Electrical conductivity of Mo6S3I6 and Mo6S4.5I4.5 nanowires

Electrical conductivity measurements on novel materials composed of bundles of Mo₆S₃I₆ and Mo₆S_4.5I_4.5 nanowires pressed in a sheet form, in the frequency range from 10 Hz to 1 MHz for temperatures between 4 K and 330 K, are reported. Temperature dependence of the quasistatic electrical conductivity show similar dependence at higher temperatures as expected for three-dimensional variable range hopping mechanism with decreasing electrical conductivity as temperature decreases. Below 50 K the electrical conductivity becomes completely temperature independent. It seems that at lower temperatures fluctuation-induced tunnelling mechanism dominates in these nanostructured materials.     

Surface immobilization of Mo6I8 octahedral cluster cores on functionalized amorphous carbon using a pyridine complexation strategy

Tetrahedral amorphous carbon (a-C) films have been grown by pulsed laser deposition to investigate a liquid phase process for surface immobilization of electroactive [Mo₆Iⁱ₈]^4 + transition metal cluster cores using a complexation reaction with a pyridine-terminated alkyl monolayer covalently bonded to the a-C surface (Py^S–alkyl/a-C). These films are stable against thermally-assisted grafting of alkene molecules and the covalent CC interface provides a robust monolayer/a-C assembly. Octahedral [Mo₆Iⁱ₈]^4 + cluster cores with iodine inner ligands and labile triflate apical ligands [Mo₆Iⁱ₈(CF₃SO₃)^a₆]^2 − have been immobilized through partial complexation in apical positions by surface pyridine groups (Py^S). The remaining CF₃SO₃⁻ apical ligands of [Mo₆Iⁱ₈ (Py^S)^a_y(CF₃SO₃)^a_6 − y] cluster units were further substituted with bromopyridine (Py-Br) to obtain air stable surface with expected final composition [Mo₆Iⁱ₈ (Py^S)^a_y(Py-Br)^a_6 − y]. The yield of the different reaction steps is followed by X-ray photoelectron spectroscopy, providing cluster coverage Σ_Mo6I8 = 9 × 10¹² cm^− 2. Each [Mo₆I₈]^4 + cluster is bound to the carbon surface through multiple anchoring metal sites (N_PYR = 3 or 4), indicating that pyridine-terminated alkyl chains are flexible enough to accommodate four bonds. Electrical transport through Hg//Mo₆I₈–Py^S–alkyl/a-C/p-Si(111) junctions shows rectifying current–voltage characteristics but does not reveal any signature of cluster immobilization.     

A novel facile synthesis and characterization of molybdenum nanowires

We describe a straightforward technique to synthesize pure Mo nanowires (NWs) from Mo₆S_yI_z (8,2 <y + z ≤ 10) NWs as precursor templates. The structural transformations occur when Mo₆S_yI_z NWs are annealed in Ar/H₂ mixture leading to the formation of pure Mo NWs with similar structures as initial morphologies. Detailed microscopic characterizations show that large diameters (>15 nm) Mo NWs are highly porous, while small diameters (<7 nm) are made of solid nanocrystalline grains. We find NW of diameter 4 nm can carry up to 30 μA current without suffering structural degradation. Moreover, NWs can be elastically deformed over several cycles without signs of plastic deformation.     

Tribological performances of Mo6S3I6 nanowires

A new nanowire-like material with the chemical formula Mo₆S₃I₆ was studied as an additive in a synthetic base oil, a polyalphaolefin (PAO). This material presents interesting friction reducing properties, friction coefficient reaching a value of 0.04 in boundary lubrication. Transmission and scanning electron microscopy, X-ray diffraction, and Raman spectroscopy were used to characterize nanowires before and after friction. The combination of these techniques gave evidence of MoS₂ formation in contact area during friction test. This structural evolution of nanowires explains their good friction reducing properties.     

