MoS2 Nanoflakes Research Articles

Coupling Long-Range Facet Junction and Interfacial Heterojunction via Edge-Selective Deposition for High-Performance Z-Scheme Photocatalyst.

The construction of photocatalytic systems that have strong redox capability, effective charge separation, and large reactive surfaces is of great scientific and practical interest. Herein, an edge‐connected 2D/2D Z‐scheme system that combines the facet junction and the interfacial heterojunction to achieve effective long‐range charge separation and large reactive surface exposure is designed and fabricated. The heterostructure is realized by the selective growth of 2D‐layered MoS2 nanoflakes on the edge‐sites of thin TiO2 nanosheets via an Au‐promoted photodeposition method. Attributed to the synergetic coupling of the facet junction and the interfacial heterojunction that assures the effective charge separation, and the tremendous but physically separated reactive sites offered by layered MoS2 and highly‐exposed (001) facets of TiO2, respectively, the artificial Z‐scheme exhibits excellent photocatalytic performance in photodegradation tests. Moreover, the junctional plasmonic Au nanoclusters not only act as electron traps to promote the edge‐selective synthesis but also generate “hot electrons” to further boost photocatalytic performance. The Z‐scheme charge‐flow direction in the heterostructure and the roles of electrons and holes are comprehensively studied using in situ irradiated X‐ray photoelectron spectroscopy and photodegradation tests. This work offers a new insight into designing high‐performance Z‐scheme photocatalytic systems.

Utilizing trapped charge at bilayer 2D MoS2/SiO2 interface for memory applications

In this work we use conductive atomic force microscopy (cAFM) to study the charge injection process from a nanoscale tip to a single isolated bilayer 2D MoS2 flake. The MoS2 is exfoliated and bonded to ultra-thin SiO2/Si substrate. Local current–voltage (IV) measurements conducted by cAFM provides insight in charge trapping/de-trapping mechanisms at the MoS2/SiO2 interface. The MoS2 nano-flake provides an adjustable potential barrier for embedded trap sites where the charge is injected from AFM tip is confined at the interface. A window of (ΔV ∼ 1.8 V) is obtain at a reading current of 2 nA between two consecutive IV sweeps. This is a sufficient window to differentiate between the two states indicating memory behavior. Furthermore, the physics behind the charge entrapment and its contribution to the tunneling mechanisms is discussed.

Size Effects in Single- and Few-Layer MoS2 Nanoflakes: Impact on Raman Phonons and Photoluminescence.

The high optical absorption and emission of bidimensional MoS2 are fundamental properties for optoelectronic and biodetection applications and the opportunity to retain these properties in high quality nano-sized flakes would bring further possibilities. Here, a large set of single-layer and few-layer (2–3 layers) MoS2 flakes with size in the range from 10 nm to 20 μm are obtained on sapphire by vapor deposition techniques and evaluated combining the information from the Raman phonons with photoluminescence (PL) and absorption bands. The flakes have triangular shape and are found to be progressively relaxed from the tensile strain imposed by the sapphire substrate as their size is reduced. An increasing hole doping as size decreases is deduced from the blue shift of the A1g phonon, related to charge transfer from adsorbed oxygen. No clear correlation is observed between defects density and size, therefore, doping would be favored by the preferential adsorption of oxygen at the edges of the flakes, being progressively more important as the edge/surface ratio is incremented. This hole doping also produces a shift of the PL band to higher energies, up to 60 meV. The PL intensity is not found to be correlated to the size but to the presence of defects. The trends with size for single-layer and for 2–3 layer samples are found to be similar and the synthesis method does not influence PL efficiency which remains high down to 40 nm being thus promising for nanoscale photonics.

Modified Polyethersulfone Ultrafiltration Membrane for Enhanced Antifouling Capacity and Dye Catalytic Degradation Efficiency

