All The Catalytic Active Sites of MoS2 for Hydrogen Evolution

Selective activation of MoS2 grain boundaries for enhanced electrochemical activity

Molybdenum disulfide (MoS2) has emerged as a promising material for catalysis and sustainable energy conversion. However, the inertness of its basal plane to electrochemical reactions poses challenges to the utilization of wafer-scale MoS2 in electrocatalysis. To overcome this limitation, we present a technique that enhances the catalytic activity of continuous MoS2 by preferentially activating its buried grain boundaries (GBs). Through mild UV irradiation, a significant enhancement in GB activity was observed that approaches the values for MoS2 edges, as confirmed by a site-selective photo-deposition technique and micro-electrochemical hydrogen evolution reaction (HER) measurements. Combined spectroscopic characterization and ab-initio simulation demonstrates substitutional oxygen functionalization at the grain boundaries to be the origin of this selective catalytic enhancement by an order of magnitude. Our approach not only improves the density of active sites in MoS2 catalytic processes but yields a new photocatalytic conversion process. By exploiting the difference in electronic structure between activated GBs and the basal plane, homo-compositional junctions were realized that improve the photocatalytic synthesis of hydrogen by 47% and achieve performances beyond the capabilities of other catalytic sites.

Precious Metal Free Hydrogen Evolution Catalyst Design and Application

The quest to identify precious metal free hydrogen evolution reaction catalysts has received unprecedented attention in the past decade. In this Review, we focus our attention to recent developments in precious metal free hydrogen evolution reactions in acidic and alkaline electrolyte owing to their relevance to commercial and near-commercial low-temperature electrolyzers. We provide a detailed review and critical analysis of catalyst activity and stability performance measurements and metrics commonly deployed in the literature, as well as review best practices for experimental measurements (both in half-cell three-electrode configurations and in two-electrode device testing). In particular, we discuss the transition from laboratory-scale hydrogen evolution reaction (HER) catalyst measurements to those in single cells, which is a critical aspect crucial for scaling up from laboratory to industrial settings but often overlooked. Furthermore, we review the numerous catalyst design strategies deployed across the precious metal free HER literature. Subsequently, we showcase some of the most commonly investigated families of precious metal free HER catalysts; molybdenum disulfide-based, transition metal phosphides, and transition metal carbides for acidic electrolyte; nickel molybdenum and transition metal phosphides for alkaline. This includes a comprehensive analysis comparing the HER activity between several families of materials highlighting the recent stagnation with regards to enhancing the intrinsic activity of precious metal free hydrogen evolution reaction catalysts. Finally, we summarize future directions and provide recommendations for the field in this area of electrocatalysis.

Structure-dependent CO2 reduction on molybdenite (MoS2) electrocatalysts

Scanning electrochemical cell microscopy (SECCM) is employed to directly identify the structure-dependent electrochemical CO2 reduction reaction (eCO2RR) activity of molybdenite (MoS2) electrocatalysts in an aqueous imidazolium-based aprotic ionic liquid electrolyte. Nanoscale defects, where the edge plane (EP) is exposed, are directly targeted, revealing heightened overall activity (eCO2RR + the competing hydrogen evolution reaction, HER) over the relatively inactive basal plane (BP). In addition, certain types of defects (e.g., step edges) only exhibit heightened activity under a CO2 atmosphere (i.e., compared to N2), indirectly confirming higher selectivity at these surface sites. Overall, this work will guide the bottom-up design of earth-abundant electrocatalysts for use in large-scale CO2 electrolysis.

Allotrope-dependent activity-stability relationships of molybdenum sulfide hydrogen evolution electrocatalysts

Molybdenum disulfide (MoS2) is widely regarded as a competitive hydrogen evolution reaction (HER) catalyst to replace platinum in proton exchange membrane water electrolysers (PEMWEs). Despite the extensive knowledge of its HER activity, stability insights under HER operation are scarce. This is paramount to ensure long-term operation of Pt-free PEMWEs, and gain full understanding on the electrocatalytically-induced processes responsible for HER active site generation. The latter are highly dependent on the MoS2 allotropic phase, and still under debate. We rigorously assess these by simultaneously monitoring Mo and S dissolution products using a dedicated scanning flow cell coupled with downstream analytics (ICP-MS), besides an electrochemical mass spectrometry setup for volatile species analysis. We observe that MoS2 stability is allotrope-dependent: lamellar-like MoS2 is highly unstable under open circuit conditions, whereas cluster-like amorphous MoS3-x instability is induced by a severe S loss during the HER and undercoordinated Mo site generation. Guidelines to operate non-noble PEMWEs are therefore provided based on the stability number metrics, and an HER mechanism which accounts for Mo and S dissolution pathways is proposed.

Semimetal 1T' phase molybdenum sulfide decorated on zinc indium sulfide with S-scheme heterojunction for enhanced photocatalytic hydrogen evolution

Heterojunction engineering is an effective strategy to improve photocatalytic performance. Two-dimensional (2D)/2D semimetal 1T' phase molybdenum sulfide/zinc indium sulfide (1T'-MoS2/ZnIn2S4) S-scheme heterojunctions with tight and stable interfaces were synthesized by a simple hydrothermal synthesis method. Under the optimal 1T'-MoS2 loading ratio (5 wt%), the hydrogen production rate of 1T'-MoS2/ZnIn2S4 composites reaches 11.42 mmol h-1 g-1, which is 3.1 and 1.4 times higher than that of pure ZnIn2S4 (2.9 mmol h-1 g-1) and ZnIn2S4/Pt (8.01 mmol h-1 g-1), and the apparent quantum efficiency (AQE) reaches 53.17 % (λ = 370 nm). Semimetal 1T' phase MoS2 on ZnIn2S4 broadens the light absorption range, enhances the light absorption ability, promotes electron transfer, and offers abundant active sites. The establishment of S-scheme heterojunctions achieves the spatial separation of photogenerated charges and increases the reduction potential. This work provides insights for the design of novel photocatalysts.

