MoS2 Edges Research Articles

Pressure-Dependent Shape and Edge Configurations of MoS2 by Kinetic Monte Carlo Simulation.

Understanding the influence of precursor pressures is crucial for optimizing the properties of MoS2 grown through the chemical vapor deposition (CVD) process. In this study, we use kinetic Monte Carlo (KMC) simulations to investigate how varying the pressures of molybdenum (PMo) and sulfur (PS) impacts the structural properties of MoS2, such as grain shape and edge configurations. The simulations differentiate three distinct regimes─growth, steady-state, and etching─each defined by specific PMo, PS, and the most probable atomic sites for filling or etching. We further explore how these regimes influence the atomic configuration of MoS2, particularly the formation of different edge structures like sulfur zigzag (ZZS), molybdenum zigzag (ZZMo), and their respective derivatives. A pressure diagram based on the equations of state and most probable atomic sites was constructed for each regime and validated by comparing predicted ZZ-derived edges to experimental observations. Additionally, the study examines the impact of etching on various line defects, providing insights into the evolution of the MoS2 edges during the CVD process. These findings underscore the importance of controlling both growth and cessation phases in the CVD process to customize edge configurations, with significant implications for chemical functionalization, catalysis, and the electronic properties of transition metal dichalcogenides.

Au – MoS2 nanoflowers sensors on interdigitated electrode for monitoring human respiration

Abstract Respiratory humidity sensors are essential for monitoring breath conditions in a variety of applications such as respiratory disease diagnosis, sleep apnea screening, and personalized medicine. With the increasing prevalence of respiratory diseases, such as asthma, chronic obstructive pulmonary disease (COPD), and sleep apnea, they have become essential tools for healthcare professionals, researchers and individuals. However, existing humidity sensors often suffer from poor sensitivity, slow response, and limited flexibility. In this paper, we report a novel Au – MoS2 respiratory humidity sensor that exhibits high resistivity, fast response, and excellent flexibility. The sensor is fabricated by drop casting a thin layer of Au decorated MoS2 on copper interdigitated electrodes (fabricated using copper clad PCB). The increase in the intensity of the peak was observed in Au – MoS2 hybrids in comparison to MoS2 analyzed using micro–Raman Spectroscopy. Using scanning electron microscopy (SEM), the morphology of MoS2 was observed, confirming its flower like structure. The Au nanoparticles were detected on the edges of MoS2, confirmed by transmission electron microscope (TEM).
The interfacial electronic interaction suggests that there is the formation of Schottky barrier between Au and MoS2 as confirmed by X–Ray photoelectron spectroscopy (XPS) and ultraviolet photoelectron spectroscopy (UPS). The improved electronic interaction results in better sensing properties. The high dielectric constant and excellent surface-to-volume ratio of the MoS2 layer make it extremely sensitive to humidity; the Au nanoparticles offer good electrical conductivity and mechanical flexibility in its place. The study tested a respiratory sensor in a simulated environment, finding it more responsive to breathing and quick to return to rest. This suggests potential for the Au-MoS2 sensor in tracking breath states due to its sensitivity and adaptability, promising applications across various fields.

Probing Electrocatalytic Synergy in Graphene/MoS2/Nickel Networks for Water Splitting through a Combined Experimental and Theoretical Lens.

