Nghiên cứu tổng hợp tio2 và khả năng ứng dụng để xử lý cr6+ trong nước thải = reseach on synthesis of tio2 and application for cr6+ treatment in wastewater

BỘ GIÁO DỤC VÀ ĐÀO TẠO
TRƯỜNG ĐẠI HỌC BÁCH KHOA HÀ NỘI

.......................................

NGUYỄN VĂN CHÚC

NGHIÊN CỨU TỔNG HỢP TIO2 VÀ KHẢ NĂNG
ỨNG DỤNG ĐỂ XỬ LÝ SR6+ TRONG NƯỚC THẢI

LUẬN VĂN THẠC SĨ KHOA HỌC

NGƯỜI HƯỚNG DẪN : TS. NGUYỄN HỒNG LIÊN

HÀ NỘI – 2010


Research on synthesis of TiO2 and application for Cr6+treatment in wastewater

ACKNOWLEDGEMENT
I would like to express my gratitude to the people that support me in the completion of
my thesis.
In the first place, I owe a special thank to my dear teachers at the Department of organic
and petrochemical technology for their passionate teaching, generosity in dealing with
students and also enthusiastic inspiration during the past four years. Their devotion to
petrochemical technology study and teaching has highly motivated me to pursue my
future career as an expert in petrochemical by all my efforts and brain power.
To my supervisors, I would like to acknowledge and extend my heartfelt gratitude to
Doctor Nguyen Hong Lien and Associated Professor, Doctor Le Minh Thang,
lecturers of the Department of organic and petrochemical technology. Within their
petrochemical course, I luckily found my interest in petrochemical technology, and then

under their guidance I continued my study in petrochemical technology in my thesis.
Indeed, I am deeply in debt of their endless patience for their correcting tiny mistakes in
my thesis and priceless encouragement that enables me to complete my thesis in due
time.
Last but not less, from deep inside, it is hard to find a suitable word to send my deep
thank and express my strong love for my family, my mother, my brothers, my sisters and
my girl friend for both their physical and spiritual support. Although I have to live far
from them, I always feel fully their images and eternal love by my side. It is the time I
realize that the family is the most wonderful thing I luckily possess.
To all, I wish you the best.
Hanoi, October 2010
Student

Nguyen Van Chuc

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Research on synthesis of TiO2 and application for Cr6+treatment in wastewater

TABLE OF CONTENTS
ACKNOWLEDGEMENT................................................................................................ 1
TABLES IN THE THESIS .............................................................................................. 4
FIGURES IN THE THESIS ............................................................................................ 5
INTRODUCTION ............................................................................................................ 7
CHAPTER I: LITERATURE REVIEW........................................................................ 8
I.1. General of wastewater .................................................................................................. 8

I.1.1. Water pollution.......................................................................................................... 8
I.1.2. Water pollution categories......................................................................................... 9
I.1.3. Causes of polluted water ......................................................................................... 10
I.1.4. Effects of polluted water ......................................................................................... 10
I.1.5. Control of water pollution ....................................................................................... 10
I.2. Cr(VI) treatment methods in wastewater ................................................................... 12
I.2.1. Ion exchange method............................................................................................... 12
I.2.2. Electrochemistry method......................................................................................... 13
I.2.3. Reduction-oxidation method and deposition method.............................................. 13
I.3. TiO2 review ................................................................................................................ 14
I.3.1. Occurrence............................................................................................................... 14
I.3.2. Physical and mechanical properties......................................................................... 14
I.3.3. Chemical properties................................................................................................. 16
I.3.4. Applications............................................................................................................. 16
I.4. Mechanism of titanium oxide photocatalytic reactions.............................................. 17
I.4.1. Band structure of semiconductors and band gap energy ......................................... 17
I.4.2. Energy structure of titanium oxide and photoeffect ................................................ 18
I.4.3. Effect of ultraviolet rays in activating titanium oxide............................................. 20
I.4.4. Decomposing power of titanium oxide photocatalyst ............................................. 21
1.4.5. The mechanism of photo-reduction of Cr(VI)........................................................ 23
I.5. Review of the used precursors in this thesis............................................................... 24
I.5.1. TiCl4 ........................................................................................................................ 24
I.5.2. Titanium isopropoxide (TTIP) ................................................................................ 28
I. 6. Literature review about using TiO2 as a photocatalyst for wastewater treatment ..... 29
I.6.1. In Vietnam ............................................................................................................... 29
I.6.2. In the world.............................................................................................................. 30
I.7. The importance and direction of thesis ...................................................................... 36
I.7.1. The importance of the thesis.................................................................................... 36
I.7.2. The direction of thesis ............................................................................................. 36
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CHAPTER II. EXPERIMENTAL ................................................................................ 37
II.1. Reagents and materials.............................................................................................. 37
II.2. Preparation of photocatalysts .................................................................................... 37
II.2.1. Sol-gel method ....................................................................................................... 37
II.2.2. Hydrolysis method ................................................................................................. 38
II.2.3. Impregnating method ............................................................................................. 39
II.3. The methods to determine the composition of chromium plating wastewater ......... 40
II.3.1. pH meter................................................................................................................. 40
II.3.2. Atomic Absorption Spectroscopy (AAS)............................................................... 41
II.4. Physico-Chemical experimental techniques ............................................................. 44
II.4.1. X-ray diffraction (XRD) ........................................................................................ 44
II.4.2. Scanning electron microscopy (SEM) ................................................................... 46
II.4.3. BET method for the determination of surface area................................................ 47
II.5. Catalytic activity test................................................................................................. 49
II.5.1. Equipment description ........................................................................................... 49
II.5.2. The analysis of the composition of reaction solution............................................. 50
CHAPTER III. RESULTS AND DISCUSSION.......................................................... 63
III.1. Composition of plating chromium wastewater........................................................ 63
III.2. Physico chemical properties of synthesized catalysts.............................................. 65
III.2.1. The specific surface areas (SSA) of synthesized catalysts ................................... 65
III.2.2. The phase composition of synthesized catalysts................................................... 66
III.3. Catalyst activity of synthesized catalysts................................................................. 70
III.3.1. The influence solar illumination and the suport to the catalytic activity.............. 70
III.3.2. The influence of pH of reaction solution to the catalytic activity ........................ 71

