Two-dimensional Transition Metal Disulfides Research Articles

1D/2D Heterostructured WS2@PANI Composite for Highly Sensitive, Flexible, and Room Temperature Ammonia Gas Sensor.

Flexible and room-temperature (RT) ammonia gas sensors are needed for exhaled breath detection and recognition. Two-dimensional transition metal disulfides are potential materials for RT gas sensing because of their low band gap and a large number of edge-exposed sites that can provide strong binding to gas molecules. In this work, a 1D/2D heterostructured composite material of 2D tungsten disulfide (WS2) modified with 1D polyaniline (PANI) was proposed. The fibrous PANI adsorbed on the edges and inserted in the interlayers of the laminated WS2 provide more diffusion channels for the ammonia gas and act as sensing sites. The WS2@PANI-based sensor shows high selectivity for ammonia with satisfying reproducibility and long-term stability. A response of 216.3% and a short response/recovery time of 25 s/39 s were achieved for 100 ppm ammonia gas. The sensing mechanism was investigated in detail via complex impedance spectra and in situ FT-IR, which was attributed to the synergistic effect of WS2 and PANI. The excellent sensing performance coupled with its resistance to thermal and humidity interference endows the WS2@PANI-based sensor with potential for human exhaled detection and wearable electronics.

A theoretical study of Lifshitz transition for 2H-TaS2.

Lifshitz transition was proposed to explain a change of the topology structure in a Fermi surface induced by continuous lattice deformation without symmetry breaking since 1960. It is well known that the anomalies of the kinetic coefficients (the coefficient of heat conduction and electrical conductivity, viscosity, sound absorption, etc.) are usually closely connected with the Lifshitz transition behavior. 2H-TaS2 is a typical representative to study its anomalies of temperature dependence of heat capacity, resistivity, Hall effect, and magnetic susceptibility. Its geometrical structure of the charge density wave (CDW) phase and layer number dependence of carrier-sign alternation upon cooling in the Hall measurements have not been well understood. The geometrical structure (T-Ts) of the CDW phase was predicted through first principles calculations for bulk and mono-layer 2H-TaS2. Driven by electron-lattice coupling, Ta atoms contract to form a partially gapped CDW phase. The CDW phase has a larger average interlayer separation of S-S atoms in the adjacent two layers compared with the metal phase, which results in a weaker chemical bonding among S-S atoms in the adjacent two layers and then a narrower bandwidth of the energy band. The narrower bandwidth of the energy band leads to a larger density of states (DOS) in the out-of-plane direction above the Fermi level for the CDW phase. As the Fermi level continually drops from the DOS region with a negative slope to that with a positive slope on cooling, the reversal of the p → n type carrier and the pocket-vanishing-type Lifshitz transition occur in the bulk 2H-TaS2. However, the Fermi level slightly drops by 6 meV and happens to be at the positions of pseudo band gaps, so the reduction of in-plane DOS and total DOS is responsible for the always p-type carrier in the mono-layer samples. Our CDW vector of the k-space separation between two saddle points is QSP ≈ 0.62 GK and can provide a theoretical support for the "saddle-point" CDW mechanism proposed by Rice and Scott. Our theoretical explanation gives a new understanding of both Lifshitz transition for symmetry breaking and reversal for the p-n carrier sign in the Hall measurements in various two-dimensional transition metal disulfides.

Tunable hydrogen evolution reaction of vacancy-defective MoSe2/WSe2 heterojunctions based on first-principle calculation

Two-dimensional transition metal disulfides (TMDs) heterojunction is a very promising non-precious metal catalyst that has been widely used as a catalyst for hydrogen evolution reactions. In this paper, the effects of different positions of Se vacancies on the electronic structure and hydrogen evolution reaction electrocatalyst of MoSe2/WSe2 heterojunction are investigated by first-principles calculations. After the introduction of vacancy, defect states appear between the conduction band bottom and the Fermi energy level of the heterojunction. This is more favorable for the electron transfer between H and the substrate. The results of the density of states calculations indicate that the defect states appear because the d orbitals of the transition metal atoms containing the Se vacancy layer hybridize with the p orbitals of the Se atoms. The differential charge density indicates that the Se vacancies promote the charge transfer of MoSe2/WSe2. In addition, calculations of the Gibbs free energy for the hydrogen evolution reaction of heterojunctions show that Se vacancies (especially the outer vacancies) enhance the electrocatalytic hydrogen precipitation performance of heterojunctions. The results provide a new way to regulate the hydrogen evolution properties of MoSe2/WSe2 heterojunctions.

