Effect of Titanium and Molybdenum Cover on the Surface Restructuration of Diamond Single Crystal during Annealing.

Diamond is an important material for electrical and electronic devices. Because the diamond is in contact with the metal in these applications, it becomes necessary to study the metal-diamond interaction and the structure of the interface, in particular, at elevated temperatures. In this work, we study the interaction of the (100) and (111) surfaces of a synthetic diamond single crystal with spattered titanium and molybdenum films. Atomic force microscopy reveals a uniform coating of titanium and the formation of flattened molybdenum nanoparticles. A thin titanium film is completely oxidized upon contact with air and passes from the oxidized state to the carbide state upon annealing in an ultrahigh vacuum at 800 °C. Molybdenum interacts with the (111) diamond surface already at 500 °C, which leads to the carbidization of its nanoparticles and catalytic graphitization of the diamond surface. This process is much slower on the (100) diamond surface; sp2-hybridized carbon is formed on the diamond and the top of molybdenum carbide nanoparticles, only when the annealing temperature is raised to 800 °C. The conductivity of the resulting sample is improved when compared to the Ti-coated diamond substrates and the Mo-coated (111) substrate annealed at 800 °C. The presented results could be useful for the development of graphene-on-diamond electronics.

Surface and Structural Characterization of PVTMS Films Treated by Elemental Fluorine in Liquid Perfluorodecalin.

Poly(vinyl trimethylsilane) (PVTMS) films were subjected to direct surface fluorination in liquid medium (perfluorodecalin). The samples were investigated using several techniques: SEM-XEDS, XPS, ATR-IR, and contact angle measurement. The methods used allowed us to estimate chemical changes occurring because of the treatment. ATR-IR showed that most of the changes occurred in the Si(CH3)3 group. Monofluorinated Si(CH3)3 groups formed in the near-surface layer (Ge crystal, 0.66 µm penetration) after 30 min of fluorination, and then di- and trifluorinated groups appeared. Oxidation of the film with oxygen was also shown with the use of ZnSe crystal (2 µm penetration). The XPS method allowed an assessment of the ratio of the main elements at the surface of the fluorinated film. Two different exponential models were proposed to fit the experimental data of SEM-XEDS. Based on the model with the intercept, the depth of fluorination was estimated to be ≤1.1 µm, which is consistent with the result from the literature for the gas-phase fluorination. Contact angle measurements showed that oxidation of the PVTMS surface prevailed for the first 45 min of fluorination (surface hydrophilization) with a subsequent fluorine content increase and hydrophobization of the surface upon 60 min of fluorination.

Highly Dispersed Ni on Nitrogen-Doped Carbon for Stable and Selective Hydrogen Generation from Gaseous Formic Acid.

Ni supported on N-doped carbon is rarely studied in traditional catalytic reactions. To fill this gap, we compared the structure of 1 and 6 wt% Ni species on porous N-free and N-doped carbon and their efficiency in hydrogen generation from gaseous formic acid. On the N-free carbon support, Ni formed nanoparticles with a mean size of 3.2 nm. N-doped carbon support contained Ni single-atoms stabilized by four pyridinic N atoms (N4-site) and sub-nanosized Ni clusters. Density functional theory calculations confirmed the clustering of Ni when the N4-sites were fully occupied. Kinetic studies revealed the same specific Ni mass-based reaction rate for single-atoms and clusters. The N-doped catalyst with 6 wt% of Ni showed higher selectivity in hydrogen production and did not lose activity as compared to the N-free 6 wt% Ni catalyst. The presented results can be used to develop stable Ni catalysts supported on N-doped carbon for various reactions.

In-Situ Electrochemical Exfoliation and Methylation of Black Phosphorus into Functionalized Phosphorene Nanosheets.

