C. Tojo

Simulation of the kinetics of nanoparticle formation in microemulsions.

Monte Carlo simulations were carried out to explain experimental results concerning the different sizes obtained for Ag and Au nanoparticles synthesized in microemulsions. Computer simulations allowed to study the interplay between the chemical reaction rate and the material interdroplet exchange, and their consequences on the mechanism and size distribution of nanoparticles synthesized in microemulsions. It has been shown that, although the material interdroplet exchange depends primarily on the flexibility of the surfactant film, a slow reaction rate leads to a more effective material interdroplet exchange for a given microemulsion. Two factors contribute to this result. Firstly, a slow reaction implies that autocatalytic growth takes place for a longer period of time, because there are available reactants. If the reaction is faster, the reactants are almost exhausted at early stages of the process. As a consequence, autocatalytic growth is only possible at the beginning. Secondly, a slow reaction rate implies the continuous production of seed nuclei, which can be exchanged between micelles due to their small size, allowing the coagulation of two nanoparticles (growth by ripening). Once again, this exchange can only take place at early stages of the synthesis when the reaction is faster. Both factors, autocatalysis and ripening, favour the slow growth of the biggest nanoparticles leading to the production of larger particles when the reaction is slower.

Surfactant Effects on Microemulsion-Based Nanoparticle Synthesis

The effect of the surfactant on the size, polydispersity, type of size distribution and structure of nanoparticles synthesized in microemulsions has been studied by computer simulation. The model simulates the surfactant by means of two parameters: the intermicellar exchange parameter, kex, related to dimer life time, and film flexibility parameter, f, related to interdroplet channel size. One can conclude that an increase in surfactant flexibility leads to bigger and polydisperse nanoparticle sizes. In addition, at high concentrations, the same reaction gives rise to a unimodal distribution using a flexible surfactant, and a bimodal distribution using a rigid one. In relation to bimetallic nanoparticles, if the nanoparticle is composed of two metals with a moderate difference in reduction potentials, increasing the surfactant flexibility modifies the nanoparticle structure, giving rise to a transition from a nanoalloy (using a rigid film) to a core-shell structure (using a flexible one).

Synthesis of Nanoparticles in Microemulsions

The technique of chemical reactions in microemulsions to produce nanoparticles has already 20 years of history behind, but the mechanisms to control the final size and the size distribution are still not well known. The knowledge of the mechanism is a crucial step in order to extend the potential applications of this technique. Nowadays there is a great interest in nanotechnologies and the developing of simple and reproducible methods to synthesize nanomaterials has attracted the interest of many researchers. The microemulsion method is a good candidate for this purpose. Microemulsions are thermodynamically stable systems composed of two inmiscible liquids (usually, water and oil) and a surfactant. Droplets of water-in-oil (W/O) or oil-in-water (O/W) are stabilized by surfactants when small amounts of water or oil are used, respectively. The size of the droplets can be controlled very precisely just by changing the ratio R = [water or oil] / [surfactant] in the nanometer range. These nanodroplets can be used as nanoreactors to carry out chemical reactions. It was initially assumed that these nanodroplets could be used as templates to control the final size of the particles, however, the research carried out in the last years has shown that besides the droplet size, several other parameters play an important role in the final size distribution. The purpose of this review is not to summarize all the results obtained in microemulsions, for which other reviews are available, but to provide a general picture of the different mechanisms involved. The influence of different parameters, such as exchange rate constant, film flexibility, reactant concentration, etc, on the final particle size will be discussed on the basis of Monte Carlo simulation results. Although this study is more focussed on reactions carried out in W/O microemulsions the general conclusions could be also applied to O/W microemulsions.

Modelling of nano-alloying and structural evolution of bimetallic core–shell nanoparticles obtained via the microemulsion route

A Monte Carlo model has been developed to describe the formation of bimetallic nanoparticles via the microemulsion route. The motivation stems from the need to understand the kinetics of nanoparticle formation in microemulsion droplets in order to determine the best experimental conditions to synthesize a nanoparticle with a given structure. We focus our study on the influence of the homogeneous and heterogeneous critical nucleus sizes of both metals on nanoparticle structure, as well as the role played by the surfactant film flexibility. The study reveals that the final structure is sensitive to changes in the critical nucleus numbers, because these parameters determine the rate of nucleation. An increase in the difference between nucleation rates of both metals gives rise to a better segregation of metals in the final nanoparticle. Likewise, as long as the formation of heterogeneous seeds is faster, the degree of alloying is greater. Finally, a fast material intermicellar exchange leads to a better mixture of metals, so the influence of the critical nucleus sizes on nanoparticle structure becomes less pronounced as the flexibility of surfactant film is increased.

Controlling Bimetallic Nanostructures by the Microemulsion Method with Subnanometer Resolution Using a Prediction Model.

We present a theoretical model to predict the atomic structure of Au/Pt nanoparticles synthesized in microemulsions. Excellent concordance with the experimental results shows that the structure of the nanoparticles can be controlled at subnanometer resolution simply by changing the reactant concentration. The results of this study not only offer a better understanding of the complex mechanisms governing reactions in microemulsions, but open up a simple new way to synthesize bimetallic nanoparticles with ad hoc controlled nanostructures.