The development of low-cost and active electrocatalysts signifies an important effort toward accelerating economical water electrolysis and overcoming the sluggish hydrogen or oxygen evolution reaction (HER or OER) kinetics. Herein, we report a scalable and rapid synthesis of inexpensive Ni and MoS2 electrocatalysts on N-doped graphene/carbon cloth substrate to address these challenges. Mesoporous N-doped graphene is synthesized by using electrochemical polymerization of polyaniline (PANI), followed by a rapid one-step photothermal pyrolysis process. The N-doped graphene/carbon cloth substrate improves the interconnection between the electrocatalyst and substrate. Consequently, Ni species deposited on an N-doped graphene OER electrocatalyst shows a low Tafel slope value of 35 mV/decade at an overpotential of 130 mV at 10 mA/cm2 current density in 1 M KOH electrolytes. In addition, Ni-doped MoS2 on N-doped graphene HER electrocatalyst shows Tafel slopes of 37 and 42 mV/decade and overpotentials of 159 and 175 mV, respectively, in acidic and alkaline electrolytes at 10 mA/cm2 current density. Both these values are lower than recently reported nonplatinum-group-metal-based OER and HER electrocatalysts. These excellent electrochemical performances are due to the high electrochemical surface area, a porous structure that improves the charge transfer between electrode and electrolytes, and the synergistic effect between the substrate and electrocatalyst. Raman spectroscopy, X-ray photoelectron spectroscopy, and density functional theory (DFT) calculations demonstrate that the Ni hydroxide species and Ni-doped MoS2 edge sites serve as active sites for OER and HER, respectively. Finally, we also evaluate the performance of the HER electrocatalyst in commercial alkaline electrolyzers.

DFT study on doping effect of different transition metals on MoS2 for enhancing the formation of S vacancies

MoS2 modification by doping transition metals is an effective way to change the Mo–S bond energy by taking advantage of the electronic variability of the d orbitals of transition metals. Herein, this study investigates the doping effect of different transition metals onto MoS2 edge sites through DFT calculations. To screen out the metal additive effect, different transition metals were doped to the MoS2 edge sites to see the effect on generating S vacancies that were generally regarded as the active sites. DFT simulation results proved that the ability of the valence electrons to populate the orbitals with the greatest overlap with the orientation of the edge Mo–S bond was a major factor in how simple it was to create S vacancies at the Mo-edge and S-edge sites. For the transition metals, the vacancy formation energy increases with the increase of periodic number for the same group, while decreases with the increase of the atomic weight for the same period. This implied that the upper right corner transition metals in the Periodic Table could be selected to modify MoS2 to obtain the minimum vacancy formation energy. Therefore, nickel and cobalt are the best choice as the doping element for modifying MoS2.

Activation of inert surface of monolayer MoS2 by transition metal and nitrogen co-doping to promote hydrogen evolution reaction

In this work, we systematically investigate the possible H adsorption sites, charge density difference, partial density of states, p-band center and Bader charge of transition metal (TM) and nitrogen (N) co-doped monolayer MoS2. The results show that the equilibrium of H adsorption and desorption can be achieved in TM-N co-doped monolayer MoS2. The ΔGH at the S site of Co-N co-doped system is −0.05 eV, which is better than that of traditional Pt site (ΔGH=-0.09 eV) and Mo site at the MoS2 edge (ΔGH=0.08 eV). This work has important implications for designing more efficient electrocatalysts.

Synthesis of MoS2/Co9S8@CoNC by impregnation and calcination for highly efficient lithium-O2 batteries

The activity of electro-catalyst on oxygen cathode is a key factor affecting the property of lithium-O2 batteries (LOBs). In this work, zeolitic imidazolate framework (ZIF–67) was impregnated in (NH4)2MoS4 solution, and then through centrifugation and calcination, nano-MoS2 and Co9S8 were grown on the surface of CoNC skeleton. The composite has mesoporous structure and abundant active groups such as Co, Co9S8, Co-N bond, Mo-N bond and active edges of MoS2 with sulfur vacancies. The coating of ultra-thin MoS2 nanosheets can improve the stability of CoNC skeleton. The Mo component induces the transformation of amorphous carbon of CoNC to graphitic carbon during calcination in order to improve conductivity of the composite. This study shows that MoS2/Co9S8@CoNC is good electro-catalyst for O2 reduction and evolution reactions (ORR/OER). The LOBs with MoS2/Co9S8@CoNC oxygen cathode show high specific discharge capacity (18162 mA h g−1) and good cycle performance (304 cycles).

