Energy Transfer Dynamics between Carbon Quantum Dots and Molybdenum Disulfide Revealed by Transient Absorption Spectroscopy

Zero-dimensional environmentally friendly carbon quantum dots(CQDs)combined with two-di-mensional materials have a wide range of applications in optoelec-tronic devices.We combined steady-state and transient absorp-tion spectroscopies to study the energy transfer dynamics between CQDs and molybdenum disulfide(MoS_(2)).Transient absorption plots showed photoinduced absorption and stimulated emission features,which involved the intrinsic and defect states of CQDs.Adding MoS_(2)to CQDs solution,the lowest unoccupied molecular orbital of CQDs transferred energy to MoS_(2),which quenched the intrinsic emission at 390 nm.With addition of MoS_(2),CQD-MoS_(2)composites quenched defect emission at 490 nm and upward absorption,which originated from another energy transfer from the defect state.Two energy transfer paths between CQDs and MoS_(2)were efficiently manipulated by changing the concentration of MoS_(2),which laid a foundation for improving device performance.

Properties of a MOS Device on Single Layer Molybdenum Disulfide

The properties of a metal-oxide-semiconductor device on a single layer MoS_(2)(molybdenum disulfide)semiconductor are determined theoretically utilizing the concept of physics that the carrier effective masses in materials are related to the intrinsic Fermi energy levels in materials by the universal mass-energy equivalence equation given as dE/E=dm/m,where E is the energy and m is the mass of the free electron.The known parameters of electron effective mass of 0.48 m and the direct bandgap of 1.8 eV for monolayer MoS_(2) semiconductor are utilized to determine the properties of the MOS(metal-oxide-semiconductor)device,with the given previous research consequence that the threshold for electron heating in SiO_(2) is 2 MV/cm-eV.

Hydrothermal Synthesis of Molybdenum Disulfide for Lithium Ion Battery Applications

Molybdenum disulfide nanoflakes were synthesized by a simple hydrothermal process using sodium molybdate and thiourea as reactants at a relatively low temperature. X-ray diffraction(XRD) and transmission elec-tron microscopy(TEM) indicate that the samples have the structure of 2H-MoS2 and the morphology of nanoflakes with the average thickness around 5-10 nm. The results of electrochemical properties indicate that the morphology and size of MoS2 particles have effects on their capacity when they are used as the anode for lithium ion battery. The as-prepared MoS2 samples have high reversible discharge capacity up to 994.6 mA·h·g-1 for the MoS2-1 elec-trode and 930.1 mA·h·g-1 for the MoS2-2 electrode and show excellent cycling performances. The MoS2-1 electrode has a better cycling stability than the MoS2-2 electrode due to their difference in the uniformity of the samples.

Developing polydopamine modified molybdenum disulfide/epoxy resin powder coatings with enhanced anticorrosion performance and wear resistance on magnesium lithium alloys

Epoxy resin powder coating has been successfully applied on the corrosion protection of magnesium lithium alloys.However,poor wear resistance and microcracks formed during the solidification have limited it extensive application.There are limited approaches to exploit such anti-corrosion and mechanical properties of magnesium lithium alloys.Herein,the epoxy resin powder coating with polydopamine modified molybdenum disulfide(MoS_(2)@PDA-EP powder coating with 0,0.1,0.2,0.5,1.0 wt.%loading)was well prepared by melt extrusion to investigate its anticorrosion performance and wear resistance.The results revealed that the addition of MoS_(2)@PDA enhanced the adhesion strength between coatings and alloys,wear resistance and corrosion protection of the powder coatings.Among them,the optimum was obtained by 0.2 wt.%MoS_(2)@PDA-EP powder coating which could be attributed to well dispersion and efficient adhesion with coating matrix.To conclude,MoS_(2)@PDA-EP powder coating is meaningfully beneficial for the anticorrosive and wear performance improvement of magnesium lithium alloys.

Molybdenum disulfide(MoS2)-based electrocatalysts for hydrogen evolution reaction:From mechanism to manipulation

Molybdenum disulfide(MoS_(2))-based materials as the non-noble metal catalysts have displayed the potential capability to drive electrocatalytic hydrogen evolution reaction(HER)for green hydrogen production along with their intrinsic activity,tunable electronic properties,low cost,and abundance reserves,which have attracted intensive attention as alternatives to the low-abundance and high-cost platinum-based catalysts.However,their insufficient catalytic HER activities and stability are the major challenges for them to become practically applicable.Hereby,the MoS_(2)-based electrocatalysts for HER are comprehensively reviewed to explain the fundamental science behind the manipulations of the crystal structure,microstructure,surface,and interface of MoS_(2) in order to enhance its catalytic performance through changing the electrical conductivity,the number of active sites,surface wettability,and the Gibbs free energy for hydrogen adsorption(ΔGH).Recent studies in surface/interface engineering,such as phase engineering,defect engineering,morphology design,and heterostructure construction,are analyzed to reveal the state-of-the-art strategies for designing and preparing the cost-effective and highperformance MoS_(2)-based catalysts through optimizing the charge transfer,surface-active sites,ΔGH,and surface hydrophilicity.Lastly,the perspectives,challenges,and future research directions of HER electrocatalysis are also given to facilitate the further research and development of HER catalysts.

