WS2 Nanoflakes Research Articles

Efficient hydrogen evolution reaction performance of Ni substituted WS2 nanoflakes

Efficient hydrogen evolution reaction performance of Ni substituted WS2 nanoflakes

Improved ammonia gas adsorption of surface engineered WS2 nanoflakes

Recent investigations into nanostructured WS2 have shown promising results in the detection of reducing gases, such as ammonia. However, this material faces limitations with respect to the vital parameters such as sensitivity, recovery, and repeatability over an extended period in its purest form. To address these challenges, a low-energy (5 keV) argon ion beam was used to irradiate nanostructured WS2 nanoflakes at various fluences, leading to observable morphological and structural changes. The irradiation of WS2 resulted in a significant enhancement in its ability to detect ammonia gas. The sensing response saw a threefold increase, and both the response and recovery times were reduced significantly compared to pristine samples. Ion beam irradiation changes the overall morphology of nanoflakes and induces a certain degree of defects as apparent from electron microscopy, X-ray diffraction, Raman spectroscopy, and X-ray photoelectron spectroscopy studies. The ion-solid interaction simulation predicts a larger amount of sulfur vacancy than tungsten due to irradiation, which is also supported by experimental results. Density functional theory predicts that sulfur vacancy leads a decisive role in better adsorption of ammonia than the pristine surface. The NH3 molecule orients towards the defective surface leading to a higher charge transfer from the NH3 molecule to the surface than the pristine surface along with a smaller adsorption height.

Ag nanoparticle functionalized vertical WS2 nanoflakes as SERS substrate for chemical sensing

Two-dimensional transition metal dichalcogenides (TMDCs) present a unique opportunity as Surface Enhanced Raman Scattering (SERS) substrates due to their superior optical properties. Here, vertically oriented tungsten disulfide (WS₂) flakes were synthesized on a silicon (Si) substrate using the pulsed laser deposition (PLD) technique. The WS₂ flakes were employed as SERS substrates for detecting low concentrations of environmental pollutants, like Rhodamine B (RhB) and Methyl Orange (MO) dyes, achieving promising enhancement factors of approximately 10⁷ with a 532 nm excitation laser. To further enhance the plasmonic activity of the vertical WS₂ flakes, silver nanoparticles (NPs) were decorated on their surface via thermal evaporation. The Ag NP-functionalized WS₂ flakes exhibited superior detection capabilities compared to pristine WS₂ flakes, achieving remarkable sensitivity at ultralow concentrations of 10⁻¹⁶ M for RhB and 10–17 M for MO dye, with enhancement factors of the order of 10⁸. The SERS performance of the WS₂-Ag substrate was also evaluated using a 633 nm laser, successfully detecting Methylene Blue (MB) dye at an ultralow concentration of 10⁻¹³ M. Prolonged exposure to RhB, MO, and MB dyes can cause cancer, neurotoxicity, and respiratory irritation. Thus, sensitive detection of these dyes at ultralow concentrations is highly crucial. A charge transfer mechanism between WS₂ and RhB dye molecules is proposed, along with the contribution of Ag NPs, to explain the enhanced detection capabilities of the nanocomposite SERS substrate.

Atomically thin-layered WS2 based resistive sensors for detection of CO and NO2 at room temperature

The number of layers present in a two-dimensional (2D) nanomaterial plays a critical role in applications that involve surface interaction, for example, gas sensing. This paper reports the synthesis of 2D WS2 nanoflakes using the facile liquid exfoliation technique. The nanoflakes were exfoliated using bath sonication (BS-WS2) and probe sonication (PS-WS2). The thickness of the BS-WS2 was found to range between 70 and 200 nm, and that of PS-WS2 varied from 0.6 to 80 nm, indicating the presence of single to few layers of WS2 when characterized using atomic force microscope. All the WS2 samples were thoroughly characterized using electron microscopes, x-ray diffractometer, Raman spectroscopy, UV–Visible spectroscopy, Fourier transform infrared spectroscope, and thermogravimetric analyser. Both the nanostructured samples were exposed to 2 ppm of NO2 at room temperature. Interestingly, BS-WS2 which comprises of a greater number of WS2 layers exhibited −14.2% response as against −3.4% response of PS-WS2, the atomically thin sample. The BS-WS2 sample was found to be highly selective towards NO2 but was slower (with incomplete recovery) as compared to PS-WS2. The PS-WS2 sample was observed to exhibit −11.9% to −27.4% response to 2–10 ppm of CO and −3.4%–35.2% response to 2–10 ppm of NO2 at room temperature, thereby exhibiting the potential to detect two gases simultaneously. These gases could be accurately predicted and quantified if the response times of the PS-WS2 sample were considered. The atomically thin WS2-based sensor exhibited a limit of detection of 131 and 81 ppb for CO and NO2, respectively.

