Edge‐Terminated MoS2 Nanoassembled Electrocatalyst via In Situ Hybridization with 3D Carbon Network

Transition metal dichalcogenides, especially MoS₂, are considered as promising electrocatalysts for hydrogen evolution reaction (HER). Since the physicochemical properties of MoS₂ and electrode morphology are highly sensitive factor for HER performance, designed synthesis is highly pursued. Here, an in situ method to prepare a 3D carbon/MoS₂ hybrid catalyst, motivated by the graphene ribbon synthesis process, is reported. By rational design strategies, the hybrid electrocatalysts with cross‐connected porous structure are obtained, and they show a high HER activity even comparable to the state‐of‐the‐art MoS₂ catalyst without appreciable activity loss in long‐term operations. Based on various physicochemical techniques, it is demonstrated that the synthetic procedure can effectively guide the formation of active site and 3D structure with a distinctive feature; increased exposure of active sites by decreased domain size and intrinsically high activity through controlling the number of stacking layers. Moreover, the importance of structural properties of the MoS₂‐based catalysts is verified by controlled experiments, validating the effectiveness of the designed synthesis approach.     

Molecular Nanowire Bonding to Epitaxial Single‐Layer MoS2 by an On‐Surface Ullmann Coupling Reaction

Lateral heterostructures consisting of 2D transition metal dichalcogenides (TMDCs) directly interfaced with molecular networks or nanowires can be used to construct new hybrid materials with interesting electronic and spintronic properties. However, chemical methods for selective and controllable bond formation between 2D materials and organic molecular networks need to be developed. As a demonstration of a self‐assembled organic nanowire‐TMDC system, a method to link and interconnect epitaxial single‐layer MoS₂ flakes with organic molecules is demonstrated. Whereas pristine epitaxial single‐layer MoS₂ has no affinity for molecular attachment, it is found that single‐layer MoS₂ will selectively bind the organic molecule 2,8‐dibromodibenzothiophene (DBDBT) in a surface‐assisted Ullmann coupling reaction when the MoS₂ has been activated by pre‐exposing it to hydrogen. Atom‐resolved scanning tunneling microscopy (STM) imaging is used to analyze the bonding of the nanowires, and thereby it is revealed that selective bonding takes place on a specific S atom at the corner site between the two types of zig‐zag edges available in a hexagonal single layer MoS₂ sheet. The method reported here successfully combining synthesis of epitaxial TMDCs and Ullmann coupling reactions on surfaces may open up new synthesis routes for 2D organic‐TMDC hybrid materials.     

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.     

Vacancy‐Driven Gelation Using Defect‐Rich Nanoassemblies of 2D Transition Metal Dichalcogenides and Polymeric Binder for Biomedical Applications

A new approach of vacancy‐driven gelation to obtain chemically crosslinked hydrogels from defect‐rich 2D molybdenum disulfide (MoS₂) nanoassemblies and polymeric binder is reported. This approach utilizes the planar and edge atomic defects available on the surface of the 2D MoS₂ nanoassemblies to form mechanically resilient and elastomeric nanocomposite hydrogels. The atomic defects present on the lattice plane of 2D MoS₂ nanoassemblies are due to atomic vacancies and can act as an active center for vacancy‐driven gelation with a thiol‐activated terminal such as four‐arm poly(ethylene glycol)–thiol (PEG‐SH) via chemisorption. By modulating the number of vacancies on the 2D MoS₂ nanoassemblies, the physical and chemical properties of the hydrogel network can be controlled. This vacancy‐driven gelation process does not require external stimuli such as UV exposure, chemical initiator, or thermal agitation for crosslinking and thus provides a nontoxic and facile approach to encapsulate cells and proteins. 2D MoS₂ nanoassemblies are cytocompatible, and encapsulated cells in the nanocomposite hydrogels show high viability. Overall, the nanoengineered hydrogel obtained from vacancy‐driven gelation is mechanically resilient and can be used for a range of biomedical applications including tissue engineering, regenerative medicine, and cell and therapeutic delivery.     