Hierarchical molybdenum disulfide nanosheet arrays stemmed from nickel-cobalt layered double hydroxide/carbon cloth for highly-efficient hydrogen evolution reaction

The hierarchical structure of molybdenum disulfide(MoS2)nanosheet arrays stemmed from nickelcobalt layered double hydroxide(NiCo-LDH)/carbon cloth was prepared by growing the MoS_(2) nanosheet arrays onto the NiCo-LDH template which was pre-deposited onto the carbon cloth substrate.In this electrode configuration,carbon cloth is the three dimensional and conductive skeleton;NiCo-LDH nanosheets,as the template,ensure the oriented growth of MoS2 nanosheet arrays.Therefore,more MoS_(2) active sites are exposed and the catalyst exhibits good hydrogen evolution reaction activity.

Strong interactions in molybdenum disulfide heterostructures boosting the catalytic performance of water splitting: A short review

Two-dimensional materials(2DMs) have attracted substantial attention due to their abundant active sites and their ultrahigh surface area for different catalytic applications due to the high lateral-longitudinal ratio. Transition metal dichalcogenides(TMDs), especially MoS2, as one of the 2DMs most often studied, have shown superior activity in electrochemical applications. Recently, combinations of different 2DMs have been widely studied, and they appear to be the most promising strategy available to develop state of the art catalysts for different reactions.In this article, we review the interactions between MoS2 and other materials as well as the novel assembly induced phase transitions of TMDs and their underlying mechanisms. Several methods for inducing the phase transition of TMDs by building MoS2-based heterostructures have been introduced. The electronic coupling between these counterparts has significantly enhanced their conductivity and optimized the energy states of the materials, thus introducing enhanced activity as compared to their original counterparts. The ideas summarized in this article may shed new light on and help to develop next-generation green energy materials by designing and constructing highly active two-dimensional catalysts for efficient water splitting.

Tunable terahertz acoustic-phonon emission from monolayer molybdenum disulfide

The acoustic-phonon emission from monolayer molybdenum disulfide(ML-MoS_(2))driven by a direct-current electric field is studied theoretically using the Boltzmann equation method.It is found that the Cerenkov emission of terahertz acoustic-phonons can be generated when a very weak electric field is applied to ML-MoS_(2).The physical mechanisms of acoustic-phonon emission are analyzed from the perspective of condensed matter physics.The acoustic-phonon emission from ML-MoS_(2)is also compared with those from graphene and GaAs.The results reveal that the frequencies of acousticphonons generated by ML-MoS_(2)are between the frequencies of those generated from GaAs and graphene.The results of this work suggest that the ML-MoS_(2)can make up for graphene and GaAs in respect of acoustic-phonon emission and be used in tunable hypersonic devices such as terahertz sound sources.

Self-dispersed molybdenum disulfide quantum dot/graphene crumpled ball as efficient high temperature lubricant additive

Inorganic nanoparticles have been proved as powerful lubricant additives at elevated temperature.However,the tribological properties are inevitably impaired due to poor dispersion and insufficient high temperature resistance of organic matter modified nanoparticles.Here,we prepare a self-dispersed molybdenum disulfide quantum dot/graphene crumpled ball(MGCB)comprising molybdenum disulfide quantum dot uniformly interspersed on the wrinkled graphene ball.The crumpled ball composite possesses excellent dispersity in polyalkylene glycol base oil without depending on surface modifiers.Compared with the conventional phosphate esters lubricant,our results indicate MGCB could vastly improve the lubrication performance of polyalkylene glycol with an extremely low concentration(0.05 wt%)at elevated temperature(150°C),showing a friction reduction of 47%and a wear reduction of 30%compared with the conventional phosphate esters lubricant(tricresyl phosphate,TCP).This is because crumpled ball potentiates synergistic lubrication effect within the boundary lubrication.Overall,we envision our designed self-dispersed MGCB has significant potential in tribological application at elevated temperature.

