Elucidation of the molecular and electronic structures of some magic silver clusters Agn (n = 8, 18, 20)

Optical and Chiroptical Stimuli-Responsive Chiral AgNPs@H-Leu-Poly(phenylacetylene) Nanocomposites in Water

Dynamic macroscopically chiral nanocomposites are prepared by combining silver nanoparticles (AgNPs) and dynamic helical poly(phenylacetylene)s (PPAs) bearing pendants functionalized with amino groups. These amino groups provide the nanocomposite with the ability to disperse in water along with high stability due to the interaction between the ammonium group and the AgNP. Moreover, the equilibrium between NH3+/NH2 produces a "blinking" contact between the PPA and the AgNPs, which allows total control of the dynamic helical behavior of the polymer. The use of acidic or neutral pH allows controlling the morphology of the nanocomposite, which consists of a nanosphere that has trapped inside it a single AgNP (pH = 2) or several AgNPs (pH = 7) with ca. 30 nm of diameter. These nanocomposites combine the optical and chiroptical stimuli-responsive properties of both components, AgNPs and PPAs. Thus, the controlled aggregation of the nanocomposite produced variations in the LSPR band of the AgNPs in a reversible manner. In turn, given that the chiral coating is selective to Ba2+, the presence of this metal ion caused a helical inversion of the chiral coating of the nanocomposite detected by electronic circular dichroism. Moreover, it is possible to distinguish between three metal ions in different oxidation states, such as Ce4+, Fe3+, and Hg2+, which produce different responses of the nanocomposite when oxidizing the AgNP to Ag+.

Accelerating Global Search of Large-Sized Silver Clusters Using Cluster Graph Attention Network

Great efforts have been devoted to understanding the stability and reactivity of silver clusters, which usually depend on geometric structures, electronic configuration, and cluster size. Despite the fact that the jellium model and Wulff construction rule have successfully rationalized the stable clusters with "magic number" behavior, some experiments imply that silver clusters with 48 valence electrons also possess puzzling enhanced stability. In this work, using a recently developed deep learning technology, i.e., cluster graph attention network (CGANet), combined with a homemade comprehensive genetic algorithm (CGA) program, we searched the global minimum (GM) structures of Agn (n = 30-60) clusters with graphics processing unit acceleration, whose efficiency is about 2 orders of magnitude higher than that of the conventional density functional theory (DFT) calculations. GM structures and some representative isomers are reported at each size, revealing the competitive structural patterns based on truncated octahedra and icosahedra as well as the icosahedra-based layer-by-layer growth mode of large-sized Ag clusters. Most importantly, the size-dependent evolution behavior of structural and electronic properties of Agn (n = 30-60) clusters can successfully explain the observed stability at Ag48. Therefore, CGANet provides a powerful tool for rapidly exploring the potential energy surface of atoms with an accuracy comparable to that of DFT.

SERS-Based Sensor Using Subnanometric Copper-Silver Mixed Clusters Ag(8-n)Cun (n = 0-8) for Pyridine: A DFT Study

Surface-enhanced Raman spectroscopy (SERS) is a powerful Raman technique that provides high selectivity and sensitivity in analyzing the intermolecular interaction of a target compound adsorbed on the surface of a noble nanomaterial, i.e., silver, gold, or copper. Although copper presents a better SERS enhancement than gold and silver, its oxidation in the air is much easier than that of gold and silver. A mixed material between these metals may potentially improve the SERS signal enhancement in this context. In this work, we evaluated the SERS spectra of pyridine (Py) adsorbed on the copper-silver mixed clusters Ag(8-n)Cun (n = 0-8) using density functional theory (DFT) at the PBE functional. The cc-pVDZ-PP basis set was chosen for Ag and Cu, while the cc-pVDZ basis set was used for C, N, and H atoms. Geometrical and electronic structures of the mixed clusters and the Py adsorption configuration on these clusters were computed. The calculated SERS spectra then revealed the influence of the Ag/Cu mixing ratio on the SERS enhancement. As a result, the substituted copper atoms on the silver cluster turned out to be favorable adsorption sites for Py. Interestingly, when the number of Cu atoms increased from n = 0 (pure Ag8 cluster) to n = 5 (Ag3Cu5 cluster), the ring stretching peak (1590 cm-1) of Py significantly increased from 20 to 120 au and then saturated around this value despite increasing the Cu atom number to 8 (pure Cu8 cluster). This observation was extended for other ligands such as pyrazine and 3H-pyrrole. TD-DFT was then employed to clarify the chemical enhancement mechanism. The results obtained hopefully provide helpful information for the design of analytical sensors with lower costs.

