Morphological Engineering of Winged Au@MoS2 Heterostructures for Electrocatalytic Hydrogen Evolution

Edge-Rich 3D Structuring of Metal Chalcogenide/Graphene with Vertical Nanosheets for Efficient Photocatalytic Hydrogen Production

Constructing pertinent nanoarchitecture with abundant exposed active sites is a valid strategy for boosting photocatalytic hydrogen generation. However, the controllable approach of an ideal architecture comprising vertically standing transition metal chalcogenides (TMDs) nanosheets on a 3D graphene network remains challenging despite the potential for efficient photocatalytic hydrogen production. In this study, we fabricated edge-rich 3D structuring photocatalysts involving vertically grown TMDs nanosheets on a 3D porous graphene framework (referred to as 3D Gr). 2D TMDs (MoS2 and WS2)/3D Gr heterostructures were produced by location-specific photon-pen writing and metal-organic chemical vapor deposition for maximum edge site exposure enabling efficient photocatalytic reactivity. Vertically aligned 2D Mo(W)S2/3D Gr heterostructures exhibited distinctly boosted hydrogen production because of the 3D Gr caused by synergetic impacts associated with the large specific surface area and improved density of exposed active sites in vertically standing Mo(W)S2. The heterostructure involving graphene and TMDs corroborates an optimum charge transport pathway to rapidly separate the photogenerated electron-hole pairs, allowing more electrons to contribute to the photocatalytic hydrogen generation reaction. Consequently, the size-tailored heterostructure showed a superior hydrogen generation rate of 6.51 mmol g-1 h-1 for MoS2/3D graphene and 7.26 mmol g-1 h-1 for WS2/3D graphene, respectively, which were 3.59 and 3.76 times greater than that of MoS2 and WS2 samples. This study offers a promising path for the potential of 3D structuring of vertical TMDs/graphene heterostructure with edge-rich nanosheets for photocatalytic applications.

Hierarchical SiC-Graphene Composite Aerogel-Supported Ni-Mo-S Nanosheets for Efficient pH-Universal Electrocatalytic Hydrogen Evolution

MoS₂ exhibits good prospects in electrocatalytic hydrogen evolution. Whereas, the electrocatalytic property of MoS₂ is restrained by its insufficient active sites, low electrical conductivity, and slow water dissociation processes. Herein, an aerogel composed of silicon carbide (SiC) and graphene (SiCnw-RGO) was constructed by growing SiC nanowires (SiCnw) in the graphene aerogel (RGO) via the CVD method, and then Ni-Mo-S nanosheets were hydrothermally synthesized on the SiCnw-RGO composite aerogel to develop an efficient pH-universal electrocatalyst. Ni-Mo-S nanosheets supported on SiCnw-RGO (Ni-Mo-S@SiCnw-RGO) exhibit an interesting hierarchical three-dimensional interconnected structure of composite aerogel. The optimal Ni-Mo-S@SiCnw-RGO electrocatalyst exhibits excellent catalytic performance with low Tafel slopes of 60 mV/dec under acidic conditions and 90 mV/dec under alkaline conditions. Density functional theory calculations demonstrate a composite catalyst exhibits advantageous hydrogen adsorption free energy and water dissociation energy barrier. This study provides a reference to design an efficient hierarchical aerogel electrocatalyst.

Combining Highly Dispersed Amorphous MoS3 with Pt Nanodendrites as Robust Electrocatalysts for Hydrogen Evolution Reaction

Surface modification of electrocatalysts to obtain new or improved electrocatalytic performance is currently the main strategy for designing advanced nanocatalysts. In this work, highly dispersed amorphous molybdenum trisulfide-anchored Platinum nanodendrites (denoted as Pt-a-MoS₃  NDs) are developed as efficient hydrogen evolution electrocatalysts. The formation mechanism of spontaneous in situ polymerization MoS₄ ^2- into a-MoS₃ on Pt surface is discussed in detail. It is verified that the highly dispersed a-MoS₃ enhances the electrocatalytic activity of Pt catalysts under both acidic and alkaline conditions. The potentials at the current density of 10 mA cm^-2 (η₁₀ ) in 0.5 m sulfuric acid (H₂ SO₄ ) and 1 m potassium hydroxide (KOH) electrolyte are -11.5 and -16.3 mV, respectively, which is significantly lower than that of commercial Pt/C (-20.2 mV and -30.7 mV). This study demonstrates that such high activity benefits from the interface between highly dispersed a-MoS₃ and Pt sites, which act as the preferred adsorption sites for the efficient conversion of hydrion (H⁺ ) to hydrogen (H₂ ). Additionally, the anchoring of highly dispersed clusters to Pt substrate greatly enhances the corresponding electrocatalytic stability.

