Interaction of Au₁₆ Nanocluster with Defects in Supporting Graphite: A Density-Functional Study

Soft-landed adsorption of Au16 on bilayered graphene is investigated using density functional theory. The orientation of the Au16 cluster and number of neighboring surface vacancies affect the overall structural and electronic properties of the cluster. The results of the PBE, vdW-DF, and vdW-DF2 exchange-correlation functionals are compared for the cluster-substrate interaction for systems with and without defects. In the presence of defects size two and greater, an Au atom adsorbs into the topmost graphene layer; this strongly influences the binding energy (>3 eV), while inducing substantial bending in the carbon plane and altering electronic properties of the system. Though the Td-symmetry and electronegative properties of the Au16 structure change in the presence of greater neighboring defects, elements of the cagelike starting structure remain throughout. The electron localization function shows that the in-plane Au–C bonds are of delocalized (metallic) nature and there is a local charge transfer to the coordinating Au. However, the net charge transfer between adsorbate and substrate is considerable only for the defect-free case (0.8e to Au16). Finally, the binding of O2 molecules to the adsorbed Au16 cluster is used to probe the potential catalytic activity of graphite and carbon nanotube systems, and in one case (for defect size two) the adsorbed O2 switches on the catalytically active superoxo-state

Superatom Model for Ag–S Nanocluster with Delocalized Electrons

Several Ag–S nanoclusters where the cluster core comprises mixed metal (main component) and sulfur atoms show superatomic orbitals in the conduction band edge. However, there are no superatomic states, i.e., delocalized electrons, in the valence band, and the clusters in question can be labeled as “zerovalent”. We show here an example of an Ag–S cluster which fulfills the superatom model and has delocalized electrons: The recently synthesized and characterized [Ag₆₂S₁₂(S<i>t</i>Bu)₃₂]²⁺ cluster has four delocalized valence electrons based on a simple counting rule, and we compare it to the zerovalent cluster [Ag₆₂S₁₃(S<i>t</i>Bu)₃₂]⁴⁺. Our electronic structure analysis confirms the existence of superatomic states in the valence and conduction bands, but the locations of these states do not agree completely with the conventional prediction based on the spherical Jellium model. [Ag₆₂S₁₂(S<i>t</i>Bu)₃₂]²⁺ displays the 1S² electronic shell closure at the Fermi energy instead of the 1S²1P² configuration as suggested by its electron count. This shift of energy levels and electron shell closing has been introduced by the core–shell structure of the cluster. Our optical absorption simulation can reproduce the features observed in the experiments, and we assign these features to the transitions involving superatomic states within the conduction band

Catalytic Activity of AuCu Clusters on MgO(100): Effect of Alloy Composition for CO Oxidation

Density functional simulations have been performed for Au7Cu23 and Au23Cu7 clusters on MgO(100) supports to probe their catalytic activity for CO oxidation. The adsorption of reactants, O2 and CO, and potential O2 dissociation have been investigated in detail by tuning the location of vacancies (F-center, V-center) in MgO(100). The total charge on Au7Cu23 and Au23Cu7 is negative on all supports, regardless of the presence of vacancies, but the effect is significantly amplified on the F-center. Au7Cu23/MgO­(100) and Au23Cu7/MgO­(100) with an F-center are the only systems to bind O2 more strongly than CO. In each case, O2 can be effectively activated upon adsorption and dissociated to 2 × O atoms. The different reaction paths based on the Langmuir–Hinshelwood (LH) and Eley–Rideal (ER) mechanisms for CO oxidation have been explored on the Au7Cu23 and Au23Cu7 clusters on F-centers, and the results are compared with the previous findings for Au15Cu15. Overall, the reaction barriers are small, but the changes in the Au:Cu ratio tune the reactant adsorption energies and sites considerably, showing also varying selectivity for CO and O2. The microkinetic model built on the basis of the above results shows a pronounced CO2 production rate at low temperature for the clusters on F-centers

