Hexagonal WS2 Research Articles

Controlled synthesis and mechanism of large-area WS2 flakes by low-pressure chemical vapor deposition

WS2 flakes have been grown successfully on SiO2(300 nm)/Si substrate via traditional low-pressure chemical vapor deposition method. We studied the controllable growth of WS2 flakes on three of the growth parameters: the time of S-precursor introduction, the temperature of WO3 precursor and the growth temperature. The as-prepared products were characterized by X-ray photoemission spectroscopy, Raman spectra and atomic force microscopy. It is found that the morphologies of WS2 flakes and the products are highly dependent on the concentration of S-precursor, W-precursor and the ratio of W atoms to S atoms, while large-area WS2 flakes up to 160 μm can be obtained. If the ratio of W/S is ≤1:2, we obtain triangular and hexagonal WS2 flakes. On the contrary, if the ratio of W/S is >1:2, besides WS2 flakes, W nanowires will be formed owing to the superfluous W atoms. This study can provide an important and practical guide to preparing large-area and high-quality two-dimensional transition metal dichalcogenides materials.

Heterogeneous Defect Domains in Single-Crystalline Hexagonal WS2.

Single-crystalline monolayer hexagonal WS2 is segmented into alternating triangular domains: sulfur-vacancy (SV)-rich and tungsten-vacancy (WV)-rich domains. The WV-rich domain with deep-trap states reveals an electron-dedoping effect, and the electron mobility and photoluminescence are lower than those of the SV-rich domain with shallow-donor states by one order of magnitude. The vacancy-induced strain and doping effects are investigated via Raman and scanning photoelectron microscopy.

Selective hydrothermally synthesis of hexagonal WS2 platelets and their photocatalytic performance under visible light irradiation

Hexagonal WS2 platelets have been synthesized via simple hydrothermal synthesis method. The hexagonal WS2 is formed by the oriented attachment (OA)-self-assembly (SA). The thickness of the WS2 platelets was between ∼20 and 100 nm. The specific surface area of these platelets is 94.63 m2 g−1. The hexagonal WS2 platelets exhibited excellent photocatalytic activity compared to irregular WS2 platelets for the degradation of rhodamine B (RhB) under visible light irradiation. This work paves the method for designing hexagonal shaped WS2 platelets with great potential for a wide spectrum of applications in photocatalysis. Moreover, this synthetic procedure may open up an opportunity to tailor the morphologies of the other nanomaterials especially transition metal sulfides.

Enhanced Catalytic Activities of Surfactant-Assisted Exfoliated WS₂ Nanodots for Hydrogen Evolution.

WS2 nanodots were prepared by liquid-phase exfoliation of bulk WS2 crystals in surfactant aqueous solution with the aid of ultrasonication. Their behaviors on catalyzing hydrogen evolution reaction (HER) were investigated after drop-casting them onto a glass carbon electrode. On the basis of the optical and electron characterizations, the nanodots were identified with a high concentration of octahedral phase of WS2 that showed better catalysis properties than the hexagonal WS2. From the polarization curve, the Tafel slope was estimated to be 51 mV per decade and the onset potential was 90 mV, indicating good catalytic performance of such nanodots. Our results suggest that surfactant-mediated exfoliation is an environmentally benign method to synthesize WS2 nanodots for improved catalyzing HER.

Fabrication of two-dimensional nanosheets via water freezing expansion exfoliation

Layered materials, if exfoliated effectively, will exhibit several unique properties, offering great potential for diverse applications. To this end, in this study, we develop a novel, universal, and environmentally friendly method named as ‘water freezing expansion exfoliation’ for producing two-dimensional nanosheets. This method exploits the expansion in the volume of water upon freezing. When the water freezing expansion condition is reproduced in layered materials, the layers exfoliate to overcome the van der Waals force between them. The expansion process is performed by repeated cycling between 4 °C and −20 °C to effectively exfoliate layered materials of graphite, hexagonal boron nitride (h-BN), MoS2 and WS2. Systematic characterization of the samples thus obtained using electron microscopy and optical studies substantiate the formation of thin flakes (graphene, h-BN, MoS2, and WS2 nanosheets). The method demonstrated in this study is cost-effective and does not demand sophisticated equipment and stringent high temperature conditions. Given this general applicability, this method holds great promise for exfoliating layered materials that are sensitive to elevated temperature.

Atomic-scale imaging and spectroscopy of defects in low-dimensional materials

Atomic defects or edge structures are of great importance for any kind of low-dimensional materials, since the interrupted periodicities strongly affect their physical and/or chemical properties. Studies of point defects in mono-layered materials have become very popular among scientists. Vacancies and topological defects in graphene are routinely examined at atomic level. Defects and edge structures in hexagonal boron nitride (h-BN) and WS2 nano-ribbons are also a hot topic among physicists. Here we describe TEM and spatially resolved EELS studies of various single-layered materials with the interrupted periodicities. Atomic defects and edge structures can be unambiguously identified with the elemental assignment. The monovacancy analysis in h-BN single-layer and the alloying and doping behaviors of MoS2/WS2 single-layers will be presented.

