BỘ MINISTRY OF EDUCATION

AND TRAINING

VIETNAM ÂCDEMY OF

SCIENCE AND TECHNOLOGY

GRADUATE UNIVERSITY OF SCIENCE AND TECHNOLOGY ……..….***…………

NGUYEN THI MAI HUONG

Study the fabrication and photocatalytic, hydrophilic

properties of TiO2/SiO2 and TiO2/PEG thin films by

sol-gel method

Major: Solid State Physics

Code: 9 44 01 04

SUMMARY OF THE THESIS

Hà Nội – 2018

The thesis is completed at: Graduate University of Sciences and Technology, Vietnam Academy of Science and Technology

Supervisors: 1) Dr. Nguyen Trong Tinh

2) Dr. Nghiem Thi Ha Lien

Reviewer 1: … Reviewer 2: … Reviewer 3: ….

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A. INTRODUCTION TiO2 is known as a photocatalytic and hydrophilic semiconductor material when excited by light. That is why TiO2 is considered to be a functional material that has the potential to create self-cleaning materials for practical applications. The hydrophilic nature of the material surface under optical excitiation is closely related to the material properties, surface configuration and stimulus. For this reason, the study on the hydrophilicity of the material is a very academically attractive subject in studying the properties as well as physical processes on the surface.

In the world, recent studies show the relationship between the hydrophilicity of the solid surface and surface energy. Exciation by light produces a change in surface energy, leading to a change in hydrophilicity.

The systematic and quantitative study of the changes in surface energy under the differentiation of TiO2 with different nano-structures promises to bring further information to the photocatalytic mechanism and super-hydrophilic effects of TiO2 material.

In Vietnam, there are a few studies related to hydrophilicity or surface energy of materials, especially hydrophilicity under the Exciation of the light. Therefore, the objectives of the thesis are presented as follows:

The objectives of the thesis:

Study on materials fabrication technology; structural - photocatalytic properties of TiO2 material, and TiO2 as the nanostructured variant. On the basis of such material system, the systematic and quantitative study on hydrophilicity or, in other words, the study of surface energy of material systems under Exciation of UV light radiation. Further clarification of the correlation between photocatalytic activity, self-cleaning and hydrophobicity of TiO2 nanostructured materials.

Research subjects: The thesis focuses on two structural systems on the basis of nanostructured and anatse-shaped TiO2: The complex nano-structure TiO2/SiO2 and Nano-porous TiO2/PEG.

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Main study contents:

Fabrication of TiO2/SiO2, TiO2/PEG material systems and experimental study on the structural properties as well as the photocatalytic properties of the two material systems.

The hydrophilicity or surface energy of TiO2/SiO2, TiO2/PEG nanostructured films is studied by contact angle measurement and semi-quantitative techniques based on micro-theoretical models of solid surface under the presence of the stimulus.

The practical and theoretical significance of the thesis

The technology of fabrication of nanostructured TiO2 material is controlled by sol-gel method. The nanostructures of TiO2 thin films are controlled. The phase transition is inhibited from the anatase configure with high photocatalytic activity of Anatase to Rutile phase into Rutile phase with low photocatalytic activity at high temperature.

A new methodology is developed for calculation and quantification of solid phase surface energy quantification based on micro theory of solid-state physics. Based on this methodology, it is possible to calculate and quantify the solid surface energy based on experimental data of measuring the liquid-solid phase contact angle by contact angle measurement technique.

Quantitative study of surface energy of nanostructured TiO2

photocatalytic film under the Exciation of UV radiation. This provides empirical evidence about a physical effect: optical Exciation can change the surface energy of the photocatalyst.

The correlation between the photocatalytic mechanism and the super-hydrophilic mechanism of the nano-structured TiO2 material system is demonstrated. Quantitative empirical data is provided, contributing to consolidate the hypothesis of the origin of the mechanism of super-hydrophilic effect of the TiO2 material system.

Layout of the thesis: The thesis consists of the introduction, 5 chapters and the conclusion. The results are published in five journals including 03 international publications and 02 national publications.

