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Haider master's thesis

  1. 1. Fabrication of TiO2 Nanotubes Using Electrochemical Anodization Republic of Iraq Ministry of Higher Education and Scientific Research University of Baghdad College of Science Fabrication of TiO2 Nanotubes Using Electrochemical Anodization A Thesis Submitted to the University of Baghdad, College of Sciences, Department of Physics as a Partial Fulfillment of the Requirements for the Degree of Master of Science in Physics By Haidar H. Hamdan Al-Eqaby B.Sc., Al Mustansiriyah University, 2007 Supervised byProf. Dr. Harith I. Jaafar Lect. Dr. Abdulkareem M. Ali 2012 AD 1433 AH 1
  2. 2. ‫‪Fabrication of TiO2 Nanotubes Using Electrochemical Anodization‬‬ ‫( َوي َْس َألونَك عَن‬ ‫َ ِ‬ ‫الروحِ‬ ‫ُّ‬ ‫الروح ِمن أَ ْمر ُقل‬ ‫ُّ ُ ْ ِ ِ‬ ‫َرِّب َو َما ُأوتِيُت ِمن‬ ‫ُْ َ‬ ‫ي‬ ‫الْ ِع ِْْل ِإ اَّل قَ ِليل)‬ ‫ا‬‫صدق الله العلي العظيم‬ ‫سورة اإلرساء‬ ‫اآلية 58‬ ‫2‬
  3. 3. Fabrication of TiO2 Nanotubes Using Electrochemical Anodization DEDICATION To: My mother My father My Brothers My Sisters My Uncle Mr. Jabbar My close friends My country beloved IraqThe martyrs of Iraq with all the love and appreciation. Haidar 3
  4. 4. Fabrication of TiO2 Nanotubes Using Electrochemical Anodization ACKNOWLEDGEMENTS Praise be to ALLAH, his majesty for his uncountable blessings, and bestprayers 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 overthe years none of this would have been possible. They have always been therefor me and I am thankful for everything they have helped me achieve. Next, I would like to thank my supervisors Prof. Dr. Harith I. Jaafarand lect. Dr. Abdulkareem M. Ali, Dr. Harith your help and guidance over theyears 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 haveyou in the department. I thank you so much for the knowledge you have passedon and I will always be grateful for having the opportunity to study under you. Iwould 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 workwould not have been possible without their help and input. I would also like to express my thanks to the deanery of the College ofSciences and head of Physics Department for their support ship to the student ofhigher education, the faculty is irreplaceable and their generosity to the studentbody is incomparable. Thank to Prof. Dr. Moohajiry (Tehran University) and his researchgroup to provide me an Opportunity to work in his respectable laboratory (SEMTechnician). 4
  5. 5. Fabrication of TiO2 Nanotubes Using Electrochemical Anodization To my fellow graduate students, thank you for the good times throughoutour years. Whether it was late nights studying or in the University, it was alwaysa good time. I wish everyone good luck in the future and hope our paths crossagain. In addition, I would like to thank my friends from Al-MustansiriyaUniversity, especially the Assistant Lecturer Ms. Marwa A. Hassan. Fromthe times that “escalated quickly” to showing me the way to “victory lane,” itseems like weve never missed a beat. Finally I would like to thank all of the other friends that I developed overthe years. I am a lucky person to have the friendships that I have. Haidar 5
  6. 6. Fabrication of TiO2 Nanotubes Using Electrochemical Anodization Supervisor Certification We certify that this thesis titled “Fabrication of TiO2 Nanotube UsingElectrochemical Anodization” was prepared by Mr. (Haidar H. Hamdan),under our supervision at Department of Physics, College of Science, Universityof Baghdad, as a partial fulfillment of the requirements for the degree of Masterof Science in Physics.Signature: Signature: (Supervisor) (Supervisor)Name: Dr. Harith I. Jaafar Name: Dr. Abdulkareem M. AliTitle: Professor Title: LecturerDate: 5 / 3 / 2012 Date: 5 / 3 / 2012 In view of the available recommendation, I forward this thesis for debateby the Examination Committee.Signature:Name: Dr. Raad M.S AL- HaddadTitle: ProfessorAddress: Head of Physics Department,Collage of Science, University of Baghdad.Date: 5 / 3 / 2012 6
  7. 7. Fabrication of TiO2 Nanotubes Using Electrochemical Anodization Examination Committee Certification We certify that we have read this thesis entitled “Fabrication of TiO2Nanotube 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 inPhysics.Signature: Signature:Name: Dr. Ikram A. Ajaj Name: Dr. Raad S. SabryTitle: Assistant Professor Title: Assistant ProfessorAddress: University of Baghdad Address: Al-Mustansiriyah UniversityDate: 25 / 4 /2012 Date: 25 / 4 /2012 (Chairman) (Member)Signature: Signature:Name: Dr. Inaam M. Abdulmajeed Name: Dr. Dr. Harith I. JaafarTitle: Assistant Professor Title: ProfessorAddress: University of Baghdad Address: University of BaghdadDate: 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. AliTitle: ProfessorAddress: Dean of the Science College,University of BaghdadDate: 27 / 4 /2012 7
  8. 8. Fabrication of TiO2 Nanotubes Using Electrochemical Anodization Abstract This thesis describes the synthesis of self-organized titanium dioxidenanotube layers by an electrochemical anodization of Ti at differentconditions (time, voltage, concentration of NH4F in electrolyte withglycerol, conductivity and water content) at room temperature (~25ºC)were investigated. In the current study, self-organized, vertically-oriented TiO2nanotubes were successfully prepared by anodization method of a pureTitanium sheet (99.5%) using anodization cell is designed for first time inIraq (Homemade) from Teflon material according to our knowledge toproduce self-ordered Titanium nanotube in organic based electrolytes(glycerol based electrolyte) an electrolyte solution containing (0.5, 1, 1.5and 2 wt.% NH4F) then added water (2 and 5Wt.% H2O) to (0.5wt.%NH4F) only with 15V. The range of anodizing time and potential werebetween 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 themorphology and structure of the obtained Titania templates, opticalinterferometer (Fizeau frings) method to tubes length measurement. For TiO2 nanotubes fabricated in non-aqueous electrolyte, theinfluence of the NH4F concentration on characteristics of nanotubes wasstudied. The results showed that when the NH4F concentration increasedfrom 0.5 to 2wt.%NH4F, the tubes diameter, tubes length and roughness ofTiO2 surface increased. Also the effects of the anodizing time and anodizing potential werestudied. The formation of TiO 2 nanotubes was very sensitive to the 8
  9. 9. Fabrication of TiO2 Nanotubes Using Electrochemical Anodizationanodizing time. Length of the tube increases withincreasing anodizing time and anodizing potential significantly. Either water content (2 and 5wt.%) with 0.5wt.% NH4F and theconductivity of electrolyte it is increasing the diameter, tube length androughness of TiO2 surface increased, but simple increasing and formationof less homogenized TiO2 nanotube. The optimal conditions for TiO2 formation was found 15V at 4hrwith 0.5wt.% NH4F due to we obtain on best results for tube diameter, tubewall thickness, tube length and more homogenized of TiO2 nanotubes. 9
  10. 10. Fabrication of TiO2 Nanotubes Using Electrochemical Anodization ContentsTitle PageDedication 3Acknowledgments 4Supervisor Certification 6Examination Committee Certification 7Abstract 8Contents 10List of Figures 13List of Tables 15List of Abbreviation 16 Chapter One (Introduction and Literature Review)Paragraph Title Page1-1 Physical and Chemical Science and Nanotechnology 191-2 Nanomaterials 191-3 Types of nano materials 201-4 Literature Review 211-5 Aim of this Work 26 Chapter Two (Theoretical Part)Paragraph Title Page2-1 Introduction to Nanotechnology 282-2 Quantum Confinement in Semiconductors 302-2-1 Quantum Dot 302-2-2 Quantum Wire 302-2-3 Quantum Well 302-3 Summary of Quantum Confinement Effect 312-4 Micro to Nano Materials Perspective 322-5 Strategies of Making Nanostructures 332-6 Properties of Titanium Dioxide (TiO2) 342-6-1 Crystal Structure of Titanium Dioxide (TiO2) 352-6-1-1 Titanium Dioxide (TiO2) in Rutile Stable Phase 352-6-1-2 Titanium Dioxide (TiO2) in Anatase Metastable Phase 362-6-1-3 Titanium Dioxide (TiO2) in Brookite Structure 372-7 Synthesis Techniques of TiO2 nanotube 392-8 Electrochemical Anodization processes 39 11
  11. 11. Fabrication of TiO2 Nanotubes Using Electrochemical Anodization2-9 Electrochemical Anodization of Metals 402-10 Mechanism of TiO2 nanotubes array formation 422-11 Factors affecting the formation of (TiO2) nanotube 462-11-1 The effect of anodization potential 462-11-2 The effect of electrolyte 472-11-3 The effect of temperature 482-11-4 The effect of annealing before and after anodizing 492-11-5 The effect of distance between electrodes 49 Chapter Three (Experimental and Methods)Paragraph Title Page3-1 Introduction 523-2 Chemicals and Instrumentations 523-2-1 Chemicals 523-2-2 Instrumentations 533-2-3 Processes flow chart of template synthesis 543-3 Electrochemical Anodization system 553-3-1 Electrochemical Anodization Cell Design 553-4 Samples preparation 563-4-1 Pretreatment of Ti samples 563-4-2 TiO2 Nanotube preparation 573-5 Characterization measurements 593-5-1 X-Ray diffraction (XRD) pattern 593-5-2 Atomic Force Microscopy (AFM) 603-5-3 Scanning Electron Microscopy(SEM) 613-5-4 Thickness measurement 62 Chapter Four (Results and Discussions)Paragraph Title Page4-1 Introduction 654-2 (I-V) characteristics of the electrochemical 65 anodization process4-2-1 Effect of NH4F concentration 654-2-2 Effect of anodizing potential 664-2-3 Effect of water content 704-2-4 Effects of conductivity 714-3 Characterization of Titania nanotubes 724-3-1 Structural and morphological characterization 73 11
