Strong Coupling of MoS2 Nanosheets and Nitrogen‐Doped Graphene for High‐Performance Pseudocapacitance Lithium Storage

Layered material MoS₂ is widely applied as a promising anode for lithium‐ion batteries (LIBs). Herein, a scalable and facile dopamine‐assisted hydrothermal technique for the preparation of strongly coupled MoS₂ nanosheets and nitrogen‐doped graphene (MoS₂/N‐G) composite is developed. In this composite, the interconnected MoS₂ nanosheets are well wrapped onto the surface of graphene, forming a unique veil‐like architecture. Experimental results indicate that dopamine plays multiple roles in the synthesis: a binding agent to anchor and uniformly disperse MoS₂ nanosheets, a morphology promoter, and the precursor for in situ nitrogen doping during the self‐polymerization process. Density functional theory calculations further reveal that a strong interaction exists at the interface of MoS₂ nanosheets and nitrogen‐doped graphene, which facilitates the charge transfer in the hybrid system. When used as the anode for LIBs, the resulting MoS₂/N‐G composite electrode exhibits much higher and more stable Li‐ion storage capacity (e.g., 1102 mAh g^?1 at 100 mA g^?1) than that of MoS₂/G electrode without employing the dopamine linker. Significantly, it is also identified that the thin MoS₂ nanosheets display outstanding high‐rate capability due to surface‐dominated pseudocapacitance contribution.     

Novel 2D Layered Molybdenum Ditelluride Encapsulated in Few‐Layer Graphene as High‐Performance Anode for Lithium‐Ion Batteries

Molybdenum ditelluride nanosheets encapsulated in few‐layer graphene (MoTe₂/FLG) are synthesized by a simple heating method using Te and Mo powder and subsequent ball milling with graphite. The as‐prepared MoTe₂/FLG nanocomposites as anode materials for lithium‐ion batteries exhibit excellent electrochemical performance with a highly reversible capacity of 596.5 mAh g^?1 at 100 mA g^?1, a high rate capability (334.5 mAh g^?1 at 2 A g^?1), and superior cycling stability (capacity retention of 99.5% over 400 cycles at 0.5 A g^?1). Ex situ X‐ray diffraction and transmission electron microscopy are used to explore the lithium storage mechanism of MoTe₂. Moreover, the electrochemical performance of a MoTe₂/FLG//0.35Li₂MnO₃·0.65LiMn_0.5Ni_0.5O₂ full cell is investigated, which displays a reversible capacity of 499 mAh g^?1 (based on the MoTe₂/FLG mass) at 100 mA g^?1 and a capacity retention of 78% over 50 cycles, suggesting the promising application of MoTe₂/FLG for lithium‐ion storage. First‐principles calculations exhibit that the lowest diffusion barrier (0.18 eV) for lithium ions along pathway III in the MoTe₂ layered structure is beneficial for improving the Li intercalation/deintercalation property.     

Lowering the Schottky Barrier Height by Graphene/Ag Electrodes for High‐Mobility MoS2 Field‐Effect Transistors

2D transition metal dichalcogenides (TMDCs) have emerged as promising candidates for post‐silicon nanoelectronics owing to their unique and outstanding semiconducting properties. However, contact engineering for these materials to create high‐performance devices while adapting for large‐area fabrication is still in its nascent stages. In this study, graphene/Ag contacts are introduced into MoS₂ devices, for which a graphene film synthesized by chemical vapor deposition (CVD) is inserted between a CVD‐grown MoS₂ film and a Ag electrode as an interfacial layer. The MoS₂ field‐effect transistors with graphene/Ag contacts show improved electrical and photoelectrical properties, achieving a field‐effect mobility of 35 cm² V^?1 s^?1, an on/off current ratio of 4 × 10⁸, and a photoresponsivity of 2160 A W^?1, compared to those of devices with conventional Ti/Au contacts. These improvements are attributed to the low work function of Ag and the tunability of graphene Fermi level; the n‐doping of Ag in graphene decreases its Fermi level, thereby reducing the Schottky barrier height and contact resistance between the MoS₂ and electrodes. This demonstration of contact interface engineering with CVD‐grown MoS₂ and graphene is a key step toward the practical application of atomically thin TMDC‐based devices with low‐resistance contacts for high‐performance large‐area electronics and optoelectronics.     

