Structure Of WS2 Research Articles

Hydrothermal synthesis of variety low dimensional WS2 nanostructures

We report a successful fabrication of tungsten disulfide (WS2) nanostructures with different morphologies including rod-like, sheet-like and fiber-like ones via a simple yet facile hydrothermal process by adding surfactants cetyltrimethyl ammonium bromide (CTAB) and polyethylene glycol (PEG). The structural features of the as-prepared WS2 are characterized systematically by the X-ray powder diffraction and scanning electron microscopy. We find that the surfactants play a critical role in producing low dimensional WS2 structures with different morphologies. We also propose growth mechanisms for the various WS2 morphologies. The simple fabrication of WS2 nanostructures can also be applied for the synthesis of other transition metal sulfides with novel nanostructures.

Decoupling reduction–sulfurization synthesis of inorganic fullerene-like WS2 nanoparticles in a particulately fluidized bed

A novel decoupling reduction–sulfurization process for the synthesis of inorganic fullerene-like (IF) WS2 nanoparticles has been developed in this work. In this strategy, the raw WO3 nanoparticles are firstly reduced by hydrogen in a fluidized bed, followed by sulfurization with H2S in a reducing atmosphere. The diffusion and reaction are the two kinetic factors playing important roles in controlling the structure of WS2. Under diffusion limitation, the porous β-W phase and inordinate WS2 structure are formed, while the α-W phase and IF-WS2 structure are usually obtained in the reaction-limited condition. The particulate fluidization of the reduced WO3 nanoparticles in the agglomerates forms is realized in this decoupling reduction–sulfurization process. The porous β-W nanoparticles and their particulate fluidization enhance the diffusion of H2S, resulting in the completion of sulfurization reaction in short time. This newly developed approach is efficient and cost-effective, and might be extended to the large-scale synthesis of other inorganic fullerene-like metal sulfides.

Synthesis of Fullerene-like WS2 Nanoparticles in a Particulately Fluidized Bed: Kinetics and Reaction Phase Diagram

A novel particulately fluidized bed (PFB) reactor was successfully applied to synthesize inorganic fullerene-like tungsten disulfide (IF-WS2) nanoparticles. The results reveal that H2S concentration is the crucial kinetic factor in regulating the structure of WS2. The H2S concentration beyond a critical value (ca. 4.3%) leads to pure IF-WS2 where the sulfurization reaction prevails over the reduction reaction, whereas H2S concentrations less than the critical value lead to bulk nanorods where the reduction reaction dominates with rapid diffusion of suboxides toward the growing surface. These two distinctly different growth modes of WS2 are attributted to the compromise in competition between the reduction and sulfidization. An IF-WS2 growth kinetics model was also proposed based on the experimental results, which provided basic data and guidance for the design of an industrial fluidized bed for synthesizing IF-WS2.

Superior Field Emission Properties of Layered WS2-RGO Nanocomposites

We report here the field emission studies of a layered WS2-RGO composite at the base pressure of ~1 × 10−8 mbar. The turn on field required to draw a field emission current density of 1 μA/cm2 is found to be 3.5, 2.3 and 2 V/μm for WS2, RGO and the WS2-RGO composite respectively. The enhanced field emission behavior observed for the WS2-RGO nanocomposite is attributed to a high field enhancement factor of 2978, which is associated with the surface protrusions of the single-to-few layer thick sheets of the nanocomposite. The highest current density of ~800 μA/cm2 is drawn at an applied field of 4.1 V/μm from a few layers of the WS2-RGO nanocomposite. Furthermore, first-principles density functional calculations suggest that the enhanced field emission may also be due to an overalp of the electronic structures of WS2 and RGO, where graphene-like states are dumped in the region of the WS2 fundamental gap.

