MINISTRY OF EDUCATION AND TRAINING
HANOI NATIONAL UNIVERSITY OF EDUCATION
----------o0o----------
PHAM KHAC VU
SYNTHESIS AND INVESTIGATION OF PHOTOCATALYTIC
PROPERTIES OF BiMO (M = V, Ti, Sn) MATERIALS
Specialization: Solid States Physics
Code: 9.44.01.04
PHYSICS DOCTORAL THESIS ABSTRACT
Hanoi 2020
THE WORK HAS BEEN COMPLETED AT
HANOI NATIONAL UNIVERSITY OF EDUCATION
Scientific Advisors
1. Assoc. Prof. Dr. Luc Huy Hoang
2. Assoc. Prof. Dr. Nguyen Van Hung
Reviewer 1: Prof. Dr. Vu Đinh Lam
Reviewer 2: Assoc. Prof. Dr. Tran Ngoc Khiem
Reviewer 3: Assoc. Prof. Dr. Nguyen Hoang Nam
The thesis will be denfended in front of Thesis Evaluation Council
at ……………………………………………………………..
on …………….h……….. , …….. / …….… / 2020.
Thesis can be found at
- National Library of Vietnam, Hanoi
- Library of Hanoi University of Education
LISTS OF PUBLICATIONS
1. Nguyen Dang Phu, Luc Huy Hoang, Pham Khac Vu, Meng-Hong Kong, Xiang-Bai Chen, Hua Chiang Wen, Wu Ching Chou. Control of crystal phase of BiVO4 nanoparticles synthesized by microwave assisted method, Journal of Materials Science: Materials in Electronics 27 (2016) 6452-6456. 2. Khac Vu Pham, Van Hung Nguyen, Dang Phu Nguyen, Danh Bich Do, Mai Oanh Le, and Huy Hoang Luc. Hydrothermal Synthesis, Photocatalytic Performance, and Phase Evolution from BiOCl to Bi2Ti2O7 in the Bi-Ti-Cl-O System, Journal of Electronic Materials 46 (2017) 6829-6833. 3. Nguyen Dang Phu, Pham Khac Vu, Dang Duc Dung, Do Danh Bich, Le Mai Oanh, Luc Huy Hoang, Nguyen Van Hung, Pham Van Hai. Temperature-dependent preparation of bismuth pyrostannate Bi2Sn2O7 and its photocatalytic characterization, Materials Chemistry and Physics 221 (2019) 197-202. 4. Phạm Khắc Vũ, Vũ Hoài Thương, Đặng Trung Đức, Nguyễn Đăng Phú, Lục Huy Hoàng và Nguyễn Văn Hùng. Tổng hợp vật liệu Bi2Sn2O7/CoFe2O4 bằng phương pháp hóa có hỗ trợ vi sóng và hoạt tính quang xúc tác, JOURNAL OF SCIENCE OF HNUE 62 (2017) 3-9. 5. Phạm Khắc Vũ, Nguyễn Văn Hùng và Lục Huy Hoàng. Ảnh hưởng của độ pH lên cấu trúc và tính chất của hạt nano Bi2Sn2O7 bằng phương pháp hóa có hỗ trợ vi sóng, Kỷ yếu Hội nghị Vật lý Chất rắn và Khoa học Vật liệu Toàn quốc – SPMS 2017 (2017) 404-407.
1 INTRODUCTION
Environmental and water pollution has been becoming a global problem. Finding the solutions to deal with environmental and water pollution has been an urgent need nowadays. By using traditional methods, only organic matter in wastewater is collected and a secondary amount of wastewater is also produced. The use of semiconductors under the influence of light to accelerate the process of decomposition of organic compounds through photocatalytic effect has many advantages such as simplicity, low cost, durability and the final products of the degradation process are CO2, H2O...., which are non-toxic substances. Therefore, methods of using photocatalytic materials for handling water pollution is a radical and environmentally friendly solution.
Family of BiMO (M = V,Ti,Sn), including Bi2M2O7 (M = Ti,Sn) and BiVO4 have been investigated so far. The published results showed that the materials of the BiMO family (M = V,Ti,Sn) with narrow band gap, are capable of photocatalysis under the irradiation of visible light. Furthermore, magnetic composite materials such as TiO2/CoFe2O4, Bi2WO6/CoFe2O4, ZnFe2O4/Bi2WO6 were also studied. The family of magnetic heterophobic materials based on Bi2M2O7 not only increases the photocatalytic efficiency by extending the lifetime of the electron-hole pair, but also can be recovered after the photocatalytic reaction by using external magnetic fields. In Vietnam, the family of BiMO (M = V,Ti,Sn) materials has not been developed yet, so there are NOT any publications of their photocatalytic properties. For these reasons, the subject of this thesis is "Synthesis and investigation of photocatalytic properties of BiMO (M = V, Ti, Sn) materials''. The purpose of this thesis is to study and synthesize BiMO materials (M = V,Ti,Sn) by microwave- assisted chemical method, hydrothermal method, sol-gel method and evaluate their physical properties. The synthesized materials (Bi2Sn2O7 and Bi2Ti2O7) were investigated the photocatalytic activity in the decomposition of RhotaminB (RhB) under visible light. The combination of Bi2Sn2O7 material with CoFe2O4 material was also studied in order to recover used materials for recycling. Moveover, the thesis proposes a technological process to control the formation of the structural phase of BiVO4 material.
Main contents of this thesis include: 1. Preparation of BiVO4 material, study on the effects of technological conditions on the structure, surface morphology, photocatalytic properties of the material; studying on controlling the form and the phase transfer of BiVO4 material.
2. Preparation of Bi2Ti2O7 material, study on the effects of technological conditions on the structure, surface morphology, photocatalytic properties and photocatalytic activities of the material; study on improving the photocatalytic activities of Bi2Ti2O7
3. Preparation of Bi2Sn2O7 nanoparticles; study on the effects of technological conditions on the structure, surface morphology, photocatalytic properties and photocatalytic activities of this material . Combination of Bi2Sn2O7 and CoFe2O4 magnetic materials for photocatalysts recovery.
