Zainab Taha Al-Abdullah

222 Pages·2013·26.77 MB·English

by  Chris Keene

A University of Sussex DPhil thesis 
Available online via Sussex Research Online: 
http://sro.sussex.ac.uk/   
This thesis is protected by copyright which belongs to the author.   
This thesis cannot be reproduced or quoted extensively from without first 
obtaining permission in writing from the Author   
The content must not be changed in any way or sold commercially in any 
format or medium without the formal permission of the Author   
When referring to this work, full bibliographic details including the 
author, title, awarding institution and date of the thesis must be given 
Please visit Sussex Research Online for more information and further details
1

The Study of the Growth Mechanism of
TiO Nanotubes and Their Applications
2

By
Zainab Taha Al-Abdullah

A Thesis Submitted for the Degree of Doctor of Philosophy
School of Life Sciences
University of Sussex

November 2012
i

Declaration

I hereby declare that this work has not been submitted in any substance to another
University for the award of any other degree or other academic or professional distinction.

Signature ……….……………….. Zainab Taha Yassin Al-Abdullah
ii

Acknowledgments

I would like to express my sincere gratitude and obligation to my supervisor Dr.
Qiao Chen for guiding me through my study. I started working with him with very little
knowledge of the subject, but his patience and support helped me gain a lot of
experimental skills and understanding. I am grateful for all the help and the support
from him and his research group. Without them, the thesis would be impossible.
I would also like to thank my second supervisor, Dr. Alaa Abdul-Sada who has
given valuable advice, help and support throughout this work.
I would like to thank the Ministry of Higher Education and Scientific Research in
Iraq for giving me this great opportunity and sponsoring my PhD study. I hope my work
will benefit the country in the future.
I would like to express my warmest thanks to Victoria Smith for her assistance in
the English language and support. I would also like to thank my colleague Rantej Kler
for the support and help with computer programmes and sharing knowledge. I am
grateful to our group members, including Daniel Lester, Fang Yuanxing, Teo Mertanen,
Giacomo Canciani, Ali Shahroozi, Brnyia Omar, Adward Lee and Jasbir Bedi, for their
help and sharing their knowledge and experiences. I specially thank Dr. Mark Osborne
and his group members Simon Cooper and Remi Boulineau for their help and support. I
wish to express my enormous gratitude to Prof. Malcolm Heggie and Dr John Turner
for their encouragement and academic support. Special thanks must goes to Dr. Julian
iii

Thorpe for TEM imaging. I would also like to thank Mr. Mick Henry in the teaching lab
for his help. I would like to thank Dr. Talib Al-Salih for encouragement and support.
Although Karen White is no longer a member of the Sussex staff, I would like to
extend my gratitude for her help and kindness throughout the process leading to the
offer of a place.
I would like to express my gratitude to my dear husband and my lovely kids for
all the patience, help and encouragement throughout the whole intense process and I am
sorry that they had to sacrifice part of their life, plans and dreams to allow me to achieve
my ambition.
I would like to express my sincere gratitude to my father for offering his house a
surety for my scholarship. This showed the confidence he had in me. Special thanks to
my mother and brothers who have always been there for me in all the difficult times, for
their love, support and encouragement, for their belief in my ability during my
experience in the UK.
Special thank for my friends Dr. Waleed Alhusabi, Dr Raghad Al-Salhi, Shatha
Sami, Youssra Al-Hilaly, Dr. Asma Mohammed, Maan Rasheed and Amran Yunis for
providing great assistance and always being available for help and suggestions.

