Quantum Dots of 1T Phase Transitional Metal Dichalcogenides Generated via Electrochemical Li Intercalation

Engineered Two-Dimensional Transition Metal Dichalcogenides for Energy Conversion and Storage

Designing efficient and cost-effective materials is pivotal to solving the key scientific and technological challenges at the interface of energy, environment, and sustainability for achieving NetZero. Two-dimensional transition metal dichalcogenides (2D TMDs) represent a unique class of materials that have catered to a myriad of energy conversion and storage (ECS) applications. Their uniqueness arises from their ultra-thin nature, high fractions of atoms residing on surfaces, rich chemical compositions featuring diverse metals and chalcogens, and remarkable tunability across multiple length scales. Specifically, the rich electronic/electrical, optical, and thermal properties of 2D TMDs have been widely exploited for electrochemical energy conversion (e.g., electrocatalytic water splitting), and storage (e.g., anodes in alkali ion batteries and supercapacitors), photocatalysis, photovoltaic devices, and thermoelectric applications. Furthermore, their properties and performances can be greatly boosted by judicious structural and chemical tuning through phase, size, composition, defect, dopant, topological, and heterostructure engineering. The challenge, however, is to design and control such engineering levers, optimally and specifically, to maximize performance outcomes for targeted applications. In this review we discuss, highlight, and provide insights on the significant advancements and ongoing research directions in the design and engineering approaches of 2D TMDs for improving their performance and potential in ECS applications.

2H-2M Phase Control of WSe2 Nanosheets by Se Enrichment Toward Enhanced Electrocatalytic Hydrogen Evolution Reaction

The phase control of transition metal dichalcogenides (TMDs) is an intriguing approach for tuning the electronic structure toward extensive applications. In this study, WSe2 nanosheets synthesized via a colloidal reaction exhibit a phase conversion from semiconducting 2H to metallic 2M under Se-rich growth conditions (i.e., increasing the concentration of Se precursor or lowering the growth temperature). High-resolution scanning transmission electron microscopy images are used to identify the stacking sequence of the 2M phase, which is distinctive from that of the 1T' phase. First-principles calculations employing various Se-rich models (intercalation and substitution) indicated that Se enrichment induces conversion to the 2M phase. The 2M phase WSe2 nanosheets with the Se excess exhibited enhanced electrocatalytic performance in the hydrogen evolution reaction (HER). In situ X-ray absorption fine structure studies suggested that the excess Se atoms in the 2M phase WSe2 enhanced the HER catalytic activity, which is supported by the Gibbs free energy (ΔGH* ) of H adsorption and the Fermi abundance function. These results provide an appealing strategy for phase control of TMD catalysts.

Coexisting Phases in Transition Metal Dichalcogenides: Overview, Synthesis, Applications, and Prospects

Over the past decade, significant advancements have been made in phase engineering of two-dimensional transition metal dichalcogenides (TMDCs), thereby allowing controlled synthesis of various phases of TMDCs and facile conversion between them. Recently, there has been emerging interest in TMDC coexisting phases, which contain multiple phases within one nanostructured TMDC. By taking advantage of the merits from the component phases, the coexisting phases offer enhanced performance in many aspects compared with single-phase TMDCs. Herein, this review article thoroughly expounds the latest progress and ongoing efforts on the syntheses, properties, and applications of TMDC coexisting phases. The introduction section overviews the main phases of TMDCs (2H, 3R, 1T, 1T', 1Td), along with the advantages of phase coexistence. The subsequent section focuses on the synthesis methods for coexisting phases of TMDCs, with particular attention to local patterning and random formations. Furthermore, on the basis of the versatile properties of TMDC coexisting phases, their applications in magnetism, valleytronics, field-effect transistors, memristors, and catalysis are discussed. Lastly, a perspective is presented on the future development, challenges, and potential opportunities of TMDC coexisting phases. This review aims to provide insights into the phase engineering of 2D materials for both scientific and engineering communities and contribute to further advancements in this emerging field.

Phase-Engineered WS2 Monolayer Quantum Dots by Rhenium Doping

Transition metal dichalcogenides (TMDs) occur in the thermodynamically stable trigonal prismatic (2H) phase or the metastable octahedral (1T) phase. Phase engineering of TMDs has proven to be a powerful tool for applications in energy storage devices as well as in electrocatalysis. However, the mechanism of the phase transition in TMDs and the synthesis of phase-controlled TMDs remain challenging. Here we report the synthesis of Re-doped WS2 monolayer quantum dots (MQDs) using a simple colloidal chemical process. We find that the incorporation of a small amount of electron-rich Re atoms in WS2 changes the metal-metal distance in the 2H phase initially, which introduces strain in the structure (strained 2H (S2H) phase). Increasing the concentration of Re atoms sequentially transforms the S2H phase into the 1T and 1T' phases to release the strain. In addition, we performed controlled experiments by doping MoS2 with Re to distinguish between Re and Mo atoms in scanning transmission electron microscopy images and quantified the concentration range of Re atoms in each phase of MoS2, indicating that phase engineering of WS2 or MoS2 is possible by doping with different amounts of Re atoms. We demonstrate that the 1T' WS2 MQDs with 49 at. % Re show superior catalytic performance (a low Tafel slope of 44 mV/dec, a low overpotential of 158 mV at a current density of 10 mA/cm2, and long-term durability up to 5000 cycles) for the hydrogen evolution reaction. Our findings provide understanding and control of the phase transitions in TMDs, which will allow for the efficient manufacturing and translation of phase-engineered TMDs.

