Mariia N. Kozlova

Colloidal 2D nanosheets of MoS2 and other transition metal dichalcogenides through liquid-phase exfoliation

This review focuses on the exfoliation of transition metal dichalcogenides MQ2 (TMD, M=Mo, W, etc., Q=S, Se, Te) in liquid media, leading to the formation of 2D nanosheets dispersed in colloids. Nowadays, colloidal dispersions of MoS2, MoSe2, WS2 and other related materials are considered for a wide range of applications, including electronic and optoelectronic devices, energy storage and conversion, sensors for gases, catalysts and catalyst supports, biomedicine, etc. We address various methods developed so far for transferring these materials from bulk to nanoscale thickness, and discuss their stabilization and factors influencing it. Long-time known exfoliation through Li intercalation has received renewed attention in recent years, and is recognized as a method yielding highest dispersed concentrations of single-layer MoS2 and related materials. Latest trends in the intercalation/exfoliation approach include electrochemical lithium intercalation, experimenting with various intercalating agents, multi-step intercalation, etc. On the other hand, direct sonication in solvents is a much simpler technique that allows one to avoid dangerous reagents, long reaction times and purifying steps. The influence of the solvent characteristics on the colloid formation was closely investigated in numerous recent studies. Moreover, it is being recognized that, besides solvent properties, sonication parameters and solvent transformations may affect the process in a crucial way. The latest data on the interaction of MoS2 with solvents evidence that not only solution thermodynamics should be employed to understand the formation and stabilization of such colloids, but also general and organic chemistry. It appears that due to the sonolysis of the solvents and cutting of the MoS2 layers in various directions, the reactive edges of the colloidal nanosheets may bear various functionalities, which participate in their stabilization in the colloidal state. In most cases, direct exfoliation of MQ2 into colloidal nanosheets is conducted in organic solvents, while a small amount of works report low-concentrated colloids in pure water. To improve the dispersion abilities of transition metal dichalcogenides in water, various stabilizers are often introduced into the reaction media, and their interactions with nanosheets play an important role in the stabilization of the dispersions. Surfactants, polymers and biomolecules usually interact with transition metal dichalcogenide nanosheets through non-covalent mechanisms, similarly to the cases of graphene and carbon nanotubes. Finally, we survey covalent chemical modification of colloidal MQ2 nanosheets, a special and different approach, consisting in the functionalization of MQ2 surfaces with help of thiol chemistry, interaction with electrophiles, or formation of inorganic coordination complexes. The intentional design of surface chemistry of the nanosheets is a very promising way to control their solubility, compatibility with other moieties and incorporation into hybrid structures. Although the scope of the present review is limited to transition metal dichalcogenides, the dispersion in colloids of other chalcogenides (such as NbS3, VS4, Mo2S3, etc.) in many ways follows similar trends. We conclude the review by discussing current challenges in the area of exfoliation of MoS2 and its related materials.

Synthesis, Crystal Structure, and Colloidal Dispersions of Vanadium Tetrasulfide (VS4)

Although many of the layered metal chalcogenides, such as MoS2, are well-studied, some other chalcogenides have received less attention by comparison. In particular, there has been an emerging interest in vanadium tetrasulfide (VS4), which displays useful properties as a component of hybrids. However, the synthetic methods and characteristics of individual VS4 are not yet well defined, and there is no report on its solution processability. Here we have synthesized VS4 by a simple and fast direct reaction between elements. Reinvestigation of the VS4 crystal structure yielded more precise atomic coordinates and interatomic distances, thereby confirming the crystallization of VS4 in the monoclinic C2/c group and its quasi-1D chainlike structure. As the chains in VS4 are only bonded by weak van der Waals forces, we further demonstrate that bulk VS4 may be ultrasonically dispersed in appropriate solvents to form colloids, similarly to the layered chalcogenides. VS4 particles in colloids retain their phase identity and rod-shaped morphology with lengths in the range of hundreds of nanometers. Isopropanol dispersion exhibited the highest concentration and stability, which was achieved owing to the repulsion caused by high negative charges on the edges of the particles.

Anionic Redox Chemistry in Polysulfide Electrode Materials for Rechargeable Batteries

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 readers 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.

A DFT study and experimental evidence of the sonication-induced cleavage of molybdenum sulfide Mo2S3 in liquids

The Mo–S phase diagram exhibits two stable compounds: well-recognized layered molybdenum disulfide MoS2 and less studied molybdenum sesquisulfide Mo2S3. Here, we build a theoretical framework for the exfoliation of Mo2S3 based on density functional theory calculations and further experimentally prove its dispersibility under ultrasonic treatment and demonstrate its potential for use in gas detectors. First, we report computational studies of both Mo2S3 exfoliation and molecular adsorption on the Mo2S3 surface. The calculations reveal the preferential direction for the fragmentation of the Mo2S3 crystal along the (01) plane and, likely, chemisorptive interaction of solvent molecules, such as H2O or DMSO, with the (01) Mo2S3 surface. Next, we experimentally study the sonication of bulk Mo2S3 samples in organic solvents and show that, indeed, they may be converted to colloidal nanosized sheets. Solid particles in the dispersions retain the initial Mo2S3 crystal structure and have the shape of plates with typical thicknesses of 5–30 nm and lateral sizes of 100–400 nm. Finally, we investigate the gas sensing properties of the Mo2S3 films deposited from the dispersions towards moisture gas and several representative volatile organic compounds.

Metal-metal bond excitation in colloidal solution of NbS3.

For the first time the comparison of the theoretical and experimental data have shown that UV-vis absorption in the region of 600nm in colloidal solution of NbS3 can be described by the d-d electronic transitions to the antibonding molecular orbital. It is proved that the process leads to excitation of metal-metal bond.

Quantum-chemical study of quasi-one-dimensional vanadium and niobium sulfides with Peierls distortion

The electronic structure and lattice parameters of several quasi-one-dimensional compounds–known VS4, NbCl4, and NbS3 and hypothetical NbS4 and VS3–are obtained using density functional theory. Comparative analysis of chemical bonding and charge distribution in all these compounds reveals the possible origin of the structural instability of NbS4 and VS3.

Tungsten dichalcogenides as possible gas-sensing elements

Possible application of tungsten dichalcogenides as gas-sensing elements is discussed in this paper. The experimental results on sensitivity of pristine and niobium doped WS2 and WSe2 to acetone and ethanol gases are presented. Polycrystalline powder specimens were obtained by high temperature solid-state synthesis from the stoichiometric mixture of pure elements. Two types of samples were studied: 1) tablets pressed at 1.5 GPa to form bulk samples and 2) thin films prepared from 35% ethanol-water colloidal dispersions by their filtration onto membrane filters (pore diameter is 20 nm). The electrical resistances of the samples were shown to be increased in the presence of ethanol and acetone gases at room temperature, thereby revealing positive response to reducing gases.