Structure Control And Photocatalytic Performance Study Of Ultrathin Graphitic Carbon Nitride

The extensive utilization of fossil energy have caused serious greenhouse effect in recent years,resulting in the global warming,the frequent occurrence of extreme weather and the rise of sea level,so that the human living environment is facing unprecedented challenges.The solar energy irradiated on the earth’s surface can fully meet human needs for energy.Converting solar energy into chemical energy as an alternative to fossil energy will improve the environment and alleviate the energy crisis.The photocatalyst can convert carbon dioxide（CO₂）into high value-added fuel or water in nature into hydrogen（H₂）energy under sunlight.At present,the efficiency of both photocatalytic hydrogen production and CO₂reduction to fuel is still very low and can not satisfy the needs of industrialization.Therefore,the development of efficient photocatalyst is the key to realize the real-life application of photocatalysis technology.Polymer base graphite carbon nitride（g-C₃N₄）is considered to be the star material in many photocatalysts because of its rich material sources,stable physicochemical properties and suitable band structure.However,g-C₃N₄synthesized by traditional thermal polymerization has some disadvantages,such as limited exposed active sites,narrow visible-light absorption range,easy recombination of photogenerated charges as well as low reduction potential and oxidation potential.As a result,the structural advantages of g-C₃N₄can not be fully exploited,resulting in low solar energy conversion efficiency.In this study,a stable g-C₃N₄photocatalytic system with excellent photocatalytic activity was synthesized by exfoliation,defect modification,non-metallic doping,isotype heterojunction and structural reorganization.The processes of photocatalytic hydrogen production and CO₂reduction were systematically studied by combining experimental and theoretical calculations.The"structure-activity"relationships between the morphology,electronic structure and band structure of g-C₃N₄and its photocatalytic performance was investigated.The specific research contents are summarized as the following five aspects:1.The g-C₃N₄ultrathin porous nanosheets with high conduction band potential were prepared by the step-by-step synergistic stripping method of HCl-assisted hydrothermal stripping and successive thermal stripping/etching in air.Compared with g-C₃N₄prepared by HCl-assisted hydrothermal stripping and g-C₃N₄prepared by thermal stripping/etching in air,the resultant g-C₃N₄prepared by two-step synergistic stripping method shows higher activity of photocatalytic CO₂reduction to CO and CH₄,and the yields of CO and CH₄were 0.695μmol g^-1h^-1and 1.636μmol g^-1h^-1,respectively.The excellent photocatalytic CO₂reduction performance is mainly attributed to the follow points:（1）Augmented specific surface area and abundant pore structure provide highly exposed active sites;（2）Ultrathin nanosheets shorten the perpendicular migration distance of photoexcited carriers to the surface and suppress recombination of photogenerated charges;（3）The high conduction band potential provides a strong driving force for photocatalytic CO₂reduction.This study will provide a reference for preparation of other two-dimensional ultrathin materials.2.A step-by-step synergistic exfoliation was used to synthesize the ultrathin and porous g-C₃N₄nanosheets with controllable concentration of carbon defects and oxygen doping.Water vapor opens the heptazine units,introduces carbon defects,and acts as an oxygen source for oxygen doping under high temperatures.Synergistic effect of controllable carbon defects and oxygen doping can continuously regulate band structures and significantly improves the separation efficiency of photogenerated charges.As a result,the prepared g-C₃N₄with vigoroso reduction potential exhibits a very high photocatalytic H₂evolution rate of 2.414 mmol g^-1h^-1under visible light and 7.414 mmol g^-1h^-1under ultraviolet-visible light,respectively,which surpasses the majority of the previously reported g-C₃N₄with well-tuned band structure.This work offers a new design idea for highly active g-C₃N₄-based photocatalysts with a well-tuned band structure.3.A vapor-assisted surface reconstruction was developed to synthesize thin-wall g-C₃N₄tubes with carbon defects.The reduced wall thickness from 144 to 25 nm boosts light-scattering ability,observably elevates the specific area,improves the mass transport as well as shortens the distance of photogenerated charges migrating towards the surface.The intentionally introduced carbon defects in the heptazine frameworks owing to the altered surface charge state,optimize the band structure and photogenerated charges transfer conduct.The resultant thin-wall g-C₃N₄tubes with powerful reduction potential shows sharp improvement in the photocatalytic H₂evolution(4.17 mmol g^-1h^-1)under visible light and approximately 27 and 11 times higher than bulk g-C₃N₄and g-C₃N₄tubes,which surpasses the majority of the previously reported g-C₃N₄tubes.This work will provide inspiration for more reasonable remodification of tubular g-C₃N₄and other polymer materials for photocatalytic energy conversion and storage.4.A simple,template-free and“bottom-up”strategy has been developed to prepare 1D/2D g-C₃N₄isotype heterojunction composed of carbon-doped nanowires and ultra-thin nanosheets.The ethanediamine（EE）grafted on melamine ensures the growth of 1D g-C₃N₄nanowires with carbon doping,and the ultra-thin g-C₃N₄nanosheets were produced through HCl-assisted hydrothermal strategy.The apparent grain boundary between 2D nanosheets and 1D carbon-doped nanowires manifests the formation of the isotype heterojunction.The built-in electric field provide potent driving force for photogenerated carriers separation.Meanwhile,the doping carbon in g-C₃N₄nanowires guarantees outstanding visible light absorption.As a result,the photocatalytic H₂evolution activity of 1D/2D g-C₃N₄isotype heterojunction is8.2 time that of the pristine g-C₃N₄,and an excellent stability is also obtained.This work provides a promising strategy to construct isotype heterojunction with different morphologies for effective photocatalytic H₂evolution.5.The ultrathin g-C₃N₄nanosheets with high degree of polymerization are synthesized via structural reorganization.Both experiments and theoretical calculations manifest that NH₃groups stabilized by hydrogen bonding networks are most easily removed and form a more stable bridged N-（C）₃,which extends conjugateπsystem,offers more electron transfer channels,causes flat distortion and activates n→π*electron transitions.The removed NH₃groups ensure effective exfoliation of g-C₃N₄,resulting in high exposure of active sites and reduced perpendicular migration distance of photogenerated charges.These advantages endow the g-C₃N₄with excellent separation and migration of photogenerated charges as well as well-tune band structure and strong visible light absorption,meantime,its surface is rich in unpaired electrons,which is conducive to CO₂activation.As expected,the g-C₃N₄demonstrates nearly100%CO₂-to-CO conversion with CO production rate of 13.26μmol g^-1h^-1underλ≥420 visible light and2μmol g^-1h^-1underλ≥500 nm visible light,respectively.This work not only provides a facile and green strategy to construct visible-light driven 2D ultrathin g-C₃N₄,but also thought-provoking insights on structure-activity relationship for photocatalytic CO₂reduction performance over g-C₃N₄.