Aromatic-Exfoliated Transition Metal Dichalcogenides: Implications for Inherent Electrochemistry and Hydrogen Evolution

Influence of Inorganic Anions on the Chemical Stability of Molybdenum Disulfide Nanosheets in the Aqueous Environment

Chemical stability is closely associated with the transformations and bioavailabilities of engineered nanomaterials and is a key factor that governs broader and long-term application. With the growing utilization of molybdenum disulfide (MoS2) nanosheets in water treatment and purification processes, it is crucial to evaluate the stability of MoS2 nanosheets in aquatic environments. Nonetheless, the effects of anionic species on MoS2 remain largely unexplored. Herein, the stability of chemically exfoliated MoS2 nanosheets (ceMoS2) was assessed in the presence of inorganic anions. The results showed that the chemical stability of ceMoS2 was regulated by the nucleophilicities and the resultant charging effects of the anions in aquatic systems. The anions promote the dissolution of ceMoS2 by triggering a shift in the chemical potential of the ceMoS2 surface as a function of the anion nucleophilicity (i.e., charging effect). Fast charging with HCO3- and HPO42-/H2PO4- was validated by a phase transition from 1T to 2H and the emergence of MoV, and it promoted oxidative dissolution of the ceMoS2. Additionally, under sunlight, ceMoS2 dissolution was accelerated by NO3-. These findings provide insight into the ion-induced fate of ceMoS2 and the durability and risks of MoS2 nanosheets in environmental applications.

Layer-Structured Anisotropic Metal Chalcogenides: Recent Advances in Synthesis, Modulation, and Applications

The unique electronic and catalytic properties emerging from low symmetry anisotropic (1D and 2D) metal chalcogenides (MCs) have generated tremendous interest for use in next generation electronics, optoelectronics, electrochemical energy storage devices, and chemical sensing devices. Despite many proof-of-concept demonstrations so far, the full potential of anisotropic chalcogenides has yet to be investigated. This article provides a comprehensive overview of the recent progress made in the synthesis, mechanistic understanding, property modulation strategies, and applications of the anisotropic chalcogenides. It begins with an introduction to the basic crystal structures, and then the unique physical and chemical properties of 1D and 2D MCs. Controlled synthetic routes for anisotropic MC crystals are summarized with example advances in the solution-phase synthesis, vapor-phase synthesis, and exfoliation. Several important approaches to modulate dimensions, phases, compositions, defects, and heterostructures of anisotropic MCs are discussed. Recent significant advances in applications are highlighted for electronics, optoelectronic devices, catalysts, batteries, supercapacitors, sensing platforms, and thermoelectric devices. The article ends with prospects for future opportunities and challenges to be addressed in the academic research and practical engineering of anisotropic MCs.

Molybdenum Diselenide and Tungsten Diselenide Interfacing Cobalt-Porphyrin for Electrocatalytic Hydrogen Evolution in Alkaline and Acidic Media

Easy and effective modification approaches for transition metal dichalcogenides are highly desired in order to make them active toward electrocatalysis. In this manner, we report functionalized molybdenum diselenide (MoSe₂) and tungsten diselenide (WSe₂) via metal-ligand coordination with pyridine rings for the subsequent covalent grafting of a cobalt-porphyrin. The new hybrid materials were tested towards an electrocatalytic hydrogen evolution reaction in both acidic and alkaline media and showed enhanced activity compared to intact MoSe₂ and WSe₂. Hybrids exhibited lower overpotential, easier reaction kinetics, higher conductivity, and excellent stability after 10,000 ongoing cycles in acidic and alkaline electrolytes compared to MoSe₂ and WSe₂. Markedly, MoSe₂-based hybrid material showed the best performance and marked a significantly low onset potential of -0.17 V vs RHE for acidic hydrogen evolution reaction. All in all, the ease and fast modification route provides a versatile functionalization procedure, extendable to other transition metal dichalcogenides, and can open new pathways for the realization of functional nanomaterials suitable in electrocatalysis.

