Haider master's thesis

Fabrication of TiO2 Nanotubes Using Electrochemical Anodization

1

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


2

صدق الله العلي العظيم

ويسألونك عن ) وح الر

وح قل من أمر الر

ربي وما أوتيت من

(العل إلا قليلا

سورة اإلرساء85اآلية


3

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


4

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 TiO2 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)

Paragraph Title Page 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)

Paragraph Title Page 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)

Paragraph Title Page

4-1 Introduction 65

4-2 (I-V) characteristics of the electrochemical

anodization process

65

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

(XRD) measurement

89

4-5-3 Results of thickness measurement 97

Chapter Five (Conclusions and Future Work)

Paragraph Title Page 5-1 Conclusions and Perspectives 100

5-2 Suggestions for Future Research 101

References 103


13

List of Figures

Figure (2-1) Density of states as a function of energy for bulk material,

quantum well, quantum wire and quantum dot.

31

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

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

45

Figure (2-8) Schematic representation of processes in (TiO2) nanotube

formation during anodization: a) in absence of fluorides; b)

in presence of fluorides

45

Figure (3-1) Flow chart of Titanium nanotube synthesis 54

Figure (3-2) Schematic and photograph of set-up illustrates of the

anodization experiment with Teflon cell

55

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

hours at 15V in the glycerol + 0.5Wt. %NH4F and glycerol

+ 1.5Wt. %NH4F

66


hours at 15 and 40V in the glycerol + 0.5Wt. %NH4F

67

Figure (4-3) Optical images of TiO2 grown on a Ti metal substrate

during 2hr of anodization at 5V (a), 10V (b), 15V (c), 25V

(d) and at 40 V (e) in 0.5wt. % NH4F.

69

Figure (4-4) The current transients recorded during 2 hours of Ti

anodization at 15V in glycerol / water / 0.5wt. %NH4F

electrolytes with different weight ratios of glycerol: water

71


hours at 15V in the glycerol + 0.5Wt. %NH4F at a different

conductivity of electrolyte

72

Figure (4-6) SEM image of Ti anodized in (0.5 wt. % NH4F) in glycerol

electrolyte at 15V for 2 h

74

Figure (4-7) SEM image of Ti anodized in (1.5 wt. % NH4F) in glycerol

electrolyte at 15V for 2 hr.

74

Figure (4-8) AFM images of Ti anodized in (0.5 wt. % NH4F +

glycerol) electrolyte at 15V for 2 h, (a) The 2D, cross

section, (b) 3D and (c) porosity normal distribution chart

75

Figure (4-9) AFM images of Ti anodized in (1.5 wt. % NH4F +

glycerol) electrolyte at 15V for 2 hr. , (a) The 2D, cross

section, (b) 3D and (c) porosity normal distribution chart

76

Figure (4-10) SEM image of Ti anodized in (0.5 Wt. % NH4F + 99.5 Wt.

% glycerol) electrolyte at 15V for 2hr

78


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78

Figure (4-12) AFM images of Ti anodized in (0.5 Wt. % NH4F + 99.5

Wt. % glycerol) electrolyte at 15V for 4hr. , (a) The 2D,

cross section, (b) 3D and (c) porosity normal distribution

chart

79



81



81

Figure (4-15) AFM images of Ti anodized in in (0.5 Wt. % NH4F + 99.5



chart

82

Figure (4-16) AFM images of Ti anodized in (0.5 Wt. % NH4F + 99.5



chart

83


% glycerol) electrolyte at 15 V for 2h

85

Figure (4-18) SEM images of Ti anodized in (0.5 Wt. % NH4F + 2 Wt. %

H2O + 97.5 Wt. % glycerol) electrolyte at 15 V for 2h

86

Figure (4-19) SEM images: (a) top-views and (b) cross-sectional images

of Ti anodized in (0.5 Wt. % NH4F + 5 Wt. % H2O + 94.5

Wt. % glycerol) electrolyte at 15 V for 2h

86

Figure (4-20) AFM images of Ti anodized in (0.5 Wt. % NH4F + 2 Wt.

% H2O + 97.5 Wt. % glycerol) electrolyte at 15 V for 2h,

(a) The 2D, cross section, (b) 3D and (c) porosity normal

distribution chart

87

Figure (4-21) AFM images of Ti anodized in (0.5% NH4F + 5% H2O +

94.5% glycerol) electrolyte at 15 V for 2h, (a) The 2D,

cross section, (b) 3D and (c) porosity y normal distribution

chart

88

Figure (4-22) XRD pattern of Titania before and after annealing at

temperatures 450°C and 3hr on Ti foil substrate

91

Figure (4-23) XRD pattern of Titania before and after annealing at

temperatures 530°C and 3hr on Ti foil substrate

94


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List of Tables

Table (2-1) Physical and chemical properties of the three TiO2

structures 38

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

added 58

Table (3-4) The condition of experimental work with water added

59

Table (4-1) Color as a variable of anodizing TiO2 thickness 69

Table (4-2) The average Roughness and Pores diameter of TiO2

nanotubes under different proportion of NH4F 77

Table (4-3) The average Roughness and Pores diameter of TiO2 nanotubes under different anodization time

79

Table (4-4) The average Roughness and pores diameter of TiO2 nanotubes under different anodization voltage

84

Table (4-5) The results of TiO2 nanotubes under different

proportion of glycerol and water content 85

Table (4-6) The average Roughness and pores diameter of TiO2

nanotubes under different proportion of glycerol and water content

89

Table (4-7) XRD results for Titania before annealing 92

Table (4-8) XRD results for Titania after annealing at temperatures 450°C and 3hr on Ti foil substrate

93

Table (4-9) XRD results for Titania before annealing 95

Table (4-10) XRD results for Titania after annealing at

temperatures 530°C and 3hr on Ti foil substrate 96

Table (4-11) Result Titania thickness measurement by optical interferometer method

97


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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


18


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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),

nanoporous Aluminum oxide (Al2O3, Alumina) [1]

and ordered macroporous

Silicon [2]

. 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


21

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

nanopores with uniform diameter and length [5]

. 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

sensors [16]

. 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 TiO2

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

and nanotechnology in the past decades [17]

. New physical and chemical

properties emerge when the size of the material becomes smaller and smaller,


22

and down to many serious environmental and pollution challenges. TiO2 also

bears tremendous hope in helping ease the energy crisis through effective

utilization of solar energy based on photovoltaic and water-splitting devices [18]

.

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 TiO2 nanomaterials [19]

.

In 1991, Zwilling et al. [20]

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.

A decade later Gong and co-workers [22]

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


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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


24

treatment of films on a photocurrent and current-voltage characteristics of

photoelectrodes were investigated.

In 2007 V. Vega et al. [27]

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.


25

In 2009 Michael et al. [29]

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.% (NH4)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 TiO2 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.% NH4F 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.% NH4F. 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.


26

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.


27


28

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

variations in solid-state properties [32]

. 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

resulting nanostructures into larger system [35]

.

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


29

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

nanometre scale [36]

. 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).


