Preparation of Bimetallic Nanoparticles

Metallic nanoparticles of definite size are easily synthesized via a "bottom-up" approach and can be surface modified with special functional groups. Nanocomposites, i.e., alloy and core-shell particles are an attractive subject mainly because of their composition-dependent optical, magnetic, and catalytic properties.

In general, bimetallic nanoparticles can be prepared by simultaneous reduction or by successive reduction of two metal ions through a suitable stabilization strategy (capping or template) [28] combating steric hindrance and static-electronic repulsive force. The former reduction methods may obtain a particle structure between core-shell and homogeneous alloy depending on the reduction condition [29-31], whereas the latter method is used for the production of core-shell particles [32]. Alloy particles may be conveniently synthesized by simultaneous reduction of two or more metal ions [23,33], whereas growth of core-shell structures may be accomplished by the successive reduction of one metal ion over the core of another [34], generally by a weak reducing agent. Otherwise, the latter process often leads to the formation of fresh nuclei of the second metal in solution, in addition to a shell around the first metal core [35], and is clearly undesirable from the application point of view. Another possible strategy to overcome this drawback could be based on immobilization of a reducing agent on the surface of the core metal, which, when exposed to the second metal ions would reduce them, thereby leading to the formation of a thin metallic shell. However, control of the reduction, nucleation, and aggregation rates of the two components maybe effective to control the size, structure, and composition distribution of bimetallic nanoparticles.

Simultaneous reduction of two kinds of precious metal ions usually gives core-shell-structured bimetallic nanoparticles [20] in which atoms of the first element form a core, and the atoms of the second cover the core to form a shell. Successive reduction of two metal salts can be considered as one of the most suitable methods to prepare core-shell-structured bimetallic particles. The deposition of one metal onto preformed monometallic nanoparticles of another metal seems to be very effective. For this purpose, however, the second element must be deposited on the surface of preformed particles, and the preformed monometallic nanoparticles must be chemically surrounded by the deposited element. The core-shell structure has been considered to be controlled by the order of redox potentials of both the ions and the coordination ability of both atoms in relation to the reducing agent. However, some difficulties are to be overcome for the preparation of bimetallic nanoparticles having a controllable core-shell structure. For instance, oxidation of the preformed core to metal ions often takes place when the other type of metal ions, added for making the shell, have a higher redox potential. This redox potential may result in the production of large islands of the shell metal on the preformed metal core and the re-reduction of the core metal ions is produced. Alternate adsorptions of one of the excess ions and further reductions of the complex by radiolytic radicals progressively build the alloyed cluster. In most of these bimetallic cluster systems, segregation occurs during reduction so that the more noble metal constitutes the core and the less noble metal the shell of a bilayered cluster. This structure stems from an intermetallic electron transfer occurring with the mixture according to the respective redox potentials of the two metals. Initially, the reduction may be equiprobable but then the less noble atoms behave as electron relays toward the other metal ions until the complete reduction of the latter, thus favoring the formation of the clusters of the more noble metal first. A few exceptions observed in intimately alloyed metal clusters prepared either by chemical reduction or by low-dose rate irradiation result from an extremely slow electron transfer, which allows the simultaneous reduction of both ions to occur under nonequilibrium kinetics.

Various methods have been reported so far for the preparation of bimetallics, for example, alcohol reduction [20-24], citrate reduction [23,36], polyol process [37], solvent extraction reduction [24,38], sonochemical method [39], photolytic reduction [40], decomposition of organometallic precursors [41], and electrolysis of a bulk metal [42]. Belloni and Henglein also proposed g radiolysis to produce bimetallic nanoparticles from two different noble metals [1,43].

The gold-silver system is the most interesting bimetallic because both metals are miscible in the bulk phase, owing to very similar lattice constants (0.408 for gold and 0.409 for silver) [44]. Especially, the size effect on the plasmon absorption in connection with the Mie theory and its modifications has been of major interest [45]. Those nanoparticles also showed two SP absorption bands originating from the individual gold and silver domains [44]. The optical properties (plasmon absorption in the visible range) are examined and compared to the calculated absorption spectra using the Mie theory. Recently, a strategy for extracting optical constants of the core and the shell material of bimetallic Ag-Au nanoparticles from their measured SP extinction spectra is reported [46]. Freeman et al. [47] and Morriss and Collins [48] prepared nanoparticles consisting of a gold core and a silver shell. Mulvaney et al. [49] deposited gold onto radiolytically prepared silver seeds by irradiation of KAu(CN)2 solution, Treguer et al. [29] prepared layered nanoparticles by radiolysis of mixed Au(III)/Ag(I) solution.

