Tantalum Clusters Research Articles

One-step deposition of diamond-like carbon films containing self-assembled metallic nanoparticles, by femtosecond pulsed laser ablation

Femtosecond pulsed laser ablation has been used for the deposition of various thin films by co-ablating pure graphite and pure metallic targets (nickel or tantalum) under vacuum conditions with an amplified Ti:sapphire laser working at 800 nm, with about 1.5 mJ per pulse at a repetition rate of 1 kHz. The plasma plume created during the femtosecond pulsed laser deposition process has been investigated, by fast imaging and spectroscopic characterizations, depending on the ablated target and deposition conditions. Particular attention has been paid to the correlation between the film nanostructure and the plasma plume properties, in terms of ejected species and energies of impinging species on the substrate. The tantalum clusters in the tantalum-doped carbon films appear under two crystalline phases: a stable phase (α-Ta) corresponding to that of the target, and a metastable phase (β-Ta), already observed at high temperatures. The presence of this metastable phase (β-Ta) is correlated to the temperature of the clusters measured from the plasma plume investigation.

Chalcogenide Clusters of Group 5—7 Metals

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Z-Contrast Tomography of Nanoscale Heterogeneous Catalysts

Most industrial catalysts are high-surface-area solids such as amorphous or crystalline oxides, onto which an active component (often clusters of a metal, metal oxide, or metal sulfide, often including high-Z contrast elements) is dispersed in the form of very small clusters or particles [1,2]. So far, metal clusters on support have been restricted almost entirely to group 8 metals but recently supported early transition metal catalysts exemplified by tantalum clusters on SiO2 have been reported [3]. Catalyst performance is sensitive to cluster size, because the surface structure, electronic properties and cluster–support interactions depend on this size. The location of metal particles and their orientation with respect to the support material are also important in determining catalytic properties. For example, partial coverage of the metal by an amorphous oxide can influence the catalytic activity and selectivity [2].

Chemistry of Tantalum Clusters in Solution and on SiO2 Supports: Analogies and Contrasts

Tantalum clusters have been synthesized from Ta(CH2Ph)5 on the surface of porous fumed SiO2. When these clusters are small, incorporating, on average, several Ta atoms, their chemistry is similar to that of molecular tantalum clusters (and other early transition-metal) clusters. For example, The Ta-Ta bonds in these small supported clusters have been characterized by extended X-ray absorption fine structure (EXAFS), IR, and UV-vis spectroscopy, being similar to those in molecular analogues. The redox reactions of the supported clusters, characterized by X-ray absorption near-edge structure, are analogous to those of early transition-metal clusters in solution. In contrast to the largest of these clusters in solution and in the solid state, those supported on SiO2 are raftlike, facilitating the substantial metal-support-oxygen bonding that is evident in the EXAFS spectra. Samples consisting of tantalum clusters on SiO2 catalyze alkane disproportionation and the conversion of methane with n-butane to give other alkanes, but catalytic properties of analogous clusters in solution have barely been explored.

Silica-supported tantalum clusters: catalysts for conversion of methane with n-butane to give ethane, propane, and pentanes

Si02-supported tantalum clusters were prepared by adsorption of the precursor Ta(CH2Ph)5 (Ph is phenyl) on the support followed by treatments in H2 at 523, 623, and 723 K. The resultant clusters, had approximate average diameters of 0.3, 0.8, and 2 nm, as determined by extended X-ray absorption fine structure (EXAFS) spectroscopy. The samples were tested as catalysts for conversion of methane with n-butane in a once-through flow reactor operated at atmospheric pressure and 523 K, and EXAFS spectroscopy was used to characterize the used catalysts. The results show that (a) the catalysts are active for the conversion of methane with n-butane to give ethane, propane, and pentanes; (b) catalytic activity decreased to nearly zero over a time on stream of 22 h; (c) the catalyst incorporating the smallest clusters exhibited the highest initial activity and that incorporating the largest clusters exhibited the lowest activity; (d) each used catalyst contained clusters of approximately the same nuclearity as the respective fresh catalyst, but with Ta–Ta bond lengths approximately 0.17 A longer than those found in the fresh catalysts. The data are consistent with catalysis by the supported clusters, and the product distributions are consistent with disproportionation of n-butane accompanied by the reaction of methane with propane to give other alkanes.

