MoS2 Surface Research Articles

Stable Liquid-Sulfur Generation on Transition-Metal Dichalcogenides toward Low-Temperature Lithium-Sulfur Batteries.

The electrochemical formation of liquid sulfur at room temperature on the basal plane of MoS2 has attracted much attention due to the high areal capacity and rapid kinetics of lithium-liquid sulfur chemistry. However, the liquid sulfur is converted to the solid phase once it contacts the solid sulfur crystals generated from the edge of MoS2. Thus, stable liquid sulfur cannot be formed on the entire MoS2 surface. Herein, we report entire liquid sulfur generation on hydrogen-annealed MoS2 (H2-MoS2), even under harsh conditions of large overpotentials and low working temperatures. The origins of the solely liquid sulfur formation are revealed to be the weakened interactions between H2-MoS2 and sulfur molecules and the decreased electrical polarization on the edges of the H2-MoS2. Progressive nucleation and droplet-merging growth behaviors are observed during the sulfur formation on H2-MoS2, signifying high areal capacities by releasing active H2-MoS2 surfaces. To demonstrate the universality of this strategy, other transition-metal dichalcogenides (TMDs) annealed in hydrogen also exhibit similar sulfur growth behaviors. Furthermore, the H2 annealing treatment can induce sulfur vacancies on the basal plane and partial oxidation on the edge of TMDs, which facilitates liquid sulfur formation. Finally, liquid sulfur can be generated on H2-MoS2 flakes at an ultralow temperature of -50 °C, which provides a possible development of low-temperature lithium-sulfur batteries. This work demonstrates the potential of a pure liquid sulfur-lithium electrochemical system using functionalized two-dimensional materials.

Cuγ (γ = 1-3)-Modified MoS2 Monolayer as a Gas Sensor for Detecting C4F7N and Its Decomposition Components.

Perfluorinated isobutyronitrile (C4F7N) is favored in electrical engineering because it is an environmentally friendly gas-insulating medium with a low greenhouse effect. Unfortunately, under the influence of electricity and over-heating, its decomposition results in the deterioration of its insulating properties, which potentially leads to partial discharge or even gas breakdown. In this paper, the adsorption behavior of C4F7N gas and its toxic decomposition product, acetonitrile (C2N2), on MoS2 surfaces doped with small copper clusters was investigated by calculating the adsorption energy and density of states, etc. The effects of multiple initial adsorption positions as well as externally applied electric fields were also taken into account. The results depict that the maximum adsorption energy of C4F7N on the Cuγ (γ = 1–3)-MoS2 surface gradually decreases with the increase in γ. The Cu3-modified MoS2 is most suitable for use as a resistive-based gas-sensitive sensor substrate. This paper provides the theoretical foundation for the maintenance of future power equipment with environmentally friendly insulating gas.

Nanoscale Control of DNA-Linked MoS2-Quantum Dot Heterostructures.

The ability to control the assembly of mixed-dimensional heterostructures with nanoscale control is key for the fabrication of novel nanohybrid systems with new functionalities, particularly for optoelectronics applications. Herein we report a strategy to control the assembly of heterostructures and tune their electronic coupling employing DNA as a linker. We functionalized MoS2 nanosheets (NSs) with biotin-terminated dsDNA employing three different chemical strategies, namely, thiol, maleimide, and aryl diazonium. This allowed us to then tether streptavidinated quantum dots (QDs) to the DNA functionalized MoS2 surface via biotin-avidin recognition. Nanoscale control over the separation between QDs and NSs was achieved by varying the number of base pairs (bp) constituting the DNA linker, between 10, 20, and 30 bp, corresponding to separations of 3.4, 6.8, and 13.6 nm, respectively. Spectroscopic data confirmed the successful functionalization, while atomic force and transmission electron microscopy were employed to image the nanohybrids. In solution steady-state and time-resolved photoluminescence demonstrated the electronic coupling between the two nanostructures, that in turn was observed to progressively scale as a function of DNA linker employed and hence distance between the two nanomoieties in the hybrids.

