MoS2 Edges Research Articles

Impact of substrate induced band tail states on the electronic and optical properties of MoS2

Substrate, environment, and lattice imperfections have a strong impact on the local electronic structure and the optical properties of atomically thin transition metal dichalcogenides. We find by a comparative study of MoS2 on SiO2 and hexagonal boron nitride (hBN) using scanning tunneling spectroscopy (STS) measurements that the apparent bandgap of MoS2 on SiO2 is significantly reduced compared to MoS2 on hBN. The bandgap energies as well as the exciton binding energies determined from all-optical measurements are very similar for MoS2 on SiO2 and hBN. This discrepancy is found to be caused by a substantial amount of band tail states near the conduction band edge of MoS2 supported by SiO2. The presence of those states impacts the local density of states in STS measurements and can be linked to a broad red-shifted photoluminescence peak and a higher charge carrier density that are all strongly diminished or even absent using high quality hBN substrates. By taking into account the substrate effects, we obtain a quasiparticle gap that is in excellent agreement with optical absorbance spectra and we deduce an exciton binding energy of about 0.53 eV on SiO2 and 0.44 eV on hBN.

Monomeric MoS42–-Derived Polymeric Chains with Active Molecular Units for Efficient Hydrogen Evolution Reaction

Molybdenum sulfides have attracted widespread attention as promising nonprecious-metal catalysts for the hydrogen evolution reaction (HER). Since the MoS2 edge was proposed as a major active site, ...

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.

Molybdenum disulfide/montmorillonite composite as a highly efficient adsorbent for mercury removal from wastewater

MoS2 and montmorillonite (MMT) nanosheets were hybridized through a simple in-situ hydrothermal synthesis in order to develop an efficient adsorbent, MoS2/MMT composite, for the removal of mercury from water. Raman, XRD, and FTIR results displayed the good hybridization of MMT and MoS2. HRTEM images indicated that the active edges of MoS2 were clearly exposed in the hybrid, demonstrating the vital role of MMT as eco-friendly, cheap and stable substrate for the growth of MoS2. The overall adsorption process of Hg2+ on MoS2/MMT proved that the composite is highly efficient for the removal of Hg2+ from water, adsorbing up to 1836 mg/g of Hg2+, which outperformed most literature reported adsorbents. The adsorption kinetic and isotherm models could be well described by pseudo-first-order and Langmuir models, respectively. Furthermore, the Langmuir dimensionless constant, RL show that Hg2+ adsorption on the composite was found favorable to occur at the temperature of 25 °C and 35 °C. The XPS analysis revealed that the S sites and slight O sites were main contributors for the remarkable adsorption performance of MoS2/MMT composite on Hg2+ removal. This work suggests that MoS2 supported on MMT substrate might be a promising adsorbent with high capacity and super efficiency for the removal of heavy metals from aqueous solution.

Ab-Initio Simulations of Monolayer InSe and MoS2 Strain Effect: From Electron Mobility to Photoelectric Effect

In this paper, we present a comprehensive study of the basic electronic properties of two typical two-dimensional materials, monolayer InSe and MoS2 in the presence of various in-plane strains. Our results demonstrate that both materials exhibit similar trends in the variation of band gaps with strain modulations. With the application of strain, a transition between indirect and direct band gap is observed in monolayer InSe. Otherwise, on the application of strains, the band edge of MoS2 shows an excursion from K point and the electron mobility can be enhanced by compressive strain. The electron mobility of InSe can be boosted by about 10% and thus up to $$ 2.24 \times 10^{3} \;{\hbox{cm}}^{2} /\left( {{\hbox{V}}\;{\hbox{s}}} \right) $$, which approaches one order of magnitude later than that of MoS2. After rebuilding the structure, we introduce illumination to our two-probe system and measure the response. The high responsivity and excellent strain flexibility of InSe rather than MoS2 make it superior as a candidate for the next generation ultrathin electronic and optoelectronic devices.

Structural and Electronic Optimization of MoS2 Edges for Hydrogen Evolution.