31

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


32

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,

http://en.wikipedia.org/wiki/Statistical_mechanics

http://en.wikipedia.org/wiki/Quantum_mechanics

http://en.wikipedia.org/wiki/Quantum

http://en.wikipedia.org/wiki/Quantum_realm

http://en.wikipedia.org/wiki/Quantum_realm


33

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

Microscope (AFM) to transfer material from the AFM tip to a surface [43]

. 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]

.

http://en.wikipedia.org/wiki/Nanoionics

http://en.wikipedia.org/wiki/Nanomechanics

http://en.wikipedia.org/wiki/Biomaterial

http://en.wikipedia.org/wiki/Biomaterial

http://en.wikipedia.org/wiki/Gold

http://en.wikipedia.org/wiki/Catalyst


34

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


35

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

figure (2-3) [47]

. The lattice parameters correspond now to a = b = 4.5933 A°

and c = 2.9592 A° with c/a ratio of 0.6442

[48].


36

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]

.


37

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/cm3). The

unit cell is composed of eight formula units of TiO2 and is formed by edge

sharing TiO2 octahedra, similar to rutile and anatase, as shown in the figure (2-

5) [47]

. Brookite belongs to the Orthorhombic crystal system its space group is

Pbca. By definition, the brookite structure is of lower symmetry than its TiO2

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]

.


38

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 = = = = = =

Density g/cm3 4.27 3.90 4.13

Melting point °C 1855 Transformed

into rutile

Transformed

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

discovered in Arkansas [49]

. 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

structure [46]

. For photocatalytic processes, anatase is the preferred structure,

http://en.wikipedia.org/wiki/Chemical_formula

http://en.wikipedia.org/wiki/Titanium

http://en.wikipedia.org/wiki/Titanium

http://en.wikipedia.org/wiki/Molar_mass

http://en.wikipedia.org/wiki/Density

http://en.wikipedia.org/wiki/Melting_point

http://en.wikipedia.org/wiki/Boiling_point

http://en.wikipedia.org/wiki/Refractive_index


39

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

Since the discovery of Carbon nanotubes in 1991 [51]

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 TiO2 [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

improved performance in photocatalysis [54]

and photovoltaics [55]

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]

.


41

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

Tsuchiya, et al. [5]

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

as an industrial catalyst and catalyst support [7]

. 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 ZrO2

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


42

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

many other forms of Titania for applications in photocatalysis [65]

and gas

sensors [66]

.

2-10- Mechanism of (TiO2) nanotubes Array Formation

Gong et al. [67]

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

controlled by tailoring the anodization parameters. Figure (2-6) [56]

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).

Jessensky et al. [68]

proposed a mechanical stress model to explain the


43

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]

.


44

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

The mechanism for the formation and growth of Titanium nanotubes

arrays by anodization method shown in figure (2-7) [70]

. 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]

.


45

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.


46

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


47

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

retards the growth of pores into nanotubes [56]

. Gong et al. [67]

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,

Paulose, Shankar and Grimes‟ review paper [74]

summarized the participation of


48

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 TiO2

nanotubes by anodization method, it can be seen that the side walls of Titanium

nanotube formed in water-based electrolyte are rough. Macak et al. [74]

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

and their thicknesses decreased. Crawford and Chawla [76]

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]

.


49

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.

Several researches [77]

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

from [78]

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]

.


51

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


52

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 (TiO2)

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

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

0.25mm

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

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

7 Deionized

distilled water Baghdad university

Conductivity 10

µs/cm


53

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

2 Polishing

Mechanical surface

polishing with paper

glass different.

Germany

1450 rpm.

240, 400, 600, 800 and 1200

grade

3 Ultrasonic cleaner Ultrasonic cleaner

USA

Bransonic 3510R-

DTH

Ultrasonic Cleaner with

Digital Timer/Heater 5L

Capacity

4 Electrochemical

cell Teflon cell Home made

100mL

5 Agitation Magnate stirrer Germany

- 220V, 50 Hz, 415 watt.

- Stirrer and heater.

- Digital Timer / Heater

6 Voltage source Power supply China Voltage maximum 60V,

3Amper

7 Current

measurement Avometer Malaysia

Measurement of ( Voltage,

Current, Resistance )

8 Conductivity

measurement Conductivity meter

(Kyoto electronic CO.,

LTD, CM-115)

Measurement of (Conductivity

in units µs/cm, range

maximum 5000 µs/cm.

9 Analysis and

characterization

AFM

AA3000, Angstrom

Advanced Inc. USA

220V, Resolution: 0.26nm

lateral,

0.1nm vertical

precision of 50nm

SEM Hitachi FE-SEM

model S-4160, Japan

0.5 - 20 kV

XRD Germany 20 kV, 30 mA

Optical

interferometer Germany

He-Ne laser of wavelength

(632.8nm) was used


54

3-2-3- Process flow chart of TiO2 nanotubes synthesis:

Cutting

Degreasing by (Ethanol and Acetone for 15min.)

Rising by DI for 15min.

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

Rising by DI and

Drying by N2 Stream for 5min.

Drying by N2 for 5min.

Pure metal Ti Foil

Anodizing process

Annealing at 500 ºC for 3hr.

SEM

Characteristics

XRD AFM

Polishing by (glass paper)

Degreasing by (Ethanol and Acetone for 15min.)

Rising by DI for 15min.

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

Annealing at 450 ºC and 530

ºC for 3hr.

Optical interferometer


55

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).

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

a b


56

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


57

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 (N2)

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).


58

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

Item NH4F wt. % Glycerol

wt. %

Voltage

(V)

Time

(hr.)

Conductivity

µ Siemens / cm

1 0.5 99.5

5

1 280

2 2

3 3

4 10

2

5 3

6 4

7

15

1

8 2

9 4

10 4 310

11 25 2

12 3

13 4

14

40

1

15 2

16 4

17 1

99

15

1 1085

18 4

19 1.5 98.5 2 1335

20 4

21 2 98 2 1600


59

2. Using (0.5%wt NH4F + glycerol + H2O) electrolyte with water at room

temperature (~25ºC) as show in Table (3-4).

Table (3-4): The conditions of experimental work with water added

Item NH4F

wt. %

Glycerol

wt. %

Water

wt. %

Voltage

(V)

Time

(hr.)

Conductivity

µ Siemens / cm

1 0.5 97.5 2

15

2 542

2 4

3 94.5 5 2 740

4 4

Three samples of (Ti) foil anodized at each conditions variable and the

other conditions are established to focus on and study the (I-V) characteristics as

well as the structural and morphological characterization.

3-5-Characterization measurements

The Characteristic measurements of this technique used to investigate the

thickness, the structural features of the Titania templates were X-ray diffraction

(XRD), scanning electron microscopy (SEM) and atomic force microscopy

(AFM).

