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Publications (10 of 15) Show all publications
Kettner, M., Duchon, T., Wolf, M. J., Kullgren, J., Senanayake, S. D., Hermansson, K., . . . Nehasil, V. (2019). Anion-mediated electronic effects in reducible oxides: Tuning the valence band of ceria via fluorine doping. Journal of Chemical Physics, 151(4), Article ID 044701.
Open this publication in new window or tab >>Anion-mediated electronic effects in reducible oxides: Tuning the valence band of ceria via fluorine doping
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2019 (English)In: Journal of Chemical Physics, ISSN 0021-9606, E-ISSN 1089-7690, Vol. 151, no 4, article id 044701Article in journal (Refereed) Published
Abstract [en]

Combining experimental spectroscopy and hybrid density functional theory calculations, we show that the incorporation of fluoride ions into a prototypical reducible oxide surface, namely, ceria(111), can induce a variety of nontrivial changes to the local electronic structure, beyond the expected increase in the number of Ce3+ ions. Our resonant photoemission spectroscopy results reveal new states above, within, and below the valence band, which are unique to the presence of fluoride ions at the surface. With the help of hybrid density functional calculations, we show that the different states arise from fluoride ions in different atomic layers in the near surface region. In particular, we identify a structure in which a fluoride ion substitutes for an oxygen ion at the surface, with a second fluoride ion on top of a surface Ce4+ ion giving rise to F 2p states which overlap the top of the O 2p band. The nature of this adsorbate F--Ce4+ resonant enhancement feature suggests that this bond is at least partially covalent. Our results demonstrate the versatility of anion doping as a potential means of tuning the valence band electronic structure of ceria.

National Category
Theoretical Chemistry Condensed Matter Physics
Identifiers
urn:nbn:se:uu:diva-393127 (URN)10.1063/1.5109955 (DOI)000478625700040 ()31370552 (PubMedID)
Funder
Swedish Research CouncileSSENCE - An eScience Collaboration
Note

M. Kettner, T. Duchoň, and M. J. Wolf contributed equally to this work.

Available from: 2019-09-24 Created: 2019-09-24 Last updated: 2019-09-24Bibliographically approved
Kim, B.-H., Kullgren, J., Wolf, M. J., Hermansson, K. & Broqvist, P. (2019). Multiscale Modeling of Agglomerated Ceria Nanoparticles: Interface Stability and Oxygen Vacancy Formation. Frontiers in Chemistry, 7, Article ID 203.
Open this publication in new window or tab >>Multiscale Modeling of Agglomerated Ceria Nanoparticles: Interface Stability and Oxygen Vacancy Formation
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2019 (English)In: Frontiers in Chemistry, E-ISSN 2296-2646, Vol. 7, article id 203Article in journal (Refereed) Published
Abstract [en]

The interface formation and its effect on redox processes in agglomerated ceria nanoparticles (NPs) have been investigated using a multiscale simulation approach with standard density functional theory (DFT), the self-consistent-charge density functional tight binding (SCC-DFTB) method, and a DFT-parameterized reactive force-field (ReaxFF). In particular, we have modeled Ce40O80 NP pairs, using SCC-DFTB and DFT, and longer chains and networks formed by Ce40O80 or Ce132O264 NPs, using ReaxFF molecular dynamics simulations. We find that the most stable {111}/{111} interface structure is coherent whereas the stable {100}/{100} structures can be either coherent or incoherent. The formation of {111}/{111} interfaces is found to have only a very small effect on the oxygen vacancy formation energy, E-vac. The opposite holds true for {100}/{100} interfaces, which exhibit significantly lower E-vac values than the bare surfaces, despite the fact that the interface formation eliminates reactive {100} facets. Our results pave the way for an increased understanding of ceria NP agglomeration.

