in-situ synthesis of tio2 nanorods by hydrothermal method

7/26/2019 In-situ Synthesis of TiO2 Nanorods by Hydrothermal Method

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UNDERGRADUATE THESIS

HANOI UNIVERSITY OF SCIENCE AND TECHNOLOGY

Center for Training of Excellent Students

Advanced Training Program


TITLE:

IN-SITU SYNTHESIS OF NANSTRUCTURED TITANIUM DIOXIDE

USING HYDROTHERMAL METHOD

Student : DO VAN LAM

Class : ELECTRONIC AND NANO MATERIALSK51

Supervisor : NGUYEN VAN QUY Ph.D

Hanoi 06-2011


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DO VAN LAMELECTRONIC AND NANO MATERIALSK51 I

ACKNOWLEDGEMENT

I would like to express my deepest thanks to my family for a lot of help

and support to finish my undergraduate thesis. They are always ready andwilling to help me through the tough times and are my biggest fans.

I am also very thankful for my supervisor Ph.D Nguyen Van Quy for

constant scientific support and many helpful discussions, but also for allowing

enough freedom to develop my own ideas and test my hypothesis with the

provided scientific materials.

I am also grateful to Assoc. Prof. Vu Ngoc Hung and Dr. Trinh Quang

Thong and the MEMS research group at ITIMS for scientific and professional

advice during my time at International Training Institute of Material Science

(ITIMS).

I would also like to thank my QCM sensor group members at ITIMS

who have helped me a lot with my research. I am particularly grateful for the

help from Vu Anh Minh, Nguyen Manh Hung, Truong Thi Hien and Bui Van

Sang.

I would like to acknowledge Ph.D Dang Thi Thanh Le for lending me

her autoclave and MSc. Le Van Minh for reviewing my thesis and advising

me with his great tips.

I also appreciate my classmates at Hanoi University of Science and

Technology and my friends for all their help and support. They give balance to

my life and never cease to astonish me with their talent, wit and friendship.Once again, I want to thank Ph.D Pham Van Quy for his financial

support and allowance to visit Vietnam National Conference of Physics 2010.


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DO VAN LAMELECTRONIC AND NANO MATERIALSK51 II

TABLE OF CONTENTS

ABBREVIATIONS ....................................................................................... IV

LIST OF TABLES ....................................................................................... VIIINTRODUCTION ............................................................................................ 1

CHAPTER 1TiO2NANORODS: PROPERTIES, SYNTHESIS AND

APPLICATIONS .............................................................................................. 3

1.1 Properties of TiO2Nanomaterials ............................................................. 3

1.1.1 Structural Properties ........................................................................... 3

1.1.2 Thermodynamic Properties ................................................................. 6

1.1.3 Electronic Properties ........................................................................... 7

1.1.4 Optical Properties ............................................................................... 8

1.2. Synthetic Methods of TiO2Nanorods .................................................... 10

1.2.1 Sol-gel Method ................................................................................. 10

1.2.2 Chemical Vapor Synthesis (CVS) .................................................... 11

1.2.3 Metal Organic Chemical Vapor Deposition (MOCVD) .................. 12

1.2.4 Template Method .............................................................................. 131.2.5 Aerosol-flame Synthesis ................................................................... 14

1.2.6 Thermal Oxidation of Ti Substrate ................................................... 15

1.2.7 Hydrothermal Method....................................................................... 16

1.3. Applications of TiO2Nanorods ............................................................. 17

1.3.1 Gas Sensors ....................................................................................... 17

1.3.2 Solar Cells ......................................................................................... 19

1.3.3 Superhydrophobic Materials ............................................................. 20

1.3.4 Photocatalysts ................................................................................... 21

CHAPTER 2EXPERIMENT ....................................................................... 23

2.1 Chemical Reagents .................................................................................. 23


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DO VAN LAMELECTRONIC AND NANO MATERIALSK51 III

2.2 Substrate Preparation .............................................................................. 23

2.3 Autoclave Cleaning ................................................................................. 24

2.4 Synthesis of Nanostructured TiO2Materials .......................................... 24

2.5 Characterization of Materials .................................................................. 27

CHAPTER 3RESULTS AND DISCUSSION ............................................. 28

3.1 A Principle for the Formation of Nanostructured TiO2Materials .......... 28

3.2 Effects of Synthesizing Parameters on the Formation of Nanostructured

TiO2Materials ............................................................................................... 30

3.2.1 Effect of Growth Time ...................................................................... 30

3.2.2 Effect of Growth Temperature .......................................................... 343.2.3 Effect of Initial Reactant Concentration ........................................... 36

3.2.4 Effect of Acidity ............................................................................... 37

3.2.5 Effect of Substrates ........................................................................... 38

CONCLUSION ............................................................................................... 41

REFERENCES ............................................................................................... 42


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DO VAN LAMELECTRONIC AND NANO MATERIALSK51 IV

ABBREVIATIONS

Symbols Meaning

ITIMS International Training Institute for Materials Science

HUST Hanoi University of Science and Technology

DSSCs Dye Sensitized Solar Cells

QCM Quartz Crystal Microbalance

MOCVD Metal Organic Chemical Vapor Deposition

TTIP Titanium isopropoxide

SEM Scanning Electronic Microscope

FE-SEM Field Emission Scanning Electronic Microscope

XRD X-ray Diffraction

FWHM Full Width at Half of Maximum

CVS Chemical Vapor Synthesis


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DO VAN LAMELECTRONIC AND NANO MATERIALSK51 V

LIST OF FIGURES

Name Ti tle of F igures Page

Figure 1.1 Lattice structures of rutile and anatase TiO2 5

Figure 1.2Arrangement of octahedrons in rutile and anatase

TiO25

Figure 1.3Changes in particle sizes of anatase and rutile

phases as a function of the annealing temperatures.7

Figure 1.4Molecular-orbital bonding structure for anatase

TiO2.

8

Figure 1.5Schematic illustration of electronic band structure:

(a) TiO2nanosheets; (b) anatase.9

Figure 1.6The schematic formation of TiO2 nanorods via

alumina template.14

Figure 1.7 Structure of a typical QCM. 19

Figure 1.8 Structure scheme of a dye-sensitized solar cell. 20

Figure 1.9FE-SEM image of TiO2nanorod film deposited on a

glass wafer.21

Figure 2.1Schematic diagram of synthesis of nanostructured

TiO2.25

Figure 2.2 Synthesizing processes of TiO2nanorods on QCM. 26


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DO VAN LAMELECTRONIC AND NANO MATERIALSK51 VI

Figure 3.1

SEM images of nanostructured TiO2grown in 10 ml

HCl, 50 ml DI water, 1 ml Ti[O(CH2)3CH3]4, at 100

oC, and in 7 h.

30

Figure 3.2

XRD pattern of TiO2nanorods grown in 10ml HCl,

50ml DI water, 1 ml Ti[O(CH2)3CH3]4 , at 100oC,

and in 7 h.

31

Figure 3.3

SEM images of nanostructured TiO2grown in 10 ml

HCl, 50 ml DI water, 1 ml Ti[O(CH2)3CH3]4, at 100o

C, and in 15 h.

