Synthesis and gas sensing properties of tio2 nanowires

ABSTRACT INTERNATIONAL TRAINING INSTITUTE FOR MATERIALS SCIENCE BATCH ITIMS 2009-2011 Title: Synthesis and gas-sensing properties of TiO2 nanowires.. List of Figures Page Figure 1.1 Hig

PHAM HONG TRANG

MINISTRY OF EDUCATION AND TRAINING HANOI UNIVERSITY OF TECHNOLOGY AND SCIENCE

MINISTRY OF EDUCATION AND TRAINING HANOI UNIVERSITY OF TECHNOLOGY AND SCIENCE

-

PHAM HONG TRANG

SYNTHESIS AND GAS-SENSING PROPERTIES

ABSTRACT INTERNATIONAL TRAINING INSTITUTE FOR MATERIALS SCIENCE

BATCH ITIMS 2009-2011

Title: Synthesis and gas-sensing properties of TiO2 nanowires

Author: Pham Hong Trang Batch: 2009-2011

Supervisor: Asso Prof Dr Nguyen Van Hieu  

Abstract: One-dimensional (1-D) TiO2 nanostructures have recently attractedsignificant attention in gas-sensor applications because of their good chemical stability, nontoxicity, and favorable electrical and optical properties Moreover, the fabrication and understanding of the growth mechanism of 1-D TiO2 nanostructures can be a building block to be realized In this study, we report the gas sensing properties of the TiO2 nanowire prepared by reactive sputtering technique and subsequently grown self-catalytically without the use of any other catalyst metals on Si(100) at a moderate growth range of temperature of (~650 to 850°C)using thermal oxidation process The oxygen source for the growing process is ethanol The shape of the TiO2 nanowires are wires which represents the tetragonal lattice structure of anatase and/or rutile TiO2 The lengths, diameters of the nanowires are ~ (80-300)nm and ~ (1.2-2.5)nm respectively

TiO2 thin films are extensively studied for application in gas sensor devices The films were first annealed at 5000C, 7000C, 9000C and exposed to different concentration of H2 gas at 100, 250, 500ppm Their gas sensing properties was obtained by measuring their resistance and were strongly dependent on synthesize technique, annealing temperature, wires size This result was compared with the sensing properties of TiO2nanopowder synthesized by hydrothermal at the same range of H2 concentration

TÓM TẮT LUẬN VĂN THẠC SĨ

VIỆN ĐÀO TẠO QUỐC TẾ VỀ KHOA HỌC VẬT LIỆU

Khóa ITIMS 2009-2011 Đềtài: Nghiên cứu chế tạo và tính chất nhạy khí của dây nano TiO2

Tácgiả: PhạmHồngTrang Khóa: 2009-2011

Ngườihướngdẫn: PGS.TS NguyễnVănHiếu

Nội dung tóm tắt: Trong những năm gần đây vật liệu cấu trúc nano TiO2 một chiều

đã thu hút sự chú ý đáng kể trong các ứng dụng cảm biến do tính chất bền hóa và không độc hại Hơn nữa, việc tìm hiểu chế tạo vật liệu cấu trúc nano TiO2 cũng được được tiến hành Trong nghiên cứu này, chúng tôi đưa ra phương pháp nghiên cứu khả năng nhạy khí của vật liệu TiO2, đề tài được thực hiện với hai nhiệm vụ chính :

- Nghiên cứu quy trình chế tạo dây TiO2

- Khảo sát tính chất nhạy khí của dây nano TiO2

Đế vật liệu được chuẩn bị bằng kỹ thuật phún xạ và sau đó tiến hành quá trình tổng hợp tự xúc tác không sử dụng kim loại làm chất xúc tác trên Si (100) trong khoảng nhiệt độ (~650-8500C) bằng cách sử dụng quá trình ôxy hóa nhiệt Nguồn ôxy cho quá trình phát triển là ethanol Các dây nano TiO2 có cấu trúc mạng tứ diện anatase

và / hoặc TiO2 Rutile Độ dài, đường kính của dây nano TiO2 lần lượt là (8-30) nm

và ~ (1,2-2,5) nm

Màng mỏng TiO2 được nghiên cứu rộng rãi cho các ứng dụng trong các thiết bị cảm biến khí Các sensor sau khi được chế tạo được ủ ở 5000C, 7000C, 9000C và được kiểm tra tính nhạy khí với các nồng độ của H2 là 100, 250, 500 ppm Các đặc tính nhậy khí thu được bằng cách đo điện trở sensor và phụ thuộc rất nhiều về kỹ thuật tổng hợp, nhiệt độ ủ, kích thước dây

Kết quả này so sánh với các thuộc tính cảm ứng của bột TiO2 chế tạo bằng phương pháp thủy nhiệt tại cùng khoảng nồng độ H2 như trên Qua so sánh nhận thấy rằng sensor trên nền vật liệu TiO2 chế tạo bằng phương pháp vật lý có độ nhạy không thua kém so với sensor chế tạo trên nền vật liệu bột TiO2 bằng phương pháp hóa học

ASSURANCE

I hereby declare that the thesis named “Synthesis and Characterization of One - Dimensional TiO2 Nanowires” is the individual study that actually based on theoretical studies and experimental work at Gas Sensor group under the supervise

of Ass.Prof Nguyen Van Hieu and the results and data presented in this thesis are true, and have never been published in other previous works

I would confirm that this assurance istrue

Hanoi, October 2011

LỜI CAM ĐOAN

Tôi xin cam đoan bản luận văn “ Nghiên cứu chế tạo và tính chất nhạy khí của dây nano TiO2” là công trình nghiên cứu thực sự của cá nhân tôi trên cơ sở nghiên cứu

lý thuyết và tiến hành thực nghiệm tại nhóm Cảm biến khí dưới sự hướng dẫn của PGS TS Nguyễn Văn Hiếu Các số liệu và kết quả được trình bày trong luận văn là trung thực và chưa được công bố trong các công trình khác trước đây

