MoS2 Platelets Research Articles

SHS of copper Chevrel phase compounds

Chevrel phase (CP) compounds are a highly multifunctional class of materials but are underused due to the high energy processing required for their synthesis. Self-propagation high-temperature synthesis (SHS) is a low energy and quick alternative for synthesizing this material system. In this investigation, copper-based CPs were synthesized via SHS using either elemental or binary precursor powders. The elemental precursors produced non-stoichiometric Cu2.76Mo6S8, and consisted mainly of grown MoS2 platelets. The binary precursor, on the other hand, produced stoichiometric Cu2Mo6S8. The influences of mixing and precursor concentration are discussed as well as the reaction mechanisms operating for the elemental system and the binary system, separately. The investigation into this synthesis process aims to provide a better understanding of the Chevrel SHS mechanism to accurately control the product.

GSH-depleting and H2O2-self-supplying hybrid nanozymes for intensive catalytic antibacterial therapy by photothermal-augmented co-catalysis

Nanozyme-based chemodynamic therapy (CDT) has shown tremendous potential in the treatment of bacterial infections. However, the CDT antibacterial efficacy is severely limited by the catalytic activity of nanozymes or the infection microenvironments such as insufficient hydrogen peroxide (H2O2) and overexpressed glutathione (GSH). Herein, a versatile hybrid nanozyme (MoS2/CuO2) is rationally constructed by simply decorating ultrasmall CuO2 nanodots onto lamellar MoS2 platelets of hydrangea-like MoS2 nanocarrier via a covalent Cu-S bond. The MoS2/CuO2 nanozyme exhibits the peroxidase-mimic activity for catalytically converting H2O2 produced by acid-triggered decomposition of the decorated CuO2 into hydroxyl radical (•OH). Meanwhile, the MoS2/CuO2 can consume GSH overexpressed in the infection sites via redox reaction mediated by polyvalent transition metal ions (Cu2+ and Mo6+) for enhanced CDT. More importantly, MoS2 support can promote the conversion of Cu2+ to Cu+ by a co-catalytic reaction based on the Mo4+/Mo6+ redox couples, and provide photonic hyperthermia (PTT) to augment the peroxidase-mimic activity. The developed MoS2/CuO2 nanozymes possesses a desirable catalytic property, as well as a remarkably improved antibacterial efficiency both in vitro and in vivo. Taken together, this study proposes a synergetic multiple enhancement strategy to successfully construct the versatile hybrid nanozymes for intensive in vivo PTT/CDT dual-mode anti-infective therapy. Statement of significanceChemodynamic therapy (CDT) has shown great potentialities in the treatment of bacterial infections, while its therapeutic efficiency is severely limited by the infection microenvironments such as insufficient hydrogen peroxide (H2O2) and overexpressed glutathione (GSH). Here, we rationally construct a hybrid nanozyme (MoS2/CuO2) with peroxidase-like activity that can enhance CDT by regulating local microenvironments, that is, simultaneously self-supplying H2O2 and consuming GSH. Importantly, MoS2 support can promote the conversion of Cu2+ to Cu+ by the Mo4+/Mo6+ redox couples, and provide photonic hyperthermia (PTT) to augment the peroxidase-mimic activity. The developed MoS2/CuO2 shows desirable PTT/CDT dual-mode antibacterial efficacy both in vitro and in vivo. This study proposes a versatile hybrid nanozyme with multiple enhancement effects for intensive in vivo anti-infective therapy.

Electrochemical deposition of PEDOT/MoS2 composite films for supercapacitors

Composite materials based on conducting polymers and transitional metal chalcogenides have attracted considerable attention as electrode materials for supercapacitors. Molybdenum disulfide is a layered transitional metal dichalcogenide extensively researched in energy storage and conversion applications, and poly(3,4-ethylenedioxythiophene) (PEDOT) is an intrinsically conducting polymer that can act as a conductive matrix. We propose a single-step electrochemical deposition of PEDOT/MoS2 composite materials from the dispersion containing commercially available MoS2 platelets and EDOT monomer. Potentiodynamic, galvanostatic, and potentiostatic modes are all suitable for synthesis of the materials with predominant content of MoS2, which is attached to the current collector via conducting PEDOT matrix. The materials can work in various water-based electrolytes and combine capacitor-like and redox pseudocapacitive response. The composites deposited on graphite foil in assembled asymmetric cells with Kynol carbon cloth counter-electrode deliver up to 870 mF cm−2 areal capacitance in LiClO4 aqueous solution at a current density of 1 mA cm−2 due to combined capacitive input of both components.

