Chemistry and Physics of Cu and H2O on ZnO Surfaces : Electron Transfer, Surface Triangles, and Theory

Matti Hellström
2015 Doctoral thesis, comprehensive summaryOmslag-MH.indd

Abstract

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, 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:nbn:se:uu:diva-236302 (URN)
978-91-554-9111-6 (ISBN)
oai:DiVA.org:uu-236302 (OAI)
diva2:765486 (DiVA)

Public defence
2015-01-09, Häggsalen, Ångströmlaboratoriet, Lägerhyddsvägen 1, Uppsala, 13:15 (English)

Opponent

  • Behler, Jörg

Supervisors

  • Hermansson, Kersti
  • Broqvist, Peter
  • Spångbeg, Daniel

List of papers

  1. Cu dimer formation mechanism on the ZnO(10(1)over-bar0) surface
  2. Small Cu Clusters Adsorbed on ZnO(10(1)over-bar0) Show Even-Odd Alternations in Stability and Charge Transfer
  3. Water-Induced Oxidation and Dissociation of Small Cu Clusters on ZnO(10-10)
  4. Cu wets the polar ZnO(0001)-Zn surface because of interaction with subsurface defects
  5. Band-Filling Correction Method for Accurate Adsorption Energy Calculations
  6. An SCC-DFTB Repulsive Potential for Various ZnO Polymorphs and the ZnO-Water System
  7. Large-scale SCC-DFTB calculations of reconstructed polar ZnO surfaces

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