Modified tio2 photocatalysts (TiO2:v, TiO2:n and TiO2 / Cnts) – synthesis and characterization

xic organic substances in the appropriate conditions. These characteristics make TiO2 become the most studied material for applications in environmental treatment technology. Photocatalytic effect of TiO2 was discovered by Fujishima and Honda in 1972, to be continued with the works of other research groups. The practical applications of TiO2 iss restricted due to following reasons: (i) TiO2 is a semiconductor with large band-gap value (about 3.2 eV for anatase phase) and (ii) the high accuracy of e- - h+ recombination. Large band-gap lead to the fact that TiO2 does not absorb visible light - the region accounted for nearly 95% of the radiant energy from the sun. Moreover, the photocatalytic activity of TiO2 come from the electrons and holes generated in the material under the illuminated light source. The recombination of electron - hole pairs in the TiO2 lower the quantum efficiency and lead to decreament of photocatalytic activity of material. Therefore, high visible-light photocatalytic activity TiO2 material become one of the most objectives of science and technology. In 2001, Asahi et al. revealed a new hope that it is possible to reduce the band-gap of TiO2 by doped N into the crystal lattice. Since then, TiO2 has been doped with diferent kind of elements such as metal, non-metal, transition metal and more to studie the effect of doping on the photocatalytic activity. Among these elements, transition metal have shown their advantages due to the decreament of band gap, increases the ability to capture electrons or reduce the recombination of the electron – hole pairs. One of the most study element in transition metal is Vanadium because of impressive result when doping in TiO2 such as (i) increase the electrical conductivity, (ii) maintain transparency and (iii) reduce the band gap of the material. To prevent the recombination of e- - h+, TiO2 has been composited with other materials such as carbon nanotubes (CNTs) and graphene. CNTs are nano-structured materials where the conductivity depends on the structure. When composited with TiO2, generated electrons in TiO2 will transfer to CNTs, reducing the recombination rate of electron – hole and improve the quantum efficiency of the material. In Vietnam, TiO2-based semiconductor materials and its application still be considered as an important research subject. However, works on this material almost concentrate on the control of particles size, reducing the band-gap value or magnetic properties. There is alittle of work concentrate on V-doped TiO2 and a complete anđ stable process to synthesize has not been done. Moreover, mosts of studies on TiO2 only consider to reduce band-gap value, not to prevent the recombination of e-h pairs in the material. To understand the mechanism of photocatalytic activity of TiO2-based materials, it is important to understand the effects of synthesis condition on the reducing of band-gap or lowering the recombination of e-h pairs. For these reasons, the chosen title of the thesis is “Modified TiO2 photocatalysts (TiO2:V, TiO2:N and TiO2-CNTs) – Synthesis and Characterization”. 2 Thesis Objectives: (i) Create the models of V-doped TiO2 materials and TiO2/CNTs composites, optimization and study their electronic structure and properties in order to understand the photocatalytic activities of materials; aim to find the suitable method to synthesize material successfully. (ii) Studying the effects of doping on the physical and photocatalytic properties of materials; complete the process to synthesize V-doped TiO2 material. (iii) Study the effects of synthesis condition on the physical and photocatalytic properties of materials; complete the process to synthesize TiO2/CNTs material. Study Subjects - V-doped and N-doped TiO2. - TiO2/CNTs composites. Research Methods: Thesis was studied using semi-empirical method, including theoretical calculations using Density Functional Theory (DFT) and experimental results. This method is useful to predict, investigate and compare the result from calculations and experiments in order to understand the photocatalytic activities of TiO2-based mateirals. Most of samples studied in thesis were sunthesized at Faculty of Physisc and Center for Nano Science and Technology, Hanoi National University of Education. The crystal tructure, and surface morphology of samples were analyzed using modern measurements as X-ray diffraction (XRD), scanning electron microscopy (SEM), and transmission electronmicroscopy (TEM and HR-TEM) images. The optical properties were analyzed using absorption spectroscopic measurements. The effects of synthesis condition on crystal structure and other properties were analyzed using Raman spectroscopic measurement, Fourier Transform Infrared (FTIR) Spectroscopy or X-ray photoelectron spectroscopy (XPS). The measurements were carried out by modern equipments with high reliability at national research centers, a few measurements were done in foreign laboratories. Electronic Structure and properties of materials were calculated using softwares base on Density Functional Theory (DFT), plane waves (PW) and pseudopotential (PP) such as Quantum ESPRESSO, Material Studio. The calculations were performed on the servers at Center for Computer Science and Faculty of Physics, Hanoi National University of Education. A part of work has been done on server at Institute of Physics, Vietnam Academy of Science and Technology, Vietnam or JAIST, Japan. Scientific Meaning and Practical Significance: The major of thesis is improving the photocatalytic activity of TiO2 anatase vy solving two problems: large band-gap value and high recombination rate of e- h pairs. The calculated results show the influence of doping on the electronic structure of doped TiO2 or TiO2/CNTs composites whereas the experimental results reveal the influence on the crystal structure, crystal lattice