Molybdenum Chalcogenides Research Articles

Electrostatic shielding effects and binding energy shifts and topological phases of bilayer molybdenum chalcogenides

AbstractBy combining binding energy and bond‐charge (BBC) model and density functional theory (DFT) calculations, the electrostatic shielding effects and binding energy shifts of bilayer MoS2, MoSe2, and MoTe2 are studied. In this study, the bilayer MoS2, MoSe2, and MoTe2 had bandgaps of 1.38, 1.28, and 1.01 eV, respectively. It is found that electrostatic shielding by electron exchange is the main cause of density fluctuation and determine the bonding state. We found that the edge polarization exactly vanishes after the bulk phase transition, just as we saw the corner‐localized modes disappear in a system with full open boundaries of bilayer MoTe2. Finally, this study establishes a method for calculating the lattice density with Green's function with energy level shift and provides new methods and ideas for the further study of the electrostatic shielding, bond states and topological phases of nanomaterials.

Prospects of an alternative superconductor technology for fusion reactors

New concepts for Tokamak fusion reactors are enabled by ReBCO high temperature superconductors, either to achieve toroidal magnetic fields in the range of 20 T, and/or by operating temperatures above 4.2 K, e.g., 20 K. The application of ReBCO tapes is challenging because common techniques for the manufacturing and quench protection of magnets, developed for classical multifilamentary superconductors, such as NbTi and Nb3Sn, cannot be applied directly. Less risky would be the use of a ternary molybdenum chalcogenide (TMC) superconductor, which was under development before the discovery of high temperature superconductors. Although a low temperature superconductor, the upper critical field is extremely high resulting in a comparable field dependence of the critical current to ReBCO. Because of the improved superconductor fraction of a multifilamentary TMC conductor, the expected engineering current density can be one order of magnitude higher. In addition, a TMC conductor has the potential for cost efficiency and a performance index around 1 $/kAm at 20 T seems be possible.

Annealing Temperature Effect on the Surface Properties of the MoSe Thin Films

2D transition metal dichalcogenides have been studied extensively in the field of electronics and photonics. Among them, the molybdenum chalcogenides have been receiving considerable attention due to their potential usage in field‐effect transistors and biosensors. Despite such promising aspects of these materials, studies regarding temperature effects on MoSe remain relatively rare. Herein, MoxSey (x = 0 ≈ 10, y = 0 ≈ 2) thin films are fabricated by radio frequency (RF) magnetron cosputtering on silicon and investigated using scanning electron microscopy for various atomic ratios and annealing temperatures from room temperature to 500 °C. Above the melting point of Se, Se evaporates and forms a layer, subsequently leading Mo to be exposed on the surface of the thin films. From the X‐ray diffraction and X‐ray photoelectron spectroscopy results, silicon peaks are observed due to the evaporation of Se. In addition, both Mo and Se are oxidized at above 300 °C. The work functions of the MoxSey thin films show the highest value at 200 °C measured by ultraviolet photoelectron spectroscopy and a Kelvin probe. Above the melting point of Se, there is a tendency for the work function to decrease due to the influence of Mo.

Chemical Diversity of Mo5S5 Clusters with Pyrazole: Synthesis, Redox and UV-vis-NIR Absorption Properties.

The chemistry of transition metal clusters has been intensively developed in the last decades, leading to the preparation of a number of compounds with promising and practically useful properties. In this context, the present work demonstrates the preparation and study of the reactivity, i.e., the possibility of varying the ligand environment, of new square pyramidal molybdenum chalcogenide clusters [{Mo5(μ3-S)i4(μ4-S)i(μ-pz)i4}(pzH)t5]1+/2+ (pzH = pyrazole, i = inner, t = terminal). The one-step synthesis starting from the octahedral Mo6Br12 cluster as well as the substitution of the apical pyrazole ligand or the selective bromination of the inner pyrazolate ligands were demonstrated. All the obtained compounds were characterized in detail using a series of physicochemical methods both in solid state (X-ray diffraction analysis, etc.) and in solution (nuclear magnetic resonance spectroscopy, mass spectrometry, etc.). In this work, redox properties and absorption in the ultraviolet-visible and near-infrared region of the obtained compounds were studied.

