| The abuse of endocrine disrupters and sulfonamide antibiotics such as Bisphenol A（BPA）,Tetrabromobisphenol A（TBBPA）,and Sulfamethazine（SMZ）has led to the continuous release of these organics into the environment at low concentrations,which has potentially adverse effects on animal and human health;for example,they cause reproductive organ damage and metabolic disorders in humans.Therefore,there is a great need for an environmentally friendly and low energy consuming novel technology to treat wastewater containing these substances;for example,they cause reproductive organ damage and metabolic disorders in humans.Thus,it is urgently needed for a new environmentally friendly and low energy consumption technology to treat wastewater containing these substances.Photocatalytic technology,as a green and low-carbon oxidation technology,can completely and efficiently degrade pollutants with no secondary pollutants.Therefore,photocatalysis technology has been gradually applied to organic wastewater treatment research.Ti O2,as a traditional semiconductor,has been widely studied in the past decade because of its strong redox ability,better anti-photocorrosion and chemical corrosion properties.However,Ti O2 has a wide band and can only be excited by UV light,which only accounts for a small portion（～5%）of sunlight,so we need to develop a material that can be excited by visible light to degrade organic pollutants.Triazine-based materials（e.g.,covalent triazine frameworks,polymeric carbon nitrogen compounds,and polyimides）are considered to be the most promising materials for photocatalytic hydrogen production and pollutant degradation because of their remarkable electronic properties,chemical stability,and cost-effectiveness properties.However,the photocatalytic activity of these triazine-based materials has the disadvantages of insufficient light absorption and short lifetime of photogenerated carriers.A large number of literatures have proved that the photocatalytic activity of triazine-based materials has been significantly improved by copolymerization,doping,and construction of heterojunctions.In this study,four categories of triazine-based photocatalysts were developed:F-Ti O2/g-C3N4 composites,triazine-based covalent organic framework materials（two prepared and named Tr COF1 and Tr COF2,respectively）,g-C3N4/Tr COF1 composites and PDI/Tr COF1 composites,and the performance of the composites was evaluated by degrading typical target pollutants（TBBPA,SMZ,and BPA）under visible light.The results of this study show that all four types of composites we prepared can degrade typical organic pollutants in water bodies with high efficiency.In order to explore the degradation performance of composites for pollutants theoretically,the paper combines DFT calculations with degradation intermediate monitoring,and radical trapping experiments to elucidate the degradation mechanism of pollutants.It is found that:the Z-type heterojunction structure formed between g-C3N4 and F-Ti O2 can effectively suppress the recombination of photogenerated electron-hole pairs and can increase the absorption light wavelength range of Ti O2.Two kinds of triazine-based composites,g-C3N4/Tr COF1and PDI/Tr COF1,formed Z-scheme and type-I heterojunctions,respectively,which significantly decreased the recombination rate of photogenerated electron-hole pairs and thus enhanced the photocatalytic activity of single triazine-based materials.The main findings are as follows:1.F-Ti O2/g-C3N4 photocatalysts with highly efficient visible light response were successfully prepared by a facile method using hydrofluoric acid,tetrabutyl titanate and melamine as precursors.Because the≡Ti-F and≡Ti:F-H on the surface of Ti O2 can act as electron capture sites and the heterojunction structure formed between g-C3N4 and Ti O2can suppress the recombination of photogenerated electron-hole pairs,control experiments showed that F-Ti O2/g-C3N4 exhibited much higher catalytic activity and visible light utilization efficiency than g-C3N4,F-Ti O2,and Ti O2/g-C3N4,and its degradation rate reached 95%in 120 min for TBBPA,98%in 60 min for SMZ under visible light irradiation.Meanwhile,F-Ti O2/g-C3N4 shows better photochemical stability and also holds higher degradation efficiency after degrading SMZ for 5 cycles.The results of the detection of degradation intermediates together with the radical trapping experiments indicate that the reactive species（·O2-,·OH,and h+）generated during photocatalysis play a crucial role.The enhanced conductivity of the excited electrons after fluorination of Ti O2 was demonstrated by DFT calculations.This work will provide new ideas to design and develop highly active photocatalysts for environmental applications.2.Two covalent organic frameworks（COFs）photocatalysts,Tr COF1 and Tr COF2,have been synthesized in a"one-step"process at room temperature,in which Tr COF1exhibits low quantum efficiency and weak photocatalytic activity under visible light irradiation,while Tr COF2 has a uniform spherical structure with ultrahigh specific surface area,which exhibits relatively good photocatalytic activity under visible light irradiation.Because Tr COF2 has a uniform spherical structure and abundant triazine-based structure,it exhibits excellent performance and photochemical stability for photocatalytic degradation of BPA under visible light irradiation,with 98%of BPA being degraded within 60 min and 90%of BPA being degraded after 5 cycles.Radical trapping experiments showed that the active species·O2-played a dominant role in the photocatalytic process,which was combined with DFT calculations to propose the catalytic mechanism of BPA degradation by Tr COF2 catalyst.The Tr COF2 material prepared in this study can serve as an efficient visible light catalyst and has great potential for the degradation and removal of pollutants from the environment.3.In view of the problem of low photocatalyst efficiency associated with the previously prepared Tr COF1 photocatalyst,we further attempted to composite it with g-C3N4 to enhance its photocatalytic performance.By heating and stirring,g-C3N4/Tr COF1composite was obtained and then heated to 400°C to improve its crystallinity.The results showed that g-C3N4/Tr COF1 exhibited high photocatalytic activity for organic pollutant BPA under visible light irradiation and could degrade BPA by 75%under 120 min light irradiation.At the same time,g-C3N4/Tr COF1 also had good photochemical stability and could degrade 68%BPA after 5 cycles.The free radical capture experiment showed that·O2-,·OH,and h+all participated in the oxidation reaction during the degradation of BPA by g-C3N4/Tr COF1 composite,and they played a synergistic role in the degradation of pollutants.In this study,two kinds of semiconductor materials were selected to construct heterostructures,which effectively enhanced the photocatalytic performance compared with a single semiconductor material.This may be because the formation of direct Z-scheme heterostructures effectively reduced the recombination rate of photogenerated electrons and holes.4.Perylenedimide（PDI）semiconductor is sensitive to visible light and is a potential photocatalyst.In this study,in order to improve the performance of the Tr COF1photocatalyst prepared previously,PDI and Tr COF1 were compounded to prepare PDI/Tr COF1 composite by heating and stirring method for the degradation of BPA.The research shows that under visible light irradiation,PDI/Tr COF1 has high photocatalytic activity on organic pollutant BPA and can degrade 70%of BPA under 120 min light irradiation.At the same time,PDI/Tr COF1 also has good photochemical stability and can degrade 60%BPA after 5 cycles.The free radical capture experiment showed that h+and·O2-played a synergistic role in the degradation of pollutants,and they participated in the oxidation reaction of BPA during the degradation of BPA by PDI/Tr COF1 composite.The PDI/Tr COF1 composite prepared in this study belongs to type-I heterojunction,which effectively reduces the recombination rate of photogenerated electrons and holes and enhances the photocatalytic performance of single semiconductor material. |
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