Unveiling the antibacterial strategies and mechanisms of MoS2: a comprehensive analysis and future directions

Antibiotic resistance is a growing problem that requires alternative antibacterial agents. MoS2, a two-dimensional transition metal sulfide, has gained significant attention in recent years due to its exceptional photocatalytic performance, excellent infrared photothermal effect, and impressive antibacterial properties. This review presents a detailed analysis of the antibacterial strategies and mechanism of MoS2, starting with its morphology and synthesis methods and focusing on the different interaction stages between MoS2 and bacteria. The paper summarizes the main antibacterial mechanisms of MoS2, such as photocatalytic antibacterial, enzyme-like catalytic antibacterial, physical antibacterial, and photothermal-assisted antibacterial. It offers a comprehensive discussion focus on recent research studies of photocatalytic antibacterial mechanisms and categorizes them, guiding the application of MoS2 in the antibacterial field. Overall, the review provides an in-depth understanding of the antibacterial mechanisms of MoS2 and presents the challenges and future directions for the improvement of MoS2 in the field of high-efficiency antibacterial materials.

Exact inversion of partially coherent dynamical electron scattering for picometric structure retrieval

The greatly nonlinear diffraction of high-energy electron probes focused to subatomic diameters frustrates the direct inversion of ptychographic data sets to decipher the atomic structure. Several iterative algorithms have been proposed to yield atomically-resolved phase distributions within slices of a 3D specimen, corresponding to the scattering centers of the electron wave. By pixelwise phase retrieval, current approaches do not only involve orders of magnitude more free parameters than necessary, but also neglect essential details of scattering physics such as the atomistic nature of the specimen and thermal effects. Here, we introduce a parametrized, fully differentiable scheme employing neural network concepts which allows the inversion of ptychographic data by means of entirely physical quantities. Omnipresent thermal diffuse scattering in thick specimens is treated accurately using frozen phonons, and atom types, positions and partial coherence are accounted for in the inverse model as relativistic scattering theory demands. Our approach exploits 4D experimental data collected in an aberration-corrected momentum-resolved scanning transmission electron microscopy setup. Atom positions in a 20 nm thick PbZr0.2Ti0.8O3 ferroelectric are measured with picometer precision, including the discrimination of different atom types and positions in mixed columns.

A semiconducting hybrid of RhOx/GaN@InGaN for simultaneous activation of methane and water toward syngas by photocatalysis

Prior to the eventual arrival of carbon neutrality, solar-driven syngas production from methane steam reforming presents a promising approach to produce transportation fuels and chemicals. Simultaneous activation of the two reactants, i.e. methane and water, with notable geometric and polar discrepancy is at the crux of this important subject yet greatly challenging. This work explores an exceptional semiconducting hybrid of RhOx/GaN@InGaN nanowires for overcoming this critical challenge to achieve efficient syngas generation from methane steam reforming by photocatalysis. By coordinating density functional theoretical calculations and microscopic characterizations, with in situ spectroscopic measurements, it is found that the multifunctional RhOx/GaN interface is effective for simultaneously activating both CH4 and H2O by stretching the C-H and O-H bonds because of its unique Lewis acid/base attribute. With the aid of energetic charge carriers, the stretched C-H and O-H bonds of reactants are favorably cleaved, resulting in the key intermediates, i.e. *CH3, *OH, and *H, to sit on Rh sites, Rh sites, and N sites, respectively. Syngas is subsequently produced via energetically favored pathway without additional energy inputs except for light. As a result, a benchmarking syngas formation rate of 8.1 mol·gcat-1·h-1 is achieved with varied H2/CO ratios from 2.4 to 0.8 under concentrated light illumination of 6.3 W·cm-2, enabling the achievement of a superior turnover number of 10,493 mol syngas per mol Rh species over 300 min of long-term operation. This work presents a promising strategy for green syngas production from methane steam reforming by utilizing unlimited solar energy.

Engineering Double Sulfur-Vacancy in CoS1.097@MoS2 Core-Shell Heterojunctions for Hydrogen Evolution in a Wide pH Range

Heterostructured nanomaterials have arisen as electrocatalysts with great potential for hydrogen evolution reaction (HER), considering their superiority in integrating different active components but are plagued by their insufficient active site density in a wide pH range. In this report, double sulfur-vacancy-decorated CoS1.097@MoS2 core-shell heterojunctions are designed, which contain a primary structure of hollow CoS1.097 nanocubes and a secondary structure of ultrathin MoS2 nanosheets. Taking advantage of the core-shell type heterointerfaces and double sulfur-vacancy, the CoS1.097@MoS2 catalyst exhibits pH-universal HER performance, achieving the overpotentials at 10 mA cm-2 of 190, 139, and 220 mV in 0.5 M H2SO4, 1.0 M KOH, and 1.0 M PBS, respectively. Systematic theoretical results show that the double sulfur-vacancy can endow the CoS1.097@MoS2 core-shell heterojunctions with promoted electron/mass transfer and enhanced reactive kinetics, thus boosting HER performance. This work clearly demonstrates an indispensable role of double sulfur-vacancy in enhancing the electrocatalytic HER performance of core-shell type heterojunctions under a wide pH operating condition.

Theoretical study of electrocatalytic properties of low-dimensional freestanding PbTiO3 for hydrogen evolution reactions

The discovery of novel materials for catalytic purposes that are highly stable is one of the main challenges nowadays for reducing our dependence on fossil fuels. Here, low-dimensional PbTiO3 is introduced as an electrocatalyst using first-principles calculations. Density-functional theory calculations indicate that 2D-PbTiO3 is dynamically and thermodynamically stable. Our results show that a single oxygen defect vacancy in 2D-PbTiO3 can play a key role in enhancing the hydrogen evolution reaction (HER), together with the Ti atoms. Our study concludes that the Volmer-Heyrovsky mechanism is a more favorable route to achieve HER than the Volmer-Tafel mechanism, including solvation and vacuum conditions.

