Synthesis Of G-C₃N₄-based Composites Photocatalytic Materials For Hydrogen Evolution Performance

In recent years,while human beings have enjoyed the convenience of industrial development and social progress,the excessive exploitation of natural resources seriously jeopardized the strategy of sustainable development.Therefore,the exploration of sustainable and environmentally-friendly energy resources is an important measure to alleviate the pressure on energy resources.It is worth noting that photocatalytic hydrogen reaction under visible light plays a key role in making full use of abundant solar energy.Compared with other energy sources,hydrogen as a clean energy not only reflects environmental protection in the production stage,but also consists with the strategy of sustainable development in the process of utilization,so it has great potential in renewable energy production and environmental issues.Semiconductor photocatalysts have become a promising material for photocatalytic hydrogen evolution.Under visible light,electrons from the valence band of semiconductors can transfer to the conduction band,thereby there are more electrons can participate in the photocatalytic hydrogen evolution reaction.So far,most photocatalysts can not fully absorb and utilize the energy of visible photons,and the conduction band bottom and valence band top potentials of the catalysts are difficult to meet the potential of the photocatalytic reaction system at the same time.So seek the materials use to photocatalytic hydrogen evolution is imminent.Since 1989,carbon nitride（C₃N₄）has begun to enter the field of scientific research,and then the existence of g-C₃N₄ has been predicted theoretically.g-C₃N₄ is a two-dimensional layered structure similar to graphite,and its band characteristics is standing out,which the top potential of g-C₃N₄ satisfies the potential requirements for oxidizing water to produce oxygen,and the bottom of the conduction band can be used for reducing water to produce hydrogen.At the same time,g-C₃N₄ has good chemical stability.However,some inherent defects such as low visible light utilization,small specific surface area,and the rapidly recombination of photo-generated carriers limit its further development.To this end,melamine and cyanuric acid are used as precursors to synthesize the hollow sphere g-C₃N₄ structure with a large specific surface area,utilizing the supermolecular self-assembly effect between melamine and cyanuric acid.Various modification methods are used improve the specific surface area,visible light absorption and the activity of photocatalysts,like the deposition of precious metal,semiconductor compound.Additionally,we analyzed the materials from morphology（SEM,TEM）,optical properties（FT-IR,UV-vis）,phase state（XRD）,electron valence state（XPS）,pore structure（BET）,and electron kinetic（TAS）and other aspects of material characterization.The main work is as follows:（1）the hollow sphere g-C₃N₄ with high specific surface area modified by highly dispersed Pd nanoparticles for photocatalytic hydrogen evolutionThe supramolecular precursor CM with a compact sheet sphere shape was formed,utilizing the supramolecular self-assembly of melamine and cyanuric acid in dimethyl sulfoxide（DMSO）solvent.The 3D hollow spherical carbon nitride（SCN）with large specific surface area were synthesize by one-step pyrolysis method in a nitrogen atmosphere.The prepared hollow sphere structure can be used as a base material for highly dispersed Pd nanoparticles,it can enhance the absorption of visible light by the photocatalytic material.Additionally,it can expose more active sites for the photocatalytic hydrogen reaction.Pd nanoparticles are implanted into the SCN as electron acceptors,thus,more photo-generated electrons can participate in the reduction reaction on the surface.In order to more intuitively show the separation and transfer of photogenerated charge carriers,we use transient absorption spectroscopy to estimate the charge separation lifetime of Pd/SCN（10 h）.It is estimated to be 1.4 ns±338.0 ps,which is only half of SCN.The decreased charge separation lifetime indicates that photogenerated electrons on the conduction band of g-C₃N₄ can transfer the surface of Pd nanoparticles.Thanks to its unique structure and excellent optical properties,Pd/SCN composites show significantly enhanced performance of photocatalytic hydrogen evolution under visible light irradiation.In particular,the photocatalytic hydrogen evolution rate of Pd/SCN（10 h）reached 267.9μmol/h,which is almost 10 times that of Pd/2D g-C₃N₄.At the same time,based on the characterization results,a possible mechanism for the photocatalytic hydrogen reaction was proposed.（2）A one-step calcination method for preparing CoO QDs/g-C3N4 heterojunction for photocatalytic hydrogen evolutionA one-step calcination method was used to prepare the hollow sphere g-C₃N₄（SCNO）/CoO heterojunction,improving the performance of photocatalytic hydrogen evolution.Supramolecular precursors were synthesized by the self-assembly of melamine and cyanuric acid.The hollow spherical carbon nitride structures（SCNN,SCNO）with large specific surface area are synthesized under different calcination atmospheres（N₂,air）,which provide sufficient conditions for anchoring CoO QDs.Meanwhile,the formation of SCNO/CoO composites not only improves the absorption of visible light,but also accelerates the separation and transfer rate of photogenerated electron-hole pairs.Using the photocatalytic hydrogen reaction as a probe,it was determined that SCNO/CoO（0.2）has the highest photocatalytic hydrogen evolution activity reaching 11495μmol/h/g.In addition,transient absorption spectrum was used to study the transfer path of photogenerated charge carriers.The results exhibit that the charge separation lifetime of SCNO/CoO（0.2）is 29.61±0.43 ps,including the transfer from the conduction band of CoO QDs to SCNO.The lifetime of the photogenerated electrons in the conduction band proves that the heterojunction formed between SCNO/CoO can significantly enhance the photocatalytic activity of the samples.Based on the characterization results of XRD,XPS,UV-vis,EIS,BET.A possible photocatalytic hydrogen reaction mechanism was proposed.