  12. 12. Fabrication of TiO2 Nanotubes Using Electrochemical Anodization of Titanium nanotubes (TiO2) in (SEM and AFM) measurement4-3-1-1 Effect of NH4F concentration 734-3-1-2 Effect of anodization time 774-3-1-3 Effect of anodizing potential 804-3-1-4 Effect of water content 844-3-1-5 Effects of conductivity 894-3-2 Structural characterization of Titania in 89 (XRD) measurement4-5-3 Results of thickness measurement 97 Chapter Five (Conclusions and Future Work)Paragraph Title Page5-1 Conclusions and Perspectives 1005-2 Suggestions for Future Research 101 References 103 12
  13. 13. Fabrication of TiO2 Nanotubes Using Electrochemical Anodization List of FiguresFigure (2-1) Density of states as a function of energy for bulk material, 31 quantum well, quantum wire and quantum dot.Figure (2-2) Schematic of nanostructure making approaches 34Figure (2-3) Rutile structure for crystalline TiO2 36Figure (2-4) Anatase metastable phase for crystalline TiO2 36Figure (2-5) Brookite structure for crystalline TiO2 37Figure (2-6) Schematic set-up of anodization experiment 44Figure (2-7) Schematic diagram of the evolution of (TiO2) nanotubes in 45 anodization: (a) oxide layer formation; (b) pore formation on the oxide layer; (c) climbs, formation between pores; (d) growth of the pores and the climbs; (e) fully developed (TiO2) nanotubes arraysFigure (2-8) Schematic representation of processes in (TiO2) nanotube 45 formation during anodization: a) in absence of fluorides; b) in presence of fluoridesFigure (3-1) Flow chart of Titanium nanotube synthesis 54Figure (3-2) Schematic and photograph of set-up illustrates of the 55 anodization experiment with Teflon cellFigure (3-3) Schematic diagram of homemade Teflon cell 56Figure (3-4) Block diagram of atomic force microscope 61Figure (3-5 a, b) Set-up and Photograph illustrates the SEM 62Figure (3-6 a, b) Experimental arrangement for observing Fizeau fringes 63Figure (4-1) The current transient recorded during anodization during 2 66 hours at 15V in the glycerol + 0.5Wt. %NH4 F and glycerol + 1.5Wt. %NH4FFigure (4-2) The current transient recorded during anodization during 2 67 hours at 15 and 40V in the glycerol + 0.5Wt. %NH4FFigure (4-3) Optical images of TiO2 grown on a Ti metal substrate 69 during 2hr of anodization at 5V (a), 10V (b), 15V (c), 25V (d) and at 40 V (e) in 0.5wt. % NH4F.Figure (4-4) The current transients recorded during 2 hours of Ti 71 anodization at 15V in glycerol / water / 0.5wt. %NH4F electrolytes with different weight ratios of glycerol: waterFigure (4-5) The current transient recorded during anodization during 4 72 hours at 15V in the glycerol + 0.5Wt. %NH4 F at a different conductivity of electrolyteFigure (4-6) SEM image of Ti anodized in (0.5 wt. % NH4F) in glycerol 74 electrolyte at 15V for 2 hFigure (4-7) SEM image of Ti anodized in (1.5 wt. % NH4F) in glycerol 74 electrolyte at 15V for 2 hr.Figure (4-8) AFM images of Ti anodized in (0.5 wt. % NH4 F + 75 glycerol) electrolyte at 15V for 2 h, (a) The 2D, cross section, (b) 3D and (c) porosity normal distribution chartFigure (4-9) AFM images of Ti anodized in (1.5 wt. % NH4 F + 76 glycerol) electrolyte at 15V for 2 hr. , (a) The 2D, cross section, (b) 3D and (c) porosity normal distribution chartFigure (4-10) SEM image of Ti anodized in (0.5 Wt. % NH4F + 99.5 Wt. 78 % glycerol) electrolyte at 15V for 2hr 13
  14. 14. Fabrication of TiO2 Nanotubes Using Electrochemical AnodizationFigure (4-11) SEM image of Ti anodized in (1.5 Wt. % NH4F + 99.5 Wt. 78 % glycerol) electrolyte at 15V for 4hrFigure (4-12) AFM images of Ti anodized in (0.5 Wt. % NH4F + 99.5 79 Wt. % glycerol) electrolyte at 15V for 4hr. , (a) The 2D, cross section, (b) 3D and (c) porosity normal distribution chartFigure (4-13) SEM image of Ti anodized in (0.5 Wt. % NH4F + 99.5 Wt. 81 % glycerol) electrolyte at 15V for 2hrFigure (4-14) SEM image of Ti anodized in (0.5 Wt. % NH4F + 99.5 Wt. 81 % glycerol) electrolyte at 40V for 2hrFigure (4-15) AFM images of Ti anodized in in (0.5 Wt. % NH4 F + 99.5 82 Wt. % glycerol) electrolyte at 15V for 2hr. , (a) The 2D, cross section, (b) 3D and (c) porosity normal distribution chartFigure (4-16) AFM images of Ti anodized in (0.5 Wt. % NH4F + 99.5 83 Wt. % glycerol) electrolyte at 40V for 2hr. , (a) The 2D, cross section, (b) 3D and (c) porosity normal distribution chartFigure (4-17) SEM image of Ti anodized in (0.5 Wt. % NH4F + 99.5 Wt. 85 % glycerol) electrolyte at 15 V for 2hFigure (4-18) SEM images of Ti anodized in (0.5 Wt. % NH4F + 2 Wt. % 86 H2O + 97.5 Wt. % glycerol) electrolyte at 15 V for 2hFigure (4-19) SEM images: (a) top-views and (b) cross-sectional images 86 of Ti anodized in (0.5 Wt. % NH4F + 5 Wt. % H2O + 94.5 Wt. % glycerol) electrolyte at 15 V for 2hFigure (4-20) AFM images of Ti anodized in (0.5 Wt. % NH4F + 2 Wt. 87 % H2O + 97.5 Wt. % glycerol) electrolyte at 15 V for 2h, (a) The 2D, cross section, (b) 3D and (c) porosity normal distribution chartFigure (4-21) AFM images of Ti anodized in (0.5% NH4 F + 5% H2O + 88 94.5% glycerol) electrolyte at 15 V for 2h, (a) The 2D, cross section, (b) 3D and (c) porosity y normal distribution chartFigure (4-22) XRD pattern of Titania before and after annealing at 91 temperatures 450°C and 3hr on Ti foil substrateFigure (4-23) XRD pattern of Titania before and after annealing at 94 temperatures 530°C and 3hr on Ti foil substrate 14
  15. 15. Fabrication of TiO2 Nanotubes Using Electrochemical Anodization List of TablesTable (2-1) Physical and chemical properties of the three TiO2 38 structuresTable (3-1) The chemicals and materials which used in process 52Table (3-2) Origin, function and specification devices 53Table (3-3) The condition of experimental work without water 58 addedTable (3-4) The condition of experimental work with water 59 addedTable (4-1) Color as a variable of anodizing TiO2 thickness 69Table (4-2) The average Roughness and Pores diameter of TiO2 77 nanotubes under different proportion of NH4FTable (4-3) The average Roughness and Pores diameter of TiO2 79 nanotubes under different anodization timeTable (4-4) The average Roughness and pores diameter of TiO2 84 nanotubes under different anodization voltageTable (4-5) The results of TiO2 nanotubes under different 85 proportion of glycerol and water contentTable (4-6) The average Roughness and pores diameter of TiO2 89 nanotubes under different proportion of glycerol and water contentTable (4-7) XRD results for Titania before annealing 92Table (4-8) XRD results for Titania after annealing at 93 temperatures 450°C and 3hr on Ti foil substrateTable (4-9) XRD results for Titania before annealing 95Table (4-10) XRD results for Titania after annealing at 96 temperatures 530°C and 3hr on Ti foil substrateTable (4-11) Result Titania thickness measurement by optical 97 interferometer method 15
  16. 16. Fabrication of TiO2 Nanotubes Using Electrochemical Anodization List of AbbreviationsSymbols Meaning0D Zero-dimensional1D One-dimensional2D Two-dimensional3D Three-dimensionalAAO Anodic Aluminum OxideATO Anodic Titanium OxideTi TitaniumTiO2 TitaniaPt PlatinumNH4F Ammonium FluorideAl AluminumAl2O3 AluminaSi SiliconHf HafniumZr ZirconiumTa TantalumNb NiobiumN2 Nitrogen GasZrO2 ZirconiaH2SO4 Sulfuric AcidNaF Sodium FluorideTa2O5 Tantalum PentoxideNa2SO4 Sodium SulfateHF Hydrofluoric AcidH3PO4 Phosphoric AcidH2O WaterF FluorideZnO Zinc OxideNaOH Sodium HydroxideDNA Deoxyribonucleic AcidPH Acidity numberDI Deionized WaterSEM Scanning Electron MicroscopyAFM Atomic Force MicroscopeXRD X-ray DiffractionASTM American Society of Testing Materialst Thickness of FilmX Fringes SpacingΔX Displacementλ Wavelength 16
  17. 17. Fabrication of TiO2 Nanotubes Using Electrochemical AnodizationUV Ultraviolet raysUV–vis Ultraviolet–visible SpectroscopyPVD Physical Vapor DepositionCVC Chemical Vapor CondensationCVD Chemical Vapor DepositionM MolarityRF Roughnesswt.% Weight Percentaged The Spacing Between Atomic Planesn Refractive Indexa Lattice Constant2Ɵ Bragg Diffraction AngleOCP Open-circuit PotentialSWNT Single Wall NanotubeMWNT Multi Wall Nanotube 17
  18. 18. Fabrication of TiO2 Nanotubes Using Electrochemical Anodization 18