Spontaneous Decoration of Ultrasmall Pt Nanoparticles on Size-Separated MoS2 Nanosheets

Liquid exfoliation can be considered as a viable approach for the scalable production of 2D materials due to its various benefits, although the polydispersity in the obtained nanosheet size hinders their straightforward incorporation. Size-separation can help alleviate these concerns, however a correlation between nanosheet size and property needs to be established to bring about size-specific applicability. Herein, size-selected aqueous nanosheet dispersions have been obtained via centrifugation-based protocols, and their chemical activity in the spontaneous reduction of chloroplatinic acid is investigated. Growth of ultrasmall Pt nanoparticles was achieved on nanosheet surfaces without a need for reducing agents, and stark differences in the nanoparticle coverage were observed as a function of nanosheet size. Defects in the nanosheets were probed via Raman spectroscopy, and correlated to the observed size-activity. Additionally, the effect of reaction temperature during synthesis was investigated. The electrochemical activity of the ultrasmall Pt nanoparticle decorated MoS2 nanosheets was evaluated for the hydrogen evolution reaction, and enhancement in performance was observed with nanosheet size, and nanoparticle decoration density. These findings shine light on the significance of nanosheet size in controlling spontaneous reduction reactions, and provide a deeper insight to intrinsic properties of liquid exfoliated nanosheets.

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.

Coupling MoS2 nanosheets with CeO2 for efficient electrocatalytic hydrogen evolution at large current densities

We developed an efficient MoS2 nanosheet electrode coupled with CeO2via a hydrothermal process to facilitate water adsorption and dissociation, which displayed good HER activity and stability at a large current density of 500 mA cm-2. In situ Raman spectroscopy confirmed the formation of hydroxide ions based on the strengthening of the Ce-O bond during the HER.

Oxygen evolution reaction on MoS2/C rods-robust and highly active electrocatalyst

Recently, water oxidation or oxygen evolution reaction (OER) in electrocatalysis has attracted huge attention due to its prime role in water splitting, rechargeable metal-air batteries, and fuel cells. Here, we demonstrate a facile and scalable fabrication method of a rod-like structure composed of molybdenum disulfide and carbon (MoS2/C) from parent 2D MoS2. This novel composite, induced via the chemical vapor deposition (CVD) process, exhibits superior oxygen evolution performance (overpotential = 132 mV at 10 mA cm-2and Tafel slope = 55.6 mV dec-1) in an alkaline medium. Additionally, stability tests of the obtained structures at 10 mA cm-2during 10 h followed by 20 mA cm-2during 5 h and 50 mA cm-2during 2.5 h have been performed and clearly prove that MoS2/C can be successfully used as robust noble-metal-free electrocatalysts. The promoted activity of the rods is ascribed to the abundance of active surface (ECSA) of the catalyst induced due to the curvature effect during the reshaping of the composite from 2D precursor (MoS2) in the CVD process. Moreover, the presence of Fe species contributes to the observed excellent OER performance. FeOOH, Fe2O3, and Fe3O4are known to possess favorable electrocatalytic properties, including high catalytic activity and stability, which facilitate the electrocatalytic reaction. Additionally, Fe-based species like Fe7C3and FeMo2S5offer synergistic effects with MoS2, leading to improved catalytic activity and durability due to their unique electronic structure and surface properties. Additionally, turnover frequency (TOF) (58 1/s at the current density of 10 mA cm-2), as a direct indicator of intrinsic activity, indicates the efficiency of this catalyst in OER. Based onex situanalyzes (XPS, XRD, Raman) of the electrocatalyst the possible reaction mechanism is explored and discussed in great detail showing that MoS2, carbon, and iron oxide are the main active species of the reaction.

The electronic structure of a strongly bound sandwich MoS2-WS2 heterobilayer

First of all, we show that two kinds of sandwich bilayers (BLs) are dynamically, thermally, and mechanically stable, which are degenerate p-type materials with intercalated Ca atoms, i.e., Nb-doped MoS₂ homobilayers (HoBLs) and Nb-doped WS₂-MoS₂ heterobilayers (HtBLs) with 25% Nb content. Specifically, their interlayer bindings are five times stronger than van der Waals interactions in their pristine counterparts. Both of them are semiconductors with indirect band gaps in the visible region within the HSE06 exchange-correlation functional. Depending upon the presence and absence of centrosymmetry, they display interesting spin-valley coupling effects in such a way that opposite hidden spin polarization or opposite spin splitting is observed at opposite k-points. They can be easily engineered into direct gap materials under compressive (>2%) strain along the zigzag direction even with an explicit consideration of giant spin splitting. Under strain, they satisfy thermodynamic conditions for bifunctional catalysis in photocatalytic water splitting. In addition, the photoholes of the BLs can be subjected to lower overpotentials than those of pristine BLs for the oxygen evolution reaction. Electrons and holes in the sandwich HtBL can be separated into different layers under photon irradiation, allowing it to be more efficient than the corresponding HoBL in solar energy harvesting.