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Scientific significance of thesis
Environmental pollution is a very serious issue which is a threat to many countries. The researches in preparation for the nanometer-scale photocatalytic materials BiMO (M = V,Ti,Sn) have great practical significance in water pollution treatment. The Bi2Sn2O7 nanoparticles exhibit highly photocatalytic activity in the decomposition of toxic, non-biodegradable organic substances. The combination of photocatalyst Bi2Sn2O7 material with CoFe2O4 material is significant in the recovery of used materials , avoiding secondary pollution. The introduction of a technological process that controls the formation and transition of the BiVO4 material is important in preparing materials for photocatalytic activity of the materials. Therefore, the results of the thesis are highly important not only in basic science but also in oriented application. Thesis content
This thesis includes 122 pages (appendix: 97 figures and 23 tables; and references are not counted), which are divided into 7 main parts (introduction, conclusion and 5 chapters)
Chapter 1: Introduce the structure, physical properties and photocatalytic activity of BiMO materials (M = V, Ti, Sn). Method for recovery of photocatalyst materials by combining with the magnetic materials to avoid the secondary pollution by the secondary substances after the use of photocatalyst materials is also introduced. Furthermore, improvement of photocatalytic activity by creating a heterojunction between two kinds of semiconductors is investigated. Chapter 2: Describe the experimental techniques used in the thesis, the experimental designs and the analytical measurements used in the research. Chapter 3: Present the results of studying the effects of technological conditions on the structure, surface morphology and optical properties of BiVO4 materials. Present the technological process to control the phase transition of the materials. Chapter 4: Present the results of studying the effect of technological conditions on structure, surface morphology, optical properties, photocatalytic activity of Bi2Ti2O7 materials. Present the method for enhancing photocatalytic activity of Bi2Ti2O7 material by combining with BiOCl material. Chapter 5: Describe the results of studying the effect of technological conditions on the structure, surface morphology, optical properties, photocatalytic activity of Bi2Sn2O7 material. The results of the combination between Bi2Sn2O7 and CoFe2O4 materials for material recovery are also presented in this chapter.
3 CHAPTER 1. OVERVIEW
1.1. Overview of Bi2Sn2O7 1.1.1. Crystal structures of Bi2Sn2O7
Pyrochlore is a fascinating mineral group having a general formula of A2B2O7, where larger cations A are in the groups 1-3 of the periodic table and B is usually transition metal ion. This structure can be interpreted as an anion-deficient derivative of fluorite structure type, with cation dislocation in combination with a relatively small displacement relative to the ideal crystal. The results are often described as two- layered oxide domains cross-sectional with a total of A2B2O6O’. The binary tungsten B2O6 components consist of the octahedral BO6 bond in three-dimensional space, with B lying in the octahedral center, surrounded by six atoms of O located at the octahedral vertices. A2O’ has O’-A-O’ straight links and anion O' appears in the tetrahedral domain. The length of the linkage A-O’ is quite short, about 2.2 to 2.3 Å, similar to the arrangement found in Cu2O. A cation A is within six atoms and forms bonds to the O2- anions, as well as two shorter bonds formed with the anions O’ perpendicular to the ring, leading to an octahedral bond distortion (Fig.1.1a).
Figure 1.1. (a) The ideal pyrochlores lattice structure along the axis (1,1,0) of
the BO6 octahedra and the conductive cations A, (b) The phase diagram of the
α-Bi2Sn2O7 phase, where Bi: Yellow, Sn: Black, O': Red and O at the corners of
the red octahedron.
Bi2Sn2O7 is a semiconductor of the Pyrochlores family (Fig. 1.1b). The
crystal structure exists in three main phases: α-Bi2Sn2O7 has a monoclinic structure
with lattice space a = 13.0502 Å; b = 15.0545 Å and c = 21.5114 Å, β = 90.038 o; β-
Bi2Sn2O7 has a facial cube structure with lattice space a = 21.4 Å; γ-Bi2Sn2O7 has a
cubic pyrochlores structure with lattice space a = 10.73 Å. Bi2Sn2O7 forms phase
structures at different temperatures, the α-Bi2Sn2O7 phase with a monoclinic structure
is formed at room temperature (T < 410 K), at a temperature of about 410 K, the β-
Bi2Sn2O7 phase appearing with the face centered cubic structure (410 K
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According to the theoretical calculation, Bi2Sn2O7 is an indirect semiconductor
1.1.2. Optical properties of Bi2Sn2O7 1.1.2.1. Electronic structure of Bi2Sn2O7 and has a band gap of 2.68 eV. 1.1.2.2. Absorption spectrum of Bi2Sn2O7 In published reports, the absorbance band of Bi2SnO2O7 material was within the visible light range, the band gap was 2.78 eV and 2.88 eV. In another study, the band gap of Bi2Sn2O7 material was between 2.362 eV and 2.462 eV. 1.1.3. Photocatalyst activities of Bi2Sn2O7 Recent results show that Bi2Sn2O7 material is capable of photocatalytic decomposition of Methyl orange (MO), Methylene blue (MB) and As (III) under the effect of visible light. 1.1.4. Studies to enhance the photocatalytic potential of the material Bi2Sn2O7 Studies to improve the photocatalytic potential of the Bi2Sn2O7 material: using the CTAB surface activator for the preparation of Bi2Sn2O7 (C-BSO), preparation of the Bi2Sn2O7-TiO2 composite materials, dispersing Bi2Sn2O7 onto the surface of Reduced graphene oxide (RGO) material. The results showed that the photocatalytic activity of the composite material was higher than that of the Bi2Sn2O7 material. 1.1.5. Studies to recover Bi2Sn2O7 Nano-scale photocatalyst materials are easily dispersed into the water, which makes the decomposition of organic substances more effective. However, after photocatalytic reactions, the separation of the catalysts from water becomes difficult, which can cause secondary environmental pollution . Moreover, the catalyst recovery issue has a great significance for reuse. One of the most efficient, environmental- friendly methods of recovery is the use of magnetic catalysts, which after the reaction, can easily be recovered through an external magnetic field. In recent publications, TiO2/CoFe2O4, Bi2WO6/CoFe2O4 and ZnFe2O4/Bi2WO6 composite materials have been successfully prepared by combining the advantage of high photocatalytic activity in the decomposition of methylene blue (MB) and Rhodamine B (RhB) with convenient magnetic retrieval. 1.2. Overview of Bi2Ti2O7 materials 1.2.1. Crystalline structure of Bi2Ti2O7
Bi2Ti2O7 material is a semiconductor of the Pyrochlores family, having a cube structure, belonging to the space group Fd3m, the crystal lattice constant is about a = 10.4 Å at ambient temperature. Bi2Ti2O7 material is unstable at high temperature, switched to Aurivillius Bi4Ti3O12 phase at temperatures above 743 K. The monocrystals Bi2Ti2O7 were diluted with double unit cells with a facial cube structure and a lattice spacing of 20.68 Å. 1.2.2. Photocatalytic properties of Bi2Ti2O7 1.2.2.1. Electronic structure of Bi2Ti2O7
A theoretical calculation showed that Bi2Ti2O7 material is an indirect semiconductor with a band gap of 2.46 eV. Another theoretical calculation showed that Bi2Ti2O7 material is a direct semiconductor with a band gap of 2.6 eV. 1.2.2.2. Absorption spectra of Bi2Ti2O7 Experimental results showed that Bi2Ti2O7 is a semiconductor having the absorption edge in the visible region. The band gap of Bi2Ti2O7 material obtained from the experiment was 2.95 eV; 2.9 eV; 2.74 eV and 2.8 eV. 1.2.3. Photocatalytic properties of Bi2Ti2O7 Recent results show that Bi2Ti2O7 is capable of photocatalytic decomposition of Methylene blue (MB), Rhodamine B (RhB) and Methyl orange (MO) under the
5 irradiation of visible light and ultraviolet light. Accordingly, the photocatalytic capacity of Bi2Ti2O7 material is much higher than that of TiO2. 1.2.4. Studies to enhance the photocatalytic potential of the material Bi2Ti2O7 In order to enhance the photocatalytic activity of Bi2Ti2O7 material under solar radiation, some studies have adjusted the fabrication technology to reduce the band gap of the material. Previous studies showed that Bi2Ti2O7 materials which have mixed with Mn, Fe shifted the absorption edge to red light, thereby improving photocatalytic activity of the material.
