Size, Stoichiometry, Dimensionality, and Ca Doping of Manganese Oxide-Based Water Oxidation Clusters: An Oxyl/Hydroxy Mechanism for Oxygen-Oxygen Coupling.

Gas-phase ion-trap reactivity experiments and density functional simulations reveal that water oxidation to H2O2 mediated by (calcium) manganese oxide clusters proceeds via formation of a terminal oxyl radical followed by oxyl/hydroxy O-O coupling. This mechanism is predicted to be energetically feasible for Mn2Oy+ (y = 2-4) and the binary CaMn3O4+, in agreement with the experimental observations. In contrast, the reaction does not proceed for the tetramanganese oxides Mn4Oy+ (y = 4-6) under these experimental conditions. This is attributed to the high fluxionality of the tetramanganese clusters, resulting in the instability of the terminal oxyl radical as well as an energetically unfavorable change of the spin state required for H2O2 formation. Ca doping, yielding a symmetry-broken lower-symmetry three-dimensional (3D) CaMn3O4+ cluster, results in structural stabilization of the oxyl radical configuration, accompanied by a favorable coupling between potential energy surfaces with different spin states, thus enabling the cluster-mediated water oxidation reaction and H2O2 formation.

The Missing Link: Au191(SPh-tBu)66 Janus Nanoparticle with Molecular and Bulk-Metal-like Properties.

Understanding the evolution of the structure and properties in metals from molecule-like to bulk-like has been a long sought fundamental question in science, since Faraday's 1857 work. We report the discovery of a Janus nanomolecule, Au191(SPh-tBu)66 having both molecular and metallic characteristics, explored crystallographically and optically and modeled theoretically. Au191 has an anisotropic, singly twinned structure with an Au155 core protected by a ligand shell made of 24 monomeric [-S-Au-S-] and 6 dimeric [-S-Au-S-Au-S-] staples. The Au155 core is composed of an 89-atom inner core and 66 surface atoms, arranged as [Au3@Au23@Au63]@Au66 concentric shells of atoms. The inner core has a monotwinned/stacking-faulted face-centered-cubic (fcc) structure. Structural evolution in metal nanoparticles has been known to progress from multiply twinned, icosahedral, structures in smaller molecular sizes to untwinned bulk-like fcc monocrystalline nanostructures in larger nanoparticles. The monotwinned inner core structure of the ligand capped Au191 nanomolecule provides the critical missing link, and bridges the size-evolution gap between the molecular multiple-twinning regime and the bulk-metal-like particles with untwinned fcc structure. The Janus nature of the nanoparticle is demonstrated by its optical and electronic properties, with metal-like electron-phonon relaxation and molecule-like long-lived excited states. First-principles theoretical explorations of the electronic structure uncovered electronic stabilization through the opening of a shell-closing gap at the top of the occupied manifold of the delocalized electronic superatom spectrum of the inner core. The electronic stabilization together with the inner core geometric stability and the optimally stapled ligand-capping anchor and secure the stability of the entire nanomolecule.

Highly Ordered Boron Nitride/Epigraphene Epitaxial Films on Silicon Carbide by Lateral Epitaxial Deposition.

The realization of high-performance nanoelectronics requires control of materials at the nanoscale. Methods to produce high quality epitaxial graphene (EG) nanostructures on silicon carbide are known. The next step is to grow van der Waals semiconductors on top of EG nanostructures. Hexagonal boron nitride (h-BN) is a wide bandgap semiconductor with a honeycomb lattice structure that matches that of graphene, making it ideally suited for graphene-based nanoelectronics. Here, we describe the preparation and characterization of multilayer h-BN grown epitaxially on EG using a migration-enhanced metalorganic vapor phase epitaxy process. As a result of the lateral epitaxial deposition (LED) mechanism, the grown h-BN/EG heterostructures have highly ordered epitaxial interfaces, as desired in order to preserve the transport properties of pristine graphene. Atomic scale structural and energetic details of the observed row-by-row growth mechanism of the two-dimensional (2D) epitaxial h-BN film are analyzed through first-principles simulations, demonstrating one-dimensional nucleation-free-energy-barrierless growth. This industrially relevant LED process can be applied to a wide variety of van der Waals materials.