Sputter Deposited Nanostructured Au–WS2 Solid Lubricant Coatings

Au-WS2 nanocomposite coatings with approximately 60 at.% Au were prepared using magnetron sputtering from high purity Au and WS2 targets. Structural characterization of the coatings using X-ray diffraction indicated the presence of (111), (200) and (220) peaks of cubic Au and broad bands corresponding to (002) and (004) planes of hexagonal WS2. Surface morphology of Au-WS2 coatings was studied using field emission scanning electron microscopy (FESEM) and atomic force microscopy which showed formation of uniformly distributed interconnected ligament-like features. High resolution transmission electron microscopy studies showed the presence of a two phase nanocomposite structure wherein nanocrystalline Au was dispersed in a matrix of nanocrystalline and amorphous WS2. The electrical resistivity of Au-WS2 coating measured using four-point-probe method was approximately 0.2 μΩm compared to Au and WS2 coatings which exhibited electrical resistivity values of approximately 0.04 μΩm and 0.057 Ωm, respectively. Ball-on-disc reciprocating tests at a load of 7 N showed that Au-WS2 nanocomposite coating exhibited a friction coefficient of 0.22 after 57,600 cycles and outperformed Au and WS2 coatings. Analyses of the ball and wear track of Au-WS2 coating using FESEM at the end of the wear test indicated formation of smooth transfer films, less accumulation of wear debris and reduced oxidation.

Synthesis of Highly Ordered Mesoporous Crystalline WS2 and MoS2 via a High-Temperature Reductive Sulfuration Route

A high-temperature reductive sulfuration method is demonstrated to synthesize highly ordered mesoporous metal sulfide crystallites by using mesoporous silica as hard templates. H2S gas is utilized as a sulfuration agent to in situ convert phosphotungstic acid H3PW12O40.6H2O to hexagonal WS2 crystallites in the silica nanochannels at 600 degrees C. Upon etching silica, mesoporous, layered WS2 nanocrystal arrays are produced with a yield as high as 96 wt %. XRD, nitrogen sorption, SEM, and TEM results reveal that the WS2 products replicated from the mesoporous silica SBA-15 hard template possess highly ordered hexagonal mesostructure (space group, p6mm) and rodlike morphology, analogous to the mother template. The S-W-S trilayers of the WS2 nanocrystals are partially oriented, parallel to the mesochannels of the SBA-15 template. This orientation is related with the reduction of the high-energy layer edges in layered metal dichalcogenides and the confinement in anisotropic nanochannels. The mesostructure can be 3-D cubic bicontinuous if KIT-6 (Iad) is used as a hard template. Mesoporous WS2 replicas have large surface areas (105-120 m2/g), pore volumes ( approximately 0.20 cm3/g), and narrow pore size distributions ( approximately 4.8 nm). By one-step nanocasting with the H3PMo12O40.6H2O (PMA) precursor into the mesochannels of SBA-15 or KIT-6 hard template, highly ordered mesoporous MoS2 layered crystallites with the 2-D hexagonal (p6mm) and 3-D bicontinuous cubic (Iad) structures can also be prepared via this high-temperature reductive sulfuration route. When the loading amount of PMA precursor is low, multiwalled MoS2 nanotubes with 5-7 nm in diameter can be obtained. The high-temperature reductive sulfuration method is a general strategy and can be extended to synthesize mesoporous CdS crystals and other metal sulfides.

Pulsed Laser Syntheses of Layer-Structured WS2 Nanomaterials in Water

We used water as an environmental friendly medium for the synthesis of hexagonal WS2 nanoparticles by the pulsed laser method. The materials collected on substrates were oriented with the 2H-WS2 basal planes parallel to the surface. The use of water, UV lasers, and large WS2 targets prevented the nanoparticles from restructuring into inorganic fullerenes, which were observed in research using hydrocarbon solvents, longer wavelength lasers, and dispersed powder targets. Fairly good dispersion of nanoparticles suggests that large surface areas are available for chemical reactivity.

Chemical and physical characterization of C(N)-doped W–S sputtered films

The load-bearing capacity of self-lubricating W–S films can be improved by doping with nitrogen or carbon. In this study, the chemical composition, the atomic bonding, the structure, and the surface and cross section morphologies of sputtered W–S–C(N) films were analyzed. The addition of the doping element leads to a progressive broadening of the x-ray diffraction (XRD) peaks indicating a loss of crystallinity. In W–S–N films, amorphous structure could be obtained. In W–S–C films, W–C compounds were detected in conjunction with the hexagonal WS2 phase. For the highest C contents, a nanocomposite structure, including those phases and graphite, was suggested for the film. X-ray photoelectron spectroscopy results showed different types of bonds in the W4f peak in good agreement with the XRD results, i.e., when W–C(N) compounds were indexed W–S, W–C, and W–N bonds are present in the W4f peak. For the highest C content film, the detection of C–C bond in the C1s peak confirmed the formation of graphite.

Search for muonium states in BN, WS2 and carbon nanotubes

A sizable missing fraction was found for semiconductors (cubic) BN and (hexagonal) WS2. A repolarization measurement at room temperature yielded a high hyperfine frequency for the dominant muonium signal in BN. The missing fraction in carbon nanotubes with average 20 nm outer diameter was less than 4% at temperatures above 5 K, with very small relaxation in transverse and longitudinal fields, indicating that such tubulenes are microscopically (semi-)metals or small-gap semiconductors, non-magnetic and non-superconducting.