Chế tạo vật liệu quang xúc tác TiO2 biến tính (TiO2:V, TiO2:N và TiO2CNTs) và nghiên cứu một số tính chất của chúng

MINISTRY OF EDUCATION AND TRAINING
HANOI NATIONAL UNIVERSITY OF EDUCATION
----------o0o----------

DUONG QUOC VAN

MODIFIED TiO2 PHOTOCATALYSTS (TiO2:V, TiO2:N AND
TiO2/CNTs) – SYNTHESIS AND CHARACTERIZATION

Specialization: Solid States Physics
Code: 62.44.01.04

PHYSICS DOCTORAL THESIS ABSTRACT

Hanoi 2017


THE WORK HAS BEEN COMPLETED AT
Faculty of Physics and Center for Science and Nano
Hanoi National University of Education

Scientific Advisors
1.

Assoc. Prof. Dr. Nguyen Minh Thuy

2.

Dr. Nguyen Huy Viet

Reviewer 1:

Prof. Dr. Nguyen Nang Dinh
University of Engineering and Technology, VNU, Hanoi

Reviewer 2:

Assoc. Prof. Dr. Nguyen Huy Dan
Institute of Material Science, VAST

Reviewer 3:

Assoc. Prof. Dr. Nguyen Van Khanh
Hanoi National University of Education

The thesis will be denfended in front of Thesis Evaluation Council at Hanoi
National University of Education on …h… , …. / … / 2017.

Thesis can be found at
-

National Library of Vietnam, Hanoi

-

Library of Hanoi University of Education


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INTRODUCTION

In recent years, TiO2 is one of the most studied and widely used material all over the world. Low
production cost, non-toxic, chemical and physical stable make TiO2 to be widely used in many areas of
life. TiO2 is a large band-gap semiconductor (~ 3.2 eV), it does not absorb visible light, has a large
refractive index and high mechanical strength. Moreover, TiO 2 is active substance which can be used to
sterilize common environments such as water or air. Due to its high photocatalytic activity, TiO 2 can be
used to decompose toxic organic substances in the appropriate conditions. These characteristics make
TiO2 become the most studied material for applications in environmental treatment technology.
Photocatalytic effect of TiO2 was discovered by Fujishima and Honda in 1972, to be continued with
the works of other research groups. The practical applications of TiO2 iss restricted due to following
reasons: (i) TiO2 is a semiconductor with large band-gap value (about 3.2 eV for anatase phase) and (ii)
the high accuracy of e- - h+ recombination. Large band-gap lead to the fact that TiO2 does not absorb
visible light - the region accounted for nearly 95% of the radiant energy from the sun. Moreover, the
photocatalytic activity of TiO2 come from the electrons and holes generated in the material under the
illuminated light source. The recombination of electron - hole pairs in the TiO2 lower the quantum
efficiency and lead to decreament of photocatalytic activity of material. Therefore, high visible-light
photocatalytic activity TiO2 material become one of the most objectives of science and technology.
In 2001, Asahi et al. revealed a new hope that it is possible to reduce the band-gap of TiO2 by doped
N into the crystal lattice. Since then, TiO2 has been doped with diferent kind of elements such as metal,
non-metal, transition metal and more to studie the effect of doping on the photocatalytic activity. Among
these elements, transition metal have shown their advantages due to the decreament of band gap, increases
the ability to capture electrons or reduce the recombination of the electron – hole pairs. One of the most
study element in transition metal is Vanadium because of impressive result when doping in TiO 2 such as
(i) increase the electrical conductivity, (ii) maintain transparency and (iii) reduce the band gap of the
material.
To prevent the recombination of e- - h+, TiO2 has been composited with other materials such as
carbon nanotubes (CNTs) and graphene. CNTs are nano-structured materials where the conductivity
depends on the structure. When composited with TiO2, generated electrons in TiO2 will transfer to CNTs,
reducing the recombination rate of electron – hole and improve the quantum efficiency of the material.
In Vietnam, TiO2-based semiconductor materials and its application still be considered as an

important research subject. However, works on this material almost concentrate on the control of particles
size, reducing the band-gap value or magnetic properties. There is alittle of work concentrate on V-doped
TiO2 and a complete anđ stable process to synthesize has not been done. Moreover, mosts of studies on
TiO2 only consider to reduce band-gap value, not to prevent the recombination of e-h pairs in the material.
To understand the mechanism of photocatalytic activity of TiO2-based materials, it is important to
understand the effects of synthesis condition on the reducing of band-gap or lowering the recombination
of e-h pairs. For these reasons, the chosen title of the thesis is “Modified TiO2 photocatalysts (TiO2:V,
TiO2:N and TiO2-CNTs) – Synthesis and Characterization”.


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Thesis Objectives: (i) Create the models of V-doped TiO2 materials and TiO2/CNTs composites,
optimization and study their electronic structure and properties in order to understand the photocatalytic
activities of materials; aim to find the suitable method to synthesize material successfully. (ii) Studying
the effects of doping on the physical and photocatalytic properties of materials; complete the process to
synthesize V-doped TiO2 material. (iii) Study the effects of synthesis condition on the physical and
photocatalytic properties of materials; complete the process to synthesize TiO 2/CNTs material.
Study Subjects
-

V-doped and N-doped TiO2.
TiO2/CNTs composites.