3D Printing of Tungstate Anion Modulated 1T‐MoS2 Composite Cathodes for High‐Performance Lithium–Sulfur Batteries

AbstractLithium–sulfur (Li–S) batteries can offer high capacity and energy‐density, but face challenges like low conductivity, lithium polysulfides (LiPSs) shuttling, and limited reaction kinetics. In this study, the electronic configuration of Mo 4d orbital in MoS2 is modulated through a one‐step method involving tungstate anion (WO42−) modulation to form a stable 1T‐MoS2/carbon composite (1T‐W‐MoS2/C). When WO42− is introduced, it causes a transfer of electrons to Mo in 2H‐MoS2, resulting in the generation of a stable 1T phase. In the composite, 1T‐MoS2 nanosheets exhibit remarkable electronic conductivity, hydrophilicity, and catalytic activity, facilitating the LiPSs adsorption and Li+ transport. Meanwhile, the WO42− modulation can create abundant adsorption/catalytic sites with defects on the basal surface and edges of MoS2, facilitating the efficient catalysis of LiPSs conversion. Furthermore, the 3D‐printed electrodes without utilization of binders and current collectors can ensure high mass loading and promote ion diffusion and electrolyte penetration. Theoretical and experimental results confirm that 1T‐W‐MoS2/C can catalyze LiPSs conversion, suppress LiPSs shuttling, and enhance sulfur reaction kinetics. Therefore, the 3D‐printed 1T‐W‐MoS2/C/S cathode exhibits a high initial capacity and excellent rate capability, achieving an areal capacity of 7.37 mAh cm−2 with a sulfur loading of 8.89 mg cm−2.

Accelerated Charge Transfer through Interface Chemical Bonds in MoS2/TiO2 for Photocatalytic Conversion of Lignocellulosic Biomass to H2.

Solar photocatalytic H2 production from lignocellulosic biomass has attracted great interest, but it suffers from low photocatalytic efficiency owing to the absence of highly efficient photocatalysts. Herein, we designed and constructed ultrathin MoS2-modified porous TiO2 microspheres (MT) with abundant interface Ti-S bonds as photocatalysts for photocatalytic H2 generation from lignocellulosic biomass. Owing to the accelerated charge transfer related to Ti-S bonds, as well as the abundant active sites for both H2 and ●OH generation, respectively, related to the high exposed edge of MoS2 and the large specific surface area of TiO2, MT photocatalysts demonstrate good performance in the photocatalytic conversion of α-cellulose and lignocellulosic biomass to H2. The highest H2 generation rate of 849 μmol·g-1·h-1 and apparent quantum yield of 4.45% at 380 nm was achieved in α-cellulose aqueous solution for the optimized MT photocatalyst. More importantly, lignocellulosic biomass of corncob, rice hull, bamboo, polar wood chip, and wheat straw were successfully converted to H2 over MT photocatalysts with H2 generation rate of 10, 19, 36, 29, and 8 μmol·g-1·h-1, respectively. This work provides a guiding design approach to develop highly active photocatalysts via interface engineering for solar H2 production from lignocellulosic biomass.

Amorphous molybdenum sulfide nanosheets composed of [Mo3S13]2- active-site motifs for enhancing conversion of Fe3+/Fe2+ in Fenton reaction under neutral condition

Molybdenum disulfide (MoS2) edges have been considered as effective active sites to promote Fenton reaction, whereas the basal plane is inert. It is dauntingly challenging to maximally exposure edge active sites and active the basal plane. Inspired that [Mo3S13]2- clusters is basic structural unit of MoS2 edge, amorphous molybdenum sulfide (a-MoSx) nanosheets composed of [Mo3S13]2- active-site motifs may be an ideal co-catalyst for Fenton reaction. Herein, accordion-like a-MoSx nanosheets were successfully designed and prepared by one step oil-heating method. The as-synthesized a-MoSx exhibited superior activity in co-catalytic Fenton degradation of tetracycline (TC, 10 mg/L) than that of MoS2, which could reach 94.73 % within 5 min under neutral condition. This is attributed to the better Mo-S coordination mode or surrounding environment to accelerate the rate-limiting step of Fe3+/Fe2+ conversion, with DFT calculations indicating for Fe3+ on Mo4+ or S2-/S22- sites in a-MoSx a higher electron aggregation density and lower adsorption energy. Impressively, all the S atoms in a-MoSx have low adsorption energy with H atom, making that the co-catalytic Fenton reaction can be performed under neutral conditions. For a scale-up experiment, 5 L of TC wastewater (50 mg/L) was almost completely degraded after 120 min in the nickel foam/a-MoSx + H2O2 + Fe2+ system.

