Vanadium and molybdenum oxide thin films on Au(111): growth and surface characterization [Elektronische Ressource] / von Sebastien Guimond

Guimond Sebastien

Informations

• Publié par: humboldt-universitat_zu_berlin
• Publié le: 01 janvier 2009
• Nombre de lectures: 74
• Langue: Deutsch
• Poids de l'ouvrage: 26 Mo

Extrait

Vanadium and molybdenum oxide thin
films on Au(111): growth and surface
characterization
Dissertation
zur Erlangung des akademischen Grades
doctor rerum naturalium
(Dr. rer. nat.)
im Fach Chemie
eingereicht an der
Mathematisch-Naturwissenschaftlichen Fakultät I
Humboldt-Universität zu Berlin
von
M.Sc. Sebastien Guimond
geboren am 10.10.1977 in Quebec, Kanada
Präsident der Humboldt-Universität zu Berlin:
Prof. Dr. Dr. h.c. Christoph Markschies
Dekan der Mathematisch-Naturwissenschaftlichen Fakultät I:
Prof. Dr. Lutz-Helmut Schön
Gutachter:
1. Prof. Dr. Hans-Joachim Freund
2. Prof. Dr. Klaus Rademann
Tag der mündlichen Prüfung: 4. Juni 2009 1 Zusammenfassung

In der vorliegenden Arbeit wurden das Wachstum und die Oberflächenstruktur von definier-
ten V2O3-, V2O5- und MoO3-Filmen auf Au(111) sowie deren Einsatz als Modellsysteme
zur Untersuchung von Elementarreaktionen auf Vanadiumoxid- und Molybdänoxid-basierten
selektiven Oxidationskatalysatoren untersucht. Im Falle von V2O3(0001) befindet sich an der
Oberfläche der Filme eine Lage von Vanadylgruppen, welche kein Bestandteil der Kristall-
struktur sind. Die O Atome der Vanadylgruppen können durch Elektronenbeschuss definiert
entfernt werden, wodurch eine partiell oder vollständig V-terminierte Oberfläche erzeugt
werden kann. Der Grad der Oberflächenreduktion wird durch die Elektronendosis festgelegt.
Dieses ermöglicht eine Untersuchung des Einflusses der Vanadylgruppen und der unterkoor-
dinierten V-Ionen auf die Reaktivität der Modellkatalysatoren. Die Präparation von defektar-
men V2O5(001)- und MoO3(010)-Filmen ist erstmals in der vorliegenden Arbeit dokumen-
tiert. Diese Filme wurden mittels Oxidation in einer Hochdruckzelle bei einem
Sauerstoffdruck von 50 mbar hergestellt. Anders als in vielen Publikationen berichtet sind
diese hochkristallinen Schichten unter UHV-Bedingungen weitgehend reduktionsbeständig.
Oberflächenverunreinigungen und Defekte scheinen aber einen großen Einfluss auf die Redu-
zierbarkeit zu haben. Die von den Strukturen der regulären Oxide abweichenden Strukturen
der Koinzidenzgitter von V2O5- und MoO3-Monolagen werden durch die Wechselwirkung
mit der Au(111)-Unterlage stabilisiert, was vermutlich durch die einfache Verschiebbarkeit
von Koordinationseinheiten in V und Mo-Oxiden erleichtert wird. Für beide Oxide beginnt
das Wachstum regulärer Oxidstrukturen erst nach Vollendung der zweiten Lage. Die für
geträgerte V2O5-Katalysatoren häufig vorgebrachte Annahme, dass V2O5-
Kristallkeimbildung direkt auf einer Monolage stattfindet, sollte somit mit Vorsicht betrachtet
werden.