Other studies have combined the Bi2Ti2O7 material with another semiconductor to form a heterogeneous composite material. Some of the materials combined with Bi2Ti2O7 improving photocatalytic activity have been published, such as: Reduced graphene oxide (RGO), TiO2, BiVO4 and Bi4Ti3O12. 1.2.5. Discussion of photocatalysis mechanism
When illuminated by appropriate light with photon energy greater than or equal to the band gap of Bi2Ti2O7 material, the electrons in the valence band stay at Bi 6s and O 2p state absorbing the photon energy of the incoming lights and transfer to the conduction band of Ti 3d state. In the valence band, the holes are positive (h+) and in the conduction band, there are negative electrons (e -). Electrons-Holes move freely to the surface or to the contact of photocatalyst material, where e- was trapped by oxygen , while the hole h+ interacts with molecules dissolved in the medium to form H2O/OH- on the surface to form OH•. acts as a strong reducing agent and OH• acts as a strong oxidizer, which interacts to decompose organic compounds in the environment. 1.3. Overview of BiVO4 materials 1.3.1.Crystalline structure of BiVO4 materials
The properties of BiVO4 are strongly dependent on many types of crystal structures, including scheelite or zircon crystal structures. The scheelite structure can have a tetragonal structure (space group: I41/a with a = b = 5.1470 Å; c = 11.7216 Å) or monoclinic (space group: I2/b with a = 5.1935 Å; b = 5.0898 Å; c = 11.6972 Å, and β = 90.3871 o), also known as clinobisvanite. The zircon crystal is a tetragonal crystal structure with space group I41/a with a = b = 7.303 Å and c = 6.584 Å. Figure 1.2B describes the monoclinic BiVO4 consisting of a VO4 tetrahedron and a BiO8 dodecahedron. Four O atoms surround V and eight O atoms surround Bi. The tetrahedron of VO4 connects to BiO8 by sharing an O atom at the top. Bi and V atoms are arranged alternately along the crystal axis, making the monoclinic BiVO4 exhibit the properties of the layered structure.
Figure 1.2. (A) Energy band diagram for (t-z) BiVO4 and (m-s) BiVO4; B) The crystal structure of monoclinic clinobisvanite BiVO4. The corresponding polyhedron structure is represented in (B) where, VO4 tetrahedron in orange, and BiO8 dodecahedron in blue.
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1.3.2. Structural variation of BiVO4
The scheelite structure of monoclinic BiVO4 has 15 optical phonons: 3Ag + 4Au + 6Bg + 4Bu, of which 9 variations (3Ag + 6Bg) are Raman active, 8 variations (4Au + 4Bu) are infrared active.
1.3.3. Optical properties of BiVO4
As mentioned above, BiVO4 has three types of structures: monoclinic scheelite (s-m), tetragonal scheelite (s-t) and tetragonal zircon (z-t). The tetragonal structure of BiVO4 has a band gap of 2.9 eV which mainly absorbs ultraviolet light. The scheelite structure of monoclinic BiVO4 has a band gap of 2.4 eV that mainly absorbs visible light. Ultraviolet light absorption for both tetragonal and monoclinic structures of BiVO4 involves the transfer of electrons from the O 2p state in the valence band to the V 3d state of the conduction band (Figure 1.38A). Meanwhile, visible light absorption is due to the transfer of electrons from the Bi 6s state or from the hybrid state between Bi 6s and O 2p in the valence band to the V 3d state of the conduction band. In addition, the Bi-O bond in the monoclinic structure of BiVO4 is distorted, which increases the electron division and the hole generated during the illumination.
1.3.4. Photocatalytic properties of BiVO4 material
BiVO4 nanoparticles have an absorption band in the visible light region, so the application of materials in the decomposition of organic matter under sunlight is of concern. Recent results show that the Bi2SnO2O7 material is capable of photocatalytic decomposition of Methylene blue (MB) and Rhodamine B (RhB) under the effect of visible light.
CHAPTER 2. EXPERIMENTAL TECHNIQUES
2.1. Procedure and method of preparation of materials
In order to prepare the proposed objectives, three methods (microwave-assisted chemical method, hydrothermal method and sol-gel method) were used in this thesis.
2.1.1. Preparation of Bi2Sn2O7 nanoparticles using microwave-assisted chemical method
The use of microwave-assisted method in synthesis has many advantages, such as: the reaction occurs faster due to the stabilization of the bulk temperature be reached during microwave processing; higher selectivity due to shorter reaction time; fewer side reactions; uniform nanoparticle size is possible to obtain; friendly with environment and large quantities of products could be reached. The process of manufacturing Bi2Sn2O7 by microwave-assisted chemical method is presented in Figure 2.1
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Figure 2.6. Preparation of BiVO4 nano- particles by microwave-assisted chemical method.
Figure 2.1. Preparation of Bi2Sn2O7 nano-particles by microwave-assisted chemical method. 2.1.2. Preparation of composite materials Bi2Sn2O7/CoFe2O4
The Bi2Sn2O7/CoFe2O4 composite material was prepared by microwave assisted chemical methods in two steps: (i) Preparation of magnetic materials CoFe2O4, (ii) Preparation of composite materials Bi2Sn2O7/CoFe2O4. The process of preparing composite materials Bi2Sn2O7/CoFe2O4 is shown in Figure 2.2 and Figure 2.3.
2.3. Preparation
Figure 2.2. Preparation of CoFe2O4 by the microwave-assisted chemical method. of the microwave-
Figure Bi2Sn2O7/CoFe2O4 by assisted chemical method.
2.1.3. Preparation of Bi2Ti2O7 nanoparticles Preparation of Bi2Ti2O7 nano-particles by the sol-gel method
Sol-gel method is the method to prepare nanoparticles based on the chemical reactions occuring at low temperatures, which is suitable for the preparation of
8 samples in powders or films, and also suitable for the working conditions in the laboratories in Vietnam. The preparation of Bi2Ti2O7 was presented in Figure 2.4.
Figure 2.5. Preparation of Bi2Ti2O7 by hydrothermal method.
