Passivated metal particles

Metal-polymer nanocomposites can be obtained by two different approaches, namely, in situ and ex situ techniques. In the in situ methods, metal particles are generated inside a polymer matrix by decomposition (e.g., thermolysis, photolysis, radiolysis, etc.) or chemical reduction of a metallic precursor dissolved into the polymer. In the ex situ approach, nanoparticles are first produced by soft-chemistry routes and then dispersed into polymeric matrices. Usually, the preparative scheme allows us to obtain metal nanoparticles whose surface has been passivated by a monolayer of -alkanethiol molecules (i.e., Crfiin+i-SH). Surface passivation has a fundamental role since it avoids aggregation and surface oxidation/contamination phenomena. In addition, passivated metal particles are hydrophobic and therefore can be easily mixed with polymers. The ex-situ techniques for the synthesis of metal/polymer nanocomposites are frequently preferred to the in situ methods because of the high optical quality that can be achieved in the final product. 

Clusters are intennediates bridging the properties of the atoms and the bulk. They can be viewed as novel molecules, but different from ordinary molecules, in that they can have various compositions and multiple shapes. Bare clusters are usually quite reactive and unstable against aggregation and have to be studied in vacuum or inert matrices. Interest in clusters comes from a wide range of fields. Clusters are used as models to investigate surface and bulk properties [2]. Since most catalysts are dispersed metal particles [3], isolated clusters provide ideal systems to understand catalytic mechanisms. The versatility of their shapes and compositions make clusters novel molecular systems to extend our concept of chemical bonding, stmcture and dynamics. Stable clusters or passivated clusters can be used as building blocks for new materials or new electronic devices [4] and this aspect has now led to a whole new direction of research into nanoparticles and quantum dots (see chapter C2.17). As the size of electronic devices approaches ever smaller dimensions [5], the new chemical and physical properties of clusters will be relevant to the future of the electronics industry. 

At the potential beyond the critical pitting potential, the passive metal electrode system turns unstable. As mentioned before, the asymmetrical fluctuations arise from the electrostatic interaction between the electrode surface and solution particles in the double layer, so that the pitting current develops rapidly, and pits grow simultaneously. 

At the end of these measurements, the electrode was polarized by sweeping the potential to -1.2 V, yielding a six-line spectrum corresponding to metallic iron with some contribution from Fe(0H)2 (curve c, Fig. 5). The potential was then scanned up to -0.3 V and a spectrum essentially identical to that recorded at -1.2 V was observed. This result clearly indicates that the iron metal particles formed by the electrochemical reduction are large enough for the contributions arising from the passivation layer to be too small to be clearly resolved. After scanning the potential several... 

Particles produced in the gas phase must be trapped in condensed media, such as on solid substrates or in liquids, in order to accumulate, stock, and handle them. The surface of newly formed metallic fine particles is very active and is impossible to keep clean in an ambient condition, including gold. The surface must be stabilized by virtue of appropriate surface stabilizers or passivated with controlled surface chemical reaction or protected by inert materials. Low-temperature technique is also applied to depress surface activity. Many nanoparticles are stabilized in a solid matrix such as an inert gas at cryogenic temperature. At the laboratory scale, there are many reports on physical properties of nanometer-sized metallic particles measured at low temperature. However, we have difficulty in handling particles if they are in a solid matrix or on a solid substrate, especially at cryogenic temperature. On the other hand, a dispersion system in fluids is good for handling, characterization, and advanced treatment of particles if the particles are stabilized. 

Despite extensive studies, the photovoltage or the solar-to-chemical energy conversion efficiency still remains relatively low. The main reason is that it is very difficult to meet all requirements for high efficiency. For example, high catalytic activity and sufficient passivation at the electrode surface are incompatible. It was found, however, that a semiconductor electrode modified with small metal particles can meet all the requirements and thus becomes an ideal type semiconductor electrode. Cu, Ag, and Au were chosen because they were reported to work as efficient electrocatalysts for the C02 reduction. p-Si electrodes modified with these metals in C02-staurated aqueous electrolyte under illumination produce mainly methane and ethylene.178 This is similar to the metal electrodes but the metal-particle-coated electrodes work at approximately 0.5 V more positive potentials, contrary to continuous metal-coated p-Si electrodes. 

The novel properties of magnetic nanoparticles are in focus of many fundamental research and practical applications [1,2], It is difficult to prevent metallic magnetic (Fe, Co, Ni) nanoparticles from oxidization under conventional experimental conditions. Several techniques, such as carbon encapsulation, reagent stabilizing, and passivation of nanoparticles, have been developed to protect metallic particles. 

Gold and silver metal particles have been investigated principally since copper particles are not stable in aqueous solutions [17, 18, 52]. Copper particles require a careful passivation to prevent tire re-oxidation of copper. Gold particles with diameters of 22 nm were found to have a hyperpolarizability magnitude of about... 

Fig. 11. Schematics of nanoenergetic materials, left) A trinary system of nanometric metal particle fuel protected by a passivation layer, suspended in a uniform bulk oxidizer matrix. right) Both fuel and oxidizer are nanoparticles, and a binder is used to generate gas that provides a working fluid.

Plasma is effective in the fabrication of nanocomposites, such as nanoparticles composed of one material and covered by a nanolayer of another. Relevant examples include, in particular, carbon-coated magnetic metal particles produced in thermal arc plasma (McHenry et al., 1994), as well as polymer-coated nanoparticles with improved adhesion, corrosion resistance, and surface passivation produced in RF plasma in a fluidized bed (Shi et al.,... 

The type of bond between the surface of the solid and adsorbate molecules determines the kind of surface processes that can take place crystal growth, growth inhibition, nucleation, corrosion, catalytic activity, and chemical passivation. Sometimes there are two types of surfaces involved in the reaction metallic and ionic (many heterogeneous catalysts consist of very small metal particles on oxidic carriers). 

---