  19. 19. Fabrication of TiO2 Nanotubes Using Electrochemical Anodization Chapter One Introduction and Literature Review1-1- Physical and Chemical Science and Nanotechnology Over the last ten years, the physics, chemistry and engineering scientistsinterested in formation of self-organized nanostructures and nanopatterns whichattracted a great scientific and technological interest due to its far-reaching andinnumerable applications. Apart from these facts, the popularity and significanceof these self- arranged nanostructures stem from the nature of their fabricationthat relies on self- regulation processes (often called self-assembly). The mainadvantage of these processes is that it can represented a ``smart´´ nano-technique. Therefore, it is not surprising that a large part of materials sciencenowadays targets these nano-scale fabrication techniques. Nanotechniques are anatural consequence of the necessity of achieving smaller and smaller electronicand 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 simpleprocess – electrochemical anodic polarization. Some important findings in thisparticular field include the growth of ordered Titanium dioxide (TiO2), [1]nanoporous Aluminum oxide (Al2O3, Alumina) and ordered macroporous [2]Silicon . Synthesis of all these materials has stimulated considerable researchefforts and given rise to many other materials to be processed in a similarfashion.1-2- Nanomaterials Nanomaterials: A materials with dimensions below 100nm and they haveat least one unique properties that is different than the bulk material and thecharacteristics can be applied in different fields such as nanoelectronics,pharmaceutical and cosmetic. Several methods have been studied in fabricating 19
  20. 20. Fabrication of TiO2 Nanotubes Using Electrochemical Anodizationthese nanostructures, which include laser ablation [3], chemical vapor deposition(CVD) [3] and template-directed growth [4]. In order to integrate one dimensionalnanomaterial into a device, a fabrication method that enables well-orderednanomaterials with uniform diameter and length is important. Template-directedgrowth is a nanomaterials fabrication method that uses a template which has [5]nanopores with uniform diameter and length . Using chemical solutions orelectro deposition, nanomaterials are filled into the nanopores of the templatesand, by etching the template, nanowires or nanotubes with similar diameter andlength as the template nanopores are obtained. Because the size and shape of thenanomaterial depends on the nanoholes of the template, fabricating a templatewith uniform pore diameters is very important. TiO2 nanotube is particularly interested with its high potential for use invarious 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 accordingtheir composition. The more classification using is the order of dimension into 0D (quantumdot), 1D (nanotube, nanowire and nanorod), 2D (nanofilm), and 3D dimensionssuch as bulk material composited by nanoparticles [9]. Nanotubes are made, sometimes, from inorganic materials such as oxidesof metals (Titanium oxide, Aluminum oxide), are similar in terms of hisstructure to the carbon nanotubes, but the heaviest of them, not the same strongas carbon nanotube. Titanium nanotube can be described as a particles of Titaniais requested about an axis, to take a cylindrical shape where both ends of theatoms associated with each slide to close the tube. 21
  21. 21. Fabrication of TiO2 Nanotubes Using Electrochemical Anodization Be one of the ends of the tube is often open and one closed in the form ofa hemisphere, as might be the wall of the tube individual atoms and is called inthis case the nanotubes and single-wall (single wall nanotube) SWNT, or two ormore named multi-wall tubes (multi wall nanotube) MWNT The tube diameterranges from less than one nm to 100 nm (smaller than the width head of hair by50,000 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 bambooand so on. The properties of these tubes are unusual in terms of strength andhardness and electrical conductivity, and others [10]. Titania nanotube is 1D type nanomaterails that is means existing only onemicro or macro dimension which represented by the length of the tube.1-4- Literature Review Since its commercial production in the early twentieth century, Titaniumdioxide (TiO2) has been widely used as a pigment [11] and in sunscreens paints[12] , ointments [13], toothpaste [14], etc. In 1972, Fujishima and Honda discoveredthe phenomenon of photocatalytic splitting of water on a TiO2 electrode underultraviolet (UV) light [15]. Since then, enormous efforts have been devoted to theresearch of TiO2 material, which has led to many promising applications in areasranging from photovoltaics and photocatalysis to photo-electrochromics and [16]sensors . These applications can be roughly divided into “energy” and“environmental” categories such as water purification, pollution prevention,antibacterial, and purify the air. Many of which depend not only on theproperties of the TiO2 material itself but also on the modifications of the TiO 2material host (e.g., with inorganic and organic dyes) and on the interactions ofTiO2 materials with the environment. An exponential growth of research activities has been seen in nanoscience [17]and nanotechnology in the past decades . New physical and chemicalproperties emerge when the size of the material becomes smaller and smaller, 21
  22. 22. Fabrication of TiO2 Nanotubes Using Electrochemical Anodizationand down to many serious environmental and pollution challenges. TiO 2 alsobears tremendous hope in helping ease the energy crisis through effective [18]utilization of solar energy based on photovoltaic and water-splitting devices .As continued breakthroughs have been made in the preparation, modification,and applications of TiO2 nanomaterials in recent years, especially after a seriesof great reviews of the subject in the 1990s. We believe that a new andcomprehensive review of TiO2 nanomaterials would further promote TiO2-basedresearch and development efforts to tackle the environmental and energychallenges that we are currently face it. Here, we focus on recent progress in thesynthesis, properties, modifications, and applications of TiO 2 nanomaterials [19]. [20] In 1991, Zwilling et al. first reported the porous surface of TiO2 filmselectrochemically formed in fluorinated electrolyte by Titanium anodization. In1999 it was reported that porous TiO2 nanostructures could be fabricated byelectrochemically anodizing a Ti sheet in an acid electrolyte containing a smallamount of hydrofluoric acid (HF) [21]. Since then, many research groups havepaid considerable attention to this field, because anodization opens up ways toeasily produce closely packed tube arrays with a self-organized verticalalignment. [22] A decade later Gong and co-workers synthesized the uniform andhighly-ordered Titanium nanotube arrays by anodization of a pure Titaniumsheet in a hydrofluoric acid (HF) aqueous electrolyte. They obtained nanotubesdirectly grew on the Ti substrate and oriented in the same directionperpendicular 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 byanodization of a pure Titanium sheet in an aqueous solution containing 0.5 to3.5 wt. % hydrofluoric acid. These tubes are well aligned and organized intohigh-density uniform arrays. While the tops of the tubes are open, the bottoms of 22
  23. 23. Fabrication of TiO2 Nanotubes Using Electrochemical Anodizationthe tubes are closed, forming a barrier layer structure similar to that of porousAlumina. The average tube diameter, ranging in size from 25 to 65 nm, wasfound to increase with increasing anodizing voltage, while the length of the tubewas found independent of anodization time. Later in 2003 Oomman K. Varghese et al. [24] used anodization with atime-dependent linearly varying anodization voltage and made films of tapered,conical-shaped Titania nanotubes. The tapered, conical-shaped nanotubes wereobtained by anodizing Titanium foil in a 0.5% hydrofluoric acid electrolyte,with the anodization voltage linearly increased from 10–23 V at rates varyingfrom 0.43- 2.0 V/min. The linearly increasing anodization voltage results in alinearly increasing nanotube diameter, with the outcome being an array ofconical-shaped nanotubes approximately 500 nm in length. Evidence providedby scanning electron-microscope images of the Titanium substrate during theinitial stages of the anodization process enabled them to propose a mechanism ofnanotube formation. In 2005 Seung-Han Oh et al. [25] a vertically aligned nanotube array ofTitanium 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 asimulated body fluid. Such TiO2 nanotube arrays and associated nanostructurescan be useful as a well-adhered bioactive surface layer on (Ti) implant metalsfor orthopaedic and dental implants, as well as for photocatalysts and othersensor applications. In 2006 Aroutiounian et al. [26] the semiconductor photoanodes made ofthin film Titanium oxide were prepared by anodization of Titanium plates inhydrofluoric acid solution at direct voltage at room temperature. The influenceof the change of Titanium oxide film growth conditions (concentration ofhydrofluoric acid, voltage, duration of anodization process) and subsequent heat 23