On the Structure of a Thiolated Gold Cluster: Au₄₄(SR)₂₈²⁻

Many thiolate-protected gold clusters prepared by wet-chemistry show abundances of certain compositions which can be explained by the shell-closing of the superatom orbitals 1S, 1P, 1D, ..., leading to magic-number series 2, 8, 18, 20, 34, 58, etc. One recently isolated such cluster, Au44(SPh)282−, is a potential candidate for the magic number 18, although its structure has not been determined. Applying the “divide-and-protect” concept and recent knowledge obtained from the structures of Au25(SR)18− and Au102(SR)44 (-SR being a thiolate group), we compare two structural models for the Au44(SR)282− cluster. We have optimized their structures, computed powder X-ray diffraction patterns and optical absorption spectra, and performed the superatom analysis on them. The model featuring the -RS-Au-SR- and -RS-Au-SR-Au-SR- motifs in the protective layer shows better energetic stability and agreement with the experimental XRD pattern than the other model which has a protective layer including longer, polymeric RS(AuSR)x motifs. However, the computed optical spectra for both models are quite different from the experimental one. Our models here can serve as benchmarks for further proposals of Au44(SR)282− cluster structures

How do Water Solvent and Glutathione Ligands Affect the Structure and Electronic Properties of Au₂₅(SR)₁₈^–?

The effects of aqueous solvent and biological ligands on the structural and electronic properties of thiolate-protected Au₂₅(SR)₁₈^– clusters have been studied by performing quantum mechanics/molecular mechanics (QM/MM) simulations. Analysis of bond distances and angles show that the solvated nanocluster experiences modest structural changes, which are reflected as flexibility of the Au core. The hydrophilic glutathione ligands shield the metallic core effectively and distort its symmetry via sterical hindrance effects. We show that the previously reported agreement between the calculated HOMO–LUMO gap of the cluster and the optical measurement is due to cancellation of errors, where the typical underestimation of the theoretical band gap compensates the effect of the missing solvent. The use of a hybrid functional results in a HOMO–LUMO gap value of 1.5 eV for the solvated nanocluster with glutathione ligands, in good agreement with optical measurements. Our results demonstrate that ligand/solvent effects should be considered for a proper comparison between theory and experiment

How do Water Solvent and Glutathione Ligands Affect the Structure and Electronic Properties of Au₂₅(SR)₁₈^–?

The effects of aqueous solvent and biological ligands on the structural and electronic properties of thiolate-protected Au₂₅(SR)₁₈^– clusters have been studied by performing quantum mechanics/molecular mechanics (QM/MM) simulations. Analysis of bond distances and angles show that the solvated nanocluster experiences modest structural changes, which are reflected as flexibility of the Au core. The hydrophilic glutathione ligands shield the metallic core effectively and distort its symmetry via sterical hindrance effects. We show that the previously reported agreement between the calculated HOMO–LUMO gap of the cluster and the optical measurement is due to cancellation of errors, where the typical underestimation of the theoretical band gap compensates the effect of the missing solvent. The use of a hybrid functional results in a HOMO–LUMO gap value of 1.5 eV for the solvated nanocluster with glutathione ligands, in good agreement with optical measurements. Our results demonstrate that ligand/solvent effects should be considered for a proper comparison between theory and experiment

How do Water Solvent and Glutathione Ligands Affect the Structure and Electronic Properties of Au₂₅(SR)₁₈^–?

