Bulk Synthesis of Inorganic Fullerene-like MS2 (M = Mo, W) from the Respective Trioxides and the Reaction Mechanism

Topochemically synthesized Nb3VS6 as a stable anode for sodium-ion batteries

Ternary metal sulfides having layered morphology are considered as potential active materials for various applications. Herein, Nb3VS6 is synthesized topochemically for the first time using a Nb-V-HDA complex having a lamellar structure by employing H2S gas as the sulfidation agent. Nb3VS6, as an anode for SIBs, exhibited a specific capacity of 101.15 mA h g-1 at 0.5 A g-1, along with excellent cycling stability with 100% capacity retention after 2500 cycles.

Nanotubes from Transition Metal Dichalcogenides: Recent Progress in the Synthesis, Characterization and Electrooptical Properties

Inorganic layered compounds (2D-materials), particularly transition metal dichalcogenide (TMDC), are the focus of intensive research in recent years. Shortly after the discovery of carbon nanotubes (CNTs) in 1991, it was hypothesized that nanostructures of 2D-materials can also fold and seam forming, thereby nanotubes (NTs). Indeed, nanotubes (and fullerene-like nanoparticles) of WS2 and subsequently from MoS2 were reported shortly after CNT. However, TMDC nanotubes received much less attention than CNT until recently, likely because they cannot be easily produced as single wall nanotubes with well-defined chiral angles. Nonetheless, NTs from inorganic layered compounds have become a fertile field of research in recent years. Much progress has been achieved in the high-temperature synthesis of TMDC nanotubes of different kinds, as well as their characterization and the study of their properties and potential applications. Their multiwall structure is found to be a blessing rather than a curse, leading to intriguing observations. This concise minireview is dedicated to the recent progress in the research of TMDC nanotubes. After reviewing the progress in their synthesis and structural characterization, their contributions to the research fields of energy conversion and storage, polymer nanocomposites, andunique optoelectronic devices are being reviewed. These studies suggest numerous potential applications for TMDC nanotubes in various technologies, which are briefly discussed.

Tailoring Triuranium Octoxide into Multidimensional Uranyl Fluoride Micromaterials

Uranium microstructured materials with controlled size and shape are relevant to the nuclear industry and have found applications as targets for medical isotope production, fuels for nuclear reactors, standards for nuclear forensics, and energy sources for space exploration. Until now, most studies at the microscale have focused on uranium microspheres (oxides, nitrides, carbides, and fluorides), while micromaterials of uranium halides, carbides, and pnictides with other morphologies are largely unknown. A promising method to shape the morphology of uranium micromaterials is the replacement of O by F atoms in oxide materials using a solid-gas reaction. Here, with the aim to elaborate unexplored uranium fluoride micromaterials, the fluorination of uranium oxide (U3O8 and UO2) microspheres (ms), microrods (mr), and microplates (mp) in an autoclave at 250 °C with HF(g) (produced from the thermal decomposition of silver bifluoride (SBF)) and with ammonium bifluoride (ABF) was evaluated. We show that the reactions between U3O8 mr and U3O8 mp and SBF provided the most efficient way to elaborate mr and mp UO2F2 micromaterials in a high yield (∼90%). The resulting UO2F2 mr (length: 3-20 μm) and UO2F2 mp (width: 1-7.5 μm) exhibited a well-defined geometry that was identical to that of the U3O8 precursors. Agglomerated (NH4)3UO2F5 and UO2F2 ms (2-3.5 μm) were prepared from the reaction of U3O8 ms with ABF. It is noted that the reaction of UO2 ms with SBF and ABF did not provide any uranium fluoride micromaterials. The successful preparation of uranium fluoride microstructures (ms, mr, and mp) developed here opens the way to novel actinide fluoride micromaterials.

Mechanism of WS2 Nanotube Formation Revealed by in Situ/ex Situ Imaging

Multiwall WS2 nanotubes have been synthesized from W18O49 nanowhiskers in substantial amounts for more than a decade. The established growth model is based on the "surface-inward" mechanism, whereby the high-temperature reaction with H2S starts on the nanowhisker surface, and the oxide-to-sulfide conversion progresses inward until hollow-core multiwall WS2 nanotubes are obtained. In the present work, an upgraded in situ SEM μReactor with H2 and H2S sources has been conceived to study the growth mechanism in detail. A hitherto undescribed growth mechanism, named "receding oxide core", which complements the "surface-inward" model, is observed and kinetically evaluated. Initially, the nanowhisker is passivated by several WS2 layers via the surface-inward reaction. At this point, the diffusion of H2S through the already existing outer layers becomes exceedingly sluggish, and the surface-inward reaction is slowed down appreciably. Subsequently, the tungsten suboxide core is anisotropically volatilized within the core close to its tips. The oxide vapors within the core lead to its partial out-diffusion, partially forming a cavity that expands with reaction time. Additionally, the oxide vapors react with the internalized H2S gas, forming fresh WS2 layers in the cavity of the nascent nanotube. The rate of the receding oxide core mode increases with temperatures above 900 °C. The growth of nanotubes in the atmospheric pressure flow reactor is carried out as well, showing that the proposed growth model (receding oxide core) is also relevant under regular reaction parameters. The current study comprehensively explains the WS2 nanotube growth mechanism, combining the known model with contemporary insight.