The activity and accessibility of MoS2 edge sites are critical to deliver high hydrogen evolution reaction (HER) efficiency. Here, a porous carbon network confining ultrasmall N-doped MoS2 nanocrystals (N-MoS2/CN) is fabricated by a self-templating strategy, which realizes synergistically structural and electronic modulations of MoS2 edges. Experiments and density functional theory calculations demonstrate that the N dopants could activate MoS2 edges for HER, while the porous carbon network could deliver high accessibility of the active sites from N-MoS2 nanocrystals. Consequently, N-MoS2/CN possesses superior HER activity with an overpotential of 114 mV at 10 mA cm-2 and excellent stability over 10 h, delivering one of best MoS2-based HER electrocatalysts. Moreover, this study opens a new venue for optimizing materials with enhanced accessible catalytic sites for energy-related applications.

Reaction mechanisms for reduction of CO2 to CO on monolayer MoS2

Since the reduction of CO2 to fuels by consuming over-generated electricity, which can establish artificial carbon cycle and energy storage at the same time, extensive studies have been devoted to developing suitable catalysts for CO2 conversion in materials science. Recently, MoS2, a typical member of transition metal dichalcogenides, has been widely investigated for its high activity and low energy cost to catalyze CO2 reduction. In this work, we simulate the microscopic dynamic process of the CO2 reduction process in the framework of density functional theory (DFT). Our results reveal that Mo exposed edges of MoS2 are inclined to adsorb CO2 molecule and tend to catalytically reduce CO2 to CO. CO2 molecule is activated by two neighboring Mo atoms and the CO double bond reconstructs in the adsorption process. The first proton/electron (H+ + e−) reaction taking place at MoS2 edges undergoes a different pathway from that on transition metal catalyst, contributing to the product selectivity towards CO. Finally, we demonstrate that desorption of CO from MoS2 edges is in virtue of unique diffusion process for adsorbed CO atoms.

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.

Basal plane oxygen exchange of epitaxial MoS2 without edge oxidation

The intentional formation of defects in transition-metal dichalcogenides, such as MoS2, is an attractive way to modify the electronic and chemical properties of this class of 2D materials. However, the mechanisms and methods available for selective doping or modification of the basal plane must be improved. Here we investigate the process of O defect formation in epitaxial single-layer MoS2 on Au(1 1 1) using scanning tunneling microscopy (STM) and ambient pressure x-ray photoelectron spectroscopy (AP-XPS) during oxidation with O2 and H2O gas from low vacuum to the mbar range. Both oxidants result in exchange of S in the upper part of the basal plane with O, in line with air exposure experiments. Temperature-dependent measurements show that this is an activated process with an experimentally estimated reaction barrier of ~0.79 ± 0.20 eV. We surprisingly find that the morphology of the MoS2 flakes and their edges remain intact in O2, even for relatively high degrees of basal plane O exchange, in contrast to the oxidation behavior of exfoliated single-layer MoS2. From analysis of atom-resolved STM images of the MoS2 edges, we can attribute this unusual stability to a passivating effect of excess edge sulfur species adsorbed under the sulfiding conditions of the MoS2 synthesis in H2S gas. We thus demonstrate that control over pre-sulfidation of the edges, temperature and pressure during oxidation can be used in a fast process to form strongly O doped single-layer MoS2 with no degradation of the initial shape and edge structure of the epitaxial MoS2 sheet.

MoS2-rGO hybrid architecture as durable support for cathode catalyst in proton exchange membrane fuel cells

Carbon black is utilized as a conventional electrocatalyst support material for proton exchange membrane fuel cells. However, this support is prone to corrosion under oxidative and harsh environments, thus limiting the durability of the fuel cells. Meanwhile, carbon corrosion would also weaken the linkage between Pt and the support material, which causes Pt agglomeration, and consequently, deterioration of the cell performance. To overcome the drawbacks of a Pt/C electrocatalyst, a hybrid support material comprising molybdenum disulfide and reduced graphene oxide is proposed and synthesized in this study to exploit the graphitic nature of graphene and the availability of the exposed edges of MoS2. TEM results show the uniform dispersion of Pt nanoparticles over the MoS2-rGO surface. Electrochemical measurements indicate higher ECSA retention and better ORR activity after 10000 potential cycles for Pt/MoS2-rGO as compared to Pt/C, demonstrating the improved durability for this hybrid support material.