Place, publisher, year, edition, pages
FRONTIERS MEDIA SA, 2019
Keywords
multiscale modeling, density functional theory, self-consistent charge density functional tight binding, reducible oxides, cerium dioxide, nanoparticles, agglomeration
National Category
Theoretical Chemistry
Identifiers
urn:nbn:se:uu:diva-385974 (URN)10.3389/fchem.2019.00203 (DOI)000468727800001 ()
Funder
Swedish Research CouncilÅForsk (Ångpanneföreningen's Foundation for Research and Development)eSSENCE - An eScience Collaboration
Available from: 2019-06-19 Created: 2019-06-19 Last updated: 2019-06-19Bibliographically approved
Wolf, M. J., Castleton, C. W. M., Hermansson, K. & Kullgren, J. (2019). STM Images of Anionic Defects at CeO2(111)-A Theoretical Perspective. Frontiers in Chemistry, 7, Article ID 212.
Open this publication in new window or tab >>STM Images of Anionic Defects at CeO2(111)-A Theoretical Perspective
2019 (English)In: Frontiers in Chemistry, E-ISSN 2296-2646, Vol. 7, article id 212Article in journal (Refereed) Published
Abstract [en]

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.

Keywords
simulated STM images, cerium dioxide (CeO2), density functional theory, anionic defects, reducible oxide
National Category
Condensed Matter Physics Physical Chemistry
Identifiers
urn:nbn:se:uu:diva-389601 (URN)10.3389/fchem.2019.00212 (DOI)000471137100001 ()
Funder
Swedish Research CouncileSSENCE - An eScience CollaborationÅForsk (Ångpanneföreningen's Foundation for Research and Development)
Available from: 2019-07-24 Created: 2019-07-24 Last updated: 2019-07-24Bibliographically approved
Du, D., Wolf, M. J., Hermansson, K. & Broqvist, P. (2018). Screened hybrid functionals applied to ceria: Effect of Fock exchange. Physical Review B, 97(23), Article ID 235203.
Open this publication in new window or tab >>Screened hybrid functionals applied to ceria: Effect of Fock exchange
2018 (English)In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 97, no 23, article id 235203Article in journal (Refereed) Published
Abstract [en]

We investigate how the redox properties of ceria are affected by the fraction of Fock exchange in screened HSE06-based hybrid density functionals, and we compare with PBE+U results, and with experiments when available. We find that using similar to 15% Fock exchange yields a good compromise with respect to structure, electronic structure, and calculated reduction energies, and represents a significant improvement over the PBE+U results. We also investigate the possibility to use a computationally cheaper HSE06//PBE+U protocol consisting of structure optimization with PBE+U, a subsequent lattice parameter rescaling step, and, finally, a single-point full hybrid calculation. We find that such a composite computational protocol works very well and yields results in close agreement with those where HSE06 was used also for the structure optimization.

National Category
Physical Sciences
Identifiers
urn:nbn:se:uu:diva-372692 (URN)10.1103/PhysRevB.97.235203 (DOI)000436035400005 ()
Available from: 2019-01-08 Created: 2019-01-08 Last updated: 2019-04-24Bibliographically approved
Kullgren, J., Wolf, M. J., Mitev, P. D., Hermansson, K. & Briels, W. J. (2017). Defect cluster at the CeO2(111) surface: A combined DFT and Monte-Carlo study. In: : . Paper presented at Annual meeting of the Swedish Chemical Society Theoretical Chemistry Section, Göteborg 16-18 August.
Open this publication in new window or tab >>Defect cluster at the CeO2(111) surface: A combined DFT and Monte-Carlo study
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2017 (English)Conference paper, Oral presentation with published abstract (Other academic)
National Category
Theoretical Chemistry
Identifiers
urn:nbn:se:uu:diva-338316 (URN)
Conference
Annual meeting of the Swedish Chemical Society Theoretical Chemistry Section, Göteborg 16-18 August
Available from: 2018-01-08 Created: 2018-01-08 Last updated: 2019-02-19Bibliographically approved
Pazoki, M., Röckert, A., Wolf, M. J., Imani, R., Edvinsson, T. & Kullgren, J. (2017). Electronic structure of organic–inorganic lanthanide iodide perovskite solar cell materials. Journal of Materials Chemistry A, 5, 23131-23138
Open this publication in new window or tab >>Electronic structure of organic–inorganic lanthanide iodide perovskite solar cell materials
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2017 (English)In: Journal of Materials Chemistry A, ISSN 2050-7488, Vol. 5, p. 23131-23138Article in journal (Refereed) Published
Abstract [en]

