二氧化鈦奈米管陣列與複合結構之形貌與結構特性及其光催化性質之研究

國 立 交 通 大 學

材 料 科 學 與 工 程 學 系

博 士 論 文

二氧化鈦奈米管陣列與複合結構之形貌

與結構特性及其光催化性質之研究

Morphological and Microstructural

Study on TiO

₂

Nanotube Arrays and

Hybrid Structure and Their

Photocatalytic Performance

研 究 生 : 徐 明 義

指導教授 : 呂 志 鵬 博士

二氧化鈦奈米管陣列與複合結構之形貌與結構特性及其光

催化性質之研究

Morphological and Microstructural Study on TiO

2

Nanotube Arrays

and Hybrid Structure and Their Photocatalytic Performance

研 究 生：徐 明 義 Student：Ming-Yi Hsu

指導教授：呂 志 鵬 博士 Advisor：Dr. Jihperng Leu

國 立 交 通 大 學

材 料 科 學 與 工 程 學 系

博 士 論 文

A Thesis

Submitted to Department of Materials Science and Engineering

College of Engineering, National Chiao Tung University

in partial Fulfillment of the Requirements

for the Degree of Doctor of Philosophy

May 2012

Hsinchu, Taiwan, Republic of China

二氧化鈦奈米管陣列與複合結構之形貌與結構特性及其光催化性質之研究

研究生: 徐明義 指導教授: 呂志鵬 博士

國立交通大學材料科學與工程學系 博士班

摘要

本研究針對使用陽極處理法製備二氧化鈦奈米管陣列(TiO2 nanotubes arrays,

TNAs)與其奈米線複合結構(TNWs/TNAs)之製程參數與後熱處理方法進行一連 串 的 顯 微 結 構 分 析 與 形 貌 演 進 之 探 討 。 首 先 ， 以 兩 種 不 同 氟 化 物 : 氫氟酸 (Hydrogen fluoride, HF)與氟化銨(Ammonium fluoride, NH4F) 製備 TNAs 後，進行

不同溫度之後熱處理，再以 X-光粉末繞射儀(XRD)、掃瞄式電子顯微鏡(SEM) 和 X 光近緣結構(XANES)進行分析討論。其結果顯示以 HF 製備出 TNAs 含 90% 非晶相(amorphous)與 10% Ti2+ (TiO) 與 Ti3+ (Ti2O3)之低氧量鈦化物。而經過

400oC 熱處理之後，其結構轉變成 93%銳鈦礦相(anatase)、6% 非晶相與 1% 低 氧鈦化物。相反地，以 NH4F 製備出 TNAs 則是含較低之非晶相 TiO2 (82%)與較 高的低氧鈦化物(18%)，其原因乃是由於電解質中只有少量的 1 wt% 水添加量， 使得溶液中氧離子供應量不足而產生較多低氧鈦化物。經過 400oC 熱處理之後， 其二氧化鈦奈米管之結晶度只增加至 86%銳鈦礦相。此低結晶度可推論是因為 NH4解離之 NH4+與 TiF62- 反應形成(NH4)2TiF6化合物所致。 另一方面，本研究利用準分子雷射以垂直式(parallel mode)與旋轉式(tilted mode)兩種方法來進行在二氧化鈦後熱處理。其研究結果發現垂直式照射試片表 面之後熱處理只能達到相較 400oC 1h 後熱處理之 50%結晶度。因為垂直式後熱 處理使得一維二氧化鈦奈米管的熱傳遞方向只能從表面開始向下產生相變化，再

加上過薄的滲透深度與太短的脈衝持續時間(25 ns)，而限制了材料之結晶度。然 而，若使用旋轉式，當角度旋轉至與雷射源呈 85o時，其結晶度可達相較 400oC 1h 後熱處理 90%之結晶度。推測因為旋轉式增加了雷射照射的面積以及較佳的雙向 熱傳導方式為其主要原因。 此外，本研究亦順利以乙二醇(ethylene glycol)與 NH4F 之含水電解質在無攪 拌環境中以一階段方式製備出二氧化鈦奈米線直接連接奈米管之複合結構 (TNWs/TNAs)，實驗中利用改變電壓與時間所得之結構形貌來推論其複合結構形 成機制包含以下四個步驟: (1) 在未攪拌系統中產生管壁厚度不均勻的 TNAs，其 管口部分受到蝕刻使得管壁漸漸變薄，(2) 較薄的管壁開始被蝕刻出小洞，且小 洞開始連結，(3) 連結的小洞將 TNAs 漸漸分開成奈米線，最後(4) 奈米線的尺 寸也受到蝕刻而隨著時間越來越細。除此之外，在光催化性質之分析結果發現: 由於較高之比表面積與電子傳輸特性，二氧化鈦奈米線/奈米管之結構相較二氧 化鈦奈米管對具有較佳之光催化特性，而且 20 nm TNWs/40 nm TNAs 可達到與 二氧化鈦奈米粉末相接近光觸媒特性。

Morphological and Microstructural Study on TiO2 Nanotube Arrays and Hybrid

Structure and Their Photocatalytic Performance

Student: Ming-Yi Hsu Advisor: Dr. Jihperng Leu

Department of Materials Science and Engineering National Chiao Tung University

Abstract

In this study, the evolution of morphology and microstructure of anodized TiO2

films by changing the anodizing parameters and post annealing process were investigated and compared. First, TiO2 nanotube arrays fabricated with HF and NH4F

electrolytes as a function of annealing temperature up to 400 oC was investigated and compared using x-ray diffraction (XRD), scanning electron microscopy (SEM), and x-ray absorption near-edge structure spectroscopy (XANES). Results showed that TiO2 nanotube arrays grown in HF electrolyte contained 90% amorphous TiO2 and

10% lower oxidation states of titanium from Ti2+ (TiO) and Ti3+ (Ti2O3) cations. After

annealing at 400oC, TiO2 nanotube arrays underwent charge transfer and phase

transformation to 93% anatase phase, 6% amorphous TiO2, and 1% suboxides. In

contrast, as-grown TiO2 nanotube arrays using NH4F electrolyte possessed less

amorphous TiO2 (82%)but more suboxides (18%) due to lower oxygen ion formation

from scanty 1wt% H2O addition. Moreover, when annealed to 400oC, the crystallinity

anatase phase could be attributed to the formation of (NH4)2TiF6 type compounds

presumably formed by the reaction of TiF62- and NH4+ ions dissociated from NH4F.

On the other hand, for TNA post annealing technology, the excimer laser annealing (ELA) were investigated as a function of the laser fluence using parallel and tilted modes. Results showed that the crystallinity of the ELA-treated TNAs reached only about 50% relative to that of TNAs treated by furnace anneal at 400oC for 1 hr. The phase transformation starts from the top surface of the TNAs with surface damage resulting from short penetration depth and limited one-dimensional heat transport from the surface to the bottom under extremely short pulse duration (25 ns) of the excimer laser. When a tilted mode was used, the crystallinity of TNAs treated by ELA at 85o was increased to 90% relative to that by the furnace anneal. This can be attributed to the increased area of the laser energy interaction zone and better heat conduction to both ends of the TNAs.

Furthermore, TiO2 nanowires connected directly with TiO2 nanotubes arrays

(TNWs/TNAs) were successfully fabricated with a mixture of ethylene glycol and water that contained NH4F electrolyte via a one-step method without mechanical

stirring. The morphology of the TNWs/TNAs structure was investigated by changing the anodizing voltage and processing time to elucidate its formation mechanism. Well-developed anodic oxide nanowires are only observed under specific anodizing voltage and processing time conditions. The evolution of TNWs follows four stages: (1) thinning of the tube wall thickness with high roughness near the TNA mouths, (2) forming strings of through holes in the upper section of the TNAs, (3) splitting into nanowires, and (4) collapsing and further thinning of nanowires. For photocatalytic application, TNWs/TNAs film demonstrated a better photocatalytic performance than

TNWs/TNAs film (20 nm wire/40 nm pore diameter) achieved a performance comparable to that of the film made from TiO2 nanoparticles.

