Basal Plane Of MoS2 Research Articles

Surface electron state engineering enhanced hydrogen evolution of hierarchical molybdenum disulfide in acidic and alkaline media

Metal sulfides (MoS2) for hydrogen evolution reaction (HER) catalysts typically suffer from inert kinetics, especially in alkaline conditions, due to its improper hydrogen adsorption/desorption capability on the basal plane S atoms. However, the hydrogen adsorption/desorption can be tuned by modulating the electron state by introducing the dopants in the basal plane of MoS2. Herein, we report a MoO2/MoS2 structure for efficient HER in alkaline and acidic electrolyte. The design of this high-performance MoO2/MoS2|P electrode is verified by density functional theory (DFT) calculation. The calculation demonstrates that the electron state of ultra-thin MoS2 can be modulated by the substitution reaction (S-O or P-O) on MoO2 surface to optimize hydrogen binding behavior and thus promote HER kinetics. Particularly, MoO2/MoS2|P (P doped) electrode exhibits an overpotential of 45 mV for 10 mA/cm2, which is a record low value among the reported transition metal dichalcogenides (TMDs) measured in the alkaline electrolyte so far.

Engineering Sulfide-Phosphide Based Double Catalysts on 3D Nickel Phosphides Framework for Electrolytic Hydrogen Evolution: Activating Short-range Crystalline MoS2 with Ni5P4-Ni2P Template

It is a universal quest to produce molecular hydrogen (H2) from sustainable green routes to diversify the heavy dependence of fossil fuels for energy consumption. Developing earth-abundant electrocatalysts for water electrolysis is a promising method to generate hydrogen via hydrogen evolution reaction (HER). In this work, we strategically promote electron transport and activate the basal planes of MoS2 via a thermal hybridization with vertically-aligned hierarchical nickel phosphide Ni5P4-Ni2P (denoted as MoS2/Ni5P4-Ni2P) for HER. The metallic-like Ni5P4-Ni2P foam promotes electron transportation into the MoS2 matrix by electron injection due to the difference of their Fermi levels. Moreover, MoS2 basal planes are activated by Ni5P4-Ni2P due to phosphorus doping effects, triggering additional actives sites and boosting the overall HER performances. Notably, the HER performance of MoS2/Ni5P4-Ni2P electrode only requires an overpotential of 96 mV to reach a geometric current density of −10 mA cm−2 with relatively low Tafel slope of 74 mV dec−1. The catalytic performances can last for at least 22 h at −10 mA cm−2 without discernible degradation, indicating the extraordinary long-term operation stability. This work provides insights into the synergistic effects of a hybridized catalyst, which may potentially serve as a next-generation electrocatalyst for efficient water splitting.

Interpretable molecular models for molybdenum disulfide and insight into selective peptide recognition.

Molybdenum disulfide (MoS2) is a layered material with outstanding electrical and optical properties. Numerous studies evaluate the performance in sensors, catalysts, batteries, and composites that can benefit from guidance by simulations in all-atom resolution. However, molecular simulations remain difficult due to lack of reliable models. We introduce an interpretable force field for MoS2 with record performance that reproduces structural, interfacial, and mechanical properties in 0.1% to 5% agreement with experiments. The model overcomes structural instability, deviations in interfacial and mechanical properties by several 100%, and empirical fitting protocols in earlier models. It is compatible with several force fields for molecular dynamics simulation, including the interface force field (IFF), CVFF, DREIDING, PCFF, COMPASS, CHARMM, AMBER, and OPLS-AA. The parameters capture polar covalent bonding, X-ray structure, cleavage energy, infrared spectra, bending stability, bulk modulus, Young's modulus, and contact angles with polar and nonpolar solvents. We utilized the models to uncover the binding mechanism of peptides to the MoS2 basal plane. The binding strength of several 7mer and 8mer peptides scales linearly with surface contact and replacement of surface-bound water molecules, and is tunable in a wide range from −86 to −6 kcal mol−1. The binding selectivity is multifactorial, including major contributions by van-der-Waals coordination and charge matching of certain side groups, orientation of hydrophilic side chains towards water, and conformation flexibility. We explain the relative attraction and role of the 20 amino acids using computational and experimental data. The force field can be used to screen and interpret the assembly of MoS2-based nanomaterials and electrolyte interfaces up to a billion atoms with high accuracy, including multiscale simulations from the quantum scale to the microscale.