Tôi xin khảng định lời cam đoan trên là đúng sự thật

Hà Nội, tháng 10 năm 2011

Tácgiả

Phạm Hồng Trang

Acknowledgments

Firstly, I am deeply grateful to my advisor Professor Nguyen Van Hieu for his direct guidance and advising me during my Master course He has believed and encouraged me by creating all the best conditions and opportunities possible for my research

I would like to express my gratitude to all the members of Gas Sensor group for their availability and willingness to share their knowledge and materials, especially their skill of doing experiments and using devices It would be difficult for me to reach this final without their helps

I also thank to Hanoi Institute of Hygiene Epidemiology - Center for Electron Microscopy, for their help in SEM, X-ray measurement

I would like to thank the committee members for their valuable comments and suggestions on my thesis

I thank all the members in the laboratory and staffs from ITIMS for their help in my life and my work Whenever I needed their support, they were pleased

Contents

ABSTRACT 2

ASSURANCE 3

Acknowledgments 4

List of tables 5

List of Figures 6

Preface 13

Chapter I Error! Bookmark not defined Introduction to TiO2 – based material Error! Bookmark not defined 1.1 Preview on TiO2 nano structure Error! Bookmark not defined 1.1.1 Crystal structure and chemical bonding Error! Bookmark not defined 1.1.2 Synthesis methods for 1-D TiO2nanostructures 20

1.2 Synthesis mechanism for 1-D TiO2 nanostructures Error! Bookmark not defined 1.2.1 Vapor – Liquid – Solid (VLS) Mechanism Error! Bookmark not defined 1.2.2 Vapor –Solid (VS) Mechanism Error! Bookmark not defined 1.2.3 Solution - Liquid –Solid (SLS) Mechanism Error! Bookmark not defined 1.3 Characterization techniques for 1-D TiO2 nanostructures Error! Bookmark not defined 1.3.1 Scanning electron microscopy Error! Bookmark not defined 1.3.2 Transmission electron microscopy Error! Bookmark not defined 1.3.3 Raman spectroscopy 30

1.3.4 X-Ray photoelectron spectroscopy (XPS) 30

1.4 Gas sensing application 31

1.4.1 Introduction to gas sensor 31

1.4.2 The characteristics of gas sensors Error! Bookmark not defined 1.4.3 Responsibility and Influencing Factors Error! Bookmark not defined Chapter II Error! Bookmark not defined

Experimental Techniques Error! Bookmark not defined

2.1 Synthesis of 1-D TiO2 nanostructures by thermal oxidation Error! Bookmark

not defined

2.1.1 Specimens and tools preparation Error! Bookmark not defined

2.1.2 Synthesis of 1-D TiO2 nanostructures of TiO2 thin film Error!

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2.2 Synthesis of 1-D TiO2 nanostructures by hydrothermal Error! Bookmark

not defined

2.2.1 Source materials and preparation methods Error! Bookmark not

defined

2.2.2 Synthesis of 1-D TiO2 nanostructures of TiO2 nano powder Error!

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3.1.1 Analysis of TiO2 sputtering layer Error! Bookmark not defined

3.1.2 Morphological analysis of TiO2 nanowire using thermal oxidation Error!

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 3.1.3 Morphological analysis of TiO2 nanopowder using hydro thermal…….73 

3.2 X-ray diffraction analysis Error! Bookmark not defined

3.2.1 X-ray diffraction analysis of TiO2 nanowires Error! Bookmark not

defined

3.2.2 X-ray diffraction analysis of TiO2 nanopowder Error! Bookmark not

defined

3.3 Gas sensing properties analysis Error! Bookmark not defined

3.3.1 Gas sensing properties analysis of TiO2 nanowires Error! Bookmark not

defined

3.3.2 Gas sensing properties analysis of TiO2 nanopowder 82

3.3.3 Gas sensing property comparison Error! Bookmark not defined Conclusions Error! Bookmark not defined

References Error! Bookmark not defined.   

List of Tables

Page

Table 2.1 The experimental modes and the numbers of experiments taken in each mode 47

List of Figures

Page Figure 1.1 High-purity (99.999%) titanium with visible crystal structure 15

Figure 1.4 The crystal structure of (a) anatase and (b) rutile 18 Figure 1.5 Unit cell of polymorphs of TiO2: (a) anatase and (b) rutile 19 Figure 1.6

The stage of alloying, nucleation and growth of nanowire synthesis arcording to the VLS growth mechanism

Pseudobinary phase diagram of semiconductor – gold system

24

Figure 1.7

Schematic describing the formation of nanowires by solution methods: (a) SLS growth mechanism, (b) growth-oriented self-assembly, (c) the growing process with the aid of surface-active substances

27

Figure 1.8 Schematic drawing of the Scanning electron microscopy 28 Figure 1.9 Schematic drawing of Transmission electron microscopy 29

Figure 1.11 Schematic drawing of -Ray photoelectron spectroscopy 31 Figure 1.12 The change of gas sensor resistance when putting test gas 35 Figure 1.13 Gas sensing mechanism of Sn2O3-doped SnO2 in the

atmosphere of (a) C2H2 and (b) C2H2 and humidity 39

Figure 2.3 Specimen container (boat) being dried on drying stove 45 Figure 2.4 Schematic drawing of the thermal CVD system for the