The MoS2 Nanotubes with Defect-Controlled Electric Properties

We describe a two-step synthesis of pure multiwall MoS₂ nanotubes with a high degree of homogeneity in size. The Mo₆S₄I₆ nanowires grown directly from elements under temperature gradient conditions in hedgehog-like assemblies were used as precursor material. Transformation in argon-H₂S/H₂ mixture leads to the MoS₂ nanotubes still grouped in hedgehog-like morphology. The described method enables a large-scale production of MoS₂ nanotubes and their size control. X-ray diffraction, optical absorption and Raman spectroscopy, scanning electron microscopy with wave dispersive analysis, and transmission electron microscopy were used to characterize the starting Mo₆S₄I₆ nanowires and the MoS₂ nanotubes. The unit cell parameters of the Mo₆S₄I₆ phase are proposed. Blue shift in optical absorbance and metallic behavior of MoS₂ nanotubes in two-probe measurement are explained by a high defect concentration.     

The synthesis and characterization of Pb5S2I6 whiskers and tubules

Pb₅S₂I₆ whiskers and tubules were synthesized from the reaction among lead chloride, thiourea, and excess sodium iodide under hydrothermal conditions at 200 °C for 20–40 h. XRD, SEM, XPS, ICP-AES, and TEM characterized the final products. Most products are whiskers with structure of 3–4 mm in length, 0.5–2.0 μm in diameter for a singular one. Meanwhile, about 10% tubules are produced in the process. The tubules are 3–6 mm in length, 8–20 μm in diameter, and 1–3 μm in thickness. Nanowhiskers were also produced in the route at 180–200 °C for 8–10 h. Raman spectra show that the Pb₅S₂I₆ crystals have complex vibrational modes of PbS and PbI₂.     

Intrinsic defects,Mo-related defects,and complexes in transition-metal carbide VC: A first-principles study

Intrinsic defects and Mo-related defects in vanadium carbide VC, as well as the defect complexes between vacancies and Mo defects were investigated by means of first-principles calculations within the framework of density functional theory. In addition, Mo diffusion in VC was also studied using LST/QST method. The formation energies of defects have clearly shown that except C vacancy (V_C) all other point defects are not energetic favorable compared to perfect VC. V_C can exist in the lattice forming nonstoichiometric carbide VC_x (x < 1), and also can stabilize the Mo-related defects (S_Mo-V, S_Mo-C, and T_Mo). Free Mo atoms have the strong tendency to enter the already formed V_V and occupy the lattice position of V atoms. Meanwhile, Mo atom in C lattice (S_Mo-C) and interstitial Mo (I_Mo) atom can also enter the V_V position stabilizing the lattice structure. S_Mo-C + V_V will transform into S_Mo-V + V_C and I_Mo + V_V will transform into S_Mo-V during optimization, and large binding energy makes Mo atom tend to exist in the interstitial position. From the perspective of energy, Mo atom tends to diffuse through the interstitial position.     

A novel 1D molybdenum oxide ribbon bridged by pyridine-4-carboxylic ligands: [(C6H4NO2) Mo2O5OH]

A novel 1D organic–inorganic hybrid compound [(C₆H₄NO₂)Mo₂O₅OH] 1 has been synthesized hydrothermally and characterized by single crystal X-ray diffraction, IR, and thermogravimetric analysis. The structure of [(C₆H₄NO₂)Mo₂O₅OH] consists of {MoO₆}octahedra and pyridine-4-carboxylic ligands. {MoO₆} octahedra share their adjacent equatorial edges, forming a new 1D ribbon-like molybdenum oxide framework. Pyridine-4-carboxylic ligands are bridged to the molybdenum sites up and down along the ribbon.     

Anhydrous organic dispersions of highly reduced chemically converted graphene

A straightforward, systematic approach to the reduction of graphene oxide (GO) that affords dispersions of chemically converted graphene (CCG) in anhydrous organic solvents with decreasing basal plane defects is reported. The extent of reduction can be controlled and optimized, resulting in highly reduced dispersible chemically converted graphene (hrCCG) having an O_1S/C_1S ratio of 0.06, which approaches that of graphite. The hrCCG dispersion in anhydrous dimethylformamide (DMF) was stable for several months at a concentration of 0.5–0.6 mg mL⁻¹. This process was found to be easily scalable and could be exploited for the large scale production of hrCCG in DMF and its dispersion in other anhydrous organic solvents. This study demonstrates that the stability of the graphene dispersion is critically dependent on the exfoliation process. The improved elimination of basal defects and the restoration of aromaticity, while maintaining dispersion stability on a large scale in an anhydrous organic solvent, greatly increase the potential of this material for a wide variety of applications.     