Controllable desulfurization in single layer MoS2 by cationic current treatment in hydrogen evolution reaction

The sulfur vacancy (Sv) generation in the MoS2 basal plane is an efficient strategy for improving the hydrogen evolution reaction (HER). By using cationic current treatment, the Sv density can be controlled by modifying the setting voltage ranges and treatment time. The Sv generation mechanism was clearly defined using Raman mapping. From the Raman mapping characterizations, for the first time, we experimentally found that Sv tends to be generated next to existing Sv by appearing around cracked/damaged areas or group formations. The S: Mo atomic ratio reduced from 2.02:1 to 1.86:1 after treatment. As a result, the current density at −0.3 V vs RHE sharply increases up to 27-fold in comparison to that with untreated MoS2, and the overpotential of treated MoS2 reaches 222 mV at 10 mA/cm2 with a Tafel slope of 96 mV/decade. Hence, the controllable Sv generation in monolayer MoS2 using a cationic current method can be considered as a fast, simple, and effective process to improve HER performance in large-scale production.

Electrochemical Hydrogen Evolution over Hydrothermally Synthesized Re-Doped MoS2 Flower-Like Microspheres.

In this research, we report a simple hydrothermal synthesis to prepare rhenium (Re)- doped MoS2 flower-like microspheres and the tuning of their structural, electronic, and electrocatalytic properties by modulating the insertion of Re. The obtained compounds were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), high-resolution transmission electron microscopy (HRTEM), Raman spectroscopy, and X-ray photoelectron spectroscopy (XPS). Structural, morphological, and chemical analyses confirmed the synthesis of poorly crystalline Re-doped MoS2 flower-like microspheres composed of few stacked layers. They exhibit enhanced hydrogen evolution reaction (HER) performance with low overpotential of 210 mV at current density of 10 mA/cm2, with a small Tafel slope of 78 mV/dec. The enhanced catalytic HER performance can be ascribed to activation of MoS2 basal planes and by reduction in charge transfer resistance during HER upon doping.

Direct Imaging of Individual Molecular Binding to Clean Nanopore Edges in 2D Monolayer MoS2.

We use annular dark-field scanning transmission electron microscopy (ADF-STEM) to study how solution-deposited molecules bind to the edges and surface regions around nanopores in MoS2 monolayers. Nanopores with clean atomically flat edges and controllable mean diameter were generated by time-dependent large-area electron beam exposure during an in situ heating process, ready for subsequent molecular attachment. An organic molecule was designed to have a dithiolane end group that binds to Mo-terminated sites and a ligand structure that incorporates a single transition metal atom (Pt) marker for ADF-STEM detection. Pt atoms were used to track molecular binding around zigzag edges of MoS2 and to predict the orientations and conformations of molecules upon binding. We found that the molecules preferred to reside on the surface of the MoS2, pointing inward when attaching to the edge, rather than dangling out from the edge into free space, which is attributed to van der Waals interactions between the aromatic core of the molecule and the MoS2 basal planes. These results help us understand the way solution-deposited single molecules attach to free-standing edges of 2D crystals and the influence of van der Waals forces in guiding molecular binding.

A feasible and environmentally friendly method to simultaneously synthesize MoS2 quantum dots and pore-rich monolayer MoS2 for hydrogen evolution reaction

MoS2 has been one of a widely researched hydrogen evolution reaction (HER) catalyst materials in recent years. However, the basal plane of MoS2 is considered to be inactive to hinder its further development. Herein, a new mass-scalable and facile intercalation method for the fabrication of multiple products, including pore-rich monolayer MoS2 (PR MoS2) and MoS2 quantum dots (MoS2 QDs), has been developed via the gas phase etching of bulk MoS2 in an acetone vapor atmosphere. The obtained monolayer MoS2 QDs with a narrow lateral size distribution (average size: 2.5 nm) present excitation-independent photoluminescence emission. Furthermore, the PR MoS2 shows a significantly enhanced HER electrocatalytic activity and good stability in 0.5 M H2SO4 solution with a small overpotential of 241 mV at a current density of 10 mA cm−2. These results demonstrate that the as-prepared PR MoS2 is very promising for the application in HER.