Regulatable Phase Manipulation-Enhanced Polarization and Conductance Loss Enabling Hierarchical 3D Microsphere-like MoS2 with Efficient Microwave Absorption

Molybdenum disulfide (MoS2) has become a new type of microwave absorption (MA) material due to the abundant functional groups and defects, high polarization effect, and controllable structural design. However, the development of MoS2 has been limited by its inherently low conductance losses and imperfect impedance matching. This study employs ammonium ion (NH4+) intercalation as a phase manipulation strategy to enhance dielectric loss and form heterogeneous structures by incorporating highly conductive 1T phase into the 2H-MoS2 crystal phase. Additionally, the implementation of CTAB as a soft template agent for constructing layered three-dimensional microsphere structures improves impedance matching. The experimental findings demonstrate that the MA performance of MoS2 can be effectively regulated by controlling the 1T phase content and morphological structure design. It is worth noting that A-MoS2-2 possesses excellent multifrequency absorption capability. A-MoS2-2 has a minimum reflection loss (RL) of -53 dB at a coating thickness of 1.99 mm and an effective absorption bandwidth (EAB) of 5.6 GHz at a thinner coating thickness of 1.77 mm. This work improves the MA properties of MoS2 by introducing metallic phases and unique structural design, which opens up new ideas for the development of MA materials.

Recent advances in defect-engineered molybdenum sulfides for catalytic applications

Electrochemical energy conversion and storage driven by renewable energy sources is drawing ever-increasing interest owing to the needs of sustainable development. Progress in the related electrochemical reactions relies on highly active and cost-effective catalysts to accelerate the sluggish kinetics. A substantial number of catalysts have been exploited recently, thanks to the advances in materials science and engineering. In particular, molybdenum sulfide (MoSx) furnishes a classic platform for studying catalytic mechanisms, improving catalytic performance and developing novel catalytic reactions. Herein, the recent theoretical and experimental progress of defective MoSx for catalytic applications is reviewed. This article begins with a brief description of the structure and basic catalytic applications of MoS2. The employment of defective two-dimensional and non-two-dimensional MoSx catalysts in the hydrogen evolution reaction (HER) is then reviewed, with a focus on the combination of theoretical and experimental tools for the rational design of defects and understanding of the reaction mechanisms. Afterward, the applications of defective MoSx as catalysts for the N2 reduction reaction, the CO2 reduction reaction, metal-sulfur batteries, metal-oxygen/air batteries, and the industrial hydrodesulfurization reaction are discussed, with a special emphasis on the synergy of multiple defects in achieving performance breakthroughs. Finally, the perspectives on the challenges and opportunities of defective MoSx for catalysis are presented.

Efficient acidic hydrogen evolution in proton exchange membrane electrolyzers over a sulfur-doped marcasite-type electrocatalyst

Large-scale deployment of proton exchange membrane (PEM) water electrolyzers has to overcome a cost barrier resulting from the exclusive adoption of platinum group metal (PGM) catalysts. Ideally, carbon-supported platinum used at cathode should be replaced with PGM-free catalysts, but they often undergo insufficient activity and stability subjecting to corrosive acidic conditions. Inspired by marcasite existed under acidic environments in nature, we report a sulfur doping-driven structural transformation from pyrite-type cobalt diselenide to pure marcasite counterpart. The resultant catalyst drives hydrogen evolution reaction with low overpotential of 67 millivolts at 10 milliamperes per square centimeter and exhibits no degradation after 1000 hours of testing in acid. Moreover, a PEM electrolyzer with this catalyst as cathode runs stably over 410 hours at 1 ampere per square centimeter and 60°C. The marked properties arise from sulfur doping that not only triggers formation of acid-resistant marcasite structure but also tailors electronic states (e.g., work function) for improved hydrogen diffusion and electrocatalysis.

One-Step Synthesis of Transition Metal Dichalcogenide Quantum Dots Using Only Alcohol Solvents for Indoor-Light Photocatalytic Antibacterial Activity

In this study, we report a one-step direct synthesis of molybdenum disulfide (MoS₂) and tungsten disulfide (WS₂) quantum dots (QDs) through a solvothermal reaction using only alcohol solvents and efficient Escherichia coli (E. coli) decompositions as photocatalytic antibacterial agents under visible light irradiation. The solvothermal reaction gives the scission of molybdenum-sulfur (Mo-S) and tungsten-sulfur (W-S) bonding during the synthesis of MoS₂ and WS₂ QDs. Using only alcohol solvent does not require a residue purification process necessary for metal intercalation. As the number of the CH₃ groups of alcohol solvents among ethyl, isopropyl, and tert(t)-butyl alcohols increases, the dispersibility of MoS₂/WS₂ increases. The CH₃ groups of alcohols minimize the surface energy, leading to the effective exfoliation and disintegration of the bulk under heat and pressure. The bulky t-butyl alcohol with the highest number of methyl groups shows the highest exfoliation and yield. MoS₂ QDs with a lateral size of about 2.5 nm and WS₂ QDs of about 10 nm are prepared, exhibiting a strong blue luminescence under 365 nm ultraviolet (UV) light irradiation. Their heights are 0.68-3 and 0.72-5 nm, corresponding to a few layers of MoS₂ and WS₂, respectively. They offer a highly efficient performance in sterilizing E. coli as the visible-light-driven photocatalyst.

Ex Situ Characterization of 1T/2H MoS2 and Their Carbon Composites for Energy Applications, a Review

The growing interest in the development of next-generation net zero energy systems has led to the expansion of molybdenum disulfide (MoS₂) research in this area. This activity has resulted in a wide range of manufacturing/synthesis methods, controllable morphologies, diverse carbonaceous composite structures, a multitude of applicable characterization techniques, and multiple energy applications for MoS₂. To assess the literature trends, 37,347 MoS₂ research articles from Web of Science were text scanned to classify articles according to energy application research and characterization techniques employed. Within the review, characterization techniques are grouped under the following categories: morphology, crystal structure, composition, and chemistry. The most common characterization techniques identified through text scanning are recommended as the base fingerprint for MoS₂ samples. These include: scanning electron microscopy (SEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and Raman spectroscopy. Similarly, XPS and Raman spectroscopy are suggested for 2H or 1T MoS₂ phase confirmation. We provide guidance on the collection and presentation of MoS₂ characterization data. This includes how to effectively combine multiple characterization techniques, considering the sample area probed by each technique and their statistical significance, and the benefit of using reference samples. For ease of access for future experimental comparison, key numeric MoS₂ characterization values are tabulated and major literature discrepancies or currently debated characterization disputes are highlighted.