Acknowledgement

本研究之得以順利完成，乃歸因眾人的支持與鼓勵。首先感謝我的指導教授 呂志鵬老師細心地指導與提攜，使學生在研究與待人處事更加精進成熟。同時也 要感謝逢甲大學材料科學與工程學系何主亮教授、中正大學光機電整合工程研究 所丁初稷教授、本校電子物理系羅志偉教授及本系張立教授在學生論文的指正及 建議，使本論文能更加的完整。 特別要感謝的是研究室車牧龍博士在SEM 微觀組織觀察上的協助。同時要 感謝王尉霖博士在TEM 晶相分析上的幫忙。另外也要感謝同步輻射中心李振民 經理及助理們，在XAS 顯微分析上的協助。 當然還有許多伴隨我度過這六年，在精神上不斷給予加油打氣的學長姐、同 學、學弟妹及好朋友們(啟仁、國原、宗琦、幸鈴、昱涵、耀庭、王智、少農、 弘恩、柏村、詩雅、婉婷、瑜修、書豪、孝謙、奎岳、沁穎、勝翔、修誠、欣熠、 丞芳、維剛與雅婷等)，由衷感謝大家的幫忙與支持使本論文得以順利完成，碩 誼濃情，深表謝忱。 最後我要感謝我夫人阡茹長久以來對我的包容與體諒，以及我摯愛雙親、哥哥及 岳父母的支持與鼓勵，您們一直是我努力向前的動力，沒有您們我可能無法順利 完成學業。最後願以此論文的榮耀與我的家人及好友們共同分享。

Contents

Page

摘要 ... I Abstract ...III Acknowledgement ...VI Contents ... VII Table Captions ...IX Figure Captions...X Explanation of abbreviations and Symbols...XIII

Chapter 1 Introduction ...1

1.1 Background...1

1.2 Objectives of the thesis ...3

1.3 Overview...4

Chapter 2 Literature Review...6

2.1 Introduction of TiO2 materials...6

2.2 Synthesis of TiO2 nanostructure ...7

2.3 Anodic oxidization technique ...8

2.3.1 The developments of anodized TiO2 nanotubes ...8

2.3.2 The growth of TiO2 nanotubes: fundamental aspects ...10

2.3.3 Applications of TiO2 nanotubes arrays ...13

2.3.4 Challenges of TiO2 nanotubes arrays ...16

2.4 Annealing of the nanotubes ...17

2.4.1 Excimer Laser Crystallization of TiO2...17

Chapter 3 Experimental Section ...30

3.1 Materials candidates...30

3.1.1 TiO2 nanotubes arrays structure (TNAs) ...30

3.1.2 TiO2 hybrid structure (TNAs/TNWs) ...30

3.2 Post annealing of TiO2...31

3.4 Characterization of key propertirs ...32

3.4.1 Morphology and microstructure characterization of the TNAs...32

3.4.2 X-ray absorption spectroscopy ...32

3.4.3 Surface areas measurement...33

3.5 Photocatalytic reaction experiments ...33

Chapter 4 The evolution of microstructure and composition of TiO2 nanotube arrays during annealing...38

4.2.1 XRD analysis ...39

4.2.2 XANES analysis ...40

4.3 Summary...47

Chapter 5 Structural and morphological transformation of TiO2 nanotube arrays induced by excimer laser treatment ...57

5.1 ELA treatment of TNAs in a parallel mode...57

51.1 XRD microstructure analysis...57

5.1.2 Morphological observation ...58

5.1.3 TEM and SAD analysis ...59

5.2 ELA treatment of TNAs in a tilted mode...61

5.2.1 XRD ...61

5.2.2 SEM ...61

5.2.3 TEM ...62

5.3 ELA treatment vs. 400oC furnace annealing...62

5.4 Summary...64

Chapter 6 TiO2 nanowires on anodic TiO2 nanotube arrays: formation mechanism and their photocatalytic performance...72

6.1 Influence of anodizing voltage...72

6.2 Influence of anodizing time ...72

6.3 Formation mechanism of the TNWs/TNAs...76

6.4 Photocatalytic reaction experiments ...78

6.5 Summary...82

Chapter 7 Conclusion ...91

References ...93

Table Captions

Page

Table 2.1 Different TiO2 polymorphs and some of their physical properties...20

Table 2.2 Average wall thickness and tube length of 10 V TiO2 nanotube arrays

anodized in HF aqueous electrolyte at different bath temperatures...21 Table 4.1 Intensity ratios of orbitals for Ti L3 edge of TNAs prepared in HF and NH4F

electrolytes: as-grown and post annealing at various temperatures, compared to those of Ti, anatase and rutile TiO2 from literatures...48

Table 4.2 Fitting results for the amount of amorphous and crystalline phases, and composition of TNAs in HF and NH4F solution as-grown and post annealed

at various temperatures ...49 Table 5.1 The XRD peak intensity ratios of TNAs annealed in conventional furnace

anneal at 400o_{C, 1 hr and excimer laser in parallel and tilted modes at a}

fluence of 125 mJ/cm2 _{for a total of 9000 shots. ...65}

Table 6.1 Dye adsorption, reaction rate constants (k), and change percentage of

Figure Captions

Figure 2.1 TiO2 different crystal structures: (a) Rutile, (b)Anatase, and (c) Brookite.22

Figure 2.2 High resolution SEM image of anodized Ti6Al4V (TA6V) in CA/HF

solution (10 V, 20 min)...23

Figure 2.3 Lateral view of the nanotubes formed in 0.1M KF, 1M H2SO4, and 0.2M citric acid solutions(25V, 20h) ...24

Figure 2.4 A comparison between SEM cross section images of nanotubes prepared in (a) an aqueous based and (b) organic electrolyte ...25

Figure 2.5 Schematic diagrams illustrating the formation mechanism of TiO2 nanotubes structures (a) oxide layer formation, (b) semicircle pores formation on the oxide film, (c) growth of the semicircle pores into scallop shaped pores, and (d) fully developed nanotube arrays...26

Figure 2.6 Schematic of the pH profile developing within the tubes . ...27

Figure 2.7 Schematic diagram of the formation of tube spatial periodicity under different conditions: (a) without stirring, (b) at medium stirring rate, and (c) at high stirring rate ...28

Figure 2.8 Mechanistic principles for the degradation of pollutants. ...29

Figure 3.1 Schematic diagram of anodization reaction system ...35

Figure 3.2 The schematic diagram of KrF excimer laser system ...36

Figure 3.3 Schematic diagrams of laser anneal systems for TNAs in (a) parallel mode and (b) tilted mode ...37

Figure 4.1 Cross-section and surface morphology of TiO2 nanotube arrays prepared by anodic oxidation in (a) HF solution for 4 hr (b) NH4F solution for 24h and (c) NH4F solution for 0.5h. ...50

Figure 4.2 XRD patterns of the TiO2 nanotube arrays prepared by anodic oxidation in (a) HF solution and (b) NH4F solution: as-grown and post-annealing at 100, 200, 300, and 400 oC...51

Figure 4.3 Ti L2,3 edge XANES spectra of TiO, Ti2O3, and TiO2 nanotube arrays prepared by anodic oxidation in HF solution: as-grown and post-annealing at 100, 200, 300, and 400 o_{C. ...52}

Figure 4.4 Ti L2,3 edge XANES spectra of TiO, Ti2O3, and TiO2 nanotube arrays

prepared by anodic oxidation in NH4F solution: as-grown and

post-annealing at 100, 200, 300, and 400 oC...53 Figure 4.5 O K edge XANES spectra of TiO, Ti2O3, and TiO2 nanotube arrays

prepared by anodic oxidation in (a) HF solution and (b) NH4F solution:

as-grown and post-annealing at 100, 200, 300, and 400 o_{C. ...54}

Figure 4.6 Fitting results for the O K-edge XANES spectra of TiO2 nanotube arrays

prepared by anodic oxidation in HF solution: (a) as-grown, and post-annealing at (b) 200 oC, (c) 300 oC, and (d) 400 oC...55 Figure 4.7 Fitting results for the O K-edge XANES spectra of TiO2 nano-tube arrays

prepared by anodic oxidation in NH4F solution: (a) as-grown, and

post-annealing at (b) 200 o_{C, (c) 300} o_{C, and (d) 400} o_C...56

Figure 5.1 XRD patterns of the TNAs prepared by anodic oxidation in NH4F solution

