High pressure effect on MoS<Subscript>2</Subscript> and MoSe<Subscript>2</Subscript> single crystals grown by CVT method

Single crystals of MoS₂ and MoSe₂ were grown by chemical vapour transport method using iodine as a transporting agent and characterized by optical microscopy, energy dispersive analysis (EDAX), X-ray powder diffraction (XRD) and Hall mobility at room temperature. The variation of electrical resistance under pressure was monitored in a Bridgman anvil set-up up to 6.5 GPa to identify occurrence of any structural transition. MoS₂ and MoSe₂ do not undergo any structural transitions under pressure.     

Out‐of‐Plane Strain Induced in a Moiré Superstructure of Monolayer MoS2 and MoSe2 on Au(111)

Making contact of transition metal dichalcogenides (TMDCs) with a metal surface is essential for fabricating and designing electronic devices and catalytic systems. It also generates strain in the TMDCs that plays significant role in both electronic and phonon structures. Therefore, detailed understanding of mechanism of the strain generation is important to fully comprehend the modulation effect for the electronic and phonon properties. Here, MoS₂ and MoSe₂ monolayers are grown on Au surface by chemical vapor deposition and it is demonstrated that the contact with a crystalline Au(111) surface gives rise to only out‐of‐plane strain in both MoS₂ and MoSe₂ layers, whereas no strain generation is observed on polycrystalline Au or SiO₂/Si surfaces. Scanning tunneling microscopy analysis provides information regarding consequent specific adsorption sites between lower S (Se) atoms in the S? Mo? S (Se? Mo? Se) structure and Au atoms via unique moiré superstructure formation for MoS₂ and MoSe₂ layers on Au(111). This observation indicates that the specific adsorption sites give rise to out‐of‐plane strain in the TMDC layers. Furthermore, it also leads to effective modulation of the electronic structure of the MoS₂ or MoSe₂ layer.     

Heterostructures of 2D Molybdenum Dichalcogenide on 2D Nitrogen-Doped Carbon: Superior Potassium-Ion Storage and Insight into Potassium Storage Mechanism

Constructing 2D heterostructure materials by stacking different 2D materials can combine the merits of the individual building blocks while eliminating their shortcomings. Dichalcogenides are attractive anodes for potassium-ion batteries (KIBs) due to their high theoretical capacity. However, the practical application of dichalcogenide is greatly hampered by the poor electrochemical performance due to sluggish kinetics of K⁺ insertion and the electrode structure collapse resulting from the large K⁺ insertion. Herein, heterostructures of 2D molybdenum dichalcogenide on 2D nitrogen-doped carbon (MoS₂, MoSe₂-on-NC) are prepared to boost their potassium storage performance. The unique 2D heterostructures possess built-in heterointerfaces, facilitating K⁺ diffusion. The robust chemical bonds (C S, C Se, C Mo bonds) enhance the mechanical strength of electrodes, thus suppressing the volume expansion. The 2D N-doped carbon nanosheets interconnected as a 3D structure offer a fast diffusion path for electrons. Benefitting from these merits, both the MoS₂-on-NC and the MoSe₂-on-NC exhibit unprecedented cycle life. Moreover, the electrochemical reaction mechanism of MoSe₂ is revealed during the process of potassiation and depotassiation.     

Synthesis of nano-MoS<Subscript>2</Subscript>/TiO<Subscript>2</Subscript> composite and its catalytic degradation effect on methyl orange

