a thesis submitted in fulfillment of the requirements for...

ANODIC OXIDATION OF TITANIUM IN SULPHURIC ACID (H2SO4) FOR BIOMEDICAL

APPLICATION

NOOR HAFIZA MOHD IDRUS

A thesis submitted in fulfillment of the requirements for the award of the Degree of

Master of Mechanical Engineering

Faculty of Mechanical and Manufacturing Engineering

Universiti Tun Hussein Onn Malaysia

December 2013

1

ABSTRACT

Titanium (Ti) is widely used in the biomedical field because it is light, strong and

biocompatible. However, Ti is non-bioactive and does not show any apatite forming

ability after being implanted into the body. Anodic oxidation is one of the traditional

methods to modify the surface of Ti into titanium oxide (TiO2), thereby enhancing

the biocompatiblity properties. Anodisation was done by supplying direct current

(d.c) through ananode and a cathode consisting of Ti substrate soaked in the

electrolyte of strong sulphuric acid solution (H2SO4) with pH ranged from 0.3 to 2.1.

Different voltages ranged from 10 to 300 V were applied to investigate its effect on

the TiO2 formation whereas the effect of H2SO4 concentrations on TiO2 production

was studied by using three different concentrations of 0.5 M, 1.5 M and 3.0 M,

respectively. Characterisation was made in terms of microstructure using field

emission scanning electron microscope (FESEM), mineralogy using x-ray diffraction

(XRD) whereas the sample color were captured using a digital camera. In vitro test

was also conducted on anodised Ti by soaking the samples in simulated body fluid

(SBF) for seven days to study the growth of hydroxyapatite (HAP). HAP growth can

be enhanced by ultraviolet (UV) irradiation. An alternative group of in vitro samples

were exposed to UV and the HAP growth rate was compared with those unexposed.

The porosity of TiO2 increased as higher voltage and H2SO4 concentration was used.

Though variable in shape, the porosity of TiO2 was found to be directly proportional

to the applied voltage and electrolyte concentration. The XRD analysis showed that

the anatase phase started at a lower voltage in higher electrolyte concentrations.

Increasing voltage contributed to more anatase formation and also induced rutile

phase. The HAP growth was enhanced by UV irradiation in the in vitro test as HAP

grew more and appeared denser on the surface of TiO2 where UV light was used as

compared to the SBF immersion in dark condition.

1

TABLE OF CONTENTS

PAGE

DECLARATION i

DEDICATION ii

ACKNOWLEDGEMENT iii

ABSTRACT iv

ABSTRAK v

TABLE OF CONTENTS vi

LIST OF TABLES ix

LIST OF FIGURES xi

CHAPTER 1 INTRODUCTION

1.1 Background 1

1.2 Problem Statement 2

1.3 Objectives 3

1.4 Scope of Study 3

CHAPTER 2 LITERATURE REVIEW

2.1 Introduction 4

2.2 Titanium and Titanium Alloy 5

2.2.1 Titanium Properties 6

2

2.2.2 Uses of Titanium and Titanium Alloy In Biomedical Application 8

2.3 Titanium Dioxide (TiO2) 112.3.1 TiO2 and Application 13

2.4 Ultra Violet (UV) Light 15

2.4.1 Reaction TiO2 with UV light 16

2.5 Oxidation 18

2.5.1 Anodic Oxidation 202.5.2 Anodic Oxidation Parameters 21

2.5.3 Applied Voltage 22

2.5.4 Time of Oxidation 26

2.5.5 H2SO4 Electrolyte 28

2.5.6 Concentration of Electrolyte 30

2.6 Biological Test (In Vitro) 31

2.6.1 Simulated Body Fluid (SBF) 32

2.6.2 Hydroxyapatite Formation 34

2.7 Effect of UV in SBF 36

CHAPTER 3 METHODOLOGY

3.1 Introduction 41

3.2 Anodic Oxidation 43

3.2.1 Preparation of Samples 443.2.2 Acid Dilution 45

3.3 Color, Morphology, and Mineralogy

Analysis 46

3.3.1 Color Analysis 473.3.2 Morphology Analysis (FESEM) 473.3.3 Phase Analysis (XRD) 48

3.4 In Vitro Test (SBF) 48

3.4.1 SBF under UV Light 53

CHAPTER 4 RESULTS AND DISCUSSION

4.1 Introduction 54

3

4.2 Colouration of Anodised Ti 554.2.1 Effect of Applied Voltage 564.2.2 Effect of H2SO4 Concentration 57

on TiO2 Colouration4.3 Microstructure of Anodised Sample 58

(TiO2)4.3.1 TiO2 Microstructure Development 62

4.4 Mineralogy of TiO2 634.4.1 Effect of Applied Voltage 634.4.2 Effect of H2SO4 Concentration 73

on TiO2 Phase Formation4.5 Biological Test (In Vitro) 754.5.1 In Vitro in SBF with and 75

without UV4.5.2 Mechanism of HAP Formation 88

and TiO2 Distribution4.5.3 Element Analysis of Anodised Ti 90

after SBF Test

CHAPTER 5 CONCLUSION AND RECOMMENDATION

5.1 Conclusion 99

5.2 Recommendation 100

REFERENCES 101

APPENDICES Appendix A: Details of Titanium and Chemicals

Appendix B: XRD Graph

Appendix C: EDS

Appendix D: Papers Published

4

LIST OF TABLES

PAGE

2.1 Basic Properties of Titanium (Lui et al., 2004) 7

2.2 Uses of titanium and its alloy in biomedical 8

application (Liu et al., 2004)