1T-MoS2 Enriched Hierarchical MoS2 /MoO3 Produced by Phase Transformation for Efficient Hydrogen Evolution Reaction

In recent years, transition metal sulfides have been widely studied in the context of their use as electrocatalysts. The electrocatalytic propensity of the classical semiconductor MoS₂ , which exists in the 1T and 2H phase structures, has attracted extensive attention. Therefore, the synthesis of highly active and stable MoS₂ -based catalysts has become the goal of many research efforts. We recently developed a method that can be utilized to prepare the MoS₂ /MoO₃ heterojunction in a phase-controlled manner. 1T-MoS₂ phase enriched MoS₂ /MoO₃ heterojunction can be generated using a simple hydrothermal and acid treatment sequence and that the heterojunction has a unique three-dimensional structure, large active surface area, and therefore achieve a low overpotential and high catalytic current density, as well as long-term stability for the hydrogen evolution reaction.

Modulation of Surface Ti-O Species in 2D-Ti3C2TX MXene for Developing a Highly Efficient Electrocatalyst for Hydrogen Evolution and Methanol Oxidation Reactions

Developing cost-effective and earth-abundant noble-metal-free electrocatalysts is imperative for the imminent electrochemical society. Two-dimensional Ti₃C₂T_X (MXene) exhibits tunable properties with high electrical conductivity and a large specific surface area, which improve its electrochemical performance. Herein, the low-temperature annealing method is used to enrich MXene with a maximum number of Ti-O terminals without formation of titanium dioxide (TiO₂) under neutral pH conditions. MXene annealed at 200 °C is found to have a large number of Ti-O termination groups, resulting in a large electrochemically active surface area and increased active sites (-O termination groups) and hence excellent electrocatalytic performance compared to other samples as well as previous reported work. The optimized sample is found to show the lowest overpotential value of 0.07 V at 10 mA cm^-2 and a Tafel slope of 0.15 V dec^-1 toward the hydrogen evolution reaction (HER), whereas for the methanol oxidation reaction (MOR), the current density is 18.08 mA cm^-2, and the onset potential is -0.51 V. In addition, it also shows long-term stability and durability toward HER as well as MOR.

Suppress Loss of Polysulfides in Lithium-Sulfur Battery by Regulating the Rate-Determining Step via 1T MoS2-MnO2 Covering Layer

The commercialization of lithium-sulfur batteries (LSBs) is obstructed by several technical challenges, the most severe of which is the irreversible loss of soluble polysulfide intermediates. These soluble polysulfides must be anchored or confined in the cathode side to maintain the long life of the LSBs. Here, 1T MoS₂-MnO₂/CC heterostructure functional covering layer is designed to regulate the rate-determining step from the liquid-to-solid reaction to solid-to-solid reaction. Rapid and uniform nucleation of solid Li₂S₂/Li₂S is therefore achieved, and the loss of soluble polysulfides is retarded. The Li-S batteries assembled with 1T MoS₂-MnO₂/CC covering layer therefore deliver outstanding rate capabilities even under high sulfur loads and large current rates. This study paves a novel way to suppress the polysulfides' "farewell effect" from the perspective of the kinetics.

Recent progress of electrochemical hydrogen evolution over 1T-MoS2 catalysts

Developing efficient and stable non-noble metal catalysts for the electrocatalytic hydrogen evolution reaction (HER) is of great significance. MoS₂ has become a promising alternative to replace Pt-based electrocatalysts due to its unique layered structure and adjustable electronic property. However, most of the reported 2H-MoS₂ materials are stable, but the catalytic activity is not very ideal. Therefore, a series of strategies such as phase modulation, element doping, defect engineering, and composite modification have been developed to improve the catalytic performance of MoS₂ in the HER. Among them, phase engineering of 2H-MoS₂ to 1T-MoS₂ is considered to be the most effective strategy for regulating electronic properties and increasing active sites. Hence, in this mini-review, the common phase modulation strategies, characterization methods, and application of 1T-MoS₂ in the HER were systematically summarized. In addition, some challenges and future directions are also proposed for the design of efficient and stable 1T-MoS₂ HER catalysts. We hope this mini-review will be helpful to researchers currently working in or about to enter the field.

Unveiling the relationship between the multilayer structure of metallic MoS2 and the cycling performance for lithium ion batteries

Molybdenum disulfide (MoS₂) with a layered structure is a desirable substitute for the graphite anode in lithium ion storage. Compared with the semiconducting phase (2H-MoS₂), the metallic polymorph (1T-MoS₂) usually shows much better cycling stability. Nevertheless, the origin of this remarkable cycling stability is still ambiguous, hindering further development of MoS₂-based anodes. Herein, we assembled multilayered 1T-MoS₂ nanosheets directly on Ti foil to investigate the Li⁺ storage mechanism. Based on experimental observation and computational simulation, we found that the cycling stability correlates with the layer number of MoS₂. Multilayered 1T-MoS₂ can accommodate inserted Li⁺ in a ternary compound Li-Mo-S through a reversible reaction, which is favorable for retaining a substantial number of MoS₂ nanodomains upon Li intercalation. These residual MoS₂ nanodomains can serve as an anchor to adhere Li_xS species, thereby suppressing the "shuttle effect" of polysulfides and enhancing cycling stability. This work sheds light on the development of high-performance anodes based on metallic MoS₂ for LIBs.

