Bulk Molybdenum Disulfide Research Articles

From Bulk Molybdenum Disulfide (MoS2) to Suspensions of Exfoliated MoS2 in an Aqueous Medium and Their Applications.

Two-dimensional (2D) layered materials like graphene, transition-metal dichalcogenides (TMDs), boron nitrides, etc., exhibit unique and fascinating properties, such as high surface-to-volume ratio, inherent mechanical flexibility and robustness, tunable bandgap, and high carrier mobility, which makes them an apt candidate for flexible electronics with low consumption of power. Because of these properties, they are in tremendous demand for advancement in energy, environmental, and biomedical sectors developed through various technologies. The production and scalability of these materials must be sustainable and ecofriendly to utilize these unique properties in the real world. Here, in this current review, we review molybdenum disulfide (MoS2 nanosheets) in detail, focusing on exfoliated MoS2 in water and the applicability of aqueous MoS2 suspensions in various fields. The exfoliation of MoS2 results in the formation of single or few-layered MoS2. Therefore, this Review focuses on the few layers of exfoliated MoS2 that have the additional properties of 2D layered materials and higher excellent compatibility for integration than existing conventional Si tools. Hence, a few layers of exfoliated MoS2 are widely explored in biosensing, gas sensing, catalysis, photodetectors, energy storage devices, a light-emitting diode (LED), adsorption, etc. This review covers the numerous methodologies to exfoliate MoS2, focusing on the various published methodologies to obtain nanosheets of MoS2 from water solutions and their use.

Molybdenum disulfide under extreme conditions: An ab initio study on its melting

Crystalline molybdenum disulfide has become a central actor in the 2D-materials community due to its promising optoelectronic and thermoelectric properties. Despite the extensive work made in investigating these properties, a vast area of knowledge remains unknown on the structure and dynamics of its disordered phases such as liquid and amorphous. Thus, the goal of this work is to investigate the melting of bulk molybdenum disulfide using ab initio molecular dynamics based on density functional theory. We employ the two-phase and Z-methods to evaluate the melting in a number of conditions. Our results at 1 bar reveal that the two-phase procedure is preferred since it predicts a melting point of 2266.92 K that is directly computed using simulations at constant pressure and energy. In contrast, this temperature is indirectly estimated at 2154.01 K with the Z-method using an interpolation of simulations at constant volume and energy. Nevertheless, we find that both methods are complementary as they allow computing different thermodynamic and structural properties. For instance, we estimate a melting heat of 0.67 eV/atom with the two-phase coexistence route, which shows very good agreement with the value of 0.75 eV/atom obtained from the difference of the internal energies of separate crystalline and liquid ensembles at the same conditions of 1 bar and 2266.92 K. In contrast, the Z-method allows us to determine the influence of pressure on the melting temperature, density, and coordination number with a lower computational cost.

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.

Exfoliation of MoS2 by zero-valent transition metal intercalation.

Exfoliation of bulk molybdenum disulfide (MoS2) into few-layered nanosheets is achieved with the assistance of zero-valent transition metal (Co0, Ni0, Cu0) intercalation. The as-prepared MoS2 nanosheets are characterized to consist of 1T- and 2H-phases with an enhanced electrocatalytic hydrogen evolution reaction (HER) activity. This work provides a novel strategy to prepare 2D MoS2 nanosheets using mild reductive reagents, which is expected to avoid the undesired structural damage from conventional chemical exfoliation.

Anisotropic slot waveguides with bulk transition metal dichalcogenides for crosstalk reduction and high-efficiency mode conversion.