32

Figure 3.4

XRD pattern of TiO2nanorods grown in 10 ml HCl,

50 ml DI water, 1 ml Ti[O(CH2)3CH3]4 , at 100

oC,and in 15 h.

33

Figure 3.5

SEM images of nanostructured TiO2 grown in 10 ml

HCl, 50 ml DI water, 1 ml Ti[O(CH2)3CH3]4, at 100

oC, and in 22 h.

33

Figure 3.6

SEM images of nanostructured TiO2 grown at

different temperatures A. 80 oC; B.100 oC; C & D.

120 oC.

35

Figure 3.7

SEM images of nanostructured TiO2 grown in

various initial reactant concentrations A. 0.5 ml; B.

ml HCl, 50 ml DI water, at 100 oC, and in 15 h.

36

Figure 3.8SEM images of nanostructured TiO2grown on QCM,in 10 ml HCl, 50 ml DI water, 1 ml

Ti[O(CH2)3CH3]4, at 100oC, and in 15 h.

38

Figure 3.9 SEM images of TiO2materials grown on Ti deposited 39


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DO VAN LAMELECTRONIC AND NANO MATERIALSK51 VII

Si substrate in 10 ml HCl acid, 50 ml DI water, 1 ml


Figure 3.10SEM images of TiO2 materials grown on FTOsubstrate in 20 ml HCl, 20ml DI water, 1ml


39

Figure 3.11

SEM images of TiO2 materials grown on FTO

substrate in 30 ml HCl, 30 ml DI water, 1 ml

Ti[O(CH2)3CH3]4, at 150 C, and in 20 h.

40

LIST OF TABLES

Name Ti tle of Tables Page

Table 1.1 Properties of bulk TiO2 4

Table 2.1 Synthesizing parameters of TiO2nanostructures 26


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DO VAN LAMELECTRONIC AND NANO MATERIALSK51 1

INTRODUCTION

In recent years nanostructured materials have received much attention

because of their superior properties which differ from those of bulk materials.

Also, there has been a great interest in controlling the structural properties of

materials and in finding superior properties of materials by using a variety of

preparative methods. Hydrothermal technique is one of the most commonly

used and effective techniques for the processing of a great variety of materials.

It is also the most prospective method to obtain nanostructured materials

where polymorphism, particle size, crystallinity and morphology could bevery well controlled. Titanium dioxide (TiO2) is one of the most extensively

studied materials due to its numerous applications. It is the most widely

accepted semiconductor for the photocatalytic reactions because of its low

cost, ease of handling and high resistance to photo-induced decomposition [1-3].

In addition, TiO2finds applications in the fields of sensors[4], solar cells [5],

electrochromic devices

[6]

, antifogging

[7]

, self-cleaning devices

[8]

, etc.Hydrothermal technique is one of the most convenient and effective methods

for the preparation of nanostructured TiO2materials and in this technique, the

required superior properties can be achieved easily by varying hydrothermal

synthesis parameters such as synthesizing time, synthesizing temperature,

initial reactant concentration, catalysts, etc. TiO2nanomaterials exist in many

forms including nanoparticles, nanorods, nanowires, nanotubes, nanobelts,

nanoporous materials, etc. Out of these nanostructured materials, TiO2

nanorods are the most concerned material because of their excellent features

to improve the efficiency of solar cells as well as the sensitivity of gas sensors

such as ultra high surface area, rapid electron transport rate and light


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scattering effect of single crystalline nanorods [58]. Therefore, choosing TiO2

nanostructures as my research goal is a right orientation. In this thesis,

nanorods of TiO2have been grown on the surface of Au-coated Si substrate

under hydrothermal conditions. Excepting for Introduction, Conclusion and

References, the organization of this thesis is presented as follow:

Chapter I TiO2Nanorods: Properties, Synthesis and Applications:

Presenting an overview of research findings about properties of TiO2

nanomaterials, synthetic methods of TiO2nanorods and their applications.

Chapter II Experiment: Describing the experimental process used tosynthesize TiO2 nanorods on Au-coated Si substrates using wet-chemical

method and apply to quartz crystal microbalance (QCM).

Chapter III Results and Discussion: Illustrating SEM images and

XRD patterns of the as-synthesized nanostructured TiO2as well as analyzing

the factors affecting to the formation of nanostructured TiO2, including growth

time, growth temperature, initial reactant concentration, acidity and substrates.


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CHAPTER 1

TiO2NANORODS: PROPERTIES, SYNTHESIS AND APPLICATIONS

Titania-based nanomaterials represent one of the most widelyinvestigated family of functional nanosystems in inorganic materials

chemistry. TiO2nanorods with ultra high surface to volume ratio and versatile

physico-chemistry are thought to have a wide range of applications such as

catalyst, energy storage, gas sensors and photovoltaics. In this thesis, the

properties of TiO2 nanomaterials are firstly introduced. Then, several

approaches to synthesize TiO2 nanorods, including aqueous sol-gel routes,chemical vapor synthesis, metal organic chemical vapor deposition, thermal

oxidation of Ti substrate, template method, aerosol-flame synthesis and

hydrothermal method are briefly given. At last, examples of early applications

of TiO2nanorods in superhydrophobic surface, photocatalysis, and solar cell

technologies are presented.

1.1 Properties of TiO2Nanomaterials

1.1.1 Structural Properties

Titanium dioxide occurs in nature as 3 main well-known minerals:

rutile, anatase and brookite. The most common form is rutile, which is also the

most stable form. Anatase and brookite both convert to rutile upon heating [9].

Table 1.1 shows the properties of bulk TiO2. Rutile, anatase and brookite all

contain six coordinated titanium atoms.

Figure 1.1 shows the unit cell structures of the rutile and anatase TiO 2.

These two structures can be described in terms of chains of TiO6octahedra,

where each Ti4+ion is surrounded by an octahedron of six O2-ions. The two


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crystal structures differ in the distortion of each octahedron and by the

assembly pattern of the octahedra chains. In rutile, the octahedron shows a

slight orthorhombic distortion; in anatase, the octahedron is significantly

distorted so that its symmetry is lower than orthorhombic. The Ti-Ti distances

in anatase are larger, whereas the Ti-O distances are shorter than those in

rutile. In the rutile structure, each octahedron is in contact with 10 neighbor

octahedrons (two sharing edge oxygen pairs and eight sharing corner oxygen

atoms), while, in the anatase structure, each octahedron is in contact with eight

neighbors (four sharing an edge and four sharing a corner) as shown in Figure

1.2. These differences in lattice structures cause different mass densities and

electronic band structures between the two forms of TiO2.

Table 1.1Properties of bulk TiO2[10].

Rutile Anatase Brookite

Crystal system tetragonal tetragonal orthorhombic

Density(kg/cm3) 4240 3830 4170

Bandgap(eV) 3,0 3,2 -

Mobility, 1 cm2/V.s 10 cm2/V.s -

Lattice

parameters

(nm)

a 0,4584 0,3733 0,5436

b - - 0,9166

c 0,2953 0,937 0,5135

a/c 0,644 2,51 0,944


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Figure 1.1Lattice structures of rutile and anatase TiO2[59].

