CHẾ tạo vật LIỆU nền Bi2WO6 có cấu TRÚC NANO và NGHIÊN cứu một số TÍNH CHẤT của CHÚNG tt tieng ang

PREFACE Fujishima and Honda have investigated photocatalytic activity of TiO in 1972 However, due to the fact that TiO2 have wide band gap semiconductor at 3.2 eV It can only absorb ultraviolet light region Currently, this is a major obstacle preventing the application of TiO in water streatment From these difficulties, seeking the kind of light – driven materials is nescessary Some materials which ability photocatalytic activity in visible light have been studied such as MnWO4, BiVO4, Ag3PO4 In among light –driven materials, Bi2WO6 material- the narrow band gap exhibited the highest photocatalytic activity in visible light (2.7 eV) The effiency of photocatalist of Bi2WO6 is stronger than that of TiO in visible light region The Bi2WO6 nanopowders have sucessfully prepared using some methods such as: Hydrothermal method, sol-gel method, coprecipitation method, sonochemical method However, there are few published paper using the microwave-assisted method Moreover, the properties and photocatalytic activities of Bi2WO6 depend on the preparation conditions of each of method, therefore, controlling conditional preparation of microwave assisted method to obtain good crystal of Bi2WO6 is necessary One another hand, to apply Bi2WO6 in water streatment, we investigate some modified Bi2WO6 materials such as compound with another semiconductor, replacement Bi, W, O by doping another elements From reason above, the Bi2WO6 material is objectives of my thesis with title ”The synthesis of the Bi2WO6 –based and the studies of its properties” Thesis Objectives: (1) to synthesize Bi2WO6 via a microwave assisted method; study effect of preparation conditions on physical properties and photocatalytic activities (2) To study enhanced photocatalytic activities Bi2WO6 material by composite with another semiconductor and doping; (3) Combine some methods to obtain Bi2WO6 with high surface and modify Bi2WO6 by doping some elements to enhance photocatalytic activities Study Objects and method approach: My thesis focus on studying physical properties and photocatalytic activity of pure Bi2WO6 and modified Bi2WO6 in term of experimental method In my thesis, the mircowave assisted method was used to synthesize Bi2WO6 with high photocatalytic activities The Bi2WO6 materials were synthesize in division of solid state physics- electronics LAB, Facculty of Physics, Hanoi National University of Education The measurements were carried out by modern equipments with high reliability at national research centers, a few measurements were done in foreign laboratories Scientific Meaning and Practical Significance: The major of thesis study experimental process to synthesis Bi2WO6 and modified Bi2WO6 materials via a microwave assisted method Investigation of effect of preparation conditions on physical properties and photocatalytic activity of Bi2WO6 via a microwave assisted method, These results will contribute to the understanding of Bi2WO6 photocatalytic in terms of basics and applicationoriented research Thesis Contents: The content of the thesis include (i) general introduction of Bi2WO6 materials; photocatalytic advantages and drawbacks of Bi2WO6; methods to improve photocatalytic activities of Bi2WO6 and some previous experimental and theoretical studies on Bi2WO6 and modified Bi2WO6; (ii) study experiment process to synthesis Bi2WO6 material via a microwave assisted method and the influence of preparation conditions on the physical and photocatalytic properties of Bi2WO6; (iii) major results of the influence of doping and composite on the photocatalytic activities of Bi2WO6 doping Gd and Bi2WO6/BiVO4 composites.(iv) new approach-microwave assisted combining hydrothermal method to synthesis high photocatalytic activity Bi2WO6 and N doped Bi2WO6, the mechanism of photocatalytic activity enhancement in the nanocomposites was investigated in detail Thesis Layout: Thesis is presented in 136 pages with 73 Figures and 21 Tables, including the heading, 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 Bi2WO6 in previous studies on understanding and improving photocatalytic properties of Bi 2WO6-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; Chapter 3: Presenting results of Bi2WO6 synthesized via a microwave assisted method, the influence of preparation conditions on structure and photocatalytic properties of Bi2WO6; the factors effect on the photocatalytic process Chapter 4: Presenting the effect of doping of Gd and composite of BiVO4 on physical and photocatalytic properties of Bi2WO6, and the mechanism of photocatalytic activity enhancement in the modified Bi2WO6 materials Chapter 5: Presenting the syntheisis of Bi2WO6 and N doped Bi2WO6 via a two step microwave assisted – hydrothermal method Influence of doping N on the physical and photocatalytic properties of Bi2WO6 Role of electrons – holes in photocatalytic reaction Conclusion: Presenting the major results of the thesis The research results of the thesis have been published in scientific works in which there are articles in international journals, articles in national journals, some results are analysized to submits on journals CHAPTER INTRODUCTION TO Bi2WO6 MATERIAL 1.1 Introduction 1.1.1 Structural characterization of Bi2WO6 Bi2WO6 belongs to the Aurivillus typewith a general formlar of: Bi 2An1Bn O3n+3, with A being Ca, Sr, Ba, Pb, Bi, Na, K B Ti, Nb,Ta, Mo, W, Fe for n=1 and B =W It is well-known that Bi 2WO6possesses both the interesting ferroelectric and ferromagnetic properties The crystal structure of Bi 2WO6belongs to orthorhombic lattice, space group P21ab, with lattice parameters a= 5.456 Å, b=16.430 Å, c=5.438 Å; α=90o, β=90o, γ=90o Bi2WO6 can be considered as (Bi 2O2)2+layers that interleaves with the perovskite-type (WO4)2-structure Figure 1.1 shows the illustrative model in which Bi, W, O atoms arranges in the unit cell of Bi 2WO6 Figure1.1: Illustration of Bi2WO6 crystal structure 1.1.2 Optical property Lie et al investigated the electronic