The superlattice structure and self-adaptive performance of C–Ti/MoS2 composite coatings

To improve the self-adaptability of MoS2 coating in different environments, the coatings were doped with functional C and Ti by unbalanced magnetron sputtering system. The clear superlattice structure with minimal modulation period was investigated by High Resolution Transmission Electron Microscope (HRTEM). The co-doped coatings have better mechanical properties due to the special structure and the formation of C–Mo, Ti–S and Ti–O bonds, and better lubrication performance in both high humidity and vacuum than those of the single-doped ones. The doped Ti not only facilitates the formation of the MoS2 (002) basal plane, but also improves the oxidation resistance of the composite film. The degree of friction-induced graphitization on the wear tracks and the quality of transfer films on the wear scars are key factors affecting the lubrication performance of the composite film. In the high-humidity environment, the reasonable doping elements can promote the formation the high-quality transfer film by interacting with H2O water molecules, which will benefit the lubrication of the coating better. Our findings deepen the understanding of MoS2 composite coating and provide a new solution for improving the self-adaptability of the coating.

Activating the MoS2 Basal Planes for Electrocatalytic Hydrogen Evolution by 2H/1T' Structural Interfaces.

Exploring highly efficient catalysts for the electrochemical hydrogen evolution reaction (HER) is highly demanded in the sustainable production of hydrogen. MoS2 is recognized as a potential candidate catalyst for HER, but its active site is mainly located at the edges, which is extremely limited. Here, we have investigated the catalytic performance of HER in the MoS2 basal planes during the structural transition from the 2H to the 1T' phase. Different kinds of 2H/1T' structural interfaces are considered, and the adsorbed H free energies (ΔGH) on these surfaces were calculated. The active site for H adsorption is on the top of S atoms at the 2H/1T' phase boundary. The zigzag 2H/1T' interfaces exhibit an optimal performance for the Volmer reaction with the ΔGH being very close to zero. The Volmer-Heyrovsky reaction is dominantly preferred to the Volmer-Tafel reaction. Our study provides a new picture to boost up the active sites of the basal plane for HER on MoS2, and this electrocatalytic mechanism is also applicable for other transition metal dichalcogenide materials.

Simultaneously Engineering Electron Conductivity, Site Density and Intrinsic Activity of MoS2 via the Cation and Anion Codoping Strategy.

The catalytic activity of 2H-MoS2 is retarded by the deficiency in active sites, inferior intrinsic activity, and slow electron transfer kinetics. However, the strategies to concurrently resolve these issues have been challenging and rarely reported. Herein, we successfully endow MoS2 with exceptional acidic HER performance by concurrently doping nitrogen and metal atoms into the basal plane of MoS2. The experimental results reveal that the N dopant that induces the intervalence charge transfer between two ions (Mo4+/Mo3+) and the atoms rearrangement can enable the successful synthesis of 1T MoS2 on reduced graphene oxides, which can concurrently increase the active-site density and facilitate the charge transfer from the substrate to the catalyst active sites. The spontaneous doping of metal cation atoms further improves the intrinsic activity of MoS2 by creating more sulfur vacancy sites and tailoring the energy level matching. The optimized electrocatalyst exhibited unprecedented activity and stability for HER with a low overpotential of 143 mV at 150 mA cm-2 and a high exchange current density of 1 mA cm-2. Therefore, our work opens up possibility to manipulate the MoS2 catalytic performance to rival Pt, which is of significant importance to both fundamental study and industry applications.