Two-dimensional quantum-sheet films with sub-1.2 nm channels for ultrahigh-rate electrochemical capacitance

Dense, thick, but fast-ion-conductive electrodes are critical yet challenging components of ultrafast electrochemical capacitors with high volumetric power/energy densities^1-4. Here we report an exfoliation-fragmentation-restacking strategy towards thickness-adjustable (1.5‒24.0 μm) dense electrode films of restacked two-dimensional 1T-MoS₂ quantum sheets. These films bear the unique architecture of an exceptionally high density of narrow (sub-1.2 nm) and ultrashort (~6.1 nm) hydrophobic nanochannels for confinement ion transport. Among them, 14-μm-thick films tested at 2,000 mV s^-1 can deliver not only a high areal capacitance of 0.63 F cm^-2 but also a volumetric capacitance of 437 F cm^-3 that is one order of magnitude higher than that of other electrodes. Density functional theory and ab initio molecular dynamics simulations suggest that both hydration and nanoscale channels play crucial roles in enabling ultrafast ion transport and enhanced charge storage. This work provides a versatile strategy for generating rapid ion transport channels in thick but dense films for energy storage and filtration applications.

Nanodots Derived from Layered Materials: Synthesis and Applications

Layered 2D materials, such as graphene, transition metal dichalcogenides, transition metal oxides, black phosphorus, graphitic carbon nitride, hexagonal boron nitride, and MXenes, have attracted intensive attention over the past decades owing to their unique properties and wide applications in electronics, catalysis, energy storage, biomedicine, etc. Further reducing the lateral size of layered 2D materials down to less than 10 nm allows for preparing a new class of nanostructures, namely, nanodots derived from layered materials. Nanodots derived from layered materials not only can exhibit the intriguing properties of nanodots due to the size confinement originating from the ultrasmall size, but also can inherit some unique properties of ultrathin layered 2D materials, making them promising candidates in a wide range of applications, especially in biomedicine and catalysis. Here, a comprehensive summary on the materials categories, advantages, synthesis methods, and potential applications of these nanodots derived from layered materials is provided. Finally, personal insights about the challenges and future directions in this promising research field are also given.

A 2H-MoS2/carbon cloth composite for high-performance all-solid-state supercapacitors derived from a molybdenum dithiocarbamate complex

A molecular complex Mo₂O₂(μ-S)₂(Et₂dtc)₂ (dtc = dithiocarbamate) is prepared and loaded onto carbon cloth (CC) through facile solvothermal treatment, followed by subsequent single-source pyrolysis. This results in a highly porous 2H-MoS₂/CC composite with a sponge-like stacked lamellar morphology. Due to its high porosity and unique nano/microstructure, the MoS₂/CC composite exhibits a specific capacitance of 550.0 F g^-1 at 1 A g^-1, outperforming some 1T-MoS₂ based electrodes. The composite is further assembled into a symmetric all-solid-state supercapacitor, which can be operated stably at a wide potential window and shows a specific capacitance of 127.5 F g^-1 at 1 A g^-1. In addition, the device delivers a high energy density of 70.8 W h kg^-1 at 1 kW kg^-1, which still remains 15.0 W h kg^-1 at 18.0 kW kg^-1. 75% of the performance of the device can be retained after 8000 cycles. Such remarkable electrochemical performance is attributed to its novel nano/microstructures with a large surface area, convenient ion transport pathways, enhanced conductivity, and improved structural stability. Thus, this work demonstrates a highly promising dithiocarbamate-based single-precursor pyrolysis route towards the fabrication of metal sulfides/carbon composites for energy storage applications.

Recent Advances on Transition Metal Dichalcogenides for Electrochemical Energy Conversion

Transition metal dichalcogenides (TMDCs) hold great promise for electrochemical energy conversion technologies in view of their unique structural features associated with the layered structure and ultrathin thickness. Because the inert basal plane accounts for the majority of a TMDC's bulk, activation of the basal plane sites is necessary to fully exploit the intrinsic potential of TMDCs. Here, recent advances on TMDCs-based hybrids/composites with greatly enhanced electrochemical activity are reviewed. After a summary of the synthesis of TMDCs with different sizes and morphologies, comprehensive in-plane activation strategies are described in detail, mainly including in-plane-modification-induced phase transformation, surface-layer modulation, and interlayer modification/coupling. Simultaneously, the underlying mechanisms for improved electrochemical activities are highlighted. Finally, the strategic evaluation on further research directions of TMDCs in-plane activation is featured. This work would shed some light on future design trends of TMDCs-based functional materials for electrochemical energy-related applications.

Enhanced Valley Splitting in Monolayer WSe2 by Phase Engineering

Lifting the valley degeneracy in two-dimensional transition metal dichalcogenides could promote their applications in information processing. Various external regulations, including magnetic substrate, magnetic doping, electric field, and carrier doping, have been implemented to enhance the valley splitting under the magnetic field. Here, a phase engineering strategy, through modifying the intrinsic lattice structure, is proposed to enhance the valley splitting in monolayer WSe₂. The valley splitting in hybrid H and T phase WSe₂ is tunable by the concentration of the T phase. An obvious valley splitting of ∼4.1 meV is obtained with the T phase concentration of 31% under ±5 T magnetic fields, which corresponds to an effective Landé g_eff factor of -14, about 3.5-fold of that in pure H-WSe₂. Comparing the temperature and magnetic field dependent polarized photoluminescence and also combining the theoretical simulations reveal the enhanced valley splitting is dominantly attributed to exchange interaction of H phase WSe₂ with the local magnetic moments induced by the T phase. This finding provides a convenient solution for lifting the valley degeneracy of two-dimensional materials.

