本科毕业设计(论文).docx

本科毕业设计(论文)题目本科毕业设计(论文)FINAL PROJECT/THESIS OF UNDERGRADUATE(2017届)Preparation and application of g-C3N4/TiO2 composite with visible light photocatalytic activity 学院材料科学与工程专业材料科学(卓越班)学生姓名学号指导教师完成日期13。承诺人(签名)：_日期： 年 月 13Abstract The collection of solar energy through photocatalysts and subsequent production of renewable energy sources (such as hydrogen) is an attractive and sustainable solution to our energy problem. Due to the potential in environmental and energy-related applications, metal oxide semiconductor photocatalysts have attracted worldwide attention.Most of the photocatalysts studied previously had the problem of low quantum efficiency, which was mainly due to the rapid recombination of charge within the photocatalyst,while the charge was slowly transferred at the interface of the photocatalyst. For example, electron-hole recombination in TiO2 usually occurs in the nanosecond time scale, whereas the interface charge transfer usually takes almost a microsecond time. This may be worse when applying internal modifications such as doping to improve their optical performance. However, when the heterojunction is formed among the different semiconductors, the charge recombination can be remarkably suppressed. g-C3N4 has been widely recognized as an ideal semiconductor for the manufacture of heterostructures with various semiconductors for the following reasons: (1) visible light absorption bandgap ( 2.75eV)，and its efficient photocatalytic performance under visible light (2) minimum conduction band (CB) (-1.12eV vs. NHE) promoting the electron to another semiconductors CB and thus its photogenerated electrons have a strong reducing ability. (3) cheap and simple synthesis which can have a wide use in production practice.TiO2 (P25) is one of the polymorphs of titania having n-type electron conductivity. Relatively open structure , a large number of surface OH groups and simple synthesis method makes a variety of modifications and large-scale production becomes feasible.For the first time, g-C3N4/TiO2 is synthesized by hydrothermal method using melamine sponge and TiO2 (P25) as the raw materials.At the same time, g-C3N4/TiO2 is also prepared by the same hydrothermal method using TiO2 and bulk g-C3N4 which is calcinated by dicyandiamide directly. On this basis, the pure physical adsorption of raw materials and calcined products is used as a control experiment. Through the change of raw material ratio,we have found the best ratio of g-C3N4/TiO2,leading an-extended-life charge which is due to the efficient separation of electrons and holes. The improvement of photocatalytic activity was proved by the control experiment and the degradation of the dye.KEYWORDS: Titanium dioxide;Nanoparticle; heterostructures;Graphitic carbon nitrideContent1. INTRODUCTION11.1 historical context of Tio221.2 TiO2 feature21.3 titanium dioxide(TiO2)-based heterogeneous photocatalysis31.3.1 a promising candidate for photocatalysis and electrocatalysisg-C3N4 31.3.2 main problem in g-C3N431.3.3 g-C3N4/TiO2 photocatalysts mechanism31.4 the key to the g-C3N4/TiO2 photocatalysts41.5 Properties of melamine sponge(MS)41.6 Synthesis of g-C3N4/TiO2 composite61.5 Subject support and main research contents102. EXPERIMENTAL DETAILS112.1 The main chemical reagents and instruments112.2 Sample preparation112.2.1 Preparation of graphene oxide112.2.2 Preparation of crumpled graphene oxide122.2.3 Material Processing-heat shock (N-doped Crumpled graphene)132.2.4 Material Processing-slow heating132.3 Characterization132.3.1 Specific surface area measurement132.3.2 Scanning electron microscopy (SEM)142.3.3 X-ray powder diffraction142.3.4 Fourier transform infrared spectroscopy142.4Electrochemical Test142.4.1 Galvanostatic charge/discharge142.4.2 Cyclic voltammograms152.4.3 Electrochemical impedance spectroscopy153. RESULTS AND DISCUSSION163.1 Influence of heat treatment process on electrochemical performance of the samples163.1.1 Influence of heat treatment process on structure of N-doped CG163.1.2 Influence of heat treatment process on electrochemical performance of N-doped CG Figure 3.7 (A) Typical Galvanostatic charge/discharge curves of symmetric ultracapacitor devices at 0.1A/g, 0.5A/g, 1A/g, 2A/g, 5A/g, 10A/g, 20A/g using urea-CG . (B) Typical charge/discharge curves of symmetric ultracapacitor devices at 0.1A/g, 0.5A/g, 1A/g, 2A/g, 5A/g, 10A/g, 