Formation mechanism of nanostructured Ag films from Ag2O particles using a sonoprocess

Nanostructured Ag films composed of nanoparticles and nanorods can be formed by the ultrasonication of ethanol solutions containing Ag₂O particles. The present work examined the formation process of these films from ethanol solutions by two different agitation methods, including ultrasonication and mechanical stirring. The mass-transfer process from Ag₂O particles to ethanol solvent is accelerated by the mechanical effects of ultrasound. Ag⁺ ions and intermediately reduced Ag clusters were released into the ethanol. These Ag⁺ ions and Ag clusters provide absorption bands at 210, 275 and 300 nm in UV-vis spectra. These bands were assigned to the absorption of Ag⁺, Ag ₄ ²⁺ and Ag_n (n?≈?3). The Ag_n clusters that readily grow to become Ag nanoparticles were formed due to the surface reaction of Ag₂O particles with ethanol under ultrasonication. The reactions of Ag⁺ ions in ethanol to form Ag nanomaterials (through the formation of Ag ₄ ²⁺ clusters) were also accelerated by ultrasonication.     

Active Ag species in MFI zeolite for direct methane conversion in the light and dark

Photo-induced methane conversion was examined over Ag-MFI zeolite at room temperature. On the oxidized Ag-MFI zeolite, containing Ag⁺ exchanged cations, huge amounts of methane were adsorbed, even in the dark, and then converted to mainly ethane upon photo-irradiation without H₂ production. It was revealed that Ag _n ⁺ small clusters were formed at the expense of Ag⁺ ion during this photoreaction, and probably hydrogen would be stored as H⁺ on the ion-exchange sites instead of Ag⁺. On the other hand, the reduced sample containing larger clusters converted methane into alkene even without photo-irradiation.     

Silver clusters: Optical absorption and ESR spectra; structure and calculation of electron transitions

The formation of Ag₃ ²⁺ and Ag₄ ⁺ clusters upon freezing out of aqueous-alcohol glassy solutions of AgClO₄ by γ-irradiation at 77 K was established by ESR and optical spectroscopies. The Ag₃ ²⁺ cluster is formed by subsequent addition of Ag⁺ ions to a silver atom, and the Ag₄ ⁺ cluster is most probably formed by the reduction of the Ag₄ ²⁺ cluster by alcohol radicals. The energies of optical transitions and energy gaps far the Ag₄ ^m+ cluster (m = 0 to 3) were calculated. The absorption bands of the clusters shift to the UV region as the charge increases, which agrees with the experimental results.     

Mass-spectrometric study of the formation of silver nanoclusters in polyethers: I. Laser desorption/ionization

Within the framework of a problem of the synthesis of silver nanoparticles and Ag _n nanoclusters in polyethers, the systems containing silver nitrate AgNO₃ and the low-molecular-weight polyethers poly(ethylene glycol) PEG-400 and oxyethylated glycerol OEG-5, in which silver ions were reduced, were studied by laser desorption/ionization mass spectrometry. The occurrence of Ag _n silver nanoclusters with n up to 35 in the systems was detected. For n > 2, the presence of ??magic numbers?? was observed; that is, positively charged Ag _n ⁺ clusters with predominantly odd values of n were detected. Negatively charged Ag _n clusters with n = 1?C3 were also detected. It was shown that one of the expected processes, namely, the formation of the stable clusters of polyether oligomers (M _m ) with the silver cation M _m · Ag⁺, took place in the test systems.     