Photopolymerization of unsaturated cyclic ethers

2,3-Dihydro-4H-pyran, DHP, and 2,3-dihydrofuran, DHF, are photopolymerized at ambient temperature with the aid of diphenyl iodonium hexafluorophosphate, Ph₂I⁺PF₆⁻ or triphenyl sulfonium hexafluorophosphate, Ph₃S⁺PF₆⁻ at λ_inc = 310 or 340 nm, respectively. DHP is photopolymerized in bulk with the aid of Ph₂I⁺PF₆⁻ and in CH₂Cl₂ solution with the aid of Ph₃S⁺PF₆⁻. The average molar mass of the polymer is low and the propagation involves both C  C bonds and ring-opening. DHF is photopolymerized in CH₂Cl₂ solution using Ph₃S⁺PF₆⁻. The average molar mass of the polymer increases with conversion (living polymerization) and can become high (4 × 10⁵ g mol⁻¹). The propagation exclusively involves C  C bonds. Dark polymerization of DHF occurs at ambient temperature upon the addition of Ph₂I⁺PF₆⁻, but Ph₃S⁺PF₆⁻ is inert against DHF. DHP does not undergo dark reactions with the two salts.     

Linear and branched polyoxide‐based copolymers: Methods to determine the CMC

Evaluation of the physical–chemical properties of aqueous solutions of nonionic surfactants based on polyoxides can be performed by different methods. Depending on the technique used, there can be a significant variation in the critical micelle concentration (CMC) found. This is related to the sensitivity of the technique regarding the unimers and micelles present in the solution as well as the structure of the surfactant evaluated. In this work, the CMC values of aqueous solutions of linear and branched poly(ethylene oxide‐polypropylene oxide) (PEO‐PPO) block copolymers were determined by tensiometry, fluorescence, and particle size analysis, using copolymers having adjacent structures (that is, hydrophilic and hydrophobic segments located adjacently in the copolymer) and alternating structures. Tensiometry was used to measure the surface tension as a function of the copolymer concentration in aqueous solution. Fluorescence was used to determine the fluorescence intensity of pyrene to plot the graphs of the I₁/I₃ and I_E/I_M relations according to the surfactant concentration. Finally, particle size analysis was used to determine the diffusion coefficient of the particles. The results showed that the fluorescence and particle size techniques provide lower (and mutually concordant) CMC values and can be considered more precise because these methods directly analyze the bulk of the solution. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

An unprecedented 2D isopolymolybdate with (3·4)(3·5·8)(3·4·5·6·8)(3·4·5·6) topology

A novel two-dimensional molybdenum oxide polymer, [Mo₄O₁₃]_n · 2nH₃O, has been prepared under specific hydrothermal conditions and characterized by IR spectroscopy and TG analysis. Single-crystal X-ray diffraction analysis reveals that compound 1 exhibits a (3, 4, 5, 6)-connected 2D layer structure with (3²·4)(3²·5³·8)(3·4²·5⁴·6·8²)(3⁴·4³·5⁴·6⁴) topology, which is constructed by two parallel molybdenum oxide dimeric chains (Mo₂O₁₀)_n bound together by the dimer of Mo₂O₉ subunits via the sharing of corners. Additionally, the electrochemistry activity of compound 1 is also reported.     

The effect of Mo2C additions on the oxidation resistance of (Ti,W)CN cermets as base material for the production of cutting tools

The effect of Mo₂C secondary carbide on the oxidation resistance of (Ti_0.93W_0.07)CN_0.3-20%Ni cermet was investigated using DSC-TG, isothermal oxidation, XRD, and SEM/EDS characterisations. Adding Mo₂C secondary carbides to (Ti, W)CN cermets is one of the methods used for improving their mechanical properties. However, Mo₂C secondary carbides bring a detrimental effect to oxidation resistance. The isothermal oxidation results of the samples at 750 °C possessing 0% and 2% MO₂C follow the parabolic law, while in the samples containing 5% and 8% Mo₂C, the rate of oxidation and the spallation of the oxide scale increased significantly. The thickness of the oxide layer in the cermet containing 5% of Mo₂C increased three times, whereas the mass per unit area of the detached oxides in the 8% Mo₂C specimen dramatically increased up to 100 times. The investigations revealed that the increase in the amount of W and Mo on the cermet material, as well as the formation of a multi-zone oxide layer, are the main reasons for oxide phase propagation.     

