Charged Silver Clusters Research Articles

Density Functional Study of Small Neutral and Charged Silver Cluster Hydrides

Small neutral, anionic, and cationic silver cluster hydrides AgnH and anionic HAgnH (n=1-7) have been studied using the PW91PW91 density functional method. It was found that the most stable structure of the AgnH complex (neutral or charged) does not always come from that of the lowest energy bare silver cluster plus an attached H atom. Among various possible adsorption sites, the bridge site is energetically preferred for the cationic and most cases of neutral Agn. For anionic Agn, the top site is preferred for smaller Agn within n<or=4 while the bridge site is preferred for bigger clusters. After binding of the second H atom, the obtained lowest energy structures HAgnH- are those with two H atoms shared with only one Ag atom in anionic clusters. Natural bond orbital analysis shows that, irrespective of the charge state, electrons always transfer from silver atoms to adsorbate. Significant odd-even alternation patterns that hydrogen atoms bind more strongly with odd-electron bare clusters than with even-electron bare clusters can be observed.

Density Functional Study of the Interaction of Carbon Monoxide with Small Neutral and Charged Silver Clusters

CO adsorption on small neutral, anionic, and cationic silver clusters Ag(n) (n = 1-7) has been studied with use of the PW91PW91 density functional theory (DFT) method. The adsorption of CO on-top site, among various possible sites, is energetically preferred irrespective of the charge state of the silver cluster. The cationic silver clusters generally have a greater tendency to adsorb CO than the anionic and neutral silver ones, except for n = 3 and 4, and the binding energies reach a local minimum at n = 5. The binding energies on the neutral clusters, instead, reach a local maximum at n = 3, which is about 0.87 eV, probably large enough to be captured in the experiments. Binding of CO to the silver clusters is generally weaker than that to the copper and gold counterparts at the same size and charge state. This is due to the weaker orbital interaction between silver and CO, which is caused by the larger atomic radius of the silver atom. In contrast, Au atoms with a larger nuclear charge but a similar atomic radius to silver owing to the lanthanide contraction are able to have a stronger interaction with CO.

Density functional study of the interaction of molecular oxygen with small neutral and charged silver clusters

O 2 adsorption on small neutral, anionic and cationic silver clusters Ag n ( n = 1–7) has been studied by using the PW91PW91 density functional method. The adsorption energies of O 2 on the anionic and neutral clusters have an odd–even alternation pattern clearly, and our calculations give the same adsorption behavior of O 2 on silver cluster anions as the experimental measurements. The adsorption energies on the cationic clusters, instead, are generally smaller than those on the anionic and neutral clusters, and reach a local maximum at Ag 4 O 2 + . Natural bond orbital analyses show that electrons mostly transfer from silver atoms to molecular O 2 except for the smallest silver cluster cations ( n = 1–3).

First-principles study of neutral and charged silver clusters

The structures of neutral and charged Agn (n=3∼6) clusters are comparably studied by the first principles methods. We find that the neutral silver clusters sustain two dimensional structures up to n=6 and the charged ones undergo a transition from two dimensional structures to three dimensional structures at n=6. The ionization potential, electron affinity and the energy gap between the highest occupied molecular orbital and the lowest unoccupied molecular orbital for all these clusters show an oscillating behavior with odd-even valence electron alteration. By calculating the dissociation energy, we have studied the possible dissociation channel of the silver clusters, the results are in agreement with the experiment.

Density functional study of the interaction of chlorine atom with small neutral and charged silver clusters

Chlorine adsorption on small neutral, anionic, and cationic silver clusters Ag(n) (n=2-7) has been studied using the PW91PW91 density functional method. It was found that the adsorption of chlorine on the lowest-energy bare clusters does not always produce the lowest-energy complexes. In addition, the binding of chlorine can greatly change the geometries of the silver clusters in some cases. Among various possible adsorption sites, bridge site is energetically preferred for the neutral Ag(n) while top site is energetically more preferred for the anionic Ag(n) with n< or =6. For cationic clusters, adsorptions on bridge and face sites have similar binding energies, which are much larger than those on top sites. Natural bond orbital analyses show that irrespective of charge state, electrons always transfer from silver atoms to adsorbate and silver acts like alkali metals in the interaction with chlorine atom. Significant odd-even alternation patterns in the properties of the complexes have been observed: Even-electron clusters often have higher ionization energies, lower electron affinities, and higher dissociation energies than their odd-electron neighbors. It was also found that chlorine atoms bind more strongly with odd-electron bare clusters than with even-electron bare clusters. These patterns reveal that even-electron clusters are more stable than odd-electron clusters.