Figure 1.2Arrangement of octahedrons in rutile and anatase TiO2[60].


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1.1.2 Thermodynamic Properties

Rutile is a stable phase at high temperatures, but anatase and brookite

are common in fine grained natural and synthetic samples. Upon heatingconcurrent with coarsening, the following transformations are all seen: anatase

to brookite to 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 [11].

Hwu et al found the crystal structure of TiO2nanoparticles depended

largely on the preparation method [12]. For small TiO2nanoparticles (< 50 nm),

anatase seemed more stable and transformed to rutile at > 700 oC. Banfield et

al found that the prepared TiO2 nanoparticles had anatase and/or brookite

structures, which transformed to rutile after reaching a certain particle size [13].

Once rutile was formed, it grew much faster than anatase. They found that

rutile became more stable than anatase for particle size > 14 nm. In a later

study, Zhang and Banfield found that the transformation sequence and

thermodynamic phase stability depended on the initial particle sizes of anatase

and brookite in their study on the phase transformation behavior of

nanocrystalline aggregates during their growth for isothermal and isochronal

reactions. They concluded that, for equally sized nanoparticles, anatase was

thermodynamically stable for sizes < 11 nm, brookite was stable for sizes

between 11 and 35 nm, and rutile was stable for sizes > 35 nm [14].

Li et alfound that only anatase to rutile phase transformation occurredin the temperature range of 700 - 800oC [15]. Both anatase and rutile particle

sizes increased with the increase of temperature, but the growth rate was

different, as shown in Figure 1.3. Rutile had a much higher growth rate than


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anatase. The growth rate of anatase leveled off at 800 C. Rutile particles,

after nucleation, grew rapidly, whereas anatase particle size remained

practically unchanged. With the decrease of initial particle size, the onset

transition temperature was decreased. The decreased thermal stability in finer

nanoparticles was primarily due to the reduced activation energy as the size-

related surface enthalpy and stress energy increased.

Figure 1.3Changes in particle sizes of anatase and rutile phases as a functionof the annealing temperatures [61].

1.1.3 Electronic Properties

In the molecular-orbital bonding diagram in Figure 1.4, a noticeable

feature can be found in the nonbonding states near the bandgap: the

nonbonding Oporbital at the top of the valence bands and the nonbonding dxy

states at the bottom of the conduction bands. In rutile, each octahedron shares

corners with eight neighbors and shares edges with two other neighbors,

forming a linear chain. In anatase, each octahedron shares corners with four


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neighbors and shares edges with four other neighbors, forming a zigzag chain

with a screw axis. Thus, anatase is less dense than rutile. Also, anatase has a

large metal-metal distance of 5.35 . As a consequence, the Ti dxyorbitals at

the bottom of the conduction band are quite isolated, while the t2gorbitals at

the bottom of the conduction band in rutile provide the metal-metal interaction

with a smaller distance of 2.96 .

Figure 1.4Molecular-orbital bonding structure for anatase TiO2: (a) atomic

levels; (b) crystal-field split levels; (c) final interaction states. The thin-solid

and dashed lines represent large and small contributions, respectively [62].

1.1.4 Optical Proper ties

The main mechanism of light absorption in pure semiconductors is

direct interband electron transitions. This absorption is especially small in


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indirect semiconductors, e.g., TiO2, where the direct electron transitions

between the band centers are prohibited by the crystal symmetry. Braginsky

and Shklover have shown the enhancement of light absorption in small TiO2

crystallites due to indirect electron transitions with momentum

nonconservation at the interface [16]. This effect increases at a rough interface

when the share of the interface atoms is larger. The indirect transitions are

allowed due to a large dipole matrix element and a large density of states for

the electron in the valence band. Considerable enhancement of the absorption

is expected in small TiO2 nanocrystals, as well as in porous and

microcrystalline semiconductors, when the share of the interface atoms is

sufficiently large. The interface absorption becomes the main mechanism of

light absorption for the crystallites that are smaller than 20 nm [16].

Figure 1.5Schematic illustration of electronic band structure: (a) TiO2

nanosheets; (b) anatase [63].


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Sato and Sakai et alshowed through calculation and measurement that

the bandgap of TiO2nanosheets was larger than the band gap of bulk TiO2,

due to lower dimensionality as shown in Figure 1.5 [17]. From the

measurement, it was found that the lower edge of the conduction band for the

TiO2nanosheet was approximately 0.1 eV higher, while the upper edge of the

valence band was 0.5 eV lower than that of anatase TiO2. The absorption of

the TiO2nanosheet colloid blue shifted (> 1.4 eV) relative to that of bulk TiO2

crystals (3.0 - 3.2 eV), due to a size quantization effect, accompanied with a

strong photoluminescence of well-developed fine structures extending into the

visible light regime [18-19].

1.2. Synthetic Methods of TiO2Nanorods

1.2.1 Sol-gel Method

Sol-gel chemistry has recently evolved as a powerful approach for

preparing low dimensional inorganic nanomaterials. It is a versatile process

used in making various ceramic materials. In sol-gel synthesis a solubleprecursor molecule is hydrolyzed to form a dispersion of colloidal particles

(the sol). Further reaction causes bonds to form between the sol particles

resulting in an infinite network of particles (the gel). The gel is then typically

heated to yield the desired material. Control over crystal structure, size, shape

and organization of TiO2 nanorods has been achieved by means of wet

chemistry. The development of Ti-O-Ti chains is favored with low content of

water, low hydrolysis rates and excess titanium alkoxide in the reaction

mixture. Due to the high reactivity of Ti precursors such as TiCl4 and Ti

alkoxides, the control of the reaction rate is a key factor in obtaining TiO 2

nanorods with the desired nanocrystalline structure and/or shapes.


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Recently, nonhydrolytic sol-gel reactions have been successfully

applied to the synthesis of nanocrystals of transition metal oxides. Cozzoli et

alhave reported the controlled growth of TiO2nanocrystals by modulation of

the hydrolysis rate of titanium tetraisopropoxide (TTIP), using oleic acid

(olea) as a stabilizing surfactant. Chemical modification of TTIP by olea is

proven to be rational strategy to tune the reactivity of the precursor toward

water. The most influential factors in shape control of the nanoparticles are

investigated by simply manipulating their growth kinetics. The presence of

tertiary amines or quaternary ammonium hydroxides as catalysts is essential to

promote fast crystallization under mild conditions [20-22].

1.2.2 Chemical Vapor Synthesis (CVS)

Two-step thermal evaporation was employed to synthesize TiO2

materials into nanorods of rutile phase using a high-frequency 350 kHz

dielectric heater at 1050 oC. The TiO2 nanorods grown in two step thermal

evaporation process under a controlled atmosphere in a tubular quartz furnace

were preceded. Because the vapor pressure of Ti is very low (10-3torr at 1577oC) and the Ti is a high melting point materials (1668 oC). Power supply was

controlled to yield a high speed heating ramp of 100 oC/minute up to 1050 oC.

During the first step process, the alumina substrate was covered with Ti

powder and thermally reacted with the substrate at 1050 oC for 30 minutes.