Phase controlled synthesis and the phase dependent photo-and electrocatalysis of CdS@CoMo2S4/MoS2 catalyst for HER

Herein we report a heterostructure with ultrathin nanosheets of Co-doped molybdenum sulfide on CdS nanorod array (donated as CdS@CoMo2S4/MoS2) by hydrothermal synthesis. Firstly, elemental Co doping MoS2 (CoMo2S4) delivers the double benefits of increased active sites and enhanced conductivity. Secondly, the structural characteristics maximally exposes the MoS2 edges and enlarges interfacial contact area between the composite catalyst and electrolyte, as well as the efficient interfacial charge transfer. The ratio of CoMo2S4/MoS2 in CdS@CoMo2S4/MoS2 plays a crucial role for the enhanced photo-assistant electrocatalytic hydrogen evolution reaction (HER). We can tune the ratio of CoMo2S4/MoS2 by controlling the preparation time or the ratio of precursor of Co/Mo. The catalyst with predominant MoS2 phase shows superior photocatalytic HER performance with a high H2 production rate of 46.60 μmol mg−1 h−1. Meanwhile, the catalyst with predominant CoMo2S4 phase exhibits not only relatively low overpotential of 172 mV at 10 mA cm−2, which outperforms most values that have been reported on catalyst supported on ITO substrate, but also possesses H2 production rate of 23.47 μmol mg−1 h−1. The superior photo-assistant electrocatalytic HER activity results from the synergistically structural and electronic modulations, as well as the proper energy band alignment between MoS2 and CdS. This investigation could provide an approach to integrate the electro- and photocatalytic activities for HER, especially the photo responding behaviour at a bias potential which is meaningful to produce H2 for actual application.

Atomic Insight into Thermolysis‐Driven Growth of 2D MoS2

AbstractUnderstanding and controlling the transformations of transition metal dichalcogenides (TMDs) from amorphous precursors into two‐dimensional (2D) materials is important for guiding synthesis, directing fabrication, and tailoring functional properties. Here, the combined effects of thermal energy and electron beam irradiation are explored on the structural evolution of 2D MoS2 flakes through the thermal decomposition of a (NH4)2MoS4 precursor inside an ultrahigh vacuum (10−9 Torr) scanning transmission electron microscope (STEM). The influence of reaction temperature, growth substrate, and the initial precursor morphology on the resulting 2D MoS2 flake morphology, edge structures, and point defects are explored. Although thermal decomposition occurs extremely fast at elevated temperatures and is difficult to capture using current STEM techniques, electron beam irradiation can induce local transformations at lower temperatures, enabling direct observation and interpretation of critical growth steps including oriented attachment and transition from single‐ to multilayer structures at atomic resolution. An increase in the number of layers of the MoS2 flakes from island growth is investigated using electron beam irradiation. These findings provide insight into the growth mechanisms and factors that control the synthesis of few‐layer MoS2 flakes through thermolysis and toward the prospect of atomically precise control and growth of 2D TMDs.

Surface partially oxidized MoS2 nanosheets as a higher efficient cocatalyst for photocatalytic hydrogen production

In this study, a novel MoO3@MoS2/TiO2 photocatalyst with higher photocatalytic hydrogen production efficiency was prepared via a hydrothermal followed by quenching method. The prepared composites were characterized by X-ray diffraction (XRD), Raman spectroscopy, UV–Vis Spectrophotometers, scanning electron microscopy (SEM), high-resolution transmission electron microscopy (HRTEM), and X-ray photoelectron spectroscopy (XPS). The photocatalytic performance for hydrogen evolution was also investigated. The results show that MoS2-340 °C/TiO2 exhibits excellent photocatalytic hydrogen production efficiency (4669 μmol g−1 h−1), which is 5.8 times higher than that of unquenched MoS2/TiO2 under the same conditions. MoO3 on the surface and edges of large layered MoS2 creates more traps adjacent to the joint. More photogenerated electrons can be enriched during the photocatalytic reaction, which achieve the purpose of exposing more active sites.

Effect of H2 Plasma Modification on the Electrocatalytic Hydrogen Evolution Performance of MoS2

