Nghiên cứu cải thiện khả năng quang xúc tác của gC3N4 biến tính với kim loại (Fe, Co, Mg, Ag) và oxit bán dẫn (TiO2, ZnO)Nghiên cứu cải thiện khả năng quang xúc tác của gC3N4 biến tính với kim loại (Fe, Co, Mg, Ag) và oxit bán dẫn (TiO2, ZnO)Nghiên cứu cải thiện khả năng quang xúc tác của gC3N4 biến tính với kim loại (Fe, Co, Mg, Ag) và oxit bán dẫn (TiO2, ZnO)Nghiên cứu cải thiện khả năng quang xúc tác của gC3N4 biến tính với kim loại (Fe, Co, Mg, Ag) và oxit bán dẫn (TiO2, ZnO)Nghiên cứu cải thiện khả năng quang xúc tác của gC3N4 biến tính với kim loại (Fe, Co, Mg, Ag) và oxit bán dẫn (TiO2, ZnO)Nghiên cứu cải thiện khả năng quang xúc tác của gC3N4 biến tính với kim loại (Fe, Co, Mg, Ag) và oxit bán dẫn (TiO2, ZnO)Nghiên cứu cải thiện khả năng quang xúc tác của gC3N4 biến tính với kim loại (Fe, Co, Mg, Ag) và oxit bán dẫn (TiO2, ZnO)Nghiên cứu cải thiện khả năng quang xúc tác của gC3N4 biến tính với kim loại (Fe, Co, Mg, Ag) và oxit bán dẫn (TiO2, ZnO)Nghiên cứu cải thiện khả năng quang xúc tác của gC3N4 biến tính với kim loại (Fe, Co, Mg, Ag) và oxit bán dẫn (TiO2, ZnO)Nghiên cứu cải thiện khả năng quang xúc tác của gC3N4 biến tính với kim loại (Fe, Co, Mg, Ag) và oxit bán dẫn (TiO2, ZnO)Nghiên cứu cải thiện khả năng quang xúc tác của gC3N4 biến tính với kim loại (Fe, Co, Mg, Ag) và oxit bán dẫn (TiO2, ZnO)Nghiên cứu cải thiện khả năng quang xúc tác của gC3N4 biến tính với kim loại (Fe, Co, Mg, Ag) và oxit bán dẫn (TiO2, ZnO)Nghiên cứu cải thiện khả năng quang xúc tác của gC3N4 biến tính với kim loại (Fe, Co, Mg, Ag) và oxit bán dẫn (TiO2, ZnO)Nghiên cứu cải thiện khả năng quang xúc tác của gC3N4 biến tính với kim loại (Fe, Co, Mg, Ag) và oxit bán dẫn (TiO2, ZnO)Nghiên cứu cải thiện khả năng quang xúc tác của gC3N4 biến tính với kim loại (Fe, Co, Mg, Ag) và oxit bán dẫn (TiO2, ZnO)Nghiên cứu cải thiện khả năng quang xúc tác của gC3N4 biến tính với kim loại (Fe, Co, Mg, Ag) và oxit bán dẫn (TiO2, ZnO)Nghiên cứu cải thiện khả năng quang xúc tác của gC3N4 biến tính với kim loại (Fe, Co, Mg, Ag) và oxit bán dẫn (TiO2, ZnO)Nghiên cứu cải thiện khả năng quang xúc tác của gC3N4 biến tính với kim loại (Fe, Co, Mg, Ag) và oxit bán dẫn (TiO2, ZnO)Nghiên cứu cải thiện khả năng quang xúc tác của gC3N4 biến tính với kim loại (Fe, Co, Mg, Ag) và oxit bán dẫn (TiO2, ZnO)Nghiên cứu cải thiện khả năng quang xúc tác của gC3N4 biến tính với kim loại (Fe, Co, Mg, Ag) và oxit bán dẫn (TiO2, ZnO)Nghiên cứu cải thiện khả năng quang xúc tác của gC3N4 biến tính với kim loại (Fe, Co, Mg, Ag) và oxit bán dẫn (TiO2, ZnO)Nghiên cứu cải thiện khả năng quang xúc tác của gC3N4 biến tính với kim loại (Fe, Co, Mg, Ag) và oxit bán dẫn (TiO2, ZnO)Nghiên cứu cải thiện khả năng quang xúc tác của gC3N4 biến tính với kim loại (Fe, Co, Mg, Ag) và oxit bán dẫn (TiO2, ZnO)Nghiên cứu cải thiện khả năng quang xúc tác của gC3N4 biến tính với kim loại (Fe, Co, Mg, Ag) và oxit bán dẫn (TiO2, ZnO)Nghiên cứu cải thiện khả năng quang xúc tác của gC3N4 biến tính với kim loại (Fe, Co, Mg, Ag) và oxit bán dẫn (TiO2, ZnO)Nghiên cứu cải thiện khả năng quang xúc tác của gC3N4 biến tính với kim loại (Fe, Co, Mg, Ag) và oxit bán dẫn (TiO2, ZnO)Nghiên cứu cải thiện khả năng quang xúc tác của gC3N4 biến tính với kim loại (Fe, Co, Mg, Ag) và oxit bán dẫn (TiO2, ZnO)
MINISTRY OF EDUCATION AND TRAINING HANOI NATIONAL UNIVERSITY OF EDUCATION - LAM THI HANG INVESTIGATING THE ENHANCEMENT OF PHOTOCATALYTIC PERFORMANCE OF g-C3N4 MODIFIED WITH METALS (Fe, Co, Mg, Ag) AND SEMICONDUCTING OXIDES (TIO2, ZnO) Specialization: Solid State Physics Code: 9.44.07.04 SUMMARY OF THE DOCTORY OF PHYSICS Hanoi, 2024 The project was completed at: HANOI NATIONAL UNIVERSITY OF EDUCATION Science instructor: 1: Prof Dr Nguyen Van Minh 2: Assoc Prof Dr Do Danh Bich Review 1: Assoc Prof Du Thi Xuan Thao – Phenikaa University Review 2: Assoc Prof Nguyen Dinh Lam – VNU University of Engineering and Technology Review 3: Assoc Prof Pham Van Hai - Hanoi National University of Education The thesis has been defended before the School-level Thesis Judging Committee meeting at Hanoi National University of Education on 2024 Thesis can be found at the library: - National Library, Hanoi - Library of Hanoi National University of Education 1 PREAMBLE In the last few decades, on planet Earth, the rapidly growing textile, dyeing, tanning, organic chemical and petrochemical industries have contributed significantly to organic pollution of water resources Organic toxins often released from these industries are pesticides, herbicides, organic dyes, etc., which mix directly with clean water and pollute water sources Synthetic organic dyes used in the textile, leather and paper industries are highly toxic, mutagenic, carcinogenic and seriously affect aquatic ecosystems and have the potential to cause health problems serious problems related to