  24. 24. Fabrication of TiO2 Nanotubes Using Electrochemical Anodizationtreatment of films on a photocurrent and current-voltage characteristics ofphotoelectrodes were investigated. [27] In 2007 V. Vega et al. synthesized Self-aligned nanoporous TiO2templates synthesized via dc current electrochemical anodization have beencarefully analyzed. The influence of environmental temperature during theanodization, ranging from 2ºC to ambient, on the structure and morphology ofthe nanoporous oxide formation, has been investigated, as well as that of the(HF) electrolyte chemical composition, its concentration and their mixtures withother acids employed for the anodization. Arrays of self-assembled Titaniananopores with inner pores diameter ranging between 50 and 100 nm, wallthickness around 20–60 nm and 300 nm in length, are grown in amorphousphase, vertical to the Ti substrate, parallel aligned to each other and uniformlydisordering distributed over all the sample surface. In 2008 Hua-Yan Si et al. [28] studied the effects of anodic voltages on themorphology, wettability and photocurrent response of the porous Titaniumdioxide films prepared by electrochemical oxidation in a hydrofluoric acid(HF)/chromic acid electrolyte have been studied. The porous Titanium dioxidefilms showed an increased surface roughness with the increasing anodizingvoltages. By controlling the films morphology and surface chemicalcomposition, the wettability of the porous Titanium dioxide films could beeasily adjusted between superhydrophilicity and superhydrophobicity. X-raydiffraction (XRD), Raman and UV–vis spectroscopy revealed that the obtainedTitanium dioxide films were in anatase phase. The Titanium dioxide filmsshowed clear photocurrent response, which decreased dramatically with theincrease of the anodizing voltages. This study demonstrates a straightforwardstrategy for preparing porous Titanium dioxide films with tunable properties,and especially emphasizes the importance of understanding theirmorphology/properties relationship. 24
  25. 25. Fabrication of TiO2 Nanotubes Using Electrochemical Anodization [29] In 2009 Michael et al. anodized Titanium-oxide containing highlyordered, vertically oriented TiO2 nanotube arrays is a nanomaterial architecturethat shows promise for diverse applications. An anodization synthesis using HF-free aqueous electrolyte solution contains 1 wt.% (NH 4)2SO4 plus 0.5 wt.%NH4F. The anodized TiO2 film samples (amorphous, anatase, and rutile) onTitanium foils were characterized with scanning electron microscopy and X-raydiffraction. Additional characterization in terms of photocurrent generated by ananode consisting of a Titanium foil coated by TiO 2 nanotubes was performedusing an electrochemical cell. A Platinum cathode was used in theelectrochemical cell. In 2010 Hun Park et al. [30] studied the properties of TiO2 nanotube arrayswhich are fabricated by anodization of (Ti) metal. Highly ordered TiO2 nanotubearrays could be obtained by anodization of (Ti foil in 0.3 wt.% NH 4F containedethylene glycol solution at 30°C. The length, pore size, wall thickness, tubediameter etc. of TiO2 nanotube arrays were analyzed by field emission scanningelectron microscopy. Their crystal properties were studied by field emissiontransmission electron microscopy and X-ray photoelectron spectroscopy. In 2011 S. Sreekantan et al. [31] formed Titanium oxide (TiO2) nanotubesby anodization of pure Titanium foil in a standard two-electrode bath consistingof ethylene glycol solution containing 5 wt.% NH 4F. The PH of the solution was∼ 7 and the anodization voltage was 60 V. It was observed that such anodizationcondition results in ordered arrays of TiO2 nanotubes with smooth surface and avery high aspect ratio. It was observed that a minimum of 1 wt. % wateraddition was required to form well-ordered TiO2 nanotubes with length ofapproximately 18.5 μm. As-anodized sample, the self-organized TiO2 nanotubeshave amorphous structure and annealing at 500oC of the nanotubes promoteformation of anatase and rutile phase. 25
  26. 26. Fabrication of TiO2 Nanotubes Using Electrochemical Anodization1-5- Aim of this Work Fabrication of forest of Titania nanotubes via electrochemical anodizingof pure Titanium foil using electrochemical Teflon cell designed for first time inIraq according to our knowledge to produce self-ordered Titanium nanotubes.Investigate the effects of some process parameters such as; time, voltage andelectrolyte composition on the diameter and length of fabricated nanotubes bynanoscopic instrument atomic force microscopy (AFM), scanning electronmicroscopy (SEM), (XRD) spectroscopy and optical interferometer method. 26
  27. 27. Fabrication of TiO2 Nanotubes Using Electrochemical Anodization 27
  28. 28. Fabrication of TiO2 Nanotubes Using Electrochemical Anodization Chapter Two Theoretical Part2-1- Introduction to Nanotechnology Nanotechnology or nanoscale science is concerned with the investigationof matter at the nanoscale, generally taken as the 1 to 100 nm range. Thebreakthrough in both academic and industrial interest in these nanoscalematerials over the past ten years has been interested because of the remarkable [32]variations in solid-state properties . The “nano” as word means dwarf (smallman) 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 thediameter 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 thethickness of a hair [33]. Nanoparticels are considered to be the building blocks for nanotechnologyand referred to particles with at least one dimension less than 100nm. Particlesin these size ranges have been used by several industries and humankind forthousands of years [34]. The nanotechnology deals with the production and application ofphysical, chemical, and biological system at scales ranging from individualatoms or molecules to submicron dimension, as well as the integration of the [35]resulting nanostructures into larger system . Nanometer–scale features are mainly built up from their elementalconstituents. Examples in chemical synthesis, the spontaneous self –assembly ofmolecular clusters (molecular self- assembly) from simple reagents in solution.The biological molecules (e.g., DNA) are used as building blocks for the 28
  29. 29. Fabrication of TiO2 Nanotubes Using Electrochemical Anodizationproduction of zero- dimensional nanostructure, and the quantum dots(nanocrystals) of arbitrary diameter (about 10 to 105 atoms). When thedimension of a material is reduced from a large size, the properties remain thesame first, and then small changes occur, until finally, when the size dropsbelow 100nm, dramatic changes in properties occur [35]. At the nanoscale, objects behave quite differently from its behave at largerscales, such as increased hardness values of metallic materials and their alloys aswell as increase the strength to face the stresses of different loads, located it,either the ceramic material increases the durability and tolerance to stressesimpact. As for the electrical properties have a great ability to connect andincrease the diffusion and interactions in nano-seconds and the speed of iontransport [36]. Nanotechnology manipulates matter for the deliberate fabrication of nano-sized materials. These are therefore “intentionally made” through a definedfabrication process. The definition of nanotechnology does not generally include“non-intentionally made nanomaterials”, that is, nano-sized particles ormaterials that belong naturally to the environment (e.g., proteins, viruses) or thatare produced by human activity [36]. A nanomaterial is an object that has at least one dimension in the [36]nanometre scale . Nanomaterials are categorized according to theirdimensions into three classes [37]: 1. Zero-dimension confinement (quantum dot). 2. One-dimension confinement (quantum wire). 3. Two-dimensions confinement (quantum well). 4. Three -dimensions confinement (bulk). 29