Structural and optical properties of the Agn-tyrosine complexes (n = 3-12): a density functional theory study

We study the optical properties of Agn (n = 3-12) neutral clusters and their coordination with a tyrosine (Tyr) molecule. A global search strategy coupled with density functional theory (DFT) computations explored the potential energy surface. Adsorption energy calculations predicted that Tyr coordination stabilizes the metal clusters, favouring the Agn-Tyr complexes with an even number of silver atoms. For the Agn low-lying isomers, the general shape and the major transitions of the calculated time dependent-DFT (TD-DFT) absorption spectra align with those of previous reports measured in an argon environment. We use the analysis of non-covalent interactions to identify the specific interactions between each silver cluster and functional groups of Tyr. The TD-DFT absorption spectra for the Agn-Tyr complexes showed that Tyr significantly modifies the optical properties of the coordinated silver clusters and affects the smaller systems to a greater extent. The optical absorption results of the bare Agn clusters and the Agn-Tyr complexes are compared and discussed in detail.

Growth pattern and electronic and magnetic properties of Cr-doped silver clusters

Growth pattern and electronic and magnetic properties of Agn Cr (n = 1-16) clusters have been investigated via density functional theory (DFT) combined with CALYPSO structure search method. The optimized geometry shows that the growth of the global minimum structures of Agn Cr clusters have obvious rule. when n > 12, silver atoms grow around an icosahedron which is almost unchanged in each structure. Analyses of electronic properties indicate that the doped Cr atom can only enhance the stability of larger silver clusters. Optical absorption and photoelectron spectra of Agn Cr isomers have been predicted and can be used for their structural identification. The icosahedral Ag12 Cr cluster with large energy level gap can be seen as a superatom. The adsorption capacity of Cr atom in Agn Cr cluster to CO is much higher than that of free Cr atom. The intensity of IR and Ramam spectra can be dramatically enhanced when CO is absorbed on Agn Cr cluster that Cr atom is encapsulated by Ag atoms. Moreover, the red shift of IR and Raman spectra of CO adsorbed on these clusters is also very small compared to free CO. Magnetism calculations show that the magnetic moment of Agn Cr clusters decreases linearly from n = 6 to 12 and increases linearly from n = 12 to 16. The total magnetic moment of Agn Cr cluster is mainly localized on the Cr atom. The change of magnetic moment of Cr atom is related to the charge transfer between Cr and Ag atoms.

Structure effects of Pt15 clusters for the oxygen reduction reaction: first-principles calculations

In the present work, the lowest energy structures and electronic properties of Pt₁₅ clusters are investigated using molecular dynamics simulations. The results showed that the most stable configuration is a capped pyramidal structure, which is 0.8 kal mol^-1 lower in energy than a layered structure previously reported [V. Kumar and Y. Kawazoe, Evolution of Atomic and Electronic Structure of Pt Clusters: Planar, Layered, Pyramidal, Cage, Cubic, and Octahedral Growth, Phys. Rev. B: Condens. Matter Mater. Phys., 2008, 77, 205418.]. The result is further confirmed by using both the PW91/cc-pVDZ-PP and PBE/PW approaches including the other representative isomers for Pt₁₅. Due to the interesting structure arrangements found, we have investigated the catalytic activities for the oxygen reduction reaction. We found that the most stable Pt₁₅ clusters are plausible catalyts for the ORR according to their interaction with oxygen species, which is consistent with experiments of Pt clusters with atomicity below 20. The results of the structure, electronic, adsorption and vibrational properties of the clusters are provided.

