Oxidation-resistant gold-55 clusters

On the kinetics of the removal of ligands from films of colloidal nanocrystals by plasmas

This paper describes the kinetic limitations of etching ligands from colloidal nanocrystal assemblies (CNAs) by plasma processing. We measured the etching kinetics of ligands from a CNA model system (spherical ZrO₂ nanocrystals, 2.5-3.5 nm diameter, capped with trioctylphosphine oxide) with inductively coupled plasmas (He and O₂ feed gases, powers ranging from 7 to 30 W, at pressures ranging from 100 to 2000 mTorr and exposure times ranging between 6 and 168 h). The etching rate slows down by about one order of magnitude in the first minutes of etching, after which the rate of carbon removal becomes proportional to the third power of the carbon concentration in the CNA. Pressure oscillations in the plasma chamber significantly accelerate the overall rate of etching. These results indicate that the rate of etching is mostly affected by two main factors: (i) the crosslinking of the ligands in the first stage of plasma exposure, and (ii) the formation of a boundary layer at the surface of the CNA. Optimized conditions of plasma processing allow for a 60-fold improvement in etching rates compared to the previous state of the art and make the timeframes of plasma processing comparable to those of calcination.

Molybdenum-Incorporated Mesoporous Silica: Surface Engineering toward Enhanced Metal-Support Interactions and Efficient Hydrogenation

In heterogeneous catalysis, strong metal-support interactions are highly desired to improve catalytic turnover on metal catalysts. Herein, molybdenum is uniformly incorporated into mesoporous silica (KIT-6) to accomplish strong interactions with iridium catalysts, and consequently, active and selective hydrogenation of carbonyl compounds. Mo-incorporated KIT-6 (Mo-KIT-6) affords electronic interactions to improve the proportion of metallic Ir⁰ species, avoiding the easy surface oxidation of ultrafine metals in silica mesocavities. Owing to the effective H₂ activation and subsequent hydrogenation on metallic Ir⁰ sites, optimal Ir/Mo-KIT-6 with a high Ir⁰/Ir^δ+ ratio delivers prominent performance in the hydrogenation of amides to amines and α,β-unsaturated aldehydes to unsaturated alcohols. As for N-acetylmorpholine hydrogenation, the Ir/Mo-KIT-6 catalyst achieves efficient turnover toward N-ethylmorpholine with high selectivity (>99%) and exhibits activity that relies on the engineered chemical state of Ir sites. Such promotion is further proved to be universal in cinnamaldehyde hydrogenation. This work will provide new opportunities for catalyst design through surface/interface engineering.

Magic number colloidal clusters as minimum free energy structures

Clusters in systems as diverse as metal atoms, virus proteins, noble gases, and nucleons have properties that depend sensitively on the number of constituent particles. Certain numbers are termed ‘magic’ because they grant the system with closed shells and exceptional stability. To this point, magic number clusters have been exclusively found with attractive interactions as present between atoms. Here we show that magic number clusters exist in a confined soft matter system with negligible interactions. Colloidal particles in an emulsion droplet spontaneously organize into a series of clusters with precisely defined shell structures. Crucially, free energy calculations demonstrate that colloidal clusters with magic numbers possess higher thermodynamic stability than those off magic numbers. A complex kinetic pathway is responsible for the efficiency of this system in finding its minimum free energy configuration. Targeting similar magic number states is a strategy towards unique configurations in finite self-organizing systems across the scales.

Magic number cluster with closed shells and increased stability often result from potential energy minimization between attractive atoms or particles. Here, Wang et al. show that such magic number clusters can also result from entropy maximization in colloidal systems with negligible interactions.

