Author Archives: Peter Broqvist

Self-Consistent-Charge Density-Functional Tight-Binding (SCC-DFTB) Parameters for Ceria in 0D to 3D

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. C2017, 121 (8), pp 4593–4607
DOI: 10.1021/acs.jpcc.6b10557

Fluorine impurities at CeO2(111): Effects on oxygen vacancy formation, molecular adsorption, and surface re-oxidation

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 1212F2 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
DOI: 10.1063/1.4973239 

Subsurface hydrogen bonds at the polar Zn-terminated ZnO(0001) surface

Authors: Matti Hellström, Igor Beinik, Peter Broqvist, Jeppe V. Lauritsen, and Kersti Hermansson

The role of hydrogen and other defects in the stabilization of polar oxide interfaces is a matter of significant fundamental and practical interest. Using experimental (scanning tunneling microscopy, x-ray photoelectron spectroscopy) and theoretical (density functional theory) surface science techniques, we find that the polar Zn-terminated ZnO(0001) surface becomes excessively Zn deficient during high-temperature annealing (780 K) in ultrahigh vacuum (UHV). The Zn vacancies align themselves into rows parallel to the [101 ̄0] direction,and the remaining surface Zn ions alternately occupy wurtzite (hcp) and zinc-blende (fcc) lattice positions, giving a characteristic “striped” c( sqrt(12) × sqrt(12))R30° surface morphology with three types of rows: wurtzite Zn, zinc-blende Zn, and Zn vacancies. Interstitial H plays a central role in such a reconstruction, as it helps to compensate the excessive Zn deficiency. We propose a model in which hydrogen occupies positions in half of the vacancy rows to form hydroxide ions that can participate in hydrogen bonds in the O subsurface layer as a result of the mixed wurtzite/zinc-blende stacking.

Phys. Rev. B, 2016, 94, 245433
DOI: 10.1103/PhysRevB.94.245433

Bismuth Iodide Perovskite Materials for Solar Cell Applications: Electronic Structure, Optical Transitions and Directional Charge Transport

Authors: Meysam Pazoki, Malin B. Johansson, Huimin Zhu, Peter Broqvist, Tomas Edvinsson, Gerrit Boschloo, and Erik M. J. Johansson

Cesium and bismuth iodides (Cs3Bi2I9 and MA3Bi2I9) are new low-toxic and air stable compounds in the perovskite solar cell family with promising characteristics. Here, the electronic structure and the nature of their optical transitions, dielectric constant, and charge carrier properties are assessed for photovoltaic applications with density functional theory (DFT) calculations and experiments. The calculated direct and indirect band gap values for Cs3Bi2I9(2.17 and 2.0 eV) and MA3Bi2I9 (2.17 and 1.97 eV) are found to be in good agreement with the experimental optical band gaps (2.2, 2.0 eV and 2.4, 2.1 eV for Cs3Bi2I9 and MA3Bi2I9, respectively) estimated for solution-processed films. There is an error cancelation in the DFT calculated band gap similar to that for lead perovskites. However, fully relativistic DFT calculations indicate that the size of the spin orbit coupling (SOC) error cancelation for bismuth perovskite (0.5 eV) is less than for lead perovskite (1 eV), and other factors are therefore also important. Band structure calculations show high effective masses of the charge carriers along the c-axis but on the other hand lower electron effective mass in the ab planes, revealing the interesting possibility for a directional charge transport. Calculations of dielectric constants, absorption coefficients, carrier effective masses, and exciton binding energies emphasize the fundamental differences between the lead and bismuth iodide perovskites and clarify the reasons behind the lower power conversion efficiency of bismuth iodide perovskite solar cells. Also the calculations show that the orientational disorder of the MA dipoles in the lattice has meaningful impacts on the near valence and conduction band edge of the electronic structure.