Ag Clusters as Active Species for HC-SCR Over Ag-Zeolites

The addition of hydrogen in the reaction atmosphere is effective in promoting the activity of Ag/alumina and Ag-zeolites on the selective reduction of NO by hydrocarbons (HC-SCR) at low temperatures. The increment of NO conversion over Ag-MFI corresponds to the periodic addition of hydrogen into C₃H₈-SCR conditions. The UV–VIS spectra of Ag-MFI have revealed that the addition of hydrogen results in the formation of Ag_n^δ+ clusters due to partial reduction and agglomeration of Ag species. The coincidence of the formation of the Ag_n^δ+ clusters and the increment of NO conversion suggests that Ag_n^δ+ clusters are the highly active species for HC-SCR. From analysis by H₂-TPR, UV–VIS, and EXAFS, the structure of Ag_n^δ+ clusters on Ag-MFI is identified as being Ag₄²⁺ on average. The formation of Ag clusters was strongly affected by the type of zeolites: The major Ag species are Ag⁺ ions for MOR, Ag_n^δ+ clusters for MFI and BEA, and relatively large metallic Ag_mparticles for Y. The sequence of Ag agglomeration (MOR < MFI < BEA < Y) is in accordance with the strength of the acid sites of zeolites. It can be expected that the interaction between the positive charge of Ag_n^δ+ clusters and acid sites, i.e., the ion-exchange site of zeolites, stabilizes Ag_n^δ+ clusters. The type of Ag species under HC-SCR conditions depends on the concentration of gas-phase oxidants (NO, O₂) and reductants (H₂, HC), and also on the number and strength of the zeolite acid sites.     

Mass-spectrometric study of the formation of silver nanoclusters in polyether media: 2. Fast atom bombardment and modeling

Within the problem of the synthesis of silver nanoclusters and nanoparticles in polyether media, systems containing silver nitrate AgNO₃ and low-molecular-weight polyethers, poly(ethylene glycol) PEG-400 or oxyethylated glycerol OEG-5, were studied by fast atom bombardment (FAB) mass spectrometry. The formation of stable clusters of polyether oligomers (M _m ) with silver cations M _m · Ag⁺ was shown, in agreement with the previous data of laser desorption/ionization. Quantum-chemical DFT calculations have shown that the M _m · Ag⁺ clusters are stabilized by wrapping of the polyether chain around the silver cation with the cation coordinating ether oxygen atoms. Silver nanoclusters were not found in the FAB mass spectra of liquid systems, but Ag _n ⁺ clusters were detected for silver nanoparticles separated from the reaction medium. No products of chemical transformations of PEG-400 or OEG-5 were observed by FAB. A plausible mechanism of the reduction of silver cations involving nitrate anions is discussed.     

Shell correction study of fission of doubly charged silver clusters

Fission of doubly charged silver clusters is investigated by the method of shell corrections. The following fission events are considered: Ag ₂₂ ²⁺ → Ag _n ⁺ + Ag _{22 ?n} ⁺ , (n=11, 10, 9, 8); Ag ₂₁ ²⁺ → Ag _n ⁺ + Ag _{21 ?n} ⁺ , (n=10, 9, 8, 7); Ag ₁₈ ²⁺ → Ag _n ⁺ + Ag _{18 ?n} ⁺ , (n=9, 8, 7, 6). It is found that the shell correction energy is comparable to or larger than the deformation energy of the liquid drop. Threshold energies for the fission events are calculated and compared with the experimental abundance spectra obtained by Katakuse et al. (1990). Correspondence between the calculated threshold energies with the shell corrections and the experimental abundance is very good, showing products from lower threshold fission channels yield more abundance. The threshold energies without the shell corrections are almost constant irrespective of the fission channels and cannot explain the experimental abundance. Abundance of some products are too small to be accounted for only by the threshold energies. The low abundance of those products may be explained by the presence of competing fission channels that have similar minimal energy paths. It is found in fission of Ag ₁₈ ²⁺ that the shell correction overwhelms the Coulomb energy and the fission channel to Ag₈ + Ag ₁₀ ²⁺ is preferred over the fission channel to Ag ₈ ⁺ + Ag ₁₀ ⁺ .     

Low-energy decay pathways of doubly charged 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⁺ .     

Electronic shell structure in multiply charged silver clusters

Silver clusters are generated by standard laser vaporization technique and ionized via multiphoton ionization. Time-of-flight mass spectrometry reveals singly, doubly and triply charged clusters, Ag _n ^z+ (z=1,2,3). The spectra show, for all charge states, intensity variations, indicating enhanced stabilities for cluster sizes with closed electronic configurations in accord with the spherical jellium model.     