Research Methods: Thesis was studied using semi-empirical method, including theoretical calculations
using Density Functional Theory (DFT) and experimental results. This method is useful to predict,
investigate and compare the result from calculations and experiments in order to understand the
photocatalytic activities of TiO2-based mateirals.
Most of samples studied in thesis were sunthesized at Faculty of Physisc and Center for Nano
Science and Technology, Hanoi National University of Education. The crystal tructure, and surface

morphology of samples were analyzed using modern measurements as X-ray diffraction (XRD), scanning
electron microscopy (SEM), and transmission electronmicroscopy (TEM and HR-TEM) images. The
optical properties were analyzed using absorption spectroscopic measurements. The effects of synthesis
condition on crystal structure and other properties were analyzed using Raman spectroscopic
measurement, Fourier Transform Infrared (FTIR) Spectroscopy or X-ray photoelectron spectroscopy
(XPS). The measurements were carried out by modern equipments with high reliability at national
research centers, a few measurements were done in foreign laboratories.
Electronic Structure and properties of materials were calculated using softwares base on Density
Functional Theory (DFT), plane waves (PW) and pseudopotential (PP) such as Quantum ESPRESSO,
Material Studio. The calculations were performed on the servers at Center for Computer Science and
Faculty of Physics, Hanoi National University of Education. A part of work has been done on server at
Institute of Physics, Vietnam Academy of Science and Technology, Vietnam or JAIST, Japan.
Scientific Meaning and Practical Significance: The major of thesis is improving the photocatalytic
activity of TiO2 anatase vy solving two problems: large band-gap value and high recombination rate of eh pairs. The calculated results show the influence of doping on the electronic structure of doped TiO 2 or
TiO2/CNTs composites whereas the experimental results reveal the influence on the crystal structure,
crystal lattice vibration, optical and photocatalytic activities. These results will contribute to the
understanding of TiO2 photocatalytic in terms of basics and application-oriented research.
The thesis contribute a comprehensive method for studying not only TiO2-based material but also
can be used to study for other photocatlytic material in general.
Thesis Contents: The content of the thesis include (i) general introduction of TiO2 materials;
photocatalytic advantages and drawbacks of TiO2; methods to improve photocatalytic activities of TiO2
and some previous experimental and theoretical studies on V-doped TiO2 and TiO2/CNTs composites; (ii)
experiment techniques and calculation methods; (iii) theoretical results of the influence of doping and
compositing on the electronic structure and photocatalytic properties of TiO2; and (iv) major results of the


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influence of experimental conditions on the photocatalytic activities of V-doped TiO2 and TiO2/CNTs
composites.

Thesis Layout: Thesis is presented in 152 pages with 110 Figures and 31 Tables, including the heading,
5 chapters, and conclusions; a list of publications, and references. Structures of the thesis as follows:
Introduction: Introducing research situation and the necessary of the thesis; the physical meaning,
the content and the structure of the thesis.
Chapter 1: Overview of physical, chemical and photocatlytic properties of TiO2 in previous studies
on understanding and improving photocatalytic properties of TiO2-based material.
Chapter 2: Experimental methods and processes to synthesize materials, basic principles of
expermental measurements used to analyze crystal structure and physical properties of materials; basics
of Denssity Functional Theory, and some of calculation techniques.
Chapter 3: Presenting calculated results of the influence of doping on electronic structure and
photocatalytic properties of TiO2; the influence CNTs on the chemical bonding, electronic structure and
photocatalytic properties of TiO2.
Chapter 4: Presenting the effect of doping of Vanadi on crystal structure, physical properties and
photocatalytic properties of TiO2.
Chapter 5: Presenting the influence of compositing of CNTs on crystal structure, physical and
photocatalytic properties of TiO2.
Conclusion: Presenting the major results of the thesis.
The research results of the thesis have been published in 16 scientific works in which there are 4
articles in international journals, 3 articles in national journals, 7 reports in national and international
conferences, 2 scientific works related to content research.


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Chapter 1: A BRIEF REVIEW ON TiO2

1.1 Overview of TiO2
1.1.1 Crystal structure and physical properties of TiO2
TiO2 is a semiconductor, exists in three polymorphs: rutile, anatase and brookite; rutile and anatase
are more stable and widely used in common.
1.1.2 Vibrations of TiO2 lattice

19
Anatase TiO2 has tetragonal structure, space group 𝐷4ℎ
(𝐼41/𝑎𝑚𝑑 ), total number of molecules per
each unit cell and primitive cell are 4 and 2, respectively. There are 10 vibration modes can be seen in
anatase lattice: 6 Raman-active modes are 𝐴1𝑔 + 3𝐸𝑔 + 2𝐵1𝑔 ; 3 infrared-active modes are 𝐴2𝑢 + mode
2𝐸𝑢 , 1 is inactive mode (for both infrared and Raman) is 𝐵2𝑢 .

1.1.3 Optical properties of TiO2
Anatase TiO2 is an indirect band gap semiconductor, bandgap energy is 𝐸𝑔  3,2 eV. The maximum
wavelength can be absorbed by TiO2 is 𝜆𝑚𝑎𝑥 = 387 𝑛𝑚.
1.1.4 Some theoretical results on anatase TiO2
Calculated results showed that anatase TiO2 has tetragonal structure with lattice parameters 𝑎 = 𝑏 =
3.692 Å; 𝑐 = 9.471 Å. TiO2 is an indirect band gap semiconductor, calculated band-gap value is around
2.0 – 2.5 eV, much smaller than experimental value 3.2 eV. Density of states (DOS) and partial density
of states (PDOS) of TiO2 proved that conduction bands of TiO2 is dominated by 3d elctrons of Ti, valence
bands of TiO2 is formed of O 2s electrons.
1.1.5 Some applications of TiO2
TiO2 materials have been used in different fields of sciences and technology. Some common used
of TiO2 can be listed as water and air treatments, electrodes for batteries or water electrolysis processes,
advanced materials synthesis.
1.2 Photocatalytic activity of anatase TiO2
When illuminated with a radiation, if radiation energy equivalent to or greater than TiO2 band gap energy
(anatase, ~3.2 eV), the electron is excited from the valence band (VB) to the conduction band (CB); generate
an e-h pair. This process may cause different effects: generate defects inside or lead to appearance of radicals
such as hydroxyl OH*, superoxide O2- on the surface of material. These defects or radicals are able to degrade
organic substances, it is origin of TiO2 photocatalytic activity.
Practical applications of TiO2 are restricted due to some drawbacks: only arbsorb a small range of Sun’s
radiation because large band-gap energy; the interaction of organic substances with material surface, and the high
recombination rate of e-h pairs.
1.3 Approaches to enhance the photocatalytic process of TiO2