Figure 2.4. Preparation of Bi2Ti2O7 by sol-gel method. Preparation of Bi2Ti2O7 nano-particles by the hydrothermal method
Hydrothermal method is one of the most effective methods to prepare small- sized materials. This method saving energy, is not toxic to the environment due to the reaction carried out in a tight/closed system. The process of preparation of Bi2Ti2O7 using the hydrothermal method is presented in Figure 2.5 2.1.4. Preparation of BiVO4 nanoparticles using microwave-assisted chemical method
Similar to the process of the preparation of Bi2Sn2O7 nanoparticles by microwave-
assisted method, the process of preparation of BiVO4 was presented in Figure 2.6
Figure 2.6. Preparation of BiVO4 nanoparticles by microwave-assisted method.
2.2. Equipments and techniques used to analyze sample features
Sample properties described thesis were in
investigated by basic the measurements, such as: SEM, TEM, HR-TEM, DTA và TGA, XRD, UV-Vis, Raman, EDX, BET, PL, M-H. Photocatalytic activities of samples were evaluated by using Rhodamine B solution as a test contaminant. Concentration of samples at initial time and after irradiation time were determined by UV-Vis measurement.
9 CHAPTER 3. SYNTHESIS AND INVESTIGATION OF THE PROPERTIES OF BiVO4 MATERIALS
Figure 3.1 presented the XRD patterns of the BiVO4 samples prepared at
3.1. Influence of the synthetic conditions on the properties of the materials 3.1.1. XRD analysis of the samples prepared at different pH different pH of the precursor solution
Fig. 3.1. XRD patterns of the BiVO4 samples prepared at different pH of the precursor solution. Fig. 3.2. SEM images of the BiVO4 samples prepared at different pH (a) pH = 7, (b) pH = 5, (c) pH = 3.
XRD patterns of Fig. 3.1 show that the BiVO4 samples prepared at pH = 7 and pH = 11 crystallize as two-phase structure tetragonal-zircon (z-t) and monoclinic - scheelite BiVO4 (s-m). The BiVO4 samples prepared at pH = 3 and pH = 5 exist in single-phase structure z-t BiVO4. The average crystallite size of the BiVO4 samples at pH = 3 and pH = 5 was calculated to be 41 and 42 nm, respectively. Hence, the solution pH has a strong effect on the crystallization and phase formation of the materials. 3.1.2. SEM analysis of the samples preprared at different pH
SEM images of the samples at different pH (Fig. 3.2) reveal that the BiVO4 samples prepared at pH = 7 and pH = 5 comprise nanoparticles with average size of dozens nm. However, for the ones at pH = 3, the surface microsphere is composed of spherical micrometer-sized and relatively uniform grains that consisted of nanometer clusters. This result implies the pH of the precursor solution has a significant impact on the morphology of BiVO4 nanoparticles. 3.1.3. UV-vis analysis of the samples prepared at different pH
Figure 3.3 shows the typical UV-Vis diffuse reflectance spectra and plot of transformed KM function for the BiVO4 samples prepared at different pH of the precursor solution. As illustrated in spectra, for the samples at pH = 7 and pH = 11, the absorption curves show two clear absorption edges; meanwhile only absorption
10 edge is observed for the ones at pH = 3 and pH = 5. All absorption edges occur at wavelengths in the visible light range.
at
Fig. 3.3. UV- Vis DRS (a) and plot of transformed KM function (b) of the BiVO4 samples preprared different pH.
3.2. Influence of the temperature on the properties of the materials 3.2.1. XRD analysis of the samples annealed at various temperatures Figure 3.4 shows the XRD patterns of the BiVO4 samples annealed at different temperatures. The samples annealed above 350 oC crystallize as single-phase structure s-m BiVO4. The samples annealed in the range of 200 oC to 325 oC have heterogeneous structures which are the composition of two phases z-t and s-m. The percentage of s-m phase in the sample can be determined approximately based on the relative proportion of intensity between characteristic peak (121) of s-m phase and characteristic peak (200) of z-t phase. We estimated that the percentage of s-m phase is 3, 8 and 60 % in the samples annealed at 200, 300 and 325 oC, respectively (Table 3.4).
Fig. 3.5. Raman scattering spectra of the BiVO4 samples as-prepared and annealed at different temperatures.
Fig. 3.4. XRD patterns of the BiVO4 samples as-prepared and annealed at different temperatures.
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Table 3.4. The effect of annealing temperature on the crystallinity and line width of nano-BiVO4 particles.
Sample Annealing temp. (oC) Eg, z-t phase (eV) Eg, s-m phase (eV)
Percentage of s-m phase (%) 0 3 8 60 100 100 100 non 200 300 325 350 375 400 - 2,51 2,45 2,42 2,40 2,40 2,40 2,85 2,83 2,75 2,74 - - -
implying an improvement improved,
As-prepared BVO 200 BVO 300 BVO 325 BVO 350 BVO 375 BVO 400 3.2.2. Raman scattering spectra analysis of the samples annealed at various temperatures Figure 3.5 demonstrates the Raman scattering spectra of the BiVO4 as-prepared sample and annealed samples at different temperatures. In the spectrum of as-prepared samples, four Raman scattering peaks corresponding to the characteristic vibrations of BiVO4 z-t phase are observed. For the nanoparticles annealed above 350 oC, these peaks disappear, while other four intensive peaks corresponding to the characteristic vibrations of BiVO4 s-m phase are detected. Particularly, for the nanoparticles annealed at 400 oC, beside those intensive peaks, a weak one was also inspected at the weight length of 709 cm-1, corresponding to the vibration of s-m phase as well. Other evidence on Raman spectra also reveals that the unannealed samples have single phase z-t crystalline, while the samples annealed above 350 oC crystallize as single phase s-m and especially at 400 oC, highly pure crystalline are obtained. These results from Raman spectra are in agreement with the conclusion from XRD experiments. For the as-prepared sample, a Raman weak-intensity peak is observed at the weight length of 200 cm-1, when the annealing temperature increases, intensity of this peak increases dramatically and shifts to the high energy field, then reaches the weight length of 210 cm-1. The Raman spectra experiments show that the as-prepared sample with s-m phase percentage below 3% can be detected by Raman scattering spectra, whereas XRD spectra could not. 3.2.3. SEM image and HRTEM analysis of the samples annealed at various temperatures Figure 3.6a-c demonstrates the FESEM images of the samples as-prepared and annealed at 325 oC and at 400 oC. FESEM images show that nano-BiVO4 particles exist as spherical grains of hundreds nm. When the samples were annealed at 400 oC, the morphology was significantly in crystallinity. The heterogeneous transition layer between two phases BiVO4 were also inspected via high-resolution transmission electron microscopy (HRTEM), which is shown in Fig. 3.6d. The formation of heterogeneous structure between two phases provides a huge advantage in separating the transmission particles, hence improving the photocatalytic activity of the materials.
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Fig. 3.7. UV-Vis (a) and plot of transformed KM function (b) of the BiVO4 samples as-prepared and annealed at different temperatures.
