Haider master's thesis

Fabrication of TiO2 Nanotubes Using Electrochemical Anodization

Republic of Iraq
Ministry of Higher Education
and Scientific Research
University of Baghdad
College of Science

Fabrication of TiO2 Nanotubes Using
Electrochemical Anodization
A Thesis
Submitted to the University of Baghdad,
College of Sciences, Department of Physics as
a Partial Fulfillment of the Requirements for the
Degree of Master of Science in Physics

By
Haidar H. Hamdan Al-Eqaby
B.Sc., Al Mustansiriyah University, 2007
Supervised by

Prof. Dr. Harith I. Jaafar Lect. Dr. Abdulkareem M. Ali
2012 AD 1433 AH

1

‫‪Fabrication of TiO2 Nanotubes Using Electrochemical Anodization‬‬

‫( َوي َْس َألونَك عَن‬
‫َ ِ‬
‫الروحِ‬ ‫ُّ‬
‫الروح ِمن أَ ْمر ُقل‬
‫ُّ ُ ْ ِ ِ‬
‫َرِّب َو َما ُأوتِيُت ِمن‬
‫ُْ َ‬ ‫ي‬
‫الْ ِع ِْْل ِإ اَّل قَ ِليل)‬
‫ا‬

‫صدق الله العلي العظيم‬
‫سورة اإلرساء‬
‫اآلية 58‬

‫2‬


DEDICATION
To:
My mother
My father
My Brothers
My Sisters
My Uncle Mr. Jabbar
My close friends
My country beloved Iraq
The martyrs of Iraq with all the love and appreciation.

Haidar

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ACKNOWLEDGEMENTS

Praise be to ALLAH, his majesty for his uncountable blessings, and best
prayers and peace be unto his best messenger Mohammed, his pure descendant,
and his family and his noble companions.

First I would like to thank my family. Without their love and support over
the years none of this would have been possible. They have always been there
for me and I am thankful for everything they have helped me achieve.

Next, I would like to thank my supervisors Prof. Dr. Harith I. Jaafar
and lect. Dr. Abdulkareem M. Ali, Dr. Harith your help and guidance over the
years which is unmeasurable and without it I would not be where I am today.
Dr. Harith, what can I say, as graduate students we are truly fortunate to have
you in the department. I thank you so much for the knowledge you have passed
on and I will always be grateful for having the opportunity to study under you. I
would like to thank Dr. Kamal H. Lateif, Dr. Baha T. Chiad, Dr. Shafiq S.
Shafiq, Dr. Fadhil I. Shrrad, Dr. Kadhim A. Aadim, Dr. Issam, Dr. Sadeem,
Dr. Qahtan, Dr. Muhammad K., Mr. Muhammad U., Mr. Issam Q., Mr.
Muhammad J., Ms. Duaa A. and Ms. Hanaa J. for their assistance. This work
would not have been possible without their help and input.

I would also like to express my thanks to the deanery of the College of
Sciences and head of Physics Department for their support ship to the student of
higher education, the faculty is irreplaceable and their generosity to the student
body is incomparable.

Thank to Prof. Dr. Moohajiry (Tehran University) and his research
group to provide me an Opportunity to work in his respectable laboratory (SEM
Technician).

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To my fellow graduate students, thank you for the good times throughout
our years. Whether it was late nights studying or in the University, it was always
a good time. I wish everyone good luck in the future and hope our paths cross
again.

In addition, I would like to thank my friends from Al-Mustansiriya
University, especially the Assistant Lecturer Ms. Marwa A. Hassan. From
the times that “escalated quickly” to showing me the way to “victory lane,” it
seems like we've never missed a beat.

Finally I would like to thank all of the other friends that I developed over
the years. I am a lucky person to have the friendships that I have.

Haidar

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Supervisor Certification
We certify that this thesis titled “Fabrication of TiO2 Nanotube Using
Electrochemical Anodization” was prepared by Mr. (Haidar H. Hamdan),
under our supervision at Department of Physics, College of Science, University
of Baghdad, as a partial fulfillment of the requirements for the degree of Master
of Science in Physics.

Signature: Signature:

(Supervisor) (Supervisor)

Name: Dr. Harith I. Jaafar Name: Dr. Abdulkareem M. Ali

Title: Professor Title: Lecturer

Date: 5 / 3 / 2012 Date: 5 / 3 / 2012

In view of the available recommendation, I forward this thesis for debate
by the Examination Committee.

Signature:

Name: Dr. Raad M.S AL- Haddad

Title: Professor

Address: Head of Physics Department,

Collage of Science, University of Baghdad.

Date: 5 / 3 / 2012

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Examination Committee Certification
We certify that we have read this thesis entitled “Fabrication of TiO2
Nanotube Using Electrochemical Anodization” as an examine committee,
examined the student Mr. (Haidar Hameed Hamdan) in its contents and that,
in our opinion meets the standard of thesis for the degree of Master of Science in
Physics.

Name: Dr. Ikram A. Ajaj Name: Dr. Raad S. Sabry
Title: Assistant Professor Title: Assistant Professor
Address: University of Baghdad Address: Al-Mustansiriyah University
Date: 25 / 4 /2012 Date: 25 / 4 /2012
(Chairman) (Member)

Name: Dr. Inaam M. Abdulmajeed Name: Dr. Dr. Harith I. Jaafar
Title: Assistant Professor Title: Professor
Address: University of Baghdad Address: University of Baghdad
Date: 25 / 4 /2012 Date: 25 / 4 /2012
(Member) (Supervisor)

Signature:
Name: Dr. Abdulkareem M. Ali
Title: Lecturer
Address: University of Baghdad
Date: 25 / 4 /2012
(Supervisor)

Approved by the Council of the College of Science.
Signature:
Name: Dr. Saleh M. Ali
Title: Professor
Address: Dean of the Science College,
University of Baghdad
Date: 27 / 4 /2012

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Abstract
This thesis describes the synthesis of self-organized titanium dioxide
nanotube layers by an electrochemical anodization of Ti at different
conditions (time, voltage, concentration of NH4F in electrolyte with
glycerol, conductivity and water content) at room temperature (~25ºC)
were investigated.
In the current study, self-organized, vertically-oriented TiO2
nanotubes were successfully prepared by anodization method of a pure
Titanium sheet (99.5%) using anodization cell is designed for first time in
Iraq (Homemade) from Teflon material according to our knowledge to
produce self-ordered Titanium nanotube in organic based electrolytes
(glycerol based electrolyte) an electrolyte solution containing (0.5, 1, 1.5
and 2 wt.% NH4F) then added water (2 and 5Wt.% H2O) to (0.5wt.%
NH4F) only with 15V. The range of anodizing time and potential were
between 1-4hr. and 5-40V, where Wt.% represent weight percentage.
Scanning electron microscopy (SEM), Atomic force microscopy
(AFM) and (XRD) X-Ray diffraction were employed to characterize the
morphology and structure of the obtained Titania templates, optical
interferometer (Fizeau frings) method to tubes length measurement.
For TiO2 nanotubes fabricated in non-aqueous electrolyte, the
influence of the NH4F concentration on characteristics of nanotubes was
studied. The results showed that when the NH4F concentration increased
from 0.5 to 2wt.%NH4F, the tubes diameter, tubes length and roughness of
TiO2 surface increased.
Also the effects of the anodizing time and anodizing potential were
studied. The formation of TiO 2 nanotubes was very sensitive to the

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anodizing time. Length of the tube increases with
increasing anodizing time and anodizing potential significantly.
Either water content (2 and 5wt.%) with 0.5wt.% NH4F and the
conductivity of electrolyte it is increasing the diameter, tube length and
roughness of TiO2 surface increased, but simple increasing and formation
of less homogenized TiO2 nanotube.
The optimal conditions for TiO2 formation was found 15V at 4hr
with 0.5wt.% NH4F due to we obtain on best results for tube diameter, tube
wall thickness, tube length and more homogenized of TiO2 nanotubes.

