V. M. Zainullina

Synthesis and photocatalytic activity of Ti[1 − x]VxO[2 − y]Cy whiskers in hydroquinone oxidation in aqueous solutions

Ti1−xVxO2−yCy (0 ≤ x ≤ 0.10 and x = 0.50) whiskers having the anatase structure were synthesized via thermolysis of vanadium-doped titanium glycolate of composition Ti1−xVx(OCH2CH2O)2 (0 ≤ x ≤ 0.10 and x = 0.50). The starting reagents used to prepare Ti1−xVx(OCH2CH2O)2 were mixtures of coprecipitated titanium and vanadyl hydroxides, which were heated in ethylene glycol at T ≤ 200°C: (1 − x)TiO(OH)2 + xVO(OH)2 + 2HOCH2CH2OH = Ti1−xVx(OCH2CH2O)2 + 3H2O↑. Thermolysis of vanadium-doped titanium glycolate in various gas media over a wide range of temperatures is useful to prepare titania samples doped with both vanadium and carbon to form a phase of the general composition Ti1 − xVxO2 − yCy whiskers prepared by thermolyzing Ti1 − xVx(OCH2CH2O)2 in air at 450°C were found to have a high photocatalytic activity in hydroquinone oxidation in aqueous solutions irradiated in the UV spectral range; the photocatalyst’s activity increases with increasing vanadium concentration. When hydroquinone was irradiated in the blue, the maximal catalytic activity was discovered in a sample of composition Ti0.50V0.50O2−yCy. Quantum-chemical calculations support experimental data that the double doping of titania (Ti1−xVxO2−yCy) enhances its photocatalytic activity compared to undoped anatase or anatase doped in one sublattice: Ti1−xVxO2 and TiO2−yCy.

ELECTRONIC STRUCTURE AND CHEMICAL BONDING IN BISMUTH SESQUIOXIDE POLYMORPHS

The electronic structure of the α-Bi2O3, Β-Bi2O3, and γ-Bi2O3 phases was investigated by the ab initio self-consistent LMTO method in a tight binding approximation and by the semiempirical Hückel method. The total and partial densities of states and Mulliken overlap populations were obtained. The stability of bismuth oxide polymorphs is discussed based on the results of the total energy calculations for crystals. An analysis of chemical bonding shows that the Bi-O interaction plays the leading role. The Bi-Bi metallic bond is absent. Mechanisms of oxygen ion migration and possible stabilization of the structure of the superionic conductor δ-Bi2O3 are discussed.

Electronic Structure and Chemical Bonding in Monoclinic and Cubic Li2-xHxTiO3 (0≤ x ≤ 2)

The linear “muffin‐tin” orbitals method in a tight binding approximation and extended Huckel theory are used to study the electronic structure and chemical bonding of lithium titanate (Li2TiO3) and its protonated analogs (Li1.75H0.25TiO3 and H2TiO3). The effect of protons on electron spectrum characteristics and bond strength are investigated for the monoclinic and cubic phases of lithium titanate. Phase stability is evaluated by cohesion energy calculations.

Effect of doping by boron, carbon, and nitrogen atoms on the magnetic and photocatalytic properties of anatase

The effect of doping of titanium dioxide with the anatase structure by boron, carbon, and nitrogen atoms on the magnetic and optical properties and the electronic spectrum of this compound has been investigated using the ab initio tight-binding linear muffin-tin orbital (TB-LMTO) band-structure method in the local spin density approximation explicitly including Coulomb correlations (LSDA + U) in combination with the semiempirical extended Hückel theory (EHT) method. The LSDA + U calculations of the electronic structure, the imaginary part of the dielectric function, the total magnetic moments, and the magnetic moments at the impurity atoms have been carried out. The diagrams of the molecular orbitals of the clusters Ti3X (X = B, C, N) have been calculated and the pseudo-space images of the molecular orbitals of the clusters have been constructed. The effect of doping on the nature and origin of photocatalytic activity in the visible spectral range and the specific features of the generation of ferromagnetic interactions in doped anatase have been discussed based on the analysis of the obtained data. It has been shown that, in the sequence TiO2 − yNy → TiO2 − yCy → TiO2 − yBy (y = 1/16), the photocatalytic activity can increase with the generation of electronic excitations with the participation of impurity bands. The calculated magnetic moments for boron and nitrogen atoms are equal to 1 μB, whereas the impurity carbon atoms are nonmagnetic.

