2H-1T Phase Research Articles

Hierarchical Spatial Confinement Unlocking the Storage Limit of MoS2 for Flexible High-Energy Supercapacitors.

Molybdenum sulfide (MoS2) is a promising electrode material for supercapacitors; however, its limited Mo/S edge sites and intrinsic inert basal plane give rise to sluggish active electronic states, thus constraining its electrochemical performance. Here we propose a hierarchical confinement strategy to develop ethylene molecule (EG)-intercalated Co-doped sulfur-deficient MoS2 (Co-EG/SV-MoS2) for efficient and durable K-ion storage. Theoretical analyses suggest that the intercalation-confined EG and lattice-confined Co can enhance the interfacial K-ion storage capacity while reducing the K-ion diffusion barrier. Experimentally, the intercalated EG molecules with mildly reducing properties induced the creation of sulfur vacancies, expanded the interlayer spacing, regulated the 2H-1T phase transition, and strengthened the structural grafting between layers, thereby facilitating ion diffusion and ensuring structural durability. Moreover, the Co dopants occupying the initial Mo sites initiated charge transfer, thus activating the basal plane. Consequently, the optimized Co-EG/SV-MoS2 electrode exhibited a substantially improved electrochemical performance. Flexible supercapacitors assembled with Co-EG/SV-MoS2 delivered a notable areal energy density of 0.51 mW h cm-2 at 0.84 mW cm-2 with good flexibility. Furthermore, supercapacitor devices were integrated with a strain sensor to create a self-powered system capable of real-time detection of human joint motion.

Non-stoichiometry induced 2H–1T phase interfaces and room-temperature ferromagnetism in defective molybdenum selenide

Magnetic semiconducting materials offer tremendous prospects for spin electronics but is challenging to achieve room-temperature ferromagnetism with unambiguous origin. Herein, a non-stoichiometry strategy is proposed to induce tunable magnetization in MoSe2−x nanoflowers via vacancy-controlled 2H–1T phase transition. The resultant MoSe2−x exhibits robust room-temperature ferromagnetism with significant positive correlation to the content of 1T phase and 2H–1T interfaces. Significant magnetic hysteresis and Curie transition above room temperature have been achieved, confirming the ferromagnetic feature of MoSe2−x. To examine the origin of ferromagnetism, formation energy and spin-polarized calculations have been conducted, indicating that the Se vacancy is beneficial for the formation of the 1T phase and interfacial spin polarization. Localized magnetic moments induced at the 2H–1T interfaces exhibit enhanced magnetism as compared to the net moments from the 1T orbital splitting, giving rise to strong coupling bound magnetic polarons. This work not only advances the understanding on the origin of magnetism in magnetic semiconductors, but also provides an effective route to generate ferromagnetism by defect and/or interface engineering that could be applied to multiferroics, spintronics, and valleytronics.

Rotation angle dependent Li intercalation and the induced phase transition in bilayer MoTe2

In this work, taking twisted bilayer MoTe2 (tb-MoTe2) as an example, density functional theory (DFT) calculations are done to investigate the interlayer rotation angle dependent lithium intercalation as well as the induced 2H → 1T phase transition in two-dimensional layered materials. Both static equipotential energy diagrams and dynamic ab initio molecular dynamics simulations (AIMD) are utilized to obtain the reliable lithium diffusion pathways and barriers. The results reveal that the interlayer rotation leads to increased interlayer separation in tb-MoTe2, and provides a stable environment for Li insertion. Moreover, the diffusion barrier is substantially reduced with increasing twist angle, promoting the fast Li diffusion. At room temperature, the mobility of Li in tb-MoTe2 with the twist angle of θ = 21.79˚ and θ = 60˚ is increased by a factor of 10 and 105, respectively. The energy barrier for the 2H → 1T phase transition is lowered from 1.23 eV in the non-twisted MoTe2 to 0.88 eV in the tb-MoTe2 with a twist angle θ = 21.79˚ for a given concentration of inserted Li. Above all, the enhanced diffusion and lower critical electron concentration facilitate the 2H-1T phase transition in tb-MoTe2. The results provide valuable insights into the non-uniform intercalation and present an opportunity to modulate the phase transition of twisted 2D systems.

