Ag13 Cluster Research Articles

Ag‐based bimetallic clusters as catalysts for p‐nitrophenol reduction by glycerol: A DFT investigation

AbstractReducing p‐nitrophenol (PNP) to p‐aminophenol is an industrially relevant synthesis. Nevertheless, only a few heterogeneous catalysts have been evaluated for the reduction of PNP by glycerol. Appropriate quantum computational studies can screen potential catalysts for this crucial green reaction. The present research investigates the catalytic activities of Pd@Ag and Ni@Ag core‐shell nanogeometries toward PNP reduction by glycerol through density functional theory (DFT) calculations. The central atom of a geometry‐optimized 13‐atom Ag cluster was replaced by Pd and Ni atoms to create the core‐shell morphologies. The interaction energies of PNP and glycerol with each of the (metal/bimetallic) clusters were evaluated by DFT calculations to find the best PNP and glycerol molecule orientation with the respective bimetallic cluster. Electrostatic potential surface and natural bond orbital analyses were performed to study the charge distribution and transfer between atomic orbitals. The frequencies of vibrational modes in isolated PNP/glycerol structures were compared to those when these molecules were in the presence of the different metal clusters to infer the effect of the interactions. All performed analyses indicated improved catalytic activity toward PNP reduction by glycerol upon Ni‐doping of the Ag13 cluster.

Ab initio density-functional studies of 13-atom Cu and Ag clusters

The putative ground-state structures of 13-atom Cu and Ag clusters have been studied using ab initio molecular-dynamics (AIMD) based on density-functional theory (DFT). An ensemble of low-energy configurations, collected along the AIMD trajectory and optimized to nearest local minimum-energy configurations, were studied. An analysis of the results suggests the existence of low-symmetric bilayer structures as strong candidates for the putative ground-state structure of Cu13 and Ag13 clusters. These bilayer structures are markedly different from a buckled bi-planar (BBP) configuration and energetically favorable, by about 0.4–0.5 eV, than the latter proposed earlier by others. Our study reveals that the structure of the resulting putative global-minimum configuration is essentially independent of the nature of basis functions (i.e., plane waves vs. pseudoatomic orbitals) employed in the calculations, for a given exchange-correlation functional. The structural configurations obtained from plane-wave-based DFT calculations show a noticeably tighter or dense first-shell of Cu and Ag atoms. A comparison of our results with recent full-potential DFT simulations is presented.

Theoretical and experimental study on adsorption of benzoic acid molecules on silver clusters

ABSTRACTThe adsorption behavior and charge transition of benzoic acid (BA) on silver nanoparticles are studied by the experiment and Density Functional Theory (DFT). The UV-visible adsorption spectrum of silver nanoparticle is investigated, and adsorption spectra of Ih-symmetry Ag13 are obtained. The main adsorption UV-visible adsorption band at 413 nm of silver colloidal with the experimental results is good agreement with that of Ag13 cluster.The surface-enhanced Raman scattering (SERS) on dried filter paper coated with silver nanoparticles and in silver aqueous colloids, indicates adsorption configurations of BA are similar. One Ag13 cluster with Ih-symmetry (Ih-Ag13 cluster) as a model of adsorbates is used to simulate Raman spectra of the adsorption configuration. Surprisingly, the calculated Raman spectra are in good agreement with experimental results. The Mulliken charge population analysis is obtained by calculation, which shows the charge characteristics of Ag13 nanoparticle.

Surface-Enhanced Raman Scattering Due to Charge-Transfer Resonances: A Time-Dependent Density Functional Theory Study of Ag13-4-Mercaptopyridine

We have used time-dependent density functional theory in conjunction with the CAM-B3LYP functional and MWB28/aug-cc-pVDZ basis set to determine non-, near-, and on-resonance Raman spectra for a complex formed by 4-mercaptopyridine (4-Mpy) binding with a Ag13 cluster via the thiolate Ag–S bond. Geometry optimizations of the Ag13-4-Mpy complex showed an on-top structure directly bound to one Ag atom with the ring of the molecule almost flat with respect to two Ag atoms of the complex. The corresponding B3LYP/MWB28/aug-cc-pVDZ geometry is also an on-top structure directly bound to one Ag atom, but the molecule is directed away from the surface. The near-resonance Raman calculations were carried out in the infinite lifetime approximation, while the on-resonant Raman excitation profiles were calculated with the complex polarization propagator (CPP) approach, introducing a half width at half-maximum spectral broadening of 0.2 eV. Calculation of the UV–vis spectra of the isolated 4-Mpy and of the Ag13-4-Mpy comp...