Interface Density Engineering on Heterogeneous Molybdenum Dichalcogenides Enabling Highly Efficient Hydrogen Evolution Catalysis and Sodium Ion Storage

Constructing active heterointerfaces is powerful to enhance the electrochemical performances of transition metal dichalcogenides, but the interface density regulation remains a huge challenge. Herein, MoO₂ /MoS₂ heterogeneous nanorods are encapsulated in nitrogen and sulfur co-doped carbon matrix (MoO₂ /MoS₂ @NSC) by controllable sulfidation. MoO₂ and MoS₂ are coupled intimately at atomic level, forming the MoO₂ /MoS₂ heterointerfaces with different distribution density. Strong electronic interactions are triggered at these MoO₂ /MoS₂ heterointerfaces for enhancing electron transfer. In alkaline media, the optimal material exhibits outstanding hydrogen evolution reaction (HER) performances that significantly surpass carbon-covered MoS₂ nanorods counterpart (η₁₀ : 156 mV vs 232 mV) and most of the MoS₂ -based heterostructures reported recently. First-principles calculation deciphers that MoO₂ /MoS₂ heterointerfaces greatly promote water dissociation and hydrogen atom adsorption via the O-Mo-S electronic bridges during HER process. Moreover, benefited from the high pseudocapacitance contribution, abundant "ion reservoir"-like channels, and low Na⁺ diffusion barrier appended by high-density MoO₂ /MoS₂ heterointerfaces, the material delivers high specific capacity of 888 mAh g^-1 , remarkable rate capability and cycling stability of 390 cycles at 0.1 A g^-1 as the anode of sodium ion battery. This work will undoubtedly light the way of interface density engineering for high-performance electrochemical energy conversion and storage systems.

Chiral nanocrystals grown from MoS2 nanosheets enable photothermally modulated enantioselective release of antimicrobial drugs

The transfer of the concept of chirality from molecules to synthesized nanomaterials has attracted attention amongst multidisciplinary teams. Here we demonstrate heterogeneous nucleation and anisotropic accumulation of Au nanoparticles on multilayer MoS₂ planes to form chiroptically functional nanomaterials. Thiol amino acids with chiral conformations modulate asymmetric growth of gold nanoarchitectures on seeds of highly faceted Au/MoS₂ heterostructures. Consequently, dendritic plasmonic nanocrystals with partial chiral morphologies are synthesized. The chirality of dendritic nanocrystals inherited from cysteine molecules refers to the structural characteristics and includes specific recognition of enantiomeric molecules. With integration of the intrinsic photothermal properties and inherited enantioselective characteristics, dendritic Au/MoS₂ heterostructures exhibit chirality-dependent release of antimicrobial drugs from hydrogel substrates when activated by exogenous infrared irradiation. A three-in-one strategy involving synthesis of chiral dendritic heterostructures, enantioselective recognition, and controlled drug release system is presented, which improves nanomaterial synthetic technology and enhances our understanding of crucial chirality information.

Recent advances in solution assisted synthesis of transition metal chalcogenides for photo-electrocatalytic hydrogen evolution

Hydrogen evolution from water splitting is considered to be an important renewable clean energy source and alternative to fossil fuels for future energy sustainability. Photocatalytic and electrocatalytic water splitting is considered to be an effective method for the sustainable production of clean energy, H₂. This perspective especially emphasizes research advances in the solution-assisted synthesis of transition metal chalcogenides for both photo and electrocatalytic hydrogen evolution applications. Transition metal chalcogenides (CdS, MoS₂, WS₂, TiS₂, TaS₂, ReS₂, MoSe₂, and WSe₂) have received intensified research interest over the past two decades on account of their unique properties and great potential across a wide range of applications. The photocatalytic activity of transition metal chalcogenides can further be improved by elemental doping, heterojunction formation with noble metals (Au, Pt, etc.), non-chalcogenides (MoS₂, In₂S₃, NiS_1-X), morphological tuning, through various solution-assisted synthesis processes, including liquid-phase exfoliation, heat-up, hot-injection methods, hydrothermal/solvothermal routes and template-mediated synthesis processes. In this review we will discuss recent developments in transition metal chalcogenides (TMCs), the role of TMCs for hydrogen production and various strategies for surface functionalization to increase their activity, different synthesis methods, and prospects of TMCs for hydrogen evolution. We have included a brief discussion on the effect of surface hydrogen binding energy and Gibbs free energy change for HER in electrocatalytic hydrogen evolution.

Molybdenum(IV) dithiocarboxylates as single-source precursors for AACVD of MoS2 thin films

Tetrakis(dithiocarboxylato)molybdenum(IV) complexes of the type Mo(S2CR)4 (R = Me, Et, iPr, Ph) were synthesized, characterized, and prescreened as precursors for aerosol assisted chemical vapor deposition (AACVD) of MoS2 thin films. The thermal behavior of the complexes as determined by TGA and GC-MS was appropriate for AACVD, although the complexes were not sufficiently volatile for conventional CVD bubbler systems. Thin films of MoS2 were grown by AACVD at 500 °C from solutions of Mo(S2CMe)4 in toluene. The films were characterized by GIXRD diffraction patterns which correspond to a 2H-MoS2 structure in the deposited film. Mo-S bonding in 2H-MoS2 was further confirmed by XPS and EDS. The film morphology, vertically oriented structure, and thickness (2.54 μm) were evaluated by FE-SEM. The Raman E12g and A1g vibrational modes of crystalline 2H-MoS2 were observed. These results demonstrate the use of dithiocarboxylato ligands for the chemical vapor deposition of metal sulfides.

