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Preparation and characterization of molybdenum disulphide catalysts
Abstract
Molybdenum disulphide has been prepared by atmospheric-pressure hydrogen reduction of molybdenum trisulphide in the temperature range 400–750 C for 4 h and characterized. Chemical analysis of the product confirmed the S to Mo ratio 2.00 as desired for MoS2, consistent with its thermogravimetric analysis data in an air atmosphere. MoS2 prepared under these reducing conditions possessed a hexagonal structure with n-type diamagnetic semiconducting behaviour and a lower surface area. The catalytic activity of the MoS2 was studied for liquid-phase hydrogenation of nitrobenzene. X-ray studies on MoS3 when reduced at 750 C for 48 h indicated that MoS2 so formed is unstable, resulting in the formation of its over-reduced products Mo2S3 and Mo.
... Molybdenum disulfide (MoS 2 ) is one of the typical transition metal dichalcogenides and has been widely used as a catalyst [1] and hydrogen storage material [2,3]. Owing to the strong in-plane interactions and weak van der Waals interactions between MoS 2 atomic layers [4,5], MoS 2 crystals have been known as an important solid lubricant for many years [6,7]. ...
... eV and the variation of the expression |M vc | 2 /ω 2 is found to be small in this range. According to Eqs. (1) and (5), the imaginary part of the dielectric function ε xx 2 ðωÞ is mainly decided by the JDOS and the transition matrix elements, this gives a similar plateau for the imaginary part of dielectric function ε xx 2 ðωÞ as compared to JDOS. ...
Abstract In this paper, the structural, electronic, and optical properties of MoS2 multilayers are investigated by employing the first-principles method. Up to six-layers of MoS2 have been comparatively studied. The covalency and ionicity in the MoS2 monolayer are shown to be stronger than those in the bulk. As the layer number is increased to two or above two, band splitting is significant due to the interlayer coupling. We found that long plateaus emerged in the imaginary parts of the dielectric function ε2xxω and the joint density of states (JDOS) of MoS2 multilayers, due to the Van Hove singularities in a two-dimensional material. One, two and three small steps appear at the thresholds of both the long plateau of ε2xxω and JDOS, for monolayer, bilayer, and trilayer, respectively. As the number of layers further increased, the number of small steps increases and the width of the small steps decreases accordingly. Due to interlayer coupling, the longest plateau and shortest plateau of JDOS are from the monolayer and bulk, respectively.
... Finally,t richalcogenides MQ 3 (M = transition metal, Q = S, Se) constitute ad iversef amily of compounds. [14] Generally,t hey are composed of trigonal prisms MX 6 stacked as to form MQ 3 trigonal prismatic chains, which extend in the direction of b-axis of the monoclinic cell (Figure 2d). Inside the MQ 6 prisms and between the neighbor- ing prisms, strong covalent bonds exist, thus, connecting the chains into layers, and the successive layers are bondedb y van der Waalsf orces. ...
... Anisotropic structures of layer-and chain-type TMPSs govern their anisotropic morphologies, and their bulk exists in the form of thin fibers, which may be exfoliated into nanosized ribbonso rr ods, similarly to nanosheetso ft ransitionm etal di- chalcogenides, [16] or intercalated by lithium. [14,17] The electronic structure of S 2À 2 groups is such that they can either donate or accept electrons. Their capability as donors is owed to the fully filled p* g orbital, which can release electrons, thus, converting S 2À 2 into S 2 À ( Figure 4). ...
The Cover Feature shows a representation of a future electric vehicle powered by Li-ion batteries with transition-metal polysulfide electrodes driven on a road. Polysulfide electrodes have an unusual operation mechanism featuring anionic redox processes in disulfide groups (S−S)²⁻. The main redox reaction is schematically drawn on the hand wheel. As an example, a fragment of the VS4 structure is displayed on the control panel. The yellow balls are S atoms connected in pairs and the blue balls are V atoms. The mountains at the backdrop are “filled” with fragments of TaS3 and NbS3 microstructures symbolizing the vast opportunities that the rich polysulfide family offers for energy conversion and storage. More information can be found in the Concept by Grayfer et al.
... Finally,t richalcogenides MQ 3 (M = transition metal, Q = S, Se) constitute ad iversef amily of compounds. [14] Generally,t hey are composed of trigonal prisms MX 6 stacked as to form MQ 3 trigonal prismatic chains, which extend in the direction of b-axis of the monoclinic cell ( Figure 2d). Inside the MQ 6 prisms and between the neighboring prisms, strong covalent bonds exist, thus, connecting the chains into layers, and the successive layers are bondedb y van der Waalsf orces. ...