Structure-HDS Reactivity Relationship of Dibenzothiophenes Based on Density Functional Theory

Perpendicular adsorption of alkyl-substituted dibenzothiophenes on a Mo₁₀S₁₈cluster was studied using density functional theory. The Mayer bond order between the sulfur atom of dibenzothiophene and the molybdenum atom of the Mo₁₀S₁₈cluster was calculated from the optimized adsorption complex. Depending on the position of the alkyl substitution in dibenzothiophene, the values of the Mayer bond order were classified into three groups, and compared with the experimental hydrodesulfurization (HDS) rate constants measured at 320 °C, 70 atm, and 1. 6 h^-1over a NiMo/Al₂O₃catalyst using a light cycle oil.     

Study on the structure of nylon 6 films iodinated before forming. II. Investigation of the crystallization behavior through thermal analysis

Two kinds of amorphous nylon 6 films iodinated before forming from the powders iodinated with 0.2N and 1.0N I₂/KI aqueous solutions were prepared by a melt‐press, and isothermally treated at 20 to 80°C for 1 day to 20 days. Thermal analyses were performed to investigate mainly the crystallization behavior on the treatment. The DSC thermograms for the treated films exhibit three temperature‐groups of endothermic peaks at 60 ～ 70°C, 105 ～ 120°C, and higher than 150°C, which may be associated with the melting of the complex crystal, the relaxed γ‐crystal, and the relaxed α‐crystal, respectively. The film containing less I₂/KI and treated at the higher temperature exhibits the peaks associated with the more stable type of crystal. The peak temperature generally increases with the treating temperature and time. On the occasion of there being two peaks associated with the γ‐crystal and the α‐crystals, ΔH for the α‐crystal increases while that for the γ‐crystal decreases with increasing the treating time. The TG curves indicate two temperature‐zones of weight loss by the volatilization of I₂ from I₅^? and the decompositions of I₃^? and nylon 6. With increasing treating temperature, the % weight loss by the volatilization of I₂ decreases, and consequently the temperature of the weight loss by the decomposition of nylon 6 increases. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 94: 1062–1069, 2004     

Phase evolution,structure, and up-/down-conversion luminescence of Li6CaLa2Nb2O12:Yb3+/RE3+ phosphors (RE = Ho,Er, Tm)

Garnet-type Li₆Ca(La_0.97Yb_0.02RE_0.01)₂Nb₂O₁₂ (RE = Ho, Er, Tm) new phosphors were successfully synthesized via solid reaction at 900°C for 5 hours, whose course of phase evolution, macroscopic/local crystal structure and up-/down-conversion (UC/DC) photoluminescence were clarified. Mechanistic study and materials characterization were attained via XRD, Rietveld refinement, DTA/TG, electron microscopy (FE-SEM/TEM), and Raman/reflectance/fluorescence spectroscopies. The phosphors were shown to exhibit UC luminescence dominated by a ~ 553 nm green band (⁵F₄/⁵S₂ → ⁵I₈ transition) for Ho³⁺, a ~ 568 nm green band (⁴S_3/2 → ⁴I_15/2 transition) for Er³⁺ and a ~ 806 nm near-infrared band (³H₄ → ³H₆ transition) for Tm³⁺ under 978 nm laser excitation, with CIE chromaticity coordinates of around (0.31, 0.68), (0.38, 0.60) and (0.17, 0.24), respectively. Analysis of the pump-power dependence of UC intensity indicated that all the emissions involve a two-photon mechanism except for the ~ 486 nm blue emission of Tm³⁺ (¹G₄ → ³H₆), which requires a three-photon process. The DC luminescence of these phosphors is featured by dominant bands at ~ 553 nm for Ho³⁺ (green, ⁵F₄/⁵S₂ → ⁵I₈ transition), ~568 nm for Er³⁺ (green, ⁴S_3/2 → ⁴I_15/2 transition) and ~ 464 nm for Tm³⁺ (blue, ¹D₂ → ³F₄ transition). The UC and DC properties were also comparatively discussed.     