vibration, optical and photocatalytic activities. These results will contribute to the understanding of TiO2 photocatalytic in terms of basics and application-oriented research. The thesis contribute a comprehensive method for studying not only TiO2-based material but also can be used to study for other photocatlytic material in general. Thesis Contents: The content of the thesis include (i) general introduction of TiO2 materials; photocatalytic advantages and drawbacks of TiO2; methods to improve photocatalytic activities of TiO2 and some previous experimental and theoretical studies on V-doped TiO2 and TiO2/CNTs composites; (ii) experiment techniques and calculation methods; (iii) theoretical results of the influence of doping and compositing on the electronic structure and photocatalytic properties of TiO2; and (iv) major results of the 3 influence of experimental conditions on the photocatalytic activities of V-doped TiO2 and TiO2/CNTs composites. Thesis Layout: Thesis is presented in 152 pages with 110 Figures and 31 Tables, including the heading, 5 chapters, and conclusions; a list of publications, and references. Structures of the thesis as follows: Introduction: Introducing research situation and the necessary of the thesis; the physical meaning, the content and the structure of the thesis. Chapter 1: Overview of physical, chemical and photocatlytic properties of TiO2 in previous studies on understanding and improving photocatalytic properties of TiO2-based material. Chapter 2: Experimental methods and processes to synthesize materials, basic principles of expermental measurements used to analyze crystal structure and physical properties of materials; basics of Denssity Functional Theory, and some of calculation techniques. Chapter 3: Presenting calculated results of the influence of doping on electronic structure and photocatalytic properties of TiO2; the influence CNTs on the chemical bonding, electronic structure and photocatalytic properties of TiO2. Chapter 4: Presenting the effect of doping of Vanadi on crystal structure, physical properties and photocatalytic properties of TiO2. Chapter 5: Presenting the influence of compositing of CNTs on crystal structure, physical and photocatalytic properties of TiO2. Conclusion: Presenting the major results of the thesis. The research results of the thesis have been published in 16 scientific works in which there are 4 articles in international journals, 3 articles in national journals, 7 reports in national and international conferences, 2 scientific works related to content research. 4 Chapter 1: A BRIEF REVIEW ON TiO2 1.1 Overview of TiO2 1.1.1 Crystal structure and physical properties of TiO2 TiO2 is a semiconductor, exists in three polymorphs: rutile, anatase and brookite; rutile and anatase are more stable and widely used in common. 1.1.2 Vibrations of TiO2 lattice Anatase TiO2 has tetragonal structure, space group 𝐷4ℎ 19 (𝐼41/𝑎𝑚𝑑), total number of molecules per each unit cell and primitive cell are 4 and 2, respectively. There are 10 vibration modes can be seen in anatase lattice: 6 Raman-active modes are 𝐴1𝑔 + 3𝐸𝑔 + 2𝐵1𝑔; 3 infrared-active modes are 𝐴2𝑢 + mode 2𝐸𝑢, 1 is inactive mode (for both infrared and Raman) is 𝐵2𝑢. 1.1.3 Optical properties of TiO2 Anatase TiO2 is an indirect band gap semiconductor, bandgap energy is 𝐸𝑔 3,2 eV. The maximum wavelength can be absorbed by TiO2 is 𝜆𝑚𝑎𝑥 = 387 𝑛𝑚. 1.1.4 Some theoretical results on anatase TiO2 Calculated results showed that anatase TiO2 has tetragonal structure with lattice parameters 𝑎 = 𝑏 = 3.692 Å; 𝑐 = 9.471 Å. TiO2 is an indirect band gap semiconductor, calculated band-gap value is around 2.0 – 2.5 eV, much smaller than experimental value 3.2 eV. Density of states (DOS) and partial density of states (PDOS) of TiO2 proved that conduction bands of TiO2 is dominated by 3d elctrons of Ti, valence bands of TiO2 is formed of O 2s electrons. 1.1.5 Some applications of TiO2 TiO2 materials have been used in different fields of sciences and technology. Some common used of TiO2 can be listed as water and air treatments, electrodes for batteries or water electrolysis processes, advanced materials synthesis. 1.2 Photocatalytic activity of anatase TiO2 When illuminated with a radiation, if radiation energy equivalent to or greater than TiO2 band gap energy (anatase, ~3.2 eV), the electron is excited from the valence band (VB) to the conduction band (CB); generate an e-h pair. This process may cause different effects: generate defects inside or lead to appearance of radicals such as hydroxyl OH*, superoxide O2- on the surface of material. These defects or radicals are able to degrade organic substances, it is origin of TiO2 photocatalytic activity. Practical applications of TiO2 are restricted due to some drawbacks: only arbsorb a small range of Sun’s radiation because large band-gap energy; the interaction of organic substances with material surface, and the high recombination rate of e-h pairs. 1.3 Approaches to enhance the photocatalytic process of TiO2 When used in practical application for environment pollution treatments, TiO2-based material has some limitations: low visible photocatalytic activity, low adsorption of organic pollutants, aggregation of particles, difficulty in distributing particles uniform, difficulty in recovery nano particles. Most of research on TiO2 are concentrated to overcome these limitations. Some common methods have been adopted by 5 previous studies: (i) modification of TiO2, (ii) optimization of synthesis process, (iii) stabilixation by support strucutres, and (iv) dispersion by magenic field. 1.4 Approaches to enhance the visible light photocatalytic activity of TiO2 To enhance the visible light photocatalytic activity of TiO2, the most common method is reducing band gap energy by doping. Previous studies show that doped TiO2 (by metal, non metal, rare earth elements,) has higher visible light photocatalytic activity. Vanadium and nitrogen are two most effective elecment in order to reduce the band gap energy of TiO2, lead to a significant shift of absorption edges of material to visible range. 