... Anisotropic structures of layer-and chain-type TMPSs govern their anisotropic morphologies, and their bulk exists in the form of thin fibers, which may be exfoliated into nanosized ribbonso rr ods, similarly to nanosheetso ft ransitionm etal dichalcogenides, [16] or intercalated by lithium. [14,17] The electronic structure of S 2À 2 groups is such that they can either donate or accept electrons. Their capability as donors is owed to the fully filled p* g orbital, which can release electrons, thus, converting S 2À 2 into S 2 À (Figure 4). ...
Classical Li-ion battery technology is based on the insertion of lithium ions into cathode materials involving metal (cationic) redox reactions. However, this vision is now being reconsidered, as many new-generation electrode materials with enhanced reversible capacities operate through combined cationic and anionic (non-metal) reversible redox processes or even exclusively through anionic redox transformations. Anionic participation in the redox reactions is observed in materials with more pronounced covalency, which is less typical for oxides, but quite common for phosphides or chalcogenides. In this Concept, we would like to draw the reader's attention to this new idea, especially, as it applies to transition-metal polychalcogenides, such as FeS2, VS4, TiS3, NbS3, TiS4, MoS3, etc., in which the key role is played by the (S−S)2−/2 S2− redox reaction. The exploration and better understanding of the anion-driven chemistry is important for designing advanced materials for battery and other energy-related applications.
... Commercial source of molybdenum metal and molybdenum compounds is MoS 2 . Molybdenum disulphide, in addition to its natural occurrence, can be prepared synthetically by several routes including direct union of elements in pure nitrogen [1], thermal decomposition of ammonium tetrathiomolybdate or molybdenum trisulphide [2][3][4][5][6][7], and by reaction between MoO 3 and H 2 S or H 2 S/H 2 mixtures [8]. Such preparative techniques result in hexagonal crystalline MoS 2 , by far the most common form, the rhombohedral form is found in nature, and may be synthesized [6]. ...
... It is possible to consider that reaction (1) is made up the following two partial reactions (2) and (3): ...
In order to achieve direct reduction of molybdenite in presence of a sulphur scavenger such as CaO such that SO2 emission is completely avoided, it is important to maximise the rate of the partial reaction involving molybdenite and hydrogen (without lime) given the low thermodynamic driving force for this reaction. Accordingly, reaction of molybdenum disulphide powders with hydrogen was investigated by thermogravimetric method. Effect of temperature and concentration on the reaction rate was studied under such conditions that resistance to mass transfer arising from external film, between particles and intra-grain was negligible. The operating temperature ranged between 973 and 1173K while the hydrogen concentration was varied between 30 and 100%. The experimental data obtained under the above conditions were analyzed by applying “the shrinking unreacted core model”. The reduction reaction was found to be first order with respect to the gaseous reactant. Pre-exponential factor and activation energy have been determined to be 3.91×103cmmin−1 and 139.0kJmol−1, respectively. Activation energy obtained from a fitted model, agreed well with the values determined from the model-free methods using isothermal measurements.
... It may be noted that Mo 2 S 3 is stable above 630 C [22] and below this temperature; Mo 2 S 3 decomposes to MoS 2 and Mo. The MoS 2 nanoparticles find applications in catalysis [22][23][24] and as a solid lubricant [25], while the Mo 2 S 3 has been widely used as a cathode material in lithium-ion based batteries [26] because of its metallic conductivity [27]. The existence of Mo 2 S 3 as the sulphide phase in equilibrium, at the temperature of 850e1200 C established first by McCabe [28] and later by Stubbles and Richardson [29]. ...
... The value of the electrical conductivity is in a range of 0.01 to 10 S/m at room temperature. The value of thermal conductivity is in a range of 0.1 to 1 W/(m*K) [5][6][7]. ...
... The O-MoS 2 can efficiently catalyse nitroarenes reduction to the corresponding arylamines with N 2 H 4 under air condition, and the used catalyst can be easily recovered and reused four times without any significant loss of activity. Different from previous MoS 2 -based catalysts with MoS 2 skeleton as the active center for the nitroarenes reduction [45][46][47][48][49][50], results of this work propose that the Mo IV O x structure implanted in O-MoS 2 works as the active center. The O-MoS 2 can catalyse the N 2 H 4 selective decomposition to the active hydrogen species in polar electronic states (H ␦− and H ␦+ ), which show high chemoselectivity toward the nitro reduction. ...
We present an efficient approach for chemoselective synthesis of various functionalized arylamines from nitroarenes over an oxygen-implanted MoS2 catalyst (O-MoS2). The HRTEM, XRD, XPS, Raman, EXAFS and NH3-TPD characterizations show the existence of MoIVOx structure and abundant coordinative unsaturated (CUS) Mo sites in the 2D-layer structure of O-MoS2. In the transfer hydrogenation of nitroarenes with hydrazine hydrate, the MoIVOx structure works as the catalytic active center. The N2H4 selectively decomposes to the active hydrogen species in polar electronic states (Hδ− and Hδ+), which show high chemoselectivity toward the nitro reduction over C[double bond; length as m-dash]C, C[double bond; length as m-dash]C, and C[triple bond; length as m-dash]N groups. The O-MoS2 catalyst can be recovered in a facile manner from the reaction mixture and recycled four times without any significant loss of activity.
