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UNDERGRADUATE THESIS
HANOI UNIVERSITY OF SCIENCE AND TECHNOLOGY
Center for Training of Excellent Students
Advanced Training Program
UNDERGRADUATE THESIS
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
Ti[O(CH2)3CH3]4, at 100oC, and in 7 h.
Figure 3.10SEM images of TiO2 materials grown on FTOsubstrate in 20 ml HCl, 20ml DI water, 1ml
Ti[O(CH2)3CH3]4, at 120oC, and in 5 h.
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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DO VAN LAMELECTRONIC AND NANO MATERIALSK51 2
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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DO VAN LAMELECTRONIC AND NANO MATERIALSK51 4
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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UNDERGRADUATE THESIS
DO VAN LAMELECTRONIC AND NANO MATERIALSK51 7
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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DO VAN LAMELECTRONIC AND NANO MATERIALSK51 9
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
water, 1 ml Ti[O(CH2)3CH3]4, at 100 C, and in 7 h.
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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
water, 1 ml Ti[O(CH2)3CH3]4, at 100 C, and in 15 h.
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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DO VAN LAMELECTRONIC AND NANO MATERIALSK51 39
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
HCl, 40 ml DI water, 1 ml Ti[O(CH2)3CH3]4, at 120 C, and in 5 h.
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
HCl, 30 ml DI water, 1 ml Ti[O(CH2)3CH3]4, at 150 C, and in 20 h.
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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