20A/g using urea-CG -60 Conclusion213.2 Influence of urea amount on electrochemical performance of the samples223.2.1 Influence of urea amount on the structure of N-doped CG223.2.2 Capacitance test of N-Doped CG prepared by different urea amounts243.2.3 Conclusion274. CONCLUSION28REFERENCES29ACKNOWLEDGMENT31Preparation and application of g-C3N4/TiO2 composite with visible light photocatalytic activity 1. IntroductionSince the beginning of the 20th century, the continuous progress of science and technology and the rapid development of industry have brought comfort and convenience to mankind.However it caused serious environmental pollution and deterioration, which has brought potential danger to human health and life. 1-3 Among the various environmental pollution, chemical pollution is the most common, dangerous and influential one.Therefore, the effective control of various chemical pollutants on the water,soil and atmospheric environment have been the focus of comprehensive environmental management. Over the years,people have been dedicated to manage environmental pollution, through such as physical,chemical or biological treatment and so on ,but there are many shortcomings. Therefore,finding a newsolution on chemical pollution has a great significance. Photocatalysis is one of the unique properties of nanocrystals.In the light irradiation, nano-semiconductor materials absorb the light and produce the photogenerated electrons and holes ,which possessing super-oxidizing capacity and reducing ability, promoting the synthesis or degradation of compounds (organic or inorganic). 4 Figure 1.1 Air Quality Indexi in Shanghai1.1 historical context of TiO2In 1972, Fujishima and Honda 5 firstly published a paper ,using TiO2 as a photocatalyst for hydrogen production, which marked the beginning of the era of photocatalysis.At that time, the energy crisis has broke out, and the development of photocatalyst and solar energy on the energy crisis has a great significance, which causing widespread concern from scientific research scholars. And then in-depth exploration on the mechanism of photocatalysis have been carried out. In 1977, Frank and Bard et al. 6 used TiO2 as a photocatalyst to decompose cyanide in water and oxidize CN to OCN ， providing a new method to the treatment of sewage. These major research have also laid the foundation for the research of catalyst in environmental purification and new energy development.1.2 TiO2 featureTiO2 ,which is non-toxic, high-catalytic-activity, good-stability and low-price, etc, is recognized as an excellent semiconductor photocatalyst. The strong oxidizing ability of the photogenerated holes of nano-TiO2 makes it possible to completely oxidize the unbiodegradable organic pollutants. A large number of studies have shown that most of the organic matter can be photocatalytic oxidation and degradation. In addition, many inorganic compounds or inorganic ions can also react with photocatalytic electrons on the surface of TiO2 to produce less toxic or non-toxic products.Thus in the atmospheric purification, antibacterial, water purification and anti-fouling,TiO2 has a broad application prospect.Figure 1.2 Nano titanium dioxide1.3 titanium dioxide(TiO2)-based heterogeneous photocatalysisOver the past few decades, titanium dioxide photocatalysis has been used for the efficient degradation of organic compounds, dyes, pesticides and drugs 7-10. However, the photocatalytic activity of TiO2 in visible light is poor ,owing to its high band gap (3.2 eV) and the recombination rate of the photo-generated electron-hole pairs, which limits its practical applications. To enhance the visible-light response and the lifetimes of its charge carriers, people have designed various strategies .11-15Whereas, it remains a long-standing problem unresolved and the efficiency remains insufficient.1.3.1 a promising candidate for photocatalysis and electrocatalysisg-C3N4Recently, the metal-free graphitic carbon nitride has attracted enormous interest due to its narrow band gap (2.7 eV), which could respond to the visible light about 460 nm.It is a promising candidate for photocatalysis and electrocatalysis owing to its unique structure and electronic properties. The two-dimensional planar structure consisted of -conjugation system facilitates the transmission of charge carriers. According to the study of Wang et al. 16, the