Nanometric metal clusters

The symbol indicates that the earliest steps of the cluster formation have been studied by pulse radiolysis. 

Non noble metal clusters are considerably more fragile to corrosion by the solvent than are noble metal clusters. Therefore the production of stable small particles results from a compromise between the smallest size and the longest stability. 

Long-lived clusters Cu , Ni , Co , Sn , Tl , Pb (Table 4) may be formed in deaerated basic medium, but Zn clusters are oxidized into zinc hydroxide within a few weeks even in the absence of Oj. Clusters Cd are better stabilized by gelatin. Ni or Co clusters display ferromagnetic behaviour. Whereas all of these non-precious metal clusters are easily oxidized in solution by O2, they may be stabilized in air after drying under inert atmosphere. 

Actually, the kinetics study of the cluster redox potential ( 4.2) mimics the process of the photographic development, except clusters are free in the solution (not fixed on AgBr crystals) and, beyond the critical nuclearity, they 

The development process may be used to select the cluster final size but it occurs also spontaneously any time an even mild reducing agent is present during the radiolytic synthesis. The specificity of this method is to combine the ion reduction successively  

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The radiolytic synthesis consists of (1) either preparing first nanometric metal clusters in solution, which are then put in contact with the support (possibly by filtration), or (2) irradiating in situ the ionic precursors after their adsorption onto the supporting material. 

Castro T ef a/1990 Size-dependent melting temperature of individual nanometre-sized metallic clusters Phys. Rev. B 42 8548... 

Gittins DI, Bethell D, Schiffrin DJ, Nichols RJ (2000) A nanometre-scale electronic switch consisting of a metal cluster and redox-addressable groups. Nature 408 67-69... 

Adsorption of ions or molecules on metal clusters markedly affects their optical properties. It was shown that the intensity and the shape of the surface plasmon absorption band of silver nanometric particles, which is close to 380 nm, change upon adsorption of various substances [125]. The important damping of the band generally observed is assigned to the change of the electron density of the thin surface layer of the... 

Small metal clusters can be incorporated into the pores of MCM-41 by encapsulating an organometallic compound by absorption and then decomposing it at low temperatures (2-300°C). Nanometre size Sn-Mo clusters have been made in this way. 

The theoretical results have also indicated that when metal atoms are bound to specific defects their chemical activity may change, in particular can increase. This is likely to be true also for small metal clusters. This has not been fully appreciated so far. In fact, even inert supports, like silica, alumina, or magnesia, can interact strongly with the supported metal if this is bound at a defect site and can have a direct role in the chemistry of the supported species. Some preliminary calculations on supported clusters, however, suggest that the effect of the defect on the cluster electronic structure is restricted to very small, really nanometric clusters of about ten atoms size [224]. Should the size of active catalysts in real applications go down to this size, the specific interaction with the substrate could no longer be ignored in the interpretation of the catalytic activity. 

The catalytic role of the metal clusters may be explained again by their influence on the reaction thermodynamics. Actually, the direct electron transfer from MV (E°(MV" /MV ) = - 0.41 Vn e) to H3O (E°(H30 /H) = -2.3 Vnhe) is thermodynamically unfavored if they are both in solution, and in contrast the reverse reaction from H to MV is fast. However, the redox potential of HjO /H when adsorbed on a nanometric cluster E°(H30, Pt /H, Pt ) is shifted to values higher than that of E°(MV /MV ) and the transfer (39-41) becomes possible. The evolution of molecular hydrogen catalyzed by clusters from radicals such as COz or (CH3)2COH which are also in solution less strong reducing agents than H radicals may be similarly explained. 

In surface-enhanced Raman spectroscopy (SERS), increased Raman signals are observed from molecules attached to metallic clusters ranging in size of the order of tens of nanometres (a. 16). Enhancements as high as lO have been observed. These enhancement factors can lead to single-molecule Raman spectroscopy (a. 17). Using a Raman microscope the probe volume can be as small as 10 picolitres. Spectra can be measured with a one-second collection time. 

Materials with surface protrusions separated by nanometric distances or nanometer-sized metallic clusters... 

Thiols and gold surfaces are ideal partners to write any kind of structure in nanometre dimensions. Artificial patterns of appropriately functionalized molecules can be used to bind metal nanoparticles specifically. Figure 25 gives an impression of the individual steps leading to a distinct cluster arrangement. 

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