When used in practical application for environment pollution treatments, TiO2-based material has
some limitations: low visible photocatalytic activity, low adsorption of organic pollutants, aggregation of
particles, difficulty in distributing particles uniform, difficulty in recovery nano particles. Most of research
on TiO2 are concentrated to overcome these limitations. Some common methods have been adopted by


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previous studies: (i) modification of TiO2, (ii) optimization of synthesis process, (iii) stabilixation by
support strucutres, and (iv) dispersion by magenic field.
1.4 Approaches to enhance the visible light photocatalytic activity of TiO2
To enhance the visible light photocatalytic activity of TiO2, the most common method is reducing
band gap energy by doping. Previous studies show that doped TiO 2 (by metal, non metal, rare earth
elements,…) has higher visible light photocatalytic activity. Vanadium and nitrogen are two most
effective elecment in order to reduce the band gap energy of TiO2, lead to a significant shift of absorption
edges of material to visible range.
1.5 Approaches to demote the e-h recombination rate
The most common and effective method to reduce e-h recombination rate is compositing TiO2 with
some materials such as CNTs, graphene, … The composite materials have ability to transfer generated
electrons from TiO2 to CNTs, graphene and lowering the recombination rate. CNTs is one of the most
material has been used to composite with TiO2.

Chapter 2: EXPERIMENTAL TECHNIQUES AND CALCULATION METHODS

2.1 Synthesis proceses of TiO2 materials
TiO2 and V-doped TiO2 samples were synthesized by hydrothermal, sol-gel and co-precipitation
methods. N-doped TiO2 films were prepared by ALD technique. TiO2/CNTs composite samples were
synthesized using hydrolysis method.
2.2 Samples-analyzed instruments and techniques
Samples were characterized using different instrument and analyzed with different techniques: Xray diffraction patterns; Raman scattering spectroscopy, Fourier transform infrared spectroscopy;
scanning electron microscopy (SEM), transmission electron microscopy (TEM); optical absorption

spectroscopy; Energy-dispersive X-ray spectroscopy (EDX), X-ray photoelectron spectroscopy (XPS),
BET.
2.3 Calculation Methods
2.3.1 Introduction to DFT
Density Functional Theory (DFT) theory is a fundamental theory for computing electronic
structures and other pproperties of materials based on first principle. The major of DFT is to solve the
Schrödinger equation for a multi-particle system in order to find the total energy through the ground state
energy. Using Born-Oppenheimer approximation and Thomas-Fermi model, the problem now turn to how
to determine ground state charge density 𝜌(𝑟⃗). It is easier to calculate system properties because the
minimum variables required to calculate for a system with N electrons has decreased from 3N (to be 4N
variable if considering both spin) down to only 3 (or 4) variables.
When used to calculate for materials, the precision of DFT depends on the form of exchangecorrelation energy. In fact, there are many functions that describe energy – exchange energy, however the
two most common are Local Density Approximation (LDA) and Generalized Gradient Approximation
(GGA). For periodic system, some computing techniques such as plane wave, supercell, or
pseudopotential are also used.


6
2.3.2 Calculation Techniques
Models were built and simulated using special softwares such as Quantum ESSPRESSO or
Materials Studio. Each calculation process can be divided into 3 steps: (i) build the unit cells; (ii) geometry
optimization and (iii) compute the characteristics of models. The output results can be export as images,
data files or other fomrs and can be used in other plotting programs.

Chapter 3: SIMULATION AND CALCULATION OF TiO2-BASED MATERIALS USING
DENSITY FUNCTIONAL THEORY

3.1 Calculated results for anatase TiO2
Table 3.3 Parameters used to calculate properties of TiO2 anatase.
Parameter

Value

Note

Approximation

GGA

Generalized Gradient Approximation

Functional

PBE

Perdew – Burke – Erzenhoff

Basis

Plane Wave

Plane Waves

Pseudopotential

Ultrasoft

Vanderbilt

Hubbard value

8.18 eV

Used for Ti 3d

Anatase TiO2 has tetragonal crystal system, total number of atoms in each unit cell are 12 (4 Ti
atoms and 8 O atoms). The Brillouin zone has shape of cuboid; the best k-points to calculate band
structure, density of states (DOS), partial DOS (PDOS) and other properties is Γ – X – M – Γ – Z – R – A
– Z; X – R; M – A. Parameter used to calculate TiO2 properties are listed in Table 3.3.
b

PDOS (eV-1)

Energy (eV)

b

Energy (eV)

Figure 3.3b Band structure of TiO2 calculated by Figure 3.5b Partial density of states of TiO2
PBE+U functional.
calculated using GGA-PBE+U.
Calculated results for anatase TiO2: tetragonal crystal system, lattice parameters 𝑎 = 𝑏 = 3.79 Å; 𝑐
= 9.72 Å; bandgap energy 3.201 eV, in a good agreement with experimental results. Calculated band
structure in Figure 3.3b showed that anatase TiO 2 is an indirect band gap semiconductor. PDOS
results in Figure 3.5b proved that Ti 3d and O 2p electrons play important roles in the formation of
conduction and valence bands of anatase TiO2.


7

3.2 Calculated results for doped TiO2
3.2.1 Doped TiO2 models
There are 8 different models for doped TiO2 have been built: pure TiO2 model (TOO), O vacancy
(TOO-v); substitutional models (TOV-s and TOV-sv); institital models (TOV-i and TOV-iv);
substitutional models (TVO-s and TVO-sv).

