Morphology Control And Doping Modification Of Carbon Nitride And Research On Photocatalytic Performance For Hydrogen Evolution

The increasing fossil fuel depletion and environmental destruction have driven humans to find alternative energy sources to solve these problems.As a carbon-free energy carrier with great energy density,hydrogen(H2)is regarded as a suitable alternative to replace traditional fossil fuels.It has been seen as a promising method for producing H2 to use photocatalytic semiconductor water splitting technology to convert solar energy into hydrogen energy through the photosynthesis system.During the development of photocatalytic science and technology,researchers have developed many semiconductor photocatalysts for photocatalytic decomposition of water.Graphitic carbon nitride(g-C3N4)is widely used in the field of photocatalytic water splitting to produce hydrogen due to its unique electronic structure,suitable band gap,high chemical stability,and low production cost.However,its application in the photocatalytic is still hampered due to the bulk g-C3N4 is insufficient absorption of visible light,low specific surface area,easy recombination of photogenerated electron-hole pairs and low quantum efficiency.Therefore,it is essential for the g-C3N4-based photocatalyst to explore excellent strategies for boosting its photoactivity.To solve the above problems,in this paper,carbon nitride materials were modified from three aspects:morphology controlling,nonmetallic elemental dopping and heterojunctions structure construction,S,P doped g-C3N4 porous microtubes(SCNP)and a series of nonmetals(S,P,F and Br)doped g-C3N4 nanosheets(x-CN)decorated with boron doped g-C3N4 nanodots(BCNDs)were synthesized to form 0D/2D BCNDs/x-CN isotype heterojunction photocatalysts were prepared,and the effects of the composition,structure and surface properties of the catalysts on the photocatalytic performance of photocatalytic water splitting to hydrogen evolution were investigated.The main contents of the discussion and the conclusions are as follows:(1)By controlling the morphology of carbon nitride,we developed a new and simple method for preparing carbon nitride porous microtubes,and systematically studied its photocatalytic hydrogen evolution performance.Using melamine and trithiocyanuric acid as raw materials via hydrothermal pretreatment method form microrods supramolecular precursor(MT),MT and NH4H2PO4 after grinding calcined under N2 was prepared from microrods to S,P doped g-C3N4 porous microtubes(SCNP)composite photocatalyst.The study showed that NH4H2PO4 plays a co-catalytic role in catalyzing the formation of porous microtubules,resulting in the morphology change of g-C3N4.The catalyst was successfully doped with S,P nonmetallic elements,which shortened the band gap and extended the absorption range of visible light.The largest specif1c surface area of SCNP(50.43 m2·g-1)was 3.3 times higher than that of the bulk g-C3N4(CN)(15.13 m2·g-1),provided more active sites.The one-dimensional porous microtubes structure could promote the transport of carriers along the one-dimensional length,and thus promote effective separation of carriers.The results showed that the maximum hydrogen evolution rate of SCNP was 4200.29 μmol·g-1.h-1,which was 18.1,7.6 and 1.6 times of that of CN(232.15 μmol·g-1·h-1),Phosphorus doped g-C3N4(PCN)(556.13μmol·g-1·h-1)and Sulphurs doped g-C3N4 microrods(SCN)(2607.64 μmol·g-1·h-1),respectively.The maximum quantum efficiency of SCNP was 40.6%at 420 nm.The SCNP was obtained by NH4H2PO4 assisted modification and sintering have optimized morphology and band structure,enhanced the visible light absorption,reduced the energy barrier,and improved the carrier separation efficiency,thus improved the photocatalytic decomposition of water performance in hydrogen production.(2)Although a lot of work has been done on regulating the photocatalytic performance of carbon nitride by single or double doping.However,integrating two kind of carbon nitride which were doped with different elements to form isotype heterojunction has been rarely reported,especially zero-dimensional(0D)/two-dimensional(2D)g-C3N4 isotype heterojunction.To this end,we report a series of nonmetals(S,P,F and Br)doped g-C3N4 nanosheets(x-CN)decorated with boron doped g-C3N4 nanodots(BCNDs)have been synthesized,and the 0D/2D BCNDs/x-CN isotype heterojunctionhas been successfully constructed.Through the systematically characterization analysis showed that the doping of nonmetallic elements could shorten the band gap and improved the visible light capture ability.In the formation of heterojunction system,the effective separation of photogenerated electron-hole pairs was promoted by built-in electric field force.The result of photocatalytic performance showed that the maximum photocatalytic hydrogen production rate of the BCNDs/S-CN,BCNDs/P-CN,BCNDs/F-CN and BCNDs/Br-CN composite could reach 3924.81 pmol·g-1·h-1,2387.62 μmol·g-1·h-1,2374.06 μmol·g-1·h-1 and 3471.56 μmol·g-1·h-1 respectively,which was about 3.3,4.3,4.0 and 3.1 times that of their counterparts,respectively.In the above 0D/2D isotype heterojunction,the photocatalytic hydrogen production rate of BCNDs/S-CN was the highest,which could be attributed to the more negative conduction band edge of S-CN compared to other nonmetals doping,which had more thermodynamically advantages,which increased the corresponding hydrogen production activity.