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Since 1995, crystal structures have been determined for many transition-metal enzymes, in particular those containing the rarely used transition metals vanadium, molybdenum, tungsten, manganese, cobalt and nickel. Accordingly, our understanding of how an enzyme uses the unique properties of a specific transition metal has been substantially increased in the past few years. The different functions of nickel in catalysis are highlighted by describing the active sites of six nickel enzymes - methyl-coenyzme M reductase, urease, hydrogenase, superoxide dismutase, carbon monoxide dehydrogenase and acetyl-coenzyme A synthase.
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PMID:Active sites of transition-metal enzymes with a focus on nickel. 991 55

Nanoscale surface patterning is of great importance for applications ranging from catalysts to biomaterials. We show the formation of ordered nanoscale dimple arrays on titanium, tungsten, and zirconium during electropolishing, demonstrating versatility of a process previously only reported for tantalum. This is a rare example of an electrochemical pattern formation process that can be translated to other materials. The dimpled surfaces have been characterized with scanning electron microscopy, transmission electron microscopy, atomic force microscopy, and X-ray photoelectron spectroscopy, and electrochemical conditions were optimized for each material. While conditions for titanium and tungsten resemble those for tantalum, zirconium requires a different type of electrolyte. Given the appropriate electropolishing chemistry, formation of these patterns should be possible on any metal surface. The process is very robust on homogeneous surfaces, but sensitive to inhomogeneities in chemical composition, such as in the case of differentially etched alloys. An alternative process for some materials such as platinum is the coating of a dimpled substrate with a thin film of the required material.
ACS Nano 2008 Dec 23
PMID:Nanopatterning of transition metal surfaces via electrochemical dimple array formation. 1920 79

The imaging properties and observation of the sterically regulated translational motion of discrete tungsten polyoxometalate Linqvist ions (i.e., [W(6)O(19)](2-)) within carbon nanotubes of specific internal diameter are reported. The translational motion of the nonspheroidal anion within the nanotube capillary is found to be impeded by its near-perfect accommodation to the internal van der Waals surface of the nanotube wall. Rotational motion of the anion about one remaining degree of freedom permits translational motion of the anion along the nanotube followed by locking in at sterically favorable positions in a mechanism similar to a molecular ratchet. This steric locking permits the successful direct imaging of the constituent octahedral cation template of individual [W(6)O(19)](2-) anions by high resolution transmission electron microscopy thereby permitting meterological measurements to be performed directly on the anion. Direct imaging of pairs of equatorial W(2) atoms within the anion reveal steric relaxation of the anion contained within the nanotube capillary relative to the bulk anion structure.
ACS Nano 2008 May
PMID:Direct imaging of the structure, relaxation, and sterically constrained motion of encapsulated tungsten polyoxometalate lindqvist ions within carbon nanotubes. 1920 94

Janus nanoparticles, characterized by their anisotropic structure and interactions, have added a new dimension to nanoscience because of their potential applications in biomedicine, sensors, catalysis, and assembled materials. The technological applications of these nanoparticles, however, have been limited as the current chemical, physical, and biosynthetic methods lack sufficient size and shape selectivity. We report a technique where gold clusters doped with tungsten can serve as a seed that facilitates the natural growth of anisotropic nanostructures whose size and shape can be controlled with atomic precision. Using ab initio simulated annealing and molecular dynamics calculations on AunW (n > 12) clusters, we discovered that the W@Au12 cage cluster forms a very stable core with the remaining Au atoms forming patchy structures on its surface. The anisotropic geometry gives rise to anisotropies in vibrational spectra, charge distributions, electronic structures, and reactivity, thus making it useful to have dual functionalities. In particular, the core-patch structure is shown to possess a hydrophilic head and a hydrophobic tail. The W@Au12 clusters can also be used as building blocks of a nanoring with novel properties.
ACS Nano 2008 Feb
PMID:Design of Janus nanoparticles with atomic precision: tungsten-doped gold nanostructures. 1920 36

Unknown parameters critical to understanding the electron-precursor-substrate interactions during electron-beam-induced deposition (EBID) have long limited our ability to fully control this nanoscale, directed assembly method. We report here values that describe the precursor-solid interaction, the precursor surface diffusion coefficient (D), the precursor sticking probability (delta), and the mean precursor surface residence time (tau), which are critical parameters for understanding the assembly of EBID deposits. Values of D = 6.4 microm(2) s(-1), delta = 0.0250, and tau = 3.20 ms were determined for a commonly used precursor molecule, tungsten hexacarbonyl W(CO)6. Space and time predictions of the adsorbed precursor coverage were solved by an explicit finite differencing numerical scheme. Evolving nanopillar surface morphology was derived from simulations considering electron-induced dissociation as the critical depletion term. This made it possible to infer the space- and time-dependent precursor coverage both on and around nanopillar structures to better understand local precursor dynamics during mass-transport-limited (MTL) and reaction-rate-limited (RRL) EBID.
ACS Nano 2010 Mar 23
PMID:Fundamental electron-precursor-solid interactions derived from time-dependent electron-beam-induced deposition simulations and experiments. 2020 41