III.3.3. The influence of catalytic synthesis methods to the catalytic activity.................. 72
III.3.4. The influence calcination temperature of catalyst to the atalytic activity ............ 73
III.3.5. The influence of amount of TiO2 doped onto Al2O3 to the catalytic activity....... 74
III.3.5. The influence of concentration of ethanol to the catalytic activity....................... 74
CONCLUSIONS ............................................................................................................. 76
References........................................................................................................................ 77

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TABLES IN THE THESIS
No

Name

Page

1

Typical physical and mechanical properties of titania

15

2

Optical properties of titania

15

3

Ultraviolet rays in ordinary surroundings

22

4

The physical properties of TiCl4

24

5

The physical properties of TTIP

29

6

Properties Computed from Structure

38

7

The amount of TiO2 loading onto Al2O3 of synthesized catalysts

40

8

The concentration of the metallic ions in wastewater

64

9

SSA of synthesized catalysts

66

10

Compositions of catalysts prepared by sol-gel, hydrolysis and
Impregnating method

69

11

The changes of concentration of Cr(VI) and COD

71

12

The effects of UV and catalyst to conversion of Cr(VI) and COD

72

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FIGURES IN THE THESIS
No
1
2
3
4
5
6
7
8
9
10

Name

20

Titanium-oxide band structure

21

Crystal structures of titanium oxide

21

Electron structure of titanium oxide

22

Oxidation mechanism

22

Reduction mechanism

39

The Sol-gel method

40

Hydroysis method

41

Impregnating method

42

pH meter
Atomic absorption spectrometer block diagram

11

Illustrates how diffraction of X-rays by crystal planes allows one to derive
lattice by using Bragg relation

12

The interaction between the primary electron beam and the sample in an
electron microscope leads to a number of detectable signals

13
14
15
16
17

Page

43
46

48
49

The BET plot

50

The reactor system
The calibration graph of Cr(VI) with 1,5 – diphenylcarbazide solution
Concentration of Cr(VI) in three reaction solutions
Concentration of Cr(VI) in reaction solution with and without
Flocculation

56
57
58

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Research on synthesis of TiO2 and application for Cr6+treatment in wastewater
18
19

The XRD patterns of the TiO2/Al2O3 samples
SEM images of the synthesized catalysts

56
68

20

The conversion of Cr(VI) deppended on pH of reaction solution

73

21

The conversion of Cr(VI) deppended on the catalytic synthesis
methods

74

22

23
24

The conversion of Cr(VI) deppended on calcination temperature of
catalyst
The conversion of Cr(VI) deppended on amount of TiO2 doped
onto Al2O3
The influence of concentration of ethanol to the catalytic activity