A nano-MoS₂/TiO₂ composite was synthesized in H₂ atmosphere by calcining a MoS₃/TiO₂ precursor, which was obtained via a quick deposition of MoS₃ on anatase nano-TiO₂ under a strong acidic condition. The obtained nano-MoS₂/TiO₂ composite was characterized by X-ray diffraction spectroscopy, Brunauer–Emmett–Teller (BET) surface area, scanning electron microscopy, high-resolution transmission electron microscopy, energy-dispersive spectrometry, ultraviolet–visible spectroscopy, and Fourier transform infrared spectroscopy. The results show that the composite had a high BET surface area because of its small size and irregularly layered structure. MoS₂ in the composite was composed of typical layered structures with thicknesses of 2–8 nm and lengths of 10–40 nm. The composite contained a wide and intensive absorption at 400–700 nm, which is in the visible light region, and presented a positive catalytic effect on removing methyl orange from the aqueous solution. The catalytic activity of the composite was influenced by the initial concentration of methyl orange, the amount of the catalyst, the pH value, and the degradation temperature. In addition, the composite catalyst could be regenerated and repeatedly used via filtration three times. The deactivating catalyst could be reactivated after catalytic reaction by heating at 450 °C for 30 min in H₂.     

Study on simultaneous detection of CO and H<Subscript>2</Subscript> with (Pd,Fe)-modified SnO<Subscript>2</Subscript> and Pt-loaded SnO<Subscript>2</Subscript> sensors

The (Pd, Fe)-modified SnO₂ (S1) and Pt-loaded SnO₂ (S2) are synthesized via a sol–gel method. As S1 has better selectivity to CO against H₂ while S2 to H₂ against CO at 400 °C. Thus S1 and S2 can be used to detect the concentration of CO and H₂, respectively. However, neither S1 nor S2 can detect the concentration of CO and H₂ when they coexist. In this paper, S1 and S2 sensors are used simultaneously for mixed gas of CO and H₂ detection, and the respective concentration of CO and H₂ is calculated. The calculation process is explained as follows: the response of S1 (R₁) and S2 (R₂) to a fixed concentration of mixed gas of CO and H₂ is obtained in experiment, respectively. So we can calculate the concentration of CO and H₂ by using simultaneous equations with the independent variable R₁ and R₂. Contrast real values with calculated values of CO and H₂ concentration, the error margin are all less than 5%, which indicates that this method may be a promising candidate for enhancing the selectivity of semiconductor-based gas sensor to two or more gases.     

MoS<Subscript>2</Subscript>/CoS<Subscript>2</Subscript> composites composed of CoS<Subscript>2</Subscript> octahedrons and MoS<Subscript>2</Subscript> nano-flowers for supercapacitor electrode materials

In pursuing excellent supercapacitor electrodes, we designed a series of MoS₂/CoS₂ composites consisting of flower-liked MoS₂ and octahedron-shaped CoS₂ through a facile one-step hydrothermal method and investigated the electrochemical performance of the samples with various hydrothermal time. Due to the coupling of two metal species and a big amount of well-developed CoS₂ and MoS₂, the results indicated that the MoS₂/CoS₂ composites electrodes exhibited the best electrochemical performance with a large specific capacitance of 490 F/g at 2 mV/s or 400 F/g at 10 A/g among all samples as the hydrothermal time reached 48 h (MCS₄₈). Furthermore, the retention of MCS₄₈ is 93.1% after 10000 cycles at 10 A/g, which manifests the excellent cycling stability. The outstanding electrochemical performance of MCS₄₈ indicates that it could be a very promising and novel energy storage material for supercapacitors in the future.     

MoS<Subscript>2</Subscript>/h-BN heterostructures: controlling MoS<Subscript>2</Subscript> crystal morphology by chemical vapor deposition

Tuning the properties of van der Waals heterostructures based on alternating layers of two-dimensional materials is an emerging field of research with implications for electronics and photonics. Hexagonal boron nitride (h-BN) is an attractive insulating substrate for two-dimensional materials as it may exert less influence on the layer’s properties than silica. In this work, MoS₂ layers were deposited by chemical vapor deposition (CVD) on thick h-BN flakes mechanically exfoliated deposited on Si/SiO₂ substrates. CVD affords the controllable, large-scale preparation of MoS₂ on h-BN alleviating shortcomings of manual mechanical assembly of such heterostructures. Electron microscopy revealed that in-plane and vertical to the substrate MoS₂ layers were grown at high yield, depending on the sample preparation conditions. Raman and photoluminescence spectroscopy were employed to assess the optical and electronic quality of MoS₂ grown on h-BN as well as the interactions between MoS₂ and the supporting substrate. Compared to silica, MoS₂ layers grown on h-BN are less prone to oxidation and are subjected to considerably weaker electronic perturbation.     

