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TiO Nanotubes and Mesosponges: 2
Towards Solar Cells and Related Applications
TiO Nanoröhren und mesoporöse Schichten: 2
Für Solarzellen und verwandte Anwendungen

Der Technischen Fakultät
der Universität Erlangen-Nürnberg
zur Erlangung des Grades

DOKTOR-INGENIEUR

Vorgelegt von Doohun Kim

Erlangen 2010





Als Dissertation genehmigt von
der Technischen Fakultät der
Universität Erlangen-Nürnberg



Tag der Einreichung: 18.05.2010
Tag der Promotion: 06.07.2010
Dekan: Prof. Dr.-Ing. Reinhard German
Berichterstatter: Prof. Dr. Patrik Schmuki
Prof. Dr.-Ing. Yongsug Tak


Acknowledgements
I would like to express my deepest appreciation to my former and present colleagues who have
helped me and shared their professional experiences.

First of all, I thank Prof. Patrik Schmuki, who introduced me to the exciting field of nanotubes
and mesosponges with his great supervision. Without his patience, guidance and support, I would never
have been able to reach this stage.

I would also like to thank my former supervisor Prof. Chongwoo Nam, Dr. Taekwan Kim, Prof.
Duck-Hyun Kim and Prof. Yongsug Tak for their helpful advice.

I am thankful to Kiyoung, Yoonchae, Nabeen and Kwonyul for being with me during the course
of my study and always making themselves available to help me for professional and personal
problems. I am thankful to Dr. Wonkeun Chang for proofreading my thesis. I would like to thank
Robert Hahn, Dr. Steffen Berger, Dr. Andrei Ghicov, Sergiu P. Albu, Dr. Poulomi Roy, Helga
Hildebrand, Martin Kolacyak, Prof. Hiroaki Tsuchiya and Prof. Sannakaisa Virtanen for their valuable
advices on numerous technical issues that I had.

Finally, I would like to express my deepest gratitude to my parents, sister, brother and other
extended family members for their constant support and encouragement.

“四年如一日”
“Last 4 year is like 1 day.”
This proverb expresses my feelings for the past few years in our lab LKO. During my PhD, I had
so many opportunities to study interesting concepts and test various novel ideas, and while doing so, I
have gained a lot of experiences in carrying out professional research projects. These would not have
been possible without the support and encouragements from the people that I have mentioned above.

To all those who contributed to the improvement and who helped me:
Thank you!!! - Doohun Kim -























ABSTRACT..............................................................................................................................................1

1 OVERVIEW.......................................................................................................................................5
1.1 INTRODUCTION................................................................................................................................5
1.2 TITANIUM DIOXIDE NANOSTRUCTURES...........................................................................................7
1.3 APPLICATIONS OF TiO2 NANOSTRUCTURES...................................................................................12
1.3.1 Photocatalysis under UV light..............................................................................................13
1.3.2 Modification of TiO2 for visible light use .............................................................................14
1.3.3 Dye-sensitized solar cell ......................................................................................................16
1.3.4 Electrochromism...................................................................................................................22
1.4 AIM OF THIS WORK .......................................................................................................................25

2 THEORY..........................................................................................................................................26
2.1 ANODIZATION OF TITANIUM.........................................................................................................26
2.1.1 Preparation of TiO nanotubes............................................................................................26 2
2.1.2 Preparation of TiO mesosponges........................................................................................34 2
2.2 DYE-SENSITIZED SOLAR CELLS......................................................................................................36
2.2.1 Fundamental of dye-sensitized solar cells.............................................................................36
2.2.2 Anode materials in DSCs......................................................................................................38
2.2.3 IPCE and IV characteristics.................................................................................................43
2.2.4 IMPS/VS measurements........................................................................................................48
2.3 RELATED APPLICATIONS...............................................................................................................55
2.3.1 Photocatalysis and photoelectrochemistry under UV or Visible light..................................55
2.3.2 Electrochromism...................................................................................................................63

3 EXPERIMENTAL AND METHODS............................................................................................67
3.1 ANODIZATION AND MODIFICATION OF NANOSTRUCTURES............................................................67
3.1.1 Anodic TiO nanostructures: nanotubes and mesosponges..................................................67 2
3.1.2 Thermal treatment...............................................................................................................68
3.1.3 Dye-sensitization...................................................................................................................68
3.2 CHARACTERIZATION OF THE NANOSTRUCTURES...........................................................................69
3.2.1 Scanning Electron Microscopy (SEM)..................................................................................69
3.2.2 Energy Dispersive X-ray spectroscopy (EDX)......................................................................69
3.2.3 X-Ray Diffraction spectroscopy (XRD).................................................................................69
3.2.4 X-ray Photoelectron Spectroscopy (XPS).............................................................................69
3.2.5 Transmission Electron Microscopy (TEM)...........................................................................69
3.3 APPLICATION OF THE NANOSTRUCTURES......................................................................................70
3.3.1 Photoelectrochemistry..........................................................................................................70
3.3.2 Solar cell construction and its efficiency measurement........................................................71
3.3.3 IMPS/VS measurements........................................................................................................73
3.3.4 Electrochromic measurement...............................................................................................74