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B. CONTENTS OF THE THESIS

Chapter 1

OVERVIEW OF TITANIUM DIOXIDE NANOMATERIALS (TIO2)

1.1. Titanium Dioxide Nanomaterials 1.1.1. Introduction.

In recent years, Nano TiO2 powder in the rutile, anatase, or mixture of rutile and anatase and brookite mixtures have been studied for use in the fields of solar cells, manufacturing electronic device, sensing head, etc. With high photocatalytic activity, TiO2

nano-material are applied in the fields of environmental treatment such as: decomposition of toxic organic compounds, water treatment, bactericidal, mildew-proof. Especially, in combination with hydrophobicity when exposed to light, TiO2 is developed as a self-cleaning material. With durable and non-toxic structure, TiO2 material is considered to be the most promising material to address many serious environmental problems and challenges of pollution.

Phase-pure TiO2 nanoparticles:

TiO2 has four forms of formation. In addition to amorphous form, it has three crystalline forms, including: anatase, rutile and brookite (Figure 1.1).

Anatase

Rutile

Brookite

Figure 1.1: The Crystal structure of TiO2

Differences in network structure lead to differences in electronic density between the two rutile and anatase forms of TiO2 and this is the cause of difference in nature between them. The nature and application of TiO2 is highly dependent on the crystalline structure of the forms and particle size of such forms.

Among the forms of TiO2, the anatase exhibits higher photocatalytic activity than the rest.

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Transformation of TiO2 forms: amorphous → anatase → rutile is significantly affected by synthetic conditions and the process of form transformation of modified TiO2 material is different from that of of pure TiO2.

1.1.2.Photocatalytic property of the TiO2 nano-material.

Photocatalytic mechanism of the TiO2 nano-material

TiO2 has an anatase band gap of 3.2eV. Therefore, under the effect of the photon energy that is greater than 3.2eV, the following process will occur:

VBCB hehTiO 2

When positive holes (h+VB) appear in the water environment,

the *OH radical formation reaction will occur: HOHOHhVB *2

OHOHhVB *

Figure 1.2: Mechanism of semiconductor photocatalysis.

On the other hand, when electrons appear on the conducting zone (e-

CB) if O2 is present in the water, the *OH radical formation reaction will occur.

Factors affecting photocatalytic properties.

There are many factors affecting the photocatalytic activity of the film such as manufacturing method, crystal crystallinity, heating temperature, effective surface area, catalytic mass, illumination intensity. However, the two major determinants of photocatalytic activity of TiO2 films are the effective surface area

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and the crystallinity of the film. In addition, for photocatalytic reactions to occur in the visible light, it is important to pay attention to the important factor known as the absorption edge of the right membrane located within this light zone.

1.1.3. Modified TiO2 nano-material. TiO2 crystals have a big band gap (3.0-3.2eV), therefore,

photocatalytic sensitivity is located only in ultraviolet light with wavelengths of less than 380nm, i.e. only 5% of solar energy in the ultraviolet zone is capable of activating photocatalytic activity.

In order to transfer the photocatalytic reaction into visible light, where there is 45% of solar energy, the methods are applied such as TiO2 doping with transitional metal elements to form intermediate states in the band gap of TiO2; attaching semiconducting photoresist or organic matter that is capable of absorbing visible light; forming the TiOx and doping nitrogen, carbon to replace oxides in TiO2 anatase crystals; forming TiO2 composites with different compounds.

The complex nano-material TiO2/SiO2

In order to increase the hydrophilicity and self-cleaningability of TiO2 material, SiO2 is doped with TiO2 to increase the acidity of the surface, which results in stronger water absorption and reduction in surface contamination.

According to Guan et al., when SiO2 is added into TiO2, meaning that silicon can enter the titanium network and replace the position of Ti4+ cations, the number of oxygenatoms associated with Si and Ti varies, creating an electrical imbalance. The result is that the acidic center (Lewis center) with a positive charge is formed on the TiO2/SiO2 complex surface. The acidity of the surface makes the TiO2/SiO2 absorb more OH-radicals. Specifically, silicon cations or saying more precisely, Ti-Si bonds can take OH- of the adsorbed H2O molecules and O2- of the complex can bind to H+ of the adsorbed water. Therefore, there is a competition of absorption of compounds in the environment and water on TiO2/SiO2 complex surface. As the acidity of the surface increases, the water (OH groups) is more strongly adsorbed and surface contamination decreases. Hydrophilicactivity causes the

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water to flow all over the surface, absorb into dirt and push it away from the surface.

Nano porous material TiO2/PEG.

PEG (PolyEthylene Glycol) is an organic polymer with a chain circuit and when being dissolved in the TiO2 sol, these chains alternate between TiO2 particles. After the fabrication, the film undergoes thermal treatment, as a result, the PEG burns and porous holes are left between the TiO2 particles. Therefore, the addition of PEG increases the volume and diameter of the porous holes of the material, leading to the increase in the surface area of the catalyst. It is hoped that this will increase the hydrophilicity of the material.

