Synthesis And Photocatalytic Performance Of G-C₃N₄-based Composites

With the rapid development of modern industry,energy crisis and environmental problems have become increasingly prominent.To solve the problems mentioned above,photocatalytic technology has attracted more and more interests and attentions.The core part of photocatalytic technology is photocatalytic material which becomes a research hotspot.Traditional photocatalytic materials represented by TiO2 are low-cost,nontoxic and high stability,but the energy gaps are broad and these materials can only absorb UV-light which restrict the application in the photocatalytic fields.Therefore,it is very significant to develop highly efficient photocatalysts with broad spectra response.The thesis has mainly studied g-C3N4 through the preparation conditions,exfoliation and construction of composites to enhance the photocatalytic activity as far as possible.At present,the application of g-C3N4 focuses on the photodegradation of organic pollutant,so photocatalytic hydrogen evolution and disinfection of modified g-C3N4 have been studied in the article to expand the application range.The detailed contents were summarized as follows:(1)The effects of preparation temperature and precursor to photocatalytic activity of g-C3N4 were studied.The g-C3N4 was synthesized at 500 ℃,550 ℃ and 600 ℃,respectively.CN-550 exhibits superior photocatalytic activities of hydrogen evolution and disinfection due to the excellent crystallinity and good separation efficiency of photogenerated carriers.The g-C3N4 was also synthesized at 550 ℃ from dif-ferent precursors(melamin,dicyandiamide and urea),and the sequence of photocatalytic activity is CN-U,CN-D and CN-M.The photocatalytic activity of CN-U is slightly better than that of CN-D,but the yield of CN-U is 3.6%which is far lower than that of CN-D(33.8%).In view of economical efficiency,dicyandiamide was selected as precursor to synthesize g-C3N4 in the following study.(2)g-C3N4 nanosheets(CNNs)were prepared by liquid ultrasonic exfoliation from bulk g-C3N4(BCN),and the photocatalytic activities of hydrogen evolution and disinfection were studied.The results of XRD and FT-IR indicate that the molecular structure of g-C3N4 has not been destroyed after exfoliation.The thickness of CNNs is much thinner than that of BCN which can be seen in SEM and TEM images.In addition,the BET specific surface area of CNNs is about 6.4 times larger than that of the BCN.The photogenerated carriers can quickly transfer from inside to surface on account of the ultrathin 2D structure of CNNs which can suppress the recombination of photogenerated carriers.The result of transient photocurrent responses shows that the separation efficiency of photogenerated carriers from CNNs is significantly superior to BCN.Under the synergistic effect of large specific surface area and high separation efficiency of photogenerated carriers,the photocatalytic activities of hydrogen evolution and disinfection of CNNs are better than that of BCN.(3)NiO/CNNs composites were prepared via an impregnation-calcination method,and the photocatalytic activities of hydrogen evolution and disinfection were studied.The electrons in conduction band of CNNs can transfer to NiO because of the unsaturated 3d orbitals in Ni2+.NiO NPs deposited on the CNNs uniformly and closely which provided favorable condition for the transportation of electrons in conduction band of CNNs to NiO.The introduction of NiO strengthens the absorption of visible light,and the transportation of electrons from CNNs to NiO restrains the recombination of photogenerated carriers.The photocatalytic activities of hydrogen evolution and disinfection of NiO/CNNs are superior to pure CNNs,and 0.5%-NiO/CNNs exhibits the optimal photocatalytic activities which is much better than 0.5%-NiO/BCN.The photocatalytic disinfection process of NiO/CNNs is mainly governed by h+,and the possible photocatalytic mechanism of NiO/CNNs is critically discussed.(4)Bi2MoO6/CNNs(BM/CNNs)composites were synthesized by an in situ solvothermal method,and the photocatalytic activities of hydrogen evolution and disinfection were studied.The ultrathin 2D structure and large specific surface area of CNNs create advantageous conditions for Bi2MoO6 loaded on CNNs uniformly and closely.Due to the matched energy band structure of Bi2MoO6 and g-C3N4,the photogenarated electrons and holes could transfer between the two which improved the separation efficiency.The photocatalytic activities of hydrogen evolution and disinfection of BM/CNNs are superior to pure CNNs,and 20%-BM/CNNs exhibits the optimal photocatalytic activities which is much better than 20%-BM/BCN.The photocatalytic disinfection process of BM/CNNs is mainly governed by h+,and it is the probable major reason of the photocatalytic enhancement that the photogenarated electrons and holes transfer between Bi2MoO6 and g-C3N4.(5)MoS2/CNNs(MS/CNNs)composites and Ag/CNNs composites were prepared by impregnation-calcination and photoreduction methods,respectively.Based on the synthesis of MS/CNNs and Ag/CNNs composites,MoS2/Ag/CNNs(MS/Ag/CNNs)composites were constructed.The samples were characterized by XRD,TEM,FT-IR,UV-Vis and PL,and the photocatalytic activities of hydrogen evolution and disinfection were studied.3%-MS/CNNs and 3%-Ag/CNNs have the best photocatalytic performance in their respective composites,and MS/Ag/CNNs exhibits more excellent photocatalytic performance of hydrogen evolution and disinfection than 3%-MS/CNNs and 3%-Ag/CNNs.Ag NPs sandwich between MoS2 nanosheets and CNNs and act as the recombination centre of the electrons from MoS2 and the holes from CNNs,which could leave the electrons in conduction band of CNNs with higher reduction ability and the holes in valence band of MoS2 with higher oxidation ability.The transportation of photogenerated carriers improves the photocatalytic activities of MS/Ag/CNNs efficiently,and the photocatalytic disinfection process of MS/Ag/CNNs is mainly governed by h+.