| 1. |
- Kullgren, Jolla, 1978-, et al.
(författare)
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Many competing ceria (110) oxygen vacancy structures : From small to large supercells
- 2012
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Ingår i: Journal of Chemical Physics. - : AIP Publishing. - 0021-9606 .- 1089-7690. ; 137:4, s. 044705-
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Tidskriftsartikel (refereegranskat)abstract
- We present periodic "DFT+U" studies of single oxygen vacancies on the CeO2(110) surface using a number of different supercells, finding a range of different local minimum structures for the vacancy and its two accompanying Ce(III) ions. We find three different geometrical structures in combination with a variety of different Ce(III) localization patterns, several of which have not been studied before. The desired trapping of electrons was achieved in a two-stage optimization procedure. We find that the surface oxygen nearest to the vacancy either moves within the plane towards the vacancy, or rises out of the surface into either a symmetric or an unsymmetric bridge structure. Results are shown in seven slab geometry supercells, p(2 x 1), p(2 x 2), p(2 x 3), p(3 x 2), p(2 x 4), p(4 x 2), and p(3 x 3), and indicate that the choice of supercell can affect the results qualitatively and quantitatively. An unsymmetric bridge structure with one nearest and one next-nearest neighbour Ce(III) ion (a combination of localizations not previously found) is the ground state in all (but one) of the supercells studied here, and the relative stability of other structures depends strongly on supercell size. Within any one supercell the formation energies of the different vacancy structures differ by up to 0.5 eV, but the same structure can vary by up to similar to 1 eV between supercells. Furthermore, finite size scaling suggests that the remaining errors (compared to still larger supercells) can also be similar to 1 eV for some vacancy structures.
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| 2. |
- Lu, Zhansheng, et al.
(författare)
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Cu-doped ceria : Oxygen vacancy formation made easy
- 2011
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Ingår i: Chemical Physics Letters. - : Elsevier BV. - 0009-2614 .- 1873-4448. ; 510:1-3, s. 60-66
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Tidskriftsartikel (refereegranskat)abstract
- DFT + U calculations of Cu-doped bulk ceria are presented. The first oxygen vacancy in Cu-doped ceria forms almost spontaneously and the second vacancy is also easily created. Whether zero, one or two oxygen vacancies, the Cu dopant is in the form Cu(+ II), and prefers to be 4-coordinated in a close to planar structure. Charge compensation, structural relaxation and available Cu-O states all play roles in lowering the O vacancy formation energies, but to different degrees when the first and second oxygen vacancies are formed. The Cu-doped ceria(1 1 1) surface system behaves in a similar fashion.
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| 3. |
- Duan, Sai, et al.
(författare)
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Thermal effects on electronic properties of CO/Pt(111) in water
- 2013
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Ingår i: Physical Chemistry, Chemical Physics - PCCP. - : Royal Society of Chemistry (RSC). - 1463-9076 .- 1463-9084. ; 15:32, s. 13619-13627
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Tidskriftsartikel (refereegranskat)abstract
- Structure and adsorption energy of carbon monoxide molecules adsorbed on the Pt(111) surfaces with various CO coverages in water as well as work function of the whole systems at room temperature of 298 K were studied by means of a hybrid method that combines classical molecular dynamics and density functional theory. We found that when the coverage of CO is around half monolayer, i.e. 50%, there is no obvious peak of the oxygen density profile appearing in the first water layer. This result reveals that, in this case, the external force applied to water molecules from the CO/Pt(111) surface almost vanishes as a result of the competitive adsorption between CO and water molecules on the Pt(111) surface. This coverage is also the critical point of the wetting/non-wetting conditions for the CO/Pt(111) surface. Averaged work function and adsorption energy from current simulations are consistent with those of previous studies, which show that thermal average is required for direct comparisons between theoretical predictions and experimental measurements. Meanwhile, the statistical behaviors of work function and adsorption energy at room temperature have also been calculated. The standard errors of the calculated work function for the water-CO/Pt(111) interfaces are around 0.6 eV at all CO coverages, while the standard error decreases from 1.29 to 0.05 eV as the CO coverage increases from 4% to 100% for the calculated adsorption energy. Moreover, the critical points for these electronic properties are the same as those for the wetting/non-wetting conditions. These findings provide a better understanding about the interfacial structure under specific adsorption conditions, which can have important applications on the structure of electric double layers and therefore offer a useful perspective for the design of the electrochemical catalysts.