Facile microwave plasma driven 3D-WSe2 2H-1T phase modulation for improving NO2 gas sensing performance

In recent years, transition metal dichalcogenides (TMDs) have been widely used for gas sensors. Here, three-dimensional (3D) WSe2 nanosheet arrays were surface treated by microwave plasma. Based on the original 3D structure, a 1T/2H hybrid phase structure was constructed by phase modulation, and Se vacancies were introduced to effectively improve its gas sensing performance. After only 60 s of treatment, the response (52.24 %), response/recovery time of the sample for 1 ppm NO2 were significantly improved with excellent stability and selectivity at room temperature. The intrinsic mechanism of its performance enhancement was elicited through various characterizations and molecular model construction. It is demonstrated that microwave plasma is a promising treatment method to improve the gas-sensitive performance of TMDs.

MoSe2 -VSe2 -NbSe2 Ternary Alloy Nanosheets to Boost Electrocatalytic Hydrogen Evolution Reaction.

Alloying of transition metal dichalcogenides (TMDs) is a pioneering method for engineering electronic structures with expanded applications. In this study, MoSe2 -VSe2 -NbSe2 ternary alloy nanosheets are synthesized via a colloidal reaction. The composition is successfully tuned over a wide range to adjust the 2H-1T phase transition. The alloy nanosheets consist of miscible atomic structures at all compositions, which is distinct from immiscible binary alloys. Compared to each binary alloy, the ternary alloys display higher electrocatalytic activity toward the hydrogen evolution reaction (HER) in an acidic electrolyte. The HER performance exhibits a volcano-type composition dependence, which is correlated with the experimental d-band center (εd ). Spin-polarized density functional theory (DFT) calculations consistently predict the homogenous atomic distributions. The Gibbs free energy of H adsorption (ΔGH* ) and the activation barrier (Ea ) support that miscible ternary alloying greatly enhances the HER performance.

WSe2-VSe2 Alloyed Nanosheets to Enhance the Catalytic Performance of Hydrogen Evolution Reaction.

Tuning the electronic structures of transition metal dichalcogenides (TMD) is essential for their implementation in next-generation energy technologies. In this study, we synthesized composition-tuned WSe2-VSe2 (W1-xVxSe2, x = 0-1) alloyed nanosheets using a colloidal reaction. Alloying the semiconducting WSe2 with VSe2 converts the material into a metallic one, followed by a 2H-to-1T phase transition at x = 0.7. Over a wide composition range, WSe2 and VSe2 are atomically immiscible and form separate ordered domains. The miscible alloy at x = 0.1 displayed enhanced electrocatalytic activity toward the hydrogen evolution reaction (HER) in an acidic electrolyte. This trend was correlated with the d-band center via a volcano-type relationship. Spin-polarized density functional theory calculations consistently predicted the atomic immiscibility, which became more significant at the 2H-1T phase transition composition. The Gibbs free energy of H adsorption on the basal planes (Se or hole sites) and the activation barriers along the Volmer-Heyrovsky reaction pathway supported the enhanced HER performance of the alloy phase, suggesting that the dispersed V-doped structures were responsible for the best HER catalytic activity. Our study demonstrates how the atomic structure of TMD alloy nanosheets plays a crucial role in enhancing catalytic activity.

Large-scale synthetic Mo@(2H-1T)-MoSe2 monolithic electrode for efficient hydrogen evolution in all pH scale ranges and seawater

