Preparation Of G-C₃N₄ Based Composites And Its Photocatalytic Performance Study

With the rapid development of the economic,two issues of the environmental pollution and energy crisis have troubled us.As known,the semiconductor photocatalysts,with the property of converting the solar energy into chemical energy under the irradiation of light,could be used as an ideal option.Those photocatalysts,with promoting the chemical reaction,completely oxidative decomposition of organic pollutants,the catalytic oxidation of water decomposition hydrogen and oxygen,reduction of carbon dioxide in the atmosphere,showed great promising in the environmental pollution and energy shortage.Graphite carbon nitride?g-C₃N₄?material with the unique band structure and good thermal stability,is widely used in decomposing water to hydrogen production,degradation of environmental pollutants and photocatalytic reduction of carbon dioxide.However,due to the disadvantages of itself,such as the smaller specific surface area,the highly recombination rate of photo-generated electron-hole pairs,the lower quantum efficiency and narrow visible light absorption and so on,which could hardly application in the field of energy in the environment widely.This paper devotes to further expand the g-C₃N₄ visible-light response composites photocatalytic materials and its application in the field of environmental energy.First,preparation of the g-C₃N₄ with enhanced photocatalytic performance by increasing the surface area and utilization ratio of visible light.Secondly,preparation of the Z-Scheme structure by combining with other semiconductor compounds such as Ag₃PO₄/Ag,which could increase the separation efficiency of photoelectron and further improve the catalytic performance of the catalyst under visible light.Finally,preparation of carbon quantum dots modified porous g-C₃N₄/TiO₂ composites.There the dual functional of carbon quantum dots with catalytic decomposition of hydrogen peroxide to produce hydrogen and oxygen,and upconversion fluorescence to convert the longer wavelengths visible light to short UV light to excite the TiO₂,could increase visible light absorption,the photogeneration of electrons and holes,decrease the combination of g-C₃N₄ photogeneration of electrons and holes to improving the photocatalytic hydrogen production catalytic activity of the g-C₃N₄.The microstructure morphology,optical properties,physical and chemical properties of the composites photocatalysts are characterized by the X-ray diffraction?XRD?,transmission electron microscope?TEM?,scanning electron microscope?SEM?,X-ray photoelectron spectroscopy?XPS?,UV-visible absorption spectrum?UV-vis?,energy dispersive spectroscopy?EDS?,fluorescence spectroscopy?PL?,all above prove that the formation of the composites material.Further,the visible light water decomposition hydrogen production are characterized by the meteorological chromatograph and the mechanism of hydrogen production enhancement are studied.The researches are as follows:1.Preparation and photocatalytic properties of the g-C₃N₄First,the bulk g-C₃N₄ obtained by the melamine were calcinated at 550 ?,then the g-C₃N₄ nanosheets and porous g-C₃N₄ nanosheets obtained by oxidizer oxidation such as concentration H₂SO₄,HNO₃.On the other hand,the g-C₃N₄ nanorods are prepared by hydrothermal method.Then the structure and performance of the materials were characterized by SEM,TEM,XRD,EDS and UV-vis.At the same time,the performance of its photocatalytic degradation of MB were characterized.2.Preparation and photocatalytic properties of the Ag₃PO₄/Ag/g-C₃N₄ nanocompositesThe hydrogen production efficiency could increase by suppressing the compound of photoelectrons and holes and improving the quantum efficiency.The g-C₃N₄ nanosheets prepared in the first chapter is used as the substrates,the Ag₃PO₄/Ag/g-C₃N₄ nanosheets composites prepared by the light reduction method and coprecipitation.The composites material exhibits a good catalytic activity and optical stability.By calculating,the visible light photocatalytic performance of this composites increase from 0.84??mol/g?h to 4.1??mol/g?h,that nearly five times than the unmodified g-C₃N₄ nanosheets,which could be ascribed to the g-C₃N₄ nanosheets and Ag₃PO₄/Ag have formed the special Z-Scheme structure,which can promote the separation of electrons and holes effectively.Further,the amount of Ag₃PO₄/Ag attached about the hydrogen production under visible light are studied.3.Preparation and photocatalytic properties of the carbon quantum dots modified porous g-C₃N₄/TiO₂ nanocompositesFirstly,carbon quantum dots were prepared by hydrothermal method.Then,via direct coupling method,carbon quantum dots were deposited on porous g-C₃N₄-TiO₂.The photocatalytic performance of this composites increase from 0.07??mol/g?h to 6.4??mol/g?h under visible light,nearly two orders of magnitude than the unmodified g-C₃N₄,it can be attributed to the up-coverted photoluminescence behaviour of carbon quantum dots and the C QDs possess catalytic activity of H₂O₂ decomposition.