Coalescence silver clusters

Cluster properties, mostly those that control electron transfer processes such as the redox potential in solution, are markedly dependent on their nuclearity. Therefore, clusters of the same metal may behave as electron donor or as electron acceptor, depending on their size. Pulse radiolysis associated with time-resolved optical absorption spectroscopy is used to generate isolated metal atoms and to observe transitorily the subsequent clusters of progressive nuclearity yielded by coalescence. Applied to silver clusters, the kinetic study of the competition of coalescence with reactions in the presence of added reactants of variable redox potential allows us to describe the autocatalytic processes of growth or corrosion of the clusters by electron transfer. The results provide the size dependence of the redox potential of some metal clusters. The influence of the environment (surfactant, ligand, or support) and the role of electron relay of metal clusters in electron transfer catalysis are discussed. 

A charged cluster may constitute an electron acceptor, but that depends on its own redox potential value, E (A -Agn) relative to the threshold imposed by the monitor potential, E°(Q -QH2). As the redox potential increases with cluster nuclearity (5, 6), a certain time after the pulse is required to allow the first supercritical clusters to be formed and their potential to reach the threshold value imposed by the hydroquinone. When time, t, is less than tc, where n < Uc, the transfer is not allowed. During this induction period, the kinetics at 380 nm correspond to pure coalescence of clusters (Figure 4), and hydroquinone is stable (the bleaching OD512 is constant). That means, obviously, that none of the silver species present at that time can react with hydroquinone, especially free Ag ions and Ag ions associated with the smallest clusters. 

The general problem of this competition has been solved by numerical Simula-tion for variable values of Xq, Sq, Wc, fed, and fet (II). The value of fed = 2 X 10 L mol s for silver clusters is taken as the same as for pure coalescence (reaction 9). The fixation of Ag" onto Agi is fast and does not interfere with... 

Stabilizing effects have been directly observed by pulse radiolysis studies of silver cluster coalescence supported on 4-nm silica colloidal particles (44). Very small oligomeric clusters absorbing at 290 nm and 330 nm are stable in the presence of oxygen and even Cu " or Ru(NH3)6Cl3 (E° = 0.2 Vnhe)-These supported the exhibition of higher redox potentials than for free oligomeric clusters. 

Colloidal support such as small Si02 particles restrict interparticle diffusion of silver atoms when formed by radiolysis of the ions at their surface. The silver oligomers absorbing at 290 and 330 nm are observed by pulse radiolysis. They are stable with respect to coalescence but they are oxidized by MV, O, Cu and RufNHj)/. Alumino-silica gels with silver ions give after irradiation optically clear xerogels containing silver clusters from 2.5 to 4.5 nm. 

When silver or gold atoms are generated from Ag CN)2 or Au (CN)2 in the presence of the methylviologen redox couple MV /MV, oxidation of the smallest clusters is also observed, because coalescence in cyanide solutions is slow (Fig. 4) [54,66]. While supercritical silver clusters ( > 6 1) (Table 3) accept electrons from MV with a progressive increase of their nuclearity, the subcritical clusters undergo a progressive oxidation by (Fig. 5). Actually, the reduced ions MV so produced act as an... 

Silver clusters can be produced by radiation in alcohols as well. The pulse radiolysis technique has been used to study the transient absorption spectra of Ag , Ag2 , and lower and higher oligomers of silver in an N2-purged 2-propanol solution of AgC104. Radiolysis of alcohols generates a number of species through different reactions. Created in the reaction, the reducing radical (CH3)2CO H has an insufficient potential to reduce Ag+ to Ag° however, it may be able to reduce species such as Ag2 or Ag3 +. The coalescence rate was faster than observed in water and methanol systems (Dey and Kishore 2005). 

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... 

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... 

Pulse radiolysis studies of the reactivity of 25 nm Agl particles with e3, in the presence of alcohol have shown that first the semiconductor spectrum at 360 nm is bleached, then silver atoms and clusters at 450-600 nm are formed.The electron transfer between the couple MV /MV and Ag clusters formed on an AgCl crystallite was studied by pulse radiolysis. The coalescence of Ag atoms at the AgCl surface is slow so that, as in the presence of CN, an electron transfer from subcritical clusters to precedes the... 

The growth of thallium clusters has been also observed by time-resolved optical spectroscopy. The coalescence steps are comparable with those of silver and the final plasmon band of Tl is located at 300 nm. ... 

If the concentration of is high, the reactions depicted by Eqs. (29)-(31) are faster than the coalescence reactions (Eqs. 10 and 11) with a fixed total concentration of atoms and the clusters now grow mostly by successive additions of supplementary reduced atoms (electron plus ion). It has been shown that once formed, a critical cluster, of silver for example,indeed behaves as a growth nucleus. Alternate reactions of electron transfer (Eqs. 32 and 34) and adsorption of surrounding metal ions (Eq. 33) make its redox potential more and more favorable to the transfer (Fig. 8), and autocatalytic growth is observed.The observation of an effective transfer therefore implies that the potential of the critical cluster is at least slightly more positive than that of the electron-donor system, i.e. °(M +/M ) > °(S/S-). 

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