Nanostructured WS2–Ni composite films for improved oxidation, resistance and tribological performance

WS2 films were prepared by radio frequency sputtering. In order to improve its mechanical properties and oxidation resistance, Ni was used as the dopant and the effect of Ni content on the microstructure, anti-oxidation capability, mechanical and tribological properties of composite films were studied by energy dispersive X-ray spectroscopy (EDS), X-ray photoelectron spectroscopy (XPS), Raman spectroscopy, grazing incidence X-ray diffraction (GIXRD), high resolution transmission electron microscope (HRTEM), field emission scanning electron microscopy (FESEM), nano-indentation tester, scratch tester and ball-on-disk tribometer. WS2 existed in nanocrystalline 2H–WS2 structure and Ni an amorphous phase. Increasing the Ni content resulted in a microstructural change from columnar platelet structure of pure WS2 film to a fiber-like structure of the composite film at low Ni content (5.0at%), and to a featureless structure at high Ni content (>10at%). Meanwhile, the films became more and more compact and showed improved anti-oxidation capability. The films represented an increase in hardness with Ni content ranging from 0 to 10.3at% due to the microstructure being densified, but exhibited high brittleness as the Ni content higher than 10at%. The composite film at low Ni content of about 5.0at% showed much better wear resistance than pure WS2 film, but became brittle and had poor wear resistances at high Ni content of above 10.3at%. The WS2–5.0at% Ni composite film exhibited the longest wear life of 5.8×105 cycles about sevenfold better than that of pure WS2 film in humid air. The wear mechanism was discussed in terms of the anti-oxidation capability of the films, morphology of the wear track and formation of transfer film.

Identification of individual and few layers of WS2 using Raman Spectroscopy

The Raman scattering of single- and few-layered WS2 is studied as a function of the number of S-W-S layers and the excitation wavelength in the visible range (488, 514 and 647 nm). For the three excitation wavelengths used in this study, the frequency of the A1g(Γ) phonon mode monotonically decreases with the number of layers. For single-layer WS2, the 514.5 nm laser excitation generates a second-order Raman resonance involving the longitudinal acoustic mode (LA(M)). This resonance results from a coupling between the electronic band structure and lattice vibrations. First-principles calculations were used to determine the electronic and phonon band structures of single-layer and bulk WS2. The reduced intensity of the 2LA mode was then computed, as a function of the laser wavelength, from the fourth-order Fermi golden rule. Our observations establish an unambiguous and nondestructive Raman fingerprint for identifying single- and few-layered WS2 films.

Morphology and atomic-scale structure of single-layer WS2 nanoclusters

Two-dimensional sheets of transition metal (Mo and W) sulfides are attracting strong attention due to the unique electronic and optical properties associated with the material in its single-layer form. The single-layer MoS2 and WS2 are already in widespread commercial use in catalytic applications as both hydrotreating and hydrocracking catalysts. Consequently, characterization of the morphology and atomic structure of such particles is of utmost importance for the understanding of the catalytic active phase. However, in comparison with the related MoS2 system only little is known about the fundamental properties of single-layer WS2 (tungstenite). Here, we use an interplay of atom-resolved Scanning Tunneling Microscopy (STM) studies of Au(111)-supported WS2 nanoparticles and calculated edge structures using Density Functional Theory (DFT) to reveal the equilibrium morphology and prevalent edge structures of single-layer WS2. The STM results reveal that the single layer S-W-S sheets adopt a triangular equilibrium shape under the sulfiding conditions of the synthesis, with fully sulfided edges. The predominant edge structures are determined to be the (101[combining macron]0) W-edge, but for the smallest nanoclusters also the (1[combining macron]010) S-edges become important. DFT calculations are used to construct phase diagrams of the WS2 edges, and describe their sulfur and hydrogen coordination under different conditions, and in this way shed light on the catalytic role of WS2 edges.