Spontaneous Reduction of Copper(II) to Copper(I) at Solid-Liquid Interface

Oxidation and reduction reactions are of central importance in chemistry as well as vital to the basic functions of life and such chemical processes are generally brought about by oxidizing and reducing agents, respectively. Herein, we report the discovery of an interfacial reduction reaction (IRR) - without the use of any external reducing agent. In course of metal-ligand coordination, spontaneous reduction of Cu(II) to Cu(I) at a solid-liquid interface was observed-unlike in a liquid-phase reaction where no reduction of Cu(II) to Cu(I) was occurred. High-quality thin films of a new coordination network compound bearing a Fe(II)-CN-Cu(I) link were fabricated by IRR and employed for efficient electro-catalysis in the form of oxygen reduction reaction. Also, thermally activated reversible structural phase transition modulated the electron transport property in thin film. This work unveils the importance of chemical reactions at solid-liquid interfaces that can lead to the development of new functional thin film materials.

Experimental and Computational Characterization of the Interaction between Gold Nanoparticles and Polyamidoamine Dendrimers

Dendrimers provide a means to control the synthesis of gold nanoparticles and stabilize their suspensions. However, design of improved dendrimers for this application is hindered by a lack of understanding how the dendrimers and synthesis conditions determine nanoparticle morphology and suspension stability. In the present work, we evaluate the effect of polyamidoamine (PAMAM) dendrimers terminated with different functional groups (-OH or -NH₃⁺) and different synthesis conditions on the morphology of the resulting gold nanoparticles and their stability in solution. We leverage molecular dynamics (MD) simulations to identify the atomic interactions that underlie adsorption of PAMAM dendrimers to gold surface and how the thermodynamics of this adsorption depends on the terminal functional groups of the dendrimers. We find that gold nanoparticles formed with hydroxyl-terminated PAMAM (PAMAM-OH) rapidly aggregate, whereas those formed with PAMAM-NH₃⁺ are stable in solution for months of storage. Synthesis under ultrasound sonication is shown to be more rapid than that under agitation, with sonication producing smaller nanoparticles. Free-energy calculations in MD simulations show that all dendrimers have a high affinity for the gold surface, although PAMAM-OH and its oxidized aldehyde form (PAMAM-CHO) have a greater affinity for the nanoparticle surface than PAMAM-NH₃⁺. Although adsorption of PAMAM-OH and PAMAM-CHO has both favorable entropy and enthalpy, adsorption of PAMAM-NH₃⁺ is driven by a strong enthalpic component subject to an unfavorable entropic component.

Experimentally Validated Structures of Supported Metal Nanoclusters on MoS2

In nanometer clusters (NCs), each atom counts. It is the specific arrangement of these atoms that determines the unique size-dependent functionalities of the NCs and hence their applications. Here, we employ a self-consistent, combined theoretical and experimental approach to determine atom-by-atom the structures of supported Pt NCs on MoS₂. The atomic structures are predicted using a genetic algorithm utilizing atomistic force fields and density functional theory, which are then validated using aberration-corrected scanning transmission electron microscopy. We find that relatively small clusters grow with (111) orientation such that Pt[11̅0] is parallel to MoS₂[100], which is different from predictions based on lattice-match for thin-film epitaxy. Other 4d and 5d transition metals show similar behavior. The underpinning of this growth mode is the tendency of the NCs to maximize the metal-sulfur interactions rather than to minimize lattice strain.

Photoinduced Glycerol Oxidation over Plasmonic Au and AuM (M = Pt, Pd and Bi) Nanoparticle-Decorated TiO₂ Photocatalysts

In this study, sol-immobilization was used to prepare gold nanoparticle (Au NP)-decorated titanium dioxide (TiO₂) photocatalysts at different Au weight % (wt. %) loading (Au_x/TiO₂, where x is the Au wt. %) and Au–M NP-decorated TiO₂ photocatalysts (Au₃M₃/TiO₂), where M is bismuth (Bi), platinum (Pt) or palladium (Pd) at 3 wt. %. The Au_x/TiO₂ photocatalysts exhibited a stronger visible light absorption than the parent TiO₂ due to the localized surface plasmon resonance effect. Increasing the Au content from 1 wt. % to 7 wt. % led to increased visible light absorption due to the increasing presence of defective structures that were capable of enhancing the photocatalytic activity of the as-prepared catalyst. The addition of Pt and Pd coupled with the Au₃/TiO₂ to form Au₃M₃/TiO₂ improved the photocatalytic activity of the Au₃/TiO₂ photocatalyst by maximizing their light-absorption property. The Au₃/TiO₂, Au₃Pt₃/TiO₂ and Au₃Pd₃/TiO₂ photocatalysts promoted the formation of glyceraldehyde from glycerol as the principle product, while Au₃Bi₃/TiO₂ facilitated glycolaldehyde formation as the major product. Among all the prepared photocatalysts, Au₃Pd₃/TiO₂ exhibited the highest photocatalytic activity with a 98.75% glycerol conversion at 24 h of reaction time.