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Contents
Title Page
Dedication 3
Acknowledgments 4
Supervisor Certification 6
Examination Committee Certification 7
Abstract 8
Contents 10
List of Figures 13
List of Tables 15
List of Abbreviation 16

Chapter One (Introduction and Literature Review)
Paragraph Title Page
1-1 Physical and Chemical Science and Nanotechnology 19
1-2 Nanomaterials 19
1-3 Types of nano materials 20
1-4 Literature Review 21
1-5 Aim of this Work 26

Chapter Two (Theoretical Part)
2-1 Introduction to Nanotechnology 28
2-2 Quantum Confinement in Semiconductors 30
2-2-1 Quantum Dot 30
2-2-2 Quantum Wire 30
2-2-3 Quantum Well 30
2-3 Summary of Quantum Confinement Effect 31
2-4 Micro to Nano Materials Perspective 32
2-5 Strategies of Making Nanostructures 33
2-6 Properties of Titanium Dioxide (TiO2) 34
2-6-1 Crystal Structure of Titanium Dioxide (TiO2) 35
2-6-1-1 Titanium Dioxide (TiO2) in Rutile Stable Phase 35
2-6-1-2 Titanium Dioxide (TiO2) in Anatase Metastable Phase 36
2-6-1-3 Titanium Dioxide (TiO2) in Brookite Structure 37
2-7 Synthesis Techniques of TiO2 nanotube 39
2-8 Electrochemical Anodization processes 39

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2-9 Electrochemical Anodization of Metals 40
2-10 Mechanism of TiO2 nanotubes array formation 42
2-11 Factors affecting the formation of (TiO2) nanotube 46
2-11-1 The effect of anodization potential 46
2-11-2 The effect of electrolyte 47
2-11-3 The effect of temperature 48
2-11-4 The effect of annealing before and after anodizing 49
2-11-5 The effect of distance between electrodes 49

Chapter Three (Experimental and Methods)
3-1 Introduction 52
3-2 Chemicals and Instrumentations 52
3-2-1 Chemicals 52
3-2-2 Instrumentations 53
3-2-3 Processes flow chart of template synthesis 54
3-3 Electrochemical Anodization system 55
3-3-1 Electrochemical Anodization Cell Design 55
3-4 Samples preparation 56
3-4-1 Pretreatment of Ti samples 56
3-4-2 TiO2 Nanotube preparation 57
3-5 Characterization measurements 59
3-5-1 X-Ray diffraction (XRD) pattern 59
3-5-2 Atomic Force Microscopy (AFM) 60
3-5-3 Scanning Electron Microscopy(SEM) 61
3-5-4 Thickness measurement 62

Chapter Four (Results and Discussions)
4-1 Introduction 65
4-2 (I-V) characteristics of the electrochemical 65
anodization process
4-2-1 Effect of NH4F concentration 65
4-2-2 Effect of anodizing potential 66
4-2-3 Effect of water content 70
4-2-4 Effects of conductivity 71
4-3 Characterization of Titania nanotubes 72
4-3-1 Structural and morphological characterization 73
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of Titanium nanotubes (TiO2) in (SEM and
AFM) measurement
4-3-1-1 Effect of NH4F concentration 73
4-3-1-2 Effect of anodization time 77
4-3-1-3 Effect of anodizing potential 80
4-3-1-4 Effect of water content 84
4-3-1-5 Effects of conductivity 89
4-3-2 Structural characterization of Titania in 89
(XRD) measurement
4-5-3 Results of thickness measurement 97

Chapter Five (Conclusions and Future Work)
5-1 Conclusions and Perspectives 100
5-2 Suggestions for Future Research 101
References 103

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List of Figures
Figure (2-1) Density of states as a function of energy for bulk material, 31
quantum well, quantum wire and quantum dot.
Figure (2-2) Schematic of nanostructure making approaches 34
Figure (2-3) Rutile structure for crystalline TiO2 36
Figure (2-4) Anatase metastable phase for crystalline TiO2 36
Figure (2-5) Brookite structure for crystalline TiO2 37
Figure (2-6) Schematic set-up of anodization experiment 44
Figure (2-7) Schematic diagram of the evolution of (TiO2) nanotubes in 45
anodization: (a) oxide layer formation; (b) pore formation
on the oxide layer; (c) climbs, formation between pores; (d)
growth of the pores and the climbs; (e) fully developed
(TiO2) nanotubes arrays
Figure (2-8) Schematic representation of processes in (TiO2) nanotube 45
formation during anodization: a) in absence of fluorides; b)
in presence of fluorides
Figure (3-1) Flow chart of Titanium nanotube synthesis 54
Figure (3-2) Schematic and photograph of set-up illustrates of the 55
anodization experiment with Teflon cell
Figure (3-3) Schematic diagram of homemade Teflon cell 56
Figure (3-4) Block diagram of atomic force microscope 61
Figure (3-5 a, b) Set-up and Photograph illustrates the SEM 62
Figure (3-6 a, b) Experimental arrangement for observing Fizeau fringes 63
Figure (4-1) The current transient recorded during anodization during 2 66
hours at 15V in the glycerol + 0.5Wt. %NH4 F and glycerol
+ 1.5Wt. %NH4F
hours at 15 and 40V in the glycerol + 0.5Wt. %NH4F
Figure (4-3) Optical images of TiO2 grown on a Ti metal substrate 69
during 2hr of anodization at 5V (a), 10V (b), 15V (c), 25V
(d) and at 40 V (e) in 0.5wt. % NH4F.
Figure (4-4) The current transients recorded during 2 hours of Ti 71
anodization at 15V in glycerol / water / 0.5wt. %NH4F
electrolytes with different weight ratios of glycerol: water
hours at 15V in the glycerol + 0.5Wt. %NH4 F at a different
conductivity of electrolyte
Figure (4-6) SEM image of Ti anodized in (0.5 wt. % NH4F) in glycerol 74
electrolyte at 15V for 2 h
Figure (4-7) SEM image of Ti anodized in (1.5 wt. % NH4F) in glycerol 74
electrolyte at 15V for 2 hr.
Figure (4-8) AFM images of Ti anodized in (0.5 wt. % NH4 F + 75
glycerol) electrolyte at 15V for 2 h, (a) The 2D, cross
section, (b) 3D and (c) porosity normal distribution chart
Figure (4-9) AFM images of Ti anodized in (1.5 wt. % NH4 F + 76
glycerol) electrolyte at 15V for 2 hr. , (a) The 2D, cross
section, (b) 3D and (c) porosity normal distribution chart
Figure (4-10) SEM image of Ti anodized in (0.5 Wt. % NH4F + 99.5 Wt. 78
% glycerol) electrolyte at 15V for 2hr

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Figure (4-12) AFM images of Ti anodized in (0.5 Wt. % NH4F + 99.5 79
Wt. % glycerol) electrolyte at 15V for 4hr. , (a) The 2D,
cross section, (b) 3D and (c) porosity normal distribution
chart
Figure (4-15) AFM images of Ti anodized in in (0.5 Wt. % NH4 F + 99.5 82
chart
Figure (4-16) AFM images of Ti anodized in (0.5 Wt. % NH4F + 99.5 83
chart
% glycerol) electrolyte at 15 V for 2h
Figure (4-18) SEM images of Ti anodized in (0.5 Wt. % NH4F + 2 Wt. % 86
H2O + 97.5 Wt. % glycerol) electrolyte at 15 V for 2h
Figure (4-19) SEM images: (a) top-views and (b) cross-sectional images 86
of Ti anodized in (0.5 Wt. % NH4F + 5 Wt. % H2O + 94.5
Wt. % glycerol) electrolyte at 15 V for 2h
Figure (4-20) AFM images of Ti anodized in (0.5 Wt. % NH4F + 2 Wt. 87
% H2O + 97.5 Wt. % glycerol) electrolyte at 15 V for 2h,
(a) The 2D, cross section, (b) 3D and (c) porosity normal
distribution chart
Figure (4-21) AFM images of Ti anodized in (0.5% NH4 F + 5% H2O + 88
94.5% glycerol) electrolyte at 15 V for 2h, (a) The 2D,
cross section, (b) 3D and (c) porosity y normal distribution
chart
Figure (4-22) XRD pattern of Titania before and after annealing at 91
temperatures 450°C and 3hr on Ti foil substrate
Figure (4-23) XRD pattern of Titania before and after annealing at 94

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List of Tables
Table (2-1) Physical and chemical properties of the three TiO2 38
structures
Table (3-1) The chemicals and materials which used in process 52
Table (3-2) Origin, function and specification devices 53
Table (3-3) The condition of experimental work without water 58
added
Table (3-4) The condition of experimental work with water 59
added
Table (4-1) Color as a variable of anodizing TiO2 thickness 69
Table (4-2) The average Roughness and Pores diameter of TiO2 77
nanotubes under different proportion of NH4F
Table (4-3) The average Roughness and Pores diameter of TiO2 79
nanotubes under different anodization time
Table (4-4) The average Roughness and pores diameter of TiO2 84
nanotubes under different anodization voltage
Table (4-5) The results of TiO2 nanotubes under different 85
proportion of glycerol and water content
Table (4-6) The average Roughness and pores diameter of TiO2 89
nanotubes under different proportion of glycerol and
water content
Table (4-7) XRD results for Titania before annealing 92
Table (4-8) XRD results for Titania after annealing at 93
Table (4-9) XRD results for Titania before annealing 95
Table (4-10) XRD results for Titania after annealing at 96
Table (4-11) Result Titania thickness measurement by optical 97
interferometer method