2.3 Refractive index, density, and crystal 11

structure of TiO2 (Carp et al., 2004)

2.4 Basic and historical information on TiO2 12

(Carp et al., 2004)

2.5 Summary of previous study on photocatalysis activity 18

of anodic oxidation TiO2 film by degrading methyl

orange (Wang et al., 2006)

2.6 Result summary of previous study on anodic 22oxidation using 1 M H2SO4 with different voltages(Yang et al., 2004)

2.7 TiO2 phase structures appear by different voltage applied 23

(Yang et al., 2004)

2.8 Surface morphology evolution on anodised cp Ti 23

(Ou et al., 2010)

2.9 Five stages of voltage effecting TiO2 microstructure 24

(Abdullah and Sorrell, 2010)

2.10 Effect of anodising time to pore diameter of TiO2 cell 26Structure (Oh et al., 2005)

2.11 Effect of time and voltage on TiO2 structure 26 (Abdullah and Sorrell, 2007)

2.12 Types of acid solution electrolyte 29

2.13 Effect of different type of electrolyte on anodised Ti 29

(Song et al., 2008)

5

2.14 The ion concentration of simulated body fluid and human 33

blood plasma (Kokubo and Takadama, 2006)

3.1 Anodic oxidation parameter 433.2 Materials and equipment necessary for anodic 43

oxidation experiment.3.3 Materials and apparatus necessary for SBF test 483.4 Reagent used to prepare SBF 504.1 Effect of voltage on TiO2 coloration 564.2 Effect of H2SO4 concentration on TiO2 colouration 574.3 Result of TiO2 surface microstructure based on FESEM 60

micro image4.4 Effect of anodizing voltage and electrolyte concentration 61

on TiO2 pore diameter4.5 ICDD number for XRD data 634.6 Mineralogy of Ti anodised in 0.5 M H2SO4 as function 64

of applied voltage4.7 Mineralogy of Ti anodised in 1.5 M H2SO4 as function 67

of applied voltage.4.8 Mineralogy of Ti anodised in 3.0 M H2SO4 as function 70

of applied voltage4.9 Comparison of phase formation between different H2SO4 73

concentration (0.5, 1.5 and 3.0 M).4.10 Summary of TiO2 microstructure observations

for Ti anodised 76in 0.5 M H2SO4 after soaking in SBF for 7 days with and without UV irradiation.

4.11 Summary of TiO2 microstructure observations


4.12 Summary of TiO2 microstructure observations


LIST OF FIGURES

PAGE

6

2.1 Jewellery made by titanium (Bartlett, 2005). 6

2.2 The artificial hip joint (Liu et al., 2004). 9

2.3 Bone screw and bone plate made from Ti 9

(Liu et al., 2004).

2.4 Artificial tooth (Liu et al., 2004). 10

2.5 Artificial heart valve (Liu et al., 2004). 10

2.6 Rutile form of titanium oxide. 11

2.7 Crystal structure of: (a) anatase, (b) rutile and 12(c) brookite (Carp et al., 2004).

2.8 Solar cell screen printing machine. 13

2.9 Tent material without any coating (left) compared 14

with TiO2 coated tent material (right)

(Hashimoto et al., 2005).

2.10 Conventional side mirror (left) compared with anti 15fogging side mirror coated with TiO2 (right) (Hashimoto et al., 2005).

2.11 Visible light and ultraviolet wavelength. 16

2.12 Photoinduced phenomena of TiO2 (Carp et al., 2004). 172.13 Schematic diagram of anodic oxidation process. 212.14 SEM micrograph of (A) Ti without any treatment and 24

anodised Ti at (B) 90 V, (C) 155 V and (D) 180 V (Yang et al., 2006).

2.15 Two types of bone structure; cortical bone (left) and 25trabecular bone (right) (Ott, 2004).

2.16 TiO2 thickness variation with anodising time 27

(Abdullah and Sorrell, 2007).

2.17 SEM micrograph of anodised Ti at 155 V in H2SO4 30concentration of (A) 0.5 M, (B) 1.0 M and (C) 3.0 M (Yang et al., 2004).

2.18 Ion migration during anodisation. 31

2.19 FESEM micrograph of hydroxyapatite precipitation 34on Ti surface anodized in H2SO4 at different voltages after soaked in SBF for 1 day and 7 days (Cui et al., 2009).

2.20 Apatite formation mechanism (Kim et al., 2003). 352.21 Hydroxyapatite precipitation during SBF immersion 37

under dark (a-d) and UV irradiation (e-h) conditions (Ueda et al., 2009).

7

2.22 Surface morphology of anodised Ti (a,d) before

38immersed in SBF, (b,e) after immersed in SBF for 2 days under dark condition, and (c,f) after immersed in SBF under UV irradiation.

2.23 SEM micrograph of CHT specimens after soaking 39in SBF; (a) specimen expose to UV in air; (b) specimen expose to UV in water condition (Uetsuki et al., 2010).

2.24 FESEM images of the unpolished surfaces of Ti 40substrates treated with NaOH and oxidised at 400 °C, 600 °C, and 800 °C, followed by soaking in SBF without and with UV irradiation for 1 day. (Abdullah and Sorrell, 2012).

3.1 Methodology flow chart. 42

3.2 Schematic diagram of anodic oxidation apparatus. 44

3.3 SBF preparation apparatus. 51

3.4 pH meter for measuring pH and temperature of SBF. 51

3.5 Schematic diagram of in vitro test of TiO2. 52

3.6 Schematic diagram of UV irradiation of anodised Ti 53

during SBF immersion.

