Hexamolybdenum Clusters Supported on Exfoliated h-BN Nanosheets for Photocatalytic Water Purification.

Nowadays, the development of new effective photocatalytic materials for the purification of real wastewaters and model systems containing organic molecules constitutes an important challenge. Here we present a preparation strategy for composite materials based on hexamolybdenum cluster complexes and exfoliated hexagonal boron nitride (h-BN) nanosheets. Cluster deposition on the nanosheet surface was achieved by impregnation of the matrix by a (Bu4N)2[{Mo6I8}(NO3)6]/acetone solution. Successful cluster immobilization and chemical composition of the samples were verified by inductively coupled plasma atomic emission spectroscopy, transmission electron microscopy with elemental mapping (TEM/EDS), X-ray photoelectron spectroscopy (XPS), and optical diffuse-reflectance spectroscopy. A small amount of water in acetone initiates the hydrolysis of a molybdenum cluster precursor with labile NO3- ligands, which are absent in the final composite, according to the XPS data. Intermediate hydrolyzed cluster forms anchor to the surface of h-BN nanosheets and promote growth of the insoluble compound [{Mo6I8}(H2O)2(OH)4]·yH2O as the final hydrolysis product. TEM/EDS proves that the cluster exists at the nanosheet surface in the form of an X-ray diffraction amorphous thin film. The samples obtained show high photocatalytic activity in the degradation of a model pollutant rhodamine B under UV- and visible-light irradiation. The materials retain their initial photocatalytic efficacy during at least six cycles without the need for recovery.

Microwave exfoliation of organic-intercalated fluorographites.

Microwave (MW) irradiation is often used in preparation of nanomaterials. Here, we investigate MW exfoliation patterns of intercalated fluorinated graphite (C2F) depending on the nature of the "guest". The resulting highly exfoliated graphites (multi-layer graphenes) show advantageous characteristics, as compared to the products of convective heating.

Pt-Decorated Boron Nitride Nanosheets as Artificial Nanozyme for Detection of Dopamine.

Over the past decade, nanosized metal oxides, metals, and bimetallic particles have been actively researched as enzyme mimetic nanomaterials. However, the common issues with individual nanoparticles (NPs) are stabilization, reproducibility, and blocking of active sites by surfactants. These problems promote further studies of composite materials, where NPs are spread on supports, such as graphene derivatives or dichalcogenide nanosheets. Another promising type of support for NPs is the few-layered hexagonal boron nitride (hBN). In this study, we develop surfactant-free nanocomposites containing Pt NPs dispersed on chemically modified hydrophilic hBN nanosheets (hBNNSs). Ascorbic acid was used as a reducing agent for the chemical reduction of the Pt salt in the presence of hBNNS aqueous colloid, resulting in Pt/hBNNS nanocomposites, which were thoroughly characterized with X-ray diffraction, transmission electron microscopy, dynamic light scattering, and X-ray photoelectron and infrared spectroscopies. Similar to graphene oxide binding the metal NPs more efficiently than pure graphene, hydrophilic hBNNSs well stabilize Pt NPs, with particle size down to around 8 nm. We further demonstrate for the first time that Pt/hBNNS nanocomposites exhibit peroxidase-like catalytic activity, accelerating the oxidation of the classical colorless peroxidase substrate 3,3',5,5'-tetramethylbenzidine (TMB) to its corresponding blue-colored oxidized product in the presence of H2O2. Kinetic and mechanism studies involving terephthalic acid and isopropanol as a fluorescent probe and an •OH radical scavenger, respectively, proved that Pt/hBNNSs assist H2O2 decomposition to active oxygen species (•OH), which are responsible for TMB oxidation. The Pt/hBNNS nanocomposite-assisted oxidation of TMB provides an effective platform for the colorimetric detection of dopamine, an important biomolecule. The presence of increased amounts of dopamine gradually inhibits the catalytic activity of Pt/hBNNSs for the oxidation of TMB by H2O2, thus enabling selective sensing of dopamine down to 0.76 µM, even in the presence of common interfering molecules and on real blood serum samples. The present investigation on Pt/hBNNSs contributes to the knowledge of hBN-based nanocomposites and discovers their new usage as nanomaterials with good enzyme-mimicking activity and dopamine-sensing properties.

Theoretical and experimental comparative study of the stability and phase transformations of sesquichalcogenides M2Q3 (M = Nb, Mo; Q = S, Se).