Recent Progress in Phase Regulation, Functionalization, and Biosensing Applications of Polyphase MoS2

The disulfide compounds of molybdenum (MoS₂ ) are layered van der Waals materials that exhibit a rich array of polymorphic structures. MoS₂ can be roughly divided into semiconductive phase and metallic phase according to the difference in electron filling state of the 4d orbital of Mo atom. The two phases show completely different properties, leading to their diverse applications in biosensors. But to some extent, they compensate for each other. This review first introduces the relationship between phase state and the chemical/physical structures and properties of MoS₂ . Furthermore, the synthetic methods are summarized and the preparation strategies for metastable phases are highlighted. In addition, examples of electronic and chemical property designs of MoS₂ by means of doping and surface modification are outlined. Finally, studies on biosensors based on MoS₂ in recent years are presented and classified, and the roles of MoS₂ with different phases are highlighted. This review offers references for the selection of materials to construct different types of biosensors based on MoS₂ , and provides inspiration for sensing performance enhancement.

Activated MoS2 by Constructing Single Atomic Cation Vacancies for Accelerated Hydrogen Evolution Reaction

Regulating the electronic structure of MoS2 by constructing cationic vacancies is an effective method to activate and improve its intrinsic properties. Herein, we synthesize the MoS2-based composite with abundant single atomic Mo cation vacancies through uniformly loading nickel-cobalt-Prussian blue analogues (NiCoPBA) (NiCoPBA-MoS2-VMo) by immersing a single Ni atom-decorated MoS2 (Ni-MoS2) into K3[Co(CN)6] solution. Subsequently, NiCoP-MoS2-VMo with improved conductivity is obtained by phosphating the composite as a high-efficiency hydrogen evolution reaction (HER) catalyst. Experiments and theoretical calculations indicate that the electrons of NiCoP are spontaneously transferred to the substrate MoS2-VMo nanosheets in NiCoP-MoS2-VMo, and the moderately oxidized NiCoP is beneficial to the adsorption of OH*. Meanwhile, the mono-atomic Mo cation vacancies of the catalyst modulate the electronic structure of S, optimizing the adsorption of hydrogen in the reaction process. Therefore, NiCoP-MoS2-VMo has enhanced chemical adsorption for H* (on MoS2-VMo) and OH*(on NiCoP), expediting the water-splitting step and HER catalysis, and benefiting from the regulation of the electronic structure induced by the construction of metallic Mo vacancies in MoS2, the as-prepared catalyst displays an overpotential of only 67 mV at 10 mA cm-2 with long-term stability (no current decay over 20 h). This work affords not only a kind of efficient HER catalysts but also a new valuable route for developing inexpensive and high-performance catalysts with single atomic cation vacancies.

A Tandem Interfaced (Ni3 S2 -MoS2 )@TiO2 Composite Fabricated by Atomic Layer Deposition as Efficient HER Electrocatalyst

Reported herein is a highly active and durable hydrogen evolution reaction (HER) electrocatalyst, which is constructed following a tandem interface strategy and functional in alkaline and even neutral medium (pH ≈ 7). The ternary composite material, consisting of conductive nickel foam (NF) substrate, Ni₃ S₂ -MoS₂ heterostructure, and TiO₂ coating, is synthesized by the hydrothermal method and atomic layer deposition (ALD) technique. Representative results include: (1) versatile characterizations confirm the proposed composite structure and strong electronic interactions among comprised sulfide and oxide species; (2) the material outperforms commercial Pt/C by recording an overpotential of 115 mV and a Tafel slope of 67 mV dec^-1 under neutral conditions. A long-term stability in alkaline electrolytes up to 200 h and impressive overall water splitting behavior (1.56 V @ 10 mA cm^-2 ) are documented; (3) implementation of ALD oxide tandem layer is crucial to realize the design concept with superior HER performance by modulating a variety of heterointerface and intermediates electronic structure.

Dual Role of MoS2 Quantum Dots in a Cross-Dehydrogenative Coupling Reaction

Modern day research focuses on the development of greener and eco-friendlier protocols to fabricate biologically relevant targets with minimal waste generation. C-C bond formation reactions are of prime importance in this regard. In a typical photocatalytic hydrogen evolution reaction, three components are used, viz, catalyst, photosensitizer, and sacrificial amine donor. Among these, the photosensitizer and sacrificial amine donors are wasted at the end of the reaction. Considering these drawbacks, in this work, we have developed a methodology targeted at the utilization of sacrificial amine donors for C-H functionalization with MoS2 quantum dots (QDs) as the catalyst as well as the photosensitizer. QDs indeed emerged to be an active participant in the heterogeneous electron transfer process. This concept opens up new possibilities in the field of nanomaterial-based photomediated organic transformations without the aid of any external photosensitizers via a clean and sustainable protocol with no side product.