1.5 Approaches to demote the e-h recombination rate The most common and effective method to reduce e-h recombination rate is compositing TiO2 with some materials such as CNTs, graphene, The composite materials have ability to transfer generated electrons from TiO2 to CNTs, graphene and lowering the recombination rate. CNTs is one of the most material has been used to composite with TiO2. Chapter 2: EXPERIMENTAL TECHNIQUES AND CALCULATION METHODS 2.1 Synthesis proceses of TiO2 materials TiO2 and V-doped TiO2 samples were synthesized by hydrothermal, sol-gel and co-precipitation methods. N-doped TiO2 films were prepared by ALD technique. TiO2/CNTs composite samples were synthesized using hydrolysis method. 2.2 Samples-analyzed instruments and techniques Samples were characterized using different instrument and analyzed with different techniques: X- ray diffraction patterns; Raman scattering spectroscopy, Fourier transform infrared spectroscopy; scanning electron microscopy (SEM), transmission electron microscopy (TEM); optical absorption spectroscopy; Energy-dispersive X-ray spectroscopy (EDX), X-ray photoelectron spectroscopy (XPS), BET. 2.3 Calculation Methods 2.3.1 Introduction to DFT Density Functional Theory (DFT) theory is a fundamental theory for computing electronic structures and other pproperties of materials based on first principle. The major of DFT is to solve the Schrödinger equation for a multi-particle system in order to find the total energy through the ground state energy. Using Born-Oppenheimer approximation and Thomas-Fermi model, the problem now turn to how to determine ground state charge density 𝜌(𝑟). It is easier to calculate system properties because the minimum variables required to calculate for a system with N electrons has decreased from 3N (to be 4N variable if considering both spin) down to only 3 (or 4) variables. When used to calculate for materials, the precision of DFT depends on the form of exchange- correlation energy. In fact, there are many functions that describe energy – exchange energy, however the two most common are Local Density Approximation (LDA) and Generalized Gradient Approximation (GGA). For periodic system, some computing techniques such as plane wave, supercell, or pseudopotential are also used. 6 2.3.2 Calculation Techniques Models were built and simulated using special softwares such as Quantum ESSPRESSO or Materials Studio. Each calculation process can be divided into 3 steps: (i) build the unit cells; (ii) geometry optimization and (iii) compute the characteristics of models. The output results can be export as images, data files or other fomrs and can be used in other plotting programs. Chapter 3: SIMULATION AND CALCULATION OF TiO2-BASED MATERIALS USING DENSITY FUNCTIONAL THEORY 3.1 Calculated results for anatase TiO2 Table 3.3 Parameters used to calculate properties of TiO2 anatase. Parameter Value Note Approximation GGA Generalized Gradient Approximation Functional PBE Perdew – Burke – Erzenhoff Basis Plane Wave Plane Waves Pseudopotential Ultrasoft Vanderbilt Hubbard value 8.18 eV Used for Ti 3d Anatase TiO2 has tetragonal crystal system, total number of atoms in each unit cell are 12 (4 Ti atoms and 8 O atoms). The Brillouin zone has shape of cuboid; the best k-points to calculate band structure, density of states (DOS), partial DOS (PDOS) and other properties is Γ – X – M – Γ – Z – R – A – Z; X – R; M – A. Parameter used to calculate TiO2 properties are listed in Table 3.3. Figure 3.3b Band structure of TiO2 calculated by PBE+U functional. Figure 3.5b Partial density of states of TiO2 calculated using GGA-PBE+U. Calculated results for anatase TiO2: tetragonal crystal system, lattice parameters 𝑎 = 𝑏 = 3.79 Å; 𝑐 = 9.72 Å; bandgap energy 3.201 eV, in a good agreement with experimental results. Calculated band structure in Figure 3.3b showed that anatase TiO2 is an indirect band gap semiconductor. PDOS results in Figure 3.5b proved that Ti 3d and O 2p electrons play important roles in the formation of conduction and valence bands of anatase TiO2. b En er gy ( eV ) b P D O S (e V -1 ) Energy (eV) 7 3.2 Calculated results for doped TiO2 3.2.1 Doped TiO2 models There are 8 different models for doped TiO2 have been built: pure TiO2 model (TOO), O vacancy (TOO-v); substitutional models (TOV-s and TOV-sv); institital models (TOV-i and TOV-iv); substitutional models (TVO-s and TVO-sv). Figure 3.6 Defect models used to calculate for TiO2: (a) TOO, (b) TOO-v, (c) TOV-s, (d) TOV-sv, (e) TOV-i, (f) TOV-iv, (g) TVO-s and (h) TVO-sv .. 3.2.2 Calculated results for V-doped TiO2 TVO-s model has smallest formation energy, indicates that V atoms substituted into positions of Ti atoms, neither into O atoms or interstitial positions. The formation energies of O vacancies of doped models are smaller than undoped models, proved that V doping lead to the increasing of possibility of O vacancies formation. DOSs of V-doped models showed that impurity energy levels created by 3d V electrons lied below the conduction bands of TiO2, reduce the optical band gap of materials. The expansion of energy levels of O 2p electron is another reason for this band gap reduction. 3.2.3 Calculated results for N-doped TiO2 TON-s model has smallest formation energy, indicates that N atoms substituted into positions of O atoms, neither into Ti atoms or interstitial positions. The formation energies of O vacancies of doped models are smaller than undoped models, proved that N doping lead to the increasing of possibility of O vacancies formation like V doping. DOSs of N-doped models showed that impurity energy levels created by 2p N electrons lied above the valence bands of TiO2, reduce the optical band gap of materials. The expansion of energy levels of O 2p electron is another reason for this band gap reduction, similar to V doping case. a b c d e f f g h Ti O V (hoặc N) Nút khuyết O 8 3.3 Calculated results for TiO2 clusters 3.3.1 Models of TiO2 clusters Models of (TiO2)n clusters (n = 1, 2, 3,4 and 5) used in were showed in Figure 3.9. n = 1 n = 2 n = 3 n = 4 n = 5 TC-1 TC-2 TC-3 TC-4 TC-5 Figure 3.9 Models of TiO2 clusters. 