| 1. |
- Migas, D. B., et al.
(författare)
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Orientation effects in morphology and electronic properties of anatase TiO2 one-dimensional nanostructures. II. Nanotubes
- 2014
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Ingår i: Physical Chemistry, Chemical Physics - PCCP. - : Royal Society of Chemistry (RSC). - 1463-9076 .- 1463-9084. ; 16:20, s. 9490-9498
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Tidskriftsartikel (refereegranskat)abstract
- In the first part [D. B. Migas et al., Phys. Chem. Chem. Phys., 2014, DOI: 10.1039/C3CP54988G] by means of ab initio calculations we have analyzed and discussed anisotropy effects on electronic properties of 001 -, 100 - and 110 - oriented anatase TiO2 nanowires. In this part we present results indicating crucial changes in morphology of anatase TiO2 nanotubes originating from TiO2 nanowires by making a hole along the wire axis. The critical wall thickness has been found to exist for the nanotubes with 001 and 110 axes: at smaller thickness their shape can be rounded, squeezed, viewed as conglomerates of nanocrystals and even represented as cylindrical and `single-walled'- like structures formed without rolling up a thin titania layer into a nanotube. In general, band dispersion near the gap region of TiO2 nanotubes is close to the one of TiO2 nanowires with the same orientation. We have also revealed that optimization of the unit cell parameter along the wire axis and consideration of quantum confinement and surface state effects are important to provide an interpretation of band-gap variation with respect to wall thickness in TiO2 nanotubes.
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| 2. |
- Popov, A. P., et al.
(författare)
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Metastable noncollinear canted states from a phenomenological model of a symmetric ferromagnetic film
- 2010
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 81:5, s. 054440-
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Tidskriftsartikel (refereegranskat)abstract
- In the present work we developed a theoretical method to find the noncollinear canted magnetic states of ferromagnetic film whose magnetic properties are treated within a simple phenomenological model that takes into account the discrete location of atomic layers. We apply this method to the description of stable and uniform in the film plane magnetic configurations of symmetric ferromagnetic film with equal anisotropy constants at both sides of a film. We demonstrated that besides the ground noncollinear canted state caused by the competition between the energies of surface and bulk anisotropy this film also exhibits metastable noncollinear canted states that are not necessarily caused by the competition between these energies. They represent various kinds of excited Bloch wall-like states with a wall parallel to the film plane. The generalization of the method developed to the case of explicit account of dipole interaction between the nearest-neighbor layers is presented. We considered the fcc and bcc films with (001) surface and demonstrated that novel metastable states discovered in the present work are not the artifact of a simple model of a film used. To prove this statement we demonstrate that these states exist when the dipole interaction between the nearest-neighbor layers is explicitly taken into account.
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| 3. |
- Ruban, Andrei V., et al.
(författare)
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Impact of magnetism on Fe under Earth's core conditions
- 2013
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 87:1, s. 014405-
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Tidskriftsartikel (refereegranskat)abstract
- Using a microscopic phenomenological model for longitudinal spin fluctuations (LSFs) based on density functional theory calculations, we demonstrate that under the Earth's core conditions (P approximate to 360 GPa, T approximate to 6000 K), Fe acquires substantial local magnetic moment, up to 1.3 mu(B), for different crystal structure modifications. We demonstrate that the LSFs produce a substantial effect on the magnetic and thermodynamic properties of iron, in particular, its equilibrium volume under solid Earth's core conditions.
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| 4. |
- Al-Zoubi, Noura, et al.