Unusual Square Pyramidal Chalcogenide Mo5 Cluster with Bridging Pyrazolate-Ligands

The family of chalcogenide molybdenum clusters is well presented in the literature by a series of compounds of nuclearity ranging from binuclear to multinuclear articulating octahedral fragments. Clusters actively studied in the last decades were shown to be promising as components of superconducting, magnetic, and catalytic systems. Here, we report the synthesis and detailed characterization of new and unusual representatives of chalcogenide clusters: square pyramidal complexes [{Mo5(μ3-Se)i4(μ4-Se)i(μ-pz)i4}(pzH)t5]1+/2+ (pzH = pyrazole, i = inner, t = terminal). Individually obtained oxidized (2+) and reduced (1+) forms have very close geometry (proven by single-crystal X-ray diffraction analysis) and are able to reversibly transform into each other, which was confirmed by cyclic voltammetry. Comprehensive characterization of the complexes, both in solid and in solution, confirms the different charge state of molybdenum in clusters (XPS), magnetic properties (EPR), and so on. DFT calculations complement the diverse study of new complexes, expanding the chemistry of molybdenum chalcogenide clusters.

Hydrothermal preparation of MoX2 (X = S, Se, Te)/TaS2 hybrid materials on carbon cloth as efficient electrocatalyst for hydrogen evolution reaction

The prominent features-based two-dimensional (2D) materials have proven themselves as efficient as well as robust electrocatalysts for the hydrogen evolution reaction (HER) owing to their low-cost, abundance and predominant conductivity. In this work, we report the synthesis of series of molybdenum chalcogenide nanostructures MoX2 (X = S, Se, Te), hybridized with TaS2 nanosheets, via a facile hydrothermal method, on self-supported carbon cloth electrode. Used as an electrocatalyst for HER, hybrid phase MoSe2/TaS2/CC electrode with a Mo/Se ratio of 1:1.5 exhibits the best HER performance, which could afford the benchmark current densities of 10 mA/cm2 at the overpotentials of 75 mV with the measured Tafel slope values of 54.7 mV/dec. In addition, the presented molybdenum dichalcogenides in this work are also complimented with robustness as determined from durability and air stability measurements. The unique aspects of these unique hybrids, such as 1T and 2H phases hybridized MoS2 and MoSe2, semimetallic nature 1T′-MoTe2 petal clusters and strong interface interaction between MoX2 (X = S, Se, Te) and conductive TaS2 nanosheets, cause superior HER catalytic performance.

Topological states in Chevrel phase materials from first principles calculations

Chevrel phase materials form a family of ternary molybdenum chalcogenides with a general chemical formula ${A}_{x}{\mathrm{Mo}}_{6}{X}_{8}$ $(A=\text{metal elements},X=\text{chalcogen})$. The variety of $A$ atoms makes a large number of family members and leads to many tunable physical properties, such as the superconductivity, thermoelectricity, and the ionic conductivity. In this work, we have further found various nontrivial band topological states in these materials by using first-principle calculations. The compounds having time-reversal symmetry, such as ${\text{BaMo}}_{6}{\text{S}}_{8}, {\text{SrMo}}_{6}{\text{S}}_{8}$, and ${\text{Mo}}_{6}{\text{S}}_{8}$, are topological insulators in both of the $R\overline{3}$ and $P\overline{1}$ phases, whereas ${\text{EuMo}}_{6}{\text{S}}_{8}$ within ferromagnetic state is an axion insulator in the $R\overline{3}$ phase and a trivial one in the $P\overline{1}$ phase. This indicates that the change of $A$ ions can modify the chemical potential, lattice distortion, and magnetic orders, which offers a unique way to influence the topological states and other properties. We hope this work can stimulate further studies of Chevrel phase materials to find more intriguing phenomena, such as topological superconducting states and Majorana modes.