Construction of MoS2-ReS2 Hybrid on Ti3C2Tx MXene for Enhanced Microwave Absorption.

Utilizing interface engineering to construct abundant heterogeneous interfaces is an important means to improve the absorbing performance of microwave absorbers. Here, we have prepared the MXene/MoS2-ReS2 (MMR) composite with rich heterogeneous interfaces composed of two-dimensional Ti3C2Tx MXene and two-dimensional transition metal disulfides through a facile hydrothermal process. The surface of MXene is completely covered by nanosheets of MoS2 and ReS2, forming a hybrid structure. MRR exhibits excellent absorption performance, with its strongest reflection loss reaching -51.15 dB at 2.0 mm when the filling ratio is only 10 wt%. Meanwhile, the effective absorption bandwidth covers the range of 5.5-18 GHz. Compared to MXene/MoS2 composites, MRR with a MoS2-ReS2 heterogeneous interface exhibits stronger polarization loss ability and superior absorption efficiency at the same thickness. This study provides a reference for the design of transition metal disulfides-based absorbing materials.

Excitons dynamic modulation by tailoring size of high-entropy Mo0.64W0.36S2 alloy

Two-dimensional transition metal disulfides with tailored optical response are vital to demonstrate versatile optoelectronic devices and nanophotonic elements. In this work, using transient absorption spectroscopy in conjunction with density functional theory calculations, we demonstrate ultrafast excitons absorption modulation via tailoring the size of a transition-metal dichalcogenide alloy Mo0.64W0.36S2 nanosheet. As the size of the nanosheet increases from 9 ± 5 to 190 ± 121 nm, peak positions of the A and B excitons vary from 666 to 675 nm and 626 to 638 nm, respectively. Furthermore, the decay lifetimes of A excitons slow down from 1.16 to 1.84 ps when the size changes from 9 ± 5 to 190 ± 121 nm. It is shown that the exciton diffusion and decay properties can be modulated by the components and structure modulation, which is beneficial for the optimal design and optimization of optoelectronic devices.

Composition-dependent activity of Mn-doping NiS2 nanosheets for boosting photocatalytic H2 evolution

Two-dimensional transition metal disulfides are excellent photocatalytic materials, which can be significantly improved by optimizing the composition and structure. Herein, Mn-doping NiS2 of (Ni1-xMnx)-S with various Ni/Mn molar ratios is proposed via a facile and low-cost solvothermal method. The optimal (Ni4/6Mn2/6)-S exhibits pinecone-like morphology composed of tiny nanosheets with enlarged active sites, which facilitates the separation of photoinduced electrons and holes, improves the electron transfer ability and conductivity, and enlarges the active sites compared with pure NiS2 and MnS. Also, the negative shift of the conduction band derived from Mott-Schottky plots and the empirical formula provides a high thermodynamic driving force for hydrogen catalytic reaction. (Ni4/6Mn2/6)-S performs an ultrahigh hydrogen evolution rate of 24.86 mmol g−1 h−1 under UV–visible light irradiation, which is 1.5 times higher than pure NiS2 (16.92 mmol g−1 h−1) and 2.3 times higher than pure MnS (10.69 mmol g−1 h−1). The outstanding repeatability of 86.7% retention and apparent quantum yield of 46.9% are also achieved. Therefore, this work offers a novel bimetallic sulfide of (Ni1-xMnx)-S to improve the conversion efficiency of solar energy to chemical energy for photocatalytic hydrogen production.

Two-Dimensional Transitional Metal Disulfides as Charge Transport Layers in Organic-Inorganic Perovskite Solar Cells.

