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Highly regio- and stereoselective hydrothiolation of a wide range of alkynes with various thiols was demonstrated in the presence of a well-defined Rh complex supported on mesoporous SBA-15 silica. The catalyst was easily recovered and reused several times without significant loss of activity or selectivity.

Replica molding with elastomeric polymers has been used routinely to replicate features less than 10 nm in size. Because the theoretical limit of this technique is set by polymer-surface interactions, atomic radii, and accessible volumes, replication at subnanometer length scales should be possible. Using polydimethylsiloxane to create a mold and polyurethane to form the replica, we demonstrate replication of elementary steps 3-5 A in height that define the minimum separation between molecular layers in the lattices of the ionic crystals potassium dihydrogen phosphate and calcite. This work establishes the operation of replica molding at the molecular scale.

Catalytic Chemistry of Hydrocarbon Conversion Reactions on Metallic Single Crystals.- Structure, Characterization and Reactivity of Pt-Sn Surface Alloys.- Catalysis at Bimetallic Electrochemical Interfaces.- Enantioselectivity on Naturally Chiral Metal Surfaces.- Chiral Expression by Organic Architectures at Metal Surfaces: the Role of Both Adsorbate and Surface in Inducing Asymmetry.- Role of C and P Sites on the Chemical Activity of Metal Carbides and Phosphides: From Clusters to Single-Crystal Surfaces.- Surface Reactions of Oxygen Containing Compounds on Metal Oxide (TiO2 and UO2) Single Crystals.- Surface Science Studies of Strong Metal-Oxide Interactions on Model Catalysts.- Surface Thermodynamics: Small Molecule Adsorption Calorimetry on Metal Single Crystals.- Surface Femtochemistry.- The Incorporation of Added Metal Atoms into Structures of Reaction Intermediates on Catalytic Metal Surfaces.- Chemical Bonding on Metal Surfaces.- From Molecular Insights to Novel Catalysts Formulation.- The Reactivity of Gas-Phase Metal Oxide Clusters: Systems for Understanding the Mechanisms of Heterogeneous Catalysts.- Catalysis by Noble Metal Nanoparticles Supported on Thin-Oxide Films.- Catalysis by Supported Size-Selected Clusters.- Catalysis by Thin Oxide Films and Oxide Nanoparticles.- Catalysis with Transition Metal Nanoparticles in Colloidal Solution: Heterogeneous or Homogeneous?.- Well-Defined Metallic and Bimetallic Clusters Supported on Oxides and Zeolites.- A Convergence of Homogeneous and Heterogeneous Catalysis: Immobilized Organometallic Catalysts.- Single-Site Heterogeneous Catalysts: Innovations, Advantages, and Future Potential in Green Chemistry and Sustainable Technology.- Molecular-Imprinted Metal Complexes for the Design of Catalytic Structures.- Heterogeneous Catalyst Design by Multiple Functional Group Positioning in Organic-Inorganic Materials: On the Route to Analogs of Multifunctional Enzymes.