human health Nowadays, the treatment of environmental pollution, especially the treatment of water pollution, has become a hot and concerned issue worldwide and the treatment of polluted water is a major persistent challenge by scientists around the world Therefore, in the field of water treatment, researchers have constantly made efforts and persistently discovered modern and effective technologies to remove toxic organic substances from polluted water In particular, the technology of decomposing toxic organic substances by photocatalysis is a widely used environmentally benign technique, using clean energy sources (natural light) to decompose substances organic pollutants into non-toxic or less toxic products and thus effectively overcome environmental pollution However, photocatalytic water treatment also faces some challenges because its effectiveness depends on many different factors such as the type of catalyst, wavelength of light, and bandgap of the substance catalysis Using semiconductor materials as catalysts in the process of treating water pollution is a highly appreciated idea in the green chemistry industry (researching chemicals to treat environmental pollution) Some popular types of materials that are currently being researched include metal oxides (TiO2, ZnO, WO3 ), ferroelectric materials with ABO3 perovskite structure (BiFeO3, BaTiO3, SrTiO3), ABO4 semiconductor compounds (ZnWO4, SnWO4) … However, most of these materials have a large band gap (> 3.2 eV), so they almost only absorb light in the ultraviolet region, accounting for about 4% of the solar spectrum Currently, finding semiconductor materials with small band gaps is a topic that attracts great attention from research groups around the world with the goal of taking advantage of sunlight sources in applications photocatalysis, helping to expand application scale, reduce costs and increase convenience Besides, narrow band semiconductor materials also have great potential in the field of energy conversion or clean fuel production such as Hydrogen and Oxygen To meet the goal of using sunlight, semiconductor materials need to meet a number of requirements such as: (i) band gap less than 3.2 eV (380nm); (ii) large contact surface area and (iii) small electron and hole recombination rate Recently, the material g-C3N4, a non-metallic organic semiconductor with unique electronic structure and optical properties with a small band gap (on the order of 2.7 eV), has received attention Extensive research by scientists around the world The g-C3N4 material possesses a number of superior physical properties such as high hardness, non- toxicity, chemical and temperature stability in different environmental conditions, large specific surface area, and high efficiency relatively high quantum and biocompatible, Therefore, this material has potential applications in a number of fields such as photoelectric conversion, temperature sensing, chemical sensing, biomedicine, and especially in the field 2 of photocatalysis to extract H2 fuel from water, decompose CO2 gas and clean organic pollution in the water environment So far, g-C3N4 materials with diverse morphologies such as nanosheets, nanowires, porous nanostructures and thin films have been researched and manufactured using different technological processes such as vapor phase deposition ( CVD and PVD), solvothermal, and pyrolysis from C- and N-rich precursors, etc Unlike metal-containing semiconductor photocatalysts, g-C3N4 can be easily synthesized by thermal polymerization from C and N rich precursors such as dicyanamide, cyanamide, melamine and urea However, research shows that g-C3N4 material also has low quantum efficiency due to the high electron-hole recombination rate; The absorption edge is at about 460 nm, so it only absorbs the blue light region of the solar spectrum Besides, g-C3N4 particles tend to cluster together, reducing the specific surface area, leading to reduced photocatalytic efficiency Recently, research on modifying g-C3N4 materials to increase the lifetime of electron-hole pairs, reduce the band gap energy and increase the specific surface area is the top priority solution for the research of g-C3N4 materials Some basic measures to improve quantum efficiency and promote photocatalytic activity of g-C3N4 materials include: (i) controlling surface morphology, creating thin nanoleaf structures, porous structures or quantum dots, quantum wires, to increase the specific surface area; (ii) combine the material with some other semiconductors to increase the lifetime of the electron-hole pair, while reducing the band gap of the material; (iii) coating the g-C3N4 surface with some metal nanoparticles that act as electron reservoirs (Pt, Ag or Au nanoparticles); (iv) doping non-metal elements (P, S, O), transition metals (Fe, Cu, Zn) to reduce the band gap while creating an electron capture center from the g-C3N4 crystal In Vietnam, research direction based on g-C3N4 materials is still quite new Currently, the material g-C3N4 has been initially deployed in the research group of Professor Dr Vo Vien belongs to Quy Nhon University The research team focuses on the technology of manufacturing g-C3N4 material from melamine precursor and doping some non-metallic elements (O, S) to enhance photocatalytic activity under visible light of material g-C3N4 In addition, the group also developed composite materials between g-C3N4 and GaN-ZnO or Ta2O5 Research results show that the photocatalytic activity of composite materials increases significantly compared to that of the component materials The research team's results supported two PhD students to successfully defend their PhD thesis in Chemistry In 2018, the research group of Prof Dr Nguyen Ngoc Ha - Department of Chemistry, Hanoi University of Education received funding from the National Foundation for Science and Technology Development (Nafosted) for material research nano composite materials based on g-C3N4 and diatomite to effectively treat reactive dyes In 2022, the PhD thesis of author Dang Thi Ngoc Hoa of Hue University also researched the synthesis of g-C3N4 composite for application in electrochemistry and photocatalysis The author focuses on researching composite materials such as ZIF-67/g-C3N4, ZIF-67/Fe2O3/g-C3N4, TiO2/g-C3N4 with precursors for making g-C3N4 is melamine and focuses on photocatalytic decomposition of Methylene Blue (MB), Diclofenac (DCF), Auramine O (AO) 3 To our knowledge, apart from the above research groups, g-C3N4 material has not yet been researched or widely announced in Vietnam In this thesis, we choose to research the production of g-C3N4 material from urea precursor using simple pyrolysis method, this is a cheap chemical, easy to find, friendly and process research Manufacturing technology to achieve thin, well-crystallized foil samples, suitable for laboratory conditions of the Department of Physics, Hanoi University of Education From there, a good quality sample was selected to conduct "Investigating the enhancement of photocatalytic performance of g-C3N4 modified with metals (Fe, Co, Mg, Ag)" These metals are cheap, chemically simple, have good conductivity and have been shown to have good results in improving the photocatalytic ability of g-C3N4 Besides, we also chose to modify with "semiconducting oxides (TiO2, ZnO)" because these are two promising photocatalytic materials for environmental applications with outstanding properties such as: Good photocatalytic properties, low cost, easy to manufacture and non-toxic Objectives of the thesis: (i) Research the influence of sample manufacturing conditions on the structure, physical properties and photocatalytic ability of g-C3N4 material, from which to select methods and conditions Suitable technological conditions to manufacture g-C3N4 thin-leaf material with good nano-crystalline size (ii) Improve the photocatalytic ability of g-C3N4 base material by modifying with metal elements (Fe, Co, Mg, Ag) and combining materials with semiconducting oxides (TiO2, ZnO) to reduce the band gap while creating an electron capture center, increasing the lifetime of the electron- hole pair From there, evaluate the influence of the concentration of modified metals as well as the percentage of combined samples on the photocatalytic ability of g-C3N4 material Research subjects: - Nano sheet material g-C3N4 - Nanomaterial g-C3N4 modified with metals Fe, Co, Mg, Ag - Nanomaterial g-C3N4 combined with semiconductors TiO2 , ZnO Research Methods: The thesis is based on experimental methods, the sample is manufactured mainly by polymerization through pyrolysis of N-rich organic precursors A number of manufacturing technologies are applied to synthesize the material materials such as pyrolysis in a noble gas environment, pyrolysis in an air environment Materials were manufactured at the Department of Physics and Center for Nano Science and Technology, Hanoi University of Education Fabricated samples are analyzed for crystal structure and physical properties using a number of techniques such as: X-ray diffraction (XRD), scanning electron microscopy (SEM, FE-SEM), electron microscopy transmittance (TEM) and high resolution transmittance (HRTEM), infrared absorption spectroscopy (FTIR), surface area and pore volume measurement (BET), UV-Vis absorption spectroscopy, fluorescence spectroscopy (PL), photoelectron spectroscopy (XPS), Raman scattering spectroscopy The fabricated samples were used to perform photocatalytic processes for decomposing 10 ppm RhB solution The concentration of remaining organic compounds was measured indirectly through UV-Vis