  30. 30. Fabrication of TiO2 Nanotubes Using Electrochemical Anodization2-2- Quantum Confinement in Semiconductors In the last few years a great effort has been devoted to the study of lowdimensional semiconductor structures. The reduction of the dimensionalitycauses several changes in the electronic and excitonic wave functions and thesefeatures can be used, at least in principle, to produce novel microelectronics [38]. In bulk semiconductor materials, the energy levels of both conductionband and valence band are continuous, with electrons and holes moving freely inall directions. As the dimensions of the material shrink, effect of quantumconfinement will be seen, this effect is seen in the objects, when size of object isless than de Broglie wavelength of electrons. Here, classical picture of electronstrapped within hard wall boundaries is not unrealistics. Three different types ofconfinement that have been realized among semiconductors materials aredescribed below [39].2-2-1- Quantum Dot Typically, the dimension is ranging from 1 to 100 nanometers. A quantumdot has the most restricted confinement in all three dimensions of the electronsand holes. It is working under the condition (λF >>Lx, Ly, Lz), where λF representthe Fermi wavelength [40]. As shown in figure (2-1). An important property of aquantum 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 areconfined in two dimensions, as shown in figure (2-1) such confinement allowsfree electrons and holes behavior in only one direction, along the length of thewire [39]. These properties give rise to produce many nanoproductions which canbe considered as a quantum wire (λF> Lx, Ly and Lx, Ly<<Lz), carbon nanotubesfor connection is example of this [40].2-2-3-Quantum Well A quantum well is a potential well that confines particles, which wereoriginally free to move in three dimensions, in two dimensions, forcing them to 31
  31. 31. Fabrication of TiO2 Nanotubes Using Electrochemical Anodizationoccupy a planar region. Their motions are confined in the directionperpendicular to the free plane. The effects of quantum confinement take placewhen the quantum well thickness becomes comparable at the de Brogliewavelength of the carriers (generally electrons and holes), leading to energylevels called “energy subbands”, i.e., the carriers can only have discrete energyvalues [41], as in the figure (2-1). In quantum well the electron are free in Z, Y directions, whereas it isconfined 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 inthe physical properties of semiconductor. The density of states g(E) is defined by the number of energy statesbetween energy E and E+dE per unit energy range, which is defined bydn(E)/dE. For electrons in a bulk semiconductor, g(E) is zero at the bottom ofthe conduction band and increases with E1/2 as the energy of the electrons in theconduction band increases. This behavior is shown in figure (2-1), which 31
  32. 32. Fabrication of TiO2 Nanotubes Using Electrochemical Anodizationcompares the density of states for electron in a quantum well (and also inquantum wire and dot), where the density of states is a step function because ofthe discreteness of the energy levels along the confinement direction [39]. The density of state for a quantum wire has an inverse energy dependenceE-1/2 for each sub-band, the density of state has a large value near Kz =0 anddecays as E-1/2 as Kz has nanozero value for that sub-band. The energy levels foran electron in a quantum dot have only discrete values, which makes the densityof 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 andappearance of discrete sub-bands corresponding to energy quantization along thedirection of confinement. As the dimensions of the material increase, the energyof the confined states decreases so the inter-band transitions shift to longerwavelengths. When the dimensions of the material are greater than de Brogliewavelength, 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 thesystem decreased. These included statistical mechanical effects, as well asquantum mechanical effects, for example the “quantum size effect”, where theelectronic 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 isreached, typically at distances of 100 nanometers or less, the so called quantumrealm. Additionally, a number of physical (mechanical, electrical, optical, etc.)properties change when compared to macroscopic systems. One example is theincrease in surface area to volume ratio altering mechanical, thermal andcatalytic properties of materials. Diffusions and reactions at nanoscale, 32
  33. 33. Fabrication of TiO2 Nanotubes Using Electrochemical Anodizationnanostructures materials and nanodevices with fast ion transport are generallyreferred to nanoionics. Mechanical properties of nanosystems are of interest inthe nanomechanics research. The catalytic activity of nanomaterials also openpotential risks in their interaction with biomaterials [42]. Materials reduced to the nanoscale can show different propertiescompared to what they exhibit on a macroscale, enabling unique applications.For instance, opaque substances become transparent (Copper); stable materialsturn combustible (Aluminum); insoluble materials become soluble (Gold). Amaterial such as Gold, which is chemically inert at normal scales, can serve as apotent chemical catalyst at nanoscales. Much of the fascination withnanotechnology stems from these quantum and surface phenomena that matterexhibits at the nanoscale [42].2-5- Strategies of Making Nanostructures There are two strategies to make nanostructures. Top-down approach andbottom-up approach. The first strategy is by start from a large chunk of materialand 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 andvapor deposition. Electrochemical dip-pen lithography utilizes an Atomic Force [43]Microscope (AFM) to transfer material from the AFM tip to a surface . Thismethod 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 thesmallest components to assemble the desired structure from the ground up,which represented by direct chemical synthesis. Usually bottom-up is associatedwith chemistry and synthesis, while Top-Down is associated with physicalprocessing techniques [43]. 33
  34. 34. Fabrication of TiO2 Nanotubes Using Electrochemical Anodization 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 thenaturally occurring oxide of Titanium, chemical formula TiO2. When used as apigment, it is called Titanium white, Pigment White 6, or CI 77891. It isnoteworthy for its wide range of applications, from paint to sunscreen to foodcolouring when it is given the E number E171. Titanium dioxide occurs innature as the well-known naturally occurring minerals rutile, anatase andbrookite. Additionally two high pressure forms, the monoclinic baddeleyiteform 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 anataseand brookite both can be converted to rutile upon heating. Rutile, anatase andbrookite all contain six coordinates Titanium. Titanium dioxide is the mostwidely used white pigment, because of its brightness and very high refractiveindex (n=2.7), in which it is surpassed only by a few other materials.Approximately 4 million tons of pigmentary TiO2 are consumed annuallyworldwide [45]. When deposited as a thin film, its refractive index and colourmake it an excellent reflective optical coating for dielectric mirrors and somegemstones, for example “mystic fire topaz”. TiO2 is also an effective pacifierin powder form, where it is employed as a pigment to provide whiteness and 34
  35. 35. Fabrication of TiO2 Nanotubes Using Electrochemical Anodizationopacity to products such as paints, coatings, plastics, papers, inks, foods,medicines (i.e. pills and tablets) as well as most toothpastes. Opacity isimproved 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 sensitivityand mainly because of its good stability in adverse environments. Titanium(IV) Oxide (II) has one stable phase, rutile (tetragonal) and two metastablepolymorph phases, brookite (orthorhombic) and anatase (tetragonal). Bothmetastable phases become rutile (stable) when submitting the material attemperatures 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 andbrookite 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 todevelop gas sensors based in thick film polycrystalline material or smallparticles. Titanium dioxide (IV) has stable phase rutile (material structure), forthe schematic rutile structure. Its unit cell contains (Ti) atoms occupy thecenter of a surrounding core composed of six Oxygen atoms placedapproximately at the corners of a quasi-regular octahedron as shown in the [47]figure (2-3) . The lattice parameters correspond now to a = b = 4.5933 A °and c = 2.9592 A° with c/a ratio of 0.6442 [48]. 35
  36. 36. Fabrication of TiO2 Nanotubes Using Electrochemical Anodization 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 phasestogether with brookite phase. For calcination processes above 700 °C allanatase structure becomes rutile. Some authors also found that 500 °C wouldbe enough for phase transition from anatase to rutile when thermal treatmenttakes place. Anatase structure is tetragonal, with two TiO2 formula units (sixatoms) per primitive cell. Lattice parameters are: a = b = 3.7710 A° and c =9.430 A° with c/a ratio of 2.5134[48], as shown in figure (2-4) [43]. Figure (2-4) Anatase metastable phase for crystalline TiO2 [47]. 36
  37. 37. Fabrication of TiO2 Nanotubes Using Electrochemical Anodization2-6-1-3- Titanium Dioxide (TiO2) in Brookite Structure The brookite structure is more complicated and has a larger cell volumethan the other two. It is also the least dense of the three forms in (g/cm 3). Theunit cell is composed of eight formula units of TiO 2 and is formed by edgesharing TiO2 octahedra, similar to rutile and anatase, as shown in the figure (2- [47]5) . Brookite belongs to the Orthorhombic crystal system its space group isPbca. By definition, the brookite structure is of lower symmetry than its TiO 2countermorphs, the dimensions of the unit cell are unequal. Also the Ti-Obond 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 thethree TiO2 structures [45]. Figure (2-5) Brookite structure for crystalline TiO2 [47]. 37