PtO2-decorated MoS2 ultrathin nanostructures for enhanced NH3 gas sensing properties

Recently, transition metal dichalcogenides (TMDs) have attracted much attention due to their high surface-to-volume ratio, various active edge sites, and adjustable bandgaps. However, in general, TMDs still exhibit low gas response and incomplete recovery. Surface modification is an effective way to facilitate the adsorption and desorption processes, thus enhancing the TMD gas sensor's sensing performance. In this study, a PtO2-decorated MoS2 nanostructure gas sensor was fabricated using a chemical reduction process. The result showed that the as-obtained nanostructures consist of accumulated ultrathin MoS2 nanosheets decorated with spherical PtO2 nanoparticles. The gas response of the as-obtained PtO2-decorated MoS2 gas sensor toward NH3 is remarkably high, with 450% upon 500 ppm NH3, which is almost 20 times higher than that of the pristine MoS2 nanostructure gas sensor in their optimal working conditions. The enhancement in gas sensing properties was attributed to the catalytic spillover effect of PtO2 nanoparticles as well as the formation of p-p heterojunction on the MoS2 surface.

High-efficient peroxymonosulfate activation for rapid atrazine degradation by FeSx@MoS2 derived from MIL-88A(Fe)

FeSx@MoS2-x (FM-x, x implied real Mo/Fe content ratios) in which FeSx derived from MIL-88A deposited on the surface of MoS2 with a tight heterogeneous interface were synthesized for peroxymonosulfate (PMS) activation to degrade atrazine (ATZ). The catalytic performance of FM-0.96 was greatly improved due to the rapid regeneration of Fe2+ resulting from the interfacial interaction. FM-0.96 could completely degrade 10.0 mg/L ATZ within 1.0 min, and the toxicities for most of its intermediates were greatly reduced. The k value of FM-0.96 was 320 and 40 times higher than that of the MoS2 and FeSx, respectively. The SO4·−, ·OH and 1O2 were mainly responsible for ATZ degradation in FM-0.96/PMS system, and the conversion pathway of 1O2 was analyzed. Furthermore, the long-term continuous operation for ATZ degradation was achieved using a fixed membrane reactor. This work provides deep insights into metal sulfide composites derived from metal-organic frameworks for removing pollutants by activating PMS.

Hydrogen spillover effect enhanced by carbon quantum dots activated MoS2

The weak hydrogen adsorption and slow catalytic kinetics are unfavorable for MoS2 as a catalyst for hydrogen evolution reaction (HER) in alkaline solution. Herein, carbon quantum dots (CQDs) with high proton adsorption energy were expected to enhance the proton adsorption of MoS2 during the HER process. The CQDs activated MoS2 can establish a suitable hydrogen adsorption gradient due to the surface charge redistribution to promote the hydrogen spillover pathway. After the CQDs were loaded on MoS2, the charge distribution would gradually transfer from the C sites of CQDs to the S sites on MoS2 surface, showing a charge gradient distribution calculated by density functional theory predictions. The adsorbed hydrogen can spill over from hydrogen-enriched CQDs to hydrogen-deficient MoS2 in HER. The process could be monitored by the quasi in-situ electrochemical impedance spectroscopy. The enhanced hydrogen spillover effect by CQDs activated MoS2 can pave the way for the design and preparation of efficient and low-cost catalysts.

Van der Waals Epitaxy of Thin Gold Films on 2D Material Surfaces for Transparent Electrodes: All-Solution-Processed Quantum Dot Light-Emitting Diodes on Flexible Substrates.

