Bulk MoS2 Research Articles

Improvement of MoS2 electrocatalytic activity for hydrogen evolution reaction by ion irradiation

Molybdenum disulfide (MoS2) is considered promising noble metal-free catalysts for the hydrogen evolution reaction (HER). Whereas the bulk MoS2 does not exhibit significant activity, the catalytic properties of various nanostructures are noticeable. Therefore we synthesized flower-like molybdenum disulfide with the simple, one-step hydrothermal method. To enhance the catalytic activity of the material, low-energy ion irradiation is employed. As-prepared MoS2 is irradiated with hydrogen and carbon ions of various energies (20–40 keV) and fluences (1014-1017 ion/cm2). Our results show that irradiation has beneficial influence on MoS2 catalytic activity toward hydrogen evolution reaction. By producing morphological changes and defects in the structure, ion irradiation also impacts the conductivity of the material, which shows predominant effect on hydrogen evolution. The increase of current density at an overpotential of 300 mV with hydrogen ion irradiation is even 6 times higher than for as-synthesized catalyst.

Magnetic‐Electric Metamirror and Polarizing Beam Splitter Composed of Anisotropic Nanoparticles

AbstractThe emergence of new materials and fabrication techniques provides progress in the development of advanced photonic and communication devices. Transition metal dichalcogenides (e.g., molybdenum disulfide, MoS2) are novel materials possessing unique physical and chemical properties promising for optical applications. In this paper, a metasurface composed of particles made of bulk MoS2 is proposed and numerically studied considering its operation in the near‐infrared range. In the bulk configuration, MoS2 has a layered structure being a uniaxial anisotropic crystal demonstrating an optical birefringence property. It is supposed that the large‐scale and uniform MoS2 layers are synthesized in a vertical‐standing morphology, and then they are patterned into a regular 2D array of disks to form a metasurface. The natural anisotropy of MoS2 is utilized to realize the splitting of electric and magnetic dipole modes of the disks while optimizing their geometric parameters to bring the desired modes into overlap. At the corresponding resonant frequencies, the metasurface behaves as either an electric or a magnetic mirror, depending on the polarization of incident light. Based on the extraordinary reflection characteristics of the proposed metasurface, it can be considered an alternative to traditional mirrors and optical splitters when designing compact and highly efficient metadevices, which provide polarization and phase manipulation of electromagnetic waves on a subwavelength scale.

Synergistic Effect of Yb3+, Tm3+, Nd3+ Doped NaYF4 Nanoparticles and MoS2 Nanosheets for Enhanced Photocatalytic Dye Degradation under Visible Light

The synergistic influence of synthesized MoS2 (syn-MoS2) nanosheets and NaYF4:Yb,Tm,Nd upconversion nanoparticles (UCNP) on effective dye degradation is demonstrated. A detailed characterization of bulk MoS2 (b-MoS2), syn-MoS2 (with enhanced photocatalytic activity compared to the commercially available b-MoS2), and UCNP was done. Optical properties revealed that emission spectra of UCNP corresponding to 450, 475, and 650 nm overlap with absorption spectra of MoS2 (∼455 and 635 nm). A comparative methylene blue degradation of 96% could be achieved with the combination of UCNP and syn-MoS2 in comparison to 56.4% for syn-MoS2 and 42% for b-MoS2. Concentration-dependent dye degradation study of UCNP and syn-MoS2 revealed a maximum dye removal efficiency of 99.6% for 150 mg L–1 of syn-MoS2 and 200 mg L–1 of UCNP within 2 hrs. Comparative degradation investigations were performed on various dyes (methylene blue, methylene orange, rhodamine B, and their mixture) and water systems (artificial river-water and artificial seawater) to assess the potential applicability of the system in environmental remediation. After five cycles of reusability studies, an efficacy of 88% dye removal was obtained. The mechanism behind this was investigated using the ESR and scavengers test, which confirmed that the primary radicals responsible for dye degradation were superoxide (•O2–) and hydroxyl (•OH). A comparative photocurrent experiment also verified that the presence of UCNP enhances the efficiency of the syn-MoS2 by 98% in the presence of visible light.

