| The alarm bell that non-renewable energy is about to be exhausted has already sounded,mankind has come a long way in exploring new energy sources.Hydrogen energy has high energy density and no pollution.It is considered one of the the ideal clean energy sources in the future.Inspired by chloroplastida photosynthesis,solar photocatalytic water splitting for hydrogen production has been extensively studied by scientists with the help of photocatalysts.In recent years,a variety of semiconductor photocatalysts have been developed through the tireless efforts of scientists.Graphitic carbon nitride(g-C3N4)is favored by many scientists because of its suitable band gap,simple preparation method,non-toxicity and good chemical stability.But the application of pure g-C3N4 in the field of photocatalytic water splitting for hydrogen evolution is severely limited due to its small specific surface area,limited visible light absorption range and severe photogenerated carrier recombination.Therefore,it is necessary to develop effective measures to solve the deficiencies of pure g-C3N4 and improve the performance of photocatalytic decomposition of water for hydrogen precipitation.In order to solve the above problems,this thesis mainly focuses on increasing the specific surface area and light absorption range,enhancing the separation and transfer efficiency of photogenerated carriers,etc.A new method of constructing heterojunction photocatalyst by heat treatment was proposed,boron doped carbon nitride nanodots(BCNDs)and carbon nitride nanosheets(CNS)heterojunction composites(BCNDs/CNS)were successfully constructed;a new method of in-situ synthesis of Ag2S nanoparticles was developed,and the Ag2S/CNS composite was successfully constructed by loading the Ag2S nanoparticles uniformly on the carbon nitride nanosheets.The effects of BCNDs/CNS and Ag2S/CNS composites on the hydrogen evolution properties of photocatalytic water splitting were investigated by characterizing the morphology,specific surface area and carrier transfer path of the composites.The main contents and conclusions of the study are as follows:(1)The construction of heterojunction catalysts by hydrothermal method has been widely reported,but we noticed that the steps of preparing heterojunction catalysts by hydrothermal method are tedious,the separation of products is difficult,and the enhancement of photocatalytic hydrogen precipitation activity is limited.Therefore,BCNDs/CNS heterojunction photocatalyst was successfully constructed by a new heat treatment method of heating a mixture of BCNDs and CNS under argon atmosphere in this paper.The systematic characterization and analysis showed that the BCNDs/CNS heterojunction can effectively promote the separation of photogenerated electron-hole.Heat treatment resulted in the elevated carbon content of CNS and enhanced light absorption performance.The hydrogen evolution performance of photocatalytic decomposition of water shows that the photocatalytic hydrogen production rate of 3BCNDs/CNS-450 reached 6260.5 μmol·g-1·h-1,which was 15.7 and 7.2 times higher than that of bulk C3N4 and CNS.The improved photocatalytic activity was mainly attributed to the heterojunction constructed between BCNDs and CNS which effectively enhanced the photogenerated electron-hole transfer and separation.The hydrogen evolution rate of BCNDs/CNS-H heterojunction photocatalysts prepared by hydrothermal method was 2019.5μmol·g-1·h-1.The photocatalytic hydrogen evolution rate of BCNDs/CNS-450 was 3.1 times higher than that of BCNDs/CNS-H.This is mainly due to the synergistic effect between the heterojunction of 3BCNDs/CNS-450 and the enhancement of light absorption due to rich carbon content.(2)Precious metals such as Pt are often used as co-catalysts in photocatalytic decomposition of water to hydrogen evolution,but due to the scarcity and high price of precious metal reserves,various metal sulfides have been developed for use as co-catalysts in photocatalytic decomposition of water.In this paper,a novel Ag2S synthesis strategy is developed.Ag2S/CNS composites were synthesized by in situ loading Ag2S nanoparticles into CNS in sulfur vapor atmosphere.The systematic study and characterization results show that Ag2S/CNS composites have better photoabsorption properties and transfer efficiency of photogenerated carriers than CNS.The results of photocatalytic decomposition of water to hydrogen showed that the highest hydrogen production rate of Ag2S/CNS reached 865.5 μmol g-1·h-1.This is mainly due to the uniform attachment of Ag2S nanoparticles to the surface of CNS,which increases the active sites of photocatalytic reaction.As the active sites of electron storage and hydrogen evolution,Ag2S nanoparticles effectively inhibit the recombination of photogenerated electron hole pairs.The activity of the in-situ Ag2S co-catalyst is almost the same as that of the same amount of Pt co-catalyst.The problems of scarcity and high price of precious metal co-catalysts were avoided successfully. |
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