Preparation Of Photocatalytic Material Based On Carbon Nitride For Clean Energy And Research To Their Hydrogen Evolution Performance

As significant material basis for the development of modern society,clean energy has a direct impact on social economy and the survival and development of human society.Since the rapid development of industry and the continuous growth of population,the conflict between energy and development is increasingly exacerbate.The rapid use of non-renewable fossil fuels has brought about energy shortages,environmental pollution and climate change,etc.Questing economical,eco-friendly and efficient renewable energy has become the research focus in the field of scientific research.As clean energy and high-density energy carrier,hydrogen has the advantages of light weight,renewable,zero pollution and abundant reserves,which can meet the requirements of sustainable energy utilization.From the perspective of green chemistry,photocatalytic hydrogen production technology can not only furnish clean energy,but also reduce fossil energy consumption.Designing and constructing photocatalytic materials with superior photocatalytic performance and stability is the momentous to expedite the fast development of hydrogen production technology.Hitherto,in many assortment of photocatalyst semiconductor materials,graphitic carbon nitride featuring adjustable electronic properties,extraordinary physical and chemical properties and simple preparation process has been widely applied in the field of hydrogen production.Therefore,the g-C₃N₄ based photocatalytic materials are deemed to be one of the best candidate materials in the domain of photocatalysis in terms of performance and application,promoting the extensive application of clean energy production and efficacious visible light-driven photocatalysts in sustainable development.However,the bulk g-C₃N₄ prepared by the conventional calcination method has shortcomings easy recombination of charge carrers,limited light absorption range,small specific surface area and few active sites,which severely astrict the development and practical application of g-C₃N₄ as photocatalytic material.For the above problems of g-C₃N₄,researchers in order to improve the performance of g-C₃N₄ and enlarge its application scope has adopted a series of measures,such as:increasing the specific surface area by improving the surface roughness of the material,adjusting the photoelectric performance by copolymerization with organic or inorganic dopants,combining with(semi)conductive composite to elevate charge carrier separation and the heterogeneous or help the catalyst to improve interfacial charge transfer,and applying the heterogeneous or homogeneous co-catalysts to improve interfacial charge transfer,etc.Aiming at the shortcomings of g-C₃N₄ in photocatalytic hydrogen production,this thesis uses molecular structure regulation and constructing heterojunction to optimize its molecular structure,photoelectric attribute and photocatalytic performance.The“structure-activity”relation between the electronic structure/heterojunction type and photocatalytic performance of g-C₃N₄ is systematically studied to realize the valid hydrogen production of g-C₃N₄-based photocatalyst materials,which provides a meaningful exploration for the further study of organic conjugated semiconductors.The staple contents of are as follows:1.Simple modification of two-dimensional structure g-C₃N₄ nanosheets and their photocatalytic hydrogen evolution(1)Fabrication of N-rich defect and?-conjugated g-C₃N₄ nanosheets and study on their photocatalytic activity towards hydrogen production from water splittingAiming at the problem of easy recombination of charge carriers and insufficient active sites,this chapter introduces a logical design that integrates N defects and?-conjugation structure into the g-C₃N₄ molecular framework.The bulk g-C₃N₄ and Na BH₄ mixture is calcined at 350?to obtain N-rich defects and?-conjugated g-C₃N₄nanosheets(DCN350).Experimental study and DFT computation confirm that DCN350 with N defects not only can shorten band gaps for expanding the light absorption range via optimizing the electronic band structure,but also act as active sites for facilitating hydrogen evolution,and the–C?N as strong electron-withdrawing functional group can make the isolated valence electrons delocalized for driving the charge spatial separation.The DCN350 shows the highest hydrogen liberating rate of1541.6?mol h^-1 g^-1,about 7.5 times improvement as compared with that of BCN(205.9?mol h^-1 g^-1).Simultaneously,it also has good photochemical stability.