Size-evolution towards metallic behavior in nano-sized gold and platinum clusters as revealed by 197Au Mössbauer spectroscopy

Four types of molecular Au₅₅ clusters, with different types of ligands, have been studied using ¹⁹⁷Au Mössbauer spectroscopy. Emission Mössbauer measurements were also performed on the larger Pt309 cluster, in which ¹⁹⁷Au was produced by neutron activation. There is a large influence of the ligands on the charge densities at the surface atoms of the metal atom cluster cores. For the Au₅₅ clusters, the inner core atoms have a different charge density at the nucleus than the bulk metal, and it depends also slightly on the type of ligands on it. For the Pt₃₀₉ compound, the inner core atoms have the same charge density at the nucleus as in the corresponding bulk metal. We, therefore, conclude that the inner core of the Pt₃₀₉ cluster is metallic with respect to the electronic charge densities, while this is not yet the case for the Au₅₅ cluster.     

Evolution of atomic and electronic structure of magnetic Gd-doped gold clusters

The evolution of atomic and electronic structure of small Au _n (n = 1–16, and 55) clusters doped with a Gd atom has been investigated using density functional theory within generalized gradient approximation for the exchange–correlation energy. Pure gold neutral clusters with n up to 15 are planar. However, with the doping of a Gd atom, the atomic structure of gold clusters changes, and there is a transition from planar-like structures to three dimensional structures at n = 10. The electronic structure of Gd-doped gold clusters shows a sharp increase in the highest occupied–lowest unoccupied molecular orbital (HOMO–LUMO) gap for certain sizes giving rise to their magic behavior. All clusters are magnetic with large magnetic moments ranging from 6 to 8 μ_B primarily due to the localized 4f electrons on Gd. This makes such clusters with large HOMO–LUMO gaps magnetic superatoms. The main interaction between gold and gadolinium atoms in the clusters is due to hybridization between Au-6s and Gd-5d6s orbitals. Our results indicate the emergence of a wheel structure for Gd@Au₇, a symmetric cage structure at n = 15 for Gd@Au₁₅ and n = 16 for Gd@Au₁₆ ⁺ and Eu@Au₁₆ corresponding to an electronic shell closing at 18 valence electrons leaving aside the f electrons on Gd while for Gd-doped Au₅₅ corresponding to 58 valence electrons, a Au₉Gd@Au₄₆ core–shell structure is obtained in which the Gd atom connects the core of Au₉ with the Au₄₆ shell. The binding energy shows odd–even oscillations with enhancement due to Gd doping compared with pure gold clusters. Such magnetic clusters of gold could have multifunctional biological applications in drug delivery, sensor, imaging, and cancer treatment.     

Polymorphism of Phosphine‐Protected Gold Nanoclusters: Synthesis and Characterization of a New 22‐Gold‐Atom Cluster

A new Au₂₂ nanocluster, protected by bis(2‐diphenyl‐phosphino)ethyl ether (dppee or C₂₈H₂₈OP₂) ligand, has been synthsized and purified with high yield. Electrospray mass spectrometry shows that the new cluster has a formula of Au₂₂(dppee)₇, containing 22 gold atoms and seven dppee ligands. The cluster is found to be stable as a solid, but metastable in solution. The new cluster has been characterized by UV‐Vis‐NIR absorption spectroscopy, collision‐induced dissociation, and ³¹P‐NMR. The properties of the new cluster have been compared with the previous Au₂₂(dppo)₆ nanocluster (dppo = 1,8‐bis(diphenyl‐phosphino)octane or C₃₂H₃₆P₂), which contains two fused Au₁₁ units. All the experimental data indicate that the new Au₂₂(dppee)₇ cluster is different from the previously known Au₂₂(dppo)₆ cluster and represents a new Au₂₂ core, which contains most likely one Au₁₁ motif with several Au₂(dppee) or Au(dppee) units. The Au₂₂(dppee)₇ cluster provides a new example of the ligand effects on the nuclearity and structural polymorphism of phosphine‐protected atom‐precise gold nanoclusters.     