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List of Abbreviations
Symbols Meaning
0D Zero-dimensional
1D One-dimensional
2D Two-dimensional
3D Three-dimensional
AAO Anodic Aluminum Oxide
ATO Anodic Titanium Oxide
Ti Titanium
TiO2 Titania
Pt Platinum
NH4F Ammonium Fluoride
Al Aluminum
Al2O3 Alumina
Si Silicon
Hf Hafnium
Zr Zirconium
Ta Tantalum
Nb Niobium
N2 Nitrogen Gas
ZrO2 Zirconia
H2SO4 Sulfuric Acid
NaF Sodium Fluoride
Ta2O5 Tantalum Pentoxide
Na2SO4 Sodium Sulfate
HF Hydrofluoric Acid
H3PO4 Phosphoric Acid
H2O Water
F Fluoride
ZnO Zinc Oxide
NaOH Sodium Hydroxide
DNA Deoxyribonucleic Acid
PH Acidity number
DI Deionized Water
SEM Scanning Electron Microscopy
AFM Atomic Force Microscope
XRD X-ray Diffraction
ASTM American Society of Testing Materials
t Thickness of Film
X Fringes Spacing
ΔX Displacement
λ Wavelength
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UV Ultraviolet rays
UV–vis Ultraviolet–visible Spectroscopy
PVD Physical Vapor Deposition
CVC Chemical Vapor Condensation
CVD Chemical Vapor Deposition
M Molarity
RF Roughness
wt.% Weight Percentage
d The Spacing Between Atomic Planes
n Refractive Index
a Lattice Constant
2Ɵ Bragg Diffraction Angle
OCP Open-circuit Potential
SWNT Single Wall Nanotube
MWNT Multi Wall Nanotube

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Chapter One
Introduction and Literature Review

1-1- Physical and Chemical Science and Nanotechnology
Over the last ten years, the physics, chemistry and engineering scientists
interested in formation of self-organized nanostructures and nanopatterns which
attracted a great scientific and technological interest due to its far-reaching and
innumerable applications. Apart from these facts, the popularity and significance
of these self- arranged nanostructures stem from the nature of their fabrication
that relies on self- regulation processes (often called self-assembly). The main
advantage of these processes is that it can represented a ``smart´´ nano-
technique. Therefore, it is not surprising that a large part of material's science
nowadays targets these nano-scale fabrication techniques. Nanotechniques are a
natural consequence of the necessity of achieving smaller and smaller electronic
and photo-devices that satisfy the actual requests of the technological evolution.
Within materials science, a highly promising approach to form self-
organized nanostructured porous oxides is essentially based on a very simple
process – electrochemical anodic polarization. Some important findings in this
particular field include the growth of ordered Titanium dioxide (TiO2),
[1]
nanoporous Aluminum oxide (Al2O3, Alumina) and ordered macroporous
[2]
Silicon . Synthesis of all these materials has stimulated considerable research
efforts and given rise to many other materials to be processed in a similar
fashion.
1-2- Nanomaterials
Nanomaterials: A materials with dimensions below 100nm and they have
at least one unique properties that is different than the bulk material and the
characteristics can be applied in different fields such as nanoelectronics,
pharmaceutical and cosmetic. Several methods have been studied in fabricating

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these nanostructures, which include laser ablation [3], chemical vapor deposition
(CVD) [3] and template-directed growth [4]. In order to integrate one dimensional
nanomaterial into a device, a fabrication method that enables well-ordered
nanomaterials with uniform diameter and length is important. Template-directed
growth is a nanomaterials fabrication method that uses a template which has
[5]
nanopores with uniform diameter and length . Using chemical solutions or
electro deposition, nanomaterials are filled into the nanopores of the templates
and, by etching the template, nanowires or nanotubes with similar diameter and
length as the template nanopores are obtained. Because the size and shape of the
nanomaterial depends on the nanoholes of the template, fabricating a template
with uniform pore diameters is very important.

TiO2 nanotube is particularly interested with its high potential for use in
various applications, e.g., being used as gas-sensor [6], self-cleaning materials [7],
and photoanode in dye-sensitized solar cells [8].

1-3-Types of nano materials
Nanomaterials can be classified by different approaches such as;
according to the X, Y and Z dimension, according to their shape and according
their composition.
The more classification using is the order of dimension into 0D (quantum
dot), 1D (nanotube, nanowire and nanorod), 2D (nanofilm), and 3D dimensions
such as bulk material composited by nanoparticles [9].
Nanotubes are made, sometimes, from inorganic materials such as oxides
of metals (Titanium oxide, Aluminum oxide), are similar in terms of his
structure to the carbon nanotubes, but the heaviest of them, not the same strong
as carbon nanotube. Titanium nanotube can be described as a particles of Titania
is requested about an axis, to take a cylindrical shape where both ends of the
atoms associated with each slide to close the tube.

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Be one of the ends of the tube is often open and one closed in the form of
a hemisphere, as might be the wall of the tube individual atoms and is called in
this case the nanotubes and single-wall (single wall nanotube) SWNT, or two or
more named multi-wall tubes (multi wall nanotube) MWNT The tube diameter
ranges from less than one nm to 100 nm (smaller than the width head of hair by
50,000 times), and has a length of up to 100 micrometers to form the nanowire.
Of several forms of nanotubes may be straight, spiral, zigzag, or conical bamboo
and so on. The properties of these tubes are unusual in terms of strength and
hardness and electrical conductivity, and others [10].
Titania nanotube is 1D type nanomaterails that is means existing only one
micro or macro dimension which represented by the length of the tube.
1-4- Literature Review

Since its commercial production in the early twentieth century, Titanium
dioxide (TiO2) has been widely used as a pigment [11] and in sunscreens paints
[12]
, ointments [13], toothpaste [14], etc. In 1972, Fujishima and Honda discovered
the phenomenon of photocatalytic splitting of water on a TiO2 electrode under
ultraviolet (UV) light [15]. Since then, enormous efforts have been devoted to the
research of TiO2 material, which has led to many promising applications in areas
ranging from photovoltaics and photocatalysis to photo-electrochromics and
[16]
sensors . These applications can be roughly divided into “energy” and
“environmental” categories such as water purification, pollution prevention,
antibacterial, and purify the air. Many of which depend not only on the
properties of the TiO2 material itself but also on the modifications of the TiO 2
material host (e.g., with inorganic and organic dyes) and on the interactions of
TiO2 materials with the environment.

An exponential growth of research activities has been seen in nanoscience
[17]
and nanotechnology in the past decades . New physical and chemical
properties emerge when the size of the material becomes smaller and smaller,

21


and down to many serious environmental and pollution challenges. TiO 2 also
bears tremendous hope in helping ease the energy crisis through effective
[18]
utilization of solar energy based on photovoltaic and water-splitting devices .
As continued breakthroughs have been made in the preparation, modification,
and applications of TiO2 nanomaterials in recent years, especially after a series
of great reviews of the subject in the 1990s. We believe that a new and
comprehensive review of TiO2 nanomaterials would further promote TiO2-based
research and development efforts to tackle the environmental and energy
challenges that we are currently face it. Here, we focus on recent progress in the
synthesis, properties, modifications, and applications of TiO 2 nanomaterials [19].
[20]
In 1991, Zwilling et al. first reported the porous surface of TiO2 films
electrochemically formed in fluorinated electrolyte by Titanium anodization. In
1999 it was reported that porous TiO2 nanostructures could be fabricated by
electrochemically anodizing a Ti sheet in an acid electrolyte containing a small
amount of hydrofluoric acid (HF) [21]. Since then, many research groups have
paid considerable attention to this field, because anodization opens up ways to
easily produce closely packed tube arrays with a self-organized vertical
alignment.
[22]
A decade later Gong and co-workers synthesized the uniform and
highly-ordered Titanium nanotube arrays by anodization of a pure Titanium
sheet in a hydrofluoric acid (HF) aqueous electrolyte. They obtained nanotubes
directly grew on the Ti substrate and oriented in the same direction
perpendicular to the surface of the electrode, forming a highly ordered nanotube-
array surface architecture.

