The Study On Modification Of G-C₃N₄ By Polyphenol With Molten Salt Post-treatment And Its Photocatalytic Hydrogen Evolution Performance

With sunlight as the driving force and semiconductor as the core,photocatalytic technology is a technology to be developed that provides sustainable energy and cannot be ignored.Graphite carbon nitride（g-C₃N₄）is a metal-free organic polymer semiconductor.As a photocatalyst,it has unique electronic structure,good physical and chemical stability,and no toxicity,which has attracted extensive concentration in the scientific research community.However,g-C₃N₄ is generally synthesized by traditional thermal polycondensation methods.The degree of condensation of g-C₃N₄ obtained by this method is generally low,and it is likely to have a block structure,resulting in shortcomings such as small specific surface area and easy recombination of carriers,which limits its photocatalytic activity.Therefore,in this paper,the precursor melamine was introduced with polyphenols as carbon to improve the conductivity of g-C₃N₄ and regulate its energy band structure.Then,after molten salt treatment,g-C₃N₄ was further polycondensated to obtain a molten salt treated polyphenol modified g-C₃N₄.Its specific surface area has been significantly increased,light absorption has also been enhanced,and carrier separation and migration efficiency has been significantly improved,effectively improving photocatalytic performance.Two methods are used to modify it:Firstly,dopamine（DA）was dissolved in a weakly alkaline solution and polymerized in situ on the surface of melamine（MA）to form a polydopamine（PDA）@MA precursor.Then,a carbon nitride（CN）core-shell structure composed of carbonized polyamine（C-PDA）was obtained by high-temperature calcination,which was recorded as C-PDA-CN.Then,a porous C-PDA-CN-ms core-shell structure was obtained by high-temperature molten salt（ms）polycondensation and etching of the C-PDA on the surface of the CN.Studies have shown that the porous core shell structure effectively increases the specific surface area,light absorption,and electrical conductivity of C-PDA-CN-ms,demonstrating excellent photocatalytic hydrogen production performance.The photocatalytic hydrogen production rate of 1 wt%C-PDA-CN-ms is the highest,reaching 3830μmol h^-1g^-1,20.8 times that of 1 wt%C-PDA-CN,and 73.6 times that of the pristine CN.In pure aqueous solution,a precursor of TA/MA was synthesized by hydrogen bonding of simple tannic acid（TA）and melamine（MA）,and a composite C-TA-CN lamellar structure of carbonized tannic acid（C-TA）was obtained by high-temperature calcination;Further processing with molten salt（ms）promotes the crystallization of C-TA-CN and fragments the lamellar structure of CN,resulting in a composite with crystalline carbon dots embedded in CN nanosheets,known as C-TA-CN-ms.The research shows that the crystallized C-TA carbon dots not only enhance the light absorption ability of CN,but also further expand theπconjugation system of CN,resulting in a significant improvement in carrier separation and transport efficiency;Compared with C-TA-CN,the specific surface area of fragmented C-TA-CN-ms treated with molten salt significantly increased,effectively promoting photocatalytic hydrogen production.The photocatalytic hydrogen production rate of 0.5 wt%C-TA-CN-ms is the highest,reaching 5912μmol h^-1g^-1,which is about 3 times higher than that of CN-ms without C-TA,12.7 times higher than that of C-TA-CN,and 22 times higher than that of the pristine CN.