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| 4. |
- Hellström, Matti, et al.
(författare)
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Water-Induced Oxidation and Dissociation of Small Cu Clusters on ZnO(101̅0)
- 2015
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Ingår i: The Journal of Physical Chemistry C. - : American Chemical Society (ACS). - 1932-7447 .- 1932-7455. ; 119:3, s. 1382-1390
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Tidskriftsartikel (refereegranskat)abstract
- The interaction between water molecules and small Cu clusters (up to a size of four atoms) adsorbed on the nonpolar ZnO(10 (1) over bar0) surface has been studied using hybrid density functional theory. We find that the water molecules can give rise to different scenarios: (i) In contrast to water adsorption on the clean ZnO(10 (1) over bar0) surface, which occurs molecularly, the first water molecule often preferentially dissociates upon adsorption on the Cu cluster, which may be a key step in the watergas shift reaction. (ii) While the adsorption of the first water molecule on the adsorbed Cu clusters is always more favorable than the adsorption on the bare ZnO surface, the opposite is true for the second molecule. (iii) As a water molecule adsorbs on the adsorbed Cu atom, it induces charge transfer between the Cu and the ZnO, so that an electron from the Cu atom populates the ZnO conduction band (giving an oxidized Cu species). (iv) Water molecule adsorption on the adsorbed Cu trimer results in a spontaneous dissociation of the Cu trimer into an adsorbed dimer and an adsorbed atom, after which the water molecule adsorbs on the atom, again resulting in the Cu-ZnO charge transfer. We also show that the use of a hybrid density functional gives qualitatively different results as compared to a semilocal density functional for this system, and we explain this in terms of the underestimation of the ZnO band gap obtained with the semilocal functional.
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| 5. |
- Hermansson, Kersti, Professor
(författare)
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DFT-based multi-scale modelling of materials and nanoparticles
- 2017
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- Redox-active metal oxide surfaces and interfaces ‒ such as electrodes, catalysts, and sensors ‒ play crucial roles in our society and in the development of new materials and greener technologies. In the scientific literature, a full arsenal of experimental methods are being used to help to characterize such materials interfaces. Simultaneously, the (ever-increasing) theoretical materials studies in the literature provide structural and mechanistic information at a detail that is difficult to beat by experiments – but are the models accurate enough? There are at least two major challenges in materials modelling: (i) how to build structural models that capture the complexity and imperfections of the real systems, and (ii) how to find good enough interaction models (say a DFT functional [1] or a force-field). Here mimicking the interactions and chemical properties of materials without explicit electrons present is a formidable task, especially when the transfer of electrons is closely coupled to the material's functionality, as is the case for redox-active metal oxides.I will discuss some of our efforts in the development of a multiscale modelling approach for surfaces and interfaces of metal oxides (e.g. CeO2, ZnO, MgO) – with and without interacting molecules (e.g. O2 and water).In summary, we combine a range of theoretical methods including DFT [2], tight-binding-DFT [3], and reactive force-field simulations [4] in a consistent multi-scale approach to examine the properties of oxide nanosystems. We generate images and spectra to make direct comparisons with the experimental counterparts (e.g. IRRAS spectra [5]), but we also generate properties that cannot be measured by experiments such as the water dipole moment enhancement on a surface (often much larger [1] than in liquid water!). I will also inform about the European Materials Modelling Council (https://emmc.info/), and our efforts to promote the use and quality of materials modelling in industry; the EMMC is open to everyone interested. References[1] G. G. Kebede, D. Spångberg, P. D. Mitev, P. Broqvist, K. Hermansson, "Comparing van der Waals DFT methods for water on NaCl(001) and MgO(001), The Journal of Chemical Physics 146, 064703 (2017). [2] M. Hellström, D. Spångberg, K. Hermansson, "Treatment of Delocalized Electron Transfer in Periodic and Embedded Cluster DFT Calculations: The Case of Cu on ZnO (10-10)", Journal of Computational Chemistry 36, 2394 (2015). [3] J. Kullgren, M. J. Wolf, K. Hermansson, Ch. Köhler, B. Aradi,Th. Frauenheim, and P. Broqvist, "Self-Consistent-Charge Density-Functional Tight-Binding (SCC-DFTB) Parameters for Ceria in 0D to 3D". J. Phys. Chem. C 121, 4593−4607 (2017). [4] P. Broqvist, J. Kullgren, M. J. Wolf, A. C. T. van Duin, K. Hermansson, "A ReaxFF force-field for ceria bulk, surfaces and nanoparticles", J. Phys. Chem. C 119, 13598 (2015). [5] S. Hu, Z. Wang, A. Mattsson, L. Österlund, K. Hermansson, "Simulation of IRRAS Spectra for Molecules on Oxide Surfaces: CO on TiO2(110)", J. Phys. Chem. C 119, 5403 (2015).