Edge-enhanced super microgenerator based on a two-dimensional Schottky junction

Super microgenerator (SMG) refers to a generator that can efficiently convert extremely weak external stimuli into electrical energy and has a small size, high power density and long lifespan, offer ground-breaking solutions for powering wearable devices, wireless distributed sensors and implanted medical equipment. However, the friction and wear between the interfaces of ordinary microgenerator results in an extremely low lifespan. Here, we present a prototype of SMGs based on a 2D–2D (graphite–MoS2) Schottky contact in the state of structural superlubricity (no wear and nearly zero friction between two contacted solid surfaces). What is even more interesting is when the graphite flake is slid from the bulk to the edge of MoS2, the output current will enhance from 31 to 56 A m−2. Through the I–V curve measurement, we found that the conductive channel across the junction can be activated and further enhanced at the edge of MoS2 compare to bulk, which provide the explanation for the above-mentioned edge enhancement of power generation. Above results provide the design principles of high-performance SMGs based on 2D–2D Schottky junctions.

High-Temperature Superlubricity in MoS2/Graphene van der Waals Heterostructures.

Achieving high-temperature superlubricity is essential for modern extreme tribosystems. Solid lubrication is the sole viable alternative due to the degradation of liquid ones but currently suffers from notable wear, instability, and high friction coefficient. Here, we report robust superlubricity in MoS2/graphene van der Waals heterostructures at high temperatures up to ∼850 K, achieved through localized heating to enable reliable friction testing. The ultralow friction of the MoS2/graphene heterostructure is found to be notably further reduced at elevated temperature and dominantly contributed by the MoS2 edge. The observation can be well described by a multi-contact model, wherein the thermally activated rupture of edge-contacts facilitates the sliding. Our results should be applicable to other van der Waals heterostructures and shed light on their applications for superlubricity at elevated temperature.

Suppression of inactive sites in NiMo@amorphous alumina catalysts for hydrodesulfurization reaction: Effect of Ga doping

Hydrodesulfurization (HDS) reaction activity depends on the formation of active NiMoS (completely sulfided multilayer type 2 sites) and inactive NiAl2O4/NixSy phases on the catalyst surface. These inactive phases not only itself inactive for the reaction but also hinder the formation of active NiMoS phases. In addition, the formation of strong Mo-O-Al bonds also lowers the reducibility of the catalyst, suggesting lesser catalytic activity (type 1 sites, partially sulfided single-layer sites). To this end, we report the formation of NiMo/Ga doped amorphous alumina (A-Al2O3) catalysts prepared by the colloidal synthesis method. A-Al2O3 was chosen as a support because it has lower surface energy, better physicochemical properties, and enhanced acidic sites than the crystalline alumina phase, which favours the formation of type 2 sites or weak metal-support interaction (completely sulfided multilayer NiMoS sites). Further, the Ga doping in A-Al2O3 support substitute the tetrahedral Al sites and form GaAl2O4 phases by reducing NiAl2O4 inactive sites, and the free Ni decorate the edges of MoS2 to form HDS active NiMoS sites. Therefore, NiMo/Ga doped A-Al2O3 catalyst (at 1 wt% Ga loading) reported enhanced HDS catalytic activity (25%), HDS reaction rate constant (kHDS-2.15 times) and turn over frequency (TOF-1.3 times) than the conventional NiMo/γ-Al2O3 catalysts.