Molybdenum Chalcogenides for Photo-Oxidative Desulfurization of Liquid Fuels Under Ambient Conditions: Process Optimization, Kinetics, and Recyclability Studies

Molybdenum Chalcogenides for Photo-Oxidative Desulfurization of Liquid Fuels Under Ambient Conditions: Process Optimization, Kinetics, and Recyclability Studies

Optimizing the Back Contact of Kesterites and Perovskites: Band Edge Design and Defect Engineering in Molybdenum Chalcogenides

AbstractBack contacts, as an important part of solar cell devices, play a critical role in efficient separation of carriers and reduction of interfacial recombination. Here, by analyzing interfacial band alignment, thickness and defect properties of back contacts, a design principle for back contacts is proposed to properly accommodate the interrelated factors of short‐circuit current (Jsc), open‐circuit voltage (Voc), series resistance (Rs), and nonradiative recombination centers (NRCs) at the same time. A preferred back contact should have (i) a high valance band maximum (VBM) and (ii) a low Fermi level relative to absorber to drive hole extraction and block electron leakage, (iii) a thin design under the premise of p+‐type characteristic to effectively reduce Rs, which also ensures a wide depletion region on the absorber side to promote hole collection, and (iv) no additional deep‐level defects as interfacial NRCs. Taking molybdenum chalcogenide as example, p+‐type MoSe2/MoS2 realized by group VB element doping satisfies the design principles and is promising as an excellent back contact for kesterite/perovskite solar cells. This study gives theoretical guidance for designing an ideal back contact and shows how to improve the photovoltaic performance of solar cells by interfacial optimization.

Charge Transport Dynamics in Microwave Synthesized One-Dimensional Molybdenum Chalcogenides

Scalable synthesis of one-dimensional molybdenum chalcogenides with tunable electronic properties may be critical for the development of nanoscale electronic device components as well as functional...

Machine Learning Guided Synthesis of Multinary Chevrel Phase Chalcogenides.

The Chevrel phase (CP) is a class of molybdenum chalcogenides that exhibit compelling properties for next-generation battery materials, electrocatalysts, and other energy applications. Despite their promise, CPs are underexplored, with only ∼100 compounds synthesized to date due to the challenge of identifying synthesizable phases. We present an interpretable machine-learned descriptor (Hδ) that rapidly and accurately estimates decomposition enthalpy (ΔHd) to assess CP stability. To develop Hδ, we first used density functional theory to compute ΔHd for 438 CP compositions. We then generated >560 000 descriptors with the new machine learning method SIFT, which provides an easy-to-use approach for developing accurate and interpretable chemical models. From a set of >200 000 compositions, we identified 48 501 CPs that Hδ predicts are synthesizable based on the criterion that ΔHd < 65 meV/atom, which was obtained as a statistical boundary from 67 experimentally synthesized CPs. The set of candidate CPs includes 2307 CP tellurides, an underexplored CP subset with a predicted preference for channel site occupation by cation intercalants that is rare among CPs. We successfully synthesized five of five novel CP tellurides attempted from this set and confirmed their preference for channel site occupation. Our joint computational and experimental approach for developing and validating screening tools that enable the rapid identification of synthesizable materials within a sparse class is likely transferable to other materials families to accelerate their discovery.

Chevrel Phase Nanoparticles as Electrocatalysts for Hydrogen Evolution

Chevrel phases (MxMo6S8) are a class of molybdenum chalcogenide materials that are attractive candidates for active non-noble metal catalysts due to the relatively low coordination of their molybdenum moieties. Conventionally, the lengthy and energy-intensive synthesis of Chevrel phases (CPs) produces highly crystalline, low surface area materials. In this work, a synthetic approach leading to a Chevrel phase with an unprecedented nanostructure is presented. The resultant material is fully characterized using a variety of spectroscopic, microscopic, and electrochemical techniques. In electrochemical testing aimed at catalyzing the hydrogen evolution reaction (HER), this nanostructured catalyst shows a substantially lower overpotential than an equivalent MoS2 phase with a similar nanostructure. Furthermore, the nanostructured Chevrel phases prove to be easily modified by electrochemical intercalation, which allows performance fine-tuning, revealing a family of versatile and tuneable catalysts.

Atomic Wires of Transition Metal Chalcogenides: A Family of 1D Materials for Flexible Electronics and Spintronics.