74

75
76

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INTRODUCTION
Water pollution has always been a major problem to the environment in the global
context. It has been supposed the leading worldwide cause of deaths and diseases
of human beings, organism in daily life. With the increasing growth of the
industrialization in major areas and urban cities, the water source just keeps
getting more polluted. In Vietnam, the water pollution is of the most serious
problems which draw the attention of all levels of society, especially,
governmental authorities, researchers, and scholars. The main reason which
caused the water pollution is suggested to originate from non-treated waterwaste
discharged from factories, and industrial plants. Therefore, wastewater treatment,
especially wastewater from coating companies containing a large amout of heavy
metal, is important task in Viet Nam in particular and in the world in general.
In this paper, the research topic focuses on the reduction of heavy metal
concentrations by using TiO2 as a photocatalyst. Cr(VI) is choosen as a poluted
agent and ethanol as a hole scavenger. The TiO2/Al2O3 system is synthesized by
variety methods and many effect agents are tested. The properties of the
sythesized catalysts are investigated by the physico-chemical method. The
catalytic activities of the sythesized catalysts are determined by the conversion of
Cr(VI) in water with the presence of ethanol.
The main content of the paper is divided into three parts. Part I discusses the
remark of water pollution and its impact to the human life, the wastewater
treatment methods, review of TiO2 and precursors, the researches about TiO2 as a
photocatalyst for wastewater treatment in the world and Viet Nam. Part II
introduces the synthesized catalytic methods, the methods to determine the
composition of plating chromium wastewater, the methods to investigate the

properties of the synthesized catalysts, the methods to calculate the reaction
results. Part III mentions the results and discussions of the experiments and the
research process.
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CHAPTER I: LITERATURE REVIEW
I.1. General of wastewater
I.1.1. Water pollution
Water pollution is currently one of the serious types of pollution facing human beings all
over the world. The issue has drawn different scales of involvement and cooperation.
A case in point is that in European Union, a high level of environmental protection and
the improvement of the quality of the water environment must be integrated into the
policies of the Union and ensured in accordance with the principle of sustainable
development. The paper will go into depth of the problem in the perspectives of
definition and causes of water pollution, and measurement of pollution and control of
water pollution
In European Charter for sustainable tourism in protected area published in 2000,
pollution of water is defined as `the discharge by man, directly or indirectly, of
substances or energy into the aquatic environment, the results of which are such as to
cause hazards to human health, harm to living resources and to aquatic eco-systems,
damage to amenities or interference with other legitimate uses of water.
There are also a variety of determinations of water pollution by different organizations.
However, they share together some common points as follows: (i) water pollution is the
contamination of water bodies such as lakes, rivers, oceans, and groundwater; (ii) all
water.

Photo I.1: Water polluted by garbage

Pollution affects organisms and plants that live in these water bodies and in almost all
cases the effect is damaging either to individual species and populations but also to the
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Research on synthesis of TiO2 and application for Cr6+treatment in wastewater
natural biological communities; (iii) it occurs when pollutants are discharged directly or
indirectly into water bodies without adequate treatment to remove harmful constituents
[46].
I.1.2. Water pollution categories
Surface water and groundwater have often been studied and managed as separate
resources, although they are interrelated [40]. Sources of surface water pollution are
generally grouped into two categories based on their origin.
a. Point source pollution
Point source pollution refers to contaminants that enter a waterway through a discrete
conveyance, such as a pipe or ditch. Examples of sources in this category include
discharges from a sewage treatment plant, a factory, or a city storm drain.
b. Non-point source pollution
Non-point source (NPS) pollution refers to diffuse contamination that does not originate
from a single discrete source. NPS pollution is often accumulative effect of small
amounts of contaminants gathered from a large area. A typical example is that the
leaching out of nitrogen compounds from agricultural land which has been fertilized.
Nutrient runoff in storm water from "sheet flow" over an agricultural field or a forest are
also cited as examples of NPS pollution.

c. Groundwater pollution
Interactions between groundwater and surface water are complex. Consequently,
groundwater pollution, sometimes referred to as groundwater contamination, is not as
easily classified as surface water pollution. By its very nature, groundwater aquifers are
susceptible to contamination from sources that may not directly affect surface water
bodies, and the distinction of point vs. nonpoint source may be irrelevant.
Groundwater accounts for 97% amount of fresh water of the Earth. However, the water
source is contaminated seriously. The pollution may derive from fixing dumping ground
unsanitarily, waste water from industrial activities, using a huge amount of fertilizer and
pesticide. The most popular ground water pollution is caused by As.
Analysis of groundwater contamination may focus on soil characteristics and hydrology,
as well as the nature of the contaminant itself.

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Research on synthesis of TiO2 and application for Cr6+treatment in wastewater
I.1.3. Causes of polluted water
The specific contaminants leading to pollution in water include a wide spectrum of
chemicals, pathogens, and physical or sensory changes such as elevated temperature and
discoloration. While many of the chemicals and substances that are regulated may be
naturally occurring (calcium, sodium, iron, manganese, etc.) the concentration is often the
key in determining what is a natural component of water, and what is a contaminant.
Many of the chemical substances are toxic. Pathogens can produce waterborne diseases
in either human or animal hosts. Alteration of water's physical chemistry includes acidity
(change in pH), electrical conductivity, temperature, and eutrophication. Eutrophication
is the fertilization of surface water by nutrients that were previously scarce.