Single-particle spectroscopic investigation on the scattering spectrum of Au@MoS2 core-shell nanosphere heterostructure

Owing to the uniform shape of the nanospheres, the Au@MoS₂ core-shell nanosphere heterostructure enables us to design nano-optoelectronic devices and nanosensors with highly tunable and reproducible optical properties. However, until now, at the single-particle level, there is still uncertainty as to how much the scattering characteristics depend on the particle size and the local environment. In this letter, we performed an in situ single-particle study of the scattering spectrum of the Au@MoS₂ core-shell nanosphere heterostructure before and after coating with the MoS₂ layer. Single-particle characterization confirms that the classic quasi-static approximation (QSA) theory can be used to predict the scattering spectra of Au@MoS₂ core-shell nanoparticles. Moreover, we have found that the A and B-exciton absorption peaks do not rely on the local refractive index change, while the position of the particle plasmon resonances does. Such features can be used as an internal reference for sensing applications against measurement errors, such as defocusing the imaging. Our results show that Au@MoS₂ core-shell nanoparticles have the potential to become one of the promising nanosensors in the field of single-particle sensing.

Au Nanoparticle Modification Induces Charge-Transfer Channels to Enhance the Electrocatalytic Hydrogen Evolution Reaction of InSe Nanosheets

Electrocatalytic water splitting for hydrogen production is an efficient, clean, and sustainable strategy to solve energy and environmental problems. As the important alternative materials for noble metals (Pt, Ir, etc.), two-dimensional (2D) materials have been widely applied for electrocatalysis, although the practical performance is restricted by low carrier mobility and slow reaction kinetics. Here, we adopt the strategy of Au nanoparticle modification to achieve the enhanced hydrogen evolution reaction (HER) performance of InSe nanosheets. Experimental results prove that the HER performance of InSe nanosheets is significantly enhanced under the modification of Au nanoparticles, and the overpotential (392 mV) and Tafel slope (59 mV/dec) are significantly reduced compared to sole InSe nanosheets (580 mV and 148.2 mV/dec). First-principles calculations have found that the InSe/Au system exhibits metallicity because the free electrons provided by the Au particles are injected into the InSe, thereby improving its conductivity. The difference charge density and localized charge density of InSe/Au show that Au nanoparticle loading can induce the formation of Au-Se electron-transfer channels with electrovalent bond characteristics, which effectively promotes the charge transfer. Meanwhile, the standard free-energy calculation of the HER process shows that the InSe/Au heterojunction has a H* adsorption/desorption Gibbs free energy [(|ΔG_H*|) = 0.59 eV] closer to the optimal value. This study reveals the theoretical mechanism of metal modification to improve the performance of electrocatalytic HER and is expected to motivate the development of a new strategy for enhancing the catalytic activity of 2D semiconductor materials.

Interface engineering of hierarchical P-doped NiSe/2H-MoSe2 nanorod arrays for efficient hydrogen evolution

Developing non-noble metal-based electrocatalysts with better activity and stability for hydrogen evolution reaction (HER) is crucial for the electrolysis of water. Herein, self-supported three-dimensional (3D) P-doped NiSe/2H-MoSe2 nanorod arrays (denoted...

Making the Most of your Electrons: Challenges and Opportunities in Characterizing Hybrid Interfaces with STEM

Inspired by the unique architectures composed of hard and soft materials in natural and biological systems, synthetic hybrid structures and associated soft-hard interfaces have recently evoked significant interest. Soft matter is typically dominated by fluctuations even at room temperature, while hard matter (which often serves as the substrate or anchor for the soft component) is governed by rigid mechanical behavior. This dichotomy offers considerable opportunities to leverage the disparate properties offered by these components across a wide spectrum spanning from basic science to engineering insights with significant technological overtones. Such hybrid structures, which include polymer nanocomposites, DNA functionalized nanoparticle superlattices and metal organic frameworks to name a few, have delivered promising insights into the areas of catalysis, environmental remediation, optoelectronics, medicine, and beyond. The interfacial structure between these hard and soft phases exists across a variety of length scales and often strongly influence the functionality of hybrid systems. While scanning/transmission electron microscopy (S/TEM) has proven to be a valuable tool for acquiring intricate molecular and nanoscale details of these interfaces, the unusual nature of hybrid composites presents a suite of challenges that make assessing or establishing the classical structure-property relationships especially difficult. These include challenges associated with preparing electron-transparent samples and obtaining sufficient contrast to resolve the interface between dissimilar materials given the dose sensitivity of soft materials. We discuss each of these challenges and supplement a review of recent developments in the field with additional experimental investigations and simulations to present solutions for attaining a nano or atomic-level understanding of these interfaces. These solutions present a host of opportunities for investigating and understanding the role interfaces play in this unique class of functional materials.

Gold nanoparticle assisted synthesis of MoS2 monolayers by chemical vapor deposition

The use of metal nanoparticles is an established paradigm for the synthesis of semiconducting one-dimensional nanostructures. In this work we study their effect on the synthesis of two-dimensional semiconducting materials, by using gold nanoparticles for chemical vapor deposition growth of two-dimensional molybdenum disulfide (MoS2). In comparison with the standard method, the employment of gold nanoparticles allows us to obtain large monolayer MoS2 flakes, up to 20 μm in lateral size, even if they are affected by the localized overgrowth of MoS2 bilayer and trilayer islands. Important modifications of the optical and electronic properties of MoS2 triangular domains are reported, where the photoluminescence intensity of the A exciton is strongly quenched and a shift to a positive threshold voltage in back-gated field effect transistors is observed. These results indicate that the use of gold nanoparticles influences the flake growth and properties, indicating a method for possible localized synthesis of two-dimensional materials, improving the lateral size of monolayers and modifying their properties.

