Bimetallic nanoparticles

Fig. 7. TEM image of starch capped copper nanoparticles 3.2 Bimetallic nanoparticles...

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. 

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. 

Recent Progress in Bimetallic Nanoparticles Their Preparation, Structures and Functions... 

Herein we briefly mention historical aspects on preparation of monometallic or bimetallic nanoparticles as science. In 1857, Faraday prepared dispersion solution of Au colloids by chemical reduction of aqueous solution of Au(III) ions with phosphorous [6]. One hundred and thirty-one years later, in 1988, Thomas confirmed that the colloids were composed of Au nanoparticles with 3-30 nm in particle size by means of electron microscope [7]. In 1941, Rampino and Nord prepared colloidal dispersion of Pd by reduction with hydrogen, protected the colloids by addition of synthetic pol5mer like polyvinylalcohol, applied to the catalysts for the first time [8-10]. In 1951, Turkevich et al. [11] reported an important paper on preparation method of Au nanoparticles. They prepared aqueous dispersions of Au nanoparticles by reducing Au(III) with phosphorous or carbon monoxide (CO), and characterized the nanoparticles by electron microscopy. They also prepared Au nanoparticles with quite narrow... 

The precise control of size, its distribution, shape, composition, and crystal structure of bimetallic nanoparticles is crucial in this field. Some strategies to prepare bimetallic nanoparticles were proposed and subsequently the corresponding methods were developed for the purpose of controlled nanoparticles. These methods enable us to find novel chemical and physical properties of bimetallic nanoparticles depending on their structures. 

Usually bimetallic nanoparticles as well as monometallic ones are characterized by many probing tools such as UV-visible (UV-Vis) spectroscopy, transmission electron microscopy (TEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), EXAFS, infrared spectroscopy of adsorbed CO (CO-IR), and so on [1,2]. 

Bimetallic nanoparticles show many interesting functions. The bimetallic nanoparticles are emerging catalysts... 

Before 2002 we have previously reviewed on bimetallic nanoparticles and their catal5dic properties [5,46,47]. Herein we concentrate on recent researches on the bimetallic nanoparticles from 2003, but also include some important researches previously reviewed by us [5,46,47]. 

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. 

M1M2 - All kinds of bimetallic nanoparticles including random alloy nanoparticles. In this case Mi and M2 should be arranged in an alphabetical order. 

Main researches on the preparation of the bimetallic nanoparticles were summarized in Table la and b. 

Figure 1. Various structure models of bimetallic nanoparticles.

Table 1. Typical preparations and characterizations of bimetallic nanoparticles reported in literatures. 

Noble metal ions can be easily reduced to the corresponding zero-valent metal atoms. Therefore, bimetallic nanoparticles consisting of two different noble metals have been extensively investigated for purpose of novel catalysts and optical materials. A simultaneous reduction of two noble metal ions with alcohol is a simple and useful technique to prepare bimetallic nanoparticles. The alcohol reduction of metal ions M + is followed by Equation (1). 

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... 

Figure 2. Formation mechanism of Ml-core/M2-shell structured bimetallic nanoparticles.

Au (or Ag) content. Decanethiol-protected AuPt alloy bimetallic nanoparticles of ca. 2.5 nm in particle size were similarly prepared [58]. The preparations of PdPt [59] and AuPd [60] bimetallic nanoparticles in water-in-oil (w/o) microemulsions can be realized in two-phase reaction system, in which a surfactant molecule itself works as a protecting agent in these cases. 

Other one-pot preparations of bimetallic nanoparticles include NOct4(BHEt3) reduction of platinum and ruthenium chlorides to provide Pto.sRuo.s nanoparticles by Bonnemann et al. [65-67] sonochemical reduction of gold and palladium ions to provide AuPd nanoparticles by Mizukoshi et al. [68,69] and NaBH4 reduction of dend-rimer—PtCl4 and -PtCl " complexes to provide dend-rimer-stabilized PdPt nanoparticles by Crooks et al. [70]. 

Late transition metal or 3d-transition metal irons, such as cobalt, nickel, and copper, are important for catalysis, magnetism, and optics. Reduction of 3d-transition metal ions to zero-valent metals is quite difficult because of their lower redox potentials than those of noble metal ions. A production of bimetallic nanoparticles between 3d-transi-tion metal and noble metal, however, is not so difficult. In 1993, we successfully established a new preparation method of PVP-protected CuPd bimetallic nanoparticles [71-73]. In this method, bimetallic hydroxide colloid forms in the first step by adjusting the pH value with a sodium hydroxide solution before the reduction process, which is designed to overcome the problems caused by the difference in redox potentials. Then, the bimetallic species... 

Ag-core/Au-shell bimetallic nanoparticles were prepared by NaBH4 reduction method [124]. UV-Vis spectra were recorded and compared with various ratios of AuAg alloy nanoparticles. The UV-Vis spectra of bimetallic nanoparticles suggested the formation of core/shell structure. Furthermore, the high-resolution transmission electron microscopy (HRTEM) image of the nanoparticles confirmed the core/shell type configuration directly. 

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]. 

Michaelis and Henglein [131] prepared Pd-core/Ag-shell bimetallic nanoparticles by the successive reduction of Ag ions on the surface of Pd nanoparticles (mean radius 4.6 nm) with formaldehyde. The core/shell nanoparticles, however, became larger and deviated from spherical with an increase in the shell thickness. The Pd/Ag bimetallic nanoparticles had a surface plasmon absorption band close to 380 nm when more than 10-atomic layer of Ag are deposited. When the shell thickness is less than 10-atomic layer, the absorption band is located at shorter wavelengths and the band disappears below about three-atomic layer. 

Srnova-Sloutfova et al. [136] prepared layered Ag-core/ Au-shell bimetallic nanoparticles by overdeposition of Au... 

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