Catalytic membranes, as a combination of heterogeneous advanced oxidation and membrane technology reaction systems, have important application prospects in the treatment of dyes and other organics. In practical applications, it is still challenging to construct catalytic membranes with excellent removal efficiency and fouling mitigation. Herein, molybdenum disulfide-iron oxyhydroxide (MoS2-FeOOH) was fabricated using iron oxide and MoS2 nanoflakes, which were synthesized by the hydrothermal method. Furthermore, by changing the concentration of MoS2-FeOOH, the MoS2-FeOOH/polyethersulfone (PES) composite ultrafiltration membrane was obtained with improved hydrophilicity, permeability, and antifouling capacity. The pure water flux of the composite membrane reached 385.3 L/(m2·h), which was 1.7 times that of the blank PES membrane. Compared with the blank membrane, with the increase of MoS2-FeOOH content, the MoS2-FeOOH/PES composite membranes had better adsorption capacity and catalytic performance, and the membrane with 3.0% MoS2-FeOOH content (M4) could be achieved at a 60.2% methylene blue (MB) degradation rate. In addition, the membrane flux recovery ratio (FRR) of the composite membrane also increased from 25.6% of blank PES membrane (M0) to more than 70% after two cycles of bovine serum albumin (BSA) filtration and hydraulic cleaning. The membrane with 2.25% MoS2-FeOOH content (M3) had the best antifouling performance, with the largest FRR and the smallest irreversible ratio (Rir). Catalytic self-cleaning of the composite membrane M3 recovered 95% of the initial flux with 0.1 mol/L H2O2 cleaning. The MoS2-FeOOH/PES composite membranes with the functions of excellent rejection and antifouling capacity have a good prospect in the treatment of printing and dyeing wastewater composed of soluble dyes.

High Performance Semiconducting Nanosheets via a Scalable Powder-Based Electrochemical Exfoliation Technique.

The liquid-phase exfoliation of semiconducting transition metal dichalcogenide (TMD) powders into 2D nanosheets represents a promising route toward the scalable production of ultrathin high-performance optoelectronic devices. However, the harsh conditions required negatively affect the semiconducting properties, leading to poor device performance. Herein we demonstrate a gentle exfoliation method employing standard bulk MoS2 powder (pressed into pellets) together with the electrochemical intercalation of a quaternary alkyl ammonium. The resulting nanosheets are produced in high yield (32%) and consist primarily of mono-, bi-, triatomic layers with large lateral dimensions (>1 μm), while retaining the semiconducting polymorph. Exceptional optoelectronic performance of nanosheet thin-films is observed, such as enhanced photoluminescence, charge carrier mobility (up to 0.2 cm2 V-1 s-1 in a multisheet device), and photon-to-current efficiency while maintaining high transparency (>80%). Specifically, as a photoanode for iodide oxidation, an internal quantum efficiency up to 90% (at +0.3 V vs Pt) is achieved (compared to only 12% for MoS2 nanosheets produced via ultrasonication). Further using a combination of fluorescence microscopy and high-resolution scanning transmission electron microscopy (STEM), we show that our gently exfoliated nanosheets possess a defect density (2.33 × 1013 cm-2) comparable to monolayer MoS2 prepared by vacuum-based techniques and at least three times less than ultrasonicated MoS2 nanoflakes. Finally, we expand this method toward other TMDs (WS2, WSe2) to demonstrate its versatility toward high-performance and fully scalable van der Waals heterojunction devices.

SnO2 nanoparticles embedded onto MoS2 nanoflakes - An efficient catalyst for photodegradation of methylene blue and photoreduction of hexavalent chromium

In this work, a molybdenum disulfide/tin oxide (MoS2/SnO2) composite was successfully prepared via a hydrothermal method. The MoS2/SnO2 composite was used as a photocatalyst for photoreduction of hexavalent chromium and photodecomposition of methylene blue. It exhibited higher photocatalytic performance under simulated solar light irradiation than MoS2 itself. The obtained material was characterized by several spectroscopic and microscopic techniques, namely X-ray photoelectron spectroscopy (XPS), energy dispersive X-ray spectroscopy (EDX), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). TEM images of the MoS2/SnO2 revealed that SnO2 nanoparticles were embedded into MoS2. Furthermore, photodegradation mechanism and the role of adsorption were discussed. The enhanced photocatalytic performance of MoS2/SnO2 composite can be directly attributed to the presence of SnO2, which may increase the separation efficiency of the electron-hole pairs. The 99.5% of MB removal was achieved after 5 minutes of illumination. To the best of our knowledge, it is one of the most photoactive MoS2-based photocatalysts for methylene blue (MB) photodecomposition. Moreover, the obtained composite was highly active against chromium (VI) photo-reduction. This work provides evidence that the MoS2/SnO2 composite possesses enormous potential in photocatalytic applications.