Co-adsorption of O2 and C2H4 on a Free Gold Dimer Probed via Infrared Photodissociation Spectroscopy.

Infrared multiple photon dissociation (IR-MPD) spectroscopy in conjunction with density functional theory (DFT) calculations has been employed to study the activation of molecular oxygen and ethylene co-adsorbed on a free gold dimer cation Au2+. Both studied complexes, Au2O2(C2H4)+ and Au2O2(C2H4)2+, show distinct features of both intact O2 and ethylene co-adsorbed on the cluster. However, the ethylene C=C double bond is activated, increasing in length by up to 0.07 Å compared with the free molecule, and the red shift of the O-O vibration frequency increases with the number of adsorbed ethylene molecules, indicating a small but increasing activation of the O-O bond. The small O2 activation and the rather weak interaction between O2 and C2H4 are also reflected in the calculated electronic structure of the co-adsorption complexes which shows only a small occupation of the empty anti-bonding O2 2π*2p orbital as well as the localization of most of the Kohn-Sham orbitals on O2 and C2H4, respectively, with only limited mixing between O2 and C2H4 orbitals. The results are compared with theoretical studies on neutral AuxO2(C2H4) (x = 3, 5, 7, 9) complexes.

Chemistry and Structure of Silver Molecular Nanoparticles.

Silver and gold molecular nanoparticles (mNPs) are a relatively new class of molecular materials of fundamental interest. They are high-nuclearity metal-organic compounds, with ligated metal cores, where the different character of bonding in the ligand shell and metal core gives rise to many of the unique properties of these materials. Research has primarily focused on gold mNPs, due to their good stability and the ease with which they may be synthesized and processed. To understand these materials as a general class, however, it will be necessary to broaden research efforts to other metals. Gold and silver are isoelectronic and have the same atomic radius, making the comparison of gold and silver mNPs attractive. The optical and chemical differences of the two metals provide useful contrasts, however, as well as a means to access a wider range of properties. In this Account, we focus on the synthesis, structure, and reactivity of silver mNPs. First, we review the origins and history of the field, from the ill-defined gas-phase metal clusters of the 1980s to the precisely defined mNPs of 1996 and onward. Next, we discuss the role of silver as a complement to gold mNPs in the effort to generalize lessons learned from either material and extend them into new metals. The synthesis of silver mNPs is covered in some detail, noting the choices made as the chemistry and the materials were developed. The importance of coordinating solvents and thermodynamic stability are also noted. The need to reduce solvent use is discussed and a new approach to achieving this goal is presented. Next, the structures of silver mNPs are discussed, including the Ag44 and Ag17 archetypes, and focusing on the successful de novo structure prediction of the latter. Structure and prediction of ligand shell motifs are also discussed. Finally, the postsynthetic chemistry and reactivity of silver mNPs are presented, including some of the first efforts to elucidate reaction mechanisms, beginning in 2012. Silver nanoparticles are gaining in popularity, particularly compared with gold, as the potential for silver to make a technological and economic impact is recognized. The superior optical properties of silver already make it a valuable material for plasmonics, but this may also translate to molecular species for nonlinear optics, sensors, and optoelectronics. The higher reactivity may also lead to a greater diversity of chemistry for silver compared to gold, including as an important broad-spectrum antimicrobial. Conversely, the "ultrastability" of the Ag44 archetype has already enabled unprecedented scale up with molecular precision, and may lead to the first industrial-scale production of metal mNPs. Clearly, silver mNPs are one of the most promising and significant new materials being studied today.

The interaction of ethylene with free gold cluster cations: infrared photodissociation spectroscopy combined with electronic and vibrational structure calculations.

