Preparation Of Crystalline Graphitic Carbon Nitride-Based Materials And Their Photocatalytic Applications

Semiconductor-based photocatalysis is considered as an ideal solution to the energy and water crises,but the preparation of efficient photocatalysts has become a key constraint to the widespread use of such advanced technology.Graphitic carbon nitride（g-C₃N₄）has attracted much attention due to its abundant precursors,nontoxicity,visible light response,high stability and suitable energy band structure.However,conventional g-C₃N₄ also suffers from low crystallinity,lots of defects in the bulk phase and rapid recombination of photogenerated charge,resulting in its low photocatalytic efficiency.Molten salts-assisted synthesis has been proved to be an efficient approach to improve the crystallinity and photocatalytic activity of g-C₃N₄,and two isotypes of crystalline g-C₃N₄ have been reported by such method,namely poly（triazine imide）（PTI）and poly（heptazine imide）（PHI）,in which PTI is of great interest due to its high crystallinity and stability.However,there are still several major problems in this area that need to be solved:（ⅰ）it is difficult to achieve the targeted preparation of lattice planes exposed PTI with uniform sizes;（ⅱ）the structural transformation mechanism during the formation of PTI in molten salts is still unclear;（ⅲ）the visible light absorption and photogenerated charge separation and transfer efficiency of PTI is low.In response to the above problems,a series of studies have been conducted in this thesis.The specific studies are as follows:1.In this thesis,a new synthetic method is proposed to achieve the simple and targeted synthesis of lattice planes exposed PTI with uniform sizes.Highly crystalline and lattice planes exposed hexagonal PTI was successfully prepared by molten salts treatment of porous nanosheets intermediate with loose layer stacking synthesized from urea（Urea-based carbon nitride,UCN）under nitrogen atmosphere,which also exhibited excellent photocatalytic overall water splitting（OWS）activity.In contrast,other precursors produced intermediates with tighter layer stacking could only produce pure PHI or PHI/PTI mixtures,which exhibited no or weak OWS activities.2.In this thesis,the mechanism of structural transformation during the formation of PTI in molten salts is investigated in detail,and the transformation pathway from melon to PHI to PTI is found.Highly crystalline g-C₃N₄ with different phases were synthesized by regulating the calcination time of UCN in the molten salts,which was caused by the difference in the progress of the structural transformation.In photocatalytic degradation tests,the stable PTI phase was less active than the intermediate state,PHI,which is probably caused by its lower visible light absorption and photogenerated charge separation and transfer efficiency.However,due to the loss of K+ within the structure of PHI in an aqueous environment,it is less stable than PTI,making the latter more suitable for further in-depth studies.3.In this thesis,CdS@PTI composite was fabricated to improve the visible light utilization and photogenerated charge separation transfer efficiency of PTI.The photocatalytic mechanism of CdS@PTI was investigated in detail as well.The experimental results show that the introduction of CdS improves the light absorption performance of PTI and forms a heterojunction at the interface between PTI and CdS,which generates a strong internal electric field（IEF）.Driven by the IEF,the recombination of photogenerated charge was efficiently prohibited,and reaches the spatial separation of charge,thereby leading to a high sterilization activity of inactivating 99.99999%of bacteria within 1 h.