3-5-1- X-Ray diffraction (XRD) pattern

XRD is a very important experimental technique that has long been used

to address all issues related to the crystal structure of solids, including lattice

constants and geometry, identification of unknown materials, orientation of

single crystals, preferred orientation of polycrystals, defects, stresses, etc. In

XRD was carried out done according to the ASTM (American Society of

Testing Materials) cards taken from Match! program version 1.9b (2011). Using

Philips pw 1050 X-ray diffractometer of 1.54 Å from Cu-k α, the XRD patterns


61

of samples were recorded in the range 2θ=10-70°. The diffractmeter was

operated at 20 kV and 30 mA, is incident on a specimen and is diffracted by the

crystalline phases in the specimen according to Bragg's law [81]

:

…………………………………………………………… (3-1)

Where d is the spacing between atomic planes in the crystalline phase and

λ is the X-ray wavelength. The intensity of the diffracted X-rays is measured as

a function of the diffraction angle 2θ and the specimen's orientation. This

diffraction pattern is used to identify the specimen's crystalline phases and to

measure its structural properties. XRD is nondestructive and does not require

elaborate sample preparation, which partly explains the wide usage of XRD

method in materials characterization [81]

.

3-5-2- Atomic Force Microscopy (AFM)

The (AFM) study carried out by (AA3000, Angstrom Advanced Inc.

USA). The AFM consists of a cantilever with a sharp tip (probe) at its end that is

used to scan the specimen surface. The cantilever is typically silicon or silicon

nitride with a tip radius of curvature on the order of nanometers. When the tip is

brought into proximity of a sample surface, forces between the tip and the

sample lead to a deflection of the cantilever according to Hooke's law, as shown

in figure (3-3) [82]

.


61

Figure (3-4) Block diagram of atomic force microscope [82]

.

Depending on the situation, forces that are measured in AFM include

mechanical contact force, van der Waals forces, capillary forces, chemical

bonding, electrostatic forces, etc. Along with force, additional quantities may

simultaneously be measured through the use of specialized types of probe.

Typically, the deflection is measured using a laser spot reflected from the top

surface of the cantilever into an array of photodiodes. Other methods that are

used include optical interferometer, capacitive sensing or piezoresistive AFM

cantilevers. These cantilevers are fabricated with piezoresistive elements that act

as a strain gauge. Using a Wheatstone bridge, strain in the AFM cantilever due

to deflection can be measured, but this method is not as sensitive as laser

deflection or interferometry [82]

.

3-5-3- Scanning Electron Microscopy (SEM)

Scanning electron microscopy (SEM) is basically a type of electron

microscope. SEM is used for various purposes;

- Topographic studies.

- Microstructure analysis.

- Elemental analysis if equipped with appropriate detector (energy/wavelength

dispersive x-rays).


62

- Chemical composition.

- Elemental mapping.

The Samples preparation by sputtering method are gold coated at 1200 V, 20

mA, using vacuum coater (Polaron E6100, UK). The SEM study carried out by

(Hitachi FE-SEM model S-4160, Japan) in University of Tehran, scanning

electron microscope equipped with Energy dispersive X-ray (EDAX); determine

the energy of the X-rays microanalysis a illustrated in figure (3-4a and b).

Figure (3-5a and b): Set-up and Photograph illustrates the SEM.

3-5-4- Thickness measurement

Titania thickness measured by optical interferometer method. An

interferometric Fizeau was used to determine the thickness of the deposited

films. The experimental setup for observing Fizeau fringes is shown

schematically in figure (3-5 a) and the Fizeau pattern is shown in figure (3-5 b).

The interferometer plates should have two surfaces, one is coated with highly

reflected semitransparent film, and the other is partially coated with the film

whose thickness is to be measured, leaving an uncoated channel across the

surface. The two plates are placed carefully in contact and inclined at a slight

wedge angle to each other. It is important that the air film between the surfaces

a b


63

should be as sell as possible. When the interferometer is illuminated by

monochromatic light from an extended source, narrow black-line Fizeau fringes

are observed on bright background. These fringes contour regions of constant

thickness between the two surfaces separated by multiple integers of half

wavelength of the monochromatic light. The thickness is obtained by measuring

the displacement of fringes in the channel from the rest of the surface. The

following equation was used to measure the thin film thickness [83]

:

………………………………………………………… (3-2)

Where X is the fringes spacing, ΔX is the displacement and λ is the

wavelength of laser light. He-Ne laser of wavelength (632.8nm) was used.

Figures (3-6a and b) experimental arrangement for observing Fizeau fringes [84]

.

Accurate thickness measurements require careful evaluation of fringe

fraction. These may be measured by a calibrated microscope eyepiece, either

way; the evaluation requires a linear measurement, the accuracy of which is

strongly dependent on the definition and sharpness of the fringes. By this

technique thickness measurement from 2-3 nm can be made routinely to an

accuracy of + 1 nm [85].

a b


64


65

Chapter Four

Results and Discussions

4-1- Introduction

The results of Titania nanotubes fabrication through electrochemical

anodization process using different parameters (time, voltage and electrolyte

concentration) as mentioned in the experimental part chapter four section (3-4-

2), will be presented and discussed in details in this chapter.

The range of anodizing time were between (1-4hr.), range of potential (5-

40V) and the elemental of; electrolyte composition were included first

electrolyte (glycerol, NH4F) without water and second electrolyte (glycerol,

NH4F and H2O) with water content each NH4F concentration and water varied to

study their effects (I-V) characteristics of the electrochemical process and the

fabricated Titania as well as the structural and morphological characterization

through the (XRD) X-ray diffraction test, SEM test, AFM test and optical

interferometer method for Titania thickness measurement.

4-2- (I-V) characteristics of the electrochemical anodization process

The current-time characteristics during the Titania formation where

recorded as shown in next sections. In general the current density starts at a high

magnitude then it reduces gradually with time then became nearly constant

(steady state).

4-2-1- Effect of NH4F concentration

It is important to compare the electrochemical data recorded during

anodization in these electrolytes since the nanotube growth was achieved over

comparably wider (F-) concentration range. Figure (4-1) shows the current

transients recorded during Titania growth in (0.5 and 1.5wt.% NH4F)

concentrations at 2 hours and 15V. The magnitude of the current is clearly

affected by increasing of the Fluorides in concentration the electrolyte due to the

increase in ions mobility and hence the conductivity of the electrolyte.


66

Figure (4-1): The current transient recorded during anodization during 2

hours at 15V in the glycerol + 0.5Wt. %NH4F and glycerol + 1.5Wt. %NH4F.

This relation is shown with decreasing F- concentration, the dissolution

rate of TiO2 becomes slower, therefore the anodizing current is smaller and the

tubes are shorter as SEM and AFM resulting shown.

4-2-2- Effect of anodizing potential

A shown in figure (4-2) increasing the anodizing potential from (25 to

40V) for the same electrolyte composition leds to increase the current of

processes, then it reduces gradually with time then became nearly constant

(steady state) due to formation of TiO2 nanotubes layer on the Ti metal surface,

as SEM and AFM resulting shown.

0

350

700

1050

1400

1750

2100

2450

2800

3150

3500

3850

0 10 20 30 40 50 60 70 80 90 100 110 120 130

Cu

rre

nt

de

nsi

ty (

µA

/ cm

2)

Anodization Time (min.)