The interaction of ethylene with free gold clusters of different sizes and charge states has been previously shown theoretically to involve two different adsorption modes of the C2H4 molecule, namely: the di-σ- and π-bonded ethylene adsorption configurations. Here, we present the first experimental investigation of the structure of a series of gas-phase gold-ethylene complexes, [Formula: see text]. By employing infrared multiple-photon dissociation spectroscopy in conjunction with first-principles calculations it is revealed that up to three C2H4 molecules preferably bind to gold cations in a π-bonded configuration. The binding of all ethylene molecules is found to be dominated by partial electron donation from the ethylene molecules to the gold clusters leading to an activation of the C-C bond. The cooperative action of multiple coadsorbed C2H4 on [Formula: see text] is shown to enable additional charge back-donation and an enhanced C-C bond activation. In contrast, the strong C-H bond is not weakened and the experimental spectra do not give any indication for C-H bond dissociation. The possible correlations of the C-C bond stretch vibration with the C-C bond length and the net charge transfer are discussed.

M4Au12Ag32(p-MBA)30 (M = Na, Cs) bimetallic monolayer-protected clusters: synthesis and structure.

Crystals of M4Au12Ag32(p-MBA)30 bimetallic monolayer-protected clusters (MPCs), where p-MBA is p-mercapto-benzoic acid and M+ is a counter-cation (M = Na, Cs) have been grown and their structure determined. The mol-ecular structure of triacontakis[(4-carboxylatophenyl)sulfanido]dodecagolddotriacontasilver, Au12Ag32(C7H5O2S)30 or C210H150Ag32Au12O60S30, exhib-its point group symmetry at 100 K. The overall diameter of the MPC is approximately 28 Å, while the diameter of the Au12Ag20 metallic core is 9 Å. The structure displays ligand bundling and inter-molecular hydrogen bonding, which gives rise to a framework structure with 52% solvent-filled void space. The positions of the M+ cations and the DMF solvent mol-ecules within the void space of the crystal could not be determined. Three out of the five crystallographically independent ligands in the asymmetric unit cell are disordered over two sets of sites. Comparisons are made to the all-silver M4Ag44(p-MBA)30 MPCs and to expectations based on density functional theory.

Controlling Ethylene Hydrogenation Reactivity on Pt13 Clusters by Varying the Stoichiometry of the Amorphous Silica Support.

Ethylene hydrogenation was investigated on size-selected Pt13 clusters supported on three amorphous silica (a-SiO2 ) thin films with different stoichiometries. Activity measurements of the reaction at 300 K revealed that on a silicon-rich and a stoichiometric film, Pt13 exhibits a similar activity to that of Pt(111), in line with the known structure insensitivity of the reaction. On an oxygen-rich film, a threefold increased rate was measured. Pulsing ethylene at 400 K, then measuring the activity at 300 K, resulted in complete loss of activity on the silicon-rich surface compared to only marginal losses on the other surfaces. The measured reactivity trends correlate with charging characteristics of a Pt13 cluster on the SiO2 films, predicted through first-principle calculations. The results reveal that the stoichiometry-dependent charging by the support can be used to tune the selectivity of reaction pathways during a catalytic hydrogenation reaction.

Structure sensitivity in the nonscalable regime explored via catalysed ethylene hydrogenation on supported platinum nanoclusters.

The sensitivity, or insensitivity, of catalysed reactions to catalyst structure is a commonly employed fundamental concept. Here we report on the nature of nano-catalysed ethylene hydrogenation, investigated through experiments on size-selected Ptn (n=8-15) clusters soft-landed on magnesia and first-principles simulations, yielding benchmark information about the validity of structure sensitivity/insensitivity at the bottom of the catalyst size range. Both ethylene-hydrogenation-to-ethane and the parallel hydrogenation-dehydrogenation ethylidyne-producing route are considered, uncovering that at the <1 nm size-scale the reaction exhibits characteristics consistent with structure sensitivity, in contrast to structure insensitivity found for larger particles. The onset of catalysed hydrogenation occurs for Ptn (n ≥ 10) clusters at T>150 K, with maximum room temperature reactivity observed for Pt13. Structure insensitivity, inherent for specific cluster sizes, is induced in the more active Pt13 by a temperature increase up to 400 K leading to ethylidyne formation. Control of sub-nanometre particle size may be used for tuning catalysed hydrogenation activity and selectivity.

