Construction Of G-C₃N₄ Based Heterojunction Photocatalyst And Study On Hydrogen Evolution Performance Of Water Decomposition Driven By Visible-light

Photocatalytic decomposition of water-to-hydrogen production can make use of abundant solar energy to obtain clean and high-energy-density hydrogen fuel,which is widely regarded as one of the most potential strategies to replace traditional fossil energy.However,the development of ideal high-performance,low-cost and environmentally friendly photocatalysts has always been full of severe challenges.Low utilization of photogenerated carriers and weak photocapture ability restrict the improvement of hydrogen production activity.Therefore,in this paper,different functional modifications based on non-metallic semiconductor graphite carbon nitride（g-C₃N₄）were carried out to improve the catalytic performance of hydrogen production by decomposition of water driven by visible light.At the same time,the photoinduced charge transfer mechanism of composite nanomaterials in eosin（EY）dye sensitized system was discussed.The main work is as follows:（1）Three-dimensional（3D）porous g-C₃N₄（UCN）was successfully prepared by hydrothermal pretreatment and annealing process,and then monoclinicβ-Ag VO₃nanorods were uniformly anchored in UCN network by physical mixing method to synthesize a novel type of UCN/β-Ag VO₃p-n heterojunction photocatalyst.The porous structure of UCN provides more active sites for the surface photocatalytic reaction and promotes the mass transfer at the contact interface between UCN andβ-Ag VO₃.UV-vis diffuse reflectance spectra and fluorescence experiments showed that UCN/β-Ag VO₃has good visible light capture ability,which not only broadens the spectral absorption range,but also prolongs the lifetime of light-induced carriers.Moreover,the existence of the built-in electric field in the barrier region established a fast electron transfer channel and significantly improved the hydrogen evolution activity.Under the condition of EY sensitization,the hydrogen production rate of optimized UCN/β-Ag VO₃reached 1716.46μmol g^-1h^-1,which was improved by 51.8 times than that of pure UCN.（2）Graphdiyne（GDY）is a new two-dimensional（2D）carbon material composed of sp and sp²hybrid carbon networks,which has attracted much attention in recent years.The surface chemical activity of GDY can be significantly improved by heteroatom（S/N）doping and in-situ growth of zero-dimensional（0D）Co₃S₄nanoparticles to modify GDY.A kind of 0D-2D/2D nanohybrid material was constructed by coupling functionalized GDY with ultra-thin 2D porous carbon defect g-C₃N₄（DCN）by simple low-temperature self-assembly method.The porous interconnected skeleton structure is beneficial to multicomponent synergistic catalysis and greatly increases the density of light absorption and active sites.Most importantly,as a charge transfer station,functionalized GDY provides a shortcut for the dual induction of electrons and holes.The charge dynamics analysis showed that the electron transfer rate of the composite is 9.27×10⁹s^-1,and the ultra-fast interfacial transfer significantly promotes the charge separation.In addition,photoelectrochemical experiments indicated that the composite nanoreactor has the lowest carrier recombination and minimum hydrogen evolution overpotential.Under the irradiation of 5 W LED light（λ>420nm）,the dye-sensitized hydrogen production rate reached 2075.67μmol g^-1h^-1,which was improved by 77.2 times than pure DCN.This work will provide a new design concept for the construction of multivariate and efficient collaborative nanocomposites.（3）Improving the utilization rate of photogenerated electrons has always been considered as an important bottleneck of high efficiency photocatalytic hydrogen production system.In this work,a ternary system was constructed using S-scheme g-C₃N₄/Co Ti O₃（CN/CTO）heterostructures modified by Co₃O₄quantum dots（CDs）.The S-scheme heterojunction maintains a good carrier separation,and the built-in modulated electric field also gives a strong driving force for charge transfer.CDs can be used as an electron acceptor to provide more active centers to realize the rapid capture of photogenerated electrons.The synergistic utilization of Step-type energy band structure and co-catalyst significantly optimizes the electron transfer path,enhances the space charge separation,and leads to more effective electrons with strong reduction ability to participate in the hydrogen evolution reaction.Using triethanolamine solution（TEOA,15vol%）as sacrificial agent and EY as photosensitizer,the average hydrogen evolution rate of optimized CN/CTO/CDs under visible light irradiation for 5 h was 1971.7μmol g^-1h^-1,which was 102.7,2.5 and 2.3 times of pure CN,CN/CTO and CN/CDs,respectively,and the corresponding apparent quantum efficiency was 3.39%.In addition,photoluminescence and photoelectrochemical experiments further verified the charge transfer mechanism of S-scheme heterojunctions and the co-catalytic performance of CDs.This work provides a valuable suggestion for the integration of multi-component photocatalytic systems.