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| 2. |
- Canovic, Sead, 1979, et al.
(författare)
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TEM and DFT investigation of CVD TiN/κ–Al2O3 multilayer coatings
- 2007
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Ingår i: Surface and Coatings Technology. - : Elsevier BV. - 0257-8972. ; 202:3, s. 522-531
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Tidskriftsartikel (refereegranskat)abstract
- This paper investigates the interfacial structure in hot-wall CVD TiN/kappa-Al2O3 multilayer coatings using both HREM and DFT modeling. Two multilayers with different thicknesses of the TiN layers (50 and 600 nm) separating the kappa-Al2O3 layers are analyzed. The general microstructure of the two multilayers is relatively similar. The TiN layer in the thicker TiN/kappa-Al2O3 coating is thick enough to be several TiN grains high. This means that epitaxial columns, which are often found in the thinner TiN/kappa-Al2O3 coatings, are not present. However, the orientation relationships at the TiN/kappa-Al2O3 interfaces are the same in both multilayers. The HREM investigations show that kappa-Al2O3 (001) planes can grow directly on flat (111) TiN faces, without any other phases or detectable amounts of impurities, such as sulphur, present. Where the TiN layers are more curved, gamma-Al2O3 can be grown, at least partly stabilized by the cube-on-cube orientation relationship between gamma-Al2O3 and the underlying TiN. The DFT calculations show very similar adsorption strengths for an 0 monolayer positioned on Ti-terminated TiC(111) and TiN(111) surfaces, with preferred adsorption in the fee site. 0 adsorption on N-terminated TiN(111) is much weaker, with preferred adsorption in the top site. Calculated elastic-energy contributions yield a higher stability for kappa-Al2O3 on TiN(111) than on TiC(111) and a higher stability for kappa-Al2O3 than for alpha-Al2O3 on both TiC and TiN. This indicates that the observed higher stability Of kappa-Al2O3 on TiC(111) than on TiN(111) is not due to the lattice mismatch, while the preferred epitaxial growth of kappa-Al2O3 over alpha-Al2O3 can be partly attributed to the mismatch.
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| 3. |
- Chakarova Käck, Svetla, 1977, et al.
(författare)
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Binding of polycyclic aromatic hydrocarbons and graphene dimers in density functional theory
- 2010
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Ingår i: New Journal of Physics. - : IOP Publishing. - 1367-2630. ; 12, s. Art. Nr. 013017-
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Tidskriftsartikel (refereegranskat)abstract
- An early van der Waals density functional (vdW-DF) described layered systems (such as graphite and graphene dimers) using a layer-averaged electron density in the evaluation of nonlocal correlations. This early vdW-DF version was also adapted to approximate the binding of polycyclic aromatic hydrocarbons (PAHs) (Chakarova S D and Schröder E 2005 J. Chem. Phys. 122 054102). In parallel to that PAH study, a new vdW-DF version (Dion M, Rydberg H, Schröder E, Langreth D C and Lundqvist B I 2004 Phys. Rev. Lett. 92 246401) was developed that provides accounts of nonlocal correlations for systems of general geometry. We apply here the latter vdW-DF version to aromatic dimers of benzene, naphthalene, anthracene and pyrene, stacked in sandwich (AA) structure, and the slipped-parallel (AB) naphthalene dimer. We further compare the results of the two methods as well as other theoretical results obtained by quantum-chemistry methods. We also compare calculations for two interacting graphene sheets in the AA and the AB structures and provide the corresponding graphene-from-graphite exfoliation energies. Finally, we present an overview of the scaling of the molecular–dimer interaction with the number of carbon atoms and with the number of carbon rings.
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| 6. |
- Valdes, A., et al.
(författare)
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Solar hydrogen production with semiconductor metal oxides: new directions in experiment and theory
- 2012
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Ingår i: Physical Chemistry Chemical Physics. - : Royal Society of Chemistry (RSC). - 1463-9084 .- 1463-9076. ; 14:1, s. 49-70
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Forskningsöversikt (refereegranskat)abstract
- An overview of a collaborative experimental and theoretical effort toward efficient hydrogen production via photoelectrochemical splitting of water into di-hydrogen and di-oxygen is presented here. We present state-of-the-art experimental studies using hematite and TiO(2) functionalized with gold nanoparticles as photoanode materials, and theoretical studies on electro and photo-catalysis of water on a range of metal oxide semiconductor materials, including recently developed implementation of self-interaction corrected energy functionals.
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| 7. |
- Vojvodic, Aleksandra, 1981, et al.