Figure 2.8 (a), (b) Comb electrodes before and (c) after attaching TiO2

Figure 2.9 TiO2nano powder synthesized by hydrothermal method 51

Figure 3.1 Scanning electron micrograph (SEM) of a cross-section of Si

Figure 3.2 Scanning electron micrograph (SEM) of TiO2 nanowires at

Figure 3.3 Scanning electron micrograph (SEM) of TiO2 nanowires at

Figure 3.4 Scanning electron micrograph (SEM) of TiO2 nanowires at

Figure 3.5 Scanning electron micrograph (SEM) of TiO2 nanowires at

Figure 3.6 Scanning electron micrograph (SEM) of TiO2 nanowires at

Figure 3.7 Scanning electron micrograph (SEM) of TiO2 nanowires at

Figure 3.8 Scanning electron micrograph (SEM) of TiO2 nanowires at

Figure 3.9 Scanning electron micrograph (SEM) of TiO2 nanowires at

Figure 3.10 Scanning electron micrograph (SEM) of TiO2 nanowires at

Figure 3.11 Scanning electron micrograph (SEM) of TiO2 nanowires at

Figure 3.12 Scanning electron micrograph (SEM) of TiO2 nanowires at

Figure 3.13 Morphology of specimens at the same concentration of 67

20sccm C2H5OH at (a) 7000C, (b) 7500C and (c) 8500magnification of 20k

C-Figure 3.14

Morphology of specimens at the same concentration of 20sccm C2H5OH at (a) 7000C, (b) 7500C and (c) 8500C- magnification of 40k

68

Figure 3.15

Morphology of specimens at the same concentration of 20sccm C2H5OH at (a) 7000C, (b) 7500C and (c) 8500C- magnification of 80k

69

Figure 3.16 Morphology of specimens heating at 700

0C with ethanol

Figure 3.17 Morphology of specimens heating at 750

0C with ethanol

Figure 3.18 Morphology of specimens heating at 850

0C with ethanol

Figure 3.19 Scanning electron micrograph (SEM) of TiO2nano powder 74 Figure3.20 X-ray diffraction patterns of TiO2 nanowire synthesized at

Figure 3.21 X-ray diffraction patterns of TiO2nanopowder without

Figure 3.22 Gas sensing characteristic of TiO2 nanowires with annealing

temp: 500°C /gas concentration : (a)100- (b)250- (c)500ppm 77 Figure 3.23

Gas sensing characteristic comparison of TiO2 nanowires with annealing temp: 500°C /gas concentration : 100- 250-500ppm

77

Figure 3.24 Gas sensing characteristic of TiO2 nanowires with annealing

temp: 700°C /gas concentration : (a)100- (b)250-(c)500ppm 79 Figure 3.25

Gas sensing characteristic comparison of TiO2 nanowires with annealing temp: 700°C /gas concentration : 100- 250-

500ppm

79

Figure 3.26 Gas sensing characteristic of TiO2 nanowires with annealing

temp: 900°C /gas concentration : (a)100- (b)250-(c)500ppm 80 Figure 3.27

Gas sensing characteristic comparison of TiO2 nanowires with annealing temp: 900°C /gas concentration : 100- 250-

Figure 3.29 Gas sensing characteristic of TiO2 nanowires with annealing

temp: 700°C /gas concentration : (a)100- (b)250-(c)500ppm 83 Figure 3.30

Gas sensing characteristic comparison of TiO2nanopowder with annealing temp: 700°C /gas concentration : 100- 250-500ppm

83 Figure 3.31 Gas sensing characteristic comparison of TiO2 nanowire 84

Preface

TiO2 nanowire materials fabricated by the physical methods is complicatedbecause of their high melting points and the absence of catalyst metals which enhance vapor-solid growth at a moderate growth temperaturebut this methods can achieve uniformity in quality and size of wires, but has the advantage of easy to make sensor series with large numbers In contrast, TiO2nanopowderwhich is made by chemical methods has the advantage of being easy to synthesized in large amount, however, it is difficult to fabricate the sensor

in large number This thesis focus on synthesize TiO2nanowires by thermal oxidation method and study the gas sensing properties of this material, besides, this also carry synthetic on TiO2 powder by hydrothermal method with the purpose of comparing advantages and disadvantages of the thermal oxidation method with hydrothermal methods to draw conclusions about the gas sensing characteristics of TiO2 naowires

In this thesis, we focus on the synthesis and characterization of 1-D TiO2nanostructures A catalyst-free method is employed using Ti thin film on Si substrate via sputtering process as seeds for TiO2 nanowires grow oxygen source such as ethanol is introduced to grow TiO2 nanowires The electrical and structural properties of the 1-D TiO2 nanostructures are studied using SEM, X-ray diffraction and electron microscopy, etc.Moreover, the gas responsibility properties of 1-D TiO2 nanostructures are also studied in details A highly responsibility to H2 gas sensor was prepared by the thermal oxidation of a Ti thin film sputtered on Si plate in C2H5OH at 600-850oC The thermally oxidized specimens were annealed at different temperatures before the sensing test to obtained the sensitivity at temperatures as high as 7500C

To receive further characteristics of gas sensing of TiO2 nanomaterial, a hydrothermal route for preparing TiO2nano powder is proposed The standard

procedure for preparation is to suspense TiO2 powder in concentrated alkaline solution

TiO2 nano scale material received from this two procedures have been characterized by the modern physicochemical such as X-ray diffraction (XRD), scanning electron microscopy (SEM) and gas sensing test The films and powder were exposed to different concentrations of H2 gas up to 500ppm Their sensitivity

to gas at high temperature of 7500C was obtained by measuring their resistance The thesis is divided into three chapters:

Chapter 1: Introduction and some preview on TiO2 and some synthesis methods and applications of 1-D TiO2 nanostructures

Chapter 2: Introduce the experimental techniques used in this work for growth and characterization of specimens

Chapter 3: The results of synthesis and characterization of TiO2 crystalline nanostructures by oxidation of Ti films and hydrothermal are introduced and discussed Finally, remarkable conclusions are also given

single-Chapter I Introduction to TiO 2 – based material

1.1 Preview on TiO 2 nano structure

1.1.1 Crystal structure and chemical bonding

Ti is a chemical element with the symbol Ti and atomic number 22 It has a low density and is a strong, lustrous, corrosion-resistant transition metal with a silver color A metallicelement, titanium is recognized for its high strength-to-weight ratio and is a strong metal with low density that is quite ductile The relatively high melting point of more than 1,650 °C makes it useful as a refractory metal It is paramagnetic and has fairly low electrical and thermal conductivity Titanium have ultimate tensile strength of about 434 MPa [1]