Structure, thermodynamics, and rearrangement mechanisms in gold clusters-insights from the energy landscapes framework

We consider finite-size and temperature effects on the structure of model AuN clusters (30 ≤ N ≤ 147) bound by the Gupta potential. Equilibrium behaviour is examined in the harmonic superposition approximation, and the size-dependent melting temperature is also bracketed using molecular dynamics simulations. We identify structural transitions between distinctly different morphologies, characterised by various defect features. Reentrant behaviour and trends with respect to cluster size and temperature are discussed in detail. For N = 55, 85, and 147 we visualise the topography of the underlying potential energy landscape using disconnectivity graphs, colour-coded by the cluster morphology; and we use discrete path sampling to characterise the rearrangement mechanisms between competing structures separated by high energy barriers (up to 1 eV). The fastest transition pathways generally involve metastable states with multiple fivefold disclinations and/or a high degree of amorphisation, indicative of melting. For N = 55 we find that reoptimising low-lying minima using density functional theory (DFT) alters their energetic ordering and produces a new putative global minimum at the DFT level; however, the equilibrium structure predicted by the Gupta potential at room temperature is consistent with previous experiments.

Unraveling Charge State of Supported Au Single-Atoms during CO Oxidation

Thermally stable Au single-atoms supported by monolayered CuO grown at Cu(110) have been successfully prepared. The charge transfer from the CuO support to single Au atoms is confirmed to play a key role in tuning the activity for CO oxidation. Initially, the negatively charged Au single-atom is active for CO oxidation with its adjacent lattice O atom depleted to generate an O vacancy in the CuO monolayer. Afterward, the Au single-atom is neutralized, preventing further CO reaction. The produced O vacancy can be healed by exposure to O₂ at 400 K and accordingly the reaction activity is restored.

Spherical harmonics based descriptor for neural network potentials: Structure and dynamics of Au147 nanocluster

We propose a highly efficient method for fitting the potential energy surface of a nanocluster using a spherical harmonics based descriptor integrated with an artificial neural network. Our method achieves the accuracy of quantum mechanics and speed of empirical potentials. For large sized gold clusters (Au147), the computational time for accurate calculation of energy and forces is about 1.7 s, which is faster by several orders of magnitude compared to density functional theory (DFT). This method is used to perform the global minimum optimizations and molecular dynamics simulations for Au147, and it is found that its global minimum is not an icosahedron. The isomer that can be regarded as the global minimum is found to be 4 eV lower in energy than the icosahedron and is confirmed from DFT. The geometry of the obtained global minimum contains 105 atoms on the surface and 42 atoms in the core. A brief study on the fluxionality in Au147 is performed, and it is concluded that Au147 has a dynamic surface, thus opening a new window for studying its reaction dynamics.

Dispersed Gold Nanoparticle Array Produced by Apoferritins Utilizing Biomineralization and Chemical Conversion

A new method for producing a dispersed gold nanoparticle (Au NP) array to anchor probe DNAs onto a DNA-sensing electrode has been developed. A homogenous gold sulfide (Au₂S) core (precursor of Au NP) was biomineralized in the cavity of a mutant apoferritin (K98E) with enhanced negative outer-surface charges. We employed a slow chemical reaction system utilizing a stable cationic gold complex. K98E could attract the gold complex, and Au₂S NPs were synthesized. K98E enabled dispersed placement of the synthesized Au₂S core onto a cationic 3-aminopropyltriethoxysilane (APTES) layer on a substrate. UV-ozone treatment eliminated the protein shells and APTES layer. X-ray photoelectron spectroscopy confirmed that the Au₂S core was reduced to Au NPs under the same treatment. Atomic force microscopy (AFM) clearly showed that the combination of apoferritin versatility, chemical system design, and UV-ozone treatment successfully produced a dispersed Au NP array on the substrate.

