Tuning the electronic properties of the golden buckyball by endohedral doping: M@Au16− (M=Ag,Zn,In)

Density functional theory study of the structure, stability, magnetic properties, and (hyper)polarizability of (sub-nm) rare-earth (RE) doped gold clusters: Au5RE with RE = Sc, Y, La-Lu

Rare-earth doped materials are of immense interest for their potential applications in linear and nonlinear photonics. There is also intense interest in sub-nanometer gold clusters due to their enhanced stability and unique optical, magnetic, and catalytic properties. To leverage their emergent properties, here we report a systematic study of the geometries, stability, electronic, magnetic, and linear and nonlinear optical properties of Au5RE (RE = Sc, Y, La-Lu) clusters using density-functional theory. Several low-energy isomers consisting of planar or non-planar configurations are identified. For most doped clusters, the non-planar configuration is energetically favored. In the case of La-, Pm-, Gd-, and Ho-doped clusters, a competition between planar and non-planar isomers is predicted. A distinct preference for the planar configuration is predicted for Au5Eu, Au5Sm, Au5Tb, Au5Tm, and Au5Yb. The distinction between the planar and non-planar configurations is highlighted by the calculated highest frequencies, with the stretching mode of the non-planar configuration predicted to be stiffer than the planar configuration. The bonding analysis reveals the dominance of the RE-d orbitals in the formation of frontier molecular orbitals, which, in turn, facilitates retaining the magnetic characteristics governed by RE-f orbitals, preventing spin-quenching of rare earths in the doped clusters. In addition, the doped clusters exhibit small energy gaps between frontier orbitals, large dipole moments, and enhanced hyperpolarizability compared to the host cluster.

Mechanisms for Catalytic CO Oxidation on SiAun (n = 1-5) Cluster

Significant progress has been made in understanding the reactivity and catalytic activity of gas-phase and loaded gold clusters for CO oxidation. However, little research has focused on mixed silicon/gold clusters (SiAun) for CO oxidation. In the present work, we performed density function theory (DFT) calculations for a SiAun (n = 1-5) cluster at the CAM-B3LYP/aug-cc-pVDZ-PP level and investigated the effects on the reactivity and catalytic activity of the SiAun cluster for CO oxidation. The calculated results show that the effect is very low for the activation barriers for the formation of OOCO intermediates on SiAu clusters, SiAu3 clusters, and SiAu5 clusters in the catalytic oxidation of CO and the activation energy barriers for the formation of OCO intermediates on OSiAu3, OSiAu4, and OSiAu5. Our calculations show that, compared with the conventional small Au cluster, the incorporation of Si enhances the catalytic performance towards CO oxidation.

Exploring the Effects of a Doping Silver Atom on Anionic Gold Clusters' Reactivity with O2

Reactivities of AgAu_n-1^- (n = 3-10) with O₂ at a low temperature were studied using an instrument combining a magnetron sputter cluster source, a microflow reactor, and a time-of-flight mass spectrometer. Their reaction products as well as size-dependent kinetic rates were nearly identical to those of corresponding Au_n^- (n = 3-10). Previous experiments showed that the Ag atom in AgAu_n-1^- (n = 3-10) was fully or partially enclosed by the gold atoms. We studied the adsorption of O₂ on these reported structures using the B3LYP theory with relatively large basis sets. The theoretical results indicate that the adsorption sites as well as the adsorption energies of O₂ on AgAu_n-1^- (n = 3-10) are nearly identical to those on the corresponding Au_n^- (n = 3-10). The O₂ adsorption on a series of proposed isomers of AgAu_n-1^- (denoted as Au_n-1Ag^-), in which the silver atom was on the protruding site, was explored using the same theoretical methods. The O₂ tends to bond with the protruding Ag atoms, and the binding energies are apparently higher than those on the corresponding Au_n^- and AgAu_n-1^-. The adsorption and activation of O₂ on Au_n^-, AgAu_n-1^-, and Au_n-1Ag^- were correlated with their global electron detachment energies (VDEs) as well as the element types of the adsorption sites. Generally, low VDE values and silver sites facilitate the O₂ adsorption, and these two factors separately dominate in various cluster species. The revealed effects of a doping silver atom in small gold clusters are helpful to understand the role of the residual silver components in many nano gold catalysts.