The hydrogen evolution reaction (HER) is the cathodic half reaction of electrocatalytic water splitting and is thus highly desirable to power the "hydrogen-based economy". Earth-abundant MoS2 has emerged as a promising HER catalyst with high electrocatalytic activity and stability due to its unique physical and chemical properties (1, 2). In the electronic structure of MoS2, the orbitals of HOMO states are mainly localized at the edged S sites, so that the localized electrons are mainly at the edge of MoS2 for charge exchange of proton (3). Extensive efforts have been devoted to developing MoS2 nanostructures to maximize the number of active edge sites, but little attention has been paid to activating the MoS2 basal planar structure. In this work, we used the H2 plasma modification to engineer the planar S-vacancies of MoS2 for enhancing the HER kinetics. Moreover, controlled plasma exposure time has been designed to explore the influence of H2 plasma on the HER performance of MoS2 catalysts. Our experimental results show that the S/Mo atomic ratios decrease with increase the H2 plasma exposure time (Figure 1a). More importantly, the HER performance is becoming better after H2 plasma modification (Figure 1b). the MoS2 H2-plasma 1200s catalyst exhibits a boosted HER activity with the overpotential as low as 92 mV at the current density of 10 mA/cm2 and striking turnover frequency per surface Mo atom (TOF) as 0.0044 s-1, better than most reported MoS2-based catalysts (Figure 1c and d). Figure Caption Figure 1. (a) Atomic ratio of S/Mo for MoS2 samples as the function of H2 plasma exposure time. (b) Polarization curves of H2 plasma treated MoS2 catalysts in 0.5 M H2SO4 solution at a scan rate of 5 mV/s. (c) Overpotential at 10 mA/cm2 and (d) TOF of the H2 plasma treated MoS2 catalysts. References X. Huang, Z. Zeng, H. Zhang, Metal dichalcogenide nanosheets: preparation, properties and applications. Chem. Soc. Rev. 42, 1934-1946 (2013).H. T. Wang et al., Electrochemical tuning of vertically aligned MoS2 nanofilms and its application in improving hydrogen evolution reaction. P. Natl. Acad. Sci. USA 110, 19701-19706 (2013).J. Hu et al., Engineering stepped edge surface structures of MoS2 sheet stacks to accelerate the hydrogen evolution reaction. Energy Environ. Sci. 10, 593-603 (2017). Figure 1

Edge‐Exposed Molybdenum Disulfide with N‐Doped Carbon Hybridization: A Hierarchical Hollow Electrocatalyst for Carbon Dioxide Reduction

AbstractElectrochemical CO2 reduction (CO2RR) is a promising technology to produce value‐added fuels and weaken the greenhouse effect. Plenty of efforts are devoted to exploring high‐efficiency electrocatalysts to tackle the issues that show poor intrinsic activity, low selectivity for target products, and short‐lived durability. Herein, density functional theory calculations are firstly utilized to demonstrate guidelines for design principles of electrocatalyst, maximum exposure of catalytic active sites for MoS2 edges, and electron transfer from N‐doped carbon (NC) to MoS2 edges. Based on the guidelines, a hierarchical hollow electrocatalyst comprised of edge‐exposed 2H MoS2 hybridized with NC for CO2RR is constructed. In situ atomic‐scale observation for catalyst growth is performed by using a specialized Si/SiNx nanochip at a continuous temperature‐rise period, which reveals the growth mechanism. Abundant exposed edges of MoS2 provide a large quantity of active centers, which leads to a low onset potential of ≈40 mV and a remarkable CO production rate of 34.31 mA cm−2 with 92.68% of Faradaic efficiency at an overpotential of 590 mV. The long‐term stability shows negligible degradation for more than 24 h. This work provides fascinating insights into the construction of catalysts for efficient CO2RR.

In situ reactivation of spent NiMoP/γ-Al2O3 catalyst for hydrodesulfurization of straight-run gas oil

Washing with xylene (X), 2,6 bis 1 hydroxy 1,1 diphenyl methyl pyridine (A) and in situ reactivation with molybdenum acetylacetonate (AceMo) on the NiMoP/Al2O3 spent surface catalyst has been investigated in the hydrodesulfurization (HDS) of straight-run gas oil. The spent catalyst was washed and dried at 120 °C before in situ reactivation. The sulfided catalysts were characterized by temperature programmed reduction (TPR), nuclear magnetic resonance (NMR), Raman spectroscopy and X-ray photoelectron spectroscopy (XPS). The HDS reactions were carried out at 5.5 MPa of H2 and the reaction temperatures at 340 °C, 360 °C and 380 °C. The CatAXAceMo displayed the highest HDS activity. The TPR results advice that CatAX presented more “rigid” MoS2 edges than CatRef catalyst. NMR results suggest that CatAXAceMo showed lower aromatic or polyaromatic hydrocarbons surface concentration deposited after HDS reaction than CatAceMo and CatRef. Raman spectroscopy revealed the formation of coke crystallites with shorter size for CatAXAceMo than others catalysts. XPS spectroscopy results exhibited that CatAXAceMo presented smaller superficial carbon and a larger concentration of MoS2 active phase. A relation between the coke crystallites size and MoS2 superficial concentration was found. A lower aromatic concentration favors the shorter coke crystallites sizes and a major availability of Mo species for the formation of MoS2 to HDS reaction.

