WS2 Surface Research Articles

Solvothermal synthesis of nickel-tungsten sulfides for 2-propanol dehydration.

The bimetallic nickel-tungsten catalysts were prepared via solvothermal method. The X-ray Diffractometer (XRD) analysis revealed that the corresponding peaks at 14°, 34°, and 58° were for tungsten disulfide (WS2 ) hexagonal phase. The catalysts displayed different crystalline phase with nickel addition, and as an effect the WS2 surface area decreased from 74.7 to 2.0 m(2) g(--1) . In this sense, high-resolution transmission electron microscopy (HRTEM) showed the layers set in direction (002) with an onion-like morphology, and in the center of the particles there is a large amount of nickel contained with 6-8 layers covering it. The catalytic dehydration of 2-propanol was selective to propene in 100% at 250 °C for the sample with 0.7 of atomic ratio of Ni/Ni + W.

UV/ozone‐treated WS2 hole‐extraction layer in organic photovoltaic cells

AbstractWS2 nanosheets obtained through a simple sonication exfoliation method are employed as a hole‐extraction layer to improve the efficiency of organic photovoltaic cells (OPVs). A reduction in the wavenumber difference in the Raman spectra, the appearance of a UV absorption peak, and atomic force microscopy images indicate that WS2 nanosheets are formed through the sonication method. The power conversion efficiency (PCE) values of OPVs with and without untreated WS2 layers are both 1.84%. After performing a UV‐ozone (UVO) treatment on the WS2 surface for 15 min, the PCE increases to 2.4%. Synchrotron radiation photoelectron spectroscopy data show that the work function of WS2 increases from 4.9 eV to 5.1 eV upon UVO treat‐ ment, suggesting that the increase in the PCE value is caused by the band alignment. Upon inserting poly(3,4‐ethylenedioxythiophene):poly(styrene‐sulfonate) (PEDOT: PSS) between the WS2 and the active layer, the PCE value of the OPV increases to 3.07%, which is superior to that of the device employing only PEDOT:PSS (2.87%). Therefore, it is considered that the use of UVO‐treated WS2 is able to improve the performance of OPV cells. (© 2014 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Density functional theory calculations of hydrogen molecule adsorption on monolayer molybdenum and tungsten disulfide

In this work, we report a theoretical study on the adsorption of molecular hydrogen (H2) on monolayer molybdenum disulfide (MoS2) and tungsten disulfide (WS2) using a first-principles van der Waals functional (vdW-DF) method. Our calculations show that the most favorable configuration for the adsorption of a H2 molecule on monolayer MoS2 is similar to that for the monolayer WS2 surface. The H2 molecule is physisorbed on the surface of the monolayers MoS2 and WS2 with adsorption energies of −131.61 and −169.44meV, respectively. Analysis of the electronic structures and charge confirmed that no significant hybridization between the respective orbitals occurs, and quantitative analysis revealed that a small interaction was obtained in terms of binding energies. Furthermore, adsorption of two H2 molecules on one and both sides of the monolayer MoS2 and WS2 was examined. The obtained results indicated that, with the adsorption of the second H2 molecule on one- and both-side of WS2, the binding energies were decreased to −155.2 and −153.6meV, respectively. However, the values for adsorption of the second H2 on monolayer MoS2 remained nearly constant compared to the adsorption of the first H2 molecule. In addition, we calculated the binding energy between the H2 molecule and the substrate (MoS2 and WS2) under axial strain of −10% to 10%, which revealed that the binding energy between the H2 molecule and the substrate was enhanced by axial strain. Our results also indicated that applying compression to monolayer WS2 yields higher reactivity with the hydrogen molecule than applying compression to monolayer MoS2.

First-principles study of graphene adsorbed on WS2 monolayer

We perform first-principles calculations to study the energetics and electronic properties of graphene adsorbed on WS2 surface (G/WS2). We find that the graphene can be bound to WS2 monolayer with an interlayer spacing of about 3.9 ´Å with a binding energy of −21–32 meV per carbon atom dependent on graphene adsorption arrangement, suggesting a weak interaction between graphene and WS2. The nearly linear band dispersion character of graphene can be preserved in G/WS2 system, with a sizable band gap, depending on graphene stacking patterns on WS2 and the distance between graphene and WS2 monolayer. More interestingly, when the interlayer spacing is larger than 3.0 ´Å, the energy-gap opening is mainly determined by the distortion of the isolated graphene peeled from WS2 surface, independent on the WS2 substrate. Further tight-binding model analysis demonstrates that the origin of semiconducting properties can be well understood by the variation of on-site energy of graphene induced by WS2 substrate.

Nanoinduced morphology and enhanced properties of epoxy containing tungsten disulfide nanoparticles

Closed‐cage (fullerene‐like) nanoparticles (NPs) of WS2 are currently produced in large amounts and were investigated as additives to thermoplastics and thermosetting polymers. The nanoinduced morphology and the resulting enhanced fracture toughness of epoxy/WS2 nanocomposites were studied. The morphology of the epoxy nanocomposites was induced by controlled WS2 surface chemistry. The WS2 NPs used were either untreated or chemically treated with acryloxy, which is compatible, and alkyl silane, which is incompatible, respectively, with the epoxy matrix. In the case where the acryloxy silane was used to treat the WS2 particles, good dispersion and compatibility were obtained in the epoxy resin. Moreover, a distinct nodular morphology was induced on fracture as a result of nucleation by the compatible NPs. In the case where the alkyl silane treatment was used cavitation morphology was induced, following mechanical loading, which is the result of incompatibility with the epoxy resin. The fracture toughness results showed an increase of 70% for nanocomposites contains alkyl‐treated WS2 compared with the neat epoxy. Modeling of the nodular morphology enabled the determination of optimal concentration of the WS2 in epoxy (0.3% by weight). Two main fracture mechanisms were observed, crack bowing around the nodular boundaries in the case of compatibility between the nanoparticle and the epoxy and particle‐induced cavitation in the case of incompatibility, respectively. These results are of significant importance both for epoxy‐based adhesives and fiber composites. POLYM. ENG. SCI., 53:2624–2632, 2013. © 2013 Society of Plastics Engineers

A DFT study on benzene adsorption over tungsten sulfides: surface model and adsorption geometries

Benzene adsorption on a WS2(100) surface was studied by ab initio periodic DFT computations. Benzene adsorption is facile on the bridge site of the bare W edge via η2 or η3 coordination. Taking into account the stable configuration at the W edge under typical hydrotreating reaction conditions (623 K, H2S/H2 = 0.01), benzene adsorption is found to be difficult, even when defective bridge sites are created.