Interface Adhesion between 2D Materials and Elastomers Measured by Buckle Delaminations

2D systems have great promise as next generation electronic materials but require intimate knowledge of their interactions with their neighbors for device fabrication and mechanical manipulation. Although adhesion between 2D materials and stiff substrates such as silicon and copper has been measured, adhesion between 2D materials and soft polymer substrates remains difficult to characterize due to the large deformability of the polymer substrates. In this work, a buckling‐based metrology for measuring the adhesion energy between few layer molybdenum disulfide (MoS₂) and soft elastomeric substrates is proposed and demonstrated. Due to large elastic mismatch, few layer MoS₂ flakes can form spontaneous wrinkles and buckle‐delaminations on elastomer substrates during exfoliation. MoS₂‐elastomer interface toughness can therefore be calculated from the buckle delamination profile measured by atomic force microscopy. The thickness of the MoS₂ flake is obtained by analyzing coexisting wrinkles on the same flake. Using this approach, adhesion of few layer MoS₂ to 10:1 Sylgard 184 polydimethylsiloxane is measured to be 18 ± 2 mJ m⁻², which is about an order of magnitude below graphene‐to‐stiff‐substrate adhesion. Finally, this simple methodology can be generalized to obtain adhesion energies between various combinations of 2D materials and deformable substrates.     

Enhanced Field‐Emission Behavior of Layered MoS2 Sheets

Field emission studies are reported for the first time on layered MoS₂ sheets at the base pressure of ～1 × 10^?8 mbar. The turn‐on field required to draw a field emission current density of 10 μA/cm² is found to be 3.5 V/μm for MoS₂ sheets. The turn‐on values are found to be significantly lower than the reported MoS₂ nanoflowers, graphene, and carbon nanotube‐based field emitters due to the high field enhancement factor (～1138) associated with nanometric sharp edges of MoS₂ sheet emitter surface. The emission current–time plots show good stability over a period of 3 h. Owing to the low turn‐on field and planar (sheetlike) structure, the MoS₂ could be utilized for future vacuum microelectronics/nanoelectronic and flat panel display applications.     

Graphitic Carbon Nitride (g‐C3N4)‐Derived N‐Rich Graphene with Tuneable Interlayer Distance as a High‐Rate Anode for Sodium‐Ion Batteries

Heteroatom‐doped carbon materials with expanded interlayer distance have been widely studied as anodes for sodium‐ion batteries (SIBs). However, it remains unexplored to further enlarge the interlayer spacing and reveal the influence of heteroatom doping on carbon nanostructures for developing more efficient SIB anode materials. Here, a series of N‐rich few‐layer graphene (N‐FLG) with tuneable interlayer distance ranging from 0.45 to 0.51 nm is successfully synthesized by annealing graphitic carbon nitride (g‐C₃N₄) under zinc catalysis and selected temperature (T = 700, 800, and 900 °C). More significantly, the correlation between N dopants and interlayer distance of resultant N‐FLG‐T highlights the effect of pyrrolic N on the enlargement of graphene interlayer spacing, due to its stronger electrostatic repulsion. As a consequence, N‐FLG‐800 achieves the optimal properties in terms of interlayer spacing, nitrogen configuration and electronic conductivity. When used as an anode for SIBs, N‐FLG‐800 shows remarkable Na⁺ storage performance with ultrahigh rate capability (56.6 mAh g^?1 at 40 A g^?1) and excellent long‐term stability (211.3 mAh g^?1 at 0.5 A g^?1 after 2000 cycles), demonstrating the effectiveness of material design.     