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| 6. |
- Hermansson, Kersti, Professor
(författare)
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Hydrogen bonds – on the move
- 2017
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- Hydrogen bonds are quite special bonds: they are strong enough to significantly modify a molecule’s properties and hold it in place in a structure, yet weak enough to be distorted or broken fairly easily by an external stimulus, an incoming reactant or a by-passing molecule. Thus, its place between the strong normal chemical bonds and the weakest van der Waals interactions makes the H-bond a very versatile and useful actor – as demonstrated by nature in numerous examples from Biology to Materials Science.This functional diversity is a unique and important feature, and an opportunity, but also a challenge since no controlled use of the remarkable properties of H-bonds can be achieved without a thorough understanding of their multi-functionality. Here computational approaches at a range of time and length scales can be of immense help as they provide results of unmatched detail, as well as the needed atomic-level understanding ‒ if the computational models and methods are accurate and realistic enough. Many such efforts can be found in the literature.*) At the same time, new powerful experimental characterization techniques emerge and new infrastructures are under development in Europe. The interplay between experiment and theory is becoming even more compelling as the dimensions of experimental and computational targets approach each other. **) *) Some published [1,2,3] and unpublished examples of our own efforts here for H-bonded systems (solutions, solids, surfaces) will also be mentioned.*) Here I will also inform about new European initiatives on this topic such as the European Materials Modelling Council. References [1] The vibrating hydroxide ion in water (Perspectives article), K. Hermansson, Ph.A. Bopp, D. Spångberg, Lj. Pejov, I. Bako, P. D. Mitev; Chemical Physics Letters 514, (2011), 1.[2] H-bond and Electric Field Correlations for Water in Highly Hydrated Crystals (invited), A. Sen, P. Mitev, A. Eriksson, K. Hermansson, Int. J. Quantum Chemistry 116, (2016), 57.[3] Comparing van der Waals DFT methods for water on NaCl(001) and MgO(001), G. G. Kebede, D. Spångberg, P.D. Mitev, P. Broqvist, and K.Hermansson, J. Chem. Phys. 146, (2017), 064703.