Massively synthesizable nickel-doped 1T-MoS2 nanosheet catalyst as an efficient tri-functional catalyst

In this study, a nickel (Ni)-doped 1T-MoS₂ catalyst, an efficient tri-functional hydrogen evolution reaction (HER), oxygen evolution reaction (OER), and oxygen reduction reaction (ORR) catalyst, was massively synthesized at high pressure (over 15 bar). The morphology, crystal structure, and chemical and optical properties of the Ni-doped 1T-MoS₂ nanosheet catalyst were characterized by transmission electron microscopy (TEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and ring rotating disk electrodes (RRDE), and the OER/ORR properties were characterized using lithium-air cells. Our results confirmed that highly pure, uniform, monolayer Ni-doped 1T-MoS₂ can be successfully prepared. The as-prepared catalysts exhibited excellent electrocatalytic activity for OER, HER, and ORR owing to the enhanced basal plane activity of Ni doping and formidable active edge sites resulting from the phase transition to a highly crystalline 1T structure from 2H and amorphous MoS₂. Therefore, our study provides a massive and straightforward strategy to produce tri-functional catalysts.

Interface prompted highly efficient hydrogen evolution of MoS2/CoS2 heterostructures in a wide pH range

Interfacial electronic characteristics are crucial for the hydrogen evolution reaction (HER), especially in nanoscale heterogeneous catalysts. In this work, we found that the synergistic activation of CoS₂ and MoS₂ (2H-MoS₂ and 1T-MoS₂) greatly enhances the HER activity in a wide pH range compared to those of each component. The Gibbs free energies for hydrogen adsorption at interfacial Co sites are as low as -0.08 (-0.25) eV and -0.20 (0.01) eV for 2H-MoS₂/CoS₂ and 1T-MoS₂/CoS₂ heterostructures in acidic (alkaline) media, respectively, which are even superior to that of Pt(111) (-0.09 eV). Moreover, the theoretical exchange current density of MoS₂/CoS₂ can reach ∼1.98 × 10^-18 A site^-1 (∼8.43 A mg^-1). Experimentally, MoS₂/CoS₂ exhibits a greatly reduced overpotential of 54 (46) mV and a Tafel slope of 42 (50) mV dec^-1 under acidic (alkaline) conditions. The improved performance mainly originates from the synergistically activated interfacial Co atoms with better electron localization and local bonding. The interfacial effect enhances the electron conductivity and improves the H adsorption characteristics, making MoS₂/CoS₂ highly valuable as efficient HER electrocatalysts.

Ultra-fast Piezocatalysts Enabled By Interfacial Interaction of Reduced Graphene Oxide/MoS2 Heterostructures

The catalytic activity has been investigated in 2D materials, and the unique structural and electronic properties contribute to their success in conventional heterogeneous catalysis. Heterojunction-based piezocatalysis has attracted increasing attention due to the band-structure engineering and the enhanced charge carrier separation by prominent piezoelectric effect. However, the piezocatalytic behavior of van der Waals (vdW) heterostructures is still unknown, and the finite active sites, catalyst poisoning, and poor conductivity are challenges for developing good piezocatalysts. Herein, a reduced graphene oxide (rGO)-MoS₂ heterostructure is rationally designed to tackle these challenges. The heterostructure shows a record-high piezocatalytic degradation rate of 1.40 × 10² L mol^-1 s^-1 , which is 7.86 times higher than MoS₂ nanosheets. Piezoresponse force microscope measurements and density functional theory calculation reveal that the coupling between semiconductive and piezoelectric properties in the vdW heterojunction is vital to break the metallic state screening effect at the MoS₂ edge for keeping the piezoelectric potential. The dynamic charges generated by MoS₂ and the fast charge transfer in rGO activate and maintain catalytically active sites for pollutant degradation with an ultra-fast rate and good stability. The working mechanism opens new avenues for developing efficient catalysts significant to wastewater treatments and other applications.

An ultrasensitive FET biosensor based on vertically aligned MoS2 nanolayers with abundant surface active sites

Molybdenum disulfide (MoS₂) nanolayers are one of the most promising two-dimensional (2D) nanomaterials for constructing next-generation field-effect transistor (FET) biosensors. In this article, we report an ultrasensitive FET biosensor that integrates a novel format of 2D MoS₂, vertically-aligned MoS₂ nanolayers (VAMNs), as the channel material for label-free detection of the prostate-specific antigen (PSA). The developed VAMNs-based FET biosensor shows two distinctive advantages. First, the VAMNs can be facilely grown using the conventional chemical vapor deposition (CVD) method, permitting easy fabrication and potential mass device production. Second, the unique advantage of the VAMNs for biosensor development lies in its abundant surface-exposed active edge sites that possess a high binding affinity with thiol-based linkers, which overcomes the challenge of molecule functionalization on the conventional planar MoS₂ nanolayers. The high binding affinity between 11-mercaptoundecanoic acid and the VAMNs was demonstrated through experimental surface characterization and theoretical calculations via density functional theory. The FET biosensor allows rapid (within 20 min) and ultrasensitive PSA detection in human serum with simple operations (limit of detection: 800 fg mL^-1). This FET biosensor offers excellent features such as ultrahigh sensitivity, ease of fabrication, and short assay time, and thereby possesses significant potential for early-stage diagnosis of life-threatening diseases.

Active States During the Reduction of CO2 by a MoS2 Electrocatalyst

Transition-metal dichalcogenides (TMDCs) such as MoS₂ are Earth-abundant catalysts that are attractive for many chemical processes, including the carbon dioxide reduction reaction (CO2RR). While many studies have correlated synthetic preparation and architectures with macroscopic electrocatalytic performance, not much is known about the state of MoS₂ under functional conditions, particularly its interactions with target molecules like CO₂. Here, we combine operando Mo K- and S K-edge X-ray absorption spectroscopy (XAS) with first-principles simulations to track changes in the electronic structure of MoS₂ nanosheets during CO2RR. Comparison of the simulated and measured XAS discerned the existence of Mo-CO₂ binding in the active state. This state perturbs hybridized Mo 4d-S 3p states and is critically mediated by sulfur vacancies induced electrochemically. The study sheds new light on the underpinnings of the excellent performance of MoS₂ in CO2RR. The electronic signatures we reveal could be a screening criterion toward further gains in activity and selectivity of TMDCs in general.

