Clusters gold-silver alloy

H.M. Lee, M. Ge, B.R. Sahu, P. Tarakeshwar, and K.S. Kim, Geometrical and electronic structures of gold, silver, and gold-silver binary clusters origins of ductility of gold and gold-silver alloy formation. J. Phys. Chem. B 107, 9994—10005, 2003. 

Figure 12 Top Maximum wavelength of the plasmon band of alloyed gold-silver clusters as a function of the mole fraction x of gold in alloyed gold-silver clusters, produced at the dose rate 7.9 MGy hr and the dose 20 kGy. Experiments, calculated values by Mie model. Bottom Extinction coefficient at the maximum of the plasmon band as a function of the mole fraction x of gold in alloyed gold-silver clusters. Experiments, calculated values from Kreibig equation [74] with r = 5 nm A with r = 3 nm. (From Ref 102.)...

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. 

The spectra of silver and gold nanoclusters are intense and distinct (Table 4). They are thus particularly suitable to detect the evolution of a cluster composition during the construction of a bimetallic cluster in mixed solution. The system studied by pulse radiolysis was the radiolytic reduction of a mixed solution of two monovalent ions, the cyano-silver and the cyano-gold ions Ag(CN)2 and Au(CN)2 (Fig- 7) [66]. Actually, the time-resolved observation demonstrated a two-step process. First, the atoms Ag and Au are readily formed after the pulse and coalesce into an alloyed oligomer. However, due to... 

For example, when the mixed solution of Ag(CN)2 and Au(CN)2 is irradiated by y-radiolysis at increasing dose, the spectrum of pure silver clusters is observed first at 400 nm, because Ag is more noble than Au due to the CN ligand. Then, the spectrum is red-shifted to 500 nm when gold is reduced at the surface of silver clusters in a bilayered structure [102], as when the cluster is formed in a two-step operation [168] (Table 5). However, when the same system is irradiated at a high dose rate with an electron beam, allowing the sudden (out of redox thermodynamics equilibrium) and complete reduction of all the ions prior to the metal displacement, the band maximum of the alloyed clusters is at 420 nm [102]. 

Similarly, at moderate dose rate for the couple Au Cl4, Ag, gold initially appears at 520 nm. Therefore, Ag ions essentially act as an electron scavenger, and as an electron relay toward more noble gold ions as far as gold ions are not totally reduced. Then silver-coated gold clusters are formed and the maximum is shifted to 400 nm, which is that of silver (Fig. 11) [102]. But at higher y- or EB dose rate (irradiation time of a few seconds), the electron transfer is too slow to compete with coalescence and the spectrum of alloyed clusters... 

Beyond 2 s, the absorbance at 520 nm decreases whereas at 400 nm it continues to increase in proportion (Fig. 5). Both absorbances reach a plateau after 20 s. The plateau at 400 nm is nearly the same as for pure silver solutions, as if the reduction equivalents of gold atoms had been all transferred to silver. Therefore, even when formed soon after the pulse, the alloyed metal clusters progressively lose, during the second time period, the zero-valent gold and are enriched in silver until the gold completely disappears, and gold ions are slowly released (Eq. 24). 

---