Polypyrrole-Modified Molybdenum Disulfide Nanocomposite Epoxy Coating Inhibits Corrosion of Mild Steel

The search for lightweight and low-cost anticorrosion coatings is particularly important in coastal environments with high salt and humidity. Graphene-based anticorrosion coatings are currently unable to provide long-lasting corrosion protection for metals because of their “corrosion-promoting activity”, and graphene-like materials, such as molybdenum disulfide (MoS2), are beginning to be anticipated its ability to protect against metals. This paper reported a simple method for preparing polypyrrole (PPy)-modified MoS2 nanomaterials from natural bulk MoS2. Their corrosion resistance behavior as fillers for epoxy (EP) resins was investigated in 3.5 wt.% NaCl solution. After the preparation of the MoS2 nanosheet dispersion by liquid-phase sonication using ethanol aqueous solution, the polypyrrole-coated molybdenum disulfide nanomaterials (MoS2@PPy) were directly obtained by adding pyrrole monomer to it in the presence of the initiator ammonium persulfate. Tafel polarization curves showed that the corrosion current of the MoS2@PPy/EP coating was 0.006 µA/cm2 after 15 days of immersion in 3.5 wt.% NaCl solution, much lower than that of pure EP coating (19.134 µA/cm2), effectively improving the anticorrosive properties of the coating. Overall, this study offered a practical method for the application of natural bulk MoS2 for corrosion protection.

In-plane heterostructured MoN/Mo2N nanosheets as high-efficiency electrocatalysts for alkaline hydrogen evolution reaction

Economical and efficient electrocatalysts are crucial to the hydrogen evolution reaction (HER) in water splitting to produce hydrogen. Heterostructured electrocatalysts generally exhibit enhanced HER catalytic activity due to the strong electron coupling effects and synergistic optimization of hydrogen adsorption–desorption. Herein, in-plane heterostructured MoN/Mo2N nanosheets are fabricated as high-efficiency HER electrocatalysts in the alkaline medium from bulk MoS2 by molten salt-assisted synthesis. Density-functional theory calculations and experiments show that the in-plane heterostructured MoN/Mo2N nanosheets facilitate interfacial electron redistribution from Mo2N to MoN, giving rise to more negative H2O adsorption energy and optimal hydrogen adsorption free energy (ΔGH* = −0.017 eV). Consequently, a low overpotential of 126 mV at 10 mA cm−2 and a small Tafel slope of 69.5 mV dec−1 are achieved in the 1M KOH electrolyte, demonstrating excellent HER characteristics. Moreover, the overpotential shows negligible change after operating at 50 mA cm−2 for 12 h, confirming the excellent stability. The results reveal a novel and effective strategy to design highly efficient 2D in-plane heterostructured HER electrocatalysts for water splitting.

Characterization of local non-equilibrium phonons in bulk MoS2 probed by temperature-dependent Raman scattering

This study presents an experimental implementation to determine the temperature dependence of Raman spectra of bulk 2H-MoS2 in the temperature range of 300–650 K. In addition, we have measured according to the anti-Stokes to Stokes integrated intensity ratio (). The results indicate that Raman shift of both the vibration modes and exhibits a linear temperature-dependent behavior in the heating and cooling processes. It is confirmed that the sample did not deteriorate by oxidization and decomposition at high temperatures. However, above 500 K, the measured phonon temperature deviated from the heat bath temperature. It is suggested that the does not correspond to the equilibrium population of optical phonons, thus indicating that the measurement was not in thermal equilibrium but in a thermal steady state under laser irradiation. We conclude that this is caused by the difference in relaxation times in the thermal steady state.