This Account reviews nanoskiving--a new technique that combines thin-film deposition of metal on a topographically contoured substrate with sectioning using an ultramicrotome--as a method of fabricating nanostructures that could replace conventional top-down techniques in selected applications. Photolithography and scanning beam lithography, conventional top-down techniques to generate nanoscale structures and nanostructured materials, are useful, versatile, and highly developed, but they also have limitations: high capital and operating costs, limited availability of the facilities required to use them, an inability to fabricate structures on nonplanar surfaces, and restrictions on certain classes of materials. Nanoscience and nanotechnology would benefit from new, low-cost techniques to fabricate electrically and optically functional structures with dimensions of tens of nanometers, even if (or perhaps especially if) these techniques have a different range of application than does photolithography or scanning beam lithography. Nanoskiving provides a simple and convenient procedure to produce arrays of structures with cross-sectional dimensions in the 30-nm regime. The dimensions of the structures are determined by (i) the thickness of the deposited thin film (tens of nanometers), (ii) the topography (submicrometer, using soft lithography) of the surface onto which the thin film is deposited, and (iii) the thickness of the section cut by the microtome (> or =30 nm by ultramicrotomy). The ability to control the dimensions of nanostructures, combined with the ability to manipulate and position them, enables the fabrication of nanostructures with geometries that are difficult to prepare by other methods. The nanostructures produced by nanoskiving are embedded in a thin epoxy matrix. These epoxy slabs, although fragile, have sufficient mechanical strength to be manipulated and positioned; this mechanical integrity allows the nanostructures to be stacked in layers, draped over curved surfaces, and suspended across gaps, while retaining the in-plane geometry of the nanostructures embedded in the epoxy. After removal of the polymer matrix by plasma oxidation, these structures generate suspended and draped nanostructures and nanostructures on curved surfaces. Two classes of applications, in optics and in electronics, demonstrate the utility of nanostructures fabricated by nanoskiving. This technique will be of primary interest to researchers who wish to generate simple nanostructures, singly or in arrays, more simply and quickly than can be accomplished in the clean-room. It is easily accessible to those not trained in top-down procedures for fabrication and those with limited or no access to the equipment and facilities needed for photolithography or scanning-beam fabrication. This Account discusses a new fabrication method (nanoskiving) that produces arrays of metal nanostructures. The defining process in nanoskiving is cutting slabs from a polymeric matrix containing embedded, more extended metal structures.

This paper describes the use of nanoskiving to fabricate complex metallic nanostructures by sectioning polymer slabs containing small, embedded metal structures. This method begins with the deposition of thin metallic films on an epoxy substrate by e-beam evaporation or sputtering. After embedding the thin metallic film in an epoxy matrix, sectioning (in a plane perpendicular or parallel to the metal film) with an ultramicrotome generates sections (which can be as thin as 50 nm) of epoxy containing metallic nanostructures. The cross-sectional dimensions of the metal wires embedded in the resulting thin epoxy sections are controlled by the thickness of the evaporated metal film (which can be as small as 20 nm) and the thickness of the sections cut by the ultramicrotome; this work uses a standard 45 degrees diamond knife and routinely generates slabs 50 nm thick. The embedded nanostructures can be transferred to, and positioned on, planar or curved substrates by manipulating the thin polymer film. Removal of the epoxy matrix by etching with an oxygen plasma generates free-standing metallic nanostructures. Nanoskiving can fabricate complex nanostructures that are difficult or impossible to achieve by other methods of nanofabrication. These include multilayer structures, structures on curved surfaces, structures that span gaps, structures in less familiar materials, structures with high aspect ratios, and large-area structures comprising two-dimensional periodic arrays. This paper illustrates one class of application of these nanostructures: frequency-selective surfaces at mid-IR wavelengths.

This Account reviews nanoskiving--a new technique that combines thin-film deposition of metal on a topographically contoured substrate with sectioning using an ultramicrotome--as a method of fabricating nanostructures that could replace conventional top-down techniques in selected applications. Photolithography and scanning beam lithography, conventional top-down techniques to generate nanoscale structures and nanostructured materials, are useful, versatile, and highly developed, but they also have limitations: high capital and operating costs, limited availability of the facilities required to use them, an inability to fabricate structures on nonplanar surfaces, and restrictions on certain classes of materials. Nanoscience and nanotechnology would benefit from new, low-cost techniques to fabricate electrically and optically functional structures with dimensions of tens of nanometers, even if (or perhaps especially if) these techniques have a different range of application than does photolithography or scanning beam lithography. Nanoskiving provides a simple and convenient procedure to produce arrays of structures with cross-sectional dimensions in the 30-nm regime. The dimensions of the structures are determined by (i) the thickness of the deposited thin film (tens of nanometers), (ii) the topography (submicrometer, using soft lithography) of the surface onto which the thin film is deposited, and (iii) the thickness of the section cut by the microtome (> or =30 nm by ultramicrotomy). The ability to control the dimensions of nanostructures, combined with the ability to manipulate and position them, enables the fabrication of nanostructures with geometries that are difficult to prepare by other methods. The nanostructures produced by nanoskiving are embedded in a thin epoxy matrix. These epoxy slabs, although fragile, have sufficient mechanical strength to be manipulated and positioned; this mechanical integrity allows the nanostructures to be stacked in layers, draped over curved surfaces, and suspended across gaps, while retaining the in-plane geometry of the nanostructures embedded in the epoxy. After removal of the polymer matrix by plasma oxidation, these structures generate suspended and draped nanostructures and nanostructures on curved surfaces. Two classes of applications, in optics and in electronics, demonstrate the utility of nanostructures fabricated by nanoskiving. This technique will be of primary interest to researchers who wish to generate simple nanostructures, singly or in arrays, more simply and quickly than can be accomplished in the clean-room. It is easily accessible to those not trained in top-down procedures for fabrication and those with limited or no access to the equipment and facilities needed for photolithography or scanning-beam fabrication. This Account discusses a new fabrication method (nanoskiving) that produces arrays of metal nanostructures. The defining process in nanoskiving is cutting slabs from a polymeric matrix containing embedded, more extended metal structures.