... MoS 2 natural crystal is n-type, with ∼ 0.2 eV activation energy [46] over a 1.23 eV indirect band-gap [11]. To the best of our knowledge, in the case of bulk crystal, the origin of the n-type character is not discussed in the literature, even if it is most likely related to the presence of native defects [47], in 1990s identified as S vacancies [48]. ...
... [5,6] MoS 2 , with a typical layered structure, has strong covalent bonds within the layers, which is very similar to graphite. [7][8][9] The spacing between neighboring layers is 0.615 nm, significantly larger than that of graphite (0.335 nm), and the weak van der Waals forces between the layers allows Li ions to diffuse without a significant increase in volume. Because of these properties, MoS 2 is more suitable for Li-ion insertion than graphite. ...
Charged up: Three-dimensional architectures constructed from graphene/MoS2 nanoflake arrays have been successfully fabricated by a one-step hydrothermal method. MoS2 nanoflakes with thicknesses less than 13 nm grow vertically on both sides of graphene sheets, which allows the architectures to be more stable during charging and discharging. Even at a high current density of 8000 mA g(-1) , their discharge capacity is still up to 516 mA h g(-1) .
... In the Mo-S system there are two stable sulfide phases: MoS 2 and Mo 2 S 3 , both possessing a layered structure (1). MoS 2 materials may have applications in catalysis (2,3) and as a lubricant (4), while the Mo 2 S 3 , owing to metallic conductivity (5), has perspective as a cathode material for lithium batteries (6). According to the Mo-S phase diagram, MoS 2 is stable until ,2000 K, and Mo 2 S 3 formation is possible from 937 to 2000 K (7). ...
Molybdenum(VI) sulfide deposited on the surface of thermally exfoliated graphite has been reduced at 1273 K in dynamic vacuum conditions. High resolution transmission electron microscopy showed formation of layered particles with an average diameter of 20 nm which, using the Fourier analysis of images, were attributed to Mo3S4 phase. Graphite surface more likely promotes the transformation of MoS3 to Mo3S4 because the unsupported particles occurred in the sample have the MoS2 structure.
... Molybdenum disulphide includes high density batteries because of its appreciable electrical conductivity and ability to reversibly intercalate lithium [7,8]. It appears to be a very promising semiconducting material for various applications such as hydrosulphurization catalysts, solid lubricants for tribological applications in high temperature and vacuum environment where the use of liquid lubricants becomes ineffective [9,10]. The spin coating, metal organic chemical vapour deposition, chemical vapour deposition, sputtering technique, sulphurization of metal, laser evaporation, pulsed electrodeposition, and microwave synthesis are some of the methods used for the deposition of Molybdenum disulphide thin films [11][12][13][14][15][16][17][18]. ...
Molybdenum disulphide thin films were deposited using chemical bath deposition method on non-conducting glass substrate using tartaric acid as a complexing agent at 363 K. The films were characterized by X-ray diffraction, scanning electron microscopy, optical absorption and electrical measurements. X-ray diffraction pattern shows that polycrystalline with hexagonal structure. The direct optical band gap was found to be 1.8 eV. Electrical measurement suggests that specific conductance was found in the order of 10−5 to 10−2 (Ω cm)−1 and showing n-type conduction mechanism.
Developing more sustainable catalytic processes for preparing N-heterocyclic compounds in a less costly, compact, and greener manner from cheap and readily available reagents is highly desirable in modern synthetic chemistry. Herein, we report a straightforward synthesis of benzimidazoles by reductive coupling of o-dinitroarenes with aldehydes in the presence of molecular hydrogen. An innovative molecular cluster-based synthetic strategy that employs Mo3S4 complexes as precursors have been used to engineer a sulfur-deficient molybdenum disulfide (MoS2)-type material displaying structural defects on both the naturally occurring edge positions and along the typically inactive basal planes. By applying this catalyst, a broad range of functionalized 2-substituted benzimidazoles, including bioactive compounds, can be selectively synthesized by such a direct hydrogenative coupling protocol even in the presence of hydrogenation-sensitive functional groups, such as double and triple carbon-carbon bonds, nitrile and ester groups, and halogens as well as diverse types of heteroarenes.