Real-Time Monitoring of Adsorption-Induced Scrolling of Colloidal Inorganic Nanosheets

Inorganic nanotubes have attracted much attention due to their unique physicochemical properties. Nanotubes can be prepared by scrolling exfoliated nanosheets under ambient conditions. However, how the nanosheet scrolled in its colloidal state has not been experimentally visualized. In this paper, we directly observed the scrolling process of nanosheets upon adsorption of organic cations. Exfoliated flat nanosheets of niobate and clay in aqueous colloids were found to scroll by adding organic cations, such as exfoliation reagents, to the colloids. Employment of cationic stilbazolium dye enabled in situ observation of the dye adsorption and scrolling by optical microscopy based on changes in color and morphology of the nanosheets. The scrolling was promoted for nanosheets adsorbed with a stilbazolium dye with a longer alkyl chain, suggesting that the interaction between the hydrophobic parts of the dye cations is the driving force of the scrolling. This finding should encourage research on the formation of nanotubes from nanosheets and also provides important guidelines for the selection of appropriate exfoliation reagents when exfoliating nanosheets from layered crystals.

Addressing Irreversibility and Structural Distortion in WS2 Inorganic Fullerene-Like Nanoparticles: Effects of Voltage Cutoff Experiments in Beyond Li+-Ion Storage Applications

Large interlayer spacing beneficially allows Na+- and K+-ion storage in transition-metal dichalcogenide (TMD)-based electrodes, but side reactions and volume change, which pulverize the TMD crystalline structure, are persistent challenges for the utilization of these materials in next-generation devices. This study first determines whether irreversibility due to structural distortion, which results in poor cycling stability, is also apparent in the case of inorganic fullerene-like (IF) tungsten disulfide (WS2) nanocages (WS2IF). To address these problems, this study proposes upper and lower voltage cutoff experiments to limit specific reactions in Na+/WS2IF and K+/WS2IF half-cells. Three-dimensional (3D) differential capacity curves and derived surface plots highlight the continuation of reversible reactions when a high upper cutoff technique is applied, thereby indirectly suggesting restricted structural dissolution. This resulted in improved capacity retention with stable performance and a higher Coulombic efficiency, laying the ground for the use of TMD-based materials beyond Li+-ion storage devices.

The Mechanical Properties Relationship of Radiation-Cured Nanocomposites Based on Acrylates and Cationic Polymerized Epoxies and the Composition of Silane-Modified Tungsten Disulfide Nanoparticles

The effect of semiconducting tungsten disulfide (WS₂) nanoparticles (NPs), functionalized by either methacryloxy, glycidyl, vinyl, or amino silanes, has been studied in photocuring of acrylate and epoxy resins (the latter photocured according to a cationic mechanism). The curing time, degree of curing (DC), thermal effects, and mechanical properties of the radiation-cured resins were investigated. X-ray photoelectron spectroscopy (XPS) and transmission electron microscopy (TEM) analyses confirmed that a silane coating was formed (1-4 nm) on the NPs' surface having a thickness of 1-4 nm. Fourier transition infrared (FTIR) was used to determine the DC of the nanocomposite resin. The curing time of the epoxy resin, at 345-385 nm wavelength, was 10 to 20 s, while for acrylate, the curing time was 7.5 min, reaching 92% DC in epoxy and 84% in acrylate. The glass transition temperature (Tg) of the photocured acrylates in the presence of WS₂ NPs increased. In contrast to the acrylate, the epoxy displayed no significant variations of the Tg. It was found that the silane surface treatments enhanced the DC. Significant increases in impact resistance and enhancement in shear adhesion strength were observed when the NPs were treated with vinyl silane. A previous study has shown that the addition of WS₂ NPs at a concentration of 0.5 wt.% is the optimal loading for improving the resin's mechanical properties. This study supports these earlier findings not only for the unmodified NPs but also for those functionalized with silane moieties. This study opens new vistas for the photocuring of resins and polymers in general when incorporating WS₂ NPs.

Hydrothermal synthesis and controlled growth of group-VIB W metal compound nanostructures from tungsten oxide to tungsten disulphide

Two-dimensional lateral group-VIB transition metal dichalcogenides (TMDs) have attracted much attention in the fast evolving field of advanced photoelectric functional materials, but their controllable fabrication is challenging. Herein, an emerging synthetic route for sulfurization of tungsten oxide was developed. During the hydrothermal reaction, the optimization of the precursor selection and synthesis parameters led to the tunable properties of WO₃-WS_xO_y-WS₂ nanostructures. The vulcanization was thermodynamically favorably at low temperatures and in an environment with a sufficient S source, wherein WO₃ was reduced by H atoms to WO_3-x, and S atoms were preferentially adsorbed on O vacancies. The WS_xO_y nanostructures have a narrow band-gap attributed to the effect of S on the valence band top and electronic density of states by density functional theory. The photocurrent response and charge transfer properties of WS_xO_y were improved due to the charge transport between WS₂ and WO₃. Understanding the formation and transformation of WS₂ nanostructures in solution contributes to the discovery of the important structure-efficiency relationship, which may be extended to other TMDs systems. Hence, extensive research efforts are still needed to develop safer and more efficient synthesis and modification methods to fully utilize the distinctive advantageous properties of TMDs in the photoelectric field.