Room Temperature Detection of H2S by Two Dimensional WS2 based Chemiresistive Sensors

H2S is a toxic gas which play important roles in different applications. This work reports detection of H2S by tungsten disulphide (WS2) nanoflakes which was synthesized using a simple liquid exfoliation method. The thickness of the nanoflakes were found to range from 0.7–2 nm while the lateral dimensions varied between 2–4 µm when observed using atomic force microscopy. The band gap of WS2 nanoflakes was found to be 2.66 eV when studied using UV-Visible spectroscopy. The total flowrate of gas was found to modulate the sensing performance of WS2. Hence, the WS2 based resistive sensor was tested with 1–27 ppm of H2S at 100 sccm flowrate and with 0.7–5 ppm H2S at 1000 sccm flowrate at room temperature (23–25°C). WS2 nanoflakes exhibited 0.86–15.36% response for 1–27 ppm H2S at 100 sccm while the response was observed to increase and vary between 0.47–15.66% for 0.7–5 ppm at 1000 sccm. The recovery of the sensors at 1000 sccm was found to be incomplete but the sensor demonstrated excellent repeatability and selectivity towards H2S. The limit of detection of the sensor was found as 253 and 53 ppb at 100 and 1000 sccm, respectively.

Electrical Properties of Electrochemically Exfoliated 2D Transition Metal Dichalcogenides Transistors for Complementary Metal‐Oxide‐Semiconductor Electronics

AbstractThe unique semiconducting characteristics of 2D materials, such as transition metal dichalcogenides (TMDs), have drawn significant interest in the field of electronic devices. However, the dependence of the device performance on the electrical properties of electrochemically exfoliated TMD nanoflakes remains unclear. In this study, the intrinsic electrical properties of diverse electrochemically exfoliated TMD nanoflakes are investigated and their applications in fully solution‐processed 2D electronics are explored. The study reveals that MoSe2 and WS2 nanoflakes are suitable candidates with moderate electron concentrations of 5.7 × 1012 and 2.5 × 1012 cm−2, respectively. These moderate electron concentrations enable a low off‐state current and high on‐state current, leading to a large on–off ratio (> 106) for the fabricated transistors. Furthermore, moderate annealing can increase the mobility by one order of magnitude by reducing the contact resistance and improving charge transport. Additionally, the trap density is significantly reduced, leading to an improved stability. Finally, fully solution‐processed 2D complementary inverters with a high voltage gain of 60 are demonstrated. This study contributes to a growing understanding of the properties of 2D materials in terms of transistor performances and promotes the development of solution‐processed devices applicable in next‐generation electronics.

Electrochemical power generation humidity sensor based on WS2 nanoflakes

The electrochemical power generation humidity sensor has received widespread attention due to its zero-power consumption. In this work, we propose an electrochemical power generation humidity sensor using WS2 nanoflakes as the humidity sensing material, without adding additional alkali metal salts (such as LiCl and NaCl) or other added materials or functional group treatments. The results show that the humidity sensor exhibits wide humidity detection range (18.7%−91.5% relative humidity (RH)), fast response/recovery speeds (8.4/5.2 s), good repeatability (20 cycles) and small humidity hysteresis (∼3.4% RH) at room temperature of 25 °C. The output voltage of the humidity sensor is 0.47 V at 91.5% RH. Combined with good water molecule adsorption and proton conductivity of the WS2 nanoflakes, the humidity sensing and power generation mechanism of the WS2 electrochemical power generation humidity sensor can be explained by the redox reactions occurring at the positive and negative electrodes (Cu and Al). Finally, we demonstrate the application of the humidity sensor in respiratory detection. This work provides a useful approach for fabricating electrochemical power generation humidity sensor with a simple single humidity sensing material.