(as-grown) and annealing by excimer laser with various fluences at 9000 shots. ...66 Figure 5.2 Surface and cross-section morphology of the TNAs annealing by excimer laser with various fluences: (a) 67 mJc/m2_{, (b) 125 mJ/cm}2_{, (c) 133 mJ/cm}2

(d) 267 mJ/cm2_{, and (e) 400 mJ/cm}2 _{at 9000 shots...67}

Figure 5.3 TEM images of the TNAs annealing by excimer laser at 400mJ/cm2 for 9000 shots: (a) full cross-section, (b) the top part, (c) middle part (d) the bottom part of corss-section, and SAD patterns of (e) the top part and (f) the bottom part of cross-section...68 Figure 5.4 The XRD patterns of TNAs annealed by conventional furnace at 400 o_{C for}

1 hr and excimer laser in parallel mode and tilted mode at a fluence of 125mJ/cm2 _{for a total of 9000 shots. ...69}

Figure 5.5 Surface morphologies of TNAs for laser annealing in (a) parallel mode, (b) 30o (c) 75o, and (d) 85o in tilted mode at a fluence of 125 mJ/cm2 for a total of 9000 shots...70 Figure 5.6 TEM images of TNAs annealing by excimer laser in 85o _{tilted mode at a}

fluence of 125 mJ/cm2 _{for a total of 9000 shots: (a) full cross-section, (b)}

the top part, (c) middle part (d) the bottom part of corss-section, and SAD patterns of (e) the top part and (f) the bottom part of cross-section. ...71

Figure 6.1 Surface morphology of the TiO2 films prepared by anodic oxidation under

different anodizing voltages: (a) 20 V, (b) 40 V, (c) 60 V, and (d) 80 V, with a constant anodizing time of 1 h. ...84 Figure 6.2 Surface morphology of the TiO2 films prepared by anodic oxidation under

different anodizing time: (a) 30 min, (b) 35 min, (c) 38 min, and (d) 40 min, with a constant anodizing voltage of 40 V. ...85 Figure 6.3 Surface (1) and cross-section (2) morphologies of the TiO2 films prepared

by anodic oxidation at different anodizing time: (a) 45 min, (b) 60 min, (c) 90 min, and (d) 120 min, with a constant anodizing voltage of 40 V; (d3) surface morphology at low magnification. ...86 Figure 6.4 (a) Conditions of required anodizing voltage and processing time (shaded zone) for forming TNWs/TNAs. (b) The pore diameter and wall thickness of TNAs top section prior to the emergence of nanowires, as a function of voltage. For cases without TNWs formation, a processing time of 30 min was used...87 Figure 6.5 Schematic diagrams along with their corresponding surface morphology

SEM images for four key stages in the TNWs/TNAs formation mechanism: (a) thinning the tube wall thickness with high roughness near the TNAs mouths, (b) forming strings of through holes in the top section of TNAs, (c) splitting into nanowires, and (d) collapsing and further thinning of nanowires. ...88 Figure 6.6 Photocatalytic degradation of MB under UV light irradiation, (C/Co) vs. reaction time plots for various TNAs, TNWs/TNAs, and TiO2

nanoparticles films. ...89 Figure 6.7 UV-visible spectra of the desorbed dye from the solution of various TNAs, TNWs/TNAs, and TiO2 nanoparticles films...90

Explanation of abbreviations

and

Symbols

I Absorption intensity Eg Band gap enegy

C Concentration ρ Density E Electric field strength τ Pulse duration time R Reflectivity Cp Specific heat

k Thermal conductivity

LT Thermal diffusion length

Dth Thermal diffusivity

λ Wavelength DSSC(s) Dye sensitized solar cell(s) EG Ethylene glycol ELA Excimer laser annealing

FIB/SEM Focused ion beam scanning electron microscope HRTEM High resolution transmission electron microscopy LFS Ligand-field splitting

MB Methylene blue 1D One dimensional FESEM Scanning electron microscopy SAD Selected area diffraction TNA(s) TiO2 nanotube arrays

TNW(s) TiO2 nanowires

TiO2 Titanium dioxide

TEM Transmission electron microscopy

XANES X-ray absorption near-edge structure spectroscopy XRD X-ray diffractometer

Chapter 1 Introduction

1.1 Background

Since the first report on electrochemical photolysis of water at titanium dioxide (TiO2) electrodes by Fujishima and Honda [1], enormous efforts have been devoted to the

research of TiO2 material, which has led to many promising applications in areas ranging

from electrochromic [2,3], photocatalytic devices [4,5,6], sensors [7,8], and solar cells [9,10,11]. The applications of TiO2 are primarily determined by its properties such as

crystalline structure, specific surface area, particle size, porosity, and thermal stability. TiO2 can exist in anatase, rutile or brookite crystalline phase [12], in addition to

amorphous phase. For photocatalytic application, the anatase phase is more active than the rutile phase, due to its larger band gap and lower electron-hole recombination probability [13]. Also, increased degree of anatase phase in crystalline TiO2 was found to

enhance the catalytic activity, for example, in the photocatalytic degradation of organic pollutants [14,15]. Such enhancement of photocatalytic activity could be attributed to the reduction of amorphous domains, defects, or impurities, which acted as the recombination centers for photogenerated electrons and holes [16,17]. For dye sensitized solar cells (DSSCs) application, it was also found that electron transport was slower in rutile phase than in anatase phase due to their difference in the extent of interparticle connectivity associated with the TiO2 particle packing density [18].

In addition, in order to conform to flexible devices regulations, low-temperature processing is an important factor. But, in TiO2 fabrication, to increase the crystallinity

of TiO2 structure after formation, post annealing is typically required to change its

structure from amorphous to anatase phase. Nevertheless, thermal annealing treatment using a conventional high temperature furnace annealing takes hours to complete the

transformation of the crystal structure of the film. To expedite the annealing time or deliver low-temperature processing for flexible devices using polymer substrates with low glass transition temperature and poor thermal stability, excimer laser annealing (ELA) is one of the preferred fast processing technologies [19,20]. It has been reported that the phase transition from amorphous to anatase or rutile phase is observed within a nanosecond time scale by laser treatment [21,22]. So far, most studies have been carried out on TiO2 powders [22] or thin films [23]. However, only a small depth of material is

processed upon ELA irradiation because most extreme heating of pulsed laser annealing is confined to the near-surface region of the sample due to the short duration of the UV laser pulse. The challenge is to overcome the limited penetration depth in thick TNAs.

In addition to the microstructure, the electron transport is another critical property influencing TiO2 applications such as DSSCs [24]. In recent years, various forms of

TiO2 nanostructure such as nanorods, nanowires, and nanotubes have attracted

significant research interests in hope to achieve higher charge carrier transport than the nanoparticles [ 25 , 26 ]. Several studies reported that one dimensional (1D) nansostructures could improve the charge-collection efficiency by promoting faster transport and slower recombination due to its axial transport path as compared to the random transport path in nanoparticles [9,27]. TiO2 nanotube arrays (TNAs) grown by

electrochemical anodization method was first reported by Zwilling et al. [28] with a length up to 500 nm (10:1 aspect ratio) using HF-based aqueous electrolyte. Moreover, several neutral electrolytes have been employed to prepare anodized TiO2 nanotubes

with higher aspect ratio [29,30]. In specific, high aspect ratio (100:1), self-organized TiO2 nanotubes could be obtained from ethylene glycol (EG) solution [30]. However,

most of studies are dedicated to one specific type of 1D TiO2 structures with less

TiO2 nanowires directly connected TiO2 nanotubes arrays structure (designated as

TNWs/TNAs) using EG and NH4F under mechanical stirring and proposed a

bamboo-splitting model. Moreover, Wang et al. [ 32 ] used a TNWs/TNAs hybrid structure, prepared by anodization and hydrothermal two-step method, for dye-sensitized solar cells application. The DSSCs with such TiO2 hybrid structure exhibited higher

photovoltaic parameters and a lower dark current [32]. Yet, the details of the formation mechanism for TNWs/TNAs still need to be clarified. Also, little work has been reported on the applications of TNWs/TNAs and TNAs for their photocatalytic properties.