optical absorption spectroscopy 4 In addition, the thesis also uses a number of software to exploit and analyze and calculate physical parameters of materials from experimental data such as Origin, UniCell, ImageJ, JCPDS standard card library Scientific and practical significance of the project: With the orientation of researching and applying g-C3N4 materials in the field of photocatalysis, the thesis has built a process for manufacturing g-C3N4 base materials using the A simple method is urea pyrolysis This is a cheap but highly effective method of using precursors This contributes to proposing a technological process for effectively manufacturing semiconductor materials that can be applied in the field of treating some organic waste in the aquatic environment Modifying the material by doping metals and combining it with other semiconductors increases the photocatalytic ability of the g-C3N4 base material The material has good photocatalytic ability to decompose some organic compounds such as RhB, oriented for application in decomposing some toxic organic substances in wastewater samples in domestic and craft villages; Actively contribute to the process of cleaning the living environment The content of the thesis includes: Overview of g-C3N4 materials, experimental techniques, research results and analysis of the effects of sample manufacturing conditions; The influence of Fe, Co, Mg, Ag metals on the structure, optical properties of materials and photocatalytic ability of g-C3N4 base materials; Results of studying the structure and properties of g-C3N4 materials combined with semiconductor TiO2 and ZnO Layout of the thesis: The thesis is presented in 145 pages with 22 tables and 109 figures, including an introduction, 5 content chapters, a conclusion, a list of research works and references As follows: Introduction: Introduces the reason for choosing the topic, the object and purpose of the research, and the scientific significance of the thesis Chapter 1: Presents an overview of the structural properties, morphology, physical properties and some research on photocatalytic orientation of g-C3N4 materials The typical properties of g-C3N4 materials are the basis for analyzing results on pure g-C3N4 and g-C3N4 model systems denatured with metals and g-C3N4 combination in chapters 3, 4 and 5 Chapter 2: Presents methods and procedures for sample fabrication, process for evaluating photocatalytic ability, principles of measurements used in analyzing material properties used in the thesis Chapter 3: Research on the effects of technological conditions on the crystal structure, physical properties and photocatalytic ability of g-C3N4 materials Chapter 4: Research on physical properties and photocatalytic ability of g-C3N4 material modified with metals Fe, Co, Mg, Ag Chapter 5: Research on physical properties and photocatalytic ability of g-C3N4 material combined with semiconductors TiO2, ZnO Conclusion: Presents the main results of the thesis The main results of the thesis have been published in 07 scientific works (including 04 articles published in international specialized journals, 03 articles published in domestic specialized journals) 5 Chapter 1 OVERVIEW 1.1 Material g-C3N4 1.1.1 Structural properties g-C3N4 crystal has a hexagonal structure, belonging to the P 6̅m2 space group According to the research results, the base unit cell of the g-C3N4 crystal has 56 atoms, including 32 N atoms and 24 C atoms a) b) Figure 1.2 (a) Unit cell and (b) AB-type layered structure of the crystal of g-C3N4 1.1.3 Surface morphology of g-C3N4 material Figure 1.6 TEM images of g-C3N4 material from different precursors The produced g-C3N4 material is usually in the form of a porous material However, the pore volume, pore size distribution and specific surface area of g-C3N4 depend on the precursor and material fabrication method 1.1.4 Optical properties of g-C3N4 materials The g-C3N4 layer unit with the gh-heptazine structure is a semiconductor with an indirect band gap Accordingly, the band gap value is 2.76 eV with the valence band maximum (VBM) at point Γ and conduction band minimum (CBM) located at point M Meanwhile, the band gap energy in real The experimental range is from 2.67 eV to 2.95 eV 1.1.5 Photocatalytic properties of g-C3N4 materials The photocatalytic ability of g-C3N4 can be applied to treat organic pollutants such as: Rhodamine B (RhB), Methylene Blue (MB), Methyl Orange (MO), Phenol, 1.2 Photocatalysis mechanism and application potential of g-C3N4 materials 1.2.1 Photocatalytic mechanism of g-C3N4 materials 1.2.2 Application potential of g-C3N4 materials 1.3 Some manufacturing methods of g-C3N4 materials 1.3.1 Sol-gel method 1.3.2 Hydrothermal method 1.3.3 Heat polymerization method 1.4 Some research directions to improve photocatalytic