High-Resolution Electrochemical Mapping of the Hydrogen Evolution Reaction on Transition-Metal Dichalcogenide Nanosheets

High-resolution scanning electrochemical cell microscopy (SECCM) is used to image and quantitatively analyze the hydrogen evolution reaction (HER) catalytically active sites of 1H-MoS₂ nanosheets, MoS₂, and WS₂ heteronanosheets. Using a 20 nm radius nanopipette and hopping mode scanning, the resolution of SECCM was beyond the optical microscopy limit and visualized a small triangular MoS₂ nanosheet with a side length of ca. 130 nm. The electrochemical cell provides local cyclic voltammograms with a nanoscale spatial resolution for visualizing HER active sites as electrochemical images. The HER activity difference of edge, terrace, and heterojunction of MoS₂ and WS₂ were revealed. The SECCM imaging directly visualized the relationship of HER activity and number of MoS₂ nanosheet layers and unveiled the heterogeneous aging state of MoS₂ nanosheets. SECCM can be used for improving local HER activities by producing sulfur vacancies using electrochemical reaction at the selected region.     

Vanadium‐Doped WS2 Nanosheets Grown on Carbon Cloth as a Highly Efficient Electrocatalyst for the Hydrogen Evolution Reaction

Two‐dimensional transition‐metal dichalcogenides have been widely studied as electrocatalysts for the hydrogen evolution reaction (HER). However, limited active sites and poor conductivity hinder their application. To solve these disadvantages, heteroatom doping has attracted wide attention because it can not only increase the active sites but also affect the intrinsic catalytic properties of the electrocatalyst. Herein, we grew vanadium‐doped WS₂ nanosheets on carbon cloth (V‐WS₂/CC) as an electrocatalyst for HER under acidic and alkaline conditions. With a proper vanadium doping concentration, the electrochemical surface areas of V_0.065‐WS₂/CC were 9.6 and 2.6 times as large as that of pure WS₂ electrocatalyst under acidic and alkaline conditions, respectively. In addition, the charge‐transfer resistance also decreased with moderate vanadium doping. Based on this, the synthesized vanadium‐doped WS₂ nanosheets exhibited good stability with high HER catalytic activity and could reach a current density of 10 mA cm⁻² at overpotentials of 148 and 134 mV in 0.5 m H₂SO₄ and 1 m KOH, respectively. The corresponding Tafel slopes were 71 and 85 mV dec⁻¹. Therefore, our synthesized vanadium‐doped WS₂ nanosheets can be a promising electrocatalyst for the production of hydrogen over a wide pH range.     

One‐pot Synthesis of CdS Nanocrystals Hybridized with Single‐Layer Transition‐Metal Dichalcogenide Nanosheets for Efficient Photocatalytic Hydrogen Evolution

Exploration of low‐cost and earth‐abundant photocatalysts for highly efficient solar photocatalytic water splitting is of great importance. Although transition‐metal dichalcogenides (TMDs) showed outstanding performance as co‐catalysts for the hydrogen evolution reaction (HER), designing TMD‐hybridized photocatalysts with abundant active sites for the HER still remains challenge. Here, a facile one‐pot wet‐chemical method is developed to prepare MS₂–CdS (M=W or Mo) nanohybrids. Surprisedly, in the obtained nanohybrids, single‐layer MS₂ nanosheets with lateral size of 4–10 nm selectively grow on the Cd‐rich (0001) surface of wurtzite CdS nanocrystals. These MS₂–CdS nanohybrids possess a large number of edge sites in the MS₂ layers, which are active sites for the HER. The photocatalytic performances of WS₂–CdS and MoS₂–CdS nanohybrids towards the HER under visible light irradiation (>420 nm) are about 16 and 12 times that of pure CdS, respectively. Importantly, the MS₂–CdS nanohybrids showed enhanced stability after a long‐time test (16 h), and 70 % of catalytic activity still remained.     

Scanning electrochemical cell microscopy: High-resolution structure−property studies of mono- and polycrystalline electrode materials

Scanning electrochemical cell microscopy (SECCM) is a nanopipette-based scanning electrochemical probe microscopy technique that utilises a mobile droplet cell to measure and visualise electrode activity with high spatiotemporal resolution. This article spotlights the use of SECCM for studying the electrochemistry of crystalline electrode materials, ranging from well-defined monocrystals (e.g., transition metal dichalcogenides: MoS₂, WS₂ and WSe₂) to structurally/compositionally heterogeneous polycrystals (e.g., polycrystalline Pt, Au, Pd, Cu, Zn, low carbon steel, boron-doped diamond) and covering the diverse areas of (photo)electrocatalysis, corrosion science, surface science and electroanalysis. In particular, it is emphasised how nanoscale-resolved information from SECCM is readily related to electrode structure and properties, collected at a commensurate scale with complementary, co-located microscopy/spectroscopy techniques, to allow structure–property relationships to be assigned directly and unambiguously.     