Insights into the hydrodesulfurization reaction mechanisms catalyzed by different K-promoted MoS2 edge sites

The K-promoted MoS2 active phase structure and its catalytic mechanism in the hydrodesulfurization reaction were investigated through the density functional theory calculations. On the basis of the calculations, the sulfur vacancy formation mechanism and the direct desulfurization of thiophene were systematically explored, and the structure–activity relationship was also discussed in detail. In the sulfur vacancy generation process, it could be found that the formation of sulfur vacancies was more favorable over the sulfur-rich edges, such as K-promoted S-edge with 100 % S coverage. In addition, the K-promoted S-edge was more preferable for the C-S bond scissions compared with the K-promoted M−edge. Furthermore, potassium atoms could prevent the adsorptions of toluene and 2-butene adsorptions, in order to reduce the excessive hydrogenation saturation of olefins and monocyclic aromatics, which would be beneficial for the octane rating maintenance during the production process of gasoline.

Water adsorption on MoS2 under realistic atmosphere conditions and impacts on tribology.

Molybdenum disulfide (MoS2) is a 2D material widely used as a dry lubricant. However, exposure to water and oxygen is known to reduce its effectiveness, and therefore an understanding of the uptake of water is important information for mitigating these effects. Here we use grand canonical Monte Carlo simulations to rigorously study water adsorption on MoS2 surfaces and edges with different concentrations of defects under realistic atmospheric conditions (i.e. various temperatures and humidity levels). We find that the amount of water adsorbed depends strongly on the number of defects. Simulations indicate that defect sites are generally saturated with water even at low ppm levels of humidity. Water binds strongly to S vacancies on interlamellar surfaces, but generally only one water molecule can fit on each of these sites. Defects on surfaces or edges of lamellae also strongly attract water molecules that then nucleate small clusters of water bonded via hydrogen bonding. We demonstrate that water preferentially binds to surface defects, but once those are saturated at a critical humidity level of about 500-1000 ppm water, water binds to edge sites where it negatively impacts the tribological performance of MoS2.

Theoretical study of the catalytic hydrodeoxygenation of furan, methylfuran and benzofurane on MoS2.

Herein, we have studied the direct deoxygenation (DDO) (without prior hydrogenation) of furan, 2-methylfuran and benzofuran on the metal edge of MoS2 with a vacancy created under pressure of dihydrogen. For the three molecules, we found that the desorption of the water molecule for the regeneration of the vacancy is the most endothermic. Based on the thermodynamic and kinetic aspects, the reactivity order of the oxygenated compounds is furan ≈ 2-methylfuran > benzofuran, which is in agreement with literature. We present the key stages of the mechanisms and highlight the effects of substituents.

Unraveling energetics and states of adsorbing oxygen species with MoS2 for modulated work function.

MoS2 and related transition metal dichalcogenides (TMDs) have recently been reported as having extensive applications in nanoelectronics and catalysis because of their unique physical and chemical properties. However, one practical challenge for MoS2-based applications arises from the easiness of oxygen contamination, which is likely to degrade performance. To this end, understanding the states and related energetics of adsorbed oxygen is critical. Herein, we identify various states of oxygen species adsorbed on the MoS2 surface with first-principles calculations. We reveal a "dissociative" mechanism through which a physisorbed oxygen molecule trapped at a sulfur vacancy can split into two chemisorbed oxygen atoms, namely a top-anchoring oxygen and a substituting oxygen, both of which show no adsorbate induced states in the bandgap. The electron and hole masses show an asymmetric effect in response to oxygen species with the hole mass being more sensitive to oxygen content due to a strong hybridization of oxygen states in the valence band edge of MoS2. Alteration of oxygen content allows modulation of the work function up to 0.5 eV, enabling reduced Schottky barriers in MoS2/metal contact. These results show that oxygen doping on MoS2 is a promising method for sulfur vacancy healing, carrier mass controlling, contact resistance reduction, and anchoring of surface electron dopants. Our study suggests that tuning the chemical composition of oxygen is viable for modulating the electronic properties of MoS2 and likely other chalcogen-incorporated TMDs, which offers promise for new optoelectronic applications.

