Study On Structure Control Of Carbon Nitride Based Photocatalytic Materials For Efficient Hydrogen Evolution

The increase of energy shortage and environmental pollution seriously has restricted the sustainable development of mankind.Photocatalytic hydrogen evolution technology is considered to be one of the effective ways to alleviate the energy crisis by converting solar energy into chemical energy.Graphite carbon nitride（g-C₃N₄）has been considered as one of the most promising photocatalytic hydrogen evolution materials because of its unique electron band structure and chemical stability.However,due to the small specific surface area,easy recombination of photogenerated carriers and low visible light utilization rate,it results in low photocatalytic efficiency.To solve these problems,this paper optimized its electronic,energy band structure,and light absorption capacity by forming heterostructures,doping hetero-elements,constructing defects,regulating the morphology,which achieves a high efficiency hydrogen evolution of g-C₃N₄-based photocatalytic materials.The main research contents of this paper are listed as follows:（1）A simple one-pot method was used to prepare sulfur-doped graphitic carbon nitride（SCN）nanosheets with anchoring MoS₂ultra-thin nanosheets（MS/SCN-x%）.The ultrathin MoS₂cocatalyst was evenly anchored on the surface of SCN through a covalent chemical bond.Benefited from its unique porous array covalent cross-linking structure,which exposed abundant active sites and enhanced the visible light absorption performance,the MS/SCN-2.5%composites exhibits excellent hydrogen evolution performance(702.53μmol·h^-1g^-1)which is superior to original MoS₂nanosheet modified by electrostatic self-assembly method(12.5μmol·h^-1g^-1),and also rivals with Pt/SCN(292.75μmol·h^-1·g^-1).（2）The ultra-thin two dimensional/two dimensional（2D/2D）g-C₃N₄/In₂Se₃heterojunction photocatalyst was prepared by the in-situ liquid phase growth method.Attributed to the heterojunction structure between the ultra-thin g-C₃N₄nanosheets and In₂Se₃interface,and the spontaneous polarization characteristic of In₂Se₃,the heterojunction g-C₃N₄/In₂Se₃composite can provide a larger number of high-speed photogenerated charge transfer channels,and the vertical intrinsic electric field of In₂Se₃would considerably inhibit the recombination of photogenerated charge.The visible light response range of the heterojunction g-C₃N₄/In₂Se₃composite was broadened from 441 nm to 550 nm by the introduction of ultra-thin In₂Se₃.The results show that the heterojunction g-C₃N₄/In₂Se₃hybrids exhibits a hydrogen evolution rate of 4.8 mmol·g^-1·h^-1,which was 5.1 times that of pure g-C₃N₄.（3）B-doped hexagonal graphite carbon nitride（BCNT）with porous structure was prepared by supramolecular self-assembly and templating method assisted by H₃BO₃.BCNT possesses one-dimensional structure and ultra-thin tube wall thickness,which is conducive to the directional transmission of photogenerated carriers and effectively inhibits the recombination of photogenerated carriers.Attributed to the doping of element B and the introduction of a new functional group cyano,the band gap value of the BCNT samples gradually decreases,and the conduction band position gradually moves upward,which broadens the visible light response range and improves the photocatalytic reduction ability.The BCNT-2 sample exhibits excellent hydrogen production performance under visible light,and the hydrogen production can reach1064.25μmol·g^-1·h^-1,which is 1.54 times that of the ultra-thin nanosheet sample（CNN）.（4）A coral-like graphitic carbon nitride（V₀CN_X-Y）with adjustable N vacancy concentration and controllable morphology was prepared by precursor reforming and alkaline earth metal Mg assisted method.Additionally,a new functional group cyano group was introduced.The introduction of N vacancy and cyano group breaks the original electronic balance of g-C₃N₄,which enhances the delocalization degree of electrons and the absorption ability of visible light,and reduces the recombination probability of photogenerated carriers.The secondary structure of coral-like structure-one-dimensional nanorods shortens the transmission path of photogenerated carriers and increases the probability of photogenerated charges reaching the surface of the material to participate in the photocatalytic reaction.The photoelectric performance test shows that the V₀CN_X-0.05 sample has the strongest photocurrent response and the smallest photogenerated electron transport resistance,which hydrogen evolution performance under visible light is 3.16 times that of bulk g-C₃N₄.