0.5 wt. % NH4F

1.5 wt. % NH4F


67

Figure (4-2): The current transient recorded during anodization during 2 hours

at 25 and 40V in the glycerol + 0.5Wt. %NH4F.

The arisen current density is found exponentially proportional to the field

strength across the oxide layer. The electric field across the oxide layer had the

vital importance for the transport of ionic species through the oxide and thus it is

responsible for the nanotube growth that also requires permanent oxide

dissolution. In order to maintain a continuous, non-disturbed growth of a

nanotube layer, the electric field should be maintained as stable, as possible.

Common to all anodizing treatments shown here, or reported elsewhere, is that

the nanotube layers are achieved by potentiostatic polarization, typically by

define ramping of the potential from the open-circuit potential (OCP) to the

constant potential value, or less frequently, by a potential step to a desired

anodization voltage. Galvanostatic anodization (under constant current

conditions) appears up to now not suitable for the nanotube growth, as it may

lead to significant voltage oscillations and destruction of the nanotube layer and

this agrees with the result in a similar work [86]

.

There is another effect of the voltage change the color of the oxide layer.

The oxide is transparent, but in potential different becomes have vivid rainbow-

0

350

700

1050

1400

1750

2100

2450

2800

3150

3500

3850

0 10 20 30 40 50 60 70 80 90 100 110 120 130

Cu

rre

nt

de

nsi

ty (

µA

/ cm

2)


25 V

40 V


68

like colors due to interference coloring i.e. for all potential color special. White

light falling on the oxide is partially reflected and partially transmitted and

refracted in the oxide film. The light that reaches the metal / oxide surface is

mostly reflected back into the oxide. Several reflections may take place. A phase

shift occurs during this process along with multiple reflections and this agrees

with the result in a similar work [87]

. The degree of absorption and number of

reflections depends on the thickness of the film. The light that was initially

reflected from the oxide surface interferes with the light that has traveled

through the oxide and has been reflected off the metal surface. Depending on the

thickness of the oxide, certain wavelengths (colors) will be in-phase and

enhanced while other wavelengths will be out of phase and dampened. Hence,

the observed color is mainly determined by the oxide thickness. The oxide

thickness is primarily voltage controlled. At any given voltage the oxide film

grows to a specific thickness and then stops thickening. However, other factors

such as material, pretreatment, anodizing solution chemistry and temperature,

load size, anode: cathode ratio, anodizing time, and tank configuration affect the

color of the anodized piece, making it somewhat difficult to predict and control

the resultant color and this agrees with the result in a similar work [87]

.

Examples of coloured TiO2 film on Ti are shown in figure (4-3). The

color will not fade, or wear off since it is produced by the electrochemical

anodization of interference at the oxide and the metal surface. However, any

coating placed on top of the oxide, such as finger prints, will affect the color.


69

Figure (4-3) Optical images of TiO2 grown on a Ti metal substrate during 2hr

of anodization at 5V (a), 10V (b), 15V (c), 25V (d) and at 40 V (e) in 0.5wt. %

NH4F.

Table (4-1) lists the color spectrum of anodized Titanium along with the

applied voltage and calculated oxide thickness (from the refractive index) as

discussed by e.g. Fujishima et al. [87]

.

Table (4-1): Color as a variable of anodizing TiO2 thickness

Sample ID Applied voltage

(V)

Color Film thickness

(nm) × 103

1 5 Light brown 1.4

2 10 Golden brown 1.86

3 15 Purple blue 2.21

4 25 Sky blue 2.71

7 40 Light olive 2.80

a b c

d e


71

………………………………………… (4-1)

The latter reaction has been associated with the filling of electron trap

sites, with an elevation of the Fermi level. These changes can be followed

spectroscopically, by an increase in the absorption of light in the wavelength

region from 380 to 600 nm. However, there has been some controversy involved

with this idea. Some workers conclude that the coloring of TiO2 films occurs as

a result of the filling of the conduction band, with the absorption of light

exciting electrons from lower to higher energy levels within the CB. This is a

more physical view, which has been advanced by Fitzmaurice and others [88]

.

Other workers have concluded that the coloring process does indeed involve

reaction (4-1) and that the absorption of light involves electronic transitions

associated with the Ti3+

ion. This is a more chemical view, which has been

advanced by Meyer and co-workers [89]

. It has been difficult to conclude which

is correct, because the absorption spectrum includes aspects that can be

explained in both ways. Specifically, if the electrons are not trapped at specific

sites, the absorption should exhibit a steadily increasing absorbance with

increasing wavelength, as was observed by Panayotov and Yates, as discussed

earlier [90]

. This is because there are many, closely spaced energy levels that are

available, with the probability being larger to absorb a smaller amount of

energy. If, on the other hand, the electrons are trapped at specific, relatively

well-defined sites, there should be specific, widely spaced energy levels, which

would lead to absorbance peaks. Cao et al. argue that, since there is a broad peak

in the absorbance at ca. 1000 nm, which corresponds to a specificabsorption

process for Ti3+

, the electrons are essentially trapped at these sites [89]

.

4-2-3-Effect of water content

The current transients recorded during anodization of Ti at 15 V for 2

hours in three different electrolytes consisting of glycerol, 0.5wt.% NH4F and


71

different amounts of water, are shown in figure (4-4). Current

density increases with increasing water content because water causes an increase

in electrolyte conductivity as well as increase in the diameter, wall thickness and

length of TiO2 nanotubes, as SEM and AFM resulting shown.

Figure (4-4): The current transients recorded during 2 hours of Ti anodization

at 15V in glycerol / water / 0.5wt. %NH4F electrolytes with different weight

ratios of glycerol: water.

The explanation is likely that the viscosity of the glycerol electrolyte (a

function of water content) and this agrees with the explanation in a previous

work [91]

, has a huge impact on the diffusion of all the species involved in the

reactions and thus on the magnitude of the field-assisted TiO2 formation and

dissolution.

4-2-4-Effects of Electrolyte Conductivity

Figure (4-5) shows the current transients recorded during anodization of

Ti at 15 V for 4 hours in two different conductivity electrolyte where the

observed conductivity of electrolyte increases almost linearly with the

increasing the number of anodization due to the increase in ions mobility and the

dissolved Ti ion. Also the same figure shows the effect of conductivities of

0

350

700

1050

1400

1750

2100

2450

2800

3150

3500

3850

4200

4550

4900

0 10 20 30 40 50 60 70 80 90 100 110 120 130

Cu

rre

nt

de

nsi

ty (

µA

/ cm

2)

Anodization Time ( min.)