The function of the first step was to form some of the TiO 2 seeds on the

surface of substrate. Due to high surface energy of the TiO2, TiO2seeds would

be good sites for growing at the second step. As a second step, new titanium

powder and alumina substrate were separated and located on the graphite boat

at high temperature (HT) and low temperature (LT) zone, respectively. The


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first step attempts to form TiO2 seeds with a higher surface energy on the

alumina substrate with lower surface energy; the second step is the growth of

TiO2 seeds gradually to form nanorods. The nanorods were 70 - 150 nm in

diameter and up to 2 m long, respectively. It was revealed that the

nanostructure associated with the TiO2morphology varied with the growing

time. Ti and TiO2 seeds were formed as nanoclusters with a high surface

energy, after the thermal reaction with the Ti powder in the first step of the

process. At growing time of 20 minutes in the second-step process, a brick-

like morphology formed. Eventually, a rod-like nanostructure was formed

after a growing time of 40 minutes. High resolution transmission electron

microscopy demonstrated that an individual nanorods exhibits a twin structure

with a bodycentered tetragonal rutile phase and grows in the [110] direction.

1.2.3 Metal Organic Chemical Vapor Deposition (MOCVD)

Well aligned rutile and anatase TiO2 nanorods have been synthesized

using a template and catalyst free MOCVD method. TiO2nanostructures can

be grown in 2 temperature zone furnace. Ti metal organic precursors, e.g.,

Ti(C10H14O5)4, placed on a Pyrex glass container was loaded into the low

temperature zone of the furnace which was controlled to vaporize the solid

reactant. The vapor carried by a N2/O2flow into the high temperature zone of

the furnace in which substrates were located. TiO2nanorods have also been

grown on tungsten carbide-cobalt (WC-Co) substrate by MOCVD using TTIP

as a precursor [24]. The presence of Co was suggested to catalyze the formation

of the TiO2nanorods. The nanorods diameter and length were about 50 - 100

nm and 0.5 - 2 mm, respectively. It appears that the presence of cobalt

catalyzes the directional growth of TiO2 and NH3 enhances this growth


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behavior. In the presence of NH3 thinner and longer nanorods growth was

observed [23]. A possible mechanism for the formation of the well aligned TiO2

nanorods by the MOCVD approach is proposed to correspond to the relative

growth rate of various crystals bounding the tetragonal TiO2nanocrystal. For a

crystal with an anisotropic crystallographic structure, the direction of the

crystal face with the corner of the coordination polyhedron occurring at the

interface posses the fast growth rate, and the directions of the crystal face with

the edge and with the face of the coordination occurring at the interface have

the second fastest and slowest growth rates, respectively. Moreover, the

growth habit of the crystal is mainly determined by the internal structures of a

given crystal and is also affected by growth conditions.

1.2.4 Template Method

The unique advantages of precise size and shape control directed by the

preformed template have leads the fabrication of TiO2 nanorods. Template

membranes used for the sol-gel synthesis of the micro and nanostructures

described are porous alumina. An important advantage of the template method

is that the nanorods prepared in this way can be diameter-controllable and well

defined. The porous alumina is prepared electrochemically from aluminum

metal. The pores are arranged in a regular hexagonal array, and pore density

as high as 1011 pores/cm2 can be achieved. Direct sol filling and sol

electrophoresis are two reported methods for the production of nanostructured

materials by combining template synthesis and sol-gel processing. The first

method used for the formation of oxide rods is direct sol filling, in which a sol

of the desired oxide material is allowed to infiltrate the pores of the template.

Sadeghzadeh et alshowed that TiO2could be formed by direct sol filling of


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template pores [24]. The schematic formation of TiO2 nanorods via alumina

template is shown in Figure 1.6. Ability to control the diameter of the wires

due to efficient filling of ultrafine pores in the membrane, control of lengths as

a function of time deposition and a high packing density resulting from higher

pH gradients from the bottom of the pores are some of the characteristic

features. Capillary action drawing the sol into the template is the driving force

forming nanorods from the sol. Some of the difficulties inherent in sol-gel

methods can be overcome by the use of electrophoresis. For instance, sol-gel

electrophoresis has been shown an effective means of making thick films.

These films often are of greater thickness, density and quality than those

formed traditionally by sol-gel methods (e.g.,dip coating, spin coating) alone.

Figure 1.6The schematic formation of TiO2nanorods via alumina templatea) anodic porous alumina prepared electrochemically from aluminum metal.

b) TiO2 nanocrystalline begin filling into the pore of alumina template

through the immersion of a template into a TiO2 sol. c) TiO2 nanorods are

formed in the pore of alumina template. d) Removing the template, the TiO2

nanorod arrays are formed [64].

1.2.5 Aerosol-flame Synthesis

Flame synthesis is a technique that can be readily scaled to produced

nanostructured materials in high volume at relatively low cost. Flame

generated materials are dominated typically by spherical primary particles and


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chain-like agglomerates, as shown even recently by in-situsynchrotron x-ray

scattering. From a fundamental perspective, the ability to selectively and

rapidly form anisotropic structures of flame-made particles in a controlled

fashion has been a unique challenge.

In contrast to the liquid phase processes, gas phase synthesis methods

are carried out at higher temperatures that result in the nanoparticles having

higher crystallinity and moderately high specific area. Process parameters can

be adjusted to produce nanoparticles with varied crystallinity and specific

surface area without the necessity of post-treatments. TiO2 particles with

various morphologies have been synthesized via aerosol assisted vapor phase

reactions [25]. Vapor source materials and/or aerosol droplets containing source

materials were fed into a quartz tube and heated in a two temperature zone

electric furnace. The reaction method meant that a combination of gas-phase

decomposition and crystal growth in the liquid droplet phase occurred. This

resulted in the controlled formation of variously shaped crystalline TiO2

nanoparticles varying from single nanoparticles to unique dendritic

nanostructures grown on a core particle.

1.2.6 Thermal Oxidation of Ti Substrate

A one step, simple method to directly synthesize large scale, uniform,

and well aligned TiO2 nanorod arrays formed on a titanium substrate using

acetone as the oxygen source in the oxidation of titanium 850 oC were

reported [27]. For comparison, TiO2films were also prepared by oxidizing Ti

substrates with pure oxygen yielded crystalline grain films, whereas the use of

argon with a low concentration of oxygen produced random nanofibers

growing from the ledges of the TiO2 grains; in contrast, highly dense, large


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scale, and well-aligned TiO2nanorod arrays have two kinds of morphology:

one is tetragonal with a height of 1 - 2 mm, a width of about 1.5 mm and a

thickness of around 100 nm; the other is columnar with size of 2 - 3 mm in

height and about 230 nm in diameter with a rough surface. Oxidation

atmosphere have a significant effect on the surface structure of the formed

TiO2 films. The use of pure oxygen yielded microcrystalline TiO2 films;

whereas the use of a mixture of Ar with a low concentration of oxygen

generated random nanofibers and the use of acetone carried by Ar produced

high density and well aligned TiO2 nanorod arrays. These remarkable

differences could be attributed to the competition of the oxygen and titanium

diffusion involved in the titanium oxidation process. With the pure oxygen,

oxygen diffusion predominates because of the high oxygen concentration; the

oxidation occurs at the Ti metal and the titanium oxide interface forming large

polycrystalline TiO2grains.