structure and density of states of Bi2WO6 using the density functional theory It was found that Bi 2WO6 is a direct-band gap semiductor with Eg~2.75 eV Experimentally, Xu et al reported the band gap of 2.8 eV from the UV-Vis spectrum, in good agreement with that determined from the theoretical calculation 1.2 Photocatalytic performance Bi2WO6 shows a photodegredation behavior for various organic compounds composed of highly stable carbon rings In addtion, Bi 2WO6 also exhibits photocatalytic characterization for antibodies, including ciprofloxacin, tetracycline hydrochloride, norfloxacin, levofloxacin Based on those studies in the literature, Bi 2WO6 could be a promising candidate for environmental applications 1.3 Synthesis methods Bi2WO6 has been successfully synthesized using a varitety of different techniques, such as hydrothermal solvothermal, sol-gel, sonochemical methods 1.4 The influence of experimental conditions on the physical properties A number of reports indicated that experimental factors, e.g pH in solution, strongly affects on the phase formation of Bi 2WO6 A sequence of chemical reaction to form Bi 2WO6 can be described in the following equations: Na2WO4.2H2O + 2HNO H2WO4 + 2NaNO3 + 2H2O Bi(NO3)3 + H2O BiONO3 + 2HNO3 BiONO3 + H2O Bi 2O2(OH)NO3 + HNO3 Bi2O2(OH)NO3 + H2WO4 Bi2WO6 + HNO3 +H2O However, when the pH is larger than 8, the reaction happens in an another route, Bi2O2(OH)NO3 + 2WO42- + 3OHBi14W2O27 + 7NO3- + 5H2O These equations suggest that, the precursors Bi(NO3)3and Na 2WO4form the Bi2WO6 or Bi14W2O27 which is dependent on the pH solution The effect of the pH solution on the morphology of Bi 2WO6was reported The authors showed that, the superstructure phase of Bi 2WO6 is unstable in the case of the pH greater than 2.5 and completely vanished if pH=7.5 The morphology of Bi2WO6 becomes rectangular sheets with width of 80 nm and length of 1-3 μm in sizes Figure 1.10 Morphology of Bi2WO6 prepared at different values of pH: pH=1 (a), pH=4.5 (b), pH=7 (c) The role of the morphology induced by the pH on the photocatalytic performance on Bi 2WO6 is shown in Fig 1.11 It can be seen that Bi 2WO6 that exists in the superstructure (low pH) has the higher photocatalytic performance compared to that in the retangular sheets (pH=7.5) Figure 1.11Dependence of photocatalytic efficiency on the morphology of Bi2WO6 For pH=7.5, but with additional surfactants like Ethylene Glycol, Xu and coworkers reported that the morphology transforms from the sheet-like shape to nano-sizedspheres 1.5 Enhance the photocatalytic performance The photodegradation performace Bi 2WO6 depends on many factors, including the electron-hole recombination, specific surface area, band gap As a result, in order to enhance its efficiency one can control these factors by modifying the original material Two common techniquesare dopands and making a composite with other compounds 1.5.1 Doping Bi2 WO6 The modification of Bi 2WO6 based on doping with various elements has been widely used in the literature: Gd, Mo, Ce, Br, Ba, Lu, Eu, Y, F, N The reuslts found that the doping of Bi 2WO6 produces a better photocatalytic performance in comparion with the pure Bi 2WO6 materials 1.5.2 Composite Bi2WO6 In addition to the doping strategy, the fabrication of Bi 2WO6 in the composite structure has attracted interest in the photocatalytic enhancement The advantage of such a method is that the composite materials possess both the physical characterization of individual components The investigations on the Bi2WO6 composite can be divided into two parts: (i) Bi 2WO6 with the semiconductor ZnWO 4, Co3O4, ZnO, BiVO4, Bi2O3, Bi2S 3, graphene oxide, WO3, g-C 3N4, TiO 2, CeO2, Ag3PO4 (ii) Bi2WO6 with metal naoparticles with a high conductivity such as: Ag, Au, Cu, Pt CHAPTER EXPERIMENTAL TECHNIQUES AND SAMPLES – ANALYZED TECHNIQUES 2.1 Synthesis Bi2WO6 The Bi2WO6 nanopowders were synthesized by microwave assisted method, a schematic diagram of experimental process as follows: Figure 2.7 The schematic diagram synthesis of Bi2WO6 nanostructure In typical synthesis: 2.5 mmol of sodium tungstate dihydrate (Na2WO4.2H2O) and mmol of bismuth nitrate (Bi(NO 3)3.5H2O) were first dissolved in 100 ml distilled water with 30 stirring at room temperature The obtained solution was heated by a Sharp- modified microwave oven with power of 750 W for 20 After microwave processing, the solution was cooled to room temperature The suspended was separated by entrifugation, washed with deionized water and acetone for several times, then dried in an oven at 70 C for 24 h To investigate the effect of experimental conditions on the physical properties, (1) the samples were finally annealed in air for h at temperatures of 400, 500, 600, and 700 oC, respectively (2) the pH of solution before heating was controlled at pH=1, 3, 5, 7, (3); the time of irradiation microwave was changed 5, 10, 15, 20 minutes 2.2 Synthesis of Bi2WO6/BiVO4 nanocomposites Bi2WO6/BiVO nanocomposites were synthesized using diagram as shown Fig 4.1 In a typical synthesis, 2.5 mmol of Na2WO4.2H2O and mmol of Bi(NO3)3.5H2O were dissolved in 100 ml distilled water with stirring at room temperature to obtain Figure 2.8 The schematic diagram synthesis of solution A; and 2.5 mmol of Bi2WO6/BiVO4 nanocomposite NH4VO3 and 2.5 mmol of Bi(NO3)3.5H2O were dissolved in 100 ml distilled water with stirring at room temperature to obtain solution B Then solution A and B were mixed with appropriate Bi 2WO6:BiVO molar ratio The mixed solution was heated by a Sanyo microwave oven with 750W for 20 After microwave processing, the solution was cooled to room temperature The resulted precipitate was separated by centrifugation, washed with deionized water and acetone for several times then dried in an oven at 70 °C for 24 h.Photocatalytic activity of the nanoparticles was evaluated by the decolorization of Rhodamine B (RhB) under visiblelight-irradiation In the experimental setup, a 300W Xe lamp was employed as the light source and a 420 nm cut-off filter was used to provide visible light