Structural and electronic properties for Be-doped Ptn (n = 1-12) clusters obtained by DFT calculations

In this work, we have performed a computational study on the structure and electronic properties for Be-doped Pt_n (n = 1-12) clusters in the framework of density functional theory (DFT). The most stable structures of the clusters are obtained by a structure search procedure based in simulated annealing. The results show that the Pt_nBe clusters adopt compact structure motifs with Be situated at the edge sites while only in Pt₁₁Be the Be atom occupies the center site. The energetic parameters showed that Pt₅Be, Pt₇Be and Pt₁₀Be are the most stable ones. The Pt_nBe clusters with (n = 5-7) have similar vertical ionization potential (vIP) and vertical electron affinity (vEA) parameters compared to the unary Pt clusters, while Pt₉Be and Pt₁₁Be have the higher vEA values. In particular, the d-band center is slightly higher for the doped clusters, suggesting an enhanced reactivity. The σ-holes are found more remarkable for the doped clusters, which are situated in the Be dopant and low coordinated Pt sites. The data on the infrared spectra of the clusters is also provided and showed a significant blue shift due to the vibrational modes of the Be atom. These results are useful for understanding the fundamental properties of Be-doped Pt_n clusters in the subnanometer region.

Silver Cluster Interactions with Tyrosine: Towards Amino Acid Detection

Tyrosine (Tyr) is involved in the synthesis of neurotransmitters, catecholamines, thyroid hormones, etc. Multiple pathologies are associated with impaired Tyr metabolism. Silver nanoclusters (Ag NCs) can be applied for colorimetric, fluorescent, and surface-enhanced Raman spectroscopy (SERS) detection of Tyr. However, one should understand the theoretical basics of interactions between Tyr and Ag NCs. Thereby, we calculated the binding energy (Eb) between Tyr and Agnq (n = 1-8; q = 0-2) NCs using the density functional theory (DFT) to find the most stable complexes. Since Ag NCs are synthesized on Tyr in an aqueous solution at pH 12.5, we studied Tyr-1, semiquinone (SemiQ-1), and Tyr-2. Ag32+ and Ag5+ had the highest Eb. The absorption spectrum of Tyr-2 significantly red-shifts with the attachment of Ag32+, which is prospective for colorimetric Tyr detection. Ag32+ interacts with all functional groups of SemiQ-1 (phenolate, amino group, and carboxylate), which makes detection of Tyr possible due to band emergence at 1324 cm-1 in the vibrational spectrum. The ground state charge transfer between Ag and carboxylate determines the band emergence at 1661 cm-1 in the Raman spectrum of the SemiQ-1-Ag32+ complex. Thus, the prospects of Tyr detection using silver nanoclusters were demonstrated.

SERS Chemical Enhancement of 2,4,5-Trichlorophenoxyacetic Acid Adsorbed on Silver Substrate

Surface-enhanced Raman spectroscopy (SERS) was employed to gain an understanding of the chemical enhancement mechanism of 2,4,5-trichlorophenoxyacetic acid (2,4,5-T), an Agent Orange, adsorbed on a silver substrate surface. Experimental measurements were performed using a micro-Raman spectrophotometer with an excitation wavelength of 532 nm and successfully detected 2,4,5-T at a relatively low concentration of 0.4 nM. Density functional theory (DFT) calculations on the interactions of the 2,4,5-T molecule with some small silver clusters, Ag_n with n = 4, 8, and 20, as well as with extended Ag surfaces, demonstrate that the most stable adsorption configuration is formed via coordination of Cl9 sites and carbonyl C═O group on the 2,4,5-T ligand to the Ag atoms on surfaces. Analyses of charge transfer mechanism and frontier orbitals distributions show an electron transfer from 2,4,5-T to the cluster in the ground state, and an inversed trend occurs for the excited singlet state process, consequently leading to a chemical enhancement of SERS signals. The obtained results are of importance for subsequent work in guiding the design of mobile sensors specifically used for services of rapid screening and detection of these toxic compounds present in the environment, as well as agricultural and food products. Extensive computations pointed out that small silver clusters, in particular of Ag₂₀ size, can be used as appropriate models for a metal nanoparticle surface.