Structural changes in bulk MoS2 as a consequence of 1.5 MeV proton micro beam irradiation

Structural changes in bulk MoS2 as a consequence of 1.5 MeV proton micro beam irradiation

The study of phase transition of MoS2 regulated by H+

The mixed-phase MoS2 (1T/2H MoS2) with heterostructure exhibited high catalytic activity. The specific ratios of 1T/2H could exhibit optimal performance in various applications. Therefore, more methods need be developed for synthesizing 1T/2H mixed-phase MoS2. Herein, a viable route was studied for the phase transition of 1T/2H MoS2 regulated by H+. Briefly, the commercially available bulk MoS2 was used to obtain 1T/2H MoS2 via chemical intercalation of Li+. Then the residual Li+ around 1T/2H MoS2 was replaced by H+ in acidic electrolytes, owing to the extremely higher charge-to-volume ratio of H+. Thus, the thermodynamically unstable 1T phase lost the protection of residual Li+ and could be re-transforming into the relatively stable 2H phase. The change of the 2H/(2H+1T) ratio was measured using novel extinction spectroscopy, which provides a rapid identification method in comparison with x-ray photoelectron spectroscopy (XPS). The experimental results revealed that the concentration of H+ influenced the phase transition velocity of MoS2. In particular, the phase transition from 1T to 2H phase in the H+ solution was faster at the beginning, and the higher the H+ concentration in an acidic solution, the faster the increase in 2H content. For an instant, the ratio of the 2H phase was increased by 7.08% in an acidic solution (CH + = 2.00 M) after 1 h, which was several times greater than the case in the distilled water. This finding provides a promising method to easily obtain different ratios of 1T/2H MoS2, which is beneficial for further development of catalytic performance especially in energy generation and storage.

Exposure to MoS2 nanosheets or bulk activated Kruppel-like factor 4 in 3D Caco-2 spheroids in vitro and mouse intestines in vivo.

MoS2 nanosheets (NSs) are novel 2D nanomaterials (NMs) being used in many important fields. Recently, we proposed the need to evaluate the influences of NMs on Kruppel-like factors (KLFs) even if these materials are relatively biocompatible. In this study, we investigated the influences of MoS2 NSs or bulk on KLF4 signaling pathway in 3D Caco-2 spheroids in vitro and mouse intestines in vivo. Through the analysis of our previous RNA-sequencing data, we found that exposure to MoS2 NSs or bulk activated KLF4 expression in 3D Caco-2 spheroids. Consistently, these materials also activated KLF4-related gene ontology (GO) terms and down-regulated a panel of KLF4-downstream genes. To verify these findings, we repeatedly exposed mice to MoS2 NSs or bulk materials via intragastrical administration (1 mg/kg bodyweight, once a day, for 4 days). It was shown that oral exposure to these materials decreased bodyweight, leading to relatively higher organ coefficients. As expected, exposure to both types of materials increased Mo elements as well as other trace elements, such as Zn, Fe, and Mn in mouse intestines. The exposure also induced morphological changes of intestines, such as shortening of intestinal villi and decreased crypt depth, which may result in decreased intestinal lipid staining. Consistent with RNA-sequencing data, we found that material exposure increased KLF4 protein staining in mouse intestines and decreased two KLF4 downstream proteins, namely extracellular signal-regulated kinase (ERK) and serine/threonine kinase (AKT). We concluded that MoS2 materials were capable to activate KLF4-signaling pathway in intestines both in vivo and in vitro.

Impact of compression on the crystal structure, electronic and magnetic properties for bulk MoS2

Impact of compression on the crystal structure, electronic and magnetic properties for bulk MoS2

Structural, elastic, electronic, optical and vibrational properties of single-layered, bilayered and bulk molybdenite MoS2-2H.

