Chemical reduction, gold clusters

Poly(amidoamine) dendrimers of the fourth generation with -OH terminal groups were used as templates to produce stable metal nanoparticles [170-172], The dendrimers (in aqueous solution) are first loaded with a predetermined amount of Cu + or Pt + metal ions following chemical reduction, metal nanoparticles are formed inside the dendrimer structure, where they are protected from agglomeration. This procedure permits both particle stability and control over particle size. Dendrimers containing Pt metal clusters were also attached to gold electrodes, and were found to be active as electrocatalysts for O2 reduction [172]. This demonstrates that the nanoparticles inside the dendrimer can mediate electron transfer processes between the electrode surface and reactants in solution. 

It is difficult to deduce what gold particle morphologies arise from heterogeneous chemical reduction of HAuCU. Understanding of the model catalysts is much easier. In brief, a) nucleation of gold clusters occurs at surface defects that act as traps b) on AI2O3, there are two kinds of traps at <0.8 and >1.6eV c) the defect density is ca. 3 x 10 sites per cm (10 monolayer) and d) when the clusters grow to >600 atoms, they leave the traps. This can explain the bimodal size distribution of the clusters. Atomistic definition of these traps is needed. 

This novel synthesis scheme has been also investigated with alloyed gold-silver and pure-silver clusters. Ag-Au-alloyed clusters obtained by simultaneous reduction and co-precipitation of two metals give results completely similar to those of the pure-gold clusters. For example, dodecanethiol completely removed PVP from the alloyed cluster surface, producing the thiol derivative. However, the hgand-exchange reaction was not complete in the case of pure-silver clusters embedded in PVP, and consequently the thiolate product isolation-purification was not possible. The success of this chemical technique with other PVP-embedded metal clusters has not be tested yet. 

To date, several methods have been developed to synthesize gold clusters with different sizes and even with a specific number of gold atoms, including chemical reduction, physical vapor deposition, electrical reduction, and so on. Among these methods, chemical reduction method has been the most frequently studied. 

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