Reaction of niobium and tantalum neutral clusters with low pressure, unsaturated hydrocarbons in a pickup cell: From dehydrogenation to Met-Car formation

Neutral niobium and tantalum clusters (Nbn and Tan) are generated by laser ablation and supersonic expansion into a vacuum and are reacted in a pickup cell with various low pressure (approximately 1 mTorr) unsaturated hydrocarbons (acetylene, ethylene, propylene, 1-butene, 1,3-butadiene, benzene, and toluene) under nearly single collision conditions. The bare metal clusters and their reaction products are ionized by a 193 nm laser and detected by a time of flight mass spectrometer. Partially and fully dehydrogenated products are observed for small (n<or=m) and large (n>or=m) neutral metal clusters, respectively, with m ranging from 2 to 5 depending on the particular hydrocarbon. In addition to primary, single collision products, sequential addition products that are usually fully dehydrogenated are also observed. With toluene used as the reactant gas, carbon loss products are observed, among which Nb8C12 and Ta8C12 are particularly abundant, indicating that the Met-Car molecule M8C12 can be formed from the neutral metal cluster upon two collisions with toluene molecules. The dehydrogenation results for low pressure reactions are compared with those available from previous studies employing flow tube (high pressure) reactors. Low pressure and high pressure cluster ion reactions are also compared with the present neutral metal cluster reactions. Reactions of unsaturated hydrocarbons and metal surfaces are discussed in terms of the present neutral cluster results.

Coexistence of ferroelectricity and ferromagnetism in tantalum clusters

The atomic and electronic structures of Ta(N) (N=2-23) clusters have been determined in the framework of pseudopotential density-functional calculations, based upon an unbiased global search with guided simulated annealing to an empirical potential. It is found that the ground-state structures of Ta(N) are very similar to those of Nb(N), showing no preference for the icosahedral growth. Also, a size- and structure-dependent ferroelectricity is found in these tantalum clusters. More importantly, it is found that the ferroelectricity and ferromagnetism can coexist in the homogeneous transition-metal cluster, offering a possibility to obtain a new type of "multiferroic" materials composed of the clusters. Finally, the far-infrared spectroscopy is suggested to be an efficient tool to distinguish the ferroelectric clusters.

Surface-Mediated Synthesis and Spectroscopic Characterization of Tantalum Clusters on Silica

Tantalum clusters were synthesized on the surface of porous silica by treatment of adsorbed Ta(CH(2)Ph)(5) in H(2) at temperatures in the range of 523-723 K. The surface species were characterized by UV-vis, far-infrared, and extended X-ray absorption fine-structure (EXAFS) spectroscopies, each of which provided evidence of Ta-Ta bonds similar to those in well-characterized molecular tantalum clusters. The Ta-Ta distance determined by EXAFS spectroscopy was 2.93 A. The chemistry of the cluster synthesis is similar to that of syntheses of similar tantalum clusters in solution. The supported clusters formed at 523 K are characterized by an EXAFS first-shell Ta-Ta coordination number of nearly 2, indicative of tri-tantalum clusters, although it is expected that a mixture of clusters was present, and reduction in H(2) at higher temperatures led to larger tantalum clusters. This is the first example of the surface-mediated synthesis of an early transition metal cluster, and the supported clusters reported here are the first to have been characterized by all three of the spectroscopic methods mentioned above. The similarity of the surface synthesis to that in solution points to opportunities to extend this new class of material to other early transition metal clusters on various supports.

