Efficient Visible Light Photocatalyst Based On Heptazine Polymer: Design,Synthesis And Catalytic Properties

Photocatalysis can convert solar energy into chemical energy,which is one of the ideal solutions to solve energy consumption and environmental pollution.One of its core issues is the design of photocatalysts.The C and N are abundant elements on earth,therefore,carbon nitrogen materials have great application prospects as photocatalysts.A large number of carbon nitride photocatalysts have been synthesized in the past decades,among which heptazine derivatives have attracted much interest due to their unique electronic structures and excellent stability.It is well known that g-C₃N₄has excellent optoelectronic properties and wide applications in the fields of catalysis,sensing,and energy.Heptazine,as the basic unit of g-C₃N₄,is the source of excellent physicochemical properties and photocatalytic activity.Therefore,it is of great theoretical and practical significance for the development of heptazine derivatives.However,at present,research on heptazine derivatives other than g-C₃N₄is still in its infancy.Based on the above,a series of novel heptazine photocatalysts were successfully prepared around the rational design and development of heptazine-based polymer photocatalysts by selecting different organic precursors and cleverly designing the synthetic routes.These heptazine polymers have good visible light absorption and excellent photocatalytic activity,and the photocatalytic activity and the related reaction mechanism have been systematically and thoroughly investigated.The main research progresses are as follows:1.Inspired by the polymerisation of urea,we have prepared a series of novel C-C bridged heptazine CNs UO_xby replacing part of urea with oxamide through high-temperature thermal polymerization,which are close to（C₆N₇）_n.The conjugated structure of UO_xwas effectively extended from an individual heptazine to the entire material.Consequently,its bandgap was reduced to 2.05 e V,and its absorption band edge was significantly extended to600 nm.Furthermore,its carrier transfer and separation were significantly enhanced,establishing its superior photocatalytic activity.Among them,the best sample UO₂has excellent photocatalytic activities for hydrogen evolution,diphenylhydrazine oxidation,and degradation of various phenols,and is a promising and efficient heptazine-based 2D carbon nitrogen.2.Most heptazine-based polymers such as g-C₃N₄are mainly prepared by high temperature thermal polymerization.In the process,high temperatures not only make it more difficult to control the degree of polymerization,but also cause thermal decomposition of organic monomers and thus make the structure of products unclear.To address the above challenges,we prepared a novel two-dimensional heptazine-based covalent organic framework material（COF-MUC）by simple solvothermal reaction using1,3,4,6,7,9-Hexaaza-trindene-2,5,8-trione（UC）and classical melem as precursors,drawing on the Schiff base reaction of carbonyl-amino groups.The energy band of COF-MUC is reduced to 0.98 e V,and the spectral absorption is greatly extended to the near-infrared region.In addition,COF-MUC is enriched with a large number of photocatalytic active sites,resulting in improved spectral absorption,carrier separation and interfacial reactions compared to conventional g-C₃N₄.Therefore,COF-MUC is endowed with ultra-high activity and selectivity for photocatalytic benzylamine coupling and oxidation of methyl phenyl sulfide.The work provides an important reference for the design of novel heptazine-based COFs and holds new promise for the construction of efficient,broad-spectrum absorbing heptazine-based polymeric photocatalysts.3.Improving the conjugation of materials is considered as one of the effective methods to expand the spectral absorption and enhance photocatalytic activity.However,excessive pursuit of conjugation may also lead to narrow band gap and reduced redox ability.In addition,it may also increase the difficulty of extracting carriers and thus reduce the photocatalytic activity.Therefore,properly improving the local conjugation of photocatalytic materials,which both enhances their spectral absorption and facilitates the separation of photogenerated carriers,becomes the key and challenge for designing and constructing new efficient photocatalysts.We successfully prepared UC units-modified g-C₃N₄CN_xby introducing the aforementioned UC monomer into the thermal polymerization process of urea.The UC monomer modulates the conjugation of g-C₃N₄while forming alternating donor-acceptor structure with the electron-deficient melon,thus leading to substantial increase in its photocatalytic activity.Among them,CN_0.02exhibited better activities than g-C₃N₄in photocatalytic hydrogen evolution,oxidation of methyl phenyl sulfide,and degradation of halogenated phenols.4.In addition to constructing D-A structures,the development of novel and efficient Z-scheme heterojunction photocatalysts is also a prospective way to solve the spectral absorption and carrier separation.Herein,a new Z-scheme photocatalyst（abbr.APM/C₃N₄）was fabricated by a simple self-assembly procedure.Transmission electron microscopy（TEM）images revealed that the 1D silver polyoxometalate nanorods loaded with Ag nanoparticles were well dispersed on the plicated 2D g-C₃N₄nanosheets.These composite catalysts exhibited an excellent and durable photocatalytic performance for the degradation of methyl orange and tetracycline and the photoreduction of Cr（VI）under visible light,which was significantly higher than that of the individual components and most of the previous reported materials.The study may provide a new comprehension into the design of polyoxometalate-based Z-scheme hybrid materials for photocatalytic applications in the removal of pollutants from wastewater.