Development Of Graphitic Carbon Nitride Based Photocatalysts For Environmental Purification Applications

The reactive oxygen species（ROS）generated by semiconductor photocatalystunder light illumination can serve as powerful oxidants to degrade organic pollutants and inactivate pathogenic microorganisms by damaging essential macromolecules.Photocatalytic reaction is driven by free and inexhaustible sunlight,and it obviates the need to continuously supply precursor chemicals.Thus,photocatalytic water treatment technology has been considered as an ideal strategy for solving the water contamination problems.Unfortunately,the real-world photocatalytic water treatment applications still face several challenging problems:（1）The photocatalytic activities of the currently-reported photocatalysts are still far from meeting the requirements of practical applications;（2）the emerging photocatalysts are becoming more complex and more difficult to synthesize,thus restricting their further applications severely;（3）various photocatalysts are solely metal-based or contain metal-containing cocatalysts,suffering from high cost and hazardous environmental impact.In this paper,novel strategies are developed for the synthesis of simple,low-cost yet highly efficient graphitic carbon nitride（g-C₃N₄）photocatalysts,and the catalysts prepared are successfully used for the degradation of organic pollutants and inactivation of pathogenic microorganisms.The main contents and conclusions are as follows.（1）A black phosphorus（BP）-g-C₃N₄heterostructure with BP acting as the cocatalyst is fabricated as an extremely active photocatalyst via a newly-developed exfoliation strategy.The results show that BP can serve as a potential high-performance cocatalyst to replace noble metals for boosting the photocatalytic activity of g-C₃N₄,that the as-prepared BP-g-C₃N₄shows an 11.1 times better decomposition rate of indomethacin（IDM）compared to the widely-studied P25 Ti O₂photocatalyst under natural sunlight illumination.The enhanced photocatalytic performance of BP-g-C₃N₄can be explained by the establishment of type I heterostructure between BP and g-C₃N₄,which can suppress the recombination of the photo-generated charge carriers,favoring the production of more abundant ROS in the photocatalytic system.The BP-g-C₃N₄also shows excellent photocatalytic performance in authentic water matrices,in scale-up application or at low IDM concentration（50μg/L）,indicating its immense potential for practical applications.（2）It is found that tuning the reaction parameter of the thermal polymerization process is an efficient strategy to enhance the photocatalytic activity of g-C₃N₄.The best g-C₃N₄synthesized under the optimal reaction condition exhibits an extraordinary photocatalytic activity,that in the absence of any additional cocatalyst,its visible-light catalytic activity is even comparable or better than that of the high-performance g-C₃N₄-based nanocomposites previously reported,with a much lower catalyst consumption.Surprisingly,the best g-C₃N₄can completely decompose rhodamine B within only 140 s of visible-light irradiation（catalyst consumption:1mg/m L）.Combining the characterization and ROS measurement results,it is concluded that the enhanced photocatalytic activity of g-C₃N₄is due to the stronger oxidizability of its photo-induced holes.Furthermore,only 5 min of ultrasonic pretreatment is required to achieve the excellent activity for the best g-C₃N₄photocatalyst,indicating its great potential for practical water treatment applications.（3）It is found that optimizing the reaction condition of the thermal polymerization process can improve the H₂O₂generation ability and disinfection activity of the g-C₃N₄samples by tuning their polymerization degree.The best g-C₃N₄prepared under the optimal reaction condition shows a 7.2 log inactivation of Escherichia coli cells within 180 min of visible-light illumination,which is equivalent to a disinfection efficiency of>99.99999%.This activity is even comparable or better than that of the g-C₃N₄-based nanocomposites previously reported.Combining the characterization,ROS measurement,and density functional theory calculation results,the disinfection activity enhancement mechanism of the best g-C₃N₄is determined:optimizing the reaction parameter of the thermal polymerization process can enhance the polymerization degree and O₂adsorption capacity of the g-C₃N₄,which is favorable for the two-electron direct reduction of O₂in the photocatalytic system to generate more abundant H₂O₂,thus improving the disinfection performance of g-C₃N₄.