Confirmation of a de novo structure prediction for an atomically precise monolayer-coated silver nanoparticle.

Fathoming the principles underpinning the structures of monolayer-coated molecular metal nanoparticles remains an enduring challenge. Notwithstanding recent x-ray determinations, coveted veritable de novo structural predictions are scarce. Building on recent syntheses and de novo structure predictions of M3Au x Ag17-x (TBBT)12, where M is a countercation, x = 0 or 1, and TBBT is 4-tert-butylbenzenethiol, we report an x-ray-determined structure that authenticates an a priori prediction and, in conjunction with first-principles theoretical analysis, lends force to the underlying forecasting methodology. The predicted and verified Ag(SR)3 monomer, together with the recently discovered Ag2(SR)5 dimer and Ag3(SR)6 trimer, establishes a family of unique mount motifs for silver thiolate nanoparticles, expanding knowledge beyond the earlier-known Au-S staples in thiol-capped gold nanoclusters. These findings demonstrate key principles underlying ligand-shell anchoring to the metal core, as well as unique T-like benzene dimer and cyclic benzene trimer ligand bundling configurations, opening vistas for rational design of metal and alloy nanoparticles.

M3Ag17(SPh)12 Nanoparticles and Their Structure Prediction.

Although silver nanoparticles are of great fundamental and practical interest, only one structure has been determined thus far: M4Ag44(SPh)30, where M is a monocation, and SPh is an aromatic thiolate ligand. This is in part due to the fact that no other molecular silver nanoparticles have been synthesized with aromatic thiolate ligands. Here we report the synthesis of M3Ag17(4-tert-butylbenzene-thiol)12, which has good stability and an unusual optical spectrum. We also present a rational strategy for predicting the structure of this molecule. First-principles calculations support the structural model, predict a HOMO-LUMO energy gap of 1.77 eV, and predict a new "monomer mount" capping motif, Ag(SR)3, for Ag nanoparticles. The calculated optical absorption spectrum is in good correspondence with the measured spectrum. Heteroatom substitution was also used as a structural probe. First-principles calculations based on the structural model predicted a strong preference for a single Au atom substitution in agreement with experiment.

Hydrogen-bonded structure and mechanical chiral response of a silver nanoparticle superlattice.

Self-assembled nanoparticle superlattices-materials made of inorganic cores capped by organic ligands, of varied structures, and held together by diverse binding motifs-exhibit size-dependent properties as well as tunable collective behaviour arising from couplings between their nanoscale constituents. Here, we report the single-crystal X-ray structure of a superlattice made in the high-yield synthesis of Na(4)Ag(44)(p-MBA)(30) nanoparticles, and find with large-scale quantum-mechanical simulations that its atomically precise structure and cohesion derive from hydrogen bonds between bundledp-MBA ligands. We also find that the superlattice's mechanical response to hydrostatic compression is characterized by a molecular-solid-like bulk modulus B(0) = 16.7 GPa, exhibiting anomalous pressure softening and a compression-induced transition to a soft-solid phase. Such a transition involves ligand flexure, which causes gear-like correlated chiral rotation of the nanoparticles. The interplay of compositional diversity, spatial packing efficiency, hydrogen-bond connectivity, and cooperative response in this system exemplifies the melding of the seemingly contrasting paradigms of emergent behaviour 'small is different' and 'more is different'.

Ultrastable silver nanoparticles.