0wt.%H2O

2wt.%H2O

5wt.%H2O


72

electrolyte on the current transients during anodization. Lower conductivity of

electrolyte reduces the speed of interaction and transmission of ions within the

electrolyte, leading to slow the growth of TiO2 nanotubes. It also decreases the

dissolution of anodized film and therefore, the transition time to reach a steady

state current value increases. The surface morphology of TiO2 structures is also

affected by conductivity of electrolyte. Either higher conductivity of electrolyte

is increase the speed of interaction and transmission of ions within the

electrolyte, leading to rapid the growth and formation of TiO2 nanotubes as

SEM and AFM resulting shown. The trend of increasing comes from the

dissolved Ti ion. The concentration of Ti ion has similar trend to a conductivity

of electrolyte and a length of TiO2 nanotube. From the potential transient graph

during anodization, we can expect the electrical behavior of anodization.

Figure (4-5): The current transient recorded during anodization

during 4h. at 15V in the glycerol + 0.5Wt. %NH4F at a different

conductivity of electrolyte.

4-3- Characterization of Titania nanotubes

In this section we show the results of structural and morphological for

Titania templates studied by (SEM, AFM and XRD) and discussed in detail.

0

50

100

150

200

250

300

350

400

450

500

550

600

650

700

0 20 40 60 80 100 120 140 160 180 200 220 240 260

Cu

rre

nt

de

nsi

ty (

µA

/ cm

2)


280 µS/cm

310 µS/cm


73

4-3-1-Structural and morphological characterization of Titanium

nanotubes (TiO2) by (SEM AFM and optical interferometer)

techniques

Scanning electron microscopy (SEM) was employed for the

morphological characterization of anodized samples. All cross-sectional images

in this thesis were taken from cracked layers after mechanical bending, cutting-

off, or scratching the samples (with a knife).

Atomic force microscopy (AFM) was used to examine TiO2 surfaces and

the pore diameter, depth and roughness factor of each sample were deduced

from picture analysis. Atomic Force Microscopy (AFM) uses a sub-nanometer

probe to scan the surface of a sample record the deflections of the tip as show in

chapter 3.

4-3-1-1-Effect of NH4F concentration

Figures (4-6) and (4-7) show the SEM images of TiO2 nanotubes prepared

by anodization treatment under 15V for 2hr in different NH4F concentration in

glycerol electrolyte. When titanium samples were anodized in different NH4F

solution, structures of the anodized titanium samples changed remarkably along

with the changing of electrolyte concentrations. From the results, it can be seen

that the formation of nanotubes is very sensitive to the concentration of NH4F.

The nanotubes are most orderly formed when anodizing Ti in 0.5 wt. % NH4F,

nanotubes are formed with 54 ± 10 nm tube diameter, 20 ± 3 nm wall thickness

and (1.76 ± 0.5 × 103) nm tube length. In contrast, the nanotubes in higher

concentration (1.5 wt. % NH4F) were observed to be relatively less orderly

formed. And dimensions nanotubes are formed with 69 ± 10 nm tube diameter,

29 ± 3 nm wall thickness and (2.71 ± 0.5× 103) nm tube length.


74

Figure (4-6): SEM image of Ti anodized in 0.5 wt. %

NH4F in glycerol electrolyte at 15V for 2 h.

Figure (4-7): SEM image of Ti anodized in 1.5 wt. %

NH4F in glycerol electrolyte at 15V for 2 hr.

Figures (4-8a,b,c) and (4-9a,b,c) show the results (AFM). Various

electrolytes determine the diameter of etching pores and the degree of ordering.

From images show the differences of Titania morphology of Titania obtained

with 0.5wt.% and 1.5wt.% NH4F. We concluded that concentration of fluoride

has a significant effect on the surface of the sample, because the increased of

concentration (F-) increases the surface etching and the generation of more

pores. According to the result, Titania etching by a low concentration of

Fluoride generated smaller pores.


75

(a) (b)

(c)

Figure (4-8): AFM images of Ti anodized in (0.5 wt. % NH4F + glycerol)

electrolyte at 15V for 2 h, (a) The 2D, cross section, (b) 3D and (c)

porosity normal distribution chart.

Porosity normal distribution chart


76

(a) (b)

(c)

Figure (4-9): AFM images of Ti anodized in (1.5 wt. % NH4F + glycerol)

electrolyte at 15V for 2 hr. , (a) The 2D, cross section, (b) 3D and (c) porosity

normal distribution chart.

The terms of diameter of the pores has been shown that when increasing

the concentration of Fluoride the diameter of the pores increased are. Increasing

the average roughness (RF) with increasing concentration of Fluoride the

surface is became more uniform, as shown in the Table (4-2).



77

Table (4-2): The average Roughness and Pores diameter of TiO2 nanotubes under

different proportion of NH4F

Concentration of

NH4F wt.% in

glycerol

Pores diameter

(nm) Average Roughness

(nm)

0.5 15.5 0.259

1.5 22 0.16

4-3-1-2-Effect of anodization time

Figures (4-10) and (4-11) shows the SEM images of TiO2 nanotubes

fabricated in (0.5 Wt. % NH4F + 99.5 Wt. % glycerol) electrolyte at 2hr. and 4hr.

under 15V. The average tube diameter, wall thickness and tube length of

nanotubes fabricated at 2hr are 54 ± 10 nm, 20 ± 3nm and (1.76 ± 0.5 × 103 ) nm

while those fabricated at 4hr. are 71 ± 10 nm, 26 ± 3 nm and (2.22 ± 0.5× 103)

nm, respectively. By comparing the dimensions of nanotubes with those

fabricated for 2h, it is noted that the anodization time (from 2hr to 4hr) does

have small effect on the diameter and wall thickness. For the effect on tube

length it is found that the extension of the anodization time is significantly

increased the tube length.


78

Figure (4-10): SEM image of Ti anodized in 0.5 Wt. %

NH4F + 99.5 Wt. % glycerol electrolyte at 15V for 2hr.



The atomic force microscopy (AFM) images of the samples shown in

figures (4-8a,b,c) and (4-12a,b,c) shows the pores size difference more clearly. It

was further observed that the thicker template in 4hr. had a rougher surface

compared to the thinner template in 2hr. As shown in Table (4-3) the average

roughness (RF) increased as the anodization time increase and the surface

became more uniform.


79

Table (4-3): The average Roughness and Pores diameter of TiO2 nanotubes under

different anodization time

Anodization time

(hr.)

Pores diameter

(nm)

Average Roughness

(nm)

2 15.5 0.259

4 29 0.16

(a) (b)

(c)

Figure (4-12): AFM images of Ti anodized in (0.5 Wt. % NH4F + 99.5 Wt. %

glycerol) electrolyte at 15V for 4hr. , (a) The 2D, cross section, (b) 3D and (c)




81

4-3-1-3-Effect of anodizing potential

In the previous results it was reported that for self-organized nanotube

growth, the best results were achieved, when a potential of (15-20 V) was used.

Interesting work by Bauer et al. [91]

however showed the possibility to achieve

the nanotube growth in H3PO4 / HF electrolytes over a range of anodizing

potentials (1 - 25V) and with the range of different tube diameters (10 - 120

nm). In this section, grown nanotube layers with even larger tube diameters by

changing the applied potential, while keeping all other conditions the same and

this agrees with the result in a similar work of others [92]

.