Effect of alloying on the stabilities and catalytic properties of Ag–Au bimetallic subnanoclusters: a theoretical investigation

Density functional theory has been applied to study the geometric and electronic structures and the catalytic properties of Ag and AgAu clusters for CO oxidation. The calculated results suggest that the doping of Au atoms improve the stability of AgAu clusters. Correspondingly, the binding energy (BE) per atom of Ag n Au is larger than that of pure Ag n+1 cluster, due to strong hybridization between the d orbitals of Au and the s orbitals of Ag in Ag n Au clusters. With the increasing Au concentration, the BE of Ag13−n Aun(n = 1–8) clusters increase smoothly, while second-order difference of energies (Δ2 E) and fragmentation energies (ΔE) show an even–odd oscillation. The AgAu clusters containing an odd n umber of gold atoms (Ag13−n Au n , n = 3, 5, 7) are relatively stable compared to their neighbor. The CO and O2 adsorption properties on the Ag13, Ag10Au3, Ag8Au5, and Ag6Au7 clusters suggest that O2 is strongly activated by the clusters, while the activation of CO is much weak. Furthermore, the activation of O2 on AgAu cluster is stronger than that on pure Ag13 cluster, especially on Ag8Au5 cluster, due to the strengthened polarization of O–O bond. Compared to Ag13, Ag10Au3, and Ag6Au7 clusters, the lower energy barriers on Ag8Au5 cluster suggest a higher catalytic activity of Ag8Au5 cluster for O2 dissociation and CO oxidation reactions. The calculated results suggest that Ag8Au5 cluster could effectively reduce the carbon monoxide poisoning and exhibits the excellent catalytic performance for CO oxidation. Our study provides atomic-scale insights into the nature of the interfacial effects that determine CO oxidation on Ag–Au cluster catalysts.

Simulation of SERS by a DFT study: a comparison of static and near-resonance Raman for 4-mercaptopyridine on small Ag clusters

We have used density functional theory at the B3LYP/LANL2DZ level to calculate the static (DFT) and frequency-dependent (TDDFT) Raman spectra of complexes of 4-mercaptopyridine (4-Mpy) binding with Ag10 and Ag13 clusters via the thiolate Ag-S bond. Geometric optimization of the Ag10-4-Mpy complexes with plus one or neutral charge had a bridging surface structure with the bond between two Ag atoms of the cluster; whereas, the minus charged Ag10-4-Mpy complex had an on-top structure directly bound to one Ag atom. A neutral Ag13-4-Mpy complex showed the on-top structure also. The static (DFT) Raman tensor for these complexes showed the spectra of the structures with bridging geometry were very similar and the static spectra of the on-top structures were also very similar. New assignments for 4-Mpy anion and 4-Mpy bound to the Ag13 cluster were made using the assignment method of Gardner and Wright (2011 J. Chem. Phys. 135 114305) which proves to be more accurate than Wilson numbers. The calculated frequencies of the normal vibrational modes were found to be sufficiently close to several experimental surface enhanced Raman scattering spectra on a variety of different Ag substrates to clarify the previous assignments. The frequency-dependent (TDDFT) Raman calculations were carried out in the infinite life-time approximation. Calculation of the UV–VIS spectra of the isolated 4-Mpy and of the Ag13-4-Mpy complex showed that binding shifts the spectra from deep in the UV to the near-IR and visible region. Calculation of the frequency-dependent (TDDFT) near-resonance Raman spectrum of this complex at 637 nm (resonance at 632 nm) showed a strong enhancement. However, within the infinite life-time approximation, the actual enhancement cannot be estimated. The transition involves a charge transfer from the 4-Mpy molecule to the Ag cluster with a weak oscillator strength, f = 0.0162. The relative intensities for the near-resonance spectrum of Ag13-4-Mpy complex are found to be very different from the static spectrum of the complex. Examination of the integrated enhancement factor, EF, as a function of the vibrational mode symmetry for near-resonance shows the order of relative enhancements to be b2 > b1 > a2 > a1. This order indicates that a charge-transfer resonance mechanism is operating with Herzberg–Teller scattering dominating Franck–Condon scattering for this simulation. Conditions prevailing in such a case are also discussed.