Transition Metal Chalcogenides as a Versatile and Tunable Platform for Catalytic CO2 and N2 Electroreduction

Group VI transition metal chalcogenides are the subject of increasing research interest for various electrochemical applications such as low-temperature water electrolysis, batteries, and supercapacitors due to their high activity, chemical stability, and the strong correlation between structure and electrochemical properties. Particularly appealing is their utilization as electrocatalysts for the synthesis of energy vectors and value-added chemicals such as C-based chemicals from the CO₂ reduction reaction (CO₂R) or ammonia from the nitrogen fixation reaction (NRR). This review discusses the role of structural and electronic properties of transition metal chalcogenides in enhancing selectivity and activity toward these two key reduction reactions. First, we discuss the morphological and electronic structure of these compounds, outlining design strategies to control and fine-tune them. Then, we discuss the role of the active sites and the strategies developed to enhance the activity of transition metal chalcogenide-based catalysts in the framework of CO₂R and NRR against the parasitic hydrogen evolution reaction (HER); leveraging on the design rules applied for HER applications, we discuss their future perspective for the applications in CO₂R and NRR. For these two reactions, we comprehensively review recent progress in unveiling reaction mechanisms at different sites and the most effective strategies for fabricating catalysts that, by exploiting the structural and electronic peculiarities of transition metal chalcogenides, can outperform many metallic compounds. Transition metal chalcogenides outperform state-of-the-art catalysts for CO₂ to CO reduction in ionic liquids due to the favorable CO₂ adsorption on the metal edge sites, whereas the basal sites, due to their conformation, represent an appealing design space for reduction of CO₂ to complex carbon products. For the NRR instead, the resemblance of transition metal chalcogenides to the active centers of nitrogenase enzymes represents a powerful nature-mimicking approach for the design of catalysts with enhanced performance, although strategies to hinder the HER must be integrated in the catalytic architecture.

Metallic Transition Metal Dichalcogenides of Group VIB: Preparation, Stabilization, and Energy Applications

Layered transition metal dichalcogenides (TMDs) of group VIB have been widely used in the realms of energy storage and conversions. Along with the existence of semiconducting states, their metallic phases have recently attracted numerous attentions owing to their fascinating physical and chemical properties. Many efforts have been devoted to obtain metallic TMDs with high purity and yield. Nevertheless, such metallic phase is thermodynamically metastable and tends to convert into semiconducting phase, which necessitates the exploration over effective strategies to ensure the stability. In this review, typical fabrication routes are introduced and those critical factors during preparation are elaborately discussed. Moreover, the stabilized strategies are summarized with concrete examples highlighting the key mechanisms toward efficient stabilization. Finally, emerging energy applications are overviewed. This review presents comprehensive research status of metallic group VIB TMDs, aiming to facilitate further scientific investigations and promote future practical applications in the fields of energy storage and conversion.

Sulfur defect rich Mo-Ni3S2 QDs assisted by O-C[double bond, length as m-dash]O chemical bonding for an efficient electrocatalytic overall water splitting

Developing earth-abundant and highly efficient electrocatalysts is critical for further development of a system. The metal (M) doping strategy and inorganic/organic composite are two common strategies to improve the performance of electrocatalysts for overall water splitting (OWS). In this paper, two strategies are subtly used to prepare Mo-Ni₃S₂ quantum dots (QDs) with rich sulfur defects through Moⁿ⁺ doping Ni₃S₂ and introduction of trisodium citrate by a two-step hydrothermal reaction. Results show that high sulfur defects can be controllably prepared as the lattice mismatch and active sites can be efficiently increased via Moⁿ⁺ doping. Moreover, the introduction of trisodium citrate with carboxyl functional groups not only enhances the degree of sulfur defects around the metal center, changes the morphology of sulfide to distribute the active centers evenly, but also endow the metal center with strong valence changing ability with organic characteristics. The in situ Raman study reveals that O-C[double bond, length as m-dash]O promotes the formation of the real active site M-OOH by the way of self-sacrifice during the OER process. Mo-Ni₃S₂ QDelectrocatalyst shows excellent performance in the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER), achieving a current density of 10 mA cm^-2 at the overpotentials of 115 mV and 222 mV with very good chemical stability, superior than that of most of the reported materials. The OWS reaction can provide a current density of 10 mA cm^-2 and 50 mA cm^-2, which only needs 1.53 V and 1.74 V with excellent industrial application prospects.

One-step synthesis of single-site vanadium substitution in 1T-WS2 monolayers for enhanced hydrogen evolution catalysis

Metallic tungsten disulfide (WS₂) monolayers have been demonstrated as promising electrocatalysts for hydrogen evolution reaction (HER) induced by the high intrinsic conductivity, however, the key challenges to maximize the catalytic activity are achieving the metallic WS₂ with high concentration and increasing the density of the active sites. In this work, single-atom-V catalysts (V SACs) substitutions in 1T-WS₂ monolayers (91% phase purity) are fabricated to significantly enhance the HER performance via a one-step chemical vapor deposition strategy. Atomic-resolution scanning transmission electron microscopy (STEM) imaging together with Raman spectroscopy confirm the atomic dispersion of V species on the 1T-WS₂ monolayers instead of energetically favorable 2H-WS₂ monolayers. The growth mechanism of V SACs@1T-WS₂ monolayers is experimentally and theoretically demonstrated. Density functional theory (DFT) calculations demonstrate that the activated V-atom sites play vital important role in enhancing the HER activity. In this work, it opens a novel path to directly synthesize atomically dispersed single-metal catalysts on metastable materials as efficient and robust electrocatalysts.