  38. 38. Fabrication of TiO2 Nanotubes Using Electrochemical AnodizationTable (2-1) Physical and chemical properties of the three TiO2 structures [45] Properties Rutile Anatase Brookite Molecular formula TiO2 === === Molar mass g/mol 79.866 Crystal System Tetragonal Tetragonal Orthorhombic Energy gap eV 3.06 3.29 Color White solid === === 3 Density g/cm 4.27 3.90 4.13 Transformed Transformed Melting point °C 1855 into rutile into rutile Boiling point 2972 Refractive index (nD) 2.609 2.488 2.583 Dielectric constant ε 110~117 48 78 Hardness (Mohs scale) 7.0~7.5 5.5~5.6 === Anatase, Rutile and Brookite have been studied for their photocatalytic,photo electrochemical and gas sensors applications. The difference in thesethree crystal structures can be attributed to various pressures and heats appliedfrom rock formations in the Earth. At lower temperatures the anatase andbrookite phases are more stable, but both will revert to the rutile phase whensubjected to high temperatures (700°C for the anatase phase and 750°C for thebrookite 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 depositsof the mineral. It was originally dubbed „arkansite‟ for the state it was [49]discovered in Arkansas . The optical properties for each phase are alsosimilar, but they have some slight difference. The absorption band gap for therutile, anatase, and brookite phases were calculated as shown in Table (2-1). Inaddition to the slight increase in the band gap, the anatase form also has aslightly higher Fermi level (0.1eV). In thin films it has been reported that theanatase structure has higher mobility for charge carriers versus the rutile [46]structure . For photocatalytic processes, anatase is the preferred structure, 38
  39. 39. Fabrication of TiO2 Nanotubes Using Electrochemical Anodizationalthough all three forms have shown to be photocatalytic. The electronicstructure of brookite is similar to anatase, based on minor differences in thelocal crystal environment [50].2-7- Synthesis Techniques of TiO2 nanotube [51] Since the discovery of Carbon nanotubes in 1991 a continuouslyincreasing research interest in one dimensional (1D) nanomaterials has beenestablished. After ten years, not only Carbon materials are widely studied, butalso a variety of metals and oxides, such as TiO 2 [52], ZnO [53], etc. The inorganicnanotubes, in particular the TiO2 ones, are of a great potential for varioustechnological applications, due to their high surface to volume ratio, enhancedelectronic properties (in comparison with nanoparticles), well-defined structuresand the possibility to precisely tailor their dimensions on the nanoscale. In the case of TiO2, several studies indicated that nanotubes have [54] [55]improved performance in photocatalysis and photovoltaics compared tocolloidal or nanoparticulate forms of TiO2. Up to now, suspensions, bundles andarrays of rather disordered TiO2 nanotubes have been produced by a variety ofdifferent methods including sol-gel, electrodeposition, sonochemical deposition,hydrothermal and solvothermal, template, chemical vapor deposition (CVD),physical vapor deposition (PVD), Chemical Vapor Condensation (CVC) andfreeze-drying .etc. [56].2-8- Electrochemical Anodization Processes The electrolytic passivation process used to increase the thickness of thenatural oxide layer on the surface of metal parts. The process is called“anodizing” because the part to be treated forms the anode electrode of anelectrical circuit. Anodizing increases corrosion resistance and wears resistance,and provides better adhesion for paint primers and glues than bare metal [53]. 39
  40. 40. Fabrication of TiO2 Nanotubes Using Electrochemical Anodization Anodization changes the microscopic texture of the surface and changes thecrystal structure of the metal near the surface. Thick coatings are normallyporous, so a sealing process is often needed to achieve corrosion resistance [57]. All metals, except gold, are unstable at room temperature in contact withOxygen at atmospheric partial pressure, and thermodynamically should tend toform an oxide. In water many metals, such as Aluminum, Titanium andTantalum, displaced Hydrogen with the production of an oxide or a salt. Thesereactions often fail to occur at any appreciable rate. The usual reason for the lackof reaction is that a thin but complete film of insoluble or slowly soluble oxide isformed. This separates the reactants and further reaction can only occur bydiffusion or migration (field-assisted movement) of metal or Oxygen ionsthrough the native oxide film. These processes are usually slow. Such transportdoes occur, thickens the film and therefore reduces the rate of reaction becauseof 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 anelectrostatic field in the oxide (or increases the field already present) andproduces continued growth of the oxide film by causing metal or Oxygen ions tobe pulled through the film. Due to this reason this kind of films are called anodicfilms.2-9-Electrochemical Anodization of Metals The electrochemical formation of self-organized nanoporous structuresproduced by the anodization of some metals have been reported .These a groupof materials rather than Aluminum and Titanium [58], have been tried to produceporous oxide templates. Porous anodic oxide films have also been achieved onsurfaces of many other metals, e.g. Hafnium [59], Niobium [60], Tantalum, InP [61] ,Tungsten [62], Vanadium, Zirconium [7] and Silicon [63]. 41
  41. 41. Fabrication of TiO2 Nanotubes Using Electrochemical Anodization Hafnium oxide has many interested properties, e.g. its high chemical andthermal stabilities, high refractive index and relatively high dielectric constant.These properties make Hafnium oxide a valuable material to be used as aprotective coating, optical coating, gas sensor or capacitor. Self-organizedporous Hafnium oxide layers were obtained successfully for the first time by [5]Tsuchiya, et al. via anodization of Hafnium at about 50 V in 1 wt.% H2SO4 +0.2 wt.% NaF at room temperature. Anodization potential was found to be a keyfactor affecting the morphology and the structure of the porous oxide [59]. Self-organized porous anodic Niobium oxide films were successfullyprepared in 1 wt.% H2SO4 + 1 wt. % HF or 1.5 wt.% HF respectively [59]. Ta2O5 has attracted intensive attention due to its application in opticaldevices. Anodization of Tantalum has been widely investigated in sulfuric,phosphoric acid, and Na2SO4 solutions and a layer of amorphous Ta2O5 with auniform thickness could be obtained [59]. Self-organized porous anodic Tantalumoxide with a reasonably narrow size distribution was fabricated via anodizingTantalum in 1 wt. % H2SO4 + 2 wt. % HF for 2 h. Zirconium oxide is an important functional material that plays a key role [7]as an industrial catalyst and catalyst support . It was reported that a compactanodic Zirconium oxide layer of up to several hundred nanometers in thicknesscan be achieved in many electrolytes. A unique feature in comparison with otheranodic metal oxides mentioned above is that the growth of the compact ZrO 2layer at room temperature directly leads to a crystalline film rather than anamorphous film as observed from other anodic metal oxides [7]. Formation ofself-organized porous Zirconium oxide layers produced by anodization of Zr at30 V in an electrolyte of 1 wt.% H2SO4 + 0.2 wt. % NH4F was reported byTsuchiya et al. [5]. Aluminum metal anodized in an acidic electrolyte and controlled undersuitable conditions, Aluminum forms a porous oxide called anodic Aluminumoxide (AAO) with very uniform and parallel cell pores. Each cell contains an 41
  42. 42. Fabrication of TiO2 Nanotubes Using Electrochemical Anodizationelongated cylindrical sub-micron or nanopore that is normal to the Aluminumsurface, extending from the surface of the oxide to the oxide/metal interface,where it is sealed by a thin barrier oxide layer with approximately hemisphericalgeometry. The structure of AAO can be described as a closely packed array ofcolumnar cells [64]. The most significant difference between typical anodic Titanium oxide(ATO) and anodic Aluminum oxide (AAO) is that the latter is a continuous filmwith a pore array while the former consist of separated nanotubes. Several recentstudies have showed that Titania nanotubes have better properties compared to [65]many other forms of Titania for applications in photocatalysis and gassensors [66].2-10- Mechanism of (TiO2) nanotubes Array Formation [67] Gong et al. first reported the formation of TiO2 nanotube arraysthrough anodization method by using Fluoride-based electrolyte. Fromcomparison with other fabrication methods, the anodization is simpler andcheaper. Moreover, the dimensions of the Titanium nanotube can be precisely [56]controlled by tailoring the anodization parameters. Figure (2-6) shows theschematic set-up of anodization experiment. In the set-up, (Ti) foil is used asan anode and inert metal, usually Platinum Pt foil, is used as a cathode.Magnetic agitation is commonly conducted to provide uniform local currentdensity and temperature condition on the surface of Ti anode. In order toachieve ordered nanotubular structures of TiO2, Fluoride ions need to bepresent in electrolytes. The as-prepared TiO2 nanotubes are annealed to formcrystal structure. The morphology and structure of the Titanium nanotube arestrongly influenced by the electrochemical conditions (such as anodizationvoltage and time) and the solution parameters (such as the composition of theelectrolyte). [68] Jessensky et al. proposed a mechanical stress model to explain the 42