Optical properties of Ag–Au nanoclusters for sulphide sensing from TDDFT calculations

A time-dependent density functional theory (TDDFT) approach is applied to investigate the optical properties of Ag–Au nanoclusters for sulphide sensing. It is found that the difference of refractive index between the cationic Ag13S and Ag13 clusters can cause the redshifts of the vertical excitation spectrum of the cationic Ag13S cluster, which is due to that sulphide can induce new electronic states in the frontier orbitals. Compared with the pure cationic Ag13 cluster, the cationic Ag12Au cluster shows enhanced sensitivity for detecting sulphide due to the hybridization between the orbitals of Ag and Au atoms, which can promote interband transition. With increasing the coverage of sulphide, more redshifts of the vertical excitation spectrum are found for the cationic Ag12Au cluster upon the adsorption of sulphide. Moreover, the cubohedral cationic Ag12Au cluster possesses more intense optical response than the icosahedral one, which is explained by the natural transition orbital analysis. It is expected that our theoretical results can provide new guidelines to design new powerful sensor for detecting sulphide using Ag–Au nanoparticles.

Silver Nanoparticles for Catalysis of Hydrogen Peroxide Decomposition: Atomistic Modeling

ABSTRACTThe interest in adopting hydrogen peroxide (H2O2) rocket propulsion systems has rekindled because H2O2 is more environmentally friendly than alternative propellants, has a high density to maximize the oxidizer-to-fuel ratio, and is able to be stored non-cryogenically. Simulations utilizing ab initio molecular dynamics have been generated to analyze the decomposition of H2O2 on the surface of a silver (Ag) metal cluster. The electronic structure for an atomic model of gaseous H2O2 molecules in the vicinity of an Ag13 cluster – one central Ag atom coordinated by the remaining twelve Ag atoms – was analyzed through density functional theory (DFT). After undergoing thermalization, the system was equilibrated at a high temperature of approximately 2000 K. The molecular dynamics did confirm that the Ag catalyst functions in facilitating the H2O2 decomposition to the final products of water and oxygen, while that the overall mechanism contains several intermediates.

Role of ligand type on the geometric and electronic properties of Ag–Au bimetallic clusters

The interactions of a set of typical ligands (–CN, –COOH, –CH3, –OH, –SH, –NH3, –NO, –NO2, N(CH3)2, and –PH3) with 13-atom icosahedral Ag–Au bimetallic clusters have been investigated by density functional theory (DFT) calculations. For the adsorption of all the ligands on the Ag13 cluster, the adsorption strength follows the order of Ag13–CN>Ag13–SH>Ag13–OH>Ag13–COOH>Ag13–CH3>Ag13–NO2>Ag13–N(CH3)2>Ag13–NO>Ag13–PH3>Ag13–NH3. Considering the composition effect, the adsorption strength of the ligands (X=–CN, –COOH, –CH3, and –NO2) on these clusters follows the order of Ag12Au–X>Ag13–X>Au12Ag–X>Au13–X. It is also found that the adsorption of the ligands (–NO, –N(CH3)2, and –PH3) on the Au-rich clusters can modify their geometric properties, and even the icosahedral structure of Au-rich clusters has been transformed into the more stable truncated octahedral one upon adsorption. Our results show that the geometric and electronic properties of Ag–Au bimetallic clusters can be tuned by the ligand type, which can provide useful insights for the preparation of ligand-stabilized Ag–Au bimetallic clusters by chemical reduction.

Experimental and Theoretical Studies on the Interaction between Isonicotinic Acid Molecules and Silver Nanoclusters

ABSTRACT The present paper describes the studies on the adsorption behavior and charge transfer from isonicotinic acid to silver nanoparticles with experiment and theory. Compared with UV–Visible adsorption spectrum, the adsorption spectrum of Ih-Ag13 cluster was quite good agreed with that of silver colloidal nanoparticle. So that one Ag13 cluster as a substrate was used to simulate Raman frequencies of the adsorption configuration. Here, it is demonstrated the calculated Raman spectra are in good agreement with experimental results. The analysis of Mulliken charge was obtained by density functional theory, which indicated the charge characteristics of Ag13 nanoparticle. Once isonicotinic acid molecules were adsorbed on sliver clusters, the charges transfer from isonicotinic acid to silver clusters, so that the surface charges of silver clusters are uneven.