(författare)
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Atomic and molecular adsorption on transition-metal carbide (111) surfaces from density-functional theory: a trend study of surface electronic factors
- 2010
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Ingår i: Journal of Physics Condensed Matter. - : IOP Publishing. - 0953-8984 .- 1361-648X. ; 22:37
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Tidskriftsartikel (refereegranskat)abstract
- This study explores atomic and molecular adsorption on a number of early transition-metal carbides (TMCs) in NaCl structure by means of density-functional theory calculations. The investigated substrates are the TM-terminated TMC(111) surfaces, of interest because of the presence of different types of surface resonances (SRs) on them and because of their technological importance in growth processes. Also, TM compounds have shown potential in catalysis applications. Trend studies are conducted with respect to both period and group in the periodic table, choosing the substrates ScC, TiC, VC, ZrC, NbC, delta-MoC, TaC, and WC (in NaCl structure) and the adsorbates H, B, C, N, O, F, NH, NH2, and NH3. Trends in adsorption strength are explained in terms of surface electronic factors, by correlating the calculated adsorption-energy values with the calculated surface electronic structures. The results are rationalized by use of a concerted-coupling model (CCM), which has previously been applied successfully to the description of adsorption on TiC(111) and TiN(111) surfaces (Ruberto et al 2007 Solid State Commun. 141 48). First, the clean TMC(111) surfaces are characterized by calculating surface energies, surface relaxations, Bader charges, and surface-localized densities of states (DOSs). Detailed comparisons between surface and bulk DOSs reveal the existence of transition-metal localized SRs (TMSRs) in the pseudogap and of several C-localized SRs (CSRs) in the upper valence band on all considered TMC(111) surfaces. The spatial extent and the dangling bond nature of these SRs are supported by real-space analyses of the calculated Kohn-Sham wavefunctions. Then, atomic and molecular adsorption energies, geometries, and charge transfers are presented. An analysis of the adsorbate-induced changes in surface DOSs reveals a presence of both adsorbate-TMSR and adsorbate-CSRs interactions, of varying strengths depending on the surface and the adsorbate. These variations are correlated to the variations in adsorption energies. The results are used to generalize the content and applications of the previously proposed CCM to this larger class of substrates and adsorbates. Implications for other classes of materials, for catalysis, and for other surface processes are discussed.
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| 8. |
- Vojvodic, Aleksandra, 1981, et al.
(författare)
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Chracterization of MoS2 Magnetic States Using Density-Functional Theory
- 2009
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Ingår i: Physical Review B - Condensed Matter and Materials Physics. - 2469-9950 .- 2469-9969. ; 80, s. 125416-
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Tidskriftsartikel (refereegranskat)abstract
- It is known that the edges of a two-dimensional slab of insulating MoS2 exhibit one-dimensional metallic edge states, the so-called “brim states.” Here, we find from density-functional theory calculations that several edge structures, which are relevant for the hydrodesulfurization process, are magnetic. The magnetism is an edge phenomenon associated with certain metallic edge states. Interestingly, we find that among the two low-index edges, only the S edge displays magnetism under hydrodesulfurization conditions. In addition, the implications of this on the catalytic activity are investigated. Despite large changes in the magnetic moments, a small influence on the adsorption energies is observed. This has implications on the suitability of magnetic measurements for monitoring the catalytic properties.
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| 9. |
- Vojvodic, Aleksandra, 1981, et al.
(författare)
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From Electronic Structure to Catalytic Activity: A Single Descriptor for Adsorption and Reactivity on Transition-Metal Carbides
- 2009
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Ingår i: Physical Review Letters. - 1079-7114 .- 0031-9007. ; 103:14, s. 146103-
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Tidskriftsartikel (refereegranskat)abstract
- Adsorption and catalytic properties of the polar (111) surface of transition-metal carbides (TMC's) are investigated by density-functional theory. Atomic and molecular adsorption are rationalized with the concerted-coupling model, in which two types of TMC surface resonances (SR's) play key roles. The transition-metal derived SR is found to be a single measurable descriptor for the adsorption processes, implying that the Brønsted-Evans-Polanyi relation and scaling relations apply. This gives a picture with implications for ligand and vacancy effects and which has a potential for a broad screening procedure for heterogeneous catalysts.
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| 10. |
- Vojvodic, Aleksandra, 1981
(författare)
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Reactivity of Transition-Metal Compounds from Electronic Structure
- 2009
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Transition-metal carbides (TMC's), nitrides, and sulfides belong to the class of materials known as transition-metal compounds (TMX's). Besides having intriguing properties, these materials are relevant for, e.g., growth and catalysis. A fundamental understanding of the reactivity of TMX surfaces is far less established than that of pure transition metal (TM) surfaces. The results of this thesis shed new light on the reactivity of TMX's. Extensive density-functional theory calculations are performed, combining substrates from the set {ScC, TiC, VC, ZrC, NbC, MoC, TaC, WC, TiN} with adsorbates from the set {H, B, C, N, O, F, Al, Si, P, S, Cl, NH, NH2, NH3}. This allows a mapping of adsorption properties, whose characteristic trends are rationalised in terms of a concerted-coupling model (CCM). In the CCM, the adsorbate frontier orbitals interact with two types of surface resonances (SR's), a TM-localised SR (TMSR) and several C-localised SR's (CSR's). The foundation of the CCM is enabled by a systematic analysis of the electronic structure, including various types of density of states, in combination with examinations of the Kohn-Sham orbitals. The developed framework makes it possible to identify a single measurable descriptor, the mean energy of the TMSR, for atomic and molecular adsorption, dissociation, and catalytic activity on TMC surfaces. The generality of this descriptor is demonstrated by applications to TM surfaces, and to ligand and defect effects. The existence of a single descriptor implies that the Bronsted-Evans-Polanyi relation and scaling relations, valuable for screening of new catalysts, apply. One-dimensional metallic surface states localised at the edges of MoS2 nanoparticles are also studied. Based on electronic structure analyses, the existence, origin, and consequences of the experimentally observed magnetism of the nanoparticles under different environmental conditions are discussed. A correlation between the edge electronic structure and the magnetism is established. A screening study of the steam reforming reaction on TMC surfaces is performed using the CCM as a tool. The results show that TMC's provide a wide spectrum of reactivities, from too reactive, via suitable, catalysts to too inert, just as TM's. Also, the importance of chemical bonds for the stability and thus the relevance of adsorption modelsfor understanding the crucial first steps in nucleation and growth of, e.g., TiX/Al2O3 multilayer coatings are shown. Taken together, the findings of this thesis show that the versatility of TMX systems and the understanding of the underlying adsorption mechanisms point towards the possibility of tuning the reactivity of TMX's.
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