Reduction of Np(V) with Fe(II) in weakly acidic solutions containing H<Subscript>2</Subscript>C<Subscript>2</Subscript>O<Subscript>4</Subscript>

The kinetics of the reaction of Np(V) with Fe(II) in dilute perchloric and nitric acid solutions containing H₂C₂O₄ was studied by spectrophotometry. In the range pH 1–2, the reaction rate is described by the equation d[Np(V)]/dt = k[Np(V)][Fe(II)][H₂C₂O₄]²[H⁺]^−1.6, k = 182 mol^−1.4 l^1.4 s⁻¹. The activation energy in the range 25–45°C is 26 kJ mol⁻¹. The reaction mechanism involves formation of Fe(II) and Np(V) oxalate complexes, followed by their reaction with the participation of the H⁺ ion.     

Microstructure and H<Subscript>2</Subscript>S gas sensing properties of nanocrystalline perovskite-type La<Subscript>0.6</Subscript>Co<Subscript>0.4</Subscript>Mn<Subscript>0.5</Subscript>Ni<Subscript>0.5</Subscript>O<Subscript>3</Subscript>

Nanocrystalline La_1−x Co _x Mn_1−y Ni _y O₃ (x = 0.2 and 0.4; y = 0.1, 0.3, and 0.5) thick films sensors prepared by sol–gel method were studied for their H₂S gas sensitivity. The structural and morphological properties have been carried out by X-ray diffraction (XRD) and transmission electron microscopy (TEM). Average particle size estimated from XRD and TEM analyses was observed to be 30–35 nm. The gas response characteristics were found to depend on the dopants concentration and operating temperature. The maximum H₂S gas response of pure LaMnO₃ was found to be at 300 °C. In order to improve the gas response, material doped with transition metals Co and Ni on A- and B-site, respectively. The La_0.6Co_0.4Mn_0.5Ni_0.5O₃ shows high response towards H₂S gas at an operating temperature 250 °C. The Pd-doped La_0.6Co_0.4Mn_0.5Ni_0.5O₃ sensor was found to be highly sensitive to H₂S at an operating temperature 200 °C. The gas response, selectivity, response time and recovery time were studied and discussed.     

Ag<Subscript>2</Subscript>Se complex nanostructures with photocatalytic activity and superhydrophobicity

Single-crystalline Ag₂Se complex nanostructures have been synthesized via a solvothermal route in which selenophene (C₄H₄Se) as a selenylation source reacts with AgNO₃ at a temperature of 240 °C. An orthorhombic phase β-Ag₂Se nanostructure was identified by X-ray diffraction (XRD), Raman spectroscopy, field emission scanning electron microscopy (FE-SEM), high resolution transmission electron microscopy (HRTEM), and photoluminescence (PL) spectroscopy. The wettability of the as-synthesized β-Ag₂Se nanostructure was studied by measurement of the water contact angle (CA). Static water CA values of over 150° were obtained, which can be attributed to the β-Ag₂Se complex nanostructure having a combination of micro- and nanostructures. The superhydrophobic Ag₂Se nanostructure may find applications in self-cleaning. Additionally, the photocatalytic activity of the as-synthesized β-Ag₂Se nanostructure was evaluated by photodegradation of rhodamine B (RhB) dye under ultraviolet (UV) light irradiation.     

Tribological properties of MoS<Subscript>2</Subscript> and carbon fiber reinforced polyimide composites

The tribological properties of carbon fiber reinforced polyimide (PI) composites with different MoS₂ containing sliding against GCr15 steel were comparatively evaluated on an M-2000 model ring-on-block test rig. The wear mechanisms were also comparatively discussed, based on scanning electron microscopic examination of the worn surface of the PI composites and the transfer film formed on the counterpart. It was found that small incorporation of MoS₂ was harmful to the improvement of friction and wear behaviors of carbon fiber reinforced PI composites. However, it was found that the increasing filler of MoS₂ significantly improved the wear resistance and decreased the friction coefficient of carbon fiber reinforced PI composites. It was also found that the tribological properties of MoS₂ and short carbon fiber reinforced PI composites were closely related with the sliding condition such as sliding rate and applied load.     

