Preparation Of G-C₃N₄-based Heterojunction Materials For Hydrogen Evolution Performance

In recent years,the progress of science and technology and the development of society have greatly promoted our demand for energy.Human activities mostly rely on coal,oil,fossil fuels and other non-renewable energy,in the process of consumption has many side effects,such as environmental pollution,global warming and so on,seriously violates the strategic needs of sustainable development of modern society,so the exploration of renewable,clean,cheap new energy has become imminent.Compared with other energy sources,hydrogen energy is produced and utilized in a clean and pollution-free process,which is consistent with the strategy of sustainable development and carbon neutrality.Therefore,hydrogen energy shows great potential among many alternative energy sources.Therefore,the exploration of efficient hydrogen production method has become a research hotspot.It is worth noting that the use of semiconductor photocatalytic decomposition of water is a simple,economical and green way to produce hydrogen.Converting solar energy into chemical energy,achieving the goal of low carbon economy and sustainable development,is the highest potential to transform energy supply.Semiconductor photocatalyst is a common material for photocatalytic hydrogen evolution.So far,semiconductor-based photocatalysts have been limited by their inadequate photocarrier separation,narrow light absorption range and limited REDOX potential.Most photocatalysts still face the challenge of lacking cheap,non-toxic and good visible light response photocatalysts.Therefore,active exploration and development of stable and efficient photocatalysts is the key and challenge of effective conversion and storage of sustainable energy.Since graphite phase carbon nitride（g-C₃N₄）and its corresponding synthetic precursors were reported,scientists have been studying the synthesis of g-C₃N₄（CN）by heat treatment of nitrogen-rich precursors such as urea,melamine and melamine.g-C₃N₄ has stable chemical properties,insoluble in acid,base or organic solvents,and suitable band structure.It is a promising metal-free semiconductor photocatalyst,which will provide great changes for photocatalytic applications.However,g-C₃N₄ has many defects,such as poor photogenerated carrier transfer efficiency and limited specific surface area.In view of the defects of g-C₃N₄,many researches are devoted to the modification of g-C₃N₄ to solve the existing problems.Therefore,in this paper,visible light absorption,active site,specific surface area and hydrogen evolution performance of g-C₃N₄ were improved by modifying morphology,heterojunction recombination and loaded quantum dots.Relevant research contents are as follows:（1）Preparation and hydrogen evolution performance of 3D/2D Zn O/g-C₃N₄Z-type heterojunction photocatalysts modified by Co P QDsIn this study,stable and efficient 3D/2D/0D Zn O/g-C₃N₄/Co P QDs（ZCNCP）multiple heterojunction photocatalysts were successfully prepared.The Zn O/g-C₃N₄（ZCN）Z-type heterojunction was constructed by precipitation polymerization of melamine on the surface of supramolecular precursor Zn O（Z）with large specific surface area flower structure.The structure can not only broaden the absorption range of visible light,but also improve the separation and mobility of photogenerated electron-hole.Co P QDs are loaded on g-C₃N₄ as electron acceptors,which can capture more photogenerated electrons.Well-dispersed Co P QDs provide more reaction sites,thus accelerating the utilization of photogenerated electrons involved in hydrogen evolution reaction.At the same time,transient absorption spectroscopy（TAS）is used to estimate that the charge separation life of ZCNCP is nearly half shorter than that of Zn O.The shorter charge separation life indicates that the transfer rate of electrons from g-C₃N₄ to Co P QDs is faster,and more photogenerated electrons participate in H⁺reduction reaction,which significantly improves the performance of photocatalytic hydrogen production.Under the experimental conditions,ZCNCP3 has the best reaction performance,which is 5638.44μmol·h^-1·g^-1,100 times that of pure CN.Moreover,the catalytic activity of ZCNCP3 is well retained after cyclic stability test.（2）Directed growth of COFs films on spongy g-C3N4 and photocatalytic hydrogen evolution propertiesThe spongiform g-C₃N₄was obtained by calcination of supramolecular self-assembly products of melamine and cyanic acid.The covalent organic framework（COFs）films were grown on the surface of g-C₃N₄by closed solvothermal method to form g-C₃N₄/COFs（CN/COFs）heterojunction photocatalyst.The heterojunction structure can broaden the light absorption range,improve the separation efficiency of photogenerated carriers,inhibit the recombination of photogenerated electron-hole pairs,and provide a channel for carrier transport.X-ray photoelectron spectroscopy showed that COFs formed a chemical bond with g-C₃N₄ surface,which made the two closely connected,thus promoting more photoelectron transfer,thus significantly improving the photocatalytic hydrogen evolution performance.Under the experimental conditions,CN/COFs-3 has the best reaction performance,and its performance reaches7788.10μmol·h^-1·g^-1,which is about 7 times of that of spongy g-C₃N₄.After 4 cycles,the hydrogen production performance of CN/COFs-3 has no significant decrease,indicating that CN/COFs-3 has high stability and sustainability.In order to demonstrate the separation and mobility of photocarriers and the photocatalytic mechanism of the samples,DFT calculation was used for simulation.According to the DFT results combined with other test results,the band structure of the two substances matches to form a heterojunction,and it can be concluded that electrons tend to switch from g-C₃N₄ to such COFs.The results showed that COFs with boric acid ring had good electron transfer to g-C₃N₄,which provided a new strategy for developing high performance COFs-derived photocatalysts.