Ru is one of the most active metals for oxygen evolution reaction, but it quickly dissolves in acidic electrolyte particularly in nanosized form. Here the authors show that coral-like solid-solution Ru‒Ir consisting of 3 nm-thick sheets with only 6 at% Ir is a long-lived catalyst with high activity

Toward Rapid-Charging Sodium-Ion Batteries using Hybrid-Phase Molybdenum Sulfide Selenide-Based Anodes

To attain both high energy density and power density in sodium-ion (Na⁺ ) batteries, the reaction kinetics and structural stability of anodes should be improved by materials optimization. In this work, few-layered molybdenum sulfide selenide (MoSSe) consisting of a mixture of 1T and 2H phases is designed to provide high ionic/electrical conductivities, low Na⁺ diffusion barrier, and stable Na⁺ storage. Reduced graphene oxide (rGO) is used as a conductive matrix to form 3D electron transfer paths. The resulting MoSSe@rGO anode exhibits high capacity and rate performance in both organic and solid-state electrolytes. The ultrafast Na⁺ storage kinetics of the MoSSe@rGO anode is attributed to the surface-dominant reaction process and broad Na⁺ channels. In situ and ex situ measurements are conducted to reveal the Na⁺ storage process in MoSSe@rGO. It is found that the MoS and MoSe bonds effectively limit the dissolution of the active materials. The favorable Na⁺ storage kinetics of the MoSSe@rGO electrode are ascribed to its low adsorption energy of -1.997 eV and low diffusion barrier of 0.087 eV. These results reveal that anion doping of metal sulfides is a feasible strategy to develop sodium-ion batteries with high energy and power densities and long life-span.

Confinement Effect of Mesopores: In Situ Synthesis of Cationic Tungsten-Vacancies for a Highly Ordered Mesoporous Tungsten Phosphide Electrocatalyst

Engineering defects in crystalline electrocatalysts is an effective approach to tailor the electronic structure and number of active sites, which are essential for the intrinsic activity of the hydrogen evolution reaction (HER). Unlike previously reported methods, we demonstrate a confinement effect using a mesoporous template for in situ fabrication of cationic W vacancies in as-prepared ordered mesoporous tungsten phosphide (WP) nanostructures by adjusting the nonstoichiometric ratio of the precursor elements. With a plenty of W vacancies and ordered mesoporosity, the as-prepared catalyst WP-Mesop exhibits better catalytic performance than the catalysts without mesopores and/or vacancies. The WP-Mesop shows an ultralow overpotential of 175 mV in acid and 229 mV in alkaline at 100 mA cm^-2 and stability of 48 h without structural collapse in both acid and alkaline media. Meanwhile, density functional theory calculations further reveal that the activation barrier for HER can be lowered by introducing cationic W vacancies. This strategy can be extended to generate cationic defects in other transition metal phosphides to improve their HER activities.

Ruthenium Nanoparticles on Cobalt-Doped 1T' Phase MoS2 Nanosheets for Overall Water Splitting

2D MoS₂ nanostructures have recently attracted considerable attention because of their outstanding electrocatalytic properties. The synthesis of unique Co-Ru-MoS₂ hybrid nanosheets with excellent catalytic activity toward overall water splitting in alkaline solution is reported. 1T' phase MoS₂ nanosheets are doped homogeneously with Co atoms and decorated with Ru nanoparticles. The catalytic performance of hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) is characterized by low overpotentials of 52 and 308 mV at 10 mA cm^-2 and Tafel slopes of 55 and 50 mV decade^-1 in 1.0 m KOH, respectively. Analysis of X-ray photoelectron and absorption spectra of the catalysts show that the MoS₂ well retained its metallic 1T' phase, which guarantees good electrical conductivity during the reaction. The Gibbs free energy calculation for the reaction pathway in alkaline electrolyte confirms that the Ru nanoparticles on the Co-doped MoS₂ greatly enhance the HER activity. Water adsorption and dissociation take place favorably on the Ru, and the doped Co further catalyzes HER by making the reaction intermediates more favorable. The high OER performance is attributed to the catalytically active RuO₂ nanoparticles that are produced via oxidation of Ru nanoparticles.

Structural-Phase Catalytic Redox Reactions in Energy and Environmental Applications

The structure-property engineering of phase-based materials for redox-reactive energy conversion and environmental decontamination nanosystems, which are crucial for achieving feasible and sustainable energy and environment treatment technology, is discussed. An exhaustive overview of redox reaction processes, including electrocatalysis, photocatalysis, and photoelectrocatalysis, is given. Through examples of applications of these redox reactions, how structural phase engineering (SPE) strategies can influence the catalytic activity, selectivity, and stability is constructively reviewed and discussed. As observed, to date, much progress has been made in SPE to improve catalytic redox reactions. However, a number of highly intriguing, unresolved issues remain to be discussed, including solar photon-to-exciton conversion efficiency, exciton dissociation into active reductive/oxidative electrons/holes, dual- and multiphase junctions, selective adsorption/desorption, performance stability, sustainability, etc. To conclude, key challenges and prospects with SPE-assisted redox reaction systems are highlighted, where further development for the advanced engineering of phase-based materials will accelerate the sustainable (active, reliable, and scalable) production of valuable chemicals and energy, as well as facilitate environmental treatment.

Recent progress of TMD nanomaterials: phase transitions and applications

Transition metal dichalcogenides (TMDs) show wide ranges of electronic properties ranging from semiconducting, semi-metallic to metallic due to their remarkable structural differences. To obtain 2D TMDs with specific properties, it is extremely important to develop particular strategies to obtain specific phase structures. Phase engineering is a traditional method to achieve transformation from one phase to another controllably. Control of such transformations enables the control of properties and access to a range of properties, otherwise inaccessible. Then extraordinary structural, electronic and optical properties lead to a broad range of potential applications. In this review, we introduce the various electronic properties of 2D TMDs and their polymorphs, and strategies and mechanisms for phase transitions, and phase transition kinetics. Moreover, the potential applications of 2D TMDs in energy storage and conversion, including electro/photocatalysts, batteries/supercapacitors and electronic devices, are also discussed. Finally, opportunities and challenges are highlighted. This review may further promote the development of TMD phase engineering and shed light on other two-dimensional materials of fundamental interest and with potential ranges of applications.