Effect of porous structural properties on lithium-ion and sodium-ion storage: illustrated by the example of a micro-mesoporous graphene1-x (MoS2) x anode.

In this work, we synthesized micro-mesoporous graphene1−x(MoS2)x with different compositional ratios via co-reduction of graphite oxide and exfoliated MoS2 platelets. We systematically studied the performance of the micro-mesoporous graphene1−x(MoS2)x as anodes in lithium-ion batteries and sodium-ion batteries. The results show that the specific surface areas of the composites decrease with introducing MoS2. The irreversible capacitance, which is related to the formation of solid electrolyte interphases, also decreases. Besides specific surface area, we found that micropores can benefit the lithiation and sodiation. We demonstrated that a specific charge capacity of 1319.02 mA h g−1 can be achieved at the 50th cycle for the graphene½(MoS2)½ anode in lithium-ion batteries. Possible relationships between such a high specific capacity and the micro-mesoporous structure of the graphene1−x(MoS2)x anode are discussed. This work may shed light on a general strategy for the structural design of electrode materials in lithium-ion batteries and sodium-ion batteries.

MoS2-on-paper optoelectronics: drawing photodetectors with van der Waals semiconductors beyond graphite.

We fabricate paper-supported semiconducting devices by rubbing a layered molybdenum disulfide (MoS2) crystal onto a piece of paper, similar to the action of drawing/writing with a pencil on paper. We show that the abrasion between the MoS2 crystal and the paper substrate efficiently exfoliates the crystals, breaking the weak van der Waals interlayer bonds and leading to the deposition of a film of interconnected MoS2 platelets. Employing this simple method, which can be easily extended to other 2D materials, we fabricate MoS2-on-paper broadband photodetectors with spectral sensitivity from the ultraviolet (UV) to the near-infrared (NIR) range. We also used these paper-based photodetectors to acquire pictures of objects by mounting the photodetectors in a homebuilt single-pixel camera setup.

Unveiling the Structure of MoS x Nanocrystals Produced upon Laser Fragmentation of MoS2 Platelets.

Transition-metal dichalcogenide MoS2 nanostructures have attracted tremendous attention due to their unique properties, which render them efficient nanoscale functional components for multiple applications ranging from sensors and biomedical probes to energy conversion and storage devices. However, despite the wide application range, the possibility to tune their size, shape, and composition is still a challenge. At the same time, the correlation of the structure with the optoelectronic properties is still unresolved. Here, we propose a new method to synthesize various morphologies of molybdenum sulfide nanocrystals, on the basis of ultrashort-pulsed laser fragmentation of MoS2 platelets. Depending on the irradiation conditions, multiple MoSx morphologies in the form of nanoribbons, nanospheres, and photoluminescent quantum dots are obtained. Besides the detailed structural analysis of the various crystals formed, the structure–property relation is investigated and discussed.

Vacuum Tribological Performance of WS2–MoS2 Composite Film Against Oil-Impregnated Porous Polyimide: Influence of Oil Viscosity

To achieve the desired levels of performance and durability for bearings applied in space, a novel solid–liquid dual lubricating system was established via the WS2–MoS2 composite films sliding against oil-impregnated porous polyimide (PPI), in which polyalphaolefin (PAO) oils with different viscosities were used, respectively. The contact angle measurement indicated that the PAO oil had good wettability on the surfaces of the composite film and the PPI. The vacuum tribological behaviors of single WS2–MoS2 composite film and the dual lubricating system were mainly evaluated under different loads and speeds. In comparison with single WS2–MoS2 composite film, the dual lubricating system exhibited the low friction coefficient and synergistic lubricating effect for the low-viscosity PAO oil under severe sliding conditions. It was concluded that the cleavages of the WS2 and MoS2 crystals along the basal plane were more sufficient and formed small and thin WS2 and MoS2 platelets in the dual lubricating system. Meanwhile, it was found that oil-impregnated PPI readily released PAO lubricating oil with low viscosity, which further decreased friction on the contact interface. The lubricating mechanism of the dual lubricating system was also revealed after correlating the tribological behaviors of the different lubricating systems.