Conventional slot waveguides (CSWs) consisting of an isotropic low-index material sandwiched by two high-index silicon wires have been extensively used in functional photonic devices, including chemical sensing, optical modulating, and all-optical signal processing, due to its significantly enhanced electric field perpendicular to the interfaces in the slot layer. However, there are two drawbacks to be improved if the CSWs are used for signal transmission in photonic integrated circuits, including the crosstalk between waveguides and direct butting mode conversion efficiency (MCE) to a silicon (Si)-strip waveguide. In this study, we propose an anisotropic SW with bulk transition metal dichalcogenide (ASWTMD) to relieve the two shortcomings by replacing the isotropic low-index slot layer with a bulk molybdenum disulfide layer having a high refractive index and giant optical anisotropy. We demonstrated the crosstalk reduction (CR) of the proposed ASWTMD by analyzing the mode profile, power confinement, and coupling strength. We also investigated the MCE by examining the mode overlap ratio and power evolution. The proposed ASWTMD shows significant CR and superior MCE for the transverse electric and transverse magnetic modes compared to those of a CSW with a SiO2-slot layer. The present design paves the possible extensibility to other transition metal dichalcogenides (TMDs) for designing state-of-the-art TMD-based photonic devices exploiting their extraordinary optical properties.

Facile and novel preparation of 2D-MoS2/carbon nanodots composite with high adsorption efficiency and capacity for dye removal

Here, hydrophobic bulk molybdenum disulfide (MoS2) was efficiently exfoliated into two dimensional (2D) nanosheets assisted with amphiphilic carbon nanodots (CNDs) in pure water. The optimized concentration of obtained 2D-MoS2 is as high as 0.41 mg/mL, which is higher than that of reported literatures. The average lateral size and layer number of 2D-MoS2 is 142.6 (±2.3) nm and 4, respectively. The characterizations and simulation demonstrated that CNDs can be instantly adsorbed on the surface of 2D-MoS2 and form 2D-MoS2/CNDs composite. Thus, after standing for 1000 h, the concentration of 2D-MoS2 still accounts for 90 % of the initial concentration with the help of CNDs. Further, this technique can also be used to prepare graphene and 2D-WS2 nanosheets. 2D-MoS2/CNDs composite as adsorbent showed excellent adsorption capacity (1136.9 mg/g) and ultrafast adsorption rate (1 min) for methylene blue removal, which benefits from abundant adsorption sites and high dispersion. This work not only develops a facile and universal technique for green preparation of 2D nanomaterials in one pot, but also provides a promising adsorbent for removing organic pollutants from aqueous solution.

Lignosulfonate functionalized nanomaterials for enhancement of the electrochemical performance of polyaniline

The electrochemical enhancement of polyaniline (PANI) by adding nanomaterials is closely related with the dispersion state of the nanomaterials in PANI. Herein, we demonstrate the fabrication of well-mixed PANI/nanomaterials composites through solid-phase exfoliation and subsequent in-situ polymerization. Bulk molybdenum disulfide (MoS2) or carbon nanotube (CNT) bundles were firstly ball-milled in the presence of calcium lignosulphonate to provide lignosulfonate (LS) noncovalently functionalized MoS2 nanosheets or CNTs, which were then used as skeletons for in-situ polymerization of aniline. The resulted nanocomposites exhibited enhanced electrochemical properties as compared with pure PANI. Typically, the ternary composite PANI/MoS2-LS/CNT-LS delivered a specific capacitance of 458.9 F/g, a 63% improvement than PANI at 1 A/g in a three-electrode system. Furthermore, excellent cycle stability (86% capacitance retention after 10, 000 charge-discharge cycles) and high specific energy (10.9 Wh/kg at specific power of 265.3 W/kg) were obtained in a coin-type symmetric supercapacitor. The study provides an efficient and facile approach for fabrication of conductive polymer/nanomaterials composite for supercapacitors with good stability and performance.