irradiation Samples with Bi2WO6:BiVO ratio 100:0; 90:10; 80:20; 70:30; 60:40; 50:50, 0:100 were indexed Bi2WO6, M 90-10, M 80-20, M 70-30, M 60-40, M 50-50, BiVO4, respectively 2.3 Synthesis of Gd doped Bi2WO6 Experimental process of Bi 2WO6/BiVO nanocomposites was shown in Fig 2.9 Figure 2.9 schematic diagram synthesis of Bi2WO6 doping Gd via a microwave assisted method The synthesis process is similar as that in our chapter Different content of Gd-doping was obtained by mixing Bi(NO 3)3·5H2O and Gd(NO3)3·6H2O with atomic ratio of Gd:Bi = 0, 1.0, 2.5, 5.0, and 7.5% 2.4 Synthesis of N doped Bi2 WO6 Experimental process to synthesis Bi2WO6 via a two step microwave assisted – hydrothermal method as follows: In the first step of microwave-assisted synthesis, 2.5 mmol Na 2WO4 2H2O and mmol Bi(NO3)3.5H2O were dissolved in 100ml distilled water with continuous stirring for two hours, and the obtained solution was heated using a 75% power Sharp microwave oven for 20 and then cooled to room temperature A range of N-doping content was obtained using molar ratios for the reagents C4HN2O and Bi(NO 3)3.5H2O of 0%, 0.1%, 0.25%, 0.5%, and 0.75% In the second step involving hydrothermal synthesis, the solution from the first step was transferred into a Teflon-lined autoclave and filled to 80% of the total volume The autoclave was then sealed into a stainless steel tank and kept at 180 C for 12 h Following this, the reactor was left to cool to room temperature naturally The resulting precipitatewas separated by centrifugation, washed several times with deionized water and ethanol, and then dried in an oven at 70 oC for 24 h in air The samples obtained using the above process are denoted here as MH:N-x (x:0, 0.1, 0.25, 0.5, 0.75) H:N-0 and M:N-0 Bi2WO6 samples were also prepared using a one-step hydrothermal method and a onestep microwave-assisted method, respectively The properties of M:N-0 Bi2WO6 nanoparticles have been investigated in chapter and In this chapter, we present a study of MH:N-x and H:N-0 Bi2WO6 nanoparticles Photocatalytic activity of the nanoparticles was evaluated by the decolorization of methylene-blue (MB) under visible-lightirradiation In the experimental setup, a 300 W Xe lamp was employed as the light source and a 420 nm cut-off filter was used to provide visible-light-irradiation In every experiment, 0.1 g of nanoparticles was added to 100 ml of MB solution (10 -5 mol/l) Before being irradiated, the suspension was magnetically stirred in the dark for h to ensure the establishment of and sorptionedesorption equilibrium between the photocatalyst and MB After a given irradiation time, the suspension was centrifuged to remove the catalyst immediately, and UV-VIS absorbance measurement was performed The decolorization of MB was monitored by the decrease of absorption peaks For reusability test, the Bi 2WO6 nanoparticles were immersed in ethanol for 3.0 h and rinsed with deionized water, and then dried at 370 K After this, the cleaned Bi 2WO6 nanoparticles were reused to test photocatalytic activity 2.5 Samples-analyzed instrume nts and techniques The crystallography of the obtained nanoparticles was analyzed using a Bruker D5005 X-ray diffractometer (XRD) The UV–VIS diffuse reflectance was performed using a Jasco V670 spectrophotometer The morphology of the nanoparticles was observed by scanning electron microscope (SEM, S4800Hitachi) High resolution transmission electron microscopy (HRTEM) images were conducted on a JEOL 2010 electron microscope operated at 200 kV The surface areas of the samples were determined by using the Brunauer–Emmett– Teller (BET) analysis of the nitrogen adsorption–desorption isotherm, which were measured at 77 K using Autosorb, Quantachrome, USA The photoluminescence (PL) measurements were carried out with a Spectrofluorometer of Horiba Jobin Yvon NanoLog using 390 nm excitation from a 500W Xenon lamp CHAPTER A STUDYING PROPERTIES, PHOTOCATALYTIC ACTIVITIES OF Bi2WO6 SYNTHESIZED VIA A MICROWAVE ASSISTED METHOD 3.1 Effect of time irradiation and pH on the physical properties of Bi2WO6 The effect of microwave irradiation on the structure of Bi 2WO6 was shown Fig 3.1 The results indicate that the good crystal Bi2WO6 was obtained at the time of microwave irradiation of 20 minutes Figure 3.1 XRD patterns of Figure 3.2 XRD patterns of Bi WO Bi2WO6 nanoparticles with microwave nanoparticles synthesized at pH=1, 3, 5, irradiation time of 5, 10, 15, 20 minutes 7, 9, 11 Fig 3.2 presents XRD patterns of Bi2WO6 synthesized at pH = 1, 3, 5, 7, 9, 11 with microwave irradiation time of 20 minutes and annealing temperature of 500 oC The XRD results indicate that single crystal phase of Bi2WO6 obtained at low pH 3.2 Effect of annealing temperature on the physical properties and photocatalytic activity of Bi2WO6 Fig 3.3 shows the XRD patterns of Bi 2WO6 annealed temperature of 400, 500, 600, and 700 oC The nanoparticles as-prepared sample would be in amorphous state, thus yield no apparent diffraction peaks With annealing treatment above 500 oC, the crystalline quality of Bi2WO6 nanoparticles gets better, and the impurity Bi 14W2O27 phase disappears The diffraction peaks of the nanoparticles obtained with annealing temperature above 500 oC can be well-indexed to pure orthorhombic Bi2WO6 phase according to the JCPDS Card (No.39-0256), as presented in Fig 3.3 This indicates that good crystalline quality Bi2WO6 nanoparticles can be synthesized by fast microwave-assisted method with low temperature (500 oC) annealing treatment Figure 3.3 XRD patterns of as-prepared Bi2WO6 and Bi2WO6 nanoparticles annealed at, 400, 500, 600 and 700 oC for h Fig 3.6 shows the SEM images of Bi 2WO6 with difference annealing temperature The increase of particle size with increasing annealing temperature is clearly observed in the SEM images The average particle size of Bi 2WO6 nanoparticles were found to be 30, 60, 80, and 400 nm for annealing temperatures of 400, 500, 600, and 700 oC, respectively For the Bi 2WO6 nanoparticles obtained with 700 oC annealing, the particle size obtained from