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Surface Reactivity and Electronic Structure of Metal Oxides
KTH, School of Information and Communication Technology (ICT), Material Physics, Material Physics, MF.
2011 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

The foci of this thesis are the metal oxides Cu2O, ZnO and Fe3O4 and their interaction with water and sulfur dioxide (SO2). The intention is to study SO2-induced atmospheric corrosion on a molecular level. All studies are based on photoelectron spectroscopy (PES) and scanning tunneling microscopy (STM) measurements. The band structure of Cu2O in the Γ-M direction has been probed by angle-resolved PES (ARPES). It reveals a more detailed picture of the bulk band structure than earlier data and gives the first experimental evidence of a dispersive hybridized Cu 3d-Cu 4s state. The experimental data is compared to band structure calculations. The structure of clean metal oxide surfaces and impact of sample preparation have been studied. Oxygen vacancies can form a (√3x√3)R30° reconstruction on Cu2O(111). Oxygen atoms adjacent to copper vacancies, steps or kinks are shown to be adsorption sites for both water and SO2. Annealing temperature influences the defect density and hydrogen content in ZnO, which can have large impact on the surface properties of ZnO(0001). Water is shown to adsorb dissociatively on ZnO(0001) and partly dissociatively on Cu2O(111). The dissociation occurs at undercoordinated oxygen sites on both surfaces. Water stays adsorbed on ZnO(0001) at room temperature but on Cu2O(111), all water has desorbed at 210 K. SO2 interacts with one or two undercoordinated O-sites on all studied oxide surfaces forming SO3 or SO4 species respectively. SO4 on Fe3O4(100) follows the (√2x√2)R45° reconstruction. On Cu2O(111) and ZnO(0001), SO2 adsorbs on defect sites. An SO3 to SO4 transition is observed on Cu2O(111) when heating an SO3 adsorbate layer from 150 K to 280K. Coadsorption of water and SO2 on ZnO(0001) and Fe3O4(100) has been studied briefly. Water blocks SO2 adsorption sites on ZnO(0001). On Fe3O4(100) and on one type of reduced ZnO(0001) sample, SO2 dissociation to atomic sulfur or sulfide occurs to a higher extent on water exposed surfaces than on clean surfaces. Water thus appears to increase the charge density on some surfaces. Further studies are needed to reveal the cause of this unexpected effect.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology , 2011. , xiv, 58 p.
Series
Trita-ICT/MAP AVH, ISSN 1653-7610 ; 2011:07
Keyword [en]
oxides, surfaces, defects, cuprous oxide, zinc oxide, magnetite, water, OH, sulfur dioxide, photoelectron spectroscopy, scanning tunneling microscopy
National Category
Other Engineering and Technologies not elsewhere specified
Identifiers
URN: urn:nbn:se:kth:diva-33667ISBN: 978-91-7415-995-0OAI: oai:DiVA.org:kth-33667DiVA: diva2:416985
Public defence
2011-05-30, Electrum, C2, Isafjordsgatan 26, Kista, 10:00 (English)
Opponent
Supervisors
Note

QC 20110516

Available from: 2011-05-16 Created: 2011-05-13 Last updated: 2012-10-30Bibliographically approved
List of papers
1. Probing the valence band structure of Cu2O using high-energy angle-resolved photoelectron spectroscopy
Open this publication in new window or tab >>Probing the valence band structure of Cu2O using high-energy angle-resolved photoelectron spectroscopy
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2007 (English)In: Physical Review B Condensed Matter, ISSN 0163-1829, E-ISSN 1095-3795, Vol. 76, no 11, 115127-1-115127-7 p.Article in journal (Refereed) Published
Abstract [en]

We present angle-resolved photoemission data along the M-Gamma-M direction from a Cu2O(111) single crystal, collected at high photon energies (h nu=619 and 891 eV) and T=100 K. Because of the high photon energies and effective background subtraction, our data give a clear picture of the bulk band structure. The results confirm the existence of a hybridized Cu 3d-Cu 4s state located between the two main Cu 3d and O 2p band regions. Several theoretical studies have predicted the existence of this band, but until now it has not been detected in any photoemission measurements. The experimentally derived band structure is compared to local density approximation calculations with and without the Hubbard potential U. The clear band dispersion in our experimental data has enabled us to extract a refined Hubbard U value, which makes it possible to achieve a better agreement between theoretically calculated bands and experimental data.

Keyword
CLOSED-SHELL INTERACTIONS; ELECTRONIC-STRUCTURE; SYNCHROTRON-RADIATION; POLAR COVALENCES; CUPROUS-OXIDE; DIRECT IMAGES; PHOTOEMISSION; CUPRITE; CUO; SPECTRA
National Category
Other Engineering and Technologies not elsewhere specified
Identifiers
urn:nbn:se:kth:diva-8063 (URN)10.1103/PhysRevB.76.115127 (DOI)000249786400046 ()2-s2.0-34848905947 (ScopusID)
Note
QC 20100702Available from: 2007-12-12 Created: 2007-12-12 Last updated: 2012-03-20Bibliographically approved
2. Atomic structure of Cu2O(111)
Open this publication in new window or tab >>Atomic structure of Cu2O(111)
2009 (English)In: Surface Science, ISSN 0039-6028, E-ISSN 1879-2758, Vol. 603, no 2, 257-264 p.Article in journal (Refereed) Published
Abstract [en]