Direct Growth of High Mobility and Low‐Noise Lateral MoS2–Graphene Heterostructure Electronics

Reliable fabrication of lateral interfaces between conducting and semiconducting 2D materials is considered a major technological advancement for the next generation of highly packed all‐2D electronic circuitry. This study employs seed‐free consecutive chemical vapor deposition processes to synthesize high‐quality lateral MoS₂–graphene heterostructures and comprehensively investigated their electronic properties through a combination of various experimental techniques and theoretical modeling. These results show that the MoS₂–graphene devices exhibit an order of magnitude higher mobility and lower noise metrics compared to conventional MoS₂–metal devices as a result of energy band rearrangement and smaller Schottky barrier height at the contacts. These findings suggest that MoS₂–graphene in‐plane heterostructures are promising materials for the scale‐up of all‐2D circuitry with superlative electrical performance.     

An Electrosynthesized 3D Porous Molybdenum Sulfide/Graphene Film with Enhanced Electrochemical Performance for Lithium Storage

Molybdenum sulfide/graphene composites are promising anode materials for lithium‐ion batteries (LIBs). In this work, MoS_x/graphene composite film with an ideal 3D porous structure is developed via a facile and straightforward electrochemical route. The MoS_x nanoparticles are uniformly anchored on the graphene nanosheets that are randomly arranged, resulting in MoS_x/graphene composites with well‐developed porous structure. Benefiting from such structure and the synergistic effect from two components, this material shows a high specific capacity over 1200 mA h g^?1, an excellent rate performance, and superior cycling stability. The dominating pseudocapacitive behavior in Li storage contributes to the outstanding rate capacity. Importantly, this kind of novel material can be easily produced as 3D microelectrodes for microscaled LIBs that are highly demanded for autonomous microelectronic systems.     

Graphene‐Like MoS2/Graphene Composites: Cationic Surfactant‐Assisted Hydrothermal Synthesis and Electrochemical Reversible Storage of Lithium

A cationic surfactant‐assisted hydrothermal route is developed for the facile synthesis of graphene‐like MoS₂/graphene (GL‐MoS₂/G) composites based on the hydrothermal reduction of Na₂MoO₄ and graphene oxide sheets with L‐cysteine in the presence of cetyltrimethylammonium bromide (CTAB), following by annealling in N₂ atmosphere. The GL‐MoS₂/G composites are characterized by X‐ray diffraction, electron microscopy, high‐resolution transmission electron microscopy, and Raman spectroscopy. The effects of CTAB concentration on the microstructures and electrochemical performances of the composites for reversible Li⁺ storage are investigated. It is found that the layer number of MoS₂ sheets decreases with increasing CTAB concentration. The GL‐MoS₂ sheets in the composites are few‐layer in the case of 0.01～0.03 mol L^?1 CTAB of hydrothermal solution and single‐layer in the case of 0.05 mol L^?1 CTAB. The GL‐MoS₂/G composites prepared with 0.01–0.02 mol·L^?1 of CTAB solution exhibit a higher reversible capacity of 940–1020 mAh g^?1, a greater cycle stability, and a higher rate capability than other samples. The exceptional electrochemical performance of GL‐MoS₂/G composites for reversible Li⁺ storage could be attributed to an effective integration of GL‐MoS₂ sheets and graphene that maximizes the synergistic interaction between them.     

Layer Dependence and Light Tuning Surface Potential of 2D MoS2 on Various Substrates

Here surface potential of chemical vapor deposition (CVD) grown 2D MoS₂ with various layers is reported, and the effect of adherent substrate and light illumination on surface potential of monolayer MoS₂ are investigated. The surface potential of MoS₂ on Si/SiO₂ substrate decreases from 4.93 to 4.84 eV with the increase in the number of layer from 1 to 4 or more. Especially, the surface potentials of monolayer MoS₂ are strongly dependent on its adherent substrate, which are determined to be 4.55, 4.88, 4.93, 5.10, and 5.50 eV on Ag, graphene, Si/SiO₂, Au, and Pt substrates, respectively. Light irradiation is introduced to tuning the surface potential of monolayer MoS₂, with the increase in light intensity, the surface potential of MoS₂ on Si/SiO₂ substrate decreases from 4.93 to 4.74 eV, while increases from 5.50 to 5.56 eV on Pt substrate. The I–V curves on vertical of monolayer MoS₂/Pt heterojunction show the decrease in current with the increase of light intensity, and Schottky barrier height at MoS₂/Pt junctions increases from 0.302 to 0.342 eV. The changed surface potential can be explained by trapped charges on surface, photoinduced carriers, charge transfer, and local electric field.     

