Study On The Synthesis And Mechanism Of Highly Active Carbon Nitride-based Photocatalysts

With the increasing urgency of environmental issues caused by energy and excess carbon dioxide（CO₂）emissions,it is very meaningful to develop a technology to convert CO₂ into energy storage materials.In recent years,photocatalytic technology has attracted more and more attention as a clean and cheap solar energy utilization technology.Among them,narrow-bandgap photocatalysts have more development prospects because they can utilize visible light as a large proportion of solar energy.Among many narrow-bandgap photocatalysts,graphite-phase carbonitride（g-C₃N₄）has attracted widespread attention due to its low cost and easy availability,reasonable position,non-toxicity and stability.However,the conventional method of g-C₃N₄ generally has a small specific surface area and poor charge separation ability,which limits its photocatalytic activity.Therefore,designing to increase its specific surface area and improve its charge separation is expected to increase its photocatalytic activity.Generally,the synthetic method can improve the specific surface area,and the regulation of electrons and holes can improve the charge separation of g-C₃N₄.Based on this,this paper plans to carry out the following two parts from the perspective of developing a synthetic method of large specific surface area g-C₃N₄ and exploring electronic and hole regulation strategies:Firstly,the synthesis method and photocatalytic performance of Mn modified large specific surface area g-C3N4 were explored.The melamine and cyanuric acid in the aqueous phase were pre-assembled,calcined to obtain a large specific surface area g-C₃N₄ nanosheet,and then high dispersed Mn oxides were in situ introduced in the as-prepared g-C₃N₄.The results show that the photo-generated charge separation and photocatalytic carbon dioxide reduction activity of g-C₃N₄ nanosheets are significantly improved compared with bulk g-C3N4,due to the large specific surface area.Based on photoluminescence spectroscopy,it was proved that Mn oxides can promote photo-generated charge separation and enhance photocatalytic carbon dioxide reduction activity.In addition,it has been proved by electrochemical tests that Mn oxides possess catalytic ability for oxygen production,and can improve photocatalytic water splitting activity.Then,the effects of co-modification of Mn and Cu oxides on the dual regulation of photo-generated holes and electrons of g-C3N4 and their effects on photocatalytic properties were investigated.Based on the method of the previous step,Mn and Cu oxides are in situ introduced in one step with high dispersion.It can be seen from the photoluminescence spectrum and the transient surface photovoltage spectrum that Mn and Cu oxides can regulate the photogenerated holes and electrons of g-C₃N₄,respectively,and improve the photo-generated charge separation.And through the electrochemical test under carbon dioxide conditions,it can be known that Cu oxides possess the catalytic ability for CO₂ reduction reaction.Through the above two factors,Cu oxides can enhance the photocatalytic carbon dioxide reduction activity of g-C₃N₄.By comparing the single and co-regulation of photo-generated charges of g-C₃N₄,it is found that there is a synergistic effect between the regulation of photogenerated electrons and holes.