The role of sodium dodecyl sulfate mediated hydrothermal synthesis of MoS2 nanosheets for photocatalytic dye degradation and dye-sensitized solar cell application

The novel properties and exciting behavior of two-dimensional nanosheet-based materials have piqued the interest of research all over the world. In this study, bulk molybdenum disulfide (bulk MoS2) and sodium dodecyl sulfate-mediated molybdenum disulfide nanosheets (MoS2-SDS NS) were synthesized via a facile sonication and hydrothermal process. The findings from the characterization revealed that the addition of sodium dodecyl sulfate (SDS) surfactant reduces the crystal phase and changes the structural morphology of bulk MoS2. Furthermore, the photocatalytic and photovoltaic performance of bulk MoS2 and MoS2-SDS NS were also investigated. The results show that by using methylene blue dye, the photocatalytic efficiency increased from 56.30% to 91.84% at 150 min under UV–Visible light irradiation, and the photo-conversion efficiency (PEC (%)) of the dye-sensitized solar cell increased from 1.47% to 3.81% for bulk MoS2 and MoS2-SDS NS, respectively. Finally, we discussed in-depth the effect of SDS surfactants on MoS2, which can improve their photovoltaic and photocatalytic performance.

Facile Preparation of MoS2 Nanocomposites for Efficient Potassium-Ion Batteries by Grinding-Promoted Intercalation Exfoliation.

Efficient exfoliations of bulk molybdenum disulfide (MoS2 ) into few-layered nanosheets in pure phase are highly attractive because of the promising applications of the resulted 2D materials in diversified optoelectronic devices. Here, a new exfoliation method is presented to prepare semiconductive 2D hexagonal phase (2H phase) MoS2 -cellulose nanocrystal (CNC) nanocomposites using grinding-promoted intercalation exfoliation (GPIE). This method with facile grinding of the bulk MoS2 and CNC powder followed by conventional liquid-phase exfoliation in water can not only efficiently exfoliate 2H-MoS2 nanosheets, but also produce the 2H-MoS2 /CNC 2D nanocomposites simultaneously. Interestingly, the intercalated CNC sandwiched in MoS2 nanosheets increases the interlayer spacing of 2H-MoS2 , providing perfect conditions to accommodate the large-sized ions. Therefore, these nanocomposites are good anode materials of potassium-ion batteries (KIBs), showing a high reversible capacity of 203 mAh g-1 at 200mA g-1 after 300 cycles, a good reversible capacity of 114 mAh g-1 at 500mA g-1 , and a low decay of 0.02% per cycle over 1500 cycles. With these impressive KIB performances, this efficient GPIE method will open up a new avenue to prepare pure-phase MoS2 and promising 2D nanocomposites for high-performance device applications.

Toxicity evaluation of bulk and nanosheet MoS2 catalysts using battery bioassays

Herein, the main aim is to study the influence of the materials’ structural properties on their ecotoxicological properties. The acute toxicity of the bulk (molybdenum disulfide) MoS2 and 2D nanosheet MoS2 was investigated using organisms of four different taxonomic groups. Ultrasound-assisted liquid-phase exfoliation method was used for preparing 2D nanosheets from bulk MoS2. Bulk and nanosheet MoS2 were characterized using scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FT-IR), and X-ray diffraction (XRD) analyses. The acute toxicity of the bulk and nanosheet MoS2 catalysts was evaluated with four different bioassays using the test organisms Vibrio fischeri (a marine photobacterium), Pseudokirchnerialla subcapitata (a freshwater microalga), Daphnia magna (a freshwater crustacean) and the freshwater duckweed Spirodela polyrhiza. The toxic effect of the materials depended on their structural/size features and the type/sensitivity of the test organism. Generally speaking, bulk MoS2 was more toxic than its nanosheet form. The freshwater crustacean Daphnia magna appeared to be the most suitable, easy-to-handle, and at the same time sensitive test organism for bulk and nanosheet MoS2 among the tested organisms.

