Core-shell metal nanoparticles

In the case of Fe as the core, there are examples of core-shell Fe-Au [100], Fe-Fe-oxide [101], Fe-oxide/Au [102], and Fe304-polymer [103] nanoparticles. The combination of Fe core/Au shell is particularly appealing because Au is not ferromagnetic, but is noble and 

The Fe-Au nanoparticles were reported to consist of metallic cores, having an average diameter of 6.1 nm, surrounded by an oxide shell, averaging 2.7 nm in thickness, for a total average particle diameter of 11.5 nm [101]. A surfactant solution is prepared with nonylphenol poly(ethoxylate) ethers. Au-coated Fe nanoparticles were also prepared in a reverse micelle formed by cetyltrimethylammonium bromide (CTAB), 1-butanol and octane as the surfactant, the co-surfactant and the oil phase, respectively [100]. The nanoparticles were prepared in aqueous solutions of micelles by reduction of Fe(II) and Au precursors with NaBH4. The typical size of the nanoparticles is about 20 nm. The existence of Fe and Au is again confirmed by energy dispersive X-ray microanalysis. 

The further study of Cho et al. [107] has showed that the structure of Fe/Au core/shell nanomaterials is somewhat complex. Mossbauer spectra were best interpreted as Fe speciation of a-Fe, Fe11, Fem and Fe-Au alloy. The Au shell was suggested to grow by nucleating from small nanoparticles on the Fe-core surface before it develops the shell structure. These nanoparticle nucleation sites form islands for the growth and coalescence 

There has been extensive work on core/shell nanoparticles where the core is magnetic Fe304, PbS and the shell is a polymer that provides biocompatibility and long-term stability [121]. PbS particles are formed in a Pb(AOT)2/polymer composite [122], according to whether this has an ordered layer structure or not, nanorods or spherical particles are obtained. 

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W. Zhang, et al. Enhancement of perovskite-based solar cells employing core-shell metal nanoparticles. Nano Letters, 2013. 13(9) p. 4505-4510. http //www.nrel.gov/ncpv/images/efficiency chart.jpg... 

Abid, J. R, M. Abid, C. Bauer, H. H. Girault, and R. F. Brevet, Controlled reversible adsorption of core—Shell metallic nanoparticles at the polarized water/l,2-dichlo-roethane interface investigated by optical second-harmonic generation, J Phys Chem C, Vol. Ill, (2007) p. 8849. 

Core-Shell Metal Fluoride, 93 MgF2 Nanoparticles as Novel Antibiofilm Agents, 95... 

F. 13 A schematic illustration of H2 evolution cm core/shell-structmed nanoparticles (with a noble metal core and a Cr203 shell) as a c[Pg.112]

Fig. 6.13 Tentative classification of bi-component hybrids most commonly reported in electro-analytical applications. A alloy and core shell metal structures, B nanoparticles (NPs) and carbon nanotubes (CNTs) encapsulated by a thin polymeric layer, C NPs grafted on the surface of CNTs and graphene, D mixture of NPs, E fullerenes included in polymeric matrices, F NPs and CNTs in polymeric matrices (Reproduced fi om Ref [169] with the permission of Springer)...

Hybrid inorganic-organic core-shell metal oxide nanoparticles from metal salts./. Mater. Chem., 14 (13), 2017-2023 ... 

Bimetallic nanoparticles, either as alloys or as core-shell structures, exhibit unique electronic, optical and catalytic properties compared to pure metallic nanopartides [24]. Cu-Ag alloy nanoparticles were obtained through the simultaneous reduction of copper and silver ions again in aqueous starch matrix. The optical properties of these alloy nanopartides vary with their composition, which is seen from the digital photographs in Fig. 8. The formation of alloy was confirmed by single SP maxima which varied depending on the composition of the alloy. 

Apart from the above described core-shell catalysts, it is also possible to coat active phases other than zeolite crystals, like metal nanoparticles, as demonstrated by van der Puil et al. [46]. More examples of applications on the micro level are given in Section 10.5, where microreactors and sensor apphcations are discussed. 

We synthesized uniform CU2O coated Cu nanoparticles from the thermal decomposition of copper acetylacetonate, followed by air oxidation. We successfully used these nanoparticles for the catalysts for Ullmann type amination coupling reactions of aryl chlorides. We synthesized core/shell-like Ni/Pd bimetallic nanoparticles from the consecutive thermal decomposition of metal-surfactant complexes. The nanoparticle catalyst was atom-economically applied for various Sonogashira coupling reactions. 