Room temperature ammonia sensor with high selectivity based on the composite materials assembled by CuO nanosheets and WS2 nanoflakes

In this work, CuO nanosheets are synthesized by one-step hydrothermal method and CuO/WS2 heterostructures are fabricated by introducing CuO doping into WS2, the preparation method was employed to enhance the performance of the gas sensor. The CuO/WS2 heterostructure nanocomposite sensor, doped with 5 % CuO, exhibited a pronounced enhancement in its response to 30 ppm of ammonia gas at ambient temperature, increasing the response value from 3.8 observed in pure WS2-based sensors to 4.7. Specifically, the response time of the CuO/WS2 heterostructure-based gas sensor decreased to 180 s, whereas the WS2-based sensor required 350 s for NH3 detection at room temperature. Furthermore, the CuO/WS2 sensor demonstrated outstanding selectivity for ammonia, maintaining stability over a 90-day test with a response fluctuation of less than 5 %. The heterojunction formed by CuO and WS2 showed superior sensitivity to NH3 compared to pristine WS2, owing to a notable enhancement in the surface catalytic effect. It also displayed sensitivity to humidity, with a response value of 6.25 at a 71 % relative humidity. At last, the gas sensing and enhanced gas sensing performance mechanisms of the composites for NH3 were elucidated. This remarkable multi-functional performance offers valuable insights for the advancement of room temperature gas sensors with diverse sensing functionalities.

Highly sensitive label-free biomolecular detection using Au-WS2 nanohybrid based SERS substrates.

Recent advancements in nanotechnology have led to the development of surface-enhanced Raman spectroscopy (SERS) based rapid and low-cost technologies for ultra-sensitive label-free detection and identification of molecular analytes. Herein, we utilized the synergistic plasmonic and chemical enhancement effects of Au-WS2 nanohybrids to attain the high-intensity Raman signals of targeted analytes. To develop these nanohybrids, a series of monodispersed Au nanoparticles (NPs) of varying diameters from 20 to 80 nm was chemically synthesized and successively blended with liquid-phase exfoliated WS2 nano-flakes of average lateral size 90 nm. They provided a maximum enhancement factor (EF) of ∼1.80 × 109 corresponding to the characteristic peaks at 1364 cm-1 and 1512 cm-1 for R6G analyte molecules. Theoretical studies based on the finite-difference time-domain simulations on Au-WS2 nanohybrid systems revealed a huge field-intensity enhancement with an EF of more than 1000 at the plasmonic hotspots, which was induced by the strong coupling of individual plasmon oscillations of the adjacent Au NPs upon light interactions. These electromagnetic effects along with the chemical enhancement effects of WS2 nanoflakes were found to be mainly responsible for such huge enhancement in Raman signals. Furthermore, these hybrids were successfully employed for achieving highly sensitive detection of the E. coli ATCC 35218 bacterial strain with a concentration of 104 CFU mL-1 in phosphate-buffered saline media, indicating their real capabilities for practical scenarios. The findings of the present study will indeed provide vital information in the development of innovative nanomaterial-based biosensors, that will offer new possibilities for addressing critical public health concerns.

Construction of WS2/NC@C nanoflake composites as performance-enhanced anodes for sodium-ion batteries.

