| 1. |
- Balaji Gopal, Chirranjeevi, et al.
(författare)
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Equilibrium oxygen storage capacity of ultrathin CeO2-δ depends non-monotonically on large biaxial strain
- 2017
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Ingår i: Nature Communications. - : Springer Science and Business Media LLC. - 2041-1723. ; 8
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Tidskriftsartikel (refereegranskat)abstract
- Elastic strain is being increasingly employed to enhance the catalytic properties of mixed ion-electron conducting oxides. However, its effect on oxygen storage capacity is not well established. Here, we fabricate ultrathin, coherently strained films of CeO2-δ between 5.6% biaxial compression and 2.1% tension. In situ ambient pressure X-ray photoelectron spectroscopy reveals up to a fourfold enhancement in equilibrium oxygen storage capacity under both compression and tension. This non-monotonic variation with strain departs from the conventional wisdom based on a chemical expansion dominated behaviour. Through depth profiling, film thickness variations and a coupled photoemission-thermodynamic analysis of space-charge effects, we show that the enhanced reducibility is not dominated by interfacial effects. On the basis of ab initio calculations of oxygen vacancy formation incorporating defect interactions and vibrational contributions, we suggest that the non-monotonicity arises from the tetragonal distortion under large biaxial strain. These results may guide the rational engineering of multilayer and core-shell oxide nanomaterials.
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| 3. |
- 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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| 4. |
- 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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| 5. |
- Juan Angel, de Gracia Triviño, 1989-
(författare)
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From Molecular Catalysts to Hybrid Electrodes: A Theoretical Guide
- 2022
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The industrial revolution thrived our society to great technological advancement and a shift from an agrarian to an industrial society. Besides this fact, the side effect has been the development of a society highly dependent on energy, and the main sources of energy are based on non-renewable fossil fuels. This issue calls for the quest for new renewable energy sources that can address the energy dependency minimizing its side effects of it. In this quest, hydrogen is a promising source due to its high energy capacity and clean sub-products.The first chapter of this thesis will revise more in deep this environmental issues and what is needed to implement sustainable hydrogen production by water splitting. As well, as how the water source is extremely relevant, and solutions for using seawater are required to scale up hydrogen production. Also, an introduction to molecular catalysts for water oxidation based on Ru will be exposed, including a historical perspective and the state of the art at this day. The first chapter will finish with the strategies explored in this thesis to overcome the limitations of molecular catalysts in water splitting devices i.e, stability and current density.This work uses an ample set of computational tools to explore the reactivity and supramolecular properties of molecular catalysts. The second chapter will start with the treatment of molecules as electronic systems utilizing molecular quantum mechanics. Wave function formalism and density functional formalism of molecular quantum mechanics will be exposed and explained to the extent that is needed to ground the results of this thesis. The next section will introduce the treatment of molecules as atomic systems employing molecular mechanics and how we derive relevant supramolecular effects such as hydrophobicity, means of attachment to electrode surfaces, solvent, and electric field effects. Finally, this chapter will revise the Empirical Valence Bond approach to study the reactivity dependence on the molecular environment.The last chapter will go over the results of this thesis that correspond to the annexed papers at the end of this work. Starting from the characterization of the oxide relay mechanism in the highly efficient catalyst Ru(tda) where a novel function for the non-coordinating carboxylate ligand is proposed. To increase the stability of the Ru(tda) an attachment to carbon surfaces has been proposed and proved to increase significantly the stability. A study of the oxide relay mechanism at the surface revealed that the water-excluded environment of the active site in the reactive intermediate does not affect the key steps of this mechanism, in agreement with the experimental results reported. Next, the Ru(bda) has been shown to effectively catalyze the formation of molecular nitrogen from ammonia in an apolar solvent. The Ru(bda) has been well studied for water oxidation due to its high efficiency and the key step has been identified as the dimerization of two complexes driven by the aqueous solvent. The study of the dimerization process in acetonitrile has revealed the crucial role of solvent in supramolecular effects since acetonitrile promotes complex-counterion pairing aiding the dimerization of the Ru(bda) and. To increase the current density is needed a strategy to increase the catalyst density at the surface. Oligomerization of the Ru(tda) has shown to be an effective strategy to increase the current density of the hybrid electroanode to levels that are comparable to commercial electrolyzers. The exploration of the ways of attachment to the carbon surface revealed high dependency on the metal center oxidation state, the solvent, and the electric field. Also, the reactivity of the oligomer has been explored using the Empirical Valence Bond approach, revealing that the O-O bond formation remains unaltered in the oligomer and the reactivity remains unaltered in this complex environment, in agreement with experimental results. Finally, the substitution of the carboxylates in the Ru(tda) by phosphonates (Ru(tPaO)) has been proved to double the efficiency of the molecular catalyst at neutral pH. Due to the similarities between carboxylates and phosphonates the oxide relay mechanism has been tested in the Ru(tPaO), revealing that the origin of the extreme reactivity comes from low barriers in all the steps. The O-O bond formation involves an intramolecular radical coupling lowering the activation barrier to 2.1 kcal/mol. This radical coupling revealed a variation of the oxide relay mechanism called the radical oxide relay mechanism.
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| 9. |
- 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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| 10. |
- 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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