Size of Elementary Clusters and Process Period in Silver Nanoparticle Formation

The time dependence of small-angle X-ray scattering (SAXS) curves for silver nanoparticle formation was followed in situ at a time resolution of 0.18 ms, which is 3 orders of magnitude higher than that used in previous reports (ca. 100 ms). The starting materials were silver nitrate solutions that were reacted with reducing solutions containing trisodium citrate. The SAXS analyses showed that silver nanoparticles were formed in three distinct periods from a peak diameter of ca. 0.7 nm (corresponding to the size of a Ag(13) cluster) during the nucleation and the early growth period. The Ag(13) clusters are most likely elementary clusters that agglomerate to form silver nanoparticles.

Biomedical applications of small silver clusters

The ground-state properties of small silver clusters Agn (2⩽n⩽24) have been studied using a linear combination of atomic Gaussian-type orbitals within the density functional theory. The results show that the Ag13 clusters, due to their noticeable magnetic moment and their considerable highest occupied-lowest unoccupied molecular orbital gap, are promising candidates for the magnetic applications of nanoparticles. In particular, our study suggests that the silver nanoclusters made out of Ag13 clusters, as building blocks, are suitable for possible future applications in biomedicine, since they could improve some present-day difficulties of magnetic nanoparticles such as toxicity and opsonization.

Synthesis, chemical, and electrochemical characterization of the [Ag13{?3-Fe(CO)4}] n? (n = 3, 4, 5) cluster anions: X-Ray structural determination of [N(PPh3)2]3 [Ag12(?12-Ag){?3Fe(CO)4}8

The oxidation of the [Fe(CO)4]2− dianion with Ag+ salts occurs through a particularinner-sphere mechanism, which involves an intermediate cascade of silver clusters stabilized by Fe(CO)4 ligands. The last detectable Ag-Fe cluster of the sequence is the [Ag13{μ-Fe(CO)4}8]3− trianion, which has been selectively obtained by using ca. 1.7 equivalents of Ag+ per mole of [Fe(CO)4]2−. The [Ag13{μ-Fe(CO)4}8]3−- trianion has been isolated in a crystalline state with several quaternary cations, and has been characterized by X-ray diffraction studies of its bis(triphenylphosphine)iminium salt. [N(PPh3)2]3 [Ag13{μ3-Fe(CO)4}8]·2(CH3)2CO, monoclinic, space group P21 (No.4),a = 16.284(2) A,b =18.767(5) A,c = 25.905(4) A,β = 90.46(1)°,V = 7916(3) A3,Z = 2,R = 0.0324. The molecular structure of the anion consists of a centered cuboctahedron of silver atoms with the triangular faces capped by Fe(CO)4 units. Chemical reduction of ( Ag13{μ3-Fe(CO)4}8]3− affords the corresponding [Ag13{μ3-Fe(CO)4)8]4−, which in turn gives [Ag13{μ3-Fe(CO)4)8]5− and [Ag6{μ3-Fe(CO)4}4]– upon further reduction. Electrochemical investigations confirm the reversibility of the [Ag13{μ3-Fe(CO)4}8]3−/4− redox change. Furthermore, in spite of some electrode poisoning effects, evidence of the existence of the [Ag13{μ3-Fe(CO)4}8]5− pentaanion was obtained. The yet structurally uncharacterized [Ag6{μ3-Fe(CO)4)4]2− dianion is quantitatively obtained by reaction of [Fe(CO)4]2− with ca. 1.5 equivalents of Ag+ or by addition of one equivalent of Ag+ to solutions of the [Ag5{Fe(CO)4}4]3− trianion. All attempts to isolate its quaternary salts as crystalline materials failed owing to formation of amorphous insoluble precipitates. The above series ofμ3-Fe(CO)4 octa-capped cuboctahedral Ag13 clusters can be envisioned as the Ag+ . Ag and Ag− cryptates of the [Ag12{μ}3-Fe(CO)4}8]4− cryptand. respectively.

Synthesis and characterization of the paramagnetic [Ag13Fe8(CO)32]4- tetraanion: a cuboctahedral Ag13 cluster stabilized by Fe(CO)4 groups behaving as 4-electron donors

The new [Ag 13 Fe 8 (CO) 32 ] 4- paramagnetic cluster anion has been obtained in a mixture with other yet uncharacterized products from the reaction of Na 2 [Fe(CO) 4 ].xTHF with ca. 1.5 equiv of AgBF 4 or AgNO 3 in tetrahydrofuran and acetonitrile solution, respectively. It has been separated from the mixture via differential solubility of the tetrasubstitutred ammonium or phosphonium salts and isolated in a crystalline state as [NMe 3 CH 2 Ph] + and [N(PPh 3 ) 2 ] + salts.