conduction band (CB) of g-C3N4 were 1.3 V and the valence band (VB) were 1.4 V at pH 7 vs the normal hydrogen electrode (NHE), respectively. Therefore, light-excited electrons in the CB of g-C3N4 have a great thermodynamic driving force to reduce O2 (E(O2/O2)= 0.16 V).Figure 1.3 Mimic diagram of g-C3N41.3.2main problem in g-C3N4Light holes in the VB of g-C3N4 is not enough to oxidize OH to hydroxyl radicals(E(-OH /OH) = 2.4V) 17. However, the photocatalytic performance of g-C3N4 isstill limited due to its weak oxidative ability of photo-generated hole and the rapid recombination of charge within the photocatalyst.1.3.3 g-C3N4/TiO2 photocatalysts mechanism To surmount the photocatalytic activity of semiconductor photocatalyst, development of g-C3N4/TiO2 photocatalysts has aroused increased attention due to its efficient separation of photogenerated electronhole pairs in two different semiconductors 18-20. There, the electrons in the CB of g-C3N4 migrate to the CB of TiO2 and recombine with the VB holes.Subsequently, the well-separated holes and electrons in the VB and CB of TiO2 and CN have stronger oxidation and reduction ability than single photocatalysts.1.4 the key to the g-C3N4/TiO2 photocatalystsAs has been mentioned above, the key to promote charge separations between heterostructures is to establish a strong interface between the two semiconductors. In the case of g-C3N4, most of the previous systems were prepared by simple mixing of g-C3N4 with TiO2, which could not guarantee large interface contact 21. On the other hand, TiO2 ,with large amounts of surface OH groups, strongly promote the formation of g-C3N4 during MS degradation. 22 More importantly, FT-IR shows that the formation of g-C3N4 will be firmly bound to the surface of TiO2, which indicates that it will establish a good interface connection. TiO2(P25) belongs to the mixed crystal type, consisting of anatase and rutile which weight ratio is about 80:20.Due to the hybrid of the two structure, the defect density in lattice of TiO2 increases, which increases carrier concentration.The number of electrons and holes increases, making it more capable of capturing the solution components (water, oxygen, organic) on the surface of TiO2. TiO2(P25) relatively open structure, a large number of surface OH groups and cheap price makes a variety of modifications and large-scale production becomes feasible. The synthesized g-C3N4/TiO2 makes the charge have an extended lifetime and the improved photocatalytic activity due to the efficient separation of electrons and holes. As far as l am concerned, this is the first report on the synthesis of g-C3N4 / TiO2 using melamine sponge(MS) condensation.1.5 Properties of melamine sponge(MS)Melamine sponge is a new type of foam with a high porosity of the three-dimensional grid structure. It has excellent sound absorption, flame retardancy, adiabaticty, thermal stability, safety and secondary processing and other comprehensive performance. These features make the product have a wide range of applications (such as heat-insulation in buildings, transportation, aviation, electromechanical commodity and household appliances, etc.) and broad market prospects.sound-absorbing: melamine sponge has a fully open three-dimensional grid structure system, which length to diameter ratio ( L / D) is between about 10 to 20.Its high porosity characteristics make the sound waves get into the deep layer of sponge easily and effectively and transformed to the vibration energy which is consumed and absorbed late.And the reflected wave is effectively eliminate. The excellent absorption of low-frequency noise exhibited by melamine sponge has aroused great interest from experts in the acoustic industry.flame retardancy: melamine sponge begins to burn in the surface only when it is contacted with the fire.Once it starts to burn, a large number of inert non-combustible gas immediately generates,which slows down the burning speed.At the same time, the surface of melamine sponge will quickly form a dense coke bed which effectively prevent the process of combustion.The fire is automatically extinguished after leaving the fire. For a scientific evaluation standard, melamine sponge can meet the standards for B1 grade low flammable materials (German standard), UL94-V0 grade high flame retardant material standard without adding flame retardants. The general