Dispersion Of MoS2 Nanosheets Research Articles

The colloidal stability of molybdenum disulfide nanosheets in different natural surface waters: Combined effects of water chemistry and light irradiation

With the increasing production and application, more molybdenum disulfide (MoS2) nanosheets could be released into environment. The aggregation and dispersion of MoS2 nanosheets profoundly impact their transport and transformation in the aquatic environment. However, the colloidal stability of MoS2 remains largely unknown in natural surface waters. This study investigated the colloidal stability of MoS2 nanosheets in six natural surface waters affected by both light irradiation and water chemistry. Compared to that of the pristine MoS2 nanosheets, the colloidal stability of MoS2 photoaged in ultrapure water declined. Light irradiation induced the formation of Mo-O bonds, the release of SO42− species, and the decrease in 1T/2H ratio, which reduced negative charge and enhanced hydrophobicity. However, the colloidal stability of MoS2 photoaged in natural surface waters was increased relative to that in ultrapure water not only for the smaller extent of photochemical transformation but more importantly the surface modification by water chemistry. Furthermore, the colloidal stability of MoS2 photoaged in natural surface waters followed the order of sea water > lake water > river water. The abundant cations (e.g., Ca2+ and Mg2+) in sea water facilitated the covalent grafting (S-C bonds) of more dissolved organic matter (DOM) on MoS2 via charge screening and cation bridging, thus inducing stronger electrostatic repulsion and steric effect to stabilize nanosheets. The crucial role of the covalent grafting of DOM was further confirmed by the positive correlation between the critical coagulation concentration values and S-C ratios (R2 = 0.82, p < 0.05). Our results highlighted the dominant role of water chemistry than light irradiation in dictating the colloidal stability of MoS2 photoaged in natural surface waters, which provided new insight into the environmental behavior of MoS2 in aquatic environment.

Rheological and shear thickening properties in MoS2 nanosheets dispersion: An integrated experimental and theoretical investigation

For the advancement of solvent processing and manufacturing procedures, understanding the rheological characteristics of suspended two-dimensional (2D) materials is crucial. Here, we synthesized MoS2 nanosheets using a simple, easy and efficient hydrothermal way. Structural and morphological studies have been carried out using X-ray diffraction and scanning/transmission electron microscopy confirming the formation of MoS2 nanosheets. The elemental composition of the as-prepared samples was studied using an energy-dispersive X-ray analyser. Optical properties were studied using FTIR spectroscopy. MoS2 nanosheets prepared at 180 °C and 200 °C were considered for rheological studies. For rheological studies, stable dispersions of MoS2 nanosheets in water were analysed. Initially, water dispersion of MoS2 nanosheets showed non-Newtonian behaviour following the Newtonian/Bingham plastic fluid model. Its viscoelastic characteristics suggest lubricating fluid application. DFT has been used to study MoS2 nanosheets under biaxial strain. This simple aqueous method can provide a reliable lubricant and improve MoS2 nanosheets rheology.

The effect of laser wavelength and concentration on the optical limiting response of exfoliated MoS2 in the NMP solvent

This study investigated the optical limiting (OL) response of the dispersion of MoS2 nanosheets in N-methyl-2-pyrrolidone (NMP) which was made by liquid-phase exfoliation. We measured the OL response of different concentrations of the samples at two wavelengths of 405 nm and 532 nm using a continuous wave (CW) laser irradiation. It was observed that the value of the OL threshold changes by changing the laser wavelength and MoS2 concentration. To understand the mechanisms underlying these results, we use the spatial self-phase modulation (SSPM) and the Z-scan methods to measure the nonlinear absorption and refraction coefficients. It was revealed that the nonlinear absorption response is too weak. These findings highlight the significant role of the nonlinear refraction effect in the spatial self-phase modulation and the OL response of MoS2 dispersion under low-power laser irradiation.