Single Atom-Engineered NIR-II Gold Clusters with Ultrahigh Brightness and Stability for Acute Kidney Injury

Near-infrared-II (NIR-II) imaging has shown great potential for monitoring the pathological progression and deep tissue imaging but is limited to present unmet NIR-II agent. Present fluorophores show a promising prospect for NIR-II imaging, but brightness and photostability are still highly challenging during real-time monitoring. In this work, atom-engineered NIR-II Au₂₄Cd₁ clusters with ultrahigh brightness, stability, and photostability are developed via single atomic Cd doping. Single atom Cd substitutions contribute to Cd 4d state in HOMO and redistribution of energy level near the gap, exhibiting 56-fold fluorescence enhancement of Au₂₄Cd₁ clusters. Meanwhile, single atomic Cd reinforces Cd Au bond energy, formation energy, and stabilized cluster structure, leading to persistent stability for up to 1 month without decay, as well as excellent photostability of 1 h without photobleaching, much longer than clinically approved indocyanine green (<5 min). In vivo imaging shows gold clusters can monitor acute kidney injury (AKI) even after 72 h of injury, enabling evaluating progression at a very long window. Meanwhile, the bioactive gold clusters can alleviate AKI-induced oxidative stress damage and acute neuroinflammation. Single atom-engineered gold clusters exhibit molecular tracking and diagnostic prospect in kidney-related diseases.     

Novel caged clusters of silicon: Fullerenes,Frank-Kasper polyhedron and cubic

We review recent findings of metal (M) encapsulated caged clusters of Si and Ge obtained from computer experiments based on an ab initio pseudopotential method. It is shown that one M atom changes drastically the properties of Si and Ge clusters and that depending upon the size of the M atom, cages of 14, 15, and 16 Si as well as Ge atoms are formed. In particular M@Si₁₆ silicon fullerene has been obtained for M= Zr and Hf, while a Frank-Kasper polyhedron has been obtained for M@X₁₆, X = Si and Ge. These clusters show high stability and large highest occupied-lowest unoccupied molecular orbital (HOMO-LUMO) gaps which are likely to make these species strongly abundant. A regular icosahedral M@X₁₂ cluster has also been obtained for X = Ge and Sn by doping a divalent M atom. Interactions between clusters are rather weak. This is attractive for developing self-assembled cluster materials.     

Oxidation state and site selection of gold in (YBa2Cu3O7 −δ)(1 − x)Au x

Recent experiments on the effect of Au substitution on the superconductivity of YBa₂Cu₃O_{7 –} (YBCO) have revealed that Au, unlike most other dopants, enhances the superconducting transition temperature, T _c, by 2 K at 10 mole % Au. In the present study. X-ray photoelectron spectra, nominally of 4, 6, 8 and 10 mole % Au-doped YBCO, have been obtained. The Au spectra display a relatively intense peak which is the oxidized component associated with the Au 4f_7/2 core level, and an additional component that is most probably due to unreacted gold. The binding energy of the oxidized component is consistent with Au⁺. This observation, together with previous studies, leads us to postulate a structural model in which Au⁺ substitutes into a two-ligand c-axis co-ordinated Cu(1) site at a Ba-O (4) surface layer.     

Metal-C60 Interface Interaction: Effects on Metal Dispersion in Co-Deposited Films

Abstract

In case of a metal finely dispersed in a C₆₀ matrix, the metal-to-C₆₀ charge transfer lowers the interface energy. The total cohesive energy per metal atom E_tot was estimated for Au clusters in fullerite. The results show that, for a charge Q_f ≥ 1 electron (e) transferred from the metal clusters to each C₆₀ cage, a minimum appears in E_tot, defining a most stable cluster size. Thereby, the equilibrium dispersion state of the metal in the fullerite matrix depends on Q_f.     

Concentration fluctuations and surface tension in liquid Au-Sn-Zn

Experimental thermodynamic data were applied to calculate the surface tension and the long-wavelength limit of the concentration-concentration fluctuations S_cc(0) in the ternary system Au-Sn-Zn. The fluctuations in the concentration of Zn and in particular those of Sn show marked maxima around Au₁₅Sn₃₀Zn₅₅ whereas for Au the fluctuations are smaller than those of a non-interacting ideal mixture. Over an extremely wide range of compositions the surface layer of the ternary is found to be primarily populated by Sn atoms. Excepting the alloys with larger amounts of Au (c_Au > 0.3) or high contents of Zn (c_Zn > 0.7) the surface tension is not very different from that of pure by Sn (0.52–0.60 Nm⁻¹). The progressive substitution of Sn by Zn at a constant value of c_Au≈ 0.3 leaves the surface tension practically constant up to c_Zn≈ 0.5. In sections of constant c_Au/c_Sn with c_Au/c_Sn > 0.3 the surface tension is observed to even pass through pronounced minima if larger amounts of Zn are added.     