The adsorption of carbon monoxide and ethylene, and their sequential adsorption, was studied over a series of Pt/SBA-15 catalysts with monodisperse particle sizes ranging from 1.7 to 7.1 nm by diffuse-reflectance infrared spectroscopy and chemisorption. Gas adsorption was dependent on the Pt particle size, temperature, and sequence of gas exposure. Adsorption of CO at room temperature on Pt/SBA-15 gives rise to a spectroscopic feature assigned to the C-O stretch: nu(CO) = 2075 cm-1 (1.9 nm); 2079 cm-1 (2.9 nm); 2082 cm-1 (3.6 nm); and 2090 cm-1 (7.1 nm). The intensity of the signal decreased in a sigmoidal fashion with increasing temperature, thereby providing semiquantitative surface coverage information. Adsorption of ethylene on Pt/SBA-15 gave rise to spectroscopic features at approximately 1340, approximately 1420, and approximately 1500 cm-1 assigned to ethylidyne, di-sigma-bonded ethylene, and pi-bonded ethylene, respectively. The ratio of these surface species is highly dependent on the Pt particle size. At room temperature, Pt particles stabilize ethylidyne as well as di-sigma- and pi-bonded ethylene; however, ethylidyne predominated on the surfaces of larger particles. Ethylidyne was the only identifiable species at 403 K, with its formation being more facile on larger particles. Co-adsorption experiments reveal that the composition of the surface layer is dependent on the order of exposure to gases. Exposure of a C2H4-covered Pt surface to CO resulted in an approximately 50% decrease in chemisorbed CO compared to a fresh Pt surface. The nu(CO) appeared at 2050 cm-1 on Pt/SBA-15 pretreated with C2H4 at room temperature. The di-sigma-bonded and pi-bonded species are the most susceptible to displacement from the surface by CO. The formation of ethylidyne appeared to be less sensitive to the presence of adsorbed carbon monoxide, especially on larger particles. Upon exposure of C2H4 to a CO-covered Pt surface, little irreversible uptake occurred due to nearly 100% site blocking. These results demonstrate that carbon monoxide competes directly with ethylene for surface sites, which will have direct implications on the poisoning of the heterogeneously catalyzed conversion of hydrocarbons.