As analogues of two-dimensional (2D) layered materials, searching for one-dimensional (1D) van der Waals wired materials as 1D Lego blocks for integration and device applications has been pursued. Motivated by the recently synthesized atomic wires of molybdenum chalcogenide, here we explored the structures and stability of 66 atomic wires of 3d, 4d, and 5d transition metal chalcogenides in the M6X6 stoichiometry (M = transition metal, X = chalcogen). After high-throughput first-principles calculations, 53 unprecedented and experimentally feasible M6X6 wires have been identified. Diverse functionalities are found in these 1D materials, including semiconductors, metals, and ferromagnets with high Young’s modulus and large fracture strain. Notably, six kinds of M6X6 wires are robust ferromagnets with Curie temperatures up to 700 K, which can be further elevated under axial strains. Moreover, these M6X6 atomic wires possess high stability and resistance to oxidation, humidity, and aggregation; both merits are desirable for device applications. This large family of 1D materials with definite structures and rich properties allows atomically precise integration for flexible electronics and spintronics.

Improving stability of organometallic-halide perovskite solar cells using exfoliation two-dimensional molybdenum chalcogenides

Organometallic-halide perovskite solar cells (PSCs) are emerging as the most promising next generation solar cell devices. However, the stability is still the main bottleneck of their further development. Here, we introduce two-dimensional (2D) molybdenum chalcogenides (MoS2 and MoSe2) (MCs) nanoflakes as a buffer layer between perovskite layer and hole transport layer (HTL) to improve the stability of the organometallic-halide PSCs. 2D MCs are obtained via liquid-phase exfoliated (LPE) approach, and Glass/FTO/compact-TiO2/ mesoporous-TiO2/FA85MA15PbI85Br15/2D MCs/Spiro-OMeTAD/Au structured solar cell devices are designed and fabricated. In this system, 2D MCs act both as a protective layer and an additional HTL of PSCs. This kind of PSCs achieve a relatively high-power conversion efficiency (PCE) of 14.9%, along with a much longer lifetime stability compared to the standard PSCs. After 1 h, PCE of the PSC adding a 2D MCs buffer layer could maintain 93.1% of initial value, while the PCE of the standard PSC dropped dramatically to 78.2% of initial efficiency. Our results pave the way towards the implementation of 2D MCs nanoflakes as a material able to boost the shelf life of PSCs and further provide the opportunity to fabricate large-area PSCs in view of their commercialization.

Recent Progress in Chemiresistive Gas Sensing Technology Based on Molybdenum and Tungsten Chalcogenide Nanostructures

AbstractChalcogens are oxygen family elements with oxygen having distinct properties than the other group members like sulfur, selenium, and tellurium. Tungsten and molybdenum chalcogenides that include mainly metal oxides (MO3; M = Mo, and W) and metal dichalcogenides (MX2; X = S, Se, and Te) are the most exciting class of inorganic materials. They have been widely investigated in various applications including lithium and sodium ion storage, supercapacitors, gas sensors, biosensors etc. due to their fascinating surface properties and ease of fabrication. Different class of nanostructures including 0D (nanoparticles, quantum dots), 1D (nanowires, nanotubes, nanoribbons, nanobelts etc.), and 2D (nanosheets, nanoflakes, nanoplates etc.) structures of these materials have been synthesized, characterized, and utilized for these applications. With recent boost in layered material research, they are also being reconsidered for future device and systems applications. In this review article, the recent developments on the chemiresistive gas sensing applications of these nanomaterials are discussed. The authors also outline the efforts which have been made over the years to improve the sensing behavior of these materials. Finally, the authors present future opportunities in realizing high‐performance gas sensors using these materials.

A general route to free-standing films of nanocrystalline molybdenum chalcogenides at a liquid/liquid interface under hydrothermal conditions

The utility of hydrothermal method applied to liquid/liquid interface for obtaining free-standing films of molybdenum chalcogenides is demonstrated. A general route for obtaining films of MoO3, MoS2 and MoSe2 at a water/toluene interface is illustrated employing a universal Mo precursor and the desired chalcogenide reactant in the two different phases. The hydrothermal conditions promote the formation of emulsions of water and toluene. A mechanism is proposed wherein the in-situ chalcogenation happens at the interfaces of tiny droplets of toluene in water and the nanosheets of the chalcogenide self-assemble at the regenerated interface during quenching to form films. The free-standing films are transferred on to various substrates for characterization. MoO3 films consist of nanobelts while MoS2 and MoSe2 films consist of a dense assembly of nanosheets. The advantage of transferring the films on arbitrary substrates is exemplified for electrochemical applications.