Fabrication of CoSe@NC nanocubes for high performance potassium ion batteries

Potassium-ion batteries (PIBs) are considered as a promising candidate for large-scale energy storage. While exploring suitable anode materials are of vital need for the practical applications of PIBs. Herein, a well-designed heterostructured anode material CoSe nanocubes wrapped by N-doped carbon (CoSe@NC), has been successfully fabricated by simple annealing ZIF-67 nanocubes followed by in-situ selenization process. It is noted that ZIF-67 nanocubes are used as an effective template for the formation of porous structure, which can facilitate the construction of heterogeneous interface between CoSe and N-doped carbon (NC), effectively stabilizing CoSe with conversion reaction product Co⁰, increasing the diffusion mobility of electrons and K⁺-ions, and alleviating huge volume change. As expected, the heterostructured CoSe@NC nanocubes exhibit excellent K⁺-storage performance, which can display a rather high initial charge capacity (388.7 mAh g^-1 at 0.1 A g^-1 with the columbic efficiency of 70%), superior cyclic stability (309.6 mA h g^-1 after 500 cycles at 2 A g^-1), and exceptional rate capability (365.9 mAh g^-1 at 2 A g^-1). In terms of the low-cost and facile production approach for CoSe@NC, which makes the CoSe@NC a promising anode material for PIBs.

Energizing periodic mesoporous organosilica (PMOS) with bismuth and cerium for photo-degrading methylene blue and methyl orange in water

This work reported an efficient catalyst to reduce the organic pollutants by using an energetic periodic mesoporous organosilica (PMOS) supported with bismuth (Bi-PMOS) and cerium (Ce-PMOS). PMOS support was designed through co-condensation of sodium silicate and 3-methacryloxypropyltrimethoxysilane on polysorbate templates. The resultant PMOSs were fabricated with bismuth and cerium oxides to formulate Bi-PMOS and Ce-PMOS, respectively. These materials showed photo-degradations of methylene blue (MB, 74.7% and 41.1% with Bi-PMOS and Ce-PMOS, respectively) and methyl orange (MO, 53.2% and 39.4% with Bi-PMOS and Ce-PMOS, respectively). Such efficient photo-degradations were attributed to the precise doping of metallic nodes of Bi₂ O₃ and CeO₂ on the porous structure of PMOS with high surface area. The results also showed that Bi and Ce were more effective in PMOS support for photo-degradation of dyes as the support provides more lifetime to photo-generated electron-hole pairs than other materials. Moreover, active reusability and high degradation efficiencies of Bi-PMOS and Ce-PMOS proved them better analytical tools to reduce organic pollutants under visible lights. PRACTITIONER POINTS: The oxides of bismuth and cerium have impressive photocatalytic characteristics. New material energizing mesoporous organosilica with bismuth and cerium for photo-degradation of methylene blue and methyl orange in water. The use of an efficient catalyst to reduce the organic pollutants by using an energetic periodic mesoporous organosilica (PMOS) supported with bismuth (Bi-PMOS) and cerium (Ce-PMOS).

Nanocrystalline NiSe2/MoS2 heterostructures for electrochemical hydrogen evolution reaction

Although it suffers from a heavy dependence on the noble platinum catalyst, the electrochemical hydrogen evolution reaction (HER) is one of the most promising methods for the production of hydrogen. After numerous efforts, it is found that MoS₂-based heterostructure may replace platinum as the electrochemical HER catalyst. In this work, the nanocrystalline NiSe₂/MoS₂ heterostructures were successfully prepared on the carbon fiber paper (CFP) substrate through electrochemical deposition and hydrothermal process. According to a series of electrochemical HER tests and a comparison with other MoS₂-based heterostructure catalysts, the CFP/NiSe₂/MoS₂ catalyst with an overpotential η ₁₀ of 143 mV and a Tafel slope of 45 mV dec^-1 exhibited an excellent electrochemical HER catalytic performance and durability. In addition, CFP/NiSe₂/MoS₂ catalyst was treated by plasma to further improve the catalytic performance of the catalyst.

Size-selective synthesis of platinum nanoparticles on transition-metal dichalcogenides for the hydrogen evolution reaction

We report a micellar system to prepare Pt-TMDs composites with tunable Pt nanoparticles (NPs, 2-6 nm in size) on single-layer TMDs (MoS2, TiS2, TaS2) nanosheets. The Pt-MoS2 composites have shown excellent performance for the hydrogen evolution reaction (HER) with the Pt NPs exhibiting a volcano-type size effect toward HER activity due to the synergistic effects between the Pt NPs and MoS2.