1.2.7 Hydrothermal Method

The hydrothermal synthesis has become one of the most powerful and

promising strategy for preparing one dimensional (1D) TiO2nanostructures.

Hydrothermal synthesis is normally conducted in autoclaves with 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 hydrothermal synthesis

of 1D nanostructured with NaOH or KOH solution shows a potential

advantage in quantity in fulfilling the requirements as electrode materials,

photocatalyst, hydrophobic surface, etc. This approach involves generation of

alkali titanate to form the hydrogen-titanates, and the thermal dehydration


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reactions in air at high temperature or hydrothermal reactions of the hydrogen-

titanate fibers to produce TiO2 nanofibers with different crystallographic

phases such as brooklite, monoclinic, anatase and rutile.

In a typical synthesis titanate nanowires were synthesized by adding the

raw TiO2nanoparticles (anatase or rutile) to a 10 M KOH aqueous solution of

NaOH or KOH. A total of 0.2 g raw particles and 20 ml of 10 M KOH

aqueous solution was mixed and after stirring for 1 h the resulting suspension

was transferred to a Teflon-lined stainless autoclave. The autoclave was

heated and stirred at 180 oC for 10 - 72 h. After it was cooled down to room

temperature, the product was repeated and ultrasonically washed by distilled

water or dilute HCl solution 0.1M and dried at 80 oC for 6 h [28-29].

TiO2 nanorods have also been synthesized with the hydrothermal

method [30-31]. Zhang et alobtained TiO2nanorods by treating a dilute TiCl4

solution at 333 - 423 K for 12 h in the presence of acid or inorganic salts. A

film of assembled TiO2nanorods deposited on a glass wafer was reported byFeng et al [32]. These TiO2 nanorods were prepared at 160 C for 2 h by

hydrothermal treatment of a titanium trichloride aqueous solution

supersaturated with NaCl.

1.3. Applications of TiO2Nanorods

1.3.1 Gas Sensors

In recent years, many research groups have reported about gas sensing

applications of TiO2 nanomaterials. Grimes et al conducted a series of

excellent studies on sensing using TiO2nanotubes[33-35]. They found that TiO2

nanotubes were excellent room-temperature hydrogen sensors not only with a


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high sensitivity but also with an ability to self-clean photoactively after

environmental contamination. Birkefeld et al found that the resistance of

anatase TiO2varied in the presence of CO and H2at temperatures above 500

C, but on doping with 10% alumina it became selective for hydrogen [36].

Carney et al found that sensors based on SnO2 - TiO2 with higher surface

areas were more sensitive to H2in the presence of O2by measuring the change

in the electrical resistance of the sensor upon exposure to different hydrogen

concentrations under a constant hydrogen gas flow rate [37]. Ruizet al found

that La-doped TiO2 nanoparticles were good sensing materials for ethanol

based on electrical resistance [38], while Cu- or Co-doped TiO2nanoparticles

were good candidates for CO sensing.

Figure 1.7 Structure of a typical QCM.

Based on the excellent sensing performance of the TiO2nanomaterials,

our research lab is aiming to the sensing properties of TiO2nanorods on QCM.

The advantages of QCM have proven very beneficial in research applications

due to their high sensitivity, real-time measurement capability, quick response

and ease of use [39-40]. A QCM is composed of a piezoelectric AT-cut quartz

crystal sandwiched between a pair of electrodes as shown in Figure 1.7. When


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the electrodes are connected to an oscillator and an AC voltage is applied over

the electrodes, the quartz crystal starts to oscillate at its resonance frequency

due to the piezoelectric effect. This oscillation is generally very stable due to

the high quality of the oscillation. A QCM works by measuring the change in

frequency of a quartz crystal resonator in corresponding to the mass change of

the surface of the large electrode. The resonance is disturbed by the addition

or removal of a small mass. If a rigid layer is evenly deposited on one or both

of the electrodes the resonant frequency will decrease proportionally to the

mass of the adsorbed layer.

1.3.2 Solar Cells

Figure 1.8Structure scheme of a dye-sensitized solar cell.

Photovoltaics based on TiO2 nanorod electrodes have been widely

studied[41-42]. A schematic presentation of the structure of a dye-sensitized

solar cell (DSSC) is given in Figure 1.8.At the heart of the system is an array

of TiO2 nanorods with the charge-transfer dye attached to its surface. The

structure is placed in contact with a redox electrolyte. Photoexcitation of the

dye injects an electron into the conduction band of TiO2. The electron can be

conducted to the outer circuit to drive the load and make electric power. The


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original state of the dye is subsequently restored by electron donation from the

electrolyte, usually an organic solvent containing a redox system, such as the

iodide/triiodide couple. The regeneration of the sensitizer by iodide prevents

the recapture of the conduction band electron by the oxidized dye. The iodide

is regenerated in turn by the reduction of triiodide at the counter electrode,

with the circuit being completed via electron migration through the external

load. The voltage generated under illumination corresponds to the difference

between the Fermi level of TiO2 and the redox potential of the electrolyte.

Overall, the device generates electric power from light without suffering any

permanent chemical transformation.

1.3.3 Superhydrophobic Materials

Figure 1.9FE-SEM images of TiO2nanorods film coated on a glass wafer[64].

The wettability of solid surfaces is a very important property. Currently,

superhydrophobic surfaces with the contact angle of water higher than 150o

are arousing much interest because they will bring great implication in daily

life and many industrial processes. Various phenomena, such as self-cleaning,

anti-fogging, contamination or oxidation, and current conduction, are expected


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to be inhibited on such a hydrophobic surface [43-45]. The TiO2nanorod films

were deposited on glass substrates by a low-temperature hydrothermal

approach. The wettability of the TiO2nanorod films was evaluated by contact

angle measurements. As shown in Figure 1.9, the water contact angle of the

as-prepared rough TiO2nanorod films is large 154, that is, the as-prepared

films show superhydrophobicity. When the as-prepared films were exposed to

UV light, their surface superhydrophobicity transformed into

superhydrophilicity; this remarkable surface wettability transition can be tuned

reversibly.

1.3.4 Photocatalysts

In TiO2-based photocatalysts, the photogenerated electrons (e-) and

holes (h+) migrate to the nanocrystal surface, where they act as redox sources,

ultimately leading to the destruction of pollutants. TiO2 is regarded as the

most efficient and environmentally friendly photocatalyst, and it has been

most widely used for photodegradation of various pollutants. In spherical

crystals, benefits arising from higher surface-to-volume ratio with decreasing

the particle size are significantly offset by the increased e-/h+recombination

probability at surface trapping sites. As a consequence, lower photocatalytic

quantum yields are observed for spherical nanocrystals smaller than a certain

dimension. Mesoporous TiO2, TiO2nanorods, and TiO2nanotubes have been

demonstrated to have high photocatalytic performance under suitable

conditions [46-48].

