Dmitry Bocharov

A first‐principles DFT study of UN bulk and (001) surface: Comparative LCAO and PW calculations

LCAO and PW DFT calculations of the lattice constant, bulk modulus, cohesive energy, charge distribution, band structure, and DOS for UN single crystal are analyzed. It is demonstrated that a choice of the uranium atom relativistic effective core potentials considerably affects the band structure and magnetic structure at low temperatures. All calculations indicate mixed metallic‐covalent chemical bonding in UN crystal with U5f states near the Fermi level. On the basis of the experience accumulated in UN bulk simulations, we compare the atomic and electronic structure as well as the formation energy for UN(001) surface calculated on slabs of different thickness using both DFT approaches.

Ab initio simulations on N and S co-doped titania nanotubes for photocatalytic applications

In this paper we present the results of quantum chemical modeling for energetically stable anatase (001) TiO2 nanotubes, undoped, doped, and codoped with N and S atoms. We calculate the electronic structure of one-dimensional (1D) nanotubes and zero-dimensional (0D) atomic fragments cut out from these nanotubes, employing hybrid density functional theory with a partial incorporation of an exact, nonlocal Hartree–Fock exchange within the formalism of the linear combination of atomic orbitals, as implemented in both CRYSTAL and NWChem total energy codes. Structural optimization of 1D nanotubes has been performed using CRYSTAL09 code, while the cut-out 0D fragments have been modelled using the NWChem code. The electronic properties of the studied systems prove that the band structure of the pristine TiO2 nanotube can be substantially modified by introducing substitutional impurity defects. The N-doped nanotube creates a midgap state that largely has a nitrogen character. The S-doped nanotube has a defect state that almost coincides with the top of the valence bond for the pristine material. For nanotubes codoped with both S and N, we observe a downward shift of the gap state of nitrogen relative to the purely N-doped state by about 0.3 eV. This results in a system with a filled gap state about 0.3 eV below the O2/H2O oxidation level, making it a very promising candidate for photocatalytic hydrogen generation under visible light, because due to the presence of sulfur, the bottom of the conduction band is only about 2.2 eV above the occupied midgap state, and also, clearly above the standard hydrogen electrode level.

Local structure of perovskites ReO3 and ScF3 with negative thermal expansion: interpretation beyond the quasiharmonic approximation

We propose an approach beyond the quasiharmonic approximation for interpretation of EXAFS and XRD data and for ab initio calculations of electronic and vibration properties of materials with negative thermal expansion. Ab initio electronic structure and lattice dynamics calculations for cubic and distorted ScF3 were performed using the linear combination of atomic orbitals (LCAO) method. The band gap obtained in calculations for ScF3 is equal to 10.54 eV and agree well with the expected value. The calculated infrared spectra of F displaced (FD) cubic ScF3 allow us to predict that its mean Sc-F-Sc angle within NTE deviates from 180 degree.

First Principles Simulations on Surface Properties and Oxidation of Nitride Nuclear Fuels

Uranium mononitride (UN) is an advanced material for the non-oxide nuclear fuel considered as a promising candidate for the use in Generation-IV fast nuclear reactors to be in operation in the next 20-30 years [1, 2]. UN reveals several advantages over a traditional UO2–type fuel (e.g., higher thermal conductivity and metal density as well as high solubility in nitric acid in the case of fuel reprocessing [2]). However, one of important problems with actinide nitrides is their effective oxidation in oxygen-containing atmosphere which can affect nuclear fuel performance [3, 4]. Thus, it is important to understand the mechanism of the initial stage of UN oxidation and to find proper solutions, in order to improve in the future the fabrication process of this nuclear fuel.

Quantum chemical simulations of titanium dioxide nanotubes used for photocatalytic water splitting

Titanium dioxide nanotubes (NTs) built from various initial 2D models of TiO2 (a promising catalyst for water splitting) are investigated via density functional theory using the B3LYP hybrid exchange-correlation functional in the localized basis set of a linear combination of atomic orbitals. For TiO2 NTs (eight different types of morphology) created from four initial 2D structures, full geometry optimization is performed and the main energy parameters, such as the band gap width, energy positions of the valence band top and the conduction band bottom, and NT formation and strain energy, are calculated. Analysis of the NT strain and formation energies enables us to choose their most stable configuration, which can further be employed to simulate NTs doped with impurity atoms capable of serving as efficient centers for the photocatalytic dissociation of water molecules.