Fig. 3.6. FESEM image of as- prepared BiVO4 sample (a), annealed at 325 oC (b), at 400 oC (c), HRTEM image of the sample annealed at 325 oC (d). 3.2.4. UV-vis analysis of the samples annealed at various temperatures
Figure 3.7a shows the UV-Vis spectra of the samples as-prepared and annealed at the various temperatures. The experiments reveal that t as-prepared he UV-Vis spectra varied regulatorily in accordance with the phase transition from z-t BiVO4 phase to s-m BiVO4 phase. Figure 3.7b is the plot of transformed KM function of the BiVO4 samples, allowing to determine the forbidden band gap Eg of the materials which are shown in Table 3.4.
The electronic structure of the semiconductor plays an important role in the photocatalytic activity of the materials. The absorption spectra of the material demonstrated that the electron structure of the BiVO4 nanoparticles was gradually modified by annealing. In addition, the absorption spectrum showed that with increasing annealing temperature, the absorption from the BiVO4 s-m phase was accelerated at the annealing temperatures of 200 and 300 ° C, very different from those obtained from XRD and Raman. This indicates that with the BiVO4 heterologous structure, its electronic structure is varied by the interaction between the two phases z -t BiVO4 and s-m BiVO4. This is consistent with the results from the FESEM and HRTEM images, suggesting that a heterogeneous structure is formed between the two phases z -t BiVO4 and s-m BiVO4 at nanoscale. The electronic structure change of BiVO4 nanoparticles is important for studying and enhancing photocatalytic activity of the materials. The photocatalytic activity of BiVO4 material will be investigated in subsequent studies.
13 CHƯƠNG 4. SYNTHESIS VÀ STUDYING ON THE PHOTOCATALYTIC MATERIAL Bi2Ti2O7
4.1. Bi2Ti2O7 material synthesized by by sol-gel method 4.1.1. Study on the structure of Bi2Ti2O7
The X-ray diffraction patterns of the Bi2Ti2O7 samples (BTO) prepared by sol-
gel method after annealing at different temperatures is shown in Figure 4.1.
annealed at
Fig. 4.1. XRD patterns of the Bi2Ti2O7 samples different temperatures.
Fig. 4.2. SEM images of the Bi2Ti2O7 samples are annealed at different temperatures: (a) 400 oC, (b) 500 oC, (c) 600 oC và (d) 700 oC.
The results obtained in Figure 4.1 show that the sample before incubation appears only at diffraction peaks of the Bi crystals. At annealing temperature of 400 °C, the diffraction peaks of the Bi2Ti2O7 cubic crystals, BiOCl crystals and Bi4Ti3O12 orthorhombic crystals appear. At annealing temperature of 500 °C, the diffraction peaks of the crystalline Bi2Ti2O7 prevailed, however, diffraction peaks of the BiOCl crystals were still present. When the annealing temperature is 600 °C, the samples crystallized as single-phase cubic Bi2Ti2O7, belonging to the space group Fd3m. However, when the annealing temperature is 700 °C, apart from the cubic crystalline Bi2Ti2O7, there is also a monoclinic crystal Bi4Ti3O12. The average crystalline grain size of Bi2Ti2O7 samples annealed at different temperatures increases along with the increasing of the temperature, varying from 19 nm to 29 nm. SEM images of Bi2Ti2O7 samples annealed at different temperatures are shown in Figure 4.2. The results of the SEM imaging show that the particles are nanometer- sized, have a spherical shape and are relatively uniform in shape. The average particle size of the samples annealed at 400 °C and 500 °C is 40 nm and 50 nm, respectively. Annealed at 600 °C, the particle size varies from 50 nm to 120 nm, when the annealing temperature is 700 °C, particle size is in the range of 100 nm to 170 nm. 4.1.2. Study on the physical properties of Bi2Ti2O7 material The Bi2Ti2O7 samples incubated at different temperatures had a clear absorption edge and the absorption edge of each sample was within the visible range, the band gap values of the samples ranged from 2.71 eV to 2.88 eV.
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4.2. Bi2Ti2O7 material prepared by hydrothermal method 4.2.1. The effect of pH on the properties of Bi2Ti2O7 material
Bi2Ti2O7 nanoparticles were prepared by hydrothermal method at the pH of the precursor solution varying from 3 to 11, hydrothermal temperature of 180 °C and for 12 hours, the product was hydrothermally dried and annealed at 600 °C for 6 hours. The X-ray diffraction patterns of the samples are shown in Figure 4.4.
Fig. 4.5. SEM images of the Bi2Ti2O7 samples prepared at different pH: (a) pH = 5, (b) pH = 11.
Fig. 4.4. XRD patterns of the Bi2Ti2O7 samples prepared at different pH of the precursor. The results obtained in Figure 4.4 show that at the pH of the solution from 3 to 9, the materials existed as the composite of the two phases Bi2Ti4O11 and the Bi2Ti2O7. When the pH of the precursor solution is 11, the materials crystallized as single-phase cubic Pyrochlore Bi2Ti2O7 form. Figure 4.5 shows that, when the pH of the precursor solution was 5, the Bi2Ti2O7 material was predominantly stick-shaped with a width of 20 nm to 30 nm and fairly evenly distributed. When the pH of the precursor solution was 11, the Bi2Ti2O7 material had a spherical shape of about 30 nm to 50 nm. The Bi2Ti2O7 samples prepared at different pH values had a clear absorption edge and absorption edges were within the visible range, the band gap values of the samples were varied from 2.63 eV to 2.89 eV. 4.2.2. The effect of the annealing conditions the properties of Bi2Ti2O7 material Bi2Ti2O7 nanoparticles were prepared by hydrothermal method with the pH value of 11 of the precursor solution kept constant, however, after hydrothermal drying the products were annealed from 400 to 600 oC. The X-ray diffraction patterns of the Bi2Ti2O7 samples annealed at different temperatures is shown in Figure 4.7. The results in Figure 4.7 show that the unannealed samples exhibited diffraction peaks of BiOCl tetragonal phase. When the samples were annealed at 400 oC and 500 oC, the material existed as a composite of BiOCl and Bi2Ti2O7. When the annealing temperature is 600 oC, it is crystallized as single-phase cubic Pyrochlore Bi2Ti2O7.
15
Fig. 4.7. XRD of the Bi2Ti2O7 samples as-prepared by hydrothermal method and annealed at different temperatures. Fig. 4.8. FE-SEM images of as-prepared sample (a) and the samples annealed at: 400 oC (b), 500 oC (c) và 600 oC (d).
Figure 4.8 shows that as the annealing temperature increases, the average particle size of the Bi2Ti2O7 increases. For the as-prepared sample, the sample annealed at 400 oC, 500 oC and 600 oC; the average particle size is 15 nm; 30 nm; 40 nm and 45 nm, respectively.