By comparison, rod-shaped TiO2 nanocrystals could lead to

considerable advantages in both technological fields, when compared to nearly

spherical particles. In nanorods, the surface to volume ratio is higher than that


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found in nanospheres, and this would guarantee a high density of active sites

available for surface reactions as well as a high interfacial charge carrier

transfer rate. Moreover, the increased delocalization of carriers in rods, where

they are free to move throughout the length of the crystal, is expected to

reduce the e-/h+ recombination probability. This could partially compensate

for the occurrence of surface trap states and ensure a more efficient charge

separation.


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CHAPTER 2

EXPERIMENT

In this chapter, the experiments of synthesizing TiO2nanorods will beexamined systematically. There are many methods to synthesize TiO2

nanorods, for example, sol-gel technique, chemical vapor synthesis, template

method, thermal oxidation, metal organic chemical vapor deposition, etc.

However, in this thesis, hydrothermal method is selected because of its

advantageous features such as facile and low-cost method, in-situ synthesis,

low temperature, controlled morphology, size uniformity, and possible large-scale fabrication.

2.1 Chemical Reagents

In this study all chemicals were of analytical grade and were used

without further purification. List of chemicals used to synthesize

nanostructured TiO2is as following:

Hydrochloric acid [HCl] 12 M (Merck KGaA, Germany)

Titanium butoxide [Ti(O(CH2)3CH3)4] (97% Aldrich)

Deionized (DI) water (produced at ITIMS)

Acetone [(CH3)2CO] (Beijing Chemicals Co. Ltd.)

Ethanol [C2H5OH] (Beijing Chemicals Co. Ltd.)

Nitric acid [HNO3] 65% and 100% (Beijing Chemicals Co. Ltd.)

2.2 Substrate Preparation

For preparation of substrates, a 100 nm of Au layer was deposited on

silicon substrate by using sputtering system. The Au-coated Si substrates were


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then divided into small pieces with size of 1 cm x 2 cm. Before synthesizing

nanostructured TiO2 materials, the Au-coated Si (Au/Si) substrates were

cleaned by a standard cleaning process that is presented as below:

The substrates were immersed in HNO3100% in 10 minutes to remove

organic contaminants such as dust, grease and silica gel from the

substrates.

Rinse in DI water in 3 minutes to neutralize acid.

The substrates were then treated in HNO365% at 110

oC in 10 minutes

to remove any heavy metal ions from the surface of the substrates.

Rinse in DI water in 3 minutes to neutralize acid.

Finally, the substrates were dried by blowing with dry air using a high-

pressure air gun.

2.3 Autoclave Cleaning

Since the synthesizing solution and substrates are kept inside the

Teflon-lined vessel of the autoclave, the vessel has to be cleaned beforecarrying out experiments. It is cleaned in a mixed solution of deionized water,

ethanol, and acetone with the volume ratio of 1:1:1 in 60 minutes with aid of

ultrasonic machine. The autoclave was then dried by a high-pressure air gun.

2.4 Synthesis of Nanostructured TiO2Materials

Figure 2.1 shows a schematic diagram of synthesis of nanostructured

TiO2materials. In a typical synthesis, 50 ml DI water was mixed with 10 ml

HCl acid to reach a total volume of 60 ml in a 100 ml Teflon-lined stainless

steel autoclave. The mixture was stirred at ambient conditions for 5 minutes

before the addition of 1 ml Ti[O(CH2)3CH3]4. After stirring for another 5


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minutes, Au-coated Si substrates were placed at an angle against the wall of

the Teflon-liner with the Au side facing down. The hydrothermal synthesis

was conducted at 100 oC in 15 h in an electric oven. After synthesis, the

autoclave was cooled to room temperature under flowing water, which took

approximately 15 minutes. The Au-coated Si substrates were then taken out,

rinsed extensively with DI water and dried at 50 oC in 30 minutes.

Figure 2.1Schematic diagram of synthesis of nanostructured TiO2.

In order to optimize the synthesizing parameters, including growth time,growth temperature, initial reactant concentration, acidity, and substrates

control experiments were examined. Table 2.1 shows the list of allsynthesizing parameters used in these experiments. Optimized parameterscould be found by changing one parameter and fixing the others.

Table 2.1: Synthesizing parameters of nanostructured TiO2.


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Figure 2.2Synthesizing processes of TiO2nanorods on QCM.

For effect of substrates, nanostructured TiO2 materials were in-situ

grown on Au electrode deposited QCM, Ti-deposited Si (Ti/Si) substrate, and

F-doped tin oxide/glass (FTO) substrate. In case of QCM, a thin film of

photoresist was fabricated on the small Au electrode of QCM by spin-coating

technique to prevent the electrode from forming nanostructured TiO2materials

Volume of HCl acid (ml) 0 10 20

Volume of DI water (ml) 60 50 40Ti butoxide (ml) 0.5 1 1.5 2

Growth T (oC) 80 100 120

Growth time (h) 7 15 22

Substrates Au/Si QCM Ti/Si FTO


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during synthesizing process. The QCM was then loaded into the autoclave

before the mixed solution of 10 ml HCl, 50 ml DI water and 1ml

Ti[O(CH2)3CH3]4was added. The experiment was carried out in 15 h, at 100oC. After the experiment had finished, the QCM was taken out, immersed into

acetone in order to remove the photoresist layer and rinsed extensively with

DI water. Figure 2.2 shows the synthesizing processes of nanostructured TiO2

on QCM.

2.5 Characterization of Materials

Structural morphology of the as-synthesized nanostructured TiO2 was

examined by FE-SEM (Hitachi S4800) at National Institute of Hygiene

Epidemiology, Hanoi.

Crystalline of the as-synthesized materials was analyzed by using

Siemens D5000 X-ray generator with Cu K radiation ( = 1.54056 ) at

Department of Chemistry in Vietnam National University, Hanoi.


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CHAPTER 3

RESULTS AND DISCUSSION

In this chapter, the formation of nanostructured TiO2and the effects ofsynthesizing parameters consisting of growth time, growth temperature, initial

reactant concentration, acidity and substrates will be discussed here.

3.1 A Principle for the Formation of Nanostructured TiO2Materials

Recently, many principles for the formation of nanostructured materials

by hydrothermal process were presented [54-57]. However, the principle

proposed by Yoldas [49] is considered to be the most suitable one for the

formation of nanostructured TiO2materials in this study.

Titanium butoxide is one of typical titanium alkoxides that is used to

synthesize nanostructured TiO2 materials. It is well known that titanium

alkoxides react vigorously with water, producing titanium hydroxides or

hydrated oxides. The reaction is often represented by the following chemicalreaction.

Ti(OR) H2O Ti(OH) nR(OH) (3.1)

where Ris an alkyl group

In reality, hydrolysis of titanium alkoxides is very complex. These

reactions produce polycondensates whose chemical compositions are a

function of their physical size and polymeric morphology. This situation arises

from the fact that, during the hydrolytic condensation, an inorganic network is

formed by a chain of hydrolysis and polymerization reactions.


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Ti OR H2O Ti OH ROH (3.2)

Ti OH RO Ti Ti O Ti R(OH) (3.3)

The oxide network extends as far as the hydrolysis conditions permit.

The polycondensed material from titanium alkoxides can never be 100% oxide

since this would require an infinite polymer with no terminal bonds. However,

the concentration of (OH)and (OR)groups and their relative rations can

be altered by the hydrolysis conditions. These conditions include

water/alkoxide ratio, molecular separation by dilution, hydrolysis medium,

catalyst, reaction temperature, and alkyl group in the alkoxide [49].