Similarly, Pd, Ag, and Pd-Ag nanoclusters on alumina have been prepared by the polyol method [230]. Dend-rimer encapsulated metal nanoclusters can be obtained by the thermal degradation of the organic dendrimers [368]. If salts of different metals are reduced one after the other in the presence of a support, core-shell type metallic particles are produced. In this case the presence of the support is vital for the success of the preparation. For example, the stepwise reduction of Cu and Pt salts in the presence of a conductive carbon support (Vulcan XC 72) generates copper nanoparticles (6-8 nm) that are coated with smaller particles of Pt (1-2 nm). This system has been found to be a powerful electrocatalyst which exhibits improved CO tolerance combined with high electrocatalytic efficiency. For details see Section 3.7 [53,369]. 

The synthesis of bimetallic nanoparticles is mainly divided into two methods, i.e., chemical and physical method, or bottom-up and top-down method. The chemical method involves (1) simultaneous or co-reduction, (2) successive or two-stepped reduction of two kinds of metal ions, and (3) self-organization of bimetallic nanoparticle by physically mixing two kinds of already-prepared monometallic nanoparticles with or without after-treatments. Bimetallic nanoparticle alloys are prepared usually by the simultaneous reduction while bimetallic nanoparticles with core/shell structures are prepared usually by the successive reduction. In the preparation of bimetallic nanoparticles, one of the most interesting aspects is a core/shell structure. The surface element plays an important role in the functions of metal nanoparticles like catal5dic and optical properties, but these properties can be tuned by addition of the second element which may be located on the surface or in the center of the particles adjacent to the surface element. So, we would like to use following marks to inscribe the bimetallic nanoparticles composed of metal 1, Mi and metal 2, M2. 

Pt/Pd bimetallic nanoparticles can be prepared by refluxing the alcohol/water (1 1, v/v) solution of palla-dium(II) chloride and hexachloroplatinic(IV) acid in the presence of poly(A-vinyl-2-pyrrolidone) (PVP) at ca. 95 °C for Ih [15,16,48]. The resulting Pd/Pt nanoparticles have a Pt-core/Pd-shell structure with a narrow size distribution and the dispersion is stable against aggregation for several years. The core/shell structure was confirmed by the technique of EAXFS. Composition of Pt/Pd nanoparticles can be controlled by the initially feed amount of two different metal ions, i.e., in this case one... 

In summary the simultaneous reduction method usually provides alloyed bimetallic nanoparticles or mixtures of two kinds of monometallic nanoparticles. The bimetallic nanoparticles with core/shell structure also form in the simultaneous reduction when the reduction is carried out under mild conditions. In these cases, however, there is difference in redox potentials between the two kinds of metals. Usually the metal with higher redox potential is first reduced to form core part of the bimetallic nanoparticles, and then the metal with lower redox potential is reduced to form shell part on the core, as shown in Figure 2. The coordination ability may play a role in some extent to form a core/shell structure. Therefore, the simultaneous reduction method cannot provide bimetallic nanoparticles with so-called inverted core/ shell structure in which the metal of the core has lower redox potential. 

Successive reduction (or two-step reduction) involves the reduction of first metal ions, followed by the reduction of second metal ions. The second metals are usually deposited on the surface of the first metals due to the formation of the strong metallic bond, resulting in core/shell structured bimetallic nanoparticles. 

Our first attempt of a successive reduction method was utilized to PVP-protected Au/Pd bimetallic nanoparticles [125]. An alcohol reduction of Pd ions in the presence of Au nanoparticles did not provide the bimetallic nanoparticles but the mixtures of distinct Au and Pd monometallic nanoparticles, while an alcohol reduction of Au ions in the presence of Pd nanoparticles can provide AuPd bimetallic nanoparticles. Unexpectedly, these bimetallic nanoparticles did not have a core/shell structure, which was obtained from a simultaneous reduction of the corresponding two metal ions. This difference in the structure may be derived from the redox potentials of Pd and Au ions. When Au ions are added in the solution of enough small Pd nanoparticles, some Pd atoms on the particles reduce the Au ions to Au atoms. The oxidized Pd ions are then reduced again by an alcohol to deposit on the particles. This process may form with the particles a cluster-in-cluster structure, and does not produce Pd-core/ Au-shell bimetallic nanoparticles. On the other hand, the formation of PVP-protected Pd-core/Ni-shell bimetallic nanoparticles proceeded by a successive alcohol reduction [126]. 

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