Evolution of Electrocatalytic CO2 Reduction Activity Induced by Charge Segregation in Atomically Precise AuAg Nanoclusters Based on Icosahedral M13 Unit 3D Assembly

The precise self-assembly of building blocks at atomic level provides the opportunity to achieve clusters with advanced catalytic properties. However, most of the current self-assembled materials are fabricated by 1/2D assembly of blocks. High dimensional (that is, 3D) assembly is widely believed to improve the performance of cluster. Herein, the effect of 3D assembly on the activity for electrocatalytic CO₂ reduction reaction (CO₂RR) is investigated by using a range of clusters (Au₈Ag₅₅, Au₈Ag₅₇, Au₁₂Ag₆₀) based on 3D assembly of M₁₃ unit as models. Although three clusters have almost the same sizes and geometric structures, Au₈Ag₅₅ exhibits the best CO₂RR performance due to the strong CO₂ adsorption capacity and effective inhibition of H₂ evolution competition reaction. The deep insight into the superior activity of Au₈Ag₅₅ is the unique electronic structure attributed to the charge segregation. This study not only demonstrates that the assembly mode greatly affects the catalytic activity, but also offers an idea for rational designing and precisely constructing catalysts with controllable activities.     

Surface nanostructures created by cluster-surface impact

This paper presents a review of the production of surface nanostructures from the controlled deposition of size-selected clusters on graphite. At high enough impact energies, the clusters can be implanted into the graphite surface to create open ‘well’ structures. Scanning tunnelling microscopy (STM) measurements, coupled with molecular dynamics (MD) simulations, for Au₇⁺, Ag₇⁺ and Si₇⁺ clusters exhibit scaling relations which reveal that the implantation depth scales linearly with the momentum of the clusters in all cases. At lower impact energies, the clusters can be pinned on the graphite surface when the impact energy exceeds a critical (threshold) value, thus allowing the fabrication of monodispersed cluster arrays which are stable at room temperature (and above). One application of such cluster arrays is to bind protein molecules; atomic force microscopy (AFM) measurements in buffer solution demonstrate dispersed arrays of both chaperonin and horseradish peroxidase molecules on graphite substrates decorated with size-selected Au_N⁺ clusters.     

Enhanced Stability,Conductance, and Catalytic Activity of Gold Nanoparticles via Oxidative Dissolution by KMnO4

The present paper reports the oxidative etching of Au nanospheres by an oxidant, KMnO₄, which resulted in the formation of new particles of different reduced sizes from single starting nanoparticles. The significant blue-shifting (536 → 527 nm) of the surface plasmon band for Au nanospheres was observed due to their size alterations after oxidative dissolution of Au⁰ to Au⁺³. This was supported by the cyclic voltammetry studies, which revealed an anodic peak at ?0.90 V in agreement with the standard electrode potential of Au⁰/Au⁺³ (?0.96 V). Dynamic light scattering (DLS) analysis showed that the hydrodynamic diameter of Au nanospheres (～41 nm) was decreased from ～26 to 4 nm as a function of KMnO₄ concentration, as further evidenced by TEM. This oxidation-driven process resulted in colloids with higher zeta potential, conductance, and enhanced catalytic activity (～2 times) than their bare nanoparticles for the reduction of p-nitrophenol to p-aminophenol attributed to the higher surface area-to-volume ratio.     

Catalysis by very small Au clusters

We review recent theoretical and experimental work on the catalytic properties of Au clusters that contain a few atoms and are supported on an oxide surface. The clusters are mass-selected and landed slowly on the oxide surface in ultra-high vacuum. STM measurements show that the clusters do not fragment and do not damage the surface when they are deposited nor do they coarsen after deposition. Their catalytic activity changes non-monotonically with the number of atoms and is sensitive to the nature of the support and to additives (hydroxyls, water, Na, Cl) present on the surface. Binary clusters (e.g. Au_nSr) can be more active than unary ones. Very recent work has managed to study catalysis by such clusters under realistic pressure conditions; their performance is very different from (and sometimes better than) that of large clusters.     

