Preparation,Modification And Photocatalytic Activity Of G-C₃N₄

In the 21st century,energy crisis and environmental pollution problems need to be solved urgently,and the harmful effects of organic pollutants on the environment have become more and more serious.Photocatalytic technology was regarded as a promising technology with good prospects for application to handle energy and environmental problems.However,common photocatalytic materials such as TiO2 and ZnO have low utilization rate of solar energy resources,which limits the promotion and application of photocatalysis technology.In recent years,g-C3N4 has shown good performance and application prospects in photocatalytic degradation of organic pollutants,water splitting into hydrogen,photocatalytic carbon dioxide reduction,etc.,and has become a hot material in the field of photocatalysis research.However,the photocatalytic performance of bulk g-C3N4 prepared by the thermal polycondensation method is poor due to many disadvantages,such as fast recombination rate of photo-generated electrons and holes,low specific surface area(SSA).In order to improve the utilization rate of sunlight,effectively reduce the recombination of photogenerated electron-hole pairs,improve the quantum yield,and thereby improve the photocatalytic efficiency of g-C3N4,doping modification,morphology control,and composite modification were used to improve the photocatalytic performance of g-C3N4.From that point,the morphology control and composite modification of g-C3N4 were studied in this paper.The structure and properties of several series of g-C3N4 samples were characterized by many modern characterization methods.The photocatalytic performances of the samples were evaluated by degrading Rhodamine B(RhB)aqueous solution(10 mg/L,100 mL)using 100 mg photocatalyst sample under visible light irradiation.Firstly,melamine was pretreated with hydrochloric acid,then melamine and acid-treated melamine were calcined at 550℃ for 2 h to obtain gCN and gX samples(where X corresponds to the acid-treating hours),effect of pretreatment time on the structure and properties of g-C3N4 were studied.It was found that the crystal and chemical structure of melamine was transformed thoroughly at the first 1 h of acid-treating,and fissure appeared on the surface of the particles.The subsequent extension of acid treatment time had little effect on the crystal structure of melamine,but the fissure gradually deepened.Compared with gCN,the crystal structure and chemical structure of g-X had little change,nitrogen vacancies and many porous agglomerates composed of granular and flaky g-C3N4 appeared in the g-X,and the specific surface area was significantly improved.The crystallinity and grain size of g-C3N4 in g-X reduced with the extension of acid treatment time.The adsorption and photocatalytic performance of g-X are greatly improved.The photocatalytic performance of g-2 sample was the best,after photocatalysis for 90 min,the degradation rate of RhB reached 99.30%,and the rate constant(k＝0.052 min’1)was 6.5 times that of gCN.The improved photocatalytic activity of g-X samples was attributed to the higher specific surface area value,outstanding photo-carrier separation efficiency and stronger light harvesting ability of g-X,and the latter two are more important.Then,the melamine treated with hydrochloric acid for 2 h was ground and mixed with an appropriate amount of ammonium chloride,and then calcined at 550℃ C for 2 h to prepare a porous photocatalytic material(p-gCN-NS)composed of g-C3N4 nanosheets.Compared with gCN,the crystal structure and chemical structure of g-C3N4 in p-gCN-NS had little change,nitrogen vacancies formed in the p-gCN-NS,the crystallinity and grain size of g-C3N4 in p-gCN-NS reduced.The average thickness of g-C3N4 nanosheets in p-gCN-NS is about 6 nm.The specific surface area of p-gCN-NS is 63.65 m2·g-1,which is about 6.7 times that of gCN,and the visible light capturing ability,photoelectron and hole separation efficiency of p-gCN-NS are also greatly improved.The adsorption performance and photocatalytic degradation performance of p-gCN-NS are much better than that of gCN.After photocatalysis for 50 min,the degradation rate of p-gCN-NS reached 99.32%,and the reaction rate constant(k = 0.09317 min-1)is 11.6 times that of gCN.Finally,the g-C3N4/rGO/TiO2 composite were prepared successfully through hydrothermal reaction using p-gCN-NS as the matrix and the modification of TiO2 and rGO.When the hydrothermal reaction temperature is between 140 to 160℃,the g-C3N4/rGO/TiO2 composites are mainly g-C3N4 and anatase,Possessing the similar photocatalytic performance and the photocatalytic performance of the 160℃ sample is slightly better.The TiO2 grains gradually grew and agglomerated with the rise of the hydrothermal reaction temperature.A little rutile appeared after the hydrothermal reaction temperature rise to 200℃,and the crystal structure of g-C3N4 changed,the number of large pores of the samples increased,the photocatalytic performance of samples decreased.When the hydrothermal temperature is 160℃,prolonging the hydrothermal reaction time has little effect on the crystal structure and chemical structure of g-C3N4 in g-C3N4/rGO/TiO2 composites,but the loading content and the grain size of TiO2 gradually increased,the photocatalytic performance of the sample gradually deteriorated.The mass ratio of g-C3N4 to TiO2 and the addition amount of GO have an effect on the structure and photocatalytic properties of g-C3N4/rGO/TiO2 composites.With the increasing of TiO2 content,the loading content of TiO2 and the specific surface area of the sample,but the photocatalytic performance reduced.The photocatalytic performance of the sample gradually lowered with the increasing of GO addition amount.When the mass ratio of g-C3N4 to TiO2 is 3:1,the amount of GO is 0.002 g,the hydrothermal reaction temperature and time are 160℃ and 6 h,respectively,the grT-r2 shows the best photocatalytic performance.RhB can be completely degraded taking 60 min and 45 min under visible light irradiation and full-spectrum irradiation,respectively,and it has good cycle stability.The grT-r2 sample has a specific surface area of 92.56 m2·g-1 and a band gap of about 2.85 eV.There is a heterojunction formed between the g-C3N4 nanosheet and the TiO2 nanocrystal,and an appropriate amount of rGO with good conductivity,so grT-r2 has higher photogenerated electron hole pair separation efficiency than gCN and p-gCN-NS,·O2-is the most active species in its photocatalytic degradation to RhB.