Modification And Photocatalytic Hydrogen Production Performance Study Of Graphitic Carbon Nitride

As the main energy resources,fossil fuels such as oil,coal,and natural gas are non-renewable resources,which will cause a series of environmental problems during use.For the sustainable development,it is necessary to develop renewable clean energy including biomass,wind and solar energy.Among them,solar energy is rich resources and cleanliness.How to efficiently use solar energy is an important subject.In recent decades,the photocatalysis that can convert solar energy into chemical energy has attracted widespread interest.As a green,high-efficiency and energy-saving technology,photocatalysis has a wide range of applications in fuel synthesis fields.Using electrons and holes generated by light excitation,water,nitrogen and carbon dioxide can be converted into clean energy or fine chemicals such as hydrogen,ammonia and methanol.As the core component of the photocatalytic reaction,the photocatalyst greatly affects the photocatalytic activity and efficiency.In recent years,graphitic carbon nitride(g-C3N4)has attracted extensive attention due to its suitable energy band structure,high physical and chemical stability,and abundant raw materials.However,because of its narrow visible light absorption range,severe photo-generated carrier recombination and few reactive sites,pristine g-C3N4 exhibits unsatisfactory photocatalytic performance.This dissertation focuses on improving the photocatalytic performance of g-C3N4 and solving some key issues in the g-C3N4 modification area.Herein,the main contents of this dissertation includes the synthesis of transition metal phosphides loaded,non-metallic element phosphorus/sulfur doped and tungsten single atom doped g-C3N4 photocatalyst.Meanwhile,the photocatalytic water splitting and organics oxidation performance of the photocatalyst,and the relationship between the performance and its morphology,composition and electronic structure are systematically explored.The main contents are summarized as follows:1.We designed and prepared CoP,FeP and NiP2 transition metal phosphides(TMPs)supported g-C3N4 hybrid photocatalysts through a simple ethylene glycol reflux and phosphating treatment,and studied the effect of different cocatalyst on the performance of photocatalytic water splitting into H2.The results show that the well-crystallized ultrafine TMP particles are uniformly distributed on the surface of the g-C3N4 matrix,forming close contact,and having excellent physical and chemical stability.From the photoelectrochemical test results,we can know that the visible light absorption,charge separation and transport of the catalyst after loading the TMP cocatalyst have been significantly improved.The photocatalytic water splitting hydrogen production experiments confirm that the photocatalytic activity of g-C3N4 is significantly improved after supporting the cocatalyst.Among them,under the optimal CoP content,the CoP/g-C3N4 can achieve a high H2 production rate of 956.8 ?mol g-1 h-1,and has excellent stability of continuous hydrogen production for 70 h.In addition,we also used the density functional theory(DFT)calculation to study the influence of the ultrafine CoP particles and the g-C3N4 matrix to form close contact on the band structure and carrier migration.2.We adopted a simple one-step synthesis strategy based on supramolecular chemical polymerization to prepare non-metallic elements(P or S)doped g-C3N4 hierarchical mesoporous microspheres(PCN-HMS and SCN-HMS)with simultaneously realizing the controllable morphology and electronic structure.During the experiment,DFT calculations were used to explore the feasibility of heteroatom doping and its possible influence on the electronic structure of g-C3N4.The photoelectrochemical measurements confirm that the as-prepared PCN-HMS and SCN-HMS exhibit an extended visible light absorption range and suppressed photogeneration charge recombination compared with the undoped g-C3N4 mesoporous microspheres and bulk g-C3N4.Photocatalysis experiments show that SCN-HMS displays significantly enhanced visible light photocatalytic hydrogen generation rate of 22.3[?mol h-1]with TEOA as sacrificial agent,which is 13.2 times higher than that of the pristine g-C3N4.More importantly,when SCN-HMS is used as dual-functional photocatalyst for hydrogen production and selective oxidation of benzyl alcohol,it can still show good activity.The generation rates of hydrogen and benzaldehyde are 3.76 and 3.87 ?mol h-1,respectively.Further research found that the improvement of photocatalytic activity can be attributed to the synergistic effect of the electronic structure modulation and hierarchical hollow structure,which enhances the visible light absorption,generates more active sites,and promotes mass transfer,and shortens the transfer path of charge carriers,so that more photogenerated carriers can participate in the surface redox reaction.As dual-function photocatalyts,the waste of sacrificial agents is avoided,and its great potential in practical applications is explored.3.We reported a simple method to synthesize tungsten single atom loaded g-C3N4 porous ultrathin nanosheets(WSA-CN-PUNS)by phosphotungstic acid-pyrrole polymerization and calcination in the air.First,the precursor of polypyrrole-polyoxometalate/g-C3N4 composite(PW12-PPy/g-C3N4)was constructed through simple pyrrole polymerization,and then WSA-CN-PUNS was obtained by high temperature oxidation and etching in air.XRD,HAADF-STEM and synchrotron radiation measurements demonstrate that the tungsten atoms in WSA-CN-PUNS are loaed on the g-C3N4 matrix in the form of single atom,and a porous structure is formed.Photoelectric test found that WSA-CN-PUNS has significantly enhanced visible light absorption,higher photocurrent response and lower electrochemical impedance compared with undoped g-C3N4.This shows the electronic structure of the photocatalyst is modulated,and the recombination of photogenerated carriers is suppressed after oxidation and etching,which was also confirmed by the DFT calculations.The photocatalytic test results show that the prepared W atom-doped g-C3N4 catalyst has significantly enhanced photocatalytic H2 evolution performance and high activity for the selective oxidation of benzyl alcohol to benzaldehyde under visible light.Among them,the WSA-CN-PUNS sample shows higher H2 production rate of 3.02 mmol h-1 g-1,which is about 4.1 and 9.7 times higher than that of the undoped g-C3N4 and the pristine g-C3N4,respectively.When benzyl alcohol is used as hole trapping agent,WSA-CN-PUNS still shows H2 and benzaldehyde generation rates of 298.7 and 305.1 ?mol h-1 g-1,which are 3 times of the pristine g-C3N4,respectively.The performance enhancement benefits from the reduced band gap and the defect levels after W doping,which improves the separation of charge carriers.This material synthesis strategy may provide a simple method for designing metal element-doped g-C3N4 photocatalysts with better photocatalytic activity.Finally,we summarize the progress of g-C3N4 based photocatalyts in photocatalytic water splitting into H2 and look forward to the next research direction.