Construction And Performances Of Highly-efficient G-C₃N₄ Photocatalysts For Water Splitting And Organic Degradation

Polymeric graphitic carbon nitride（g-C₃N₄）is a visible-light driven semiconductor photocatalytic material.The conjugated polymerization structure endows g-C₃N₄outstanding optical properties,unique electronic structure and good physicochemical stability.Therefore,g-C₃N₄ has become a research hotspot in the application field of solar energy conversion and environmental remediation.In this thesis,g-C₃N₄ based photocatalytic materials are modified based on the intrinsic problems of g-C₃N₄ in photocatalytic reactions,such as low photogenerated electron-hole separation and migration efficiency,small specific surface area,serious lamella stacking,limited visible light response and weak interface coupling.Defect engineering,functional composite construction of heterojunctions,nanostructure design,element co-doping and other methods are targeted to optimize g-C₃N₄-based photocatalytic materials to maximize the realization of efficient visible-light photocatalysis of different systems oriented to green energy conversion or environmental remediation.The connection between improved photocatalytic performance and structure regulation of g-C₃N₄ is studied,which provides a new guiding idea for the design and construction of novel g-C₃N₄-based photocatalytic systems.In view of the inherent problems of g-C₃N₄ with photo-generated electron-hole recombination and low photoexcited carrier separation efficiency,nitrogen defects are reasonably introduced into the g-C₃N₄ framework.The graphitic carbon nitride modified with nitrogen defects was successfully prepared by in situ thermal polymerization of dicyandiamide assisted by insoluble carbonate（Ba CO₃）.The addition of Ba CO₃ will affect the thermal polycondensation process of g-C₃N₄ and the periodic arrangement of g-C₃N₄ interlayer chains.The heptazine ring structure of g-C₃N₄ is optimized.Accordingly,the opened units of heptazine ring of g-C₃N₄ are modified with cyano groups（-C≡N）,and nitrogen vacancies are formed at the position of N_2C.Cyano groups serve as the reactive sites of photocatalytic reactions,and the surface charge polarization caused by these strong electron-trapping sites help to generate more effective photoexcited carriers,greatly improving the separation efficiency of photogenerated electrons and holes,thus enabling efficient multifunctional photocatalytic applications.The concentration of surface nitrogen defect states can be effectively regulated by changing the mass of auxiliary additive Ba CO₃,thereby further regulating the photocatalytic performance of materials.The sample with the best performance achieved a reaction rate of 71.57μmol h^-1 in the photoreduction-driven catalytic water splitting hydrogen evolution reaction under visible light irradiation,which was about 13.5 times that of pristine g-C₃N₄.The photocatalytic degradation kinetic rate constant of methyl orange（MO）reaches 5 times that of pristine g-C₃N₄,thus realizing the photocatalytic reduction and oxidation applications of water splitting for hydrogen evolution and degradation of organic pollutants.Besides the high probability of photo-excited carrier recombination and the low separation efficiency of photogenerated electron and holes,the small specific surface area,insufficient exposure of the reactive area and limited visible light response range of bulk g-C₃N₄ also limit its versatile photocatalytic applications.A one-pot method was developed to fabricate the direct Z-scheme MoO₃/g-C₃N₄ heterojunction by adopting the strategy of construction of hybrid materials,realizing the simultaneous optimization of material morphology,band gap structure and redox ability.The particle size of one-component can be adjusted by changing the concentration of precursors.The obtained structure of heterojunction interface via the adopted method significantly alleviates the packing of bulk g-C₃N₄,thereby the specific surface area is significantly increased(64.3m² g^-1),which exposes more active sites to form oxygen vacancies.The research demonstrates that the existence of oxygen vacancies helps to narrow the intrinsic band gap and expand the visible light absorption range.Therefore,MoO₃/g-C₃N₄ possesses obvious improved ability to capture visible light,thereby increasing the utilization efficiency of solar energy.Meanwhile,MoO₃/g-C₃N₄ follows the Z-scheme charge transfer mechanism during photocatalytic reactions,which endows MoO₃/g-C₃N₄ with high redox ability.The properties that strong reduction driving force at conduction band（CB）position and strong oxidability at VB position make it the best candidate for versatile photocatalytic materials.Compared with pristine g-C₃N₄,the photocatalytic hydrogen evolution activity of MoO₃/g-C₃N₄ is significantly improved by nearly 17 times,while the photocatalytic degradation activity of MO is significantly improved to a greater extent.The enhancement of interfacial coupling performance plays a crucial role in improving the photocatalytic activities.g-C₃N₄ combined with suitable electron transfer materials（such as carbon dots,CDs for short）can improve the photocatalytic activity,and the constructed heterojunction forms an internal electronic field for accelerating space charge separation.Nevertheless,the problem of weak interfacial coupling needs to be addressed urgently.A cooperative coupling strategy is utilized to fabricate the S,Cl co-doped g-C₃N₄/CDs heterojunction hybrid materials（hereafter referred to as CSCl-C₃N₄）with the interface coupling in the form of the strongest chemical covalent bond.The photocatalytic performance of CSCl-C₃N₄ can be regulated by the concentration of CDs.The direct calcination method for composites makes 0.5CSCl-C₃N₄ represent as ultrathin 2D nanosheets with the thickness of about 3.12 nm,rich pore structure and significantly increased specific surface area.Confirmed by a series of theoretical calculations and experimental characterization results,the optimization of the electronic structure and optical response via co-doping and CDs conjointly narrow the band gap of g-C₃N₄ and extend the visible light absorption range,thereby significantly accelerating the charge transfer between CDs and heteroatom-doped g-C₃N₄.Uniformly distributed CDs that act as both electron acceptors and donors,heteroatom doping that promotes delocalization ofπ-conjugated electrons,and tightly connected heterostructures greatly facilitate the efficient charge transfer,migration and separation of photogenerated carriers.The photocatalytic water splitting of 0.5CSCl-C₃N₄ with suitable CDs concentration reaches the optimal H₂ evolution rate of 192.49μmol h^-1,which is about 25 times that of pristine carbon nitride(7.74μmol h^-1).The kinetic rate constant k(0.28 L mg^-1 min^-1)toward photodegradation of the antibiotic tetracycline was 15.3 times that of pristine g-C₃N₄(0.018 L mg^-1 min^-1).