In recent years, transition metal dichalcogenides have received great attention since they can be prepared as two-dimensional semiconductors, presenting heterodesmic structures incorporating strong in-plane covalent bonds and weak out-of-plane interactions, with an easy cleavage/exfoliation in single or multiple layers. In this context, molybdenite, the mineralogical name of molybdenum disulfide, MoS2, has drawn much attention because of its very promising physical properties for optoelectronic applications, in particular a band gap that can be tailored with the material's thickness, optical absorption in the visible region and strong light-matter interactions due to the planar exciton confinement effect. Despite this wide interest and the numerous experimental and theoretical articles in the literature, these report on just one or two specific features of bulk and layered MoS2 and sometimes provide conflicting results. For these reasons, presented here is a thorough theoretical analysis of the different aspects of bulk, monolayer and bilayer MoS2 within the density functional theory (DFT) framework and with the DFT-D3 correction to account for long-range interactions. The crystal chemistry, stiffness, and electronic, dielectric/optical and phonon properties of single-layered, bilayered and bulk molybdenite have been investigated, to obtain a consistent and detailed set of data and to assess the variations and cross correlation from the bulk to single- and double-layer units. The simulations show the indirect-direct transition of the band gap (K-K' in the first Brillouin zone) from the bulk to the single-layer structure, which however reverts to an indirect transition when a bilayer is considered. In general, the optical properties are in good agreement with previous experimental measurements using spectroscopic ellipsometry and reflectivity, and with preliminary theoretical simulations.

Green solvent exfoliation of few layers 2D-MoS2 nanosheets for efficient energy harvesting and storage application

In this study, a facile, cost-effective, and green solvent exfoliation route with bio extract was derived from Cissus quadrangularis (MP) and it was used to exfoliate to get the few layers of MoS2. Compared to conventional chemical exfoliation, the green solvent exfoliation method showed quick delamination from the bulk MoS2. To compare the effectiveness of the exfoliation in green solvents, the exfoliation analysis of the bulk MoS2 was conducted in N-Methyl-2-pyrrolidone and another bio-extract with low Hansen properties was derived from Mussa acuminate. The MP-derived green extract showed excellent delamination of the few layers of MoS2 nanosheets with a minimal processing time of 2 h. It was confirmed that the layered structure with a lateral size of 150 nm and a d-spacing was 0.63 nm by HRTEM, and also Raman spectrum revealed the minimum frequency difference (24.6 cm−1) for MP, which confirmed the presence of the few-layer MoS2. The obtained few-layered MoS2 nanosheets were further examined as an electrode material for dye-sensitized solar cells (DSSC) and supercapacitor applications. The fabricated DSSC with MP-electrode showed increased Voc with a photo conversion efficiency (PCE) of 5.14 %. As a supercapacitor electrode, it exhibited a significant potential window expansion of 0.1 V with a specific capacitance of 183 F/g at a scan rate of 5 mVs−1. This work opens up a simple, inexpensive, and eco-friendly green solvent exfoliation methodology to obtain the layers of 2D materials.

Phase engineering of layered anode materials during ion-intercalation in Van der Waal heterostructures

Transition metal dichalcogenides (TMDs) are a class of 2D materials demonstrating promising properties, such as high capacities and cycling stabilities, making them strong candidates to replace graphitic anodes in lithium-ion batteries. However, certain TMDs, for instance, MoS2, undergo a phase transformation from 2H to 1T during intercalation that can affect the mobility of the intercalating ions, the anode voltage, and the reversible capacity. In contrast, select TMDs, for instance, NbS2 and VS2, resist this type of phase transformation during Li-ion intercalation. This manuscript uses density functional theory simulations to investigate the phase transformation of TMD heterostructures during Li-, Na-, and K-ion intercalation. The simulations suggest that while stacking MoS2 layers with NbS2 layers is unable to limit this 2H → 1T transformation in MoS2 during Li-ion intercalation, the interfaces effectively stabilize the 2H phase of MoS2 during Na- and K-ion intercalation. However, stacking MoS2 layers with VS2 is able to suppress the 2H → 1T transformation of MoS2 during the intercalation of Li, Na, and K-ions. The creation of TMD heterostructures by stacking MoS2 with layers of non-transforming TMDs also renders theoretical capacities and electrical conductivities that are higher than that of bulk MoS2.

High-Order Harmonics Generation in MoS2 Transition Metal Dichalcogenides: Effect of Nickel and Carbon Nanotube Dopants.