1.2. Hydrophilic effects of TiO2.

1.2.1. Hydrophilic mechanism under the light Exciation for the TiO2 nano-material

Fingre.1.3: Schematic representation of photo-induced hydrophilicity

In the presence of UV light, some electrons and holes participating in redox reactions with oxygen molecules and water adsorbed on the TiO2 surface to produce the free oxygen radicals with strong oxidation and destruction of organic impurities. Other electrons involved in deoxidizing the Ti4+ catrions into Ti3+ catrions and the hole oxidizes the anions to release the atomic oxygen and produce oxygen-free locations on the TiO2 surface. Water in the air will occupy this position and create an OH-

absorption group on the TiO2 surface. The OH- absorption groups form hydrogen bonds with water, therefore, the surface is hydrophilic (Figure 1.3).

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The hydrophilicity of the material is measured by the contact angle value of the water drop with the material surface; the smaller the contact angle is, the greater the hydrophilicity is.

Chapter 2. FABRICATION TECHNOLOGY, EXPERIMENTAL

PROCESSES AND RESEARCH METHODS

2.1. Fabrivation technology

The thesis selects sol - gel method and centrifugal spin – coating method for fabrication of materials and thin films on nanostructured TiO2 base. Fabrication technology is based on two processes:

Hydrolysis process:

Condensation process:

2.2. Study methods of photocatalytic properties for TiO2

nano-material. Methods of measuring decomposition of organic pigments

which determine the speed of the photocatalytic reaction.

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The Methylene Blue (MB) solution has an initial concentration of C0 decomposed on contact with the optically catalytic active surface due to the UV radiation, resulting in a discoloration of the solution.

The Ct concentration of the solution is determined at equal intervals during the measurement from the UV-VIS absorption spectra. Ln (C0/Ct) = kt, in which k: constant of reaction speed, t: Reaction time.

Measurement method of bactericidal of photocatalytic effect.

Photocatalytic materials can destroy biological materials such as bacteria, viruses and mildew. The germicidal mechanism is mainly formed by photobiological holes; photobiological electrons on the catalytic surface will destroy or deform the cell wall, break down the DNA chain of such biological materials, making them inoperable or dead.

The principle of the method is to evaluate the number of live bacteria over time as it comes into contact with the material and then to evaluate the photocatalytic activity of the material.

Method of measurement of hydrophilic properties by contact angle technique.

The device includes functional blocks as shown in the figure.

Figure 2.1: Schematic diagram of the contact angle device

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2.3. Technique of hydrophilicity evaluation Method of evaluation of a hydrophobic, super-hydrophobic,

hydrophobic or super-hydrophobic surface is based on the value of the contact angle measured by dropping water on it.

Figure 2.2 below is the corresponding exposure/contact angle value for quantitative evaluation on hydrophilicity of a surface.

Figure 2.2: Hydrophilic and hydrophobic surfaces.

However, to have more quantitative results on the hydrophilicity of the surface, we should carry out studies on the surface tension and the free surface of the material. Specifically, the approaches through micro-physics models of the liquid and solid's surface interaction should be used.

Chapter 3.

SURFACE ENERGY OF THE SOLID AND CONTACT ANGLE OF SOLID-LIQUID PHASE MODEL OF SURFACE

ENERGY CALCULATION FOR TIO2 MATERIAL

Chapter 3 presents an overview of some approaches to the micro interaction model in solid-liquid transition related to the contact angle. On this basis, a specific approach and calculation method will be developed for TiO2 surface free energy in this thesis. 3.1. Free surface energy of the solids and its relationship with liquid drop contact angle on the solid surface.

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Surface free energy and surface tension of the solids.

Surface energy is the energy to create a unit of material surface area in equilibrium with the surrounding vacuum. Another opinion of surface energy is that it is related to the effort for cutting a sample block in order to create new surfaces in an area unit. Therefore, the unit of surface energy in the SI is J/ m2.

Surface Tension of liquid.

Surface tension is the tensile force among surfaces in a tangential direction of the surface in equilibrium with the environment where the surface is formed.

Surface energy = Energy/Area = J/m2 = (Nx m)/m2 = N/m = Force/length = Surface tension.

Relationship between solid-liquid phase contact angle and surface energy.

