Monodispersed tantalum disulfide and adsorption complexes with cations

Two-dimensional inorganic nanosheets: production and utility in the development of novel electrochemical (bio)sensors and gas-sensing applications

This review (with 178 references) focuses on inorganic layered materials (ILMs) and the use of their two-dimensional nanosheets in the development of novel electrochemical (bio)sensors, analytical devices, and gas-phase sensing applications. The text is organized in three main sections including the presentation of the most important families of ILMs, a comprehensive outline of various "bottom-up", "top-down," and hydro(solvo)thermal methods that have been used for the production of ILM nanosheets, and finally an evaluative survey on their utility for the determination of analytes with interest in different sectors of contemporary analysis. Critical discussion on the effect of the production method on their electronic properties, the suitability of each nanomaterial in different sensing technologies along with an assessment of the performance of the (bio)sensors and devices that have been proposed within the last 5 years, is enclosed. The perspectives of further improving the utility of 2D inorganic nanosheets in sensing applications, in real-world samples, are also discussed.

TiS2 Monolayer as an Emerging Ultrathin Bifunctional Catalyst: Influence of Defects and Functionalization

We have envisaged the hydrogen evolution and oxygen evolution reactions (HER and OER) on a two-dimensional (2D) noble-metal-free titanium disulfide (TiS₂ ) monolayer, which belongs to the exciting family of transition metal dichalcogenides (TMDCs). Our theoretical investigation to probe the HER and OER on both the H and T phases of 2D TiS₂ is based on electronic-structure calculations witihin the framework of density functional theory (DFT). Since TiS₂ is the lightest compound among the group-IV TMDCs, it is worth exploring the catalytic activity of a TiS₂ monolayer through the functionalization at the anion (S) site, substituting with P, N, and C dopants as well as by incorporating single sulfur vacancy defects. We have investigated the effect of functionalization and vacancy defects on the structural, electronic, and optical response of a TiS₂ monolayer by determining the density of states, work-function, and optical absorption spectra. We have determined the HER and OER activities for the functionalized and defective TiS₂ monolayers based on the reaction coordinate, which can be constructed from the adsorption free energies of the intermediates (H*, O*, OH* and OOH*, where * denotes the adosrbed state) in the HER and OER mechanisms. Finally, we have shown that TiS₂ monolayers are emerging as a promising material for the HER and OER mechanisms under the influence of functionalization and defects.

Exfoliation of crystals

The review is concerned with progress in methods for exfoliation of crystals, from mechanical exfoliation using sticky tape to modern techniques involving sonication-assisted exfoliation, shear exfoliation in liquids using intercalating agents and stabilizers, direct liquid exfoliation and cosolvent exfoliation. The potential of methods of osmotic swelling in water and in organic dispersion media with constant and variable chemical composition of nanosheets, chemical and electrochemical intercalation, exfoliation by hydrazine (including versions resulting in changes in the chemical composition of nanosheets), ionic liquids and supercritical fluids is discussed. Methods for size sorting of nanosheets by density-gradient and cascade centrifugation and the possibility of nanosheet size control are analyzed.

The bibliography includes 136 references.

Preparation of 2D material dispersions and their applications

A comprehensive review on the exfoliation of layer materials into 2D materials, their assembly, and applications in electronics and energy.

Emerging Trends in Phosphorene Fabrication towards Next Generation Devices

The challenge of science and technology is to design and make materials that will dominate the future of our society. In this context, black phosphorus has emerged as a new, intriguing two-dimensional (2D) material, together with its monolayer, which is referred to as phosphorene. The exploration of this new 2D material demands various fabrication methods to achieve potential applications- this demand motivated this review. This article is aimed at supplementing the concrete understanding of existing phosphorene fabrication techniques, which forms the foundation for a variety of applications. Here, the major issue of the degradation encountered in realizing devices based on few-layered black phosphorus and phosphorene is reviewed. The prospects of phosphorene in future research are also described by discussing its significance and explaining ways to advance state-of-art of phosphorene-based devices. In addition, a detailed presentation on the demand for future studies to promote well-systemized fabrication methods towards large-area, high-yield and perfectly protected phosphorene for the development of reliable devices in optoelectronic applications and other areas is offered.