Phototherapy with layered materials derived quantum dots

Quantum dots (QDs) originating from two-dimensional (2D) sheets of graphitic carbon nitride (g-C₃N₄), graphene, hexagonal boron nitride (h-BN), monoatomic buckled crystals (phosphorene), germanene, silicene and transition metal dichalcogenides (TMDCs) are emerging zero-dimensional materials. These QDs possess diverse optical properties, are chemically stable, have surprisingly excellent biocompatibility and are relatively amenable to surface modifications. It is therefore not difficult to see that these QDs have potential in a variety of bioapplications, including biosensing, bioimaging and anticancer and antimicrobial therapy. In this review, we briefly summarize the recent progress of these exciting QD based nanoagents and strategies for phototherapy. In addition, we will discuss about the current limitations, challenges and future prospects of QDs in biomedical applications.

Voltage issue of aqueous rechargeable metal-ion batteries

Over the past two decades, a series of aqueous rechargeable metal-ion batteries (ARMBs) have been developed, aiming at improving safety, environmental friendliness and cost-efficiency in fields of consumer electronics, electric vehicles and grid-scale energy storage. However, the notable gap between ARMBs and their organic counterparts in energy density directly hinders their practical applications, making it difficult to replace current widely-used organic lithium-ion batteries. Basically, this huge gap in energy density originates from cell voltage, as the narrow electrochemical stability window of aqueous electrolytes substantially confines the choice of electrode materials. This review highlights various ARMBs with focuses on their voltage characteristics and strategies that can effectively raise battery voltage. It begins with the discussion on the fundamental factor that limits the voltage of ARMBs, i.e., electrochemical stability window of aqueous electrolytes, which decides the maximum-allowed potential difference between cathode and anode. The following section introduces various ARMB systems and compares their voltage characteristics in midpoint voltage and plateau voltage, in relation to respective electrode materials. Subsequently, various strategies paving the way to high-voltage ARMBs are summarized, with corresponding advancements highlighted. The final section presents potential directions for further improvements and future perspectives of this thriving field.

Self-templated construction of 1D NiMo nanowires via a Li electrochemical tuning method for the hydrogen evolution reaction

NiMo based materials have been widely recognized as the most promising alternatives to noble Pt electrocatalysts used in alkaline electrolytes for the hydrogen evolution reaction. However, it is difficult to construct a nanostructure, especially 1D morphology, for NiMo materials via an electrochemical method. Herein, a novel Li electrochemical tuning method, for the first time, is introduced to synthesize 1D NiMo nanowires by insertion of lithium ions into parent NiMoO₄ nanorods. The as-prepared NiMo catalyst exhibits high HER activity in 1 M KOH, in terms of low overpotential (73 mV) at a current density of 10 mA cm^-2 and a small Tafel slope (37.2 mV dec^-1) and charge transfer resistance (11.3 Ω). Furthermore, no decay in catalytic performance is observed for this material when it is operated at -0.125 V (vs. RHE) for 1250 min and a high Faraday efficiency (96%) is achieved. The high activity of NiMo is ascribed to the synergistic interplay between Ni and Mo and its unique nanostructure, which can expose more active sites and facilitate the mass transfer and hydrogen bubble release.

The Origin of High Activity of Amorphous MoS2 in the Hydrogen Evolution Reaction

Molybdenum disulfide (MoS₂ ) and related transition metal chalcogenides can replace expensive precious metal catalysts such as Pt for the hydrogen evolution reaction (HER). The relations between the nanoscale properties and HER activity of well-controlled 2H and Li-promoted 1T phases of MoS₂ , as well as an amorphous MoS₂ phase, have been investigated and a detailed comparison is made on Mo-S and Mo-Mo bond analysis under operando HER conditions, which reveals a similar bond structure in 1T and amorphous MoS₂ phases as a key feature in explaining their increased HER activity. Whereas the distinct bond structure in 1T phase MoS₂ is caused by Li⁺ intercalation and disappears under harsh HER conditions, amorphous MoS₂ maintains its intrinsic short Mo-Mo bond feature and, with that, its high HER activity. Quantum-chemical calculations indicate similar electronic structures of small MoS₂ clusters serving as models for amorphous MoS₂ and the 1T phase MoS₂ , showing similar Gibbs free energies for hydrogen adsorption (ΔG_H* ) and metallic character.

Two-dimensional MoS2/Fe-phthalocyanine hybrid nanostructures as excellent electrocatalysts for hydrogen evolution and oxygen reduction reactions

Two-dimensional (2D) MoS2 nanostructures have been extensively investigated in recent years because of their fascinating electrocatalytic properties. Herein, we report 2D hybrid nanostructures consisting of 1T' phase MoS2 and Fe-phthalocyanine (FePc) molecules that exhibit excellent catalytic activity toward both the hydrogen evolution reaction (HER) and oxygen reduction reaction (ORR). X-ray absorption spectra revealed an increased Fe-N distance (2.04 Å) in the hybrid complex relative to the isolated FePc. Spin-polarized density functional theory calculations predicted that the Fe center moves toward the MoS2 layer and induces a non-planar structure with an increased Fe-N distance of 2.05 Å, which supports the experimental results. The experiments and calculations consistently show a significant charge transfer from FePc to stabilize the hybrid complex. The excellent HER catalytic performance of FePc-MoS2 is characterized by a low Tafel slope of 32 mV dec-1 at a current density of 10 mA cm-2 and an overpotential of 0.123 V. The ORR catalytic activity is superior to that of the commercial Pt/C catalyst in pH 13 electrolyte, with a more positive half-wave potential (0.89 vs. 0.84 V), a smaller Tafel slope (35 vs. 87 mV·dec-1), and a much better durability (9.3% vs. 40% degradation after 20 h). Such remarkable catalytic activity is ascribed to the HER-active 1T' phase MoS2 and the ORR-active nonplanar Fe-N4 site of FePc.