SEM is more than 10 times of the particle size estimated from XRD We suggest that this extreme difference would be mainly correlated with the layered structure of Bi 2WO6 nanoparticles: the XRD results would mainly indicate the thickness of layered nanoparticles, while SEM images would mainly indicate the plane size of layered nanoparticles The layered structure of the Bi2WO6 nanoparticles is confirmed by HRTEM image, as shown in Fig 3.6(e) This image was taken for the Bi 2WO6 nanoparticles obtained with annealing temperature of 500 oC Fig 3.6(e) shows that the average space between adjacent planes is 0.31 nm, which is assigned to the (131) planes of orthorhombic layered structure of Bi 2WO6 Figure 3.6 SEM images of the nanoparticles obtained with annealing temperatures of 400 (a), 500 (b), 600 (c), 700 oC (d) and TEM image of Bi2WO6 nanoparticles obtained with annealing temperature of 500 OC (e) Figure 3.7 Nitrogen absorption and Figure 3.10 Uv-vis diffuse reflectance desorption curve of Bi 2WO6 at spectra of Bi 2WO6 with annealing annealing temperature of 400 500, 600 temperatures of 400, 500, 600, and o and 700 C 700 oC The surface areas of the Bi2WO6 nanoparticles were estimated by BET experiments, the results are shown in Fig 3.7 With increasing annealing temperature, the particle surface area deceases gradually For the Bi2WO6 nanoparticles obtained with annealing temperatures of 600 and 700 oC, their surface area is similar, although the SEM images indicated very different particle size This further suggests that the SEM images indicated only the plane size of the layered nanoparticles The thickness of the layered nanoparticles would be similar for the Bi2WO6 nanoparticles obtained with annealing temperatures of 600 and 700 oC, thus their surface areas are similar The above results show that with increasing annealing temperature, the crystalline quality of Bi2WO6 nanoparticles gets better, which improves the visible-light-absorption This would result in more photo-generated electrons and holes, thus be helpful to improve photocatalytic activity However, with increasing annealing temperature, the surface area of the Bi2WO6 nanoparticles gets smaller This would result in less active sites, thus decrease the photocatalytic activity Whether visible-light-absorption or surface-area plays 10 major role for improving photocatalytic activity would depend on the efficiency of transportation of photogenerated charges to active sites If surface area plays a major role, this would indicate high efficiency of transportation of photogenerated charges to active sites Then, these nanoparticles would be promising for achieving high photocatalytic activity Fig 3.10 shows the UVVIS diffuse reflectance spectra of the nanoparticles obtained with annealing temperatures of 400, 500, 600, and 700 oC According to the spectra, the samples present photo-absorption properties from UV to visible light shorter than 450 nm, which implied the possibility of good photocatalytic activity under visible light-irradiation.The band gaps of the Bi2WO6 nanoparticles obtained with annealing temperature of 500, 600 and 700 oC were estimated to be 2.93, 2.89 and 2.83 eV, respectively For the nanoparticles obtained with 400 o C annealing, there are two phases of Bi2WO6 and Bi14W2O27 These two phases would have different contribution for the absorption spectrum, thus the above formula could be not simply applied to one spectrum for obtaining the band gaps of two phases Figure 3.15 Cycling runs of the Figure 3.13 Absorbance change of MB photocatalytic degradation of MB at 665 nm as a function of visible light under visible light in the presence of irradiation time in the presence of Bi2WO6 nanoparticles annealed at 500 Bi2WO6 nanoparticles annealed at o C different temperatures To compare the photocatalytic activities of the nanoparticles annealed at different temperatures The absorbance time variations (A t/Ao) of the peaks at 668 nm for nanoparticles annealed at 400, 500, 600, and 700 oC are plotted in Fig 3.13 (At is time-dependent absorbance and Ao is initial absorbance) Fig 3.13 shows that the nanoparticles obtained with annealing temperature of 400 o C have the lowest photocatalytic activity This would be correlated with the significant impurity phase of Bi 14W2O27 nanoparticles These nanoparticles have similar visible-light absorption with Bi2WO6 nanoparticles, and they have bigger surface area Thus the low photocatalytic activity would indicate very low efficiency of transportation of photo-generated charges to active sites in Bi14W2O27 nanoparticles For the Bi2WO6 nanoparticles obtained with annealing temperature above 500 oC, the photocatalytic activity decreases gradually with 11 increasing annealing temperature We have shown that the visible-light absorption of the Bi2WO6 nanoparticles increases gradually with increasing annealing temperature; while the surface area of the nanoparticles decreases gradually with increasing annealing temperature Thus, for the Bi2WO6 nanoparticles, the surface area plays more important role than visible-light absorption for enhancing photocatalytic activity This indicates efficient transportation of photo-generated charges to active sites in the Bi2WO6 nanoparticles obtained with annealing at 500 oC Considering practical application, it is important and necessary to investigate the reusability and stability of a photocatalyst To confirm the reusability and stability of the photocatalytic performance of the Bi2WO6 nanoparticles, circulating runs in the photocatalytic degradation of MB under visible-light irradiation were checked As shown in Fig 3.15, comparing with the first run, the photocatalytic activity losses ~5%, 6%, and 10% for second, third, and fourth run, respectively CHAPTER STUDY OF MODIFIED Bi2WO6 NANOPOWDERS VIA A MICROWAVE ASSISTED METHOD 4.1 Results of synthesis and study properties of Bi2WO6/BiVO4 nanocomposites Figure 4.3 presents the XRD patterns of pure Bi2WO6 nanoparticles, Bi 2WO6/BiVO4 nanocomposites, and pure BiVO4 nanoparticles The characteristic