Structural Tuning Of Graphitic Carbon Nitrides With Highly Improved Photocatalytic Performance

Owing to the ability of converting low density solar energy to high density chemical fuel,such as hydrogen evolution from water splitting,organic fuel production from CO2 reduction,and so on,and eliminating organic pollutants in water and air,the photocatalysis technology based on semiconductors has been widely considered as one of the most promising way to alleviate the ever-increasing worldwide energy crisis and environmental issues by using the sustainable solar energy.The practical application of the photocatalyst technology depends strongly on the photocatalyst.Nevertheless,many drawbacks like the high cost,limited visible-light adsorption,secondary pollution,and low photocatalytic efficiency under visible light of the traditional transition metal-containing photocatalysts seriously impede their practical applications.Owing to the extraordinary advantages of metal-free composition,harvesting visible light,excellent chemical and thermal stability and ease availability,polymeric graphitic carbon nitride(g-C3N4 or GCN)has demonstrated to be one of the most fascinating sunlight-driven photocatalysts.However,a low specific surface area,insufficient solar-light absorption,and fast recombination of photogenerated carriers caused by the ?–? conjugated electronic system severely limit the photocatalytic performance and practical application of the conventional bulk GCN.In this context,this dissertation aims to improve the photocatalytic performance of GCN by many effective ways of structural regulation including microstructures,phase structures,and energy band structures,and carefully studies the intrinsic relationship between the preparation procedures,microstructures,and the photocatalytic activities in detail.This will provide much guidance for designing efficient photocatalysts based on GCN.As a new concept,we have shown the construction of a macroscopic 3D porous graphitic carbon nitride monolith(PCNM)by a one-step thermal polymerization of urea inside the framework of a commercial melamine sponge.The resultant PCNM had a freestanding structure,abundant pores,and a high specific surface area of 78 m2 g-1.More importantly,PCNM exhibits excellent photocatalytic activity,which is 2.84 times higher than that of GCN powder under visible light.Then,we achieve the preparation of holey graphitic carbon nitride nanosheets(HGCN)with abundant in-plane holes by thermally treating bulk GCN(BGCN)under an ammonia atmosphere.The as-obtained HGCN had a high specific surface area of 196 m2 g-1,and was found to be self-modified with abundant carbon vacancies that may be produced from the reaction between GCN and NH3.As such,HGCN had a much higher photocatalytic hydrogen production rate of nearly 20 times the rate of BGCN.Furthermore,we developed a cell cracker assisted exfoliation method to prepare the small-sized monolayer graphitic carbon nitride nanosheets(SMGCN)with an average lateral dimension and thickness of 55 and 0.3 nm,respectively,by using the protonated layered graphitic carbon nitride nanosheets as the precursor.The as-prepared SMGCN showed remarkably enhanced photocatalytic performance,and can be also used as good fluorescent probes for cell imaging due to the excellent fluorescent properties.Meanwhile,we also produced monolayer graphitic carbon nitride quantum dots with the average size of 3 nm.In addition,crystalline carbon nitride(CCN)constructed from the units of poly(triazine imides)intercalated with Li+,which is also one of the promising photocatalyst,were synthesized in an easy manner by heating a mixture of melamine and lithium halides.This method has the advantages of low cost,scalable production,and high efficiency.Importantly,CCN exhibited an enhanced photocatalytic performance compared to conventional GCN under visible-light irradiation because of their higher harvesting ability and charge carrier separation efficiency.And on this base we proposed and achieve the synthesis of a complex polymeric carbon nitride(CPCN)composed of CCN and GCN.It is found that the phase heterojunction formed between CCN and GCN can efficiently promote the separation of photoinduced electron and holes.More significantly,the photocatalytic activity of CPCN was 5.3 and 10.6 times as that of CCN and GCN under visible light.