Production of mono- to few-layer MoS2 nanosheets in isopropanol by a salt-assisted direct liquid-phase exfoliation method

Here, we report a facile salt-assisted direct liquid-phase exfoliation method for mass production of MoS2 nanosheets. We choose organic solvent isopropanol (IPA) as exfoliation media and potassium ferrocyanide, potassium sodium tartrate, or sodium tartrate as salt, the assistant. The selected salts show universal and efficient assistant effect for the exfoliation of MoS2 in IPA. Especially, potassium ferrocyanide (K4Fe(CN)6) can enhance the exfoliation efficiency up to 73 times and a dispersion of MoS2 nanosheets with concentration as high as 0.240 mg mL−1 can be easily obtained in IPA-K4Fe(CN)6 system. Transmission electron microscopy, atomic force microscopy (AFM), and Raman spectroscopy show that bulk MoS2 has been successfully exfoliated into mono- to few-layer MoS2 nanosheets. AFM analysis indicates that nearly 60% flakes are monolayer in MoS2 dispersion.

Constructing hierarchical polymer@MoS2 core-shell structures for regulating thermal and fire safety properties of polystyrene nanocomposites

Two dimensional MoS2 acting as reinforcing fillers had attracted intense interests in recent years. However, the homogeneous dispersion of MoS2 nanosheets in polymer matrix and exact interface control were still difficult to achieve due to potent van der Waals forces and surface inactiveness. In this work, hierarchical polystyrene@MoS2 core-shell structures were constructed by combining latex technology and self-assembly of oppositely charged MoS2 nanosheets onto the surface of polystyrene spheres. The formed core-shell structures were characterized by X-ray diffraction, Fourier transform infrared, thermogravimetric analysis, scanning electron microscopy, transmission electron microscopy and further employed as reinforcing fillers in a host polystyrene matrix by solvent blending method. It was clearly observed that the construction of polystyrene@MoS2 core-shell structures not only improved the dispersion of MoS2 in the matrix and the interfacial adhesion between MoS2 and polystyrene, but also obviously improved the thermal stability and fire safety of polystyrene. The method proposed here had proved to be an efficient and facile approach to fabricate polymer/MoS2 nanocomposites with good dispersion and markedly improved properties.

Homogenous Dispersion of MoS2 Nanosheets in Polyindole Matrix at Air-Water Interface Assisted by Langmuir Technique.

Two-dimensional (2D) inorganic layered materials when embedded in organic polymer matrix exhibit exotic properties that are grabbing contemporary attention for various applications. Here, nanosheet morphology of molybdenum disufide (MoS2) synthesized via one-pot facile hydrothermal reaction are exfoliated in benign aqueous medium in the presence of indole to obtain a stable dispersion. These exfoliated nanosheets then act as host to template the controlled polymerization of indole. The preassembled MoS2-polyindole (MoS2-PIn) nanostructures are reorganized at the air-water interface using the Langmuir method to facilitate maximum interfacial interaction between nanosheet and polymer. This report emphasizes large area, homogeneous dispersion of uniform-sized MoS2 nanosheets (40-60 nm diameter) in the PIn matrix and the formation of stable and uniform film via the Langmuir-Schaefer (LS) method. These self-assembled, MoS2 decorated PIn LS films are characterized using atomic force microscopy (AFM) and transmission electron microscopy (TEM). The fabricated LS films in sandwiched structure Al/MoS2-PIn/ITO as the Schottky diode portrayed remarkable enhancements in charge transport properties. Our study illustrates the potential of the MoS2-PIn LS film in electronic applications and opens a new dimension for uniform dispersion of 2D materials in other polymers via the Langmuir method for device fabrication and enhancement of electrical properties.