The transition metal dichalcogenides have instigated a lot of interest as harmonic generators due to their exceptional nonlinear optical properties. Here, the molybdenum disulfide (MoS2) molecular structures with dopants being in a plasma state are used to demonstrate the generation of intense high-order harmonics. The MoS2 nanoflakes and nickel-doped MoS2 nanoflakes produced stronger harmonics with higher cut-offs compared with Mo bulk and MoS2 bulk. Conversely, the MoS2 with nickel nanoparticles and carbon nanotubes (MoS2-NiCNT) produced weaker coherent XUV emissions than other materials, which is attributed to the influence of phase mismatch. The influence of heating and driving pulse intensities on the harmonic yield and cut-off energies are investigated in MoS2 molecular structures. The enhanced coherent extreme ultraviolet emission at ~32 nm (38 eV) due to the 4p-4d resonant transitions is obtained from all aforementioned molecular structures, except for MoS2-NiCNT.

Effects of pH on the Structure and Optical Property of Molybdenum Disulfide Nanocrystals Synthesized by Hydrothermal Method

Molybdenum disulphide (MoS2) is emerging as one of the most attractive two-dimensional (2D) materials amongst the transition metal dichalcogenides (TMDs) group. MoS2 exhibits a thickness-dependent band gap that changes from indirect band gap of ~1.3 eV for bulk MoS2 to direct band gap of ~1.9 eV in monolayer form. Such indirect-to-direct gap transition due to quantum confi nement results in giant enhancement in photoluminescence quantum yield. Recent study demonstrated that the co-existence of 2H- and 1T-MoS2 synthesized by hydrothermal method exhibited superior electronic conductivity to those with 2H semiconducting phase. Metallic 1T phase of MoS2 is attracting much attention due to its high electronic conductivity and potential applications in supercapacitors, thermoelectric energy harvesting, and memristors. In this paper, we investigated a hybrid structure between 2H and 1T of MoS2 by a hydrothermal process and investigated its optical property via photoluminescence spectra with changes in pH values. Our results indicated that photoluminescence occurred in the visible light region and the band gap was determined to be ~1.96–2.50 eV. In addition, pH of the precursor solution strongly affected the fi nal structure of MoS2 nanocrystals, which in turn, affected their photoluminescence property wherein pH = 4–5 was identifi ed as optimum for 2H–1T-MoS2 synthesis.

Thermal Annealing Effects on the Raman and Photoluminescence Properties of Mono and Few-Layer MoS<sub>2</sub> Films

In this study, we report that the thermal treatment effects on the Raman and photoluminescence (PL) spectra of mono and few-layer MoS2 films by annealing in the vacuum and air at 300°C, respectively. The MoS2 film samples were prepared on silicon substrate by exfoliating from a bulk MoS2 crystal with a micromechanical exfoliation. For characterization of structural properties of the MoS2 films and identification of the Raman active modes, Raman spectrometer equipped with a He-Ne laser source and an optical microscope has been used. The results show that the vacuum annealing 7L MoS2 decreases the Full Width at Half Maximum (FWHM) of the Raman active modes as E12g, A1g and the vacuum annealing 1L MoS2 increases the PL intensity and peak energy, for 60% and 13.3meV, respectively also air annealing bilayer MoS2 increased the PL intensity (IA) and peak energy (EA), respectively for 85% and 15.4 meV (300°C for 40 min). After thermal annealing (vacuum and air), we observe that the indirect bandgap of the few-layer MoS2 was changed.

Gamma-Ray Irradiation Induced Ultrahigh Room-Temperature Ferromagnetism in MoS2 Sputtered Few-Layered Thin Films

Defect engineering is of great interest to the two-dimensional (2D) materials community. If nonmagnetic transition-metal dichalcogenides can possess room-temperature ferromagnetism (RTFM) induced by defects, then they will be ideal for application as spintronic materials and also for studying the relation between electronic and magnetic properties of quantum-confined structures. Thus, in this work, we aimed to study gamma-ray irradiation effects on MoS2, which is diamagnetic in nature. We found that gamma-ray exposure up to 9 kGy on few-layered (3.5 nm) MoS2 films induces an ultrahigh saturation magnetization of around 610 emu/cm3 at RT, whereas no significant changes were observed in the structure and magnetism of bulk MoS2 (40 nm) films even after gamma-ray irradiation. The RTFM in a few-layered gamma-ray irradiated sample is most likely due to the bound magnetic polaron created by the spin interaction of Mo 4d ions with trapped electrons present at sulfur vacancies. In addition, density functional theory (DFT) calculations suggest that the defect containing one Mo and two S vacancies is the dominant defect inducing the RTFM in MoS2. These DFT results are consistent with Raman, X-ray photoelectron spectroscopy, and ESR spectroscopy results, and they confirm the breakage of Mo and S bonds and the existence of vacancies after gamma-ray irradiation. Overall, this study suggests that the occurrence of magnetism in gamma-ray irradiated MoS2 few-layered films could be attributed to the synergistic effects of magnetic moments arising from the existence of both Mo and S vacancies as well as lattice distortion of the MoS2 structure.