A sandwich-like configuration with a signal amplification strategy using a methylene blue/aptamer complex on a heterojunction 2D MoSe2/2D WSe2 electrode: Toward a portable and sensitive electrochemical alpha-fetoprotein immunoassay

Liver cancer is one of the most common global health problems that features a high mortality rate. Alpha-fetoprotein (AFP) is a potential liver cancer biomarker for the diagnosis of liver cancer. The quantitative detection of AFP at an ultratrace level has important medical significance. Using the reaction of the antibody-antigen pair in an immunosensor enables the sensitive and selective AFP assay. Finding a strategy in signal generation and amplification is challenging to fabricate new sensitive electrochemical immunosensors for AFP detection. This study demonstrates the construction of a simple, reliable, and label-free immunosensor for the detection of AFP on a smart phone. Exfoliated two-dimensional (2D) molybdenum diselenide (MoSe2) and 2D tungsten diselenide (WSe2) were employed to modify the disposable screen-printed carbon electrode (SPCE) to use as the electrochemical platform, which is affixed to a small potentiostat connected to a smart phone. The modified electrode offers antibody immobilization and allows detection of AFP via an immunocomplex forming a sandwich-like configuration with the AFP-corresponding aptamer. A heterojunction 2D MoSe2/2D WSe2 composite improves the SPCE's reactivity and provides a large surface area and good adsorption capacity for the immobilizing antibodies. The signal generation for the immunosensor is from the electrochemical response of methylene blue (MB) intercalating into the aptamer bound on the electrode. The response for the proposed sandwich-like immunosensor is proportional to the AFP concentration (1.0-50,000 pg ml-1). The biosensor has potential for the development of a simple and robust point-of-care diagnostic platform for the clinical diagnosis of liver cancer, achieving a low limit of detection (0.85 pg ml-1), high sensitivity, high selectivity, good stability, and excellent reproducibility.

Exfoliation of MoS2 Nanosheets Enabled by a Redox-Potential-Matched Chemical Lithiation Reaction

Ion intercalation assisted exfoliation is the oldest and most popular method for the scalable synthesis of molybdenum disulfide (MoS₂) nanosheets. The commonly used organolithium reagents for Li⁺ intercalation are n-butyllithium (n-BuLi) and naphthalenide lithium (Nap-Li); however, the highly pyrophoric nature of n-BuLi and the overly reducing power of Nap-Li hinder their extensive application. Here, a novel organolithium reagent, pyrene lithium (Py-Li), which has intrinsic safe properties and a well-matched redox potential, is reported for the intercalation and exfoliation of MoS₂. The redox potential of Py-Li (0.86 V vs Li⁺/Li) is located just between the intercalation (1.13 V) and decomposition (0.55 V) potentials of bulk MoS₂, thus allowing precise Li⁺ intercalation to form a lamellar LiMoS₂ compound without undesirable structural damage. The lithiation reaction can be accomplished within 1 h at room temperature and the exfoliated nanosheets are almost single layer. This method also offers the advantages of low cost, high repeatability, and ease in realizing large-scale production.

Template free-synthesis of cobalt-iron chalcogenides [Co0.8Fe0.2L2, L = S, Se] and their robust bifunctional electrocatalysis for the water splitting reaction and Cr(vi) reduction

The ease of production of materials and showing multiple applications are appealing in this modern era of advanced technology. This paper reports the synthesis of a pair of novel cobalt–iron chalcogenides [Co_0.8Fe_0.2S₂ and Co_0.8Fe_0.2Se₂] with enhanced electro catalytic activities. These ternary metal chalcogenides were synthesized by a one-step template-free approach via a hexamethyldisilazane (HMDS)-assisted synthetic method. Transient photocurrent (TPC) studies and electrochemical impedance spectra (EIS) of these materials showed free electron mobility. Their bifunctional activities were verified in both the electrochemical oxygen evolution reaction (OER) and in the electrochemical reduction of toxic inorganic heavy metal ions [Cr(vi)] in polluted water. The materials showed robust catalytic ability in the oxygen evolution reaction with minimum possible over potential (345 and 350 mV @ η10) as determined by linear sweep voltammetry and the lower Tafel values (52.4 and 84.5 mV dec⁻¹) for Co_0.8Fe_0.2Se₂ and Co_0.8Fe_0.2S₂ respectively. Surprisingly, both the materials also showed an excellent activity towards electrochemical Cr(vi) reduction to Cr(iii). Besides the maximum current achieved for Co_0.8Fe_0.2S₂, a minimum value for the Limit of detection (LOD) was obtained for Co_0.8Fe_0.2S₂ (0.159 μg L⁻¹) compared to Co_0.8Fe_0.2Se₂ (0.196 μg L⁻¹). We tested the durability of catalysts, the critical factor for the prolonged use of catalysts, through the recyclability measurements of these materials as catalysts. Both the catalysts presented outstanding durability and balanced electro catalytic activities for up to 1500 CV cycles, and chronoamperometry studies also confirmed exceptional stability. The enhanced catalytic activities of these materials are ascribed to the free electron movement, evidenced by the increased TPC measured and EIS. Therefore, the template-free synthesis of these electro catalysts containing non-noble metal illustrates the practical approach to develop such types of catalysts for multiple functions.