Surface and Interface Engineering: Molybdenum Carbide-Based Nanomaterials for Electrochemical Energy Conversion

Molybdenum carbide (Mo_x C)-based nanomaterials have shown competitive performances for energy conversion applications based on their unique physicochemical properties. A large surface area and proper surface atomic configuration are essential to explore potentiality of Mo_x C in electrochemical applications. Although considerable efforts are made on the development of advanced Mo_x C-based catalysts for energy conversion with high efficiency and stability, some urgent issues, such as low electronic conductivity, low catalytic efficiency, and structural instability, have to be resolved in accordance with their application environments. Surface and interface engineering have shown bright prospects to construct highly efficient Mo_x C-based electrocatalysts for energy conversion including the hydrogen evolution reaction, oxygen evolution reaction, nitrogen reduction reaction, and carbon dioxide reduction reaction. In this Review, the recent progresses in terms of surface and interface engineering of Mo_x C-based electrocatalytic materials are summarized, including the increased number of active sites by decreasing the particle size or introducing porous or hierarchical structures and surface modification by introducing heteroatom(s), defects, carbon materials, and others electronic conductive species. Finally, the challenges and prospects for energy conversion on Mo_x C-based nanomaterials are discussed in terms of key performance parameters for the catalytic performance.

Hierarchically Ordinated Two-Dimensional MoS2 Nanosheets on Three-Dimensional Reduced Graphene Oxide Aerogels as Highly Active and Stable Catalysts for Hydrogen Evolution Reaction

Hydrogen gas (H2) is being intensively proposed as a next-generation clean energy owing to the depletion of fossil fuels. Electrochemical water splitting is one of the most promising processes for hydrogen production. Furthermore, many efforts focusing on electrochemical water splitting have been made to develop low-cost, electrochemically active, and stable catalysts for efficient hydrogen production. MoS2 has emerged as an attractive material for developing catalysts for the hydrogen evolution reaction (HER). Hence, in this study, we design hierarchically ordinated two-dimensional (2D) MoS2 nanosheets on three-dimensional (3D) reduced graphene oxide (rGO) (H-2D/3D-MoS2-rGO) aerogel structures as a new class of electrocatalysts for the HER. We use the one-pot hydrothermal synthesis route for developing high-performance electroactive materials for the HER. The as-prepared H-2D/3D-MoS2-rGO contains a unique 3D hierarchical structure providing large surface areas owing to the 3D porous networks of rGO and more active sites owing to the many edge sites in the MoS2 nanosheets. In addition, the H-2D/3D-MoS2-rGO structure exhibits remarkable electrochemical properties during the HER. It shows a lower overpotential than pure MoS2 and excellent electrochemical stability owing to the large number of active sites (highly exposed edge sites) and high electrical conductivity from the rGO structure.

Topology of transition metal dichalcogenides: the case of the core-shell architecture

Non-planar architectures of the traditionally flat 2D materials are emerging as an intriguing paradigm to realize nascent properties within the family of transition metal dichalcogenides (TMDs). These non-planar forms encompass a diversity of curvatures, morphologies, and overall 3D architectures that exhibit unusual characteristics across the hierarchy of length-scales. Topology offers an integrated and unified approach to describe, harness, and eventually tailor non-planar architectures through both local and higher order geometry. Topological design of layered materials intrinsically invokes elements highly relevant to property manipulation in TMDs, such as the origin of strain and its accommodation by defects and interfaces, which have broad implications for improved material design. In this review, we discuss the importance and impact of geometry on the structure and properties of TMDs. We present a generalized geometric framework to classify and relate the diversity of possible non-planar TMD forms. We then examine the nature of curvature in the emerging core-shell architecture, which has attracted high interest due to its versatility and design potential. We consider the local structure of curved TMDs, including defect formation, strain, and crystal growth dynamics, and factors affecting the morphology of core-shell structures, such as synthesis conditions and substrate morphology. We conclude by discussing unique aspects of TMD architectures that can be leveraged to engineer targeted, exotic properties and detail how advanced characterization tools enable detection of these features. Varying the topology of nanomaterials has long served as a potent methodology to engineer unusual and exotic properties, and the time is ripe to apply topological design principles to TMDs to drive future nanotechnology innovation.

Layered MoS2 defect-driven in situ synthesis of plasmonic gold nanocrystals visualizes the planar size and interfacial diversity

Current defect theories significantly guide broad research progress, whereas the recognition of defect status remains challenging. Herein, MoS2 defect type, density and exposed state are visually identified with a reagent indicator of HAuCl4. Mo-terminated defects spontaneously reduce [AuCl4]- anions and oxidized Mo species are dissociated. Consequently, MoS2 edges guide the epitaxial branch of Au nanocrystals (NCs), followed by sequential growths at their planar defects. The size-evolution processes of LaMer growth and planar packages of the aggregative growth of Au/MoS2 nanoseeds result in the occupation of Au atomic layers on heterostructures. Consequently, shell-core hybrids are presented with localized surface plasmon resonance characteristics. The mechanism is systematically explored via the discriminated performance of plasmonic characteristics of Au nanostructures on semiconducting MoS2 substrates. With plasmonic identification, defect-associated size and interfacial diversities of MoS2 are visually information-rich. Tunable morphologies and synergistic optical characteristics of plasmonic semiconductor heterostructures inspire many more applications through the edge and planar defects intrinsic in layered MoS2.