1.2 Objectives of the thesis

Based on the above description, there are still some weaknesses and controversies to be resolved and the primary aim of this thesis is to investigate some of them and to provide new insights on the subject. First, we are interested in the microstructures and composition of TiO2 nanotube arrays as prepared by anodic oxidation in HF and NH4F

electrolytes. In addition to conventional X-ray diffraction (XRD), X-ray absorption near-edge structure spectroscopy (XANES) was employed to probe element-specific partial density of empty electronic states in different polymorphs of TiO2 and any lower

oxidation states such as TiO and Ti2O3 [33,34,35]. Furthermore, the evolution of

microstructure and suboxides (TiO and Ti2O3) in the TiO2 nanotubes prepared by HF

and NH4F electrolytes as a function of annealing temperature was investigated and

compared. The mechanism responsible for their difference in XANES spectra of different amount of crystalline, amorphous phases and lower oxidation states of titanium in TiO2 nanotube arrays will be proposed.

Also, excimer laser annealing treatment on TNAs has been optimized for the production of crystalline structure. The evolution of morphology and microstructure in TNAs were investigated as functions of laser fluence. The mechanisms of phase transition and the changes of the surface of TNAs induced by laser are proposed. Furthermore, laser annealing in a tilted mode has been developed to resolve the limited penetration depth in a parallel mode. Its difference in the structural and morphological transformation of TNAs will be examined and discussed.

Furthermore, this study proposes a one-step method for the fabrication of a TNWs-covered TNAs hybrid structure, using a mixture of EG and water containing NH4F electrolyte without mechanical stirring. The morphology of the TNWs/TNAs

structure was then examined by changing the anodizing voltage and processing time, to elucidate the detailed formation mechanism of TNWs/TNAs. The photocatalytic degradation of methylene blue (MB) using various TNWs/TNAs and TNAs structures was investigated and compared with the film made of TiO2 nanoparticles.

1.3 Overview

The goal of this thesis is to study the structural and morphological transformation of TNAs and TiO2 hybrid structure using electrochemical anodization techniques, and

furthermore approach some photocatalytic applications of these materials. The important electrochemical parameters controlling the growth of the hybrid structure have been extensively studied and optimized. Also, appropriate post-fabrication processing conditions such as heat treatment conditions have been optimized for the high crystallinity TiO2. Chapter 2 offers a literature survey on the TiO2 materials,

development of fabrication methods for TiO2 nanotubes, and annealing method for TiO2

Chapter 4 discusses the microstructure and composition of TiO2 nanotube arrays

fabricated with HF and NH4F electrolytes and their evolution during annealing. Chapter 5

discuses the structural and morphological transformation of TiO2 induced by excimer

laser annealing. Chapter 6 describes the formation mechanism of TiO2 hybrid structure

(TNWs/TNAs), and presents the photocatalysis applications. Finally, Chapter 7 provides a summary of the key results in this study.

Chapter 2

Literature Review

This chapter is a review of journals and references available in literature that are relevant to the research topic. Section 2.1 is an introduction of TiO2 materials. Section 2.2

is synthesis of TiO2 nanostructure, and section 2.3 is a discussion on anodic oxidization

technique. Finally, the annealing method for TNAs crystallization is described in section 2.4.

2.1 Introduction of TiO

2

materials

Titanium dioxide (TiO2) is the most commonly used compound of titanium. Since

its commercial production in the early twentieth century, TiO2 has been widely used as

a pigment in sunscreens, paints, ointments, and toothpaste. TiO2 powder is chemically

inert, stable under sunlight, and is very opaque: This allows it to impart a pure and brilliant white color to the brown or gray chemicals that form the majority of household plastics. However, in 1972, Fujishima and Honda discovered the phenomenon of photocatalytic splitting of water on a TiO2 electrode under ultraviolet (UV) light. Since

then, enormous efforts have been devoted to the research of TiO2 [36].

TiO2 can exist in three distinct crystalline polymorphs: anatase, rutile, or brookite

crystalline phase, respectively shown in Fig. 2.1 [37]. From Fig. 2.1, all three crystal structures are made up of distorted octahedra, each one representing a TiO6 unit, where

each Ti4+ is at the centre of the unit and coordinates six O2- ions. The manner in which the octahedra assemble to form a TiO6 based chain is different and characteristic of each

polymorph. In these three phases, rutile and anatase are the most commonly synthesized phases. Anatase and brookite are metastable phases and convert into rutile at high

temperature, usually above 600 °C [38]. Table 2.1 lists some of the key properties [37,39] of the three TiO2 polymorphs. Both rutile and anatase have a tetragonal crystal structure,

whereas brookite has an orthorhombic symmetry. Rutile is the densest phase and has the highest refractive index, while anatase is characterised by the widest band-gap (~3.2eV) [39]. The properties (density, band-gap and refractive index) of brookite fall between those of rutile and anatase.

2.2 Synthesis of TiO

2

nanostructure

The wide ranging properties of TiO2 have generated a great deal of interest in many

different fields. Many researchers have focused their studies on the production and processing of nanostructure TiO2. One of the most obvious advantages provided by

nano-materials is that of the greatly increased surface area offered by small nanoparticles. For example, in DSSCs applications, the active electrode in the DSSC is composed of a high surface area TiO2 nanoparticle film that carries an anchored organic dye [2]. However, as

the injected electrons, which excited from dye, diffuse through the TiO2 particle network

to the collecting transparent conducting oxide (TCO) substrate, due to the randomly packed TiO2 nanoparticle (NP) films, it has been inferred that transport is limited by the

residence time of electrons in traps. In recent years, in order to reduce random-walk effects and suppress potential recombination at grain boundaries, 1D morphologies such as nanowires, nanorods and nanotubes have been explored as an alternative to nanoparticle based films. Thus, various approaches such as template synthesis [40,41,42], chemical vapor deposition (CVD) [43], hydrothermal reactions [44,45], and anodic oxidization [28, 46 , 47 , 48 ] have been developed for preparing TiO2

nanostructure. Among these approaches, anodic oxidization is a relatively low cost process and represents a simple technique that can be easily automated for preparing TiO2

nanostructure. The following part of the chapter reviews the most relevant achievements regarding the research on anodic oxidization TiO2 nanotubes, along with a discussion of

current ideas and understanding of the process.

2.3 Anodic oxidization technique

2.3.1 The developments of anodized TiO2 nanotubes

First generation of TiO2 nanotubes

In 1999, Zwilling and co-workers anodized Ti and Ti–6Al–4V(TA6V) alloy in an electrolyte containing 0.5 mol/l chromic acid and 0.095 mol/l HF [28], while the first report on anodized TiO2 nanotubes (called first generation). A typical porous structure

obtained in TA6V is shown in Fig. 2.3. It was clear that the nanoporous structure observed only formed when sufficient HF was added to the electrolyte mixture, as pure chromic acid (CA) was leading to the formation of a thin but stable oxide layer with no apparent pore structure. However, unlike anodized alumina, where tube length increases indefinitely with anodizing time, TiO2 nanotubes reach a steady state length when

anodized. That is, after typically 10 to 20 minutes of anodization, the etching rate equals the dissolution rate so that the tube length does not show any further increase with additional anodizing time [28].

Second generation of TiO2 nanotubes

In subsequent work, Grimes and co-workers overcome this limitation since they used other fluorine salts (as fluorine ion source besides HF) and combined buffers, bases and milder acids to adjust the pH and fluorine ion content. Salts like KF, NH4F or NaF totally dissociate in aqueous solution and then hydrolyze with water to form HF [49,50]. Moreover, HF is a relatively mild acid and in acidic solutions (pH<3.45) more than 50%

of the fluorine exists in the form of HF. As a result pH and fluorine ion concentration are closely related (and solutions with KF, NaF or NH4F and no additional acid are basic.)