Comment on 'Structural characterization, reactivity and vibrational properties of silver clusters: a new global minimum for Ag16' by P. L. Rodríguez-Kessler, A. R. Rodríguez-Domínguez, D. MacLeod Carey and A. Muñoz-Castro, Phys. Chem. Chem. Phys., 2020, 22, 27255, DOI: D0CP04018E

A recent paper by Rodríguez-Kessler et al., Phys. Chem. Chem. Phys., 2020, 22, 27255-27262, reported not only results of quantum chemical computations (using the PW91 density functional) on Ag₁₆ clusters as emphasized in the article's title, but also on the Ag₁₅ size. These authors confirmed previous results obtained by McKee and Samokhvalov (J. Phys. Chem. A, 2017, 121, 5018-5028 using the M06 density functional) that the most stable isomer of Ag₁₅ is a C_2v structure. We wish to point out that two low symmetry isomers of Ag₁₅ that have a similar energy content are even lower in energy than their reported C_2v global minimum. The relative energies between low-lying Ag₁₅ isomers were again found to be method-dependent, and within the expected accuracy of DFT and CCSD(T) methods they could be considered as energetically degenerate, and likely coexist in a molecular beam. The new lower-energy Ag₁₅ isomers appear to fit more consistently within the structural evolution of small silver clusters.

Theoretical Study of the Binding of the Thiol-Containing Cysteine Amino Acid to the Silver Surface Using a Cluster Model

Computational approaches within the framework of density functional theory (DFT) were employed to elucidate the binding mechanism of the cysteine amino acid on silver nanoparticles using several small silver clusters Ag_n with n = 2-10 as surface models. The long-range corrected LC-BLYP functional and correlation consistent basis sets cc-pVTZ-PP and cc-pVTZ were used to determine the structural features, energetics, and spectroscopic and electronic properties of the resulting complexes. In vacuum and highly acidic conditions, cysteine molecules prefer to adsorb on silver clusters via their amine group. In aqueous solution, the thiolate head turns out to be the most energetically favorable binding site. The cysteine affinity of silver clusters is greatly altered in different conditions, i.e., acidic solution < vacuum < aqueous solution, and is strongly dependent on the cluster size. As compared to free clusters, the frontier orbital energy gap of the ones capped by cysteine is significantly improved, which corresponds to stronger stability, especially in aqueous solution. The analysis of frontier orbitals also reveals that both forward and backward electron donations exhibit comparable contributions to the enhancement of stabilizing interactions. As for an application, a chemical enhancement mechanism of the surface-enhanced Raman scattering (SERS) procedure of cysteine by silver clusters was also analyzed.

On the structure and reactivity of PtnCun (n = 1-7) alloy clusters

The structure, electronic and reactivity properties of Pt_nCu_n (n = 1-7) clusters are investigated in the framework of density functional theory (DFT). The most stable forms of the clusters are obtained by a structure search procedure based in simulated annealing. The results show that the Pt_nCu_n cluster alloys adopt layered structure motifs with segregation of the Cu and Pt species. The total magnetic moments of the clusters adopt the low spin configuration. The bimetallic cluster reactivity is investigated by using the ionization potential, electron affinity, and the d-band center, respectively. The results show that the Pt_nCu_n clusters with (n = 5-7) have similar vIP and vEA parameters compared to the unary Pt clusters, but the d-band center is slightly higher suggesting an enhanced reactivity for the bimetallic clusters. On the other hand, the molecular electrostatic potential shows that the Cu species increase the available active sites on the cluster surface. The data on the infrared spectra of the clusters is also provided. These results are useful to understand the fundamental properties of Pt-M bimetallic alloys in the subnanometer region.