Ultrathin WS2 Nanoflakes as a High‐Performance Electrocatalyst for the Hydrogen Evolution Reaction

Much has been done to search for highly efficient and inexpensive electrocatalysts for the hydrogen evolution reaction (HER), which is critical to a range of electrochemical and photoelectrochemical processes. A new, high‐temperature solution‐phase method for the synthesis of ultrathin WS₂ nanoflakes is now reported. The resulting product possesses monolayer thickness with dimensions in the nanometer range and abundant edges. These favorable structural features render the WS₂ nanoflakes highly active and durable catalysts for the HER in acids. The catalyst exhibits a small HER overpotential of approximately 100 mV and a Tafel slope of 48 mV/decade. These ultrathin WS₂ nanoflakes represent an attractive alternative to the precious platinum benchmark catalyst and rival MoS₂ materials that have recently been heavily scrutinized for the electrocatalytic HER.     

WS<Subscript>2</Subscript> grafted on silicon and nano-silicon particles etched: a high-performance electrocatalyst for hydrogen evolution reaction

Most of research has been carried out for the development of electrocatalysts for hydrogen evolution reaction (HER), which are high activity and low cost. In this study, a practical, usable, highly active, cheap, and none noble metal catalyst was developed for HER. To this end, tungsten disulfide supported on silicon (WS₂/Si) and on silicon nanoparticles (WS₂/nano-Si) were prepared. To increase the catalytic activity of WS₂/nano-Si, chemical etching was used to prepare WS₂/nano-Si etched. The synthesized electrocatalysts were characterized using Fortier transform infrared spectroscopy, field emission scanning electron microscopy, X-ray photoelectron spectroscopy, and X-ray diffraction methods. To evaluate the electrochemical attributes of WS₂/Si and WS₂/n-Si before and after chemical etching, electrochemical impedance spectroscopy, linear sweep, and cyclic voltammetry were used. The electrochemical measurements indicated an intense activity of the WS₂/nano-Si/etched, through a high density of the current and low overpotential for HER, with a small overpotential of 0.14 V, Tafel slopes as small as 45 mV dec^?1, and large cathodic currents. These results show that through etching process of silicon in HF the quantities of the active sites have been changed and increased considerably.     

Ultrathin MoS2 Nanosheets Supported on N‐doped Carbon Nanoboxes with Enhanced Lithium Storage and Electrocatalytic Properties

Molybdenum disulfide (MoS₂) has received considerable interest for electrochemical energy storage and conversion. In this work, we have designed and synthesized a unique hybrid hollow structure by growing ultrathin MoS₂ nanosheets on N‐doped carbon shells (denoted as C@MoS₂ nanoboxes). The N‐doped carbon shells can greatly improve the conductivity of the hybrid structure and effectively prevent the aggregation of MoS₂ nanosheets. The ultrathin MoS₂ nanosheets could provide more active sites for electrochemical reactions. When evaluated as an anode material for lithium‐ion batteries, these C@MoS₂ nanoboxes show high specific capacity of around 1000 mAh g^?1, excellent cycling stability up to 200 cycles, and superior rate performance. Moreover, they also show enhanced electrocatalytic activity for the electrochemical hydrogen evolution.     

Correlative Electrochemical Microscopy of Li‐Ion (De)intercalation at a Series of Individual LiMn2O4 Particles

The redox activity (Li‐ion intercalation/deintercalation) of a series of individual LiMn₂O₄ particles of known geometry and (nano)structure, within an array, is determined using a correlative electrochemical microscopy strategy. Cyclic voltammetry (current–voltage curve, I–E) and galvanostatic charge/discharge (voltage–time curve, E–t) are applied at the single particle level, using scanning electrochemical cell microscopy (SECCM), together with co‐location scanning electron microscopy that enables the corresponding particle size, morphology, crystallinity, and other factors to be visualized. This study identifies a wide spectrum of activity of nominally similar particles and highlights how subtle changes in particle form can greatly impact electrochemical properties. SECCM is well‐suited for assessing single particles and constitutes a combinatorial method that will enable the rational design and optimization of battery electrode materials.     