The experiment worked found that they could grow nanotubes up to 4.4 m (Fig. 2.4) using a solution of 0.1M KF as fluorine source, 1M H2SO4 as acid, 0.2 M citric acid

presumably serving as buffer and NaOH as base to be added until the desired pH of 4.5 was obtained [50]. Later in 2005 Grimes and coworkers reported even longer nanotubes of up to 6 m, over 17 to 20 h of anodization using the same electrolyte as before [51,52].

Third generation of TiO2 nanotubes

The third generation of nanotubes refers to smooth tubes (i.e. no ripples along the wall), prepared in organic electrolytes (some almost water-free), where the dissolution rate of the forming oxide is minimized. For clarity a comparison of SEM cross section images, taken from literature [30], of nanotubes grown respectively in an aqueous and in an organic environment are shown, respectively, in Fig.2.5. It can be observed that tubes obtained in water are much rougher an irregular (i.e. ripples along the wall) than the smooth tubes grown in organic solution. The reason was to use a viscous electrolyte, where ion diffusion is slower than in water, to increase the pH gradient between the bottom and the top of the tubes. This led to the formation of TiO2 nanotubes up to 7 m

thick (compared to 0.5 m for first generation). They also attributed the smoothness and the regular morphology of the tube walls to the lower diffusion coefficient of the electrolyte which suppresses pH bursts at the pore bottom which occur when working in aqueous media. Over the last few years, TiO2 nanotube arrays with lengths of up to

approximately 1000 m were achieved using a non-aqueous, polar organic electrolyte such as formamide, dimethyl sulfoxide, ethylene glycol or diethylene glycol [53,54,55]. In 2007, Grimes and co-workers published the synthesis of 0.36 mm long nanotubes [53],

practically demonstrating that the nanotube lengths was only limited by the initial titanium foil thickness. The following section will discuss the fundamental aspects and chemistry of the growth of TiO2 nanotubes by anodization.

2.3.2 The growth of TiO2 nanotubes: fundamental aspects

Formation mechanism of TiO2 nanotube arrays

The formation mechanism of TNAs structures is similar to anodic aluminum oxide (AAO), which is the result of competition between field-assisted anodic oxidation, defined as the formation of the anodic layer under an applied electric field by Eqs. (2.1)-(2.3) and chemical/field assisted dissolution of the forming oxide by Eq. (2.4), which can be regarded as dissolution promoted by the presence of fluoride ions (chemical dissolution) and by the electric field weakening the bond between Ti and O (field assisted dissolution) [56] :

Electrochemical reactions of anodic titanium oxide At Ti/Ti oxide interface:

 _ Ti e Ti 2 2 (2.1)  _  O H O H 2 4 2 2 2 (2.2)   _{ O TiO  e} Ti2 2 2 ₂ 2 (2.3)

At TiO2/solution interface:

 _{ }   HF TiF H O H TiO 6 2 2 2 2 6 2 (2.4)

Meanwhile, the formation mechanism of the TiO2 nanotubes at various stages is

schematically illustrated by Figs. 2.6(a)-(d). Initially, field-enhanced oxidation occurs at the Ti/Ti oxide interface by Eqs. (2.5)-(2.7) when oxygen ions diffusion to the Ti layer as shown in Figs. 2.6(a). At the same time, competing field-enhanced oxide dissolution occurs at TiO2/solution interface illustrated by Fig. 2.6(b). Specifically,

fluoride-containing electrolyte reacts with TiO2 to form TiF62- as described by Eqs. (2.4).

Moreover, small pores are generated and spread uniformly over the surface of the film under an electric field. When the pore to pore distance achieves a suitable value at

which the electric field of each pore would not affect to each other, the distribution of electric-field strength would change. As a consequent, increased in local field strength at the bottom of the pore in conjunction with low dissolution rate at sidewall, highly-order pore structures were formed as shown by Fig. 2.6(c). Finally, in the growth stage, fully developed TiO2 nanotube arrays are shown in Fig. 2.6(d). Field-enhanced

dissolution developed the depth of pore and, therefore, the steady-state nanotubes morphology was created.

Key parameters for controlling the growth of the nanotubes

Summarizing these observations, the key parameters to be taken into account when growing anodized TiO2 nanotubes are the following:

Electrolyte

The electrolyte plays a crucial role in the growth of anodized TiO2 nanotubes, as

previously discussed. The main distinction is between aqueous and organic-based, where the water content is the important rule to limit dissolution of the oxide. Moreover, the pH of the solution is also important, considering the higher dissolution rate of the oxide in an acidic environment. As shown schematically in Fig. 2.7. While the pore bottom is at a low pH, the pore mouth (top of the pores/tubes) remains under a protective environment (higher pH) by using chemical buffer species [NH4F/(NH4)2SO4]. The rapid rate of TiO2 dissolution in the first generation of

nanotubes was reduced by replacing the HF acid with less aggressive solutions containing fluoride salts, raising the maximum thickness up to 2-3m [29]. This is one of the reasons why dissolution of the anodic oxide is at its lowest when using fluoride salts (some of them have basic hydrolysis) instead of hydrofluoric acid.

Mechanical stirring system

The mechanical stirring was sometimes used to accelerate the reaction rate for nanotube growth. In addition, mechanical stirring also affect the inner tube morphology of TNAs. Fig. 2.8 shows the schematic diagram of the formation of tube spatial periodicity under different conditions: (a) without stirring; (b) at medium stirring rate; (c) at high stirring rate [57]. The smoothing effect of the tube inner surface and the acceleration of the growth are due to the redistribution of the F− anions in the nanotubes. Based on the experimental results, the mechanism has been discussed with the consideration of the local reactions and transport processes of the main reaction species. Under this interpretation, the current oscillation and the morphology change in the pore are attributed to the redistribution of the ionic species by the fluctuation in the tube layer when there is no stirring. They can be significantly influenced by the convection above the tube layer and the slow transport process in the tubes with the existence of mechanical stirring.

Temperature

The temperature of the electrolyte affects the chemical dissolution and electrochemical etching rate in the growth of nanotube arrays via anodic oxidation of titanium. For example Grimes reported nanotube arrays were grown with a constant 10 V anode potential in an electrolyte of acetic acid plus 0.5% HF mixed in 1:7 ratio and kept at each of four different electrolyte bath temperatures: 5oC, 25oC, 35oC and 50oC. Table 2.2 shows the variation in 10 V wall thicknesses and tube length as a function of anodization temperature. Results show that with decreasing anodization bath temperature, the length of the nanotubes increases from 120 nm at 50oC to 224 nm at 5oC. Also, with decreasing anodization temperature the wall thickness increases from 9 nm at 50oC to 34 nm at 5oC, confirming the trend of increasing nanotube wall thickness

with lower anodization temperature [58].

2.3.3 Applications of TiO2 nanotubes arrays

The main motivation behind research on anodized TiO2 nanotubes is the possibility

to investigate their impact on a wide range of technologies, including photocatalysis [59,60], chemical sensing [61,62,63] and photovoltaic devices such as DSSCs [64]. As a result, these applications are described in detail in the following sections.

Photocatalysis

Since the pioneering work by Honda and Fujishima [1], photocatalysis using various semiconductors has received much attention for their potential in the utilization of light energy. In particular, TiO2 photocatalysts have been extensively studied due to relative

cheap, high chemical stability and high reactivity of photo-generated holes. Most work on TiO2 photocatalysts has been devoted to the study of reactions associated with the

photodecomposition of H2O into H2 and O2. The principle of the photocatalysis is shown

in Fig. 2.9. According to this simplified scheme, electron-hole pair generated upon UV excitation is trapped at the surface as spatially separated redox centers. The reactive electron reduces O2 from air initially to a superoxide and finally to hydrogen peroxide and

an OH radical, whereas the reactive hole oxidizes the pollutant to its radical cation either directly or through a primarily formed OH radical produced by the oxidation of ubiquitous water. Thus, both the reductive and oxidative interfacial electron transfer processes lead to strong oxidizing agents which can induce mineralization of organic and inorganic pollutants and kill bacteria.