Synergisms and antagonisms between MoS2 nanotubes and representative oil additives under various contact conditions

Abstract MoS2 nanotubes are known to enhance the tribological properties of lubricants thanks to support friction-reducing and anti-wear properties. However, in fully-formulated lubricants particularly for steel elements, other properties such as oxidation and corrosion protection are also necessary to provide a comprehensive lubricating performance and protection against oil degradation. As a consequence, the coexistence of MoS2 nanotubes with other oil additives is unavoidable in immediate future lubrication technologies. This study investigates the influence of representative oil additives such as anti-wear (AW), extreme pressure (EP), detergents and dispersants on the rheological and tribological performance of lubricant mixtures containing MoS2 nanotubes (NTs). The results obtained provide a wide overview on which of the current commonly used oil additives have a place in future nanolubricant formulations and which additives need to be reconsidered. In general, the synergy between the oil additives and the MoS2 nanotubes strongly depends on the contact conditions. The reason lies in the interaction mechanisms between the MoS2 nanotubes and the lubricated surfaces under the presence of the supplementary additive. MoS2 have an excellent synergy with AW additives under mixed rolling/sliding conditions due to the exfoliation of MoS2 platelets on top of the AW tribofilm. Under reciprocating sliding, the interaction between the MoS2 nanotubes and dispersant leads to higher antagonisms. Under extreme pressure unidirectional sliding, their synergy is better with S containing EP additives.

Synergistic effect of MoS2 and diamond nanoparticles in electrochemical sensors: determination of the anticonvulsant drug valproic acid.

The authors describe an electrochemical sensor based on the use of diamond nanoparticles (DNPs) and molybdenum disulfide (MoS2) platelets. The sensor was applied to the voltammetric determination of the anticonvulsant valproic acid which was previously derivatized with ferrocene. The MoS2 platelets were obtained by an exfoliation method, and the DNPs were directly dispersed in water and subsequently deposited on a glassy carbon electrode (GCE). The sensor response was optimized in terms of the solvent employed for dispersing the MoS2 nanomaterial and the method for modifying the GCE. Sensors consisting of a first layer of MoS2 dispersed in ethanol/water and a second layer of DNPs give better response. The single steps of sensor construction were characterized by atomic force microscopy and electrochemical impedance spectroscopy. The differential pulse voltammetric response of the GCE (measured at +0.18 V vs. Ag/AgCl) was compared to that of sensors incorporating only one of the nanomateriales (DNPs or MoS2). The formation of a hybrid MoS2-DNP structure clearly improves performance. The GCE containing both nanomaterials exhibits high sensitivity (740 µA ⋅ mM-1 ⋅ cm-2), a 0.27 μM detection limit, and an 8% reproducibility (RSD). The sensor retained 99% of its initial response after 45 days of storage. Graphical abstract Electrochemical sensor by co-immobilization of MoS2 and diamond nanoparticles (DNP). The formation of a hybrid MoS2-DNP structure enhances the performance of the sensor towards valproic acid derivatized with a ferrocene group, when compared with sensors incorporating only DNP or MoS2.

Structure and tribological behavior of Pb-Ti/MoS2 nanoscaled multilayer films deposited by magnetron sputtering method

The Pb-Ti/MoS2 nanoscaled multilayer films with different bilayer period were deposited by unbalanced magnetron sputtering system. The morphology, microstructure, mechanical and tribological properties of the films were investigated. It was found that the film changed from multilayer structure to composite structure as the bilayer period decreased from 25nm to 6nm, due to the diffusion effect. The multilayer film showed a pronounced (002) diffraction peak, the growth of the MoS2 platelets below the interface were affected by Pb and Ti, and the c-axis of MoS2 platelets were inclined to the substrate at an angle of −30° to 30°. The hardness of the film ranged from 5.9 to 7.2GPa depending on the bilayer period. The tribological behavior of the films was performed under vacuum, and the friction coefficient were typically below 0.25. Furthermore, the nanoscale multilayer film with a bilayer period of 20nm exhibits much better mechanical and tribological properties than pure MoS2. The result indicates that the nanoscale multilayer is a design methodology for developing high basal plane oriented and vacuum solid lubricating MoS2 based materials.