Understanding the Independent and Interdependent Role of Water and Oxidation on the Tribology of Ultrathin Molybdenum Disulfide (MoS2)

AbstractIn this work, the tribological behavior of ultrathin MoS2 is investigated to understand the independent roles of water and oxidation. Water adsorption is identified as the primary interfacial mechanism for both SiO2/pristine‐MoS2 and SiO2/graphene interfaces, however, tribological behavior of pristine‐MoS2 is observed to be more sensitive to presence of water due to stronger MoS2–water interaction. Comparison of pristine‐MoS2 and oxidized‐MoS2 reveals that the oxidation of MoS2 significantly increases its friction and sensitivity to water by playing a more detrimental role. The specific effect of oxygen on friction via chemical interactions is studied in isolation through density functional theory simulations of a tip sliding on MoS2 basal planes and over edges before and after oxidation. The maximum change in energy, or energy barrier correlating with friction, as the tip moves across the surface, increases after oxidation by up to 66% for the basal plane and by 25% at the edge. Charge density analysis suggests that the more localized and nonuniform interfacial charge distribution on oxygen‐rich surfaces, as compared to pristine surfaces, leads to higher resistance to sliding. This confirms that oxygen presence alone increases friction and when coupled with the presence of water, both effects are additive in increasing friction.

Improving Electrochemical Pb2+ Detection Using a Vertically Aligned 2D MoS2 Nanofilm.

Recent advancements in MoS2 nanofilms have aided in the development for important water-related environmental applications. However, a MoS2 nanofilm-coated sensor has yet to have been applied for heavy metal detection in water-related environmental samples. In this study, a novel vertically aligned two-dimensional (2D) MoS2 (edge exposed) nanofilm was applied for in situ lead ion (Pb2+) detection. The developed sensor showed an excellent linear relationship toward Pb2+ between 0 and 20 ppb at -0.45 V vs Ag/AgCl using square wave anodic stripping voltammetry (SWASV) with the improved limit of detection (LOD) of 0.3 ppb in a tap water environment. The vertically aligned 2D MoS2 sensor exhibited improved detection sensitivity (2.8 folds greater than a previous metallic [Bi] composite electrode) with lower relative standard deviation for repetitive measurements (n = 11), indicating enhanced reproducibility for Pb2+ detection. The vertically aligned 2D MoS2 layers exhibited 2.6 times higher sensitivity than horizontally aligned 2D MoS2 (basal plane exposed). Density functional theory calculations demonstrated that adsorption energy of Pb on the MoS2 side edge was much higher (4.11 eV) than those on the basal plane (0.36 and 0.07 eV). In addition, the band gap center of vertical MoS2 was found to be higher than the Pb2+ → Pb reduction potential level and capable of reducing Pb2+. Overall, the newly developed vertically aligned 2D MoS2 sensor showed excellent performance for detecting Pb2+ in a real drinking water environment with good reliability.

Activating MoS2 basal planes for hydrogen evolution through the As doping and strain

As a non-precious catalyst for the electrochemical hydrogen evolution reaction (HER), the two-dimensional MoS2 has been widely studied. To activate the MoS2 inert basal plane to enable optimal activity, high defect concentration of sulfur vacancies is needed. Herein, based on the first-principles calculations we demonstrate that the HER of MoS2 can be greatly enhanced by As doping and biaxial strain. We show that the As-doping sites are new catalytic sites and the bonding of H can be greatly enhanced. Moreover, the relative hydrogen adsorption free energy (ΔΔGH) can be further manipulated by the strain effect, which efficiently adjusts the catalytic activity. With the synergy of the biaxial strain (2%–3%) and the uniform doping of the As atoms (3.125% concentration), the ΔΔGH can be modulated to zero. Our findings provide a way to achieve the high intrinsic HER activity among molybdenum-sulfide-based catalysts.

Stimulated Electrocatalytic Hydrogen Evolution Activity of MOF-Derived MoS2 Basal Domains via Charge Injection through Surface Functionalization and Heteroatom Doping.