Figure 1.1 High-purity (99.999%) titanium with visible crystal structure

The metal is a dimorphic allotrope whose hexagonal alpha form changes into a body-centered cubic (lattice) β form at 882 °C (1,620 °F) The specific heat of the alpha form increases dramatically as it is heated to this transition temperature but then falls and remains fairly constant for the β form regardless of temperature This phase

is usually hexagonal (ideal) or trigonal (distorted) and can be viewed as being due to

a soft longitudinal acoustic phonon of the β phase causing collapse of (111) planes of atoms The most noted chemical property of titanium is its excellent resistance to corrosion; it is almost as resistant as platinum, capable of withstanding attack by

dilute sulfuric acid and hydrochloric acid as well as chlorine gas, chloride solutions, and most organic acids However, it is soluble in concentrated acids However, it is slow to react with water and air, because it forms a passive and protective oxide coating that protects it from further reaction

Figure 1.2 Electron configuration of Titanium

TiO2 materials is a wide band gap semiconductor of (3.2-3.8)eV, having the outstanding properties such as a high refractive index as well as an excellent transmittance in the visible and near-infrared range, a high dielectric constant with physical, chemical stability, and non-toxicity TiO2 forms in the three different structures: rutile, anatase and brookite Rutile is the stable phase at high temperatures, but anatase and brookite are common in fine grained (nanoscale) natural and synthetic samples The band gap of anatase and rutile is 3.2 and 3.0 eV, respectively Via thermal annealing, the following transformations are all seen: anatase to brookiteto rutile, brookite to anatase to rutile, anatase to rutile, and brookite to rutile These transformation sequences imply very closely balanced energetics as a function of particle size The minerals rutile and brookite as well as anatase all have the same chemistry, TiO2, but they have different structures At higher temperatures, about 915 0C, anatase will automatically revert to the rutile structure Rutile is the more common and the more well-known mineral of the three, while anatase is the rarest Anatase shares many of the same or nearly the same properties as rutile such as luster, hardness and density However due to structural differences anatase and rutile differ slightly in crystal habit and more distinctly in cleavage

Figure 1.3Titanium dioxide

Anatase and rutile have the same symmetry, tetragonal 4/m 2/m 2/m, despite having different structures In Rutile, the structure is based on octahedrons of titanium oxide which share two edges of the octahedron with other octahedrons and form chains It is the chains themselves which are arranged into a four-fold symmetry In anatase, the octahedrons share four edges hence the four fold axis Crystals of anatase are very distinctive and are not easily confused with any other mineral They form the eight faced tetragonal dipyramids that come to sharp elongated points The elongation is pronounced enough to distinguish this crystal form from octahedral crystals, but there is a similarity In fact anatase is wrongly called "octahedrite" in spite of the difference in forms

The surface enthalpies of the three polymorphs are sufficiently different that crossover in thermodynamic stability can occur under conditions that preclude coarsening, with anatase and/or brookite stable at small particle size [1,2] The crystal structures properties of TiO2 are summarized in the table 1.1 [3] The crystal diagrams and unit cell of two most stable structures of TiO2, anatase and rutile phases are shown in Figure 1.4 [4] and 1.5, respectively

Since its commercial production in the early twentieth century, titanium dioxide (TiO2) has been widely used as a pigment [5] and in sunscreens [6], paints [7], ointments, toothpaste [8], etc Since Fujishima and Honda discovered the phenomenon of photocatalytic splitting of water on a TiO2 electrode under

ultraviolet (UV) light in 1972 [9], TiO2 material has been extensive researched for many promising applications such as photovoltaic, photocatalysis, photo-/electrochromics and sensors [10-13] These applications can be roughly divided into “energy” and “environmental” categories, many of which depend not only onthe properties of the TiO2 material itself but also on the modifications of the TiO2 material host (e.g., with inorganic and organic dyes) and on the interactions

of TiO2 materials with the environment [14]

Recently, the challenges for the development of future technologies of electronics is the realization of devices that control not only the electron charge, as

in present electronics, but also its spin, setting the basis for future spintronics Spintronics represents the concept of the synergetic and multifunctional use of charge and spin dynamics of electrons, aiming to go beyond the traditional dichotomy of semiconductor electronics and magnetic storage technology

Figure 1.4 The crystal structure of (a) anatase, (b) rutile and (c) brookite

In the emerging field of nanotechnology, a goal is to make nanostructures with special properties with respect to those of bulk or single-particle species An exponential growth of research activities has been seen in the past decades [21-25] The unique physical and chemical properties exhibit when the size of the material becomes smaller and smaller, and down to the nanometer scale Among the unique properties of nanomaterials, the movement of electrons and holes in semiconductor nanomaterials is primarily governed by the well-known quantum confinement, and the transport properties related to phonons and photons are largely affected by the size and geometry of the materials [21-24] The specific surface area and surface-to-volume ratio increase dramatically as the size of a material decreases [21,26]

Figure 1.5 Unit cell of polymorphs of TiO2: (a) anatase and (b) rutile

As the most promising photocatalyst [12-14], TiO2materials are expected to play an important role in helping solve many serious environmental and pollution challenges TiO2also bears tremendous hope in helping ease the energy crisis through effective utilization of solar energy based on photovoltaic and water-splitting devices [10,27,28]

However, to further promote TiO2-based development efforts to solve the environmental and energy challenges would require the fabrication of one-dimensional (1-D) TiO2 nanostructures such as nanobelts and nanowires Furthermore, the seamless integration of future spintronic devices into nanodevices would also require the fabrication of 1-D DMS nanostructures in well-defined architectures

Table 1.1 TiO2 phases and their lattice constants

Hydrothermal synthesis is normally used a steel pressure vessel which called autoclave with or without teflon liners under controlled temperature and/or pressure with the reaction in aqueous solutions The temperature can be elevated above the boiling point of water, reaching the pressure of vapor saturation The temperature and the amount of solution added to the autoclave largely determine the internal pressure produced It is a method that is widely used for the production of small particles in the ceramics industry Many groups have used the hydrothermal method to prepare 1-D TiO2nanostructures such as nanorods[29,30], nanowires [31,32], and nanotubes [33-36]