Redox Chemistry of Tantalum Clusters on Silica Characterized by X-ray Absorption Spectroscopy

SiO(2)-supported clusters of tantalum were synthesized from adsorbed Ta(CH(2)Ph)(5) by treatment in H(2) at 523 K. The surface species were characterized by X-ray absorption spectroscopy (extended X-ray absorption fine structure (EXAFS) spectroscopy and X-ray absorption near edge spectroscopy (XANES)) and ultraviolet-visible spectroscopy. The EXAFS data show that SiO(2)-supported tantalum clusters were characterized by a Ta-Ta coordination number of approximately 2, consistent with the presence of tritantalum clusters, on average. When these were reduced in H(2) and reoxidized in O(2), the cluster nuclearity remained essentially unchanged, although reduction and oxidation occurred, respectively, as shown by XANES and UV-vis spectra; in the reoxidation, the tantalum oxidation state change was approximately two electronic charges per tritantalum cluster. The data demonstrate an analogy between the chemistry of group 5 metals on the SiO(2) support and their chemistry in solution, as determined by the group of Cotton.

Silica-supported tantalum clusters: catalyst for alkane conversion

Silica-supported tantalum clusters (on average, approximately tritantalum) were formed by the treatment, in either H(2) or ethane, of adsorbed Ta(CH(2)Ph)(5); the supported catalyst is active for ethane conversion to methane and propane at 523 K, with the used catalyst containing clusters of the same average nuclearity as the precursor.

The structure of small Ta clusters

The structure of small tantalum clusters is investigated by using moleculardynamics simulations. A structural evolution from polytetrahedral structuresto layered Frank–Kasper-type structures is revealed as cluster size increases toN∼100 atoms. The lowest-energy structures have been located for clusters withN≤78. The bulk-like (bcc) structure becomes the most stable structure beyondN∼100. The stabilized structure strongly depends on the cooling rate. A structure similar toβ-Ta,a σ-type Frank–Kasper structure, can be obtained by rapid cooling. The structural propertiesof small Ta clusters presented in the paper provide insight into the formation and origin ofβ-phase Ta.The growth of β-Ta films in practice may be due to the nucleation of Ta clusters with layeredFrank–Kasper-type structure during the initial stage of film growth.

Nanocomposite tantalum–carbon-based films deposited by femtosecond pulsed laser ablation

Nanostructured coatings of metal (tantalum) containing diamond-like carbon (a-C:Ta) have been prepared by femtosecond pulsed laser deposition (PLD). The films, containing 15 at.% tantalum, have been deposited by ablating sequentially graphite and metallic tantalum in vacuum conditions with an amplified Ti:sapphire laser. The coatings have been investigated by X-ray photoelectron spectroscopy, grazing angle X-ray diffraction, energy filtered transmission electron microscopy, scanning and high resolution transmission electron microscopies. Evidence of metallic α-Ta and β-Ta particles (diameter in the 100 nm range) and smaller quasi-amorphous tantalum clusters embedded in the carbonaceous matrix have been shown. A thin tantalum carbide interface between the carbon matrix and the top surface of the tantalum nodules has also been identified. The ability of femtosecond pulsed laser deposition to synthetize nanocomposite carbon-based films and to control their nanostructure is discussed.

Catalytic dehydrogenation of aliphatic amines to nitriles, imines, or vinylamines and dealkylation of tertiary aliphatic amines over halide cluster catalysts of group 5 and 6 transition metals

A molybdenum halide cluster, (H 3O) 2[(Mo 6Cl 8)Cl 6] ⋅ 6H 2O, with an octahedral metal framework was used as a catalyst in a gas-flow reactor under hydrogen or helium. Diethylamine started to react above 300 °C, and dehydrogenation to N-ethylideneethylamine proceeded selectively above 350 °C. Dibutylamine yielded the corresponding dehydrogenation product, and pyrrolidine and piperidine were dehydrogenated to 1-pyrroline and 2,3,4,5-tetrahydropyridine. Dehydrogenation of primary amines yielded the corresponding nitriles and the condensation product N-alkylidenealkylamine. Tripropylamine and tributylamine were dehydrogenated to yield, for the most part, N-vinyldialkylamines. In contrast, triethylamine was dealkylated to diethylamine. Thus, the halide cluster exhibited a characteristic catalysis for aliphatic amines: nitrogen-bonded hydrogens were removed completely to yield nitrile or imine, and when there was no such hydrogen, a neighboring hydrogen or the nitrogen-bonded alkyl group was removed. Niobium, tantalum, and tungsten chloride clusters of the same metal framework were also active catalysts for dealkylation and dehydrogenation of diethylamine.