First Principle Evaluation of Photocatalytic Suitability for TiO2-Based Nanotubes

Water splitting under the influence of solar light on semiconducting electrodes Im‐ mersed in aqueous electrolyte is a potentially clean and renewable source for hydrogen fuel production. Its efficiency depends on relative position of the band gap edges (the visible light interval between infrared and ultraviolet (UV) ranges of electromagnetic spectrum corresponds to gap widths 1.5–2.8 eV) accompanied by a proper band alignment relative to both reduction (H/H2) and oxidation (O2/H2O) potentials (−4.44 eV and −5.67 eV on energy scale for vacuum, respectively) which must be positioned inside the band gap. Its width for TiO2 anatase-structured bulk is experimentally found to be 3.2 eV, which corresponds to photocatalytic activity under UV light possessing only ~1% efficiency of sunlight energy conversion. Noticeable growth of this efficiency can be achieved by by adjusting the band gap edges for titania bulk through nanoscale transformation of its morphology to anatase-type nanotubes (NTs) (formed by folding of (001) or (101) nanothin TiO2 sheets consisting of 9 or 6 atomic layers and possessing either (n,0) or (−n,n) chiralities, respectively) accompanied by partial substitution of pristine atoms by CO, FeTi, NO and SO single dopants as well as NO+SO codopants. In the latter case, the band gap can be reduced down to 2.2 eV while the efficiency is achieved up to ~15%. The energy differences between the edges of band gap (VB and CB), the highest occupied and lowest unoccu‐ pied impurity levels inside the band gap (HOIL and LUIL, respectively) induced in doped NTs, while preserving the proper disposition of these levels relatively to the redox potentials, so that εVB <εHOIL <εO2/H2O <εH+/H2 <εLUIL <εCB, thus reducing the photon energy required for dissociation of H2O molecule. In this chapter, we analyze applicabil‐ ity of large-scale first principle calculations on the doped single-wall titania NTs of different morphologies with the aim of establishment of their suitability for photocatalytic water splitting.

Interpretation of the U L3-edge EXAFS in uranium dioxide using molecular dynamics and density functional theory simulations

X-ray absorption spectroscopy is employed to study the local structure of pure and Cr-doped UO2 at 300 K. The U L3-edge EXAFS spectrum is interpreted within the multiplescattering (MS) theory using the results of the classical and ab initio molecular dynamics simulations, allowing us to validate the accuracy of theoretical models. The Cr K-edge XANES is simulated within the full-multiple-scattering formalism considering a substitutional model (Cr at U site). It is shown that both unrelaxed and relaxed structures, produced by ab initio density functional theory (DFT) calculations, fail to describe the experiment.

Ab initio molecular dynamics simulations of the Sc K-edge EXAFS of scandium trifluoride

Scandium fluoride ScF3 has a simple cubic structure and attracts attention due to its large negative thermal expansion (NTE) over a wide range of temperatures (0-1100 K). In this study we present ab initio molecular dynamics (AIMD) simulations of ScF3 and their validation using the Sc K-edge EXAFS spectra in the temperature range from 300 K to 1000 K measured at the XAFS beamline of ELETTRA. The obtained results allow an assessement of the employed AIMD model and provide insight into the local structure and the lattice dynamics of ScF3 beyond the harmonic approximation. A strong anisotropy of the fluorine atom vibrations in the direction orthogonal to the -Sc-F-Sc- chain is observed. An expansion of the ScF6 octahedra is found upon increasing temperature despite of the overall NTE of the crystal.

Local structure studies of SrTi16O3 and SrTi18O3

In this work we report on the local structure of Ti in SrTi 16 O3 (STO16) and SrTi 18 O3 (STO18) investigated in the low temperature range (6‐300K) by extended x-ray absorption fine structure and x-ray absorption near edge structure (XANES) spectroscopy at Ti K-edge and by optical second harmonic generation (SHG). By comparing XANES of STO16 and STO18 we have identified the isotopic effect which produces at T < 100K a noticeable difference in the measured mean square relative displacements (MSRD) of Ti‐O1 bonds: while STO16 follow the expected Einstein-like behavior, for STO18 we have measured an increase of MSRD values with decreasing temperature. This is an indication of an increasing off-center position of the Ti atoms in the TiO6 octahedra.

Water Adsorption on Clean and Defective Anatase TiO2 (001) Nanotube Surfaces: A Surface Science Approach

We use ab initio molecular dynamics simulations to study the adsorption of thin water films with 1 and 2 ML coverage on anatase TiO2 (001) nanotubes. The nanotubes are modeled as 2D slabs, which consist of partially constrained and partially relaxed structural motifs from nanotubes. The effect of anion doping on the adsorption is investigated by substituting O atoms with N and S impurities on the nanotube slab surface. Due to strain-induced curvature effects, water adsorbs molecularly on defect-free surfaces via weak bonds on Ti sites and H bonds to surface oxygens. While the introduction of an S atom weakens the interaction of the surface with water, which adsorbs molecularly, the presence of an N impurity renders the surface more reactive to water, with a proton transfer from the water film and the formation of an NH group at the N site. At 2 ML coverage, a further surface-assisted proton transfer takes place in the water film, resulting in the formation of an OH- group and an NH2+ cationic site on the surface.