WS2 Nanoflakes Research Articles

Enhanced ammonia sensing properties of rGO/WS2 heterojunction based chemiresistive sensor by marginal sulfonate decoration

Abstract Two-dimensional reduced graphene oxide (rGO) nanosheets were pre-sulfonated and further incorporated by WS2 nanoflakes to form S-rGO/WS2 heterojunctions by hydrothermal synthesis. The sulfonation and WS2 incorporation in S-rGO/WS2 heterojunction have been confirmed by EDS, XPS and FTIR. The sensing performance of rGO-based chemiresistive-type sensor to ammonia at room temperature has been significantly improved by the sulfonation and incorporation of WS2 nanoflakes. The chemiresistive sensor based on S-rGO/WS2 exhibits enhanced response signals to 10∼50 ppm ammonia, while the response/recovery time is significantly shortened compared to rGO/WS2 reported previously at room temperature. It benefits from both the extra introduced active acid centers sensitive to ammonia molecules and the better desorption-ability facilitated significantly by sulfonate groups (S O3H). The adsorption energy calculation of ammonia molecule on the sulfonated graphene sheet based on density functional theory (DFT) has been performed to illustrate the adsorption capacity of S O3H group. The selectivity and stability of gas sensor based on S-rGO/WS2 have also been studied.

Highly symmetric and delayed excitonic emission response and space charge-limited current transport in β-irradiated WSe2 and WS2 nanoflakes

We report on the highly symmetric excitonic emission, manifested Raman active modes, and electrical properties of WSe2 and WS2 nanoflakes subjected to energetic β-particles. Raman spectra have revealed numerous mixed modes and a declining trend of the E12g-to-A1g intensity ratio with radiation dose (0–4.8 kGy). The nanoscale WSe2 exhibits a remarkable luminescence peak, located at ~ 675 nm and characterized by a combinatorial effect of both 2s and 2p excitons, whereas the nanoflakes of WS2 offered a testimony to charged excitons (trions) in the range ~ 624–630 nm. Moreover, a moderate (from ~ 2.29 to ~ 2.88 ns) and a nearly sevenfold enhancement in the slow life-time decay parameters have been observed for the irradiated WSe2 and WS2 nanoflakes, respectively. Furthermore, the rectifying nature of the I–V trends suggests formation of metal–semiconductor nanojunctions with transport characteristics described by parameters such as ideality factor (η) and barrier height (φb), both of which gave a declining trend with increasing β-dose. As substantiated by transport characteristics and first principle calculations, a semi-metallic behavior of WSe2 was also realized due to the introduction of excessive chalcogen defects at the highest radiation dose. The results would strengthen our insights prior to their deployment in next-generation nanodevices integrating nano-electronics and nano-photonics at large.

Structural, optical, magnetic and electrochemical properties of hydrothermally synthesized WS2 nanoflakes

Transition metal dichalcogenides (TMDCs) have emerged as highly intriguing materials due to their unique tunable band gap properties with layer number, finding applications in optoelectronic devices. TMDCs have been considered as potential hydrogen evolution reaction (HER) catalysts. Here we describe the hydrothermal synthesis of WS2, a promising TMDC material for efficient HER, under various conditions, and its structural, optical and magnetic and electrochemical studies are discussed in detail. The WS2 nanoflakes have good optical absorption property in region of 400–700 nm and the photoluminescence spectra show the excitation-dependent emission from two peaks corresponding to A and B excitons due to the polydispersity of nanoflakes. WS2 nanoflakes show room temperature ferromagnetism, which is attributed to the presence of zigzag edges in the magnetic ground state at the grain boundaries. The electrochemical results make them a potential candidate in energy conversion. The tunable optical and magnetic properties of WS2 could empower a great deal of flexibility in designing atomically thin optoelectronic and spintronic devices.