4 RESULTS AND DISCUSSION......................................................................................................75
4.1 SELF-ORGANIZED TiO2 NANOTUBES IN DSCs................................................................................75
4.1.1 Morphological aspect...........................................................................................................75
4.1.2 Crystalline structure.............................................................................................................80
4.1.3 Dye-sensitization...................................................................................................................83
4.2 MORPHOLOGICAL MODIFICATION OF TiO2 NANOTUBES FOR DSCs..............................................84
4.2.1 Nanograss eliminated TiO nanotubes.................................................................................84 2
4.2.2 Bamboo type TiO nanotubes................................................................................................88 2
4.2.3 TiO nanoparticle decoration on TiO nanotubes................................................................92 2 2
4.3 TiO2 MESOSPONGES IN DSCs........................................................................................................96
4.3.1 Formation of TiO mesosponge and its use in DSCs............................................................96 2
4.3.2 Efficiency enhancing by TiCl treatment............................................................................103 4
4.4 PHOTO-ELECTROCHEMISTRY OF TiO2 NANOTUBES AND OTHER APPLICATIONS...........................107
4.4.1 Electrochemistry of N-doped TiO nanotubes grown on TiN alloys...................................107 2
4.4.2 Electrochromism of TiO and WO nanotube composites grown on TiW alloys................112 2 3
4.4.3 Capillary effect and wetting behavior of TiO2 nanotubes...................................................120

CONCLUSIONS..................................................................................................................................124
REFERENCES.....................................................................................................................................128
LIST OF SYBOLS...............................................................................................................................143
LIST OF PUBLICATIONS.................................................................................................................146
























ABSTRACT
There has been a considerable interest in nano-oxide materials owing to their remarkable
characteristics in electrical, optical and chemical properties which have led to various applications.
Among the various oxides, titanium dioxide (TiO ) is the most widely studied material, because of its 2
promising applications in photocatalysts, biomedical devices and solar cells with its non-toxic nature,
high chemical stability and relatively low production cost. However, photoinduced processes in TiO 2
typically require UV light irradiation due to its comparably high bandgap energy. This does not allow
an efficient use of solar energy, because only 2~5% of the solar spectrum are in the UV range.
Therefore, considerable efforts have been made to engineer the bandgap by doping of TiO 2
photoelectrode with suitable species or by sensitization with visible light absorbers to enhance the solar
light conversion efficiency in the visible range.
In many applications, TiO layers have been prepared by a variety of techniques such as sol-2
gel, e-beam evaporation, magnetron sputtering, anodization, etc. Especially, anodization is a simple and
exquisite method for synthesizing TiO nanostructures, and thus anodization is a promising method that 2
can be considered as a convenient and cost-effective process. Since the last decade, a significant body
of work on anodization has been dedicated to form self-organized and highly ordered nanotube oxide
layers. Their morphologies (e.g. nanotube length, diameter and geometrical modification) and
crystallinities (e.g. amorphous, anatase and rutile) can be tuned and adjusted to their applications by
tailoring the anodization and annealing conditions. In this work, the electrochemical single step
anodization process in fluoride-containing electrolytes is employed to prepare vertically oriented TiO 2
nanotubes. Furthermore, we introduce an entirely novel anodization approach to prepare a mesoporous
TiO nanostructure. Using the anodization in a hot phosphate electrolyte, an anodic oxide layer with 2
several tens of micrometers thickness is formed, and by subsequent selective etching treatment of this
layer, a connected, ordered and mesoporous TiO network (so called TiO mesosponge layer) is 2 2
obtained.
Over the past 30 years, most of the works for TiO nanostructures have been investigated on 2
conventionally sintered nanoparticles that are typically prepared by chemical synthesis. It is necessary
to study anodic TiO layers in the applications for photoactive electrodes. In this work, the use of TiO 2 2
nanotubes and mesosponges mainly in dye-sensitized solar cells (DSCs) are investigated to understand
their physical, chemical and electrochemical behaviors. Additionally, for other related applications
1
based on photoelectrochemistry, three different approaches were addressed with nanotube layers grown
i) on TiN to activate photocatalysis in the visible range, ii) on TiW to use in electrochromism and iii) on
pure Ti to make an UV induced hydrophilic capillary.
In the first part of this work, a Ru-complex photosensitized TiO nanotube layers are used in 2
DSCs. A range of fundamental geometries, crystallinities and chemical dye absorptions of TiO 2
nanotube layers are systematically investigated.
In the second part, we introduce three approaches to prepare modified TiO nanotubes with 2
various ideas such as i) anodization of polished Ti foil to eliminate an undesired morphology-
“nanograss”, ii) applying alternated voltage to prepare bamboo-type nanotubes and iii) decorating TiO 2
particles on TiO nanotubes. By these simple variations, the geometry and surface properties of the 2
nanotube layers can be altered over a wide range. When these highly aligned, ordered and reinforced
geometries are used in DSCs, significant increase in the conversion efficiencies can be achieved - i.e.,
efficiency improvements of 23% for “nanograss free nanotubes”, 53% for “nanobamboos” and 30% for
“particle decorated nanotubes” were achieved.
In the third part, we introduce an entirely novel anodization/etching process to form thick
mesoporous oxide layers (TiO mesosponge layers). By using the simple anodization process followed 2
by the chemical etching step, highly regular and robust mesoporous TiO structures with thicknesses 2
greater than 50 μm can be directly formed on Ti. These layers consist of nanoscopic pores and strong
interlinkages of TiO nano-features, thus providing significantly high specific surface area, which can 2
be exploited to DSCs more efficiently. Moreover, nanoparticles can be decorated uniformly on the
walls of TiO mesosponges over the entire layer thickness by TiCl treatment. With a higher specific 2 4
dye-loading on the layer, a significant efficiency improvement of 36% for the particle decorated
mesosponges was achieved.
In the final part, we show a significant visible-photoresponse of N-doped TiO nanotubes 2
prepared by anodization of a TiN alloy. Moreover, we show the use of highly ordered TiO -WO 2 3
nanotube layers that are formed by anodization of TiW alloys, which show drastically enhanced
intercalation and electrochromic properties. Additionally, we address effects associated with the
electrolyte penetration kinetics into TiO nanotube layers. This effect is verified by photoresponse 2
measurements and can be exploited to achieve photo-induced filling of the nanotubes by a secondary
material.
2

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