Microemulsions as Nanoreactors to Obtain Bimetallic Nanoparticles

*Concha Tojo, David Buceta and M. Arturo López-Quintela*

## **Abstract**

Microemulsions are frequently used as nanoreactors for the synthesis of bimetallic nanoparticles. The ability to manipulate the metal distribution in bimetallic nanoparticles is essential for optimizing applications, and it requires a deeper understanding of how compartmentalization of reaction medium affects nanoparticle synthesis. A simulation model was developed to predict the atomic structure of bimetallic nanoparticles prepared via microemulsion in terms of metals employed and microemulsion composition. The model was successfully proved by comparing theoretical and experimental Au/Pt STEM profiles. On this basis, the model becomes a strong tool to further enhance our knowledge of the complex mechanisms governing reactions in microemulsions and its impact on final nanostructures. The purpose of this study is to perform a comprehensive kinetic analysis of coreduction of different couple of metals in the light of the interplay between three kinetic parameters: intermicellar exchange rate, chemical reduction rates of the two metals, and reactants concentration. The particular combination of these factors determines the reaction rate of each metal, which in turn determines the final metal arrangement.

**Keywords:** bimetallic nanoparticles, microemulsions, reduction rate, intermicellar exchange rate, nanocatalysts

### **1. Introduction**

From the pioneering work of Boutonnet et al. [1], the synthesis of nanoparticles in microemulsions has been widely investigated with a variety of technical applications in catalysis [2–4], photonics [5], and energy conversion and storage devices [6–8]. The microemulsion route allows to control the size and composition of nanoparticles. A microemulsion consists of nanometer-sized water droplets dispersed in the oil phase and stabilized by a surfactant film. Reactants can be dissolved in the nano-sized water droplets or reverse micelles and can be exchanged between them by direct material transfer during an interdroplet collision [9]. The intermicellar exchange allows the reactants to be carried by the same droplet, so the chemical reaction can proceed inside the nanoreactor. Due to the space limitation inside the micelle, nucleation and growth of the particle are restricted, so it can result in the formation of size-controlled particles. In spite of the complexity of the reaction medium, microemulsion route has several advantages when compared to traditional methods. The first one is that nanoparticle size is directly controlled by the water/surfactant ratio, so narrow size distributions can be obtained. Another advantage is that surfactants around the nanoparticles can be removed with ease

and nanoparticles can be prepared at room temperature. In addition, the confinement of reactants inside micelles induces important changes in reactant concentrations, which strongly affect the reaction rates. Finally, in relation to catalysis, nanoparticles obtained by the microemulsion route present an improved catalytic behavior than particles with the same composition which are synthesized by traditional procedures [10, 11].

A variety of nanomaterials, ranging from metals [12–14], bimetallic structures [15–17], other inorganic nanoparticles [18–20], and organic compounds [21, 22], has been prepared by this approach. In the field of catalysis, microemulsion approach was successfully used to prepare different nanostructured catalytic materials [2, 10, 17, 23–25].

Nevertheless, microemulsion route present a challenge due to the difficulty in managing the material intermicellar exchange. As mentioned above, reactants are distributed in separate nanoreactors, so the whole process (chemical reaction, nucleation, and subsequent growth to build up final particles) is conditioned by the material exchange between them. This exchange is mainly dictated by the surfactant, which is located on the interface between water and oil phases. The hydrophilic portion of the surfactant is anchored into water and the lipophilic one into oil, forming a film which surrounds the micelle surface. It is believed that, when a micelle-micelle collision is violent enough, the surfactant film breaks up, allowing the material exchange. As a consequence, the rate of intermicellar exchange controls the reactants encounter and therefore plays a key role in chemical kinetics in microemulsions. The ease with which intermicellar channels are established as well as their size and stability are determined by the microemulsion composition, which in turn has been shown to affect final nanoparticle properties [26–28].

