Study On Defects And Alkali Cations For Enhanced Photocatalytic Performance Of Carbon Nitride

Fossil fuels consumption is increasing along with increasingly serious environmental pollution has raised global highly concern for finding alternatives.Solar energy is abundant,widespread,and renewable energy for an ideal alternative to fossil fuels.The development of photocatalytic materials for efficient conversion of solar-tochemical energy and storing as sustainable energy carriers(such as H2)that helps to overcome current energy and environmental issues.Among various photocatalytic materials,polymer carbon nitride(PCN),a new two-dimensional semiconductor material,has been captured widely concerns because of its high stability,facile fabrication,appealing band structure and raw material abundant.However,PCN as a 2D conjugated material features many intrinsic shortcomings,such as low charge separation efficiency,mobility,and the number of accessible surface-active sites,leading to severely limited photocatalytic performance.In this paper,based on a fully understanding of PCN materials and combining it with the intrinsic advantages,a series of efficient PCN-based photocatalytic catalysts were prepared by defect engineering and alkali ions doping to ensure high photocatalytic performance.These catalysts have been successfully applied in photocatalytic water or seawater splitting for hydrogen production,and synthesis of high value-added chemicals.In addition,this study further revealed the mechanism by which nitrogendefects moieties and alkali metal ions enhance the photocatalytic performance at the atomic-,molecular-level by synchrotron radiation and in-situ X-ray photoelectron spectroscopy(XPS),etc.More importantly,we discovered the dynamic evolution of πconjugated electronic structure(C=N/C-N)in PCN catalyst under photoexcitation conditions by in-situ XPS characterization,which provides a new insight to further development of highly efficient PCN-based photocatalysts.The main researches are as follows:1.Aim at some shortcomings of pristine PCN,such as low electron-hole separation efficiency,mobility,and the number of accessible surface-active sites.This study successfully prepared nitrogen defects and alkali metal ion modified materials(KCNv)by potassium chloride hard template method.The findings show that the nitrogen defect sites and alkali metal ions in PCN system significantly enhance the electron-hole separation ability and improve the charge mobility.Impressively,a high H2 yield(4132.1 μmol g-1 h-1)was achieved in KCN-v,which is 15 times higher than the pristine PCN.2.A NaCl/KCl two-step interfacial polymerization strategy was developed to controllable fabricate v-CN-KNa that contained varying levels of nitrogen defects and alkali ions.The alkali ions and abundant edge defect sites such as cyano groups,hydroxyl groups,were introduced into PCN by stepwise interfacial polymerization method.It not only improves the dispersibility of PCN in water,but also enhances the capability of electron-hole separation and charge transfer,thereby exhibiting excellent photocatalytic HER activity,especially in real seawater,which shows great potential for practical applications.These results were further confirmed by theoretical calculations that nitrogen defects sites improve the charge separation,while alkali metal ions in v-CN-KNa enhance the charge carrier mobility,resulting in efficient charge separation and transfer.Additionally,the v-CN-KNa as a photocatalyst enables simultaneously produce H2 and valuable N-Benzylidenebenzylamine without using sacrificial agents.3.Based on the above research basis,we systematically studied the mechanism of the introduction of alkali metal ions into PCN.It was found that alkali ions show a unique tailoring effect in PCN synthesis process.Accordingly,we further developed Rb single-atom modified PCN photocatalysts(PCN-Rb).The structural evolution in prepare process was discussed in depth,indicating that the modification of PCN with alkali metal ions can not only construct surface defect sites by tailoring 3-s-triazine ring structure,but also clear suppress the agglomeration of the cocatalyst Pt.This work obtained highly dispersed Pt nanoparticles and single atoms in PCN framework.Theoretical calculations and experimental results confirm that the introduction of alkali cations and defects significantly modulates the PCN surface electronic structure,improving the mobility and charge separation of charge carriers to ensure excellent photocatalytic performance.More importantly,this work reveals the dynamic evolution of π-conjugated electronic structure(C=N/C-N)in PCN catalyst under photoexcitation conditions by in-situ characterization,which provides an important reference for understanding the photocatalytic mechanism of PCN-based catalysts.