(författare)
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Tetragonality of carbon-doped ferromagnetic iron alloys : A first-principles study
- 2012
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 85:1, s. 014112-
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Tidskriftsartikel (refereegranskat)abstract
- Using density-functional theory in combination with the exact muffin-tin orbital (EMTO) method and coherent potential approximation, we investigate the alloying effect on the tetragonality of Fe-C solid solution forming the basis of steels. In order to assess the accuracy of our approach, first we perform a detailed study of the performance of the EMTO method for the Fe(16)C(1) binary system by comparing the EMTO results to those obtained using the projector augmented wave method. In the second step, we introduce different substitutional alloying elements (Al, Cr, Co, Ni) into the Fe matrix and study their impact on the structural parameters. We demonstrate that a small amount of Al, Co, and Ni enhances the tetragonal lattice ratio of Fe(16)C(1) whereas Cr leaves the ratio almost unchanged. The obtained trends are correlated with the single-crystal elastic parameters calculated for carbon-free alloys.
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| 5. |
- Amft, Martin, et al.
(författare)
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A Molecular Mechanism for the Water-Hydroxyl Balance during Wetting of TiO2
- 2013
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Ingår i: The Journal of Physical Chemistry C. - : American Chemical Society (ACS). - 1932-7447 .- 1932-7455. ; 117:33, s. 17078-17083
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Tidskriftsartikel (refereegranskat)abstract
- We show that the formation of the wetting layer and the experimentally observed continuous shift of the H2O-OH balance toward molecular water at increasing coverage on a TiO2(110) surface can be rationalized on a molecular level. The mechanism is based on the initial formation of stable hydroxyl pairs, a repulsive interaction between these pairs, and an attractive interaction with respect to water molecules. The experimental data are obtained by synchrotron radiation photoelectron spectroscopy and interpreted with the aid of density functional theory calculations and Monte Carlo simulations.
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| 6. |
- Amft, Martin, et al.
(författare)
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Adsorption of Cu, Ag, and Au atoms on graphene including van der Waals interactions
- 2011
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Ingår i: Journal of Physics. - : IOP Publishing. - 0953-8984 .- 1361-648X. ; 23:39
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Tidskriftsartikel (refereegranskat)abstract
- We performed a systematic density functional (DF) study of the adsorption of copper, silver, and gold adatoms on pristine graphene, especially accounting for van der Waals (vdW) interactions by the vdW-DF and PBE + D2 methods. In particular, we analyze the preferred adsorption site (among top, bridge, and hollow positions) together with the corresponding distortion of the graphene sheet and identify diffusion paths. Both vdW schemes show that the coinage metal atoms do bind to the graphene sheet and that in some cases the buckling of the graphene layer can be significant. Only the results for silver are qualitatively at variance with those obtained with the generalized gradient approximation, which gives no binding in this case. However in all three cases, we observe some quantitative differences between the vdW-DF and PBE + D2 methods. For instance the adsorption energies calculated with the PBE + D2 method are systematically higher than the ones obtained with vdW-DF. Moreover, the equilibrium distances computed with PBE + D2 are shorter than those calculated with the vdW-DF method.
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| 7. |
- Amft, Martin, et al.
(författare)
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Catalytic activity of small MgO-supported Au clusters towards CO oxidation : A density functional study
- 2010
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Ingår i: Physical Review B Condensed Matter. - 0163-1829 .- 1095-3795. ; 81:19, s. 195443-
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Tidskriftsartikel (refereegranskat)abstract
- In order to explain the experimentally found catalytic characteristics of Au1-4 /MgO (100) we have performed a comprehensive density functional study of these systems and their ability to (co)adsorb CO and O2 molecules. Starting from the carefully determined ground-state structures we have analyzed binding mechanisms, the influence of spin-orbit coupling, and charge redistributions in Au1-4 /MgO+CO (O2). Experimentally Au1,2 /MgO were found to be inactive under a mixed atmosphere. We show that O2 strongly binds to Au1 /MgO that prevents coadsorption. Although a catalytic reaction cycle towards CO oxidation, analogous to the gas phase reaction involving Au 2-, is energetically possible for Au2 /MgO, the cluster will get blocked by a strongly bound CO. On the other hand, the catalytic activity of Au3,4 /MgO could be explained by their ability to coadsorb CO and O2, hence indicating the occurrence of a Langmuir- Hinshelwood-type reaction mechanism for these clusters.