Platinum nanoparticles in the size range of 1.7-7.1 nm were produced by alcohol reduction methods. A polymer (poly(vinylpyrrolidone), PVP) was used to stabilize the particles by capping them in aqueous solution. The particles were characterized by X-ray diffraction (XRD) and transmission electron microscopy (TEM). TEM investigations demonstrate that the particles have a narrow size distribution. Mesoporous SBA-15 silica with 9-nm pores was synthesized by a hydrothermal process and used as a catalyst support. After incorporation into mesoporous SBA-15 silica using low-power sonication, the catalysts were calcined to remove the stabilizing polymer from the nanoparticle surface and reduced by H2. Pt particle sizes determined from selective gas adsorption measurements are larger than those determined by bulk techniques such as XRD and TEM. Roomtemperature ethylene hydrogenation was chosen as a model reaction to probe the activity of the Pt/SBA-15 materials. The reaction was shown to be structure insensitive over a series of Pt/SBA-15 materials with particle sizes between 1.7 and 3.6 nm. The hydrogenolysis of ethane on Pt particles from 1.7 to 7.1 nm was weakly structure sensitive with smaller particles demonstrating higher specific activity. Turnover rates for ethane hydrogenolysis increased monotonically with increasing metal dispersion, suggesting that coordinatively unsaturated metal atoms present in small particles are more active for C 2H6 hydrogenolysis than the low index planes that dominate in large particles. An explanation for the structure sensitivity is suggested, and the potential applications of these novel supported nanocatalysts for further studies of structure -activity and structure-selectivity relationships are discussed.

The surface science of heterogeneous metal catalysis uses model systems ranging from single crystals to monodispersed nanoparticles in the 1–10 nm range. Molecular studies reveal that bond activation (C–H, H–H, C–C, C O) occurs at 300 K or below as the active metal sites simultaneously restructure. The strongly adsorbed molecules must be mobile to free up these sites for continued turnover of reaction. Oxide–metal interfaces are also active for catalytic turnover. Examples using C–H and C O activation are described to demonstrate these properties. Future directions include synthesis, characterization, and reaction studies with 2D and 3D monodispersed metal nanoclusters to obtain 100% selectivity in multipath reactions. Investigations of the unique structural, dynamic, and electronic properties of nanoparticles are likely to have major impact in surface technologies. The fields of heterogeneous, enzyme, and homogeneous catalysis are likely to merge for the benefit of all three.

A novel high surface area heterogeneous catalyst based on solution phase colloidal nanoparticle chemistry has been developed. Monodisperse platinum nanoparticles of 1.7-7.1 nm have been synthesized by alcohol reduction methods and incorporated into mesoporous SBA-15 silica during hydrothermal synthesis. Characterization of the Pt/SBA-15 catalysts suggests that Pt particles are located within the surfactant micelles during silica formation leading to their dispersion throughout the silica structure. After removal of the templating polymer from the nanoparticle surface, Pt particle sizes were determined from monolayer gas adsorption measurements. Infrared studies of CO adsorption revealed that CO exclusively adsorbs to atop sites and red-shifts as the particle size decreases suggesting surface roughness increases with decreasing particle size. Ethylene hydrogenation rates were invariant with particle size and consistent with a clean Pt surface. Ethane hydrogenolysis displayed significant structure sensitivity over the size range of 1-7 nm, while the apparent activation energy increased linearly up to a Pt particle size of approximately 4 nm and then remained constant. The observed rate dependence with particle size is attributed to a higher reactivity of coordinatively unsaturated surface atoms in small particles compared to low-index surface atoms prevalent in large particles. The most reactive of these unsaturated surface atoms are responsible for ethane decomposition to surface carbon. The ability to design catalytic structures with tunable properties by rational synthetic methods is a major advance in the field of catalyst synthesis and for the development of accurate structure-function relationships in heterogeneous reaction kinetics.

A synthetic method for the design of heterogeneous catalysts incorporating monodisperse platinum nanoparticles into ordered mesoporous oxide frameworks has been developed. Nanoparticles were synthesized in solution in the presence of surface templating polymer and encapsulated into mesoporous oxide matrices. After polymer removal by calcination, ethylene hydrogenation rates were consistent with previously reported results, while superior activity of low coordination atoms in small crystallites during ethane hydrogenolysis was demonstrated. Changes in reaction selectivity with particle size during the hydrogenation/dehydrogenation of cyclohexene were attributed to a hydrogen coverage effect influencing the hydrogenation pathway and the apparent structure sensitivity of the dehydrogenation pathway.