The emergence of highly efficient lead halide perovskite solar cell materials makes the exploration and engineering of new lead free compounds very interesting both from a fundamental perspective as well as for potential use as new materials in solar cell devices. Herein we present the electronic structure of several lanthanide (La) based materials in the metalorganic halide perovskite family not explored before. Our estimated bandgaps for the lanthanide (Eu, Dy, Tm, Yb) perovskite compounds are in the range of 2.0–3.2 eV showing the possibility for implementation as photo-absorbers in tandem solar cell configurations or charge separating materials. We have estimated the typical effective masses of the electrons and holes for MALaI3 (La= Eu, Dy, Tm, Yb) to be in the range of 0.3–0.5 and 0.97–4.0 units of the free electron mass, respectively. We have shown that the localized f-electrons within our DFT+U approach, make the dominant electronic contribution to the states at the top of the valence band and thus have a strong impact on the photo-physical properties of the lanthanide perovskites. Therefore, the main valence to conduction band electronic transition for MAEuI3 is based on inner shell f-electron localized states within a periodic framework of perovskite crystal by which the optical absorption onset would be rather inert with respect to quantum confinement effects. The very similar crystal structure and lattice constant of the lanthanide perovskites to the widely studied CH3NH3PbI3 perovskite, are prominent advantages for implementation of these compounds in tandem or charge selective contacts in PV applications together with lead iodide perovskite devices

National Category
Physical Chemistry Engineering and Technology
Identifiers
urn:nbn:se:uu:diva-332883 (URN)10.1039/C7TA07716E (DOI)000415070100029 ()
Available from: 2017-11-02 Created: 2017-11-02 Last updated: 2019-02-19Bibliographically approved
Wolf, M. J., Hermansson, K., Mitev, P. D., Briels, W. & Kullgren, J. (2017). Fluorine clusters at CeO2(111) - A DFT+U and Monte Carlostudy. In: : . Paper presented at Towards Reality in Nanoscale Materials IX Nanoscale Materials for Warfare Agent Detection: Nanoscience for Security 13th – 16th February 2017 Levi, Finland.
Open this publication in new window or tab >>Fluorine clusters at CeO2(111) - A DFT+U and Monte Carlostudy
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2017 (English)Conference paper, Oral presentation with published abstract (Other academic)
Abstract [en]

STM experiments on CeO2(111) reveal depressions in the surface oxygen sub-lattice which are observed to form clusters of various shapes and sizes [1].While these depressions were assumed to be oxygen vacancies, subsequent DFTcalculations have indicated that clusters of oxygen vacancies are energeticallyunstable [2-4]. Recently, we showed theoretically that fluorine impurities shouldappear almost identical to oxygen vacancies in STM experiments, but that theirproperties are more in line with those of the defects observed in experiments [5].Here, I will present the results of a further investigation into the distribution ofF impurity clusters at CeO2(111), using a combination of DFT+U calculations,and Monte Carlo sampling based on a simple but accurate pair potential whichwas fitted to the DFT results. The distribution is characterised in terms of thenumber of clusters of a certain size, and also on their topology, i.e. whetherthey are compact or open/linea r. Our results compare favourably with theexperiments, and also exhibit some interesting physics in their own right.

[1] F. Esch et al., Science 309, 752 (2005).[2] J. Conesa, Cat. Today 143, 315 (2009).[3] C. Zhang et al., Phys. Rev. B 79, 075433 (2009).[4] X.-P. Wu & X.-Q. Gong, Phys. Rev. Lett. 116, 086102 (2016).[5] J. Kullgren, M. J. Wolf et al., Phys. Rev. Lett. 112, 156102 (2014).