(2)Fabrication of g-C₃N₄ nanosheets with donor-acceptor(D-A)structure and study on their photocatalytic activity towards hydrogen production from water splittingTo solve the problem of low intramolecular charge transfer efficiency of bulk phase g-C₃N₄,a simple one-step calcination method is used to doped benzene ring into the g-C₃N₄ molecular skeleton to construct the D-A structures.Characterizations such as ¹³C NMR,FT-IR and XPS verifies that the benzene ring is successfully implanted into the framework of g-C₃N₄ nanosheets.Studies have shown that the prepared benzene ring doped two-dimensional g-C₃N₄ nanosheets(2D-BDCN)exhibit excellent photocatalytic hydrogen production activity under visible light excitation,which is attributed to the fact that the D-A structure enables the fixed-point transfer of electrons in the g-C₃N₄molecule from donor to acceptor segments,and the density functional theory(DFT)calculation shows that the benzene ring not only plays the role of donor to promote intramolecular charge transfer,but also can broaden the visible light response range and improve the visible light utilization efficiency.The hydrogen production rate of 5BDCN is 2529.2?mol h^-1g^-1,about 7.3 times improvement as compared with that of g-C₃N₄(344.8?mol h^-1g^-1),indicating that benzene ring doping g-C₃N₄ can effectively improve the intramolecular charge transport and extending the electronic life.2.Fabrication of g-C₃N₄-based heterojunction and their photocatalytic hydrogen evolution(1)Fabrication of g-C₃N₄ nanotube@p DA/Ni Co-LDH and study on their photocatalytic activity towards hydrogen production from water splittingFor further encouraging the carrier separation of single-component g-C₃N₄,this chapter uses the calcination-hydrothermal synthesis and self-polymerization process to synthesize g-C₃N₄ nanotubes@p DA/Ni Co-LDH(LPC)heterojunction photocatalysts.In the LPC composite system,the 1D g-C₃N₄ nanotubes possesses larger specific surface area can afford a substantial amount of active sites,which conduce to shorten the charge transmission path,as well as the high-speed mass transfer behavior of the nanotube can quicken the photocatalytic reaction course.The g-C₃N₄/Ni Co-LDH type-II heterojunction can efficaciously stimulate the spatial separation of photo-produced charge.In addition,polydopamine(p DA)as heterojunction metal-free interface mediums can provide multiple action(?-?*electron delocalization effect,adhesive action and photosensitization).The experimental results of photocatalytic hydrogen evolution show that,the optimized LPC nanocomposite has the highest hydrogen production rate(1555.1?mol h^-1g^-1),which is about 6.7 and 3.2 times higher than that of g-C₃N₄(232.5?mol h^-1g^-1)and CN@p DA(485.4?mol h^-1g^-1),respectively,the LPC also has good stability and cyclicity.(2)Fabrication of 2D/2D WO₃/Pt/g-C₃N₄Schottky-Ohmic junction and study on their photocatalytic activity towards hydrogen production from water splittingIn view of the slow reaction kinetics of the Z-scheme heterojunction and the low electron utilization caused by the recombination of some electrons and holes,this chapter further optimizes the hydrogen evolution activity of the g-C₃N₄ heterojunction photocatalyst by constructing Schottky-Ohmic junction.We elaborately select 2D ultra-thin g-C₃N₄ and WO₃ as electron donors,and Pt with larger work function as electron acceptor in this work.The Schottky-Ohmic junction and Z-scheme heterojunction can be controllable prepared via regulating the load position of Pt.The 2D/2D WPC Schottky-Ohmic junction showcases admirable photocatalytic performance than that of2D/2D WCP Z-scheme heterojunction.The optimized WPC Schottky-Ohmic junction photocatalyst exhibits remarkable photocatalytic H₂-release performance with ability to hydrogen production rate reaches 5197.2?mol h^-1g^-1?mol upon exposure to visible light,which is about 1.2 and 11.5 times higher than that of WO₃/g-C₃N₄/Pt(WCP)(4477.6?mol h^-1g^-1)and pure CN(452.8?mol h^-1g^-1),respectively.This remarkable enhancement of photocatalytic performance is ascribed to:(i)Schottky-Ohmic junction can strikingly expedite spatial charge separation and elongate electron lifetime,(ii)the2D/2D structure can shorten the charge transportation distance,(iii)Pt with rich electron density can stably adsorb H⁺.(3)Fabrication of 2D/2D ZnIn₂S₄-S/g-C₃N₄Van der Waals heterojunction and study on their photocatalytic activity towards hydrogen production from water splittingAiming at the problem that the charge carrier transport of traditional heterojunctions composed of random morphological structures is ordinarily affected by factors such as lattice matching degree and interfacial charge transfer resistance,this chapter adopts in-situ growth method to construct 2D/2D ZnIn₂S₄-S/g-C₃N₄ van der Waals heterojunction with sulfur vacancies(ZIS-S/CN VDW)photocatalysts.The study unravels that the prepared 2D/2D 30ZIS-S/CN VDW heterojunction possesses enviable photocatalytic H₂ evolution property with capability to generate H₂ rate up to3215.0?mol h^-1g^-1,which is approximately 2.1 and 6.0 times high than that of ZIS-S(1504.9?mol h^-1g^-1)and g-C₃N₄(532.8?mol h^-1g^-1),respectively.The synergistic effect of VDW heterojunction and sulfur vacancies is propitious for improve the photocatalytic hydrogen production activity of 30ZIS-S/CN VDW heterojunction.DFT calculations elucidate that 2D/2D VDW heterojunction with larger contact area and strong electron coupling effect stimulate photogenerated charge kinetics,and the sulfur vacancies act as photosensitization units for ameliorating the light absorption capacity and as active sites for trapping electrons to elongate charge lifetime.This research not only enriches the 2D/2D heterojunction photocatalytic system based on g-C₃N₄,but also provides new ideas for the design of heterojunction photocatalysts with excellent performance.