Gas-phase cluster cations generated by direct laser ablation on AgBr/S mixture

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Controlling the Reactivity of the [P8W48O184]40− Inorganic Ring and Its Assembly into POMZite Inorganic Frameworks with Silver Ions

The construction of pure‐inorganic framework materials with well‐defined design rules and building blocks is challenging. In this work, we show how a polyoxometalate cluster with an integrated pore, based on [P₈W₄₈O₁₈₄]^40? (abbreviated as {P₈W₄₈}), can be self‐assembled into inorganic frameworks using silver ions, which both enable reactions on the cluster as well as link them together. The {P₈W₄₈} was found to be highly reactive with silver ions resulting in the in situ generation of fragments, forming {P₉W₆₃O₂₃₅} and {P₁₀W₆₆O₂₅₁} in compound ( 1 ) where these two clusters co‐crystallize and are connected into a POMZite framework with 11 Ag⁺ ions as linkers located inside clusters and 10 Ag⁺ linking ions situated between clusters. Decreasing both the concentration of Ag⁺ ions, and the reaction temperature compared to the synthesis of compound ( 1 ), leads to {P₈W₅₁O₁₉₆} in compound 2 where the {P₈W₄₈} clusters are linked to form a new POMZite framework with 9 Ag⁺ ions per formula unit. Further tuning of the reaction conditions yields a cubic porous network compound ( 3 ) where {P₈W₄₈} clusters as cubic sides are joined by 4 Ag⁺ ions to give a cubic array and no Ag⁺ ions were found inside the clusters.     

Shell effects in singly and multiply charged silver and gold clusters

The mass spectra of silver- and gold-clusters, generated by a gas aggregation technique and ionized by electron impact, reveal anomalies in the relative abundance of both singly and multiply charged clusters. Concentration maxima for singly charged species Ag _n ⁺ and Au _n ⁺ (n=3, 9, 19, (21), 35) are in agreement with experimental data of Katakuse and the predictions from the electronic shell model. The observed anomalies in the abundance spectra of doubly charged silver and gold clusters as well as triply charged silver cluster ions are explained in terms of electronic shell closing.     

The reduction process of phytic acid–silver ion system: A pulse radiolysis study

Reduction of silver ion in a silver–phytic acid (1:1 ratio) system has been studied using pulse radiolysis technique. Time-resolved transformation of the intermediates, Ag⁺→Ag⁰→Ag₂⁺→Ag₃²⁺, has been clearly observed in the reduction of silver–phytic acid (1:1) system. The effect of phytic acid on the formation and decay of initial silver clusters has been also studied. The surface plasmon absorption band of stable silver nanoparticle (410 nm) and dynamic light scattering technique has been used to characterize the nanoparticles and measure the average size (R_av=100 nm).     

The Geometric Structure of Silver‐Doped Silicon Clusters

Cationic silver‐doped silicon clusters, Si_nAg⁺ (n=6–15), are studied using infrared multiple photon dissociation in combination with density functional theory computations. Candidate structures are identified using a basin‐hopping global optimizations method. Based on the comparison of experimental and calculated IR spectra for the identified low‐energy isomers, structures are assigned. It is found that all investigated clusters have exohedral structures, that is, the Ag atom is located at the surface. This is a surprising result because many transition‐metal dopant atoms have been shown to induce the formation of endohedral silicon clusters. The silicon framework of Si_nAg⁺ (n=7–9) has a pentagonal bipyramidal building block, whereas the larger Si_nAg⁺ (n=10–12, 14, 15) clusters have trigonal prism‐based structures. On comparing the structures of Si_nAg⁺ with those of Si_nCu⁺ (for n=6–11) it is found that both Cu and Ag adsorb on a surface site of bare Si_n⁺ clusters. However, the Ag dopant atom takes a lower coordinated site and is more weakly bound to the Si_n⁺ framework than the Cu dopant atom.     

Quantum-chemical study of the effect of oxygen on the formation of active sites of silver clusters during the selective adsorption of hydrocarbons

Density functional theory (PBE with a modified Dirac-Coulomb-Breit Hamiltonian) is used to simulate the adsorption of hydrocarbons (C₂H₂, C₂H₄, C₂H₆) on the surface of a sorbent containing Ag⁰, Ag^δ+, and AgO sites. The dynamics of change in the structural characteristics of Ag _n (n ≤ 10) is analyzed and the adsorption of oxygen on Ag₈ and Ag₁₀ is studied to select the adsorption site model. Studying the interaction of hydrocarbons with Ag₈, Ag₁₀, Ag ₁₀ ⁺ , Ag₁₀O, and Ag₁₀O₂ clusters reveals that the presence of oxygen leads to an increase in the activation of unsaturated hydrocarbons, and the adsorption energy of C₂H₂ increases tenfold. It is found that the role of adsorbed oxygen is not only to form adsorption sites of hydrocarbons (Ag^δ+) but also to bind C₂H₂ and C₂H₄ directly to the sorbent’s surface.     