Inorganic two-dimensional materials are gradually becoming resources for modern electronic device manufacturing. Fascinate of 2D transition metal dicholgonides (TMDs) are particularly high. TMDS are very great potential for their characteristics and band gap structure in optoelectronic devices. In TMDs, 2D MoS2 is most researchable material due to its good performance and its adequacy of electronic and optoelectronic application. Here we demonstrate MoS2 thin film for various temperature such as 400, 450, 500, 550°C via Jet Nebulizer Spray Pyrolysis (JNSP) technique for PN diode application. XRD pattern revealed that the polycrystalline nature of MoS2 films with hexagonal crystal structure. The elongated irregular rod-like structures were revealed through FESEM. Elemental confirmation studies of Mo and S were done through EDX. The MoS2 films deposited at 550°C exhibit minimum band gap. The average conductivity values were found to be increased from 1.730×10⁻⁸ to 3.877×10⁻⁷ S/cm with substrate temperature. A positive photo conducting-nature of p-Si/n-MoS2 diode have been fabricated. Remarkably, the p-Si/n-MoS2 diode fabricated at 550 °C revealed minimum n values of 2.23.
2D Nanolayered molybdenum disulfide cobalt promoted materials (Co-Mo-S) have been stablished as chemoselective catalysts for the hydrogenation of nitroarenes under relatively mild conditions. Co-Mo-S catalysts have been prepared by a one-pot hydrothermal synthesis that allows for obtaining unsupported catalysts with a high number of active sites per unit volume. Applying these catalysts, the hydrogenation of the nitro functionality has been carried out selectively in the presence of double and triple bonds, aldehydes, ketones as well as carboxylic acid derivatives groups, thus affording the corresponding anilines in good to excellent yields. Interestingly, the partial hydrogenation of some dinitroarenes has also been successfully accomplished. In addition, its catalytic performance has been evaluated for the preparation of the bio-active compound paracetamol through a one-pot direct hydrogenative amidation reaction.
Herein, we describe an atom efficient and general protocol for the chemoselective hydrogenation of nitroarenes to anilines catalyzed by well-defined diimino and diamino cubane-type Mo3S4 clusters. The novel diimino [Mo3S4Cl3(dnbpy)3]+ ([5]+) (dnbpy= 4,4'-dinonyl-2,2'-dipyridyl, L1) trinuclear complex was synthesized in high yields by simple ligand substitution reactions starting from the thiourea (tu) [Mo3S4(tu)8(H2O)]Cl4·4H2O (3) precursor. This strategy has also been successfully adapted for the isolation of the diamino [Mo3S4Cl3(dmen)3](BF4) ([6](BF4)), (dmen= N,N'-dimethylethylenediamine) salt. Applying these catalysts, high selectivity in the hydrogenation of functionalized nitroarenes has been accomplished. Over thirty anilines bearing synthetically functional groups have been synthesized in 70 to 99% yield. Notably, the integrity of the cluster core is preserved during catalysis. Based on kinetic studies on the hydrogenation of nitrobenzene and other potential reaction intermediates, the direct reduction to aniline is the preferential route.
Molybdenum disulfide (MoS2) thin films were grown directly on SiO2 covered wafers by atomic layer deposition (ALD) at the deposition temperatures ranging from 175 to 225 °C using molybdenum hexacarbonyl [Mo(CO)6] and H2S plasma as the precursor and reactant, respectively. Self-limited film growth on the thermally-grown SiO2 substrate was observed with both the precursor and reactant pulsing time. The growth rate was ∼0.05 nm/cycle and a short incubation cycle of around 13 was observed at a deposition temperature of 175 °C. The MoS2 films formed nanocrystalline microstructure with a hexagonal crystal system (2H-MoS2), which was confirmed by X-ray diffraction and transmission electron microscopy. Single crystal MoS2 nanosheets, ∼20 nm in size, were fabricated by controlling the number of ALD cycles. The ALD-MoS2 thin films exhibited good stoichiometry with negligible C impurities, approximately 0.1 at.% from Rutherford backscattering spectrometry (RBS). X-ray photoelectron spectroscopy confirmed the formation of chemical bonding from MoS2. The step coverage of ALD-MoS2 was approximately 75% at a 100 nm sized trench. Overall, the ALD-MoS2 process made uniform deposition possible on the wafer-scale (4 in. in diameter).