It has been known that TiO2 with high surface area and crystallinity can enhance

catalytic abilities [65,66,67], because high surface area increases the adsorption of reactants more efficiently and crystallinity decreases the electron-hole recombination

sites. Therefore, many efforts have been directed to create high surface area structure with crystallinity, nanostructured TiO2. TiO2 nanotubes have been also studied for

dehydrogenation of ethanol [59], decomposition of gaseous isopropanol into acetone and carbon dioxide [60].

Chemical sensing

Semiconducting metal-oxides such as ZnO, SnO2 and TiO2 have been widely utilized for applications in gas sensors. Their gas sensing properties are largely based on the surface reaction between the metal-oxides and adsorbed gas species. The charge transfer interactions on the surface of such metal oxides, i.e., the adsorption of negatively charged oxygen and the oxidative/reductive interaction between target gases and adsorbed oxygen, lead to the significant variation in electrical conductivity upon exposure to analyte gases.

To meet the requirements of environmental and air-quality monitoring, there have been significant efforts to enhance the sensitivity of gas sensors. Recently, many nano technological approaches have been employed to enlarge the surface area or improve the charge-collection efficiency. From this perspective, sensing materials with one-dimensional (1D) geometry are very promising for their high sensitivity and fast response speed. Thus far, various promising 1D materials such as carbon nanotubes and Si nanowires have been studied for the fabrication of high sensitivity chemical sensors. For example, Comini et al. [61] used nanobelts for CO and NO2 sensing. The nanobelt sensors detected a few ppb levels of NO2, which was difficult to monitor with

conventional sensors. Vargese et al. [62] used TiO2 nanotubes for high sensitivity sensors

and the sensitivity of nanotube sensors reached up to 103. Ryu et al. [63] created TiO2 nano-honeycomb structure by using photoelectron chemical etching and applied it as a H2

sensor. They reported that the nano-honeycomb structure enhanced the response time as well as the sensitivity.

Dye-Sensitized Solar Cells (DSSC)

Dye-sensitized solar cell (DSSC) is low cost alternative to inorganic semiconductor photovoltaic devices. Energy conversion in a DSSC is based on the injection of an electron from a photo-excited state of the sensitizer dye (typically a bipyridine metal complex) into the conduction band of semiconductor (TiO2 is by far the most employed

semiconductor). The original state of the dye is subsequently restored by electron donation from the electrolyte, usually an iodide/triiodide redox couple system. The iodide is regenerated at the counter electrode by electrons passed through a load.

The large band gap semiconductor, TiO2, in DSSC is usually fabricated in the form

of nanoporous structure in order to increase the contact of dye-TiO2 and simultaneously allow dye to directly contact with the redox electrolyte. Also, light absorption at the dye on nanoporous TiO2 is higher than that on flat TiO2 surface which only harvests a

negligibly small fraction of the incoming light [68].

Lately, research has been directed toward synthesizing structures with a high degree of order than the random assembly of nanoparticles. A desirable morphology of the films would have the mesoporous channels or nanorods aligned parallel to each other and vertically with respect to the glass substrate. This would facilitate charge diffusion in the pores and the mesoporous film, giving easier access to the film surface, avoiding grain boundaries and allowing the junction to be formed under better control. One approach to fabricate such oxide structures is based on anodized TiO2 nanotubes by

by Frank and co-workers [64]. They reported that the nanotubes and the nanoparticles in conventional DSSCs have similar transport properties (i.e. the electron has to diffuse

through the oxide and reach the electrode to feed the circuit), however lower recombination occurs in the nanotubes because of a higher charge collection efficiency. In addition, the nanotubes harvest the light more efficiently than conventional DSSCs, because of stronger light scattering effects, however their use in DSSCs is far from optimized.

2.3.4 Challenges of TiO2 nanotubes arrays

From the literature, it has know crystalline nanotube or nanowire based TiO2 , in contrast with the random transport path in nanoparticle, have been investigated to improve electron collection. But, there still have challenges need to overcome. One key challenge of using vertically aligned 1-D nanostructures is that, compared to mesoporous films, the 1-D nanostructures typically have a low internal surface area. For example, in DSSC application, the reported efficiency of TiO2 NT based DSSC is generally much lower than that of DSSCs based on nanoparticles and amounted to 0.61%–2.9% [10,11,27,69]. The possible reason is that the internal surface area of NT based photoanode is much smaller than that of NPs, with a lower dye loading and sunlight absorption.

It seems that the high surface area and the good electron transport cannot be satisfied simultaneously in DSSC based on simple TiO2 nanostructures of first

generation. In order to satisfy both requirements we should design more complex nanostructures with a multiscale organization, in which small nanoparticles, nanowires or nanotubes are organized around long central cores connected directly to the electrode. This is a next generation of TiO2 nanostructures that has a great potential to

2.4 Annealing of the nanotubes

Factors such as crystal structure, degree of crystallinity of the anodized TiO2

nanotubes have to be taken into account in order to optimize their use in any possible application. For example the crystal structure is very important in photovoltaic applications, since it affects the ability to separate and transport charges (i.e. therefore affecting the performance of the device). However, As-prepared anodized TNAs are usually reported to be amorphous phase. To increase the crystallinity, post annealing is required to change its structure.

Nevertheless, thermal annealing treatment using a conventional furnace annealing takes hours and involves high temperature to complete the transformation of the crystal structure of the film. In order to expedite the annealing time or deliver low-temperature processing for flexible devices using polymer substrates, excimer laser annealing is one of the preferred fast processing technologies. In the followed section, excimer laser annealing for TiO2 crystallization would be described.

2.4.1 Excimer Laser Crystallization of TiO2

Laser-Solid Interaction

Electromagnetic radiation with wavelength ranging from ultraviolet to infrared interacts exclusively with electrons, as atoms are too heavy to respond significantly to the high frequencies (ν > 1013 Hz) [70]. Therefore, the optical properties of material are determined by the energy states of its valence electrons. Bond electrons normally weakly respond to the external electromagnetic wave and affect only its phase velocity. However, free electrons can be accelerated and therefore extract energy from the field. Since the field is periodically changing, the oscillating electrons reradiate their kinetic energy or collide with the atoms, giving their energy to the lattice.

Absorption of incident energy fundamentally dictates the resultant thermal state of the material and therefore is a suitable point to begin an analysis of laser-solid interactions.

The mechanisms involved in absorption of incident radiation in materials are defined by the electronic structure of the material, and therefore it is useful to discuss exclusively semiconductors. In semiconductors, five distinct mechanisms for the absorption of light can be identified [71].

(1) Photons with energy (hν) much less than the band-gap energy (Eg) can excite

lattice vibrations directly.

(2) Free or nearly free carriers can be excited by absorption of light with hν < Eg;

such carriers will always be present as a result of finite temperature and doping.

(3) An induced metallic-like absorption due to free carriers generated by the laser radiation itself can occur.

(4) For photon energies are larger than Eg, absorption will take place by direct and

indirect (photon-assisted) excitation of electron-hole pairs.

(5) Absorption induced by broken symmetry of the crystalline lattice is possible. Here, the largest contributions to absorption of laser radiation with hν > Eg by crystalline or amorphous TiO2 are found in mechanisms (4) and (5), respectively.

Eximer laser crystallization of TiO2

The basic mechanism of laser heating proceeds is through photon absorption and the subsequent rapid transfer of energy from the electrons to the lattice. During laser annealing a beam of photons is focused on a sample. Simply put, the photons interact with the electrons in the sample which then transfer the energy to the lattice. This causes localized heating in the area where the photons hit the sample. More specifically, the

wavelength of this light determines how the energy will be absorbed in the TiO2. The

energy of the beam, or incident photon energy, is determined by the Eq. (2.5): 

hc/

E _(2.5)

where h is the Planck’s constant (6.626×10-34 Js), c is the speed of light (3.00×108 m/s), and λ is the wavelength of the laser. Based on this equation, the photon energy from KrF excimer laser (λ=248 nm) is 5 eV. With the bandgap of TiO2 around 3.0 eV,

laser energy greater than this bandgap. When a beam of photons of energy hν > Eg is absorbed in a semiconductor, excited carriers, which results in lattice heating [72], is a complicated process and a field of active research [73]. Excited carrier relaxation times on the order of pico-seconds. As incident radiation is converted to increasing lattice temperatures, the thermophysical properties of the material dictate temperature distribution and phase changes. This aspect of laser annealing area has been actively investigated [74,75,76].

