The Preparation And Photocatalytic Performance Of G-C₃N₄ Based Heterojunction

Using semiconductor catalytic materials to convert solar energy into chemical energy is considered to be an important way to solve environmental pollution and energy crisis.g-C₃N₄ has attracted extensive attention in photocatalysis due to its low cost,nontoxicity,visible light response and suitable band gap.However,the low specific surface area of pristine g-C₃N₄,the easy recombination of photogenerated carriers and the weak oxidation ability of holes limit its further development.The construction of heterojunctions have the advantage of（1）enhances the light absorption ability;（2）promote the separation of photogenerated carriers;（3）reducing the overpotential of the reaction;（4）enhancing the redox ability of photogenerated electron-hole pairs are considered as one of the main strategies to improve the efficiency of photocatalytic reactions.In this paper,two-dimensional g-C₃N₄ is used as the matrix to provide a larger specific surface area for the reaction,subsequently appropriate materials and methods are selected to successfully construct binary and ternary heterojunctions and explore their related photocatalytic activities.The main research contents are as follows:（1）Mo Ox precursors were obtained by hydrothermal method,then the uniformly dispersed Mo C quantum dots（Mo C QDs-C）on carbon sheets were obtained by treating precursors at high temperature,and finally Mo C QDs-C/g-C₃N₄ composite photocatalysts were prepared by ultrasonic self-assembly technology.The experimental results show that the 2D carbon layer acts as a“bridge”connecting g-C₃N₄ nanosheets and Mo C quantum dots,promoting the separation and migration of charges,thereby inhibiting the recombination of charge carriers.Mo C QDs as non-noble metal cocatalysts not only can expand the light absorption range but also accelerate surface reactions.The maximum hydrogen evolution rate of the optimized sample 40 wt%Mo C QDs-C/g-C₃N₄（MCCN）was 69.6 and 1.7 times higher than that of pure g-C₃N₄ and Pt/g-C₃N₄,respectively.（2）Z-type g-C₃N₄/Mo O₃ heterojunction was prepared by in-situ hydrothermal method.The mechanism was explored by adding a capture agent and EPR technology,it is found that the mechanism conforms to the typical Z-mechanism electron transfer path.The experimental results show that the construction of the heterojunction can not only effectively promote the separation and migration of photogenerated charges,but also improve its photogenerated hole ability.On the g-C₃N₄/Mo O₃-2 photocatalyst,H₂O₂ is generated by photo-oxidation of water by holes and photoreduction of oxygen by electrons,so that the best sample g-C₃N₄/Mo O₃-2 had the highest H₂O₂ yield of 874μM within 180 min and its AQE at 420 nm was measured to be 8.9%.（3）The ternary g-C₃N₄/Mo O₃/Cu₂O composite photocatalyst was prepared by in-situ growth of g-C₃N₄/Mo O₃ and then electrodeposition of Cu₂O.It is found that the ternary composite photocatalyst follows the double Z-type charge transfer mechanism.The close contact between the components and their charge transfer paths enable the photogenerated electrons and holes in the sample to maintain high redox capacity,promote the effective separation of charges,and thus improve the degradation performance of tetracycline.So the performance test found that the optimal sample 1.5g-C₃N₄/Mo O₃/Cu₂O-100 s can achieve 97.75%tetracycline degradation rate within 150min and its apparent rate constant（k）is 0.02048 min^-1,which is 3.4 and 5.8 times of that of 1.5 g-C₃N₄/Mo O₃ and g-C₃N₄,respectively.It also showed excellent cyclic stability.The ternary g-C₃N₄/Mo O₃/Cu₂O composite photocatalyst was prepared by in-situ growth of g-C₃N₄/Mo O₃ and then electrodeposition of Cu₂O.The charge separation efficiency and tetracycline degradation performance of g-C₃N₄/Mo O₃/Cu₂O were significantly improved because of the intense contact of the components and the double Z-mechanism electron transfer pathway.The optimal sample 1.5 g-C₃N₄/Mo O₃/Cu₂O-100 s can achieve 97.75%tetracycline degradation rate within 150 min and its apparent rate constant（k）is 0.02048 min^-1,which is 3.4 and 5.8 times of that of 1.5 g-C₃N₄/Mo O₃and g-C₃N₄,respectively.At the same time,the stability of the sample was tested,and it was found that after four cycles,the performance of the sample hardly decreased,indicating that it exhibited excellent cycle stability.It also showed excellent cyclic stability.