The ease of production of materials and showing multiple applications are appealing in this modern era of advanced technology. Cobalt–iron chalcogenides showing multiple application is reported.

Impact of the number of surface-attached tungsten diselenide layers on cadmium sulfide nanorods on the charge transfer and photocatalytic hydrogen evolution rate

The selection of layered number and time-course destruction of layers may affect the charge transfer between 2D-to-1D heterostructure, making it possible to improve the efficiency of solar-to-hydrogen evolution. Herein, we demonstrate a simple, low-cost systematic protocol of 2D-WSe₂ nanolayer numbers ranging from 7 to 60 aiding the ultrasonication time-course. The resultant nanolayers were assembled on the surface of 1D-CdS nanorods, which demonstrated an improved surface shuttling property. Consequently, a drastic improvement in photocatalytic solar-driven hydrogen evolution was observed (103.5 mmol h^-1 g^-1) with seven-layered WSe₂ (few-layered WSe₂) attached on CdS nanorods surface. This enhanced photocatalytic performance is attributed to the selection of layers on CdS surface that expose abundant active sites; along with suitable energy levels, this can facilitate increased charge transfer leading to feasible photocatalytic reactions. Significantly, the present study proposes an efficient and sustainable process to produce hydrogen and demonstrates the potential of numbered WSe₂ nanosheets as a co-catalyst material.

Bimetallic cobalt-iron diselenide nanorod modified glassy carbon electrode: an electrochemical sensing platform for the selective detection of isoniazid

The increasing demand of a sensitive and portable electrochemical sensing platform in pharmaceutical analysis has developed widespread interest in preparing electrode materials possessing remarkable properties for the electrochemical determination of target drug analytes. Herein, we report the synthesis, characterization and application of bimetallic cobalt-iron diselenide (FeCoSe₂) nanorods as electrode modifiers for the selective detection of a commonly used anti-tuberculosis drug Isoniazid (INZ). We prepared FeCoSe₂ nanorods by a simple hydrothermal route and characterized these by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy dispersive X-ray spectroscopy (EDX) and temperature-programmed reduction (TPR) techniques. The electrochemical characterization of FeCoSe₂ modified GCE was performed by cyclic voltammetry (CV) and square wave anodic stripping voltammetry (SWASV). Under optimized experimental conditions, a linear current-concentration response was obtained for INZ in the range of 0.03–1.0 μM, with very low limit of detection 1.24 × 10⁻¹⁰ M. The real applicability of the designed FeCoSe₂/GCE sensing platform was adjudicated by the detection of INZ in biological samples.

FeCoSe₂ bimetallic nanorods were synthesized by hydrothermal method. The modified electrode responded excellently towards isoniazid detection with LOD of 1.24 × 10⁻¹⁰ M. FeCoSe₂/GCE showed applicability for INZ detection in real samples.

Local electrochemical activity of transition metal dichalcogenides and their heterojunctions on 3D-printed nanocarbon surfaces

Transition metal dichalcogenides (TMDs) have shown to be promising catalysts for the electrochemical hydrogen evolution reaction (HER) and 3D-printing enables fast prototyping and manufacturing of water splitting devices. However, the merging of TMDs with complex 3D-printed surfaces and nanostructures as well as their localized characterization remains challenging. In this work, electrodeposition of MoS2 and WS2 and their heterojunctions are used to modify thermally activated 3D-printed nanocarbon structures. Their electrochemical performance for the HER is investigated macroscopically by linear sweep voltammetry and microscopically by scanning electrochemical microscopy. This study demonstrates different local HER active sites of MoS2 and WS2 within the 3D-printed nanocarbon structure that are not solely located at the outer surface, but also in the interior up to ∼150 μm for MoS2 and ∼300 μm for WS2.