3.3.2 Charges transfers in cluster (TiO2)n + When electrons are added to (TiO2)n cluster, they tend to settle around Ti atoms, occupying the empty d orbitals of the Ti atoms. The added electrons concentrate at Ti atoms with smaller coordination numbers (empty d orbitals), most of them are Ti atoms on the surface of the TiO2 nanoparticle. + When electrons are pulled out of (TiO2)n clusters, the lost electrons are usually single electrons in O 2p orbitals, especially from the O atoms which have the coordinate number 1. When electrons are taken in photoelectric reactions, the holes tend to settle around the O atoms on the material surface, which has smaller coordination number than other O atoms inside material. 3.4 Calcualted results for TiO2/CNTs materials 3.4.1 Models of TiO2/CNTs The unit cell of pseudoperiodic lattices used to calculate for TiO2/CNTs has lattice constants 𝑎 = 𝑏 = 30.0 Å; 𝑐 = 17.0 Å and 𝛼 = 𝛽 = 𝛾 = 90o. Each unit cell include a (10,0) single-wall CNTs and one of five clusters in Figure 3.9; contain 160 C atoms, 𝑛 Ti atoms and 2𝑛 O atoms (n = 1, 2, 3, 4 and 5). 3.4.2 Geometry and bonding in TiO2/CNTs Different models of TiO2/CNTs were built, optimized and calculated. The final conclusions are: + Optimized structures of TiO2/CNTs models are not influented by initial positions of (TiO2)2 on CNTs surfaces. + The Ti-C bonding lengths are in the range of 2.58 to 3.47 Å, does not depend on the relative postions between TiO2 and CNTs. + The motion directions or final positions of Ti atoms are influented by coordinate numbers of Ti atoms in (TiO2)n clusters. The adsorption anergies of all models are nearly equal to eachother, indicate that the adsorption of TiO2 on CNTs are chemical adsorprion. The calculated results also suggest that interaction between TiO2 and CNTs are caused by interaction between d orbitals of Ti and 𝜋 electrons of CNTs surfaces. The redistribution of electron density when TiO2 adsopted on CNTs surfaces on Figure 3.14 showed that the electron density in the middle zone - between TiO2 and CNTs - is increased significantly. This increment is related to the formation of chemical bonding on CNTs surface. 9 CTO-1 a. Cross-section. b. Parallel-section. CTO-2 c. Config. A d. Config. B CTO-3 e. Cross-section f. Parallel-section Figure 3.14 Electron density redistribution of TiO2/CNTs models. 3.4.3 Photocatalytic activity of TiO2/CNTs Figure 3.17 PDOS of TC-2 cluster and CNT in CTO-2 model. Figure 3.18b DOS of C atoms on the CNTs surface. Figure 3.17 showed that the band gap of TiO2/CNTs were the overlay of PDOS of TiO2 and CNTs. This overlay make the charge transfers between TiO2 and CNTs happen easier, especially for photocatalytic reactions. Ti O C b P D O S (e V -1 ) Energy (eV) 10 Figure 3.18b shows the PDOSs of different C atoms on CNTs surfaces. When the bonding between C atoms and TiO2 were formed, new states appear in the range of 0 to 2.0 eV in the band gap of C atoms. These new states increase the charges exchange processes between TiO2 and CNTs. The influcence of TiO2 on C atoms are not localized, it can increase the conduction of CNTs. The high conductivity of CNTs improve the capable of charges exchange between TiO2 and CNTs, increase the photocatalytic activity of materials, in a good agreement with previous studie. The results can be used to orient methods to improve the photocatalytic activity of TiO2 by doping or compositing in chapter 4 and 5. Chapter 4: THE INFLUENCE OF DOPING ON THE PHOTOCATALYTIC ACTIVITY OF TiO2 Table 4.1 V-doped TiO2 samples series. 1 V-doped TiO2 samples fabricated by different methods H V , C V a n d S V S er ie s Fabricated Methods V concentration (% at.) 0 0.1 0.3 0.5 0.7 0.9 Hydrothermal HV0 HV1 HV3 HV5 HV7 HV9 Co-precipitation CV0 CV1 CV3 CV5 CV7 CV9 Sol-gel SV0 SV1 SV3 SV5 SV7 SV9 2 V-doped TiO2 samples synthesized by hydrothermal method with different time H V T S e r ie Samples HVT0 HVT1 HVT3 HVT5 HVT7 HVT9 V concentration (% at.) 0 0.5 0.5 0.5 0.5 0.5 Hydrothermal Times (hrs.) 5 1 3 5 7 9 3 V-doped TiO2 samples synthesized by hydrothermal method with different solvents H S S e r ie Samples HWAT HCLA HOXA HOLA Precursors TiCl4 + Ethanol TiCl4 TiCl4 + Ethanol TiCl4 + Ethanol Solvents V2O5 + H2O V2O5 + HCl V2O5 + OXA V2O5 + OLA 4 V-doped TiO2 samples using solvents with different OLA concentration H A S e r ie Samples HA1420 HA1520 HA1620 HA1820 Precursors TiCl4 + Ethanol Solvents V2O5 + OLA TiCl4 : OLA : Ethanol 1 : 4 : 20 1 : 5 : 20 1 : 6 : 20 1 : 8 : 20 4.1 The influence of synthesis methods and doping concentration on V-doped TiO2 samples 4.1.1 Crystal structures of V-doped TiO2 Figure 4.1 shows the XRD patterns of V-doped TiO2 samples synthesized by different methods: hydrothermal, co-precipitation and sol-gel. All patterns show the characteristic peaks of anaatase TiO2, 11 consistent with JCPDS 21-1272 standard card in ICDD library. TiO2 has tetragonal crystal system, space group I41/amd; lattice constant 𝑎 = 𝑏 = 3.7852 Å, 𝑐 = 9.5143 Å and 𝛼 = 𝛽 = 𝛾 = 90°. Figure 4.1 XRD patterns of (a) HV, (b) CV and (c) SV series. 4.1.2 Optical properties of V-doped TiO2 samples Figure 4.2 shows the UV-Vis absorption spectra of HV serie, the inset shows the band gap energy of samples calculated from UV-Vis spectra. The absorption edge of undoped sample HV0 is 390 nm, consistent with experimental result. V-doped samples show a redshift of absorption edges, caused from the substitution of V4+ ions into TiO2 lattice. The calculated results show a limited concentration of V to reduce band gap energy of doped samples. In this thesis, the limited concentration is 0.5% at. and is used for latter study a In te n si ty ( a. u .) b In te n si ty ( a. u .) c In te n si ty ( a. u .) 