Carbon Nitride Molecule Structure Regulation And Photocatalytic Water Splitting To Produce Hydrogen,Carbon Dioxide Reduction And Nitrogen Fixation

The sustainable development of society and economy have severely restricted by the global crisis of energy shortages and environmental pollution,making the development of renewable energy a global consensus.Hydrogen is an efficient clean and renewable energy carrier,and an ideal substitute for fossil fuels such as coal and petroleum.Since the use of fossil fuels to obtain hydrogen sources is prone to produce a large amount of greenhouse gases or other pollutants to be emitted,more importantly,water is very abundant on the earth.Therefore,obtaining hydrogen from water is a sustainable,clean and environmentally friendly way.The ways to obtain hydrogen in water are divided into direct cracking to produce hydrogen and indirect use of hydrogen atoms in water as a hydrogen source.At present,photocatalytic water splitting to produce hydrogen is the most ideal way.And using hydrogen atoms in water as a hydrogen source is also the most environmentally friendly way to synthesize valuable hydrogen-containing chemicals.Therefore,the development of high-efficiency,stable and low-cost photocatalysts to regulate the production of hydrogen from hydrogen atoms in water and to synthesize valuable hydrogen-containing chemicals has received widespread attention.This thesis is mainly devoted to regulating molecule structure of photocatalytic material graphite carbon nitride（g-C₃N₄）and it applied to water splitting to produce hydrogen,carbon dioxide reduction and nitrogen fixation.The main researches are summarized as follows:（1）Two-type cyanamide defects g-C₃N₄（CCN）was prepared through the thermal polymerization of thiourea in the presence of KCl.Stable potassium isothiocyanate was in situ generated via thiourea isomerization and then reacted with different amino groups（–NH₂ and=NH）in tri-s-triazine rings to obtain two-type cyanamide defects.Theoretical calculations and experiment results confirm that the ratio of the two-type cyanamide defects could be adjusted by KCl dosage,accompanying tunable energy levels of CCN.The charge carrier transfer and separation of CCN was greatly improved.Furthermore,the existence of cyanamide defects hindered the formation of intermolecular hydrogen bonds among g-C₃N₄,which facilitated the formation of porous structure and exposed more active sites for photocatalytic hydrogen evolution reaction（HER）.As a result,the optimized photocatalyst showed a high HER rate of 4.0 mmolg^-1h^-1,which was 5 times higher than 0.8 mmolg^-1h^-1 for pristine g-C₃N₄.And the apparent quantum efficiency reached up to 10.95%at 450 nm.The findings deepen the understanding on precise molecular tuning of g-C₃N₄.（2）Crystalline carbon nitride with cyanamide defect edges（crystalline CCN）was synthesized by one step polymerization of urea in the presence of KCl.The texture and electronic band structure of carbon nitrides could be facilely tailored by changing KCl dosage.The light absorption edge of crystalline CCN extended to 736 nm due to n→π*electron transition.The enhanced dielectric constants of crystalline carbon nitrides promoted exciton polarization dissociation.The small effective electron mass（me*）in crystalline CCN facilitated me*diffusion.The efficient separation of electrons and holes benefited the formation of internal electric field,showing an 8.56-fold promotion in electron transfer compared to pristine CN.Significantly,femtosecond time-resolved transient absorption demonstrated that the surface electron density on crystalline CCN was enhanced in the presence of salt ions（Na Cl）.As a result,crystalline CCN exhibited 14.9 times higher HER rate than pristine CN under visible light irradiation.The apparent quantum yield for H₂ evolution on crystalline CCN reached to 42%at 420 nm and 9%at 500 nm.This study gets a comprehensive understanding of photocatalytic HERs using carbon nitride photocatalysts.（3）Hydroxyl-grafted oxygen-linked tri-s-triazine-based polymer（HGONTP）is achieved through the polycondensation of h ydrothermally pretreated dicyandiamide（DCDA）.The content of C-O-C linkers and terminal-OH groups in HGONTP can be regulated by the cyclization and hydrolysis degrees of DCDA through the replacement of the pendant-NH₂ groups with-OH groups.The HGONTP photocatalyst exhibits an outstanding light absorption from UV to near-IR,possessing a narrow band gap of2.18 e V,a hydrophilic surface,a large specific surface area of 96.1 m² g^-1 and reduced charge recombination.As a result,HGONTP exhibits a 27.7-fold higher hydrogen evolution rate than that for pristine g-C₃N₄.The apparent quantum yield reaches up to12.6%at 420 nm and 4.1%at 500 nm.The proposed strategy paves a new avenue to design high-performance polymeric photocatalysts used in water.（4）A Ru-Co bimetal center at the interface of Ru/Co Sx with S-vacancy is constructed on graphitic carbon nitride nanosheets（Ru-Vs-Co S/CN）.Two N atoms in N₂are bridged to the Ru-Co center,highly polarizing N≡N bond elongated to double bond order.Because of plasmonic electric-field-enhancing effect,Ru/Co Sx interface boosts energetic electron generation,performing electron transfer from Co Sx via Ru for asymmetrical electron donation to N₂ adsorbate.The Ru-end bound N at Ru-Co center is preferentially hydrogenated.The NH₃ production rate reaches up to 0.438mmol g^-1 h^-1,demonstrating a high apparent quantum efficiency of 1.28%at 400 nm and solar-to-ammonia efficiency of 0.042%in pure water under AM1.5G light irradiation.