Enhancement of nitrogen physisorption in coadsorption with oxygen on free, positively charged silver clusters

The coadsorption of molecular nitrogen and oxygen on small cationic silver clusters in the gas phase is experimentally studied. The presence of oxygen enhances the adsorption probabilities of N2. This indicates a partial charge transfer out of the finite free electron reservoir of the small silver particles into the chemisorbed oxygen molecule.

Formation of long-lived clusters and silver nucleation in the γ-irradiation of aqueous silver perchlorate solutions containing polyphosphate

It was found that long-lived positively charged and neutral silver clusters were formed by the radiation-chemical reduction of Ag+ ions in aqueous solutions containing sodium polyphosphate. The nuclearity of the clusters increased with absorbed dose; then, quasi-metal particles were formed. The process culminated in the formation of silver nanoparticles.

Constitution and Properties of Nanocomposites Prepared by Thermal Decomposition of Silver Salts Sorbed by Polyacrylate Matrix

Constitution and dispersity of the products of thermal decomposition of silver nitrate ammonia complex sorbed by polyacrylate matrix are studied by the methods of small- and wide-angle X-ray scattering, optical, photoelectron and Auger-electron spectroscopies. It is shown that, at temperatures of 140–150°C, the complete decomposition of the complex occurs with the formation of nanoparticles and charged silver clusters in polymer bulk. No initial or intermediate products were observed. The average size of obtained nanoparticles is equal to 5 nm. The particles with the size less than 5 nm are amorphous according to X-ray data. The stabilization of nanoparticles occurs due to the adsorption of acrylic copolymer (presumably, via oxygen atoms) on their surfaces. Upon long-term storage in air, the self-diffusion of silver particles and clusters takes place from the surface to composite bulk caused by the detachment of oxygen-containing groups occurring at the metal–polymer interface.

Production of dianionic and trianionic noble metal clusters in a Penning trap

Metal clusters, Au n −, Ag n −, and Cu n −, from a laser vaporization source are transferred to a Penning trap and subjected to a bath of simultaneously stored electrons. After some reaction period multiply charged anionic gold, silver, and copper clusters are observed if the clusters under investigation are large enough. The cluster sizes for the onset of the formation of dianions and trianions are compared to a charged sphere model. The description of the experimental findings is largely improved if the repulsive Coulomb barrier is taken into account which prevents autodetachment of the surplus electrons on the experimental time scale of a second.

Dissociation energies of silver clusters Ag n+, n=14, 15, 16, 18

A recently developed method to determine dissociation energies has been applied to positively charged silver clusters of size n=14, 15, 16 and 18. The method uses a combination of sequential and single step decays. It requires an uncalibrated thermometer which here is provided by the evaporation rate constants of the product clusters. For this purpose, earlier measurements [J. Chem. Phys. 57 (1998) 2786] are reanalyzed with the new method. The resulting dissociation energies are compared with the liquid drop values and the measured decay rate constants with expected rate constants from detailed balance theory.

Silver Ion-Exchanged Zeolites Y, X, and Low-Silica X: Observations of Thermally Induced Cation/Cluster Migration and the Resulting Effects on the Equilibrium Adsorption of Nitrogen

Silver is known to strongly affect the adsorptive properties of some zeolites. It is also known that thermal vacuum dehydration of some argentiferous zeolites leads to the formation of charged silver clusters within the zeolite. In this work we have synthesized silver zeolites of the types Y, X, and low-silica X. The zeolites were treated in such a way as to promote the formation of intracrystalline charged silver clusters. Equilibrium room-temperature isotherms were measured for adsorption of nitrogen for each of the zeolites after various heat treatments and dehydration. These materials were structurally characterized via Rietveld refinement using neutron powder diffraction data. Color changes upon heat treatment and subsequent X-ray photoemission spectroscopy confirmed some reduction of Ag+ → Ag0. The effects of various dehydration conditions, including the time, temperature, and atmosphere, on the room-temperature adsorption of nitrogen are discussed. Structural characterization, along with valence bond calculations, revealed the presence of cations in site II*, which are more active in Ag−LSX samples that were vacuum dehydrated at 450 °C as compared to those that were vacuum dehydrated at 350 °C.

Fission barriers of doubly charged silver clusters

The monomer evaporation energies and fission barriers of doubly charged silver cluster ions in the size range 9≤n≤25 are measured by collision induced dissociation. They are compared to the dissociation energies of singly charged silver clusters. A macroscopic liquid drop model combined with empirical microscopic corrections successfully describes the measured fission barriers.