I.1.4. Effects of polluted water
There are various effects of water pollution.
• Spread of disease: Drinking polluted water can cause cholera or typhoid infections,
along with diarrhea.
• Affects aody organs: The consumption of highly contaminated water can cause injury
to the heart and kidneys.
• Harms the food chain: Toxins within water can harm aquatic organisms, thus
breaking a link in the food chain.
• Causes algae in water: Urea, animal manure and vegetable peelings are food for
algae. Algae grow according to how much waste is in a water source. Bacteria feed off
the algae, decreasing the amount of oxygen in the water. The decreased oxygen causes
harm to other organisms living in the water.
• Flooding: The erosion of soil into waterways causes flooding, especially with heavy
rainfall.
• Harms animals: Birds that get into oil-contaminated water die from exposure to cold
water and air due to feather damage. Other animals are affected when they eat dead fish
in contaminated streams.
• The effects of water pollution are not always immediate. They are not always seen at
the point of contamination. They are sometimes never known by the person responsible
for the pollution. However, water pollution has a huge impact on our lives.
I.1.5. Control of water pollution
a. Domestic sewage
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Research on synthesis of TiO2 and application for Cr6+treatment in wastewater
In urban areas, domestic sewage is typically treated by centralized sewage treatment

plants. Cities with sanitary sewer overflows or combined sewer overflows employ one or
more engineering approaches to reduce discharges of untreated sewage, including:
• Utilizing a green infrastructure approach to improve stormwater management capacity

throughout the system [42].
• Repair and replacement of leaking and malfunctioning equipment [43].
• Increasing overall hydraulic capacity of the sewage collection system (often a very
expensive option)
b. Industrial wastewater
Some industrial facilities generate ordinary domestic sewage that can be treated by
municipal facilities. Industries that generate wastewater with high concentrations of
conventional pollutants (e.g. oil and grease), toxic pollutants (e.g. heavy metals, volatile
organic compounds) or other nonconventional pollutants such as ammonia, need
specialized treatment systems. Some of these facilities can install a pre-treatment system
to remove the toxic components, and then send the partially-treated wastewater to the
municipal system. Industries generating large volumes of wastewater typically operate
their own complete on-site treatment systems.
Some industries have been successful at redesigning their manufacturing processes to
reduce or eliminate pollutants, through a process called pollution prevention.
Heated water generated by power plants or manufacturing plants may be controlled with:
• Cooling ponds, man-made bodies of water designed for cooling by evaporation,
convection, and radiation
• Cooling towers, which transfer waste heat to the atmosphere through evaporation
and/or heat transfer
• Cogeneration, a process where waste heat is recycled for domestic and/or industrial
heating purpose
c. Agricultural wastewater
Nutrients (nitrogen and phosphorus) are typically applied to farmland as commercial
fertilizer; animal manure; or spraying of municipal or industrial wastewater (effluent) or
sludge. Nutrients may also enter runoff from crop residues, irrigation water, wildlife, and

atmospheric deposition. Farmers can develop and implement nutrient management plans
to reduce excess application of nutrients.

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Research on synthesis of TiO2 and application for Cr6+treatment in wastewater
Pollution prevention practices include low impact development techniques, installation of
green roofs and improved chemical handling (e.g. management of motor fuels & oil,
fertilizers and pesticides) [44]. Runoff mitigation systems include infiltration basins,
bioretention systems, constructed wetlands, retention basins and similar devices.
d. Thermal pollution
Thermal pollution from runoff can be controlled by stormwater management facilities
that absorb the runoff or direct it into groundwater, such as bioretention systems and
infiltration basins. Retention basins tend to be less effective at reducing temperature, as
the water may be heated by the sun before being discharged to a receiving stream.
I.2. Cr(VI) treatment methods in wastewater
There are some methods to treat Cr(VI) in wastewater in the world.
I.2.1. Ion exchange method
Whenever an ion is removed out of an aqueous solution and is replaced by another ionic
species, this is what we generally refer to as “ion exchange”. There are synthetic
materials available that have been specially designed to enable ion exchange operations at
high performance levels. Among many other applications, these so called “ion
exchangers” can be used in processes of environmental protection such as purification,
decontamination, recycling or even for the design of new environment-friendly
production processes. Synthetic and industrially produced ion exchange resins consist of
small, porous beads that are insoluble in water and organic solvents. The most widely

