Substoichiometric Molybdenum Sulfide Phases with Catalytically Active Basal Planes

Unprecedented Mo3S4 cluster-catalyzed radical C-C cross-coupling reactions of aryl alkynes and acrylates

A new method for the generation of benzyl radicals from terminal aromatic alkynes has been developed, which allows the direct cross coupling with acrylate derivatives. Our additive-free protocol employs air-stable diamino Mo3S4 cubane-type cluster catalysts in the presence of hydrogen. A sulfur-centered cluster catalysis mechanism for benzyl radical formation is proposed based on catalytic and stoichiometric experiments. The process starts with the cluster hydrogen activation to form a bis(hydrosulfido) [Mo3(μ3-S)(μ-S)(μ-SH)2Cl3(dmen)3]+ intermediate. The reaction of various aromatic terminal alkynes containing different functionalities with a series of acrylates affords the corresponding Giese-type radical addition products.

Hydrogen production, storage, and transportation: recent advances

One such technology is hydrogen-based which utilizes hydrogen to generate energy without emission of greenhouse gases. The advantage of such technology is the fact that the only by-product is water. Efficient storage is crucial for the practical application of hydrogen. There are several techniques to store hydrogen, each with certain advantages and disadvantages. In gaseous hydrogen storage, hydrogen gas is compressed and stored at high pressures, requiring robust and expensive pressure vessels. In liquid hydrogen storage, hydrogen is cooled to extremely low temperatures and stored as a liquid, which is energy-intensive. Researchers are exploring advanced materials for hydrogen storage, including metal hydrides, carbon-based materials, metal-organic frameworks (MOFs), and nanomaterials. These materials aim to enhance storage capacity, kinetics, and safety. The hydrogen economy envisions hydrogen as a clean energy carrier, utilized in various sectors like transportation, industry, and power generation. It can contribute to decarbonizing sectors that are challenging to electrify directly. Hydrogen can play a role in a circular economy by facilitating energy storage, supporting intermittent renewable sources, and enabling the production of synthetic fuels and chemicals. The circular economy concept promotes the recycling and reuse of materials, aligning with sustainable development goals. Hydrogen availability depends on the method of production. While it is abundant in nature, obtaining it in a clean and sustainable manner is crucial. The efficiency of hydrogen production and utilization varies among methods, with electrolysis being a cleaner but less efficient process compared to other conventional methods. Chemisorption and physisorption methods aim to enhance storage capacity and control the release of hydrogen. There are various viable options that are being explored to solve these challenges, with one option being the use of a multilayer film of advanced metals. This work provides an overview of hydrogen economy as a green and sustainable energy system for the foreseeable future, hydrogen production methods, hydrogen storage systems and mechanisms including their advantages and disadvantages, and the promising storage system for the future. In summary, hydrogen holds great promise as a clean energy carrier, and ongoing research and technological advancements are addressing challenges related to production, storage, and utilization, bringing us closer to a sustainable hydrogen economy.

Phase-Regulated Active Hydrogen Behavior on Molybdenum Disulfide for Electrochemical Nitrate-to-Ammonia Conversion

Electrochemical reduction of nitrate waste is promising for environmental remediation and ammonia preparation. This process includes multiple hydrogenation steps, and thus the active hydrogen behavior on the surface of the catalyst is crucial. The crystal phase referred to the atomic arrangements in crystals has a great effect on active hydrogen, but the influence of the crystal phase on nitrate reduction is still unclear. Herein, enzyme-mimicking MoS2 in different crystal phases (1T and 2H) are used as models. The Faradaic efficiency of ammonia reaches ≈90 % over 1T-MoS2 , obviously outperforming that of 2H-MoS2 (27.31 %). In situ Raman spectra and theoretical calculations reveal that 1T-MoS2 produces more active hydrogen on edge S sites at a more positive potential and conducts an effortless pathway from nitrate to ammonia instead of multiple energetically demanding hydrogenation steps (such as *HNO to *HNOH) performed on 2H-MoS2 .

Anomalous Dynamics of Defect-Assisted Phonon Recycling in Few-Layer Mo0.5W0.5S2

Alloying has emerged as a new strategy to tune the function of 2D transition metal dichalcogenides (TMDCs). However, the lack of research on the electrical and structural properties of these alloys limits their practical applications. Here, femtosecond transient absorption spectroscopy with pump pulse tunability is performed to elucidate the ultrafast carrier dynamics in the few-layer Mo_0.5W_0.5S₂ prepared by the liquid phase exfoliation method. An anomalous rebleaching of the ground state is observed at high pump fluence by 3.1 eV excitation. We ascribe this rebleaching of the ground state to the mechanism that the carriers trapped in the defect are thermally excited back to the untrapped exciton state due to the phonon recycling, which hinders the dissipation of nonradiative energy, through comparative experiments and global analysis. Our findings demonstrate a novel energy transfer channel assisted by defect in few-layer TMDCs which is critical for their advanced applications.

Low-Temperature Synthesis Strategy for MoS2 Slabs Supported on TiO2(110)

MoS2 supported on oxides like TiO2 has a broad range of applications. The atomic structure of this system is therefore very useful to study. Previous research work in this area has made use of high-temperature synthesis methods, while the preparation of an MoS2/TiO2 in very important applications, such as catalysis, makes use of a low-temperature synthesis method. In this work, we investigate a low-temperature synthesis strategy for MoS2 slabs supported on rutile TiO2(110). Using scanning tunneling microscopy and X-ray photoelectron spectroscopy, we demonstrate that not only flat MoS2 slabs with irregular shapes but also MoSx stripes with a large number of coordinatively unsaturated Mo atoms are formed. In particular, it becomes evident that, for atomic structural characterization of MoS2/TiO2 and similar oxide-supported systems grown by low-temperature synthesis methods, the surface structure of the support becomes highly relevant.