diffraction peaks in Fig 4.3a can be well-indexed to pure orthorhombic Bi 2WO6 phase according to the JCPDS No.39-0256 The characteristic diffraction peaks in Fig 4.3e can be wellindexed to pure monoclinic scheelite Figure 4.3 XRD patterns of pure BiVO4 phase according to JCPDS No.75- Bi2WO6 (a), M 80-20 (b), M 70-30 1867 The diffraction peaks of (c), 50-50 (d) pure BiVO (e) Bi2WO6/BiVO (80–20) nanocomposites in Fig 4.3 have contributions from both orthorhombic Bi2WO6 and monoclinic scheelite BiVO4 phases, and no impurity peaks were found When BiVO content increases from 20 to 50%, the intensities of diffraction peaks of BiVO increases significantly, and the intensities of diffraction peaks of Bi2WO6 decreases significantly (Figs 4.3b–d) The diffraction peak positions of nanocomposites match precisely to values of pure nanoparticles, indicating there would be no incorporation of W and V ions in the Bi 2WO6/BiVO nanocomposites 12 Figure 4.8 Raman spectra of pure Bi2WO6, BiVO4 and Bi2WO6/BiVO4 samples Figure 4.5 HRTEM images and EDX spectra of M 70-30 sample The HRTEM image of 70–30 Bi2WO6/BiVO nanocomposites is shown in Fig 4.5 The lattice spacing of 0.31 nm corresponds to the d spacing between adjacent (131) crystallographic planes of Bi 2WO6, while the fringes of 0.27 nm match the (024) planes of m- BiVO4 The Raman spectra and TEM images indicate that in the nanocomposites, the Bi 2WO6 and BiVO4 nanoparticles would be in close contact, which could be helpful for separation of photogenerated free carriers, thus improving photocatalytic activity Figure 4.7 presents the diffuse reflection spectra of pure Bi 2WO6 nanoparticles, Bi 2WO6/BiVO4 nanocomposites, and pure BiVO4 nanoparticles The pure Bi2WO6 nanoparticles show photo-absorption from UV to visible light shorter than 450 nm The absorption range of the Bi2WO6/BiVO nanocomposites is substantially extended toward visible light, which is because the pure BiVO4 Figure 4.7 UV–Vis diffuse nanoparticles have photo-absorption from reflectance spectra of pure UV–VIS light up to 550 nm The Bi2WO6, BiVO4 and significant increase of visible light Bi2WO6/BiVO4 absorption could be very helpful for improving the photocatalytic activity Figure 4.7 indicated that the absorption of Bi2WO6/BiVO nanocomposites is not a simple linear summation of the absorption of pure Bi2WO6 and BiVO4 nanoparticles This indicates that in the nanocomposites, the Bi2WO6 and BiVO4 nanoparticles would be in close contact and there would be charge transfer between Bi2WO6 and BiVO4 nanocrystallites, consistent with the SEM and TEM results To compare the photocatalytic activity of the samples, the absorbance time variation (At/Ao) of the 544 nm peak of RhB in the presence of pure Bi2WO6, 80–20 Bi2WO6/BiVO4, 70–30 Bi2WO6/BiVO4, 50–50 Bi2WO6/BiVO 4, and pure 13 BiVO4 nanoparticles are plotted in Fig 4.7 (At is timedependent absorbance, and Ao is initial absorbance) In order to quantitatively analyze RhB degradation, the Langmuir– Hinshelwood model was applied: ln(At/Ao) = kt, where k is the reaction Figure 4.7 Absorbance change of 553 rate constant The fitting results are nm peak as a function of irradiation presented in Fig 4.7b Figure 4.7b time in the presence of pure BiVO 4, indicated that the photocatalytic Bi2WO6 and Bi2WO6/BiVO4 activity of pure Bi2WO6 nanoparticles nanocomposite is much higher than that of pure BiVO4 nanoparticles The photocatalytic activity of all Bi2WO6/BiVO nanocomposites is higher than that of pure Bi2WO6 nanoparticles The photocatalytic activity of 70–30 and 50–50 Bi2WO6/BiVO nanocomposites is higher than that of pure Bi2WO6 nanoparticles, while the photocatalytic activity of 80–20 Bi2WO6/BiVO4 nanocomposites is lower than that of pure Bi2WO6 nanoparticles For the Bi2WO6/BiVO nanocomposites, as the content of BiVO4 increases, the photocatalytic activity first shows a significant increase, then, slowly decreases; and the 70–30 Bi2WO6/BiVO nanocomposites have the highest photocatalytic degradation efficiency These phenomena indicate complex mechanism of photocatalytic process in the Bi2WO6/BiVO4 nanocomposites The recombination rate of photogenerated electron-hole pairs of the Bi2WO6/BiVO nanocomposite samples was investigated by PL emission experiment, and the results are presented in Fig 4.8 In the figure, the PL intensities of all the samples have been corrected taking into account the different absorption of 390 nm excitation light PL emission is mainly resulted from the recombination of free carriers Figure 4.8 PL emission spectra Therefore, PL experiment is a useful of pure Bi WO , BiVO and M technique to survey the recombination rate 80-30;M 70-30 M 50-50 of photogenerated electron–hole pairs in a samples nanocomposite semiconductor: in general, the lower the corrected PL intensity, the lower the recombination rate of photogenerated electron–hole pairs As can be seen in Fig 4.8, all the Bi 2WO6/BiVO4 nanocomposites have lower recombination rate of free carriers than that of pure Bi2WO6 and BiVO nanoparticles This indicates that there would be charge 14 transfer between the Bi 2WO6 and BiVO4 nanocrystallites, and free carriers have been separated in these two semiconductors, in agreement with the absorption result in Fig 4.8 Charge transfer could occur if nanoparticles are in close contact, it is much easier than energy transfer process, which is strongly depending on the separation between the centers of nanoparticles Therefore, our results support that in the nanocomposites, charge transfer would be happed between the Bi 2WO6 and BiVO4 nanocrystallites 4.2 Results of synthesis and study properties of Bi2WO6/BiVO4 nanocomposites The XRD patterns of pure and Gddoped Bi2WO6 nanoparticles are presented in Fig 4.9 As can be seen in Fig 4.9, all these diffraction peaks match well with orthorhombic Bi 2WO6 (JCPDS Card No 73-2020), and no peaks of impurity phases can be observed This indicates Gd-doping did not modify the host crystalline structure and also did not lead to the generation of any new crystal phase Figure 4.9 XRD patterns of Since the