Response of RF-sputtered MoS2 composite films to LEO space environment

The rf-sputtered MoS2-Au composite films were exposed in real low earth orbit (LEO) space environment by a space environment exposure device (SEED) aboard China Shenzhou-7 manned spaceship. The ability of the composite films resistant to atomic oxygen (AO) was investigated using X-ray photoelectron spectroscope (XPS), X-ray diffraction (XRD) and field emission scanning electron microscope (FESEM) equipped with an X-ray energy dispersive spectroscopy (EDS). The results showed that the microstructure of the composite film was densified because the growth of MoS2 platelets was suppressed to some content due to the addition of Au, although it was still characterized by a typical dendrite-like morphology. No obvious change in the morphology, phase structure, element composition and friction property was observed from the space exposed and non-exposed composite films, indicating that the composite film exhibited a better anti-oxidation ability as exposed in LEO environment. The improved anti-oxidation ability was attributed to the more compact microstructure and a passivation effect that the dangling bonds at the edge planes of MoS2 platelets were probably occupied by the partial doping atoms.

A spray-freezing approach to reduced graphene oxide/MoS2 hybrids for superior energy storage

A three-dimensional (3D) architectural hybrid, composed of reduced graphene oxide (RGO) and ultrathin MoS2 layers, is fabricated by a facile spray-freezing method. The spray-freezing to liquid nitrogen rapidly freezes the precursor droplets which avoids phase separation and restacking of MoS2 and RGO platelets, and the following drying/annealing results in the porous 3D structure. The as-prepared 3D architectural RGO/MoS2 hybrid has a high surface area of 128m2g−1, a porous structure and a good electrical conductivity. In LIBs, the capacity of RGO/MoS2 anode (with an optimized MoS2 content of 55wt%) remains 1197mAhg−1 after 400 cycles of measurement at a current density of 1Ag−1 and it remains 892 mAh g−1 over 400 cycles at a current density of 2Ag−1. A capacity of 723mAhg−1 is obtained at a current of 10Ag−1. As for the anode (with an optimized MoS2 content of 74wt%) in SIBs, a high initial discharge capacity of 1315mAhg−1, a superior rate capacity of 470mAhg−1 at 1Ag−1 and an excellent cycling stability (518mAhg−1 after 200 cycles at 0.5Ag−1) are demonstrated.

Stability, Molecular Sieving, and Ion Diffusion Selectivity of a Lamellar Membrane from Two-Dimensional Molybdenum Disulfide.

Two-dimensional (2D) subnanometer channels allow unique mass transport promising for molecular sieving. New 2D channels of MoS2 nanosheets allow one to understand molecular transmission and separation, unlike the graphene oxide counterpart containing various defects and cationic metal contaminants. Membranes from layered MoS2 platelets show extraordinary stability in an aqueous environment and compatibility with polymer filters, both beneficial to efficient manufacturing. Sharing gas-tightness and unimpeded water vapor permeation with a graphene oxide membrane, our lamellar MoS2 membrane demonstrates a molecular sieving property for organic vapor for the first time. The MoS2 membrane also reveals diffusion selectivity of aqueous ions, attributable to the energy penalty in bulk-to-2D dimensional transition. These newly revealed properties of the lamellar membrane full of angstrom-sized 2D channels point to membrane technology applications for energy and environment.