Thermally driven homonuclear-stacking phase of MoS2 through desulfurization

Engineering phase transitions or finding new polymorphs offers tremendous opportunities for developing functional materials. We reveal that the thermally driven desulfurization of single-crystalline MoS2 samples improves transport properties by reducing the band gap and further induces metallization. Semi-desulfurization, i.e., removal of the topmost S layer, results in the placement of the exposed Mo layers directly on top of the following sub-layers, together with the bottom S layer of the top layer. This homonuclear (AA) stacking derived from the AA' stacking of the hexagonal (2H) phase is retained even after further desulfurization of the remaining bottom S layer, i.e., full desulfurization of the top layer. Our findings fundamentally explain why the 2H phase of TMDs is characterized by AA' stacking.

Controlling a Chemical Coupling Reaction on a Surface: Tools and Strategies for On-Surface Synthesis

On-surface synthesis is appearing as an extremely promising research field aimed at creating new organic materials. A large number of chemical reactions have been successfully demonstrated to take place directly on surfaces through unusual reaction mechanisms. In some cases the reaction conditions can be properly tuned to steer the formation of the reaction products. It is thus possible to control the initiation step of the reaction and its degree of advancement (the kinetics, the reaction yield); the nature of the reaction products (selectivity control, particularly in the case of competing processes); as well as the structure, position, and orientation of the covalent compounds, or the quality of the as-formed networks in terms of order and extension. The aim of our review is thus to provide an extensive description of all tools and strategies reported to date and to put them into perspective. We specifically define the different approaches available and group them into a few general categories. In the last part, we demonstrate the effective maturation of the on-surface synthesis field by reporting systems that are getting closer to application-relevant levels thanks to the use of advanced control strategies.

Hydrogen Evolution Catalyzed by a Molybdenum Sulfide Two-Dimensional Structure with Active Basal Planes

Molybdenum disulfide has been demonstrated as a promising catalyst for hydrogen evolution reaction (HER). However, its performance is limited by fractional active edge sites and the strong dependence on hydrogen coverage. In this study, we find an enhanced HER performance in a two-dimensional substoichiometric molybdenum sulfide. Both first-principles calculations and experimental results demonstrate that the basal plane is catalytically active toward HER, as evidenced by an optimum Gibbs free energy and a low reaction overpotential. More interestingly, the HER performance is insensitive to hydrogen coverage and can be improved under compressive in-plane biaxial strains. Our results suggest not only an improved HER performance of substoichiometric molybdenum sulfide due to its chemical reactive basal plane but also a way to tune the performance.

Sulfur-driven switching of the Ullmann coupling on Au(111)

We demonstrate a method to selectively switch the Ullmann coupling reaction of 2,8-dibromodibenzothiophene on a Au(111) support. The Ullmann coupling reaction is effective already at low temperature, but the complete inhibition of the same reaction can be achieved on Au(111) pre-exposed to H2S. The marked difference in reactivity of pretreated Au(111) is explained by the S-passivation of free Au atoms emerging from reconstruction sites. The inhibited state can be fully lifted by removing the S via hydrogen gas post-exposure.

Exploitation of the Large-Area Basal Plane of MoS2 and Preparation of Bifunctional Catalysts through On-Surface Self-Assembly

The development of nonprecious electrochemical catalysts for water splitting is a key step to achieve a sustainable energy supply for the future. Molybdenum disulfide (MoS2) has been extensively studied as a promising low-cost catalyst for hydrogen evolution reaction (HER), whereas HER is only catalyzed at the edge for pristine MoS2, leaving a large area of basal plane useless. Herein, on-surface self-assembly is demonstrated to be an effective, facile, and damage-free method to take full advantage of the large ratio surface of MoS2 for HER by using multiscale simulations. It is found that as supplement of edge sites of MoS2, on-MoS2 M(abt)2 (M = Ni, Co; abt = 2-aminobenzenethiolate) owns high HER activity, and the self-assembled M(abt)2 monolayers on MoS2 can be obtained through a simple liquid-deposition method. More importantly, on-surface self-assembly provides potential application for overall water splitting once the self-assembled systems prove to be of both HER and oxygen evolution reaction activities, for example, on-MoS2 Co(abt)2. This work opens up a new and promising avenue (on-surface self-assembly) toward the full exploitation of the basal plane of MoS2 for HER and the preparation of bifunctional catalysts for overall water splitting.

Hydrogen physisorption based on the dissociative hydrogen chemisorption at the sulphur vacancy of MoS2 surface

We provide a new insight that the sulphur-depleted MoS₂ surface can store hydrogen gas at room temperature. Our findings reveal that the sulphur-vacancy defects preferentially serve as active sites for both hydrogen chemisorption and physisorption. Unexpectedly the sulphur vacancy instantly dissociates the H₂ molecules and strongly binds the split hydrogen at the exposed Mo atoms. Thereon the additional H₂ molecule is adsorbed with enabling more hydrogen physisorption on the top sites around the sulphur vacancy. Furthermore, the increase of the sulphur vacancy on the MoS₂ surface further activates the dissociative hydrogen chemisorption than the H₂ physisorption.