Molybdenum disulfide nanosheets: From exfoliation preparation to biosensing and cancer therapy applications

Over the past few decades, nanotechnology has developed rapidly. Various nanomaterials have been gradually applied in different fields. As a kind of two-dimensional (2D) layered nanomaterial with a graphene-like structure, molybdenum disulfide (MoS₂) nanosheets have broad research prospects in the fields of tumor photothermal therapy, biosensors and other biomedical fields because of their unique band gap structure and physical, chemical and optical properties. In this paper, the latest research progress on MoS₂ is briefly summarized. Several commonly used exfoliation methods for the preparation of MoS₂ nanosheets are reviewed based on the studies in the past five years. Additionally, the current research status of MoS₂ nanosheets in the field of biomedicine is introduced. At the end of this review, a brief overview of the limitations of MoS₂ research and its future prospects in the field of biomedicine is also provided.

Se-Rich MoSe2 Nanosheets and Their Superior Electrocatalytic Performance for Hydrogen Evolution Reaction

Two-dimensional MoSe₂ has emerged as a promising electrocatalyst for the hydrogen evolution reaction (HER), although its catalytic activity needs to be further improved. Herein, we report Se-rich MoSe₂ nanosheets synthesized using a hydrothermal reaction, displaying much enhanced HER performance at the Se/Mo ratio of 2.3. The transition from the 2H to the 1T' phase occurred as Se/Mo exceeded 2. Structural analysis revealed the presence of Se adatoms as well as the formation of Se-Se bonding. Based on first-principles calculations, we propose two equally stable Se-rich structures. In the first one, excess Se atoms bridge two MoSe₂ layers via the interlayer Se-Se bonds. In the second one, the Se atoms substitute for the Mo atoms, and extra Se atoms are added closest to the Mo-substituted Se. Calculation of Gibbs free energy along the reaction path indicates that the Se adatoms of the second model are the most active sites for HER.

A layered Ge2Sb2Te5 phase change material

In this study, a universal Ge₂Sb₂Te₅ phase change material was sputtered to obtain a layered structure. The crystalline phase of this material was prepared by annealing. SEM (scanning electron microscopy) and HRTEM (high-resolution transmission electron microscopy) images give confirmed that the sputtered Ge₂Sb₂Te₅ thin film in crystalline phase has multiple layers. The layers can be exfoliated by acetone. The thicknesses of acetone-exfoliated crystalline and amorphous flakes are approx. 10-60 nm.

MoS2 quantum dots chemically modified with porphyrin for solid-state broadband optical limiters

MoS₂ quantum dots (MQDs) with unique electronic and optical properties are promising broadband nonlinear optical (NLO) materials for ultrafast optical applications. It would be very interesting and challenging to functionalize MQDs with another hotspot optoelectronically active molecule "porphyrin". Herein, by treating MQDs with tetraphenylporphyrin (TPP) diazonium salts, we synthesized a novel nanohybrid material, MQD-TPP, in which TPP was covalently functionalized to the surface of MQDs via a C-S linkage. To explore its solid-state broadband NLO application, the MQD-TPP nanohybrid was encapsulated into a poly(methyl methacrylate) (PMMA) matrix for the open-aperture Z-scan measurements at 532 and 1064 nm. In contrast to MQDs/PMMA and TPP/PMMA, the MQD-TPP/PMMA film exhibited superior nonlinear optical and optical limiting responses with the largest nonlinear coefficients (β_eff) and the lowest optical limiting (OL) thresholds of about 1059.17 cm GW^-1 and 1.62 J cm^-2 at 532 nm and 831.13 cm GW^-1 and 1.97 J cm^-2 at 1064 nm, respectively.

Recent advances in two-dimensional materials and their nanocomposites in sustainable energy conversion applications

Two-dimensional (2D) materials have a wide platform in research and expanding nano- and atomic-level applications. This study is motivated by the well-established 2D catalysts, which demonstrate high efficiency, selectivity and sustainability exceeding that of classical noble metal catalysts for the oxygen reduction reaction (ORR), oxygen evolution reaction (OER), and/or hydrogen evolution reaction (HER). Nowadays, the hydrogen evolution reaction (HER) in water electrolysis is crucial for the cost-efficient production of a pure hydrogen fuel. We will also discuss another important point related to electrochemical carbon dioxide and nitrogen reduction (ECR and N2RR) in detail. In this review, we mainly focused on the recent progress in the fuel cell technology based on 2D materials, including graphene, transition metal dichalcogenides, black phosphorus, MXenes, metal-organic frameworks, and metal oxide nanosheets. First, the basic attributes of the 2D materials were described, and their fuel cell mechanisms were also summarized. Finally, some effective methods for enhancing the performance of the fuel cells based on 2D materials were also discussed, and the opportunities and challenges of 2D material-based fuel cells at the commercial level were also provided. This review can provide new avenues for 2D materials with properties suitable for fuel cell technology development and related fields.

