Mixed Ions of Precious Metals

Coreduction of mixed ions is the simplest but smart method to synthesize bimetallic nanoclusters. However, this method is not always successful. Au/Pt bimetallic nanoclusters were prepared by citrate reduction by Miner et al, from the corresponding two metal salts, such as tetrachloroauric(III) acid and hexachloropla-tinic(IV) acid. Reduction of the metal ions is completed within 4 h after the addition of citrate. They studied the formation of colloidal dispersion by ultraviolet-visible (UV-Vis) spectroscopy, which showed that the spectrum is not the simple sum of those of the two monometallic nanoclusters, indicating that the bimetallic nanoclusters have an alloy structure. 

An alcohol reduction method in which alcohol like ethanol can work both as a reductant and a solvent has been applied to the synthesis of polymer-capped bimetallic nanoclusters. They have been prepared by the simultaneous reduction of the two corresponding metal ions with refluxing alcohol. For example, colloidal dispersions of Pd/Pt bimetallic nanoclusters can be prepared by refluxing 

Coordination numbers of PVP-capped Pd/Pt (4/1) bimetallic nanoclusters and those calculated for the models 

These core/shell structures were found to be very common in the PVP-capped bimetallic nanoclusters of precious metals prepared by the coreduction of precious metal ions with alcohol. For example, PVP-capped Au/Pd and Pt/Rh bimetallic nanoclusters prepared by alcohol coreduction in solution were also investigated in a similar way and have been proved to have a core/shell structure where Au and Pt form the core, and Pd and Rh form the shell, respectively. There appears to be a trend among the precious metals in ease of formation of the core or shell  

The metal on the left side more easily forms a core than that on the right side and vice versa for a shell. Thus, in the combination of Au and Pd, for example, Au forms the core and Pd the shell, and so on. The question that arises then is how the order of this trend is determined. The question was answered by the in situ UV-Vis spectra measured during the coreduction of Au and Pt ions in alcohol in the presence of PVP. According to precise analyses of spectra, the formation mechanism of core/shell-structured bimetallic nanoclusters has been proposed as shown in Fig. 3.20. Here the difference in the redox potential of both elements and that in the strength of coordination bonds between polymer and atom of both elements are the factors that determine the order of ease in forming the core or shell. 

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