Edge-Enriched 2D MoS2 Thin Films Grown by Chemical Vapor Deposition for Enhanced Catalytic Performance

Chemical vapor deposition (CVD) is used to grow thin films of 2D MoS2 with nanostructure for catalytic applications in the hydrogen evolution reaction (HER). Tailoring of the CVD parameters results in an optimized MoS2 structure for the HER that consists of large MoS2 platelets with smaller layered MoS2 sheets growing off it in a perpendicular direction, which increases the total number of edge sites within a given geometric area. A surface area to geometric area ratio of up to ∼340 is achieved, benefiting from the edge-exposed high-porosity network structure. The optimized thickness of the MoS2 film is determined for maximum performance, revealing that increasing thickness leads to increased impedance of the MoS2 film and reduced current density. The current density of the optimum sample reaches as high as 60 mA/cm2geo (normalized by geometric area) at an overpotential of 0.64 V vs RHE (in 0.5 M H2SO4), with a corresponding Tafel slope of ∼90 mV/dec and exchange current density of 23 μA/cm2geo. The lower...

Pulsed cathodoluminescence and Raman spectra of MoS2 and WS2 nanocrystals and their combination MoS2/WS2 produced by self-propagating high-temperature synthesis

Molybdenum and tungsten disulfide nanoplates were produced by self-propagating high-temperature synthesis in argon atmosphere. This method provides an easy way to produce MoS2 and WS2 from nanoplates up to single- and several layers. The Raman peak intensities corresponding to in-plane E12g and out-of-plane A1g vibration modes and their shifts strongly depend on the thicknesses of the MoS2 and WS2 platelets indicating size-dependent scaling laws and properties. An electron beam irradiation of MoS2 and WS2 powders leads to an occurrence of pulsed cathodoluminescence (PCL) spectra at 575 nm (2.15 eV) and 550 nm (2.25 eV) characteristic to their intrinsic band gaps. For the combination of MoS2 and WS2 nanopowders, a PCL shoulder at 430 nm (2.88 eV) was observed, which is explained by the radiative electron-hole recombination at the MoS2/WS2 grain boundaries. The luminescence decay kinetics of the MoS2/WS2 nanoplates appears to be slower than for individual MoS2 and WS2 platelets due to a spatial separation of electrons and holes at MoS2/WS2 junction resulting in extension of recombination time.

Tribological performance of TiN, TiAlN and CrN hard coatings lubricated by MoS2 nanotubes in Polyalphaolefin oil

Hard coatings deposited by PVD (physical vapor deposition) tend to have a relatively high coefficient of friction, which means that in applications they are lubricated with conventional lubricants. Molybdenum disulfide (MoS2) is a lubricant additive and a friction modifier, and when using the recently developed MoS2 multi-wall nanotubes it is possible to decrease this coefficient of friction even further. For this reason the tribological behavior of MoS2 nanotubes added to polyalphaolefin (PAO) synthetic oil was investigated for the lubrication of cold-work tool steel AISI D2 coated with TiN, TiAlN and CrN hard coatings. The experiments were performed with a ball-on-flat reciprocating machine under a contact pressure of 1.0–2.0GPa (Hertz, max) and a sliding velocity of 0.5–1.0cm/s, with a 100Cr6 steel counterpart body. The results were compared to pure PAO oil and to a mixture of commercially available MoS2 platelets with PAO. It was found that the addition of the MoS2 nanotubes leads to a significant reduction in the friction and an improvement in the wear behavior.

An Investigation on the Reduced Ability of IF-MoS2 Nanoparticles to Reduce Friction and Wear in the Presence of Dispersants

Inorganic fullerene-like molybdenum disulfide (IF-MoS2) nanoparticles are known to exhibit great friction and wear-reducing abilities in severe boundary lubrication regimes, when added to a base oil alone. Their use in fully formulated lubricants was investigated in this study, and the tribological benefits attributed to the IF-MoS2 nanoparticles were found to be lost in the presence of dispersants. Various experimental techniques were used on three reference oils (base oil containing only IF-MoS2, only dispersants and both IF-MoS2 and dispersants) in order to understand the effect of succinimide-based dispersants on the three phases needed for effective nanoparticle-based lubrication, namely (1) the passing of the nanoparticles through the contact (2) the exfoliation of the IF-MoS2 inside the contact and (3) the adhesion of the released MoS2 platelets on the friction surfaces. The dispersants were shown to improve the dispersion of the nanoparticles in the oil by reducing their agglomeration, but prevented the adhesion of a low-friction MoS2 tribofilm on the steel surfaces. In-situ contact visualization revealed that the well-dispersed nanoparticles passed through the contact and exfoliated nanoparticles were observed after tribological testing. These results imply that nanoparticle dispersion itself does not seem to be an issue concerning nanoparticle effectiveness, even though the reduced agglomerate size and inertia may have affected nanoparticle flow near the contact, as well as entrapment and exfoliation conditions inside the contact. The use of succinimide-based dispersants may, however, have affected the tribochemistry of the contact, by an excessive adsorption on the steel surfaces and/or by encapsulating the released MoS2 platelets, preventing tribofilm adhesion. A balance was finally found between nanoparticle dispersion and friction reduction, but for very low dispersant concentrations and after a running-in period. The role of succinimide-based dispersants and their effect on nanoparticle lubrication were discussed in the light of these results.