Electrochemical exfoliation of ultrathin ternary molybdenum sulfoselenide nanosheets to boost the energy-efficient hydrogen evolution reaction

Developing low-cost and highly efficient transition metal dichalcogenides as alternative electrocatalysts has become an urgent need for the hydrogen evolution reaction (HER). However, the inert basal planes of transition metal dichalcogenides severely suppress their practical applications. Herein, we developed ultrathin ternary molybdenum sulfoselenide (MoSe_xS_2-x) nanosheets by using the cathodic electrochemical exfoliation approach in non-aqueous electrolytes. The exfoliated MoSe_xS_2-x nanosheets demonstrated high structural integrity with lateral dimensions up to ∼1.5 μm and an average thickness of about 3 nm. Owing to the unique ultrathin structure and immensely exposed active sites, the ternary MoSe_xS_2-x nanosheets supported on Ni foam demonstrated a greatly enhanced electrocatalytic activity for the HER in 1.0 M KOH with an overpotential of 123 mV at a current density of 10 mA cm^-2 and high stability, superior to majority of the previously published MoS₂-based electrocatalysts. Furthermore, the ternary MoSe_xS_2-x nanosheets as a highly active bifunctional electrocatalyst contributed to enhanced energy-efficient hydrogen production and electrocatalytic synthesis of ammonia.

Two-Dimensional Materials on the Rocks: Positive and Negative Role of Dopants and Impurities in Electrochemistry

Two-dimensional (2D) materials, such as graphene and transition-metal chalcogenides, were shown in many works as very potent catalysts for industrially important electrochemical reactions, such as oxygen reduction, hydrogen and oxygen evolution, and carbon dioxide reduction. We critically discuss here the development in the field, showing that not only dopants but also impurities can have dramatic effects on catalysis. Note here that the difference between dopant and impurity is merely semantic-dopant is an impurity deliberately added to the material. We contest the general belief that all doping has a positive effect on electrocatalysis. We show that in many cases, dopants actually inhibit the electrochemistry of 2D materials. This review provides a balanced view of the field of 2D materials electrocatalysis.

Direct solution-phase synthesis of 1T' WSe2 nanosheets

Crystal phase control in layered transition metal dichalcogenides is central for exploiting their different electronic properties. Access to metastable crystal phases is limited as their direct synthesis is challenging, restricting the spectrum of reachable materials. Here, we demonstrate the solution phase synthesis of the metastable distorted octahedrally coordinated structure (1T’ phase) of WSe₂ nanosheets. We design a kinetically-controlled regime of colloidal synthesis to enable the formation of the metastable phase. 1T’ WSe₂ branched few-layered nanosheets are produced in high yield and in a reproducible and controlled manner. The 1T’ phase is fully convertible into the semiconducting 2H phase upon thermal annealing at 400 °C. The 1T’ WSe₂ nanosheets demonstrate a metallic nature exhibited by an enhanced electrocatalytic activity for hydrogen evolution reaction as compared to the 2H WSe₂ nanosheets and comparable to other 1T’ phases. This synthesis design can potentially be extended to different materials providing direct access of metastable phases.

1T’ phases of transition metal dichalcogenides show promise for electrocatalysis, energy storage, and spintronic applications but are difficult to obtain. Here the authors synthesize 1T’ WSe2 few-layered nanosheets by kinetically-controlled colloidal synthesis, and test their electrocatalytic activity.