New insights into APCVD grown monolayer MoS2 using time-domain terahertz spectroscopy

In modern era, wireless communications at ultrafast speed are need of the hour and search for its solution through cutting edge sciences is a new perspective. To address this issue, the data rates in order of terabits per second (TBPS) could be a key step for the realization of emerging sixth generation (6G) networks utilizing terahertz (THz) frequency regime. In this context, new class of transition metal dichalcogenides (TMDs) have been introduced as potential candidates for future generation wireless THz technology. Herein, a strategy has been adopted to synthesize high-quality monolayer of molybdenum di-sulfide (MoS2) using indigenously developed atmospheric pressure chemical vapor deposition (APCVD) set-up. Further, the time-domain transmission and sheet conductivity were studied as well as a plausible mechanism of terahertz response for monolayer MoS2 has been proposed and compared with bulk MoS2. Hence, the obtained results set a stepping stone to employ the monolayer MoS2 as potential quantum materials benefitting the next generation terahertz communication devices.

HBN Encapsulation Effects on the Phonon Modes of MoS2 with a Thickness of 1 to 10 Layers

AbstractInterfaces with surrounding materials, where charged impurities and surface roughness are present, have a significant impact on the electrical and optical properties of 2D materials. In the change of the phonon modes of MoS2 accompanied by thickness variation, the portion caused by intrinsic factors and the portion caused by the interface effect are separated by examining the result of encapsulation with hexagonal boron nitride (hBN). For instance, the frequency of the A1g peak of MoS2 supported by SiO2 decreases by ≈4 cm−1 in air for a thickness reduction from ten layers to monolayer. Of this decrease, roughly 2 cm−1 is attributable to the weakening of the van der Waals interlayer interaction, while the remaining 2 cm−1 is due to the interface effect. The interface state, that is, the types and concentrations of impurities at the interface, between MoS2 and SiO2 is estimated to be similar to that between MoS2 and air because the Raman properties when one surface of MoS2 is in contact with SiO2 and with air are identical within the measurement error. When entirely encapsulated with hBN, the width of the A1g peak of few‐layer MoS2 is significantly reduced, becoming comparable or equal to that of bulk MoS2.

Trapped mode control in metasurfaces composed of particles with the form birefringence property.

Progress in developing advanced photonic devices relies on introducing new materials, discovered physical principles, and optimal designs when constructing their components. Optical systems operating on the principles of excitation of extremely high-quality factor trapped modes (also known as the bound states in the continuum, BICs) are of great interest since they allow the implementation of laser and sensor devices with outstanding characteristics. In this paper, we discuss how one can utilize the anisotropic properties of novel materials (transition metal dichalcogenides, TMDs), particularly, the bulk molybdenum disulfide (MoS2), to realize the excitation of trapped modes in dielectric metasurfaces. The bulk MoS2 is a thin-film structure in which the light wave behaves the same way as that in the uniaxial anisotropic material with the form birefringence property. Our metasurface is composed of an array of disk-shaped nanoparticles (resonators) made of the MoS2 material under the assumption that the anisotropy axis of MoS2 can be tilted to the rotation axis of the disks. We perform a detailed analysis of eigenwaves and scattering properties of such anisotropic resonators as well as the spectral features of the metasurface revealing dependence of the excitation conditions of the trapped mode on the anisotropy axis orientation of the MoS2 material used.