Controllable Preparation Of Hierarchical Carbon Nitride And Its Photocatalytic Property For Hydrogen Production

As a kind of polymer semiconductor with suitable band structure,stable physical and chemical properties,friendly environment and easy preparation,graphite g-C₃N₄ has been developed and evolved for a long decade since it was first used for photocatalytic water splitting in 2009.People's interest in it has never been reduced and researches on it have been gradually deepened.At present,fast recombination of photogenerated carriers,small specific surface area,and the lack of active sites are still the most difficult problems in the field of g-C₃N₄.Although researchers have tried various physical and chemical methods,little effect has been achieved.To solve this problem,we controlled the morphology and crystal structure of g-C₃N₄ by precursor modification method,and the number of active sites and energy band structure were improved.Meanwhile,heterostructure was constructed to facilitate electron flow between g-C₃N₄ and other semiconductors,thus achieving the efficient separation of charges.The specific research content is as follows:?1?Hydrothermal pretreatment of dicyandiamide?DCD?was used to form organic small molecule solutions with uniform composition.Then,Coral-like g-C₃N₄ composed of one-dimensional hollow tubes was obtained by rapid freezing of liquid nitrogen and thermal polymerization.On this basis,phytic acid?PA?was added for phosphorus doping,and the visible light absorption of the product was not only enhanced,but the specific surface area and the microstructure of the tube surface also changed with the increase of doping amount.As a result,its photocatalytic hydrogen production rate in visible light was 22.4 times higher than that of g-C₃N₄ prepared by traditional methods,and its apparent quantum efficiency?AQY?at 420 nm was also excellent,reaching 4.32%.The hollow tube,which is composed of one-dimensional tube wall and two-dimensional shell sheet,constitutes a g-C₃N₄ homogeneous junction with the same type and different properties,which can effectively realize the spatial separation of charges and greatly improve the photocatalytic performance.?2?On the basis of previous experiment,dilute hydrochloric acid was added to the hydrothermal dicyandiamide solution to change the functional groups of small organic molecules,and the nanocage-like g-C₃N₄ with nitrogen-deficient was obtained by further thermal polymerization.Owning to the functional groups of the precursor,the chemical composition,polymerization pathway and morphology of the products were changed.The pore structure of g-C₃N₄ with large specific surface area,clear visibility and wide pore size distribution is conducive to the transportation of reactants and light reflection,and the existence of nitrogen defects at the pore edge provides the active site for light reaction.Meanwhile,g-C₃N₄ homogeneous junction caused by different polymerization modes makes more effective charge separation.With the dual promotion of structure and defect,its photocatalytic ability in visible light was 20.3 times higher than that of g-C₃N₄prepared by traditional methods,and its apparent quantum efficiency?AQY?at 420 nm also reached 3.01%.?3?g-C₃N₄/TiO₂@Ni_x P ternary composite photocatalytic materials were prepared by hydrothermal and photo-deposition methods.g-C₃N₄ and TiO₂ combined in a lamellar form,which present a three-dimensional spatial structure as a whole,increasing the utilization of light and enhancing the charge transfer in the heterogeneous interface.As a cocatalyst,Ni_xP was loaded on TiO₂ under the action of photo-deposition,which not only solved the interface stability caused by physical mixing,but also avoided the confusion of interface charge transfer,greatly accelerating the light reaction.Under the full spectrum,its highest yield of hydrogen is about 5438.0 mol·g^-1·h^-1.