In the past decade, organic-inorganic perovskite solar cells (PSCs) have received significant attentions due to their high efficiencies and low costs. However, the commercialization of PSCs is stilled hindered by several issues such as device performance (especially for large-area cells) and stability. Recently, two-dimensional (2D) transition metal disulfides (TMDs) show great potentials in solving aforementioned problems due to their unique morphological structure and electrical properties. Herein, we summarize the advancements in the recent applications of various TMDs materials as charge transport layers in PSCs. Although some progress have been made, there are considerable issues to be tackled in this field. The challenges and development directions of these 2D TMDs materials for PSCs are also clarified. Lastly, the most recent advancements about TMDs materials in some other electronic (or optoelectronic) fields are also summarized and discussed.

Promoting sensitivity and selectivity of NO2 gas sensor based on (P,N)-doped single-layer WSe2: A first principles study

Two-dimensional transition metal disulfides(TMDs) has attracted considerable attention on gas sensor due to its excellent physicochemical properties. But the disadvantage is that the interaction between TMDs and gas is too weak, so that the response of sensor is difficult to be detected.In this study, the sensing ability of single-layer P and N atom doped WSe2 for common gases have been evaluated, including CO, SO2, H2O, NH3 and NO2.The molecular model of the adsorption systems is established, and the changes of electronic structure and adsorption behavior of the gas after doping are discussed from the binding energy, adsorption energy, band structure and density of states (DOS) by using density functional theory (DFT).The results show that dopants can improve the adsorption strength between gas molecules and substrates, among which the adsorption effect of NO2 gas molecule is the best, and the adsorption energy of WSe2 doped with single-layer P atoms is the largest.The calculation also shows that the P and N atom doped system has shorter adsorption distance and more charge transfer than the single-layer pristine WSe2.It can be inferred that single-layer N-doped WSe2 can be used as an effective, potential and highly selective NO2 gas sensor.

Enhanced electrocatalytic hydrogen evolution performance of 2D few-layer WS2 nanosheets via piezoelectric effects

Two-dimensional transition metal disulfides are considered a great candidate for non-noble metal catalyst because of their abundant reserves and excellent electrochemical activity in hydrogen evolution reaction (HER). Herein, two-dimensional (2D) few-layer WS2 nanosheets were synthesized via a facile liquid phase stripping technique. The piezoelectricity of the synthesized WS2 nanosheets was verified by piezoelectric force microscope (PFM) and used to promote the piezo-catalytic hydrogen generation, where the piezoelectric charges generated under ultrasound vibration are utilized for hydrogen evolution. Moreover, the effect of various mechanical vibration conditions on the piezo-catalytic HER of WS2 was also discussed. The results show that under the same overpotential, the hydrogen production rate with ultrasound vibration (40 kHz, 40 W) is 6 times higher and the impedance is 4 times lower than that without ultrasound. The catalytic efficiency of WS2 can be significantly increased through this method, which paves the way towards non-noble metal HER catalysts to produce hydrogen with aid of piezoelectricity.

Carbon-embedded hierarchical and dual-anion C@MoSP heterostructure for efficient capacitive deionization of saline water

Recently, two-dimensional transition-metal disulfides (TMDs) with amazing chemical and physical superiorities are emerging as promising faradic ion intercalation materials for effective capacitive deionization (CDI) application. Herein, a carbon-embedded hierarchical and dual-anion C@MoSP nanoflowers was synthesized by a facile hydrothermal reaction accompanied by the phosphorization treatment, and explored as an advanced CDI electrode material. Benefiting from the specific microstructure (e.g., hierarchically porous arrangement, enriched porosity and highly open pore network) and the strong synergistic effect of highly conductive carbon and low-electronegativity P dopant, the C@MoSP electrode demonstrated high surface area, enrich accessible interlayer, superior ion diffusion pathway and good electric conductivity for effective salty ion intercalation/deintercalation; Accordingly, the C@MoSP electrode realized an outstanding gravimetric adsorption capacity of 25.43 mg/g, excellent intercalation rate, and good durability in a NaCl aqueous solution with an initial concentration of 500 mg/L at an operation voltage of 1.2 V, which were evidently enhanced in comparison to MoSP and pristine MoS2 ones.