4.1 TiO2 colours as function of applied voltage and electrolyte 55

concentration.

4.2 FESEM micro-images of Ti surface after being anodised 58

at different voltage (20 – 100 V) and in different

electrolyte concentrations (0.5 – 3.0 M).

4.3 FESEM micro-images of Ti surface after being anodized 59

at different voltage (120 – 200 V) and in different

electrolyte concentrations (0.5 – 3.0 M).

4.4 Schematic diagram of TiO2 microstructure development. 62

4.5 XRD pattern of Ti anodised in 0.5 M H2SO4 at low 65

(20 – 40 V) and intermediate (60 – 100 V) voltage. 4.6 XRD pattern of Ti anodised in 0.5 M H2SO4 at high 66

(120 – 200 V) voltage.


(20 – 40 V) and intermediate (60 – 100 V) voltage.4.8 XRD pattern of Ti anodised in 1.5 M H2SO4 at high 69

(120 – 200 V) voltage.


8

(20 – 40 V) and intermediate (60 – 100 V) voltage.4.10 XRD pattern of Ti anodised in 3.0 M H2SO4 at high 72

(120 – 200 V) voltage.4.11 Ion concentration in different H2SO4 molar. 74

4.12 FESEM micro-images of Ti anodised in 0.5 M H2SO4

77

at 20 V, 60 V and 80 V, followed by soaking in SBF

without and with UV irradiation for 7 days.


78

at 100 V and 140 V, followed by soaking in SBF



79




81




82




83



4.18 FESEM micro-images of Ti anodised in 3.0 M H2SO4 85







9



4.21 Effect of H2SO4 concentrations on HAP formation and 88

distribution.

4.22 Schematic diagram of hydroxyapatite formation mechanism 89

in this study as modified from work by Kim et al., 2003

and Ueda et al., 2009.

4.23 EDS result of sample anodised at 160 V in 1.5 M H2SO4 90

after soaked in SBF without UV.

4.24 EDS result of sample anodised at 200 V in 3.0 M H2SO4 91

after soaked in SBF with UV.

4.25 Ca element of hydroxyapatite on Ti samples anodised 92

in 0.5 M H2SO4 after in vitro test with and without UV.

4.26 P element of hydroxyapatite on Ti samples anodised 93










4.31 Schematic diagram of HAP formation under UV

irradiation as modified from work by Ueda et al., 2009. 97

1

CHAPTER 1

INTRODUCTION

1.1 Background

Anodic oxidation is a process of producing and increasing the natural oxide layer on

a metal substrate (Abdullah and Sorrell, 2007). It is traditional method used to

modify surface and properties of metal substrate. The process involved direct current

flow through the electrolytic solution while the substrates immersed in the solution.

The voltage of direct current used in anodic oxidation in various solutions usually

may range from 1 to 300 V. It can perform in acid solution and alkaline solution

either, but acid solution (H2SO4) usually being used because it result to thicker

titanium dioxide layer (Diamanti and Pedeferri, 2006). However the thickness may

vary depends on parameters used such as voltage, concentrations of acid and time.

The application of anodic oxidation contributes to the development of

biomaterial used in medical applications. Anodic oxidation is used to modify the

surface structure and properties of titanium and titanium alloys to form titanium

dioxide in order to enhance biocompatibility and long term stability of implant

grafted onto bone. Anodisation was employed to produce calcium phosphate coating

in metallic implant. Titanium dioxide been used in medical application such dental

implant, artificial medical implant and bone replacement (Liu et al., 2004). Ultra

violet also knows as UV light is used to enhance the hydroxyapatite on TiO2 surface.

2

1.2 Problem Statement

Sintered hydroxyapatite and glass ceramic are bioactive materials which form

bioactive bonding with the living bone after they are implanted in bone site. Their

fracture resistance however, not enough to replace bones under load bearing

condition (Yang et al., 2004). So, at the time being, metals such as stainless steel,

Co-Cr alloys, titanium and titanium alloy are used for this purpose due to their high

fracture toughness. These metal however are non bioactive and they show no apatite

forming ability after being implanted into bone.

These metal will undergoes corrosion process which it leads to form oxides

layer or hydroxide when exposing to the atmosphere and reacts with the present of

water vapour and air. This phenomenon can actually protect the surface for further

alterations. By using anodic oxidation, titanium surface will be modified where the

oxide and properties can be controlled for desired biomedical application. This study

is tended to improve the biocompatibility of titanium and to produce different surface

on titanium substrate.

The In Vitro test in SBF can predict the growth of HAP on anodised Ti after

implanted. TiO2 also is one of photocatalyst candidate which can react with UV light

to enhance some chemical reaction including HAP formation. The SBF are done

under UV light to enhance the reaction and formation of HAP on anodised Ti.

3

1.3 Objectives

The Objectives of this research are:

1. To produce titanium dioxide (TiO2) on titanium (Ti) substarte using anodic

oxidation.2. To investigate the effect of applied voltage and H2SO4 electrolyte

concentration on morphology and mineralogy of TiO2.3. To assess the in vitro bioactivity of the TiO2 layer in simulated body fluid

(SBF).4. To observe the effect of ultra violet (UV) irradiation on hydroxyapatite

precipitation on anodised Ti.