Although molybdenum selenide with controllable phase and morphology is pivotal for high-efficiency hydrogen evolution reaction (HER), at present, the MoSe2-based HER electrodes widely applied to all pH scale ranges and seawater are remaining tremendous challenge, due to the absence of active sites, poor conductivity of MoSe2 and the limitation of synthetic method. In this work, the large-scale Mo@(2H-1T)-MoSe2 monolithic electrode was successfully fabricated by etching Mo mesh in Na2SeO3 solution via a hydrothermal technique. Continuous phase and morphology modulation of MoSe2 on Mo mesh and the optimal synthetic condition are described to fabricate a state-of-the-art electrode Mo@(2H-1T)-MoSe2 with the piezo-flexoelectric coupling effect and heterostructure. The HER tests of Mo@(2H-1T)-MoSe2 in all pH ranges and seawater exhibit an enhanced HER activity, high stability and cycle durability, which benefit from the synergistic interaction of the piezo-flexoelectric coupling effect of hybrid 2H-1T phase of MoSe2 nanosheets and heterojunction. Operando EIS and theoretical calculations also revealed this viewpoint. This strategy can also be extended arbitrarily to the large-scale preparation of other heterostructured monolithic electrodes for high-efficiency HER, such as M@MSex (M = Ni, Cu, W, etc.). This study provides a simple, effective and low-cost approach to develop the state-of-the-art metal-based heterostructured electrodes for other energy-related applications.

Phase-Transition Mo1-xVxSe2 Alloy Nanosheets with Rich V-Se Vacancies and Their Enhanced Catalytic Performance of Hydrogen Evolution Reaction.

Alloys of transition-metal dichalcogenide can display distinctive phase evolution because of their two-dimensional structures. Herein, we report the colloidal synthesis of Mo1-xVxSe2 alloy nanosheets with full composition tuning. Alloying led to a phase transition at x = 0.7 from the semiconducting 2H phase MoSe2 to the metallic 1T phase VSe2. It also produced significant V and Se vacancies, which became the richest in the 2H phase at x = 0.3-0.5. Extensive spin-polarized density functional theory calculations consistently predicted the 2H-1T phase transition at x = 0.7, in agreement with the experimental results. The vacancy formation energy also supports the formation of V and Se vacancies. Alloying in the 2H phase enhanced the electrocatalytic performance toward hydrogen evolution reaction (HER) at x = 0.3 (in 0.5 M H2SO4) or 0.4 (in 1 M KOH). The Gibbs free energy along the HER pathway indicates that this maximum performance is due to the highest concentration of active V and Se vacancy sites.

2D MoWS2 Alloy As Multifunctional Electrocatalyst for High Performance Cathode in Li-S Batteries

The lithium sulfur (Li-S) battery is an energy storage device beyond Li-ion batteries which presents unique polysulfide chemistry resulting in high theoretical capacity of 2,600 Wh/kg. However, the unwanted shuttling of soluble lithium polysulfides (LiPSs) such as Li2S8 and Li2S6, corrodes the Li anode by forming an inactive sulfide layer and limit the life cycle of the Li-S battery. In previous studies, pristine 2D materials were used as the catalyst to anchor the polysulfides in cathodes and prevent the shuttling effect. The weight loading and concentration of catalysts in cathode was too high for the practical application in Li-S battery. In our study, we synthesized novel lightweight 2D Mo0.5W0.5S2 catalyst on CNT paper as a sulfur host cathode to efficiently catalyze the polysulfide reactions. Our catalyst shows mixed 2H-1T phase resulting in enhanced polysulfide binding, conversion and electron transfer resulting in improved specific capacity of 1,228 mAh/g at 0.1 C. The accelerated transformation of the dissolved polysulfides to the insoluble LiPSs resulted in an improved cyclic stability upto 400 cycles, as compared to 100 cycles for pristine cathode. Our study shows novel approach with binder-free ultra-light weight catalytic cathodes for practical application of Li-S battery.

Molybdenum Disulfide Nanosheet/Quantum Dot Dynamic Memristive Structure Driven by Photoinduced Phase Transition.

MoS2 2D nanosheets (NS) with intercalated 0D quantum dots (QDs) represent promising structures for creating low-dimensional (LD) resistive memory devices. Nonvolatile memristors based 2D materials demonstrate low power consumption and ultrahigh density. Here, the observation of a photoinduced phase transition in the 2D NS/0D QDs MoS2 structure providing dynamic resistive memory is reported. The resistive switching of the MoS2 NS/QD structure is observed in an electric field and can be controlled through local QD excitations. Photoexcitation of the LD structure at different laser power densities leads to a reversible MoS2 2H-1T phase transition and demonstrates the potential of the LD structure for implementing a new dynamic ultrafast photoresistive memory. The dynamic LD photomemristive structure is attractive for real-time pattern recognition and photoconfiguration of artificial neural networks in a wide spectral range of sensitivity provided by QDs.