Preparation of MoS2‐graphene Hybrid Nanosheets and Simultaneously Electrochemical Determination of Levodopa and Uric Acid

MoS₂ nanoflakes were prepared by exfoliating commercial MoS₂ powders with the assistance of ultrasound and graphene foam was synthesized by chemical vapor deposition using nickel foam as the template. MoS₂‐graphene hybrid nanosheets were developed through the combination of MoS₂ nanoflakes and graphene nanosheets by ultrasonic dispersion. The hybrid nanosheets were sprayed onto the ITO coated glass, which acts as an electrode for the simultaneously electrochemical determination of levodopa and uric acid. The MoS₂‐graphene hybrid nanosheets were characterized by scanning electron microscopy, X‐ray diffraction and Raman spectroscopy. The results show that the hybrid nanosheets are composed of MoS₂ and graphene with a sheet‐like morphology. The sensitivity of the electrode for levodopa and uric acid is 0.36 μA μM⁻¹ and 0.39 μA μM⁻¹, respectively. The electrode also shows low limit of detection, good selectivity, reproducibility and stability. And it is potential for use in clinical research.     

Free‐standing and Flexible MoS2/rGO Paper Electrode for Amperometric Detection of Folic Acid

The development of flexible electrodes is of considerable current interest because of the increasing demand for modern electronics, portable medical products, and compact devices. We report a new type of flexible electrochemical sensor fabricated by integrating graphene and MoS₂ nanosheets. A highly flexible and free‐standing conductive MoS₂ nanosheets/reduced graphene oxide (MoS₂/rGO) paper was prepared by a two‐step process: vacuum filtration and chemical reduction treatment. The MoS₂/graphene oxide (MoS₂/GO) paper obtained by a simple filtration method was transformed into MoS₂/rGO paper after a chemical reduction process. The obtained MoS₂/rGO paper was characterized by scanning electron microscopy, X‐ray diffraction spectroscopy, X‐ray photoelectron spectroscopy, Raman spectroscopy, electrochemical impedance spectroscopy. The electrochemical behavior of folic acid (FA) on MoS₂/rGO paper electrode was investigated by cyclic voltammetry and amperometry. Electrochemical experiments indicated that flexible MoS₂/rGO composite paper electrode exhibited excellent electrocatalytic activity toward the FA, which can be attributed to excellent electrical conductivity and high specific surface area of the MoS₂/rGO paper. The resulting biosensor showed highly sensitive amperometric response to FA with a wide linear range.     

Superior Electrical Conductivity in Hydrogenated Layered Ternary Chalcogenide Nanosheets for Flexible All‐Solid‐State Supercapacitors

As the properties of ultrathin two‐dimensional (2D) crystals are strongly related to their electronic structures, more and more attempts were carried out to tune their electronic structures to meet the high standards for the construction of next‐generation smart electronics. Herein, for the first time, we show that the conductive nature of layered ternary chalcogenide with formula of Cu₂WS₄ can be switched from semiconducting to metallic by hydrogen incorporation, accompanied by a high increase in electrical conductivity. In detail, the room‐temperature electrical conductivity of hydrogenated‐Cu₂WS₄ nanosheet film was almost 10¹⁰ times higher than that of pristine bulk sample with a value of about 2.9×10⁴ S m^?1, which is among the best values for conductive 2D nanosheets. In addition, the metallicity in the hydrogenated‐Cu₂WS₄ is robust and can be retained under high‐temperature treatment. The fabricated all‐solid‐state flexible supercapacitor based on the hydrogenated‐Cu₂WS₄ nanosheet film shows promising electrochemical performances with capacitance of 583.3 F cm^?3 at a current density of 0.31 A cm^?3. This work not only offers a prototype material for the study of electronic structure regulation in 2D crystals, but also paves the way in searching for highly conductive electrodes.     

Solution‐Processed Two‐Dimensional MoS2 Nanosheets: Preparation,Hybridization, and Applications

As one member of the emerging class of ultrathin two‐dimensional (2D) transition‐metal dichalcogenide (TMD) nanomaterials, the ultra‐thin MoS₂ nanosheet has attracted increasing research interest as a result of its unique structure and fascinating properties. Solution‐phase methods are promising for the scalable production, functionalization, hybridization of MoS₂ nanosheets, thus enabling the widespread exploration of MoS₂‐based nanomaterials for various promising applications. In this Review, an overview of the recent progress of solution‐processed MoS₂ nanosheets is presented, with the emphasis on their synthetic strategies, functionalization, hybridization, properties, and applications. Finally, the challenges and opportunities in this research area will be proposed.     