Silver colloid with gold reduced in the surface layer was prepared by Chen and Nickel [50] by mixing a solution of HAuCl4 with a silver colloid and by addition of a reductant (p-phenylenediamine) in the second step. A two-step wet radiolytic synthesis resulting in a size-dependent spontaneous alloying within Aucore-Agshell nanoparticles and a photochemical approach to Aucore-Agshen nanoparticle preparation were reported [40,51]. Since Ag and Au are miscible in all proportions, but differ in both redox potentials and surface energies, the results of a particular preparative strategy with respect to formation of either the alloyed or layered nanoparticle composition are not always readily predictable, and characterization of the composition of the resulting nanoparticles is thus of key importance. Liz-Marzan et al. [52] used inorganic fibers in aqueous solution for the stabilization of gold-silver particles with diameters of 2-3 nm after the simultaneous reduction of gold and silver salts by sodium borohydride. Mulvaney et al. [53] as well as Sinzig et al. [54] prepared silver nanoparticles coated with an overlayer of gold (core-shell nanoparticles). These particles have two distinct plasmon absorption bands and their relative intensities depend on the thickness of the shell. But also alloy formation within the shell was suggested on the basis of the optical absorption spectra. Similarly, gold-silver composite colloids (30-150 nm in diameter) consisting of mixtures of gold and silver domains were obtained by irradiating aqueous solutions of gold and silver ions with 253.7 nm UV light [55]. Alloy nanoparticles, on the other hand, have mainly been studied because of their catalytic effects [56,57].

However, for numerous other couples of metal ions, the radiation-induced reduction of mixed ions does not produce eventually solid solutions but a segregation of the metals in the core-shell structure, such as for the case of Agcore-Cushell [58] or of Aucore-Ptshell [30]. The absorption spectrum changes in all these systems from the SP spectrum of the first metal to that of the second, suggesting that the latter is coated on the clusters formed first. Since both ions have almost the same initial probabilities of encountering the radiolytic reducing radicals and of being reduced, the results were interpreted as a consequence of an electron transfer from the less noble metal, as soon as it is reduced to one of its lower valency states, to the ions of the more noble metal, which is obviously favored by this displacement and is reduced first [1,2,30,58]. The role of nuclei played by more noble metals such as Pt, Pd, or Cu is efficient to reduce the metal ions of Ni, Co, Fe, Pb, and Hg, which otherwise do not easily yield stable monometallic clusters through irradiation [30]. The bimetallic character of Fe-Cu clusters is attested by their ferromagnetic properties and the change of the optical spectrum relative to pure copper clusters. Similarly, a chemical reduction of a mixture of two ions among Au(III), Pt(IV), and Pd(II) (with decreasing order of redox potentials) yields bilayered clusters of Au-Pd, Au-Pt, or Pt-Pd [59]. Composite clusters of Ag-Pd have been also characterized [60,61]. Bilayered Au-Pt, Ag-Pt, and Ag-Au supported on megalith fibers have been studied by optical absorptions [52]. Photochemical reduction of precursor salts in the presence of a suitable surfactant has now become a worthy addition to this field of research.

As gold has low catalytic activity compared to platinum or palladium, the structural and catalytic changes have been examined for the admixture of platinum or palladium to gold [20,62-64]. Turkevich et al. [65] have synthesized the Au-Pd bimetallic particles and described their morphologies. Toshima et al. [20] have described the catalytic activity and analyzed the structure of the poly(N-vinyl-2-pyrolidone)-protected Au-Pd bimetallic clusters prepared by the simultaneous reduction of HAuCl4 and PdCl2 in the presence of poly(N-vinyl-2-pyrolidone) (PVP). Other research groups have reported the formation of the Au-Pd bimetallic particles with a palladium rich shell by the simultaneous alcoholic reduction method [64]. In contrast, successive alcoholic reduction did not give the core-shell products but, instead, gave the "cluster-in-cluster" product based on the coordination number of the constituents [22]. Mizukoshi et al. [66] reported the preparation and structure of Au-Pd bimetallic nanoparticles by sonochemical reduction of the Au(III) and Pd(II) ions. Au-Pt bimetallic nanoparticles were prepared by citrate reduction by Miner et al. from the corresponding metal salts [67]. By the same method, citrate-stabilized Pd-Pt bimetallic nanoparticles can also be prepared [67]. Colloidal dispersions of Pd-Pt bimetallic nanoparticles can be prepared by refluxing the alcohol water mixture in the presence of PVP [68]. Toshima et al. have succeeded in the preparation of polymer protected various nanoscopic bimetallic colloids of noble metals by co-reduction of the corresponding metal ions in a refluxing mixture of water and alcohol, as well as the preparation of Cu-Pd, Cu-Pt bimetallic colloids with well-defined alloy structures by a cold alloying process [17,20,60,68,69]. The resulting bimetallic crystal particles depend on the ratio of the ionic precursors; the structures Cu-Pd and Cu3-Pd [59,70], Ni-Pt [70,71], Cu-Au, and Cu3-Au [70,71], have been found. Alloyed Ag-Pt clusters in ethylene glycol have also been prepared by chemical reduction of silver bis(oxalato)platinate and characterized by a homogeneous (111) lattice spacing [72]. The properties of both the metals maybe somewhat different in relation to their redox reactions that control their formation through reduction. The alloying takes place upon irradiation [73] through fast association between atoms or clusters and excess ions, which in the case of mixed solutions yield bimetallic complexes.

