Ultrathin two-dimensional (2D) nanosheets of layered transition metal dichalcogenides (TMDs), such as MoS2, TiS2, TaS2, WS2, MoSe2, WSe2, etc., are emerging as a class of key materials in chemistry and electronics due to their intriguing chemical and electronic properties. The ability to prepare these TMD nanosheets in high yield and large scale via various methods has led to increasing studies on their hybridization with other materials to create novel functional composites, aiming to engineer their chemical, physical and electronic properties and thus achieve good performance for some specific applications. In this critical review, we will introduce the recent progress in hybrid nanoarchitectures based on 2D TMD nanosheets. Their synthetic strategies, properties and applications are systematically summarized and discussed, with emphasis on those new appealing structures, properties and functions. In addition, we will also give some perspectives on the challenges and opportunities in this promising research area.

Two-dimensional transition metal dichalcogenide nanosheet-based composites.

Advanced electrodes with a high energy density at high power are urgently needed for high-performance energy storage devices, including lithium-ion batteries (LIBs) and supercapacitors (SCs), to fulfil the requirements of future electrochemical power sources for applications such as in hybrid electric/plug-in-hybrid (HEV/PHEV) vehicles. Metal sulfides with unique physical and chemical properties, as well as high specific capacity/capacitance, which are typically multiple times higher than that of the carbon/graphite-based materials, are currently studied as promising electrode materials. However, the implementation of these sulfide electrodes in practical applications is hindered by their inferior rate performance and cycling stability. Nanostructures offering the advantages of high surface-to-volume ratios, favourable transport properties, and high freedom for the volume change upon ion insertion/extraction and other reactions, present an opportunity to build next-generation LIBs and SCs. Thus, the development of novel concepts in material research to achieve new nanostructures paves the way for improved electrochemical performance. Herein, we summarize recent advances in nanostructured metal sulfides, such as iron sulfides, copper sulfides, cobalt sulfides, nickel sulfides, manganese sulfides, molybdenum sulfides, tin sulfides, with zero-, one-, two-, and three-dimensional morphologies for LIB and SC applications. In addition, the recently emerged concept of incorporating conductive matrices, especially graphene, with metal sulfide nanomaterials will also be highlighted. Finally, some remarks are made on the challenges and perspectives for the future development of metal sulfide-based LIB and SC devices.

Nanostructured metal sulfides for energy storage

Mo- and W-dichalcogenide compounds have a two-dimensional monolayer form that differs from graphene in an important respect: it can potentially have more than one crystal structure. Some of these monolayers exhibit tantalizing hints of a poorly understood structural metal-to-insulator transition with the possibility of long metastable lifetimes. If controllable, such a transition could bring an exciting new application space to monolayer materials beyond graphene. Here we discover that mechanical deformations provide a route to switching thermodynamic stability between a semiconducting and a metallic crystal structure in these monolayer materials. Based on state-of-the-art density functional and hybrid Hartree–Fock/density functional calculations including vibrational energy corrections, we discover that MoTe2 is an excellent candidate phase change material. We identify a range from 0.3 to 3% for the tensile strains required to transform MoTe2 under uniaxial conditions at room temperature. The potential for mechanical phase transitions is predicted for all six studied compounds. 2D transition metal dichalcogenide materials can potentially exist in more than one monolayer crystal structure, a feature likely to be useful for electronics that is absent in graphene. Here, the authors present calculations showing that such phase transitions may be readily accessible in MoTe2.

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Structural phase transitions in two-dimensional Mo- and W-dichalcogenide monolayers.

Using first-principles calculations and deformation potential theory, we investigate the intrinsic carrier mobility (μ) of monolayer MoS2 sheet and nanoribbons. In contrast to the dramatic deterioration of μ in graphene upon forming nanoribbons, the magnitude of μ in armchair MoS2 nanoribbons is comparable to its sheet counterpart, albeit oscillating with ribbon width. Surprisingly, a room-temperature transport polarity reversal is observed with μ of hole (h) and electron (e) being 200.52 (h) and 72.16 (e) cm2 V–1 s–1 in sheet, and 49.72 (h) and 190.89 (e) cm2 V–1 s–1 in 4 nm nanoribbon. The high and robust μ and its polarity reversal are attributable to the different characteristics of edge states inherent in MoS2 nanoribbons. Our study suggests that width reduction together with edge engineering provide a promising route for improving the transport properties of MoS2 nanostructures.