Tungsten disulfide catalysts from tetraalkylammonium thiotungstates by ex situ activation, their properties and HDS activity

The role of carbon coming from tetraalkylammonium thiotungstate precursors during the ex situ activation of WS2 catalysts was herein studied through the comparison of two different thiosalts, ammonium tetrathiotungstate (ATT, without carbon) and cetyltrimethylammonium tetrathiotungstate (CTATT, with carbon). The influence on the resulting WS2 catalysts, of a N2 vs a H2S/H2 (20% H2S) atmosphere during ex situ activation was also evaluated. Freshly obtained catalysts were characterized by N2 adsorption–desorption isotherms, X-ray diffraction (XRD), transmission electron microscopy (TEM) and energy-dispersive X-ray spectroscopy (EDX), while their catalytic properties were evaluated in the hydrodesulfurization of dibenzothiophene at 3.1MPa and 623K. No significant difference was found in the textural and morphological properties of the final WS2 catalysts derived from thermal decomposition of ATT under N2 flow, compared to 20% H2S/H2 flow. In contrast, the textural, morphological and catalytic properties of WS2 catalysts derived from CTATT were strongly influenced by the nature of the atmosphere. Samples of WS2 derived from ATT showed an absence of porosity, while WS2 catalysts formed from CTATT presented a mesoporous organization characterized by type IV N2 isotherms. XRD and TEM showed that the use of CTATT induces a strong destacking of the WS2 particles and the excessive formation of amorphous carbon, thus yielding less active WS2 catalysts which confirm the negative role of carbon-containing tetraalkylammonium precursors in the activation of WS2 catalysts, as opposed to their positive influence in the activation of MoS2 catalysts. The very high activity of the WS2 catalyst obtained from ATT is attributed to an optimized incorporation of structural carbon inside the WS2 structure. Finally, selectivity results show the depleted hydrogenating character of the WS2 catalysts formed from CTATT.

Electric transport properties and 1H NMR study of the fullerene‐like WS2 nanoparticles

AbstractElectrical resistivity and Hall measurements, of pellets compacted from IF ‐WS2 nanoparticles and 2H‐WS2 powder were done. Electrical transport measurements were carried out on pellets by the van der Pauw method in a wide temperature range. Arrhenius plots for conductivities of the WS2 samples (2H, IF and IF +treatment) exhibit marked variations in (∂ ln σT –1)/∂T –1 with temperature. Resistivity of IF ‐WS2 pellets is higher than that of 2H‐WS2 pellets. It was found that the electrical properties of IF ‐WS2 powder vary with aposteriory heat treatment under vacuum. The 1H NMR measurements show that the prepared product contains water (and possibly some hydrogen) molecules that occupy the voids in the central part of the fullerene‐like nanoparticles and the nanopores between the adhering IF ‐WS2 particles. Defects in the IF ‐WS2 structure, arising due to the strain release during the folding of the layers, may result in additional sites for the absorbed water. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Synthesis of inorganic fullerene(MS2,M = Zr, Hf and W)phases using H2S and N2/H2 microwave-induced plasmas

Microwave-induced plasmas (MIPs) ofH2S andN2/H2 have been used tosulfidize and reduce WO3 and MS3 (where M = Zr and Hf) phases respectively. Characterization by powder x-ray diffraction andelectron microscopy techniques shows that inorganic fullerene structures ofWS2 andnanorods/nanotubes of ZrS2 can be synthesized. In all cases the structures appear to exhibit a higher concentration ofdefects and final morphologies are reflective of the precursor used. Furthermore, forZrS2 and HfS2 phases significant post-synthesis oxidation is observed.

Synthesis and structure of multi-layered WS2(CoS), MoS2(Mo) nanocapsules and single-layered WS2(W) nanoparticles

Multi-layered MoS2 (or WS2) nanocages stuffed with Mo (or CoS/CoO) nanocrystals have been synthesized by using the reaction between metal nanoparticles and sulfur powders. This simple synthesis method, different from the conventional methods for synthesizing pure inorganic fullerenes, is also potentially important for large-scale synthesis of nanoparticles of other metal dichalcogenide. Besides the multi-layered WS2 nanocapsules, we have successfully fabricated nanocapsules with a single-layered WS2 sheet encapsulating W by using the arc-discharge method. We discuss possible mechanisms for the formation of the unique core-shell structured nanocapsules. (c) 2005 Springer Science + Business Media, Inc.

