Research On The Approaches And Mechanism For Improving The Photocatalytic Performance Of Graphite Carbon Nitride

With the rapid development of social economy,the energy shortage and environmental pollution become more and more serious.It has significance to develop efficient solar energy-chemical energy conversion technology for clean energy and remediation of environmental pollution.Photocatalytic hydrogen evolution and degradation of organic pollutants have become the forefront and hotspot(?)current research.Graphite carbon nitride(g-C3N4),as a nonmetallic polymer semiconductor,is an ideal photocatalytic material because it has a series of advantages such as the narrow band gap(Eg=2.7 eV),high visible light absorption,manageable electronic structure and high physical and chemical stability.However,the low quantum efficiency,which is attributable to its low conductivity and serious carrier recombination efficiency,keeping back the practical application of g-C3N4.This is an urgent problem to be solved.In this paper,triazine/heptazine-based g-C3N4 heterojunction,K+and H+doped g-C3N4 and supramolecular modified g-C3N4 are produced to regulate surface/interface structure,band structure and optical properties of g-C3N4.The photocatalytic hydrogen production and degradation of gaseous acetaldehyde are used as typical reaction cases to explore the path and mechanism of the improvement of photocatalytic performance.The detail content is as follows:(1)Microwave molten-salt strategy for the synthesis of triazine/heptazine g-C3N4 heterojunction with efficient charge separation:The g-C3N4 with triazine/heptazine heterojunction obtained by using precursors(melamine and thiourea)though microwave molten-salt process.The ratio of triazine and heptazine in heterojunction is controlled by changing the molar ratio of the precursors.When the molar ratio of melamine/thiourea is 2:1,the optimum triazine/heptazine heterojunction g-C3N4 has an excellent hydrogen evolution reaction(HER)rate.Through a series of photochemical characterization analysis,this unique heterojunction structure has a narrowed band gap position,broadened photo-absorption edge and the enhanced separation efficiency of electron-hole pairs,is benefit to improve the photocatalytic hydrogen evolution preformance.(2)Precursor-engineering coupled microwave molten-salt for the regulating effect of triazine/heptazine g-C3N4 heterojunction:The effects of microwave,molten salt and precursor regulation on the g-C3N4 heterojunction structure obtained by thiourea/dicyandiamide,urea/melamine or urea/dicyandiamide as precursors coupled microwave and molten-salt were further studied.It is found that microwave plays an important role in controlling the ratio of triazine/heptazine g-C3N4 heterojunction,and molten salt is a necessary condition for the formation of triazine g-C3N4.It was found that the heptazine g-C3N4 was synthesized by thiourea or urea as a single precursor,while triazine g-C3N4 was synthesized by melamine or dicyandiamide as a single precursor.Microwave plays an important role in controlling the ratio of triazine/heptazine g-C3N4 heterojunction,and molten-salt is a necessary condition for the formation of triazine g-C3N4.In the process of heterojunction formation,the triazine g-C3N4 is formed first,and in the molten salt environment,the triazine g-C3N4 is further formed,so the complete triazine/hepazine g-C3N4 heterojunction is formed.The hybrid precursor combined with microwave molten-salt method is an effective universal method to control the heterojunction of triazine/heptazine g-C3N4.(3)Potassium or hydrogen ions doping g-C3N4 for the different photocatalytic oxidaition/reduction behavior though the band gap regulation:K+doped g-C3N4 was successfully synthesized by molten-salt method,and then H+ doped g-C3N4 was synthesized by acid treatment of K+ doped g-C3N4.It was found that K+doped g-C3N4 has strong reducibility for photocatalytic HER,while H+ doped g-C3N4 has strong oxidizability for photodegradation performance of gaseous acetaldehyde under visible light and simulated sun light.The difference in photocatalytic oxidation/reduction performance of g-C3N4 is because the regulation of energy band structures of K+ and H+ doped g-C3N4.Specifically,K+ doped g-C3N4 has excellent visible light absorption ability and more negative conduction band,which is more beneficial to the photocatalytic reduction reaction.While H+doped g-C3N4 has the more positive valence band,and its valence band holes and electron-generated reactive oxygen radicals of ·O2-and ·OH have higher oxidation activity,which is more favorable for photocatalytic oxidation reaction.(4)Supramolecule modified g-C3N4 for the efficient charge separation:The incomplete polymerized precursor or intermediate in g-C3N4 was successfully synthesized into the supramolecule of melamine-cyanuric acid(MCA)by hydrothermal treatment of g-C3N4,and by introducing salicylic acid in the hydrothermal process further regulate the synthesized supramolecule.The supramolecular modified g-C3N4 shows excellent performance in photocatalytic hydrogen evolution.The supramolecular modified g-C3N4 has more negative conduction band,higher carrier separation and transport efficiency,and promotes more carriers to participate in the photocatalytic reaction process,which is favorable for the photocatalytic hydrogen evolution.