Preparation of MoS2‐Coated Three‐Dimensional Graphene Networks for High‐Performance Anode Material in Lithium‐Ion Batteries

A novel composite, MoS₂‐coated three‐dimensional graphene network (3DGN), referred to as MoS₂/3DGN, is synthesized by a facile CVD method. The 3DGN, composed of interconnected graphene sheets, not only serves as template for the deposition of MoS₂, but also provides good electrical contact between the current collector and deposited MoS₂. As a proof of concept, the MoS₂/3DGN composite, used as an anode material for lithium‐ion batteries, shows excellent electrochemical performance, which exhibits reversible capacities of 877 and 665 mAh g^?1 during the 50^th cycle at current densities of 100 and 500 mA g^?1, respectively, indicating its good cycling performance. Furthermore, the MoS₂/3DGN composite also shows excellent high‐current‐density performance, e.g., depicts a 10^th‐cycle capacity of 466 mAh g^?1 at a high current density of 4 A g^?1.     

Powder,Paper and Foam of Few‐Layer Graphene Prepared in High Yield by Electrochemical Intercalation Exfoliation of Expanded Graphite

A facile and high‐yield approach to the preparation of few‐layer graphene (FLG) by electrochemical intercalation exfoliation (EIE) of expanded graphite in sulfuric acid electrolyte is reported. Stage‐1 H₂SO₄‐graphite intercalation compound is used as a key intermediate in EIE to realize the efficient exfoliation. The yield of the FLG sheets (<7 layers) with large lateral sizes (tens of microns) is more than 75% relative to the total amount of starting expanded graphite. A low degree of oxygen functionalization existing in the prepared FLG flakes enables them to disperse effectively, which contributes to the film‐forming characteristics of the FLG flakes. These electrochemically exfoliated FLG flakes are integrated into several kinds of macroscopic graphene structures. Flexible and freestanding graphene papers made of the FLG flakes retain excellent conductivity (≈24 500 S m^?1). Three‐dimensional (3D) graphene foams with light weight are fabricated from the FLG flakes by the use of Ni foams as self‐sacrifice templates. Furthermore, 3D graphene/Ni foams without any binders, which are used as supercapacitor electrodes in aqueous electrolyte, provide the specific capacitance of 113.2 F g^?1 at a current density of 0.5 A g^?1, retaining 90% capacitance after 1000 cycles.     

Controllable Design of MoS2 Nanosheets Anchored on Nitrogen‐Doped Graphene: Toward Fast Sodium Storage by Tunable Pseudocapacitance

Transition‐metal disulfide with its layered structure is regarded as a kind of promising host material for sodium insertion, and intensely investigated for sodium‐ion batteries. In this work, a simple solvothermal method to synthesize a series of MoS₂ nanosheets@nitrogen‐doped graphene composites is developed. This newly designed recipe of raw materials and solvents leads the success of tuning size, number of layers, and interplanar spacing of the as‐prepared MoS₂ nanosheets. Under cut‐off voltage and based on an intercalation mechanism, the ultrasmall MoS₂ nanosheets@nitrogen‐doped graphene composite exhibits more preferable cycling and rate performance compared to few‐/dozens‐layered MoS₂ nanosheets@nitrogen‐doped graphene, as well as many other reported insertion‐type anode materials. Last, detailed kinetics analysis and density functional theory calculation are also employed to explain the Na⁺‐ storage behavior, thus proving the significance in surface‐controlled pseudocapacitance contribution at the high rate. Furthermore, this work offers some meaningful preparation and investigation experiences for designing electrode materials for commercial sodium‐ion batteries with favorable performance.     

