Stable zigzag edges of transition-metal dichalcogenides with high catalytic activity for oxygen reduction

Observation of Sub-10 nm Transition Metal Dichalcogenide Nanocrystals in Rapidly Heated van der Waals Heterostructures

Two-dimensional materials, such as transition metal dichalcogenides (TMDCs), have the potential to revolutionize the field of electronics and photonics due to their unique physical and structural properties. This research presents a novel method for synthesizing crystalline TMDCs crystals with <10 nm size using ultrafast migration of vacancies at elevated temperatures. Through in situ and ex situ processing and using atomic-level characterization techniques, we analyzed the shape, size, crystallinity, composition, and strain distribution of these nanocrystals. These nanocrystals exhibit electronic structure signatures that differ from the 2D bulk: i.e., uniform mono- and multilayers. Further, our in situ, vacuum-based synthesis technique allows observation and comparison of defect and phase evolution in these crystals formed under van der Waals heterostructure confinement versus unconfined conditions. Overall, this research demonstrates a solid-state route to synthesizing uniform nanocrystals of TMDCs and lays the foundation for materials science in confined 2D spaces under extreme conditions.

Elucidating the Volcanic-Type Catalytic Behavior in Lithium-Sulfur Batteries via Defect Engineering

Defects are generally considered to be effective and flexible in the catalytic reactions of lithium-sulfur batteries. However, the influence of the defect concentration on catalysis remains ambiguous. In this work, molybdenum sulfide with different sulfur vacancy concentrations is comprehensively modulated, showing that the defect level and the adsorption-catalytic performance result in a volcano relationship. Moreover, density functional theory and in situ experiments reveal that the optimal level of sulfur defects can effectively increase the binding energy between molybdenum sulfide and lithium polysulfides (LiPSs), lower the energy barrier of the LiPS conversion reaction, and promote the kinetics of Li2S bidirectional catalytic reaction. The lower bidirectional catalytic performance incited by excessive or deficient sulfur defects is mainly due to the deformed geometrical structures and reduced adsorption of key LiPSs on the catalyst surface. This work underscores the imperative of controlling the defect content and provides a potential approach to the commercialization of lithium-sulfur batteries.

Effect of MoSe2 nanoribbons with NW30 edge reconstructions on the electronic and catalytic properties by strain engineering

Monolayer transition metal dichalcogenides (TMDs), typical two-dimensional semiconductors, have been extensively studied for their extraordinary physical properties and utilized for nanoelectronics and optoelectronics. However, the finite samples and discontinuity in the synthesis process of TMD materials definitely induce defect edges in nanoribbons and greatly influence the device performance. Here, we systematically studied the atomic structures, energetic and mechanical stability, and electronic and catalytic properties of MoSe₂ nanoribbons on the basis of experiments. Clear benefits of ZZSe-Mo-NW30 edged nanoribbons were found to evidently increase the dynamic stability according to our first-principles calculations. Meanwhile, unsaturated Mo atoms at the edge sites induced local magnetic moments up to 0.54 μ_B and changed the chemical environments of adjacent Se atoms, which acted as active sites for the hydrogen evolution reaction (HER) with a lower onset potential of -0.04 eV. The external tensile strain on these nanoribbons can have negligible effects on the electronic and catalytic properties. The onset potential of the ZZSe-Mo-NW30 edged nanoribbons only changed 0.03 eV under critical tensile strain. The atomic-scale research of edge reconstructions in TMD materials provides new opportunities to modulate the synthesis mechanism for experiments and defect-engineering applications in electrochemical catalysts.

Structural engineering brings new electronic properties to Janus ZrSSe and HfSSe monolayers

Interfacing effects within emergent two-dimensional (2D) materials are of fundamental interest and are at the center of applications in nanoelectronics. Thus, out-of-plane and in-plane heterostructures as well as electronic heterostructures with phase boundaries and large-angle (60°) grain boundaries (GBs) of Janus ZrSSe and HfSSe are studied in this work using first-principles calculations. The out-of-plane heterostructures of T-ZrSSe and T-HfSSe illustrate quite weak interfacing interactions, thus the electronic properties are, unusually, more like the superposition of individual monolayers. The in-plane heterostructures of T-ZrSSe and T-HfSSe, interestingly, exhibit an indirect-direct band gap transition and type-II band alignment, which correspond to boosted optical properties and spatially separated excitons. For the in-plane electronic heterostructures that are constituted by T-ZrSSe and H-ZrSSe, semiconductor-metal crossover occurs due to the polar discontinuity across the T-H phase boundary, and they behave as one-dimensional metallic wires embedded in otherwise semiconducting Janus ZrSSe, creating a one-dimensional electron/hole gas. This also indicates a strategy for stabilizing the unstable and/or metastable monolayer via the phase boundary. As a result of the zero formal bulk polarization of the T-phase ZrSSe, the metallicity of 60° GBs originates mainly from the edge atoms rather than from the polar discontinuity.