The alloying of tungsten filament when using 1,1,3,3-tetramethyl-1,3-disilacyclobutane (TMDSCB) in a hot-wire chemical vapor deposition reactor was systematically studied by scanning electron microscopy, Auger electron spectroscopy, analysis of the power consumed by the filament, and in situ mass spectrometric measurements of the gas-phase species produced in the process. Only carburization of the W filament was observed. The carburization is mainly caused by the interaction of methyl radicals with the filament. Graphite as well as both WC and W2C alloys can form on the filament surface, depending on the filament temperatures and source gas pressures. Both WC and graphite are converted to W2C with the diffusion of C into the filament. It is shown that filament carburization affects the consumption rate of the source gas and the intensities of gas-phase reaction products. Gas-phase reactions dominate at T < or = 1400 degrees C. The carburization rate increases with increasing filament temperatures and dominates at T > or = 1800 degrees C.
ACS Appl Mater Interfaces 2009 Sep
PMID:Carburization of tungsten filaments in a hot-wire chemical vapor deposition process using 1,1,3,3-tetramethyl-1,3-disilacyclobutane. 2035 15

This article describes a novel and simple route to preparing VO(2) thermochromic films by using a VOCl(2) solution with poly(vinylpyrrolidone) (PVP). X-ray diffraction and Raman spectra showed that the VO(2) films deposited with PVP consisted of a nearly pure monoclinic/rutile (M/R) phase. Conversely, films prepared without PVP contained obviously impure crystalline phases. The as-prepared films with PVP showed excellent optical properties compared to those prepared by common gas-phase methods: an integral visible transmittance of 54.5% and an IR reduction (change in transmittance) of 41.5% at 2000 nm. The phase-transition temperatures were adjusted from 69 to 54 degrees C by tungsten doping. Equipment analyses revealed that PVP plays two roles in the film formation. First, it fundamentally acts as a film-forming promoter to improve physical gelation via interactions among oppositely charged carbonyl groups and amine groups of the polymer. Second, the negatively charged carbonyl groups can interact with VO(2+) to form a uniform mixed-gel film after solvent evaporation. Thus, the addition of PVP can stabilize the solution and improve the as-prepared film quality and phase purity. The current study suggests that the process has promise in applications of smart windows.
ACS Appl Mater Interfaces 2009 Oct
PMID:A novel solution process for the synthesis of VO2 thin films with excellent thermochromic properties. 2035 55

Solid lubricants (SLs) characterized by low coefficients of friction (mu) and wear rates (w) drastically improve the life span of instruments that undergo extreme frictional wear. However, the performance of SLs such as sputtered or nanoparticulate molybdenum disulfide (MoS(2)), tungsten disulfide (WS(2)), or graphite deteriorates heavily under extreme operational conditions such as elevated temperatures and high humidity. Here, we present our preliminary results, which demonstrate that composites of carbon nanotubes (CNTs) and MoS(2) produced by electrodeposition of MoS(2) on vertically aligned CNT films have low mu ( approximately 0.03) and w (approximately 10(-13) mm(3)/N.mm) even at 300 degrees C, which are about 2 orders of magnitude better than those of nanoparticulate MoS(2)-based coatings. The high load-bearing capacity of CNTs provides a strong enduring support to MoS(2) nanoclusters and is responsible for their ultralow w. The incorporation of these composites in liquid lubricants reduces the friction coefficient of the liquid lubricants by approximately 15%. The technique described here to produce SL coatings with extremely appealing frictional properties will provide valuable solutions for a variety of tribological applications where the coatings encounter high temperature, reduced pressure, and/or low- and high-humidity conditions.
ACS Appl Mater Interfaces 2009 Mar
PMID:Carbon nanotube-MoS2 composites as solid lubricants. 2035 96

Zero- and one-dimensional metal nanocrystals were successfully fabricated with accurate control in size, shape, and position on semiconductor surfaces by using a novel in situ fabrication method of the nanocrystal with a biasing tungsten tip in transmission electron microscopy. The dominant mechanism of nanocrystal formation was identified mainly as local Joule heating-assisted electromigration through the direct observation of formation and growth processes of the nanocrystal. This method was applied to extracting metal atoms with an exceedingly faster growth rate ( approximately 10(5) atoms/s) from a metal-oxide thin film to form a metal nanocrystal with any desired size and position. By real-time observation of the microstructure and concurrent electrical measurements, it was found that the nanostructure formation can be completely controlled into various shapes such as zero-dimensional nanodots and one-dimensional nanowires/nanorods.
ACS Nano 2010 Jun 22
PMID:Direct fabrication of zero- and one-dimensional metal nanocrystals by thermally assisted electromigration. 2046 20

The novel type of an electron field emitter is demonstrated by welding a single carbon "onion" onto the end of a tungsten tip inside a high-resolution transmission electron microscope. Such merged structure is found to markedly reduce the onset voltage peculiar to a standard tungsten field emitter due to the small size of the onion and its highly curved surface. Similar to short carbon nanotubes, individual C-onion emitters can sustain large emission currents, more than 100 muA, and exhibit good long-term emission stability. Moreover the insertion of a high electrical resistance in series can suppress the current fluctuation to only 1.9%. All these properties make these newly created field emitters promising candidates for the advanced point electron sources.
ACS Nano 2010 Aug 24
PMID:Carbon "onions" as point electron sources. 2073 25


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