used base-materials are polystyrene and polyacrylate. The diameter of the beads is in a
range of 0.3 to 1.3 mm. The beads contain around 50% of water, which is dispersed in the
gel-structured compartments of the material. Since water is dispersed homogenously
through the bead, water soluble materials can move freely, in and out.
To each of the monomer units of the polymer, so called “functional groups” are attached.
These functional groups can interact with water soluble species, especially with ions.
Ions are either positively (cat ions) or negatively (anions) charged. Since the functional
groups are also charged, the interaction between ions and functional groups is exhibited
via electrostatic forces. Positively charged functional groups (e.g. a quarternary amine)
interact with anions and negatively charged functional group (e.g. a sulfonic, phosphonic
or carboxylic acid group) will interact with cations. The binding force between the
functional group and the attached ion is relatively loose. The exchange can be reversed
by another ion passing across the functional group.
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Research on synthesis of TiO2 and application for Cr6+treatment in wastewater
Then another exchange reaction can take place and so on and so forth. One exchange
reaction can follow another [45].
I.2.2. Electrochemistry method
Principles: The method based on the redox processes to remove metals doped onto the
electrodes from wastewater when a direct current beam went through the electrode.
The unsoluble anode was made by graphite or lead oxide, the cathode was made by
tungsten, iron or nickel. Metallic ions were reduced at cathode to less poisonous forms or
metal.
Mem+ + (m-n)e+

Men+ (m>n≥0)

(m,n: oxidation number of metal Me)
I.2.3. Reduction-oxidation method and deposition method
Principles:
-The reduction-oxidation method used oxidation agents (Cl2, O2,…) or reduction agents
(Na2SO3, FeSO4,…) to change Cr(VI) and polutants to less poisonous forms.
-The deposition method used agents combining with metallic ions in wastewater to
deposition form at a suitable pH. The depositions were easly removed from wastewater
by decanting method.
I.2.4. Photocatalytic method
All of the extensive knowledge that was gained during the development of semiconductor
photoelectrochemistry during the 1970 and 1980s has greatly assisted the development of
photocatalysis. In particular, it turned out that TiO2 is excellent for photocatalytically
breaking down organic compounds. For example, if one puts catalytically active TiO2
powder into a shallow pool of polluted water and allows it to be illuminated with
sunlight, the water will gradually become purified. Ever since 1977, when Frank and
Bard first examined the possibilities of using TiO2 to decompose cyanide in water, there
has been increasing interest in environmental applications. These authors quite correctly
pointed out the implications of their result for the field of environmental purification.
Their prediction has indeed been borne out, as evidenced by the extensive global efforts
in this area.One of the most important aspects of environmental photocatalysis is the
availability of a material such as titanium dioxide, which is close to being an ideal
photocatalyst in several respects. For example, it is relatively inexpensive, highly stable

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Research on synthesis of TiO2 and application for Cr6+treatment in wastewater
chemically, and the photogenerated holes are highly oxidizing. In addition,
photogenerated electrons are reducing enough to produce superoxide from dioxygen.
I.3. TiO2 review
I.3.1. Occurrence
Titanium dioxide occurs in nature as well-known minerals rutile, anatase and brookite,
and additionally as two high pressure forms, a monoclinic baddeleyite-like form and an
orthorhombic α-PbO2-like form, both found recently at the Ries crater in Bavaria. The
most common form is rutile, which is also the most stable form. Anatase and brookite
both convert to rutile upon heating. Rutile, anatase and brookite all contain six
coordinated titanium.[48]
I.3.2. Physical and mechanical properties
Physical and mechanical properties of sintered titania are summarised in table 1, while
optical properties of titania are provided in table 2.

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Table I.1. Typical physical and mechanical properties of titania. [47]
Property
Density

4 gcm-3

Porosity

0%

Modulus of Rupture

140MPa

Compressive Strength

680MPa

Poisson’s Ratio

0.27

Fracture Toughness

3.2 Mpa.m-1/2

Shear Modulus

90GPa

Modulus of Elasticity

230GPa

Microhardness (HV0.5)

880

Resistivity (25°C)

1012 ohm.cm

Resistivity (700°C)

2.5x104 ohm.cm

Dielectric Constant (1MHz)

85

Dissipation factor (1MHz)

5x10-4

Dielectric strength

4 kVmm-1

Thermal expansion (RT-1000°C)

9 x 10-6
11.7 WmK-1

Thermal Conductivity (25°C)

Table I.2. Optical properties of titania. [47]
Phase

Refractive
Index

Density
(g.cm-3)

Crystal
Structure

Anatase

2.49

3.84

Tetragonal

Rutile

2.903

4.26

Tetragonal

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I.3.3. Chemical properties
TiO2 is nonreactive substance, did not react with water, did not disolve in low acid
solution (except HF) and base solution.
TiO2

+ 6 HF

=

H2TiF6

+

2H2O

TiO2

+ 2 NaOH

=

Na2TiO3

+

H2O

TiO2

+ Na2CO3

= Na2TiO3

+

CO2

TiO2

+ 2K2S2O7

=

Ti(SO4)2 +

2K2SO4

TiO2 could be reduced to Ti2O3 by C at 870oC or with TiCl4 by H2 at 1400oC.
3TiO2