Probing the structural, electronic, and adsorptive properties of Au16O2- clusters

Great progress has been made in O₂ adsorption on gold clusters. However, systematic investigations of O₂ adsorption on [Formula: see text] clusters have not been reported. Here, we present a systematic study of the structural, electronic, and adsorptive properties of [Formula: see text] clusters by density functional theory (DFT) calculations coupled with stochastic kicking method. Global minimum searches for [Formula: see text] reveal that exohedral derivatives are more favored. Furthermore, the obtained ground-state structure exhibits significant stability, as judged by its larger adsorption energy (1.16 eV) and a larger HOMO-LUMO gap (0.57 eV). The simulated photoelectron spectra (PES) of [Formula: see text] isomers will be instructive to identify the structures in future experiments. There are three interesting discoveries in the present paper: (1) O₂ undergoes chemical adsorption onto the parent [Formula: see text] clusters, but the amount of the adsorption energy is related to the parent [Formula: see text] clusters; (2) the process that O₂ undergoes dissociative adsorption onto the parent [Formula: see text] clusters is exothermic; (3) [Formula: see text] isomers show smaller X-A energy gaps than those of parent [Formula: see text] clusters, reflecting that their geometric and electronic structures are distorted remarkably due to dissociative adsorption of O₂.

Structural exploration of AuxM− (M = Si, Ge, Sn; x = 9–12) clusters with a revised genetic algorithm

We used a revised genetic algorithm (GA) to explore the potential energy surface (PES) of Au_xM⁻ (x = 9–12; M = Si, Ge, Sn) clusters. The most interesting finding in the structural study of Au_xSi⁻ (x = 9–12) is the 3D (Au₉Si⁻ and Au₁₀Si⁻) → quasi-planar 2D (Au₁₁Si⁻ and Au₁₂Si⁻) structural evolution of the Si-doped clusters, which reflects the competition of Au–Au interactions (forming a 2D structure) and Au–Si interactions (forming a 3D structure). The Au_xM⁻ (x = 9–12; M = Ge, Sn) clusters have quasi-planar structures, which suggests a lower tendency of sp³ hybridization and a similarity of electronic structure for the Ge or Sn atom. Au₉Si⁻ and Au₁₀Si⁻ have a 3D structure, which can be viewed as being built from Au₈Si⁻ and Au₉Si⁻ with an extra Au atom bonded to a terminal gold atom, respectively. In contrast, the quasi-planar structures of Au_xM⁻ (x = 9–12; M = Ge, Sn) reflect the domination of the Au–Au interactions. Including the spin–orbit (SO) effects is very important to calculate the simulated spectrum (structural fingerprint information) in order to obtain quantitative agreement between theoretical and future experimental PES spectra.

We used a revised genetic algorithm (GA) to explore the potential energy surface (PES) of Au_xM⁻ (x = 9–12; M = Si, Ge, Sn) clusters.

The stability of binary Al12X nanoclusters (X = Sc and Ti): superatom or Wade's polyhedron

Binary nanoclusters (NCs) exhibit strong potential as building blocks for tailor-made scientific materials based on the precise tuning of their electron countings and spin states along with the synergistic effects that originate from the constituent elements. Herein, we studied the electronic and geometric structures of transition metal (TM) doped aluminum (Al) Al₁₂X NCs (X  =  Sc and Ti), which are binary systems that extend from representative superatom [Formula: see text] anions. On the basis of the photoelectron spectroscopy (PES) and density functional theory (DFT) calculations, Al₁₂X anion and neutral structures are characterized as vertex-replaced icosahedron. The highly stable exohedral Al₁₂X icosahedron is described based on an electron counting rule derived from the coupling of Wade-Mingos' rule and the jellium model.