Few-layered MoS2 vertically aligned on 3D interconnected porous carbon nanosheets for hydrogen evolution

MoS2 has been considered as a non-precious alternative to platinum based electrocatalyst for electrochemical hydrogen evolution. Since most of the active site exists on the edges of MoS2, a material design that could increase the exposure of the edges could improve its catalytic activity. In this work, we prepared small sized and few-layered MoS2 that is vertically aligned on three-dimensional interconnected porous carbon nanosheets (IPC). Benefiting from exposure of MoS2 edges, MoS2/IPC composite exhibits a high catalytic activity (225 mA cm−2 at −250 mV vs. RHE, and Tafel slope of 38 mV dec−1) and maintains steady performance for several hours. Since the procedure to prepare MoS2/IPC composites is scalable and cost-effective, our method of preparing few-layered MoS2 on porous carbon shows great potential as a competitive electrocatalysts for HER.

High-Performance Photodetectors Based on Lead-Free 2D Ruddlesden-Popper Perovskite/MoS2 Heterostructures.

Two-dimensional (2D) Ruddlesden-Popper perovskites have attracted great interest for their promising applications in high-performance optoelectronic devices owing to their greatly tunable band gaps, layered characteristics, and better environmental stability over three-dimensional (3D) perovskites. Here, we for the first time report on photodetectors based on few-layer MoS2 (n-type) and lead-free 2D perovskite (PEA)2SnI4 (p-type) heterostructures. The heterojunction device is capable of sensing light over the entire visible and near-infrared wavelength range with a tunable photoresponse peak. By using few-layer graphene flakes as the electrical contact, the performance of the heterostructures can be improved with a responsivity of 1100 A/W at 3 V bias, a fast response speed of ∼40 ms under zero bias, and an excellent rectification ratio of 500. Importantly, the quantum efficiency can achieve 38.2% at zero bias, which is comparable or even higher than that of 3D perovskite/2D material photodetectors. Importantly, the spectral response peak of heterojunctions gradually shifts in a wide spectral range from the band edge of MoS2 toward that of (PEA)2SnI4 with the external bias. We believe these 2D perovskite/2D material heterostructures with a great diversity represent an interesting system for investigating the fundamental optoelectronic properties and open up a new pathway toward 2D perovskite-based optoelectronic devices.

One Pot Assembly of Vertical Embedded MoS2/Graphene Heterostructure and Its High Performance for Hydrogen Evolution Reaction

Two-dimensional molybdenum disulfide has great potential to replace rare precious metals for hydrogen evolution reaction. Herein, we report a one pot assembly strategy for in situ growing vertical embedded MoS2 nanocrystals on graphene surface. The resultant sandwiched architecture could not only expose more active sites within MoS2 edge but also reduce stacking tendency of the layer-structured graphene support, leading to a significant improvement in HER performance. The materials showed an overpotential of 142 mV at 10 mA·cm–2 and a Tafel slope of 68 mV·dec–1, which are among the best results ever obtained by the similar materials. Moreover, this method is facile, environmentally friendly, and suitable for mass production, making it attractive for practical applications.

Mo–S–Ti–C Nanocomposite Films for Solid-State Lubrication

A Mo–S–Ti–C nanocomposite film with both high-crystalline MoS2 in (002) basal planes and high hardness was successfully synthesized in the condition of pure MoS2 films growing with edge-terminated surface. The evolution of their structure and the transformation of crystal orientation during film growth were investigated by X-ray diffraction (XRD) patterns and focused ion beam combining with the transmission electron microscopy (FIB/TEM). TEM revealed that the MoS2 edge sites usually considered as catalytic materials in catalysis field can provide active sites for titanium and carbon atoms aggregating in deposition. This aggregation of dopants not only largely passivates the activated MoS2 edge sites and induces the initial (100) textured MoS2 lamellar gradually transforming into (002) orientation but also effectively reduces the damage the dopants do to the highly ordered MoS2 crystallites. As a result, a Mo–S–Ti–C composite film with well-crystallized MoS2 in (002) basal planes was formed. The nanoindent...