Gas-sensing properties of Ptn-doped WSe2 to SF6 decomposition products

Two-dimensional transition metal disulfides (TMDs) has attracted considerable attention due to its excellent physicochemical properties. In order to detect the SF6 decomposition products, transition metal Pt cluster doping was chosen to enhance the adsorption property of intrinsic WSe2 monolayer. The adsorption of SO2, SOF2 and SO2F2 on Ptn (n=1–3) doped WSe2 monolayer is studied based on the first-principle calculation. The adsorption energy, charge transfer, and density of states of the interaction between the target gas molecules and Ptn-WSe2 were studied. The calculation results showed that Pt3 doping dramatically enhances the adsorption of WSe2 to SO2 and SOF2 and SO2F2 molecules. Meanwhile, electrons transfer from Ptn-WSe2 surface to these three kinds of target gas molecules, reducing the conductivity of the adsorption system in different degrees. The results of this study not only have important significance for explaining the sensing mechanism of Ptn-WSe2 adsorption to SF6 gas decomposition products, but also provide a potential material for further development of gas sensing sensors.

Plasmonic Effect on the Magneto-Optical Property of Monolayer WS2 Studied by Polarized-Raman Spectroscopy

In recent years, the magneto-optical properties of two-dimensional transition metal disulfides have attracted more and more attention due to their further device applications in spintronics and valleytronics. However, to our knowledge, the plasmonic effect on the magneto-optical properties of WS2 has not been studied. In this work, monolayer WS2 transferred on SiO2/Si substrate and Au film were investigated respectively using polarized-Raman spectroscopy at 4 K under different magnetic fields. Prominent magnetic field–induced variations in the Raman intensities of WS2 samples were observed, which also exhibited significant differences in the spectral evolution versus magnetic field. The resonance magnetic field was 5 T and 5.5 T for the WS2 on SiO2/Si substrate and Au film, respectively. Remarkably, the magneto-optical Raman intensities of A1′ and 2LA(M) modes for WS2 on Au film were reduced to approximately 60% compared with that of WS2 on SiO2/Si. These results suggest that the plasmonic effect–induced charge transfer plays an important role in the magneto-optical Raman effect of WS2.

Effects of Phase Selection on Gas-Sensing Performance of MoS2 and WS2 Substrates.

Two-dimensional transition metal disulfides such as MoS2 and WS2 exhibit multiple phases. Altering their phase makes it possible to change their chemical and physical properties significantly. Although several phase-induced modification mechanisms have been reported, their effects on the gas-sensing performance of these substrates remain unknown. Here, the effects of phase selection on the gas-sensing characteristics of 1T′ and 2H monolayer MoS2 and WS2 were explored using a density functional theory-based first-principles approach. The theoretical computations took into account the binding energy, band structure, theoretical recovery time, density of states, electron difference density, and total electron density. The results showed that there is a significant change in the density of states near the Fermi level as well as greater charge transfer between the gas in question and the substrate when the gas is adsorbed onto 1T′ MoS2 and WS2. Thus, phase selection is important for improving the gas-sensing performance of monolayer MoS2 and WS2. This study provides theoretical evidence for increasing the sensing performance of polymorph films of these materials.

Hierarchical and Self-Supported Vanadium Disulfide Microstructures@Graphite Paper: An Advanced Electrode for Efficient and Durable Asymmetric Capacitive Deionization

Two-dimensional transition-metal disulfides (TMDs) have aroused boosting attention for the capacitive deionization (CDI) application because of their amazing chemical and physical superiorities. Nevertheless, besides the unsatisfying desalination capacity, the reported TMD-based CDI electrodes mostly relied on the highly time-consuming and polymer binder-assisted slurry-coating electrode processing. Thus, the exploring of high-performance active materials accompanied by the simplified electrode processing remains a great challenge. Herein, a novel hierarchical and self-supported VS2@graphite paper (GP) electrode consisting of numerous VS2 nanosheets vertically grown on the GP substrate was rationally proposed as an advanced CDI electrode. The superior microstructure framework (i.e., hierarchical pore arrangement, enriched open void spaces, and free binder) enabled the electrode with a high specific surface area, optimized ion diffusion pathway, excellent electric conductivity, and good wettability. Accordingly, the binder-free self-supported VS2@GP electrode demonstrated a remarkable specific capacitance of 241.75 F/g. More importantly, when coupling with the activated carbon for the asymmetrical CDI unit, the constructed VS2@GP electrode delivered an outstanding salt adsorption capacity of 31.19 mg/g, excellent adsorption rate, and great long-term durability, which were significantly improved compared to those of the traditional powder electrode.

