Preparation And Photocatalytic Performance Of G-C₃N₄-Based Micro-nano Composites

Nowadays,among the various possibilities for exploring attractive sustainable energy sources and technologies,photocatalytic technology is considered as one of the most appealing and promising technologies to directly harvest and convert or store renewable solar energy for the green sustainable energy and environmental applications.As a special kind of organic semiconductor conjugated polymer,graphite phase carbon nitride（g-C₃N₄）has attracted people’s attention due to its advantages of visible light response,non-toxicity,low cost,excellent thermal and chemical stability.However,g-C₃N₄ was limit its development in the field of photocatalysis for industrial applications by its small specific surface area,low quantum yield,high carrier recombination efficiency and narrow visible light response range.In this thesis,hybridization of g-C₃N₄with BiOX（X=Cl,Br,I）was performed to solve these issues.The formation of heterojunction structures could enhance the separation of the photo-generated charge carriers and promote the photocatalytic activities.The mechanisms of g-C₃N₄ based composite photocatalysis were also studied briefly.The details were shown as follows:（1）A series of g-C₃N₄/BiOBr_0.2I_0.8 composites with different ratios(defined as xCN/BiOBr_0.2I_0.8,x=5,10,20 and 30)were prepared using a thermal polycondensation-solvothermal method.By the way,the bulk g-C₃N₄ was prepared by urea pyrolysis for comparisons.The morphology,structure and photoelectrochemical properties of the composites were characterized by XRD,TEM,FT-IR,SEM,XPS,DRS,PL and electrochemical tests.These composite photocatalysts have unique visible-light absorption property and excellent photogenerated-carrier separation capability.Specifically,the 10CN/BiOBr_0.2I_0.8 hybrid photocatalyst with g-C₃N₄ weight ratio of 10wt.%showed highest visible-light photocatalytic degradation ability towards pollutant removal in water with a degradation rate of 0.0695 min^-1,which was 3.2 and 5.0 times higher than that of pure BiOBr_0.2I_0.8.8 and bulk g-C₃N₄,respectively.Such significantly enhanced stable photocatalytic activity could be attributed to the effective charge separation and transfer on the interfaces between g-C₃N₄ nanosheets and BiOBr_0.2I_0.8microspheres,indicating that constructing inorganic-organic composites provide a useful methodology for developing efficient visible-light photocatalysts.（2）The g-C₃N₄/BiOCl_xBr_1-x composites were synthesized by simple chemical impregnation and high temperature pyrolysis.The composites were characterized using XRD,SEM,XPS,UV-vis and PL.The photocatalytic activities of g-C₃N₄/BiOCl_xBr_1-x-x composites were analyzed by the TC degradation reaction.The photocatalytic activity of g-C₃N₄/BiOCl_xBr_1-x composite was higher than that of pure g-C₃N₄,and 10 wt.%is determined as the optimal weight ratio to prepare the highest g-C₃N₄/BiOCl_xBr_1-x-x composite photocatalysts.The photocurrent and electrochemical impedance spectroscopy results indicate that the charge transfer efficiency of g-C₃N₄/BiOCl_xBr_1-x composite has a significant increase related with increased surface area and formation of heterojunction structures.（3）The K-doped g-C₃N₄/BiOBr photocatalysts were synthesized by combination with the methods of the K-doping and the BiOBr-loading to investigate the coupling effects on photocatalytic performances.The morphology,structure and photoelectrochemical properties of the composites were also characterized by XRD,TEM,FT-IR,SEM,XPS,DRS,PL and electrochemical measurements.The K-doped g-C₃N₄/BiOBr shows the increased specific surface areas,significant enhanced the visible-light absorption,effective charge separation and more react sites,resulting in enhanced photocatalytic performances.This research presents a new potential route to further improve the photocatalytic performances of g-C₃N₄ based composite photocatalysts for the green sustainable energy and environmental applications.