Surface chemistry of group IB metals and related oxides

Catalytic surface chemistry of IB metals are reviewed with an attempt to bridge model catalysts and powder catalysts.

Radical attached aluminum nanoclusters: an alternative way of cluster stabilization

The stability and electronic structure of radical attached aluminum nanoclusters are investigated using density functional theory (DFT). A detailed investigation shows good correlation between the thermodynamic stability of radical attached clusters and the stability of the attached radical anions. All other calculated parameters like HOMO-LUMO gap and charge transfer are also found to be consistent with the observed thermodynamic stabilities of the complexes. Investigation of the electronic structure of radical attached complexes further shows the presence of jellium structures within the core similar to the ligated clusters. Comparison with available experimental and theoretical data also proves the validity of superatomic complex theory for the radical attached clusters as well. Based on the evaluated thermodynamic parameters, selected radical attached clusters are observed to be more thermodynamically stable in comparison with experimentally synthesized ligated clusters. Stabilization of small metal clusters is one of the greatest challenges in current cluster science and the present investigation confirms the fact that radical attached clusters can provide a viable alternative to ligated clusters in the future.

Relative edge energy in the stability of transition metal nanoclusters of different motifs

When a structure is reduced to a nanometer scale, the proportion of the edge atoms increases significantly, which can play a crucial role in determining both their geometric and electronic properties, as demonstrated by the recently established generalized Wulff construction principle [S. F. Li, et al., Phys. Rev. Lett., 2013, 111, 115501]. Consequently, it is of great interest to clarify quantitatively the role of the edge atoms that dominate the motifs of these nanostructures. In principle, establishing an effective method valid for determining the absolute value of the surface energy and particularly the edge energy for a given nanostructure is expected to resolve such a problem. However, hitherto, it is difficult to obtain the absolute edge energy of transition metal clusters, particularly when their sizes approach the nanometer regime. In this paper, taking Ru nanoclusters as a prototypical example, our first-principles calculations introduce the concept of relative edge energy (REE), reflecting the net edge atom effect over the surface (facet) atom effect, which is fairly powerful to quasi-quantitatively estimate the critical size at which the crossover occurs between different configurations of a given motif, such as from an icosahedron to an fcc nanocrystal. By contrast, the bulk effect should be re-considered to rationalize the power of the REE in predicting the relative stability of larger nanostructures between different motifs, such as fcc-like and hcp-like nanocrystals.

Interplay between the spin-selection rule and frontier orbital theory in O2 activation and CO oxidation by single-atom-sized catalysts on TiO2(110)

Distinct chemical activities of a Pd monomer and a Pd₂ dimer supported on TiO₂(110) for O₂ activation.

In situ studies on controlling an atomically-accurate formation process of gold nanoclusters

Knowledge of the molecular formation mechanism of metal nanoclusters is essential for developing chemistry for accurate control over their synthesis. Herein, the "top-down" synthetic process of monodisperse Au13 nanoclusters via HCl etching of polydisperse Aun clusters (15 ≤ n ≤ 65) is traced by a combination of in situ X-ray/UV-vis absorption spectroscopy and time-dependent mass spectrometry. It is revealed experimentally that the HCl-induced synthesis of Au13 is achieved by accurately controlling the etching process with two distinctive steps, in sharp contrast to the traditional thiol-etching mechanism through release of the Au(i) complex. The first step involves the direct fragmentation of the initial larger Aun clusters into metastable intermediate Au8-Au13 smaller clusters. This is a critical step, which allows for the secondary size-growth step of the intermediates toward the atomically monodisperse Au13 clusters via incorporating the reactive Au(i)-Cl species in the solution. Such a secondary-growth pathway is further confirmed by the successful growth of Au13 through reaction of isolated Au11 clusters with AuClPPh3 in the HCl environment. This work addresses the importance of reaction intermediates in guiding the way towards controllable synthesis of metal nanoclusters.