MoS<Subscript>2</Subscript>/C/C nanofiber with double-layer carbon coating for high cycling stability and rate capability in lithium-ion batteries

MoS₂ has attracted a lot of interest in the field of lithium-ion storage as an anode material owing to its high capacity and two-dimensional (2D)-layer structure. However, its electrochemical properties, such as rate capability and cycling stability, are usually limited by its low conductivity, volume variation, and polysulfide dissolution during lithiation/delithiation cycling. Here, a designed two-layer carbon-coated MoS₂/carbon nanofiber (MoS₂/C/C fiber) hybrid electrode with a double-layer carbon coating was achieved by a facile hydrothermal and subsequent electrospinning method. The double carbon layer (inner amorphous carbon and outer carbon fiber) shells could efficiently increase the electron conductivity, prevent the aggregation of MoS₂ flakes, and limit the volume change and polysulfide loss during long-term cycling. The as-prepared MoS₂/C/C fiber electrode exhibited a high capacity of up to 1,275 mAh/g at a current density of 0.2 A/g, 85.0% first cycle Coulombic efficiency, and significantly increased rate capability and cycling stability. These results demonstrate the potential applications of MoS₂/C/C fiber hybrid material for energy storage and may open up a new avenue for improving electrode energy storage performance by fabricating hybrid nanofiber electrode materials with double-layer carbon coatings.

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Atomic‐Level Customization of 4 in. Transition Metal Dichalcogenide Multilayer Alloys for Industrial Applications

Despite many encouraging properties of transition metal dichalcogenides (TMDs), a central challenge in the realm of industrial applications based on TMD materials is to connect the large‐scale synthesis and reproducible production of highly crystalline TMD materials. Here, the primary aim is to resolve simultaneously the two inversely related issues through the synthesis of MoS_{2(1?x )}Se_2x ternary alloys with customizable bichalcogen atomic (S and Se) ratio via atomic‐level substitution combined with a solution‐based large‐area compatible approach. The relative concentration of bichalcogen atoms in the 2D alloy can be effectively modulated by altering the selenization temperature, resulting in 4 in. scale production of MoS_1.62Se_0.38, MoS_1.37Se_0.63, MoS_1.15Se_0.85, and MoS_0.46Se_1.54 alloys, as well as MoS₂ and MoSe₂. Comprehensive spectroscopic evaluations for vertical and lateral homogeneity in terms of heteroatom distribution in the large‐scale 2D TMD alloys are implemented. Se‐stimulated strain effects and a detailed mechanism for the Se substitution in the MoS₂ crystal are further explored. Finally, the capability of the 2D alloy for industrial application in nanophotonic devices and hydrogen evolution reaction (HER) catalysts is validated. Substantial enhancements in the optoelectronic and HER performances of the 2D ternary alloy compared with those of its binary counterparts, including pure‐phase MoS₂ and MoSe₂, are unambiguously achieved.     

Proton Conductivity and Thermal Properties of Ba(H<Subscript>2</Subscript>PO<Subscript>4</Subscript>)<Subscript>2</Subscript>

We have grown single crystals of barium dihydrogen phosphate and studied its thermal transformations during heating to 500°C and its electrotransport properties. Ba(H₂PO₄)₂ (Pccn) has been shown to undergo no phase transitions up to its dehydration temperature. The thermal decomposition of Ba(H₂PO₄)₂, accompanied by dehydration, involves two steps, with maximum rates at ~265 and 370°C, and results in the formation of barium dihydrogen pyrophosphate and barium metaphosphate, respectively. The total enthalpy of the endothermic dehydration events is–244.6 J/g. Using impedance spectroscopy, we have studied in detail the proton conductivity of polycrystalline and single-crystal Ba(H₂PO₄)₂ samples in a controlled atmosphere. Adsorbed water has been shown to have a significant effect on the proton conductivity of Ba(H₂PO₄)₂ up to 130°C. The proton conductivity of the Ba(H₂PO₄)₂ single crystals has been shown to be anisotropic. The conductivity anisotropy correlates with specific structural features of the salt. Higher conductivity values, 3 × 10^–9 to 2 × 10^–7 S/cm in the range 60–160°C, have been observed in the [100] crystallographic direction, exceeding the conductivity along [010] by an order of magnitude. The activation energy for proton conduction is 0.80 eV.     

