Synthesis Of Graphitic Carbon Nitride-Based Nanocomposites And Study On Their Photocatalytic Performance

Photocatalytic hydrogen production based on semiconductors has become an effective method of converting solar energy into hydrogen energy due to its low cost and good industrial prospects,which is of great importance.Among numerous semiconductors,inorganic semiconductors dominate the field of photocatalytic hydrogen production because of their outstanding performance.However,inorganic semiconductor photocatalysts with high activity often contain rare or toxic metals,limiting their practical application.Therefore,the development of low toxic or non-toxic photocatalysts with excellent activity has become the focus of research.Graphitic carbon nitride（CN）has become the star material due to its visible light response,easily tunable structure and properties,high stability,earth-abundant and low cost.Nevertheless,the pristine CN mainly suffers from the challenges including low visible-light utilization,high photogenerated carrier recombination efficiency and poor surface active sites,which cannot meet the needs of practical applications.In this dissertation,a series of CN-based photocatalysts were successfully prepared.On the one hand,element doping and morphology control were employed to optimize the composition and structure of the CN.On the other hand,cocatalyst loading and interfacial modifications were carried out to efficiently enhance the separation and extraction of photogenerated carriers.The optimized photocatalyst performance successfully surpassed that of CN/3 wt%Pt,demonstrating its potential practical application.The major contents and research results are listed below:（1）Design and synthesis of CN composite photocatalysts with honeycomb-like porous structures.In this work,carbon polymer dots（CPDs）with nitrogen-doped and conjugated structure were prepared and embedded into CN to form honeycomb-like porous CN/R-CPDs composite photocatalysts.The honeycomb-like porous structure provides more active sites for photocatalytic reactions and a large number of channels for electron transfer.Meanwhile,the abundant functional groups on the surface of R-CPDs improve the hydrophilicity of CN/R-CPDs and facilitate the adsorption of water molecules.The optimized sample CN/R-CPDs-2 exhibits a photocatalytic hydrogen production performance of 3465μmol g^-1 h^-1 under simulated sunlight with 3 wt%Pt as cocatalyst.This photocatalytic hydrogen production performance is superior to that obtained by embedding CPDs with only nitrogen-doped structures or GQDs（i.e.with only highly conjugated structures）in CN,confirming the synergistic effect of element doping and conjugation effects.In addition,in-depth electrochemical analysis showed that both the high electrical conductivity and the strong electron extraction ability of R-CPDs facilitate the rapid extraction and transport of photogenerated carriers,accelerating the occurrence of hydrogen production reactions.（2）Design and synthesis of Mo-based nanoparticles/CN composite photocatalysts.The high cost and low abundance of noble metal cocatalysts（Pt）limit their large-scale practical applications.In this work,on the basis of element doping and morphology modulation,1T-MoS₂ nanoparticles with high conductivity and more active sites were in-situ grown on B-doped carbon nitride（CNB）nanosheets via a one-step hydrothermal method,which successfully replaced the precious metal cocatalyst Pt.The doping sites of boron atoms in the CN structure are determined by theoretical calculations.In the same time,B-doping reduces the band gap and improves the utilization ratio of sunlight.The smaller size of1T-MoS₂ NPs exposes more catalytically active sites and possesses high conductivity.Furthermore,the in-situ loading method is more advantageous to the transfer of carriers from CNB to the 1T-MoS₂,thereby promoting the separation of photogenerated electron-hole pairs.The 1T-MoS₂/CNB2 hydrogen generation activity reaches 5334μmol g^-1 h^-1,which is 167times higher than that of pure CN and 137 times higher than that of CNB,and superior to most of the CN-based composite photocatalysts reported at that time.Femtosecond（fs）-time-resolved transient absorption（TA）spectroscopy measurements are carried out to prove the effective charge transfer from CNB to the MoS₂.（3）Design and synthesis of Mo-based hollow nanospheres/CN composite photocatalysts.Although the 1T-MoS₂ nanoparticle cocatalyst achieved decent photocatalytic performance,to further enhance its stability,in this work,stable Ni nanoparticle-modified Mo₂C hollow nanospheres were synthesized using a template-free method and loaded on CN nanosheets to obtain composite photocatalysts（CN/Mo₂C-Ni）.The hollow structure of Mo₂C not only enhances light absorption but also provides more reaction sites for the photocatalytic reaction.The introduction of Ni nanoparticles regulate the electronic structure of Mo₂C,weakening the strong interaction force of Mo atoms on H atoms and promoting the desorption of hydrogen.The obtained CN/Mo₂C-Ni-10 composite photocatalyst exhibits a hydrogen production rate twice that of the CN/Mo₂C sample,with an AQE of 13.1%at 400 nm.Electrochemical analysis tests show that the overpotential of CN/Mo₂C is lower than that of CN,while the introduction of Ni nanoparticles further reduces the overpotential and favors the hydrogen production reaction.In addition,the Tafel slope confirms that the CN/Mo₂C-Ni-10 has a faster reaction rate.（4）Design and synthesis of bimetallic Mo-based nanoparticles/CN composite photocatalysts.Compared with the corresponding single-metal phosphide,bimetallic phosphide generally has a smaller electron transfer resistance and a lower hydrogen evolution overpotential.In addition,the synergy between the bimetals is more advantageous for the hydrogen evolution reaction performance.In this work,the bimetallic phosphide NiMoP₂nanoparticles were loaded onto the CN surface by in-situ method to construct Schottky heterojunctions.The synergy between the bimetallic Ni and Mo promotes the decomposition of water and the desorption of hydrogen.The formed Schottky barrier hinders the return flow of electrons and facilitates the extraction and transport of photogenerated carriers.The in-situ synthesis method ensures excellent stability of the samples.The optimized sample CN/3NiMoP₂ demonstrates a comparable photocatalytic hydrogen evolution performance to that of CN/3 wt%Pt(770μmol g^-1 h^-1)and satisfactory stability（21 h）,suggesting that NiMoP₂ has enormous potential in the field of photocatalytic hydrogen production.（5）Constructing charge transfer bridge based on CPDs for interfacial modulation of Mo-based cocatalysts with CN.In the previous work,element doping,morphology modulation and cocatalyst loading methods were employed to modify CN to facilitate carrier migration.To further accelerate the separation and transport of photogenerated carriers between interfaces,in this work,a unique shell based on carbonized polymer dots（CPDs）was fabricated to serve as electron’s path for charge transfer between carbon nitrides（CN）and metal-based cocatalysts.This CPDs shell homogenously fixes each CPD on the catalyst surface through metal-N bonding,and possesses strong electron extraction ability by masterly controlling the pyridinic N structure.As a result,the obtained CN/MoP@CPDs-200 achieves an outstanding apparent quantum efficiency（18.0%at 400 nm）,which is superior to CN/3wt%Pt.Importantly,in-depth transient photo-induced voltage studies confirm the excellent charge extraction ability of CPDs shell,and thorough apparent kinetic analysis reveals that the photocatalytic hydrogen production process undergoes a Volmer-Heyrovsky mechanism,which provides valuable experimental results and theoretical direction for CPDs applied in the field of photocatalysis.