Quantum Mechanical Modeling of the Interactions between Noble Metal (Ag and Au) Nanoclusters and Water with the Effective Fragment Potential Method

Explicit solvent interactions can significantly alter the physical and chemical properties of noble metal (e.g., gold and silver) nanoclusters. In order to compute these solvent interactions at a reasonable computational cost, a quantum mechanical (QM)/molecular mechanics (MM) approach, where the metal nanocluster is treated with full QM and the water molecules are treated with a MM force field, can be used. However, classical MM force fields were typically parameterized using molecules containing main group elements as the reference. The accuracy of noble metal-solvent interactions obtained with these force fields therefore remains unpredictable. The effective fragment potential (EFP) force field, designed to model explicitly solvated systems, represents an attractive method to simulate solvated noble metal nanoclusters because it is derived from first principles and contains few or no fitted parameters, depending on implementation. At the density functional theory-optimized geometries, good correlation is obtained between the nanocluster-water interaction energies computed with EFP and those computed with the reference coupled cluster singles, doubles, and perturbative triples method. It is shown that the EFP method gives qualitatively accurate interaction energies at medium-large intermolecular distances for various molecular configurations. In order to achieve higher quantitative accuracy, the first solvation shell should be treated with full QM, if possible. EFP is therefore a promising method for the QM modeling of explicitly solvated silver and gold nanoclusters.

Structural characterization, reactivity, and vibrational properties of silver clusters: a new global minimum for Ag16

In the present work, the lowest energy structures and electronic properties of Agn clusters up to n = 16 are investigated using a successive growth algorithm coupled with density functional theory calculations (DFT).

Study of odd-even effects in physisorption and chemisorption of Ar, N2, O2 and NO on open shell Ag11-13+ clusters by means of self-consistent van der Waals density functional calculations

We have studied the adsorption and coadsorption properties of one or more X = Ar, N₂, O₂, and NO adsorbates on cationic silver clusters Ag_11-13⁺, whose sizes are in the open shell region of metal clusters, aiming to understand the observed odd-even effects in the abundance spectra of Ag_11-13⁺·mX complexes. All calculations were performed self-consistently using a non-local van der Waals correlation functional, covering the different nature of the interactions between the silver substrate and the several adsorbates, which range from dispersion (London) forces for Ar, non covalent π-π interactions for N₂, charge-transfer interactions for O₂ and NO, and the covalent Ag-Ag bond in the nude silver cluster. Despite the wide interval of adsorption energies, spanning two orders of magnitude, we have been able to explain the following experimental facts. For X = Ar, N₂, and O₂ reactions with Ag_11-13⁺, it was observed in the mass spectra an abundance peak at n = 12 [M. Schmidt, et al., ChemPhysChem, 2015, 16, 855]. In addition it was observed the competitive adsorption of two or more N₂ molecules, and the cooperative effect of adsorbing N₂ together with O₂ molecules. For X = NO, an abundance peak at n = 12 has been also observed [J. Ma, et al., Phys. Chem. Chem. Phys., 2016, 18, 12819]. We find that the main factors determining these properties are the different core motifs of the cluster geometry (pentagonal bipiramid for Ag₁₁⁺ and Ag₁₃⁺, but triangular prism for Ag₁₂⁺) and, on the other hand, the odd number of valence electrons for Ag₁₂⁺, leading to a smaller HOMO-LUMO gap than those of its neighbours. Further details about the preferred adsorption sites, dipole moments, and dipole polarizabilities are also discussed.

Comment on "Theoretical Investigations on Geometrical and Electronic Structures of Silver Clusters"

We would like to make a few comments on the results reported in a recent paper in this journal by Tsuneda, J. Comput. Chem, 2019, 40, 206. The structures of some pure silver clusters were not correctly assigned. © 2019 Wiley Periodicals, Inc.