Defect Modulation Of Layered Carbon Nitride (g-C₃N₄) And Its Composite For Photocatalytic Hydrogen Generation

With the increasingly serious energy crisis,the energy problem has gradually become the focus of global attention.Hydrogen energy is considered to be the most ideal clean and sustainable renewable energy in the future.Semiconductor-based photocatalysis technology can directly convert solar energy into hydrogen energy using photocatalysts,which is considered as promising method for obtaining hydrogen energy.Therefore,finding suitable photocatalytic materials is the main research goal of the photocatalytic technology.At present,the most attractive semiconductor material is the graphite-phase carbon nitride(g-C₃N₄).The reason is that g-C₃N₄ has visible light response,stable physical and chemical properties,facile procedures for synthesis,easy to obtain raw materials.Most importantly,the carbon nitride is comprised by only two non-metallic elements,carbon and nitrogen,thus it is regarded as a green and environmentally friendly material.Since it was found that graphite-phase carbon nitnitide can decompose water to produce hydrogen through photocatalytic technology,researchers have conducted intensive studies on it.However,the intrinsic drawbacks of g-C₃N₄,such as poor visible light absorption,fast recombination of electrons and holes,slow charge transfer rate,and deficient sites of redox reaction,limit its application in practical sense.Based on the current research results and our understanding on the process of photocatalytic hydrogen generation,multiple non-metallic element doping and defect engineering via physical and chemical method might be an effective protocol to address the shortages of g-C₃N₄.In this paper,pristine g-C₃N₄ was used as stardting material to synthesize non-metallic elements doped g-C₃N₄,and to construct heterojunctions with forign semiconductor.The influence of multiple doped and defects on the chemical and electronic structure of g-C₃N₄ was intensivel studied.The benefits of doping and defects to the heterojunction construction and Pt cocatalyst modification were discussed in detals as well.The optical properties,behaviros of photogenerated charge carriers,as well as performance of photocatalytic hydrogen generation of pristine and target g-C₃N₄ were compared and discussed.The specific research contents are summarized as follows:In the first part,dicyandiamide was used as raw material to synthesize bulk carbon nitride(denoized as CN)by solid phase thermal polymerization.N and P were incorporated into the structure of carbon nitride by gas phase reaction to generate N and C defect site,respectively.N-doped carbon nitride(denoted as N-CN)was synthesized with ammonia as N source.P-doped carbon nitride(denoted as P-CN)was synthesized with phosphine as P source.At the same time,N and P co-doped carbon nitride(denoted as NP-CN)was successfully synthesized by two-step gas phase method;that is,firstly annealing CN in ammonia and followed by phosphine.Through the characterization and related measusrements,it is found that although the doped samples retain the main structure of carbon nitride,the introduced non-metallic defects notably change the feature of it,such as altered band structure,accelerated separation of charge carriers,increased surface area,and enhanced visible light absorption,all of which synergistically improve the performance of carbon nitride.The hydrogen evolution rates of N-CN,P-CN and NP-CN are 1.5,1.1,and 3 times higher than that of CN,respectively.After deposition with 2 wt%Pt,the hydrogen evolution rate of all the samples are elevated.Compared to Pt/CN,Pt/N-CN,Pt/P-CN and Pt/NP-CN exhibit 4.99,5.3 and 7.8 times higher efficiency.The experimental results show that the N/P co-doping strategy is effective to improve the performance of photocatalytic hydrogen evolution of carbon nitride.The second part uses nitrogen-doped carbon nitride as a starting(denoted as NCN)material to loads silver onto the surface of NCN by sodium borohydride reduction method.The obtained Ag/NCN was mixed with Cd Zn S-X(with different molar ratio between Cd and Zn)under sonication and stirring to give Z-type composite,in which Ag acts as carrier transfer mediator.In the visible-light-driven hydrogen evolution test,it was found that the Z-type composite with a Cd/Zn molar ratio of 2/8 had a better hydrogen evolution rate of 1279.45?mol·h^-1·g^-1,which was 4.4 and 5.8 times higher than that of NCN and Cd Zn S-2.The compositon,morphology,microstructure,optical properties,and photocatalytic performance of the composite were studied by SEM,TEM,UV-Vis,FT-IR,PL,BET and electrochemical methods.The results showed that the construction of Z-type composites not only enhanced the absorption capacity of NCN for visible light,but also effectively slowed down the recombination rate of electrons and holes,which promoted the charge separation of a single component and in turn boosting the performance of photocatalytic hydrogen production.