Study On Photocatalytic Reduction Of CO₂ Over Graphitic Carbon Nitride And Its 0D/2D Heterostructures

Rapid consumption of fossil fuels has not only brought about energy crisis,but also caused environmental problems due to the increasing concentration of CO₂ in air by fossil fuel combustion.CO₂ conversion into chemicals with solar energy is a promising approach to address the energy and climate issues.The design and synthesis of photocatalyst is the key to realize the effective conversion of CO₂ driven by solar energy.Graphitic carbon nitride（g-C₃N₄）has been attracted much attention in the field of photocatalysis due to its absorption of visible light,strong redox ability,non-toxic nature,low price,and high stability.In this thesis,g-C₃N₄ and its zero-dimensional/two-dimensional（0D/2D）heterojunctions with high activity and stability were designed and prepare to study the influence of defects of g-C₃N₄,the interfacial charge transfer and transition metal coupled 0D/2D heterojunctions on the reaction performance of CO₂ photoreduction.The main findings from this thesis are as follows:g-C₃N₄ nanosheets with N defects were prepared by tartaric acid-assisted dicyandiamide polymerization.Defects were located at both three-coordinate N atoms and uncondensed terminal NHx species in g-C₃N₄ framework.Owing to enhanced light absorption,high separation efficiency of photo-generated charge induced by N defects,and large specific surface area and pore volumec,deficient g-C₃N₄ exhibited CO evolution of 285 μmol g^-1 in photocatalytic CO₂ reduction reaction,which was 8 times higher than g-C₃N₄ prepared by dicyandiamide under the same conditions.Charge dynamics studies found that the N defects in g-C₃N₄ could act as the trapped sites of photoinduced charges,which prolonged the lifetimes of charge carriers and enhanced the efficiency of charge separation.The decreased interlayer stacking distance in g-C₃N₄ was in favor of mobility of charge carriers to the surface of the semiconductor.Building on the above findings on effective g-C₃N₄ nanosheets,oxygen vacancy-rich 0D/2D TiO₂/g-C₃N₄ heterostructure was prepared by using NH2-MIL-125（Ti）and melamine as precursors through two-step in-situ pyrolysis strategy.An intimate interface was formed between semiconductors by chemical bonds in in-situ pyrolysis process.TiO₂ quantum dots with high crystallinity were successfully formed during the secondary heat treatment,which plays a role of exfoliation on g-C₃N₄,making it transform from bulk to nano-sheet shape.Due to large surface area and porosity,high CO₂ adsorption capacity,enhanced visible light absorption,and efficient charge separation,0D/2D TiO₂/g-C₃N₄ showed enhanced photocatalytic performance of CO₂ reduction with 389μmol g^-1 of CO formation.Charge dynamics studies demonstrated that interfacial charge transfer was taken place from 2D g-C₃N₄ to 0D TiO₂ in sub-picosecond time.The ultrafast charge transfer significantly promoted separation of charge carriers.At the same time,charge transfer at interface led to fast decay of charge carries,suggesting shallow trapped states of charges,which was beneficial to CO₂ adsorption and activation.Based on the above work,the transition metal was introduced to modify the 0D/2D TiO₂/g-C₃N₄ heterojunction.0D/2D TiO₂/Co/g-C₃N₄ heterostructure was prepared using ZIF-67 and NH2-MIL-125（Ti）as precursors with urea through in-situ pyrolysis.The yield of photocatalytic CO₂ reduction to CO reached 1437 μmol g^-1.Structural analysis results indicated that Co was doped in 0D/2D TiO₂/g-C₃N₄ structure and acted as a bridge between 0D TiO₂ and 2D g-C₃N₄ by chemical bond at interface.Co functioned as the electron transport medium accelerating the charge transfer at interface in the heterogeneous.In addition,mechanism studies demonstrated that[Co（bpy）3]2+in the reaction system had not any activity under visible light.Electron transfer occurred from heterogeneous photocatalyst to[Co（bpy）3]2+,which acted as the active sites to catalyze CO₂ reduction into CO during the reaction.In summary,this present work found that the interfaces of heterostructure with transition metal bonds significantly improve electron transfer rate,resulting into enhancement of photocatalytic CO₂ reduction activity.And it provided a platform for the design and synthesis of multi-dimensional g-C₃N₄ heterojunctions into the efficient photocatalytic reduction of CO₂.