Synthesis and characterization of large-sized monolayer MoS2 nanoflakes by sulfurization of exfoliated MoO3 powder

We demonstrate a novel approach based on sulfurization of MoO3 powder pre-exfoliated on the SiO2/Si substrate to synthesize high crystalline quality and large-sized monolayer MoS2 films uniformly covering the whole substrate. The surface morphology, thickness, crystalline quality, and luminescence mechanism of the MoS2 films were characterized by the optical microscopy, Raman spectroscopy, and photoluminescence spectroscopy. The results reveal that MoS2 films directly synthesized on the bare SiO2/Si substrate by our method have high surface coverage superior over those of one-step chemical vapor deposition (CVD) method and exhibit uniform monolayer with larger lateral length up to ∼120 µm compared to those of the traditional two-step CVD method. Our proposed growth strategy opens up new pathways for CVD synthesis of high crystalline quality and large-sized uniform monolayer MoS2 films and can also be utilized for synthesizing other two-dimensional materials.

A new 2-methylimidazole-assisted liquid-exfoliation method for a rapid scalable fabrication of chemically pure MoS2 nanosheets

A new liquid exfoliation method of pristine MoS2 in a hot solution of 2-methylimidazole for a rapid fabrication of 1–2 nm-thick MoS2 nanoflakes is developed. The method makes it possible to produce large amounts of stable MoS2 hydrosols with concentrations 0.21–0.53 mg/ml within 2–4 h. Energy-dispersive X-ray spectroscopies and thermogravimetric analysis confirmed chemical purity of MoS2 after the removal of 2-methylimidazole with water. The as-formed MoS2 nanoflakes have a relatively narrow size distribution (50 ± 25 nm) and exhibit a strong aggregation-dependent luminescence at 420–475 nm. The thin films obtained from the MoS2 nanoflakes by spin-coating possess good semiconductor properties showing a non-ohmic linear current-voltage dependence and a surface resistivity up to 7.9 × 10–2 Ω·сm. The reported method may provide an affordable, experimentally simple and ecology-friendly alternative to commonly used techniques for fabricating MoS2 nanosheets with a potentially useful combination of size and properties for further applications in hybrid and composite materials.

Effect of Se content on the oxygen evolution reaction activity and capacitive performance of MoSe2 nanoflakes

In this work, a facile and cost-effective method is proposed to synthesize ultrathin MoSe2 nanoflakes. To get high purity MoSe2 nanoflakes, the reaction parameters are optimized. The TEM and Raman analysis confirmed the formation of few-layered MoSe2 nanoflakes consisting of 2 to 3 layers. The influence of Se content on the OER and capacitive performance of MoSe2 are studied in detail. The catalyst with optimal electrochemical performance (MS1.8) shows outstanding OER activity and capacitive performance. It requires a low overpotential of only 320 mV to reach a current density of 10 mA cm−2 with a low Tafel slope of 45.3 mV dec−1. Also, it exhibited a specific capacity of 25 mAh g−1 at a current density of 1 A g−1. The excellent OER activity of the MoSe2 nanoflakes can be attributed to the few-layered structure, which provides a high electrochemical surface area. Moreover, the prepared MoSe2 nanoflakes show excellent cyclic stability up to 5000 CV cycles. Our findings indicate that the amount of Se in MoSe2 plays a significant role in achieving maximum electrochemical performance from MoSe2. The method used in this study may provide a low-cost and simple technique to synthesize non-noble MoSe2 for a wide-scale application.

Controlled decoration of nanoceria on the surface of MoS2 nanoflowers to improve the biodegradability and biocompatibility in Drosophila melanogaster model

In recent years, nanozymes based on two-dimensional (2D) nanomaterials have been receiving great interest for cancer photothermal therapy. 2D materials decorated with nanoparticles (NPs) on their surface are advantageous over conventional NPs and 2D material based systems because of their ability to synergistically improve the unique properties of both NPs and 2D materials. In this work, we report a nanozyme based on flower-like MoS2 nanoflakes (NFs) by decorating their flower petals with NCeO2 using polyethylenimine (PEI) as a linker molecule. A detailed investigation on toxicity, biocompatibility and degradation behavior of fabricated nanozymes in wild-type Drosophila melanogaster model revealed that there were no significant effects on the larval size, morphology, larval length, breadth and no time delay in changing larvae to the third instar stage at 7–10 d for MoS2 NFs before and after NCeO2 decoration. The muscle contraction and locomotion behavior of third instar larvae exhibited high distance coverage for NCeO2 decorated MoS2 NFs when compared to bare MoS2 NFs and control groups. Notably, the MoS2 and NCeO2-PEI-MoS2 NFs treated groups at 100 μg ml−1 covered a distance of 38.2 mm (19.4% increase when compared with control) and 49.88 mm (no change when compared with control), respectively. High-resolution transmission electron microscopy investigations on the new born fly gut showed that the NCeO2 decoration improved the degradation rate of MoS2 NFs. Hence, nanozymes reported here have huge potential in various fields ranging from biosensing, cancer therapy and theranostics to tissue engineering and the treatment of Alzheimer’s disease and retinal therapy.