𝟐𝜽 (degree) Figure 4.2 UV-Vis absorption spectra of HV serie. A b so rp ta n ce ( a. u .) Wavelength (nm) 12 4.1.3 Photocatalytic activity of V-doped samples 4.1.3.1 The influence of synthesis methods on photocatalytic activity of V-doped TiO2 Table 4.2 Conditions of photocatalytic reaction of V-doped samples. TiO2 samples V concen. (%at) Illuminated Light Names HV0 0 Room HT+R HV0 0 Lamp HT+L HV5 0.5 Lamp HV+L SV5 0.5 Lamp SV+L CV5 0.5 Lamp CV+L The photocatalytic activity of V-doped samples were tested by photodegration of 5.10-6 mol/litre phenol solution; illuminated by a 100 W light bub. The photocatalytic conditions were listed in table 4.2. Photocatalytic activity of samples were showed in Figure 4.4. All samples are able to photodegrade phenol, the efficiency is higher when illuminated by light bub. V-doped samples have higher photocatalytic activity than undoped samples; HV sample (synthesized by hydrothermal method) has the highest photocatalytic activity. SEM images of V-doped samples were showed in the figure 4.5. CV sample (synthesized by co- precipitation method) has largest particles size, HV sample (synthesized by hydrothermal method) has smallest particle sizes. The smaller particle sizes, the larger specific surface area of sample and lower electron – hole recombination rate; increase the photocatalytic activity of sample. Figure 4.5 SEM images of (a) HV5, (b) CV5 and (c) SV5 samples. BET results of HV0 and HV5 show that both samples contain average size pores, specific surface area of two samples are similar; indicate that specific surface areas do not play an important role in the increment of photocatalytic activity of doped samples. XPS results of HV0 and HV5 sampes werw showed in figure 4.8. The Ti 2p peak of HV0 is a high symmetric shape, imply that there are only Ti4+ ions in samples. For HV5 sample, the existent of V lead to the formation of O vacancies and so Ti3+ can be formed; make the peak to be unsymmetric. The O 1s peaks of samples indicate that new peak at 534 eV appear in doped sample, which is belonged to V-O a 200 nm b 300 nm c 200 nm Figure 4.4 Photodegradation of phenol in different conditions. P h e n o l C o n ce n tr at io n ( % ) Illuminated Time (min.) 13 bonding. This result suggests that V atoms have substituted into Ti atoms positions; constient with previous results in chapter 3. Figure 4.8 XPS peaks of (a) Ti 2p and (b) O 1s of HV0 and HV5; Gaussian fitting of O 1s of (c) HV0 and (d) HV5 samples. 4.1.3.2 The influence of doping concentration on photocatalytic activity of TiO2 Figure 4.9 Photodegradation of phenol of HV serie. Figure 4.10. UV-Vis absorption spectra of HVT serie. Figure 4.9 show the result of photodegration of phenol by V-doped samples with different impurity concentration. HV5 sample contain 0.5% at. V show the highest photocatalytic activity in the visible range; also the same value for CV and SV series. It mean that the best concentration of V in the sample for highest photocatalytic activity is 0.5% at. This concentration is used for all samples in latter studies in this thesis. a In te n si ty ( a. u .) Binding Energy (eV) b In te n si ty ( a. u .) Binding Energy (eV) c In te n si ty ( a. u .) Binding Energy (eV) d In te n si ty ( a. u .) Binding Energy (eV) P h e n o l C o n ce n tr at io n ( % ) Illuminated Time (min.) A b so rp ta n ce ( a. u .) Wavelength (nm) 14 4.2 The influence of hydrothermal parameters on V-doped TiO2 samples 4.2.1 The influence of hydrothermal time on V-doped TiO2 samples Figure 4.10 show the UV-Vis absorption spectra of HVT serie – 0.5% at. V-doped TiO2 samples with different hydrothermal time (tH). The absorption edges of doped samples shifted to the longer wavelength zone, the absorptances of all samples are increase. When the hydrothermal time is smaller than 7 hours, the absorptance of samples increase when the tH raise up; howerver, when tH is larger than 7 hours, the absorptance decrease when tH increase. Figure 4.11 shows that when tH reachs to 9 hours, V-doped sample contain amount of rutile phase. Figure 4.12 also show that the increment of tH change the growth direction of TiO2 crystal in sample. So the chosen hydrothermal time is 7 hours. Figure 4.11 XRD patterns of HT, HVT5 and HVT7 samples. Figure 4.12 Relative intensity of XRD peaks of HVT5 and HVT9 samples. 4.2.2 The influence of solvents on photocatalytic activity of V-doped TiO2 In hydrothermal methods, samples properties will affected by another parameter – hydrothermal solvents. The change in solvent polarity lead to the change in properties of samples. Figure 4.13 show that all samples are monophase, independent of the polarity of solvents. Two samples using solvents contain organic acids – HOLA and HOXA – have smaller crystal sizes than the other two, HCLA and HWAT. It is consistent with SEM and Raman results. UV-Vis absorption spectra in figure 4.15 point out that the absorption edges of samples do not perform any redshift but HOLA and HOXA samples have larger absoptances of than HWAT and HCLA samples. The increment of absorptances can be explained by two reasons: (i) the substitution of V atom into TiO2 lattice and (ii) the formation of polarized layer on the surface of TiO2 nano particles. Figure 4.17 show that HOLA and HOXA samples have higher photocatalytic activity due to two reasons: (i) smaller crystal sizes and (ii) higher absorptances. In te n si ty ( a. u .) 𝟐𝜽 (degree) In te n si ty ( a. u .) Peaks Figure 4.13 XRD patterns of HVS serie. In te n si ty ( a. u .) 𝟐𝜽 (degree) 15 Figure 4.15 UV-Vis absorption spectra of HWAT, HCLA, HOXA and HOLA samples. Figure 4.17 Photodegradation of MB solutions of HS serie. 