Simple Synthesis Of Modification Polymeric Carbon Nitride And Research On Their Photo Catalytic Performance

With global population growth and the industry development,consumption of fossil energy increases year by year,which causes more and more serious energy crisis and environmental pollution.One of the most potential means to address these issues is effectively utilizing extensive,inexhaustible,and renewable resources like water and solar energy.However,because of the low energy density and inhomogeneous,discontinuous,and instable distribution of solar energy irradiation on the earth’s surface,development of relevant technologies with efficient utilization of solar energy is very slow yet.Among masses of ways of utilizing solar energy,the photocatalytic technology can converse solar energy with low energy density into chemical energy with high energy density,and possesses potential value for application in water splitting to produce hydrogen,degradation of organic pollutants,reduction of carbon dioxide,nitrogen fixation,and selective organic transformation,etc.This technique shows promise for relieving growing energy and environmental crises and thus attracts extensive attention of scientists.Photocatalysts are the core of the photocatalytic technology,and polymeric carbon nitride(CN)has attracted great interest of researchers as a metal-free and visible light responsive photocatalyst with excellent characteristics,such as non-toxicity,low cost,high stability,suitable energy band structures.However,the photoactivity of CN is far from meeting the request of industrial application,owing to its high recombination efficiency of photogenerated charge carriers,low specific surface area,and narrow spectral absorption range(λ<460 nm).Morphological tuning,element doping,defect engineering,and fabrication of hetero/homo-junction are common modification strategies to enhance the photoactivity of CN,but they possess shortcomings like harsh experimental conditions,serious pollution,or high cost of products.Base on these,simple,efficient,and novel strategies are developed in this thesis to structure modify CN and effective improve its photocatalytic performance,with the mechanism for potoactivity enhancement systematically expounded.Main research contents in this thesis are listed as follows:(1)Mesoporous melamine(MMA)comprising nanorods was prepared via dispersing commercial MA(MA)in acid solution under stirring,with-40 times increase in the specific surface area.By using the mixture of MMA and the gas template(NH4Br)as the precursor,hierarchically mesoporous CN(HMCN)was successfully synthesized after simple calcination,with much thinner nanosheets,～4 times higher specific surface area,stronger photoabsorption,and faster charge separation than bulk CN.On this account,HMCN exhibits much enhanced photoactivity in H2 production and pollution degradation,and its visible-light photocatalytic H2 evolution and RhB degradation rates are～8.9 and～11.2 times that of CN,respectively,with prominently higher generation rates of reactive oxygen species(e.g.,·O2-).Chemical stability of HMCN is high and its preparation method may guide synthesis of other mesoporous CN-based photocatalysts.(2)Mesopores CN(MCN)homojunctions were successfully synthesized by calcining cyanuric acid(CYA)particles-loaded nanobelts-like melamine which was prepared by stirring ethanol suspension of melamine and trichloroisocyanuric acid at room temperature to hydrolyze trichloroisocyanuric acid(water from residue in ethanol and air).MCN exhibits a higher photogenerated charge separation efficiency,larger specific surface areas,stronger visible light absorption,five times higher photocatalytic hydrogen evolution rate,faster singlet oxygen generation,and higher photoactivity in organic degradation than CN.Construction of the MCN homojunctions arise from coexistence of massive and few mesopores regions in MCN with different and matched energy band structure.This can theoretically direct construction of CN homojunctions.(3)Mesopore-rich CN with nitrogen vacancies(mCNv)was successfully synthesised by mechanical ball-milling of the intermediate(melem)with succedent calcination.The formation of mCNv arises from the ball-milling induced structure distortion and morphological variation of the intermediate with massive intermolecular hydrogen bonds ruptured,affecting molecular polycondensation in the calcination process.Synergism of mesopores and vacancies render the as-prepared mCNv exhibits enlarged specific surface area,considerably thinned nanosheets,extended visible light absorption,enhanced photoexcited charge separation and transfer,and thus～6.3-fold improved photocatalytic H2 evolution activity under visible light irradiation,compared with the bulk CN.This work provides a novel,and industrializable technique for simultaneous creation of defects and mesopores in CN.