Ti

a

b

c

d

e

f f

g

h

O

V (hoặc N)

Nút khuyết O

Figure 3.6 Defect models used to calculate for TiO2: (a) TOO, (b) TOO-v, (c) TOV-s, (d) TOV-sv, (e)
TOV-i, (f) TOV-iv, (g) TVO-s and (h) TVO-sv ..
3.2.2 Calculated results for V-doped TiO2
TVO-s model has smallest formation energy, indicates that V atoms substituted into positions of Ti
atoms, neither into O atoms or interstitial positions. The formation energies of O vacancies of doped models
are smaller than undoped models, proved that V doping lead to the increasing of possibility of O vacancies
formation.
DOSs of V-doped models showed that impurity energy levels created by 3d V electrons lied below the
conduction bands of TiO2, reduce the optical band gap of materials. The expansion of energy levels of O 2p
electron is another reason for this band gap reduction.
3.2.3 Calculated results for N-doped TiO2
TON-s model has smallest formation energy, indicates that N atoms substituted into positions of O
atoms, neither into Ti atoms or interstitial positions. The formation energies of O vacancies of doped models
are smaller than undoped models, proved that N doping lead to the increasing of possibility of O vacancies
formation like V doping.
DOSs of N-doped models showed that impurity energy levels created by 2p N electrons lied above the
valence bands of TiO2, reduce the optical band gap of materials. The expansion of energy levels of O 2p
electron is another reason for this band gap reduction, similar to V doping case.


8
3.3 Calculated results for TiO2 clusters
3.3.1 Models of TiO2 clusters
Models of (TiO2)n clusters (n = 1, 2, 3,4 and 5) used in were showed in Figure 3.9.
n=1
n=2
n=3
n=4

TC-1

TC-2

TC-3

TC-4

n=5

TC-5

Figure 3.9 Models of TiO2 clusters.
3.3.2 Charges transfers in cluster (TiO2)n
+ When electrons are added to (TiO2)n cluster, they tend to settle around Ti atoms, occupying the
empty d orbitals of the Ti atoms. The added electrons concentrate at Ti atoms with smaller coordination
numbers (empty d orbitals), most of them are Ti atoms on the surface of the TiO2 nanoparticle.
+ When electrons are pulled out of (TiO2)n clusters, the lost electrons are usually single electrons in
O 2p orbitals, especially from the O atoms which have the coordinate number 1. When electrons are taken
in photoelectric reactions, the holes tend to settle around the O atoms on the material surface, which has
smaller coordination number than other O atoms inside material.
3.4 Calcualted results for TiO2/CNTs materials
3.4.1 Models of TiO2/CNTs
The unit cell of pseudoperiodic lattices used to calculate for TiO2/CNTs has lattice constants 𝑎 = 𝑏
= 30.0 Å; 𝑐 = 17.0 Å and 𝛼 = 𝛽 = 𝛾 = 90o. Each unit cell include a (10,0) single-wall CNTs and one of
five clusters in Figure 3.9; contain 160 C atoms, 𝑛 Ti atoms and 2𝑛 O atoms (n = 1, 2, 3, 4 and 5).
3.4.2 Geometry and bonding in TiO2/CNTs
Different models of TiO2/CNTs were built, optimized and calculated. The final conclusions are:
+ Optimized structures of TiO2/CNTs models are not influented by initial positions of (TiO2)2 on
CNTs surfaces.
+ The Ti-C bonding lengths are in the range of 2.58 to 3.47 Å, does not depend on the relative

postions between TiO2 and CNTs.
+ The motion directions or final positions of Ti atoms are influented by coordinate numbers of Ti
atoms in (TiO2)n clusters. The adsorption anergies of all models are nearly equal to eachother, indicate
that the adsorption of TiO2 on CNTs are chemical adsorprion. The calculated results also suggest that
interaction between TiO2 and CNTs are caused by interaction between d orbitals of Ti and 𝜋 electrons of
CNTs surfaces.
The redistribution of electron density when TiO2 adsopted on CNTs surfaces on Figure 3.14 showed
that the electron density in the middle zone - between TiO2 and CNTs - is increased significantly. This
increment is related to the formation of chemical bonding on CNTs surface.


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CTO-1

a. Cross-section.

b. Parallel-section.

c. Config. A

d. Config. B

e. Cross-section

f. Parallel-section

CTO-2

CTO-3

C

Ti
O
Figure 3.14 Electron density redistribution of TiO2/CNTs models.
3.4.3 Photocatalytic activity of TiO2/CNTs

PDOS (eV-1)

b

Energy (eV)
Figure 3.17 PDOS of TC-2 cluster and CNT in Figure 3.18b DOS of C atoms on the CNTs
CTO-2 model.
surface.
Figure 3.17 showed that the band gap of TiO2/CNTs were the overlay of PDOS of TiO2 and CNTs. This
overlay make the charge transfers between TiO2 and CNTs happen easier, especially for photocatalytic
reactions.


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Figure 3.18b shows the PDOSs of different C atoms on CNTs surfaces. When the bonding between
C atoms and TiO2 were formed, new states appear in the range of 0 to 2.0 eV in the band gap of C atoms.
These new states increase the charges exchange processes between TiO 2 and CNTs. The influcence of
TiO2 on C atoms are not localized, it can increase the conduction of CNTs.
The high conductivity of CNTs improve the capable of charges exchange between TiO2 and CNTs,
increase the photocatalytic activity of materials, in a good agreement with previous studie. The results can
be used to orient methods to improve the photocatalytic activity of TiO 2 by doping or compositing in
chapter 4 and 5.

Chapter 4: THE INFLUENCE OF DOPING ON THE PHOTOCATALYTIC ACTIVITY OF TiO2

Table 4.1 V-doped TiO2 samples series.

HV, CV and SV Series

1

Fabricated Methods

HS Serie

3

0.1

0.3

0.5

0.7

0.9

Hydrothermal

HV0

HV1

HV3

HV5

HV7

HV9

Co-precipitation

CV0

CV1

CV3

CV5

CV7

CV9

Sol-gel

SV0

SV1

SV3

SV5

SV7

SV9

V-doped TiO2 samples synthesized by hydrothermal method with different time
Samples

HVT0

HVT1

HVT3

HVT5

HVT7

HVT9

V concentration (% at.)