With the assistance of van der Waals (vdW) epitaxy, nanometer-thick and highly conductive gold films are deposited onto MoS2 surfaces for use as transparent anode electrodes in quantum dot light-emitting diodes (QLEDs) on poly(ethylene terephthalate) (PET) substrates. After transferring wafer-scale and monolayer MoS2 to PET substrates, 10 nm thick gold (Au) films are deposited onto the two-dimensional (2D) material surfaces as anode electrodes. Bounded only by weak vdW forces on 2D material surfaces, the diffusive Au adatoms tend to facilitate lateral growth and lead to the formation of continuous and highly conductive thin metal films in the nanometer regime. The Au film exhibits excellent tensile bending stability for its sheet resistance, which is superior to that of rigid indium-tin oxide (ITO) films on PET substrates. Thermally stable CdSe@CdZnS/ZnS QLEDs are fabricated on the PET substrate. Compared with devices fabricated on sapphire substrates, the phenomenon of sub-bandgap turn-on is observed for the flexible device. Based on our demonstrations, the high conductivity and robust durability toward substrate bending make the nanometer-thick Au film grown on 2D material surfaces a promising candidate to replace current ITO anode electrodes for flexible device applications.

Thermal stability and high-temperature photoluminescence of chemical vapor deposited MoS2 in different atmosphere

Understanding the high-temperature behaviors of two-dimensional (2D) transition metal dichalcogenides under different environments will provide a directive guide for the fabrication and application of their nanoelectronic devices. In this study, we investigate the high-temperature properties of chemical vapor deposition grown molybdenum disulfide (MoS2) in different atmospheres through the ex situ and in situ morphological and spectroscopical characterizations. When the MoS2 is exposed to Ar, dry O2, and wet O2 at the temperature of 500, 350, and 300 °C, the etching process starts at the edge and inner regions simultaneously, and the etched trenches are opposite to the outline of the MoS2 triangular shape. We observe a clear redshift of MoS2 in the Raman peak position and the photoluminescence peak position by in situ spectroscopical characterizations as the temperature increases. Moreover, a strong photoluminescence enhancement of 24-fold of MoS2 is observed when exposing to O2 at 300 °C. This should be due to the chemical and physical adsorbed oxygen on the MoS2 surface. Moreover, we find that physical adsorption can be eliminated after vacuum annealing. This study provides a real-time strategy to study the morphology and property evolution of 2D materials at high temperature in different atmospheres. These results will contribute to the applications in future electronic and optoelectronic functional devices of 2D materials.

Construction Si–O–Mo bond via etching method: enhancing selective adsorption capacity of MoS2/montmorillonite to Pb2+

The inert substrate surface of MoS2 leads to unsatisfactory adsorption performance. Herein, an inert substrate surface activation strategy is proposed, and the resulted MoS2/montmorillonite (MMT) prepared via the hydrothermal method is immersed with NaF (F–Mt). The intercalation of Na + contributes to inducing the transformation of MoS2 from 2H phase to 1 T phase, thereby providing a large number of S vacancies and exposing available Mo4+. Meanwhile, the surface of MMT is activated due to the fact that partially [Si4O10]4- are etched by F-, which facilitates forming Si–O–Mo bonds to promote the charge transfer. The improved charge transfer rate can make S2- in MoS2 accumulate more negative charges, greatly enhancing the adsorption capacity to Pb2+. Moreover, the natural negative potential on the surface of MMT solves the problem of easy stacking for nanosheets, improving the availability of active sites on the edge, which are also benefit from the etching effect on MoS2 nanosheets with NaF. The maximum adsorption capacity achieves 97.09 mg/g. Additionally, density functional theory (DFT) calculations confirm the resulted S vacancies in MoS2 are in favor of removing Pb2+, which is also contributed by the electrostatic and chemical adsorption with MMT. Therefore, it suggests that F–Mt is an excellent adsorbent for Pb2+.