J. Phys. Chem. C, 2016, 120 (51), pp 29039–29046
DOI: 10.1021/acs.jpcc.6b11745

Enhanced wetting of Cu on ZnO by migration of subsurface oxygen vacancies

Authors: Igor Beinik, Matti Hellström, Tomas N. Jensen, Peter Broqvist, and Jeppe V. Lauritsen

ncommMetal adhesion on metal oxides is strongly controlled by the oxide surface structure and composition, but lack of control over the surface conditions often limits the possibilities to exploit this in opto- and micro-electronics applications and heterogeneous catalysis where nanostructural control is of utmost importance. The Cu/ZnO system is among the most investigated of such systems in model studies, but the presence of subsurface ZnO defects and their important role for adhesion on ZnO have been unappreciated so far. Here we reveal that the surface-directed migration of subsurface defects affects the Cu adhesion on polar ZnO(0001) in the technologically interesting temperature range up to 550K. This leads to enhanced adhesion and ultimately complete wetting of ZnO(0001) by a Cu overlayer. On the basis of our experimental and computational results we demonstrate a mechanism which implies that defect concentrations in the bulk are an important, and possibly controllable, parameter for the metal-on-oxide growth.

Nature Communications 6, Article number: 8845

DOI: 10.1038/ncomms9845

ReaxFF Force-Field for Ceria Bulk, Surfaces, and Nanoparticles

Authors: Peter BroqvistJolla KullgrenMatthew J. WolfAdri C. T. van Duin, and Kersti Hermansson

reaxff

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. C2015119 (24), pp 13598–13609

Water-Induced Oxidation and Dissociation of Small Cu Clusters on ZnO(101̅0)

Authors: Matti Hellström, Daniel Spångberg, Peter Broqvist, and Kersti Hermansson

matti1

The interaction between water molecules and small Cu clusters (up to a size of four atoms) adsorbed on the nonpolar ZnO(101̅0) 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(101̅0) 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 water–gas 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.

J. Phys. Chem. C, 2015119 (3), pp 1382–1390 

DOI: 10.1021/jp509501z

Formic Acid on TiO2–x (110): Dissociation, Motion, and Vacancy Healing

Authors: Shuanglin Hu, Phillipe A. Bopp, Lars Österlund, Peter Broqvist, and Kersti Hermansson

shThe adsorption and dissociation of a formic acid molecule (HCOO) on a partially reduced rutile TiO2–x (110) surface and the subsequent transformations of the adsorbed fragments are studied via quantumechanical molecular dnamics simulations and climbing-image nudged elastic band (CI-NEB) calculations. The electronic structure methods used are self-consistent-charge density functional tight binding (SCC-DFTB) and DFT+U calculations. We address the apparent lack of consensus in the literature regarding the formic acid adsorbate species that heal the O vacancies, where different experiments have suggested the occurrence of one, two, or no such species types. From our calculations, we propose that the formic acid molecule quickly dissociates on the surface into a formate ion and a proton. If no mechanism exists by which the dissociation products can migrate away from each other, three formate species will coexist on the partially reduced TiO2 surface: one majority species bound to the Ti rows and two minority species healing the O vacancies. However, if such a diffusion mechanism does exist, our barrier calculations show that one of the minority species will transform into the other, and only two adsorbate types can be expected on the surface. We also identify a new adsorbate configuration (which we denote C′), where the formate is located on the row of two-coordinated oxygen atoms, healing an O vacancy and accepting an H-bond from the detached H atom.

J. Phys. Chem. C 118, 14876 (2014). 

DOI: 10.1021/jp500771d

Large-scale SCC-DFTB calculations of reconstructed polar ZnO surfaces

Authors: Stefan E. Huber, Matti Hellström, Michael Probst, Kersti Hermannson, and Peter Broqvist

We present a theoretical study of a range of surface znodefects for the most abundant polar ZnO(0001) surfaces using a tight binding approach with self-consistent charges (SCC-DFTB). We find that a combination of triangular pits at the Zn-terminated surface and a strongly ordered hexagonal defect pattern at the O-terminated surface constitutes a very stable reconstruction, in excellent agreement with experimental findings. On the whole, the SCC-DFTB method describes the polar surfaces of ZnO very well, and at a low computational cost which allows for the investigation of larger – and more realistic – surface structures compared to previous studies. Such large-scale calculations show that, at the Zn-terminated surface, the reconstruction results in a high density of one-layer deep triangular pit-like defects and surface vacancies which allow for a high configurational freedom and a vast variety of defect motifs. We also present extensive tests of the performance of the SCC-DFTB method in comparison with DFT results.

Surface Science 628, 51 (2014)

DOI: http://dx.doi.org/10.1016/j.susc.2014.05.001

Reactive oxygen species in stoichiometric ceria: Bulk and low-index surfaces

Authors: Jolla Kullgren, Kersti Hermansson, and Peter Broqvist

pss

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). 

DOI: 10.1002/pssr.201409099