Visualization of the electrocatalytic activity of three-dimensional MoSe<Subscript>2</Subscript>@reduced graphene oxide hybrid nanostructures for oxygen reduction reaction

Developments of nanostructured transition metal dichalcogenides (TMDs) materials as novel electrocatalyst candidates for oxygen reduction reaction (ORR) is a new strategy to promote the developments of non-precious metal ORR catalysts. In this work, a three-dimensional (3D) hybrid of rosebud-like MoSe₂ nanostructures supported on reduced graphene oxide (rGO) nanosheets was successfully synthesized through a facile hydrothermal strategy. The prepared MoSe₂@rGO hybrid nanostructure showed enhanced electrocatalytic activity for the ORR in alkaline medium compared to that of the pure MoSe₂, rGO, and their simple physical mixture, which could benefit from the excellent oxygen adsorption ability of the abundantly exposed active edge sites of the ultrathin MoSe₂ layers, the conductivity and aggregation-limiting effect of the rGO platform, as well as the unique 3D rosebud-like architecture of the hybrid material. The electrocatalytic activity of the MoSe₂@rGO hybrid towards ORR was comparable to that of commercial Pt/C catalysts. And the promoted reaction was revealed to involve a nearly four-electron-dominated ORR process by analysis of the obtained Koutecky–Levich plots. The scanning electrochemical microscopy (SECM) technique, with the advantages of investigating of the local catalytic activity of samples with high spatial resolution and simultaneously evaluating activities of different catalysts in a single experiment, was further applied to investigate the local ORR electrocatalytic activity of MoSe₂@rGO and compare it with those of other catalyst samples through applying different sample potentials. The excellent stability and methanol tolerance of the 3D nanostructured MoSe₂@rGO hybrid against methanol further prove the 3D nanostructured MoSe₂@rGO hybrid as a promising ORR electrocatalyst in alkaline solution for potential applications in fuel cells and metal–air batteries.

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Synthesis and structure of multi-layered WS<Subscript>2</Subscript>(CoS), MoS<Subscript>2</Subscript>(Mo) nanocapsules and single-layered WS<Subscript>2</Subscript>(W) nanoparticles

Multi-layered MoS₂ (or WS₂) nanocages stuffed with Mo (or CoS/CoO) nanocrystals have been synthesized by using the reaction between metal nanoparticles and sulfur powders. This simple synthesis method, different from the conventional methods for synthesizing pure inorganic fullerenes, is also potentially important for large-scale synthesis of nanoparticles of other metal dichalcogenide. Besides the multi-layered WS₂ nanocapsules, we have successfully fabricated nanocapsules with a single-layered WS₂ sheet encapsulating W by using the arc-discharge method. We discuss possible mechanisms for the formation of the unique core-shell structured nanocapsules.     

Thermodynamic properties and homogeneity regions of Tl<Subscript>6</Subscript>SCl<Subscript>4</Subscript> and Tl<Subscript>5</Subscript>Se<Subscript>2</Subscript>Cl

In this activity system Tl-Tl₂X-X (X = S, Se)are studied using emf measurements of concentration chains relative thallic electrode. The solid phase diagrams of these systems are clarified, homogeneity areas of the compounds Tl₆SCl₄ and Tl₅Se₂Cl are determined. On the basis of emf measurement results, relative partial molar functions of thallium in alloys and standard integral thermodynamic functions (ΔG ⁰(298 K), ΔH ⁰ (298 K), ΔS ⁰ (298 K)) of the ternary compounds Tl₆SCl₄ and Tl₅Se₂ Cl and phases of variable composition based on the latter are calculated.     

