TiO₂-based Nanoheterostructure Construction And Its Performance In Photocatalytic Degradation Of Organic Pesticides

In recent decades,many countries around the world have been using organic pesticides in large quantities to increase food production.Triazine herbicides are the earliest and most heavily used class of organic pesticides and are gaining attention because they are highly stable in the environment and do not degrade easily by nature.Studies have shown that triazine herbicides have environmental endocrine disruptor properties and are carcinogenic,mutagenic,bioaccumulative and highly toxic to humans and animals.Current research shows that residues of triazine herbicides in soil and water bodies are already high and its high leachability can make it easier to flow into groundwater and surface water,leading to more serious contamination.The treatment of triazine herbicides has become an urgent environmental problem to be solved.Photocatalysis,which makes effective use of solar energy,is expected to be a key technology in solving the problem of organic pollution in such water environments.However poor charge separation,short charge lifetime and low visible light utilisation have been serious constraints on the development of photocatalysts.The construction of nano-heterojunction photocatalyst systems that can effectively promote photogenerated charge separation and simultaneously improve the performance of photocatalytic degradation of pesticide contaminants in the environment is a feasible strategy.With triazine herbicides as the research target,this paper focuses on the classical photocatalytic materials of inorganic semiconductors（TiO₂）and organic semiconductors（chlorophyll,carbon dots）to obtain highly active photocatalysts by constructing heterojunctions,exploring the effects of the construction of heterojunctions on the charge separation process,the activity of photocatalytic degradation of organic pesticides and the degradation mechanism,and providing references for further construction of new photocatalytic systems for efficient photocatalytic degradation of environmental organic pesticides to better solve environmental problems.The thesis work is focused on two main areas:Aiming at the key scientific problems of pyropheophorbide-a photocatalysts such as easy recombination of photogenerated charges,poor stability,lack of oxygen activation centers to generate ROS,insufficient oxygen activation capacity,and unclear degradation mechanism of atrazine,Ppa/（001）TiO₂ nanosheets were constructed by carbonyl bonding one-step self-assembly method for the study of visible light degradation of atrazine.The results showed that the Ppa/（001）TiO₂ heterostructured nanocatalysts exhibited higher performance in photocatalytic degradation of atrazine,41 times higher than that of pure Ppa.The results of photophysical and photochemical tests showed that（001）TiO₂platform can maintain the thermodynamic lifetime of photogenerated electrons,and its surface rich in F residues can effectively promote surface oxygen adsorption,effectively using photogenerated electrons to activate oxygen to form·O₂^-radicals,which in turn promotes charge separation,thus enhancing the activity of photocatalytic degradation of atrazine,and finally achieving a dual radical attack pathway for the synergistic degradation of atrazine by·O₂^-and·OH.By using liquid chromatography-tandem mass spectrometry,radical trapping experiment and EPR test,etc.,the active groups and intermediate products were analyzed,and it was concluded that·O₂^-radical played a key role in the initial photocatalytic degradation,and the·OH radicals achieved further mineralization of atrazine,and the two synergistically improved the catalytic degradation activity of visible light.To address the key scientific issues such as poor charge separation,poor oxygen activation and unclear mechanism pattern of degrading triazine herbicides in N-doped TiO₂（NT）photocatalysts,carbon quantum dot and N-TiO₂ heterojunction composites（CQD/NT）with high visible photocatalytic activity were prepared by a simple hydrothermal and calcination method.On this basis,H₃PO₄-CQD/NT nanocomposite photocatalysts were constructed by modifying a certain proportion of phosphoric acid groups onto the surface of CQD/NT by wet chemical and calcination methods for the visible light-catalyzed degradation of triazines（Ametryn,Metribuzin and atrazine）.The photocatalytic results showed that heterojunctions constructed with carbon dots and the appropriate ratio of phosphoric acid modified H₃PO₄-CQD/NT nanocomposite photocatalysts exhibited higher photocatalytic degradation of organic pesticides,approximately 6 times that of NT and 15 times that of 0.75 CQD/NT,respectively.Using photophysical and photochemical tests,the mechanism of the improved performance of the H₃PO₄-CQD/NT nanocomposite photocatalyst was revealed,that is,the heterojunction constructed by carbon quantum dots and NT effectively promoted charge separation,while the modification of phosphoric acid,promoted oxygen adsorption and oxygen activation,generated·O₂^-radicals and formed a·OH and·O₂^-double radical synergistic attack degradation pathway.A detailed analysis and comparison of the degradation mechanisms and pathways of three triazine ring herbicides by liquid chromatography-tandem mass spectrometry and free radical capture experiments revealed that·O₂^-and·OH synergistic degradation attacked the triazine ring selectively,with·O₂^-preferring to attack alkyl groups first and·OH preferring to attack groups with lone pairs of electrons（thiomethyl or chlorine）first,and the synergistic action of the two effectively improved the performance of photocatalytic degradation of organic pesticides.