sponge such as polyolefins, polystyrene, polyurethane, etc. often need flame retardants for their own flammable characteristic.Many of flame retardant will be thermally decomposed at high temperatures, and release the poisonous gas,which causes secondary disasters and environmental pollution.The two important indicators in material combustion evaluation, which is often overlooked by the user, is flowing drop propertyand density of smoke. General sponge ,such as polyethylene, polystyrene, polyurethane, etc. are burning in the combustion and produce droplets, which will quickly cause the spread of fire. Flame retardant modification to these materials is useless in the above flaws. At the same time, polystyrene, polyurethane in the combustion process can also produce a large number of toxic fumes, flame retardant modification and the addition of smoke inhibitors tend to bring side effects such as increasing toxicity and cost. The melamine sponge does not generate droplets when burning.The amount of smoke generated by melamine sponge is much lower than the above materials, and so the toxicity is.Thermal stability: Melamine sponge is thermosetting plastic, with a high degree of three-dimensional network of cross-linked structure system.Compared with thermoplastic materials such as polyethylene,polystyrene and polyurethane with lower degree of contact, melamine sponge has a high thermal stability and aging resistance. Melamine sponge can work at 150 for a long time,and can work a at the temperature of 180 for a short time without decomposition and deformation. It will be obviously decomposed only above 400 . While,the operating temperature of polyolefin, polystyrene, polyurethane and other sponge is only 80 , the deformation and decomposition will occure above 80 .Thermal insulation： melamine sponge is light material, with 810kg/m3 density. Its opening rate is up to 99% .Three-dimensional grid structure of the melamine sponge inhibits convective heat transfer effectively, coupled with its unique thermal stability, making it a great lightweight thermal insulation materials.Safety: melamine sponge ,made in high temperature,does not have any residual free formaldehyde.Its stable chemical structure and crosslinking system makes it having a unique chemical stability,which can meet the requirement of food sanitation . These features make it used widely in the commodity, interior decoration and transportation noise reduction and other fields. secondary processing: melamine sponge can be processed by cutting, lathing and other means to shape it into a cone,wedge and etc.,which can meet the requirements of sound absorption. Melamine sponge can be surface treated by spraying to improve their surface properties. Surface treated by Haipulong and urethane, which can adjust its hygroscopicity, melamine sponges surface self-cleaning will improve.1.6 Synthesis of g-C3N4/TiO2 compositeFreeze - drying method：technically known as lyophilisation, lyophilization, or cryodesiccationis a dehydration process typically used to preserve a perishable material or make the material more convenient for transport. Freeze-drying works by freezing the material and then reducing the surrounding pressure to allow the frozen water in the material to sublimate directly from the solid phase to the gas phase. There are four stages in the complete drying process: pretreatment, freezing, primary drying, and secondary drying.1.Pretreatment: Pretreatment includes any method of treating the product prior to freezing. This may include concentrating the product, formulation revision (i.e., addition of components to increase stability, preserve appearance, and/or improve processing), decreasing a high-vapor-pressure solvent, or increasing the surface area. In many instances the decision to pretreat a product is based on theoretical knowledge of freeze-drying and its requirements, or is demanded by cycle time or product quality considerations.2.Freezing: In a lab, this is often done by placing the material in a freeze-drying flask and rotating the flask in a bath, called a shell freezer, which is cooled by mechanical refrigeration, dry ice in aqueous methanol, or liquid nitrogen. On a larger scale, freezing is usually done using a freeze-drying machine. In this step, it is important to cool the material below its triple point, the lowest temperature at which the solid and l