ionic radius of Gd 3+ (0.094 nm) Bi2WO6 doping Gd (0, 1.0, 2.5, is smaller than that of Bi 3+ (0.103 nm), 5.0, 7.5, and 10.0 %) the lattice parameters of Bi 2WO6 nanoparticles would decrease after Gd 3+ ions replacing Bi3+ ions Figure 4.11 The lattice of Bi2WO6 doping Gd (0, 1.0, 2.5, 5.0, 7.5 %) Figure 4.14 Raman spectra of Gd doped Bi2WO6 Fig 4.11 shows the lattice parameters of orthorhrombic of Bi 2WO6, the results indicate that with further increasing Gd-doping to 7.5%, the lattice parameters not show further decreasing This may suggest that at higher Gd concentration, only a partial of Gd 3+ ions could substitute the Bi 3+ ions in the Bi2WO6 host lattice Moreover, the peaks of 790 cm-1 also shift forward longer wave number and with increasing Gd doping ro 5.7%, these peaks were not shifted futher The Raman results agree with that XRD patterns 15 Table 4.1 initial Gd concentrations and Gd concentration from EDXS analysis Initial Gd0% 1% 2.5% 5% 7.5% doped Gd concentration 0.98 2.04 2.20 2.29 from EDXS Figure 4.16 XPS analysis of 2.5% Gd-doped Bi2WO6 nanoparticles: (a) Bi 4f (b) W 4f (c) O1s and (d) Gd 4d To analyze the Gd concentration in Gd- doped Bi2WO6 nanoparticles, EDXS measurements were performed Table 4.6 list the initial Gd-doped Bi2WO6 nanoparticles and the Gd concentration results from EDXS analyses for all Gd-doped samples The EDXS analysis indicated that with initial Gddoping of 1.0, 2.5, 5.0, and 7.5%, the obtained nanoparticles have the final Gd concentration of about 0.98, 2.04, 2.20, and 2.29%, respectively This is in good agreement with the XRD results The XRD and EDXS results indicate that through fast microwave assisted synthesis, only with initial Gd-doping up to about 2.5%, Gd3+ ions can successfully substitute the Bi 3+ ions in the Bi 2WO6 host lattice; while with further Hình 4.15 UV–Vis diffuse increasing Gd-doping, amorphous oxidation reflectance spectra of pure, state of Gd could be formed, which would 1.0, 2.5, 5.0 and 7.5% Gd make the understanding of these samples doped Bi2WO6 nanoparticles difficult Figure presents the chemical state of 2.5% Gd-doped Bi2WO6 nanoparticles The high resolution XPS spectra of the three primary elements Bi 4f, W 4f, and O1s, are shown in Fig 4.16a–c, respectively The results showed that the binding energies of Bi 4f7/2, Bi 4f5/2, W 4f7/2, W 4f5/2, and O1s are 159.7, 165.2, 35.8, 37.9, and 530.7 eV, respectively Figure 4.16 d is the XPS spectrum of Gd 4d The characteristic peaks of Gd 4d region are at 141.8 and 149.4 eV, which would be correlated to Gd 4d5/2 and Gd 4d 3/2, respectively These results indicate that Gd modified Bi 2WO6 nanoparticles have been successfully obtained, in good agreement with XRD results 16 Figure 4.15 presents the DRS spectra of pure, 1.0 and 2.5% Gd-doped Bi2WO6 nanoparitcles As can be seen in Fig 4.15, all the pure and Gd-doped Bi2WO6 nanoparitcles show similar photo-absorption property: having light absorption from UV to visible light ~ 450 nm The DRS spectra of these samples displayed steep edges, inferring that the visible light absorption should be caused by bandgap transition but not by transition from impurity level With 1.0 and 2.5% Gd-doping, the absorption edge has a weak redshift to visible region, and the visible absorption intensity has a weak increase The red-shift of absorption edge and increase of visible absorption intensity would be helpful for the enhancement of visible-light-driven photocatalytic activity of Gd-doped Figure 4.17 Absorbance change of Bi2WO6 nanoparticles 553 nm peak as a function of irradiation time in the presence of The time variations of absorbance of the pure and Gd -doped Bi WO peak at 553 nm for pure, 1.0 and 2.5% Gd- nanoparticles doped Bi2WO6 nanoparticles are plotted in Fig It shows in Fig that the photocatalytic activity of Bi 2WO6 nanoparticles can be remarkably improved with Gd-doping After 120 visible-lightirradiation, the RhB decolorization rate for pure Bi2WO6 nanoparticles is only about 45%; with 2.5% Gd doping, it can be increased to 100% This shows that Gd doping is very helpful to improve the Figure 4.19 PL emission photocatalytic activity of Bi 2WO6 spectra of pure, 1.0 and 2.5% nanoparticles Gd3+ has high stability due to Gd-doped Bi2WO6 the half-filled electronic configuration at nanoparticles outermost electron shell Thus, when Bi 3+ ions are substituted by more stable Gd3+ ions, photogenerated electrons can be more easily transferred to the active sizes on the surface of nanoparticles Therefore, the recombination rate of electron–hole pairs would decrease and photocatalytic activityof nanoparticles would increase with Gd-doping Figure 4.19 shows the PL emission spectra of pure, Gd-doped Bi2WO6 nanoparitcles The emission peak around 470 nm can be assigned to the intrinsic luminescence of Bi 2WO6, which would be correlated with the direct electron–hole recombination of band transition from the hybrid orbit of Bi 6s and O2p (VB) to the empty W5d orbit (CB) in the WO6 2− complex As can be seen in 17 Fig 4.19, the PL emission spectra of pure and Gd-doped Bi2WO6 nanoparticles displayed the main PL peaks at similar positions but with significant different intensities Figure 4.19 showed that these samples displayed similar absorption intensity Therefore, the significant difference of PL intensity would be correlated with very different recombination rate of electron–hole pairs in these nanoparticles This significant modification of recombination rate of electron– hole pairs by Gd-doing would be of great importance for improving photocatalytic activity of Bi 2WO6 nanoparticles CHAPTER STUDY OF NITROGEN DOPING Bi2WO6 VIA A MICROWAVE ASSISTED- HYDROTHERMAL METHOD 5.1 Effect of experimental method on physical and photocatalytic properties of Bi2WO6 Fig 5.1 shows XRD patterrns of Bi2WO6 samples synthesisized by microwave assisted method, hydrothermal method and microwave assisted – hydrothermal method Figure 5.1 XRD patterrns of Bi2WO6 samples synthesisized by microwave assisted method, hydrothermal method and microwave assisted – hydrothermal