Size and Shape's Effects on the High-Pressure Behavior of WS2 Nanomaterials

Exploring the behavior of nanocrystals with varying shapes and sizes under high pressure is crucial to understanding the relationship between the morphology and properties of nanomaterials. In this study, we investigated the compression behaviors of WS2 nanotubes (NT-WS2) and fullerene-like nanoparticles (IF-WS2) by in situ high-pressure X-ray diffraction (XRD) and Raman spectroscopy. It was found that the bulk modulus of NT-WS2 is 81.7 GPa, which is approximately twice as large as that of IF-WS2 (46.3 GPa). This might be attributed to the fact that IF-WS2 with larger d-spacing along the c-axis and higher defect density are more compressible under isotropic pressure than NT-WS2. Thus, the slender NT-WS2 possess a more stable crystal structure than the IF-WS2. Our findings reveal that the effects of morphology and size play crucial roles in determining the high-pressure properties of WS2 nanoparticles, and provide significant insight into the relationship between structure and properties.

Photocatalytic activity of layered MoS2 in the reductive degradation of bromophenol blue

Molybdenum disulphide (MoS₂) is a layered material with interesting photocatalytic properties. In this study, a layered MoS₂ was produced using a hydrothermal method. The obtained material was characterised by XRD (X-ray diffraction), XPS (X-ray photoelectron spectroscopy), SEM (scanning electron microscopy), UV-Vis spectroscopy, DLS (dynamic light scattering), and zeta potential analysis. For the evaluation of the photocatalytic properties of layered MoS₂, a solution of bromophenol blue (BPB) and the catalyst was illuminated for 120 minutes. According to the experimental results, MoS₂ exhibited excellent catalytic activity in BPB degradation. The MoS₂ preparation method enabled improved light harvesting, avoided fast charge recombination (related to bulk MoS₂), and created a large number of suitable electron transfer sites for photocatalytic reactions. Simulation of BPB decay and bromide production was carried out for a further understanding of MoS₂ photocatalytic action. The simulation results proved the reduction mechanism of BPB photodegradation.

Molybdenum disulphide (MoS₂) is a layered material with interesting photocatalytic properties.

Influence of the choice of precursors on the synthesis of two-dimensional transition metal dichalcogenides

The interest in transition metal dichalcogenides (TMDCs; MEy/2; M = transition metal; E = chalcogenide, y = valence of the metal) has grown exponentially across various science and engineering disciplines due to their unique structural chemistry manifested in a two-dimensional lattice that results in extraordinary electronic and transport properties desired for applications in sensors, energy storage and optoelectronic devices. Since the properties of TMDCs can be tailored by changing the stacking sequence of 2D monolayers with similar or dis-similar materials, a number of synthetic routes essentially based on the disintegration of bulk (e.g., chemical exfoliation) or the integration of atomic constituents (e.g., vapor phase growth) have been explored. Despite a large body of data available on the chemical synthesis of TMDCs, experimental strategies with high repeatability of control over film thickness, phase and compositional purity remain elusive, which calls for innovative synthetic concepts offering, for instance, self-limited growth in the z-direction and homogeneous lateral topography. This review summarizes the recent conceptual advancements in the growth of layered van der Waals TMDCs from both mixtures of metal and chalcogen sources (multi-source precursors; MSPs) and from molecular compounds containing metals and chalcogens in one starting material (single-source precursor; SSPs). The critical evaluation of the strengths, limitations and opportunities of MSP and SSP approaches is provided as a guideline for the fabrication of TMDCs from commercial and customized molecular precursors. For example, alternative synthetic pathways using tailored molecular precursors circumvent the challenges of differential nucleation and crystal growth kinetics that are invariably associated with conventional gas phase chemical vapor transport (CVT) and chemical vapor deposition (CVD) of a mixture of components. The aspects of achieving high compositional purity and alternatives to minimize competing reactions or side products are discussed in the context of efficient chemical synthesis of TMDCs. Moreover, a critical analysis of the potential opportunities and existing bottlenecks in the synthesis of TMDCs and their intrinsic properties is provided.