Photocurrent enhancement of hybrid perovskite CsGeBr3 assisted two-dimensional WS2 nano-flakes based on electron-hole mobility improvement

The high-performance photocurrent generations of tungsten disulfide Nano-flakes (WS2 NFs) are studied for optoelectronic applications and transistor structures. Moreover, a lead-free inorganic perovskite is formed by replacing of Lead component with germanium at CsPbX3 (X = Cl, Br, I) to achieve an excellent performance of the WS2 based photodetector. The results display that role of the perovskite on the CsGeBr3/WS2 hetero-structure is excellent to enhance photocurrent of the hetero-structure. The external quantum efficiency of the CsGeBr3/WS2 hetero-structure is obtained to be ~151%, and a detectivity is about 4.72 × 108 Jones. Indeed, the external quantum efficiency and detectivity enhance about two times after drop casting of the CsGeBr3 on the WS2 NFs device under green illumination. This finding results from highly efficient photo-excited carrier separation in the interface of the WS2 and CsGeBr3. Drain-source photocurrent in terms of electrical field displays two regimes related to direct and indirect excitons. Indeed, the possibility of exciton indirect formation enhances with increasing of the electrical field due to transfer momentum to electron.

Nanomechanical Gas Sensing with Laser Treated 2D Nanomaterials

Abstract2D nanomaterials such as graphene oxide (GO), molybdenum disulfide (MoS2), and tungsten disulfide (WS2) are viable candidates for use in chemical gas sensors due to their large specific surface area available for analyte adsorption. In this work, these 2D materials are treated with a femtosecond laser process to intentionally introduce defects, dopants, and functional groups to the material for improved gas adsorption properties. The materials are coated onto a nanomechanical membrane‐type surface stress sensor (MSS) to evaluate their sensing capability toward a select group of volatile organic compounds. By utilizing the MSS platform, the approach avoids the need for 2D materials with conductive properties typically required in chemoresistive sensors. The results show that a longer laser treatment time for graphene oxide increases the sensor response, which is attributed to an increase in defects and oxygen functional groups. Doping of graphene oxide with boron nitride improves sensor response, likely due to the introduction of pyrrolic nitrogen groups with high chemical activity. Additionally, the graphene oxides demonstrate partial selectivity toward the detection of toluene, attributable to π–π interactions. MoS2 and WS2 nanoflakes also show enhanced sensor response attributed to the formation of apical/bridging sulfur bonds with high catalytic activity.

Improving the electrical and mechanical performances of embedded capacitance materials by introducing tungsten disulfide nanoflakes into the dielectric layer

Embedded capacitance materials have attracted tremendous attention because of their prominent merits in comparison with discrete capacitors. However, the currently used embedded capacitance materials are suffering technical problems such as limited electrical and mechanical performances, which can hardly adapt to the rapid developments of next-generation electronics. Here, we present an effective approach for improving the electrical and mechanical performances by adding two-dimensional (2D) transition metal dichalcogenides (TMDC) tungsten disulfide (WS2) nanoflakes into the barium titanate/epoxy resin (BT/ER) system. The anti-sedimentation capability, viscosity and film-formation capability of pastes synthesized by different kinds and amounts of fillers were investigated. The introduction of semiconducting WS2 nanoflakes can work as inner micro-electrodes and is beneficial to the dispersion of BT particles in ER matrix. The dielectric constant and the peel strength can be improved to 22.54 at 1 kHz and 9.4 N/mm, respectively. The restricted dielectric loss below 0.07 at 1 kHz can be accounted by the lower conductivity of WS2 nanoflakes. The introduction of WS2 nanoflakes into BT/ER system may provide new avenues for fabrication of advanced embedded capacitance materials. Further performance improvements may be achieved by optimizing the variety, structure and chemical properties of TMDCs.

Flexible and low power CO gas sensor with Au-functionalized 2D WS2 nanoflakes

A flexible and selective CO gas sensor was fabricated from Au-functionalized two-dimensional (2D) WS2 nanoflakes on polyamide substrate. Au-functionalized 2D WS2 nanoflakes gas sensors fabricated on polyamide can be operated under self-heating mode (1-5 V) with good selectivity to CO gas. Gas sensors fabricated on polyamide not only showed flexibility, but also demonstrated good stability and repeatability. In particular, even after 1000 times tilting, the optimized gas sensor could detect low concentrations of CO gas. The underlying sensing mechanism is discussed in detail. The results indicated the feasibility of realization of low-energy consumption and flexible gas sensors using Au-functionalization on 2D WS2 nanoflakes.