The development of layered metal sulfides with stable structure and accessible active sites is of great importance for sodium-ion batteries (SIBs). Herein, a simple liquid-mixing method is elaborately designed to immobilize WS2 nanoflakes on N-doped carbon (NC), then further coat carbon to produce WS2/NC@C. In the formation process of this composite, the presence of NC not only avoids the overlap and improves the dispersion of WS2 nanoflakes, but also creates a connection network for charge transfer, where the wrapped carbon provides a stable chemical and electrochemical reaction interface. Thus, the composite of WS2/NC@C exhibits the desired Na+ storage capacity as anticipated. The reversible capacity reaches the high value of 369.8 mA h g-1 at 0.2 A g-1 after 200 cycles, while excellent rate performances and cycle life are also acquired in that capacity values of 256.7 and 219.6 mA h g-1 at 1 and 5 A g-1 are preserved after 1000 cycles, respectively. In addition, the assembled sodium-ion hybrid capacitors (SIHCs, AC//WS2/NC@C) exhibit an energy/power density of 68 W h kg-1 at 64 W kg-1, and capacity retention of 82.9% at 1 A g-1 after 2000 cycles. The study provides insight into developing layered metal sulfides with eminent performance of Na+ storage.

Judicious Selection of Precursors with Suitable Chemical Valence State for Controlled Growth of Transition Metal Chalcogenides

AbstractTransition metal chalcogenides (TMCs) have attracted wide attentions as a class of promising material for both fundamental investigations and electronic applications due to their atomic thin thickness, dangling bond‐free surface, and excellent electronic properties. Specifically, TMCs show outstanding properties such as good thermal conductivity, robust mechanical properties, and extraordinary electronical characteristics, bestowing them utility in both fundamental research and applications. Recently, the development of post‐Moore electronics based on TMCs calls for their large‐size and single‐crystal growth. However, researchers about synthesis usually focus on controlling several growth parameters (such as growth temperature, flow rate, and time). Herein, it is reported that the chemical valence states of transition metal precursors play an important role in controlling the lateral size and crystal quality for TMCs. The study discusses the valence states‐dependent growth mechanism for WS2 and MoS2 from four factors: evaporation temperature, skipping of reaction steps, atomic binding energy of the precursors, and formation energy. In addition, the as‐grown WS2 and MoS2 nanoflakes exhibit good photoelectric response properties. For EuS, the growth results are obviously different by using EuBr3 and EuBr2 as precursors. The studies provide a unique perspective and also new knowledge to controllably grow large‐size and good crystal quality TMCs.

Fully Flexible MXene-based Gas Sensor on Paper for Highly Sensitive Room-Temperature Nitrogen Dioxide Detection.

Flexible chemiresistive gas sensors have attracted growing interest due to their capability in real-time and rapid detection of gas. However, the performance of gas sensors has long been hindered by the poor charge transfer ability between the conventional metal electrode and gas sensing semiconductors. Herein, for the first time, a fully flexible paper-based gas sensor integrated with the Ti3C2Tx-MXene nonmetallic electrode and the Ti3C2Tx/WS2 gas sensing film was designed to form Ohmic contact and Schottky heterojunction in a single gas sensing channel. Ti3C2Tx/WS2 has outstanding physical and chemical properties for both Ti3C2Tx and WS2 nanoflakes, showing high conductivity, effective charge transfer, and abundant active sites for gas sensing. The response of the gas sensor to NO2 (1 ppm) at room temperature is 15.2%, which is about 3.2 and 76.0 times as high as that of the Au interdigital electrode integrated with the Ti3C2Tx/WS2 sensor (4.8%) and the MXene electrode integrated with the Ti3C2Tx sensor (0.2%), respectively. Besides, this design performed at a limit of detection with 11.0 ppb NO2 gas and displayed excellent stability under high humidities. Based on first-principles density functional theory calculation results, the improvement of the gas sensing performance can be mainly attributed to the heterojunction regulation effect, work function matching, and suppressing metal-induced gap states. This work provides a new approach for the design of flexible gas sensors on paper with MXene-based conductive electrodes and gas sensing materials.

