Graphene MoS2 Research Articles

Gravure printing of hybrid MoS2@S-rGO interdigitated electrodes for flexible microsupercapacitors

In this letter, we demonstrated gravure printing of hybrid MoS2@S-rGO consisting of sulfonated reduced graphene oxide (S-rGO) and MoS2 nanoflowers to obtain a highly porous pattern of interdigitated electrodes, leading to a microsupercapacitor on a flexible polyimide substrate. The in-plane interdigital design of the printed microelectrodes is effective in increasing accessibility of electrolyte ions into the large active surface area. The optimized MoS2@S-rGO microsupercapacitor achieved a high specific capacitance (6.56 mF/cm2), energy density (0.58 mWh/cm3), and power density (13.4 mW/cm3), respectively. In addition, the printed microsupercapacitor lost only 9% of the maximum capacity after 1000 cycles, indicating that the printed hybrid MoS2@S-rGO microsupercapacitors are quite stable for potential flexible device applications.

Reductive hybridization route with exfoliated graphene oxide and MoS2 nanosheets to efficient electrode materials

Mesoporous MoS2–graphene nanohybrid with excellent electrode activity can be synthesized by the reductive hybridization of exfoliated MoS2 and graphene oxide (G-O) nanosheets under hydrothermal condition. The use of G-O precursor is crucial in synthesizing homogeneously hybridized MoS2–graphene materials with mesoporous stacking structure. The MoS2–graphene nanohybrid prepared with G-O shows promising electrode performance for lithium ion batteries with large discharge capacity of ∼1400mAhg−1 for the 100th cycle and excellent rate performance, which is much superior to that of the homologue prepared with rG-O precursor and also restacked MoS2 nanosheets. The present results clearly demonstrate the usefulness of G-O precursor in the hydrothermal synthesis of graphene-based hybrid materials.

Tunable Electrical and Optical Characteristics in Monolayer Graphene and Few-Layer MoS2 Heterostructure Devices.

Lateral and vertical two-dimensional heterostructure devices, in particular graphene-MoS2, have attracted profound interest as they offer additional functionalities over normal two-dimensional devices. Here, we have carried out electrical and optical characterization of graphene-MoS2 heterostructure. The few-layer MoS2 devices with metal electrode at one end and monolayer graphene electrode at the other end show nonlinearity in drain current with drain voltage sweep due to asymmetrical Schottky barrier height at the contacts and can be modulated with an external gate field. The doping effect of MoS2 on graphene was observed as double Dirac points in the transfer characteristics of the graphene field-effect transistor (FET) with a few-layer MoS2 overlapping the middle part of the channel, whereas the underlapping of graphene have negligible effect on MoS2 FET characteristics, which showed typical n-type behavior. The heterostructure also exhibits a strongest optical response for 520 nm wavelength, which decreases with higher wavelengths. Another distinct feature observed in the heterostructure is the peak in the photocurrent around zero gate voltage. This peak is distinguished from conventional MoS2 FETs, which show a continuous increase in photocurrent with back-gate voltage. These results offer significant insight and further enhance the understanding of the graphene-MoS2 heterostructure.

Atomically thin resonant tunnel diodes built from synthetic van der Waals heterostructures.

Vertical integration of two-dimensional van der Waals materials is predicted to lead to novel electronic and optical properties not found in the constituent layers. Here, we present the direct synthesis of two unique, atomically thin, multi-junction heterostructures by combining graphene with the monolayer transition-metal dichalcogenides: molybdenum disulfide (MoS2), molybdenum diselenide (MoSe2) and tungsten diselenide (WSe2). The realization of MoS2–WSe2–graphene and WSe2–MoS2–graphene heterostructures leads to resonant tunnelling in an atomically thin stack with spectrally narrow, room temperature negative differential resistance characteristics.

Structural stability, electronic and magnetic properties of MoS2 quantum dots based on the first principles

Abstract The properties of MoS2 quantum dots are mainly ruled by the edge structures and shapes. We systematically explore structural stability, shapes and magnetic properties of different MoS2 quantum dots with different chemical potential of sulfur using the first principles. Three possible stable edge structures corresponding to different chemical potential of sulfur are confirmed. These edge structures are combined into different shapes of MoS2 quantum dots which is changed with sulfur chemical potential. The electronic structure calculation and the partial charge analysis indicate that the MoS2 quantum dot with armchair edge might have better catalyze ability. These MoS2 quantum dots show different magnetic behavior, which can adapt to different needs of spin devices. Given the recent achievement of graphene quantum dot structures and MoS2 quantum dots experimental study, we believe that our calculated results are suitable for experimental verification and can be useful to preparing different shapes of MoS2 quantum dots.