method Figure 5.2 SEM images of Bi2WO6 samples synthesisized by microwave assisted method, hydrothermal method and microwave assisted – hydrothermal method and specific surface area The XRD results indicate that the samples have good crystal structure Fig 5.2 show the SEM images and specific surface area of samples It can be seen that mophorlogy of Bi2WO6 synthesized by microwave assisted method is nanoparticles, the Bi2WO6 synthesized by hydrothermal method have with border not clearly while superstructure flower of Bi2WO6 synthesized by microwave assisted-hydrothermal method consist of small nanoparticles The results of specific surface area of Bi2WO6 synthesized by microwave assisted method is higher than that of Bi2WO6 synthesized by other method.Specific surface area plays importance roles in photocatalytic process Therefor, Bi2WO6 18 synthesized by Microwave assisted – hydrothermal method can be improved photocatalytic activity 5.2 Effect of N doping on the physical and photocatalytic properties of Bi2WO6 Fig 5.4 shows the XRD patterns of H:N-0, MH:N-0, MH:N-0.1, MH:N0.25, MH:N-0.5, and MH:N-0.75 nanoparticles Five diffraction peaks at 28.3, 32.7, 47.0, 55.9, and 58.6 can be clearly observed in all the samples These peaks match very well with the orthor hombic phase of Bi2WO6 (JCPDS 390256) Thus, N-doping does not change the phase structure of the crystalline Bi2WO6 host The inset of Fig 5.5 shows that the diffraction peak position undergoes a very weak shift to a lower angle with increasing N doping The change in lattice parameters from N-doping can be attributed to nitrogen replacing the common oxygen in the crystal lattice of Bi2WO6 The XPS results below also indicate that oxygen sites are replaced by nitrogen ions The ion radius of nitrogen is slightly smaller than that of oxygen; thus, when some of the oxygen sites are replaced by nitrogen ions, the lattice parameters slightly increase Moreover, all XRD patterns in Fig 5.5 show sharp and high intensity peaks, indicating the high crystalline quality of the samples Figure 5.4 XRD patterns of samples H:N-0, MH:N-0, MH:N-0.1, MH:N0.25, MH:N-0.5, MH:N-0.75 Figure 5.5 XPS spectra of sample MH:N-0.5 Fig 5.5 presents the XPS spectra of the MH:N-0.5 nanoparticles The XPS spectra show that the binding energies of Bi 4f7/2, Bi 4f5/2, W 4f7/2, W 4f5/2, Ov, and O1s are 164.2, 158.9, 38.0, 35.8, 532.0 and 530.0 eV, respectively These values are consistent with previously reported results Fig 5.6d shows the XPS spectrum of the doping element N1s The N1s peak was found only at 398.9 eV; no other peaks appeared The binding energy at 398.9 eV can be attributed to nitrogen replacing the common oxygen in the crystal lattice of Bi2WO6 to form the O-Bi-N-W-O bond This binding energy cannot represent O-Bi-N or O-W-N alone, since both the binding energies of Bi 4f and W 4f have changed to some extent, as shown in Fig 5.6 (a) and (b); the common oxygen ion in the O-Bi-O-W-O bond is therefore likely to have been replaced by a nitrogen ion The XPS results indicate that N-doped Bi2WO6 was 19 successfully prepared using the two-step microwave-assisted and hydrothermal method, and show good agreement with the XRD results Figure 5.6 SEM images of samples MH:N-0 (a), MH:N-0.1(b), MH:N-0.25(c), MH:N0.5(d), MH:N-0.75(e),MH:N-0.25 with scale bar 5μm (f) Figure 5.7 UV-VIS spectra of samples MH:N-0, MH:N-0.1, MH:N-0.25, MH:N-0.5, MH:N0.75 Fig 5.7 shows the DRS spectra of the H:N-0, MH:N-0, MH:N-0.1, MH:N-0.25, MH:N-0.5 and MH:N-0.75 nanoparticles These DRS spectra indicate that all of the samples have steep edges of visible light absorption at about 450 nm This absorption originates from the bandgap transition, rather than the impurity level The two-step method slightly increases absorption intensity and causes a weak redshift of the absorption edge With N-doping, the intensity of absorption undergoes a further weak increase, and the edge of absorption has a further weak redshift N-doping is therefore not very helpful for improving the visible light absorption of Bi2WO6 nanoparticles, and mainly helps to decrease therecombination rate of electron-hole pairs Figure 5.8 Absorbance change of Figure 5.9 First-order kinetics plot for 553 nm peak as a function of RhB degradation of samples H:N-0, irradiation time in the presence of MH:N-0, MH:N-0.1, MH:N-0.25, H:N-0, MH:N-0, MH:N-0.1, MH:N-0.5, MH:N-0.75 MH:N-0.25, MH:N-0.5, MH:N0.75 The photocatalytic activities of the nanoparticles were investigated using the visible light irradiation of RhB in an aqueous solution Fig 5.8 shows the temporal evolution of UV-VIS absorption of an RhB solution with MH:N-0.5 nanoparticles With increasing irradiation time, the intensity of all absorption 20 peaks of RhB decreases rapidly, indicating very good photocatalytic activity for the MH:N-0.5 nanoparticles In addition to the rapid decrease in intensity, the absorption peak at 545 nm is blue-shifted and broadened, correlating with the N-demethylation and deethylation processes It can be seen that the MH:N-0 nanoparticles shows much higher photocatalytic activity than the H:N-0 nanoparticles This is well explained by the much larger surface area of MH:N0 nanoparticles compared to the H:N-0 nanoparticles Interestingly, all N-doped Bi2WO6 nanoparticles show much better photocatalytic activity than pure Bi2WO6 nanoparticles This enhancement of photocatalytic activity with Ndoping mainly correlates with the decrease in the recombination rate of photogenerated electron-hole pairs With increasing N-doping, the photocatalytic activity first significantly increases, then quickly decreases with further increases in Ndoping, and the highest photocatalytic activity is obtained for the MH:N-0.5 nanoparticles This agrees well with the PL results in Fig 5.10 With increasing N-doping, the recombination rate of electron-hole pairs first decreases, and then quickly increases with further increases in N-doping, and lowest recombination rate is obtained for the MH:N-0.5 nanoparticles After being irradiated for 30 min, about 17%, 23%, 51%, 