MnFe2O4/MoS2 nanocomposite as Oxidase-like for electrochemical simultaneous detection of ascorbic acid, dopamine and uric acid

In this paper, we adopt a one-pot solvothermal synthesis method to prepare MnFe2O4/MoS2 nanocomposite (MMF) as an artificial enzyme for simultaneous electrochemical detection of ascorbic acid (AA), dopamine (DA) and uric acid (UA) by screen-printed carbon electrode (SPCE). The exfoliated MoS2 surface contains numerous S defects and the spinel MnFe2O4 possesses non-homogeneous catalystic properties, high abundance of oxygen vacancies and surface. It provides high adsorption properties through the replacement of cations with large amounts of active clusters on the surface of nano-impurity composites, as well as the high surface activity of MoS2. Finally, we found that MMF can effectively and sensitively detect AA, DA and UA, and the peak current increases significantly in a certain scanning speed range. The electrochemical sensing performance of the synthetic nanocomposite for the analytes was better than that of individually synthesized MoS2 and MnFe2O4, respectively. The obtained limit of detection (LOD) of MMF for AA, DA and UA were 0.17 mM, 0.41 μM and 0.14 μM, respectively. In addition, the LOD for simultaneous detection of AA, DA and UA were as low as 0.90 mM, 0.16 μM and 3.05 μM, respectively. The prepared MMF showed good anti-interference ability and reproducibility for the analytes sensing.

Observation of robust superlubricity of MoS2 on Au(111) in ultrahigh vacuum

The structural and superlubric properties of single layer MoS2 on Au(111) forming moiré superlattice structures have been investigated by means of ultrahigh vacuum atomic force microscope with bimodal and contact modes. We synthesize epitaxial monolayer MoS2 flakes on the Au(111) surface in ultrahigh vacuum. Using friction force microscopy, atomic friction measurements indicate a superlubric regime between the tip apex and the moiré corrugated MoS2 surface in which the friction force remains at an ultralow value and is independent from normal load. Superlubricity conditions are observed for different loads and velocities which indicates the absence of out-of-plane deformations. We find that the MoS2 layer including the moiré superlattice modulation originating from the natural misfit between MoS2 and the Au(111) substrate is relatively rigid. We also demonstrate a low friction coefficient of the MoS2 surface crossing a single Au(111) step. Our results open up a new avenue for minimizing friction in nanoscale electronic devices and other dry rigid contacts used in aerospace lubrication.

Surface strain-enhanced MoS2 as a high-performance cathode catalyst for lithium–sulfur batteries

Lithium–sulfur batteries (LSBs) are one of the main candidates for the next generation of energy storage systems. To improve the performance of LSBs, we herein propose the use of strained MoS2 (s-MoS2) as a catalytically active sulfur host. The introduction of strain in the MoS2 surface, which alters its atomic positions and expands the S–Mo–S angle, shifts the d-band center closer to the Fermi level and provides the surface with abundant and highly active catalytic sites; these enhance the catalyst's ability to adsorb lithium polysulfides (LiPS), accelerating its catalytic conversion and promoting lithium-ion transferability. Strain is generated through the synthesis of core–shell nanoparticles, using different metal sulfides as strain-inducing cores. s-MoS2 nanoparticles are supported on carbon nanofibers (CNF/s-MoS2), and the resulting electrodes are characterized by capacities of 1290 and 657 ​mAh g−1 ​at 0.2 and 5 ​C, respectively, with a 0.05% capacity decay rate per cycle at 8 ​C during 700 cycles. Overall, this work not only provides an ingenious and effective strategy to regulate LiPS adsorption and conversion through strain engineering, but also indicates a path toward the application of strain engineering in other energy storage and conversion fields.

Revealing local structural properties of an atomically thin MoSe2 surface using optical microscopy

Using a triangular molybdenum diselenide (MoSe2) flake as surface-enhanced Raman spectroscopy (SERS) platform, we demonstrate the dependency of the Raman enhancement on laser beam polarization and local structure using copper phthalocyanine (CuPc) as probe. Second harmonic generation (SHG) and photoluminescence spectroscopy and microscopy are used to reveal the structural irregularities of the MoSe2 flake. The Raman enhancement in the focus of an azimuthally polarized beam, which possesses exclusively an in-plane electric field component is stronger than the enhancement by a focused radially polarized beam, where the out-of-plane electric field component dominates. This phenomenon indicates that the face-on oriented CuPc molecules strongly interact with the MoSe2 flake via charge transfer and dipole–dipole interaction. Furthermore, the Raman scattering maps on the irregular MoSe2 surface show a distinct correlation with the SHG and photoluminescence optical images, indicating the relationship between local structure and optical properties of the MoSe2 flake. These results contribute to understand the impacts of local structural properties on the Raman enhancement at the surface of the 2D transition-metal dichalcogenide.