Silver particles having a gold layer were prepared and the UV-Vis absorption spectra of these bimetallic nanoparticles were intensively investigated. Several metal ions were deposited onto silver sols to produce bimetallic nanoparticles [1,74]. Mercury ions can be reduced in the presence of silver sol, which results in the formation of Agcore-Hgshell bimetallic nanoparticles [75]. Ligand-stabilized Au-Pd [76] and Au-Pt [77] bimetallic nanoparticles were prepared by Schmid et al. by successive reduction. In an earlier study, Turkevich and Kim proposed gold-layered palladium nanoparticles [65]. Three types of Au-Pd bimetallic nanoparticles such as Aucore-Pdsheu, Pdcore-Aushell, and random alloyed particles were prepared by the application of the successive reduction technique [78].

Reduction of the corresponding double salts is one of the important techniques for the bimetallic nanoparticle synthesis. Torigoe and Esumi proposed silver(I)bis(oxalato)palladate(II) as a precursor of Ag-Pd bimetallic nanoparticles stabilized by PVP [65]. Preparation of PVP-stabilized Ag-Pt bimetallic nanoparticles by borohydride reduction from silver(I)bis(oxalato)platinate(I) was also reported [72].

Instead of chemical reduction, an electrochemical process can be used to create metal atoms from bulk metal. Reetz and Helbig proposed an electrochemical method including both oxidation of bulk metal and reduction of the metal ions for the size-selective metal nanoparticles [79]. The particle size can be controlled by the current density. The Pd-Pt, Ni-Pd, Fe-Co, and Fe-Ni bimetallic nanoparticles have also been obtained by this method [80].

8.3.1 Characterization

For more than a decade, many efforts have been made for the preparation and characterization of nanosized materials. To understand the special properties of nanoparticle systems, it has become increasingly important to develop techniques for characterizing such materials at the nanometer level. Their prominence stems from the recognition that nanophase systems often possess dramatically different properties because they may show different characteristics compared to conventional bulk materials or atoms, the smallest units of matter. Characterization is most important for the bimetallic nanoparticles. Characterization of colloids (hydrosols) of bimetallic nanoparticles constituting various combinations of noble metals was the subject of numerous papers, e.g., Au-Pd [65,81], Au-Pt [77,81], Ag-Pd [62,82], Ag-Pt [52,72], and Ag-Au [29,48,49,83].

The first question asked about metal nanoparticles is concerned with aggregation state, size, and morphology. Amongst the technique commonly used, transmission electron microscopy (TEM) is indispensable for metal nanoparticle studies. Structural information is obtained from TEM and highresolution TEM (HRTEM). TEM is a key tool in the quest to understand nanophase systems even though there remain many challenges in applying modern microscopy techniques to small particles [84]. The TEM emphasizes the intensity contrast. Further, HRTEM can now provide information not only on the particle size and shape but also on the crystallography of the monometallic and bimetallic nano-particles. High-resolution phase contrast microscopy is well suited to determine the lattice spacing of thin crystals. This method enables one to distinguish core-shell structures on the basis of observing different lattice spacing. For large crystalline metal nanoparticles, HRTEM suggests the area composition by the fringe measurement, using crystal information of nanoparticles observed in the particle images [77,85,86]. It is easier to measure the lattice spacing on particles of well-defined shape in exact zone-axis orientations. Furthermore, in the case of supported metal nanoparticles, particle growth can be directly seen by in situ TEM observation. It is also necessary to measure the lattice changes and surface relaxations in nanoparticles, which may cause significant changes in the properties of nanophase materials. In bimetallic systems, small changes in lattice spacing may be related to the formation of alloy phases. Accurate determination of the lattice spacing in tiny crystallites is one area of importance for nanoparticle systems. When energy-dispersive x-ray microanalysis (EDX) is used in conjunction with TEM, localized elemental information can be obtained [77,87]. Urban et al. have conducted a number of studies on small well-defined clusters in zone-axis orientations [88].

