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The most distinctive feature of nanoscale materials is the size dependence often displayed by these properties. Size dependence is usually quantitative: the optical response, the catalytic reactivity, or the magnetic moment exhibit variations that vary with the nanoparticle size, smoothly at first in the so-called scalable regime, then nonmonotonically when the particle becomes small enough. Under some circumstances, the dependence is more qualitative and can arise from some changes in chemical bonding.
Besides its size, the shape of a nanoparticle can also affect its property, opening interesting avenues of research, e. Mixing several metals together provides another opportunity for tuning a physical or chemical property at the nanoscale. This ambition is rooted in the achievements of early metallurgists from the Bronze age who found several millennia ago that the strength and durability of their materials could be enhanced by mixing different metals.
Metal alloys at the nanoscale are a prime example of nanoalloys in which the relative composition is a new variable to be varied, expectantly having a profound influence on the desired property along with size itself. However, it should be made clear here that nanoalloys do not only refer to mixed, nanoscale alloys. For sake of a general definition and perhaps by lack of a more rigorous term , nanoalloys are currently understood as multimetallic nanoparticles, with no assumption about the chemical order within them.
Adding the dimension of composition to the existing roles of size and structure entails a significant complexity, which could only be addressed after research on pure metal nanoparticles had reached some level of maturity.
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Tackling this complexity by experimental or theoretical means requires dedicated tools that build upon methods available for monometallic systems, as well as methods more specific to the presence of several metals. One first objective of the present book is to provide a broad introduction to such methods, either for synthesis purposes or for fundamental investigations. Beyond fundamentals, and although a relatively young discipline, nanoalloys have also started to receive a considerable attention for their potential interest in several applied fields, for energy production, magnetic storage, or biomedicine.
These topics are closely related to specific chemical or physical issues but deserved dissertations of their own. This book was designed with the aim to present and discuss the major topics of relevance for nanoalloys, at a time where literature on the subject remains scarce. The book is organized into 11 chapters covering the most fundamental aspects of nanoalloys related to their synthesis Chapters 1 and 2 and characterization Chapter 4 , as well as their theoretical study Chapter 3.
Aspects related to their thermodynamics Chapter 5 and kinetics Chapter 6 are covered as well.
Nanoalloys From Fundamentals To Emergent Applications
The gear then moves to more specific topics, including optics Chapter 7 , magnetism Chapter 8 , and catalysis Chapter 9 , and finally to biomedical applications Chapter 10 and the technologically relevant issue of self-assembly Chapter The contributors of the book are all world experts in their respective fields, and it is a pleasure to thank them for their fine work. Stephen Berry, who has pioneered the study of the physics and chemistry of atomic clusters, is also gratefully acknowledged for his foreword. The dependence of the properties of nanoscale materials on the size, shape and composition of the nanocrystal is a phenomenon of both fundamental scientific interest and many practical and technological applications [1—5].
These properties are often different, and sometimes superior, to those of the corresponding bulk materials. The origins of the differences can be explained by the high surface-to-volume ratio, dispersion factors and the quantum size effects unique to a specific length scale [1—7]. The confinement of electrons within a semiconductor nanocrystal results in the shift of the band gap to higher energy with smaller crystalline size.
This effect is known as the " quantum size effect " [1—7]. In the strong confinement regime, the actual size of the semiconductor particle determines the allowed energy levels and thus the optical and electronic properties of the material. The characterization of the unique properties of nanocrystals can ultimately lead to identifying many potential uses and applications, ranging from catalysis, ceramics, microelectronics, sensors, pigments, and magnetic storage to drug delivery and biomedical applications [1—7].
The applications of nanoparticles are thus expected to enhance many fields of advanced technology particularly in the areas of catalysis, chemical and biological sensors, optoelectronics, drug delivery, and media storage. Considerable research interest has been focused on the study of bimetallic nanoalloys due to the additional new properties that may arise from the combination of different compositions of metals on the nanoscale [ 8—17].
The chemical and physical properties of nanoalloys can be tuned by varying the type of metal, composition, degree of chemical ordering, as well as the size and shape of the nanoalloy crystals. At a fundamental level, information on the evolution of the electronic structures of bimetallic nanoparticles as a function of size, composition and shape, and the associated changes in the optical, catalytic and magnetic properties continue to be a major goal of research in nanostructured materials.