+ TiCl4 + 2 H2 =

2Ti2O3 +

4HCl

TiO2 was reduced to Ti3O5 by H2

3TiO2

+ H2

= Ti3O5 + H2O

∆H= 8.9 Kcal.mol-1

I.3.4. Applications
Applications for sintered titania are limited by its relatively poor mechanical properties. It
does however find a number of electrical uses in sensors and electrocatalysis. By far its
most widely used application is as a pigment, where it is used in powder form, exploiting
its optical properties.
a. Pigments
The most important function of titanium dioxide however is in powder form as a pigment
for providing whiteness and opacity to such products such as paints and coatings
(including glazes and enamels), plastics, paper, inks, fibres and food and cosmetics.
Titanium dioxide is by far the most widely used white pigment. Titania is very white and
has a very high refractive index – surpassed only by diamond. The refractive index
determines the opacity that the material confers to the matrix in which the pigment is
housed. Hence, with its high refractive index, relatively low levels of titania pigment are
required to achieve a white opaque coating.
The high refractive index and bright white colour of titanium dioxide make it an effective
opacifier for pigments. The material is used as an opacifier in glass and porcelain
enamels, cosmetics, sunscreens, paper, and paints. One of the major advantages of the
material for exposed applications is its resistance to discoloration under UV light [47].
b. Photocatalysis
Titanium dioxide, particularly in the anatase form, is a photocatalyst under ultraviolet
(UV) light. Recently it has been found that titanium dioxide, when spiked with nitrogen
ions or doped with metal oxide like tungsten trioxide, is also a photocatalyst under either

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visible or UV light. The strong oxidative potential of the positive holes oxidizes water to
create hydroxyl radicals. It can also oxidize oxygen or organic materials directly.
Titanium dioxide is thus added to paints, cements, windows, tiles, or other products for
its sterilizing, deodorizing and anti-fouling properties and is used as a hydrolysis catalyst.
It is also used in dye-sensitized solar cells, which are a type of chemical solar cell (also
known as a Graetzel cell). The photocatalytic properties of titanium dioxide were
discovered by Akira Fujishima in 1967 and published in 1972. The process on the surface
of the titanium dioxide was called the Honda-Fujishima effect. Titanium dioxide has
potential for use in energy production as a photocatalyst.[48]
c. Oxygen Sensors
Even in mildly reducing atmospheres titania tends to lose oxygen and become sub
stoichiometric. In this form the material becomes a semiconductor and the electrical
resistivity of the material can be correlated to the oxygen content of the atmosphere to
which it is exposed. Hence titania can be used to sense the amount of oxygen (or
reducing species) present in an atmosphere. [47]
d. Antimicrobial Coatings
The photocatalytic activity of titania results in thin coatings of the material exhibiting self
cleaning and disinfecting properties under exposure to UV radiation. These properties
make the material a candidate for applications such as medical devices, food preparation
surfaces, air conditioning filters, and sanitaryware surfaces. [47]
I.4. Mechanism of titanium oxide photocatalytic reactions
I.4.1. Band structure of semiconductors and band gap energy
If the nucleus of an atom were the sun in our solar system, the electrons revolving
around the nucleus would be the orbiting planets. The path that an electron travels is

referred to as an "orbit." There is a limit to the number of electrons that can occupy one
orbit. Electrons in the outermost orbit are referred to as "valence electrons." Valence
electrons are responsible for the bonding of atoms. When there are few atoms, the energy
values of electrons in orbits are scattered. However, when the number of bonded atoms
increases, the values become continuous within a certain range, rather than being
scattered. This range is referred to as an "energy band." The area between two energy
bands, where there is no electron energy, is referred to as a "forbidden band." Among the
bands filled with electrons, the one with the highest energy level (the electron orbit
farthest from the nucleus) is referred to as the "valence band," and the band outside of
this is referred to as the "conduction band." The energy width of the forbidden band
between the valence band and the conduction band is referred to as the "band gap."
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The band gap is like a wall that electrons must jump over in order to become free. The
amount of energy required to jump over the wall is referred to as the "band-gap energy."
Only electrons that jump over the wall and enter the conduction band (which are referred
to as "conduction electrons") can move around freely. In the case of silicon, the band gap
energy is approximately 1.1 eV, which is equal to approximately 1100 nm when
converted to the wavelength of light. When rutile type titanium oxide and anatase type
titanium oxide are irradiated with light of 413 nm or lower, or 388 nm or lower,
respectively, valence band electrons move up to the conduction band. At the same time,
as many positive holes as the number of electrons that have jumped to the conduction
band are created. [49]
I.4.2. Energy structure of titanium oxide and photoeffect
In a compound semiconductor consisting of different atoms, the valence band and