A Comparative Study on the Anti-Friction Performance of Amorphous Silicon Films Enhanced by WS2 Nanoflakes

A Comparative Study on the Anti-Friction Performance of Amorphous Silicon Films Enhanced by WS2 Nanoflakes

Understanding the linear and nonlinear optical responses of few-layer exfoliated MoS2 and WS2 nanoflakes: experimental and simulation studies

Understanding the linear and nonlinear optical (NLO) responses of two-dimensional nanomaterials is essential to effectively utilize them in various optoelectronic applications. Here, few-layer MoS2 and WS2 nanoflakes with lateral size less than 200 nm were prepared by liquid-phase exfoliation, and their linear and NLO responses were studied simultaneously using experimental measurements and theoretical simulations. Finite-difference time-domain (FDTD) simulations confirmed the redshift in the excitonic transitions when the thickness was increased above 10 nm indicating the layer-number dependent bandgap of nanoflakes. WS2 nanoflakes exhibited around 5 times higher absorption to scattering cross-section ratio than MoS2 nanoflakes at various wavelengths. Open aperture Z scan analysis of both the MoS2 and WS2 nanoflakes using 532 nm nanosecond laser pulses reveals strong nonlinear absorption activity with effective nonlinear absorption coefficient (β eff) of 120 cm GW−1 and 180 cm GW−1, respectively, which was attributed to the combined contributions of ground, singlet excited and triplet excited state absorption. FDTD simulation results also showed the signature of strong absorption density of few layer nanoflakes which may be account for their excellent NLO characteristics. Optical limiting threshold values of MoS2 and WS2 nanoflakes were obtained as ∼1.96 J cm−2 and 0.88 J cm−2, respectively, which are better than many of the reported values. Intensity dependent switching from saturable absorption (SA) to reverse SA was also observed for MoS2 nanoflakes when the laser intensity increased from 0.14 to 0.27 GW cm−2. The present study provides valuable information to improve the selection of two-dimensional nanomaterials for the design of highly efficient linear and nonlinear optoelectronic devices.

Photoresponse properties of thin films of vertically grown WS2 nanoflakes

The photoresponse properties of Tungsten disulphide (WS2)/Silicon (Si) thin films, synthesised by the e-beam evaporation technique, have been reported. The synthesised films show preferential orientation along (001) direction of hexagonal WS2. The synthesised films were composed of vertically grown nanoflakes that were a few-layered thick. The films were highly absorbent in the visible range and exhibited A, B, and C excitonic transitions in the absorbance spectra. A type-II heterojunction between WS2/Si was obtained. Photodetection studies gave a high responsivity of 1.17 AW−1 at an illumination of 1.6 mWcm−2 of white light.

Construction of two-dimensional Ag/WS2 hybrid membranes with self-cleaning ability by photocatalysis for efficient water filtration

Two-dimensional Ag/WS2 hybrid membranes were developed to improve separation performance of WS2 membrane by in situ self-reduction of AgNO3 on WS2 nanoflakes. With the increase of Ag content in Ag/WS2 hybrid membrane, water flux increased as well because of increased interlayer spacing and more irregular stacking of WS2 nanoflakes, while the rejection rates for dye molecules decreased. The hybrid membrane containing 6.92 wt% of Ag nanoparticles (Ag/WS2-2) presented optimal separation performance, exhibiting a EB rejection rate around 90% and a water permeance higher than 500 L/m2·h·bar. Moreover, Ag/WS2-2 membrane revealed excellent UV-driven self-cleaning ability by photodegradation of contaminant on membrane surface, and its water flux recovery rate was higher than 99.0%. The formation of Ag/WS2 heterostructure can assist separation of photoinduced electrons and holes by transferring photoinduced electrons to Ag nanoparticles with enhanced photocatalytic activity.

Ab Initio Study of CO2 Activation on Pristine and Fe-Decorated WS2 Nanoflakes.