0

0.5

0.5

0.5

0.5

0.5

Hydrothermal Times (hrs.)

5

1

3

5

7

9

V-doped TiO2 samples synthesized by hydrothermal method with different solvents
Samples
Precursors
Solvents

HA Serie

4

V concentration (% at.)

0

HVT
Serie

2

V-doped TiO2 samples fabricated by different methods

HWAT

HCLA

HOXA

HOLA

TiCl4 +
Ethanol

TiCl4

TiCl4 +
Ethanol

TiCl4 +
Ethanol

V2O5 + H2O

V2O5 + HCl

V2O5 + OXA

V2O5 + OLA

V-doped TiO2 samples using solvents with different OLA concentration
Samples

HA1420

Precursors

HA1620

HA1820

TiCl4 + Ethanol

Solvents
TiCl4 : OLA : Ethanol

HA1520

V2O5 + OLA
1 : 4 : 20

1 : 5 : 20

1 : 6 : 20

1 : 8 : 20

4.1 The influence of synthesis methods and doping concentration on V-doped TiO2 samples
4.1.1 Crystal structures of V-doped TiO2
Figure 4.1 shows the XRD patterns of V-doped TiO2 samples synthesized by different methods:
hydrothermal, co-precipitation and sol-gel. All patterns show the characteristic peaks of anaatase TiO2,


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consistent with JCPDS 21-1272 standard card in ICDD library. TiO2 has tetragonal crystal system, space
group I41/amd; lattice constant 𝑎 = 𝑏 = 3.7852 Å, 𝑐 = 9.5143 Å and 𝛼 = 𝛽 = 𝛾 = 90°.

Intensity (a.u.)

a

Intensity (a.u.)

b

Intensity (a.u.)

c

𝟐𝜽 (degree)
Figure 4.1 XRD patterns of (a) HV, (b) CV and (c) SV series.

Figure 4.2 shows the UV-Vis absorption spectra
of HV serie, the inset shows the band gap energy of

samples calculated from UV-Vis spectra. The
absorption edge of undoped sample HV0 is 390 nm,
consistent with experimental result. V-doped samples
show a redshift of absorption edges, caused from the
substitution of V4+ ions into TiO2 lattice.
The calculated results show a limited
concentration of V to reduce band gap energy of doped
samples. In this thesis, the limited concentration is 0.5%
at. and is used for latter study

Absorptance (a.u.)

4.1.2 Optical properties of V-doped TiO2 samples

Wavelength (nm)
Figure 4.2 UV-Vis absorption spectra of HV
serie.


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4.1.3 Photocatalytic activity of V-doped samples
4.1.3.1 The influence of synthesis methods on photocatalytic activity of V-doped TiO2
Table 4.2 Conditions of photocatalytic reaction of V-doped samples.
TiO2 samples

V concen. (%at)

Illuminated Light

Names

HV0

0

Room

HT+R

HV0

0

Lamp

HT+L

HV5

0.5

Lamp

HV+L

SV5

0.5

Lamp

SV+L

CV5

0.5

Lamp

CV+L

Photocatalytic activity of samples were showed
in Figure 4.4. All samples are able to photodegrade
phenol, the efficiency is higher when illuminated by
light bub. V-doped samples have higher photocatalytic
activity than undoped samples; HV sample
(synthesized by hydrothermal method) has the highest
photocatalytic activity.

Phenol Concentration (%)

The photocatalytic activity of V-doped samples were tested by photodegration of 5.10-6 mol/litre
phenol solution; illuminated by a 100 W light bub. The photocatalytic conditions were listed in table 4.2.

SEM images of V-doped samples were showed
in the figure 4.5. CV sample (synthesized by coIlluminated Time (min.)
precipitation method) has largest particles size, HV Figure 4.4 Photodegradation of phenol in
sample (synthesized by hydrothermal method) has different conditions.
smallest particle sizes. The smaller particle sizes, the
larger specific surface area of sample and lower electron – hole recombination rate; increase the

photocatalytic activity of sample.
a

b

200 nm

c

300 nm

200 nm

Figure 4.5 SEM images of (a) HV5, (b) CV5 and (c) SV5 samples.
BET results of HV0 and HV5 show that both samples contain average size pores, specific surface
area of two samples are similar; indicate that specific surface areas do not play an important role in the
increment of photocatalytic activity of doped samples.
XPS results of HV0 and HV5 sampes werw showed in figure 4.8. The Ti 2p peak of HV0 is a high
symmetric shape, imply that there are only Ti 4+ ions in samples. For HV5 sample, the existent of V lead
to the formation of O vacancies and so Ti3+ can be formed; make the peak to be unsymmetric. The O 1s
peaks of samples indicate that new peak at 534 eV appear in doped sample, which is belonged to V-O


13
bonding. This result suggests that V atoms have substituted into Ti atoms positions; constient with
previous results in chapter 3.
b

Intensity (a.u.)

Intensity (a.u.)

a

Binding Energy (eV)

Binding Energy (eV)

c

Intensity (a.u.)

Intensity (a.u.)

d

Binding Energy (eV)

Binding Energy (eV)

Figure 4.8 XPS peaks of (a) Ti 2p and (b) O 1s of HV0 and HV5; Gaussian fitting of O 1s of (c) HV0
and (d) HV5 samples.

Absorptance (a.u.)

Phenol Concentration (%)

4.1.3.2 The influence of doping concentration on photocatalytic activity of TiO2

Illuminated Time (min.)