The isothermal nitrogen adsorption /desorption measurements to determine the specific surface area of the material were carried out, the results are shown in Table 4.5. The results showed that the as-prepared sample had the highest surface area, when the annealing temperature increased, the surface area decreased. Table 4.5. The specific surface area of the as-prepared sample and the annealed samples at different temperatures.
Sample As-prepared BTO HT400 BTO HT500 BTO HT600
66.2 52.9 22.4 13.1 Specific Surface area (m2/g)
Absorption spectra of as-prepared sample and annealed samples at different temperatures are shown in Figure 4.9a. As-prepared sample has an absorption edge of below 400 nm. The absorption edge of Bi2Ti2O7 material is about 455 nm, implying that the material is capable of well absorbing visible light. The absorption edge of the annealed samples at 400 oC and 500 oC was shifted towards the red light compared to the as-prepared sample and annealed samples at 600 oC. Figure 4.9b is the plot of transformed KM function from absorption spectra of the as-prepared Bi2Ti2O7 sample and the samples annealed at different temperatures. The band gap values of as- prepared sample and annealed samples at 400 oC, 500 oC and 600 oC are 2.81 eV; 2.57 eV; 2.57 eV and 2.63 eV, respectively. Thus, the Bi2Ti2O7/BiOCl composite materials have the smallest band gap, which suggests that they would have high photocatalytic activity in the visible light region.
16
(b) of
Fig. 4.9. UV-Vis (a) of and plot KM transformed function the Bi2Ti2O7 as-prepared sample and annealed samples at different temperatures 400 oC, 500 oC and 600 oC.
4.3. The effect of technological conditions and synthetic method on the photocatalytic activity of Bi2Ti2O7 material 4.3.1. The effect of the annealing temperatures on the photocatalytic activity of Bi2Ti2O7 material prepared by sol-gel method The photocatalytic activity of Bi2Ti2O7 material prepared by sol-gel method was evaluated by the decrease of RhB concentration over time when illuminated. Results showed that the Bi2Ti2O7 sample was prepared with annealing temperature at 400 °C afforded the best photocatalytic activity, after 180 minutes of irradiation, almost RhB was degraded completely. The slope value k’ of the photocatalytic degradation of RhB with the presence of Bi2Ti2O7 annealed at 400 °C was the largest (k’ = 0.024) and the smallest for the single-phase Bi2Ti2O7 crystalline annealed at 600 °C (k’= 0.008). 4.3.2. The effect of pH values on the photocatalytic activity of Bi2Ti2O7 material prepared by hydrothermal method Figure 4.10 shows that the Bi2Ti2O7 samples prepared at pH = 7 for the precursor solution resulted in the best photocatalytic activity; after 180 minutes of irradiation, almost RhB was degraded completely, the slope value k’ of the photocatalytic degradation of RhB with the presence of Bi2Ti2O7 synthesized at pH = 7 is the largest (k’= 0.028) and the smallest for single-phase Bi2Ti2O7 crystalline prepared at pH = 11 (k’ = 0.006)
Fig. 4.10. Plots demonstrating the change of the intensity of absorption peak at 554 nm over time of RhB solution under the RhB-degradative photocatalytic activity of Bi2Ti2O7 materials which were synthesized by hydrothermal method at different pH values (a) and the change of RhB concentration during the photocatalysis of Bi2Ti2O7 according to Langmuir-Hinshelwood model.
17 4.3.3. The effect of the annealing temperature on the photocatalytic activity of Bi2Ti2O7 material synthesized by hydrothermal method
Figure 4.11 shows that photocatalytic activity of the samples annealed at 400 °C is the highest, after 120 minutes of irradiation by Xenon lamp, up to 95 % of the RhB concentration was degraded, the constant rate of photocatalytic reactions of the Bi2Ti2O7/BiOCl composite is 2.5 and 2 times greater than that of BiOCl and Bi2Ti2O7, respectively.
layer effectively suppresses transition
Fig. 4.11. (a) The plot demonstrating the decreasing of RhB concentration caused by visible light and by the photocatalytic activity of the as-prepared sample and annealed samples at 400 oC, 500 oC and 600 oC; (b) The plot demonstrating the decreasing of RhB concentration due to the photocatalytic activity of the as-prepared sample and annealed samples at 400 oC, 500 oC and 600 oC according to langmuir- Hinshelwood model. Bi2Ti2O7/BiOCl composite material exhibits a broader absorption band than that of Bi2Ti2O7 material, which results in the enhancement of photocatalytic activity in the visible light region of the composite material. Furthermore, when Bi2Ti2O7 and BiOCl are close together a heterogeneous structure is formed (Figure 4.12). When the heterogeneous transition layer is illuminated by visible light, both Bi2Ti2O7 and BiOCl semiconductors are excited and produce electron-hole pairs. Due to the overlapping band structure, the photons on CB of Bi2Ti2O7 can be easily migrated to the BiOCl (CB), and reversely, the BiOCl (VB) hole can move freely to the VB of the Bi2Ti2O7. This heterogeneous recombination of the charge, and thus increases the photocatalytic activity.
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Fig. 4.12. Schematic diagram of the mechanism of enhanced photocatalytic activity of the Bi2Ti2O7/BiOCl composite. Fig. 4.13. Photoluminescence spectra of the samples: (a) the pure Bi2Ti2O7 sample (annealed at 600 oC), (b) the Bi2Ti2O7/BiOCl composite (annealed at 400 oC).
To determine that the electron-hole pair separation as a photocatalytic mechanism, the photoluminescence spectrum (PL) was characterized by Bi2Ti2O7 material and Bi2Ti2O7/BiOCl composite material. The results shown in Figure 4.13 indicate that the photoluminescence intensity of the Bi2Ti2O7/BiOCl composite material is significantly lower than that of the pure Bi2TiO7 material, which suggests that the combination of Bi2TiO7 and BiOCl facilitates the electron-hole pair separation, resulting in enhanced photocatalytic activity.
CHAPTER 5. RESULTS AND STUDIES ON PHOTOCATALYST MATERIALS Bi2Sn2O7
5.1. The effects of technological conditions on the properties of materials 5.1.1.The effects of pH
In order to investigate the crystallization process of prepared Bi2Sn2O7, the analytic results of DTA and TGA measurements are shown in Figure 5.1. Accordingly, at a temperature of 345 °C there is a rather high thermal absorption peak, which is the phase-forming temperature of Bi2Sn2O7 material. Thus, under atmospheric pressure, the structural phase transition temperature of the Bi2Sn2O7 must be greater than 345 °C.
Figure 5.1. DTA curve and TGA curve of prepared Bi2Sn2O7 at heating rate of Figure 5.2. X-ray diffraction spectrum of Bi2Sn2O7 materials prepared at different pH.
19
10∘C.min−1.