National Category
Nano Technology
Identifiers
urn:nbn:se:uu:diva-338364 (URN)
Conference
Towards Reality in Nanoscale Materials IX Nanoscale Materials for Warfare Agent Detection: Nanoscience for Security 13th – 16th February 2017 Levi, Finland
Available from: 2018-01-08 Created: 2018-01-08 Last updated: 2019-02-19Bibliographically approved
Wolf, M. J., Kullgren, J., Broqvist, P. & Hermansson, K. (2017). Fluorine impurities at CeO2(111): Effects on oxygen vacancy formation, molecular adsorption, and surface re-oxidation. Journal of Chemical Physics, 146(4), Article ID 044703.
Open this publication in new window or tab >>Fluorine impurities at CeO2(111): Effects on oxygen vacancy formation, molecular adsorption, and surface re-oxidation
2017 (English)In: Journal of Chemical Physics, ISSN 0021-9606, E-ISSN 1089-7690, Vol. 146, no 4, article id 044703Article in journal (Refereed) Published
Abstract [en]

We investigate the effects of anion doping with fluorine impurities on the chemistry of the CeO2 (111) facet, using the results of DFT+U calculations. We consider three prototypical processes: the formation of oxygen vacancies, the adsorption of O-2 and H2O molecules, and the re-oxidation of the surface with fragments of the two molecules. We find that the first two of these processes are not strongly affected, but that the presence of F lowers the energy gained in the re-oxidation of the surface in comparison to the healing of an oxygen vacancy, by 1.47 eV in the case of O-2 (provided that the F is part of a cluster) and by 0.92 eV in the case of H2O. Based on these results, we suggest that F could enhance the redox chemistry of ceria by toggling between being in the surface and on the surface, effectively facilitating the release of lattice O by acting as a "place holder" for it. Finally, we find that the desorption of F as either 1/2 F-2 or HF is energetically unfavourable, suggesting that F doped ceria should be stable in the presence of O-2 and H2O.

Place, publisher, year, edition, pages
AMER INST PHYSICS, 2017
National Category
Chemical Sciences
Identifiers
urn:nbn:se:uu:diva-319652 (URN)10.1063/1.4973239 (DOI)000394520200039 ()28147533 (PubMedID)
Funder
Swedish Research CouncilThe Swedish Foundation for International Cooperation in Research and Higher Education (STINT)eSSENCE - An eScience CollaborationEU, European Research Council, CM1104
Available from: 2017-04-07 Created: 2017-04-07 Last updated: 2019-02-19Bibliographically approved
Kettner, M., Duchon, T., Wolf, M., Kullgren, J., Kus, P., Sevcikova, K., . . . Nehasil, V. (2017). Modification of valence band of ceria via anion doping with fluorine. Paper presented at 254th National Meeting and Exposition of the American-Chemical-Society (ACS) on Chemistry's Impact on the Global Economy, AUG 20-24, 2017, Washington, DC. Abstract of Papers of the American Chemical Society, 254
Open this publication in new window or tab >>Modification of valence band of ceria via anion doping with fluorine
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2017 (English)In: Abstract of Papers of the American Chemical Society, ISSN 0065-7727, Vol. 254Article in journal, Meeting abstract (Other academic) Published
Place, publisher, year, edition, pages
AMER CHEMICAL SOC, 2017
National Category
Chemical Sciences
Identifiers
urn:nbn:se:uu:diva-357075 (URN)000429525601837 ()
Conference
254th National Meeting and Exposition of the American-Chemical-Society (ACS) on Chemistry's Impact on the Global Economy, AUG 20-24, 2017, Washington, DC
Available from: 2018-08-13 Created: 2018-08-13 Last updated: 2019-02-19Bibliographically approved
Kim, B.-H., Hermansson, K., Wolf, M. J., Kullgren, J. & Broqvist, P. (2017). Multiscale modelling of CeO2 nano-interfaces. In: : . Paper presented at Nanoscale Materials for Warfare Agent Detection: Nanoscience for Security 13th – 16th February 2017 Levi, Finland.
Open this publication in new window or tab >>Multiscale modelling of CeO2 nano-interfaces
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2017 (English)Conference paper, Poster (with or without abstract) (Other academic)
National Category
Nano Technology
Identifiers
urn:nbn:se:uu:diva-338369 (URN)
Conference
Nanoscale Materials for Warfare Agent Detection: Nanoscience for Security 13th – 16th February 2017 Levi, Finland
Available from: 2018-01-08 Created: 2018-01-08 Last updated: 2019-02-19
Organisations
Identifiers
ORCID iD: ORCID iD iconorcid.org/0000-0001-5902-4187

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