Stabilizing sulfur vacancy defects by performing "click" chemistry of ultrafine palladium to trigger a high-efficiency hydrogen evolution of MoS2

Defect engineering is widely applied in transition metal dichalcogenides to produce high-purity hydrogen. However, the instability of vacancy states on catalysis still remains a considerable challenge. Here, our first-principles calculations showed that, by optimizing the asymmetric S vacancy in the highly asymmetric 1T' crystal of layered bitransition metal dichalcogenides (Co-MoS2) in light of Pd modulation, the relative amount of metastable phase and the quantity of active sites in the structure can be reduced and increased, respectively, leading to a further boosted hydrogen evolution reaction (HER) activity toward layered bi-transition metal dichalcogenides. Thus, we then used a "click" chemistry strategy to make such a catalyst with engineered unsaturated sulfur edges via a strong coupling effect between ultrafine Pd ensembles and Co-MoS2 nanosheets. As expected, the Pd-modulated Co-MoS2 nanosheets exhibited a very low overpotential of 60 mV at 10 mA cm-2 with a small Tafel slope (56 mV dec-1) for the HER in 1.0 M PBS, comparable to those of commercial Pt/C. In addition, their high HER activity was retained in acidic and alkaline conditions. Both the theoretical and experimental results revealed that Pd ensembles can efficiently activate and stabilize the inert basal plane S sites during HER processes as a result of the formation of Pd-S in Co-MoS2. This work not only provides a deeper understanding of the correlation between defect sites and intrinsic HER catalytic properties for transition metal chalcogenide (TMD)-based catalysts, but also offers new insights into better designing earth-abundant HER catalysts displaying high efficiency and durability.

Emerging One-/Two-Dimensional Heteronanostructure Integrating SiC Nanowires with MoS2 Nanosheets for Efficient Electrocatalytic Hydrogen Evolution

MoS₂ has emerged as a good application prospect in the electrocatalytic hydrogen evolution reaction (HER). Nevertheless, the catalytic activity of MoS₂ is greatly restricted by its inferior electrical conductivity, inadequate exposure of active edge sites, and sluggish water dissociation dynamics. Herein, a 1D/2D heteronanostructure composed of SiC nanowires wrapped with MoS₂ nanosheets was prepared via the hydrothermal synthesis of MoS₂ on highly connected SiC nanowires (SiCnw). The nanocomposites exhibit an emerging tectorum-like morphology with interface connections of C-Mo bonds, which benefit the efficient interfacial transmission of electrons. Due to the synergetic catalytic effects of MoS₂ nanosheets and SiC nanowires, the MoS₂/SiCnw nanocomposites possess efficient catalytic performance with a low Tafel slope (55 mV/dec). SiC nanocrystals could reduce the activated water dissociation energy barrier, and the morphologies of connected nanowires could improve the active site exposure and charge transport. The nanocomposites possess favorable hydrogen adsorption free energy from density functional theory (DFT) calculations. The electrocatalytic performance of MoS₂/SiCnw nanocomposites could be further improved by assembling the nanocomposites on a carbon fiber paper to enhance the electronic transmission efficiency.

Covalently Connected Nb4N5-xOx-MoS2 Heterocatalysts with Desired Electron Density to Boost Hydrogen Evolution

Rational design and controllable synthesis of efficient and robust electrocatalysts for hydrogen evolution reaction (HER) remain a critical challenge for the renewable energy economy. Herein, heterostructured Nb₄N_5-xO_x-MoS₂ (0 < x < 1) anchored on N-doped graphene (defined as Nb₄N_5-xO_x-MoS₂/NG) is synthesized by hydrothermal and chemical vapor deposition (CVD) approaches. During the CVD process, MoS₂ nanosheets are etched into small pieces and covalently interconnected with Nb₄N_5-xO_x to form fine Nb₄N_5-xO_x-MoS₂ heterostructures, which possess abundant interfaces and fully exposed edge active sites. The as-prepared Nb₄N_5-xO_x-MoS₂ heterostructures with Nb-(N,S)-Mo bridges provide desired electron density, which exhibit excellent chemisorption ability for both H and water, significantly improving the intrinsic HER activity. Meanwhile, the covalently connected Nb₄N_5-xO_x-MoS₂ heterostructures together with chemical coupling of Nb₄N_5-xO_x-MoS₂ and N-doped graphene improve the structural stability and ensure fast electron transfer in the Nb₄N_5-xO_x-MoS₂/NG nanocomposite, further supporting the H₂ generation and stability.

Hybridizing Plasmonic Materials with 2D-Transition Metal Dichalcogenides toward Functional Applications

Recently, 2D transition metal dichalcogenides (TMDs) have become intriguing materials in the versatile field of photonics and optoelectronics because of their strong light-matter interaction that stems from the atomic layer thickness, broadband optical response, controllable optoelectronic properties, and high nonlinearity, as well as compatibility. Nevertheless, the low optical cross-section of 2D-TMDs inhibits the light-matter interaction, resulting in lower quantum yield. Therefore, hybridizing the 2D-TMDs with plasmonic nanomaterials has become one of the promising strategies to boost the optical absorption of thin 2D-TMDs. The appeal of plasmonics is based on their capability to localize and enhance the electromagnetic field and increase the optical path length of light by scattering and injecting hot electrons to TMDs. In this regard, recent achievements with respect to hybridization of the plasmonic effect in 2D-TMDs systems and its augmented optical and optoelectronic properties are reviewed. The phenomenon of plasmon-enhanced interaction in 2D-TMDs is briefly described and state-of-the-art hybrid device applications are comprehensively discussed. Finally, an outlook on future applications of these hybrid devices is provided.

