| 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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| 2. |
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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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| 6. |
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| 7. |
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| 8. |
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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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| 11. |
- 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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| 12. |
- 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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| 13. |
- 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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| 14. |
- Vojvodic, Aleksandra, 1981
(författare)
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Steam Reforming on Transition-Metal Carbides from Density-Functional Theory
- 2012
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Ingår i: Catalysis Letters. - : Springer Science and Business Media LLC. - 1572-879X .- 1011-372X. ; 142:6, s. 728-735
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Tidskriftsartikel (refereegranskat)abstract
- A screening study of the steam reforming reaction on clean and oxygen covered early transition-metal carbides surfaces is performed by means of density-functional theory calculations. It is found that carbides provide a wide spectrum of reactivities, from too reactive via suitable to too inert. Several molybdenum-based systems are identified as possible steam reforming catalysts. The findings suggest that carbides provide a playground for reactivity tuning, comparable to the one for pure metals.
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| 15. |
- Vojvodic, Aleksandra, 1981
(författare)
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Towards a Chemisorption Model for Transition-Metal Nitrides and Carbides from Density-Functional Theory: A TiN and TiC Case Study
- 2006
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Since the 1960's, a number of models for chemisorption on metals has beenpresented, most notably the Anderson-Grimley-Newns model for metals and thed-band model of Hammer and Nørskov for transition metals. The intention of this thesis is to explore the adsorption on transition-metal nitrides andcarbides, technologically interesting materials (in, e.g., cutting tools,microelectronics, and bio-implants, and as promising catalyst materials)due to their combination of metallicity, covalency, and ionicity. Theaim of this study is to identify and understand the key microscopic mechanisms behind the adsorption on the polar and reactive (111)surfaces of titanium nitride and carbide, which have long been consideredprototypes for the early transition-metal nitrides and carbides. The chemisorption of atoms from the first three periods in the periodic table(H, B, C, N, O, F, Al, Si, P, S, and Cl) on the TiN(111) surface is studiedin a systematic manner by density-functional theory (DFT) and compared with corresponding results on TiC(111). The adsorption on the TiX's (X = C, N) isfound to be more complex and versatile than on transition-metal surfaces. Thetrends in adsorption energies Eads have similar characteristics onboth the Ti-terminated TiX(111) surfaces. The Eads trends for period2 and 3 adatoms show a great variation and are found to vary along theperiods in a Sabatier-like pyramidic way, with an extraordinarily strongchemisorption for the O atom. The preference for adsorption on TiC vs. TiN varies depending on the adsorbate element, with TiC showing astronger chemisorption than TiN for all elements except B and Al. Thisvariety in the calculated adsorption energies is explained with the concerted-coupling model (CCM), which originally was proposed for adsorption on the Ti-terminated TiC(111) surface and here is shown to be generalisable to adsorption on Ti-terminated TiN(111). The model describes the chemisorption to be based on the concerted action of two different types ofcouplings: (i) between the adatom state and a Ti-localised surface resonance(TiSR), and (ii) between the adatom state and X-localised surface resonances(XSR's). The CCM is also able to explain the similarities and differences inthe Eads trends between TiN(111) and TiC(111). In addition, it is shown that the theoretical stabilities in vacuum of the N-and Ti-terminated TiN(111) surfaces are very close. Therefore, adsorption onthe N-terminated TiN(111) surface is also considered, revealing an``inverted'' pyramid-like adsorption energy trend for period 2 and 3 adatomsand indicating a concerted-coupling mechanism on this surface as well.
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| 16. |
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| 17. |
- Vojvodic, Aleksandra, 1981, et al.