Dry reforming of methane to synthesis gas over supported molybdenum carbide catalysts

The dry reforming of methane at elevated pressure over supported molybdenum carbide catalysts, prepared from oxide precursors using ethane TPR, has been studied. The relative stability of the catalysts is Mo₂C/Al₂O₃>Mo₂C/ZrO₂>Mo₂C/SiO₂>Mo₂C/TiO₂, and calcination of the oxide precursor for short periods was found to be beneficial to the catalyst stability. Although the support appears to play no beneficial role in the methane dry reforming reaction, the alumina-supported material was stable for long periods of time; this may be important for the production of pelletised industrial catalysts. The evidence suggests that the differences in the stabilities may be due to interaction at the precursor stage between MoO₃ and the support, while catalyst deactivation is due to oxidation of the carbide to MoO₂, which is inactive for methane dry reforming.     

Preparation of the mixed sulfide Nb2Mo3S10 catalyst from the mixed oxide precursor

Mixed sulfide Nb₂Mo₃S₁₀ was obtained by reaction of H₂S with the mixed oxide Nb₂Mo₃O₁₄ at 873 K. High‐resolution electron microscopy and X‐ray diffraction show the formation of homogeneous lamellar mixed sulfide. Due to the increased lability of oxygen, the sulfidation of the mixed oxide is easier compared to that of Nb₂O₁₅ oxide. As follows from the characterization of transient solid products, the transformation from the oxide to the sulfide includes the intermediate formation of a partially reduced “bronze” compound. For the thiophene hydrodesulfurization model reaction, the catalytic properties of Nb₂Mo₃S₁₀ mixed sulfide are intermediate between those of the individual MS₂ sulfides (M = Mo, Nb), but closer to those of the Nb compound. This revised version was published online in July 2006 with corrections to the Cover Date.     

Synthesis and properties of block polypseudorotaxanes by threading cucurbit[6]uril

A novel water-soluble block polypseudorotaxane (4) was synthesized in water from cucurbit[n]uril (n?=?6), designated as CB[6], and a block copolymer of methoxy poly(ethylene glycol)-b-poly-[N ¹-(4-vinylbenzyl)-pentane-1,5-diamine dihydrochloride]-b-methoxy poly(ethylene glycol) (P3). Driven by the hydrophobic/hydrophobic interaction and the association between the diamine and glycoluril units, CB[6] beads are localized on the pentamethylene units in the side chains of P3 as found by NMR studies. The degree of threading, i.e., the average number of CB[6] beads per repeat recognition unit of P3, can be controlled from 0.25 to 1.0 by varying the amount of CB[6] added. This molecular feature leads to interesting aggregation behavior of the polypseudorotaxanes in aqueous solutions, as revealed by dynamic light scattering measurements, transmission electron microscopy observations, ultraviolet/visible spectroscopy (UV?Cvis) and fluorescence spectrometry. The average hydrodynamic radius (R _h), the intensity of UV?Cvis absorption band and the fluorescence intensity (I _f) of the block polypseudorotaxanes in solution increase with the increasing of threaded CB[6]. With the solution temperature increasing, the size of the aggregates in water increased and the fluorescence intensity (I _f) of the solution decreased, which shows the polypseudorotaxanes can sensitively respond to temperature changes. This opens the door for the application of the block polypseudorotaxanes in various fields such as drug delivery and controlled release.     

Crystal structure,impedance and broadband dielectric spectra of ordered scheelite-structured Bi(Sc1/3Mo2/3)O4 ceramic

Bi(Sc_1/3Mo_2/3)O₄ ceramics were prepared via solid state reaction method. It crystallized with an ordered scheelite-related structure (a?=?16.9821(9)?Å, b?=?11.6097(3)?Å, c?=?5.3099(3)?Å and β?=?104.649(2)°) with a space group C12/C1, in which Bi³⁺, Sc³⁺ and Mo⁶⁺ are ?8, ?6 and ?4 coordinated, respectively. Bi(Sc_1/3Mo_2/3)O₄ ceramics were densifiedat 915?°C, giving a permittivity (ε_r) ～24.4, quality factor (Qf, Q?=?1/dielectric loss, f?=?resonant frequency) ～48, 100?GHz and temperature coefficient of resonant frequency (TCF)?～??68?ppm/°C. Impedance spectroscopy revealed that there was only a bulk response for conductivity with activation energy (E_a) ～0.97?eV, suggesting the compound is electrically and chemically homogeneous. Wide band dielectric spectra were employed to study the dielectric response of Bi(Sc_1/3Mo_2/3)O₄ from 20?Hz to 30?THz. ε_r was stable from 20?Hz to the GHz region, in which only ionic and electron displacive polarization contributed to the?ε_r.