, and 8.7 nm, respectively. The rise of the total number of microemulsion produced smaller particles. Particles produced by microemulsion procedure were smaller in size and showed higher saturation magnetization, compared to the particles obtained by Massart's procedure.

Shen et al. [19] prepare CTAB-stabilized gold nanowires and nanoparticles with a networked structure and different shapes using in situ *n*-butanol reduction in CTAB/n-butanol/n-heptane/HAuCl4(aq) through microwave dielectric heating (XRD, UV-vis, and TEM). The shape of the hydrophobic gold nanocrystals was effectively modulated tuning of CTAB/HAuCl4 ratios. Anisotropic gold nanostructure formation mechanism was deliberated, which confirmed that CTAB (**Figure 3**) perform a preeminent key role in the drummed-up gold nanowires.

With the decrease of *W*, the shapes of gold nanocrystals obtained by this method changed from sphere-like decahedron nanoparticles to nanowires with a networked structure. It was revealed that AuCl4− cation and ion of surfactant CTAB played key roles on formation and stabilization of the shape of gold nanowires. The attracting force among gold nanoparticles, which also caused an orienting growth of gold nucleus, was caused by the preferential adsorption of AuCl4− and the selective adsorption of CTAB on the facets of preliminary gold particle surface. This method novel and simple for synthesizing gold nanowires with a networked structure is expected to be appropriate to the synthesis of other metals to attain novel nanostructures.

Sharma et al. [20] reported a detailed and complete study on the growth kinetics of iron oxalate nanorods as they formed throughout several days inside the water pool of CTAB microdroplets (MDs) in isooctane. Authors underlined the novelty of this experimental study on nanostructure preparation in microemulsion-based reactions that could explain the key role of droplet interplay in the reaction kinetics. DLS, TEM, and FCS characterization was completed throughout some days to follow the whole growth kinetics of iron oxalate nanorods beginning from their particle nucleation. Considering the FCS data with an appropriate kinetic model, the droplet-fusion time or dimer lifetime 28 μs was obtained along with the droplet melting rate and the equilibrium constant of the chemical reaction. The droplet association rate exhibited a noteworthy time dependency that straight relates it to the growth mechanism. Combining FCS, DLS, and TEM, three different periods in the complete nanorod growth kinetics appeared: (i) a prolonged nucleationdominant NP growth period, then (ii) a short period where isotropic NPs shifted to anisotropic growth to form nanorods, and (iii) finally the period where dropletfusion-assisted elongation of nanorods was proved. The detailed methodologies discussed in this study could be applied to understand the growth kinetics of nanostructures, which are required for various applications in nanoscience and nanotechnology.

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**Figure 6.**

*Brij 97 chemical structure.*

**Figure 4.**

**Figure 5.**

*Triton X-100 chemical structure.*

*IGEPAL CO-520 chemical structure.*

*Synthesis of NPs by Microemulsion Method DOI: http://dx.doi.org/10.5772/intechopen.80633*

Here I review the literature about the use of nonionic surfactants as stabilizer/ emulsifier of the microemulsion involved on the synthesis of MNPs by microemul-

Sanchez-Dominguez et al. [21] reported a novel and direct approach to synthesize inorganic NPs at ambient conditions using *o/w* microemulsions to dissolve organometallic precursors. Addition of reducing or oxidizing/precipitating agents implies the formation of metallic or metal oxide nanoparticles, respectively. The authors chose nonionic aggregates and several strategic compositions for nanoparticle synthesis at 25°C. HREM shown that small metal NPs (Pd, Pt, and Rh) and nanocrystalline metal oxide NPs (Ce(IV) oxide with cubic crystalline structure confirmed by XRD) of <7 nm can be obtained at atmospheric pressure and 25°C.

Capek reviewed water-in-oil microemulsion method to prepare metal NPs [22]. This technique allows preparation of ultrafine metal particles ranging in the size diameter between 5 and 50 nm. Author reviewed the previous literature on particle synthesis of various metals such as silver, copper, cadmium, cobalt, nickel, cadmium, and gold in the reverse microemulsion systems. The precursor metal salts and reducing agents are mainly water-soluble molecules, and consequently the MNP nuclei formation progresses in the water microdroplets. The rate of the nuclei formation of the particle depends on the percolation degree of microemulsion droplets. Effects of stabilizer nature and concentration, the oil phase nature, reducing agent, and additive on the NP synthesis are shortened and assessed. The impact of numerous factors such as temperature, the metal salt nature, the incident light, and reaction conditions were also revised. Tan et al. [23] reported the microemulsion application to prepare silica-coated

iron oxide NPs using nonionic surfactants (Triton X-100, Igepal CO-520, and Brij-97, their chemical structure can be observed in **Figures 4**–**6**, respectively) for the preparation of microemulsions. Iron oxide NPs are formed by co-precipitation.

**2.3 Nonionic surfactants**

sion method.

**Figure 3.** *CTAB chemical structure.*
