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Chemistry and Physics of Cu and H2O on ZnO Surfaces: Electron Transfer, Surface Triangles, and Theory
Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Chemistry - Ångström, Structural Chemistry.
2015 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

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.

When a Cu cluster on a ZnO surface becomes positively charged, this happens because it donates an electron to the ZnO conduction band. Hence, it is necessary to use a method which describes the ZnO band gap correctly, and we show that a hybrid density functional, which includes a fraction of Hartree-Fock exchange, fulfills this requirement. When the ZnO conduction band becomes populated by electrons from Cu, band-filling occurs, which affects the adsorption energy. The band-filling correction is presented as a means to extrapolate the calculated adsorption energy under periodic boundary conditions to the zero coverage (isolated adsorbate, infinite supercell) limit.

A part of this thesis concerns the parameterization of the computationally very efficient SCC-DFTB method (density functional based tight binding with self-consistent charges), in a multi-scale modeling approach. Our findings suggest that the SCC-DFTB method satisfactorily describes the interaction between ZnO surfaces and water, as well as the stabilities of different surface reconstructions (such as triangularly and hexagonally shaped pits) at the polar ZnO(0001) and ZnO(000-1) surfaces.

 

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2015. , 50 p.
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 1207
Keyword [en]
catalysis, density functional theory, SCC-DFTB, band-filling correction
National Category
Chemical Sciences
Identifiers
URN: urn:nbn:se:uu:diva-236302ISBN: 978-91-554-9111-6 (print)OAI: oai:DiVA.org:uu-236302DiVA: diva2:765486
Public defence
2015-01-09, Häggsalen, Ångströmlaboratoriet, Lägerhyddsvägen 1, Uppsala, 13:15 (English)
Opponent
Supervisors
Available from: 2014-12-18 Created: 2014-11-17 Last updated: 2015-02-03
List of papers
1. Cu dimer formation mechanism on the ZnO(10(1)over-bar0) surface
Open this publication in new window or tab >>Cu dimer formation mechanism on the ZnO(10(1)over-bar0) surface
2012 (English)In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 86, no 23, 235302- p.Article in journal (Refereed) Published
Abstract [en]

The formation of Cu dimers on the ZnO(10 (1) over bar0) 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(10 (1) over bar0) surface with small barriers of migration (0.3-0.4 eV) along ZnO[1 (2) over bar 10], repeatedly switching their oxidation states, while the barrier along ZnO[0001] 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.

National Category
Natural Sciences
Identifiers
urn:nbn:se:uu:diva-191772 (URN)10.1103/PhysRevB.86.235302 (DOI)000311806300007 ()
Available from: 2013-01-15 Created: 2013-01-14 Last updated: 2017-12-06Bibliographically approved
2. Small Cu Clusters Adsorbed on ZnO(10(1)over-bar0) Show Even-Odd Alternations in Stability and Charge Transfer
Open this publication in new window or tab >>Small Cu Clusters Adsorbed on ZnO(10(1)over-bar0) Show Even-Odd Alternations in Stability and Charge Transfer
2014 (English)In: The Journal of Physical Chemistry C, ISSN 1932-7447, E-ISSN 1932-7455, Vol. 118, no 12, 6480-6490 p.Article in journal (Refereed) Published
Abstract [en]

Using hybrid density functional theory, we investigate structural and electronic properties of small Cu-n clusters (with n <= 9) adsorbed on the nonpolar ZnO(10 (1) over bar0) 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 duster stability (for n = 1-6) and cluster charge, as a function of the number of atoms. Even-numbered dusters 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 dusters are lower than for even-numbered ones. The most stable adsorbed odd-numbered clusters are neutral and planar for n <= 3 and positively charged and polyhedral for n >= 7. For n = 5, both neutral planar and positively charged polyhedral configurations are similarly stable.