1.4 Scope of study

1. Preparation of TiO2 on Ti substrate using anodic oxidation method at different

applied voltage (10 - 200 V).2. Acid sulphuric solution (H2SO4) is use as electrolyte at different concentration

(0.5, 1.5, and 3.0 M).3. Biological test was done on anodised titanium using simulated body fluid

(SBF) with and without UV light for 7 days.4. The characterisation of TiO2 will be performed using:

a) X-ray diffraction (XRD - mineralogyb) Field emission scanning electron microscope (FESEM) -

microstructurec) Digital camera – colour

d) Element Dispersive Spectroscopy EDS – HAP element (Calcium and

Phosphorus).

1

CHAPTER 2

LITERATURE REVIEW

2.1 Introduction

Titanium (Ti) is widely used as biomedical implant especially in orthopedic and

dental implant because it has excellent mechanical properties and biocompatibility.

Titanium dioxide (TiO2) produced by several surface modification techniques helps

to improve the bonding between natural bone and implant. Anodic oxidation

technique performed on Ti substrate was reported to produce stronger adhesion

between the oxide and substrate (Schvezov et al., 2010 and Anpo et al., 2005). The

parameter used during anodic oxidation such as applied voltage, types and

concentration of electrolyte, anodisation time, and current density determine the

microstructure and crystalline composition of TiO2 coating obtained by this method.

Simulated body fluid (SBF) test is done to obtain and evaluate in vitro result, thus in

vivo can be predicted. By immersing anodised Ti in SBF for several days,

hydroxyapatite (HAP) formations are observed and the mechanisms are discussed

based on the result of different anodising parameter. An approach of irradiating UV

light during SBF immersion had been reported to induce apatite forming ability. By

exposing to UV light, a better result might be obtained with more HAP growth on

TiO2 surface. A review on previous studies and some fundamental on each topic are

discussed.

2

2.2 Titanium and Titanium Alloy

The history of titanium was started as early as 1970 by William Gregor who had

discovered titanium but was named by Martin Heinrich Klaproth in 1795, referring to

the mythological first sons of the earth, Titans (Liu et al., 2004). The two most useful

properties of titanium in metal form are corrosion resistance and the highest strength-

to-weight ratio of any metal. In its unalloyed condition, titanium is as strong as some

steels and 45% lighter which make it suitable to be used as medical implant for

biomedical applications.

Due to its lightweight properties, Ti and its alloy have been considered for

variety of applications to high speed commercial aircraft structure (Brewer et al.,

1988). The high specific strength, unique corrosion resistance and non magnetic

properties of titanium resulted in wide application of titanium in chemical and ship

building industries. Over 35 years, titanium has been used in creating jewellery.

Interesting and unusual jewellery was created by colouring titanium using heating or

anodising method (Bartlett, 2005). Figure 2.1 shows some interesting jewellery made

by titanium metal since 1982.

Titanium and titanium alloys are widely used in biomedical devices and

components, especially as hard tissue replacements such as in cardiac and

cardiovascular applications because of their desirable properties; relatively low

modulus, good fatigue strength, formability, machinability, corrosion resistance, and

biocompatibility (Donachie, 1988).

Titanium as per other metal with exception of gold naturally have a thin layer

of oxides (1.5-10 nm) form on their surface because of the reaction with aqueous

vapour present in the air (Diamanti et al., 2006; Kuromoto et al., 2007). Further

alteration is restricted by this thin layer oxide on titanium surface. Titanium alloys

cannot meet all of the clinical requirements for applications and so has to be

modified in order to improve its biological properties. Surface modification

techniques such as plasma spraying, blasting, etching, and sintering, anodic plasma-

chemical treatment, anodic oxidation under galvanostatic and potentiostatic mode as

well as ion implantation are often performed to obtain rougher surface, thereby

expanding the use of titanium and its alloys in the biomedical field (Kuromoto et al.,

2007).

3

Figure 2.1: Jewellery made by titanium (Bartlett, 2005).

2.2.1 Titanium Properties

Titanium is a transition element which means it is a chemical element with valence

electrons in two shells instead of only one. Titanium belongs in Group IV of Period 4

in the periodic table.

It has an atomic number of 22 and atomic weight of 47.9. Titanium had been

widely used because of its most useful properties such as corrosion resistance and

4

highest strength to weight ratio compared to other metals. Some other useful

properties are lower modulus, low density, lightweight, strong, and superior

biocompatibility (Liu et al., 2004). Table 2.1 shows some basic physical properties of

unalloyed or pure titanium.

Table 2.1: Basic Properties of Titanium (Liu et al., 2004).

Property ValueAtomic numberAtomic weight (g/mol)Density (g cm-3)Thermal conductivity (W/(m K))Melting temperature ( o C)Boiling temperature (estimated) ( o C)Transformation temperature ( o C)Modulus of elasticity (GPa)Yield strength (MPa)Ultimate strength (MPa)

2247.94.5419.216683260882.5105692785

The low Young modulus of Ti alloys is one of most crucial properties

believed to reduce stress shielding effect whereas among Ti alloys, β-type Ti alloys is

ideal for biomedical application because it has the lowest Young’s modulus of

various Ti alloys. β-Ti can be stabilized by addition of niobium, molybdenum and

vanadium. Ou et al. have concluded from their study that anodising behavior of Ti-

30N-1Fe-1Hf alloy composed of CA and β-GP was similar to that of commercial

pure Ti (Ou et al., 2011).