Surface curvature-confined strategy to ultrasmall nickel-molybdenum sulfide nanoflakes for highly efficient deep hydrodesulfurization

Size-controlled synthesis of two-dimensional (2D) catalysts with low stacking numbers and small nanoflake lengths is crucial for promoting the catalytic performance in diverse heterogeneous catalysis. Herein, we report a facile and general “surface curvature-confined synthesis” strategy to modulate the slab lengths and stacking numbers of 2D transition metal sulfides by controlling the strain induced by different surface curvature of supports. An efficient NiMo sulfide with shorter slab length (average 3.71 nm), less stacking number (1–2 layers) and more edge active sites is synthesized onto ZSM-5 zeolites with the average size of 100 nm, which shows superior kHDS value of dibenzothiophene (14.05 × 10−7 mol/(g·s)), enhanced stability up to 80 h, and high direct desulfurization selectivity (> 95%). This design concept is also proved to be generally applicable to modulate the slab lengths and stacking numbers of other 2D catalysts such as MoS2 and WS2 nanoflakes, which shows great potentials for developing more ultrasmall 2D catalysts with controlled sizes and excellent catalytic activities.

Ruthenium-decorated tungsten disulfide quantum dots for a CO2 gas sensor

In this work, a selective chemiresistive gas sensor for carbon dioxide gas detection at room temperature (∼25 °C) was successfully fabricated, where ruthenium-decorated tungsten disulfide (Ru@WS2) quantum dots (QDs) have been used as the sensing material. A mixed solvent of lithium hydroxide (LiOH · H2O) and N-methyl-2-pyrrolidone (NMP) was used to obtain the Ru-decorated WS2 QDs from the exfoliated WS2 nanoflakes. Then, the prepared WS2 QDs and Ru@WS2 QDs were confirmed using different material characterization techniques. The gas sensors were prepared by spraying the WS2 QDs and Ru@WS2 QDs on gold interdigitated electrodes (IDE), and were then exposed to various concentrations of CO2 gas in dry air conditions. Also, the effect of humidity on both sensors in 5000 ppm CO2 gas has been studied. The Ru@WS2 QD based sensor showed superior sensitivity and good selectivity to CO2 gas in comparison with isopropanol, acetone, ethanol, methanol and benzene at room temperature than the WS2 QD. The sensor showed an increase in resistance when exposed to CO2 gas ranging from 500 to 5000 ppm, indicating p-type characteristics. The Ru@WS2 QD shows less effects at different humid conditions compared to WS2 QD as a CO2 gas sensor.

Photoluminescence quenching of WS2 nanoflakes upon Ga ion irradiation

Modification of the electronic structure by defect engineering is crucial towards controlling the semiconductor band gap. In this report, the WS2 nanoflakes produced by self-sustaining high temperature synthesis are irradiated by Ga ion beam. Intensities of the Raman phonon modes and the photoluminescence (PL) are decreased upon increasing dose of Ga ion beam irradiation from 2·1013 to 1016 cm−2. This leads to the defect generation in the nanoflakes and their thinning. The maximum irradiation dose at 1016 cm−2 results in the degradation of the WS2 nanoflakes, thus to total quenching of the PL signals.

Triboelectrification of Two-Dimensional Chemical Vapor Deposited WS2 at Nanoscale

Triboelectric properties of chemical vapor deposited WS2 nanoflakes have been characterized in nano-range by atomic force microscopy (AFM) and Kelvin force microscopy (KFM). The triboelectric process is dependent on the thickness of WS2 nanoflakes, and it is sensitive to the adsorbates like water molecules, as well as transferred Pt from the tip on the sample. The density of tribo-charge can be modified by applying various biases to the conductive Pt-coated tip during the frictional process. Tunneling of the tribo-charge into the gap between WS2 and the underlying substrate results in a long lifetime, which is about 100 times longer than conventional triboelectric charges. Moreover, we observe a positive correlation between the layer number and resistance to charge dissipation. Our finding can become the driving force for a new category of two-dimensional (2D) WS2 triboelectrically controllable nanodevices.

Thickness-dependent electrochemical response of plasma enhanced atomic layer deposited WS2 anodes in Na-ion battery