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.

Vertical heterostructure of graphite-MoS2 for gas sensing.

2D materials, given their form-factor, high surface-to-volume ratio, and chemical functionality have immense use in sensor design. Engineering 2D heterostructures can result in robust combinations of desirable properties but sensor design methodologies require careful considerations about material properties and orientation to maximize sensor response. This study introduces a sensor approach that combines the excellent electrical transport and transduction properties of graphite film with chemical reactivity derived from the edge sites of semiconducting molybdenum disulfide (MoS2) through a two-step chemical vapour deposition method. The resulting vertical heterostructure shows potential for high-performance hybrid chemiresistors for gas sensing. This architecture offers active sensing edge sites across the MoS2 flakes. We detail the growth of vertically oriented MoS2 over a nanoscale graphite film (NGF) cross-section, enhancing the adsorption of analytes such as NO2, NH3, and water vapor. Raman spectroscopy, density functional theory calculations and scanning probe methods elucidate the influence of chemical doping by distinguishing the role of MoS2 edge sites relative to the basal plane. High-resolution imaging techniques confirm the controlled growth of highly crystalline hybrid structures. The MoS2/NGF hybrid structure exhibits exceptional chemiresistive responses at both room and elevated temperatures compared to bare graphitic layers. Quantitative analysis reveals that the sensitivity of this hybrid sensor surpasses other 2D material hybrids, particularly in parts per billion concentrations.

Seeing nanoscale electrocatalytic reactions at individual MoS2 particles under an optical microscope: probing sub-mM oxygen reduction reaction.

MoS2 is a promising electrocatalytic material for replacing noble metals. Nanoelectrochemistry studies, such as using nanoelectrochemical cell confinement, have particularly helped in demonstrating the preferential electrocatalytic activity of MoS2 edges. These findings have been accompanied by considerable research efforts to synthesize edge-abundant nanomaterials. However, to fully apprehend their electrocatalytic performance, at the single particle level, new instrumental developments are also needed. Here, we feature a highly sensitive refractive index based optical microscopy technique, namely interferometric scattering microscopy (iSCAT), for monitoring local electrochemistry at single MoS2 petal-like sub-microparticles. This work focuses on the oxygen reduction reaction (ORR), which operates at low current densities and thus requires high-sensitivity imaging techniques. By employing a precipitation reaction to reveal the ORR activity and utilizing the high spatial resolution and contrast of iSCAT, we achieve the sensitivity required to evaluate the ORR activity at single MoS2 particles.

One-pot hydrothermal synthesis of self-assembled MoS2/WS2 nanoflowers for chemiresistive room-temperature NO2 sensors

Transition metal dichalcogenides (TMDs) based heterostructures are becoming hotspots of gas-sensing applications due to the high electronic mobility, high surface-volume ratio, and abundant adsorption sites. However, the preparation of TMDs-based heterostructures is usually required multiple steps or higher synthesis temperatures (≥ 500 °C). Herein, the flower-like MoS2/WS2 composites were firstly prepared by a facile one-pot hydrothermal method to effectively detect NO2 at room temperature. The WS2 molecules prefer to grow on the edge of as-nucleated MoS2 to form the MoS2/WS2 nanosheets, and then they can grow by layer-by-layer self-assembly to form the MoS2/WS2 heterostructure. The optimized MoS2/WS2 heterostructured composite exhibits the very high sensitivity to NO2 at room temperature, involving high response, good reproducibility, excellent selectivity and long-term stability. Specially, the responses (ΔR/Rg) of MoS2/WS2-0.5 composite to 5 and 100 ppb NO2 are 6.1% and 23.1%, respectively. Moreover, a plausible gas sensing mechanism was proposed from viewpoints of electron transfer and gas adsorption by combining multiple experiments and DFT calculations. This work provides a facile process for synthesis of TMDs-based heterostructures, but also a prospective gas sensing material for room-temperature NO2 detection.