Mechanism of silver nucleation upon the radiation-induced reduction of its ions in polyphosphate-containing aqueous solutions

Long-lived (hours to days) silver clusters, Ag ₄ ²⁺ , Ag ₄ ⁺ , Ag ₈ ²⁺ , etc., are formed upon the radiation-induced reduction of Ag⁺ ions in aqueous solutions containing sodium polyphosphate. The efficiency of the cluster formation decreases and the stability of the clusters increase with a rise in the concentration of the polymeric stabilizer. In the course of the aggregation of clusters, their sizes increase, quasi-metallic particles emerge, and the process terminates with the formation of silver nanoparticles. The mechanism of silver nucleation upon the radiation-induced reduction of silver ions in aqueous solutions is discussed.     

Electron impact and single photon ionization cross sections of neutral silver clusters

Neutral silver atoms and small clusters Ag _n (n=1...4) were generated by sputtering, i.e. by bombarding a polycrystalline silver surface with Ar⁺ ions of 5 keV. The sputtered particles were ionized by a crossed electron beam and subsequently detected by a quadrupole mass spectrometer. In alternative to the electron impact ionization, the same neutral species were also ionized by single photon absorption from a pulsed VUV laser (photon energy 7.9 eV), and the photoionization cross sections were evaluated from the laser intensity dependence of the measured signals. By in situ combining both ionization mechanisms, absolute values of the ratio σ _e (Ag _n )/σ _e (Ag) between the electron impact ionization cross sections of silver clusters and atoms could be determined for a fixed electron energy of 46 eV. These values can then be used to calibrate previously measured relative ionization functions. By calibrating the results using literature data measured for silver atoms, we present absolute cross sections for electron impact ionization of neutral Ag₂, Ag₃ and Ag₄ as a function of the electron energy between threshold and 125 eV.     

Solvation and lon association in the system AgNO3−CH3CN: A Raman and infrared spectral study

Detailed studies of the Raman and infrared line parameters of AgNO₃?CH₃CN systems ranging from dilute solutions to 9M are reported. A concentration quotient of 1.1M ^?1 was obtained for the formation of the Ag⁺NO ₃ ^? ion pair when C<4M. The complex appears to have point group C_s(σ_V) with Ag⁺ in a “roll-on” position. The Ag⁺ ion is solvated by four molecules of CH₃CN; nitrate ion replaces three of these when bound to Ag⁺. When C>4M, multiple ion aggregates form in the solution. A low-frequency 110 cm^?1 line is ascribed to librational motions of NO ₃ ^? , probably bound to Ag⁺.     

Regulation of mixed Ag valence state by non-thermal plasma for complete oxidation of formaldehyde

Formaldehyde is an important air pollutant and its removal is essential to protect human health and meet environmental regulations. Ag-based catalyst has a considerable potential for HCHO oxidation in low temperature range. The valence state of Ag is one of the key roles in formaldehyde catalytic oxidation.However, its effect on activity is still ambiguous. Non-thermal plasma and conventional calcination were employed to regulate Ag valence state in this study. Three Ag-Co/CeO₂ cataly...     

Ionic conductivity of and Raman spectroscopy investigation in binary oxosalts (1 − x)AgPO3xAg2SO4 glasses

(1 ? x)AgPO₃xAg₂SO₄ homogeneous glasses obtained by quenching of a melt of the two salts are pure ionic Ag⁺ conductors. The RT conductivity is increased from 2.5 × 10^?7 to 4 × 10^?6 (Ω cm)^?1 when the ratio of Ag₂SO₄ is increased from 0 to 0.3. Raman spectroscopy shows that no modifications of the (PO₃)^∞ chain skeleton occur by adding Ag₂SO₄. The low-frequency Raman band lying at about 55 cm^?1 is quantitatively correlated to Ag⁺ oscillations, the hopping distance decreasing from 3.0 to 2.7 Å if a jump process between regular Ag⁺ sites is considered.