Electronic Properties of Defective Janus MoSSe Monolayer

Two-dimensional (2D) transition-metal dichalcogenides (TMDCs) hold great promise in electronics and optoelectronics due to their novel electronic and optical properties. In TMDCs, structural defects are inevitable and might play a decisive role in device performance. In this work, point defects, line vacancies, and 60° grain boundaries (GBs) are explored in 2D Janus MoSSe, a new member to the family of TMDCs, by means of the first-principles calculations. S and Se vacancies are found to be the most favorable point defects, and they tend to aggregate along the zigzag direction to form line vacancies. Comparing with isolated point defects, line vacancies induced in-gap states are more dispersive. In particular, 60° GBs behave as one-dimensional metallic quantum wires, as a consequence of the polar discontinuity. Thus, effectively controlling the formation of defects at nanoscale brings new electronic characteristics, providing new opportunities to broaden the applications of 2D TMDCs.

Water dissociation and association on mirror twin boundaries in two-dimensional MoSe2: insights from density functional theory calculations

The adsorption and dissociation of water molecules on two-dimensional transition metal dichalcogenides (TMDs) is expected to be dominated by point defects, such as vacancies, and edges. At the same time, the role of grain boundaries, and particularly, mirror twinboundaries (MTBs), whose concentration in TMDs can be quite high, is not fully understood. Using density functional theory calculations, we investigate the interaction of water, hydroxyl groups, as well as oxygen and hydrogen molecules with MoSe₂ monolayers when MTBs of various types are present. We show that the adsorption of all species on MTBs is energetically favorable as compared to that on the basal plane of pristine MoSe₂, but the interaction with Se vacancies is stronger. We further assess the energetics of various surface chemical reactions involving oxygen and hydrogen atoms. Our results indicate that water dissociation on the basal plane should be dominated by vacancies even when MTBs are present, but they facilitate water clustering through hydroxyl groups at MTBs, which can anchor water molecules and give rise to the decoration of MTBs with water clusters. Also, the presence of MTBs affects oxygen reduction reaction (ORR) on the MoSe₂ monolayer. Unlike Se vacancies which inhibit ORR due to a high overpotential, it is found that the ORR process on MTBs is more efficient, indicating their important role in the catalytic activity of MoSe₂ monolayer and likely other TMDs.

A supramolecular hybrid sensor based on cucurbit[8]uril, 2D-molibdenum disulphide and diamond nanoparticles towards methyl viologen analysis

We develop an electrochemical sensor by using 2D-transition metal dichalcogenides (TMD), specifically MoS₂, and nanoparticles stabilized with cucurbit[8]uril (CB[8]) incorporated together with them. Two different nanoparticles are assayed: diamond nanoparticles (DNPs) and gold nanoparticles (AuNp). 0D materials, together with TMD, provide increased conductivity and active surface while the macrocycle CB[8] affords selectivity towards the guest methyl viologen (MV²⁺), also named paraquat. Glassy Carbon (GC) electrodes are modified by drop-casting of suspensions of MoS₂, followed by either a CB[8]-DNPs hybrid dispersion or a CB[8]-AuNp suspension. Atomic force microscopy is employed for the morphological characterization of the electrochemical sensor surface while cyclic voltammetry and electrochemical impedance spectroscopy techniques allow the electrochemical characterization of the sensor. The well-stablished signals of CB[8]-encapsulated MV²⁺ arise in voltammetric measurements when the macrocycle modifies the 0D-materials. Once the sensor construction and differential pulse voltammetry parameters have been optimized for quantification purposes, calibration procedures are performed with the platform GC/MoS₂/CB[8]-DNPs. This sensing platform shows linear relations between peak intensity and the MV²⁺ concentration in the linear concentration range of (0.73-8.0) · 10^-6 M with a limit of detection of 2.2 · 10^-7 M. Analyses of river water samples fortified with MV²⁺ at the μM level shows recoveries of 100% with RSD values of 6.4% (n = 3).