An active MoS2 with Pt-doping and sulfur vacancy for strengthen CO2 adsorption and fast Capture: A DFT approach

In this paper, MoS2_1VS_1PtMo was constructed to synergically enhance carbon dioxide adsorption on MoS2. DFT calculations show that MoS2_1VS_1PtMo can effectively break the transport barrier between MoS2 surface and CO2 molecules. S vacancies and Pt atoms act as electron transport bridges and simultaneously transport electrons to CO2 molecules. In MoS2_1VS_1PtMo, S vacancies and Pt atoms transfer 0.06 e to CO2 molecules, respectively, which transfer more electrons than MoS2_1VS and MoS2_1PtMo. It indicates that MoS2_1VS_1PtMo can synergistically enhance the electron transport efficiency, exhibiting the effect of 1 + 1 > 2. Meanwhile, the adsorption energy of CO2 molecules in MoS2_1VS_1PtMo is − 0.44 eV, which is higher than MoS2_1VS and MoS2_1PtMo, and its adsorption energy is 1.83 times higher than pristine MoS2. This indicates that sulfur vacancies and Pt doping can significantly enhance the CO2 adsorption performance, which provides a reliable idea for enhancing the carbon capture performance of Mo-based material systems.

Lithium-induced intralayer rearrangement of molybdenum disulfide: Effect of graphene coating

Nanoscale composites consisting of molybdenum disulfide and graphene (MoS2/graphene) are promising anode materials for lithium-ion batteries. In this work, to gain insight into the role of graphene coating in the interaction of lithium with MoS2, we used in situ X-ray photoelectron spectroscopy (XPS) experiments. The deposition of lithium was carried out in a high vacuum simultaneously on the surface of polycrystalline MoS2 and MoS2 coated with a few-layer graphene. The XPS Mo 3d and S 2p spectra of lithiated samples exhibited the appearance of low-energy components assigned to MoMo bonds and weakened S–Mo bonds according to the density functional theory calculations. Such an atomic rearrangement in the MoS2 layer was much more significant for MoS2/graphene due to excess lithium in the interface. Annealing of the samples at 500 °C revealed that the lithium-induced intralayer MoS2 rearrangement is irreversible despite the removal of lithium. Moreover, at 800 °C, the rearranged MoS2 regions containing the residual lithium decomposed to form metallic molybdenum and Li2S2. This finding can help to understand the processes occurring in MoS2/graphene electrodes and to develop advanced materials for efficient lithium-ion batteries.

Severe carbon accumulation on black phosphorous flakes induced by synchrotron x-ray radiation

Two-dimensional (2D) materials, such as black phosphorus (BP) and transition metal dichalcogenide, recently gained much interest due to their unique electronic, photonic, and mechanical properties. Synchrotron-based scanning photoelectron microscopy (s-SPEM) with a spatial resolution (down to 70 nm) has actively applied for exploring the relationship between chemical information and electronic structure of 2D materials on concerned local surface area. Here, we compare carbon accumulation on the surface of BP and MoS2 during synchrotron beam irradiation. During the BP characterization, carbon was rapidly accumulated on BP surface as irradiation progress, while MoS2 demonstrated better resistance to carbon accumulation in comparison to BP. Our work suggests that the higher surface reactivity of BP results in the formation of even BP-carbide structure upon the irradiation, while MoS2 with relatively low reactivity is rather stable against carbon accumulation. This photon-induced carbon accumulation causes the misleading result of surface chemical information and electronic structure in highly reactive 2D materials or nanostructures.

Enhanced Light Absorption and Efficient Carrier Collection in MoS2 Monolayers on Au Nanopillars.