Moiré induced organization of size-selected Pt clusters soft landed on epitaxial graphene

Two-dimensional hexagonal arrays of Pt nanoparticles (1.5 nm diameter) have been obtained by deposition of preformed and size selected Pt nanoparticles on graphene. This original self-organization is induced, at room temperature, by the 2D periodic undulation (the moiré pattern) of graphene epitaxially grown on the Ir(111) surface. By means of complementary techniques (scanning tunneling microscopy, grazing incidence X ray scattering), the Pt clusters shapes and organization are characterized and the structural evolution during annealing is investigated. The soft-landed clusters remain quasi-spherical and a large proportion appears to be pinned on specific moiré sites. The quantitative determination of the proportion of organized clusters reveals that the obtained hexagonal array of the almost spherical nanoparticles is stable up to 650 K, which is an indication of a strong cluster-surface interaction.

Magic-number gold nanoclusters with diameters from 1 to 3.5 nm: relative stability and catalytic activity for CO oxidation

Relative stability of geometric magic-number gold nanoclusters with high point-group symmetry ((Ih), D(5h), O(h)) and size up to 3.5 nm, as well as structures obtained by global optimization using an empirical potential, is investigated using density functional theory (DFT) calculations. Among high-symmetry nanoclusters, our calculations suggest that from Au(147) to Au(923), the stability follows the order Ih > D(5h) > Oh. However, at the largest size of Au(923), the computed cohesive energy differences among high-symmetry I(h), D(5h) and O(h) isomers are less than 4 meV/atom (at PBE level of theory), suggesting the larger high-symmetry clusters are similar in stability. This conclusion supports a recent experimental demonstration of controlling morphologies of high-symmetry Au(923) clusters ( Plant, S. R.; Cao, L.; Palmer, R. E. J. Am. Chem. Soc. 2014, 136, 7559). Moreover, at and beyond the size of Au(549), the face-centered cubic-(FCC)-based structure appears to be slightly more stable than the Ih structure with comparable size, consistent with experimental observations. Also, for the Au clusters with the size below or near Au(561), reconstructed icosahedral and decahedral clusters with lower symmetry are slightly more stable than the corresponding high-symmetry isomers. Catalytic activities of both high-symmetry and reconstructed I(h)-Au(147) and both Ih-Au(309) clusters are examined. CO adsorption on Au(309) exhibits less sensitivity on the edge and vertex sites compared to Au(147), whereas the CO/O2 coadsorption is still energetically favorable on both gold nanoclusters. Computed activation barriers for CO oxidation are typically around 0.2 eV, suggesting that the gold nanoclusters of ∼ 2 nm in size are highly effective catalysts for CO oxidation.

Fluorescence and room temperature activity of Y2O3:(Eu3+,Au3+)/palygorskite nanocomposite

Palygorskite supported Y₂O₃:(Eu³⁺,Au³⁺) nanocomposites show simultaneous fluorescence and catalytic activities.

Gold nanoparticles decorated on a graphene-periodic mesoporous silica sandwich nanocomposite as a highly efficient and recyclable heterogeneous catalyst for catalytic applications

Ultra-small Au nanoparticles (Au NPs) were immobilized on HS-functionalized sandwich-like periodic mesoporous silica (PMS) coated graphene (G) to form the novel nanocomposite catalyst Au NPs@HS-G-PMS.

Au encapsulated into Al-MCM-41 mesoporous material: in situ synthesis and electronic structure

A facile one-step technique is proposed for the successful synthesis of highly ordered Au/Al-MCM-41. The charge state of Au³⁺ in the mesoporous framework was partially reduced due to the accompanying Al when clay was used as source.