Deducing the apparent flat-band position Vafb and the doping level of large area single layer graphene MOS capacitors

A capacitance–voltage (CV) study on large area CVD single layer graphene MOS capacitors has been carried out. The CV features are carefully examined to reveal the electronic origins of the observed frequency and bias dependence. In this study we investigate the frequency dispersion, propose the definition and extraction of the apparent flat-band Vafb, and finally perform the low temperature CV measurement to deduce the doping level of the graphene MOSCAPs.

Probing Defect‐Induced Midgap States in MoS2 Through Graphene–MoS2 Heterostructures

Crystalline defects in MoS2 may induce midgap states, resulting in low carrier mobility. These midgap states are usually difficult to probe by conventional transport measurement. The quantum capacitance of single‐layer graphene is sensitive to defect‐induced states near the Dirac point, at which the density of states is extremely low. It is reported that the hexagonal‐boron nitride/graphene/MoS2 sandwich structure facilitates the exploration of the properties of those midgap states in MoS2. Comparative results of the quantum capacitance of pristine graphene indicate the presence of several midgap states with distinct features. Some of these states donate electrons while some states lead to localization of electrons. It is believed that these midgap states originate from intrinsic point defects such as sulfur vacancies, which have a significant impact on the property of the MoS2/graphene interface. They are responsible for the contact problems of metal/MoS­2 interfaces.

A self-powered graphene–MoS2 hybrid phototransistor with fast response rate and high on–off ratio

Here, we present a self-powered graphene–MoS2 hybrid phototransistor that simultaneously exhibits high responsivity, fast response rate and high on–off ratio. In this phototransistor, fewer-layer MoS2 serves as a sensitizer to absorb light, and high mobility graphene serves as an expressway for carrier transport, an asymmetric metal contact (titanium and palladium) design is adopted to provide a strong internal electric-field in the phototransistor channel. Under light illumination, a photovoltage arises at the two asymmetric graphene/electrode interface with photodiode mechanism, which act as a bias voltage driving the large quantity of photocarriers injected from MoS2 layer to transport in graphene channel with photoconductor mechanism. With this ingenious design, the hybrid phototransistor achieves a response rate of 0.13ms and on–off ratio up to 1428 together with responsivity exceeding 3.0A/W under zero bias operation. The hybrid phototransistor takes full the advantage of fast response rate of the photodiode and high responsivity of the photoconductor, which makes it has great potential for wide applications in optoelectronics, such as optical logic, optical computing and optical communication.

Electronic Structure of Twisted Bilayers of Graphene/MoS2 and MoS2/MoS2

Vertically stacked two-dimensional multilayer structures have become a promising prototype for functionalized nanodevices due to their wide range of tunable properties. In this paper we performed first-principles calculations to study the electronic structure of nontwisted and twisted bilayers of hybrid graphene/MoS2 (Gr/MoS2) and MoS2/MoS2. Both twisted bilayers of Gr/MoS2 and MoS2/MoS2 show significant differences in band structures from the nontwisted ones with the appearance of the crossover between direct and indirect band gap and gap variation. More interestingly, the band structures of twisted Gr/MoS2 with different rotation angles are very different from each other, while those of MoS2/MoS2 are very similar. The variation of band structure with rotation angle in Gr/MoS2 is, indeed, originated from the misorientation-induced lattice strain and the sensitive band-strain dependence of MoS2.

Roles of MoS2 and Graphene as Cocatalysts in the Enhanced Visible‐Light Photocatalytic H2 Production Activity of Multiarmed CdS Nanorods

AbstractThe incorporation of cocatalysts into semiconductors is proved to be an effective approach to improving the efficiency of the photocatalytic H2 production. Noble metals such as Pt have been widely used as cocatalysts and can significantly improve the performance of photocatalytic H2 production. However, owing to the high cost and low abundance, the use of Pt in practical applications is restricted. Herein, we report two well‐known 2 D layered materials, MoS2 and graphene, as highly active cocatalysts for H2 production in CdS‐based photocatalytic systems. The CdS–MoS2 and CdS‐MoS2–graphene nanocomposites were prepared by using a facile two‐step solvothermal method, and the morphologies of CdS and MoS2 can be well controlled. The as‐prepared binary CdS–MoS2 nanocomposite exhibits the enhanced visible‐light photocatalytic activity for H2 production in lactic acid aqueous solution compared with a CdS–graphene nanocomposite and a conventional platinized CdS photocatalyst. Moreover, the ternary CdS–MoS2–graphene nanocomposite achieves the highest visible‐light photocatalytic H2 production activity of 621.3 μmol h−1 and the apparent quantum efficiency of 54.4 % at λ=420 nm. The enhanced photocatalytic activity of the CdS–MoS2–graphene nanocomposite can be primarily attributed to the positive synergistic effect between graphene sheets and thin MoS2 nanoplates. The graphene sheets can accelerate the efficient electron transfer from CdS nanorods to the active edge sites of MoS2 nanoplates, and the nanosized MoS2 can facilitate the photogenerated electrons participating in the photocatalytic H2 production. The mechanisms for improving the photocatalytic performance of the MoS2‐ and/or graphene‐modified CdS nanocomposites were proposed by using the electrochemical analysis and photoluminescence measurement.