properties of g-C3N4 materials 1.4.1 Combination of g-C3N4 with other materials g-C3N4 can be combined with many other semiconductors to create heterosemiconductor materials such as: TiO2, WO3, ZnO, Ag2WO4 Studies show that the modification of g-C3N4 materials by uniform combination aims to reduce the recombination of electron-hole pairs in the material to enhance the photocatalytic ability 6 1.4.2 Modification of g-C3N4 with metallic elements Modification of g-C3N4 material by doping elements Fe, Co, Mg, Cu, Na, K, Zr, Mn or coating metal nanoparticles such as Au, Ag, Pt on the surface material surface g-C3N4 has been studied by several groups These studies show that modifying g-C3N4 material by uniform doping gives better photocatalytic ability than pure g-C3N4 material Chapter 2 EXPERIENCE 2.1 Material manufacturing process 2.1.1 C fabrication of pure g-C3N4 material 2.1.2 Fabrication of g-C3N4 doped Fe/Co/Mg 2.1.3 Fabrication of g-C3N4 materials coated with Ag metal nanoparticles 2.1.4 The model systems are fabricated and studied in the thesis Table 2.1 Symbols of the model systems used in the thesis Pure g-C3N4 model system was made in Ar gas environment Temper 450 o C 500 o C 550 o C 600 o C 650 o C ature change gCN(Ar450 gCN(Ar)500 gCN(Ar)550 gCN(Ar)600 gCN(Ar)650 Change 0.5 h 1.0 h 1.5 h 2.0 h 2.5 h gCN(Ar)1.0h gCN(Ar)1.5h gCN(Ar)2.0h gCN(Ar)2.5h time gCN(Ar)0.5h Sample system g-C3N4 purified in air environment Temper 450 o C 500 o C 550 o C 600 o C 650 o C ature gCN-650 change gCN-450 gCN-500 gCN-550 gCN-600 Change 0.5 h 1.0 h 1.5 h 2.0 h 2.5 h time gCN-0.5h gCN-1.0h gCN-1.5h gCN-2.0h gCN-2.5h Model system g-C3N4 doped with Fe/Co/Mg metal Doped 0% 3% 5% 7% 10% with Fe g-C3N4 FeCN3 FeCN5 FeCN7 FeCN10 Doping 0% 7% 8% 10% 12% Co g-C3N4 CoCN7 CoCN8 CoCN10 CoCN12 Mg 0% 7% 8% 10% 12% doping g-C3N4 MgCN7 MgCN8 MgCN10 MgCN12 Model system g-C3N4 coated with Ag/Au metal Coated 0.00M 0.005M 0.007M 0.01M 0.03M 0.05M 0.1M with Ag g-C3N4 gCN/Ag nanopar gCN/Ag gCN/Ag gCN/Ag gCN/Ag gCN/Ag ticles 0.005M 0.007M 0.01M 0.03M 0.05M 0.1M Au 0.00M 0.001M 0.003M 0.005M 0.007M 0.009M nanopar g-C3N4 gCN/Au gCN/Au gCN/Au gCN/Au gCN/Au ticle 0.001M 0.003M 0.005M 0.007M 0.009M coating 2.2 Photocatalytic testing of organic matter decomposition 2.3 Methods for investigating the physical properties of model systems Measurements taken to analyze the properties of materials include: X-ray diffraction measurements; Scanning electron microscopy measurement; High resolution transmission and transmission electron microscopy; Raman scattering 7 spectroscopy; Infrared absorption spectrometry; Method of measuring absorption spectroscopy; X-ray photoelectron spectroscopy method; Fluorescence spectroscopy method; Differential thermal analysis method; Nitrogen adsorption-desorption isotherm method Chapter 3 RESEARCH IN FABRICATION OF GRAPHITIC CARBON NITRIDE MATERIALS g-C3N4 3.1 The g-C3N4 system was manufactured in an Ar atmosphere 3.1.1 Effect of calcination temperature 3.1.1.1 Crystal structure Figure 3.1 (a) XRD diagram of g-C3N4 sample system made from Urea precursor in Ar atmosphere at different temperatures ; (b) Change in crystal lattice constant according to sample heating temperature The XRD pattern shows 3 diffraction peaks at the angle 2θ about 12.47°; 24.59°and 27.17° The diffraction intensity increased sharply from the calcination temperature of 450 oC to 550 oC and gradually decreased as the calcination temperature continued to increase 3.1.1.2 Surface morphology Figure 3.2 SEM of g-C3N4 sample system fabricated from Urea in Ar atmosphere at varying temperatures (a) 450, (b) 500, (c) 550 and (d) 600 C °for 2 time now 8 Figure 3.2 shows that the sample calcined at a temperature of 450 °C has a morphology similar to a large, uneven membrane with holes and many folds on the surface Figure 3.3 (a) Nitrogen adsorption-desorption isotherm and (b) Barrett-Joyner- Halenda (BJH) pore volume distribution curve of g-C3N4 material c) Chemical composition analysis Figure 3.4a presents the composite XPS spectrum of sample gCN(Ar)550 showing characteristic peaks of elements C, N and O at energies of 288 eV, 400 eV and 533 eV Figure 3.4 X-ray photoelectron spectroscopy of g-C3N4 material fabricated in Ar environment at 550 oC for 2 hours: (a) synthesized XPS spectrum and high-resolution XPS spectrum of (b) N1s state, (c) ) C1s and (d)O1s e) Absorption properties 11 Chapter 4 INCREASING THE PHOTOCATATIC ABILITY OF g-C3N4 MATERIALS BY METAL DOPING 4.1 The g-C3N4 system doped with Fe metal 4.1.1 Structural properties All samples did not exhibit any diffraction peaks of Fe crystals The (101) and (002) diffraction peaks shift slightly to the left as the Fe concentration increases (Figure 4.2b) The lattice constants are calculated as (a = b = 4.97 Å, c = 6.47 ) and (a = b = 4.98 Å, c = 6.48 ) for the doped samples, respectively FeCN3 and FeCN5 The increase in crystal structure parameters shows a certain change of the g-C3N4 crystal upon Fe doping, leading to a less dense structural pattern in the crystal lattice This change can be due to the alternating doping configuration of large radius Fe ions in the g-C3N4 crystal by chemically bonding with the six unpaired electron-paired nitrogen atoms as shown in Figure 4.2, leading to crystal lattice expansion 4.1.2 Fluctuating nature The intensity of all absorption peaks increased as the Fe content increased Magnification of the FTIR absorption peaks (Figure 4.3b) shows a slight shift of the 814 cm−1 peak toward higher wavenumbers as the Fe content increases Magnification of the FTIR absorption peaks (Figure 4.3b) shows a slight shift of the 814 cm-1 peak toward higher wavenumbers as the Fe content increases, to 812.1, 813, 813, and 813.9 cm-1 for the g-C3N4, FeCN5 and FeCN7 samples Meanwhile, the peaks at 1240 cm-¹ and 1320 cm-¹ almost do not change position This further shows that the influence of Fe impurity on the g-C3N4 lattice structure , although very small, leads to a slight expansion of the benzene ring as observed in the XRD analysis 4.1.3 Nitrogen BET adsorption - desorption spectroscopy results The BET surface areas are 91, 100, 132 and 104 m2/g for g-C3N4, FeCN5, FeCN7 and FeCN10, respectively This result shows that the specific surface area increases slightly when doping Fe into the g-C3N4 crystal lattice This result shows that the specific surface area 12 increases slightly when doping Fe into the g-C3N4 crystal lattice Because a large specific surface area is beneficial for photocatalytic activity, we predict that the FeCN7 sample with the largest BET surface area will have high photocatalytic activity Figure 4.4b also shows that the average pore size of all samples is about 35-40 nm 4.1.4 Optical absorption properties The absorbance of this tail gradually increases with increasing Fe content, which can be reasonably explained by the incorporation of Fe into the g-C3N4 lattice , leading to the formation of impurity energy levels in the restricted area Table 4.2 Band gap energy values of g-C3N4 samples doped with Fe with different concentrations Sample gC 3 N 4 FeCN3 FeCN5 FeCN7 FeCN10 Eg ( eV) 2.92 2.83 2.8 2.81 2.8 4.1.5 Luminescent properties It is clear that the PL intensity of the Fe-doped g-C3N4 sample is significantly reduced compared to that of the pure g- C3N4 nanoparticles Because the luminescence intensity reflects the recombination rate of electron-hole pairs, the lower the PL intensity, the slower the recombination rate The sharp decrease in PL intensity indirectly shows that the recombination rate of electron-hole pairs is low, which is necessary for improving photocatalytic performance The reason for reducing the recombination rate of electron-hole pairs may be due to the presence of Fe2+/Fe³+ ion in the g-C3N4 crystal lattice, which acts as an electron capture center When excited, the electron receives energy from a photon and jumps from the top of the valence band to the bottom of the conduction band to become a free electron Then, the electron easily moves to the impurity level of Fe3+ due to its location in the forbidden band As a result, the lifetime of the electron-hole pair increases, which is beneficial for the photocatalytic process 4.1.6 Chemical composition analysis Figure 4.7 XPS spectra of Fe-doped g-C3N4 samples with different concentrations 13 The pure g-C3N4 material exhibits characteristic peaks of C, N and O at 284 eV, 397 eV and 532 eV respectively while the FeCN7 sample also shows a sharp XPS peak at 710 eV energy 4.1.7 Photocatalytic properties of RhB degradation The photocatalytic performance was significantly improved for all Fe-doped g-C3N4 samples The RhB decomposition rate gradually increases with the doping Fe content, reaching the highest value for FeCN7 and then decreasing when increasing the Fe content to 10% RhB solution was almost completely decomposed after 30 minutes for sample FeCN7 while sample FeCN5 needed about 50 minutes and sample FeCN10 needed about 60 minutes A first-order kinetic model is used to determine the photocatalytic reaction rate, ln(Co /C) = kt, where the rate constant k is calculated from the slope of the linear relationship of plot of ln(Co/C) versus reaction time (Figure 4.8b) The FeCN7 sample exhibits the greatest reaction rate constant (k~0.117), which is about 10 times larger than that of pure g-C3N4 (k~0.012) The order of samples with strong to weak photocatalytic ability is FeCN7, FeCN5, FeCN10, FeCN3, g-C3N4 The photocatalytic ability of the FeCN7 sample has good stability, the photodegradation rate of RhB is about 95% after three cycles of reuse 4.2 Model system g-C3N4 doped with Co