Construction Of Ternary Z-scheme Heterojunction Based On G-C₃N₄ And Its Application In Visible Light Photocatalysis

With the continuous consumption of global oil and natural gas,the energy shortage and environmental pollution crisis are becoming more and more serious.Thus,it is urgent to find a new alternative energy source.Hydrogen,as a sustainable fuel with high energy density and pollution-free,has drawn the increasing attention of researchers.Photocatalytic water splitting by capturing abundant solar energy is an economical and environmentally friendly way to produce hydrogen.Among many photocatalysts,g-C₃N₄ as a metal-free polymer n-type semiconductor material,is widely used in the field of photocatalytic water splitting to produce hydrogen due to its unique photoelectrochemical properties and suitable band structure.Unfortunately,the shortcomings of pure g-C₃N₄,such as small specific surface area,easy and rapid recombination of photogenerated electrons and holes,and narrow response range of visible light,hinder its practical application.Therefore,it is necessary to improve the preparation method of g-C₃N₄ and construct a composite photocatalyst based on g-C₃N₄.In addition,because the Z-scheme heterojunction photocatalyst that simulates the photosynthesis mechanism of natural green plants has unique advantages,it is worth to exploring the construction of Z-scheme heterojunction based on g-C₃N₄.This paper mainly focuses on the mechanism of Z-scheme photocatalysis and the research background,improvement methods,hydrogen production performance and reaction mechanism of Z-scheme photocatalyst based on g-C₃N₄.By improving the preparation method of g-C₃N₄,ultrathin two-dimensional porous g-C₃N₄ nanosheets with different pore sizes were obtained,which increased its specific surface area and provided more reactive sites.Based on this result,two novel g-C₃N₄-based ternary Z-scheme heterojunctions with further improved visible light utilization and photogenerated carrier separation efficiency were successfully constructed,leading to an increase in the efficiency and stability of photocatalytic hydrogen production.This work can be used for reference in the design of efficient and stable g-C₃N₄-based Z-scheme photocatalyst.（1）The top-down and bottom-up template-free methods were used to synthesize ultrathin two-dimensional porous g-C₃N₄ nanosheets with different pore diameters.Through systematic characterization,their morphology and structure were compared and their photoelectrochemical properties were explored.The formation of a two-dimensional porous structure is not only conducive to the absorption and utilization of visible light,but also provides more reactive sites that act as traps to reduce the rate of photogenerated carrier recombination.（2）One-dimensional WO₃ nanotubes（WNT）,synthesized by simple hydrothermal method,were embedded into P-doped two-dimensional porous g-C₃N₄ nanosheets（PCNS）by electrostatic self-assembly method.The above-mentioned photocatalyst was further loaded with r GO as a co-catalyst by hydrothermal method.Due to the synergistic effect of the 1D/2D structural advantage and the Z-scheme electron transfer mechanism,the r GO/PCNS/WNT photocatalyst exhibits enhanced photocatalytic activity and excellent stability under simulated sunlight irradiation.（3）A novel porous Fe₂O₃ nanoplate was synthesized by a simple hydrothermal method,which was further coupled with ultrathin porous g-C₃N₄ nanoplates and r GO to form a ternary Z-scheme heterojunction through self-assembly.The photocatalytic hydrogen production activity of r GO/g-C₃N₄/Fe₂O₃ under visible light is significantly improved.The characterization results show that the formation of the Z-scheme electron transfer mechanism makes the photocatalytic system exhibit stronger carrier separation efficiency and extended carrier lifetime.As an electronic medium,r GO further enriches the photogenerated electrons in g-C₃N₄,increasing the efficiency of the photocatalytic reaction.Finally,the overall photocatalytic performance of r GO/g-C₃N₄/Fe₂O₃ is improved.