Preparation Of Graphitic Carbon Nitride Based Photocatalyst And Tailoring Its Catalytic Performance

Because of the rapid improvement of people's life quality in 21 st century,the global environmental pollution and energy shortages have become increasingly serious,and gradually become important factors threatening the sustainable development of mankind.Semiconductor photocatalysis using solar irradiation has attracted enormous attentions due to its fantastic properties,such as nontoxicity,safety and renewability.However,the high cost,narrow spectral region and poor chemical stability of common photocatalysts limit the practical application in photocatalysis.As a metal-free photocatalyst,graphitic carbon nitride(CN)is a fascinating polymeric organic semiconductor photocatalyst of tri-s-triazine units with distinct advantages,for instance the low-cost precursors,facile synthetic process,high chemical stability and metal-free nature.Unfortunally,CN has small surface area and low charge separation efficiency,suppressing its industrial application.Therefore,the researchers usually construct CN-based heterojunctions and change the morphology of CN to improve their photocatalytic performance.In this dissertation,we investigated the catalytic activity of CN-based catalysts under visible light irradiation by employing graphene quantum dots(GQDs)to delaminate and sensitize CN.Their catalytic performances,stability and universality were also investigated by degrading organic pollution.Addtionally,we controlled the surface charge and morphology of CN to prepared protonated CN nanosheets(P-CNNS)that exhibited superior performance for photocatalytic reduction of p-nitrophenol(PNP)to p-aminophenol(PAP).The correlative works were summarized as follows:1.In this study,zero-dimensional/two-dimensional N-doped GQDs(N-GQDs)/CNNS heterojunction catalysts were prepared by the ultrasonication of N-GQDs and CN.N-GQDs can be successfully intercalated in the interlayer space of CN through the ?-? interaction between N-GQDs and CN.The band energy of N-GQDs and CN are well interlaced,so their interfaces are in close contact to form n-n junctions,which can effectively separate photo-generated electrons and holes.The photocatalytic performance of N-GQDs/CNNS composites has been significantly improved.The effect of N-GQDs loading on the photocatalytic performance was analyzed through the photodegradation tests of N-GQDs/CNNS samples with different N-GQDs contents under visible light.Among them,0.5 wt% N-GQDs/CNNS had the best performance,and its degradation rate was 2.4 and336.7 times that of CN and N-GQDs respectively.Also,the composite material has good stability and the photocatalytic degradation performance has no apparent decrease after four cycles.Besides,the photocatalytic mechanism of N-GQDs/CNNS was proposed through experimental results and theoretical calculations.In addition,the active species capture experiments suggested that the main active species in the photocatalytic process of N-GQDs/CNNS were superoxide radical(·O2-)and hydroxyl radical(·OH).2.The photocatalytic performance of N-GQDs/CNNS is improved due to the interlaced band structure.In order to promote the practical application of GQDs/CN based photocatalysts,herein we tailored the semiconductive behavior of GQDs and transformed the n-type hydrothermally synthesized GQDs to p-type via the modification of P=O groups with an electron-withdrawing ability.The obtained highly dispersed and stable P-doped GQDs(P-GQDs)had uniform size and thickness,high crystallinity,and wide visible-light absorption region.The subsequent assembly with the n-type CN formed a stable metal-free photocatalyst because of the ?-? interaction between conjugated GQDs sheets and CN layers and the possible hydrogen bonds as well.Due to the formation of p-n junction on the P-GQDs/CN interface,the photogenerated charges were efficiently separated which resulted in an enhanced photocatalytic performance.This strategy illuminates the GQDs applications by controlling the electronic structure of GQDs based on the detailed application,not only for photocatalysis but also for many other fields such as solar cells,catalysis,electrocatalysis,and so on.3.Photocatalytic reduction of PNP to PAP is of significant importance because of the high toxicity of PNP and the wide application of PAP.CN is an excellent photocatalyst for various photo-reduction reactions,but inefficient for photo-reduction of PNP due to the electrostatic exclusion.In this work,we controlled the morphology and surface property of CN and achieved significantly enhanced activity.The obtained P-CNNS material has positively charged surface that can adsorb p-nitrophenolate anions,therefore facilitating the transfer of photo-generated electrons from catalyst to PNP.Its reaction rate is 1626 times higher than that of the CN.Besides the surface charge,the morphology of photocatalyst also has important effect on activity,which is demonstrated by the relatively low activity of protonated CN in comparison to P-CNNS.The P-CNNS photocatalyst has an excellent stability and superior catalytic universality for photo-reduction of various nitroaromatic compounds.This work does not only expand the application of a well-known material,but also highlights the importance of understanding photocatalytic mechanism for designing photocatalyst.