Synthesis of monometallic nanoclusters

The quite negative value of E°(M /M ) and the dependence of the cluster redox potential on the nuclearity have cmcial consequences in the formation of early nuclei, their possible corrosion or their growth. As an example, the faster the coalescence, the lower is the probability of corrosion by the medium. Another consequence is that when clusters of different sizes encounter, even if they are prevented from coalescing by a polymer coating, the smaller, which has a more negative potential, can still donate successively electrons to the ions adsorbed on the larger one, up to the complete oxidation of the former. The process results in a narrower size dispersity but also to a further growth. However, various applications require the synthesis of small clusters and the coalescence should be somewhat prevented by a stabilizer or a support. 

Similarly, when PtClg ions are irradiated in the presence of PA which prevents coalescence, most of Pt atoms are found by STM imaging in the form of very small platinum oligomers of 3-7 atoms only. In the presence of PP, PtCls irradiated solutions present a UV band with a maximum at 215 nm. About 20 % of the initial signal is lost by oxidation when the sample is exposed to air, the rest being stable. This result again confirms that the redox potential of the smallest clusters is markedly shifted to the negative values. 

The synthesis is selective and m is controlled by adjusting the dose (m decreases at high doses). The mechanism of the reduction has been observed recently by pulse radiolysis. Reduction by pulse-induced radicals of all [PtjCCO) clusters yields the transient [Pt3(CO)Jj, which then quickly dimerizes into [Pt3(CO)J, 2 before undergoing cleavage into 3 x [Pt3(CO)6]4 . If the initial reduction is only partial, further addition of [Pt3(CO)j]4 with excess [Pt3(CO)6]g leads to 2 x [Pt3(CO)j5 which is also the product observed by y-radiolysis. Molecular clusters [Pt3(CO)6]5 have been observed by STM.  

Other molecular metal clusters have been obtained by y-radiolysis, such as Ru3(CO),2 and H2Ru6(CO) g, or Fe(CO 5 and Fe3(CO),2 at higher [M]/[L ratio. Similarh 

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