Electrodeposition

Electrodeposition is commonly employed to produce a coating, usually metallic, on a surface by the action of reduction at the cathode The substrate to be coated is used as cathode and immersed into a solution which contains a salt of the metal to be deposited The metallic ions are attracted to the cathode and reduced

to metallic form With the use of the template of an AAM, TiO2 nanowires can be obtained by electrodeposition [37-38]

Vapor deposition refers to any process in which materials in a vapor state are condensed to form a solid-phase material Recently, they have been widely explored to fabricate various nanomaterials Vapor deposition processes usually take place within a vacuum chamber If no chemical reaction occurs, this process

is called physical vapor deposition (PVD); otherwise, it is called chemical vapor deposition (CVD) In CVD processes, thermal energy heats the gases in the coating chamber and drives the deposition reaction The CVD method has been used to fabrication 1-D TiO2 nanostructures such as nanorods, nanowalls, nanosheets and nanobelts [39-41]

Physical vapor deposition

In physical vapor deposition (PVD), materials are first evaporated and then condensed to form a solid material The primary PVD methods include thermal deposition, ion plating, ion implantation, sputtering, laser vaporization, and laser

surface alloying TiO2 nanowire arrays have been fabricated by a simple PVD method or thermal deposition [42-44]

Direct oxidation method

1-D TiO2 nanostructures can be obtained by oxidation of titanium metal using oxidants or under anodization Crystalline TiO2nanorods have been obtained by direct oxidation of a titanium metal plate with hydrogen peroxide [45-46] At high temperature, 1-D TiO2 nanostructures can be obtained by oxidizing a Ti plate with acetone, methanol, ethanol, etc as a good oxygen source [47-48] Highly dense and well-aligned TiO2nanorod arrays were formed when acetone was used as the oxygen source, and only crystal grain films or grains with random nanofibers growing from the edges were obtained with pure oxygen

or argon mixed with oxygen The competition of the oxygen and titanium diffusion involved in the titanium oxidation process largely controlled the morphology of the TiO2 With pure oxygen, the oxidation occurred at the Ti metal and the TiO2 interface, since oxygen diffusion predominated because of the high oxygen concentration When acetone was used as the oxygen source, Ti diffused to the oxide surface and reacted with the adsorbed acetone species As extensively studied, TiO2 nanotubes can be obtained by anodic oxidation of titanium foil [49-50]

1.2 Synthesis mechanism for 1-D TiO 2 nanostructures

1.2.1 Vapor – Liquid – Solid (VLS) Mechanism

Nanowires are commonly grown using vapor, solution or electrodeposition methods.High temperature growth from the vapor phase is often preferred due to the high crystal quality that can be obtained and the ability to grow large quantities of wires at once A key factor in most vapor- and solution-based methods is the presence of smallmetal droplets during synthesis Analysis of the growth of silicon whiskersfrom the vapor phase using gold catalyst particles lead to the postulation of the Vapor-Liquid-Solid (VLS) mechanism of growth.The VLS mechanism consists

of three stages which are illustrated in figure 1.6 A First, a metal particle absorbs semiconductor material and forms an alloy In this step the volume of the particle increases and the particle often transitions from a solid to a liquid state Second, the alloy particle absorbs more semiconductor material until it is saturated The saturated alloy droplet becomes in equilibrium with the solid phase of the semiconductor and nucleation occurs (i.e solute/solid phase transition) During the final phase, as steady state is formed in which a semiconductor crystal grows at the solid/liquid interface The precipitated semiconductor material grows as a wire because it is energetically more favorable than extension of the solid-liquid interface That semiconductor material is precipitated at the existing solid/liquid interface as opposed to the formation of a new interface In the VLS mechanism, the wire diameter is determined by the diameter of the alloy particle which is in turn determined by the low temperature size of the metal particle and the temperature The wire length is determined by the growth rate and time

Figure 1.6(A) The stage of (a,b) alloying, (c) nucleation and (d) growth of

nanowire synthesis according to the VLS growth mechanism (B) Pseudo binary phase diagram of semiconductor – gold system The arrows indicate the subsequent phases when a gold drop absorbers semiconductor metal at a constant temperature (I) At first, the gold particle is the solid or liquid state By absorbing semiconductor material, a liquid alloy is formed (III) Subsequent absorption of more semiconductor material allows the liquid alloy to be equilibrium with the solid semiconductor

When the system is cooled, the alloy droplet solidifies at the wire tip To examine the feasibility of VLS wire growth from a certain semiconductor/metal combination it is essential to study the binary phase diagram (figure 1.6B); the metal should form an alloywith the semiconductor at a temperature that also allows the semiconductor to exist in the solid phase [51]

1.2.2 Vapor –Solid (VS) Mechanism

VS mechanism occurs when the crystal nanowires are grown directly from the condensing vapor from the material without using a catalyst Catalytic growth process itself has many parameters and complex dynamics should be modeled Nanowires with uniform cross-section, flat surface atoms and the tip pyramid are the characteristics typical of VS mechanism with the help of pre-oxide nano crystals

Vapor-solid growth process without the help of metal catalysts that are mainly used to synthesize metal oxide semiconductor often called self-catalytic process to grow nanostructures grown directly vapor phase Reasonable growth mechanism is anisotropic growth, growth based on disability (such as growing through the deflection torsion) and self-catalytic growth is proposed based on observations on the electron microscope According to classical theory, the process

of growing crystals from liquid or vapor phase, the initial growth plays a major role

in determining the deposition of atoms There are two kinds of surface structures: (a) rough surface created by multiple layers of atoms are not arranged The deposited atoms can bind to the surface and the crystal continues to grow with the continued power of the atom deposition on substrate, (2) atoms on the surface automatically sorted