Low-energy electron diffraction and scanning tunneling microscopy have been used to probe the surface atomic structure Of Cu2O(111) after various sample preparations. Annealing in oxygen gives a stoichiometric (1 x 1) oxygen terminated surface and further annealing in ultra-high vacuum results in a clear (root 3 x root 3)R30 degrees reconstruction and surface faceting. Tunneling from filled states in the reconstructed surface reveals a hexagonal pattern of large protrusions, which show an internal structure. The reconstruction is believed to be due to one-third of a monolayer of ordered oxygen vacancies. At areas on the surface where the large features are missing, another smaller type of protrusions is visible, which is associated with the ideal (1 x 1) surface. The relative position of the two types of features gives two possible models of the (111) surface. In the first model, the (1 x 1) surface is the ideal bulk terminated surface and coordinatively unsaturated oxygen ions are missing in the reconstructed surface. The second model agrees with the first model with the exception that coordinatively unsaturated copper ions in the outmost copper layer are missing in both the (1 x 1) and the reconstructed surface. The latter model is supported by previous surface free energy calculations. Since the undercoordinated copper ions have been suggested to be the catalytic active sites Of Cu2O(111), the presence or absence of these cations could be of great importance for the fundamental understanding of the surface reactivity Of Cu2O and of copper-based catalysts.

Keyword
Catalysis, Copper oxides, Low-energy electron diffraction (LEED), Scanning tunneling microscopy, Scanning tunneling spectroscopies, Single crystal surfaces, Surface defects, Surface structure, scanning-tunneling-microscopy, energy synchrotron-radiation, temperature steam conversion, plasma-chemical preparation, density-functional theory, closed-shell interactions, electronic-structure, nanostructured catalysts, polar covalences, carbon-monoxide
Identifiers
urn:nbn:se:kth:diva-18173 (URN)10.1016/j.susc.2008.10.048 (DOI)000263384500003 ()2-s2.0-58249115242 (ScopusID)
Note
QC 20100525Available from: 2010-08-05 Created: 2010-08-05 Last updated: 2011-05-16Bibliographically approved
3. Role of defects in surface chemistry on Cu2O(111)
Open this publication in new window or tab >>Role of defects in surface chemistry on Cu2O(111)
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(English)Manuscript (preprint) (Other academic)
Identifiers
urn:nbn:se:kth:diva-33744 (URN)
Note
QS 2011Available from: 2011-05-16 Created: 2011-05-16 Last updated: 2011-05-16Bibliographically approved
4. Water Adsorption on ZnO(0001): Transition from Triangular Surface Structures to a Disordered Hydroxyl Terminated phase
Open this publication in new window or tab >>Water Adsorption on ZnO(0001): Transition from Triangular Surface Structures to a Disordered Hydroxyl Terminated phase
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2010 (English)In: The Journal of Physical Chemistry C, ISSN 1932-7447, E-ISSN 1932-7455, Vol. 114, no 25, 11157-11161 p.Article in journal (Refereed) Published
Abstract [en]

We present room temperature scanning tunneling microscopy and photoemission spectroscopy studies of water adsorption on the Zn-terminated ZnO(0001) surface. Data indicates that the initial adsorption is dissociative leaving hydroxyl groups on the surface. At low water coverage, the adsorption occurs next to the oxygen-terminated step edges, where water is believed to bind to zinc cations leaving off hydrogen atoms to under-coordinated oxygen anions. When increasing the water dose, triangular terraces grow in size and pits diminish until the surface is covered with wide irregular terraces and a large number of small pits. Higher water exposure (20 Langmuir) results in a much more irregular surface. Hydrogen, which is produced in the dissociation reaction is believed to have an important role in the changed surface structure at high exposures. The fact that adsorbed water completely changes the structure of ZnO(0001) is an important finding toward the understanding of this surface at atmospheric conditions.

National Category
Physical Chemistry
Identifiers
urn:nbn:se:kth:diva-27268 (URN)10.1021/jp1004677 (DOI)000278982300022 ()2-s2.0-77954066762 (ScopusID)
Note
QC 20101216Available from: 2010-12-16 Created: 2010-12-09 Last updated: 2011-05-16Bibliographically approved
5. SO2 Interaction with Zn(0001) and ZnO(0001) and the Influenceof Water
Open this publication in new window or tab >>SO2 Interaction with Zn(0001) and ZnO(0001) and the Influenceof Water
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(English)Manuscript (preprint) (Other academic)
Identifiers
urn:nbn:se:kth:diva-33746 (URN)
Note
QS 2011Available from: 2011-05-16 Created: 2011-05-16 Last updated: 2011-05-16Bibliographically approved
6. High resolution spectroscopic and microscopic signatures of ordered growth of ferrous sulfate in SO2 assisted corrosion of Fe3O4(100)
Open this publication in new window or tab >>High resolution spectroscopic and microscopic signatures of ordered growth of ferrous sulfate in SO2 assisted corrosion of Fe3O4(100)
2007 (English)In: Applied Physics Letters, ISSN 0003-6951, E-ISSN 1077-3118, Vol. 91, no 9Article in journal (Refereed) Published
Abstract [en]

The authors present a high-resolution core-level photoemission study of a Fe3O4(100) surface exposed to 50 L (1 L=10(-6) mbar s) of H2O and 50 L of SO2. S 2p core-level spectra reveal the presence of SO3 and SO4 species. An additional peak in the Fe 3p core-level spectrum shows that they bond with iron from the substrate. Complementary scanning tunneling microscopy of the same surface demonstrates formation of a long-range ordered sulfate locked in the (root 2x root 2)R45 degrees-surface potential.

Keyword
scanning-tunneling-microscopy, sulfuric-acid-solution, in-situ, adsorbed sulfate, zno powders, adlayer, photoemission, electrodes, fe3o4(001), adsorption
National Category
Physical Sciences
Identifiers
urn:nbn:se:kth:diva-16921 (URN)10.1063/1.2776854 (DOI)000249156100099 ()2-s2.0-34548439418 (ScopusID)
Note

QC 20100525

Available from: 2010-08-05 Created: 2010-08-05 Last updated: 2013-01-16Bibliographically approved

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