Preparation And Photocatalytic Performance Of Doped Carbon Nitride For Hydrogen Evolution

Photocatalytic water splitting hydrogen generation is one of the most promising technologies for alleviating environmental pollution and energy crisis.Therein,photocatalysts are the bottleneck factor restricting the development and application of this technology.Therefore,the design and development of cheap and efficient photocatalysts for hydrogen evolution is of great significance to solve the two major demands of energy and environment.Graphitic carbon nitride（g-C₃N₄）is a promising photocatalytic material since the advantages of simple preparation,suitable visible light response,ideal band gap and excellent thermal stability.However,the weak redox potential,fast carrier recombination rate and small specific surface area of g-C₃N₄seriously restrict its application in the field of photocatalysis.The effective strategies to improve the photocatalytic performance of g-C₃N₄are morphology regulation,surface modification,heteroatoms-doping and heterojunction construction.Among them,heterogeneous element doping can not only promote the separation efficiency of photogenerated carriers,but also realize the synchronous optimization of g-C₃N₄electron and band structure.Therefore,it is necessary to improve the photocatalytic performance of g-C₃N₄significantly by employing doping engineering.Based on the above analysis,sulfur and potassium are selected as the representatives of non-metallic and metallic elements,respectively,to conduct elemental doping research on the optimization of the photocatalytic performance of g-C₃N₄.The specific contents are as follows:（1）Sulfur doped g-C₃N₄with rich carbon defects（S-g-C₃N₄-D）was successfully prepared by one-step thermal polymerization by using urea,sulfur powder and DMF as the precursor.Among them,the gas generated by DMF decomposition contributes to the stripping of g-C₃N₄,making its specific surface area up to 135.7 m²g^-1.The mechanism of catalytic reaction is described in detail theoretically.In this work,S doping can not only achieve effective regulation of g-C₃N₄band structure,but also significantly enhance the light absorption capacity.Meanwhile,the existence of free radicals can also accelerate the separation efficiency of photogenerated carriers.In addition,the separation and transfer mechanism of photogenerated carriers was revealed by ultraviolet electron spectroscopy（UPS）.Therefore,S-g-C₃N₄-D exhibits a photocatalytic H₂evolution of 3190μmol h^-1g^-1,which is 8.6 times that of bulk g-C₃N₄.（2）Sulfur-doped g-C₃N₄nanosheets（S-g-C₃N₄）were in situ synthesized by high-temperature thermal polymerization of urea with the presence of sulfur powder and water.The thermal stripping process of g-C₃N₄was significantly promoted by high temperature evaporation of water and thermal decomposition of urea,thus the specific surface area of g-C₃N₄can even reach up to 198.8 m²g^-1,since the existent of abundant pores.The photochemical activity data show that the sulfur doping can not only regulate the band gap of g-C₃N₄and shorten the migration path of photogenerated carriers,but also significantly improve the separation efficiency of photogenerated carriers and enhance the absorption capacity of visible light.Therefore,S-g-C₃N₄possesses an efficient photocatalytic evolution hydrogen rate of 3461.6μmol h^-1g^-1,which is nearly10 times higher than that of bulk g-C₃N₄.Most importantly,this work also provides a n novel and sustainable way for the construction of efficient g-C₃N₄-based photocatalysts.（3）Potassium doped porous g-C₃N₄nanosheets（K-g-C₃N₄）were successfully prepared by two-step calcination method under the dual effects of ammonium chloride foaming effect and potassium chloride molten salt intercalation.Meanwhile,the morphology,structure,composition and photocatalytic property of K-g-C₃N₄were systematically characterized.Thereinto,the visible light absorption capacity of g-C₃N₄was significantly enhanced by interlayer insertion effect of K.In addition,ammonia and hydrogen chloride produced from the pyrolysis of ammonium chloride can promote the stripping of g-C₃N₄,making its specific surface area more than twice that of bulk g-C₃N₄.Under the combined action of ammonium chloride autogas foaming and potassium chloride melt intercalation,K-g-C₃N₄exhibits faster photocurrent response,lower charge transfer impedance,and longer photocarrier lifetime than those of bulk g-C₃N₄.Therefore,the K-g-C₃N₄possesses a photocatalytic H₂evolution activity of 8700μmol h^-1g^-1,showing a good application prospect for photocatalytic water splitting for hydrogen evolution.