Preparation And Catalytic Performance Of Hybrid Photocatalysts Based On G-C₃N₄

Semiconductor based photocatalysis is widely recognized as a promising strategy for solving energy and environmental problems due to its unique characteristics of low cost and environmental friendly.At present,the surface modification of semiconductor materials,the preparation of large specific surface area nanostructures and the construction of semiconductor heterostructures are the three most basic means to improve the photocatalytic performance of semiconductor materials.In this thesis,graphite phase carbon nitride(g-C3N4)based photocatalyst was designed and prepared throuth dye sensitization,thermal stripping,and heterojunction construction with B4C,respectively.The obtained hybrid photocatalyst has shown excellent performance in degradation of organic pollution and nitrogen fixation.The main innovative research results are summarized as follows(1)g-C3N4 obtained by pyranolysis,polyaddition and polycondensation of cyanamide is sensitized with 3,4,9,10-tetradecanoic acid dianhydride,copper phthalocyanine and cobalt green dyes alone or in combination.The light absor:ption properties of dye sensitized g-C3N4 were characterized by UV-Vis DRS.The results have shown that the higher the sensitization temperature,the better the sensitization effect on the premise that the dye does not decompose We have realized the broadening of the light absorption range ofg-C3N4 up to about 1000 nm by dye sensitization,which significantly improved the utilization efficiency of light.It is confirmed that dye sensitization can change the response range to visible light of some semiconductor photocatalysts,broad the absorption of light,make up for the shortcomings of semiconductors with broden band gap in solar energy utilization,and improve the utilization efficiency of visible ligt.(2)The thermally-exfoliated two-dimensional nanosheets of g-CN4(TE-g-C3N4)were prepared by H2SO4 intercalation and thermal treatment to overcome the shortcomings of conventional g-C3N4,which not only are prone to aggregate,but aslo expose insufficient surface active sites.The obained TE-g-C3N4 was characterized by XRD,XPS,FTIR,SEM,BET,UV-Vis DRS and PL,respectively.The the catalytic and photoelectrochemical performance of TE-g-C3N4 were evaluated by photo-Fenton reaction and photo electrochemical test,respectively.The results have shown that the surface area of TE-g-C3N4 reaches 59 m2g-1,which is more than 2 times higher than that of pristine g-C3N4,thus provide more active sites than traditional,g-C3N4.Photocatalytic measurement demonstrated that TE-g-C3N4 has better performance for RhB degradation than g-C3N4.The rate of RhB degradation on TE-g-C3N4 was 2 times faster than that on g-C3N4,and the rate was further accelerated by addition of Fe3+.At the same time,the TE-g-C3N4 possesses good stability.The photocatalytic activity remained above 80%of the initial value,even after five times of recycling.It is particularly important to note that,unlike conventional photo-Fenton reaction,which requires both light illumination and addition of H2O2,this reaction system here exhibits high photofentenson activity without any H2O2.The mechanism show that TE-g-C3N4 can produce H2O2 in-situ under light illumination,and then drive Fenton-like reaction,which further accelerates the reaction rate.Meanwhile,TE-g-C3N4 also exhibited excellent photoelectrochemical performance.The photocurrent density from the TE-g-C3N4 photoanode reaches 4.3 ?A/cm2 at 0.4 V vs.Ag/AgCl,which is more than 2.3-folders higher than that of the conventional g-C3N4 and the stability of the PEC device is also very good.After continuous illumination for 800 s,The photocurrent density can maintain more than 90%of the initial value.(3)In order to overcome the shortcomings of easy recombination of photogenerated electron-hole pairs and low mobility of charge carriers in g-C3N4,B4C/g-C3N4 heterojunction was prepared by a simple solvent evaporation method and characterized by XRD,XPS,FTIR,SEM,BET,UV-Vis DRS and PL,respectively.The obtained B4C/g-C3N4 was used for photocatalytic nitrogen fixation at room temperature and atmospheric pressure and its photoelectrochemical performance was also analyzed.The results have shown that when the mass ratio of B4C to g-C3N4 is 2:3,the coresponding B4C/g-C3N4 heterojunction has the best photocatalytic nitrogen fixation performance.,and the NH4+ yield reaches 2378 ?mol/g/h during 1 hour of operation,which is 4.5 times and 2.6 times higher than that of B4C and g-C3N4,respectively.After 5 cycles of repeat operation,the NH4+ yield can be maintained above 95%of the initial value,implying the good stability of this kind of catalyst.At the same time,B4C/g-C3N4 also exhibited excellent photoelectrocatalytic performance.The photocurrent density of the B4C/lg-C3N4 photoanode reaches 7.5 ?A/cm2 at 0.4 V vs.Ag/AgCl,which is more than 25-folders higher and 4-folders higher than that of the g-C3N4 and B4C,respectively.And the stability of the PEC devices are also very good.Mechanism studies show that the formation of heterojunction provides a direct path for the transport of photogenerated charge carriers,increases charge mobility,and extends the lifetime of photogenerated electrons.