I dedicate this thesis to my dear Bassam
iv

University of Sussex
Zainab Taha Yassin Al-Abdullah
D.Phil
The Study of the Growth Mechanism of TiO Nanotubes and Their
2
Applications
Summary
This research project focused on the creation of nanomaterials and their applications. The
main aim was to control the growth of TiO nanotubes with various morphologies and to
2
investigate potential applications for controlled drug release and for photocatalytic water
splitting.
The electrochemical anodisation process in fluoride-containing organic electrolytes was
employed to prepare vertically aligned TiO nanotubular arrays, with inner diameters of
2
individual nanotubes ranging from 50 to 150 nm. A variety of morphologies was created by
precise control of experimental conditions and parameters. The formation of crystal phases in
the TiO nanotubes was controlled by the annealing temperature (in air) and monitored by
2
powder X-ray diffraction (XRD).
The fundamental anodisation parameters affecting the morphologies, such as anodisation
voltage, electrolyte composition, stirring and the effect of magnetic fields were investigated.
Various processing procedures that affect the anodisation process have been studied. The
influence of hydroxide islands on the growth mechanism was shown by analysis of anodisation
current-time profiles, contact angle measurements and SEM observations.
The effect of pre-patterns on the Ti substrate was also studied. The substrate was
patterned either mechanically or by Electron Beam Lithography (EBL) with
polymethylmethacrylate (PMMA) as a positive photoresist. Instead of circular nanotubes,
polygonal TiO nanotubes were formed from the mechanically patterned substrate whereas
2
rectangular and tube-in-tube TiO nanotubes were formed by using EBL.
2
v

The TiO nanotubes were used as photoanodes for photocatalytic water splitting using a
2
photoelectrochemical cell for generating hydrogen gas. The effects of nanotube morphology and
crystal structure on the efficiency of the conversion of photon energy to chemical energy were
studied on samples annealed at various temperatures, and with a range of organic hole
scavengers.
In addition, control of the morphology was realised by surface passivation with organic
thin films and by the control of the anodisation parameters. With stepwise control, bottle shaped
nanotubes (nanobottles) were designed and created for their application in controlled drug
release.
Scanning and transmission electronic microscopy (SEM and TEM) were used to examine
the structure and morphology of the nanotubes. The surface composition was studied by X-ray
Photo-electron Spectroscopy (XPS) and Energy Dispersive X-ray Spectroscopy (EDX). Crystal
phases were identified by XRD.
vi

Table of contents
Declaration ....................................................................................................................... I
Acknowledgments .......................................................................................................... II
Summary ....................................................................................................................... IV
List of Figures ............................................................................................................. XIV
List of Tables ............................................................................................................... XX
Chapter 1 Introduction ................................................................................................... 1
1.1 An Overview of TiO Nanotube Structure and Synthesis ........................................... 1
2
1.1.1 The Electronic Structure of the Semiconductor ............................................. 4
1.1.2 Fermi Level and Doping ................................................................................. 4
1.1.3 The Fundamentals of Synthesis of TiO Nanotubular Arrays ........................ 6
2
1.1.4 Study of Anodisation Parameters ................................................................. 10
1.1.4.1 Anodisation Voltage .............................................................................. 10
1.1.4.2 The Distance between the Electrodes .................................................... 13
1.1.4.3 Electrolyte .............................................................................................. 14
1.1.4.4 Fluoride Concentration .......................................................................... 14
1.1.4.5 pH........................................................................................................... 15
1.1.4.6 Temperature ........................................................................................... 15
1.1.5 The Development of the Anodisation Method for Creating TiO Nanotubes
2
..................................................................................................................................... 16
1.1.5.1 The First Generation of TiO Nanotubes (using Aqueous Electrolytes) 16
2
1.1.5.2 The Second Generation of TiO Nanotubes (using Buffered Solutions)
2
................................................................................................................................. 16
1.1.5.3 The Third Generation of TiO Nanotubes (using Polar Organic
2
compounds) .............................................................................................................. 17
1.1.5.4 The Fourth Generation of TiO Nanotubes from (Non-Fluoride, Acid
2
Base Electrolytes) .................................................................................................... 18
1.2 Titanium Dioxide: A Broad Range of Applications ................................................. 18
1.2.1 TiO in Photocatalysis .................................................................................. 19
2
1.2.2 Water Splitting by TiO Photocatalysis ........................................................ 22
2
1.2.2.1 The Mechanism of Hydrogen Production by a Photoelectrochemical
Cell (PEC) ................................................................................................................ 23
vii