3D Mesoporous van der Waals Heterostructures for Trifunctional Energy Electrocatalysis

The emergence of van der Waals (vdW) heterostructures of 2D materials has opened new avenues for fundamental scientific research and technological applications. However, the current concepts and strategies of material engineering lack feasibilities to comprehensively regulate the as‐obtained extrinsic physicochemical characters together with intrinsic properties and activities for optimal performances. A 3D mesoporous vdW heterostructure of graphene and nitrogen‐doped MoS₂ via a two‐step sequential chemical vapor deposition method is constructed. Such strategy is demonstrated to offer an all‐round engineering of 2D materials including the morphology, edge, defect, interface, and electronic structure, thereby leading to robustly modified properties and greatly enhanced electrochemical activities. The hydrogen evolution is substantially accelerated on MoS₂, while the oxygen reduction and evolution are significantly improved on graphene. This work provides a powerful overall engineering strategy of 2D materials for electrocatalysis, which is also enlightening for other nanomaterials and energy‐related applications.     

Highly Flexible and Transparent Multilayer MoS2 Transistors with Graphene Electrodes

A highly flexible and transparent transistor is developed based on an exfoliated MoS₂ channel and CVD‐grown graphene source/drain electrodes. Introducing the 2D nanomaterials provides a high mechanical flexibility, optical transmittance (～74%), and current on/off ratio (>10⁴) with an average field effect mobility of ～4.7 cm² V^?1 s^?1, all of which cannot be achieved by other transistors consisting of a MoS₂ active channel/metal electrodes or graphene channel/graphene electrodes. In particular, a low Schottky barrier (～22 meV) forms at the MoS₂/graphene interface, which is comparable to the MoS₂/metal interface. The high stability in electronic performance of the devices upon bending up to ±2.2 mm in compressive and tensile modes, and the ability to recover electrical properties after degradation upon annealing, reveal the efficacy of using 2D materials for creating highly flexible and transparent devices.     

Surface Tension Components Based Selection of Cosolvents for Efficient Liquid Phase Exfoliation of 2D Materials

A proper design of direct liquid phase exfoliation (LPE) for 2D materials as graphene, MoS₂, WS₂, h‐BN, Bi₂Se₃, MoSe₂, SnS₂, and TaS₂ with common cosolvents is carried out based on considering the polar and dispersive components of surface tensions of various cosolvents and 2D materials. It has been found that the exfoliation efficiency is enhanced by matching the ratio of surface tension components of cosolvents to that of the targeted 2D materials, based on which common cosolvents composed of IPA/water, THF/water, and acetone/water can be designed for sufficient LPE process. In this context, the library of low‐toxic and low‐cost solvents with low boiling points for LPE is infinitely enlarged when extending to common cosolvents. Polymer‐based composites reinforced with a series of different 2D materials are compared with each other. It is demonstrated that the incorporation of cosolvents‐exfoliated 2D materials can substantially improve the mechanical and thermal properties of polymer matrices. Typically, with the addition of 0.5 wt% of such 2D material as MoS₂ nanosheets, the tensile strength and Young's modulus increased up to 74.85% and 136.97%, respectively. The different enhancement effect of 2D materials is corresponded to the intrinsic properties and LPE capacity of 2D materials.     

Aligned Heterointerface‐Induced 1T‐MoS2 Monolayer with Near‐Ideal Gibbs Free for Stable Hydrogen Evolution Reaction

1T‐phase molybdenum disulfide (1T‐MoS₂) exhibits superior hydrogen evolution reaction (HER) over 2H‐phase MoS₂ (2H‐MoS₂). However, its thermodynamic instability is the main drawback impeding its practical application. In this work, a stable 1T‐MoS₂ monolayer formed at edge‐aligned 2H‐MoS₂ and a reduced graphene oxide heterointerface (EA‐2H/1T/RGO) using a precursor‐in‐solvent synthesis strategy are reported. Theoretical prediction indicates that the edge‐aligned layer stacking can induce heterointerfacial charge transfer, which results in a phase transition of the interfacial monolayer from 2H to 1T that realizes thermodynamic stability based on the adhesion energy between MoS₂ and graphene. As an electrocatalyst for HER, EA‐2H/1T/RGO displays an onset potential of ?103 mV versus RHE, a Tafel slope of 46 mV dec^?1 and 10 h stability in acidic electrolyte. The unexpected activity of EA‐2H/1T/RGO beyond 1T‐MoS₂ is due to an inherent defect caused by the gliding of S atoms during the phase transition from 2H to 1T, leading the Gibbs free energy of hydrogen adsorption (ΔG_H*) to decrease from 0.13 to 0.07 eV, which is closest to the ideal value (0.06 eV) of 2H‐MoS₂. The presented work provides fundamental insights into the impressive electrochemical properties of HER and opens new avenues for phase transitions at 2D/2D hybrid interfaces.     

