Silver clusters electronic properties

Bonacic-Koutecky, V., Burda, J., Mitric, R., Ge, M., Zampella, G. and Fantucci, P. (2002) Density functional study of structural and electronic properties of bimetallic silver - gold clusters Comparison with pure gold and silver clusters./oumol of Chemical Physics, 117, 3120-3131. 

Abstract Silver clusters, composed of only a few silver atoms, have remarkable optical properties based on electronic transitions between quantized energy levels. They have large absorption coefficients and fluorescence quantum yields, in common with conventional fluorescent markers. But importantly, silver clusters have an attractive set of features, including subnanometer size, nontoxicity and photostability, which makes them competitive as fluorescent markers compared with organic dye molecules and semiconductor quantum dots. In this chapter, we review the synthesis and properties of fluorescent silver clusters, and their application as bio-labels and molecular sensors. Silver clusters may have a bright future as luminescent probes for labeling and sensing applications. 

Silver clusters 2.5 nm in diameter displayed unusual electrocatalytic properties in Wolff rearrangements of diazoketones.67 The reaction proceeds with electron transfer to and from the silver cluster. The presence of an a-ketocarbene/ketene was confirmed using pyridine as a nucleophilic probe and by UV-visible spectroscopy. Electrochemistry was used to support the role of the silver particles in the rearrangement. 

The electronic properties of small silver clusters chemisorbed on AgBr have been calculated by Baetzold (66) using MO theory. This problem deals with catalysis since, as Hamilton and Urbach (67) have described, the silver centers are catalysts for the chemical reduction of AgBr grains. By using various experimental techniques, they indicate that a minimum size cluster of 4 Ag atoms is required for the catalysis. This suggests that some properties of 4 bonded silver atoms are different from atomic and perhaps like bulk properties, which could account for the catalysis. 

Rather dramatic alterations in the electronic properties and relaxation dynamics of supported silver atoms and clusters have been traced to extremely subtle differences in ground and excited state guest-host interaction potentials. For Ag° and Ag2+ in faujasite zeolites, pronounced changes in their optical... 

Pulse radiolysis experiments also allowed the study of the reactivity and the redox properties of transient species such as silver atoms and silver clusters. Ag° reacts faster than Agj and both species act as strong electron donors since their respective redox potentials are... 

This figure presents a partial analysis of the electrochemical properties of surface silver clusters of discrete sizes, and it established the preferential stability of the small clusters which contain an even number of electrons (as does Ag, ). 

Theoretical studies of varying sizes of neutral and anionic gold (An) and silver (Ag) clusters indicate that they exhibit an even-odd oscillation in their stability and electronic properties (Fig. 34.5) [17]. Thus, clusters which have an even number of atoms tend to be more stable in the neutral state, while those having an odd number of atoms tend to be more stable in the anionic state. Since the 6s orbital energy of Au is almost as low as 5d orbitals, the strong s-d hybridization in Au favours ID and 2D structures in the case of the gold 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. 

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). 

The aim of this work is to extend the kinetics study of electron transfer to monitor donors of more positive redox potential than previously studied, toward silver clusters, Ag ", as acceptors and thus to approach the domain where clusters get metal-hke properties (13). The selected donor is the naphta-zarin hydroquinone, with properties similar to those of the hydroquinone used as a developer in photography. Its redox potential depends on pH, so that different monitor potentials are available through control of pH. Moreover, the reactivity of the donor may be followed by variation of absorbance when naphtazarin hydroquinone, almost transparent in the visible, is replaced by oxidized quinone with an intense absorption band. 

The development of the super-atom model for the description of electronic properties of metal clusters arose from the attempt to understand and interpret experimental data by W. Schulze and co-workers Figure 1.1 shows the absorption of small silver particles... 

Li and Be aggregates [140] C particles [141], Fe chains [142], Ni [143] Ag [144] [14S] and Na aggregates [146]. In all cases the electronic properties of the small particles are different from bulk properties. It was found with silver that the LP. decreases from its single atom value towards the bulk work function as cluster size increases. In the case of Na particles [146] it was found that smaller particles exhibit the higher ionisation potential and excitation energy but lower bond energy. 

Reversible formation of Ag nanoparticles onto transparent polymer films using an electrochemical technique has been reported (Black et al. 2007). It is known that Ag nanoparticles possess special optical properties (El-Noura et al. 2010). During the nanoparticle formation process, some colored Ag clusters appear, which can be photogenerated and stabilized inside a polymer matrix. An optically transparent film is mandatory for controlling the process of particle formation inside polymer film by means of spectroelectrochemistry. Based on this aspect, Ag nanoparticles were prepared by photoreduction of Ag+ ions in transparent cross-linked films made up of poly(vinyl alcohol) and poly(acrylic acid) (Chalal et al. 2012). For achieving nanoparticles with greater reversibility, a redox mediator (TMB +) was used. The principal role of these species is that they supply electrons that help oxidize silver clusters (Black et al. 2007). 

Silver clusters are known to have exceptional absorption properties as we have also discussed above. Furthermore, the Ags cluster has already been used successfully as an intracellular marker, which makes the complex an interesting model system. The study has been performed using DFT to calculate the structural properties and TDDFT to calculate the absorption spectra. In both cases an 11-electron relativistic effective core potential has been employed. The temperature-broadened spectra were obtained by performing molecular-dynamics simulations on a semiempirical AMI level and sampling spectra over time. 

Original methods of synthesis of highly dispersed silver catalysts (based upon the application of strong reducing properties of electrons solvated in soldium-ammonia solutions, the adsorption-contact method of drying and a weak solubility in nitric acid of the Si02-supported small silver clusters) allowed us to synthesize Si02-supported silver particles of sizes less than 6 nm. It makes possible some unusual catalytic and other physico-chemical properties of these particles to be discovered. 

Attention has been given to the synthesis of bimetallic silver-gold clusters [71] due to their effective catalytic properties, resistance to poisoning, and selectivity [72]. Recently molecular materials with gold and silver nanoclusters and nanowires have been synthesized. These materials are considered to be good candidates for electronic nanodevices and biosensors [73]. 

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... 

Adsorption of ions or molecules on metal clusters markedly affects their optical properties. It was shown that the intensity and the shape of the surface plasmon absorption band of silver nanometric particles, which is close to 380 nm, change upon adsorption of various substances [125]. The important damping of the band generally observed is assigned to the change of the electron density of the thin surface layer of the... 

---