In 2001 Dawei Gong et al. [23] fabricated Titanium dioxide nanotubes by
anodization of a pure Titanium sheet in an aqueous solution containing 0.5 to
3.5 wt. % hydrofluoric acid. These tubes are well aligned and organized into
high-density uniform arrays. While the tops of the tubes are open, the bottoms of
22


the tubes are closed, forming a barrier layer structure similar to that of porous
Alumina. The average tube diameter, ranging in size from 25 to 65 nm, was
found to increase with increasing anodizing voltage, while the length of the tube
was found independent of anodization time.
Later in 2003 Oomman K. Varghese et al. [24] used anodization with a
time-dependent linearly varying anodization voltage and made films of tapered,
conical-shaped Titania nanotubes. The tapered, conical-shaped nanotubes were
obtained by anodizing Titanium foil in a 0.5% hydrofluoric acid electrolyte,
with the anodization voltage linearly increased from 10–23 V at rates varying
from 0.43- 2.0 V/min. The linearly increasing anodization voltage results in a
linearly increasing nanotube diameter, with the outcome being an array of
conical-shaped nanotubes approximately 500 nm in length. Evidence provided
by scanning electron-microscope images of the Titanium substrate during the
initial stages of the anodization process enabled them to propose a mechanism of
nanotube formation.
In 2005 Seung-Han Oh et al. [25] a vertically aligned nanotube array of
Titanium oxide fabricated on the surface of titanium substrate by anodization.
The nanotubes were then treated with NaOH solution to make them bioactive,
and to induce growth of hydroxyapatite (bone-like calcium phosphate) in a
simulated body fluid. Such TiO2 nanotube arrays and associated nanostructures
can be useful as a well-adhered bioactive surface layer on (Ti) implant metals
for orthopaedic and dental implants, as well as for photocatalysts and other
sensor applications.
In 2006 Aroutiounian et al. [26] the semiconductor photoanodes made of
thin film Titanium oxide were prepared by anodization of Titanium plates in
hydrofluoric acid solution at direct voltage at room temperature. The influence
of the change of Titanium oxide film growth conditions (concentration of
hydrofluoric acid, voltage, duration of anodization process) and subsequent heat

23


treatment of films on a photocurrent and current-voltage characteristics of
photoelectrodes were investigated.
[27]
In 2007 V. Vega et al. synthesized Self-aligned nanoporous TiO2
templates synthesized via dc current electrochemical anodization have been
carefully analyzed. The influence of environmental temperature during the
anodization, ranging from 2ºC to ambient, on the structure and morphology of
the nanoporous oxide formation, has been investigated, as well as that of the
(HF) electrolyte chemical composition, its concentration and their mixtures with
other acids employed for the anodization. Arrays of self-assembled Titania
nanopores with inner pores diameter ranging between 50 and 100 nm, wall
thickness around 20–60 nm and 300 nm in length, are grown in amorphous
phase, vertical to the Ti substrate, parallel aligned to each other and uniformly
disordering distributed over all the sample surface.
In 2008 Hua-Yan Si et al. [28] studied the effects of anodic voltages on the
morphology, wettability and photocurrent response of the porous Titanium
dioxide films prepared by electrochemical oxidation in a hydrofluoric acid
(HF)/chromic acid electrolyte have been studied. The porous Titanium dioxide
films showed an increased surface roughness with the increasing anodizing
voltages. By controlling the films morphology and surface chemical
composition, the wettability of the porous Titanium dioxide films could be
easily adjusted between superhydrophilicity and superhydrophobicity. X-ray
diffraction (XRD), Raman and UV–vis spectroscopy revealed that the obtained
Titanium dioxide films were in anatase phase. The Titanium dioxide films
showed clear photocurrent response, which decreased dramatically with the
increase of the anodizing voltages. This study demonstrates a straightforward
strategy for preparing porous Titanium dioxide films with tunable properties,
and especially emphasizes the importance of understanding their
morphology/properties relationship.

24


[29]
In 2009 Michael et al. anodized Titanium-oxide containing highly
ordered, vertically oriented TiO2 nanotube arrays is a nanomaterial architecture
that shows promise for diverse applications. An anodization synthesis using HF-
free aqueous electrolyte solution contains 1 wt.% (NH 4)2SO4 plus 0.5 wt.%
NH4F. The anodized TiO2 film samples (amorphous, anatase, and rutile) on
Titanium foils were characterized with scanning electron microscopy and X-ray
diffraction. Additional characterization in terms of photocurrent generated by an
anode consisting of a Titanium foil coated by TiO 2 nanotubes was performed
using an electrochemical cell. A Platinum cathode was used in the
electrochemical cell.
In 2010 Hun Park et al. [30] studied the properties of TiO2 nanotube arrays
which are fabricated by anodization of (Ti) metal. Highly ordered TiO2 nanotube
arrays could be obtained by anodization of (Ti foil in 0.3 wt.% NH 4F contained
ethylene glycol solution at 30°C. The length, pore size, wall thickness, tube
diameter etc. of TiO2 nanotube arrays were analyzed by field emission scanning
electron microscopy. Their crystal properties were studied by field emission
transmission electron microscopy and X-ray photoelectron spectroscopy.
In 2011 S. Sreekantan et al. [31] formed Titanium oxide (TiO2) nanotubes
by anodization of pure Titanium foil in a standard two-electrode bath consisting
of ethylene glycol solution containing 5 wt.% NH 4F. The PH of the solution was
∼ 7 and the anodization voltage was 60 V. It was observed that such anodization
condition results in ordered arrays of TiO2 nanotubes with smooth surface and a
very high aspect ratio. It was observed that a minimum of 1 wt. % water
addition was required to form well-ordered TiO2 nanotubes with length of
approximately 18.5 μm. As-anodized sample, the self-organized TiO2 nanotubes
have amorphous structure and annealing at 500oC of the nanotubes promote
formation of anatase and rutile phase.

25


1-5- Aim of this Work
Fabrication of forest of Titania nanotubes via electrochemical anodizing
of pure Titanium foil using electrochemical Teflon cell designed for first time in
Iraq according to our knowledge to produce self-ordered Titanium nanotubes.
Investigate the effects of some process parameters such as; time, voltage and
electrolyte composition on the diameter and length of fabricated nanotubes by
nanoscopic instrument atomic force microscopy (AFM), scanning electron
microscopy (SEM), (XRD) spectroscopy and optical interferometer method.

26


27


Chapter Two
Theoretical Part

2-1- Introduction to Nanotechnology
Nanotechnology or nanoscale science is concerned with the investigation
of matter at the nanoscale, generally taken as the 1 to 100 nm range. The
breakthrough in both academic and industrial interest in these nanoscale
materials over the past ten years has been interested because of the remarkable
[32]
variations in solid-state properties . The “nano” as word means dwarf (small
man) in Greek, nano as SI unit refers amount of 10-9, such as nanometer,
nanolitter and nanogram [33].

As such a nanometer is 10-9 meter and it is 10,000 times smaller than the
diameter of a human hair. A human hair diameter is about 50000 nm (i.e., 50×
10-9 meter) in size, meaning that a 50 nanometer object is about 1/1000th of the
thickness of a hair [33].

Nanoparticels are considered to be the building blocks for nanotechnology
and referred to particles with at least one dimension less than 100nm. Particles
in these size ranges have been used by several industries and humankind for
thousands of years [34].

The nanotechnology deals with the production and application of
physical, chemical, and biological system at scales ranging from individual
atoms or molecules to submicron dimension, as well as the integration of the
[35]
resulting nanostructures into larger system .

Nanometer–scale features are mainly built up from their elemental
constituents. Examples in chemical synthesis, the spontaneous self –assembly of
molecular clusters (molecular self- assembly) from simple reagents in solution.
The biological molecules (e.g., DNA) are used as building blocks for the

28


production of zero- dimensional nanostructure, and the quantum dots
(nanocrystals) of arbitrary diameter (about 10 to 105 atoms). When the
dimension of a material is reduced from a large size, the properties remain the
same first, and then small changes occur, until finally, when the size drops
below 100nm, dramatic changes in properties occur [35].

At the nanoscale, objects behave quite differently from its behave at larger
scales, such as increased hardness values of metallic materials and their alloys as
well as increase the strength to face the stresses of different loads, located it,
either the ceramic material increases the durability and tolerance to stresses
impact. As for the electrical properties have a great ability to connect and
increase the diffusion and interactions in nano-seconds and the speed of ion
transport [36].

Nanotechnology manipulates matter for the deliberate fabrication of nano-
sized materials. These are therefore “intentionally made” through a defined
fabrication process. The definition of nanotechnology does not generally include
“non-intentionally made nanomaterials”, that is, nano-sized particles or
materials that belong naturally to the environment (e.g., proteins, viruses) or that
are produced by human activity [36].

A nanomaterial is an object that has at least one dimension in the
[36]
nanometre scale . Nanomaterials are categorized according to their
dimensions into three classes [37]:

1. Zero-dimension confinement (quantum dot).
2. One-dimension confinement (quantum wire).
3. Two-dimensions confinement (quantum well).
4. Three -dimensions confinement (bulk).