2.2.2 Uses of Titanium and Titanium Alloy in Biomedical Application

5

Hard tissues especially in orthopedics and dentals are often damaged due to

accidents, aging, diseases and other causes. In biomedical field, surgically

substituting the damaged hard tissues with artificial replacements is often practiced

(Liu et al., 2004). The titanium dioxide resulting from different technique of

surface modification process is biocompatible to be implemented as artificial

body replacement since the properties of titanium surface has been modified

to be bioactive to react with body fluid.

Titanium and titanium alloy is widely used as artificial implant such as hip

joint implant, artificial tooth, and artificial heart valve and even for osteosynthesis

implant (Liu et al., 2004). The uses of titanium and its alloy in biomedical

application are summarised in Table 2.2.

Table 2.2: Uses of titanium and its alloy in biomedical application (Liu et al., 2004).

Field Application FigureOrthopedics 1. Total hip joint

replacement (femoral head, cup, and stem).

2. Bone screws.3. Bone plate.

Figure 2.2

Figure 2.3

Dental implant Three types:1. Subsperiosteal2. Prosthesis3. Transosteal

Figure 2.4

Cardiovascular Artificial heart valve Figure 2.5

6

Figure 2.2: The artificial hip joint (Liu et al., 2004).

Figure 2.3: Bone screw and bone platemade from Ti (Liu et al., 2004).

(TiO2)

7

Figure 2.4: Artificial tooth (Liu et al., 2004).

Figure 2.5: Artificial heart valve (Liu et al., 2004).

Ti

TiO2

Ti

8

2.3 Titanium Dioxide (TiO2)

About 95% of titanium ore extracted from the Earth is destined for refinement into

titanium dioxide. Titanium dioxide, also known as titanium (IV) oxide or titania, is

the naturally occurring oxide of titanium. Naturally TiO2 exists in three crystalline

phase, anatase and rutile are stable phases while brookite is non-stable (Goresy et al.,

2001, and Kuromoto et al., 2007). From the result of biological evaluation in

simulated body fluid (SBF), the anodic TiO2 substrateshowed bioactive activity.

Optical properties of titanium dioxide are summarised in Table 2.3, while

some basic and historical information are given in Table 2.4. Rutile form of titanium

dioxide is shown in Figure 2.6 whereas the crystal structure of TiO2 can be seen in

Figure 2.7.

Table 2.3: Refractive index, density, and crystal structure of TiO2

(Carp et al., 2004)

Phase

RefractiveIndex

(grand/petit)

Density(kg.m-3)

SystemStructure

ng np

Anatase 2.5688

2.6584

3830 Tetragonal

Rutile 2.9467

2.6506

4240 Tetragonal

Brookite 2.8090

2.6770

4170 Rhombohedral

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

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

9

Figure 2.6: Rutile form of titanium dioxide.

Table 2.4: Basic and historical information on TiO2(Carp et al., 2004)

TiO2 Phase Discovery history ColourAssociateminerals

Rutile - Discovered by Werner in Spain (1803)

- Its name derived from Latin word, rutilus meaning red, referring to red deep colour observed in some specimens during transmitted light view.

- Reddish brown- Sometimes

yellowish, bluish or violet, being transparent to opaque.

- Quartz- Chromium- Barite- Hematite- Silicates

Brookite - Discovered by A. Levy at Snowen (Pays De Gales, England) (1825)

- Named in honour of English mineralogist, H.J. Brook.

- Dark brown to greenish black opaque

- Anatase- Rutile- Feldspar- Chalcopyrite- Hematite- Sphene

Anatase - Named by R.J Hauy from the Greek word, anatasis means extension due to itslonger vertical axis compare to rutile

- Black, reddish brown, yellowish brown, dark blue, gray.

- Rock crystal- Fledspar- Axinite in

cravices in granite

Figure 2.7: Crystal structure of: (a) anatase, (b) rutile and (c) brookite(Carp et al., 2004).

2.3.1 TiO2 and Application

10

TiO2 is widely used as a white pigment in paint with total production of 51%, 19% in

plastic, and 17% in paper. Furthermore, in the last few years, the usage of TiO2 as

white pigment has increased in some sectors including textile, food, leather, and

pharmaceuticals (Carp et al., 2004) due to some reasons such as high refractive

index, it can be produced at high purity and the appropriate particle range (Abdullah

et al., 2007).TiO2 powder has been used to fabricate screen printing paste for dye

sensitised solar cells (Ito et al., 2007). Dye sensitised solar cell has commercial

interest as low cost alternatives to conventional solid state devices. Figure 2.8 shows

a solar cell screen printing machine.

Figure 2.8: Solar cell screen printing machine.

TiO2 coating material has a self-cleaning function and is anti-fogging due to its

photo catalytic ability which induces its superhydrophilicity. Furthermore, TiO2

coated materials has been commercialized as photo catalytic building materials such

as exterior tiles, glass, aluminium walls, and PVC fabric as shown in Figure 2.9

(Hashimoto et al., 2005). For example, Chubu International Airport in Japan has

11

been installed with photo catalytic exterior glass. Stains on TiO2 coated material or

building can easily washed by water or rainfall.

Figure 2.9: Tent material without any coating (left) compared with TiO2 coated tent material (right) (Hashimoto et al., 2005).

Water droplets occur on mirror or glass area during fogging weather when the

haze condensate. On TiO2 coated glass, no water droplets are forms because the

surface is highly hydrophilic but thin film of water is formed in replacement,

preventing fogging and resulting in clearer vision through the glass surface

(Hashimoto et al., 2005). Moreover, Japan automobile industry has installed anti

fogging side mirror to their cars (Figure 2.10).