Laser-sculptured ultrathin transition metal carbide layers for energy storage and energy harvesting applications

Ultrathin transition metal carbides with high capacity, high surface area, and high conductivity are a promising family of materials for applications from energy storage to catalysis. However, large-scale, cost-effective, and precursor-free methods to prepare ultrathin carbides are lacking. Here, we demonstrate a direct pattern method to manufacture ultrathin carbides (MoC_x, WC_x, and CoC_x) on versatile substrates using a CO₂ laser. The laser-sculptured polycrystalline carbides (macroporous, ~10–20 nm wall thickness, ~10 nm crystallinity) show high energy storage capability, hierarchical porous structure, and higher thermal resilience than MXenes and other laser-ablated carbon materials. A flexible supercapacitor made of MoC_x demonstrates a wide temperature range (−50 to 300 °C). Furthermore, the sculptured microstructures endow the carbide network with enhanced visible light absorption, providing high solar energy harvesting efficiency (~72 %) for steam generation. The laser-based, scalable, resilient, and low-cost manufacturing process presents an approach for construction of carbides and their subsequent applications.

Transition metal carbides are attractive for electrochemical energy storage and catalysis, but cost effective preparation on a large scale is challenging. Here the authors use a direct pattern method to fabricate transition metal carbides for supercapacitors and solar energy harvesting for steam generation.

Two dimensional MoS2 meets porphyrins via intercalation to enhance the electrocatalytic activity toward hydrogen evolution

Two-dimensional MoS₂ meets porphyrin molecules to form unique 1T' phase intercalated complexes via a one-step procedure of hydrothermal reactions. The resultant Mn-porphyrin-MoS₂ exhibits excellent electrocatalytic activity toward the hydrogen evolution reaction, with a Tafel slope of 35 mV dec^-1 and 10 mA cm^-2 at an overpotential of 0.125 V. Spin-polarized density functional theory calculations confirmed that the intercalation of Mn-porphyrin into 1T'-MoS₂ is quite favourable due to strong charge transfer from Mn metals. Their outstanding catalytic performance could be ascribed to the high electron concentration as well as the low activation barrier of the Heyrovsky reaction.

Highly Ambient-Stable 1T-MoS2 and 1T-WS2 by Hydrothermal Synthesis under High Magnetic Fields

The phase-controlled synthesis of metallic and ambient-stable 2D MX₂ (M is Mo or W; X is S) with 1T octahedral coordination will endow these materials with superior performance compared with their semiconducting 2H coordination counterparts. We report a clean and facile route to prepare 1T-MoS₂ and 1T-WS₂ through hydrothermal processing under high magnetic fields. We reveal that the as-synthesized 1T-MoS₂ and 1T-WS₂ are ambient-stable for more than 1 year. Electrochemical measurements show that 1T-MoS₂ performs much better than 2H-MoS₂ as the anode for sodium ion batteries. These results can provide a clean and facile method to prepare ambient-stable 1T-phase MX₂.

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.

Edge, size, and shape effects on WS2, WSe2, and WTe2 nanoflake stability: design principles from an ab initio investigation

The magic nanoflakes, obtained by the evaluation of the relative stability function, are n = 9 and 14 for all chemical compositions, whereas n = 12 is a magic number for WS₂ and WSe₂.

Self-Assembly of Large-Area 2D Polycrystalline Transition Metal Carbides for Hydrogen Electrocatalysis

Low-dimensional (0/1/2 dimension) transition metal carbides (TMCs) possess intriguing electrical, mechanical, and electrochemical properties, and they serve as convenient supports for transition metal catalysts. Large-area single-crystalline 2D TMC sheets are generally prepared by exfoliating MXene sheets from MAX phases. Here, a versatile bottom-up method is reported for preparing ultrathin TMC sheets (≈10 nm in thickness and >100 μm in lateral size) with metal nanoparticle decoration. A gelatin hydrogel is employed as a scaffold to coordinate metal ions (Mo⁵⁺ , W⁶⁺ , Co²⁺ ), resulting in ultrathin-film morphologies of diverse TMC sheets. Carbonization of the scaffold at 600 °C presents a facile route to the corresponding MoC_x , WC_x , CoC_x , and to metal-rich hybrids (Mo_{2- x} W_x C and W/Mo₂ C-Co). Among these materials, the Mo₂ C-Co hybrid provides excellent hydrogen evolution reaction (HER) efficiency (Tafel slope of 39 mV dec^-1 and 48 mVj = 10 mA cm-2 in overpotential in 0.5 m H₂ SO₄ ). Such performance makes Mo₂ C-Co a viable noble-metal-free catalyst for the HER, and is competitive with the standard platinum on carbon support. This template-assisted, self-assembling, scalable, and low-cost manufacturing process presents a new tactic to construct low-dimensional TMCs with applications in various clean-energy-related fields.

Differentiating Polymorphs in Molybdenum Disulfide via Electron Microscopy

The presence of rich polymorphs and stacking polytypes in molybdenum disulfide (MoS₂ ) endows it with a diverse range of electrical, catalytic, optical, and magnetic properties. This has stimulated a lot of interest in the unique properties associated with each polymorph. Most techniques used for polymorph identification in MoS₂ are macroscopic techniques that sample averaged properties due to their limited spatial resolution. A reliable way of differentiating the atomic structure of different polymorphs is needed in order to understand their growth dynamics and establish the correlation between structure and properties. Herein, the use of electron microscopy for identifying the atomic structures of several important polymorphs in MoS₂ , some of which are the subjects of mistaken assignment in the literature, is discussed. In particular, scanning transmission electron microscopy-annular dark field imaging has emerged as the most effective and reliable approach for identifying the different phases in MoS₂ and other 2D materials because its images can be directly correlated to the atomic structures. Examples of the identification of polymorphs grown under different conditions in molecular beam epitaxy or chemical vapor deposition, for example, 3R, 1T, 1T'-phases, and 1T'-edges, are presented, including their atomic structures, fascinating properties, growth methods, and corresponding thermodynamic stabilities.