Advanced 2D molybdenum disulfide for green hydrogen production: Recent progress and future perspectives

The development of renewable and affordable energy is crucial for building a sustainable society. In this context, establishing a sustainable infrastructure for renewable energy requires the integration of energy storage, specifically use of renewable hydrogen. The hydrogen evolution reaction (HER) of electrochemical water splitting is a promising method for producing green hydrogen. Recently, two-dimensional nanomaterials have shown great promise in promoting the HER in terms of both fundamental research and practical applications due to their high specific surface areas and tunable electronic properties. Among them, molybdenum disulfide (MoS2), a non-noble metal catalyst, has emerged as a promising alternative to replace expensive platinum-based catalysts for the HER because MoS_(2)has a high inherent activity, low cost, and abundant reserves. At present, greatly improved activity and stability are urgently needed for MoS_(2)to enable wide deployment of water electrolysis devices. In this regard, efficient strategies for precisely modifying MoS_(2)are of interest. Herein, the progress made with MoS_(2)as an HER catalyst is reviewed, with a focus on modification strategies, including phase engineering, morphology design, defect engineering, heteroatom doping, and heterostructure construction. It is believed that these strategies will be helpful in designing and developing high-performance and low-cost MoS2-based catalysts by lowering the charge transfer barrier, increasing the active site density, and optimizing the surface hydrophilicity. In addition, the challenges of MoS_(2)electrocatalysts and perspectives for future research and development of these catalysts are discussed.

Covalent-architected molybdenum disulfide arrays on Ti_(3)C_(2)T_(x) MXene fiber towards robust capacitive energy storage

Ti_(3)C_(2)T_(x)MXene fiber has shown extraordinary potential for supercapacitor electrode in wearable elec-tronics and textile energy storage,but realizing high energy density and practical-powered applications remains a great challenge.Here,we report a covalent-architected molybdenum disulfide-Ti_(3)C_(2)T_(x)(MoS_(2)-Ti_(3)C_(2)T_(x))core-shell fiber for high-performance supercapacitor.Benefiting from the microfluidic and micro-reaction strategies,the ordered MoS_(2)arrays are strongly bridged on Ti_(3)C_(2)T_(x)fiber via Ti-O-Mo bond,re-sulting in large exposed surface,enhanced porosity and excellent interfacial conduction for charges high diffusion and faradaic transfer.The MoS_(2)-Ti_(3)C_(2)T_(x)fiber exhibits ultra-large capacitance of 2028 F cm^(-3)and admirable reversibility in 1 M H_(2)SO_(4)aqueous electrolyte.Meanwhile,MoS_(2)-Ti_(3)C_(2)T_(x)fiber-based solid-state supercapacitor presents high energy density of 23.86 mWh cm^(-3),capacitance of 1073.6 F cm^(-3)and superior cycling ability of 92.13%retention after 20,000 cycles,which can realize stable energy supply for wearable watch,LEDs,electric fans,toy ship and self-powered devices.Our work may provide an insight-ful guidance for the advanced design of structural fiber towards robust new energy and next-generation wearable industry.

Oriented molybdenum disulfide-silica/hydrogenated nitrile butadiene rubber composites:Effects of nanosheets on mechanical and dielectric properties

A series of non-covalently functionalized molybdenum disulfide-silica(f-MoS_(2)-SiO_(2))nanocomposites was prepared by an in-situ assembled method and used to fabricate the oriented molybdenum disulfide-SiO_(2)/Hydrogenated Nitrile Butadiene Rubber(f-MoS2-SiO_(2)/HNBR)composites.The characterization results show the synergistic dispersion between the functionalized molybdenum disulfide(f-MoS2)nanosheets and SiO_(2)nanoparticles.The addition of f-MoS2 nanosheets can improve the dispersion of fillers in the rubber matrix and weaken the filler network.The non-covalently functionalization improves the interface interaction between f-MoS_(2)nanosheets and the rubber matrix.Furthermore,the tensile strength of f-MoS2-SiO_(2)/HNBR is 65.9%higher than that of SiO_(2)/HNBR by adding 1.0wt%of f-MoS_(2).At the same time,the dielectric constant of f-MoS2-SiO_(2)/HNBR is increased by 23.7%compared to SiO_(2)/HNBR due to the micro-capacitor structure of parallel f-MoS2 nanosheets in the rubber matrix.Our work provides new ideas for the development of high-performance elastomer materials.