metal 4.2.1 Structural properties The position of the (002) diffraction peak shifts slightly toward the smaller 2 theta angle when doped with Co The (002) diffraction peak shifts toward the small 2 theta angle, demonstrating the decrease of the value 𝑑in the formula 2𝑑𝑠𝑖𝑛𝜃 = 𝑛𝜆 From there, it can be inferred that the lattice constant 𝑐increases slightly with this peak shift This can be explained by the large radius Co2+ ions filling the interstitial space between the heptazine units, causing a slight stretch of the lattice, leading to an increase in the lattice constant 𝑎, 𝑏, 𝑐 4.2.2 Vibration properties g-C3N4 sample and the Co-doped samples both exhibit scattering peaks at the 14 same positions The analysis results of the peak position of 706 cm-1 corresponding to the oscillations of heptazine units are shown in Figure 4.12b, showing that the peak position of the Co-doped samples has a slight shift towards the wave number smaller than the pure sample This observation can be attributed to the presence of Co impurities interspersing the vacancies between the heptazine units in the g-C3N4 lattice that affects the oscillations of these units With different impurity concentrations, the effect is also different This is consistent with the results of X-ray diffraction analysis 4.2.3 Chemical composition analysis Figure 4.15 shows typical XPS spectra of the C1s and N1s states The C1s peak of pure g- C3N4 is analyzed into 3 peaks at positions with binding energies of: 283.0 eV representing the C-C bond, 284.6 eV representing the C=N bond and 286.2 eV characterize the N-C=N bond Regarding the characteristic peak C1s of sample CoCN10, we found that the peak is also decomposed into 3 component peaks at 281.6; 283.1 and 285.0 eV However, based on the intensity and shape of the spectral peak, we determine that the 285.0 eV peak corresponds to the C=N bond Accordingly, the XPS peak tends to shift slightly toward higher binding energy (horizontal arrow) Similarly, the characteristic XPS spectrum of the N1s state in Figure 4.15b also shows a separation into three component peaks corresponding to C-N=C bonds at 395.9; N-(C)3 at 397.2 and C-H-N at 399.2 eV These peaks also tend to shift slightly toward higher binding energies when doped with Co The characteristic peak of the Co2p state includes two component peaks at 780.7 eV and 794.7 eV corresponding to two energy states with different spin levels Co2p3/2 and Co2p1/2 These are the binding energy levels corresponding to the ionic state of the Co impurity that exists in the CoCN10 sample 4.2.4 Optical properties 15 Research shows that Co doping does not change the optical absorption properties of the g-C3N4 material The fluorescence peak intensity of Co-doped samples tends to decrease gradually compared with pure samples Detailed analysis of the position of the component fluorescence peaks (Figure 4.18b) of the 2 samples g-C3N4 and CoCN10 revealed a slight shift of the fluorescence peak towards the low wavelength for the P3 and P4 peaks This is relatively consistent with the observation in the UV- vis spectrum, that the Co impurity does not significantly affect the energy band structure of the g-C3N4 material, however, changes the shape of the absorption base The gradual decrease in fluorescence intensity indirectly reflects the reduced amount of electron-hole recombination, which is favorable for photocatalysis Sample CoCN10 has the lowest fluorescence intensity, promising for the best photocatalytic performance 4.2.5 Test of photocatalytic activity The order of samples with photocatalytic ability from strong to weak is CoCN10 > CoCN12 > CoCN8 > CoCN7 > g- C3N4 and is the same for both cases using different light sources 4.3 Sample system g- C3N4 metal doped Magnesium Mg 4.3.2 Analysis of the chemical composition on the surface Characteristic XPS spectrum analysis of the Mg2p state is presented in Figure 4.21, showing the existence of the impurity element Co in the Co2+ state on the surface of the g-C3N4 crystal 4.3.3 Test of photocatalytic ability Observing Figure 4.23 it was found that the photocatalytic treatment results under sunlight and Xenon lamp light were almost the same Sample MgCN10 has the best photocatalytic performance; RhB solution is almost completely decomposed after 60 minutes of illumination with the catalyst 16 MgCN10 While at the same time, the catalyst, the sample g-C3N4, only decomposed more than 50% of the RhB solution of the same concentration The order of samples with catalytic ability from strong to weak is MgCN10, MgCN8, MgCN12, MgCN7, g-C3N4 4.4 Results of studying the properties of Ag-coated g-C3N4 materials 4.4.1 Research results on structural properties and grain morphology of g-C3N4 coated with Ag Figure 4.24a presents the diffraction pattern (XRD) of the fabricated Ag-coated