Heterojunction Construction And Structural Defect Modulation On G-C₃N₄ Based Photocatalysts

The growing energy crisis and environmental pollution problem have jeopardized human health and the sustainable development of society.Addressing energy crisis and environmental pollution issues properly are the key to the sustainable development of human society.Photocatalytic materials can convert solar energy into chemical energy or produce reactive groups to mineralize organic pollutants,aiming to solve energy crisis and environmental problems.Graphite phase carbon nitride（g-C₃N₄）has suitable band structure and visible light response activity,rendering the potential application prospects in the field of photocatalysis.However,the lower light absorption and quantum efficiency of pure g-C₃N₄ lead to poor photocatalytic activity,thus pure g-C₃N₄ cannot meet the requirements of practical solar energy conversion applications.Therefore,in order to meet application standards of photocatalysis technology,pure g-C₃N₄ need be structurally adjusted or modified.This paper mainly studied,designed and prepared g-C₃N₄ heterojunctions and modified g-C₃N₄ materials by structural defects,aiming to solve the key technical problems about low visible light absorption capacity and easy recombination of photogenerated electron-hole pairs,and conducted a series of characterization tests.Firstly,C-TiO₂/g-C₃N₄ heterojunction materials were prepared via heat-treating the mixture of TiC and urea.Combination of g-C₃N₄ and C-TiO₂ enhanced absorption ability toward visible light and decreased recombination rate of photoexcited electron-hole pairs.When the content of C-TiO2 is 0.330%,C-TiO₂/g-C₃N₄ heterojunction has the optimal photocatalytic activity,5.728 mmol/g,which is 2.5 times higher than that of the pure g-C₃N₄photocatalyst,and its photocatalytic stability is also significantly improved.C-TiO₂/WS₂/g-C₃N₄ three-phase heterojunction materials were prepared by impregnation method and post-heat treatment.Three-phase heterojunction system improves light absorption ability and reduces band gap.Its band gap structure is beneficial to the occurrence of water splitting.The photocatalytic water decomposition activity of the three-phase heterojunction is higher than that of the original g-C₃N₄,C-TiO₂/g-C₃N₄ and WS₂/g-C₃N₄,and also shows better photocatalytic ability to decompose seawater.It can be found that the photoexcited electrons on C-TiO₂ and WS₂ transfer into g-C₃N₄,while holes on g-C₃N₄ migrate into C-TiO₂ and WS₂,which realizes efficient separation of photogenerated electrons and holes.Bi₂O₂CO₃/g-C₃N₄ heterojunction materials were prepared by calcining the mixture of bismuth citrate and urea.Bi atoms in Bi₂O₂CO₃ nanoparticles form weak Bi-C and Bi-N bonds with C and N atoms in g-C₃N₄,increasing the stability of heterojunction.When the content of Bi₂O₂CO₃ in the heterojunction is 0.64 wt.%,photocatalytic hydrogen rate reaches a maximum,965μmol?g^-1?h^-1,which is about 3 times higher than that of pure g-C₃N₄.In fact,Bi₂O₂CO₃ and g-C₃N₄ are not a traditional heterojunction system but a Z-type heterojunction system.Two structural defects of O atom and cyano group were successfully introduced into the g-C₃N₄ lattice by heat-treating the mixture of ascorbic acid and urea.O atoms replace N atom in g-C₃N₄ crystal lattice to form N-C-O bond and C=O bond and cyano group links with the edge N atom.The co-modified g-C₃N₄ materials have a strong visible light absorption ability,and a large absorption shoulder shows in the visible light range that greatly extends visible light absorption range.The hydrogen production of co-modified g-C₃N₄ during 5 h is 3.5 times higher than that of g-C₃N₄,reaching to 6.7 mmol/g.It is due to the good shunting action of the photogenerated electrons and holes by O atoms and cyano group,enabling its spatial separation on the g-C₃N₄ nanosheets.Finally,the co-modified g-C₃N₄ with two structural defects of S atom and cyano group was prepared by second treatment method,and Zero-valence S atoms were also deposited on g-C₃N₄ nanosheets.S atoms replace N atom in the g-C₃N₄ lattice to form a C-S bond,which becomes a structural defect.On the other hand,the incomplete oxidation of H₂S produces a zero-valence S atom.The doped S atom and cyano group promote the spatial separation of photogenerated electrons and holes on g-C₃N₄ and the zero-valence S atoms expose more active sites,which leads to the outstanding photocatalytic performance of co-modified g-C₃N₄.