Electronic structure and chemical bonding in oxygen conductors β-Bi4V2O11 and γ-Bi4V2O11

Ab initio calculations of the electron density are fulfilled for bismuth vanadate polymorphs: β-Bi4V2O11 and γ-Bi4V2O11. A semiconductor-type spectrum is obtained for both phases. The calculated bandgap value decreases in the direction from the β to γ phase. Chemical bond analysis shows an increase in the strength of covalent interactions in the series of Bi-V, Bi-O, and V-O bonds. The V-O bond strength in β-Bi4V2O11 is appreciably higher than in the γ phase. The relatively weak binding of oxygen in the tetragonal structure of γ-Bi4V2O11 confirms the possibility of easier oxygen transport in the γ phase than in the β phase. The stability of the bismuth vanadate polymorphs and the oxygen-disorder features in the γ-Bi4V2O11 structure are discussed using the results of calculation of the total energies of the phases. Bond overlap population analysis explains the existence of the homogeneous range and the feasibility of doping β-Bi4V2O11 and γ-Bi4V2O11. The possibility of using four-and five-charged d cations as stabilizers of the crystal structure of bismuth vanadates is discussed.

Investigation of the influence of nonstoichiometry and doping with carbon and nitrogen on the electronic spectrum of rutile by the coherent potential method

The electronic spectra and magnetic properties of rutile (TiO2), nonstoichiometric rutile (TiO2 − δ), and stoichiometric and nonstoichiometric rutiles doped with carbon or nitrogen (TiO2 − y − δCy and TiO2 − y − δNy, y(δ) = 0, 0.03, 0.06) have been calculated using the coherent potential method. The used method of investigation has made it possible for the first time to calculate the disordered arrangement of impurity atoms and vacancies in the oxygen sublattice with their arbitrary concentration. The changes in the electronic spectrum and magnetic properties with variations in the concentration of dopants have been investigated. The possible photocatalytic activity of considered compositions has been analyzed.

Electronic structure of the high-temperature cubic phase of SrFeO2.5

The electronic structure and magnetic properties of the high-temperature cubic phase of strontium ferrite SrFeO2.5 with the disordered location of oxygen vacancies have been calculated using the coherent potential method. The semiconductor character of the electronic spectrum of SrFeO2.5 is observed only at certain types of disordering of vacancies over the oxygen sublattice, which makes it possible to choose among structural models of the high-temperature phase proposed on the basis of experiments.

Ground state of BaCoS2 as a set of energy-degenerate orbital-ordered configurations of Co2+ ions

The linear muffin-tin orbital method in the local density approximation (LDA + U) explicitly considering Coulomb correlations has been applied to calculate the electronic structure, magnetic moments, and parameters of the Heisenberg exchange interaction for cobalt ions in BaCoS2. Five solutions close in total energy with various orbital ordering of Co2+ ions and almost identical spin magnetic moments μ = 2.32μB of the Co2+ ion 3d-shell have been found. The BaCoS2 ground state can be considered as a set of energy-degenerate orbital-ordered configurations of Co2+ ions in the high-spin state.

Genesis of the electronic spectrum and magnetic properties of a high-temperature phase of nonstoichiometric strontium ferrite SrFeO3–δ (0 ≤ δ ≤ 0.5)

The genesis of the electronic spectrum and magnetic properties of a high-temperature cubic phase of SrFeO3–δ with the disordered distribution of oxygen vacancies under the variation of their concentration has been studied in the coherent-potential approximation within the LDA + U formalism. Experimental tendencies of the fundamental electron-energy and magnetic characteristics in SrFeO3–δ under the variation of oxygen nonstoichiometry within the homogeneity region, 0 ≤ δ ≤ 0.5, have been described.

Electronic structure of rutile simultaneously doped with carbon and nitrogen atoms in the coherent potential approximation

The coherent potential method has been used for calculating the electronic structure and magnetic properties of rutile with a disordered arrangement of impurity carbon and nitrogen atoms in the oxygen sublattice: TiO2 − x − yCxNy, x(y) = 0, 0.03, and 0.06. The tendencies to changes in the magnetic moment and photocatalytic activity with variations in the carbon-nitrogen composition of codoped rutile have been analyzed using the obtained data.