Two-dimensional black phosphorus (BP) has attracted great attention as a perspective material for various applications. The chemical functionalization of BP is an important pathway for the preparation of materials with improved stability and enhanced intrinsic electronic properties. Currently, most of the methods for BP functionalization with organic substrates require either the use of low-stable precursors of highly reactive intermediates or the use of difficult-to-manufacture and flammable BP intercalates. Herein we report a facile route for simultaneous electrochemical exfoliation and methylation of BP. Conducting the cathodic exfoliation of BP in the presence of iodomethane makes it possible to generate highly active methyl radicals, which readily react with the electrode's surface yielding the functionalized material. The covalent functionalization of BP nanosheets with the P-C bond formation has been proven by various microscopic and spectroscopic methods. The functionalization degree estimated by solid-state 31P NMR spectroscopy analysis reached 9.7%.

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.

Nano TiO2 and Molybdenum/Tungsten Iodide Octahedral Clusters: Synergism in UV/Visible-Light Driven Degradation of Organic Pollutants.

Emissions of various organic pollutants in the environment becomes a more and more acute problem in the modern world as they can lead to an ecological disaster in foreseeable future. The current situation forces scientists to develop numerous methods for the treatment of polluted water. Among these methods, advanced photocatalytic oxidation is a promising approach for removing organic pollutants from wastewater. In this work, one of the most common photocatalysts-titanium dioxide-was obtained by direct aqueous hydrolysis of titanium (IV) isopropoxide and impregnated with aqueous solutions of octahedral cluster complexes [{M6I8}(DMSO)6](NO3)4 (M = Mo, W) to overcome visible light absorption issues and increase overall photocatalytic activity. XRPD analysis showed that the titania is formed as anatase-brookite mixed-phase nanoparticles and cluster impregnation does not affect the morphology of the particles. Complex deposition resulted in the expansion of the absorption up to ~500 nm and in the appearance of an additional cluster-related band gap value of 1.8 eV. Both types of materials showed high activity in the photocatalytic decomposition of RhB under UV- and sunlight irradiation with effective rate constants 4-5 times higher than those of pure TiO2. The stability of the catalysts is preserved for up to 5 cycles of photodegradation. Scavengers' experiments revealed high impact of all of the active species in photocatalytic process indicating the formation of an S-scheme heterojunction photocatalyst.

Hydrophobic-Hydrophilic Properties and Characterization of PIM-1 Films Treated by Elemental Fluorine in Liquid Perfluorodecalin.

A direct fluorination technique was applied for the surface treatment of PIM-1 films in a liquid phase (perfluorodecalin). The fluorinated samples were analyzed by various instrumental techniques. ATR-IR spectroscopy showed that the fluorination predominantly takes place in methylene- and methyl-groups. Cyano-groups, aromatic hydrogens and the aromatic structure of the PIM-1 repeat unit were shown to be relatively stable at the fluorination conditions. XPS confirmed that the concentration of fluorine, as well as oxygen, in the near surface layer (~1 nm) increases with fluorination time. C1s and O1s surface spectra of the fluorinated PIM-1 samples indicated an appearance of newly-formed C-F and C-O functional groups. Scanning electron microscopy and X-ray energy-dispersive spectroscopy of the fluorinated PIM-1 samples showed an increase of the fluorine concentration at the surface (~0.1-1 µm) with the treatment duration. Analysis of the slices of the PIM-1 films demonstrated a decline of the fluorine content within several microns of the film depth. The decline increased with the fluorination time. A model of fluorine concentration dependence on the film depth and treatment duration was suggested. A change in the specific free surface energy as a result of PIM-1 fluorination was revealed. The fluorination time was shown to affect the surface energy (γSV), providing its shift from a low value (25 mJ∙m-2), corresponding to tetrafluoroethylene, up to a relatively high value, corresponding to a hydrophilic surface.

Optical Properties of Tricarboxylic Acid-Derived Carbon Dots.

Herein, we report the characterization of two types of luminescent carbon dots (CDs) synthesized by the hydrothermal treatment of citric acid and trans-aconitic acid by using ammonia solution as a nitrogen dopant. The lateral size range of nanoparticles for CDs lies in the range of 3-15 nm. The intense blue photoluminescence (PL) was emitted by the CDs at around 409-435 nm under the excitation of 320 nm. The PL quantum yield of the synthesized CDs ranged from 26.4 to 51%. Our results of the structural and optical properties of CDs imply that molecular fluorophores are an important part of the structure; in particular, the main contribution to the PL is carried by the fluorophores based on citrazinic acid derivatives, which formed during the synthesis of CDs.

