Metal alloy nanocrystals

An interesting approach to synthesize metal alloy nanocrystals is the use of simultaneous salt reduction and thermal decomposition processes. Sun et al. [18] reported on the synthesis of iron-platinum (FePt) nanoparticles through the reduction of platinum acetylacetonate by a diol, and decomposition of iron pentacarbonyl (Fe(CO)5) in the presence of a surfactant mixture (oleic acid and oleyl amine). On the basis of a similar approach, Chen and Nikles [217] synthesized ternary alloy nanoparticles (FC cCo3,Ptioo x-y), using a simultaneous reduction of acetylacetonate and platinum acetylacetonate and thermal decomposition of Fe(CO)5 and obtaining an average particle diameter of 3.5 nm and narrow particle size distribution. 

This mechanism has been adopted as the explanation of CO-tolerant H2 oxidation on other Pt-M alloy catalysts. Although practically all transition metals are oxidized in acid solutions, the fact is that only a few metals alloyed with platinum show certain activity for oxidation of H2/CO mixtures, as such metals have to be able to provide OH at low potential (e.g., near the hydrogen reversible potential). Research has been conducted on both Pt-M bimetallic nanocrystals such as Pt-Ru, Pt-Sn, and Pt-Mo [20, 21, 35, 113] and M-decorated Pt single crystals involving Ru [16-20, 26, 44- 7, 70, 81, 106-108, 114-122, 123-128], Mo [24, 70, 126-128, 129-131], Sn [26,29-31, 125] and other transition metals [42, 114, 129, 130]. The bifunctional mechanism is generally acknowledged as the dominant effect for single-crystal Pt decorated by other metals, particularly for Ru on Pt(l 11) [46, 106-108, 124]. 

Recently, the VLS growth method has been extended beyond the gas-phase reaction to synthesis of Si nanowires in Si-containing solvent (Holmes et al, 2000). In this case 2.5-nm Au nanocrystals were dispersed in supercritical hexane with a silicon precursor (e.g., diphenylsilane) under a pressure of 200-270 bar at 500°C, at which temperature the diphenylsilane decomposes to Si atoms. The Au nanocrystals serve as seeds for the Si nanowire growth, because they form an alloy with Si, which is in equilibrium with pure Si. It is suggested that the Si atoms would dissolve in the Au crystals until the saturation point is reached then they are expelled from the particle to form a nanowire with a diameter similar to the catalyst particle. This method has an advantage over the laser-ablated Si nanowire in that the nanowire diameter can be well controlled by the Au particle size, whereas liquid metal droplets produced by the laser ablation process tend to exhibit a much broader size distribution. With this approach, highly crystalline Si nanowires with diameters ranging from 4 nm to 5 nm have been produced by Holmes et al. (2000). The crystal orientation of these Si nanowires can be controlled by the reaction pressure. 

In our relativistic density-functional study of mixed Pt-M nanoparticle surfaces is represented by a two-layer cluster with seven surface and three second-layer atoms, Ptio-nMn(7,3) [6]. The subnano cluster model does not simulate bulk surface properties because of its limited size and undercoordinated metal atoms. However, the model is suitable for simulating the properties of nanoscale particle catalysts, e.g., Pt-Ru alloy nanoparticles wife an fee surface. Catalytically much more active than bulk metal surfaces, these nanocrystals exhibit a transition from metallic to insulator properties [48]. The cluster model is also suitable for rough Pt-M electrode surfaces that exhibit a high surface density of reactive Pt-M sites [49]. 

This review of STM studies of thin anodic oxide (passive) films formed on metals and alloys shows that important results have been obtained by direct imaging of the sur ce structure, providing direct evidence on (for example), the crystallinity of passive films and the nature of defects. The fully crystalline character of the film on Ni has been demonstrated by STM. The nature of defects (steps, kinks, vacancies, points of reduced thickness) has been elucidated. This is important for a better understanding of the breakdown of passive films. The unique protectiveness of fire film on Cr may be related to the observed structure with oxide nanocrystals cemented by a noncrystalline hydroxide. Many more results are expected to be produced, in the future, on the atomic structure of passive films, including the local interactions of impurities and anions with passive films and especially with surface defects, file local conductivity of passive films derived fi om I-V curves at specific sites, and chemical features derived fixim spectroscopic imaging. All these data should drastically improve our understanding of the relation between structure and properties of passive films. 

In addition, ILs could also be used as both solvent and electrolyte for the electrodeposition of copper [35, 36], aluminum [37, 38], tantalum [4], platinum [39], silver [40, 41], gold [40-42], and silicon [43]. For example, Endres et al. have reported the electrodeposition of nanocrystalline metals and alloys, such as aluminum from ILs, which previously could not be electrodeposited from aqueous or organic solutions. This method enabled the synthesis of aluminum nanocrystals with average grain sizes of about 10 nm, Al-Mn alloys, as well as Fe and Pd nanocrystals [4] [as shown in Fig. 4.2). 

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