Figure 2.10: Conventional side mirror (left) compared with anti fogging side mirror coated with TiO2 (right) (Hashimoto et al., 2005).

12

2.4 Ultra Violet (UV) Light

In our solar system, the sun emits electromagnetic radiation of different energy or

wavelengths. Some of the radiation is visible and can be seen by human eyes such as

the radiation that makeup colours of rainbows. The radiation which cannot be seen

by human eyes, beyond the violet or end of visible light is called ultraviolet light.

The wavelength of UV light spans the range between 200 and 400 nm. It is divided

into three groups (Jiang et al., 2009):

1. UVA (320–400 nm)2. UVB (290–320 nm)3. UVC (200 – 290 nm)

UV light that penetrates our skin is made up of two type of radiation which is UVA

and UVB. UVA is the less energetic portion of the UV spectrum. Before reaching to

ground level, it is only absorbed slightly by ozone layer. Some studies showed that

UVA is hazardous because it causes skin damage, pre mature photo aging, wrinkling,

and also eye damage (Heisler and Grant, 2000; Kimlin et al., 2002).

UVB comprises the most energetic wavelengths and it reaches the ground

level even though it is strongly absorbed by stratospheric ozone. UVB has harmful

effects in some biological systems such as disrupted proteins (DNA), causes skin

sunburn, skin cancer and eye cataracts in humans, loss of productivity and other

harsh effects in plants (Setlow, 1974; WHO, 2002; Parisi et al., 2007). However,

UVB can also be beneficial. It can initiate the production of vitamin D that helps

build and maintain human bone and it could prevent certain types of cancer (Turnbull

et al., 2005).

UVC is the high energetic portion of the UV spectrum and it is all absorbed

by ozone layer and others atmospheric constituents and does not reach the earth

surface (Jacovides et al., 2012). Figure 2.11 illustrates visible light, UVA and UVB

wavelength.

13

Figure 2.11: Visible light and ultraviolet wavelength.

2.4.1 Reaction TiO2 with UV Light

Photo catalysis is a reaction in which light is used to activate a substance which

modifies the rate of a chemical reaction without being involved itself. When photons

have higher energy, the band would gap as an electron is promoted to conduction

band leaving the hole in valence band and the excited electron can be used to derive

a chemical reaction (Carp et al., 2004). It is anatase and rutile with band gap value of

3.20eV and 3.02eV which needed by UV light to activate its photo catalytic

performances (Carp et al., 2004; Mills and Hunte, 1997; Diamanti et al., 2011).

Figure 2.12 shows a photo-induced phenomenon.

14

Figure 2.12: Photo-induced phenomena of TiO2 (Carp et al., 2004)).

TiO2 is an ideal photo catalyst option to be used in industry due to its useful

properties such as highest stability, low cost, virulence, anti-photo corruption, high

photo catalysis, and strong oxidation ability (Hashimoto et al., 2005; Wang et al.,

2007). TiO2 can induce and accelerate some chemical reaction when exposed to UV

light irradiation (Hashimoto et al., 2005). Anatase is the most active photo catalyst of

the three phase of TiO2. The photo catalytic ability of TiO2 was found to be able to increase the

degradation of hazardous chemical in wastewater or in air (Mantzavinos and

Psillakis, 2004; Nakano et al., 2003; Chiron et al., 2000; Frank and Bard, 1977) such

as endocrine disruptors, injurious volatile compounds, and organic pollutant

(Masahashi et al., 2008). Wang et al., have studied the photo catalytic activity of

porous TiO2 films prepared by anodic oxidation (Wang et al., 2006). Photo catalysis

of TiO2 produced under different condition was studied by degrading the methyl

orange and the result are summarised in Table 2.5.

Table 2.5: Summary of previous study on photo catalysis activity of anodic oxidationTiO2 film by degrading methyl orange (Wang et al., 2006).

Parameter Value/Type Main observation Finding

15

Anodisationvoltage

80- 140 VDegradation rate increased by increasing voltage

-The amount of anatase increased by increasing voltage leads to increase in photo catalysis activity.-Rutile formed at 150V lower the degradation rate.150 V

Worse degradation rate

pH of methylorange

AcidDegradation rate is highest at pH 1

-Hydroxyl in acid has strong oxidation ability which is important factor inphoto catalysis.

Alkaline

Degradation rate lowest at pH 9 and continue to increase from 9

Type ofelectrolyte

HydrofluoricAcid

Low degradation rate

-TiO2 film produced in H2SO4 is thicker than that inHF. Therefore, more TiO2 can facilitate to improve photo catalysis.

Sulphuric AcidHigh degradation rate

2.5 Oxidation

Oxidation is a process in which oxygen molecules interact or combine with a

substance that will change its chemical properties. It is precisely defined as the loss

of electrons in the atoms when two or more substances interact. Commonly,

oxidation is assumed to be bad or harmful such as causing rust or corrosion in

automobiles. The oxide layers build up as surface elements lose their electrons while

the negatively charged elements react with oxygen in air or with water molecules.

Oxidation also brings benefits in some areas, especially in the medical field.

Oxidation is used in surface modification of metal to be implanted in the human

body. In this study, the oxidation process occurs on titanium surface to produce

titanium dioxide for uses biomedical uses. Specifically the method of anodic

oxidation will be implemented on titanium surface modification to improve its

biocompatibility. Examples of a few types of oxidation are listed below:

1. Sol gel oxidation - Hydrolysis and condensation of alkoxide-based

precursors. Sol-gel process can be used to produce a wide range of oxides in

16

various forms, including powders, fibers, coatings and thin films, and porous

membranes.

