Study the fabrication and photocatalytic, hydrophilic properties of TiO2 / SiO2 and TiO2 / PEG thin films by sol - Gel method

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: . - 1 - 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. - 2 - 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. - 3 - 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. - 4 - 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 - 5 - 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 O 2- 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 - 6 - 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). - 7 - 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. - 8 - 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 - 9 - 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. - 10 - 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. - 11 - 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: - 12 - 2)(21cos svlve lv 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. - 13 - 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. - 14 - X-ray diffraction spectra of TiO2/SiO2(0÷50%) sintered at 500 oC. X-ray diffraction spectra of TiO2/SiO2 (0÷50%) sintered 800 oC 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. - 15 - 4.1.4. Findings on photocatalytic properties based on the