| The shortage of fossil fuels and the environmental pollution are hot issues of global concern.Hydrogen energy,as a kind of"zero pollution,zero carbon,high energy density,environmentally friendly and widely sourced"sustainable clean energy,plays an important role in global energy distribution.Photocatalytic technology has been widely concerned by researchers,which can utilize natural solar energy as the driving force to realize photocatalytic decomposition of water to produce hydrogen and degradation of organic pollutants,Therefore,in order to use hydrogen energy as a renewable energy bridge and efficiently remove organic pollutants from water,it is urgent to find a cheap and efficient photocatalyst.Recently,graphite phase carbon nitride?g-C3N4?has been favored by many researchers because of its wide sources,only composed of carbon and nitrogen,environmental protection,low cost and easy access,good thermal stability and corrosion resistance,band gap width of about 2.7 eV,and strong reduction ability.However,the photogenerated electrons-holes pairs are easy to recombine when pure g-C3N4 is excited by light,which is due to the fact that photogenerated electrons and holes are located in the same 3,s-triazine ring?holes are located in N and electrons are located in C?.Additionaly,g-C3N4 possesses small specific surface area and low solar energy utilization,which greatly limits the application of g-C3N4 in the field of photocatalysis.Therefore,it is of great significance to improve the performance of g-C3N4 in photocatalytic decomposition of water to produce hydrogen and degradation of organic pollutants by regulating the morphology,introducing the carbon medium and designing the donor-acceptor?D-A?conjugated polymer to inhibit the recombination of charge carriers and improve the utilization rate of light energy.In this thesis,the effects of band structure,physical/chemical properties and photoelectric properties on photocatalytic activities and mechanism were studied by adjusting the morphology of g-C3N4,constructing carbon media coupling g-C3N4-based heterojunction and g-C3N4-based D-A conjugated polymer.The main contents of this thesis are as follows:1.Controllable preparation of g-C3N4 with different morphologies and its behavior and mechanism of enhancing photocatalytic activity?1?The ultrathin graphitic carbon nitride?ug-C3N4?is prepared by intercalated hydrogen bond effect of NO3-for the first time reported.The thickness,specific surface area,nitrogen vacancy intensity and band gap energy of ug-C3N4 nanosheets can be controlled by the concentration of NO3-in the inserted layer.Compared with bulk g-C3N4?bg-C3N4?,the ug-C3N4 nanosheet not only possesses large specific surface area,but also has nitrogen vacancy-rich that provides more active sites and speed up the photogenerated charge transfer.Meanwhile,the optimized photocatalytic activity of ug-C3N4-7 for hydrogen evolution(836.3?mol h-11 g-1)and degradation of2-Mercaptobenzothiazole?MBT,84%,2h?is about 4.0 and 1.75 times of that of bg-C3N4.?2?The g-C3N4 microtubes?mg-C3N4?with adjustable orifice closure and spatial anisotropic of charge are established by using precursor reforming strategy to overcome hydrogen bonding.Compared with bg-C3N4 and ug-C3N4,the photocatalytic hydrogen evolution rate of mg-C3N4 has been obviously enhanced.Particularly,the mg-C3N4-24 shows the best photocatalytic hydrogen evolution activity(957.9?mol h-11 g-1),which is over 18.72 and 3.77 times higher than the bg-C3N4 and ug-C3N4,respectively.In addition,the experimental results of selective photodeposition reveal the carrier migration behavior,that the photogenerated electrons migrate to the shell,and the holes tend to migrate to the inner shell of mg-C3N4,so as to achieve efficient space anisotropic charge separation.2.Construction of carbon media coupling g-C3N4-based composites and study on its photocatalytic properties and mechanism?1?We have successfully prepared 2D/2D Cu2WS4/g-C3N4 heterojunction modified by yeast-derived carbon?YC?spheres as bridge of charge carriers by simple hydrothermal method.The introduction of YC sphere is not only advantageous in accelerating charge transfer,but also broadening the range of light absorption.Moreover,the YC spheres can also increase specific surface area and surface roughness to enhance the adsorption capacity of to pollutant molecules,which provide