Photolysis of Fluorinated Graphites with Embedded Acetonitrile Using a White-Beam Synchrotron Radiation.

Fluorinated graphitic layers with good mechanical and chemical stability, polar C-F bonds, and tunable bandgap are attractive for a variety of applications. In this work, we investigated the photolysis of fluorinated graphites with interlayer embedded acetonitrile, which is the simplest representative of the acetonitrile-containing photosensitizing family. The samples were continuously illuminated in situ with high-brightness non-monochromatized synchrotron radiation. Changes in the compositions of the samples were monitored using X-ray photoelectron spectroscopy and near-edge X-ray absorption fine structure (NEXAFS) spectroscopy. The NEXAFS N K-edge spectra showed that acetonitrile dissociates to form HCN and N2 molecules after exposure to the white beam for 2 s, and the latter molecules completely disappear after exposure for 200 s. The original composition of fluorinated matrices CF0.3 and CF0.5 is changed to CF0.10 and GF0.17, respectively. The highly fluorinated layers lose fluorine atoms together with carbon neighbors, creating atomic vacancies. The edges of vacancies are terminated with the nitrogen atoms and form pyridinic and pyrrolic units. Our in situ studies show that the photolysis products of acetonitrile depend on the photon irradiation duration and composition of the initial CFx matrix. The obtained results evaluate the radiation damage of the acetonitrile-intercalated fluorinated graphites and the opportunities to synthesize nitrogen-doped graphene materials.

Single-Walled Carbon Nanotubes with Red Phosphorus in Lithium-Ion Batteries: Effect of Surface and Encapsulated Phosphorus.

Single-walled carbon nanotubes (SWCNTs) with their high surface area, electrical conductivity, mechanical strength and elasticity are an ideal component for the development of composite electrode materials for batteries. Red phosphorus has a very high theoretical capacity with respect to lithium, but has poor conductivity and expends considerably as a result of the reaction with lithium ions. In this work, we compare the electrochemical performance of commercial SWCNTs with red phosphorus deposited on the outer surface of nanotubes and/or encapsulated in internal channels of nanotubes in lithium-ion batteries. External phosphorus, condensed from vapors, is easily oxidized upon contact with the environment and only the un-oxidized phosphorus cores participate in electrochemical reactions. The support of the SWCNT network ensures a stable long-term cycling for these phosphorus particles. The tubular space inside the SWCNTs stimulate the formation of chain phosphorus structures. The chains reversibly interact with lithium ions and provide a specific capacity of 1545 mAh·g-1 (calculated on the mass of phosphorus in the sample) at a current density of 0.1 A·g-1. As compared to the sample containing external phosphorus, SWCNTs with encapsulated phosphorus demonstrate higher reaction rates and a slight loss of initial capacity (~7%) on the 1000th cycle at 5 A·g-1.

Enhancement of Volumetric Capacitance of Binder-Free Single-Walled Carbon Nanotube Film via Fluorination.

Robust electrode materials without the addition of binders allow increasing efficiency of electrical storage devices. We demonstrate the fabrication of binder-free electrodes from modified single-walled carbon nanotubes (SWCNTs) for electrochemical double-layer capacitors (EDLCs). Modification of SWCNTs included a sonication in 1,2-dichlorobenzene and/or fluorination with gaseous BrF3 at room temperature. The sonication caused the shortening of SWCNTs and the splitting of their bundles. As a result, the film prepared from such SWCNTs had a higher density and attached a larger amount of fluorine as compared to the film from non-sonicated SWCNTs. In EDLCs with 1M H2SO4 electrolyte, the fluorinated films were gradually defluorinated, which lead to an increase of the specific capacitance by 2.5-4 times in comparison with the initial values. Although the highest gravimetric capacitance (29 F g-1 at 100 mV s-1) was observed for the binder-free film from non-modified SWCNT, the fluorinated film from the sonicated SWCNTs had an enhanced volumetric capacitance (44 F cm-3 at 100 mV s-1). Initial SWCNT films and defluorinated films showed stable work in EDLCs during several thousand cycles.