4.3 The influence of concentration of solvents on morphology of V-doped TiO2 4.3.1 Crystal structures XRD patterns of HA serie - used solvents with different OLA concentration - were showed in figure 4.19. Almost samples are anatase + brookite multiphase; the ratio of brookite increase with the increment of OLA concentration; or in other hands, the increment of OLA concentration reduce the monophase of sample. The monophase samples can be received with ratio of TiCl4 : OLA : H2O equals to 1 : 8 : 20. Figure 4.19 XRD patterns of HA1420 ÷ HA1820 samples. 4.3.2 The influence of solvent on morphology of V-doped TiO2 SEM and TEM images of 0.5% at. V-doped samples in figure 4.20 show the affection of OLA concentration on the V-doped samples’s morphology. The ratio TiCl4 : OLA : C2H5OH increase from 1 : 4 : 20 to 1 : 8 : 20, the particles shape change from spheroid-like to cubic-like, similar rod-like and spherical-like. A b so rp ta n ce ( a. u .) Wavelength (nm) M B C o n ce n tr at io n ( % ) Illuminated Time (min.) In te n si ty ( a. u .) 𝟐𝜽 (degree) 16 Figure 4.20 SEM and HR-TEM images of (a,e) HA1420, (b,f) HA1520, (c,g) HA1620 and (d,h) HA1820. a 200 nm e 20 nm b 300 nm f 20 nm c 200 µm g 20 nm d 500 nm h 20 nm 17 4.4 Fabrication of N-doped TiO2 films N-doped TiO2 thin films were fabricated using Atomic Layder Deposition (ALD) method. Two seires have been fabricated: ANN serie - samples with different NH3 flow rate from 0 to 20 cm 3/min, denoted as ANN0, ANN10 và ANN20. ANT serie – samples with same NH3 flow rate and different treted temperature: 300, 400 và 500 oC; denoted as ANT300, ANT400 và ANT500. AFM image of undoped sample ANN0 sample were showed in figure 4.22. The sample surface is quite uniform, the particles are in the shape of pyramidal, similar to equilibrium shape of TiO2 crystal. The treated temperature does not affect to samples’ surface. For N-doped samples, the conclusions are similar to undoped samples. The absorption spectra of N-doped samples show that the band gap energies decrease with the increment of impurity concentration. The reason come from new energy levels in the band gaps of material, caused by substitution of N atoms into TiO2 lattice. The higher N concentrations, the wider impurity levels and the smaller band gap energy of doped samples. Figure 4.26 O 1s peaks of (a) ANN0, (b) ANN10 and (c) ANN20 samples. XPS results in figure 4.27 show the formation of TiO2 bonding in all samples, Ti has valence of +4, O has valence of -2. N 1s peaks only show the appearance of Ti-N bonding peaks, imply that N atoms substituted in to O atoms positions. Figure 4.27 N 1s peak of (a) ANN10 and (b) ANN20 samples. a Binding Energy (eV) In te n si ty ( a. u .) b Binding Energy (eV) In te n si ty ( a. u .) c Binding Energy (eV) In te n si ty ( a. u .) a In te n si ty ( a. u .) Binding Energy (eV) b In te n si ty ( a. u .) Binding Energy (eV) Figure 4.22 AFM image of ANN0. 18 Chapter 5: THE INFLUENCE OF SYNTHESIS METHODS ON THE PROPERTIES OF TiO2/CNTs MATERIALS Table 5.1 Lists of TiO2/CNTs samples. 1 TiO2/CNTs samples using CNTs oxidized by different methods T C -O S e r ie Samples Oxidation Solution Solvents TC-0-0 None TTiP + Ethanol + H2O TC-1-I HNO3 TTiP + Isopropanol TC-1-BA HNO3 TTiP + Ethanol + H2O + BA TC-2-I HNO3 : H2SO4 TTiP + Isopropanol TC-2-BA HNO3 : H2SO4 TTiP + Ethanol + H2O + BA 2 TiO2/CNTs samples using CNTs oxidized by different BA concentration solvents T C -B A S e r ie Samples Oxidation Solution VBA (ml) Solvents TC-EHB-5 Ethanol + H2O + BA 5 TTiP + Ethanol + H2O TC-EHB-10 Ethanol + H2O + BA 10 TTiP + Ethanol + H2O TC-EHB-20 Ethanol + H2O + BA 20 TTiP + Ethanol + H2O TC-EB-10 Ethanol + BA 10 TTiP + Ethanol + H2O 3 TiO2/CNTs samples with different 𝑚𝑇𝑖𝑂2: 𝑚𝐶𝑁𝑇𝑠 ratio T C -m Samples TC1 TC3 TC30 TC80 TC500 TC1000 𝑚𝑇𝑖𝑂2: 𝑚𝐶𝑁𝑇𝑠 1 : 1 3 : 1 30 : 1 80 : 1 500 : 1 1000 : 1 5.1 The influence of CNTs oxidation on TiO2/CNTs properties 5.1.1 The formation of TiO2-CNTs contact layer Figure 5.1 SEM images of TC-0-0. CNTs have been oxidized by HNO3 65% solution and H2SO4 : HNO3 mixture; BA were used to aid the oxidation process of CNTs.  For samples used unoxidized CNTs, TiO2 nanoparticles do not adhere on CNTs surfaces and 500 nm 200 nm 19 aggregated.  For samples used CNTs oxidized by HNO3 solution, TiO2 nanoparticles dispersed in samples, adhere to CNTs surface. The dispersion is not uniform and TiO2 nanopartilce still aggregated. HNO3 solution oxidize CNTs better than H2SO4 : HNO3 mixture.  BA play an important role in the oxidation of CNTs, the adherence of TiO2 on CNTs surface is better. Figure 5.2 SEM images of (a) TC-1-I, (b) TC-1-BA,(c) TC-2-I and (d) TC-2-BA. 5.1.2 The influence of BA concentration SEM images of TC-BA serie in Figure 5.3 show that (i) TiO2 nanoparticle dispersed uniformly in samples and adhere on CNTs surfaces, (ii) the amount of BA around 10 ml give the best sample. Figure 5.4 XRD patterns of TC-BA serie. XRD patterns of TC-BA serie in figure 5.4 show that the characteristic peaks of CNTs do not appear clearly, even for (002) peaks at 26.6o. The existence of (002) peaks can be seen by the expansion of (101) a 500 nm b 500 nm c 1000 d 500 nm 𝟐𝜽 (degree) In te n si ty ( a. u .) 20 peaks of TiO2; the larger expansion, the higher ratio of CNTs in the sample. The results can be reconfirmed in absorption spectra in fig ure 5.6. When the amount of CNTs increase, the absorptance in visible range increase. Figure 5.7 show the photodegradation of MB in solutions using TC-BA serie. All of TiO2/CNTs samples show higer photocatalytic activity than TiO2, CNTs or self-degradation of MB; imply that TiO2- CNTs interaction increase the photocatalytic activity of samples. The influence of oxidation can be seen in figure 5.6. TC-EHB-10 and TC-EB-10 samples show higher photocatalytic activity than TC-EHB-5 or TC-EHB-20 samples, TC-1-I sample has lowest photocatalytic activity. The reason can be explained by the formation of TiO2-CNTs contact layers. The more contact layers, ther higher photocatalytic activity. Figure 5.6 UV-Vis spectra of TC-BA. Figure 5.6 Photocatalytic RhB degradation curves of TC-BA serie. 