The effects of aqueous solvent and biological ligands on the structural and electronic properties of thiolate-protected Au₂₅(SR)₁₈^– clusters have been studied by performing quantum mechanics/molecular mechanics (QM/MM) simulations. Analysis of bond distances and angles show that the solvated nanocluster experiences modest structural changes, which are reflected as flexibility of the Au core. The hydrophilic glutathione ligands shield the metallic core effectively and distort its symmetry via sterical hindrance effects. We show that the previously reported agreement between the calculated HOMO–LUMO gap of the cluster and the optical measurement is due to cancellation of errors, where the typical underestimation of the theoretical band gap compensates the effect of the missing solvent. The use of a hybrid functional results in a HOMO–LUMO gap value of 1.5 eV for the solvated nanocluster with glutathione ligands, in good agreement with optical measurements. Our results demonstrate that ligand/solvent effects should be considered for a proper comparison between theory and experiment

Improving the Adsorption of Au Atoms and Nanoparticles on Graphite via Li Intercalation

Supported nanoclusters have an important future in chemical processes such as catalysis. However, to optimize the properties of supported nanoclusters, attention must be paid to the electronic properties of both adsorbate and substrate materials. Highly ordered pyrolytic graphite is commonly used as a substrate for Au nanoclusters; however, cluster functionality and mobility is a problem on this inert surface. Therefore, we have designed a model for Li-doped graphite and investigated the electronic properties of adsorbed Au atoms and nanoclusters on this material using density functional theory (DFT). We find that increasing the concentration of Li atoms in the substrate results in improved adsorption for both Au atoms and Au₁₆ nanoclusters onto the surface, with adsorption energies up to 0.96 and 1.50 eV, respectively, when using the Perdew, Burke, and Ernzerhof (PBE) exchange-correlation functional. In the case of the Au₁₆ nanocluster, charge transfer of >1 <i>e</i> is computed, which should make this supported system functionally suitable for reactions such as CO oxidation. Furthermore, a pseudoionic bond is observed in some cases for atomic Au over a surface C atom, though the presence of such chemical interaction is dependent on the exchange-correlation functional used

Silver Sulfide Nanoclusters and the Superatom Model

The superatom model of electron-shell closings has been widely used to explain the stability of noble-metal nanoclusters of few nanometers, including thiolate-protected Au and Ag nanoclusters. The presence of core sulfur atoms in silver sulfide (Ag–S) nanoclusters renders them a class of clusters with distinctive properties as compared to typical noble-metal clusters. Here, it is natural to ask whether the superatom model is still applicable for the Ag–S nanoclusters with mixed metal and nonmetal core atoms. To address this question, we applied density functional simulations to analyze a series of Ag–S nanoclusters: Ag₁₄S­(SPh)₁₂(PPh₃)₈, Ag₁₄(SC₆H₃F₂)₁₂(PPh₃)₈, Ag₇₀S₁₆(SPh)₃₄(PhCO₂)₄(triphos)₄, and [Ag₁₂₃S₃₅(S<i>t</i>Bu)₅₀]³⁺. We observed that superatomic orbitals are still present in the conduction band of these Ag–S clusters where the cluster cores comprise mostly silver atoms. Our Bader charge analysis illustrates that thiolates play a significant role in withdrawing charge (electron density) from the core Ag atoms. The simulated optical absorption properties of the selected Ag–S clusters reflect the substantial band gaps associated with typical molecular orbitals on both sides. Apart from Ag₁₄S­(SPh)₁₂(PPh₃)₈, which has a central sulfur atom in the cluster core, superatomic orbitals of the Ag–S clusters can have contributions for individual transitions in the conduction band

Coexisting Honeycomb and Kagome Characteristics in the Electronic Band Structure of Molecular Graphene

We uncover the electronic structure of molecular graphene produced by adsorbed CO molecules on a copper (111) surface by means of first-principles calculations. Our results show that the band structure is fundamentally different from that of conventional graphene, and the unique features of the electronic states arise from coexisting honeycomb and Kagome symmetries. Furthermore, the Dirac cone does not appear at the K-point but at the Γ-point in the reciprocal space and is accompanied by a third, almost flat band. Calculations of the surface structure with Kekulé distortion show a gap opening at the Dirac point in agreement with experiments. Simple tight-binding models are used to support the first-principles results and to explain the physical characteristics behind the electronic band structures