Authors: Jolla Per Kullgren, Matthew Jason Wolf, Kersti Hermansson, Christof Köhler, Bálint Aradi, Thomas Frauenheim, and Peter Broqvist
Reducible oxides such as CeO2 are challanging to describe
with standard density functional theory (DFT) due to the mixed valence states of the cations, and often require the use of additional correction schemes, an
d/or more computationally expen- sive methods. This adds a new layer of complexity when it comes to the generation of Slater-Koster tables and the corresponding repulsive potentials for self-consistent density functional based tight binding (SCC-DFTB) calculations of such materials. In this work, we provide guidelines for how to set up a parameterisation scheme for mixed valence oxides within the SCC-DFTB framework, with a focus on reproducing structural and electronic properties as well as redox reaction energies calculated using a reference DFT method. This parameterisation procedure has been used to generate parameters for Ce–O interactions, with Ce in its +III or +IV formal oxidation states. The generated parameter set is validated through comparison to DFT calculations for various ceria (CeO2) and reduced ceria (CeO2−x ) systems of different dimensionalities ranging from 0D (nano-particles) to 3D (bulk). As oxygen vacancy defects in ceria are of crucial importance to many technological applications, special focus is directed towards the capability of describing such defects accurately.
J. Phys. Chem. C, 2017, 121 (8), pp 4593–4607
Authors: Matthew J. Wolf, Jolla Kullgren, Peter Broqvist, and Kersti Hermansson
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 O2 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 O2 (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 surfaceand 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 1212
F2 or HF is energetically unfavourable, suggesting that F doped ceria should be stable in the presence of O2 and H2O.
J. Chem. Phys. 2017, 146, 044703
Authors: Peter Broqvist, Jolla Kullgren, Matthew J. Wolf, Adri C. T. van Duin, and Kersti Hermansson
We have developed a reactive force-field of the ReaxFF type for stoichiometric ceria (CeO2) and partially reduced ceria (CeO2–x). We describe the parametrization procedure and provide results validating the parameters in terms of their ability to accurately describe the oxygen chemistry of the bulk, extended surfaces, surface steps, and nanoparticles of the material. By comparison with our reference electronic structure method (PBE+U), we find that the stoichiometric bulk and surface systems are well reproduced in terms of bulk modulus, lattice parameters, and surface energies. For the surfaces, step energies on the (111) surface are also well described. Upon reduction, the force-field is able to capture the bulk and surface vacancy formation energies (Evac), and in particular, it reproduces the Evac variation with depth from the (110) and (111) surfaces. The force-field is also able to capture the energy hierarchy of differently shaped stoichiometric nanoparticles (tetrahedra, octahedra, and cubes), and of partially reduced octahedra. For these reasons, we believe that this force-field provides a significant addition to the method repertoire available for simulating redox properties at ceria surfaces.
J. Phys. Chem. C, 2015, 119 (24), pp 13598–13609
Authors: Jolla Kullgren, Kersti Hermansson, and Peter Broqvist
We have calculated the stabilities of some reactive oxygen species (ROS) in stoichiometric bulk ceria and at the low-index (111) and (110)-surfaces, both in vacuum and in the presence of additional O2molecules. We find that the formation of intrinsic ROS, here oxygen superoxides (O2–) and peroxides (O22–), is always endothermic at vacuum conditions and that the superoxide formation always leads to a higher formation energy than the peroxide formation. In the presence of additional O2molecules, intrinsic peroxide formation becomes exothermic at the (110)-surface in conjunction with the formation of extrinsic superoxide ions from adsorbed O2 molecules. This coexistence of intrinsic and extrinsic ROS species is anticipated to be stable at low temperatures, and can be important for understanding the ROS chemistry for nanoceria used in low-temperature applications.
Physica Status Solidi (RLL) – Rapid Research Letters 8, 600 (2014).
Authors: J. Kullgren, M. J. Wolf, C. W. M. Castleton, P. D. Mitev, W. J. Briels, and K. Hermansson
We propose a resolution to the puzzle presented by the surface defects observed with STM at the (111) surface facet of CeO2 single crystals. In the seminal paper of Esch et al. [Science 309, 752 (2005)] they were identified with oxygen vacancies, but the observed behavior of these defects is inconsistent with the results of density functional theory (DFT) studies of oxygen vacancies in the literature. We resolve these inconsistencies via DFT calculations of the properties of both oxygen vacancies and fluorine impurities at CeO2(111), the latter having recently been shown to exist in high concentrations in single crystals from a widely used commercial source of such samples. We find that the simulated filled-state STM images of surface-layer oxygen vacancies and fluorine impurities are essentially identical, which would render problematic their experimental distinction by such images alone. However, we find that our theoretical results for the most stable location, mobility, and tendency to cluster, of fluorine impurities are consistent with experimental observations, in contrast to those for oxygen vacancies. Based on these results, we propose that the surface defects observed in STM experiments on CeO2 single crystals reported heretofore were not oxygen vacancies, but fluorine impurities. Since the similarity of the simulated STM images of the two defects is due primarily to the relative energies of the 2p states of oxygen and fluorine ions, this confusion might also occur for other oxides which have been either doped or contaminated with fluorine.
Phys. Rev. Lett. 112, 156102
Authors: Jolla Kullgren, Kersti Hermansson and Peter Broqvist
We use theoretical simulations to study how oxidative and humid environments affect the chemical composition, shape and structure of ceria nanoparticles. Based on our calculations, we predict that small stoichiometric ceria nanoparticles will have a very limited stability range when exposed to these environments. Instead, we find that reduced ceria nanoparticles are stabilized without changing their inherent shape through the adsorption of oxygen molecules in the form of superoxo species and in the form of hydroxo species. Based on our results, we propose a redox-cycle for metastable ceria nanoparticles without the formation of explicit oxygen vacancies, which is important for understanding the low-temperature oxygen chemistry of ceria at the nanoscale.
Authors: Matti Hellström, Kjell Jorner, Maria Bryngelsson, Stefan Ernest Huber, Jolla Per Kullgren, Thomas Frauenheim, and Peter Broqvist
We have developed an efficient scheme for the generation of accurate repulsive potentials for self-consistent charge density-functional based tight-binding calculations, which involves energy-volume scans of bulk polymorphs with different coordination numbers.
The scheme was used to generate an optimized parameter set for various ZnO polymorphs. The new potential was subsequently tested for ZnO bulk, surface, and nano-wire systems as well as for water adsorption on the low-index wurtzite (10-10) and (11-20) surfaces. By comparison to results obtained at the density functional level of theory, we show that the newly generated repulsive potential is highly transferable and capable of capturing most of the relevant chemistry of ZnO and the ZnO/water interface.
J. Phys. Chem. C, 117, 17004 (2013).
Authors: Jolla Kullgren, Kersti Hermansson, and Peter Broqvist
We provide an explanation for the experimental finding of a dramatically enhanced low-temperature oxygen storage capacity for small ceria nanoparticles. At low temperature, small octahedral ceria nanoparticles will be understoichiometric at both oxidizing and reducing conditions without showing explicit oxygen vacancies. Instead, rather than becoming stoichiometric at oxidizing conditions, such particles are stabilized through oxygen adsorption forming superoxo (O2–) ions and become in this way supercharged with oxygen. The supercharging effect is size-dependent and largest for small nanoparticles where it gives a direct increase in the oxygen storage capacity and simultaneously provides a source of active oxygen species at low temperatures.
J. Phys. Chem. Lett., 2013, 4 (4), pp 604–608