Young's equation.

In 1805, Thomas Young reported on the relationship between contact angle and surface energy. The contact surface of a liquid drop on a solid surface is determined by the mechanical equilibrium of the water falling under the surface of the energy of the three phases, the solid-liquid energy sl , the solid-vapor energy sv , and

the liquid-vapor energy lv described in Figure 3.1 below.

Figure3.1:Diagram showing the relationship for the three surface

tensions (surface free energies) for a droplet of liquid resting on a solid substrate at the three-phase point

coslvslsv

3.2. The thesis's methodology of TiO2 photocatalytic surface energy calculation.

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From the hypothesis of the TiO2 surface under the effect of UV radiation upon contact with water, to separate the different physicochemical interaction components on the surface, the fairly complex chemical experiments are requested. In fact, the empirical data of the thesis mainly include:

- The contact angle of various liquids such as H2O, alcohol, Triton X, Ethylene Glycol, Glycerol, etc. on TiO2 membrane surface is experimentally measured.

- The structure of TiO2 film form is made by different method (photocatalytic properties depends on TiO2 membrane configuration).

- TiO2 film is stimulated by UV radiation over illumination time and recovery time to their initial state (State dynamics under Exciation and recovery of the TiO2 photocatalytic film).

In order to calculate the surface energy of the TiO2 photocatalytic film, the thesis will use the semi-empirical approach as follows:

- Assuming that the surface energy of the TiO2 photocatalytic film is the sum of the components involving in the interaction at the solid-liquid contact;

- Using the Young's equation, considering the modification of dynamic interaction coefficient due to the contact among the three phases solid - liquid - vapor at the location of contact point calculation. This approach was used by Good for calculating surface energy from contact angle data:

svlvsvlvsl 2

Developing Li's approach on the basis of Good Fowkes' theory of transforming the interaction coefficient Φ into the expanel dynamic coefficient (e-exponential function) that contains the parameters γLV, γSV and the experimental ratio β depending on the solid.

2)(2 svlvesvlvsvlvsl

With this approach, Li leads to the contact angle dependence

on the surface energy quantities in Young type as follows:

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2)(21cos svlvelv

sv

In case of using different liquids (with known surface tension

value γlv), we have set the dependent function Cosθ in the γlv with the different liquids. In this case, γsv and β will be constants in the above equation.

By using the approximation method with a parameter γlv going from at least 4 points (4 different types of liquis), we can calculate the constants β and γsv of the solid surface (TiO2). The Matlab tool is used in the approximation method.

After calculating the γsv of the TiO2 surface, Young's equation can be used to calculate the solid-liquid transition energy γsl of TiO2 and water.

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Chapter 4. FINDINGS ON MANUFACTURING TECHNOLOGY,

STRUCTURAL PROPERTIES AND PHOTOCATALYTIC PROPERTIES OF TIO2/SIO2 AND TIO2/PEG MATERIALS

4.1. The complex nano-material TiO2/SiO2. 4.1.1. Result of TiO2/SiO2 material fabrication

Figure 4.1: Sol TiO2/SiO2(0-50%) fabrication process.

4.1.2. Crystalline phase structure of TiO2/SiO2 material.

The findings on the crystalline phase structure gives a very important comment that when SiO2 is introduced, the crystalline phase structure of the TiO2 material is not transferred to the Rutile phase when the material is sintered at high temperature.

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X-ray diffraction spectra of TiO2/SiO2(0÷50%) sintered at 500oC.

X-ray diffraction spectra of

TiO2/SiO2 (0÷50%) sintered 800oC

4.1.3.Structure of TiO2/SiO2thin film.

TiO2 /SiO2 (0%) 500oC.15->25nm

TiO2 /SiO2 (0%) 600oC.15->30nm

TiO2 /SiO2 (0%) 700oC.30->60nm

TiO2 /SiO2 (0%) 800oC.40->90nm

TiO2 /SiO2 (10%) 800oC.15->30nm

TiO2 /SiO2 (40%) 800oC.15->30nm

According to the findings on the film surface form and particle size, the particle size of pure TiO2 gradually increases with the annealing temperature. However, when the annealing temperature increases to 8000C, the particle size does not increase.

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4.1.4. Findings on photocatalytic properties based on the results of methylene blue (MB) decomposition

Figure 4.2: MB concentration by time of illumination.