g- C3N4 samples The results show that, g-C3N4 and g-C3N4 nanosheets coated with Ag clusters with different Ag+ concentrations exhibit similar diffraction peaks The pure g- C3N4 sample exhibits three distinct diffraction peaks at about 13.00; 24.93 and 27.65o correspond to the diffraction planes (100), (101) and (002) of the hexagonal phase of the graphite carbon nitride crystal (JCPDS tag number 87-1526) It can be seen that the XRD intensity of Ag-coated g-C3N4 samples gradually decreases as the Ag+ concentration in the initial solution increases In addition, the (002) peak position has a slight shift toward the larger 2θ angle To clearly observe the shift of the (002) peak position, we normalized this peak intensity and then fit it using a Gaussian function Figure 4.24b shows the fitting curves of the (002) peak of the fabricated samples and the displacement of the peak position can be clearly seen These observations indicate that the Ag clusters had some influence on the crystal structure of g-C3N4 However, the XRD patterns of the samples also showed that no diffraction peak corresponding to Ag crystals was observed in all Ag- coated g-C3N4 samples Figure 4.25 TEM images of pure samples g-C3N4 (ab) and g-C3N4 /Ag 0.01M (cd) The inset of figure (c) shows the Ag nanocluster diameter histogram 17 Figures 4.25c and 4.25d demonstrate the presence of densely distributed and uniformly distributed small sphere-like Ag NPs decorated on the surface of g-C3N4 nanosheets The inset of Figure 4.25c shows a histogram of Ag NP diameters with particle sizes ranging from 3 to 5 nm (average diameter is 4 nm) 4.4.2 Results of X-ray photoelectron spectroscopy (XPS) research Figure 4.26 XPS spectrum (a) and XPS spectrum of C1s (b), N1s (c) and Ag3d (d) atoms of samples g-C3N4 and g-C3N4 /Ag 0.01M Elemental Ag was detected in a 0.01M gCN/Ag sample at a binding energy of about 368 eV 4.4.3 Research results on optical properties g-C3N4 nanosheets pure show the absorption edge at about 430 nm and the absorption edges of the g-C3N4 samples coated with Ag nanoparticles shifted slightly to longer wavelength, indicating a narrowing band gap Accordingly, the bandgap energy estimated using the graph of Tauc (inner part of Figure 4.27a) for the indirect semiconductor was reduced from 2.88 eV for the 4 crystalline g-C3N4 nanosheet purity down to 2.82 eV for gCN/Ag 0.01M sample Furthermore, Figure 4.27a also exhibits an increase in absorbance around 400 nm (arrows pointing upwards) for Ag-coated g-C3N4 samples, which can be assigned to the absorption due to plasmon resonance surface (SPR) of Ag NPs 18 4.4.4 Results of research on photocatalytic ability g-C3N4 samples showed enhancement in both adsorption and photocatalytic efficiency significantly Although after 120 min of irradiation using pure g- C3N4 nanoplates About 25% RhB still existed, but some Ag-coated samples such as gCN/Ag 0.01M, gCN/Ag 0.03M and gCN/Ag 0.1M decomposed almost 100% RhB after only 60 minutes The 0.01M gCN/Ag sample exhibited the strongest photocatalytic activity in the decomposition of RhB, as evidenced by the largest slope of the C/Co curve The order of samples with increasing photocatalytic efficiency is g-C3N4, gCN/Ag0.005M, gCN/Ag0.007M, gCN/Ag0.05M, gCN/Ag0.1M, gCN/Ag0.03M, gCN/Ag0.01M and gCN/Ag0.01M Figure 4.29b shows the obvious change of RhB concentration in the UV-vis absorption spectrum of the 0.01M gCN/Ag heterodimer as a function of time After 50 min of Xenon irradiation, the 554 nm absorption peak of RhB not only disappeared completely but also changed from 554 nm to 530 nm, demonstrating the decomposition of the conjugated RhB structure From the photocatalytic results of RhB decomposition of the obtained material systems, we compared the ability to decompose RhB of pure g-C3N4 samples, FeCN7, CoCN10, MgCN10 and gCN/Ag 0.01M as shown in Figure 4.30 The results showed that, after only 30 minutes of irradiation, FeCN7 samples decomposed 100% of RhB, CoCN10, MgCN10 and gCN/Ag 0.01M samples decomposed about 75, 82 and 91%, respectively, while g-C3N4 samples decomposed g-C3N4 can only decompose about 52% of RhB The first-order kinetic model is also used to determine the photocatalytic reaction rate of the above samples , ln(C o /C) = kt, where the rate constant k is calculated from the slope of the bond linear relationship of the graph ln(Co/C) compared to reaction time (Figure 4.30b) The complete decomposition time of the 10 ppm RhB solution and the k rate of pure g-C3N4, FeCN7, CoCN10, MgCN10 and 0.01M gCN/Ag samples are listed in Table 4.5 Table 4.5 shows that the material g-C3N4 doped with 7% Fe (FeCN7) has a reaction rate of 0.117 , 9.75 times higher than pure g-C3N4 (k = 0.012 ) Samples of g-C3N4 doped with Co, Mg 10% (CoCN10, MgCN10) and samples of g-C3N4 coated with 0.01M Ag