The catalytically inactive components of a film have been converted, through an operando method of synthesis, to produce a catalyst for the reaction that the film is catalyzing. Specifically, thin films of molybdenum diselenide have been synthesized using a two-step wet-chemical method, in which excess sodium selenide was first added to a solution of ammonium heptamolydbate in aqueous sulfuric acid, resulting in the spontaneous formation of a black precipitate that contained molybdenum triselenide (MoSe3), molybdenum trioxide (MoO3), and elemental selenium. After purification and after the film had been drop cast onto a glassy carbon electrode, a reductive potential was applied to the precipitate-coated electrode. Hydrogen evolution occurred within the range of potentials applied to the electrode, but during the initial voltammetric cycle, an overpotential of ∼400 mV was required to drive the hydrogen-evolution reaction at a benchmark current density of −10 mA cm–2. The overpotential required to evolve hydrogen at the benchmark rate progressively decreased with subsequent voltammetry cycles, until a steady state was reached at which only ∼250 mV of overpotential was required to pass −10 mA cm–2 of current density. During the electrocatalysis, the catalytically inactive components in the as-prepared film were (reductively) converted to MoSe2 through an operando method of synthesis of the hydrogen-evolution catalyst. The initial film prepared from the precipitate was smooth, but the converted film was completely covered with pores ∼200 nm in diameter. The porous MoSe2 film was stable while being assessed by cyclic voltammetry for 48 h, and the overpotential required to sustain 10 mA cm–2 of hydrogen evolution increased by <50 mV over this period of operation.Keywords: hydrogen-evolution reaction; synthesis of molybdenum diselenide; wet-chemical synthesis of layered electrocatalysts; mesoporous catalysts; synthesis of group VI dichalcogenides
The Powder Mixture of Thermally Exfoliated Graphite (TEG) and Amorphous Molybdenum (VI) Sulfide MoS3 Has Been Annealed in Vacuum at 723, 1073, and 1273 K. The Obtained Samples Were Examined Using Transmission Electron Microscopy (TEM), X-Ray Diffraction (XRD), and Raman Spectroscopy. XRD and Raman Spectroscopy Detected that the Main Product of MoS3 Annealing Is MoS2 Nanoparticles. TEM Images Showed the Formation of a Continuous Coating on the Surface of TEG Platelets at 1273 K and Fourier Analysis of the High Resolution TEM Image Revealed that this Coating Corresponds to the Mo2S3 Layers.
Thin films of MoSx
have been prepared on silicon substrates by spin coating and thermolysis of 0.5 M solutions of alkyldiammonium tetrathiomolybdates in 1,2-ethanediamine (EDA) and 1,2-propanediamine (12PDA). The films have been heat treated in air at temperatures between 80 and 250°C and under N2 atmosphere at temperatures between 300 and 800°C. X-ray diffraction shows a restricted crystallisation and amorphous residues in both kind of films. EDA-based films exhibit a high tendency to crystallise whereas 12PDA-based films form associated structures with the solvent preventing precursor crystallisation. An insight into the processes occurring in film formation is gained by infrared spectroscopy which indicates a beginning of the decomposition of the 12PDA-based film at temperatures as low as 80°C with incorporation of the diamine solvent. In contrast, the EDA-based films show first signs of a decomposition at 150°C. The decomposition of the intermediate MoS3 in both cases starts between 250 and 300°C. By means of SNMS depth profiles carbon contents up to 21 and 32 atom-% were found in EDA- and 12PDA-based films, respectively. The films show a significant deficit of sulphur which is compensated by the carbon. Near the surface of the coatings a loss of carbon is observed.
The sulphides and selenides of Mo and Ware layered compounds that possess a hexagonal strueture. The electronic structure of these layered transition metal dichalcogenide compounds, e.g. MoS2, WS2, MoSe2, WSe2, etc. [1-6], is of fundamental interest primarily because of the unusual crystal structure of these compounds. Within a layer the bonds are strong; while between adjacent layers, they are weak. As a result, the compounds exhibit marked anisotropy in most of their physical properties, which accounts for the great interest in this family of materials. The layer type structure also facilitates the process of intercalation by foreign metal atoms and organic and inorganic molecules, thereby allowing a convenient method of changing the strueture and electronic properties of the host crystals [7]. These compounds can form a wide fange of solid solutions, with either mixed metals or mixed chalcogenides [8, 9]. The main applications of these types of compounds are in hydrogen storage devices [7], in the field of catalysis [10], high temperature high pressure lubricants [1], long-life photochemical solar cells [11], etc. We have prepared a series of mixed metal-chalcogens solid solutions based on the Mo-W-S-Se quaternary system, Mo0.sW0.sSxSe2-x (0 ~< x ~< 2); whose characterization, thermal and magnetic behaviour have been reported earlier [12]. The aim of the present smdy is to determine some microstructural parameter, like particle size, r.m.s, strain, etc.; and, owing to the strong intralayer and weak interlayer forces, layer disorders are expected. We have thus calculated the variability of interlayer spacing, g, the fraction of planes affected by such defects, 7, and the dislocation density, p, etc. The estimations have been made using the method of variance, Fourier and integral width of the X-ray diffraction (XRD) line profiles of the (0 0 1) reflections. The compounds were synthesized directly from the elements, and the method of preparation is quite similar to that reported earlier [13]. The sample was then ground at room temperature (32 °C) by hand without any appreciable change in temperature and were passed through a 150 mesh sieve. The XRD intensities of the compounds were taken on a Philips 1729 diffractometer using CuKa radiation. Assuming that the broadening of the X-ray diffraction (XRD) line profile is due to presence of size broadening, strain broadening and broadening
Molybdenum disulphide, MoS2, thin films have been deposited by chemical bath deposition method on glass and quartz substrate using ammonium tetrathiomolybdate as a single source precursor for Mo and S and subjected to vacuum heat treatment at different temperatures. X-ray diffraction of as-deposited film indicated its amorphous character and showed the development of poorly crystalline MoS2 thin film on increasing annealing temperature. The film has been characterized by energy dispersive X-ray analysis, X-ray photoelectron spectroscopy, scanning electron micrograph and the optical properties also have been studied.