The design of MoS2‐based electrocatalysts with exceptional reactivity and robustness remains a challenge due to thermodynamic instability of active phases and catalytic passiveness of basal planes. This study details a viable in situ reconstruction of zinc–nitrogen coordinated cobalt–molybdenum disulfide from structure directing metal–organic framework (MOF) to constitute specific heteroatomic coordination and surface ligand functionalization. Comprehensive experimental spectroscopic studies and first‐principle calculations reveal that the rationally designed electron‐rich centers warrant efficient charge injection to the inert MoS2 basal planes and augment the electronic structure of the inactive sites. The zinc–nitrogen coordinated cobalt–molybdenum disulfide shows exceptional catalytic activity and stability toward the hydrogen evolution reaction with a low overpotential of 72.6 mV at −10 mA cm−2 and a small Tafel slope of 37.6 mV dec−1. The present study opens up a new opportunity to stimulate catalytic activity of the in‐plane MoS2 basal domains for enhanced electrochemistry and redox reactivity through a “molecular reassembly‐to‐heteroatomic coordination and surface ligand functionalization” based on highly adaptable MOF template.

Unveiling Electrochemical Reaction Pathways of CO2 Reduction to CN Species at S-Vacancies of MoS2.

Although C1 species such as CO and CH4 constitute the majority of CO2 reduction (CO2 R) products on known catalysts, recent experiments showed that 1-propanol with two C-C bonds is produced as the main CO2 R product on MoS2 single crystals in aqueous electrolytes. Herein, the CO2 R mechanism on MoS2 is investigated by using first-principle calculations. Focusing on S-vacancies (VS ) as the catalytic site, potential free-energy pathways to various CO2 R products are obtained by means of a computational hydrogen electrode model. The results underline the role of HCHO, which is one of the elemental C1 products, in opening pathways to CN species for N>1. Key steps to increase C-C bonds are the adsorption of HCHO at the VS site and binding of another HCHO to the adsorbed one. The predicted products and theoretical working potentials to open their pathways are consistent with experiments, which indicates that VS is an important active site for CO2 R on the MoS2 basal plane.

Facile chemical-vapour-deposition synthesis of vertically aligned co-doped MoS2 nanosheets as an efficient catalyst for triiodide reduction and hydrogen evolution reaction

A combination of high surface area, fast-speed charge transportation, excellent intrinsic activity, and low material cost is desired for electrocatalysts’ applications, such as hydrogen production, counter electrodes of electrochemical solar cells, etc. In this regard, we originally develop a vertically aligned Co-doped MoS2 nanosheet array via a facile chemical vapour deposition (CVD) approach that utilizes the reaction between drop-coated CoCl2-MoCl5 precursor film and sulfur vapour released from elemental sulfur powder. Such structure that exposes primarily edge sites of the nanosheets provides reaction with more active centers and guarantees that electrons transport almost along high-electron-mobility basal plane of MoS2. Simultaneously, the catalytic activity of the in-plane S atoms of MoS2, exposing at the splits of MoS2 nanosheets, can be triggered via Co atom-doping. Such an array that is in-situ grown on graphite foil substrate performs as an efficient electrocatalyst for hydrogen evolution reaction with an overpotential of 185 mV at a current density of 10 mA·cm−2 and an extremely high TOF 0.56 s−1 at 200 mV overpotential, meanwhile, sponsoring as a counter electrode for efficient (8.99%) dye-sensitized solar cells.

Enhanced Gas Sensing Performance of Surface‐Activated MoS2 Nanosheets Made by Hydrothermal Method with Excess Sulfur Precursor

Defect engineering of two‐dimensional (2D) nanostructures is of significant importance for achieving enhanced surface properties in optoelectronic, sensors, and catalytic applications. In this work, the authors study the gas‐sensing properties of 2D molybdenum sulfide (MoS2) nanosheets fabricated using a hydrothermal synthetic route with various nominal S/Mo precursor ratios, in order to generate nano‐domains on the surface. The effect of the excess S precursor on the morphology and the resulting gas sensing performance are investigated by varying the S/Mo precursor ratio in the range of 2–12. Nano‐domains are generated on the MoS2 basal plane for all samples, and their evolution became significant as the S/Mo precursor ratio increases. The presence of the nano‐domains provides additional active edge sites, enhancing the surface‐to‐mass ratio and the resulting gas‐sensing properties. Resistive type gas sensing tests using such 2D MoS2 show that, when the S/Mo precursor ratio is increased from 2 to 12, the sensitivity to 500 ppm NO2 gas at room temperature increased from 27 to 213%.