Figures (4-13) and (4-14) show a SEM images taken from Ti samples

anodized at (25 and 40V) in a mixture of 99.5 Wt. % glycerol with 0.5Wt. %

NH4F electrolyte. All samples have been anodized for 2 hours. As we can see

the formation of a self-organized and uniform nanotube layers with different

tube diameters are possible achieved. To the best of our knowledge, this glycerol

/ NH4F electrolyte is the only electrolyte up to now that allows growth of such a

wide variety of nanotube diameters. The higher the applied potential the larger is

the tube diameter. When anodizing Ti in 25V, nanotubes are formed with 74 ±

10 nm tube diameter, 25 ± 3 nm wall thickness and (1.22 ± 0.5 × 103) nm tube

length. And when anodizing Ti in 40V, nanotubes are formed with 80 ± 10 nm

tube diameter, 26 ± 3 nm wall thickness and (3.6 ± 0.5× 103) nm tube length.

From the results, it can be seen that the formation of nanotubes is very sensitive

to the anodization voltage. Titania nanotubes can be formed over a wider range

of anodizing potential. Furthermore, when the voltage is increased, the diameter

and the length of nanotubes also increase because of the increase in the chemical

etching rate at a high anodizing potential.


81

Figure (4-13): SEM image of Ti anodized in (0.5 Wt. %

NH4F + 99.5 Wt. % glycerol) electrolyte at 25V for 2hr.



Measuring by (AFM) shows that TiO2 topography varies with applied

voltage because it affects the etching rate. An increase in the magnitude of

applied voltage causes an increase in pores diameter and thickness of template

as shown in figures (4-15a,b,c) and (4-16a,b,c). In 40V it is evident that well-

patterned dimples existed in the entire Ti sheet compared with the 25V. The

dimple size is dependent on the applied potential with bigger potentials resulting


82

in larger dimples. Increasing the potential will favor the formation of thick

Titania film with wide pore diameters. Changing the potential may vary the rate

of the chemical reactions that lead to the formation of the Titanium oxide.

(a) (b)

(c)

Figure (4-15): AFM images of Ti anodized in in (0.5 Wt. % NH4F + 99.5 Wt.

% glycerol) electrolyte at 25V for 2hr. , (a) The 2D, cross section, (b) 3D and

(c) porosity normal distribution chart.



83

(a) (b)

(c)

Figure (4-16): AFM images of Ti anodized in (0.5 Wt. % NH4F + 99.5 Wt. %

glycerol) electrolyte at 40V for 2hr. , (a) The 2D, cross section, (b) 3D and (c)


The variation of the rate of formation of Titania could influence the

arrangement of Titanium oxide molecules on the surface of Titanium foil. The

variation of roughness and pores diameter with voltage is shown in Table (4-4).



84

Table (4-4): The average Roughness and pores diameter of TiO2

nanotubes under different anodization voltage

Anodization Voltage

(V)

pores diameter

(nm)

Average Roughness

(nm)

25 37 0.587

40 80 6.13

4-3-1-4-Effect of water content

In this section, the influence of the water content in the inorganic

electrolytes is demonstrated. It will be shown that the addition of even a small

amount of water has an extraordinary effect on the formation of the nanotubular

layers.

Figures (4-17), (4-18) and (4-19a, b) SEM shows the results from a set of

anodization experiments at 15V for 2hr. using (0.5 Wt. % NH4F + 99.5 Wt. %

glycerol) electrolyte, (0.5 Wt. % NH4F + 2 Wt. % H2O + 97.5 Wt. % glycerol)

electrolyte and (0.5% NH4F + 5% H2O + 94.5% glycerol) respectively. The tube

diameters, wall thickness and lengths are presented in Table (4-5). From the

results, it can be determined that the diameter and thickness of tubes vary a

simple with the water content. Still, the data show the trend that decreasing the

water content decreases the diameter and decreases the thickness of the tubes.

On the other hand, the length of tubes is strongly sensitive to the water content.

The length of tubes formed in the mixture of glycerol and water (2 Wt. % H2O)

is shorter than that formed in the mixture of glycerol and water (5 Wt. % H2O)

by 260 nm.


85

Table (4-5): The results of TiO2 nanotubes under different proportion of

glycerol and water content

Water content in

glycerol / H2O /

0.5Wt.%NH4F

mixture /Wt. %

Tube diameter

(nm)

Wall thickness

(nm)

Tube length

(nm) × 103

0 54±10 20±3 1.76±0.5

2 79±10 26±3 2.86±0.5

5 89±10 30±3 3.12±0.5

For the pure glycerol electrolyte, the tubes have the smallest diameter,

wall thickness and length. When the proportion of water added to the electrolyte,

the tube length and diameter have slightly increased. This could be a result of

the added H2O that helps form more TiO2 (→ increased oxidation), as compared

with the nanotubes grown in "pure" glycerol and this agrees with the result in a

similar work [92,93]

.

Figure (4-17): SEM images of Ti anodized in (0.5 Wt. % NH4F +

99.5 Wt. % glycerol) electrolyte at 15 V for 2h.


86

Figure (4-18): SEM images of Ti anodized in (0.5 Wt. % NH4F + 2

Wt. % H2O + 97.5 Wt. % glycerol) electrolyte at 15 V for 2h.

(a) (b)

Figure (4-19): SEM images: (a) top-views and (b) cross-sectional images of Ti anodized

in (0.5 Wt. % NH4F + 5 Wt. % H2O + 94.5 Wt. % glycerol) electrolyte at 15 V for 2h.

In (AFM) measurement the surface morphology of the formation (TiO2)

nanotube by anodization has been subjected to extensive study. Figures (4-

8a,b,c), (4-20a,b,c) and (4-21a,b,c) show the surface evolution with increasing

the water content 2 and 5 Wt. % H2O to 0.5wt.%NH4F respectively . It has been

observed that the tubes density increases and proportional with addition of the

water.


87

The bright yellow color represents the wall of tubes while darker yellow

represents pores. From the pictures it can be concluded that as the sample is

anodized, by adding more water, the surface of the sample averages out.

(a) (b)

(c)

Figure (4-20): AFM images of Ti anodized in (0. 5 Wt. % NH4F + 2 Wt. %

H2O + 97.5 Wt. % glycerol) electrolyte at 15 V for 2h, (a) The 2D, cross

section, (b) 3D and (c) porosity normal distribution chart.



88

(a) (b)

(c)

Figure (4-21): AFM images of Ti anodized in (0.5% NH4F + 5% H2O +

94.5% glycerol) electrolyte at 15 V for 2h, (a) The 2D, cross section, (b)

3D and (c) porosity normal distribution chart.

Furthermore, the roughness average (RF) increased by adding water and

the same behavior for the diameter of the pores, as well as the sample surface

becomes more uniform, as shown in the Table (4-6).



