Authors: Vanessa Quaranta, Matti Hellström, Jörg Behler, Jolla Kullgren, Pavlin D. Mitev, and Kersti Hermansson
Unraveling the atomistic details of solid/liquid interfaces, e.g., by means of vibrational spectroscopy, is of vital importance in numerous applications, from electrochemistry to heterogeneous catalysis. Water-oxide interfaces represent a formidable challenge because a large variety of molecular and dissociated water species are present at the surface. Here, we present a comprehensive theoretical analysis of the anharmonic OH stretching vibrations at the water/ZnO(10-10) interface as a prototypical case. Molecular dynamics simulations employing a reactive high-dimensional neural network potential based on density functional theory calculations have been used to sample the interfacial structures. In the second step, one-dimensional potential energy curves have been generated for a large number of configurations to solve the nuclear Schrödinger equation. We find that (i) the ZnO surface gives rise to OH frequency shifts up to a distance of about 4 Å from the surface; (ii) the spectrum contains a number of overlapping signals arising from different chemical species, with the frequencies decreasing in the order ν(adsorbed hydroxide) > ν(non-adsorbed water) > ν(surface hydroxide) > ν(adsorbed water); (iii) stretching frequencies are strongly influenced by the hydrogen bond pattern of these interfacial species. Finally, we have been able to identify substantial correlations between the stretching frequencies and hydrogen bond lengths for all species.
The Journal of Chemical Physics, 148, 241720 (2018);
Authors: Igor Beinik, Matti Hellström, Tomas N. Jensen, Peter Broqvist, and Jeppe V. Lauritsen
Metal 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 550 K. 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
Authors: Matti Hellström, Daniel Spångberg, and Kersti Hermansson
We assess the consequences of the interface model—embedded-cluster or periodic-slab model—on the ability of DFT calculations to describe charge transfer (CT) in a particularly challenging case where periodic-slab calculations indicate a delocalized charge-transfer state. Our example is Cu atom adsorption on ZnO(10 0), and in fact the periodic slab calculations indicate three types of CT depending on the adsorption site: full CT, partial CT, and no CT. Interestingly, when full CT occurs in the periodic calculations, the calculated Cu atom adsorption energy depends on the underlying ZnO substrate supercell size, since when the electron enters the ZnO it delocalizes over as many atoms as possible. In the embedded-cluster calculations, the electron transferred to the ZnO delocalizes over the entire cluster region, and as a result the calculated Cu atom adsorption energy does not agree with the value obtained using a large periodic supercell, but instead to the adsorption energy obtained for a periodic supercell of roughly the same size as the embedded cluster. Different density functionals (of GGA and hybrid types) and basis sets (local atom-centered and plane-waves) were assessed, and we show that embedded clusters can be used to model Cu adsorption on ZnO(100), as long as care is taken to account for the effects of CT.
J. Comput. Chem. 2015, 36,2394–2405.
Authors: Matti Hellström, Daniel Spångberg, Peter Broqvist, and Kersti Hermansson
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, 2015, 119 (3), pp 1382–1390
2015 Doctoral thesis, comprehensive summary
This thesis discusses the chemistry and physics of Cu and H2O on ZnO surfaces, based primarily on results from quantum chemical calculations. The underlying context is heterogeneous catalysis, where Cu/ZnO-mixtures are used in the industrial synthesis of methanol and in the water gas shift reaction. Electron transfer between small Cu clusters and ZnO is central to this thesis, as are the design and use of models that can describe realistic and very large-scale ZnO surface structures while still retaining the electronic nature of the system. Method and model enhancements as well as tests and validations constitute a large part of this thesis.
The thesis demonstrates that the charges of small Cu clusters, adsorbed on the non-polar ZnO(10-10) surface, depend on whether the Cu clusters contain an even or odd number of atoms, and whether water is present (water can induce electron transfer from Cu to ZnO). On the polar Zn-terminated ZnO(0001) surface, Cu becomes negatively charged, which causes it to attract positively charged subsurface defects and to wet the ZnO(0001) surface at elevated temperatures. Read more
Authors: Stefan E. Huber, Matti Hellström, Michael Probst, Kersti Hermannson, and Peter Broqvist
We present a theoretical study of a range of surface defects 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)
Authors: Matti Hellström, Daniel Spångberg, Kersti Hermansson and Peter Broqvist
Using hybrid density functional theory, we investigate structural and electronic properties of small Cun clusters (with n ≤ 9) adsorbed on the nonpolar ZnO(101̅0) surface. The Cu clusters grow in a planar fashion up to a size of six atoms, after which the clusters take on a polyhedral shape. We find even–odd alternations with respect to both cluster stability (for n = 1–6) and cluster charge, as a function of the number of atoms. Even-numbered clusters are always charge-neutral, while odd-numbered clusters can become positively charged by donation of an electron to the ZnO conduction band, which can be traced back to the fact that the ionization energies of odd-numbered gas-phase Cu clusters are lower than for even-numbered ones. The most stable adsorbed odd-numbered clusters are neutral and planar forn ≤ 3 and positively charged and polyhedral for n ≥ 7. For n = 5, both neutral planar and positively charged polyhedral configurations are similarly stable.
J. Phys. Chem. C, 2014, 118 (12), pp 6480–6490
Authors: Matti Hellström, Daniel Spångberg, Kersti Hermansson and Peter Broqvist
We present a simple method, the “band-filling correction”, to calculate accurateadsorption energies (Eads) in the low coverage limit from finite-size supercell slab calculations using DFT.
We show that it is necessary to use such a correction if charge transfer takes place between the adsorbate and the substrate, resulting in the substrate bands either filling up or becoming depleted. With this correction scheme, we calculate Eads of an isolated Cu atom adsorbed on the ZnO(101̅0) surface. Without the correction, the calculated Eads is highly coverage-dependent, even for surface supercells that would typically be considered very large (in the range from 1 nm × 1 nm to 2.5 nm × 2.5 nm). The correction scheme works very well for semilocal functionals, where the corrected Eads is converged within 0.01 eV for all coverages. The correction scheme also works well for hybrid functionals if a large supercell is used and the exact exchange interaction is screened.
J. Chem. Theory Comput. 9, 4673 (2013).
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).
Matti Hellström, Daniel Spångberg, Kersti Hermansson, and Peter Broqvist
The formation of Cu dimers on the ZnO(101̅ 0) surface has been studied using hybrid density functional theory. Depending on the adsorption site, Cu atoms are found to adsorb with either oxidation state 0 or +1. In the latter case, the Cu atom has donated an electron to the ZnO conduction band. The two modes of adsorption display similar stability at low coverages, while at higher coverages the neutral species is more stable. Single Cu atoms diffuse across the ZnO(101̅ 0) surface with small barriers of migration (0.3–0.4 eV) along ZnO[12̅ 10], repeatedly switching their oxidation states, while the barrier along ZnO is significantly higher (>1.5 eV). The formation of a Cu dimer from two adsorbed Cu atoms is energetically favorable with two competing structures of similar stability, both being charge neutral. The minimum energy paths for Cu atom diffusion and dimer formation are characterized by at least one of the two Cu atoms being in oxidation state 0.
Phys. Rev. B, 2012, 86, 235302