5.2 The influence of mass ratio on TiO2/CNTs properties 5.2.1 Crystal structures Figure 5.7 XRD patterns of TC1÷TC1000 samples. XRD patterns of TC-m serie – TiO2/CNTs samples used CNTs oxidized by HNO3 solution – were showed in figure 5.7. Anatase TiO2 peaks were observed for all samples, consistent with JCPDS 21-1272 cards in ICDD library. CNTs peaks or unknown peaks were not observed. The influence of of 𝑚𝑇𝑖𝑂2: 𝑚𝐶𝑁𝑇𝑠 ratio on samples properties can be seen in figure 5.8: the A b so rp ta n ce ( a. u .) Wavelength (nm) M B C o n ce n tr at io n ( % ) Illuminated Time (hrs) 𝟐𝜽 (degree) In te n si ty ( a. u .) 21 increment of CNTs mass expanse the width of (101) peaks and shift it to larger 2𝜃 region. The reason is explained by the overlap of CNTs (002) peaks and TiO2 (101) peaks. 5.2.2 Morphology of TiO2/CNTs samples SEM and HR-TEM images of TC1 in figure 5.8 show that TiO2 nanoparticles dispersed and adhere on CNTs surface. The dispersion is not uniform, TiO2 nanoparticles still be aggregated. The aggregation is proportional to the amount of CNTs in the samples. Figure 5.8 (a) SEM and (b) HR-TEM images of TC1. Figure 5.11 XPS results of TC10. BET results showed that specific surface area of TiO2 and TC1 are approximated 100 m 2/g and 150 m2/g, respectively; indicate that when composited with CNTs, the specific surface area of samples increase. XPS results show that only C-O-Ti bonding peaks can be observed, the Ti-C bonding peaks can not a 100 nm b 100 nm a In te n si ty ( a. u .) Binding Energy (eV) b In te n si ty ( a. u .) Binding Energy (eV) c In te n si ty ( a. u .) Binding Energy (eV) d In te n si ty ( a. u .) Binding Energy (eV) 22 be found. The interaction between TiO2 and CNTs are formed by indirect C-O-Ti bonding, not by direct bonding Ti-C. These O atoms can be form by oxidation processes, reconfirm the role of CNts oxidation in the formation of TiO2-CNTs contact layers. 5.2.3 Optical properties of TiO2/CNTs UV-Vis absorption spectra of TC-m serie – TiO2/CNTs samples with different 𝑚𝑇𝑖𝑂2: 𝑚𝐶𝑁𝑇𝑠 ratio – infigure 5.12 show that the absorption edges do not change but the absorptance in visible range increase with the increment of CNTs mass. Figure 5.12 UV-Vis absorption spectra of TC-m serie. Figure 5.13 Photodegradation of MB by TC-m serie. 5.2.4 Photocatalytic activity of TiO2/CNTs Photodegradation of MB of TC-m serie were showed in figure 5.14. All composite samples have higher photocatalytic activity than TiO2 sample, the TC3 sample with 𝑚𝑇𝑖𝑂2: 𝑚𝐶𝑁𝑇𝑠 = 3 : 1 has highest photocatalytic activity. This imply that CNTs has an important role in the increment of photocatalytic activity of samples. Figure 5.14 Electron transfer mechanism. Figure 5.15 Photon absorption mechanism. Two reasons can be used to explain the increment of photocatalytic activity of TiO2/CNTs samples: (i) the reduction of e – h recombination rate and (ii) the increment of specific surface area. There are two mechanisms to explain the role of CNTs: elctron transfer mechanism and photon absorption mechanism. In electron transfer mechanism in figure 5.14, photoelectron in TiO2 when illuminated will transfer to CNTs, reduce the e – h recombination rate; consistent with previous result in chapter 3. Experimental results in chapter 5 show that the absorptance in visible range increase, consistent with photon absorption mechanism in figure 5.15. In TiO2/CNTs materials, CNTs play two roles – photon absorber and electron conductor; both of them increase the photocatalytic activity of TiO2/CNTs materials. A b so rp ta n ce ( a. u .) Wavelength (nm) M B C o n ce n tr at io n ( % ) Illuminated Time (hrs) 23 CONCLUSION A. On the material simulation using DFT 1. DFT has been used to simulate and calculate for TiO2 and doped TiO2 materials, TiO2/CNTs materials. The chosen parameters to calculate for TiO2: generalized gradient approximation (GGA), PBE exchange-correlation function, plane wave basis, Vanderbilt pseudopotential and Hubbard potential 8.18 eV (used for d orbital electrons). The calculated results are in good agreement with experimental and other theoretical studies. 2. A systematic research on V-doped and N-doped TiO2 has been performed. The doping of V, N lead to the formation of O vacancies and extra electrons; these electron localized around Ti atoms near O vacancies. These electrons play an important role in the improvement of photocatalytic activity of materials. The mechanism of visible phototcatalytic activities of doped TiO2 has been proposed. 3. Properties of (TiO2)n has been simulated and investigated successfully: surface O atoms play an important roles in the photochemical processes associated with holes tranfered to TiO2 nanoparticles surface whereas Ti atoms take part in processes associated with electrons transferred to TiO2 nanoparticles. These results are in a good agreement with previous researchs and can be used to predict of TiO2-CNTs interaction and the enhancement of photocatalytic activity of TiO2- CNTs materials. 4. Adsorption processes of TiO2 on CNTs surfaces has been simulated and calculated, the result show that they are stable, weak chemical adsorption processes. The TiO2-CNTs bondings are formed through the interaction of empty d orbitals of Ti atoms and 𝜋 electrons on the CNTs surface. The partial densities of states C atoms on CNTs surface raise up, increase the ability of electron exchange between TiO2 and CNTs, improve the photocatalytic activity of TiO2. B. On the fabrication methods and material properties 1. TiO2 and doped TiO2 were prepared by hydrothermal, sol-gel and co-precipitation methods; the hydrothermal sample has highest photocatalytic activity. The chosen synthesis conditions: hydrothermal method, 0.5% V doping concentration, and hydrothermal time is 5 hours, oleic acid (OLA) contained solvents. 2. Characteristics of 0.5%V-doped TiO2: monophase anatase material, average particle sizes of 10 ÷ 20 nm, 2.90 eV band gap values, high photocatalytic activity (photodegrade more than 90% of phenol in tested solution after 3 hours of visible light illumination). The XPS results indicate that V atoms are substituted into the Ti atoms postions, consistent with theoretical calculations. 