Efficient Hydrogen Evolution via 1T-MoS2 /Chlorophyll-a Heterostructure: Way Toward Metal Free Green Catalyst

Electrocatalytic hydrogen evolution reaction (HER) is regarded as a sustainable and green way for H₂ generation, which faces a great challenge in designing highly active, stable electrocatalysts to replace the state-of-art noble metal-platinum catalysts. 1T MoS₂ is highly promising in this regard, but the synthesis and stability of this is a particularly pressing task. Here, a phase engineering strategy has been proposed to achieve a stable, high-percentage (88%) 1T MoS₂ /chlorophyll-a hetero-nanostructure, through a photo-induced donation of anti-bonding electrons from chlorophyll-a (CHL-a) highest occupied molecular orbital to 2H MoS₂ lowest unoccupied molecular orbital. The resultant catalyst has abundant binding sites provided by the coordination of magnesium atom in the CHL-a macro-cycle, featuring higher binding strength and low Gibbs-free energy. This metal-free heterostructure exhibits excellent stability via band renormalization of Mo 4d orbital which creates the pseudogap-like structure by lifting the degeneracy of projected density of state with 4S in 1T MoS₂ . It shows extremely low overpotential, toward the acidic HER (68 mV at the current density of 10 mA cm^-2 ), very close to the Pt/C catalyst (53 mV). The high electrochemical-surface-area and electrochemical turnover frequency support enhanced active sites along with near zero Gibbs free energy. Such a surface-reconstruction strategy provides a new avenue toward the production of efficient non-noble-metal-catalysts for the HER with the aim of green-hydrogen production.

In Situ Porousized MoS2 Nano Islands Enhance HER/OER Bifunctional Electrocatalysis

2D molybdenum disulfide (MoS₂ ) is developed as a potential alternative non-precious metal electrocatalyst for energy conversion. It is well known that 2D MoS₂ has three main phases 2H, 1T, and 1T'. However, the most stable 2H-phase shows poor electrocatalysis in its basal plane, compared with its edge sites. In this work, a facile one-step hydrothermal-driven in situ porousizing of MoS₂ into self-supporting nano islands to maximally expose the edges of MoS₂ grains for efficient utilization of the active stable sites at the edges of MoS₂ is reported. The results show that such active, aggregation-free nano islands greatly enhance MoS₂ 's hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) bifunctional electrocatalytic activities. At a low overpotential of 248 and 300 mV, the porous MoS₂ nano islands can generate a current density of 10 mA cm^-2 in HER and OER, which is much better than typical nanosheet morphology. Surprisingly, the porous MoS₂ nano islands even exhibit better performance than the current commercial RuO₂ catalyst in OER. This discovery will be another effective strategy to promote robust 2H-phase, instead of 1T/1T'-phase, MoS₂ to achieve efficient endurable bifunctional HER/OER, which is expected to further replace precious metal catalysts in industry.

Moiré Superlattice Structure in Two-Dimensional Catalysts: Synthesis, Property and Activity

Two-dimensional (2D) layered materials have been widely used as catalysts due to their high specific surface area, large fraction of uncoordinated surface atoms, and high charge carrier mobility. Moiré superlattice emerges in 2D layered materials with twist angle or lattice mismatch. By manipulating the moiré superlattice structure, 2D layered materials present modulated electronic band structure, topological edge states, and unconventional superconductivity which are tightly associated with the performance of catalysts. Hence, engineering moiré superlattice structures are proposed to be an important technology in modifying 2D layered materials for improved catalytic properties. However, currently, the investigation of moiré superlattice structure in a catalytic application is still in its infancy. This perspective starts with the discussion of structural features and fabrication strategy of 2D materials with moiré superlattice structure. Afterward, the catalytic applications, including electrocatalytic and photocatalytic applications, are summarized. In particular, the promotion mechanism of the catalytic performance caused by the moiré superlattice structure is proposed. Finally, the perspective is concluded by outlining the remaining challenges and possible solutions for the future development of 2D materials with moiré superlattice structure towards the catalytic applications.

MoS2/PDA@Cu composite as a peroxidase-mimicking enzyme with high-effect antibacterial and anticancer activity

Since nanozymes were proposed, their applications have become more and more extensive. As a research hotspot in recent years, MoS₂ also shows many enzyme-like properties. However, as a novel peroxidase, MoS₂ has the disadvantage of a low maximum reaction rate. In this study, the MoS₂/PDA@Cu nanozyme was synthesized by a wet chemical method. The modification of PDA on the surface of MoS₂ achieved the uniform growth of small-sized Cu Nps. The obtained MoS₂/PDA@Cu nanozyme displayed excellent peroxidase-like activity and antibacterial properties. The minimum inhibitory concentration (MIC) of the MoS₂/PDA@Cu nanozyme against S. aureus reached 25 μg mL^-1. Furthermore, it showed a more pronounced inhibitory effect on bacterial growth with the addition of H₂O₂. The maximum reaction rate (V_max) of the MoS₂/PDA@Cu nanozyme is 29.33 × 10^-8 M s^-1, which is significantly higher as compared to that of HRP. It also exhibited excellent biocompatibility, hemocompatibility and potential anticancer properties. When the concentration of the nanozyme was 160 μg mL^-1, the viabilities of 4T1 cells and Hep G2 cells were 45.07% and 32.35%, respectively. This work indicates that surface regulation and electronic transmission control are good strategies for improving peroxidase-like activity.