  43. 43. Fabrication of TiO2 Nanotubes Using Electrochemical Anodizationformation of hexagonally ordered pore arrays of nanotybes. The model isexplained the general self-ordering mechanism for Aluminum and Titaniumas: 1. The oxidation takes place at the entire metal/oxide interface mainly by the migration of Oxygen containing ions from the electrolyte. 2. The dissolution and thinning of the oxide layer is mainly due to the hydration reaction of the formed oxide layer. 3. In the case of barrier oxide growth without pore formation, all metal ions reaching the electrolyte/oxide interface contribute to oxide formation. On the other hand, porous metal oxide is formed when metal ions drift through the oxide layer. Some of them are ejected into the electrolyte without contributing to the oxide formation. 4. Pores grow perpendicular to the surface when the field-enhanced dissolution at the electrolyte/oxide interface is equilibrated with oxide growth at the oxide/metal interface. 5. The volume of the anodized metal is expanded by difference of density between metal and metal oxide. 6. This volume expansion leads to compressive stress during the oxide formation in the oxide/metal interface. The expansion in the vertical direction pushes the pore walls upwards [69]. 43
  44. 44. Fabrication of TiO2 Nanotubes Using Electrochemical Anodization [56] Figure (2-6) Schematic set-up of anodization experiment The mechanism for the formation and growth of Titanium nanotubes [70]arrays by anodization method shown in figure (2-7) . At the beginning ofthe process, electrochemical etching is dominated. Due to the aid of electricfield, oxide is grown on the metal surface. Where O2- ions from H2O migratedvia the oxide layer and reacted with the metal at the metal/oxide interface,while Ti4+ cations are ejected from metal/oxide interface to oxide/electrolyteinterface, as show in (Eq. 2-1) [56]. …………………..….…………..…. (2-1) Fluoride ions in the electrolyte helped the formation of nanotubes on theTi surface. Therefore without F- ions, the electric field will be reduced as theoxide keep on going, which leads to the exponential current decay, as show infigure (2-8) [71]. 44
  45. 45. Fabrication of TiO2 Nanotubes Using Electrochemical Anodization 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)xOylayer. All these conditions retard the formation of oxide layer. The presence of(F-) ions, on the other hand, possess different mechanism which mainly due tochemical dissolution of TiO2 in the Fluoride ions containing electrolyte, asshow in (Eq. 2-2). [56]. [ ] ……………………....………………. (2-2) The dissolution of TiO2 leads to the random formation of small pores. 45
  46. 46. Fabrication of TiO2 Nanotubes Using Electrochemical AnodizationThese pores keep on growing as the oxide layer moves inward at the porebottom. 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 watersoluble TiF62- , as show in (Eq. 2-3) [56]. [ ] ………………………………………..…….. (2-3) As time proceeds, more and more pores are formed and grow. Eachindividual pore starts completing for the available current with other pores.Under optimum conditions, the pores share equal amount of available currentand spread uniformly under steady state conditions. The thickness and depthof the pores continue to grow to form nanotube structure when the rate ofoxide growth at the metal/oxide interface is higher than that of the oxidedissolution at the pore-bottom/electrolyte interface. However, the thicknessceases to increase when the two rates ultimately become the same; while thenanotube length remains unchanged thereafter the electrochemical etching rateequals 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 thethickness of nanotube i.e. led to increase the Titania thickness. While theeffective concentration of Ammonium Fluoride leads to increase etching. Also,an applied voltage was found high affected on Titania size (pore, diameter, wallthickness and length of tube) and the uniformity [56].2-11-1-The effect of anodization potential Chemical etching rate is determined by the anodization potential.Nanotube structures are only found within a certain range of anodization 46
  47. 47. Fabrication of TiO2 Nanotubes Using Electrochemical Anodizationpotential. If the voltage is too low, the electrochemical etching rate is lower thanthat of chemical dissolution rate, which means that the rate of oxide growth atthe metal/electrolyte interface is lower than that of oxide dissolution at the pore-bottom/electrolyte interface. As a result, the thickness of the barrier layerdecreases and the pores form, which cannot grow into nanotubes. On the otherhand, if the voltage is too high, the electrochemical etching rate is much higherthan that of chemical dissolution rate, the thickness of the barrier layer increasesvery fast, which leads to the reduction of the electrochemical etching rate and [56] [67]retards the growth of pores into nanotubes . Gong et al. studied theinfluence of anodization potential on the formation of TiO2 nanotube arraysunder 0.5%wt. HF aqueous solution at room temperature. He showed that at lowanodization potential (voltage ~3V), only pores are found without the formationof a clear tube. When the voltage is increased to 10V, nanotube structure startsto appear. As the voltage further increases, the thickness, length and the innerdiameter of the nanotubes increase. However, such nanotube structuredisappears when the voltage is greater than 23V. Liang and Li‟s [73] found theeffect of voltage on the TiO2 morphology and shows similar trend. Under 0.1wt.%NH4F aqueous solution at room temperature, complete well-aligned nanotubearrays are found when the anodizing voltage is within 18V to 25V. The samelength of nanotube increase as voltage increases.2-11-2-The effect of electrolyte Electrolyte plays a crucial role for the TiO2 nanotubes formation sincechemical dissolution rate which is affected by the composition of the electrolyteand it is direct influential factor in the nanotubes formation. By increasing (F-)concentration is the chemical dissolution rate increased. Since nanotubes cannotbe formed when the chemical dissolution is too high or too low, only certain (F-)concentration range is in favor of the formation of nanotubes. Mor, Varghese, [74]Paulose, Shankar and Grimes‟ review paper summarized the participation of 47
  48. 48. Fabrication of TiO2 Nanotubes Using Electrochemical Anodizationother researchers on the effect of (F-) concentration on TiO2 nanotube formationand states that nanotube structures are found when the (F-) concentration wasbetween 0.05 to 0.3wt. %. The water content or viscosity of the electrolyte also affects themorphology of nanotube formation. From most studies on the formation of TiO 2nanotubes by anodization method, it can be seen that the side walls of Titanium [74]nanotube formed in water-based electrolyte are rough. Macak et al. inferredthat the distance between ridges on the Titanium nanotube side walls werecaused by the current transients during anodization. Hydrolysis reaction of the(Ti4+) cations is driven by the applied current.2-11-3-The effect of temperature Increasing anodization temperature are the Titanium nanotube lengths [76]and their thicknesses decreased. Crawford and Chawla studied the current-time behavior during anodization of (Ti) samples under different temperature.They are noticed that the rate of chemical dissolution increases with increasingelectrolyte temperature. As a result, the growth of the barrier layer is offset byhigher chemical dissolution rate and thinner nanotube walls are resulted.Besides, they study also reflects that nanotube formation occurs very rapidlywith 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 ispreferred. However, if the temperature is too low, the walls will be too thick thatthey fill the voids in the inter-pore areas, leading the tube-like structuresapproach a nanoporous structure in appearance [74]. 48