Figure 4.3: The MB decay rate constant depends on the SiO2

Figure: 4.3 demonstrates the decomposition rate constants of TiO2/SiO2 film samples (0 ÷ 50%), showing the effect of % SiO2 on the decomposition rate. The TiO2/SiO2 sample (40%) has the fastest MB decomposition rate.

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4.2. Nano porous TiO2/PEG. 4.2.1.Results of material fabrication

Figure 4.4: TiO2/PEG fabrication process

4.2.2.Crystalline phase structure of TiO2/PEG material

(a) (b) (c)

Hình 4.5: X-ray diffraction spectra of TiO2/PEG (0÷50%)

sintered at 5000C (a), 6500C (b) và 8000C (c) Thus, the percentage of introduced PEG affects the phase

transition from anatase to rutile when the sample is sintered at high

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temperature (6500C). However, when the sintering temperature is raised to 8000C for TiO2/PEG samples (0%, 30% and 50%) (Figure 4.5), the whole crystalline phase structure has been transformed into a rutile form. This is an undesirable phase for TiO2

photocatalyst. 4.2.3. Structure of TiO2 / PEG film surface form

Hình 4.6: SEM image of TiO2/PEG (0÷50%) thin films

Surface area of nano porous material TiO2/PEG.

Sample Surface area(m2/g) TiO2 - 0%PEG 41,5 TiO2- 10%PEG 47,1 TiO2- 20%PEG 63,2 TiO2- 30%PEG 68,5 TiO2- 40%PEG 86,7 TiO2 - 50%PEG 54,3

Table4.1: Surface area of TiO2/PEG (0÷50%)

According to the result of the surface form structure and surface area measurement, when PEG is added to TiO2 solution,

20%

30% 40% 50%

0% 10%

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resulting in a change in film porosity and the optimum level at the percentage of PEG in the sol of about 40%.

4.2.4. The findings on photocatalytic properties of nano porous material TiO2/PEG.

Figure 4.7: MB concentration by time of illumination.

Figure 4.8: The MB decay rate constant depends on the PEG

Figure 4.8 demonstrated the decomposition rate of TiO2/PEG film samples (0 ÷ 50%), showing the effect of PEG percentage on the decomposition rate. Of which, the TiO2/PEG sample (40%) has the fastest MB decomposition rate.

Chapter 5

FINDINGS ON HYDROPHIBILITY AND SURFACE ENERGY OF TWO PHOTOCATALYTIC MATERIAL

SYSTEMS TIO2/SIO2, TIO2/PEG

There are many applications in life directly related to wetting such as the industries of printing, painting, detergents, weaving, dyeing, self-cleaning materials, textiles and so on. The biomedical sector also has applications related to the wetting such as ability of absorption of protein, interaction on the cell surface, etc.

Therefore, the study on the wetting (hydrophilicity, hydrophobicity) or, in other words, the study on surface energy is very useful and of big concern.

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In a normal way, surface energy is denoted by γ, but there is rarely an absolutely ideal surface, in fact the contact surface is always between two different phases or two different substances.

It is very important to determine the interface energy of two solid-vapor (γsv) phases and two solid-liquid (γsl) phases in pure science and in application. Direct measurement of energy among phases is very difficult. At present, there is a series of indirect approaches to obtain these values. Determination of the surface energy through the contact angle from the Young’s equation ( .cossl sv lv ) is one of the simplest methods since the contact

angle is a value that can be easily determined by experiment.

In order to change the surface energy, physicochemical agents have been used such as changing the coating with surfactants or mechanical-physical-thermal effects, as well as fabrication technology, changing the position of atoms, molecules in structure, etc. However, recently, there are other methods. In this thesis, we use experiment to prove that it is possible to use light Exciation to change surface energy of TiO2 photocatalyst. And we have also started to study the properties and rules of photocatalytic effects that affect surface energy. This is a kind of physically pure agent, which is different from known physicochemical agents.

5.1. Hydrophilicity and surface energy of complex nano-material TiO2/SiO2.

5.1.1. Hydrophilicity of of complex nano-material TiO2/SiO2.

TiO2/SiO2 thin films (0 ÷ 50%) are applied to the sintered glass substrate at a temperature of 5000C. The film is UV-irradiated (365 nm wavelength), the light intensity measured on the sample surface is 1mW/cm2.

The graph demonstrates the contact angle of the TiO2/SiO2 samples at illumination time shown in Figure 5.1. In all samples, the contact angles of the water drop decrease with the illumination, reaching a saturation value.