Twist-Angle-Dependent Optoelectronics in a Few-Layer Transition-Metal Dichalcogenide Heterostructure

Lattice matching has been supposed to play an important role in the coupling between two materials in a vertical heterostructure (HS). To investigate this role, we fabricated a heterojunction device with a few layers of p-type WSe₂ and n-type MoSe₂ with different crystal orientation angles. The crystal orientations of WSe₂ and MoSe₂ were estimated using high-resolution X-ray diffraction. Heterojunction devices were fabricated with twist angles of 0, 15, and 30°. The I- V curve of the sample with the twist angle of 0° under the dark condition showed a diodelike behavior. The strong coupling due to lattice matching caused a well-established p-n junction. In cases of 15 and 30° samples, the van der Waals gap was built because of lattice mismatching, which resulted in the formation of a potential barrier. However, when the light-emitting diode light of 365 nm (3.4 eV) was illuminated, it was possible for excited electrons and holes to jump beyond the potential barrier and the current flowed well in both forward and reverse directions. The effects of the twist angle were analyzed by spectral responsivity and external quantum efficiency, where it was found that the untwisted HS exhibited higher sensitivity under IR illumination, whereas the twisting effect was not noticeable under UV illumination. From photoluminescence and Raman spectroscopy studies, it was confirmed that the twisted HS showed a weak coupling because of the lattice mismatch.

Metallic MoS2 nanosheets: multifunctional electrocatalyst for the ORR, OER and Li-O2 batteries

Lithium-oxygen batteries (LOBs) possess the highest theoretical specific density among all types of lithium batteries, making them ideal candidates to replace the current Li ion batteries for next-generation electric vehicle applications. However, designing highly active catalysts with high electronic conductivities to kinetically accelerate the sluggish oxygen reduction/evolution reactions (ORR/OER) is still a big challenge. This work was dedicated to developing two-dimensional (2D) trigonal phase MoS₂ (1T-MoS₂) nanosheets as a highly active electrocatalyst for LOBs for the first time. Metallic 1T-MoS₂ prepared via in situ liquid-redox intercalation and exfoliation was hybridized with functionalized carbon nanotubes (CNTs) to form freestanding, binder-free oxygen electrodes. The 1T-MoS₂/CNT electrode exhibited excellent electrochemical performances with a high reversible capacity of 500 mA h g^-1 at a current density of 200 mA g^-1 for more than 100 cycles owing to the catalytically active surfaces of 1T-MoS₂ accessible by Li⁺ ions and O₂. Density functional theory (DFT) calculations identified the catalytically active basal planes in 1T-MoS₂ during the ORR as well as the initial ORR path during LOB cycles. The results based on the rotational ring disk electrode (RRDE) experiments also supported the findings from the DFT calculations, where the 1T-MoS₂ basal planes are active for both the ORR and OER, not the semiconducting hexagonal MoS₂ (2H-MoS₂) whose edges are only electrocatalytically active. This study sheds light on the use of metallic 1T-MoS₂ as a multifunctional oxygen electrocatalyst for LOB applications with enhanced ORR and OER activities.

Moiré-templated strain patterning in transition-metal dichalcogenides and application in twisted bilayer MoS2

To take full advantage of the electronic properties of transition-metal dichalcogenides and their vdW layered structures, it will be necessary to control the local electronic structure, on which the effect of lattice deformation is significant. Nevertheless, a general approach to programming nanoscale morphology in TMD materials, which would permit local strain engineering, has proven elusive. In this work, we propose a general moiré-templated nanoscale morphology engineering method based on bilayer TMDs. The moiré superlattice plays the key role in enforcing in-plane periodical variations in local interlayer spacing and potential energy. Upon global in-plane compression, the high-energy, large-interlayer-separation stacking domains serve as periodic buckling initiation sites. The buckled features can be thus precisely correlated to the moiré periodicity. The spatial profile of the buckled morphology and strain field are possible to be pre-determined, providing a bridge to the electronics and optoelectronics design. We take twisted bilayer MoS2 to demonstrate our approach. We further demonstrate how the morphology can modulate band gap and optical absorption of a MoS2 monolayer, envisioned as a potential constituent layer in a Moiré-templated, strain-engineered vdW heterostructure of TMDs. The robustness and programmable nature of our approach arise from superlattice symmetry, energetics and mechanics. Our approach provides a new strategy for on-demand design of morphology and local strain in TMDs under mechanical deformation.

