Bimetallic Nanoparticles

The concept of the coordination complex enabled Werner to make a great sense of discovery but there was no scheme in the concept for direct bonding between metal atoms. Later on, in complex compounds the existence of metal-to-metal bonding has been proved beyond doubt and so is the case in metallic clusters in the nanoregime. Bimetallic nanoparticles, composed of two different elements, have been reported to show outstanding characters different from the corresponding monometallic ones [16-18]. In the nanoregime, when two metals are combined within a single nanoparticle (bimetallic nanoparticles), the optical, electronic, and magnetic properties of the bimetallic particles are directed by a combination of the properties (dielectric constants) of both metals. Such a combination strongly depends on the microscopic arrangement of the metals within the particle, i.e., whether an alloy, a perfect core-shell structure, or something in between is obtained, but in any of these cases, there is direct interaction between the metals. These features maybe enhanced, modified, or suppressed in the case of bimetallic and multimetallic nanoparticles, because of intermetallic interactions arising from their constitutional and morphological combinations. Totally new functions may be created by overcoming disadvantages of single-component nanoparticles. Unique features expected for multimetallic nanoparticles may include (1) physical and chemical interactions among different atoms and phases that lead to novel functions, (2) altered miscibility and interactions unique to nanometer dimensions (macroscopic phase property may not apply), and (3) morphological variations that are related to new properties.

Bimetallic clusters and colloids are of special interest as their chemical and physical properties may be tuned by varying the atomic ordering, composition, and size and also due to their superior catalytic properties [19-21] than those of single metallic components. They may serve as models to study the formation of different alloys. It is now possible to save precious metals, by optimizing the synthetic conditions so that only very thin surface layers occur. Bimetallic nanoparticles have played an important role in improving the catalytic quality [20-22], changing the surface plasmon (SP) band [23,24], and regulating the magnetic properties [25,26]. Because of all the above special properties that are brought about by the changes on surface and structure caused by alloying, control of composition distribution of bimetallic nanoparticles is crucial to the improvement of particle properties. It has been found that bimetallic structures with fivefold symmetry have a number of structural variants and some unexpected pentagonal shapes that cannot at all be obtained by simply rotating the main symmetry axis, but can be obtained by displacements of the fivefold axis from the center of the particle, which generates asymmetric structures [27]. Perfectly monodispersed nanoparticles are of course ideal, but special properties are to be expected even if the ideality is not perfectly realized. Mass production of uniform nanoparticles is most important to realize, as most chemical or physical properties of nanosize materials have not been elucidated yet. Much attention is now being paid to this area.

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