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Hình 5.1: Contact angle by the time illumination of the TiO2/SiO2

(0÷50%) thin films

Hình 5.2:Saturation angle of TiO2/SiO2(0÷50%) thin films

It can be commented that the wetting increases (ie, the wetting angle decreases) when the ratio of SiO2 increases, however, when the ratio of SiO2 is up to 50%, the wetting decreases. The optimum ratio of SiO2 is at about 40%.

This changing law is in line with the changing law of photocatalytic properties discussed in Chapter 4. It can be deduced that photocatalytic activity and the wetting are created by the same origin.

Hình 5.3: contact angle is restored _ TiO2/SiO2 (0÷50%)

Surface acidity produces surface hydroxyl groups. Such stable hydroxyl groups are beneficial for maintaining hydrophilicity. This explains why the contact angle of water slowly increases and remains at low value for a long time in the dark for complex films.

5.1.2. Energy surface of TiO2/SiO2 thin films.

When a liquid drop is placed on the surface of a solid, it is easy to determine the contact angle through the measurement.

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However, the important thing is that the contact angle holds important information about the surface energy of the solid γsl and interface energy of the liquid γsl through the Young's equation:

coslvslsv

Surface energy (γsv) value of TiO2/SiO2 film.

The liquids are selected as in Table 5.1.

Table5.1: Surface energy (γsv) value of liquids.

Liquids lv (mJ.m-2) Liquids lv (mJ.m-2)

Ethanol 22,39 Ethyleneglycol 47,3 TritonX 33 Glycerol 63,4 PEG 600 44,5 Nước 72,29

From the results of the contact angle of different solutions on the TiO2/SiO2 film (0 ÷ 50%) according to the illumination time by UV light (365nm). The illumination intensity at the sample surface is 1mW/cm2. Apply the surface energy calculation model with TiO2 material presented in Chapter 3:

2)(21cos svlve

lv

sv

We can calculate the surface energy value γsv of TiO2/SiO2 films (0 ÷ 50%).

Bảng 5.2: Surface energy value γsv of TiO2/SiO2 (0÷50%) thin films at times0, 30, 60, 90,120 minute

TiO2/SiO2

(0÷50%) γsv(mJ.m-2)

0 minute 30 minute 60 minute 90 minute 120 minute

0% 43,5 51,0 59,9 60,7 60,8

10% 42,6 59,8 60,6 60,8 60,9

20% 46,5 60,3 61,1 61,3 61,6

30% 44,8 60,2 61,5 61,4 61,6

40% 48,6 61,2 62 62,1 62,1

50% 45,7 59,3 60,2 60,9 61,5

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Hình 5.4: Surface energy γsvof TiO2/SiO2(0÷50%) thin films by time of

illumination.

Figure 5.4 demonstrates the dependence of the surface energy γsv of the TiO2/SiO2 film (0 ÷ 50%) by the illumination time. We have comment that the γsv of the samples increases according to the illumination time to the saturation value.

The magnitude of the change in energy value γsv from the moment of non- illumination to the saturation value is about 20%.

The saturated energy values among samples with different SiO2 ratios are different but insignificant. Of which, the TiO2/SiO2 sample (40%) had the highest saturation value γsv.

Value of the interface energy (γsl) between water and TiO2/SiO2 film.

With the γsv of each kind of TiO2/SiO2 film (0 ÷ 50%), by substituting the value γsv in the Young’s equation

coslvslsv , for each value of the contact angle θ

Illumination time (Minute)

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changing at the illumination time, it is possible to calculate the interface energy between water and thin films.

coslvsvsl

Table 5.3: Contact angle θ of the water, the surface energy γsv and the interface energy between water and TiO2/SiO2(0÷50%) thin films

illumination time

(minute)

TiO2/SiO2 (0%) TiO2/SiO2 (10%)

θ γ

sv γsl θ

γsv γ

sl

0 33,7 43,5 -16,7 29,3 42,6 -20,5 30 25,2 51 -14,4 22,3 59,8 -7,1 60 17,4 59,9 -9,1 13,8 60,6 -9,6 90 16,3 60,7 -8,7 13,1 60,8 -9,6

120 17,4 60,8 -8,2 14,6 60,9 -9,1 illumination

time (minute)

TiO2/SiO2 (20%) TiO2/SiO2 (30%)