  49. 49. Fabrication of TiO2 Nanotubes Using Electrochemical Anodization2-11-4-The effect of Titanium annealing before and after anodizing The advantage of the annealing before the anodizing process is to get ridof defects and stresses in Titanium that you get during the cutting process orbefore. Freshly-prepared Titanium nanotube is amorphous. Post-thermaltreatment is essential for the crystallization of Titanium nanotube. Annealingtemperature has significant effect on the formation of different crystal types. [77]Several researches studied the relationship between them. Due to differentpreparation conditions during anodization, the as-prepared nanotubes might beincorporated with different impurities which affect the rate of phasetransformation. As a result, the similar phase pattern may not occur under sameannealing temperature range. However, the phase transformation follows thesame trend as annealing temperature increases. Taking in account the results [78]from as example, they show that when annealing temperature is below280°C, the Titanium nanotube remains amorphous; at about 300°C, smalldiffraction peak of anatase is detected except for the peaks of (Ti); as theannealing temperature increases, the peak intensity of anatase phase becomestronger and sharper; at approximately 430°C, apart from the peaks of (Ti) andantase, rutile phase appears with peaks of low intensity; as temperatureincreases, the rutile peaks grows while the anatase peak diminishes; beyond680°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 thecurrent density at steady stage decreases gradually as the distance increases, sodoes the electric field strength because of the resistance drop in organicelectrolyte [79]. 49
  50. 50. Fabrication of TiO2 Nanotubes Using Electrochemical Anodization The increment in the separation distance between the electrodes leds tosmall amounts of anode ions that can be mobilize and migrate since the electricfield is not strong enough due to the distance of separation [80]. 51
  51. 51. Fabrication of TiO2 Nanotubes Using Electrochemical Anodization 51
  52. 52. Fabrication of TiO2 Nanotubes Using Electrochemical Anodization Chapter Three Experimental and Methods3-1- Introduction This chapter is dedicated to the experimental work, which includes thepreparation of (TiO2) nanotube using electrochemical anodization method atdifferent conditions (time, voltage, electrolyte concentration and conductivity),as shows in section (3-4-2), and study the structural characterization of (TiO 2)nanotube of manufacturer during the measurements (XRD, SEM AFM andOptical interferometer). This method using electrolyte which consists of Fluoride and viscousorganic electrolyte (glycerol) were introduced.3-2- Chemicals and InstrumentationsIn 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 processItem Material Original Specification 99.7% ; thickness 1 Ti Foil Alfa Aesar A Johnson Mat they company 0.25mm 99.7% ; thickness 2 Pt Foil Sigma Aldrich company, Germany 0.25mm 3 NH4F BDH Chemicals Ltd pool England 99.5% 4 Glycerol BDH Chemicals Ltd pool England 99.5% 5 Ethanol China 99.7% pure 6 Acetone China 99.7% pure Deionized Conductivity 10 7 Baghdad university distilled water µs/cm 52
  53. 53. Fabrication of TiO2 Nanotubes Using Electrochemical Anodization3-2-2- Instrumentations Different types of instruments and apparatuses are used in process asshow in Table (3-2). Table (3-2) Origin, function and specification devicesItem Function Device type Original Specification 1 Cutting Commercial machine China Mechanical surface 1450 rpm. 2 Polishing polishing with paper Germany 240, 400, 600, 800 and 1200 glass different. grade USA Ultrasonic Cleaner with 3 Ultrasonic cleaner Ultrasonic cleaner Bransonic 3510R- Digital Timer/Heater 5L DTH Capacity Electrochemical 100mL 4 Teflon cell Home made cell - 220V, 50 Hz, 415 watt. 5 Agitation Magnate stirrer Germany - Stirrer and heater. - Digital Timer / Heater Voltage maximum 60V, 6 Voltage source Power supply China 3Amper Current Measurement of ( Voltage, 7 Avometer Malaysia measurement Current, Resistance ) Measurement of (Conductivity Conductivity (Kyoto electronic CO., 8 Conductivity meter in units µs/cm, range measurement LTD, CM-115) maximum 5000 µs/cm. 220V, Resolution: 0.26nm AA3000, Angstrom lateral, AFM Advanced Inc. USA 0.1nm vertical precision of 50nm Analysis and 9 Hitachi FE-SEM 0.5 - 20 kV characterization SEM model S-4160, Japan XRD Germany 20 kV, 30 mA Optical He-Ne laser of wavelength Germany interferometer (632.8nm) was used 53
  54. 54. Fabrication of TiO2 Nanotubes Using Electrochemical Anodization3-2-3- Process flow chart of TiO2 nanotubes synthesis: Pure metal Ti Foil Cutting Annealing at 500 ºC for 3hr. Degreasing by (Ethanol and Acetone for 15min.) Polishing by (glass paper) Rising by DI for 15min. Degreasing by (Ethanol and Acetone for 15min.) Rising by DI for 15min. Drying by N2 for 5min. Anodizing process Anodizing in Teflon Cell Using Electrolyte (NH4F, H2O and Glycerol) 0.5-2wt.% NH4F with Glycerol 0.5wt.% NH4F+ 2 and 5wt.%H2O with Glycerol Rising by DI and Drying by N2 Stream for 5min. Annealing at 450 ºC and 530 ºC for 3hr. Characteristics XRD AFM SEM Optical interferometer 54
  55. 55. Fabrication of TiO2 Nanotubes Using Electrochemical Anodization Figure (3-1) Flow chart of TiO2 nanotube synthesis3-3- Electrochemical Anodization System A schematic diagram of anodization for Titanium oxidation system isshown in Figure (3-2a and b). This system consists of electrochemical Tefloncell, two electrodes (cathode and anode), power supply (DC current), magneticstirrer, Avometer, and suitable electrolyte for process. These electrochemicalprocesses were performed at room temperature (~25ºC). a b Figure (3-2a and b) Schematic and photograph of set-up illustrates of the anodization experiment with Teflon cell3-3-1-Electrochemical Anodization Cell Design Anodization cell is designed for first time in Iraq according to ourknowledge to produce self-ordered Titanium nanotubes. The main parts of thecell are composed of two electrodes and a stirrer. An anode was made from abrass plate to hold and to allow current through the sample Titanium andPlatinum foil (1cm × 0.5 cm) was used to be a cathode, and the distance betweenthe anode and cathode was 3 cm. The advantage of Pt used in our system is to beinert against the solutions which were used during of the processelectrochemical anodization as electrolytes. All of the components were set up and put them into the cell. During theprocess, the electrolyte is stirred vigorously by using magnetic. “The aims of 55
  56. 56. Fabrication of TiO2 Nanotubes Using Electrochemical Anodizationvigorously stirring electrolyte are to prevent local heat occurring on the surfaceof titanium and also to make sure that heat and electrolyte distribute uniformly”.The cell was used during Titanium anodization process; it is homemadedesigned in Baghdad See figure (3-3). A cell made of Teflon material rectangular hollow length of 10 cm andwidth 5 cm and a capacity of 100 ml, it has open two sides, the first side theupper end, which enters through the cathode and the electrolyte, either thesecond side, O-ring sealed hole equipped with a copper disk to support Titaniumanode electrode and facilitate electrical connection, and is installed by screws ofiron and put between the anode and the electrolyte washer (from rubber) toprevent the flow of the solution outside the cell as show in figure (3-3). Figure (3-3) Schematic diagram of homemade Teflon cell3-4- Samples preparation This section shows how to prepare (TiO2) nanotubes and the operationsconducted 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 conductivebrass in the base holder of a Teflon cell figure (3-2). The sample was degreased 56
  57. 57. Fabrication of TiO2 Nanotubes Using Electrochemical Anodizationby sonication in a solution of acetone and ethanol for 15 min respectively, andthen washed in (DI) deionized water. Before anodizing, the Titanium sampleswere annealed at 500ºC for 3 h to remove the mechanical stress and to enhancethe grain size, and then cooled in air. The titanium foil Ti surface wasmechanical surface polished with glass papers starting from 240 and increasingto 400, 600, 800 and 1200 with diamond paste. Intermittently after polishingwith different SiC papers, the surface was washed with (DI) deionized water torinse off any particles generated while polishing. Ultrasonic cleaning in acetone,ethanol and (DI) deionized water respectively for about 15 minutes was doneafter polishing to clean the surface more effectively then dried with (N 2)nitrogen stream; After mechanical polishing process is completed, sample put inTeflon 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 andPlatinum served as the counter electrode. The anodizing process was preparedwith the conditions as shown below: 1. Using (NH4F + glycerol) electrolyte without water at room temperature (~25ºC) using different composition, where increasing the number of anodizing electrolyte conductivity increases as show in Table (3-3). 57