Noble-metal nanoparticles have had a substantial impact across a diverse range of fields, including catalysis, sensing, photochemistry, optoelectronics, energy conversion and medicine. Although silver has very desirable physical properties, good relative abundance and low cost, gold nanoparticles have been widely favoured owing to their proved stability and ease of use. Unlike gold, silver is notorious for its susceptibility to oxidation (tarnishing), which has limited the development of important silver-based nanomaterials. Despite two decades of synthetic efforts, silver nanoparticles that are inert or have long-term stability remain unrealized. Here we report a simple synthetic protocol for producing ultrastable silver nanoparticles, yielding a single-sized molecular product in very large quantities with quantitative yield and without the need for size sorting. The stability, purity and yield are substantially better than those for other metal nanoparticles, including gold, owing to an effective stabilization mechanism. The particular size and stoichiometry of the product were found to be insensitive to variations in synthesis parameters. The chemical stability and structural, electronic and optical properties can be understood using first-principles electronic structure theory based on an experimental single-crystal X-ray structure. Although several structures have been determined for protected gold nanoclusters, none has been reported so far for silver nanoparticles. The total structure of a thiolate-protected silver nanocluster reported here uncovers the unique structure of the silver thiolate protecting layer, consisting of Ag2S5 capping structures. The outstanding stability of the nanoparticle is attributed to a closed-shell 18-electron configuration with a large energy gap between the highest occupied molecular orbital and the lowest unoccupied molecular orbital, an ultrastable 32-silver-atom excavated-dodecahedral core consisting of a hollow 12-silver-atom icosahedron encapsulated by a 20-silver-atom dodecahedron, and the choice of protective coordinating ligands. The straightforward synthesis of large quantities of pure molecular product promises to make this class of materials widely available for further research and technology development.

STEM Electron Diffraction and High Resolution Images Used in the Determination of the Crystal Structure of 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.

Bare clusters derived from protein templates: Au25(+), Au38(+) and Au102(+).

A discrete sequence of bare gold clusters of well-defined nuclearity, namely Au25(+), Au38(+) and Au102(+), formed in a process that starts from gold-bound adducts of the protein lysozyme, were detected in the gas phase. It is proposed that subsequent to laser desorption ionization, gold clusters form in the gas phase, with the protein serving as a confining growth environment that provides an effective reservoir for dissipation of the cluster aggregation and stabilization energy. First-principles calculations reveal that the growing gold clusters can be electronically stabilized in the protein environment, achieving electronic closed-shell structures as a result of bonding interactions with the protein. Calculations for a cluster with 38 gold atoms reveal that gold interaction with the protein results in breaking of the disulfide bonds of the cystine units, and that the binding of the cysteine residues to the cluster depletes the number of delocalized electrons in the cluster, resulting in opening of a super-atom electronic gap. This shell-closure stabilization mechanism confers enhanced stability to the gold clusters. Once formed as stable magic number aggregates in the protein growth medium, the gold clusters become detached from the protein template and are observed as bare Au(n)(+) (n=25, 38, and 102) clusters.

Au(67)(SR)(35) nanomolecules: characteristic size-specific optical, electrochemical, structural properties and first-principles theoretical analysis.