In spite of the promising future of the Sodium (Na)-ion batteries (NIBs), it still suffers with low specific capacity and stability mostly owing to the slow kinetics associated with Na ion. In this regard, transition metal sulfides (TMSs) are considered to be one of the best anode materials that can efficiently store Na ions not only owing to their graphite-like layered structure but also through conversion reactions facilitated by their multi-oxidation states. However, the poor cyclic stability of these TMSs attributed to the low electronic conductivity of the TMSs hinders the practical use. Therefore, understanding on an optimized mass loading (or physical dimensions) and configuration of such active electrode material are essential to improve the kinetics associated with Na-ion and electron pathway. To study this, plasma-enhanced atomic layer deposition (PEALD) is employed to grow WS2 using tungsten hexacarbonyl [W(CO)6] and H2S plasma as a precursor and reactant, respectively. The thin films of WS2 deposited directly on stainless steel coin with varying ALD cycles (200–600) are then tested as anode in NIBs without any binder or conducting carbon. Two-stage growth mode is observed with increasing number of ALD cycles which leads to WS2 nano-flakes formation on top of a two-dimensional film of the same. Reversible conversion and intercalation reactions from the cyclic voltammetry measurements are evident for the electrochemical stability of these pristine-WS2 films. While the highest areal capacity of ∼58.8 μAh/cm2 at a current density of 50 μA/cm2, after 50 charge-discharge cycles, is achieved with 400 ALD cycles, the highest capacity retention (∼72.5%) is observed for the film deposited with minimum (200) ALD cycles under same conditions. However, the capacity as well as its retention degrades drastically when the number of ALD cycles is further increased beyond 400. In this study, we address a critical issue associated with WS2 as an anode material for NIBs which should be similarly true for other TMSs as well.

WS2/CsPbBr3 van der Waals heterostructure planar photodetectors with ultrahigh on/off ratio and piezo-phototronic effect-induced strain-gated characteristics

High-performance, low-power and multifunction-integrated devices are crucial in emerging technologies. Herein, we demonstrate WS2/CsPbBr3 van der Waals heterostructure (vdWH) planar photodetectors combining the high mobility of mechanically exfoliated 2D WS2 nanoflakes with the remarkable optoelectronic properties of 1D single-crystalline CsPbBr3 nanowires and the strain-gated and strain-sensing characteristics induced by the piezo-phototronic effect. Owing to the effective charge carrier transfer and channel depletion originating from the appropriate energy band alignment, collaborative improvement of the photocurrent and dark current is realized, thus, an ultrahigh on/off ratio of 109.83 is achieved. The highest responsivity and detectivity at Vd = 2 V are 57.2 A W−1 and 1.36 × 1014 Jones, respectively. Even with a lower Vd of 0.5 V, decent performance is still obtained. Furthermore, the interfacial carrier transfer can be manipulated through the piezo-phototronic effect induced by CsPbBr3 monocrystal nanowires. Thus, when constructed on a flexible PEN substrate, the WS2/CsPbBr3 vdWH planar photodetector presents strain-gated photocurrent and responsivity, modulated by a factor of 11.3 with strain application, and a strain-sensing function is simultaneously realized due to the linear dependence of the photocurrent on strain. This unprecedented device design opens up a new avenue toward not only high-performance and low-power but also multifunction-integrated devices realized by the direct effect of mechanical inputs on charge carriers.

In-plane Aligned Colloidal 2D WS2 Nanoflakes for Solution-Processable Thin Films with High Planar Conductivity

Two-dimensional transition-metal dichalcolgenides (2D-TMDs) are among the most intriguing materials for next-generation electronic and optoelectronic devices. Albeit still at the embryonic stage, building thin films by manipulating and stacking preformed 2D nanosheets is now emerging as a practical and cost-effective bottom-up paradigm to obtain excellent electrical properties over large areas. Herein, we exploit the ultrathin morphology and outstanding solution stability of 2D WS2 colloidal nanocrystals to make thin films of TMDs assembled on a millimetre scale by a layer-by-layer deposition approach. We found that a room-temperature surface treatment with a superacid, performed with the precise scope of removing the native insulating surfactants, promotes in-plane assembly of the colloidal WS2 nanoflakes into stacks parallel to the substrate, along with healing of sulphur vacancies in the lattice that are detrimental to electrical conductivity. The as-obtained 2D WS2 thin films, characterized by a smooth and compact morphology, feature a high planar conductivity of up to 1 μS, comparable to the values reported for epitaxially grown WS2 monolayers, and enable photocurrent generation upon light irradiation over a wide range of visible to near-infrared frequencies.