Observation of electronic and geometric shell structures of small silver clusters

Singly charged silver clusters Open image in new window in the size range n = 17 to n = 78 have been captured and stored in a Penning trap, size selected and subjected to an electron beam which leads to further ionization as well as dissociation. The resulting abundance spectra of doubly and triply charged clusters show several features: (1) A critical size for the production of doubly charged clusters which can be understood in terms of the decay pathways as previously investigated by collision induced dissociation, (2) an odd-even alternation in the abundance of singly and doubly charged clusters which inverses sign with change of charge state, (3) prominent signals for particular doubly and triply charged species that indicate both electronic and geometric influences on the clusters’ stability.

Spheroidal shell model for fission of doubly charged silver clusters

Fission of doubly charged silver clusters ${\mathrm{Ag}}_{n}^{2+}$ is studied in the size range of $12&lt;~n&lt;~22.$ It is assumed that two fragment clusters are in contact at the transition state. The shape of each fragment is an energetically optimized spheroid. The energy barrier for various fission channels is calculated by using the shell correction method. This simple model closely reproduces the fragment size distribution measured by Katakuse et al. It is confirmed that the shell effects govern the distribution of fission fragments, while the liquid droplet model cannot explain the distribution. In particular, the even-odd oscillations observed in the distribution are found to be due to deformations of fragment clusters. Competition between fission and evaporation is discussed.

Generation of large gas-phase silver cluster ions by laser desorption/ionization of silver-containing salts

This work is the first to demonstrate the direct generation of large silver cluster ions through laser desorption/ionization of silver-containing salts without the assistance of a molecular beam. Both positively and negatively charged silver clusters up to 100-mer have been produced using this approach. It was observed that the silver cluster distributions produced by this method are similar to those produced using the beam-based approaches, namely there are two special features in the mass spectra. The first is that the cluster intensity distribution displays an odd–even alternation pattern. The second is a steep decrease of ion intensities after some clusters called `magic numbers'.

Experimental dissociation energies of metal cluster dications and their interpretation in a liquid-drop model with empirical corrections

The dissociation energies of doubly charged silver cluster ions in the size range $9&lt;~n&lt;~25$ are measured by multiple collision induced dissociation. They are compared to the dissociation energies of singly charged clusters. To this end, the latter are used to calculate shell corrections in a macroscopic-microscopic model. Good agreement between the resulting predictions of the dissociation energies of the doubly charged systems and the experimental values is found, which indicates the strong influence of electronic effects on the stability of small silver clusters.

Multiple-collision induced dissociation of trapped silver clusters Agn+ (2⩽n⩽25)

The dissociation energies of singly charged silver cluster cations, Agn+ (2⩽n⩽25), are determined by multiple-collision induced dissociation (MCID) in a Penning trap. The fragment yield is analyzed in terms of a linearized impulsive collision theory for the energy transfer in the multicollisional process and the delayed decay as predicted by the Rice–Ramsperger–Kassel (RRK) model. Previous photofragmentation experiments performed in the size range (9⩽n⩽21) are found to be in good agreement with the present results. Theoretical predictions agree for most clusters sizes.

Collision induced dissociation of doubly charged stored metal cluster ions

Singly charged noble metal cluster ions from an external laser vaporization source are accumulated in a Penning trap. They are subjected to electron bombardment for further ionization. The doubly charged clusters are size selected and their low energy fragmentation pathways are studied by multiple-collision-induced dissociation. The reaction products are analysed by time-of-flight detection. In general, large clusters tend to evaporate neutral atoms while small clusters fission into two singly charged fragments, one of which is a trimer. The details, however, vary and depend on the specific elements and cluster sizes as exemplified by the fragmentation of doubly charged copper, silver, gold and platinum clusters of size around n = 16. © 1997 John Wiley & Sons, Ltd.

Low-energy decay pathways of doubly charged silver clusters $Ag_{n}^{2+}$ (n = 9 - 24)

The low-energy dissociation channels of mass selected silver cluster ions Ag n 2+ (n = 9–24) are determined by collision induced dissociation (CID) in a Penning trap. While all clusters of the size n ≥ 17 evaporate neutral monomers, most smaller clusters undergo asymmetric fission of the form Ag n 2+ → Ag n−3 + + Ag {3} + . However, Ag 15 2+ and Ag 11 2+ emit monomers which indicates shell or odd-even effects. The observed fragmentation pathways are different from previous reports of measurements with sputtered Ag n 2+ .

Collision induced dissociation of doubly charged silver clusters Ag n 2+ , n=21,22,23

Doubly charged silver clusters Agn 2+, n=21, 22, 23, are produced by electron bombardment of an Agn + ensemble stored in a Penning trap. After mass selection the clusters are subjected to collision induced dissociation. The fragmentation channels are determined by time-of-flight mass spectrometry after ejection of the resulting ion ensemble from the trap. Monomer evaporation is the only decay path observed.