Electrochemically exfoliated Ni-doped MoS2 nanosheets for highly efficient hydrogen evolution and Zn-H2O battery

Thanks to tunable physical and chemical properties, two-dimensional (2D) materials have received intensive interest, endowing their excellent electrocatalytic performances for applications in energy conversion. However, their catalytic activities are largely determined by poor adsorption energy and limited active edge sites. Herein, a one-step electrochemical exfoliation strategy was developed to fabricate 2D Ni-doped MoS₂ nanosheets (Ni-EX-MoS₂) with a lateral size of ～500 nm and thickness of ～3.5 nm. Profiting from high electrical conductivity and abundant exposing active sites, Ni-EX-MoS₂ catalyst displayed an admirable performance for electrochemical hydrogen evolution reaction (HER) with a low overpotential of 145 mV at 10 mA/cm² as well as a small Tafel slope of 89 mV/dec in alkaline media, which are superior to those of the most reported MoS₂-based electrocatalysts. The formed Ni species with tuning electronic structure played a crucial role as primary active center of Ni-EX-MoS₂, as well as the forming stable 1T/2H phase MoS₂ interface demonstrated a synergistic effect on electrocatalytic HER performance. Further, Ni-EX-MoS₂ was employed as a cathode electrode for alkaline Zn-H₂O battery, which displayed a high power density of 3.3 mW/cm² with excellent stability. This work will provide a simple and effective guideline for design of electrochemically exfoliated transition metal-doped MoS₂ nanosheets to inspire their practical applications in energy catalytic and storage.     

Bifunctional Electrocatalyst with 0D/2D Heterostructure for Highly Efficient Hydrogen and Oxygen Generation

A novel MoS₂ quantum dots/CoSe₂ nanosheet (MoS₂ QDs/CoSe₂) hybrid with 0D/2D heterostructure has been developed. The CoSe₂ nanosheets (NSs) enable an excellent oxygen evolution reaction (OER) activity with increasing vacancy configuration on one hand, while the MoS₂ QDs serve as an eminent hydrogen evolution reaction (HER) catalyst on the other. By integrating MoS₂ QDs and CoSe₂ NSs, the hybrid exhibits excellent electrocatalytic performances in HER and OER. The unique 0D/2D hetero‐interface increases the exposed active sites and facilitates electron transfer, thereby boosting the electrocatalytic activity. Relatively low overpotentials of 82 mV and 280 mV are required to drive the current density of 10 mA/cm² for HER and OER, with corresponding Tafel slopes of 69 and 75 mV/dec, respectively. As such, this work provides an efficient yet simple approach to construct bifunctional electrocatalysts with enhanced activity and stability.     

Side‐Chain‐Directed Dispersion of MoS2 Nanosheets by V‐Shaped Polyaromatic Compounds

Bulk molybdenum disulfide (MoS₂) itself is virtually insoluble in common organic solvents because of the tight stacks of multiple MoS₂ nanosheets. Here we report that V‐shaped polyaromatic compounds with non‐ionic side chains can efficiently exfoliate and disperse the inorganic nanosheets. Simple grinding and sonication (less than total 1 h) of MoS₂ powder with the V‐shaped compounds gave rise to large MoS₂ nanosheets highly dispersed in NMP through efficient host‐guest S–π interactions. DLS and AFM analyses revealed that the lateral sizes (ca. 150–270 nm) and thicknesses (ca. 2–8 nm) of the products depend on the identity of the non‐ionic side chains on the V‐shaped dispersant.     

Self‐Optimization of the Active Site of Molybdenum Disulfide by an Irreversible Phase Transition during Photocatalytic Hydrogen Evolution

The metallic 1T‐MoS₂ has attracted considerable attention as an effective catalyst for hydrogen evolution reactions (HERs). However, the fundamental mechanism about the catalytic activity of 1T‐MoS₂ and the associated phase evolution remain elusive and controversial. Herein, we prepared the most stable 1T‐MoS₂ by hydrothermal exfoliation of MoS₂ nanosheets vertically rooted into rigid one‐dimensional TiO₂ nanofibers. The 1T‐MoS₂ can keep highly stable over one year, presenting an ideal model system for investigating the HER catalytic activities as a function of the phase evolution. Both experimental studies and theoretical calculations suggest that 1T phase can be irreversibly transformed into a more active 1T′ phase as true active sites in photocatalytic HERs, resulting in a “catalytic site self‐optimization”. Hydrogen atom adsorption is the major driving force for this phase transition.     