Role of size and shape selectivity in interaction between gold nanoclusters and imidazole: a theoretical study

We present a theoretical study on the structure, stability, spectra and electronic properties of imidazole (Im) adsorbed on gold nanoclusters (Aun, n = 2, 4, 6, 8, 10, and 20). These computations were performed using various density functional theories with and without inclusion of Grimme's (D3) dispersion correction. For small clusters, we also carried out wavefunction-based ab initio (MP2 and SCS-MP2) computations for comparison. Vibrational, atoms in molecules (AIM) and natural bond orbital (NBO) analyses clearly reveal the occurrence of charge transfer (CT) through covalent (N1-Au) and noncovalent interactions that play important roles in the stability of the Im@Aun complexes with anchor assisted H-bonds (Cα-H · Au). Therefore, gold clusters can act as H-bond acceptors with biomolecules for development of new materials and applications. Our study establishes also the ability and reliability of PBE0 and M05-2X functionals compared to B3LYP and PBE for an accurate description of covalent and noncovalent interactions between Im and gold clusters since they lead to close agreement with MP2. Finally, we show that the Au8 cluster may be viewed as large enough to mimic the 3D gold surface.

Synergetic approach for simple and rapid conjugation of gold nanoparticles with oligonucleotides

Attaching thiolated DNA on gold nanoparticles (AuNPs) has been extremely important in nanobiotechnology because DNA-AuNPs combine the programmability and molecular recognition properties of the biopolymers with the optical, thermal, and catalytic properties of the inorganic nanomaterials. However, current standard protocols to attach thiolated DNA on AuNPs involve time-consuming, tedious steps and do not perform well for large AuNPs, thereby greatly restricting applications of DNA-AuNPs. Here we demonstrate a rapid and facile strategy to attach thiolated DNA on AuNPs based on the excellent stabilization effect of mPEG-SH on AuNPs. AuNPs are first protected by mPEG-SH in the presence of Tween 20, which results in excellent stability of AuNPs in high ionic strength environments and extreme pHs. A high concentration of NaCl can be applied to the mixture of DNA and AuNP directly, allowing highly efficient DNA attachment to the AuNP surface by minimizing electrostatic repulsion. The entire DNA loading process can be completed in 1.5 h with only a few simple steps. DNA-loaded AuNPs are stable for more than 2 weeks at room temperature, and they can precisely hybridize with the complementary sequence, which was applied to prepare core-satellite nanostructures. Moreover, cytotoxicity assay confirmed that the DNA-AuNPs synthesized by this method exhibit lower cytotoxicity than those prepared by current standard methods. The proposed method provides a new way to stabilize AuNPs for rapid and facile loading thiolated DNA on AuNPs and will find wide applications in many areas requiring DNA-AuNPs, including diagnosis, therapy, and imaging.

Gold Catalysis in the Complete Oxidation or Decomposition of Small Molecule Pollutants

Supported gold catalysts are useful for the elimination of small molecule pollutants at low temperature. Catalytic oxidation and decomposition are ways to eliminate these air pollutants. The complete oxidation of CO, ethylene and formaldehyde to CO2 over supported gold catalysts, which can be achieved at room temperature or lower, has been studied widely and in depth. Some research has focused on the decomposition of ozone, N2O and NO over supported gold catalysts. The mechanism of catalysis by supported gold material has been elucidated for the above mentioned reactions.

Enhancing catalytic performance of Au catalysts by noncovalent functionalized graphene using functional ionic liquids

New catalyst, prepared through Au nanoparticles anchored on the Ionic Liquid of 3,4,9,10-perylene tetracarboxylic acid-noncovalent functionalized graphene (Au/PDIL-GS), was fabricated using a facile and environment-friendly approach. The information of the morphologies, sizes, dispersion of Au nanoparticles (NPs) and chemical composition for the as-prepared catalysts was verified by systematic characterizations, including transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), Raman spectra, X-ray diffraction (XRD) and X-Ray photoelectron spectroscopy (XPS). As a new catalyst, the resulting Au/PDIL-GS exhibited excellent catalytic activity in the reduction of 4-nitrophenol because of the synergistic effect between the PDIL-GS and Au NPs. The facile and environment-friendly approach provides a green way to effectively synthesize low cost Au-based catalysts for 4-NP reduction and is promising for the development of other useful materials.