A hydrolytic polycondensation reaction equation which would include

the variability of the oxide content and the polymeric nature of the

condensates can be written:

nTi(OR) (4 n x y)H2O TiO[2(+)/2](OH)(OR)

(4 n y)R(OH) (3.4)

where is the number of titanium ions polymerized in a given condensation,

and and are the numbers of OHand ORgroups in the molecule. The

number of titanium ions along with the nature of terminal bonds xandy, in

TiO[2(+)/2](OH)(OR) determine the oxide content when this

compound is decomposed to the oxide:

TiO[2(+)/2](OH)(OR) nTiO2 xH2O yR(OH) (3.5)

Analysis of the above equation shows an increase in the equivalent

oxide content with increasing. The initial increase in occurs rapidly, and

then levels off. The presence of HCl in this study acts as a catalyst to modulate

hydrolysis rate of titanium butoxide.


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3.2 Effects of Synthesizing Parameters on the Formation of

Nanostructured TiO2Materials

3.2.1 Effect of Growth Time

In this study, hydrothermal synthesis of nanostructured TiO2 on Au-

coated Si substrate was carried out in various time from 7 h to 22 h. Figure

3.1 shows SEM images of the nanostructured TiO2 synthesized in 7 h. The

nanostructured materials are uniformly distributed on the substrate and have a

lily-like shape. The Figure 3.1 (A) shown that the nanostructured TiO2with

lily-like shape was evenly distributed on Au-coated Si substrate. The tips of

the lily-like shape of nanostructured TiO2 materials consist of many step

edges, which are predicted to be TiO2nanorods grown in the same direction as

shown in high magnification SEM image of Figure 3.1 (B).

Figure 3.1SEM images of nanostructured TiO2grown in 10 ml HCl, 50 ml DI

water, 1 ml Ti[O(CH2)3CH3]4, at 100 C, and in 7 h.

Figure 3.2 shows an XRD pattern of the as-synthesized nanostructured

TiO2 on the Au-coated Si substrate synthesized in 7 h. The XRD pattern


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indicated that the array of nanostructured TiO2deposited on the Au-coated Si

substrate is titanium dioxide rutile type. All diffraction peaks (110), (101),

(200), (111), (210) and (211) that appear at 2 = 27.30, 35.69, 38.85,

40.80, 43.60, and 53.78, respectively, agree well with the tetragonal rutile

phase (JCPDS No: 77-0445, a = b = 0.46255 nm, c = 0.29825 nm). The XRD

pattern also shows the appearance of Au peaks, which confirm that the TiO2

nanorods were grown on Au layer of the Si substrate.

20 30 40 50 60 70

0

50

100

150Rutile TiO

2- 7h

In

tensity

(a.u.

)

2(degree)

(110)

TiO2

(10

1)

TiO2

(111)

Au

(200)TiO

2

(111)

TiO2

(210)

TiO2

(200)Au

(211)

TiO2

(220)

TiO2

(002)

TiO2

(220)

Au

Figure 3.2XRD pattern of TiO2nanorods grown in 10ml HCl, 50ml DI



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For purpose of increasing length of TiO2 nanorods, the synthesizing

time was extended to 15 h. The as-synthesized TiO2nanorods are shown in

Figure 3.3. The SEM images indicated that the entire surface of the Au-coated

Si substrate is uniformly covered with TiO2 nanorods. In comparison with

TiO2nanorods synthesized in 7 h, the length of TiO2nanorods synthesized in

15 h was increased several times. It is clearly shown in high magnification

SEM image of Figure 3.3 (B).

Figure 3.3SEM images of nanostructured TiO2 grown in 10 ml HCl, 50 ml

DI water, 1 ml Ti[O(CH2)3CH3]4, at 100 C, and in 15 h.

Figure 3.4 shows an XRD pattern of the as-synthesized nanomaterials

on the Au-coated Si substrate in 15 h. The XRD pattern once again appears

the diffraction peaks at 2 = 27.30, 35.69, 38.85, 40.80, 43.60, and

53.78, which indicated that the as-synthesized TiO2material is rutile phase.

There are no remarkable peaks detected on the pattern excepting for the peaks

of Au, which deposited on Si substrate, appearing at 2 = 38.05, 44.40, and

64.90, it confirmed that the density and length of TiO2 nanorods were

enhanced significantly when synthesis time was increased.


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20 30 40 50 60 70

0

50

100

150

TiO2

(002)

Rutile TiO2- 15h

Intensity

(a.u.

)

2(degree)

(110)

TiO2

(101)

TiO2

(111)

Au(200)

TiO2

(111)

TiO2

(210)

TiO2

(200)

Au

(211)

TiO2

(220)

TiO2

(220)

Au

Figure 3.4XRD pattern of TiO2nanorods grown in 10 ml HCl, 50 ml DI


Figure 3.5SEM images of nanostructured TiO2 grown in 10 ml HCl, 50 ml

DI water, 1 ml Ti[O(CH2)3CH3]4, at 100 C, and in 22 h.


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Figure 3.5 shows SEM images of TiO2nanorods synthesized on the Au-

coated Si substrate in 22 h. The TiO2nanorods are comparatively uniform and

evenly distributed on Au surface of the Au-coated Si substrate. The dimension

of TiO2nanorods are about 450 to 550 nm in length and 50 nm in width. In

comparison with the nanorods synthesized in 15 h, the size of TiO2nanorods

synthesized in 22 h was significantly increased. The images at different

locations and magnifications reveal that the entire surface of the Au/Si

substrate is uniformly covered with TiO2 nanorods. The homogeneous

nanostructures were produced on a very large-scale on the substrate.

3.2.2 Effect of Growth Temperature

Growth time directly affects on rate of growth and morphology of TiO2

nanorods. In this study, we tried to decrease growth temperature to reduce

energy supply and easily control morphology of TiO2 nanorods. Effect of

temperature on the growth of TiO2nanorods was investigated in the range of

80 oC to 120 oC. Other synthesizing parameters were fixed in 10 ml HCl acid,

50 ml DI water, 1 ml Ti[O(CH2)3CH3]4 and 15 h. Initially, nanostructured

TiO2materials synthesized at 80oC in 15 h as shown in Figure 3.6 (A) have

the same structure of theirs grown at 100 oC in 7 h (Figure 3.1). When the

temperature was elevated to 100 oC, the nanostructured TiO2materials were

narrower and sharper as shown in Figure 3.6 (B). Further increasing

temperature to 120 oC, a film of TiO2materials was formed on the surface of

the substrate as in Figure 3.6 (C). However, the film was not firmly attached

to the Au-coated Si substrate. It peeled easily off the substrate after drying at

50 oC for a few minutes. Figure 3.6 (D) shows surface of substrate after

removing the thin film of TiO2. It was surprising that there is a sparse array of


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Figure 3.6 SEM images of nanostructured TiO2 grown at different

temperatures A. 80 C; B.100 C; C & D. 120 C.

TiO2nanorods firmly attached to the Au surface of the Au-coated Si substrate.

