Study On Preparation And Photocatalytic Properties Of Graphitic Carbon Nitride-based Heterojunctions

Aiming at the low specific surface area,rapid charge carrier recombination and low sunlight utilization rate caused by the narrow visible light response of graphitic carbon nitride（g-C₃N₄）,various materials were incorporated to construct a series of g-C₃N₄-based heterojunctions using g-C₃N₄nanosheets as the substrate material.Extended light absorption range and improved separation efficiency of photogenerated electrons and holes together improved their photocatalytic performance for hydrogen generation as well as pollutant degradation and transformation.For these g-C₃N₄-based heterojunctions,the separation and transfer mechanism of photogenerated charge carriers in S-scheme,Z-scheme and Schottky heterojunctions was explored via the regulation of band structures and photoelectrical properties.The main research contents are as follows:Firstly,layered g-C₃N₄/Sn S₂S-scheme heterojunctions were prepared by the in-situ growth of Sn S₂nanosheets on the surface of ultrathin g-C₃N₄nanosheets.The experimental and theoretical calculation results revealed that the difference of Fermi levels between g-C₃N₄and Sn S₂induced the formation of the in-built electric field at the interface,which caused the band bending effect.As such,a part of charge carriers were consumed through the S-scheme electron transfer path and the photogenerated electrons and holes with strong REDOX abilities were retained on g-C₃N₄and Sn S₂,respectively.In the field of pollutant removal,as-prepared g-C₃N₄/Sn S₂heterojunctions were employed in the simultaneous photoreduction and photooxidation reaction.Under visible light irradiation,the composites could remove hexavalent chromium（Cr（VI））and 2,4-dichlorophenol（2,4-DCP）at the same time.From the perspective of band structure,this part of work explored the formation process of S-scheme heterojunctions,which paved the way for the construction of other S-scheme heterojunctions in the subsequent work.In order to further extend the solar light absorption range of g-C₃N₄-based heterojunctions,g-C₃N₄/Mo O_3-xheterojunctions were prepared by in-situ growing nonstoichiometric Mo O_3-xnanosheets on the surface of ultrathin g-C₃N₄nanosheets on the basis of the basic principle of constructing S-scheme heterojunctions.The introduction of oxygen vacancies not only extended the light absorption to near infrared（NIR）region but also produced the new defect energy level,which further promoted the separation process of charge carriers.To further improve the utilization efficiency of electrons and holes retained in the S-scheme heterojunctions,the composites were used for photocatalytic activation of persulfate to remove the organic pollutant phenol.Under full light irradiation,as-prepared g-C₃N₄/Mo O_3-xheterojunctions could effectively remove phenol via the synergy effect of the direct photodegradation reaction dominated by holes and the radical/non-radical pathways led by persulfate.Taking the advantages of the S-scheme electron transfer path and oxygen vacancies in photocatalytic reaction,WO_xwith abundant oxygen vacancies and suitable band structure was selected to construct g-C₃N₄/WO_xheterojunctions.Then,in order to further improve the light absorption ability and accelerate the transfer of photogenerated carriers,nitrogen-doped carbon dots（NCDs）and tungsten disulfide（WS₂）were incorporated as electron conductors in g-C₃N₄/WO_xheterojunctions,respectively,which produced g-C₃N₄/NCDs/WO_xand g-C₃N₄/WO_x/WS₂heterojunctions.For g-C₃N₄/NCDs/WO_xheterojunctions,a part of NCDs located between g-C₃N₄and WO_x.The excellent conductivity could accelerate the electron transfer at the interface,which realized the construction of all-solid Z-scheme heterojunctions.In addition,the up-converted fluorescence property of NCDs further improved the utilization rate of solar light.Under visible or NIR light irradiation,g-C₃N₄/NCDs/WO_xheterojunctions exhibited obviously improved photocatalytic hydrogen generation activity.As for g-C₃N₄/WO_x/WS₂heterojunctions,WS₂could be used as an effective electron acceptor to receive photogenerated electrons from g-C₃N₄,which further inhibited the recombination of electrons and holes.The role of noble metal Pt was replaced initially.Therefore,in addition to the degradation of organic pollutants,the introduction of WS₂successfully realized the photocatalytic water splitting towards hydrogen generation without any noble metal co-catalyst,which provided the possibility for the application of noble-metal-free photocatalyst.Finally,considering the substitution role of WS₂for noble metal co-catalyst,the influence of other tungsten-based compounds such as W₂C,W₂N and WP on the photocatalytic performance of g-C₃N₄was further explored.The poor dispersity and stability of single tungsten-based compounds were improved by interface engineering and in-situ carbon coating.W₂C/W₂N and WP@NC composites were obtained.Then,the two composites were loaded on the surface of g-C₃N₄nanosheets through mechan-chemical grinding and heat treatment process,respectively.Obtained g-C₃N₄/W₂C/W₂N and g-C₃N₄/WP@NC Schottky heterojunctions exhibited enhanced visible light absorption,lower carrier recombination rate,abundant heterogeneous interfaces and more active sites,which together improved their photocatalytic performances.With the absence of noble metal co-catalyst,the photocatalytic hydrogen generation rate of the two heterojunctions increased significantly,which reached more than a hundred times higher than that of g-C₃N₄and even better than that of g-C₃N₄/Pt.Real noble-metal-free photocatalyst was constructed and applied successfully.In addition,apart from the previous studies on the degradation of phenolic pollutants,we further extended the application of g-C₃N₄-based heterojunctions to the conversion of phenolic pollutants.Under visible light irradiation,with sodium borohydride（Na BH₄）as the reductant,the rapid removal of p-nitrophenol（4-NP）was realized by two pathways of photooxidation reaction and photo-assisted hydrogenation reduction reaction.Both of the two heterojunctions exhibited significantly improved 4-NP removal efficiency,whose kinetic constants increased by tens of times compared with that of pristine g-C₃N₄.