Polarity-reversed robust carrier mobility in monolayer MoS₂ nanoribbons.

In this work, uniform molybdenum disulfide (MoS2)/tungsten disulfide (WS2) quantum dots are synthesized by the combination of sonication and solvothermal treatment of bulk MoS2/WS2 at a mild temperature. The resulting products possess monolayer thickness with an average size about 3 nm. The highly exfoliated and defect-rich structure renders these quantum dots plentiful active sites for the catalysis of hydrogen evolution reaction (HER). The MoS2 quantum dots exhibit a small HER overpotential of ≈120 mV and long-term durability. Moreover, the strong fluorescence, good cell permeability, and low cytotoxicity make them promising and biocompatible probes for in vitro imaging. In addition, this work may provide an alternative facile approach to synthesize the quantum dots of transition metal dichalcogenides or other layered materials on a large scale.

One‐Pot, Facile, and Versatile Synthesis of Monolayer MoS 2 /WS 2 Quantum Dots as Bioimaging Probes and Efficient Electrocatalysts for Hydrogen Evolution Reaction

Ammoniated MS2 (M = Mo,W) have been synthesized by reacting LixMS2 with a saturated solution of ammonium chloride. While, largely neutral NH3 is present in the interlayer of ammoniated MoS2, equal amounts of NH3 and NH4+ ions are present in the tungsten analog. The ammoniated MS2 exfoliate readily in a variety of polar solvents with exfoliation being best in water. Ammoniated WS2 forms a more stable colloidal dispersion compared to the Mo analog because the ammonium ions do not deintercalate easily from the layers even on exfoliation. The dispersions are comprised of large nanosheets of MS2 with lateral dimensions in the order of micrometers. The layers could be restacked from the colloidal dispersions by evaporating the solvent. While the colloidal dispersions of ammoniated MoS2 yield NH3-free MoS2 on restacking, the dispersions of ammoniated WS2 yield WS2 intercalated with NH4+ ions. Co-stacking of MoS2 and WS2 nanosheets from a mixture of both the colloidal dispersions results in MoS2–WS2 hybrids in wh...

Two-Dimensional Nanosheets and Layered Hybrids of MoS2 and WS2 through Exfoliation of Ammoniated MS2 (M = Mo,W)

Preparation of MoS2–reduced graphene oxide (rGO) hybrid paper for catalytic applications by simple exfoliation–costacking

WS2 nanoribbons have been synthesized by chemical unzipping of WS2 nanotubes. Lithium atoms are intercalated in WS2 nanotubes by a solvothermal reaction with n-butyllithium in hexane. The lithiated WS2 nanotubes are then reacted with various solvents--water, ethanol, and long chain thiols. While the tubes break into pieces when treated with water and ethanol, they unzip through longitudinal cutting along the axes to yield nanoribbons when treated with long chain thiols, 1-octanethiol and 1-dodecanethiol. The slow diffusion of the long chain thiols reduces the aggression of the reaction, leading to controlled opening of the tubes.

Chemical unzipping of WS2 nanotubes.

Chemical unzipping of WS2 nanotubes

Various alkylamine intercalated MS2 (M = Mo, W) have been prepared by reacting LixMS2 with aqueous solutions of the amines. The amine intercalated MS2 exfoliate readily in alcohols. While the shorter amine intercalated MS2 exfoliate better in lower alcohols the longer amine intercalated MS2 exfoliate better in higher alcohols. The longer amine intercalated MS2 exfoliate well even in a nonpolar solvent, toluene. Up to 9 millimoles of MS2 per liter could be dispersed through exfoliation in organic solvents. The dispersions are quite stable and comprise 2D-nanosheets of MS2. Hybrids of MoS2 and WS2 could be prepared by evaporating the solvent from a mixture of colloidal dispersions of amine intercalated MoS2 and WS2. The hybrid exhibits features of a heterostructure in its photoluminescence spectrum.

A. Anto Jeffery

Papers

Two-Dimensional Nanosheets and Layered Hybrids of MoS2 and WS2 through Exfoliation of Ammoniated MS2 (M = Mo,W)

Preparation of MoS2–reduced graphene oxide (rGO) hybrid paper for catalytic applications by simple exfoliation–costacking

Chemical unzipping of WS2 nanotubes.

Chemical unzipping of WS2 nanotubes

Scalable large nanosheets of transition metal disulphides through exfoliation of amine intercalated MS2 [M = Mo, W] in organic solvents