Mechanically exfoliated MoS2 nanoflakes for optimizing the thermoelectric performance of SrTiO3-based ceramic composites

As a semiconducting material with relatively low thermal conductivity, MoS2 nanoflake has the potential to serve as a modulator for optimizing the performance of thermoelectric (TE) materials. However, the low yield of MoS2 nanoflakes prepared by conventional methods has constrained the development of MoS2 optimized TE materials. We propose a mechanical exfoliation method for mass production of MoS2 nanoflakes using attrition mill. After mixed with La and Nb co-doped SrTiO3 (SLNT) powder, the MoS2/SLNT composites are fabricated by spark plasma sintering. It is found that the heterojunctions formed at MoS2/SLNT interfaces with proper band offset can effectively scatter the low-energy electrons, resulting in enhanced Seebeck coefficient without significantly undermining the electrical conductivity. The power factor of composites is improved when the MoS2 content is lower than 1.5 vol%. Meanwhile, the thermal conductivity of composites is significantly decreased due to the phonon scattering induced large thermal resistance at MoS2/SLNT interfaces, which is much higher than that in graphene embedded SrTiO3 composites. Consequently, a maximum ZT = 0.24 is obtained at 800 K in 1.5 vol% MoS2/SLNT composite, which is ∼26 % higher compared with pristine matrix. This work paves the way for application of TE materials modulated by transition metal dichalcogenides.

Label-free ultra-low level electrochemical detection of mercury(II) ions in aqueous medium via direct covalent functionalization of single-stranded DNA with MoS2 nanoflakes

Label-free ultra-low level electrochemical detection of mercury(II) ions in aqueous medium via direct covalent functionalization of single-stranded DNA with MoS2 nanoflakes

Ultra-durable and highly-efficient hybrid capacitive deionization by MXene confined MoS2 heterostructure

Capacitive deionization (CDI) is an emerging desalination technology, offering an advisable route to fetch clean water due to its low energy consumption and environmental friendliness. Hybrid CDI (HCDI), proceeding through a faradic ion storage mechanism, is thought as the next generation of CDI due to its high desalination capacity and charge efficiency. However, current HCDI system is plagued by its low desalination rate and poor long-term stability due to the structural instability of the faradic materials. Herein, we developed an effective strategy via combining 2D MoS2 nanoflakes with 2D MXene to construct a 3D “mutually supported” network architecture, in which MoS2 nanoflakes could act as the “pillar” of the intrinsic structure of MXene, while MXene could offer structural reinforcement to prevent the structural agrgregation and enhance the electrical conductivity of the heterostructure. As a result, the HCDI system based on MoS2/MXene heterostructure electrode displays an outstanding desalination performance with a high desalination capacity of 23.98 mg g−1, outstanding desalination rate of 4.6 mg g−1 min−1 and superior cycling stability with only 4% desalination capacity degradation for over 100 cycles, indicating that our strategy should be a promising approach to achieve highly efficient HCDI.

Boundary Slip of Oil Molecules at MoS2 Homojunctions Governing Superlubricity.

Molybdenum disulfide (MoS2) nanoflakes are widely used as nano-additives in oil for the excellent lubrication performance. However, the molecular mechanism of MoS2 nanoflakes in oil governing the friction properties remains elusive. In this study, MoS2 homojunctions were constructed by combining the fabricated MoS2 probe and MoS2 crystal with an atomic force microscope (AFM), and the superlubricity with an ultralow friction coefficient of approximately 0.003 at MoS2 homojunctions was attained after the formation of a confined oil layer, exhibiting a 67% reduction of the friction coefficient in comparison to that under a nitrogen atmosphere. The boundary slip of oil molecules on the MoS2 crystal with a small energy barrier was observed, causing the shear to occur at the interface of oil/MoS2 crystal with an extremely low shear strength, which contributes to the achievement of superlubricity. This boundary slip of oil molecules at MoS2 homojunctions can be extended to the macroscale for friction reduction, supplying a fundamental insight into the lubrication mechanism of MoS2 nanoflakes in oil, which has potential applications for designing an efficient lubrication system with nano-additives.