There is an intense race by the scientific community to identify materials with potential applications for the conversion of carbon dioxide (CO2) into new products. To extend the range of possibilities and explore new effects, in this work, we employ density functional theory calculations to investigate the presence of edge effects in the adsorption and activation of CO2 on pristine and Fe-decorated (WS2)16 nanoflakes. We found that Fe has an energetic preference for hollow sites on pristine nanoflakes, binding with at least two two-fold edge S atoms and one or two three-fold core S atoms. Fe adsorption on the bridge sites occurs only at the edges, which is accompanied by the breaking of W-S bonds in most cases (higher energy configurations). CO2 activates on (WS2)16 with an OCO angle of about 129° only at higher energy configurations, while CO2 binds via a physisorption mechanism, linear structure, in the lowest energy configuration. For CO2 on Fe/(WS2)16, the activation occurs at lower energies only by the direct interaction of CO2 with Fe sites located near to the nanoflake edges, which clearly indicates the enhancement of the catalytic activity of (WS2)16 nanoflakes by Fe decoration. Thus, our study indicates that decorating WS2 nanoflakes with TM atoms could be an interesting strategy to explore alternative catalysts based on two-dimensional materials.

Lithium/Sodium‐Ion Batteries: In Situ, Atomic‐Resolution Observation of Lithiation and Sodiation of WS2 Nanoflakes: Implications for Lithium‐Ion and Sodium‐Ion Batteries (Small 24/2021)

In article number 2100637, Vinayak P. Dravid, Jinsong Wu, and co-workers perform a systematic study of WS2 nanoflakes as the anode for lithium-ion and sodium-ion batteries. It is found that compared to LIBs, SIBs exhibit a higher reversible sodium storage capacity and superior cyclability. The results show a direct correlation between the reaction dynamics of lithiated/sodiated WS2 nanoflakes and their electrochemical performance.

In Situ, Atomic-Resolution Observation of Lithiation and Sodiation of WS2 Nanoflakes: Implications for Lithium-Ion and Sodium-Ion Batteries.

WS2 nanoflakes have great potential as electrode materials of lithium-ion batteries (LIBs) and sodium-ion batteries (SIBs) because of their unique 2D structure, which facilitates the reversible intercalation and extraction of alkali metal ions. However, a fundamental understanding of the electrochemical lithiation/sodiation dynamics of WS2 nanoflakes especially at the nanoscale level, remains elusive. Here, by combining battery electrochemical measurements, density functional theory calculations, and in situ transmission electron microscopy, the electrochemical-reaction kinetics and mechanism for both lithiation and sodiation of WS2 nanoflakes are investigated at the atomic scale. It is found that compared to LIBs, SIBs exhibit a higher reversible sodium (Na) storage capacity and superior cyclability. For sodiation, the volume change due to ion intercalation is smaller than that in lithiation. Also, sodiated WS2 maintains its layered structure after the intercalation process, and the reduced metal nanoparticles after conversion in sodiation are well-dispersed and aligned forming a pattern similar to the layered structure. Overall, this work shows a direct interconnection between the reaction dynamics of lithiated/sodiated WS2 nanoflakes and their electrochemical performance, which sheds light on the rational optimization and development of advanced WS2 -based electrodes.

Improving graphene gas sensors via a synergistic effect of top nanocatalysts and bottom cellulose assembled using a modified filtration technique

Improving graphene properties with nanosized catalysts is of interest for numerous applications, especially chemical sensors. Thus, the aim of this study is to prepare nanocatalysts and assemble them on reduced graphene oxide (RGO) for sensing nitric oxides (NOx). The nanocatalysts used in this study include TiO2 nanoparticles (commercial) with WO3, WS2, and MoS2 nanoflakes (mechanically synthesized). Their decoration on the RGO active channel (forming nanohybrids) was obtained via a modified vacuum filtration technique, in which a small syringe and USB-powered pump were employed to surmount the limitation of the conventional filtration setup in the patternable fabrication of device arrays. Sensing measurements expose a co-effect of the nanocatalysts and the cellulose substrate on our RGO-based paper sensors. That can boost the sensitivity of RGO toward NO (1-10 ppm) and NO2 (0.7–3.5 ppm) but weaken it toward NH3 (2–20 ppm). Particularly among the nanocatalysts, MoS2 nanoflakes made our RGO nanohybrid possessing the fastest response/recovery and the highest selectivity toward NOx at room temperature (25 °C). Generally, this study reveals the synergistic effect of MoS2 and cellulose substrate on the improvement of RGO sensing ability and confirms the aptitude of our sensors for flexible and disposable NOx sensing applications.