Various MoS2-, WS2- and C-Based Micro- and Nanoparticles in Boundary Lubrication

Various solid lubricant particles have been experimentally evaluated as possible additives to oils. However, information in terms of a direct comparison of their tribological properties is still missing. In this study, we have compared the tribological properties of seven different solid lubricant micro- and nanoparticles as additives in polyalphaolefin (PAO) oil: MoS2 nanotubes, MoS2 platelets (2 and 10 μm), WS2 nanotubes, WS2 fullerene-like nanoparticles, graphite platelets (20 μm) and multi-walled carbon nanotubes. The experiments were performed in the boundary lubrication regime under a contact pressure of 1 GPa (Hertz, max) using a ball-on-disc tribotester. In general, the particles significantly decreased the friction and wear compared to the base PAO oil. We found that it was the material of the particles that largely determined their tribological performance. The effect of the size of the particles was much less important, and the morphology (shape) of the particles had little or no influence. We have also investigated the effect of ultrasonication during suspension preparation on particle damage and found that the solid lubricant particles were not notably affected, except the MoS2 and WS2 nanotubes, which became somewhat shorter.

Nanolubrication: How Do MoS2-Based Nanostructures Lubricate?

Inorganic MoS2 nanotubes under high pressure have been investigated at the atomistic scale using Born−Oppenheimer molecular dynamics simulations. We show that the low friction coefficient at high loads is not due to the ball-bearing effect. Instead the nanostructures burst under high loads and form nanoplatelets which attach themselves at the material surfaces of closest contact, effectively leading to local “nanocoating” which results in friction coefficients identical to those of lubricants based on MoS2 platelets. The breakup process always starts from the innermost tube, which opens the possibility to tune the nanomaterials to be effective at well-defined loads.

Modification of contact surfaces by fullerene-like solid lubricant nanoparticles

Recently, it has been established that WS2 and MoS2 nanoparticles (IF) mixed in oil, and impregnated into porous matrix of powdered materials appear to enhance the tribological properties in definite loading range in comparison to typical metal dichalcogenides particles. Modification of the contact surface by IF-WS2 nanoparticles from oil, grease and from pores of densified powdered samples is considered in this work. The role of the IF solid lubricants as a part of a third body is discussed also in this work. The effect of the IF in oil was studied using a pin-on-disk tester. The study of IF in greases and impregnated into the pores of densified matrices was performed using a ring-block tester. The state of the mating surfaces and the nanoparticles before and after the friction test has been studied by TEM, SEM and XPS. It was established that the IF nanoparticles form a protective film allowing an increase in the load capacity of rubbed pairs. The tribological behavior of lithium grease with IF was found to be appreciably better than that of a commercially available grease incorporating MoS2 platelets (2H). It is found that the IF nanoparticles lead to improved tribological behavior of greases with the IF nanoparticles under very high loads. Impregnation of the IF-WS2 into the pores of densified powdered materials provides a reduced friction coefficient and wear rate over a wide load range in comparison to 2H solid lubricant. The main advantages of the IF nanoparticles is ascribed to the release and furnishing of the nanoparticles from the open pores onto the metal surface and their confinement at the interface, which is not possible with dense flat surfaces. It is expected that the spherical shape of IF nanoparticles facilitate their sliding/rolling between the rubbed surfaces.