3D hybrids based on WS2/N, S co-doped reduced graphene oxide: Facile fabrication and superior performance in supercapacitors

To further enhance electrochemical performance of WS2-based materials in supercapacitors, a 3D architecture with WS2 nanoflakes interconnected and quantum dots anchored on N and S co-doped reduced graphene oxide (WS2/N,S-rGO) crumpled nanosheets is fabricated for the first time via a novel, facile and scalable approach, which is based on rapid solution combustion synthesis of the precursor and subsequent gas-solid phase sulfurization, and needs no such tedious processes as filtration, washing and drying. Significantly, the WS2/N,S-rGO hybrid possesses a specific capacitance of 1562.5 F g−1 at 1 A g−1 and a rate capability of 780 F g−1at 40 A g−1, which are superior to the currently-reported WS2-based electrode materials. As to the WS2/N,S-rGO hybrid, the 3D hierarchical architecture, synergistic effect between WS2 and N,S-rGO nanosheets as well as nitrogen and sulfur co-doping on rGO offer a variety of advantages such as larger contact surface area, shorter ion diffusion path, excellent charge transport and especially the enhanced pseudo-capacitance of rGO, and thus result in huge improvement in electrochemical activity, which demonstrate the WS2/N,S-rGO hybrid to be a superb anode material in supercapacitors.

Mechanistic insight into the formation of colloidal WS2 nanoflakes in hot alkylamine media.

Developing convenient and reliable synthetic methodologies for solution processable 2D layered ultrathin nanostructures with lateral size control is one of the major challenges for practical applications. In this study, a rational understanding a long-chain amphiphilic surfactant assisted non-hydrolytic synthesis that is able to generate dimension-controllable 2D-WS2 nanocrystal flakes in a single-step protocol is proposed. The evolution of the starting soft organic–inorganic lamellar template into ultrathin few-layer 2D-WS2 nanostructures with lateral size modulation over a range between 3 and 30 nm is monitored. The initial formation of WS2 nanoseeds occurs in a self-assembled sacrificial precursor source, acting as a template, where larger two-dimensional nanostructures can grow without undergoing significant thickness variation. Overall, the chemical nature and steric hindrance of the alkylamines are essential to modulate the reactivity of such WS2 nanoclusters, which correlate with the lateral size of the resulting nanoflakes.

Hierarchical architecture of coupling graphene and 2D WS2 for high-performance supercapacitor

A hierarchical architecture of coupling graphene and 2D WS2 for high-performance supercapacitors is fabricated by a facile ice-template approach. The hybridization content of WS2 in graphene architecture strongly affects the morphology structure and surface chemical nature of the electrode, and can significantly enhance electrochemical behaviors. As proposed, the hierarchical WS2/graphene architectures (WGA) showing rough and wrinkled surface can facilitate electrolyte diffusion and increase the wettability during the measurement. Meanwhile, the formation of 1T WS2 and covalent bonds at WS2/graphene interface are component for the improvement of pseudocapacitive behavior of WGA; A high performance like a superior specific capacitance (383.6 F g−1), good rate capability (79.9%) and excellent cyclic stability (102.5%) is found due to the fast proton insertion into the WS2 nanoflakes. This work provides an efficient and facile strategy to construct the hierarchical architectures for pseudocapacitive capacitors.

Low-temperature one-pot synthesis of WS2 nanoflakes as electrocatalyst for hydrogen evolution reaction

Transition metal dichalcogenides have unique physicochemical properties. Herein, a low-temperature facile method is demonstrated to synthesize ultrathin tungsten disulfide nanoflakes. They are loosely stacked between layers with highly exposed edges, which provide lots of active sites for electrochemical applications. The by-product of crystalline carbon improves their conductivity, which also enhances their performance in hydrogen evolution reaction.

Reduced graphene oxide hybridized with WS2 nanoflakes based heterojunctions for selective ammonia sensors at room temperature

Hybrid of the two dimensional nanostructured reduced graphene oxide (rGO) and WS2 has been investigated for a room temperature ammonia sensor. The formed rGO/WS2 heterojunctions prepared by one-step hydrothermal synthesis indicated a good sensitivity to different concentrations of ammonia from 10 ppm to 50 ppm at room temperature. The WS2 nanoflakes doped in the heterojunction plays significant role in the enhanced response through the introduction of more hydroxyls in rGO and the extra Lewis acid active centers. The sensor also shows an excellent selectivity to NO2, alcohols, formaldehyde, acetone and benzene and a good long term stability indicating a potential to be employed as a room temperature NH3 sensor.

Lithium Electrochemistry of WS2 Nanoflakes Studied by In-situ TEM

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