We fabricated hybrid nanostructures consisting of MoS2 monolayers and Au nanopillar (Au-NP) arrays. The surface morphology and Raman spectra showed that the MoS2 flakes transferred onto the Au-NPs were very flat and nonstrained. The Raman and photoluminescence intensities of MoS2/Au-NP were 3- and 20-fold larger than those of MoS2 flakes on a flat Au thin film, respectively. The finite-difference time-domain calculations showed that the Au-NPs significantly concentrated the incident light near their surfaces, leading to broadband absorption enhancement in the MoS2 flakes. Compared with a flat Au thin film, the Au-NPs enabled a 6-fold increase in the absorption in the MoS2 monolayer at a wavelength of 615 nm. The contact potential difference mapping showed that the electric potential at the MoS2/Au contact region was higher than that of the suspended MoS2 region by 85 mV. Such potential modulation enabled the Au-NPs to efficiently collect photogenerated electrons from the MoS2 flakes, as revealed by the uniform positive surface photovoltage signals throughout the MoS2 surface.

Atom elimination strategy for MoS2 nanosheets to enhance photocatalytic hydrogen evolution

The regulation of the basic properties of atom-economic catalysts at the atomic scale and atomic-level insights into the underlying mechanism of catalysis are less explored. We engineer the surface of vertical immobilized MoS2 on dispersible TiO2 nanofibers via atomic subtraction to precisely manipulate active sites at the atomic level. The photocatalytic performances of TiO2@MoS2 after H2 reduction towards the hydrogen production under visible light irradiation (>420 nm) are about 4 times that of TiO2@MoS2 before H2 reduction. Importantly, the enhanced stability of TiO2@MoS2 lasts for at least 30 h. Promising catalytic activity that is attributed to omnidirectional exposed active sites located defects, edges, corners that are transformed from the subtractive atomic sites could be exhumed comprehensively. This work will provide an intriguing and effective approach on tuning electronic structures for optimizing the catalytic activity at the atomic level by atom elimination strategy. To get rid of a few atomics on the surface of atomically-thin MoS2 nanosheet could be a prudent avenue for enabling the basal plane of MoS2 catalytically active.

Frenkel-defected monolayer MoS2 catalysts for efficient hydrogen evolution

Defect engineering is an effective strategy to improve the activity of two-dimensional molybdenum disulfide base planes toward electrocatalytic hydrogen evolution reaction. Here, we report a Frenkel-defected monolayer MoS2 catalyst, in which a fraction of Mo atoms in MoS2 spontaneously leave their places in the lattice, creating vacancies and becoming interstitials by lodging in nearby locations. Unique charge distributions are introduced in the MoS2 surface planes, and those interstitial Mo atoms are more conducive to H adsorption, thus greatly promoting the HER activity of monolayer MoS2 base planes. At the current density of 10 mA cm−2, the optimal Frenkel-defected monolayer MoS2 exhibits a lower overpotential (164 mV) than either pristine monolayer MoS2 surface plane (358 mV) or Pt-single-atom doped MoS2 (211 mV). This work provides insights into the structure-property relationship of point-defected MoS2 and highlights the advantages of Frenkel defects in tuning the catalytic performance of MoS2 materials.

Effect of radical on defect and molecular structure of monolayer MoS2 by low temperature plasma treatment

The aim of this study is to form the sulfur defects on monolayer molybdenum disulfide (MoS2) by low temperature microwave plasma treatment suppressing disturbance of molecular structure. CVD-grown and plasma treated multilayer MoS2 surface were analyzed to investigate the effects of H2 and Ar plasma treatment on sulfur defects and molecular structure. It was found that the disturbance of molecular structure was suppressed in the H2 plasma treatment compared to the Ar plasma treatment. Varying the incident ratio of hydrogen ions H+ and radicals H*, the influences of H2 plasma treatment with high and low H*/H+ ratio on monolayer MoS2 structure were discussed. As a result of X-ray photoelectron spectroscopy, Raman spectroscopy and photoluminescence analysis, sulfur defects increased with the increase in total amount of radical incident on MoS2. In addition, it is speculated that the etching with radical contributed to form sulfur defects suppressing the disturbance of molecular structure.