X-ray diffraction patterns of Bi2Sn2O7 samples prepared from different pH of precursor solutions and then incubated at 700 °C are shown in Figure 5.2. Accordingly, the samples which were prepared at pH = 11 and 12 crystallized under single crystalline form of Bi2Sn2O7, in cubic Pyrochlore structure, belonging to space group Fd3m. Meanwhile, samples were prepared at pH = 3 to 9 crystallized under non-monomeric crystals. Thus, the pH of the solution strongly influences the phase formation of the Bi2SnO2O7 material.
Bi2Sn2O7 material was prepared with a pH-adjusted precursor solution consisting of medium sized spherical particles of 30 to 50 nm, distributed uniformly. Samples were prepared at different pH having similar morphology. The results showed that the pH of the precursor solution did not significantly affect the morphology of the Bi2Sn2O7 material.
Bi2Sn2O7 samples all had a clear absorption edge, which was in the visible light
region, the values of the band gap of the samples ranged from 2.67 eV to 2.74 eV. 5.1.2. The effect of the annealing temperature on the structure and optical properties of the materials
The X-ray diffraction patterns of the Bi2Sn2O7 samples prepared at pH 12 before and after annealing from 300 °C to 700 °C is shown in Figure 5.5. The results showed that upon microwave irradiation, the samples appeared weak intensity diffraction peaks corresponding to the BiOCl phase formation. When the annealing temperature was 400 °C, the diffraction peaks of the Bi2Sn2O7 phase appeared but in very weak intensity, most of the material is amorphous. When the annealing temperature increased to 500 °C, the intensity of the diffraction peaks increased, whereas the full width at half maximum decreased implying a good crystallization of the crystalline phase. Thus, the nanomaterial adopted in the form the single-phase cubic pyrochlore Bi2Sn2O7 crystalline while annealing above 400 °C.
annealed at
Fig. 5.5. XRD patterns of the Bi2Sn2O7 samples prepared at different annealing temperatures (pH = 12).
Fig. 5.6. SEM images of the Bi2Sn2O7 samples different temperatures: 400 oC (a), 500 oC (b), 600 oC (c) and 700 oC (d).
The average crystalline grain size of Bi2Sn2O7 samples annealed at different
temperatures varied significantly over the annealing temperatures (Table 5.3).
20 Table 5.3. Specific surface area and grain size of the Bi2Sn2O7 nanoparticles obtained at different annealing temperatures 400, 500, 600 and 700 oC.
400 6 500 8 600 11 700 15
51 62
Annealing temperature (°C) Crystalline grain size XRD (nm) Grain size SEM (nm) Specific surface area (m2/g) 40 94.8 ± 0.4 45 55.8 ± 0.2 31.7 ± 0.1 16.2 ± 0.1
The SEM images of the Bi2Sn2O7 samples in Figure 5.6 shows that the average particle size increases from 40 nm to 60 nm when the annealing temperature increases from 400 to 700 °C. The nitrogen adsorption/desorption isotherms were performed for various samples to evaluate the effects of the annealing temperature on the specific surface area of the Bi2Sn2O7 nanoparticles. The results are shown in Table 5.3.
UV-Vis diffuse reflectance spectra and its plot of transformed KM function for the as-prepared sample and annealed Bi2Sn2O7 samples at different temperatures are shown in Figure 5.7. The results show that the as-prepared sample has an absorption edge in the ultraviolet region, which is the absorption edge of BiOCl material. After annealing, the absorption edges of all samples shifted to the visible region. When the annealing temperature was increased from 400 to 700 oC, the band gap values of Bi2Sn2O7 samples ranged from 2.4 eV to 2.67 eV.
Fig. 5.7. UV-Vis (a) and plot of transformed KM function (b) of the nano- Bi2Sn2O7 as- particles sample prepared annealed and at samples different temperature.
to 740 nm. Figure 5.8b is
The photoluminescence spectra at room temperature of Bi2Sn2O7 nanoparticles annealed at different temperatures are shown in Figure 5.8a. In Figure 5.8a, the photoluminescence spectra of all Bi2Sn2O7 annealed at different temperatures has a spectrum of emission ranging from 450 the photoluminescence peaks of the Bi2Sn2O7 annealed at 400 oC was corrected by the Lorentzian function. The results show that the highest photoluminescence peak at 528 nm (~2.35 eV) can be attributed to the fundamental shift (region/region) of the electron. The Bi2Sn2O7 sample annealed at 500 oC had the lowest average photoluminescence intensity, which indirectly reflected that the lifetime of electron
21 and optical hole is the least possibility, in other words, the lifetime of the electron and the optical hole of the sample was the longest. The Bi2Sn2O7 annealed at 400 oC had the highest photoluminescence intensity, indirectly reflecting the lifetime of the electron and the optical hole was the shortest.
(a) 5.8.
curve
Fig Photoluminescence spectra of the Bi2Sn2O7 samples annealed at different temperatures (b) Lorentzian Fitting function in photoluminescence spectra of the Bi2Sn2O7 sample annealed at 400 oC.
5.2. The effect of technological conditions on the photocatalytic activity of the materials 5.2.1. The photocatalytic activity of Bi2Sn2O7 material synthesized at different pH values of the precursor solution
The photocatalytic potential of the Bi2Sn2O7 material was evaluated by RhB concentration reduction over time when illuminated. The results showed that the Bi2Sn2O7 sample was synthesized at pH = 12 afforded the best photocatalytic activity. After 6 hours of irradiation, the RhB concentration decreased to about 15 %. 5.2.2. The effect of the annealing temperature on the photocatalytic activity
Figure 5.9 shows that the Bi2Sn2O7 sample annealed at 500 oC gave the best photocatalytic activity, after 3 hours illuminated, the remaining RhB concentration was about 2.3 %. For the Bi2Sn2O7 samples annealed at 400, 600, and 700 oC, after 3 hours, the remaining RhB concentration was 21.2, 10.6, and 27.6 %, respectively.
Bi2Sn2O7
the change of RhB concentration in
Fig. 5.9. Plot demonstrating the intensity change of the absorption peak at 554 nm over time of RhB solution in the RhB-degradative photocatalysis of Bi2Sn2O7 (a) the and photocatalysis of Bi2Sn2O7 annealed at different temperature according to Langmuir-Hinshelwood model (b). Fig. 5.10. Recycling runs of the particle annealed at 500 oC for the the RhB degradation by photocatalysis of materials when the irradiation by visible light.
22
The photocatalytic efficiency of the Bi2Sn2O7 sample annealed at 500 °C for the RhB degradation after 3 successive reuse is shown in Figure 5.10. The results show that, compared to the first use, the reduction of photoactivity of nanoparticles Bi2Sn2O7 annealed at 500 °C is about 1 %; 2.7 % and 4 % respectively for second, third and fourth use. Therefore, it can be concluded that Bi2Sn2O7 nanoparticles at 500 oC exhibit relatively high photocatalytic activity and high stability. Thus, the Bi2Sn2O7 nanoparticles prepared at pH = 12 and annealed at 500 °C for 3 hours exhibited the best photocatalytic activity in the degradation of RhB under the visible light. 5.3. Synthesis và study on the composite material of Bi2Sn2O7 with magnetic material CoFe2O4 5.3.1. Physical properties of the composite material
The X-ray diffraction patterns of the Bi2Sn2O7/CoFe2O4 (BSO/CFO) composite samples were prepared using the microwave assisted chemical method shown in Figure 5.11. The results show that in the XRD spectrum of the BSO/CFO composite sample, there are no new peaks and no peak shifting except the diffusion peaks of BSO and CFO.
