The ground state electronic structure and the formation energies of both TiO 2 and SrTiO 3 nanotubes (NTs) containing C O , N O , S O , and Fe Ti substitutional impurities are studied using first-principles calculations. We observe that N and S dopants in TiO 2 NTs lead to an enhancement of their visible-light-driven photocatalytic response, thereby increasing their ability to split H 2 O molecules. The differences between the highest occupied and lowest unoccupied impurity levels inside the band gap (HOIL and LUIL, respectively) are reduced in these defective nanotubes down to 2.4 and 2.5 eV for N and S doping, respectively. The band gap of an N O +S O co-doped titania nanotube is narrowed down to 2.2 eV (while preserving the proper disposition of the gap edges relatively to the reduction and oxidation potentials, so that HOIL < O 2 /H 2 O < H + /H 2 < LUIL ), thus decreasing the photon energy required for splitting of H 2 O molecule. For C-and Fe-doped TiO 2 NTs, some impurity levels lie in the interval between both redox potentials, which would lead to electron-hole recombination. Our calculations also reveal in sulfur-doped SrTiO 3 NTs a suitable band distribution for the oxygen evolution reaction, although the splitting of water molecules would be hardly possible due to an unsuitable conduction band position for the hydrogen reduction reaction. KeywordsDensity Functional Theory, SrTiO 3 and TiO 2 nanotubes, single-and double-atom dopants, atomic structure, electronic structure, photocatalytic properties. the influence of solar light on semiconducting photoelectrodes in aqueous electrolyte is a potentially clean and renewable source for hydrogen fuel. The process is often considered as artificial photosynthesis, and as such is an attractive and challenging research topic in the field of chemistry and renewable energy. 1-3The efficiency of the water splitting reactions 4 depends on the relative position of the semiconductor band edges (hole and electron energies) with respect to the redox levels, which are defined as measure of the affinity of the semiconducting substance for electrons (its electronegativity) compared with hydrogen. Redox couples in electrochemical reactions are characterized by molecules or ions in a solution which can be reduced and oxidized by a pure electron transfer. 5 This requires the semiconductors to exhibit a proper band alignment relative to the water redox potentials, e.g., the conduction band minimum of the p-type photocathode should be higher than the water reduction potential H + /H 2 , while the valence band maximum of the n-type photoanode, should be lower than the water oxidation potential O 2 /H 2 O. 6 Major limitations for the solar light conversion by photocatalysis relate to the band gap position in the corresponding photocatalytic materials and their stability in an aqueous environment.A number of binary and ternary metal oxide semiconductors have been intensively studied so far. 3,4,6 SrTiO 3 and, especially, TiO 2 (which distinguishes itself due to its superior chemica...

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.

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PACS numbers: 68.43. Bc, 73.20.At, 71.15.Mb Fabrication, handling and disposal of nuclear fuel materials require comprehensive knowledge of their surface morphology and reactivity. Due to unavoidable contact with air components (even at low partial pressures), UN samples contain considerable amount of oxygen impurities affecting fuel properties. The basic properties of O atoms adsorbed on the UN(001) surface are simulated here combining the two first principles calculation methods based on the plane wave basis set and that of the localized atomic orbitals.The actinide nitrides and carbides, e.g., uranium mononitride (UN) with a face centered cubic (fcc) rock salt structure, belong to the family of non-oxide ceramic nuclear fuels considered as promising candidates for the use in Generation-IV fast nuclear reactors. These materials reveal several advantages over traditional UO 2 fuel (e.g., higher thermal conductivity and metal density) [1]. One of the problems with nitride and carbide fuels is their active interaction with the oxygen which results in an effective fuel oxidation and degradation [2]. This could affect the fabrication process as well as the fuel performance and safety. First experimental studies on O in UN were performed in 80ies ([1] and references therein). These activities were continued recently combining several techniques ([2] and references therein). However, understanding of the atomistic mechanism of fuel oxidation needs first principles theoretical modeling. Thus, to shed more light on this problem, we study here theoretically the interaction of atomic oxygen with the UN(001) surface.Theoretical simulations of uranium compounds are especially complicated due to a relativistic character of an electron motion in the U atomic core and the strong electronelectron correlation. Moreover, UN is characterized by a mixed metal-covalent chemical bonding. Physical and chemical properties of light actinides are determined by partly localized 5f electrons, which determine a number of properties, such as mixed valence, magnetism, etc. A series of first principles DFT calculations on pure and defective UO 2 were performed recently (e.g., [3][4][5][6][7][8]) whereas a number of similar calculations on the nitride fuels is still much more limited [9][10][11][12][13][14][15]. In our recent paper [15] the methodology was proposed for LCAO calculations of the UN surface properties. The first results on the pure UN surfaces were presented therein using two approaches based on the basis sets of atomic orbitals (AO) and plane waves (PW), respectively. Use of the two different methods greatly increases the reliability of the results obtained.To simplify modeling of the oxygen interaction with UN powder surface, we study here only the (001) surface which according to Tasker [16] has the lowest energy. To simulate the perfect UN(001) substrate as well as its interaction with oxygen, we have * Corresponding author: e-mail: bocharov@latnet.lv 2 employed the DFT-PW computer code VASP 4.6 [17] based on the use of ...

The incorporation of oxygen atoms has been simulated into either nitrogen or uranium vacancy at the UN(001) surface, sub-surface or central layers. For calculations on the corresponding slab models both the relativistic pseudopotentials and the method of projector augmented-waves (PAW) as implemented in the VASP computer code have been used. The energies of O atom incorporation and solution within the defective UN surface have been calculated and discussed. For different configurations of oxygen ions at vacancies within the UN(001) slab, the calculated density of states and electronic charge re-distribution was analyzed. Considerable energetic preference of O atom incorporation into the N-vacancy as compared to U-vacancy indicates that the observed oxidation of UN is determined mainly by the interaction of oxygen atoms with the surface and sub-surface N vacancies resulting in their capture by the vacancies and formation of O-U bonds with the nearest uranium atoms.