Stabilization of golden cages by encapsulation of a single transition metal atom

Golden cage-doped nanoclusters have attracted great attention in the past decade due to their remarkable electronic, optical and catalytic properties. However, the structures of large golden cage doped with Mo and Tc are still not well known because of the challenges in global structural searches. Here, we report anionic and neutral golden cage doped with a transition metal atom MAu16 (M = Mo and Tc) using Saunders 'Kick' stochastic automation search method associated with density-functional theory (DFT) calculation (SK-DFT). The geometric structures and electronic properties of the doped clusters, MAu16q (M = Mo and Tc; q = 0 and -1), are investigated by means of DFT theoretical calculations. Our calculations confirm that the 4d transition metals Mo and Tc can be stably encapsulated in the Au16- cage, forming three different configurations, i.e. endohedral cages, planar structures and exohedral derivatives. The ground-state structures of endohedral cages C2v Mo@Au16--(a) and C1 Tc@Au16--(b) exhibit a marked stability, as judged by their high binding energy per atom (greater than 2.46 eV), doping energy (0.29 eV) as well as a large HOMO-LUMO gap (greater than 0.40 eV). The predicted photoelectron spectra should aid in future experimental characterization of MAu16- (M = Mo and Tc).

Geometric and electronic properties of Si-atom doped Al clusters: robustness of binary superatoms against charging

Chemically stabilized binary superatoms are formed with Si-atom doping into Al superatoms.

Insights into the structural, electronic and magnetic properties of V-doped copper clusters: comparison with pure copper clusters

The structural, electronic and magnetic properties of Cu_n+1 and Cu_nV (n = 1–12) clusters have been investigated by using density functional theory. The growth behaviors reveal that V atom in low-energy Cu_nV isomer favors the most highly coordinated position and changes the geometry of the three-dimensional host clusters. The vibrational spectra are predicted and can be used to identify the ground state. The relative stability and chemical activity of the ground states are analyzed through the binding energy per atom, energy second-order difference and energy gap. It is found that that the stability of Cu_nV (n ≥ 8) is higher than that of Cu_n+1. The substitution of a V atom for a Cu atom in copper clusters alters the odd-even oscillations of stability and activity of the host clusters. The vertical ionization potential, electron affinity and photoelectron spectrum are calculated and simulated for all of the most stable clusters. Compare with the experimental data, we determine the ground states of pure copper clusters. The magnetism analyses show that the magnetic moments of Cu_nV clusters are mainly localized on the V atom and decease with the increase of cluster size. The magnetic change is closely related to the charge transfer between V and Cu atoms.

Probing the structures of gold-aluminum alloy clusters AuxAly(-): a joint experimental and theoretical study

Besides the size and structure, compositions can also dramatically affect the properties of alloy nanoclusters. Due to the added degrees of freedom, determination of the global minimum structures for multi-component nanoclusters poses even greater challenges, both experimentally and theoretically. Here we report a systematic and joint experimental/theoretical study of a series of gold-aluminum alloy clusters, AuxAly(-) (x + y = 7,8), with various compositions (x = 1-3; y = 4-7). Well-resolved photoelectron spectra have been obtained for these clusters at different photon energies. Basin-hopping global searches, coupled with density functional theory calculations, are used to identify low-lying structures of the bimetallic clusters. By comparing computed electronic densities of states of the low-lying isomers with the experimental photoelectron spectra, the global minima are determined. It is found that for y ≥ 6 there is a strong tendency to form the magic-number square bi-pyramid motif of Al6(-) in the AuxAly(-) clusters, suggesting that the Al-Al interaction dominates the Au-Au interaction in the mixed clusters. A closely related trend is that for x > 1, the gold atoms tend to be separated by Al atoms unless only the magic-number Al6(-) square bi-pyramid motif is present, suggesting that in the small-sized mixed clusters, Al and Au components do not completely mix with one another. Overall, the Al component appears to play a more dominant role due to the high robustness of the magic-number Al6(-) square bi-pyramid motif, whereas the Au component tends to be either "adsorbed" onto the Al6(-) square bi-pyramid motif if y ≥ 6, or stays away from one another if x < y < 6.