3D MoS2–Graphene Microspheres Consisting of Multiple Nanospheres with Superior Sodium Ion Storage Properties

A novel anode material for sodium‐ion batteries consisting of 3D graphene microspheres divided into several tens of uniform nanospheres coated with few‐layered MoS2 by a one‐pot spray pyrolysis process is prepared. The first discharge/charge capacities of the composite microspheres are 797 and 573 mA h g−1 at a current density of 0.2 A g−1. The 600th discharge capacity of the composite microspheres at a current density of 1.5 A g−1 is 322 mA h g−1. The Coulombic efficiency during the 600 cycles is as high as 99.98%. The outstanding Na ion storage properties of the 3D MoS2–graphene composite microspheres may be attributed to the reduced stacking of the MoS2 layers and to the 3D structure of the porous graphene microspheres. The reduced stacking of the MoS2 layers relaxes the strain and lowers the barrier for Na+ insertion. The empty nanospheres of the graphene offer voids for volume expansion and pathways for fast electron transfer during repeated cycling.

Synthesis of Few‐Layer MoS2–Graphene Composites with Superior Electrochemical Lithium‐Storage Performance by an Ionic‐Liquid‐Mediated Hydrothermal Route

AbstractFew‐layer MoS2–graphene (FL‐MoS2/G) composites are successfully fabricated by using a facile hydrothermal approach with the assistance of a small amount of ionic liquid (1‐butyl‐3‐methylimidazolium tetrafluoroborate, [BMIM]BF4). The effects of the ionic liquid on the microstructure and electrochemical performance of the FL‐MoS2/G composites are investigated. It is found that the composites have quasi‐3D architectures composed of the interconnected graphene, on which the few‐layer MoS2 sheets with more edges and defects are anchored. Electrochemical tests demonstrate that the FL‐MoS2/G delivers a high reversible capacity of approximately 1200 mA h g−1 with excellent cycle performance and enhanced high‐rate capability. The remarkable electrochemical performance can be attributed to novel features such as the robust quasi‐3D heterostructural architecture and the synergetic interaction between FL‐MoS2 sheets and graphene.

Thermal transport in a graphene–MoS2 bilayer heterostructure: a molecular dynamics study

The in-plane and out-of-plane thermal transport properties of the graphene–MoS2 bilayer are investigated with several influencing factors being considered.

Architecturally designed Pt-MoS2 and Pt-graphene composites for electrocatalytic methanol oxidation.

Thin films consisting of platinum nanoparticles (Pt NPs) with uniform size and distribution have been successfully prepared at a liquid-liquid interface. Apart from the usual substrates like glass, Si etc. the films were also deposited on the surfaces of MoS2 thin films and graphene nanosheets (GNS) respectively, by using a layer-by-layer (LbL) deposition technique to form Pt-MoS2 and Pt-GNS composites. The loading concentration of Pt NPs on MoS2 and GNS can be adjusted by selecting the number and sequence of the component layers during LbL deposition. The Pt thin films, Pt-MoS2 and Pt-GNS nanocomposite thin films are characterized using transmission electron microscopy (TEM), high resolution transmission electron microscopy (HRTEM), energy dispersive X-ray spectrometry (EDS), X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). TEM results of the composites show that Pt NPs with sizes in the range of 1 to 3 nm are uniformly dispersed on the MoS2/GNS surface. The catalytic activities of Pt and Pt-composites for the reaction of methanol oxidation are studied using cyclic voltammetry and chronoamperometry. Electrochemical studies reveal that both the Pt-MoS2 and Pt-GNS nanocomposites show excellent electrocatalytic activity towards methanol oxidation. Pt-MoS2 and Pt-GNS nanocomposite electrodes show excellent stability for reuse of the catalyst. A probable mechanism of catalysis has been discussed. We propose that the similar architecture reported here would be promising for the synthesis of high performance catalysts for fuel cells, gas phase reactions, and other applications such as sensors.

MoS₂ decoration by Mo-atoms and the MoS₂-Mo-graphene heterostructure: a theoretical study.