Hollow nanocrystals can be synthesized through a mechanism analogous to the Kirkendall Effect, in which pores form because of the difference in diffusion rates between two components in a diffusion couple. Starting with cobalt nanocrystals, we show that their reaction in solution with oxygen and either sulfur or selenium leads to the formation of hollow nanocrystals of the resulting oxide and chalcogenides. This process provides a general route to the synthesis of hollow nanostructures of a large number of compounds. A simple extension of the process yielded platinum–cobalt oxide yolk-shell nanostructures, which may serve as nanoscale reactors in catalytic applications.

Decomposition of Co2(CO)8 in the presence of ca. 4 nm × 20 nm oleic acid stabilized brookite TiO2 nanorods was performed according to a prior report in the literature in which Co–TiO2 hybrid nanocrystals had been observed. The hybrid nanocrystals could not be duplicated; however, we report a procedure that consistently led to a mixture of blue Co(II)–TiO2 nanorods and Co precipitate. The Co(II)–TiO2 nanorods have single-site Co(II) ions selectively attached to the TiO2 nanocrystal surface. Transmission electron microscopy and powder X-ray diffraction data show the crystal phase and morphology of the nanorod is unchanged on addition of Co(II) and no new crystal phases or particulate domains are associated with the colloid. A combination of UV-visible, X-ray photoelectron, and X-ray absorption spectroscopic analysis indicates Co(II) sites are present on the surface of the TiO2 nanorods in octahedral and tetrahedral coordination in ∼1 : 1 ratio. A mechanism is proposed in which the Co–Co bond of the precursor undergoes heterolysis followed by disproportionation of Co(I) to yield Co(II) and Co(0) precipitate. The Co(II)–TiO2 nanorods were shown to exhibit greater activity than TiO2 nanorods in the degradation of 5,8-dihydroxy-1,4-naphthoquinone dye under visible light irradiation.

Highly active and selective aerobic cross-dehydrogenative coupling of terminal alkynes with thiols to construct alkynyl sulfides catalyzed by Cu(I) using molecular oxygen as the oxidant has been demonstrated under mild reaction conditions. The process is applicable to a wide range of alkynes and various thiols and is compatible with a variety of functional groups on both alkyne and thiol coupling partners.

The article by Teck L. Tan and Kewu Bai [1] was published after the authors had informed the MRS Proceedings editor that they wished to withdraw their article. The publisher and the MRS apologize to the authors for this oversight, and this notice is published online in place of the article itself.

Catalyst T RED (K) CO uptakea (μmol g−1) COT/CuT COirr/CuT ‘O’ uptakeb (μmol g−1) Cu0 disp. dc (nm) Total Irreversible 4.96% Cu/AC–HNO3 423 170.0 62.0 0.218 0.079 12.1 0.031 35.5 473 35.4 6.1 0.045 0.008 18.3 0.047 23.5 573 19.6 0.0 0.025 0.0 16.5 0.042 26.0 4.89% Cu/AC–ASIS 423 54.0 0.0 0.070 0.0 0.0 0.0 0.0 473 54.6 0.0 0.071 0.0 8.8 0.023 48.1 573 51.6 0.0 0.067 0.0 16.6 0.043 25.5 5.01% Cu/AC–HTT–H2 423 79.4 5.7 0.101 0.007 54.0 0.137 8.0 473 83.5 5.6 0.106 0.007 58.9 0.149 7.4 573 59.1 0.0 0.075 0.0 67.3 0.171 6.4 0.98% Cu/AC–HTT–H2 473 52.5 1.1 0.340 0.007 11.1 0.144 7.6 573 53.2 0.0 0.345 0.0 8.9 0.115 9.5 0.63% Cu/GF-IE 573 0.0 0.0 0.0 0.0 0.9d 0.018 60.6 Cu powder 573 – – – – 1.8d 2.3 × 10−4 ∼4800 Cu chromite 573 55.0 18.3 0.009 0.003 181.5 0.056 19.6 a Volumetric uptakes at 75 Torr corrected for irreversible uptake on support. b Uptakes at 363 K and 75 Torr N2O determined gravimetrically. c Cu crystallite size based on d = 1.1/(2Oad/CuT). d Determined volumetrically.