Fig. 5.12. Magnetic hysteresis of the composite BSO/CFO samples and CFO samples (inserted figure). Fig. 5.11. XRD patterns of the pure BSO, pure CFO and BSO/CFO composite samples.
The energy dispersive X-ray spectroscopy (EDX) of the BSO/CFO composite samples with CFO/BSO mass ratio 10 % and 15 %, respectively, characterized the peaks corresponding to the elements: Bi; Fe; Co; Sn and O proving that the synthesized material has the desired chemical composition.
The SEM images of the pure BSO sample and BSO/CFO composite samples showed that in the pure BSO samples, nanoparticles were spherical, fairly evenly distributed with an average particle size of 40 nm. The BSO/CFO composite samples were not clear in shape due to the contribution of components from the CFO in the sample.
Magnetic properties of the BSO/CFO composite samples and CFO samples were investigated by magnetic hysteresis. Figure 5.12 is the magnetic hysteresis of the BSO/CFO composite material and the CFO material. The results on Figure 5.12 showed that the saturation of the CFO sample was about 45 emu/g, the saturation of the BSO/CFO composite materials were of 0; 0.9; 1.8; 2.6; 4.4 and 6 emu/g for samples with CFO mass ratio of 2.5; 5; 7.5; 10; 12.5 and 15 % respectively. Thus, the composite BSO/CFO is ferromagnetic, the higher the CFO mass ratio is, the stronger the ferromagnetism of the material was observed.
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The absorption edges of the BSO samples and BSO/CFO composite were detected in the visible light region. The band gap value of BSO is 2.79 eV. The band gap values of the BSO/CFO samples varied from 2.42 eV to 2.74 eV when CFO mass proportion in the sample increased from 2.5 % to 15 %. 5.3.2. Photocatalytic activity and examination of the recycling feasibility by the magnetic of the composite samples
Figure 5.13 shows the recycling ability by magnetic field of the composite material BSO 10 CFO (10 % mass ratio of CFO). The results show that the BSO (yellow solution) is not affected by the magnetic field and the solution is still homogeneous; the BSO 10 CFO composite in the solution is attracted to the magnet and the remaining solution appears to be transparent. This demonstrates that the BSO 10 CFO composite has a combination between two components: BSO and CFO.
Fig. 5.13. Recycling potential by magnetic field of the composite material BSO/CFO. Figure 5.14 shows the intensity reduction of 554 nm peak compared to the initial intensity over time by the photocatalytic activity of the pure BSO sample and the BSO/CFO composite samples with different CFO mass ratios. The results showed that the BSO/CFO composite samples with a CFO mass ratio of 2.5 to 10 % exhibited slightly lower photocatalytic activity than the pure BSO sample. Thus, the role of the CFO in the BSO/CFO composite is to bind with BSO and is likely to enable the recovery ability by external magnetic fields.
degradation by
Fig. 5.14. Plot demonstrating the intensity change of absorption peak at 554 nm over time of the RhB solution under the RhB-degradative photocatalytic activity (a) and the change of RhB concentration during photocatalysis of BSO and the BSO/CFO composite samples according to the Langmuir-Hinshelwood model (b).
Fig. 5.15. Recycling runs of the BSO 10 CFO composite of the RhB the photocatalytic activity in the presence of material while irradiation by the visible light. The photocatalytic efficiency of the samples after recovery and three-time reusing for the degradation of RhB is shown in Figure 5.15. The results show that the BSO 10 CFO composite retains good photocatalytic activity in the decomposition of RhB after 180 minutes of illumination by the visible light. After initial use, up to 86 % of RhB was degraded, 75 % of RhB was degraded after the first reuse, about 62 % of RhB was decomposed after the second reuse and only 50 % of RhB was degraded after the third reuse.
24 CONCLUSION 1. BiVO4 particles have been successfully synthesized by microwave-assisted chemical method, there is an influence of pH of the precursor solution and annealing temperature on the structure, morphology and optical property. The annealing temperature of BiVO4 influences the phase formation and phase transitions of the nanomaterials. The as-prepared sample crystallized as single phase z-t BiVO4 (pH = 3), the phase transition from z-t BiVO4 to s-m BiVO4 was observed upon increasing the annealing temperature. The complete phase transition was clearly marked at 350 oC.
2. Nano-Bi2Ti2O7 particles have been successfully synthesized via sol-gel and hydrothermal method. The pH of precursor solution and annealing temperature have an impact on the structure, specific surface area and optical property of the nanomaterials. They exist in monoclinic α cubic Pyrochlore structure at 600 oC annealing temperature after drying sol-gel during sol-gel process. With hydrothermal method, at pH = 11 of the precursor solution and 600 oC annealing temperature, the nanomaterials adopt monoclinic α cubic Pyrochlore Bi2Ti2O7. 3. The preparing methods, pH of the precursor solution and the annealing temperature have an impact on the photocatalytic activity of the nanomaterials. The samples synthesized by hydrothermal method at pH = 7 and 600 oC annealing temperature are the composites of Bi2Ti2O7/Bi2Ti4O11, exhibiting highest photocatalytic activity (k’ = 0,028). While the samples prepared by hydrothermal method crystallize as single phase Bi2Ti2O7 showing lowest photocatalytic activity (k’ = 0,006).
4. Bi2Sn2O7 nanoparticles have been successfully synthesized in optimal conditions at pH = 12 of the precursor solution and annealing temperature 500 oC, showing the highest photocatalytic efficiency and high stability photocatalytic activity after many times of recycling. After four recycling runs of the degradation of RhB revealed that the photocatalytic activity losses were around 1 %, 2.7 % and 4 % for the second, third and fourth run, respectively.
5. Bi2Sn2O7/CoFe2O4 composites have been successfully synthesized by two steps via microwave-assisted chemical method. Effect of weight percentage of CoFe2O4 to Bi2Sn2O7 (from 2.5 to 15 %) on the structure, the optical property, the magnetic activity, the photocatalytic activity and reusability of the material was investigated. The Bi2Sn2O7/CoFe2O4 composite sample with 10 % CoFe2O4 by weight still remained high photocatalytic activity, exhibiting degradation rate of RhB up to 86 %, 75 %, 62 % and 50 % for the first, second, third and forth run, respectively, after 180 mins visible light irradiation. The reusability of this sample could reach 78 % after 3 recycling runs. Therefore, it has great potential in practical application.