Here we propose a completely new covalent heterostructure based on graphene and self-decorated MoS2 monolayers. Detailed investigation of the decoration process of the MoS2 surface by Mo adatoms was performed using first principles DFT methods. Comparison between valence-only and semicore pseudopotentials was performed to correctly describe the interaction between Mo adatoms and the MoS2 surface. It was found that self-decoration by Mo atoms is favorable from an energetic point of view. We studied in detail various decoration paths of Mo atoms on the MoS2 surface. The strong variation of electronic properties after the decoration of MoS2 was found. The impact of the presence of Mo adatoms on the electronic properties of the graphene/MoS2 heterostructure was shown.

MoS2–graphene nanosheet–CNT hybrids with excellent electrochemical performances for lithium-ion batteries

A flower-like MoS2–graphene nanosheet–CNT (MoS2–GNS–CNT) nanocomposite is successfully prepared by a facile hydrothermal process for fast lithium storage.

Graphene–MoS2 hybrid nanostructures enhanced surface plasmon resonance biosensors

In this work, we propose a new configuration of surface plasmon resonance (SPR) sensor that is based on graphene–MoS2 hybrid structures for ultrasensitive detection of molecules. The proposed system displays a phase-sensitivity enhancement factor of more than 500-fold when compared to the SPR sensing scheme without the graphene–MoS2 coating or with only graphene coating. Our hypothesis is that the monolayer MoS2 has a much higher optical absorption efficiency (∼5%) than that of the graphene layer (∼2.3%). Based on our findings, the electron energy loss of MoS2 layer is comparable to that of graphene and this will allow a successful (∼100%) of light energy transfer to the graphene–MoS2 coated sensing substrate. Such process will lead to a significant enhancement of SPR signals. Our simulation shows that a quasi-dark point of the reflected light can be achieved under this condition and this has resulted in a steep phase jump at the resonance angle of our newly proposed SPR system. More importantly, we found that phase interrogation detection approach of the graphene–MoS2 hybrid structures-based sensing system is more sensitive than that of using the regularly angular interrogation method and our theoretical analysis indicates that 45nm of Au film thickness and 3 coating layers of MoS2 nanosheet are the optimized parameters needed for the proposed SPR system to achieve the highest detection sensitivity range.

Innovative preparation of MoS2–graphene heterostructures based on alginate containing (NH4)2MoS4 and their photocatalytic activity for H2 generation

Films and particles of MoS2 on graphene (G) showing synergy for photocatalytic H2 evolution due to the strong interaction between the two layered components have been obtained by pyrolysis at 900°C under argon flow of ammonium alginate, films or powders, containing variable proportions of (NH4)2MoS4. X-ray diffraction shows that under these conditions (NH4)2MoS4 decomposes to MoS2, while simultaneously alginate forms few layers G or graphitic carbon residues. Sonication of MoS2–G powders in water produces exfoliation of the material leading to few-layers platelets of MoS2–G heterostructures. MoS2 is considered as an alternative of noble metals for H2 evolution in dye-sensitized photocatalytic systems. In the present case, MoS2–G heterostructures exhibit more than double photocatalytic activity for H2 generation than pristine MoS2 particles.

Ultrafast molecule separation through layered WS(2) nanosheet membranes.

Two-dimensional layered materials have joined in the family of size-selective separation membranes recently. Here, chemically exfoliated tungsten disulfide (WS2) nanosheets are assembled into lamellar thin films and explored as an ultrafast separation membrane for small molecules with size of about 3 nm. Layered WS2 membranes exhibit 5- and 2-fold enhancement in water permeance of graphene oxide membranes and MoS2 laminar membranes with similar rejection, respectively. To further increase the water permeance, ultrathin nanostrands are used as templates to generate more fluidic channel networks in the WS2 membrane. The water permeation behavior and separation performance in the pressure loading-unloading process reveal that the channels created by the ultrathin nanostrands are cracked under high pressure and result in a further 2-fold increase of the flux without significantly degrading the rejection for 3 nm molecules. This is supported by finite-element-based mechanical simulation. These layered WS2 membranes demonstrate up to 2 orders of magnitude higher separation performance than that of commercial membranes with similar rejections and hold the promising potential for water purification.

Graphene/MoS2 hybrid structure and its photoresponse property

A novel graphene/MoS2 hybrid structure composed of reduced graphene oxide and MoS2 nanosheets has been successfully prepared via a facile one-step hydrothermal method. Under the hydrothermal treatment, pristine graphene oxide is reduced and tightly covered with liquid-exfoliated MoS2 nanosheets. The microstructures and properties have been examined by X-ray diffraction, scanning electron microscopy and Raman spectra. It is found that ultrathin MoS2 nanosheets with high crystallinity are located on the reduced graphene oxide. The obtained MoS2/graphene hybrid structure exhibits preferable photoresponse activity under the illumination of ultraviolet light compared to hybrid materials of MoS2 bulk and reduced graphene oxide. The improved photoresponse can be attributed to the extremely efficient charge separation, as well as MoS2 nanostructures of the hybrid structure.