2Abstract

The growth and the surface structure of well-ordered V2O3, V2O5 and MoO3 thin films have
been investigated in this work. These films are seen as model systems for the study of ele-
mentary reaction steps occurring on vanadia and molybdena-based selective oxidation cata-
lysts. It is shown that well-ordered V2O3(0001) thin films can be prepared on Au(111). The
films are terminated by vanadyl groups which are not part of the V2O3 bulk structure. Elec-
tron irradiation specifically removes the oxygen atoms of the vanadyl groups, resulting in a V-
terminated surface. The fraction of removed vanadyl groups is controlled by the electron dose.
Such surfaces constitute interesting models to probe the relative role of both the vanadyl
groups and the undercoordinated V ions at the surface of vanadia catalysts. The growth of
well-ordered V2O5(001) and MoO3(010) thin films containing few point defects is reported
here for the first time. These films were grown on Au(111) by oxidation under 50 mbar O2 in
a dedicated high pressure cell. Contrary to some of the results found in the literature, the films
are not easily reduced by annealing in UHV. This evidences the contribution of radiation and
surface contamination in some of the reported thermal reduction experiments. The growth of
ultrathin V2O5 and MoO3 layers on Au(111) results in formation of interface-specific
monolayer structures. These layers are coincidence lattices and they do not correspond to any
known oxide bulk structure. They are assumed to be stabilized by electronic interaction with
Au(111). Their formation illustrates the polymorphic character and the ease of coordination
units rearrangement which are characteristic of both oxides. The formation of a second layer
apparently precedes the growth of bulk-like crystallites for both oxides. This observation is at
odds with a common assumption that crystals nucleate as soon as a monolayer is formed dur-
ing the preparation of supported vanadia catalysts.

3 Acknowledgements

First and foremost I would like to express my gratitude towards Prof. Hans-Joachim
Freund for his support and for giving me the opportunity to pursue this PhD work in the
Chemical Physics department of the Fritz-Haber-Institut. I would also like to thank Prof.
Klaus Rademann for his help and for accepting to be my thesis director at the Humboldt
University. Many thanks to Dr. Helmut Kuhlenbeck, who supervised my work and has been
of invaluable help throughout this thesis.

I had the chance to meet a lot of great persons during my stay at the Fritz-Haber-
Institute. Either as invaluable colleagues, friends or both, they all contributed to make this
stay a very enjoyable period of my life. Specials thanks go to Helder Marchetto, Doron
Lahav, Marta Borasio, Celine Lemire, Mohammad Abu Haija, Yuriy Romanyshyn, Daniel
Göbke, Alexander Uhl and Fank Höbel. I would also like to acknowledge the support of
Manuela Misch and Erika Popovic. I am especially grateful for the technical support of
Matthias Naschitzki, which greatly contributed to my work.

Last but not the least; I thank my friends and my family for their unfaltering support and
their encouragements. This work is dedicated to them. Audrée: thanks for your support over
all those years.

I am deeply grateful to Stefanie for being a part of my life and for accompanying me in
the last stressful part of my work. Thanks for your patience.

4Content:
1 Introduction....................................................................................................................... 6
2 Experimental ..................................................................................................................... 9
2.1 Experimental setup .............................................................................................................. 9
2.1.1 UHV systems .................................................................................................................................. 9
2.1.2 High pressure cell.......................................................................................................................... 12
2.1.3 Sample setup.. 13
2.1.4 Au(111) substrate 14
2.2 Experimental Techniques.................................................................................................. 16
2.2.1 Low energy electron diffraction (LEED) ...................................................................................... 16
2.2.2 Scanning tunneling microscopy (STM)......................................................................................... 17
2.2.3 Photoelectron spectroscopy (PES) ................................................................................................ 18
2.2.4 Near-edge X-ray absorption fine structure (NEXAFS)................................................................. 19
3 Growth and surface reduction of V O (0001) thin films ............................................... 20 2 3
3.1 Introduction........................................................................................................................ 20
3.2 Preparation Method........................................................................................................... 24
3.3 Results and discussion ....................................................................................................... 25
3.3.1 Vanadyl-terminated V O (0001)/Au(111): growth and surface structure..................................... 25 2 3
3.3.2 Surface reduction with electrons: formation of a V-termination................................................... 33
3.3.3 Thermal stability of V-terminated V O (0001)............................................................................. 40 2 3
3.4 Summary............................................................................................................................. 43
4 Growth of V O thin films............................................................................................... 45 2 5
4.1 Introduction........................................................................................................................ 45
4.2 Preparation Method........................................................................................................... 47
4.3 Results and discussion ....................................................................................................... 48
4.3.1 Structure of the first layers............................................................................