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: C. M. Yim, M. B. Watkins, M. J. Wolf, C. L. Pang, K. Hermansson, and G. Thornton
Polarons in metal oxides are important in processes such as catalysis, high temperature superconductivity, and dielectric breakdown in nanoscale electronics. Here, we study the behavior of electron small polarons associated with oxygen vacancies at rutile TiO2(110), using a combination of low temperature scanning tunneling microscopy (STM), density functional theory, and classical molecular dynamics calculations. We find that the electrons are symmetrically distributed around isolated vacancies at 78 K, but as the temperature is reduced, their distributions become increasingly asymmetric, confirming their polaronic nature. By manipulating isolated vacancies with the STM tip, we show that particular configurations of polarons are preferred for given locations of the vacancies, which we ascribe to small residual electric fields in the surface. We also form a series of vacancy complexes and manipulate the Ti ions surrounding them, both of which change the associated electronic distributions. Thus, we demonstrate that the configurations of polarons can be engineered, paving the way for the construction of conductive pathways relevant to resistive switching devices.
Phys. Rev. Lett. 117, 116402, (2016)
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: Matthew J. Wolf, Alexander L. Shluger
We present an overview of the structure and properties of clean steps at the surfaces of binary oxides, utilising recent data from scanning probe and spectroscopic experiments, and theoretical calculations. We review and discuss their atomic structure, electronic structure, and interactions with prototypical point defects, using examples from studies on technologically important oxides such as MgO, CeO2, TiO2 and ZrO2. We also review methods of calculating the step formation energy, and discuss reasons for their limited success in explaining the step structures observed in experiments.
Defects at Oxide Surfaces , Springer Series in Surface Sciences Volume 58 , 2015, pp 191-214.
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
The conference “Oorgandagarna – Inorganic Days” was held in Åhus in Sweden from 17-19 June 2013. Dalton Transactions sponsored two poster prizes, and the two winners were Erik Lewin and Matthew Wolf from Uppsala University. Congratulations to Erik and Matthew, and I hope all the conference attendees had a great time!