Thin films of MoS2 and amorphous MoS3 have been prepared on various glass and stainless steel substrates by thermolysis of spin cast solutions of (NH4)2MoS4 in an organic diamine (ethylenediamine and 1,2-diaminopropane). After deposition the resulting single-source precursor film [(H3NCnH2nNH3)MoS4] was dried at 250°C in air and subsequently heat treated at temperatures between 300 and 800°C under inert nitrogen (N2) atmosphere yielding films of MoSx (2≤x≤3) with carbon impurities. The resulting films with thicknesses up to 160 nm (800°C) and 250 nm (300°C) are highly homogeneous with a metallic lustre and appear brown in transmission. While the films are amorphous to X-ray diffraction up to temperatures of 400°C, the crystallinity increases for higher temperatures due to the formation of MoS2 microcrystallites. Furthermore, the composition and the microstructure strongly depend on the used diamine solvent. Both X-ray and electron diffraction reveal a partially preferred orientation of the crystallites with their basal planes parallel to the substrate (⊥c, type II). After heat treatment at 800°C under N2 the MoS2 layer stacks consist of three to four layers with a length of 5–8 nm. The optical characterisation of the thin films shows a strong absorption in the visible part of the spectrum characteristic of MoS2 with an absorption coefficient α550 of 2×105 cm−1. Tribological measurements up to 5000 cycles against steel reveal the highest wear-life with a friction coefficient μ
Selenium substituted indium intercalation compounds of tungsten disulphide, In1/3WS2−xSex (0≤x≤2) have been studied for microstructural characterization using X-ray line profile analysis to find out information about crystallite size, r.m.s. strain, dislocation density, variability of interlayer spacing, fraction of planes affected by such defects, stacking fault probability, crystallite size anisotropy etc. Scanning electron microscopic (SEM) and scanning tunneling microscopic (STM) studies are also reported herewith. These results have also been compared with respect to the pure WS2−xSex (0≤x≤2).
The present paper deals with the preparation of indium intercalated compounds of tungsten sulphoselenide, In1/3WSxSe2−x (0 ≤x≤ 2) and their characterization by various physicochemical methods. The X-ray studies confirmed that all these compounds possess hexagonal symmetry and there exist no anisotropy towards the crystallite size. Room temperature magnetic susceptibility, thermoelectric power experiments and electrical conductivity measurements in the temperature range 25–350°C indicated that In1/3WS2 and In1/3WSxSe2 − x (0 ≤x≤ 2) compounds exhibit p- and n-type diamagnetic semiconducting behaviour respectively and results are explained on the basis of existing band model. Thermal stability of these compounds in air and nitrogen has also been studied and formation of tungsten bronze based on X-ray data of oxidized products has been proposed.
The present work deals with preparation and studies on structure determination, layer disorder, microstructural parameter and a few other properties of tantalum substituted W0.65Mo0.35−xTaxSe2, viz. W0.65Mo0.35−xTaxSe2 (0≤x≤0.35). X-ray analysis shows that all these compounds possess MoS2 type structure. The particle size, r.m.s. strain, dislocation density, variability of interlayer spacing and fraction of planes affected by such defects have also been calculated for all compounds. It has been observed that W0.65Mo0.20Ta0.15Se2 compounds possess small size and high values for other parameters. Room temperature magnetic susceptibility, thermoelectric power experiments and two probe electrical conductivity measurements confirmed that all these compounds are diamagnetic p-type metals except the parent compound W0.65Mo0.35Se2, which is a p-type semiconductor. Thermal stability behavior of these compounds in air atmosphere has also been studied and based on weight loss in TG and X-ray data of the oxidized products, formation of the respective oxidized products has been proposed. Scanning electron microscopy studies of these compounds have also been made.