1.2.3 Solution - Liquid –Solid (SLS) Mechanism

The serendipitously discovered solution-liquid-solid (SLS) mechanism has been refined into a nearly general synthetic method for semiconductor nanowires Purposeful control of diameters and diameter distributions is achieved The synthesis proceeds by a solution-based catalyzed-growth mechanism in which nanometer-scale metallic droplets catalyze the decomposition of metallo-organic precursors and crystalline nanowire growth Related growth methods proceeding by the analogous vapor-liquid-solid (VLS) and supercritical solution – liquid - solid mechanisms are known, and the relative attributes of the methods are compared In short, the VLS method is most general and appears to afford nanowires of the best crystalline quality The SLS method appears to be advantageous for producing the smallest nanowire diameters and for variation and control of surface ligation

The process of nanowires growing from a solution is of two steps: (1) crystal nucleation formation, (2) crystal growth by aggregation of the monomer into seed, and (3) stable surface by active substances So far more anisotropic growth mechanism of nano crystals in solution have been proposed Here presented three processes:

Solution-liquid-solid (SLS) Growth mechanism from the sead

During the SLS reaction, monomer is generated from the decomposition of the precursor molecules at high temperatures The metal particles catalyze these reactions are extremely small and thus easily activated at low temperatures The monomer reacts with the metal to form germ nano-alloy drops of super saturation Semiconductor nanowires such as Si and Ge from Au particles under supercritical flow were made using this method Furthermore, ultra-thin nanowires with diameters of 2-3 nm were fabricated by this method and has many attractive optical properties The nanowire diameter cannot be easily created by means of classical VLS from vapor phase

Self-organized growth process

The process of growing self-organized based on the characteristics of the nanoparticles in solution has the huge ratio of the surface arias and volume The particles are apart to reduce the surface energy and thus reduce the total energy The assembly in one direction is to implement this process Penn and Banfield first observed the formation mechanism of nanowires follow this mechanism as synthetic TiO2nano-crystalline by hydrolysis Cut TiO2 crystal in short by three main surface {001}, {121} and {101}, because {001} surface energy associated with the highest surface so it will move following the high-energy surface when the thermodynamics is favorable During the docking orientation, nano crystals along the direction [001] except {001} surface

The crystal anisotropy growth by dynamics control

Anisotropic crystal growth by the differences in surface energy causes the formation of most long-nano crystals However, different surface energy (depending

on the intrinsic characteristics of the crystal) is not large enough to cause highly anisotropic growth along the nanowire By adding surface active substances, it was found that the surface energy can be transformed and the active molecule selected and restricted surface for crystal nucleation (Figure 1.7c)

Figure 1.7 Schematic describing the formation of nanowires by solution methods:

(a) SLS growth mechanism, (b) growth-oriented self-assembly, (c) the growing process with the aid of surface-active substances [52]

1.3 Characterization techniques for 1-D TiO 2 nanostructures

1.3.1 Scanning electron microscopy

Scanning electron microscopy (SEM) is one of the most widely used techniques used in characterization of nano materials and nanostructures The resolution of the SEM approaches a few nanometers, and the instruments can operate at magnifications that are easily adjusted from ~10 to over 300,000 It also provides the chemical composition information near the surface

In a typical SEM, a source of electrons is focused into a beam, with a very fine spot size of ~ 5 nm and having energy ranging from a few hundred eV to 50 KeV, that is projected over the surface of specimen by deflection coils The schematic of SEM are shown in Fig.1.8[53] Combining with chemical analytical capabilities, SEM not only provides the image of the morphology of nanostructured materials and devices, but can also provide detailed information of chemical composition and distribution

Generally, SEM instrument is combined with the electron probe micro analyzer (EPMA) The EPMA can yield both qualitative identification and quantitative compositional information from regions of a sample as small as a micrometer in diameter In this thesis, the SEM is used to analyze microstructural characteristics of the 1-D TiO nanostructures About EPMA, this instrument is

utilized to define the composition of the elements in the 1-D TiO2 nanostructures with and without metal doping

Figure 1.8 Schematic drawing of the Scanning electron microscopy

1.3.2 Transmission electron microscopy

Transmission electron microscopy (TEM) is one of the most powerful and versatile techniques for the characterization of nanostructured systems It is unique characteristics allow us to achieve atomic resolution of crystal lattices as well as obtain chemical and electronic at the sub-nanometter scale

As well known, the interaction of an electron beam with a solid specimen results in several elastic or inelastic scattering phenomena (backscattering or reflection, emission of secondary electrons, X rays or optical photons, and transmission of the

no deviated beam along with beam deviated as a consequence of elastic – single atom scattering, diffraction – or inelastic phenomena) The TEM technique is dedicated to the analysis of the transmitted or forward-scattered beam Such a beam

is passed through a series of lenses, among which the objective lens mainly determines the image resolution, in order to obtain the magnified image In low resolution TEM, the objective apertures will be adjusted for selection of the central

beam (containing the less-scattered electrons) or of a particular diffracted (or scattered in any form) beam to form the bright-field or dark-field image, respectively In high-resolution TEM (HRTEM), which is usually performed in bright-field mode, the image is formed of collecting a few diffracted beams in addition to the central one The schematic diagrams of the electron and x-ray optics

of TEM are shown in Fig.1.9 [54]

Figure 1.9Schematic drawing of Transmission electron microscopy[54]

Although TEM has no inherent ability to distinguish atomic species, electron scattering is exceedingly sensitive to the target element and various spectroscopies are developed for the chemical composition analysis The used of scanning TEM (STEM) in which a fine convergent electron probe is scanned over the sample (the resolution being related to the probe size that can be as small as ~0.1 nm) is particularly useful for this purpose Energy-dispersive X-ray Spectroscopy (EDS) and Electron Energy Loss Spectroscopy (EELS) are most commonly used in chemical microanalysis [55,56] Connecting the electrons scattered at high angles in

a STEM instrument has proven most useful to obtain images of a clusters in catalysts or even single (relatively heavy) atoms or point defects