The reason of this phenomenon could be explained as the rapid reaction rate

of reagents in autoclave at high synthesized temperature. There could be a

threshold point where the amount of HCl acid became insufficient to keep the

hydrolysis rate of titanium butoxide normal at a certain temperature and

pressure. Below this point, nanostructured materials were formed and had the

rod-like structures as Figure 3.6 (D). Beyond this point, the hydrolysis rate of

titanium butoxide increased rapidly and a homogeneous structure of TiO2

clusters was formed and deposited on the surface of the Au-coated Si substrate

as Figure 3.6 (C). The substrate used to synthesize nanostructured TiO2


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materials at 120 C could experience the two processes: the initial process

occurred below the threshold point and the second process occurred beyond

this point. The dimension of the remained nanorods was about 300 nm in

length and 50 nm in width. In comparison with the nanorods grown at 100 C,

the nanorods grown at 120 C were about a half in size. Thus, we can

conclude that increment in temperature could increase the grown reaction rate

and decrease the size of nanostructures. This result has reported in a recent

article about growth of TiO2nanorods on FTO substrates[50].

3.2.3 Effect of Initial Reactant Concentration

Figure 3.7 SEM images of nanostructured TiO2 grown in various initialreactant concentrations A. 0.5 ml; B. ml HCl, 50ml DI water, at 100C, and

in 15 h.

For purpose of increasing density of TiO2nanorods, the initial titanium

precursor concentration was varied from 0.5 ml to 2 ml. Figure 3.7 shows

SEM images of the nanostructured TiO2materials grown in different initialreactant concentrations. It can be recognized that there is an increment in

density of TiO2 nanorods when the concentration of titanium precursor

increases from 0.5 ml to 1 ml. In addition, the size of nanorods also increases


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along with the density of nanorods. Further increasing the amount of titanium

butoxide to 2 ml causes rapid hydrolysis and homogeneous precipitation as

soon as it is added to the growth solution. The growth solution remains turbid

even after prolonged stirring. As a result, only a thick film of TiO2was found

instead of TiO2 nanorods. Even after the film had peeled off the substrate,

there was nothing left on the Au surface of the substrate.

3.2.4 Effect of Acidity

The previous studies of Aydil et al [50] and Xingzhao et al [51] have

shown dependence of acidity on the hydrolysis reaction rate of titaniumalkoxide. In this research, the growth of TiO2 nanorods on Au-coated Si

substrate was favored at 100 C when a mixed solution containing 10 ml HCl

acid and 50 ml DI water was used. Increasing the volume of deionized water

with respect to the volume of HCl acid while keeping the total volume of the

growth solution constant increased the hydrolysis rate of titanium butoxide. In

fact, when titanium butoxide is introduced into 60 ml DI water, TiO2

precipitates immediately. There were no nanorods found on Au-coated Si

substrate after hydrothermal growth in this solution. Thus, in the absence of

HCl acid or at low HCl acid concentrations, the entire titanium precursor

precipitates and settles to the bottom of the reaction vessel as TiO2, and none

remains available for the growth of nanorods. Although high acid

concentration can suppress the hydrolysis of titanium butoxide, it can cause

damage to the Au layer deposited on Si substrate. When a growth solution of

20 ml HCl acid and 40 ml DI water were used, the solution remained clear

after hydrothermal reaction and TiO2 did not form on the Au-coated Si

substrate or in the homogeneous phase. Thus, we can conclude that growth of


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TiO2nanorods requires slow hydrolysis of titanium butoxide in a fairly acidic

aqueous medium.

3.2.5 Effect of Substrates

In this study, the effect of various substrates including QCMs, Ti

deposited Si substrates, and FTO substrates were examined. After synthesized,

the small Au electrode of QCM covered with the photoresist was totally

unaffected by the synthesizing solution and the large Au electrode was

overlaid by a white film of TiO2. Figure 3.8 shows SEM images of TiO2

nanorods grown on QCM in 15 h. The TiO2 nanorods were uniformlydistributed and evenly covered the entire Au electrode. This could be a good

signal for gas sensing measurement in future as reported in some of the recent

articles [52-53]about the sensitivity of TiO2-based QCM sensors.

Figure 3.8 SEM images of nanostructured TiO2 grown on QCM, in 10 ml

HCl, 50 ml DI water, 1 ml Ti[O(CH2)3CH3]4, at 100 C, and in 15 h.

SEM images of the as-synthesized TiO2materials on the Ti deposited Si

substrate is shown in Figure 3.9. There were only a few flower-like structures

found on the Ti deposited Si substrate. In case of FTO substrate, a similar

flower-like structure was found, however, the structure was more uniform and


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much higher in density as shown in Figure 3.10. However, the size of these

flower-like structures was unfavorable with diameter of about 2 m.

Figure 3.9SEM images of TiO2materials grown on Ti deposited Si substrate

in 10 ml HCl acid, 50 ml DI water, 1 ml Ti[O(CH2)3CH3]4, at 100 C, and in 7

h.

Figure 3.10SEM images of TiO2materials grown on FTO substrate in 20 ml


For purpose of decreasing the dimension of the flower-like TiO2

structure on FTO substrate, an experiment with synthesizing parameters of 30

ml HCl, 30 ml DI water, 1 ml Ti[O(CH2)3CH3]4were carried out at 150 C in


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20 h. Because of the high synthesis temperature, the acid concentration was

increased to 30 ml to suppress the rapid hydrolysis rate of titanium butoxide.

Figure 3.11 shows SEM images of as-synthesized nanorods on the FTO

substrate. The nanorods have square top facets, which are the expected growth

habit for the tetragonal crystal structure. The nanorods are uniformly

distributed and vertically oriented to the substrate, which refer to great

potential in dye-sensitized solar cells.

Figure 3.11SEM images of TiO2materials grown on FTO substrate in 30 ml


Owing to time limitation, I do not intend to further study on Ti

deposited Si and FTO substrates so that the SEM images of the substrates are

just an illustration about the formation of nanostructured TiO2 materials on

various substrates.


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CONCLUSION

During my study, I have achieved some results as following:

Successful synthesis of nanostructured TiO2materials on Au-coated Sisubstrates using hydrothermal method.

As-synthesized TiO2nanomaterials indicate titanium dioxide rutile type.

As-synthesized TiO2nanorods have strong adhesion with substrate and

oriented vertical alignment.

Synthesizing parameters including growth time, growth temperature,

initial reactant concentration, acidity, and type of substrates could be

selectively chosen to prepare rutile nanostructured TiO2with the desired

lengths and densities.

As-synthesized TiO2 nanorods have dimension of about 500 nm in

length and 50 nm in diameter with the optimum hydrothermal

parameters in 10 ml HCl, 50 ml DI water, 1 ml Ti[O(CH2)3CH3]4, at

100oC and in 22 h.

Because of difficulties and challenges in gas measurement systems for

QCM, I have not had a chance to investigate gas sensitivity of as-synthesized

TiO2nanorods. In future, I will continue studying to improve morphology of

TiO2nanorods by adding surfactants, changing catalysts, studying on different

substrates, and examining the gas sensitivity of the TiO2nanostructures.


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