Synthesis of ultrathin WSe2 nanosheets and their high-performance catalysis for conversion of amines to imines

Tungsten diselenide (WSe2) is the material with the lowest thermal conductivity in the world. Most physical methods are used for the synthesis of tungsten diselenide. Here, a simple colloidal method is reported for the synthesis of WSe2 nanosheets. The composition, valence, size, morphology and properties of the samples were characterized and measured. Results showed that the obtained WSe2 nanosheets with a thickness of 0.7 nm had strong blue fluorescence. Significantly, the synthesized WSe2 nanosheets exhibited excellent catalytic activity for the aerobic coupling of amines to imines, with 100% yield under visible light irradiation and air atmosphere. As a photocatalyst, it exhibited excellent recyclability, and maintained a high yield after 5 cycles. It was found that this reaction could also happen in the presence of natural light by slightly extending the reaction time. Moreover, H2O was used as a solvent in the catalytic process, avoiding expensive and toxic organic solvents. This work provides an efficient, economical and sustainable process for the synthesis of imines and shows the great potential of WSe2 nanosheets as photocatalysts for organic synthesis.

Molybdenum Sulfide Electrocatalysis is Dramatically Influenced by Solvents Used for Its Dispersions

Transition metal dichalcogenides, especially MoS₂ and related MoS₃, have attracted attention as potential replacement of platinum for electrochemical energy applications. These materials are typically treated before the use in solvents. It is assumed that these solvents do not influence follow-up electrochemistry. Here, we show that the oxygen reduction overpotentials as well as inherent electrochemistry of MoS₃ is dramatically influenced by solvents used, them being water, acetonitrile, dimethylformamide, or ethanol. This has a profound impact on the interpretation of the electrochemical studies and the choice of MoS _x solvent treatment.

Layered transition metal dichalcogenide electrochemistry: journey across the periodic table

Studies on layered transition metal dichalcogenides (TMDs), in particular for Group VIB TMDs like MoS2 and WS2, have long reached a crescendo in the realms of electrochemical applications initiated by their remarkable catalytic and electronic properties. One area that garnered considerable attention is the fervent pursuit of layered TMDs as electrocatalysts for hydrogen evolution reaction (HER), driven by global efforts towards reducing carbon footprint and attaining hydrogen economy. This Tutorial Review captures the essence of electrochemistry of different classes of layered TMDs and metal chalcogenides across the period table and showcases their tuneable electrochemical and HER catalytic attributes that are governed by the elemental composition, structure and anisotropy. Of interest to the assiduously studied Group VIB TMDs, we describe the role of elemental constituents and material purity in aspects of surface composition and structure, on their electrochemistry. Across families of layered TMDs in the periodic table, we highlight the apparent trends in their electrochemical and electrocatalytic properties through diligent comparison. Inevitably, these trends vary according to the type of chalcogen or transition metal that constitutes the eventual TMD. Beyond layered TMDs, we discuss the electrochemistry and recent progress in HER electrocatalysis of other layered metal chalcogenides that are overshadowed by the success of Group VIB TMDs. At the pinnacle of the emergent applications of layered TMDs, it is prudent to demystify the intrinsic electrochemical behaviour that originates from the participation of the elemental constitution of transition metal or chalcogen. Moreover, knowledge of the catalytic and electronic properties of the various TMD families and emerging trends across the period or down the group is of paramount importance when introducing or refining their prospective uses. The annotations in this Tutorial Review are envisioned to promote discourse into the catalytic and electrochemical trends of TMDs that is currently absent.

The effect of varying solvents for MoS2 treatment on its catalytic efficiencies for HER and ORR

MoS₂ has been investigated intensively in the field of catalysis for the hydrogen evolution reaction (HER) in particular. Much effort has been made by various research teams worldwide to look into the specific catalyst design such as nano-structuring, defect engineering or hybrid structures. But what evades us is the fundamental preparation method for the dispersion of powdered MoS₂. Individual research teams with their best practices might be subjective and not validated by extensive experimental results. In this report, we find that the overpotential for the catalysis of HER varies from 0.57 to 0.72 V (freshly prepared) when different dispersion media are used, such as acetonitrile, N,N-dimethylformamide, ethanol, methanol and water. In terms of oxygen reduction reaction (ORR) catalysis, less significant differences were found. With both HER and ORR pertinent to the fuel cell industry, this report would serve as an insight to readers when comparing the results of MoS₂ catalysis across the literature from different research groups when different solvents were used as the dispersion medium.

An electrochemical anodization strategy towards high-activity porous MoS2 electrodes for the hydrogen evolution reaction

An electrochemical anodization strategy was developed for the formation of MoS₂ electrodes for the hydrogen evolution reaction.