The preparation of gold nanomolecules with sizes other than Au(25)(SR)(18), Au(38)(SR)(24), Au(102)(SR)(44), and Au(144)(SR)(60) has been hampered by stability issues and low yields. Here we report a procedure to prepare Au(67)(SR)(35), for either R = -SCH(2)CH(2)Ph or -SC(6)H(13), allowing high-yield isolation (34%, ~10-mg quantities) of the title compound. Product high purity is assessed at each synthesis stage by rapid MALDI-TOF mass-spectrometry (MS), and high-resolution electrospray-ionization MS confirms the Au(67)(SR)(35) composition. Electronic properties were explored using optical absorption spectroscopy (UV-visible-NIR regions) and electrochemistry (0.74 V spacing in differential-pulsed-voltammetry), modes of ligand binding were studied by NMR spectroscopy ((13)C and (1)H), and structural characteristics of the metal atom core were determined by powder X-ray measurements. Models featuring a Au(17) truncated-decahedral inner core encapsulated by the 30 anchoring atoms of 15 staple-motif units have been investigated with first-principles electronic structure calculations. This resulted in identification of a structure consistent with the experiments, particularly, the opening of a large gap (~0.75 eV) in the (2-) charge-state of the nanomolecule. The electronic structure is analyzed within the framework of a superatom shell model. Structurally, the Au(67)(SR)(35) nanomolecule is the smallest to adopt the complete truncated-decahedral motif for its core with a surface structure bearing greater similarity to the larger nanoparticles. Its electronic HOMO-LUMO gap (~0.75 eV) is nearly double that of the larger Au(102) compound and it is much smaller than that of the Au(38) one. The intermediary status of the Au(67)(SR)(35) nanomolecule is also reflected in both its optical and electrochemical characteristics.

Total structure and electronic properties of the gold nanocrystal Au36(SR)24.

A golden opportunity: the total structure of a Au(36)(SR)(24) nanocluster reveals an unexpected face-centered-cubic tetrahedral Au(28) kernel (magenta). The protecting layer exhibits an intriguing combination of binding modes, consisting of four regular arch-like staples and the unprecedented appearance of twelve bridging thiolates (yellow). This unique protecting network and superatom electronic shell structure confer extreme stability and robustness.

Size-selected monodisperse nanoclusters on supported graphene: bonding, isomerism, and mobility.

Soft-landing of size-selected Pd(n) (n ≤ 20) nanoclusters on a Moiré-patterned surface of graphene adsorbed on Ru(0001) leads to controlled formation of a truly monodisperse cluster-assembled material. Combined scanning tunneling microscopy and first-principles calculations allow identification of selective adsorption sites, characterization of size-dependent cluster isomers, and exploration of interconversion processes between isomeric forms that manifestly influence cluster surface mobility. Surface-assembled cluster superstructures may be employed in nanocatalytic applications, as well as in fundamental investigations of physical factors controlling bonding, structure, isomerism, and surface mobilities of surface-supported clusters.

The superstable 25 kDa monolayer protected silver nanoparticle: measurements and interpretation as an icosahedral Ag152(SCH2CH2Ph)60 cluster.

A cluster obtained in high yield from the reduction of a silver-thiolate precursor, Ag-SCH(2)CH(2)Ph, exhibited a single sharp peak near 25 kDa in the matrix-assisted laser desorption mass spectrum (MALDI MS) and a well-defined metal core of ~2 nm measured with transmission electron microscopy (TEM). The cluster yields a single fraction in high-performance liquid chromatography (HPLC). Increased laser fluence fragments the cluster until a new peak near 19 kDa predominates, suggesting that the parent cluster-Ag(152)(SCH(2)CH(2)Ph)(60)-evolves into a stable inorganic core-Ag(152)S(60). Exploiting combined insights from investigations of clusters and surface science, a core-shell structure model was developed, with a 92-atom silver core having icosahedral-dodecahedral symmetry and an encapsulating protective shell containing 60 Ag atoms and 60 thiolates arranged in a network of six-membered rings resembling the geometry found in self-assembled monolayers on Ag(111). The structure is in agreement with small-angle X-ray scattering (SAXS) data. The protective layer encapsulating this silver cluster may be the smallest known three-dimensional self-assembled monolayer. First-principles electronic structure calculations show, for the geometry-optimized structure, the development of a ~0.4 eV energy gap between the highest-occupied and lowest-unoccupied states, originating from a superatom 90-electron shell-closure and conferring stability to the cluster. The optical absorption spectrum of the cluster resembles that of plasmonic silver nanoparticles with a broad single feature peaking at 460 nm, but the luminescence spectrum shows two maxima with one attributed to the ligated shell and the other to the core.