Engineering MoS2 Basal Planes for Hydrogen Evolution via Synergistic Ruthenium Doping and Nanocarbon Hybridization.

Promoting the intrinsic activity and accessibility of basal plane sites in 2D layered metal dichalcogenides is desirable to optimize their catalytic performance for energy conversion and storage. Herein, a core/shell structured hybrid catalyst, which features few‐layered ruthenium (Ru)‐doped molybdenum disulfide (MoS2) nanosheets closely sheathing around multiwalled carbon nanotube (CNT), for highly efficient hydrogen evolution reaction (HER) is reported. With 5 at% (atomic percent) Ru substituting for Mo in MoS2, Ru‐MoS2/CNT achieves the optimum HER activity, which displays a small overpotential of 50 mV at −10 mA cm−2 and a low Tafel slope of 62 mV dec−1 in 1 m KOH. Theoretical simulations reveal that Ru substituting for Mo in coordination with six S atoms is thermodynamically stable, and the in‐plane S atoms neighboring Ru dopants represent new active centers for facilitating water adsorption, dissociation, and hydrogen adsorption/desorption. This work provides a multiscale structural and electronic engineering strategy for synergistically enhancing the HER activity of transition metal dichalcogenides.

Vertically Grown Few-Layer MoS2 Nanosheets on Hierarchical Carbon Nanocages for Pseudocapacitive Lithium Storage with Ultrahigh-Rate Capability and Long-Term Recyclability.

Molybdenum disulfide (MoS2 ) is an intensively studied anode material for lithium-ion batteries (LIBs) owing to its high theoretical capacity, but it is still confronted by severe challenges of unsatisfactory rate capability and cycle life. Herein, few-layer MoS2 nanosheets, vertically grown on hierarchical carbon nanocages (hCNC) by a facile hydrothermal method, introduce pseudocapacitive lithium storage owing to the highly exposed MoS2 basal planes, enhanced conductivity, and facilitated electrolyte access arising from good hybridization with hCNC. Thus, the optimized MoS2 /hCNC exhibits reversible capacities of 1670 mAh g-1 at 0.1 A g-1 after 50 cycles, 621 mAh g-1 at 5.0 A g-1 after 500 cycles, and 196 mAh g-1 at 50 A g-1 after 2500 cycles, which are among the best for MoS2 -based anode materials. The specific power and specific energy, which can reach 16.1 kW and 252.8 Wh after 3000 cycles, respectively, indicate great potential in high-power and long-life LIBs. These findings suggest a promising strategy for exploring advanced anode materials with high reversible capacity, high-rate capability, and long-term recyclability.

Design of basal plane active MoS2 through one-step nitrogen and phosphorus co-doping as an efficient pH-universal electrocatalyst for hydrogen evolution

The exploration of low-cost, stable, and highly active noble-metal-free electrocatalyst for hydrogen evolution reaction (HER) in a wide pH range is crucial but still challenging task for renewable energy techniques. MoS2-based materials have been considered as a promising electrocatalyst for HER. However, corresponding studies have been hampered by the lack of effective routes to fully utilize the large number of inert basal plane for catalyzing HER, especially under alkaline media. Herein, a novel ammonia ions-guided-nitrogenization-phosphorization strategy is developed to prepare N and P co-doped MoS2 with active basal plane for efficient catalyzing HER with a quite low overpotential of 116 and 78 mV in 0.5 M H2SO4 and 1.0 M KOH to achieve a current density of 10 mA cm−2, respectively. Experimental studies and theoretical calculations confirm Mo-N-P sites in the basal plane of MoS2 can not only accelerate HER kinetics, but also result in energetic favorability and structure stability. Furthermore, outstanding performances are also obtained under both sea and river water, vastly broadening the application prospects.