The wide angle X-ray diffraction on W0·65Mo5−x
Tax
Se2 (0≤x≤0·35) compounds have been used for calculation of the crystallite size by method of variance and Fourier technique and for microstructural parameters purposes. The crystallite size showed the decreasing trend with increasing composition up tox=0·15 and after that it increased sharply. On the contrary, mean fractional change in interlayer spacing, fractions of the planes affected by defects, dislocation density, and root mean square strain showed the opposite trend. The crystallite size anisotropy and stacking fault probability showed 1:1 correspondence in their variation with composition. The radial distribution analysis have also been employed to find out the information about the inter atomic distances, coupling constants and mean square displacements for different pairs of atoms. These results have been interpreted in terms of difference in packing of layers in these compounds.
The present paper deals with the preparation of tungsten-substituted molybdenum disulfide, Mo1−xWxS2 (0≤x≤1) compounds and their characterization by various physicochemical methods. X-ray studies confirmed that these compounds crystallized in a layer type of hexagonal structure. X-ray line profile analysis techniques such as the method of variance and Fourier analysis have been used for microstructural characterization of these compounds to find out information about particle size, r.m.s. strain, variability of interlayer spacing, fraction of planes affected by such defects, dislocation density, stacking fault probability and radial distribution function analysis for calculating coupling constants and mean-square displacements, etc. Room-temperature magnetic susceptibility measurements and thermoelectric power experiments and temperature variation of conductivity (25–350°C) confirmed, respectively, diamagnetic n- and p-type semiconducting behaviour for Mo1−xWxS2. Thermal stability behaviour of these compounds in air and inert atmosphere has also been carried out up to 1000°C. These studies indicated that the compounds are nearly stable up to 1000°C in inert atmosphere, whereas, in air, oxidative degradation of these compounds started at 400°C. The nature of the oxidized products has also been studied by X-ray analysis. Scanning electron micrograph studies are also reported here.
It is known that the edges of a two-dimensional slab of insulating MoS2 exhibit one-dimensional metallic edge states, the so-called “brim states.” Here, we find from density-functional theory calculations that several edge structures, which are relevant for the hydrodesulfurization process, are magnetic. The magnetism is an edge phenomenon associated with certain metallic edge states. Interestingly, we find that among the two low-index edges, only the S edge displays magnetism under hydrodesulfurization conditions. In addition, the implications of this on the catalytic activity are investigated. Despite large changes in the magnetic moments, a small influence on the adsorption energies is observed. This has implications on the suitability of magnetic measurements for monitoring the catalytic properties.
Microstructural characterization of indium intercalation compounds of molybdenum sulphoselenide, (In1/3MoS2-xSex (0≤x≤2) compounds, was carried out by X-ray line profile analysis techniques. The compounds were also subjected to scanning electron microscopic and scanning tunneling microscopic studies. The surface features as recorded by STM revealed the formation of island type morphology of MoS2 measuring around 300 angstrom. In general as evidenced by the decrease of roughness upon intercalation, indium intercalated compounds show potential as better lubricant.
The transition metal dichalcogenides of groups IVB, VB, and VIB exhibit considerable diversity in their physical properties, spanning the fields of metals, semiconductors, insulators, and superconductors [1]. An important feature of these materials is that they crystallize in a quasi two-dimensional type of layered structure which imparts substantial anisotrophy to many of their properties. In addition, the layered structure facilitates the process of intercalation, or insertion of foreign atoms and molecules into these process of intercalation, or insertion of foreign atoms and molecules into these materials, thus allowing a convenient method for altering the structure and electronic behavior.
The transition metal dichalcogenides are about 60 in number. Two-thirds of these assume layer structures. Crystals of such materials can be cleaved down to less than 1000 Å and are then transparent in the region of direct band-to-band transitions. The transmission spectra of the family have been correlated group by group with the wide range of electrical and structural data available to yield useful working band models that are in accord with a molecular orbital approach. Several special topics have arisen; these include exciton screening, d-band formation, and the metal/insulator transition; also magnetism and superconductivity in such compounds. High pressure work seems to offer the possibility for testing the recent theory of excitonic insulators.
The present work reports a detailed investigation on crytal structure, electrical properties, and thermal stability for indium intercalation compounds of molybdenum disulphide. InxMoS2 (0⩽x⩽1). X-ray analyses show that intercalated compounds, like the host material 2HMoS2, also possess hexagonal symmetry with a small but continuous increase in interlayer spacing; there also exists a particle size anisotropy. Room temperature thermoelectric power experiments and low temperature (150 – 300 K) conductivity measurements indicate that intercalated compounds also exhibit n-type semiconducting behaviour similar to that of the base material MoS2. Thermal stability behaviour of these compounds in air and argon atmosphere has also been studied.