3. The shapes and sizes of the nanoparticles can be controlled by changing the polarization of solvents. Samples synthesized using solvent with TiCl4 : OLA : Ethanol = 1 : 8 : 20 have smallest particle sizes and highest photocatalytic activity. 4. TiO2 and N-doped TiO2 thin films prepared by atomic layer deposition have uniform thickness, average particle sizes of 20 nm, absorption edges have been shifted to visible range. The XPS results indicate that in TiO2 sample, O has valence of +2, Ti has valence of +4, and no other valence states exists. When doped into TiO2 lattice, N atoms substituted into O atoms positions, do not substitute into the position of Ti or interstitial positions, in a good agreement with theoretical calculations. 24 5. TiO2/CNTs composites were successfully synthesized by hydrolysis method using CNTs activated by different processes. The compatible synthesis conditions: CNTs activated solution is ethanol + water + BA mixture, mass ratio 𝑚𝑇𝑖𝑂2: 𝑚𝐶𝑁𝑇𝑠 is in the range of 3 to 30. TiO2 particles adhere to the surface of CNTs, observable diameter of CNTs is around of 50 ÷ 70 nm and for TiO2 is around of 10 ÷ 20 nm. The specific surface area of TiO2/CNTs samples approximate 150 m 2/g, 1.5 times larger than that of TiO2. TiO2/CNTs samples show the characteristics of CNTs and anatase TiO2 phase and the existence of CNts-TiO2 interactions, consistent with the theoretical results. 6. Visible light photocatalytic activity TiO2/CNTs composite is approximate 5 times higher than that of the initial TiO2 materials. The origins of photocatalytic activity improvement have been explained, in a good agreement with previous studies. LISTS OF PUBLICATIONS 1. Duong Quoc Van, Le Minh Thu and Nguyen Minh Thuy, V-doped TiO2 Anatase: Calculation and Experiment, Journal of Science of HNUE, Vol. 62, Iss. 8, 2017, pp. 127-134, doi: 10.18173/2354- 1059.2017-0040. 2. Duong Quoc Van, Le Minh Thu, Nguyen Manh Nghia, Nguyen Minh Thuy, A DFT Study on N-doped Anatase TiO2, Journal of Science of HNUE, Vol. 61, No. 7, 2016, pp. 157-164, doi: 10.18173/2354- 1059.2016-0045. 3. Duong Quoc Van, Nguyen Minh Thuy, Nguyen Thi Ngoc Minh and Nguyen Huy Viet, Electronic Structure of TiO2 Multilayer Films, The 9th Conference on Solid State Physics and Materials Science, 2015. 4. Minh Thuy Nguyen, Cao Khang Nguyen, Thi Mai Phuong Vu, Quoc Van Duong, Tien Lam Pham and Tien Cuong Nguyen, A study on carbon nanotube titanium dioxide hybrids: experiment and calculation, Advances in Natural Sciences: Nanoscience and Nanotechnology, Vol. 5, 2014, doi: 10.1088/2043- 6262/5/4/045018. 5. Nguyen Minh Thuy, Duong Quoc Van and Le Thi Hong Hai, The Visible Light Activity of the TiO2 and TiO2:V 4+ Photocatalysts, Nanomaterials and Nanotechnology, Vol. 2, 2012, doi: 10.5772/55318. 6. Nguyen Minh Thuy, Duong Quoc Van, Le Thi Hong Hai, Nguyen Manh Nghia and Nguyen Hong Quan, The Solvent Influent on the Properties of TiO2:V 4+ Nanoparticles Prepared by Hydrothermal Method, Advanced Materials Research, Vol. 548, pp. 105-109, 2012; doi: 10.4028/www.scientific.net/AMR.548.105. 7. Duong Quoc Van, Nguyen Minh Thuy and Nguyen Huy Viet, A DFT Study on Structural and Electronic Properties of N-doped Anatase TiO2 Layers, Journal of Science of HNUE, Vol. 59, No. 7, 2014, pp. 150-156. 8. Nguyen Minh Thuy, Duong Quoc Van and Trinh Hai Dang, Influence of The Preparation Parameters on the Properties of the TiO2 and TiO2:N Thin Films, Journal of Science and Technology, Vol. 52 (3B), 2014, pp. 174 – 182. 9. Duong Quoc Van, Nguyen Minh Thuy, Electronic Structure of N-doped TiO2: DFT and DFT+U Calculations, Proceeding of the International Symposium on Nano-Materials, Technology and Applications (NANOMATA2014), 2014. 10. Duong Quoc Van, Nguyen Minh Thuy, Dang Thi Thu Hoai and Nguyen Huy Viet, Electronic Structure of Ideal N-doped TiO2 Films, Tuyển tập các báo cáo Hội nghị Vật lý chất rắn và Khoa học vật liệu toàn quốc lần thứ 8, 2013, pp. 152-156. 11. Nguyen Minh Thuy, Duong Quoc Van, Pham Van Hai and Le Thi Hong Hai, An Improvement of Photocatalyst of TiO2 and TiO2:0.5%V 4+ Nanoparticles: Experiment and Calculation, Proceeding of The 6th International Workshop on Advanced Materials Science and Nanotechnology (IWAMSN2012), 2012. 12. Duong Quoc Van, Nguyen Minh Thuy, Bui Thanh Liem and Nguyen Huy Viet, Investigation of Anatase TiO2 Properties Using Generalized Gradient Approximation, Advances in Optics, Photonics, Spectroscopy & Applications VII, 2012, pp. 247-252, ISSN 1589 – 4271. 13. Nguyen Minh Thuy, Duong Quoc Van, Nguyen Thanh Vinh, Nguyen Thi Thu Minh, Nguyen Hong Quan, Trinh Hai Dang and Le Thi Hong Hai, An Improvement of Photocatalyst of TiO2:V 4+ Nanoparticles, Advances in Optics, Photonics, Spectroscopy & Applications VII, 2012, pp. 378-384, ISSN 1589- 4271. 14. Nguyen Minh Thuy, Le Thi Hong Hai, Duong Quoc Van and Bui Thi Hau, A Visible Light Activity of TiO2 Based Photocatalysts, Journal of Science of HNUE, Vol. 56, No. 1, 2011, pp. 11-20. 15. Nguyen Cao Khang, Duong Quoc Van, Nguyen Minh Thuy, Nguyen Van Minh, Phan Ngoc Minh, Remarkably enhanced photocatalytic activity by sulfur-doped titanium dioxide in nanohybrids with carbon nanotubes, Journal of Physics and Chemistry of Solids, Vol. 99, pp. 119-123, 2016, doi: 10.1016/j.jpcs.2016.06.011. 16. Phung Thi Len, Nguyen Manh Nghia, Nguyen Cao Khang, Duong Quoc Van and Nguyen Thi Hue, Enhanced photocatalytic efficiency of TiO2 with doped Ni-immobilized on silica gel, Journal of Science of HNUE, Vol. 61, No. 7, 2016, pp. 151-156, doi: 10.18173/2354-1059.2016-0044. 17. Minh Thuy Nguyen, Tien Lam Pham, Minh Huong Nguyen, Quoc Van Duong and Tien Cuong Nguyen, A Study of CO Adsorption on the anatase TiO2 (001) Surface, Proceeding of the 2nd International Conference on Advanced Materials and Nanotechnology (ICAMN2014), 2014.