(författare)
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Trends in bulk electron-structural features of rocksalt early transition-metal carbides
- 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
- A detailed and systematic density-functional theory (DFT) study of a series of early transition-metal carbides (TMCs) in the NaCl structure is presented. The focus is on the trends in the electronic structure and nature of bonding, which are essential for the understanding of the reactivity of TMCs. The employed approach is based on a thorough complementary analysis of the electron density differences, the density of states (DOS), the band structure and the real-space wavefunctions to gain an insight into the bonding of this class of materials and get a more detailed picture of it than previously achieved, as the trend study allows for a systematic identification of the bond character along the different bands. Our approach confirms the presence of both the well-known TM-C and TM-TM bonds and, more importantly, it shows the existence and significance of direct C-C bonds in all investigated TMCs, which are frequently neglected but have been identified in some cases (Zhang et al 2002 Solid State Commun. 121 411; Ruberto et al 2007 Phys. Rev. B 75 235438). New information on the spatial extent of the bonds, their k-space location within the band structure and their importance for the bulk cohesion is provided. Trends in covalency and ionicity are presented. The resulting electron-structural trends are analyzed and discussed within a two-level model.
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| 18. |
- Walton, Alex S., et al.
(författare)
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Interface Controlled Oxidation States in Layered Cobalt Oxide Nanoislands on Gold
- 2015
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Ingår i: ACS Nano. - : American Chemical Society (ACS). - 1936-086X .- 1936-0851. ; 9:3, s. 2445-2453
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Tidskriftsartikel (refereegranskat)abstract
- Layered cobalt oxides have been shown to be highly active catalysts for the oxygen evolution reaction (OER; half of the catalytic "water splitting" reaction), particularly when promoted with gold. However, the surface chemistry of cobalt oxides and in particular the nature of the synergistic effect of gold contact are only understood on a rudimentary level, which at present prevents further exploration. We have synthesized a model system of flat, layered cobalt oxide nanoislands supported on a single crystal gold (111) substrate. By using a combination of atom-resolved scanning tunneling microscopy, X-ray photoelectron and absorption spectroscopies and density functional theory calculations, we provide a detailed analysis of the relationship between the atomic-scale structure of the nanoislands, Co oxidation states and substrate induced charge transfer effects in response to the synthesis oxygen pressure. We reveal that conversion from CO2+ to Co3+ can occur by a facile incorporation of oxygen at the interface between the nanoisland and gold, changing the islands from a Co-O bilayer to an O-Co-O trilayer. The O-Co-O trilayer islands have the structure of a single layer of beta-CoOOH, proposed to be the active phase for the OER, making this system a valuable model in understanding of the active sites for OER. The Co oxides adopt related island morphologies without significant structural reorganization, and our results directly demonstrate that nanosized Co oxide islands have a much higher structural flexibility than could be predicted from bulk properties. Furthermore, it is clear that the gold/nanoparticle interface has a profound effect on the structure of the nanoislands, suggesting a possible promotion mechanism.
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| 19. |
- Öberg, Henrik, et al.
(författare)
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Stability of Pt-Modified Cu(111) in the Presence of Oxygen and Its Implication on the Overall Electronic Structure
- 2013
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Ingår i: The Journal of Physical Chemistry C. - : American Chemical Society (ACS). - 1932-7447 .- 1932-7455. ; 117:32, s. 16371-16380
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Tidskriftsartikel (refereegranskat)abstract
- The electronic structure and stability of Cu(111)-hosted Pt overlayers with and without the presence of atomic oxygen have been studied by means of core-level spectroscopy and density functional theory (DFT). Because of lattice mismatch, Pt(111) overlayers grown on Cu(111) are compressively strained, and hard X-ray photoelectron spectroscopy together with Pt L-3-edge X-ray absorption spectroscopy (XAS) reveals a pronounced downshift of the Pt d-band owing to the increased overlap of the d-orbitals, an effect also reproduced theoretically. Exposure to oxygen severely alters the surface composition; the O-Cu binding energy largely exceeds that of O-Pt, and DFT calculations predict surface segregation of Cu atoms. Comparing the adsorbate electronic structure for O on unstrained Pt(111) with that of O on Pt-modified Cu(111) using O K-edge XAS and X-ray emission spectroscopy salient differences are observed and calculations show that Cu-segregation to the topmost layer is required to reproduce the measured spectra. It is proposed that O is binding in a hollow site constituted by at least two Cu atoms and that up to 75% of the Pt atoms migrate below the surface.
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