1.3.3 Raman spectroscopy

Raman spectroscopy is a light scattering technique, and can be thought of in its simplest form as a process where a photon of light interacts with a sample to produce scattered radiation of different wavelengths Raman spectroscopy is extremely information rich It can be used for applications as wide ranging as chemical identification, characterization of molecular structures, effects of bonding, environment and stress on a sample, pharmaceuticals, forensic science, polymers, thin films, semiconductors and even for the analysis of fullerene structures and carbon nano-materials [57]

1.3.4 X-Ray photoelectron spectroscopy (XPS)

In photoemission techniques, we use either X-rays or ultraviolet photons to bombard the surface of a sample The incident photons cause the emission of electrons (photoelectrons) from atoms in the near-surface region (about the top 4nm) of the sample In X-ray photoelectron spectroscopy, the photoelectrons have energies characteristic of the atom they came from, allowing us to make elemental and chemical determinations

Figure 1.10Schematic

drawing of Raman [57]

Figure 1.11Schematic drawing of -Ray photoelectron spectroscopy[58]

Ultraviolet photons used for UV photoelectron spectroscopy have lower energies and probe the electrons in the outermost valence levels of the surface atoms, yielding surface electronic structure information such as work function and valence band-edge It can be used to evaluate bonding environments of surface layers via XPS, and valence states via UPS Ultrahigh vacuum facilitates sample cleanliness Aluminum, magnesium, and monochromatic Al Kα radiation allow a variety of analytical conditions A high-resolution electron energy analyzer minimizes the spread of photoelectrons, allowing accurate determination of energies and enhanced peak separation for accurate chemical identifications XPS also uses to quantitative identification of elements with detection levels down to 0.5 at.%, and combining with argon ion gun equipped for sputter cleaning of the surface, it can also provide detailed information of limited compositional depth profiles of layers <1 µm thick [58]

1.4 Gas sensing application

1.4.1 Introduction to gas sensor

A gas sensor is defined as a device, which one or more of its physical properties (e.g mass, electrical conductivity or capacitance) changes upon its exposure towards a gas species A change in these properties can be measured and quantified directly or indirectly A typical gas sensor comprises of a sensing layer integrated

with a transducing platform, which is in direct contact with the environment (gas) Gas molecules interact chemically with the sensing layer, which result in a change

of the sensor’s physical/chemical properties The transducer then measures these changes and produces an electrical output signal Gas sensors have vast applications such as the ones, which will be presented in the next section These different applications require various expectations regarding the sensor’s performance with

respect to the sensitivity, selectivity, reliability and other parameters

Gas detection is important for controlling industrial and vehicle emissions, household security and environmental monitoring In recent decades many devices have been developed for detecting CO2, CO, SO2, O2, O3, H2, Ar, N2, NH3, H2O and several organic vapors However, the low selectivity or the high operation temperatures required when most gas sensors are used have prompted the study of new materials and the new properties that come about from using traditional materials in a nanostructured mode In this thesis, I have reviewed the main research studies that have been made of gas sensors that use nanomaterials The main quality characteristics of these new sensing devices have enabled us to make

a critical review of the possible advantages and drawbacks of these nanostructured material-based sensors

Semiconductor nanostructures such as nanowires, nanotubes and nanorodshas attracted much considerable interest owing to their unique electrical, optical, magnetic, catalytic and chemical properties as well as their potential applications in nanodevices [59] Among the semiconductor nanomaterials, TiO2 nanowires and nanoparticles have been extensively studied because of a broad spectrum of potential applications such as dye-sensitized solar cells [60-61], hydrogen storage [62], gas sensor [63] Importantly, due to its outstanding gas sensing properties, non-toxicity, low cost, and chemical stability, TiO2nanostructures is the most promising material in photocatalysis for environmental applications [64-68] Current semiconductor processing techniques are quickly reaching their physical limitations One promising solution is

nanoelectronics, where semiconducting nanowires can be used as a building block for electronic devices In particular, TiO2 nanowires have many potential applications including sensors and solar cell devices In order to develop and optimize future nanoelectronics devices, characterization of the electronic properties of the nanowire is vital Also, The gas sensing reaction mechanisms of TiO2 are widely studied [12,14,69,70]

Conductometric semiconducting metal oxide such as TiO2gas sensors have been widely used and investigated in the detection of gases Investigations have indicated that the gas sensing process is strongly related to surface reactions, so one

of the important parameters of gas sensors, the sensitivity of the metal oxide based materials, will change with the factors influencing the surface reactions, such as chemical components, surface-modification and microstructures of sensing layers, temperature and humidity

Gas species: H2 has been investigated in this research It is well known that this gas is affecting the environment, and also more importantly, human health if over exposure occurs The following subsections will introduce the fundamentals

of this gas The major applications for sensing them will be presented below to rationalize the author’s choice for sensing this by developing the nanostructured based sensors as per the objectives of this thesis

Hydrogen, a colorless and odorless gas, has attracted a great deal of attention due to the fact that it can be used as a clean and renewable source of energy especially in fuel cells The primary physical hazards associated with hydrogen gas are its flammability and potential for explosions Hydrogen gas is explosive when mixed with air at concentrations as low as 4% and very low energy is needed to ignite hydrogen-air mixture [71] Furthermore, there are safety concerns in hydrogen use, storage in a large amounts, handling and transport Manipulation and storage of hydrogen are associated with danger of leakage, which can lead to

explosion Therefore, sensors are needed to detect hydrogen leaks to warn of explosion hazard

Hydrogen is a major cause of metal-corrosion whereby it weakens metals internally This is especially significant at elevated temperatures Due to its small size, hydrogen molecules can penetrate into metals and affect their mechanical properties such as strength and durability Hydrogen sensors are also widely used

in different applications such as monitoring of process control systems in industries such as glass, aerospace, chemical, metallurgy (steel) and petroleum and even in biomedical applications

1.4.2 The characteristics of gas sensors

Each type of sensor has specific parameters to be evaluated For gas sensors, the parameters such as sensitivity, speed of response, recovery time, selectivity and stability are used to assess the quality of the sensor

R

R

(1.3) Where: Rair:film sensor resistance in the air (Ra)