2. High temperature oxidation - Corrosion process involving the reaction

between the metal and the atmospheric oxygen at elevated temperatures.

3. Micro arc oxidation - An electrochemical surface treatment process for

generating oxide coatings on metals. Similar to anodic oxidation, but it

employs higher potentials, discharges occur and resulting plasma which

modifies the structure of the oxide layer.

4. Thermal oxidation – A process that produces thin layer of oxide, usually

silicon dioxide, on the surface of a wafer. This method forces an oxidizing

agent to diffuse into the water at high temperature and react with it.

5. Anodic oxidation – Electrolytic process used to increase the thickness of the

natural oxide layer on the metal surface.

2.5.1 Anodic Oxidation

Anodic oxidation is a reaction where the atoms in an element lose electrons and the

valence of the element is correspondingly increased or a process in which a

substance combined with oxygen. It is also known by the British as electrolytic

passivation process used to increase the thickness of the natural oxide layer on the

surface of metal parts. It is a well-established method to produce different types of

protective oxide coating on metals. The process is called "anodic" because the part to

be oxidised from the anode electrode of an electrical circuit.

The anodised metal oxide is grown by passing a direct current through an

electrolytic solution, with the metal substrate serving as the anode, the positive

electrode. The current releases hydrogen at the cathode the negative electrode and

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

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

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

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

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

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

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

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

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

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oxygen at the surface of the metal film anode, creating a build-up of metal oxide.

Alternating current and pulsed current is also possible but rarely used. There are

three substrates or metal base commonly been anodise which are aluminium,

titanium and magnesium. In this research, titanium will be anodised.

Anodic oxidation is one of the surface modification techniques which are

necessary to improve chemical bond between Ti substrate with bone tissue. An

alternative of anodic oxidation, plasma spraying hydroxyapatite (HAP) is also

accepted extensively in clinic involving coating of bioactive material (HAP) on Ti

substrate. This method however, has some disadvantages such as weak adhesion

between HAP layer for implantation and plasma spray, high temperature process and

difficulty to control HA layer composition and crystallinity based on desired

application (Ou et al., 2010).

The schematic diagram of anodic oxidation process is shown in Figure 2.13.

Anodic oxidation of titanium metal in simple electrolyte would produce anatase or

rutile on their surfaces by conditioning the anodic oxidation process, which might be

sufficient for preparation of titanium metal with high apatite-forming ability.

Figure 2.13: Schematic diagram of anodic oxidation process.

Power supply

Cathode (-)Anode (+)Electrons flow

Titaniumsubstrate

Oxidelayer Electrolyte

http://en.wikipedia.org/w/index.php?title=Pulsed_current&action=edit&redlink=1

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

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

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2.5.2 Anodic Oxidation Parameters

Process parameters are some aspect to be controlled during the experiment (anodic

oxidation) on titanium substrate to produce titanium dioxide. Titanium dioxide may

present differently in both amorphous and crystalline structures, depending on

process parameters. Crystalline oxides, that is, anatase and rutile, present several

peculiar features, such as photo catalytic behavior, superhydrophilicity and

biocompatible properties (Diamanti and Pedeferri, 2006).

The different parameters applied, will result to different surface and different

thickness of titanium dioxide. The growth behavior of the titanium dioxide also can

be varied over quite a wide range by altering the anodic parameters (Diamanti and

Pedeferri, 2006; Kuromoto et al., 2007), such as:

1. Applied voltage2. Time3. Concentrations of electrolyte4. Current density5. Agitation speed6. Surface area ratios of cathode to anode

2.5.3 Applied Voltage

The different porosity for implant can be obtained by applying different voltages

during anodisation. The amount of voltage applied will somehow affect the result.

The higher the voltage applied, the thicker the surface of titanium dioxide being

produced. Commonly the amount of voltage applied in anodic oxidation may range

from 1 to 300 volt. The low potentials which range between 1 to 130 volt allow the

obtaining of a smooth, amorphous oxide, about 3–100 nm thick (Diamanti and

19

Pedeferri, 2006). The porosity and the pore size increase with the increasing voltage.

The porous region would form due to dielectric breakdown as the film breaks down

locally while the voltage increases. The formation of TiO2 occurs when O2- ions

migrate into the metal-film interface and Ti4+ ions migrate from metal Ti to film-

electrolyte boundary. Larger crates or pores can also be observed at higher voltage

due to pores interconnection with each other (Kuromoto et al., 2006).

Table 2.6: Result summary of previous study on anodic oxidation using 1 M H2SO4 with different voltages (Yang et al., 2004).

H2SO4 concentration(Molar, M)

Voltage range (V) Description

1

90 - 180 Porous surface>105 Spark discharge occurred

90 - 155Porosity and pore size

increase

155 - 180Porosity and structure not

change

Table 2.6 summarises the result of previous study done by Yang et al, on

anodic oxidation using 1 mol H2SO4 with dissimilar voltages (Yang et al., 2004).

They reported pore size increased by increasing in applied voltage. However the

porosity somehow becomes fixed at 155 V. At applied voltage 90V, some other

researchers reported the beginning of anatase formation (Yang et al., 2004; Daimanti

and Pedeferri, 2006). The intensity of anatase increased by increasing voltage up to

155 V and some amount of rutile started to appear (Table 2.7). However, at very high

voltage only rutile appeared on the surface. In another study, Cui et al., reported

anatase phase converted into rutile with the increase of applied voltage from 100 to

150V (Cui et al., 2009).