Realistic Surface Science Models of Hydrodesulfurization Catalysts on Planar Thin-Film Supports: The Role of Chelating Agents in the Preparation of CoW/SiO2catalysts

Realistic surface science models of supported molybdenum- and tungsten-based hydro-treating catalysts have been made by impregnating planar thin films of silica and alumina with aqueous solutions of the normally utilized precursor salts of active elements. These models have enabled detailed studies of catalyst preparation and hydrodesulfurization kinetics. Optimization of the catalysts is possible by applying appropriate chelating agents for the promoting elements cobalt and nickel. These complexes work by stabilizing the promoting element such that during sulfidation MoS2 and WS2 structures form before the promoters are released so that these have a higher chance to end up at the edges of MoS2 and WS2 instead of forming the less active Co9S8 and Ni2S3 phases. The extent to which sulfidation of the promoter cobalt is delayed correlates with the activity of the catalyst for thiophene hydrodesulfurization.

Structure of Restacked and Pillared WS2: An X-ray Absorption Study

Exfoliated and restacked tungsten disulfide were characterized by X-ray absorption at the sulfur K edge and tungsten L3 edge. The local organization around W atoms was probed by EXAFS, by applying a nonGaussian distance distribution model in order to take into account the high disorder in the structure. On the basis of previous structural determinations performed by electron microdiffraction and pair-distribution functions deduced from neutron diffraction, a structural model was tested by comparing the theoretical expected EXAFS signal deduced from this model to the experimental signal. All results converge to prove that upon exfoliation and restacking the WS2 structure evolves toward a clusterization that gives zigzag chains. Moreover, the local shift of the W atoms is in tandem with a periodic shift of the S atoms along the stacking axis, which causes the layer to deviate from full planarity. Further pillaring with various metal clusters does not affect this structural evolution.

New Crystal Structures of WS2: Microtubes, Ribbons, and Ropes

New Crystal Structures of WS2: Microtubes, Ribbons, and Ropes

New Crystal Structures of WS2: Microtubes, Ribbons, and Ropes

WS2 microtubes, ribbons, and ropes are some of the unusual forms of inorganic crystals – grown by a chemical transport reaction – for which evidence is presented here. The Figure shows a WS2 rope composed of four nanotubes with one nanotube periodically wound around the widest tube. A further example, of two nanotubes wound around each other, can be found on the front cover.

Excitons in WS2 films pressure and temperature effects

AbstractA technique for producing thin film samples WS2 is reported. Exciton peak positions are measured as a function of hydrostatic pressure to 3500 atm at 90 K and temperatures in the range 90 to 300 K. Values for (∂E/∂P)T for the A‐ and B‐peaks are 5.9 × 10−6 and 7.2 × 10−6 eV/atm, respectively. Peak C shifts 0.3 × 10−6 eV/atm, while values of (∂E/∂T)P are −1.8 × 10−4, −2.8 × 10−4, and −0.3 × 10−4 eV/K for peaks A, B, and C, respectively. The value of (∂E/∂T)V is estimated at 1 × 10−4 eV/K. A comparison of these results with those for MoS2 is made, and comments are made concerning the nature of the band structure of WS2.

Physical and chemical properties of WS2 catalysts

1. An x-ray structural investigation was made of WS2 catalysts with different W: S ratios and different catalytic activities. 2. Regardless of the W: S ratio (within the limits of WS2.30- WS1.862), the WS2 catalysts consisted of only one crystalline phase, WS2, with identical lattice parameters. 3. WS2 catalysts with excess sulfur above the stoichiometric amount for the formula WS2, did not contain the sulfide of hexavalent tungsten WS2. 4. The excess sulfur above the stoichiometric amount was not incorporated into the crystal structure of WS2 as a result of the deficit of tungsten atoms in it. 5. The crystal structure of the WS2 phase was disordered; it consisted of small, fine blocks of WS2 layers, randomly displaced relative to each other. When WS2 catalysts were briefly heated, appreciable ordering of the crystal structure only occurred above 700°.