Preparation Of Carbon Nitride With Sheet And Hexagonal Prism-Shaped And Photocatalytic Reduction Of CO₂

In recent years,the two main challenges as the shortage of the energy supply and the problem of disastrous environmental pollution have been recognized by researchers.It is a better way to efficiently and inexpensively convert solar energy into chemical fuels by developing an artificial photosynthetic system because solar fuels are high density energy carriers with long-term storage capacity.Recently,graphitic carbon nitride（g-C₃N₄）,a polymeric semiconductor has been widely used as metal-free,low-cost and stable visible-light-active photocatalyst in the sustainable utilization of solar energy.This has attracted worldwide attention from photocatalytic.However,C₃N₄ photocatalytic have some defects,such as low quantum efficiency and high recombination of electrons and holes,greatly restrict the photocatalytic application of C₃N₄.Therefore,the control and optimization of microstructure is the most effective means to improve the photocatalytic activity.In our work,C₃N₄ was modified by many ways to enhance its photocatalytic performance with different microstructures without destroying the basic chemical structures.The resultant catalyst powders were characterized by field-emission scanning electron microscopy（FESEM）,X-ray diffractometry（XRD）,transmission electron microscopy（TEM）,UV-vis diffuse reflectance spectroscopy（UV-vis DRS）,X-ray photoelectron spectroscopy（XPS）,and Brunauer-Emmett-Teller（BET）N2 adsorptionto probe the significant dependence of catalytic activity on physicochemical properties of the catalysts,combining with the photocatalytic activity of C₃N₄ in the reduction of CO₂ under visible light irradiation.U-C₃N₄ is easily formed by the condensation（heating）of nitrogen-rich organic compounds（urea）,U-C₃N₄ was stirred with H2SO4（98 wt%）for different time,and then poured into deionized water and sonicated for exfoliation,obtained products were U-C₃N₄ sheets（C₃N₄-acid-t）.Compared with U-C₃N₄,the C₃N₄-acid-t exhibited markedly improved yield,quantum yield（QY）,energy returned on energy invested（EROEI）,and turnover number（TON）for CO₂ reduction to CO,CH4 and CH3 OH under visible-light irradiation.In this study,the optimal stirred time with H2SO4（98 wt%）was determined to be 8 h.After 4 h of reaction,the CO,CH4 and CH3 OH yield was 7.99,9.29 and 1.40 μmol g-1,corresponding to a QY and EROEI of 0.58‰ and 0.17‰,respectively.The intercalation of H2SO4 into bulk g-C₃N₄ destroies via van der Waals The enhanced photocatalytic performance can beattributed to high-surface area and the increased active sites.A hexagonal prism-shaped carbon nitride（H-C₃N₄）was first synthesized from urea-derived C₃N₄（U-C₃N₄）using an alkaline hydrothermal process.Considering the samples of the specific surface area,morphology and photocatalytic activity,finding an optimal alkaline hydrothermal time for C₃N₄ modification U-C₃N₄ decomposition followed by hydrogen bond rearrangement of hydrolyzed products leads to the formation of a hexagonal prism-shaped structure.The H-C₃N₄ catalysts displayed superior activity in the photoreduction of CO₂ with H2 O compared to U-C₃N₄.The enhanced photocatalytic activities can be attributed to the promotion of incompletely coordinated nitrogen atom formation in the C₃N₄ molecules,-NH2 and-NH-served as the mainactive sites for CO₂ activation and photoconversion.In a addition,the catalyst maintained stable performance throughout five successive recyclability test runs.