Andrey A. Rempel

Preparation of disordered and ordered highly nonstoichiometric carbides and evaluation of their homogeneity

The features of solid-phase synthesis of highly nonstoichiometric carbides with controlled concentration of structural vacancies in the carbon sublattice have been considered. Four main stages are distinguished in the synthesis of nonstoichiometric carbides by the solid-phase sintering technique. It is shown that the inhomogeneity of highly nonstoichiometric compounds leads to the broadening of diffraction reflections. The analytical relation between the broadening and the inhomogeneity is obtained for the first time, and the method based on diffraction measurements is proposed for the quantitative evaluation of the degree of homogeneity of nonstoichiometric compounds. As an illustration of the advanced approach, the degree of homogeneity of nonstoichiometric niobium carbide is evaluated from the experimental data on the width of x-ray diffraction peaks. The heat treatment conditions providing the preparation of nonstoichiometric Group IV and V transition metal carbides in the disordered and ordered states are determined.

Non-periodicity in nanoparticles with close-packed structures.

Experimentally it is observed that nanomaterials from II-VI compounds like CdS have a high density of stacking faults. It is argued that these are not crystal defects but rather that they represent a characteristic feature of nanomaterials.

Aerobic oxidative C–H/C–H coupling of azaaromatics with indoles and pyrroles in the presence of TiO2 as a photocatalyst

In this paper we wish to report a highly selective and benign method for the double C–H/C–H coupling of azaaromatics with indoles or pyrrole in the presence of air oxygen/TiO2, as an effective photocatalytic oxidative system. It has been shown that this versatile approach can be applied for direct C–H functionalisation of a variety of azaaromatic systems, such as mono-, di- and triazines, substituted and unsubstituted azines and their benzo-annelated analogues.

Nanostructure and atomic ordering in vanadium carbide

Nanostructured nonstoichiometric vanadium carbide VC0.87 was obtained in powdered form using the ordering effect. The composition, structure, and properties of the carbide were studied by chemical and thermogravimetric analysis, gas chromatography, x-ray diffraction, optical and electronic microscopy, electron-positron annihilation, magnetic susceptibility, and microhardness methods. Nanostructured vanadium carbide VC0.87 possesses the crystal structure of the cubic ordered phase V8C7 with space group P4332. Vanadium carbide nanocrystallites are shaped in the form of 400–600 nm in diameter and 15–20 nm thick curved petals. The surface layer of the nanocrystallites contains defects of the vacancy agglomerate type. The microhardness of vanadium carbide, obtained by vacuum sintering of VC0.87 nanopowder was 60–80 GPa, which is 3–4 times greater than the microhardness of coarse-grained vanadium carbide with the same composition and close to the hardness of diamond.

Polymorphic transformation in nanocrystalline silver sulfide

The α-Ag2S acanthite–β-Ag2S argentite phase transformation in nanocrystalline silver sulfide has been studied in situ for the first time using high-temperature X-ray diffraction and scanning electron microscopy. The formation of argentite has been proved by differential thermal analysis. The acanthite–argentite transformation occurs at a temperature of ∼449–450 K, and its enthalpy is ∼3.7–3.9 kJ mol–1. The thermal expansion coefficients of acanthite and argentite have been estimated.

Titania synthesized through regulated mineralization of cellulose and its photocatalytic activity

Cellulose used for thousands of years has been rediscovered recently as a novel smart material for various nanotechnological applications. Its insoluble fibrils are functionalized by using mineralization methods developed in nanochemistry. Here they are coated by titania synthesized in one stage by a new green approach. It consists of controlling the localization of very fast hydrolysis and condensation reactions. Cellulose fibrils are placed in ethylene glycol with such an amount of water that is absorbed entirely by the hygroscopic polysaccharide. This hydrating water works as a reaction centre when the precursor reaches it. Instant hydrolysis and following condensation reactions proceeding mainly on the fibrils provide their mineralization. Titania prepared at ambient conditions is in an amorphous state. It is transferred in crystalline forms under a variety of conditions including moderate temperature (80 °C), calcination in air and cellulose carbonization in an inert atmosphere. These treatments result in photocatalytic activity. Even cellulose treated at the moderate conditions demonstrates significant self-cleaning ability consisting of fast degradation of methylene blue under outdoor sunlight irradiation. Photocatalytic activity of carbon–titania hybrids includes a side reaction of the oxidation of the carbonized fibrils. Photocatalytic properties of some of the calcinated samples, not containing organics, were comparable with a commercial photocatalyst.

Probabilities of octahedral clusters depending on long-range order parameters and composition in nonstoichiometric titanium monoxide TiO y

A method for calculating the probabilities of cluster configurations in ordered superstructures of strongly nonstoichiometric compounds depending on the composition and the order parameter is described using the Ti5O5 superstructure of nonstoichiometric titanium monoxide TiOy. Analytic expressions are derived for the dependences of the probabilities of the main cluster configurations in the Ti5O5 superstructure on the fraction of atomic positions in the titanium and oxygen sublattices and on the long-range order parameter. The probabilities of configurations are calculated for various long-range order parameters taking into account experimental data on the concentration of structural vacancies in the titanium and oxygen sublattices. The dependences of the probabilities of cluster configurations on the short-range order parameter are established from the relations between the superstructure long-range and short-range order parameters.

Relationship between short- and long-range orders in nonstoichiometric titanium monoxide (TiOy)

The relationship between the short- and long-range orders in various phases of nonstoichiometric titanium monoxide (TiOy) has been analyzed for the first time. The types of the local environment of lattice sites in the metal and nonmetal (oxygen) sublattices of Ti5O5, Ti3O2, Ti2O3, and Ti4O5 superstructures are described. It is established that, in phases where ordering takes place simultaneously in both sublattices, all parameters of the superstructural short-range order determining the positions of atoms and vacancies in the first three coordination spheres can be uniquely expressed via the long-range order parameters. If the ordering takes place only in one sublattice, then five of the six short-range order parameters vanish. It is shown that, using data on the maximum absolute values of six short-range order parameters and on the fractions of occupied atomic positions in titanium and oxygen sublattices, it is possible to predict the type of ordered phase expected to form in the nonstoichiometric titanium monoxide TiOy.

A study of cadmium sulfide nanocrystalline films by grazing incidence X-ray diffraction

Thin cadmium sulfide films were prepared on a monocrystalline-crystal silicon substrate by chemical deposition from aqueous solutions. Grazing incidence X-ray diffraction revealed that the cadmium sulfide films are comprised of nanocrystal particles, with 80% of the particles having a size of 5 ± 1 nm. Some nanocrystals have a wurtzite structure, while others, a sphalerite one. The presence of cubic phase in the films is indicative of a nonequilibrium state of the nanocrystalline films. Thirty minutes after the onset of the formation of cadmium sulfide, the size and crystal structure of the constituent particles of the film become independent of the deposition time—only the film thickness increases. In addition, the initial stage of the formation of the cadmium sulfide film is accompanied by the deposition of cadmium hydroxide Cd(OH)2.

Silver sulfide nanoparticles with a carbon-containing shell

Silver sulfide nanoparticles have been synthesized through chemical deposition from aqueous solutions of silver nitrate and sodium sulfide in the presence of sodium citrate as a complexing agent and stabilizer. The nanoparticles have a Ag2S core with a monoclinic crystal structure, covered with a carbon-containing citrate shell. Varying initial reactant concentrations, we can obtain core/shell nanoparticles with a tailored Ag2S core size and carbon-containing shell thickness.