Exploring the Formation of Black Phosphorus Intercalation Compounds with Alkali Metals

Black phosphorus intercalation compounds (BPICs) with alkali metals (namely: K and Na) have been synthesized in bulk by solid‐state as well as vapor‐phase reactions. By means of a combination of in situ X‐ray diffraction, Raman spectroscopy, and DFT calculations the structural behavior of the BPICs at different intercalation stages has been demonstrated for the first time. Our results provide a glimpse into the very first steps of a new family of intercalation compounds, with a distinct behavior as compared to its graphite analogues (GICs), showing a remarkable structural complexity and a dynamic behavior.

Colloidal 2D nanosheets of MoS2 and other transition metal dichalcogenides through liquid-phase exfoliation

This review focuses on the exfoliation of transition metal dichalcogenides MQ₂ (TMD, M=Mo, W, etc., Q=S, Se, Te) in liquid media, leading to the formation of 2D nanosheets dispersed in colloids. Nowadays, colloidal dispersions of MoS₂, MoSe₂, WS₂ and other related materials are considered for a wide range of applications, including electronic and optoelectronic devices, energy storage and conversion, sensors for gases, catalysts and catalyst supports, biomedicine, etc. We address various methods developed so far for transferring these materials from bulk to nanoscale thickness, and discuss their stabilization and factors influencing it. Long-time known exfoliation through Li intercalation has received renewed attention in recent years, and is recognized as a method yielding highest dispersed concentrations of single-layer MoS₂ and related materials. Latest trends in the intercalation/exfoliation approach include electrochemical lithium intercalation, experimenting with various intercalating agents, multi-step intercalation, etc. On the other hand, direct sonication in solvents is a much simpler technique that allows one to avoid dangerous reagents, long reaction times and purifying steps. The influence of the solvent characteristics on the colloid formation was closely investigated in numerous recent studies. Moreover, it is being recognized that, besides solvent properties, sonication parameters and solvent transformations may affect the process in a crucial way. The latest data on the interaction of MoS₂ with solvents evidence that not only solution thermodynamics should be employed to understand the formation and stabilization of such colloids, but also general and organic chemistry. It appears that due to the sonolysis of the solvents and cutting of the MoS₂ layers in various directions, the reactive edges of the colloidal nanosheets may bear various functionalities, which participate in their stabilization in the colloidal state. In most cases, direct exfoliation of MQ₂ into colloidal nanosheets is conducted in organic solvents, while a small amount of works report low-concentrated colloids in pure water. To improve the dispersion abilities of transition metal dichalcogenides in water, various stabilizers are often introduced into the reaction media, and their interactions with nanosheets play an important role in the stabilization of the dispersions. Surfactants, polymers and biomolecules usually interact with transition metal dichalcogenide nanosheets through non-covalent mechanisms, similarly to the cases of graphene and carbon nanotubes. Finally, we survey covalent chemical modification of colloidal MQ₂ nanosheets, a special and different approach, consisting in the functionalization of MQ₂ surfaces with help of thiol chemistry, interaction with electrophiles, or formation of inorganic coordination complexes. The intentional design of surface chemistry of the nanosheets is a very promising way to control their solubility, compatibility with other moieties and incorporation into hybrid structures. Although the scope of the present review is limited to transition metal dichalcogenides, the dispersion in colloids of other chalcogenides (such as NbS₃, VS₄, Mo₂S₃, etc.) in many ways follows similar trends. We conclude the review by discussing current challenges in the area of exfoliation of MoS₂ and its related materials.

Tailored performance of layered transition metal dichalcogenides via integration with low dimensional nanostructures

Transition metal dichalcogenides (TMDs) with a unique sandwich structure have attracted tremendous attention in recent years due to their distinctive electrical and optical properties.

A comparative study of hydrogen evolution reaction on pseudo-monolayer WS2 and PtS2: insights based on the density functional theory

In this study, we investigated the catalytic activity of ultrathin PtS₂ and WS₂ nanostructures for the hydrogen evolution reaction by electronic structure calculations based on the spin-polarised density functional theory.

Exfoliation of WS2in the semiconducting phase using a group of lithium halides: a new method of Li intercalation

Lithium halide assisted high yield synthesis of few layers of 2H phase semiconducting WS₂in organic solvents is reported.