Preparation of MoS2 nanosheets to support Pd species for selective steerable hydrogenation of acetylene

Selective hydrogenation of acetylene to ethylene while avoiding over-hydrogenation into ethane is challenging for polymer industry. For highly steerable catalyst design of Metal-MoS2, we reported a few-layer nano-MoS2 supported Pd catalyst, in which the support was prepared by the exfoliation method of bulk molybdenum disulfide (MoS2) in formamide solution. The results from TEM, SAED, AFM and Raman spectra analysis confirmed that the few-layer nano-MoS2 was produced successfully. The XPS result indicates Pd charge transferred to MoS2, proving strong Pd-MoS2 interactions. Pd/nano-MoS2 provided a good catalytic performance for the hydrogenation of acetylene to ethylene, and the ethylene selectivity can reach up to 75% while acetylene conversion can achieve nearly 100%. Comparing with the commercial 5% Pd/C catalyst reported at similar conditions, the Pd/nano-MoS2 synthesized in this study presents better activity and selectivity. In all, this work shows potential application of MoS2 nanosheets used as catalyst support to achieve selective steerable reactions.

Enhanced Mechanical Toughness of Isotactic Polypropylene Using Bulk Molybdenum Disulfide.

Herein, we report the use of bulk molybdenum disulfide (MoS2) as the reinforcing agent to enhance the toughness of isotactic polypropylene (iPP). The iPP-MoS2 nanocomposites with varying amounts of MoS2 (0.1 to 5 wt %) were prepared by a one-step melt extrusion method, and the effects of MoS2 on the morphology, thermal, and mechanical properties were evaluated by different instrumental techniques such as Raman, ATR-FTIR, UTM, TEM, TGA, and DSC. TEM images showed the uniform dispersion of multilayer MoS2 in the polymer matrix, and XRD results suggested the formation of the β phase when a low amount of MoS2 is loaded in the composites. Mechanical tests revealed a significant increase in the toughness and elongation at break (300–400%) in the composites containing low amounts of MoS2 (0.25 to 0.5 wt %). Enhanced toughness and elongation in iPP could be related to the combined effect of the β phase and the exfoliation of bulk MoS2 under applied stress. The thermal stability of the composites was also improved with the increase in MoS2 loading. Direct utilization of bulk MoS2 and one-step melt extrusion process could be a cost-effective method to induce high elasticity and toughness in iPP.

Speed dependence of friction on single-layer and bulk MoS2 measured by atomic force microscopy

We perform atomic force microscopy (AFM) experiments on mechanically exfoliated, single-layer and bulk molybdenum disulfide (MoS2) in order to probe friction forces as a function of sliding speed. The results of the experiments demonstrate that (i) friction forces increase logarithmically with respect to sliding speed, (ii) there is no correlation between the speed dependence of friction and the number of layers of MoS2, and (iii) changes in the speed dependence of friction can be attributed to changes in the physical characteristics of the AFM probe, manifesting in the form of varying contact stiffness and tip-sample interaction potential parameters in the thermally activated Prandtl–Tomlinson model. Our study contributes to the formation of a mechanistic understanding of the speed dependence of nanoscale friction on two-dimensional materials.

Facile synthesis of rGO–MoS2–Ag nanocomposites with long-term antimicrobial activities

The abuse of antibiotics has led to the emergence of numerous super resistant bacteria, which pose a serious threat to public health. Developing nanomaterials with novel modes of bactericidal activity offers the promise of fighting pathogens without the risk of causing drug resistances. Here, we used reduced graphene oxide (rGO), bulk molybdenum disulfide (MoS2) and silver nitrate (AgNO3) to synthesize a ternary nanocomposite, rGO–MoS2–Ag, via a simple one-pot method. The resulting rGO–MoS2–Ag presented as crumpled and sheet-like structures decorated with Ag nanoparticles. The minimum inhibitory concentration and minimum bactericidal concentration of rGO–MoS2–Ag against Escherichia coli were 50 and 100 μg ml−1, while Staphylococcus aureus reacted only to twice higher concentrations of 100 and 200 μg ml−1, respectively. Notably, rGO–MoS2–Ag exhibited better antibacterial activity towards E. coli and S. aureus than rGO, MoS2, or rGO–MoS2. This result can be attributed to the excellent performance of rGO–MoS2–Ag in destroying the bacterial cell membrane and inducing the generation of reactive oxygen species. The Ag+ ion release of rGO–MoS2–Ag was delayed, endowing the nanocomposite with long-term antibacterial capabilities and better biosafety. Our results indicate that the as-prepared rGO–MoS2–Ag has promising potential for application in biomedicine and public health.