Ethylene glycol solvent induced expansion of interplanar spacing and 2H-1T phase transformation of molybdenum disulfide nanocomposites for enhanced lithium storage capability

Molybdenum disulfide (MoS2), has aroused people's much research interest as an anode candidate for next generation of Li-ion batteries because of special layered structure and high theoretical capacity. However, MoS2 suffers from a poor cycling stability and an inferior rate capability upon charge/discharge processes. So we have successfully synthesized a more stable expanded nanocomposite with superior electrical conductivity by intercalating moderate amorphous carbon between two adjacent S–Mo–S interlayer via a simple method which exhibits excellent lithium storage performance with high capacity (858.9 mA h g−1 at 1 A g−1 for 1000 cycles), and superior rate capability (518 mA h g−1 at 4 A g−1). This superior electrochemical performance is attributed to the special structure (few layers, 2H phase, expanded interlayer spacing, amorphous carbon between S–Mo–S layers, C–O–Mo bond in the layer), small size and evenly distributed MoS2 nanosheets. Besides, we study the 2H-1T phase transformation mechanism by controlling the ratio of ethylene glycol to water and O–CO bond plays an important role in promoting 2H-1T phase transformation.

Strong Charge Transfer at 2H-1T Phase Boundary of MoS2 for Superb High-Performance Energy Storage.

Transition metal dichalcogenides exhibit several different phases (e.g., semiconducting 2H, metallic 1T, 1T') arising from the collective and sluggish atomic displacements rooted in the charge-lattice interaction. The coexistence of multiphase in a single sheet enables ubiquitous heterophase and inhomogeneous charge distribution. Herein, by combining the first-principles calculations and experimental investigations, a strong charge transfer ability at the heterophase boundary of molybdenum disulfide (MoS2 ) assembled together with graphene is reported. By modulating the phase composition in MoS2 , the performance of the nanohybrid for energy storage can be modulated, whereby remarkable gravimetric and volumetric capacitances of 272 F g-1 and 685 F cm-3 are demonstrated. As a proof of concept for energy application, a flexible solid-state asymmetric supercapacitor is constructed with the MoS2 -graphene heterolayers, which shows superb energy and power densities (46.3 mWh cm-3 and 3.013 W cm-3 , respectively). The present work demonstrates a new pathway for efficient charge flow and application in energy storage by engineering the phase boundary and interface in 2D materials of transition metal dichalcogenides.

Boundary activated hydrogen evolution reaction on monolayer MoS2

Recently, monolayer molybdenum disulphide (MoS2) has emerged as a promising and non–precious electrocatalyst for hydrogen evolution reaction. However, its performance is largely limited by the low density and poor reactivity of active sites within its basal plane. Here, we report that domain boundaries in the basal plane of monolayer MoS2 can greatly enhance its hydrogen evolution reaction performance by serving as active sites. Two types of effective domain boundaries, the 2H-2H domain boundaries and the 2H-1T phase boundaries, were investigated. Superior hydrogen evolution reaction catalytic activity, long-term stability and universality in both acidic and alkaline conditions were achieved based on a multi-hierarchy design of these two types of domain boundaries. We further demonstrate that such superior catalysts are feasible at a large scale by applying this multi-hierarchy design of domain boundaries to wafer-scale monolayer MoS2 films.

Re doping induced 2H-1T phase transformation and ferromagnetism in MoS2 nanosheets

In this paper, Re-doped MoS2 nanosheets were synthesized by a facile hydrothermal reaction and an annealing process. Through structural characterization, it was observed that the doping of Re could lead to 2H-1T transformation in both the Re and Mo coordinating structures, and the 1T ratios of Mo and Re atoms increased with the Re-doping concentration. Hysteresis loops demonstrated intrinsic room temperature ferromagnetism in the Re-doped MoS2 nanosheets, and the value of saturated magnetization reached 8 memu/g when the doping concentration of Re was 17.12 at. %. The magnetic properties resulted from the increasing net magnetic moments of Re atoms and 1T-coordinated Mo atoms. Additionally, the 1T-incorporated 2H structure and magnetism were thermally stable in these samples. These results explored a new path for MoS2 nanosheets to be applied in spintronic devices in the future.