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| 8. |
- Amft, Martin, et al.
(författare)
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Does H2O improve the catalytic activity of Au1−4/MgO towards CO oxidation?
- 2010
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Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- The present density functional theory study addresses the question whether the presence of H2O influences the catalytic activity of small gold clusters, Au1-4/MgO(100), towards the oxidation of carbon monoxide. To this end, we studied the (co-)adsorption of H2O and CO/O2 on these gold clusters. The ground state structures in the presence of all three molecular species, that we found, are Au1O2/MgO and Au2-4CO/MgO with H2O adsorbed on the surface in the proximity of the clusters-molecule complex. In this configuration the catalytic activity of Au1-4/MgO is indifferent to the presence of H2O. We also found that a stable, highly activated hydroperoxyl-hydroxyl complex, O2H ·· OH, can be formed on Au1,3/MgO. For the catalytic active system Au8/MgO, it has been predicted that this complex opens an alternative catalytic reaction pathway towards CO oxidation. Our results suggest that this water mediated catalytic cycle is unlikely to occur on Au1,3/MgO. In the case of Au1/MgO the cycle is interrupted by the dissociation of the remaining (OH)2 complex after forming the first CO2 molecule. On Au3 /MgO the O2H ·· OH complex is likely to decompose upon the impact of a CO molecule, since three of its bond dissociation energies are comparable to the reaction barrier of the CO to CO2 oxidation.
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| 9. |
- Amft, Martin, et al.
(författare)
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Influence of the cluster dimensionality on the binding behavior of CO and O(2) on Au(13)
- 2012
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Ingår i: Journal of Chemical Physics. - : AIP Publishing. - 0021-9606 .- 1089-7690. ; 136:2, s. 024312-
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Tidskriftsartikel (refereegranskat)abstract
- We present an ab initio density functional theory study of the binding behavior of CO and O(2) molecules to two-and three-dimensional isomers of Au(13) in order to investigate the potential catalytic activity of this cluster towards low-temperature CO oxidation. First, we scanned the potential energy surface of Au(13) and studied the effect of spin-orbit coupling on the relative stabilities of the 21 isomers we identified. While spin-orbit coupling increases the stability of the three-dimensional more than the two-dimensional isomers, the ground state structure at 0 K remains planar. Second, we systematically studied the binding of CO and O(2) molecules onto the planar and three-dimensional structures lowest in energy. We find that the isomer dimensionality has little effect on the binding of CO to Au(13). O(2), on the other hand, binds significantly to the three-dimensional isomer only. The simultaneous binding of multiple CO molecules decreases the binding energy per molecule. Still, the CO binding remains stronger than the O(2) binding. We did not find a synergetic effect due to the co-adsorption of both molecular species. On the three-dimensional isomer, we find O(2) dissociation to be exothermic with an dissociation barrier of 1.44 eV.
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| 10. |
- Bondarenko, Nina, et al.
(författare)
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Hole bipolaron formation at (100) MgO/CaO epitaxial interface
- 2014
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 89:12, s. 125118-
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Tidskriftsartikel (refereegranskat)abstract
- Hole localization accompanying the formation of a cation vacancy in bulk MgO and CaO and at the (100) MgO/CaO interface is described using the Heyd-Scuseria-Ernzerhof hybrid functionals and DFT + U method. The ground state is found to be the O-1-O-1 bipolaronic configuration both in bulk oxides and at their interfaces. The ground-state magnetic configuration is a triplet, which has an energy only about 1-2 meV lower than that of the singlet state. The one-centered O-2-O-0 bipolaron was found to be metastable with its stability being enhanced at the interfaces compared to that in bulk oxides. Possible transition between different biplaronic configurations at the interface are analyzed for chosen configurations.
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