Hemispherical shell-thin lamellar WS2 porous structures composited with CdS photocatalysts for enhanced H2 evolution

Two-dimensional transition metal disulfides (2D TMDs) with intriguing electrical and optical properties have been extensively applied in the field of catalysis. Tungsten disulfide (WS2) as one of the 2D TMDs plays an important role in cocatalyst for photocatalytic reactions. In this work, hemispherical shell-thin lamellar WS2 porous structures have been designed to make use of synergistic effect between slow photo effect of WS2 pore and high electron transfer of 2D WS2 for promoting photocatalyst activity. Porous WS2 structures derived from the ultrasonic-assisted high-temperature calcination are composited with cadmium sulfide (CdS) photocatalyst through hydrothermal method. The formed porous WS2/CdS composite shows superior photocatalytic H2 evolution activity compared with non-porous WS2/CdS composite or pure CdS under the visible-light irradiation. The H2 evolution rate of the WS2/CdS composite is 58.1 mmol h−1 g−1, being 38 times higher than that of CdS. Controlled experiments and structural characterizations reveal that the enhanced photocatalytic activity of the porous WS2/CdS composite is closely related to its unique configuration with coexistence of hemispherical shell and thin lamellar structures. Heterojunction structure between WS2 and CdS accelerates transfer of photo-excited electrons. Slow photo effect from WS2 porous structure provides more chance for light utilization of CdS in the pores to improve the visible-light absorption and photocurrent. The unique porous 2D TMDs materials are expected as a new class of efficient cocatalysts for H2 evolution catalysts.

Recent Advance in Gas Sensing by Using Two-Dimensional Transition Metal Disulfides Materials

Recent Advance in Gas Sensing by Using Two-Dimensional Transition Metal Disulfides Materials

Free-radical gases on two-dimensional transition-metal disulfides (XS2, X = Mo/W): robust half-metallicity for efficient nitrogen oxide sensors

The detection of single gas molecules is a highly challenging work because it requires sensors with an ultra-high level of sensitivity. By using density functional theory, here we demonstrate that the adsorption of a paramagnetic unpaired free radical gas (NO) on a monolayer of XS2 (X = Mo, W) can trigger the transition from semiconductor to half metal. More precisely, the single-layer XS2 (X = Mo, W) with NO adsorbed on it would behave like a metal in one spin channel while acting as a semiconductor in the other spin orientation. The half-metallicity is robust and independent of the NO concentration. In contrast, no half-metallic feature can be observed after the adsorption of other free radical gases such as NO2. The unique change in electronic properties after the adsorption of NO on transition-metal sulfides highlights an effective strategy to distinguish NO from other gas species by experimentally measuring spin-resolved transmission. Our results also suggest XS2 (X = Mo, W) nanosheets can act as promising nanoscale NO sensors.

Two-Dimensional Transition Metal Disulfides for Chemoresistive Gas Sensing: Perspective and Challenges

Transition metal disulfides have been attracting significant attentions in recent years. There are extensive applications of transition metal disulfides, especially on gas sensing applications, due to their large specific surface-to-volume ratios, high sensitivity to adsorption of gas molecules and tunable surface functionality depending on the decoration species or functional groups. However, there are several drawbacks such as poor gas selectivity, sluggish recovery characteristics and difficulty in the fabrication of large-scale devices. Here, we provide a review of recent progress on the chemoresistive gas sensing properties of two-dimensional transition metal disulfides. This review also provides various methods to enhance the gas sensing performance of two-dimensional disulfides, such as surface functionalization, decoration receptor functions and developing nanostructures.