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From photocatalysis to optoelectronic switches [Elektronische Ressource] : studies of visible light active photoelectrodes based on surface-modified titanium dioxide / vorgelegt von Radim Beránek

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151 pages
From Photocatalysis to Optoelectronic Switches: Studies of Visible Light Active Photoelectrodes Based on Surface-Modified Titanium Dioxide Den Naturwissenschaftlichen Fakultäten der Friedrich-Alexander-Universität Erlangen-Nürnberg zur Erlangung des Doktorgrades vorgelegt von Radim Beránek aus D ěčín (Tschechien) Als Dissertation genehmigt von den Naturwissenschaftlichen Fakultäten der Universität Erlangen-Nürnberg Tag der mündlichen Prüfung: 13. Juli 2007 Vorsitzender der Promotionskommission: Prof. Dr. Eberhard Bänsch Erstberichterstatter: Prof. Dr. Horst Kisch Zweitberichterstatter: Dr. Ulrich Nickel Acknowledgements I wish to thank Prof. Dr. Horst Kisch for the supervision of this work and many fruitful discussions. I am particularly grateful for his advice, his trust in me, and his generous support of my work. Parts of this work would not have been possible without the help of several people. Dr. Marcin Janczarek and Dr. Shanmugasundaram Sakthivel are acknowledged for the synthesis of powders used in Chapter 3. Bernhard Neumann (Hahn-Meitner-Institut, Berlin) is gratefully acknowledged for surface photovoltage measurements, their interpretation and fruitful discussions. The work presented in Chapter 8 has profited immensely from discussions with Dr. Wojciech Macyk and Dr. Konrad Szaci łowski. I thank Dr.
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From Photocatalysis to Optoelectronic Switches:
Studies of Visible Light Active Photoelectrodes Based on
Surface-Modified Titanium Dioxide






Den Naturwissenschaftlichen Fakultäten
der Friedrich-Alexander-Universität Erlangen-Nürnberg
zur
Erlangung des Doktorgrades

vorgelegt von
Radim Beránek
aus D ěčín (Tschechien)


Als Dissertation genehmigt von den Naturwissenschaftlichen Fakultäten der
Universität Erlangen-Nürnberg




















Tag der mündlichen Prüfung: 13. Juli 2007
Vorsitzender der Promotionskommission: Prof. Dr. Eberhard Bänsch
Erstberichterstatter: Prof. Dr. Horst Kisch
Zweitberichterstatter: Dr. Ulrich Nickel

Acknowledgements
I wish to thank Prof. Dr. Horst Kisch for the supervision of this work and many
fruitful discussions. I am particularly grateful for his advice, his trust in me, and his
generous support of my work.
Parts of this work would not have been possible without the help of several people.
Dr. Marcin Janczarek and Dr. Shanmugasundaram Sakthivel are acknowledged for the
synthesis of powders used in Chapter 3. Bernhard Neumann (Hahn-Meitner-Institut,
Berlin) is gratefully acknowledged for surface photovoltage measurements, their
interpretation and fruitful discussions. The work presented in Chapter 8 has profited
immensely from discussions with Dr. Wojciech Macyk and Dr. Konrad Szaci łowski. I
thank Dr. Marc Gärtner for his help with many experiments especially in the early
stage of my research, for valuable discussions and for correcting the German version
of the Summary. Dariusz Mitoraj is acknowledged for photocatalytic experiments and
for discussions. I thank also Christina Wronna for elemental analyses, Susanne
Hoffman for XRD measurements, Regina Müller for TGA analyses, Siegfried Smolny
for surface area measurements, Helga Hildebrand for XPS measurements, Anja
Friedrich for SEM and EDX analyses, Prof. Patrik Schmuki and
Prof. Sannakaisa Virtanen for allowing to perform the impedance measurements in
their labs, and Jan M. Macak and Dr. Julia Kunze for discussions. Manfred Weller,
Peter Igel and their colleagues from the “Werkstatt” are acknowledged for the
construction of photoelectrochemical cells and assistance with technical problems.
I am also indebted to Dr. Matthias Moll for his manifold help, Uwe Reißer for his help
with electronic equipment, Ronny Wiefel for glass work, and to Dr. Jörg Sutter for
computer assistance.
Many thanks to all my colleagues for contributing to the very good atmosphere in
the group – Marc, Marcin, Gerald, Müge, Ayyappan, Sakthi, Joachim, Sina, Dai,
Marco, Guillermo, Long, Darek, Przemek and Francesco – and also to all my friends
outside the institute, especially from the KHG.
I am very grateful towards my parents for their lifelong love and encouragement,
and I dedicate this work to them.
Last but not least, I would like to thank Ewa for her love and support.