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| 7. |
- Hermansson, Kersti, Professor
(författare)
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Multiscale modelling of metal oxide interfaces and nanoparticles
- 2017
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- Redox-active metal oxide surfaces and interfaces ‒ such as electrodes, catalysts, and sensors ‒ play crucial roles in our society and in the development of new materials and greener technologies. In the scientific literature, a full arsenal of experimental methods are being used to help characterize such interfaces. At the same time, the number of theoretical studies in the literature steadily increases, providing mechanistic information at a detail that is hard to beat by experiment. Are such theoretical results accurate enough? Here the major challenges are (i) how to build a structural model that captures the complexity and imperfections of the real system at hand, and (ii) how to find an interaction model/a materials relation (say a DFT functional [1] or a force-field) that is good enough.A 5 nm metal oxide nanoparticle may be very small to an experimentalist, but it contains many thousand atoms, making standard quantum-mechanical (e.g. regular DFT) methods totally unfeasible. Can force-field calculations be used instead? Well, mimicking the interactions and chemical properties without explicit electrons present is a formidable task, especially when the transfer of electrons is closely coupled to the material's functionality, as is the case for redox-active metal oxides. I will discuss some of our efforts in the development of a multiscale modelling approach for surfaces and interfaces of metal oxides (e.g. CeO2, ZnO, MgO) – with and without interacting molecules (e.g. O2 and water).In summary, we combine a range of theoretical methods including DFT [2], tight-binding-DFT [3], and reactive force-field simulations [4] in a consistent multi-scale approach to examine the properties of oxide nanosystems. We generate images and spectra to make direct comparisons with the experimental couterparts (e.g. IRRAS spectra [5] and a new unpublished approach to predict vibrational spectra for OH-covered metal oxides), but we also generate properties that cannot be measured by experiments such as the water dipole moment enhancement on a surface (oftem much larger than in liquid water!). I will also inform about the European Materials Modelling Council (https://emmc.info/), and our efforts to promote the use of materials modelling in industry and the quality of the modelling results; the EMMC is open to everyone interested.References:[1] G. G. Kebede, D. Spångberg, P. D. Mitev, P. Broqvist, K. Hermansson, "Comparing van der Waals DFT methods for water on NaCl(001) and MgO(001), The Journal of Chemical Physics 146, 064703 (2017). [2] M. Hellström, D. Spångberg, K. Hermansson, "Treatment of Delocalized Electron Transfer in Periodic and Embedded Cluster DFT Calculations: The Case of Cu on ZnO (10-10)", Journal of Computational Chemistry 36, 2394 (2015). [3] J. Kullgren, M. J. Wolf, K. Hermansson, Ch. Köhler, B. Aradi,Th. Frauenheim, and P. Broqvist, "Self-Consistent-Charge Density-Functional Tight-Binding (SCC-DFTB) Parameters for Ceria in 0D to 3D". J. Phys. Chem. C 121, 4593−4607 (2017). [4] P. Broqvist, J. Kullgren, M. J. Wolf, A. C. T. van Duin, K. Hermansson, "A ReaxFF force-field for ceria bulk, surfaces and nanoparticles", J. Phys. Chem. C 119, 13598 (2015). [5] S. Hu, Z. Wang, A. Mattsson, L. Österlund, K. Hermansson, "Simulation of IRRAS Spectra for Molecules on Oxide Surfaces: CO on TiO2(110)", J. Phys. Chem. C 119, 5403 (2015).
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| 8. |
- Hermansson, Kersti, Professor
(författare)
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Multiscale modelling of reactive metal oxide interfaces
- 2017
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- Chemically active metal oxide surfaces and interfaces ‒catalysts, sensors, electrodes‒ play crucial roles in our society and in the development of new technologies. Modelling such complex systems is by no means easy, and the computational scientist needs to make shrewd decisions about both the choice of structural model for the interface and the choice of total-energy method. This presentation concerns static and dynamic condensed-matter chemistry modelling of metal oxide surfaces, interfaces, nanoparticles. I will discuss some of our efforts to develop multiscale modelling protocols for metal oxide surfaces, nanoparticles and interfaces (e.g. CeO2 and ZnO) – with and without interacting molecules. We combine a range of theoretical methods including DFT, tight-binding-DFT, and reactive force-field models. A key question here is whether it is really possible to model redox-active metal oxides without including the electrons? Adequate models for post-processing of simulation data is as important as the data generation itself, since the post-processing links directly to experimental methods for e.g. surface characterization, such as spectra and images. I will also discuss some of our efforts in this field. Chemically