29


2-2- Quantum Confinement in Semiconductors
In the last few years a great effort has been devoted to the study of low
dimensional semiconductor structures. The reduction of the dimensionality
causes several changes in the electronic and excitonic wave functions and these
features can be used, at least in principle, to produce novel microelectronics [38].
In bulk semiconductor materials, the energy levels of both conduction
band and valence band are continuous, with electrons and holes moving freely in
all directions. As the dimensions of the material shrink, effect of quantum
confinement will be seen, this effect is seen in the objects, when size of object is
less than de Broglie wavelength of electrons. Here, classical picture of electrons
trapped within hard wall boundaries is not unrealistics. Three different types of
confinement that have been realized among semiconductors materials are
described below [39].
2-2-1- Quantum Dot
Typically, the dimension is ranging from 1 to 100 nanometers. A quantum
dot has the most restricted confinement in all three dimensions of the electrons
and holes. It is working under the condition (λF >>Lx, Ly, Lz), where λF represent
the Fermi wavelength [40]. As shown in figure (2-1). An important property of a
quantum dot is the large surface to volume ratio [39].
2-2-2- Quantum Wire
A quantum wire is a structure in which the electrons and holes are
confined in two dimensions, as shown in figure (2-1) such confinement allows
free electrons and holes behavior in only one direction, along the length of the
wire [39]. These properties give rise to produce many nanoproductions which can
be considered as a quantum wire (λF> Lx, Ly and Lx, Ly<<Lz), carbon nanotubes
for connection is example of this [40].
2-2-3-Quantum Well
A quantum well is a potential well that confines particles, which were
originally free to move in three dimensions, in two dimensions, forcing them to

31


occupy a planar region. Their motions are confined in the direction
perpendicular to the free plane. The effects of quantum confinement take place
when the quantum well thickness becomes comparable at the de Broglie
wavelength of the carriers (generally electrons and holes), leading to energy
levels called “energy subbands”, i.e., the carriers can only have discrete energy
values [41], as in the figure (2-1).
In quantum well the electron are free in Z, Y directions, whereas it is
confined in the X direction. When λF>Lx, and Lx <<Ly, Lz [40].

Figure (2-1): Density of states as a function of energy for bulk material,
quantum well, quantum wire and quantum dot [41].

2-3- Summary of Quantum Confinement Effect
Quantum confinement introduces a number of important modifications in
the physical properties of semiconductor.
The density of states g(E) is defined by the number of energy states
between energy E and E+dE per unit energy range, which is defined by
dn(E)/dE. For electrons in a bulk semiconductor, g(E) is zero at the bottom of
the conduction band and increases with E1/2 as the energy of the electrons in the
conduction band increases. This behavior is shown in figure (2-1), which
31


compares the density of states for electron in a quantum well (and also in
quantum wire and dot), where the density of states is a step function because of
the discreteness of the energy levels along the confinement direction [39].
The density of state for a quantum wire has an inverse energy dependence
E-1/2 for each sub-band, the density of state has a large value near Kz =0 and
decays as E-1/2 as Kz has nanozero value for that sub-band. The energy levels for
an electron in a quantum dot have only discrete values, which makes the density
of states a series of delta functions at each of the allowed energy value, i.e. g(E)
= δ(E-En) (n=1, 2, …).

Quantum confinement also induces a blue shift in the band gap and
appearance of discrete sub-bands corresponding to energy quantization along the
direction of confinement. As the dimensions of the material increase, the energy
of the confined states decreases so the inter-band transitions shift to longer
wavelengths. When the dimensions of the material are greater than de Broglie
wavelength, the inter-band transition energy finally approaches the bulk value
[39]
.
2-4- Micro to Nano Materials Perspective
A number of physical phenomena became pronounce as the size of the
system decreased. These included statistical mechanical effects, as well as
quantum mechanical effects, for example the “quantum size effect”, where the
electronic properties of solids are altered with great reductions in particle size.
This effect does not come into play by going from macro to micro dimensions.
However, quantum effects become dominant when the nanometer size range is
reached, typically at distances of 100 nanometers or less, the so called quantum
realm. Additionally, a number of physical (mechanical, electrical, optical, etc.)
properties change when compared to macroscopic systems. One example is the
increase in surface area to volume ratio altering mechanical, thermal and
catalytic properties of materials. Diffusions and reactions at nanoscale,

32


nanostructures materials and nanodevices with fast ion transport are generally
referred to nanoionics. Mechanical properties of nanosystems are of interest in
the nanomechanics research. The catalytic activity of nanomaterials also open
potential risks in their interaction with biomaterials [42].
Materials reduced to the nanoscale can show different properties
compared to what they exhibit on a macroscale, enabling unique applications.
For instance, opaque substances become transparent (Copper); stable materials
turn combustible (Aluminum); insoluble materials become soluble (Gold). A
material such as Gold, which is chemically inert at normal scales, can serve as a
potent chemical catalyst at nanoscales. Much of the fascination with
nanotechnology stems from these quantum and surface phenomena that matter
exhibits at the nanoscale [42].
2-5- Strategies of Making Nanostructures
There are two strategies to make nanostructures. Top-down approach and
bottom-up approach. The first strategy is by start from a large chunk of material
and by cut it and trim it till getting nanosized architecture as shown in figure (2-
2) [43]. It includes methods such as electrochemical dip-pen nanolithography and
vapor deposition. Electrochemical dip-pen lithography utilizes an Atomic Force
[43]
Microscope (AFM) to transfer material from the AFM tip to a surface . This
method is able to create nanowires down to 1nm but it is quite slow (Ophir,
2004) [44].
The second strategy is a bottom-up procedure where are start from the
smallest components to assemble the desired structure from the ground up,
which represented by direct chemical synthesis. Usually bottom-up is associated
with chemistry and synthesis, while Top-Down is associated with physical
processing techniques [43].

33


Figure (2-2) Schematic of nanostructure making approaches [43].

2-6- Properties of Titanium Dioxide (TiO2)
Titanium dioxide, also known as Titanium (IV) oxide or Titania [45], is the
naturally occurring oxide of Titanium, chemical formula TiO2. When used as a
pigment, it is called Titanium white, Pigment White 6, or CI 77891. It is
noteworthy for its wide range of applications, from paint to sunscreen to food
colouring when it is given the E number E171. Titanium dioxide occurs in
nature as the well-known naturally occurring minerals rutile, anatase and
brookite. Additionally two high pressure forms, the monoclinic baddeleyite
form and the orthorhombic form have been found at the Ries crater in Bavaria.
The most common form is rutile, which is also the most stable form anatase
and brookite both can be converted to rutile upon heating. Rutile, anatase and
brookite all contain six coordinates Titanium. Titanium dioxide is the most
widely used white pigment, because of its brightness and very high refractive
index (n=2.7), in which it is surpassed only by a few other materials.
Approximately 4 million tons of pigmentary TiO2 are consumed annually
worldwide [45]. When deposited as a thin film, its refractive index and colour
make it an excellent reflective optical coating for dielectric mirrors and some
gemstones, for example “mystic fire topaz”. TiO2 is also an effective pacifier
in powder form, where it is employed as a pigment to provide whiteness and

34


opacity to products such as paints, coatings, plastics, papers, inks, foods,
medicines (i.e. pills and tablets) as well as most toothpastes. Opacity is
improved by optimal sizing of the Titanium dioxide particle [45].

2-6-1-Crystal Structure of Titanium Dioxide (TiO2)

TiO2 is extensively used in gas sensing because of its desirable sensitivity
and mainly because of its good stability in adverse environments. Titanium
(IV) Oxide (II) has one stable phase, rutile (tetragonal) and two metastable
polymorph phases, brookite (orthorhombic) and anatase (tetragonal). Both
metastable phases become rutile (stable) when submitting the material at
temperatures above 700 °C (in pure state, when no additives have been added)
[46]
. A brief sum up of crystal and structural properties of rutile, anatase and
brookite phases can be presented in the following sections.

2-6-1-1-Titanium Dioxide (TiO2) in Rutile Stable Phase

TiO2 owing to its chemical and mechanical stabilities, Titanium dioxide
(TiO2), which were a wide energy gap n-type semiconductor, has been used to
develop gas sensors based in thick film polycrystalline material or small
particles. Titanium dioxide (IV) has stable phase rutile (material structure), for
the schematic rutile structure. Its unit cell contains (Ti) atoms occupy the
center of a surrounding core composed of six Oxygen atoms placed
approximately at the corners of a quasi-regular octahedron as shown in the
[47]
figure (2-3) . The lattice parameters correspond now to a = b = 4.5933 A °
and c = 2.9592 A° with c/a ratio of 0.6442 [48].

35


Figure (2-3) Rutile structure for crystalline TiO2 [47].

2-6-1-2-Titanium Dioxide (TiO2) in Anatase Metastable Phase

The anatase polymorph of TiO2 is one of its two metastable phases
together with brookite phase. For calcination processes above 700 °C all
anatase structure becomes rutile. Some authors also found that 500 °C would
be enough for phase transition from anatase to rutile when thermal treatment
takes place. Anatase structure is tetragonal, with two TiO2 formula units (six
atoms) per primitive cell. Lattice parameters are: a = b = 3.7710 A° and c =
9.430 A° with c/a ratio of 2.5134[48], as shown in figure (2-4) [43].

Figure (2-4) Anatase metastable phase for crystalline TiO2 [47].