Gas-Phase "Prehistory" and Molecular Precursors in Monolayer Metal Dichalcogenides Synthesis: The Case of MoS2

Two-dimensional MoS₂ is one of the most promising materials for nanoelectronics due to its semiconducting nature and plethora of extraordinary properties. The main method for mass production of large-scale, high-quality MoS₂ monolayers is chemical vapor deposition (CVD). Yet, the details of the chemistry occurring during the synthesis remain largely unknown, hindering process optimization. Combining ab initio molecular dynamics (AIMD) simulations and first-principles calculations allows us to explore the complete processes of MoS₂ monolayer growth at the atomic level. We find that solid MoO₃ precursor sublimates forming ringlike molecules, such as Mo₃O₉, which can later be regarded as gas-phase Mo-carrier reactants, undergoing sulfurization in three main stages: ring opening, chain breaking as the rate-limiting step, and further sulfurization. The fully sulfurized MoS₆ molecule emerges as an immediate gas precursor to the crystal growth, as it reacts to join the MoS₂-layer edge, with the release of a S₄ molecule. Our comprehensive study provides detailed insights into the microscopic reaction mechanisms of MoS₂ CVD growth and guidance for optimizing the synthesis parameters for transition metal dichalcogenides.

Employment of Micro- and Nano-WS2 Structures to Enhance the Tribological Properties of Copper Matrix Composites

Friction and wear are responsible for around 23% of the energy consumption in transportation, manufacturing, power generation, and residential sectors. Employed components are exposed to a wide range of operational conditions, therefore a suitable material design is fundamental to decreasing tribological issues, energy consumption, costs, and environmental impact. This study aims to analyze the effect of different solid lubricants on the suitability of copper matrix composites (CuMCs) as a potential solution to reduce the depletion of sliding electrical contacts working under extreme conditions. CuMCs samples are produced by cold-pressing and sintering to merge a high electrical conductivity with the lubricant effect supplied by different species, namely tungsten disulfide micro-powder (WS2), inorganic fullerene-like (IF) tungsten disulfide nanoparticles, and graphene nanoplatelets (GNP). The crystalline structure of the pristine and composite materials is characterized via XRD. The electrical tests show a small decrease of conductivity compared to pure copper, due to the insulating effect of WS2; however, the measured values are still adequate for conduction purposes. Micro-scratch and wear tests highlight the positive effect of the combination of WS2 structures and GNP. The friction coefficient reduction leads to the possibility of extending the lifetime of the components.

Solving the "MoS2 Nanotubes" Synthetic Enigma and Elucidating the Route for Their Catalyst-Free and Scalable Production

This study solves a more than two-decades-long "MoS₂ Nanotubes" synthetic enigma: the futile attempts to synthesize inorganic nanotubes (INTs) of MoS₂ via vapor-gas-solid (VGS) reaction. Among them was replication of the recently reported pure-phase synthesis of the analogous INT-WS₂. During these years, successful syntheses of spherical nanoparticles of WS₂ and MoS₂ were demonstrated as well. All these nanostructures were obtained by VGS reaction of corresponding oxides with H₂/H₂S gases, at elevated temperatures (>800 °C), in a fluidized bed reactor (FBR) and a one-pot process. This success and apparent similarity between the two compounds "hid" from us the option of looking for the INT-MoS₂ reaction parameters in entirely different regimes. The main challenge in the synthesis of INT-MoS₂ via VGS was the instability of the in situ prepared suboxide nanowhiskers against over-reduction and recrystallization at high temperatures. The elucidated growth mechanism dictates separation of the reaction into five steps, as properties of the intermediate products are not consistent with a single process and require individual conditions for each step. A horizontal reactor with a porous-quartz reaction cell, which creates proper quasi-static (contrary to the FBR) conditions for the reaction involving sublimation, was imperative for the effective nanofabrication of INT-MoS₂. These findings render a reproducible synthetic route for the production of highly crystalline pure-phase MoS₂ nanotubes via a multistep VGS process, without the assistance of a catalyst and in a scalable fashion. Being a semiconductor, flexible, and strong, INT-MoS₂ offers a platform for much research and numerous potential applications, particularly in the field of optoelectronics and reinforcement of polymer composites.

Surface-diffusion-limited growth of atomically thin WS2 crystals from core-shell nuclei

Atomically thin transition metal dichalcogenides (TMDs) have recently attracted great attention since the unique and fascinating physical properties have been found in various TMDs, implying potential applications in next-generation devices. The progress towards developing new functional and high-performance devices based on TMDs, however, is limited by the difficulty in producing large-area monolayer TMDs due to a lack of knowledge of the growth processes of monolayer TMDs. In this work, we have investigated the growth processes of monolayer WS2 crystals using a thermal chemical vapor deposition method, in which the growth conditions were adjusted in a systematic manner. It was found that, after forming WO3-WS2 core-shell nanoparticles as nucleation sites on a substrate, the growth of three-dimensional WS2 islands proceeds by ripening and crystallization processes. Lateral growth of monolayer WS2 crystals subsequently occurs by the surface diffusion process of adatoms toward the step edge of the three-dimensional WS2 islands. Our results provide understanding of the growth processes of monolayer WS2 by using chemical vapor deposition methods.