conduction band formation processes are complicated, but the principles involved are the
same. For example, it is known that the valence band of titanium oxide is comprised of
the 2p orbital of oxygen (O), while the conduction band is made up of the 3d orbital of
titanium (Ti). In a semiconductor with a large band gap, electrons in the valence band
cannot jump up to the conduction band. However, if energy is applied externally,
electrons in the valence band can rise (this is referred to as "excitation") to the conduction
band. Consequently, as many electron holes (holes left behind by the electrons moving up
to the conduction band) as the number of excited electrons are created in the valence
band. This is equivalent to the movement of electrons from the bonding orbital to the
antibonding orbital. In other words, the photoexcited state of a semiconductor is generally
unstable and can easily break down.
Titanium oxide, on the other hand, remains stable even when it is photoexcited. This is
one of the reasons that titanium oxide makes an excellent photocatalyst. The following
three factors pertaining to the band structure of semiconductors have the greatest effect
on photocatalytic reactions:
(1) Band gap energy
(2) Position of the lowest point in the conduction band
(3) Position of the highest point in the valence band
In photocatalytic reactions, the band gap energy principally determines which light
wavelength is most effective, and the position of the highest point in the valence band is
the main determinant of oxidative decomposing power of photocatalyst. [49]

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Fig.I.1 Titanium-oxide band structure [49]
I.4.2. Crystal structures and photocatalytic activity of titanium oxide
There are three types of crystal structures in natural titanium oxide: the rutile type, the
anatase type, and the brookite type. All three of these types are expressed using the same
chemical formula (TiO2); however, their crystal structures are different. Titanium oxide
absorbs light having an energy level higher than that of the band gap, and causes
electrons to jump to the conduction band to create positive holes in the valence band.
Despite the fact that the band gap value is 3.0 eV for the rutile type and 3.2 eV for the
anatase type, they both absorb only ultraviolet rays. However, the rutile type can absorb
the rays that are slightly closer to visible light rays.
As the rutile type can absorb light of a wider range, it seems logical to assume that the
rutile type is more suitable for use as a photocatalyst. However, in reality, the anatase
type exhibits higher photocatalytic activity. One of the reasons for this is the difference in
the energy structure between the two types. In both types, the position of the valence
band is deep, and the resulting positive holes show sufficient oxidative power. However,
the conduction band is positioned near the oxidation-reduction potential of the hydrogen,
indicating that both types are relatively weak in terms of reducing power. It is known that
the conduction band in the anatase type is closer to the negative position than in the rutile
type; therefore, the reducing power of the anatase type is stronger than that of the rutile
type. Due to the difference in the position of the conduction band, the anatase type
exhibits higher overall photocatalytic activity than the rutile type. [49]

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.
Fig. I.2 Crystal structures of titanium oxide [49]
I.4.3. Effect of ultraviolet rays in activating titanium oxide
The band gap of anatase type titanium oxide is 3.2 eV, which is equivalent to a
wavelength of 388 nm. The absorption of ultraviolet rays shorter than this wavelength
promotes reactions. These ultraviolet rays are near-ultraviolet rays contained in the
sunlight reaching the earth and emitted by room lights, and they have a very limited range
of weak light throughout the spectrums of sunlight and room lights.
The development of a visible-light photocatalyst may be considered as a solution, but no
substance superior to titanium oxide as a material for photocatalysts has yet been
discovered. One major reason for this is that a semiconductor with a smaller band gap
than that of titanium oxide results in autolysis if it receives light in the presence of water.
In titanium oxide, the absorption of ultraviolet rays with a wavelength of 388 nm or
shorter promotes reactions; however, it is known that 254-nm rays having a greater
energy level, which are used in germicidal lamps, are absorbed by the DNA of living
organisms and form pyrimidine dimers, thereby damaging the DNA.
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Titanium oxide photocatalyst does not require ultraviolet rays that have an energy level
as high as 254 nm and are hazardous to humans. It also allows reactions to be initiated by
the near-ultraviolet rays with relatively long wavelengths contained in sunlight and
emitted by fluorescent lamps. [49]
Table I.3. Ultraviolet rays in ordinary surroundings [49]

I.4.4. Decomposing power of titanium oxide photocatalyst

When light is absorbed by titanium oxide, two carrier electrons (e-) and positive holes
(h+) are formed. In ordinary substances, electrons and positive holes recombine quickly;
however, in titanium oxide photocatalyst they recombine more slowly. The percentage of
carrier recombination has a major effect on the photocatalytic efficiency.

Fig.I.3. Electron structure of titanium oxide [49]
One of the notable features of titanium oxide is the strong oxidative decomposing power
of positive holes, which is greater than the reducing power of electrons excited to the
conduction band. The surface of a photocatalyst contains water, which is referred to as
"absorbed water." When this water is oxidized by positive holes, hydroxy radicals (• OH),
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which have strong oxidative decomposing power, are formed. Then, the hydroxy radicals
react with organic matter. If oxygen is present when this process takes place, the
intermediate radicals in the organic compounds and oxygen molecules can undergo
radical chain reactions and consume oxygen in some cases. In such a case, the organic
matter eventually decomposes, ultimately becoming carbon dioxide and water. Under
some conditions, organic compounds can react directly with the positive holes, resulting
in oxidative decomposition. Meanwhile, the reduction of oxygen contained in the air
occurs as a pairing reaction. As oxygen is an easily reducible substance, if oxygen is
present, the reduction of oxygen takes place instead of hydrogen generation. The
reduction of oxygen results in the generation of superoxide anions (• O2-). Superoxide
anions attach to the intermediate product in the oxidative reaction, forming peroxide or
changing to hydrogen peroxide and then to water.