Understanding the exfoliation and dispersion of MoS2 nanosheets in pure water

Exfoliation and dispersion of two dimensional (2D) transition metal dichacogenides (TMDCs) such as MoS2 in water are highly demanded. In this paper, we exfoliate MoS2 nanosheets in pure water via ultrasonication and attempt to understand the exfoliation process and the dispersion behavior of hydrophobic MoS2. Cavitation-induced exfoliation leads to thinning and fragmentation of MoS2 flakes. Formation of mesoporous MoS2 sheets suggests that the exfoliation initiates from the basal planes. As for the dispersion of MoS2 nanosheets in water, we find that higher centrifugation rate and thus smaller lateral size, results in improved stability. This can be attributed to the enhanced edge effects. Fragmentation of MoS2 flakes generates many edges, to which hydrophilic and ionizable groups are attached. Herein, the edge effects include two aspects. On the one hand, edge-attached polar and hydrophilic groups promote interactions of MoS2 nanosheets with water molecules and result in better wettability, which can be explained by the hemi-wicking model. On the other hand, dissociation of these groups makes MoS2 sheets negatively charged. It is found that edge charge dominates. As the flake size reduces, attractive van der Waals force and hydrophobic force can be overcome by electrostatic repulsions, which prevent MoS2 nanosheets from aggregation. This study opens a new vista on exfoliating and dispersing hydrophobic 2D nanomaterials in water.

Dye adsorption and photo-induced recycling of hydroxypropyl cellulose/molybdenum disulfide composite hydrogels

A novel environmental friendly dye adsorbent composited of hydroxypropyl cellulose (HPC) and molybdenum disulfide (MoS2) were fabricated, which can be recycled and reused by illumination due to the photo-catalytic properties of MoS2. In order to promote a more homogeneous dispersion of MoS2 nanosheets in HPC, MoS2 was functionalized with HPC chains through esterification. The successful functionalization of MoS2 was confirmed by FTIR, XRD, XPS characterizations. The morphologies of the HPC and HPC-MoS2/HPC hydrogels were compared by SEM. The obtained HPC-MoS2/HPC hydrogels exhibited enhanced adsorption performance toward methylene blue. An adsorption improvement of HPC hydrogel had been brought by the incorporation of HPC-MoS2 sheets. Adsorption kinetics was then imitated by Lagergren pseudo-first-order and pseudo-second-order models. Adsorption isotherms were imitated by Langmuir isotherm and Freundlich isotherm. The photo-regeneration experiments of HPC-MoS2/HPC hydrogel after absorption showed that the absorbent can be activated easily by illumination with little loss of dye adsorbing capacity.

The effective reinforcements of functionalized MoS2 nanosheets in polymer hybrid composites by sol-gel technique

In this work, siloxane group was grafted on the surface of exfoliated MoS2 nanosheets by chemical conjugation with 3-mercaptopropyl trimethoxysilane and then the functionalized MoS2 was incorporated into poly(vinyl alcohol) (PVA) matrix as reinforcements using sol-gel technique. The results of FTIR, TGA, XPS and TEM indicated that 3-mercaptopropyl trimethoxysilane was successfully grafted onto the surface of exfoliated MoS2 nanosheets. The functionalized MoS2 was well dispersed in PVA and no obvious aggregation of MoS2 nanosheets was observed. The incorporation of functionalized MoS2 nanosheets significantly enhanced the thermal properties, flame retardancy and mechanical properties of PVA films. The half weight loss degradation temperature of PVA films was increased from 298 to 416°C and a 12°C increment in the glass transition temperature was achieved with only 1.0wt% functionalized MoS2. Furthermore, the peak heat release rate and the tensile strength of PVA films was decreased by 52.5% and improved by 98.4%, respectively, compared to that of neat PVA. These excellent reinforcements were mainly attributed to well dispersion of MoS2 nanosheets in the polymer matrix and strong interfacial adhesion between the two components. This work demonstrated herein will provide a promising route to fabricate MoS2-based polymer nanocomposites with excellent performances.

One-pot synthesis of hierarchical Ni2P/MoS2 hybrid electrocatalysts with enhanced activity for hydrogen evolution reaction

