Ligand Effects on the Structure and the Electronic Optical Properties of Anionic Au₂₅(SR)₁₈ Clusters

This study addresses how ligands module the structure and the electronic optical properties of a large set of the experimentally known anionic thiolate-protected gold clusters, Au₂₅(SR)₁₈^[1−]. Starting from the experimental crystal structure, computational density functional theory calculations reveal that low-polarity R groups do not disturb the Au₂₅S₁₈ framework significantly, such that the inversion symmetry­(<i>C</i>_<i>i</i>) of the crystalline state is retained. In the case of <i>p</i>-thiolphenolate ligands, <i>p</i>-SPhX, a major distortion of the Au₂₅S₁₈ framework, destroys the inversion symmetry, the distortion increasing in the order given X = H, Cl, NO₂ and CO₂H. For branched R groups, linking −CH₃ or −NH₂ groups at the two-position of the phenylethylthiolate ligand, the inversion symmetry is retained and lost, respectively; similarly, the <i>N</i>-acetyl-cysteine ligand also distorts the framework. These results demonstrate a systematic preference of inversion-symmetric versus nonsymmetric framework depending on the ligand-type. The more distorted structures also exhibit significantly reduced HOMO–LUMO gap values and affect the optical absorption spectra accordingly. This study correlates the distortion of the Au₂₅S₁₈ framework with the structure, electronic, and optical properties among the studied clusters

La Tierra de Segovia : diario independiente: Año I Número 135 - 1919 octubre 21

Copia digital. Madrid : Ministerio de Cultura. Subdirección General de Coordinación Bibliotecaria, 200

Metallic Two-Dimensional Nanoframes: Unsupported Hierarchical Nickel–Platinum Alloy Nanoarchitectures with Enhanced Electrochemical Oxygen Reduction Activity and Stability

Electrochemical oxygen reduction reaction (ORR) catalysts that have both high activities and long-term stabilities are needed for proton-exchange membrane fuel cells (PEMFCs) and metal–air batteries. Two-dimensional (2D) materials based on graphene have shown high catalytic activities, however, carbon-based materials result in significant catalyst degradation due to carbon oxidation that occurs at high electrochemical potentials. Here, we introduce the synthesis and electrochemical performance of metallic 2D nanoframes which represent a new approach to translate 2D materials into unsupported (carbon-free) electrocatalysts that have both significantly higher ORR catalytic activities and stabilities compared with conventional Pt/carbon electrocatalysts. Metallic Ni–Pt 2D nanoframes were synthesized by controlled thermal treatments of Pt-decorated Ni­(OH)₂ nanosheets. The nanoframes consist of a hierarchical 2D framework composed of a highly catalytically active Pt–Ni alloy phase with an interconnected solid and pore network that results in three-dimensional molecular accessibility. The inclusion of Ni within the Pt structure resulted in significantly smaller Pt lattice distances compared to those of Pt nanoparticles. On the basis of its unique local and extended structure, the ORR specific activity of Ni–Pt 2D nanoframes (5.8 mA cm_Pt^–2) was an order of magnitude higher than Pt/carbon. In addition, accelerated stability testing at elevated potentials up to 1.3 V_RHE showed that the metallic Ni–Pt nanoframes exhibit significantly improved stability compared with Pt/carbon catalysts. The nanoarchitecture and local structure of metallic 2D nanoframes results in high combined specific activity and elevated potential stability. Analysis of the ORR electrochemical reaction kinetics on the Ni–Pt nanoframes supports that at low overpotentials the first electron transfer is the rate-determining step, and the reaction proceeds via a four electron reduction process. The ability to create metallic 2D structures with 3D molecular accessibility opens up new opportunities for the design of high activity and stability carbon-free catalyst nanoarchitectures for numerous electrocatalytic and catalytic applications

Quantitative Analysis of Structure and Bandgap Changes in Graphene Oxide Nanoribbons during Thermal Annealing

Graphene oxide nanoribbons (GONRs) are wide bandgap semiconductors that can be reduced to metallic graphene nanoribbons. The transformation of GONRs from their semiconductive to the metallic state by annealing has attracted significant interest due to its simplicity. However, the detailed process by which GONRs transform from wide-bandgap semiconductors to semimetals with a near zero bandgap is unclear. As a result, precise control of the bandgap between these two states is not currently achievable. Here, we quantitatively examine the removal of oxygen-containing groups and changes in the bandgap during thermal annealing of GONRs. X-ray photoelectron spectroscopy measurements show the progressive removal of oxygen-containing functional groups. Aberration-corrected scanning transmission electron microscopy reveals that initially small graphene regions in GONRs become large stacked graphitic layers during thermal annealing. These structural and chemical changes are correlated with progressive changes in the electrochemical bandgap, monitored by cyclic voltammetry. These results show that small changes in the thermal annealing temperature result in significant changes to the bandgap and chemical composition of GONRs and provide a straightforward method for tuning the bandgap in oxidized graphene structures

ESI-MS Identification of Abundant Copper–Gold Clusters Exhibiting High Plasmonic Character

The protected noble-metal structures comprising 145 metal-atom sites and 60 ligands are among the frequently identified larger metal-cluster systems exploited in many avenues of research. Herein we report a comparative electrospray ionization-mass spectrometry (ESI-MS) investigation of the 60-fold thiolated Au₁₄₄ and CuAu₁₄₄ clusters, in various positive charge-states, in conjunction with a density-functional theoretical (DFT) analysis based upon the icosahedral Pd₁₄₅-structure-type applicable to these systems. Samples rich in the hexanethiolate-protected CuAu₁₄₄ clusters are obtained via a single-phase reduction process. The predicted electronic structure of the vacancy-centered Au₁₄₄(SR)₆₀ system provided a simple rationale for the limiting [4+] charge-state observed of Au₁₄₄, whereas the maximal [3+] charge detected on the CuAu₁₄₄(SR)₆₀ cluster can be explained if the 145th atom occupies the central site. Occupancy of the center-site stabilizes the superatomic 3S-orbital, and thereby shifts the shell-closing count from 82 to 84 free electrons. The DFT-calculated energetics also predicts a strong (0.65 eV) preference for placing the smaller Cu ion in this central site. Remarkably, the optical absorption spectra of dilute tetrahydrofuran (THF) solutions feature a broad band centered near 2.3 eV, in contrast to the previously reported “nonplasmonic” response of sub-2.0-nm all-gold or -copper clusters. Other methods (matrix-assisted laser desorption ionization mass spectrometry and high-resolution electron microscopy) were used to investigate whether aggregation phenomena might account for this observed plasmon emergence. This unusual result points to the need to obtain highly purified samples of copper-doped gold clusters of ca. 145 atoms total

STEM Electron Diffraction and High-Resolution Images Used in the Determination of the Crystal Structure of the Au₁₄₄(SR)₆₀ Cluster

Determination of the total structure of molecular nanocrystals is an outstanding experimental challenge that has been met, in only a few cases, by single-crystal X-ray diffraction. Described here is an alternative approach that is of most general applicability and does not require the fabrication of a single crystal. The method is based on rapid, time-resolved nanobeam electron diffraction (NBD) combined with high-angle annular dark field scanning/transmission electron microscopy (HAADF-STEM) images in a probe corrected STEM microscope, operated at reduced voltages. The results are compared with theoretical simulations of images and diffraction patterns obtained from atomistic structural models derived through first-principles density functional theory (DFT) calculations. The method is demonstrated by application to determination of the structure of the Au₁₄₄(SCH₂CH₂Ph)₆₀ cluster