National Category
Natural Sciences
Identifiers
urn:nbn:se:uu:diva-224359 (URN)10.1021/jp412694y (DOI)000333578300057 ()
Available from: 2014-05-12 Created: 2014-05-09 Last updated: 2017-12-05Bibliographically approved
3. Water-Induced Oxidation and Dissociation of Small Cu Clusters on ZnO(101̅0)
Open this publication in new window or tab >>Water-Induced Oxidation and Dissociation of Small Cu Clusters on ZnO(101̅0)
2015 (English)In: The Journal of Physical Chemistry C, ISSN 1932-7447, E-ISSN 1932-7455, Vol. 119, no 3, 1382-1390 p.Article in journal (Refereed) Published
Abstract [en]

The interaction between water molecules and small Cu clusters (up to a size of four atoms) adsorbed on the nonpolar ZnO(10 (1) over bar0) 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(10 (1) over bar0) 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 watergas 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.

National Category
Theoretical Chemistry Physical Chemistry Materials Chemistry
Identifiers
urn:nbn:se:uu:diva-236298 (URN)10.1021/jp509501z (DOI)000348491900013 ()
Available from: 2014-11-17 Created: 2014-11-17 Last updated: 2017-12-05Bibliographically approved
4. Cu wets the polar ZnO(0001)-Zn surface because of interaction with subsurface defects
Open this publication in new window or tab >>Cu wets the polar ZnO(0001)-Zn surface because of interaction with subsurface defects
Show others...
(English)Article in journal (Refereed) Submitted
National Category
Materials Chemistry Theoretical Chemistry Inorganic Chemistry Physical Chemistry Condensed Matter Physics
Identifiers
urn:nbn:se:uu:diva-236300 (URN)
Available from: 2014-11-20 Created: 2014-11-17 Last updated: 2015-01-14
5. Band-Filling Correction Method for Accurate Adsorption Energy Calculations: A Cu/ZnO Case Study
Open this publication in new window or tab >>Band-Filling Correction Method for Accurate Adsorption Energy Calculations: A Cu/ZnO Case Study
2013 (English)In: Journal of Chemical Theory and Computation, ISSN 1549-9618, E-ISSN 1549-9626, Vol. 9, no 11, 4673-4678 p.Article in journal (Refereed) Published
Abstract [en]

We present a simple method, the “band-filling correction”, to calculate accurate adsorption 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.

Place, publisher, year, edition, pages
Columbus, Ohio: American Chemical Society (ACS), 2013
National Category
Natural Sciences
Identifiers
urn:nbn:se:uu:diva-213142 (URN)10.1021/ct400645v (DOI)000327044500001 ()
Available from: 2013-12-18 Created: 2013-12-18 Last updated: 2017-12-06Bibliographically approved
6. An SCC-DFTB Repulsive Potential for Various ZnO Polymorphs and the ZnO-Water System
Open this publication in new window or tab >>An SCC-DFTB Repulsive Potential for Various ZnO Polymorphs and the ZnO-Water System
Show others...
2013 (English)In: The Journal of Physical Chemistry C, ISSN 1932-7447, E-ISSN 1932-7455, Vol. 117, no 33, 17004-17015 p.Article in journal (Refereed) Published
Abstract [en]

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 nanowire systems as well as for water adsorption on the low-index wurtzite (10 (1) over bar0) and (11 (2) over bar0) 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.

National Category
Natural Sciences
Identifiers
urn:nbn:se:uu:diva-208368 (URN)10.1021/jp404095x (DOI)000323593100028 ()
Available from: 2013-09-30 Created: 2013-09-30 Last updated: 2017-12-06Bibliographically approved
7. Large-scale SCC-DFTB calculations of reconstructed polar ZnO surfaces
Open this publication in new window or tab >>Large-scale SCC-DFTB calculations of reconstructed polar ZnO surfaces
Show others...
2014 (English)In: Surface Science, ISSN 0039-6028, E-ISSN 1879-2758, Vol. 628, 50-61 p.Article in journal (Refereed) Published
Abstract [en]

We present a theoretical study of a range of surface defects for the most abundant polar ZnO(0001)/(000 (1) over bar) 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.

Keyword
ZnO, Polar surfaces, SCC-DFTB, DFT
National Category
Condensed Matter Physics Chemical Sciences
Identifiers
urn:nbn:se:uu:diva-230913 (URN)10.1016/j.susc.2014.05.001 (DOI)000340221200008 ()
Available from: 2014-09-05 Created: 2014-09-01 Last updated: 2017-12-05Bibliographically approved

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