3D Interconnected and Multiwalled Carbon@MoS2@Carbon Hollow Nanocables as Outstanding Anodes for Na‐Ion Batteries

Currently, the specific capacity and cycling performance of various MoS₂/carbon‐based anode materials for Na‐ion storage are far from satisfactory due to the insufficient structural stability of the electrode, incomplete protection of MoS₂ by carbon, difficult access of electrolyte to the electrode interior, as well as inactivity of the adopted carbon matrix. To address these issues, this work presents the rational design and synthesis of 3D interconnected and hollow nanocables composed of multiwalled carbon@MoS₂@carbon. In this architecture, (i) the 3D nanoweb‐like structure brings about excellent mechanical property of the electrode, (ii) the ultrathin MoS₂ nanosheets are sandwiched between and doubly protected by two layers of porous carbon, (iii) the hollow structure of the primary nanofibers facilitates the access of electrolyte to the electrode interior, (iv) the porous and nitrogen‐doping properties of the two carbon materials lead to synergistic Na‐storage of carbon and MoS₂. As a result, this hybrid material as the anode material of Na‐ion battery exhibits fast charge‐transfer reaction, high utilization efficiency, and ultrastability. Outstanding reversible capacity (1045 mAh g^?1), excellent rate behavior (817 mAh g^?1 at 7000 mA g^?1), and good cycling performance (747 mAh g^?1 after 200 cycles at 700 mA g^?1) are obtained.     

Generalized Scheme for High Performing Photodetectors with a p‐Type 2D Channel Layer and n‐Type Nanoparticles

A generalized scheme for the fabrication of high performance photodetectors consisting of a p‐type channel material and n‐type nanoparticles is proposed. The high performance of the proposed hybrid photodetector is achieved through enhanced photoabsorption and the photocurrent gain arising from its effective charge transfer mechanism. In this paper, the realization of this design is presented in a hybrid photodetector consisting of 2D p‐type black phosphorus (BP) and n‐type molybdenum disulfide nanoparticles (MoS₂ NPs), and it is demonstrated that it exhibits enhanced photoresponsivity and detectivity compared to pristine BP photodetectors. It is found that the performance of hybrid photodetector depends on the density of NPs on BP layer and that the response time can be reduced with increasing density of MoS₂ NPs. The rising and falling times of this photodetector are smaller than those of BP photodetectors without NPs. This proposed scheme is expected to work equally well for a photodetector with an n‐type channel material and p‐type nanoparticles.     

Self‐Formed Channel Devices Based on Vertically Grown 2D Materials with Large‐Surface‐Area and Their Potential for Chemical Sensor Applications

2D layered materials with sensitive surfaces are promising materials for use in chemical sensing devices, owing to their extremely large surface‐to‐volume ratios. However, most chemical sensors based on 2D materials are used in the form of laterally defined active channels, in which the active area is limited to the actual device dimensions. Therefore, a novel approach for fabricating self‐formed active‐channel devices is proposed based on 2D semiconductor materials with very large surface areas, and their potential gas sensing ability is examined. First, the vertical growth phenomenon of SnS₂ nanocrystals is investigated with large surface area via metal‐assisted growth using prepatterned metal electrodes, and then self‐formed active‐channel devices are suggested without additional pattering through the selective synthesis of SnS₂ nanosheets on prepatterned metal electrodes. The self‐formed active‐channel device exhibits extremely high response values (>2000% at 10 ppm) for NO₂ along with excellent NO₂ selectivity. Moreover, the NO₂ gas response of the gas sensing device with vertically self‐formed SnS₂ nanosheets is more than two orders of magnitude higher than that of a similar exfoliated SnS₂‐based device. These results indicate that the facile device fabrication method would be applicable to various systems in which surface area plays an important role.     

2D GeP: An Unexploited Low‐Symmetry Semiconductor with Strong In‐Plane Anisotropy

Germanium phosphide (GeP), a new member of the Group IV–Group V compounds, is introduced into the fast growing 2D family with experimental and theoretical demonstration of strong anisotropic physical properties. The indirect band gap of GeP can be drastically tuned from 1.68 eV for monolayer to 0.51 eV for bulk, with highly anisotropic dispersions of band structures. Thin GeP shows strong anisotropy of phonon vibrations. Moreover, photodetectors based on GeP flakes show highly anisotropic behavior with anisotropic factors of 1.52 and 1.83 for conductance and photoresponsivity, respectively. This work lays the foundation and ignites future research interests in Group IV–Group V compound 2D materials.