36


2-6-1-3- Titanium Dioxide (TiO2) in Brookite Structure

The brookite structure is more complicated and has a larger cell volume
than the other two. It is also the least dense of the three forms in (g/cm 3). The
unit cell is composed of eight formula units of TiO 2 and is formed by edge
sharing TiO2 octahedra, similar to rutile and anatase, as shown in the figure (2-
[47]
5) . Brookite belongs to the Orthorhombic crystal system its space group is
Pbca. By definition, the brookite structure is of lower symmetry than its TiO 2
countermorphs, the dimensions of the unit cell are unequal. Also the Ti-O
bond lengths vary more so than in the rutile or anatase phases, as do the O-Ti-
O bond angles. Table (2-1) shows the physical and chemical properties of the
three TiO2 structures [45].

Figure (2-5) Brookite structure for crystalline TiO2 [47].

37


Table (2-1) Physical and chemical properties of the three TiO2 structures [45]

Properties Rutile Anatase Brookite
Molecular formula TiO2 === ===
Molar mass g/mol 79.866
Crystal System Tetragonal Tetragonal Orthorhombic
Energy gap eV 3.06 3.29
Color White solid === ===
3
Density g/cm 4.27 3.90 4.13
Transformed Transformed
Melting point °C 1855
into rutile into rutile
Boiling point 2972
Refractive index (nD) 2.609 2.488 2.583
Dielectric constant ε 110~117 48 78
Hardness (Mohs scale) 7.0~7.5 5.5~5.6 ===
Anatase, Rutile and Brookite have been studied for their photocatalytic,
photo electrochemical and gas sensors applications. The difference in these
three crystal structures can be attributed to various pressures and heats applied
from rock formations in the Earth. At lower temperatures the anatase and
brookite phases are more stable, but both will revert to the rutile phase when
subjected to high temperatures (700°C for the anatase phase and 750°C for the
brookite phase). Although rutile is the most abundant of the three phases,
many quarries and mines containing only the anatase or brookite form exist.
Brookite was first discovered in 1849 in Magnet Cove, a site of large deposits
of the mineral. It was originally dubbed „arkansite‟ for the state it was
[49]
discovered in Arkansas . The optical properties for each phase are also
similar, but they have some slight difference. The absorption band gap for the
rutile, anatase, and brookite phases were calculated as shown in Table (2-1). In
addition to the slight increase in the band gap, the anatase form also has a
slightly higher Fermi level (0.1eV). In thin films it has been reported that the
anatase structure has higher mobility for charge carriers versus the rutile
[46]
structure . For photocatalytic processes, anatase is the preferred structure,

38


although all three forms have shown to be photocatalytic. The electronic
structure of brookite is similar to anatase, based on minor differences in the
local crystal environment [50].

2-7- Synthesis Techniques of TiO2 nanotube
[51]
Since the discovery of Carbon nanotubes in 1991 a continuously
increasing research interest in one dimensional (1D) nanomaterials has been
established. After ten years, not only Carbon materials are widely studied, but
also a variety of metals and oxides, such as TiO 2 [52], ZnO [53], etc. The inorganic
nanotubes, in particular the TiO2 ones, are of a great potential for various
technological applications, due to their high surface to volume ratio, enhanced
electronic properties (in comparison with nanoparticles), well-defined structures
and the possibility to precisely tailor their dimensions on the nanoscale.
In the case of TiO2, several studies indicated that nanotubes have
[54] [55]
improved performance in photocatalysis and photovoltaics compared to
colloidal or nanoparticulate forms of TiO2. Up to now, suspensions, bundles and
arrays of rather disordered TiO2 nanotubes have been produced by a variety of
different methods including sol-gel, electrodeposition, sonochemical deposition,
hydrothermal and solvothermal, template, chemical vapor deposition (CVD),
physical vapor deposition (PVD), Chemical Vapor Condensation (CVC) and
freeze-drying .etc. [56].
2-8- Electrochemical Anodization Processes
The electrolytic passivation process used to increase the thickness of the
natural oxide layer on the surface of metal parts. The process is called
“anodizing” because the part to be treated forms the anode electrode of an
electrical circuit. Anodizing increases corrosion resistance and wears resistance,
and provides better adhesion for paint primers and glues than bare metal [53].

39


Anodization changes the microscopic texture of the surface and changes the
crystal structure of the metal near the surface. Thick coatings are normally
porous, so a sealing process is often needed to achieve corrosion resistance [57].

All metals, except gold, are unstable at room temperature in contact with
Oxygen at atmospheric partial pressure, and thermodynamically should tend to
form an oxide. In water many metals, such as Aluminum, Titanium and
Tantalum, displaced Hydrogen with the production of an oxide or a salt. These
reactions often fail to occur at any appreciable rate. The usual reason for the lack
of reaction is that a thin but complete film of insoluble or slowly soluble oxide is
formed. This separates the reactants and further reaction can only occur by
diffusion or migration (field-assisted movement) of metal or Oxygen ions
through the native oxide film. These processes are usually slow. Such transport
does occur, thickens the film and therefore reduces the rate of reaction because
of a decreased concentration gradient or electrostatic field [57].

Usually, an oxide coated metal is made on the anode of an electrolytic cell
(with a solution that does not dissolve the oxide), the applied current sets up an
electrostatic field in the oxide (or increases the field already present) and
produces continued growth of the oxide film by causing metal or Oxygen ions to
be pulled through the film. Due to this reason this kind of films are called anodic
films.

2-9-Electrochemical Anodization of Metals
The electrochemical formation of self-organized nanoporous structures
produced by the anodization of some metals have been reported .These a group
of materials rather than Aluminum and Titanium [58], have been tried to produce
porous oxide templates. Porous anodic oxide films have also been achieved on
surfaces of many other metals, e.g. Hafnium [59], Niobium [60], Tantalum, InP [61]
,
Tungsten [62], Vanadium, Zirconium [7] and Silicon [63].

41


Hafnium oxide has many interested properties, e.g. its high chemical and
thermal stabilities, high refractive index and relatively high dielectric constant.
These properties make Hafnium oxide a valuable material to be used as a
protective coating, optical coating, gas sensor or capacitor. Self-organized
porous Hafnium oxide layers were obtained successfully for the first time by
[5]
Tsuchiya, et al. via anodization of Hafnium at about 50 V in 1 wt.% H2SO4 +
0.2 wt.% NaF at room temperature. Anodization potential was found to be a key
factor affecting the morphology and the structure of the porous oxide [59].
Self-organized porous anodic Niobium oxide films were successfully
prepared in 1 wt.% H2SO4 + 1 wt. % HF or 1.5 wt.% HF respectively [59].
Ta2O5 has attracted intensive attention due to its application in optical
devices. Anodization of Tantalum has been widely investigated in sulfuric,
phosphoric acid, and Na2SO4 solutions and a layer of amorphous Ta2O5 with a
uniform thickness could be obtained [59]. Self-organized porous anodic Tantalum
oxide with a reasonably narrow size distribution was fabricated via anodizing
Tantalum in 1 wt. % H2SO4 + 2 wt. % HF for 2 h.
Zirconium oxide is an important functional material that plays a key role
[7]
as an industrial catalyst and catalyst support . It was reported that a compact
anodic Zirconium oxide layer of up to several hundred nanometers in thickness
can be achieved in many electrolytes. A unique feature in comparison with other
anodic metal oxides mentioned above is that the growth of the compact ZrO 2
layer at room temperature directly leads to a crystalline film rather than an
amorphous film as observed from other anodic metal oxides [7]. Formation of
self-organized porous Zirconium oxide layers produced by anodization of Zr at
30 V in an electrolyte of 1 wt.% H2SO4 + 0.2 wt. % NH4F was reported by
Tsuchiya et al. [5].
Aluminum metal anodized in an acidic electrolyte and controlled under
suitable conditions, Aluminum forms a porous oxide called anodic Aluminum
oxide (AAO) with very uniform and parallel cell pores. Each cell contains an

41


elongated cylindrical sub-micron or nanopore that is normal to the Aluminum
surface, extending from the surface of the oxide to the oxide/metal interface,
where it is sealed by a thin barrier oxide layer with approximately hemispherical
geometry. The structure of AAO can be described as a closely packed array of
columnar cells [64].
The most significant difference between typical anodic Titanium oxide
(ATO) and anodic Aluminum oxide (AAO) is that the latter is a continuous film
with a pore array while the former consist of separated nanotubes. Several recent
studies have showed that Titania nanotubes have better properties compared to
[65]
many other forms of Titania for applications in photocatalysis and gas
sensors [66].