89

Table (4-6): The average Roughness and pores diameter of TiO2

nanotubes under different proportion of glycerol and water content

Water content in glycerol

/ H2O / 0.5Wt.%NH4F

mixture /Wt. %

Pores diameter

(nm) Average Roughness

(nm)

0 15.5 0.259

2 26 0.511

5 39 0.668

4-3-1-5- Effects of Electrolyte Conductivity

Anodization of Ti is affected by conditions of electrolyte, such as fluoride

source and composition of solution. When the anodization was repeatedly

executed in the same bath, the conductivity of electrolyte was increased. The

conductivity of electrolyte increases almost linearly with the increase of number

of anodization. It also indicates that higher conductivity results in the increases

of TiO2 nanotubes length at the same anodizing conditions which fabricated in

(0.5 Wt. % NH4F + 99.5 Wt. % glycerol) electrolyte at and 4hr. under 15V. Two

different solutions, which have 280 µS/cm and 310 µS/cm at room temperature

(~25 oC).

4-3-2-Structural characterization of Titania in (XRD) measurement

(TiO2) layers were studied by X-ray diffraction (XRD) techniques. It is a

noncontact and nondestructive technique used to identify the crystalline phases

present in materials and to measure the structural properties of these phases. In

XRD was carried out done according to the ASTM (American Society of

Testing Materials) cards taken from Match! Program version 1.9b (2011).


91

We used the measured (X-Ray) to determine the crystal structure that

appears to us so that we can determine the knowledge and its applications, as

they prefer the Rutile in photovoltaic applications for its ability to reflect light as

it is more stable than Anatase. Either Anatase is preferred in the applications of

optical catalysts because it has the ability to transfer a higher mobility of electric

charges.

We took a sample under conditions (0.5wt.%NH4F + 99.5wt.% glycerol)

electrolyte at 15V for 2hr and we had a measurement of (XRD). Figure (4-22)

shows the XRD measurement result of TiO2 nanotubes formed before and after

annealing. Before annealing, it can be seen that the Titania is a poly-crystalline

nature, as indicated in Table (4-7) peaks sites that have emerged in the

diffraction pattern and the corresponding phases compared with the (ASTM)

standards, where it shows that before annealing shows us a combination of

phases (Anatase and Brookite).


91

Figure (4-22): XRD pattern of Titania before and after annealing at temperatures

450°C for 3hr on Ti foil substrate.


92

Table (4-7): The XRD results for Titania before annealing

2ӨExp.

[Degree]

d Exp.

[Å]

I/I0Exp.

%

d ASTM.

[Å]

I/I0

ASTM.

%

(hkl) phase Card No.

35.58 2.523 16 2.5785 2.4 (002) Brookite 96-900-4140

38.96 2.312 57

2.2977 38.6 (400) Brookite 96-900-4140

1.3188 53.9 (220) Anatase 96-101-0943

40.70 2.217 100 2.2487 152.3 (202) Brookite 96-900-4140

53.94 1.700 4.5 1.6929 211.5 (230) Brookite 96-900-4140

63.54 1.464 14 1.4624 102.8 (521) Brookite 96-900-4140

71.85 1.314 2.6 1.3142 18.9 (323) Brookite 96-900-4140

76.79 1.241 31 1.2413 1.1 (204) Brookite 96-900-4140

77.20 1.235 19 1.2325 22.6 (031) Anatase 96-101-0943

77.99 1.225 8.7 1.2236 3.5 (513) Brookite 96-900-4140

After annealing at temperatures 450°C for 3hr on Ti foil substrate,

appeared to us a new peaks which present a new phase of the (TiO2) is the

(Rutile) phase most systematic and stability from the phases (Anatase and

Brookite). Because of the heat, disappeared phase (Anatase) due to its

transformation completely into phase (Rutile) and transformation part of the

phases (Brookite) also into (Rutile). Also, the intensity and sharpness of almost

all the peaks increased considerably after annealing and the peaks become


93

narrower, as shown in figure (4-22) and Table (4-8) with the comparison with

(ASTM).

Table (4-8): The XRD results for Titania after annealing at temperatures 450°C for 3hr

on Ti foil substrate.

2ӨExp.

[Degree]

d Exp.

[Å]

I/I0Exp.

%

d ASTM.

[Å]

I/I0

ASTM.

%


34.90 2.570 37 2.508 43 (101) Rutile 96-900-4145

38.31 2.350 100 2.311 6.3 (200) Rutile 96-900-4145

39.90 2.260 82 2.205 17.7 (111) Rutile 96-900-4145

52.86 1.732 48 1.699 48.4 (211) Rutile 96-900-4145

62.80 1.480 24

1.475 1.5 (610) Brookite 96-900-4140

1.493 6.5 (002) Rutile 96-900-4145

71.22 1.324 35 1.313 0.8 (311) Rutile 96-900-4145

76.81 1.241 14 1.241 0.1 (204) Brookite 96-900-4140

77.21 1.235 95 1.210 0.8 (212) Rutile 96-900-4145

78.00 1.225 6 1.223 0.3 (513) Brookite 96-900-4140

Also we took other sample under conditions (0.5wt.%NH4F + 99.5wt.%

glycerol) electrolyte at 15V for 4hr and we had a measurement of (XRD). Figure

(4-23) shows the XRD measurement result of TiO2 nanotube formed before and

after annealing. Before annealing, it can be seen that the Titania is a poly-


94

crystalline nature, as indicated in Table (4-9) peaks sites that have emerged in

the diffraction pattern and the corresponding phases compared with the (ASTM)

standards, where it shows that before annealing shows us a combination of

phases ( Brookite and Anatase) .

Figure (4-23): XRD pattern of Titania before and after annealing at temperatures

530°C for 3hr on Ti foil substrate.


95

Table (4-9): The XRD results for Titania before annealing.

2ӨExp.

[Degree]

d Exp.

[Å]

I/I0Exp.

%

d ASTM.

[Å]

I/I0

ASTM.

%


35.13 2.554 10 2.578 0.2 (002) Brookite 96-900-4140

38.44 2.341 18 2.342 15 (004) Anatase 96-101-0943

40.16 2.245 100 2.248 15 (202) Brookite 96-900-4140

52.90 1.731 29 1.736 0.02 (222) Brookite 96-900-4140

62.80 1.480 25 1.475 1.5 (610) Brookite 96-900-4140

70.59 1.334 51 1.335 0.1 (413) Brookite 96-900-4140

76.87 1.240 9.5 1.241 0.1 (204) Brookite 96-900-4140

77.20 1.236 8.2

1.238 2 (133) Brookite 96-900-4140

1.232 2.2 (031) Anatase 96-101-0943

77.98 1.225 6.6 1.224 0.35 (513) Brookite 96-900-4140

After annealing at temperatures 530°C for 3hr on Ti foil substrate,

appeared to us a new peaks which present a new phase of the (TiO2) is the

(Rutile) phase most systematic and stability from the phases (Anatase and

Brookite). Appearance of new peaks is result of transformation of phases

(Anatase and Brookite) to phase (Rutile) with heat and the appearance others

new peaks for the (Anatase and Brookite), as shown in figure (4-23). However,

these new peaks that appeared stronger than the peaks that appeared after

annealing at 450 ºC, as in the Table (4-10) with the comparison with (ASTM).