1.2.2.2 The Efficiency of TiO in a PEC: Problems and Solutions ................... 25
2
1.2.3 Photovoltaic Application .............................................................................. 27
1.2.3.1 Solar Energy for photoexcitation ........................................................... 29
1.2.3.2 Current-Voltage (I-V) Classification ..................................................... 30
1.2.3.3 Fill Factor ............................................................................................... 31
1.2.3.4 The Efficiency........................................................................................ 32
1.2.4 Drug Delivery ............................................................................................... 32
1.3 Other Applications of TiO ....................................................................................... 33
2
1.4 Thesis Aims ............................................................................................................... 34
Chapter 2 Experimental ............................................................................................... 37
2.1 Abstract ..................................................................................................................... 37
2.2 Sample Preparation ................................................................................................... 37
2.3 Anodising Unit .......................................................................................................... 37
2.4 Characterisation by Electron Microscope Techniques .............................................. 38
2.5 Scanning Electron Microscopy (SEM) ..................................................................... 39
2.5.1 Stigmatism and Resolution ........................................................................... 43
2.5.2 Working Distance and Resolution ................................................................ 44
2.5.3 Working Distance and Depth of Field .......................................................... 44
2.5.4 Accelerating Voltage and Resolution ........................................................... 45
2.5.5 Preparation of SEM Specimens of Anodised TiO ...................................... 47
2
2.5.5.1 The Secondary Electron Image (SEI) .................................................... 48
2.6 Energy Dispersive X-ray Spectroscopy (EDX) ........................................................ 49
2.7 Transmission Electron Microscopy (TEM) .............................................................. 49
2.8 X-Ray Diffraction ..................................................................................................... 50
2.9 The photoelectrochemical Cell (PEC) ...................................................................... 54
2.10 Annealing ................................................................................................................ 55
2.11 Electron Beam Lithography (EBL) ......................................................................... 55
Chapter 3 A New Approach to the Catalytic Growth of Anodised TiO Nanotubes
2
......................................................................................................................................... 58
3.1 Abstract ..................................................................................................................... 58
3.2 Introduction ............................................................................................................... 58
viii

3.3 Experimental Details ............................................................................................ 59
3.4 Results and Discussion .............................................................................................. 60
3.4.1 Creating the Catalytic Hydroxide Layer ...................................................... 61
3.4.2 Contact Angle Measurements ...................................................................... 64
3.4.3 Time Dependent SEM Study at Different Stages of Anodisation ................ 66
3.4.4 Change of Current during Anodisation ........................................................ 71
3.5 The Effect of Processing Parameters on Nanotube Morphology .............................. 79
3.5.1 Influence of Anodisation Voltage on Tube Length ...................................... 79
3.5.2 Influence of Anodisation Voltage on the Tube Diameter ............................ 80
3.5.3 Influence of the Fluoride Concentration on the Film Morphology .............. 81
3.5.4 Control of Nanotube Morphology by Mechanical Stirring .......................... 83
3.5.5 Control of Nanotube Morphology by Magnetic Fields ................................ 88
3.6 Conclusion ................................................................................................................ 93
Chapter 4 Formation of Polygonal TiO Nanotubes .................................................. 95
2
4.1 Abstract ..................................................................................................................... 95
4.2 Introduction ............................................................................................................... 95
4.3 Experimental Details ................................................................................................. 97
4.4 Results and Discussion .............................................................................................. 97
4.4.1 Morphology Evolution during the Anodisation ........................................... 99
4.4.2 The Effect of Initial Surface Texture on the Growth Mechanism .............. 101
4.4.3 Crystallinity of TiO Nanotubes: Circular vs Polygonal ............................ 101
2
4.5 Conclusion .............................................................................................................. 103
Chapter 5 Control of the Morphology of TiO Nanotubes by Patterning with
2
Electron Beam Lithography ....................................................................................... 105
5.1 Abstract ................................................................................................................... 105
5.2 Introduction ............................................................................................................. 105
5.3 Experimental Details ............................................................................................... 107
5.4 Results and Discussion ............................................................................................ 108
5.4.1The Thickness of the Resist ......................................................................... 108
5.4.2 EBL Parameters .......................................................................................... 109
5.4.3 Development of Patterns ............................................................................ 110