Symmetry breaking of in-plane Raman scattering by elliptically polarized light in MoS2

The breaking of the doubly degenerate in-plane lattice vibrations in monolayer, bilayer, and bulk molybdenum disulfide (${\mathrm{MoS}}_{2}$) are oftentimes ignored in the analysis of the material's lattice behaviors due to the lack of variation of polarization for the excitation light. In this work, we have observed variations in the relative intensity of the two most dominant Raman peaks of ${\mathrm{MoS}}_{2}$ via polarized micro-Raman spectroscopy using elliptically polarized incident light. The use of elliptically polarized light has demonstrated distinct properties in its incident Raman polarization vector. This gives an additional degree of freedom affecting the relationship between the $x$ (${E}_{2gx}^{1}$) and $y$ (${E}_{2gy}^{1}$) components of the material's in-plane lattice vibrations. The different ratio of the magnitudes for the ${E}_{2gx}^{1}$ and ${E}_{2gy}^{1}$ in the lattice vibrations can be induced by changing the polarization state of the incident light. This work investigates the material's unexplored fundamental phonon property that may enlighten past and future studies involving phonon behaviors.

The Modifications of Electronic and Optical Properties of Bulk Molybdenum Disulfide by Oxygen Substitution

We study the electronic and optical properties of bulk MoS2 and MoSO systems using the plane-wave method within the generalized gradient approximation. The structural properties show that the O substitution at S sites tunes the z-axis Wyckoff positions of all atoms and the bond angles. The electronic band structures show that the substitution tunes the conduction band minimum at 0.8 Σ to M and significantly promotes more localized valence states compared to that of MoS2 system. The localization, mainly applying to Mo 4d and S 3p states, is more pronounced above −4 eV. Below −4 eV, O 2p states are dominant indicating that they are more stable than S 3p states. However, the substitution slightly increases the indirect and direct (K → K) bandgap of MoS2 system. From the optical properties, both systems show the strong optical dichroism. By means of the substitution, the σ1 width significantly enhances, while the plasmonic-state energy levels decrease. Our result emphasizes that the O substitution significantly tunes the electronic and optical properties of bulk MoS2 system.

Efficient Mechanochemical Preparation of Graphene-Like Molybdenum Disulfide and Graphene-Based Composite Electrocatalysts for Hydrogen Evolution Reaction

Graphene-like MoS2 and graphene-based (gMoS2/Gr, Gr@gMoS2, and gMoS2@Gr) nanocomposites, which are able to operate as electrocatalysts of water splitting with hydrogen evolution, were prepared by solventless mechanochemical delamination of bulk molybdenum disulfide and graphite. It was shown that the sequence of nanostructuring significantly affects both the morphology and the electrocatalytic properties of the prepared nanocomposites. The material prepared by sequential mechanochemical treatment of molybdenum disulfide and graphite—gMoS2@Gr—is characterized by significant delamination of both components with nearly complete disruption of the layer ordering, few-layer MoS2 nanoparticles with higher degree of defectiveness, the graphene component with sufficiently large ordered regions, due to which it exhibited the best electrocatalytic performance with the Tafel slope of 60 mV/dec and the overvoltage of 195 mV at the current density of 10 mA/cm2 (or specific current ~ 17 A/g). It was shown that gMoS2@Gr nanocomposite as HER electrocatalyst can be used as an effective counter electrode in photoelectrochemical cells, providing the photocurrent of 1 mA/cm2 at the voltage of 365 mV. As a whole, the presented data show that gMoS2@Gr nanocomposite is an efficient HER electrocatalyst attractive due to the simplicity and cheapness of its preparation provided by the mechanochemical approach.