Polycrystalline molybdenum disulphide has been grown by high pressure (about 54.88 × 106 N m−2) reduction of molybdenum trisulphide and characterized by various physicochemical methods. The chemical analysis of the compound established a sulphur to molybdenum ratio of 2, which was also confirmed by thermogravimetric analysis in air. The X-ray data was found to be consistent with 2H-MoS2, n-type conductivity (S = −275 μV °C−1) and diamagnetic behaviour ( c.g.s. e.m.u.). It is a semiconducting material with a room temperature conductivity of of and an activation energy of 0.124 eV in the temperature range 150–300 K. A band gap of 0.95 ± 0.02 eV was found from optical absorption studies on a thin MoS2 film. Thermal studies of MoS2 in an argon atmosphere has shown its stability up to 1200 °C and thereafter it decomposes to Mo2S3. A model has been proposed which describes the thermal behaviour of MoS2 when heated at various temperatures up to 1200 °C and is based on the appearance of lines in X-ray powder diffractogram of the product resulting from the crystallization of MoS2. Scanning electron microscope studies have also been made.
Holes, photogenerated in d-energy bands of covalent semiconducting layer-type group VI-transition metal dichalcogenides react electrochemically differently from holes generated in semiconductors with valence bands based on p-orbitals (e. g. , CdS, ZnO, CdSe, GaAs). The chemical character of these holes as missing d-electrons, on the other hand, gives rise to very specific electrochemical surface reactions with electron donors such as I** minus , Br** minus , and even OH** minus , for example.
Molybdenum sulfide samples of varying surface areas were prepared by the reduction in H2 of MoS3 at 250, 350, 450, 550 and 800 °C. From their X-ray diffractograms, changes occurring in their crystal structure are deduced. These deductions are supported by observed changes in the surface area. Together, they make possible some speculations concerning the dependence of the catalytic activity of molybdenum sulfide on the temperature of pretreatment.
The present work deals with an investigation on synthesis, structure and properties of indium intercalation compounds of tungsten diselenide ; InxWSe2 (0⩽x⩽1). X-ray analysis shows that these intercalated compounds like the host 2HWSe2 also possess hexagonal symmetry with a small but continuous increase in c-parameter. However, a new phase appeared in and InWSe2 diffractograms due to In2Se3. Room temperature magnetic susceptibility, thermoelectric power experiments and two-probe conductivity measurements in the temperature range 25 −225°C indicated that intercalated compounds also exhibit n-type semiconducting behaviour similar to the host WSe2 and results are explained on the basis of existing band model. Thermal stability behaviour of these compound in atmosphere has also been studied and based on X-ray data of oxidized product formation of tungsten bronzes has been proposed.
Catalytic effects of Mo and W compounds in various chemical reactions have been known for decades. Supported catalysts containing Mo or W components as essential active ingredients are the most widely used catalysts in refinery practice. In spite of this, the forms of active Mo or W species and their intimate roles in catalytic reactions are not well understood, Although the presence of sulfides such as MoS2 and WS2 is frequently reported in these materials, their participation in catalytic reactions is a matter of speculation. In recent years, significant progress has been made in this respect as the result of applying modern techniques, particularly spectroscopy.
Tungsten dichalcogenides constitute a well defined family of compounds which crystallize in a layer type structure. These compounds find a wide range of applications in the field of catalysis and as a lubricant at high temperatures and pressures. They have also been investigated successfully as cathode and anode materials in photoelectrochemical cells for solar energy conversion. The layered tungsten dichalcogenides also exhibit superconducting behaviour when intercalated with alkali or alkaline earth metals and different divalent rare earth metals. In the present paper an attempt has been made to review the preparation, crystal structure and band models of tungsten dichalcogenides. Furthermore, we have tried to incorporate the physical, chemical, optical and electrical properties along with intercalation, thermal stability and uses of these compounds.
A detailed discussion on crystal structure of indium intercalated compounds of molybdenum disulphide, InxMoS2 (0⩽x⩽1) based on x-ray studies is reported. It shows that intercalated compounds, like the host 2HMoS2 also possess hexagonal symmetry with a small but continuous increase in c-parameter, unlike their alkali and alkaline earth metals analogs. This has been explained on the basis of the fact that there may be no increase in c-parameter at all in a given intercalation process and that this does not have purely geometric origin but also comes from electronic effects. Results of SEM study are also reported.
Sulphide Catalysts, Their Preparation and Applications
- O Weisser
- S Landa
- O. Weisser
Zaklodni Pochody Organicke Synthesy
- P H Groggins
Reactioner in Und and gesten Stoffen
- K Hauffe
- K. Hauffe
Intercalated Layered Materials
- M S Whittingham
- M. S. Whittingham
Zaklodni Pochody Organicke Synthesy” (Unit Process in Organic Synthesis
- P H Groggins
- P. H. Groggins