Synthesis Of G-C₃N₄/TiO₂ Nanocomposites And Their Photocatalytic Performance

Graphitic carbon nitride?g-C₃N₄?,as a novel metal-free semiconductor material with visible light response,is widely used in the fields of photocatalytic water splitting for hydrogen evolution,water and gas waste treatment,organic pollutants degradation and so on.On the other hand,g-C₃N₄ has important research significance due to the advantages of high physicochemical stability,adjustable morphology,non-toxicity,as well as easy synthesis procedure via simple thermal polymerization using low-cost carbon and nitrogen-rich precursors.However,the photocatalytic activity of g-C₃N₄ strictly limited due to small specific surface area and serious recombination of photoinduced charge carries in bulk g-C₃N₄.To address above issues,we constructed an efficient g-C₃N₄/TiO₂ heterojunction to promote the separation and transfer of photogenerated electron-hole carriers.Also,we improved the morphological design of the heterojunction to increase its specific surface area and photocatalytic active sites.The photocatalytic activity of g-C₃N₄ is significantly enhanced for hydrogen evolution via these two methods,the detailed research contents and conclusions are listed as follows:?1?g-C₃N₄ nanosheets were prepared by thermal polymerization of melamine and ammonium chloride mixture.Then,two-dimensional/two-dimensional?2D/2D?g-C₃N₄/TiO₂ heterojunction was successfully fabricated via in-situ growth of TiO₂ nanosheets on the surface of g-C₃N₄nanosheets by solvothermal method.After annealing at 420?C,the ultrathin TiO₂ nanosheets are distributed uniformly and vertically on the surfaces of g-C₃N₄ nanosheet,forming a three-dimensional?3D?hierarchical structure.The 2D/2D g-C₃N₄/TiO₂ heterojunction possesses large specific surface area and close contact interface,providing sufficient high-speed carrier transfer channels to promote the separation of photogenerated carriers.Compared to pure g-C₃N₄nanosheets and pure TiO₂ nanoflowers,the 2D/2D g-C₃N₄/TiO₂ heterojunction shows significantly improved photocatalytic activity under simulated sunlight.Furthermore,g-C₃N₄/TiO₂heterojunction catalysts with different ratios were prepared by adjusting g-C₃N₄ content to investigate the effect of g-C₃N₄ content on photocatalytic activity.The results demonstrate that the optimal sample with 20 wt%g-C₃N₄?0.2-CT?exhibits the highest photocatalytic hydrogen evolution rate,as high as 26.22 mmol·h^-1·g^-1.?2?2D/2D g-C₃N₄/TiO₂ composites with 20 wt%g-C₃N₄ were prepared by the same procedures as described in previous part,but they were subjected to different annealing temperatures to investigate the effects of the annealing temperature on the morphological and photocatalytic properties of 2D/2D g-C₃N₄/TiO₂.The experimental results demonstrate that the annealing temperature affects the morphology,crystallinity and light absorption intensity of the composites,resulted in various photocatalytic hydrogen evolution efficiency.The sample subjected to 420?C annealing?CT420?exhibits the highest photocatalytic hydrogen evolution rate,as high as 26.22 mmol·h^-1·g^-1,about 7.3 times of that of g-C₃N₄ nanosheets under simulated sunlight irradiation,which is attributed to the synergistic effects of enhanced to enhanced crystallinity,specific surface area and light absorption of g-C₃N₄/TiO₂ composites via annealing.?3?The g-C₃N₄ nanoparticles?NPs?/TiO₂ nanoflower heterojunctions were successfully prepared by an immersion and one-step annealing method using melamine as g-C₃N₄ precursor and TiO₂ nanoflowers with larger specific surface area as the base material.g-C₃N₄ NPs are uniformly distributed in the interspaces of TiO₂ nanoflowers during thermal polymerization.The porous flower-like structure has large specific surface area,high photo-generated carrier separation and transfer efficiency,therefore,the photocatalytic performance is significantly enhanced.The photocatalytic water splitting hydrogen evolution rate reaches as high as 20.75 mmol·g^-1·h^-1 under simulated sunlight.To investigate the effects of different g-C₃N₄ precursors on the morphology and photocatalytic property of heterojunction catalysts,melamine was replaced with thiourea and urea during sample preparation following the identical procedure,respectively.Compared to melamine as g-C₃N₄ precursor,the nanoflower structure of TiO₂ was seriously destroyed when thiourea or urea is used as g-C₃N₄ precursor,leading to g-C₃N₄ nanosheet/TiO₂ nanoparticle heterojunctions with relatively low specific surface area,and high photogenerated electron-hole recombination rate,therefore,the photocatalytic performance is seriously degraded.To conclude,melamine is an excellent g-C₃N₄ precursor to successfully synthesize g-C₃N₄/TiO₂ nanoflower heterojunctions with superior photocatalytic performance,and it can be expected to extend its application in other heterojunction photocatalyst construction.