Controllable Construction Of Highly Crystalline G-C₃N₄ And Its Photocatalytic Properties For Hydrogen Production

With the rapid development of economy and society,the extensive use of fossil fuels has brought energy crisis and environmental pollution which can’t be ignored.Therefore,the development of environmentally friendly new energy is the only way for mankind to achieve sustainable development.And hydrogen energy is regarded as the most promising clean energy in the 21st century.Water resources are abundant in nature,and sunlight resources are more abundant.Using these two kinds of renewable resources,a new energy production method for photocatalytic water splitting to produce hydrogen under visible light has been developed.Photocatalysts is the key to photocatalytic water splitting for hydrogen.Graphitic carbon nitride（g-C₃N₄）is a promising photocatalyst with strong chemical stability and suitable band structure.However,the original g-C₃N₄ has shortcomings in the photocatalytic process,such as low utilization rate of visible light and low efficiency of photogenerated carrier separation and transmission,which limits its further development.In the preparation of g-C₃N₄,thermal condensation is the most commonly used method because of its simple operation and suitable for mass production.However,the problems of low visible light absorption capacity and high photogenerated electron-hole pair recombination rate of original g-C₃N₄ prepared by traditional thermal polymerization are mainly due to its low crystallization with the adverse electronic structure and electron transport channel.The low crystallization of g-C₃N₄ is caused by the disordered accumulation of precursors and reaction intermediates during thermal polymerization.Therefore,starting from improving the thermal polymerization process,it is of great significance to explore a way to improve the crystallization performance of g-C₃N₄ from the root to improve the visible photocatalytic hydrogen production capacity of the product.In response to the above problems,we have designed the following two parts of work:1.Preparation of imidazole-modified g-C₃N₄（CN-IMx）and its photocatalytic hydrogen production application.In this part,we introduce heterocyclic conjugated small molecule imidazole to participate in the hot polymerization process,by forming supramolecular co-assembly with urea precursors and complex reaction intermediates during the reaction to induce ordered polymerization and generate high crystalline g-C₃N₄.Using urea and different amounts of imidazole mixture as the starting reactant,a series of highly crystalline g-C₃N₄ with different composition were successfully prepared by one-pot polymerization.SEM,BET,TG and XRD tests proved that the introduction of imidazole can help improve the crystallinity of CN-IMx without changing the basic morphological structure by forming ordered supramolecular assemblies with reaction precursors and intermediates.FT-IR,Raman,XPS,solid-state ¹³C MAS NMR analysis and optical and photoelectrochemical tests such as UV-Vis DRS,EIS,photocurrent,etc.,show that CN-IMx has an extendedπconjugate structure and enhanced visible light absorption,while the separation and transmission efficiency of photogenerated carriers is also significantly improved.The band gap and band structure tests also show that CN-IMx（Eg=2.76 e V,CB=-1.67 e V vs.NHE）is more suitable for photocatalytic hydrogen production than the original CN（Eg=2.91 e V,CB=-1.57 e V vs.NHE）in the band gap and conduction band position.Visible photocatalytic hydrogen production test shows that the photocatalytic performance of CN-IMx has been significantly improved which is closely related to the change of imidazole addition amount,and the hydrogen production rate can be increased by up to 3 times.And CN-IM300 is a high crystalline product with the best electronic structure and hydrogen production performance when the ratio of urea to imidazole dosage is 5 g:300mg,.This study proves that it is effective to regulate the crystallization degree and electronic structure of g-C₃N₄promoting copolymerization by adding small molecules to generate co-assembly for improving the photocatalytic hydrogen production performance of g-C₃N₄.2.Preparation of pyrazole modified g-C₃N₄（CN-Py-x）and its application of photocatalytic hydrogen production.In the first part of the work,we found that the introduction of small molecules can improve the crystallinity and enhance the hydrogen production activity of the photocatalyst via synergistic effect.In this section,we introduce pyrazole which is the isomeride of imidazole to participate in copolymerization for further studying the effects of small molecules with different structures on the crystallinity and photocatalytic performance of g-C₃N₄.SEM and XRD tests showed that the same amount of two molecules with different structure had different degrees of crystallinity regulation,and only the addition of an appropriate amount of pyrazole could obtain a high crystalline CN-Py-x.FT-IR and XPS results show that CN-Py-x still retains the basic structure of g-C₃N₄,and prove that the difference in regulatory effects of imidazole and pyrazole is due to their structural difference.Optical and photoelectrochemical tests such as UV-vis DRS,EIS and photocurrent show that CN-Py-x has better visible light utilization and photogenerated electron-hole separation and transmission capabilities than the original CN.The band gap and band structure tests also show that CN-Py-x（Eg=2.30 e V,CB=-1.74 e V vs.NHE）is more suitable for photocatalytic hydrogen production than the original CN（Eg=2.91 e V,CB=-1.57 e V vs.NHE）in the band gap and conduction band position.The visible photocatalytic hydrogen production test shows that the catalytic performance of CN-Py-x has been significantly improved which is closely related to the change of pyrazole addition,and the hydrogen production rate can be increased by up to4 times.And CN-Py-300 is a high crystalline product with the best electronic structure and hydrogen production performance which can be obtained when the ratio of urea to pyrazole dosage is 5 g:300 mg.This study proves that by changing the structure of copolymeric small molecules,it is feasible to further improve the photocatalytic hydrogen production performance of g-C₃N₄ improving the synergy of high crystallinity and electronic structure.