Constructing The Electronic Transmission Path In Carbon Nitride And Its Application In Photocatalysis

As a kind of non-metallic and polymeric semiconductor material,g-C₃N₄ has lots of advantages including corrosion resistance,good stability and easily controllable structure.Meanwhile,narrow bandgap makes it response to visible light,thus it becomes a research hotspot in the field of photocatalysis.However,shortcomings including small surface area,lots of defects on the surface,fast electron-hole recombination ability and low visible light response ability seriously influence the process of industrialization.Thus,this thesis aims at improving the photocatalytic performance of carbon nitride by enlarging surface area,repairing surface defects,regulating the molecular structure of dye-sensitive carbon nitride,and finally widening the scope of its application.The main contents and results of this research are as follows.?1?mpg-C₃N₄ prepared by template method has larger surface area?221.3m²/g?,however,more functional defects on the surface of mpg-C₃N₄ formed with increasing surface area.Amino and cyano have been proved the main defects on the surface of as-made mpg-C₃N₄ via chemical methods,i.e.cyano reduction method and deamination method.Melamine with amino groups and triazine structure was selected as‘little patch'to passivate and remedy various defects inside mpg-C₃N₄.Furthermore,the melamine remediation can result in g-C₃N₄/mpg-C₃N₄ junctions,which also favor the electron transfer and charge separation during the photocatalystic reaction.In order to explore feasible applications,R-C₃N₄ was used asphotocatalystforthephotooxidationreactionof1,4-dihydro-2,6-dimethylpyridine-3,5-dicarboxylate?1,4-DHP?and simultaneous H₂evolution.The conversion rates of 1,4-DHP and H₂ production catalyzed by R-C₃N₄ were enhanced 2 and 6.5 times,respectively.This smart design is beneficial for the conversion of1,4-DHP during the preparation of bioactive compounds and the clean hydrogen production at the same time.?2?Photooxidation plays an important role in organic synthesis.However,deep oxidation and degradation could reduce the product selectivity and the final yield of target product,which is a crucial issue in the photooxidation reactions.Decreasing the conduction band?CB?position and keeping the valence band?VB?position of C₃N₄ is a relaible way to solve the above mentioned problem.Herein,with proper highest occupied molecular orbital?HOMO?and lowest occupied molecular orbital?LUMO?energy levels,pure terephthalic acid?PTA?was selected as the monomer candidate to tune the band structure of C₃N₄?P-CN?,which was predicted based on the DFT calculations.P-CN was prepared via a facile copolymerization shows almost unchanged VB position of C₃N₄,and its CB position significantly reduces.Thus visible light response of C₃N₄ could be effectively enhanced,and the deep oxidation or degradation could be inhibited.Thus the photocatalytic efficiency of P-CN in the photooxidation reaction of 1,4-DHP is improved 17-fold compared with g-C₃N₄.More importantly,the pure product could be obtained by simple filtration without any purification,and there is no obviously deep oxidation or degradation.?3?Dye-sensitized graphite carbon nitride can effectively broaden the light response range and boost the electron transfer rate of g-C₃N₄.However,the limited suitable dyes hinder their large application.Boron-dipyrromethene?BODIPY?fluorophores?without noble metals?is one kind of stable photosensitizer and can efficiently response to visible light.This work firstly synthesized corresponding BODIPY derivatives designed by gaussian calculation via SUZUKI coupling reaction.The as-prepared BODIPY sensitized carbon nitride system was applied into hydrogen evolution reaction.Results show that redox potential of BODIPY derivatives,CB and VB of carbon nitride,and the oxidation potential of sacrifice agent jointly influence the reactivity of dye sensitization system.Reaction mechanism of hydrogen evolution from water splitting via dye sensitized carbon nitride system can be clarified.This strategy can provide a new method for leveling the photocatalytic performance of carbon nitride and also for dye sensitized semiconductor system.