Hydrated silver clusters, redox potentials

The dependence of cluster potential on nuclearity was obtained by changing the reference potential in a series of redox monitors (Table 5). The redox potentials of hydrated silver clusters are seen to increase with n. The data in Fig. 11 indicate that, at least for the redox properties of silver clusters, the transition between the meso-... 

Fig. 6 compares the nuclearity effect on the redox potentials [19,31,63] of hydrated Ag+ clusters E°(Ag /Ag )aq together with the effect on ionization potentials IPg (Ag ) of bare silver clusters in the gas phase [67,68]. The asymptotic value of the redox potential is reached at the nuclearity around n = 500 (diameter == 2 nm), which thus represents, for the system, the transition between the mesoscopic and the macroscopic phase of the bulk metal. The density of values available so far is not sufficient to prove the existence of odd-even oscillations as for IPg. However, it is obvious from this figure that the variation of E° and IPg do exhibit opposite trends vs. n, for the solution (Table 5) and the gas phase, respectively. The difference between ionization potentials of bare and solvated clusters decreases with increasing n as which corresponds fairly well to the solvation free energy of the cation deduced from the Born solvation model [45] (for the single atom, the difference of 5 eV represents the solvation energy of the silver cation) [31]. 

Radiolysis has been used successfully in order to synthesize various noble (such as silver, gold and platinum) and non-noble (such as nickel and iron) metal nanoparticles in aqueous solution and also in other solvents such as alcohols. Due to their relatively low redox potential compared to that of the bulk, metal clusters can be oxygen-sensitive. However, the deoxygenation (by bubbling with an inert gas such as argon or nitrogen) of the solutions prior to irradiation and their study under inert atmosphere prevent their oxidation. Moreover, since water radiolysis leads to the formation of protons in addition to that of hydrated electrons, radio-induced acidification of the medium may lead to non-noble metal clusters corrosion. Therefore, to avoid the oxidation by protons, the solutions can be prepared in slightly basic medium. 

Electron emission from illuminated oligomeric silver clusters has also been observed. Quantum yields larger than 0.1 were found, which is understood in terms of the rather negative redox potentials of these clusters. Figure 9 describes a typical laser flash experiment (a) is the spectrum of the solution it contains several absorption bands of stabilized clusters, and (b) is the difference spectrum immediately after a 308 nm laser flash. The difference spectrum contains the broad absorption at longer wavelengths of the hydrated electron. In addition, it contains negative absorptions where the bands of the clusters were positioned. The latter effect indicates that the clusters (which are positively... 

---