An overview of the recent advances in inorganic nanotubes

Advanced nanomaterials play a prominent role in nanoscience and nanotechnology developments, opening new frontiers in these areas. Among these nanomaterials, due to their unique characteristics and enhanced chemical and physical properties, inorganic nanotubes have been considered one of the most interesting nanostructures. In recent years, important progress has been achieved in the production and study of these nanomaterials, including boron nitride, transition metal dichalcogenide nanotubular structures, misfit-based nanotubes and other hybrid/doped nanotubular objects. This review is devoted to the in-depth analysis of recent studies on the synthesis, atomic structures, properties and applications of inorganic nanotubes and related nanostructures. Particular attention is paid to the growth mechanism of these nanomaterials. This is a crucial point for the challenges ahead related to the mass production of high-quality defect-free nanotubes for a variety of applications.

WS2 Nanotubes, 2D Nanomeshes, and 2D In-Plane Films through One Single Chemical Vapor Deposition Route

We demonstrate a versatile, catalyst free chemical vapor deposition process on insulating substrates capable of producing in one single stream one-dimensional (1D) WO_{3- x} suboxides leading to a wide range of substrate-supported 2H-WS₂ polymorphs: a tunable class of out-of-plane (of the substrate) nanophases, with 1D nanotubes and a pure WS₂, two-dimensional (2D) nanomesh (defined as a network of webbed, micron-size, few-layer 2D sheets) at its extremes; and in-plane (parallel to the substrate) mono- and few-layer 2D domains. This entails a two-stage approach in which the 2WO₃ + 7S → 2WS₂ + 3SO₂ reaction is intentionally decoupled. First, various morphologies of nanowires or nanorods of high stoichiometry, WO_2.92/WO_2.9 suboxides (belonging to the class of Magnéli phases) were formed, followed by their sulfurization to undergo reduction to the aforementioned WS₂ polymorphs. The continuous transition of WS₂ from nanotubes to the out-of-plane 2D nanomesh, via intermediary, mixed 1D-2D phases, delivers tunable functional properties, for example, linear and nonlinear optical properties, such as reflectivity (linked to optical excitations in the material), and second harmonic generation (SHG) and onset of saturable absorption. The SHG effect is very strong across the entire tunable class of WS₂ nanomaterials, weakest in nanotubes, and strongest in the 2D nanomesh. Furthermore, a mechanism via suboxide (WO_{3- x}) intermediate as a possible path to 2D domain growth is demonstrated. 2D, in-plane WS₂ domains grow via "self-seeding and feeding" where short WO_2.92/WO_2.9 nanorods provide both the nucleation sites and the precursor feedstock. Understanding the reaction path (here, in the W-O-S space) is an emerging approach toward controlling the nucleation, growth, and morphology of 2D domains and films of transition-metal dichalcogenides.

Flatlands in the Holy Land: The Evolution of Layered Materials Research in Israel

The experimental identification of fullerenes in 1985, carbon nanotubes in 1991, inorganic nanotubes in 1992, and graphene in 2004 are cornerstone events that have marked the beginning of the layered nanostructures era of materials science. Nowadays, the synthesis of such low-dimensional systems is a routine practice allowing the controlled fabrication of 0-, 1-, and 2D layered structures of diverse chemical compositions. These systems possess unique physical properties that stem from their structural anisotropy characterized by strong intralayer covalent bonding and weaker interlayer dispersive interactions. This, in turn, results in promising functionality that attracts the attention of scientists from many disciplines including chemists, physicists, material scientists, engineers, as well as life scientists that are interested in both their basic and applied science aspects. Here, a short review of the contribution of the Israeli scientific community to this effort over the past 3 decades, is provided.

Topotactic Growth of Edge-Terminated MoS2 from MoO2 Nanocrystals

Layered transition metal dichalcogenides have distinct physicochemical properties at their edge-terminations. The production of an abundant density of edge structures is, however, impeded by the excess surface energy of edges compared to basal planes and would benefit from insight into the atomic growth mechanisms. Here, we show that edge-terminated MoS₂ nanostructures can form during sulfidation of MoO₂ nanocrystals by using in situ transmission electron microscopy (TEM). Time-resolved TEM image series reveal that the MoO₂ surface can sulfide by inward progression of MoO₂(202̅):MoS₂(002) interfaces, resulting in upright-oriented and edge-exposing MoS₂ sheets. This topotactic growth is rationalized in the interplay with density functional theory calculations by successive O-S exchange and Mo sublattice restructuring steps. The analysis shows that formation of edge-terminated MoS₂ is energetically favorable at MoO₂(110) surfaces and provides a necessary requirement for the propensity of a specific MoO₂ surface termination to form edge-terminated MoS₂. Thus, the present findings should benefit the rational development of transition metal dichalcogenide nanomaterials with abundant edge terminations.