Van der Waals interfacial reconstruction in monolayer transition-metal dichalcogenides and gold heterojunctions

The structures and properties of van der Waals (vdW) heterojunctions between semiconducting two-dimensional transition-metal dichalcogenides (2D TMDs) and conductive metals, such as gold, significantly influence the performances of 2D-TMD based electronic devices. Chemical vapor deposition is one of the most promising approaches for large-scale synthesis and fabrication of 2D TMD electronics with naturally formed TMD/metal vdW interfaces. However, the structure and chemistry of the vdW interfaces are less known. Here we report the interfacial reconstruction between TMD monolayers and gold substrates. The participation of sulfur leads to the reconstruction of Au {001} surface with the formation of a metastable Au4S4 interfacial phase which is stabilized by the top MoS2 and WS2 monolayers. Moreover, the enhanced vdW interaction between the reconstructed Au4S4 interfacial phase and TMD monolayers results in the transition from n-type TMD-Au Schottky contact to p-type one with reduced energy barrier height.

A Critical Review on Enhancement of Photocatalytic Hydrogen Production by Molybdenum Disulfide: From Growth to Interfacial Activities

Ultrathin 2D molybdenum disulfide (MoS₂ ), which is the flagship of 2D transition-metal dichalcogenide nanomaterials, has drawn much attention in the last few years. 2D MoS₂ has been banked as an alternative to platinum for highly active hydrogen evolution reaction because of its low cost, high surface-to-volume ratio, and abundant active sites. However, when MoS₂ is used directly as a photocatalyst, contrary to public expectation, it still performs poorly due to lateral size, high recombination ratio of excitons, and low optical cross section. Besides, simply compositing MoS₂ as a cocatalyst with other semiconductors cannot satisfy the practical application, which stimulates the pursual of a comprehensive insight into recent advances in synthesis, properties, and enhanced hydrogen production of MoS₂ . Therefore, in this Review, emphasis is given to synthetic methods, phase transitions, tunable optical properties, and interfacial engineering of 2D MoS₂ . Abundant ways of band edge tuning, structural modification, and phase transition are addressed, which can generate the neoteric photocatalytic systems. Finally, the main challenges and opportunities with respect to MoS₂ being a cocatalyst and coherent light-matter interaction of MoS₂ in photocatalytic systems are proposed.

Ru x Se@MoS2 hybrid as a highly efficient electrocatalyst toward hydrogen evolution reaction

Alkaline hydrogen evolution reaction (HER) requires highly efficient and stable catalytic materials, the engineering of which needs overall consideration of the water dissociation process as well as the intermediate hydrogen adsorption process. Herein, a Ru x Se@MoS2 hybrid catalyst was synthesized by the decoration of MoS2 with Ru x Se nanoparticles through a two-step hydrothermal reaction. Due to the bifunctionality mechanism in which Ru promotes the water dissociation and the nearby Se atoms, unsaturated Mo and/or S atoms act as active sites for the intermediate hydrogen adsorption, the hybrid catalyst exhibits an exceptional HER performance in basic media with a rather low overpotential of 45 mV at a current density of 10 mA cm-2 and a small Tafel slope of 42.9 mV dec-1. The synergetic effect between Ru x Se and MoS2 not only enables more catalytically active sites, but also increases the inherent conductivity of the hybrid catalyst, leading to more favorable HER kinetics under both alkaline and acidic conditions. As a result, Ru x Se@MoS2 also demonstrates an enhanced catalytic activity toward HER in 0.5 M H2SO4 in comparison with pure Ru x Se and MoS2, which requires an overpotential of 120 mV to deliver a 10 mA cm-2 current density and gives a Tafel slope of 72.2 mV dec-1. In addition, the hybrid electrocatalyst also exhibits superior electrochemical stability during the long-term HER process in both acidic media and alkaline media.

MoS2 Liquid Cell Electron Microscopy Through Clean and Fast Polymer-Free MoS2 Transfer

Two dimensional (2D) materials have found various applications because of their unique physical properties. For example, graphene has been used as the electron transparent membrane for liquid cell transmission electron microscopy (TEM) due to its high mechanical strength and flexibility, single-atom thickness, chemical inertness, etc. Here, we report using 2D MoS₂ as a functional substrate as well as the membrane window for liquid cell TEM, which is enabled by our facile and polymer-free MoS₂ transfer process. This provides the opportunity to investigate the growth of Pt nanocrystals on MoS₂ substrates, which elucidates the formation mechanisms of such heterostructured 2D materials. We find that Pt nanocrystals formed in MoS₂ liquid cells have a strong tendency to align their crystal lattice with that of MoS₂, suggesting a van der Waals epitaxial relationship. Importantly, we can study its impact on the kinetics of the nanocrystal formation. The development of MoS₂ liquid cells will allow further study of various liquid phenomena on MoS₂, and the polymer-free MoS₂ transfer process will be implemented in a wide range of applications.

Fabrication of uniform Ru-doped NiFe2O4 nanosheets as an efficient hydrogen evolution electrocatalyst

Ru-doped NiFe₂O₄ nanosheets exhibit outstanding electrocatalytic hydrogen evolution activity and stability.