59%, 81%, and 45% of the RhB is degraded by the H:N-0, MH:N-0, MH:N-0.1, MH:N-0.25, MH:N0.5 and MH:N-0.75 nanoparticles, respectively.This suggests that for these samples, both the surface area and the recombination rate of the electron-hole pairs are important in improving the rate of decolorization of RhB under visible light irradiation, and that the recombination rate is more significant For the nanoparticles prepared using the two-step microwave-assisted and hydrothermal method, the surface area is significantly improved over the hydrothermal method, although if the recombination rate is relatively high, the ability to quickly decolorize RhB is still limited This indicates that the surface area of the N-doped Bi2WO6 nanoparticles is still relatively small Thus, a more significant improvement in surface area would be helpful in the further enhancement of photocatalytic activity Graphene has very large surface area and superb optical properties, and we are currently applying graphene in preparing N-doped Bi2WO6 nanoparticles with a larger surface area and superb optical properties, to achieve a better enhancement of photocatalytic activity Fig 5.10 shows the PL spectra of MH:N-0, MH:N-0.1, MH:N-0.25, MH:N-0.5, and MH:N-0.75 nanoparticles The emission peak at ~544 nm originates from the transition of the Bi 6s and O2p hybrid orbit (VB) to the W 5d orbit (CB) in the WO 62- complex It was found that the emission intensities of all N-doped Bi2WO6 nanoparticles were lower than those of pure Bi2WO6 nanoparticles, suggesting a slower recombination rate of excited electron-hole pairs in the N-doped Bi2WO6 nanoparticles Interestingly, the MH:N-0.5 nanoparticles show the lowest emission intensity of all the N-doped Bi2WO6 21 nanoparticles, and are thus the most promising in terms of achieving high photocatalytic activity Figure 5.10 PL spectra of samples MH:N-0, MH:N-0.1, MH:N-0.25, MH:N-0.5, MH:N-0.75 Figure 5.11 Degradation efficiency of RhB by MH:N-0.5 nanoparticles alone and with the addition of IPA and KI In the investigation of the photocatalytic mechanism in MH:N-0.5 nanoparticles, trapping experiments were performed to determine the main active species in the photocatalytic process The results are presented in Fig 5.11 Increasing the addition of isopropyl alcohol as a scavenger of photogenerated electrons causes the photocatalytic activity to be very weakly affected, indicating that photogenerated electrons not contribute a great deal to the degradation of RhB Upon addition of potassium iodide as a scavenger of photogenerated holes, the photocatalytic activity decreased significantly, and with increasing concentration of potassium iodide, the photocatalytic activity is almost quenched This indicates that photogenerated holes are the dominant active species in the degradation of RhB 22 CONCLUSION The Bi2WO6 was successfully synthesized by microwave assisted method The good crystal Bi2WO6 was obtained at pH=1, microwave irradiation time of 20 mintes and annealing temperature of 500 oC.Our results indicated that surface area of the nanoparticles plays major role for increasing photocatalytic activity: the photocatalytic activities increases about linearly with increasing particle surface area; but weakly affected by crystal quality This suggested high efficiency of transportation of photo-generated electrons and holes to the active sites before recombination The Bi2WO6 modified with Gd doping and composite with BiVO4 was also synthesisized via a microwave assisted method The Gd-doped Bi2WO6 nanoparticles exhibit significantly higher visible-light-driven photocatalytic activity than pure Bi2WO6 nanoparticles, and with about 2.5% Gd doping.The enhancement of photocatalytic performance of Gd doped Bi2WO6 nanoparticle would be mainly correlated with the decrease of recombination rate of hotogenerated electron–hole pairs with Gddoping The Bi2WO6 composite with BiVO was also synthesisized via a microwave assisted method The Bi2WO6/BiVO nanocomposite could enhance the photocatalytic activity of Bi 2WO6 nanoparticles, and the 70– 30 Bi2WO6/BiVO nanocomposites exhibited highest photocatalytic degradation efficiency of RhB under visible Mechanism of photocatalytic activity of Bi 2WO6/BiVO4 suggested that particle surface area plays the most important role for improving photocatalytic activity, and the recombination rate of photogenerated electron-hole pairs plays more important role than the amount of light absorbed by nanoparticles for improving photocatalytic activity A two-step microwave-assisted and hydrothermal method is presented here for the synthesis of Bi 2WO6 nanoparticles The results show that this two-step method can significantly enlarge the surface area of nanoparticles compared with a conventional hydrothermal approach Ndoped Bi2WO6 was also synthesized by microwave assisted – hydrothermal method Nitrogen not only replace Oxygen in host Bi2WO6 N doped Bi2WO6 can significantly decrease the recombination rate of photogenerated electron-hole pairs, and the lowest recombination rate can be achieved by N-doping with an atomic ratio of N:Bi = 0.5 It is also shown that both the surface area and recombination rate of photogenerated electron-hole pairs are important in improving the photocatalytic activity of Bi2WO6 nanoparticles Furthermore, trapping experiments indicated that the photogenerated holes are the dominant active species in the photocatalytic process 23 ... photocatalytic Bi2WO6 and Bi2WO6/ BiVO4 activity of pure Bi2WO6 nanoparticles nanocomposite is much higher than that of pure BiVO4 nanoparticles The photocatalytic activity of all Bi2WO6/ BiVO nanocomposites... MODIFIED Bi2WO6 NANOPOWDERS VIA A MICROWAVE ASSISTED METHOD 4.1 Results of synthesis and study properties of Bi2WO6/ BiVO4 nanocomposites Figure 4.3 presents the XRD patterns of pure Bi2WO6 nanoparticles,... pure Bi2WO6 nanoparticles The photocatalytic activity of 70–30 and 50–50 Bi2WO6/ BiVO nanocomposites is higher than that of pure Bi2WO6 nanoparticles, while the photocatalytic activity of 80–20 Bi2WO6/ BiVO4