Molybdenum disulfide@nickel phyllosilicate hybrid for improving the flame retardancy and wear resistance of epoxy composites

In this study,nickel phyllosilicate was synthesized based on molybdenum disulfide(MoS2@NiPS)by the sol-gel method,and then MoS2@NiPS was used to prepare epoxy composites.The thermal stability,flame retardancy,and frictional performances of epoxy composites were studied.With the addition of 3 wt%MoS2@NiPS,the epoxy composite increased the limiting oxygen index from 23.8%to 26.1%and reduced the vertical burning time from 166 s for epoxy resin to 35 s.The residual char of the epoxy composite increased from 11.8 to 20.2 wt%.MoS2@NiPS promoted the graphitization of the residual char,and facilitated the formation of a dense and continuous char layer,thereby improving the fire safety of epoxy resin.The epoxy composite with 3 wt%MoS2@NiPS had excellent wear resistance property with a wear rate of 2.19×10^(‒5) mm^(3)·N^(-1)·m^(-1),which was 68.8%lower than that of epoxy resin.This study presented a practical approach to improve the frictional and fire resistance of epoxy composites.

Highly sensitive piezoresistive pressure sensors based on laser-induced graphene with molybdenum disulfide nanoparticles

Wearable pressure sensors have drawn significant attention because of their extensive applications in motion detection, tactile sensing, and health monitoring. However, the complex manufacturing process and high cost of active materials make low-cost,large-scale production elusive. In this work, we report a flexible piezoresistive pressure sensor assembled with two 3D laserinduced graphene(LIG) foam electrodes on a polyimide thin film from a simple laser scribing process in the ambient environment. The design of the air gap between the two foam electrodes allows the sensor to showcase a low limit of detection of 0.274 Pa, which provides favorable sensing performance in motion detection and wrist pulse monitoring. The addition of spherical MoS2 nanoparticles between the two foam electrodes further enhances the sensitivity to 88 k Pa-1 and increases the sensing range to significantly outperform the previous literature reports. The demonstrated LIG pressure sensors also exhibit fast response/recovery rates and excellent durability/repeatability.

Boosting pH-Universal Hydrogen Evolution of Molybdenum Disulfide Particles by Interfacial Engineering

The design of high-efficiency non-noble and earth-abundant electrocatalysts for hydrogen evolution reaction(HER)is highly paramount for water splitting and renewable energy systems.Molybdenum disulfide(MoS_(2))with abundant edge sites can be utilized as a promising alternative,but its catalytic activity is greatly related to the pH values,especially in an alkaline environment due to the extremely high energy barriers for water adsorption and dissociation steps.Here we report an exceptionally efficient and stable electrocatalyst to improve the sluggish HER process of layered MoS_(2)particles in different pH electrolytes,especially in base.The electrocatalyst is constructed by in situ growing selenium-doped MoS_(2)(Se-MoS_(2))nanoparticles on three-dimensional cobalt nickel diselenide(mCo_(0.2)Ni_(0.8)Se_(2))nanostructured arrays.Due to the large number of active edge sites of Se-MoS_(2)particles exposed at the surface,robust electrical conductivity and large surface area of mCo_(0.2)Ni_(0.8)Se_(2)support,and strong interfacial interactions between Se-MoS_(2)and mCo_(0.2)Ni_(0.8)Se_(2),this hybrid catalyst shows very outstanding catalytic HER properties featured by low overpotentials of 30 and 122 mV at 10 and 100 mA/cm^(2)with good operational stability in base,respectively,which outperforms most of inexpensive catalysts consisting of layered MoS_(2),transition metal selenides and sulfides,and it performs as well as noble Pt catalysts.Meanwhile,this electrocatalyst is also very active in neutral and acidic electrolytes,requiring low overpotentials of 93 and 94 mV at 10 mA/cm^(2),respectively,demonstrating its superb pH universality as a HER electrocatalyst with excellent catalytic durability.This study provides a straightforward strategy to construct an efficient non-noble electrocatalyst for driving the HER kinetics in different electrolytes.

Revisiting lithium-storage mechanisms of molybdenum disulfide

Molybdenum disulfide(MoS_(2)),a typical two-dimensional transition metallic layered material,attracts tremendous attentions in the electrochemical energy storage due to its excellent physicochemical properties.However,with the deepening of the research and exploration of the lithium storage mechanism of these advanced MoS_(2)-based anode materials,the complex reaction process influenced by internal and external factors hinders the exhaustive understanding of the lithium storage process.To design stable anode material with high performance,it is urgent to review the mechanisms of reported anode materials and summarize the related factors that influence the reaction processes.This review aims to dissect all possible side reactions during charging and discharging process,uncover internal and external factors inducing various anode reactions and finally put forward strategies of controlling high cycling capacity and super-stable lithium storage capability of MoS_(2).This review will be helpful to the design of MoS_(2)-based lithium-ion batteries(LIBs) with excellent cycle performance to enlarge the application fields of these advanced electrochemical energy storage devices.