2-10- Mechanism of (TiO2) nanotubes Array Formation
[67]
Gong et al. first reported the formation of TiO2 nanotube arrays
through anodization method by using Fluoride-based electrolyte. From
comparison with other fabrication methods, the anodization is simpler and
cheaper. Moreover, the dimensions of the Titanium nanotube can be precisely
[56]
controlled by tailoring the anodization parameters. Figure (2-6) shows the
schematic set-up of anodization experiment. In the set-up, (Ti) foil is used as
an anode and inert metal, usually Platinum Pt foil, is used as a cathode.
Magnetic agitation is commonly conducted to provide uniform local current
density and temperature condition on the surface of Ti anode. In order to
achieve ordered nanotubular structures of TiO2, Fluoride ions need to be
present in electrolytes. The as-prepared TiO2 nanotubes are annealed to form
crystal structure. The morphology and structure of the Titanium nanotube are
strongly influenced by the electrochemical conditions (such as anodization
voltage and time) and the solution parameters (such as the composition of the
electrolyte).
[68]
Jessensky et al. proposed a mechanical stress model to explain the

42


formation of hexagonally ordered pore arrays of nanotybes. The model is
explained the general self-ordering mechanism for Aluminum and Titanium
as:

1. The oxidation takes place at the entire metal/oxide interface mainly by
the migration of Oxygen containing ions from the electrolyte.
2. The dissolution and thinning of the oxide layer is mainly due to the
hydration reaction of the formed oxide layer.
3. In the case of barrier oxide growth without pore formation, all metal
ions reaching the electrolyte/oxide interface contribute to oxide
formation. On the other hand, porous metal oxide is formed when
metal ions drift through the oxide layer. Some of them are ejected into
the electrolyte without contributing to the oxide formation.
4. Pores grow perpendicular to the surface when the field-enhanced
dissolution at the electrolyte/oxide interface is equilibrated with oxide
growth at the oxide/metal interface.
5. The volume of the anodized metal is expanded by difference of density
between metal and metal oxide.
6. This volume expansion leads to compressive stress during the oxide
formation in the oxide/metal interface. The expansion in the vertical
direction pushes the pore walls upwards [69].

43


[56]
Figure (2-6) Schematic set-up of anodization experiment

The mechanism for the formation and growth of Titanium nanotubes
[70]
arrays by anodization method shown in figure (2-7) . At the beginning of
the process, electrochemical etching is dominated. Due to the aid of electric
field, oxide is grown on the metal surface. Where O2- ions from H2O migrated
via the oxide layer and reacted with the metal at the metal/oxide interface,
while Ti4+ cations are ejected from metal/oxide interface to oxide/electrolyte
interface, as show in (Eq. 2-1) [56].

…………………..….…………..…. (2-1)

Fluoride ions in the electrolyte helped the formation of nanotubes on the
Ti surface. Therefore without F- ions, the electric field will be reduced as the
oxide keep on going, which leads to the exponential current decay, as show in
figure (2-8) [71].

44


Figure (2-7) Schematic diagram of the evolution of (TiO2) nanotubes in
anodization: (a) oxide layer formation; (b) pore formation on the oxide layer;
(c) climbs, formation between pores; (d) growth of the pores and the climbs;
(e) fully developed (TiO2) nanotubes arrays [70]

Figure (2-8) Schematic representation of processes in (TiO2) nanotube
formation during anodization: a) in absence of Fluorides; b) in
presence of Fluorides [71]

Moreover, Ti4+ cations ejected may formed a precipitate Ti (OH)xOy
layer. All these conditions retard the formation of oxide layer. The presence of
(F-) ions, on the other hand, possess different mechanism which mainly due to
chemical dissolution of TiO2 in the Fluoride ions containing electrolyte, as
show in (Eq. 2-2). [56].

[ ] ……………………....………………. (2-2)

The dissolution of TiO2 leads to the random formation of small pores.

45


These pores keep on growing as the oxide layer moves inward at the pore
bottom. Since the growth of pores increases the active area for oxide to form,
the current increases. Furthermore, instead of forming Ti(OH)xOy precipitate,
Ti4+ ions arriving at the oxide/solution interface react with F- to form water
soluble TiF62- , as show in (Eq. 2-3) [56].

[ ] ………………………………………..…….. (2-3)

As time proceeds, more and more pores are formed and grow. Each
individual pore starts completing for the available current with other pores.
Under optimum conditions, the pores share equal amount of available current
and spread uniformly under steady state conditions. The thickness and depth
of the pores continue to grow to form nanotube structure when the rate of
oxide growth at the metal/oxide interface is higher than that of the oxide
dissolution at the pore-bottom/electrolyte interface. However, the thickness
ceases to increase when the two rates ultimately become the same; while the
nanotube length remains unchanged thereafter the electrochemical etching rate
equals to the chemical dissolution rate of the top surface of the nanotubes [72].

2-11-Factors affecting the formation of (TiO2) nanotube

There are several factors affected the formation of (TiO2) nanotubes,
(time of anodizing, electrolyte composition (water content) and voltage) on the
thickness of nanotube i.e. led to increase the Titania thickness. While the
effective concentration of Ammonium Fluoride leads to increase etching. Also,
an applied voltage was found high affected on Titania size (pore, diameter, wall
thickness and length of tube) and the uniformity [56].

2-11-1-The effect of anodization potential

Chemical etching rate is determined by the anodization potential.
Nanotube structures are only found within a certain range of anodization

46


potential. If the voltage is too low, the electrochemical etching rate is lower than
that of chemical dissolution rate, which means that the rate of oxide growth at
the metal/electrolyte interface is lower than that of oxide dissolution at the pore-
bottom/electrolyte interface. As a result, the thickness of the barrier layer
decreases and the pores form, which cannot grow into nanotubes. On the other
hand, if the voltage is too high, the electrochemical etching rate is much higher
than that of chemical dissolution rate, the thickness of the barrier layer increases
very fast, which leads to the reduction of the electrochemical etching rate and
[56] [67]
retards the growth of pores into nanotubes . Gong et al. studied the
influence of anodization potential on the formation of TiO2 nanotube arrays
under 0.5%wt. HF aqueous solution at room temperature. He showed that at low
anodization potential (voltage ~3V), only pores are found without the formation
of a clear tube. When the voltage is increased to 10V, nanotube structure starts
to appear. As the voltage further increases, the thickness, length and the inner
diameter of the nanotubes increase. However, such nanotube structure
disappears when the voltage is greater than 23V. Liang and Li‟s [73] found the
effect of voltage on the TiO2 morphology and shows similar trend. Under 0.1wt.
%NH4F aqueous solution at room temperature, complete well-aligned nanotube
arrays are found when the anodizing voltage is within 18V to 25V. The same
length of nanotube increase as voltage increases.

2-11-2-The effect of electrolyte

Electrolyte plays a crucial role for the TiO2 nanotubes formation since
chemical dissolution rate which is affected by the composition of the electrolyte
and it is direct influential factor in the nanotubes formation. By increasing (F-)
concentration is the chemical dissolution rate increased. Since nanotubes cannot
be formed when the chemical dissolution is too high or too low, only certain (F-)
concentration range is in favor of the formation of nanotubes. Mor, Varghese,
[74]
Paulose, Shankar and Grimes‟ review paper summarized the participation of

47


other researchers on the effect of (F-) concentration on TiO2 nanotube formation
and states that nanotube structures are found when the (F-) concentration was
between 0.05 to 0.3wt. %.

The water content or viscosity of the electrolyte also affects the
morphology of nanotube formation. From most studies on the formation of TiO 2
nanotubes by anodization method, it can be seen that the side walls of Titanium
[74]
nanotube formed in water-based electrolyte are rough. Macak et al. inferred
that the distance between ridges on the Titanium nanotube side walls were
caused by the current transients during anodization. Hydrolysis reaction of the
(Ti4+) cations is driven by the applied current.

2-11-3-The effect of temperature

Increasing anodization temperature are the Titanium nanotube lengths
[76]
and their thicknesses decreased. Crawford and Chawla studied the current-
time behavior during anodization of (Ti) samples under different temperature.
They are noticed that the rate of chemical dissolution increases with increasing
electrolyte temperature. As a result, the growth of the barrier layer is offset by
higher chemical dissolution rate and thinner nanotube walls are resulted.
Besides, they study also reflects that nanotube formation occurs very rapidly
with increasing temperature and reaches its equilibrium thickness earlier. Hence,
the nanotube length decreases with increasing temperature. In order to obtain
(TiO2) nanotubes with thicker wall and longer length, the lower temperature is
preferred. However, if the temperature is too low, the walls will be too thick that
they fill the voids in the inter-pore areas, leading the tube-like structures
approach a nanoporous structure in appearance [74].