Crystal Growth and Magneto-transport of Bi<Subscript>2</Subscript>Se<Subscript>3</Subscript> Single Crystals

In this letter, we report on the growth and characterization of bulk Bi ₂Se ₃ single crystals. The studied Bi ₂Se ₃ crystals are grown by the self-flux method through the solid-state reaction from high-temperature (950 °C) melt of constituent elements and slow cooling (2 ℃/h). The resultant crystals are shiny and grown in the [00l] direction, as evidenced from surface XRD. Detailed Reitveld analysis of powder X-ray diffraction (PXRD) of the crystals showed that these are crystallized in the rhombohedral crystal structure with a space group of R3m (D5), and the lattice parameters are a = 4.14 (2), b = 4.14 (2), and c = 28.7010 (7) Å. Temperature versus resistivity (ρ?T) plots revealed metallic conduction down to 2 K, with typical room temperature resistivity (ρ _{300 K}) of around 0.53 m Ω-cm and residual resistivity (ρ _{0 K}) of 0.12 m Ω-cm. Resistivity under magnetic field [ ρ(T)H] measurements exhibited large + ve magneto-resistance right from 2 to 200 K. Isothermal magneto-resistance [ ρH] measurements at 2, 100, and 200 K exhibited magneto-resistance (MR) of up to 240 %, 130 %, and 60 %, respectively, at 14 T. Further, the MR plots are nonsaturating and linear with the field at all temperatures. At 2 K, the MR plots showed clear quantum oscillations at above say 10 T applied field. Also, the Kohler plots, i.e., Δρ/ ρ _oversus B/ ρ, were seen consolidating on one plot. Interestingly, the studied Bi ₂Se ₃ single crystal exhibited the Shubnikov-de Haas (SdH) oscillations at 2 K under different applied magnetic fields ranging from 4 to 14 T.     

Synthesis,characterization and formation mechanism of monodispersed Gd<Subscript>2</Subscript>O<Subscript>2</Subscript>S:Eu<Superscript>3+</Superscript> nanocrystals

Monodispersed Gd₂O₂S:Eu³⁺ nanostructures with tunable morphologies have been selectively fabricated by solvothermal method in the presence of stable inorganic precursors avoiding metalorganic precursors. The size and morphology of the products were controlled successfully by adjusting the reaction conditions. The products were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), selected-area electron diffraction (SAED) and X-ray photoelectron spectroscopy (XPS). The corresponding UV absorption and photoluminescence excitation spectra show a significant blue-shift confirming the quantum confinement effect. A possible growth mechanism for the formation of monodispersed Gd₂O₂S:Eu³⁺ nanocrystals has been proposed. The luminescence mechanism and the size dependence of their fluorescence properties are also discussed.     

Improved photocatalytic degradation of organic dye using Ag<Subscript>3</Subscript>PO<Subscript>4</Subscript>/MoS<Subscript>2</Subscript> nanocomposite

Highly efficient Ag₃PO₄/MoS₂ nanocomposite photocatalyst was synthesized using a wet chemical route with a low weight percentage of highly exfoliated MoS₂ (0.1 wt.%) and monodispersed Ag₃PO₄ nanoparticles (~5.4 nm). The structural and optical properties of the nanocomposite were studied using various characterization techniques, such as XRD, TEM, Raman and absorption spectroscopy. The composite exhibits markedly enhanced photocatalytic activity with a low lamp power (60 W). Using this composite, a high kinetic rate constant (k) value of 0.244 min^-1 was found. It was observed that ~97.6% of dye degrade over the surface of nanocomposite catalyst within 15 min of illumination. The improved photocatalytic activity of Ag₃PO₄/MoS₂ nanocomposite is attributed to the efficient interfacial charge separation, which was supported by the PL results. Large surface area of MoS₂ nanosheets incorporated with well dispersed Ag₃PO₄ nanoparticles further increases charge separation, contributing to enhanced degradation efficiency. A possible mechanism for charge separation is also discussed.