Synthesis of 2D Metal Chalcogenide Thin Films through the Process Involving Solution-Phase Deposition

2D metal chalcogenide thin films have recently attracted considerable attention owing to their unique physicochemical properties and great potential in a variety of applications. Synthesis of large-area 2D metal chalcogenide thin films in controllable ways remains a key challenge in this research field. Recently, the solution-based synthesis of 2D metal chalcogenide thin films has emerged as an alternative approach to vacuum-based synthesis because it is relatively simple and easy to scale up for high-throughput production. In addition, solution-based thin films open new opportunities that cannot be achieved from vacuum-based thin films. Here, a comprehensive summary regarding the basic structures and properties of different types of 2D metal chalcogenides, the mechanistic details of the chemical reactions in the synthesis of the metal chalcogenide thin films, recent successes in the synthesis by different reaction approaches, and the applications and potential uses is provided. In the last perspective section, the technical challenges to be overcome and the future research directions in the solution-based synthesis of 2D metal chalcogenides are discussed.

Nanocomposites based on tubular and onion nanostructures of molybdenum and tungsten disulfides: inorganic design, functional properties and applications

The review concerns the development and the state-of-the-art in studies on the surface modification methods aimed at fabricating promising nanocomposites based on multilayer inorganic tubular and onion (fullerene-like) MoS2 and WS2 nanostructures. The synthetic details and structural features of these materials are considered. Considerable attention is paid to targeted functionalization of molybdenum and tungsten disulfide nanostructures and to fundamental principles that underlie their ability to chemical interactions. The functional properties and applications of the obtained materials are described.

The bibliography includes 183 references.

Optically Active 1D MoS2 Nanobelts

Transition metal dichalcogenides can be synthesized in a wide range of structures. 1D geometries, including nanotubes and nanowires, are especially intriguing due to enhanced light-matter interactions stemming from both the thickness and width possessing subwavelength dimensions. In this letter, we demonstrate the synthesis of 1D MoS₂ nanobelts through chemical vapor deposition and examine the mechanism driving the formation of this material. We also report enhanced light scattering within these structures. Finally, we investigate the phototransistor behavior of MoS₂ nanobelts and observed a photoresponsivity around 1.5 A/W, an order of magnitude greater than analogous multilayer 2D MoS₂ sheets reported previously.

Photoluminescence nonuniformity from self-seeding nuclei in CVD-grown monolayer MoSe2

We present optical spectroscopy (photoluminescence and Raman spectrum) studies of monolayer transition metal dichalcogenide MoSe₂, with spatial location, temperature and excitation power dependence. The investigated spectra show location-dependent behavior with an increase in photoluminescence and Raman intensity and a blue-shift in photoluminescence peak position in the inner region. The observed behaviors of a large shift in the photoluminescence peak position at the edge and biexciton emissions in the inner region confirm that the monolayer MoSe₂ crystals grow from nucleation centers during the CVD process. Temperature activated energy and dependence of the peak position are attributed to residual oxygen during the growth. Investigating this information provides a basis for precisely controlling the synthesis of TMDCs and their application in advanced optoelectronics.

Template conversion of MoO3 to MoS2 nanoribbons: synthesis and electrochemical properties

Electrochromic α-MoO₃ nanoribbons were prepared by a hydrothermal method and converted to MoS₂ with the same morphology.

Phosphine-free synthesis and shape evolution of MoSe2 nanoflowers for electrocatalytic hydrogen evolution reactions

Free-standing colloidal MoSe₂ nanoflowers were synthesized by a phosphine-free solution-processing approach, which showed good electrocatalytic activities.

How the Toughest Inorganic Fullerene Cages Absorb Shockwave Pressures in a Protective Nanocomposite: Experimental Evidence from Two In Situ Investigations

Nanocomposites fabricated using the toughest caged inorganic fullerene WS₂ (IF-WS₂) nanoparticles could offer ultimate protection via absorbing shockwaves; however, if the IF-WS₂ nanomaterials really work, how they behave and what they experience within the nanocomposites at the right moment of impact have never been investigated effectively, due to the limitations of existing investigation techniques that are unable to elucidate the true characteristics of high-speed impacts in composites. We first fabricated Al matrix model nanocomposites and then unlocked the exact roles of IF-WS₂ in it at the exact moment of impact, at a time resolution that has never been attempted before, using two in situ techniques. We find that the presence of IF-WS₂ reduced the impact velocity by over 100 m/s and in pressure by at least 2 GPa against those Al and hexagonal WS₂ platelet composites at an impact speed of 1000 m/s. The IF-WS₂ composites achieved an intriguing inelastic impact and outperformed other reference composites, all originating from the "balloon effect" by absorbing the shockwave pressures. This study not only provides fundamental understanding for the dynamic performance of composites but also benefits the development of protective nanocomposite engineering.