Phase Instability in van der Waals In2 Se3 Determined by Surface Coordination

van der Waals In₂ Se₃ has attracted significant attention for its room-temperature 2D ferroelectricity/antiferroelectricity down to monolayer thickness. However, instability and potential degradation pathway in 2D In₂ Se₃ have not yet been adequately addressed. Using a combination of experimental and theoretical approaches, we here unravel the phase instability in both α- and β'-In₂ Se₃ originating from the relatively unstable octahedral coordination. Together with the broken bonds at the edge steps, it leads to moisture-facilitated oxidation of In₂ Se₃ in air to form amorphous In₂ Se_3-3x O_3x layers and Se hemisphere particles. Both O₂ and H₂ O are required for such surface oxidation, which can be further promoted by light illumination. In addition, the self-passivation effect from the In₂ Se_3-3x O_3x layer can effectively limit such oxidation to only a few nanometer thickness. The achieved insight paves way for better understanding and optimizing 2D In₂ Se₃ performance for device applications.

High Resolution Electrochemical Imaging for Sulfur Vacancies on 2D Molybdenum Disulfide

Molybdenum disulfide (MoS₂ ) is considered as one of the most promising non-noble-metal catalysts for hydrogen evolution reaction (HER). To achieve practical application, introducing sulfur (S) vacancies on the inert basal plane of MoS₂ is a widely accepted strategy to improve its HER activity. However, probing active sites at the nanoscale and quantitatively analyzing the related electrocatalytic activity in electrolyte aqueous solution are still great challenges. In this work, utilizing high-resolution scanning electrochemical microscopy, optimized electrodes and newly designed thermal drift calibration software, the HER activity of the S vacancies on an MoS₂ inert surface is in situ imaged with less than 20-nm-radius sensitivity and the HER kinetic data for S vacancies, including Tafel plot and onset potential, are quantitatively measured. Additionally, the stability of S vacancies over the wide range of pH 0-13 is investigated. This study provides a viable strategy for obtaining the catalytic kinetics of nanoscale active sites on structurally complex electrocatalysts and evaluating the stability of defects in different environments for 2D material-based catalysts.

Wafer-scale controlled growth of MoS2by magnetron sputtering: from in-plane to inter-connected vertically-aligned flakes

Recently, Molybdenum disulfide (MoS₂) has attracted great attention due to its unique characteristics and potential applications in various fields. The advancements in the field have substantially improved at the laboratory scale however, a synthesis approach that produces large area growth of MoS₂on a wafer scale is the key requirement for the realization of commercial two-dimensional (2D) technology. Herein, we report tunable MoS₂growth with varied morphologies via radio frequency magnetron sputtering by controlling growth parameters. The controlled growth from in-plane to vertically-aligned (VA) MoS₂flakes has been achieved on a variety of substrates (Si, Si/SiO₂, sapphire, quartz, and carbon fiber). Moreover, the growth of VA MoS₂is highly reproducible and is fabricated on a wafer scale. The flakes synthesized on the wafer show high uniformity, which is corroborated by the spatial mapping using Raman over the entire 2-inch Si/SiO₂wafer. The detailed morphological, structural, and spectroscopic analysis reveals the transition from in-plane MoS₂to VA MoS₂flakes. This work presents a facile approach to directly synthesize layered materials by sputtering technique on wafer scale. This paves the way for designing mass production of high-quality 2D materials, which will advance their practical applications by integration into device architectures in various fields.

Retrieving Grain Boundaries in 2D Materials

The coalescence of randomly distributed grains with different crystallographic orientations can result in pervasive grain boundaries (GBs) in 2D materials during their chemical synthesis. GBs not only are the inherent structural imperfection that causes influential impacts on structures and properties of 2D materials, but also have emerged as a platform for exploring unusual physics and functionalities stemming from dramatic changes in local atomic organization and even chemical makeup. Here, recent advances in studying the formation mechanism, atomic structures, and functional properties of GBs in a range of 2D materials are reviewed. By analyzing the growth mechanism and the competition between far-field strain and local chemical energies of dislocation cores, a complete understanding of the rich GB morphologies as well as their dependence on lattice misorientations and chemical compositions is presented. Mechanical, electronic, and chemical properties tied to GBs in different materials are then discussed, towards raising the concept of using GBs as a robust atomic-scale scaffold for realizing tailored functionalities, such as magnetism, luminescence, and catalysis. Finally, the future opportunities in retrieving GBs for making functional devices and the major challenges in the controlled formation of GB structures for designed applications are commented.

Layer-Structured Anisotropic Metal Chalcogenides: Recent Advances in Synthesis, Modulation, and Applications

The unique electronic and catalytic properties emerging from low symmetry anisotropic (1D and 2D) metal chalcogenides (MCs) have generated tremendous interest for use in next generation electronics, optoelectronics, electrochemical energy storage devices, and chemical sensing devices. Despite many proof-of-concept demonstrations so far, the full potential of anisotropic chalcogenides has yet to be investigated. This article provides a comprehensive overview of the recent progress made in the synthesis, mechanistic understanding, property modulation strategies, and applications of the anisotropic chalcogenides. It begins with an introduction to the basic crystal structures, and then the unique physical and chemical properties of 1D and 2D MCs. Controlled synthetic routes for anisotropic MC crystals are summarized with example advances in the solution-phase synthesis, vapor-phase synthesis, and exfoliation. Several important approaches to modulate dimensions, phases, compositions, defects, and heterostructures of anisotropic MCs are discussed. Recent significant advances in applications are highlighted for electronics, optoelectronic devices, catalysts, batteries, supercapacitors, sensing platforms, and thermoelectric devices. The article ends with prospects for future opportunities and challenges to be addressed in the academic research and practical engineering of anisotropic MCs.