θ γ

sv γsl θ

γsv γ

sl

0 28,3 46,5 -17,2 26,9 44,8 -19,7 30 20,6 60,3 -7,4 19,2 60,2 -8,1 60 12,1 61,1 -9,6 9 61,5 -9,9 90 11,2 61,3 -9,6 7,6 61,4 -10,3

120 13,2 61,6 -8,8 8,2 61,6 -10,0 illumination

time (minute)

TiO2/SiO2 (40%) TiO2/SiO2 (50%)

θ γ

sv γsl θ

γsv γ

sl

0 24,7 48,6 -17,1 30,5 45,7 -16,6 30 15,6 61,2 -8,4 21,1 59,3 -8,1 60 5,1 62 -10,0 14,5 60,2 -9,8 90 4,8 62,1 -9,9 12 60,9 -9,8

120 3,9 62,1 -10,0 13,1 61,5 -8,9

Figure 5.5 demonstrates the dependence of the interface energy value between the TiO2/SiO2 film surface (0 ÷ 50%) and water γsl at the illumination time.

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Illumination time (Minute)

Figure 5.5: Interface energy TiO2/SiO2(0÷50%) thin films – Water by time of illumination.

We have comments that the interface energy γsl of the film samples with water increases by the illumination time up to the saturation value. The magnitude of the change in the value of γsl from the time of no illumination ~-18mJ.m2 to the saturation value ~ -9mJ.m-2 is about 50%.

The saturation magnitude γsl is not significantly different among samples with different SiO2 content. The saturation setting time of samples is approximately the same, after about 30 minutes of illumination.

5.2. Nano porous material TiO2/PEG. Unlike the TiO2/SiO2 system that is a two-component

complex, the TiO2/PEG system studied in this part has only one physical component, nanoTiO2, while the mixed PEG is burned out

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after being sintered and holes are left on the film structure, resulting in a TiO2/PEG porous film. The PEG ratio here corresponds to the porosity of the obained material as indicated in Chapter 4. It means that feffective surface will be different for each percentage of PEG. The following studies and measurements are performed in the same way as for TiO2/SiO2 samples.

5.2.1. Findings on hydrophilic properties of nano porous material TiO2/PEG.

The contact angle of the water drop will be measured on the TiO2/PEG film samples (0 ÷ 50%) under UV light (365nm) at the same intensity of 1mW/cm2 by time and when the film sample is placed in the darkness. Thereby, we will see the influences of factors such as film porosity and thickness through the percentage of PEG added into solution on the hydrophilic effect.

Effect of porosity and thickness on hydrophilicity of

TiO2/PEG films.

Decrease the contact angle when illuminated film thickness ~0,042μm film thickness ~0,092μm film thickness ~0,14μm

Recovery process of contact angle in dark

Figure 5.6: Contact angle by the time illumination and the recovery of

contact angle in dark of TiO2/PEG (0÷50%) thin films.

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In all the samples, the contact angle reduces by illumination time. However, there is an influence of PEG percentage or porosity on the hydrophilicity. Specifically, the increase in porosity increases the area of the internal surface, leading to better water absorption of the film and creating more OH groups. The OH group will make hydrogen bonds; therefore, when reaching the film surface, the water will easily spread over the surface.

We also commented that the thinner the film, the smaller the amount of TiO2 particles on the film, meaning that the smaller the internal surface area. When the film is lighted by light with a higher energy than that of band gap, the number of electron-hole pairs is produced in less and slower manner; On the other hand, due to the narrow internal surface area, the possibility of contact with the environmental factors is low, which makes the free OH group produced less than that of the film samples produced from strong fluid or spin-coating for more times. Therefore, the hydrophobicity of the thinner film sample is poorer than that of the thicker film samples. The influence of thickness is similar in the recovery process.

5.2.2. Surface energy of TiO2/PEG thin films.

Surface energy value (γsv) of TiO2/PEG thin films.

The calculations will be similarly implemented to the TiO2/SiO2 film system.

Table 5.4: Surface energy value γsv of TiO2/PEG(0÷50%) thin films at 0, 30, 60, 90,12, 150 (minute)

TiO2/PEG

(0÷50%) γSV(mJ.m-2)

0 minute 30 minute 60 minute 90 minute 120 minute 150 minute

0% 40,2 47,4 55,7 59,6 60,5 60,7

10% 40,8 49,6 56,8 60,2 61 61,1

20% 42,7 49,6 57,5 60,6 61,3 61,4

30% 42,8 49,7 58,1 61 61,5 61,6

40% 45,5 51,1 60,3 61,6 62,1 61,9

50% 42,5 49,4 58 60,6 61,5 61,3

- 27 -

Figure 5.7: Interface energy TiO2/PEG(0÷50%) thin films – Water by time of illumination..