Effect of Toluene Addition in an Electric Arc on Morphology, Surface Modification, and Oxidation Behavior of Carbon Nanohorns and Their Sedimentation in Water.

Carbon nanohorns (CNHs) are attractive for various applications, where a high specific surface area and long dispersion stability in water are important. In the present work, we study these parameters of CNHs prepared by arc evaporation of graphite depending on the conditions of the synthesis and subsequent oxidation in air. It is shown that the addition of toluene in the reactor during the arcing allows obtaining CNHs functionalized with -CHx groups. Heating of CNHs in air at 400 °C leads to substitution of -CHx groups for oxygen-containing groups. Moreover, the CNH endcaps are opened at 500 °C, and as a result, the specific surface area of CNHs increases 4 times. Aqueous suspensions with a concentration of oxidized CNHs of 100 µg/mL are stable for 8 months.

Insight into the thermal decomposition of ammonium hexahalogenoiridates(IV) and hexachloroiridate(III).

Thermal decomposition of (NH4)3[IrCl6]·H2O, (NH4)2[IrCl6] and (NH4)2[IrBr6] in reductive and inert atmospheres has been investigated in situ using quick-EXAFS and temperature-resolved powder X-ray diffraction. For the first time, (NH4)2[Ir(NH3)Cl5] and (NH4)2[Ir(NH3)Br5] have been proven as intermediates of thermal decomposition of (NH4)3[IrCl6]·H2O, (NH4)2[IrCl6] and (NH4)2[IrBr6]. Thermal degradation of (NH4)2[IrCl6] and (NH4)2[IrBr6] is a more complex process as suggested previously and includes simultaneous formation of (NH4)2[Ir(NH3)Cl5] and (NH4)2[Ir(NH3)Br5] intermediates mixed with metallic iridium. In the inert atmosphere, complexes (NH4)[Ir(NH3)2Cl4] and (NH4)[Ir(NH3)2Br4] as well as [Ir(NH3)3Br3] were proposed as possible intermediates before formation of metallic iridium particles.

Chemiresistive Properties of Imprinted Fluorinated Graphene Films.

The electrical conductivity of graphene materials is strongly sensitive to the surface adsorbates, which makes them an excellent platform for the development of gas sensor devices. Functionalization of the surface of graphene opens up the possibility of adjusting the sensor to a target molecule. Here, we investigated the sensor properties of fluorinated graphene films towards exposure to low concentrations of nitrogen dioxide NO2. The films were produced by liquid-phase exfoliation of fluorinated graphite samples with a composition of CF0.08, CF0.23, and CF0.33. Fluorination of graphite using a BrF3/Br2 mixture at room temperature resulted in the covalent attachment of fluorine to basal carbon atoms, which was confirmed by X-ray photoelectron and Raman spectroscopies. Depending on the fluorination degree, the graphite powders had a different dispersion ability in toluene, which affected an average lateral size and thickness of the flakes. The films obtained from fluorinated graphite CF0.33 showed the highest relative response ca. 43% towards 100 ppm NO2 and the best recovery ca. 37% at room temperature.

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.

Bimetallic Pt,Ir-containing coatings formed by MOCVD for medical applications.