Wavelength (nm)

Figure 4.9 Photodegradation of phenol of HV Figure 4.10. UV-Vis absorption spectra of HVT
serie.
serie.
Figure 4.9 show the result of photodegration of phenol by V-doped samples with different impurity
concentration. HV5 sample contain 0.5% at. V show the highest photocatalytic activity in the visible
range; also the same value for CV and SV series. It mean that the best concentration of V in the sample
for highest photocatalytic activity is 0.5% at. This concentration is used for all samples in latter studies in
this thesis.


14
4.2 The influence of hydrothermal parameters on V-doped TiO2 samples
4.2.1 The influence of hydrothermal time on V-doped TiO2 samples
Figure 4.10 show the UV-Vis absorption spectra of HVT serie – 0.5% at. V-doped TiO2 samples
with different hydrothermal time (tH). The absorption edges of doped samples shifted to the longer
wavelength zone, the absorptances of all samples are increase. When the hydrothermal time is smaller
than 7 hours, the absorptance of samples increase when the t H raise up; howerver, when tH is larger than
7 hours, the absorptance decrease when tH increase.

Intensity (a.u.)

Intensity (a.u.)

Figure 4.11 shows that when tH reachs to 9 hours, V-doped sample contain amount of rutile phase.
Figure 4.12 also show that the increment of t H change the growth direction of TiO2 crystal in sample. So
the chosen hydrothermal time is 7 hours.

Peaks

𝟐𝜽 (degree)

Figure 4.11 XRD patterns of HT, HVT5 and HVT7 Figure 4.12 Relative intensity of XRD peaks of
samples.
HVT5 and HVT9 samples.

In hydrothermal methods, samples properties will
affected by another parameter – hydrothermal solvents.
The change in solvent polarity lead to the change in
properties of samples. Figure 4.13 show that all samples
are monophase, independent of the polarity of solvents.
Two samples using solvents contain organic acids –
HOLA and HOXA – have smaller crystal sizes than the
other two, HCLA and HWAT. It is consistent with SEM
and Raman results.

Intensity (a.u.)

4.2.2 The influence of solvents on photocatalytic activity of V-doped TiO2

UV-Vis absorption spectra in figure 4.15 point out
𝟐𝜽 (degree)
that the absorption edges of samples do not perform any
Figure 4.13 XRD patterns of HVS serie.
redshift but HOLA and HOXA samples have larger
absoptances of than HWAT and HCLA samples. The increment of absorptances can be explained by two
reasons: (i) the substitution of V atom into TiO2 lattice and (ii) the formation of polarized layer on the
surface of TiO2 nano particles. Figure 4.17 show that HOLA and HOXA samples have higher

photocatalytic activity due to two reasons: (i) smaller crystal sizes and (ii) higher absorptances.


MB Concentration (%)

Absorptance (a.u.)

15

Wavelength (nm)

Illuminated Time (min.)

Figure 4.15 UV-Vis absorption spectra of HWAT, Figure 4.17 Photodegradation of MB solutions of
HCLA, HOXA and HOLA samples.
HS serie.
4.3 The influence of concentration of solvents on morphology of V-doped TiO2
4.3.1 Crystal structures

Intensity (a.u.)

XRD patterns of HA serie - used solvents with different OLA concentration - were showed in figure
4.19. Almost samples are anatase + brookite multiphase; the ratio of brookite increase with the increment
of OLA concentration; or in other hands, the increment of OLA concentration reduce the monophase of
sample. The monophase samples can be received with ratio of TiCl4 : OLA : H2O equals to 1 : 8 : 20.

𝟐𝜽 (degree)
Figure 4.19 XRD patterns of HA1420 ÷ HA1820 samples.
4.3.2 The influence of solvent on morphology of V-doped TiO2
SEM and TEM images of 0.5% at. V-doped samples in figure 4.20 show the affection of OLA

concentration on the V-doped samples’s morphology. The ratio TiCl4 : OLA : C2H5OH increase from 1 :
4 : 20 to 1 : 8 : 20, the particles shape change from spheroid-like to cubic-like, similar rod-like and
spherical-like.


16
e

a

200 nm

20 nm

f

b

20 nm

300 nm

g

c

200 µm

20 nm

h

d

500 nm

20 nm

Figure 4.20 SEM and HR-TEM images of (a,e) HA1420, (b,f) HA1520, (c,g) HA1620 and (d,h) HA1820.


17
4.4 Fabrication of N-doped TiO2 films
N-doped TiO2 thin films were fabricated using Atomic Layder Deposition (ALD) method. Two
seires have been fabricated: ANN serie - samples with different NH3 flow rate from 0 to 20 cm3/min,
denoted as ANN0, ANN10 và ANN20. ANT serie – samples with same NH3 flow rate and different treted
temperature: 300, 400 và 500 oC; denoted as ANT300, ANT400 và ANT500.
AFM image of undoped sample ANN0 sample were
showed in figure 4.22. The sample surface is quite uniform,
the particles are in the shape of pyramidal, similar to
equilibrium shape of TiO2 crystal. The treated temperature
does not affect to samples’ surface. For N-doped samples,
the conclusions are similar to undoped samples.
The absorption spectra of N-doped samples show that
the band gap energies decrease with the increment of
impurity concentration. The reason come from new energy
levels in the band gaps of material, caused by substitution of
N atoms into TiO2 lattice. The higher N concentrations, the
wider impurity levels and the smaller band gap energy of
doped samples.

Binding Energy (eV)

Binding Energy (eV)

c

Intensity (a.u.)

b

Intensity (a.u.)

Intensity (a.u.)

a

Figure 4.22 AFM image of ANN0.

Binding Energy (eV)

Figure 4.26 O 1s peaks of (a) ANN0, (b) ANN10 and (c) ANN20 samples.
XPS results in figure 4.27 show the formation of TiO2 bonding in all samples, Ti has valence of +4,
O has valence of -2. N 1s peaks only show the appearance of Ti-N bonding peaks, imply that N atoms
substituted in to O atoms positions.
b

Intensity (a.u.)