Atomic‐Precision Gold Clusters for NIR‐II Imaging

Near‐infrared II (NIR‐II) imaging at 1100–1700 nm shows great promise for medical diagnosis related to blood vessels because it possesses deep penetration and high resolution in biological tissue. Unfortunately, currently available NIR‐II fluorophores exhibit slow excretion and low brightness, which prevents their potential medical applications. An atomic‐precision gold (Au) cluster with 25 gold atoms and 18 peptide ligands is presented. The Au₂₅ clusters show emission at 1100–1350 nm and the fluorescence quantum yield is significantly increased by metal‐atom doping. Bright gold clusters can penetrate deep tissue and can be applied in in vivo brain vessel imaging and tumor metastasis. Time‐resolved brain blood‐flow imaging shows significant differences between healthy and injured mice with different brain diseases in vivo. High‐resolution imaging of cancer metastasis allows for the identification of the primary tumor, blood vessel, and lymphatic metastasis. In addition, gold clusters with NIR‐II fluorescence are used to monitor high‐resolution imaging of kidney at a depth of 0.61 cm, and the quantitative measurement shows 86% of the gold clusters are cleared from body without any acute or long‐term toxicity at a dose of 100 mg kg^?1.     

Au Nanoclusters Sensitized Black TiO2−x Nanotubes for Enhanced Photodynamic Therapy Driven by Near‐Infrared Light

The low reactive oxygen species production capability and the shallow tissue penetration of excited light (UV) are still two barriers in photodynamic therapy (PDT). Here, Au cluster anchored black anatase TiO_2?x nanotubes (abbreviated as Au₂₅/B‐TiO_2?x NTs) are synthesized by gaseous reduction of anatase TiO₂ NTs and subsequent deposition of noble metal. The Au₂₅/B‐TiO_2?x NTs with thickness of about 2 nm exhibit excellent PDT performance. The reduction process increased the density of Ti³⁺ on the surface of TiO₂, which effectively depresses the recombination of electron and hole. Furthermore, after modification of Au₂₅ nanoclusters, the PDT efficiency is further enhanced owing to the changed electrical distribution in the composite, which forms a shallow potential well on the metal–TiO₂ interface to further hamper the recombination of electron and hole. Especially, the reduction of anatase TiO₂ can expend the light response range (UV) of TiO₂ to the visible and even near infrared (NIR) light region with high tissue penetration depth. When excited by NIR light, the nanoplatform shows markedly improved therapeutic efficacy attributed to the photocatalytic synergistic effect, and promotes separation or restrained recombination of electron and hole, which is verified by experimental results in vitro and in vivo.     

Synthesis of Au microwires by selective oxidation of Au–W thin-film composition spreads

Abstract

We report on the stress-induced growth of Au microwires out of a surrounding Au–W matrix by selective oxidation, in view of a possible application as ‘micro-Velcro’. The Au wires are extruded due to the high compressive stress in the tungsten oxide formed by oxidation of elemental W. The samples were fabricated as a thin-film materials library using combinatorial sputter deposition followed by thermal oxidation. Sizes and shapes of the Au microwires were investigated as a function of the W to Au ratio. The coherence length and stress state of the Au microwires were related to their shape and plastic deformation. Depending on the composition of the Au–W precursor, the oxidized samples showed regions with differently shaped Au microwires. The Au₄₈W₅₂ composition yielded wires with the maximum length to diameter ratio due to the high compressive stress in the tungsten oxide matrix. The values of wire length (35 μm) and diameter (2 μm) achieved at the Au₄₈W₅₂ composition are suitable for micro-Velcro applications.     