Superlubricity of molybdenum disulfide subjected to large compressive strains

The friction between a molybdenum disulphide(MoS2)nanoflake and a MoS2 substrate was analyzed using a modified Tomlinson model based on atomistic force fields.The calculations performed in the study suggest that large deformations in the substrate can induce a dramatic decrease in the friction between the nanoflake and the substrate to produce the so-called superlubricity.The coefficient of friction decreases by 1–4 orders of magnitude when a high strain exceeding 0.1 is applied.This friction reduction is strongly anisotropic.For example,the reduction is most pronounced in the compressive regime when the nanoflake slides along the zigzag crystalline direction of the substrate.In other sliding directions,the coefficient of friction will reduce to its lowest value either when a high tensile strain is applied along the zigzag direction or when a high compressive strain is applied along the armchair direction.This anisotropy is correlated with the atomic configurations of MoS_(2).

Highly sensitive detection of mercury（Ⅱ） ions with few-layer molybdenum disulfide

Two-dimensional （2D） layered transition metal dichalcogenide （TMD） materials （e.g., MoS2） have attracted considerable interest due to their atomically thin geometry and semiconducting electronic properties. With ultrahigh surface to volume ratio, the electronic properties of these atomically thin semiconductors can be readily modulated by their environment. Here we report an investigation of the effects of mercury（II） （Hg^2＋） ions on the electrical transport properties of few-layer molybdenum disulfide （MoS2）. The interaction between Hg^2＋ ions and few-layer MoS2 was studied by field-effect transistor measurements and photoluminescence. Due to a high binding affinity between Hg2. ions and the sulfur sites on the surface of MoS2 layers, Hg^2＋ ions can strongly bind to MoS2. We show that the binding of Hg^2＋ can produce a p-type doping effect to reduce the electron concentration in n-type few-layer MoS2. It can thus effectively modulate the electron transport and photoluminescence properties in few-layer MoS2. By monitoring the conductance change of few-layer MoS2 in varying concentration Hg2~ solutions, we further show that few-layer MoS2 transistors can function as highly sensitive sensors for rapid electrical detection of Hg^2＋ ion with a detection limit of 30 pM.

Interlayer engineering of molybdenum disulfide toward efficient electrocatalytic hydrogenation

Electrocatalytic hydrogenation(ECH)enables the sustainable production of chemicals under ambient condition;however,suffers from serious competition with hydrogen(H2)evolution and the use of precious metals as electrocatalysts.Herein,molybdenum disulfide is for the first time developed as an efficient and noble-metal-free catalyst for ECH via in situ intercalation of ammonia or alkyl-amine cations.This interlayer engineering regulates phase transition(2H→1 T),and effectively ameliorates electronic configurations and surface hydrophobicity to promote the ECH of biomass-derived oxygenates,while prohibiting H2 evolution.The optimal one intercalated by dimethylamine(MoS_(2)-DMA)is capable of hydrogenating furfural(FAL)to furfuryl alcohol with high Faradaic efficiency of 86.3%–73.3%and outstanding selectivity of>95.0%at−0.25 to−0.65 V(vs.RHE),outperforming MoS_(2) and other conventional metals.Such prominent performance stems from the enhanced chemisorption and surface hydrophobicity.The chemisorption of H intermediate and FAL,synchronously strengthened on the edge-sites of MoS_(2)-DMA,accelerates the surface elementary step following Langmuir-Hinshelwood mechanism.Moreover,the improved hydrophobicity benefits FAL affinity to overcome diffusion limitation.Discovering the effective modulation of MoS_(2) from a typical H2 evolution electrocatalyst to a promising candidate for ECH,this study broadens the scope to exploit catalysts used for electrochemical synthesis.

Gate-tunable linear magnetoresistance in molybdenum disulfide field-effect transistors with graphene insertion layer

Molybdenum disulfide (MoS2) holds great promise as atomically thin two-dimensional (2D) semiconductor for future electronics and opto-electronics. In this report, we study the magnetoresistance (MR) of MoS2 field-effect transistors (FETs) with graphene insertion layer at the contact interface. Owing to the unique device structure and high-quality contact interface, a gate-tunable linear MR up to 67% is observed at 2 K. By comparing with the MRs of graphene FETs and MoS2 FETs with conventional metal contact, it is found that this unusual MR is most likely to be originated from the contact interfaces between graphene and MoS2, and can be explained by the classical linear MR model caused by spatial fluctuation of carrier mobility. Our study demonstrates large MR responses in MoS2-based systems through heterojunction design, shedding lights for the future magneto-electronics and van der Waals heterostructures.