Bokwon Yoon

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.

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.

Control and Manipulation of Gold Nanocatalysis: Effects of Metal Oxide Support Thickness and Composition

Control and tunability of the catalytic oxidation of CO by gold clusters deposited on MgO surfaces grown on molybdenum, Mo(100), to various thicknesses are explored through temperature-programmed reaction measurements on mass-selected 20-atom gold clusters and via first-principles density functional theory calculations. Au(20) was chosen because in the gas phase it is characterized as an extraordinarily stable tetrahedral-pyramidal structure. Dependencies of the catalytic activities and microscopic reaction mechanisms on the thickness and stoichiometry of the MgO films and on the dimensionalities and structures of the adsorbed gold clusters are demonstrated and elucidated. Langmuir-Hinshelwood mechanisms and reaction barriers corresponding to observed low- and high-temperature CO oxidation reactions are calculated and analyzed. These reactions involve adsorbed O(2) molecules that are activated to a superoxo- or peroxo-like state through partial occupation of the antibonding orbitals. In some cases, we find activated, dissociative adsorption of O(2) molecules, adsorbing at the cluster peripheral interface with the MgO surface. The reactant CO molecules either adsorb on the MgO surface in the cluster proximity or bind directly to the gold cluster. Along with the oxidation reactions on stoichiometric ultrathin MgO films, we also study reactions catalyzed by Au(20) nanoclusters adsorbed on relatively thick defect-poor MgO films supported on Mo and on defect-rich thick MgO surfaces containing oxygen vacancy defects.

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.

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.

Hydrogen-promoted oxygen activation by free gold cluster cations.

Small gas-phase gold cluster cations are essentially inert toward molecular oxygen. Preadsorption of molecular hydrogen, however, is found to cooperatively activate the binding of O(2) to even-size Au(x)(+) (x = 2, 4, 6) clusters. Measured temperature- and reaction-time-dependent ion intensities, obtained by ion trap mass spectrometry, in conjunction with first-principles density-functional theory calculations, reveal promotion and activation of molecular oxygen by preadsorbed hydrogen. These processes lead to the formation of a hydroperoxo intermediate on Au(4)(+) and Au(6)(+) and culminate in the dissociation of O(2) via the release of H(2)O. Langmuir-Hinshelwood reaction mechanisms involving the coadsorption of both of the reactant molecules are discussed for both cluster sizes, and an alternative Eley-Rideal mechanism involving hydrogen molecules adsorbed on a Au(6)(+) cluster reacting with an impinging gaseous oxygen molecule is analyzed. Structural fluctionality of the gold hexamer cation, induced by the adsorption of hydrogen molecules, and resulting in structural isomerization from a ground-state triangular structure to an incomplete hexagonal one, is theoretically predicted. Bonding of H(2) on cationic gold clusters is shown to involve charge transfer to the clusters. This serves to promote the bonding of coadsorbed oxygen through occupation of the antibonding 2pi* orbitals, resulting in excess electronic charge accumulation on the adsorbed molecule and weakening of the O-O bond. The theoretical results for hydrogen saturation coverages and reaction characteristics between the coadsorbed hydrogen and oxygen molecules are found to agree with the experimental findings. The joint investigations provide insights regarding hydrogen and oxygen cooperative adsorption effects and consequent reaction mechanisms.

Morphology control of the supported islands grown from soft-landed clusters

Abstract The morphology of islands grown on surfaces from soft-landed clusters has been investigated by electron microscopy. Compact islands have been observed on amorphous carbon surfaces, whereas an evolution from compact to ramified shapes occurs on graphite surfaces as the mean size of deposited clusters increases. Moreover, by increasing the surface defect density on graphite, a continuous variation of the island morphology is observed, from extended ramified shapes to small compact shapes. In order to account for the island morphologies observed, we propose a crude model involving a competition between the time for aggregated clusters to coalesce and the time interval between successive arrivals of clusters to grow the islands. It shows that there exists a critical island size R 0 dividing island shapes into compact shapes for R R 0 and into ramified shapes for R > R 0 . This critical size R 0 varies as a function of the incident cluster size. Relying on our experimental results, we show how the morphology of the islands can be controlled by the size of the incident clusters and the presence of surface defects.

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.

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.

Fundamental Insight into the Substrate-Dependent Ripening of Monodisperse Clusters

Sintering is one of the major routes for the deactivation of surface‐supported catalytic particles in heterogeneous catalysis. The two mechanisms that are responsible for such coarsening phenomena are Ostwald ripening, in which larger clusters tend to grow at the expense of smaller ones, and Smoluchowski ripening, in which entire clusters diffuse and coalesce. The sintering properties of cluster‐assembled materials can be influenced by tuning the interactions between the particle and the substrate. To explore the fundamental factors that control cluster‐ripening mechanisms, we deposited truly monodisperse Pd clusters onto three different model catalysts: bare Rh(1 1 1), graphene‐Moiré films that were grown on Rh(1 1 1) and Ru(0 0 0 1), and a hexagonal boron‐nitride film that was grown on Rh(1 1 1). The evolution of particle size and density was tracked by high‐resolution scanning tunneling microscopy. The principal microscopic mechanisms that govern the ripening processes on each of these three substrates have been determined from thorough analyses of the cluster heights and size distributions. The ripening mechanisms were related to the distinct cluster‐adsorption and atom‐detachment energies that were obtained from first‐principle calculations. These results elucidate the ripening processes and underlie the formulation of a strategy for optimizing cluster stability against ripening, where both, the binding of the clusters to the surface and that of the individual atoms, must be controlled. Such tuning of the interactions may be achieved through the judicious selection of surfaces with laterally modulated wettability.