Scalable Exfoliation of Bulk MoS₂ to Single- and Few-Layers Using Toroidal Taylor Vortices.

The production of a large amount of high-quality transition metal dichalcogenides is critical for their use in industrial applications. Here, we demonstrate the scalable exfoliation of bulk molybdenum disulfide (MoS2) powders into single- or few-layer nanosheets using the Taylor-Couette flow. The toroidal Taylor vortices generated in the Taylor-Couette flow provide efficient mixing and high shear stresses on the surfaces of materials, resulting in a more efficient exfoliation of the layered materials. The bulk MoS2 powders dispersed in N-methyl-2-pyrrolidone (NMP) were exfoliated with the Taylor-Couette flow by varying the process parameters, including the initial concentration of MoS2 in the NMP, rotation speed of the reactor, reaction time, and temperature. With a batch process at an optimal condition, half of the exfoliated MoS2 nanosheets were thinner than ~3 nm, corresponding to single to ~4 layers. The spectroscopic and microscopic analysis revealed that the exfoliated MoS2 nanosheets contained the same quality as the bulk powders without any contamination or modification. Furthermore, the continuous exfoliation of MoS2 was demonstrated by the Taylor-Couette flow reactor, which produced an exfoliated MoS2 solution with a concentration of ~0.102 mg/mL. This technique is a promising way for the scalable production of single- or few-layer MoS2 nanosheets without using hazardous intercalation materials.

Comparable investigation of unsupported MoS2 hydrodesulfurization catalysts prepared by different techniques: Advantages of support leaching method

Unsupported molybdenum disulfide catalysts were synthesized from ammonium tetrathiomolybdate with chitosan, hexadecylammonium and Triton X100 as structural agents by leaching of MoS2/Al2O3 and MoS2/C/Al2O3 catalyst supports with HF acid and covering of “FeS” seeds by the MoS2 shell. Activation of the solids was performed under the H2S/H2 flow or by the decomposition of precursors in the presence of a hydrocarbon solvent in an autoclave reactor with varied H2 pressure. X-ray diffraction, N2 adsorption, temperature-programmed reduction, X-ray photoelectron spectroscopy and transmission electron microscopy techniques were used to characterize their structure and morphology. The nature of organic additives and activation conditions affect the surface area, pore size distribution, and morphological characteristics of MoS2 particles. Hydrodesulfurization of dibenzothiophene was performed in an autoclave reactor over unsupported MoS2based catalysts. The catalytic behavior of bulk MoS2 catalysts showed that the hydrogenation pathway prevailed over the direct desulfurization pathway. Higher HYD/DDS selectivity for dibenzothiophene hydrogenation was observed with the bulk molybdenum disulfide catalyst formed by etching of the MoS2/Al2O3 catalyst support. Unsupported Mo sulfide catalysts exhibited high hydrogenation activity in dibenzothiophene hydrodesulfurization and, therefore, might find wide applications in hydroprocessing of heavy feed and co-hydrotreating of petroleum fractions and plant oils.

Energy–Momentum Cathodoluminescence Spectroscopy of Dielectric Nanostructures

Precise knowledge of the local density of optical states (LDOS) is fundamental to understanding nanophotonic systems and devices. Complete LDOS mapping requires resolution in energy, momentum, and space, and hence a versatile measurement approach capable of providing simultaneous access to the LDOS components is highly desirable. Here, we explore a modality of cathodoluminescence spectroscopy able to resolve, in single acquisitions, the dispersion in energy and momentum of the radiative LDOS. We perform measurements on a titanium nitride diffraction grating, bulk molybdenum disulfide, and silicon to demonstrate that the technique can probe and disentangle the dispersion of coherent and incoherent cathodoluminescence signals. The approach presented raises cathodoluminescence spectroscopy to a versatile tool for subwavelength design and optimization of nanophotonic devices in the reciprocal space.