The extensive family of transition metal chalcogenides has been comprehensively investigated owing to their diverse useful properties. However, even among them, there are ones that have received comparatively less attention; in particular, these are molybdenum and niobium sulfides and selenides with the composition of M : Q = 2 : 3 (M = Mo, Nb; Q = S, Se). Mo or Nb chalcogenides with this stoichiometry may adopt one of two structures: (i) sesquichalcogenides M2Q3, where important structural elements are infinite metal chains, or (ii) self-intercalated compounds M1.33Q2, in which extra M atoms are inserted between MQ2 layers. Depending on the M-Q combination, in practice, either none, one, or both of them may exist. The reasons for chemical dissimilarity in the series of seemingly related compounds haven't been addressed until the present work. Here, we present the first generalized comparative study of these chalcogenides by quantum-chemical computations verified by laboratory experiments. High-temperature phases of Mo2S3 and Nb2Se3 may be stably isolated at room temperature, while "Nb2S3" and "Mo2Se3" had not been obtained, nor were they expected to exist from DFT data. The structure-determining motifs of sesquichalcogenides M2Q3 are metallic chains, and thus, apparently, if metal's electron deficiency (or excess) prevents the formation of M-M chains, then the M2Q3-type structure cannot form. If the metal has an adequate electron density and the structure does form at high temperature (as it happens for Mo2S3 and Nb2Se3), then it can be kinetically stabilized by quenching, and stored under laboratory conditions for long times. However, if Nb2Se3 is left to cool down slowly, it undergoes phase transition to iso-stoichiometric intercalate Nb1.333Se2, in good agreement with DFT predictions of the close values of their free energies. Isostructural intercalate Nb1.333S2 is found to be the only experimental product in the Nb-S system, in full accordance with DFT prediction. Effective stabilization of self-intercalated phases is provided by significant charge transfer from intercalated Nb atoms to the NbQ2 layers, as confirmed by DFT. The obtained data may serve to get insight into polymorphism of some less-studied transition metal chalcogenides and to promote their use for future functional materials.

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 reader's attention to this new idea, especially, as it applies to transition-metal polychalcogenides, such as FeS2 , VS4 , TiS3 , NbS3 , TiS4 , MoS3 , etc., in which the key role is played by the (S-S)2- /2 S2- redox reaction. The exploration and better understanding of the anion-driven chemistry is important for designing advanced materials for battery and other energy-related applications.

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.

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.

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.

Synthesis, properties, and dispersion of few-layer graphene fluoride.

We have fluorinated few-layer graphene (FLG) by using a low-temperature fluorination route with gaseous ClF3. The treatment process resulted in a new graphene derivative with a finite approximate composition of C2F. TEM studies showed that the product consisted of thin transparent sheets with no more than 10 fluorographene layers stacked together. Spectroscopic methods revealed a predominantly covalent nature of the C-F bonds in the as-synthesized product and we found no evidence for the existence of so-called "semi-ionic" C-F bonds, as observed in bulk C(x)F. In contrast to the case of graphite and typical (thick) expanded graphites, fluorination of FLG did not lead to the intercalation of ClF3 molecules, owing to the lack of a 3D layered structure. The approximate "critical" number of graphene layers that were necessary to form a phase of intercalated compound was estimated to be more than 12, thus providing a "chemical proof" of the difference between the properties of few-layered graphenes and bulk graphites. Fluorographene C2F was successfully delaminated into thinner layers in organic solvents, which is an important property for its integration into electronic devices, nanohybrids, etc.

Photoluminescent nanographitic/nitrogen-doped graphitic hollow shells as a potential candidate for biological applications.

Water-dispersible graphitic hollow spheres were synthesized using a soft chemical route under hydrothermal conditions by glucose carbonization using a magnetite/silica-encapsulated core-shell sphere as a template. Carbonization on the templates happens as the magnetite core is partially or completely eliminated depending on the reaction conditions. Therefore, nano-sized graphitic hollow spheres or magnetite-core-encapsulated graphitic shells could be obtained. Also nitrogen-doped graphitic spheres were synthesized by a hydrothermal reaction. The graphitic and nitrogen-doped graphitic spheres show wavelength dependent photoluminescence in 300-600 nm range. The photoluminescence seems to depend on the fraction of the sp2 domains and N-doping, therefore, tunable PL emission can be achieved by controlling the nature of sp2 sites. In addition the cellular uptake of the graphitic hollow spheres was evaluated in human HeLa cells, demonstrating its main localization in the cytoplasm. A blue fluorescence signal was the most intensively observed in the cellular uptake process, although some green and red fluorescence was also observed. Since the cores of Fe3O4 could be completely or partly eliminated in a controllable way, it can be used as a magnetic resonance imaging agent. In addition, their easily modifiable hydrophilic surfaces for multi-functionality and hydrophobic voids covered by oxidized graphite make them promising candidates for applications in cellular photo-imaging and targeted drug delivery.

Functionalization and dispersion of hexagonal boron nitride (h-BN) nanosheets treated with inorganic reagents.

A mixture of bulk hexagonal boron nitride (h-BN) with hydrazine, 30% H(2)O(2), HNO(3)/H(2)SO(4), or oleum was heated in an autoclave at 100 °C to produce functionalized h-BN. The product formed stable colloid solutions in water (0.26-0.32 g L(-1)) and N,N-dimethylformamide (0.34-0.52 g L(-1)) upon mild ultrasonication. The yield of "soluble" h-BN reached about 70 wt%. The dispersions contained few-layered h-BN nanosheets with lateral dimensions in the order of several hundred nanometers. The functionalized dispersible h-BN was characterized by IR spectroscopy, X-ray photoelectron spectroscopy (XPS), Raman spectroscopy, UV/Vis spectroscopy, X-ray diffraction (XRD), dynamic light scattering (DLS), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and atomic force microscopy (AFM). It is shown that h-BN preserves its hexagonal structure throughout the functionalization procedure. Its exfoliation into thin platelets upon contact with solvents is probably owing to the attachment of hydrophilic functionalities.