Zinc Single Atom Confinement Effects on Catalysis in 1T-Phase Molybdenum Disulfide

Active sites are atomic sites within catalysts that drive reactions and are essential for catalysis. Spatially confining guest metals within active site microenvironments has been predicted to improve catalytic activity by altering the electronic states of active sites. Using the hydrogen evolution reaction (HER) as a model reaction, we show that intercalating zinc single atoms between layers of 1T-MoS2 (Zn SAs/1T-MoS2) enhances HER performance by decreasing the overpotential, charge transfer resistance, and kinetic barrier. The confined Zn atoms tetrahedrally coordinate to basal sulfur (S) atoms and expand the interlayer spacing of 1T-MoS2 by ∼3.4%. Under confinement, the Zn SAs donate electrons to coordinated S atoms, which lowers the free energy barrier of H* adsorption-desorption and enhances HER kinetics. In this work, which is applicable to all types of catalytic reactions and layered materials, HER performance is enhanced by controlling the coordination geometry and electronic states of transition metals confined within active-site microenvironments.

Low-dimensional transition metal sulfide-based electrocatalysts for water electrolysis: overview and perspectives

Hydrogen prepared by electrocatalytic decomposition of water ("green hydrogen") has the advantages of high energy density and being clean and pollution-free, which is an important energy carrier to face the problems of the energy crisis and environmental pollution. However, the most used commercial electrocatalysts are based on expensive and scarce precious metals and their alloy materials, which seriously restricts the large-scale industrial application of hydrogen energy. The development of efficient non-precious metal electrocatalysts is the key to achieving the sustainable development of the hydrogen energy industry. Transition metal sulfides (TMS) have become popular non-precious metal electrocatalysts with great application potential due to their large specific surface area, unique electronic structure, and rich regulatory strategies. To further improve their catalytic activities for practical application, many methods have been tried in recent years, including control of morphology and crystal plane, metal/nonmetal doping, vacancy engineering, building of self-supporting electrocatalysts, interface engineering, etc. In this review, we introduce firstly the common types of TMS and their preparation. Additionally, we summarize the recent developments of the many different strategies mentioned above for efficient water electrolysis applications. Furthermore, the rationales behind their enhanced electrochemical performances are discussed. Lastly, the challenges and future perspectives are briefly discussed for TMS-based water dissociation catalysts.

A wide-angle X-ray scattering laboratory setup for tracking phase changes of thin films in a chemical vapor deposition chamber

The few-layer transition metal dichalcogenides (TMD) are an attractive class of materials due to their unique and tunable electronic, optical, and chemical properties, controlled by the layer number, crystal orientation, grain size, and morphology. One of the most commonly used methods for synthesizing the few-layer TMD materials is the chemical vapor deposition (CVD) technique. Therefore, it is crucial to develop in situ inspection techniques to observe the growth of the few-layer TMD materials directly in the CVD chamber environment. We demonstrate such an in situ observation on the growth of the vertically aligned few-layer MoS₂ in a one-zone CVD chamber using a laboratory table-top grazing-incidence wide-angle X-ray scattering (GIWAXS) setup. The advantages of using a microfocus X-ray source with focusing Montel optics and a single-photon counting 2D X-ray detector are discussed. Due to the position-sensitive 2D X-ray detector, the orientation of MoS₂ layers can be easily distinguished. The performance of the GIWAXS setup is further improved by suppressing the background scattering using a guarding slit, an appropriately placed beamstop, and He gas in the CVD reactor. The layer growth can be monitored by tracking the width of the MoS₂ diffraction peak in real time. The temporal evolution of the crystallization kinetics can be satisfactorily described by the Avrami model, employing the normalized diffraction peak area. In this way, the activation energy of the particular chemical reaction occurring in the CVD chamber can be determined.

A combined QTAIM/IRI topological analysis of the effect of axial/equatorial positions of NH2 and CN substituents in the [(PY5Me2)MoO]+ complex

By means of the Interaction Region Indicator (IRI) and Quantum Theory of Atoms in Molecules (QTAIM), the influence exerted by NH₂ (amino) and CN (cyano) as electron donor and electron acceptor substituent groups, respectively, located at para-positions of axial and equatorial pyridine rings of derivatized complexes coming from the [(PY₅Me₂)MoO]⁺ complex during the hydrogen molecular release in the gas phase was analyzed. In any case, a H-H covalent bond is forming at the transition state, with a strengthening of the electron density of 5.5% when the substituent group involved is NH₂ at the para-position of the axial pyridine ring. However, there was no difference between NH₂ and CN when these substituent groups are located at the para-positions of the equatorial pyridine rings. The topological properties of electron densities from the QTAIM are not perturbed by the electron donor and electron acceptor nature of the substituents, even when these substituent groups are located at the axial or equatorial pyridine rings of the Mo-based complex.

Impact Electrochemistry of MoS2: Electrocatalysis and Hydrogen Generation at Low Overpotentials

MoS₂ materials have been extensively studied as hydrogen evolution reaction (HER) catalysts. In this study nanoparticulate MoS₂ is explored as a HER catalyst through impact voltammetry. The onset potential was found to be -0.10 V (vs RHE) at pH 2, which was confirmed to be due to HER by scale-up of the impact experiment to generate and collect a sufficient volume of the gas to enable its identification as hydrogen via gas chromatography. This is in contrast to electrodeposited MoS₂, which was found to be stable in pH 2 sulfuric acid solution with an onset potential of -0.29 V (vs RHE), in good agreement with literature. XPS was used to categorize the materials and confirm the chemical composition of both nanoparticles and electrodeposits, with XRD used to analyze the crystal structure of the nanoparticles. The early onset of HER was postulated from kinetic analysis to be due to the presence of nanoplatelets of about 1-3 trilayers participating in the impact reactions, and AFM imaging confirmed the presence of these platelets.