Understanding the Metal Distribution in Core-Shell Nanoparticles Prepared in Micellar Media

The factors that govern the reaction rate of Au/Pt bimetallic nanoparticles prepared in microemulsions by a one-pot method are examined in the light of a simulation model. Kinetic analysis proves that the intermicellar exchange has a strong effect on the reaction rates of the metal precursors. Relating to Au, reaction rate is controlled by the intermicellar exchange rate whenever concentration is high enough. With respect to Pt, the combination of a slower reduction rate and the confinement of the reactants inside micelles gives rise to an increase of local Pt salt concentration. Two main consequences must be emphasized: On one hand, Pt reduction may continue independently whether or not a new intermicellar exchange takes place. On the other hand, the accumulation of Pt reactants accelerates the reaction. As the reactant accumulation is larger when the exchange rate is faster, the resulting Pt rate increases. This results in a minor difference in the reduction rate of both metals. This difference is reflected in the metal distribution of the bimetallic nanoparticle, which shows a greater degree of mixture as the intermicellar exchange rate is faster.

Kinetic Study on the Formation of Bimetallic Core-Shell Nanoparticles via Microemulsions

Computer calculations were carried out to determine the reaction rates and the mean structure of bimetallic nanoparticles prepared via a microemulsion route. The rates of reaction of each metal were calculated for a particular microemulsion composition (fixed intermicellar exchange rate) and varying reduction rate ratios between both metal and metal salt concentration inside the micelles. Model predictions show that, even in the case of a very small difference in reduction potential of both metals, the formation of an external shell in a bimetallic nanoparticle is possible if a large reactant concentration is used. The modification of metal arrangement with concentration was analyzed from a mechanistic point of view, and proved to be due to the different impact of confinement on each metal: the reaction rate of the faster metal is only controlled by the intermicellar exchange rate but the slower metal is also affected by a cage-like effect.

The impact of the confinement of reactants on the metal distribution in bimetallic nanoparticles synthesized in reverse micelles

Summary A kinetic study on the formation of bimetallic nanoparticles in microemulsions was carried out by computer simulation. A comprehensive analysis of the resulting nanostructures was performed regarding the influence of intermicellar exchange on reactivity. The objects of this study were metals having a difference in standard reduction potential of about 0.2–0.3 V. Relatively flexible microemulsions were employed and the concentration of the reactants was kept constant, while the reaction rate of each metal was monitored as a function of time using different reactant proportions. It was demonstrated that the reaction rates depend not only on the chemical reduction rate, but also on the intermicellar exchange rate. Furthermore, intermicellar exchange causes the accumulation of slower precursors inside the micelles, which favors chemical reduction. As a consequence, slower reduction rates strongly correlate with the number of reactants in this confined media. On the contrary, faster reduction rates are limited by the intermicellar exchange rate and not the number of reactants inside the micelles. As a result, different precursor proportions lead to different sequences of metal reduction, and thus the arrangement of the two metals in the nanostructure can be manipulated.

Core-shell nanocatalysts obtained in reverse micelles: structural and kinetic aspects

Ability to control the metal arrangement in bimetallic nanocatalysts is the key to improving their catalytic activity. To investigate how metal distribution in nanostructures can be modified, we developed a computer simulation model on the synthesis of bimetallic nanoparticles obtained in microemulsions by a one-pot method. The calculations allow predicting the metal arrangement in nanoparticle under different experimental conditions. We present results for two couples of metals, Au/Pt (Δe = 0.26 V) and Au/Ag (Δe = 0.19 V), but conclusions can be generalized to other bimetallic pairs with similar difference in standard reduction potentials. It was proved that both surface and interior compositions can be controlled at nanometer resolution easily by changing the initial reactant concentration inside micelles. Kinetic analysis demonstrates that the confinement of reactants inside micelles has a strong effect on the reaction rates of the metal precursors. As a result, the final nanocatalyst shows a more mixed core and a better defined shell as concentration is higher.

Bimetallic nanoparticles synthesized in microemulsions: A computer simulation study on relationship between kinetics and metal segregation

Computer simulations were carried out to study the origin of the different metal segregation showed by bimetallic nanoparticles synthesized in microemulsions. Our hypothesis is that the kinetics of nanoparticle formation in microemulsions has to be considered on terms of two potentially limiting factors, chemical reaction itself and the rate of reactants exchange between micelles. From the kinetic study it is deduced that chemical reduction in microemulsions is a pseudo first-order process, but not from the beginning. At the initial stage of the synthesis, redistribution of reactants between micelles is controlled by the intermicellar exchange rate, meanwhile the core and middle layers are being built. This exchange control has a different impact depending on the reduction rate of the particular metal in relation to the intermicellar exchange rate. For the case of Au/Pt nanoparticles, the kinetic constant of Au (fast reduction) is strongly dependent on intermicellar exchange rate and reactant concentration. On the contrary, the kinetic constant of Pt (slower reduction) remains constant. Therefore, the fact that the reaction takes place in a microemulsion affects more or less depending on the reduction rate of the metals. As a consequence, the final nanostructure not only depends on difference between the reduction rates of both metals, but also on the reduction rate of each metal in relation to the intermicellar exchange rate.