Table 2.1 Different TiO2 polymorphs and some of their physical properties.

Rutile Anatase Brookite

Crystal System tetragonal tetragonal Orthorhombic Density (g/cm3)[37, 39] 4.13-4.26 3.79-3.84 3.99-4.11

Band-Gap (eV)[39] 3.0 3.2 3.11 Refractive Index [39] 2.72 2.52 2.63

Table 2.2 Average wall thickness and tube length of 10 V TiO2 nanotube arrays anodized

in HF aqueous electrolyte at different bath temperatures [58]

Anozization temperatures Wall thickness (nm) Tube length (nm) 5o C 34 224 25o C 24 176 35o C 13.5 156 50o C. 9 120

Figure 2.1 TiO2 different crystal structures: (a) Rutile, (b)Anatase, and (c) Brookite

Figure 2.2 High resolution SEM image of anodized Ti6Al4V (TA6V) in CA/HF solution

Figure 2.3 Lateral view of the nanotubes formed in 0.1M KF, 1M H2SO4, and 0.2M

Figure 2.4 A comparison between SEM cross section images of nanotubes prepared in (a) an aqueous based and (b) organic electrolyte [30].

Figure 2.5 Schematic diagrams illustrating the formation mechanism of TiO2 nanotubes

structures (a) oxide layer formation, (b) semicircle pores formation on the oxide film, (c) growth of the semicircle pores into scallop shaped pores, and (d) fully developed nanotube arrays.

Figure 2.7 Schematic diagram of the formation of tube spatial periodicity under different conditions: (a) without stirring, (b) at medium stirring rate, and (c) at high stirring rate [57].

Chapter 3 Experimental Section

3.1 Materials candidates

3.1.1 TiO2 nanotubes arrays structure (TNAs)

Titanium foil 99.9% purity, 0.5 mm thickness and sample size 1 × 1.5 cm2 was used as the substrate for forming TiO2 layer by anodic oxidation. Prior to anodization, Ti foil

was ultrasonically cleaned by distilled water, rinsed by acetone, and then dried by a purging N2 gas. The schematic diagram of anodization reaction system is illustrated in

Fig. 3.1. All anodization experiments were carried out at room temperature using a two-electrode electrochemical cell consisting of a stainless steel foil (SS304) as the cathode and a Ti foil as the anode, at a constant dc potential. Two different electrolyte compositions and their anodization conditions are (1) 0.25 wt % hydrofluoric acid

solution 50% aqueous solution; samples anodized at 20 V for 4 h and (2) 0.5 wt % NH4F

dissolved in the ethylene glycol (EG) solution with 1 wt % H2O; samples anodized at 20

V for 0.5 h. The height of TNAs prepared by HF and NH4F electrolytes in this study was

fixed at 500 nm, unless stated otherwise.

3.1.2 TiO2 hybrid structure (TNAs/TNWs)

The TiO2 nanotube arrays and nanowires were fabricated by using electrolytes

consisting of EG and water (99:1 in wt%) with 0.5 wt% NH4F. The process conditions of

anodizing voltage and processing time were selected to elucidate the formation mechanism of TNWs/TNAs structures. First, the anodizing voltage was varied from 20 to 80 V, while the processing time was maintained at 1 h. Then, an anodizing voltage of 40 V was used, while the anodization time was increased from 30 min to 120 min.

3.2 Post annealing of TiO

2

After fabrication, the samples were annealed by conventional furnace and excimer laser annealing; Following section will described the detail of two annealed technologies.

3.2.1 Conventional furnace annealing

In conventional furnace annealing, the TiO2/Ti samples were annealed in ambient.

A tungsten wire heater was rolled around the quartz tube to create a homogeneous temperature in the furnace. After TNAs fabrication, thermal annealing was performed in ambient air at 400 oC for 1 h under a heating rate of 2 oC/min.

3.2.2 Excimer laser annealing

The Laser annealing system was performed with a KrF excimer laser operated at a 248 nm wavelength (Lambda Physik Complex 201), 25 ns (FWHM) pulse width and 10 Hz repetition rate. The schematic diagram of KrF excimer laser system are illustrated in Fig. 3.2. The diameter of the laser beam was adjusted to 0.5 × 1.5 cm2 to ensure full coverage onto the TNAs samples (sample size is 1 × 1.5 cm2) by two separate exposures. The laser annealing conditions were selected to elucidate the structural and morphological transformation of TNAs by changing the laser fluence from 67, 125, 133, 267 to 400 mJ/cm2 under the same number of 9000 shots. In this study, we also used two laser-sample irradiation modes for laser annealing of TNAs: (1) parallel mode and (2) tilted mode as schematically illustrated in Figs. 3.3(a) and 3.3(b), respectively. In the parallel mode (Fig. 3.3(a)), the angle α between the laser beam and sample was set at 90o with a fixed substrate holder. In the tilted mode (Fig. 3.3(b)), the angle α (0o - 90o) was varied from 30o to 85o in this study. In addition, the sample was rotated 40o manually around the axis of the laser beam for every 1000 shots to deliver a total of 9000 shots with

uniformity. In order to compare any difference in the structure and morphology between laser annealing and conventional annealing, conventional annealing of TNAs samples was carried out using a furnace in air atmosphere at 400oC for 1 hr.

3.3 Characterization of key propertirs

3.3.1 Morphology and microstructure characterization of the TNAs

The surface and cross-section morphology of TNAs were examined using a field emission scanning electron microscope (FESEM) (JOEL JSM-6700) ), which was operated at an accelerating voltage of 15.0 kV, and a focused ion beam scanning electron microscope (FIB/SEM) (FEI Nova-200), which was operated at an accelerating voltage of 5.0 kV. An X-ray diffractometer (XRD) (Siemens Diffractometer D5000) with Cu Kα (λ=1.5405Å) radiation was employed to analyze the crystal structure of the TNAs films. In addition, transmission electron microscope (TEM) images and selected area diffraction (SAD) patterns of TNAs were obtained by a high-resolution transmission electron microscope (HRTEM) (JEOL 2010) at 200 kV.

3.3.2 X-ray absorption spectroscopy

The X-ray absorption near-edge structure spectroscopy (XANES) measurements

were performed using the beam line 20A of National Synchrotron Radiation Research Center (NSRRC), Hsinchu, Taiwan. The XANES spectra were collected in the vicinity of titanium L-edge (450-490 eV) and oxygen K-edge (520-570 eV) regions. All spectra in this paper were measured in total electron yield mode using a high-energy spherical grating monochrometer with energy resolution ~70 meV or better for 6m-HSGM beamline at 400 eV [77]. All spectra were collected at room temperature, while the chamber pressure was kept at about 2 × 10−8 Torr or better. The incoming radiation flux was monitored by the total photocurrent produced in a clean Au mesh inserted into the

beam. The energy scale in the XAS spectra was calibrated by using the known peak positions of a SrTiO3. The resolution of the Ti L2,3 and O K-edge XAS spectra was ~100

meV. In order to compare different oxidation states of titanium, we have also measured the Ti L-edge and O K-edge of TiO (99.9%, Strem Chemical) and Ti2O3 (99.8%, Alfa

Aesar). Quantitative analysis of crystalline, amorphous phases, and suboxides in TNAs were obtained from the intensity ratios of relevant orbitals in the O K-edge based on the published spectra [78,79,80,81]. Specifically, a commercial curve-fitting software (PeakFit v4.12, SeaSolve, USA) was used to determine the intensities of TiO, Ti2O3, TiO2

(anatase), and TiO2 (amorphous) patterns by using a Gaussian-Lorentzian function. The

error of the fitting was minimized by repeating the curve fitting procedure to yield a coefficient of determination (R2) greater than 0.96.