"Nano Killers" Activation by permonosulfate enables efficient anaerobic microorganisms disinfection

The development of effective nanomaterials for killing anaerobic bacteria is essential for human health and economic development. Here, we propose a new bactericidal mechanism where theoretical calculations are in good agreement with experimental results. The "poison arrow-head" of MoS₂ nanosheets enables the vigorous extraction of lipids from the cell membrane. Based on density functional calculations, oxidation active species (OAS) are generated due to the strong adsorption energy between S vacancies in MoS₂ and chemical substrates (permonosulfate (PMS) and H₂O). These OAS can be visualized as numerous moving "nano killers", constantly oxidizing the lipids around MoS₂; thereby, re-releasing the surface of the sharp knife. The process of physical extraction collaborated with chemical oxidation not only precisely positions the cell membrane but also allows for continuous sterilization. This work digs into the mechanism of anaerobic bacterial sterilization, which sheds significant light on biological analysis, antibacterial, cancer therapy, and anti microbiologically influenced corrosion.

Phosphorus-Rich Ruthenium Phosphide Embedded on a 3D Porous Dual-Doped Graphitic Carbon for Hydrogen Evolution Reaction

Metal phosphides have recently emerged as promising electrocatalysts for hydrogen evolution reaction (HER). Herein, we report the synthesis of ruthenium diphosphide embedded on a dual-doped graphitic carbon by pyrolyzing chitosan beads impregnated with ruthenium chloride and phosphorus pentoxide. The as-synthesized RuP₂@N-P-C displays a good electrocatalytic activity in acidic, neutral and alkaline media. We show that the HER activity of the electrocatalyst can be tuned by varying the concentration of Li⁺ cations. Co-diffusion effects on H⁺ exerted by Li⁺ on HER in the porous carbon matrix have been observed.

Fe-Doped MoS2 Nanozyme for Antibacterial Activity and Detoxification of Mustard Gas Simulant

The peroxidase-like catalytic activity of various nanozymes was extensively applied in various fields. In this study, we have demonstrated the preparation of Fe-doped MoS₂ (Fe@MoS₂) nanomaterials with enhanced peroxidase-like activity of MoS₂ in a co-catalytic pathway. In view of Fenton reaction, the peroxidase-like Fe@MoS₂ nanozyme prompted the decomposition of hydrogen peroxide (H₂O₂) to a reactive hydroxyl radical (·OH). The efficient decomposition of H₂O₂ in the presence of Fe@MoS₂ has been employed toward the antibacterial activity and detoxification of mustard gas simulant. The combined effect of Fe@MoS₂ and H₂O₂ showed remarkable antibacterial activity against the drug-resistant bacterial strain methicillin-resistant Staphylococcus aureus and Escherichia coli with the use of minimal concentration of H₂O₂. Fe@MoS₂ was further applied for the detoxification of the chemical warfare agent sulfur mustard simulant, 2-chloroethyl ethyl sulfide, by selective conversion to the nontoxic sulfoxide. This work demonstrates the development of a hybrid nanozyme and its environmental remediation from harmful chemicals to microbes.

First-principles modeling of the highly dynamical surface structure of a MoS2 catalyst with S-vacancies

Vacancy sites, e.g., S-vacancies, are essential for the performance of MoS₂ catalysts. As earlier studies have revealed that the size and shape of the S-vacancies may affect the catalytic activity, we have studied the behavior and mobility of such vacancies on MoS₂ using DFT calculations and kinetic Monte-Carlo (kMC) simulations. The diffusion barriers for the S-vacancies are highly dependent on the immediate environment: isolated single S-vacancies are found to be immobile. In contrast, small nS-vacancies formed from n = 2 to 5 neighboring S-vacancies are often highly dynamic systems that can move within a confined area. Large extended nS-vacancies are generally unstable and transform quickly into alternating patterns of S-atoms and vacancy sites. These results illustrate the importance of recognizing MoS₂ (but also other catalysts) as dynamic structures when trying to tune their catalytic performances by introducing specific defect structures.

Magnetic Field Alignment and Optical Anisotropy of MoS2 Nanosheets Dispersed in a Liquid Crystal Polymer

Molybdenum disulfide (MoS₂) nanosheets exhibit anisotropic optical and electronic properties, stemming from their shape and electronic structure. Unveiling this anisotropy for study and usage in materials and devices requires the ability to control the orientation of dispersed nanosheets, but to date this has proved a challenging proposition. Here, we demonstrate magnetic field driven alignment of MoS₂ nanosheets in a liquid crystal (LC) polymer and unveil the optical properties of the resulting anisotropic assembly. Nanosheet optical anisotropy is observed spectroscopically by Raman and direction-dependent photoluminescence (PL) measurements. Resulting data indicate significantly lower PL emission due to optical excitation with electric field oscillation out of plane, parallel to the MoS₂ c-axis, than that associated with perpendicular excitation, with the dichroic ratio I_perp/I_par = 3. The approach developed here provides a useful route to elucidate anisotropic optical properties of MoS₂ nanosheets and to utilize such properties in new materials and devices.

Atomic and structural modifications of two-dimensional transition metal dichalcogenides for various advanced applications

Two-dimensional (2D) transition metal dichalcogenides (TMDs) and their heterostructures have attracted significant interest in both academia and industry because of their unusual physical and chemical properties. They offer numerous applications, such as electronic, optoelectronic, and spintronic devices, in addition to energy storage and conversion. Atomic and structural modifications of van der Waals layered materials are required to achieve unique and versatile properties for advanced applications. This review presents a discussion on the atomic-scale and structural modifications of 2D TMDs and their heterostructures via post-treatment. Atomic-scale modifications such as vacancy generation, substitutional doping, functionalization and repair of 2D TMDs and structural modifications including phase transitions and construction of heterostructures are discussed. Such modifications on the physical and chemical properties of 2D TMDs enable the development of various advanced applications including electronic and optoelectronic devices, sensing, catalysis, nanogenerators, and memory and neuromorphic devices. Finally, the challenges and prospects of various post-treatment techniques and related future advanced applications are addressed.