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Die vorliegende Arbeit entstand in der Zeit von Oktober 2003 bis Mai 2007 am
Institut für Anorganische Chemie der Universität Erlangen-Nürnberg unter Anleitung
von Herrn Prof. Dr. Horst Kisch.
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Contents
Acknowledgements.............................................................................................. 1
Contents................................................................................................................ 3
Symbols and Abbreviations................................................................................ 5
1. Introduction .................................................................................................. 9
2. Literature .................................................................................................... 15
2.1 Fundamentals of Semiconductor Physics .......................................................15
2.1.1 Energy Band Model..........................................................................................15
2.1.2 Optical Properties of Semiconductors ..............................................................19
2.2 Photoelectrochemistry of Semiconductors .....................................................23
2.2.1 Semiconductor/Electrolyte Interface................................................................23
2.2.1.1 The Model of Gerischer23
2.2.1.2 Charge and Potential Distribution at the Interface..................................................26
2.2.1.3 The Energy Position of Band Edges .......................................................................30
2.2.2 Photoinduced Processes at Semiconductors in Electrolytes.............................32
2.2.2.1 Compact Semiconductor Electrodes33
2.2.2.2 Nanostructured Semiconductor Electrodes.............................................................35
2.2.2.3 Semiconductor Photocatalysis ................................................................................38
2.3 Nitrogen Doping/Modification of TiO ..........................................................40 2
2.3.1 Preparation Methods.........................................................................................40
2.3.2 Nitrogen Doping and the Origin of Visible-Light Activity..............................41
2.3.3 Photoelectrochemical Studies...........................................................................43
2.3.4 A Novel Low-Bandgap Titania ........................................................................44
3. Photocurrent and Surface Photovoltage Investigations of Low-Bandgap
Nitrogen-Modified TiO .................................................................................... 46 2
3.1 Introduction.....................................................................................................46
3.2 Experimental...................................................................................................46
3.3 Results and Discussion ...................................................................................49
3.3.1 Optical and Structural Characterization ...........................................................49
3.3.2 Surface Photovoltage Measurements................................................................50
3.3.3 Photocurrent Measurements.............................................................................53
3.3.4 Mechanism of Visible-Light Formic Acid Degradation...................................58
3.4 Conclusions60
4. Surface-Modified Anodic TiO Films for Visible Light Photocurrent 2
Response............................................................................................................. 61
4.1 Introduction.....................................................................................................61
4.2 Experimental...................................................................................................61
4.3 Results and Discussion ...................................................................................65
4.3.1 Structure and Composition ...............................................................................65
34.3.2 Photocurrent Measurements.............................................................................67
4.3.3 Flatband Potential Determination ....................................................................70
4.3.4 Mechanism of Photocurrent Generation ..........................................................73
4.4 Conclusions ....................................................................................................74
5. Tuning the Optical and Photoelectrochemical Properties of Surface-
Modified TiO 75 2
5.1 Introduction75
5.2 Experimental...................................................................................................76
5.3 Results and Discussion...................................................................................79
5.3.1 Optical and Structural Characterization........................................................... 79
5.3.2 Determination of Band Edges..........................................................................89
5.3.3 Photocurrent Measurements.............................................................................94
5.4 Conclusions ..................................................................................................100
6. Surface-Modified Transparent Nanocrystalline Films......................... 101
6.1 Introduction101
6.2 Experimental.................................................................................................101
6.3 Results and Discussion.................................................................................103
6.3.1 Structural and Optical Properties ...................................................................103
6.3.2 Spectroelectrochemical Measurements..........................................................104
6.3.3 Photocurrent Measurements...........................................................................108
6.4 Conclusions111
7. Comparative Discussion 112
7.1 Preparation Procedures112
7.2 Optical Properties .........................................................................................113
7.3 XPS Investigations .......................................................................................114
7.4 Quasi-Fermi Level........................................................................................114
7.5 Photocurrent Generation Efficiency.............................................................115
7.6 Conclusions ..................................................................................................116
8. A Wavelength-Controlled Optoelectronic Switch................................. 117
8.1 Introduction117
8.2 Experimental.................................................................................................118
8.3 Results and Discussion.................................................................................120
8.3.1 Determination of Bandgap Energy and Band Edges .....................................120
8.3.2 Photocurrent Response and the Mechanism of Switching.............................122
8.4 Conclusions126
9. Summary ................................................................................................... 127
10. Zusammenfassung 132
11. References .............................................................................................. 138