Hydrogen Chemical Configuration and Thermal Stability in Tungsten Disulfide Nanoparticles Exposed to Hydrogen Plasma

The chemical configuration and interaction mechanism of hydrogen adsorbed in inorganic nanoparticles of WS₂ are investigated. Our recent approaches of using hydrogen activated by either microwave or radiofrequency plasma dramatically increased the efficiency of its adsorption on the nanoparticles surface. In the current work we make an emphasis on elucidation of the chemical configuration of the adsorbed hydrogen. This configuration is of primary importance as it affects its adsorption stability and possibility of release. To get insight on the chemical configuration, we combined the experimental analysis methods with theoretical modeling based on the density functional theory (DFT). Micro-Raman spectroscopy was used as a primary tool to elucidate chemical bonding of hydrogen and to distinguish between chemi- and physisorption. Hydrogen adsorbed in molecular form (H₂) was clearly identified in all the plasma-hydrogenated WS₂ nanoparticles samples. It was shown that the adsorbed hydrogen is generally stable under high vacuum conditions at room temperature, which implies its stability at the ambient atmosphere. A DFT model was developed to simulate the adsorption of hydrogen in the WS₂ nanoparticles. This model considers various adsorption sites and identifies the preferential locations of the adsorbed hydrogen in several WS₂ structures, demonstrating good concordance between theory and experiment and providing tools for optimizing of hydrogen exposure conditions and the type of substrate materials.

Synthesis of Ultrathin WS2 Nanosheets and Their Tribological Properties as Lubricant Additives

In this paper, ultrathin WS₂ nanosheets with thickness of about 5 nm were successfully prepared by a facile solid phase reaction method. The as-synthesized samples were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). On the basis of experimental results obtained under different reaction durations, a possible formation mechanism of WS₂ nanosheets is proposed. The tribological performance of ultrathin WS₂ nanosheets as additives in the 500SN base oil was tested by an UMT-2 ball-on-disc tribotester, and the worn surface of the steel disc was investigated by a non-contact optical profile testing instrument and SEM. The results showed that the friction coefficient and anti-wear property of base oil can be improved strikingly by adding ultrathin WS₂ nanosheets. Especially, when the concentration of WS₂ nanosheets was 1.0 wt.%, the corresponding lubricating oil exhibited the best tribological properties. Moreover, according to the investigation of the wear scar, an anti-friction and anti-wear mechanism is proposed. It is believed that the reduction of friction and wear must come from the addition of ultrathin WS₂ nanosheets which can penetrate and enter the friction interface and form a continuous tribofilm on the rubbing face.

Growth Mechanism of Transition Metal Dichalcogenide Monolayers: The Role of Self-Seeding Fullerene Nuclei

Due to their unique optoelectronic properties and potential for next generation devices, monolayer transition metal dichalcogenides (TMDs) have attracted a great deal of interest since the first observation of monolayer MoS2 a few years ago. While initially isolated in monolayer form by mechanical exfoliation, the field has evolved to more sophisticated methods capable of direct growth of large-area monolayer TMDs. Chemical vapor deposition (CVD) is the technique used most prominently throughout the literature and is based on the sulfurization of transition metal oxide precursors. CVD-grown monolayers exhibit excellent quality, and this process is widely used in studies ranging from the fundamental to the applied. However, little is known about the specifics of the nucleation and growth mechanisms occurring during the CVD process. In this study, we have investigated the nucleation centers or "seeds" from which monolayer TMDs typically grow. This was accomplished using aberration-corrected scanning transmission electron microscopy to analyze the structure and composition of the nuclei present in CVD-grown MoS2-MoSe2 alloys. We find that monolayer growth proceeds from nominally oxi-chalcogenide nanoparticles which act as heterogeneous nucleation sites for monolayer growth. The oxi-chalcogenide nanoparticles are typically encased in a fullerene-like shell made of the TMD. Using this information, we propose a step-by-step nucleation and growth mechanism for monolayer TMDs. Understanding this mechanism may pave the way for precise control over the synthesis of 2D materials, heterostructures, and related complexes.

WS2–3D graphene nano-architecture networks for high performance anode materials of lithium ion batteries

Tungsten disulfide nanoflakes grown on plasma activated three dimensional graphene networks. The work features a simple growth of TMDs-based LIBs anode materials that has excellent rate capability, high specific capacity and long cycling stability.

Thioether functionalised gallium and indium alkoxides in materials synthesis

Thermolysis of thioether functionalised metal alkoxides leads to the incorporation of sulphides in materials synthesized by CVD and hot-injection method.

Solar Synthesis of PbS-SnS2 Superstructure Nanoparticles

We report the synthesis and supporting density-functional-theory computations for a closed-cage, misfit layered-compound superstructure from PbS-SnS2, generated by highly concentrated sunlight from a precursor mixture of Pb, SnS2, and graphite. The unique reactor conditions created in our solar furnace are found to be particularly conducive to the formation of these nanomaterials. Detailed structural and chemical characterization revealed a spontaneous inside-out formation mechanism, with a broad range of nonhollow fullerene-like structures starting at a diameter of ∼20 nm and a wall thickness of ∼5 layers. The computations also reveal a counterintuitive charge transfer pathway from the SnS2 layers to the PbS layers, which indicates that, in contrast to binary-layered compounds where it is principally van der Waals forces that hold the layers together, polar forces appear to be as important in stabilizing superstructures of misfit layered compounds.

Synthesis and properties of molybdenum disulphide: from bulk to atomic layers

This review paper aims at a general introduction on up-to-date studies on the synthesis and properties of two-dimensional atomic layers of molybdenum disulphide, along with a brief overview on the bulk and three polytypes.