The structure and stability of β-Ta thin films

Ta films with tetragonal crystalline structure (β-phase), deposited by magnetron sputtering on different substrates (steel, silicon, and silicon dioxide), have been studied. In all cases, very highly preferred (001) orientation was observed in X-ray diffraction measurements. All diffraction data revealed two weak reflections corresponding to d-spacing of 0.5272 and 0.1777 nm. The presence of the two peaks, attributed to (001) and (003) reflections, indicates that β-Ta films exhibit a high preference for the space group of P-42 1 m over P4 2 /mnm, previously proposed for β-Ta. Differences in relative intensities of (00 l) reflections, calculated for single crystal β-Ta σ-type Frank–Kasper structure and those measured in the films, are attributed to defects in the films. Molecular dynamics simulations performed on tantalum clusters with six different initial configurations using the embedded-atom-method potential revealed the stability of β-Ta, which might explain its growth on many substrates under various deposition conditions.

Tantalum Clusters in Oxidic Matrices — Synthesis, Crystal Structure and Magnetic Properties of Eu1.83Ta15O32.

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Tantalum Clusters in Oxidic Matrices — Synthesis, Crystal Structure and Magnetic Properties of Eu1.83Ta15O32

AbstractThe mixed‐valent oxotantalate Eu1.83Ta15O32 was prepared from a compressed mixture of Ta2O5 and the metals in a sealed Ta ampoule at 1400 °C. The crystal structure was determined by means of single crystal X‐ray diffraction: space group R3¯, a = 777.2(6) pm and c = 3523.5(3) pm, Z = 3, 984 symmetrically independent reflections, 83 variables, RF = 0.027 for I &gt; 2σ (I). The structure is isotypic to Ba2Nb15O32. The salient feature is a [Ta(+8/3)6O12iO6a] cluster consisting of an octahedral Ta6 core bonded to 12 edge‐bridging inner and six outer oxygen atoms. The clusters are arranged to slabs which are sandwiched by layers of [Ta(+5)3O13] triple octahedra. Additional Ta(+5) and Eu(+2) atoms provide the cohesion of these structural units. Twelve‐fold coordinated Eu(+2) atoms are situated on a triply degenerate position 33 pm displaced from the threefold axis of symmetry. A depletion of the Eu(+2) site from 6 to 5.5 atoms per unit cell reduces the number of electrons available for Ta‐Ta bonding from 15 to 14.67 electrons per cluster. Between 125 and 320 K Eu1.83Ta15O32 is semi‐conducting with a band gap of 0.23 eV. The course of the magnetization is consistently described with the Brillouin function in terms of a Mmol/(NAμB) versus B/T plot in the temperature range 5 K — 320 K and at magnetic flux densities 0.1 T — 5 T. At moderate flux densities (&lt; 1 T) the magnetic moment agrees fairly well with the expected value of 7.94 μB for free Eu (2+) ions with 4f7 configuration in 8S7/2 ground state. Below 5 K, anisotropic magnetization measurements at flux densities B &lt; 1 T point to an onset of an antiferromagnetic ordering of Eu spins within the layers and an incipient ferromagnetic ordering perpendicular to the layers.

Phasing at high resolution using Ta6Br12 cluster.