A simple one-pot synthesis method has been used to fabricate novel Ni2P/MoS2 hybrid electrocatalysts for hydrogen evolution reaction (HER). Owing to the weak conductivity and layered structure of MoS2, Ni2P nanoparticles with excellent conductivity and activity have been doped into MoS2 for improving the electrocatalytic performances for HER. The structure and morphology of the as-prepared Ni2P/MoS2 hybrid nanostructures are characterized. XRD and XPS show the elemental composition and valence of Ni2P/MoS2. SEM and TEM confirm that the close interaction of the hybrid materials and good dispersion of Ni2P nanoparticles. The as-synthesized Ni2P/MoS2 hybrid electrocatalysts exhibit excellent activity with onset overpotential of 75mV and Tafle slope of 76mVdec−1, which are much better than that of pure MoS2. The enhanced stability of the as-prepared Ni2P/MoS2 for HER has also been observed. The improved performances for HER may be ascribed to the better conductivity and dispersion of MoS2 nanosheets in Ni2P/MoS2 hybrid electrocatalysts. The small size and good dispersion of Ni2P nanoparticles also contributed to the enhancement of HER activity. Compared with mechanically mixed MoS2 and Ni2P (Ni2P-MoS2), Ni2P/MoS2 hybrid materials demonstrate better electrochemical performances for HER, implying the existence of synergistic effect between Ni2P and MoS2 on HER activity.

Graphene-mediated highly-dispersed MoS2 nanosheets with enhanced triiodide reduction activity for dye-sensitized solar cells

Novel composites made of two dimensional (2D) MoS2 nanosheets and graphene (G) have been fabricated by a combined approach of chemical vapor deposition and hydrothermal technique. The G film helps to mediate the growth and dispersion of MoS2 nanosheets, of which the unique role of G in the MoS2 growth is revealed by density functional theory study. The results show that the hexagonal lattice carbon of the G film can easily interact with sulfur species derived from reaction precursors, which favors the uniform growth of 2D MoS2. The unique function of the G film demonstrated here can be extended to other carbon substrates for growing 2D MoS2 nanosheets. The G-MoS2 composites consisting of two types of 2D materials are tested as the binder-free counter electrodes (CEs) for dye-sensitized solar cells, showing a high power conversion efficiency of 7.1% that is comparable to the expensive Pt CEs. The 2D G film in the hybrids has two functions: the active sites for dispersing the electrochemically active MoS2 crystals and the high electrical conducting matrix for fast charge transfer. This synergistic effect may help to shed a light on the functionalization of other inorganic materials to G for advanced energy applications.

Edge and confinement effects allow in situ measurement of size and thickness of liquid-exfoliated nanosheets.

Two-dimensional nanomaterials such as MoS2 are of great interest both because of their novel physical properties and their applications potential. Liquid exfoliation, an important production method, is limited by our inability to quickly and easily measure nanosheet size, thickness or concentration. Here we demonstrate a method to simultaneously determine mean values of these properties from an optical extinction spectrum measured on a liquid dispersion of MoS2 nanosheets. The concentration measurement is based on the size-independence of the low-wavelength extinction coefficient, while the size and thickness measurements rely on the effect of edges and quantum confinement on the optical spectra. The resultant controllability of concentration, size and thickness facilitates the preparation of dispersions with pre-determined properties such as high monolayer-content, leading to first measurement of A-exciton MoS2 luminescence in liquid suspensions. These techniques are general and can be applied to a range of two-dimensional materials including WS2, MoSe2 and WSe2.

High‐Concentration Aqueous Dispersions of MoS2

AbstractMolybdenum disulfide (MoS2) nanosheets have been attracting increasing research interests due to their unique material properties. However, the lack of a reliable large‐scale production method impedes their practical applications. Here a facile, efficient, and scalable method for the fabrication of high‐concentration aqueous dispersion of MoS2 nanosheets using combined grinding and sonication is reported. The 26.7 ± 0.7 mg/mL concentration achieved is the highest concentration in an aqueous solution reported up to now. Grinding generates pure shear forces to detach the MoS2 layers from the bulk materials. Subsequent sonication further breaks larger crystallites into smaller crystallites, which promotes the dispersion of MoS2 nanosheets in ethanol/water solutions. The exfoliation process establishes a new paradigm in the top‐down fabrication of 2D nanosheets in aqueous solution. In the meantime, MoS2‐based sensing film produced using this approach has successfully demonstrated the feasibility of a low‐cost and efficient NH3 gas sensor using inkjet printing as a viable method.