Production of Two-Dimensional Nanomaterials via Liquid-Based Direct Exfoliation

Tremendous efforts have been devoted to the synthesis and application of two-dimensional (2D) nanomaterials due to their extraordinary and unique properties in electronics, photonics, catalysis, etc., upon exfoliation from their bulk counterparts. One of the greatest challenges that scientists are confronted with is how to produce large quantities of 2D nanomaterials of high quality in a commercially viable way. This review summarizes the state-of-the-art of the production of 2D nanomaterials using liquid-based direct exfoliation (LBE), a very promising and highly scalable wet approach for synthesizing high quality 2D nanomaterials in mild conditions. LBE is a collection of methods that directly exfoliates bulk layered materials into thin flakes of 2D nanomaterials in liquid media without any, or with a minimum degree of, chemical reactions, so as to maintain the high crystallinity of 2D nanomaterials. Different synthetic methods are categorized in the following, in which material characteristics including dispersion concentration, flake thickness, flake size and some applications are discussed in detail. At the end, we provide an overview of the advantages and disadvantages of such synthetic methods of LBE and propose future perspectives.

Active edge sites in MoSe2 and WSe2 catalysts for the hydrogen evolution reaction: a density functional study

MoSe2 and WSe2 nanofilms and nanosheets have recently been shown to be active for electrochemical H2 evolution (HER). In this work, we used periodic density functional theory to investigate the origin of the catalytic activity on these materials. We determined the relevant structures of the Mo/W-edges and the Se-edges under HER conditions and their differential hydrogen adsorption free energies. The Mo-edge on MoSe2 and the Se-edge on both MoSe2 and WSe2 are found to be the predominantly active facets for these catalysts, with activity predicted to be comparable to or better than MoS2. On the other hand, the (0001) basal planes are found to be inert. We further explain the enhanced activity at the edges in terms of localized edge states, which provide insight into the trends in HER activity seen between the two catalysts. Our results thus suggest that an optimal catalyst design should maximize the exposure of edge sites. Comparisons are also made between the transition metal selenide catalysts and their sulfide counterparts in order to understand the consequences of having either Mo/W or Se/S atoms. It is found that linear scaling relations describe the S/Se binding onto the edge and the H binding onto the S/Se.

Synthesis and defect investigation of two-dimensional molybdenum disulfide atomic layers

CONSPECTUS: The unique physical properties of two-dimensional (2D) molybdenum disulfide (MoS2) and its promising applications in future optoelectronics have motivated an extensive study of its physical properties. However, a major limiting factor in investigation of 2D MoS2 is its large area and high quality preparation. The existence of various types of defects in MoS2 also makes the characterization of defect types and the understanding of their roles in the physical properties of this material of critical importance. In this Account, we review the progress in the development of synthetic approaches for preparation of 2D MoS2 and the understanding of the role of defects in its electronic and optical properties. We first examine our research efforts in understanding exfoliation, direct sulfurization, and chemical vapor deposition (CVD) of MoS2 monolayers as main approaches for preparation of such atomic layers. Recognizing that a natural consequence of the synthetic approaches is the addition of sources of defects, we initially focus on identifying these imperfections with intrinsic and extrinsic origins in CVD MoS2. We reveal the predominant types of point and grain boundary defects in the crystal structure of polycrystalline MoS2 using transmission electron microscopy (TEM) and understand how they modify the electronic band structure of this material using first-principles-calculations. Our observations and calculations reveal the main types of vacancy defects, substitutional defects, and dislocation cores at the grain boundaries (GBs) of MoS2. Since the sources of defects in two-dimensional atomic layers can, in principle, be controlled and studied with more precision compared with their bulk counterparts, understanding their roles in the physical properties of this material may provide opportunities for changing their properties. Therefore, we next examine the general electronic properties of single-crystalline 2D MoS2 and study the role of GBs in the electrical transport and photoluminescence properties of its polycrystalline counterparts. These results reveal the important role played by point defects and GBs in affecting charge carrier mobility and excitonic properties of these atomic layers. In addition to the intrinsic defects, growth process induced substrate impurities and strain induced band structure perturbations are revealed as major sources of disorder in CVD grown 2D MoS2. We further explore substrate defects for modification and control of electronic and optical properties of 2D MoS2 through interface engineering. Self-assembled monolayer based interface modification, as a versatile technique adaptable to different conventional and flexible substrates, is used to promote significant tunability in the key MoS2 field-effect device parameters. This approach provides a powerful tool for modification of native substrate defect characteristics and allows for a wide range of property modulations. Our results signify the role of intrinsic and extrinsic defects in the physical properties of MoS2 and unveil strategies that can utilize these characteristics.

Microstructure and properties of carbon nanosheet/copper composites processed by particle-assisted shear exfoliation

Graphite was exfoliated to CNSs with the help of copper particles by going through the narrow gaps between the stator and rotor of the stator–rotor mixer.