abundant active sites for photocatalytic reactions.The g-C3N4/YC/Cu2WS4heterojunction dispaled the efficient photocatalytic activity for reducing hexavalent chromium Cr???and degradation of tetracycline?TC?under visible light.Meanwhile,the acute toxicity of Daphnia magna?D.magna?gradually decreases with the conversion of Cr???to Cr???in solution.In addition,the possible intermediate products and the photocatalytic reaction mechanism are revealed in depth.?2?The Z-scheme g-C3N4/Bi2WO6 heterojunctions modified by nitrogen-doped graphene quantum dots?NGQDs?was successfully prepared by a simple hydrothermal method,which dramatically enhance the electron-hole separation efficiency and light harvest ability.NGQDs modified Z-scheme g-C3N4/Bi2WO6heterojunctions exhibited outstanding universal photocatalytic degradation activity for removing the various antibiotics.NGQDs can be used as an effective active site and electron transport medium to significantly improve the photocatalytic degradation of TC,ciprofloxacin?CIP?and oxytetracycline?OTC?.In addition,using electron spin resonance?ESR?technology and active species trapping experiments,we found that the main active species of NGQDs modified Z-scheme g-C3N4/Bi2WO6heterojunctions in photocatalytic degradation of organic pollutants are holes,superoxide radicals?·O2-?and hydroxyl radicals?·OH?.?3?Urea formaldehyde resin-carbonized nitrogen doped carbon?UFR-NC?ribbons modified g-C3N4 nanosheets were prepared by a facile thermal treatment method.The g-C3N4/UFR-NC composites not only show larger specific surface area,better crystallinity and outstanding stability but also have high separation efficiency of charge carriers.Additionally,g-C3N4/UFR-NC composites possess superior optical adsorption from visible to near-infrared region?NIR?light and the band gap can be easily adjusted by changing the content of UFR-NC ribbons.The g-C3N4/UFR-NC0.02exhibits the highest photocatalytic hydrogen evolution activity(1686.4?mol h-1 g-1),which is over 54.75 and 6.51 times higher than that of the g-C3N4 obtained by direct calcination of melamine?g-C3N4-M?and direct calcination of urea?g-C3N4-U?under visible light.In addition,the g-C3N4/UFR-NC0.02.02 displays an enhanced photocatalytic hydrogen evolution activity of 531.9?mol h-1 g-1 and 9.0?mol h-1 g-1 under the blue visible??=475 nm?and NIR-light irradiation??>800 nm?.3.Design of g-C3N4-based D-A conjugated polymer and study on the properties and mechanism of photocatalytic decomposition of water to produce hydrogen?1?A novel intramolecular g-C3N4-MFx donor-receptor?D-A?conjugated copolymers with a porous structure and large specific surface area have been facilely prepared by copolymerizing urea with melamine-formaldehyde?MF?resin.The as-prepared porous intramolecular g-C3N4-MFx D-A conjugated copolymers not only enlarge the light utilization but also accelerate the separation of charge carriers.Compared to the pure g-C3N4,the specific surface area of g-C3N4-MF100 is clearly increased,and the conduction band is significantly shifted up.Compared to the pure g-C3N4,the porous g-C3N4-MF100 D-A conjugated copolymer achieves the best photocatalytic hydrogen evolution activity(3612.65?mol h-11 g-1).In addition,the apparent quantum efficiency of the porous intramolecular g-C3N4-MF100 D-A conjugated copolymer reaches 8.6%at 420 nm.?2?Ultrathin g-C3N4-BIx D-A conjugate polymer was prepared by copolymerization of benzoyl isothiocyanate?BI?with urea.The ultrathin g-C3N4-BIx D-A conjugated copolymers not only represent effective suppression of charge reverses recombination but also broaden the range of light absorption.Additionally,ultrathin g-C3N4-BIx D-A conjugated copolymers has higher crystallinity and the conduction band position is up-shifts compared with pure g-C3N4.And the transient absorption spectroscopy also indicates that ultrathin g-C3N4-BI400 D-A conjugated copolymer shows the effectively forward transfer ability of charge carriers.Ultimately,compared with other synthesized samples,ultrathin g-C3N4-BI400 D-A conjugated copolymer achieves the best photocatalytic hydrogen evolution activity(5442.74?mol h-1 g-1).And the AQE of 23.7%and 7.0%is achieved at 420 nm and 450 nm that far exceeds majority of the previously reported g-C3N4-based D-A conjugated structures and ultrathin g-C3N4 nanosheets. |
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