Synthesis Of Carbon Nitride Based Photocatalysts And Study On Their Photocatalytic Properties For Hydrogen Evolution

Hydrogen（H₂）energy,a substitute for traditional fossil energy,is of great significance for solving the problems of energy shortage and environmental pollution worldwide.Photocatalytic water splitting for hydrogen production is recognized as one of the effective green methods for producing H₂.Graphitic carbon nitride（g-C₃N₄ ）has the advantages of low cost and excellent visible light response,which makes it a promising catalyst in photocatalytic hydrogen evolution reaction.However,due to its low specific surface area and high resistivity,the bulk g-C₃N₄ （B-C₃N₄ ）leads to high photogenerated charge recombination rates,which severely reduces the performance of g-C₃N₄ for photocatalytic hydrogen evolution reaction.In order to enhance photocatalytic hydrogen precipitation activity by improving photogenerated charge separation efficiency in g-C₃N₄ systems,the structure design of g-C₃N₄ was carried out,and the influence of the structure relationship of platinum（Pt）-like cocatalyst in g-C₃N₄ -based photocatalyst on hydrogen evolution performance was systematically studied.The main research content of this thesis is as follows:1)To strip B-C₃N₄ more easily and efficiently,porous carbon nitride nanosheets（NS-C₃N₄ ）were prepared bottom-up by a copolymer intercalation method,which makes up for the shortcomings of traditional stripping B-C₃N₄ .Compared with B-C₃N₄ ,NS-C₃N₄ has larger specific surface area(61 m²·g^-1)and abundant porous structure.The hydrogen evolution rate reached 4061.8μmol·h^-1·g^-1after loading Pt cocatalyst,which was 37.5 times that of B-C₃N₄ .Study on the morphology and energy band structure of NS-C₃N₄ found that it can play a better role in photocatalytic hydrogen evolution reaction because of its various properties,such as the large specific surface area and porous structure can provide more reactive sites and reduce the photogenic carrier recombination rate,and the narrower band gap（2.12 e V）broadening its visible light absorption range.2)In-situ composite photocatalyst（In-situ WS₂/g-C₃N₄ ）was obtained by in-situ vulcanization reaction,in which tungsten sulfide nanosheets（WS₂NS）were anchored on the surface of g-C₃N₄ nanosheets（g-C₃N₄ NS）instead of Pt as cocatalyst.The effects of different loading modes of WS₂NS on hydrogen evolution performance of composite photocatalyst were also studied systematically.The photocatalytic hydrogen evolution efficiency of In-situ WS₂/g-C₃N₄ exhibited an initial increase followed by a subsequent decrease with increasing cocatalyst load.And In-situ WS₂/g-C₃N₄ has an optimal photocatalytic hydrogen evolution efficiency(219.9μmol·h^-1·g^-1)at an optimal ratio of 5 wt.%.It is 5.8 times that of WS₂/g-C₃N₄ .WS₂NS,the cocatalyst,acts as an electron trap capturing electrons and participating in photocatalytic hydrogen evolution.Besides,in-situ vulcanization made WS₂NS and g-C₃N₄ NS bind more tightly,which,compared with the physically supported ones,enhanced the interaction between heterojunction,reduced the recombination rate of photogenerated carriers.In addition,the charge transfer process was calculated with density functional theory（DFT）after WS₂NS and g-C₃N₄ NS formed close contact.3)In order to further investigate the effect of morphology and structure of non-noble metal cocatalyst on hydrogen evolution efficiency of composite photocatalyst.Molybdenum carbide（Mo₂C）with two-dimensional（2D）structure was synthesized by precursor intercalation method in this thesis,and the difficult problem of preparation of 2D Mo₂C was successfully solved.2D/2D Mo₂C/g-C₃N₄ heterojunction photocatalysts were prepared by ultrasound-assisted self-assembly.The results show that the photocatalytic hydrogen evolution rate of 7 wt.%2D/2D Mo₂C/g-C₃N₄ is 675.3μmol·h^-1·g^-1,which is 5.1 times and 3.5 times of 7 wt.%B/2D Mo₂C/g-C₃N₄ and 0.5 wt.%Pt/g-C₃N₄ ,respectively.This is because the 2D heterojunction formed by Mo₂C and g-C₃N₄ can not only increase the contact area between the main catalyst and the cocatalyst,but also enable the electrons between the heterojunction to migrate rapidly and be captured by Mo₂C,thus inhibiting the recombination of photogenerated electrons and holes.This fully demonstrates that the performance of cocatalyst can be enhanced by designing and synthesizing it according to the structural properties of the main catalyst,so as to maximize the photocatalytic efficiency of the composite photocatalyst.Finally,the theoretical models of 2D/2D Mo₂C/g-C₃N₄ and B/2D Mo₂C/g-C₃N₄ were established by DFT,and the charge transfer processes of both were calculated,which strengthened the theoretical basis of 2D heterojunction.