Metal particles core/shell structured bimetallic

Successive Reduction of Metal Ions. Successive reduction of two metal salts can be considered as one of the most suitable methods to prepare core/shell structured bimetallic particles (Fig. 9.1.3). In 1970, Turkevich and Kim tried to grow gold on Pd nanoparticles to obtain gold-layered Pd nanoparticles (39). The deposition of one... 

Sacrificial Hydrogen Reduction. In the previous session, successive reduction of metal ions is sometimes successful to synthesize core/shell structured bimetallic fine particles. However, sometimes it is not successful. For example, Schmid and coworkers could synthesize Au/Pd bimetallic fine particles, while we could not. Compared to these, the size of the Pd core particles is much larger in Schmid s case than in our case. During the reduction of AuCH-, the oxidation of Pd° atoms may also occur in the system of Schmid and coworkers. However, because of their large size, before the decomposition of Pd core particles, generated Au atoms may precipitate on them, thus also protecting Pd from further oxidation. 

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. 

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]. 

In 1989, we developed colloidal dispersions of Pt-core/ Pd-shell bimetallic nanoparticles by simultaneous reduction of Pd and Pt ions in the presence of poly(A-vinyl-2-pyrrolidone) (PVP) [15]. These bimetallic nanoparticles display much higher catalytic activity than the corresponding monometallic nanoparticles, especially at particular molecular ratios of both elements. In the series of the Pt/Pd bimetallic nanoparticles, the particle size was almost constant despite composition and all the bimetallic nanoparticles had a core/shell structure. In other words, all the Pd atoms were located on the surface of the nanoparticles. The high catalytic activity is achieved at the position of 80% Pd and 20% Pt. At this position, the Pd/Pt bimetallic nanoparticles have a complete core/shell structure. Thus, one atomic layer of the bimetallic nanoparticles is composed of only Pd atoms and the core is completely composed of Pt atoms. In this particular particle, all Pd atoms, located on the surface, can provide catalytic sites which are directly affected by Pt core in an electronic way. The catalytic activity can be normalized by the amount of substance, i.e., to the amount of metals (Pd + Pt). If it is normalized by the number of surface Pd atoms, then the catalytic activity is constant around 50-90% of Pd, as shown in Figure 13. 

An alternative approach to the preparation of bulk Pt bimetallic particles has been the preparation of surface modification of Pt particles with second metals. This approach has been used to prepare partially coated and fully coated Pt particles to give "core-shell" structures. Core-shell structures of Pt or PtM bimetal-lies on alternative metal cores have also been prepared. This approach argues... 

Nanoparticles are obtained by two general approaches top down and bottom up. In top down methods, bulk metals are mechanically ground to the nanosize and stabilized by using a suitable stabilizer. - The problem with this method is difficulty in achieving the narrow size distribution and control over the shape of the particles. Moreover, bimetallic nanoparticles with core shell structures cannot be obtained by this method. 

The bimetallic particles will be in a core-shell structure or alloy form, depending on the preparation route, miscibility and reductiOT kinetics of metal ions. As an example, bimetallic particles like Au/Ag are reported to exhibit Ag core and Ag/Au alloyed-shell type of stracture by the seed-growth method [2]. However, if the reductions are simultaneous in presence of a capping agent, Au/Ag nanoparticles form homogeneous alloy [3], When they are reduced simultaneously by the microemulsion method, Au/Ag nanoparticles can form homogeneous alloy [4] or an enriched in Au core-enriched in Ag shell structure [5], depending on the preparation conditions. 

Kan et al. reported preparation of Au-core/Pd-shell bimetallic nanoparticles by successive or simultaneous sonochemical irradiation of their metal precursors in ethylene glycol, respectively. In the successive method, Pd clusters or nanoparticles are first formed by reduction of Pd(N03)2, followed by adding HAUCI4 solution. As a result, Au-core/Pd-shell structured particles are formed, although Pd-core/Au-shell had been expected. In their investigations, the successive method was more effective than the simultaneous one in terms of the formation of the Au-core/Pd-shell nanoparticles [143]. 

We have extended the seed-mediated technique for the synthesis of bimetallic nanoparticles, having core-shell type structure appending photoreduction of metal ions. It has been proved that the deposition of a less noble metal (M) as shell on a preformed nobler nanoparticle core (M ) seems to be very effective by UV activation. Using this seed-mediated method we were able to synthesize Aucore Agsheii particles. First for the preparation of gold seeds (S), TX-lOO (10 M) and HAuC (5.0 x 10 %) were taken in a quartz cuvette so that the final concentration of Au(III) ion remained 5.0 x 10 M. Then the... 

The above synthetic strategy leads to easy generation of [R(Ag°)(Cu )] H and [R(Au°)(Pd°)] Cl nanocomposites with their inverted structures. The order of deposition of the bimetallic shells on the polystyrene beads can be altered by the successive immobilization of their corresponding precursors. Matrixes such as [R (Pd°)(Pt°)] Cl and [R(Ag°)(Au°)]+Cl were also synthesized from their corresponding metal chloride precursors. The layer-by-layer deposition technique has been widely used to fabricate core-shell particles because of its convenience to tailor the thickness and composition of the shells. The thickness can be controlled by varying the number of cycles of operation immobilization and subsequent reduction. In this way, we can deposit more than two metals on any kind of charged polystyrene bead. 

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