3.3.3 Surface areas measurement

The surface areas of the TNWs/TNAs films were measured by dye (N719 dye, Solaronix) adsorption, which is a commonly used method in DSSC applications [82].Specifically, the amount of dye adsorption was determined by desorbing the dye from the TiO2 films into 5 mM NaOH aqueous solution. The quantification was based

on the dye’s maximum absorption values at 515 nm in the dye-desorbed NaOH solutions as measured by an UV-visible light spectrometer (Evolution 300), using a dye solution of concentration 8×10-2 mM as a reference.

3.4 Photocatalytic reaction experiments

For the photocatalytic reaction experiments, the TNWs/TNAs film on Ti substrate of 2×1 cm2 size was immersed in a quartz cuvette containing 10 ml methylene blue (MB) (C16H18ClN3S, Acros Organics) solution with an initial concentration of 2.5×10-5 M. The

Denki Co., Japan). The decomposition rate of the MB in the solution along with TNWs/TNAs films, can be obtained by monitoring its absorbance periodically (every 4 h) using an UV/Vis spectrometer at wavelength 650 nm. The total height of the TNWs/TNAs was maintained at 12 ± 0.5 m, in which the TNAs tube length was ~11 m and the TNWs was ~1 m in cross-section with a wire length of ~5-10 m.

Figure 3.3 Schematic diagrams of laser anneal systems for TNAs in (a) parallel mode and (b) tilted mode

Chapter 4

The evolution of microstructure and composition of TiO

2

nanotube arrays during annealing

4.1 Morphological observation

We first examined the surface and cross-section morphology of TNAs prepared by anodic oxidation in (a) HF solution and (b) NH4F solution using SEM. As shown in Fig.

4.1(a) for HF solution system, the self-organized regular porous TiO2 structure consists

of pore arrays with a uniform pore diameter of approximately 70 nm and a wall thickness of 20 nm. TNAs of 500 nm (7:1 aspect ratio) in length were obtained in a 0.25 wt% HF solution at room temperature and 20V working voltage for 4 h. However, the length (~500 nm) of nanotubes did not increase with anodization time in HF solution because the high hydroxyl concentrations increased the electrochemical etching rate [28].

In order to obtain TiO2 nanotubes with high aspect ratio, we used a high-viscosity

ethylene glycol electrolyte with 0.5 wt% NH4F and 1 wt% H2O. Surface morphology of

TiO2 nanotube structure with treatment time of 24 hours shows smaller tube diameter of

40 nm, same wall thickness (~20 nm), and tube length of 7 m (170:1 high aspect ratio), as illustrated in Fig. 4.1(b). This clearly shows that the electrolyte is the crucial factor controlling the nanotube morphology and its growth rate. The key for achieving high-aspect-ratio growth was to adjust the ionic diffusion coefficient of electrolyte, which was responsible for maintaining a high H+ concentration at the pore bottom with a protective environment maintained along the pore walls and at the pore mouth during chemical drilling [29]. For the rest of this study, TNAs with 500 nm height were used

for comparison. Surface morphology of TNAs prepared by NH4F electrolyte illustrated

in Fig. 4.1(c) shows tube diameter of 40 nm, wall thickness of 20 nm, and tube length of 500 nm after treatment time of only 0.5h.

4.2 Microstructure and composition of TNAs

4.2.1 XRD analysis

Following morphological observation, the microstructure of TNAs was examined by XRD analysis. Figure 4.2 shows that TNAs prepared in (a) HF solution and (b) NH4F solution as-grown and annealing to 100oC are both in fully amorphous phase. In

the HF system, the anatase phase was formed when annealing temperature was  200o_C,

and the intensity of anatase (101) peak increased with increasing temperature up to 400oC. In contrast, in the NH4F system, when TNAs was annealed up to 200oC, the

spectrum remained the same as that of as-grown, showing no change in amorphous TiO2. There was no evidence for the presence of either rutile or anatase phase for

annealing temperature at 200oC. This was noticeably different from TNAs as-grown in HF solution, which was converted into crystalline TiO2 after annealing at 200oC.

However, when the annealing temperature was raised to 300oC, the anatase phase appeared and the intensity of anatase (101) peak increased with increasing temperature from 300 to 400oC. This illustrated that a sufficient annealing energy, i.e. annealing temperature, must be imparted to transform as-grown amorphous TiO2 into anatase

phase, and that the amount of anatase phase in TiO2 nanotube films increased with

increasing annealing temperature. However, XRD cannot distinguish any suboxides in the amorphous TiO2 structure as-grown by HF or NH4F system at room temperature and

analyze the electronic states in different polymorphs and oxidation states of TiO2 array

nanotube in the following section.

4.2.2 XANES analysis

4.2.2.1 Ti L2,3 edge XANES spectra

Figure 4.3 illustrates Ti L2,3-edge XANES spectra of TNAs as-grown in HF

solution and post annealed to various temperatures ranging from 100 to 400 oC. There were four dominant features, which could be attributed to excitations of Ti 2p3/2

(L3-edge) and Ti 2p1/2 (L2-edge) core levels into empty Ti 3d states. For TNAs as-grown

in HF solution at room temperature, the L2,3-edge showed broad features with low

intensities in t2g and structureless eg, which were indicative of amorphous TiO2 [83]. The

peak broadening was attributed to a loss of the long-range order due to the effects of interactions of titanium with second-neighbor atoms [35]. When as-grown TiO2 was

annealed to 100, 200, 300, and 400 oC, the spectra instead showed definite crystalline structures as indicated by the sharpness and higher t2g orbitals and double features of eg

orbitals in L3 edge. In addition, the leading edge of the Ti L-edge shifted 0.3 eV to higher

energy. The increase in Ti 2p3/2 binding energy implied that Ti had changed from lower

charge states Ti0, Ti+2 or Ti+3 to Ti4+ (TiO2) [84].This suggests that TNAs prepared at

room temperature contain not only amorphous TiO2 but also some lower oxidation states

of titanium such as TiO and Ti2O3, which undergo oxidation during annealing, thus

causing the Ti L-edge shift to higher energy.

Table 4.1 summarizes the intensity ratios of orbitals for Ti L3 edge of TNAs

prepared by anodic oxidation as-grown in HF and NH4F solutions and subsequently

annealed to 100, 200, 300, and 400oC. As illustrated in Fig. 4.3 and Table 4.1, for all TiO2 polymorphs, the intensity ratio of I(L3-t2g)/I(L3-eg) increases from 0.78 to

0.98-1.01 as the annealing temperature is raised to 200, 300, and 400oC. Since the intensity of the L-edge features varied with the density of empty d-states, an increase of I(L3-t2g)/I(L3-eg) intensity ratio implied an empty state in t2g orbitals, which was

consistent with an increase in Ti4+ cations [85]. Thus, annealing in oxygen atmosphere led to enhance oxidation of Ti cations to form TiO2. On the other hand, the eg-related

peak of the L3 edge was split into two peaks (b1 and b2) at 461 eV as the annealing

temperature is increased to 200oC. This showed that the major differences between Ti L2,3-edge spectra of amorphous and crystalline phases of TiO2 were significant changes

in positions, intensities, and widths of eg-related peaks b1 and b2. For instance, in

anatase, the intensity of peak b1 is substantially stronger than that of peak b2; while in

rutile, the intensity of peak b2 is substantially stronger than that of peak b1 [84]. When

the annealing temperature was further raised from 200 to 400oC, the I(b1)/I(b2) intensity

ratio was increased from 1.01 to 1.10. Based on I(t2g)/I(eg) and I (b1)/I(b2) intensity

ratios in the published literature [85], we can conclude that with increasing annealing temperature, more TNAs were transformed towards anatase TiO2.

The Ti L2,3 edge XANES spectra of TNAs as-grown in NH4F solution and

annealed to various temperatures ranging from 100 to 400oC are shown in Fig. 4.4. As-grown nanotube arrays at room temperature showed no obvious difference from the TNAs prepared in HF solution. Both Ti L2,3 edge spectra showed the characteristics of

amorphous TiO2 with broad structures, low intensity of t2g, and structureless eg. When

TNAs were annealed at 200oC, the spectrum remained the same as that of as-grown at room temperature, showing no change in amorphous TiO2 characteristics upon anneal

under oxygen environment. This was in striking contrast to TNAs as-grown in HF solution, which had been converted into crystalline TiO2 after annealing at 200oC.