Size-controlled metal nanoparticles

Figure 1. Graphical model for the generation of size-controlled metal nanoparticles inside metallated resins, (a) Pd is homogeneously dispersed inside the polymer framework (b) Pd is reduced to Pd (c) Pd atoms start to aggregate in subnanoclusters (d) a single 3 nm nanocluster is formed and blocked inside the largest mesh present in that slice of polymer framework (Reprinted from Ref [5], 2004, with permission from Wiley-VCH.)...

By using other templates, the size of metal nanoparticles can be also controlled. Chen et al. reported the sonochemical reduction of Au(III), Ag(I) and Pd(II) and synthesis of Au, Ag and Pd nanoparticles loaded within mesoporous silica [48,49]. Zhu et al. also reported the sonochemical reduction of Mn04 to Mn02 and synthesis of Mn02 nanoparticles inside the pore channels of ordered mesoporous cabon [50]. Taking into account these reports, the rigid pore of inorganic materials can be used as a template for the size controlled metal nanoparticle synthesis even in the presence of ultrasound. 

Neumann and coworkers have developed a novel method to prepare size-controlled metal nanoparticles [159]. The method is based on the coupling of the two reactions as shown in (6.11) and (6.12) ... 

The final method used to generate size-controlled metal nanoparticles involves ligand displacement from organometallic complexes with zero-valent metals. 

From the viewpoint of size control, bimetallic nanoparticles naturally have a strong tendency to provide monodispersed particles, compared with monometallic nanoparticles [49]. This tendency cannot be completely understood yet, but redox properties between two metals might result in this advantageous properties of bimetallic nanoparticles. 

The potential of morphologically controlled metal nanoparticles should be expanded by further improvement of their preparation method. It is highly required to develop preparation methods to obtain a better morphological control, i.e., perfect facet control on the particles of optional size. Better morphological control of metal nanoparticles is expected to be achieved in near future and the obtained metal particles will find new exciting applications, not only in catalysis but also in other technically important fields. 

The solvent-free controlled thermolysis of metal complexes in the absence or presence of amines is the simple one-pot synthesis of the metal nanoparticles such as gold, silver, platinum, and palladium nanoparticles and Au-Ag, Au-Pt, and Ag-Pd alloy nanoparticles. In spite of no use of solvent, stabilizer, and reducing agent, the nanoparticles produced by this method can be well size regulated. The controlled thermolysis in the presence of amines achieved to produce narrow size dispersed small metal nanoparticles under milder condition. This synthetic method may be highly promising as a facile new route to prepare size-regulated metal nanoparticles. Finally, solvent-free controlled thermolysis is widely applicable to other metal nanoparticles such as copper and nickel... 

Recently, microgel-stabilized, size-controlled metal nanoclusters have found promising applications in the field of catalysis. In particular, microgel systems can work as active carriers for the metal nanoparticles, which allows us to modulate the catalytic activity of nanoparticles by a thermodynamic transition that takes place within the carrier system [24, 69], The principle is shown in Fig. 8 Metallic... 

In order to control the size of metal nanoparticles and to avoid cluster coalescence into large colloids, which eventually precipitate, stabilizing... 

Pulsed laser ablation of metal samples in liquid environments by combination (in coincidence or in sequence) of two laser beams at different wavelengths has been examined in order to clarify a possibility of formation of size-selected metal nanoparticles. It has been shown that dual laser ablation technique in transparent liquids is suitable for fabrication of nanoparticles of metals where the size of particles can be controlled. The mean diameter of silver particles fabricated in water was typically in the range of 15-20 nm. 

CNT networks were used as a template and the deposition of Pt was performed by stepping the potential from 0.0 V (where there was no eleetrode reaetion) to -0.4 V (vs Ag/AgCl) for a period of 30 s and then back to an open circuit. This procedure allows the density and size of metal nanoparticles to be controlled by careful choice of the applied potential and deposition time. Figure 14.11 shows FE-SEM images taken after Pt deposition. A high density of nanoparticles was deposited close to the gold contact electrode within a distance of approximately 10 pm from the edge. 

Scheme 1 Illustration of the general synthetic method followed in our group for the synthesis of metal nanoparticles i decomposition of the precimsor, nucleation ii first growth process in ripening or coalescence leading to size and shape controlled objects through addition of stabilizers which prevent the full precipitation of the metal (iv)...

In summary, we found that Ugands indeed coordinate at the surface of nanoparticles and that they can be firmly or loosely attached to this surface according to their chemical nature. Furthermore, the hgands influence the reactivity of the metal nanoparticles. This is important in catalysis but, as we will see later in this paper, is also important for the control of the growth of metal nanoparticles of defined size and shape. 

Methods for the design of size- and even shape-controlled [186,190,191,370-372] metallic nanoparticles have reached a rather mature stadium thanks to the contributions of the pioneer groups of the last 25 years. Applications in a number of fields of practical Nanotechnology are now moving fast into the focus of R D [203,373]. For an overview on the potential application of metal nanoparticles in the rapidly growing fields of quantum dots, self-assembly, and electrical properties, the reader is advised to consult recently published specialist review articles, e.g.. Refs. [160,281] and book chapters (cf Chapters 2, 4, and 5 in Ref. [60]). In the following three sub-sections the authors restrict themselves to a brief summary of a few subjects of current practical interest in fields with which they are most familiar. 

From the viewpoint of size control, bimetallic systems are usually very convenient to produce monodispersed metal nanoparticles [49]. Although the exact reason is not clear yet, this is probably attributed to the redox equilibrium between the two elements. 

Recently, however, the development of nanotechnology may provide the changes on the research and development of practical catalysts. As mentioned in the previous section we can now design and synthesize a metal nanoparticle with not only various sizes and shapes, but also with various combinations of elements and their locations. Thus, we can now design the synergetic effect of two elements. In the case of core/shell structured bimetallic nanoparticles, the shell element can provide a catalytic site and the core element can give an electronic effect (a ligand effect) on the shell element. Since only the atoms on the surface can be attached by substrates, the thickness of the shell should be an important factor to control the catalytic performance. 

It is said that the 21st century is the age of nanotechnology since nanoparticles are applicable to an increasing number of areas. Therefore, this research field will occupy the much attention of scientists. Precisely controlling the primary size and structure of metallic nanoparticles, i.e., size, shape, crystal structure, and composition, however, is... 

The identification of structure sensitivity would be both impossible and useless if there did not exist reproducible recipes able to generate metal nanoparticles on a small scale and under controlled conditions, that is, with narrow size and/or shape distribution onto supports. Metal nanoparticles of controlled size, shape, and structure are attractive not only for catalytic applications, but are important, for example in optics, data storage, or electronics (c.f. Chapter 5). In order not to anticipate other chapters of this book (esp. Chapter 2), remarks will therefore be confined to few examples. 

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