Preparation,characterization And Application Study Of G-C₃N₄ Based Nanocomposites

Graphite carbon nitride（g-C₃N₄）,as a new type of non-metallic semiconductor,has stable physical and chemical properties and visible light response,and has attacted much attention from the researchers recently.However,due to the low specific surface area and the high recombination rate in the bulk g-C₃N₄,its photocatalytic activity is limited.In view of the shortcomings of g-C₃N₄ itself,other semiconductors,such as SnO_2-x,rod-likeα-FeOOH andβ-FeOOH,were combined with g-C₃N₄ to try to construct the Z-scheme heterojunction photocatalysts.The photoelectric properties,microstructures and photocatalytic application of these synthesized heterojunction photocatalysts were tested,characterized and analyzed,respectively.The main findings of the research are summarized as follows:（1）The flower-like SnO₂/g-C₃N₄ composite photocalyst was prepared by combining the hydrothermal method and solid-phase method.To obtain an optimal catalytic efficiency,the mass ratio of the two precursors（SnO₂ and g-C₃N₄）in the composite was regulated through the degradation experiment of Rh B under visible light and the optimized sample was named 15%SnO₂/g-C₃N₄.In order to further improve the photocatalytic activity,the 15%SnO₂/g-C₃N₄ sample was annealed in nitrogen atmosphere at different temperatures to modulate the Sn²⁺defect content,and the flower-like SnO_2-x/g-C₃N₄ composites containing different content of Sn²⁺active sites were successfully prepared.The optical characterization shows that compared with g-C₃N₄,the absorption ability of SnO_2-x/g-C₃N₄ composite is obviously enhanced in visible light range.Under visible light irradiation,the Rh B degradation rate of 15%SnO_2-x/g-C₃N₄-2 Rh B prepared under optimized annealing condition is 5.1 and 1.8times than that of g-C₃N₄ precursor and 15%SnO₂/g-C₃N₄ composite,respectively.Moreover,without any co-catalyst（such as platinum and other precious metals）,the hydrogen production efficiency of 15%SnO_2-x/g-C₃N₄-2 composite photocatalyst is nearly 200μmol g^-1 h^-1,which is～13 times than that of g-C₃N₄ precursors.Therefore,the Sn²⁺defect-rich 15%SnO_2-x/g-C₃N₄-2 sample shows superior photoreduction ability.Through the detection and characterization of superoxide radical(·O^2-),and hydroxyl radical（·OH）,it is verified that the flower-like SnO_2-x/g-C₃N₄ composite satisfies the Z-scheme charge transfer mechanism in the photocatalytic reaction process.In addition,a series of theoretical calculations show that the charge redistribution and effective carrier separation/transport in Sn²⁺defect-rich SnO_2-x play a leading role in improving the photocatalytic performance of the composite photocatalysts.（2）Using melamine and ferric chloride as raw materials,the rod-likeβ-FeOOH/g-C₃N₄ composite photocatalyst was firstly synthesized through a hydrothermal method.Within this rod-likeβ-FeOOH/g-C₃N₄composite,it was found that the photocatalytic activity decreased with the increase of rod-likeβ-FeOOH amount.It is speculated that rod-likeβ-FeOOH and g-C₃N₄ form the traditional II heterojunction,that is,the holes in the valence band of rod-likeβ-FeOOH migrate to the valence band of g-C₃N₄.While the holes in the valence band of g-C₃N₄ have limited oxidation capacity,which is insufficient to produce main active substance·OH.Secondly,rod-likeα-FeOOH/g-C₃N₄ composite photocatalyst was prepared by combining the hydrothermal and solid phase method.The optical,electrical and morphological characterizations of rod-likeα-FeOOH/g-C₃N₄ samples were performed.It was found that compared with pure g-C₃N₄,the absorption range of rod-likeα-FeOOH/g-C₃N₄ composite photocatalyst was significantly expanded and enhanced.The electron spin resonance（ESR）test showed that the signal intensity of the active species(·O^2-and·OH)of the composite material significantly higher than that ofα-FeOOH and g-C₃N₄,which proved that the recombination rate of the carrier was significantly reduced.The constructed Z-scheme heterojunction was beneficial to improve the photocatalytic activity.The results showed that 15%rod-likeα-FeOOH/g-C₃N₄ exhibited the best photocatalytic activity under visible light irradiation,with a first-order rate constant k value of 0.246min^-1,which was 2.6 times and 17.6 times than that of g-C₃N₄ and rod-likeα-FeOOH,respectively.Without the addition of other co-catalysts,under visible light irradiation the photocatalytic hydrogen production rate of the 15%rod-likeα-FeOOH/g-C₃N₄ sample reached 250μmol g^-1 h^-1,which was～20times than that of pure g-C₃N₄.