In the paper at hand, we are focused on the study of Pt/M (M = Au, Rh) nanoparticles synthesized in microemulsions. Platinum-based nanoparticles (NPs) exhibit remarkable electrocatalytic activity in many important chemical and electrochemical reactions including oxygen reduction reaction (ORR) and direct methanol oxidation [29]. Apart from the inherent chemical and physical properties of the constitutive metals, the catalytic activity, which is one of the more relevant applications of bimetallic nanoparticles, relies notably on the metal distribution, that is, on the intraparticle nanoarrangement [30]. Bimetallic nanoparticles can show four main mixing patterns: (a) core-shell structures, in which one metal forms the core and the second metal covers the first one forming the surrounding shell; (b) mixed structures, which are often called alloys; (c) multilayer structures [31]; and (d) sub-cluster segregated structures, characterized by a small number of heteroatomic bonds [12]. So, the control of bimetallic intrastructure, mainly within the first atomic layers from the surface [25, 32], is key for performance enhancement of bimetallic catalysts. Furthermore, the optimal metal distribution depends on the particular chemical reaction. Au-core/Pt-shell nanocatalyst exhibits an improved activity to catalyze formic acid electro-oxidation [33] or oxygen reduction reaction [34, 35]. On the contrary, an alloyed Pt-Au is better for electro-oxidation of methanol [36]. Therefore, an in-depth study aimed at tailoring well-defined structures will be of great interest.

Although the simultaneous reduction of the two metals by the microemulsion route is one of the most common procedures to control the size and composition of bimetallic nanoparticles [24, 37], the prediction of the resulting metal arrangement is complicated, as far as the current state-of-the-art is concerned. As a matter of fact, many studies designed to produce new nanoarrangements via microemulsions come from trial-and-error experiments, mainly due to the high number of involved synthetic variables and to their interaction with the inherent complexity of the reaction media. A robust tool for elucidating the interplay between the different

**61**

(ε 0

*Microemulsions as Nanoreactors to Obtain Bimetallic Nanoparticles*

each metal, which in turn defines the final metal arrangement.

factors concerning final bimetallic nanoarrangements is computer simulation. With the aim of understanding the different factors affecting final nanostructures, we perform a comprehensive kinetic analysis of coreduction of different couple of metals in the light of the interplay between three kinetic parameters: intermicellar exchange rate, chemical reduction rates of the two metals, and reactants concentration. The particular combination of these factors determines the reaction rate of

A model was developed to simulate the kinetic course of the two chemical reductions (see Ref. [38] for details). The reaction medium is a microemulsion, which is described as a set of micelles. The one-pot method is reproduced by mixing equal volumes of three microemulsions, each of which contains one of the three reactants (two metal precursors and the reducing agent R). This pattern of mixing reactants recreates the one-pot method, by which the two metal salts are simultaneously

Reactants are initially distributed throughout micelles using a Poisson distribution, that is, the occupation of all micelles is not similar. In this study, we present results using different values of metal precursors concentration, but keeping a pro-

−〉 = 〈cPtCl6

Micelles move and collide with each other. The intermicellar collision is a key feature in kinetics in microemulsions, because upon collision micelles are able to establish a water channel, which allows the exchange of their contents (metal precursors, reducing agent, metallic atoms, and/or growing particles). The material intermicellar exchange makes possible the reactant encounter inside micelles and, as a consequence, it is determinant of chemical reactions to occur. The intermicellar collision is simulated by choosing a 10% of micelles at random. These selected micelles collide, fuse (allowing material intermicellar exchange), and then redisperse. One Monte Carlo step begins in each intermicellar collision and ends when the quantity of species carried by colliding micelles is revised in agreement to the

The reduction rate of a metal A (vA) can be related to the standard potential

precursors in each micelle, which corresponds to 0.01, 0.08, 0.16, and 0.40 M, respectively, in a micelle with a radius of 4 nm. Au and Rh precursors (AuCl4

tions, that is, reducing agent R is in excess: (〈cR〉 = 10〈cPtCl6

<sup>2</sup>−〉 = 〈c〉 = 2, 16, 32, and 64 metal

. Calculations have been made under isolation condi-

<sup>2</sup>−〉).

<sup>−</sup> and

(1)

*DOI: http://dx.doi.org/10.5772/intechopen.80549*

**2. The model**

reduced.

RhCl6

**2.1 Initial reactants concentration**

portion 1:1 of the two metals: 〈cAuCl4

<sup>3</sup>−) are represented by M+

exchange criteria described below.

**2.3 Metal characterization: reduction rate ratio**

A) by means of the Volmer equation:

**2.2 Microemulsion dynamics and time unit**

*Microemulsions as Nanoreactors to Obtain Bimetallic Nanoparticles DOI: http://dx.doi.org/10.5772/intechopen.80549*

factors concerning final bimetallic nanoarrangements is computer simulation. With the aim of understanding the different factors affecting final nanostructures, we perform a comprehensive kinetic analysis of coreduction of different couple of metals in the light of the interplay between three kinetic parameters: intermicellar exchange rate, chemical reduction rates of the two metals, and reactants concentration. The particular combination of these factors determines the reaction rate of each metal, which in turn defines the final metal arrangement.