Composite electrocatalyst Mo<Subscript>x</Subscript>W<Subscript>1-x</Subscript>S<Subscript>2</Subscript> nanosheets on carbon fiber paper for highly efficient hydrogen evolution reaction

Molybdenum disulfide (MoS₂) or tungsten disulfide (WS₂), as a promising catalyst, is widely investigated for hydrogen evolution reaction (HER). In this work, a composite electrocatalysts Mo_xW_1-xS₂ is successfully decorated on carbon fiber paper (CFP) through a facile hydrothermal method. The three-dimensional porous CFP can enable the diffusion and penetration of electrolyte. Comparing with MoS₂ and WS₂ catalyst, the composite electrocatalyst Mo_xW_1-xS₂ nanosheets can expose the large number of electrochemically active sites. Hence, the as-prepared Mo_xW_1-xS₂/CFP (3:1) exhibit the outstanding HER catalytic activity with the small Tafel slope of 68 mV dec^?1 and the low overpotential of ??178.4?±?0.5 mV at a current density of 10 mA cm^?2. Chronoamperometric current test for 18 h confirm the long-term stability of the composite electrocatalyst.     

Hierarchical Mesoporous SnO Microspheres as High Capacity Anode Materials for Sodium‐Ion Batteries

Mesoporous SnO microspheres were synthesised by a hydrothermal method using NaSO₄ as the morphology directing agent. Field emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM) and high‐resolution transmission electron microscopy (HRTEM) analyses showed that SnO microspheres consist of nanosheets with a thickness of about 20 nm. Each nanosheet contains a mesoporous structure with a pore size of approximately 5 nm. When applied as anode materials in Na‐ion batteries, SnO microspheres exhibited high reversible sodium storage capacity, good cyclability and a satisfactory high rate performance. Through ex situ XRD analysis, it was found that Na⁺ ions first insert themselves into SnO crystals, and then react with SnO to generate crystalline Sn, followed by Na–Sn alloying with the formation of crystalline NaSn₂ phase. During the charge process, there are two slopes corresponding to the de‐alloying of Na–Sn compounds and oxidisation of Sn, respectively. The high sodium storage capacity and good electrochemical performance could be ascribed to the unique hierarchical mesoporous architecture of SnO microspheres.     

Size‐Dependent Enhancement of Electrocatalytic Oxygen‐Reduction and Hydrogen‐Evolution Performance of MoS2 Particles

MoS₂ particles with different size distributions were prepared by simple ultrasonication of bulk MoS₂ followed by gradient centrifugation. Relative to the inert microscale MoS₂, nanoscale MoS₂ showed significantly improved catalytic activity toward the oxygen‐reduction reaction (ORR) and hydrogen‐evolution reaction (HER). The decrease in particle size was accompanied by an increase in catalytic activity. Particles with a size of around 2 nm exhibited the best dual ORR and HER performance with a four‐electron ORR process and an HER onset potential of ?0.16 V versus the standard hydrogen electrode (SHE). This is the first investigation on the size‐dependent effect of the ORR activity of MoS₂, and a four‐electron transfer route was found. The exposed abundant Mo edges of the MoS₂ nanoparticles were proven to be responsible for the high ORR catalytic activity, whereas the origin of the improved HER activity of the nanoparticles was attributed to the plentiful exposed S edges. This newly discovered process provides a simple protocol to produce inexpensive highly active MoS₂ catalysts that could easily be scaled up. Hence, it opens up possibilities for wide applications of MoS₂ nanoparticles in the fields of energy conversion and storage.     

Electrochemical sulfidation of WS2 nanoarrays: Strong dependence of hydrogen evolution activity on transition metal sulfide surface composition

The activity of transition metal sulfides for the hydrogen evolution reaction (HER) can be increased by sulfur-enrichment of active metal-sulfide sites. In this report, we investigate the electrochemical sulfidation of atmospherically aged WS₂ nanoarrays with respect to enhancing HER activity. In contrast to MoS₂, it is found that sulfidation diminishes HER activity. Electrochemical and XPS experiments suggest the involvement of insoluble tungsten oxides in the altered HER and electron transfer properties. This demonstrates the strong dependence of the transition metal dichalcogenide (TMD) composition with the successful sulfur incorporation and subsequent HER activity.