Scalable production of large quantities of defect-free few-layer graphene by shear exfoliation in liquids

To progress from the laboratory to commercial applications, it will be necessary to develop industrially scalable methods to produce large quantities of defect-free graphene. Here we show that high-shear mixing of graphite in suitable stabilizing liquids results in large-scale exfoliation to give dispersions of graphene nanosheets. X-ray photoelectron spectroscopy and Raman spectroscopy show the exfoliated flakes to be unoxidized and free of basal-plane defects. We have developed a simple model that shows exfoliation to occur once the local shear rate exceeds 10(4) s(-1). By fully characterizing the scaling behaviour of the graphene production rate, we show that exfoliation can be achieved in liquid volumes from hundreds of millilitres up to hundreds of litres and beyond. The graphene produced by this method performs well in applications from composites to conductive coatings. This method can be applied to exfoliate BN, MoS2 and a range of other layered crystals.

Optical properties of exfoliated MoS2 coaxial nanotubes - analogues of graphene

We report on the first exfoliation of MoS2 coaxial nanotubes. The single-layer flakes, as the result of exfoliation, represent the transition metal dichalcogenides' analogue of graphene. They show a very low degree of restacking in comparison with exfoliation of MoS2 plate-like crystals. MoS2 monolayers were investigated by means of electron and atomic force microscopies, showing their structure, and ultraviolet-visible spectrometry, revealing quantum confinement as the consequence of the nanoscale size in the z-direction.

The Art of the Possible: An Overview of Catalyst Specimen Preparation Techniques for TEM Studies

Obtaining useful microstructural information about catalysts requires appropriate procedures for preparing specimens for the transmission electron microscope. Unfortunately, most descriptions of catalyst specimen preparation are scattered throughout numerous journal articles or are unavailable. Traditional techniques for preparing heterogeneous catalyst powders include primarily dilute dispersion and ultramicrotomy. The advantages and disadvantages of these will be discussed in terms of information obtainable and possible artifacts. In addition, techniques for preparing layered materials, as well as some novel approaches and model systems, will be presented. With these, as with more traditional approaches, the best method for a specific material will be arrived at only through experimentation. Our aim is to describe a variety of possibilities for getting an already synthesized catalyst into the microscope suitably neat, thin, and artifactfree.

Layer-by-Layer Assembly and Spontaneous Flocculation of Oppositely Charged Oxide and Hydroxide Nanosheets into Inorganic Sandwich Layered Materials

Exfoliated oxide nanosheets such as Ti0.91O2 and Ca2Nb3O10 and layered double hydroxide (LDH) nanosheets of Mg2/3Al1/3(OH)2 were restacked into inorganic sandwich layered materials. Sequential adsorption of these oppositely charged nanosheets from their colloidal suspensions yielded multilayer ultrathin films while their simple mixing produced lamellar flocculates. Eliminating carbonate ions from the reaction system was found to be essential for successfully achieving the sandwich structures. The flocculated materials as well as the films were characterized by atomic force microscopy (AFM), UV-visible absorption spectroscopy, X-ray photoelectron spectroscopy, X-ray diffraction (XRD), transmission electron microscopy (TEM), and chemical analysis, which all supported the formation of the ordered sandwich structures. AFM observations revealed alternate dense tiling of LDH nanosheets and oxide nanosheets onto a substrate surface. UV-visible absorption spectra exhibited progressive enhancement of optical density due to oxide nanosheets as a function of deposition cycles, providing strong evidence for regular growth of multilayer films. The combinations of Mg2/3Al1/3(OH)2/Ti0.91O2 and Mg2/3Al1/3(OH)2/Ca2Nb3O10 produced XRD Bragg peaks having multilayer spacings of 1.2 and 2.0 nm, respectively. These basal spacing values are compatible with the sum of thickness of LDH nanosheets and corresponding oxide nanosheets. TEM images of flocculated samples displayed lamellar features with two different constituent layers appearing alternately.

Inclusion Systems of Organic Molecules in Restacked Single-Layer Molybdenum Disulfide

Novel materials have been obtained by restacking single-layer molybdenum disulfide (MoS(2)) with organic molecules included between the layers. A large variety of organic molecules can be included between layers of MoS(2) and other transition-metal dichalcogenides. The films with the included organics are formed at the interface between an aqueous suspension of the MoS(2) and a water-immiscible organic liquid. The organic molecules are not necessarily electron donors. A highly oriented, conducting film of restacked MoS(2) containing ferrocene is presented as an example.