active metal oxide surfaces and interfaces ‒catalysts, sensors, electrodes‒ play crucial roles in our society and in the development of new technologies. Modelling such complex systems is by no means easy, and the computational scientist needs to make shrewd decisions about both the choice of structural model for the interface and the choice of total-energy method. . I will discuss some of our efforts to develop multiscale modelling protocols for metal oxide surfaces, nanoparticles and interfaces (e.g. Ceria and ZnO) – with and without interacting molecules. We combine a range of theoretical methods including DFT, tight-binding-DFT, and reactive force-field models. A key question here is: Is it possible to model redox-active metal oxides without including the electrons? Adequate models for post-processing of simulation data is as important as the data generation itself, since the post-processing links directly to experimental methods for, e.g., surface characterization, such as spectra and images. I will also discuss some of our efforts in this field.References: [1] M. Hellström, K. Jorner, M. Bryngelsson, S.E. Huber, J. Kullgren, Th. Frauenheim, P. Broqvist, "An SCC-DFTB Repulsive Potential for Various ZnO Polymorphs and the ZnO-Water System", J. Phys. Chem. C, 2013, 117, 17004.[2] P. Broqvist, J. Kullgren, M. J. Wolf, A. C. T. van Duin, K. Hermansson, "A ReaxFF force-field for ceria bulk, surfaces and nanoparticles", J. Phys. Chem. C, 2015, 119, 13598.[3] M. Hellström, D. Spångberg, K. Hermansson, "Treatment of Delocalized Electron Transfer in Periodic and Embedded Cluster DFT Calculations: The Case of Cu on ZnO (10-10)", Journal of Computational Chemistry, 2015, 36, 2394.[4] S. Hu, Z. Wang, A. Mattsson, L. Österlund, K. Hermansson, "Simulation of IRRAS Spectra for Molecules on Oxide Surfaces: CO on TiO2(110)", J. Phys. Chem. C, 2015, 119, 5403.
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| 9. |
- Wolf, Matthew J., et al.
(författare)
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Oxygen chemistry of halogen-doped CeO2(111)
- 2021
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Ingår i: Physical Chemistry, Chemical Physics - PCCP. - : Royal Society of Chemistry. - 1463-9076 .- 1463-9084. ; 23:35, s. 19375-19385
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Tidskriftsartikel (refereegranskat)abstract
- We study substitutional fluorine, chlorine and bromine impurities at CeO2(111), and their effects on the oxygen chemistry of the surface, using density functional theory. We find that impurity formation results in a halide ion and one Ce3+ ion for all three halogens, although the formation energy depends strongly on the identity of the halogen; however, once formed, all three halogens exhibit a similar propensity to form impurity-impurity pairs. Furthermore, while the effects of halogen impurities on oxygen vacancy formation are marginal, they are more significant for oxygen molecule adsorption, due to electron transfer from the Ce3+ ion which results in an adsorbed superoxide molecule. We also consider the displacement of a halide ion on to the surface by half of an oxygen molecule, and find that the energy required to do so depends strongly not only on the identity of the halogen, but also on whether or not a second halogen impurity, with its associated Ce3+ ion, is present; if it is, then the process is greatly facilitated. Overall, our results demonstrate the existence of a rich variety of ways in which the oxygen chemistry of CeO2(111) may be modified by the presence of halogen dopants.
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| 10. |
- Wolf, Matthew J., et al.
(författare)
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STM Images of Anionic Defects at CeO2(111)-A Theoretical Perspective
- 2019
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Ingår i: Frontiers in Chemistry. - : FRONTIERS MEDIA SA. - 2296-2646. ; 7
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Tidskriftsartikel (refereegranskat)abstract
- We present a theoretically oriented analysis of the appearance and properties of plausible candidates for the anionic defects observed in scanning tunneling microscopy (STM) experiments on CeO2(111). The simulations are based on density functional theory (DFT) and cover oxygen vacancies, fluorine impurities and hydroxyl groups in the surface and sub-surface layers. In the surface layer, all three appear as missing spots in the oxygen sublattice in filled state simulated STM images, but they are distinguishable in empty state images, where surface oxygen vacancies and hydroxyls appear as, respectively, diffuse and sharp bright features at oxygen sites, while fluorine defects appear as triangles of darkened Ce ions. In the sub-surface layer, all three defects present more complex patterns, with different combinations of brightened oxygen ion triangles and/or darkened Ce ion triangles, so we provide image maps to support experimental identification. We also discuss other properties that could be used to distinguish the defects, namely their diffusion rates and distributions.
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