Gold, silver, and copper (group IB metal) nanoparticles all have characteristic colors related with their particle size. Thus for these metals, observation of the UV-Vis spectra can be a useful complement to other methods in characterizing metal particles. Optical properties of bimetallic nanoparticles comprising Ag and Au are thus the subject of considerable interest. Comparison of the calculated and measured SP extinction spectra was frequently employed as one of the criteria of distinguishing between an alloyed and layered (core-shell) structure of the bimetallic Ag-Au nanoparticles. Comparison of spectra of bimetallic nanoparticles with the spectra of physical mixture of the respective monometallic particle dispersions can confirm a bimetallic structure for the nanoparticles [67,89]. The UV-Vis spectral changes during the reduction can provide quite important information [60]. Moreover, the UV-Vis absorption spectra of the Au-Ag bimetallic particles show substantial differences between the alloy and a core-shell structure, which has been studied explicitly by experimental and theoretical measurements [90].

Extended x-ray absorption fine structure (EXAFS) has been especially useful in providing structural information about the nanocrystalline, as well as the crystalline materials. The analysis of EXAFS allows determination of local structural parameters, such as interatomic distance and coordination number, which are difficult to measure by any other method. In contrast, colloidal dispersions of nanoparticles can be made at a low concentration of metal and the particles in the dispersions can be small and uniform. EXAFS samples can be obtained by concentrating the dispersions without aggregation, to give data of high quality. Sinfelt et al. carried out major studies on the EXAFS of supported bimetallic nanoparticles [91,92]. Bradley et al. prepared Cu-Pd bimetallic nanoparticles by deposition of zerovalent Cu atoms onto preformed Pd nanoparticles [93].

Infrared spectroscopy (IR) has been widely applied to the investigation of the surface chemistry of adsorbed small molecules. By comparison of IR spectra of CO on a series of bimetallic nanoparticles at various metal compositions, one can elucidate the surface microstructure of bimetallic nanoparticles [21,93].

X-ray methods are informative for nondestructive elemental and structural analyses. X-ray diffraction (XRD) gives structural information of nanoparticles, including qualitative elemental information. For monometallic nanoparticles, the phase changes with increasing diameter of nanoparticles can be investigated with XRD. The presence of bimetallic particles as opposed to a mixture of monometallic particles can also be demonstrated by XRD, since the diffraction pattern of the physical mixtures consists of overlapping lines of the two individual monometallic nanoparticles and is clearly different from that of the bimetallic nanoparticles. The structural model of bimetallic nanoparticles can be proposed by comparing the observed XRD spectra and the computer-simulated ones [94].

For a rationalization of their catalytic properties, the surface composition and structure is indispensable information and quantitative x-ray photoelectron spectroscopy (XPS) analysis is a powerful tool in the elucidation of the surface composition. From the quantitative analysis of XPS data of bimetallic nanoparticles, one can note which elements are present in the surface region. For the core-shell structure, the XPS data gives the peak of the binding energy corresponding to the metal in the shell. On the other hand, for alloy nanoparticles, a separate peak is assigned that is different from both the constituting metal.

One of the most revealing analytical methods for the composition of bimetallic nanoparticles is EDX, which is usually coupled with a transmission electron microscope with high resolution [62,77,95]. EDX is a kind of electron probe microanalysis (EPMA) or x-ray microanalysis (XMA) method, which has higher sensitivity than the usual EPMA or XMA techniques. This method provides analytical data that cannot be obtained by the other three methods mentioned above.

NMR spectroscopy of metal isotopes [96] is a powerful technique for understanding the electronic environment of metal atoms in metallic particles by virtue of the NMR shifts caused by free electrons (Knight shifts) [97]. The NMR spectra of metal nanoparticles, having Pauli paramagnetic properties, are governed both by the density of energy levels at the Fermi energy and by the corresponding wave function intensities of each site: the local density of states (LDOS). Further, the electronic properties of metal nanoparticles may be informative for investigating the catalytic properties of the metal nanoparticles.

All the tools at hand comfortably provide enough information about the nanostructural material ordering but quantitative evaluation of the thickness of the add-layer over the core structure still remains a debate.

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