Controlled Synthesis And Photocatalytic Water Splitting Performance Of G-C₃N₄-based Materials With Efficient Separation Of Photogenerated Carriers

Energy and environmental issues at a global level are two challenging problems due to the strong dependence on the fossil fuels.Among various technologies,the direct conversion of solar to chemical energy and photodegradation pollutants using photocatalysts have received significant attention because photocatalytic method is considered to be one of the most cost-effective and effective strategies to solve future energy demand and environmental pollution problems.The development of efficient,stable,inexpensive photocatalysts that can effectively absorb visible light is the key to realize the practical application of photocatalysis.In addition to metal-based semiconductors,it has been demonstrated recently that non-metallic semiconductor g-C₃N₄ also shows significant promise due to its visible light absorption,high chemical stability and thermal stability as well as its simple synthesis method using inexpensive and accessible precursors.Traditionally,the bandgap of g-C₃N₄ is about2.7 e V,and its highest occupied molecular orbital（HOMO）and lowest unoccupied molecular orbital（LUMO）are located at+1.4 V and-1.3 V（vs.NHE,p H=7）.Thus,g-C₃N₄ has been generally considered as very promising photocatalyst driven by visible light.However,bulk g-C₃N₄ exhibits the low photocatalytic performance due to its high carrier recombination,low light absorption efficiency and small specific surface area.To overcome these shortcomings,this thesis focuses on the modified surface/interface and changing morphology of g-C₃N₄,and its composites to improve the carrier separation efficiency and optimize their photocatalytic performance.The research achievements mainly include the following four aspects:1.Synthesis of hydroxyl-rich porous C₃N₄:a facile two-step approach was adopted to synthesize hydroxyl-rich porous C₃N₄.It includes the following steps:（a）precursor was prepared via freeze-drying the mixture of urea,thiourea and NH₄Cl,and（b）the precursor was thermally condensed.Systematic characterization for the samples demonstrated the formation of C₃N₄ featured by unique hydroxyl-rich porous structure with an extended tri-s-triazine unit.When applied as photocatalyst for the water splitting under UV-vis light irradiation,the resulted sample displays a high hydrogen evolution rate of 243.4μmol/g/h,about 4 times higher than that of C₃N₄prepared by conventional urea decomposition method.Such a performance enhancement could be due to the porous structure and surface hydroxyl-rich functional groups that promoted multiple-times reflection and light absorption,increased the active site numbers,and improved carrier transfer/separation efficiency.This study might provide a new insights for design and synthesis of high-performance photocatalysts.2.Layer-by-layer assembly into a bulk-like g-C₃N₄:a bulk-like g-C₃N₄ was firstly achieved through initiating a layer-by-layer assembly,which involves important processes of artificially protonated and oxygen doped g-C₃N₄ nanosheets.When using as photocatalyst under UV-visible light irradiation,the obtained sample exhibited excellent photocatalytic H₂ production rate of 1538.3μmol/h/g,about 8times higher than pristine bulk g-C₃N₄,which may be due to the effective migration/separation of the photogenerated carriers.This work may offer a general pathway,initiating a layer-by-layer assembly,to synthesize other bulk-like nanocomposites for novel applications.3.Synthesis of transition metal phosphides（TMPs,M=Fe,Co,Ni）and bulk g-C₃N₄（b-g-C₃N₄）composites:the composites of TMPs/b-g-C₃N₄ with different transition metal compositions were prepared by electrostatic adsorption and phosphating procedure in Ar atmosphere.The relationship between the photocatalytic activity of TMPs/b-g-C₃N₄ and the d-orbital electron arrangement of transition metals was studied for the first time.The results indicated that photocatalytic hydrogen evolution activity is highly dependent on the number of unpaired d electrons（n）which can adjust the Eads.Among the examined catalysts,Fe Co Ni P/b-g-C₃N₄exhibited the highest photocatalytic hydrogen production activity of 196μmol/g/h,which can be comparable with Pt/b-g-C₃N₄,and be better than mono-（Fe P,Co P,Ni P）,bi-（Fe Co P,Co Ni P,Fe Ni P）TMPs/b-g-C₃N₄ composites.Moreover,the catalyst also showed excellent photodegradation performance of 4-nitrophenol,and better OER performance due to the synergistic effect of Fe Co Ni P and b-g-C₃N₄.It is expected to be an effective method to improve the activity of transition metal composite catalysts by adjusting the electron concentration of unpaired d electrons.4.Synthesis of Fe Co Ni P/Zn_0.86Fe_2.14O4+δQDs/g-C₃N₄ heterojunction:Fe Co Ni P,Zn_0.86Fe_2.14O4+δquantum dots（QDs）with different particle sizes and g-C₃N₄ were coupled to form a heterojunction through mixing and calcining method for the first time.Zn_0.86Fe_2.14O4+δnanoparticles with different particle sizes were synthesized through the hydrothermal method,and the growth mechanism and structure of Zn_0.86Fe_2.14O4+δnanoparticles on the particle size dependence were studied.Zn_0.86Fe_2.14O4+δnanoparticles underwent distinct growth processes at different reaction temperature:at 200 ^oC,the growth mechanism follows an Ostwald ripening model with activation energy 124.7 k J/mol,while at 120 ^oC,the particle growth is governed by aggregation and coalescence,Ostwald ripening mechanism with the activation energy 27.5 k J/mol and digestive ripening process.Furthermore,the dependence of the photocatalytic hydrogen production activity in Fe Co Ni P/Zn_0.86Fe_2.14O4+δQDs/g-C₃N₄ heterojunction on the particle size of Zn_0.86Fe_2.14O4+δQDs was studied.The heterojunction with Zn_0.86Fe_2.14O4+δQDs in 5.1nm gave the optimal photocatalytic hydrogen production activity,and it exhibited the hydrogen production with 976μmol/h for 10 h.