Intensity (a.u.)

a

Binding Energy (eV)
Binding Energy (eV)
Figure 4.27 N 1s peak of (a) ANN10 and (b) ANN20 samples.


18
Chapter 5: THE INFLUENCE OF SYNTHESIS METHODS ON THE PROPERTIES OF
TiO2/CNTs MATERIALS

Table 5.1 Lists of TiO2/CNTs samples.
1

TiO2/CNTs samples using CNTs oxidized by different methods

TC-O Serie

Samples

2

TC-BA Serie
TC-m

Solvents

TC-0-0

None

TTiP + Ethanol + H2O

TC-1-I

HNO3

TTiP + Isopropanol

TC-1-BA

HNO3

TTiP + Ethanol + H2O + BA

TC-2-I

HNO3 : H2SO4

TTiP + Isopropanol

TC-2-BA

HNO3 : H2SO4

TTiP + Ethanol + H2O + BA

TiO2/CNTs samples using CNTs oxidized by different BA concentration solvents
Samples

3

Oxidation Solution

Oxidation Solution

VBA (ml)

Solvents

TC-EHB-5

Ethanol + H2O + BA

5

TTiP + Ethanol + H2O

TC-EHB-10

Ethanol + H2O + BA

10

TTiP + Ethanol + H2O

TC-EHB-20

Ethanol + H2O + BA

20

TTiP + Ethanol + H2O

TC-EB-10

Ethanol + BA

10

TTiP + Ethanol + H2O

TiO2/CNTs samples with different 𝑚𝑇𝑖𝑂2 : 𝑚𝐶𝑁𝑇𝑠 ratio
Samples

TC1

TC3

TC30

TC80

TC500

TC1000

𝑚𝑇𝑖𝑂2 : 𝑚𝐶𝑁𝑇𝑠

1:1

3:1

30 : 1

80 : 1

500 : 1

1000 : 1

5.1 The influence of CNTs oxidation on TiO2/CNTs properties
5.1.1 The formation of TiO2-CNTs contact layer

500 nm

200 nm

Figure 5.1 SEM images of TC-0-0.
CNTs have been oxidized by HNO3 65% solution and H2SO4 : HNO3 mixture; BA were used to aid
the oxidation process of CNTs.


For samples used unoxidized CNTs, TiO2 nanoparticles do not adhere on CNTs surfaces and


19




aggregated.
For samples used CNTs oxidized by HNO3 solution, TiO2 nanoparticles dispersed in samples, adhere
to CNTs surface. The dispersion is not uniform and TiO2 nanopartilce still aggregated. HNO3 solution
oxidize CNTs better than H2SO4 : HNO3 mixture.
BA play an important role in the oxidation of CNTs, the adherence of TiO2 on CNTs surface is better.
a

b

500 nm
c

500 nm
d

1000

500 nm

Figure 5.2 SEM images of (a) TC-1-I, (b) TC-1-BA,(c) TC-2-I and (d) TC-2-BA.
5.1.2 The influence of BA concentration

Intensity (a.u.)

SEM images of TC-BA serie in Figure 5.3 show that (i) TiO2 nanoparticle dispersed uniformly in
samples and adhere on CNTs surfaces, (ii) the amount of BA around 10 ml give the best sample.

𝟐𝜽 (degree)

Figure 5.4 XRD patterns of TC-BA serie.
XRD patterns of TC-BA serie in figure 5.4 show that the characteristic peaks of CNTs do not appear
clearly, even for (002) peaks at 26.6o. The existence of (002) peaks can be seen by the expansion of (101)


20
peaks of TiO2; the larger expansion, the higher ratio of CNTs in the sample. The results can be reconfirmed
in absorption spectra in fig ure 5.6. When the amount of CNTs increase, the absorptance in visible range
increase.
Figure 5.7 show the photodegradation of MB in solutions using TC-BA serie. All of TiO2/CNTs
samples show higer photocatalytic activity than TiO2, CNTs or self-degradation of MB; imply that TiO2CNTs interaction increase the photocatalytic activity of samples.

MB Concentration (%)

Absorptance (a.u.)

The influence of oxidation can be seen in figure 5.6. TC-EHB-10 and TC-EB-10 samples show
higher photocatalytic activity than TC-EHB-5 or TC-EHB-20 samples, TC-1-I sample has lowest
photocatalytic activity. The reason can be explained by the formation of TiO 2-CNTs contact layers. The
more contact layers, ther higher photocatalytic activity.

Wavelength (nm)

Illuminated Time (hrs)

Figure 5.6 UV-Vis spectra of TC-BA.

Figure 5.6 Photocatalytic RhB degradation curves
of TC-BA serie.

5.2 The influence of mass ratio on TiO2/CNTs properties

Intensity (a.u.)

5.2.1 Crystal structures

𝟐𝜽 (degree)
Figure 5.7 XRD patterns of TC1÷TC1000 samples.
XRD patterns of TC-m serie – TiO2/CNTs samples used CNTs oxidized by HNO3 solution – were
showed in figure 5.7. Anatase TiO2 peaks were observed for all samples, consistent with JCPDS 21-1272
cards in ICDD library. CNTs peaks or unknown peaks were not observed.
The influence of of 𝑚𝑇𝑖𝑂2 : 𝑚𝐶𝑁𝑇𝑠 ratio on samples properties can be seen in figure 5.8: the


21
increment of CNTs mass expanse the width of (101) peaks and shift it to larger 2𝜃 region. The reason is
explained by the overlap of CNTs (002) peaks and TiO2 (101) peaks.
5.2.2 Morphology of TiO2/CNTs samples
SEM and HR-TEM images of TC1 in figure 5.8 show that TiO2 nanoparticles dispersed and adhere
on CNTs surface. The dispersion is not uniform, TiO2 nanoparticles still be aggregated. The aggregation
is proportional to the amount of CNTs in the samples.
a

b

100 nm

100 nm

Figure 5.8 (a) SEM and (b) HR-TEM images of TC1.