The Ta(6)Br(12)(2+) cluster compound is known to be a powerful reagent for derivatization of crystals of large macromolecules at low resolution. The cluster is a regular octahedron of six Ta atoms with 12 bridging Br atoms at the edges of the octahedron. The cluster is compact, of approximately spherical shape, with a radius of about 6 A. Both tantalum and bromine display a significant anomalous diffraction signal at their absorption edges at 1.25 and 0.92 A, respectively. At resolutions lower than 5 A the tantalum cluster behaves as a super-atom and provides very large isomorphous and anomalous signals, which significantly diminish at about 4 A. However, beyond 3 A the individual Ta atoms can be resolved and the phasing power of the cluster increases again. The Ta(6)Br(12)(2+) cluster has been used for phasing four different proteins at high resolution. Ta(6)Br(12)(2+) appeared to be a mild derivatization reagent and, despite partial incorporation, led to a successful solution of crystal structures by the single-wavelength anomalous diffraction (SAD) approach.

Cluster-Crosslinked Inorganic-Organic Hybrid Polymers: Influence of the Cluster Type on the Materials Properties

AbstractMethacrylate-substituted tetranuclear tantalum, zirconium and titanium oxide clusters (Zr4O2(OMc)12, Ti4O2(OPri)6(OMc)6, and Ta4O4(OEt)8(OMc)4) were prepared by reaction of the alkoxides with methacrylic acid. The clusters were then polymerized with methyl methacrylate as co-monomers in different molar ratios (0.5–2 mol% of the functionalized cluster). In the resulting inorganic-organic hybrid polymers, the clusters crosslink the polymer chains very efficiently. The physical properties and structural features of the hybrid polymers, investigated by their swelling behavior, thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), 13C MAS NMR and impedance spectroscopy, exhibit a clear dependence on the cluster portion in the polymer and on the cluster type.

Preparation of bistrimethylsilylmethylniobiumtetrafluoride and the application of KHF 2 and n-Bu 4NHF 2 as fluorinating reagents

BisTiCl 3 ( 1), BisNbCl 4 ( 2) and BisNbBr 4 ( 3) were prepared by alkylation of NbCl 5, NbBr 5 and TiCl 4 with Bis 2Zn ( 4) (Bis=(SiMe 3) 2CH). Compounds 2 and 3 were obtained as solids in moderate yields and are sensitive to moisture and air. In addition, the corresponding fluoride derivative of 2, i.e. BisNbF 4 ( 5) was prepared in moderate yield via chlorine–fluorine metathesis employing Me 3SnF ( 6) as a fluorinating reagent. In another attempt Ti(O i-Pr) 4 was reacted with KHF 2 to give the fluorine-containing titanium cluster μ 3-fluoro-μ 3-oxo-tris(μ- isopropoxy)-hexakis( isopropoxy)-tri-titanium ( 7). The fluorine-free tantalum cluster bis(μ 3-oxo)-hexakis(μ-ethoxy)-octakis(μ-oxo)-tetradecakis(ethoxy)-octa-tantalum ( 8) was obtained from n-Bu 4NHF 2 and Ta(OEt) 5. The identity of all new compounds was documented by analytical/spectroscopic (IR, 1H-, 29Si-, 19F-NMR, MS) data. The crystal structures of 2, 7 and 8 were determined by single crystal X-ray diffraction.

Structural Investigation of Palladium Clusters on γ-AlO3(111)/NiAl(110) with Transmission Electron Microscopy

For Pd clusters on γ-Al 2 O 3 epitaxially grown on NiAl(110) the lattice constant has been determined as a function of the cluster size by evaluating patterns induced by double diffraction involving the NiAl substrate and the cluster lattice. Similar to results reported previously for platinum and tantalum clusters on the same substrate (Surf. Sci. 1997, 391, 27; Surf. Sci. 1998, 413, 192), we observe a reduction of the distances within the lattice with decreasing cluster size. The highest observed reduction is 5% for clusters with a diameter of about 12 A. Within the limits of the experimental errors the reduction of the lattice distances is isotropic. Although the samples have been exposed to air prior to investigation, the clusters are only weakly oxidized as concluded from XPS data.