2D SiP nanoflakes as new high-performance lubricant additive for steel/steel sliding contact

Two-dimensional (2D) materials have been studied as lubricant materials for their unique structural characteristics. Here, a 2D SiP single crystal has been grown and its lubricant properties as hexadecane-based additive have been systematically researched. 2D SiP nanoflakes were obtained by using the liquid-phase exfoliation method. The tribological results show that the COF and the wear volume of hexadecane can be remarkably decreased by 30% and 73% under 90 N by adding the 25 mg/L of SiP nanoflakes, respectively. The hexadecane with SiP nanoflakes has larger extreme pressure (3265 MPa) than MoS2 nanoflakes (2709 MPa). Furthermore, it was determined that the weak interlaminar shear interaction and the formation of tribofilm between the sliding interfaces enhance the anti-friction and anti-wear properties.

TLM Analysis of Metal Contacts for 2D MoS2 Nanoflakes Deposited by Chemical Vapour Deposition-free technique

In the past decade, two-dimensional (2D) nanomaterials are gaining significant attention due to their novel properties in electrical, optical, mechanical which are different from their bulk forms. Therefore, it becomes one of the most promising materials for sensing applications. However, to connect the 3D world with 2D materials require excellent communication links which is a contact that renders remarkable electrical properties and specifically 2D nanomaterials-based sensor has been suffering from high resistance and low sensitivity due to the loss caused by poor contact between the 2D nanomaterials and metal interface. This work explores the electrical properties of metal/2D contacts by the rectangular transmission line method (TLM). The critical resistance parameters such as sheet and specific contact resistance were determined by characterizing the current-voltage (IV) relation. The behavior of different metals (silver and gold) contacts to 2D MoS2 nanoflakes deposited on Si substrate by low-cost spray coating technique are observed. These metals are chosen based on the work function relative to the electron’s affinity, χ of MoS2. UV-Vis’s analysis is carried out to estimate the final concentration of MoS2 solution.

MoS2 Nanoflake and ZnO Quantum Dot Blended Active Layers on AuPd Nanoparticles for UV Photodetectors

MoS₂ Nanoflake and ZnO Quantum Dot Blended Active Layers on AuPd Nanoparticles for UV Photodetectors

Exciton-dielectric mode coupling in MoS2 nanoflakes visualized by cathodoluminescence.

Two-dimensional (2D) transition metal dichalcogenides (TMDCs), possessing unique exciton luminescence properties, have attracted significant attention for use in optical and electrical devices. TMDCs are also high refractive index materials that can strongly confine the electromagnetic field in nanoscale dimensions when patterned into nanostructures, thus resulting in complex light emission that includes exciton and dielectric resonances. Here, we use cathodoluminescence (CL) to experimentally visualize the emission modes of single molybdenum disulfide (MoS2) nanoflakes and to investigate luminescence enhancement due to dielectric resonances in nanoscale dimensions, by using a scanning transmission electron microscope. Specifically, we identify dielectric modes whose resonant wavelength is sensitive to the shape and size of the nanoflake, and exciton emission peaks whose energies are insensitive to the geometry of the flakes. Using a four-dimensional CL method and boundary element method simulations, we further theoretically and experimentally visualize the emission polarization and angular emission patterns, revealing the coupling of the exciton and dielectric resonant modes. Such nanoscopic observation provides a detailed understanding of the optical responses of MoS2 including modal couplings of excitons and dielectric resonances which play a crucial role in the development of energy conversion devices, single-photon emitters, and nanophotonic circuits with enhanced light-matter interactions.

A Thin Composite Coating for Threads Protection: Synergistic Effect of Modified Mos2 and F-Ceo2 Nanoflakes

A Thin Composite Coating for Threads Protection: Synergistic Effect of Modified Mos2 and F-Ceo2 Nanoflakes

Insights into the multifunctional applications of strategically Co doped MoS2 nanoflakes

Simultaneous tuning of magnetic, transport and electrochemical properties through strategic doping of cobalt (Co) ions in hydrothermally treated multi-layered MoS2 nanoflakes (NFs).