Synthesis of heat-resistant oxygen-free coatings in the Nb‒Cr‒Mo‒Si‒B system from compositions modified with chalcogenide compounds of refractory metals of groups IV‒VIa

The results of studies on the modification of the composition of compositions in the Nb‒Cr‒Mo‒Si‒B system with molybdenum chalcogenides are presented. The positive effect of using MoS 2 and MoSe 2 as an integral part of multicomponent oxygen-free compositions associated with the formation of a highly active form of elemental molybdenum as a result of their dissociation, which determines the high heat resistance of the coating formed on their basis, is shown. Ill. 4. Ref. 45. Tab. 3.

Performance evaluation of molybdenum dichalcogenide (MoX2; X= S, Se, Te) nanostructures for hydrogen evolution reaction

Hydrogen evolution reaction (HER) using transition metal dichalcogenides (TMDs) have gained interest owing to their low-cost, abundancy and predominant conductivity. However, forthright comparisons of transition metal chalcogenides for HER are scarcely conducted. In this work, we report the synthesis of series of molybdenum chalcogenide nanostructures MoX2 (X = S, Se, Te) via a facile hydrothermal method. Used as an electrocatalyst for HER, MoS2 nanograins, MoSe2 nanoflowers and MoTe2 nanotubes could afford the benchmark current densities of 10 mA cm−2 at the overpotentials of −173 mV, −208 mV and −283 mV with the measured Tafel slope values of 109.81 mV dec−1, 65.92 mV dec−1 and 102.06 mV dec−1, respectively. Besides other factors influencing HER, the role of electronic conductivity in HER of these molybdenum dichalcogenides are elucidated. In addition, the presented molybdenum dichalcogenides in this work are also complimented with robustness as determined from high-current density stability measurements.

In Situ Synchrotron Powder Diffraction Study of Cd Intercalation into Chevrel Phases: Crystal Structure and Kinetic Effect.

Chevrel phases are molybdenum chalcogenides of formula M xMo6X8 (where M is a cation and X is a chalcogen) that present a complex and captivating intercalation chemistry that has drawn the interest of the solid-state chemistry community since their discovery. This property has a huge potential for applied science and device development for energy storage and pollutant removal and detection, but a deeper knowledge of the intercalation processes and chemistry is still necessary. In the present work, the intercalation of Cd2+ in aqueous solution has been studied, taking advantage of the complementarity of electrochemical characterization and synchrotron powder diffraction acquired during an in situ combined experiment. During the experiment, industrially adequate electrochemical conditions (room temperature and reduced process time) were applied, allowing a better understanding of the intercalation processes. The intercalated phases obtained by electrochemistry have been characterized ex situ, and for the first time the structures of Cd2Mo6X8 (X = S, Se) have been determined. Unexpectedly, Cd2Mo6Se8 presents a trigonal crystal structure with only cavity 2 occupied, which has not been encountered before for Chevrel phases.

Synthesis and Exploration of the Lubricating Behavior of Nanoparticulated Mo15S19 in Linseed Oil.

Molybdenum chalcogenides present interesting properties beyond their superconducting critical temperatures and upper critical magnetic fields, making them suitable for potential applications in tribology, batteries, catalysis, or thermopower. In this study, Mo15S19 nanoparticles with an average diameter of 10 nm were synthesized via the reaction of ammonium molybdate with hydrochloric acid and elemental sulfur as reducers at 245 °C. The oxidation to MoO3 in air was efficiently avoided by using linseed oil as a reaction medium and dispersant. Scanning electron microscopy (SEM) micrographs of the as-prepared samples revealed the presence of few-micron-size aggregates, while transmission electron microscopy (TEM) characterization evidenced that the samples were polynanocrystalline with a high degree of homogeneity in size (standard deviation of 2.7 nm). The absence of the first-order (00l) reflection in the X-ray diffraction pattern was also indicative of the absence of Mo3S4 stacking, suggesting that it was a non-layered material. A dispersion of the nanoparticles in linseed oil has been studied as a lubricant of steel–steel sliding contacts, showing the formation of a surface layer that reduces wear and mean friction coefficients with respect to the base oil.