Design And Photocatalytic Hydrogen Production Performance Of Graphitic Carbon Nitride Composites

With the continuous growth of the world's population,traditional non-renewable resources are diminishing,and the deterioration of the environment and the energy crisis have become increasingly serious social problems.It is of great practical significance to develop new clean and environmentally friendly energy.Photocatalytic technology has attracted much attention in the field of environmental protection and energy conversion,because it can directly use solar energy.Preparation of economical and environmentally friendly photocatalysts with excellent performance and high stability is the current research focus.In recent years,graphitic carbon nitride(g-C3N4)has attracted many scholars at home and abroad with its unique electronic configuration,favorable stability,and the ability to absorb and use visible light.However,pristine g-C3N4 has the disadvantages of small specific surface area,low photogenerated carrier separation efficiency and low quantum efficiency.Therefore,improving the separation efficiency of g-C3N4 photogenerated carriers and the utilization rate of visible light is the key to broadening its application range.In this thesis,g-C3N4 was used as the research object,and the heterojunction photocatalyst was constructed by combining with other semiconductors to enhance the photocatalytic performance of g-C3N4.The physical and chemical properties,photoelectric properties and photocatalytic properties of the resulting materials were investigated by XRD,FT-IR,XPS,SEM,TEM,DRS and PL etc.The specific research contents are as follows(1)The polymer precursor conversion method was used to synthesize the SiOC particles,and then the SiOC particles were loaded on the g-C3N4 layer by ultrasonic dispersion-heat treatment to synthesize SiOC/g-C3N4 composites.The characterization results showed that SiOC and g-C3N4 form a heterojunction at the interface;compared with g-C3N4,the visible light response capability of SiOC/g-C3N4 composites was significantly enhanced,the specific surface area was also increased,separation and transport efficiency of photogenerated carriers had also been significantly improved.The 9.1%SiOC/g-C3N4 photocatalyst exhibited the highest photocatalytic activity and good cycle stability.The hydrogen production rate was 1020 ?mol h-1g-1,which was 3.2 and 25.5 times of g-C3N4 and SiOC materials,respectively,and the AQE was 6.1%(?=420 nm)(2)The SiC nanowires were prepared by vapor deposition method,and the prepared SiC nanowires were mixed with urea to synthesize SiC/g-C3N4 composite photocatalysts by calcination method.The characterization results showed that the SiC nanowires were uniformly embedded in g-C3N4 and form a heterojunction structure;the introduction of SiC enhances the visible light response of the SiC/g-C3N4 composites and expedites the separation and migration of photogenerated carriers,and has a larger specific surface area compared with g-C3N4.The experimental data of visible photocatalytic hydrogen production showed that 16.9%SiC/g-C3N4 photocatalyst had the highest photocatalytic activity,the hydrogen production rate was 1983?mol h-1g-1,which was 6.2 and 29.2 times of g-C3N4 and SiC monomer,respectively,and the AQE was 6.8%(?=420 nm).SiC/g-g-C3N4 photocatalyst also showed good stability in the cycle experiment(3)Using Ag as the electron transport medium,Z-type ternary Ag/SiC/g-C3N4 composites were prepared by one-step calcination and chemical reduction method.The results of photocatalytic hydrogen production showed that the photocatalytic activity of 3%Ag/SiC/g-C3N4 was the best(2971 ?mol h-1g-1),which was 8.8 and 1.5 times of g-C3N4 and SiC/g-C3N4,respectively,and the AQE was 7.3%(?=420 nm).Moreover,the ternary Ag/SiC/g-C3N4 composites also showed good repeatability in the cycle experiment.This is because the combination of SiC and g-C3N4 expands the visible light absorption range of g-C3N4,and the presence of heterojunctions promotes the migration of carriers.The Schottky junction formed by Ag due to the plasma resonance effect further promotes carrier separation,thereby impeding the recombination rate of photogenerated electron-hole pairs,making Ag/SiC/g-C3N4 have excellent photocatalytic activity.