48


2-11-4-The effect of Titanium annealing before and after anodizing

The advantage of the annealing before the anodizing process is to get rid
of defects and stresses in Titanium that you get during the cutting process or
before. Freshly-prepared Titanium nanotube is amorphous. Post-thermal
treatment is essential for the crystallization of Titanium nanotube. Annealing
temperature has significant effect on the formation of different crystal types.
[77]
Several researches studied the relationship between them. Due to different
preparation conditions during anodization, the as-prepared nanotubes might be
incorporated with different impurities which affect the rate of phase
transformation. As a result, the similar phase pattern may not occur under same
annealing temperature range. However, the phase transformation follows the
same trend as annealing temperature increases. Taking in account the results
[78]
from as example, they show that when annealing temperature is below
280°C, the Titanium nanotube remains amorphous; at about 300°C, small
diffraction peak of anatase is detected except for the peaks of (Ti); as the
annealing temperature increases, the peak intensity of anatase phase become
stronger and sharper; at approximately 430°C, apart from the peaks of (Ti) and
antase, rutile phase appears with peaks of low intensity; as temperature
increases, the rutile peaks grows while the anatase peak diminishes; beyond
680°C, (Ti) and anatase completely transformed to rutile phase.

2-11-5-The effect of distance between electrodes
The distance between electrodes affects current density. Clearly the
current density at steady stage decreases gradually as the distance increases, so
does the electric field strength because of the resistance drop in organic
electrolyte [79].

49


The increment in the separation distance between the electrodes leds to
small amounts of anode ions that can be mobilize and migrate since the electric
field is not strong enough due to the distance of separation [80].

51


51


Chapter Three
Experimental and Methods
3-1- Introduction
This chapter is dedicated to the experimental work, which includes the
preparation of (TiO2) nanotube using electrochemical anodization method at
different conditions (time, voltage, electrolyte concentration and conductivity),
as shows in section (3-4-2), and study the structural characterization of (TiO 2)
nanotube of manufacturer during the measurements (XRD, SEM AFM and
Optical interferometer).
This method using electrolyte which consists of Fluoride and viscous
organic electrolyte (glycerol) were introduced.
3-2- Chemicals and Instrumentations
In this section show the Chemicals and Instrumentations used in the work.
3-2-1- Chemicals: Table (3-1) shows the Chemicals and materials which used
in process.
Table (3-1): The chemicals and materials used in process
Item Material Original Specification

99.7% ; thickness
1 Ti Foil Alfa Aesar A Johnson Mat they company
0.25mm

99.7% ; thickness
2 Pt Foil Sigma Aldrich company, Germany
0.25mm

3 NH4F BDH Chemicals Ltd pool England 99.5%

4 Glycerol BDH Chemicals Ltd pool England 99.5%

5 Ethanol China 99.7% pure

6 Acetone China 99.7% pure

Deionized Conductivity 10
7 Baghdad university
distilled water µs/cm

52


3-2-2- Instrumentations
Different types of instruments and apparatuses are used in process as
show in Table (3-2).
Table (3-2) Origin, function and specification devices
Item Function Device type Original Specification

1 Cutting Commercial machine China

Mechanical surface 1450 rpm.
2 Polishing polishing with paper Germany 240, 400, 600, 800 and 1200
glass different. grade

USA Ultrasonic Cleaner with
3 Ultrasonic cleaner Ultrasonic cleaner Bransonic 3510R- Digital Timer/Heater 5L
DTH Capacity

Electrochemical 100mL
4 Teflon cell Home made
cell

- 220V, 50 Hz, 415 watt.
5 Agitation Magnate stirrer Germany - Stirrer and heater.
- Digital Timer / Heater

Voltage maximum 60V,
6 Voltage source Power supply China
3Amper

Current Measurement of ( Voltage,
7 Avometer Malaysia
measurement Current, Resistance )

Measurement of (Conductivity
Conductivity (Kyoto electronic CO.,
8 Conductivity meter in units µs/cm, range
measurement LTD, CM-115)
maximum 5000 µs/cm.

220V, Resolution: 0.26nm
AA3000, Angstrom
lateral,
AFM Advanced Inc. USA
0.1nm vertical
precision of 50nm
Analysis and
9 Hitachi FE-SEM 0.5 - 20 kV
characterization SEM
model S-4160, Japan

XRD Germany 20 kV, 30 mA

Optical He-Ne laser of wavelength
Germany
interferometer (632.8nm) was used

53


3-2-3- Process flow chart of TiO2 nanotubes synthesis:
Pure metal Ti Foil

Cutting

Annealing at 500 ºC for 3hr.

Degreasing by (Ethanol and Acetone for 15min.)

Polishing by (glass paper)

Rising by DI for 15min.

Degreasing by (Ethanol and Acetone for 15min.)

Rising by DI for 15min.

Drying by N2 for 5min.

Anodizing process

Anodizing in Teflon Cell Using Electrolyte
(NH4F, H2O and Glycerol)

0.5-2wt.% NH4F with Glycerol 0.5wt.% NH4F+ 2 and 5wt.%H2O with Glycerol

Rising by DI and
Drying by N2 Stream for 5min.

Annealing at 450 ºC and 530 ºC for 3hr.

Characteristics

XRD AFM SEM Optical interferometer

54


Figure (3-1) Flow chart of TiO2 nanotube synthesis

3-3- Electrochemical Anodization System
A schematic diagram of anodization for Titanium oxidation system is
shown in Figure (3-2a and b). This system consists of electrochemical Teflon
cell, two electrodes (cathode and anode), power supply (DC current), magnetic
stirrer, Avometer, and suitable electrolyte for process. These electrochemical
processes were performed at room temperature (~25ºC).

a b

Figure (3-2a and b) Schematic and photograph of set-up illustrates of the anodization
experiment with Teflon cell

3-3-1-Electrochemical Anodization Cell Design
Anodization cell is designed for first time in Iraq according to our
knowledge to produce self-ordered Titanium nanotubes. The main parts of the
cell are composed of two electrodes and a stirrer. An anode was made from a
brass plate to hold and to allow current through the sample Titanium and
Platinum foil (1cm × 0.5 cm) was used to be a cathode, and the distance between
the anode and cathode was 3 cm. The advantage of Pt used in our system is to be
inert against the solutions which were used during of the process
electrochemical anodization as electrolytes.
All of the components were set up and put them into the cell. During the
process, the electrolyte is stirred vigorously by using magnetic. “The aims of
55


vigorously stirring electrolyte are to prevent local heat occurring on the surface
of titanium and also to make sure that heat and electrolyte distribute uniformly”.
The cell was used during Titanium anodization process; it is homemade
designed in Baghdad See figure (3-3).
A cell made of Teflon material rectangular hollow length of 10 cm and
width 5 cm and a capacity of 100 ml, it has open two sides, the first side the
upper end, which enters through the cathode and the electrolyte, either the
second side, O-ring sealed hole equipped with a copper disk to support Titanium
anode electrode and facilitate electrical connection, and is installed by screws of
iron and put between the anode and the electrolyte washer (from rubber) to
prevent the flow of the solution outside the cell as show in figure (3-3).

Figure (3-3) Schematic diagram of homemade Teflon cell

3-4- Samples preparation
This section shows how to prepare (TiO2) nanotubes and the operations
conducted on the samples before preparation.
3-4-1- Pretreatment of Ti foil samples
0.25 mm thick Titanium foil was cut into suitable shape for conductive
brass in the base holder of a Teflon cell figure (3-2). The sample was degreased

56


by sonication in a solution of acetone and ethanol for 15 min respectively, and
then washed in (DI) deionized water. Before anodizing, the Titanium samples
were annealed at 500ºC for 3 h to remove the mechanical stress and to enhance
the grain size, and then cooled in air. The titanium foil Ti surface was
mechanical surface polished with glass papers starting from 240 and increasing
to 400, 600, 800 and 1200 with diamond paste. Intermittently after polishing
with different SiC papers, the surface was washed with (DI) deionized water to
rinse off any particles generated while polishing. Ultrasonic cleaning in acetone,
ethanol and (DI) deionized water respectively for about 15 minutes was done
after polishing to clean the surface more effectively then dried with (N 2)
nitrogen stream; After mechanical polishing process is completed, sample put in
Teflon cell and it is prepared to next electrochemical process.
3-4-2-TiO2 Nanotube preparation
For electrochemical process the prepared Ti as the working electrode and
Platinum served as the counter electrode. The anodizing process was prepared
with the conditions as shown below:
1. Using (NH4F + glycerol) electrolyte without water at room temperature
(~25ºC) using different composition, where increasing the number
of anodizing electrolyte conductivity increases as show in Table (3-3).

57


Table (3-3): The conditions of experimental work without water added

Item NH4F wt. % Glycerol Voltage Time Conductivity
wt. % (V) (hr.) µ Siemens / cm

1 0.5 99.5 1 280

2 5 2

3 3

4 10 2

5 3

6 4

7 1

8 2
15
9 4

10 4 310

11 25 2

12 3

13 4

14 1

15 40 2

16 4

17 1 99 1 1085

18 4
15
19 1.5 98.5 2 1335

20 4

21 2 98 2 1600

58

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