Sputtering of thin films of bariated molecules of arachidic acid by large noble gas clusters

Coarse-grained molecular dynamics computer simulations have been employed to investigate the sputtering process of a multilayer organic system composed of long, well-organized linear molecules induced by an impact of slow clusters composed of large number of noble gas atoms. The organic system is represented by Langmuir–Blodgett multilayers formed from bariated molecules of arachidic acid. The sputtering yield, surface modifications and the angular distributions of ejected species have been analyzed as a function of the kinetic energy of an Ar₈₇₂ cluster projectile and the thickness of the deposited multilayer. It has been shown that the physics of ejection by these large and slow clusters is distinct from the ejection events stimulated by the popular SIMS clusters: C₆₀, Au₃ and SF₅. The organic molecules are not ejected by interaction with the energized substrate but by direct interactions with the projectile atoms.     

NH3 and H2S adsorption on Au3Pt3 cluster studied by a first principles calculation

The equilibrium atomic structures of the Au₃Pt₃ cluster and NH₃, H₂S molecules are determined by Density Functional Theory calculations. The adsorption of NH₃ and H₂S molecules on Au₃Pt₃ cluster is then studied. The energy levels and the corresponding charge densities, adsorption energies, charge transfer and magnetization are calculated for different adsorption sites. We find that the adsorption generally modifies the structure of the Au₃Pt₃ cluster and the adsorbate. There exists strong interaction between Pt and N atoms. We observe that H₂S dissociate at a specific site for the local density approximation (LDA) and one of the H atoms binds to Pt. In general, there is charge transfer from the molecules NH₃ and H₂S to Au₃Pt₃ cluster, for almost all adsorption sites. This makes the Au₃Pt₃ cluster semiconducting.     

Thin film interdiffusion of Au and In at room temperature

Results on the thin film interdiffusion of Au and In at room temperature are presented. Indium films 56–3000 Å thick were deposited by vacuum evaporation onto previously deposited gold films of thicknesses ranging from 200 to 1600 Å. The interdiffusion of Au and In occurs quickly at room temperature and the intermetallic compounds AuIn₂, AuIn, Au₇In₃ and Au₄In are formed. Which compound will be formed depends on the Au:In ratio in the layer and on the diffusion time.     

Two distinct fluorescent quantum clusters of gold starting from metallic nanoparticles by pH-dependent ligand etching

Two fluorescent quantum clusters of gold, namely Au₂₅ and Au₈, have been synthesized from mercaptosuccinic acid-protected gold nanoparticles of 4–5 nm core diameter by etching with excess glutathione. While etching at pH ∼3 yielded Au₂₅, that at pH 7–8 yielded Au₈. This is the first report of the synthesis of two quantum clusters starting from a single precursor. This simple method makes it possible to synthesize well-defined clusters in gram quantities. Since these clusters are highly fluorescent and are highly biocompatible due to their low metallic content, they can be used for diagnostic applications. Electronic Supplementary Material  Supplementary material is available for this article at and is accessible for authorized users. This article is published with open access at Springerlink.com     

Metal‐Cluster‐Decorated TiO2 Nanotube Arrays: A Composite Heterostructure toward Versatile Photocatalytic and Photoelectrochemical Applications

Recent years have witnessed increasing interest in the solution‐phase synthesis of atomically precise thiolate‐protected gold clusters (Au_x); nonetheless, research on the photocatalytic properties of Au_x–semiconductor nanocomposites is still in its infancy. In this work, recently developed glutathione‐capped gold clusters and highly ordered nanoporous layer‐covered TiO₂ nanotube arrays (NP‐TNTAs) are employed as nanobuilding blocks for the construction of a well‐defined Au_x/NP‐TNTA heterostructure via a facile electrostatic self‐assembly strategy. Versatile photocatalytic performances of the Au_x/NP‐TNTA heterostructure which acts as a model catalyst, including photocatalytic oxidation of organic pollutant, photocatalytic reduction of aromatic nitro compounds and photoelectrochemical (PEC) water splitting under simulated solar light irradiation, are systematically exploited. It is found that synergistic interaction stemming from monodisperse coverage of Au_x clusters on NP‐TNTAs in combination with hierarchical nanostructure of NP‐TNTAs reinforce light absorption of Au_x/NP‐TNTA heterostructure especially within visible region, hence contributing to the significantly enhanced photocatalytic and PEC water splitting performances. Moreover, photocatalytic and PEC mechanisms over Au_x/NP‐TNTA heterostructure are elucidated and corresponding reaction models were presented. It is anticipated that this work could boost new insight for photocatalytic properties of metal‐cluster‐sensitized semiconductor nanocomposites.