On a practical level, the unique properties of metallic and bimetallic nanoparticles are exploited for a variety of applications, including nanocatalysis particularly for efficient selective catalysts, sensors, optical markers and filters, fuel and oil additives for energy enhancement and surface modifications as well as many other applications [18—50]. The synthesis and characterization of metallic nanocrystals and bimetallic nanoalloys with controlled size and shape have attracted rapidly growing interest both for fundamental scientific interest and many practical and technological applications [ 1—50].
A wide range of scientifically interesting and technologically important nanoparticles have been produced by both chemical and physical methods [1—17]. Examples of these approaches include solvothermal methods, template-assisted, kinetic growth control, sonochemical reactions, thermolysis of single-source precursor in ligating solvents, and microwave irradiation MWI methods [51—68].
The synthesis of nanocrystals by colloidal methods involves nucleation the initial formation of the appropriate semiconductor bond , growth the formation of a highly crystalline core and passivation of the nanocrystal surface [7,56,69]. The passivation step is important in stabilizing the colloid and controlling the growth of the nanoparticles, preventing the agglomeration and fusing of the particles, and allowing the solubility of the nanoparticles in common solvents [7,56,69].
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In this chapter, we describe several chemical methods for the synthesis of nanoalloys with more focus on the MWI methods [ 59—68]. The MWI approach provides simple and fast routes to the synthesis of nanomaterials since no high temperature or high pressure is needed. The heating of a substance by MWI depends on the ability of the material solvent or reagent to absorb microwave radiation and convert it into heat.
This is based on two principal mechanisms: dipole rotation and ionic conduction, that is, by reversal of solvent dipoles and the resulting replacement of charged ions of a solute [79,80]. Polar reactants with a high microwave extinction coefficient can be excited by direct absorption of microwaves.
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Due to the difference in the solvent and reactant dielectric constants, selective dielectric heating can provide significant enhancement in reaction rates. By using metal precursors that have large microwave absorption cross-sections relative to the solvent, very high effective reaction temperatures can be achieved. The rapid transfer of energy directly to the reactants faster than they are able to relax causes an instantaneous internal temperature rise.
Thus, the activation energy is essentially decreased as compared with conductive heating and the reaction rate increases accordingly. As a consequence, reactions might be performed at lower temperatures and hotspots or other temperature inhomogeneities can be prevented. Furthermore, reaction parameters such as temperature, time, and pressure can be controlled easily.
This also allows the rapid decomposition of the precursors thus creating highly supersaturated solutions where nucleation and growth can take place to produce the desired nanocrystalline products. These conditions lead to the formation of very small nanocrystals since the higher the supersaturation the smaller the critical size required for nucleation.
For the formation of nanoalloys, the experimental conditions must be chosen to yield binary nucleation events where the initial nuclei contain both metals with compositions that reflect the compositions of the two metal precursors. This requires careful choice of the two metal precursors with almost identical decomposition profiles to ensure the occurrence of binary nucleation events.
These conditions are different from those involved in the formation of core—shell nanoparticles, where the shell atoms heterogeneously nucleate on the preexisting core nuclei.
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Following the nucleation events, the growth of the nanoalloy nanocrystals can be effectively inhibited by the adsorption of ligating organic surfactants that bind strongly to the nanocrystals, thus stabilizing and passivating the surface. In nanoalloys, depending on the surface composition of the binary nanocrystal faces, selective adsorption of the capping agents can result in controlling the final shape of the grown nanocrystal. The organization of the chapter is as follows.
First, we present a brief overview of the classical nucleation theory CNT since nucleation and growth greatly influence the control of the size and shape of nanoparticles which consequently determine the unique properties that may characterize nanoparticles [ 80—84]. Second, we describe conventional chemical methods for the synthesis of nanalloys using reduction and precipitation of mixed-metal hydroxide solutions and chemical reduction of mixed metal cations.
Third, we present detailed description of the MWI method as a general procedure for the synthesis of a variety of high quality, crystalline bimetallic nanoalloys with controlled size and shape. Finally, we discuss the application of bimetallic nanocrystals in the area of nanocatalysis and describe the synthesis of selected nanoalloys supported on ceria nanoparticles as nanocatalysts for CO oxidation.
This demonstrates another advantage of using the MWI approach where a supported nanoalloy catalyst can be prepared using a facile one-pot synthesis. Nucleation of liquid droplets from the vapor phase can occur homogeneously or heterogeneously. Thirteen Festivals:a ritual year in Bengal. Bhagavad Gita. Essential of sociology:a down to-earth approach. Keeping the jewel in the crown. Predicting turning points in the interest rate cycle.
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