4 Symbols and Abbreviations
Symbols and Abbreviations
A electron acceptor species
A (1) electron affinity
(2) absorbance
APCE absorbed photon to current efficiency
ASTM American Society for Testing and Materials
α absorption coefficient
a activity of oxidized species ox
a activity of reduced species red
BET Brunauer-Emmett-Teller
CB conduction band
C capacitance
C capacitance of the Helmholtz double layer H
C capacitance of the Gouy-Chapman layer G
C capacitance of the space charge layer SC
c concentration of oxidized species ox
c concentration of reduced species red
D electron donor species
DFT density functional theory
D (E) density of states for oxidized species in a solution ox
D (E) density of states for reduced species in a solution red
d thickness
E energy
E applied potential appl
E conduction band edge C
E conduction band edge at the surface C,S
0E conduction band edge at the surface at U = 0 C,S H
EDX energy dispersive X-ray spectroscopy
eE energy of free electrons on the hydrogen scale
E Fermi level F
E flatband potential FB
*E quasi-Fermi level of electrons Fn
5Symbols and Abbreviations
 
*E quasi-Fermi level of holes Fp
E Fermi level of a redox couple F,redox
E bandgap energy g
E open-circuit potential OC
0E Fermi level of oxidized species ox
E phonon energy p
EPR electron paramagnetic resonance
0E Fermi level of reduced species red
E electrochemical potential of electrons in solution redox
0E standard reduction potential redox
E valence band edge V
End edge at the surface V,S
– e electron
–e electron in the conduction band CB
ε relative permittivity
ε permittivity of vacuum 0
F Faraday constant
FE-SEM field-emission scanning electron microscope
F(R) Kubelka-Munk function ∞
FTO fluorine doped tin oxide
FWHM full width at half maximum
f frequency
f(E) Fermi-Dirac distribution
ϕ potential
ΔG Gibbs free energy
+ h hole
hν energy of light
I (1) intensity of light
(2) ionization energy
IHP inner Helmholtz plane
IPCE incident photon to current efficiency
IPCE incident photon to current efficiency for back-side illumination BS
IPCEciency for front-side illumination FS
6 Symbols and Abbreviations
ITO indium tin oxide
i imaginary unit
i photocurrent density ph
k pH dependence constant
kT thermal energy
L( λ) penetration depth of light
L diffusion length of the minority carriers P
/L h of the majority carriers P
λ (1) reorganization energy
(2) wavelength
N doping concentration
N density of acceptor states A
N density of donor states D
N(E) density of states in the semiconductor
NHE normal hydrogen electrode
n (1) electron concentration in the conduction band
(2) constant depending on the nature of the optical transition
(3) number of electrons transferred in a redox reaction
n (E) energy density of electrons in the conduction band E
OHP outer Helmholtz plane
Ox oxidized species
P light power density
p (1) hole concentration in the valence band
(2) pressure
p (E) energy density of holes in the valence band E
pH isoelectric point IEP
q elementary charge
R universal gas constant
R diffuse reflectance of the sample relative to the reflectance of a standard ∞
Red reduced species
ρ volume charge density
SPV surface photovoltage
T absolute temperature
7Symbols and Abbreviations
 
TGA thermal gravimetric analysis
TOC total organic carbon
U potential drop in the Gouy-Chapman layer G
U potential drop in the Helmholtz layer H
U potential drop in the depletion layer S
VB valence band
W width of the depletion layer
W (E) distribution function for oxidized species in a solution ox
W (E) distribution function for reduced species in a solution red
X Sanderson electronegativity
XPS X-ray photoelectron spectroscopy
XRD X-ray diffractometry
x distance
Z imaginary part of impedance im


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