Surface energy γsv of the TiO2/PEG film (0 ÷ 50%) increases by the illumination time to the saturation value.

However, the change in saturated energy γsv by PEG percentage is insignificant (from the minimal value γSV of the TiO2/PEG (0%) sample = 60,70mJ.m-2 to the sample with the maximal value γsv of the TiO2/PEG sample (40%) = 62,1mJ.m-2).

The saturated energy setting time of the post-illumination samples are similar, after about 60 minutes of illumination.

The results are consistent with the photocatalytic survey results and hydrophilicity studies.

Illumination time (minute)

- 28 -

Value of the interface energy between water and TiO2/PEG film (γsl)

Table 5.5: Contact angle θ of the water, the surface energy γsv and the interface energy between the water and TiO2/PEG(0÷50%) thin films

illumination time

(minute)

TiO2/PEG (0%) TiO2/PEG (10%)

θ γ

sv γsl θ

γsv γ

sl

0 41,2 40,2 -14,2 39,5 40,8 -15,0

30 34,2 47,4 -12,4 31,3 49,6 -12,2

60 22 55,7 -11,3 21,8 56,8 -10,3

90 18,1 59,6 -9,1 14,3 60,2 -9,9

120 16,2 60,5 -8,9 12,1 61 -9,7

150 14,1 60,7 -9,4 12,2 61,1 -9,6 illumination

time (minute)

TiO2/PEG (20%) TiO2/PEG (30%)

θ γ

sv γsl θ

γsv γ

sl

0 35,1 42,7 -16,5 34,1 42,8 -17,1

30 30,1 49,6 -13,0 28,4 49,7 -13,9

60 19,4 57,5 -10,7 17,5 58,1 -10,8

90 15,6 60,6 -9,0 13,2 61 -9,4

120 11,4 61,3 -9,6 10,2 61,5 -9,6

150 13,6 61,4 -8,9 9,7 61,6 -9,7 illumination

time (minute)

TiO2/PEG (40%) TiO2/PEG (50%)

θ γ

sv γsl θ

γsv γ

sl

0 35,7 45,5 -13,2 37,2 42,5 -15,1

30 29,1 51,1 -12,1 33,4 49,4 -11,0

60 16,2 60,3 -9,1 21,3 58 -9,4

90 10,5 61,6 -9,5 17,3 60,6 -8,4

120 8,5 62,1 -9,4 12,4 61,2 -9,4

150 7,4 61,9 -9,8 14 61,3 -8,8

- 29 -

Figure 5.8: Interface energy TiO2/PEG(0÷50%) thin films – Water by time of illumination.

The interface energy of the film with water γsl increases by the illumination time to the saturation value. The magnitude of the change in value γsl from the time of no illumination ~ -15mJ.m-2 to the saturation value ~ -9.5mJ.m-2 is about 35%.

The saturation magnitude of γsl is not significantly different among samples with different PEG percentage. Saturation setting time γsl of samples is approximately the same after about 60 minutes of illumination.

Illumination time (minute)

- 30 -

CONCLUSIONS

The thesis has new contributions that can be mentioned as follows:

- Titanium Dioxide Anantase phase and modified nanostructure materials were successfully fabricated by sol-gel method. Nanostructures of the TiO2 thin films were well-controlled. The undesired crystalline phases transition at high temperatures from Anatase (high photocatalytic activity phase) to Rutile (lower photocatalytic activity phase) has been inhibited.

- The methodology based on the micro theory of materials for quantitative calculation energy of the solid surface and solid-liquid interface has been developed. The implementation of this calculation methodology to experimental data from contact angle measurements has been applied. It is possible to quantity calculate the energy of a solid surface and solid-liquid interface from experimental data.

- The quantitative data of surface and solid-liquid interface energy of the TiO2 nanostructured photocatalytic film under the excitation of UV radiation has been done. The calculated data provides empirical evidence for an interesting effect: the light excitation can vary the surface energy of photocatalyst material such as Titanium dioxide.

- The relationship between photocatalytic and hydrophilic properties in the nanostructured TiO2 materials has been cleared. Quantitative empirical data provided the contribution to hypothesis of the mechanism formation of super hydrophilic effect in Titanium dioxide nanostructure thin film system.