Ultrasonic‐Ball Milling: A Novel Strategy to Prepare Large‐Size Ultrathin 2D Materials

Large‐size ultrathin 2D materials, with extensive applications in optics, medicine, biology, and semiconductor fields, can be prepared through an existing common physical and chemical process. However, the current exfoliation technologies still need to be improved upon with urgency. Herein, a novel and simple “ultrasonic‐ball milling” strategy is reported to effectively obtain high quality and large size ultrathin 2D materials with complete lattice structure through the introduction of moderate sapphire (Al₂O₃) abrasives in a liquid phase system. Ultimately numerous high‐quality ultrathin h‐BN, graphene, MoS₂, WS₂, and BCN nanosheets are obtained with large sizes ranging from 1–20 µm, small thickness of ≈1–3 nm and a high yield of over 20%. Utilizing shear and friction force synergistically, this strategy provides a new method and alternative for preparing and optimizing large size ultrathin 2D materials.     

Graphene‐Contacted Ultrashort Channel Monolayer MoS2 Transistors

2D semiconductors are promising channel materials for field‐effect transistors (FETs) with potentially strong immunity to short‐channel effects (SCEs). In this paper, a grain boundary widening technique is developed to fabricate graphene electrodes for contacting monolayer MoS₂. FETs with channel lengths scaling down to ≈4 nm can be realized reliably. These graphene‐contacted ultrashort channel MoS₂ FETs exhibit superior performances including the nearly Ohmic contacts and excellent immunity to SCEs. This work provides a facile route toward the fabrication of various 2D material‐based devices for ultrascaled electronics.     

Unique 1D Cd1−xZnxS@O‐MoS2/NiOx Nanohybrids: Highly Efficient Visible‐Light‐Driven Photocatalytic Hydrogen Evolution via Integrated Structural Regulation

Development of noble‐metal‐free photocatalysts for highly efficient sunlight‐driven water splitting is of great interest. Nevertheless, for the photocatalytic H₂ evolution reaction (HER), the integrated regulation study on morphology, electronic band structures, and surface active sites of catalyst is still minimal up to now. Herein, well‐defined 1D Cd_1?xZn_xS@O‐MoS₂/NiO_x hybrid nanostructures with enhanced activity and stability for photocatalytic HER are prepared. Interestingly, the band alignments, exposure of active sites, and interfacial charge separation of Cd_1?xZn_xS@O‐MoS₂/NiO_x are optimized by tuning the Zn‐doping content as well as the growth of defect‐rich O‐MoS₂ layer and NiO_x nanoparticles, which endow the hybrids with excellent HER performances. Specifically, the visible‐light‐driven (>420 nm) HER activity of Cd_1?xZn_xS@O‐MoS₂/NiO_x with 15% Zn‐doping and 0.2 wt% O‐MoS₂ (CZ_0.15S‐0.2M‐NiO_x) in lactic acid solution (66.08 mmol h^?1 g^?1) is about 25 times that of Pt loaded CZ_0.15S, which is further increased to 223.17 mmol h^?1 g^?1 when using Na₂S/Na₂SO₃ as the sacrificial agent. Meanwhile, in Na₂S/Na₂SO₃ solution, the CZ_0.15S‐0.2M‐NiO_x sample demonstrates an apparent quantum yield of 64.1% at 420 nm and a good stability for HER under long‐time illumination. The results presented in this work can be valuable inspirations for the exploitation of advanced materials for energy‐related applications.