96

Table (4-10): The XRD results for Titania after annealing at temperatures 530°C for 3hr

on Ti foil substrate.

2ӨExp.

[Degree]

d Exp

[Å]

I/I0Exp

.

%

d ASTM.

[Å]

I/I0

ASTM.

%


36.13 2.486 8

2.483 23 (102) Brookite 96-900-4140

2.508 44 (101) Rutile 96-900-4145

39.30 2.292 2.5

2.297 4 (400) Brookite 96-900-4140

2.311 6 (200) Rutile 96-900-4145

2.298 7.5 (112) Anatase 96-101-0943

41.01 2.201 100 2.205 17 (111) Rutile 96-900-4145

53.81 1.703 37 1.699 48 (211) Rutile 96-900-4145

63.69 1.461 9.5

1.462 10 (521) Brookite 96-900-4140

1.462 6 (130) Rutile 96-900-4145

1.674 16 (015) Anatase 96-101-0943

71.50 1.319 91

1.320 2.7 (041) Brookite 96-900-4140

1.319 5.3 (220) Anatase 96-101-0943

76.89 1.239 27

1.238 2 (133) Brookite 96-900-4140

1.2460 83.4 (125) Anatase 96-101-0943

78.31 1.221 8.5

1.2236 3.5 (513) Brookite 96-900-4140

1.2104 8.7 (212) Rutile 96-900-4145


97

4-3-3- Results of thickness measurement

All Titania thickness measurement by optical interferometer method and

some are measured by (SEM). The results were close as show in Table (4-11).

Table (4-11) Titania thickness measurements by optical interferometer method

Using electrolyte (NH4F + glycerol) without water

Item NH4F wt.

%

Glycerol

wt. %

Voltage

(V)

Time

(hr.)

Thickness (nm

×103)

Conductivity

µ Siemens / cm

1 0.5 99.5

5

1 1.4

280

2 2 1.76

3 3 1.22

4 10

2 1.86

5 3 2.3

6 4 3.6

7

15

1 2.13

8 2 2.77

9 4 2.21

10 4 2.75

310

11 25 2 1.94

12 3 1.66

13 4 1.32

14

40

1 2.22

15 2 2.63

16 4 2.84

17 1

99

15

1 1.73

1085

18 4 1.55

19 1.5 98.5 2 2.43

1335

20 4 2.71

21 2 98 2 1.4

1600


98

Using electrolyte (NH4F + glycerol) with water

Item NH4F wt.

%

Glycerol

wt. %

Water

wt. %

Voltage

(V)

Time

(hr.)

Thickness

(nm ×103 )

Conductivity

µ Siemens / cm

1

0.5

97.5 2

15

2 1.85 542

2 4 2. 86

3 94.5

5 2 1.76 740

4 4 3.12

From results we observe that thickness of the layer (TiO2) nanotubes

depend on the time and applied voltage, this means increasing the thickness of

layer (TiO2) nanotubes increasing time and applied voltage and this agrees with

the result in a similar work [56]

.

Either the effect of the other parameters on the layer (TiO2) nanotubes, we

can recognize a slight increase in the film thickness appeared.


99


111

Chapter Five

Conclusions and Future Work

5-1- Conclusions and Perspectives

The focus of the present study is to investigate the interaction of different

anodization parameters and the morphology of TiO2 nanotubes, introduce a

fabrication of TiO2 nanotubes grown on Ti substrate. Titania nanotubes were

successfully prepared by anodization method in organic based electrolytes

(glycerol based electrolytes). The summarized results from this work are the

following:

1. Adding water (2 and 5wt. %) to the electrolyte (NH4F + glycerol) led to

formation of less homogenized TiO2 nanotube which wider diameter.

2. Main functional for Fluoride ions in the process of anodizing is

the etching and pores formed to the tubes that grow on a regular basis,

also increasing of Fluoride concentration affects increase the diameter of

the pores, the wall thickness and tube length, but this increase is not

large compared to other factors.

3. Increasing the applied voltage increases the pores diameter and

significantly increases the thickness of Titania layer and changing the

voltage change colors of oxide formed on foil Titanium.

4. The optimal conditions for TiO2 formation was found at 15V for 4hr with

0.5wt.% NH4F due to best results for diameter, wall thickness, length and

more homogenized of TiO2 nanotubes.

5. Length of the tube increases with increasing anodizing time significantly,

with the longer anodizing time whenever we get a longer tube.

6. All the TiO2 nanotube layers synthesized in this work have an unstable

structure (Anatase and Brookite) phases. This unstable structure can be


111

converted to a crystalline structure more stable (Rutile) phase by

annealing.

5-2- Suggestions for Future Research

The present work can be extended to include the following suggested subject:

1. Application of Titania nanotubes such as developing new solar cell and

chemical sensors.

2. Using electrolyte contenting (1, 1.5,2 wt.%NH4F ) with different amount

of water at different applied voltage.

3. Preparation of Titania nanotubes via aqueous electrolyte such HF.

4. Preparation of separated TiO2 nanotubes.


112


113

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115

Curriculum Vitae (Bibliography)

Haidar H. Hamdan Name

10-4-1984 / Baghdad - IRAQ Born

male SEX

single Marriage status

Assistant lecturer address

009647712999836 mobile

Baghdad University, College of Science, Department of Physics. Work Address

[email protected] E-mail address

Education

2007 B.Sc. (Honors) in physics, Al -Mustansiriyah University, College of

Science, Department of Physics.

2012 M.Sc. (Honors) in physics, Baghdad University, College of

Science, Department of Physics.

Professional Experience

2003- 2007 Under graduate studies at Al- Mustansiriyah University,

College of Science, Department of Physics.

mailto:[email protected]


116

2010- 2012 Graduate studies at Baghdad University, College of

Science, Department of Physics, M.Sc. Thesis (Fabrication of

TiO2 Nanotubes Using Electrochemical Anodization)

Professional interest

Solar cell fabrication and characterization

Semiconductors science (thin film, characterization, application,

device, etc.)

Gas sensors

Nanotechnology science

Publications

1. Effect of Irradiation time on Optical Characteristics of Indium

Oxide Thin Films (Proceedings of the 4th International Scientific

Conference of Salahaddin University-Erbil, October 18-20, 2011 Erbil,

Kurdistan, Iraq)

2. Preparation and characterization of p-Ag2O/n-Si Heterojunction

devices produced by rapid thermal oxidation (Proceedings at Clean

Energy Solutions for Sustainable Environment February 16-19, 2012 –

Beirut, Lebanon)

3. Preparation and characterization of MIS device for optoelectronic

Application (Proceedings at The 2nd International Conference on Renewable

Energy: Generation and Applications March 4-7, 2012 United Arab Emirates

University, Al Ain, UAE)


117

4. Palladium–Doped SnO2 Nanostructure Thin Film Prepared Using

SnCl4 Precursor for Gas Sensor Application (Proceedings of the 4th

International Conference on Nanostructures (ICNS4) 12-14 March 2012,

Kish Island, I.R. Iran).