Nonresonant chemical mechanism in surface-enhanced Raman scattering of pyridine on M@Au12 clusters

By employing density functional theory (DFT), this study presents a detailed analysis of nonresonant surface-enhanced Raman scattering (SERS) of pyridine on M@Au12 (M = V(-), Nb(-), Ta(-), Cr, Mo, W, Mn(+), Tc(+), and Re(+))-the stable 13-atom neutral and charged gold buckyball clusters. Changing the core atom in M@Au12 enabled us to modulate the direct chemical interactions between pyridine and the metal cluster. The results of our calculations indicate that the ground-state chemical enhancement does not increase as the binding interaction strengthens or the transfer charge increases between pyridine and the cluster. Instead, the magnitude of the chemical enhancement is governed, to a large extent, by the charged properties of the metal clusters. Pyridine on M@Au12 anion clusters exhibits strong chemical enhancement of a factor of about 10(2), but the equivalent increase for pyridine adsorbed on M@Au12 neutral and cation clusters is no more than 10. Polarizability and deformation density analyses clearly show that compared with the neutral and cation clusters, the anion clusters have more delocalized electrons and occupy higher energy levels in the pyridine-metal complex. Accordingly, they produce larger polarizability, leading to a stronger nonresonant enhancement effect.

Adsorption of gas molecules on Gd@Aun (n = 14, 15) clusters and their implication for molecule sensors

The Gd@Au₁₅ cluster as an excellent gas sensor for NO and NO₂ detection.

Endohedrally doped gold nanocages: efficient catalysts for O2 activation and CO oxidation

Gold nanocages are the most attractive catalytic materials as all the atoms in the cage type clusters reside on the surface, making them available for chemisorption by reacting molecules.

Structural evolution of (Au2S)n (n = 1–8) clusters from first principles global optimization

First principles global optimization reveals the structural evolution and novel geometries of (Au₂S)_n nanoclusters at n = 1–8.

Structure identification of endohedral golden cage nanoclusters

The endohedral structures of MAu₁₆⁻ (M = Y, Zr and Nb) nanoclusters.

Superatom-atom super-bonding in metallic clusters: a new look to the mystery of an Au20 pyramid

Using the super valence bond model, a generalized chemical picture for the electronic shells of an Au20 pyramid is given. It is found that Au20 can be viewed to be a superatomic molecule, of which its superatomic 16c-16e core (T) is in D(3)S hybridization bonded with four vertical Au atoms for the molecule-like (TAu4) electronic shell-closure. Based on such a superatom-atom bonding model, TX4 (X = F, Cl, or Br) are predicted to be very stable. Such a superatom-atom T-Au/T-X bonding enriches the scope of chemistry.

Nature of valence transition and spin moment in Ag(n)V(+) clusters

Evolution in the atomic structure, bonding characteristics, stability, and the spin magnetic moment of neutral and cationic AgnV clusters has been investigated using first-principles density functional approach with gradient corrected functional. It is shown that at small sizes, the V 4s states hybridize with Ag states to form 1S and 1P like superatomic orbitals, whereas the 3d states are localized on V giving the V atom an effective valence of 1 or 2. Starting from Ag8V(+), the V 3d states begin to participate in the bonding by hybridizing with the nearly free electron gas to form 1D superatomic orbitals increasing the V atom effective valence toward 5. For the cationic clusters, this changing valence results in three shell closures that lead to stable species. These occur for cationic clusters containing 5, 7, and 14 Ag atoms. The first two stable species correspond to filled 1S and 1P shells in two and three dimensions with a valence of 2 for V, whereas the closure at 14 Ag atoms correspond to filled 1S, 1P, and 1D shells with V site exhibiting a valence of 5. The transition from filled 1S and 1P shells to filled 1S, 1P, and 1D shells is confirmed by a quenching of the spin magnetic moment. The theoretical findings are consistent with the observed drops in intensity in the mass spectrum of AgnV(+) clusters after 5, 7, and 14 Ag atoms.