The sequential continuous-flow hydrothermal synthesis of molybdenum disulphide

MoS₂ has been prepared by the sequential formation of thiomolybdates, conversion to MoS₃ and decomposition to MoS₂ in a continuous-flow hydrothermal process.

Biocompatibility of tungsten disulfide inorganic nanotubes and fullerene-like nanoparticles with salivary gland cells

Impaired salivary gland (SG) function leading to oral diseases is relatively common with no adequate solution. Previously, tissue engineering of SG had been proposed to overcome this morbidity, however, not yet clinically available. Multiwall inorganic (tungsten disulfide [WS2]) nanotubes (INT-WS2) and fullerene-like nanoparticles (IF-WS2) have many potential medical applications. A yet unexplored venue application is their interaction with SG, and therefore, our aim was to test the biocompatibility of INT/IF-WS2 with the A5 and rat submandibular cells (RSC) SG cells. The cells were cultured and subjected after 1 day to different concentrations of INT-WS2 and were compared to control groups. Growth curves, trypan blue viability test, and carboxyfluorescein succinimidyl ester (CFSE) proliferation assay were obtained. Furthermore, cells morphology and interaction with the nanoparticles were observed by light microscopy, scanning electron microscopy and transmission electron microscopy (TEM), and energy dispersive X-ray spectroscopy. The results showed no significant differences in growth curves, proliferation kinetics, and viability between the groups compared. Moreover, no alterations were observed in the cell morphology. Interestingly, TEM images indicated that the nanoparticles are uptaken by the cells and accumulate in cytoplasmic vesicles. These results suggest promising future medical applications for these nanoparticles.

One-step gas-solid reaction synthesis of W@WS2 nanorattles and their novel catalytic activity

W@WS2 nanorattles were synthesised by heating a mixture of WO3 nanoparticles and S at a relatively low temperature between 750 and 950 °C in H2/Ar. In addition to the temperature, the competition between the H2 reduction and the S sulphidation sub-reactions dominated the reaction mechanisms and the morphologies of final products. Based on this, several types of nanostructures, including WS2 nanoflakes, inorganic fullerene-like nanoparticles and W@WS2 nanorattles (with desirable core size and shell thickness), could be selectively prepared by simply tailoring the processing parameters. Moreover, it was found for the first time that as-prepared W@WS2 nanorattles exhibited excellent catalytic activities which were close to or even better than their much more costive Au-based counterparts.

Nanoparticle conversion chemistry: Kirkendall effect, galvanic exchange, and anion exchange

Conversion chemistry is a rapidly maturing field, where chemical conversion of template nanoparticles (NPs) into new compositions is often accompanied by morphological changes, such as void formation. The principles and examples of three major classes of conversion chemical reactions are reviewed: the Kirkendall effect for metal NPs, galvanic exchange, and anion exchange, each of which can result in void formation in NPs. These reactions can be used to obtain complex structures that may not be attainable by other methods. During each kind of conversion chemical reaction, NPs undergo distinct chemical and morphological changes, and insights into the mechanisms of these reactions will allow for improved fine control and prediction of the structures of intermediates and products. Conversion of metal NPs into oxides, phosphides, sulphides, and selenides often occurs through the Kirkendall effect, where outward diffusion of metal atoms from the core is faster than inward diffusion of reactive species, resulting in void formation. In galvanic exchange reactions, metal NPs react with noble metal salts, where a redox reaction favours reduction and deposition of the noble metal (alloying) and oxidation and dissolution of the template metal (dealloying). In anion exchange reactions, addition of certain kinds of anions to solutions containing metal compound NPs drives anion exchange, which often results in significant morphological changes due to the large size of anions compared to cations. Conversion chemistry thus allows for the formation of NPs with complex compositions and structures, for which numerous applications are anticipated arising from their novel catalytic, electronic, optical, magnetic, and electrochemical properties.

Component-controllable WS(2(1-x))Se(2x) nanotubes for efficient hydrogen evolution reaction

Owing to the excellent potential for fundamental research and technical applications in optoelectronic devices and catalytic activity for hydrogen evolution reaction (HER), transition metal dichalcogenides have recently attracted much attention. Transition metal sulfide nanostructures have been reported and demonstrated promising application in transistors and photodetectors. However, the growth of transition metal selenide nanostructures and their applications has still been a challenge. In this work, we successfully synthesized high-quality WSe2 nanotubes on carbon fibers via selenization. More importantly, through optimizing the growth conditions, ternary WS2(1–x)Se2x nanotubes were synthesized and the composition of S and Se can be systematically controlled. The as-grown WS2(1–x)Se2x nanotubes on carbon fibers, assembled as a working electrode, revealing low overpotential, high exchange current density, and small series resistance, exhibit excellent electrocatalytic properties for hydrogen evolution reaction. Our study provides the experimental groundwork for the synthesis of low-dimensional transition metal dichalcogenides and may open up exciting opportunities for their application in electronics, photoelectronics, and catalytic electrochemical reactions.