Metal clusters electronic effects

The cluster-size dependence has not yet been explained it may reflect an intrinsically low activity of the clusters, but it might also be a consequence of increasing removal of residual ligands such as C from the clusters with increasing severity of treatment in H2. Other possibilities include a steric effect of the support, limiting adsorption of the reactants on the metal— such an effect would be greatest for the smallest clusters. Electronic effects should not be ruled out. 

Coadsorption phenomena in heterogeneous catalysis and surface chemistry quite commonly consider competitive effects between two reactants on a metal surface [240,344]. Also cooperative mutual interaction in the adsorption behavior of two molecules has been reported [240]. Recently, this latter phenomenon was found to be very pronounced on small gas-phase metal cluster ions too [351-354]. This is mainly due to the fact that the metal cluster reactivity is often strongly charge state dependent and that an adsorbed molecule can effectively influence the metal cluster electronic structure by, e.g., charge transfer effects. This changed electronic complex structure in turn might foster (or also inhibit) adsorption and reaction of further reactant molecules that would otherwise not be possible. An example of cooperative adsorption effects on small free silver cluster ions identified in an ion trap experiment will be presented in the following. 

In 1977 we reported a method based on graph theory for study of the skeletal bonding topology in polyhedral boranes, carboranes, and metal clusters Q). Subsequent work has shown this method to be very effective In relating electron count to cluster shape for diverse metal clusters using a minimum of computation. Discrete metal clusters treated effectively by this method Include post-transition metal clusters (, ) > osmium carbonyl clusters (O, gold clusters, platinum carbonyl clusters (J., 7 ) > and... 

The spherical shell model can only account for tire major shell closings. For open shell clusters, ellipsoidal distortions occur [47], leading to subshell closings which account for the fine stmctures in figure C1.1.2(a ). The electron shell model is one of tire most successful models emerging from cluster physics. The electron shell effects are observed in many physical properties of tire simple metal clusters, including tlieir ionization potentials, electron affinities, polarizabilities and collective excitations [34]. 

A.Y. Stakheev, and L.M. Kustov, Effects of the support on the morphology and electronic properties of supported metal clusters modem concepts and progress in 1990s, Appl. Catal. A 188, 3-35 (1999). 

The electrochemical behavior of heterometallic clusters has been reviewed clsewbcre."" The interest in examining clusters stems from their potential to act as "electron sinks " in principle, an aggregate of several metal atoms may be capable of multiple redox state changes. The incorporation of heterometals provides the opportunity to tune the electrochemical response, effects which should be maximized in very mixed"-metal clusters. Few very mixed -metal clusters have been subjected to detailed electrochemical studies the majority of reports deal with cyclic voltammetry only. Table XII contains a summary of electrochemical investigations of "very mixed"-metal clusters. 

Besides these many cluster studies, it is currently not knovm at what approximate cluster size the metallic state is reached, or when the transition occurs to solid-statelike properties. As an example. Figure 4.17 shows the dependence of the ionization potential and electron affinity on the cluster size for the Group 11 metals. We see a typical odd-even oscillation for the open/closed shell cases. Note that the work-function for Au is still 2 eV below the ionization potential of AU24. Another interesting fact is that the Au ionization potentials are about 2 eV higher than the corresponding CUn and Ag values up to the bulk, which has been shown to be a relativistic effect [334]. A similar situation is found for the Group 11 cluster electron affinities [334]. 

There is no doubt that metallic nanoparticles that have defined sizes and shapes will become key components of a number of novel, highly sophisticated products, the prototypes of which are currently emerging from the industrial R D departments. The outlook is promising for the industrial production of defined 1.4nm metal clusters for use as single electron switches or transistors, for the cost-effective fabrication of ultrapure metallic nanomaterials needed for dye solar cells or sensors, and for the reproducible production of (particularly) efficient and durable... 

This mode of calculation has been called the EAN rule (effective atomic number rule). It is valid for arbitrary metal clusters (closo and others) if the number of electrons is sufficient to assign one electron pair for every M-M connecting line between adjacent atoms, and if the octet rule or the 18-electron rule is fulfilled for main group elements or for transition group elements, respectively. The number of bonds b calculated in this way is a limiting value the number of polyhedron edges in the cluster can be greater than or equal to b, but never smaller. If it is equal, the cluster is electron precise. 

The information available is discussed in light of the effects of excitation energy and the environment on the photofragmentation process of several transition metal cluster complexes. The photochemical information provides a data base directly relevant to electronic structure theories currently used to understand and predict properties of transition metal complexes (1,18,19). 

It is important to note that in none of the geometric forms are the five metal atoms equivalent. It follows that M-M distances will vary within a given cluster and may reflect geometric or symmetry constraints as well as simple electronic effects. 

In addition, the rate of Oz reduction, forming 02 by electron, is of importance in preventing carrier recombination during photocatalytic processes utilizing semiconductor particles. 02 formation may be the slowest step in the reaction sequence for the oxidation of organic molecules by OH radicals or directly by positive holes. Cluster deposition of noble metals such as Pt, Pd, and Ag on semiconductor surfaces has been demonstrated to accelerate their formation because the noble metal clusters of appropriate loading or size can effectively trap the photoinduced electrons [200]. Therefore, the addition of a noble metal to a semiconductor is considered as an effective method of semiconductor surface modification to improve the separation efficiency of photoinduced electron and hole pairs. 

Fig. 1 The effect of size on metals. Whereas bulk metal and metal nanoparticles have a continuous band of energy levels, the limited number of atoms in metal clusters results in discrete energy levels, allowing interaction with light by electronic transitions between energy levels. Metal clusters bridge the gap between single atoms and nanoparticles. Even though in the figure the energy levels are denoted as singlets, we must remark that the spin state of the silver clusters is not yet firmly established...

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