New Insight in Sodium Intercalation in MoS2 at Different Stages and the Optimized Cycle Performance for Sodium Ion Batteries

Due to the high abundance and low cost of the sodium (Na), Na-ion batteries are an alternative to lithium-ion (Li-ion) batteries for large-scale energy storage, such as electric vehicles and stationary storage for renewable energies. Similar to the successful lithium intercalation chemistry, Na-ion batteries have recalled much attention recently and a significant achievement have been made. However, the large size of the ionic diameter and ionization potential of the Na ions make it difficult in structural stability and ion diffusion, usually resulting in different reaction mechanisms1. Transition metal dichalcogenides MS2 (M=Mo, W) and their intercalated compounds belong to a large class of the so-called 2D layered solids. The layers of these materials consist of three interconnected, hexagonally coordinated atomic sheets (S-Mo-S)2. The weak interlayer van der Waals force permits the possible intercalation of some foreign atoms between the layers, such as alkali metal. Li-ion intercalated MoS2 (LixMoS2) has been widely explored and known as intercalation (x<0.5), 2H-1T phase transition (0.5<x<1) and conversion (x>1)3. However, few studies have been focused on the structural change of the host when Na ions are inserted into the MoS2 lattice. Moreover, the electrochemical performance of MoS2for Na-ion insertion needs to be improved. In this work, in situ and ex situ techniques were combined to monitor the structural changes of MoS2 at atomic levels at various intercalation stages and recognize the reaction products. By optimizing the binder and the cut-off potential, we improve the cycling performance of MoS2 for Na-ion battery by a large margin. When discharged to 0.2 V, the cell shows an initial discharge capacity of 241 mAh g-1 and maintains at 165 mAh g-1 after 70 cycles. However, intercalation of too many Na ions in the host leads to the decomposition of MoS2 into metallic Mo and NaxS2 (x£2). The performance of the cell becomes deteriorated in the subsequent cycles due to large volume expansion and the electrolyte reduction at low potentials (0.01 V) though a reversible capacity up to 440 mAh g-1 can be obtained.(1) Palomares, V.; Serras, P.; Villaluenga, I.; Hueso, K. B.; Carretero-Gonzalez, J.; Rojo, T. Energy Environ. Sci. 2012, 5, 5884-5901.(2) Zak, A.; Feldman, Y.; Lyakhovitskaya, V.; Leitus, G.; Popovitz-Biro, R.; Wachtel, E.; Cohen, H.; Reich, S.; Tenne, R. J. Am. Chem. Soc. 2002, 124, 4747-4758.(3) Fang, X. P.; Hua, C. X.; Guo, X. W.; Hu, Y. S.; Wang, Z. X.; Gao, X. P.; Wu, F.; Wang, J. Z.; Chen, L. Q. Electrochim. Acta 2012, 81, 155-160.

Atomic mechanism of dynamic electrochemical lithiation processes of MoS₂ nanosheets.

Layered molybdenum disulfide (MoS2) has been studied for decades for its diversity of structure and properties, where the structural dynamic evolution during lithium intercalation is an important but still indistinct, controversial topic. Here the electrochemical dynamic process of MoS2 nanosheets upon lithium intercalation has been systematically investigated by in situ high-resolution transmission electron microscopy. The results indicate that the lithiated MoS2 undergoes a trigonal prismatic (2H)-octahedral (1T) phase transition with a lithium ion occupying the interlayer S-S tetrahedron site in the 1T-LiMoS2. A pseudoperiodic structural modulation composed of polytype superlattices is also revealed as a consequence of the electron-lattice interaction. Furthermore, the shear mechanism of the 2H-1T phase transition has been confirmed by probing the dynamic phase boundary movement. The in situ real-time characterization at atomic scale provides a great leap forward in the fundamental understanding of the lithium ion storage mechanism in MoS2, which should be also of help for other transition metal dichalcogenides.