Table 2.7: TiO2 phase structures appear by different voltage applied(Yang et al., 2004).

Voltage Applied (V) TiO2 Structure90 Anatase155 Anatase and rutile180 Only rutile

20

Table 2.7 shows the phase structure of titanium dioxide, produced by

different voltage applied to anodic oxidation process. The surface morphology

evolution of anodised cp Ti obtained by Ou et al., in their study was summarised in

Table 2.8 (Ou et al., 2010). From their result, obviously TiO2 thickness increased

with increasing voltage (Ou et al., 2010).

Table 2.8: Surface morphology evolution on anodised cp Ti (Ou et al., 2010).

Voltage, (V) Surface morphology

100Scratches formed during mechanical

grounding remained on surface

200Several irregular crates resulted from

sparking were observed

300Surface was completely covered with

crates of 1~2 µm

Figure 2.14: SEM micrograph of (A) Ti without any treatment and anodized Ti at (B)90V, (C) 155V and (D) 180V (Yang et al., 2006).

Figure 2.14 shows SEM micrograph of untreated Ti (Fig 1(A)) and porous

surface of anodised Ti with increasing pore size (Fig 1 (B- D)) when applied voltage

21

was increased. Abdullah and Sorrell have specified the effect of voltage on TiO2

microstructure into five stages as per Table 2.9 (Abdullah and Sorrell, 2010).

Table 2.9: Five stages of voltage effecting TiO2 microstructure(Abdullah and Sorrell, 2010).

Voltage Range Type ofMicrostructure

RelevantPhenomenonType Volts

Very Low 5 Incomplete Onset of film growthLow 10 Dense and smooth Monolayer formation

Intermediate 20 - 70 Dense and unevenIncreasing film

growthHigh 100 Variable porosity Onset of arcing

Very High 150 - 350 Consistent porosity Subsurface arcingThe applied voltage is very important parameter because it is the main

parameter effecting TiO2 structure in anodic oxidation. It is also important to design

bone implant based on its original structure. The bone structure consist of two types

(Figure 2.15); cortical bone which is the outer part of long bone isvery dense and stiff

while trabecular bone- the inner part is soft, foamed structure that is also porous and

filled with osseous medulla (Mano et al., 2004). The density of bone decreases from

outer to core and its structure continuously adapt to stress applied on it. So, the

implant material must be compatible and designed nearly as per original living bone

and concern the surrounding of living tissue. The bone like porous structure can be

implemented on implant by applying different voltage during anodisation.

Increasing voltage

22

Figure 2.15: Two types of bone structure; cortical bone (left) and trabecular bone (right) (Ott, 2004).

2.5.4 Time of Oxidation

Time is another important parameter in anodic oxidation process. The electrolyte

might need certain amount of time to completely anodise titanium plate immersed in

the solution. The longer the process, the more the surface area being anodises

(Diamanti and Pedeferri, 2006). Beside, as time increases, the oxide thickness

produced on the surface area becomes thicker (Table 2.10) and well crystallised

(Table 2.11). The growth of pore diameter of TiO2 cell structure with anodic time

increases rapidly in the beginning stage of anodisation and the increment became

linear after 30 minutes.

More porous

23

Table 2.10: Effect of anodising time to pore diameter of TiO2 cell structure(Oh et al., 2005).

Anodising Time(minutes)

Pore Diameters of TiO2 Cell Structure(μm)

5 0.3230 0.56

Table 2.11: Effect of time and voltage on TiO2 structure (Abdullah and Sorrell, 2007).

Time(min)

Voltage (V) TiO2 structure

1 90,180 Low volume of anatase and not well crystallised.

50 90Higher anatase volume and much better

crystallised

50 180 Higher anatase and crytallinity.

Abdullah and Sorrell have demonstrated anodic oxidation of Ti in different

mixing solution and they have obtained the graph of TiO2 thickness with function of

time for all four different solution: (1) 1.5 M H2SO4, (2) 1.5 M H2SO4 + 0.3 M H2O2,

(3) 1.5 M H2SO4 + 0.3 M H3PO4, and (4) 1.5 M H2SO4 + 0.3 M H3PO4 + 0.3 M H2O2

(Abdullah and Sorrell, 2007). Figure 2.16 shows graph of TiO2 thickness versus

anodising time.

24

Figure 2.16 :TiO2 thickness variation with anodising time (Abdullah andSorrell, 2007).

The thickness increased as anodising time increased until 30 minutes for all

solution. After 30 minutes, TiO2 thickness anodised in solution 1, 2, and 3 have

stopped rising, but for solution 4 the thickness continued to increase until 50 minutes.

The result was similar with previous work by Oh et al., 2005. However, for solution

4, the effect of type of electrolyte might have taken over influencing the thickness

and was beneficial at this point in obtaining thicker TiO2 coating.

2.5.5 H2SO4 Electrolyte

Another parameter that is most effective in manipulating the oxide characteristics is

the type of electrolyte solution. Water, particularly salted or acidic water where when

ions are added, can be electrolysed. The electrolyte is usually a solution of water or

other solvents in which ions are dissolved. The most frequently used electrolytes in

titanium anodisation are phosphoric (H3PO4) and sulphuric acids (H2SO4),

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

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

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

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

1

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