Intensity (a.u.)

b

Intensity (a.u.)

a

Binding Energy (eV)

Binding Energy (eV)
d

Intensity (a.u.)

Intensity (a.u.)

c

Binding Energy (eV)
Binding Energy (eV)
Figure 5.11 XPS results of TC10.
BET results showed that specific surface area of TiO2 and TC1 are approximated 100 m2/g and 150
m2/g, respectively; indicate that when composited with CNTs, the specific surface area of samples
increase.
XPS results show that only C-O-Ti bonding peaks can be observed, the Ti-C bonding peaks can not


22

be found. The interaction between TiO2 and CNTs are formed by indirect C-O-Ti bonding, not by direct
bonding Ti-C. These O atoms can be form by oxidation processes, reconfirm the role of CNts oxidation
in the formation of TiO2-CNTs contact layers.
5.2.3 Optical properties of TiO2/CNTs

Absorptance (a.u.)

MB Concentration (%)

UV-Vis absorption spectra of TC-m serie – TiO2/CNTs samples with different 𝑚𝑇𝑖𝑂2 : 𝑚𝐶𝑁𝑇𝑠 ratio
– infigure 5.12 show that the absorption edges do not change but the absorptance in visible range increase
with the increment of CNTs mass.

Wavelength (nm)

Illuminated Time (hrs)

Figure 5.12 UV-Vis absorption spectra of TC-m Figure 5.13 Photodegradation of MB by TC-m
serie.
serie.
5.2.4 Photocatalytic activity of TiO2/CNTs
Photodegradation of MB of TC-m serie were showed in figure 5.14. All composite samples have
higher photocatalytic activity than TiO2 sample, the TC3 sample with 𝑚𝑇𝑖𝑂2 : 𝑚𝐶𝑁𝑇𝑠 = 3 : 1 has highest
photocatalytic activity. This imply that CNTs has an important role in the increment of photocatalytic
activity of samples.

Figure 5.14 Electron transfer mechanism.

Figure 5.15 Photon absorption mechanism.

Two reasons can be used to explain the increment of photocatalytic activity of TiO2/CNTs samples:
(i) the reduction of e – h recombination rate and (ii) the increment of specific surface area. There are two
mechanisms to explain the role of CNTs: elctron transfer mechanism and photon absorption mechanism.
In electron transfer mechanism in figure 5.14, photoelectron in TiO2 when illuminated will transfer to
CNTs, reduce the e – h recombination rate; consistent with previous result in chapter 3. Experimental
results in chapter 5 show that the absorptance in visible range increase, consistent with photon absorption
mechanism in figure 5.15. In TiO2/CNTs materials, CNTs play two roles – photon absorber and electron
conductor; both of them increase the photocatalytic activity of TiO 2/CNTs materials.


23

CONCLUSION
A. On the material simulation using DFT
1.

2.

3.

4.

DFT has been used to simulate and calculate for TiO2 and doped TiO2 materials, TiO2/CNTs
materials. The chosen parameters to calculate for TiO2: generalized gradient approximation (GGA),
PBE exchange-correlation function, plane wave basis, Vanderbilt pseudopotential and Hubbard
potential 8.18 eV (used for d orbital electrons). The calculated results are in good agreement with
experimental and other theoretical studies.
A systematic research on V-doped and N-doped TiO2 has been performed. The doping of V, N lead
to the formation of O vacancies and extra electrons; these electron localized around Ti atoms near
O vacancies. These electrons play an important role in the improvement of photocatalytic activity

of materials. The mechanism of visible phototcatalytic activities of doped TiO2 has been proposed.
Properties of (TiO2)n has been simulated and investigated successfully: surface O atoms play an
important roles in the photochemical processes associated with holes tranfered to TiO 2
nanoparticles surface whereas Ti atoms take part in processes associated with electrons transferred
to TiO2 nanoparticles. These results are in a good agreement with previous researchs and can be
used to predict of TiO2-CNTs interaction and the enhancement of photocatalytic activity of TiO2CNTs materials.
Adsorption processes of TiO2 on CNTs surfaces has been simulated and calculated, the result show
that they are stable, weak chemical adsorption processes. The TiO 2-CNTs bondings are formed
through the interaction of empty d orbitals of Ti atoms and 𝜋 electrons on the CNTs surface. The
partial densities of states C atoms on CNTs surface raise up, increase the ability of electron
exchange between TiO2 and CNTs, improve the photocatalytic activity of TiO2.

B. On the fabrication methods and material properties
1.

2.

3.

4.

TiO2 and doped TiO2 were prepared by hydrothermal, sol-gel and co-precipitation methods; the
hydrothermal sample has highest photocatalytic activity. The chosen synthesis conditions:
hydrothermal method, 0.5% V doping concentration, and hydrothermal time is 5 hours, oleic acid
(OLA) contained solvents.
Characteristics of 0.5%V-doped TiO2: monophase anatase material, average particle sizes of 10 ÷
20 nm, 2.90 eV band gap values, high photocatalytic activity (photodegrade more than 90% of
phenol in tested solution after 3 hours of visible light illumination). The XPS results indicate that
V atoms are substituted into the Ti atoms postions, consistent with theoretical calculations.
The shapes and sizes of the nanoparticles can be controlled by changing the polarization of solvents.

Samples synthesized using solvent with TiCl4 : OLA : Ethanol = 1 : 8 : 20 have smallest particle
sizes and highest photocatalytic activity.
TiO2 and N-doped TiO2 thin films prepared by atomic layer deposition have uniform thickness,
average particle sizes of 20 nm, absorption edges have been shifted to visible range. The XPS results
indicate that in TiO2 sample, O has valence of +2, Ti has valence of +4, and no other valence states
exists. When doped into TiO2 lattice, N atoms substituted into O atoms positions, do not substitute
into the position of Ti or interstitial positions, in a good agreement with theoretical calculations.