Rgas :membrane resistance with the appearance of test gas (Rg)

Figure 1.10 shows the resistance change of gas sensor (based on n-type semiconductor materials) when putting test gas

Response speed and recovery time

Speed of response is the time from the begin time of the test gas until the resistance

of the sensor reaches a stable value Rg

Recovery time is the time from when the gas off until the resistance of the sensor back to its original state

For a gas sensor, the smaller the speed of response and recovery time is, the higher performance of the sensor get

Figure 1.12.The change of gas sensor resistance when putting test gas

Stability

Stability of a sensor is the repeatability (stability) of that after long use The results of the measurements give constant value in the working environment of the sensor

1.4.3 Responsibility and Influencing Factors

Many metal oxides are suitable for detecting combustible, reducing, or oxidizing gases by conductive measurements The following oxides show a gas response in their conductivity: Cr2O3, Mn2O3, Co3O4, NiO, CuO, SrO, In2O3, WO3,

TiO2, V2O3, Fe2O3, GeO2, Nb2O5, MoO3, Ta2O5, La2O3, CeO2, Nd2O3 [21] Metal oxides selected for gas sensors can be determined from their electronic structure The range of electronic structures of oxides is so wide that metal oxides were

divided into two the following categories [72]:

(1) Transition-metal oxides (Fe2O3, NiO, Cr2O3, etc.)

(2) Non-transition-metal oxides, which include (a) pre-transition-metal oxides (Al2O3, etc.) and (b) post-transition-metal oxides (ZnO, SnO2, etc.)

Pre-transition-metal oxides (MgO, etc.) are expected to be quite inert, because they have large band gaps Neither electrons nor holes can easily be formed They are seldom selected as gas sensor materials due to their difficulties in electrical conductivity measurements Transition-metal oxides behave differently because the energy difference between a cationdn configuration and either a dn+1 or dn−1 configurations is often rather small [92] They can change forms in several different kinds of oxides So, they are more sensitive than pre-transition-metal oxides to environment However, structure instability and non-optimality of other parameters important for conductometric gas sensors limit their field of application Only transition-metal oxides with d0 and d10 electronic configurations find their real gas sensor application The d0 configuration is found in binary transition-metal oxides such as TiO2, V2O5, WO3 d10configuration is found in post-transition-metal oxides, such as ZnO, SnO2

The controlled synthesis of nano- or microsized particles with different shape and morphology has attracted considerable interest, because the properties of nano- and microcrystals depend not only on their composition, but also on their structure, phase, shape, size, and size distribution

a Surface of material

Numerous researchers have shown that the reversible interaction of the gas with the surface of the material is a characteristic of conductometric semiconducting metal oxide gas sensors [72] This reaction can be influenced by many factors, including internal and external causes, such as natural properties of base materials, surface

areas and microstructure of sensing layers, surface additives, temperature and humidity, etc Many papers about metal oxide gas sensors have been published in recent years [72-91] As one of the important parameters of gas sensors, sensitivity has been attracting more and more attention and much effort has been made to enhance the sensitivity of gas sensors There is not a uniform definition for gas sensor sensitivity now Usually, sensitivity (S) can be defined as Ra/Rg for reducing gases or Rg/Ra for oxidizing gases, where Ra stands for the resistance of gas sensors

in the reference gas (usually the air) and Rg stands for the resistance in the reference gas containing target gases Both Ra and Rg have a significant relationship with the surface reaction(s) taking place Although there are many reviews in this field, to the best of our knowledge there were no special reviews about the factors influencing sensitivity

b Chemical Composition:

Semiconducting metal oxides have been investigated extensively at elevated temperatures for the detection of simple gases There are many parameters of materials for gas sensor applications, for example, adsorption ability, catalytic activity, sensitivity, thermodynamic stability, etc Many different metal oxide materials appear favorable in some of these properties, but very few of them are suitable to all requirements For example, the composite ZnO-SnO2 sensors exhibited significantly higher sensitivity than sensors constructed solely from tin dioxide or zinc oxide when tested under identical experimental conditions Sensors based on the two components mixed together are more sensitive than the individual components alone suggesting a synergistic effect between the two components

In addition to the synergistic effect, heterojunction interface between two or more components also contributes to the enhancement of the composite gas sensor performance The principle of formation of heterojunction barriers in air ambient and their disruption on exposure to target gas is employed So, the resistance and proportion of p-n heterojunctions in the composite gas sensor becomes a control factor to the gas sensor performance Furthermore, it has shown that changing the

proportions of each material in the composite yields a wide range of sensor materials with very different sensing characteristics

c Surface Modification:

In many gas sensors, the conductivity response is determined by the efficiency of catalytic reactions with detected gas participation, taking place at the surface of gas-sensing material Therefore, control of catalytic activity of gas sensor material is one of the most commonly used means to enhance the performances of gas sensors However, in practice, the widely used gas sensing metal oxide material such as TiO2, ZnO, SnO2, Cu2O, Ga2O3, Fe2O3, are the least active with catalytic point of view

Noble metals are high-effective oxidation catalysts and this ability can be used to enhance the reactions on gas sensor surfaces A wide diversity of methods, including impregnation, sol-gel, sputtering and thermal evaporation, has been used for introducing noble metal additives into oxide semiconductors Different doping states can be obtained by different methods Mixture of noble metal particles and metal oxides may be obtained by sol-gel method, while metal oxides modified by noble metal particles on the surface are possibly obtained by sputtering or thermal evaporation

d Microstructure

The operating characteristics of solid state gas sensors are determined by both receptor and transducer functions The last function is very important, because it determines the efficiency of chemical interactions’ conversion into electrical signal Therefore, it is very important to synthesize metal oxides with optimal morphology and crystallographic structure

A sensor’s sensitivity can be significantly increased by using materials with very small grains sixes, and this simulated result agrees well with the experimental observation For small grains and narrow necks, when the grain size is less than twice the thickness of surface charge layers, the grain is fully involved in the space-charge layer [3] Then a surface influence on free charge carrier’s mobility should be