Clusters, nuclearity-dependent properties

For more than two decades, extensive research work has been devoted to the unique properties of clusters. They are made of a small number (or nucleaiity) of atoms or molecules only, and therefore constitute a new state of matter, or mesoscopic phase, between the atom or molecule and the crystal. New methods have been developed in physics and chemistry for their synthesis, their direct observation, the study of their properties, and of their crucial role in number of processes, such as phase transition, catalysis, surface phenomena, imaging. Owii to its specific approach, radiation chemistry offei first the opportunity to reveal the existence of nuclearity-dependent properties of clusters and has then proven to be a powerfid method to study the mechanisms of cluster formation and reactivity in solution. 

It was also observed, in 1973, that the fast reduction of Cu ions by solvated electrons in liquid ammonia did not yield the metal and that, instead, molecular hydrogen was evolved [11]. These results were explained by assigning to the quasi-atomic state of the nascent metal, specific thermodynamical properties distinct from those of the bulk metal, which is stable under the same conditions. This concept implied that, as soon as formed, atoms and small clusters of a metal, even a noble metal, may exhibit much stronger reducing properties than the bulk metal, and may be spontaneously corroded by the solvent with simultaneous hydrogen evolution. It also implied that for a given metal the thermodynamics depended on the particle nuclearity (number of atoms reduced per particle), and it therefore provided a rationalized interpretation of other previous data [7,9,10]. Furthermore, experiments on the photoionization of silver atoms in solution demonstrated that their ionization potential was much lower than that of the bulk metal [12]. Moreover, it was shown that the redox potential of isolated silver atoms in water must... 

Stability means that clusters do not undergo coalescence nor corrosion by the medium, at least in the absence of oxygen. The quite negative value of ii°(MVM ) and the dependence of the cluster redox potential on the nuclearity have crucial consequences in the formation of early nuclei, their possible corrosion or their growth. As an example, the faster the coalescence, the lower is the probability of corrosion of the small clusters by the medium. The property of stability offers the means to apply to these clusters a larger amount of suitable characterization techniques than to transient oligomers. 

One of the important applications of mono- and multimetallic clusters is to be used as catalysts [186]. Their catalytic properties depend on the nature of metal atoms accessible to the reactants at the surface. The possible control through the radiolytic synthesis of the alloying of various metals, all present at the surface, is therefore particularly important for the catalysis of multistep reactions. The role of the size is twofold. It governs the kinetics by the number of active sites, which increase with the specific area. However, the most crucial role is played by the cluster potential, which depends on the nuclearity and controls the thermodynamics, possibly with a threshold. For example, in the catalysis of electron transfer (Fig. 14), the cluster is able to efficiently relay electrons from a donor to an acceptor, provided the potential value is intermediate between those of the reactants [49]. Below or above these two thresholds, the transfer to or from the cluster, respectively, is thermodynamically inhibited and the cluster is unable to act as a relay. The optimum range is adjustable by the size [63]. 

Evidently, the redox potential values of clusters must be comprised between the limit values of the atom and the bulk metal. As the other physicochemical properties, redox potentials of clusters depend on their nuclearity, mostly at low n (Fig. Kinetic studies,... 

Important consequences result from the increase of the redox potential of metal clusters with their nuclearity. Indeed, independently of the metal, the smaller clusters are more sensitive to oxidation and can undergo corrosion even by mild oxidizing agents. Moreover, size-dependent redox properties explain the catalytic efficiency of colloidal particles during electron transfer processes. Their redox potentials control their role as electron relays the required potential being intermediate between the thresholds of the potentials of the donor (more negative) and of the acceptor (more positive). Catalytic properties of the nanoparticles are thus size-dependent. Haruta and co-workers reported that gold nanoparticles smaller than 5 nm have potential applications in catalysis as they are very active in... 

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. 

The increase of the redox potential of a metal cluster in a solvent with its nuclearity is now well established 1-4). The difference between the single atom and the bulk metal potentials is large (more than 2 V, for example, in the case of silver (3)). The size dependence of the redox potential for metal clusters of intermediate nuclearity plays an important role in numerous processes, particularly electron transfer catalysis. Although some values are available for silver clusters (5, 6), the transition of the properties from clusters (mesoscopic phase) to bulk metal (macroscopic phase) is unknown except for the gas phase (7-9). 

Although the properties of a bare metal cluster in the gas phase, e.g. the ionization potential, depend solely on the nuclearity, it is clear that any other molecule interacting with the metal atoms influences their behavior. 

It was emphasized that cluster redox properties depended on the nuclearity, mostly at low n values. The oligomers are spontaneously unstable with respect to coalescence and the determination of the redox properties of these transient oligomers is again accessible only by means of a kinetic approach. The clusters are formed as above by using a pulse to induce atoms which then coalesce during the coalescence they can react with an added reactant. Depending on the chemical properties of the reactant and on their nuclearity n, the clusters may behave as electron acceptors or donors. 

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