Probing the electronic properties and structural evolution of anionic gold clusters in the gas phase

Gold nanoparticles have been discovered to exhibit remarkable catalytic properties in contrast to the chemical inertness of bulk gold. A prerequisite to elucidate the molecular mechanisms of the catalytic effect of nanogold is a detailed understanding of the structural and electronic properties of gold clusters as a function of size. In this review, we describe joint experimental studies (mainly photoelectron spectroscopy) and theoretical calculations to probe the structural properties of anionic gold clusters. Electronic properties and structural evolutions of all known Au(n)(-) clusters as experimentally confirmed to date are summarized, covering the size ranges of n = 3-35 and 55-64. Recent experimental efforts in resolving the isomeric issues of small gold clusters using Ar-tagging, O(2)-titration and isoelectronic substitution are also discussed.

Adsorption properties of chalcogen atoms on a golden buckyball Au16(-) from first principles

Using first-principles density functional theory, we investigate the adsorption properties of chalcogen elements (oxygen and sulfur) on an anionic golden nanocage Au(16)(-) and its effects on the structural and electronic properties of the golden cage. In particular, we find that when a sulfur atom is encapsulated inside Au(16)(-), its bonding character with Au atoms appears ionic due to electron transfer from sulfur to the gold nanocage. In contrast, the exohedrally adsorbed S atom tends to have strong orbital hybridization with the golden nanocage. For an oxygen adsorption case, electrons from the golden cage tend to be shared with the adsorbed O atom exhibiting strong orbital hybridization, regardless of its adsorption sites. To investigate the transition behaviors between the most stable exohedral and endohedral adsorption configurations, we calculate the activation and reaction energies in the transition. The oxygen atom experiences a lower energy barrier than the sulfur atom due to its smaller atomic radius. Finally, we explore the vibrational properties of S- or O-adsorbed Au(16)(-) buckyballs by calculating their infrared spectra.

Icosahedral crown gold nanocluster au(43)cu(12) with high catalytic activity

Structural and catalytic properties of the gold alloy nanocluster Au(43)Cu(12) are investigated using a density-functional method. In contrast to the pure Au(55) nanocluster, which exhibits a low-symmetry C(1) structure, the 55-atom "crown gold" nanocluster exhibits a multishell structure, denoted by Au@Cu(12)@Au(42), with the highest icosahedral group-symmetry. In addition, density functional calculations suggest that this geometric magic-number nanocluster possesses comparable catalytic capability as a small-sized Au(10) cluster for the CO oxidation, due in part to their low-coordinated Au atoms on vertexes. The gold alloy nanocluster also shows higher selectivity for styrene oxidation than the bare Au(111) surface.

Structural transition of gold nanoclusters: from the golden cage to the golden pyramid

How nanoclusters transform from one structural type to another as a function of size is a critical issue in cluster science. Here we report a study of the structural transition from the golden cage Au(16)(-) to the pyramidal Au(20)(-). We obtained distinct experimental evidence that the cage-to-pyramid crossover occurs at Au(18)(-), for which the cage and pyramidal isomers are nearly degenerate and coexist experimentally. The two isomers are observed and identified by their different interactions with O(2) and Ar. The cage isomer is observed to be more reactive with O(2) and can be preferentially "titrated" from the cluster beam, whereas the pyramidal isomer has slightly stronger interactions with Ar and is favored in the Au(18)Ar(x)(-) van der Waals complexes. The current study allows the detailed structural evolution and growth routes from the hollow cage to the compact pyramid to be understood and provides information about the structure-function relationship of the Au(18)(-) cluster.