Fig. I.4. Oxidation mechanism[49]

Fig.I.5. Reduction mechanism[49]
As reduction tends to occur more easily in organic matter than in water, when the
concentration of organic matter becomes high, the possibility of positive holes being used
in the oxidative reactions with organic matter increases, thus reducing the rate of carrier
recombination. It is believed that, under conditions in which positive holes are
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sufficiently consumed, the process of electrons transferring to oxygen molecules on the
reduction side determines the reaction speed of the entire photocatalytic reaction. In other
words, by enabling easier transfer of electrons to oxygen molecules, the efficiency of
photocatalytic reactions can be improved. This can be achieved by allowing titanium
oxide to carry a metal as a support.[49]
1.4.5. The mechanism of photo-reduction of Cr(VI)
The photo-reduction of Cr(VI) toCr(III) can be achieved via a photocatalytic process with
a simplified mechanism as follows:
TiO2 +hν→ h+ +e− (1)
Cr2O72− +14H+ +6e− → 2Cr3+ +7H2O (2)
2H2O + 4h+ → O2 +4H+ (3)
H2O + h+ → •OH + H+ (4)
•

OH + Organics → ··· → CO2 +H2O (5)

h+ +Organics → ··· → CO2 +H2O (6)
UV light illumination on TiO2 produces hole–electron pairs (reaction (1)) at the surface
of the photocatalyst. After the hole–electron pairs being separated, the electrons can
reduce Cr(VI) to Cr(III) (reaction (2)), and the holes may lead to generation of O2 in the
absence of any organics (reaction (3)).
Therefore, in a completely inorganic aqueous solution, the net photocatalytic reaction is
the three-electron-reduction of Cr(VI) to Cr(III) with oxidation of water to oxygen, which
is a kinetically slow four-electron process. And hence the photocatalytic reduction of
Cr(VI) alone is quite slow. Alternatively, the photocatalytic reduction of Cr(VI) can be
carried out in couple with the photocatalytic oxidation of organic pollutants by adding
some amount of organic pollutants in solution. In the presence of degradable organic
pollutants, the holes can produce •OHradicals (reaction (4)), which can further degrade
the organics to CO2 and H2O (reaction (5)). Of course, the holes can also directly oxidize
the organic molecules (reaction (6)). In otherPositive holes (h+) that cause oxidative
reaction have very strong oxidative power. They directly oxidize water and produce a
highly reactive compound [OH]. In some cases, they directly oxidize organic matter
attached to the surface.Radical chain reactions also occur between the radicals and the
oxygen molecules (reaction (6)). In other words, in the presence of organic species, the
photogenerated holes are rapidly scavenged from the TiO2 particles, suppressing
electron–hole recombination on TiO2 and accelerating the reduction of Cr(VI) by
photogenerated electron. One of the important strategies of promoting the photocatalytic
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reduction of Cr(VI) (and the photocatalytic degradation of organic pollutants) is
enhancing the charge separation, which can be achieved by improving the structure of the

photocatalyst and by introducing scavengers of holes and/or electrons in the solution.[60]
I.5. Review of the used precursors in this thesis
I.5.1. TiCl4
Titanium tetrachloride is the inorganic compound with the formula TiCl4. It is an
important intermediate in the production of titanium metal and the pigment titanium
dioxide. TiCl4 is an unusual example of a metal halide that is highly volatile. Upon
contact with humid air, it forms spectacular opaque clouds of titanium dioxide (TiO2) and
hydrogen chloride (HCl).
a. Properties and structure
Table I.4. The physical properties of TiCl4
Physical Properties
Molecular formula

TiCl4

Molar mass

189.71 g/mol

Appearance

Colourless fuming liquid

Density

1.726 g/cm3

Melting point

-24.8 °C

Boiling point

136.4 °C

Viscosity

8.27×10-4 Pa·s

Solubility in water

Reacts to form TiO2 and HCl

Solubility

Soluble in ethanol

TiCl4 is a dense, colourless distillable liquid, although crude samples may be yellow or
even red-brown. It is one of the rare transition metal halides that is a liquid at room
temperature, VCl4 being another example. This property reflects the fact that TiCl4 is
molecular; that is, each TiCl4 molecule is relatively weakly associated with its
neighbours. Most metal chlorides are polymers, wherein the chloride atoms bridge
between the metals. The attraction between the individual TiCl4 molecules is weak,
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