Biocompatible PtxIr(1-x) layers combining high mechanical strength of the iridium component and outstanding corrosion resistance of the platinum component providing reversible charge transfer reactions in the living tissue are one of the important materials required for implantable medical electrodes. The modern trend to complicate the shape and reduce the electrode dimensions includes the challenge to develop precise methods to obtain such bimetallic coatings with enhanced surface area and advanced electrochemical characteristics. Herein, PtxIr(1-x) coatings were firstly obtained on cathode and anode pole tips of endocardial electrodes for pacemakers using chemical vapor deposition technique. To deposit PtxIr(1-x) coatings with a wide range of metal ratios (x = 0.5-0.9) the combination of acetylacetonate-based volatile precursors with compatible thermal characteristics was used for the first time. The expected metal ratio in the coatings was regulated by a partial pressure of the precursor vapors in the reaction zone and was in the good agreement with its real value measured by various methods, including energy-dispersive and wavelength dispersive spectroscopy, X-ray photoelectron spectroscopy. According to the X-ray powder diffraction analysis, PtxIr(1-x) coatings consisted of fcc-PtxIr(1-x) solid solution phases. The microscopy data confirmed the formation of PtxIr1-x coatings with the enhanced surface areas. The effect of electrochemical activation on the surface composition and morphology of the samples was studied. The electrochemical characteristics of samples were estimated from cyclic voltammetry and electrochemical impedance spectroscopy data. The charge storage capacity (CSC) values of activated samples were in the range of 19-108 mCcm-2 (phosphate buffer saline solution, 100 mV/s).

From oxide to a new type of molecular tungsten compound: formation of bitetrahedral cluster complexes [{W6(µ4-O)2(µ3-CCN)4}(CN)16]10- and [{W6(µ4-O)2(µ3-As)4}(CN)16]10.

Tungsten trioxide has been found to be a convenient precursor for the synthesis of metal cluster compounds with new types of cluster cores. The reaction between WO3 and KCN led to the formation of the cluster complex [{W6(µ4-O)2(µ3-CCN)4}(CN)16]10-. Unexpectedly, it includes the fully deprotonated form of acetonitrile, the CCN3- anion, as a µ3-bridging ligand coordinated to the trigonal faces of the bitetrahedral W6 metallocluster. A similar complex [{W6(µ4-O)2(µ3-As)4}(CN)16]10- containing µ3-As3- ligands instead of µ3-CCN3- ones has been synthesized by the reaction between WO3, As and KCN.

XPS experimental and DFT investigations on solid solutions of Mo1-xRexS2 (0 < x < 0.20).

The synthesis, characterization, experimental X-ray photoelectron spectra (XPS) and density-functional theory (DFT) investigations on solid solutions of Mo1-xRexS2 (x = 0.05, 0.10, 0.15 and 0.20) are reported herein. It is shown that even at a low concentration of dopant Re atoms, clustering occurs. At an Re concentration of 5% the formation of dimer-like impregnations is observed. An increase in the dopant concentration leads to an increase in the amount of clustered rhenium atoms and to the formation of rhombic clusters. The absence of magnetism within the studied Mo1-xRexS2 solid solutions allowed us to suggest a mechanism for the distribution of rhenium inside molybdenum disulphide through the initial formation of rhenium disulphide and its subsequent spreading.

Anisotropy of chemical bonding in semifluorinated graphite C2F revealed with angle-resolved X-ray absorption spectroscopy.

Highly oriented pyrolytic graphite characterized by a low misorientation of crystallites is fluorinated using a gaseous mixture of BrF(3) with Br(2) at room temperature. The golden-colored product, easily delaminating into micrometer-size transparent flakes, is an intercalation compound where Br(2) molecules are hosted between fluorinated graphene layers of approximate C(2)F composition. To unravel the chemical bonding in semifluorinated graphite, we apply angle-resolved near-edge X-ray absorption fine structure (NEXAFS) spectroscopy and quantum-chemical modeling. The strong angular dependence of the CK and FK edge NEXAFS spectra on the incident radiation indicates that room-temperature-produced graphite fluoride is a highly anisotropic material, where half of the carbon atoms are covalently bonded with fluorine, while the rest of the carbon atoms preserve π electrons. Comparison of the experimental CK edge spectrum with theoretical spectra plotted for C(2)F models reveals that fluorine atoms are more likely to form chains. This conclusion agrees with the atomic force microscopy observation of a chain-like pattern on the surface of graphite fluoride layers.